E-cigarettes induce in vivo toxicological effects that can raise the cancer risk

Introduction: Electronic cigarettes (e-cigs) are devices thought to deliver nicotine in a vaping solution without tobacco combustion. Perceived as a safer alternative to conventional cigarettes, e-cigs are aggressively marketed as lifestyle-choice consumables, thanks to few regulatory restrictions. E-cigs have also gained popularity among never-smokers and teenagers, becoming an emergent public health issue. Despite the burgeoning worldwide consumption, their safety remains largely unproven and it is unknown whether these devices could cause in vivo toxicological effects contributing to cancer. Materials and methods: Male Sprague Dawley rats were randomly split into two groups: non-exposed and whole-body mode exposed (10 animals per group). Treated animals consumed 1 mL/day of e-liquid containing 18 mg/mL of nicotine. During the experiment the e-cigarette voltage was set at 5.5 (and the wattage was about 15). Animals were exposed for 4 hours/day for 5 consecutive days/week, and for 4 consecutive weeks. Results: Toxic aldehydes production, due to thermal decomposition of glycerol, were detected in e-cig aerosol. In exposed lung, we observed a significant increase in phase I CYP1A1/2, CYP2B1/2, CY2C11 and CYP3A enzymes. The overproduction of reactive oxygen species resulting from CYP induction, together with the free radicals present in the aerosol, was confirmed by means “Electron paramagnetic resonance-radical probe” technique. Notably, we observed that the lung antioxidant enzymes catalase, DT-diaphorase, superoxide dismutase and the conjugating phase II glutathione S-transferases were all markedly reduced in animal treated. In addition, we found a markedly reduced FRAP (ferric reducing antioxidant power) value in the lung of e-cig rodents. Moreover, plasma FRAP levels and measurement of carbonyl residues, as biomarkers of oxidative injury to proteins, were inversely correlated in e-cig vapour-exposed rats. We also found that 8-hydro-2′-deoxyguanosine markedly increased in the lungs of treated rats. Interestingly, e-cigs produce extensive DNA damage in leukocytes (Comet test). We also found that e-cig vapour determines an increase in the percentage of immature micronucleated reticulocytes over normal reticulocyte. Next, the urine of e-cig-exposed animals induced a dose-dependent increase in the number of S. typhimurium revertants in different strains. Discussion and conclusion: We found that e-cigs have a powerful booster effect on phase-I carcinogen-bioactivating enzymes, and increase oxygen free radical production and DNA oxidation to 8-hydroxy- 2′-deoxyguanosine. Furthermore, we found that e-cigs damage DNA not only at chromosomal level in peripheral blood, such as strand breaks in leucocytes and micronuclei formation in reticulocytes, but also at gene level such as point mutations in urine. As these detrimental phenomena are typically induced by conventional cigarettes, the erroneous belief that e-cigs are safe should be retracted and suitable measures implemented to protect public health. Our study should be seen as the starting point for further investigations designed to confirm the harmful health impact of e-cigs, particularly after long-term exposure and under different usage conditions.