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Microbiology (2015), 161, 1150–1160
Review
DOI 10.1099/mic.0.000066
Inflammation and proliferation – a causal event of
host response to Helicobacter pylori infection
Vinod Vijay Subhash and Bow Ho
Correspondence
Bow Ho
Department of Microbiology, Yong Loo Lin School of Medicine, National University of Singapore,
117545, Singapore
[email protected]
Helicobacter pylori is a major aetiological agent in the development of various gastroduodenal
diseases. Its persistence in gastric mucosa is determined by the interaction between various host,
microbial and environmental factors. The bacterium colonizes the gastric epithelium and induces
activation of various chemokine mediators, including NFkB, the master regulator of inflammation.
H. pylori infection is also associated with an increase in expression of cell cycle regulators,
thereby leading to mucosal cell hyper-proliferation. Thus, H. pylori-associated infections manifest
activation of key host response events, which inadvertently could lead to the establishment of
chronic infection and neoplastic progression. This article reviews and elaborates the current
knowledge in H. pylori-induced activation of various host signalling pathways that could promote
cancer development. Special focus is placed on the inflammatory and proliferative responses that
could serve as suitable biomarkers of infection, since a sustained cell proliferation in an
environment rich in inflammatory cells is characteristic in H. pylori-associated gastric
malignancies. Here, the role of ERK and WNT signalling in H. pylori-induced activation of
inflammatory and proliferative responses respectively is discussed in detail. An in depth analysis of
the underlying signalling pathways and interacting partners causing alterations in these crucial
host responses could contribute to the development of successful therapeutic strategies for the
prevention, management and treatment of H. pylori infection.
Introduction
Helicobacter pylori infects the human gastric mucosa and
plays an important role in the pathogenesis of chronic gastritis, peptic ulcer disease and gastric cancer (Blaser, 1998;
Kuipers et al., 1995). Studies have shown a strong association between H. pylori infection and the development
of gastric adenocarcinoma and mucosa-associated gastric
lymphoma (Parsonnet et al., 1991; Uemura et al., 2001).
Although colonization with H. pylori occurs in a vast
majority of the population, only a minority of individuals
develop clinical manifestations of the infection. Thus, acquisition does not directly mean infection but is a condition
that could enhance the relative risk of developing various
clinical disorders of the upper gastrointestinal tract. For
instance, the lifetime risk of developing peptic ulcer disease
in H. pylori positive subjects is estimated to be 10–20 %
(Kuipers et al., 1995), which is three to fourfold higher
than in non-infected individuals (Nomura et al., 1994). H.
pylori manifests many mechanisms of pathogenicity, which
include changes in host gene expression, infection-induced
inflammation and cell proliferation, loss of epithelial cell
Abbreviations: EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; MAPK, mitogen activated protein
kinase; OMP, outer-membrane protein; PAI, pathogenicity island.
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polarity and cell elongation (Yamaoka, 2010). The primary
disorder that accompanies H. pylori colonization is chronic
acute gastritis. The intra-gastric severity of this chronic
inflammatory process depends on a variety of factors, such
as host genetics and immune response, diet, level of acid
production and importantly, virulence characteristics of
the infecting strain (Kusters et al., 1997). Studies on the
differential pathogenic properties of H. pylori indicate that
the increased pathogenicity correlates with the ability of the
strain to induce morphological changes, infection-associated
inflammation, vacuole formation and successive degeneration of in vitro cultured cells (Kusters et al., 2006). Some of
the established virulence factors that are likely to play a
crucial role in determining the outcome of H. pylori infection are as follows.
cag Pathogenicity island (cag PAI)
The cag PAI is a 40 kb DNA insertion element that contains 27 to 31 genes flanked by 31 bp direct repeats. It
encodes one of the most intensely investigated H. pylori
proteins, CagA, and its expression is strongly associated
with peptic ulceration (Censini et al., 1996; Crabtree et al.,
1991). Eighteen of the cag PAI-encoded proteins serve as
building blocks of a type IV secretory system that functions
in exporting bacterial proteins, like CagA, across the
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H. pylori-induced inflammatory and proliferative responses
bacterial membrane and into host gastric epithelial cells
(Christie & Vogel, 2000). Due to its association with
gastroduodenal diseases, the cag PAI is now a well characterized H. pylori virulence determinant, and CagA is
frequently used as an indicator of the presence of the
entire cag PAI (Wroblewski et al., 2010).
al., 2000). The presence of some of these OMPs is associated
with enhanced mucosal inflammation and colonization
ability of H. pylori (Yamaoka et al., 2002) resulting in
mucosal injury and increased risk of gastroduodenal diseases
(Sugimoto et al., 2011).
