Cell Line-Dependent Cell Death Pathways Induced by Thymoquinone in Colorectal Cancer Cells

Colorectal cancer (CRC) remains a leading cause of cancer-related mortality, with resistance to 5-fluorouracil (5-FU) representing a major therapeutic challenge. Thymoquinone (TQ), a bioactive constituent of Nigella sativa, exhibits anticancer activity; however, the mechanisms underlying TQ-induced cell death appear to be highly context dependent. This study aimed to characterize cell line-specific death pathways triggered by TQ in colorectal cancer models with distinct molecular backgrounds and differential responsiveness to 5-FU. Human CRC cell lines RKO (5-FU-sensitive) and SW1116 (poorly responsive), along with normal colon epithelial cells (CCD-841CoN), were treated with TQ, 5-FU, or their combination for 24 h. Cell viability, DNA fragmentation, caspase-3/7, -8, and -9 activity, cell death phenotypes, and expression of apoptosis- and necroptosis-related genes were evaluated using MTT assays, ELISA, luminescent assays, flow cytometry, and RT-qPCR. TQ significantly reduced viability in both CRC cell lines while exerting minimal cytotoxicity toward normal cells. In RKO cells, characterized by microsatellite instability (MSI), TQ induced DNA fragmentation, caspase activation, and transcriptional upregulation of pro-apoptotic genes, consistent with apoptosis-associated signaling. In contrast, SW1116 cells, which exhibit chromosomal instability (CIN) and reduced responsiveness to 5-FU, displayed decreased viability accompanied by suppressed caspase activity and predominant features of caspase-independent necrotic cell death. This differential response may be attributed to the CIN phenotype, which has been associated with impaired apoptotic signaling and enhanced tolerance to cytotoxic stress. Combined TQ and 5-FU treatment did not produce synergistic cytotoxicity, as confirmed by Bliss independence analysis, but revealed distinct, cell line-dependent death programs. These findings demonstrate that TQ modulates cell death execution in a molecular context-dependent manner rather than enhancing 5-FU efficacy through pharmacological synergy.