RhoA activity is required for fibronectin assembly and counteracts beta1B integrin inhibitory effect in FRT epithelial cells

Fine tuning of Rac1 and RhoA alters cuspal shapes by remolding the cellular geometry

The anatomic and functional combinations of cusps and lophs (ridges) define the tooth shape of rodent molars, which distinguishes species. The species-specific cusp patterns result from the spatiotemporal induction of enamel knots (EKs), which require precisely controlled cellular behavior to control the epithelial invagination. Despite the well-defined roles of EK in cusp patterning, the determinants of the ultimate cuspal shapes and involvement of epithelial cellular geometry are unknown. Using two typical tooth patterns, the lophodont in gerbils and the bunodont in mice, we showed that the cuspal shape is determined by the dental epithelium at the cap stage, whereas the cellular geometry in the inner dental epithelium (IDE) is correlated with the cuspal shape. Intriguingly, fine tuning Rac1 and RhoA interconvert cuspal shapes between two species by remolding the cellular geometry. Either inhibition of Rac1 or ectopic expression of RhoA could region-distinctively change the columnar shape of IDE cells in gerbils to drive invagination to produce cusps. Conversely, RhoA reduction in mice inhibited invagination and developed lophs. Furthermore, we found that Rac1 and RhoA modulate the choices of cuspal shape by coordinating adhesion junctions, actin distribution, and fibronectin localization to drive IDE invagination.

ATX-LPA1 axis contributes to proliferation of chondrocytes by regulating fibronectin assembly leading to proper cartilage formation

The lipid mediator lysophosphatidic acid (LPA) signals via six distinct G protein-coupled receptors to mediate both unique and overlapping biological effects, including cell migration, proliferation and survival. LPA is produced extracellularly by autotaxin (ATX), a secreted lysophospholipase D, from lysophosphatidylcholine. ATX-LPA receptor signaling is essential for normal development and implicated in various (patho)physiological processes, but underlying mechanisms remain incompletely understood. Through gene targeting approaches in zebrafish and mice, we show here that loss of ATX-LPA₁ signaling leads to disorganization of chondrocytes, causing severe defects in cartilage formation. Mechanistically, ATX-LPA₁ signaling acts by promoting S-phase entry and cell proliferation of chondrocytes both in vitro and in vivo, at least in part through β1-integrin translocation leading to fibronectin assembly and further extracellular matrix deposition; this in turn promotes chondrocyte-matrix adhesion and cell proliferation. Thus, the ATX-LPA₁ axis is a key regulator of cartilage formation.

Rho GTPases as pathogen targets: Focus on curable sexually transmitted infections

Pathogens have evolved highly specialized mechanisms to infect hosts. Several microorganisms modulate the eukaryotic cell surface to facilitate their engulfment. Once internalized, they hijack the molecular machinery of the infected cell for their own benefit. At different stages of phagocytosis, particularly during invasion, certain pathogens manipulate pathways governed by small GTPases. In this review, we focus on the role of Rho proteins on curable, sexually transmitted infections caused by Chlamydia trachomatis, Neisseria gonorrhoeae, Trichomonas vaginalis and Treponema pallidum. Despite the high, worldwide frequencies of these sexually-transmitted diseases, very little is known about the strategies developed by these microorganisms to usurp key eukaryotic proteins that control intracellular signaling and actin dynamics. Improved knowledge of these molecular mechanisms will contribute to the elucidation of how these clinically important pathogens manipulate intracellular processes and parasitize their hosts.

The polyphenols (-)-epigallocatechin-3-gallate and luteolin synergistically inhibit TGF-β-induced myofibroblast phenotypes through RhoA and ERK inhibition

The presence of reactive stroma, predominantly composed of myofibroblasts, is directly associated with and drives prostate cancer progression. We have previously shown that (−)-Epigallocatechin-3-gallate (EGCG), in the form of Polyphenon E, significantly decreases serum levels of HGF and VEGF in prostate cancer patients. Given that HGF and VEGF are secreted from surrounding tumor myofibroblasts, these observations suggested that EGCG may inhibit prostate cancer-associated myofibroblast differentiation. Herein, we demonstrate that micromolar combinations of EGCG and a second polyphenol, luteolin, synergistically inhibit TGF-β-induced myofibroblast phenotypes in prostate fibroblast cell lines, as observed primarily by potentiation of fibronectin expression. Functionally, EGCG and luteolin inhibited TGF-β-induced extracellular matrix contraction, an enhancer of tumor cell invasion. EGCG and luteolin inhibited downstream TGF-β-induced signaling, including activation of ERK and AKT, respectively, but mechanistically, only ERK appeared to be necessary for TGF-β-induced fibronectin expression. Furthermore, neither EGCG nor luteolin affected Smad signaling or nuclear translocation. Rho signaling was found to be necessary for TGF-β-induced fibronectin expression and EGCG and luteolin each reduced RhoA activation. Finally, EGCG and luteolin were shown to reverse TGF-β-induced fibronectin expression, implicating that these natural compounds may be useful not only in preventing but also in treating already activated myofibroblasts and the diseases they cause, including cancer. The ability of EGCG and luteolin to synergistically target myofibroblasts suggests that combined clinical use of these compounds could prevent or reverse cancer progression through targeting the tumor microenvironment, in addition to the tumor itself.

