Structure, Tumorigenicity, Microsomal Metabolism, and DNA Binding of 7-Nitrodibenz[a,h]anthracene

Abstract

It has been previously proposed that a nitropolycyclic aromatic hydrocarbon (nitro-PAH) with its nitro functional group perpendicular or nearly perpendicular to the aromatic moiety exhibits lower tumorigenicity than the corresponding parent aromatic hydrocarbon. We also hypothesized that reduction of the nitro group is not involved, or contributed less significantly in the metabolic activation of this class of nitro-PAHs. To verify this hypothesis, we selected 7-nitrodibenz[a,h]anthracene (7-NDB[a,h]A) for study. The X-ray crystallographic structure of 7-NDB[a,h]A was determined and indicated that the dihedral angle between the nitro functional group and the aromatic dibenz[a,h]anthracenyl moiety was 80.6 degrees, indicating the nitro group preferentially adopts a nearly perpendicular orientation. The tumorigenicity of 7-NDB[a,h]A and dibenz[a,h]anthracene (DB[a,h]A) was determined in the male B6C3F1 neonatal mouse. Mice were administered ip injections of 1/7, 2/7, and 4/7 of the total dose of 7-NDB[a,h]A (400 nmol in 35 microL of DMSO per mouse) within 24 h of birth and at 8 and 15 days of age, respectively, and sacrificed at 12 months of age. DB[a,h]A induced 78 and 96% hepatocellular adenomas and carcinomas, respectively. However, 7-NDB[a,h]A induced only 50 and 8% hepatocellular adenomas and carcinomas compared with the 8 and 4% hepatocellular adenomas and carcinomas induced by the solvent vehicle, DMSO. Aerobic metabolism of 7-NDB[a,h]A by liver microsomes of 15-day old male B6C3F1 neonatal mice resulted in trans-3,4-dihydroxy-3, 4-dihydro-7-nitrodibenz[a,h]anthracene (7-NDB[a,h]A trans-3, 4-dihydrodiol) and trans-10,11-dihydroxy-10, 11-dihydro-7-nitrodibenz[a,h]anthracene (7-NDB[a,h]A trans-10, 11-dihydrodiol) as predominant metabolites. Under anaerobic conditions, 7-NDB[a,h]A was not metabolized (nitroreduced). The DNA adduct levels in liver and lung tissues of male B6C3F1 mice treated with 7-NDB[a,h]A and sacrificed 24 h and 6 days after final dosing were determined by 32P-postlabeling/TLC. In all cases, the DNA adducts derived from 7-NDB[a,h]A trans-3,4-dihydrodiol and 7-NDB[a, h]A trans-10,11-dihydrodiol were formed. These results suggest that both of the metabolites, 7-NDB[a,h]A trans-3,4-dihydrodiol and 7-NDB[a,h]A trans-10,11-dihydrodiol, are involved in the metabolic activation of 7-NDB[a,h]A, leading to tumor induction in the neonatal mouse. Thus, our results described in this paper support our hypotheses that a nitro-PAH with a perpendicular nitro orientation exhibits lower tumorigenicity than the corresponding parent PAH and that nitroreduction contributes less significantly in the metabolic activation.

Effect of Nitro Orientation on Ras -Protooncogene Mutation in Liver Tumors from 7-Nitrodibenz[ a,h ]anthracene-Treated Mice

“…The mutations from mice treated with DB[a,h]A, and the ras mutations from 7-NDB[a,h]A mice were evenly distributed between K-ras codon 13 and H-ras codon 61. The nitro group of 7-NDB[a,h]A preferentially adopts a perpendicular orientation, with a 80.6 • dihedral angle between the nitro functional group and the aromatic ring (5). In our previous studies (1)(2)(3)(4)(5), we have demonstrated that a nitro substituent perpendicular or nearly perpendicular to the aromatic plane results in low DNA binding, weak or no direct-acting bacterial mutagenicity, and low levels of reductive metabolism.…”

“…The nitro group of 7-NDB[a,h]A preferentially adopts a perpendicular orientation, with a 80.6 • dihedral angle between the nitro functional group and the aromatic ring (5). In our previous studies (1)(2)(3)(4)(5), we have demonstrated that a nitro substituent perpendicular or nearly perpendicular to the aromatic plane results in low DNA binding, weak or no direct-acting bacterial mutagenicity, and low levels of reductive metabolism. Observations from the present study indicate that this nitro group orientation may also result in a low tumorigenic response, a low frequency of tumor ras-oncogene mutation, and a low frequency of the K-ras codon 13 GGC → CGC mutation that appears to be common for tumors induced by PAHs and substituted PAHs.…”

“…We have been interested in the structure-activity relationships of PAHs and nitro-PAHs (1)(2)(3)(4)(5). Nitro-PAHs comprise a class of genotoxic environmental contaminants that can be considered separately or as derivatives of PAHs.…”

Environmental nitration processes enhance the mutagenic potency of aromatic compounds

“…These molecules react, directly or upon esterification of the N‐hydroxy function, with DNA to form covalent adducts (Ritter et al, 2002). The reduction rate increases with the number and the position of the nitro groups in the same way (Fu et al, 1998). This type of reaction promotes also the formation of reactive oxygen species (ROS) which could lead to oxidative DNA damage (Nagy et al, 2005).…”

Carbocations from dibenz[a,j]anthracene and dibenz[a,h]anthracene, their methylated derivatives, and oxidized metabolites: A stable ion and DFT study

“…14 The 7-nitro-DB[a,h]A, with a buttressed nitro group at the meso-position, is also metabolized to the 3,4-and 10,11-dihydrodiols which formed covalent DNA adducts. 15 As for DB[a,c]A, it is an active tumorigen that formed the 10,11-, 3,4-and the 1,2-dihydrodiols in metabolic studies. 16 Several studies have dealt with the synthesis of these dihydrodiols.…”