The plant-derived bioactive molecule thymoquinone negatively regulates colorectal cancer stem cells’ self-renewal capacity -

Abstract

Cancer relapse following therapy remains a leading cause of death among humans worldwide. Colorectal cancer, after a potentially curative surgery, has the tendency to relapse after 5 years on average. It is believed that cancer stem cells (CSCs), which are a subpopulation of cancer cells that retain the ability of self-renewal and differentiation into different mature cells, are the main factor in cancer relapse. Although CSCs pose a significant problem for cancer relapse, they provide a target for drug therapy. Thymoquinone (TQ) is a promising anticancer molecule shown to inhibit cancer cell growth and progression in numerous cancer systems both in vitro and in vivo. We hypothesize that TQ targets colon cancer cells with stem-like properties by inhibiting their self-renewal property. Here, we investigated the effect of TQ on colon cancer cells with stem-like properties using two isogenic HCT116 human colon cancer cell lines that differ in their p53 status. We also investigated the relevance of stem cell markers profiling in 2-dimensional (2-D) monolayer cultured cells versus 3-dimensional (3-D) colonospheres. Sphere-formation and propagation assays were used to assess TQ effect on self-renewal potential of enriched CSC populations. Inhibition of self-renewal required 10-fold lower concentrations of TQ in 3-D HCT116 colonospheres when compared to those required to inhibit cell growth in two-dimensional 2-D monolayers. Colonospheres which survived TQ treatment at 1 and 3µM TQ were propagated from generation 1 to 5 and showed, at every generation, similar dose-dependent decrease in sphere viability upon TQ treatment. Colonospheres treated with 1µM TQ showed a consistent decrease in sphere viability over serial passages from generation 1 to 5 regardless of their p53 status. We also showed that a pure CSC population with wild type p53 is less responsive to TQ treatment when compared to a CSC population with p53 deletion. TQ significantly decreased HCT116 p53+-+ sphere size at generation 1 but not in H