An acidic microenvironment promotes lymphatic metastasis of melanoma by Thy-1 in endothelial cells and integrin αvβ3 in tumor cells

Melanoma tissues exhibit an acidic microenvironment compared with that of surrounding normal tissues. However, the effects of acidic conditions on the lymphatic metastasis, a crucial prognostic factor for patients with melanoma, are unclear. In the present study, we aimed to investigate the role of the acidic microenvironment in the function of lymphatic endothelial cells. We first conducted gene expression profiling using human dermal lymphatic endothelial cells (HDLECs) treated with low pH media. Based on these results, we focused on Thy-1/CD90, whose expression increased in a time-dependent manner in HDLECs under acidic conditions. Immunohistochemical analysis of primary tumor tissues in a mouse melanoma model revealed an increased expression of Thy-1 in lymphatic endothelial cells. The expression of integrin αvβ3, a receptor for Thy-1, was also up-regulated in melanoma cells under acidic conditions. The adhesion of HDLECs to melanoma cells was accelerated under acidic conditions, which was reduced by Thy-1 knockdown in HDLECs. Furthermore, lymphatic metastasis was significantly attenuated in a mouse melanoma metastasis model when inoculated with integrin αv-silenced melanoma cells. These results suggest that acid-induced Thy-1 in lymphatic endothelial cells, as well as integrin αvβ3 in melanoma cells, may promote their mutual cellular adhesion, contributing to lymphatic metastasis.