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Background and Purpose cAMP as a second messenger stimulates expression of microphthalmia-associated transcription factor (MITF) or the tyrosinase gene in UVB-induced skin pigmentation. Diphenylmethylene hydrazinecarbothioamide (QNT 3-80) inhibits α-melanocyte-stimulating hormone (α-MSH)-induced melanin production in B16 murine melanoma cells but its molecular basis remains to be defined. Here, we investigated the mechanism underlying the amelioration of skin hyperpigmentation by QNT 3-80. Experimental Approach We used melanocyte cultures with raised levels of cAMP and UVB-irradiated dorsal skin of guinea pigs for pigmentation assays. Immunoprecipitation, kemptide phosphorylation, fluorescence analysis and docking simulation were applied to elucidate a molecular mechanism of QNT 3-80. Key Results QNT 3-80 inhibited melanin production in melanocyte cultures with elevated levels of cAMP, including those from human foreskin. This compound also ameliorated hyperpigmentation in vivo in UVB-irradiated dorsal skin of guinea pigs. As a mechanism, QNT 3-80 directly antagonized cAMP binding to the regulatory subunit of PKA, nullified the dissociation and activation of inactive PKA holoenzyme in melanocytes and fitted into the cAMP-binding site on the crystal structure of human PKA under the most energetically favourable simulation. QNT 3-80 consequently inhibited cAMP- or UVB-induced phosphorylation (activation) of cAMP-responsive element-binding protein in vitro and in vivo, thus down-regulating expression of genes for MITF or tyrosinase in the melanogenic process. Conclusions and Implications Our data suggested that QNT 3-80 could contribute significantly to the treatment of skin disorders with hyperpigmented patches with the cAMP-binding site of PKA as its molecular target.

Ultraviolet (UV) radiation under sunlight stimulates skin pigmentation through immediately affecting the oxidative modification of existing melanin pigments and the spatial redistribution of pigmented melanosomes followed by the up-regulation of melanogenic genes in delayed kinetics. However, abnormal accumulation and synthesis of melanin biopolymers are responsible for skin disorders with more pigmented patches. Chemical-based regulation of the hyperpigmented disorders has been a long-standing goal for cosmetic and pharmaceutical applications. A large number of the chemicals with antimelanogenic activity have met with limited or no success in the treatment of skin patients, since they may not overcome the challenge of penetrating the skin barrier. Guinea pig skin displays similar kinetic parameters to human skin in the transdermal absorption of numerous chemicals, thus can serve as the surrogate for human skin. Here, we provide a concise review of our current understanding of the chemical-based therapy against skin hyperpigmentation in UV-irradiated guinea pig models, suggest molecular mechanisms of the action and emphasize the translation from preclinical outcomes to skin patients.