A Dominant-Negative PPARgamma Mutant Promotes Cell Cycle Progression and Cell Growth in Vascular Smooth Muscle Cells.

PPARgamma ligands have been shown to have antiproliferative effects on many cell types. We herein report that a synthetic dominant-negative (DN) PPARgamma mutant functions like a growth factor to promote cell cycle progression and cell proliferation in human coronary artery smooth muscle cells (CASMCs). In quiescent CASMCs, adenovirus-expressed DN-PPARgamma promoted G1-->S cell cycle progression, enhanced BrdU incorporation, and increased cell proliferation. DN-PPARgamma expression also markedly enhanced positive regulators of the cell cycle, increasing Rb and CDC2 phosphorylation and the expression of cyclin A, B1, D1, and MCM7. Conversely, overexpression of wild-type (WT) or constitutively-active (CA) PPARgamma inhibited cell cycle progression and the activity and expression of positive regulators of the cell cycle. DN-PPARgamma expression, however, did not up-regulate positive cell cycle regulators in PPARgamma-deficient cells, strongly suggesting that DN-PPARgamma effects on cell cycle result from blocking the function of endogenous wild-type PPARgamma. DN-PPARgamma expression enhanced phosphorylation of ERK MAPKs. Furthermore, the ERK specific-inhibitor PD98059 blocked DN-PPARgamma-induced phosphorylation of Rb and expression of cyclin A and MCM7. Our data thus suggest that DN-PPARgamma promotes cell cycle progression and cell growth in CASMCs by modulating fundamental cell cycle regulatory proteins and MAPK mitogenic signaling pathways in vascular smooth muscle cells (VSMCs).

Vascular effects of TZDs: new implications.

The incidence of diabetes, now affecting more than 170 million individuals is growing rapidly. Type 2 diabetes, which accounts for 90% of all diabetes cases, is associated with increased cardiovascular morbidity and mortality. Thiazolidinediones (TZDs), used for the treatment of patients with type 2 diabetes improve insulin sensitivity and endothelial dysfunction and exert beneficial effects on the lipid profile by activating the peroxisome proliferator-activated receptor gamma (PPAR-gamma). Moreover, a large body of evidence indicates that TZDs exhibit antiatherogenic effects independent of their antidiabetic and lipid-lowering properties by modulating inflammatory processes. This review will focus on the role of PPAR-gamma agonists in the vessel wall and summarize their effects on C-reactive protein (CRP), plasminogen activator inhibitor type-1 (PAI-1), matrix metalloproteinase-9 (MMP-9), adiponectin and ATP-binding cassette transporter A1 (ABCA1) and their implications for treatment of advanced stages of atherosclerosis, particularly in a setting of type 2 diabetes.

Activation of peroxisome proliferator-activated receptor gamma suppresses telomerase activity in vascular smooth muscle cells.

Activation of the peroxisome proliferator-activated receptor (PPAR) gamma, the molecular target for insulin sensitizing thiazolidinediones used in patients with type 2 diabetes, inhibits vascular smooth muscle cell (VSMC) proliferation and prevents atherosclerosis and neointima formation. Emerging evidence indicates that telomerase controls key cellular functions including replicative lifespan, differentiation, and cell proliferation. In the present study, we demonstrate that ligand-induced and constitutive PPARgamma activation inhibits telomerase activity in VSMCs. Telomerase reverse transcriptase (TERT) confers the catalytic activity of telomerase, and PPARgamma ligands inhibit TERT expression through a receptor-dependent suppression of the TERT promoter. 5'-deletion analysis, site-directed mutagenesis, and transactivation studies using overexpression of Ets-1 revealed that suppression of TERT transcription by PPARgamma is mediated through negative cross-talk with Ets-1-dependent transactivation of the TERT promoter. Chromatin immunoprecipitation assays further demonstrated that PPARgamma ligands inhibit Ets-1 binding to the TERT promoter, which is mediated at least in part through an inhibition of Ets-1 expression by PPARgamma ligands. In VSMCs overexpressing TERT, the efficacy of PPARgamma ligands to inhibit cell proliferation is lost, indicating that TERT constitutes an important molecular target for the antiproliferative effects of PPARgamma ligands. Finally, we demonstrate that telomerase activation during the proliferative response after vascular injury is effectively inhibited by PPARgamma ligands. These findings provide a previously unrecognized mechanism for the antiproliferative effects of PPARgamma ligands and support the concept that PPARgamma ligands may constitute a novel therapeutic approach for the treatment of proliferative cardiovascular diseases.

