Heat-induced proteasomic degradation of HSF1 in serum-starved human fibroblasts aging in vitro

The exposure of human fibroblasts (HF) aging in vitro to heat shock resulte d in an attenuated expression of the heat shock-inducible HSP70. When late passage cells were cultured in the continuous presence of serum, we observe d a reduced accumulation of the cytoplasmic polyadenylated HSP70 mRNA. The levels of HSF1 activation and nuclear HSP70 mRNA were comparable to those o f early passage cells (M. A. Bonelli et al., Exp. Cell Res. 252, 20-32, 199 9). When late passage cells were serum-starved overnight, we observed a red uced activation of HSF1 and a decreased level of HSP70 mRNA during heat sho ck. However, at 37 degreesC the levels of HSF1 differed little between late passage HF and early passage cells, irrespective of the presence of serum. Interestingly, during heat shock a marked decrease in the level and, conse quently, in the binding activity of HSF1 was noted only in serum-starved, l ate passage HF. The decrease in the level of HSF1 was counteracted by back addition of serum to the cells during heat shock. Addition of the specific proteasome inhibitor MG132 blocked a decrease in HSF1 during heat shock, ma intaining levels observed in late passage cells and HSF1 activity comparabl e to that of early passage HF. The recovery of the level and activity of HS F1 observed in late passage HF incubated in the presence of MG132 suggests that heat shock unmasks a latent proteasome activity responsible for HSF1 d egradation. (C) 2001 Academic Press.