Nicotine, a major component in tobacco, has been implicated as a potential factor that promotes the development of lung cancer. Studies have shown that 80–90% of inhaled nicotine is absorbed systemically during smoking, 1 mg from a single cigarette, resulting in plasma concentrations of about 15 ng/mL immediately after smoking. In vivo studies demonstrate nicotine promotes the growth of solid tumors, suggesting that nicotine may contribute to cell proliferation, invasion, and angiogenesis. Nicotine itself is not capable of causing tumors, because it does not have a chemical reactivity such as to cause mutagenic lesions to nucleic acids. However, it can fall into a class of molecules called promoters or co-carcinogens. How this was possible was a question that arose in the early 1990s, when the role of protein kinase C (PKC) in the induction of cell proliferation induced by promoters called phorbolic esters was being studied. Initially it was thought that nicotine could activate this protein directly, it was later confirmed that the action is mediated by the regular binding of nicotine to its surface receptors.
Persistent exposure of mouse epithelial cells to nicotine elicits Ras signaling and subsequent Raf/MAP kinase activity mediated by the class of nicotinic acetylcholine receptors (nAChRs), accompanied by a significant increase in cyclin D1 protein expression needed for cell replication. The sequence of events starts from the Ras/MAP kinase signaling to catch on the AP-1 transcription factor. Protein kinase C and PI-3-kinase are transiently activated after the treatment and the Ras-ERK1/2 signaling becomes sustained after long term exposure to nicotine. Furthermore, upon nicotine treatment, the cells do not arrest in the G(1) phase of the cell cycle following serum starvation, meaning that this substance is able to sustain cell replication in a relative absence of cell growth factors. The series of events that nicotine triggers starting from what has been said has been the subject of in-depth analysis over the last 25 years and has not been without surprises. A role for cellular chromatin remodeling has also been demonstrated.
For example, experiments on the effects of nicotine on the phases of the cell cycle in normal lung cells have shown that the role of the free radical-oncogenic axis is crucial in preparing the cell for subsequent tumor alterations. The most known example iof a regular alteration in lung cancer cells is the amplification of the DHFR gene. This encodes the dihydro-folate reductase enzyme, which is essential for the synthesis of nucleotide bases. But generally about preventing the whole, there is the famous p53 tumor suppressor gene, so much so that lung cells treated with nicotine and also exposed to the E6 protein of the papilloma virus to neutralize p53, are unable to amplify the DHFR gene. The process depends on PKC kinase, which explains why researchers originally thought nicotine could directly activate this protein. Following the activation of multiple transcription factors, nicotine is capable of doing much more. In fact, not only AP-1 is involved, but other DNA-binding proteins such as E2F1, Id-1, Elk-1, AP2a and even other transcription factors that should normally activate are in the embryonic state.
This latter process seems to take place regularly already within the tumor mass, especially in cells with stem cell potential. In these, the intracellular signal of nicotine converges on transcription factors such as Sox-2, Oct-4, GATA4 and GATA6, which are essential for maintaining the ability to self-renew. The last two, among the battery of genes under their control, provide for the proper expression of the nicotinic receptor, creating a biochemical maintenance loop. These results will also be relevant to many patients with small cell lung cancer exposed to nicotine via second-hand smoke, electronic cigarettes, and patches or gums to quit smoking. Over the past few years, e-cigarettes have been promoted as healthier alternatives to traditional cigarette smoking as they do not contain tobacco. Ecigs do surely not contain polycyclic aromatics and other carcinogenic nitrogen derivatives found in tobacco smoke, this is why they are deemed almost safe and not directly carcinogenic. Yet nicotine is still in the flavoured liquid letting people think this way nicotine alone would not cause cancer.
Not being able to cause direct DNA mutations that would accelerate malignant transformation, nicotine presumably has to act by persisting on conventional cellular transduction pathways to achieve the same goal. But it does it anyway, so much so that in vitro treatment of NHBE lung epithelial cells with nicotine transforms them from flat and fibroblast-like to neuronal-like. The effect appears to be mediated by the crosstalk between the nicotinic receptor and that of the EGF receptor, which converges in the activation of transcription factors typical of nerve cells, such as N-REST and Pax-3. The subsequent finding of the expression of a neuronal marker such as N-CAM cadherin confirms the process. Just a couple of week ago, a new report described how The process is more subtle than one might think because it even involves the remodeling of cellular biological rhythms. This phenomenon has recently been investigated in the context of another type of cancer, one of the established risk factors is digaretta smoke.
Worldwide, more than 1 million women are diagnosed with breast cancer every year and more than 410,000 die of the disease. Large cohort studies performed in the United States, Japan and China indicate that the risk of breast cancer is associated with active and passive smoking. By exposing normal mammary epithelium cells to nicotine in vitro, the cells reacted by varying two distinct gene batteries positive and negative. The skimming of the analysis data highlighted five genes with the highest expression (HPCAL4, TREM1, EFNA2, SPATA22 and KRT85) and five genes with the highest elimination (NPAS3, DEFB129, NEAT1, WDR96 and VRK2). Scientists were attracted by the very highest (HPCAL-4) and the very lowest (NPAS3) of the gene list. HPCAL4 is a relatively under-studied gene and is basically an calcium channel from neural origins, but it is reported to be overexpressed in lung carcinoma. It belongs to hippocalcin-like channels, being all of them ion channels that have been reported to enhance cell proliferation in brain cancers like glioblastoma.
The chromosome region 1p34.2, where HPCAL4 is located, is also associated with many types of cancer including breast, lung, neuroblastoma, and colorectal. The most down-regulated gene, NPAS3, instead has demonstrated tumor suppressive activity and, therefore, its down regulation in nicotine stressed cells is suggestive of NPAS3’s possible unknown role in increased cancer susceptibility from nicotine stress. However, it belongs to the large family of nuclear HLH-bZIP receptors, which also includes the transcription factors that regulate circadian rhythms, (CLOCK, NPAS2, BMAL1, BMAL2, PER1, PER2, PER3, CRY1 and CRY2. These nuclear factors allow or suppress the gene batteries responsible for maintaining daily circadian rhythms. The action of the dimer NPAS2-BMAL1 is regulated in a dependent manner on the levels of oxidative stress because these proteins want heme as a cofactor, the oxygen-carrying prosthetic group for hemoglobin, cytochromes and other respiratory or enzymatic proteins.
A correct ratio of reduced pyridine cofactors (NADH and NADPH) facilitates their work on gene expression. On the opposite, carbon monoxide counteracts their activity by binding to the iron atom of the heme. Needless to say, one of the major gaseous products of cigarette combustion is carbon monoxide. Nobody had ever known raised the possibility that carbon monoxide could contribute to the carcinogenic effect of the cigarette, as studies on its effects have always bee dedicated on cardiovascular health. Probably the carbon monoxide role in either lung or breast cancer is time-dependent and is only cooperative. Though not direct, the fact that this toxin suppresses the work of clock proteins whose network has been demostrated to be subverted in cancer, confirms that any form of smoking would be noxious to our health. About nicotine, in the contrary, there are more biological proofs underneath. In conclusion, conventional tobacco smoking for sure would reach the top damage, while tobacco-heated options could flat the probability.
About the e-cigs we keep on ourselves.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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