Abstract 2602: AKT1-mediated activation of HSF1 by phosphorylation and an association with metastasis-free survival
Authors: Lu Wen-Cheng , Ray Haimanti , Omari Ramsey , Cao Sha , Carpenter Richard 
Published: August 2019

Breast cancer is the most common cancer affecting women worldwide. The overwhelming majority of deaths from breast cancer are caused by metastasis. The transcription factor heat shock factor (HSF1), which classically regulates the heat shock response, has been linked to tumor progression and metastasis. Our previous data indicate HSF1 is activated in approximately 50% of patient breast tumors, independent of subtype and is associated with promoting epithelial-to-mesenchymal transition (EMT). Our laboratory has shown that AKT1 phosphorylates Ser326 of HSF1, leading to HSF1 activation in breast cancer cells and EMT. However, it remains unclear what phosphorylation sites are most critical for HSF1 transcriptional activity and whether AKT1 also regulates other HSF1 sites. To answer this question, HSF1 was subjected to an in vitro kinase assay. We observed Ser326 phosphorylation via immunoblotting. These phosphorylated HSF1 proteins were further subjected to mass spectrometry wherein we identified three novel sites of AKT1-mediated phosphorylation in Thr142, Ser230, and Thr527. All of these sites, along with Ser326, have previously been shown to promote HSF1 transcriptional activity. Interestingly, incubation with a pan-AKT inhibitor also suppressed the heat shock response by HSF1. However, when HSF1 protein was incubated with AKT2 or AKT3, there was no phosphorylation at Ser326. Furthermore, transcriptional activity of HSF1 and expression of Hsp70, a known HSF1 target gene, were increased by AKT1 but not by AKT2 or AKT3. Thus, we have further identified that AKT1 phosphorylates several activating residues on the HSF1 protein whereas AKT2 and AKT3 do not phosphorylate HSF1 to promote its activation. The importance of promoting HSF1 transcriptional activity by AKT1 cannot be understated as the PI3K pathway has been found to be genetically activated in ~77% of breast cancer. Additionally, it has recently been reported that this pathway is activated in almost 40% of all human tumors. However, whether HSF1 activity has any relation to EMT or metastasis in these other tumor types is unknown. To address this, we developed a gene expression signature for HSF1. We found this gene signature was strongly associated with metastasis-free survival in a broad range of solid tumors including breast cancer, lung cancer, ovarian cancer, melanoma, pancreatic cancer, and prostate cancer. In summary, we established a more definitive mechanism by which AKT1 phosphorylates and activates the HSF1 protein by phosphorylation of several activating sites. Furthermore, due to the seemingly ubiquitous nature of PI3K/AKT signaling across human tumors, we found that HSF1 is also potentially activated in these tumor types and has a strong association with patient outcomes, in particular metastasis.

Citation Format: Wen-Cheng Lu, Haimanti Ray, Ramsey Omari, Sha Cao, Richard Carpenter. AKT1-mediated activation of HSF1 by phosphorylation and an association with metastasis-free survival [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2602.