Angiotensin II stimulates tyrosine phosphorylation of phospholipase C-gamma-associated proteins. Characterization of a c-Src-dependent 97-kD protein in vascular smooth muscle cells

Next-Generation Sequencing in the Assessment of the Transcriptomic Landscape of DNA Damage Repair Genes in Abdominal Aortic Aneurysm, Chronic Venous Disease and Lower Extremity Artery Disease

Vascular diseases are one of the most common causes of death and morbidity. Lower extremity artery disease (LEAD), abdominal aortic aneurysm (AAA) and chronic venous disease (CVD) belong to this group of conditions and exhibit various presentations and courses; thus, there is an urgent need for revealing new biomarkers for monitoring and potential treatment. Next-generation sequencing of mRNA allows rapid and detailed transcriptome analysis, allowing us to pinpoint the most pronounced differences between the mRNA expression profiles of vascular disease patients. Comparison of expression data of 519 DNA-repair-related genes obtained from mRNA next-generation sequencing revealed significant transcriptomic marks characterizing AAA, CVD and LEAD. Statistical, gene set enrichment analysis (GSEA), gene ontology (GO) and literature analyses were applied and highlighted many DNA repair and accompanying processes, such as cohesin functions, oxidative stress, homologous recombination, ubiquitin turnover, chromatin remodelling and DNA double-strand break repair. Surprisingly, obtained data suggest the contribution of genes engaged in the regulatory function of DNA repair as a key component that could be used to distinguish between analyzed conditions. DNA repair-related genes depicted in the presented study as dysregulated in AAA, CVD and LEAD could be utilized in the design of new biomarkers or therapies associated with these diseases.

Sinner or Saint?: Nck Adaptor Proteins in Vascular Biology

The Nck family of modular adaptor proteins, including Nck1 and Nck2, link phosphotyrosine signaling to changes in cytoskeletal dynamics and gene expression that critically modulate cellular phenotype. The Nck SH2 domain interacts with phosphotyrosine at dynamic signaling hubs, such as activated growth factor receptors and sites of cell adhesion. The Nck SH3 domains interact with signaling effectors containing proline-rich regions that mediate their activation by upstream kinases. In vascular biology, Nck1 and Nck2 play redundant roles in vascular development and postnatal angiogenesis. However, recent studies suggest that Nck1 and Nck2 differentially regulate cell phenotype in the adult vasculature. Domain-specific interactions likely mediate these isoform-selective effects, and these isolated domains may serve as therapeutic targets to limit specific protein-protein interactions. In this review, we highlight the function of the Nck adaptor proteins, the known differences in domain-selective interactions, and discuss the role of individual Nck isoforms in vascular remodeling and function.

Divergent modulation of Rho-kinase and Ca(2+) influx pathways by Src family kinases and focal adhesion kinase in airway smooth muscle

Background and Purpose

The importance of tyrosine kinases in airway smooth muscle (ASM) contraction is not fully understood. The aim of this study was to investigate the role of Src‐family kinases (SrcFK) and focal adhesion kinase (FAK) in GPCR‐mediated ASM contraction and associated signalling events.

Experimental Approach

Contraction was recorded in intact or α‐toxin permeabilized rat bronchioles. Phosphorylation of SrcFK, FAK, myosin light‐chain‐20 (MLC₂₀) and myosin phosphatase targeting subunit‐1 (MYPT‐1) was evaluated in cultured human ASM cells (hASMC). [Ca²⁺]_i was evaluated in Fura‐2 loaded hASMC. Responses to carbachol (CCh) and bradykinin (BK) and the contribution of SrcFK and FAK to these responses were determined.

Key Results

Contractile responses in intact bronchioles were inhibited by antagonists of SrcFK, FAK and Rho‐kinase, while after α‐toxin permeabilization, they were sensitive to inhibition of SrcFK and Rho‐kinase, but not FAK. CCh and BK increased phosphorylation of MYPT‐1 and MLC₂₀ and auto‐phosphorylation of SrcFK and FAK. MYPT‐1 phosphorylation was sensitive to inhibition of Rho‐kinase and SrcFK, but not FAK. Contraction induced by SR Ca²⁺ depletion and equivalent [Ca²⁺]_i responses in hASMC were sensitive to inhibition of both SrcFK and FAK, while depolarization‐induced contraction was sensitive to FAK inhibition only. SrcFK auto‐phosphorylation was partially FAK‐dependent, while FAK auto‐phosphorylation was SrcFK‐independent.

