Regulation of Rac and Cdc42 pathways by G(i) during lysophosphatidic acid-induced cell spreading

Lysophosphatidic Acid Regulates Rho Family of GTPases in Lungs

The bio-active lipid, lysophosphatidic acid (LPA) interacts with various lysophosphatidic acid receptors (LPARs) to affect a variety of cellular functions, including proliferation, differentiation, survival, migration, morphogenesis and others. The Rho family of small GTPases, is well-known downstream signaling pathways activated by LPA. Among the Rho GTPases, RhoA, Rac1, and Cdc42 are best characterized and LPA-induced activation of the GTPases RhoA, Rac1, and Cdc42 influences a wide range of cellular processes and functions such as cell differentiation, contractile movements, cellular migration, or infiltration. In this review, we will briefly discuss the interplay between LPA and each of these three Rho family proteins, summarizing the main interactions between them. Our discussion will focus mainly on their interplay within lung endothelial and epithelial cells, drawing attention to how these interactions may contribute to pro-inflammatory processes.

Ovarian Cancer Dissemination-A Cell Biologist's Perspective

Epithelial ovarian cancer (EOC) comprises multiple disease states representing a variety of distinct tumors that, irrespective of tissue of origin, genetic aberrations and pathological features, share common patterns of dissemination to the peritoneal cavity. EOC peritoneal dissemination is a stepwise process that includes the formation of malignant outgrowths that detach and establish widespread peritoneal metastases through adhesion to serosal membranes. The cell biology associated with outgrowth formation, detachment, and de novo adhesion is at the nexus of diverse genetic backgrounds that characterize the disease. Development of treatment for metastatic disease will require detailed characterization of cellular processes involved in each step of EOC peritoneal dissemination. This article offers a review of the literature that relates to the current stage of knowledge about distinct steps of EOC peritoneal dissemination, with emphasis on the cell biology aspects of the process.

Gut microbial metabolite butyrate protects against proteinuric kidney disease through epigenetic- and GPR109a-mediated mechanisms

Butyrate is a short-chain fatty acid derived from the metabolism of indigestible carbohydrates by the gut microbiota. Butyrate contributes to gut homeostasis, but it may also control inflammatory responses and host physiology in other tissues. Butyrate inhibits histone deacetylases, thereby affecting gene transcription, and also signals through the metabolite-sensing G protein receptor (GPR)109a. We produced an mAb to mouse GPR109a and found high expression on podocytes in the kidney. Wild-type and Gpr109a^-/- mice were induced to develop nephropathy by a single injection of Adriamycin and treated with sodium butyrate or high butyrate-releasing high-amylose maize starch diet. Butyrate improved proteinuria by preserving podocyte at glomerular basement membrane and attenuated glomerulosclerosis and tissue inflammation. This protective phenotype was associated with increased podocyte-related proteins and a normalized pattern of acetylation and methylation at promoter sites of genes essential for podocyte function. We found that GPR109a is expressed by podocytes, and the use of Gpr109a^-/- mice showed that the protective effects of butyrate depended on GPR109a expression. A prebiotic diet that releases high amounts of butyrate also proved highly effective for protection against kidney disease. Butyrate and GPR109a play a role in the pathogenesis of kidney disease and provide one of the important molecular connections between diet, the gut microbiota, and kidney disease.-Felizardo, R. J. F., de Almeida, D. C., Pereira, R. L., Watanabe, I. K. M., Doimo, N. T. S., Ribeiro, W. R., Cenedeze, M. A., Hiyane, M. I., Amano, M. T., Braga, T. T., Ferreira, C. M., Parmigiani, R. B., Andrade-Oliveira, V., Volpini, R. A., Vinolo, M. A. R., Mariño, E., Robert, R., Mackay, C. R., Camara, N. O. S. Gut microbial metabolite butyrate protects against proteinuric kidney disease through epigenetic- and GPR109a-mediated mechanisms.

Protease-activated receptor 2 induces migration and promotes Slug-mediated epithelial-mesenchymal transition in lung adenocarcinoma cells

Protease-activated receptor 2 (PAR2), a G protein-coupled receptor for trypsin, contributes to growth, anti-apoptosis, and migration in lung cancer. Given that PAR2 activation in airway epithelial cells compromises the airway epithelium barrier by disruption of E-cadherin adhesion, PAR2 may be involved in epithelial-mesenchymal transition (EMT) in lung adenocarcinoma cells. Although PAR2 is known to promote the migration of lung cancer cells, the detailed mechanism of this event is still not clear. Here, we found that PAR2 is highly expressed in several lung adenocarcinoma cell lines. In two lung adenocarcinoma cell lines, CL1-5 and H1299 cells, activation of PAR2 induces migration and Slug-mediated EMT. The underlying mechanisms involved in PAR2-induced migration and EMT in CL1-5 cells were further investigated. We showed that PAR2-induced migration of CL1-5 cells is mediated by the Src/p38 mitogen-activated protein kinase (p38 MAPK) signaling pathway. β-arrestin 1, not G protein, is involved in this PAR2-mediated Src/p38 MAPK signaling pathway. PAR2-induced EMT in CL1-5 cells is dependent on the activation of extracellular-signal-regulated kinase 2 (ERK2). The activation of ERK2 further mediates Slug stabilization through suppressing the activity of glycogen synthase kinase 3β. In addition, a poor prognosis was observed in lung adenocarcinoma patients with a high expression of PAR2. Thus, PAR2 regulates migration through β-arrestin 1-dependent activation of p38 MAPK and EMT through ERK2-mediated stabilization of Slug in lung adenocarcinoma cells. Our finding also suggests that PAR2 might serve as a therapeutic target for metastatic lung adenocarcinoma and a potential biomarker for predicting the prognosis of lung adenocarcinoma.

RA and ω-3 PUFA co-treatment activates autophagy in cancer cells

Retinoic acid (RA), is a promising therapeutic agent for the treatment of breast cancer. However, metabolic disorders and drug resistance reduce the efficacy of RA. In this study, we found that RA and ω-3 polyunsaturated fatty acids (ω-3 PUFAs) synergistically induced cell death in vitro and in vivo and autophagy activation. Moreover, RA-induced hypercholesterolemia was completely corrected by ω-3 PUFA supplementation. In addition, we demonstrated that the effects of this combination on the autophagic flux were independent of the two major canonic regulatory complexes controlling autophagic vesicle formation. The treatment activated Gαq-p38 MAPK signaling pathways, which resulted in autophagy of breast cancer cells. Knockdown of Gαq or P38 expression prevented RA and ω-3 PUFAs from inducing autophagy. Data indicated that Gαq-p38activation was mediated by the co-activation of GPR40 and RARα in lipid rafts, rather than by the activation of GPR120, RARβ, or RARγ. The results of this study suggest that hyperlipidemic drug side effects may be ameliorated by the administration of ω-3 PUFAs. Thus, the therapeutic indexes of the corresponding drugs may be increased.

PTEN controls glandular morphogenesis through a juxtamembrane β-Arrestin1/ARHGAP21 scaffolding complex

PTEN controls three-dimensional (3D) glandular morphogenesis by coupling juxtamembrane signaling to mitotic spindle machinery. While molecular mechanisms remain unclear, PTEN interacts through its C2 membrane-binding domain with the scaffold protein β-Arrestin1. Because β-Arrestin1 binds and suppresses the Cdc42 GTPase-activating protein ARHGAP21, we hypothesize that PTEN controls Cdc42 -dependent morphogenic processes through a β-Arrestin1-ARHGAP21 complex. Here, we show that PTEN knockdown (KD) impairs β-Arrestin1 membrane localization, β-Arrestin1-ARHGAP21 interactions, Cdc42 activation, mitotic spindle orientation and 3D glandular morphogenesis. Effects of PTEN deficiency were phenocopied by β-Arrestin1 KD or inhibition of β-Arrestin1-ARHGAP21 interactions. Conversely, silencing of ARHGAP21 enhanced Cdc42 activation and rescued aberrant morphogenic processes of PTEN-deficient cultures. Expression of the PTEN C2 domain mimicked effects of full-length PTEN but a membrane-binding defective mutant of the C2 domain abrogated these properties. Our results show that PTEN controls multicellular assembly through a membrane-associated regulatory protein complex composed of β-Arrestin1, ARHGAP21 and Cdc42.

