In our previous studies we demonstrated that cigarette smoke or its extracts significantly inhibit mucosal cell proliferation in human and animal mucosal cells, associated with the reduction of EGF and polyamine release (29,30). EGF plays an important role in mucosal cell proliferation and modulates mucosal integrity. During ulceration, the synthesis of EGF, as well as its expression are markedly upregulated in epithelial cells adjacent to the ulcer crater (31). Data from our previous study also demonstrated that cigarette smoke significantly inhibited EGF synthesis and its mRNA expression in salivary glands and gastric mucosa in rats with acetic acid ulcers (29). In addition, gastric ulcer healing was also delayed along with a reduced mucosal cell proliferation, suggesting that the delay of ulcer healing induced by cigarette smoke is possibly caused by the reduction of EGF release at the ulcer site.

Polyamines are found to be associated with mucosal cell proliferation during the ulcer healing process (32,33). Polyamines are involved in EGF-mediated cell proliferation and acid secretion in the stomach (34). Ornithine decarboxylase (ODC) is the primary enzyme for the biosynthesis of polyamines, including putrescine, spermine and spermidine. To further elucidate the association between smoking and peptic ulcer disease, in previous studies, we also examined ODC activity, which is crucial for promoting mucosal growth and has gastroprotective effects during gastric ulcer healing. Following the intragastric administration with cigarette smoke extracts once daily for three days, ulcer sizes were markedly enlarged and the myeloperoxidase activity was also increased. Cigarette smoke also significantly inhibited cell migration and cell proliferation with a reduction in ODC activity in an in vitro wound model. Moreover, the inhibitory effect on cell proliferation and ODC activity induced by cigarette smoke may be reversed by exogenous spermidine, indicating that the delayed wound healing in the stomach induced by cigarette smoke was at least in part due to a reduction in polyamine synthesis (30,32).

Under normal conditions, large amounts of hydrochloric acid exist in the stomach, which help to break down food into smaller particles for further digestion in the digestive tract. Gastric acid is neutralized in the duodenum by sodium bicarbonate produced by the pancreas. The increased secretion of stomach acid and/or a reduction in sodium bicarbonate production in the pancreas can interfere with the protective mechanisms of the gastric mucosa and the inner layer of the duodenum, where ulcers are normally formed. Ample evidence suggests that smoking can increase the production of gastric acid, accompanied by a reduction in bicarbonate production. The role of cigarette smoke and its active compounds, such as nicotine on acid production and sodium bicarbonate production has been reviewed (35). Researchers have found that the intravenous injection of nicotine hydrogen tartrate (0.012–0.020 mg/kg body weight) increases the concentration of hydrogen and chloride ions in the gastric juice (36). In an early study, Ligny et al (37) demonstrated that the magnitude of acid secretion was associated with the number of cigarettes smoked. They also found that tobacco smoking over a long period of time stimulated vagus nerves and induced functional parietal cells to increase pentagastrin-induced acid output in smokers.

Bile is a digestive fluid produced by the liver, and normally flows into the duodenum, where it digests fats and removes toxins. Bile salts also function as detergents and damage the mucosal barrier. The pylorus is a one-way valve between the stomach and the duodenum that prevents bile and other contents of the small intestine going back into the stomach (38). In a clinical study, it was demonstrated that cigarette smoking induces pyloric incompetence and increases the duodenogastric reflux (39), which may be due to the reduction in basal pyloric pressure induced by smoking (40), leading to mucosal injury in the stomach.

Pancreatic bicarbonate plays an important role in neutralizing extra acid coming from the stomach. The increased secretion of gastric acid, as well as a reduction in sodium bicarbonate production would interfere with the protective mechanisms in the stomach and the duodenum, possibly leading to the development of ulcers in these organs. Several clinical studies have shown that the secretion of bicarbonate is diminished after cigarette smoking (41,42). Furthermore, the degree of inhibition on basal pancreatic secretion has been shwon to have a good correlation with the blood nicotine concentrations in humans (43).