Conclusions and Implications

SrcFK mediates Ca²⁺‐sensitization in ASM, while SrcFK and FAK together and individually influence multiple Ca²⁺ influx pathways. Tyrosine phosphorylation is therefore a key upstream signalling event in ASM contraction and may be a viable target for modulating ASM tone in respiratory disease.

Role of nonreceptor protein tyrosine kinases during phospholipase C-gamma 1-related uterine contractions in the rat

Activated phospholipase C1, produced in response to tyrosine phosphorylation, appears to play an important role during uterine contractions. These studies sought to determine which non-receptor protein tyrosine kinases are involved in the activation of phospholipase C1 in rat uterine tissue. In vitro contraction studies were performed utilizing isoform specific protein tyrosine kinase inhibitors. Western blots were performed utilizing antibodies to phosphotyrosine-phospholipase C1, total phospholipase C1, c-Src kinase and Lck kinase. Spontaneous, stretch-stimulated, and bpV(phen) (tyrosine phosphatase inhibitor) enhanced uterine contractions were significantly suppressed in response to Damnacanthal (Lck kinase inhibitor) and PP1 (c-Src kinase inhibitor). Damnacanthal and PP1 also significantly suppressed bpV(phen)-enhanced tyrosine phosphorylation of phospholipase C1. Western blots confirmed expression of Lck kinase and c-Src kinase in uterine tissue. In conclusion, the Lck and c-Src kinases appear to play an important role in regulating tyrosine phosphorylation of phospholipase C1 and contractile activity in the rat uterus.

Role of phospholipase Cgamma1 in cell spreading requires association with a beta-Pix/GIT1-containing complex, leading to activation of Cdc42 and Rac1

The significance of multiprotein signaling complexes in cell motility is becoming increasingly important. We have previously shown that phospholipase Cgamma1 (PLCgamma1) is critical for integrin-mediated cell spreading and motility (N. Jones et al., J. Cell Sci. 118:2695-2706, 2005). In the current study we show that, on a basement membrane-type matrix, PLCgamma1 associates with the adaptor protein GIT1 and the Rac1/Cdc42 guanine exchange factor beta-Pix; GIT1 and beta-Pix form tight complexes independently of PLCgamma1. The association of PLCgamma1 with the complex requires both GIT1 and beta-Pix and the specific array region (gammaSA) of PLCgamma1. Mutations of PLCgamma1 within the gammaSA region reveal that association with this complex is essential for the phosphorylation of PLCgamma1 and the progression to an elongated morphology after integrin engagement. Short interfering RNA (siRNA) depletion of either beta-Pix or GIT1 inhibited cell spreading in a fashion similar to that seen with siRNA against PLCgamma1. Furthermore, siRNA depletion of PLCgamma1, beta-Pix, or GIT1 inhibited Cdc42 and Rac1 activation, while constitutively active forms of Cdc42 or Rac1, but not RhoA, were able to rescue the elongation of these cells. Signaling of the PLCgamma1/GIT1/beta-Pix complex to Cdc42/Rac1 was found to involve the activation of calpains, calcium-dependent proteases. Therefore, we propose that the association of PLCgamma1 with complexes containing GIT1 and beta-Pix is essential for its role in integrin-mediated cell spreading and motility. As a component of this complex, PLCgamma1 is also involved in the activation of Cdc42 and Rac1.

