The functional interplay of Helicobacter pylori factors with gastric epithelial cells induces a multi-step process in pathogenesis
Cell Communication and Signaling
The functional interplay of Helicobacter pylori factors with gastric epithelial cells induces a multi-step process in pathogenesis
Gernot Posselt 0
Steffen Backert 1
Silja Wessler 0
0 Division of Molecular Biology, Department of Microbiology, Paris-Lodron University , Salzburg , Austria
1 Department of Biology, Institute for Microbiology, Friedrich Alexander University Erlangen/Nuremberg , Erlangen , Germany
Infections with the human pathogen Helicobacter pylori (H. pylori) can lead to severe gastric diseases ranging from chronic gastritis and ulceration to neoplastic changes in the stomach. Development and progress of H. pylori-associated disorders are determined by multifarious bacterial factors. Many of them interact directly with host cells or require specific receptors, while others enter the host cytoplasm to derail cellular functions. Several adhesins (e.g. BabA, SabA, AlpA/B, or OipA) establish close contact with the gastric epithelium as an important first step in persistent colonization. Soluble H. pylori factors (e.g. urease, VacA, or HtrA) have been suggested to alter cell survival and intercellular adhesions. Via a type IV secretion system (T4SS), H. pylori also translocates the effector cytotoxin-associated gene A (CagA) and peptidoglycan directly into the host cytoplasm, where cancer- and inflammation-associated signal transduction pathways can be deregulated. Through these manifold possibilities of interaction with host cells, H. pylori interferes with the complex signal transduction networks in its host and mediates a multi-step pathogenesis.
The interaction between pathogens and tissue- or
organspecific target cells in their host determines the
establishment and development of infectious diseases. Therefore,
pathogens must expose adapted, but specialized factors to
overcome the host defense mechanisms at the tissue
surface. In the digestive tract, the gastric mucosa is covered
by a thick mucus layer protecting the epithelium from
protein-lysing enzymes, gastric acid and finally chyme,
which can also contain unwanted bacteria and pathogens.
Forming this first effective barrier, epithelial cells show
an apico-basolateral organization, which is primarily
maintained by tight junctions, adherence junctions and a strictly
regulated actin cytoskeleton [1,2]. Functional tight
junctions are crucial for the maintenance of epithelial polarity
and cell-to-cell adhesion, and form a paracellular barrier
that precludes the free passage of molecules. Tight
junctions are composed of several types of transmembrane
proteins (e.g. occludin, claudins, junctional adhesion
molecules [JAMs]) that bind to cytoplasmic peripheral proteins
(e.g. zonula occludens [ZO] protein-1, -2 and 3, cingulin
or multi-PDZ protein-1 [MUPP1]) and link the
transmembrane proteins to the actin cytoskeleton. Adherence
junctions mediate intercellular adhesions between neighboring
cells, control the actin cytoskeleton and, therefore, exhibit
anti-tumor properties. They consist of the transmembrane
protein E-cadherin that bridges adjacent epithelial cells
with the intracellular actin cytoskeleton. This involves a
signaling complex composed of -catenin, p120-catenin,
-catenin and epithelial protein lost in neoplasm (EPLIN),
which is recruited to the intracellular domain of E-cadherin.
These dynamic intercellular junctions are crucial for the
integrity of the gastric epithelium and protect against
intruding pathogens [1,2].
Helicobacter pylori (H. pylori) is a bacterial class-I
carcinogen  that specifically colonizes the gastric epithelium
of humans as a unique niche, where it can induce
inflammatory disorders (e.g. ulceration, chronic gastritis, etc.) and
malignant neoplastic diseases (mucosa-associated lymphoid
tissue [MALT] lymphoma and gastric cancer) [4,5]. To
resist the hostile environment in the stomach, H. pylori has
developed highly sophisticated mechanisms to establish
life-long infections in the stomach if not therapeutically
eradicated. This is why it is considered as one of the most
successful bacterial pathogens. H. pylori induces gastritis
in all infected patients, but only a minority of
approximately 10-15% suffers from clinical symptoms. The reason
for the different responses to H. pylori is not clearly
understood, but many reports point to individual genetic
susceptibilities of the host to H. pylori-associated disorders.
