Type I γ-GT mRNA is expressed in B16 melanoma and levels correlate with pigmentation

γ-Glutamyl transpeptidase (γ-GT), an ectoenzyme involved mainly in glutathione metabolism, is expressed in B16 melanoma cells. B16 melanoma cells under continuous culture conditions show a phenotypic drift from melanotic to amelanotic and re-melanotic stages. We have investigated the regulation of γ-GT in B16 melanoma cells under such different pigmentary conditions. High levels of γ-GT messenger RNA (mRNA) and activity were detected in pigmented B16 melanoma cells, whereas in amelanotic B16 melanoma cells the levels were very low. Treatment with lactic acid, a known inhibitor of tyrosinase gene expression, also led to the down-regulation of γ-GT mRNA and activity. Thus our results indicate that γ-GT regulation depends on the pigmentation status in pigment cells. We have also assessed the levels of γ-GT in normal murine melanocytes (melan-a cells). It was seen that melan-a cells express very low levels of γ-GT. As γ-GT is known to be regulated in a tissue-specific manner, and is expressed from as many as six promoters giving rise to six different types of mRNAs each having unique 5′ ends, we have further investigated the type of γ-GT mRNA expressed in B16 melanoma and melan-a cells. In this study, we have conclusively demonstrated that type I mRNA transcript of γ-GT is expressed in B16 melanoma and melan-a cells.