Alterations of melanin synthesis in human melanoma cells selected in vitro for multidrug resistance.

Abstract

Previous data showing the correlation of multidrug resistance (MDR) and differentiation in tumor cell populations (Melloni et al. 1988; Stavrovskaya et al. 1990) suggest that: 1) isolation of MDR cells by cytostatic drugs leads to the selection of more differentiated cell variants and 2) in more differentiated cell variants the activity of MDR-related P-glycoprotein (Pgp) is more prominent than in less differentiated cells. Here we used human melanoma cell line mS and two variants selected from mS population: a) MDR variant of mS selected by colchicine (mS-0.5) and b) mS-trRAR/2--variant obtained by introduction of expressing retinoic acid receptor RAR-alpha cDNA into mS cell. The differentiation status, expression of MDR1 gene and Pgp functioning were compared in wild-type cells and mS variants. Electron microscopic examination of melanosomes showed that the mS-0.5 subline comprised more differentiating cells in the population than parental mS cultures and that these cells were at later stages of melanogenesis. The increase in the degree of differentiation in mS-0.5 population coincided with MDR1 gene overexpression, occurrence of Pgp molecules on the cell membrane and acceleration of Pgp-mediated Rhodamine 123 (Rh123) efflux. mS-trRAR/2, proved to be more differentiated than mS cells. The MDR1 mRNA level and Rh123 efflux were not elevated in mS-trRAR/2 cells, however, retinoic acid (RA) treatment increased both the degree of differentiation and Rh123 efflux in mS-trRAR/2 to a greater extent than in mS cultures. Thus, the data obtained in this study are in favor of the suppositions mentioned above. The mechanisms of coordinated alterations of differentiation and Pgp activity in MDR cells are discussed.