Involvement of pertussis toxin-sensitive and -insensitive G proteins in alpha-thrombin signalling on cultured human vascular smooth muscle cells

Multiple Signaling Pathways Contribute to the Thrombin-induced Secretory Phenotype in Vascular Smooth Muscle Cells

We attempted to investigate molecular mechanisms underlying phenotypic change of vascular smooth muscle cells (VSMCs) by determining signaling molecules involved in chemokine production. Treatment of human aortic smooth muscle cells (HAoSMCs) with thrombin resulted not only in elevated transcription of the (C-C motif) ligand 11 (CCL11) gene but also in enhanced secretion of CCL11 protein. Co-treatment of HAoSMCs with GF109230X, an inhibitor of protein kinase C, or GW5074, an inhibitor of Raf-1 kinase, caused inhibition of ERK1/2 phosphorylation and significantly attenuated expression of CCL11 at transcriptional and protein levels induced by thrombin. Both Akt phosphorylation and CCL11 expression induced by thrombin were attenuated in the presence of pertussis toxin (PTX), an inhibitor of Gi protein-coupled receptor, or LY294002, a PI3K inhibitor. In addition, thrombin-induced production of CCL11 was significantly attenuated by pharmacological inhibition of Akt or MEK which phosphorylates ERK1/2. These results indicate that thrombin is likely to promote expression of CCL11 via PKC/Raf-1/ERK1/2 and PTX-sensitive protease-activated receptors/PI3K/Akt pathways in HAoSMCs. We propose that multiple signaling pathways are involved in change of VSMCs to a secretory phenotype.

Differential signaling by protease-activated receptors: implications for therapeutic targeting

Protease-activated receptors (PARs) are a family of four G protein-coupled receptors that exhibit increasingly appreciated differences in signaling and regulation both within and between the receptor class. By nature of their proteolytic self-activation mechanism, PARs have unique processes of receptor activation, "ligand" binding, and desensitization/resensitization. These distinctive aspects have presented both challenges and opportunities in the targeting of PARs for therapeutic benefit-the most notable example of which is inhibition of PAR1 on platelets for the prevention of arterial thrombosis. However, more recent studies have uncovered further distinguishing features of PAR-mediated signaling, revealing mechanisms by which identical proteases elicit distinct effects in the same cell, as well as how distinct proteases produce different cellular consequences via the same receptor. Here we review this differential signaling by PARs, highlight how important distinctions between PAR1 and PAR4 are impacting on the progress of a new class of anti-thrombotic drugs, and discuss how these more recent insights into PAR signaling may present further opportunities for manipulating PAR activation and signaling in the development of novel therapies.

Heterotrimeric G proteins, focal adhesion kinase, and endothelial barrier function

Ligands by binding to G protein coupled receptors (GPCRs) stimulate dissociation of heterotrimeric G proteins into Gα and Gβγ subunits. Released Gα and Gβγ subunits induce discrete signaling cues that differentially regulate focal adhesion kinase (FAK) activity and endothelial barrier function. Activation of G proteins downstream of receptors such as protease activated receptor 1 (PAR1) and histamine receptors rapidly increases endothelial permeability which reverses naturally within the following 1-2 h. However, activation of G proteins coupled to the sphingosine-1-phosphate receptor 1 (S1P1) signal cues that enhance basal barrier endothelial function and restore endothelial barrier function following the increase in endothelial permeability by edemagenic agents. Intriguingly, both PAR1 and S1P1 activation stimulates FAK activity, which associates with alteration in endothelial barrier function by these agonists. In this review, we focus on the role of the G protein subunits downstream of PAR1 and S1P1 in regulating FAK activity and endothelial barrier function.

