Role of Helicobacter pylori infection in pathogenesis of atherosclerosis

Abstract

Though a century old hypothesis, infection as a cause for atherosclerosis is still a debatable issue. Epidemiological and clinical studies had shown a possible association but inhomogeneity in the study population and study methods along with potential confounders have yielded conflicting results. Infection triggers a chronic inflammatory state which along with other mechanisms such as dyslipidemia, hyper-homocysteinemia, hypercoagulability, impaired glucose metabolism and endothelial dysfunction, contribute in pathogenesis of atherosclerosis. Studies have shown a positive relations between Cytotoxic associated gene-A positive strains of Helicobacter pylori and vascular diseases such as coronary artery disease and stroke. Infection mediated genetic modulation is a new emerging theory in this regard. Further large scale studies on infection and atherosclerosis focusing on multiple pathogenetic mechanisms may help in refining our knowledge in this aspect.

Key Words: Atherosclerosis; Coronary artery disease; Helicobacter pylori; Infection; Stroke

Core tip: Though a century old hypothesis, infection as a cause of atherosclerosis is still a debatable issue. Clinical and epidemiological studies had shown a possible association, however in-homogeneity in the study population and methodology has yielded conflicting results. We performed a literature search on MEDLINE electronic database using keywords such as Helicobacter pylori (H. pylori), infection, atherosclerosis, coronary artery disease, myocardial infarction, stroke, cerebro-vascular disease and peripheral arterial disease using MeSH terms, to review this subject. The association between H. pylori and atherosclerosis is not strong and a causal role is not yet established. Large scale studies on infection and atherosclerosis focusing on multiple pathogenetic mechanisms may help in refining our knowledge in this aspect.

INTRODUCTION

Though a century old hypothesis, infection is still debated as a cause of atherosclerosis[1]. Infection triggers a chronic inflammatory state which along with other mechanisms such as dyslipidemia, hyper-homocysteinaemia, hypercoagulability, impaired glucose metabolism and endothelial dysfunction contribute in pathogenesis of atherosclerosis. Studies have shown a positive relations between Cytotoxic associated gene-A (Cag-A) positive Helicobacter pylori (H. pylori) strains with vascular diseases such as coronary artery disease (CAD) and stroke. Infection mediated genetic modulation is a new emerging theory in this regard. Minick and Fabricant’s work on infection and atherosclerosis in animal model had made the ground for revolutionary research in this field[2,3]. Chronic infection triggers T1 Helper cell (Th1) mediated inflammatory reaction, which plays a crucial role in atherosclerosis. Markers of infection and inflammation were also studied as the risk factors for atherosclerosis[4-6]. An association between infection and atherosclerosis was established following detection of infectious agents from arterial vessels, positive immune-histochemistry studies, detection of microbial DNA sequences in atherosclerotic plaques by PCR method, positive serological response with higher titres in infected patients, and a positive correlation of infection with atherosclerotic burden and dyslipidaemia[7-23]. The microbial agents that have been implicated in the etio-pathogenesis of atherosclerosis are presented in Table 1, Figure 1.

Table 1 Microbial agents associated with atherosclerosis.

Bacteria	Viruses
Chlamydia pneumonia	H simplex virus type 1 and 2
Helicobacter pylori	Cytomegalovirus
Helicobacter cinaedi	Epstein- Barr virus
Hemophilus influenzaMycoplasma pneumonia

Figure 1 Theories of infection related atherosclerosis.

This review has been divided into two parts. Part I elucidates different mechanisms of H. pylori related atherosclerosis and relevant studies. Part II reviews the literature about H. pylori association with atherosclerotic diseases such as CAD, stroke and peripheral arterial disease (PAD).

MECHANISMS OF H. PYLORI RELATED ATHEROSCLEROSIS

Development of CAD in patients without conventional risk factors suggests a possible role of an additional unexplored mechanism. The evolution of atherosclerosis in the background of chronic inflammatory milieu involves multiple pathways (Table 2, Figure 1). Some of these pathways will be discussed in following section.

Table 2 Mechanisms of Helicobacter pylori related atherosclerosis.

Induction of inflammatory response secondary to chronic infectious state

Endothelial damage

Chronic low grade activation of coagulation cascade

Dysregulation of lipid metabolism resulting in increased total cholesterol and triglyceride levels and reduced high density lipoprotein levels

Hyperhomocysteinaemia

While the proponents support the possible association[52,70,104], the opponents refute this hypothesis[46,71,105].

H. pylori and endothelial dysfunction

Infection related chronic vascular inflammation can result in endothelial dysfunction. Tousoulis et al[24] first proposed an inflammatory mechanism for endothelial dysfunction. C-reactive protein (CRP) and inflammatory adhesion molecule such as intracellular adhesion molecule-1 (ICAM-1) are elevated in patients with H. pylori infection, suggesting a possible link between infection and endothelial dysfunction[25]. Chronic infection triggers release of inflammatory cytokines such as interleukin (IL)-1, IL-6 and tumor necrosis factor-α (TNF-α), which affects microvascular vasomotor functions, resulting into vasoconstriction and endothelial dysfunction. Coskun et al[26] studied a possible relation between H. pylori infection in children and endothelial dysfunction as a precursor for future atherosclerosis. There was no significant association between H. pylori seropositivity and CRP levels with flow mediated vasodilation. Another evidence is about increase prevalence of slow flow in the major epicardial coronary arteries in patients with H. pylori infection[27]. The possible mechanism of slow flow was endothelial dysfunction secondary to raised homocysteine levels. H. pylori infection causes malabsorption of vitamin B12 and folic acid and thus increases serum homocysteine levels. Evrengul et al[27] reported a mean TIMI frame count of coronary flow as 46.3 ± 8.7 and 24.3 ± 2.9 in patients with and without H. pylori infection, respectively. An association between H. pylori infection and functional vascular disorders such as cardiac syndrome-X, migraine and primary Reynaud phenomenon provides evidence about its role in endothelial dysfunction and atherosclerosis[28-32].

Chronic inflammation

Presence of chronic, persistent inflammation provides a vital clue for infectious theory of CAD. Chronic H. pylori infection induces a pro-inflammatory state, resulting into an increase in cytokines levels such as TNF-α, Interleukins (IL-1, IL-6, IL-8), gamma interferon, coagulant factors - fibrinogen, thrombin and soluble adhesion molecules such as intercellular adhesion molecule (ICAM-1), vascular cell adhesion molecule (VCAM-1)[33-35]. Eradication of H. pylori infection by use of antibiotics leads to reduction in cytokines levels[34,36]. These evidences suggest that H. pylori induced inflammatory cascade plays an active role in atherosclerosis. Activated T lymphocytes and macrophages following cytokines release induce proliferation of smooth muscle cells and extracellular matrix, which plays a crucial role in pathogenesis of atherosclerosis. It also stimulates metalloproteinases production, which causes rupture of atheroma cap and leads to acute coronary syndromes. However, a large population based study failed to support the association between H. pylori and increased inflammatory cytokines[37].

Recent research has unveiled novel molecular mechanisms of H. pylori mediated inflammation[38-41]. H. pylori infection exerts an immune-inflammatory reaction by activating cyclooxygenase enzyme-2 (COX-2), which causes increase production of prostaglandin (PGE₂) and nitric oxide (NO). H. pylori cell wall lipopolysaccharide (LPS) triggers toll-like receptor-4, which activates various secondary mediators such as mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase, c-Jun N-terminal kinase (JNK) and p38 kinase resulting in enhanced stimulation of NOS and COX-2 gene expression[38,39]. LPS-induced activation of MAPK cascade is also associated with epidermal growth factor receptor (EGFR) transactivation which is a key protein regulating cellular proliferation, differentiation, migration and modulation of apoptosis[41]. Gherlin, a peptide hormone activates NO synthase, thereby inhibiting H. pylori LPS induced activation of COX-2 and other inflammatory pathways[40].

H. pylori and hyper-homocysteinaemia

H. pylori causes atrophic gastritis, which is associated with malabsorption of vitamin B12 and folic acid. Deficiency of these vitamins causes hyper-homocysteinaemia due to interruption of re-methylation pathway[42-45]. Hence, it may have a role in the pathogenesis of premature atherosclerosis[45]. In a study by Kutluana et al[45], carotid intima media thickness was found to be higher in patients with H. pylori related atrophic gastritis. In this study, H. pylori positive patients had significantly higher homocysteine levels compared to controls (14.17 ± 9.24 μmol/L vs 9.81 ± 3.42 μmol/L, P = 0.01). Senmaru et al[46] reported a higher prevalence of CAD in atrophic gastritis (5.8% vs 2.8%). Torisu et al[47] had shown an association between increased pulse wave velocity, a preclinical marker of atherosclerosis with atrophic gastritis. Apart from hyper-homocysteinaemia, other mechanisms are reduced ghrelin levels and induction of chronic pro-inflammatory cascade resulting into endothelial damage[46,47]. However, Bloemenkamp et al[48] did not support the hypothesis about H. pylori infection induced hyper-homocysteinemia and atherosclerosis.

H. pylori and dyslipidemia

H. pylori infection is associated with lower HDL cholesterol (HDL-C) and higher total cholesterol (TC), LDL cholesterol (LDL-C) and triglyceride levels. Higher apolipoprotein-B and lower apolipoprotein-A (apo-A) levels were also reported[11]. Murray et al[49] demonstrated that women with H. pylori infection had lower HDL-C (P = 0.006). Another study had also shown significantly lower HDL-C levels in infected patients[11]. Niemelä et al[50] and Laurila et al[22] reported an increase triglyceride levels in H. pylori positive patients. These alterations in lipid homeostasis proved to be significant even after adjusting co-variables such as socioeconomic class, body weight, age and diabetic status[22,51]. de Luis et al[52] showed that eradication of H. pylori decreases apo-A and increases HDL-C. Other studies had also shown reduction in TC, LDL-C levels and increase in HDL-C, apo-AI and apo-AII levels following H. pylori eradication[53-55]. However, this association was not supported by few other authors[56-59].

H. pylori, impaired glucose metabolism and metabolic syndrome

Gillum et al[60] reported a significant association of H. pylori seropositivity with CAD in diabetic males. de Luis et al[51] showed that CAD and cerebrovascular diseases were significantly more seen in H. pylori infected diabetic patients. Yoshikawa et al[61] suggested that H. pylori seropositivity increases brachial-ankle pulse wave velocity, a marker of atherosclerosis, in patients with impaired glucose metabolism. Aydemir et al[62] reported that H. pylori positive subjects had higher homeostatic model assessment-insulin resistance (HOMA-IR) levels (2.56 ± 1.54 vs 1.73 ± 1.1, P < 0.05), a surrogate of insulin resistance, as compared to H. pylori negative controls. Aslan et al[63] had shown that paraoxanase, a marker of oxidative stress is well correlated with HOMA-IR levels and is significantly elevated in H. pylori positive patients. Regarding role of H. pylori eradication therapy in improvement of glucose tolerance, Gen et al[64] reported that HOMA-IR level significantly reduced after successful therapy, whereas Park et al[65] did not show any significant reduction. Polyzos et al[66] in his systematic review concluded that available evidences indicate a potential association between H. pylori infection and insulin resistance. Gunji et al[67] reported that H. pylori infection was significantly and independently associated with metabolic syndrome. A recent study by Ando et al[68] revealed that eradication of H. pylori increases circulating adiponectin levels and might be helpful in prevention of metabolic syndrome. Naja et al[69] suggested no association between H. pylori infection and metabolic syndrome or impaired glucose tolerance.

H. pylori, hypertension and arterial stiffness

Migneco et al[70] demonstrated a significant reduction in blood pressure after eradication of H. pylori in hypertensive subjects. The possible association of H. pylori with arterial stiffness was initially reported by Adachi and Yoshikawa. Adachi et al[71] reported that carotid pulse wave velocity was higher in seropositive subjects. Yoshikawa et al[61] similarly reported a higher brachial-ankle pulse wave velocity in seropositive patients with impaired glucose metabolism. The possible association of H. pylori and arterial stiffness tends to be more in younger subjects, whereas in the elderly arterial stiffness is more often due to aging[72]. Honda et al[73] demonstrated that H. pylori infection did not affect the age related progression of arteriosclerosis over a 4 years follow-up period.

EVIDENCE OF ASSOCIATION BETWEEN H. PYLORI AND ATHEROSCLEROSIS

H. pylori and CAD

Demonstration of an association between H. pylori and CAD is always challenging. Both conditions are more prevalent in the population, increases with age and are related to socioeconomic status. The following section reviews the evidence of H. pylori association with CAD.

Numerous studies have shown that CAD patients have a higher prevalence of H. pylori infection[74-77]. Vijayvergiya et al[77] demonstrated that CAD patients had higher IgG seropositivity as compared to controls (42% vs 23%, P = 0.06). Franceschi et al[78] found that H. pylori Cag-A was significantly associated with acute coronary events (OR = 1.34; 95%CI: 1.15-1.58, P = 0.0003). Niemelä et al[50] showed that the association between CAD and H. pylori infection was not strong. A meta-analysis revealed that there is a little association between H. pylori infection and stroke, but the strength of association was greater for Cag-A positive strains[79]. H. pylori was shown to be associated with premature CAD even in patients without conventional cardiovascular risk factors[80,81]. A number of studies had shown a negative association between H. pylori and CAD which include serological[82,83] and histological studies[84-86]. A negative association is even reported in long term follow-up studies[87]. The Australian Busselton health study comprising of 1612 healthy subjects demonstrated negative association between infection and CAD or stroke[88]. Danesh et al[89] in his meta-analysis of five prospective studies reported no significant association of H. pylori infection with CAD (RR = 1.13). Association of H. pylori infection and outcome of CAD treatment had also been studied. Schiele et al[90] found that H. pylori infection was not a risk factor for restenosis after percutaneous coronary angioplasty. Limnell et al[91] had shown an inverse relationship between H. pylori infection and coronary bypass graft occlusion. Results from Caerphilly heart disease study suggested that Cag-A seropositivity had no relations with CAD or CAD related mortality[92].

H. pylori has been associated with cardiac syndrome X, i.e., angina pectoris with normal epicardial coronaries[28-30]. The proposed mechanism is chronic endothelial dysfunction. Eskandrian et al[28] reported a higher prevalence of H. pylori positivity in syndrome X patients compared to controls (95% vs 47.5%). Patients with syndrome X were found to be more commonly associated with H. pylori Cag-A positivity and elevated IL-1 and TNF-α[93]. Lanza et al[94] has also described association of inflammation, infectious burden and vascular dysfunction. Assadi et al[30] reported 15% of patients with syndrome X had urea breath test (UBT) positivity for H. pylori while none of the patients with chronic stable angina or controls had UBT positivity.

H. pylori and acute myocardial infarction

H. pylori induced inflammatory reaction is possibly responsible for plaque instability and platelet aggregation in acute coronary syndrome patients. Danesh et al[95] demonstrated a higher prevalence of H. pylori infection (42% vs 24%, OR = 1.75) in young acute myocardial infarction (AMI) survivors. Alkout et al[96] showed a higher titre of H. pylori IgG titre in patients who died of AMI (151 ng/mL vs 88 ng/mL, p= 0.034). Kahan et al[97] reported a higher prevalence of H. pylori seropositivity in recent myocardial infarction patients as compared to controls (68% vs 53%, OR = 1.36). This remained significant even after adjusting for other CAD risk factors like age, sex, smoking and hypertension. Kinjo et al[98] suggested that H. pylori infection was significantly associated with AMI in younger patients (age < 55 years, OR = 2.7) but not in those with age of > 55 years. Frazer et al showed a higher prevalence of H. pylori infection in AMI patients compared to control (41.6% vs 34.5%; P = 0.038)[99].

Similar to CAD, negative associations is also been reported between H. pylori and myocardial infarction. Zhu et al[100] hypothesised that H. pylori infection could not lead to CAD or myocardial infarction. Murray et al[101] had shown a negative association between H. pylori and risk for myocardial infarction. Pellicano et al[102] reported a negative association between cytotoxic H. pylori strains and myocardial infarction, with insignificant anti-Cag-A antibody seropositivtiy between cases and controls (33.8% vs 26.8%).

