# Electronic Cigarettes' Toxicity: From Periodontal Disease to Oral Cancer
## 4 Chemical Composition of e-Liquids and Vapors and the Implications in Oral Carcinogenesis
### Nicotine  
Nicotine remains one of the main components responsible for the toxicity of electronic cigarettes. Nicotine represents a significant source of concern, because 45% of the nicotine released from e-cigarettes is deposited in the oral cavity, and its concentrations in saliva are 10.5 times higher than those in plasma [1]. Nicotine can be found in the saliva of all tobacco products users, and its absorption is closely connected to the salivary pH. More alkaline saliva determines higher rates of nicotine absorption [41]. Figure 4: Carcinogens found in e-cigarettes’ liquids and vapors. E-liquids with nicotine demonstrated a higher degree of cytotoxicity in comparison to nicotine-free samples. In one study, human gingival fibroblasts exposed to nicotine presented vacuolization of cytoplasm, the endoplasmic reticulum was heavily dilated, and the mitochondria were also affected. These changes in the morphology of the cells are considered a sign of lysosome metabolism alteration, impairing the digestive capacity. Nicotine is a lysosomesotropic amine, that accumulates into lysosomes, and it raises the pH above the optimum level for hydrolases activity. By affecting intracellular autophagy, nicotine alters the cell's capacity to degrade dysfunctional and unnecessary components, a skill that is essential for cell survival [52]. There is evidence that nicotine may directly promote carcinogenesis by stimulating lesion development, angiogenesis, and by inhibiting apoptosis [53; 54]. Nicotine is metabolized into cotinine and nitrosamines. Nitrosamines are compounds that lead to the appearance of methylation intermediates in DNA. Nicotine and NNK (nicotine-derived nitrosamine ketone) also activate serine/threonine kinase Akt, a member of the oncogene families involved in tumorigenesis. Alterations in Akt activity modify vital cellular processes (glucose metabolism and protein synthesis, cell growth, cell survival and apoptosis, cell migration, angiogenesis), and cause major cellular perturbations, ultimately leading to cancer [26; 55]. Nicotine interacts with nicotinic acetylcholine receptors (nAChR) that are found in the central nervous system, but also in non-neuronal cells, where they regulate cellular growth, differentiation, and migration. Nicotinic acetylcholine receptors (nAChRs) are involved in accelerated cell proliferation, tumor invasion, and angiogenesis. Nicotine and tobacco-specific nitrosamines (NNK, NNN) are nAChRs agonists with a high affinity. Nicotine alters the expression levels and the sensitivity of these receptors, which play an important role in carcinogenesis. The activation of these receptors activates proliferative signaling pathways in cancer cells (eq. PI3K/AKT, Raf/MEKK/ERK1/2, NF-kB), which inhibits cell apoptosis and elevates the risk of cancer. Chronic nicotine and tobacco-specific nitrosamine exposure also activates STAT3, which is associated with perturbations in the cell cycle, and chemoresistance [56; 57]. Cytokine-inducible SH2 containing protein (CISH) is a member of the suppressors of cytokine signaling (SOCS) proteins and it regulates STAT3 activity. [PERSON] et al. demonstrated that STAT3 can be activated through a miR-944-mediated CISH down-expression pattern. The results of their study clearly showed that NNK-induced TP63/miR-944 co-expression is an important factor in oral carcinogenesis [35]. The proliferative effects of nAChRs activated by nicotine and NNK were first reported for lung cancer cell lines, but these effects were observed in various normal and cancerous cells. nAChRs are also found on extraneuronal cells, including oral epithelium and oral keratinocytes, where the expressed subunits are \(\alpha\)3, \(\alpha\)5, \(\alpha\)7, \(\alpha\)9, and \(\beta\)2. In the cells, nAChRs are located at two levels: on the cell membrane and the mitochondrial membrane [57].