Proteinases and signalling: pathophysiological and therapeutic implications via PARs and more

Proteinases like thrombin, trypsin and tissue kallikreins are now known to regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) via exposure of a tethered receptor-triggering ligand. On their own, short synthetic PAR-selective PAR-activating peptides (PAR-APs) mimicking the tethered ligand sequences can activate PARs 1, 2 and 4 and cause physiological responses both in vitro and in vivo. Using the PAR-APs as sentinel probes in vivo, it has been found that PAR activation can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (both central and peripheral nervous system) and can promote cancer metastasis and invasion. In general, responses triggered by PARs 1, 2 and 4 are in keeping with an innate immune inflammatory response, ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased or decreased nociception. Further, PARs have been implicated in a number of disease states, including cancer and inflammation of the cardiovascular, respiratory, musculoskeletal, gastrointestinal and nervous systems. In addition to activating PARs, proteinases can cause hormone-like effects by other signalling mechanisms, like growth factor receptor activation, that may be as important as the activation of PARs. We, therefore, propose that the PARs themselves, their activating serine proteinases and their associated signalling pathways can be considered as attractive targets for therapeutic drug development. Thus, proteinases in general must now be considered as 'hormone-like' messengers that can signal either via PARs or other mechanisms.

Human vascular smooth muscle cells express functionally active endothelial cell protein C receptor

The endothelial cell protein C receptor (EPCR) is expressed on endothelial cells and regulates the protein C anticoagulant pathway via the thrombin-thrombomodulin complex. Independent of its anticoagulant activity, activated protein C (APC) can directly signal to endothelial cells and upregulate antiapoptotic and antiinflammatory genes. Here we show that vascular smooth muscle cells (SMCs) also express EPCR. EPCR protein on SMCs was detected by flow cytometry and Western blotting. EPCR mRNA was identified by quantitative RT-PCR. To examine the functionality of EPCR, intracellular signaling in APC-stimulated SMCs was analyzed by determination of intracellular free calcium transients using confocal laser scanning microscopy. Phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK-1/2) was detected by immunoblotting. APC-induced ERK-1/2 phosphorylation was inhibited by an anti-EPCR antibody and by a cleavage site blocking anti-PAR-1 antibody, indicating that binding of APC to EPCR and cleavage of protease-activated receptor-1 (PAR-1) were involved. APC elicited an increase in [(3)H]-thymidine incorporation. The mitogenic effect of APC was significantly enhanced in the presence of thrombin. EPCR expression was also detected in SMCs in the fibrous cap of human carotid artery plaques. The present data demonstrate functionally active EPCR in SMCs and suggest that EPCR-bound APC might modulate PAR-1-mediated responses of SMCs to vascular injury.

Mini-dystrophin expression down-regulates overactivation of G protein-mediated IP3 signaling pathway in dystrophin-deficient muscle cells

We present here evidence for the enhancement of an inositol 1,4,5-trisphosphate (IP3) mediated calcium signaling pathway in myotubes from dystrophin-deficient cell lines (SolC1(−)) as compared to a cell line from the same origin but transfected with mini-dystrophin (SolD(+)). With confocal microscopy, we demonstrated that calcium rise, induced by the perifusion of a solution containing a high potassium concentration, was higher in SolC1(−) than in SolD(+) myotubes. The analysis of amplitude and kinetics of the calcium increase in SolC1(−) and in SolD(+) myotubes during the exposure with SR Ca²⁺ channel inhibitors (ryanodine and 2-APB) suggested the presence of two mechanisms of SR calcium release: (1) a fast SR calcium release that depended on ryanodine receptors and (2) a slow SR calcium release mediated by IP3 receptors. Detection analyses of mRNAs (reverse transcriptase [RT]-PCR) and proteins (Western blot and immunolocalization) demonstrated the presence of the three known isoforms of IP3 receptors in both SolC1(−) and SolD(+) myotubes. Furthermore, analysis of the kinetics of the rise in calcium revealed that the slow IP3-dependent release may be increased in the SolC1(−) as compared to the SolD(+), suggesting an inhibitory effect of mini-dystrophin in this signaling pathway. Upon incubation with pertussis toxin (PTX), an inhibitory effect similar to that of the IP3R inhibitor (2-APB) was observed on K⁺-evoked calcium release. This result suggests the involvement of a Gi protein upstream of the IP3 pathway in these stimulation conditions. A hypothetical model is depicted in which both Gi protein and IP3 production could be involved in K⁺-evoked calcium release as well as a possible interaction with mini-dystrophin. Our findings demonstrate the existence of a potential relationship between mini-dystrophin and SR calcium release as well as a regulatory role of mini-dystrophin on intracellular signaling.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

