Peroxisome proliferator-activated receptors (PPARs) and their role in the vessel wall: possible mediators of cardiovascular risk?

Peroxisome proliferator-activated receptors (PPARs) are nuclear transcription factors that regulate gene expression in response to activation by specific ligands including insulin sensitizing thiazolidinedione and lipid-lowering fibrates. Recent work has identified PPARalpha and PPARgamma expression in vascular cells and established a role for these receptors in the regulation of genes relevant to atherogenesis. The following review will focus on the role of PPAR activation in the vasculature and discuss the potential clinical implications of these findings.

PPARalpha activators inhibit tissue factor expression and activity in human monocytes

BACKGROUND

Tissue factor (TF), expressed on the surface of monocytes and macrophages in human atherosclerotic lesions, acts as the major procoagulant initiating thrombus formation in acute coronary syndromes. Peroxisome proliferator-activated receptor-alpha (PPARalpha), a nuclear receptor family member, regulates gene expression in response to certain fatty acids and fibric acid derivatives. Given that some of these substances reduce TF activity in patients, we tested whether PPARalpha activators limit TF responses in human monocytic cells.

METHODS AND RESULTS

Pretreatment of freshly isolated human monocytes or monocyte-derived macrophages with PPARalpha activators WY14643 and eicosatetraynoic acid (ETYA) led to reduced lipopolysaccharide (LPS)-induced TF activity in a concentration-dependent manner (maximal reduction to 43+/-8% with 250 micromol/L WY14643 [P:<0.05, n=5] and to 42+/-12% with 30 micromol/L ETYA [P:>0.05, n=3]). Two different PPARgamma activators (15-deoxy(_Delta12,14)-prostaglandin J(2) and BRL49653) lacked similar effects. WY14643 also decreased tumor necrosis factor-alpha protein expression in supernatants of LPS-stimulated human monocytes. Pretreatment of monocytes with WY14643 inhibited LPS-induced TF protein and mRNA expression without altering mRNA half-life. Transient transfection assays of a human TF promoter construct in THP-1 cells revealed WY14643 inhibition of LPS-induced promoter activity, which appeared to be mediated through the inhibition of nuclear factor-kappaB but not to be due to reduced nuclear factor-kappaB binding.

CONCLUSIONS

PPARalpha activators can reduce TF expression and activity in human monocytes/macrophages and thus potentially reduce the thrombogenicity of atherosclerotic lesions. These data provide new insight into how PPARalpha-activating fibric acid derivatives and certain fatty acids might influence atherothrombosis in patients with vascular disease.

Emerging concepts in metabolic abnormalities associated with coronary artery disease

To an increasing degree, cardiology and endocrinology are finding a broadening interface. There is little doubt that atherosclerosis is in many ways a metabolic disorder, just as it is becoming increasingly clear that diabetes is a vascular disease. Framing such notions is evidence of diabetes as a risk equivalent for coronary disease, and clinical cardiovascular trials demonstrate the impact of altering lipid metabolism. Although the focus has been on statins and LDL, data continues to emerge for other therapies for triglycerides and HDL. These issues are discussed, as are future directions for metabolic therapeutic interventions for vascular disease.

Peroxisome proliferator-activated receptors in vascular biology and atherosclerosis: emerging insights for evolving paradigms

Peroxisome Proliferator-Activated Receptors (PPARs), members of the steroid hormone nuclear receptor superfamily, act as ligand-activated transcription factors controlling the expression of specific target genes. Known PPAR isoforms include PPAR gamma, important in adipogenesis and lipid metabolism, PPAR alpha, implicated in fatty acid metabolism, and PPAR delta, about which the least is known. Recent work implicates PPAR alpha and gamma in vascular biology and atherosclerosis, and will be reviewed here. Such effects may have clinical implications given PPAR agonists in use as pharmacologic agents (eg, thiazolidinediones as insulin sensitizers [gamma] and fibric acids as lipid lowering agents [alpha]).

