MUTANT FREQUENCY OF LACI IN TRANSGENIC MICE FOLLOWING BENZO[A]PYRENE TREATMENT AND PARTIAL-HEPATECTOMY

The Big Blue(R) transgenic mouse provides an in vivo mutation system t hat permits the study of pharmacodynamic parameters on mutant frequenc y (MF) following xenobiotic exposure. We have studied the effects of c ellular proliferation on the frequency of mutations in the loci transg ene by evaluating the MF in the liver of male C57B1/6 Big Blue(R) mice following treatment with benzo[a]pyrene (B[a]P) and a partial hepatec tomy. Mice received either 40 mg/kg of B[a]P in corn oil or corn oil a lone by i.p. injection on three consecutive days, followed by a partia l hepatectomy on the fourth day. Three days later (i.e., 7 days follow ing the initial B[a]P injection), the animals were sacrificed and the MF in the liver was compared to the MF observed in the liver of the sa me mouse at the time of hepatectomy. Induction of cytochrome P-450 1A (CYP1A) following B[a]P treatment was evident by Western blot analysis . The MF in untreated control animals was not significantly different at hepatectomy (4.7 +/- 0.8 x 10(-5)) and 3 days later, at sacrifice ( 3.0 +/- 0.4 x 10(-5)). Neither was the MF observed in the B[a]P-treate d mice at the time of sacrifice (12.0 +/- 2.1 x 10(-5)) significantly different from the MF observed at the time of hepatectomy (10.6 +/- 5. 3 x 10(-5)). However, B[a]P-treatment resulted in a 4.0-fold increase in MF at sacrifice which was significantly different (p < 0.05), when compared to the untreated controls. The B[a]P-treated mice at hepatect omy showed a modest 2.2-fold increase in MF which was not statisticall y significantly different from the untreated controls. In addition, bo th control and B[a]P-treated tissues gave sectored mutant plaques. The sectored plaque frequency (SPF) was significantly elevated (p < 0.05) in the B[a]P-treated mice at hepatectomy (4.2 +/- 1.0 x 10(-5)) and s acrifice (7.3 +/- 2.4 x 10(-5)) as compared to the respective frequenc y in the control mice at hepatectomy (1.9 +/- 0.7 x 10(-5)) and sacrif ice (1.4 +/- 0.2 x 10(-5)). One explanation for this data is the persi stence of the B[a]P adducts in the mouse genomic DNA that was packaged into the lambda phage, and ultimately fixed as mutations in Escherich ia coli.