The involvement of gremlin-1 in external stress-induced melanogenesis

External stress such as ultraviolet (UV) radiation and pollutants enhances skin pigmentation. In response to external stressors, skin cells such as keratinocytes, melanocytes, and fibroblasts, release secretory proteins to deliver signals to neighboring cells. Along with communication among skin cells, melanin production is a critical step for protecting skin cells against external stressors. It is well known that a variety of mediators such as alpha-melanocyte stimulating hormone (α-MSH), stem cell factor (SCF), keratinocyte growth factor, and fibroblast growth factor stimulate melanin synthesis [1].