Hypoxia enhances vascular cell proliferation and angiogenesis in vitro via rapamycin (mTOR)-dependent signaling

Angiogenesis and vascular cell proliferation are pivotal in physiological and pathological processes including atherogenesis, restenosis, wound healing, and cancer development. Here we show that mammalian target of rapamycin (mTOR) signaling plays a key role in hypoxia-triggered smooth muscle and endothelial proliferation and angiogenesis in vitro. Hypoxia significantly increased DNA synthesis and proliferative responses to platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) in rat and human smooth muscle and endothelial cells. In an in vitro 3-dimensional model of angiogenesis, hypoxia increased PDGF- and FGF-stimulated sprout formation from rat and mouse aortas. Hypoxia did not modulate PDGF receptor mRNA, protein, or phosphorylation. PI3K activity was essential for cell proliferation under normoxic and hypoxic conditions. Activities of PI3K-downstream target PKB under hypoxia and normoxia were comparable. However, mTOR inhibition by rapamycin specifically abrogated hypoxia-mediated amplification of proliferation and angiogenesis, but was without effect on proliferation under normoxia. Accordingly, hypoxia-mediated amplification of proliferation was further augmented in mTOR-overexpressing endothelial cells. Thus, signaling via mTOR may represent a novel mechanism whereby hypoxia augments mitogen-stimulated vascular cell proliferation and angiogenesis.