Ultraviolet-B-irradiated mouse epidermis releases mediators that stimulate melanoma cells

We determined whether the exposure of mouse epidermal cells to ultraviolet radiation (UVR) induces the production of factors that can stimulate the in vitro and in vivo growth of syngeneic melanoma cells. Epidermal sheets isolated from the back skin of C3H/HeN mice were placed into sterile medium and exposed to different doses of UVB radiation; 18 h later, culture supernatants were harvested. Supernatants of epidermal sheets not exposed to UVR served as controls. Supernatants from UVR-treated epidermal sheets, but not control supernatants, stimulated the in vitro and in vivo growth of some murine melanomas, but not cells of a fibrosarcoma, a hepatocellular carcinoma, or a squamous carcinoma. Supernatants of UVR-treated epidermal cells injected into the ears of syngeneic mice stimulated epidermal cell proliferation in a short-term assay. We conclude that UVR may contribute to the incidence of cutaneous melanoma by its ability to cause the release from epidermal cells of diffusible factors that promote the outgrowth of small numbers of melanoma cells.