Cigarette smoking is one of the risk factors for esophageal cancer (62–65). Recently, a cohort study with a 20-year follow-up period conducted by Japanese researchers found that individuals who began smoking at a younger age and consumed larger amounts of alcohol more had a higher risk of developing esophageal cancer compared with the normal population. The esophageal cancer mortality risk was as high as 9.33 (95% CI, 2.55–34.2) for smokers who began smoking between the ages of 10 and 19 years and consuming three units of alcohol per day (64).

Supporting data have demonstrated that cigarette smoking is a major risk factor for cancer of the oral cavity (66–68). A recent study demonstrated that the odds ratios for current smokers and former smokers were 11.8 (95% CI, 8.6–16.3) and 2.2 (95% CI, 1.6–3.1), respectively when compared to non-smokers. The risk of developing cancer of the oral cavity increased with the quantity and duration of cigarette smoking (66). In addition, the risk of developing cancer of the oral cavity in former smokers decreased with time. The buccal mucosa and the floor of the mouth were the most sensitive sites with lesions induced by smoking (67).

The relationship between the occurrence of stomach cancer and cigarette smoking has been studied since the 1950s. Cigarette smoking has been considered as one of the key risk factors for gastric cancer, which increases the incidence of the disease by approximately 1.5- to 2.5-fold among current smokers (69). Nicotine, the active compound in cigarette smoke, has been demonstrated to be capable of promoting gastric tumor growth and neovascularization (3). In a 20-year follow-up study involving 18,244 middle-aged and older men conducted in Shanghai, China, researchers found that the risk of gastric cancer was statistically significantly higher in ever smokers [hazard ratio (HR), 1.59; 95% CI, 1.27–1.99] than in non-smokers (70). Furthermore, among the non-drinkers, the ever smokers experienced an 80% higher risk of gastric cancer (HR, 1.81; 95% CI, 1.36–2.41). All these observations indicate that cigarette smoking may exert independent effects on the development of gastric cancer.

Tobacco consumption is considered an established risk factor for pancreatic cancer (71,72). In a 10-year cohort study, researchers found that cigarette smoking was related to an increased risk of the disease [relative risk (RR), 1.7; 95% CI, 1.6–1.9] and mortality (RR, 1.6; 95% CI, 1.4–1.7) in patients with pancreatic cancer (72).

Phipps et al (74) carried out a study in 1,968 patients with stage III colon cancer in order to examine the relationship between smoking and cancer outcome. They found that smoking history was significantly associated with a shorter disease-free survival (DFS), and time to recurrence (73) in patients with colon cancer (74). Compared with never-smokers, ever smokers experienced a significantly shorter DFS with a three-year DFS proportion of 70 vs. 74% (HR, 1.21; 95% CI, 1.02–1.42). Compared with never-smokers, participants who were former or current smokers were older and were more likely to be male, and to have colon tumors that were dMMR and/or BRAF mutated (74).

nAChRs are a family of ligand gate ion channels that function as the key regulators of nicotinic and cholinergic signaling in cells (75). nAChRs are known to participate in cellular adhesion and migration through the interactions with rapsyn and herparan sulphate proteoglycan (76,77). Increasing evidence suggests that nicotine and its derivatives, such as N-nitrosonornicotine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanonee can directly activate nAChRs to promote cell growth and angiogenesis and inhibit the drug-induced apoptosis of cancer cells (75).

A recent study demonstrated that nicotine activates Yes-associated protein 1 (YAP1) through nAChR-mediated signaling in esophageal squamous cell cancer (ESCC) (76). Zhao et al (76) reported that nicotine administration increased cell proliferation and migration, and promoted resistance to apoptosis in ESCC. In addition, nicotine administration was also found to induce the nuclear translocation and activation of YAP1 in ESCC. Nicotine signaling can also inhibit the interaction of YAP1 with p63 that contributes to the inhibitory effects of nicotine on cell apoptosis. The association between cigarette smoking and YAP1 activation was also observed in clinical esophageal cancer samples (76). These results suggest that nicotine may be the active compound in cigarette smoke responsible for carcinogenesis in the esophagus.

In our previous studies, we found that nicotine in cigarette smoke may be the most active ingredient responsible for the tumorigenesis of colon cancer cells (78,79). Nicotine was demonstrated to stimulate the proliferation of human colon adenocarcinoma HT-29 cells through the activation of α7-nAChR followed by the catecholamine-synthesis pathway and adrenaline release and finally, β-adrenergic activation (78). Furthermore, in an animal study, nicotine was shown to promote tumor growth, mainly by activating the β-adrenoceptors and the subsequent expression of cyclooxygenase-2, prostaglandin E2, and VEGF in tumor tissues (79). These results demonstrate for the first time the contributory role of α7-bAChR and β-adrenoceptors in the tumorigenesis of colon cancer with significant involvement of some stress hormones.

Many toxic compounds in cigarette smoke can interact with DNA to form DNA adducts, which are believed to be another important mechanism for carcinogenesis induced by cigarette smoke (80). Among various ingredients in cigarette smoke, TNSAs are known as the responsible compounds for the formation of DNA adducts (81). In an early clinical study, Dyke et al (81) found that in males only, DNA adducts in gastric tumor tissues from smokers were significantly higher than in those from non-smokers. Nitrosamines and other nitroso compounds in cigarette smoke are capable of covalently interacting with DNA, which alters the normal biological function of DNA and eventually induces carcinogenesis in the GI tract and in urinary bladder (81,82). To date, the formation of DNA adducts has been found in cancer tissues from the oral cavity (83), esophagus (84), stomach (81), pancreas (85) and colon (86).

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) has been demonstrated to be one of contributors for smoke-induced pancreatic cancers. It is clear that NNK can react with DNA to form DNA methyl and pyridyloxobutyl adducts (87). These DNA adducts can induce an activating point mutation of the Ki-ras gene in codon 12, which is common in human pancreatic adenocarcinomas (88,89). Askari et al (89) further demonstrated that NNK induced the transactivation of the EGF receptor, increased the accumulation of intracellular cyclic AMP, and activated the phosphorylation of mitogen-activated protein kinase (MAPK) and ERK1/2. These results indicate that the NNK-mediated β-adrenergic cAMP-dependent signaling pathway may contribute to the development of pancreatic carcinogenesis in smokers.

The relationship between cancer and inflammation was perceived as early as the 19th century. It is clear that chronic inflammation predisposes to cancer at the proximity of the site of inflammation (90). Chronic inflammation in the GI tract can be caused by H. pylori infection, autoimmune diseases, such as IBD and inflammatory conditions, such as peptic ulcers. Various types of immune cells are involved in the formation of the tumor inflammatory microenvironment, such as macrophages, neutrophils, mast cells and lymphocytes. During the inflammatory process, various inflammatory components acting as messengers of inflammation are released by the immune cells and tumor cells in the microenvironment of the neoplastic tissues. These include cytokines, such as tumor necrosis factor-α (TNF-α), interleukin (IL)-1 and IL-6, and chemokines, such as CXCL8. Cigarette smoking is a risk factor for the development of chronic inflammation in the GI tract as reviewed in the previous section, which also promotes inflammation-associated adenoma/adenocarcinoma formation (91).

