Date Published: June 13, 2019
Publisher: Public Library of Science
Author(s): E. L. Durham, C. Balog, R. N. Howie, M. A. Boyce, J. R. Arand, G. Warren, A. C. LaRue, J. J. Cray, Sakamuri V. Reddy.
Nicotine is known to affect cell proliferation and differentiation, two processes vital to proper development of the mandible. The mandible, the lower jaw in mammals and fish, plays a crucial role in craniofacial development. Malformation of the jaw can precipitate a plethora of complications including disrupting development of the upper jaw, the palate, and or impeding airway function. The purpose of this study was to test the hypothesis that in utero nicotine exposure alters the development of the murine mandible in a dose dependent manner. To test this hypothesis, wild type C57BL6 mice were used to produce in utero nicotine exposed litters by adding nicotine to the drinking water of pregnant dams at concentrations of 0 μg/ml (control), 50 μg/ml (low), 100 μg/ml (medium), 200 μg/ml (high) throughout pregnancy to birth of litters mimicking clinically relevant nicotine exposures. Resultant pups revealed no significant differences in body weight however, cephalometric investigation revealed several dimensions affected by nicotine exposure including mandibular ramus height, mandibular body height, and molar length. Histological investigation of molars revealed an increase in proliferation and a decrease in apoptosis with nicotine exposure. These results demonstrate the direct effects of nicotine on the developing mandible outside the context of tobacco use, indicating that nicotine use including tobacco alternatives, cessation methods, and electronic nicotine delivering products may disrupt normal growth and development of the craniofacial complex.
As of 2014, more than 3% of adults in the United States (US) (4% men, 3.5% women) used e-cigarettes every day or some days . This is similar to the 3.4% of the US population that uses smokeless tobacco, but still pales in comparison to the 26.1% of the US population that uses smoking tobacco. Altogether, more than 1 in 4 adults in the US are regularly exposed to nicotine . Nicotine, a powerful psychoactive stimulant drug and the primary compound found in tobaccos, most nicotine replacement therapeutics (NRT), as well as electronic nicotine delivering products (ENDS) has been linked to alterations of many cellular processes including cell proliferation, age-related diseases, and birth defects [3–7]. Despite the link between adverse birth outcomes of pre- and peri-natal nicotine exposure, research suggests 11% of US women continue to smoke or use alternative nicotine products through the third trimester of pregnancy [8, 9]. Nicotine has been observed to cross the placenta during pregnancy allowing for circulation and concentration in developing fetal tissues .
Our data indicate that in utero nicotine exposure negatively effects mandibular development, however, there was not a clear dose dependent response. Our confirmation of the exposure model by measuring the cotinine levels of the pre-treated dams highlights the vast range of cotinine levels that indicate active nicotine exposure [18, 19, 26]. The observed reduction in cotinine in the highest dose of nicotine (200 μg/ml) may indicate less consumption of water in those individuals, however animals were monitored for dehydration daily throughout the pretreatment, breeding, and pregnancy and no indication of dehydration was observed. This reduction in cotinine may also be related to variable metabolism of nicotine[17–19, 27]. Unlike in the human population, we did not observe a reduction in weight of the nicotine exposed animals 15 days post-natal, indicating that if there was a reduction in birth weight associated with in utero nicotine, it was regained quickly after birth in this murine model [3, 28].
These results demonstrate the direct effects of nicotine on the developing mandible, outside the context of tobacco use. The significant alteration to the growth of the ramus, body, and mandibular molars could have additional consequences as individuals continue to grow post-natally. Importantly, these data indicate that tobacco alternatives, including cessation methods and ENDS that incorporate nicotine, which may be marketed as safer than cigarette use, also disrupt normal growth and development of the craniofacial complex.