Obesity, peroxisome proliferator-activated receptor, and atherosclerosis in type 2 diabetes.

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear hormone receptor superfamily. The 3 PPAR isotypes, PPAR-alpha, PPAR-gamma, and PPAR-delta, play a key role in the regulation of lipid and glucose metabolism. Obesity and the interrelated disorders of the metabolic syndrome have become a major worldwide health problem. In this review, we summarize the critical role of PPARs in regulating inflammation, lipoprotein metabolism, and glucose homeostasis and their potential implications for the treatment of obesity, diabetes, and atherosclerosis.

Transcriptional repression of ATP-binding cassette transporter A1 gene in macrophages: a novel atherosclerotic effect of angiotensin II.

Angiotensin II (Ang II) is a powerful accelerator of atherosclerosis. Herein, we describe a novel transcription mechanism through which Ang II inhibits macrophage expression of the ATP-binding cassette transporter A1 (ABCA1), a key regulator of reverse cholesterol transport. We demonstrate that chronic Ang II infusion substantially promotes macrophage infiltration, foam cell formation, and atherosclerosis in low-density lipoprotein receptor-deficient mice and significantly reduces ABCA1 expression in peripheral macrophages. Administration of the Ang II type 1 receptor blocker valsartan inhibited Ang II-induced ABCA1 mRNA repression, macrophage cholesterol accumulation, and atherosclerosis. Ang II treatment reduced ABCA1 promoter activity of in vitro cultured mouse peritoneal macrophages, inducing fos-related antigen 2 (Fra2) protein binding to an ABCA1 promoter E-box motif, a site known to negatively regulate macrophage ABCA1 transcription. Valsartan pretreatment blocked Fra2 binding to the ABCA1 promoter, and Fra2 small interfering RNA pretreatment attenuated Ang II-mediated ABCA1 transcriptional inhibition, confirming the role of Fra2 in this process. This new evidence suggests that Ang II, a well-known proinflammatory and pro-oxidative factor, alters macrophage cholesterol homeostasis by repressing ABCA1 to promote foam cell formation and atherosclerosis.

Liver x receptor agonists inhibit cytokine-induced osteopontin expression in macrophages through interference with activator protein-1 signaling pathways.

Osteopontin (OPN) is a proinflammatory cytokine and adhesion molecule implicated in the chemoattraction of monocytes and in cell-mediated immunity. We have recently reported that genetic OPN-deficiency attenuates the development of atherosclerosis in apoE-/- mice identifying OPN as potential target for pharmacological intervention in atherosclerosis. Synthetic agonists for the Liver X Receptor (LXR), members of the nuclear hormone receptor superfamily, prevent the development of atherosclerosis by regulating cholesterol homeostasis and suppressing inflammatory gene expression in macrophages. We demonstrate here that LXR ligands inhibit cytokine-induced OPN expression in macrophages. Two synthetic LXR ligands, T0901317 and GW3965, inhibited TNF-alpha, IL-1beta, INF-gamma and lipopolysaccharide induced OPN mRNA and protein expression in RAW 264.7 macrophages. Transient transfection experiments revealed that LXR ligands suppress cytokine-induced OPN promoter activity. Deletion analysis, heterologous promoter assays, and site-directed mutagenesis identified an activator protein-1 (AP-1) consensus site at -76 relative to the initiation site that supports OPN transcription in macrophages and mediates the effects of LXR ligands to inhibit OPN transcription. Electrophoretic mobility shift and chromatin immunoprecipitation assays indicated that LXR agonists inhibit cytokine-induced c-Fos and phospho-c-Jun binding to this AP-1 site. Cytokine-induced c-Fos and phospho-c-Jun protein expression was inhibited by LXR ligands and overexpression of c-Fos and c-Jun reversed the inhibitory effect of LXR ligands on OPN promoter activity in transactivation assays. Finally, treatment of C57BL/6J mice with LXR ligands inhibited OPN expression in peritoneal macrophages indicating that the observed effects of LXR ligands to inhibit OPN expression are applicable in vivo. These observations identify the regulation of macrophage OPN expression as a mechanism whereby LXR ligands may impact macrophage inflammatory responses and atherosclerosis. The full text of this article is available online at http://circres.ahajournals.org.

