Growth factor-induced phosphoinositide 3-OH kinase/Akt phosphorylation in smooth muscle cells: induction of cell proliferation and inhibition of cell death

Melittin inhibits vascular smooth muscle cell proliferation through induction of apoptosis via suppression of nuclear factor-kappaB and Akt activation and enhancement of apoptotic protein expression

In the present study, we have investigated the bee venom (BV) and melittin (a major component of BV)-mediated antiproliferative effect and defined its mechanisms of action in cultured rat aortic vascular smooth muscle cell(s) (VSMC). BV and melittin ( approximately 0.4-0.8 microg/ml) effectively inhibited 5% fetal bovine serum-induced and 50 ng/ml platelet-derived growth factor BB (PDGF-BB)-induced VSMC proliferation. The regulation of apoptosis has attracted much attention as a possible means of eliminating excessively proliferating VSMC. In the present study, the treatment of BV and melittin strongly induced apoptosis of VSMC. To investigate the antiproliferative mechanism of BV and melittin, we examined the effect of melittin on nuclear factor kappaB (NF-kappaB) activation, the PDGF-BB-induced IkappaBalpha phosphorylation, and its degradation were potently inhibited by melittin and whether DNA binding activity and nuclear translocation of NF-kappaB p50 subunit in response to the action of PDGF-BB were potently attenuated by melittin. In further investigations, melittin markedly inhibited the PDGF-BB-induced phosphorylation of Akt and weakly inhibited phosphorylation of extracellular signal-regulated kinase 1/2, upstream signals of NF-kappaB. Treatment of melittin also potently induced proapoptotic protein p53, Bax, and caspase-3 expression but decreased antiapoptotic protein Bcl-2 expression. These results suggest the antiproliferative effects of BV and melittin in VSMC through induction of apoptosis via suppressions of NF-kappaB and Akt activation and enhancement of apoptotic signaling pathway.

Aging alters vascular mechanotransduction: pressure-induced regulation of p70S6k in the rat aorta

Physical forces are important regulators of vascular structure and function though it is unknown how aging may affect the ability of the vasculature to respond to mechanical stimuli. We investigated the pressure-induced activation of ribosomal S6-kinase (p70S6k) and its pathway-related proteins (Akt, GSK-3beta, SHP-2, PTEN) in aortae from young adult (6 month), aged (30 month), and very aged (36 month) Fischer 344 x Brown Norway F1 hybrid rats. With aging, the aortic tissue content of Akt. SHP-2, and PTEN was significantly increased while total p70S6k and GSK-3beta were unchanged. By comparison, the basal phosphorylation of p70S6k at Thr 389 and Thr 421/Ser 424 was increased ( approximately 40%) and unchanged, respectively, while Akt decreased (approximately 37%), GSK-3beta was unchanged, SHP-2 increased (approximately 73.5%), and PTEN increased (approximately 120%) in the aortae of very aged rats. Acute pressurization of aortae resulted in similar increases in phosphorylation of Akt among the different age groups. By comparison, pressure-induced phosphorylation of p70S6k at Thr 389, GSK-3beta and SHP-2 decreased; whereas, PTEN dephosphorylation was increased in 36-month versus 6-month aortae. The results indicate marked alterations in the p70S6k signaling pathway with aging. The implications of these findings on age-associated vessel remodeling are discussed.

Sustained orbital shear stress stimulates smooth muscle cell proliferation via the extracellular signal-regulated protein kinase 1/2 pathway

OBJECTIVE

Nonlaminar shear stress stimulates smooth muscle cell (SMC) proliferation and migration in vivo, especially after an endothelial-denuding injury. To determine whether sustained shear stress directly stimulates SMC proliferation in vitro, the effect of orbital shear stress on SMC proliferation, phenotype, and extracellular signal-regulated protein kinase 1/2 (ERK1/2) phosphorylation was examined.

METHODS

Bovine SMCs were exposed to orbital shear stress (210 rpm) for up to 10 days, with and without the ERK1/2 upstream pathway inhibitor PD98059 (10 microM) or the p38 pathway inhibitor SB203580 (10 microM). Proliferation was directly counted and assessed with proliferation cell nuclear antigen. Western blotting was used to assess activation of SMC ERK1/2 and SMC phenotype markers.

RESULTS

SMCs exposed to sustained orbital shear stress (10 days) had 75% increased proliferation after 10 days compared with static conditions. Expression of markers of the contractile phenotype (alpha-actin, calponin) was decreased, and markers of the synthetic phenotype (vimentin, beta-actin) were increased. ERK1/2 was phosphorylated in the presence of orbital shear stress, and orbital shear-stress-stimulated SMC proliferation was inhibited in the presence of PD98059 but sustained in the presence of SB203580. Orbital shear-stress-induced changes in SMC phenotype were also inhibited in the presence of PD98059.

