SIRT1 Regulation of the Heat Shock Response in an HSF1-Dependent Manner and the Impact of Caloric Restriction

The heat shock response (HSR) is the cell\u27s molecular reaction to protein damaging stress and is critical in the management of denatured proteins. Activation of HSF1, the master transcriptional regulator of the HSR, results in the induction of molecular chaperones called heat shock proteins (HSPs). Transcription of hsp genes is promoted by the hyperphosphorylation of HSF1, while the attenuation of the HSR is regulated by a dual mechanism involving negative feedback inhibition from HSPs and acetylation at a critical lysine residue within the DNA binding domain of HSF1, which results in a loss of affinity for DNA. SIRT1 is a NAD+-dependent histone deacetylase that has been reported to deacetylate HSF1, thus promoting stress-induced HSF1 DNA binding ability and increasing HSP expression (Westerheide, Anckar et al. 2009). While an abundance of research is aimed to investigate SIRT1 substrate regulation, the mechanism in which SIRT1 itself is regulated is less understood (Haigis and Sinclair 2010). Positive and negative modulators of SIRT1 include AROS and DBC1, respectively, and have yet to be investigated in relation to SIRT1-dependent regulation of the HSR. In addition, metabolic stress such as caloric restriction has been shown to modulate SIRT1 activity in yeast (Rahat, Maoz et al. 2011), but the effect of caloric restriction on the HSR is unknown. Using cell-based assays, we have investigated how the HSR may be controlled by factors influencing SIRT1 activity. We found that heat shock results in an increase in the cellular NAD+/NADH ratio and an increase in recruitment of SIRT1 to the hsp70 promoter. Furthermore, we found that the SIRT1 modulators, AROS and DBC1, impact hsp70 transcription, HSF1 acetylation status, and HSF1 recruitment to the hsp70 promoter. The nematode Caenorhabditis elegans is a useful model organism for testing the relationship between the HSR and metabolism, as these animals can easily be calorically-restricted via bacterial limitation and possess the mammalian SIRT1 homolog, Sir2.1. Using C. elegans, we demonstrate that caloric restriction and heat shock have a synergistic effect on the HSR in a sir2.1-dependent manner. We show that caloric restriction increases the ability of heat shock to promote thermotolerance and fitness in wild-type animals and to preserve movement in a polyglutamine toxicity neurodegenerative disease model and that this effect is dependent on sir2.1. These studies provide insight into SIRT1-dependent regulation of the HSR and the impact of metabolism on this response. We highlight the SIRT1 modulators AROS and DBC1 as two new targets available for therapeutic regulation of the HSR and add caloric restriction as another HSR activator that can synergize with heat shock