The Ang II-induced growth of vascular smooth muscle cells involves a phospholipase D-mediated signaling mechanism

Angiotensin (Ang) II acts as a mitogen in vascular smooth muscle cells (VSMC) via the activation of multiple signaling cascades, including phospholipase C, tyrosine kinase, and mitogen-activated protein kinase pathways. However, increasing evidence supports signal-activated phospholipases A(2) and D (PLD) as additional mechanisms. Stimulation of PLD results in phosphatidic acid (PA) formation, and PA has been linked to cell growth. However, the direct involvement of PA or its metabolite diacylglycerol (DAG) in Ang II-induced growth is unclear. PLD activity was measured in cultured rat VSMC prelabeled with [(3)H]oleic acid, while the incorporation of [(3)H]thymidine was used to monitor growth. We have previously reported the Ang II-dependent, AT(1)-coupled stimulation of PLD and growth in VSMC. Here, we show that Ang II (100 nM) and exogenous PLD (0.1-100 units/mL; Streptomyces chromofuscus) stimulated thymidine incorporation (43-208% above control). PA (100 nM-1 microM) also increased thymidine incorporation to 135% of control. Propranolol (100 nM-10 microM), which inhibits PA phosphohydrolase, blocked the growth stimulated by Ang II, PLD, or PA by as much as 95%, an effect not shared by other beta-adrenergic antagonists. Propranolol also increased the production of PA in the presence of Ang II by 320% and reduced DAG and arachidonic acid (AA) accumulation. The DAG lipase inhibitor RHC-80267 (1-10 microM) increased Ang II-induced DAG production, while attenuating thymidine incorporation and release of AA. Thus, it appears that activation of PLD, formation of PA, conversion of PA to DAG, and metabolism of DAG comprise an important signaling cascade in Ang II-induced growth of VSMC.     

Anti-proliferative effect of rosiglitazone on angiotensin II-induced vascular smooth muscle cell proliferation is mediated by the mTOR pathway

VSMC (vascular smooth muscle cell) proliferation contributes significantly to intimal thickening in atherosclerosis, restenosis and venous bypass graft diseases. Ang II (angiotensin II) has been implicated in VSMC proliferation though the activation of multiple growth-promoting signals. Although TZDs (thiazolidinediones) can inhibit VSMC proliferation and reduce Ang II-induced fibrosis, the mechanism underlying the inhibition of VSMC proliferation and fibrosis needs elucidation. We have used primary cultured rat aortic VSMCs and specific antibodies to investigate the inhibitory mechanism of rosiglitazone on Ang II-induced VSMC proliferation. Rosiglitazone treatment significantly inhibited Ang II-induced rat aortic VSMC proliferation in a dose-dependent manner. Western blot analysis showed that rosiglitazone significantly lowered phosphorylated ERK1/2 (extracellular-signal-regulated kinase 1/2), Akt (also known as protein kinase B), mTOR (mammalian target of rapamycin), p70S6K (70 kDa S6 kinase) and 4EBP1 (eukaryotic initiation factor 4E-binding protein) levels in Ang II-treated VSMCs. In addition, PPAR-γ (peroxisome-proliferator-activated receptor γ) mRNA increased significantly and CTGF (connective tissue growth factor), Fn (fibronectin) and Col III (collagen III) levels decreased significantly. The results demonstrate that the rosiglitazone directly inhibits the pro-atherosclerotic effect of Ang II on rat aortic VSMCs. It also attenuates Ang II-induced ECM (extracellular matrix) molecules and CTGF production in rat aortic VSMCs, reducing fibrosis. Importantly, PPAR-γ activation mediates these effects, in part, through the mTOR-p70S6K and -4EBP1 system.     