Characteristics of H. pylori infection
Cytotoxin-associated gene A (CagA)
The 132 kDa oncoprotein CagA is encoded by the gene
cagA and is present in approximately 60–70 % of H. pylori
strains (Caleman Neto et al., 2014). It is translocated into
the host cell through the type IV secretory system and is
tyrosine phosphorylated at glutamate-proline-isoleucinetyrosine-alanine (EPIYA) motifs (Noto & Peek, 2012).
Once translocated into host cells, CagA induces actin
cytoskeleton and cell morphological changes, termed a
‘hummingbird’ phenotype (Hatakeyama, 2008). This virulence factor has been studied extensively, especially its role
in H. pylori-induced inflammation. Phospho-CagA activates a eukaryotic tyrosine phosphatase (SHP-2), leading to
sustained activation of extracellular signal-regulated kinases
1 and 2 (ERK1/2). Additionally, ectopic expression of CagA
in gastric epithelial cells induces NFkB nuclear translocation
and IL-8 production, and was shown to be mediated
through the ERK1/2 pathway (Backert & Naumann, 2010).
Vacuolating cytotoxin A (VacA)
VacA is a highly immunogenic 95 kDa protein that induces
massive vacuolization in epithelial cells in vitro (Cover &
Blaser, 1992). VacA is reported to play a role in gastric
colonization of H. pylori (Oertli et al., 2013). Although all
strains carry a functional vacA gene, there is considerable
variation in vacuolating activities among strains (Yamaoka,
2009). The vacuolation process requires the presence of
weak bases like ammonia (Cover et al., 1992). Many of the
VacA-mediated effects arise either directly or indirectly
from its membrane binding and pore formation. However,
VacA is known to also enter the cytosol of host cell and
interferes with cytoskeleton-dependent cell functions,
induction of apoptosis, and immune modulation (Cover
et al., 2003; Cover & Blanke, 2005; Hellmig et al., 2005).
Outer-membrane proteins (OMPs)
Adhesion of the bacterium to the gastric epithelium is critical
for the establishment of a stable infection. At least 32 OMPs
have been identified in H. pylori, the majority of which are
involved in adhesion of the bacterium to the gastric epithelial
cell surface (Tomb et al., 1997). Some of the widely studied
OMPs of H. pylori include AlpA, OipA, BabA, BabB, SabA,
SabB and HopZ, for which the fucosylated ABO blood group
antigens and the related sialyl-Lewis x and sialyl-Lewis a
antigens (sLe x and sLe a) have been identified as functional
receptors (Yamaoka et al., 2006; Yamaoka, 2008). Association
of peptic ulcer with increased expression of Lewis antigens in
H. pylori has been reported in the Asian population (Zheng et
http://mic.sgmjournals.org
The development of chronic gastritis associated with H.
pylori is a multifactorial process and is characterized by
gastric epithelial cell injury and infiltration of the mucosa
with both acute and chronic inflammatory cells (Smoot
et al., 1999; Yang et al., 2012a). The virulence factors produced by H. pylori inflict direct damage on gastric epithelial
cells together with significant increases in inflammatory
cytokine production, epithelial cell proliferation and apoptosis. In H. pylori-infected patients, gastric abnormalities
are likely to be due to a combination of direct mucosal
injury from bacterial virulence factors and indirect injury
from the strong inflammatory response stimulated by
infection. Interestingly, inflammation and proliferation
show an invariable correlation in vivo and the increase in
inflammation-induced epithelial cell injury was shown
to be associated with a reflex increase in proliferation of
uninjured cells (Tao et al., 2007). This appears to be a
characteristic feature of H. pylori infection, as gastric
mucosal cell proliferation is increased in H. pyloriassociated chronic gastritis but not in chronic gastritis
where the organism is absent (Lynch et al., 1995). Study by
Munoz et al. (2007) also suggests proliferative activity as
the epithelial regenerative response to cell damage caused
by inflammatory mediators. The initiation of an active
inflammatory response. concomitant with the failure to
regulate proliferation and suppression of apoptosis, are the
minimal requirements for a cell to become cancerous
(Green & Evan, 2002; Hipfner & Cohen, 2004). Among the
H. pylori virulence factors, the role of CagA in particular
has been well-defined in contributing to disease severity,
and was shown to stimulate proliferation, apoptosis induction and inflammation in infected cells (Backert & Selbach,
2008; Boonyanugomol et al., 2011; Smoot et al., 1999).
