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production in human vascular smooth muscle cells
Division of Nephrology, Department of Medicine, New England Medical Center Hospitals and Tufts University School of Medicine, Boston, Massachusetts 02111
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
O2 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 O2
environment (20% O2) or in
chambers containing low (~1%) or moderate (5%)
O2. Levels of IL-1
and IL-1
mRNA increased in human VSMC after 24-48 h of incubation in low
O2 compared with levels in
normoxic cells and then decreased upon subsequent reoxygenation. Levels
of cell-associated IL-1
also increased progressively after 24-48 h in low O2; however,
detectable IL-1
was not released from the cells in the media.
IL-1
was detectable in cell lysates and supernatants; however, the
levels were not affected by exposure to low
O2. mRNA encoding for tumor
necrosis factor-
(TNF-
), 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%
O2 enhanced proliferation of human
VSMC but did not induce IL-1
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
O2 or 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 O2 tension induces VSMC
proliferation, independent of IL-1, whereas exposure to very low
O2 tension induces production of
IL-1
.
oxygen tension; tumor necrosis factor; interleukin-1-receptor antagonist
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