Downregulation of androgen receptor expression by luteolin causes inhibition of cell proliferation and induction of apoptosis in human prostate cancer cells and xenografts

BACKGROUND. Androgen receptor (ARs) play a crucial role in the development and progression of prostate cancer. Recent studies have suggested that prostate cancer cell proliferation is inhibited by AR downregulation. Our aim was to investigate how luteolin, a natural flavonoid, affects cell growth and AR expression in prostate cancer cells and xenografts. METHODS. We assessed prostate cancer cell (LNCaP, DU145, and PC-3) proliferation and apoptosis by MTT assay, flow cytometric analysis, and Western analysis. AR function was measured by evaluating the AR target molecule, prostate-specific antigen (PSA), by RT-PCR, Western blotting, and enzyme-linked immunosorbent assay. We determined the mechanism of AR downregulation with cycloheximide chase assays, proteasome inhibitor, and coimmunoprecipitation experiments. The effects of luteolin on growth inhibition in vivo were examined by LNCaP xenografts in SCID mice. RESULTS. Luteolin significantly repressed prostate cancer cell proliferation and induced apoptosis in LNCaP cells. PC-3 and DU145 cells were less susceptible to luteolin-mediated growth inhibition. Luteolin simultaneously suppressed intracellular and secreted PSA levels and repressed AR mRNA and protein expression in a dose- and time-dependent manner. Luteolin reduced the association between AR and heat-shock protein 90, causing AR degradation through a proteasome-mediated pathway in a ligand-independent manner. Luteolin also suppressed LNCaP xenograft tumor growth in SCID mice. CONCLUSION. Luteolin-mediated AR downregulation contributes to the inhibition of cell proliferation and the induction of apoptosis in LNCaP human prostate cancer cells, suggesting that AR is a molecular target for luteolin-mediated anticancer activity. Luteolin may act as a chemopreventive or chemotherapeutic agent for prostate cancer.