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

Insulin regulates human mammosphere development and function

Assessing the role of lactogenic hormones in human mammary gland development is limited due to issues accessing tissue samples and so development of a human in vitro three-dimensional mammosphere model with functions similar to secretory alveoli in the mammary gland can aid to overcome this shortfall. In this study, a mammosphere model has been characterised using human mammary epithelial cells grown on either mouse extracellular matrix or agarose and showed insulin is essential for formation of mammospheres. Insulin was shown to up-regulate extracellular matrix genes. Microarray analysis of these mammospheres revealed an up-regulation of differentiation, cell-cell junctions, and cytoskeleton organisation functions, suggesting mammosphere formation may be regulated through ILK signalling. Comparison of insulin and IGF-1 effects on mammosphere signalling showed that although IGF-1 could induce spherical structures, the cells did not polarise correctly as shown by the absence of up-regulation of polarisation genes and did not induce the expression of milk protein genes. This study demonstrated a major role for insulin in mammary acinar development for secretory differentiation and function indicating the potential for reduced lactational efficiency in women with obesity and gestational diabetes.

The role of tight junction proteins in ovarian follicular development and ovarian cancer

Tight junctions (TJ) are protein structures that control the transport of water, ions and macromolecules across cell layers. Functions of the transmembrane TJ protein, occluding (OCLN) and the cytoplasmic TJ proteins, tight junction protein 1 (TJP1; also known as zona occludens protein-1), cingulin (CGN) and claudins (CLDN) are reviewed, and current evidence of their role in the ovarian function is reviewed. Abundance of OCLN, CLDNs and TJP1 mRNA changed during follicular growth. In vitro treatment with various growth factors known to affect ovarian folliculogenesis indicated that CGN, OCLN and TJP1 are hormonally regulated. The summarized studies indicate that expression of TJ proteins (i.e., OCLN, CLDN, TJP1 and CGN) changes with follicle size in a variety of vertebrate species but whether these changes in TJ proteins are increased or decreased depends on species and cell type. Evidence indicates that autocrine, paracrine and endocrine regulators, such as fibroblast growth factor-9, epidermal growth factor, androgens, tumor necrosis factor-α and glucocorticoids may modulate these TJ proteins. Additional evidence presented indicates that TJ proteins may be involved in ovarian cancer development in addition to normal follicular and luteal development. A model is proposed suggesting that hormonal downregulation of TJ proteins during ovarian follicular development could reduce barrier function (i.e., selective permeability of molecules between theca and granulosa cells) and allow for an increase in the volume of follicular fluid as well as allow additional serum factors into the follicle that may directly impact granulosa cell functions.

Minireview: Steroid/nuclear receptor-regulated dynamics of occluding and anchoring junctions

A diverse set of physiological signals control intercellular interactions by regulating the structure and function of occluding junctions (tight junctions) and anchoring junctions (adherens junctions and desmosomes). These plasma membrane junctions are comprised of multiprotein complexes of transmembrane and cytoplasmic peripheral plasma membrane proteins. Evidence from many hormone-responsive tissues has shown that expression, modification, molecular interactions, stability, and localization of junctional complex-associated proteins can be targeted by nuclear hormone receptors and their ligands through transcriptional and nontranscriptional mechanisms. The focus of this minireview is to discuss molecular, cellular, and physiological studies that directly link nuclear receptor- and ligand-triggered signaling pathways to the regulation of occluding and anchoring junction dynamics.

The role of tight junctions in mammary gland function

Tight junctions (TJ) are cellular structures that facilitate cell-cell communication and are important in maintaining the three-dimensional structure of epithelia. It is only during the last two decades that the molecular make-up of TJ is becoming unravelled, with two major transmembrane-spanning structural protein families, called occludin and claudins, being the true constituents of the TJ. These TJ proteins are linked via specific scaffolding proteins to the cell's cytoskeleton. In the mammary gland TJ between adjacent secretory epithelial cells are formed during lactogenesis and are instrumental in establishing and maintaining milk synthesis and secretion, whereas TJ integrity is compromised during mammary involution and also as result of mastitis and periods of mammary inflamation (including mastitis). They prevent the paracellular transport of ions and small molecules between the blood and milk compartments. Formation of intact TJ at the start of lactation is important for the establishment of the lactation. Conversely, loss of TJ integrity has been linked to reduced milk secretion and mammary function and increased paracellular transport of blood components into the milk and vice versa. In addition to acting as a paracellular barrier, the TJ is increasingly linked to playing an active role in intracellular signalling. This review focusses on the role of TJ in mammary function of the normal, non-malignant mammary gland, predominantly in ruminants, the major dairy producing species.

Structural and functional alteration of corneal epithelial barrier under inflammatory conditions

PURPOSE

The aim of the study was to determine the effect of inflammatory conditions on the expression of tight junction (TJ) and adherens junction (AJ) proteins between human corneal epithelial cells and, consequently, on corneal epithelial barrier integrity.

MATERIALS AND METHODS

Zonula occludens proteins ZO-1 and ZO-2, claudin-1 and -2 (CLDN-1 and CLDN-2), occludin (OCLN) as well as E-cadherin (E-cad) expression were analyzed in a human corneal epithelial cell line (HCE) at basal conditions and after stimulation with inflammatory cytokines (TNFα, TGFβ, IL-10, IL-13, IL-17, IL-6), using real time RT-PCR, Western blotting and immunofluorescence. Actin cytoskeleton staining was performed after all stimulations. Transepithelial electrical resistance (TER) and fluorescein transepithelial permeability (TEP) were measured as barrier integrity functional assays.

