Impaired adaptive vascular growth in hypercholesterolemic rabbit

Therapeutic Angiogenesis for Peripheral Artery Disease: Lessons Learned in Translational Science

Peripheral arterial disease (PAD) is a major health care problem. There have been limited advances in medical therapies, and a huge burden of symptomatic patients with intermittent claudication and critical limb ischemia who have limited treatment options. Angiogenesis is the growth and proliferation of blood vessels from existing vasculature. For approximately 2 decades, "therapeutic angiogenesis" has been studied as an investigational approach to treat patients with symptomatic PAD. Despite literally hundreds of positive preclinical studies, results from human clinical studies thus far have been disappointing. Here we present an overview of where the field of therapeutic angiogenesis stands today and examine lessons learned from previously conducted clinical trials. The objective is not to second-guess past efforts but to place the lessons in perspective to allow for trial success in the future to improve agent development, trial design, and ultimately, clinical outcomes for new therapeutics for PAD.

Impact of chronic anticholesterol therapy on development of microvascular rarefaction in the metabolic syndrome

OBJECTIVE

The obese Zucker rat (OZR) model of the metabolic syndrome is partly characterized by moderate hypercholesterolemia, in addition to other contributing comorbidities. Previous results suggest that vascular dysfunction in OZR is associated with chronic reduction in vascular nitric-oxide (NO) bioavailability and chronic inflammation, both frequently associated with hypercholesterolemia. As such, we evaluated the impact of chronic cholesterol-reducing therapy on the development of impaired skeletal muscle arteriolar reactivity and microvessel density in OZR and its impact on chronic inflammation and NO bioavailability.

MATERIALS AND METHODS

Beginning at seven weeks of age, male OZR were treated with gemfibrozil, probucol, atorvastatin, or simvastatin (in chow) for 10 weeks. Subsequently, plasma and vascular samples were collected for biochemical/molecular analyses, while arteriolar reactivity and microvessel network structure were assessed by using established methodologies after 3, 6, and 10 weeks of drug therapy.

RESULTS

All interventions were equally effective at reducing total cholesterol, although only the statins also blunted the progressive reductions to vascular NO bioavailability, evidenced by greater maintenance of acetylcholine-induced dilator responses, an attenuation of adrenergic constrictor reactivity, and an improvement in agonist-induced NO production. Comparably, while minimal improvements to arteriolar wall mechanics were identified with any of the interventions, chronic statin treatment reduced the rate of microvessel rarefaction in OZR. Associated with these improvements was a striking statin-induced reduction in inflammation in OZR, such that numerous markers of inflammation were correlated with improved microvascular reactivity and density. However, using multivariate discriminant analyses, plasma RANTES (regulated on activation, normal T-cell expressed and secreted), interleukin-10, monocyte chemoattractant protein-1, and tumor necrosis factor alpha were determined to be the strongest contributors to differences between groups, although their relative importance varied with time.

CONCLUSIONS

While the positive impact of chronic statin treatment on vascular outcomes in the metabolic syndrome are independent of changes to total cholesterol, and are more strongly associated with improvements to vascular NO bioavailability and attenuated inflammation, these results provide both a spatial and temporal framework for targeted investigation into mechanistic determinants of vasculopathy in the metabolic syndrome.

Human studies of angiogenic gene therapy

Despite significant advances in medical, interventional, and surgical therapy for coronary and peripheral arterial disease, the burden of these illnesses remains high. To address this unmet need, the science of therapeutic angiogenesis has been evolving for almost two decades. Early preclinical studies and phase I clinical trials achieved promising results with growth factors administered as recombinant proteins or as single-agent gene therapies, and data accumulated through 10 years of clinical trials indicate that gene therapy has an acceptable safety profile. However, more rigorous phase II and phase III clinical trials have failed to unequivocally demonstrate that angiogenic agents are beneficial under the conditions and in the patients studied to date. Investigators have worked to understand the biology of the vascular system and to incorporate their findings into new treatments for patients with ischemic disease. Recent gene- and cell-therapy trials have demonstrated the bioactivity of several new agents and treatment strategies. Collectively, these observations have renewed interest in the mechanisms of angiogenesis and deepened our understanding of the complexity of vascular regeneration. Gene therapy that incorporates multiple growth factors, approaches that combine cell and gene therapy, and the administration of "master switch" agents that activate numerous downstream pathways are among the credible and plausible steps forward. In this review, we examine the clinical development of angiogenic gene therapy, summarize several of the lessons learned during the conduct of these trials, and suggest how this prior experience may guide the conduct of future preclinical investigations and clinical trials.

