Inhibition mechanism of HSP70 induction in murine FM3A cells maintained at low culture temperature

Abstract

We have shown previously that induction of HSP70 synthesis in murine FM3A and its mutant ts85 cells by heat shock is somehow modulated by culture temperature. In this study, we further examined activation of heat shock transcription factor (HSF) and induction kinetics of HSP70 synthesis and HSP70 mRNA in FM3A and ts85 cells maintained at 37°C (37°C-cells) and 33°C (33°C-cells). Upon exposure to heat shock, HSF was activated to a high level in 37°C-FM3A cells, whereas HSF was activated only to a low level in the 33°C-cells. The induction of HSP70 mRNA and HSP70 synthesis occurred successively in the 37°C-cells but not in the 33°C-cells. On the other hand, in both 37 and 33°C-ts85 cells, activation of HSF, induction of HSP70 mRNA, and HSP70 synthesis occurred successively. Characteristically, protein synthesis in both 33°C-FM3A and ts85 cells was significantly lower than in the respective 37°C-cells, but constitutive HSP73 levels were similar among both the 37 and 33°C-cells. Furthermore, inhibition of protein synthesis of FM3A cells did not influence the activation of HSF, but accelerated inactivation of the activated HSF. We discuss the possible inhibition mechanisms of activation of HSF in 33°C-FM3A cells, regarding the function of HSP70 in both protein synthesis and repression of HSF.