Peroxisome proliferator-activated receptor-gamma activators inhibit IFN-gamma-induced expression of the T cell-active CXC chemokines IP-10, Mig, and I-TAC in human endothelial cells

Peroxisome proliferator-activated receptor-gamma (PPARgamma), a member of the nuclear hormone receptor superfamily originally shown to play an important role in adipocyte differentiation and glucose homeostasis, is now known to regulate inflammatory responses. Given the importance of endothelial cell (EC)-derived chemokines in regulating leukocyte function and trafficking, we studied the effects of PPARgamma ligands on the expression of chemokines induced in ECs by the Th1 cytokine IFN-gamma. Treatment of ECs with PPARgamma activators significantly inhibited IFN-gamma-induced mRNA and protein expression of the CXC chemokines IFN-inducible protein of 10 kDa (IP-10), monokine induced by IFN-gamma (Mig), and IFN-inducible T-cell alpha-chemoattractant (I-TAC), whereas expression of the CC chemokine monocyte chemoattractant protein-1 was not altered. PPARgamma activators decreased IFN-inducible protein of 10 kDa promoter activity and inhibited protein binding to the two NF-kappaB sites but not to the IFN-stimulated response element ISRE site. Furthermore, PPARgamma ligands inhibited the release of chemotactic activity for CXC chemokine receptor 3 (CXCR3)-transfected lymphocytes from IFN-gamma-stimulated ECs. These data suggest that anti-diabetic PPARgamma activators might attenuate the recruitment of activated T cells at sites of Th1-mediated inflammation.

A review of metabolic and cardiovascular effects of oral antidiabetic agents: beyond glucose-level lowering

It has become evident that cardiovascular disease is the major cause of morbidity and mortality in type 2 diabetes mellitus. This raises the possibility that glucose lowering agents with other nonglucose-lowering effects, might have added benefits. In this review, we focus on the metabolic and cardiovascular effects of oral antidiabetic agents that go beyond glucose-level lowering. Such effects include lipid modifying actions, antithrombotic and profibrinolytic activities, and direct action at the level of the vessel wall to improve endothelial function or prevent smooth muscle hyperplasia. These additional activities, particularly those seen with the newer oral antidiabetic agents, hold the promise of reducing cardiovascular complications beyond that achievable by glucose lowering alone.

Effect of lipid-lowering therapy on vasomotion and endothelial function

The recent clinical trials of lipid lowering have established the benefit of this therapy in men and women with, or at high risk for, cardiovascular disease. It is now thought that most of the reduction in the risk of clinical events is due to functional rather than anatomic changes in atherosclerotic arteries. Cholesterol-lowering drugs improve endothelial vasomotor function and vascular nitric oxide in patients with coronary artery disease over several months. These changes in vasomotor function may reflect other beneficial changes that are regulated by nitric oxide such as the reduced recruitment and activation of inflammatory cells and a shift in the coagulation balance to favor thrombolysis. These mechanisms may contribute to the reduction in myocardial ischemia and clinical events observed with lipid lowering in patients with vascular disease. Lipid-lowering therapy decreases cardiovascular events and is an important adjunct to coronary revascularization most likely because an improvement in endothelial function prevents the development and destabilization of new atherosclerotic lesions and subsequent ischemic events.

Atherosclerotic plaque rupture: emerging insights and opportunities

Reduction in acute,coronary events requires interventions that affect the mechanisms leading to formation of atherosclerotic lesions, as well as the molecular events that precipitate acute myocardial infarction. Data from clinical trials indicate that it is the vulnerability of atherosclerotic plaque to rupture, rather than the degree of atherosclerosis, that is the primary determinant of thrombosis-mediated acute coronary events. The characteristics of a plaque that is vulnerable to rupture include a thin fibrous cap separating the circulation from procoagulants in the plaque's lipid core; increased numbers of inflammatory cells (e.g., macrophages and T cells); and a relative paucity of vascular smooth muscle cells (VSMC). Plaque stability reflects various dynamic factors: interaction of inflammatory cells, VSMC production of the extracellular matrix that is the bulwark of the fibrous cap, inhibition of this process by certain cytokines, and increased degradation of the matrix by matrix metalloproteinases. There is growing interest in the concept that intervention in the inflammatory processes of atherogenesis might reduce lesion formation and/or progression. There has also been substantial progress in understanding the transcriptional regulation of proteins that are critically involved in atherogenesis. Recently, peroxisomal proliferator-activated receptors (PPARs) have been identified as a potential link between insulin resistance and atherosclerosis. This concept is supported by the discovery through drug screening of thiazolidinediones (troglitazone, rosiglitazone), compounds that are not only ligands for PPARgamma, a nuclear receptor involved in adipogenesis, but also are antidiabetic agents.

PPARalpha activators inhibit cytokine-induced vascular cell adhesion molecule-1 expression in human endothelial cells

BACKGROUND

Adhesion molecule expression on the endothelial cell (EC) surface is critical for leukocyte recruitment to atherosclerotic lesions. Better understanding of transcriptional regulation of adhesion molecules in ECs may provide important insight into plaque formation. Peroxisome proliferator-activated receptor-alpha (PPARalpha), a member of the nuclear receptor family, regulates gene expression in response to certain fatty acids and fibric acid derivatives. The present study investigated PPARalpha expression in human ECs and their regulation of vascular cell adhesion molecule-1 (VCAM-1).

