Tobacco smoking, E-cigarettes, and nicotine harm

Tobacco smoking has caused more than 20 million premature American deaths in the 50 y after the first US Surgeon General Report of Smoking and Health (1). The recognition of this major health hazard has led to efforts to both prevent the initiation of smoking and aid smokers in quitting, and most recently to an announced strategy of lowering nicotine in cigarettes and pushing those addicted to nicotine toward harm-reduction products. Nicotine is the addictive component in the >7,000 compounds and more than 70 identified chemical carcinogens in tobacco smoke. Nicotine alone has not been shown to be carcinogenic in animal bioassays, but can be metabolized to form carcinogenic tobacco-specific N-nitrosamines, including N-nitrosonornicotine (NNN) and a nicotine-derived nitrosamine ketone (NNK) (2). One could argue that without the addictive properties of nicotine, tobacco smoking would neither be popular nor a health problem, and lung cancer would not be the major lethal cancer worldwide (3). The delivery of nicotine in skin patches and chewing gum are considered as less-harmful alternatives to tobacco smoking and can aid tobacco smokers to quit (4). E-cigarettes are gaining in popularity not only as a purported smoking cessation strategy, but are also being promoted as being a “cool” habit in commercial and online media, much like tobacco cigarettes were promoted before commercials and other advertising were banned in the United States and other countries (https://en.wikipedia.org/wiki/Public_Health_Cigarette_Smoking_Act) (5). E-cigarettes contain nicotine dissolved into solvents, such as glycerol and propylene glycol. The nicotine is released in the aerosolized vapor produced by electronic heating within the E-cigarette. In PNAS, Lee et al. (6) present data showing that nicotine metabolites form DNA adducts and inhibit the DNA repair proteins XPC and OGG1/2 in human bronchial epithelial and urothelial cells in vitro and in the lungs, bladder, and heart of a mouse model inhaling E-cigarette vapor (ECV) in vivo.

Key hallmarks of chemical carcinogenesis include the metabolism of a chemical into an electrophilic metabolite that binds covalently to DNA and, if not enzymatically repaired, mutations occur. The accumulation of such mutations and other changes in the genomic landscape can lead to cancer (7). Many chemical carcinogens also damage DNA repair enzymes, so that the chemical-DNA adducts are not efficiently repaired (8). Nicotine is rapidly metabolized in vivo to cotinine and other metabolites, including a small portion of N-nitrosamines that may be further metabolized to methyl diazohydroxide (MDOH) and pyridyl-butyl derivatives (PBD) (2). Lee et al. (6) report that the MDOH- and PBD-derived electrophiles covalently bind to DNA in the lung, bladder, and heart of mice intermittently inhaling ECV for 12 wk at a dose equivalent to light E-cigarette smoking for 10 y. The inhibition of DNA repair proteins XPC and OGG1/2 in the lung was inversely related to the levels of the DNA adducts formed by PBD and MDOH (i.e., γ-OH-PdG and O6-medG). The next series of experiments utilized human bronchial epithelial and urothelial cells. Significantly, nicotine and NNK each induce the same types of γ-OH-PdG and O6-medG adducts and inhibit DNA repair in these human cell types. The results provide evidence of cross-animal species similarity in the pattern of DNA damage and increased human and public health relevance. Lee et al. then show that either nicotine or NNK enhanced the mutational susceptibility to either hydrogen peroxide or UV light and cell transformation of human lung or urothelial cells.

E-Cigarettes and Genetic Damage

The Lee et al. (6) study provides convincing data that nicotine in ECV can be metabolized to form DNA adducts, γ-OH-PdG and O6-medG, and can inhibit key enzymes involved in nucleotide excision repair and base excision repair. The next step in these evidence-based investigations would be the analysis of these nicotine-derived DNA adducts in human bronchial epithelial cells obtained by bronchoscopy or sputum from E-cigarette users. The methodology used in this study may be specific and sensitive enough for such studies. The measurement of the effects of DNA repair enzymes may also be possible. Although Lee et al. provide evidence that either nicotine or NNK increased the mutational susceptibility of human lung and bladder cells to UV light or hydrogen peroxide in vitro, mutagenicity of nicotine exposure alone was not detected. Additional studies using more sensitive mutagenesis assays are warranted. Whole-genome sequencing of the lung DNA from the ECV-exposed mice may reveal a mutational signature that could be compared with those found in human lung cancers in smokers and nonsmokers (9). The cell transformation assay (i.e., increased cell growth in soft agar) is also limited in predicting the carcinogenicity, if any, of nicotine. Longer-term exposure to ECV in animal models should also be further investigated.

Implications Toward Risk Assessment of E-Cigarettes

The implications of this study and others on the potential mechanisms of toxicity of E-cigarette vapor are of current interest to the Food and Drug Administration as it moves forward on its announced harm-reduction policy that includes use of noncombustible products. Because the Food and Drug Administration-Center for Tobacco Products uses a public health standard, such a policy must consider the risks and benefits of these products to the entire population. While the use of E-cigarettes as an alternative to tobacco smoking may benefit users of traditional tobacco, the increasing use of E-cigarettes among adolescent nonsmokers is a significant concern because evidence is accumulating that it can be a “nicotine gateway” to smoking tobacco (10, 11) and is associated with chronic bronchitis symptoms (12). One major issue is how best to interpret the results of research like that by Lee et al. (6) in risk assessment analysis. The findings of Lee et al. suggest that inhaling vapor from E-cigarettes, potentially for decades, may not fully mitigate the risk for cancer compared with the established increase in mortality and morbidity of conventional tobacco cigarettes in cancer epidemiological studies. Such epidemiological studies of E-cigarettes will require following cohorts using E-cigarettes in future decades. In the meantime, while more studies are needed, this kind of in vivo/in vitro approach will have value in the decision making related to an overall public health benefit standard of E-cigarettes.