Pro-apoptotic effects of streptochlorin isolated from Streptomyces sp. in human leukemic U937 cells

Abstract

Streptochlorin is a small molecule produced by marine Streptomyces sp.(strain 04DH110) that is known to have anti-angiogenic and anti-cancer properties. However, the mechanism by which streptochlorin exerts is effects is not well understood. In this study, we investigated the proapoptotic effect of streptochlorin in human leukemic U937 cells. Streptochlorin treatment resulted in concentration- and time-dependent growth inhibition of U937 cells by inducing apoptosis, as evidenced by the formation of apoptotic bodies, DNA fragmentation and the accumulation ofcells in the sub-G1 phase. The increase in apoptosis that was induced by streptochlorin was correlated with down-regulation of the anti-apoptotic Bcl-2 expression, up-regulation of pro-apoptotic Bax and FasL, a decrease in the mitochondrial membrane potential (MMP, Δψm), activation of caspases and degradation of poly-(ADP-ribose) polymerase (PARP) and phospholipase C (PLC)-g1 protein. Both the cytotoxic effects and apoptotic characteristics induced by streptochlorin were significantly inhibited by z-DEVD-fmk, a caspase-3 inhibitor, which demonstrates the important role that caspase-3 played in the observed cytotoxic effects. Furthermore, Bcl-2 overexpression significantly reversed the streptochlorin-induced growth inhibitory effects via inhibition of the MMP collapse and caspases activation and effectively attenuated the apoptotic response to streptochlorin. However, the elevated levels of FasL expression induced by streptochlorin were not reduced by Bcl-2 overexpression. Taken together, these findings demonstrate that the pro-apoptotic effect of streptochlorin is mediatedthrough activation of caspases and mitochondria in U937 cells.