Thymoquinone induces apoptosis in human colon cancer HCT116 cells through inactivation of STAT3 by blocking JAK2- and Src-mediated phosphorylation of EGF receptor tyrosine kinase

Thymoquinone (TQ), a compound isolated from black seed oil (Nigella sativa), has been reported to possess anti-inflammatory and anticancer activities. However, the molecular mechanisms underlying the anticancer effects of TQ remain poorly understood. In the present study, we found that TQ significantly reduced the viability of human colon cancer HCT116 cells in a concentration- and time-dependent manner. Treatment of cells with TQ induced apoptosis, which was associated with the upregulation of Bax and inhibition of Bcl-2 and Bcl-xl expression. TQ also activated caspase-9,-7, and -3, and induced the cleavage of poly-(ADP-ribose) polymerase (PARP). Pretreatment with a pan-caspase inhibitor, z-VAD-fmk, abrogated TQ-induced apoptosis by blocking the cleavage of caspase-3 and PARP. Treatment of cells with TQ also diminished the constitutive phosphorylation, nuclear localization and the reporter gene activity of signal transducer and activator of transcription-3 (STAT3). TQ attenuated the expression of STAT3 target gene products, such as survivin, c-Myc, and cyclin-D1, -D2, and enhanced the expression of cell cycle inhibitory proteins p27 and p21. Treatment with TQ attenuated the phosphorylation of upstream kinases, such as Janus-activated kinase-2 (JAK2), Src kinase and epidermal growth factor receptor (EGFR) tyrosine kinase. Pharmacological inhibition of JAK2 and Src blunted tyrosine phosphorylation of EGFR and STAT3, while treatment with an EGFR tyrosine kinase inhibitor gefitinib inhibited phosphorylation of STAT3 without affecting that of JAK2 and Src in HCT116 cells. Collectively, our study revealed that TQ induced apoptosis in HCT116 cells by blocking STAT3 signaling via inhibition of JAK2- and Src-mediated phosphorylation of EGFR tyrosine kinase.