Abstract 3379: HSF1: Essential for myeloma cell survival and a promising therapeutic target
Authors: Fok Jacqueline H L , Hedayat Somaieh , Zhang Lei , Aronson Lauren I. , Mirabella Fabio , Eccles Suzanne A. , Davies Faith E. 
Published: August 2015

Heat shock factor 1 (HSF1), a transcription factor known as the “master regulator” of the heat shock response (HSR), is emerging as an exciting therapeutic target in cancer due to its role in regulating cellular responses to proteotoxic stress as well as various oncogenic processes. Myeloma is hypothesized to be susceptible to therapies targeting protein homeostasis as it is characterised by the overproduction of immunoglobulin which is linked to significant ER stress. However, clinical responses to inhibitors of HSP90, a molecular chaperone overexpressed in myeloma patients, have been modest. This is partly due to HSF1-mediated transcriptional upregulation of anti-apoptotic HSP72. We are seeking to determine whether targeting the HSR through HSF1 may be an effective therapeutic strategy.

HSF1 knockdown using shRNA downregulated levels of anti-apoptotic HSP72 and HSP27 and significantly reduced viability in human myeloma cell lines. This was associated with apoptosis as evidenced by increases in caspase 3/7 activity and PARP cleavage which were rescued by treatment with z-VAD-FMK, a pan-caspase inhibitor. We next treated human myeloma cell lines with KRIBB-11, a direct small molecule inhibitor of HSF1, and observed growth inhibition with GI50s of between 3.0 - 8.5 μM. Notably, KRIBB-11 was cytotoxic to myeloma cell lines but not to peripheral blood mononuclear cells from healthy donors.

To shed light on the mechanisms by which myeloma cells rely on HSF1 for survival, we carried out microarray analysis on HSF1 knockdown cells and found that XIAP-associated factor 1 (XAF1), a pro-apoptotic tumour suppressor gene known to be transcriptionally repressed by HSF1, was upregulated (&gt;2.63 log-fold increase, p&lt;0.0001). qRT-PCR and Western blotting confirmed XAF1 upregulation with HSF1 knockdown. An increase in XAF1, PARP cleavage and decrease in HSP72 and HSP27 levels were also observed with KRIBB-11 treatment, confirming that the loss or inhibition of HSF1 in myeloma modulates its downstream target genes and leads to caspase-mediated cell death.

These results suggest a mechanism whereby HSF1 overexpression in myeloma cells represses XAF1 expression to evade apoptotic stresses and suggest XAF1 as a possible biomarker for HSF1 inhibition in myeloma. Interestingly, unlike HSP90 inhibition, HSF1 knockdown in myeloma cell lines did not modulate other key myeloma protein handling mechanisms such as aggresome formation, autophagy, the ubiquitin-proteasome pathway or ER stress. Efficacy testing and biomarker analyses of HSF1 inhibitors in s.c. human myeloma xenograft models are underway and early results are promising.

To summarise, using both genetic and pharmacological inhibition, our data demonstrate that HSF1 maintains myeloma cell survival during proteotoxic stress by protecting cells from apoptosis via XAF1. More importantly, we demonstrate the potential efficacy and therapeutic window in targeting HSF1 for myeloma treatment.

Citation Format: Jacqueline H L Fok, Somaieh Hedayat, Lei Zhang, Lauren I. Aronson, Fabio Mirabella, Suzanne A. Eccles, Faith E. Davies. HSF1: Essential for myeloma cell survival and a promising therapeutic target. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3379. doi:10.1158/1538-7445.AM2015-3379