150-kDa oxygen-regulated protein (ORP150) suppresses hypoxia-induced apoptotic cell death

To determine the contribution of 150-kDa oxygen-regulated protein (ORP150) to cellular processes underlying adaptation to hypoxia, a cell line stably transfected to overexpress ORP150 antisense RNA was created. In human embryonic kidney (HEK) cells stably overexpressing ORP150 antisense RNA, ORP150 antigen and transcripts were suppressed to low levels in normoxia and hypoxia, whereas wild-type cells showed induction of ORP150 with oxygen deprivation. Inhibition of ORP150 in antisense transfectants was selective, as hypoxia-mediated enhancement of glucose-regulated protein (GRP) 78 and GRP94 was maintained. However, antisense ORP150 transfectants displayed reduced viability when subjected to hypoxia, compared with wild-type and sense-transfected HEK cells. In contrast, diminished levels of ORP150 had no effect on cytotoxicity induced by other stimuli, including oxygen-free radicals and sodium arsenate. Although cellular ATP content was similar in hypoxia, compared with ORP150 antisense transfectants and wild-type HEK cells, suppression of ORP150 expression was associated with accelerated apoptosis. Hypoxia-mediated cell death in antisense HEK transfectants did not cause an increase in caspase activity or in cytoplasmic cytochrome c antigen. A well recognized inducer of apoptosis in HEK cells, staurosporine, caused increased caspase activity and cytoplasmic cytochrome c levels in both wild-type and antisense cells. These data indicate that ORP150 has an important cytoprotective role in hypoxia-induced cellular perturbation and that ORP150-associated inhibition of apoptosis may involve mechanisms distinct from those triggered by other apoptotic stimuli.