Human cell-mediated cytotoxicity, mutagenicity, and DNA adduct formation of 7H-dibenzo(c,g)carbazole and its N-methyl derivative in diploid human fibroblasts.

The aromatic N-heterocyclic 7H-dibenzo(c,g)carbazole (DBC), like polynuclear aromatic hydrocarbons, is a potent inducer of local sarcomas, papillomas, and respiratory tumors, but unlike such compounds it also induces hepatomas. The N-methyl derivative of DBC, N-methyl-dibenzo(c,g)carbazole (MeDBC), also induces sarcomas, papillomas, and respiratory tumors but at a lower frequency than DBC. However, MeDBC lacks hepatocarcinogenic potential, suggesting that DBC undergoes metabolic activation at its carbon atoms and also at the nitrogen position and that the N-7 position plays a role in liver carcinogenesis by DBC. We compared the cytotoxic and mutagenic potential of DBC and MeDBC using a human epithelial cell-mediated activation assay. Repair-deficient, diploid human fibroblasts derived from a xeroderma pigmentosum (XP) patient were used as target cells, and a human hepatoma cell line, Hs703T, was used as a source of exogenous metabolism. Resistance to 6-thioguanine served as the genetic marker for mutations. The Hs703T cells, but not the target XP cells, activated DBC and MeDBC into forms capable of interacting with DNA. In the cell-mediated assay, both DBC and MeDBC induced cytotoxicity and mutations in the target XP cells in a dose-dependent manner. However, DBC was effective at 2-fold lower concentrations than MeDBC. DNA adduct analysis using a 32P-postlabeling assay showed that at biologically significant low doses DBC was 2.5 times more effective than MeDBC in covalent binding to DNA. At higher doses, the difference in ability to bind to DNA was 1.3-fold. In both XP and Hs703T cells, DBC produced three major adducts, while MeDBC produced two major adducts, one of which was the same as one detected in the DBC adduct pattern. The number of DBC- and MeDBC-induced DNA adducts corresponding to a particular level of cytotoxicity and mutagenicity in the XP cells was 10 times lower than that for (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene.