Carcinogenicity of nicotine and signal pathways in cancer progression: a review

Smoking is a major environmental factor causing nearly a third of cancer-related deaths. Nicotine accounts for 95% of tobacco’s alkaloid content and 1.5% of the commercial cigarette products weight. With most cigarettes containing 1.2–2.9 mg of nicotine, heavy smokers can absorb 20–40 mg/day, resulting in plasma concentrations of 23–35 ng/ml. Nicotine exert potential oncogenic effects primarily through nicotinic acetylcholine receptors, without involving signal pathways or other receptors. Recent reviews have recognized the relationship between nicotine and a certain type of tumor. However, there is no systematic overview on the signal pathways related to nicotine via receptors or non-receptors in a variety of tumors. This review discusses emerging evidence of the relationship between cancer progression and nicotine. The list of cancers reportedly connected to nicotine is expanding and presently contains lung carcinoma, as well as cervical, colon, gastric, pancreatic, breast, liver, bladder, head and neck cancers including nasopharynx, tongue and oral cavity. There is growing evidence through the use of animal xenograft models and cell culture systems, that (1) nicotine's carcinogenic role stems from multiple signaling mechanisms, primarily involving both non-receptor-mediated actions and receptor-mediated effects, including nicotinic acetylcholine receptors, β-adrenergic receptors and epidermal growth factor receptors, as well as transforming growth factor β receptors; (2) nicotine could induce chromosomal abnormalities, DNA damage, and micronuclei formation; (3) nicotine also can enhance oxidative stress, leading to tumor initiation or progression due to excessive production of reactive oxygen species. Based on these findings, nicotine seems to be a potent oncogenic agent in modulating tumor cell proliferation, invasion and migration by various signaling pathways associated with chemical carcinogenicity.