Extragastric manifestations of Helicobacter pylori infection: Possible role of bacterium in liver and pancreas diseases

Abstract

Helicobacter pylori (H. pylori) is an ancient microorganism that has co-evolved with humans for over 60000 years. This bacterium typically colonizes the human stomach and it is currently recognized as the most common infectious pathogen of the gastroduodenal tract. Although its chronic infection is associated with gastritis, peptic ulcer, dysplasia, neoplasia, MALT lymphoma and gastric adenocarcinoma, it has been suggested the possible association of H. pylori infection with several extragastric effects including hepatobiliary and pancreatic diseases. Since a microorganism resembling H. pylori was detected in samples from patients with hepatobiliary disorders, several reports have been discussed the possible role of bacteria in hepatic diseases as hepatocellular carcinoma, cirrhosis and hepatic encephalopathy, nonalcoholic fatty liver disease and fibrosis. Additionally, studies have reported the possible association between H. pylori infection and pancreatic diseases, especially because it has been suggested that this infection could change the pancreatic physiology. Some of them have related a possible association between the microorganism and pancreatic cancer. H. pylori infection has also been suggested to play a role in the acute and chronic pancreatitis pathogenesis, autoimmune pancreatitis, diabetes mellitus and metabolic syndrome. Considering that association of H. pylori to liver and pancreas diseases needs further clarification, our work offers a review about the results of some investigations related to the potential pathogenicity of H. pylori in these extragastric diseases.

Core tip: Helicobacter pylori (H. pylori) has been associated with several extragastric manifestations, including liver and pancreas diseases. Evidence for its role in the pathogenesis of chronic liver diseases and liver carcinoma is supported by several clinical and experimental studies. Furthermore, epidemiologic and serology-based works have reported a possible association between the microorganism and pancreatic cancer. H. pylori infection has also been linked to the acute and chronic pancreatitis pathogenesis and it could be related to the development of autoimmune pancreatitis, diabetes mellitus and metabolic syndrome. This review summarizes recent findings on the possible role of H. pylori infection in the etiology of liver and pancreas disorders.

INTRODUCTION

Helicobacter pylori (H. pylori) is an ancient organism that has co-evolved with humans for over 60000 years[1]. This bacterium typically colonizes the human stomach and it is currently recognized as the most common infectious pathogen of the gastroduodenal tract[2]. Most infected individuals do not develop disease, leading to the hypothesis that some H. pylori strains are harmless or even beneficial[3]. However, its chronic infection is associated with increased risk for several disease outcomes including gastritis, peptic ulcer, dysplasia, neoplasia, mucosa associated lymphoid tissue lymphoma and invasive gastric adenocarcinoma[4].

In recent years, it has been suggested the possible role of H. pylori infection with several extragastric effects including neurodegenerative, metabolic and cardiovascular conditions, as well as hepatobiliary, pancreatic and colorectal diseases[5-7]. Moreover, studies indicate that this bacterium may be related to the development of skin diseases such as urticaria as well as rheumatic disorders[8,9].

Since Helicobacter spp. have been isolated from the liver samples of a variety of mammals, it was suggested that bacteria may be involved in the pathogenesis of chronic liver diseases (CLD) and liver carcinoma[10-12]. Regarding to human studies, a microorganism resembling H. pylori was firstly detected in resected gallbladder mucosa of patient with gallstone[13]. Nilsson et al[14] reported the detection of H. pylori using molecular biology techniques such as polymerase chain reaction (PCR), hybridization and partial DNA sequencing in liver samples from patients with primary sclerosing cholangitis and primary biliary cirrhosis (PBC).

Subsequent studies have verified the possible association of H. pylori infection in the development of other liver diseases, particularly hepatocellular carcinoma (HCC), in different geographic areas. Some authors have reported a very high prevalence of antibodies to H. pylori and the prevalence of bacteria in patients with cirrhosis, compared with controls[15-18]. Although Helicobacter spp. DNA was successfully detected in liver samples of individuals with primary liver carcinoma[19-23], the remaining question was whether these findings corresponded to the true liver colonization by bacteria or whether H. pylori DNA could result from the retrograde transfer of this DNA from the duodenum to liver. Nevertheless, the isolation of H. pylori in culture medium using liver samples was crucial for supporting the true bacterial colonization[24,25].

