Post-transcriptional down-regulation of ROCKI/Rho-kinase through an MEK-dependent pathway leads to cytoskeleton disruption in Ras-transformed fibroblasts

Multi-OMICs landscape of SARS-CoV-2-induced host responses in human lung epithelial cells

COVID-19 pandemic continues to remain a global health concern owing to the emergence of newer variants. Several multi-Omics studies have produced extensive evidence on host-pathogen interactions and potential therapeutic targets. Nonetheless, an increased understanding of host signaling networks regulated by post-translational modifications and their ensuing effect on the cellular dynamics is critical to expanding the current knowledge on SARS-CoV-2 infections. Through an unbiased transcriptomics, proteomics, acetylomics, phosphoproteomics, and exometabolome analysis of a lung-derived human cell line, we show that SARS-CoV-2 Norway/Trondheim-S15 strain induces time-dependent alterations in the induction of type I IFN response, activation of DNA damage response, dysregulated Hippo signaling, among others. We identified interplay of phosphorylation and acetylation dynamics on host proteins and its effect on the altered release of metabolites, especially organic acids and ketone bodies. Together, our findings serve as a resource of potential targets that can aid in designing novel host-directed therapeutic strategies.

Regulation of smooth muscle contraction by monomeric non-RhoA GTPases

Smooth muscle contraction in the cardiovascular system, airways, prostate and lower urinary tract is involved in the pathophysiology of many diseases, including cardiovascular and obstructive lung disease plus lower urinary tract symptoms, which are associated with high prevalence of morbidity and mortality. This prominent clinical role of smooth muscle tone has led to the molecular mechanisms involved being subjected to extensive research. In general smooth muscle contraction is promoted by three major signalling pathways, including the monomeric GTPase RhoA pathway. However, emerging evidence suggests that monomeric GTPases other than RhoA may be involved in signal transduction in smooth muscle contraction, including Rac GTPases, cell division control protein 42 homologue, adenosine ribosylation factor 6, Ras, Rap1b and Rab GTPases. Here, we review these emerging functions of non-RhoA GTPases in smooth muscle contraction, which has now become increasingly more evident and constitutes an emerging and innovative research area of high clinical relevance.

Oncogenic Signaling Alters Cell Shape and Mechanics to Facilitate Cell Division under Confinement

To divide in a tissue, both normal and cancer cells become spherical and mechanically stiffen as they enter mitosis. We investigated the effect of oncogene activation on this process in normal epithelial cells. We found that short-term induction of oncogenic RasV12 activates downstream mitogen-activated protein kinase (MEK-ERK) signaling to alter cell mechanics and enhance mitotic rounding, so that RasV12-expressing cells are softer in interphase but stiffen more upon entry into mitosis. These RasV12-dependent changes allow cells to round up and divide faithfully when confined underneath a stiff hydrogel, conditions in which normal cells and cells with reduced levels of Ras-ERK signaling suffer multiple spindle assembly and chromosome segregation errors. Thus, by promoting cell rounding and stiffening in mitosis, oncogenic RasV12 enables cells to proliferate under conditions of mechanical confinement like those experienced by cells in crowded tumors.

The Ras-ERK pathway modulates cytoskeleton organization, cell motility and lung metastasis signature genes in MDA-MB-231 LM2

MDA-MB-231 LM2 (herein referred to as LM2) is a derivative of MDA-MB-231 cells that was selected for its ability to metastasize to lung tissue in vivo. We investigated cellular properties of LM2 including actin cytoskeleton organization, motility and signaling pathways that drive the expression of genes associated with the lung metastasis signature. Parental cells exhibit well-developed stress fibers, whereas LM2 had poorly organized stress fibers. LM2 exhibited higher levels of K-Ras protein and corresponding higher levels of phosphorylated ERK compared with parental cells. The Ras-ERK pathway was responsible for the disruption of stress fibers because inhibition of MEK with UO126 or small interfering RNA (siRNA) against K-Ras or ERK1/2 resulted in restoration of stress fibers and focal adhesions. We observed that the K-Ras-ERK pathway is important for the expression of genes associated with the lung metastasis signature. Paradoxically, inhibition of the Ras-ERK pathway did not result in inhibition of cell motility but was accompanied by activation of the phosphatidylinositol 3-kinase (PI3K) pathway. Inhibition of both ERK and PI3K pathways was required to inhibit motility of LM2 cells. These results suggest that both ERK and PI3K pathways drive motile functions of metastatic LM2 cells and genes associated with the lung metastasis signature.

Crosstalk of Ras and Rho: activation of RhoA abates Kras-induced liver tumorigenesis in transgenic zebrafish models

RAS and Rho small GTPases are key molecular switches that control cell dynamics, cell growth and tissue development through their distinct signaling pathways. Although much has been learnt about their individual functions in both cell and animal models, the physiological and pathophysiological consequences of their signaling crosstalk in multi-cellular context in vivo remain largely unknown, especially in liver development and liver tumorigenesis. Furthermore, the roles of RhoA in RAS-mediated transformation and their crosstalk in vitro remain highly controversial. When challenged with carcinogens, zebrafish developed liver cancer that resembles the human liver cancer both molecularly and histopathologically. Capitalizing on the growing importance and relevance of zebrafish (Danio rerio) as an alternate cancer model, we have generated liver-specific, Tet-on-inducible transgenic lines expressing oncogenic Kras(G12V), RhoA, constitutively active RhoA(G14V) or dominant-negative RhoA(T19N). Double-transgenic lines expressing Kras(G12V) with one of the three RhoA genes were also generated. Based on quantitative bioimaging and molecular markers for genetic and signaling aberrations, we showed that the induced expression of oncogenic Kras during early development led to liver enlargement and hepatocyte proliferation, associated with elevated Erk phosphorylation, activation of Akt2 and modulation of its two downstream targets, p21Cip and S6 kinase. Such an increase in liver size and Akt2 expression was augmented by dominant-negative RhoA(T19N), but was abrogated by the constitutive-active RhoA(G14V). Consequently, induced expression of the oncogenic Kras in adult transgenic fish led to the development of hepatocellular carcinomas. Survival studies further revealed that the co-expression of dominant-negative RhoA(T19N) with oncogenic Kras increased the mortality rate compared with the other single or double-transgenic lines. This study provides evidence of the previously unappreciated signaling crosstalk between Kras and RhoA in regulating liver overgrowth and liver tumorigenesis. Our results also implicate that activating Rho could be beneficial to suppress the Kras-induced liver malignancies.