Phosphatidylinositide 3-kinase regulates angiotensin II-induced cytosolic phospholipase A2 activity and growth in vascular smooth muscle cells

Angiotensin (Ang) II via the AT(1) receptor acts as a mitogen in vascular smooth muscle cells (VSMC) through stimulation of multiple signaling mechanisms, including tyrosine kinases and mitogen-activated protein kinase (MAPK). In addition, cytosolic phospholipase A(2)(cPLA(2))-dependent release of arachidonic acid (AA) is linked to VSMC growth and we have reported that Ang II stimulates cPLA(2) activity via the AT(1) receptor. The coupling of Ang II to the activation of cPLA(2) appears to involve mechanisms both upstream and downstream of MAPK such that AA stimulates MAPK activity which phosphorylates cPLA(2) to further enhance AA release. However, the upstream mechanisms responsible for activation of cPLA(2) are not well-defined. One possibility includes phosphatidylinositide 3-kinase (PI3K), since PI3K has been reported to participate in the upstream signaling events linked to activation of MAPK. However, it is not known whether PI3K is involved in the Ang II-induced activation of cPLA(2) or if this mechanism is associated with the Ang II-mediated growth of VSMC. Therefore, we used cultured rat VSMC to examine the role of PI3K in the Ang II-dependent phosphorylation of cPLA(2), release of AA, and growth induced by Ang II. Exposure of VSMC to Ang II (100 nM) increased [(3)H]thymidine incorporation, cell number, and the release of [(3)H]AA. Also, using Western analysis, Ang II increased the phosphorylation of MAPK and cPLA(2) which were blocked by the MAPK kinase inhibitor PD98059 (10 microM/L). Similarly, the PI3K inhibitor LY294002 (10 microM/L) abolished the Ang II-mediated increase in MAPK phosphorylation, as well as phosphoserine-PLA(2). Further, inhibition of PI3K blocked the Ang II-induced release of AA and VSMC mitogenesis. However, exogenous AA was able to restore VSMC growth in the presence of LY294002, as well as reverse the inhibition of MAPK and cPLA(2) phosphorylation by LY294002. Thus, it appears from these data that Ang II stimulates the PI3K-sensitive release of AA which stimulates MAPK to phosphorylate cPLA(2) and enhance AA release. This mechanism may play an important role in the Ang II-induced growth of VSMC.     

The effects of thiazolidinediones on vascular smooth muscle cell activation by angiotensin II

Angiotensin II (Ang II) stimulates the activation of extracellular signal-regulated kinase (ERK), a subgroup of the mitogen-activated protein kinase (MAPK) family, in cultured vascular smooth muscle cells (VSMC). This ERK activation was recently shown to be a critical regulatory factor for Ang II-mediated migration and growth. It has been demonstrated that the thiazolidinedione troglitazone (TRO) blocked Ang II-induced DNA synthesis and migration in VSMC. Here we provide evidence for TRO to inhibit Ang II-induced ERK activation which was suggested to constitute the mechanism by which this agent blocks Ang II-induced VSMC growth and migration. We have found that pretreatment with PD98059, which selectively blocks the activity of ERK pathway at the level of MAPK kinase, decreased Ang II-induced AP-1 activation and that TRO is capable of inhibiting Ang II-induced AP-1 activation. On the other hand, the other thiazolidinediones pioglitazone (PIO) and rosiglitazone (ROSI) had little effect on Ang II-induced activation of ERK or AP-1, suggesting the inhibitory effects of TRO on VSMC activation by Ang II be independent of the peroxisome proliferator-activated receptor-gamma (PPARgamma) for which thiazolidinediones are ligands. Ang II-induced ERK activation was inhibited by protein kinase C (PKC)-specific inhibitor GF109203X, while TRO was also able to block PKC activator phorbol 12 myristate 13-acetate (PMA)-induced ERK activation. Accordingly, TRO may inhibit Ang II-induced MAPK activation at least partly by an inhibition of PKC. These results support the assumption that by targeting MAPK activation, TRO may inhibits the critical signaling steps leading to restenosis and atherosclerosis that may result in part from dysregulated VSMC growth and migration induced by Ang II.     

Phosphoinositide and calcium signalling responses in smooth muscle cells: comparison between lipoproteins, Ang II, and PDGF.

