Acetylsalicylic acid (aspirin; ASA) is one of the leading non-steroidal anti-inflammatory drugs (NSAIDs) and is known to act by directly suppressing the activity of cyclooxygenase (COX), the key enzyme catalyzing the biosynthesis of prostaglandins. ASA has wide range of effects and the most well known pharmacological effect is reducing pain or fever.1) Previous reports showed ASA and some other NSAIDs were able to prevent the development of cancer of the colon, breast, stomach, and lung.2-5) These biological effects of NSAIDs are the results of their anti-cancer ability, by inducing apoptosis and cell cycle arrest. 6) Moreover, ASA has been shown to be effective as an antiplatelet drug in preventing heart attacks, stroke, and cerebral thrombosis. 1,[7][8][9] Skin pigmentation results from melanin synthesis by melanocytes and is caused by exposure to UV radiation. Melanin plays an important role in the prevention of suninduced skin injury, and is a major determinant of skin color. 10,11) Tyrosinase is a key enzyme in melanin synthesis that catalyzes three different chemical reactions; the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA), the oxidation of DOPA to DOPAquinone, and the oxidation of 5,6-dihydroxyindole (DHI) to indole-quinone.12) In the absence of thiols, DOPAquinone changes to DOPAchrome and then to DHI or indole 5,6-quinone 2-carboxylic acid (DHICA). There are two other factors in this melanogenic pathway, one is tyrosinase-related protein-2 (TRP-2; DOPAchrome tautomerase), which catalyzes the conversion of DOPAchrome to DHICA, and the other is TRP-1 (DHICA oxidase) that catalyzes the oxidation of DHICA.13-17) Cyclic AMP (cAMP) is a key messenger in melanin synthesis. 18) cAMP inducers such as a-melanocyte stimulating hormone (a-MSH), forskolin, and isobutylmethylxanthine (IBMX) enhance melanogenesis. 19) Investigation of the depigmenting mechanisms and clinical aspects of skin-whitening agents is very important. In addition, increased production and accumulation of melanin leads to many hyperpigmentation disorders such as melasma, postinflammatory pigmentation, solar lentigo, etc., which become prominent with aging. Many chemicals have been demonstrated to show inhibitory effects on melanogenesis through inhibition of the enzymatic activity of tyrosinase, but effects on related gene expression, protein degradation, glycosylation, melanosome transfer, and regulation of cellular signaling are also reported to control melanogenesis (reviewed by Solano et al. 20) and Briganti et al. 21)). Among these studies the effect of ASA on the pigmentary system is as yet unreported. Therefore, we conducted an investigation on the depigmenting mechanisms of ASA in murine B16 melanoma cells. In this study, we showed that ASA inhibited a-MSH-enhanced melanogenesis in B16 melanoma and this depigmenting mechanism was attributed to the downregulation of tyrosinase expression and/or the increase of tyrosinase degradation. MATERIALS AND METHODSCell Culture B16F1 murine melanoma cells were cultured in Dul...