Role of Heat Shock Factor 2 in proteasome subunits genes expression

Abstract

Heat Shock Factors (HSF) form the main family of transcription factors involved in response to proteotoxic stress. Five HSF are currently described in vertebrate, but only HSF1 and HSF2 are for the moment, well documented. HSF1 is considered as the bona-fide stress-induced transcription factor, whereas HSF2 is more described for its implication in gametogenesis and embryonic development. However, recent studies have shown that HSF2 can have additional roles in response to protein denaturation1,2 and in bookmarking of several stress-related genes 3,4. We have studied the specific role of HSF1 and HSF2 in the cellular response to proteasome inhibition. Using immortalized Mouse Embryonic Fibroblast (iMEF) derived from knock-out Hsf1 and/or Hsf2 mice, we found that cells deleted for either HSF1 or HSF2 are more sensitive to proteasome inhibition than wild type cells. This result suggests that both HSFs are important for cellular response to proteasome inhibitor treatment. To establish the relationship between the proteasome and these transcription factors, several tests were performed using knock-out cells. First, we found a significant decrease of the chymotrypsin-like activity of the proteasome in Hsf2-/- and in Hsf1-/- & Hsf2-/- iMEF. Secondly, the basal expression of ten proteasome subunits was measured by real time PCR. In agreement with our measure of proteasome activity, we found that two catalytic subunits, PSMB2 and PSMB5 which support trypsin-like and chymotrypsin-like activity respectively, were present at lower level in HSF2 deleted iMEF. Moreover, PSMA1, PSMC4 and PSMD10 also display a lower expression in absence of HSF2, whereas the other proteasome subunits tested are not affected. In conclusion, our data showed that HSF2 is involved in some proteasome subunits basal expression. In HSF2 deficient iMEF, decrease expression of these subunits have a direct influence in cell physiology with a decrease of catalytic activity of the proteasome. These results could open a new field in cancer therapy: development of HSF2 inhibitors could improve the action and reduce chemoresistance to proteasome inhibitors such as bortezomib.