UDP-glucuronosyltransferases (UGTs) are cytoprotective and may also be genoprotective. Since over 10% of the population have hereditary deficiencies in UGTs, this family of enzymes could constitute an important determinant of susceptibility to chemical carcinogenesis, teratogenesis, and neurodegeneration. Fibroblasts contain Phase I and II drug-metabolizing enzymes, including UGTs, and undergo mitosis, rendering them susceptible to xenobiotic genotoxicity associated with micronucleus formation, which is thought to reflect carcinogenic initiation. Accordingly, skin fibroblasts may provide an accessible model for elucidating genoprotective mechanisms in both animals and humans and for characterizing the potential role of UGTs as determinants of individual toxicological susceptibility. To test this hypothesis, the carcinogen/teratogen benzo(a)pyrene [B(a)P], or its noncarcinogenic B(e)P isomer, was incubated with cultured skin fibroblasts obtained from male RHA-J/J rats. These rats have a hereditary homozygous deficiency in bilirubin UGT and demonstrate reduced xenobiotic glucuronidation, enhanced cytochrome P-450-catalyzed bioactivation, covalent binding, and toxicity of acetaminophen and B(a)P. Control fibroblasts were cultured from UGT-normal congenic homozygous male RHA-+/+ rats and male Wistar rats. The cells were incubated with 10 µm B(a)P or B(e)P either for assessment of micronucleus formation or for quantifying the bioactivation and covalent binding of B(a)P and the glucuronidation of its hydroxylated metabolites. Compared to control fibroblasts incubated only with buffer, micronucleus formation was not enhanced by either DMSO vehicle or B(e)P. In contrast, B(a)P significantly enhanced micronucleus formation in all cells, and UGT-deficient cells (RHA-J/J) had a >2-fold higher B(a)P-initiated micronucleus formation compared to UGT-normal cells (RHA-+/+) (P < 0.05). Glucuronidation of total B(a)P metabolites was 10% lower in RHA-J/J UGT-deficient fibroblasts, and the covalent binding of B(a)P to protein, reflective of an electrophilic reactive intermediate and DNA-alkylating agent, was up to 3-fold higher in RHA-J/J UGT-deficient fibroblasts or fibroblast homogenates compared to UGT-normal controls (P < 0.05). In fibroblast homogenates, addition of the UGT cosubstrate UDP-glucuronic acid reduced B(a)P covalent binding, corroborating the cytoprotective importance of UGTs. There was a highly significant correlation between decreasing glucuronidation of B(a)P metabolites and increasing bioactivation and covalent binding of B(a)P (r = -0.889; P = 0.018) in fibroblasts from RHA-J/J and RHA-+/+ rat strains, indicating an important genoprotective role for UGT. These results provide the first evidence that hereditary UGT deficiencies may enhance susceptibility to chemical carcinogenesis and suggest that skin fibroblasts may provide a useful and highly sensitive model for human risk assessment.
This research was supported by a grant to P. G. W. from the Medical Research Council of Canada. D. S. V. was supported in part by a Merck Frosst Postgraduate Fellowship Award provided jointly by Merck Frosst Canada, Inc., and the Medical Research Council of Canada. Preliminary reports of this work were presented at the annual meeting of the Society of Toxicology, New Orleans, LA, March 1993, and at the 5th North American Meeting of the International Society for the Study of Xenobiotics, Tucson, AZ, October 1993.