Joseph Bloom, Ph.D.
Project Summary:
Tobacco use is the largest cause of preventable mortality in the U.S. and worldwide. Given the public health consequences of cigarette smoking, improved success in quitting can have a huge positive impact on longevity and quality of life for millions of people. Altered nicotine metabolism, including natural variation in human nicotine-metabolism genes, is associated with differences in smoking-related behaviors. Specifically, we have shown that variation in the FMO3 gene, related to the nicotine N-oxidation metabolism pathway, is associated with differences in the levels of nicotine dependence among smokers. But the mechanism of FMO3’s effect on nicotine dependence is not understood.
Final Report:
The main objective of this project was to explore altered nicotine N-oxidation in a mouse model of nicotine-related phenotypes including nicotine withdrawal. We attempted to manipulate nicotine N-oxidation and nicotine-N-oxide levels in mice by three methods: 1) using an FMO-gene knockout mouse, 2) exposing mice to 3,3’-diindolymethane (DIM), and 3) by administering exogenous nicotine-N-oxide. DIM is a natural derivative of cruciferous vegetables that has been shown to alter FMO activity in humans and rats. However, we found that DIM’s effect on nicotine metabolism is very different in mice than in rats. In rats, DIM drastically reduces nicotine N-oxidation without altering another nicotine metabolism pathway, nicotine C-oxidation. In mice, however, DIM did not affect N-oxidation while increasing C-oxidation. This was true in both male and female mice from multiple mouse strains. Because differences in nicotine C-oxidation also impact cigarette consumption in smokers, we chose to pursue further experiments in DIM-fed mice despite this unexpected result.
Nicotine withdrawal was measured in both male and female mice of different strains under different experimental conditions using a variety of phenotypes, including changes in respiration, somatic signs (tremor, paw shaking, etc.), pain sensitivity, locomotion, anxiety, and conditioned place preference. Initial experiments were also performed under most conditions using acute nicotine as well as nicotine withdrawal. Unfortunately, although differences in some phenotypes (notably acute nicotine’s effect on respiration) could be detected between mouse strains, significant differences were not found between FMO knockout mice and sibling controls, or between DIM-fed mice and controls. Exogenous nicotine-N-oxide administration also did not alter nicotine withdrawal phenotypes. Therefore, we concluded that, in light of DIM’s unexpected effects, and our failure to detect significant differences between FMO knockout and control mice, the mouse is not an optimal model to study the effects of altered nicotine N-oxidation using the assays currently in our hands.
Given the exceptional public health consequences of cigarette smoking, improved cessation would have a huge positive impact on human longevity and quality of life. Variation in the FMO3 gene, responsible for nicotine N-oxidation, is associated with differences in liability to nicotine dependence among smokers.
