Cytostatic effects of 3,3'-diindolylmethane in human endometrial cancer cells result from an estrogen receptor-mediated increase in transforming growth factor-alpha expression

3,3'-Diindolylmethane (DIM), a major in vivo product of indole-3-carbinol (I3C), is a promising anticancer agent derived from vegetables of the Brassica genus including broccoli, Brussels sprouts and cabbage. We report here that DIM has a potent cytostatic effect in cultured human Ishikawa endometrial cancer cells. A combination of northern blot and quantitative PCR analyses revealed that DIM induced the level of TGF-alpha transcripts by approximately 4-fold within 24 h of indole treatment. DIM also induced a 4-fold increase in the activity of the estrogen response marker, alkaline phosphatase (AP). Co-treatment of cells with the estrogen receptor (ER) antagonist ICI, or with the inhibitor of PKA-mediated activation of the ER, H89, ablated the DIM induction of both TGF-alpha expression and AP activity. Furthermore, DIM increased the maximum stimulatory effect of estrogen on TGF-alpha expression. Co-treatment with the protein synthesis inhibitor, cycloheximide, abolished the inductive effects of DIM, indicating differences in the mechanistic requirements of DIM and estrogen. DIM treatment also stimulated levels of secreted TGF-alpha protein by >10-fold. The ectopic addition of TGF-alpha inhibited the growth of Ishikawa cells, whereas incubation with a TGF-alpha antibody partially reversed the growth inhibitory effects of DIM. Taken together, these results extend our previous findings of the ligand independent estrogen receptor agonist activity of DIM, and uncover an essential role for the stimulation in TGF-alpha expression and the TGF-alpha activated signal transduction pathway in the potent cytostatic effects of DIM in endometrial cancer cells.

ENaC- and CFTR-dependent ion and fluid transport in mammary epithelia

Mammary epithelial 31EG4 cells (MEC) were grown as monolayers on filters to analyze the apical membrane mechanisms that help mediate ion and fluid transport across the epithelium. RT-PCR showed the presence of cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial Na(+) channel (ENaC) message, and immunomicroscopy showed apical membrane staining for both proteins. CFTR was also localized to the apical membrane of native human mammary duct epithelium. In control conditions, mean values of transepithelial potential (apical-side negative) and resistance (R(T)) are -5.9 mV and 829 Omega x cm(2), respectively. The apical membrane potential (V(A)) is -40.7 mV, and the mean ratio of apical to basolateral membrane resistance (R(A)/R(B)) is 2.8. Apical amiloride hyperpolarized V(A) by 19.7 mV and tripled R(A)/R(B). A cAMP-elevating cocktail depolarized V(A) by 17.6 mV, decreased R(A)/R(B) by 60%, increased short-circuit current by 6 microA/cm(2), decreased R(T) by 155 Omega x cm(2), and largely eliminated responses to amiloride. Whole cell patch-clamp measurements demonstrated amiloride-inhibited Na(+) currents [linear current-voltage (I-V) relation] and forskolin-stimulated Cl(-) currents (linear I-V relation). A capacitance probe method showed that in the control state, MEC monolayers either absorbed or secreted fluid (2--4 microl x cm(-2) x h(-1)). Fluid secretion was stimulated either by activating CFTR (cAMP) or blocking ENaC (amiloride). These data plus equivalent circuit analysis showed that 1) fluid absorption across MEC is mediated by Na(+) transport via apical membrane ENaC, and fluid secretion is mediated, in part, by Cl(-) transport via apical CFTR; 2) in both cases, appropriate counterions move through tight junctions to maintain electroneutrality; and 3) interactions among CFTR, ENaC, and tight junctions allow MEC to either absorb or secrete fluid and, in situ, may help control luminal [Na(+)] and [Cl(-)].

Indole-3-carbinol inhibits CDK6 expression in human MCF-7 breast cancer cells by disrupting Sp1 transcription factor interactions with a composite element in the CDK6 gene promoter

Indole-3-carbinol (I3C), a compound naturally occurring in Brassica vegetables, can induce a G(1) cell cycle arrest of human MCF-7 breast cancer cells that is accompanied by the selective inhibition of cyclin-dependent kinase 6 (CDK6) expression. Reverse transcriptase-polymerase chain reaction analysis of CDK6 mRNA decay rates revealed that I3C had no effect on CDK6 transcript stability. We report the first identification and functional characterization of the CDK6 promoter in order to determine whether I3C inhibits CDK6 transcription. In MCF-7 cells stably transfected with CDK6 promoter-linked luciferase reporter plasmids, I3C inhibited CDK6 promoter activity in an I3C-specific response that was not a consequence of the growth-arrested state of the cells. Deletion analysis revealed a 167-base pair I3C-responsive region of the CDK6 promoter between -805 and -638. Site-specific mutations within this region revealed that both Sp1 and Ets-like sites, which are spaced 5 base pairs apart, were necessary for I3C responsiveness in the context of the CDK6 promoter. Electrophoretic mobility shift analysis of protein-DNA complexes formed with nuclear proteins isolated from I3C-treated and -untreated cells, in combination with supershift assays using Sp1 antibodies, demonstrated that the Sp1-binding site in the CDK6 promoter forms a specific I3C-responsive DNA-protein complex that contains the Sp1 transcription factor. Taken together, our results suggest that I3C down-regulates CDK6 transcription by targeting Sp1 at a composite DNA site in the CDK6 promoter.

Tissue-specific expression of the transcriptionally regulated serum and glucocorticoid-inducible protein kinase (Sgk) during mouse embryogenesis

In situ hybridization of mouse embryo whole mounts and sagittal sections revealed a tissue- and stage-specific expression pattern of the transcriptionally regulated serum and glucocorticoid-inducible protein kinase (sgk) during embryogenesis. Sgk expression is first observed at embryonic day 8.5 (E8.5) in the decidua and yolk sac, and then during developmental stages E9.5 through E12.5 this kinase is highly localized in the heart chamber, otic vesicle, blood vessels surrounding the somites, and lung buds. At the later stages of mouse embryogenesis, E13.5 through E16.5, sgk expression becomes highly concentrated in brain (choroid plexus), distal epithelium and the terminal bronchi/bronchioles, adrenal gland, liver, thymus and intestines, remains high in heart tissue, and is expressed at a low level in the other embryonic tissues.

Aldosterone induces rapid apical translocation of ENaC in early portion of renal collecting system: possible role of SGK

Aldosterone controls sodium reabsorption and potassium secretion in the aldosterone-sensitive distal nephron (ASDN). Although clearance measurements have shown that aldosterone induces these transports within 30--60 min, no early effects have been demonstrated in vivo at the level of the apical epithelial sodium channel (ENaC), the main effector of this regulation. Here we show by real-time RT-PCR and immunofluorescence that an aldosterone injection in adrenalectomized rats induces alpha-ENaC subunit expression along the entire ASDN within 2 h, whereas beta- and gamma-ENaC are constitutively expressed. In the proximal ASDN portions only, ENaC is shifted toward the apical cellular pole and the apical plasma membrane within 2 and 4 h, respectively. To address the question of whether the early aldosterone-induced serum and glucocorticoid-regulated kinase (SGK) might mediate this apical shift of ENaC, we analyzed SGK induction in vivo. Two hours after aldosterone, SGK was highly induced in all segment-specific cells of the ASDN, and its level decreased thereafter. In Xenopus laevis oocytes, SGK induced ENaC activation and surface expression by a kinase activity-dependent mechanism. In conclusion, the rapid in vivo accumulation of SGK and alpha-ENaC after aldosterone injection takes place along the entire ASDN, whereas the translocation of alpha,beta,gamma-ENaC to the apical plasma membrane is restricted to its proximal portions. Results from oocyte experiments suggest the hypothesis that a localized activation of SGK may play a role in the mediation of ENaC translocation.

SGK integrates insulin and mineralocorticoid regulation of epithelial sodium transport

The epithelial Na+ channel (ENaC) constitutes the rate-limiting step for Na+ transport across tight epithelia and is the principal target of hormonal regulation, particularly by insulin and mineralocorticoids. Recently, the serine-threonine kinase (SGK) was identified as a rapidly mineralocorticoid-responsive gene, the product of which stimulates ENaC-mediated Na+ transport. Like its close relative, protein kinase B (also called Akt), SGK's kinase activity is dependent on phosphatidylinositol 3-kinase (PI3K), a key mediator of insulin signaling. In our study we show that PI3K is required for SGK-dependent stimulation of ENaC-mediated Na+ transport as well as for the production of the phosphorylated form of SGK. In A6 kidney cells, mineralocorticoid induction of the phosphorylated form of SGK preceded the increase in Na+ transport, and specific inhibition of PI3K inhibited both phosphorylation of SGK and mineralocorticoid-induced Na+ transport. Insulin both augmented SGK phosphorylation and synergized with mineralocorticoids in stimulating Na+ transport. In a Xenopus laevis oocyte coexpression assay, SGK-stimulated ENaC activity was also markedly reduced by PI3K inhibition. Finally, in vitro-translated SGK specifically interacted with the ENaC subunits expressed in Escherichia coli as glutathione S-transferase fusion proteins. These data suggest that SGK is a PI3K-dependent integrator of insulin and mineralocorticoid actions that interacts with ENaC subunits to control Na+ entry into kidney collecting duct cells.

Involvement of the helix-loop-helix protein Id-1 in the glucocorticoid regulation of tight junctions in mammary epithelial cells

Mammary epithelial cell-cell junctions undergo morphological and structural differentiation during pregnancy and lactation, but little is known about the transcriptional regulators that are involved in this process. In Con8 mammary epithelial tumor cells, we have previously documented that the synthetic glucocorticoid, dexamethasone, induces the reorganization of the tight junction and adherens junction and stimulates the monolayer transepithelial electrical resistance (TER), a reliable in vitro measurement of tight junction sealing. Western blots demonstrated that dexamethasone treatment rapidly and strongly stimulated the level of the Id-1 protein, which is a serum-inducible helix-loop-helix transcriptional repressor. The steroid induction of Id-1 was robust by 4 h of treatment and maintained over a 24-h period. Isopropyl-1-thio-beta-d-galactopyranoside-inducible expression of exogenous Id-1 in Con8 cells was shown to strongly facilitate the dexamethasone induction of TER in the absence of serum without altering the dexamethasone-dependent reorganization of ZO-1, beta-catenin, or F-actin. Ectopic overexpression of Id-1 in the SCp2 nontumorigenic mammary epithelial cells, which does not undergo complete dexamethasone-dependent tight junction reorganization, enhanced the dexamethasone-induced ZO-1 tight junction localization and stimulated the monolayer TER. Moreover, antisense reduction of Id-1 protein in SCp2 cells prevented the apical junction reorganization and dexamethasone-stimulated TER. Our results implicate Id-1 as acting as a critical regulator of mammary epithelial cell-cell interactions at an early step in the glucocorticoid-dependent signaling pathway that controls tight junction integrity.