PPARgamma-mediated insulin sensitization: the importance of fat versus muscle.

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear hormone receptor that functions as a transcriptional regulator in a variety of tissues. PPARgamma activation, e.g., through binding of the synthetic glitazones or thiazolidinediones (TZD), results in a marked improvement in type 2 diabetic patients of insulin and glucose parameters resulting from an improvement of whole body insulin sensitivity. The role of different metabolic tissues (fat, skeletal muscle, liver) in mediating PPARgamma function in glucose and insulin homeostasis is still unclear. Recently, the function of PPARgamma in adipose tissue and skeletal muscle has been intensively characterized by using targeted deletion of PPARgamma in those tissues. In those studies, adipose PPARgamma has been identified as an essential mediator for the maintainance of whole body insulin sensitivity. Two major mechanisms have been described. 1) Adipose PPARgamma protects nonadipose tissue against excessive lipid overload and maintains normal organ function (liver, skeletal muscle); and 2) adipose PPARgamma guarantees a balanced and adequate production of secretion from adipose tissue of adipocytokines such as adiponectin and leptin, which are important mediators of insulin action in peripheral tissues. In contrast to studies in adipose-specific PPARgamma-deficient mice, the data in muscle-specific PPARgamma(-/-) mice demonstrate that whole body insulin sensitivity is, at least in part, relying on an intact PPARgamma system in skeletal muscle. Finally, these early and elegant studies using tissue-specific PPARgamma knockout mouse models pinpoint adipose tissue as the major target of TZD-mediated improvement of hyperlipidemia and insulin sensitization.

Liver x receptors: potential novel targets in cardiovascular diseases.

The Liver X Receptors, LXRalpha and LXRbeta are members of the nuclear hormone receptor superfamily which have recently been implicated as novel pharmacological targets for the treatment of cardiovascular diseases. The identification of natural and synthetic ligands for LXRs and the generation of LXR-deficient mice have been crucial for our understanding of the function of these receptors and for the identification of LXR-regulated target genes, particularly with respect to the role of LXRs in regulating cholesterol homeostasis. Synthetic LXRalpha/beta agonists induce cholesterol efflux and reverse cholesterol transport, improve glucose metabolism, inhibit macrophage-derived inflammation, and suppress the proliferation of vascular smooth muscle cells. By regulating the expression of multiple genes involved in these pathways, LXR agonists prevent the development and progression of atherosclerosis and inhibit neointima formation following angioplasty of the arterial wall. In this review, we will summarize the important roles of LXR in metabolism and vascular biology and discuss its implications as potential molecular drug target for the treatment of cardiovascular diseases.

Liver X receptor agonists suppress vascular smooth muscle cell proliferation and inhibit neointima formation in balloon-injured rat carotid arteries.

The liver X receptors alpha and beta (LXRalpha and LXRbeta) are important regulators of cholesterol homeostasis in liver and macrophages. Synthetic LXR ligands prevent the development of atherosclerosis in murine models; however, the potential functional relevance of LXRs in vascular smooth muscle cells (VSMCs) has not been investigated. In the present study, we demonstrate that LXRs are expressed and functional in primary human coronary artery VSMCs (CASMCs). LXR ligands inhibited mitogen-induced VSMC proliferation and G1-->S phase progression of the cell cycle. Inhibition of G1 exit by LXR ligands was accompanied by a dose-dependent inhibition of retinoblastoma protein (Rb) phosphorylation, which functions as the key switch for G1-->S cell cycle progression. LXR ligands suppressed mitogen-induced degradation of the cyclin-dependent kinase inhibitor p27Kip1, attenuated cyclin D1 and cyclin A expression, and inhibited the expression of S phase-regulatory minichromosome maintenance protein 6. Stabilization of p27kip1 by LXR ligands was mediated by supressing the transcriptional activation of the S phase kinase-associated protein 2 (Skp2), an F-box protein that targets p27Kip1 for degradation. Inhibition of Rb phosphorylation and G1-->S cell cycle progression by LXR ligands was reversed in VSMCs overexpressing Skp2, indicating that Skp2 as an upstream regulator of p27Kip1 degradation plays a central role in LXR ligand-mediated inhibition of VSMC proliferation. Furthermore, adenovirus-mediated overexpression of the S phase transcription factor E2F, which is released after Rb phosphorylation, reversed the inhibitory effect of LXR ligands on VSMC proliferation and S phase gene expression, suggesting that the primary mechanisms by which LXR ligands inhibit VSMC proliferation occur upstream of Rb phosphorylation. Finally, neointima formation in a model of rat carotid artery balloon injury was significantly attenuated after treatment with the LXR ligand T1317 compared with vehicle-treated animals. These data demonstrate that LXR ligands inhibit VSMC proliferation and neointima formation after balloon injury and suggest that LXR ligands may constitute a novel therapy for proliferative vascular diseases. The full text of this article is available online at http://circres.ahajournals.org.

Will the potential of peroxisome proliferator-activated receptor agonists be realized in the clinical setting?

Drugs targeting both peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists (the thiazolidinediones) and PPAR-alpha (the fibrates) have already been developed for clinical use. However, the thiazolidinediones, currently prescribed to treat hyperglycemia and improve peripheral insulin resistance, may also have cardiovascular benefits that have yet to be fully realized. Animal models of atherosclerosis have shown that the thiazolidinediones reduce the extent of atherosclerotic lesions and inhibit macrophage accumulation. Clinical studies have also shown that these drugs improve the lipid profile of patients at risk of developing atherosclerosis and reduce circulating levels of inflammatory markers. This combination of lower lipid concentrations and reduced inflammation may explain the cardiovascular benefits of this class of drugs. Early trials in patients with coronary stents have reported promising findings, with restenosis rates being greatly reduced with thiazolidinedione therapy. It is hoped that the results of future clinical trials will continue to be encouraging, so that the thiazolidinediones' cardiovascular benefits can be fully realized in the clinic.

C-reactive protein induces apoptosis in human coronary vascular smooth muscle cells.

BACKGROUND: Accumulating evidence suggests that C-reactive protein (CRP), in addition to being a predictor of coronary events, may have direct actions on the vessel wall in the evolution of atherosclerosis. Although accumulation of vascular smooth muscle cells (VSMCs) in the intima is a key event in the development of arterial lesions, apoptosis of VSMCs also plays an important role in progression of atherosclerotic lesions and contributes to increased plaque vulnerability. METHODS AND RESULTS: In the present study we demonstrate that CRP induces caspase-mediated apoptosis of human coronary VSMCs. DNA microarray analysis was used to identify CRP-regulated genes. The growth arrest- and DNA damage-inducible gene 153 (GADD153) mRNA expression was prominently upregulated by CRP. As confirmed by Northern blot analysis, CRP induced a time- and dose-dependent increase of GADD153 mRNA expression. GADD153, a gene involved in growth arrest and apoptosis in vascular and nonvascular cells, is regulated at both transcriptional and posttranscriptional levels. CRP regulation of GADD153 mRNA expression in VSMCs occurs primarily at the posttranscriptional level by mRNA stabilization. Small interfering RNA (siRNA) specifically targeted to GADD153 reduced CRP-induced apoptosis. GADD153 also specifically colocalized to apoptotic VSMCs in human coronary lesions, further supporting a functional role for GADD153 in CRP-induced cell death. CONCLUSIONS: These results demonstrate that GADD153 is a CRP-regulated gene in human VSMCs and plays a causal role in CRP-induced apoptosis. Pharmacological targeting of CRP expression or action may provide a novel therapy for atherosclerosis.

Osteopontin modulates angiotensin II-induced fibrosis in the intact murine heart.

