Abstract
Apigenin is a non-toxic, non-mutagenic compound that is widely present in common fruits and vegetables and has proven anti-inflammatory and anti-carcinogenic effects. Apigenin has shown a marked effect on cell cycle checkpoint controls and cell survival and/or apoptosis in various cancer cells, through mechanisms that have not been properly elucidated. Here, for the first time, we report on a novel mechanism of chemoprotection by apigenin in human prostate cancer cells through inhibition of the chromatin modifying enzymes, histone deacetylases (HDACs). HDACs are part of a transcriptional co-repressor complex that influences various tumor suppressor genes. Because of the significant roles played by HDACs in various human cancers, HDAC inhibitors have emerged as a new class of chemotherapeutic agents. Exposure of human prostate cancer 22Rv1 and PC-3 cells to physiologically relevant doses of apigenin (20- and 40-\#956;M), resulted in inhibition of total HDAC enzyme activity in both cell lines, with more pronounced effects in PC-3 cells. Analysis of PC-3 cells indicates that HDAC3 is the major histone deacetylase inhibited by apigenin. HDAC inhibition was accompanied by a concomitant increase in acetylated histones with an enhanced interaction of acetylated histone H4 with the promoter region of the cell cycle regulator p21 gene and the pro-apoptotic Bax gene as ascertained by chromatin immunoprecipitation assay. A corresponding increase was observed for p21Cip1/Waf1 and Bax protein and mRNA expression after apigenin exposure, consistent with the use of HDAC inhibitor, trichostatin A. In vivo studies of PC-3 xenografts in athymic nude mice further demonstrated that feeding apigenin orally at doses of 20- and 50-\#956;g/mouse/day over an 8 week period resulted in a marked reduction in total HDACs enzymatic activity. While no difference in HDAC1 protein expression was observed, HDAC3 expression was decreased at both doses of apigenin. An increase in p21 expression was also observed in apigenin-exposed mice, compared to the control group. Furthermore, apigenin intake caused a significant decrease in Bcl2 expression with concomitant increase in Bax, with overall shifting of the Bax/Bcl2 ratio in favor of apoptosis. Our findings confirm for the first time that apigenin is an inhibitor of HDAC, and exposure to apigenin results in reversal of aberrant epigenetic events that promote malignancy. These findings provide new insight into the epigenetic mechanisms of apigenin activity in human prostate cancer cell lines and in a pre-clinical model of prostate cancer, and provide additional evidence for the potential utitity of apigenin as a chemopreventive and/or chemotherapeutic agent in the management of prostate cancer.
Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2990.
100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO