Low Levels of Sox2 are required for Melanoma Tumor-Repopulating Cell Dormancy

Tumorigenic cells, when facing a hostile environment, may enter a dormant state, leading to long-term tumor survival, relapse, and metastasis. To date, the molecular mechanism of tumor cell dormancy remains poorly understood. Methods: A soft, 3-dimentional (3D) fibrin gel culture system was used to mechanically select and grow highly malignant and tumorigenic melanoma tumor-repopulating cells (TRCs). We cultured control melanoma TRCs, TRCs with Sox2 knockdown, TRCs with Sox2 knockout, and a 2D control for in vitro and in vivo experiments. Western blotting, immunofluorescence, and flow cytometry analysis were performed to examine TRC dormancy and exit from dormancy. Results: Under a low-expression condition, we show that Sox2, a stemness molecule participates in dormancy regulation of highly tumorigenic cells that can repopulate a tumor (TRCs). Intriguingly, complete depletion of Sox2 via knockout results in dormancy exit and growth resumption of melanoma TRCs in culture and elevation of melanoma TRC apoptosis. Mice that are injected subcutaneously with Sox2-depleted melanoma TRCs do not form tumors and survive much longer than those injected with melanoma TRCs. We found that complete depletion of Sox2 promotes nuclear translocation of phosphorylated STAT3, where it binds to the p53 gene promoter, thus activating the p53-caspase3 cascade. Conclusion: These findings provide a novel insight into the role of the Sox2 gene in tumor cell stemness, tumor dormancy, and apoptosis.