Paradoxical effects of thymoquinone in prostate cancer cells: Induction of reactive oxygen species.

Abstract

Thymoquinone (TQ) a well known anti‐oxidant is an active ingredient of black seed oil. It has been shown to inhibit growth of prostate cancer (PCa) cells by inhibiting expression of androgen receptor (AR) and E2F‐1 in parental LNCaP cells and its isogenic variant, hormone independent AR expressing C4‐2B cells. However, the precise mechanism of action of TQ remains unexplored, especially in AR naïve (PC‐3) cells as models of hormone refractory disease. Our results demonstrated that TQ inhibits growth of both androgen receptor positive (C4‐2B) and androgen receptor naïve (PC‐3) PCa cells with an IC50 value of 80 and 50 µM at 48 hr, respectively. TQ (100 µM) induced a 3 and 4 fold increase in ROS generation and a decrease in GSH levels by 35% and 45% in C4‐2B and PC‐3 cells, respectively. Treatment of PCa cells with TQ did not activate caspases but resulted in an increase in the levels of apoptosis inducing factor (AIF) indicating a caspase‐independent cell death mechanism. Treatment of PC‐3 cells with TQ (50 µM) up‐regulated GADD45α and down‐regulated BCL‐2 (2 fold) and BAG‐1 (1.5 fold). However, TQ‐induced inhibition of the expression and transactivational activity of AR in C4‐2B cells was not restored upon simultaneous co‐exposure with N‐acetylcysteine (NAC). In both cell types, (NAC) completely inhibited TQ‐induced ROS generation and protected the cells from cytotoxicity suggesting that ROS generation by TQ was primarily responsible for growth inhibition.