Ras and Rho GTPases: a family reunion

Activation of Ras-Ral Pathway Attenuates p53-independent DNA Damage G2 Checkpoint

Earlier we have found that in p53-deficient cells the expression of activated Ras attenuates the DNA damage-induced arrest in G(1) and G(2). In the present work we studied Ras-mediated effects on the G(2) checkpoint in two human cell lines, MDAH041 immortalized fibroblasts and Saos-2 osteosarcoma cells. The transduction of the H-Ras mutants that retain certain functions (V12S35, V12G37, and V12C40 retain the ability to activate Raf or RalGDS or phosphatidylinositol 3-kinase, respectively) as well as the activated or dominant-negative mutants of RalA (V23 and N28, respectively) has revealed that the activation of Ras-RalGEFs-Ral pathway was responsible for the attenuation of the G(2) arrest induced by ethyl metanesulfonate or doxorubicin. Noteworthy, the activated RalA V23N49 mutant, which cannot interact with RLIP76/RalBP1 protein, one of the best studied Ral effectors, retained the ability to attenuate the DNA damage-induced G(2) arrest. Activation of the Ras-Ral signaling affected neither the level nor the intracellular localization of cyclin B1 and CDC2 but interfered with the CDC2 inhibitory phosphorylation at Tyr(15) and the decrease in the cyclin B/CDC2 kinase activity in damaged cells. The revealed function of the Ras-Ral pathway may contribute to the development of genetic instability in neoplastic cells.

Rac1 and PAK1 are upstream of IKK-epsilon and TBK-1 in the viral activation of interferon regulatory factor-3

The anti-viral type I interferon (IFN) response is initiated by the immediate induction of IFN beta, which is mainly controlled by the IFN-regulatory factor-3 (IRF-3). The signaling pathways mediating viral IRF-3 activation are only poorly defined. We show that the Rho GTPase Rac1 is activated upon virus infection and controls IRF-3 phosphorylation and activity. Inhibition of Rac1 leads to reduced IFN beta promoter activity and to enhanced virus production. As a downstream mediator of Rac signaling towards IRF-3, we have identified the kinase p21-activated kinase (PAK1). Furthermore, both Rac1 and PAK1 regulate the recently described IRF-3 activators, I kappa B kinase- and TANK-binding kinase-1, establishing a first canonical virus-induced IRF-3 activating pathway.

Differential regulation of actin stress fiber assembly and proplatelet formation by alpha2beta1 integrin and GPVI in human megakaryocytes

The actin cytoskeleton plays a major role in platelet function. In contrast, its precise role in the function of megakaryocytes (MKs) is less understood but may be important for a chemoattractive response and an efficient proplatelet formation. In the marrow microenvironment, mature MKs are in contact with the extracellular matrix, including fibrillar collagen type I. MKs express alpha2beta1 integrin and the immunoglobulin superfamily member glycoprotein VI (GPVI), the main receptors for collagen. Using function-blocking antibodies or specific ligands, we investigated in primary human MKs how alpha2beta1 integrin and GPVI regulate stress fiber formation, the primary actin structures needed for cell contraction. Stress fiber assembly requires synergistic activation of the MAPK/Erk1/2 pathway and the small guanosine triphosphatase Rho via its effector, Rho-associated coiled-coil kinase (ROCK). alpha2beta1 integrin is crucial for stress fiber formation, whereas GPVI triggers rapid and sustained activation of the Erk1/2 pathway. Strikingly, after a longer adhesion time, proplatelet formation was significantly inhibited by the engagement of alpha2beta1 integrin, not by GPVI, likely through the Rho/ROCK pathway. Thus, proplatelet formation in human MKs could be tightly regulated by differential interactions with their collagen receptors. We propose that this interaction with collagen prevents proplatelet formation within the marrow.

Selective inhibition of vascular endothelial growth factor receptor-2 (VEGFR-2) identifies a central role for VEGFR-2 in human aortic endothelial cell responses to VEGF

Vascular endothelial growth factor receptors (VEGFR) are considered essential for angiogenesis. The VEGFR-family proteins consist of VEGFR-1/Flt-1, VEGFR-2/KDR/Flk-1, and VEGFR-3/Flt-4. Among these, VEGFR-2 is thought to be principally responsible for angiogenesis. However, the precise role of VEGFRs1-3 in endothelial cell biology and angiogenesis remains unclear due in part to the lack of VEGFR-specific inhibitors. We used the newly described, highly selective anilinoquinazoline inhibitor of VEGFR-2 tyrosine kinase, ZM323881 (5-[[7-(benzyloxy) quinazolin-4-yl]amino]-4-fluoro-2-methylphenol), to explore the role of VEGFR-2 in endothelial cell function. Consistent with its reported effects on VEGFR-2 [IC(50) < 2 nM], ZM323881 inhibited activation of VEGFR-2, but not of VEGFR-1, epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), or hepatocyte growth factor (HGF) receptor. We studied the effects of VEGF on human aortic endothelial cells (HAECs), which express VEGFR-1 and VEGFR-2, but not VEGFR-3, in the absence or presence of ZM323881. Inhibition of VEGFR-2 blocked activation of extracellular regulated-kinase, p38, Akt, and endothelial nitric oxide synthetase (eNOS) by VEGF, but did not inhibit p38 activation by the VEGFR-1-specific ligand, placental growth factor (PIGF). Inhibition of VEGFR-2 also perturbed VEGF-induced membrane extension, cell migration, and tube formation by HAECs. Vascular endothelial growth factor receptor-2 inhibition also reversed VEGF-stimulated phosphorylation of CrkII and its Src homology 2 (SH2)-binding protein p130Cas, which are known to play a pivotal role in regulating endothelial cell migration. Inhibition of VEGFR-2 thus blocked all VEGF-induced endothelial cellular responses tested, supporting that the catalytic activity of VEGFR-2 is critical for VEGF signaling and/or that VEGFR-2 may function in a heterodimer with VEGFR-1 in human vascular endothelial cells.

Involvement of Ras Activation in Human Breast Cancer Cell Signaling, Invasion, and Anoikis

Although mutated forms of ras are not associated with the majority of breast cancers (<5%), there is considerable experimental evidence that hyperactive Ras can promote breast cancer growth and development. Therefore, we determined whether Ras and Ras-responsive signaling pathways were activated persistently in nine widely studied human breast cancer cell lines. Although only two of the lines harbor mutationally activated ras, we found that five of nine breast cancer cell lines showed elevated active Ras-GTP levels that may be due, in part, to HER2 activation. Unexpectedly, activation of two key Ras effector pathways, the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase and phosphatidylinositol 3'-kinase/AKT signaling pathways, was not always associated with Ras activation. Ras activation also did not correlate with invasion or the expression of proteins associated with tumor cell invasion (estrogen receptor alpha and cyclooxygenase 2). We then examined the role of Ras signaling in mediating resistance to matrix deprivation-induced apoptosis (anoikis). Surprisingly, we found that ERK and phosphatidylinositol 3'-kinase/AKT activation did not have significant roles in conferring anoikis resistance. Taken together, these observations show that Ras signaling exhibits significant cell context variations and that other effector pathways may be important for Ras-mediated oncogenesis, as well as for anoikis resistance, in breast cancer. Additionally, because ERK and AKT activation are not strictly associated with Ras activation, pharmacological inhibitors of these two signaling pathways may not be the best approach for inhibition of aberrant Ras function in breast cancer treatment.

Galectin-1(L11A) predicted from a computed galectin-1 farnesyl-binding pocket selectively inhibits Ras-GTP

Ras biological activity necessitates membrane anchorage that depends on the Ras farnesyl moiety and is strengthened by Ras/galectin-1 interactions. We identified a hydrophobic pocket in galectin-1, analogous to the Cdc42 geranylgeranyl-binding cavity in RhoGDI, possessing homologous isoprenoid-binding residues, including the critical L11, whose RhoGDI L77 homologue changes dramatically on Cdc42 binding. By substituting L11A, we obtained a dominant interfering galectin-1 that possessed normal carbohydrate-binding capacity but inhibited H-Ras GTP-loading and extracellular signal-regulated kinase activation, dislodged H-Ras(G12V) from the cell membrane, and attenuated H-Ras(G12V) fibroblast transformation and PC12-cell neurite outgrowth. Thus, independently of carbohydrate binding, galectin-1 cooperates with Ras, whereas galectin-1(L11A) inhibits it.