RESULTS

ZO-1, ZO-2, CLDN-1, CLDN-2, OCLN and E-cad were detected in HCE cell membranes at basal conditions. Cytokine stimulation resulted in significant changes in the expression of TJ and AJ proteins, both at mRNA and protein level, a remarkable change in their localization pattern, as well as a reorganization of actin cytoskeleton. Pro-inflammatory cytokines TNFα, TGFβ, IL-13, IL-17 and IL-6 induced a structural and functional disruption of the epithelial barrier, while IL-10 showed a barrier protective effect.

CONCLUSION

Simulated inflammatory conditions lead to an alteration of corneal barrier integrity by modulating TJ, and to a lesser extent also AJ, protein composition, at least In Vitro. The observed barrier protective effects of IL-10 support its well-known anti-inflammatory functions and highlight a potential therapeutic perspective.

TGF-β1 mediates sirolimus and cyclosporine A-induced alteration of barrier function in renal epithelial cells via a noncanonical ERK1/2 signaling pathway

The immunosuppressant drugs cyclosporine A (CsA) and sirolimus (SRL) used in combination demonstrated beneficial effects in organ transplantation, but this combination can also result in increased adverse effects. We previously showed that not only CsA treatment but also its combination with SRL decreased paracellular permeability in renal proximal tubular cells by modification of the tight junction proteins, claudins, through ERK1/2 signaling pathway. In this present study, evidence is presented that not only CsA but also the combination of CsA/SRL may have adverse effects on the barrier function of renal proximal cells, at least in part, through the expression of the cytokine transforming growth factor (TGF)-β1. CsA treatment upregulated TGF-β1 gene expression and this upregulation was enhanced when CsA and SRL were applied together. Addition of TGF-β1 (5 ng/ml) altered the barrier function with increased transepithelial electrical resistance (TER) and claudin-1 expression. Use of a TGF-β1-blocking antibody or blockage of TGF-β1 receptor kinase activity with SD208 prevented the CsA- and CsA/SRL-induced increase in TER. No evidence was found in the present studies to indicate that CsA or CsA/SRL treatment activated the TGF-β1 Smad canonical signaling pathway, whereas addition of TGF-β1 (5 ng/ml) did activate the Smad pathway. Addition of the ERK1/2 signaling inhibitor U0126 was able to prevent the TGF-β1-mediated increase in TER and claudin expression. It is most likely that the CsA- and CsA/SRL-induced increases in TGF-β1 expression may not be sufficient to trigger the Smad pathway but however may trigger other TGF-β1 receptor-mediated signaling including the ERK1/2 signaling pathway.

Stromal upregulation of lateral epithelial adhesions: gene expression analysis of signalling pathways in prostate epithelium

Background

Stromal signalling increases the lateral cell adhesions of prostate epithelial cells grown in 3D culture. The aim of this study was to use microarray analysis to identify significant epithelial signalling pathways and genes in this process.

Methods

Microarray analysis was used to identify genes that were differentially expressed when epithelial cells were grown in 3D Matrigel culture with stromal co-culture compared to without stroma. Two culture models were employed: primary epithelial cells (ten samples) and an epithelial cell line (three experiments). A separate microarray analysis was performed on each model system and then compared to identify tissue-relevant genes in a cell line model.

Results

TGF beta signalling was significantly ranked for both model systems and in both models the TGF beta signalling gene SOX4 was significantly down regulated. Analysis of all differentially expressed genes to identify genes that were common to both models found several morphology related gene clusters; actin binding (DIAPH2, FHOD3, ABLIM1, TMOD4, MYH10), GTPase activator activity (BCR, MYH10), cytoskeleton (MAP2, MYH10, TMOD4, FHOD3), protein binding (ITGA6, CD44), proteinaceous extracellular matrix (NID2, CILP2), ion channel/ ion transporter activity (CACNA1C, CACNB2, KCNH2, SLC8A1, SLC39A9) and genes associated with developmental pathways (POFUT1, FZD2, HOXA5, IRX2, FGF11, SOX4, SMARCC1).

Conclusions

In 3D prostate cultures, stromal cells increase lateral epithelial cell adhesions. We show that this morphological effect is associated with gene expression changes to TGF beta signalling, cytoskeleton and anion activity.

Inhibition of basal p38 or JNK activity enhances epithelial barrier function through differential modulation of claudin expression

Tight junctions (TJs) form a barrier to the paracellular diffusion of ions and solutes across epithelia. Although transmembrane proteins of the claudin family have emerged as critical determinants of TJ permeability, little is known about the signaling pathways that control their expression. The aim of this study was to assess the role of three mitogen-activated protein kinases (MAPKs), i.e., extracellular signal-regulated kinase-1/2 (ERK1/2), c-Jun NH(2)-terminal kinases (JNKs), and p38 kinases, in the regulation of epithelial barrier function and claudin expression in mammary epithelial cells. Addition of either PD169316 (a p38 inhibitor) or SP600125 (a JNK inhibitor) induced formation of domes (a phenomenon dependent on TJ barrier function) and enhanced transepithelial electrical resistance, whereas U0126 (an inhibitor of the ERK1/2 activators MEK1/MEK2) had no significant effect. Similar results were obtained using mechanistically unrelated p38 or JNK inhibitors. PD169316 increased the expression of claudin-4 and -8, whereas SP600125 increased claudin-4 and -9 and downregulated claudin-8. Silencing of p38alpha by isoform-specific small interfering RNAs increased claudin-4 and -8 mRNAs, whereas silencing of p38beta only increased claudin-4 mRNA. Silencing of either JNK1 or JNK2 increased claudin-9 mRNA expression while decreasing claudin-8 mRNA. Moreover, selective silencing of JNK2 increased claudin-4 and -7 mRNAs. Finally, both PD169316 and SP600125 inhibited the paracellular diffusion of Na(+) and Cl(-) across epithelial monolayers. Collectively, these results provide evidence that inhibition of either p38 or JNK enhances epithelial barrier function by selectively modulating claudin expression, implying that the basal activity of these MAPKs exerts a tonic effect on TJ ionic permeability.