METHODS AND RESULTS

Immunohistochemistry revealed that human carotid artery ECs express PPARalpha. Pretreatment of cultured human ECs with the PPARalpha activators fenofibrate or WY14643 inhibited TNF-alpha-induced VCAM-1 in a time- and concentration-dependent manner, an effect not seen with PPARgamma activators. Both PPARalpha activators decreased cytokine-induced VCAM-1 mRNA expression without altering its mRNA half-life. Transient transfection of deletional VCAM-1 promoter constructs and electrophoretic mobility shift assays suggest that fenofibrate inhibits VCAM-1 transcription in part by inhibiting NF-kappaB. Finally, PPARalpha activators significantly reduced adhesion of U937 cells to cultured human ECs.

CONCLUSIONS

Human ECs express PPARalpha, a potentially important regulator of atherogenesis through its transcriptional control of VCAM-1 gene expression. Such findings also have implications regarding the clinical use of lipid-lowering agents, like fibric acids, which can activate PPARalpha.

PPARgamma activation in human endothelial cells increases plasminogen activator inhibitor type-1 expression: PPARgamma as a potential mediator in vascular disease

Plasminogen activator inhibitor type-1 (PAI-1) is a major physiological inhibitor of fibrinolysis, with its plasma levels correlating with the risk for myocardial infarction and venous thrombosis. The regulation of PAI-1 transcription by endothelial cells (ECs), a major source of PAI-1, remains incompletely understood. Adipocytes also produce PAI-1, suggesting possible common regulatory pathways between adipocytes and ECs. Peroxisomal proliferator-activated receptor-gamma (PPAR)gamma is a ligand-activated transcription factor that regulates gene expression in response to various mediators such as 15-deoxy-Delta12, 14-prostaglandin J2 (15d-PGJ2) and oxidized linoleic acid (9- and 13-HODE). The present study tested the hypotheses that human ECs express PPARgamma and that this transcriptional activator regulates PAI-1 expression in this cell type. We found that human ECs contain both PPARgamma mRNA and protein. Immunohistochemistry of human carotid arteries also revealed the presence of PPARgamma in ECs. Bovine ECs transfected with a PPAR response element (PPRE)-luciferase construct responded to stimulation by the PPARgamma agonist 15d-PGJ2 in a concentration-dependent manner, suggesting a functional PPARgamma in ECs. Treatment of human ECs with 15d-PGJ2, 9(S)-HODE, or 13(S)-HODE augmented PAI-1 mRNA and protein expression, whereas multiple PPARalpha activators did not change PAI-1 levels. Introduction of increasing amounts of a PPARgamma expression construct in human fibroblasts enhanced PAI-1 secretion from these cells in proportion to the amount of transfected DNA. Thus, ECs express functionally active PPARgamma that regulates PAI-1 expression in ECs. Our results establish a role for PPARgamma in the regulation of EC gene expression, with important implications for the clinical links between obesity and atherosclerosis.

Peroxisome proliferator-activated receptor gamma activators inhibit gene expression and migration in human vascular smooth muscle cells

Migration of vascular smooth muscle cells (VSMCs) plays an important role in atherogenesis and restenosis after arterial interventions. The expression of matrix metalloproteinases (MMPs), particularly MMP-9, contributes to VSMC migration. This process requires degradation of basal laminae and other components of the arterial extracellular matrix. Peroxisome proliferator-activated receptors (PPARs), members of the nuclear receptor family, regulate gene expression after activation by various ligands. Recent studies have suggested opposing effects of PPAR gamma (PPARgamma) activation on atherogenesis. The present study tested the hypotheses that human VSMCs express PPAR alpha (PPARalpha) and PPARgamma and that PPAR agonists in VSMCs modulate MMP-9 expression and activity, as well as VSMC migration. Human VSMCs expressed PPARalpha and PPARgamma mRNA and protein. Treatment of VSMCs with the PPARgamma ligands troglitazone and the naturally occurring 15-deoxy-Delta12, 14-prostaglandin J2 (15d-PGJ2) decreased phorbol 12-myristate 13-acetate-induced MMP-9 mRNA and protein levels, as well as MMP-9 gelatinolytic activity in the supernatants in a concentration-dependent manner. Six different PPARalpha activators lacked such effects. Addition of prostaglandin F2alpha, known to limit PPARgamma activity, diminished the MMP-9 inhibition seen with either troglitazone or 15d-PGJ2, further implicating PPARgamma in these effects. Finally, troglitazone and 15d-PGJ2 inhibited the platelet-derived growth factor-BB-induced migration of VSMCs in vitro in a concentration-dependent manner. PPARgamma activation may regulate VSMC migration and expression and activity of MMP-9. Thus, PPARgamma activation in VSMCs, via the antidiabetic agent troglitazone or naturally occurring ligands, may act to counterbalance other potentially proatherosclerotic PPARgamma effects.