Galectin-3 augments K-Ras activation and triggers a Ras signal that attenuates ERK but not phosphoinositide 3-kinase activity

Depending on the cellular context, Ras can activate characteristic effectors by mechanisms still poorly understood. Promotion by galectin-1 of Ras activation of Raf-1 but not of phosphoinositide 3-kinase (PI3-K) is one such mechanism. In this report, we describe a mechanism controlling selectivity of K-Ras4B (K-Ras), the most important Ras oncoprotein. We show that galectin-3 acts as a selective binding partner of activated K-Ras. Galectin-3 co-immunoprecipitated significantly better with K-Ras-GTP than with K-Ras-GDP, H-Ras, or N-Ras and colocalized with green fluorescent protein-K-Ras(G12V), not with green fluorescent protein-H-Ras(G12V), in the cell membrane. Co-transfectants of K-Ras/galectin-3, but not of H-Ras/galectin-3, exhibited enhanced and prolonged epidermal growth factor-stimulated increases in Ras-GTP, Raf-1 activity, and PI3-K activity. Extracellular signal-regulated kinase (ERK) activity, however, was attenuated in K-Ras/galectin-3 and in K-Ras(G12V)/galectin-3 co-transfectants. Galectin-3 antisense RNA inhibited the epidermal growth factor-stimulated increase in K-Ras-GTP but enhanced ERK activation and augmented K-Ras(G12V) transformation activity. Thus, unlike galectin-1, which prolongs Ras activation of ERK and inhibits PI3-K, K-Ras-GTP/galectin-3 interactions promote, in addition to PI3-K and Raf-1 activation, a third inhibitory signal that attenuates active ERK. These experiments established a novel and specific mechanism controlling the duration and selectivity of signals of active K-Ras, which is extremely important in many human tumors.

Role of Rho-family proteins in cell adhesion and cancer

Rho-family proteins control signalling pathways that regulate a wide range of biological processes. In vitro studies implicating Rho proteins in cell adhesion, migration, transcriptional activation, cell-cycle progression and transformation suggested roles for these proteins in the formation and progression of tumours in vivo. Studies using different recombinant mouse models have recently confirmed this idea. Rho signalling pathways crosstalk with different oncogenic signalling cascades, including those downstream of Ras and Wnt, and contribute to various aspects of tumourigenesis, including survival, growth and progression of tumour cells.

Overexpression of FAK promotes Ras activity through the formation of a FAK/p120RasGAP complex in malignant astrocytoma cells

Focal adhesion kinase (FAK) signaling may be mediated through the modulation of Ras activity. We have shown previously that grade III malignant astrocytoma biopsy samples exhibit elevated levels of FAK, and that overexpression of FAK in U-251MG malignant astrocytoma cells promotes the phosphorylation of Shc, a potential upstream mediator of Ras activity. Here, we report that overexpression of FAK promotes Ras activity in U-251MG malignant astrocytoma cells cultured in aggregate suspension or as monolayers adherent to vitronectin. The overexpression of FAK also promoted the association of FAK with p120RasGAP, which is a negative regulator of Ras activity, in the U-251MG cells cultured in aggregate suspension, with this association being abrogated upon plating of the cells onto vitronectin. An association of FAK with p120RasGAP also was observed in malignant astrocytoma biopsy samples, but not in normal brain samples. As overexpression of FAK in U-251MG cells in aggregate suspension culture reduced the amount of p120RasGAP complexed with active Ras, we hypothesize that the association of FAK with p120 RasGAP may facilitate Ras activity. The overexpression of a mutated FAK in which the Y397 had been mutated to F did not result in the formation of the FAK/p120RasGAP complex and did not promote Ras activity, indicating that the Y397 residue of FAK plays a role in the formation of this complex and in the activation of Ras. Moreover, the overexpression of mutated FAK (397F) was found to inhibit anchorage-independent growth. These data provide the basis for a previously undescribed mechanism in which the elevated expression of FAK can promote Ras activity through its competitive recruitment of p120RasGAP, thereby diminishing the association of p120RasGAP with active Ras.

Inverted signaling hierarchy between RAS and RAC in T-lymphocytes

In order to generate coherent biological responses to extracellular stimuli, cells have established synergistic and antagonistic crosstalk between pathways with similar or opposing functions, respectively. Two routes cooperating in the generation of mitogenic and cytoskeletal functions are those induced by Ras and Rho/Rac GTPases. In these signaling interactions, Rho/Rac proteins have been always placed in a downstream position respect to Ras in all cell systems analysed so far. In this report, we describe that such signaling hierarchy does not apply to T-lymphocytes. Thus, we show that both Rac1 GDP/GTP exchange factors such as Vav and constitutively active versions of Rac1 can promote the effective stimulation of the Ras pathway in T-lymphocytes. The molecular link for this new type of pathway interconnectivity is RasGRP1, a diacylglycerol-dependent GDP/GTP exchange factor for Ras that translocates to the plasma membrane in a Vav- and Rac1-dependent manner. The effect of the Vav/Rac1 pathway on the Ras pathway is highly dependent on the activity of phospholipase C-gamma, the key cellular supplier of intracellular diacylglycerol. Signaling experiments suggest that this crosstalk represents a signaling strategy used by the T-cell receptor to promote robust biological responses of both the Rac/Rho and Ras pathways upon antigen engagement.

Rab5 is a signalling GTPase involved in actin remodelling by receptor tyrosine kinases

Rab5 is a small GTPase involved in the control of intracellular trafficking, both at the level of receptor endocytosis and endosomal dynamics. The finding that Rab5 can be activated by receptor tyrosine kinases (RTK) raised the question of whether it also participates in effector pathways emanating from these receptors. Here we show that Rab5 is indispensable for a form of RTK-induced actin remodelling, called circular ruffling. Three independent signals, originating from Rab5, phosphatidylinositol-3-OH kinase and Rac, respectively, are simultaneously required for the induction of circular ruffles. Rab5 signals to the actin cytoskeleton through RN-tre, a previously identified Rab5-specific GTPase-activating protein (GAP). Here we demonstrate that RN-tre has the dual function of Rab5-GAP and Rab5 effector. We also show that RN-tre is critical for macropinocytosis, a process previously connected to the formation of circular ruffles. Finally, RN-tre interacts with both F-actin and actinin-4, an F-actin bundling protein. We propose that RN-tre establishes a three-pronged connection with Rab5, F-actin and actinin-4. This may aid crosslinking of actin fibres into actin networks at the plasma membrane. Thus, we have shown that Rab5 is a signalling GTPase and have elucidated the major molecular elements of its downstream pathway.

The small GTP-binding protein RhoA regulates c-jun by a ROCK-JNK signaling axis

RhoA regulates the actin cytoskeleton and the expression of genes associated with cell proliferation. This includes c-fos and c-jun, which are members of the AP1 family of transcription factors that play a key role in normal and aberrant cell growth. Whereas RhoA stimulates the c-fos SRE by a recently elucidated mechanism that is dependent on actin treadmilling, how RhoA regulates c-jun is still poorly understood. We found that RhoA stimulates c-jun expression through ROCK, but independently from the ability of ROCK to promote actin polymerization. Instead, we found that ROCK activates JNK, which then phosphorylates c-Jun and ATF2 when bound to the c-jun promoter. Thus, ROCK represents a point of signal divergence downstream from RhoA, as it promotes actin reorganization and the consequent expression from the c-fos SRE, while a parallel pathway connects ROCK to JNK, thereby stimulating c-jun expression. Ultimately, these pathways converge in the nucleus to regulate AP1 activity.

