Ameliorating effects and mechanisms of tuna peptides on UVB-induced photoaging and melanogenesis targeting oxidative stress, apoptosis, and tyrosinase inactivation.

Ultraviolet B (UVB) radiation causes skin photodamage and excessive melanogenesis, but effective multi-target protective agents remain limited. Antioxidant peptides derived from marine by-products are promising; however, it remains unclear whether specific peptides from skipjack tuna skin can simultaneously alleviate UVB-induced photodamage and inhibit melanogenesis, and the underlying molecular mechanisms are poorly defined. Therefore, the primary objective of this study was to systematically investigate the protective efficacy and molecular mechanisms of three antioxidant peptides derived from skipjack tuna skin-Met-Pro-Ala-Gly-Glu (MPAGE), Leu-Asn-Pro-Gly-Ala (LNPGA), and Gly-Thr-Gly-Pro-Ala (GTGPA)-against UVB-induced skin photodamage and excessive melanogenesis. We hypothesized that these peptides exert synergistic photoprotective and anti-melanogenic effects via multi-target regulation of Nrf2-mediated antioxidant pathways, inflammatory and apoptotic cascades, and tyrosinase inhibition. In UVB-irradiated HaCaT cells, MPAGE, LNPGA, and GTGPA significantly upregulate nuclear factor erythroid 2-related factor 2 (Nrf2) protein expression and promote its nuclear translocation, increasing the levels of NAD(P)H quinone dehydrogenase 1 (NQO1), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (CAT) levels, thereby scavenging reactive oxygen species (ROS), reducing malondialdehyde (MDA), and alleviating oxidative damage. Mechanistically, molecular docking predictions suggested that these peptides may activate Nrf2 by interacting with the Kelch domain of kelch-like ECH-associated protein 1 (Keap1) and competitively occupying its ligand-binding pockets. Additionally, the peptides also attenuated apoptosis by reducing bcl-2-associated X protein/b-cell lymphoma 2 (Bax/Bcl-2) ratio and cysteine-dependent aspartate-specific protease 3 (Caspase-3) level. Moreover, they potently inhibited the β-galactosidase and collagenase production and reduced matrix metalloproteinase-1 (MMP-1) and matrix metalloproteinase-3 (MMP-3) levels, suppressing UVB-induced collagen degradation and mitigating skin photoaging. Enzyme-Linked Immunosorbent Assay (ELISA) revealed a marked reduction in interleukin-6 (IL-6), interleukin-1 β (IL-1β), and tumor necrosis factor α (TNF-α) levels, demonstrating significant anti-inflammatory effects. Finally, MPAGE, LNPGA, and GTGPA exhibited dose-dependent inhibitory effects on melanogenesis in B16-F10 cells. In silico docking analysis further predicted that these peptides could deeply embed in the active pocket of tyrosinase by hydrogen bondings interactions (<6 Å) with key catalytic residues. In summary, this work provides the first comprehensive mechanistic characterization of MPAGE, LNPGA, and GTGPA, demonstrating that they alleviate UVB-induced photodamage through synergistic antioxidant, anti-aging, anti-inflammatory, and anti-apoptotic mechanisms, while potentially inhibiting melanogenesis through computationally predicted tyrosinase suppression. These findings provide a basis for further experimental validation of these peptides as multi-target skin-protective agents.