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

C/EBP transcription factors in the lung epithelium

During recent years, the biological roles of CCAAT/enhancer binding proteins (C/EBPs) in the lung have started to be uncovered. C/EBPs form a family within the basic region-leucine zipper class of transcription factors. In the lung epithelium C/EBPalpha, -beta, and -delta are expressed. Lung-specific target genes for these transcription factors include the surfactant proteins A and D, the Clara cell secretory protein, and the P450 enzyme CYP2B1. As more information is gathered, a picture is emerging in which C/EBPalpha has a role in regulating proliferation as well as differentiation-dependent gene expression, whereas C/EBPbeta and -delta, in addition to a partly overlapping role in regulating expression of differentiation markers, also seem to be involved in responses to injury and hormones.

Differential inhibition of thrombin- and EGF-stimulated human cultured airway smooth muscle proliferation by glucocorticoids

The present study compared the effects of glucocorticoids on thrombin- and EGF-stimulated proliferation in human cultured airway smooth muscle (ASM) to identify pathways that may be differentially regulated by glucocorticoids. Mitogenic responses to thrombin were inhibited by extracellular-regulated kinase (ERK 1/2) and phosphoinositide 3-kinase (PI3K) inhibitors, whereas mitogenic responses to EGF were inhibited by ERK 1/2 and PI3K inhibitors as well as by the p38 mitogen activated protein kinase inhibitor, SB203580 (10 microM). Mitogenic responses to thrombin were more sensitive to inhibition by dexamethasone (Dex) or fluticasone propionate (FP) than were those to EGF. Elevated cyclin D1 protein and mRNA levels induced by thrombin and EGF were attenuated equally by glucocorticoids. The protein or mRNA levels of the cyclin-dependent kinase inhibitors (cdki) p21(Cip1), p27(Kip1) were unaffected by Dex treatment of ASM cells treated with mitogens. The resistance of EGF-induced proliferation to inhibition by glucocorticoids is not associated with a failure to regulate cyclin D1 induction, nor does it appear to be explained by differential regulation of the levels of the cdki's, p21(Cip1) and p27(Kip1).

Glucocorticoid receptor functions as a potent suppressor of mouse skin carcinogenesis

Glucocorticoids are effective inhibitors of epidermal proliferation and skin tumorigenesis. Glucocorticoids affect cellular functions via glucocorticoid receptor (GR), a well-known transcription factor. Recently, we generated skin-targeted transgenic mice overexpressing GR under control of the keratin5 promoter (K5-GR mice). To test the hypothesis that GR plays a role as a tumor suppressor in skin, we bred K5-GR transgenic mice with Tg.AC transgenic mice, which express v-Ha-ras oncogene in the skin, and compared the susceptibility of F1 offspring to TPA-induced skin carcinogenesis. GR overexpression in the epidermis dramatically inhibited skin tumor development. In K5-GR/ras+ double transgenic mice papillomas developed later and the average number of tumors per animal was 15% (in males) and 40% (in females) of the number seen in wild type (w.t./ras+) littermates. In addition, the papillomas in w.t./ras+ animals were eight to nine times larger. GR overexpression resulted in a decrease in keratinocyte proliferation combined with a modest increase in apoptosis and differentiation of keratinocytes in K5-GR/ras+ papillomas. Our data clearly indicate that interference of GR transgenic protein with nuclear factor kappa B (NF-kappaB) transcription factor had resulted in NF-kappaB blockage in K5-GR/ras+ tumors. We discuss the role of NF-kappaB blockage in tumor-suppressor effect of GR.

Identification of a functional link for the p53 tumor suppressor protein in dexamethasone-induced growth suppression

Serine/threonine phosphatase 5 (PP5) can act as a suppresser of p53-dependent growth suppression and has been reported to associate with several proteins, including the glucocorticoid receptor/heat-shock protein-90 complex. Still, the physiological/pathological roles of PP5 are unclear. To characterize the relationship of PP5, glucocorticoid receptor activation and p53, here we describe the development of chimeric antisense oligonucleotides that potently inhibit human p53 expression. This allowed us to regulate the expression of either p53 (e.g. with ISIS 110332) or PP5 (e.g. with ISIS 15534) in genetically identical cells. Studies with ISIS 110332 revealed that the suppression of p53 expression is associated with a decrease in the basal expression of the cyclin-dependent kinase inhibitor protein, p21(WAF1/Cip1), and a concomitant increase in the rate of cell proliferation. Suppression of p53 also blocks dexamethasone-induced p21(WAF1/Cip1) expression and G(1)-growth arrest. Furthermore, treatment with ISIS 110332, but not the mismatched controls, ablates the suppression of growth produced by prior treatment with dexamethasone. Additional studies revealed that dexamethasone-dependent p21(WAF1/Cip1) expression occurs without an apparent change in p53 protein levels or the phosphorylation status of p53 at Ser-6, -37, or -392. However, dexamethasone treatment is associated with an increase in p53 phosphorylation at Ser-15. Suppression of PP5 expression with ISIS 15534 also results in the hyperphosphorylation of p53 at Ser-15. Together, these findings indicate that the basal expression of p53 plays a functional role in a glucocorticoid receptor-mediated response regulating the expression of p21(Waf1/Cip1) via a mechanism that is suppressed by PP5 and associated with the phosphorylation of p53 at Ser-15.

Regulation of macrophage migration inhibitory factor expression by glucocorticoids in vivo

Glucocorticoid hormones are important anti-inflammatory agents because of their anti-inflammatory and proapoptotic action within the immune system. Their clinical usefulness remains limited however by side effects that result in part from their growth inhibitory action on sensitive target tissues. The protein mediator, macrophage migration inhibitory factor (MIF), is an important regulator of the host immune response and exhibits both glucocorticoid-antagonistic and growth-regulatory properties. MIF has been shown to contribute significantly to the development of immunopathology in several models of inflammatory disease. Although there is emerging evidence for a functional interaction between MIF and glucocorticoids in vitro, little is known about their reciprocal influence in vivo. We investigated the expression of MIF in rat tissues after ablation of the hypothalamic-pituitary-adrenal axis and after high-dose glucocorticoid administration. MIF expression is constitutive and independent of the influence of adrenal hormones. Hypophysectomy and the attendent loss of pituitary hormones, by contrast, decreased MIF protein content in the adrenal gland. Administration of dexamethasone was found to increase MIF protein expression in those organs that are considered to be sensitive to the growth inhibitory effects of glucocorticoids (immune and endocrine tissues, skin, and muscle). This increase was most likely because of a posttranscriptional regulatory effect because tissue MIF mRNA levels were not influenced by dexamethasone treatment. Finally, MIF immunoneutralization enhanced lymphocyte egress from blood during stress-induced lymphocyte redistribution, consistent with a functional interaction between MIF and glucocorticoids on immune cell trafficking in vivo. These findings suggest a role for MIF in both the homeostatic and physiological action of glucocorticoids in vivo.

Redundant cooperative interactions for assembly of a human U6 transcription initiation complex

The core human U6 promoter consists of a proximal sequence element (PSE) located upstream of a TATA box. The PSE is recognized by the snRNA-activating protein complex (SNAP(c)), which consists of five types of subunits, SNAP190, SNAP50, SNAP45, SNAP43, and SNAP19. The TATA box is recognized by TATA box binding protein (TBP). In addition, basal U6 transcription requires the SANT domain protein Bdp1 and the transcription factor IIB-related factor Brf2. SNAP(c) and mini-SNAP(c), which consists of just SNAP43, SNAP50, and the N-terminal third of SNAP190, bind cooperatively with TBP to the core U6 promoter. By generating complexes smaller than mini-SNAP(c), we have identified a 50-amino-acid region within SNAP190 that is (i) required for cooperative binding with TBP in the context of mini-SNAP(c) and (ii) sufficient for cooperative binding with TBP when fused to a heterologous DNA binding domain. We show that derivatives of mini-SNAP(c) lacking this region are active for transcription and that with such complexes, TBP can still be recruited to the U6 promoter through cooperative interactions with Brf2. Our results identify complexes smaller than mini-SNAP(c) that are transcriptionally active and show that there are at least two redundant mechanisms to stably recruit TBP to the U6 transcription initiation complex.

Dexamethasone causes sustained expression of mitogen-activated protein kinase (MAPK) phosphatase 1 and phosphatase-mediated inhibition of MAPK p38

The stress-activated protein kinase p38 stabilizes a number of mRNAs encoding inflammatory mediators, such as cyclooxygenase 2 (Cox-2). In HeLa cells the anti-inflammatory glucocorticoid dexamethasone destabilizes Cox-2 mRNA by inhibiting p38 function. Here we demonstrate that this effect is phosphatase dependent. Furthermore, in HeLa cells dexamethasone induced the sustained expression of mitogen-activated protein kinase phosphatase 1 (MKP-1), a potent inhibitor of p38 function. The inhibition of p38 and the induction of MKP-1 by dexamethasone occurred with similar dose dependence and kinetics. No other known p38 phosphatases were induced by dexamethasone, and other cell types which failed to express MKP-1 also failed to inhibit p38 in response to dexamethasone. The proinflammatory cytokine interleukin 1 (IL-1) induced MKP-1 expression in a p38-dependent manner and acted synergistically with dexamethasone to induce MKP-1 expression. In HeLa cells treated with IL-1 or IL-1 and dexamethasone, the dynamics of p38 activation mirrored the expression of MKP-1. These observations suggest that MKP-1 participates in a negative-feedback loop which regulates p38 function and that dexamethasone may inhibit proinflammatory gene expression in part by inducing MKP-1 expression.

Interaction between glucocorticoids and beta2 agonists on bronchial airway smooth muscle cells through synchronised cellular signalling

BACKGROUND

Increased airway smooth muscle bulk is a pathological feature of asthma. Asthma is well controlled by the combined inhalation of glucocorticoids and beta2-adrenoceptor agonists. The basic molecular mechanism of the interaction of the two drugs on proliferation of airway smooth muscle cells is yet to be identified. Our aim was to elucidate how glucocorticoids and beta2 agonists affect the growth of human bronchial airway smooth muscle cells.

METHODS

We assessed the effect of formoterol and budesonide on the activation and function of transcription factors by immunohistochemistry, western blotting, DNA mobility shift assay, and a luciferase reporter gene assay. The effect of the drugs and the involvement of specific transcription factors on cell proliferation was ascertained by direct cell count and confirmed by thymidine incorporation.

FINDINGS

Both classes of drugs (10(-8) mol/L) activated C/EBP-alpha and the glucocorticoid receptor with different kinetic profiles, and inhibited proliferation. The combination of lower doses of drugs (10(-12) to 10(-9) mol/L) resulted in a synchronised activation of the transcription factors and an enhanced antiproliferative effect. The action of the drugs alone or in combination on transcription-factor activity and proliferation was suppressed by either depletion of C/EBP-alpha or in the presence of a glucocorticoid-receptor blocker.

INTERPRETATION

Our findings could provide one explanation for the interaction of beta2 agonists and glucocorticoids at a molecular level, and indicate that the concentration of inhaled glucocorticoids can be reduced when combined with beta2 agonists, minimising the side-effects of the drugs.

The effect of topical corticosteroids on Ki67 and p53 expression in vulval lichen sclerosus

BACKGROUND

Topical corticosteroids have become the treatment of choice for genital lichen sclerosus (LS) and are believed to be required for long-term relief of symptoms.

OBJECTIVE

To compare vulval LS that had been treated with topical corticosteroids, vulval LS that had not received topical corticosteroids, and histologically normal vulval skin.

METHODS

We used immunohistochemistry to look for Ki67 expression and abnormal p53 expression.

RESULTS

We found a statistically significant difference for p53 overexpression, with increased levels seen when comparing corticosteroid-treated LS with normal genital skin (P = 0.011). Ki67 expression was also significantly higher in the corticosteroid-treated group compared with normal genital skin (P = 0.001), and increased levels were also found in the treated group compared with untreated LS (P = 0.05).

CONCLUSIONS

Our data suggest that topical corticosteroids have an effect on cell cycle proteins in genital skin and, in particular, genital skin with LS changes.

Glucocorticoids inhibit lung cancer cell growth through both the extracellular signal-related kinase pathway and cell cycle regulators

Glucocorticoids inhibit the proliferation of various cell types, but the mechanism of this inhibition remains unclear. We investigated the effect of dexamethasone on non-small cell lung cancer cell growth and cell cycle progression. We showed that dexamethasone suppresses the proliferation of A549 and Calu-1 cells, with accumulation of cells in G1/G0 stage of the cell cycle, as determined by fluorescence-activated cell sorter analysis. Western blot analysis confirmed that this is associated with hypophosphorylation of retinoblastoma protein. Using Western blot analysis and in vitro kinase assays, we found that dexamethasone results in decreased activity of CDK2 and 4, decreased levels of cyclin D, E2F, and Myc, and increased levels of the CDK inhibitor p21(Cip1). In addition, we found that dexamethasone decreases activity of extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK). The kinetics of all these changes indicate that inhibition of the ERK/MAPK pathway precedes the cell cycle effects, suggesting that regulation of this MAPK-signaling pathway may be an alternative mechanism for glucocorticoid-induced cell cycle arrest and growth inhibition.

Enhancement of p53 activity and inhibition of neural cell proliferation by glucocorticoid receptor activation

In analyzing the molecular mechanisms underlying glucocorticoid-induced apoptosis in neural cells, we observed that dexamethasone, by activating glucocorticoid receptors, causes arrest of HT-22 cells in the G1 phase of the cell cycle; upon withdrawal of the agonist, cells resume proliferation. Our investigations revealed that glucocorticoid treatment, although having no effects on endogenous p53 protein stability, induces rapid translocation of p53 to the nucleus and enhances its transcriptional activity. Consistently, transfection studies with p53-responsive promoters revealed a substantial stimulation of the trans-activation potential of exogenous p53 by dexamethasone. Cells arrested in G1 failed to show signs of apoptosis even after overexpression of p53. Although dexamethasone induced transcription of the proapoptotic gene bax, there was no increase of Bax protein levels. We conclude that glucocorticoid receptor-induced neural cell cycle arrest is associated with an increase in nuclear translocation and transcriptional activity of p53, and suggest that potentiation of p53 may serve as a brake on cell proliferation and may prime cells for differentiation or death induced by other signals.

New ideas about aldosterone signaling in epithelia

The systemic actions of aldosterone are well documented; however, in comparison, our understanding of the cellular and molecular mechanisms by which aldosterone orchestrates these actions is rudimentary. Aldosterone exerts most of its physiological actions by modifying gene expression. It is now apparent that aldosterone represses almost as many genes as it induces. Several aldosterone-sensitive genes, including serum and glucocorticoid-inducible kinase (sgk) and small, monomeric Kirsten Ras GTP-binding protein (Ki-ras) have recently been identified. The molecular mechanisms and elements bestowing corticosteroid sensitivity on these and many other genes are becoming clear. Induction of Ki-Ras and Sgk is necessary and sufficient for some portion of aldosterone action in epithelia. These two signaling factors are components of a converging pathway with phosphatidylinositol 3-kinase positioned between them that enables both stabilizing the epithelial Na(+) channel (ENaC) in the open state as well as increasing the number of ENaC in the apical membrane. This aldosterone-induced signaling pathway contains many potential sites for feedback regulation and cross talk from other cascades and potentially impinges directly on the activity of transport proteins and/or cellular differentiation to modify electrolyte transport.