Regulation of anoikis by Cdc42 and Rac1

Ras family small GTPases play a critical role in malignant transformation, and Rho subfamily members contribute significantly to this process. Anchorage-independent growth and the ability to avoid detachment-induced apoptosis (anoikis) are hallmarks of transformed epithelial cells. In this study, we have demonstrated that constitutive activation of Cdc42 inhibits anoikis in Madin-Darby canine kidney (MDCK) epithelial cells. We showed that activated Cdc42 stimulates the ERK, JNK, and p38 MAPK pathways in suspension condition; however, inhibition of these signaling does not affect Cdc42-stimulated cell survival. However, we demonstrated that inhibition of phosphatidylinositol 3-kinase (PI3K) pathway abolishes the protective effect of Cdc42 on anoikis. Taking advantage of a double regulatory expression system, we also showed that Cdc42-stimulated cell survival in suspension condition is, at least in part, mediated by Rac1. We also provide evidence for a positive feedback loop involving Rac1 and PI3K. In addition, we show that the survival functions of both constitutively active Cdc42 and Rac1 GTPases are abrogated by Latrunculin B, an actin filament-depolymerizing agent, implying an important role for the actin cytoskeleton in mediating survival signaling activated by Cdc42 and Rac1. Together, our results indicate a role for Cdc42 in anchorage-independent survival of epithelial cells. We also propose that this survival function depends on a positive feedback loop involving Rac1 and PI3K.

Rho family GTPases cooperate with p53 deletion to promote primary mouse embryonic fibroblast cell invasion

The Rho family GTPases Rac1, RhoA and Cdc42 function as molecular switches that transduce intracellular signals regulating multiple cell functions including gene expression, adhesion, migration and invasion. p53 and its regulator p19Arf, on the other hand, are tumor suppressors that are critical in regulating cell cycle progression and apoptosis. Previously, we have demonstrated that the Rho proteins contribute to the cell proliferation, gene transcription and migration phenotypes unleashed by p19Arf or p53 deletion in primary mouse embryo fibroblasts (MEFs). To further investigate their functional interaction in the present study, we have examined the involvement of Rho signaling pathways in p53-mediated cell invasion. We found that in primary MEFs (1) p53 or p19Arf deficiency led to a marked increase in the number of focal adhesion plaques and fibronectin production, and RhoA, Rac1 and Cdc42 contribute to the p53- and p19Arf-mediated focal adhesion regulation, but not fibronectin synthesis; (2) although endogenous Rac1 activity was required for the p19Arf or p53 deficiency-induced migration phenotype, hyperactive Rho GTPases could not further enhance cell migration, rather they suppressed cell-cell adhesion of p53-/- MEFs; (3) expression of the active mutant of RhoA, Rac1 or Cdc42, but not Ras, promoted an invasion phenotype of p53-/-, not p19Arf-/-, cells; (4) although ROCK activation can partially recapitulate Rho-induced invasion phenotype, multiple pathways regulated by RhoA, in addition to ROCK, are required to fully cooperate with p53 deficiency to promote cell invasion; and (5) extracellular proteases produced by the active RhoA-transduced cells are also required for the invasion phenotype of p53-/- cells. Combined with our previous observations, these results strongly suggest that mitogenic activation of Rho family GTPases can cooperate with p53 deficiency to promote primary cell invasion as well as transformation and that multiple signaling components regulated by the Rho proteins are involved in these processes.

Rho GTPases: potential candidates for anticancer therapy

Low molecular weight Rho GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell proliferation, apoptosis, lipid metabolism, cytoarchitecture, adhesion, migration, cell polarity, and transcriptional regulation. The high incidence of overexpression of some members of the Rho family of GTPases in human tumors suggests that these proteins are important in the carcinogenic process, and therefore potential candidates for a therapeutic intervention. In recent years, the characterization of downstream effectors to Rho GTPases has increased our understanding of the general cellular effects that permit aberrant proliferation and motility of tumor cells. In addition, several transcription factors have been identified to play important roles at various levels of Rho-induced tumorigenesis. Accordingly, drugs that specifically alter Rho signaling display antineoplastic properties both at the level of tumor growth and tumor metastasis. In this review, a brief summary of the progress made in understanding the biological functions elicited by Rho GTPases that contribute to tumor biology will be made. In addition, a description of new drugs available targeted to specific elements of Rho signaling with antineoplastic or antimetastatic activity is included.

Postgenomic global analysis of translational control induced by oncogenic signaling

It is commonly assumed that developmental and oncogenic signaling achieve their phenotypic effects primarily by directly regulating the transcriptional profile of cells. However, there is growing evidence that the direct effect on transcription may be overshadowed by differential effects on the translational efficiency of specific existing mRNA species. Global analysis of this effect using microarrays indicates that this mechanism of controlling protein production provides a highly specific, robust, and rapid response to oncogenic and developmental stimuli. The mRNAs so affected encode proteins involved in cell-cell interaction, signal transduction, and growth control. Furthermore, a large number of transcription factors capable of secondarily rearranging the transcriptional profile of the cell are controlled at this level as well. To what degree this translational control is either necessary or sufficient for tumor formation or maintenance remains to be determined.

p27Kip1 modulates cell migration through the regulation of RhoA activation

The tumor suppressor p27(Kip1) is an inhibitor of cyclin/cyclin-dependent kinase (CDK) complexes and plays a crucial role in cell cycle regulation. However, p27(Kip1) also has cell cycle-independent functions. Indeed, we find that p27(Kip1) regulates cell migration, as p27(Kip1)-null fibroblasts exhibit a dramatic decrease in motility compared with wild-type cells. The regulation of motility by p27(Kip1) is independent of its cell-cycle regulatory functions, as re-expression of both wild-type p27(Kip1) and a mutant p27(Kip1) (p27CK(-)) that cannot bind to cyclins and CDKs rescues migration of p27(-/-) cells. p27(-/-) cells have increased numbers of actin stress fibers and focal adhesions. This is reminiscent of cells in which the Rho pathway is activated. Indeed, active RhoA levels were increased in cells lacking p27(Kip1). Moreover, inhibition of ROCK, a downstream effector of Rho, was able to rescue the migration defect of p27(-/-) cells in response to growth factors. Finally, we found that p27(Kip1) binds to RhoA, thereby inhibiting RhoA activation by interfering with the interaction between RhoA and its activators, the guanine-nucleotide exchange factors (GEFs). Together, the data suggest a novel role for p27(Kip1) in regulating cell migration via modulation of the Rho pathway.

CD29 integrin- and LIMK1/cofilin-mediated actin reorganization regulates the migration of haematopoietic progenitor cells underneath bone marrow stromal cells

Migration and successive homing of haematopoietic stem/progenitor cells (HS/PCs) into haematopoietic microenvironments are critical to their proliferation and differentiation. To investigate molecular mechanisms underlying HS/PC migration, we used a human erythroleukaemia (HEL) cell line which has been characterized as a haematopoietic progenitor cell line and displays high migratory properties underneath the haematopoietic-supportive stromal cell line, HESS-M28. HEL cell migration is mediated by the adhesion of the CD29 integrin on HEL cells to HESS-28 cells which leads to the localization of filamentous actin and formation of cell polarity at membrane protrusions via actin cytoskeleton reorganization. HEL cell migration is inhibited by both dominant negative forms of the Rho-GTPase family members and a cell permeable inhibitor of LIMK1, S3 peptide. Expression of constitutively active- or inactive-forms of cofilin also inhibits HEL cell migration and phosphorylated cofilin is localized to the front protrusions of HEL cells. These results suggest that cytoskeleton reorganization mediated by a Rho-GTPase/LIMK1/cofilin pathway plays a critical role in the migration of HEL cells underneath HESS-M28 cells.

C-terminal SRC kinase controls acute inflammation and granulocyte adhesion

To establish whether the widely expressed regulator of Src family kinases Csk contributes to the control of acute inflammation in vivo, we inactivated csk in granulocytes by conditional mutagenesis (Cre/loxP). Mutant mice (Csk-GEcre) developed acute multifocal inflammation in skin and lung. Animals were protected from the disease in a microbiologically controlled environment, but remained hypersensitive to LPS-induced shock. Csk-deficient granulocytes showed enhanced spontaneous and ligand-induced degranulation with hyperinduction of integrins. This hyperresponsiveness was associated with hyperadhesion and impaired migratory responses in vitro. Hyperphosphorylation of key signaling proteins such as Syk and Paxillin in mutant granulocytes further supported breakdown of the activation threshold set by Csk. By enforcing the need for ligand engagement Csk thus prevents premature granulocyte recruitment while supporting the motility of stimulated cells through negative regulation of cell adhesion.