Autophosphorylation-dependent degradation of Pak1, triggered by the Rho-family GTPase, Chp

The Paks (p21-activated kinases) Pak1, Pak2 and Pak3 are among the most studied effectors of the Rho-family GTPases, Rac, Cdc42 (cell division cycle 42) and Chp (Cdc42 homologous protein). Pak kinases influence a variety of cellular functions, but the process of Pak down-regulation, following activation, is poorly understood. In the present study, we describe for the first time a negative-inhibitory loop generated by the small Rho-GTPases Cdc42 and Chp, resulting in Pak1 inhibition. Upon overexpression of Chp, we unexpectedly observed a T-cell migration phenotype consistent with Paks inhibition. In line with this observation, overexpression of either Chp or Cdc42 caused a marked reduction in the level of Pak1 protein in a number of different cell lines. Chp-induced degradation was accompanied by ubiquitination of Pak1, and was dependent on the proteasome. The susceptibility of Pak1 to Chp-induced degradation depended on its p21-binding domain, kinase activity and a number of Pak1 autophosphorylation sites, whereas the PIX- (Pak-interacting exchange factor) and Nck-binding sites were not required. Together, these results implicate Chp-induced kinase autophosphorylation in the degradation of Pak1. The N-terminal domain of Chp was found to be required for Chp-induced degradation, although not for Pak1 activation, suggesting that Chp provides a second function, distinct from kinase activation, to trigger Pak degradation. Collectively, our results demonstrate a novel mechanism of signal termination mediated by the Rho-family GTPases Chp and Cdc42, which results in ubiquitin-mediated degradation of one of their direct effectors, Pak1.

Epidermal growth factor stimulates Rac1 and p21-activated kinase in vascular smooth muscle cells

Epidermal growth factor (EGF) has been shown to be a potent mitogen for vascular smooth muscle cells (VSMC) both in vitro and in vivo, thus contributing to the development of atherosclerosis and hypertension. Stimulation of Rho-family GTPases Rac/Cdc42 exerts pleiotropic cellular effects and have been demonstrated to contribute to EGF-induced proliferation in other cell systems. However, the effect of EGF on Rac/Cdc42 activation is unknown for VSMC. In the present report, we evaluated stimulation of Rac/Cdc42 by EGF in VSMC performing PAK-PBD binding assay. EGF treatment of VSMC induced time and concentration dependent binding of GTP-bound Rac1 to PAK-PBD peaking at 1 min and showing sustained activation up to 15 min. However, stimulation of Cdc42 could not be demonstrated. To further evaluate downstream effectors of Rac1 stimulation of p21-activated kinase (PAK) and c-Jun N-terminal kinase (JNK) by EGF was determined. In VSMC, EGF sequentially stimulated PAK, peaking at 5 min, and JNK, peaking at 15 min. Pretreatment of VSMC by EGF receptor specific tyrosine kinase inhibitor AG1478 and non-specific tyrosine kinase inhibitor genistein inhibited EGF-induced activation of Rac1, PAK and JNK, whereas tyrosine kinase inhibitors specific for Src (PP1) and specific for platelet-derived growth factor (AG1296) had no effect. Specific inhibition or Rac1 by NSC23766 attenuated EGF-induced [(3)H] thymidine incorporation in VSMC. Our data provide evidence for EGF-induced Rac1 activation and implicate PAK and JNK as downstream targets of Rac1 in EGF signal transduction in VSMC.

The role of phospholipase Cgamma1 tyrosine phosphorylation during phasic myometrial contractions

OBJECTIVE

Phospholipase Cgamma1 (PLCgamma1) is expressed in myometrium and is activated by tyrosine phosphorylation. These studies sought to determine the association between PLCgamma1 tyrosine phosphorylation and spontaneous uterine contractions.

STUDY DESIGN

In vitro contraction studies were performed with spontaneously contracting rat uterine strips along with strips that were treated with potassium bisperoxo(1,10 phenanthroline)oxovanadate (bpV(phen), a protein tyrosine phosphatase inhibitor. Additional studies were performed with phenylarsine oxide (a PLCgamma inhibitor) and other inhibitors. Western blots were performed to determine the phosphotyrosine PLCgamma1 levels.

RESULTS

Spontaneous contractile activity and tyrosine phosphorylation of PLCgamma1 (but not PLCgamma2) were increased significantly in response to bpV(phen); in contrast, oxytocin and thrombin produced comparable contractile activity but did not alter phosphotyrosine-PLCgamma1. Phenylarsine oxide and neomycin significantly decrease bpV(phen)-stimulated contractions and PLCgamma1 tyrosine phosphorylation; other inhibitors only suppressed contractions.