CONCLUSION

Orbital shear stress directly stimulates SMC proliferation in long-term culture in vitro and is mediated, at least partially, by the ERK1/2 pathway. The ERK1/2 pathway may also mediate the orbital shear-stress-stimulated switch from SMC contractile to synthetic phenotype. These results suggest that shear-stress-stimulated SMC proliferation after vascular injury is mediated by a pathway amenable to pharmacologic manipulation.

The role of reactive oxygen species in insulin signaling in the vasculature

Although there is an abundance of evidence suggesting that insulin resistance plays a significant role in the vasculature, the precise mechanistic role involved still remains unclear. In this review, we discuss the current background of insulin resistance in the context of insulin signaling and action in the vasculature. Also, studies suggest that insulin resistance, diabetes, and cardiovascular disease all share a common involvement with oxidative stress. Recently, we reported that lysophosphatidylcholine, a major bioactive product of oxidized low-density lipoprotein, and angiotensin II, a vasoactive hormone and a potent inducer of reactive oxygen species (ROS), negatively regulate insulin signaling in vascular smooth muscle cells (VSMCs). In endothelial cells, insulin stimulates the release of nitric oxide, which results in VSMC relaxation and inhibition of atherosclerosis. Other data suggest that angiotensin II inhibits the vasodilator effects of insulin through insulin receptor substrate-1 phosphorylation at Ser312 and Ser616. Moreover, ROS impair insulin-induced vasorelaxation by neutralizing nitric oxide to form peroxynitrite. Thus, evidence is growing to enable us to better understand mechanistically the relationship between insulin/insulin resistance and ROS in the vasculature, and the impact they have on cardiovascular disease.

On How Insulin May Influence Ageing and Become Atherogenic throughout the Insulin-Like Growth Factor-1 Receptor Pathway: In vitro Studies with Human Vascular Smooth Muscle Cells

BACKGROUND

It is known that growth factors play a role in ageing and atherogenesis, and insulin develops mitogenic activity in vitro.

OBJECTIVES

This study focuses on the pathway by which insulin induces proliferation and mobility in vascular smooth muscle cells (SMCs) compared with that of insulin-like growth factor-1 (IGF-1), because they are two basic phenomena for atherogenesis that could also help to understand the role of insulin in the ageing process.

METHODS

Bromodeoxyuridine DNA incorporation, chemotaxis and the appearance of membrane ruffles were measured in cultured SMCs after incubation with insulin or IGF-1 in the presence of insulin or IGF-1 receptor-blocking antibodies.

RESULTS

Insulin-induced SMC proliferation through the IGF-1 receptors; indeed, the blockade of insulin receptors does not inhibit the mitogenic influence of insulin. On the contrary, insulin-induced cell migration was inhibited by blocking the insulin receptor but not the IGF-1 receptor. Nevertheless, in less differentiated SMCs from non-confluent cultures, the migratory response was significantly higher and insulin lost its receptor specificity. It was stimulated through receptors both for insulin and IGF-1. In these cases the IGF-1 action was similar. Insulin-induced F-actin rearrangements took place through both types of receptors, but IGF-1 was a little more specific through its own receptors.

CONCLUSION

The pathway activated by insulin to induce SMC proliferation is not different from that of IGF-1, whereas the unspecific mechanism inducing mobility in growing cells seems to be related to a higher sensitivity response. Cells with the highest mitotic activity have the highest mobility in which stimulation of receptor specificity is lost for either insulin or IGF-1. Extrapolating these results to in vivo, insulin could become relevant for inducing stabilization and also side effects in ageing.

Systematic Evaluation of Anti-apoptotic Growth Factor Signaling in Vascular Smooth Muscle Cells

Peptide growth factors contribute to the pathogenesis of cardiovascular diseases by inducing a variety of cellular responses including anti-apoptotic effects. Several of the signaling molecules that are activated by growth factor receptors such as Src family kinases (Src), phosphatidylinositol 3'-kinase (PI3K), phospholipase Cgamma (PLCgamma), Ras, and SHP-2 were shown to mediate survival signals. We systematically investigated the relative contribution of each signaling molecule for growth factor-dependent cell survival in vascular smooth muscle cells (VSMC). Our approach was the use of mutated plateletderived growth factor (PDGF) beta-receptors (betaPDGFR) in which the tyrosine residues required for binding of each signaling molecule were individually mutated to phenylalanine. To bypass endogenous PDGFR in VSMC we used chimeric receptors (ChiRs), containing the extracellular domain of the macrophage colony-stimulating factor (M-CSF) receptor and the cytoplasmic domain of the wild type (WT) or mutated betaPDGFR. Selective activation of the ChiR-WT with M-CSF significantly reduced apoptosis to the same extent as PDGF-BB in non-transfected cells. Deletion of the binding site for PI3K, but not for Src, RasGAP, SHP-2, or PLCgamma, completely abolished the anti-apoptotic effect. Consistently, a ChiR mutant that only binds PI3K was fully able to mediate cell survival as efficiently as the ChiR-WT. Furthermore, the PDGF-dependent anti-apoptotic effect in non-transfected cells was completely abolished by the PI3K inhibitor wortmannin, whereas inhibitors of Src, PLCgamma, ERK, or p38 MAP kinase had no effect. The exploration of downstream signaling events revealed that PDGF-BB activates the anti-apoptotic Akt signaling pathway in a PI3K-dependent manner. Moreover, Akt phosphorylates and thus inactivates the pro-apoptotic proteins BAD and Forkhead transcription factors (FKHR, FKHRL1). We conclude that growth factor-dependent cell survival in VSMC is mediated only by activation of the PI3K/Akt pathway, whereas all other receptor-associated signaling molecules do not play a significant role.

Insulin Signaling in Arteries Prevents Smooth Muscle Apoptosis

OBJECTIVE

Insulin is an antiapoptotic factor of cultured vascular cells, but it is not clear whether it also exerts antiapoptotic effects on vascular cells in vivo. We studied insulin receptor signaling in the arteries of normal and diabetic rats to establish whether insulin exhibits antiapoptotic activity toward vascular smooth muscle cells in vivo as well as in vitro.

METHODS AND RESULTS

Western blot analysis and real-time polymerase chain reaction revealed alpha- and beta-subunits of the insulin receptor in association with insulin receptor substrate-1 and phosphatidylinositol 3-kinase in the media of the aorta and carotid artery. The insulin receptor signaling pathway was partially activated under physiological conditions, further activated by intravenous insulin injection, and was attenuated in streptozotocin-induced diabetic rats. Lipopolysaccharide injection induced more apoptosis of vascular smooth muscle cells in diabetic rats than in control rats, whereas insulin prevented apoptosis in the aortic wall. An in vitro study suggested that the antiapoptotic effect of insulin was mediated by phosphatidylinositol 3-kinase.

CONCLUSIONS

Insulin is an antiapoptotic factor of vascular smooth muscle cells in vitro and in vivo. Decreased insulin activity on the artery may increase smooth muscle cell death and cause unstable plaque formation associated with diabetes.

Inhibition of cell cycle progression and migration of vascular smooth muscle cells by prostaglandin D2 synthase: resistance in diabetic Goto-Kakizaki rats

The regulation of vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis plays a clear role in the atherosclerotic process. Recently, we reported on the inhibition of the exaggerated growth phenotype of VSMCs isolated from hypertensive rats by lipocalin-type prostaglandin D2 synthase (L-PGDS). In the present study, we report the differential effects of L-PGDS on VSMC cell cycle progression, migration, and apoptosis in wild-type VSMCs vs. those from a type 2 diabetic model. In wild-type VSMCs, exogenously added L-PGDS delayed serum-induced cell cycle progression from the G1 to S phase, as determined by gene array analysis and the decreased protein expressions of cyclin-dependent kinase-2, p21Cip1, and cyclin D1. Cyclin D3 protein expression was unaffected by L-PGDS, although its gene expression was stimulated by L-PGDS in wild-type cells. In addition, platelet-derived growth factor-induced VSMC migration was inhibited by L-PGDS in wild-type cells. Type 2 diabetic VSMCs, however, were resistant to the L-PGDS effects on cell cycle progression and migration. L-PGDS did suppress the hyperproliferation of diabetic cells, albeit through a different mechanism, presumably involving the 2.5-fold increase in apoptosis and the concomitant 10-fold increase of L-PGDS uptake we observed in these cells. We propose that in wild-type VSMCs, L-PGDS retards cell cycle progression and migration, precluding hyperplasia of the tunica media, and that diabetic cells appear resistant to the inhibitory effects of L-PGDS, which consequently may help explain the increased atherosclerosis observed in diabetes.

Activation of phosphatidylinositol 3-kinase, Akt, and mammalian target of rapamycin is necessary for hypoxia-induced pulmonary artery adventitial fibroblast proliferation

In contrast to cell types in which exposure to hypoxia causes a general reduction of metabolic activity, a remarkable feature of pulmonary artery adventitial fibroblasts is their ability to proliferate in response to hypoxia. Previous studies have suggested that ERK1/2, phosphatidylinositol 3-kinase (PI3K), Akt, and mammalian target of rapamycin (mTOR) are activated by hypoxia and play a role in a variety of cell responses. However, the pathways involved in mediating hypoxia-induced proliferation are largely unknown. Using pharmacological inhibitors, we established that PI3K-Akt, mTOR-p70 ribosomal protein S6 kinase (p70S6K), and EKR1/2 signaling pathways play a critical role in hypoxia-induced adventitial fibroblast proliferation. We found that exposure of serum-starved fibroblasts to 3% O2 resulted in a time-dependent activation of PI3K and transient phosphorylation of Akt. However, activation of PI3K was not required for activation of ERK1/2, implying a parallel involvement of these pathways in the proliferative response of fibroblasts to hypoxia. We found that hypoxia induced significant increases in mTOR, p70S6K, 4E-BP1, and S6 ribosomal protein phosphorylation, as well as dramatic increases in p70S6K activity. The activation of p70S6K/S6 pathway was sensitive to inhibition by rapamycin and LY294002, indicating that mTOR and PI3K/Akt are upstream signaling regulators. However, the magnitude of hypoxia-induced p70S6K activity and phosphorylation suggests involvement of additional signaling pathways. Thus our data demonstrate that hypoxia-induced adventitial fibroblast proliferation requires activation and interaction of PI3K, Akt, mTOR, p70S6K, and ERK1/2 and provide evidence for hypoxic regulation of protein translational pathways in cells exhibiting the capability to proliferate under hypoxic conditions.

