Abstract 3100: Hsp90 inhibition results in GR, AR and JAK protein degradation, decreased triple-negative breast cancer proliferation and increased paclitaxel sensitivity

Glucocorticoid receptor (GR) activity in triple-negative breast cancer (TNBC) is associated with cell survival and chemotherapy resistance. Targetable molecular drivers for TNBC have been difficult to identify and therefore standard treatment remains limited to chemotherapy. Recently, new-generation small molecule Hsp90 inhibitors (e.g. ganetespib and NVP-AUY922) have proven to be both potent inhibitors of Hsp90 client proteins and far less toxic than previous Hsp90 inhibitors. Moreover, favorable clinical responses have been observed in some TNBC patients enrolled in a phase II clinical trial of ganetespib monotherapy. A ganetespib and paclitaxel clinical trial is ongoing in breast cancer; however, it is not known which Hsp90 client proteins contribute to increased tumor chemotherapy sensitivity. Recent studies in our lab found that inhibition of glucocorticoid receptor (GR) activity plays an important role in Hsp90 inhibitor-mediated sensitization of TNBC cells to paclitaxel-induced cytotoxicity. However, we also observed that following significant shRNA-mediated GR depletion, ganetespib still provided some sensitization of TNBC cells to paclitaxel. This suggests that inactivation of additional anti-apoptotic Hsp90 client proteins play a role in mediating ganetespib effects. Proteins closely associated with GR function that are also key Hsp90 inhibitor client proteins include the androgen receptor (AR, implicated in TNBC proliferation and apoptosis) and JAK, a component of JAK/STAT signaling pathway implicated in TNBC tumorigenesis and chemoresistance. Here we hypothesize that AR and JAK/STAT signaling have overlapping but distinct roles in Hsp90-mediated chemotherapy sensitization of TNBCs. Western blot analysis of TNBC xenografts treated with ganetespib +/- paclitaxel showed depletion of AR as well as JAK1 and 2. Ganetespib treatment also led to decreased levels of TMPRSS2 gene expression, an AR target gene. Additionally Western blot analysis showed decreased levels of activated phosphorylated STAT3 transcriptional factor, a downstream mediator of JAK, following ganetespib treatment of TNBC cell lines. These results suggest the involvement of AR and JAK in mediating ganetespib mechanism of action. We are currently beginning to determine how AR and JAK/STAT signaling pathways may interact with GR signaling by establishing CRISPR/Cas9 knockouts of GR, AR, and/or JAK in TNBC cells treated with ganetespib +/- paclitaxel. Understanding the relative roles of Hsp90 client proteins in TNBC is predicted to lead to a more rational selection of patients with TNBCs likely to benefit from Hsp90-inhibitor treatment. Citation Format: Abena S. Agyeman, Wesley J. Jun, Suzanne D. Conzen. Hsp90 inhibition results in GR, AR and JAK protein degradation, decreased triple-negative breast cancer proliferation and increased paclitaxel sensitivity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3100.