Inflammatory response
A key patho-physiological event in H. pylori infection is the
initiation and continuance of an inflammatory response.
Gastric inflammation is a characteristic outcome in patients
infected with H. pylori and represents the host immune
response to the organism. Histologically, H. pylori-associated chronic gastritis is characterized by surface epithelial
degeneration and infiltration of the gastric mucosa by
neutrophils, macrophages, T- and B-lymphocytes. Infiltration
by these chronic inflammatory cells results in upregulation of
numerous pro-inflammatory cytokines including IL-1b, IL6,
IL-8, IL-10, IL-23, TNF-a and transforming growth factor
beta (TGF-b) (Crabtree et al., 1991; Noach et al., 1994). H.
pylori is also known to induce a strong T-helper (Th)17
immune response leading to upregulation of IL-17A (also
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V. V. Subhash and B. Ho
known as IL-17), IL-17F, IL-21 and IL-22 (Sorelli-Lee et al.,
2012; Zhuang et al., 2010). The persistence of Th17 has been
shown to be a consequence of IL-1b levels, which remain
elevated in the gastric mucosa. This might favour the proliferation of Th17 cells previously recruited into the gastric
mucosa during active H. pylori infection (Figueiredo et al.,
2014). However, these inflammatory and immune responses
may not be effective in clearing the infection, but could leave
the host prone to complications resulting from chronic
inflammation (Naito & Yoshikawa, 2002). Qualitative or
quantitative differences in H. pylori-induced gastric mucosal
inflammation may play a pivotal role in determining the
varied clinical outcomes of infection (Bodger & Crabtree,
1998).
Mechanism of inflammation. Gastric inflammation in H.
pylori infection may occur through two different mechanisms. Firstly, the organism may interact with the surface
epithelial cells, either producing direct cell damage or causing
the liberation of epithelial derived pro-inflammatory mediators (chemokines). Secondly, H. pylori virulence factors (e.g.
CagA, VacA) may gain access to the gastric mucosa, thereby
stimulating specific and non-specific immune responses
involving the liberation of a variety of cytokine messengers as
illustrated in Table 1. The role of chemokines as mediators of
gastric inflammation is well established. Many studies have
shown that the gastric epithelium is an important source of
chemokines. Interestingly, a contributory role of H. pylori
virulence factors, especially CagA, has previously been
reported in chemokine activation, especially IL-8, a potent
neutrophil-activating chemotactic cytokine or chemokine
(Crabtree et al., 1995; Eck et al., 2000; Watanabe et al., 1997).
IL-8 is also upregulated by IL-17, which is induced in
H. pylori-infected gastric mucosa (Sebkova et al., 2004).
Furthermore, histological severity of H. pylori-induced
gastritis (Ando et al., 1998) and gastric ulcers (Shimizu
Table 1. Mediators of inflammation in H. pylori gastritis
Cytokine
IL-1a/b
IL-6
IL-7
IL-8
IL-10
IL-12
IL-17
IL-23
IFN-c
TGF-b
TNFa
GM-CSF
GRO-a
RANTES
MIP-1a
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Function
Pro-inflammatory (activation of leukocytes)
Pro-inflammatory (B- and T-cell activation/
differentiation)
T- and B-cell regulation
Neutrophil recruitment and activation
Immune downregulation
Stimulation of Th1 response
Neutrophil or mononuclear cell activation
Promotes IL-17-secreting T cells
Pro-inflammatory (especially cellular immunity)
Neutrophil recruitment
Pro-inflammatory (activation of leukocytes)
Pro-inflammatory, maturation factor
Neutrophil recruitment and activation
Mononuclear cell recruitment and activation
Mononuclear cell recruitment and activation
et al., 1999) correlates well with increased IL-8 levels in
patient gastric mucosa. Similarly, in vitro co-culturing of H.
pylori with gastric epithelial cells showed an increase in IL-8
secretion (Crabtree et al., 1995; Sharma et al., 1995). It was
therefore no surprise that the whole genome profiling of H.
pylori-infected gastric epithelial cells revealed IL-8 to be the
most significantly upregulated gene (Eftang et al., 2012). IL-8
released by infected gastric epithelial cells is instrumental in
regulating neutrophil infiltration of the gastric mucosa in H.
pylori-associated gastritis (Nozawa et al., 2002). IL-8 is
established as a chemotactic and activating peptide for T
lymphocytes that also induces the production of reactive
oxygen species (Naito & Yoshikawa, 2002). If defence mechanisms fail and chronic infection results, continued upregulation of IL-8, coupled with the activation of neutrophils and
lymphocytes, could lead to mucosal damage and increased
free radical formation, thereby instituting IL-8 as playing a
pivotal role in the immunopathogensis of H. pylori infection.
Intracellular signalling in gastric inflammation. The
adherence of H. pylori to gastric epithelial cells induces tyrosine phosphorylation of host proteins, cytoskeletal reorganization and activation of several intracellular signalling events
that have further downstream effects via activation of the
transcription factor NFkB. In gastric and intestinal epithelial
cells, NFkB has a central role in regulating genes that govern
the onset of mucosal inflammatory responses following
microbial infections (Lamb & Chen, 2010; Orlowski &
Baldwin, 2002; Yang et al., 2012a). NFkB activation is
effected through a series of phosphorylation and transactivation events (Malinin et al., 1997; Nemoto et al., 1998)
triggering a downstream signalling pathway that contributes
to gastric inflammation in H. pylori-infected individuals
(Bhattacharyya et al., 2002). Activation of NFkB leads to
upregulation of expression of a variety of inflammatory
mediators including IL-8 (De Luca & Iaquinto, 2004; Keates
et al., 1999; Naito & Yoshikawa, 2002). This event is essential
for the activation of innate and adaptive immune responses
against pathogens (Lamb & Chen, 2013). Stimulation and
activation of NFkB does not require protein synthesis, and
therefore its effect is particularly important in immune,
inflammatory and acute phase responses where rapid
initiation of host response following exposure to pathogens
is critical in facilitating survival (Naito & Yoshikawa, 2002).
However, sustained and constitutive NFkB activation results
in the progression of many diseases, including chronic
inflammation, infectious diseases and cancer (Hayden &
Ghosh, 2008). H. pylori strains harbouring the cag PAI (cag
PAI+) can cause direct signalling in gastric epithelial cells to
activate NFkB, leading to the release of IL-8 (Naumann,
2005). The role of bacterial virulence factors, especially CagA,
in NFkB induction has also been identified previously
(Papadakos et al., 2013).
In resting cells, NFkB activity is inhibited by its association
with the IkB (IkBa, IkBb) proteins in the cytoplasm
(Verma et al., 1995). Activation of NFkB requires phosphorylation of two conserved serine residues within the
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Microbiology 161
H. pylori-induced inflammatory and proliferative responses
N-terminal domain of IkBa (Karin & Ben-Neriah, 2000).
H. pylori infection results in the activation of specific
intracellular signalling pathways with subsequent activation
of the IkB kinase (IKK) complex. This complex comprises
two catalytic subunits (IKKa and IKKb) and a regulatory
subunit (IKKc), and can phosphorylate IkBa. Phosphorylation of IkBa is followed by its ubiquitination and proteosomal degradation, thereby resulting in NFkB nuclear
translocation (Karin & Ben-Neriah, 2000; Maeda et al., 2000).
Nuclear translocation of NFkB is followed by increased IL-8
mRNA and protein levels (Sharma et al., 1998) which, when
secreted extracellularly, are critical in the initiation of an
inflammatory response. Protein kinases are also required for
optimal NFkB activation, by targeting functional domains of
NFkB protein itself. More importantly, phosphorylation and
activation of these key subunits play an important role in
determining both the strength and duration of the NFkBmediated transcriptional response (Karin & Ben-Neriah, 2000).
