Effects of hyperosmotic NaCl and glycerol stress on stress response of human HeLa cells

The maintenance of intracellular osmotic pressure is of fundamental importance for cell survival. Since osmoregulatory processes are important to all living organisms, large fluctuations in environmental osmolarity may elicit or modulate stress responses. We examined whether hyperosmotic stress induced or modulated stress responses in human HeLa cells. When HeLa cells were incubated in medium supplemented with 50-150 mM NaCl or 0.2-1.0 M glycerol for 3-27 h, the stress response, analyzed at the levels of HSP70 synthesis, HSP70 mRNA accumulation, and heat shock transcription factor (HSF) activation, was not induced by either hyperosmotic stress. In hyperosmotic 150 mM NaCl or 1.0 M glycerol medium, the stress response to heat shock was inhibited at the levels of HSF activation, HSP70 mRNA accumulation, and HSP70 synthesis. In vitro activation of HSF showed that inhibition of this activation by hyperosmotic NaCl or glycerol stress was not irreversible. Furthermore, addition of the physiological osmolyte betaine to medium reversed the inhibition of heat-induced HSP70 synthesis under hyperosmotic NaCl stress but not under hyperosmotic glycerol stress. The effect of betaine against hyperosmotic NaCl stress was observed mainly at the translation level, and betaine seemed to enable cells to translate HSP70 mRNA specifically under heat shock conditions by restoring protein synthetic ability.