Bone marrow-derived cells are not the origin of the cancer stem cells in ultraviolet-induced skin cancer.

Several lines of evidence have demonstrated that various cancers are derived from cancer stem cells (CSCs), which are thought to originate from either tissue stem or progenitor cells. However, recent studies have suggested that the origin of CSCs could be bone marrow-derived cells (BMDCs); for example, gastric cancer, which follows persistent gastric inflammation, appears to originate from BMDCs. Although our previous research showed the capability of BMDCs to differentiate into epidermal keratinocytes, it has yet to be determined whether skin CSCs originate from BMDCs. To assess the possibility that BMDCs could be the origin of CSCs in skin squamous cell carcinoma (SCC), we used a mouse model of UVB-induced skin SCC. We detected a low percentage of BMDCs in the lesions of epidermal dysplasia (0.59%), SCC in situ (0.15%), and SCC (0.03%). Furthermore, we could not find any evidence of clonal BMDC expansion. In SCC lesions, we also found that most of the BMDCs were tumor-infiltrating hematopoietic cells. In addition, BMDCs in the SCC lesions lacked characteristics of epidermal stem cells, including expression of stem cell markers (CD34, high alpha6 integrin) and the potential retention of BrdU label. These results indicate that BMDCs are not a major source of malignant keratinocytes in UVB-induced SCC. Therefore, we conclude that BMDCs are not the origin of CSCs in UVB-induced SCC.