Abstract 2928: Regulation of MST1 and MST2 by mTOR and androgen receptor signaling in prostate cancer

The serine/threonine kinases MST1 and MST2 (hippo in Drosophila) were originally identified as pro-apoptotic proteins. In addition to their pro-apoptotic functions, evidence suggests that deficiency in MST1 and MST2 and their downstream effectors has been linked to developmental defects, tissue overgrowth, and cancer, including prostate cancer (PCa). However, the molecular mechanism that may modulate MST1 or MST2 signaling in PCa cells remains unknown. Here we investigate the impact of androgen receptor (AR) and PI3K-AKT-mTOR signaling on MST1 or MST2. We used biochemical, mutagenesis, histochemical, and pharmacological approaches to carry out this investigation. First, we demonstrate that AR and MST1 co-localize and form protein complexes in cell nuclei. Androgen further stimulates the interaction between the two proteins and reduces MST1 kinase activity in LNCaP PCa cells, suggesting that AR may sequester MST1 in an inactive form. Second, we showed that MST1 at Threonine120 (T120) was exclusively phosphorylated in cell nuclei, whereas MST2 at Threonine117 (T117), corresponding to MST1-T120, was phosphorylated only in the cytoplasm, although MST1 and MST2 were found in both locations. Similar phosphorylation patterns were also observed in prostate cancer tissue samples. Third, we have found that the T120 is not a physiologic target of AKT kinase in vivo, because the inhibition of AKT activity by a specific PI3K inhibitor, LY294002, had no effect on the phosphorylation of MST1 at the T120 site, but it did inhibit MST1-T117 phosphorylation. Furthermore, attenuation of mTOR activity by Ku0063794, a specific mTOR1 and mTOR2 inhibitor, abolished MST2-T117 phosphorylation, but without having an effect on the levels of phospho-MST1-Thr120. Lastly, the use of combined LY294002 and Ku0063794 compounds synergistically inhibited PCa cell growth, relative to a single compound. Our data suggest that AR and mTOR pathway signaling may oppose MST1 and MST2 in discrete cell locations. Thus, understanding the functional relationship between these signal networks in the context of cell proliferation and cell death may have important therapeutic implications in prostate cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2928. doi:10.1158/1538-7445.AM2011-2928