B Lymphocyte-Specific Loss of Ric-8A Results in a Gα Protein Deficit and Severe Humoral Immunodeficiency

Resistance to inhibitors of cholinesterase 8A (Ric-8A) is a highly evolutionarily conserved cytosolic protein initially identified in Caenorhabditis elegans, where it was assigned a regulatory role in asymmetric cell divisions. It functions as a guanine nucleotide exchange factor for Gαi, Gαq, and Gα12/13 and as a molecular chaperone required for the initial association of nascent Gα subunits with cellular membranes in embryonic stem cell lines. To test its role in hematopoiesis and B lymphocytes specifically, we generated ric8 (fl/fl) vav1-cre and ric8 (fl/fl) mb1-cre mice. The major hematopoietic cell lineages developed in the ric8 (fl/fl) vav1-cre mice, notwithstanding severe reduction in Gαi2/3, Gαq, and Gα13 proteins. B lymphocyte-specific loss of Ric-8A did not compromise bone marrow B lymphopoiesis, but splenic marginal zone B cell development failed, and B cells underpopulated lymphoid organs. The ric8 (fl/fl) mb1-cre B cells exhibited poor responses to chemokines, abnormal trafficking, improper in situ positioning, and loss of polarity components during B cell differentiation. The ric8 (fl/fl) mb1-cre mice had a severely disrupted lymphoid architecture and poor primary and secondary Ab responses. In B lymphocytes, Ric-8A is essential for normal Gα protein levels and is required for B cell differentiation, trafficking, and Ab responses.

Rho GTPases in platelet function

The Rho family of GTP binding proteins, also commonly referred to as the Rho GTPases, are master regulators of the platelet cytoskeleton and platelet function. These low-molecular-weight or 'small' GTPases act as signaling switches in the spatial and temporal transduction, and amplification of signals from platelet cell surface receptors to the intracellular signaling pathways that drive platelet function. The Rho GTPase family members RhoA, Cdc42 and Rac1 have emerged as key regulators in the dynamics of the actin cytoskeleton in platelets and play key roles in platelet aggregation, secretion, spreading and thrombus formation. Rho GTPase regulators, including GEFs and GAPs and downstream effectors, such as the WASPs, formins and PAKs, may also regulate platelet activation and function. In this review, we provide an overview of Rho GTPase signaling in platelet physiology. Previous studies of Rho GTPases and platelets have had a shared history, as platelets have served as an ideal, non-transformed cellular model to characterize Rho function. Likewise, recent studies of the cell biology of Rho GTPase family members have helped to build an understanding of the molecular regulation of platelet function and will continue to do so through the further characterization of Rho GTPases as well as Rho GAPs, GEFs, RhoGDIs and Rho effectors in actin reorganization and other Rho-driven cellular processes.

R-Ras is required for murine dendritic cell maturation and CD4+ T-cell priming

R-Ras is a member of the RAS superfamily of small GTP-binding proteins. The physiologic function of R-Ras has not been fully elucidated. We found that R-Ras is expressed by lymphoid and nonlymphoid tissues and drastically up-regulated when bone marrow progenitors are induced to differentiate into dendritic cells (DCs). To address the role of R-Ras in DC functions, we generated a R-Ras-deficient mouse strain. We found that tumors induced in Rras(-/-) mice formed with shorter latency and attained greater tumor volumes. This finding has prompted the investigation of a role for R-Ras in the immune system. Indeed, Rras(-/-) mice were impaired in their ability to prime allogeneic and antigen-specific T-cell responses. Rras(-/-) DCs expressed lower levels of surface MHC class II and CD86 in response to lipopolysaccharide compared with wild-type DCs. This was correlated with a reduced phosphorylation of p38 and Akt. Consistently, R-Ras-GTP level was increased within 10 minutes of lipopolysaccharide stimulation. Furthermore, Rras(-/-) DCs have attenuated capacity to spread on fibronectin and form stable immunologic synapses with T cells. Altogether, these findings provide the first demonstration of a role for R-Ras in cell-mediated immunity and further expand on the complexity of small G-protein signaling in DCs.

LPA(1) -induced migration requires nonmuscle myosin II light chain phosphorylation in breast cancer cells

The enhanced migration found in tumor cells is often caused by external stimuli and the sequential participation of cytoskeleton-related signaling molecules. However, until now, the molecular connection between the lysophosphatidic acid (LPA) receptor and nonmuscle myosin II (NM II) has not been analyzed in detail for LPA-induced migration. Here, we demonstrate that LPA induces migration by activating the LPA(1) receptor which promotes phosphorylation of the 20 kDa NM II light chain through activation of Rho kinase (ROCK). We show that LPA-induced migration is insensitive to pertussis toxin (PTX) but does require the LPA(1) receptor as determined by siRNA and receptor antagonists. LPA activates ROCK and also increases GTP-bound RhoA activity, concomitant with the enhanced membrane recruitment of RhoA. LPA-induced migration and invasion are attenuated by specific inhibitors including C3 cell-permeable transferase and Y-27632. We demonstrate that NM II plays an important role in LPA-induced migration and invasion by inhibiting its cellular function with blebbistatin and shRNA lentivirus directed against NM II-A or II-B. Inhibition or loss of either NM II-A or NM II-B in 4T1 cells results in a decrease in migration and invasion. Restoration of the expression of NM II-A or NM II-B also rescued LPA-induced migration. Taken together, these results suggest defined pathways for signaling through the LPA(1) receptor to promote LPA-mediated NM II activation and subsequent cell migration in 4T1 breast cancer cells.

Cdc42 regulates extracellular matrix remodeling in three dimensions

Extracellular matrix (ECM) actively participates in normal cell regulation and in the process of tumor progression. The Rho GTPase Cdc42 has been shown to regulate cell-ECM interaction in conventional two-dimensional culture conditions by using dominant mutants of Cdc42 in immortalized cell lines that may introduce nonspecific effects. Here, we employ three-dimensional culture systems for conditional gene targeted primary mouse embryonic fibroblasts that better simulate the reciprocal and adaptive interactions between cells and surrounding matrix to define the role of Cdc42 signaling pathways in ECM organization. Cdc42 deficiency leads to a defect in global cell-matrix interactions reflected by a decrease in collagen gel contraction. The defect is associated with an altered cell-matrix interaction that is evident by morphologic changes and reduced focal adhesion complex formation. The matrix defect is also associated with a reduction in synthesis and activation of matrix metalloproteinase 9 (MMP9) and altered fibronectin deposition patterning. A Cdc42 mutant rescue experiment found that downstream of Cdc42, p21-activated kinase (PAK), but not Par6 or WASP, may be involved in regulating collagen gel contraction and fibronectin organization. Thus, in addition to the previously implicated roles in intracellular regulation of actin organization, proliferation, and vesicle trafficking, Cdc42 is essential in ECM remodeling in three dimensions.

