Abstract: Objective To establish a method to quantitatively assess melanosome transfer with incorporation of 14C- thiouracil （TU） into nascent melanin. Methods To characterize whether 14C-TU was exclusively incorporated into melanin-producing cells, the same number of mouse melan-a or SP-1 keratinocytes were labeled with 14C-TU for 12 hours and 48 hours, respectively, followed by the measurement of radioactivity. Mouse melan-a melanocytes were pre-labeled with 1 Ci/mL 14C-TU, and cocultured with mouse SP1 keratinocytes to develop an assay system for melanosome transfer to keratinocytes. Following co-culture, the keratinocytes with transferred radioactivity were separated from melanocytes at different time points via two times of differential trypsinization. Transferred radioactivity in keratinocytes, denoting the amount of melanosome transfer, was measured with liquid scintillation counting. Meanwhile, the effects of forskolin, a PKA activator, and nicotinamide on melanosome transfer were also investigated with this assay system. Results The incorporated radioactivity in melan-a cells was 66- or 80-fold as high as that in SP-1 cells, indicating that 14C-TU would be a suitable tracer for melanosome transfer in co-culture with keratinocytes. A purity of 84.5% was achieved for keratinocytes with transferred radioactivity by twice differential trypsinization. As shown by this assay, there was an approximately 0.67-fold decrease in melanosome transfer with the treatment of 1 g/L nicotinamide and 2.3-fold increase with 20 μmol/L forskolin treatment. After coculture with SP1 cells for 8-12 hours, melan-a cells developed well-extending dendrites with detectable melanosome transfer, while no proliferation of melan-a cells induced by forskolin was seen. Conclusion An optimized protocol for selective incorporation of 14C-TU into nascent melanin has been successfully applied to the quantitative measurement of melanosome transfer from melanocytes to keratinocytes induced by forskolin or nicotinamide.

Key words: melanosome transfer, coculture, isotope