Heat shock factor 2 is required for maintaining proteostasis against febrile-range thermal stress and polyglutamine aggregation

Heat shock response is characterized by the induction of heat shock proteins (HSPs), which facilitate protein folding, and non-HSP proteins with diverse functions, including protein degradation, and is regulated by heat shock factors (HSFs). HSF1 is a master regulator of HSP expression during heat shock in mammals, as is HSF3 in avians. HSF2 plays roles in development of the brain and reproductive organs. However, the fundamental roles of HSF2 in vertebrate cells have not been identified. Here we find that vertebrate HSF2 is activated during heat shock in the physiological range. HSF2 deficiency reduces threshold for chicken HSF3 or mouse HSF1 activation, resulting in increased HSP expression during mild heat shock. HSF2-null cells are more sensitive to sustained mild heat shock than wild-type cells, associated with the accumulation of ubiquitylated misfolded proteins. Furthermore, loss of HSF2 function increases the accumulation of aggregated polyglutamine protein and shortens the lifespan of R6/2 Huntington's disease mice, partly through αB-crystallin expression. These results identify HSF2 as a major regulator of proteostasis capacity against febrile-range thermal stress and suggest that HSF2 could be a promising therapeutic target for protein-misfolding diseases.     

Neuroprotective drug riluzole amplifies the heat shock factor 1 (HSF1)- and glutamate transporter 1 (GLT1)-dependent cytoprotective mechanisms for neuronal survival

Heat shock factor 1 (HSF1) mediates the cellular response to stress to increase the production of heat shock protein (HSP) chaperones for proper protein folding, trafficking, and degradation; failure of this homeostatic mechanism likely contributes to neurodegeneration. We show that the neuroprotective drug riluzole increased the amount of HSF1 in NG108-15 neuroprogenitor cells by slowing the specific turnover of HSF1 and supporting a more robust and sustained activation of HSF1. Using Hsp70-luciferase as a functional readout of the activity of HSF1, we show that riluzole amplified the heat shock induction of the reporter gene with an optimal increase at 1 μM. Immunocytochemical staining and Western blot quantitation of HSP70 in NG108-15 neuroprogenitor cells and embryonic spinal cord neurons provided corroborative evidence that riluzole amplified the HSF1-dependent regulation of HSP70 expression. Parallel studies on the GLT1 glutamate transporter showed that riluzole increased GLT1-reporter and GLT1 protein expression and that the increase was enhanced by heat shock and coincident with the increased expression of HSP70 and HSP90. This result is consistent with the anti-glutamatergic profile of riluzole and the presence of multiple heat shock elements on the GLT1 gene promoter, suggesting that riluzole may modulate GLT1 expression through HSF1. The increased HSP chaperones and GLT1 transporter blunted glutamate-induced and N-methyl D-aspartate receptor-mediated excitotoxic death. In summary, we show that riluzole increased the amount and activity of HSF1 to boost the expression of HSPs and GLT1 for neuroprotection under stress.     

Regulatory interfaces between the stress protein response and other gene expression programs in the cell

The stress protein response involves the immediate reprogramming of gene expression in cells exposed to proteomic insult leading to massive synthesis of heat shock proteins (HSP). We have examined the outcome when cells are induced to activate two other gene expression programs--the acute inflammatory response and entry of quiescent cells into the cell cycle--and then exposed to protein stress. We find that these responses are mutually antagonistic with, on the one hand, heat shock factor 1 (HSF1) inhibition through the phosphorylation of inhibitory serine residues after inflammatory or mitogenic stimulus and, on the other hand, after stress, HSF1 directly repressing the promoters of genes that mediate acute inflammation and mitogenesis. The expression of the stress protein response during periods of acute protein damage was shown to lead to efficient activation of HSF1 and HSP expression accompanied by repression of other gene expression programs.