An examination of heme action in gene expression: heme and heme deficiency affect the expression of diverse genes in erythroid k562 and neuronal PC12 cells

The aim of this study was to identify key genes whose expression is altered by heme and heme deficiency in the human erythroleukemia K562 cells and in the NGF-induced rat pheochromocytoma neuronal PC12 cells, respectively. By quantitative RT-PCR, Northern blotting, and Western blotting analyses, we found that the expression of the CDK inhibitors p18 and p21 was upregulated at the early and late stages of heme-induced erythroid differentiation of K562 cells, respectively, while the expression of cyclin D1 was downregulated. Data from succinyl acetone and desferrioxamine treatments suggest that these effects of heme in K562 cells were specific. Further, by microarray expression analysis, we found that inhibition of heme synthesis by succinyl acetone in NGF-induced PC12 cells drastically altered the expression of several groups of important neuronal genes, including the structural genes encoding neurofilament proteins and synaptic vesicle proteins, regulatory genes encoding signaling components beta-arrestin and p38 MAPK, and stress-response genes encoding hsp70. These results show that heme and heme deficiency affect the expression of diverse genes in a cell-type specific manner in mammalian cells, and that heme, although needed at different levels, is critical for both erythropoiesis and neurogenesis. These studies provide insights into how heme may act to control diverse regulatory processes in mammals.

CCAAT/enhancer binding protein alpha uses distinct domains to prolong pituitary cells in the growth 1 and DNA synthesis phases of the cell cycle

BACKGROUND

A number of transcription factors coordinate differentiation by simultaneously regulating gene expression and cell proliferation. CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic/leucine zipper transcription factor that integrates transcription with proliferation to regulate the differentiation of tissues involved in energy balance. In the pituitary, C/EBPalpha regulates the transcription of a key metabolic regulator, growth hormone.

RESULTS

We examined the consequences of C/EBPalpha expression on proliferation of the transformed, mouse GHFT1-5 pituitary progenitor cell line. In contrast to mature pituitary cells, GHFT1-5 cells do not contain C/EBPalpha. Ectopic expression of C/EBPalpha in the progenitor cells resulted in prolongation of both growth 1 (G1) and the DNA synthesis (S) phases of the cell cycle. Transcription activation domain 1 and 2 of C/EBPalpha were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPalpha remained competent for G1 and S phase prolongation. C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.

CONCLUSION

We found that C/EBPalpha utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells. G1 and S phase prolongation did not require that C/EBPalpha remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPalpha transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

C/EBPalpha triggers proteasome-dependent degradation of cdk4 during growth arrest

CCAAT/enhancer binding protein alpha (C/EBPalpha) causes growth arrest via direct interaction with the cyclin-dependent kinases cdk2 and cdk4. In this paper, we present evidence showing that C/EBPalpha enhances a proteasome-dependent degradation of cdk4 during growth arrest in liver of newborn mice and in cultured cells. Overexpression of C/EBPalpha in several biological systems leads to a reduction of cdk4 protein levels, but not mRNA levels. Experiments with several tissue culture models reveal that C/EBPalpha enhances the formation of cdk4-ubiquitin conjugates and induces degradation of cdk4 through a proteasome-dependent pathway. As a result, the half-life of cdk4 is shorter and protein levels of cdk4 are reduced in cells expressing C/EBPalpha. Gel filtration analysis of cdk4 complexes shows that a chaperone complex cdk4-cdc37-Hsp90, which protects cdk4 from degradation, is abundant in proliferating livers that lack C/EBPalpha, but this complex is weak or undetectable in livers expressing C/EBPalpha. Our studies show that C/EBPalpha disrupts the cdk4-cdc37-Hsp90 complex via direct interaction with cdk4 and reduces protein levels of cdk4 by increasing proteasome-dependent degradation of cdk4.

Interaction of C/EBPalpha and the glucocorticoid receptor in vivo and in nontransformed human cells

Belonging to the family of steroid hormones, glucocorticoids are essential for development and survival of vertebrates. The cellular response to glucocorticoids is attributed to the glucocorticoid receptor, which functions as a transcription factor. However, the majority of glucocorticoid-modulated genes lack a DNA binding site for the glucocorticoid receptor, raising the question of which mechanism mediates the responses to glucocorticoids. It has been suggested that besides direct DNA binding of the glucocorticoid receptor, interaction with members of other transcription factor families modulates the effect of the glucocorticoid receptor. However, the significance of such transcription factor interaction is not clear. In cultured human mesenchymal cells and peripheral blood leukocytes of human volunteers treated with glucocorticoids, we detected the formation of a complex between the GR and the CCAAT/enhancer binding protein alpha. In in vitro experiments, this interaction turned out to be responsible for the inhibitory action of glucocorticoids on lymphocytic and mesenchymal cell proliferation. Our results suggest that complex formation of the GR with C/EBPalpha accounts for a novel pathway of glucocorticoid action.

Conditional gene targeting for cancer gene therapy

Current treatment of solid tumors is limited by severe adverse effects, resulting in a narrow therapeutic index. Therefore, cancer gene therapy has emerged as a targeted approach that would significantly reduce undesired side effects in normal tissues. This approach requires a clear understanding of the molecular biology of both the malignant clone and the biological vectors that serve as vehicles to target cancer cells. In this review we discuss novel approaches for conditional gene expression in cancer cells. Targeting transgene expression to malignant tissues requires the use of specific regulatory elements including promoters based on tumor biology, tissue-specific promoters and inducible regulatory elements. We also discuss the regulation of both replication and transgene expression by conditionally-replicative viruses. These approaches have the potential to restrict the expression of transgenes exclusively to tissues of interest and thereby to increase the therapeutic index of future vectors for cancer gene therapy.

Transcriptional regulation of the lung fatty acid synthase gene by glucocorticoid, thyroid hormone and transforming growth factor-beta 1

Fatty acid synthase (FAS) is a key enzyme in the biosynthesis of lung surfactant. FAS expression in fetal lungs is increased by glucocorticoids and this effect is largely due to increased transcription. The stimulatory effect of glucocorticoid on FAS expression is antagonized by thyroid hormone and transforming growth factor-beta 1 (TGF-beta 1). To determine the glucocorticoid responsive regions of the FAS gene we employed deletion analysis and reporter gene assays. A549 cells were transfected with various FAS gene constructs ligated to the firefly luciferase gene and cultured with dexamethasone (Dex) for 24 h after which luciferase activity was measured. Dex increased luciferase expression in response to a fragment in the promoter and 5'-flanking region of the FAS gene, from -1592 to +65 bp. This increase was antagonized by triiodothyronine (T(3)) and TGF-beta 1. Serial deletions showed that the full response to Dex and T(3) were retained in the 89 bp -33/+56 bp fragment whereas the response to TGF was mediated by the immediately upstream -104/-34 bp sequence. The Dex responsive region of the FAS gene could not be separated from the minimal promoter showing that they are intimately associated. The extents of Dex stimulation and antagonism by T(3) and TGF in A549 cells were similar to those noted on parameters of FAS expression in fetal lung explants. These data show that the effects of Dex, T(3) and TGF on FAS expression are mediated by DNA sequences in the promoter region of the gene.