ESX induces transformation and functional epithelial to mesenchymal transition in MCF-12A mammary epithelial cells

ESX is an epithelial-restricted member of a large family of transcription factors known as the Ets family. ESX expression has been shown to be correlated with Her2/neu proto-oncogene amplification in highly aggressive breast cancers and induced by Her2/neu in breast cell lines, but its role in tumorigenesis is unknown. Previously, we have shown that ESX enhances breast cell survival in colony-formation assays. In order to determine whether ESX can act as a transforming gene, we stably transfected MCF-12A human mammary epithelial cells with the ESX expression vector, pCGN2-HA-ESX. The MCF-12A cell line is immortalized, but nontransformed, and importantly, these cells fail to express endogenous ESX protein. We used pCGN2-HA-Ets-2 and pSVRas expression vectors as positive controls for transformation. Like HA-Ets-2 and V12-Ras, stable expression of ESX induced EGF-independent proliferation, serum-independent MAPK phosphorylation and growth in soft agar. Additionally, stable ESX expression conferred increased cell adhesion, motility and invasion in two-dimensional and transwell filter assays, and an epithelial to mesenchymal morphological transition. In three-dimensional cultures, parental and vector control (pCGN2) cells formed highly organized duct-like structures with evidence of cell polarity, ECM adhesion-dependent proliferation and cell survival, and lack of cellular invasion into surrounding matrix. Remarkably, the ESX stable cells formed solid, disorganized structures, with lack of cell polarity, loss of adhesion junctions and cytokeratin staining and loss of dependence on ECM adhesion for cell proliferation and survival. In addition, ESX cells invaded the surrounding matrix, indicative of a transformed and metastatic phenotype. Taken together, these data show that ESX expression alone confers a transformed and in vitro metastatic phenotype to otherwise normal MCF-12A cells.

Selenium attenuates lipopolysaccharide-induced oxidative stress responses through modulation of p38 MAPK and NF-kappaB signaling pathways

Lipopolysaccharide (LPS) produces reactive oxygen species (ROS) and nitric oxide (NO) in macrophages. These molecules are involved in inflammation associated with endotoxic shock. Selenium (Se), a biologically essential trace element, modulates the functions of many regulatory proteins involved in signal transduction and affects a variety of cellular activities, including cell growth and survival. We demonstrate that Se attenuated LPS-induced ROS and NO production in murine macrophage cultures in vitro. This Se-decreased production of NO was demonstrated by decreases in both mRNA and protein expression for inducible NO synthase (iNOS). The preventive effects of Se on iNOS were p38 mitogen-activated protein kinase- and nuclear factor-kappaB-dependent. Se specifically blocked the LPS-induced activation of p38 but not that of c-jun-N-terminal kinase and extracellular signal-regulated kinase; the p38-specific pathway was confirmed using p38 inhibitor SB 203580. These results suggest that the mechanism by which Se may act as an anti-inflammatory agent and that Se may be considered as a possible preventive intervention for endotoxemia, particularly in Se-deficient locations. However, the efficacy and safety of Se need to be further investigated, because long-term intake > 0.4 mg Se/day in adults can produce adverse effects.

Statins inhibit osteoblast migration by inhibiting Rac-Akt signaling

Cell migration is a key event in repair and remodeling of skeletal tissues, but the mechanism of osteoblast migration has not been resolved. Statins, which are inhibitors of 3-hydroxy-3-methylglutaryl CoA reductase, increase bone. However, the effect of statins on osteoblast migration remains to be clarified. We investigated the effect of fluvastatin and mevastatin on platelet-derived growth factor (PDGF)-induced migration of osteoblastic MC3T3-E1 cells. PDGF promoted osteoblast migration, while the statins inhibited PDGF-induced migration, and mevalonate and geranylgeranylpyrophosphate but not farnesylpyrophosphate abolished the effect of statins. Dominant-negative Rac severely inhibited PDGF-induced osteoblast migration and reduced Akt phosphorylation. Further, fluvastatin reduced Akt phosphorylation and dominant-negative Akt inhibited PDGF-induced osteoblast migration. These results demonstrate that statins inhibit PDGF-induced osteoblast migration and Rac-Akt signaling plays an important role in the osteoblast migration, and suggest that statins restrain Rac function by inhibiting geranylgeranylation of Rac, which leads to the reduction in Akt activation and osteoblast migration.

Involvement of Rho family GTPases in p19Arf- and p53-mediated proliferation of primary mouse embryonic fibroblasts

The Rho family GTPases Rac1, RhoA, and Cdc42 function as molecular switches that transduce intracellular signals regulating gene expression and cell proliferation as well as cell migration. p19(Arf) and p53, on the other hand, are tumor suppressors that act both independently and sequentially to regulate cell proliferation. To investigate the functional interaction and cooperativeness of Rho GTPases with the p19(Arf)-p53 pathway, we examined the contribution of Rho GTPases to the gene transcription and cell proliferation unleashed by deletion of p19Arf or p53 in primary mouse embryo fibroblasts. We found that (i) p19(Arf) or p53 deficiency led to a significant increase in PI 3-kinase activity, which in turn upregulated RhoA and Rac1 activities; (ii) deletion of p19Arf or p53 led to an increase in cell growth rate that was in part dependent on RhoA, Rac1, and Cdc42 activities; (iii) p19(Arf) or p53 deficiency caused an enhancement of the growth-related transcription factor NF-kappa B and cyclin D1 activities that are partly dependent on RhoA or Cdc42 but not on Rac1; (iv) forced expression of the activating mutants of Rac1, RhoA, or Cdc42 caused a hyperproliferative phenotype of the p19Arf(-/-) and p53(-/-) cells and promoted transformation of both cells; (v) RhoA appeared to contribute to p53-regulated cell proliferation by modulating cell cycle machinery, while hyperactivation of RhoA further suppressed a p53-independent apoptotic signal; and (vi) multiple pathways regulated by RhoA, including that of Rho-kinase, were required for RhoA to fully promote the transformation of p53(-/-) cells. Taken together, these results provide strong evidence indicating that signals through the Rho family GTPases can both contribute to cell growth regulation by p19Arf and p53 and cooperate with p19Arf or p53 deficiency to promote primary cell transformation.

Dominant active Rac and dominant negative Rac revert the dominant active Ras phenotype in Colletotrichum trifolii by distinct signalling pathways

The small G-protein superfamily is an evolutionarily conserved group of GTPases that regulate diverse signalling pathways including pathways for growth and development in eukaryotes. Previously, we showed that dominant active mutation in the unique Ras gene (DARas) of the fungal phytopathogen Colletotrichum trifolii displays a nutrient-dependent phenotype affecting polarity, growth and differentiation. Signalling via the MAP kinase pathway is significantly impaired in this mutant as well. Here we describe the cloning and functional characterization of Rac (Ct-Rac1), a member of the Rho family of G proteins. Ct-Rac1 expression is downregulated by DARas under limiting nutrition. Co-expression of DARas with dominant active Rac (DARac) stimulates MAPK activation and restores the wild-type phenotype. Inhibition of MAPK activation suppresses phenotypic restoration suggesting Rac-mediated MAPK activation is responsible for reversion to the wild-type phenotype. We also examined the role of reactive oxygen species (ROS) in these genetic backgrounds. The DARas mutant strain generates high levels of ROS as determined by DCFH-DA fluorescence. Co-expression with DNRac decreases ROS generation to wild-type levels and restores normal fungal growth and development. Pretreatment of DARas with antioxidants or a cytosolic phospholipase A2 inhibitor also restores the wild-type phenotype. These findings suggest that Ras-mediated ROS generation is dependent on a Rac-cPLA(2)-linked signalling pathway. Taken together, this study provides evidence that Rac functions to restore the hyphal morphology of DARas by regulating MAPK activation and intracellular ROS generation.