Novel bradykinin signaling in adult rat cardiac myocytes through activation of p21-activated kinase

Although bradykinin (BK) is known to exert effects on the myocardium, its intracellular signaling pathways remain poorly understood. Experiments in other cell types indicated that p21-activated kinase-1 (Pak1), a Ser/Thr kinase downstream of small monomeric G proteins, is activated by BK. We previously reported that the expression of active Pak1 in adult cardiac myocytes induced activation of protein phosphatase 2A and dephosphorylation of myofilament proteins (Ke et al. Circ Res 94: 194–200, 2004). In experiments reported here, we tested the hypothesis that BK signals altered protein phosphorylation in adult rat cardiac myocytes through the activation and translocation of Pak1. Treatment of myocytes with BK resulted in the activation of Pak1 as demonstrated by increased autophosphorylation at Thr423 and a diminished striated localization, which is present in the basal state. BK induced dephosphorylation of both cardiac troponin I and phospholamban. Treatment of isolated myocytes with BK also blunted the effect of isoproterenol to enhance peak Ca²⁺ and relaxation of Ca²⁺ transients. Protein phosphatase 2A was demonstrated to associate with both Pak 1 and phospholamban. Our studies indicate a novel signaling mechanism for BK in adult rat cardiac myocytes and support our hypothesis that Pak 1 is a significant regulator of phosphatase activity in the heart.

Structure and function of heterotrimeric G protein-regulated Rho guanine nucleotide exchange factors

Activation of certain classes of G protein-coupled receptors (GPCRs) can lead to alterations in the actin cytoskeleton, gene transcription, cell transformation, and other processes that are known to be regulated by Rho family small-molecular-weight GTPases. Although these responses can occur indirectly via cross-talk from canonical heterotrimeric G protein cascades, it has recently been demonstrated that Dbl family Rho guanine nucleotide exchange factors (RhoGEFs) can serve as the direct downstream effectors of heterotrimeric G proteins. Heterotrimeric Galpha(12/13), Galpha(q), and Gbetagamma subunits are each now known to directly bind and regulate RhoGEFs. Atomic structures have recently been determined for several of these RhoGEFs and their G protein complexes, providing fresh insight into the molecular mechanisms of signal transduction between GPCRs and small molecular weight G proteins. This review covers what is currently known about the structure, function, and regulation of these recently recognized effectors of heterotrimeric G proteins.

Evaluating Cellular Impedance Assays for Detection of GPCR Pleiotropic Signaling and Functional Selectivity

G-protein—coupled receptors can couple to different signal transduction pathways in different cell types (termed cell-specific signaling) and can activate different signaling pathways depending on the receptor conformation(s) stabilized by the activating ligand (functional selectivity). These concepts offer potential for developing pathway-specific drugs that increase efficacy and reduce side effects. Despite significant interest, functional selectivity has been difficult to exploit in drug discovery, in part due to the burden of multiple assays. Cellular impedance assays use an emerging technology that can qualitatively distinguish Gs, Gi/o, and Gq signaling in a single assay and is thereby suited for studying these pharmacological concepts. Cellular impedance confirmed cell-specific Gs and Gq coupling for the melanocortin-4 receptor and dual Gi and Gs signaling with the cannabinoid-1 (CB1) receptor. The balance of Gi versus Gs signaling depended on the cell line. In CB1-HEKs, Giand Gs-like responses combined to yield a novel impedance profile demonstrating the dynamic nature of these traces. Cellspecific signaling was observed with endogenous D1 receptor in U-2 cells and SK-N-MC cells, yet the pharmacological profile of partial and full agonists was similar in both cell lines. We conclude that the dynamic impedance profile encodes valuable relative signaling information and is sufficiently robust to help evaluate cell-specific signaling and functional selectivity. ( Journal of Biomolecular Screening 2009:246-255)

Regulation of cardiac excitation and contraction by p21 activated kinase-1

Cardiac excitation and contraction are regulated by a variety of signaling molecules. Central to the regulatory scheme are protein kinases and phosphatases that carry out reversible phosphorylation of different effectors. The process of beta-adrenergic stimulation mediated by cAMP dependent protein kinase (PKA) forms a well-known pathway considered as the most significant control mechanism in excitation and contraction as well as many other regulatory mechanisms in cardiac function. However, although dephosphorylation pathways are critical to these regulatory processes, signaling to phosphatases is relatively poorly understood. Emerging evidence indicates that regulation of phosphatases, which dampen the effect of beta-adrenergic stimulation, is also important. We review here functional studies of p21 activated kinase-1 (Pak1) and its potential role as an upstream signal for protein phosphatase PP2A in the heart. Pak1 is a serine/threonine protein kinase directly activated by the small GTPases Cdc42 and Rac1. Pak1 is highly expressed in different regions of the heart and modulates the activities of ion channels, sarcomeric proteins, and other phosphoproteins through up-regulation of PP2A activity. Coordination of Pak1 and PP2A activities is not only potentially involved in regulation of normal cardiac function, but is likely to be important in patho-physiological conditions.

Beta-Bungarotoxin induction of neurite outgrowth in NB41A3 cells

In this study, different concentrations of beta-Bgt were used to treat cultured NB41A3 cells. Inverted phase contrast microscopy was then used 24h after treatment to observe the outgrowth of neurite. We found a clear outgrowth of neurite at beta-Bgt concentrations of 357 nM. However, using a cytotoxicity assay to study apoptosis, we found no significant difference in the rate of cell death in cell cultures treated with either 357 nM or 714 nM. Western blotting showed that after treatment with beta-Bgt, there was a notable decrease in small G protein Cdc42 and a marked increase in RhoA protein. Flow cytometry revealed that beta-Bgt did not alter the calcium influx in NB41A3 cells. The neurite outgrowth induced by beta-Bgt was not affected by extracellular EGTA, suggesting that the internalization of beta-Bgt from extracellular was independent of phospholipase. Taken together, our results suggest the beta-Bgt-induced outgrowth of neurite from NB41A3 cells may be mediated by small G proteins.

Sphingosine-1-phosphate and endothelin-1 induce the expression of rgs16 protein in cardiac myocytes by transcriptional activation of the rgs16 gene

The expression of the negative Regulator of G protein signaling 16 (RGS16) is rapidly induced in cardiomyocytes by various stimuli. To identify the promoter of the mouse RGS16 gene, a 1.8-kb deoxyribonucleic acid fragment 5' of the RGS16-coding region was subcloned into a firefly-luciferase reporter vector and four overlapping fragments were analyzed. The luciferase production was quantified in neonatal rat cardiac myocytes (NRCM). A 0.6-kb fragment that induced a tenfold increase in luciferase activity contained the minimal promoter sequence. Its activity was twofold stimulated by fetal calf serum, endothelin-1 (ET-1), and sphingosine 1-phosphate (S1P), which stimuli also elevated the level of RGS16 protein. Stimulation of NRCM with ET-1 induced activation of the monomeric GTPases RhoA and Rac1, whereas S1P and the selective S1P1 receptor agonist SEW2871 only induced a pronounced activation of Rac1. In accordance, the treatment with the Rho-, Rac-, and Cdc42-inactivating Clostridium difficile Toxin B (TcdB) 10463 inhibited ET-1 and S1P-induced transcriptional activation. The ET-1-induced activation was insensitive to pertussis toxin but selectively suppressed by the RhoA-C-specific C2I-C3 ADP-ribosyl transferase and the ET(B) receptor antagonist BQ788. The S1P-induced activation was specifically inhibited by pertussis toxin and the Rac-inactivating TcdB 1470. All stimulated transcriptional activity was abolished by the negative transcription factor Yin Yang 1 (YY1), which binds to a consensus sequence within the minimal promoter. Taken together, our data show that most likely ET(B)- and S1P1-receptors induce RGS16 protein expression in cardiac myocytes by increasing the transcriptional activity of the rgs16 gene. This activation is mediated by heterotrimeric G proteins, Rho GTPases, and is under negative control of the transcription factor YY1.