Macrophages in human atheroma contain PPARgamma: differentiation-dependent peroxisomal proliferator-activated receptor gamma(PPARgamma) expression and reduction of MMP-9 activity through PPARgamma activation in mononuclear phagocytes in vitro

Mononuclear phagocytes play an important role in atherosclerosis and its sequela plaque rupture in part by their secretion of matrix metalloproteinases (MMPs), including MMP-9. Peroxisomal proliferator-activated receptor gamma (PPARgamma), a transcription factor in the nuclear receptor superfamily, regulates gene expression in response to various activators, including 15-deoxy-delta12,14-prostaglandin J2 and the antidiabetic agent troglitazone. The role of PPARgamma in human atherosclerosis is unexplored. We report here that monocytes/macrophages in human atherosclerotic lesions (n = 12) express immunostainable PPARgamma. Normal artery specimens (n = 6) reveal minimal immunoreactive PPARgamma. Human monocytes and monocyte-derived macrophages cultured for 6 days in 5% human serum expressed PPARgamma mRNA and protein by reverse transcription-polymerase chain reaction and Western blotting, respectively. In addition, PPARgamma mRNA expression in U937 cells increased during phorbol 12-myristate 13 acetate-induced differentiation. Stimulation of PPARgamma with troglitazone or 15-deoxy-delta12,14-prostaglandin J2 in human monocyte-derived macrophages inhibited MMP-9 gelatinolytic activity in a concentration-dependent fashion as revealed by zymography. This inhibition correlates with decreased MMP-9 secretion as determined by Western blotting. Thus, PPARgamma is present in macrophages in human atherosclerotic lesions and may regulate expression and activity of MMP-9, an enzyme implicated in plaque rupture. PPARgamma is likely to be an important regulator of monocyte/macrophage function with relevance for human atherosclerotic disease.

Upregulation of endothelial nitric oxide synthase by HMG CoA reductase inhibitors

BACKGROUND

Oxidized low-density lipoprotein (ox-LDL) causes endothelial dysfunction in part by decreasing the availability of endothelial nitric oxide (NO). Although HMG CoA reductase inhibitors restore endothelial function by reducing serum cholesterol levels, it is not known whether they can also directly upregulate endothelial NO synthase (ecNOS) activity.

METHODS AND RESULTS

Human saphenous vein endothelial cells were treated with ox-LDL (50 microg/mL thiobarbituric acid reactive substances 12 to 16 nmol/mg) in the presence of HMG CoA reductase inhibitors simvastatin and lovastatin. In a time-dependent manner, ox-LDL decreased ecNOS mRNA and protein levels (91+/-4% and 67+/-8% reduction after 72 hours, respectively). Both simvastatin (1 micromol/L) and lovastatin (10 micromol/L) upregulated ecNOS expression by 3.8-fold and 3.6-fold, respectively, and completely prevented its downregulation by ox-LDL. These effects of simvastatin on ecNOS expression correlated with changes in ecNOS activity. Although L-mevalonate alone did not affect ecNOS expression, cotreatment with L-mevalonate completely reversed ecNOS upregulation by simvastatin. Actinomycin D studies revealed that simvastatin stabilized ecNOS mRNA (tau1/2, 43 versus 35 hours). Nuclear run-on assays and transient transfection studies with a -1.6 kb ecNOS promoter construct showed that simvastatin did not affect ecNOS gene transcription.

CONCLUSIONS

Inhibition of endothelial HMG CoA reductase upregulates ecNOS expression predominantly by posttranscriptional mechanisms. These findings suggest that HMG CoA reductase inhibitors may have beneficial effects in atherosclerosis beyond that attributed to the lowering of serum cholesterol by increasing ecNOS activity.