Dose-Dependent Effects of Platelet-Derived Growth Factor-B on Glial Tumorigenesis

Platelet-derived growth factor (PDGF) is expressed in many different tumors, but its precise roles in tumorigenesis remain to be fully defined. Here, we report on a mouse model that demonstrates dose-dependent effects of PDGF-B on glial tumorigenesis. By removing inhibitory regulatory elements in the PDGFB mRNA, we are able to substantially elevate its expression in tumor cells using a retroviral delivery system. This elevation in PDGF-B production results in tumors with shortened latency, increased cellularity, regions of necrosis, and general high-grade character. In addition, elevated PDGF-B in these tumors also mediates vascular smooth muscle cell recruitment that supports tumor angiogenesis. PDGF receptor (PDGFR) signaling appears to be required for the maintenance of these high-grade characteristics, because treatment of high-grade tumors with a small molecule inhibitor of PDGFR results in reversion to a lower grade tumor histology. Our data show that PDGFR signaling quantitatively regulates tumor grade and is required to sustain high-grade oligodendrogliomas.

Induction of gicerin/CD146 in the rat carotid artery after balloon injury

Gicerin is a cell adhesion molecule belonging to the immunoglobulin superfamily. It is reported that the human homologous molecule, CD146, is expressed in the endothelial cells. Here, we found that the expression of gicerin was increased in the rat carotid arteries after balloon injury. Immunohistochemical analysis demonstrated that the expression of gicerin protein was increased in the medial smooth muscle cells prior to the formation of neointima one week after the injury and was also increased in the luminal edge of the neointima after two weeks. We employed A10 cells, a cell line derived from rat aortic smooth muscle cell, and examined the effect of growth factors on the expression of gicerin, such as IGF-1, PDGF-BB, and bFGF. We found that IGF-1, but not PDGF-BB and bFGF, significantly increases the expression of gicerin protein in A10 cells. These suggest gicerin might be involved in the arteriosclerotic neointima formation in the artery.

Age-Dependent Decline of In Vitro Migration (Basal and Stimulated by IGF-1 or Insulin) of Human Vascular Smooth Muscle Cells

Since biological aging causes a decrease in functions such as cell proliferation, we have studied the possible effect of age on the migration capacity of human vascular smooth muscle cells (SMCs). To this aim, the migration activity of cultured SMCs from arteries of male human donors ranging in age from 43-77 years was determined in a Boyden chamber, under basal conditions and after insulin-like growth factor-1 (IGF-1) or insulin stimulation. Migration activity decreased with donor age (r2 = 87%, 85%, and 78%, respectively). IGF-1 and insulin significantly reduced the age-dependent relationship observed in basal conditions, so that, comparing young with old, both IGF-1 and insulin stimulated SMC migration similarly, although the effect of age remained in absolute terms. In this article, we conclude that the age-dependent decline of migration activity--similar to what has already been shown for SMC proliferation--may be part of the biological ageing phenotype, which is not overcome by hormone stimulation.

Insulin stimulates glucose transport via nitric oxide/cyclic GMP pathway in human vascular smooth muscle cells

OBJECTIVE

In cultured human vascular smooth muscle cells, insulin increases cyclic GMP production by inducing nitric oxide (NO) synthesis. The aim of the present study was to determine whether in these cells the insulin-stimulated NO/cyclic GMP pathway plays a role in the regulation of glucose uptake.

METHODS AND RESULTS

Glucose transport in human vascular smooth muscle cells was measured as uptake of 2-deoxy-d-[3H]glucose, cyclic GMP synthesis was checked by radioimmunoassay, and GLUT4 recruitment into the plasma membrane was determined by immunofluorescence. Insulin-stimulated glucose transport and GLUT4 recruitment were blocked by an inhibitor of NO synthesis and mimicked by NO-releasing drugs. Insulin- and NO-elicited glucose uptake were blocked by inhibitors of soluble guanylate cyclase and cyclic GMP-dependent protein kinase; furthermore, glucose transport was stimulated by an analog of cyclic GMP.

CONCLUSIONS

Our results suggest that insulin-elicited glucose transport (and the corresponding GLUT4 recruitment into the plasma membrane) in human vascular smooth muscle cells is mediated by an increased synthesis of NO, which stimulates the production of cyclic GMP and the subsequent activation of a cyclic GMP-dependent protein kinase.

