Rapid activation of the heat shock transcription factor, HSFl, by either HYPO- or hyper-osmotic stress in mammalian cells

Osmoregulation is important for the survival of living organisms. We have demonstrated that exposure of mammalian cells to a hypo-osmotic stress prominently induced the binding activity of the heat shock transcription factor (HSFl) (Biochem. .1. 307, 347-352,1995). We have also shown that this hypo osmotic stress-induced activation of HSF binding can be demonstrated in Hel.a cells maintained in a binary sorbitol solution. We have now examined the stress response of HeLa cells in the sorbitol solution over a concentration range from 0.1 M and 0.90 M. We found that HSF binding activity could be prominently induced not only under hypo-osmotic condition (0.1 M and 0.25 M sorbitol) but also at sorbitol concentration between 0.50 M and 0.90 M (hyper-osmotic) The hyper-osmotic stress-induced HSF activation shared all the biochemical characteristics as that induced by hypo-osmotic stress. In both cases. HSF activation could be observed within 5 min after osmotic stress. The activation was accompanied by HSF trimerization, nuclear translocation, and appeared to be independent of protein synthesis. This activation was rapidly reversed once the cells under osmotic stress were returned to iso-osmotic condition (0.3 0.4 M sorbitol), with A half-life of 25 min or less. Intriguingly, this rapid turnover was inhibited by protein synthesis inhibitor cycloheximide. Tnlike heat shock and other stresses, activation of HSF by either hypo- or hyper-osmotic stress did not lead to an accumulation of HSP70 mRNA.