Connective tissue growth factor antagonizes transforming growth factor-β1/Smad signalling in renal mesangial cells

The critical involvement of TGF-β1 (transforming growth factor-β1) in DN (diabetic nephropathy) is well established. However, the role of CTGF (connective tissue growth factor) in regulating the complex interplay of TGF-β1 signalling networks is poorly understood. The purpose of the present study was to investigate co-operative signalling between CTGF and TGF-β1 and its physiological significance. CTGF was determined to bind directly to the TβRIII (TGF-β type III receptor) and antagonize TGF-β1-induced Smad phosphorylation and transcriptional responses via its N-terminal half. Furthermore, TGF-β1 binding to its receptor was inhibited by CTGF. A consequent shift towards non-canonical TGF-β1 signalling and expression of a unique profile of differentially regulated genes was observed in CTGF/TGF-β1-treated mesangial cells. Decreased levels of Smad2/3 phosphorylation were evident in STZ (streptozotocin)-induced diabetic mice, concomitant with increased levels of CTGF. Knockdown of TβRIII restored TGF-β1-mediated Smad signalling and cell contractility, suggesting that TβRIII is key for CTGF-mediated regulation of TGF-β1. Comparison of gene expression profiles from CTGF/TGF-β1-treated mesangial cells and human renal biopsy material with histological diagnosis of DN revealed significant correlation among gene clusters. In summary, mesangial cell responses to TGF-β1 are regulated by cross-talk with CTGF, emphasizing the potential utility of targeting CTGF in DN.

Cutaneous human papillomavirus type 38 E7 regulates actin cytoskeleton structure for increasing cell proliferation through CK2 and the eukaryotic elongation factor 1A

We previously reported that the oncoproteins E6 and E7 from cutaneous human papillomavirus type 38 (HPV38) can immortalize primary human keratinocytes in vitro and sensitize transgenic mice to develop skin cancer in vivo. Immunofluorescence staining revealed that human keratinocytes immortalized by HPV38 E6 and E7 display fewer actin stress fibers than do control primary keratinocyte cells, raising the possibility of a role of the viral oncoproteins in the remodeling of the actin cytoskeleton. In this study, we show that HPV38 E7 induces actin stress fiber disruption and that this phenomenon correlates with its ability to downregulate Rho activity. The downregulation of Rho activity by HPV38 E7 is mediated through the activation of the CK2-MEK-extracellular signal-regulated kinase (ERK) pathway. In addition, HPV38 E7 is able to induce actin fiber disruption by binding directly to eukaryotic elongation factor 1A (eEF1A) and abolishing its effects on actin fiber formation. Finally, we found that the downregulation of Rho activity by HPV38 E7 through the CK2-MEK-ERK pathway facilitates cell growth proliferation. Taken together, our data support the conclusion that HPV38 E7 promotes keratinocyte proliferation in part by negatively regulating actin cytoskeleton fiber formation through the CK2-MEK-ERK-Rho pathway and by binding to eEF1A and inhibiting its effects on actin cytoskeleton remodeling.

Sorafenib targets dysregulated Rho kinase expression and portal hypertension in rats with secondary biliary cirrhosis

BACKGROUND AND PURPOSE

Extrahepatic vasodilation and increased intrahepatic vascular resistance represent attractive targets for the medical treatment of portal hypertension in liver cirrhosis. In both dysfunctions, dysregulation of the contraction-mediating Rho kinase plays an important role as it contributes to altered vasoconstrictor responsiveness. However, the mechanisms of vascular Rho kinase dysregulation in cirrhosis are insufficiently understood. They possibly involve mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK)-dependent mechanisms in extrahepatic vessels. As the multikinase inhibitor sorafenib inhibits ERK, we tested the effect of sorafenib on haemodynamics and dysregulated vascular Rho kinase in rats with secondary biliary cirrhosis.

EXPERIMENTAL APPROACH

Secondary biliary cirrhosis was induced by bile duct ligation (BDL). Sorafenib was given orally for 1 week (60 mg.kg(-1).d(-1)). Messenger RNA levels were determined by quantitative real time polymerase chain reaction, protein expressions and protein phosphorylation by Western blot analysis. Aortic contractility was studied by myographic measurements, and intrahepatic vasoregulation by using livers perfused in situ. In vivo, haemodynamic parameters were assessed invasively in combination with coloured microspheres.

KEY RESULTS

In BDL rats, treatment with sorafenib decreased portal pressure, paralleled by decreases in hepatic Rho kinase expression and Rho kinase-mediated intrahepatic vascular resistance. In aortas from BDL rats, sorafenib caused up-regulation of Rho kinase and an improvement of aortic contractility. By contrast, mesenteric Rho kinase remained unaffected by sorafenib.

CONCLUSIONS AND IMPLICATIONS

Intrahepatic dysregulation of vascular Rho kinase expression is controlled by sorafenib-sensitive mechanisms in rats with secondary biliary cirrhosis. Thus, sorafenib reduced portal pressure without affecting systemic blood pressure.

An essential role for p120-catenin in Src- and Rac1-mediated anchorage-independent cell growth

p120-catenin regulates epithelial cadherin stability and has been suggested to function as a tumor suppressor. In this study, we used anchorage-independent growth (AIG), a classical in vitro tumorigenicity assay, to examine the role of p120 in a different context, namely oncogene-mediated tumorigenesis. Surprisingly, p120 ablation by short hairpin RNA completely blocked AIG induced by both Rac1 and Src. This role for p120 was traced to its activity in suppression of the RhoA-ROCK pathway, which appears to be essential for AIG. Remarkably, the AIG block associated with p120 ablation was completely reversed by inhibition of the downstream RhoA effector ROCK. Harvey-Ras (H-Ras)-induced AIG was also dependent on suppression of the ROCK cascade but was p120 independent because its action on the pathway occurred downstream of p120. The data suggest that p120 modulates oncogenic signaling pathways important for AIG. Although H-Ras bypasses p120, a unifying theme for all three oncogenes is the requirement to suppress ROCK, which may act as a gatekeeper for the transition to anchorage independence.

