Activation of the heat shock transcription factor by hypoxia in mammalian cells.

Members of the stress protein family such as HSP70 are induced in ischemic tissues and may contribute to the ability of cells to survive episodes of transient circulatory insufficiency. However, the biochemical events that lead to this induction, and their degree of similarity with pathways triggered by heat stress, have not been defined. In this study, we demonstrate that transient exposure of cultured C2C12 mouse myogenic cells to a hypoxic atmosphere stimulates DNA binding activity of the heat shock transcription factor through mechanisms that are independent of new protein synthesis. Activation of heat shock transcription factor in hypoxic cells is temporally associated with induction of endogenous HSP70 gene transcription and with induction of a heterologous reporter gene controlled by the human HSP70 promoter. Furthermore, induction of the human HSP70 promoter by hypoxia requires an intact heat shock element, indicating that other cis-acting transcriptional control elements contained within this complex promoter are not sufficient to transduce signals generated within hypoxic cells. These findings provide strong evidence that hypoxia and heat shock induce expression of the HSP70 gene by similar, if not identical, mechanisms.