Findings reported in this journal supplement provide an important contribution to the better understanding of the smoking of mentholated cigarettes. To characterize fully the role of menthol in tobacco addiction and related health outcomes, pathways involved in the biological effects resulting from the use of mentholated cigarettes will also need to be elucidated. Research questions based on mechanisms suggested by which menthol could increase exposure and harm by enhancing initiation or inhibiting cessation have been well articulated. While some evidence may provide support for these hypotheses, the results are inconclusive and much additional research work remains.
A considerable body of research has examined the effects of menthol when it is administered alone and unburned . Menthol stimulates TRPM8 receptors, which are involved in its cooling and local anesthetic effects, as well as other members of the transient receptor potential (TRP) family of ion channels, TRPA1 and TRPV3. The TRPM8 receptors are also of interest because TRPM8 channel activation plays a major role in physiopathology . TRPM8 expression increases in the early stages of prostate cancer, as well as in other malignancies. Because TRPs are intimately linked with intracellular Ca2+ signaling, they are implicated in the control of cell cycle progression, cell migration and programmed cell death. Menthol may also regulate TRPM8-independent Ca2+ transport and cellular processes . Other effects of menthol, such as bronchodilation and respiration changes, have also been attributed to the effects of menthol on calcium conductance resulting in the inhibition of smooth muscle contraction and inhibition of sensory afferents . However, the continuing challenge is to determine the effects of menthol when administrated in the complex matrix of a cigarette smoke mixture. One of the unanswered questions is what happens to menthol in cigarette smoking? What new substances are emitted in cigarette smoke owing to the addition of menthol to the cigarette, and what are the effects of these substances? Little is known about the toxicity profile of menthol when it is burned in conjunction with tobacco, and the available data are conflicting. In one study, the pyrolysis of menthol produced a potent carcinogen, benzo[a]pyrene . In contrast, another study reported that little pyrolysis and combustion of menthol occurred during a cigarette puffing procedure . Further research is warranted using the more sophisticated technology now available.
The exact impact of menthol on smoking behavior remains to be understood. The preponderance of evidence is that menthol makes it harder to quit, and menthol makes it easier to start. However, a biological mechanism of action has not been determined. Clinical studies are providing some insights. Menthol inhibits metabolism of nicotine, resulting in a reduction in nicotine clearance . Inhibition of nicotine metabolism occurs both by slower oxidative metabolism to cotinine and by slower glucuronide conjugation. In vitro analysis has also shown that menthol inhibits the microsomal oxidation of nicotine to cotinine and the P450 2A6-mediated 7-hydroxylaton of coumarin . Okuyemi et al. , in a study assessing the efficacy of sustained-release bupropion for smoking cessation, found that menthol smokers had lower smoking cessation rates after 6 weeks of treatment. Can menthol interact with pharmacotherapeutic agents? More research is needed to elucidate the mechanisms by which menthol and nicotine interact with each other or other agents and potentially modify the tobacco user's exposure to one or the other, or additional compounds found in tobacco. Menthol can also activate several TRP excitatory ion channels affecting calcium conductance. If occurring in nerve cells containing neurotransmitters involved in drug reinforcement, then menthol may affect smoking behavior by enhancing the reinforcing effects of tobacco. More research on menthol receptor sites, as well as identification of central nervous system-mediated actions, will be useful in determining the mechanisms of action of menthol when present in cigarette smoke. Changes in gene expression in brain reward regions are thought to contribute to the pathogenesis and persistence of drug addiction . Increasing evidence suggests that these gene expression changes in neurons are mediated in part by epigenetic mechanisms that alter chromatin structure on specific gene promoters . Molecular and bioinformatic approaches are being used to understand the complex epigenetic regulation of gene expression by drugs of abuse. This approach provides an intriguing area of study for cigarette smoking and the effects of mentholated cigarettes. In vitro analysis has shown that nicotine can induce epigenetic changes . Using genome-wide techniques involving hybridizing immunoprecipitated chromatin to genome-wide promoter microarrays (ChIP-chip) or to high-throughput sequencing, a wealth of new information can be uncovered about epigenetic regulation in specific brain regions, as well as novel gene targets that control behavioral responses to mentholated cigarette smoking.
A limited number of epidemiological studies have been conducted examining the relationship between menthol cigarette smoking and disease risk [12,13]. Most of these studies have found no association with increased deaths from cancer, coronary heart disease or other cardiovascular diseases. However, several limitations have been noted, suggesting that the results are not yet conclusive. One issue has been the difficulty in classifying subjects as exclusively menthol or non-menthol smokers for a long enough time-period. Studies have not been designed to detect a small effect size that would be expected in comparing the risk for cancer from one cigarette brand or formulation to another. Parsing out the additional harm associated with smoking menthol cigarettes might require very large sample sizes or the exclusive use of menthol cigarettes for long periods. An interesting and helpful discussion that is developing is the need to expand the concept of harm; that is, to expand the perspective from that of increased toxicity, morbidity and/or mortality associated with menthol to one that considers other outcomes, such as birth outcomes, asthma, chronic obstructive pulmonary disease (COPD) or other ‘quality of life’ health outcomes. Studies of various cardiovascular parameters, for example, are emerging that demonstrate a greater effect in menthol smokers . Epidemiological studies have also tended to group people into categories that may be too heterogeneous to be informative. Studies are needed to identify population subgroups that may be of higher risk from mentholated cigarette smoking and to understand the determining factors. Another important consideration is that mentholated cigarette smoking does not occur in isolation. Therefore, studies, although complex, should include examining its interactive effects with other life-style and environmental factors, such as diet/nutrition, obesity, alcoholic intake, psychosocial stress, occupational exposures and the presence of other diseases/illnesses .
An alternate approach to assessing the hazards associated with menthol smoking has focused on its effect on the metabolism of tobacco smoke carcinogens. Although studies have been limited, activities of several metabolizing enzymes, including CYP3A4, N-acetyltransferase and UDP-glucuronosyltransferases, can be affected. An important observation was the finding that the 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL)-Gluc/NNAL ratio, a possible indicator of lung cancer risk, was significantly lower in menthol versus non-methanol smokers . In subsequent human liver microsomal studies, menthol inhibited the rate of NNAL-O-glucuronidation and NNAL-N-glucuronidation. Epigenetic mechanisms, including DNA methylation and histone modification, are also important in disease induction resulting from exposure to cigarette smoking [17,18]. Increased attention should be given to examining whether differential effects of non-mentholated versus mentholated cigarette smoking can be mediated through this biological pathway. Additionally, the epigenome is far more sensitive and responsive than the genome to environmental influences, suggesting that this pathway may be important in understanding potential interactive effects of life-style and environmental factors.
The use of the technologies of transcriptomics, proteomics and metabolomics should also be encouraged. Important leads could be gained from comparative studies involving gene expression profiling of cultured cells exposed to non-mentholated or mentholated cigarette smoke or tissues from smokers using microarray technology. Studies are beginning to demonstrate that current smokers can be distinguished from former smokers and non-smokers by their metabolic profiles . Can mentholated cigarette smokers be distinguished? As another area of interest, the availability of improved model systems will permit the development and testing of new hypotheses. Given the potential harm of mentholated cigarette smoking and associated unanswered questions, our research efforts should increase.