IDENTIFICATION OF DITHIOLETHIONE-INDUCIBLE GENE-1 AS A LEUKOTRIENE B-4 12-HYDROXYDEHYDROGENASE - IMPLICATIONS FOR CHEMOPREVENTION

Cancer chemoprevention is inhibition of neoplastic disease by naturall y occurring or synthetic chemical agents. Dithiolethiones inhibit prod uction of experimentally produced tumors by elevating the expression o f several genes that encode for known cytoprotective enzymes. In an ef fort to discover additional molecular mechanisms mediating chemopreven tion, cDNA clones representing a gene that is transcriptionally activa ted by dithiolethiones, hence named dithiolethione-inducible gene-1 (D IG-1), were isolated from rat liver via differential hybridization scr eening. The deduced amino acid sequence of DIG-1 was found to have 80% identity with the human liver enzyme leukotriene B-4 (LTB4) 12-hydrox ydehydrogenase. DIG-1, purified >400-foId from the liver of rats dosed with 1,2-dithiole-3-dithiolethione, possessed an NADP(+)-dependent ac tivity to convert LTB4 to 12-oxo-LTB4. Kinetic analysis of DIG-1 revea led apparent K-m and V-max values of 28 mM and 8.1 nmol 12-oxo-LTB4 fo rmed/min/mg purified protein respectively. Since LTB4 is a potent chem otactic factor and stimulator of production of reactive oxygen species from neutrophils, the effects of DIG-1 on these LTB4-mediated process es were examined. Pre-incubation of LT]B4 with purified rat hepatic DI G-1 greatly diminished LTB4-stimulated migration of neutrophils. In ad dition, preincubation of LTB4 with purified rat hepatic DIG-1 reduced LTB4-stimulated production of superoxide anions in neutrophils, as evi denced by decreased lucigenin-derived chemiluminescence. These results suggest that DIG-1-catalyzed dehydrogenation of LTB4 to 12-oxo-LTB4 i nhibits the proinflammatory actions of LTB4. Consequently, elevation o f LTB4 catabolism via enhanced DIG-1 activity may suppress inflammator y processes implicated in tumorigenesis.