Accordingly, genetic polymorphisms associated with an
elevated risk for gastric cancer have been identified in
genes encoding interleukins (e.g. IL-1), tumor necrosis
factor (TNF), cyclooxygenase-2 (COX2), and other host
factors [6,7]. Aside from host factors, H. pylori isolates
harbor different patterns of genetic elements encoding for
bacterial factors that are crucially involved in persistent
colonization and pathogenesis. Some of these have already
been defined as virulence factors , while others might
serve as important niche and colonization determinants
 or are still under investigation for their pathological
In the last three decades, remarkable progress has
been made in the understanding of pathogenicity-related
factors of H. pylori and their functional interaction
with gastric epithelial cell components. These
virulencerelated factors are either secreted, membrane-associated,
or translocated into the cytosol of host cells, where they
can directly interfere with host cell functions (Figure 1).
As a consequence of their different locations during the
infection process, H. pylori is able to exploit a plurality
of mechanisms to manipulate host cellular processes and
to deregulate signaling cascades. The influence of H. pylori
on these signaling pathways results in adherence, induction
of proinflammatory responses through
cytokine/chemokine release, apoptosis, proliferation, and a pronounced
motogenic response as characterized in vitro. Taken
together, these eventually result in persistent colonization,
severe inflammation, disruption of the epithelial barrier
function, and possibly gastric cancer (Figure 1). These
effects originate from selective pathogenhost interactions,
which have been summarized in this review to give a
comprehensive overview of the large number of specialized
bacterial factors and how H. pylori utilizes them to
manipulate the gastric epithelium. Many of these factors act
cooperatively, eventually leading to a complex scenario of
pathogenesis-related signaling events.
Membrane-associated factors: adhesins and beyond
Despite gastric peristalsis and transportation of chyme,
H. pylori establishes a strong interaction with epithelial
cells. In fact, adhesion of H. pylori is considered to be
the first important step in pathogenesis in the stomach. The
large group of outer membrane proteins (OMPs) contains
some adhesins (e.g. blood-group-antigen-binding adhesin
[BabA], sialic acid binding adhesin [SabA],
adherenceassociated lipoprotein A and B [AlpA/B], and outer
inflammatory protein A [OipA]) that mediate binding of H. pylori
to the host cell membrane, and other factors (e.g.
lipopolysaccharide [LPS] and flagellin) that are able to trigger
inflammatory responses in host tissues (Figure 2a).
Although bacterial adherence is crucially important for
H. pylori pathogenesis, data showing direct effects of the
above adherence factors on signaling pathways are
scarce. This indicates that canonical adhesins may not
Figure 1 Cellular responses to H. pylori upon colonization of a polarized epithelium. H. pylori expresses membrane-bound factors, secretes
factors and exploits a type IV secretion system (T4SS) to inject effectors. These contribute to adhesion or induce signal transduction pathways
leading to the induction of proinflammatory cytokine release, apoptosis, cell motility or proliferation. This network of diverse signaling pathways
and cellular responses are involved in the establishment of persistent infection, inflammation and disruption of the epithelial polarity and integrity
contributing to the development of gastritis, ulceration and gastric malignancies.
Figure 2 Model of H. pylori factors interacting with host cells. (a) At the apical side of the polarized epithelium H. pylori establishes the first
adherence. SabA, BabA, AlpA/B, OipA, HopZ, HorB, etc. are considered as important adhesins that bind to host cell receptors (e.g. Leb, sLex,
laminin) and might contribute to NF-B or MAPK signaling. (b) H. pylori secretes VacA, which forms pores in the host membranes and localizes to
mitochondria where it can interfere with apoptosis-related processes. Furthermore, VacA might influence the cellular barrier function by affecting
tight junctions; an effect which has also been proposed for soluble urease. Together with H. pylori-secreted HtrA, which directly cleaves the
adherence junction molecule E-cadherin, H. pylori efficiently disrupts the epithelial barrier. The T4SS injects the bacterial factor CagA. At the apical
side of polarized cells, CagA might translocate via phosphatidylserine and cholesterol. In the cytosol of H. pylori-infected cells, CagA exhibits
inhibitory effects on VacA-mediated apoptosis and the integrity of tight and adherence junctions. HtrA-triggered E-cadherin cleavage might be
enhanced through H. pylori-induced MMPs and could increase the destabilization of the adherence complex composed of intracellular -catenin
and p120-catenin. Disruption of the E-cadherin complex might contribute to tumor-associated target gene expression in the nucleus and/or to
the regulation of the actin cytoskeleton during cell morphological changes and motility. (c) Integrins are expressed at the basolateral side of a
polarized epithelium and could be contacted by the T4SS adhesin CagL upon disruption of the intercellular adhesions. CagA translocates across
51-integrins and becomes rapidly tyrosine phosphorylated. Phosphorylated CagA then deregulates signal transduction pathways, leading to
alterations in gene expression, and strongly interferes with the cytoskeletal rearrangement, which is important for the motogenic response to
H. pylori. Peptidoglycan is considered to be another effector that binds Nod1, thereby activating the NF-B signaling pathways.
