Computational insights into the mutagenicity of two tobacco-derived carcinogenic DNA lesions

Brain distribution and metabolic profiling of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in rats investigated by UHPLC-HRMS/MS following peripheral administration

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is known to be a tobacco-specific N-nitrosamine and has peripheral carcinogenic properties. It can also induce oxidative stress, glial cell activation, and neuronal damage in the brain. However, the distribution and metabolic characteristics of NNK in the central nervous system are still unclear. Here, a sensitive and effective UHPLC-HRMS/MS method was established to identify and investigate the metabolites of NNK and their distribution in the rat brain. In addition, the pharmacokinetic profiles were simultaneously investigated via blood-brain synchronous microdialysis. NNK and its seven metabolites were well quantified in the hippocampus, cortex, striatum, olfactory bulb, brain stem, cerebellum, and other regions of rat brain after peripheral exposure (5 mg/kg, i.p.). The average content of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) in all brain regions was at least threefold higher than that of NNK, indicating a rapid carbonyl reduction of NNK in the brain. Lower concentrations of pyridine N-oxidation products in the cortex, olfactory bulb, hippocampus, and striatum might be related to the poor detoxification ability in these regions. Compared to α-methyl hydroxylation, NNK and NNAL were more inclined to the α-methylene hydroxylation pathway. Synchronous pharmacokinetic results indicated that the metabolic activity of NNK in the brain was different from that in the blood. The mean α-hydroxylation ratio in the brain and blood was 0.037 and 0.161, respectively, which indicated poor metabolic activity of NNK in the central nervous system.

Multiscale computational investigations of the translesion synthesis bypass of tobacco-derived DNA adducts: critical insights that complement experimental biochemical studies

Among the numerous agents that damage DNA, tobacco products remain one of the most lethal and result in the most diverse set of DNA lesions. This perspective aims to provide an overview of computational work conducted to complement experimental biochemical studies on the mutagenicity of adducts derived from the most potent tobacco carcinogen, namely 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (nicotine-derived nitrosaminoketone or NNK). Lesions ranging from the smallest methylated thymine derivatives to the larger, flexible pyridyloxobutyl (POB) guanine adducts are considered. Insights are obtained from density functional theory (DFT) calculations and molecular dynamics (MD) simulations into the damaged nucleobase and nucleoside structures, the accommodation of the lesions in the active site of key human polymerases, the intrinsic base pairing potentials of the adducts, and dNTP incorporation opposite the lesions. Overall, the computational data provide atomic level information that can rationalize the differential mutagenic properties of tobacco-derived lesions and uncover important insights into the impact of adduct size, nucleobase, position, and chemical composition of the bulky moiety.

Metabolism and DNA Adduct Formation of Tobacco-Specific N-Nitrosamines

The tobacco-specific N-nitrosamines 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) always occur together and exclusively in tobacco products or in environments contaminated by tobacco smoke. They have been classified as "carcinogenic to humans" by the International Agency for Research on Cancer. In 1998, we published a review of the biochemistry, biology and carcinogenicity of tobacco-specific nitrosamines. Over the past 20 years, considerable progress has been made in our understanding of the mechanisms of metabolism and DNA adduct formation by these two important carcinogens, along with progress on their carcinogenicity and mutagenicity. In this review, we aim to provide an update on the carcinogenicity and mechanisms of the metabolism and DNA interactions of NNK and NNN.

Structural Rationalization for the Nonmutagenic and Mutagenic Bypass of the Tobacco-Derived O4-4-(3-Pyridyl)-4-oxobut-1-yl-thymine Lesion by Human Polymerase η: A Multiscale Computational Study

Tobacco-derived pyridyloxobutyl (POB) DNA adducts are unique due to the large size and flexibility of the alkyl chain connecting the pyridyl ring to the nucleobase. Recent experimental work suggests that the O4-4-(3-pyridyl)-4-oxobut-1-yl-T (O4-POB-T) lesion can undergo both nonmutagenic (dATP) and mutagenic (dGTP) insertion by the translesion synthesis (TLS) polymerase (pol) η in human cells. Interestingly, the mutagenic rate for O4-POB-T replication is reduced compared to that for the smaller O4-methylthymine (O4-Me-T) lesion, and O4-POB-T yields a different mutagenic profile than the O2-POB-T variant (dTTP insertion). The present work uses a combination of density functional theory calculations and molecular dynamics simulations to probe the impact of the size and flexibility of O4-POB-T on pol η replication outcomes. Due to changes in the Watson-Crick binding face upon damage of canonical T, O4-POB-T does not form favorable hydrogen-bonding interactions with A. Nevertheless, dATP is positioned for insertion in the pol η active site by a water chain to the template strand, which suggests a pol η replication pathway similar to that for abasic sites. Although a favorable O4-POB-T:G mispair forms in the pol η active site and DNA duplexes, the inherent dynamical nature of O4-POB-T periodically disrupts interstrand hydrogen bonding that would otherwise facilitate dGTP insertion and stabilize damaged DNA duplexes. In addition to explaining the origin of the experimentally reported pol η outcomes associated with O4-POB-T replication, comparison to structural data for the O4-Me-T and O2-POB-T adducts highlights an emerging common pathway for the nonmutagenic replication of thymine alkylated lesions by pol η, yet underscores the broader impacts of bulky moiety size, flexibility, and position on the associated mutagenic outcomes.

Recent Studies on DNA Adducts Resulting from Human Exposure to Tobacco Smoke

DNA adducts are believed to play a central role in the induction of cancer in cigarette smokers and are proposed as being potential biomarkers of cancer risk. We have summarized research conducted since 2012 on DNA adduct formation in smokers. A variety of DNA adducts derived from various classes of carcinogens, including aromatic amines, polycyclic aromatic hydrocarbons, tobacco-specific nitrosamines, alkylating agents, aldehydes, volatile carcinogens, as well as oxidative damage have been reported. The results are discussed with particular attention to the analytical methods used in those studies. Mass spectrometry-based methods that have higher selectivity and specificity compared to 32P-postlabeling or immunochemical approaches are preferred. Multiple DNA adducts specific to tobacco constituents have also been characterized for the first time in vitro or detected in vivo since 2012, and descriptions of those adducts are included. We also discuss common issues related to measuring DNA adducts in humans, including the development and validation of analytical methods and prevention of artifact formation.