The effects of low density lipoprotein (LDL) and high density lipoprotein (HDL3) on second messenger systems were investigated in cultured human vascular smooth muscle cells (VSMC) and compared with those of angiotensin II (Ang II) and platelet-derived growth factor (PDGF-BB). Phosphoinositide metabolism was studied in myo-[2-3H]-inositol prelabelled VSMC using high performance liquid anion-exchange chromatography. The spectra of inositol phosphate isomers increased after stimulation with either Ang II, LDL, HDL3 or PDGF-BB were qualitatively identical. Major increases occurred in 4-IP1, 1,4-IP2, 1,3,4-IP3 and 1,3,4,5-IP4. These are metabolic conversion products of 1,4,5-IP3 for which only a minor increase was found. Thus lipoproteins, like Ang II and PDGF-BB, activate polyphosphatidylinositol-specific phospholipase C. Intracellular Ca2+ concentrations ([Ca2+]i) were studied in fura-2 loaded VSMC. In monolayer cultures LDL and HDL3 increased [Ca2+]i with kinetics comparable to those for Ang II. Relative to the effects of these agonists, the PDGF-BB-induced increase in [Ca2+]i was slower in onset and the decay from peak [Ca2+]i levels more gradual. Fluorescence recordings from single cells exposed to LDL and HDL3 revealed a prolonged series of transient oscillations of [Ca2+]i, a phenomenon typical for calcium-mobilizing hormones. Additionally, as found for Ang II, preincubation of VSMC with either phorbol 12-myristate, 13-acetate, forskolin or 8-bromo-cyclic GMP inhibited LDL- and HDL-induced accumulation of [3H]-inositol monophosphate. We propose that LDL and HDL3 stimulate signal transduction in VSMC via mechanisms analogous to those of Ca(2+)-mobilizing hormones.     

Implication of multiple signaling pathways in the regulation of angiotensin II induced enhanced expression of Giα proteins in vascular smooth muscle cells

We have previously shown that A10 vascular smooth muscle cells (VSMC) exposed to angiotensin II (Ang?II) exhibited overexpression of Giα proteins. In the present study, we examined the involvement of different signaling pathways in regulating Ang II induced enhanced expression of Giα proteins in VSMC by using pharmacological inhibitors. Ang II induced increased expression of Giα proteins in A10 VSMC was markedly attenuated by actinomycin D, losartan (an AT(1) receptor antagonist), dibutyryl cAMP, phospholipase C (PLC) inhibitor U73122, protein kinase C (PKC) inhibitors staurosporine and GP109203X, but not by PD123319 (an AT(2) receptor antagonist). In addition, BAPTA-AM and TMB-8 (chelators of intracellular Ca(2+)); and nifedipine (a blocker of L-type Ca(2+) channels) significantly inhibited Ang II induced enhanced expression of Giα proteins. On the other hand, extracellular Ca(2+) chelation using EGTA did not affect the Ang II evoked enhanced levels of Giα proteins. Furthermore, pretreatment of A10 VSMC with calmidazolium (an inhibitor of calmodulin), or KN93 (an inhibitor of CaM kinase), or genistein (an inhibitor of protein tyrosine kinase, PTK), also attenuated the increased levels of Giα proteins induced by Ang?II. These results suggest that Ang II induced enhanced expression of Giα proteins may be regulated by different signaling pathways through AT(1) receptors in A10 VSMC.     

MicroRNA-199a-5p aggravates angiotensin II–induced vascular smooth muscle cell senescence by targeting Sirtuin-1 in abdominal aortic aneurysm

Vascular smooth muscle cells (VSMCs) senescence contributes to abdominal aortic aneurysm (AAA) formation although the underlying mechanisms remain unclear. This study aimed to investigate the role of miR-199a-5p in regulating VSMC senescence in AAA. VSMC senescence was determined by a senescence-associated β-galactosidase (SA-β-gal) assay. RT-PCR and Western blotting were performed to measure miRNA and protein level, respectively. The generation of reactive oxygen species (ROS) was evaluated by H2DCFDA staining. Dual-luciferase reporter assay was used to validate the target gene of miR-199a-5p. VSMCs exhibited increased senescence in AAA tissue relative to healthy aortic tissue from control donors. Compared with VSMCs isolated from control donors (control-VSMCs), those derived from patients with AAA (AAA-VSMCs) exhibited increased cellular senescence and ROS production. Angiotensin II (Ang II) induced VSMC senescence by promoting ROS generation. The level of miR-199a-5p expression was upregulated in the plasma from AAA patients and Ang II–treated VSMCs. Mechanistically, Ang II treatment significantly elevated miR-199a-5p level, thereby stimulating ROS generation by repressing Sirt1 and consequent VSMC senescence. Nevertheless, Ang II–induced VSMC senescence was partially attenuated by a miR-199a-5p inhibitor or Sirt1 activator. Our study revealed that miR-199a-5p aggravates Ang II–induced VSMC senescence by targeting Sirt1 and that miR-199a-5p is a potential therapeutic target for AAA.     