Akt controls vascular smooth muscle cell proliferation in vitro and in vivo by delaying G1/S exit

Constitutive activation of serine/threonine kinase Akt causes uncontrolled cell-cycle progression in different cell types and in malignancy. To investigate how Akt activation modulates cell-cycle progression in vascular smooth muscle cells (SMCs) in vitro and in the intact animal, we inhibited Akt-dependent signaling by adenovirus-mediated transfection of a dominant-negative Akt mutant (AA-Akt). We observed reduced proliferation rate (P<0.01), DNA synthesis (P<0.01), and a significant arrest in G1/S exit (P<0.01) both in vitro in response to serum stimulation and in vivo after vascular injury. In vivo transfection of the balloon-injured vessel with AA-Akt reduced SMC proliferation, resulting in decreased neointima compared with control virus (P<0.01). These effects were at least in part modulated, both in vitro and in vivo, by increased p21Cip1 expression, as demonstrated by lack of effect of AA-Akt on cell proliferation in p21-/- mouse SMCs. In conclusion, this study demonstrates that Akt-dependent signaling enhances cell-cycle progression of nontransformed SMCs in vitro and in response to vascular injury in the intact animal. These results suggest a role for Akt signaling in modulating the response of normal tissues to stress and the response of the arterial wall to acute and possibly repetitive injuries that ultimately contribute to restenosis and atherosclerosis.

A proatherogenic role for cGMP-dependent protein kinase in vascular smooth muscle cells

Nitric oxide (NO) exerts both antiatherogenic and proatherogenic effects, but the cellular and molecular mechanisms that contribute to modulation of atherosclerosis by NO are not understood completely. The cGMP-dependent protein kinase I (cGKI) is a potential mediator of NO signaling in vascular smooth muscle cells (SMCs). Postnatal ablation of cGKI selectively in the SMCs of mice reduced atherosclerotic lesion area, demonstrating that smooth muscle cGKI promotes atherogenesis. Cell-fate mapping indicated that cGKI is involved in the development of SMC-derived plaque cells. Activation of endogenous cGKI in primary aortic SMCs resulted in cells with increased levels of proliferation; increased levels of vascular cell adhesion molecule-1, peroxisome proliferator-activated receptor gamma, and phosphatidylinositol 3-kinase/Akt signaling; and decreased plasminogen activator inhibitor 1 mRNA, which all are potentially proatherogenic properties. Taken together, these results highlight the pathophysiologic significance of vascular SMCs in atherogenesis and identify a key role for cGKI in the development of atherogenic SMCs in vitro and in vivo. We suggest that activation of smooth muscle cGKI contributes to the proatherogenic effect of NO and that inhibition of cGKI might be a therapeutic option for treating atherosclerosis in humans.

Platelet-derived growth factor-BB-mediated activation of Akt suppresses smooth muscle-specific gene expression through inhibition of mitogen-activated protein kinase and redistribution of serum response factor

Platelet-derived growth factor (PDGF) inhibits expression of smooth muscle (SM) genes in vascular smooth muscle cells and blocks induction by arginine vasopressin (AVP). We have previously demonstrated that suppression of SM-alpha-actin by PDGF-BB is mediated in part through a Ras-dependent pathway. This study examined the role of phosphatidylinositol 3-kinase (PI3K)y and its downstream effector, Akt, in regulating SM gene expression. PDGF caused a rapid sustained activation of Akt, whereas AVP caused only a small transient increase. PDGF selectively caused a sustained stimulation of p85/p110 alpha PI3K. In contrast, p85/110 beta PI3K activity was not altered by either PDGF or AVP, whereas both agents caused a delayed activation of Class IB p101/110 gamma PI3K. Expression of a gain-of-function PI3K or myristoylated Akt (myr-Akt) mimicked the inhibitory effect of PDGF on SM-alpha-actin and SM22 alpha expression. Pretreatment with LY 294002 reversed the inhibitory effect of PDGF. Expression of myr-Akt selectively inhibited AVP-induced activation of c-Jun N-terminal kinase and p38 mitogen-activated protein kinases, which we have shown are critical for induction of these genes. Nuclear extracts from PDGF-stimulated or myr-Akt expressing cells showed reduced serum response factor binding to SM-specific CArG elements. This was associated with appearance of serum response factor in the cytoplasm. These data indicate that activation of p85/p110 alpha/Akt mediates suppression of SM gene expression by PDGF.

Dual role of SRC homology domain 2-containing inositol phosphatase 2 in the regulation of platelet-derived growth factor and insulin-like growth factor I signaling in rat vascular smooth muscle cells

Src homology domain 2 (SH2)-containing inositol phosphatase 2 (SHIP2) possesses 5-phosphatase activity and an SH2 domain. The role of SHIP2 in platelet-derived growth factor (PDGF) and IGF-I signaling was studied by expressing wild-type (WT-) and a catalytically defective (Delta IP-) SHIP2 into rat aortic smooth muscle cells by adenovirus-mediated gene transfer. PDGF- and IGF-I-induced tyrosine phosphorylation of their respective receptors and phosphatidylinositol 3-kinase (PI3-kinase) activity were not affected by the expression of either WT- or Delta IP-SHIP2. SHIP2 possessed 5'-phosphatase activity to hydrolyze the PI3-kinase product phosphatidylinositol 3,4,5-trisphosphate in vivo. Akt and glycogen synthase kinase 3beta are known to be downstream molecules of PI3-kinase, leading to the antiapoptotic effect. Overexpression of WT-SHIP2 inhibited PDGF- and IGF-I-induced phosphorylation of these molecules and the protective effect of poly(ADP-ribose) polymerase degradation, whereas these phosphorylations and the protective effect were enhanced by the expression of Delta IP-SHIP2, which functions in a dominant negative fashion. Regarding the Ras-MAPK pathway, PDGF- and IGF-I-induced tyrosine phosphorylation of Shc was not affected by the expression of either WT- or Delta IP-SHIP2, whereas both expressed SHIP2 associated with Shc. Importantly, PDGF and IGF-I stimulation of Shc/Grb2 binding, MAPK activation, and 5-bromo-2'-deoxyuridine incorporation were all decreased in both WT- and Delta IP-SHIP2 expression. These results indicate that SHIP2 plays a negative regulatory role in PDGF and IGF-I signaling in vascular smooth muscle cells. As the bifunctional role, our results suggest that SHIP2 regulates PDGF- and IGF-I-mediated signaling downstream of PI3-kinase, leading to the antiapoptotic effect via 5-phosphatase activity, and that SHIP2 regulates the growth factor-induced Ras-MAPK pathway mainly via the SH2 domain.

Prostaglandin D2 synthase inhibits the exaggerated growth phenotype of spontaneously hypertensive rat vascular smooth muscle cells

Lipocalin-type prostaglandin D2 synthase (L-PGDS) has recently been linked to a variety of pathophysiological cardiovascular conditions including hypertension and diabetes. In this study, we report on the 50% increase in L-PGDS protein expression observed in vascular smooth muscle cells (VSMC) isolated from spontaneously hypertensive rats (SHR). L-PGDS expression also increased 50% upon the differentiation of normotensive control cells (WKY, from Wistar-Kyoto rats). In addition, we demonstrate differential effects of L-PGDS treatment on cell proliferation and apoptosis in VSMCs isolated from SHR versus WKY controls. L-PGDS (50 microg/ml) was able to significantly inhibit VSMC proliferation and DNA synthesis and induce the apoptotic genes bax, bcl-x, and ei24 in SHR but had no effect on WKY cells. Hyperglycemic conditions also had opposite effects, in which increased glucose concentrations (20 mm) resulted in decreased L-PGDS expression in control cells but actually stimulated L-PGDS expression in SHR. Furthermore, we examined the effect of L-PGDS incubation on insulin-stimulated Akt, glycogen synthase kinase-3beta (GSK-3beta), and ERK phosphorylation. Unexpectedly, we found that when WKY cells were pretreated with L-PGDS, insulin could actually induce apoptosis and failed to stimulate Akt/GSK-3beta phosphorylation. Insulin-stimulated ERK phosphorylation was unaffected by L-PGDS pretreatment in both cell lines. We propose that L-PGDS is involved in the balance of VSMC proliferation and apoptosis and in the increased expression observed in the hypertensive state is an attempt to maintain a proper equilibrium between the two processes via the induction of apoptosis and inhibition of cell proliferation.

Oscillatory shear stress increases smooth muscle cell proliferation and Akt phosphorylation

PURPOSE

Hemodynamic forces affect smooth muscle cell (SMC) proliferation and migration both in vitro and in vivo. However, the effects of oscillatory shear stress (SS) on SMC proliferation and signal transduction pathways that control survival are not well described.

METHODS

Bovine aortic SMC were exposed to arterial levels of oscillatory SS (14 dyne/cm(2)) with an orbital shaker; control cells were exposed to static conditions (0 dyne/cm(2)). Cell number and (3)[H]thymidine incorporation were measured after 1, 3, or 5 days of SS. Activation of the Akt pathway was assessed with the Western blot technique. Specificity of the phosphatidylinositol 3-kinase (PI3K) pathway was determined with the Western blot technique with the inhibitors LY294002 (10 micromol/L) or wortmannin (25 nmol/L).

RESULTS

Arterial levels of oscillatory SS increased SMC cell number by 20.1 +/- 3.7% and (3)[H]thymidine incorporation by 33.4% +/- 6.8% at 5 days. To identify whether SS increased activity of the SMC survival pathway, Akt activation was measured. SMC exposed to SS demonstrated increased Akt phosphorylation compared with control cells, with maximal phosphorylation at 60 minutes. Both PI3K inhibitors specifically inhibited the increase in Akt phosphorylation in SMC exposed to oscillatory SS.

CONCLUSION

SMC directly respond to oscillatory SS by increasing DNA synthesis, proliferation, and activation of the PI3K-Akt signal transduction pathway. These results suggest a mechanism of SMC survival and proliferation in response to endothelial-denuding arterial injury.