Hyperosmotic stress stimulates promoter activity and regulates cellular utilization of the serum- and glucocorticoid-inducible protein kinase (Sgk) by a p38 MAPK-dependent pathway

We have established that the serum- and glucocorticoid-inducible protein kinase (Sgk) is a new component of the hyperosmotic stress response. Treatment of NMuMg mammary epithelial cells with the organic osmolyte, sorbitol, caused the stable accumulation of Sgk transcripts and protein after an approximately 4-h lag. Transient transfection of a series of sgk-CAT reporter plasmids containing either 5' deletions or continuous 6-base pair substitutions identified a hyperosmotic stress-regulated element that is GC-rich and is necessary for the sorbitol stimulation of sgk gene promoter activity. Gel shift analysis identified four major DNA-protein complexes in the hyperosmotic stress-regulated element that, by competition with excess consensus wild type and mutant oligonucleotides and by antibody supershifts, contains the Sp1 transcription factor. Several lines of evidence suggest that the p38 MAPK signaling pathway mediates the hyperosmotic stress stimulation of sgk gene expression. Treatment with pharmacological inhibitors of p38 MAPK or with a dominant negative form of MKK3, an upstream regulator of p38 MAPK, significantly reduced or ablated the sorbitol induction of sgk promoter activity or protein production. Using an in vitro peptide transphosphorylation assay, sorbitol treatment activates either endogenous or exogenous Sgk that is localized to the cytoplasmic compartment. Thus, we propose that the stimulated expression of enzymatically active Sgk after sorbitol treatment is a newly defined component of the p38 MAPK-mediated response to hyperosmotic stress.

Follicle-Stimulating hormone (FSH) stimulates phosphorylation and activation of protein kinase B (PKB/Akt) and serum and glucocorticoid-lnduced kinase (Sgk): evidence for A kinase-independent signaling by FSH in granulosa cells

FSH stimulates in ovarian granulosa cells diverse, differentiation-dependent responses that implicate activation of specific cellular signaling cascades. In these studies three kinases were investigated to determine their relationship to FSH, cAMP, and A kinase signaling: protein kinase B (PKB/Akt), serum and glucocorticoid-induced kinase (Sgk), and p38 mitogen-activated protein kinase (p38MAPK). The phosphorylation (activation) of these kinases was analyzed by using selective agonists/inhibitors: forskolin/H89 for cAMP-dependent protein kinase (A kinase), insulin-like growth factor I (IGF-I)/LY294002 and wortmannin for phosphatidylinositol-dependent kinase (PI3-K), and phorbol myristate (PMA)/GF109203X for diacylglycerol and Ca++-dependent kinases (C kinases). An inhibitor (PD98059) of MEK1, which regulates extracellular regulated kinases (ERKs), and SB203580, which inhibits p38MAPK, were also used. In addition, we analyzed the expression of the recently described, cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFI and GEFII) that impact Ras-related GTPases and Raf kinases, known regulators of various protein kinase cascades. We provide evidence that FSH, forskolin, and 8-bromo-cAMP stimulate phosphorylation of PKB by mechanisms involving PI3-K (LY294002/wortmannin sensitive) not A kinase (H89 insensitive), a pattern of response mimicking that of IGF-I. In contrast, FSH induction and phosphorylation of Sgk protein requires A kinase (H89 sensitive) but also involves PI3-K (LY294002 sensitive) as well as p38MAPK (SB203580 sensitive) pathways. PMA (C kinase) abolished FSH-mediated (but not IGF-I-mediated) phosphorylation of PKB at a step(s) upstream of PI3-K and independent of A kinase. Lastly, FSH-mediated phosphorylation of p38MAPK is negatively affected by A kinase and PI3-K, suggesting that it may be downstream of specific members of the cAMP-GEF/Rap/Raf pathway. We propose that cAMP activation of A kinase is obligatory for transcription of Sgk in granulosa cells whereas cAMP (IGF-I-like)-mediated phosphorylation (activation) of PKB and Sgk (via PI3-K), as well as p38MAPK, involves other cellular events. These results provide new and exciting evidence that cAMP acts in granulosa cells by A kinase-dependent and -independent mechanisms, each of which controls specific kinase cascades.

Ligand-independent activation of estrogen receptor function by 3, 3'-diindolylmethane in human breast cancer cells

3,3'-Diindolylmethane (DIM), a major in vivo product of acid-catalyzed oligomerization of indole-3-carbinol (I3C), is a promising anticancer agent present in vegetables of the Brassica genus. We investigated the effects of DIM on estrogen-regulated events in human breast cancer cells and found that DIM was a promoter-specific activator of estrogen receptor (ER) function in the absence of 17beta-estradiol (E(2)). DIM weakly inhibited the E(2)-induced proliferation of ER-containing MCF-7 cells and induced proliferation of these cells in the absence of steroid, by approximately 60% of the E(2) response. DIM had little effect on proliferation of ER-deficient MDA-MB-231 cells, suggesting that it is not generally toxic at these concentrations. Although DIM did not bind to the ER in this concentration range, as shown by a competitive ER binding assay, it activated the ER to a DNA-binding species. DIM increased the level of transcripts for the endogenous pS2 gene and activated the estrogen-responsive pERE-vit-CAT and pS2-tk-CAT reporter plasmids in transiently transfected MCF-7 cells. In contrast, DIM failed to activate transcription of the simple E(2)- and diethylstilbesterol-responsive reporter construct pATC2. The estrogen antagonist ICI 182780 (7alpha-[9-[(4,4,5,5, 5-pentafluoropentyl)sulfonyl]nonyl]-estra-1,3,5(10)-triene-3, 17beta-diol) was effective against DIM-induced transcriptional activity of the pERE-vit-CAT reporter, which further supports the hypothesis that DIM is acting through the ER. We demonstrated that ligand-independent activation of the ER in MCF-7 cells could be produced following treatment with the D1 dopamine receptor agonist SKF-82958 [(+/-)6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepinehydrobromide]. We also demonstrated that the agonist effects of SKF-82958 and DIM, but not of E(2), could be blocked by co-treatment with the protein kinase A (PKA) inhibitor H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide). These results have uncovered a promoter-specific, ligand-independent activation of ER signaling for DIM that may require activation by PKA, and suggest that this major I3C product may be a selective activator of ER function.

The major cyclic trimeric product of indole-3-carbinol is a strong agonist of the estrogen receptor signaling pathway

Indole-3-carbinol (I3C), a component of Brassica vegetables, is under study as a preventive agent of cancers of the breast and other organs. Following ingestion, I3C is converted to a series of oligomeric products that presumably are responsible for the in vivo effects of I3C. We report the effects of the major trimeric product, 5,6,11,12,17,18-hexahydrocyclonona[1,2-b:4,5-b':7,8-b' ']triindole (CTr), on the estrogen receptor (ER) signaling pathways. Tumor-promoting effects of high doses of I3C may be due to activation of aryl hydrocarbon receptor (AhR)-mediated pathways; therefore, we also examined the effects of CTr on AhR activated processes. We observed that CTr is a strong agonist of ER function. CTr stimulated the proliferation of estrogen-responsive MCF-7 cells to a level similar to that produced by estradiol (E(2)) but did not affect the growth of the estrogen-independent cell line, MDA-MD-231. CTr displaced E(2) in competitive-binding studies and activated ER-binding to an estrogen responsive DNA element in gel mobility shift assays with EC(50)s of about 0.1 microM. CTr activated transcription of an E(2)-responsive endogenous gene and exogenous reporter genes in transfected MCF-7 cells, also with high potency. CTr failed to activate AhR-mediated pathways, consistent with the low-binding affinity of CTr for the AhR reported previously. Comparisons of the conformational characteristics of CTr with other ER ligands indicated a remarkable similarity with tamoxifen, a selective ER antagonist used as a breast cancer therapeutic agent and suggest an excellent fit of CTr into the ligand-binding site of the ER.

Expression of human papilloma virus E7 protein causes apoptosis and inhibits DNA synthesis in primary hepatocytes via increased expression of p21(Cip-1/WAF1/MDA6)

The impact of human papilloma virus (HPV16) E7 proteins and retinoblastoma (RB) antisense oligonucleotides upon mitogen-activated protein kinase (MAPK)-mediated inhibition of DNA synthesis via p21(Cip-1/WAF1/MDA6) (p21) was determined in primary hepatocytes. Prolonged activation of the MAPK pathway in p21(+/+) or p21(-/-) hepatocytes caused a large decrease and increase, respectively, in DNA synthesis. Either transfection with RB antisense oligonucleotides, expression of wild type E7, or RB binding mutant E7 (C24S) proteins increased p21 levels and reduced DNA synthesis in p21(+/+) hepatocytes. RB antisense oligonucleotides and E7 proteins increased apoptosis in p21(+/+), but not p21(-/-), hepatocytes. Expression of wild type E7 increased DNA synthesis above control levels in p21(-/-) cells, which was additive with prolonged MAPK activation. In contrast, expression of mutant E7 did not alter DNA synthesis above control levels in p21(-/-) cells and was supra-additive with prolonged MAPK activation. Antisense ablation of RB in p21(-/-) hepatocytes had a weak stimulatory effect upon DNA synthesis itself but enhanced the capacity of mutant E7 protein to stimulate DNA synthesis to the same level observed using wild type E7. The ability of prolonged MAPK activation to stimulate DNA synthesis in the presence of mutant E7 and antisense RB was additive. Collectively, the present data demonstrate that loss of RB function together with loss of p21 function plays an important role in the E7- and MAPK-dependent modulation of apoptosis and DNA synthesis in primary hepatocytes.