H. pylori Cag-A positivity - Is the risk greater?

Cag-A positivity has raised a curiosity in the infectious theory of atherosclerosis. Several studies had shown a significant relationship between Cag-A strain and CAD or stroke. Carriers of Cag-A positive strains had a higher risk for stroke (OR = 2.99) and carotid plaque instability (OR = 8.42)[103]. De Bastiani et al[104] showed increased prevalence of Cag-A seropositivity and ischemic stroke. Rasmi et al[93] reported a positive relation between Cag-A seropositivity and cardiac syndrome-X. Huang et al[105] revealed that Cag-A positive strains enhanced atherosclerosis in CAD patients by modifying oxidised LDL levels and high sensitive C-reactive protein (hsCRP) levels. Kowalski[36] showed that Cag-A positivity was significantly associated with greater coronary artery lumen loss and restenosis after percutaneous coronary artery stenting. He also demonstrated that H. pylori eradication significantly attenuate reduction in coronary artery lumen after coronary artery stenting[36]. But various authors had denied the excess risk of Cag-A positive strains with atherosclerosis. Koenig et al[106] demonstrated a similar prevalence of Cag-A seropositivity in CAD patients and healthy subjects. Whincup et al[107] in his prospective study comprising of 505 patients and 1025 healthy subjects had clearly shown that there was no significant association of seropositivity with CAD. Murray et al[101] reported negative association between the virulent H. pylori Cag-A strains and acute myocardial infarction.

H. pylori and stroke

By catalysing atherosclerotic pathways, H. pylori infection may be a risk factor for ischemic stroke. Single infectious agent is weakly linked to stroke but cumulative chronic infectious exposures, or “infectious burden”, have been associated with the risk of stroke. The adjusted hazard ratio demonstrating the risk of association between H. pylori and stroke was 1.13, whereas that of infectious burden and stroke was 1.39[108]. The possible mechanisms include macrophage activated plaque destabilization, increased expression of various adhesion molecules and inflammatory cytokines, localized hypercoagulability, altered gene expression, and a molecular mimicry. Markus et al[109] found a higher prevalence of H. pylori seropositivity in stroke cases compared to controls. There was an association between H. pylori infection and large vessel disease and lacunar stroke irrespective of other confounding factors. Another study by Grau et al[110] demonstrated an association between H. pylori seropositivity and ischemic stroke. Elkind et al[111] suggested that that chronic infectious burden results in increase carotid plaque thickness and stroke. A retrospective study reported higher incidence of ischemic stroke in patients with H. pylori infection than in non-infected group (14.8 vs 8.45 per 1000 person years)[112]. Diomedi et al[113] showed that Cag-A positive H. pylori infection was associated with poorer short term clinical outcomes and greater carotid intima media thickness in stroke patients. Increased risk of stroke in Cag-A positive H. pylori patients may be due to enhanced plaque vulnerability[103,114]. In one of the studies, the positive correlation between H. pylori and stroke was confounded by socioeconomic class[115]. A study on chronic bacterial infection and stroke demonstrated that elevated anti- H. pylori antibody was not significantly associated with ischemic stroke[116].

H. pylori and peripheral arterial disease

Studies about association of H. pylori infection with peripheral arterial disease (PAD) are limited. Bloemenkamp et al[117] demonstrated infection as a novel risk factor for PAD in young women. A case control study on infection and PAD in young women suggested that H. pylori infection was positively correlated with PAD only in those with high CRP levels[118]. Sawayama et al[119] reported a significantly higher prevalence of H. pylori infection in PAD cases than in controls (79.7% vs 44.8%; P < 0.01).

CONCLUSION

Overall the association between H. pylori and CAD is not strong and a causal role is yet to be established. Future studies on larger scale may possibly establish a stronger link between the two. If it gets established, there can be drastic reduction in burden of CAD by managing H. pylori infection. Proponents of infectious theory will have a real challenge in the years to come because establishing a definite causal role of H. pylori in CAD will be a nightmare due to the existence of numerous confounding factors. Opponents may continue to criticise the infectious theory of CAD because of lack of strong scientific evidence.