MECHANISMS OF MELANOGENESIS INHIBITION BY TUMOR-NECROSIS-FACTOR-ALPHAIN B16 F10 MOUSE MELANOMA-CELLS/

The inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) is p resent in the dermal and epidermal layers of normal skin [Kilgus, O., Payer, E., Schreiber, S., Elbe, A., Strohal, R. & Stingl, G. (1993) J. Invest. Dermatol. 100, 674-680]. Its local concentrations are modifie d by several stimuli, including wound healing and ultraviolet irradiat ion. Moreover, TNF-alpha inhibits melanogenesis in normal melanocytes [Swope, V., Abdel-Malek, Z., Kassem, L. & Norlund, J. (1991) J. Invest . Dermatol. 96, 180-185], and is, therefore, a potential autocrine/par acrine regulator of pigmentation. We have analyzed the mechanisms of t his effect using B16/F10 melanoma cells as a model. Nanomolar concentr ations of TNF-alpha inhibit the tyrosine hydroxylase and dopa oxidase activities of B16/F10 melanocytes, to less than 30% control levels, wi thout effects on tyrosinase-related protein 2/dopachrome tautomerase ( TRP2/DCT). The 50% inhibition was obtained at 1 nM TNF-alpha and 48 h treatment. The effect of TNF-alpha was noticeable after 6 h treatment, and maximal after 24 h. This inhibition is explained by decreased int racellular levels of tyrosinase and tyrosinase-related protein 1 (TRP1 ), but not of TRP2/DCT as detected by Western blotting. Northern-blot experiments showed that the inhibitory effect is partially explained b y a reduced accumulation of the corresponding mRNAs, that dropped to a bout 50% of control values (48 h treatment, 5 nM TNF-alpha). Moreover, the tyrosine hydroxylase and dopa oxidase activities decreased more r apidly in TNF-alpha-treated cells than in control cells, under conditi ons of inhibition of protein synthesis. This suggests a TNF-mediated r eduction of tyrosinase half-life. However, the possibility of an inhib itory post-translational modification of the enzyme induced by TNF can not be ruled out. Therefore, the inhibitory effect of TNF-alpha on tyr osinase and TRP-1 results from combined effect on mRNA levels and enzy matic activity or protein stability.