OBJECTIVES: Osteopontin (OPN) is upregulated in left ventricular hypertrophy and is stimulated by angiotensin II (AngII). Our objective was to determine whether mice deficient in OPN would be protected from AngII-induced cardiac fibrosis. BACKGROUND: Interstitial fibrosis can lead to myocardial dysfunction and ultimately heart failure. Osteopontin activates integrins that regulate cell adhesion, migration, and growth, thus implicating OPN in the process of cardiac fibrosis. METHODS: Osteopontin null (OPN(-/-)) mice (n = 18) and wild-type controls (n = 20) were infused with AngII (2.5 or 3.0 microg/kg/min) for four days or three weeks via osmotic mini-pumps. Hearts were assessed morphometrically and histologically, including quantitative assessment of fibrosis via optical microscopic imaging analysis. Cardiac fibroblasts derived from these mice were evaluated for adhesion and proliferation. Cardiac transcript expression for cytokines, extracellular matrix (ECM), integrin, and atrial natriuretic peptide were assessed. RESULTS: Osteopontin(-/-) mice exhibited less cardiac fibrosis (0.7%) than wild-type mice (8.0%) (p < 0.01) and lowered heart/body weight ratios (0.10% vs. 0.23%) (p < 0.01) after three weeks of AngII infusion. Expression of transforming growth factor-beta, fibronectin, and collagen was not different between OPN(-/-) and wild-type mice, despite the decrease in ECM accumulation in the OPN(-/-) mice. Adhesion to ECM substrates decreased by 30% to 50% in cardiac fibroblasts of OPN(-/-) mice but was restored in OPN(-/-) cells by the addition of recombinant osteopontin. CONCLUSIONS: Osteopontin mediates cardiac fibrosis, probably through the modulation of cellular adhesion and proliferation. Because OPN is increased in cardiac hypertrophy and its lack attenuates fibrosis, understanding of OPN function is essential to extend our knowledge about molecular determinants of cardiac hypertrophy and failure.

Signaling pathways involved in induction of GADD45 gene expression and apoptosis by troglitazone in human MCF-7 breast carcinoma cells.

We previously reported that the PPARgamma agonist troglitazone (TRO) inhibits proliferation and induces apoptosis in human MCF-7 breast carcinoma cells. To understand the mechanisms of antiproliferative and pro-apoptotic effects of TRO, we screened a limited DNA array containing 23 genes involved in regulating either the cell cycle and/or apoptosis. Four of the 23 genes screened exhibited regulation by TRO, with growth arrest and DNA damage-inducible gene 45 (GADD45) being the most strongly upregulated. TRO induced GADD45 mRNA expression in a time- and dose-dependent manner. Depletion of GADD45 by siRNA abrogated TRO-induced apoptosis in MCF-7 cells demonstrating the physiological relevance of GADD45 upregulation. Signaling pathways mediating TRO-induced GADD45 were also investigated. Several mitogen-activated protein kinase (MAPK) pathways were involved in the induction of GADD45 by TRO. Inhibition of the c-jun N-terminal kinase MAPK pathway by SP600125 partially abolished TRO-induced GADD45 mRNA, and protein expression and apoptosis. In contrast, inhibition of the p38 MAPK pathway by SB203580, or through overexpression of a dominant-negative mutant of p38 MAPK, augmented GADD45 mRNA induction and GADD45 promoter activation as well as cell apoptosis by TRO. Blockade of the extracellular signal-regulated kinase MAPK pathway by PD98059 also enhanced TRO's effects on GADD45 and apoptosis. Two other PPARgamma agonists pioglitazone and rosiglitazone did not induce GADD45 expression. Our finding of GADD45 induction by TRO may provide a new insight concerning the mechanisms for TRO's antiproliferative and pro-apoptotic effects in breast cancer cells.

Angiotensin II, PPAR-gamma and atherosclerosis.

Atherosclerosis is a complex, chronic disease state that usually arises from the converging action of several pathogenic processes, including hypertension, hyperlidemia, obesity and insulin resistance. Significantly, due to the increasing incidence of type 2 diabetes worldwide, several aspects of the renin-angiotensin system, including the capacity for angiotensin II synthesis and binding are increased in human and animal models of type II diabetes, and potentiate vascular lesion formation. Angiotensin II, an important vasoactive peptide of the renin-angiotensin system, profoundly accelerates atherosclerosis in animal models of diabetes. Conversely, in both human and animal studies, inhibition of angiotensin II synthesis or activity has been shown to significantly reduce atherosclerosis and cardiovascular mortality. Cardiovascular protection is independent of blood pressure and baseline activity of the renin-angiotensin system, suggesting an important and direct role for the vascular renin-angiotensin system in atherosclerotic progression. Angiotensin II appears to accelerate atherosclerosis through activation of several distinct signal transduction pathways, and via these mechanisms can function as a vascular growth and migration factor, a pro-inflammatory cytokine and an oxidative stress agent. Thiazolidinediones, a class of oral insulin-sensitizing agents in broad clinical use for the treatment of type 2 diabetes, have been shown to ameliorate cardiovascular disease in animal trials and clinical studies. Thiazolidinediones also appear to regulate angiotensin II signaling at multiple levels, significantly reducing the expression of the angiotensin II type 1 receptor and repressing signal transduction through this receptor to suppress vascular remodeling, lesion formation, and oxidative stress.