Role of protease-activated receptors in the vascular system

Thrombin is one of the key molecules involved in the development of vascular diseases. Thrombin does not only serve as a coagulation factor, but it also exerts cellular effects by activating protease (proteinase)-activated receptors (PARs), a family of seven-transmembrane G protein-coupled receptors. This study focused on the role of PARs in the vascular system. Among the four members so far identified, PAR-1 and PAR-2 were found to play an important role in the vascular system, while the functional roles of PAR-3 and PAR-4 appear to be mostly limited to platelets. The endothelial cells play a primary role in mediating the vascular effects of PARs under physiological conditions, while PARs of the smooth muscle cells can be induced under pathological conditions, and therefore play a more pathophysiological role. PAR-1 and PAR-2 mediate various vascular effects including regulation of vascular tone, proliferation and hypertrophy of smooth muscle and angiogenesis. Since proteases are activated under pathological conditions such as hemorrhage, tissue damage, and inflammation, PARs are suggested to play a critical role in the development of functional and structural abnormality in the vascular lesion. Understanding the functional role of PARs in the vascular system can thus help in the development of new strategies for the prevention and therapy of vascular diseases.

Alpha-thrombin-mediated phosphatidylinositol 3-kinase activation through release of Gbetagamma dimers from Galphaq and Galphai2

Chinese hamster embryonic fibroblasts (IIC9 cells) express the Galpha subunits Galphas, Galphai2, Galphai3, Galphao, Galpha(q/11), and Galpha13. Consistent with reports in other cell types, alpha-thrombin stimulates a subset of the expressed G proteins in IIC9 cells, namely Gi2, G13, and Gq as measured by an in vitro membrane [35S]guanosine 5'-O-(3-thio)triphosphate binding assay. Using specific Galpha peptides, which block coupling of G-protein receptors to selective G proteins, as well as dominant negative xanthine nucleotide-binding Galpha mutants, we show that activation of the phosphatidylinositol 3-kinase/Akt pathway is dependent on Gq and Gi2. To examine the role of the two G proteins, we examined the events upstream of PI 3-kinase. The activation of the PI 3-kinase/Akt pathway by alpha-thrombin in IIC9 cells is blocked by the expression of dominant negative Ras and beta-arrestin1 (Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2000) J. Biol. Chem. 275, 18046-18053, and Goel, R., Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2002) J. Biol. Chem. 277, 18640-18648), indicating a role for Ras and beta-arrestin1. Interestingly, inhibition of Gi2 and Gq activation blocks Ras activation and beta-arrestin1 membrane translocation, respectively. Furthermore, expression of the Gbetagamma sequestrant, alpha-transducin, inhibits both Ras activation and membrane translocation of beta-arrestin1, suggesting that Gbetagamma dimers from Galphai2 and Galphaq activate different effectors to coordinately regulate the PI 3-kinase/Akt pathway.

The proteinase-activated receptor-2 mediates phagocytosis in a Rho-dependent manner in human keratinocytes

Recent work shows that the G-protein-coupled receptor proteinase activated receptor-2 activates signals that stimulate melanosome uptake in keratinocytes in vivo and in vitro. The Rho family of GTP-binding proteins is involved in cytoskeletal remodeling during phagocytosis. We show that proteinase-activated receptor-2 mediated phagocytosis in human keratinocytes is Rho dependent and that proteinase-activated receptor-2 signals to activate Rho. In contrast, Rho activity did not affect either proteinase-activated receptor-2 activity or mRNA and protein levels. We explored the signaling mechanisms of proteinase-activated receptor-2 mediated Rho activation in human keratinocytes and show that activation of proteinase-activated receptor-2, either through specific proteinase-activated receptor-2 activating peptides or through trypsinization, elevates cAMP in keratinocytes. Proteinase-activated receptor-2 mediated Rho activation was pertussis toxin insensitive and independent of the protein kinase A signaling pathway. These data are the first to show that proteinase-activated receptor-2 mediated phagocytosis is Rho dependent and that proteinase-activated receptor-2 signals to Rho and cAMP in keratinocytes. Because phagocytosis of melanosomes is recognized as an important mechanism for melanosome transfer to keratinocytes, these results suggest that Rho is a critical signaling intermediate in melanosome uptake in keratinocytes.

alpha-Thrombin induces rapid and sustained Akt phosphorylation by beta-arrestin1-dependent and -independent mechanisms, and only the sustained Akt phosphorylation is essential for G1 phase progression

