Abstract P4-16-05: Hsp90 inhibition potentiates chemotherapy effectiveness in triple negative breast cancer: Role of glucocorticoid receptor signaling

Abstract

Background: Few promising molecular targets exist for chemoresistant triple negative breast cancer (TNBC). Our laboratory has previously demonstrated that a subset of TNBCs express high glucocorticoid receptor (GR) levels. In breast epithelial cells, GR transcriptional activity directly upregulates the expression of several anti-apoptotic target genes including serum/glucocorticoid regulated kinase 1 (SGK1). High GR and/or SGK1 expression are both associated with decreased chemotherapy-induced cell death in TNBC models. A meta-analysis of patients with early-stage TNBC found that high primary tumor GR (NR3C1) expression significantly associates with earlier relapse compared to GR-non expressing TNBC. Inhibiting GR/SGK1 activity is therefore a testable strategy for improving TNBC responsiveness to chemotherapy. Both GR and SGK1 require Hsp90 for their activity; therefore, Hsp90 inhibition is predicted to disrupt both GR and SGK1 cell survival signaling in TNBC. Employing a second-generation Hsp90 inhibitor, ganetespib (Synta Pharmaceuticals), we determined the effectiveness of Hsp90 inhibition on GR/SGK1 activity and chemotherapy sensitivity using in vitro and in vivo TNBC models. Hypothesis: Hsp90 inhibition will decrease GR and SGK1 activity in TNBC in association with improved chemotherapy effectiveness. Results: Using TNBC cell lines treated with ganetespib at clinically relevant concentrations, we found that total GR and the relative proportion of phosphorylated Ser211 GR were significantly reduced beginning 2 hours following treatment. Reduced glucocorticoid-induced (SGK1) mRNA expression was also seen when cells were pre-treated with ganetespib. We next used fluorescent microscopy and live cell imaging to measure TNBC cell death over a 96 hour time course following chemotherapy +/- ganetespib therapy. Addition of ganetespib to paclitaxel treatment of MDA-MB-231 cells led to significant in vitro cytotoxic synergy; as hypothesized, this synergy was diminished in GR-depleted MDA-MB-231 cells. Cytotoxic synergy following ganetespib and chemotherapy treatment was also seen in vivo in MDA-MB-231 xenografts. Interestingly, tumor GR expression following Hsp90 inhibitor and paclitaxel treatment was significantly reduced compared to tumors from mice treated with paclitaxel alone. Using GR-depleted or control MDA-MB-231 xenografts, studies are currently underway to further characterize the role of GR/SGK1 activity as a target of Hsp90 inhibition to reverse TNBC chemoresistance. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-16-05.