Rosmarinic acid induces melanogenesis through protein kinase A activation signaling

Melanogenesis is a physiological process that results in the synthesis of melanin pigments, which play a crucial protective role against skin photocarcinogenesis. In order to determine the effects of rosmarinic acid on melanogenesis and elucidate the molecular events of melanogenesis induced by rosmarinic acid, several experiments were performed in B16 melanoma cells. In this study, we showed that the melanin content and tyrosinase expression were increased by rosmarinic acid in a concentration-dependent manner. In addition, after the melanin content was increased by rosmarinic acid, it was reduced by H-89 and KT 5720, protein kinase A (PKA) inhibitors, but not by SB203580, a p38^mapk inhibitor, or Ro-32-0432, a PKC inhibitor, which suggests the involvement of PKA in rosmarinic acid-induced melanogenesis. Consistent with this, rosmarinic acid induced the phosphorylation of CRE-binding protein (CREB), but had no effect on the phosphorylation of p38^mapk or the inhibition of Akt phosphorylation. Additionally, rosmarinic acid induced the activation of cAMP response element (CRE) without having any effect on cAMP production, which suggests that rosmarinic acid-induced melanogenesis is mediated by PKA, which occurs downstream of cAMP production. This result was further confirmed by the fact that rosmarinic acid-induced phosphorylation of CREB was inhibited by H-89, but not by PD98059, a MEK1 inhibitor, or by LY294002, a phosphatidylinositol-3-kinase (PI3K) inhibitor. Rosmarinic acid-induced expression of tyrosinase protein was attenuated by H-89. Based on these results, we report for the first time that rosmarinic acid induces melanogenesis through PKA activation signaling.