Anti-Melanogenic Effects of L-Theanine on B16F10 Cells and Zebrafish.

L-Theanine, a natural amino acid found in green tea () leaves, is known for its diverse psychotropic effects. This study aimed to evaluate the inhibitory effect of L-theanine on melanin production and uncover its regulatory mechanism. We evaluated the anti-melanogenic activities of L-theanine in vitro and in vivo. In B16F10 murine melanoma cells induced by α-melanocyte-stimulating hormone, melanin content and intracellular tyrosinase activity were determined, and melanogenesis-related protein expression and signaling pathways were analyzed by Western blotting. Melanin reduction was further assessed using the zebrafish () test. L-Theanine reduced the intracellular tyrosinase activity and melanin content of B16F10 cells. It also attenuated the expression of melanogenesis-related proteins, such as microphthalmia- associated transcription factor, tyrosinase (TYR), TYR-related protein-1, and dopachrome tautomerase. L-Theanine modulated the protein kinase A (PKA), cAMP responder element binding protein (CREB), phosphorylation of/protein kinase B (Akt), glycogen synthase kinase-3β (GSK-3β), and β-catenin. The antimelanogenic activity of L-theanine (<2 mg/mL) was further confirmed using zebrafish larvae. L-Theanine inhibited melanogenesis by downregulating the PKA/CREB and Akt/GSK-3β/β-catenin signaling pathways. In summary, L-theanine shows potential as a skin-whitening compound, warranting further investigation for its possible applications in cosmetic and pharmaceutical products.