Roles of adipocytes and M2-macrophages in high-fat diet-stimulated lymph node metastasis of B16F10 melanoma

Abstract

The tumor microenvironment consists of a number of distinct cell types including tumor cells, fibroblasts, endothelial cells, immune cells, and adipocytes, and produces a wide variety of factors, which contribute to tumor progression. We previously observed that high-fat diet (HFD) feeding stimulates solid tumor growth and lymph node (LN) metastasis in mice subcutaneously injected with B16F10 melanoma cells. We also observed that HFD feeding increases the number of lipid vacuoles and M2-macrophage (M2-Mφ)s as well as the expression of various cytokines/growth factors in tumor tissues. In the present study, we noted that HFD increased the expression of CCR7 in the tumor. The concentrations of CCL19 and CCL21, the CCR7 ligands, were significantly higher in the LN than those in tumor, and HFD feeding increased the concentrations of these ligands in the LN with negligible changes in the tumor tissue creating higher concentration gradients of these two ligands between the LN and tumor. In order to explore the interactions between tumor cells, adipocytes, lymphatic endothelial cell (LEC)s, and/or M2-Mφs, we established in vitro co-culture models involving 3 distinct cell types. The mRNA levels of cytokines and growth factors were increased in B16F10 cells and M2-Mφs when B16F10 cells, M2-Mφs, and/or mature adipocyte (MA)s were co-cultured in a transwell system. Among these cytokines/growth factors, the mRNA levels of MCP-1 were synergistically increased in B16F10 cells when co-cultured with M2-Mφs and MAs. The migration of monocytes was tremendously elevated when media conditioned by B16F10 cells/M2-Mφs/MAs co-cultures was used as a chemoattractant. Furthermore, MAs increased the expression of CCR7 mRNA in B16F10 cells and the expression of CCL19 and CCL21 mRNAs in LECs. These results indicate that in HFD-fed mice, adipocytes induce MCP-1 expression and monocyte infiltration which play important roles in solid tumor growth and LN metastasis. Additionally, adipocyte-stimulated activation of CCL19, CCL21/CCR7 axis in lymphatic endothelial cells and tumor cells helps tumor cells to enter the lymphatic vessels and increased ligand concentration gradients between the LN and tumor facilitate tumor cell migration to the LN.