Thymoquinone, A Bioactive Component of Black Caraway Seeds, Causes G1 Phase Cell Cycle Arrest and Apoptosis in Triple-Negative Breast Cancer Cells with Mutant p53

Thymoquinone (TQ) from black caraway seeds has several anticancer activities; however, its effect on triple-negative breast cancer (TNBC) cells that lack functional tumor suppressor p53 is not known. Here, we explored the growth inhibitory effect of TQ on 2 TNBC cell lines with mutant p53. Cell metabolism assays showed that TQ inhibited TNBC cell growth without affecting normal cell growth. Flow cytometric analyses of TQ-treated TNBC cells showed G1 phase cell cycle arrest and apoptosis characterized by the loss of mitochondrial membrane integrity. Western blots of lysates from TQ-treated TNBC cells showed cytochrome c and apoptosis-inducing factor in the cytoplasm, as well as caspase-9 activation consistent with the mitochondrial pathway of apoptosis. Caspase-8 was also activated in TQ-treated TNBC cells, although the mechanism of activation is not clear at this time. Importantly, TQ-induced apoptosis was only partially inhibited by zVAD-fmk, indicating a role for caspase-independent effector molecules. Poly(ADP-ribose) polymerase cleavage and increased γH2AX, as well as reduced Akt phosphorylation and decreased expression of X-linked inhibitor of apoptosis, were evident in TQ-treated cells. Finally, TQ enhanced cisplatin- and docetaxel-induced cytotoxicity. These findings suggest that TQ could be useful in the management of TNBC, even when functional p53 is absent.