New research suggests that, during tubulointerstitial fibrosis, α-smooth muscle actin (SMA)-expressing mesenchymal cells might derive from the tubular epithelium via epithelial-mesenchymal transition (EMT). Although transforming growth factor-β1(TGF-β1) plays a key role in EMT, the underlying cellular mechanisms are not well understood. Here we characterized TGF-β1-induced EMT in LLC-PK1 cells and examined the role of the small GTPase Rho and its effector, Rho kinase, (ROK) in the ensuing cytoskeletal remodeling and SMA expression. TGF-β1 treatment caused delocalization and downregulation of cell contact proteins (ZO-1, E-cadherin, β-catenin), cytoskeleton reorganization (stress fiber assembly, myosin light chain phosphorylation), and robust SMA synthesis. TGF-β1induced a biphasic Rho activation. Stress fiber assembly was prevented by the Rho-inhibiting C3 transferase and by dominant negative (DN) ROK. The SMA promoter was activated strongly by constitutively active Rho but not ROK. Accordingly, TGF-β1-induced SMA promoter activation was potently abrogated by two Rho-inhibiting constructs, C3 transferase and p190RhoGAP, but not by DN-ROK. Truncation analysis showed that the first CC(A/T)richGG (CArG B) serum response factor-binding cis element is essential for the Rho responsiveness of the SMA promoter. Thus Rho plays a dual role in TGF-β1-induced EMT of renal epithelial cells. It is indispensable both for cytoskeleton remodeling and for the activation of the SMA promoter. The cytoskeletal effects are mediated via the Rho/ROK pathway, whereas the transcriptional effects are partially ROK independent.

It is becoming clear that interconnected functional gene networks, rather than individual genes, govern stem cell self-renewal and differentiation. To identify epigenetic factors that impact on human epidermal stem cells we performed siRNA-based genetic screens for 332 chromatin modifiers. We developed a Bayesian mixture model to predict putative functional interactions between epigenetic modifiers that regulate differentiation. We discovered a network of genetic interactions involving EZH2, UHRF1 (both known to regulate epidermal self-renewal), ING5 (a MORF complex component), BPTF and SMARCA5 (NURF complex components). Genome-wide localization and global mRNA expression analysis revealed that these factors impact two distinct but functionally related gene sets, including integrin extracellular matrix receptors that mediate anchorage of epidermal stem cells to their niche. Using a competitive epidermal reconstitution assay we confirmed that ING5, BPTF, SMARCA5, EZH2 and UHRF1 control differentiation under physiological conditions. Thus, regulation of distinct gene expression programs through the interplay between diverse epigenetic strategies protects epidermal stem cells from differentiation.

Hyperosmotic stress initiates adaptive responses, including phosphorylation of myosin light chain (MLC) and concomitant activation of Na ϩ -K ϩ -Cl Ϫ cotransporter (NKCC). Because the small GTPase Rho is a key regulator of MLC phosphorylation, we investigated 1) whether Rho is activated by hyperosmotic stress, and if so, what the triggering factors are, and 2) whether the Rho/Rho kinase (ROK) pathway is involved in MLC phosphorylation and NKCC activation. Rho activity was measured in tubular epithelial cells by affinity pulldown assay. Hyperosmolarity induced rapid (Ͻ1 min) and sustained (Ͼ20 min) Rho activation that was proportional to the osmotic concentration and reversed within minutes upon restoration of isotonicity. Both decreased cell volume at constant ionic strength and elevated total ionic strength at constant cell volume were capable of activating Rho. Changes in [Na ϩ ] and [K ϩ ] at normal total salinity failed to activate Rho, and Cl Ϫ depletion did not affect the hyperosmotic response. Thus alterations in cellular volume and ionic strength but not individual ion concentrations seem to be the critical triggering factors. Hyperosmolarity induced mono-and diphosphorylation of MLC, which was abrogated by the Rho-family blocker Clostridium toxin B. ROK inhibitor Y-27632 suppressed MLC phosphorylation under isotonic conditions and prevented its rise over isotonic levels in hypertonically stimulated cells. ML-7 had a smaller inhibitory effect. In contrast, it abolished the hypertonic activation of NKCC, whereas Y-27632 failed to inhibit this response. Thus hyperosmolarity activates Rho, and Rho/ROK pathway contributes to basal and hyperosmotic MLC phosphorylation. However, the hypertonic activation of NKCC is ROK independent, implying that the ROK-dependent component of MLC phosphorylation can be uncoupled from NKCC activation. cell volume; ionic strength; small GTPases; Y-27632; ML-7; myosin light chain; NaOSMOTIC STRESS induces a variety of compensatory responses that can be classified into three major categories: activation of ion transport systems; changes in the transcription of osmosensitive genes and, consequently, in the expression of osmolyte-generating enzymes or solute carriers; and reorganization of the cytoskeleton (for reviews, see Refs. 28,35,38,and 53). Whereas considerable knowledge has accumulated about these vital adaptive responses, comparatively little is known about the signaling processes that link them to the initial osmotic insult. Regarding the cytoskeletal effects, our previous studies (8, 29) have shown that Rac and Cdc42, two members of the Rho family small GTPases, are stimulated by hyperosmotic stress and that their activation contributes to the osmotically provoked cytoskeleton remodeling. Specifically, these small GTPases appear to play a role in the reinforcement of the cortical cytoskeleton by facilitating peripheral de novo F-actin assembly (8, 29) and by inducing translocation of cortactin (8), a protein that potentiates actin polymerization and stabilizes newly gener...