Heterotrimeric G protein betagamma subunits stimulate FLJ00018, a guanine nucleotide exchange factor for Rac1 and Cdc42

We previously reported that Gbetagamma signaling regulates cell spreading or cell shape change through activation of a Rho family small GTPase, suggesting the existence of a Gbetagamma-regulated Rho guanine-nucleotide exchange factor (RhoGEF). In this study we examined various RhoGEF clones, found FLJ00018 to beaGbetagamma-activated RhoGEF, and investigated the molecular mechanism of Gbetagamma-induced activation of Rho family GTPases. Co-expression of the genes for FLJ00018 and Gbetagamma enhanced serum response element-mediated gene transcription in HEK-293 cells. Combined expression of Gbetagamma and FLJ00018 significantly induced activation of Rac and Cdc42 but not RhoA. FLJ00018 also enhanced gene transcription induced by carbachol-stimulated m2 muscarinic acetylcholine receptor, and this enhancement was blocked by pertussis toxin. Furthermore, we demonstrated Gbetagamma to interact directly with the N-terminal region of FLJ00018 and the N-terminal fragment of this molecule to inhibit serum response element-dependent transcription induced by Gbetagamma/FLJ00018 and carbachol. In NIH3T3 cells, FLJ00018 enhanced lysophosphatidic acid-induced cell spreading, which was also blocked by the N-terminal fragment of FLJ00018. These results provide evidence for a signaling pathway by which G(i)-coupled receptor specifically induces Rac and Cdc42 activation through direct interaction of Gbetagamma with FLJ00018.

Inhibition of endothelial barrier dysfunction by P21-activated kinase-1

We investigated the activity of P21-activated kinase-1 (Pak1) on myosin light chain phosphorylation and on thrombin-induced barrier dysfunction in human endothelial cells (HMEC). HMEC were infected with recombinant adenoviruses that express constitutively active Pak1, LacZ, wild-type, and a mutant myosin regulatory light chain, mMLC20 (Thr18Ala, Ser19Ala). Expression of the recombinant Pak1 mediated by adenovirus in HMEC was regulated. Active Pak1 induced dephosphorylation of MLC20 in HMEC, but not in smooth muscle cells. Active Pak1 significantly inhibited thrombin-induced endothelial barrier dysfunction. Expression of the unphosphorylatable MLC20 also inhibited thrombin-induced endothelial barrier dysfunction. Constitutively active Pak1 associated with phosphatase 2A and induced a post-translational modification of the phosphatase. Our data provide novel evidence indicating that Pak1 regulates endothelial barrier function through activation of phosphatase 2A.

p21-Activated kinase-1 and its role in integrated regulation of cardiac contractility

We review here a novel concept in the regulation of cardiac contractility involving variations in the activity of the multifunctional enzyme, p21-activated kinase 1 (Pak1), a member of a family of proteins in the small G protein-signaling pathway that is activated by Cdc42 and Rac1. There is a large body of evidence from studies in noncardiac tissue that Pak1 activity is key in regulation of a number of cellular functions, including cytoskeletal dynamics, cell motility, growth, and proliferation. Although of significant potential impact, the role of Pak1 in regulation of the heart has been investigated in only a few laboratories. In this review, we discuss the structure of Pak1 and its sites of posttranslational modification and molecular interactions. We assemble an overview of the current data on Pak1 signaling in noncardiac tissues relative to similar signaling pathways in the heart, and we identify potential roles of Pak1 in cardiac regulation. Finally, we discuss the current state of Pak1 research in the heart in regard to regulation of contractility through functional myofilament and Ca(2+)-flux modification. An important aspect of this regulation is the modulation of kinase and phosphatase activity. We have focused on Pak1 regulation of protein phosphatase 2A (PP2A), which is abundant in cardiac muscle, thereby mediating dephosphorylation of sarcomeric proteins and sensitizing the myofilaments to Ca(2+). We present a model for Pak1 signaling that provides a mechanism for specifically affecting cardiac cellular processes in which regulation of protein phosphorylation states by PP2A dephosphorylation predominates.

GPR92 as a new G12/13- and Gq-coupled lysophosphatidic acid receptor that increases cAMP, LPA5

The signaling effects of lysophospholipids such as lysophosphatidic acid (LPA) are mediated by G protein-coupled receptors (GPCRs). There are currently four LPA receptors known as LPA(1-4). Genetic deletion studies have identified essential biological functions for LPA receptors in mice. However, these studies have also revealed phenotypes consistent with the existence of as yet unidentified receptors. Toward identifying new LPA receptors, we have screened collections of GPCR cDNAs using reverse transfection and cell-based assays. Here we report an interim result of one screen to identify receptors that produced LPA-dependent changes in cell shape: the orphan receptor GPR92 has properties of a new LPA receptor. Sequence analyses of human GPR92 and its mouse homolog have approximately 35% amino acid identity with LPA4/GPR23. The same cell-based approaches that were used to identify and/or characterize LPA(1-4), particularly heterologous expression in B103 cells or RH7777 cells, were utilized and compared with known LPA receptors. Retroviral-mediated expression of epitope-tagged receptors was further combined with G protein minigenes and pharmacological intervention, along with calcium imaging and whole-cell patch clamp electrophysiology. LPA-dependent receptor internalization following exposure to LPA but not related lysophospholipids was observed. Furthermore, LPA induced concentration-dependent activation of G(12/13) and G(q) and increased cAMP levels. Specific [3H]LPA binding was detected in cell membranes heterologously expressing GPR92 but not control membranes. Northern blot and reverse transcriptase-PCR studies indicated a broad low level of expression in many tissues including embryonic brain and enrichment in small intestine and sensory dorsal root ganglia, as well as embryonic stem cells. These results support GPR92 as a fifth LPA receptor, LPA5, which likely has distinct physiological functions in view of its expression pattern.

Insights into seven and single transmembrane-spanning domain receptors and their signaling pathways in human natural killer cells

Human natural killer (NK) cells are important cells of the innate immune system. These cells perform two prominent functions: the first is recognizing and destroying virally infected cells and transformed cells; the second is secreting various cytokines that shape up the innate and adaptive immune re-sponses. For these cells to perform these activities, they express different sets of receptors. The receptors used by NK cells to extravasate into sites of injury belong to the seven transmembrane (7TM) family of receptors, which characteristically bind heterotrimeric G proteins. These receptors allow NK cells to sense the chemotactic gradients and activate second messengers, which aid NK cells in polarizing and migrating toward the sites of injured tissues. In addition, these receptors determine how and why human resting NK cells are mainly found in the bloodstream, whereas activated NK cells extravasate into inflammatory sites. Receptors for chemokines and lysophospholipids belong to the 7TM family. On the other hand, NK cells recognize invading or transformed cells through another set of receptors that belong to the single transmembrane-spanning domain family. These receptors are either inhibitory or activating. Inhibitory receptors contain the immune receptor tyrosine-based inhibitory motif, and activating receptors belong to either those that associate with adaptor molecules containing the immune receptor tyrosine-based activating motif (ITAM) or those that associate with adaptor molecules containing motifs other than ITAM. This article will describe the nature of these receptors and examine the intracellular signaling pathways induced in NK cells after ligating both types of receptors. These pathways are crucial for NK cell biology, development, and functions.

Rho kinases regulate endothelial invasion and migration during valvuloseptal endocardial cushion tissue formation

Rho-associated kinase (ROCK) is a downstream effector of small Rho-GTPases, and phosphorylates several substrates to regulate cell functions, including actin cytoskeletal reorganization and cellular motility. Endothelial-mesenchymal transformation (EMT) is a critical event in the formation of valves and septa during cardiogenesis. It has been reported that ROCK plays an important role in the regulation of endocardial cell differentiation and migration during mouse cardiogenesis (Zhao and Rivkees [2004] Dev. Biol. 275:183-191). Immunohistochemistry showed that, during chick cardiogenesis, ROCK1 and -2 were expressed in the transforming and migrating endothelial/mesenchymal cells in the outflow tract (OT) and atrioventricular (AV) canal regions from which valvuloseptal endocardial cushion tissue would later develop. Treatment with Y27632, a specific ROCK inhibitor, of cultured AV explants or AV endothelial monolayers of stage 14-minus heart (preactivated stage for EMT) on three-dimensional collagen gel perturbed the seeding of mesenchymal cells into the gel lattice. In these experiments, Y27632 did not suppress the expression of an early transformation marker, smooth muscle alpha-actin. Moreover, Y27632 inhibited the mesenchymal invasion in stage 14-18 AV explants, in which endothelial cells had committed to undergo EMT. ML-9, a myosin light chain kinase inhibitor, also inhibited the mesenchymal invasion in cultured AV explants. These results suggest that ROCKs have a critical role in the mesenchymal cell invasion/migration that occurs at the late onset of EMT.