Identification of a human src homology 2-containing protein-tyrosine-phosphatase: a putative homolog of Drosophila corkscrew

src homology 2 (SH2) domains direct binding to specific phosphotyrosyl proteins. Recently, SH2-containing protein-tyrosine-phosphatases (PTPs) were identified. Using degenerate oligonucleotides and the PCR, we have cloned a cDNA for an additional PTP, SH-PTP2, which contains two SH2 domains and is expressed ubiquitously. When expressed in Escherichia coli, SH-PTP2 displays tyrosine-specific phosphatase activity. Strong sequence similarity between SH-PTP2 and the Drosophila gene corkscrew (csw) and their similar patterns of expression suggest that SH-PTP2 is the human corkscrew homolog. Sequence comparisons between SH-PTP2, SH-PTP1, corkscrew, and other SH2-containing proteins suggest the existence of a subfamily of SH2 domains found specifically in PTPs, whereas comparison of the PTP domains of the SH2-containing PTPs with other tyrosine phosphatases suggests the existence of a subfamily of PTPs containing SH2 domains. Since corkscrew, a member of the terminal class signal transduction pathway, acts in concert with D-raf to positively transduce the signal generated by the receptor tyrosine kinase torso, these findings suggest several mechanisms by which SH-PTP2 may participate in mammalian signal transduction.

Chromosomal localization of an SH2-containing tyrosine phosphatase (PTPN6)

We have used panels of somatic cell hybrids and fluorescent in situ hybridization to determine the chromosomal localization of the novel nontransmembrane tyrosine phosphatase PTPN6 (protein tyrosine phosphatase, nonreceptor type 6), which contains two SH2 domains. PTPN6 maps to 12p13, a region commonly involved in leukemia-associated chromosomal abnormalities. Since PTPN6 is expressed at high levels in hematopoietic cells of all lineages and its expression is induced early in hematopoietic differentiation, altered expression and/or structure of PTPN6 may play a role in leukemogenesis.

Isolation of a src homology 2-containing tyrosine phosphatase

Tyrosine phosphorylation is controlled by the opposing actions of tyrosine kinases and phosphotyrosine phosphatases (PTPs). src homology 2 domains (SH2) are found in several types of signaling proteins, including some tyrosine kinases. These domains bind phosphotyrosyl proteins and thus help promote signal transduction. Using mixed oligonucleotide-directed polymerase chain reactions, two previously undescribed rat PTP cDNA fragments were generated. Through subsequent screening of rat megakaryocyte and human erythroleukemia libraries, we obtained a full-length coding sequence for one of these fragments. This cDNA, SH-PTP1, encodes a tyrosine phosphatase containing two highly conserved SH2 domains. SH-PTP1, with a 2.4-kilobase mRNA, a predicted open reading frame of 595 amino acids, and a structure suggesting a nontransmembrane protein, is expressed primarily in hematopoietic and epithelial cells. When expressed in Escherichia coli, SH-PTP1 possesses PTP activity. The structure of SH-PTP1 establishes an additional branch of the tyrosine phosphatase family and suggests mechanisms through which tyrosine phosphatases might participate in signal transduction pathways.

Evolution and organization of the human protein C gene

We have isolated overlapping phage genomic clones covering an area of 21 kilobases that encodes the human protein C gene. The gene is at least 11.2 kilobases long and is made up of nine exons and eight introns. Two regions homologous to epidermal growth factor and transforming growth factor are encoded by amino acids 46-91 and 92-136 and are precisely delimited by introns, as is a similar sequence in the genes for coagulation factor IX and tissue plasminogen activator. When homologous amino acids of factor IX and protein C are aligned, the positions of all eight introns correspond precisely, suggesting that these genes are the product of a relatively recent gene duplication. Nevertheless, the two genes are sufficiently distantly related that no nucleic acid homology remains in the intronic regions and that the size of the introns varies dramatically between the two genes. The similarity of the genes for factor IX and protein C suggests that they may be the most closely related members of the serine protease gene family involved in coagulation and fibrinolysis.

The structure and evolution of a 461 amino acid human protein C precursor and its messenger RNA, based upon the DNA sequence of cloned human liver cDNAs

Human liver cDNA coding for protein C has been synthesized, cloned and sequenced. The abundance of protein C message is approximately 0.02% of total mRNA. Three overlapping clones contain 1,798 nucleotides of contiguous sequence, which approximates the size of the protein's mRNA, based upon Northern hybridization. The cDNA sequence consists of 73 5'-noncoding bases, coding sequence for a 461 amino acid nascent polypeptide precursor, a TAA termination codon, 296 3'-noncoding bases, and a 38 base polyadenylation segment. The nascent protein consists of a 33 amino acid "signal", a 9 amino acid propeptide, a 155 amino acid "light" chain, a Lys-Arg connecting dipeptide, and a 262 amino acid "heavy" chain. Human protein C and Factor IX and X precursors possess about one third identical amino acids (59% in the gamma-carboxyglutamate domain), including two forty-six amino acid segments homologous to epidermal growth factor. Human protein C also has similar homology with prothrombin in the "leader", gamma-carboxyglutamate and serine protease domains, but lacks the two "kringle" domains found in prothrombin.