Uptake of melanosomes and increased melanin formation by cultured melanoma cells after treatment with isolated melanosomes.

Melanosomes isolated from a subcutaneous Harding-Passey mouse melanoma and purified by density gradient centrifugation were labelled in vitro with 14C-tyrosine or 3H-dihydroxyphenylalanine in the melanin portion. Incubation of monolayer cultures of Harding-Passey melanoma cells during exponential growth phase (wherin cells contained relatively few melanosomes) with isolated and labelled melanosomes during a time-period of up to 3 days resulted in rapid cellular uptake of label (reaching a saturation level after about half a day). Following a lag period of several hours, the melanin content rose near-linearly in the course of 3 days. Comparison of curves of uptake of radioactivity and melanin concentration indicates that the latter rise is due primarily to newly formed melanin. Ultrastructural studies revealed a strikingly increased number of melanosomes in melanosome-treated cells. Some of these appeared to be the result of phagocytotic uptake. In fact, invagigations of the plasma membrane containing exogenous melanosomes were observed. Since most intracellular melanosomes were localized directly in the cytoplasmic matrix, dissolution of the phagosome membrane appeared to have taken place. Aggregates of melanosomes either surrounded by a membrane or free in the cytoplasm were also observed. These bodies might represent phagolysosomes or/and centres for the formation of new melanosomes. The combined biochemical and ultrastructural findings suggest stimulated melanogenesis induced by phagocytized melanosomes.