GenesfromFibroblasts AreTransactivated after Chromosomal Transfer intoMelanomaCells

Humanandmouse fibroblast chromosomes carrying tyrosinase or b-locus geneswere introduced, by microcell hybridization, into pigmented Syrian hamster melanoma cells, andthemicrocell hybrids weretested fortransactivation ofthefibroblast tyrosinase andb-locus genes.Byusing species-specific PCRamplification todistinguish fibroblast andmelanoma cDNAs,itwas demonstrated thatthepreviously silent fibroblast tyrosinase andb-locus geneswere transactivated following chromosomal transfer into pigmented melanoma cells. However, transactivation ofthemouse fibroblast tyrosinase gene was unstable inmicrocell hybrid subclones andpossibly dependent on asecond fibroblast locus that could havesegregated inthesubclones. This second locus was notnecessaryfortransactivation ofthefibroblast b-locus gene,thusdemonstrating noncoordinate transactivation offibroblast tyrosinase andb-locus genes.Transactivation ofthefibroblast tyrosinase gene inmicrocell hybrids apparently isdependent on theabsence ofa putative fibroblast extinguisher locus fortyrosinase gene expression, whichpresumably isresponsible fortheextinction of pigmentation inhybrids between karyotypically complete fibroblasts andmelanoma cells. Thegeneration ofdistinct cell typesinmulticellular animalsonlyrecently hasbeguntobeunderstood atthegenetic andmolecular levels. Muchemphasis hasbeenplaced on factors regulating transcription andon theDNA sequences interacting withsuchfactors. However, thecontrol mechanismsdetermining whichregulatory factors will beactive andwhentheywill beexpressed arestill largely unknown, as arethemechanisms whichmaintain thedifferentiated state. Studies on themolecular control ofpigment cell differentiation arestill atan early stage,since pigment-cell-specific geneswere cloned more recently than genesassociated with a numberofother tissue types. Pigmentation inmammalsresults fromthesynthesis of melanin inmelanocytes oftheskin, hairbulbs, andpigmentedepithelium oftheeye.Pigmentation isa complex biochemical andgenetic process,with, inthe mouse, over50 distinct genetic loci involved (41). Thesynthesis ofmelanin inmelanocytes requires theenzyme tyrosinase, encoded by themouse albino (c)locus. Inaddition, other enzymes which regulate thetypeofmelanin synthesized haverecently been identified (for a review see reference 19).Another locus encoding an enzyme related totyrosinase isthemouse brown(b)locus, whichencodes apigment-cell-specific cata