Postulating that serotonin (5-HT), released from smoking-activated platelets could be involved in smoking-induced vascular modifications, we studied its catabolism in a series of 115 men distributed as current smokers (S), never smokers (NS) and former smokers (FS) who had stopped smoking for a mean of 13 years. 5-HT, monoamine oxidase (MAO-B) activities and amounts were measured in platelets, and 5-hydroxyindolacetic acid (5-HIAA)—the 5-HT/MAO catabolite—in plasma samples. Both platelet 5-HT and plasma 5-HIAA levels were correlated with the 10-year cardiovascular Framingham relative risk (P<0.01), but these correlations became non-significant after adjustment for smoking status, underlining that the determining risk factor among those taken into account in the Framingham risk calculation was smoking. Surprisingly, the platelet 5-HT content was similar in S and NS but lower in FS with a parallel higher plasma level of 5-HIAA in FS. This was unforeseen since MAO-B activity was inhibited during smoking (P<0.00001). It was, however, consistent with a higher enzyme protein concentration found in S and FS than in NS (P<0.001). It thus appears that MAO inhibition during smoking was compensated by a higher synthesis. To investigate the persistent increase in MAO-B protein concentration, a study of the methylation of its gene promoter was undertaken in a small supplementary cohort of similar subjects. We found that the methylation frequency of the MAOB gene promoter was markedly lower (P<0.0001) for S and FS vs. NS due to cigarette smoke-induced increase of nucleic acid demethylase activity. This is one of the first reports that smoking induces an epigenetic modification. A better understanding of the epigenome may help to further elucidate the physiopathology and the development of new therapeutic approaches to tobacco addiction. The results could have a larger impact than cardiovascular damage, considering that MAO-dependent 5-HT catabolism is also involved in addiction, predisposition to cancer, behaviour and mental health.

Smoking is a major risk factor of morbidity and mortality. It is well established that monoamine oxidase (MAO) activity is decreased in smokers. Serotonin (5-HT), a major substrate for MAO that circulates as a reserve pool stored in platelets, is a marker of platelet activation. We recently reported that smoking durably modifies the platelet 5-HT/MAO system by inducing a demethylation of the MAO gene promoter resulting in high MAO protein concentration persisting more than ten years after quitting smoking. The present data enlarges the results to another MAO substrate, norepinephrine (NE), further confirming the central role of MAO in tobacco use-induced diseases. Thus, MAO could be a readily accessible and helpful marker in the risk evaluation of smoking-related diseases, from cardiovascular and pulmonary diseases to depression, anxiety and cancer. The present review implements the new finding of epigenetic regulation of MAO and suggests that smoking-induced MAO demethylation can be considered as a hallmark of smoking-related cancers similarly to other aberrant DNA methylations.