Despite the countless destructive health consequences and carcinogenic properties of cigarette smoke, the CDC estimates that 40 million adults in the United States currently smoke cigarettes. The toxic chemicals are capable of causing damage to nearly every organ in the body and can lead to the development of cardiovascular disease, respiratory disease, and cancer. Researchers continue to add to the large body of evidence of the detrimental effects nicotine exposure has on cellular processes and our health. Now, they’re beginning to illuminate the potential epigenetic effects cigarette smoking may have on male reproduction using bisulfite conversion and sequencing techniques. In a recent study published in Biochemical and Biophysical Research Communications, scientists utilized a mouse model to demonstrate how nicotine may lead to critical epigenetic changes and cell death in mice testes.
In addition to the plethora of well-known health risks posed by cigarette smoking, research has shown that inhaling cigarette smoke can cause germ cell apoptosis, or programmed cell death, and actually reduce the quality of semen. Researchers from Shanghai Jiao Tong University and Shanghai Jiao Tong University Affiliated Sixth People’s Hospital sought to measure alterations to telomeres in mice testes and changes in the expression of genes related to apoptosis as a result of exposure to nicotine. The loss of telomeres, or the end structures that protect chromosomes, has been linked to apoptosis as well as aging.
In this study, forty male mice were given nicotine or saline injections for 5 weeks. The nicotine-treated mice were given a daily dose of nicotine that mimicked the levels seen in heavy smokers. After the treatments, the researchers collected testes and spermatozoa samples in order to detect telomerase activity and calculate apoptosis rate. They also investigated the expression of Nme2 promoter, which has been associated with reduction in telomerase activity and telomere length in vivo.
The team of researchers found that the apoptosis rate in mouse testes increased significantly as a result of exposure to nicotine. In other words, cells in the testes of nicotine-treated mice were dying at a faster rate compared to normal mice. They also reported that the nicotine group had an overall decrease in telomerase activity and shortened telomere length. As the rate of apoptosis increased, the telomere length and telomerase activity decreased, hinting that shorter telomere length may be more likely to lead to apoptosis.
The researchers then performed bisulfite conversion of genomic DNA followed by bisulfite sequencing to assess the DNA methylation changes of Nme2 promoter. Previous studies have shown that DNA methylation, a common epigenetic mechanism, can regulate the expression of genes in various ways as a result of tobacco smoking or nicotine. The researchers used the Methylamp DNA Modification Kit from EpiGentek to efficiently convert purified genomic DNA from the mice testes. This kit converts DNA so that 5-methylcytosine (5-mC), the methylated form of cytosine, can be detected in downstream applications for gene-specific DNA methylation analysis. In this case, the researchers successfully utilized bisulfite conversion of DNA and sequencing to analyze the methylation status of Nme2.
The research team uncovered hypomethylation in the Nme2 promoter, which was induced by nicotine exposure (Fig. 1). Ultimately, the group of scientists concluded that “nicotine treatment could upregulate Nme2 expression in murine testes via Nme2 promoter hypomethylation” which can inhibit the telomerase activity and shorten telomere length. These abnormal telomeres may then lead to apoptosis of testes cells.
This study exposes the potential epigenetic impact smoking cigarettes might have on the male reproductive system. But, the exact mechanisms of how nicotine epigenetically impacts gene expression or induces apoptosis in various cells have yet to be determined. By now, the detrimental effects of smoking on the body are well known, even by those who smoke. This research reinforces that understanding, adding to the growing list of reasons to kick this harmful habit, particularly while trying to conceive.
Source: Gu. Y. et al. (2016). Nicotine induces Nme2-mediated apoptosis in mouse testes. Biochemical and Biophysical Research Communications, in press.