Expression and localization of serum/glucocorticoid-induced kinase in the rat ovary: relation to follicular growth and differentiation

Expression of serum/glucocorticoid-inducible kinase (Sgk), one member of an inducible serine/threonine kinase family, is induced by FSH/cAMP in rat granulosa cells cultured in defined medium. The FSH-stimulated pattern of sgk expression is biphasic, and transcriptional activation of the sgk gene depends on an intact Sp1/Sp3 binding site within the proximal promoter. To determine whether sgk was expressed in a hormone-dependent and physiologically relevant manner in vivo, the cellular levels of sgk messenger RNA (mRNA) and protein as well as the subcellular localization of this kinase were analyzed in ovaries containing follicles and corpora lutea at specific stages of differentiation. To stimulate follicular development and luteinization, hypophysectomized (H) rats were treated with estradiol (E; HE) and FSH (FSH; HEF) followed by hCG (hCG; HEF/hCG). To analyze Sgk in functional corpora lutea, PRL was administered to HEF/hCG rats, or ovaries of pregnant rats were obtained on day 7, 15, or 22 of gestation. In situ hybridization indicated that sgk mRNA was low/undetectable in granulosa cells of H and HE rats. An acute injection (i.v.) of FSH to HE rats rapidly increased sgk mRNA at 2 and 8 h. Sgk mRNA was also elevated in granulosa cells of preovulatory follicles of HEF rats and in luteal cells of HEF/hCG and pregnant rats. Northern blots and Western blots confirmed the in situ hybridization data, indicating that the amount and cellular localization Sgk protein were related to that of sgk mRNA. When the subcellular localization of this kinase was analyzed by immunohistochemistry, Sgk protein was nuclear in granulosa cells and some thecal cells of large preovulatory follicles. In contrast, Sgk protein was cytoplasmic in luteal cells as well as some cells within the stromal compartment. Intense immunostaining was also observed in oocytes present in primordial follicles, but not in growing follicles. Collectively, these results show that FSH and LH stimulate marked increases in the cellular content of Sgk, as well as dramatic changes in the subcellular distribution of this kinase. The specific nuclear vs. cytoplasmic compartmentalization of Sgk in granulosa cells and luteal cells, respectively, indicates that Sgk controls distinct functions in proliferative vs. terminally differentiated granulosa cells.

Requirement for Ras and phosphatidylinositol 3-kinase signaling uncouples the glucocorticoid-induced junctional organization and transepithelial electrical resistance in mammary tumor cells

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates the remodeling of the apical junction (tight and adherens junctions) and the transepithelial electrical resistance (TER), which reflects tight junction sealing. Indirect immunofluorescence revealed that dexamethasone induced the recruitment of endogenous Ras and the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase to regions of cell-cell contact, concurrently with the stimulation of TER. Expression of dominant-negative RasN17 abolished the dexamethasone stimulation in TER, whereas, dexamethasone induced the reorganization of tight junction and adherens junction proteins, ZO-1 and beta-catenin, as well as F-actin, to precise regions of cell-cell contact in a Ras-independent manner. Confocal microscopy revealed that RasN17 and the p85 regulatory subunit of PI 3-kinase co-localized with ZO-1 and F-actin at the tight junction and adherens junction, respectively. Treatment with either of the PI 3-kinase inhibitors, wortmannin or LY294002, or the MEK inhibitor PD 098059, which prevents MAPK signaling, attenuated the dexamethasone stimulation of TER without affecting apical junction remodeling. Similar to dominant-negative RasN17, disruption of both Ras effector pathways using a combination of inhibitors abolished the glucocorticoid stimulation of TER. Thus, the glucocorticoiddependent remodeling of the apical junction and tight junction sealing can be uncoupled by their dependence on Ras and/or PI 3-kinase-dependent pathways, implicating a new role for Ras and PI 3-kinase cell signaling events in the steroid control of cell-cell interactions.

Functional and subcellular changes in the A-kinase-signaling pathway: relation to aromatase and Sgk expression during the transition of granulosa cells to luteal cells

The responsiveness of granulosa cells to FSH (cAMP) changes as these cells switch from the proliferative stage in growing follicles to the terminally differentiated, nonproliferating stage after LH-induced luteinization. To analyze this transition, two well characterized culture systems were used. 1) Granulosa cells isolated from immature rats were cultured in serum-free medium, a system that permits analysis of dynamic, short-term responses to hormones/cAMP. 2) Granulosa cells from preovulatory (PO) follicles that have been exposed in vivo to surge concentrations of hCG (PO/ hCG) were cultured in medium containing 1% FBS, a system that permits analyses of cells that have undergo irreversible, long-term changes associated with luteinization. To analyze the biochemical basis for the switch in cAMP responsiveness, the localization of A-kinase pathway components was related to the expression of two cAMP target genes, aromatase (CYP19) and serum-and glucocorticoid-induced kinase (Sgk). Components of the A-kinase pathway were analyzed by Western blotting and indirect immunofluorescence using specific antibodies to the C subunit, RIIalpha/beta subunits, CREB (cAMP-regulatory element binding protein), phospho-CREB, CBP (CREB binding protein), and Sgk. Cellular levels of C subunit and CREB were similar in all cell types and hormone treatments. CREB and CBP were nuclear; RIIalpha/beta was restricted to a cytoplasmic basket-like structure. Addition of FSH to immature granulosa cells caused rapid nuclear import of C subunit within 1 h. Nuclear C subunit decreased by 6 h after FSH but could be rapidly reimported to the nucleus by the addition of forskolin at 6, 24, or 48 h. Nuclear C subunit was associated with the rapid but transient increases in phospho-CREB. FSH induced Sgk in a biphasic manner in which the protein was nuclear at 1 h and cytoplasmic at 48 h. Aromatase mRNA was only expressed at 24-48 h after FSH, a pattern that was not altered by phosphodiesterases or phosphatases. In the luteinized (PO/hCG) granulosa cells, immunoreactive C subunit was localized in a punctate pattern in the nucleus as well as to a cytoplasmic basket-like structure, a distribution pattern not altered by forskolin. Aromatase, Sgk, and phospho-CREB were expressed at elevated levels in a non-forskolin-responsive manner. Most notable, both phospho-CREB and Sgk were preferentially localized in a punctate pattern within the cytoplasm and not altered by forskolin. Collectively, these data indicate that when granulosa cells differentiate to luteal cells the subcellular localization (nuclear vs. cytoplasmic) of A-kinase pathway components changes markedly. Thus, either the mechanisms of nuclear import and export or the presence of distinct docking sites (and functions ?) dictate where A-kinase, phospho-CREB and Sgk are localized in granulosa cells compared with the terminally differentiated luteal cells.

Serum and glucocorticoid-inducible kinase (SGK) is a target of the PI 3-kinase-stimulated signaling pathway

Serum and glucocorticoid-inducible kinase (SGK) is a novel member of the serine/threonine protein kinase family that is transcriptionally regulated. In this study, we have investigated the regulatory mechanisms that control SGK activity. We have established a peptide kinase assay for SGK and present evidence demonstrating that SGK is a component of the phosphoinositide 3 (PI 3)-kinase signaling pathway. Treatment of human embryo kidney 293 cells with insulin, IGF-1 or pervanadate induced a 3- to 12-fold activation of ectopically expressed SGK. Activation was completely abolished by pretreatment of cells with the PI 3-kinase inhibitor, LY294002. Treatment of activated SGK with protein phosphatase 2A in vitro led to kinase inactivation. Consistent with the similarity of SGK to other second-messenger regulated kinases, mutation of putative phosphorylation sites at Thr256 and Ser422 inhibited SGK activation. Cotransfection of PDK1 with SGK caused a 6-fold activation of SGK activity, whereas kinase-dead PDK1 caused no activation. GST-pulldown assays revealed a direct interaction between PDK1 and the catalytic domain of SGK. Treatment of rat mammary tumor cells with serum caused hyperphosphorylation of endogenous SGK, and promoted translocation to the nucleus. Both hyperphosphorylation and nuclear translocation could be inhibited by wortmannin, but not by rapamycin.

Indole-3-carbinol and tamoxifen cooperate to arrest the cell cycle of MCF-7 human breast cancer cells

The current options for treating breast cancer are limited to excision surgery, general chemotherapy, radiation therapy, and, in a minority of breast cancers that rely on estrogen for their growth, antiestrogen therapy. The naturally occurring chemical indole-3-carbinol (I3C), found in vegetables of the Brassica genus, is a promising anticancer agent that we have shown previously to induce a G1 cell cycle arrest of human breast cancer cell lines, independent of estrogen receptor signaling. Combinations of I3C and the antiestrogen tamoxifen cooperate to inhibit the growth of the estrogen-dependent human MCF-7 breast cancer cell line more effectively than either agent alone. This more stringent growth arrest was demonstrated by a decrease in adherent and anchorage-independent growth, reduced DNA synthesis, and a shift into the G1 phase of the cell cycle. A combination of I3C and tamoxifen also caused a more pronounced decrease in cyclin-dependent kinase (CDK) 2-specific enzymatic activity than either compound alone but had no effect on CDK2 protein expression. Importantly, treatment with I3C and tamoxifen ablated expression of the phosphorylated retinoblastoma protein (Rb), an endogenous substrate for the G1 CDKs, whereas either agent alone only partially inhibited endogenous Rb phosphorylation. Several lines of evidence suggest that I3C works through a mechanism distinct from tamoxifen. I3C failed to compete with estrogen for estrogen receptor binding, and it specifically down-regulated the expression of CDK6. These results demonstrate that I3C and tamoxifen work through different signal transduction pathways to suppress the growth of human breast cancer cells and may, therefore, represent a potential combinatorial therapy for estrogen-responsive breast cancer.

Cell cycle and hormonal control of nuclear-cytoplasmic localization of the serum- and glucocorticoid-inducible protein kinase, Sgk, in mammary tumor cells. A novel convergence point of anti-proliferative and proliferative cell signaling pathways

The serum- and glucocorticoid-inducible kinase (sgk) is a novel serine/threonine protein kinase that is transcriptionally regulated in rat mammary tumor cells by serum under proliferative conditions or by glucocorticoids that induce a G1 cell cycle arrest. Our results establish that the subcellular distribution of Sgk is under stringent cell cycle and hormonal control. Sgk is localized to the perinuclear or cytoplasmic compartment as a 50-kDa hypophosphorylated protein in cells arrested in G1 by treatment with the synthetic glucocorticoid dexamethasone. In serum-stimulated cells, Sgk was transiently hyperphosphorylated and resided in the nucleus. Laser scanning cytometry, which monitors Sgk localization and DNA content in individual mammary tumor cells of an asynchronously growing population, revealed that Sgk actively shuttles between the nucleus (in S and G2/M) and the cytoplasm (in G1) in synchrony with the cell cycle. In cells synchronously released from the G1/S boundary, Sgk localized to the nucleus during progression through S phase. The forced retention of exogenous Sgk in either the cytoplasmic compartment, using a wild type sgk gene, or the nucleus, using a nuclear localization signal-containing sgk gene (NLS-Sgk), suppressed the growth and DNA synthesis of serum-stimulated cells. Thus, our study implicates the nuclear-cytoplasmic shuttling of sgk as a requirement for cell cycle progression and represents a novel convergence point of anti-proliferative and proliferative signaling in mammary tumor cells.

