Keratinocyte Extracellular Matrix-Mediated Regulation of Normal Human Melanocyte Functions

Active roles of cell‐cell interaction between melanocytes and neighboring keratinocytes for the regulation of melanocyte functions in the skin have been suggested. We examined substantial regulatory mechanisms of keratinocyte extracellular matrix (kECMs) for normal human melanocyte functions without direct cell‐cell contact. We specially devised kECMs from proliferating or differentiating keratinocytes and further treated them with environmental stimulus ultraviolet B (UVB) for skin pigmentary system. Normal human melanocytes (NHM) were cultured on the various keratinocyte ECMs and initially the effects of the kECMs upon melanocyte morphology (dendrite formation and extension), growth, melanin production and expressions of pigmentation‐associated protein (MEL‐5) and proliferation‐associated protein (proliferating cell nuclear antigen; PCNA/cyclin) were studied. Then we compared the effects of these cell‐matrix interactions with those of direct melanocyte‐keratinocyte, cell‐cell contact in co‐culture on melanocyte functions. Melanocytes cultured on any types of the kECMs that were tested significantly extended dendrites more than that on plastic cell culture dish without kECM (control). Melanocytes cultured on the kECM prepared from UVB irradiated differentiating keratinocytes resulted in 219% increase in the number of dendrites. The growth of melanocytes on kECMs was also stimulated up to 280% of control. The kECM produced by proliferating keratinocytes had a more significant effect on the growth than kECM from differentiating keratinocytes. This melanocyte growth stimulating effect was decreased with kECM from UVB treated differentiating keratinocytes. The melanin content per melanocyte was constant on any of the kECMs. Expression of pigmentation‐associated protein detected by monoclonal antibody, MEL‐5, was not changed on the kECM, while it was increased in melanocytes in co‐culture with keratinocytes. Expression of PCNA/cyclin in melanocytes cultured on kECMs was generally downregulated on kECM and in co‐culture compared to that in a control culture. We demonstrated that the kECMs play important roles in the melanocyte morphology and proliferation. These observations suggest that environmental (UVB) and physiological (Ca++) stimuli can regulate melanocyte functions through the keratinocyte extracellular matrix in vivo.