Rohit Khurana

Role of Angiogenesis in Cardiovascular Disease A Critical Appraisal

The role of angiogenesis in atherosclerosis and other cardiovascular diseases has emerged as a major unresolved issue. Angiogenesis has attracted interest from opposite perspectives. Angiogenic cytokine therapy has been widely regarded as an attractive approach both for treating ischemic heart disease and for enhancing arterioprotective functions of the endothelium; conversely, a variety of studies suggest that neovascularization contributes to the growth of atherosclerotic lesions and is a key factor in plaque destabilization leading to rupture. Here, we critically review the evidence supporting a role for angiogenesis and angiogenic factors in atherosclerosis and neointima formation, emphasizing the problems raised by some of the landmark studies and the suitability of animal models of atherosclerosis and neointimal thickening for investigating the role of angiogenesis. Because many of the relevant studies have focused on the role of vascular endothelial growth factor (VEGF), we consider this work in the wider context of VEGF biology and in light of recent experience from clinical trials of VEGF and other angiogenic cytokines for ischemic heart disease. Also discussed are recent findings suggesting that, although angiogenesis may contribute to neointimal growth, it is not required for the initiation of intimal thickening. Our assessment of the evidence leads us to conclude that, although microvessels are a feature of advanced human atherosclerotic plaques, it remains unclear whether angiogenesis either plays a central role in the development of atherosclerosis or is responsible for plaque instability. Furthermore, current evidence from clinical trials of both proangiogenic and antiangiogenic therapies does not suggest that inhibition of angiogenesis is likely to be a viable therapeutic strategy for cardiovascular disease.

Angiogenesis-Dependent and Independent Phases of Intimal Hyperplasia

Background—Neointimal vascular smooth muscle cell (VSMC) proliferation is a primary cause of occlusive vascular disease, including atherosclerosis, restenosis after percutaneous interventions, and bypass graft stenosis. Angiogenesis is implicated in the progression of early atheromatous lesions in animal models, but its role in neointimal VSMC proliferation is undefined. Because percutaneous coronary interventions result in induction of periadventitial angiogenesis, we analyzed the role of this process in neointima formation. Methods and Results—Local injury to the arterial wall in 2 different animal models induced periadventitial angiogenesis and neointima formation. Application of angiogenesis stimulators vascular endothelial growth factor (VEGF-A165) or a proline/arginine-rich peptide (PR39) to the adventitia of the injured artery induced a marked increase in neointimal thickening beyond that seen with injury alone in both in vivo models. Inhibition of either VEGF (with soluble VEGF receptor 1 [sFlt1]) or fibroblast growth factor (FGF) (with a dominant=negative form of FGF receptor 1 [FGF-R1DN]), respectively, signaling reduced adventitial thickening induced by VEGF and PR39 to the level seen with mechanical arterial injury alone. However, neither inhibitor was effective in preventing neointimal thickening after mechanical injury when administered in the absence of angiogenic growth factor. Conclusions—Our findings indicate that adventitial angiogenesis stimulates intimal thickening but does not initiate it.

Placental Growth Factor Promotes Atherosclerotic Intimal Thickening and Macrophage Accumulation

Background—Placental growth factor (PlGF) has been implicated in the pathophysiological angiogenesis and monocyte recruitment that underlie chronic inflammatory disease, but its role in atherosclerosis has not been examined. We investigated the effects of exogenous PlGF, delivered by adenoviral gene transfer, on atherogenic intimal thickening and macrophage accumulation induced by collar placement around the rabbit carotid artery and examined the effects of PlGF deficiency on atherosclerosis in apolipoprotein E–deficient (apoE−/−) mice. Methods and Results—Periadventitial transfer of PlGF2-encoding adenoviruses significantly increased intimal thickening, macrophage accumulation, endothelial vascular cell adhesion molecule-1 expression, and adventitial neovascularization in the collared arteries of hypercholesterolemic rabbits and increased the intima-to-media ratio in rabbits fed a normal diet. Neointimal macrophages were associated with increased expression of the PlGF receptor Flt-1. The size and macrophage content of early atherosclerotic lesions were reduced in mice deficient in both apoE and PlGF compared with apoE-deficient mice. Conclusions—Local adenoviral PlGF2 delivery promotes atherogenic neointima formation in hypercholesterolemic rabbits, and PlGF is required for macrophage infiltration in early atherosclerotic lesions in apoE−/− mice. These findings support a novel role for PlGF in the pathogenesis of atherosclerotic disease.

Yin Yang-1 Inhibits Vascular Smooth Muscle Cell Growth and Intimal Thickening by Repressing p21WAF1/Cip1 Transcription and p21WAF1/Cip1-Cdk4-Cyclin D1 Assembly

Vascular injury initiates a cascade of phenotype-altering molecular events. Transcription factor function in this process, particularly that of negative regulators, is poorly understood. We demonstrate here that the forced expression of the injury-inducible GLI-Krüppel zinc finger protein Yin Yang-1 (YY1) inhibits neointima formation in human, rabbit and rat blood vessels. YY1 inhibits p21WAF1/Cip1 transcription, prevents assembly of a p21WAF1/Cip1-cdk4-cyclin D1 complex, and blocks downstream pRbSer249/Thr252 phosphorylation and expression of PCNA and TK-1. Conversely, suppression of endogenous YY1 elevates levels of p21WAF1/Cip1, PCNA, pRbSer249/Thr252 and TK-1, and increases intimal thickening. YY1 binds Sp1 and prevents its occupancy of a distinct element in the p21WAF1/Cip1 promoter without YY1 itself binding the promoter. Additionally, YY1 induces ubiquitination and proteasome-dependent degradation of p53, decreasing p53 immunoreactivity in the artery wall. These findings define a new role for YY1 as both an inducer of p53 instability in smooth muscle cells, and an indirect repressor of p21WAF1/Cip1 transcription, p21WAF1/Cip1-cdk4-cyclin D1 assembly and intimal thickening.

