Enhancing pigmentation via activation of A3 adenosine receptors in B16 melanoma cells and in human skin explants.

A3 adenosine receptor, A3AR, belongs to the Gi proteins coupled receptors. Activation of A3AR by its agonist, IB-MECA, decreases cAMP and was expected to reduce melanin level. Unexpectedly, B16 melanoma cells exposed to IB-MECA increased melanin levels in a dose-dependent manner. Human skin explants exposed to IB-MECA showed an increase in DOPA positive cells and in melanin deposition in keratinocytes. The agonist induced AKT phosphorylation, leading to a rapid translocation of the transcription factor MiTF towards the nucleus. DOPA oxidase activity and melanin levels induced by IB-MECA were further enhanced by PD98509, an inhibitor ERK signalling pathway. Our study shows that IB-MECA decreases cAMP while inducing melanogenesis. The proposed mechanism involves activation of PI3K/AKT signalling pathway by β/γ subunits of the G protein coupled to A3AR. The increase in melanin level in human skin explants suggests that IB-MECA may be a potential candidate to the treatment of hypopigmentation of skin.