Loss of FoxO3a prevents aortic aneurysm formation through maintenance of VSMC homeostasis

Vascular smooth muscle cell (VSMC) phenotypic switching plays a critical role in the formation of abdominal aortic aneurysms (AAAs). FoxO3a is a key suppressor of VSMC homeostasis. We found that in human and animal AAA tissues, FoxO3a was upregulated, SM22α and α-smooth muscle actin (α-SMA) proteins were downregulated and synthetic phenotypic markers were upregulated, indicating that VSMC phenotypic switching occurred in these diseased tissues. In addition, in cultured VSMCs, significant enhancement of FoxO3a expression was found during angiotensin II (Ang II)-induced VSMC phenotypic switching. In vivo, FoxO3a overexpression in C57BL/6J mice treated with Ang II increased the formation of AAAs, whereas FoxO3a knockdown exerted an inhibitory effect on AAA formation in ApoE^−/− mice infused with Ang II. Mechanistically, FoxO3a overexpression significantly inhibited the expression of differentiated smooth muscle cell (SMC) markers, activated autophagy, the essential repressor of VSMC homeostasis, and promoted AAA formation. Our study revealed that FoxO3a promotes VSMC phenotypic switching to accelerate AAA formation through the P62/LC3BII autophagy signaling pathway and that therapeutic approaches that decrease FoxO3a expression may prevent AAA formation.Subject terms: Cell biology, Diseases     

Resveratrol attenuates angiotensin II-induced senescence of vascular smooth muscle cells

Resveratrol (3,5,4'-trihydroxystilbene), a polyphenol abundant in red wine, is known to extend the life span of diverse species. On the contrary, it was reported that angiotensin (Ang) II enhances senescence of vascular smooth muscle cells (VSMCs). We, therefore, examined whether resveratrol attenuates Ang II-induced senescence of VSMC. Senescence-associated β-galactosidase (SA β-gal) assay showed that Ang II induced senescence of VSMC. The Ang II-induced senescence was inhibited by losartan, an Ang II type 1 receptor (AT1R) antagonist but not by PD123319, Ang II type 2 receptor antagonist, indicating that AT1R is responsible for the induction of senescence. Resveratrol suppressed Ang II-induced senescence of VSMC in a dose-dependent manner. In addition, resveratrol suppressed Ang II-induced induction of p53 and its downstream target gene p21, both of which play an important role in the induction of senescence. Resveratrol suppressed senescence of VSMC possibly through inhibition of AT1R-dependent induction of p53/p21. Suppression of p53 induction may be involved in the longevity by resveratrol.     

Down-regulation by antisense oligonucleotides establishes a role for the proline-rich tyrosine kinase PYK2 in angiotensin ii-induced signaling in vascular smooth muscle