NC-1059: a channel-forming peptide that modulates drug delivery across in vitro corneal epithelium

PURPOSE

The goal of this study was to determine whether a synthetic peptide, NC-1059, can modulate the corneal epithelium to increase the permeation of therapeutic agents across this barrier.

METHODS

An in vitro system employing transformed human corneal epithelial (THCE) cells was optimized for this study. Culture conditions were identified to promote formation of a confluent monolayer that rapidly develops a substantial transepithelial electrical resistance. Electrical parameters were measured with a modified Ussing flux chamber, and solute flux was quantified with fluorescently labeled compounds.

RESULTS

NC-1059 causes a concentration-dependent increase in short-circuit current and an increase in transepithelial electrical conductance when assessed in modified Ussing chambers. The effect of NC-1059 on transepithelial electrical resistance was reversible. To test for paracellular permeability and size exclusion, FITC-labeled dextran ranging in size from 10 to 70 kDa was used. Dextran permeated the corneal cell monolayer in the presence, but not the absence, of NC-1059. Fluorescein sodium and carboxyfluorescein were then used as low molecular weight markers with similar NC-1059-modulated kinetics being observed. Maximum permeation for the fluorescein derivatives occurred 30 to 90 minutes after a 5-minute NC-1059 exposure. A prototypical drug, methotrexate, also exhibited increased permeation in the presence of NC-1059.

CONCLUSIONS

NC-1059 enhances drug permeation across cultured corneal epithelial cell monolayers by transiently affecting the paracellular pathway. Thus, NC-1059 is a lead compound for development of cotherapeutic agents to enhance access and effectiveness of ophthalmic compounds.

Glucocorticoid-mediated apoptosis resistance of solid tumors

More than a quarter of a century ago, the phenomenon of glucocorticoid-induced apoptosis in the majority of hematological cells was first recognized. More recently, glucocorticoid-induced antiapoptotic signaling associated with apoptosis resistance towards cytotoxic therapy has been identified in cells of epithelial origin, most of malignant solid tumors and some other tissues. Despite these huge amounts of data demonstrating differential pro- and anti-apoptotic effects of glucocorticoids, the underlying mechanisms of cell type-specific glucocorticoid signaling are just beginning to be described. This review summarizes our present understanding of cell type-specific pro- and anti-apoptotic signaling induced by glucocorticoids. We shortly introduce mechanisms of glucocorticoid resistance of hematological cells. We highlight and discuss the emerging molecular evidence of a general induction of survival signaling in epithelial cells and carcinoma cells by glucocorticoids. We give a summary of our current knowledge of decreased proliferation rates in response to glucocorticoid pre- and combination treatment, which are suspicious to be involved not only in protection of normal tissues, but also in protection of solid tumors from cytotoxic effects of anticancer agents.

Mechanism of cytokine modulation of epithelial tight junction barrier

Cytokines play a crucial role in the modulation of inflammatory response in the gastrointestinal tract. Pro-inflammatory cytokines including tumor necrosis factor-alpha, interferon-gamma, interleukin-1beta?IL-1beta?, and interleukin-12 are essential in mediating the inflammatory response, while anti-inflammatory cytokines including interleukin-10 and transforming growth factor-beta are important in the attenuation or containment of inflammatory process. It is increasingly recognized that cytokines have an important physiological and pathological effect on intestinal tight junction (TJ) barrier. Consistent with their known pro-inflammatory activities, pro-inflammatory cytokines cause a disturbance in intestinal TJ barrier, allowing increased tissue penetration of luminal antigens. Recent studies indicate that the inhibition of cytokine induced increase in intestinal TJ permeability has an important protective effect against intestinal mucosal damage and development of intestinal inflammation. In this review, the effects of various pro-inflammatory and anti-inflammatory cytokines on intestinal TJ barrier and the progress into the mechanisms that mediate the cytokine modulation of intestinal TJ barrier are reviewed.

