To optimize skin pigmentation in order to help body prevention against UV radiation, the mechanism of
melanin pigment transfer from melanocytes to keratinocytes must be elucidated.
Melanin transfer to keratinocytes requires specific recognition between keratinocytes and melanocytes or melanosomes. Cell surface
sugar-specific receptor (membrane
lectin) expression was studied in human C32
melanoma cells, an
amelanotic melanoma, by flow cytometry analysis of neoglycoprotein binding as an approach to the molecular specificity.
Sugar receptors on melanocytes are mainly specific for alpha-L-
fucose. Their expression is enhanced upon treatment by the
diacylglycerol analogue
1-oleoyl-2-acetylglycerol, which can induce
melanin synthesis in amelanotic human
melanoma cells in a dose-dependent manner. Flow cytometry analyses showed a small-sized population of vesicles distinguishable from large cells by their fluorescence properties upon neoglycoprotein binding. Sorting indicated that the small-sized subpopulation is composed of vesicles produced by melanocytic cells. Upon vesicle formation, a selective concentration of
sugar receptors specific for 6-phospho-beta-D-galactosides appears in the resulting melanocytic vesicles. Vesicles are recognized and taken up by cultured keratinocytes and a partial inhibitory effect was obtained upon cell incubation in the presence of
neoglycoproteins, indicating a possible participation of
sugar receptors in this recognition. The validity for such a model to help in understanding the natural
melanin transfer by melanosomes is confirmed by electron microscopy, which demonstrates the presence of
melanin inside keratinocytic cells upon incubation with melanocytic vesicles.