Another extragastric digestive organ indicated as a possible target for H. pylori is the pancreas and some mechanisms by which H. pylori infection may influence pancreatic physiology have been object of several studies. It has been suggested that some aggressive factors produced by this microorganism such as ammonia and lypopolysaccharides (LPSs), as well as the production of inflammatory cytokines, could induce the pancreas damage. These conditions associated with the activation of leukocytes could be responsible for the clinical outcome of the pancreas diseases[26].

Considering these aspects, several epidemiologic and serology-based studies have reported a possible association between the microorganism and pancreatic cancer[27-30] and, in addition, epidemiological studies have examined if there could be any association between peptic ulcers development and risk of pancreatic cancer. Results from cohort studies with large number of pancreatic cancer cases and detailed information on type of peptic ulcers (i.e., gastric vs duodenal) observed positive associations with gastric ulcers, but not duodenal ulcers[31,32]. H. pylori infection has also been suggested to play a role in the acute and chronic pancreatitis (CP) pathogenesis[33-35] and it has been reported that this infection could be related to autoimmune pancreatitis (AIP) mainly through induction of autoimmunity, by molecular mimicry, and apoptosis[36]. Finally, a possible role of H. pylori in diabetes mellitus (DM)[37,38] and metabolic syndrome have been investigated[39,40].

However, the interpretation of so different results obtained from the mentioned studies makes the interpretation of their relevance be inconclusive. H. pylori more virulent strains, such as the ones that harbor the CagA pathogenicity island, together with the host characteristics, could be important for the clinical expression of pancreatic diseases, and other systemic disorders. In some cases, H. pylori might be a minor factor contributing to disease development owing to the persistent inflammatory state of the gastric mucosa[41]. Considering these findings and that, the role of H. pylori infection in the liver and pancreatic diseases remains controversial, in the review we attempt to discuss about the results of some investigations related to the potential pathogenicity of H. pylori in these extra gastric diseases.

ROLE OF H. PYLORI IN LIVER DISEASES

HCC

The HCC is the main primary malignant tumor of the liver and the fifth most common type of cancer. It is estimated that more than 50000 cases are diagnosed annually and that the disease is the third leading cause of cancer-related death in the world population[42-47]. The cases of death due to HCC vary 250000-1 million individuals per year and its prevalence varies according to geographic location, gender, age and ethnicity[48].

The development of HCC is attributed to several factors such as alcoholism, exposure to mycotoxins (aflatoxins), hereditary hemochromatosis, PBC, deficiency of α-1-anti-trypsin, Wilson’s disease and microcystin[21,49-51]. Chronic viral infections are considered the main risk factor of HCC in 75%-80% of cases[47], wherein hepatitis B virus (HBV) infection is responsible by 50%-55% of cases[52] and hepatitis C virus (HCV) infection occurs in 25%-30% of patients[53]. However, the detection of H. pylori in patients with HCC may suggest the role of bacteria in the pathogenesis of this disease.

Several studies have tried to correlate the H. pylori infection in the development of HCC. Avenaud et al[20] detected H. pylori in 100% (8/8) of the liver tissue employed in their study. In 2001, Swedish researchers identified Helicobacter spp. in patients with HCC and cholangiocarcinoma[54]. Evaluating patients with chronic hepatitis, cirrhosis and HCC, Dore et al[21] used the serology and PCR to verify the presence of H. pylori infection. The results showed that 54% of patients were positive for the bacteria, and the prevalence of infection was higher in patients with HCC (73%) compared with patients with cirrhosis (58%) and chronic hepatitis (39%). Helicobacter DNA was detected in 17% of liver cirrhosis patients and 55% of individuals with HCC.