Abnormal vascular smooth muscle cell (VSMC) growth plays a key role in the pathogenesis of hypertension and atherosclerosis. Angiotensin II (Ang II) elicits a hypertrophic growth response characterized by an increase in protein synthesis in the absence of DNA synthesis and cell proliferation. Intracellular signaling mechanisms linking angiotensin type I receptor activation to protein synthesis in VSMC have not been fully characterized. The present study investigates the role of the nonreceptor proline-rich tyrosine kinase 2 (PYK2) in Ang II-induced VSMC protein synthesis and in the regulation of two signaling pathways that have been implicated in the control of protein synthesis, the extracellular signal-regulated kinase (ERK1/2) and the phosphatidylinositol 3-kinase/Akt pathways. PYK2 antisense oligonucleotides were used to down-regulate PYK2 expression in cultured VSMC. An 80% down-regulation in PYK2 expression resulted in an approximately 80% inhibition of ERK1/2 (3.8 +/- 1.3 versus 16.6 +/- 1.8), p70S6 kinase (1.03 +/- 0.03 versus 3.8 +/- 0.5), and Akt activation (3.0 +/- 0.8 versus 16.0 +/- 1.0) by Ang II. Furthermore, PYK2 down-regulation resulted in a complete inhibition of Ang II-induced VSMC protein synthesis. These data conclusively identify PYK2 as an upstream regulator of both the ERK1/2 and the phosphatidylinositol 3-kinase/Akt pathways that are involved in Ang II-induced VSMC protein synthesis.     

Ribozyme to human TGF-beta1 mRNA inhibits the proliferation of human vascular smooth muscle cells

Transforming growth factor-beta (TGF-beta) has been reported to be involved in the pathogenesis of cardiovascular proliferative diseases such as hypertensive vascular disease, atherosclerosis, and arterial restenosis after angioplasty. We designed a 38-base DNA-RNA chimeric hammerhead ribozyme to cleave human TGF-beta1 mRNA as a gene therapy for human arterial proliferative diseases. In the presence of MgCl(2), synthetic ribozyme to human TGF-beta1 mRNA cleaved the synthetic target RNA into two RNA fragments of predicted size. A control mismatch ribozyme, with one different base in the catalytic loop region, was inactive. DNA-RNA chimeric ribozyme (0. 01-1.0 microM) significantly inhibited angiotensin II (Ang II)-stimulated DNA synthesis in a dose-dependent manner in human vascular smooth muscle cells (VSMC). The mismatch ribozyme did not affect Ang II-stimulated DNA synthesis in the cells. DNA-RNA chimeric ribozyme (1.0 microM) inhibited the proliferation of human VSMC in the presence of Ang II. DNA-RNA chimeric ribozyme (1.0 microM) significantly inhibited Ang II-stimulated TGF-beta1 mRNA and protein expression in human VSMC. These results indicate that the designed DNA-RNA chimeric hammerhead ribozyme targeted to human TGF-beta1 mRNA can effectively and potentially inhibit growth of human VSMC by cleaving the TGF-beta1 mRNA. This finding suggests that this ribozyme will be useful in the gene therapy of arterial proliferative diseases.     

The GTPase ARF6 Controls ROS Production to Mediate Angiotensin II-Induced Vascular Smooth Muscle Cell Proliferation

High reactive oxygen species (ROS) levels and enhanced vascular smooth muscle cells (VSMC) proliferation are observed in numerous cardiovascular diseases. The mechanisms by which hormones such as angiotensin II (Ang II) acts to promote these cellular responses remain poorly understood. We have previously shown that the ADP-ribosylation factor 6 (ARF6), a molecular switch that coordinates intracellular signaling events can be activated by the Ang II receptor (AT₁R). Whether this small GTP-binding protein controls the signaling events leading to ROS production and therefore Ang II-dependent VSMC proliferation, remains however unknown. Here, we demonstrate that in rat aortic VSMC, Ang II stimulation led to the subsequent activation of ARF6 and Rac1, a key regulator of NADPH oxidase activity. Using RNA interference, we showed that ARF6 is essential for ROS generation since in conditions where this GTPase was knocked down, Ang II could no longer promote superoxide anion production. In addition to regulating Rac1 activity, ARF6 also controlled expression of the NADPH oxidase 1 (Nox 1) as well as the ability of the EGFR to become transactivated. Finally, ARF6 also controlled MAPK (Erk1/2, p38 and Jnk) activation, a key pathway of VSMC proliferation. Altogether, our findings demonstrate that Ang II promotes activation of ARF6 to controls ROS production by regulating Rac1 activation and Nox1 expression. In turn, increased ROS acts to activate the MAPK pathway. These signaling events represent a new molecular mechanism by which Ang II can promote proliferation of VSMC.     

