Tumor necrosis factor alpha-mediated inhibition of melanogenesis is dependent on nuclear factor kappa B activation

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Abstract
Melanogenesis is a physiological process resulting in the synthesis of melanin pigments which play a crucial protective role against skin photocarcinogenesis. In vivo, solar ultraviolet light triggers the secretion of numerous keratinocyte-derived factors that are implicated in the regulation of melanogenesis. Among these, tumor necrosis factor α (TNFα), a cytokine implicated in the pro-inflammatory response, down-regulates pigment synthesis in vitro. In this report, we aimed to determine the molecular mechanisms by which this cytokine inhibits melanogenesis in B16 melanoma cells. First, we show that TNFα inhibits the activity and protein expression of tyrosinase which is the key enzyme of melanogenesis. Further, we demonstrate that this effect is subsequent to a down-regulation of the tyrosinase promoter activity in both basal and cAMP-induced melanogenesis. Finally, we present evidence indicating that the inhibitory effect of TNFα on melanogenesis is dependent on nuclear factor kappa B (NFκB) activation. Indeed, overexpression of this transcription factor in B16 cells is sufficient to inhibit tyrosinase promoter activity. Furthermore, a mutant of inhibitory kappa B (IκB), that prevents NFκB activation, is able to revert the effect of TNFα on the tyrosinase promoter activity. Taken together, our results clarify the mechanisms by which TNFα inhibits pigmentation and point out the key role of NFκB in the regulation of melanogenesis.

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