(-)-Epicatechin acts as a potent agonist of the membrane androgen receptor, ZIP9 (SLC39A9), to promote apoptosis of breast and prostate cancer cells

(-)-Epicatechin, a flavonoid present in high concentrations in foods such as green tea and cocoa, exerts beneficial and protective effects in numerous disease models, including anti-tumorigenesis and Apoptosis in human breast and prostate Cancer cells. Potential interactions of (-)-epicatechin and (+)-catechin with the membrane Androgen Receptor, ZIP9 (SLC39A9), which mediates androgen induction of Apoptosis in these Cancer cells, were investigated. Both (-)-epicatechin and (+)-catechin were effective competitors of [³H]-testosterone binding to PC-3 prostate Cancer cells (nuclear androgen receptor-negative, nAR-null) overexpressing ZIP9 (PC3-ZIP9), with relative binding affinities of 75 % and 28 % that of testosterone, respectively. (-)-Epicatechin (200 nM) mimicked the effects of 100 nM testosterone in inducing Apoptosis of PC3-ZIP9 cells, whereas (+)-catechin (concentration range 200 nM-1000 nM) did not significantly increase Apoptosis and instead blocked the apoptotic response to testosterone. (-)-Epicatechin also activated androgen-dependent ZIP9 signaling pathways, inducing decreases in cAMP production and elevating intracellular free zinc levels, while (+)-catechin typically lacked these actions. Both (-)-epicatechin and (+)-catechin also bound to cell membranes of MDA-MB-468 breast Cancer cells (nAR-null, high ZIP9 expression). MDA-MB-468 cells showed similar apoptotic, cAMP, and free zinc signaling responses to (-)-epicatechin to those observed in PC3-ZIP9 cells, as well as antagonism by (+)-catechin of testosterone-induced Apoptosis and modulation of cAMP and Caspase-3 levels. Moreover, knockdown of ZIP9 expression in MDA-MB-468 cells with siRNA decreased specific [³H]-testosterone binding of both catechins and blocked the apoptotic and free zinc responses to testosterone and (-)-epicatechin. The results indicate (-)-epicatechin is a potent ZIP9 agonist in breast and prostate Cancer cells.