Inhibition of AT1 receptor internalization by concanavalin A blocks angiotensin II-induced ERK activation in vascular smooth muscle cells. Involvement of epidermal growth factor receptor proteolysis but not AT1 receptor internalization

Recent studies of beta(2)-adrenergic receptor suggest that agonist-promoted receptor internalization may play an important role in extracellular signal-regulated kinase (ERK) activation by G protein-coupled receptors. In the present study, we explored the effects of angiotensin II (Ang II) type-1 receptor (AT(1)) internalization on Ang II-induced activation of ERK using the receptor internalization blocker concanavalin A (ConA) and the carboxyl terminus-truncated receptor mutants with impaired internalization. ConA inhibited AT(1) receptor internalization without affecting ligand binding to the receptor, Ang II-induced generation of second messengers, and activation of tyrosine kinases Src and Pyk2 in vascular smooth muscle cells (VSMC). ConA blocked ERK activation evoked by Ang II and the calcium ionophore A23187. Impairment of AT(1) receptor internalization by truncating the receptor carboxyl terminus did not affect Ang II-induced ERK activation. ConA induced proteolytic cleavage of the epidermal growth factor (EGF) receptor at carboxyl terminus and abolished Ang II-induced transactivation of the EGF receptor, which is critical for ERK activation by Ang II in VSMC. ConA also induced proteolysis of erbB-2 but not platelet-derived growth factor receptor. Thus, ConA blocks Ang II-induced ERK activation in VSMC through a distinct mechanism, the ConA-mediated proteolysis of the EGF receptor.     

Angiotensin II triggers release of leukotriene C4 in vascular smooth muscle cells via the multidrug resistance-related protein 1

The multidrug resistance-related protein-1 (MRP1) is important for the management of oxidative stress in vascular cells in vivo. Substrates of MRP1 are, among others, glutathione and the leukotriene C₄ (LTC₄), an eicosanoid and mediator of inflammation. Angiotensin (Ang) II infusion results in MRP1^?/? mice compared to wild-type mice in improved endothelial function and reduced reactive oxygen species (ROS) formation. However, the interaction between Ang II, LTC₄ and MRP1 is not completely understood and has never been investigated in vitro. Ang II induced in vascular smooth muscle cells (VSMC) the release of LTC₄ and the generation of ROS. Pharmacologic inhibition of MRP1 via MK 571 significantly reduced Ang II-induced ROS release (L012-luminescence) in VSMC. The release of ROS after Ang II stimulation is inhibited, to a comparable degree, by blockade of the Cys-LT1 receptor with montelukast. Incubation of VSMC with recombined LTC₄ and Ang II caused enhanced rates of proliferation in VSMC. This effect can be rescued by either MRP1 or Cys-LT1 receptor inhibition. Accordingly, stimulation of VSMC with LTC₄ reduces intracellular levels of glutathione, but does not affect apoptosis. LTC₄ stimulation results in a significant activation of MRP1, but does not alter MRP1 expression. These findings indicate a connection between Ang II, MRP1 and LTC₄. Both, MRP1 and LTC₄, are potentially promising targets for atheroprotective therapy.     

Human-derived vascular smooth muscle cells produce angiotensin II by changing to the synthetic phenotype

We investigated whether vascular smooth muscle cells (VSMC)-derived from human produce angiotensin (Ang) II upon change from the contractile phenotype to the synthetic phenotype by incubation with fibronectin (FN). Expression of alpha-smooth muscle (SM) actin, apparent in the contractile phenotype, was decreased by FN. Expressions of matrix Gla and osteopontin, apparent in the synthetic phenotype, were increased by FN. Ang II measured by radioimmunoassay (RIA) was significantly increased in human VSMC by FN. Expression of mRNAs for Ang II-generating proteases cathepsin D, cathepsin G, ACE, and chymase was increased by FN. Expressions of cathepsin D and cathepsin G proteins were also increased by FN. Ang I-generating activity, which was inhibited by an aspartyl protease inhibitor pepstatin A, was readily detected in the conditioned medium from human VSMC. Antisense oligodeoxynucleotides (ODNs) that hybridize with cathepsin D and cathepsin G significantly inhibited FN-increased Ang II in conditioned medium and cell extracts. In VSMC conditioned medium, FN-induced elevation of Ang II was significantly inhibited by temocapril but not by chymostatin. Ang II type 1 receptor antagonist CV11974 completely, and antisense cathepsin D and cathepsin G ODNs partially inhibited the FN-stimulated growth of human VSMC. These results indicate that the change of homogeneous cultures of human VSMC from the contractile to the synthetic phenotype sequentially increases expression of proteases cathepsin D, cathepsin G, and ACE, production of Ang II and productions of growth factors, culminating in VSMC proliferation. These findings implicate a new mechanism for the pathogenesis of human vascular proliferative diseases.     

