Proteasome inhibitors lactacystin and MG132 inhibit the dephosphorylation of HSF1 after heat shock and suppress thermal induction of heat shock gene expression

Recently, we have shown that two proteasome inhibitors, MG132 and lactacyst in, induce hyperphosphorylation and trimerization of HSF1, and transactivat e heat shock genes at 37 degrees C. Here, we examined the effects of these proteasome inhibitors and, in addition, a phosphatase inhibitor calyculin A (CCA) on the activation of HSF1 upon heat shock and during post-heat-shock recovery, with emphasis on HSF1 hyperphosphorylation and the ability of HS F1 to transactivate heat shock genes. When lactacystin, MG132, or CCA was p resent after heat shock, HSF1 remained hyperphosphorylated during post-heat -shock recovery at 37 degrees C. Failure of HSF1 to recover to its preheate d dephosphorylated state correlated well with the suppression of the heat-i nduced hsp70 expression. In vitro, HSF1 from heat-shocked cells, when depho sphorylated, showed an increase in HSE-binding affinity. Taken together, th ese data suggest that phosphorylation of HSF1 plays an important role in th e negative regulation of heat-shock response. Specifically, during post-hea t-shock recovery phase, prolonged hyperphosphorylation of HSF1 suppresses h eat-induced expression of heat shock genes. (C) 1999 Academic Press.