CONCLUSION

These studies support the hypothesis that spontaneous myometrial contractions are associated with tyrosine phosphorylation of PLCgamma1; both of which are further enhanced by the inhibition of protein tyrosine phosphatase activity.

The multifunctional GIT family of proteins

The G-protein-coupled receptor (GPCR)-kinase-interacting proteins 1 and 2 (GIT1 and GIT2) are ubiquitous multidomain proteins involved in diverse cellular processes. They traffic between three distinct cellular compartments (cytoplasmic complexes, focal adhesions and the cell periphery) through interactions with proteins including ARF, Rac1 and Cdc42 GTPases, p21-activated kinase (PAK), PAK-interacting exchange factor (PIX), the kinase MEK1, phospholipase Cgamma (PLCgamma) and paxillin. GITs and PIX cooperate to form large oligomeric complexes to which other proteins are transiently recruited. Activation of Rac1 and Cdc42 drives association of PAK with these oligomers, which unmasks the paxillin-binding site in GITs that recruits them to focal complexes. There, they regulate cytoskeletal dynamics by feedback inhibition of Rac1. GITs also participate in receptor internalization by regulating membrane trafficking between the plasma membrane and endosomes, targeting ARF GTPases through their ARF GTPase-activating protein (ARF-GAP) activity. Furthermore, GITs act as scaffolds to control spatial activation of several signaling molecules. Finally, recent results suggest pathogenic roles for GIT proteins in Huntington's disease and HIV infection.

SHP protein tyrosine phosphatase expression in rat uterine tissue

OBJECTIVE

Enhanced tyrosine phosphorylation of phospholipase C-gamma1 (PLCgamma1) is associated with increased spontaneous contractile activity. PLCgamma1 phosphorylation is regulated by cellular protein tyrosine kinases and tyrosine phosphatases (PTPs). The studies in this report were undertaken to characterize the expression of two PTPs known to bind to PLCgamma1: Src-homology phosphatase type-1 (SHP-1) and type-2 (SHP-2).

METHODS

Uterine and other tissues were obtained from non-pregnant (estrus) and pregnant (gestational day 12 through day 1 postpartum) Sprague-Dawley rats. PTP activity in myometrial homogenates was determined using an in vitro fluorometric PTP assay with and without bpV(phen) (a nonselective PTP inhibitor), or PTP-Inhibitor 1 (PTP-I1, a SHP selective inhibitor). Western blots were performed using polyclonal antibodies to SHP-1 and SHP-2. Immunoprecipitation studies were performed to demonstrate an association between PLCgamma1 and the SHP proteins.

RESULTS

The in vitro PTP assays demonstrated comparable enzyme activity in myometrium from estrus and pregnant animals. BpV(phen) produced a 93% reduction in PTP activity (P <.05); similarly, PTP-I1 produced an 86% reduction in enzyme activity (P <.05). Western blots confirmed robust expression of both SHP-1 and SHP-2 protein in rat uterus. SHP-1 expression decreased significantly at the end of gestation; in contrast, SHP-2 levels remained stable. Immunoprecipitation studies confirmed an association between the SHP proteins and PLCgamma1.

CONCLUSION

These studies have demonstrated that SHP-1 and SHP-2 are expressed in rat myometrium and appear to be responsible for the PTP activity in this tissue, thereby providing a molecular mechanism for the modulation of PLCgamma1 phosphotyrosine levels in the rat uterus.