Epithelial sodium channel regulated by aldosterone-induced protein sgk

Sodium homeostasis in terrestrial and freshwater vertebrates is controlled by the corticosteroid hormones, principally aldosterone, which stimulate electrogenic Na+ absorption in tight epithelia. Although aldosterone is known to increase apical membrane Na+ permeability in target cells through changes in gene transcription, the mechanistic basis of this effect remains poorly understood. The predominant early effect of aldosterone is to increase the activity of the epithelial sodium channel (ENaC), although ENaC mRNA and protein levels do not change initially. Rather, the open probability and/or number of channels in the apical membrane are greatly increased by unknown modulators. To identify hormone-stimulated gene products that modulate ENaC activity, a subtracted cDNA library was generated from A6 cells, a stable cell line of renal distal nephron origin, and the effect of candidates on ENaC activity was tested in a coexpression assay. We report here the identification of sgk (serum and glucocorticoid-regulated kinase), a member of the serine-threonine kinase family, as an aldosterone-induced regulator of ENaC activity. sgk mRNA and protein were strongly and rapidly hormone stimulated both in A6 cells and in rat kidney. Furthermore, sgk stimulated ENaC activity approximately 7-fold when they were coexpressed in Xenopus laevis oocytes. These data suggest that sgk plays a central role in aldosterone regulation of Na+ absorption and thus in the control of extracellular fluid volume, blood pressure, and sodium homeostasis.

Glucocorticoid down-regulation of fascin protein expression is required for the steroid-induced formation of tight junctions and cell-cell interactions in rat mammary epithelial tumor cells

Glucocorticoid hormones, which are physiological regulators of mammary epithelium development, induce the formation of tight junctions in rat Con8 mammary epithelial tumor cells. We have discovered that, as part of this process, the synthetic glucocorticoid dexamethasone strongly and reversibly down-regulated the expression of fascin, an actin-bundling protein that also interacts with the adherens junction component beta-catenin. Ectopic constitutive expression of full-length mouse fascin containing a Myc epitope tag (Myc-fascin) in Con8 cells inhibited the dexamethasone stimulation of transepithelial electrical resistance, disrupted the induced localization of the tight junction protein occludin and the adherens junction protein beta-catenin to the cell periphery, and prevented the rearrangement of the actin cytoskeleton. Ectopic expression of either the carboxyl-terminal 213 amino acids of fascin, which includes the actin and beta-catenin-binding sites, or the amino-terminal 313 amino acids of fascin failed to disrupt the glucocorticoid induction of tight junction formation. Mammary tumor cells expressing the full-length Myc-fascin remained generally glucocorticoid responsive and displayed no changes in the levels or protein-protein interactions of junctional proteins or the amount of cytoskeletal associated actin filaments. However, a cell aggregation assay demonstrated that the expression of Myc-fascin abrogated the dexamethasone induction of cell-cell adhesion. Our results implicate the down-regulation of fascin as a key intermediate step that directly links glucocorticoid receptor signaling to the coordinate control of junctional complex formation and cell-cell interactions in mammary tumor epithelial cells.

Dysfunctional glucocorticoid receptor with a single point mutation ablates the CCAAT/enhancer binding protein-dependent growth suppression response in a steroid-resistant rat hepatoma cell variant

We used glucocorticoid-resistant and -sensitive hepatoma cell variants to characterize the mechanism of hepatoma cell resistance to the growth inhibitory effects of glucocorticoids. BDS1 hepatoma cells express transcriptionally active glucocorticoid receptors and undergo a stringent G1 cell cycle arrest in response to glucocorticoids that is dependent on the induced expression of the CCAAT/enhancer binding protein alpha (C/EBPalpha) transcription factor. In contrast, EDR1 hepatoma cells, which express normal levels of glucocorticoid receptors, fail to growth arrest or express C/EBPalpha when treated with glucocorticoids. Ectopic expression of wild-type rat glucocorticoid receptors into EDR1 cells restored the growth suppression response, suggesting a defect in the EDR1 receptor. DNA sequence analysis revealed a single point mutation causing a cysteine-to-tyrosine substitution at amino acid position 457 (C457Y-GR) in the zinc finger region of the glucocorticoid receptor that mediates both receptor-DNA and receptor-protein interactions. Glucocorticoid activation of the alpha1-acid glycoprotein (AGP) promoter, a liver acute-phase response gene, requires receptor-DNA binding as well as an interaction with C/EBPalpha. In contrast to the wild-type glucocorticoid receptor, ectopic expression of C/EBPalpha in EDR1 cells, or coexpression of C/EBPalpha along with the C457Y-GR into receptor-deficient EDR3 cells was required to partially restore glucocorticoid responsiveness of the AGP promoter by the EDR1 glucocorticoid receptor. Constitutive expression of the wild-type glucocorticoid receptor, but not the C457Y-GR mutant, was sufficient to restore the glucocorticoid growth suppression response to receptor-deficient EDR3 cells. Thus, we have identified a glucocorticoid-resistant hepatoma cell variant with a single point mutation in the zinc finger region of the glucocorticoid receptor gene that ablates the glucocorticoid growth suppression response and attenuates transcriptional activation of the AGP promoter.

Indole-3-carbinol inhibits the expression of cyclin-dependent kinase-6 and induces a G1 cell cycle arrest of human breast cancer cells independent of estrogen receptor signaling

Indole-3-carbinol (I3C), a naturally occurring component of Brassica vegetables such as cabbage, broccoli, and Brussels sprouts, has been shown to reduce the incidence of spontaneous and carcinogen-induced mammary tumors. Treatment of cultured human MCF7 breast cancer cells with I3C reversibly suppresses the incorporation of [3H]thymidine without affecting cell viability or estrogen receptor (ER) responsiveness. Flow cytometry of propidium iodide-stained cells revealed that I3C induces a G1 cell cycle arrest. Concurrent with the I3C-induced growth inhibition, Northern blot and Western blot analyses demonstrated that I3C selectively abolished the expression of cyclin-dependent kinase 6 (CDK6) in a dose- and time-dependent manner. Furthermore, I3C inhibited the endogenous retinoblastoma protein phosphorylation and CDK6 phosphorylation of retinoblastoma in vitro to the same extent. After the MCF7 cells reached their maximal growth arrest, the levels of the p21 and p27 CDK inhibitors increased by 50%. The antiestrogen tamoxifen also suppressed MCF7 cell DNA synthesis but had no effect on CDK6 expression, while a combination of I3C and tamoxifen inhibited MCF7 cell growth more stringently than either agent alone. The I3C-mediated cell cycle arrest and repression of CDK6 production were also observed in estrogen receptor-deficient MDA-MB-231 human breast cancer cells, which demonstrates that this indole can suppress the growth of mammary tumor cells independent of estrogen receptor signaling. Thus, our observations have uncovered a previously undefined antiproliferative pathway for I3C that implicates CDK6 as a target for cell cycle control in human breast cancer cells. Moreover, our results establish for the first time that CDK6 gene expression can be inhibited in response to an extracellular antiproliferative signal.

Role of the CCAAT/enhancer binding protein-alpha transcription factor in the glucocorticoid stimulation of p21waf1/cip1 gene promoter activity in growth-arrested rat hepatoma cells

The preceding paper (Cha, H. H., Cram, E. J., Wang, E. C., Huang, A. J., Kasler, H. G., and Firestone, G. L. (1998) J. Biol. Chem. 273, 0000-0000(478563) defined a glucocorticoid responsive region within teh promoter of the p21 CDK inhibitor gene that contains a putative DNA-binding site for the transcription factor CCAAT/ enhancer binding protein-alpha (C/EBP alpha). Wild type rat BDS1 hepatoma cells as well as as4 hepatoma cells, which express antisense sequences to C/EBP alpha and ablate its protein production, were utilized to investigate the role of this transcription factor in the glucocorticoid regulation of p21 gene expression. The stimulation of p21 protein levels and promoter activity, as well as inhibition of CDK2-mediated retinoblastoma protein phosphorylation, by the synthetic glucocorticoid, dexamethasone, required the expression of C/EBP alpha. Overexpression of C/EBP alpha in as4 cells rescued the dexamethasone responsiveness of the p21 promoter. Site-directed mutagenesis of the p21 promoter revealed that dexamethasone stimulation of p21 promoter activity required the C/EBP consensus DNA-binding site. Furthermore, in glucocorticoid receptor-defective EDR1 hepatoma cells, dexamethasone failed to stimulate C/EBP alpha and p21 protein expression and promoter activities. Our results have established a functional link between the glucocorticoid receptor signaling pathway that mediates a G1 cell cycle arrest of rat hepatoma cells and the transcriptional control of p21 by a cascade that requires the steroid induction of C/EBP alpha gene expression.

Glucocorticoids stimulate p21 gene expression by targeting multiple transcriptional elements within a steroid responsive region of the p21waf1/cip1 promoter in rat hepatoma cells

Glucocorticoids can induce a G1 arrest in the cell cycle progression of BDS1 rat hepatoma cells. In these cells, dexamethasone, a synthetic glucocorticoid, stimulated a rapid and selective increase in expression of the p21 cyclin-dependent kinase (CDK) inhibitor mRNA and protein and virtually abolished CDK2 phosphorylation of the retinoblastoma protein. Expression of the p27 CDK inhibitor, and other G1-acting cell cycle proteins, remained unaffected. Dexamethasone stimulated p21 promoter activity in a p53-independent manner that required functional glucocorticoid receptors. Transforming growth factor-beta, which also induced a G1 cell cycle arrest of the hepatoma cells, failed to elicit this response. Analysis of 5' deletions of the p21 promoter uncovered a glucocorticoid responsive region between nucleotides -1481 and -1184, which does not contain a canonical glucocorticoid response element but which can confer dexamethasone responsiveness to a heterologous promoter. Fine mapping of this region uncovered three distinct 50-60-base pair transcriptional elements that likely function as targets of glucocorticoid receptor signaling. Finally, ectopic expression of p21 had no effect on hepatoma cell growth in the absence of glucocorticoids but facilitated the ability of dexamethasone to inhibit cell proliferation. Thus, our results have established a direct transcriptional link between glucocorticoid receptor signaling and the regulated promoter activity of a CDK inhibitor gene that is involved in the cell cycle arrest of hepatoma cells.

Follicle stimulating hormone-regulated expression of serum/glucocorticoid-inducible kinase in rat ovarian granulosa cells: a functional role for the Sp1 family in promoter activity

Recently, a family of novel, serine/threonine protein kinases has been identified. One of these transcriptionally inducible, immediate-early genes encodes serum/glucocorticoid inducible-protein kinase, sgk. By in situ hybridization, we show that sgk expression in the rat ovary is selectively localized to granulosa cells. In culture, FSH or forskolin, activators of the protein kinase A (PKA) pathway, rapidly (2 h) and transiently increased sgk mRNA levels in undifferentiated granulosa cells. Sgk mRNA exhibited a biphasic expression pattern, with maximal levels observed at 48 h of FSH/forskolin as granulosa cells differentiate to the preovulatory phenotype. Deletion analyses using sgk promoter-reporter constructs (-4.0 kb to -35 bp) identified a region between -63 and -43 bp that mediated FSH and forskolin-responsive transcription in undifferentiated and differentiated granulosa cells. This G/C-rich region 1) conferred both basal and inducible transcription to the minimal -35 sgk promoter chloramphenicol acetyltransferase reporter construct, 2) specifically bound Sp1 and Sp3 present in granulosa cell extracts, and 3) bound recombinant Sp1. Mutation of 2 bp in this region not only prevented Sp1 and Sp3 binding, but also abolished the PKA-mediated transactivation observed when using the wild type construct. Sp1 and Sp3 DNA-binding activity and protein levels did not change significantly during sgk induction. Collectively, these data indicate that Sp1/Sp3 transactivation of the sgk promoter likely involves regulated, phosphorylation-dependent interaction with other factors. Thus the novel, biphasic induction of sgk that correlates with granulosa cell progression from proliferation to differentiation appears to involve sequential, coordinated actions of FSH, PKA, and transcription factors, including Sp1 and Sp3.

