Exposure to electronic-cigarette (E-cig) smoke caused mice to develop lung cancer, a new study finds.

Published online October 7 in Proceedings of National Academy of Sciences (PNAS), the study found that 9 of 40 mice (22.5 percent) exposed to E-cig smoke with nicotine for 54 weeks developed lung adenocarcinomas. None of the 20 mice from the study exposed to the same E-cig smoke without nicotine developed cancer.

Led by Moon-shong Tang, PhD, of NYU School of Medicine, the study also found that 23 of 40 mice (57.5 percent) exposed to E-cig smoke developed bladder hyperplasia, genetic changes that make cells more likely to multiply, and a step toward abnormal tissue growth seen in cancer.

Only one of the 17 mice exposed to the zero-nicotine E-cig smoke developed hyperplasia.

Tang acknowledges the study's limitations, including that it was conducted in a relatively small number of mice susceptible to developing cancer over their lifetime (one-year study period designed to offset age-related cancer).

The study mice also did not inhale smoke like a human would, but, instead, were surrounded by it (whole-body exposure).

"Tobacco smoke is among the most dangerous environmental agents to which humans are routinely exposed, but the potential of E-cig smoke as a threat to human health is not yet fully understood," says Tang, a professor in the Departments of Environmental Medicine, Medicine, and Pathology. "Our study results in mice were not meant to be compared to human disease, but instead argue that E-cig smoke must be more thoroughly studied before it is deemed safe or marketed that way."

Inside Cells

The question of whether nicotine itself, separate from tobacco smoke, causes cancer is controversial because of conflicting study results over time that used oft-criticized methods. Almost all researchers agree, however, that chemicals added during the curing of tobacco - nitrate and nitrite - can cause a reaction called nitrosation (the addition of a particle called a nitrosonium ion), the authors say.

This is known to convert nicotine into nitrosamines such as NNN (N-nitrosonoricotine) and NNK (nicotine-derived nitrosamine ketone), proven carcinogens in mice and humans.

Conventional thinking, says Tang, has been that smoke from cured tobacco deposits nitrosamines into a smoker's organs and blood, with nitrosamine blood tests the best measure of their potential to cause cancer.

Such tests in a 2017 study found that levels of a compound related to NNK, called NNAL, were 95 percent lower in E-cig smokers than in tobacco smokers, leading some experts to conclude that a switch to E-cigs might save millions of lives. Partly as a result of such public messaging, 3.6 million junior high and high school students having embraced E-cigs, say the authors.

Against this backdrop, the new study finds that mammalian cells contain their own nitrosonium ions, which directly react with nicotine to form nitrosamines, including NNK. Many studies have also shown that human and mouse cells also have ample supplies of cytochrome p450, which further converts NNN and NNK into compounds (e.g. formaldehyde and CH3N=NOH) that can react with DNA to form damaging adducts (e.g. gamma-OH-PdG and O6-methyl-dG), the researchers say.

Tang's team had shown in a February 2018 PNAS article that E-cig smoke induces DNA damage in the mouse lung and bladder, and that nitrosation in cultured human lung and bladder cells converts nicotine into derivatives that increase DNA code changes (mutations) with the potential to transform normal cells into cancer cells. Specifically, the earlier study found that nicotine is transformed into nitrosamines, then into DNA damaging agents, which ultimately form DNA adducts.

The current study results confirm that nicotine from E-cig smoke can cause cancer in the lungs, and precancerous growth in the bladders, of mice. Furthermore, the results argue that nicotine, once inside cells, is converted into nitrosamines that do not leave cells and, therefore, could never be captured by tests that measure nitrosamine levels outside of cells (e.g. blood tests), says Tang.

"Our results support the argument that the nicotine-derived DNA adducts are likely the main causes for carcinogenesis in mice exposed to E-cig smoke," says study author Herbert Lepor, MD, the Martin Spatz Chair of Urology at NYU Langone Health. "Our next step in this line of work will be to expand the number of mice studied, to shorten and prolong E-cigarette exposure time, and to further investigate the genetic changes caused by E-cigarette smoke."

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Along with Tang and Lepor, study authors were Xue-Ru Wu, Hyun-Wook Lee, Yong Xia, Fang-Ming Deng, Andre Moreira, Lung-Chi Chen, and William Huang in the departments of Environmental Medicine, Medicine, Pathology and Urology at NYU School of Medicine.

The study was funded by National Institutes of Health grants R01CA190678, 1P01CA165980, P30CA16087 and ES00260.