Activation of mitogen activated protein kinases
(MAPKs). The MAPKs constitute an important group of
serine and threonine signalling kinases that transduce a
variety of extracellular stimuli through a cascade of protein
phosphorylations leading to the activation of transcription
factors. The MAPK signal transduction pathway plays a
crucial role in many aspects of immune-mediated inflammatory responses. The involvement of MAPK pathways in
NFkB activation has been well documented (Malinin et al.,
1997; Nemoto et al., 1998). There are three principal MAPK
family members: (i) p46 and p54 c-Jun N-terminal kinase
(JNK), or stress-activated protein kinase, with multiple
subisoforms, (ii) p38 MAPK, with a, b, c, and d isoforms, and
(iii) p42 and p44 ERK (Bhattacharyya et al., 2002). Previous
studies have shown that exposure of gastric epithelial cell lines
to H. pylori strains induced the production of IL-8 through
the activation of MAPK signalling pathways (Keates et al.,
1999; Meyer-ter-Vehn et al., 2000), identified marked
stimulation of p38, ERK and JNK pathways in the event of
H. pylori infection and showed that inhibition of these
pathways showed differential attenuation of H. pyloriinduced IL-8 secretion. Among the MAPKs, ERK was
shown to play a prominent role in signal transmission for
the efficient activation of H. pylori-induced NFkB activation,
resulting in the production of IL-8 (Nozawa et al., 2002).
ERK exerts its multiple biological effects by phosphorylating membrane or cytoskeletal proteins and is activated
upon phosphorylation by dual specificity kinases MEK1
and MEK2 (Roberts & Der, 2007). A study by Nozawa et al.
(2002) indicated that the stimulation of the ERK signalling
pathway by H. pylori may be directly responsible for NFkB
activation and subsequent synthesis of IL-8. Moreover,
when phosphorylated, ERK (pERK) translocates to the
nucleus, which can result in the activation of other transcription factors, including activator protein-1, which when
bound to the IL-8 promoter region would lead to maximal
gene expression (Asim et al., 2010; Hoffmann et al., 2002).
The role of CagA in ERK activation has been studied
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previously (Zhao et al., 2010). As shown in Fig. 1, CagA
contributes to ERK phosphorylation and resultant IL-8
secretion, via phosphorylation-dependent and -independent
pathways (Nguyen et al., 2008). Once translocated into the
host cell, CagA binds to and activates the SHP-2 protein,
which in turn, activates the ERK signalling cascade (Higashi
et al., 2002, 2004). Furthermore, gene expression analysis of in
vitro H. pylori-infected cells revealed a dominant role of the
ERK/NFkB signalling cascade in regulating the host response,
as it mediates the regulation of a large majority of the genes
having crucial implications in carcinogenesis (Keates et al.,
2007; Shibata et al., 2005).
Ras–Raf signalling pathway in ERK activation. The compo-
nents of this pathway, Ras and Raf, are proto-oncogenes
that contribute to ERK activation through complex protein–
protein interactions (Kolch, 2000) (Fig. 1). Infection of
gastric epithelial cells with H. pylori activates Ras proteins by
inducing the exchange of GDP with GTP, which converts Ras
into its active conformation. During this process, the
archetypal Ras exchange factor, SOS (son of sevenless), is
Type IV secretory system
CagA+unknown factors
CagA
CagA
CagA-P
Grb2
SHP-2
SOS
MEK1/2
Raf
Ras
P-ERK
NFκ B
IL-8 secretion
Nucleus
Fig. 1. Intracellular events in H. pylori-induced IL-8 secretion: H.
pylori virulence factors are translocated into the host cells through
a type IV secretion system. Among others, CagA is shown to play a
key role in IL-8 secretion, through the activation of ERK signalling
kinases. Both phosphorylated (CagA-P) and unphosphorylated
CagA contribute to IL-8 secretion via SHP-2-mediated and Ras–
Raf-mediated pathways, respectively.
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V. V. Subhash and B. Ho
towed to the membrane by the growth-factor-receptorbound protein 2 (GRB-2) adaptor protein, which recognizes
tyrosine phosphate docking sites located on the receptors
themselves or on receptor substrate proteins. Activated Ras
binds to Raf with high affinity and induces its phosphorylation
(Moodie & Wolfman, 1994). Phosphorylayted Raf activates
MEK1/2;, which then leads to the activation of downstream
ERKs (Kolch, 2000). H. pylori CagA was found to potentiate
the activation of NFkB via the RasARafAMEKAERK
pathway (Brandt et al., 2005). This process appears to involve
phosphorylation-independent binding of CagA to GRB-2, the
upstream effector of Ras (Mimuro et al., 2002).
EGFR in ERK activation. Epidermal growth factor receptors
(EGFRs) are receptor linked tyrosine kinases, over-expression of which is prevalent in gastric cancer (Normanno et al.,
2006). H. pylori infection triggers the ectodomain shedding
of the EGFR ligand HB-EGF, which is required for EGFR
activation through its phosphorylation at tyrosine residues
(Fig. 2). Tyrosine kinase activity of the cytoplasmic domain
of the EGFR also leads to its over-expression through an
autocrine loop of EGFR transactivation (Keates et al., 2007).
Phosphorylated EGFR (P-EGFR) binds to the GRB-2 docking
protein and forms a complex with the guanine nucleotide
exchange factor SOS (Zarich et al., 2006). This assembly
promotes Ras activation. Activated Ras directly interacts with
and activates Raf, which phosphorylates and activates MEK,
which in turn phosphorylates and activates ERKs, leading to
downstream activation of NFkB and IL-8 secretion (Avruch
et al., 2001).
Cell proliferation
Another characteristic feature of H. pylori-induced gastric
malignancies arises from its effects on cell proliferation.
H. pylori-induced EGF
P-EGFR
Ras
Raf
Aberrant regulation of proliferation together with the
suppression of apoptosis is the minimal requirement for a
cell to become cancerous (Green & Evan, 2002; Hipfner &
Cohen, 2004). H. pylori infection of the gastric mucosa has
been associated with an increase in gastric epithelial cell
proliferation (Peek et al., 1999; Schneider et al., 2011), both
in vitro and in vivo (Bechi et al., 1996; Brenes et al., 1993;
Fan et al., 1996). This effect in proliferation has an important pathogenic significance, because increased cell proliferation may elevate gastric mutation rates, and could well
be a predisposing factor for gastric neoplasia. A study by
Smoot et al. (1999) suggests that direct contact by H. pylori
could inhibit gastric cell proliferation, supporting the
theory that increased cell proliferation seen in vivo with H.
pylori-associated gastritis is a reflex response to cell injury,
and not directly caused by bacterial contact. Successful
treatment of H. pylori infection was shown to reduce the
hyper-proliferation rates of gastric epithelial cells to normal
(Cahill et al., 1995; Lynch et al., 1995). Intriguingly, in vitro
studies with cagA positive strains exhibited a pro-proliferative effect compared with the cagA negative strains (Smoot
et al., 1999). This effect of CagA in cell proliferation, together
with its contribution to inflammation, may explain in part
the stronger association of cagA positive strains with gastric
cancer (Cabral et al., 2007; Suzuki et al., 2009).
Signalling events in cell cycle regulation and
proliferation. Several pathways have been proposed to
explain the mechanisms underlying the alteration of cell
proliferation during H. pylori infection. In mammalian
cells, cellular proliferation is regulated in a cell cycle that is
governed by the sequential formation and degradation of
cyclins and cyclin-dependent kinases (Hirata et al., 2001).
Among various cyclins, cyclin D1 regulates passage through
the restriction point and entry into the S phase (Sherr, 1996).
It was also found that over-expression of cyclin D1 shortens
the G1 phase and increases the rate of cellular proliferation
(Resnitzky & Reed, 1995; Robles et al., 1996). Various factors,
such as the MAPK cascade (Lavoie et al., 1996) and the
Wnt signalling pathway (Shtutman et al., 1999; Tetsu &
McCormick, 1999), are known regulators of cyclin D1
expression (Hirata et al., 2001). Among these, activation of
the Wnt pathway was shown to play a key role in cell
proliferation (Zhang & Xue, 2008) and alteration in its
signalling components has been described in about 30 % of
gastric cancer patients (Clements et al., 2002).
