Mechanism of heat shock protein 72 induction in primary cultured astrocytes after oxygen-glucose deprivation

Induction of stress proteins is thought to be important in the protection of cells from a variety of environmental insults including heat, hypoxia and ischemia. The aim of this study was to compare the mechanism of induction of heat shock protein 72 (HSP72) in primary cultures of murine cortical astrocytes by heat and combined oxygen-glucose deprivation (OGD), a model of in vitro ischemia. S-35-methionine labeling and immunoblotting showed increased HSP72 synthesis and accumulation lasting for up to 24 h following heat or OGD. Heat induced a markedly greater amount of HSP72 mRNA and protein than did OGD. We then sought evidence of heat shock transcription factor-1 (HSF-1) activation. An increase in apparent molecular weight of nuclear HSF-1 after heat or OGD was observed consistent with increased phosphorylation. To seek an explanation of the difference between heat and OGD as inducers of HSP72 we examined the binding activity of HSP72+13 to other proteins. More cellular protein was found to co-immunoprecipitate with HSP72+73, and more HSP72+73 was found in the pellet fraction after heat shock compared to OGD. These results suggest that HSP72 induction is regulated in astrocytes at least in part at the level of HSF activation, by both heat and OGD. Reduced availability of free HSP72+73 in heated cells could be responsible for the greater magnitude of HSP72 induction after heat compared to OGD.