TGF-beta inhibits prolactin-induced expression of beta-casein by a Smad3-dependent mechanism

Transforming growth factor-beta (TGF-beta) is a multifunctional growth factor, affecting cell proliferation, apoptosis, and extracellular matrix homeostasis. It also plays critical roles in mammary gland development, one of which involves inhibition of the expression of milk proteins, such as beta-casein, during pregnancy. Here we further explore the underlying signaling mechanism for it. Our results show that TGF-beta suppresses prolactin-induced expression of beta-casein mRNA and protein in primary mouse mammary epithelial cells, but its effect on protein expression is more evident. We also find out that this inhibition is not due to the effect of TGF-beta on cell apoptosis. Furthermore, inhibition of TGF-beta type I receptor kinase activity by a pharmacological inhibitor SB431542 or overexpression of dominant negative Smad3 substantially restores beta-casein expression. By contrast, inhibition of p38 and Erk that are known to be activated by TGF-beta does not alleviate the inhibitory effect of TGF-beta. These results are consistent with our other observation that Smad but not MAPK pathway is activated by TGF-beta in mammary epithelial cells. Lastly, we show that prolactin-induced tyrosine phosphorylation of Jak2 and Stat5 as well as serine/threonine phosphorylation of p70S6K and S6 ribosomal protein are downregulated by TGF-beta, although the former event requires considerably long exposure to TGF-beta. We speculate that these events might be involved in repressing transcription and translation of beta-casein gene, respectively. Taken together, our results demonstrate that TGF-beta abrogates prolactin-stimulated beta-casein gene expression in mammary epithelial cells through, at least in part, a Smad3-dependent mechanism.

The role of glucocorticoids in secretory activation and milk secretion, a historical perspective

In this review we present our current understanding of the role of glucocorticoids in secretory activation and milk secretion by looking at the literature from a historical perspective. We begin with the early endocrine ablation experiments and continue from there to show that glucocorticoids are not just necessary for secretory activation and milk secretion--but mandatory. Specifically, we discuss the importance of glucocorticoids to: (1) induce the formation of ultrastructural components necessary to support milk synthesis and secretion, including rough endoplasmic reticulum and tight junction sealing; (2) regulate milk protein gene expression; and (3) prevent the second phase of involution, possibly by preventing the breakdown of the extracellular matrix.

Glucocorticoid-induced degradation of glycogen synthase kinase-3 protein is triggered by serum- and glucocorticoid-induced protein kinase and Akt signaling and controls beta-catenin dynamics and tight junction formation in mammary epithelial tumor cells

Glucocorticoid hormones stimulate adherens junction and tight junction formation in Con8 mammary epithelial tumor cells and induce the production of a stable nonphosphorylated beta-catenin protein localized exclusively to the cell periphery. Glycogen synthase kinase-3 (GSK3) phosphorylation of beta-catenin is known to trigger the degradation of this adherens junction protein, suggesting that steroid-activated cascades may be targeting this protein kinase. We now demonstrate that treatment with the synthetic glucocorticoid dexamethasone induces the ubiquitin-26S proteasome-mediated degradation of GSK3 protein with no change in GSK3 transcript levels. In transfected cells, deletion of the N-terminal nine amino acids or mutation of the serine-9 phosphorylation site on GSK3-beta prevented its glucocorticoid-induced degradation. Expression of stabilized GSK3 mutant proteins ablated the glucocorticoid-induced tight junction sealing and resulted in production of a nonphosphorylated beta-catenin that localizes to both the nucleus and the cell periphery in steroid-treated cells. Serine-9 on GSK3 can be phosphorylated by Sgk (serum- and glucocorticoid-induced protein kinase) and by Akt. Expression of dominant-negative forms of either Sgk- or Akt-inhibited glucocorticoid induced GSK3 ubiquitination and degradation and disrupted the dexamethasone-induced effects on beta-catenin dynamics. Furthermore, the steroid-induced tight junction sealing is attenuated in cells expressing dominant-negative forms of either Sgk or Akt, although the effect of blunting Sgk signaling was significantly greater. Taken together, we have uncovered a new cellular cascade in which Sgk and Akt trigger the glucocorticoid-regulated phosphorylation, ubiquitination, and degradation of GSK3, which alters beta-catenin dynamics, leading to the formation of adherens junctions and tight junction sealing.

Role for TGF-β in Cyclosporine-Induced Modulation of Renal Epithelial Barrier Function

It was previously shown that cyclosporine A (CsA) increases transepithelial resistance in MDCK cells. Activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) mitogen-activated protein kinase (MAPK) cascade seems to be pivotal to the CsA-induced increase in transepithelial electrical resistance (TER). This study examined the role played by TGF-beta in mediating the CsA-induced activation of ERK1/2 and the resulting increase in TER in MDCK cells. Paracellular permeability across MDCK monolayers after various treatments was assessed by measurement of TER. TGF-beta secretion was measured by Western blot and ELISA. Activation of the ERK1/2 pathway and tight junction protein expression were also assessed by Western blot analysis. CsA increased production and secretion of TGF-beta and expression of the TGF-beta receptor II. Exogenous addition of TGF-beta1 activated ERK1/2 and increased TER across MDCK monolayers, both of which were attenuated by the MEK inhibitor U0126. Neutralizing antibodies against TGF-beta1 and the TGF-beta receptor II significantly reduced the CsA-induced increase in TER. Both CsA and TGF-beta1 increased expression of tight junction proteins claudin-1 and zonula occludens 2. Inhibition of the p38 MAPK pathway also attenuated the TGF-beta1-induced increase in TER. The results presented here suggest that the CsA-induced modulation of paracellular permeability may be mediated, at least in part, by an increase in TGF-beta production.