B-RAF regulation of Rnd3 participates in actin cytoskeletal and focal adhesion organization

The actin cytoskeleton controls multiple cellular functions, including cell morphology, movement, and growth. Accumulating evidence indicates that oncogenic activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 (MEK/ERK1/2) pathway is accompanied by actin cytoskeletal reorganization. However, the signaling events contributing to actin cytoskeleton remodeling mediated by aberrant ERK1/2 activation are largely unknown. Mutant B-RAF is found in a variety of cancers, including melanoma, and it enhances activation of the MEK/ERK1/2 pathway. We show that targeted knockdown of B-RAF with small interfering RNA or pharmacological inhibition of MEK increased actin stress fiber formation and stabilized focal adhesion dynamics in human melanoma cells. These effects were due to stimulation of the Rho/Rho kinase (ROCK)/LIM kinase-2 signaling pathway, cumulating in the inactivation of the actin depolymerizing/severing protein cofilin. The expression of Rnd3, a Rho antagonist, was attenuated after B-RAF knockdown or MEK inhibition, but it was enhanced in melanocytes expressing active B-RAF. Constitutive expression of Rnd3 suppressed the actin cytoskeletal and focal adhesion effects mediated by B-RAF knockdown. Depletion of Rnd3 elevated cofilin phosphorylation and stress fiber formation and reduced cell invasion. Together, our results identify Rnd3 as a regulator of cross talk between the RAF/MEK/ERK and Rho/ROCK signaling pathways, and a key contributor to oncogene-mediated reorganization of the actin cytoskeleton and focal adhesions.

Vascular dysfunction in human and rat cirrhosis: role of receptor-desensitizing and calcium-sensitizing proteins

In cirrhosis, vascular hypocontractility leads to vasodilation and contributes to portal hypertension. Impaired activation of contractile pathways contributes to vascular hypocontractility. Angiotensin II type 1 receptors (AT1-Rs) are coupled to the contraction-mediating RhoA/Rho-kinase pathway and may be desensitized by phosphorylation through G-protein-coupled receptor kinases (GRKs) and binding of beta-arrestin-2. In the present study, we analyzed vascular hypocontractility to angiotensin II in cirrhosis. Human hepatic arteries were obtained during liver transplantation. In rats, cirrhosis was induced by bile duct ligation (BDL). Contractility of rat aortic rings was measured myographically. Protein expression and phosphorylation were analyzed by Western blot analysis. Immunoprecipitation was performed with protein A-coupled Sepharose beads. Myosin light chain (MLC) phosphatase activity was assessed as dephosphorylation of MLCs. Aortas from BDL rats were hyporeactive to angiotensin II and extracellular Ca2+. Expression of AT1-R and Galphaq/11,12,13 remained unchanged in hypocontractile rat and human vessels, whereas GRK-2 and beta-arrestin-2 were up-regulated. The binding of beta-arrestin-2 to the AT1-R was increased in hypocontractile rat and human vessels. Inhibition of angiotensin II-induced aortic contraction by the Rho-kinase inhibitor Y-27632 was pronounced in BDL rats. Basal phosphorylation of the ROK-2 substrate moesin was reduced in vessels from rats and patients with cirrhosis. Analysis of the expression and phosphorylation of Ca(2+)-sensitizing proteins (MYPT1 and CPI-17) in vessels from rats and patients with cirrhosis suggested decreased Ca2+ sensitivity. Angiotensin II-stimulated moesin phosphorylation was decreased in aortas from BDL rats. MLC phosphatase activity was elevated in aortas from BDL rats.

CONCLUSION

Vascular hypocontractility to angiotensin II in cirrhosis does not result from changes in expression of AT1-Rs or G-proteins. Our data suggest that in cirrhosis-induced vasodilation, the AT1-R is desensitized by GRK-2 and beta-arrestin-2 and that changed patterns of phosphorylated Ca(2+) sensitizing proteins decrease Ca(2+) sensitivity.

Molecular mechanism of cofilin dephosphorylation by ouabain

We previously reported that phosphorylated cofilin-triosephosphate isomerase (TPI) complex interacts with Na,K-ATPase and enhances the pump activity through the phosphorylation of cofilin via Rho-mediated signaling pathway. In this study, we tested the hypothesis that the dephosphorylation of cofilin may be induced through Na,K-ATPase inhibition by ouabain. The phosphorylation level of cofilin by ouabain which decreases in a time- and dose-dependent manner in various human cell lines, remains unchanged by pretreatment with Src inhibitor, PP2; epidermal growth factor receptor (EGFR) inhibitor, AG1478; Raf-1 kinase (Raf) inhibitor, GW5074; and ERK kinase (MEK) inhibitor, PD98059, and by transfection of Ras dominant negative mutant (RasN17). This suggests that ouabain dephosphorylates cofilin through the Src/EGFR/Ras/Raf/MEK pathway. Ouabain activates Ras/Raf/MEK pathway, but down-regulates Rho kinase (ROCK)/LIM kinase (LIMK)/cofilin pathway, implying that there may be a cross-talk by ouabain between the Ras/Raf/MEK and the ROCK/LIMK/cofilin pathways. Immunofluorescence and flow cytometry suggest that ouabain-induced active form of cofilin may be involved in cytoskeletal reorganization and cell volume regulation. Thus, these findings demonstrate a new molecular mechanism for the dephosphorylation of cofilin through the inhibition of Na,K-ATPase by ouabain.

ALK5 promotes tumor angiogenesis by upregulating matrix metalloproteinase-9 in tumor cells

Transforming growth factor beta 1 (TGF-beta1) is a potent tumor suppressor but, paradoxically, TGF-beta1 enhances tumor growth and metastasis in the late stages of cancer progression. This study investigated the role of TGF-beta type I receptor, ALK5, and three mitogen-activated protein kinases (MAPKs) in metastasis by breast cancer cell line MDA-MB-231. We show that autocrine TGF-beta signaling in MDA-MB-231 cells is required for tumor cell invasion and tumor angiogenesis. Expression of kinase-inactive ALK5 reduces tumor invasion and formation of new blood vessels within the tumor orthotopic xenografts in severe combined immunodeficiency (SCID) mice. In contrast, constitutively active ALK5-T204D enhances tumor invasion and angiogenesis by stimulating expression of matrix metalloproteinase MMP-9/gelatinase-B. Ablation of MMP-9 in ALK5-T204D cells by RNA interference (RNAi) reduces tumor invasion and tumor growth. Importantly, RNAi-MMP-9 reduces tumor neovasculature and increases tumor cell death. Induction of MMP-9 by TGF-beta-ALK5 signaling requires MEK-ERK but not JNK, p38 MAPK or Smad4. Dominant-negative MEK blocks and constitutively active MEK1 enhances MMP-9 expression. However, all three MAPK cascades (ERK, JNK and p38 MAPK) are required for TGF-beta-mediated cell migration. Collectively, our results show that TGF-beta-ALK5-MAPK signaling in tumor cells promotes tumor angiogenesis and MMP-9 is an important component of this program.

