Mechanism underlying inhibitory effect of six dicaffeoylquinic acid isomers on melanogenesis and the computational molecular modeling studies.

Dicaffeoylquinic acid (DCQA), which contain 2 caffeic acids and a quinic acid, is 6 isomeric compounds (1,3-, 1,4-, 1,5-, 3,4-, 3,5-, and 4,5-DCQA). In this study, the mechanism underlying the inhibitory effect of DCQA isomers on melanogenesis in B16F1 murine melanoma cells stimulated by melanocyte stimulating hormone (α-MSH) was evaluated. DCQA isomers showed inhibitory effects on melanogenesis in α-MSH-stimulated B16F1 cells. Furthermore, the anti-melanogenesis activities of 1,5-DCQA and 4,5-DCQA were 61% and 84%, respectively, which were greater than that of arbutin (35%). For cell-free tyrosinase, 3,4-DCQA and 4,5-DCQA indicated high inhibitory effects, similar to the activity to arbutin (35%) at 25 μM. DCQA isomers inhibited the melanogenic enzymes including tyrosinase and dopachrome tautomerase (DCT) on α-MSH-stimulated B16F1 cells. Interestingly, 4,5-DCQA, the most potent inhibitor of melanogenesis among the six DCQA isomers, significantly downregulated the expression of microphthalmia-associated transcription factor (MITF), tyrosinase-related protein 1 (TRP1) containing tyrosinase, and DCT. In particular, the inhibitory mechanism of 4,5-DCQA on MITF expression was elucidated, revealing that 4,5-DCQA inhibits the phosphorylation of cAMP response element-binding protein (CREB) by attenuating cAMP generation during melanogenesis. A molecular docking study was conducted to elucidate the inhibitory mechanism of 4,5-DCQA on cAMP production. DCQA isomers dock to the residues of adenylyl cyclase with a distance of <3 Å, except for 1,3-DCQA. Especially, 4,5-DCQA showed Full Fitness of -1304.68 kcal/mol and △G of -8.33 kcal/mol, as well as H-bonding with adenylyl cyclase at ILE953 and LYS930 residues. In conclusion, DCQA isomers have different effects on melanogenesis depending on their structure. Especially, 4,5-DCQA has depigmentation activity through the inhibitory effect on cellular tyrosinase directly and binding effect on adenylyl cyclase, resulting in the downregulation of MITF protein, thereby reducing the expression of melanogenic enzymes.