Regulation of differential pro- and anti-apoptotic signaling by glucocorticoids

More than a quarter of a century ago, the phenomenon of glucocorticoid-induced apoptosis in the majority of hematological cells was first recognized. More recently, glucocorticoid-induced antiapoptotic signaling associated with apoptosis resistance has been identified in cells of epithelial origin, most of malignant solid tumors and some other tissues. Despite these huge amount of data demonstrating differential pro- and anti-apoptotic effects of glucocorticoids, the underlying mechanisms of cell type specific glucocorticoid signaling are just beginning to be described. This review summarizes our present understanding of cell type-specific pro- and anti-apoptotic signaling induced by glucocorticoids. In the first section we give a summary and update of known glucocorticoid-induced pathways mediating apoptosis in hematological cells. We shortly introduce mechanisms of glucocorticoid resistance of hematological cells. We highlight and discuss the emerging molecular evidence of a general induction of survival signaling in epithelial cells and carcinoma cells by glucocorticoids. We provide a model for glucocorticoid-induced resistance in cells growing in a tissue formation. Thus, attachment to the extracellular matrix and cell-cell contacts typical for e.g. epithelial and tumor cells may be crucially involved in switching the balance of several interacting pathways to survival upon treatment with glucocorticoids.

Selective glucocorticoid control of Rho kinase isoforms regulate cell-cell interactions

The two Rho kinase isoforms ROCK1 and ROCK2 are downstream effectors of the small GTPase RhoA, although relatively little is known about potential isoform specific functions or the selective control of their cellular activities. Using Con8 rat mammary epithelial cells, we show that the synthetic glucocorticoid dexamethasone strongly stimulates the level of ROCK2 protein, which accounts for the increase in total cellular ROCK2 activity, whereas, steroid treatment down-regulated ROCK1 specific kinase activity without altering ROCK1 protein levels. In Con8 cells, the glucocorticoid induced formation of tight junctions requires the steroid-mediated down-regulation RhoA and function of the RhoA antagonist Rnd3. Treatment with the ROCK inhibitor Y-27632 ablated both the glucocorticoid-induced and Rnd3-mediated stimulation in tight junction sealing. Taken together, our results demonstrate that the expression and activity of ROCK1 and ROCK2 can be uncoupled in a signal-dependent manner, and further implicate a new function for ROCK2 in the steroid control of tight junction dynamics.

Bioenergetic remodeling during cellular differentiation: changes in cytochrome c oxidase regulation do not affect the metabolic phenotype

Myogenesis induces mitochondrial proliferation, a decrease in reactive oxygen species (ROS) production, and an increased reliance upon oxidative phosphorylation. While muscles typically possess 20%-40% excess capacity of cytochrome c oxidase (COX), undifferentiated myoblasts have only 5%-20% of the mitochondrial content of myotubes and muscles. We used two muscle lines (C2C12, Sol8) and 3T3-L1 pre-adipocytes to examine if changes in COX regulation or activity with differentiation cause a shift in metabolic phenotype (i.e., more oxidative, less glycolytic, less ROS). COX activity in vivo can be suppressed by its inhibitor, nitric oxide, or sub-optimal substrate (cytochrome c) concentrations. Inhibition of nitric oxide synthase via L-NAME had no effect on the respiration of adherent undifferentiated cells, although it did stimulate respiration of myoblasts in suspension. While cytochrome c content increased during differentiation, there was no correlation with respiratory rate or reliance on oxidative metabolism. There was no correlation between COX specific activity and oxidative metabolism between cell type or in relation to differentiation. These studies show that, despite the very low activities of COX, undifferentiated myoblasts and pre-adipocytes possess a reserve of COX capacity and changes in COX with differentiation do not trigger the shift in metabolic phenotype.

Estimation of paracellular conductance of primary rat alveolar epithelial cell monolayers

Freshly isolated rat type II pneumocytes, when grown on permeable tissue culture-treated polycarbonate filters, form confluent alveolar epithelial cell monolayers (RAECM). Cells in RAECM undergo transdifferentiation, exhibiting over time morphological and phenotypic characteristics of type I pneumocytes in vivo. We recently reported that transforming growth factor-β1 (TGF-β1) decreases overall monolayer resistance ( Rte) and stimulates short-circuit current in a dose-dependent manner. In this study, we investigated the effects of TGF-β1 (50 pM) or 10% newborn bovine serum (NBS) on modulation of paracellular passive ion conductance and its contribution to total passive ion conductance across RAECM. On days 5–7 in culture, tight-junctional resistance ( Rtj, kΩcm2) of RAECM, cultured in minimally defined serum-free medium (MDSF) with or without TGF-β1 or NBS, was estimated from the relationship between observed transmonolayer voltage and resistance after addition of gramicidin D to apical potassium isethionate Ringer solution under open-circuit conditions. NaCl Ringer solution bathed the basolateral side throughout the experimental period. Results showed that transmonolayer conductance (1/ Rte) and tight-junctional conductance (1/ Rtj) are 0.59 and 0.14 mS/cm2 for control monolayers in MDSF, 1.59 and 0.38 mS/cm2 for monolayers exposed to TGF-β1, and 0.38 and 0.18 mS/cm2 for monolayers grown in the presence of NBS. The contributions to total transepithelial ion conductance by the paracellular pathway are estimated to be 23, 23, and 47% for control, TGF-β1-exposed, and newborn bovine serum (NBS)-treated RAECM, respectively.