High-density lipoprotein inhibits migration of vascular smooth muscle cells through its sphingosine 1-phosphate component

High-density lipoprotein (HDL) is a well-established anti-risk factor against atherosclerosis, but the mechanism of its anti-atherogenic actions is not fully understood. Here, we examined the role of the HDL-associated sphingosine 1-phosphate (S1P), a lysolipid mediator, in the lipoprotein-induced actions in rat vascular smooth muscle cells (VSMCs). Both HDL and S1P inhibited platelet-derived growth factor-induced migration of VSMCs. The inhibitory effect was associated with an inhibition of cell spreading and these responses were reversed by a desensitization of VSMCs with S1P. HDL and S1P also inhibited migration of Chinese hamster ovary cells and this effect was enhanced by overexpressing S1P2 receptor. Finally, we showed that, even though S1P promoted DNA synthesis, HDL and S1P did not increase cell number of VSMCs. These findings suggest a novel mechanism for anti-atherogenic actions of HDL through its S1P component.

Galphaq/11 signaling induces apoptosis through two pathways involving reduction of Akt phosphorylation and activation of RhoA in HeLa cells

We have previously reported that expression of the constitutively active mutant of Galpha11 or stimulation of m1 muscarinic acetylcholine receptor induced proteolytic activation of Rho-associated kinase (ROCK-I) by caspase and apoptosis in HeLa cells. In this study, we investigate the molecular mechanisms of Galphaq/11-induced apoptosis in m1 muscarinic acetylcholine receptor-expressing HeLa cells. Overexpression of Bcl-2 inhibited carbachol-induced ROCK-I cleavage, indicating a mitochondrial apoptotic pathway. Overexpression of the constitutively active mutant of Akt that delivers an anti-apoptotic survival signal had a similar influence. Insulin, a major survival factor in many cells, strongly increased phosphorylation of Akt, which was completely blocked by carbachol. This latter effect was partially inhibited by treatment with the tyrosine phosphatase inhibitors, orthovanadate and pervanadate. In parallel with these observations, carbachol attenuated insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1, an effect eliminated by orthovanadate. On the other hand, carbachol induced rapid stimulation of endogenous RhoA, and expression of a constitutively active mutant of RhoA increased ROCK-I cleavage. Orthovanadate and the dominant negative mutant of RhoA partially, and their combination completely, inhibited carbachol-induced ROCK-I cleavage and apoptosis. These results demonstrate that Gq/11 signaling induces apoptosis by reducing insulin-stimulated Akt phosphorylation through tyrosine dephosphorylation and activating RhoA in HeLa cells.

Activation of p21-activated kinase 1 is required for lysophosphatidic acid-induced focal adhesion kinase phosphorylation and cell motility in human melanoma A2058 cells

Lysophosphatidic acid (LPA), one of the naturally occurring phospholipids, stimulates cell motility through the activation of Rho family members, but the signaling mechanisms remain to be elucidated. In the present study, we investigated the roles of p21-activated kinase 1 (PAK1) on LPA-induced focal adhesion kinase (FAK) phosphorylation and cell motility. Treatment of human melanoma cells A2058 with LPA increased phosphorylation and activation of PAK1, which was blocked by treatment with pertussis toxin and by inhibition of phosphoinositide 3-kinase (PI3K) with an inhibitor LY294002 or by overexpression of catalytically inactive mutant of PI3Kgamma, indicating that LPA-induced PAK1 activation was mediated via a Gi protein and the PI3Kgamma signaling pathway. In addition, we demonstrated that Rac1/Cdc42 signals acted as upstream effector molecules of LPA-induced PAK activation. However, Rho-associated kinase, MAP kinase kinase 1/2 or phospholipase C might not be involved in LPA-induced PAK1 activation or cell motility stimulation. Furthermore, PAK1 was necessary for FAK phosphorylation by LPA, which might cause cell migration, as transfection of the kinase deficient mutant of PAK1 or PAK auto-inhibitory domain significantly abrogated LPA-induced FAK phosphorylation. Taken together, these findings strongly indicated that PAK1 activation was necessary for LPA-induced cell motility and FAK phosphorylation that might be mediated by sequential activation of Gi protein, PI3Kgamma and Rac1/Cdc42.

Matrix metalloproteinase 1 interacts with neuronal integrins and stimulates dephosphorylation of Akt

Several studies have demonstrated that matrix metalloproteinases (MMPs) are cytotoxic. The responsible mechanisms, however, are not well understood. MMPs may promote cytotoxicity through their ability to disrupt or degrade matrix proteins that support cell survival, and MMPs may also cleave substrates to generate molecules that stimulate cell death. In addition, MMPs may themselves act on cell surface receptors that affect cell survival. Among such receptors is the alpha(2)beta(1) integrin, a complex that has previously been linked to leukocyte death. In the present study we show that human neurons express alpha(2)beta(1) and that pro-MMP-1 interacts with this integrin complex. We also show that stimulation of neuronal cultures with MMP-1 is associated with a rapid reduction in the phosphorylation of Akt, a kinase that can influence caspase activity and cell survival. Moreover, MMP-1-associated dephosphorylation of Akt is inhibited by a blocking antibody to the alpha(2) integrin, but not by batimastat, an inhibitor of MMP-1 enzymatic activity. Such dephosphorylation is also stimulated by a catalytic mutant of pro-MMP-1. Additional studies show that MMP-1 causes neuronal death, which is significantly diminished by both a general caspase inhibitor and anti-alpha(2) but not by batimastat. Together, these results suggest that MMP-1 can stimulate dephosphorylation of Akt and neuronal death through a non-proteolytic mechanism that involves changes in integrin signaling.

Intracellular localization and functional effects of P21-activated kinase-1 (Pak1) in cardiac myocytes

We investigated intracellular localization and substrate specificity of P21-activated kinase-1 (Pak1) in rat cardiac myocytes. Pak1 is a serine/threonine protein kinase that is activated by Rac1/Cdc42 and important in signaling of stress responses. Yet the localization and in vivo function of Pak1 in heart cells is poorly understood. Studies reported here indicate that Pak1 physically interacts with protein phosphatase 2a and localizes to the Z-disk, cell membrane, intercalated disc, and nuclear membrane of adult rat heart myocytes. We compared levels of phosphorylation of cardiac troponin I (cTnI) in control myocytes with phosphorylation of cTnI and myosin binding protein C (C-protein) in myocytes with increased Pak1 activity. The increase in activity was induced by infection of myocytes with a recombinant adenovirus (AdPak1) containing cDNA for a constitutively active Pak1. Control cells were infected with a virus (AdLacZ) containing LacZ. Basal levels of phosphorylation of cTnI and C-protein were relatively high in the myocytes infected with AdLacZ. However, phosphorylation of cTnI and C-protein in cells expressing constitutively active Pak1 was significantly reduced compared with those expressing LacZ. Measurement of Ca2+ tension relations in single myocytes demonstrated that this reduction in phosphorylation of cTnI and C-protein was associated with the predicted increase in sensitivity to Ca2+. Our data provide evidence for a novel pathway of phosphatase regulation in cardiac myocytes.

