THE HEAT-SHOCK TRANSCRIPTION FACTOR HSF1 IS RAPIDLY ACTIVATED BY EITHER HYPER-OSMOTIC OR HYPOOSMOTIC STRESS IN MAMMALIAN-CELLS

Osmoregulation, the cellular response to environmental changes of osmo larity and ionic strength, is important for the survival of living org anisms. We have demonstrated previously that an exposure of mammalian cells to hypo-osmotic stress, either in growth medium (30% growth medi um and 70% water) or in binary solution containing sorbitol and water, prominently induced the DNA-binding activity of the heat-shock transc ription factor (HSF1) [Huang, Caruccio, Liu and Chen (1995) Biochem. J . 307, 347-352]. Since hyperosmotic and hypo-osmotic stress usually el icit opposite biological responses, we wondered what would be the effe ct of hyperosmotic stress on HSF activation. In this study we have exa mined the HSF DNA-binding activity in HeLa cells maintained in the sor bitol/water binary solution over a wide concentration range (0.1-0.9M) and in Dulbecco's medium supplemented with sorbitol or NaCl. We found that HSF-binding activity could be induced prominently under both hyp o-osmotic (0.1-0.25 M) and hyperosmotic conditions (0.50-0.90 M). In b oth cases, HSF activation was observed within 5 min after changing the osmotic pressure. The activation was accompanied by both HSF trimeriz ation and nuclear translocation, and appeared to be independent of pro tein synthesis. The effects of hypo-or hyper-osmotic stress on HSF act ivation could be reversed once the cells were returned to iso-osmotic conditions (0.30 M) with a half-life (t(1/2) of 25 min or less. This r apid turnover of the osmotic-stress-induced HSF-binding activity was i nhibited by cycloheximide, a potent inhibitor of protein synthesis. Un like heat shock, activation of HSF by either hypo-or hyper-osmotic str ess did not lead to an accumulation of heat-shock protein 70 (HSP70) m RNA in HeLa cells. We propose that HSF activation during osmotic stres s may serve physiological functions independent of the synthesis of he at-shock proteins.