AP-1-derived peptides impact melanin production and melanosome biogenesis. (a) Melanin content estimation of MNT-1 cells treated with EK-11, QK-5, GN-11, SS-5 (10 µM), or H2O as control; (b) Conventional electron microscopy of H2O- or QK-5-treated MNT-1 cells. Black arrows: immature melanosomes (stages I/II); black arrowheads: maturing stage III melanosomes; white arrowheads: pigmented stage IV melanosomes. (c) Quantification of immature melanosomes (stages I or II) and mature melanosomes (stages III or IV) per condition; (d) Melanin content estimation of 3D-HRPE (phototype VI) treated with 30 µM of QK-5, QA-3, or AK-3 peptides or H2O as control. Data are the average presented as a mean ± standard deviation. ns, non-significant; * p < 0.05; ** p < 0.01; *** p < 0.001. Bars, 1 µm.