Thymoquinone inhibits cell proliferation through regulation of G1/S phase cell cycle transition in N-nitrosodiethylamine-induced experimental rat hepatocellular carcinoma

Dysregulated cell proliferation and tumorigenesis is frequently encountered in several cancers including hepatocellular carcinogenesis (HCC). Thus, agents that inhibit cell proliferation and restrain hepatic tumorigenesis through cell cycle regulation have a beneficial effect in the treatment of hepatocellular carcinogenesis. The present study was aimed to investigate the efficacy of thymoquinone (TQ), an active compound derived from the medicinal plant Nigella sativa, on N-nitrosodiethylamine (NDEA) 0.01% in drinking water for 16 weeks]-induced hepatocarcinogenesis in experimental rats. After experimental period, the hepatic nodules, liver injury markers and tumor markers levels were substantially increased in NDEA induced liver tumors in rats. However, TQ (20 mg/kg body weight) treatment greatly reduced liver injury markers and decreased tumor markers and prevented hepatic nodule formation and reduced tumor multiplicity in NDEA induced hepatic cancer bearing rats and this was evident from argyrophilic nucleolar organizer region (AgNORs) staining. Moreover, the uncontrolled cell proliferation was assessed by specific cell proliferative markers proliferating cell nuclear antigen (PCNA) and Ki67] by immunofluorescence, immunoblot and analysis of mRNA expression. Simultaneously, we assessed the activity of TQ on G1/S phase cell cycle regulation with specific cell cycle proteins (p21^WAF1/CIP1, CDK4, Cyclin D1 and Cyclin E) by immunoprecipitation in experimental rats. Treatment with TQ significantly reduced the detrimental alterations by abrogating cell proliferation, which strongly induced G1/S arrest in cell cycle transition. In conclusion, our results suggest that TQ has a potent anti proliferative activity by regulating the G1/S phase cell cycle transition and exhibit a beneficial role in the treatment of HCC.