Enhanced Melanization of Harding-Passey Mouse Melanoma Cells Following Treatment with Exogenous Melanosomes in Monolayer Culture

Abstract

Purified melanosomes isolated from subcutaneously growing 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.