Pivotal involvement of the CX3CL1-CX3CR1 axis for the recruitment of M2-TAMs in skin carcinogenesis

We previously revealed the crucial roles of CX3CL1-CX3CR1 axis in skin wound healing. Although repeated wounds frequently develop into skin cancer, the roles of CX3CL1 in skin carcinogenesis remain elusive. Here, we proved that CX3CL1 protein expression and CX3CR1+ macrophages were observed in human skin cancer tissues. Similarly, we observed the enhancement of CX3CL1 expression and the abundant accumulation of CX3CR1+ tumor-associated macrophages (TAMs) with M2 phenotypes in the skin carcinogenesis process induced by the combined treatment with 7,12-dimethylbenz-(a)anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA). In this mouse skin carcinogenesis process, CX3CR1+ TAMs exhibited M2 phenotypes with the expression of Wnt3a and angiogenic molecules including vascular endothelial growth factor (VEGF) and matrix metalloproteinase (MMP)-9. Compared to wild-type mice, CX3CR1-deficient mice showed fewer numbers of skin tumors with a lower incidence. Concomitantly, M2-macrophage numbers and neovascularization reduced with the depressed expression of angiogenic factors and Wnt3a. Thus, the CX3CL1-CX3CR1 axis can crucially contribute to skin carcinogenesis by regulating the accumulation and functions of TAMs. Thus, this axis can be a good target for preventing and/or treating skin cancers.