Dissecting focal adhesions in cells differentially expressing calreticulin: a microscopy study

BACKGROUND INFORMATION

Our previous studies have shown that calreticulin, a Ca2+-binding chaperone located in the endoplasmic reticulum, affects cell-substratum adhesions via the induction of vinculin and N-cadherin. Cells overexpressing calreticulin contain more vinculin than low expressers and make abundant contacts with the substratum. However, cells that express low levels of calreticulin exhibit a weak adhesive phenotype and make few, if any, focal adhesions. To date, the identity of the types of focal adhesions made by calreticulin overexpressing and low expressing cells has not been dissected.

RESULTS

The results of the present study show that calreticulin affects fibronectin matrix assembly in L fibroblast cell lines that differentially express the protein, and that these cells also differ profoundly in focal adhesion formation. Although the calreticulin overexpressing cells generate numerous interference-reflection-microscopy-dark, vinculin- and paxillin-containing classical focal contacts, as well as some fibrillar adhesions, the cells expressing low levels of calreticulin generate only a few weak focal adhesions. The fibronectin receptor was found to be clustered in calreticulin overexpressing cells, but diffusely distributed over the cell surface in low expressing cells. Plating L fibroblasts on fibronectin-coated substrata induced extensive spreading in all cell lines tested. However, although calreticulin overexpressing cells were induced to form classical vinculin-rich focal contacts, the low calreticulin expressing cells overcame their weak adhesive phenotype by induction of many tensin-rich fibrillar adhesions, thus compensating for the low level of vinculin in these cells.

CONCLUSIONS

We propose that calreticulin affects fibronectin production and, thereby, assembly, and it indirectly influences the formation and/or stability of focal contacts and fibrillar adhesions, both of which are instrumental in matrix assembly and remodelling.

The promotion of chondrogenesis in adipose-derived adult stem cells by an RGD-chimeric protein in 3D alginate culture

The dynamic regulation of integrin-binding peptides is crucial for chondrogenic differentiation. Here, we revealed the feasibility for flexible modification of RGD by embedding a large molecular weight and slightly charged (isoelectric point, 6-6.25) RGD-chimeric protein (CBD-RGD) with cellulose-binding domain (CBD) in three dimensional (3D) alginate beads to evaluate the chondrogenesis of adipose-derived adult stem cells (ADAS). The binding of CBD-RGD with cells and its diffusion from alginate beads were studied on fluorescein isothiocyanate (FITC)-conjugated CBD-RGD. The increases in gene expression (Sox9, Aggrecan, fibronectin and collagen II), accumulation of chondrogenic matrices and decrease of collagen X gene expression during TGF-beta3 induction were only observed for those beads containing 10mg/g CBD-RGD initially, with 20.18+/-0.73% of that released in a week. The contrary was observed for beads with CBD-RGD 20mg/g initially and having higher persistence (only 8.6+/-2.17% released in a week). The 10mg/g CBD-RGD-mediated enhancement was demonstrated via the activation of integrin alpha5 and beta1-dependent pathway, and especially related to the upregulation of Sox9 gene and the temporary block of fibronectin expression as well as sustained inhibition of RhoA activity in the early differentiation stage. Thus, we speculated that the dynamic mobility of CBD-RGD may account for the enhanced chondrogenesis. It was concluded that the CBD-RGD-alginate culture system promoted the chondrogenesis of mesenchymal stem cells coordinated with TGF-beta3 induction in an RGD dose-dependent manner.