An MLCK-dependent window in late G1 controls S phase entry of proliferating rodent hepatocytes via ERK-p70S6K pathway

We show that MLCK (myosin light chain kinase) plays a key role in cell cycle progression of hepatocytes: either chemical inhibitor ML7 or RNA interference led to blockade of cyclin D1 expression and DNA replication, providing evidence that MLCK regulated S phase entry. Conversely, inhibition of RhoK by specific inhibitor Y27632 or RhoK dominant-negative vector did not influence progression in late G1 and S phase entry. Inhibition of either MLCK or RhoK did not block ERK1/2 phosphorylation, whereas MLCK regulated ERK2-dependent p70S6K activation. In addition, DNA synthesis was reduced in hepatocytes treated with p70S6K siRNA, demonstrating the key role played by the kinase in S phase entry. Interestingly, after the G1/S transition, DNA replication in S phase was no longer dependent on MLCK activity. We strengthened this result by ex vivo experiments and evidenced an MLCK-dependent window in late G1 phase of regenerating liver after two-thirds partial hepatectomy. In conclusion, our results underline an MLCK-dependent restriction point in G1/S transition, occurring downstream of ERK2 through the regulation of p70S6K activation, and highlighting a new signaling pathway critical for hepatocyte proliferation.

RhoA/Rho-kinase in erectile tissue: mechanisms of disease and therapeutic insights

Penile erection is a complicated event involving the regulation of corpus cavernosal smooth muscle tone. Recently, the small monomeric G-protein RhoA and its downstream effector Rho-kinase have been proposed to be important players for mediating vasoconstriction in the penis. RhoA/Rho-kinase increases MLC (myosin light chain) phosphorylation through inhibition of MLCP (MLC phosphatase) thereby increasing Ca2+ sensitivity. This review will outline the RhoA/Rho-kinase signalling pathway, including the upstream regulators, guanine nucleotide exchange factors, GDP dissociation inhibitors and GTPase-activating proteins. We also summarize the current knowledge about the physiological roles of RhoA/Rho-kinase in both male and female erectile tissues and its aberrations contributing to erectile dysfunction in several disease states. Understanding the RhoA/Rho-kinase signalling pathway in the regulation of erection is important for the development of therapeutic interventions for erectile dysfunction.

Oncogenic K-Ras down-regulates Rac1 and RhoA activity and enhances migration and invasion of pancreatic carcinoma cells through activation of p38

Activating mutations in the K-ras gene are genetic alterations frequently found in human carcinomas, particularly in pancreatic adenocarcinomas. Mutation of the K-ras gene is thought to be an early and important event in pancreatic tumor initiation, but the precise role of the mutant K-Ras proteins in neoplastic progression is still unknown. In the present study, we have characterized the influence of oncogenic K-Ras on the phenotype and on the signal transduction of epitheloid PANC-1 pancreatic carcinoma cells by generating PANC-1 cell clones, which stably express EGFP(enhanced green fluorescent protein)-K-Ras (V12). EGFP-K-Ras (V12)-expressing cells exhibited a more fibroblastoid cellular phenotype with irregular cell shape and disorganized cytokeratin filaments. Moreover, these cells showed a marked enhancement of their migratory and invasive properties. Stable expression of EGFP-K-Ras (V12) down-regulated the activity of Rac1 and RhoA, resulting in reduced subcortical actin filaments and stress fibers, which might contribute to the epithelial dedifferentiation. Characterization of the activity of mitogen-activated protein kinases revealed that EGFP-K-Ras (V12) enhanced the activity of p38, but did not affect the activities of the Raf/MEK/ERK cascade and JNK. While inhibition of either MEK or JNK activity had no effect on EGFP-K-Ras (V12)-induced migration, inhibition of p38 activity markedly reduced EGFP-K-Ras (V12)-induced migration. Collectively, the results suggest that oncogenic K-Ras enhances the malignant phenotype and identify the mitogen-activated protein kinase p38 as a target to inhibit oncogenic K-Ras-induced pancreatic tumor cell migration.

Resensitization of breast cancer cells to anoikis by Tropomyosin-1: role of Rho kinase-dependent cytoskeleton and adhesion

Two most common properties of malignant cells are the presence of aberrant actin cytoskeleton and resistance to anoikis. Suppression of several key cytoskeletal proteins, including tropomyosin-1 (TM1), during neoplastic transformation is hypothesized to contribute to the altered cytoskeleton and neoplastic phenotype. Using TM1 as a paradigm, we have shown that cytoskeletal proteins induce anoikis in breast cancer (MCF-7 and MDA MB 231) cells. Here, we have tested the hypothesis that TM1-mediated cytoskeletal changes regulate integrin activity and the sensitivity to anoikis. TM1 expression in MDA MB 231 cells promotes the assembly of stress fibers, induces rapid anoikis via caspase-dependent pathways involving the release of cytochrome c. Further, TM1 inhibits binding of MDA MB 231 cells to collagen I, but promotes adhesion to laminin. Inhibition of Rho kinase disrupts TM1-mediated cytoskeletal reorganization and adhesion to the extracellular matrix components, whereas the parental cells attach to collagen I, spread and form extensive actin meshwork in the presence of Rho kinase inhibitor, underscoring the differences in parental and TM1-transduced breast cancer cells. Further, treatment with the cytoskeletal disrupting drugs rescues the cells from TM1-induced anoikis. These new findings demonstrate that the aberrant cytoskeleton contributes to neoplastic transformation by conferring resistance to anoikis. Restoration of stress fiber network through enhanced expression of key cytoskeletal proteins may modulate the activity of focal adhesions and sensitize the neoplastic cells to anoikis.

Silencing of the Tropomyosin-1 gene by DNA methylation alters tumor suppressor function of TGF-beta

Loss of actin stress fibers has been associated with cell transformation and metastasis. TGF-beta induction of stress fibers in epithelial cells requires high molecular weight tropomyosins encoded by TPM1 and TPM2 genes. Here, we investigated the mechanism underlying the failure of TGF-beta to induce stress fibers and inhibit cell migration in metastatic cells. RT-PCR analysis in carcinoma cell lines revealed a significant reduction in TPM1 transcripts in metastatic MDA-MB-231, MDA-MB-435 and SW620 cell lines. Treatment of these cells with demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) increased mRNA levels of TPM1 with no effect on TPM2. Importantly, 5-aza-dC treatment of MDA-MB-231 cells restored TGF-beta induction of TPM1 and formation of stress fibers. Forced expression of TPM1 by using Tet-Off system increased stress fibers in MDA-MB-231 cells and reduced cell migration. A potential CpG island spanning the TPM1 proximal promoter, exon 1, and the beginning of intron 1 was identified. Bisulfite sequencing showed significant cytosine methylation in metastatic cell lines that correlated with a reduced expression of TPM1. Together these results suggest that epigenetic suppression of TPM1 may alter TGF-beta tumor suppressor function and contribute to metastatic properties of tumor cells.