Hypoxia-inducible factor 1alpha polymorphism and coronary collaterals in patients with ischemic heart disease

STUDY OBJECTIVES

Marked variability exists in coronary artery collaterals in patients with ischemic heart disease. Although multiple factors are thought to play a role in collateral development, the contribution of genetic factors is largely unknown. Hypoxia inducible factor 1 (HIF-1), a transcriptional activator that functions as a master regulator of oxygen homeostasis, is one possible genetic factor that could play an important role in modulating collateral development.

DESIGN, SETTING, AND PARTICIPANTS

Collateral vessels were determined in 100 patients with > or = 70% narrowing of at least one coronary artery without acute myocardial infarction or prior revascularization. DNA was genotyped for the presence of a single nucleotide (C to T) polymorphism that changes residue 582 of HIF-1alpha from proline to serine.

MEASUREMENTS AND RESULTS

The frequency of the T allele was significantly higher among patients without collaterals compared to patients with collaterals (0.188 vs 0.037, p < 0.001). In multivariate analyses, two variables affecting collateral formation were detected: two- or three-vessel coronary artery disease was a significant positive predictor (odds ratio [OR], 4.17; 95% confidence interval [CI], 1.61 to 10.8; p = 0.001), whereas the presence of HIF-1alpha genotype CT or TT was a negative predictor (OR, 0.19; 95% CI, 0.04 to 0.84; p = 0.03).

CONCLUSIONS

These data suggest that variations in HIF-1alpha genotype may influence development of coronary artery collaterals in patients with significant coronary artery disease.

Poor coronary collateral circulation is associated with higher concentrations of soluble adhesion molecules in patients with single-vessel disease

OBJECTIVE

As the endothelium and inflammatory cells play a crucial role in the development of collaterals after a sudden or slowly progressing stenosis of coronary arteries, the levels of soluble endothelial adhesion molecules (CAMs) including vascular cell adhesion molecule (VCAM-1) intercellular adhesion molecule-1 (ICAM-1) and E-selectin were compared between patients with poor coronary collaterals and patients with well-developed collaterals.

METHODS

In the study, 97 non-diabetic subjects with single-vessel disease were included. Collateral supply to the stenotic coronary artery was determined by angiographic grading system of 0-3 (Rentrop et al. J Am Coll Cardiol 1985; 5:587-592). Serum levels of adhesion molecules were measured by enzyme-linked immunosorbent assay.

RESULTS

Patients were divided into two groups according to the collateral degree (group A: 50 patients with grade 0 and 1; group B: 47 patients with grade 2 and 3 collaterals). The groups were well matched with respect to baseline clinical and angiographic characteristics. Levels of soluble VCAM-1 (mean+/-SEM; 875+/-26.6 versus 742.7+/-35.1 ng/ml; P=0.004), ICAM-1 (322.4+/-12.4 versus 269.4+/-13.3 ng/ml; P=0.005), and E-selectin (43.6+/-2.6 versus 33+/-2.4 ng/ml; P=0.004) were found to be significantly higher in group A in comparison with group B. In addition, when patients were divided into four groups according to the collateral degree, patients with grade 0 collaterals had the highest values and those with grade 3 collaterals had the lowest values for all these molecules.

CONCLUSIONS

We concluded that poor collateral circulation is associated with increased levels of soluble CAMs in patients with obstructive coronary artery disease. However, further studies are needed to elucidate the exact role of these inflammatory markers in the setting of poor collateral circulation.

