Posttranslational modifications of glucocorticoid receptor associated with drug-resistance in prostate cancer

PROJECT SUMMARY (ABSTRACT) An evolving concept that contributes to our understanding of resistance to androgen deprivation therapy (ADT) or androgen receptor (AR) antagonists in advanced prostate cancer (PCa) is the ability of cells to evade AR blockade and turn on compensatory hormone receptor signaling for survival. Preclinical and

clinical studies by us and others have demonstrated that induction of glucocorticoid receptor (GR) expression confers resistance to AR-targeted therapy. Elegant studies by others in the hormone receptors field have shown that posttranslational modifications (PTM) on nuclear hormone receptors can amplify or alter their canonical activity to favor cancer cell survival or drug resistance. One such PTM is,

phosphorylation. We published that pan-AKT inhibition blocks GR induction/activity and overcomes GR- mediated resistance to AR-targeted therapy (ARi). We observed this in a setting of both ligand-dependent and ligand-independent GR signaling. Furthermore, preliminary proteomics studies show that GR phosphorylation is expressed with progression to castrate-resistant PCa (CRPC) and increases

significantly in enzalutamide-resistant CRPC. This increase also correlated with an increase in AKT1 expression. We found that AKT1 is directly bound to GR, which validates the finding by others. Pharmacologic inhibition specific for AKT1 or knockdown of AKT1 significantly decreased pGR(s134) but not total GR. In addition, inhibition of AKT1 inhibited pGR(s134) expression in the nucleus with or without

ligand. When cells were exposed to a GR-specific modulator (GRM), CORT134, in combination with enzalutamide and the presence/absence of ligands, we only observed suppression of cell proliferation in CRPC but not very effective in enzalutamide-resistant CRPC. Conversely, AKT1 inhibition decreased pGR(s134) in the nucleus, with/without ligand, and was also associated with inhibition of cell proliferation.

Phospho-site mutation on pGR(s134) was also associated with suppressed cell proliferation. Our central hypothesis is that induction of oncogenic GR in enzalutamide-resistant CRPC is modulated by PTM on GR, and inhibition of pGR, would re-sensitize treatment-resistant CRPC to enzalutamide. We will test our

central hypothesis and accomplish the objective of this application by pursuing the following specific aims: (i) Characterize ligand-dependent driven GR vs. PTM-driven GR expression and activity in patient- derived xenograft models using multi-omic assays, (ii) Perform functional studies to determine the

mechanism of PTM driven GR vs. ligand-dependent GR activity in lethal PCa (iii) Determine the therapeutic efficacy and benefit of targeting AKT-GR-AR as a novel therapeutic approach for lethal prostate cancers. Impact: The results from this study will significantly expand our knowledge of the mechanism of PTMs in advanced PCa and the role their unique signatures play in disease progression

and resistance to AR and/or GR targeted therapy. Currently, clinical trials are testing the therapeutic effects of ARi with GRM or ARi with AKT1-specific inhibitors in PCa. Our findings will be of immediate clinical relevance as they would guide what stage patients with advanced disease would benefit from

GRMs and at what stage alternative approaches (targeting GR PTMs) be used in GR-driven lethal disease.