In Chinese hamster embryonic fibroblasts (IIC9 cells) alpha-thrombin activates the MAPK(ERK) and phosphatidylinositol 3-OH-kinase (PI 3-kinase)/Akt pathways, and both are essential for progression through the G(1) phase of the cell cycle. We investigated in IIC9 cells, the role of beta-arrestin1 in alpha-thrombin signaling to these pathways. alpha-Thrombin stimulates rapid and sustained PI 3-kinase and Akt activities. Expression of a dominant negative beta-arrestin1 (beta-arrestin1(V53D)) inhibits rapid but not sustained PI 3-kinase and Akt activities. Surprisingly, expression of beta-arrestin1(V53D) does not block activation of the MAPK(ERK) pathway. PI 3-kinase and Akt activities are also inhibited by expression of a beta-arrestin1 mutant, which impairs binding to c-Src (beta-arrestin1(P91G-P121E)), indicating the involvement of c-Src in the rapid stimulation of the PI 3-kinase/Akt pathway. Consistent with these results, PP1, a selective inhibitor of c-Src family kinases, prevents alpha-thrombin-stimulated Akt phosphorylation. Expression of beta- arrestin1(V53D) does not prevent G(1) progression, as its expression has no effect on [(3)H]thymidine incorporation into DNA. In agreement with the ineffectiveness of beta-arrestin1(V53D) to block G(1) progression, cyclin D1 protein amounts and CDK4-cyclin D1 activity is unaffected by expression of beta-arrestin1(V53D). Thus in IIC9 cells, alpha-thrombin activates rapid beta-arrestin1-dependent and sustained beta-arrestin1-independent Akt activity, suggesting that two mechanisms are involved. Furthermore, although blocking the beta-arrestin1-independent PI 3-kinase/Akt pathway prevents G(1) progression, inhibition of the beta-arrestin1-dependent pathway does not, indicating different roles for the rapid and sustained activities.

Evidence for functionally active protease-activated receptor-4 (PAR-4) in human vascular smooth muscle cells

1. This study investigates, whether in addition to the protease-activated receptor-1 (PAR-1), PAR-4 is present in vascular smooth muscle cells (SMC) of the human saphenous vein and whether this receptor is functionally active. PAR-1 and PAR-4 are stimulated by thrombin and by the synthetic peptides SFLLRN and GYPGQV, respectively. 2. mRNAs for both, PAR-1 and PAR-4, were detected in the SMC by using reverse transcriptase polymerase chain reaction (RT - PCR). 3. Treatment of the SMC with GYPGQV (200 microM) resulted in a transient increase in free intracellular calcium. This calcium signal was completely abolished after a preceding challenge with thrombin (10 nM), indicating homologous receptor desensitization. 4. Stimulation of the SMC with 10 nM thrombin or 200 microM SFLLRN caused a time-dependent activation of the extracellular signal-regulated kinases-1/2 (ERK-1/2) with a maximum at 5 min. In contrast, 100 nM thrombin as well as 200 microM of GYPGQV induced a prolonged phosphorylation of ERK-1/2 with a maximum at 60 min. These data suggest that PAR-1 and PAR-4 are activated by thrombin at distinct concentrations and with distinct kinetics. 5. GYPGQV stimulated [(3)H]-thymidine incorporation in SMC. At 500 microM, the peptide increased DNA synthesis 2.5 fold above controls. A comparable mitogenic effect was obtained after stimulation of the SMC by 10 nM thrombin or 100 microM SFLLRN, respectively. 6. These data indicate that a functionally active PAR-4 is present in SMC and, in addition to PAR-1, might contribute to thrombin-induced mitogenesis.

Synergy between thrombin and serotonin in inducing vascular smooth muscle cell proliferation

