GST
isoforms have been extensively studied in adult tissues but little is known about the composition and levels of these
enzymes in fetal tissues. As part of our ongoing studies to determine the potential role of metabolic
enzymes in mediating the differential susceptibility of different strains of mice to lung
tumorigenesis following in utero exposure to
3-methylcholanthrene (MC), we screened for GST
enzyme activity and for expression of the individual GSTalpha, pi, mu, and theta
isoforms in murine fetal lung and liver tissues isolated from the parental strains and F1 crosses between C57BL/6 (B6) and BALB/c (C) mice. Using
1-chloro-2,4-dinitrobenzene (CDNB) as a substrate, we found that treatment with MC had no effect on the levels of GST
enzyme activity in either the fetal lung or liver in either of the two parental strains or their F1 crosses. Low levels of expression of each of the four
enzymes were detected by Western blotting in both fetal lung and liver tissues in all four strains. A statistically significant 3.5-fold induction was observed only for GSTmu in the fetal lung of the parental strain of BALB/c mice 48 h after exposure to MC. None of the other
enzymes showed any significant differences in the levels of expression following exposure to MC. Although strain-specific differences in the expression of the
GSTs that were independent of MC treatment were observed, they could not account for the differences previously observed in either the Ki-ras mutational spectrum or lung
tumor incidence in the different strains of mice. Similar results were obtained when the maternal metabolism of MC was assayed in liver microsomal preparations. The results are consistent with previous studies showing low levels and poor inducibility of phase II
enzymes during gestation, and demonstrate for the first time that all four of the major GST
enzymes are expressed in fetal tissues. While the high inducibility of activating
enzymes, such as
Cyp1a1, and low, uninducible levels of phase II conjugating
enzymes probably account for the high susceptibility of the fetus to transplacentally induced
tumor formation, the results also suggest that factors other than metabolism may account for the strain-specific differences in susceptibility to
carcinogen-mediated lung
tumor induction following in utero exposure to chemical
carcinogens.