Imaging filopodia dynamics in the mouse blastocyst

During mammalian development, the first cell lineage diversification event occurs in the blastocyst, when the trophectoderm (TE) and the inner cell mass (ICM) become established. Part of the TE (polar) remains in contact with the ICM and differs from the mural TE (mTE) which is separated from the ICM by a cavity known as the blastocoele. The presence of filopodia connecting ICM cells with the distant mural TE cells through the blastocoelic fluid was investigated in this work. We describe two types of actin-based cell projections found in freshly dissected and in vitro cultured expanding blastocysts: abundant short filopodia projecting into the blastocoelic cavity that present a continuous undulating behavior; and long, thin traversing filopodia connecting the mural TE with the ICM. Videomicroscopy analyses revealed the presence of vesicle-like structures moving along traversing filopodia and dynamic cytoskeletal rearrangements. These observations, together with immunolocalization of the FGFR2 and the ErbB3 receptors to these cell extensions, suggest that they display signal transduction activity. We propose that traversing filopodia are employed by mitotic mTE cells to receive the required signals for cell division after they become distant to the ICM.

Inhibition of Intimal Thickening in the Rat Carotid Artery Injury Model by a Nontoxic Ras Inhibitor

BACKGROUND

Neointimal formation with and without previous vascular injury is common after balloon dilation and in transplant arteriosclerosis. It involves proliferation and migration of medial smooth muscle cells and inflammation, processes that are regulated by Ras proteins and their down-stream effectors. Farnesylthiosalicylate (FTS) is a Ras inhibitor that interferes with Ras membrane anchorage and affects Ras proteins in their active state. In the present study, we tested the hypothesis that systemic administration of FTS will suppress intimal thickening in the rat carotid injury model.

METHODS AND RESULTS

The effects of FTS on rat vascular smooth muscle cells (VSMC) and splenocytes proliferation were evaluated in vitro. The in vivo effects of FTS on the neointima of balloon-injured male Wistar rats, treated daily for 2 weeks with FTS (5 mg/kg weight, intraperitoneally) were evaluated by determination of Ras, Ras-GTP, and active ERK levels (3 days after injury), and by quantitative determination of the extent of intimal thickening and immunohistochemistry for Ras, iNOS, NFkB, and Ki-67 (2 weeks after injury). FTS inhibited VSMC and splenocyte proliferation as well as interferon-gamma secretion by splenocytes in a dose-dependent manner. Compared with controls, FTS treatment resulted in a strong decrease in Ras-GTP and active ERK, and it significantly reduced intimal thickening after the injury. Ras expression appeared predominantly at areas of neointima regardless of the treatment group. NFkB and iNOS-positive cell numbers were reduced in sections of FTS treated rats.

CONCLUSIONS

FTS appears to act as a potent inhibitor of intimal thickening in a model of experimental arterial injury.

Nuclear Accumulation of Globular Actin as a Cellular Senescence Marker

We evaluated the nuclear actin accumulation as a new marker of cellular senescence, using human diploid fibroblast (HDF), chondrocyte primary cultures, Mv1Lu epithelial cells, and Huh7 cancer cells. Nuclear accumulation of globular actin (G-actin) and dephosphorylated cofilin was highly significant in the senescent HDF cells, accompanied with inhibition of LIM kinase (LIMK) -1 activity. When nuclear export of the actin was induced by 12-O-tetradecanoylphorbol-13-acetate, DNA synthesis of the senescent cells increased significantly, accompanied with changes of morphologic and biochemical profiles, such as increased RB protein phosphorylation and decreased expressions of p21(WAF1), cytoplasmic p-extracellular signal-regulated kinase 1/2, and caveolins 1 and 2. Significance of these findings was strengthened additionally by the fact that nuclear actin export of young HDF cells was inhibited by the treatment with leptomycin B and mutant cofilin transfection, whose LIMK-1 phosphorylation site was lost, and the old cell phenotypes were duplicated with nuclear actin accumulation, suggesting that nuclear actin accumulation was accompanied with G1 arrest during cellular senescence. The aforementioned changes were observed not only in the replicative senescence but also in the senescence induced by treatment of HDF cells, Mv1Lu, primary culture of human chondrocytes, or Huh7 cells with H-ras virus infection, hydroxyurea, deferoxamine, or H(2)O(2). Nuclear actin accumulation was much more sensitive and an earlier event than the well-known, senescence-associated beta-galactosidase activity.

Requirement for Ras guanine nucleotide releasing protein 3 in coupling phospholipase C-gamma2 to Ras in B cell receptor signaling

Two important Ras guanine nucleotide exchange factors, Son of sevenless (Sos) and Ras guanine nucleotide releasing protein (RasGRP), have been implicated in controlling Ras activation when cell surface receptors are stimulated. To address the specificity or redundancy of these exchange factors, we have generated Sos1/Sos2 double- or RasGRP3-deficient B cell lines and determined their ability to mediate Ras activation upon B cell receptor (BCR) stimulation. The BCR requires RasGRP3; in contrast, epidermal growth factor receptor is dependent on Sos1 and Sos2. Furthermore, we show that BCR-induced recruitment of RasGRP3 to the membrane and the subsequent Ras activation are significantly attenuated in phospholipase C-gamma2-deficient B cells. This defective Ras activation is suppressed by the expression of RasGRP3 as a membrane-attached form, suggesting that phospholipase C-gamma2 regulates RasGRP3 localization and thereby Ras activation.

Epithelial-mesenchymal transition and its implications for fibrosis

Epithelial to mesenchymal transition (EMT) is a central mechanism for diversifying the cells found in complex tissues. This dynamic process helps organize the formation of the body plan, and while EMT is well studied in the context of embryonic development, it also plays a role in the genesis of fibroblasts during organ fibrosis in adult tissues. Emerging evidence from studies of renal fibrosis suggests that more than a third of all disease-related fibroblasts originate from tubular epithelia at the site of injury. This review highlights recent advances in the process of EMT signaling in health and disease and how it may be attenuated or reversed by selective cytokines and growth factors.

Epithelial-mesenchymal transition and its implications for fibrosis

Epithelial to mesenchymal transition (EMT) is a central mechanism for diversifying the cells found in complex tissues. This dynamic process helps organize the formation of the body plan, and while EMT is well studied in the context of embryonic development, it also plays a role in the genesis of fibroblasts during organ fibrosis in adult tissues. Emerging evidence from studies of renal fibrosis suggests that more than a third of all disease-related fibroblasts originate from tubular epithelia at the site of injury. This review highlights recent advances in the process of EMT signaling in health and disease and how it may be attenuated or reversed by selective cytokines and growth factors.

Enhanced cathepsin L expression is mediated by different Ras effector pathways in fibroblasts and epithelial cells

Ras expression induces increased expression and altered targeting of lysosomal proteases in multiple cell types, but the specific downstream cytoplasmic signaling pathways mediating these changes have not been identified. In this study, we compared the involvement of 3 major Ras effectors, Raf, phosphatidylinositol 3-kinase (PI3K) and Ral guanine nucleotide exchange factor (RalGEF) in the Ras-mediated alteration of lysosomal protease protein expression and targeting in rat 208F fibroblasts and rat ovarian surface epithelial (ROSE) cells. Effector domain mutants of Ras, constitutively activated variants of Raf, PI3K and RalGEF and pharmacologic inhibitors of MEK and PI3K were utilized to determine the role of these downstream pathways in mediating fibroblast transformation and lysosomal protease regulation in the fibroblasts and epithelial cells. We found that Raf activation of the ERK mitogen-activated protein kinase pathway alone was sufficient to cause morphologic and growth transformation of the fibroblasts and was necessary and sufficient to alter cathepsin L expression and targeting. In contrast, transformation and upregulation of cathepsin L expression in the epithelial cells required the activity of all 3 Ras effectors. Increased protease secretion from the epithelial cells was not observed on ectopic expression of Ras, as it was from the fibroblasts, consistent with the utilization of different signaling pathways in the 2 cell types. In neither cell type did Ras expression increase the expression, processing or secretion of 2 other major lysosomal proteases, cathepsin B and cathepsin D. Thus, Ras utilizes different effectors to mediate transformation and to deregulate cathepsin L expression and secretion in fibroblast and epithelial cells.

RhoA Binds to the Amino Terminus of MEKK1 and Regulates Its Kinase Activity

MEKK1 is a mitogen-activated protein kinase kinase kinase (MAP3K) that can regulate the c-Jun amino-terminal kinase (JNK) MAP kinase cascade. MEKK1 is comprised of a kinase domain and a long amino-terminal regulatory domain. This amino-terminal domain has a scaffold function in that it can assemble modules of the JNK and ERK MAP kinase cascades. Recently, we have demonstrated that MEKK1 binds to p115 Rho GTPase-activating protein, which has GTPase-activating protein activity toward RhoA. Thus, we tested whether Rho GTPases interact with the regulatory domain of MEKK1. RhoA, but not Rac or Cdc42, binds to a site in the aminoterminal one-third of MEKK1, which includes its PHD domain. The interaction is prevented by mutation of the essential cysteine in the MEKK1 PHD domain. Rho-GTP stimulates the kinase activity of full-length MEKK1 as much as 10-fold toward MEK4 but does not appear to be ubiquitinated by MEKK1 under conditions that result in modification of ERK2. In summary, we have characterized a novel point at which Rho GTPases impinge upon the regulation and function of MEKK1.

Chromosome 13q12 encoded Rho GTPase activating protein suppresses growth of breast carcinoma cells, and yeast two-hybrid screen shows its interaction with several proteins

We have characterized the cDNA for a Rho GTPase activating protein (GAP) mapping to chromosome 13q12. The cDNA was characterized by determining the complete sequence of a 4.8 kb cDNA clone that represents the 5' untranslated region (UTR), the translated region, and the 3' UTR. The protein has a sterile alpha-motif (SAM), a distinct GAP domain, and a conserved START (StAR related lipid transfer) domain. The cDNA has 5 instability motifs (ATTTA) in the 3' UTR and one motif in the translated region between GAP and START domains. The RhoGAP transcript is truncated in some breast carcinoma cell lines and it has low expression in other breast cancer cell lines as compared to a normal breast cell line. We have previously observed the absence of RhoGAP transcript in a breast tumor specimen. A GST-fusion of the RhoGAP was tested for its specificity on RhoA, Cdc42, and Rac1. The protein was most active for RhoA. Transfection of RhoGAP into MCF7 cells significantly inhibited cell growth. The introduction of the RhoGAP construct into MDAMB231 cells that had previously been transfected with a p21 construct did not affect cell proliferation, indicating the involvement of p21 in Rho-mediated proliferation of cancer cells. NIH3T3 cells overexpressing RhoGAP showed considerable inhibition of stress fiber formation. Several cDNAs were identified as RhoGAP interactors by using the yeast two-hybrid assay system. These cDNAs correspond to SWI/SNF, alpha-tubulin, HMG CoA reductase, and TAX1 binding protein (TAX1BP1). The interaction with HMG CoA reductase may partially explain the growth inhibition of breast carcinoma cells by statin class of cholesterol lowering drugs. The biological significance of the interacting proteins is discussed in the context of their involvement in tumorigenesis. Our results indicate that loss of RhoGAP or its altered activity suppresses the growth of breast tumor cells. The presence of various motifs in RhoGAP and its interaction with several other proteins suggest that the protein may regulate Rho signaling in multiple ways and possibly function in a Rho-independent manner.

TSH-activated signaling pathways in thyroid tumorigenesis

Thyrotropin (TSH) is considered the main regulator of thyrocyte differentiation and proliferation. Thus, the characterization of the different signaling pathways triggered by TSH on these cells is of major interest in order to understand the mechanisms implicated in thyroid pathology. In this review we focus on the different signaling pathways involved in TSH-mediated proliferation and their role in thyroid transformation and tumorigenesis. TSH mitogenic activities are mediated largely by cAMP, which in turn may activate protein kinase (PKA)-dependent and independent processes. We analyze the effects of increased cAMP levels and PKA activity during cell cycle progression and the role of this signaling pathway in thyroid tumor initiation. Alternative pathways to PKA in the cAMP-mediated proliferation appear to involve the small GTPases Rap1 and Ras. We analyze the Ras effectors (PI3K, RalGDS and Raf) that are thought to mediate its oncogenic activity, as well as the ability of Ras to induce apoptosis in thyrocytes. Finally, we discuss the activation of the PLC/PKC cascade by TSH in thyroid cells and the role of this signaling pathway in the TSH-mediated proliferation and tumorigenesis.

Regulation of cell polarity and protrusion formation by targeting RhoA for degradation

The Rho family of small guanosine triphosphatases regulates actin cytoskeleton dynamics that underlie cellular functions such as cell shape changes, migration, and polarity. We found that Smurf1, a HECT domain E3 ubiquitin ligase, regulated cell polarity and protrusive activity and was required to maintain the transformed morphology and motility of a tumor cell. Atypical protein kinase C zeta (PKCzeta), an effector of the Cdc42/Rac1-PAR6 polarity complex, recruited Smurf1 to cellular protrusions, where it controlled the local level of RhoA. Smurf1 thus links the polarity complex to degradation of RhoA in lamellipodia and filopodia to prevent RhoA signaling during dynamic membrane movements.

Coactivation of Rac1 and Cdc42 at lamellipodia and membrane ruffles induced by epidermal growth factor

A major function of Rho-family GTPases is to regulate the organization of the actin cytoskeleton; filopodia, lamellipodia, and stress fiber are regarded as typical phenotypes of the activated Cdc42, Rac, and Rho, respectively. Using probes based on fluorescent resonance energy transfer, we report on the spatiotemporal regulation of Rac1 and Cdc42 at lamellipodia and membrane ruffles. In epidermal growth factor (EGF)-stimulated Cos1 and A431 cells, both Rac1 and Cdc42 were activated diffusely at the plasma membrane, followed by lamellipodial protrusion and membrane ruffling. Although Rac1 activity subsided rapidly, Cdc42 activity was sustained at lamellipodia. A critical role of Cdc42 in these EGF-induced morphological changes was demonstrated as follows. First, phorbol 12-myristate 13-acetate, which activated Rac1 but not Cdc42, could not induce full-grown lamellipodia in Cos1 cells. Second, a GTPase-activating protein for Cdc42, KIAA1204/CdGAP, inhibited lamellipodial protrusion and membrane ruffling without interfering with Rac1 activation. Third, expression of the Cdc42-binding domain of N-WASP inhibited the EGF-induced morphological changes. Therefore, Rac1 and Cdc42 seem to synergistically induce lamellipodia and membrane ruffles in EGF-stimulated Cos1 cells and A431 cells.

Phosphatidylinositol 3-kinase and mTOR signaling pathways regulate RNA polymerase I transcription in response to IGF-1 and nutrients

Regulation of ribosomal RNA gene transcription by RNA polymerase I (Pol I) is fundamental to ribosome biogenesis and therefore protein translation capacity and cell growth, yet little is known of the key signaling cascades involved. We show here that insulin-like growth factor-1 (IGF-1)-induced Pol I transcription in HEK293 cells is entirely dependent on phosphatidylinositol 3-kinase (PI3K) activity and, additionally, is modulated by the mammalian target of rapamycin (mTOR), which coordinates Pol I transcription with the availability of amino acids. The mitogen-activated protein kinase (MAPK) pathway is weakly stimulated by IGF-1 in these cells and partly contributes to Pol I transcription regulation. Activation of Pol I transcription by IGF-1 results from enhancement of the activity of the Pol I transcription machinery and increased occupancy by SL1 of the endogenous tandemly repeated ribosomal promoters in vivo. The inputs from PI3K, mTOR, and MAPK pathways converge to direct appropriate rRNA gene expression by Pol I in the nucleolus of mammalian cells in response to environmental cues, such as growth factors and nutrients.

The polybasic region of Ras and Rho family small GTPases: a regulator of protein interactions and membrane association and a site of nuclear localization signal sequences

Many small GTPases in the Ras and Rho families have a C-terminal polybasic region (PBR) comprised of multiple lysines or arginines. The PBR controls diverse functions of these small GTPases, including their ability to associate with membranes, interact with specific proteins, and localize in subcellular compartments. Different signaling pathways mediated by Ras and Rho family members may converge when the small GTPases are directed by their PBRs to shared binding sites in specific proteins or at cell membranes. The PBR promotes the interactions of small GTPases with SmgGDS, which is a nucleocytoplasmic shuttling protein that stimulates guanine nucleotide exchange by small GTPases. The PBR of Rac1 was recently found to have a functional nuclear localization signal (NLS) sequence, which enhances the nuclear accumulation of protein complexes containing SmgGDS and Rac1. Sequence analysis demonstrates that canonical NLS sequences (K-K/R-x-K/R) are present in the PBRs of additional Ras and Rho family members, and are evolutionarily conserved across several phyla. These findings suggest that the PBR regulates the nucleocytoplasmic shuttling of some Ras and Rho family members when they are in protein complexes that are too large to diffuse through nuclear pores. These diverse functions of the PBR indicate its critical role in signaling by Ras and Rho family GTPases.