Identification and implantation stage-specific expression of an interferon-alpha-regulated gene in human and rat endometrium

Implantation of the developing blastocyst is regulated by multiple effectors, such as steroid hormones, growth factors, and cytokines. To understand how these diverse signaling pathways interact to modulate uterine gene expression, we employed a gene expression screen technique to identify the molecules that are induced in the periimplantation rat uterus. Here we report the isolation of a complementary DNA representing a novel gene, interferon-regulated gene 1 (IRG1). This gene exhibits significant homology to interferon (IFN)-alpha/beta-inducible human genes p27 and 6-16, indicating that these genes may belong to the same family. Consistent with this finding, expression of IRG1 messenger RNA (mRNA) in rat uterus increased about 20-fold in response to IFNalpha. Uterine expression of IRG1 was also stimulated by estrogen and was partially inhibited by an antiestrogen, ICI 182,780. In pregnant rats, IRG1 expression was high on day 1, but declined on days 2 and 3. The level of IRG1 mRNA again rose transiently on day 4 immediately preceding implantation. In situ hybridization analysis localized the IRG1 mRNA expression in the endometrial epithelium and the surrounding stroma. Interestingly, the expression of p27, which shows high homology to IRG1, was strongly enhanced in human endometrium during the midsecretory phase of the menstrual cycle, overlapping the putative window of implantation. Both IRG1 and p27 mRNAs are therefore induced in the endometrium in an implantation stage-specific manner. We also observed a synergistic interaction between IFNalpha and estrogen receptor signaling pathways that led to maximal induction of p27 mRNA in Ishikawa cells. Although the functional roles of IRG1 and p27 remain unclear, we describe for the first time, identification of a gene family regulated by IFNalpha in both rodent and human uteri. More importantly, our studies reveal that a complex interplay between the steroid hormone and IFN pathways regulates the expression of these genes in the endometrium at the time of implantation.

Glucocorticoids stimulate p21(CIP1) in mesangial cells and in anti-GBM glomerulonephritis

BACKGROUND

Glucocorticoids are widely used for the treatment of glomerulonephritis, but the mechanism of cell cycle inhibition by glucocorticoids is poorly understood at a molecular level.

METHODS

The effects of dexamethasone on cell cycle progression were examined in rat mesangial cells. To investigate the mechanisms of cell cycle inhibition by dexamethasone, we transfected the -2.3 kb p21(CIP1) promoter-CAT construct to mesangial cells using an electroporation

METHOD

We also examined whether glucocorticoids stimulate the expression of p21(CIP1) and inhibit cell proliferation in glomeruli of anti-glomerular basement membrane (GBM) glomerulonephritis in rats.

RESULTS

Dexamethasone inhibited 3H-thymidine uptake and the percentages of S and G2/M phases in rat mesangial cells. Dexamethasone stimulated CAT activity of the p21(CIP1) promoter 4.5-fold. Deletion analysis of the p21(CIP1) promoter revealed that the glucocorticoid-responsive region (GRE) is present between -1.4 and -1.1 kb upstream of the transcription initiation site. Dexamethasone inducibility of p21(CIP1) promoter activity requires the presence of the C/EBP alpha DNA binding site in the GRE of the p21(CIP1) promoter and C/EBP alpha protein. Intravenous injection of anti-GBM antibody caused mesangial proliferation, crescent formation, and proteinuria in rats. Ten days of administration of prednisolone (1 mg/kg/day) reduced proteinuria and inhibited mesangial cell proliferation and crescent formation. The glomerular-sieving method revealed that prednisolone increased p21(CIP1) expression in glomeruli.

CONCLUSION

These data suggest that the cell cycle arrest of mesangial cells is mediated by a functional link between the glucocorticoid receptor and the transcriptional control of p21(CIP1) not only in vitro but also in vivo. Our observations provide new insights into the molecular mechanisms of glucocorticoid action in glomerulonephritis.

The bronchial epithelium in chronic and severe asthma

Although patients with severe, steroid-refractory asthma represent a minor proportion of the asthmatic population, they consume a disproportionate amount of healthcare costs and have a greatly impaired quality of life. They respond poorly to conventional anti-inflammatory therapy and frequently exhibit a component of fixed airflow obstruction that has been linked to airway wall remodeling. In addition to its classic barrier function, the bronchial epithelium responds to changes in the external environment by secreting cytoprotective molecules and mediators that signal to cells of the immune system. In asthma, the bronchial epithelium is stressed and damaged, with shedding of the columnar cells into the airway lumen. This damage and ensuing repair responses are proposed to orchestrate airway inflammation and remodeling via activation of myofibroblasts in the underlying lamina reticularis. This allows the two cell types to work as a trophic unit, propagating and amplifying the response at the cell surface into the submucosa. Because wound healing involves inflammation, repair, and remodeling processes, this review considers the evidence that exaggerated inflammation and remodeling of the airways arise as a consequence of abnormal injury and repair responses coordinated by the bronchial epithelium, highlighting, where possible, steroid-insensitive components.

p53-independent upregulation of KILLER/DR5 TRAIL receptor expression by glucocorticoids and interferon-gamma

KILLER/DR5 is a death-domain-containing proapoptotic receptor that binds to the cytotoxic ligand TRAIL. It was originally reported that induction of KILLER/DR5 mRNA following DNA damage was p53-dependent, but some drugs that induce apoptosis can upregulate KILLER/DR5 mRNA expression in cell lines with mutated p53. We further extend those findings by classifying the capability of various apoptosis-inducing drugs to increase the expression of KILLER/DR5 mRNA in a p53-independent manner. beta-Lapachone, a topoisomerase inhibitor, increased KILLER/DR5 mRNA in colon cancer cell lines with wild-type p53 but not with mutant p53. In contrast, betulinic acid, a novel chemotherapeutic compound, induced apoptosis and KILLER/DR5 mRNA in melanoma and glioblastoma cells through a p53-independent mechanism. The synthetic glucocorticoid dexamethasone elevated KILLER/DR5 mRNA in glioblastoma, ovarian cancer, and colon cancer cell lines with mutant p53 undergoing apoptosis, and this induction was inhibited by the transcriptional inhibitor actinomycin D. Although another glucocorticoid, prednisolone, also induced apoptosis, it did not increase KILLER/DR5 mRNA. Finally, the cytokine interferon-gamma (IFN-gamma) induced apoptosis and KILLER/DR5 in cell lines with mutant p53, and the induction of KILLER/DR5 mRNA by IFN-gamma was delayed in cells lacking wild-type STAT1, a transcription factor implicated in IFN-gamma signaling. Similarly, the induction of KILLER/DR5 mRNA by the cytokine TNF-alpha was also delayed in cell lines with mutated STAT1. These findings suggest that KILLER/DR5 may play a role in p53-independent apoptosis induced by specific drugs and warrants further investigation as a novel target for chemotherapy of tumors lacking wild-type p53.

Increased expression of cyclin D1, cyclin E and p21(Cip1) associated with decreased expression of p27(Kip1) in chemically induced rat mammary carcinogenesis

We induced rat mammary tumors in 7-week-old female Sprague-Dawley rats by intragastric administration of 7,12-dimethylbenz(a)anthracene (DMBA), and analyzed by immunohistochemistry the expression of cyclin D1, cyclin E, p21(Cip1), and p27(Kip1) in carcinomas, atypical tumors, and benign tumors as well as normal mammary glands from the control group. Proliferation status was assessed by immunohistochemistry using bromodeoxyuridine (BrdU). A sequential increase in cyclin D1-, cyclin E-, and p21(Cip1)-positive epithelial cells was observed from normal mammary glands, to atypical tumors, to carcinomas. In contrast, carcinomas showed a significantly lower number of epithelial cells immunoreactive to p27(Kip1) when compared with atypical tumors, benign tumors and normal mammary glands. The immunoreactivities of BrdU, cyclin D1, cyclin E, and p21(Cip1) were positively correlated, whereas that of p27(Kip1) appeared inversely correlated to those of the others. Reverse transcriptase-polymerase chain reaction (RT-PCR) and western blot analysis were also performed to determine the mRNA and protein levels of cyclins and cyclin-dependent kinase inhibitors in tumors and normal mammary glands. The protein levels for cyclin D1, cyclin E and p21(Cip1) in carcinomas and atypical tumors were significantly higher than those in benign tumors, while normal mammary glands showed negligible expression. On RT-PCR, tumors showed higher mRNA levels of cyclin D1 and cyclin E than those of normal mammary glands. Our results suggest that rat mammary carcinogenesis involves increased expression of cyclin D1, cyclin E, and p21(Cip1), associated with decreased expression of p27(Kip1).

