Identification of dithiolethione-inducible gene-1 as a leukotriene B4 12-hydroxydehydrogenase: implications for chemoprevention.

Cancer chemoprevention is inhibition of neoplastic disease by naturally occurring or synthetic chemical agents. Dithiolethiones inhibit production of experimentally produced tumors by elevating the expression of several genes that encode for known cytoprotective enzymes. In an effort to discover additional molecular mechanisms mediating chemoprevention, cDNA clones representing a gene that is transcriptionally activated by dithiolethiones, hence named dithiolethione-inducible gene-1 (DIG-1), were isolated from rat liver via differential hybridization screening. The deduced amino acid sequence of DIG-1 was found to have 80% identity with the human liver enzyme leukotriene B 4 (LTB 4 ) 12-hydroxydehydrogenase. DIG-1, purified >400-fold from the liver of rats dosed with 1,2-dithiole-3-dithiolethione, possessed an NADP + -dependent activity to convert LTB 4 to 12-oxo-LTB 4 . Kinetic analysis of DIG-1 revealed apparent K m and V max values of 28 mM and 8.1 nmol 12-oxo-LTB 4 formed/min/mg purified protein respectively. Since LTB 4 is a potent chemotactic factor and stimulator of production of reactive oxygen species from neutrophils, the effects of DIG-1 on these LTB 4 -mediated processes were examined. Pre-incubation of LTB 4 with purified rat hepatic DIG-1 greatly diminished LTB 4 -stimulated migration of neutrophils. In addition, pre-incubation of LTB 4 with purified rat hepatic DIG-1 reduced LTB 4 -stimulated production of superoxide anions in neutrophils, as evidenced by decreased lucigenin-derived chemi-luminescence. These results suggest that DIG-1-catalyzed dehydrogenation of LTB 4 to 12-oxo-LTB 4 inhibits the pro-inflammatory actions of LTB 4 . Consequently, elevation of LTB 4 catabolism via enhanced DIG-1 activity may suppress inflammatory processes implicated in tumorigenesis.