GIT1 is a scaffold for ERK1/2 activation in focal adhesions

GIT1 (G protein-coupled receptor kinase-interacting protein 1) has been shown to regulate focal adhesion disassembly. We previously reported that GIT1 associates with MEK1 and acts as a scaffold to enhance ERK1/2 activation. Here, we show that GIT1 co-localizes with ERK1/2 in focal adhesions and regulates cell migration in vascular smooth muscle cells, HEK293 cells, and HeLa cells. Immunofluorescence showed that GIT1 co-localized with phospho-ERK1/2 in focal adhesions after epidermal growth factor stimulation. Because Src is required for both GIT1 tyrosine phosphorylation and focal adhesion disassembly, we studied the effects of Src on GIT1-ERK1/2 interactions. PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) inhibited association of GIT1 with ERK1/2, and their co-localization in focal adhesions was dramatically decreased in SYF-/- cells. GIT1 small interfering RNA significantly inhibited ERK1/2 recruitment to and activation in focal adhesions. GIT1 small interfering RNA and mutated GIT1 lacking the MEK1 binding domain significantly decreased epidermal growth factor-stimulated cell spreading and migration, suggesting that GIT1-mediated events such as ERK1/2 activation are required for spreading and migration. In summary, the present study further supports a key role for GIT1 (a MEK1-binding protein) as a scaffold for signal transduction in focal adhesions.

Identification of Nck interacting proteins in vascular smooth muscle cells

The adaptor molecule Nck has been demonstrated to mediate Angiotensin II (AngII)-induced stimulation of p21-activated kinase (PAK) and c-Jun NH2-terminal kinase (JNK) in vascular smooth muscle cells (VSMC). We have previously demonstrated, that immunoprecipitation of Nck from VSMC stimulated by AngII yielded an unidentified 100 kD phosphotyrosine (pTyr) protein. The present study was aimed at identifying the Nck-associated 100 kD pTyr protein in VSMC. Several candidate proteins of appropriate size, that had been shown previously either to bind to Nck or had been implicated in signal transduction pathways leading to activation of PAK or JNK were tested for association with Nck in VSMC. The first candidate protein we tested was Git1, which did not bind to Nck in VSMC upon stimulation by AngII. However, we identified dynamin as a 100 kD protein that was bound to Nck in VSCM via interaction with the third Nck-SH3 domain. However, dynamin was not tyrosine phosphorylated by AngII treatment and seemed to be distinct from the 100 kD phosphotyrosine protein that was found in Nck immunoprecipitates. Future work will now have to identify the Nck-associated 100 kD pTyr protein and functional studies will have to address its role in AngII signaling.

GIT1 functions as a scaffold for MEK1-extracellular signal-regulated kinase 1 and 2 activation by angiotensin II and epidermal growth factor

Activation of the mitogen-activated protein kinase pathway represented by extracellular signal-regulated kinases (ERK1/2) and activation of the upstream kinase (MEK1) are critical events for growth factor signal transduction. c-Src has been proposed as a common mediator for these signals in response to both G protein-coupled receptors (GPCRs) and tyrosine kinase-coupled receptors (TKRs). Here we show that the GPCR kinase-interacting protein 1 (GIT1) is a substrate for c-Src that associates with MEK1 in vascular smooth-muscle cells and human embryonic kidney 293 cells. GIT1 binding via coiled-coil domains and a Spa2 homology domain is required for sustained activation of MEK1-ERK1/2 after stimulation with angiotensin II and epidermal growth factor. We propose that GIT1 serves as a scaffold protein to facilitate c-Src-dependent activation of MEK1-ERK1/2 in response to both GPCRs and TKRs.

GIT1 mediates Src-dependent activation of phospholipase Cgamma by angiotensin II and epidermal growth factor

Critical events for vasoconstrictor and growth factor signal transduction include stimulation of phospholipase Cgamma (PLCgamma) and elevation of intracellular calcium. c-Src has been proposed as a common mediator for these signals activated by both G protein-coupled receptors (GPCRs) and tyrosine kinase-coupled receptors (TKRs). Here we show that the GPCR kinase-interacting protein-1 (GIT1) is a substrate for c-Src that undergoes tyrosine phosphorylation in response to angiotensin II (AngII) and EGF in vascular smooth muscle and 293 cells. GIT1 associates with PLCgamma via the PLCgamma Src homology 2 and 3 domains constitutively, and the interaction is unaltered by AngII and EGF. GIT1 interaction with PLCgamma is required for PLCgamma activation based on inhibition of tyrosine phosphorylation and calcium mobilization after GIT1 knockdown with antisense GIT1 oligonucleotides. GIT1 interacts with PLCgamma via a novel Spa homology domain (SHD) and a coiled-coil domain. Deletion mutation analysis showed that GIT1(SHD) is required for AngII- and EGF-mediated PLCgamma activation (measured by phosphorylation of Tyr783 and inositol 1,4,5-trisphosphate formation). We propose that GIT1 is a novel regulator of PLCgamma function that mediates PLCgamma activation by c-Src and integrates signal transduction by GPCRs and TKRs.