Thiazolidinedione regulation of smooth muscle cell proliferation.

Vascular smooth muscle cells (VSMCs) in the media of adult arteries are normally quiescent, proliferate at low frequency, and are arrested in the G(0)/G(1) phase of the cell cycle. Proliferation of VSMCs occurs in response to arterial injury and plays a crucial role in the atherosclerotic process and in the pathogenesis of restenosis. Patients with type 2 diabetes mellitus are at increased risk for postangioplasty restenosis, which results from excessive intimal hyperplasia. Insulin sensitizers of the thiazolidinedione (TZD) class inhibit growth of VSMCs by attenuating the activity of important cell-cycle regulators. The TZDs inhibit progression from G(1) to S phase in the cell cycle by blocking growth factor-induced phosphorylation of retinoblastoma tumor suppressor protein (Rb). In animal models of restenosis, TZDs inhibit intimal hyperplasia after mechanical injury in both insulin-sensitive and insulin-resistant vessels. Preliminary clinical studies using troglitazone demonstrate less intimal hyperplasia with this TZD after implantation of coronary stents in individuals with type 2 diabetes. Further large trials are needed to confirm that treatment with a TZD can protect against postangioplasty restenosis.

Angiotensin II-accelerated atherosclerosis and aneurysm formation is attenuated in osteopontin-deficient mice.

Osteopontin (OPN) is expressed in atherosclerotic lesions, particularly in diabetic patients. To determine the role of OPN in atherogenesis, ApoE-/-OPN+/+, ApoE-/-OPN+/-, and ApoE-/-OPN-/- mice were infused with Ang II, inducing vascular OPN expression and accelerating atherosclerosis. Compared with ApoE-/-OPN+/+ mice, ApoE-/-OPN+/- and ApoE-/-OPN-/- mice developed less Ang II-accelerated atherosclerosis. ApoE-/- mice transplanted with bone marrow derived from ApoE-/-OPN-/- mice had less Ang II-induced atherosclerosis compared with animals receiving ApoE-/-OPN+/+ cells. Aortae from Ang II-infused ApoE-/-OPN-/- mice expressed less CD68, C-C-chemokine receptor 2, and VCAM-1. In response to intraperitoneal thioglycollate, recruitment of leukocytes in OPN-/- mice was impaired, and OPN-/- leukocytes exhibited decreased basal and MCP-1-directed migration. Furthermore, macrophage viability in atherosclerotic lesions from Ang II-infused ApoE-/-OPN-/- mice was decreased. Finally, Ang II-induced abdominal aortic aneurysm formation in ApoE-/-OPN-/- mice was reduced and associated with decreased MMP-2 and MMP-9 activity. These data suggest an important role for leukocyte-derived OPN in mediating Ang II-accelerated atherosclerosis and aneurysm formation.

Expression of minichromosome maintenance proteins in vascular smooth muscle cells is ERK/MAPK dependent.

