Nicotine-derived nitrosamine ketone (NNK) is one of the key

tobacco-specific nitrosamines Tobacco-specific nitrosamines (TSNAs) comprise one of the most important groups of carcinogens in tobacco products, particularly cigarettes (traditional and electronic) and fermented dipping snuff. Background These nitrosamine carcinogens are fo ...

derived from nicotine. It plays an important role in

carcinogenesis Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cell (biology), cells are malignant transformation, transformed into cancer cells. The process is characterized by changes at the cellular, G ...

. The conversion of

nicotine Nicotine is a natural product, naturally produced alkaloid in the nightshade family of plants (most predominantly in tobacco and ''Duboisia hopwoodii'') and is widely used recreational drug use, recreationally as a stimulant and anxiolytic. As ...

to NNK entails opening of the

pyrrolidine Pyrrolidine, also known as tetrahydropyrrole, is an organic compound with the molecular formula (CH2)4NH. It is a cyclic secondary amine, also classified as a saturated heterocycle. It is a colourless liquid that is miscible with water and most ...

ring.

Synthesis and occurrence

NNK can be produced by standard methods of

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

Tobacco

NNK is both found in cured tobacco and is produced during its burning (pyrolysis). The amount of NNK delivered in cigarette smoke ranged from 30 to 280 ng/cigarette in one study and 12 to 110 ng/cigarette in another. Sun-cured tobaccos ("Oriental") contain very little NNK and other TSNAs due to low-nitrate soil, lack of nitrate fertilizer, and sun-curing. Flue-cured tobacco (a.k.a. "Virginia" tobacco), especially when using an open flame, contains most of the NNK in American blended tobaccos although Marlboro's "virginia blend" had the lowest levels of NNK per nicotine out of many tested with the exception of Natural American Spirit.

e-Cigarettes

e-Cigarettes do not convert nicotine to NNK due to their lower operating temperatures. The amount of NNK delivered by e-cigarettes reaches 2.8 ng per 15 puffs (approximately 1 cigarette). NNK was found in 89% of Korean

e-cigarette liquids An electronic cigarette is a handheld battery-powered vaporizer that simulates smoking, but without tobacco combustion. E-cigarette components include a mouthpiece (drip tip), a cartridge (liquid storage area), a heating element/ atomizer, a mi ...

. Concentrations range from 0.22 to 9.84 μg/L. For the product that had the highest amount, if 1 ml is equivalent to 20 cigarettes, there would be 9.84/20 = 0.5 ng NNK per e-cig cigarette dose. Cigarettes with 1 gram of tobacco average about 350 ng.

Biology

Metabolism

NNK is initially a procarcinogen that needs activation to exert its effects. The activation of NNK is done by enzymes of the cytochrome pigment (CYP) multigene family. These enzymes catalyze hydroxylation reactions. Besides the CYP family, NNK can also be activated by metabolic genes, like myeloperoxidase (MPO) and epoxide hydrolase (EPHX1). NNK can be activated by two different routes, the oxidative path and the reductive path. In the oxidative metabolism NNK undergoes an α-hydroxylation catalyzed by

cytochrome P450 Cytochromes P450 (P450s or CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that mostly, but not exclusively, function as monooxygenases. However, they are not omnipresent; for examp ...

. This reaction can be done by two pathways, namely by α-methylhydroxylation or by α-methylenehydroxylation. Both pathways produce the carcinogenic metabolized isoform of NNK, NNAL. In the reductive metabolism NNK undergoes either a carbonyl reduction or a pyridine N-oxidation, both producing NNAL. NNAL can be detoxified by

glucuronidation Glucuronidation is often involved in drug metabolism of substances such as drugs, pollutants, bilirubin, androgens, estrogens, mineralocorticoids, glucocorticoids, fatty acid derivatives, retinoids, and bile acids. These linkages involve gly ...

, producing non-carcinogenic compounds known as NNAL-Glucs. The glucuronidation can take place on the oxygen next to the ring (NNAL-O-Gluc), or it takes place on the nitrogen inside the ring(NNAL-N-Gluc). The NNAL-Glucs are then excreted by the kidneys into the urine.

Signaling pathways

Once NNK is activated, it initiates a cascade of signaling pathways (for example ERK1/2,

NF-κB Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a family of transcription factor protein complexes that controls transcription (genetics), transcription of DNA, cytokine production and cell survival. NF-κB is found i ...

, PI3K/Akt,

MAPK A mitogen-activated protein kinase (MAPK or MAP kinase) is a type of serine/threonine-specific protein kinases involved in directing cellular responses to a diverse array of stimuli, such as mitogens, osmotic stress, heat shock and proinflamm ...

, FasL,

K-Ras ''KRAS'' (Kirsten rat sarcoma virus) is a gene that provides instructions for making a protein called K-Ras, a part of the RAS/MAPK pathway. The protein relays signals from outside the cell to the cell's nucleus. These signals instruct the cell ...

