The exposure of 3T3 cells to a medium made hypertonic by the addition of NaCl induced activation of a heat-shock transcription factor (HSF). This activation, as monitored by gel-mobility-shift assays, occurred within 10 min of hypertonic shock and was dose-dependent in relation to the osmotic strength of the medium up to 0.7 osM. Competition analysis indicated that the effect of hypertonic shock on HSF binding activity was specific. The magnitude of the heat-shock element (HSE)-HSF binding induced by incubating the cells in a 0.7 osM medium was comparable in intensity and time course with that induced by a 44 °C heat shock. Following removal of the stressors, the decrease in HSF-HSE binding was more rapid in hypertonicity-shocked than in heat-shocked cells. Treatment of the cells with cycloheximide did not inhibit HSF-HSE binding, indicating that the activation was independent of new protein synthesis. By using a specifically directed polyclonal serum, HSF1 was identified as the transcription factor involved in the hypertonicity-induced activation. HSF was also activated when a membrane-impermeable osmolyte such as sucrose was used to increase the osmolarity of the medium. However, no HSF-HSE binding was observed after addition of glycerol (a freely membrane-permeable osmolyte) in excess. There was a temporal relationship between the hypertonicity-induced volume decrease, the increase in the intracellular K+ concentration and the induction of HSF-HSE binding. In contrast, an increase in the intracellular Na+ concentration was not required to induce HSF-HSE binding. However, unlike the heat-shock response, the activation of HSF by hypertonic shock did not lead to elongation of the RNA transcript of heat-shock protein 70.

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