Negative cross-talk between p53 and the glucocorticoid receptor and its role in neuroblastoma cells

The tumour suppressor p53 and the glucocorticoid receptor (GR) respond to different types of stress. We found that dexamethasone-activated endogenous and exogenous GR inhibit p53-dependent functions, including transactivation, up- (Bax and p21(WAF1/CIP1)) and down- (Bcl2) regulation of endogenous genes, cell cycle arrest and apoptosis. GR forms a complex with p53 in vivo, resulting in cytoplasmic sequestration of both p53 and GR. In neuroblastoma (NB) cells, cytoplasmic retention and inactivation of wild-type p53 involves GR. p53 and GR form a complex that is dissociated by GR antagonists, resulting in accumulation of p53 in the nucleus, activation of p53-responsive genes, growth arrest and apoptosis. These results suggest that molecules that efficiently disrupt GR-p53 interactions would have a therapeutic potential for the treatment of neuroblastoma and perhaps other diseases in which p53 is sequestered by GR.

Wild-type p53-dependent etoposide-induced apoptosis mediated by caspase-3 activation in human glioma cells

OBJECT

In an attempt to understand the roles of several apoptosis-related genes in human glioma cells, the authors investigated the relationship of wild-type p53, interleukin-1beta-converting enzyme (ICE), caspase-3 (CPP32), bax, and bcl-2 to the apoptotic response of three glioma cell lines after treatment with etoposide.

METHODS

A human glioma cell line (U-87MG) that expresses wild-type p53, one that expresses mutant p53 (T-98G), and a T-98G derivative (T-98G/p53) that was transfected with a wild-type p53 expression vector (pCDM8-p53/neo) were used. Cell growth inhibition in response to etoposide was quantified using a modified methylthiazol tetrazolium colorimetric assay. Induction of apoptosis was evaluated using Hoechst 33258 staining and a DNA fragmentation assay. To study the expression of the apoptosis-related proteins and messenger RNAs in the three glioma cell lines, Western blotting and polymerase chain reaction were performed. A caspase assay and Western blot analysis were used to assess CPP32 and ICE protease activity. A CPP32 inhibition assay was used to determine whether a specific CPP32 inhibitor, DEVD-CHO, affects the apoptosis induced by etoposide in malignant glioma cells. Etoposide significantly inhibited the growth of U-87MG and T-98G/p53 cells in a dose-dependent manner compared with the growth of the T-98G cells. Treatment with low concentrations of etoposide resulted in the increased expression of wild-type p53; it also initiated CPP32 activity and induced apoptosis in the U-87MG cells. Apoptosis was not induced in T-98G cells at low concentrations of etoposide, although it was induced at high concentrations. Furthermore, low concentrations of etoposide also induced apoptosis in the T-98G/p53 cells by enhancing the expression of transfected wild-type p53, decreasing the expression of bcl-2, and activating CPP32 activity. However, etoposide did not alter the expression of bax and did not initiate ICE activity in these three glioma cell lines. Etoposide-induced apoptosis can be suppressed by the CPP32 inhibitor DEVD-CHO.

CONCLUSIONS

These findings indicate that wild-type p53, CPP32, and bcl-2 may mediate apoptosis induced by etoposide. Forced expression of wild-type p53 increases etoposide cytotoxicity in human glioma cells by inducing apoptosis and may have important therapeutic implications.

C/EBPalpha inhibits cell growth via direct repression of E2F-DP-mediated transcription

Using an inducible transcription system which allows the regulated expression of C/EBP isoforms in tissue culture cells, we have found that the ectopic expression of C/EBPalpha, at a level comparable to that found in normal liver tissue, has a pronounced antimitogenic effect in mouse L cells and NIH 3T3 cells. The inhibition of cell division by C/EBPalpha in mouse cells cannot be reversed by simian virus 40 T antigen, by oncogenic ras, or by adenovirus E1a protein. When expressed in thymidine kinase-deficient L cells or 3T3 cells, C/EBPalpha is detected in a protein complex which binds to the E2F binding sites found in the promoters of the genes for E2F-1 and dihydrofolate reductase (DHFR). Bacterially expressed C/EBPalpha has no affinity for these E2F sites, but when recombinant C/EBPalpha is added to nuclear extracts from mouse fibroblasts, a new E2F binding activity appears, which contains the C/EBPalpha protein. Using an E2F-DP1-responsive promoter linked to a reporter gene, it can be shown that C/EBPalpha directly inhibits the induction of this promoter by E2F-DP1 in transient-transfection assays. Furthermore, C/EBPalpha can be shown to inhibit the S-phase induction of the E2F and DHFR promoters in permanent cell lines. These findings delineate a straightforward mechanism for C/EBPalpha-mediated cell growth arrest through repression of E2F-DP-mediated S-phase transcription.

Glucocorticoids inhibit developmental stage-specific osteoblast cell cycle. Dissociation of cyclin A-cyclin-dependent kinase 2 from E2F4-p130 complexes

Unique cell cycle control is instituted in confluent osteoblast cultures, driving growth to high density. The postconfluent dividing cells share features with cells that normally exit the cell cycle; p27(kip1) is increased, p21(waf1/cip1) is decreased, free E2F DNA binding activity is reduced, and E2F4 is primarily nuclear. E2F4-p130 becomes the predominant E2F-pocket complex formed on E2F sites, but, unlike the complex that typifies resting cells, cyclin A and CDK2 are also present. Administration of dexamethasone at this, but not earlier stages, results in reduction of cyclin A and CDK2 levels with a parallel decrease in the associated kinase activity, dissociation of cyclin A-CDK2 from the E2F4-p130 complexes, and inhibition of G(1)/S transition. The glucocorticoid-mediated cell cycle attenuation is also accompanied by, but not attributable to, increased p27(kip1) and decreased p21(waf1/cip1) levels. The attenuation of osteoblast growth to high density by dexamethasone is associated with severe impairment of mineralized extracellular matrix formation, unless treatment commences in cultures that have already grown to high density. Both the antimitotic and the antiphenotypic effects are reversible, and both are antagonized by RU486. Thus, glucocorticoids induce premature attenuation of the osteoblast cell cycle, possibly contributing to the osteoporosis induced by these drugs in vivo.

Two-month glucocorticoid treatment increases proliferation in the stomach and large intestine of rats

BACKGROUND/AIMS

It has been shown that acute or short-term treatments with glucocorticoids lead to a marked decrease in proliferation in the stomach and large intestine. The effects of more prolonged glucocorticoid treatment on cell renewal in these organs are not known. The present work was therefore undertaken to examine the proliferative activity in the stomach and colon during 2 months of glucocorticoid treatment in comparison with shorter treatments.