Sertoli-Sertoli and Sertoli-germ cell interactions and their significance in germ cell movement in the seminiferous epithelium during spermatogenesis

Spermatogenesis is the process by which a single spermatogonium develops into 256 spermatozoa, one of which will fertilize the ovum. Since the 1950s when the stages of the epithelial cycle were first described, reproductive biologists have been in pursuit of one question: How can a spermatogonium traverse the epithelium, while at the same time differentiating into elongate spermatids that remain attached to the Sertoli cell throughout their development? Although it was generally agreed upon that junction restructuring was involved, at that time the types of junctions present in the testis were not even discerned. Today, it is known that tight, anchoring, and gap junctions are found in the testis. The testis also has two unique anchoring junction types, the ectoplasmic specialization and tubulobulbar complex. However, attention has recently shifted on identifying the regulatory molecules that "open" and "close" junctions, because this information will be useful in elucidating the mechanism of germ cell movement. For instance, cytokines have been shown to induce Sertoli cell tight junction disassembly by shutting down the production of tight junction proteins. Other factors such as proteases, protease inhibitors, GTPases, kinases, and phosphatases also come into play. In this review, we focus on this cellular phenomenon, recapping recent developments in the field.

Activation of ERK1/2 MAP kinase pathway induces tight junction disruption in human corneal epithelial cells

Activation of protein kinase C (PKC) by exposure of cultured human corneal epithelial cells to phorbol 12-myristate 13-acetate (PMA) resulted in an increase in paracellular permeability as evidenced by a decrease in transepithelial electrical resistance (TER). A change in the membrane distribution of the tight junction protein ZO-1 was also observed in the PMA-treated cells. In contrast, when the cells were treated with PMA in the presence of PD98059, a specific inhibitor of mitogen-activated protein kinase (MAPK) kinase, all barrier characteristics were preserved, suggesting that PKC induces tight junction disruption through the activation of MAPK. The role of this signaling pathway in the regulation of epithelial permeability was further elucidated by the use of corneal epithelial-derived cell lines expressing constitutively activated (ca) or dominant-negative (dn) mutants of MAPK kinase-1 (MEK1). Transfectants of caMEK1, when compared to parental cells, had higher levels of phosphorylated extracellular regulated protein kinase (ERK), altered distribution of ZO-1 and occludin, and much reduced TER. On the other hand, dnMEK1 transfectants had lower but detectable levels of ERK phosphorylation, more flattened morphology, and, most importantly, significantly higher TER when compared to parental cells. Our study demonstrates that activation of PKC causes the disruption of tight junctions through activation of MAP kinase and that the MAP kinase signaling pathway plays a key role in the regulation of epithelial cell morphology and barrier function in the cornea.

TGF-betas: their role in testicular function and Sertoli cell tight junction dynamics

Transforming growth factor-betas (TGF-betas) are known to regulate multiple physiological functions in the testis, which include spermatogenesis, Leydig cell steroidogenesis, extracellular matrix synthesis and testis development. More recent studies have shown that TGF-beta3 also regulates Sertoli cell tight junction (TJ) dynamics in vitro via the p38 mitogen-activated protein (MAP) kinase pathway, suggesting that this cytokine plays a crucial role in regulating the opening and closing of the blood-testis barrier (BTB). This in turn regulates the passage of pre-leptotene and leptotene spermatocytes across the BTB at stages VIII-XI of the seminiferous epithelial cycle. This review summarizes recent advances of studies on TGF-betas in the testis, highlighting their regulatory role in TJ dynamics.

Glucocorticoid down-regulation of RhoA is required for the steroid-induced organization of the junctional complex and tight junction formation in rat mammary epithelial tumor cells

In Con8 mammary epithelial tumor cells, we have documented previously that the synthetic glucocorticoid dexamethasone induces the reorganization of the tight junction and adherens junction (apical junction) and stimulates the monolayer transepithelial electrical resistance (TER), which is a reliable in vitro measurement of tight junction sealing. Western blots demonstrated that dexamethasone treatment down-regulated the level of the RhoA small GTPase prior to the stimulation of the monolayer TER. To test the role of RhoA in the steroid regulation of apical junction dynamics functionally, RhoA levels were altered in Con8 cells by transfection of either constitutively active (RhoA.V14) or dominant negative (RhoA.DN19) forms of RhoA. Ectopic expression of constitutively active RhoA disrupted the dexamethasone-stimulated localization of zonula occludens-1 and beta-catenin to sites of cell-cell contact, inhibited tight junction sealing, and prevented the complete formation of the F-actin ring structure at the apical side of the cell monolayer. In a complementary manner, dominant negative RhoA caused a precocious organization of the tight junction, adherens junction, and the F-actin rings in the absence of steroid, whereas the monolayer TER remained glucocorticoid-responsive. Taken together, our results demonstrate that the glucocorticoid down-regulation of RhoA is a required step in the steroid signaling pathway which controls the organization of the apical junctional complex and the actin cytoskeleton in mammary epithelial cells.

