Effect of hepatocyte growth factor on assembly of zonula occludens-1 protein at the plasma membrane

Physiological Signaling and Structure of the HGF Receptor MET

The "hepatocyte growth factor" also known as "scatter factor", is a multifunctional cytokine with the peculiar ability of simultaneously triggering epithelial cell proliferation, movement and survival. The combination of those proprieties results in the induction of an epithelial to mesenchymal transition in target cells, fundamental for embryogenesis but also exploited by tumor cells during metastatization. The hepatocyte growth factor receptor, MET, is a proto-oncogene and a prototypical transmembrane tyrosine kinase receptor. Inhere we discuss the MET molecular structure and the hepatocyte growth factor driven physiological signaling which coordinates epithelial proliferation, motility and morphogenesis.

HGF and the regulation of tight junctions in human prostate cancer cells

Hepatocyte growth factor (HGF) may impact the metastasis of prostate cancer via its action on prostate stem cells or their progeny. Tight junctions (TJs) are crucial to the process of metastasis and have been previously shown to be regulated by HGF. The present study aimed to evaluate the effect of HGF on the function of TJs in human prostate epithelial, prostate stem cell-like and prostate cancer cell lines. Four human prostate cancer cell lines (PC-3, DU-145, PZHPV-7, CaHPV-10), normal adult prostate parental epithelial cells (RWPE-1) and a stem cell-like derivative of RWPE-1 (WPE-STEM) were used to assess HGF-induced changes in TJs. A significant difference was noted in the behaviour between the WPE-STEM, RWPE-1 and the cancer cell lines which was HGF concentration-dependent. However, in the WPE-STEM cells, the effect was biphasic, with the cells seemingly resistant to HGF-modulated TJ disruption. Closer examination revealed that HGF affected the redistribution of ZO-1, ZO-2 and ZO-3 away from the TJs of confluent cells with concurrent loss of claudin-1 and claudin-5, and western blot analysis revealed a loss in TJ protein expression of ZO-1 and ZO-2. We demonstrated for the first time that HGF regulates TJ function in human prostate cells. Moreover, this regulation was dependent on the tumourigenicity of the cells, with the most aggressive cells most susceptible and the stem cell-like cells least susceptible. These data offer an intriguing glimpse of how TJs affect the behaviour of prostate cancer cells and how HGF modulates the expression and function of the molecules maintaining TJ structure and function.

Hepatocyte growth factor stimulates the migration of gastric epithelial cells by altering the subcellular localization of the tight junction protein ZO-1

BACKGROUND

Hepatocyte growth factor (HGF) is essential for epithelial restitution, a process in which epithelial cells rapidly migrate to cover desquamated epithelium after mucosal injury in the gastrointestinal tract. In this study, we aimed to elucidate the molecular mechanisms of the HGF-mediated reconstitution of gastric epithelial structures by analyzing the expression and subcellular dynamics of tight junction proteins.

METHODS

We treated human gastric epithelial MKN74 cells with HGF, and examined the effects of HGF on cell migration and proliferation, and the expression and subcellular dynamics of tight junction proteins; as well, we investigated the effect of HGF on paracellular permeability to macromolecules (using fluorescein isothiocyanate [FITC]-dextran).

RESULTS

HGF significantly stimulated the migration of MKN74 cells, but not their proliferation, in a dose-dependent manner. HGF did not affect the expression of tight junction proteins, including claudin-1, -3, -4 and -7; occludin; and zonula occludens (ZO)-1. However, fluorescence immunostaining revealed that, in the cell membrane, the levels of ZO-1, but not those of occludin or claudin-4, were transiently decreased 1 h after HGF treatment. The results were further confirmed by western blotting: HGF reduced the amount of ZO-1 protein in the cell membrane fraction concomitantly with an increase in cytoplasmic ZO-1. Furthermore, HGF reduced the interaction between ZO-1 and occludin, and induced the tyrosine phosphorylation of occludin, whereas the phosphorylation status of ZO-1 was not affected by exposure to HGF. Despite a decrease in the ZO-1/occludin interaction, HGF did not affect paracellular permeability to macromolecules.

CONCLUSIONS

HGF alters the subcellular localization of ZO-1, probably through the tyrosine phosphorylation of occludin, which may induce cell dispersion during epithelial restitution.

Altered expression of ZO-1 and ZO-2 in Sertoli cells and loss of blood-testis barrier integrity in testicular carcinoma in situ

Carcinoma in situ (CIS) is the noninvasive precursor of most human testicular germ cell tumors. In normal seminiferous epithelium, specialized tight junctions between Sertoli cells constitute the major component of the blood-testis barrier. Sertoli cells associated with CIS exhibit impaired maturation status, but their functional significance remains unknown. The aim was to determine whether the blood-testis barrier is morphologically and/or functionally altered. We investigated the expression and distribution pattern of the tight junction proteins zonula occludens (ZO) 1 and 2 in normal seminiferous tubules compared to tubules showing CIS. In normal tubules, ZO-1 and ZO-2 immunostaining was observed at the blood-testis barrier region of adjacent Sertoli cells. Within CIS tubules, ZO-1 and ZO-2 immunoreactivity was reduced at the blood-testis barrier region, but spread to stain the Sertoli cell cytoplasm. Western blot analysis confirmed ZO-1 and ZO-2, and their respective mRNA were shown by RT-PCR. Additionally, we assessed the functional integrity of the blood-testis barrier by lanthanum tracer study. Lanthanum permeated tight junctions in CIS tubules, indicating disruption of the blood-testis barrier. In conclusion, Sertoli cells associated with CIS show an altered distribution of ZO-1 and ZO-2 and lose their blood-testis barrier function.

