Activation of heat shock factor 1 by hyperosmotic or hypo-osmotic stress is drastically attenuated in normal human fibroblasts during senescence

We have previously reported that osmotic stress prominently induces the DNA binding activity of the heat shock transcription factor 1 (HSF1). In the p resent study, we examined the effects of medium osmolarity on both the acti vation of HSF1 and the programmed cell death in normal human fibroblasts du ring cellular senescence. The activation of HSF1 occurred rapidly in presen escent (early passage) IMR-90 cells when exposed to either hypo-osmotic or hyperosmotic stress. in contrast, the activation of HSF1 was significantly attenuated in senescent cells. Western blot analysis indicated that equal a mounts of HSF1 were present as monomers in the cytoplasm of both presenesce nt and senescent cells in normal growth medium. Under Either hypo-osmotic o r hyperosmotic stress, trimerization and nuclear localization of HSF1 occur red in presenescent cells but not in senescent cells. More than 80% of HSF1 in senescent cells remained as monomers in the cytoplasm under osmotic str ess, suggesting a defect in the signal transduction pathways that lead to H SF1 trimerization or a dysfunction in the HSF1 protein itself. Possible inv olvement of mitogen-activated protein kinase (MAPK) signal transduction pat hways in the activation HSF1 was investigated by monitoring the activation of the three MAPKs, ERK1/2, JNK1/2, and p38, in cells exposed to hypo-osmot ic or hyperosmotic stress. All three MAPKs were activated by hyperosmotic s tress but not hypo-osmotic stress, suggesting that the MAPK signal transduc tion pathways may not be directly linked to the osmotic stress-induced acti vation of HSF1. In contrast to the rapid heat shock transcription factor (H SF) activation, apoptosis occurred only after long-term exposure to hypoosm otic or hyperosmotic stress. Despite the prominent induction of HSF1 activa tion, the presenescent cells were more sensitive than the senescent cells t o the osmotic stress-induced apoptosis. J. Cell. Physiol. 184:183-190, 2000 . (C) 2000 Wiley-Liss, Inc.