Angiotensin II signaling pathways mediated by tyrosine kinases

Angiotensin II (AngII) plays a critical role in control of cardiovascular and renal homeostasis. In addition to its physiological action as a vasoconstrictor, growing evidence supports the notion that AngII contributes to cardiovascular diseases such as hypertension, atherosclerosis, and heart failure. The physiological and pathological actions of AngII in adults are mediated largely via the AngII type 1 receptor (AT1R), a heterotrimeric G-protein-coupled receptor (GPCR). Besides coupling with heterotrimeric G proteins to activate phospholipase C-beta (PLC-beta), AT1R also activates receptor tyrosine kinases (PDGF-R, EGF-R and IGF-R) and non-receptor tyrosine kinases (Src, Fyn, Yes, proline-rich tyrosine kinase 2 (Pyk2), focal adhesion kinase (FAK) and JAK2). These tyrosine kinases play critical roles in AngII-stimulated cell signal events.

Mechanism of vascular smooth muscle cells activation by hydrogen peroxide: role of phospholipase C gamma

BACKGROUND

Hydrogen peroxide (H2O2) formation is a critical factor in processes involving ischaemia/ reperfusion. However, the precise mechanism by which reactive oxygen species (ROS) induce vascular damage are insufficiently known. Specifically, activation of phospholipase C gamma (PLCgamma) is a probable candidate pathway involved in vascular smooth muscle cells (VSMC) activation by H2O2.

METHODS

The activation of human venous VSMC was measured as cytosolic free calcium mobilization, shape change and protein phosphorylation, focusing on the role of tyrosine phosphorylation-activated PLCgamma.

RESULTS

The exposure of VSMC to exogenous H2O2 caused a rapid increase in cytosolic free calcium concentration ([Ca2+]i), and induced a significant VSMC shape change. Both effects were dependent on a tyrosine kinase-mediated mechanism, as determined by the blockade of short-term treatment of VSMC with the protein tyrosine kinase inhibitor, genistein. Giving further support to the putative role of phospholipase C (PLC)-dependent pathways, the [Ca2+]i and VSMC shape change response were equally inhibited by the specific PLC blocker, 1-(6-((17-beta-methoxyestra-1,3,5(10)trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122). In addition, U73122 had a protective effect against the deleterious action (24 h) of H2O2 on non-confluent VSMC. As a further clarification of the specific pathway involved, the exposure to H2O2 significantly stimulated the tyrosine phosphorylation of PLCgamma with a concentration- and time-profile similar to that of [Ca(2+)](i) mobilization.

CONCLUSIONS

The present study reveals that H(2)O(2) activates PLCgamma on VSMC through tyrosine phosphorylation and that this activation has a major role in rapid [Ca(2+)](i) mobilization, shape-changing actions and damage by H(2)O(2) in this type of cells. These findings have direct implications for understanding the mechanisms of the vascular actions of H(2)O(2) and may help to design pharmacologically protective strategies.

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.

Coupling of M(2) muscarinic receptors to Src activation in cultured canine colonic smooth muscle cells

The purpose of this study was to determine whether Src tyrosine kinases are one of the signaling intermediaries linking M(2) receptor stimulation to extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) in cultures of canine colonic smooth muscle cells (CSMC). RT-PCR studies demonstrate expression of multiple Src tyrosine kinases, including Src, Fyn, and Yes, in CSMC. Muscarinic stimulation of CSMC with 10 microM ACh results in a twofold increase in Src activity within 10 min but does not increase the activity of Fyn. Treatment with the M(2) antagonist AF-DX 116 (10 microM) blocks ACh-stimulated Src activation in primary CSMC cultures that express both M(2) and M(3) receptors and in first-passage CSMC cultures that express predominantly M(2) receptors. Alkylation of M(3) receptors with 100 nM N,N-dimethyl-4-piperidinyl diphenylacetate mustard has no effect on Src activity. Treatment with the pyrazolopyrimidine Src inhibitor PP1 (10 microM) or AF-DX 116 (10 microM) blocks ACh-stimulated ERK phosphorylation. Together these results indicate that M(2) receptors are coupled to Src tyrosine kinase and subsequent activation of ERK in cultured CSMC.