Reciprocal interactions between cells and extracellular matrix during remodeling of tissue constructs

Cells remodel extracellular matrix during tissue development and wound healing. Similar processes occur when cells compress and stiffen collagen gels. An important task for cell biologists, biophysicists, and tissue engineers is to guide these remodeling processes to produce tissue constructs that mimic the structure and mechanical properties of natural tissues. This requires an understanding of the mechanisms by which this remodeling occurs. Quantitative measurements of the contractile force developed by cells and the extent of compression and stiffening of the matrix describe the results of the remodeling processes. Not only do forces exerted by cells influence the structure of the matrix but also external forces exerted on the matrix can modulate the structure and orientation of the cells. The mechanisms of these processes remain largely unknown, but recent studies of the regulation of myosin-dependent contractile force and of cell protrusion driven by actin polymerization provide clues about the regulation of cellular functions during remodeling.

Different molecular motors mediate platelet-derived growth factor and lysophosphatidic acid-stimulated floating collagen matrix contraction

Fibroblast-collagen matrix contraction has been used as a model system to study how cells organize connective tissue. Previous work showed that lysophosphatidic acid (LPA)-stimulated floating collagen matrix contraction is independent of Rho kinase, whereas platelet-derived growth factor (PDGF)-stimulated contraction is Rho kinase-dependent. The current studies were carried out to learn more about the molecular motors responsible for LPA- and PDGF-stimulated contraction. We found that neither PDGF nor LPA-dependent contractile mechanisms require myosin II regulatory light chain kinase or increased phosphorylation of myosin II regulatory light chain (measured as diphosphorylation). Low concentrations of the specific myosin II inhibitor blebbistatin blocked PDGF-stimulated matrix contraction and LPA-stimulated retraction of fibroblast dendritic extensions but not LPA-stimulated matrix contraction. These data suggest that PDGF- and LPA-stimulated floating matrix contraction utilize myosin II-dependent and -independent mechanisms, respectively. LPA-dependent, Rho kinase-independent force generation also was detected during fibroblast spreading on collagen-coated coverslips.

Phosphatidic acid has a potential to promote hair growth in vitro and in vivo, and activates mitogen-activated protein kinase/extracellular signal-regulated kinase kinase in hair epithelial cells

Phospholipids have recently been discovered to play an important role in cellular regulation. In this study, we focused on phosphatidic acid and lysophosphatidic acid, which are phospholipids known to possess growth-hormonal effects on several types of cells, and examined their growth-promoting effects on murine hair epithelial cells. We discovered that phosphatidic acid possesses intensive growth-promotional effects on hair epithelial cells and epidermal keratinocytes. In contrast, lyso-phosphatidic acid showed lower growth-promoting effects on hair epithelial cells relative to phosphatidic acid and showed minimal or no growth-promoting activity on epidermal keratinocytes. Phosphatidic acid was also shown to have hair-growing activity to induce the anagen phase of the hair cycle in the in vivo murine model. For the purpose of examining the hair-growing mechanisms of phosphatidic acid, we examined its relationship to the mitogen-activated protein kinase cascade linked to cell proliferation and the transforming growth factor beta signal pathway known to be a regulator of catagen induction. We confirmed that phosphatidic acid activates MEK-1/2 and upregulates the expression of MEK-1/2 in cultured murine hair epithelial cells. Addition of transforming growth factor beta1 to hair epithelial cell cultures concentration-dependently decreased cell growth and induced apoptosis; however, addition of phosphatidic acid to the culture neutralized the growth-inhibiting effects of transforming growth factor beta1 and protected the cells from apoptosis. We speculate that the hair-growing activity of phosphatidic acid is at least linked to its growth-promoting effects on hair epithelial cells that follow mitogen-activated protein kinase/extracellular signal-regulated kinase kinase activation and its protective action on transforming-growth-factor-beta1-induced apoptosis that is assumed to trigger catagen induction in the hair cycle.

CCR7 ligands induce rapid endocytosis in mature dendritic cells with concomitant up-regulation of Cdc42 and Rac activities

Although chemokines are well known to function in chemotaxis, additional roles for these molecules in the immune system are not well understood. Dendritic cells (DCs) developmentally regulate the expression of chemokine receptors to facilitate their migration from the peripheral tissues to regional lymph nodes. Expressions of CCR1 and CCR5 on immature DCs are down-regulated on maturation, whereas CCR7 is selectively expressed on mature DCs. In the present study, we examined the effects of CCL19 and CCL21, 2 CCR7 ligands, on endocytosis of fluorescein isothiocyanate (FITC)-dextran by murine DCs. Both CCL19 and CCL21 markedly induced rapid uptake of FITC-dextran by mature DCs but not immature DCs. In contrast, CCL3, a ligand of CCR1 and CCR5, induced rapid uptake of FITC-dextran by immature DCs but not mature DCs. CCL19-induced endocytosis could be completely blocked by Clostridium difficile toxin B, which inhibits the Rho guanosine triphosphatase proteins, Rho, Rac, and Cdc42. This process was not abrogated by Y-27632, a specific inhibitor of Rho-associated kinase. In addition, CCL19 rapidly enhanced Cdc42 and Rac activity in mature DCs. These findings demonstrate that certain chemokines induce rapid endocytosis in each relevant DC population. It is suggested that CCR7 ligands activate Cdc42 and Rac, thereby inducing the endocytosis in mature DCs.

Activation of Gbetagamma signaling downstream of Wnt-11/Xfz7 regulates Cdc42 activity during Xenopus gastrulation

Wnt-11/Xfz7 signaling plays a major role in the regulation of convergent extension movements affecting the dorsal marginal zone (DMZ) of gastrulating Xenopus embryos. In order to provide data concerning the molecular targets of Wnt-11/Xfz7 signals, we have analyzed the regulation of the Rho GTPase Cdc42 by Wnt-11. In animal cap ectoderm, Cdc42 activity increases as a response to Wnt-11 expression. This increase is inhibited by pertussis toxin, or sequestration of free Gbetagamma subunits by exogenous Galphai2 or Galphat. Activation of Cdc42 is also produced by the expression of bovine Gbeta1 and Ggamma2. This process is abolished by a PKC inhibitor, while phorbol esther treatment of ectodermal explants activates Cdc42 in a PKC-dependent way, implicating PKC downstream of Gbetagamma. In activin-treated animal caps and in the embryo, interference with Gbetagamma signaling rescues morphogenetic movements inhibited by Wnt-11 hyperactivation, thus phenocopying the dominant negative version of Cdc42 (N(17)Cdc42). Conversely, expression of Gbeta1gamma2 blocks animal cap elongation. This effect is reversed by N(17)Cdc42. Together, our results strongly argue for a role of Gbetagamma signaling in the regulation of Cdc42 activity downstream of Wnt-11/Xfz7 in mesodermal cells undergoing convergent extension. This idea is further supported by the observation that expression of Galphat in the DMZ causes severe gastrulation defects.

Overexpression of WISP-1 down-regulated motility and invasion of lung cancer cells through inhibition of Rac activation

Wnt-induced-secreted-protein-1 (WISP-1) is a cysteine-rich, secreted factor belonging to the CCN family. These proteins have been implicated in the inhibition of metastasis; however, the mechanisms involved have not been described. We demonstrated that overexpression of WISP-1 in H460 lung cancer cells inhibited lung metastasis and in vitro cell invasion and motility. We investigated the possibility that WISP-1 may regulate activation of Rac, a small GTPase important for cytoskeletal reorganizations during motility. In an indirect assay, WISP-1-expressing cells exhibited marked reduction in Rac activation compared with control cells. Blocking antibodies to alpha(v)beta(5) and alpha(1) integrins restored Rac activation in WISP-1 cells, suggesting that the inhibitory effect of WISP-1 on Rac lies downstream of integrins. Constitutively activated Rac mutant (RacG12V) was transfected into WISP-1 cells to restore Rac activation and these WISP-1/RacG12V transfectants were used for further studies. We performed microarray and real-time PCR analyses to identify genes involved in invasion that may be differentially regulated by WISP-1. Here, we showed decreased expression of metalloproteinase-1 (MMP-1) in WISP-1 cells compared with controls but increased expression in WISP-1/RacG12V cells. In an invasion assay across collagen I, an MMP-1 target matrix, WISP-1 cells were significantly less invasive compared with controls, whereas WISP-1/RacG12V cells showed elevated invasion levels. This work illustrates a negatively regulated pathway by WISP-1 involving integrins and Rac in the down-regulation of invasion.

