Aging‐induced collateral impairment: role of arterial rarefaction, decreased eNOS expression/signaling, and increased susceptibility of endothelial cells to apoptosis

Abstract

Despite positive animal studies, clinical angiogenesis trial outcomes have been disappointing. Because 1) aging impairs ischemia‐induced collaterogenesis in animals, and 2) patients targeted for angiogenesis therapy are older, we determined whether aging‐induced changes in collateral phenotype could contribute to the disappointing clinical results.ObjectiveTo determine a) whether aging alters collateral phenotype such that they are more resistant to therapeutic interventions and, if so, b) what are the contributing molecular mechanisms.Methods and ResultsMicro‐computed tomography (MicroCT) was used to assess vessel phenotype in young (4 month) and old (20 month) wildtype C57 mice under baseline conditions. Old mice had approximately 60% fewer pre‐existing arterioles ≤25μm in the calf and thigh, approximately 50% fewer arterioles >25μm in the calf, and decreased baseline endothelial nitric oxide synthase (eNOS) protein and phosphorylation at serine‐1177 (indicating both decreased eNOS expression and signaling) than young mice. To determine if this association reflected causality, we examined vessel phenotype in eNOS knockout (KO) mice; young eNOS KO mice exhibited significantly fewer arterioles than young wild‐type mice, similar to the findings in old wild‐type mice. Interestingly, endothelial cells obtained from older mice a) exhibited decreased eNOS protein and serine‐117 phosphorylation, and b) were more sensitive to an apoptotic stimulus, findings that may contribute mechanistically to vessel dropout.ConclusionsAging leads to arteriolar rarefaction, an effect contributed to by decreased eNOS expression and signaling, which predisposes endothelial cells to undergo apoptosis. This microvasculature rarefaction may importantly contribute to the thus‐far disappointing results of clinical angiogenesis trials.This work was supported by NIH RO1 HL085003‐01A2.