The MEK1 scaffolding protein MP1 regulates cell spreading by integrating PAK1 and Rho signals

How the extracellular signal-regulated kinase (ERK) cascade regulates diverse cellular functions, including cell proliferation, survival, and motility, in a context-dependent manner remains poorly understood. Compelling evidence indicates that scaffolding molecules function in yeast to channel specific signals through common components to appropriate targets. Although a number of putative ERK scaffolding proteins have been identified in mammalian systems, none has been linked to a specific biological response. Here we show that the putative scaffold protein MEK partner 1 (MP1) and its partner p14 regulate PAK1-dependent ERK activation during adhesion and cell spreading but are not required for ERK activation by platelet-derived growth factor. MP1 associates with active but not inactive PAK1 and controls PAK1 phosphorylation of MEK1. Our data further show that MP1, p14, and MEK1 serve to inhibit Rho/Rho kinase functions necessary for the turnover of adhesion structures and cell spreading and reveal a signal-channeling function for a MEK1/ERK scaffold in orchestrating cytoskeletal rearrangements important for cell motility.

Human diploid fibroblasts are resistant to MEK/ERK-mediated disruption of the actin cytoskeleton and invasiveness stimulated by Ras

Ras-induced transformation is characterized not only by uncontrolled proliferation but also by drastic morphological changes accompanied by the disruption of the actin cytoskeleton. Previously, we reported that human fibroblasts are more resistant than rodent fibroblasts to Ras-induced transformation. To explore the molecular basis for the difference in susceptibility to Ras-induced transformation, we investigated the effect of activated H-Ras on the actin cytoskeleton in human diploid fibroblasts and in rat embryo fibroblasts, both of which are immortalized by SV40 early region. We demonstrate here that Ras-induced morphological changes, decreased expression of tropomyosin isoforms, and suppression of the ROCK/LIMK/Cofilin pathway observed in the rat fibroblasts were not detected in the human fibroblasts even with high expression levels of Ras. We also show that activation of the MEK/ERK pathway sufficed to induce all of these alterations in the rat fibroblasts, whereas the human fibroblasts were refractory to these MEK/ERK-mediated changes. In addition to morphological changes, we demonstrated that the expression of activated Ras induced an invasive phenotype in the rat, but not in the human fibroblasts. These studies provide evidence for the existence of human-specific mechanisms that resist Ras/MEK/ERK-mediated transformation.

Oncogenic transformation of human cells: shortcomings of rodent model systems

Long-standing difficulties in the in vitro transformation of human cells have been overcome. Using telomerase, several successful oncogene-mediated transformations of human cells have been reported and the following cellular requirements for human cell transformation have been proposed: the maintenance of telomere sequences, the inactivation of Rb and p53 pathways, the perturbation of protein phosphatase 2A (PP2A) and the expression of activated Ras. Even when all of these requirements are fulfilled, however, the transformed phenotypes of human cells seem to be much less malignant than those of rodent cells meeting the same requirements. This suggests the existence of undefined cell-autonomous mechanisms that render human cells resistant to malignant transformation.

Raf-1 regulates Rho signaling and cell migration

Raf kinases relay signals inducing proliferation, differentiation, and survival. The Raf-1 isoform has been extensively studied as the upstream kinase linking Ras activation to the MEK/ERK module. Recently, however, genetic experiments have shown that Raf-1 plays an essential role in counteracting apoptosis, and that it does so independently of its ability to activate MEK. By conditional gene ablation, we now show that Raf-1 is required for normal wound healing in vivo and for the migration of keratinocytes and fibroblasts in vitro. Raf-1-deficient cells show a symmetric, contracted appearance, characterized by cortical actin bundles and by a disordered vimentin cytoskeleton. These defects are due to the hyperactivity and incorrect localization of the Rho-effector Rok-alpha to the plasma membrane. Raf-1 physically associates with Rok-alpha in wild-type (WT) cells, and reintroduction of either WT or kinase-dead Raf-1 in knockout fibroblasts rescues their defects in shape and migration. Thus, Raf-1 plays an essential, kinase-independent function as a spatial regulator of Rho downstream signaling during migration.

The small G-protein RalA stimulates metastasis of transformed cells

RAS oncogenes play a critical role in oncogenic transformation and metastases formation. Here we show that Ha-ras greatly stimulates spontaneous metastatic activity of transformed cells through the Ras/RalGDS/RalA intracellular signaling pathway. Introduction of RalA alone leads to a drastic increase of metastatic activity of transformed cells. We demonstrate that metastatic ability of cells could be dramatically enhanced by RalA stimulation or, conversely, hampered by RalA suppression. Furthermore, we found that during in vivo selection cells acquire high metastatic properties as a result of endogenous RalA activation. The ability of RalA to induce metastasis was demonstrated in spontaneously transformed as well as in virus transformed fibroblasts.

Myosin light chain kinase and acto-myosin contractility modulate activation of the ERK cascade downstream of oncogenic Ras

The actin cytoskeleton is recognized as an important component of both adhesion- and growth factor-dependent signaling, but its role in oncogene-dependent signaling has received much less attention. In this study, we investigated the role played by the acto-myosin cytoskeleton and its main regulators, i.e., myosin light chain kinase and Rho kinase, in oncogenic Ki-Ras-induced signaling. We found that activation of the ERK cascade by Ras is dependent on acto-myosin contractility, under the regulation of myosin light chain kinase but not Rho kinase. Inhibition of myosin II or myosin light chain kinase caused a complete loss of ERK phosphorylation in a time- and dose-dependent manner, but proved dispensable for activation of the PI3K pathway. We also provide evidence that the target of myosin light chain kinase lays at the level of Raf activation. Since myosin light chain kinase is a target of ERK, these results suggest a previously uncharacterized signaling pathway involving Ras-mediated alterations of the actin cytoskeleton, which might play a critical role in ERK activation by the Ras oncogene and contribute to aberrant signaling and enhanced cell motility. In addition, restoration of stress fibers following ectopic expression of tropomyosin 2 resulted in reduced levels of ERK phosphorylation. Finally, these studies suggest that myosin light chain kinase but not Rho kinase plays an essential role in the generation of ERK signaling in transformed cells and indicate distinct cellular roles for Rho-kinase and myosin light chain kinase-dependent functions involving the regulation of acto-myosin contractility.

