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.

Osmotic stress is known to affect the cytoskeleton; however, this adaptive response has remained poorly characterized, and the underlying signaling pathways are unexplored. Here we show that hypertonicity induces submembranous de novo F-actin assembly concomitant with the peripheral translocation and colocalization of cortactin and the actin-related protein 2/3 (Arp2/3) complex, which are key components of the actin nucleation machinery. Additionally, hyperosmolarity promotes the association of cortactin with Arp2/3 as revealed by coimmunoprecipitation. Using various truncation or phosphorylation-incompetent mutants, we show that cortactin translocation requires the Arp2/3- or the F-actin binding domain, but the process is independent of the shrinkage-induced tyrosine phosphorylation of cortactin. Looking for an alternative signaling mechanism, we found that hypertonicity stimulates Rac and Cdc42. This appears to be a key event in the osmotically triggered cytoskeletal reorganization, because 1) constitutively active small GTPases translocate cortactin, 2) Rac and cortactin colocalize at the periphery of hypertonically challenged cells, and 3) dominant-negative Rac and Cdc42 inhibit the hypertonicity-provoked cortactin and Arp3 translocation. The Rho family-dependent cytoskeleton remodeling may be an important osmoprotective response that reinforces the cell cortex.

Cell volume affects diverse functions including cytoskeletal organization, but the underlying signaling pathways remained undefined. We have shown previously that shrinkage induces Fyn-dependent tyrosine phosphorylation of the cortical actin-binding protein, cortactin. Because FER kinase was implicated in the direct phosphorylation of cortactin, we investigated the osmotic responsiveness of FER and its relationship to Fyn and cortactin. Shrinkage increased FER activity and tyrosine phosphorylation. These effects were abolished by the Src family inhibitor PP2 and strongly mitigated in Fyn-deficient but not in Src-deficient cells. FER overexpression caused cortactin phosphorylation that was further enhanced by hypertonicity. Exchange of tyrosine residues 421, 466, and 482 for phenylalanine prevented cortactin phosphorylation by hypertonicity and strongly decreased it upon FER overexpression, suggesting that FER targets primarily the same osmo-sensitive tyrosines. Because constituents of the cell-cell contacts are substrates of Fyn and FER, we investigated the effect of shrinkage on the adherens junctions. Hypertonicity provoked Fyn-dependent tyrosine phosphorylation in ␤-catenin, ␣-catenin, and p120Cas and caused the dissociation of ␤-catenin from the contacts. This process was delayed in Fyn-deficient or PP2-treated cells. Thus, FER is a volume-sensitive kinase downstream from Fyn, and the Fyn/FER pathway may contribute to the cell size-dependent reorganization of the cytoskeleton and the cell-cell contacts.Changes in cell volume or cell shape are known to affect a variety of functions including the activity of ion transporters, the organization of the cytoskeleton, and the transcription of certain genes (1-3). Although these responses are important for the maintenance of cellular homeostasis and integrity, little is known about the volume-dependent signaling pathways that convert the initial alteration to the compensatory or regulatory effector functions.We and others have noted previously that one of the earliest cellular responses to hyperosmotic stress is a dramatic increase in protein tyrosine phosphorylation (4, 5). This phosphotyrosine accumulation is triggered by cell shrinkage and not by hyperosmolarity per se, and in fibroblastic cell lines it occurs predominantly in bands of 80 -90 and 110 -130 kDa. Our recent studies aimed at the identification of targets of the hypertonic phosphorylation and the respective tyrosine kinases showed that one of the major substrates for osmotic phosphorylation is cortactin (6), an 80 -85-kDa cortical actin filament cross-linking protein, and preferred substrate for Src family kinases (7-10). Cortactin has a unique structure composed of six and a half tandem repeats responsible for actin binding, followed by a serine-threonine-rich domain, a tyrosine-rich sequence, and a C-terminal SH3 domain (8, 11). These features, together with the fact that upon cell stimulation cortactin localizes to peripheral membrane structures such as ruffles and lamellipodia (8,12), suggest that...

Hypertonicity inhibits a variety of neutrophil functions through poorly defined mechanisms. Our earlier studies suggest that osmotically induced actin polymerization and cytoskeleton remodeling is a key component in the hypertonic block of exocytosis and cell movement. To gain insight into the signaling mechanisms underlying the hyperosmotic F-actin response, we investigated whether hypertonicity stimulates Rac and Cdc42 and, if so, whether their activation contributes to the hypertonic rise in F-actin. Using a recently developed pull-down assay that specifically captures the active forms of these small GTPases, we found that hypertonicity caused an ~2.5- and ~7.2-fold activation of Rac and Cdc42, respectively. This response was rapid and sustained. Small GTPase activation was not mediated by the osmotic stimulation of Src kinases, heterotrimeric G proteins, or phosphatidylinositol 3-kinase. Interestingly, an increase in intracellular ionic strength was sufficient to activate Rac even in the absence of cell shrinkage. Inhibition of Rac and Cdc42 by Clostridium difficile toxin B substantially reduced but did not abolish the hypertonicity-induced F-actin response. Thus hypertonicity is a potent activator of Rac and Cdc42, and this effect seems to play an important but not exclusive role in the hyperosmolarity-triggered cytoskeleton remodeling.