Keratinocyte-melanocyte interactions during melanosome transfer

The epidermal-melanin unit is composed of one melanocyte and approximately 36 neighboring keratinocytes, working in synchrony to produce and distribut e melanin. Melanin is synthesized in melanosomes, transferred to the dendri te tips, and translocated into keratinocytes, forming caps over the keratin ocyte nuclei. The molecular and cellular mechanisms involved in melanosome transfer and the keratinocyte-melanocyte interactions required for this pro cess are not yet completely understood. Suggested mechanisms of melanosome transfer include melanosome release and endocytosis, direct inoculation ('i njection'), keratinocyte-melanocyte membrane fusion, and phagocytosis. Stud ies of the keratinocyte receptor protease-activated receptor-2 (PAR-2) supp ort the phagocytosis theory. PAR-2 controls melanosome ingestion and phagoc ytosis by keratinocytes and exerts a regulatory role in skin pigmentation. Modulation of PAR-2 activity can enhance or decrease melanosome transfer an d affects pigmentation only when there is keratinocyte-melanocyte contact. Moreover, PAR-2 is induced by UV irradiation and inhibition of PAR-2 activa tion results in the prevention of UVB-induced tanning. The role of PAR-2 in mediating UV-induced responses remains to be elucidated.