Induction of apoptosis by ethanol extract of Citrus unshiu Markovich peel in human bladder cancer T24 cells through ROS-mediated inactivation of the PI3K/Akt pathway.

Citrus unshiu peel has been used to prevent and treat various diseases in traditional East-Asian medicine including in Korea. Extracts of C. unshiu peel are known to have various pharmacological effects including antioxidant, anti-inflammatory, and antibacterial properties. Although the possibility of their anti-cancer activity has recently been reported, the exact mechanisms in human cancer cells have not been sufficiently studied. In this study, the inhibitory effect of ethanol extract of C. unshiu peel (EECU) on the growth of human bladder cancer T24 cells was evaluated and the underlying mechanism was investigated. The present study demonstrated that the suppression of T24 cell viability by EECU is associated with apoptosis induction. EECU-induced apoptosis was found to correlate with an activation of caspase-8, -9, and -3 in concomitance with a decrease in the expression of the inhibitor of apoptosis family of proteins and an increase in the Bax:Bcl-2 ratio accompanied by the proteolytic degradation of poly(ADP-ribose) polymerase. EECU also increased the generation of reactive oxygen species (ROS), collapse of mitochondrial membrane potential, and cytochrome c release to the cytosol, along with a truncation of Bid. In addition, EECU inactivated phosphatidylinositol 3-kinase (PI3K) as well as Akt, a downstream molecular target of PI3K, and LY294002, a specific PI3K inhibitor significantly enhanced EECU-induced apoptosis and cell viability reduction. However, N-acetyl cysteine, a general ROS scavenger, completely reversed the EECU-induced dephosphorylation of PI3K and Akt, as well as cell apoptosis. Taken together, these findings suggest that EECU inhibits T24 cell proliferation by activating intrinsic and extrinsic apoptosis pathways through a ROS-mediated inactivation of the PI3K/Akt pathway.