A critical role of tropomyosins in TGF-beta regulation of the actin cytoskeleton and cell motility in epithelial cells

We have investigated transforming growth factor beta (TGF-beta)-mediated induction of actin stress fibers in normal and metastatic epithelial cells. We found that stress fiber formation requires de novo protein synthesis, p38Mapk and Smad signaling. We show that TGF-beta via Smad and p38Mapk up-regulates expression of actin-binding proteins including high-molecular-weight tropomyosins, alpha-actinin and calponin h2. We demonstrate that, among these proteins, tropomyosins are both necessary and sufficient for TGF-beta induction of stress fibers. Silencing of tropomyosins with short interfering RNAs (siRNAs) blocks stress fiber assembly, whereas ectopic expression of tropomyosins results in stress fibers. Ectopic-expression and siRNA experiments show that Smads mediate induction of tropomyosins and stress fibers. Interestingly, TGF-beta induction of stress fibers was not accompanied by changes in the levels of cofilin phosphorylation. TGF-beta induction of tropomyosins and stress fibers are significantly inhibited by Ras-ERK signaling in metastatic breast cancer cells. Inhibition of the Ras-ERK pathway restores TGF-beta induction of tropomyosins and stress fibers and thereby reduces cell motility. These results suggest that induction of tropomyosins and stress fibers play an essential role in TGF-beta control of cell motility, and the loss of this TGF-beta response is a critical step in the acquisition of metastatic phenotype by tumor cells.

B-Raf acts via the ROCKII/LIMK/cofilin pathway to maintain actin stress fibers in fibroblasts

Recent data have shown that the BRAF gene is mutated at a high frequency in human malignancies. We have analyzed the migratory characteristics of B-raf(-/-) mouse embryonic fibroblasts (MEFs) and compared these with the organization of the actin cytoskeleton and the activity of signaling pathways that are known to influence this organization. Disruption of B-raf significantly reduced the levels of phospho-ERK1/2 and, surprisingly, induced an approximately 1.5-fold increase in cell migration. Consistent with these findings, the high level of actin stress fibers normally present in MEFs was considerably reduced following disruption of B-raf, and the F-actin content of B-raf(-/-) cells was less than half that of B-raf(+/+) cells. Phosphorylation of the myosin light chain on Thr18/Ser19 residues was not reduced in B-raf(-/-) cells. Rather, reduced ROCKII expression and attenuated phosphorylation of ADF/cofilin on serine 3 occurred. Normal stress fiber and phosphocofilin levels were restored by the expression of human B-Raf and catalytically active MEK and by the overexpression of LIM kinase (LIMK). These results have important implications for the role of the B-Raf/ERK signaling pathway in regulating cell motility in normal and malignant cells. They suggest that B-Raf is involved in invasiveness by regulating the proper assembly of actin stress fibers and contractility through a ROCKII/LIMK/cofilin signaling pathway.

Role of the p66Shc isoform in insulin-like growth factor I receptor signaling through MEK/Erk and regulation of actin cytoskeleton in rat myoblasts

To investigate the role of Shc in IGF action and signaling in skeletal muscle cells, Shc protein levels were reduced in rat L6 myoblasts by stably overexpressing a Shc cDNA fragment in antisense orientation (L6/Shcas). L6/Shcas myoblasts showed marked reduction of the p66Shc protein isoform and no change in p52Shc or p46Shc proteins compared with control myoblasts transfected with the empty vector (L6/Neo). When compared with control, L6/Shcas myoblasts demonstrated 3-fold increase in Erk-1/2 phosphorylation under basal conditions and blunted Erk-1/2 stimulation by insulin-like growth factor I (IGF-I), in the absence of changes in total Erk-1/2 protein levels. Increased basal Erk-1/2 activation was paralleled by a greater proportion of phosphorylated Erk-1/2 in the nucleus of L6/Shcas myoblasts in the absence of IGF-I stimulation. The reduction of p66Shc in L6/Shcas myoblasts resulted in marked phenotypic abnormalities, such as rounded cell shape and clustering in islets or finger-like structures, and was associated with impaired DNA synthesis in response to IGF-I and lack of terminal differentiation into myotubes. In addition, L6/Shcas myoblasts were characterized by complete disruption of actin filaments and cell cytoskeleton. Treatment of L6/Shcas myoblasts with the MEK inhibitor PD98059 reduced the abnormal increase in Erk-1/2 activation to control levels and restored the actin cytoskeleton, re-establishing the normal cell morphology. Thus, the p66Shc isoform exerts an inhibitory effect on the mitogen-activated protein kinase signaling pathway in rodent myoblasts, which is necessary for maintenance of IGF responsiveness of the MEK/Erk pathway and normal cell phenotype.

N Terminus Is Essential for Tropomyosin Functions

Down-regulation of several key actin-binding proteins, such as alpha-actinin, vinculin, gelsolin, and tropomyosins (TMs), is considered to contribute to the disorganized cytoskeleton present in many neoplastic cells. TMs stabilize actin filaments against the gel severing actions of proteins such as cofilin. Among multiple TMs expressed in non-muscle cells, tropomyosin-1 (TM1) isoform induces stress fibers and functions as a suppressor of malignant transformation. However, the molecular mechanisms of TM1-mediated cytoskeletal effects and tumor suppression remain poorly understood. We have hypothesized that the ability of TM1 to stabilize microfilaments is crucial for tumor suppression. In this study, by employing a variant TM1, which contains an N-terminal hemagglutinin epitope tag, we demonstrate that the N terminus is a key determinant of tropomyosin-1 function. Unlike the wild type TM1, the modified protein fails to restore stress fibers and inhibit anchorage-independent growth in transformed cells. Furthermore, the N-terminal modification of TM1 disorganizes the cytoskeleton and delays cytokinesis in normal cells, abolishes binding to F-actin, and disrupts the dimeric associations in vivo. The functionally defective TM1 allows the association of cofilin to stress fibers and disorganizes the microfilaments, whereas wild type TM1 appears to restrict the binding of cofilin to stress fibers. TM1-induced cytoskeletal reorganization appears to be mediated through preventing cofilin interaction with microfilaments. Our studies provide in vivo functional evidence that the N terminus is a critical determinant of TM1 functions, which in turn determines the organization of stress fibers.

Cytoplasmic p21Cip1 is involved in Ras-induced inhibition of the ROCK/LIMK/cofilin pathway

Accumulating evidence suggests that p21(Cip1) located in the cytoplasm might play a role in promoting transformation and tumor progression. Here we show that oncogenic H-RasV12 contributes to the loss of actin stress fibers by inducing cytoplasmic localization of p21(Cip1), which uncouples Rho-GTP from stress fiber formation by inhibiting Rho kinase (ROCK). Concomitant with the loss of stress fibers in Ras-transformed cells, there is a decrease in the phosphorylation level of cofilin, which is indicative of a compromised ROCK/LIMK/cofilin pathway. Inhibition of MEK in Ras-transformed NIH3T3 results in restoration of actin stress fibers accompanied by a loss of cytoplasmic p21(Cip1), and increased phosphorylation of cofilin. Ectopic expression of cytoplasmic but not nuclear p21(Cip1) in Ras-transformed cells was effective in preventing stress fibers from being restored upon MEK inhibition and inhibited phosphorylation of cofilin. p21(Cip1) was also found to form a complex with ROCK in Ras-transformed cells in vivo. Furthermore, inhibition of the PI 3-kinase pathway resulted in loss of p21(Cip1) expression accompanied by restoration of phosphocofilin, which was not accompanied by stress fiber formation. These results suggest that restoration of cofilin phosphorylation in Ras-transformed cells is necessary but not sufficient for stress fiber formation. Our findings define a novel mechanism for coupling cytoplasmic p21(Cip1) to the control of actin polymerization by compromising the Rho/ROCK/LIMK/cofilin pathway by oncogenic Ras. These studies suggest that localization of p21(Cip1) to the cytoplasm in transformed cells contributes to pathways that favor not only cell proliferation, but also cell motility thereby contributing to invasion and metastasis.

The role of collagen structure in mitogen stimulation of ERK, cyclin D1 expression, and G1-S progression in rat hepatocytes

Adhesion to type 1 collagen can elicit different cellular responses dependent upon whether the collagen is in a fibrillar form (gel) or monomeric form (film). Hepatocytes adherent to collagen film spread extensively, express cyclin D1, and increase DNA synthesis in response to epidermal growth factor, whereas hepatocytes adherent to collagen gel have increased differentiated function, but lower DNA synthesis. The signaling mechanisms by which different forms of type I collagen modulate cell cycle progression are unknown. When ERK MAP kinase activation was analyzed in hepatocytes attached to collagen film, two peaks of ERK activity were demonstrated. Only the second peak, which correlated with an increase of cyclin D1, was required for G1-S progression. Notably, this second peak of ERK activity was absent in cells adherent to collagen gel, but not required in the presence of exogenous cyclin D1. Expression of activated mutants of the Ras/Raf/MEK signaling pathway in cells adherent to collagen gel restored ERK phosphorylation and DNA synthesis, but differentially affected cell shape. Although Ras, Raf, and MEK all increased expression of cyclin D1 on collagen film, only Ras and Raf significantly up-regulated cyclin D1 levels on collagen gel. These results demonstrate that adhesion to polymerized collagen induces growth arrest by inhibiting the Ras/ERK-signaling pathway to cyclin D1 required in late G1.

Effects of rho kinase and actin stress fibers on sustained extracellular signal-regulated kinase activity and activation of G(1) phase cyclin-dependent kinases

We recently reported that Rho kinase is required for sustained ERK signaling and the consequent mid-G(1) phase induction of cyclin D1 in fibroblasts. The results presented here indicate that these Rho kinase effects are mediated by the formation of stress fibers and the consequent clustering of alpha5beta1 integrin. Mechanistically, alpha5beta1 signaling and stress fiber formation allowed for the sustained activation of MEK, and this effect was mediated upstream of Ras-GTP loading. Interestingly, disruption of stress fibers with ML-7 led to G(1) phase arrest while comparable disruption of stress fibers with Y27632 (an inhibitor of Rho kinase) or dominant-negative Rho kinase led to a more rapid progression through G(1) phase. Inhibition of either MLCK or Rho kinase blocked sustained ERK signaling, but only Rho kinase inhibition allowed for the induction of cyclin D1 and activation of cdk4 via Rac/Cdc42. The levels of cyclin E, cdk2, and their major inhibitors, p21(cip1) and p27(kip1), were not affected by inhibition of MLCK or Rho kinase. Overall, our results indicate that Rho kinase-dependent stress fiber formation is required for sustained activation of the MEK/ERK pathway and the mid-G(1) phase induction of cyclin D1, but not for other aspects of cdk4 or cdk2 activation. They also emphasize that G(1) phase cell cycle progression in fibroblasts does not require stress fibers if Rac/Cdc42 signaling is allowed to induce cyclin D1.

Rocks: multifunctional kinases in cell behaviour

ROCKs, or Rho kinases, are serine/threonine kinases that are involved in many aspects of cell motility, from smooth-muscle contraction to cell migration and neurite outgrowth. Recent experiments have defined new functions of ROCKs in cells, including centrosome positioning and cell-size regulation, which might contribute to various physiological and pathological states.

RhoE binds to ROCK I and inhibits downstream signaling

RhoE belongs to the Rho GTPase family, the members of which control actin cytoskeletal dynamics. RhoE induces stress fiber disassembly in a variety of cell types, whereas RhoA stimulates stress fiber assembly. The similarity of RhoE and RhoA sequences suggested that RhoE might compete with RhoA for interaction with its targets. Here, we show that RhoE binds ROCK I but none of the other RhoA targets tested. The interaction of RhoE with ROCK I was confirmed by coimmunoprecipitation of the endogenous proteins, and the two proteins colocalized on the trans-Golgi network in COS-7 cells. Although RhoE and RhoA were not able to bind ROCK I simultaneously, RhoE bound to the amino-terminal region of ROCK I encompassing the kinase domain, at a site distant from the carboxy-terminal RhoA-binding site. Overexpression of RhoE inhibited ROCK I-induced stress fiber formation and phosphorylation of the ROCK I target myosin light chain phosphatase. These data suggest that RhoE induces stress fiber disassembly by directly binding ROCK I and inhibiting it from phosphorylating downstream targets.

Overexpression of kinase suppressor of Ras upregulates the high-molecular-weight tropomyosin isoforms in ras-transformed NIH 3T3 fibroblasts

The down-regulation of the high-molecular-weight isoforms of tropomyosin (TM) is considered to be an essential event in cellular transformation. In ras-transformed fibroblasts, the suppression of TM is dependent on the activity of the Raf-1 kinase; however, the requirement for other downstream effectors of Ras, such as MEK and ERK, is less clear. In this study, we have utilized the mitogen-activated protein kinase scaffolding protein Kinase Suppressor of Ras (KSR) to further investigate the regulation of TM and to clarify the importance of MEK/ERK signaling in this process. Here, we report that overexpression of wild-type KSR1 in ras-transformed fibroblasts restores TM expression and induces cell flattening and stress fiber formation. Moreover, we find that the transcriptional activity of a TM-alpha promoter is decreased in ras-transformed cells and that the restoration of TM by KSR1 coincides with increased transcription from this promoter. Although ERK activity was suppressed in cells overexpressing KSR1, ERK inhibition alone was insufficient to upregulate TM expression. The KSR1-mediated effects on stress fiber formation and TM transcription required the activity of the ROCK kinase, because these effects could be suppressed by the ROCK inhibitor, Y27632. Overexpression of KSR1 did not directly regulate ROCK activity, but did permit the recoupling of ROCK to the actin polymerization machinery. Finally, all of the KSR1-induced effects were mediated by the C-terminal domain of KSR1 and were dependent on the KSR-MEK interaction.