Subepithelial fibroblasts in intestinal villi: roles in intercellular communication

Ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes in the gut. Intestinal villi are motile, equipped with chemosensors and mechanosensors, and transduce signaling to sensory neurons, but the exact mechanisms have not yet been elucidated. Subepithelial fibroblasts located under the villous epithelium form contractile cellular networks via gap junctions. The networks ensheathe lamina propria and are in close contact with epithelium, neural and capillary networks, smooth muscles, and immune cells. Unique characteristics of subepithelial fibroblasts have been revealed by primary cultures isolated from rat duodenal villi. They include rapid reversal changes in cell shape by cAMP reagents and endothelins, cell shape-dependent mechanosensitivity that induces ATP release as a paracrine mediator, contractile ability, and expression of various receptors for vasoactive and neuroactive substances. Herein, we review these characteristics that play a key role in the villi. They serve as a barrier/sieve, flexible mechanical frame, mechanosensor, and signal transduction machinery in the intestinal villi, which are regulated locally and dynamically by rapid cell shape conversion.

Albumin marks pseudopodia of astrocytoma cells responding to hepatocyte growth factor or serum

It is well accepted that dysfunction in the blood brain barrier (BBB) allows permeation of albumin from the bloodstream into astrocytic brain tumors, especially glioblastomas, the most aggressive astrocytomas. In vitro, bovine serum albumin (BSA) aids functional cell assays by maintaining cytokines and growth factors in solution and delivering its cargo of fatty acids. Earlier, we showed that BSA was prominent in lysates prepared from pseudopodia formed by U87 astrocytoma cells. The present studies investigated the association of albumin with pseudopodia formed by U87 and LN229 astrocytoma cells. With hepatocyte growth factor (HGF) stimulation, cell migration was enhanced and BSA, especially its dimerized form, was prominent in pseudopodia compared to unmigrated cells on one-dimensional gels and immunoblots. When lysates were equalized for levels of glyceraldehyde-3-phosphate dehydrogenase, the rise for BSA levels in pseudopodia vs migrated cells was comparable or greater than levels noted for established pseudopodial proteins, beta-actin and ezrin. The increase for dimerized BSA in pseudopodia compared to unmigrated cells was greater than the rise in levels of beta-actin, ezrin, HGF, and phosphorylated Met when pseudopodia were harvested from filters with 1 mum pores using either cell line. Fluorescein (F)-labeled BSA co-localized with HGF on actin-rich cellular protrusions and with CM-DiI labeled pseudopodial plasma membranes. The F-BSA highlighted small, individual pseudopodial profiles more so than complex pseudopodial networks (reticulopodia) or unmigrated cells. Labeled human serum albumin also decorated pseudopodia preferentially. Albumin's association with pseudopodia may help to explain its selective accumulation in astrocytomas in vivo. The leaky BBB permits serum albumin to enter the microenvironment of astrocytomas thus allowing their invasive cells contact with serum albumin as a source of fatty acids that would be useful for remodeling cell membranes in pseudopodia. Thus, albumin potentially aids and marks invasion as it accumulates in these tumors.

Interferon-gamma induces internalization of epithelial tight junction proteins via a macropinocytosis-like process

Increased epithelial permeability is observed in inflammatory states. However, the mechanism by which inflammatory mediators such as IFN-gamma increase epithelial permeability is unknown. We recently observed that IFN-gamma induces disassembly of tight junctions (TJ); in this study we asked whether such TJ disassembly is mediated by endocytosis of junctional proteins. The role of three major internalization pathways in disruption of TJ in IFN-gamma-treated intestinal epithelial cells was analyzed using selective inhibitors and markers of the pathways. No role for the clathrin- and caveolar-mediated endocytosis in the IFN-gamma-induced internalization of TJ proteins was observed. However, inhibitors of macropinocytosis blocked internalization of TJ proteins and junctional proteins colocalized with macropinocytosis markers, dextran and phosphatidylinositol-3,4,5-trisphosphate. Internalized TJ proteins were identified in early and recycling endosomes but not in late endosomes/lysosomes. These results for the first time suggest that IFN-gamma produces a leaky epithelial barrier by inducing macropinoytosis of TJ proteins.

