Anti-melanogenic Effects of Lactuca sativa L. Stem Extract through the Inhibition of Tyrosinase Activity and the PKA-CREB/MITF Signaling Pathway in B16F10 Melanoma Cells

Purpose: To examine the anti-melanogenic effects and underlying molecular mechanisms of <i>Lactuca sativa</i> L. stem extract (LSSE) in B16F10 murine melanoma cells. Methods: A LSSE was prepared using 70% ethanol as the solvent. Cell viability was determined using a water-soluble tetrazolium salt (WST-1)-based assay. To assess the anti-melanogenic effects of LSSE, intracellular melanin content, intracellular and mushroom tyrosinase activity, and the expression of melanogenesis-related genes and proteins were measured by reverse transcription-polymerase chain reaction and immunoblot analysis. Results: LSSE showed no significant cytotoxicity up to a concentration of 2.0%. Upon α-MSH stimulation, LSSE reduced melanin synthesis and cellular tyrosinase activity in a dose-dependent manner. Interestingly, LSSE also directly inhibited mushroom tyrosinase activity using L-tyrosine and L-DOPA as substrates. A marked reduction in the mRNA and protein expression of melanogenic enzymes, such as tyrosinase, tyrosinase-related Protein 1 (TRP-1), tyrosinase-related Protein 2 (TRP-2) was observed along with their transcription factor, microphthalmia-associated transcription factor (MITF). This was accompanied by downregulation of phosphorylated-protein kinase A (p-PKA) and -cAMP response element-binding protein (p-CREB) without altered total PKA and CREB protein levels. This indicates suppression of the PKA-CREB signaling pathway. Conclusion: LSSE attenuates melanogenesis by directly inhibiting tyrosinase activity and downregulating the PKA-CREB-MITF signaling pathway. LSSE shows potential as a safe and effective anti-melanogenic agent for cosmetic applications.