IGF-I promotes a shift in metabolic flux in vascular smooth muscle cells

13C-nuclear magnetic resonance (NMR) spectroscopy was used to test our hypothesis that insulin-like growth factor I (IGF-I) stimulates glucose flux into both nonoxidative and oxidative pathways in vascular smooth muscle cells (VSMC). Rat VSMC were exposed to uniformly labeled [13C]glucose ([U-13C]glucose; 5.5 mM) and [3-13C]pyruvate (1 mM) in the presence and absence of IGF-I (100 ng/ml). IGF-I increased glucose flux through glycolysis and the tricarboxylic acid (TCA) cycle as well as total anaplerotic flux into the TCA cycle. Previous work in our laboratory identified an increase in GLUT1 content and glucose metabolism in neointimal VSMC that was sufficient to promote proliferation and inhibit apoptosis. To test whether IGF-I could potentiate the GLUT1-induced increased flux in the neointima, we utilized VSMC harboring constitutive overexpression of GLUT1. Indeed, IGF-I markedly potentiated the GLUT1-induced increase in glucose flux through glycolysis and the TCA cycle. Taken together, these findings demonstrate that upregulation of glucose transport through either IGF-I or increased GLUT1 content stimulates glucose flux through both nonoxidative and oxidative pathways in VSMC.

PI3K is required for proliferation and migration of human pulmonary vascular smooth muscle cells

Human vascular smooth muscle cell proliferation and migration contribute to vascular remodeling in pulmonary hypertension and atherosclerosis. The precise mechanisms that regulate structural remodeling of the vessel wall remain unknown. This study tests the hypothesis that phosphatidylinositol 3-kinase (PI3K) activation is both necessary and sufficient to mediate human pulmonary vascular smooth muscle (PVSM) cell proliferation and migration. Microinjection of human PVSM cells with a dominant-negative class IA PI3K inhibited platelet-derived growth factor (PDGF)-induced DNA synthesis by 65% (P < 0.001; chi(2) analysis) compared with cells microinjected with control plasmid, whereas microinjection of cells with a constitutively active class IA PI3K (p110*-CA) was sufficient to induce DNA synthesis (mitotic index of p110*-CA-microinjected cells was 15% vs. 3% in control cells; P < 0.01). Transfection of PVSM cells with p110*-CA was also sufficient to promote human PVSM cell migration. In parallel experiments, stimulation of human PVSM cells with PDGF induced PI3K-dependent activation of Akt, p70 S6 kinase, and ribosomal protein S6 but not mitogen-activated protein kinase. PDGF-induced proliferation and migration was inhibited by LY-294002. These results demonstrate that PI3K signaling is both necessary and sufficient to mediate human PVSM cell proliferation and migration and suggest that the activation of PI3K may play an important role in vascular remodeling.

Essential role of AKT-1/protein kinase B alpha in PTEN-controlled tumorigenesis

PTEN is mutated at high frequency in many primary human cancers and several familial cancer predisposition disorders. Activation of AKT is a common event in tumors in which the PTEN gene has been inactivated. We previously showed that deletion of the murine Pten gene in embryonic stem (ES) cells led to increased phosphatidylinositol triphosphate (PIP(3)) accumulation, enhanced entry into S phase, and better cell survival. Since PIP(3) controls multiple signaling molecules, it was not clear to what degree the observed phenotypes were due to deregulated AKT activity. In this study, we mutated Akt-1 in Pten(-/-) ES cells to directly assess the role of AKT-1 in PTEN-controlled cellular processes, such as cell proliferation, cell survival, and tumorigenesis in nude mice. We showed that AKT-1 is one of the major downstream effectors of PTEN in ES cells and that activation of AKT-1 is required for both the cell survival and cell proliferation phenotypes observed in Pten(-/-) ES cells. Deletion of Akt-1 partially reverses the aggressive growth of Pten(-/-) ES cells in vivo, suggesting that AKT-1 plays an essential role in PTEN-controlled tumorigenesis.

Ceramide-induced inhibition of Akt is mediated through protein kinase Czeta: implications for growth arrest

We recently demonstrated that ceramide-coated balloon catheters limit vascular smooth muscle cell (VSMC) growth after stretch injury in vivo. In that study, inhibition of VSMC growth was correlated with a decrease in phosphorylation of the cell survival kinase Akt (protein kinase B). Utilizing cultured A7r5 VSMCs, we have now examined the mechanism by which ceramide inhibits Akt phosphorylation/activation. Our initial studies showed that ceramide-induced inhibition of Akt phosphorylation was not mediated through diminution in phosphoinositide 3-kinase activity. As we have previously demonstrated that protein kinase Czeta (PKCzeta) is a target of ceramide, we proposed an alternative signaling mechanism by which ceramide induces inhibition of Akt through activation of PKCzeta. We demonstrate that C(6)-ceramide (but not the inactive analog dihydro-C(6)-ceramide) induced PKCzeta activity and also caused a selective increase in the association between Akt and PKCzeta, without affecting PKCepsilon, in A7r5 cells. In addition, the ability of ceramide to significantly decrease platelet-derived growth factor-induced Akt phosphorylation or cell proliferation was abrogated in A7r5 cells overexpressing a dominant-negative mutant of PKCzeta. Taken together, these data suggest that ceramide-mediated activation of PKCzeta leads to diminished Akt activation and consequent growth arrest in VSMCs. The therapeutic potential for ceramide to limit dysregulated VSMC growth has direct applicability to vascular diseases such as restenosis and atherosclerosis.