Antagonism of glucocorticoid receptor transactivity and cell growth inhibition by transforming growth factor-beta through AP-1-mediated transcriptional repression

We have examined the interaction of the glucocorticoid receptor (GR) and transforming growth factor-beta (TGF-beta) signal pathways because of their mutual involvement in the regulation of cell growth, development and differentiation. Most studies of this cross-talk event have focused on the effects of glucocorticoids (GCs) on TGF-beta responses. In this work, we show that TGF-beta can antagonize dexamethasone (Dex)-mediated growth suppression in mouse fibrosarcoma L929 cells. TGF-beta also repressed GR-mediated reporter (pMMTV-CAT) gene expression in a concentration-dependent manner, with an IC(50) of 5 ng/ml of TGF-beta. Maximal inhibition (76%) was observed at 10 ng/ml of TGF-beta. Conversely, Dex inhibited TGF-beta-mediated promoter (p3TP-Lux) activity in these same cells. As TGF-beta inhibition of GR-mediated gene expression occurred after Dex-mediated nuclear translocation of GR, we conclude that TGF-beta inhibition of GR signaling occurs at the level of GR-mediated transcription activity. However, TGF-beta did not repress GR-mediated gene expression using the pGRE(2)E1B-CAT minimal promoter construct, suggesting that TGF-beta did not inhibit intrinsic GR activity but, rather, required DNA-binding factor(s) distinct from GR. As the MMTV promoter contains several putative AP-1 binding sites, we hypothesized that AP-1, a transcription factor composed of c-jun and c-fos proteins, might be involved in the TGF-beta inhibition of GR functions. Curcumin, a potent inhibitor of AP-1 expression, completely abolished the inhibitory effect of TGF-beta on GR-mediated gene expression without affecting GR activity in the absence of TGF-beta, and this drug blocked TGF-beta-induced binding of AP-1 to a response element derived from the MMTV sequence. Furthermore, curcumin abolished TGF-beta inhibition of Dex-induced growth suppression. Taken as a whole, our data suggest that TGF-beta can antagonize the growth inhibitory properties of GR by blocking GR transactivity at complex promoters through a mechanism involving transcriptional repression by DNA-bound AP-1.

Etk/Bmx activation modulates barrier function in epithelial cells

Etk/Bmx is a member of the Tec family of cytoplasmic non-receptor tyrosine kinases known to express in epithelial cells. We demonstrate herein that Etk activation in stably Etk-transfected epithelial Pa-4 cells resulted in a consistently increased transepithelial resistance (TER). After 24 h of hypoxic (1% O2) exposure, the TER and equivalent active ion transport rate ( I eq) were reduced to <5% of the normoxia control in Pa-4 cells, whereas both TER and I eqwere maintained at comparable and 60% levels, respectively, relative to their normoxic controls in cells with Etk activation. Moreover, Pa-4 cells exhibited an abundant actin stress fiber network with a diffuse distribution of β-catenin at the cell periphery. By contrast, Etk-activated cells displayed a redistribution of actin to an exclusively peripheral network, with a discrete band of β-catenin also concentrated at the cell periphery, and an altered occludin distribution profile. On the basis of these findings, we propose that Etk may be a novel regulator of epithelial junctions during physiological and pathophysiological conditions.

TGF-beta and colorectal carcinogenesis

There is substantial evidence to support the contention that the Smad portion of the TGF-beta signal transduction pathway provides an important tumor-suppressor function. Mutational loss of function of Smad pathway members have been associated with the development of human cancers and appear to be causative in selected rodent carcinogenesis models. TGF-beta also has multiple other actions that appear to be independent of the growth-inhibitory/tumor suppressor effects. The predominant effect of TGF-beta appears to be dependent on the context of the responding cell. Once transformation has occurred, TGF-beta effects may be detrimental and may actually promote tumor cell survival, invasion, and metastasis. Recent work suggests that these effects may involve TGF-beta regulation of COX-2 and other pathways that may contribute to tumor cell aggressiveness. In gaining a better understanding of the mechanisms by which TGF-beta may promote tumor progression, it is likely that new therapeutic strategies may be developed that preserve tumor-suppressor function of TGF-beta while inhibiting the tumor-promoting effects.

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.

Transport of milk constituents by the mammary gland

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.

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.

Glucocorticoid receptor inhibits transforming growth factor-beta signaling by directly targeting the transcriptional activation function of Smad3

The transforming growth factor-beta (TGF-beta) family of cytokines and glucocorticoids regulate diverse biological processes through modulating the expression of target genes. Here we report that glucocorticoid receptor (GR) represses TGF-beta transcriptional activation of the type-1 plasminogen activator inhibitor (PAI-1) gene in a ligand-dependent manner. Similarly, GR represses TGF-beta activation of the TGF-beta responsive sequence containing Smad3/4-binding sites. Using mammalian two-hybrid assays, we demonstrate that GR inhibits transcriptional activation by both Smad3 and Smad4 C-terminal activation domains. Finally, we show that GR interacts with Smad3 both in vitro and in vivo. These results suggest a molecular basis for the cross-regulation between glucocorticoid and TGF-beta signaling pathways.

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.

Tight junction regulation in the mammary gland

Tight junctions form a narrow, continuous seal that surrounds each endothelial and epithelial cell at the apical border, and act to regulate the movement of material through the paracellular pathway. In the mammary gland, the tight junctions of the alveolar epithelial cells are impermeable during lactation, and thus allow milk to be stored between nursing periods without leakage of milk components from the lumen. Nonetheless mammary epithelial tight junctions are dynamic and can be regulated by a number of stimuli. Tight junctions of the mammary gland from the pregnant animal are leaky, undergoing closure around parturition to become the impermeable tight junctions of the lactating animal. Milk stasis, high doses of oxytocin, and mastitis have been shown to increase tight junction permeability. In general changes in tight junction permeability in the mammary gland appear to be the results of a state change and not assembly and disassembly of tight junctions. Both local factors, such as intramammary pressure and TGF-beta, and systemic factors, such as prolactin, progesterone, and glucocorticoids, appear to play a role in the regulation of mammary tight junctions. Finally, the tight junction state appears to be closely linked to milk secretion. An increase in tight junction permeability is accompanied by decrease in the milk secretion rate, and conversely, a decrease in tight junction permeability is accompanied by an increase in the milk secretion rate.

