Thymoquinone, a component of the herb Nigella sativa, suppresses proliferation and viability of prostate cancer cells by down-regulating androgen receptor and E2F-1

Abstract

20038 Background: Prostate cancer (CaP) that relapses following androgen-ablation therapy is hormone refractory, yet its growth continues to depend on androgen receptor (AR). Therefore, a curative strategy must include structural elimination of AR. Thymoquinone (TQ) is a component of Nigella sativa, an herb that has been used for thousands of years for a variety of diseases including cancer. We examined TQ effect on AR and cell cycle regulatory proteins required for proliferation and viability of CaP cells. Methods: Inhibitory effect of TQ on DNA synthesis and proliferation of exponentially growing LNCaP cells was assessed by 3H-thymidine incorporation into DNA and MTS assay respectively. We examined TQ effect on AR and cell cycle regulatory proteins by Western blot analysis. The effect of TQ on cell cycle progression was determined using synchronized LNCaP cells. Western blots were then used to analyze equal amounts of protein isolated from cells at regular intervals during their progression through the cell cycle in the presence or absence of TQ. Results: We observed a dose-dependent increase in the inhibitory effect of TQ on DNA synthesis and proliferation of LNCaP cells. We also observed a significant decrease in AR and E2F-1, and a dramatic increase in p21, p27, and pRB, and Bax. Synchronized LNCaP cells treated with TQ failed to enter S phase. Western blot analysis revealed that TQ treatment of synchronized cells resulted in: (i) a significant decrease in AR, that normally increases in early- to late-G1phase, and E2F-1, that normally increases in late-G1phase (ii) down-regulation of cyclins A and E, Cdks-2 and 4, and Cdc-6 (iii) induction of p53 and p27. Conclusion: These observations demonstrate that TQ inhibits proliferation and viability of prostate cancer cells by decreasing AR and E2F-1 levels and inducing the proteins that cause cell cycle arrest and promote apoptotic events. Thus TQ may prove to be an effective treatment of hormone refractory CaP and a promising chemo- and/or radiation- sensitizing agent in adjuvant therapy as well. This could represent a novel approach for treatment of prostate cancer by an herbal agent that was shown to spare normal cells in previous studies. No significant financial relationships to disclose.