Hyperglycemia-induced apoptosis in human umbilical vein endothelial cells: inhibition by the AMP-activated protein kinase activation

Apoptosis has been observed in vascular cells, nerve, and myocardium of diabetic humans and experimental animals, although whether it contributes to or is a marker of complications in these tissues is unclear. Previous studies have shown that incubation of human umbilical vein endothelial cells (HUVECs) with 30 vs. 5 mmol/l glucose for 72 h causes a significant increase in apoptosis, possibly related to an increase in oxidative stress. We report here that this increase in apoptosis (assessed morphologically by TdT-mediated dUTP nick- end labeling staining) is preceded (24 h of incubation) by inhibition of fatty acid oxidation, by increases in diacylglycerol synthesis, the concentration of malonyl CoA, and caspase-3 activity, and by decreases in mitochondrial membrane potential and cellular ATP content. In addition, the phosphorylation of Akt in the presence of 150 microU/ml insulin was impaired. No increases in ceramide content or its de novo synthesis were observed. AMP-activated protein kinase (AMPK) activity was not diminished; however, incubation with the AMPK activator 5-aminoimidazole-4-carboxamide-riboside increased AMPK activity twofold and completely prevented all of these changes. Likewise, expression of a constitutively active AMPK in HUVEC prevented the increase in caspase-3 activity. The results indicate that alterations in fatty-acid metabolism, impaired Akt activation by insulin, and increased caspase-3 activity precede visible evidence of apoptosis in HUVEC incubated in a hyperglycemic medium. They also suggest that AMPK could play an important role in protecting the endothelial cell against the adverse effects of sustained hyperglycemia.

TNFalpha inhibits insulin's antiapoptotic signaling in vascular smooth muscle cells

Tumor necrosis factor alpha (TNFalpha) interferes with insulin signaling in adipose tissue and may promote insulin resistance. Insulin resistance is associated with vascular injury, but little is known about the interaction of TNFalpha and insulin in the vasculature. By activating the Insulin receptor (IR) --> IRS-1 --> phosphatidylinositol-3-kinase (PI3K) --> Akt-pathway, insulin protects vascular smooth muscle cells (VSMC) from undergoing apoptosis. We therefore investigated the effect of TNFalpha on insulin's antiapoptotic signaling in rat aortic VSMC. Insulin induced rapid tyrosine-phosphorylation of the IR and IRS-1 and caused a 2.8-fold increase of IRS-1-bound PI3K. TNFalpha had no effect on insulin-induced tyrosine-phosphorylation of IR or IRS-1, but inhibited insulin-stimulated IRS-1/PI3K-association by 84%. Insulin-induced phosphorylation of Akt downstream of PI3K was inhibited by TNFalpha in a similar pattern. We next examined the effect of TNFalpha on insulin's protective actions on H(2)O(2)-induced apoptosis. Insulin alone prevented 72.8% of H(2)O(2)-induced apoptosis, which was significantly inhibited by TNFalpha. TNFalpha alone did not induce apoptosis. In contrast, TNFalpha had no effect on PDGF-induced antiapoptotic signal transduction via Akt. Thus, TNFalpha selectively interferes with insulin's antiapoptotic signaling in VSMC by inhibiting the association of IRS-1/PI3K and the downstream activation of Akt.

Enhanced stimulation of Akt-3/protein kinase B-gamma in human aortic smooth muscle cells

Growth factor-induced activation of Akt (protein kinase B) is implicated in the proliferation of vascular smooth muscle cells (VSMC) in addition to antiapoptotic signaling. Although previous studies have documented increases in total Akt or Akt-1 activity in rodent VSMC, little is known about the regulation of Akt-2 or Akt-3 kinase activity in VSMC from any species. In the present study, reverse transcriptase-polymerase chain reaction revealed the expression of all three Akt isoforms in human aortic VSMC. In vitro kinase assays using immunoprecipitated Akt isoforms showed robust increases in Akt-3 activity after stimulation of human aortic VSMC with platelet-derived growth factor (PDGF), insulin, and insulin-like growth factor-1. In contrast, these growth factors produced modest and marginal increases in Akt-1 and Akt-2 kinase activity, respectively. Pretreatment of VSMC with a phosphoinositide-3kinase (PI-3K) inhibitor, LY294002, led to significant inhibition of growth factor(s)-induced increases in Akt-3 activity and DNA synthesis. The present findings provide the first direct evidence that the Akt-3 isoform is predominantly activated in human aortic VSMC. Moreover, these data suggest that PI-3K-dependent activation of Akt-3 may play a major role in VSMC proliferation.