Wnt pathway in cell proliferation. The canonical Wnt
MEK
P-ERK
Fig. 2. Schematic representation of the mechanism involved in
EGFR-mediated ERK activation.
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signalling pathway plays a crucial role in regulating the
proliferation and homeostasis of gastrointestinal epithelia.
Aberrant activation of Wnt signalling was reported in H.
pylori-induced gastritis, which could lead to enhanced
proliferation, thereby leading to carcinogenesis (Yang et al.,
2012b). A critical event in this process involves the nuclear
translocation of b-catenin, where it forms heterodimers
with lymphocyte enhancer factor (LEF) and T-cell factor
(TCF) (Korinek et al., 1997). Association of b-catenin
with LEF/TEF proteins could result in the transcriptional
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Microbiology 161
H. pylori-induced inflammatory and proliferative responses
upregulation of target genes that are involved in cellular
processes like cell cycle control and proliferation (Brabletz
et al., 1999; He et al., 1998; Lustig et al., 2002). Detection of
b-catenin nuclear translocation provides an estimate for
the frequency of Wnt activation in a large number of gastric
cancers. Previous studies have shown increased nuclear bcatenin expression in between 27 and 31 % of tumours (Tong
et al., 2001; Woo et al., 2001). These findings are in tandem
with the previous reports that showed Wnt activation in
nearly one-third of patients with gastric cancers (Clements
et al., 2002). Recent studies demonstrated that infection of
epithelial cells with H. pylori stimulates transcriptional
activity of b-catenin. H. pylori was shown to induce activation
of WNT target genes including cyclin D1, c-myc and Axin2
(Murata-Kamiya et al., 2007; Neal et al., 2013). Also, in vivo
studies have shown activation and nuclear accumulation of
b-catenin in gastric epithelium of patients infected with H.
pylori (Franco et al., 2005), suggesting the aberrant activation
of b-catenin as a preceding factor for the development of
gastric cancer. Increased b-catenin expression was reported in
up to 50 % of H. pylori-infected gastric adenocarcinoma
specimens when compared with non-transformed gastric
mucosa (Tsukashita et al., 2003).
Mutations that constitutively activate b-catenin signalling
can lead to the development of cancer (Logan & Nusse,
2004). In non-stimulated cells, the protein level of free bcatenin is kept low by a so-called destruction complex (Fig.
3) consisting of adenomatous polyposis coli (APC), Axin,
casein kinase 1a (CK1a) and glycogen synthase kinase 3b
(GSK3b). GSK3b induces phosphorylation of b-catenin
leading to its poly-ubiquitinylation and subsequent degradation (Kimelman & Xu, 2006). Wnt ligands bind to their
Frizzled family of serpentine receptors and to their coreceptors LDL-related protein (LRP) 5/6, which impairs
b-catenin (Gnad et al., 2010). Binding of Wnt ligands to
LRP 6 activates the dishevelled homologue-1 (Dv1) phosphoprotein. Dv1 activation leads to a sequence of events in
which the C terminus of LRP6 becomes phosphorylated by
GSK3b and CK1c, resulting in bringing Axin together with
Dvl to the plasma membrane (Davidson et al., 2005; Zeng
et al., 2005). As a consequence, the degradation complex is
no longer functional and b-catenin translocates to and
accumulates in the nucleus (Polk & Peek, 2010). Through
sequential mutational analysis, Kurashima et al. (2008)
showed that a funtional CagA EPIYA-repeat region enhances
b-catenin membranous translocalization. H. pylori CagA
physically interacts with E-cadherin and disrupts E-cadherin
and b-catenin complex formation, which also triggers cytoplasmic and nuclear accumulation of b-catenin. Together,
these studies provide important insights into the mechanism
of H. pylori, and in particular CagA, in the deregulation of the
Wnt/b-catenin pathway and the promotion of gastric cancer.