Reorganization of actin cytoskeleton by the phosphoinositide metabolite glycerophosphoinositol 4-phosphate

Glycerophosphoinositol 4-phosphate (GroPIns-4P) is a biologically active, water-soluble phospholipase A metabolite derived from phosphatidylinositol 4-phosphate, whose cellular concentrations have been reported to increase in Ras-transformed cells. It is therefore important to understand its biological activities. Herein, we have examined whether GroPIns-4P can regulate the organization of the actin cytoskeleton, because this could be a Ras-related function involved in cell motility and metastatic invasion. We find that in serum-starved Swiss 3T3 cells, exogenously added GroPIns-4P rapidly and potently induces the formation of membrane ruffles, and, later, the formation of stress fibers. These actin structures can be regulated by the small GTPases Cdc42, Rac, and Rho. To analyze the mechanism of action of GroPIns-4P, we selectively inactivated each of these GTPases. GroPIns-4P requires active Rac and Rho, but not Cdc42, for ruffle and stress fiber formation, respectively. Moreover, GroPIns-4P induces a rapid translocation of the green fluorescent protein-tagged Rac into ruffles, and increases the fraction of GTP-bound Rac, in intact cells. The activation of Rac by GroPIns-4P was near maximal and long-lasting. Interestingly, this feature seems to be critical in the induction of actin ruffles by GroPIns-4P.

Cdc42 and Rac1 are necessary for autotaxin-induced tumor cell motility in A2058 melanoma cells

Autotaxin (ATX) is a strong motogen that can increase invasiveness and angiogenesis. In the present study, we investigated the signal transduction mechanism of ATX-induced tumor cell motility. Unlike N19RhoA expressing cells, the cells expressing N17Cdc42 or N17Rac1 showed reduced motility against ATX. ATX activated Cdc42 and Rac1 and increased complex formation between these small G proteins and p21-activated kinase (PAK). Furthermore, ATX phosphorylated focal adhesion kinase (FAK) that was not shown in cells expressing dominant negative mutants of Cdc42 or Rac1. Collectively, these data strongly indicate that Cdc42 and Rac1 are essential for ATX-induced tumor cell motility in A2058 melanoma cells, and that PAK and FAK might be also involved in the process.

CCL19 induces rapid dendritic extension of murine dendritic cells

Dendritic cells (DCs) possess numerous dendrites that may be of great advantage to interaction with T cells. However, it has been poorly understood how the dendritic morphology of a DC is controlled. In the present study, using a murine spleen-derived DC line, we analyzed effects of CCR7 ligands, CCL19 and CCL21, on dendritic morphology. Mature DCs, but not immature DCs, showed vigorous migration to either CCL19 or CCL21. CCL19 also rapidly (within 30 minutes) induced marked extension of dendrites of mature DCs that was maintained at least for 24 hours. On the other hand, CCL21 failed to induce rapid dendritic extension, even though a modest dendritic extension of mature DCs, compared to that by CCL19, was induced 8 or 24 hours after treatment with CCL21. In addition, pretreatment with a high concentration of CCL21 significantly inhibited the rapid dendritic extension induced by CCL19. Thus, it is suggested that CCL19 and CCL21 exert agonistic and antagonistic influences on the initiation of dendritic extension of mature DCs. The CCL19-induced morphologic changes were completely blocked by Clostridium difficile toxin B that inhibits Rho guanosine triphosphatase proteins such as Rho, Rac, and Cdc42, but not by Y-27632, a specific inhibitor for Rho-associated kinase. These findings suggest that Rac or Cdc42 (or both), but not Rho, are involved in the CCL19-induced dendritic extension of mature DCs.

G-protein alpha(olf) subunit promotes cellular invasion, survival, and neuroendocrine differentiation in digestive and urogenital epithelial cells

The heterotrimeric G-protein subunits Galpha and Gbetagamma are involved in cellular transformation and tumor development. Here, we report the expression of Galpha(olf) in human digestive and urogenital epithelial cells using RT-PCR and Western blot. When the constitutively activated form of Galpha(olf)Q214L (AGalpha(olf)) was stably transfected in canine kidney MDCKts.src and human colonic HCT-8/S11 epithelial cells, it induced cellular invasion in collagen gels. AGalpha(olf)-mediated invasion was abrogated by agonists of platelet activating factor receptors (PAF-R) and protease-activated receptors -1 (PAR-1), pharmacological inhibitors of PI3'-Kinase (wortmannin), protein kinase C (Gö6976 and GF109203X), Rho GTPase (C3T exoenzyme), but was independent of protein kinase A. Accordingly, the invasive phenotype induced by AGalpha(olf) in HCT-8/S11 cells was reversed by the RhoA antagonist RhoD (G26V). Although AGalpha(olf) protected MDCKts.src cells against serum starvation-mediated apoptosis via a Rho-independent pathway, both AGalpha(olf) and Rho inhibition by C3T induced neuroendocrine-like differentiation linked to extensive neurite outgrowth and parathyroid hormone-related protein expression in human prostatic LNCaP-AGalpha(olf) cells. Since prostate tumors with a larger neuroendocrine cell population display increased invasiveness, persistent activation of the G-protein alpha(olf) may exert convergent adverse effects on cellular invasion and survival in solid tumors during the neoplastic progression towards metastasis. doi:10.1038/sj.onc.1205498

Lysophosphatidic acid stimulates p21-activated kinase in vascular smooth muscle cells

Lysophosphatidic acid (LPA) has been shown to be a potent mitogen for vascular smooth muscle cells. Src-dependent transactivation of receptor tyrosine kinases has been previously demonstrated to mediate LPA-induced activation of MAP kinase ERK1/2. Furthermore, generation of reactive oxygen species (ROS) by LPA is also known to contribute to MAP kinase activation. Rho family small G-proteins Rac and Cdc42, and their immediate downstream effector p21-activated kinase (PAK), have been demonstrated to mediate important effects on the cytoskeleton that are relevant for cell migration and proliferation. In the present report we evaluated stimulation of PAK by LPA in rat aortic vascular smooth muscle cells (VSMC) by PAK immunocomplex MBP in-gel kinase assay. LPA increased PAK activity 3-fold, peaking at 5 min and showing sustained activation up to 45 min. Inhibition of tyrosine kinases by pretreatment of VSMC with genistein or specific inhibition of Src by PP1 greatly diminished LPA-induced PAK activation, whereas specific inhibition of PDFG- and EGF receptor kinase by tyrphostin AG1296 and AG1478 had no effect. Furthermore, inhibition of Galpha(i) by pertussis toxin and inhibition of NADH/NADPH oxidase by diphenylene iodonium also diminished LPA-induced stimulation of PAK. This is the first study to demonstrate that LPA activates PAK. In VSMC, PAK activation by LPA is mediated by Galpha(i) and is dependent on Src, whereas EGF- or PDGF receptor transactivation are not involved. Furthermore, generation of ROS is required for LPA-induced activation of PAK.