METHODS

Rats were treated with either the glucocorticoid triamcinolone acetonide or vehicle for 63, 33 or 3 days. Proliferation was assessed in the glandular epithelium of the fundal part of the stomach and in the epithelium of the colonic crypts using three criteria: the mitotic index; the bromodeoxyuridine labelling index, and the proliferating cell nuclear antigen-labelling index (percentage of mitotic or labelled cells).

RESULTS

Treatment with glucocorticoid for 63 days resulted in a very significant increase in all proliferative parameters tested in the gastric mucosa and the colonic crypts. On the contrary, treatments with glucocorticoid for 3 or 33 days had a marked inhibitory influence on proliferation in these tissues.

CONCLUSION

As opposed to treatments for 3 or 33 days, glucocorticoid treatment for 2 months leads to an increase in the number of cycling cells in the gastric and colonic mucosae.

Novel mechanism of steroid action in skin through glucocorticoid receptor monomers

Glucocorticoids (GCs), important regulators of epidermal growth, differentiation, and homeostasis, are used extensively in the treatment of skin diseases. Using keratin gene expression as a paradigm of epidermal physiology and pathology, we have developed a model system to study the molecular mechanism of GCs action in skin. Here we describe a novel mechanism of suppression of transcription by the glucocorticoid receptor (GR) that represents an example of customizing a device for transcriptional regulation to target a specific group of genes within the target tissue, in our case, epidermis. We have shown that GCs repress the expression of the basal-cell-specific keratins K5 and K14 and disease-associated keratins K6, K16, and K17 but not the differentiation-specific keratins K3 and K10 or the simple epithelium-specific keratins K8, K18, and K19. We have identified the negative recognition elements (nGREs) in all five regulated keratin gene promoters. Detailed footprinting revealed that the function of nGREs is to instruct the GR to bind as four monomers. Furthermore, using cotransfection and antisense technology we have found that, unlike SRC-1 and GRIP-1, which are not involved in the GR complex that suppresses keratin genes, histone acetyltransferase and CBP are. In addition, we have found that GR, independently from GREs, blocks the induction of keratin gene expression by AP1. We conclude that GR suppresses keratin gene expression through two independent mechanisms: directly, through interactions of keratin nGREs with four GR monomers, as well as indirectly, by blocking the AP1 induction of keratin gene expression.

Ras induces p21Cip1/Waf1 cyclin kinase inhibitor transcriptionally through Sp1-binding sites

p21Cip1/Waf1 cyclin-dependent kinase inhibitor (p21) is inducible by Raf and mitogen-activated protein kinase kinase (MAPKK), but the level of regulation is unknown. We show here by conditional and transient Ras-expression models that Ras induces p21. Induction of p21 in conditionally Ras-expressing cells is posttranscriptional utilizing mitogen-activated protein kinase (MAPK) pathway. Transient, high-level Ras-expression induces transcriptional activation of p21 mediated by a GC-rich region in p21 promoter -83-54 bp relative to the transcription initiation site containing binding sites for Sp1-family transcription factors. Mutation of either Sp1-binding site 2 or 4 in this region decreases the magnitude of induction of promoter activity by Ras, but only the simultaneous mutation of both sites abolishes fully the induction. Electrophoretic mobility shift assays using an oligonucleotide corresponding to Sp1-binding site 2 indicate that both Sp1 and Sp3 transcription factors bind to this region. The results demonstrate that the central cytosolic growth regulator Ras is a potent transcriptional and posttranscriptional inducer of the nuclear growth inhibitor p21.

p57Kip2, a glucocorticoid-induced inhibitor of cell cycle progression in HeLa cells

Glucocorticoids exert antiproliferative effects on a number of cell types, including the HeLa cervical carcinoma cell line. However, the mechanism responsible for the antiproliferative effect is poorly understood. In this report we have investigated the role of the recently identified cyclin-dependent kinase inhibitor (CDI) p57Kip2 in the antiproliferative effect conferred by glucocorticoids. When HeLa cells were treated with the synthetic glucocorticoid dexamethasone (DEX), the doubling time of exponentially growing cells increased 2-fold. Within 11 h of DEX treatment, this was accompanied by an accumulation of cells in the G1 phase of the cell cycle with a corresponding decreased proportion of cells in the S phase and decreased CDK2 activity. DEX treatment of the HeLa cells dramatically induced the protein and mRNA expression of the CDI p57Kip2. This induction was seen within 4 h of DEX treatment, preceding a major DEX-induced accumulation of cells in the G1 phase. DEX-induced mRNA expression of p57Kip2 did not require de novo protein synthesis, and the transcription of the p57Kip2 gene was increased as determined by a run-on transcription assay. Furthermore, DEX induction of p57Kip2 was not a consequence of the cell cycle arrest, since other growth inhibition signals did not result in strong p57Kip2 induction. Overexpression of p57Kip2 using HeLa cells stably transfected with a tetracycline-inducible vector showed that p57Kip2 is sufficient to reconstitute an antiproliferative effect similar to that seen in DEX-treated cells. Selective p57Kip2 expression by the tetracycline analog doxycycline to levels comparable to those observed on DEX induction resulted in a 1.7-fold increase in the doubling time and a shift of HeLa cells to the G1 phase as well as a decrease in CDK2 activity. Taken together, these results suggest that glucocorticoid treatment directly induces transcription of the p57Kip2 gene and that the p57Kip2 protein is involved in the glucocorticoid-induced antiproliferative effect.

Glucocorticoids have pleiotropic effects on small intestinal crypt cells

Glucocorticoids have long been known to accelerate maturation of the intestinal tract, but the molecular mechanisms that account for their physiological function in the epithelium remain poorly characterized. Using rat intestinal epithelial cell lines (IEC-6, IEC-17, and IEC-18) as models, we have characterized glucocorticoid receptors in crypt cells and documented striking morphological, ultrastructural, and functional alterations induced by these hormones in intestinal cells. They include arrest of growth, formation of tight junctions, appearance of long, slender microvilli, reorganization of the endoplasmic reticulum and trans-Golgi network, and downregulation of the cell cycle regulatory proteins cyclin-dependent kinase 6 and p27(Kip1). These effects are consistent with the activation or modulation of multiple genes important in the physiological function of absorptive villous cells but are probably not directly involved in the induction of cell differentiation.

Dual signaling role of the protein tyrosine phosphatase SHP-2 in regulating expression of acute-phase plasma proteins by interleukin-6 cytokine receptors in hepatic cells

One of the major actions of interleukin-6 (IL-6) is the transcriptional activation of acute-phase plasma proteins (APP) genes in liver cells. Signaling by the IL-6 receptor is mediated through the signal transducing subunit gp130 and involves the activation of Janus-associated kinases (JAKs), signal transducer and activator of transcription 3 (STAT3), and mitogen-activated protein (MAP) kinase. Functional analysis of gp130 in rat hepatoma cells by using transduced chimeric G-CSFR-gp130 receptor constructs demonstrates that SHP-2, the Src homology 2 (SH2) domain-containing protein tyrosine phosphatase, acts as a negative regulator of the JAK/STAT signaling in part by downregulating JAK activity, thereby indirectly moderating the induction of STAT3-dependent APP genes. This study shows that in hepatoma cells, the recruitment and tyrosine phosphorylation of SHP-2, but not SHC, is the primary signaling event associated with the activation of MAP kinases (ERK1/2) by gp130. Overexpression of truncated SHP-2 that lacks Grb2-interacting sites, but not the full-length catalytically inactive SHP-2, reduces ERK activation by IL-6, confirming the signal-mediating role of SHP-2. Activation of ERK1/2 is correlated with induction of the immediate-early response genes. Stimulation of the c-fos, c-jun, and egr-1 genes is essentially absent in cells expressing gp130 with a Y759F mutation, which is unable to recruit SHP-2. Interestingly, both JAK/STAT and SHP-2 pathways regulate the induction of the junB gene. Moreover, disengagement of SHP-2 from gp130 signaling not only enhances APP gene induction but also further reduces cell proliferation, in part correlated with the attenuated expression of immediate-early response genes. These results suggest that IL-6 regulation of APP genes is affected by SHP-2 in two ways: SHP-2 acts as a phosphatase on the JAK/STAT pathway and serves as linker to the MAP kinase pathway, which in turn moderates APP production.