Angiotensin II induces c-fos mRNA in aortic smooth muscle. Role of Ca2+ mobilization and protein kinase C activation

Vasoconstrictors such as angiotensin II (Ang II) play an important role in the pathogenesis of hypertension. These agonists may be responsible for the abnormal vascular smooth muscle cell (VSMC) growth seen in hypertension, either indirectly as a consequence of elevating blood pressure or directly as a result of receptor-mediated effects on VSMC growth. To investigate whether Ang II might directly initiate or modulate some of the "early" genetic programs associated with growth in VSMC, the expression of the proto-oncogene c-fos was studied in cultured rat aortic VSMC. Ang II rapidly induced the accumulation of c-fos mRNA, with maximal levels occurring at approximately 30 min. Induction of c-fos mRNA by Ang II was concentration-dependent, with a maximal response at 100 nM. Ang II induction of c-fos mRNA was blocked by its competitive inhibitor, [sarcosine 1,isoleucine 8]angiotensin II. Induction of c-fos mRNA was not dependent upon Ang II-stimulated intracellular alkalinization or activation of Na+/H+ exchange, but was dependent upon mobilization of intracellular Ca2+ and protein kinase C activation. Epidermal growth factor, a VSMC mitogen, also induced c-fos mRNA in VSMC, but by a mechanism different from that of Ang II. These results demonstrate that the vasoconstrictor hormone Ang II induces in VSMC one of the earliest genes, c-fos, associated with the proliferative response.     

Role of the actin cytoskeleton in angiotensin II signaling in human vascular smooth muscle cells

Angiotensin II (Ang II) regulates vascular smooth muscle cell (VSMC) function by activating signaling cascades that promote vasoconstriction, growth, and inflammation. Subcellular mechanisms coordinating these processes are unclear. In the present study, we questioned the role of the actin cytoskeleton in Ang II mediated signaling through mitogen-activated protein (MAP) kinases and reactive oxygen species (ROS) in VSMCs. Human VSMCs were studied. Cells were exposed to Ang II (10-7 mol/L) in the absence and presence of cytochalasin B (10-6 mol/L, 60 min), which disrupts the actin cytoskeleton. Phosphorylation of p38MAP kinase, JNK, and ERK1/2 was assessed by immuno blotting. ROS generation was measured using the fluoroprobe chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (4 micromol/L). Interaction between the cytoskeleton and NADPH oxidase was determined by evaluating the presence of p47phox in the Triton X-100 insoluble membrane fraction. Ang II significantly increased phosphorylation of p38MAP kinase, JNK, and ERK1/2 (two- to threefold above control, p < 0.05). Cytochalasin B pretreatment attenuated p38MAP kinase and JNK effects (p < 0.05) without altering ERK1/2 phosphorylation. ROS formation, which was increased in Ang II stimulated cells, was significantly reduced by cytochalasin B (p < 0.01). p47phox, critically involved in NADPH oxidase activation, colocalized with the actin cytoskeleton in Ang II stimulated cells. Our data demonstrate that Ang II mediated ROS formation and activation of p38MAP kinase and JNK, but not ERK1/2, involves the actin cytoskeleton in VSMCs. In addition, Ang II promotes interaction between actin and p47phox. These data indicate that the cytoskeleton is involved in differential MAP kinase signaling and ROS generation by Ang II in VSMCs. Together, these studies suggest that the cytoskeleton may be a central point of crosstalk in growth- and redox-signaling pathways by Ang II, which may be important in the regulation of VSMC function.