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Motility of normal and transformed cells within and across tissues requires specialized subcellular structures, e.g. membrane ruffles, lamellipodia and podosomes, which are generated by dynamic rearrangements of the actin cytoskeleton. Because the formation of these sub-cellular structures is complex and relatively poorly understood, we evaluated the role of the adapter protein SH3PXD2B [HOFI, fad49, Tks4], which plays a role in the development of the eye, skeleton and adipose tissue. Surprisingly, we find that SH3PXD2B is requisite for the development of EGF-induced membrane ruffles and lamellipodia, as well as for efficient cellular attachment and spreading of HeLa cells. Furthermore, SH3PXD2B is present in a complex with the non-receptor protein tyrosine kinase Src, phosphorylated by Src, which is consistent with SH3PXD2B accumulating in Src-induced podosomes. Furthermore, SH3PXD2B closely follows the subcellular relocalization of cortactin to Src-induced podosomes, EGF-induced membrane ruffles and lamellipodia. Because SH3PXD2B also forms a complex with the C-terminal region of cortactin, we propose that SH3PXD2B is a scaffold protein that plays a key role in regulating the actin cytoskeleton via Src and cortactin.

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Myosin-based contractility plays important roles in the regulation of epithelial functions, particularly paracellular permeability. However, the triggering factors and the signaling pathways that control epithelial myosin light chain (MLC) phosphorylation have not been elucidated. Herein we show that plasma membrane depolarization provoked by distinct means, including high extracellular K(+), the lipophilic cation tetraphenylphosphonium, or the ionophore nystatin, induced strong diphosphorylation of MLC in kidney epithelial cells. In sharp contrast to smooth muscle, depolarization of epithelial cells did not provoke a Ca(2+) signal, and removal of external Ca(2+) promoted rather than inhibited MLC phosphorylation. Moreover, elevation of intracellular Ca(2+) did not induce significant MLC phosphorylation, and the myosin light chain kinase (MLCK) inhibitor ML-7 did not prevent the depolarization-induced MLC response, suggesting that MLCK is not a regulated element in this process. Instead, the Rho-Rho kinase (ROK) pathway is the key mediator because 1) depolarization stimulated Rho and induced its peripheral translocation, 2) inhibition of Rho by Clostridium difficile toxin B or C3 transferase abolished MLC phosphorylation, and 3) the ROK inhibitor Y-27632 suppressed the effect. Importantly, physiological depolarizing stimuli were able to activate the same pathway: L-alanine, the substrate of the electrogenic Na(+)-alanine cotransporter, stimulated Rho and induced Y-27632-sensitive MLC phosphorylation in a Na(+)-dependent manner. Together, our results define a novel mode of the regulation of MLC phosphorylation in epithelial cells, which is depolarization triggered and Rho-ROK-mediated but Ca(2+) signal independent. This pathway may be a central mechanism whereby electrogenic transmembrane transport processes control myosin phosphorylation and thereby regulate paracellular transport.

Sec14 protein was first identified in Saccharomyces cerevisiae, where it serves as a phosphatidylinositol transfer protein that is essential for the transport of secretory proteins from the Golgi complex. A protein domain homologous to Sec14 was identified in several mammalian proteins that regulates Rho GTPases, including exchange factors and GTPase activating proteins. P50RhoGAP, the first identified GTPase activating protein for Rho GTPases, is composed of a Sec14-like domain and a Rho-GTPase activating protein (GAP) domain. The biological function of its Sec14-like domain is still unknown. Here we show that p50RhoGAP is present on endosomal membranes, where it colocalizes with internalized transferrin receptor. We demonstrate that the Sec14-like domain of P50RhoGAP is responsible for the endosomal targeting of the protein. We also show that overexpression of p50RhoGAP or its Sec14-like domain inhibits transferrin uptake. Furthermore, both P50RhoGAP and its Sec14-like domain show colocalization with small GTPases Rab11 and Rab5. We measured bioluminescence resonance energy transfer between p50RhoGAP and Rab11, indicating that these proteins form molecular complex in vivo on endosomal membranes. The interaction was mediated by the Sec 14-like domain of p50RhoGAP. Our results indicate that Sec14-like domain, which was previously considered as a phospholipid binding module, may have a role in the mediation of protein-protein interactions. We suggest that p50RhoGAP provides a link between Rab and Rho GTPases in the regulation of receptor-mediated endocytosis.Endocytosis has a crucial role in the delivery of extracellular molecules to various intracellular compartments. The regulation of endocytosis is still poorly understood. Several routes and mechanisms of endocytosis were identified in eukaryotic cells (1). Among them, receptormediated endocytosis involves the internalization of receptors and their ligands by clathrin-coated pits. This process is essential for the efficient uptake of nutrients such as iron or lipids, and it is also involved in cellular signaling. Internalized receptors first enter the early endosome, from where they can either return to the plasma membrane or enter the endocytic recycling compartment (ERC).2 Through endocytic recycling, receptors are re-utilized several times, and recycling is also essential for maintaining the proper lipid composition of the plasma membrane. ERC is a complex network of vesicles and tubules that is often localized around the microtubule-organizing center (2, 3). Sorting in the ERC is a complex process where most molecules return to the plasma membrane, although transport also occurs to the trans-Golgi network.Small GTPases, including members of the Rab and Rho families, are involved in the organization of receptor-mediated endocytosis. Rab GTPases regulate the budding, trafficking, and fusion of endocytic vesicles at the different compartments (4, 5). Rab5, one of the best characterized Rab GTPases, has a role in the formation of sorting endosomes. Rab5 togeth...