Human melanoma cells generate leukotrienes B₄ and C₄ from leukotriene A₄

Abstract We examined the synthesis of leukotrienes (LTs) in human melanoma cells in order to assess the function of LTs in human melanocytes. LTA₄ hydrolase, which catalyzes the conversion of LTA₄ to LTB₄, was detected in the supernatant of cultured human melanoma (MeWo) cells and melanoma cells obtained from patients. Immunoblotting analysis using an antihuman LTA₄ hydrolase antibody showed LTA₄ hydrolase to be a 70-kDa protein in both MeWo and melanoma cells. Considerable activity of LTC₄ synthase, which catalyzes the conversion of LTA₄ to LTC₄, was detected in the microsomal fraction of both MeWo and melanoma cells. The HPLC profile of the LTC₄ synthase reaction products revealed that LTC₄ was the main product. LTD₄ was not detected under these conditions, indicating that the microsomal fraction of human melanoma cells lacks the membrane-bound γ-glutamyl transferase that converts LTC₄ to LTD₄. LTC₄ synthase activity was inhibited by the additon of MK-886, and was not altered by treatment with N-ethylmaleimide or 1-chloro-2,4-dinitrobenzene. These results indicate that the enzyme responsible for the conversion of LTA₄ to LTC₄ in human melanoma cells is LTC₄ synthase rather than a nonspecific or microsomal glutathione-S-transferase. These results also suggest that human melanoma cells can generate LTB₄ and LTC₄ from LTA₄, and that this process is catalyzed by two enzymes: LTA₄ hydrolase and LTC₄ synthase.