Thymoquinone from Nigella Sativa Seeds Promotes the Antitumor Activity of Topotecan in Human Colorectal Cancer Cell Lines

Abstract

Thymoquinone (TQ), the bioactive constituents of the volatile oil of Nigella Sativa seeds, has shown promising beneficial pharmacological effects such as inhibiting the proliferation and inducing apoptosis of several types of cancer cells. Topotecan, a topoisomerase I inhibitor, is a widely used anticancer drugs with primary dose-limiting toxicity. It is necessary to identify other chemotherapeutic agents that can increase the efficacy of topotecan while limiting its toxicity. The aim of this study was to investigate the potential synergistic effect of thymoquinone with topotecan. HT 29cCells were incubated with different topotecan and thymoquinone concentrations for 24 and 48 hours in order to determine the IC 50 for each compound. The reduction in proliferation was significantly dose- and time- dependent. After determining the best combination (40µM thymoquinone and 0.6 µM topotecan ), the expression levels of Bax, Bcl2, p53, p21, caspase-9, -3 and -8 was studied by western blots. In addition, cell cycle analysis and annexin/PI staining were performed. Both drugs induced apoptosis through a p53 independent mechanism, the expression of p21 was only seen in thymoquinone treatment and cell cycle arrest in the S phase was detected upon applying each compound separately, whereas combined treatment only increased the production of fragmented DNA. Both compounds induced apoptosis through the extrinsic pathway at 24 hours while at 48 hours, the intrinsic pathway was activated upon topotecan treatment only. As a conclusion thymoquinone was found to increase the sensitivity of the chemotherapeutic reagent topotecan by inhibiting proliferation and lowering toxicity through p53 and bax/bcl2 independent mechanisms.