Helicobacter pylori Infection and Dietary Factors Act Synergistically to Promote Gastric Cancer.

However, the incidence of gastric cancer (GC) has been decreased in past decades; GC is the second cause of cancer related death in the world. Evidence has illustrated that several factors including Helicobacter pylori (H. pylori) infection, host genetics, and environmental factors (smoking and particularly diet) may play a crucial role in gastric carcinogenesis. It has been demonstrated that high consumption of fresh fruits, vegetables, high level of selenium and zinc in drinking water, sufficient iron, and cholesterol protect against GC, while; smoked , pickled, and preserved foods in salt, and nitrites increase the risk of GC. Epidemiological studies have also proved that H. pylori infection and a high salt diet could independently induce atrophic gastritis and intestinal metaplasia. Recently, studies have been demonstrated that dietary factors directly influence H. pylori virulence. The use of appropriate diet could reduce levels of H. pylori colonization or virulence and prevent or delay development of peptic ulcers or gastric carcinoma. This is attractive from a number of perspectives including those of cost, treatment tolerability, and cultural acceptability. This review will describe new insights into the pathogenesis of H. pylori in relation to environmental factors, especially dietary, not only to find the developed means for preventing and treating GC, but also for understanding the role of chronic inflammation in the development of other malignancies.


Introduction
The incidence of gastric cancer (GC) has been declined over time, but it is the second cause of death from cancer in the world (Jemal et al., 2011;de Martel et al., 2012;Cover and Peek, 2013;Ferlay et al., 2013;Epplein et al., 2014). H. pylori infects and persists within the stomach of most of the world's population causing chronic inflammation within stomach that can result in gastritis, peptic ulcer disease and cancer. This bacterium is the major cause

Helicobacter pylori Infection and Dietary Factors Act Synergistically to Promote Gastric Cancer
Negin Raei 1 *, Bahador Behrouz 2 *, Saber Zahri 1 , Saeid Latifi-Navid 1 occurrence of this malignancy, individuals who are at high risk, and developing means for GC prevention (Cover and Peek, 2013). The gastric adenocarcinoma has remarkably variable incidence worldwide. The high incidence areas are East Asia, Central America, parts of South America, and Eastern Europe (Sonnenberg, 2010;de Martel et al., 2012;Soerjomataram et al., 2012). Over the past century the incidence of tumors in the distal stomach has been decreased, despite the decreasing of gastric adenocarcinomas of the distal stomach over time, a steady increase has been reported about the proximal stomach and gastroesophageal junction in the United States and Europe (Wu et al., 2007;Sonnenberg, 2010;Haile et al., 2012). Diet has long been thought to be correlated with GC risk, most of the published studies have been reported that there is a synergistic interaction between the dietary factors and the H. pylori infection in relation to GC risk (Yamaguchi and Kakizoe, 2001).

H. pylori Virulence Factors Promote GC
Studies have been shown that there is a genetic variation among isolates of H. pylori (Blaser and Berg, 2001). H. pylori harbors a 40 kb chromosomal region, which is known as the cag pathogenicity island (PAI). (Akopyants et al., 1998). The cag PAI is a pathogenicity island in the H. pylori genome and encodes numerous genes that, upon cell contact, are expressed and assembled into the needle-like type 4 secretion system (T4SS). H. pylori T4SS binds the integrin β1 receptor that is located on the basal membrane and transfers the cytotoxin associated gene A (cagA) which is also encoded by the cag PAI (Shaffer et al., 2011). Once inside the cell, cagA is tyrosin phosphorylated on specific EPIYA motifs by host kinases, and phosphorylated cagA activates host cellular phosphatase (SHP-2) that results in humming bird phenotype within host epithelial cells (Higashi et al., 2002).
Non-phosphorylated cagA manipulates PAR1b and promotes loss of cell polarity. Moreover, it interacts with the epithelial tight-junction scaffolding protein ZO-1 and the transmembrane protein junctional adhesion molecule-A (JAM-A) resulting in aberrations in tight junction function at bacterial adherence site (Amieva et al., 2003), and also activates β-catenin which upregulates the genes which are involved in cancer progression (Franco et al., 2005). Due to the mentioned activities cagA has been known as bacterial oncoprotein (Murata-Kamiya et al., 2007).
Another H. pylori virulence factor that is associated with the development of GC is the secreted vacuolating cytotoxin A (VacA) toxin (Cover and Blanke, 2005; Boquet and Ricci, 2012). All strains of H. pylori harbor vacA gene but only in 40% of them VacA might be expressed. The vacA gene displays an allelic diversity in five various regions: the single region s (alleles: s1a, s1b, s1c and s2), the medium region m (alleles: m1 and m2), the intermediate region i (alleles: i1 and i2), the deletion region d (alleles: d1, no deletion and d2, with deletion of 69 to 81 bp) and the c region (alleles: c1, with deletion of 15 bp and c2, no deletion) (Rhead et al., 2007;Ogiwara et al., 2009). The combination of mentioned sequences results in VacA protein, which the polymorphism of VacA is crucial in toxin activity: e.g., the s1 allele, especially in combination with the m1 allele, is highly associated with the risk of developing PU and GC (Ferreira et al., 2012). These extensive polymorphisms are therefore strongly associated with the risk of PU (gastric ulcer or duodenal ulcer) and gastric precancerous lesions (non-atrophic gastritis or intestinal metaplasia) or GC (Gonzalez et al., 2011;Winter et al., 2014;Honarmand-Jahromy et al., 2015). VacA induces apoptosis in gastric epithelial cells, through stimulating the release of cytochrome C from mitochondria, which results in the activation of caspase-3 in apoptosis pathway (Cover et al., 2003). VacA can also lead to changes in several immune cells involving, T cells, B cells, neutrophils, mast cells and macrophages (Torres et al., 2007).
H. pylori colonizes in the gastric mucus, a small fraction of bacteria can reach to the host cells and adhere via the expression of adhesions. The H. pylori genome involves about 30 hop genes, which code the outer membrane proteins (OMPs); their cellular localization hints that these proteins are involved in adherence or transport of nutrients. One of these genes that has been known to be correlated with GC risk is Blood group Antigen Binding Adhesin, BabA which binds sialyle-lewis b (Suzuki et al., 2012). It has been proved that BabA is expressed more commonly by cag PAI positive strains (Hennig et al., 2004). Furthermore, patients infected with H. pylori harboring cag PAI are at high risk of GC (Cover and Peek, 2013).

Smoking
Tobacco use is associated with an increased risk for GC. A meta-analysis concluded that the cumulative risk of GC risk in male smokers was more than female smokers, in comparison to individuals who never smoke. The analysis showed that relative risk (RR) was increased from 1.3 for the lowest intake to 1.7 for the smoking of about 30 cigarettes per day, it was demonstrated that smoking was remarkably associated with both cardia and noncardia cancer and the RR was 1. 87 and 1.60, respectively (Ladeiras-Lopes et al., 2008;Shikata et al., 2008). Another extensive review by International Agency for Research on Cancer (IARC) has been shown that there was a consistent and causal association of stomach cancer with cigarette smoking in both men and women. There was more than 40% of the reduction in male cancer deaths between 1991 and 2003 contributed to the decline in smoking in the last half century. It has been proved that smoking had a remarkable correlation with both gastric cardia cancer and non-cardia cancer in Ardabil province, Iran (Pourfarzi et al., 2009;Babaei et al., 2010). The association between H. pylori infection and smoking is controversial. In an study into the factors related with H. pylori infection in Glasgow, U.K., found that ex-smokers (71%) and current cigarette smokers (73%) have higher occurrence than never-smokers (59%), but similar prevalence to each other (Woodward et al., 2000). After adjustment for age, social class, sex, and number of siblings, smokers and ex-smokers were at a higher risk for the infection than subjects who never smoked (Odd ratio (OR): 1.5; 95% confidence interval (95% CI): 1.1-1.9 and OR: 1.4; (Woodward et al., 2000). Other reports from the literature showed no association of smoking with H. pylori infection (Camargo et al., 2004). Smokers often share cigarettes; therefore, the act of smoking involves inserting orally the tip of a cigarette, which may well have been handled by other people ( Tyas and Pederson, 1998). However, smoking is also characteristic of low social status, and the smoking effect could simply be a proxy for other socioeconomic factors such as income and education (Fiedorek et al., 1991).

Alcohol Consumption
First, self-reporting of alcohol consumption is unreliable as the measurement of the quantity of alcohol consumption is not standardized. Secondly, recall bias in case-control study may affect the result significantly. Lastly, alcohol effect may be so small that the effect could not be shown in past studies. It has been shown ingestion of alcohol is associated with development of distal, but not proximal GC (Tramacere et al., 2012). Recently, DOI:http://dx.doi.org/10.7314/APJCP.2016.17.3.917 H. pylori and Dietary Factors Act Synergistically to Promote Gastric Cancer two prospective large cohort studies reported positive association between alcohol consumption and GC risk. In the European Prospective Investigation into Cancer and Nutrition study, heavy consumption of alcohol (>60 g/day) was found to be positively associated with the risk of intestinal type non-cardia GC in men with HR (95%CI) 1.65 (1.06-2.58) (Duell et al., 2011). In a population based cohort study in Shanghai, China, heavy drinkers were found to have significantly increased risk of GC with HR (95% CI) of 1.46 (1.05-2.04) (Moy et al., 2010).

Dietary Risk Factors for GC
Epidemiologic studies have been shown an association between diet and GC risk worldwide. Substantial evidence from ecological, case control, and cohort studied have been proved that the diets that are highly linked to GC risk are those rich in salt, pickled, smoked or poorly preserved foods, those with a high quantity of meat, low fruit, and vegetable content (Epplein et al., 2008;Kim et al., 2010;Gonzalez et al., 2012;Ren et al., 2012)

Salt Intake
The high consumption of salt, has been demonstrated to be linked with an increased GC risk ( Tsugane and Sasazuki, 2007). Dietary salt intake varies among populations, in some populations with a high incidence of GC it is reported that average salt consumption is 49 gram per day (You et al., 1988). One study on urinary sodium exertion has been shown an intense correlation between the salt consumption and GC morality rates (Joossens et al., 1996). Experiments have also been shown that H. pylori gene expression has changed in response to the level of sodium chloride (Gancz et al., 2008;Loh et al., 2012). The high level of sodium chloride leads to changes in H. pylori cells shape from a typical spiral to elongated form and an upregulation of cagA gene in some strains. Epidemiological studies have also indicated that H. pylori infection and a high salt diet could independently and significantly induce atrophic gastritis and intestinal metaplasia (Gancz et al., 2008;Brenner et al., 2009;Wang et al., 2009;D'Elia et al., 2012).

Fruit and Vegetable Consumption
Meta-analyses of case-control studies have illustrated that a high consumption of fruits and non-starchy vegetables are remarkably against the development of stomach cancer (OR=0.62 and 95% CI=0.46), which is an impact that is stronger in Asia than in United States or Europe (Wiseman, 2008). Particularly, flavonoid consumption is significantly correlated with a reduction in GC risk in women (Wiseman, 2008). Vitamin C, probably through its antioxidant effect, has also known as beneficial factor against the development of GC (Jenab et al., 2006). Over the last century, vast number of changes have occurred in methods of keeping and preserving food in developed countries, the availability of refrigeration led to an increased intake of fresh food (Correa, 2013). The gradual decline in the incidence of GC may be because of the changes in diet that have accompanied refrigeration (Cover and Peek, 2013).

Iron Deficiency
Iron deficiency have been associated with an increased risk for GC and neoplasms that arise elsewhere in the gastrointestinal tract (Ioannou et al., 2002;Pra et al., 2009;van Lee et al., 2011). Colonization by H. pylori strains has been correlated with hemorrhagic gastritis and finally loss of iron (Yip et al., 1997). Long-term H. pylori infection can lead to the development of hypochlorydia, reduced ascorbic acid levels, and subsequently reduced absorption of iron (Waring et al., 1996). Case-control studies have shown an inverse association between dietary iron intake and gastric adenocarcinoma, experiments on serum ferritin have also been shown an inverse association with GC which was more sever in cases diagnosed within 15 years of examination (P-value= 0.02) (Knekt et al., 1994;Harrison et al., 1997). Based on these studies it is demonstrated that the iron deficiency from both blood loss and a low-iron diet is relevant.

Cholesterol Intake
H. pylori is auxotrophic for cholesterol, and so the dietary quantity of cholesterol could have an effect on colonization of H. pylori in the stomach is related to gastric disease. In vitro studies have been shown that H. pylori extracts lipid from plasma membranes of epithelial cells for subsequent glycosylation. Extra cholesterol can increase the level of phagocytosis of H. pylori by antigenpresenting cells, including macrophages and dendritic cells, and promotes antigen-specific T cell responses (Wunder et al., 2006). A diet rich in cholesterol results in T-cell-dependent reduction of the H. pylori in the gastric, therefore a cholesterol-rich diet may decrease the risk of subsequent inflammation and injury caused by H. pylori (Cover and Peek, 2013).

Other Risk Factors
It has been indicated that higher concentration of selenium and zinc in drinking water may be as protective factors against gastric carcinogenesis (Nakaji et al., 2001) It has been demonstrated that high exposure to radon, radium or natural uranium in drinking water has been associated with the risk of GC (Auvinen et al., 2005).
The other environmental carcinogens that might be responsible for the incidence of GC is the high level of nitrate in food particularly in agricultural products that needs more research (Derakhshan et al., 2008).

Conclusion
Several factors including properties of the H. pylori, host genetics, and environmental exposure particularly diet may play a remarkable role in gastric carcinogenesis; each has significant role on the level of long-term interactions between H. pylori and humans. It has been proved that approximately 50% of the world's population is infected with H. pylori but a small proportion develops GC. There is great interest in recognition of subpopulations at high risk for disease. Our future goals are to gain deeper insight into the pathogenesis of H. pylori in relation to environmental factors especially dietary factors, not only to find the developed means for preventing and treating GC, but also for understanding the role of chronic inflammation in the development of other malignancies. The use of appropriate diet could reduce levels of H. pylori colonization or virulence and prevent or delay development of peptic ulcer or gastric carcinoma, is attractive from a number of perspectives including those of cost, treatment tolerability, and cultural acceptability.