Angiotensin II-induced stimulation of p21-activated kinase and c-Jun NH2-terminal kinase is mediated by Rac1 and Nck

p21-activated kinase (PAK) has been shown to be an upstream mediator of JNK in angiotensin II (AngII) signaling. Little is known regarding other signaling molecules involved in activation of PAK and JNK by AngII. Rho family GTPases Rac and Cdc42 have been shown to enhance PAK activity by binding to p21-binding domain of PAK (PAK-PBD). In vascular smooth muscle cells (VSMC) AngII stimulated Rac1 binding to GST-PAK-PBD fusion protein. Pretreatment of VSMC by genistein inhibited AngII-induced Rac1 activation, whereas Src inhibitor PP1 had no effect. Inhibition of protein kinase C by phorbol 12,13-dibutyrate pretreatment also decreased AngII-mediated activation of Rac1. The adaptor molecule Nck has been shown previously to mediate PAK activation by facilitating translocation of PAK to the plasma membrane. In VSMC AngII stimulated translocation of Nck and PAK to the membrane fraction. Overexpression of dominant-negative Nck in Chinese hamster ovary (CHO) cells, stably expressing the AngII type I receptor (CHO-AT1), inhibited both PAK and JNK activation by AngII, whereas it did not affect ERK1/2. Finally, dominant-negative Nck inhibited AngII-induced DNA synthesis in CHO-AT1 cells. Our data provide evidence for Rac1 and Nck as upstream mediators of PAK and JNK in AngII signaling and implicate JNK in AngII-induced growth responses.

Superoxide anion-induced formation of inositol phosphates involves tyrosine kinase activation in smooth muscle cells from rat mesenteric artery

Our previous studies have demonstrated an enhanced production of inositol phosphates (IPs) induced by superoxide in smooth muscle cells (SMCs). The mechanisms for this effect, however, remained largely unknown. In the present study, it was found that superoxide increased IP production in SMCs from rat mesenteric arteries in a time-dependent manner. The effect of superoxide on IP formation was significantly inhibited by the antioxidants n-acetylcysteine or alpha-lipoic acid. Genistein and tyrphostin A25, two tyrosine kinase inhibitors, also inhibited the superoxide-induced IP formation. The application of monoclonal antibody against phospholipase Cgamma (PLCgamma) significantly inhibited the superoxide-induced IP formation. Finally, the expression level of PLCgamma proteins was increased 6 hrs after exposing SMCs to superoxide. The present findings demonstrate that superoxide activates the tyrosine kinase pathway and suggest that the tyrosine kinase-mediated IP formation may represent a novel mechanism underlying the signalling role of superoxide in rat mesenteric artery SMCs.

MAP kinases and vascular smooth muscle function

The mitogen-activated protein (MAP) kinase family members are ubiquitously expressed protein kinases activated in response to a variety of extracellular stimuli and shown to be involved in cell growth, transformation, differentiation and apoptosis. MAP kinases have been implicated in both growth and apoptosis of vascular smooth muscle cells (VSMC) which suggests that they play important roles in cardiovascular diseases such as essential hypertension, atherosclerosis, and restenosis followed angioplasty. The MAP kinases are themselves components of specific kinase cascades characterized by activation by specific stimuli, families of related serine and threonine kinases and downstream substrates that include other kinases, transcription factors, membrane receptors and other cell mediators. Cross-talk among the different MAP kinases results in direct modulation of signal transduction. In addition, increased expression and activation of MAP kinase phosphatases plays an important role in MAP kinase inactivation. Our laboratory has used angiotensin II (AngII), a potent activator of all MAP kinases in VSMC, to study mechanisms by which MAP kinases are regulated by vasoactive peptides. In this review, we describe the mechanisms by which AngII activates MAP kinases, and potential roles for MAP kinases in AngII-dependent effects on VSMC function.