Verhoef et al[55] revealed the presence of Helicobacter DNA in 45% of liver samples from patients with HCC in contrast to 10% positive samples in the control group. Sequence analysis indicated that fragments had similarity with DNA of H. pylori. In this study the authors also noted the similarity among three samples of gastric biopsies of patients with HCC who were positive for the liver culture, suggesting that gastric colonization with H. pylori strains may be associated with the induction of HCC.

In 2004, Pellicano et al[56] confirmed the presence of Helicobacter spp. DNA in 17 (85%) of 20 liver samples from patients with HCC compared with 33% of positive Helicobacter cases in the control samples. Recently, more studies emphasize the detection of bacteria in hepatic tissue from patients with HCC, even suggesting its possible role in the progression of CLD due to higher prevalence of H. pylori in more advanced stages of liver disease as cirrhosis and HCC[25,57].

In addition, several researchers have described the association between H. pylori and HBV or HCV in the development of HCC. In fact, the prevalence of anti-H. pylori antibodies in patients infected with HBV is significantly higher when compared to subjects without viral infection[17,18,58-63]. In an attempt to verify the role of H. pylori in the progression of CLD in patients infected with HCV, investigators have reported that Helicobacter spp. DNA was detected in 4.2% of controls and 3.5% of individuals with noncirrhotic chronic hepatitis, compared with 61%-68% in cirrhotic liver and 90% in HCC tumoral tissue[22]. These results reinforced that prevalence of H. pylori infection may be associated with later stages of CLD and suggested that disease increased with the severity of the cancer. Then, it may be possible that H. pylori co-infection with HBV or HCV results in the progression from cirrhosis to cancer, reinforcing the synergistic cooperation between H. pylori and hepatitis virus in the development of HCC[47].

The mechanism by which H. pylori colonizes the human liver is not fully understood. Some researchers hypothesized that H. pylori DNA detection in liver tissue can result from bacterial translocation from the stomach into the blood through the portal system, especially in the advanced stages of chronic liver disease, when occurs the portal hypertension[47,64]. Furthermore, bacteria can reach the liver via circulating phagocytes and macrophages or retrograde transfer from the duodenum[24]. However, reports that involved H. pylori culture from the liver samples of patients with HCC support a true hepatic colonization by bacteria, discarding the possibility of retrograde contamination[25,65]. Additionally, no other bacteria in the digestive tract are associated with human liver carcinogenesis[66,67].

The experimental murine models of H. hepaticus infection show that bacteria are able to induce chronic active hepatitis and HCC in various strains of animals[11,68]. Moreover, enteric Helicobacter species are capable of producing toxins which can cause hepatocellular injury in vivo[69]. Furthermore, Helicobacter spp. may induce the production of proinflammatory chemokines and cytokines which contributes to the development of liver cancer by DNA damage, growth stimulation, increase of survival, angiogenesis and invasion into host tissue[70].

In a report employing human hepatocytes culture infected with H. pylori, Ito et al[71] have shown that bacteria are able to adhere and penetrate into these cells. The authors suggest that the process of bacterial internalization can be an H. pylori strategy to avoid the host immune reaction and remain in the liver, resulting in morphological and physiological changes in the hepatocytes. Analyzing the in vitro proliferation, adhesion and invasion responses of the hepatic tumor cell lines to LPS authors demonstrated that these characteristics were increased in response to LPS, which may be related to increased gene expression of interleukin-8 (IL-8) and transforming growth factor-beta 1 (TGF-β1). Considering that H. pylori has LPS, they inferred that bacteria may be ignored by host immune system and directly promote adhesion and invasion of hepatoma cells mediated by LPS[72].

In addition to the host immune system evasion mechanisms, H. pylori virulence factors detected in liver samples from patients with HCC, such as vacA and cagA genes, are supposed to be involved in liver carcinogenesis[21,63,73]. These findings were recently supported by Esmat et al[74] who found that the positivity of cagA gene was directly proportional to the severity of liver disease. They studied patients infected with HCV in the presence and absence of cirrhosis and HCC. The cagA positivity occurred in 75% of patients with cirrhosis and HCC, 52.9% of cirrhosis subjects without HCC and 32% of individuals with chronic hepatitis. The authors have also shown significant differences when compared METAVIR system among groups which confirmed H. pylori association with later stages of fibrosis.

Cirrhosis and hepatic encephalopathy

Cirrhosis is a major health problem with high incidence and prevalence worldwide[75]. Considering that patients with cirrhosis are more likely to develop gastrointestinal mucosal lesions, with increased risk for peptic ulcer disease (PUD)[76], it was suggested that H. pylori infection has an important role in the pathogenesis of PUD in cirrhotic patients and may be related to the development of hepatic encephalopathy (HE) and hyperammonemia[77]. In fact, a recent review has demonstrated that eradication of H. pylori infection in patients with cirrhosis may have a positive effect on the control of hyperammonemia and HE[78].

In a meta-analysis, researchers have described that prevalence of H. pylori infection in individuals with cirrhosis has increased significantly worldwide, especially in Europe and America, due to viral cirrhosis and PBC[79]. However, Pellicano et al[80] related that some papers which concluded that there was no association between the prevalence of bacteria and PBC[81,82] were not reported in this meta-analysis. Another report demonstrated that there is a significant association between H. pylori infection and portal hypertensive gastropathy (PHG) in cirrhotic patients which is also related to the severity of PHG, suggesting that eradication of H. pylori must be considered in cirrhotic patients with PHG[75].

HE is a frequent complication of liver cirrhosis and manifests itself as a wide variety of neuropsychiatric symptoms[83]. It is admitted that ammonia is the most relevant substance in the pathogenesis of HE. Then, reduction of ammonia production in the gastrointestinal tract is a current treatment strategy for HE[78]. Considering that H. pylori urease hydrolyses urea present in the gastric juice into ammonia and carbon dioxide, and the amount of ammonia produced in the gastric mucosa could increase blood ammonia levels in cirrhotic patients, the possible participation of H. pylori infection in the pathogenesis of HE has been studied.

The current data associating H. pylori infection to the pathophysiology of HE are inconclusive. Initially, some reports have demonstrated that H. pylori contributes to hyperammonemia in cirrhosis, and the eradication of bacteria may reduce the blood ammonia[84-86]. After that, other studies have not found a significant difference in the ammonia levels between cirrhotic patients with and without H. pylori infection[87,88]. In a review, Zullo et al[89] confirmed that gastric ammonia production by H. pylori urease appears to be inadequate to clinically affect ammonia levels in the majority if cirrhotic patients.

In a recent review, it was demonstrated that positivity of H. pylori infection was higher in HE patients compared to non-HE individuals, particularly in the older subjects. However, there are no strong evidences for an effect of bacteria on increasing blood ammonia level, nor there is strong evidence to support the hypothesis that H. pylori eradication can reduce blood ammonia level and improve HE symptoms[90].

However, authors have described that HE is not fully reversible and H. pylori might contribute to persistent cognitive impairment even after resolution of symptoms[91]. Furthermore, it is believed that inflammatory cytokines produced during H. pylori infection may cross the blood-brain barrier and contribute to the pathogenesis of cognitive dysfunction associated with cirrhosis[92].

Nonalcoholic fatty liver disease

The nonalcoholic fatty liver disease (NAFLD) is currently considered the most common liver disease in Western countries, affecting up to 25%-30% of subjects[93-95]. NAFLD includes a broad spectrum of liver disorders, which range from nonalcoholic fatty liver to nonalcoholic steatohepatitis (NASH) which may progress to cirrhosis and HCC without significant alcohol consumption[96-98].

Authors have shown the possible participation of H. pylori infection in the pathogenesis of insulin resistance (IR)[39]. Considering that IR is implicated in the etiology of NAFLD, the evaluation of H. pylori infection as a risk factor for IR may help clarify its effect on NAFLD[99].

