The adaptive responses in skin to ultraviolet (UV) radiation include increased cornification of keratinocytes and increased synthesis and distribution of melanin by melanocytes. The possible involvement of paracrine factors in the generation of these responses was studied in a novel two-stage culture model. Human melanocytes or keratinocytes were first irradiated or sham-irradiated and then the conditioned media collected from these cells after 24 h were used to treat unirradiated skin cells. Immunofluorescent staining for transforming growth factor (TGF)-beta1 was increased in UV-irradiated keratinocytes compared with sham-irradiated cells. Increased TGF-beta1 was also detected in the culture media of irradiated keratinocytes. Treatment of unirradiated keratinocytes with conditioned media collected from UV-irradiated keratinocytes resulted in increased absolute numbers and percentages of cornified envelopes per well compared with treatment with conditioned media from sham-irradiated keratinocytes. The magnitude of this effect increased with increased dose of initial irradiation. The effects of conditioned media from UVR-treated cells were mimicked by authentic TGF-beta1. Treatment of conditioned media from irradiated cells with an antibody shown to neutralise the effects of TGF-beta1 but not with a non-immune antibody of similar isotype, abolished this bioactivity of the conditioned media from UV-irradiated cells. Immunofluorescent staining for ACTH was also increased in UV-irradiated keratinocytes. Conditioned media from UV-irradiated keratinocytes increased tyrosinase activity of unirradiated melanocytes, an effect which was mimicked by authentic ACTH. This bioactivity of conditioned media from irradiated keratinocytes was abolished in the presence of an antibody which neutralised the activity of ACTH but not MSH. These results provide evidence to support the involvement of TGF-beta1 and ACTH in the cornification and pigmentary responses respectively of skin cells after UV exposure.

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