directly activate signaling, but rather mediate a tight
interaction between H. pylori and the host target cell,
probably paving the way for additional bacterial factors
to interact with their cognate receptors. In addition to
OMPs and adhesins, flagellin and LPS have been widely
investigated to address their role in H. pylori
pathogenesis. In general, flagellin and LPS are important factors
in many other bacterial infections, but it is unclear to
what extent both factors contribute to H. pylori-induced
signaling events. In contrast to the flagellin of other
bacterial pathogens, H. pylori flagellin has only a very
low capacity to stimulate toll-like receptor 5
(TLR5)dependent interleukin-8 (IL-8) release . This has
been confirmed by the finding that purified H. pylori
flagellin is a poor ligand for TLR5 . Little information
is available on the effects of H. pylori LPS on epithelial
cells, indicating a yet undefined role in the H.
pyloriinfected epithelium as well. However, it has been
suggested that H. pylori LPS might be a TLR2 agonist in
gastric MKN45 cells, contributing to the activation of
nuclear factor kappa B (NF-B) and chemokine
expression independently of the canonical LPS receptor TLR4
. However, several factors have been well established
as H. pylori adhesins that have the potential to alter
signal transduction pathways, either by binding directly to
cell surface receptors or acting indirectly, bringing other
bacterial factors in a position to interact with cell surface
structures which normally lack the capacity for signal
Blood-group-antigen-binding adhesin (BabA)
H. pylori adhesion has been correlated with the presence
of fucosylated blood group antigens  and the OMP
BabA was subsequently identified as the first adhesin of
H. pylori that binds to the fucosylated blood group 0
antigens Lewis B (Leb) and the related H1 on the
epithelium . However, the binding specificity of BabA to
blood group 0 antigens is restricted to certain H. pylori
strains, termed specialist strains, while BabA from
generalist strains equally binds fucosylated blood group
A antigens . Recently, Globo H
hexaglycosylceramide was suggested as an additional BabA binding
partner that might play a role in the infection of
nonsecretor individuals . Interestingly, specialist strains
were found predominantly in South American countries,
where the blood group 0 phenotype predominates in the
local population. This adaptability in the binding
specificity of BabA could be attributed to the loss of selective
pressure on blood group A and B binding, rather than
active selection of specialist strains, for binding affinities
in specialist strains do not excel those of generalist
strains . The analysis of the genetic basis of BabA
revealed two BabA loci (BabA1 and BabA2, of which
BabA1 is not expressed ) and a closely related
paralogous BabB locus . It has been suggested that
BabA expression is regulated via phase variation and
recombination events with the BabB locus, as several
studies have shown loss- and gain-of-function mutations
in vitro and in vivo [14,18-20]. Additionally, the genetic
configuration of the bab genes has been shown to
correlate with preferential localization in the stomach and
the BabA/B setting correlates with the highest risk for
gastric cancer .
BabA-mediated adhesion of H. pylori to gastric epithelial
cells might enhance CagA translocation and the induction
of inflammation . Furthermore, triple-positive clinical
H. pylori isolates (BabA+, VacAs1+, CagA+) show greater
colonization densities, elevated levels of gastric
inflammation and a higher incidence of intestinal metaplasia in
H. pylori-infected patients as compared to VacAs1+,
CagA+ double-positive variants . Epidemiologically,
triple-positive strains are correlated with the highest
incidence of ulceration and gastric cancer .
Sialic acid-binding adhesin (SabA)
Independently of the adherence to fucosylated blood
group antigens via BabA, H. pylori binds to sialic
acidmodified glycosphingolipids, in particular sialyl-Lewis x/a
(sLeX and sLea), via the bacterial adhesin SabA .
