Melanosome Clusters and Apoptotic Melanocytes Internalized by Dermal Fibroblasts Impairs Cellular Function

Skin pigmentation disorders with dermal melanin involvement are therapeutically challenging due to poor responsiveness to conventional treatments and unclear pathogenesis. While melanin in the dermis is typically cleared by macrophages, human dermal fibroblasts (HDFs) can also internalize extracellular melanosome clusters (EMC) and apoptotic melanocytes that have occasionally been observed in the dermis, which may contribute to persistent pigment-related abnormalities. However, the functional impact of phagocytosis for fibroblasts remains unclear. In this study, HDFs were incubated with EMC or apoptotic MNT-1 melanoma cells (ApoMNT-1)-both exposing the phagocytic "eat-me" signal phosphatidylserine-confirming their internalization via Annexin V and TEM assays. Phagocytosis of EMC and ApoMNT-1 significantly upregulated the mRNA expression of IL-6, IL-8, IL-1β, PTGS2, and NF-κB, as well as the protein levels. Additionally, HDF proliferation was markedly inhibited, and the mRNA and protein levels of COL-1 and COL-3 were reduced. Furthermore, HDF mitochondrial function was impaired by the phagocytosis of EMC or ApoMNT-1, as indicated by decreases in basal respiration, ATP production, glycolysis, glycolytic capacity, and mitochondrial membrane potential. These results suggest that HDF internalization of EMC and apoptotic melanocytes triggers pro-inflammatory responses, inhibits cell proliferation and collagen synthesis, and impairs mitochondrial function. These findings provide novel insights into the biological mechanisms of dermal melanin-related processes and the functional changes of dermal fibroblasts after melanin phagocytosis, which lays a foundation for further in vivo studies on dermal hyperpigmentation.