Targeted combinatorial therapy of androgen receptor in androgen-independent prostate cancer cells.

Abstract

29 Background: Gain of function of androgen receptor (AR) and PI3K/Akt/mTOR pathway due to loss of PTEN function are interrelated determinants of AIPC progression and resistance to antiandrogens. We determined that histone deacetylase inhibitors (HDACi), known to regulate AR expression and transcriptional activity, restore sensitivity to bicalutamide (Bic) of AIPC cells and the combination is synergistic. We postulate that adding a dual inhibitor of PI3K/mTOR would potentiate growth inhibition and apoptotic response. Methods: Androgen-independent, PTEN null AI-LNCaP (AI-cells) and wt PTEN RV1-cell lines were treated for 72h with the HDACi Panobinostat (PAN), the dual PI3K/mTOR inhibitor BEZ235 (E) and Bic as single agents or in doublet or triplet combinations. Growth was measured by MTT assay, Akt/mTOR pathway target proteins p-Akt, p-4EBP1 and p-S6K by western blot. Activation of ligand-independent AR downstream targets: CDC20, IDI1, CDK1, UBE2C, PRDM4 by qRT-PCR. Apoptosis by oligonucleosome quantification by ELISA was compared to docetaxel. Results: In both cell lines, the IC50s were markedly reduced in response to the doublet combinations and further decreased by the triplet. The isobolograms showed synergy. The apoptosis response increased by two fold in AI and RV1 cells in response to the doublet PAN+E and by four fold with the addition of Bic in AI cells. Only E decreased the protein levels of p473-akt, p-4EBP1, p-S6K markedly while only PAN decreased AR and these levels were not affected by the combinations. However, the triplet but not the doublets decreased by 2-fold the expression of AR targets in AI-cells but not in RV1 cells. Conclusions: The combination of PAN+E is highly effective in AIPC cells regardless of PTEN status but the addition of Bic is essential to maximize the effect in PTEN null AI-cells, suggesting they are more dependent on aberrant activation of AR. [Table: see text] No significant financial relationships to disclose.