Antagonistic regulation of cell migration by epidermal growth factor and glucocorticoid in human gastric carcinoma cells

Epidermal growth factor (EGF) induced the disruption and scattering of colonies of TMK-1, a cell line derived from a human gastric carcinoma. A stimulatory action of EGF on cell migration was also observed as determined by a wound assay. However, these actions of EGF were inhibited if the cells were pretreated with dexamethasone, a synthetic glucocorticoid. Dexamethasone increased cell adhesion to collagen type IV and laminin, but not to poly-L-lysine and fibronectin. In contrast, EGF did not affect cell adhesion to these extracellular matrices whether dexamethasone was present or not. Dexamethasone enhanced the protein levels of both alpha1 and beta1 integrin subunits, and that of the alpha1 beta1 heterodimer. Further, flow cytometric analysis revealed that dexamethasone increased the expression of beta1 and alpha1 integrin subunits at the cell surface, whereas EGF increased expression of beta1 and alpha2 subunits at the cell surface. Antibodies against alpha1 and beta1 integrin subunits inhibited the increased cell adhesion seen in the presence of dexamethasone. An immunofluorescence study indicated that dexamethasone increased the formation of focal adhesions along the entire edges of cell colonies. In contrast, EGF led to the formation of focal adhesions preferentially at the cell front, and this EGF-induced preferential formation was not observed if the cells were pretreated with dexamethasone. These results suggest that glucocorticoid increased cell adhesion to the extracellular matrix via alpha1 beta1 integrin, and thereby antagonized EGF-induced cell migration.

Remodeling of junctional complexes during the development of the outer blood-retinal barrier

BACKGROUND

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by separating the neural retina from fenestrated capillaries in the choroid. The barrier depends upon tight junctions within the apical junctional complexes that bind neighboring cells. During development, permeability decreases as the apical junctional complex gradually matures. To investigate this process, the composition of the apical junctional complex was monitored during RPE development in chicken embryos.

METHODS

Permeability was monitored by incubating freshly isolated RPE/choroid in medium containing horseradish peroxidase followed by histochemical staining and electron microscopy. The expression of the tight junction proteins, ZO-1 and occludin, was determined by immunofluorescence and immunoblotting. Development of the RPE apical junctional complex was to compared to the homologous complex that forms the outer limiting membrane of the neural retina.

RESULTS

The apical junctional complex of the RPE was permeable to horseradish peroxidase until embryonic day 10-12. Two putative forms of ZO-1 had approximately the same molecular mass as mammalian ZO-1 and were present in the apical junctional complexes at different stages of development. We identified one form as ZO-1, because it was present in mature RPE and shared an epitope with the rodent isoforms, ZO-1 alpha+ and ZO-1 alpha-. The second form lacked this epitope but was identified by a polyclonal antibody to ZO-1. It was designated the ZO-1-like protein (ZO-1LP). On embryonic day 3, occludin and ZO-1LP were observed along the apical surface of the neuroepithelium that gave rise to the RPE and the neural retina. In the neural retina, occludin expression decreased just before inner segments were formed, but ZO-1LP expression continued in the outer limiting membrane throughout development. During RPE development, occludin expression was constant or increased slightly. By contrast, ZO-1LP was gradually replaced by ZO-1 and total ZO-1 immunoreactive proteins decreased more than 10x.

CONCLUSIONS

A gradual change in the composition of the apical junctional complexes accompanied the period of barrier formation. In RPE, ZO-1 gradually replaced ZO-1LP, but the decrease in ZO-1 expression suggests its functions during junction formation are not directly related to junction permeability. By contrast, occludin was lost and ZO-1LP retained where an adherens junction forms the permeable, outer limiting membrane.

A synthetic peptide corresponding to the extracellular domain of occludin perturbs the tight junction permeability barrier

Occludin, the putative tight junction integral membrane protein, is an attractive candidate for a protein that forms the actual sealing element of the tight junction. To study the role of occludin in the formation of the tight junction seal, synthetic peptides (OCC1 and OCC2) corresponding to the two putative extracellular domains of occludin were assayed for their ability to alter tight junctions in Xenopus kidney epithelial cell line A6. Transepithelial electrical resistance and paracellular tracer flux measurements indicated that the second extracellular domain peptide (OCC2) reversibly disrupted the transepithelial permeability barrier at concentrations of < 5 microM. Despite the increased paracellular permeability, there were no changes in gross epithelial cell morphology as determined by scanning EM. The OCC2 peptide decreased the amount of occludin present at the tight junction, as assessed by indirect immunofluorescence, as well as decreased total cellular content of occludin, as assessed by Western blot analysis. Pulse-labeling and metabolic chase analysis suggested that this decrease in occludin level could be attributed to an increase in turnover of cellular occludin rather than a decrease in occludin synthesis. The effect on occludin was specific because other tight junction components, ZO-1, ZO-2, cingulin, and the adherens junction protein E-cadherin, were unaltered by OCC2 treatment. Therefore, the peptide corresponding to the second extracellular domain of occludin perturbs the tight junction permeability barrier in a very specific manner. The correlation between a decrease in occludin levels and the perturbation of the tight junction permeability barrier provides evidence for a role of occludin in the formation of the tight junction seal.