A role for the Rho-p160 Rho coiled-coil kinase axis in the chemokine stromal cell-derived factor-1alpha-induced lymphocyte actomyosin and microtubular organization and chemotaxis

The possible involvement of the Rho-p160ROCK (Rho coiled-coil kinase) pathway in the signaling induced by the chemokine Stromal cell-derived factor (SDF)-1alpha has been studied in human PBL. SDF-1alpha induced activation of RhoA, but not that of Rac. RhoA activation was followed by p160ROCK activation mediated by RhoA, which led to myosin light chain (MLC) phosphorylation, which was dependent on RhoA and p160ROCK activities. The kinetics of MLC activation was similar to that of RhoA and p160ROCK. The role of this cascade in overall cell morphology and functional responses to the chemokine was examined employing different chemical inhibitors. Inhibition of either RhoA or p160ROCK did not block SDF-1alpha-induced short-term actin polymerization, but induced the formation of long spikes arising from the cell body, which were found to be microtubule based. This morphological change was associated with an increase in microtubule instability, which argues for an active microtubule polymerization in the formation of these spikes. Inhibition of the Rho-p160ROCK-MLC kinase signaling cascade at different steps blocked lymphocyte migration and the chemotaxis induced by SDF-1alpha. Our results indicate that the Rho-p160ROCK axis plays a pivotal role in the control of the cell shape as a step before lymphocyte migration toward a chemotactic gradient.

In vivo rho GTPase-activating protein activity of Pseudomonas aeruginosa cytotoxin ExoS

ExoS is a bifunctional type III cytotoxin secreted by Pseudomonas aeruginosa, which comprises a C-terminal ADP ribosyltransferase domain and an N-terminal Rho GTPase-activating protein (GAP) domain. In vitro, ExoS is a Rho GAP for Rho, Rac, and Cdc42; however, the in vivo modulation of Rho GTPases has not been addressed. Using a transient transfection system and delivery by P. aeruginosa, interactions were examined between the Rho GAP domain of ExoS and Rho GTPases in CHO cells. Rho GTPases were expressed as green fluorescent protein (GFP) fusion proteins to facilitate quantitation. GFP fusions of wild-type and dominant active Rho, Rac, and Cdc42 localized to discrete regions of CHO cells and appeared functional based upon their modulation of the actin cytoskeleton. Coexpression of the Rho GAP domain of ExoS changed the intracellular distribution of GFP-Rac and GFP-Cdc42 from a predominately membrane location to a cytosolic location. Coexpression of the Rho GAP domain of ExoS did not change the distribution of GFP-Rho, which was primarily in the cytosol. Coexpression of dominant active Rac (DARac) and DACdc42 inhibited actin reorganization by the Rho GAP domain but did not maintain the formation of actin stress fibers, which indicated that Rho had been inactivated. Similar results were observed when ExoS was delivered into CHO cells by P. aeruginosa. These data indicate that in vivo the Rho GAP activity of ExoS stimulates the reorganization of the actin cytoskeleton by inhibition of Rac and Cdc42 and stimulates actin stress fiber formation by inhibition of Rho.

Differential regulation of Rho and Rac through heterotrimeric G-proteins and cyclic nucleotides

Platelets were used to study the activation of Rho and Rac through G-protein-coupled receptors and its regulation by cyclic nucleotides. The thromboxane A(2) (TXA(2)) mimetic rapidly activated both small GTPases independently of integrin alpha(IIb)beta(3) activation., which leads to the activation of G(12)/G(13) and G(q) did not induce Rac activation in G alpha(q)-deficient platelets but was able to activate Rho, to stimulate actin polymerization and phosphatidylinositol 4,5-bisphosphate formation, and to induce shape change. Rac activation by in wild-type platelets could be blocked by chelation of intracellular Ca(2+) and was partially sensitive to apyrase and AR-C69931MX, an antagonist of the G(i)-coupled ADP receptor. Cyclic AMP, which completely blocks platelet function, inhibited the -induced activation of G(q) and G(12)/G(13) as well as of Rac and Rho. In contrast, cGMP, which has no effect on platelet shape change blocked only activation of G(q) and Rac. These data demonstrate that Rho and Rac are differentially regulated through heterotrimeric G-proteins. The G(12)/G(13)-mediated Rho activation is involved in the shape change response, whereas Rac is activated through G(q) and is not required for shape change. Cyclic AMP and cGMP differentially interfere with -induced Rho and Rac activation at least in part by selective effects on the regulation of individual G-proteins through the TXA(2) receptor.

Protein kinase A regulates Rac and is required for the growth factor-stimulated migration of carcinoma cells

Members of the Rho family of small GTPases, such as Rho and Rac, are required for actin cytoskeletal reorganization during the migration of carcinoma cells. Phosphodiesterases are necessary for this migration because they alleviate cAMP-dependent protein kinase (PKA)-mediated inhibition of RhoA (O'Connor, K. L., Shaw, L. M., and Mercurio, A. M. (1998) J. Cell Biol. 143, 1749-1760; O'Connor K. L., Nguyen, B.-K., and Mercurio, A. M. (2000), J. Cell Biol. 148, 253-258). In this study, we report that the migration of breast and squamous carcinoma cells toward either lysophosphatidic acid or epidermal growth factor involves not only phosphodiesterase activity but also cooperative signaling from PKA. Furthermore, we demonstrate that Rac1 activation in response to chemoattractant or beta(1) integrin clustering is regulated by PKA and that Rac1 is required for this migration. Also, we find that beta(1) integrin signaling stimulates the rapid and transient activation of PKA. A novel implication of these findings is that carcinoma cell migration is controlled by cAMP-dependent as well as cAMP inhibitory signaling mechanisms.

Galpha11 induces caspase-mediated proteolytic activation of Rho-associated kinase, ROCK-I, in HeLa cells

Expression of the constitutively active mutant of Galpha(11) (Galpha(11)QL) induces the formation of vinculin-containing focal adhesion-like structures in HeLa cells. This was found to be inhibited by Y-27632, a specific inhibitor of Rho-associated kinases (ROCK), but not by co-expression with a dominant negative mutant of RhoA, suggesting Rho-independent activation of ROCK by Galpha(11)QL. Investigation of trypan blue exclusion and immunocytochemistry with an antibody against cleaved caspase revealed the cellular phenotype of Galpha(11)QL-expressing cells to be identical to that displayed by cells undergoing apoptosis, and the caspase inhibitor zVAD-fmk blocked all morphological changes induced by Galpha(11)QL. Transfection of Galpha(11)QL induced cleavage of ROCK-I, and this proteolysis was also prevented by zVAD-fmk. ROCK-I C-terminally truncated at its authentic caspase sites also induced the formation of vinculin-containing focal adhesion-like structures. In addition, cleavage of ROCK-I was observed when cells overexpressing m1 muscarinic acetylcholine receptors were stimulated with carbachol. These results suggest that Galpha(11) induces proteolytic activation of ROCK-I by caspase and thereby regulates the actin cytoskeleton during apoptosis.

Cdc42, dynein, and dynactin regulate MTOC reorientation independent of Rho-regulated microtubule stabilization

In migrating adherent cells such as fibroblasts and endothelial cells, the microtubule-organizing center (MTOC) reorients toward the leading edge [1-3]. MTOC reorientation repositions the Golgi toward the front of the cell [1] and contributes to directional migration [4]. The mechanism of MTOC reorientation and its relation to the formation of stabilized microtubules (MTs) in the leading edge, which occurs concomitantly with MTOC reorientation [3], is unknown. We show that serum and the serum lipid, lysophosphatidic acid (LPA), increased Cdc42 GTP levels and triggered MTOC reorientation in serum-starved wounded monolayers of 3T3 fibroblasts. Cdc42, but not Rho or Rac, was both sufficient and necessary for LPA-stimulated MTOC reorientation. MTOC reorientation was independent of Cdc42-induced changes in actin and was not blocked by cytochalasin D. Inhibition of dynein or dynactin blocked LPA- and Cdc42-stimulated MTOC reorientation. LPA also stimulates a Rho/mDia pathway that selectively stabilizes MTs in the leading edge [5, 6]; however, activators and inhibitors of MTOC reorientation and MT stabilization showed that each response was regulated independently. These results establish an LPA/Cdc42 signaling pathway that regulates MTOC reorientation in a dynein-dependent manner. MTOC reorientation and MT stabilization both act to polarize the MT array in migrating cells, yet these processes act independently and are regulated by separate Rho family GTPase-signaling pathways.