Repression of glucocorticoid receptor transactivation and DNA binding of a glucocorticoid response element within the serum/glucocorticoid-inducible protein kinase (sgk) gene promoter by the p53 tumor suppressor protein

sgk is a novel member of the serine/threonine protein kinase family that is transcriptionally regulated by serum and glucocorticoids in Rat2 fibroblasts and in mammary epithelial cells. 5'-Deletion analysis of the sgk promoter, using a series of sgk-CAT. (chloramphenicol acetyltransferase) chimeric reporter gene plasmids, defined a glucocorticoid-responsive region that contains a glucocorticoid response element (sgkGRE) between -1000 and -975 bp. The sgkGRE is specifically bound by glucocorticoid receptors and is sufficient to confer glucocorticoid responsiveness to a heterologous promoter in several cell lines. Strikingly, cotransfection of either the murine or human wild type p53, but not a mutant p53, repressed the dexamethasone-stimulated transactivation of reporter plasmids containing either the sgkGRE or a consensus GRE. Gel shift analysis revealed that in vitro synthesized p53 prevented binding of the glucocorticoid receptor both to the sgkGRE as well as to a consensus GRE. The p53-mediated repression of dexamethasone-induced sgkGRE activity required both the DNA binding and transactivation functions of the p53 protein. Activation of endogenous p53, by exposure to UV light, repressed the glucocorticoid receptor transactivation of a consensus GRE-CAT reporter plasmid in transfected cells. Conversely, activated glucocorticoid receptors suppressed the transactivation function of p53, while transrepression by p53 was largely unaffected. The presented data demonstrate that sgk is a primary glucocorticoid-responsive protein kinase gene that implicates a new pathway of cross-talk between steroid receptor signaling and cellular phosphorylation cascades. In addition, our study provides the first evidence of mutual interference of transactivation functions of p53 and the glucocorticoid receptor, possibly through their direct interaction.

Glucocorticoid-stimulated CCAAT/enhancer-binding protein alpha expression is required for steroid-induced G1 cell cycle arrest of minimal-deviation rat hepatoma cells

By genetic correlation with the growth-suppressible phenotype and direct functional tests, we demonstrate that the glucocorticoid-stimulated expression of the CCAAT/enhancer-binding protein alpha (C/EBP alpha) transcription factor is required for the steroid-mediated G1 cell cycle arrest of minimal-deviation rat hepatoma cells. Comparison of C/EBP alpha transcript and active protein levels induced by the synthetic glucocorticoid dexamethasone in glucocorticoid growth-suppressible (BDS1), nonsuppressible receptor-positive (EDR1) and nonsuppressible receptor-deficient (EDR3) hepatoma cell proliferative variants revealed that the stimulation of C/EBP alpha expression is a rapid, glucocorticoid receptor-mediated response associated with the G1 cell cycle arrest. Consistent with the role of C/EBP alpha as a critical intermediate in the growth suppression response, maximal induction of transcription factor mRNA occurred within 2 h of dexamethasone treatment whereas maximal inhibition of [3H] thymidine incorporation was observed 24 h after steroid treatment. As a direct functional approach, ablation of C/EBP alpha protein expression and DNA-binding activity by transfection of an antisense C/EBP alpha expression vector blocked the dexamethasone-induced G1 cell cycle arrest of hepatoma cells but did not alter general glucocorticoid responsiveness. Transforming growth factor beta induced a G1 cell cycle arrest in C/EBP alpha antisense transfected cells, demonstrating the specific involvement of C/EBP alpha in the glucocorticoid growth suppression response. Constitutive expression of a conditionally activated form of C/EBP alpha caused a G1 cell cycle arrest of BDS1 hepatoma cells in the absence of glucocorticoids. In contrast, overexpression of C/EBP beta or C/EBP delta had no effect on hepatoma cell growth. Taken together, these results demonstrate that the steroid-induced expression of C/EBP alpha is necessary to mediate the glucocorticoid G1 cell cycle arrest of rat hepatoma cells and implicates a role for this transcription factor in the growth control of liver-derived epithelial tumor cells.

p53 stimulates promoter activity of the sgk. serum/glucocorticoid-inducible serine/threonine protein kinase gene in rodent mammary epithelial cells

sgk is a novel member of the serine/threonine protein kinase gene family that is transcriptionally regulated by serum and glucocorticoids in mammary epithelial cells. To functionally determine if the sgk promoter is regulated by the p53 tumor suppressor protein in mammary cells, a series of sgk promoter fragments with 5'-deletions were linked to the bacterial chloramphenicol acetyltransferase gene (sgk-CAT) and transiently co-transfected into nontumorigenic NMuMG or transformed Con8Hd6 mammary epithelial cells with p53 expression plasmids. Wild-type p53, but not mutant p53, strongly stimulated sgk promoter activity in both mammary epithelial cell lines. These effects were mediated by specific regions within the sgk promoter containing p53 DNA-binding sites. The sgk p53 sequence at-1380 to-1345 (site IV) was sufficient to confer p53-dependent transactivation to a heterologous promoter, and p53 was capable of binding to this sequence in vitro as assessed by gel shift analysis. In the nontumorigenic NMuMG epithelial cell line, cotransfection of wild-type p53 strongly stimulated the activities of both the sgk promoter and the well characterized p53-responsive p21/Waf1 promoter, whereas in Rat-2 fibroblasts, wild-type p53 repressed the basal activities of both promoters, revealing that sgk and p21/Waf1 are similarly regulated in a cell type-specific manner. Taken together, these results demonstrate that sgk is a new transcriptional target of p53 in mammary epithelial cells and represent the first example of a hormone-regulated protein kinase gene with a functionally defined p53 promoter recognition element.

Antagonistic regulation of tight junction dynamics by glucocorticoids and transforming growth factor-beta in mouse mammary epithelial cells

The synthetic glucocorticoid, dexamethasone, stimulated the transepithelial electrical resistance and suppressed the DNA synthesis of 31EG4 nontransformed mouse mammary epithelial cells. The addition of transforming growth factor-beta 1 (TGF-beta) to mammary cells simultaneously with or up to 24 h after dexamethasone treatment prevented the steroid induction of transepithelial electrical resistance and stimulated the incorporation of [3H]thymidine. However, the TGF-beta inhibition of tight junction formation did not require de novo DNA synthesis. Confocal microscopy revealed that the organized immunostaining pattern of the tight junction protein, ZO-1, and F-actin at the cell periphery was disrupted by TGF-beta, resulting in disorganized and diffuse staining patterns throughout the cell. Western blot analysis demonstrated that TGF-beta did not alter the protein levels of ZO-1. In contrast to cells not treated or pretreated with steroid for up to 24 h, TGF-beta had no effect on cells pretreated with dexamethasone for 48 h. Transfection of chimeric reporter genes containing promoters responsive to either glucocorticoid or TGF-beta demonstrated that the mutual antagonism of tight junction dynamics by dexamethasone and TGF-beta occurs in the presence of intact signaling pathways. Taken together, our results establish for the first time that glucocorticoids and TGF-beta can antagonistically regulate tight junction formation in a nontransformed mammary cell line.

Glucocorticoid-induced functional polarity of growth factor responsiveness regulates tight junction dynamics in transformed mammary epithelial tumor cells

The synthetic glucocorticoid, dexamethasone, induces the "normal-like" differentiated property of tight junction formation and suppresses growth of the Con8 mammary epithelial tumor cell line, derived from a 7,12-dimethylbenz(alpha)anthracene-induced rat mammary adenocarcinoma. Characterization of the transepithelial electrical resistance of Con8 mammary tumor cells cultured on permeable supports revealed that a novel response to dexamethasone is the generation of a polarized cell monolayer with respect to epidermal growth factor receptor responsiveness. Administration of transforming growth factor-alpha (TGF-alpha) to the basolateral, but not the apical, plasma membrane compartment disrupted the glucocorticoid-stimulated tight junction barrier. Confocal immunofluorescence microscopy revealed that dexamethasone caused the ZO-1 tight junction-associated protein to localize exclusively to the apical border of laterally adjacent membranes of the cell periphery, whereas basolateral administration of TGF-alpha caused the redistribution of ZO-1 back to disorganized aggregates along the cell periphery. In contrast, TGF-alpha was able to exert its mitogenic effects equally on both sides of the cell monolayer independent of its polarized disruption of tight junction formation. Our results represent the first evidence for a functional polarization of the epidermal growth factor receptor and strongly implicate the glucocorticoid-regulated formation of tight junctions in policing the polarized responsiveness of mammary cells to growth factors.

Transforming growth factor beta down-regulation of CKShs1 transcripts in growth-inhibited epithelial cells

CKShs1 is a mammalian homologue of the yeast suc1 and CKS1 genes, for which the null mutation leads to arrest in both the G1 and G2 phases of the cell cycle in Saccharomyces cerevisiae. Northern blot analysis revealed that transcript levels of CKShs1 are strongly down-regulated in mink lung cells and moderately down-regulated in BALB keratinocytes within 10 h of exposure to transforming growth factor beta (TGF-beta), whereas growth arrest of both cell lines requires at least 15 h of TGF-beta treatment. As a genetic test for the potential role of CKShs1 in TGF-beta growth regulation, we analyzed a stably transfected derivative of mink lung cells that constitutively overexpresses a truncated form of the type 2 TGF-beta receptor and is resistant to TGF-beta growth inhibition; CKShs1 transcripts are not down-regulated by TGF-beta in this mutant cell line. TGF-beta down-regulation of CKShs1 transcripts is specific, since mRNA levels of mammalian G1 cyclins D1, D2, and D3 do not change in response to TGF-beta in either cell line. Cyclin D1 and cyclin D2 transcripts are strongly induced by epidermal growth factor, and beta 2-microglobulin transcripts are strongly induced by TGF-beta in BALB keratinocytes released from quiescence by addition of epidermal growth factor. Our results suggest a role for CKShs1 gene products in TGF-beta growth arrest of epithelial cells.