Listeria monocytogenes invades the epithelial junctions at sites of cell extrusion

Listeria monocytogenes causes invasive disease by crossing the intestinal epithelial barrier. This process depends on the interaction between the bacterial surface protein Internalin A and the host protein E-cadherin, located below the epithelial tight junctions at the lateral cell-to-cell contacts. We used polarized MDCK cells as a model epithelium to determine how L. monocytogenes breaches the tight junctions to gain access to this basolateral receptor protein. We determined that L. monocytogenes does not actively disrupt the tight junctions, but finds E-cadherin at a morphologically distinct subset of intercellular junctions. We identified these sites as naturally occurring regions where single senescent cells are expelled and detached from the epithelium by extrusion. The surrounding cells reorganize to form a multicellular junction that maintains epithelial continuity. We found that E-cadherin is transiently exposed to the lumenal surface at multicellular junctions during and after cell extrusion, and that L. monocytogenes takes advantage of junctional remodeling to adhere to and subsequently invade the epithelium. In intact epithelial monolayers, an anti-E-cadherin antibody specifically decorates multicellular junctions and blocks L. monocytogenes adhesion. Furthermore, an L. monocytogenes mutant in the Internalin A gene is completely deficient in attachment to the epithelial apical surface and is unable to invade. We hypothesized that L. monocytogenes utilizes analogous extrusion sites for epithelial invasion in vivo. By infecting rabbit ileal loops, we found that the junctions at the cell extrusion zone of villus tips are the specific target for L. monocytogenes adhesion and invasion. Thus, L. monocytogenes exploits the dynamic nature of epithelial renewal and junctional remodeling to breach the intestinal barrier.

Inhibition of Pkhd1 impairs tubulomorphogenesis of cultured IMCD cells

Fibrocystin/polyductin (FPC), the gene product of PKHD1, is responsible for autosomal recessive polycystic kidney disease (ARPKD). This disease is characterized by symmetrically large kidneys with ectasia of collecting ducts. In the kidney, FPC predominantly localizes to the apical domain of tubule cells, where it associates with the basal bodies/primary cilia; however, the functional role of this protein is still unknown. In this study, we established stable IMCD (mouse inner medullary collecting duct) cell lines, in which FPC was silenced by short hairpin RNA inhibition (shRNA). We showed that inhibition of FPC disrupted tubulomorphogenesis of IMCD cells grown in three-dimensional cultures. Pkhd1-silenced cells developed abnormalities in cell-cell contact, actin cytoskeleton organization, cell-ECM interactions, cell proliferation, and apoptosis, which may be mediated by dysregulation of extracellular-regulated kinase (ERK) and focal adhesion kinase (FAK) signaling. These alterations in cell function in vitro may explain the characteristics of ARPKD phenotypes in vivo.

Modulation of epithelial morphology, monolayer permeability, and cell migration by growth arrest specific 3/peripheral myelin protein 22

Peripheral myelin protein 22 (PMP22) is associated with a subset of hereditary peripheral neuropathies. Although predominantly recognized as a transmembrane constituent of peripheral nerve myelin, PMP22 is localized to epithelial and endothelial cell-cell junctions, where its function remains unknown. In this report, we investigated the role of PMP22 in epithelial biology. Expression of human PMP22 (hPMP22) slows cell growth and induces a flattened morphology in Madin-Darby canine kidney (MDCK) cells. The transepithelial electrical resistance (TER) and paracellular flux of MDCK monolayers are elevated by hPMP22 expression. After calcium switch, peptides corresponding to the second, but not the first, extracellular loop of PMP22 perturb the recovery of TER and paracellular flux. Finally, subsequent to wounding, epithelial monolayers expressing hPMP22 fail to migrate normally. These results indicate that PMP22 is capable of modulating several aspects of epithelial cell biology, including junctional permeability and wound closure.

Breaking into the epithelial apical-junctional complex--news from pathogen hackers

The epithelial apical-junctional complex is a key regulator of cellular functions. In addition, it is an important target for microbial pathogens that manipulate the cell to survive, proliferate and sometimes persist within a host. Out of a myriad of potential molecular targets, some bacterial and viral pathogens have selected a subset of protein targets at the apical-junctional complex of epithelial cells. Studying how microbes use these targets also teaches us about the inherent physiological properties of host molecules in the context of normal junctional structure and function. Thus, we have learned that three recently uncovered components of the apical-junctional complex of the Ig superfamily--junctional adhesion molecule, Nectin and the coxsackievirus and adenovirus receptor--are important regulators of junction structure and function and represent critical targets of microbial virulence gene products.

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.

Membrane-associated HB-EGF modulates HGF-induced cellular responses in MDCK cells

In MDCK cells, hepatocyte growth factor/scatter factor (HGF/SF) induces epithelial cell dissociation, scattering, migration, growth and formation of branched tubular structures. By contrast, these cells neither scatter nor form tubular structures in response to the epidermal growth factor (EGF) family of growth factors. Heparin-binding EGF-like growth factor (HB-EGF) is a member of the EGF family of growth factors and is synthesized as a membrane-associated precursor molecule (proHB-EGF). ProHB-EGF is proteolytically cleaved to release a soluble ligand (sHB-EGF) that activates the EGF receptor. Although recent studies suggest possible physiological functions, the role of proHB-EGF remains largely undefined. Using MDCK cells stably expressing proHB-EGF, a noncleavable deletion mutant of proHB-EGF or soluble HB-EGF, we show that epithelial cell functions differ depending on the form of HB-EGF being expressed. Expression of noncleavable membrane-anchored HB-EGF promoted cell-matrix and cell-cell interactions and decreased cell migration, HGF/SF-induced cell scattering and formation of tubular structures. By contrast, expression of soluble HB-EGF induced increased cell migration, decreased cell-matrix and cell-cell interactions and promoted the development of long unbranched tubular structures in response to HGF/SF. These findings suggest that HB-EGF can not only modulate HGF/SF-induced cellular responses in MDCK cells but also that membrane-bound HB-EGF and soluble HB-EGF give rise to distinctly different effects on cell-cell and cell-extracellular matrix interactions.

Hepatocyte growth factor induces MDCK cell morphogenesis without causing loss of tight junction functional integrity

Hepatocyte growth factor (HGF) induces mitogenesis, motogenesis, and tubulogenesis of cultured Madin-Darby canine kidney (MDCK) epithelial cells. We report that in addition to these effects HGF stimulates morphogenesis of tight, polarized MDCK cell monolayers into pseudostratified layers without loss of tight junction (TJ) functional integrity. We tested TJ functional integrity during formation of pseudostratified layers. In response to HGF, the TJ marker ZO-1 remained in morphologically complete rings and functional barriers to paracellular diffusion of ruthenium red were maintained in pseudostratified layers. Transepithelial resistance (TER) increased transiently two- to threefold during the morphogenetic transition from monolayers to pseudostratified layers and then declined to baseline levels once pseudostratified layers were formed. In MDCK cells expressing the trk/met chimera, both HGF and NGF at concentrations of 2.5 ng/ml induced scattering. However, 2.5 ng/ml HGF did not affect TER. The peak effect of HGF on TER was at a concentration of 100 ng/ml. In contrast, NGF at concentrations as high as 25 μg/ml had no effect on TER or pseudostratified layer morphogenesis of trk/met-expressing cultures. These results suggest that altered presentation of the stimulus, such as through HGF interaction with low-affinity sites, may change the downstream signaling response. In addition, our results demonstrate that HGF stimulates pseudostratified layer morphogenesis while inducing an increase in TER and maintaining the overall tightness of the epithelial layer. Stimulation of epithelial cell movements by HGF without loss of functional TJs may be important for maintaining epithelial integrity during morphogenetic events such as formation of pseudostratified epithelia, organ regeneration, and tissue repair.

Proinflammatory cytokines disrupt epithelial barrier function by apoptosis-independent mechanisms

It is well known that inflammatory conditions of the intestinal mucosa result in compromised barrier function. Inflammation is characterized by an influx into the mucosa of immune cells that influence epithelial function by releasing proinflammatory cytokines such as IFN-gamma and TNF-alpha. Mucosal barrier function is regulated by the epithelial apical junctional complex (AJC) consisting of the tight junction and the adherens junction. Since the AJC regulates barrier function, we analyzed the influence of IFN-gamma and TNF-alpha on its structure/function and determined the contribution of apoptosis to this process using a model intestinal epithelial cell line, T84, and IFN-gamma and TNF-alpha. AJC structure/function was analyzed by confocal microscopy, biochemical analysis, and physiologic measurement of epithelial gate/fence function. Apoptosis was monitored by determining cytokeratin 18 cleavage and caspase-3 activation. IFN-gamma induced time-dependent disruptions in epithelial gate function that were potentiated by coincubation with TNF-alpha. Tight junction fence function was somewhat disrupted. Cytokine treatment was associated with internalization of AJC transmembrane proteins, junction adhesion molecule 1, occludin, and claudin-1/4 with minimal effects on the cytoplasmic plaque protein zonula occludens 1. Detergent solubility profiles of junction adhesion molecule 1 and E-cadherin and their affiliation with "raft-like" membrane microdomains were modified by these cytokines. Inhibition of cytokine-induced apoptosis did not block induced permeability defects; further emphasizing their primary influence on the epithelial AJC structure and barrier function. Our findings for the first time clearly separate the proapoptotic effects of IFN-gamma and TNF-alpha from their abilities to disrupt barrier function.

Osteoactivin expressed during cirrhosis development in rats fed a choline-deficient, L-amino acid-defined diet, accelerates motility of hepatoma cells

BACKGROUND/AIMS

Hepatocellular carcinoma (HCC) is closely associated with chronic liver diseases, particularly cirrhosis. However, the genes involved in hepatocarcinogenesis in the context of developing cirrhosis remain unknown. This study aims to identify genes associated with early cirrhosis-associated hepatocarcinogenesis.

METHODS

We examined genes differentially expressed between the livers of normal rats and rats fed a choline-deficient, L-amino acid-defined (CDAA) diet using suppression subtractive hybridization. We examined both the expression in the liver and HCC tissues of osteoactivin (OA), isolated in this screen, and its effect on invasiveness and metastasis.

RESULTS

OA mRNA was strongly expressed in the livers of rats fed the CDAA diet for 1-3 months. Moderate expression was sustained for 18 months. OA overexpression increased the invasiveness and metastasis of rat hepatoma cells in vitro and in vivo. In humans, OA expression was not detectable in normal liver tissues. While OA transcripts were detectable in cirrhotic nontumorous liver tissues surrounding HCCs, the majority of HCC tissue samples exhibited higher levels of OA expression than the surrounding normal tissue.

CONCLUSIONS

These results indicate that OA is a novel factor involved in the progression of HCC via stimulation of tumor invasiveness and metastatic potential.