Soluble Human Complement Receptor 1 Limits Ischemic Damage in Cardiac Surgery Patients at High Risk Requiring Cardiopulmonary Bypass

Background—This study was undertaken to determine whether soluble human complement receptor type 1 (TP10), a potent inhibitor of complement activation, would reduce morbidity and mortality in high-risk patients undergoing cardiac surgery on cardiopulmonary bypass (CPB). Methods—This was a randomized multicenter, prospective, placebo-controlled, double-blind study in which 564 high-risk patients undergoing cardiac surgery on CPB received an intravenous bolus of TP10 (1, 3, 5, 10 mg/kg) or placebo immediately before CPB. The primary endpoint was the composite events of death, myocardial infarction (MI), prolonged (≥24 hours) intra-aortic balloon pump support (IABP), and prolonged intubation. Results—TP10 significantly inhibited complement activity after 10 to 15 minutes of CPB and this inhibition persisted for 3 days postoperatively. However, there was no difference in the primary endpoint between the 2 groups (33.7% placebo versus 31.4% TP10; P=0.31). The primary composite endpoint was, however, reduced in all male TP10 patients by 30% (P=0.025). TP10 reduced the incidence of death or MI in males by 36% (P=0.026), the incidence of death or MI in CABG males by 43% (P=0.043) and the need for prolonged IABP support in male CABG and valve patients by 100% (P=0.019). There was, however, no improvement seen in female TP10 patients. There were no significant differences in adverse events between the groups. Conclusion—TP10 effectively inhibits complement activation during CPB; however, this was not associated with an improvement in the primary endpoint of the study. Nevertheless, TP10 did significantly decrease the incidence of mortality and MI in male patients.

Vascular Endothelial Growth Factor Synergistically Enhances Induction of E-Selectin by Tumor Necrosis Factor-α

Objective—The regulation of endothelial cell adhesion molecules (CAMs) by vascular endothelial growth factor (VEGF) was investigated in cell cultures and in a rabbit model of atherogenic neointima formation. Methods and Results—VEGF regulation of vascular CAM-1 (vascular cell adhesion molecule), intercellular CAM-1 (intercellular adhesion molecule), and E-selectin were investigated in human umbilical vein endothelial cells using quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, and flow cytometry, and in the rabbit collar model of atherogenic macrophage accumulation by immunostaining. VEGF alone caused no significant induction of vascular cell adhesion molecule-1, intercellular adhesion molecule-1, or E-selectin compared with tumor necrosis factor-α. In both hypercholesterolemic and normal rabbits, adenoviral VEGF-A165 expression caused no increase in endothelial vascular cell adhesion molecule-1 or E-selectin. In contrast, pretreatment of human umbilical vein endothelial cells with VEGF significantly increased E-selectin expression induced by tumor necrosis factor-α, compared with tumor necrosis factor-α alone, whereas vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 were unaffected. VEGF similarly enhanced IL-1β–induced E-selectin upregulation. VEGF also synergistically increased tumor necrosis factor-α–induced E-selectin mRNA and shedding of soluble E-selectin. Synergistic upregulation of E-selectin expression by VEGF was mediated via VEGF receptor-2 and calcineurin signaling. Conclusions—VEGF alone does not activate endothelium to induce CAM expression; instead, VEGF “primes” endothelial cells, sensitizing them to cytokines leading to heightened selective pro-inflammatory responses, including upregulation of E-selectin.

Vascular Endothelial Growth Factor Gene Transfer Inhibits Neointimal Macrophage Accumulation in Hypercholesterolemic Rabbits

Objective—This study aims to determine the effects of periadventitial vascular endothelial growth factor (VEGF) gene transfer on neointima formation and macrophage accumulation induced by collar placement around the carotid artery in hypercholesterolemic rabbits. Methods and Results—Collar placement around the carotid artery in cholesterol-fed rabbits induced intimal thickening with increased neointimal macrophage content. Liposome-mediated VEGF gene transfer, confirmed by transgene-specific RT-PCR, caused a marked inhibition of both intimal thickening and macrophage accumulation compared with a lacZ control gene. VEGF gene transfer was not accompanied by a significant increase in adventitial neovascularization. Collaring of carotid arteries in hypercholesterolemic rabbits also upregulated endothelial VCAM-1 expression. Inhibition of neointimal macrophage infiltration in VEGF-transduced, collared arteries was associated with decreased endothelial VCAM-1. Conclusions—VEGF gene transfer inhibits collar-induced intimal thickening, macrophage accumulation, and VCAM-1 expression in cholesterol-fed rabbits. These findings support the concept that low-level VEGF expression can exert arterioprotective effects in the presence of high blood cholesterol.

Endothelial progenitor cells: Precursors for angiogenesis

The presence of endothelial progenitor cells has been demonstrated in the bone marrow and systemic circulation of adults, thus raising the possibility of a novel strategy to induce therapeutic angiogenesis for ischemic arterial disease. Successful incorporation into sites of actively occurring angiogenesis in numerous animal models has accelerated the enthusiasm for exploiting their therapeutic capacity in humans and has led to the recent use of putative endothelial precursor cells in phase I feasibility and safety studies. However, key biological issues remain ill defined. The relative contribution of these cells to postnatal physiological and pathological neovascularization has not been fully characterized. Furthermore, the molecular phenotype of the putative endothelial progenitor cell and the processes leading to their mobilization from the bone marrow and homing to sites of angiogenesis have yet to be elucidated. This review addresses these fundamental issues that warrant further basic investigation before the full therapeutic potential of these cells can be achieved within appropriate target patients.