Sericin-Induced Melanogenesis in Cultured Retinal Pigment Epithelial Cells Is Associated with Elevated Levels of Hydrogen Peroxide and Inflammatory Proteins

We previously demonstrated that the silk protein sericin promotes pigmentation of retinal pigment epithelium (RPE) by activating the NF-κB pathway. Among numerous agents, NF-κB can be activated by hydrogen peroxide. In the present study, we explored possible associations between reactive oxygen species and sericin-induced melanogenesis in RPE. The proteome of human fetal RPE cultured for seven days with or without 1% sericin was analyzed using ingenuity pathway analysis (IPA). The proteomic data was verified by immunofluorescence and immunoblotting. Light microscopy and scanning electron microscopy were used to assess morphology. Dihydroethidium (DHE) and dihydrorhodamine (DHR) assays were used to measure superoxide and hydrogen peroxide species. Expression levels of proteins related to inflammation, differentiation, cell survival and cell adhesion were higher in cells cultured in Dulbecco's Modified Eagle Medium (DMEM) with 1% sericin, whereas cells cultured in DMEM alone showed higher expression levels of proteins associated with Bruch's membrane and cytoskeleton. Despite upregulation of inflammatory proteins, sericin co-cultured RPE yielded significantly higher cell viability compared to cells cultured without sericin. Addition of sericin to culture media significantly increased hydrogen peroxide-levels without significantly affecting superoxide-levels. We suggest that sericin-induced melanogenesis in cultured RPE is associated with elevated levels of superoxide dismutase, hydrogen peroxide and inflammatory proteins.