RhoA/Rho kinase signaling in the cumulus mediates extracellular matrix assembly

Cumulus cells surround the oocyte and regulate the production and assembly of the extracellular matrix (ECM) around the cumulus-oocyte complex for its timely interaction with sperm in the oviduct. We recently found that C-C chemokines such as CCL2, CCL7, and CCL9 are produced and stimulate integrin-mediated ECM assembly in the postovulatory cumulus to protect eggs and that prostaglandin E(2)-EP2 signaling in the cumulus cells facilitates fertilization by suppressing this chemokine signaling, which otherwise results in fertilization failure by preventing sperm penetration through the cumulus ECM. However, it remains unknown as to what mechanisms underlie chemokine-induced cumulus ECM assembly. Here we report that inhibition of EP2 signaling or addition of CCL7 augments RhoA activation and induces the surface accumulation of integrin and the contraction of cumulus cells. Enhanced surface accumulation of integrin then stimulates the formation and assembly of fibronectin fibrils as well as induces cumulus ECM resistance to hyaluronidase and sperm penetration. These changes in the cumulus ECM as well as cell contraction are relieved by the addition of Y27632 or blebbistatin. These results suggest that chemokines induce integrin engagement to the ECM and consequent ECM remodeling through the RhoA/Rho kinase/actomyosin pathway, making the cumulus ECM barrier resistant to sperm penetration. Based on these results, we propose that prostaglandin E(2)-EP2 signaling negatively regulates chemokine-induced Rho/ROCK signaling in cumulus cells for successful fertilization.

Mislocalized scaffolding by the Na-H exchanger NHE1 dominantly inhibits fibronectin production and TGF-beta activation

Secretion and assembly of the extracellular matrix protein fibronectin regulates a number of normal cell and tissue functions and is dysregulated in disease states such as fibrosis, diabetes, and cancer. We found that mislocalized scaffolding by the plasma membrane Na-H exchanger NHE1 suppresses fibronectin expression, secretion, and assembly. In fibroblasts, wild-type NHE1 localizes to the distal margin of membrane protrusions or lamellipodia but a mutant NHE1-KRA2 lacking binding sites for PI(4,5)P2 and the ERM proteins ezrin, radixin, and moesin is mislocalized and found uniformly along the plasma membrane. Although NHE1 regulates intracellular pH homeostasis, fibronectin production is not regulated by changes in intracellular pH, nor is it attenuated in NHE1-deficient cells, indicating fibronectin expression is independent of NHE1 activity. However, fibronectin production is nearly absent in cells expressing NHE1-KRA2 because scaffolding by NHE1 is mislocalized. Additionally, secretion of active but not latent TGF-beta is reduced and exogenous TGF-beta restores fibronectin secretion and assembly. Our data indicate that scaffolding by NHE1-KRA2 dominantly suppresses fibronectin synthesis and TGF-beta activation, and they suggest that NHE1-KRA2 can be used for obtaining a mechanistic understanding of how fibronectin production is regulated and speculatively for therapeutic control of dysregulated production in pathological conditions.

Intestinal epithelial CD98: an oligomeric and multifunctional protein

The intestinal epithelial cell-surface molecule, CD98 is a type II membrane glycoprotein. Molecular orientation studies have demonstrated that the C-terminal tail of human CD98 (hCD98), which contains a PDZ-binding domain, is extracellular. In intestinal epithelial cells, CD98 is covalently linked to an amino-acid transporter with which it forms a heterodimer. This heterodimer associates with beta(1)-integrin and intercellular adhesion molecular 1 (ICAM-1) to form a macromolecular complex in the basolateral membranes of polarized intestinal epithelial cells. This review focuses on the multifunctional roles of CD98, including involvement in extracellular signaling, adhesion/polarity, and amino-acid transporter expression in intestinal epithelia. A role for CD98 in intestinal inflammation, such as Intestinal Bowel Disease (IBD), is also proposed.

Inadequate activation of the GTPase RhoA contributes to the lack of fibronectin matrix assembly in von Hippel-Lindau protein-defective renal cancer cells

The von Hippel-Lindau (VHL) tumor suppressor gene regulates extracellular matrix deposition. In VHL negative renal cancer cells, VHL(-), the lack of fibronectin matrix assembly is thought to promote and maintain tumor angiogenesis allowing vessels to infiltrate tumors. Therefore, and considering the importance of this process in tumor growth, we aimed to study why VHL(-) renal cancer cells fail to form a proper extracellular matrix. Our results showed that VHL(-) cells were not defective in fibronectin production and that the fibronectin produced by these cells was equally functional in promoting cell adhesion and matrix assembly as that produced by VHL+ cells. We have previously reported that VHL(-) cells fail to form beta1 integrin fibrillar adhesions and have a diminished organization of actin stress fibers; therefore, we aimed to study if the small GTPase family is involved in this process. We found that activation of the RhoA GTPase was defective in VHL(-) cells, and this was possibly mediated by an increased activation of its inhibitor, p190RhoGAP. Additionally, the expression of constitutively active RhoA in VHL(-) cells resulted in formation of a fibronectin matrix. These results strongly suggest an important role for RhoA in some of the defects observed in renal cancer cells.