Transforming growth factor beta3 regulates the dynamics of Sertoli cell tight junctions via the p38 mitogen-activated protein kinase pathway

Earlier studies have implicated the significance of transforming growth factor-beta3 (TGFbeta3) in the regulation of Sertoli cell tight junction (TJ) dynamics, possibly via its inhibitory effects on the expression of occludin, claudin-11, and zonula occludens-1 (ZO-1). Yet the mechanism by which TGFbeta3 regulates the Sertoli cell TJ-permeability barrier is not known. Using techniques of semiquantitative reverse transcription-PCR (RT-PCR), immunoblotting, immunohistochemistry, and inhibitors against different kinases coupled with physiological techniques to assess the Sertoli cell TJ barrier function, it was shown that this TGFbeta3-induced effect on Sertoli cell TJ dynamics is mediated via the p38 mitogen-activated protein (MAP) kinase pathway. First, the assembly of the Sertoli cell-TJ barrier was shown to be associated with a transient but significant decline in both the TGFbeta3 production and expression by Sertoli cells. Furthermore, addition of TGFbeta3 to Sertoli cell cultures during TJ assembly indeed perturbed the TJ barrier with an IC50 at approximately 9 pM. Second, the TGFbeta3-induced disruption of the TJ barrier was associated with a transient induction in MEKK2 but not the other upstream signaling molecules that mediate TGFbeta3 action, such as Smad2, Cdc42, Rac2, and N-Ras, suggesting this effect might be mediated via the p38 MAP kinase pathway. This postulate was confirmed by the observation that TGFbeta3 also induced the protein level of the activated and phosphorylated form of p38 MAP kinase at the time the TJ barrier was perturbed. Third, and perhaps the most important of all, this TGFbeta3-mediated inhibitory effect on the TJ barrier and the TGFbeta3-induced p-p38 MAP kinase production could be blocked by SB202190, a specific p38 MAP kinase inhibitor, but not U0126, a specific MEK1/2 kinase inhibitor. These results thus unequivocally demonstrate that TGFbeta3 utilizes the p38 MAP kinase pathway to regulate Sertoli cell TJ dynamics.

Expression of kinase-inactive c-Src delays oxidative stress-induced disassembly and accelerates calcium-mediated reassembly of tight junctions in the Caco-2 cell monolayer

The activity of Src kinases appears to play a role in both assembly and disassembly of tight junction. However, the role of a specific isoform of Src kinase in regulation of tight junction is not known. In the present study the role of c-Src in regulation of epithelial tight junction was investigated in Caco-2 cell monolayers. Oxidative stress (xanthine oxidase + xanthine) induced an activation and membrane translocation of c-Src. The oxidative stress-induced decrease in transepithelial electrical resistance, increase in inulin permeability, and redistribution of occludin and ZO-1 from the intercellular junctions were prevented by PP2. The rates of oxidative stress-induced activation of c-Src, tyrosine phosphorylation of ZO-1 and beta-catenin, decrease in resistance, increase in permeability to inulin, and redistribution of occludin and ZO-1 were significantly greater in cells transfected with wild type c-Src, whereas it was low in cells transfected with kinase-inactive c-SrcK297R mutant, when compared with those in empty vector-transfected cells. The rates of recovery of resistance, increase in barrier to inulin, and reorganization of occludin and ZO-1 into the intercellular junctions during the calcium-induced reassembly of tight junction were much greater in Caco-2 cells transfected with c-SrcK297R as compared with those in cells transfected with empty vector or wild type c-Src. These results show that the dominant-negative expression of kinase-inactive c-Src delays the oxidative stress-induced disruption of tight junction and accelerates calcium-induced assembly of tight junction in Caco-2 cells and demonstrate that oxidative stress-induced disruption of tight junction is mediated by the activation of c-Src.

Platelet-derived growth factor mediates tight junction redistribution and increases permeability in MDCK cells

Increased tissue permeability is a common characteristic of a number of diseases such as pulmonary edema, inflammatory bowel disease, several kidney diseases, diabetic retinopathy, and tumors. We hypothesized that growth factors increase permeability by redistribution of tight junction proteins away from the cell border. To investigate mechanisms of growth factor-mediated permeability, we examined the effect of platelet derived growth factor (PDGF) on Madin-Darby canine kidney (MDCK) cell tight junction protein distribution and on permeability. PDGF altered the cellular distribution of occludin and ZO-1 from the cell border to the cytoplasm and increased permeability to 70 kDa dextran in a concentration-dependent manner. Treatment of MDCK cells with PDGF prior to fixation allowed binding of the lectin concanavalin A to the basement membrane of fixed cells, while binding was prevented in untreated control monolayers, implying that PDGF induced the formation of a paracellular transport pathway. Cell fractionation experiments with PDGF-treated cells revealed a novel occludin-containing low-density, detergent resistant subcellular structure, which increased in the buoyant fractions relative to occludin in the pellet in a time- and concentration-dependent manner. Immunocytochemistry revealed that a pool of internalized occludin co-labels with the early endosome marker, EEA1, suggesting that PDGF may stimulate occludin to enter an endosomal pathway. PDGF may act as a permeabilizing agent by moving tight junction proteins away from the cell border in discrete microdomains, and the effects of PDGF on permeability and tight junction protein distribution may model the regulation of epithelial and endothelial barrier properties by other peptide growth factors.

