Thymoquinone Radiosensitizes Human Colorectal Cancer Cells in 2D and 3D Culture Models

SIMPLE SUMMARY: Radiotherapy is a standard of care therapy that kills cancer cells but has limited efficacy in targeting the resistant cancer stem cells (CSCs). The use of radiosensitizers in experimental studies and clinical practice is a successful strategy for eradicating CSCs. Here, we investigated the radiosensitizing potential of thymoquinone (TQ), a natural compound with known anti-cancer activity, in 2D and 3D cultures of colorectal cancer, and in patient-derived organoids. We show that TQ sensitized cancer cells and stem/progenitor cells to radiation mainly through the inhibition of cell survival, DNA repair, and stemness in addition to regulating major pathways implicated in this process. Thus, TQ could be used as a sensitizer to effectively target and kill colorectal cancer cells. ABSTRACT: Resistance of cancer cells and normal tissue toxicity of ionizing radiation (IR) are known to limit the success of radiotherapy. There is growing interest in using IR with natural compounds to sensitize cancer cells and spare healthy tissues. Thymoquinone (TQ) was shown to radiosensitize several cancers, yet no studies have investigated its radiosensitizing effects on colorectal cancer (CRC). Here, we combined TQ with IR and determined its effects in two-dimensional (2D) and three-dimensional (3D) culture models derived from HCT116 and HT29 CRC cells, and in patient-derived organoids (PDOs). TQ sensitized CRC cells to IR and reduced cell viability and clonogenic survival and was non-toxic to non-tumorigenic intestinal cells. TQ sensitizing effects were associated with G2/M arrest and DNA damage as well as changes in key signaling molecules involved in this process. Combining a low dose of TQ (3 µM) with IR (2 Gy) inhibited sphere formation by 100% at generation 5 and this was associated with inhibition of stemness and DNA repair. These doses also led to ~1.4- to ~3.4-fold decrease in organoid forming ability of PDOs. Our findings show that combining TQ and IR could be a promising therapeutic strategy for eradicating CRC cells.