Regulation of RhoA-dependent ROCKII activation by Shp2

Contractile forces mediated by RhoA and Rho kinase (ROCK) are required for a variety of cellular processes, including cell adhesion. In this study, we show that RhoA-dependent ROCKII activation is negatively regulated by phosphorylation at a conserved tyrosine residue (Y722) in the coiled-coil domain of ROCKII. Tyrosine phosphorylation of ROCKII is increased with cell adhesion, and loss of Y722 phosphorylation delays adhesion and spreading on fibronectin, suggesting that this modification is critical for restricting ROCKII-mediated contractility during these processes. Further, we provide evidence that Shp2 mediates dephosphorylation of ROCKII and, therefore, regulates RhoA-induced cell rounding, indicating that Shp2 couples with RhoA signaling to control ROCKII activation during deadhesion. Thus, reversible tyrosine phosphorylation confers an additional layer of control to fine-tune RhoA-dependent activation of ROCKII.

TGFbeta-induced RhoA activation and fibronectin production in mesangial cells require caveolae

Glomerular sclerosis of diverse etiologies is characterized by mesangial matrix accumulation, with transforming growth factor-beta (TGFbeta) an important pathogenic factor. The GTPase RhoA mediates TGFbeta-induced matrix accumulation in some settings. Here we study the role of the membrane microdomain caveolae in TGFbeta-induced RhoA activation and fibronectin upregulation in mesangial cells (MC). In primary rat MC, TGFbeta1 time dependently increased RhoA and downstream Rho kinase activation. Rho pathway inhibition blocked TGFbeta1-induced upregulation of fibronectin transcript and protein. TGFbeta1-induced RhoA activation was prevented by disrupting caveolae with cholesterol depletion and rescued by cholesterol repletion. Compared with wild types, RhoA/Rho kinase activation was absent in MC lacking caveolae. Reexpression of caveolin-1 (and caveolae) restored these responses. Phosphorylation of caveolin-1 on Y14, effected by Src kinases, has been implicated in signaling responses. Overexpression of nonphosphorylatable caveolin-1 Y14A prevented TGFbeta1-induced RhoA activation. TGFbeta1 also activated Src, and its inhibition blocked RhoA activation. Furthermore, TGFbeta1 led to association of RhoA and caveolin-1. This was prevented by Src or TGFbeta receptor I inhibition, and by caveolin-1 Y14A overexpression. Last, fibronectin upregulation by TGFbeta1 was blocked by Src inhibition, not seen in caveolin-1 knockout MC, and restored by caveolin-1 reexpression in the latter. TGFbeta1-induced collagen I accumulation also required caveolae. TGFbeta1-mediated Smad2/3 activation, however, did not require caveolae. We conclude that RhoA/Rho kinase mediates TGFbeta-induced fibronectin upregulation. This requires caveolae and caveolin-1 interaction with RhoA. Interference with caveolin/caveolae or RhoA signaling thus represents a potential target for the treatment of fibrotic renal disease.

Analysis of the suitability of calreticulin inducible HEK cells for adhesion studies: microscopical and biochemical comparisons

Calreticulin is a Ca(2+)-buffering ER chaperone that also modulates cell adhesiveness. In order to study the effect of calreticulin on the expression of adhesion-related genes, we created a calreticulin inducible Human Embryonic Kidney (HEK) 293 cell line. We found that fibronectin mRNA and both intra- and extra-cellular fibronectin protein levels increased following calreticulin induction. However, despite this increase in fibronectin, HEK293 cells did not assemble an extracellular fibrillar fibronectin matrix regardless of the level of calreticulin expression. Furthermore, HEK293 cells exhibited a poorly organized actin cytoskeleton, did not have clustered fibronectin receptors at the cell surface, and did not form focal contacts. This likely accounts for the lack of fibronectin matrix deposition by these cells regardless of calreticulin expression level. Vinculin abundance did not appreciably increase upon calreticulin induction and the level of active c-Src, a regulatory kinase of focal contacts, was found to be abundant and unregulated by calreticulin induction in these cells. The inability to form stable focal contacts and to commence fibronectin fibrillogenesis due to high c-Src activity may be responsible for the poor adhesive phenotype of HEK 293 cells. Thus, we show here that HEK293 cells are not suitable for microscopical studies of cell-substratum adhesions, but are best suited for biochemical studies.

