MUTAGENIC RESPONSE OF THE ENDOGENOUS HPRT GENE AND LACL TRANSGENE IN BENZO[A]PYRENE-TREATED BIG BLUE(TM) B6C3F1 MICE

Big Blue(TM) (BB) and generic B6C3F1 mice were given one to three i.p. injections of 50 mg/kg benzo[a]pyrene (B[a]P) in DMSO every other day to achieve cumulative doses of 50 to 150 mg/kg. Three weeks after tre atment, the mutation Frequency at the endogenous hprt gene and lacl tr ansgene was measured in splenic T cells. Generic mice given 50, 100, a nd 150 mg/kg B[a]P displayed induced hprt(-) Frequencies (observed hpr t(-) frequency minus control frequency) of 5.5 +/- 1.0, 11 +/- 2.0, an d 19 +/- 2.6 x 10(-6), respectively (average +/- SEM). In contrast, BB mice given 50 and 150 mg/kg B[a]P displayed induced hprt(-) frequenci es of 0.9 +/- 0.6 and 9.1 +/- 1.5 x 10(-6). P-32 postlabeliing reveale d that the lower hprt response in BE mice correlated with lower amount s of BP-DNA adducts in spleen, liver, and lung 24 hours after B[a]P ex posure. Western blot analysis of liver samples from B[a]P-treated mice suggests that the reduced adduct load in turn may be due to lower P45 0 1A1 levels in BE mice. The frequency of induced, nonsectored blue pl aques (observed blue plaque frequency minus control frequency) in BE m ice receiving 50 and 150 mg/kg B[a]P was 41 +/- 9 and 134 +/- 10 x 10( -6) (15- to 40-fold higher than the induced hprt(-) frequency in the s ame treated animals). Sectored plaques were observed in both control a nd B[a]P groups but their frequency showed no relationship to dose (se ctored frequency in ail groups was approximately 20 x 10(-6)). To test whether persistent DNA adducts in the packaged lambda vector were con tributing to the observed blue plaque frequency, purified lambda-LIZ D NA was treated in vitro with B[a]P diet epoxide (BPDE), packaged, and plated on E. coil lawn cells. Treatment with BPDE did not produce sign ificant increases in homogeneous blue plaques, suggesting that the maj ority of mutants obtained from B[a]P-treated BE mice occurred in vivo. These results indicate that B[a]P exposure produces many more mutatio ns at the lacl transgene than at the endogenous hprt locus. (C) 1996 W iley-Liss. Inc.