TNF-α, as a pro-inflammatory cytokines, can not only induce hemorrhagic necrosis of tumors, but also has protumoral functions. It has been found that a high dose of TNF can destroy the tumor vasculature and cut-off the supplement of O₂ and nutrition for tumor growth to exert necrotic effects in tumors (92). However, TNF-α can also induce DNA damage (93), suppress DNA repair (93) and promote the growth of tumor cells (94). Increasing evidence also suggests that TNF-α enhances tumor growth and invasion, angiogenesis, leukocyte recruitment and facilitates epithelial to mesenchymal transition (90). The bidirectional role of TNF-α in tumor progression and cell death is due to the fact that TNF can bind to different membrane-bound homotrimeric receptors, TNFRI and TNFRII, to trigger opposite pathways (95). As regards tumor promotion, TNF-α can inhibit the expression of glycogen synthase kinase-3β, and consequently activate the Wnt/β-catenin signaling pathway to induce tumor development (96). Furthermore, TNF family members can also suppress the immune response in the tumor environment, which may be due to the inhibition of the major histocompatibility complex class II in tumor-associated macrophages through the decoy receptor-3 (97).

IL-6 plays an important role in tumor development, such as colorectal cancer, in the GI tract (98). Clinical data have shown that IL-6 serum levels from patients with colorectal cancers are significantly increased and positively correlate with tumor load, including tumor size and liver metastases (98). It was demonstrated that two major signaling pathways, the signal transducers and activators of transcription 1 and 3 (STAT1/3) and the Src-homology tyrosine phosphatase 2 (SHP2)-Ras-ERK, are involved in the IL-6-mediated proliferation of intestinal epithelial cells (99). In addition, IL-6 can also promote tumor growth by increasing the colony formation of human colon carcinoma cells (100). These biological actions of IL-6 in colorectal cancer progression were further elucidated by mediating through the soluble IL-6 receptors derived from tumor cells rather than from the membrane-bound receptors (101).

IL-1β has been found to be capable of promoting tumor cells to metastasize, by activating the cancer-related inflammation cascade (102,103). In models of 3-methylcholanthrene-induced carcinogenesis, it was IL-1β, rather than IL-1α in the tumor microenvironment that was capable of determining the invasive potential of malignant cells, including increased tumor adhesion and invasion, angiogenesis and immune suppression (104). Microenvironmental IL-1β is a required factor for tumor invasiveness and angiogenesis, which may contribute to the production of TNF-α and vascular endothelial cell growth factor by IL-1β (105). Recently, Carmi et al (106) found that myeloid cells released IL-1β and induced endothelial cells to produce proangiogenic factors, such as VEGF, and subsequently provided the inflammatory microenvironment for tumor progression and angiogenesis. Furthermore, they also observed that IL-1β inhibition significantly reduced tumor growth by suppressing inflammation and inducing the maturation of immature myeloid cells into M1 macrophages (106).

Chemokines in the tumor microenvironment are another important factors for modifying tumor growth, and promoting angiogenesis (107) and tumor metastatic spread (108). CXCL1 (growth-regulated oncogene α) for example, produced by human colorectal cancer cells is capable of inducing microvascular endothelial cell migration and tube formation in vitro. PGE2-induced CXCL1 in the tumor microenvironment has also been found to increase microvessel density and stimulate LS-174T cell proliferation in an in vivo model (109). Together with CXCL-1, the angiogenic chemokine CXCL8 (IL-8) was also significantly unregulated in tumor tissues from patients with colorectal cancer (110). CXCL8 signals are mainly activated through the interaction with CXCR1 and CXCR2 present in cancer cells and other cells. To date, CXCR1 and CXCR2 receptors are widely expressed in cancer cells, tumor-associated macrophages, neutrophils and endothelial cells (111). Therefore, the increased CXCL8 levels caused by cigarette smoking could nurture the tumor microenvironment to promote cancer growth (112). Studies have shown that CXCL8 induces cell proliferation by the activation of classical MAPK and downstream phosphorylation of ERK1/2 in neutrophils and cancer cells (113,114). CXCL8 also regulates angiogenesis by the induction of matrix metalloproteinase 9 (MMP-9) through the activation of VEGFR-2 in endothelial cells, and subsequently promotes cancer growth and metastasis (115).