Previous studies have indicated that apart from playing an important role in hemostasis and thrombosis, thrombin may also contribute to the development of postangioplasty restenosis caused by the stimulation of vascular smooth muscle cell (VSMC) proliferation. Because thrombin generation in vivo is accompanied by platelet activation and release of smooth muscle cell (SMC) growth factors such as serotonin, we examined the possible interaction between these two compounds on VSMC proliferation. Thrombin (0.01 to 100 nmol/L), thrombin receptor-activating peptide (0.1 to 1000 micromol/L), and serotonin (5HT; 0.1 to 1000 micromol/L) increased tritiated thymidine incorporation into the DNA of canine aortic VSMCs in a dose-dependent manner. When thrombin and 5HT were added together at sub-threshold concentrations, they acted synergistically in inducing tritiated thymidine incorporation. These findings were paralleled by a 90%+/-5% increase in the cell number at 48 hours, as compared with a 37%+/-2% increase with 50 micromol/L serotonin and a 13%+/-3% increase with 0.1 nmol/L thrombin. We also demonstrated that a brief exposure to thrombin (1 hour) is sufficient to show its potentiating effect on serotonin. The mitogenic effect of serotonin and its synergistic interaction with thrombin on VSMC proliferation was abolished by serotonin type 2 receptor antagonist LY281067. Similarly, gamma-hirudin--a direct thrombin inhibitor--blocked the mitogenic effect of thrombin and its synergistic interaction with serotonin. When LY281067 and gamma-hirudin were used together, they abolished the mitogenic effects of both the agonists. Because clot-bound active thrombin can escape inactivation by anti-thrombin, this thrombin may potentiate the mitogenic effect of serotonin and keep the SMCs in a proliferative state for a long period of time. These findings support the use of 5HT2 receptor antagonists in combination with thrombin inhibitors in the prevention of SMC proliferation after coronary angioplasty.

Thrombin activates transcription factors sp1, NF-kappaB, and CREB: importance of the use of phosphatase inhibitors during nuclear protein extraction for the assessment of transcription factor DNA-binding activities

Thrombin, a serine protease, is an important effector of many cellular processes and has been shown to up-regulate the expression of several genes. The mechanisms underlying thrombin-mediated regulation of gene transcription remain poorly understood. The original aim of this work was to study the effects of thrombin on the activation of transcription factors, Sp1, NF-kappaB, and CREB by means of electrophoretic mobility-shift assays (EMSA). However, an inconsistent pattern of results was observed. We raised the possibility that some EMSA results may have been erroneous by the fact that during the nuclear protein extraction and EMSA procedure, transcription factors are dephosphorylated by cellular phosphatases and hence their DNA-binding capacities are modified. Therefore, we have altered the original nuclear extraction protocol by including a mixture of phosphatase inhibitors during protein extraction and subsequent EMSA steps. We show here that this simple measure led to significant changes in both basal and thrombin-induced levels of activation of Sp1 and CREB, but not of NF-kappaB. In light of the data presented here, it would be important to reexamine the conclusions of many reports in which EMSA was used to assess the basal and agonist-induced levels of transcription factor DNA-binding activities.

Selectively enhanced cellular signaling by Gi proteins in essential hypertension. G alpha i2, G alpha i3, G beta 1, and G beta 2 are not mutated

Recent studies have shown an enhanced signaling capacity of receptors coupled to pertussis toxin (PTX)-sensitive guanine nucleotide-binding proteins (G proteins) in immortalized B lymphoblasts from patients with essential hypertension. In the present study, we analyzed (1) whether such alterations would also be expressed in nontransformed cells of these individuals and (2) whether other G protein-mediated signaling pathways were also altered. Therefore, we established primary cultures of skin fibroblasts from previously characterized normotensive and hypertensive individuals (NT and HT cells, respectively). [Ca2+]i rises induced by lyso-phosphatidic acid (LPA), thrombin, and sphingosine-1-phosphate as well as the formation of inositol 1,4,5-trisphosphate and [3H]thymidine incorporation evoked by LPA were PTX sensitive and enhanced twofold in HT fibroblasts. In contrast, cellular responses induced by bradykinin, endothelin-1, and angiotensin II (all PTX insensitive) were similar in NT and HT cells. Formation of cAMP induced by stimulation of Gs with isoproterenol was identical in NT and HT cells. Western blot analysis yielded no evidence for an overexpression of G alpha i2, G alpha i3, G beta 2, and G beta 4. Furthermore, sequencing of cDNAs encoding for the ubiquitously expressed PTX-sensitive G protein subunits G alpha i2, G alpha i3, G beta 1, and G beta 2 from NT and HT cell lines yielded no evidence for mutations in these genes. Although the molecular mechanisms remain to be defined, these data support the concept of a selective enhancement of signal transduction via PTX-sensitive G proteins in essential hypertension.