Interestingly, sLeX is absent in the healthy non-inflamed gastric
mucosa, and therefore SabA-mediated adhesion becomes
a relevant factor in bacterial persistence after successful
colonization and establishment of inflammatory processes
in the stomach . Accordingly, Marcos and colleagues
 were able to show that H. pylori-induced
inflammation leads to elevated expression of the glycosyltransferase
3GnT5, which acts as an important factor in the
biosynthesis of the sLeX antigen. The induction of 3GnT5
was dependent on tumor necrosis factor alpha (TNF-),
but not IL-8, and cells expressing ectopic 3GnT5 gave
higher adhesion rates for SabA-positive H. pylori strains
. Like the situation with OipA and BabA, expression
of SabA is subject to phase variation and gene conversion
with its paralog SabB . Additionally, acid-responsive
signaling in H. pylori limits SabA transcription, which
indicates that H. pylori adhesion is a dynamic and regulated
Adherence-associated lipoprotein A and B (AlpA/B)
The OMPs AlpA and AlpB were initially described as
proteins that facilitate binding of H. pylori to Kato-3 cells and
the apical surface of gastric tissue sections [30,31]. AlpA
and AlpB share a high degree of homology and are
cotranscribed from the same operon. Moreover, both
proteins are necessary for H. pylori-mediated adhesion to
gastric biopsies . In contrast to other adhesins, AlpA
and AlpB are not subjected to phase variation and virtually
all clinical isolates express both Alp proteins [32,33].
Importantly, deletion mutants lacking AlpA/B showed severe
colonization defects in mouse and guinea pig animal
models [33,34]. In sharp contrast, a recent study in
Mongolian gerbils suggests that AlpA/B-deficient strains
lead to exuberant gastric inflammation, as compared to
the isogenic gerbil-adapted wildtype strain . The
reason for these conflicting results in different experimental
settings remains unclear.
Interestingly, Lu et al. described significant differences in
the activation of signaling pathways (mitogen-activated
protein kinases [MAPKs], c-Fos, and c-Jun-, cAMP
response element-binding protein [CREB]-, activator
protein-1 [AP-1]-, and NF-B-related signaling) induced
by H. pylori AlpA/B deletion mutants . These data
imply that AlpA/B-mediated adherence facilitates a
stronger activation of certain signal transduction pathways.
However, injection and phosphorylation of CagA, as well
as IL-8 induction, were not significantly affected by AlpA/
B deletion . H. pylori has been shown to bind
components of the extracellular matrix (ECM), especially collagen
IV and laminin , which have been proposed as
candidate host factors acting as receptors. In this context,
AlpA/B has been implicated in the adhesion to laminin
. As one of the major components of the ECM,
laminin binds to integrin; hence, it would be interesting to
investigate whether AlpA/B can indirectly modulate
integrin signaling through binding to laminin.
Outer inflammatory protein A (OipA)
OipA also belongs to the OMP group, and has been
suggested to amplify IL-8 secretion via interferon-stimulated
responsive element (ISRE) acting in parallel to the
cagPAIdependent mechanisms [38,39]. This is in contrast to
other re-complementation studies indicating that OipA
primarily functions in H. pylori adhesion to host cells,
while the IL-8 level remains unaffected [36,40]. The
reason for these opposing observations is not clear.
Yamaoka and co-workers have reported that the
expression of functional OipA in H. pylori is phase-variable,
and can be switched on or off by a slipped strand
mispairing mechanism during chromosomal replication
[39,41,42]. The OipA expression status is often associated
with the presence of cagPAI, VacAs1, and VacAm1 allelic
variants in western-type clinical isolates [40,43,44].
Therefore, it is difficult to provide relevant correlations between
OipA status and clinical manifestation, for the OipA status
does not seem to be completely independent of other
disease-relevant genetic factors of the bacterium.
However, like other adhesins, OipA appears to be an
important factor in the Mongolian gerbil infection model,
since OipA-deficient strains failed to establish an infection
and did not induce chronic inflammation and gastric
metaplasia [45,46]. To date, no specific receptor or surface
molecule for OipA binding has been described.