Transforming growth factor-alpha abrogates glucocorticoid-stimulated tight junction formation and growth suppression in rat mammary epithelial tumor cells

The glucocorticoid and transforming growth factor-alpha (TGF-alpha) regulation of growth and cell-cell contact was investigated in the Con8 mammary epithelial tumor cell line derived from a 7,12-dimethylbenz(alpha)anthracene-induced rat mammary adenocarcinoma. In Con8 cell monolayers cultured on permeable filter supports, the synthetic glucocorticoid, dexamethasone, coordinately suppressed [3H]thymidine incorporation, stimulated monolayer transepithelial electrical resistance (TER), and decreased the paracellular leakage of [3H]inulin or [14C]mannitol across the monolayer. These processes dose dependently correlated with glucocorticoid receptor occupancy and function. Constitutive production of TGF-alpha in transfected cells or exogenous treatment with TGF-alpha prevented the glucocorticoid growth suppression response and disrupted tight junction formation without affecting glucocorticoid responsiveness. Treatment with hydroxyurea or araC demonstrated that de novo DNA synthesis is not a requirement for the growth factor disruption of tight junctions. Immunofluorescence analysis revealed that the ZO-1 tight junction protein is localized exclusively at the cell periphery in dexamethasone-treated cells and that TGF-alpha caused-ZO-1 to relocalize from the cell periphery back to a cytoplasmic compartment. Taken together, our results demonstrate that glucocorticoids can coordinately regulate growth inhibition and cell-cell contact of mammary tumor cells and that TGF-alpha, can override both effects of glucocorticoids. These results have uncovered a novel functional "cross-talk" between glucocorticoids and TGF-alpha which potentially regulates the proliferation and differentiation of mammary epithelial cells.

Relationship of serine/threonine phosphorylation/dephosphorylation signaling to glucocorticoid regulation of tight junction permeability and ZO-1 distribution in nontransformed mammary epithelial cells

The synthetic glucocorticoid dexamethasone regulates tight junction permeability resulting in an increased transepithelial electrical resistance (TER) of cultured 31EG4 mammary epithelial cells. Inhibition of cellular type 1 and type 2A protein phosphatase activity by okadaic acid reduced the TER of dexamethasone-treated monolayers of 31EG4 cells to basal levels within 24 h. Coincident with the increase in tight junction permeability, immunofluorescence revealed that okadaic acid caused a partial cellular redistribution of the ZO-1 tight junction-associated protein. The potent glucocorticoid antagonist RU486 had no effect on TER or ZO-1 distribution, indicating that the effects of okadaic acid are not a result of disrupting glucocorticoid receptor function. Immunoprecipitation of 32P-labeled cells and V8 protease peptide mapping demonstrated that dexamethasone did not alter ZO-1 phosphorylation. However, consistent with the changes in TER, dexamethasone induced a 2.3-fold stimulation in ZO-1 protein levels which was reduced to 73% of basal levels by okadaic acid. No effects on ZO-1 transcript levels were observed. Monolayers grown in the presence of glucocorticoids had only 28% less junction density and 16.5% more linear junction/cell, which cannot account for the observed increases of TER and ZO-1 protein levels. Taken together, our results have shown that a disruption of phosphorylation/dephosphorylation activity overrides the glucocorticoid regulation of tight junction permeability in 31EG4 mammary cells.

Glucocorticoid-regulated trafficking of mouse mammary tumor virus proteins in permeabilized hepatoma cells. Requirements of intracellular membrane transport for maturation of the cytoplasmic phosphorylated polyprotein

Glucocorticoids coincidentally regulate the localization of mouse mammary tumor virus (MMTV) glycoproteins and maturation of viral phosphoproteins in viral infected rat hepatoma cells. To test for a functional interaction between MMTV transmembrane glycoproteins and cytoplasmic phosphoproteins, the bacterial cytolysin streptolysin-O was utilized to selectively permeabilize the plasma membrane and reconstitute exocytic trafficking. Streptolysin-O-permeabilized M1.54 cells pretreated with glucocorticoids retained the capability for proteolytic processing, cell surface delivery, and externalization of MMTV glycoproteins as determined by immunoprecipitation and immunofluorescence microscopy. The efficient maturation of MMTV phosphoproteins indicated that these viral proteins are properly transported near or to the plasma membrane in permeabilized cells. These maturation events in semi-intact cells were dependent on the addition of cell cytosol and were specifically inhibited by the membrane impermeant GTP analog guanosine 5'3-O-(thio)triphosphate, an agent known to impede vesicular transport of membrane proteins, but which has not previously been shown to alter cytoplasmic protein maturation or transport. The addition of anti-MMTV antibodies directed against the cytoplasmic domain of the glycoprotein precursor to transport competent semi-intact M1.54 cells resulted in the dramatic inhibition of both MMTV glycoprotein and phosphoprotein maturation. These results were not obtained using either preimmune sera or antiserum specific for the luminal portion of the glycoprotein precursor. Our findings suggest that the functional interaction of cytosolic MMTV phosphoproteins with the cytoplasmic domain of the viral membrane glycoprotein is required for the efficient transport and processing of each class of proteins in glucocorticoid-treated cells and provides the first evidence for the involvement of vesicular transport in the delivery and maturation of cytoplasmic viral proteins at the plasma membrane or the pericellular region.

Glucocorticoids induce a G1/G0 cell cycle arrest of Con8 rat mammary tumor cells that is synchronously reversed by steroid withdrawal or addition of transforming growth factor-alpha

Con8 mammary tumor cells are an epithelial cell line derived from the 7,12-dimethylbenz(alpha)anthracene-induced 13762NF rat mammary adenocarcinoma. The synthetic glucocorticoid dexamethasone suppresses the growth of Con8 cells, and after 5 days of treatment with this steroid, Con8 cells undergo less than 0.5 population doublings. This growth arrest is accompanied by a 30-fold elevation in c-jun transcript levels, no change in c-fos expression, and a moderate increase in total AP-1 transcriptional activity. Dexamethasone inhibited DNA synthesis within one cell cycle, and flow cytometry of propidium iodide-stained nuclei demonstrated that dexamethasone growth-suppressed cells had a DNA content indicative of a specific cell cycle block in either G1 or G0. Consistent with a G1/G0 arrest of the cell cycle, dexamethasone did not prevent Con8 cells from entering the S phase after release from synchronization at the G1/S boundary by a double thymidine block. Analysis of [3H]thymidine incorporation and autoradiography of [3H]thymidine-labeled nuclei revealed that after either dexamethasone withdrawal or the addition of transforming growth factor-alpha (TGF alpha), Con8 cells synchronously reinitiate cell cycle progression. Northern blot analysis demonstrated that an induction of transcripts for the G1 marker genes c-myc and cyclin D1 occurs before cells enter the S-phase. After dexamethasone withdrawal, c-myc and cyclin D1 expression transiently peak at 2 and 4 h, respectively. In contrast, c-myc expression peaked at 0.5-1 h, whereas cyclin D1 expression was induced at 2 h and maintained at a high level after the addition of TGF alpha. Our results demonstrate that glucocorticoids induce a specific block of the cell cycle progression of a rat mammary tumor cell, and that after synchronous progression through the cell cycle, the temporal expression pattern for c-myc and cyclin D1 is distinct for dexamethasone release vs. the addition of TGF alpha to glucocorticoid-suppressed cells.

Hormonal regulation of the polarized function and distribution of Na/H exchange and Na/HCO3 cotransport in cultured mammary epithelial cells

The time course for development of polarized function and morphological distribution of pH regulatory mechanisms has been examined in a mouse mammary epithelial cell line (31EG4). Monolayers grown on permeable supports had tight junctions when grown 3-4 days in the presence of the lactogenic hormones dexamethasone (D, a synthetic glucocorticoid) and insulin (I), or in D, I, and prolactin (P), but there were no tight junctions in the absence of D. Microspectrofluorimetry of the pH-sensitive dye BCECF was used to measure pH (pHi) in cells mounted in a two-sided perfusion chamber to distinguish pH regulatory activity at the apical and basolateral membranes. Na/H exchange was assayed as the Na-dependent, amiloride-sensitive component of pHi recovery from an acid load induced by a pulse of NH3/NH4-containing solution. When monolayers were grown 3-4 d in the presence of P, D, and I, Na/H exchange was restricted to the basolateral membrane. In contrast, in the absence of P, Na/H exchange was present on both the apical and basolateral membranes. After 5-6 days, in the presence or absence of P, Na/H exchange was present only on the basolateral membrane. An antibody to the NHE-1 isoform of the Na/H exchanger was used to determine its morphological distribution. In all hormone conditions the antibody recognized a protein of approximately 110 kD (Western blot), and confocal immunofluorescence microscopy of this antibody and of an anti-ZO-1 (the marker of the tight junctions) antibody showed that the morphological distribution of the Na/H exchanger was similar to the functional distribution under all hormonal treatments. In addition, a putative Na/HCO3 cotransport system was monitored as a Na-dependent, amiloride-insensitive pHi recovery mechanisms that was inhibited by 200 microM H2DIDS. After treatment with D+I (but not with I alone) cotransport appeared exclusively on the basolateral membrane, and the polarized expression of this transporter was not altered by P. We conclude that when mammary cells are grown in D+I-containing media, the Na/H exchanger is expressed initially (i.e., after 3-4 d) on both the apical and basolateral membranes and later (5-6 d) on only the basolateral membrane. P (in the presence of D+I) selectively speeds this polarization, which is determined by polarized distribution of the exchanger to the apical and/or basal membrane and not by the activation and/or inactivation of transporters.(ABSTRACT TRUNCATED AT 400 WORDS)

Immediate-early transcriptional regulation and rapid mRNA turnover of a putative serine/threonine protein kinase

Serine/threonine protein kinases are important regulators of diverse cellular processes including metabolism, proliferation, and differentiation. We have previously identified the cDNA for a 49-kDa serine/threonine kinase, designated sgk, which is transcriptionally responsive to glucocorticoid hormones and serum in epithelial cells. We report here that sgk expression is also rapidly induced by dexamethasone or serum in Rat2 fibroblasts. Nuclear run-on and Northern blot analysis revealed that the induction of sgk mRNA is an immediate-early transcriptional response to serum stimulation of quiescent fibroblasts, which occurs just after the peak in c-jun transcription. In contrast to the glucocorticoid-stimulated sgk expression in Rat2 fibroblasts, the transcriptional induction of sgk by serum was transient and sgk transcripts decayed with a particularly rapid half-life of 20 min. The rapid turnover of sgk, in combination with its immediate-early transcriptional response to serum, suggests a novel mechanism for responding to mitogenic signals during G0 to S transition and entry into the cell cycle.