The role of proteases in fibronectin matrix remodeling in thyroid epithelial cell monolayer cultures

Fischer rat thyroid (FRT) cells organize a matrix of extracellular fibronectin (FN) fibrils, which undergoes extensive remodeling according to cell culture confluence. In non-confluent cells FN forms a fibrillar array associated with the ventral cell surface. However, basal FN is progressively removed in confluent cultures and substituted by non-fibrillar FN deposits at lateral cell domains in regions of cell-cell contacts. FRT cells secrete and expose on the plasma membrane the tissue-type plasminogen activator and, in serum-free cultures, plasminogen induces a rapid loss of FN fibrils. Incubation with plasmin inhibitors greatly reduces this effect. FRT cells also express annexin II, a plasminogen receptor, suggesting that plasmin activity is associated with the pericellular enviroment. This is in agreement with the observation that a great reduction in FN degradation is observed if the cells are pre-incubated with carboxypeptidase B, which prevents plasminogen binding to the cells. A gelatinolytic activity with a molecular weigth equivalent to MMP-2 has been demonstrated by zymography of culture media, and the presence of MMP-2 and MT1-MMP on the cell plasma membrane has been detected by immunofluorescence. These results indicate that in the FN remodeling process, occurring during FRT epithelium maturation, both plasmin-dependent (tPA activated) and plasmin-independent proteolytic activities are involved.

CD98-mediated links between amino acid transport and beta 1 integrin distribution in polarized columnar epithelia

In non-polarized cells, CD98 has been shown to both influence beta(1) integrins and heterodimerize with LAT-2, which confers amino acid transport capability on the LAT-2/CD98 heterodimer. Since LAT-2 is most heavily expressed in intestine and CD98 associates with the beta(1) integrin splice form selectively found in such epithelia, we investigated the relationship and polarity of these proteins using the intestinal epithelial model Caco2-BBE. CD98 was found to selectively coimmunoprecipitate with both LAT-2 and beta(1) integrin, and, logically, all three proteins were polarized to the same (basolateral) domain. Furthermore, expression of CD98 in polarized epithelia lacking human CD98 (MDCK cells) disrupted beta(1) integrin surface distribution and cytoskeletal architecture, suggesting that CD98 can influence integrin function. Expression of a CD98 mutant lacking the specific residues conferring LAT-2 binding similarly affected cells, confirming that the latter effect was not due to LAT-2 sequestration. Use of CD98 truncation mutants suggest that a 10-amino acid domain located at the putative cytoplasmic tail/transmembrane domain interface was necessary and sufficient to induce the phenotype change. We conclude that the CD98/LAT-2 amino acid transporter is polarized to the same domain on which beta(1) integrin resides. CD98 appears to associate with beta(1) integrin and, in doing so, may influence its function as revealed by disruption of the outside-in signaling that confers cytoskeletal organization. Furthermore, such findings suggest a link between classic transport events and a critical element of barrier function: integrin-mediated influences on cytoskeletal organization.

Loss of alpha3beta1 integrin function results in an altered differentiation program in the mouse submandibular gland

Mammalian submandibular gland (SMG) development leads to the establishment of highly organized secretory acinar and nonsecretory ductal epithelial cells. The ability of maturing salivary epithelial cells to attain their differentiated state has been shown to depend, in part, on interactions between extracellular matrix (ECM) proteins and their integrin receptors. In a search for key regulators of salivary cell lineage, we have studied alpha3beta1 integrin, a receptor for the basement membrane protein laminin, by characterizing embryonic day 18 (E18) SMGs isolated from mice carrying a targeted mutation in the alpha3 integrin gene. Transmission electron microscopy studies showed that the mutant SMGs exhibited an aberrant differentiation phenotype with defects in the apical-basal polarity axis and in the basement membrane. Based on immunohistochemistry and Western blot analyses, the alpha3beta1-deficient SMGs had altered expression and/or localization of several ECM and adhesive molecules, including laminin beta1, fibronectin, alpha5 integrin, and E-cadherin. These changes correlated with alterations in the activation state of Ras-extracellular signal-regulated kinase (ERK), as well as the expression and/or localization of Cdc42 and RhoA, two Rho GTPases that regulate the organization of the actin cytoskeleton. We conclude that alpha3beta1 is required for normal salivary cell differentiation and that its absence affects multiple components of adhesive complexes and their associated signalling pathways.