Inhibition of melanin synthesis by cystamine in human melanoma cells

In studies to determine whether pigmentation can be regulated physiological ly by thiols, human melanoma cells (MM418c5) and melanocytes were found to become depigmented when cultured continuously in 50 mu M cystamine. Cystami ne was depleted from the culture medium and the treatment was nontoxic and reversible. Cysteamine, dithiothreitol, and phenylthiourea were less effect ive, and glutathione, cysteine, and cystine were inactive. Tyrosinase (dopa oxidase) activity was not greatly affected except for induction of a lag p eriod. In contrast, tyrosinase activity in an amelanotic melanoma cell line (MM96L) was rapidly inhibited without consumption of cystamine/cysteamine, in association with the generation of free thiol in the culture medium, an d could be enhanced by the cystine transport inhibitor, glutamate. Tyrosina se expressed by a recombinant vaccinia virus was inhibited by cystamine tre atment of MM96L and HeLa cells. Cystamine treatment lowered the degree of c ross-linking of the pigmentation antigen gp75/TRP-1 in MM418c5 cells. Tyros inase protein and mRNA levels in MM418c5 cells were not affected by cystami ne. The results show that cystamine at a concentration close to physiologic levels has multiple effects on the melanogenic pathway. In amelanotic cell s, tyrosinase has a short half-life and is readily inhibited by cystamine/c ysteamine whereas tyrosinase in the more mature melanosomes of the pigmente d cell appears to be less accessible to proteolytic and thiol attack. Inhib ition of melanin synthesis in the latter cell type may arise to a significa nt degree from reduction of cystamine to cysteamine, which sequesters quino nes.