Novel growth factors involved in the pathogenesis of proliferative vitreoretinopathy

AIMS

To determine whether hepatocyte growth factor (HGF) and connective tissue growth factor (CTGF) are expressed in human specimens of proliferative vitreoretinopathy (PVR) and to propose a model of PVR pathogenesis based upon the known activities of these growth factors. Methods Immunohistochemical methods (ABC Elite) were used to demonstrate the presence of HGF and CTGF in cryostat sections of five human PVR membranes.

RESULTS

In each of the five PVR membranes, stromal cells were immunohistochemically positive for both HGF and CTGF. Based upon this information and the known actions of these growth factors, a model of PVR pathogenesis was developed. In this model, injury of the retina induces an inflammatory response that upregulates HGF expression inducing the formation of multilayered groups of migratory retinal pigment epithelial cells (RPE). These RPE, present in a provisional extracellular matrix, come in contact with vitreous containing TGF-beta. The TGF-beta is activated, upregulating expression of CTGF. Under the influence of TGF-beta and CTGF, RPE become myofibroblastic and fibrosis ensues. Retinal traction induces further detachment continuing the cycle of retinal injury.

CONCLUSIONS

HGF and CTGF are expressed in PVR membranes and may play important roles in the pathogenesis of PVR. The expression and function of these growth factors should be critically examined in human PVR specimens, in in vitro cultures of RPE, and in animal models of PVR.

Hepatocyte growth factor alters renal epithelial cell susceptibility to uropathogenic Escherichia coli

The urinary tract is frequently the source of Escherichia coli bacteremia. Bacteria from the urinary tract must cross an epithelial layer to enter the bloodstream. Hepatocyte growth factor (HGF) alters the polarity of Madin-Darby canine kidney (MDCK) epithelial cells. The role of cell polarity in determining renal epithelial resistance to Escherichia coli invasion is not well known. A model of polarized and HGF-treated MDCK epithelial cells grown on filters was used to study the role of epithelial cell polarity during the interaction of nonvirulent (XL1-Blue) and uropathogenic (J96) strains of Escherichia coli with renal epithelium. Basolateral exposure of MDCK cells to J96, but not XL1-Blue, resulted in loss of transepithelial resistance (TER), which was due to epithelial cytotoxicity and not degradation of epithelial junctional proteins by bacterial proteases. Apical exposure to both J96 and XL1-Blue did not alter TER. Pretreatment of polarized MDCK cell monolayers with HGF renders the cells sensitive to loss of TER and cytotoxicity by apical exposure to J96. Analysis by confocal microscopy demonstrated that HGF treatment of MDCK cell monolayers also greatly enhances adherence of J96 to the apical surface of the cell monolayer. These data demonstrate that the basolateral surface of polarized epithelia is more susceptible to J96 cytotoxicity. The data also support the hypothesis that processes that alter epithelial cell polarity increase sensitivity of epithelia to bacterial injury and adherence from the apical compartment.

HGF upregulates and modifies subcellular distribution of proteins in colon cancer cell enterocytic differentiation

Hepatocyte growth factor (HGF) and its receptor, c-Met, are involved in cell transformation. To study their role in intestinal cell differentiation, we used Caco-2 colon cancer cells, which differentiate spontaneously into enterocytes during culture. Cells grown continuously in the presence of HGF reached confluence more quickly than control cells. Markers of enterocytic differentiation, such as alkaline phosphatase and sucrase-isomaltase activities, adhesion molecules, and structural proteins such as E-cadherin, villin, and F-actin were upregulated by HGF throughout the 35 days of culture, and actin fibers were reorganized. HGF also stimulated expression and tyrosine phosphorylation of c-Met and Gab-1 as well as protein kinase C (PKC)-alpha expression. PKC-alpha has been shown to be involved in intestinal differentiation. We therefore investigated the possibility that increases in PKC-alpha protein levels were responsible for the HGF-promoted events. We did this by incubating cells with Gö-6976, an inhibitor of PKC-alpha and -beta1, concomitantly with HGF. This inhibitor abolished the HGF-induced increase in villin levels before, but not after, confluence. Thus HGF accelerates Caco-2 cell differentiation and stimulates the metabolic and structural events accompanying this process. These HGF-promoted events may be mediated partly by Gab-1, and the effects of HGF on villin before confluence seem to involve PKC.

Enteropathogenic Escherichia coli activates ezrin, which participates in disruption of tight junction barrier function

Enteropathogenic Escherichia coli (EPEC) is an important human intestinal pathogen, especially in infants. EPEC adherence to intestinal epithelial cells induces the accumulation of a number of cytoskeletal proteins beneath the bacteria, including the membrane-cytoskeleton linker ezrin. Evidence suggests that ezrin can participate in signal transduction. The aim of this study was to determine whether ezrin is activated following EPEC infection and if it is involved in the cross talk with host intestinal epithelial cells. We show here that following EPEC attachment to intestinal epithelial cells there was significant phosphorylation of ezrin, first on threonine and later on tyrosine residues. A significant increase in cytoskeleton-associated ezrin occurred following phosphorylation, suggesting activation of this molecule. Nonpathogenic E. coli and EPEC strains harboring mutations in type III secretion failed to elicit this response. Expression of dominant-negative ezrin significantly decreased the EPEC-elicited association of ezrin with the cytoskeleton and attenuated the disruption of intestinal epithelial tight junctions. These results suggest that ezrin is involved in transducing EPEC-initiated signals that ultimately affect host physiological functions.