), resulting in uncontrolled cellular proliferation and tumorigenesis. NNK activates μ en m-calpain kinase, which induces lung metastasis via the ERK1/2 pathway. This pathway upregulates cellular myelocytomatosis (

c-Myc ''Myc'' is a family of regulator genes and proto-oncogenes that code for transcription factors. The ''Myc'' family consists of three related human genes: ''c-myc'' ( MYC), ''l-myc'' ( MYCL), and ''n-myc'' ( MYCN). ''c-myc'' (also sometimes ...

) and B cell leukemia/lymphoma 2 (

Bcl-2 Bcl-2, encoded in humans by the ''BCL2'' gene, is the founding member of the Bcl-2 family of regulator proteins. BCL2 blocks programmed cell death (apoptosis) while other BCL2 family members can either inhibit or induce it. It was the first a ...

) in which the two oncoproteins are involved in cellular proliferation, transformation and apoptosis. Also NNK promotes cell survival via phosphorylation with cooperation of c-Myc and Bcl-2 causing cellular migration, invasion and uncontrolled proliferation. The ERK1/2 pathway also phosphorylates NF-κB causing an upregulation of

cyclin D1 Cyclin D1 is a protein that in humans is encoded by the ''CCND1'' gene. Gene expression The CCND1 gene encodes the cyclin D1 protein. The human CCND1 gene is located on the long arm of chromosome 11 (band 11q13). It is 13,388 base pairs lo ...

, a G1 phase regulator protein. When NNK is present it directly involves cellular survival dependent on NF-κB. Further studies are needed to better understand NNK cellular pathways of NF-κB. The phosphoinositide 3-kinase (PI3K/Akt) pathway is also an important contributor to NNK-induced cellular transformations and metastasis. This process ensures the proliferation and survival of tumorigenic cells. The ERK1/2 and Akt pathways show consequential changes in levels of protein expression as a result of NNK-activation in the cells, but further research is needed to fully understand the mechanism of NNK-activated pathways.

Pathology

Toxicity

NNK is known as a

mutagen In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer in ...

, which means it causes polymorphisms in the human genome. Studies showed that NNK induced gene polymorphisms in cells that involve in cell growth, proliferation and differentiation. There are multiple NNK dependent routes that involve cell proliferation. One example is the cell route that coordinates the downregulation of

retinoic acid receptor beta Retinoic acid receptor beta (RAR-beta), also known as NR1B2 (nuclear receptor subfamily 1, group B, member 2) is a nuclear receptor that in humans is encoded by the ''RARB'' gene. Function This gene encodes retinoic acid receptor beta, a memb ...

(RAR-β). Studies showed that with a 100 mg/kg dose of NNK, several

point mutations A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product—consequences ...

were formed in the RAR-β gene, inducing

tumorigenesis Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abn ...

in the lungs. Other genes affected by NNK include

sulfotransferase In biochemistry, sulfotransferases (SULTs) are transferase enzymes that catalyze the transfer of a sulfo group () from a donor molecule to an acceptor alcohol () or amine (). The most common sulfo group donor is 3'-phosphoadenosine-5'-phosphos ...

1A1 (SULT1A1),

transforming growth factor beta Transforming growth factor beta (TGF-β) is a multifunctional cytokine belonging to the transforming growth factor superfamily that includes three different mammalian isoforms (TGF-β 1 to 3, HGNC symbols TGFB1, TGFB2, TGFB3) and many other ...

(TGF-β), and

angiotensin II Angiotensin is a peptide hormone that causes vasoconstriction and an increase in blood pressure. It is part of the renin–angiotensin system, which regulates blood pressure. Angiotensin also stimulates the release of aldosterone from the ...

(AT2). NNK plays a very important role in

gene silencing Gene silencing is the regulation of gene expression in a cell to prevent the expression of a certain gene. Gene silencing can occur during either Transcription (genetics), transcription or Translation (biology), translation and is often used in res ...

, modification, and functional disruption which induce

Inhibition

Chemical compounds derived from

cruciferous vegetable Cruciferous vegetables are vegetables of the family Brassicaceae (also called Cruciferae) with many genera, species, and cultivars being raised for food production such as cauliflower, cabbage, kale, garden cress, bok choy, broccoli, Brussels sp ...

s and

EGCG Epigallocatechin gallate (EGCG), also known as epigallocatechin-3-gallate, is the ester of epigallocatechin and gallic acid, and is a type of catechin. EGCG – the most abundant catechin in tea – is a polyphenol under basic research for its ...

inhibit lung

by NNK in

animal model An animal model (short for animal disease model) is a living, non-human, often genetic-engineered animal used during the research and investigation of human disease, for the purpose of better understanding the disease process without the risk of ha ...

s. Whether these effects have any relevance to human health is unknown and is a subject of ongoing research.

References

{{reflist

External links

MSDS
Nitrosamines 3-Pyridyl compounds IARC Group 1 carcinogens Aromatic ketones