Enhanced melanization of Harding-Passey mouse melanoma cells following treatment with exogenous melanosomes in monolayer culture.

Purified melanosomes isolated from subcutaneously growting Harding-Passey melanomas of NMRI-mice were labeled either in vitro with [14C]tyrosine or [14C]DOPA in the melanin portion, or in vivo in the melanin and protein portion following i. p. injection of [14C]tyrosine. Treatment of monolayer cultures of Harding-Passey melanoma cells (HPM-73 line) with such labeled melanosomes resulted in rapid uptake of label during the first 4 h which leveled off thereafter. A portion of the "incorporated" label could be removed by a 15 min chase with unlabeled melanosomes. Uptake of labeled melanosomes by HPM-73 cells was followed by increased cellular melanization which was not only due to melanin derived from incorporated melanosomes but primarily to newly formed melanin. Tyrosinase activity was elevated in melanosome-treated cells. Tyrosinase activity of control cells was significantly reduced following a 24 h exposure to actinomycin D or cycloheximide. On the other side, the same inhibitor treatment of melanosome-pretreated cells resulted in less inhibition of tyrosinase activity. The present findings suggest "melanophagic" properties of cultured melanoma cells resulting in enhanced melanogenesis after phagocytotic uptake of functionally active exogenous melanosomes.