In 2008, researchers have detected H. pylori 16S rDNA in hepatic tissue collected from a 44-year-old woman with NASH[100]. After that, in a study employing liver samples from patients with CLD, H. pylori DNA was amplified in 45.5% (5/11) of samples obtained from subjects with NAFLD[57].

The case report described by Abenavoli et al[101] related the improvement in IR and fatty liver indices after H. pylori eradication therapy, reinforcing the possible association among H. pylori infection, IR and NAFLD. In the same year, another study revealed that prevalence of anti-H. pylori IgG titers, together with lower circulating adiponectin and higher tumor necrosis factor-α levels, was higher in individuals with NAFLD compared with control group[102]. Moreover, using logistic regression analysis model, researchers mentioned that both H. pylori infection and Homeostasis Model of Assessment-Insulin Resistance which is the marker of the metabolic syndrome, were considered independent variables to predict NAFLD. In addition, no correlation was found between H. pylori infection and progression to NASH. Then, it was suggested that presence of H. pylori may be involved to early-stage NAFLD.

However, Sumida et al[103] performed a recent study which was the first one to show that NASH is more prevalent in H. pylori-positive patients than in non-infected individuals. Histopathologic analysis revealed that bacteria was associated with hepatocyte ballooning; however, there was no association of H. pylori with steatosis or liver fibrosis. Authors still mentioned that although the exact pathogenic mechanisms involved in hepatocyte ballooning as well as its role in NAFLD remain unclear, it is considered as a key histologic feature of NASH.

Considering that the prevalence of NAFLD is increasing worldwide and that H. pylori infection may present a role in its pathogenesis, further studies on this area are needed, in order to provide better understanding of the role of H. pylori infection in NAFLD. However, once this association is confirmed, it is possible that H. pylori eradication regimens might have therapeutic implications on NAFLD[39].

Fibrosis

As mentioned before, Helicobacter spp. infection is more prevalent in advanced stages of liver diseases, reinforcing the possible association of bacteria with the progression of chronic hepatitis to cirrhosis and HCC[49].

In order to elucidate these findings, researchers have induced experimental hepatic fibrosis with carbon tetrachloride (CCl4) administration in mice and rats orally challenged with H. pylori. Authors verified a significant increase in the fibrotic score in H. pylori-positive animals treated with CCl4 when compared with non-infected animals treated with CCl4. Furthermore, they observed that alpha-smooth muscle actin and TGF-β1 also enhanced in H. pylori infected animals[104].

After that, authors have suggested that increased liver fibrosis in H. pylori infection may occur through increased TGF-β1 induced pro-inflammatory signaling pathways in hepatic stellate cell line (HSC). They still mentioned that H. pylori infection may be involved in increased risk TGF-β1-mediated tumorigenesis by disturbing the balance between apoptosis and proliferation of hepatocytes[105]. Another group has also described an increase in activated kupffer cells and hydrogen peroxide levels in H. pylori infection which might result in activation of HSC alone or in combination with TGF-β1, amplifying hepatic inflammation via release of proinflammatory cytokines[106,107].

More recently, Esmat et al[74] studied liver samples from patients with HCV-related chronic hepatitis and cirrhosis in the presence or absence of HCC in order to verify the possible role of H. pylori infection in the disease progression. They showed that prevalence of cagA gene was directly proportional to severity of liver disease and was more positive in advanced stages of fibrosis (28.2%) compared to early stages (5.9%); it was still suggested by authors that H. pylori can produce toxins that may interfere with hepatic cells.

One aspect that deserves further analysis of hepatology experts is the analysis of a possible role of H. pylori infection in the liver fibrosis determined by schistosomiasis.

Therefore, these findings confirm that hepatocytes can be stimulated by H. pylori infection, resulting in collagen accumulation and, consequently, in hepatic fibrosis[108].

ROLE OF H. PYLORI IN PANCREAS DISEASES

Acute and CP

In the last decades, acute pancreatitis, defined as an acute inflammatory response from unregulated activation of pancreatic enzymes, has demonstrated an increase in its incidence[109,110]. This disorder, which can present a persistent hypovolemia, besides a decreased intravascular volume, can lead to extrapancreatic complications[111]. In 2007, a publication revealed an important frequency of patients with acute pancreatitis that also presented acute gastrointestinal mucosal damage[112] and it suggests that this bacterium infection could somewhat influence the acute pancreatitis progression.

Two important mechanisms-hypergastrinemia and duodenal acidification-together with the translocation of the microorganism or its toxins into the pancreas, have been cited as important mechanisms by which H. pylori infection could have an effect on the acute pancreatitis progression[26]. Warzecha et al[33] observed the effect of H. pylori infection of the gastric mucosa on the clinical evaluation of the disease in a model of ischaemia/reperfusion-induced in rats. Their results have suggested an evidence of damage effect of H. pylori infection of the stomach in patients diagnosed with acute pancreatitis. Lee et al[113] trying to evaluate the relationship between PUD and acute pancreatitis, studied 78 patients with acute pancreatitis, and 41 of them suffered from peptic ulcer disease, but only 31.7% of these 41 patients were infected by H. pylori. They concluded that PUD is associated with severe acute pancreatitis and the treatment for PUD should be considered for patients with that disease.

As regards to the possible association between CP and H. pylori infection, different hypothesis have been suggested. According to Manes et al[26], there are three possible roles of this infection and the evolution of CP: The influence of this microorganism infection in the pathogenesis and evolution in idiopathic forms of CP, the influence of this infection on the exocrine pancreatic secretion in individuals with CP, and the possibility of CP influences the gastrointestinal physiology and, consequently, the pathogen colonization.

Besides, important alterations in the gastric function found in CP could demonstrate the presence of H. pylori infection in the stomach rather than changes induced by the pancreatic disease. Study developed by Manes et al[35] reported that the prevalence of H. pylori infection in patients with CP is similar that of two other studied groups of individuals, one with patients with alcoholic liver cirrhosis and other with healthy subjects, but the frequency and severity of H. pylori negative chronic gastritis in the antrum was significantly higher in individuals with CP than in the other groups. Similar results were found by Niemann et al[34]. In their study, the prevalence of H. pylori infection was investigated in individuals with CP, with and without duodenal ulcers, in comparison to a control group which only included patients with duodenal ulcer. The results suggested that the bacterium infection can contribute to the CP development, but no as the main cause for the disease development.

Pancreatic cancer

The potential role of H. pylori infection in pancreatic cancer (exocrine pancreatic carcinoma or pancreatic ductal adenocarcinoma) has also been suggested. This cancer is the fifth leading cause of cancer related death worldwide. Its high degree of death incidence is especially due to the diagnosis generally done in the advanced stage and to the poor responses to current treatments[114-117].

Lowenfels et al[118] considered that similarly pathologic consequences of gastric tissue due to H. pylori chronic infection could be observed in CP and consequently in pancreatic cancer.

A meta-analysis of six cohort studies and one case-control study found that the pooled relative risk estimate for pancreatic cancer among patients with CP is 13.3[119]. Duell et al[120] analyzed 5048 patients with pancreatic cancer in ten case-control studies and found only a small association between pancreatic cancer and antecedent CP, although this study presented some limitations. Despite of these findings, it has been considered that CP is a rare cause of pancreatic cancer[121].

Other possible mechanisms for the association between H. pylori and pancreatic cancer include changes in gastrin (increased secretion) and somatostatin (low number of antral somatostatin cells) resulting from H. pylori gastritis[28,122-124], increased DNA synthesis, increased formation of N-nitroso components (due to bacterial overgrowth), and chronic inflammation properly, aspect that can be responsible by itself for initiating the carcinogenesis process[125-127].

In addition, H. pylori chronic infection, as responsible for the production of proinflammatory cytokines and reactive oxygen species, as well as of other inflammatory mediators, may induce the tissue inflammation. In consequence, the increase on genomic DNA damage and cell proliferation (aspects that may lead to an inactivation of tumor-suppressor genes), are factors which may contribute the malignant transformation of pancreatic cells[128]. Considering it, Takayama et al[129] reported that activities of nuclear factor-κb, activator protein-1, and serum response element of human pancreatic cancer cells were shown to be increased by H. pylori infection, as well as serum levels of IL-8, suggesting that the development of pancreatic cancer could be similar to the gastric carcinogenesis. Consequently, environmental aspects such as dietary habits, smoking and alcohol consumption can contribute for the development of pancreatic cancer[130], as well as they contribute in gastric cancer.

Finally, other important meta-analysis concluded that H. pylori infection can be considered a significantly factor to the pancreatic cancer development, also considering that regional aspects can be important for this. Xiao et al[29] reported countries regional aspects reporting that the association between H. pylori infection and pancreatic cancer development is more evident in Europe and East Asia, and decreases in North America. They also suggested that H. pylori CagA positive strains are not possibly associated with pancreatic cancer development. Despite of it, meta-analysis conducted by Wang et al[30] concluded that H. pylori infection and CagA positive strains are associated with a decreased risk of pancreatic cancer in Eastern populations but have no significant associations in Western countries. Lindkvist et al[131], in a prospective study, also concluded that no association between this pathogen infection and the risk for pancreatic cancer was found in their nested case-control study within a population based cohort. Nevertheless, recent study with 56 cases of pancreatic cancer analyzed anti-Hp IgG (H. pylori-specific antibodies), Hp IgM (H. pylori antibodies) and CagA-Hp-IgG (H. pylori serotoxin-associated protein A antibody), comparing the results with a control group. The results obtained demonstrated that H. pylori infection rate in the patients group was significantly higher than that in the control group (P < 0.01). Besides, the positive rate of CagA-Hp in the observation group was 38.88%, and 21.53% in the control group (P < 0.05). The researchers concluded that H. pylori infection, especially with CagA positive strains, besides smoking history and the history of CP, is one of the risk factors for pancreatic cancer development[132].

AIP

AIP has been recognized as a form of CP, which is always associated with autoimmune manifestations[133]. It is defined as an inflammatory process of the pancreas characterized by hypergammaglobulinemia, enlargement of the organ, fibrotic changes with lymphocytic infiltrations and presence of autoantibodies, among other alterations, all of them contributing to the tissue destruction possibly by apoptosis[36].

The coexistence of AIP with other autoimmune diseases has been reported in the literature. Among them, they can be cited Sjögren’s syndrome, PBC, autoimmune hepatitis, Hashimoto’s thyroiditis and gastric ulcer, among others[134-136].

As regards to the mechanisms by which H. pylori infection could trigger AIP, the molecular mimicry between bacterial antigens and human ones has been reported as the most plausible hypothesis, based on epidemiological studies[137,138].

According to Kountouras et al[36], bacterial heat shock proteins (Hsps), particularly Hsp-60 or Hsp-70 of H. pylori, may represent major target antigens responsible for molecular mimicry causing autoreactivity between this microorganism and the host’s immune gastric tissue, being probably responsible for the humoral and/or cellular (T-cell) response against these proteins and consequently influencing the pathogenesis of autoimmune diseases such as AIP[139,140].

Guarneri et al[141] reported the existence of two molecules possibly involved in the molecular mimicry (CA-II and α-HpCA), that are homologous to the HLA molecule DRB1*0405, reported as a risk factor for the development of AIP[142]. Considering it, and the importance of α-HpCA for gastric colonization, the host immune response against this molecule could turn against the autoantigen CA-II, promoting the appearance of AIP in genetically predisposed individuals[142,143].

Besides, HLA-DR antigens are expressed on the pancreatic duct cells as well as on CD4⁺ suggesting an autoimmune mechanism involved in inflammation[134,144], event that, in conjunction with epithelial cells apoptosis, can be upregulated by H. pylori infection and, consequently, be important to the development of AIP[140].

