Heme oxygenases (HOs) -1 and -2 catalyze the breakdown of
heme to release
carbon monoxide,
biliverdin, and ferrous
iron, which may preserve cell function during oxidative stress. HO-1 levels decrease in endothelial cells exposed to
hypoxia, whereas the effect of
hypoxia on HO-2 expression is unknown. The current study was carried out to determine if
hypoxia alters HO-2
protein levels in human endothelial cells and whether this
enzyme plays a role in preserving their viability during hypoxic stress. Human umbilical vein endothelial cells (HUVECs), human aortic endothelial cells (HAECs), and human blood outgrowth endothelial cells were exposed to 21% or 1% O(2) for 48 or 16 h in the presence or absence of
tumor necrosis factor-alpha (10 ng/ml) or H(2)O(2) (100 microm). In all three endothelial cell types HO-1
mRNA and
protein levels were decreased following hypoxic incubation, whereas HO-2
protein levels were unaltered. In HUVECs HO-2 levels were maintained during
hypoxia despite a 57% reduction in steady-state HO-2
mRNA level and a 43% reduction in total
protein synthesis. Polysome profiling revealed increased HO-2 transcript association with polysomes during
hypoxia consistent with enhanced translation of these transcripts. Importantly, inhibition of HO-2 expression by small interference RNA increased oxidative stress, exacerbated mitochondrial membrane depolarization, and enhanced
caspase activation and apoptotic cell death in cells incubated under hypoxic but not normoxic conditions. These data indicate that HO-2 is important in maintaining endothelial viability and may preserve local regulation of vascular tone,
thrombosis, and inflammatory responses during reductions in systemic
oxygen delivery.