Mutagenicity of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the mammary gland of Big Blue rats on high-and low-fat diets.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a food-borne mutagen and mammary gland carcinogen in female rats. A high-fat diet has been shown to increase the incidence of PhIP-induced mammary gland tumors. The current study used Big Blue rats harboring the lambda lacI mutational reporter transgene, to address whether the promotional effect of a high-fat diet is mediated via modulation in mammary gland mutagenesis. Big Blue rats were given 10 doses of PhIP (75 mg/kg, p.o.) and placed on defined low-fat (5% corn oil) or high-fat (23.5% corn oil) diet for 6 weeks prior to collecting mammary glands. The lacI mutant frequency (mean +/- standard error, n = 3 rats) was 231 +/- 15 (x10(-6)) and 193 +/- 12 (x10(-6)) in the low-and high-fat group, respectively. Values were increased 12-fold over control but were not significantly different between the two diets. In a parallel study, diet did not alter the mutant frequency induced by 7,12-dimethylbenz[a]anthracene (DMBA) (125 mg/kg, p.o.) in the mammary gland. The findings suggest that the promotion by the high-fat diet is not mediated via an increase in mutations. Consistent with the high potency of DMBA as a mammary carcinogen, the mutant frequency was 20-30% higher with DMBA than with PhIP. Sixty-nine and 56 PhIP-induced lacI mutants were sequenced from the low-and high-fat diet groups, respectively. While the percentage of various types of mutations was identical between the diet groups, some difference in the distribution of mutations along the lacI gene was observed. The mutation spectrum in the mammary gland from rats on both diets was consistent with the formation of PhIP-guanine adducts which were detected by a (32)P-post-labeling assay. Guanine base substitutions accounted for approximately 85% of all mutations irrespective of diet. Single base pair deletions at guanine occurred in 11-17% of mutants. G:C to T:A transversions were the predominant base substitution mutation accounting for 35-43% of all mutations. The majority of all guanine mutations (74%) occurred at guanine bases adjacent to another G:C pair. Five out of 125 (4%) mutations involved a guanine deletion in the 5'-GGGA-3' sequence, a PhIP signature mutation reported previously. Twelve out of 125 (10%) mutations involved the guanine base in the sequence 5'-CAG(Purine)-3' (Pu). The findings from these studies suggest that 5'-CAG(Pu)-3' is an additional characteristic target site for PhIP-guanine adduct-induced mutations in vivo in the mammary gland.