Nevertheless, based on infections with an oipA
deletion mutant, OipA has been suggested to induce
phosphorylation of focal adhesion kinase (FAK), leading
to downstream activation of the MAPKs extracellular
signal-regulated kinases 1 and 2 (Erk1/2) and the
formation of actin stress fibers . Collectively, these data
indicate a host cell receptor with the capability of
transmitting signal transduction in response to OipA; hence,
it would be interesting to investigate whether
recombinant OipA can bind to a host cell receptor and induce
FAK signaling. As implied by a genomic knock-out
mutant, OipA-mediated FAK activation might be a
consequence of altered epidermal growth factor receptor
(EGFR) signaling [47,48]. However, activation of EGFR
has been convincingly shown to require a functional
T4SS  and recombinant CagL alone is able to
activate EGFR . Additionally, an oipA-knock-out
mutant of H. pylori was not able to trigger the EGFR
signaling cascade involving phosphatidylinositide
3kinases (PI3K) phosphoinositide-dependent kinase-1
(PDK1) Akt, which has been suggested to contribute
to the regulation of FoxO forkhead transcription factor
activity  and finally to the induction of IL-8
secretion . In a recent study, it was proposed that EGFR/
FAK/Akt signaling leads to phosphorylation of the focal
adhesion protein paxillin, which then causes cytoskeletal
reorganization and, subsequently, cell elongation .
In summary, OipA is an interesting H. pylori adhesion
factor since it possibly interferes directly with signal
transduction pathways that are predominantly activated
by T4SS/CagA factors. This might indicate that OipA
contributes to T4SS-dependent cellular responses, either
through the direct activation of a yet unidentified
receptor or indirectly through mediating tight adhesion
between H. pylori and the host cell, leading to stronger
T4SS/CagA-mediated signaling. In this context, it would
be interesting to investigate whether the available oipA
mutants still express fully functional T4SS pili.
Other putative adhesins
In addition to the well described group of adhesion
molecules, several other factors have been implicated in
H. pylori adhesion to the gastric mucosa. The
phasevariable protein HopZ has been suggested to play a role
in bacterial adhesion  and recent studies have been
able to demonstrate a role in the early phase of
colonization. Re-isolates from a healthy volunteer challenged
with HopZ off H. pylori showed a strong in vivo
selection for the HopZ on status . Another report by
Snelling and co-workers proposed an adhesion-related
function for HorB . As an additional OMP, HopQ
might also have an influence on bacterial adhesion. In a
subset of tested H. pylori strains, hopQ deletion
increased H. pylori adherence to AGS cells and led to a
hyperadherent phenotype and subsequently to increased
CagA phosphorylation, while IL-8 induction was not
affected . Accordingly, HopQ significantly decreased
CagA injection in co-infection experiments in gastric
epithelial cells . The question of whether HopQ
interferes with the function of other adhesins in certain
H. pylori strains is still to be answered. Hence, recent
findings showing that a HopQ knock-out mutant in another
H. pylori isolate did not affect bacterial adhesion are
not necessarily contradictory. The expression of HopQ
contributed to cagPAI-dependent signaling and CagA
injection, as these could be restored through hopQ
re-expression . These data suggest that H. pylori
adhesins might act in two ways, either in a cooperating or
in a masking manner.
H. pylori-secreted urease, VacA and HtrA: priming factors
Secreted factors exhibit a high potential since they can act
at the very beginning of microbial infections without
requiring direct contact or adhesion to the host cells. In
secretome analyses of H. pylori, a wide range of secreted or
extracellular factors has been identified [59-61]. Although
most extracellular proteins from H. pylori remain largely
uncharacterized, our knowledge of -glutamyl
transpeptidase (GGT), H. pylori neutrophil-activating protein
(HP-NAP), urease, vacuolating cytotoxin A (VacA), and
high temperature requirement A (HtrA) is steadily
increasing. For example, GGT has been identified in the soluble
fraction of H. pylori , and has been shown to enhance
colonization of mice . Interestingly, recombinant GGT
can induce apoptosis and cell cycle arrest in AGS cells
[63,64], but the molecular mechanism has not yet been
elucidated. HP-NAP is a chemotactic factor of H. pylori
that mainly attracts and activates neutrophils ;
however, it does not play a prominent role during interactions
with epithelial cells. Moreover, various direct effects of
urease, VacA, and HtrA on gastric epithelial cells have
been described, including induction of apoptosis and
weakened integrity of intercellular adhesions (Figure 2b).
