Surviving ischemia: adaptive responses mediated by hypoxia-inducible factor 1 (original) (raw)

Ischemia and infarction occur when tissue perfusion is insufficient to meet metabolic demands. In animal models of coronary artery occlusion, myocardial ischemia induces VEGF expression and collateral blood vessel development (11, 12). In humans, many patients with coronary artery stenosis fail to develop collateral vessels. Because there is an inverse correlation between infarct size and collateral blood flow (13, 14), such individual variation in adaptive responses to ischemia may determine the risk and severity of myocardial infarction. I will describe the angiogenic response potential as manifested in an animal model, discuss molecular mechanisms underlying the limited angiogenic response in humans, and consider strategies to circumvent these limitations.

To study the effects of hypoxia in the developing heart in vivo, near-term fetal sheep were subjected to a daily partial exchange transfusion in which blood was replaced with saline (15). Under these conditions, the hematocrit and arterial oxygen content are gradually reduced to one-third of normal levels within one week. Concomitantly, cardiac output increases by 50% and the heart/body weight ratio increases by 30%. To maintain myocardial oxygenation under these circumstances, myocardial blood flow increases fivefold. Capillary density and minimal capillary diameter are significantly increased and intercapillary distance is decreased in the hearts of anemic compared with control fetuses (15). Underlying these differences are three- to fourfold increases in the expression of VEGF protein and mRNA, as well as HIF-1α protein. In these anemic sheep fetuses, increased cardiac output provides systemic compensation for decreased oxygen-carrying capacity. The increased cardiac work and mass increase myocardial oxygen consumption, thus providing a hypoxic stimulus for increased HIF-1α expression and HIF-1–mediated VEGF gene transcription. Temporo-spatial correlations between HIF-1α protein and VEGF mRNA expression have also been demonstrated in the context of developmental and ischemic vascularization of the mouse retina (16).

Why does this response pathway fail to provide sufficient neovascularization in patients with coronary artery disease to prevent ischemia? One major difference is that the fetal sheep heart is a growing organ that is actively engaged in angiogenesis. Although this suggests differences in angiogenic responses as a function of developmental stage, aging in general may be an important factor. The absence of collateralization in some patients with symptomatic coronary artery disease may reflect impaired ability to produce VEGF. Ischemia-induced VEGF expression is likely to be influenced by a combination of genetic and environmental factors. Recent studies described below suggest that both aging and individual variability in responses to ischemia may influence the outcome of atherosclerotic vascular disease.

Collateral vessel development following femoral artery ligation is significantly reduced in older mice and rabbits (17). The impairment in vascularization is multifactorial, as older animals exhibit impaired VEGF production, vascular endothelial cell dysfunction, and reduced lymphocytic infiltration of ischemic tissues. Aortic smooth muscle cells from old rabbits manifest impaired hypoxia-induced VEGF expression due to a reduction in HIF-1 DNA-binding activity (18). HIF-1 DNA-binding activity is also reduced in brain, kidney, liver, and lung tissue from old versus young mice subjected to hypoxia (19).

These results do not address why individuals of similar age and degree of coronary occlusion vary in the extent of collateralization. Peripheral blood mononuclear cells isolated from patients with no angiographic evidence of coronary collateralization manifest significantly reduced induction of VEGF mRNA in response to hypoxia compared with cells from individuals with collateral development (20). This difference is maintained after analysis of multiple covariates. Thus, genetic or environmental factors or both may determine the physiological response to ischemia in each individual. VEGF production may be modulated via changes in the expression or activity of HIF-1. If so, it may be possible to identify individuals with suboptimal ischemia-induced VEGF expression; such individuals in particular may be candidates for therapeutic angiogenesis.

Clinical trials involving VEGF protein administration or VEGF gene therapy are underway (reviewed in ref. 21). It is unclear whether VEGF alone is sufficient to induce development of normally-functioning collateral vessels that will correct perfusion defects in ischemic myocardium. Whereas transgenic mice overexpressing VEGF exhibit excessive vascular permeability, mice overexpressing both VEGF and angiopoietin-1 manifest normal vascular permeability (22). HIF-1α gene therapy may have the advantage of inducing the expression not only of VEGF, but also of other hypoxia-induced angiogenic or survival factors (such as angiopoietin-2 and IGF-2) and their receptors on endothelial cells (such as FLT-1). In a recent study, HIF-1α gene therapy was as effective as VEGF in stimulating therapeutic angiogenesis in a rabbit hindlimb ischemia model (23). Furthermore, transgenic mice expressing HIF-1α in the skin demonstrated increased vascular density, as in the case of VEGF transgenic mice, but whereas VEGF transgenic mice demonstrate increased capillary leakage in response to an inflammatory stimulus, in HIF-1α transgenic mice there was actually decreased capillary leakage compared with nontransgenic littermates (J. Arbeit, personal communication). Whether this property is due to HIF-1–mediated expression of angiopoietin-2, which is known to be induced by hypoxia (24), remains to be established.