Hypoxia stimulates proliferation and interleukin-1α production in human vascular smooth muscle cells

Several lines of evidence indicate that hypoxia is a stimulus to vascular smooth muscle cell (VSMC) proliferation that occurs in pulmonary hypertension. The present study tested the hypothesis that low O-2 tension directly stimulates human VSMC proliferation by inducing them to produce interleukin (IL)-1, a potent autocrine growth factor for human VSMC. Human VSMC derived from pulmonary artery, aorta, or saphenous vein were incubated in either a normal in vitro O-2 environment (20% O-2) or in chambers containing low (similar to 1%) or moderate (5%) O-2. Levels of IL-1 alpha and IL-1 beta mRNA increased in human VSMC after 24-48 h of incubation in low O-2 compared with levels in normoxic cells and then decreased upon subsequent reoxygenation. Levels of cell-associated IL-1 alpha also increased progressively after 24-48 h in low O-2; however, detectable IL-1 alpha was not released from the cells in the media. IL-1 beta was detectable in cell lysates and supernatants; however, the levels were not affected by exposure to low O-2. mRNA encoding for tumor necrosis factor-alpha (TNF-alpha), a related cytokine and VSMC mitogen, was not detectable in human VSMC exposed to either low or 20% O-2. Proliferation of human VSMC was not stimulated during exposure to low O-2, despite the fact that cells remained responsive to the mitogenic effect of exogenous IL-1. Interestingly, however exposure to 5% O-2 enhanced proliferation of human VSMC but did not induce TL-1 alpha production. Inhibition of IL-1 binding to the type I IL-1 receptor by exogenous addition of IL-1-receptor antagonist (10 mu g/ml) did not attenuate the proliferation rates of human VSMC incubated in 20%, 5%, or low O-2 or in human VSMC that were reoxygenated after exposure to low O-2. These results demonstrate two direct and distinct effects of hypoxia on VSMC. Exposure to moderately low O-2 tension induces VSMC proliferation, independent of IL-1, whereas exposure to very low O-2 tension induces production of IL-1 alpha.