Induction of DNA damage and p21-dependent senescence by Riccardin D is a novel mechanism contributing to its growth suppression in prostate cancer cells in vitro and in vivo

Our previous studies had shown that Riccardin D (RD) exhibited cytotoxic effects by induction of apoptosis and inhibition of angiogenesis and topoisomerase II. Here, we reported that apoptosis is not the sole mechanism by which RD inhibits tumor cell growth because low concentrations of RD caused cellular senescence in prostate cancer (PCa) cells. Low concentrations of RD were used to treat PCa cells in vitro and in vivo, and senescence-associated beta-galactosidase activity, DNA damage response markers, and/or colony-forming ability, cell cycle were analyzed, respectively. We then used siRNA knockdown to identify key factor in RD-triggered cellular senescence. RD treatment caused growth arrest at G0/G1 phase with features of cellular senescence phenotype such as enlarged and flattened morphology, increased senescence-associated-beta-galactosidase staining cells, and decreased cell proliferation in PCa cells. Induction of cellular senescence by RD occurred through activation of DNA damage response including increases in the phosphor-H2AX, inactivation of Chk1/2, and suppression of repair-related Ku70/86 and phosphor-BRCA1 in PCa cells in vitro and in vivo. Analysis of expression levels of p53, p21(CIP1), p16(INK4a), p27(KIP1), pRb and E2F1 and genetic knockdown of p21(CIP1) demonstrated an important role of p21(CIP1) in RD-triggered cellular senescence. Involvement of the DNA damage response and p21(CIP1) defines a novel mechanism of RD action and indicates that RD could be further developed as a promising anticancer agent for cancer therapy.