Besides, important researches have indicate that apoptosis is a mechanism of cell death in several important H. pylori-associated upper gastrointestinal damages and in extradigestive disorders. These studies hypothesized that H. pylori could change the expression of some genes including that ones encoding growth factors, transcription factors, and apoptosis proteins, among others, consequently contributing to the development both of gastrointestinal and extradigestive diseases. In addition, some virulence factors of H. pylori, such as urease, could contribute to cell apoptosis probably to activation of T cells[36]. Finally, in addition, microcirculatory changes promoted by H. pylori infection through platelet and platelet-leukocyte aggregation could promote the amplification of the pancreatic injury[142,145,146].

Obviously the relationship between H. pylori infection and AIP development has to better studied, but it can be considered that various autoimmune and apoptotic sequelae induced by this chronic and long-term infection appear to influence the pathophysiology of AIP.

DM and metabolic disorders

Type 1 or type 2 DM development have also been reported to a high prevalence of H. pylori positive individuals[147]. In a meta-analysis, Zhou et al[148] found a high prevalence of this microorganism infection in individuals with DM, particularly type 2. Jeon et al[149] reported similar results and concluded in their sampling that individuals who were seropositive for H. pylori were 2.7 times more at risk to develop DM than seronegative patients. Nevertheless, this association remains controversial[150].

Gunji et al[151] evaluated the association between IR and H. pylori infection and suggested that this infection could really contribute even in an independent way to promoting this condition. Besides, study developed by So et al[152] in China concluded that H. pylori infection could be an independent predictor for hyperglycemia and reduced insulin sensitivity and this fact has shown to be important for the high prevalence of type 2 DM in this country population.

Metabolic syndrome, one of the most prevalent global health problems that predisposes to type 2 DM and it is linked to IR, has also be proposed to be associated to H. pylori infection[153,154]. Polyzos et al[39], through a quantitative homeostatic model, studied this possible association, but data concerning this relationship remain contradictory. In anyway, the eradication of the infection appears to prevent negative metabolic effects in the pathogenesis of DM[155].

CONCLUSION

The higher prevalence of H. pylori infection in subjects with hepatitis, cirrhosis, HE, NAFLD and HCC may suggest its possible role in the pathogenesis of CLD. Moreover, many studies have found a synergistic association between this bacterium and hepatitis viruses, mainly HCV, suggesting that H. pylori may represent a co-risk factor in the progression of liver diseases, especially HCC. However, high quality prospective studies in non-cirrhotic HCC patients co-infected with HCV are needed to confirm these findings.

Another important fact to consider is that patients with severe CLD are more likely to develop bacterial infection. Then, H. pylori infection could be related to immunological and inflammatory changes in the liver. In this context, bacterium would not be considered a risk factor for HCC and the colonization of hepatic tissue could result from tumor process.

Complex interactions in the gastric mucosal changes determined by the pathogen infection could contribute to the pancreatic cancer development, especially by N-nitrosamine exposure of the host, added by dietary and smoking habits. Important meta-analyses have reported an increased risk for pancreatic cancer development in subjects contaminated by H. pylori. In addition, it has also been suggested that H. pylori causes AIP due to molecular mimicry between bacterium and enzymes that are highly expressed in the pancreatic ductal and pancreatic cells, in addiction of apoptosis process. Finally, H. pylori infection has been associated to both type 1 and type 2 DM development, as well as metabolic syndrome, interaction that has not been determined. Despite some of these studies present some limitations, including a small number of patients and only hypothesis which have not been elucidated yet, the role played by H. pylori in the pathogenesis of such conditions have a substantial impact of healthcare and obviously its infection and the relationship with all the conditions described above has to be subject for further investigation.

Considering the worldwide liver and pancreatic diseases burden, as well as the possible association between H. pylori infection, which is commonly chronic and reported to poor sanitary conditions, besides the widespread use of some medicines that can mask the real condition of the infection, a complete elucidation of the role played by H. pylori in the pathogenesis of such conditions have certainly a substantial impact of healthcare.