Abstract 446: HSP90 inhibitors disrupt a transient HSP90-HSF1 interaction and identify a noncanonical model of HSP90-mediated HSF1 regulation

Heat shock factor 1 (HSF1) is an evolutionarily conserved transcription factor that initiates the cytoprotective heat shock response (HSR) and mediates a cancer-specific transcriptional program, which is comprised of over 500 genes and has been associated with poor clinical outcomes. HSF1 is thought to be regulated by molecular chaperones, including Heat Shock Protein 90 (HSP90). The purpose of our study was to describe the interactions between molecular chaperones, such as HSP90 and HSP70, and HSF1 and to determine the mechanism(s) by which HSP90 inhibitors promote the HSR in mammalian cells. Using a combination of techniques including IP-WB, ChIP, and RT-qPCR analysis, we demonstrated that HSP90 binding to HSF1 depends on HSP90 conformation and is only readily visualized for the ATP-dependent, N-domain dimerized chaperone, a conformation rarely sampled by mammalian HSP90. We also showed that ATP-competitive, N-domain targeted HSP90 inhibitors disrupt this interaction and enhance HSF1 phosphorylation, 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, as has been previously proposed, 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. Citation Format: Megan L. Tigue, Toshiki Kijima, Thomas L. Prince, Kendrick H. Yim, Harvey Schwartz, Kristin Beebe, Sunmin Lee, Marek A. Budzynski, Heinric Williams, Jane B. Trepel, Lea Sistonen, Stuart Calderwood, Len Neckers. HSP90 inhibitors disrupt a transient HSP90-HSF1 interaction and identify a noncanonical model of HSP90-mediated HSF1 regulation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 446.