Two groups of S-[2-(N,N-dialkylamino)ethyl]isothiourea derivates which depigmented
melanoma cells either with inhibition of
tyrosinase (group 1, R = methyl, isopropyl) or without inhibition of
tyrosinase (group 2, R = benzyl, phenyl) were studied. Treatment of human
melanoma cells with non-lethal doses of group 1 drugs led to a reduction in the levels of
mRNA for the pigmentation genes
tyrosinase,
tyrosinase-related protein-1 and Pmel 17. The group 1
drug S-[2-N,N-diisopropylamino)ethyl[isothiourea] (
DINOR) (R = isopropyl) produced only moderate inhibition of
DNA,
RNA and
protein synthesis in three cell lines during the first 24 hr of treatment, and there was no correlation between the extent of inhibition and long-term toxicity. A group 2
drug (R = benzyl) rapidly inhibited
DNA synthesis in an
amelanotic melanoma cell line (MM96E) sensitive to killing by the
drug; association of the latter with inhibition of
RNA or
protein synthesis was less clear. MM96E cells were also sensitive to killing by
reactive oxygen species. In pigmented
melanoma cells (MM418), incorporation of [125I]
thiouracil, a false precursor of
melanin, increased during the first 24 hr of treatment with
DINOR whereas a group 2
drug (R = phenyl) inhibited incorporation of [125I]
thiouracil. Cells depigmented by treatment with drugs from either group suffered the same amount of DNA damage as pigmented cells after UVB irradiation, as judged by inhibition of
DNA synthesis, but did not recover as well as pigmented cells, whether or not
drug was present during recovery. The results suggested that (1) group 1 agents down-regulated message for several pigmentation genes, possibly at the transcriptional level; (2) the toxicity of group 2 drugs was related to
reactive oxygen species; and (3)
melanin protected cells from UVB by enhancing cellular recovery.