Glucocorticoids coordinately disrupt a transforming growth factor alpha autocrine loop and suppress the growth of 13762NF-derived Con8 rat mammary adenocarcinoma cells

We have demonstrated previously that the synthetic glucocorticoid dexamethasone suppresses the growth of Con8 rat mammary tumor cells, which are derived from the 13762NF transplantable, hormone-responsive rat mammary adenocarcinoma. Dexamethasone inhibited [3H]thymidine incorporation into Con8 cells at high cell density under both serum and serum-free conditions. Fractionation in nonreducing sodium dodecyl sulfate-polyacrylamide gels of proteins secreted from dexamethasone-treated and untreated Con8 mammary tumor cells revealed two size classes of glucocorticoid inhibited mitogenic activities; a larger M(r) 27,000-33,000 and a smaller M(r) 5,000-12,000 activity. Both size classes of mitogens restimulated the growth of glucocorticoid-suppressed Con8 cells suggesting that they can act in an autocrine fashion. The smaller mitogen was identified as transforming growth factor alpha (TGF-alpha) since this activity competed with 125I-epidermal growth factor (EGF) for EGF receptor binding and was selectively immunodepleted with monoclonal TGF-alpha antibodies but not with EGF antibodies. Western blots and radioreceptor assay of Con8-secreted proteins revealed that glucocorticoids inhibited the production of a M(r) 5500 immunoreactive TGF-alpha protein by 10-fold. Consistent with a steroid effect on the level of TGF-alpha production, rather than on its activity, the specific mitogenic activities of the TGF-alpha s secreted by dexamethasone-treated and untreated Con8 cells were identical to that of recombinant human TGF-alpha. Treatment of intact cells with suramin, which dissociates ligand-receptor complexes, revealed that the EGF receptor-mediated mitogenic response is functional in both glucocorticoid-treated and untreated cells. Taken together, our results demonstrate that glucocorticoids suppress Con8 mammary tumor cell growth and disrupt a potential TGF-alpha autocrine loop which results in a dramatic reduction in the level of extracellular TGF-alpha.

Overexpression of transforming growth factor alpha overrides the glucocorticoid-mediated suppression of Con8 mammary tumor cell growth in vitro and in vivo

In a preceding paper (D. B. Alexander et al., Cancer Res., 53: 1808-1815, 1993), we demonstrated that the in vitro glucocorticoid inhibition of Con8 mammary tumor cell growth is accompanied by the disruption of a transforming growth factor alpha (TGF-alpha) autocrine loop. This growth suppression response functions in vivo since proliferation of Con8-derived tumors was inhibited in rats treated with the synthetic glucocorticoid, dexamethasone. The effect of dexamethasone on Con8-derived tumor growth was reversible in that tumors rapidly grew at the site of inoculation after discontinuing injections of dexamethasone. To test the in vivo relationship between the glucocorticoid growth suppression response and the TGF-alpha autocrine loop, Con8 cells were transfected with a TGF-alpha expression vector and single cell-derived neomycin-resistant subclones were recovered. [3H]Thymidine incorporation of cultured monolayers of transfected Con8 mammary cells and measurement of tumor diameters in rats revealed that dexamethasone failed to suppress the in vitro proliferation or in vivo tumor growth of Con8-derived cells producing high constitutive levels of secreted TGF-alpha. In contrast, both the in vivo and in vitro growth of Con8 cells transfected with vector controls were fully suppressible by glucocorticoids. Consistent with our in vitro observations, these results demonstrate that the regulation of TGF-alpha production plays a key role in the in vivo glucocorticoid suppression of Con8-derived mammary tumor growth.

Characterization of sgk, a novel member of the serine/threonine protein kinase gene family which is transcriptionally induced by glucocorticoids and serum

A novel member of the serine/threonine protein kinase gene family, designated sgk, for serum and glucocorticoid-regulated kinase, was identified in a differential screen for glucocorticoid-inducible transcripts expressed in the Con8.hd6 rat mammary tumor cell line. sgk encodes a protein of 49 kDa which has significant sequence homology (45 to 55% identity) throughout its catalytic domain with rac protein kinase, the protein kinase C family, ribosomal protein S6 kinase, and cyclic AMP-dependent protein kinase. sgk mRNA is expressed in most adult rat tissues, with the highest levels in the thymus, ovary, and lung, as well as in several rodent and human cell lines. sgk mRNA was stimulated by glucocorticoids and by serum within 30 min, and both inductions were independent of de novo protein synthesis. The transcriptional regulation by glucocorticoids is a primary response, since the promoter of sgk contains a glucocorticoid response element consensus sequence 1.0 kb upstream of the start of transcription which is able to stimulate chloramphenicol acetyltransferase reporter gene activity in a dexamethasone-dependent manner. Antibodies that specifically recognize sgk-encoded protein on an immunoblot were generated. This protein was shown to increase in abundance with glucocorticoid treatment in a manner which paralleled the mRNA accumulation. This is the first report of a presumed serine/threonine protein kinase that is highly regulated at the transcriptional level by glucocorticoid hormones and suggests a novel interplay between glucocorticoid receptor signalling and a protein kinase of the second messenger family.

Glucocorticoids reversibly arrest rat hepatoma cell growth by inducing an early G1 block in cell cycle progression

We have previously documented that glucocorticoids suppress the proliferation of BDS1 hepatoma cells, a rat epithelial tumor cell line derived from minimal deviation Reuber H35 hepatoma cells. Flow cytometry demonstrated that, after treatment with the synthetic glucocorticoid dexamethasone, the growth of an asynchronous population of BDS1 cells was arrested within one cell cycle which resulted in an accumulation of cells with a G1-G0-like DNA content. Consistent with a glucocorticoid-induced block early in the G1 phase of the cell cycle, propidium iodide flow cytometry revealed that addition of dexamethasone up to 2 h after release from contact inhibition prevented BDS1 hepatoma cells from entering S phase, whereas dexamethasone treatment after 2 h had no effect on the entry of cells into S phase. Moreover, dexamethasone treatment did not prevent BDS1 cells from entering S phase after release from synchronization at the G1-S boundary by a double thymidine block. Analysis of DNA content, [3H]-thymidine incorporation, and autoradiography of [3H]-thymidine-labeled nuclei revealed that, after release from dexamethasone, BDS1 cells synchronously reinitiated cell cycle progression and entered S phase 8 h after hormone withdrawal. Northern blot analysis demonstrated that the level of transcripts encoding the G1 marker genes CYL-1 and CYL-2 G1 cyclins peaked 4 h after dexamethasone withdrawal. Dexamethasone induced a 20-fold increase in the level of c-jun mRNA which was reversed after hormone withdrawal, whereas expression of c-fos transcripts remained at a low level during the time course of hormone treatment and withdrawal. Transient transfections with a collagenase-chloramphenicol acetyltransferase reporter gene showed that dexamethasone inhibited 12-O-tetradecanoylphorbol-13-acetate-inducible, but not basal, AP-1 transcription factor activity. Our results demonstrate that glucocorticoids reversibly induce an early G1 block in cell cycle progression of an epithelial tumor cell line that occurs with a coordinate elevation in the expression of c-jun transcripts.

Trafficking of wild-type and an endoproteolytic-site mutant of the mouse mammary tumor virus glycoprotein

We have utilized a mouse mammary tumor virus (MMTV) glycoprotein gene containing a mutation in the endoproteolytic cleavage site to investigate the biological significance of processing, the structural requirements for and the events involved in the proteolytic maturation of MMTV. Using oligonucleotide-directed mutagenesis, the endoproteolytic cleavage site within the MMTV glycoprotein was mutated from Arg-Ala-Lys-Arg to Arg-Ala-Asn-Gln and both the wild-type and mutated genes were transfected and expressed in HTC rat hepatoma cells. Indirect immunofluorescence, steady state radiolabeling and pulse-chase kinetic experiments demonstrated that this mutated glycoprotein was transported to the cell surface with the same efficiency as the wild-type maturation products; however, proteolytic cleavage and fusogenic activity were almost completely abolished. Consistent with the lack of cleavage, the endoglycosidase H-resistant precursor, gp78, accumulated on the cell surface and in the extracellular environment. When HTC cells expressing the wild-type MMTV glycoprotein were treated with the Golgi mannosidase I and II inhibitors, deoxymannojirimycin and swainsonine respectively, the resulting endogycosidase H-sensitive glycoproteins were processed efficiently. Taken together, these results suggest that proteolytic processing of the MMTV glycoprotein most likely occurs in the trans Golgi or at a later step in the exocytic pathway and occurs after the formation of an endoglycosidase H-resistant, terminally sialylated intermediate. Moreover, the acquisition of endoglycosidase H-resistant oligosaccharides is not a prerequisite for recognition by the cellular proteases to generate the viral maturation products. Our evidence also suggests that the processing of the MMTV envelope glycoprotein is required for the functional exposure of the fusion domain which is involved in viral infectivity.

Glucocorticoid-regulated stability of a constitutively expressed mouse mammary tumor virus glycoprotein

Glucocorticoids regulate the transport and processing of mouse mammary tumor virus (MMTV) glycoproteins in viral-infected HTC rat hepatoma cells. To begin to determine the role of cellular components involved in this steroid-mediated response, a constitutively expressed MMTV glycoprotein gene containing a mutation in the endoproteolytic cleavage site was used to simplify the viral maturation products. Expression of the uncleavable MMTV glycoprotein gene in transfected HTC rat hepatoma cells demonstrated that treatment with the synthetic glucocorticoid dexamethasone resulted in a 5-fold increase in the steady state level of the intracellular and cell surface MMTV glycoproteins. Under these conditions, dexamethasone did not alter the level of MMTV glycoprotein transcripts. Pulse-chase radiolabeling with [35S]methionine demonstrated that dexamethasone did not affect the apparent rate of MMTV glycoprotein translation, and an analysis of oligosaccharide side-chain structure by endoglycosidase-H digestion revealed that glucocorticoids did not alter the 45-min endoplasmic reticulum to Golgi transit time. Pulse-chase kinetic analysis of 4-h pulse-labeled cells revealed that the half-life of the mature glycosylated MMTV polyprotein, gp78, was 105 min in glucocorticoid-treated cells and 45 min in untreated cells. Taken together, our results suggest that glucocorticoids increase the stability of MMTV glycoproteins by a posttranslational mechanism and that this effect may be occurring relatively early in the exocytic pathway.

Glucocorticoid-induced formation of tight junctions in mouse mammary epithelial cells in vitro

Phenotypically stable cultures of untransformed mouse mammary epithelial cells (denoted 31EG4) were established and utilized to investigate the lactogenic hormone (glucocorticoids, insulin, and prolactin) regulation of tight junction formation. When 31EG4 cells were grown on permeable supports for 4 days in medium containing the synthetic glucocorticoid dexamethasone and insulin, confluent cell monolayers obtained a transepithelial electrical resistance (TER) of 1000-3000 omega.cm2. In contrast, over the same time period, confluent monolayers treated with insulin or insulin and prolactin maintained a low TER (35-150 omega.cm2). Consistent with the formation of tight junctions, apical to basolateral paracellular permeability was decreased from 12% to 1% for [14C]mannitol and 3.3% to 0.3% for [3H]inulin when cells were cultured in dexamethasone. This effect of dexamethasone on TER required extracellular calcium, de novo protein synthesis, dose-dependently correlated with glucocorticoid receptor occupancy, and was not due to an increase in cell density. As shown by direct and indirect immunofluorescence microscopy, dexamethasone treatment did not modulate the production or location of filamentous actin, the tight junction protein ZO-1, or the cell adhesion protein E-cadherin. Our results suggest that glucocorticoids play a fundamental role in the function and maintenance of cell-cell contact in the mammary epithelia by inducing the formation of tight junctions.

