Pigmented cell lines of mouse albino melanocytes containing a tyrosinase cDNA with an inducible promoter.

Melanocyte cell lines, with characteristic dendritic morphology and melanosomes, were established from young mice of wild-type (C57BL/6) and of two albino (C57BL/6-c2J/c2J and BALB/c) inbred strains. The albino cells were cotransfected with two plasmids: pMTtyr1, containing the full-length tyr1 cDNA for tyrosinase encoded by the c locus, under the control of the inducible mouse metallothionein-I (MT-I) promoter; and pSVneo beta, allowing selection of transformants by G418 resistance. The intrinsic albino defect was corrected by the tyr1 cDNA in transfected cells, thereby validating the coding capability of tyr1 for tyrosinase. Black melanin was formed in the genetically black (B/B) C57BL/6-c2J/c2J cells and brown melanin in the genetically brown (b/b) BALB/c cells. Pigment was produced even without adding heavy metal (for induction of the MT-I promoter), thus obviating the need for adding it, but was formed more rapidly upon addition of ZnSO4 up to 100 microM. Stable transfected albino melanocyte lines with active tyrosinase and melanization were obtained. Addition of ZnSO4 at 200 microM was lethal to the cells. However, this toxicity--attributable at least in part to melanin precursors--was prevented if the cells sojourned at 100 microM ZnSO4 for two weeks before being exposed to the 200 microM level. Adaptation was lost when the cells were removed from 200 microM ZnSO4 for one week and then returned to it. Avoidance of toxicity under these conditions is thus the result of physiological detoxification mechanisms rather than selection for a genetic change.