HGF/SF induces mesothelial cell migration and proliferation by autocrine and paracrine pathways

Mesothelial repair differs from that of other epithelial-like surfaces as healing does not occur solely by centripetal in-growth of cells as a sheet from the wound margins. Mesothelial cells lose their cell-cell junctions, divide, and adopt a fibroblast-like morphology while scattering across and covering the wound surface. These features are consistent with a cellular response to hepatocyte growth factor/scatter factor (HGF/SF). In this study, we examined the ability of mesothelial cells to secrete HGF/SF and investigated its possible role as an autocrine regulator of mesothelial cell motility and proliferation. We found that human primary mesothelial cells expressed HGF/SF mRNA and secreted active HGF/SF into conditioned medium as determined by ELISA and in a scattering bioassay. These cells also expressed the HGF/SF receptor, Met, as shown by RT-PCR and by Western blot analysis and immunofluorescence. Incubation of mesothelial cells with neutralizing antibodies to HGF/SF decreased cell migration to 25% of controls, whereas addition of HGF/SF disrupted cell-cell junctions and induced scattering and enhanced mesothelial cell migration. Furthermore, HGF/SF showed a small but significant mitogenic effect on all mesothelial cell lines examined. In conclusion, HGF/SF is produced by mesothelial cells and induces both motility and proliferation of these cells. These data are consistent with HGF/SF playing an autocrine role in mesothelial healing.

HGF regulates tight junctions in new nontumorigenic gastric epithelial cell line

The regulation of intercellular adhesion by hepatocyte growth factor (HGF) was examined on a novel nontumorigenic gastric epithelial cell line (IMGE-5) derived from H-2Kb-tsA58 transgenic mice. IMGE-5 cells constitutively expressed cytokeratin 18 and HGF receptors. Under permissive conditions (33 degrees C + interferon-gamma), IMGE-5 cells proliferated rapidly but did not display membrane expression of adherens and tight junction proteins. Under nonpermissive conditions, their proliferation was decreased and they displayed a strong, localized membrane expression of E-cadherin/beta-catenin and occludin/ZO-1. HGF treatment largely prevented the targeting of ZO-1 to the tight junction and induced a significant decrease of the transepithelial resistance measured across a confluent IMGE-5 cell monolayer. HGF rapidly increased the tyrosine phosphorylation of ZO-1 and decreased its association with occludin in a phosphatidylinositol 3-kinase (PI 3-kinase)-dependent manner. PI 3-kinase was also involved in HGF-induced migration of IMGE-5 cells. Our results demonstrate that 1) HGF prevents the appearance of ZO-1 in the membrane during epithelial cell differentiation; 2) HGF causes partial relocalization of ZO-1 to the cytoplasm and nucleus and concomitantly stimulates cell dissociation and migration; and 3) IMGE-5 cells offer a useful model for the study of gastric epithelial cell differentiation.

Molecular physiology and pathophysiology of tight junctions. IV. Regulation of tight junctions by extracellular stimuli: nutrients, cytokines, and immune cells

The epithelial lining of the gastrointestinal tract forms a regulated, selectively permeable barrier between luminal contents and the underlying tissue compartments. Permeability across the epithelium is, in part, determined by the rate-limiting barrier of the paracellular pathway-the most apical intercellular junction referred to as the tight junction (TJ). The TJ is composed of a multiprotein complex that affiliates with the underlying apical actomyosin ring. TJ structure and function, and therefore epithelial permeability, are influenced by diverse physiological and pathological stimuli; here we review examples of such stimuli that are detected at the cell surface. For example, luminal glucose induces an increase in paracellular permeability to small molecules. Similarly, but by other means, cytokines and leukocytes in the vicinity of the epithelium also regulate TJ structure and paracellular permeability by influencing the TJ protein complex and/or its association with the underlying actin cytoskeleton.

MAGUK proteins: structure and role in the tight junction

ZO-1, ZO-2 and ZO-3 are tight junction (TJ)-associated proteins that belong to the MAGUK family. In addition to the presence of the characteristic MAGUK modules (PDZ, SH3 and GK), ZOs have a distinctive carboxyl terminal with splicing domains, acidic- and proline-rich regions. The modular organization of these proteins allows them to function as scaffolds, which associate to transmembrane TJ proteins, the cytoskeleton and signal transduction molecules. ZOs shuttle between the TJ and the nucleus, where they may regulate gene expression.

