HSP90 inhibitors disrupt a transient HSP90-HSF1 interaction and identify a noncanonical model of HSP90-mediated HSF1 regulation

Abstract

Heat shock factor 1 (HSF1) initiates a broad transcriptional response to proteotoxic stress while also mediating a cancer-specific transcriptional program. HSF1 is thought to be regulated by molecular chaperones, including Heat Shock Protein 90 (HSP90). HSP90 is proposed to sequester HSF1 in unstressed cells, but visualization of this interaction in vivo requires protein crosslinking. In this report, we show that HSP90 binding to HSF1 depends on HSP90 conformation and is only readily visualized for the ATP-dependent, N-domain dimerized chaperone, a conformation only rarely sampled by mammalian HSP90. We have used this mutationally fixed conformation to map HSP90 binding sites on HSF1. Further, we show that ATP-competitive, N-domain targeted HSP90 inhibitors disrupt this interaction, resulting in the increased duration of HSF1 occupancy of the hsp70 promoter and significant prolongation of both the constitutive and heat-induced HSF1 transcriptional activity. While our data do not support a role for HSP90 in sequestering HSF1 monomers to suppress HSF1 transcriptional activity, our findings do identify a noncanonical role for HSP90 in providing dynamic modulation of HSF1 activity by participating in removal of HSF1 trimers from heat shock elements in DNA, thus terminating the heat shock response.

Protein level variability determines phenotypic heterogeneity in proteotoxic stress response

“…While HSP90 exhibits weakly interaction with HSF1 without stress Zou et al , 1998, recent results suggest that sequestration may not be the important role of HSP90 in HSF1 regulation Kijima et al , 2018. Instead HSP90 interacts with transcriptionally active HSF1 trimers Conde et al , 2009 and newly synthetized HSP90 may regulate HSF1 by attenuating its ability to promote transcription when bound to HSE in DNA Kijima et al , 2018. Interestingly, at the single cell level, we do not find correlation between HSRN activation and HSP90 copy number (data not shown).…”

“…We have tested whether CCRV could also arise from level variations of HSP90, another important chaperone. While HSP90 exhibits weakly interaction with HSF1 without stress Zou et al , 1998, recent results suggest that sequestration may not be the important role of HSP90 in HSF1 regulation Kijima et al , 2018. Instead HSP90 interacts with transcriptionally active HSF1 trimers Conde et al , 2009 and newly synthetized HSP90 may regulate HSF1 by attenuating its ability to promote transcription when bound to HSE in DNA Kijima et al , 2018.…”

HSF1: Primary Factor in Molecular Chaperone Expression and a Major Contributor to Cancer Morbidity

“…What is now becoming apparent is that each HSP regulates HSF1 in a unique manner. Hsp70 and the constitutively expressed paralog Hsc70 bind all along the unstructured domains of HSF1 and can interact with both the "closed" monomer and "open" trimer conformations of HSF1 [171]. These data suggest that Hsp70 and Hsc70 may repress HSF1 in multiple ways, including: (i) The binding and stabilization of inactive monomers, (ii) disassembly of active trimers, (iii) blockade of key transcription-related protein-protein interactions by binding to the TAD.…”

“…One of the major hypotheses regarding HSF1 activation is that the factor is constitutively repressed by its products Hsp70 and Hsp90, an elegant pathway of feedback inhibition [169,170], although a recent report questions the inferences of these early studies [171]. What is now becoming apparent is that each HSP regulates HSF1 in a unique manner.…”

Dietary restriction induces post-transcriptional regulation of longevity genes

“…The daf-21 gene encodes the ortholog of the mammalian molecular chaperone HSP90. Despite its title as a heat shock protein, HSP90 is a pro-growth chaperone that inhibits activity of the heat shock transcription factor (Kijima et al, 2018). Lowering its expression was recently shown to increase lifespan in wild-type animals in a separate study investigating C. elegans orthologs of human genes that are differential expressed with age in blood (Sutphin et al, 2017).…”