Modulation de la néovascularisation post-ischémique en présence de facteurs de risque cardiovasculaire

Atherosclerosis is the main cause of myocardial infarction, sudden cardiac death, stroke and lower limb ischemia. It is responsible for nearly half of all deaths in industrialized countries. When atherosclerotic arterial obstructions are so important that direct revascularization techniques cannot be successfully performed, the severity of residual tissue ischemia depends on the ability of the organism to spontaneously develop new blood vessels (neovascularization). Postnatal neovascularization is the result of two phenomena: the formation of new bloods vessels from the existing vasculature (angiogenesis) and vessel formation from progenitor cells (vasculogenesis). Our laboratory has demonstrated that several cardiovascular risk factors (smoking, aging, and hypercholesterolemia) also impair the angiogenic response after ischemia. However, the precise mechanisms involved in that pathophysiology are still unknown. A common feature of all the cardiovascular risk factors is increased oxidative stress. Therefore, the purpose of the present work was to elucidate the influence of cardiovascular risk factors and oxidative stress on neovascularization. Our results demonstrate that exposure to cigarette smoke and aging are associated with impaired neovascularization in response to ischemia, and that this is at least in part due to increased oxidative stress. In addition, we demonstrate that fatty acids derived from the diet can modulate the response to tissue ischemia.  The first study of the research project evaluated the effect of cigarette smoke exposure on neovascularization in response to ischemia, and the effect of an antioxidant therapy. Exposure to cigarette smoke was associated with a significant reduction in the recovery of blood flow perfusion and vessel density in ischemic muscles. However, a complete recovery of neovascularization was demonstrated in mice exposed to cigarette smoke that were treated with probucol or antioxidant vitamins. We found that antioxidant therapy in mice exposed to cigarette smoke was associated with a significant reduction of oxidative stress levels in the plasma and in ischemic muscles. In addition, endothelial progenitor cells (EPCs) exposed to cigarette smoke extracts in vitro showed a significant decrease in their angiogenic activities (migration, adhesion and homing into ischemic tissues) that was completely rescued by probucol and antioxidants vitamins.  The goal of the second study was to investigate the potential role of NADPH oxidase (Nox2) in the modulation of ischemia-induced neovascularization in the context of aging. We found that the expression of Nox2 is increased by aging in ischemic muscles of control mice. This is associated with a significant reduction of blood flow recovery after ischemia in older compared to young control mice. We also demonstrated that the density of capillaries and arterioles is significantly reduced in ischemic muscles of older animals, whereas oxidative stress levels are increased. Nox-2 deficiency reduces oxidative stress levels in ischemic tissues and improves blood flow recovery and vascular densities in older animals. We also demonstrated that the functional activities of EPCs (migration and adhesion to mature endothelial cells) were significantly reduced in older compared to young mice. However, Nox2 deficiency is associated with preserved EPCs functional activities in older animals. We also demonstrated an age-dependent pathological increase of oxidative stress in EPCs that is not found in Nox2-deficient animals.  The third study of the research project investigated the effect of fatty acids derived from the diet on postnatal neovascularization. To this end, mice received a diet containing either 20% corn oil (rich in omega-6) or 20% fish oil (rich in omega-3). Our results demonstrate that an omega-3 rich diet increases neovascularization in response to ischemia at the macrovascular, microvascular and clinical level compared to an omega-6 rich diet. This increased postnatal neovascularization is associated with decreased total cholesterol/HDL cholesterol ratio in the serum and improved VEGF/NO pathway in ischemic tissues. In addition, the omega-3 rich diet is associated with a significant increase of central (bone marrow) and peripheral (spleen) EPCs. We also show that the functional activities of EPCs (migration and incorporation into tubules) are improved and oxidative stress level in EPCs is reduced by the omega-3 rich diet. In conclusion, our studies have clarified the impact of cardiovascular risk factors (smoking and aging) and fatty acids derived from the diet (omega-3) on ischemia-induced neovascularization. We have also identified several mechanisms involved in that physiopathology. Globally, our studies should contribute to a better understanding of the effects of cigarette smoking, aging and omega-3 on the evolution of ischemic vascular diseases.

Keyphrases: Nox2, Endothelial progenitor cells, Oxidative stress, Cigarette smoke, Antioxidants, Aging, Omega-3, Neovascularization