Differential transport and processing of variant mouse mammary tumor virus glycoproteins

The transport and proteolytic processing of two individual gene isolates of the mouse mammary tumor virus (MMTV) glycoprotein were compared in transfected rat HTC hepatoma cells. Plasmids were constructed such that the MMTV glycoprotein genes were constitutively expressed from the promoter within the Rous Sarcoma Virus 5' Long Terminal Repeat in the absence of other MMTV proteins. An isolate of the GR strain MMTV glycoprotein was efficiently transported and processed resulting in the localization of MMTV glycoproteins at the cell surface and in the extracellular environment. Moreover, the kinetics of acquisition of endoglycosidase H resistant oligosaccharide side chains and the rate of endoproteolytic cleavage of the glycosylated polyprotein expressed in transfected cells were virtually identical to that observed in viral-infected rat hepatoma cells. In contrast, a natural variant of the C3H strain MMTV glycoprotein expressed in transfected cells was retained in an intracellular compartment by a heavy chain binding protein (BiP)-independent pathway in an endoglycosidase H sensitive and uncleaved form. This MMTV glycoprotein isolate was retained early in the exocytic pathway and displayed a half-life of approximately 45 min in transfected cells. Only a minor fraction of the expressed C3H variant glycoprotein was detected at the cell surface but was not externalized. Our results suggest that the variant C3H MMTV glycoprotein contains one or more mutations that preclude its efficient transport through the exocytic pathway.

Disruptions in intracellular membrane trafficking and structure preclude the glucocorticoid-dependent maturation of mouse mammary tumor virus proteins in rat hepatoma cells

We have previously shown that glucocorticoids regulate the trafficking and processing of mouse mammary tumor virus (MMTV) proteins in viral-infected M1.54 rat hepatoma cells. To examine the role of intracellular membrane integrity on MMTV protein maturation, brefeldin A (BFA) was utilized to disrupt membrane flow between the endoplasmic reticulum and Golgi. Immunoprecipitation and immunofluorescence microscopy revealed that in the presence of dexamethasone, BFA inhibited the proteolytic processing, cell surface delivery, and externalization of MMTV glycoproteins. Glycosidase digestion and inhibitors of protein glycosylation confirmed that the observed differences in apparent sizes of MMTV glycoprotein products are due to BFA-induced changes in oligosaccharide processing. BFA treatment inhibited the proteolytic processing of the MMTV phosphoprotein precursor, which normally associates with the cytoplasmic face of intracellular membranes. Similarities in salt extraction efficiency revealed that BFA did not affect the membrane affinity of the uncleaved phosphorylated precursor. In a complementary approach, proteolytic processing of the phosphorylated polyprotein did not occur in glucocorticoid-treated HTC cells transfected with a mutant MMTV provirus encoding a normal phosphorylated precursor, but which express a truncated MMTV glycoprotein missing its transmembrane domain and cytoplasmic tail. These results suggest that the MMTV glycoproteins and phosphoproteins may interact at a late step in the transport pathway in a manner required for their mutual processing in response to glucocorticoids and establishes the importance of functional interactions with intracellular membranes for maturation of the cytoplasmic MMTV phosphoproteins.

Glucocorticoid growth suppression response in 13762NF adenocarcinoma-derived Con8 rat mammary tumor cells is mediated by dominant trans-acting factors

The in vitro and in vivo growth of Con8 cells, a single cell-derived subclone of the 13762NF-transplantable rat mammary adenocarcinoma, is strongly suppressed by glucocorticoid hormones. Hybrids were formed between glucocorticoid-suppressible Con8.hD6 mammary tumor cells (Con8 transfected with the histidinol dehydrogenase selectable marker) and either glucocorticoid-resistant 8RUV7 mammary tumor cells (derived from Con8) or MCT-HTC rat hepatoma cells. Both of the glucocorticoid-resistant 8RUV7 and MCT-HTC fusion partners express functional glucocorticoid receptors, since hormone-responsive genes such as plasminogen activator inhibitor are fully dexamethasone inducible. Karyotypic analyses revealed that the hybrid cell populations possessed the appropriate number of chromosomes for a fusion between the glucocorticoid-suppressible and either of the two resistant cell types. Moreover, Northern blots showed that the intertissue hybrids expressed transcripts for both the milk fat globule membrane protein gene originating from the parental Con8.hD6 mammary tumor cells as well as mouse mammary tumor virus glycoprotein sequences which had been transfected into the MCT-HTC hepatoma cells as a molecular tag. Analysis of DNA content and [3H]thymidine incorporation demonstrated that growth of both the intratissue (Con8.hD6 x 8RUV7) and intertissue (Con8.hD6 x MCT-HTC) hybrids was glucocorticoid suppressible, even though the absolute rates of proliferation differed depending on the parental cells. Analysis of conditioned medium isolated from glucocorticoid-treated and untreated Con8.hD6 cells indicated that the growth suppression response is not mediated through the elaboration of an extracellular growth inhibitor. Taken together, our results demonstrate that the glucocorticoid-suppressible phenotype of Con8 rat mammary tumor cells is dominant, suggesting the existence of intracellular regulatory factors under glucocorticoid control that may function as trans-acting suppressors of tumor cell growth.

Altered effects of glucocorticoids on the trafficking and processing of mouse mammary tumor virus glycoproteins constitutively expressed in rat hepatoma cells in the absence of nonglycosylated viral components

We have documented previously that glucocorticoid hormones modulate the posttranslational localization of cell surface mouse mammary tumor virus (MMTV) glycoproteins in the viral-infected M1.54 rat HTC hepatoma cell line. To determine whether glucocorticoids affect the trafficking of individually synthesized MMTV glycoproteins, HTC cells were transfected with a constitutively expressed MMTV glycoprotein gene lacking the viral phosphoprotein and polymerase genes. This construct also allows equivalent levels of MMTV glycoproteins to be compared in the presence or absence of glucocorticoids. Indirect immunofluorescence and immunoprecipitation of radiolabeled cells revealed that in transfected cells the transmembrane MMTV glycoproteins are efficiently expressed, transported to the cell surface, and proteolytically cleaved in the presence or in the absence of the synthetic glucocorticoid dexamethasone. Cell surface immunoprecipitation of [35S]methionine-labeled cells showed that the level of plasma membrane gp78 appeared to be stimulated 2-fold after dexamethasone treatment, even though fluorescence-activated cell sorting revealed no discernible change in the total concentration of cell surface MMTV glycoproteins. Analysis of oligosaccharide side chain maturation through a pulse-chase radiolabeling revealed that the rate of rough endoplasmic reticulum-Golgi transport was essentially identical in dexamethasone-treated and untreated transfected cells and was similar to that observed in dexamethasone-treated M1.54 cells. Thus, in contrast to viral-infected hepatoma cells, mostly constitutive cellular machinery mediates the trafficking and maturation of cell surface MMTV glycoproteins expressed outside of the proviral context. Taken together, our results suggest that the glucocorticoid-stimulated synthesis of nonglycosylated viral components may contribute to or be responsible for the regulated trafficking of MMTV glycoproteins observed in viral-infected rat hepatoma cells.

Expression of mouse mammary tumor virus glycoprotein truncations defines roles for the transmembrane domain and ectodomain hydrophobic region in constitutive exocytic trafficking and proteolytic processing

A mutational analysis was used to identify structural domains that are important for exocytic transport and proteolytic cleavage of the mouse mammary tumor virus (MMTV) glycoprotein, which is expressed as a multidomain polyprotein. Rat HTC hepatoma cells were transfected with the MMTV glycoprotein gene driven by the constitutive Rous sarcoma virus promoter, with mutant genes encoding a series of polypeptide truncations or with a defective MMTV provirus containing a premature termination codon in the viral glycoprotein gene. Efficient proteolytic maturation and transport of MMTV glycoproteins to the cell surface or extracellular environment required the presence of the transmembrane domain but not the cytoplasmic tail. Two stable truncations retaining the hydrophobic region of the ectodomain in the absence of the transmembrane domain and cytoplasmic tail (trgp67 and trgp58) remained in endoglycosidase H sensitive and uncleaved forms. One of these truncations, trgp58, appeared to be tightly associated with intracellular membranes and strongly bound by heavy chain binding protein, whereas the other truncation, trgp67, was a soluble component of the lumen and persists intracellularly by a heavy chain binding protein-independent pathway. The truncated MMTV glycoprotein additionally lacking the hydrophobic region of the ectodomain was efficiently secreted. Taken together, our results demonstrate that the hydrophobic transmembrane domain of the MMTV glycoprotein is required for proper transport and proteolytic processing, whereas, in the absence of the transmembrane domain, the presence of a hydrophobic region of the ectodomain correlated with retention at an early step in the exocytic pathway.

Identification of the glucocorticoid suppressible mitogen from rat hepatoma cells as an angiogenic platelet-derived growth factor A-chain homodimer

We have previously shown that BDS.1 rat hepatoma cells are hypersensitive to the antiproliferative effects of glucocorticoids, and secrete a glucocorticoid suppressible mitogenic activity (denoted GSM). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that GSM purified to near homogeneity migrated as a 28-kDa protein under nonreducing conditions and as a single 15-kDa polypeptide in the presence of sulfhydryl reducing agents suggesting a homodimeric structure. Anti-platelet-derived growth factor (PDGF) A-chain specific antibodies selectively immunodepleted the mitogenic activity which can be extracted from nonreducing gels in the 26-30-kDa fraction and, in Western blots, recognized the 15-kDa reduced form of GSM. Western blot analysis further showed that dexamethasone suppressed the level of secreted PDGF A-chain protein in BDS.1 cells but not in glucocorticoid receptor-deficient hepatoma cells. Northern blots revealed that dexamethasone reduced expression of the PDGF A-chain 2.3- and 1.7-kilobase transcripts in proportion to the level of detectable PDGF-AA protein. Similarly to PDGF-AA, the hepatoma cell-derived GSM has a potent angiogenic activity. Taken together, our results demonstrate that the predominant glucocorticoid suppressible mitogen secreted from rat hepatoma cells is a PDGF A-chain homodimer and suggest that in vivo glucocorticoids may potentially regulate hepatoma growth by modulating PDGF-stimulated tumor vascularization.