Luteolin was gamma irradiated at doses of 0, 15, 30, 50, 70, and 100 kGy. We observed that the luteolin peak decreased simultaneously with the appearance of new radiolytic peaks, using high-performance liquid chromatography (HPLC). The highest new radiolytic peak (GLM) of radiolytic product in gamma-irradiated luteolin was observed at a dose of 70 kGy, and the GLM was identified by nuclear magnetic resonance and high-performance-liquid-chromatography-quadrupole-time-of-flight (HPLC-Q-TOF) mass spectrometry. We examined whether 70 kGy gamma-irradiated luteolin has more effective anti-melanogenic effects than intact luteolin. Seventy kilograys of gamma-irradiated luteolin inhibited melanin synthesis and intracellular tyrosinase activity without cytotoxicity, whereas the intact luteolin-treated group did not show anti-melanogenic activity in 3-isobutyl-1-methylxanthine-stimulated B16BL6 melanoma cells. The expression of melanogenic enzymes, such as tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2, was decreased by 70 kGy gamma-irradiated luteolin treatment, owing to the suppression of microphthalamia-associated transcription factor and 3',5'-cyclic adenosine monophosphate (cAMP) response element binding protein. In addition, gamma-irradiated luteolin decreased the phosphorylation of phosphoinositide 3-kinase (PI3K)/Akt and extracellular regulated kinase (ERK). The anti-melanogenic effects of 70 kGy gamma-irradiated luteolin were attenuated by the treatment of two specific inhibitors (PD98059 and LY294002), and these results indicate that the anti-melanogenic effects were mediated by ERK and PI3K signaling pathways. Therefore, our findings suggest that gamma-irradiated luteolin can be a potential cosmeceutical agent for skin whitening.
Keywords: anti-melanogenesis; gamma irradiation; luteolin; microphthalamia-associated transcription factor; tyrosinase.