Concerted regulation of cell dynamics by BNIP-2 and Cdc42GAP homology/Sec14p-like, proline-rich, and GTPase-activating protein domains of a novel Rho GTPase-activating protein, BPGAP1

RhoA, Cdc42, and Rac1 are small GTPases that regulate cytoskeletal reorganization leading to changes in cell morphology and cell motility. Their signaling pathways are activated by guanine nucleotide exchange factors and inactivated by GTPase-activating proteins (GAPs). We have identified a novel RhoGAP, BPGAP1 (for BNIP-2 and Cdc42GAP Homology (BCH) domain-containing, Proline-rich and Cdc42GAP-like protein subtype-1), that is ubiquitously expressed and shares 54% sequence identity to Cdc42GAP/p50RhoGAP. BP-GAP1 selectively enhanced RhoA GTPase activity in vivo although it also interacted strongly with Cdc42 and Rac1. "Pull-down" and co-immunoprecipitation studies indicated that it formed homophilic or heterophilic complexes with other BCH domain-containing proteins. Fluorescence studies of epitope-tagged BPGAP1 revealed that it induced pseudopodia and increased migration of MCF7 cells. Formation of pseudopodia required its BCH and GAP domains but not the proline-rich region, and was differentially inhibited by coexpression of the constitutively active mutant of RhoA, or dominant negative mutants of Cdc42 and Rac1. However, the mutant without the proline-rich region failed to confer any increase in cell migration despite the induction of pseudopodia. Our findings provide evidence that cell morphology changes and migration are coordinated via multiple domains in BPGAP1 and present a novel mode of regulation for cell dynamics by a RhoGAP protein.

Neurotrophin 3 activation of TrkC induces Schwann cell migration through the c-Jun N-terminal kinase pathway

During development and nerve injury, complex interactions between glial cells and neurons are essential for establishing proper nerve function. Neurotrophins play multiple roles in the developing nervous system, including cell survival, growth, and differentiation. Here we show that migration of Schwann cells, isolated from sciatic nerves, is significantly enhanced by neurotrophin 3, but not by nerve growth factor or brain-derived neurotrophic factor. The neurotrophin-3-induced cell migration was also observed in Schwann cells isolated from sciatic nerves of p75NTR-/- mice, indicating that neurotrophin 3 enhances cell migration through TrkC. This effect was blocked by K252a, an inhibitor of the Trk receptor family. Additionally, the neurotrophin-3-induced cell migration depended on Rho GTPases (Rac1 and Cdc42) and c-Jun N-terminal kinase. We obtained the same results with Cos-7 cells expressing TrkC. Taken together, these results suggest that neurotrophin 3 activation of TrkC induces Schwann cell migration through the c-Jun N-terminal kinase signaling pathway.

DroVav, the Drosophila melanogaster homologue of the mammalian Vav proteins, serves as a signal transducer protein in the Rac and DER pathways

Mammalian Vav signal transducer proteins couple receptor tyrosine kinase signals to the activation of the Rho/Rac GTPases, leading to cell differentiation and/or proliferation. The unique and complex structure of mammalian Vav proteins is preserved in the Drosophila melanogaster homologue, DroVav. We demonstrate that DroVav functions as a guanine-nucleotide exchange factor (GEF) for DRac. Drosophila cells overexpressing wild-type (wt) DroVav exhibited a normal morphology. However, overexpression of a truncated DroVav mutant (that functions as an oncogene when expressed in NIH3T3 cells) results in striking changes in the actin cytoskeleton, resembling those usually visible following Rac activation. Dominant-negative DRac abrogated these morphological changes, suggesting that the effect of the truncated DroVav mutant is mediated by activation of DRac. In Drosophila cells, we find that stimulation of the Drosophila EGF receptor (DER) increases tyrosine phosphorylation of DroVav, which in turn associates with tyrosine-phosphorylated DER. In addition, the following results imply that DroVav participates in downstream DER signalling, such as ERK phosphorylation: (a) overexpression of DroVav induces ERK phosphorylation; and (b) 'knockout' of DroVav by RNA interference blocks ERK phosphorylation induced by DER stimulation. Unlike mammalian Vav proteins, DroVav was not found to induce Jnk phosphorylation under the experimental circumstances tested in fly cells. These results establish the role of DroVav as a signal transducer that participates in receptor tyrosine kinase pathways and functions as a GEF for the small RhoGTPase, DRac.

Inhibition of protein geranylgeranylation induces apoptosis in myeloma plasma cells by reducing Mcl-1 protein levels

HMG-CoA reductase is the rate-limiting enzyme of the mevalonate pathway leading to the formation of cholesterol and isoprenoids such as farnesylpyrophosphate (FPP) and geranylgeranylpyrophosphate (GGPP). The inhibition of HMG-CoA reductase by lovastatin induced apoptosis in plasma cell lines and tumor cells from patients with multiple myeloma. Here we show that cotreatment with mevalonate or geranylgeranyl moieties, but not farnesyl groups, rescued myeloma cells from lovastatin-induced apoptosis. In addition, the inhibition of geranylgeranylation by specific inhibition of geranylgeranyl transferase I (GGTase I) induced the apoptosis of myeloma cells. Apoptosis triggered by the inhibition of geranylgeranylation was associated with reduction of Mcl-1 protein expression, collapse of the mitochondrial transmembrane potential, expression of the mitochondrial membrane protein 7A6, cytochrome c release from mitochondria into the cytosol, and stimulation of caspase-3 activity. These results imply that protein geranylgeranylation is critical for regulating myeloma tumor cell survival, possibly through regulating Mcl-1 expression. Our results show that pharmacologic agents such as lovastatin or GGTase inhibitors may be useful in the treatment of multiple myeloma.

Akt phosphorylation of serine 21 on Pak1 modulates Nck binding and cell migration

The p21-activated protein kinases (Paks) regulate cellular proliferation, differentiation, transformation, and survival through multiple downstream signals. Paks are activated directly by the small GTPases Rac and Cdc42 and several protein kinases including Akt and PDK-1. We found that Akt phosphorylated and modestly activated Pak1 in vitro. The major site phosphorylated by Akt on Pak1 mapped to serine 21, a site originally shown to be weakly autophosphorylated on Pak1 when Cdc42 or Rac activates it. A peptide derived from the region surrounding serine 21 was a substrate for Akt but not Pak1 in vitro, and Akt stimulated serine 21 phosphorylation on the full-length Pak1 much better than Rac did. The adaptor protein Nck binds Pak near serine 21, and its association is regulated by phosphorylation of this site. We found that either treatment of Pak1 in vitro with Akt or coexpression of constitutively active Akt with Pak1 reduced Nck binding to Pak1. In HeLa cells, green fluorescent protein-tagged Pak1 was concentrated at focal adhesions and was released when Akt was cotransfected. A peptide containing the Nck binding site of Pak1 fused to a portion of human immunodeficiency virus Tat to allow it to enter cells was used to test the functional importance of Nck/Pak binding in Akt-stimulated cell migration. This Tat-Nck peptide reduced Akt-stimulated cell migration. Together, these data suggest that Akt modulates the association of Pak with Nck to regulate cell migration.

