Human melanoma cells generate leukotrienes B4 and C4 from leukotriene A4

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 gamma-glutamyl transferase that converts LTC4 to LTD4. LTC4 synthase activity was inhibited by the addition 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.