Distinct glucocorticoid receptor transcriptional regulatory surfaces mediate the cytotoxic and cytostatic effects of glucocorticoids

Glucocorticoids act through the glucocorticoid receptor (GR), which can function as a transcriptional activator or repressor, to elicit cytostatic and cytotoxic effects in a variety of cells. The molecular mechanisms regulating these events and the target genes affected by the activated receptor remain largely undefined. Using cultured human osteosarcoma cells as a model for the GR antiproliferative effect, we demonstrate that in U20S cells, GR activation leads to irreversible growth inhibition, apoptosis, and repression of Bcl2. This cytotoxic effect is mediated by GR's transcriptional repression function, since transactivation-deficient mutants and ligands still bring about apoptosis and Bcl2 down-regulation. In contrast, the antiproliferative effect of GR in SAOS2 cells is reversible, does not result in apoptosis or repression of Bcl2, and is a function of the receptor's ability to stimulate transcription. Thus, the cytotoxic versus cytostatic outcome of glucocorticoid treatment is cell context dependent. Interestingly, the cytostatic effect of glucocorticoids in SAOS2 cells involves multiple GR activation surfaces. GR mutants and ligands that disrupt individual transcriptional activation functions (activation function 1 [AF-1] and AF-2) or receptor dimerization fail to fully inhibit cellular proliferation and, remarkably, discriminate between the targets of GR's cytostatic action, the cyclin-dependent kinase inhibitors p21(Cip1) and p27(Kip1). Induction of p21(Cip1) is agonist dependent and requires AF-2 but not AF-1 or GR dimerization. In contrast, induction of p27(Kip1) is agonist independent, does not require AF-2 or AF-1, but depends on GR dimerization. Our findings indicate that multiple GR transcriptional regulatory mechanisms that employ distinct receptor surfaces are used to evoke either the cytostatic or cytotoxic response to glucocorticoids.

Glucocorticoid inhibits oxidized LDL-induced macrophage growth by suppressing the expression of granulocyte/macrophage colony-stimulating factor

Glucocorticoid, an anti-inflammatory agent, inhibits the development of atherosclerosis in various experimental animal models. This is partially explained by its ability to inhibit smooth muscle cell migration and proliferation in the intima and to reduce chemotaxis of circulating monocytes and leukocytes into the subendothelial spaces. We have recently demonstrated that oxidized LDL (Ox-LDL) has a mitogenic activity for macrophages in vitro in which Ox-LDL-induced granulocyte/macrophage colony-stimulating factor (GM-CSF) production plays an important role. Proliferation of cellular components is one of the characteristic events in the development and progression of atherosclerotic lesions. In the present study, we investigated the effects of glucocorticoids on Ox-LDL-induced macrophage growth. Dexamethasone, prednisolone, and cortisol inhibited Ox-LDL-induced thymidine incorporation into macrophages by 85%, 70%, and 50%, respectively. Ox-LDL induced a significant production of GM-CSF by macrophages, which was effectively inhibited by dexamethasone, prednisolone, and cortisol by 80%, 65%, and 50%, respectively. Dexamethasone-mediated inhibition of Ox-LDL-induced GM-CSF mRNA expression and macrophage growth was significantly abrogated by RU-486, a glucocorticoid receptor antagonist. Our results suggest that the inhibitory effects of glucocorticoids on macrophage growth may be due to the inhibition of Ox-LDL-induced GM-CSF production through transactivation of the glucocorticoid receptor.

Glucocorticoid block of protein kinase C signalling in mouse pituitary corticotroph AtT20 D16:16 cells

1. The regulation of large conductance calcium- and voltage-activated potassium (BK) currents by activation of the protein kinase C (PKC) and glucocorticoid signalling pathways was investigated in AtT20 D16:16 clonal mouse anterior pituitary corticotroph cells. 2. Maximal activation of PKC using the phorbol esters, 4beta-phorbol 12-myristate, 13-acetate (PMA), phorbol 12, 13 dibutyrate (PDBu) and 12-deoxyphorbol 13-phenylacetate (dPPA) elicited a rapid, and sustained, inhibition of the outward steady-state voltage- and calcium- dependent potassium current predominantly carried through BK channels. 3. The effect of PMA was blocked by the PKC inhibitors bisindolylmaleimide I (BIS; 100 nM) and chelerythrine chloride (CHE; 25 microM) and was not mimicked by the inactive phorbol ester analogue 4alpha-PMA. 4. PMA had no significant effect on the 1 mM tetraethylammonium (TEA)-insensitive outward current or pharmacologically isolated, high voltage-activated calcium current. 5. PMA had no significant effect on steady-state outward current in cells pre-treated for 2 h with 1 microM of the glucocorticoid agonist dexamethasone. Dexamethasone had no significant effect on steady-state outward current amplitude or sensitivity to 1 mM TEA and did not block PMA-induced translocation of the phorbol ester-sensitive PKC isoforms, PKCalpha and PKCepsilon, to membrane fractions. 6. Taken together these data suggest that in AtT20 D16:16 corticotroph cells BK channels are important targets for PKC action and that glucocorticoids inhibit PKC signalling downstream of PKC activation.

Induced Differentiation of U937 Cells by 1,25-dihydroxyvitamin D3 Involves Cell Cycle Arrest in G1 That Is Preceded by a Transient Proliferative Burst and an Increase in Cyclin Expression

The hormonal form of vitamin D, 1,25-dihydroxyvitamin D3[1,25(OH)2D3], is a potent inhibitor of cellular proliferation as well as an inducer of differentiation of myeloid leukemic cells to macrophages. We have previously reported that a number of genes are upregulated by 1,25(OH)2D3 during myeloid differentiation, including the cyclin-dependent kinase (CDK) inhibitors p21, p27, 15, and p18, suggesting that cell cycle arrest and differentiation are tightly linked processes. We further explore here the relationship between growth inhibition and differentiation. We report that, upon 1,25(OH)2D3 treatment, U937 cells exhibited an early proliferative burst followed by growth inhibition and subsequent differentiation. Although CDK levels remain constant throughout, this transient increase in proliferation was accompanied by increases in cyclin A, D1, and E protein levels. p21 and p27 levels were also elevated during both the proliferative burst and subsequent inhibition of cell growth. Ectopic overexpression of p21 and/or p27 in U937 cells, in the absence of hormone, resulted in an induction of the expression of monocyte/macrophage-specific markers, whereas overexpression of p15 and p18 had no effect, suggesting that a subset of CDK inhibitors are important for both growth arrest and differentiation and that an early increase in proliferation is somehow a prerequisite for subsequent differentiation. However, no such biphasic behavior was detected in cells that are growth inhibited by 1,25(OH)2D3but do not differentiate, such as MCF-7 cells. Taken together, these results indicate that both growth stimulation and subsequent inhibition precede differentiation and involve induction of both cyclins and p21 and p27, whereas cell cycle arrest of differentiated cells can be achieved simply by elevations in CDK inhibitors.

