The tumor suppressor ING1b is a novel corepressor for the androgen receptor and induces cellular senescence in prostate cancer cells

The androgen receptor (AR) signaling is critical for prostate cancer (PCa) progression to the castration-resistant stage with poor clinical outcome. Altered function of AR-interacting factors may contribute to castration-resistant PCa (CRPCa). Inhibitor of growth 1 (ING1) is a tumor suppressor that regulates various cellular processes including cell proliferation. Interestingly, ING1 expression is upregulated in senescent primary human prostate cells; however, its role in AR signaling in PCa was unknown. Using a proteomic approach by surface-enhanced laser desorption ionization-mass spectrometry (SELDI-MS) combined with immunological techniques, we provide here evidence that ING1b interacts in vivo with the AR. The interaction was confirmed by co-immunoprecipitation, in vitro GST-pull-down, and quantitative intracellular colocalization analyses. Functionally, ING1b inhibits AR-responsive promoters and endogenous key AR target genes in the human PCa LNCaP cells. Conversely, ING1b knockout (KO) mouse embryonic fibroblasts (MEFs) exhibit enhanced AR activity, suggesting that the interaction with ING1b represses the AR-mediated transcription. Also, data suggest that ING1b expression is downregulated in CRPCa cells compared with androgen-dependent LNCaP cells. Interestingly, its ectopic expression induces cellular senescence and reduces cell migration in both androgen-dependent and CRPCa cells. Intriguingly, ING1b can also inhibit androgen-induced growth in LNCaP cells in a similar manner as AR antagonists. Moreover, ING1b upregulates different cell cycle inhibitors including p27 KIP1, which is a novel target for ING1b. Taken together, our findings reveal a novel corepressor function of ING1b on various AR functions, thereby inhibiting PCa cell growth.