Abstract: A high-throughput screening of 708 natural monomers was performed for their inhibitory activity against melanin synthesis in zebrafish. S-Equol, an intestinal metabolite of soy isoflavones, was identified as having potent anti-melanogenic agent. The dose-dependent inhibitory effect of S-equol on melanogenesis in zebrafish was evaluated. Its impact on the expression and functions of key melanogenic genes was analyzed by using quantitative polymerase chain reaction (qPCR) as well as determining tyrosinase (TYR) activity and melanin content, and its potential signaling network was explored by transcriptome sequencing. S-Equol (5–20 μmol/L) exhibited no obvious toxicity and dose-dependently reduced surface pigmentation, decreasing the area of melanin deposition by approximately 60% at 20 μmol/L. It significantly down-regulated the expression of key genes including those encoding dopachrome tautomerase (dct), tyrosinase (tyr), and microphthalmia- associated transcription factor a (mitfa), inhibited TYR activity, and reduced the melanin content to 20.65% and 19.79% of that of the control at 10 and 20 μmol/L, respectively. Transcriptome analysis revealed that S-equol down-regulated the expression of Ras homolog gene family member A (RhoA)/Rho-associated coiled-coil containing protein kinase (ROCK) signaling pathway genes, including rho and rock1. These results indicate that S-equol regulates melanogenesis at the gene expression, enzymatic, and signaling pathway levels, providing a scientific basis for the development of gut microbiota- targeted functional factors derived from soy isoflavones for skin whitening.

Key words: S-equol; soy isoflavone metabolite; tyrosinase; Ras homolog gene family member A/Rho-associated coiled-coil containing protein kinase signaling pathway; anti-melanogenesis; zebrafish