Rho-Associated Protein Kinase Contributes to Early Atherosclerotic Lesion Formation in Mice

Members of the Rho family of small GTPases have been recently implicated in inflammatory signaling. We examined the effect of in vivo inhibition of Rho kinase on atherogenesis in mice. Low-density lipoprotein receptor (LDLR) knockout (KO) mice fed a cholate-free high-fat diet received daily intraperitoneal injection of saline (n=8, control group) or Y-27632 (30 mg/kg, n=9), a specific Rho kinase inhibitor. After 9 weeks, Y-27632 treatment resulted in significant in vivo inhibition of Rho kinase activity ( P =0.004). Body weights, arterial blood pressures, and plasma cholesterol levels were comparable in both groups. Atherosclerotic lesion size in the aortic sinus and thoracic aorta of mice treated with Y-27632 was reduced by respectively 35% and 29% in comparison with the saline-treated animals ( P =0.006 and P =0.03, respectively). This was associated with a significant reduction in T lymphocyte accumulation ( P =0.035) and expression of p65 subunit of NF-κB within plaques ( P <0.05). In vitro, treatment with Y-27632 inhibited p65 phosphorylation and degradation of IκBα in mouse peritoneal macrophages and significantly inhibited concanavalin A-induced proliferation of spleen-derived T cells ( P <0.001). In conclusion, inhibition of Rho kinase significantly limits early atherosclerotic plaque development in the LDLR KO mice. This study identifies Rho kinase inhibitors as potential candidates for the treatment of atherosclerosis.

Rac2 specificity in macrophage integrin signaling: potential role for Syk kinase

Herein we report that, despite the similarity of Rac2 to Rac1 (92% amino acid identity), macrophages derived from Rac2-/- mice, which continue to express Rac1, display a marked defect in alphavbeta3/alphavbeta5 and alpha4beta1 integrin-directed migration measured on vitronectin and fibronectin fragments (FN-H296), respectively. In contrast, mouse embryo fibroblasts derived from the Rac2 knockout mice utilize Rac1 for migration via alphavbeta3/alphavbeta5 and alpha4beta1. The genetic reconstitution of bone marrow-derived macrophages (BMM) with Rac2 restores the integrin-dependent migration of Rac2-deficient macrophages on vitronectin (VN) and FN-H296. The levels of GTP-Rac2 generated upon specific integrin engagement in wild type macrophages parallels the phenotypic defect observed in Rac2-deficient macrophages; i.e. FN-H296, alpha4beta1 > VN, alphavbeta3/alphavbeta5 > FN-CH271, alpha5beta1 > intact FN. In a COS7 cell system, the expression of Syk kinase alone is sufficient to convert the alpha4beta1 migration response to Rac2 dependence. Therefore, we present the first evidence that the alpha4beta1 receptor in blood cells has evolved a Syk-Rac2 signaling axis to transmit signals required for integrin-directed migration suggesting that Syk kinase in part encodes myeloid Rac2 specificity in vivo.

The eps8 family of proteins links growth factor stimulation to actin reorganization generating functional redundancy in the Ras/Rac pathway

Sos-1, a guanine nucleotide exchange factor (GEF), eps8 and Abi1, two signaling proteins, and the lipid kinase phosphoinositide 3-kinase (PI3-K), assemble in a multimolecular complex required for Rac activation leading to actin cytoskeletal remodeling. Consistently, eps8 -/- fibroblasts fail to form membrane ruffles in response to growth factor stimulation. Surprisingly, eps8 null mice are healthy, fertile, and display no overt phenotype, suggesting the existence of functional redundancy within this pathway. Here, we describe the identification and characterization of a family of eps8-related proteins, comprising three novel gene products, named eps8L1, eps8L2, and eps8L3. Eps8Ls display collinear topology and 27-42% identity to eps8. Similarly to eps8, eps8Ls interact with Abi1 and Sos-1; however, only eps8L1 and eps8L2 activate the Rac-GEF activity of Sos-1, and bind to actin in vivo. Consistently, eps8L1 and eps8L2, but not eps8L3, localize to PDGF-induced, F-actin-rich ruffles and restore receptor tyrosine kinase (RTK)-mediated actin remodeling when expressed in eps8 -/- fibroblasts. Thus, the eps8Ls define a novel family of proteins responsible for functional redundancy in the RTK-activated signaling pathway leading to actin remodeling. Finally, the patterns of expression of eps8 and eps8L2 in mice are remarkably overlapping, thus providing a likely explanation for the lack of overt phenotype in eps8 null mice.

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.

Ras induces vascular smooth muscle cell senescence and inflammation in human atherosclerosis

BACKGROUND

Vascular cells have a finite cell lifespan and eventually enter an irreversible growth arrest, cellular senescence. The functional changes associated with cellular senescence are thought to contribute to human aging and age-related vascular disorders. Ras, an important signaling molecule involved in atherogenic stimuli, is known to promote aging in yeast and cellular senescence in primary human fibroblasts. The aim of this study was to investigate the role of Ras-induced vascular smooth muscle cell (VSMC) senescence in atherogenesis.

METHODS AND RESULTS

We introduced an activated ras allele (H-rasV12) into human VSMCs using retroviral infection. Introduction of H-rasV12 induced a growth arrest with phenotypic characteristics of cellular senescence, such as enlarged cell shapes and increases in expression of cyclin-dependent kinase inhibitors and senescence-associated beta-galactosidase (SA-beta-gal) activity. Activation of Ras drastically increased expression of proinflammatory cytokines, in part through extracellular signal-regulated kinase activation. To determine whether Ras activation induces cellular senescence in vivo, we transduced the adenoviral vector encoding H-rasV12 into rat carotid arteries injured by a balloon catheter. Introduction of Ras into the arteries enhanced vascular inflammation and senescence compared with mock-infected injured arteries. Moreover, SA-beta-gal-positive VSMCs were detected in the intima of advanced human atherosclerotic lesions and exhibited increased levels of extracellular signal-regulated kinase activity and proinflammatory cytokine expression.

CONCLUSIONS

Our results suggest that atherogenic stimuli mediated by Ras induce VSMC senescence and vascular inflammation, thereby contributing to atherogenesis. This novel mechanism of atherogenesis may provide insights into a new antisenescence treatment for atherosclerosis.

Skeletal muscle signaling pathway through the dystrophin glycoprotein complex and Rac1

The dystrophin glycoprotein complex has been proposed to be involved in signal transduction. Here we have shown that laminin binding causes syntrophin to recruit Rac1 from the rabbit skeletal muscle. Laminin-Sepharose and syntrophin-Sepharose bind a protein complex containing Rac1 from the muscle membranes. The presence of heparin, which inhibits laminin interactions, prevents recruitment of Rac1. The dystrophin glycoprotein complex recruits Rac1 via syntrophin through a Grb2.Sos1 complex. A syntrophin antibody also prevents recruitment of Rac1, suggesting that the signaling complex requires syntrophin. PAK1 is in turn bound by Rac1. c-Jun NH2-terminal kinase-p46 is phosphorylated and activated only when laminin is present, and the p54 isoform is activated when laminin is depleted or binding is inhibited with heparin. In the presence of laminin, c-Jun is activated in both skeletal muscle microsomes and in C2C12 myoblasts, and proliferation increases in C2C12 myoblasts. We postulate that this pathway signals muscle homeostasis and hypertrophy.

Protein profile of tax-associated complexes

Infection with human T-cell leukemia virus type 1 (HTLV-1) results in adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. Tax, a 40-kDa protein, regulates viral and cellular transcription, host signal transduction, the cell cycle, and apoptosis. Tax has been shown to modulate cellular CREB and NFkappaB pathways; however, to date, its role in binding to various host cellular proteins involved in tumorigenesis has not been fully described. In this study, we describe the Tax-associated proteins and their functions in cells using several approaches. Tax eluted from a sizing column mostly at an apparent molecular mass of 1800 kDa. Following Tax immunoprecipitation, washes with high salt buffer, two-dimensional gel separation, and mass spectrometric analysis, a total of 32 proteins was identified. Many of these proteins belong to the signal transduction and cytoskeleton pathways and transcription/chromatin remodeling. A few of these proteins, including TXBP151, have been shown previously to bind to Tax. The interaction of Tax with small GTPase-cytoskeleton proteins, such as ras GAP1m, Rac1, Cdc42, RhoA, and gelsolin, indicates how Tax may regulate migration, invasion, and adhesion in T-cell cancers. Finally, the physical and functional association of Tax with the chromatin remodeling SWI/SNF complex was assessed using in vitro chromatin remodeling assays, chromatin remodeling factor BRG1 mutant cells, and RNA interference experiments. Collectively, Tax is able to bind and regulate many cellular proteins that regulate transcription and cytoskeletal related pathways, which might explain the pleiotropic effects of Tax leading to T-cell transformation and leukemia in HTLV-1-infected patients.