The PDZ domains of zonula occludens-1 induce an epithelial to mesenchymal transition of Madin-Darby canine kidney I cells. Evidence for a role of beta-catenin/Tcf/Lef signaling

The integrity of cell-cell contacts such as adherens junctions (AJ) and tight junctions (TJ) is essential for the function of epithelia. During carcinogenesis, the increased motility and invasiveness of tumor cells reflect the loss of characteristic epithelial features, including cell adhesion. While beta-catenin, a component of AJ, plays a well characterized dual role in cell adhesion and signal transduction leading to epithelial cell transformation, little is known about possible roles of tight junction components in signaling processes. Here we show that mutants of the TJ protein zonula occludens protein-1 (ZO-1), which encode the PDZ domains (ZO-1 PDZ) but no longer localize at the plasma membrane, induce a dramatic epithelial to mesenchymal transition (EMT) of Madin-Darby canine kidney I (MDCKI) cells. The observed EMT of these MDCK-PDZ cells is characterized by a repression of epithelial marker genes, a restricted differentiation potential and a significantly induced tumorigenicity. Intriguingly, the beta-catenin signaling pathway is activated in the cells expressing the ZO-1 PDZ protein. Ectopic expression of the adenomatous polyposis coli tumor suppressor gene, known to down-regulate activated beta-catenin signaling, reverts the transformed fibroblastoid phenotype of MDCK-PDZ cells. Thus, cytoplasmic localization of the ZO-1 PDZ domains induces an EMT in MDCKI cells, most likely by modulating beta-catenin signaling.

Transformation of Madin-Darby canine kidney (MDCK) epithelial cells by Epstein-Barr virus latent membrane protein 1 (LMP1) induces expression of Ets1 and invasive growth

The Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) has a significant role in initiating EBV-associated lymphoproliferative disease and EBV-related malignancies. In view of clinical features related to the type of EBV latency, LMP1 may influence invasiveness of EBV associated tumors categorized as types II and III as represented on nasopharyngeal carcinoma (NPC). To screen for genes associated with invasion of epithelial cells transformed by LMP1, Madin-Darby canine kidney (MDCK) epithelial cells were transformed by LMP1. Stable transfection of a LMP1 gene into MDCK cells induced morphological change from cobblestone to a long spindle-shape, reduced cell-cell adhesion and caused high cell motility. Parental MDCK cells, which form spherical cysts in three-dimensional collagen gel matrix, form branching tubules following exposure to hepatocyte growth factor (HGF). MDCK cells transformed by LMP1 showed invasive growth to form branching tubules into collagen gel without HGF-treatment. mRNA differential display and Northern hybridization identified plasminogen activator inhibitor-1 (PAI-1), urokinase type plasminogen activator (uPA) and ets1 as genes upregulated during transformation by LMP1. Expression of a dominant negative type of Etsl in LMP1-transformed cells downregulated uPA expression and cell motility. Deletion of LMP1 cytoplasmic carboxy-terminal activating region 1 (CTAR1) domain abolished transformation, but a deletion mutant lacking CTAR2 domain still retained transforming and uPA-inducing ability. Expression of Ets1 was immunolocalized in tumor cells of NPC tissue which frequently express LMP1. Taken together, it is suggested that LMP1 induces expression of Ets1 which may contribute to invasion of NPC by stimulating cell motility and uPA expression.

Hepatocyte growth factor/scatter factor decreases the expression of occludin and transendothelial resistance (TER) and increases paracellular permeability in human vascular endothelial cells

Hepatocyte growth factor/scatter factor (HGF/SF), a multi function cytokine has been shown to regulate the expression of cell adhesion molecules in endothelial cells. In the current study, we examined the effects of HGF/SF on the function of tight junctions and the expression of occludin in these cells. Stimulation of human vascular endothelial cells with HGF/SF resulted in a concentration dependent increase in the paracellular permeability as measured using fluorescence labelled dextran but a decrease in the transendothelial cell resistance (TER) of the endothelial cells. Western blotting revealed that HGF/SF decreased the level of occludin in the cells, a primary tight junction forming protein. Immunofluorescence study indicated that shortly after treatment with HGF/SF there was a disturbance of the distribution of occludin and then a reduction in the staining of the molecule. It is concluded that HGF/SF decreases the expression of occludin, resulting in the functional change of tight junction.

Phenotypic control of gap junctional communication by cultured alveolar epithelial cells

We examined phenotype-specific changes in gap junction protein [connexin (Cx)] expression and function by cultured rat alveolar type II cells. Type II cells cultured on extracellular matrix in medium containing keratinocyte growth factor (KGF) and 2% fetal bovine serum (FBS; KGF/2) retained expression of surfactant protein C and the 180-kDa lamellar body membrane protein (lbm180). These markers were lost when cells were cultured in medium containing 10% FBS (MEM/10). With RT-PCR, cells cultured in MEM/10 showed transient increases in Cx43 and Cx46 mRNA expression, whereas Cx32 and Cx26 decreased and Cx30.3 and Cx37 were unchanged. Transient changes in Cx32, Cx43, and Cx46 protein expression were confirmed by immunoblot. In contrast, cells cultured in KGF/2 retained expression of Cx32 and showed increased expression of Cx30.3 and Cx46 mRNAs, compared with that in day 0 cells. With immunofluorescence microscopy, Cx32 and Cx43 were at the plasma membrane of cells grown in KGF/2, whereas Cx46 was exclusively intracellular. Type II cells cultured in MEM/10 showed approximately 3- to 4-fold more intercellular transfer of microinjected lucifer yellow through gap junctions than cells grown in 2% FBS. Thus type II cells dynamically alter gap junctional communication, and distinct alveolar epithelial cell phenotypes express different connexins.