Induced Differentiation of U937 Cells by 1,25-dihydroxyvitamin D3 Involves Cell Cycle Arrest in G1 That Is Preceded by a Transient Proliferative Burst and an Increase in Cyclin Expression

The hormonal form of vitamin D, 1,25-dihydroxyvitamin D3[1,25(OH)2D3], is a potent inhibitor of cellular proliferation as well as an inducer of differentiation of myeloid leukemic cells to macrophages. We have previously reported that a number of genes are upregulated by 1,25(OH)2D3 during myeloid differentiation, including the cyclin-dependent kinase (CDK) inhibitors p21, p27, 15, and p18, suggesting that cell cycle arrest and differentiation are tightly linked processes. We further explore here the relationship between growth inhibition and differentiation. We report that, upon 1,25(OH)2D3 treatment, U937 cells exhibited an early proliferative burst followed by growth inhibition and subsequent differentiation. Although CDK levels remain constant throughout, this transient increase in proliferation was accompanied by increases in cyclin A, D1, and E protein levels. p21 and p27 levels were also elevated during both the proliferative burst and subsequent inhibition of cell growth. Ectopic overexpression of p21 and/or p27 in U937 cells, in the absence of hormone, resulted in an induction of the expression of monocyte/macrophage-specific markers, whereas overexpression of p15 and p18 had no effect, suggesting that a subset of CDK inhibitors are important for both growth arrest and differentiation and that an early increase in proliferation is somehow a prerequisite for subsequent differentiation. However, no such biphasic behavior was detected in cells that are growth inhibited by 1,25(OH)2D3but do not differentiate, such as MCF-7 cells. Taken together, these results indicate that both growth stimulation and subsequent inhibition precede differentiation and involve induction of both cyclins and p21 and p27, whereas cell cycle arrest of differentiated cells can be achieved simply by elevations in CDK inhibitors.

C/EBPalpha regulates formation of S-phase-specific E2F-p107 complexes in livers of newborn mice

We previously showed that the rate of hepatocyte proliferation in livers from newborn C/EBPalpha knockout mice was increased. An examination of cell cycle-related proteins showed that the cyclin-dependent kinase (CDK) inhibitor p21 level was reduced in the knockout animals compared to that in wild-type littermates. Here we show additional cell cycle-associated proteins that are affected by C/EBPalpha. We have observed that C/EBPalpha controls the composition of E2F complexes through interaction with the retinoblastoma (Rb)-like protein, p107, during prenatal liver development. S-phase-specific E2F complexes containing E2F, DP, cdk2, cyclin A, and p107 are observed in the developing liver. In wild-type animals these complexes disappear by day 18 of gestation and are no longer present in the newborn animals. In the C/EBPalpha mutant, the S-phase-specific complexes do not diminish and persist to birth. The elevation of levels of the S-phase-specific E2F-p107 complexes in C/EBPalpha knockout mice correlates with the increased expression of several E2F-dependent genes such as those that encode cyclin A, proliferating cell nuclear antigen, and p107. The C/EBPalpha-mediated regulation of E2F binding is specific, since the deletion of another C/EBP family member, C/EBPbeta, does not change the pattern of E2F binding during prenatal liver development. The addition of bacterially expressed, purified His-C/EBPalpha to the E2F binding reaction resulted in the disruption of E2F complexes containing p107 in nuclear extracts from C/EBPalpha knockout mouse livers. Ectopic expression of C/EBPalpha in cultured cells also leads to a reduction of E2F complexes containing Rb family proteins. Coimmunoprecipitation analyses revealed an interaction of C/EBPalpha with p107 but none with cdk2, E2F1, or cyclin A. A region of C/EBPalpha that has sequence similarity to E2F is sufficient for the disruption of the E2F-p107 complexes. Despite its role as a DNA binding protein, C/EBPalpha brings about a change in E2F complex composition through a protein-protein interaction. The disruption of E2F-p107 complexes correlates with C/EBPalpha-mediated growth arrest of hepatocytes in newborn animals.

Ligand-independent activation of the glucocorticoid receptor by beta2-adrenergic receptor agonists in primary human lung fibroblasts and vascular smooth muscle cells

The glucocorticoid receptor (GR) is a ubiquitously expressed transcription factor present in most cell types. Upon ligand binding, the GR is activated and translocates into the nucleus where it transmits the anti-inflammatory actions of glucocorticoids. Here, we describe the ligand-independent activation of GR by the beta2-adrenergic receptor (beta2-AR) agonists, salbutamol and salmeterol, in primary human lung fibroblasts and vascular smooth muscle cells. Immunohistochemistry demonstrated expression of GR and the beta2-AR by fibroblasts and vascular smooth muscle cells. Treatment of the cells with the beta2-AR agonists, salbutamol or salmeterol, resulted in translocation of GR into the nucleus beginning at 30 min, as shown by immunohistochemistry and Western blotting of cytosolic and nuclear cell extracts. In comparison, activation of GR induced by the corticosteroids dexamethasone and fluticasone occurred at the same time after treatment (30 min) but resulted in a more complete depletion of GR from the cytosolic compartment. Electrophoretic mobility shift assays confirmed that nuclear GR, activated by both beta2-AR agonists and glucocorticoids, actively bound to the GR consensus sequence (GR element). Functional activation of the GR was confirmed by a Luciferase reporter gene assay, using a GR driven promoter fragment from the p21((WAF1/CIP1)) gene. The effects of the beta2-AR agonists, salbutamol and salmeterol, were dependent upon binding to the beta2-AR, because blocking of beta2-AR with propranolol abrogated GR activation. GR activation appeared to involve cAMP. In summary, these data show that beta2-AR agonists are potent activators of GR. Ligand-independent activation of GR by beta2-AR agonists may substantially mediate the anti-inflammatory actions of these drugs observed in vitro and in vivo.

The C/EBP family of transcription factors in the liver and other organs

Members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors are pivotal regulators of liver functions such as nutrient metabolism and its control by hormones, acute-phase response and liver regeneration. Recent progress in clarification of regulatory mechanisms for the C/EBP family members gives insight into understanding the liver functions at the molecular level.

Phosphorylation and inhibition of rat glucocorticoid receptor transcriptional activation by glycogen synthase kinase-3 (GSK-3). Species-specific differences between human and rat glucocorticoid receptor signaling as revealed through GSK-3 phosphorylation

Transcriptional activation by the glucocorticoid receptor (GR) is regulated by both glucocorticoid binding and phosphorylation. The rat GR N-terminal transcriptional regulatory domain contains four major phosphorylation sites: threonine 171 (Thr171), serine 224 (Ser224), serine 232 (Ser232), and serine 246 (Ser246). We have previously demonstrated that Ser224 and Ser232 are phosphorylated by cyclin-dependent kinases, while Ser246 is phosphorylated by the c-Jun N-terminal kinase. We report here that the remaining GR phosphorylation site, Thr171, is a target for glycogen synthase kinase-3 (GSK-3) in vitro and in cultured mammalian cells. Increasing GSK-3 activity through its overexpression in cultured cells inhibits GR transcriptional enhancement, an effect dependent upon Thr171. Correspondingly, overexpression of a constitutively active form of the GSK-3 inhibitor, protein kinase B/Akt, increases GR transcriptional enhancement. Overexpression of GSK-3 had no effect on GR-mediated transcriptional repression of AP1-dependent gene expression. Importantly, transcriptional activation by the human GR (hGR), which contains an alanine (Ala150) at the position equivalent to Thr171 in rat GR, is not affected by GSK-3 overexpression. Introduction of a threonine residue at this position (A150T) establishes GSK-3-mediated inhibition of hGR transcriptional activation. These findings demonstrate species-specific differences in GR signaling, as revealed through GSK-3 phosphorylation, which suggests that GR function in rodents may not fully recapitulate receptor action in humans and that hGR is capable of adopting the GSK-3 signaling pathway through a somatic mutation.

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.