Proliferation of vascular smooth muscle cells (VSMC) represents a key event for the pathogenesis of postangioplasty restenosis. Minichromosome maintenance proteins (MCM) form essential components of the prereplicative complex at DNA replication origins and are regulated by E2F. The present studies were designed to investigate the signal transduction pathways controlling the expression of MCM6 and MCM7 in VSMC in response to mitogenic stimuli. MCM6 and MCM7 expression was substantially increased after stimulation with platelet-derived growth factor-BB and insulin. Pretreatment with PD98059, a specific inhibitor of the extracellular signal-regulated kinases (ERK)-mitogen-activated protein kinase (MAPK), competely inhibited the mitogen-induced MCM6 and MCM7 mRNA and protein expression, demonstrating a critical role for this pathway in transmitting transmembrane signals required for the initiation of DNA replication. The p38MAPK inhibitor SB203580, the phosphatidylinositol 3 kinase (PI3-kinase) pathway inhibitor wortmannin, and the protein kinase C pathway (PKC) inhibitor Gö 6976 did not significantly affect mitogen-induced MCM6 and MCM7 expression. Transient transfection experiments revealed that PD98059 inhibited mitogen-induced MCM6 and MCM7 transcriptional activation. In addition, blockade of ERK/MAPK signaling with PD98059 strongly inhibited phosphorylation of the retinoblastoma protein (Rb) and activity of a luciferase reporter plasmid driven by multiple E2F elements. Inhibition of mitogen-induced MCM6 and MCM7 expression by PD98059 was reversed by ectopic overexpression of E2F, indicating that ERK/MAPK signaling is required for events that occur upstream of E2F release from phosphorylated Rb. In combination, these data demonstrate that the ERK/MAPK signal transduction pathway plays a central role in regulating E2F-dependent MCM expression and DNA replication in VSMC.

Regulation of the growth arrest and DNA damage-inducible gene 45 (GADD45) by peroxisome proliferator-activated receptor gamma in vascular smooth muscle cells.

Peroxisome proliferator-activated receptor (PPAR) gamma is activated by thiazolidinediones (TZDs), widely used as insulin-sensitizing agents for the treatment of type 2 diabetes. TZDs have been shown to induce apoptosis in a variety of mammalian cells. In vascular smooth muscle cells (VSMCs), proliferation and apoptosis may be competing processes during the formation of restenotic and atherosclerotic lesions. The precise molecular mechanisms by which TZDs induce apoptosis in VSMCs, however, remain unclear. In the present study, we demonstrate that the TZDs rosiglitazone (RSG), troglitazone (TRO), and a novel non-TZD partial PPARgamma agonist (nTZDpa) induce caspase-mediated apoptosis of human coronary VSMCs. Induction of VSMC apoptosis correlated closely with an upregulation of growth arrest and DNA damage-inducible gene 45 (GADD45) mRNA expression and transcription, a well-recognized modulator of cell cycle arrest and apoptosis. Using adenoviral-mediated overexpression of a constitutively active PPARgamma mutant and the irreversible PPARgamma antagonist GW9662, we provide evidence that PPARgamma ligands induce caspase-mediated apoptosis and GADD45 expression through a receptor-dependent pathway. Deletion analysis of the GADD45 promoter revealed that a 153-bp region between -234 and -81 bp proximal to the transcription start site, containing an Oct-1 element, was crucial for the PPARgamma ligand-mediated induction of the GADD45 promoter. PPARgamma activation induced Oct-1 protein expression and DNA binding and stimulated activity of a reporter plasmid driven by multiple Oct-1 elements. These findings suggest that activation of PPARgamma can lead to apoptosis and growth arrest in VSMCs, at least in part, by inducing Oct-1-mediated transcription of GADD45. The full text of this article is available online at http://www.circresaha.org.

PGE(2) is generated by specific COX-2 activity and increases VEGF production in COX-2-expressing human pancreatic cancer cells.

In some cancers cyclooxygenase (COX) inhibition appears to be anti-mitogenic and anti-angiogenic, but the actions of COX-derived prostaglandins in pancreatic cancer (PaCa) are unknown. In this study COX-2 was detected in three of six PaCa cell lines while COX-1 was identified in all cell lines. COX-2 expression correlated with basal and arachidonic acid (AA) stimulated PGE(2) production. PGE(2) production was inhibited by the COX-2 inhibitor nimesulide. In COX-2 expressing cells, exogenous AA and PGE(2) increased VEGF synthesis via the EP(2) receptor. Whereas PGE(2) stimulated intracellular cAMP formation in COX-2 positive and negative cells, 8-bromo cAMP stimulated VEGF production only in COX-2 expressing cells. Stimulating COX-2 expressing PaCa cell lines with AA enhanced migration of endothelial cells, an effect which was inhibited by a COX-2 inhibitor and EP(2) receptor antagonist. These data identify a subset of human PaCa cell lines that express functional COX-2 enzyme. PGE(2) generated by specific COX-2 activity increases VEGF secretion in human PaCa cells through an autocrine mechanism.