Oncogenic Ras leads to Rho activation by activating the mitogen-activated protein kinase pathway and decreasing Rho-GTPase-activating protein activity

Transformation by oncogenic Ras requires signaling through Rho family proteins including RhoA, but the mechanism(s) whereby oncogenic Ras regulates the activity of RhoA is (are) unknown. We examined the effect of Ras on RhoA activity in NIH 3T3 cells either stably transfected with H-Ras(V12) under control of an inducible promoter or transiently expressing the activated H-Ras. Using a novel method to quantitate enzymatically the GTP bound to Rho, we found that expression of the oncogenic Ras increased Rho activity approximately 2-fold. Increased Rho activity was associated with increased plasma membrane binding of RhoA and decreased activity of the Rho/Ras-regulated p21(WAF1/CIP1) promoter. RhoA activation by oncogenic Ras could be explained by a decrease in cytosolic p190 Rho-GAP activity and translocation of p190 Rho-GAP from the cytosol to a detergent-insoluble cytoskeletal fraction. Pharmacologic inhibition of the Ras/Raf/MEK/ERK pathway prevented Ras-induced activation of RhoA and translocation of p190 Rho-GAP; expression of constitutively active Raf-1 kinase or MEK was sufficient to induce p190 Rho-GAP translocation. We conclude that in NIH 3T3 cells oncogenic Ras activates RhoA through the Raf/MEK/ERK pathway by decreasing the cytosolic activity and changing the subcellular localization of p190 Rho-GAP.

Inhibitors of Ras/Raf-1 interaction identified by two-hybrid screening revert Ras-dependent transformation phenotypes in human cancer cells

The interaction of activated Ras with Raf initiates signaling cascades that contribute to a significant percentage of human tumors, suggesting that agents that specifically disrupt this interaction might have desirable chemotherapeutic properties. We used a subtractive forward two-hybrid approach to identify small molecule compounds that block the interaction of Ras with Raf. These compounds (MCP1 and its derivatives, 53 and 110) reduced serum-induced transcriptional activation of serum response element as well as Ras-induced transcription by way of the AP-1 site. They also inhibited Ras-induced Raf-1 activation in human embryonic kidney 293 cells, Raf-1 and mitogen-activated protein kinase kinase 1 activities in HT1080 fibrosarcoma cells, and epidermal growth factor-induced Raf-1 activation in A549 lung carcinoma cells. The MCP compounds caused reversion of ras-transformed phenotypes including morphology, in vitro invasiveness, and anchorage-independent growth of HT1080 cells. Decreased level of matrix metalloproteinases was also observed. Further characterization showed that MCP compounds restore actin stress fibers and cause flat reversion in NIH 3T3 cells transformed with H-Ras (V12) but not in NIH 3T3 cells transformed with constitutively active Raf-1 (RafDeltaN). Finally, we show that MCP compounds inhibit anchorage-independent growth of A549 and PANC-1 cells harboring K-ras mutation. Furthermore, MCP110 caused G(1) enrichment of A549 cells with the decrease of cyclin D level. These results highlight potent and specific effects of MCP compounds on cancer cells with intrinsic Ras activation.

MEK mediates v-Src-induced disruption of the actin cytoskeleton via inactivation of the Rho-ROCK-LIM kinase pathway

Cellular transformation by v-Src is believed to be caused by aberrant activation of signaling pathways that are normally regulated by cellular Src. Using normal rat kidney cells expressing a temperature-sensitive mutant of v-Src, we examined the role of the Raf/MEK/ERK, phosphatidylinositol 3-kinase/Akt, and Rho pathways in morphological transformation and cytoskeletal changes induced by v-Src. Activation of v-Src elicited a loss of actin stress fibers and focal contacts. A decrease in the phosphorylation level of cofilin was detected upon v-Src activation, which is indicative of attenuated Rho function. Inhibition of MEK using U0126 prevented v-Src-induced disruption of the cytoskeleton as well as dephosphorylation of cofilin, whereas treatment with a phosphatidylinositol 3-kinase inhibitor had no protective effect. In normal rat kidney cells stably transformed by v-Src, we found that the chronic activation of MEK induces down-regulation of ROCK expression, thereby uncoupling Rho from stress fiber formation. Taken together, these results establish MEK as an effector of v-Src-induced cytoskeleton disruption, participating in v-Src-induced antagonism of the cellular function of Rho.

Expression of BRG1, a human SWI/SNF component, affects the organisation of actin filaments through the RhoA signalling pathway

The human BRG1 (brahma-related gene 1) protein is a component of the SWI/SNF family of the ATP-dependent chromatin remodelling complexes. We show here that expression of the BRG1 protein, but not of an ATPase-deficient BRG1 protein, in BRG1-deficient SW13 cells alters the organisation of actin filaments. BRG1 expression induces the formation of thick actin filament bundles resembling stress-fibres, structures that are rarely seen in native SW13 cells. BRG1 expression does not influence the activity state of the RhoA-GTPase, which is involved in stress-fibre formation. We find that RhoA is equally activated by stimuli, such as serum, in BRG1-expressing cells,ATPase-deficient BRG1-expressing cells and native SW13 cells. However, the activation of RhoA by lysophosphatidic acid and serum does not trigger the formation of stress-fibre-like structures in SW13 cells. Activation of the RhoA-GTPase in BRG1-expressing cells induces stress-fibre-like structures,indicating that the BRG1 can couple RhoA activation to stress-fibre formation. At least two downstream effectors are involved in stress-fibre formation,Rho-kinase/ROCK and Dia. BRG1 expression, but not the expression of the ATP-deficient BRG1, increases the protein level of ROCK1, one form of the Rho-kinase/ROCK. That this is of importance is supported by the findings that an increased Rho-kinase/ROCK activity in SW13 cells, obtained by overexpressing wild-type ROCK1 and ROCK2, induces stress-fibre formation. No specificity between the two Rho-kinase/ROCK forms exists. Our results suggest that the BRG1 protein affects the RhoA pathway by increasing the protein level of ROCK1, which allows stress-fibre-like structures to form.