A recent study by Sokolova et al. (2008) demonstrated that
infection of epithelial cells with H. pylori suppresses GSK3b
activity via the phospho-inositol kinase 3 (PI3K)/AKT
pathway. This would lead to b-catenin nuclear translocation,
Canonical
L
R
P
6
Non-canonical
Frizzled
Frizzled
L
R
P CK1γ
6
Dvl
P
GSK3β
P
Wnt
Axin
GSK3β
APC
CK1α
APC
β-Catenin
CK1α
Axin
Degradation
β-Catenin
Nucleus
β-Catenin
LEF/TCF
Fig. 3. Wnt pathway in activation of b-catenin: in the absence of Wnt activation, cytosolic b-catenin remains bound within a
multi-protein inhibitory complex composed of GSK3b and APC tumour suppressor protein and Axin, where b-catenin is
constitutively phosphorylated (P) by GSK3b, ubiquitylated and degraded. Binding of Wnt to Frizzled activates Dvl and Wnt coreceptors, low density lipoprotein receptor-related protein 5 (LRP5) and LRP6, which then interact with Axin and other
members of the inhibitory complex, leading to GSK3b dephosphorylation. These events inhibit the degradation of b-catenin,
leading to its nuclear accumulation and resulting in the transcriptional activation of target genes that influence carcinogenesis
(Polk & Peek, 2010).
http://mic.sgmjournals.org
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1155
V. V. Subhash and B. Ho
and results in the upregulation of cyclin D1 in a CagA- and
T4SS-independent manner.
other potential virulence factors and their clinical prevalence
could further the understanding of H. pylori pathogenicity.
MAPKs in cell proliferation. MAPK cascades have also been
shown to play a role in regulation of cell proliferation in
mammalian cells in a manner inextricable from other
signal transduction systems, by sharing substrate and crosscascade interaction (Chen et al., 2006). The activated ERKs
translocate to the nucleus and transactivate various transcription factors changing gene expression to promote
growth, differentiation or mitosis (Zhang & Liu, 2002).
The interaction among the MAPK members (ERK, P38 and
JNK), which may be important in controlling different
signal cascades during bacterial infection, has not been
determined in gastric epithelial cells. However, studies have
demonstrated a cross-talk among MAPK members that may
be important in modulating the downstream molecules,
regulating cell cycle and proliferation (Cargnello & Roux,
2011). Thus, the interaction between bacterial infection and
MAPK activation is likely to contribute to H. pylori-induced
alterations in cell cycle control and proliferation of gastric
epithelial cells (Ding et al., 2008).
phospho-c-Fos c-Jun activator protein-1 complex that causes
apoptosis in macrophages. J Biol Chem 285, 20343–20357.
Conclusions
Baud, V. & Karim, M. (2009). Is NF-kB a good target for cancer
therapy? Hopes and pitfalls. Nat Rev Drug Discov 8, 33–40.
H. pylori infection is characterized by a host environment
rich with infiltration of inflammatory cells coupled with
hyper-proliferation. These characteristics could have greater
impacts in contributing to prolonged and enhanced survival
of infected cells, hence favouring cancer progression. Targeting
of signalling pathways and interacting partners causing
alterations in inflammatory and proliferative responses could
contribute to the development of successful therapeutic
strategies for the prevention, management and treatment of
H. pylori infection. Efforts are under way to develop specific
inhibitors that can be used in molecularly targeted therapy
to prevent NFkB activation without any effects on other
signalling pathways, and be more active in infected/
malignant cells than in normal cells (Baud & Karin, 2009).
These could help in better management of H. pylori-infected
individuals, as the persistence of chronic inflammation in
gastric mucosa and elevated H. pylori antibodies after
successful eradication therapy are common (Veijola et al.,
2007). However, excessive and prolonged NFkB inhibition
can be detrimental due to its important role in innate
immunity, and hence should be transient and highly reversible
to avoid long-term immunosuppression. Prospective studies
should also explore altered host signalling and molecular
markers in H. pylori-infected individuals, since the value of H.
pylori genotypes as predictors of disease outcome is limited.
Alterations in Wnt signalling leading to aberrant b-catenin
expression at a precancerous stage were reported previously in
H. pylori-infected patients (Chow et al., 2012). Since early
aberrations in b-catenin expression could potentially be
predictive of a severe disease outcome, future studies are
required to validate the clinical utility of b-catenin expression
as an early diagnostic marker for H. pylori-induced gastric
malignancies. By the same token, studies aimed at identifying
1156
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