Haptoglobin phenotype and coronary artery collaterals in diabetic patients

Cross-cultural epidemiological studies of incident cardiovascular disease in the diabetic patient have demonstrated marked differences in susceptibility that may be due to a genetic factor. The coronary artery collateral circulation is the chief determinant of the size of a myocardial infarction and is highly variable between patients. We recently demonstrated that a functional allelic polymorphism in the haptoglobin gene is correlated with a number of diabetic vascular complications. We thus set out to test the hypothesis that haptoglobin phenotype is associated with collateral formation in the setting of diabetes. We correlated the Hp phenotype (1-1, 2-1 or 2-2) as determined by polyacrylamide electrophoresis with the presence or absence of coronary collaterals by angiography in 82 consecutive diabetic patients and 138 consecutive non-diabetic patients undergoing catheterization. We found that diabetic patients with the Hp phenotype 2-1 were more likely to have collaterals than diabetic patients with the Hp phenotype 2-2 (P=0.007). There was no correlation between Hp phenotypes and the presence of collaterals in non-diabetic patients. Hp phenotype thus appears to be associated with the development of the coronary collateral circulation in diabetic patients with coronary artery disease. Haptoglobin 2-2 may predispose to less compensation for coronary artery stenosis in diabetic patients, and thereby portend a worse prognosis.

Oxidized low-density lipoprotein downregulates endothelial basic fibroblast growth factor through a pertussis toxin-sensitive G-protein pathway: mediator role of platelet-activating factor-like phospholipids

BACKGROUND

Oxidized LDL (oxLDL) inhibits angiogenesis in part by downregulating endothelial basic fibroblast growth factor (bFGF). To determine the mechanism of the downregulation, we investigated the signal transduction pathway involving potential phospholipid mediators.

METHODS AND RESULTS

Cultured bovine aortic endothelial cells were incubated with PBS (lipoprotein-free control), LDL, or copper oxLDL under serum-free conditions. At 24 hours, oxLDL (50 microg/mL) decreased bFGF mRNA (Northern blot), bFGF protein (Western blot and ELISA), and concomitant DNA synthesis, all by 40% to 50% compared with PBS. LDL had no effect. Pretreating the cells with 100 ng/mL pertussis toxin (PTX) for 18 hours before oxLDL exposure almost completely blocked the inhibitory effects of oxLDL. In contrast, inhibiting other major cellular signal transduction pathways with PD-98059 (mitogen-activated protein kinase kinase inhibitor), HA-1004 (inhibitor of cGMP- and cAMP-dependent protein kinase), or Ro-31-8220 (protein kinase C inhibitor) or chelating intracellular Ca(2+) with BAPTA-AM failed to attenuate any of the oxLDL effects assayed. Addition to the cultures of WEB 2086, a specific antagonist of the PTX-sensitive G protein-coupled platelet-activating factor (PAF) receptor, blocked the action of oxLDL. Whereas PAF dispersed in the culture medium failed to produce oxLDL-like effects, degradation of PAF and PAF-like phospholipids accumulated in oxLDL with a recombinant human PAF acetylhydrolase eliminated the inhibitory effects of oxLDL on bFGF expression and DNA synthesis.

CONCLUSIONS

OxLDL suppresses endothelial bFGF expression and DNA synthesis through a PTX-sensitive heterotrimeric G-protein pathway involving mediator phospholipids similar, but not identical, to PAF.

Oxidized low-density lipoproteins inhibit endothelial cell proliferation by suppressing basic fibroblast growth factor expression

BACKGROUND

Hyperlipidemia inhibits proliferation of endothelial cells (ECs) in culture and angiogenesis in vivo and in arterial explants. Elucidation of the mechanisms may suggest novel therapies against atherosclerosis.

METHODS AND RESULTS

Basic fibroblast growth factor (bFGF) expression and mitogenic effects were assessed in bovine aortic ECs incubated with oxidized LDL (ox-LDL). Compared with native LDL and lipoprotein-free controls, ox-LDL reduced bFGF mRNA levels in a time- and concentration-dependent manner, 100 microg/mL producing a maximum reduction of 40% to 50% within 24 to 48 hours. There were commensurate reductions in intracellular and extracellular bFGF concentrations, DNA and total RNA syntheses, and cell replication. FGF receptor 1 and beta-actin mRNA levels were unchanged. Ox-LDL accelerated bFGF mRNA degradation in actinomycin D-treated cells. However, inhibition of bFGF expression by ox-LDL was attenuated by cyclohexamide, indicating a requirement for continuous new protein synthesis for posttranscriptional destabilization. Reduced syntheses of DNA and total RNA were completely restored by bFGF but not by vascular endothelial growth factor. Inhibition of total RNA synthesis achieved by exposing cells to a bFGF-neutralizing antibody was similar in magnitude to that induced by ox-LDL.

CONCLUSIONS

Cytotoxic effects of ox-LDL on ECs are attributable in part to suppression of bFGF expression.