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BACKGROUND: Melanin pigment plays a major role in the expression of normal human skin color as well as in the photoprotection against ultraviolet damage. Melanin produced in melanocytes is transferred via dendrites to surrounding keratinocytes, and this anatomical relationship is termed as epidermal melanin unit. The rates of pigment synthesis and transfer by melanocytes appear to be influenced by ultraviolet light, though the precise factors regulating human epidermal pigmentation remain unelucidated. It has been reported that keratinocytes in vitro release factors that could modulate melanocyte behavior. Ultraviolet irradiation was also been known to enhance the release of various kinds of cytokine from keratinocytes in vivo and in vitro. OBJECTIVE: We postulated that keratinocytes rather than melanocytes could play a primary role in UVB-induced pigmentation, and keratinocytes, when irradiated with UVB, release substances that could modulate or stimulate melanin synthesis from melanocytes. The fact that keratinocytes are located efficiently for direct sunlight irradiation at the top of melanocytes, that they release various biological factors known to simulate melanin synthesis from melanocytes and that they constitute the majority of epidermal cells supported this possibility. To investigate this possibility, we evaluated the effect of supernatant from UVB-irradiated cultured keratinocytes on the growth, melanin content, and tyrosinase activity of human melanocytes. METHODS: Human cultured keratinocytes were irradiated with UVB(30, 60, or 120mj/cm2)once, and after 24 hours, supernatant of the keratinocytes were collected and added to a growth medium of melanocytes for 5 days in concentration of 15, 25 or 35%, We observed numeric and morphologic changes as well as melanin content and tyrosinase activity in situ of cultured human melanocytes. RESULTS: 1. When cultured melanocytes were incubated with supernatant of non-irradiated keratinocytes, the number of melanocytes, amount of melanin and tyrosinase activity increased in groups added with 25% or35% concentration of supernatant. 2. The number of melanocytes incubated with 15% or 25% concentrations of supernatant from cultured keratinocytes irradiated with UVB increased in both 30 and 60mj/cm2 of UVB irradiated groups and decreased in 120mJ/cm2of UVB irradiated groups. 3. The melanin content of melanocytes incubated with 15% concentration of supernatant from UVB-irradiated cultured keratinocytes increased in 120mJ/cm2 of UVG irradiated groups. 4. The tyrosinase activity of melanocytes incubated with 15% concentration of supernatant from UVB-irradiated cultured keratinocytes increased in 120mJ/cm2 of UVB irradiated groups and the tyrosinase activity of melanocytes incubated with 25% concentration of supernatant from UVB-irradiated cultured keratinocytes increased with 35% supernatant concentration of supernatant from UVB-irradiated keratinocytes, the tyrosinase activity increased in 30mJ/cm2of UVB irradiated groups. CONCLUSION: The above results suggest that UVB-irradiated kerationcytes release soluble or photoactivated factors which could modulate the growth and melanization of melanocytes, and that keratinocytes play an important or primary role in the regulation of UVB induced pigmentation.
Biological Factors ; Dendrites ; Humans* ; Keratinocytes* ; Melanins* ; Melanocytes* ; Monophenol Monooxygenase* ; Pigmentation ; Skin ; Sunlight ; Ultraviolet Rays

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