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

Abstract

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 O2tension 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 O2environment (20% O2) or in chambers containing low (∼1%) or moderate (5%) O2. Levels of IL-1α and IL-1β mRNA increased in human VSMC after 24–48 h of incubation in low O2compared with levels in normoxic cells and then decreased upon subsequent reoxygenation. Levels of cell-associated IL-1α also increased progressively after 24–48 h in low O2; however, detectable IL-1α was not released from the cells in the media. IL-1β was detectable in cell lysates and supernatants; however, the levels were not affected by exposure to low O2. mRNA encoding for tumor necrosis factor-α (TNF-α), a related cytokine and VSMC mitogen, was not detectable in human VSMC exposed to either low or 20% O2. Proliferation of human VSMC was not stimulated during exposure to low O2, despite the fact that cells remained responsive to the mitogenic effect of exogenous IL-1. Interestingly, however, exposure to 5% O2enhanced proliferation of human VSMC but did not induce IL-1α production. Inhibition of IL-1 binding to the type I IL-1 receptor by exogenous addition of IL-1-receptor antagonist (10 μg/ml) did not attenuate the proliferation rates of human VSMC incubated in 20%, 5%, or low O2or in human VSMC that were reoxygenated after exposure to low O2. These results demonstrate two direct and distinct effects of hypoxia on VSMC. Exposure to moderately low O2tension induces VSMC proliferation, independent of IL-1, whereas exposure to very low O2tension induces production of IL-1α.