Abstract
BACKGROUND: Most pharmacological depigmenting agents and cosmetic skin-brightening products achieve their effects by suppressing melanogenesis. However, the fate of melanin after melanosome transfer to keratinocytes-and the mechanisms governing its intracellular clearance-remains insufficiently explored. AIMS: This study aimed to elucidate the intracellular mechanism of melanin degradation in keratinocytes and to establish a simplified and operable experimental strategy for evaluating melanin clearance beyond melanogenesis inhibition. METHODS: A simplified in vitro model was established in which human epidermal keratinocytes phagocytosed isolated melanosomes, allowing investigation of melanin degradation independent of melanocyte activity. In parallel, a cell-free oxidative system consisting of ferrous ions and hydrogen peroxide was employed to chemically induce hydroxyl radical-mediated melanin degradation. Lysosomal activity, intracellular oxidative status, hydroxyl radical (•OH) generation, melanin content, and pH dependence were assessed using fluorescence imaging and biochemical assays. RESULTS: Keratinocytes exhibited a two-step melanin degradation process involving lysosomal proteolysis followed by oxidative breakdown mediated by •OH. Treatment with hydrolyzed conchiolin protein (HCP) enhanced melanin degradation by promoting lysosomal activation and modulating intracellular oxidative conditions. Fluorescence imaging demonstrated partial colocalization of •OH signals with lysosomes and suggested alterations in lysosomal pH following HCP exposure. Chemical assays further revealed that alkaline conditions more effectively promoted hydroxyl radical-mediated melanin degradation. CONCLUSIONS: This study identifies an intracellular melanin degradation pathway operating within keratinocytes and presents a simplified experimental framework integrating cellular and cell-free models. HCP emerges as a modulator of lysosomal-oxidative pigment clearance, offering an alternative pigmentation control strategy beyond melanogenesis inhibition and supporting the development of skin-brightening approaches that preserve physiological pigment homeostasis.