Background: Prostate cancers (PCa) rely on androgens for growth and survival, and androgen ablation is a standard treatment for advanced PCa. However, a majority of tumors invariably relapse to an androgen depletion-independent (ADI) disease. Importantly, studies on patient specimens show that the androgen receptor (AR) and its target gene prostate-specific antigen (PSA) are expressed in nearly all cancers of the prostate both before and after androgen depletion therapy. Findings from our laboratory and others demonstrate that activation of the AR by residual (castration) levels of androgens or nonandrogenic factors including growth factors and cytokines is one of the key mechanisms promoting ADI progression of PCa. These findings highlight that the AR is a viable therapeutic target for ADI PCa.O-class forkhead box protein-1 (FOXO1) has been shown to possess tumor suppressor functions through its transcriptional regulation of expression of genes involved in cell cycle arrest, apoptosis, DNA damage repair, and oxidative stress detoxification. Our preliminary studies demonstrate that in addition to the effect on androgenic activation of the AR, FOXO1 inhibits androgen-independent activation of the AR in a manner independent of its function as a transcription factor. We further demonstrate that this transcription-independent function of FOXO1 requires its nuclear localization and that nuclear exclusion of FOXO1 abolishes its inhibitory effect on AR. Moreover, FOXO1 inhibition of AR was blocked by treatment of PCa cells with the histone deacetylase (HDAC) inhibitor trichostatin A, and forced expression of HDAC3 but not HDAC1 and HDAC2 specifically enhanced the inhibition of the AR by FOXO1. Our findings reveal a novel corepressor function of FOXO1 in inhibition of androgen-independent activation of the AR in PCa cells.Objective/Hypothesis: Based upon our preliminary data and the fact that inactivation of FOXO1 by its deletion, nuclear exclusion, or degradation often occurs during PCa progression, we hypothesize that FOXO1 functions as a key corepressor molecule in prohibiting androgen-independent activation of the AR and that inactivation of FOXO1 promotes aberrant activation of the AR and ADI progression of PCa.Specific Aims: (1) Evaluate the impact of FOXO1 on activation of the AR and growth of prostate tumors in castrated animals. (2) Delineate the regions in FOXO1 and AR required for FOXO1 inhibition of AR activation. (3) Determine the role of HDAC3 in FOXO1 inhibition of androgen-independent activation of the AR and growth of PCa cells.Study Design: The finding that FOXO1 inhibits androgen-independent activation of the AR and growth of ADI PCa cells in vitro prompts us to assess the impact of FOXO1 on AR activation and PCa progression in vivo using ADI orthotopic xenograft models in castrated mice (Aim 1). In Aim 2, an array of assays, including recombinant protein purification, in vitro protein binding, luciferase reporter gene assays, real-time PCR, and proliferation assays, will be performed to delineate the regions in FOXO1 and AR that are required for FOXO1 inhibition of AR activation and to define the functional domain(s) in AR responsible for FOXO1 interaction and inhibition of AR transactivation. Finally, the importance of HDAC3 in FOXO1 inhibition of androgen-independent activation of the AR and impact of HDAC3 on FOXO1 inhibition of ADI PCa growth will be examined (Aim 3).Innovation: This project stems from our preliminary finding that FOXO1 inhibits androgen-independent activation of the AR through a previously uncharacterized transcription-independent mechanism. One key innovation of the proposal is to investigate FOXO1 as an intrinsic corepressor of AR, a novel study concept. Another key innovation of this proposal is to test the role of FOXO1-mediated inhibition of the AR in development of resistance of human prostate tumors to androgen depletion treatment in xenograft models. The third innovation is to use a newly developed bioluminescence-based imaging technique to monitor the influence of FOXO1 on AR activation and PCa progression under in vivo prostatic microenvironment in real time.Impact: The indispensible role of the AR in development of resistance to androgen depletion therapy has accentuated the need to uncover the molecular mechanism underlying the promiscuous activation of the AR under androgen depletion conditions. The focus of this proposal is to test the anti-AR and anti-cancer efficacy of FOXO1 in orthotopic PCa mouse models and to elucidate the molecular basis underlying FOXO1-mediated inhibition of androgen-independent activation of the AR. Realization of this project will not only advance our understanding of the molecular basis underlying the regulation and function of the AR in ADI PCa but also provide opportunities for the development of new therapeutics for this disease.
|Effective start/end date||1/1/08 → 4/30/13|
- Congressionally Directed Medical Research Programs: $401,097.00
- U.S. Department of Defense: $473,292.00