Genetic Differences in the Metabolic Activation of Benzo[a]Pyrene in Mice

Abstract

The carcinogenesis index of subcutaneous benzo[a]pyrene-initiated fibrosarcomas in the genetically ‘responsive’ C3H inbred mouse strain is more than five times higher, and about 15 times higher, than that in the ‘responsive’ C57BL/6 and the ‘nonresponsive’ DBA/2 strains, respectively. Carcinogenesis indices involving F₁ hybrids of these strains indicate that additional genes besides the Ah locus may cause a particular inbred strain to be more sensitive, or resistant, to benzo[a]pyrene-initiated tumors than would be expected solely on the basis of aryl hydrocarbon (benzo[a]pyrene) hydroxylase (EC 1.14.14.2) inducibility. The DNA-bound benzo[a]pyrene metabolite complexes generated by mouse liver or skin microsomes in vitro can be resolved into at least nine distinct peaks by elution of a Sephadex LH-20 column with a water-methanol gradient. Eight peaks, shown previously to be associated with increased hepatic cytochrome Pi-450 content, are greater with liver microsomes from genetically responsive C3H and C57BL/6 inbred strains and the (C57BL/6)(C3H)F1, (C3H)(DBA/2)F₁ and (C57BL/6)(DBA/2)Fι hybrids than from the genetically nonresponsive DBA/2 inbred strain. All nine peaks are greater with skin microsomes in vitro from C3H and C57BL/6 than from DBA/2 mice. Benzo[a]pyrene mutagenicity in vitro with Salmonella typhimurium tester strain TA98 is increased 5- to 6-fold with liver microsomes from C3H and C57BL/6 inbred mice and the three F₁ hybrids mentioned above, compared with liver microsomes from DBA/2 mice. Similar genetic differences in benzo[a]pyrene mutagenesis with the bacterial tester strain TA100 are also seen. DNA ‘repair’ – as measured by the rate at which DNA-bound benzo[a]ρyrene 4,5-oxide and the 7,8-diol-9,10-epoxides are removed from mouse skinDNA – is not significantly different between C3H and C57BL/6 mice. The K-region oxide appears to be removed from DNA nucleosides, however, at least twice as rapidly as the 7,8-diol-9,10-epoxides. Our data thus demonstrate a strikingly good correlation between genetically determined increases in peaks representing Pι-450-catalyzed benzo[a]pyrene metabolites bound to DNA and benzo[a]pyrene mutagenesis in vitro. Neither of these in vitro parameters, nor DNA repair in mouse skin in vivo, however, explains the 5- to 6-fold difference in benzo[a]pyrene carcinogenesis index between the two genetically responsive strains, C3H and C57BL/6.