The urease complex has often been described as a
surface-presented virulence factor of H. pylori. The
primary function of the urease machinery is buffering the
acidic pH by converting urea to CO2 and ammonia,
which is required for neutralizing the gastric acid around
the bacteria. It has long been assumed that urease is
secreted or surface-localized and contributes significantly
to H. pyloris ability to colonize and persist in the
stomach, since it is actually considered to be an acid-sensitive
bacterium . The importance of urease for successful
colonization has been highlighted in several studies
[66-68]; however, an individual report indicates that
urease-negative H. pylori strains are still able to colonize
Mongolian gerbils .
The various sequenced genomes of H. pylori contain a
urease gene cluster, which consists of seven conserved
genes (UreAB and EI). UreA and UreB represent the
structural subunits of a Ni2+-dependent hexameric enzyme
complex. UreE, UreF, UreG and UreH are accessory
proteins involved in nickel incorporation and enzyme
assembly. Together with arginase, UreI is responsible for a
sustained supply of urea under acidic environmental
conditions . In contrast to the hypothesis of surface-localized
urease, another current model assumes that the main
urease activity resides in the bacterial cytoplasm .
Apart from its role in the successful colonization of
H. pylori, urease might also indirectly interfere with host
cell functions. Urease-dependent ammonia production
contributes to the loss of tight junction integrity in the
epithelium, as demonstrated by decreased trans-epithelial
electric resistance (TEER) and enhanced occludin
processing and internalization in in vitro cultures .
Apparently, disruption of the tight junction integrity was
independent of VacA and CagA in these studies, which is
in sharp contrast to previous reports [73,74]. The effect of
urease on tight junctions has been confirmed by another
report showing that ureB deletion abrogates H. pyloris
ability to disturb tight junctions as a CagA- or
VacAindependent process. By regulating the myosin regulatory
light chain kinase (MLCK) and Rho kinase, UreB
expression seems to be required for phosphorylation of MLC
. Even if the detailed mechanism through which
H. pylori urease activates this signaling pathway remains
unclear, these data can explain how urease contributes to
the inflammatory responses that accompany the
disruption of the epithelial barrier.
Vacuolating cytotoxin A (VacA)
First evidence for a secreted vacuole-inducing toxin was
found in experiments using filtrated H. pylori broth
culture in 1988 . This toxin was later identified as VacA
[77,78]. The cellular responses to VacA range from
vacuolization and apoptosis to the inhibition of T cell functions
[79,80]. Due to these diverse cellular responses, VacA is
considered to be a multifunctional toxin. However, in
recent years it has become increasingly clear that most
effects are due to the anion-channel function of VacA in
multiple subcellular compartments and different cell
types. Within the gene sequence, diversity of the signal
sequence (allele types s1 or s2), intermediate region (allele
types i1 or i2) and mid-region (allele types m1 or m2) has
been observed [81,82]. As a consequence of its mosaic
gene structure, the VacA protein is very heterogeneous
and exists in different variants with differing activities.
VacA is expressed as a 140 kDa protoxin with an
Nterminal signal region, a central toxin-forming region of
88 kDa (p88), and a C-terminal autotransporter domain,
which is required for secretion of the toxin . Upon
secretion, VacA is further processed into two subunits,
termed VacAp33 and VacAp55 according to their
respective molecular weight, which form membrane-spanning
hexamers [84,85]. It has been proposed that the VacAp55
domain is primarily responsible for target cell binding
, while vacuolization requires a minimal sequence
composed of the entire VacAp33 and the first ~100 amino
acids of VacAp55 [87,88].
The precise mechanism of VacA entry into target cells is
still divisive, reflected by the fact that several putative
receptors have been described. Presented on epithelial cells,
EGFR might serve as a potential candidate to bind VacA
prior to its internalization [89,90]. Further, receptor
protein tyrosine phosphatases RPTP  and RPTP 
have been described as VacA receptors that promote
VacA-dependent vacuolization. VacA binding to
sphingomyelin in lipid rafts has also been shown to be an
important event in VacA-mediated vacuolization . In
contrast to the induction of large vacuoles, VacA also
promotes the formation of autophagosomes in gastric
epithelial cells, which requires its channel-forming activity .
The low-density lipoprotein receptor-related protein-1
(LRP1) has been proposed to act as a receptor that
interacts with VacA to promote autophagy and apoptosis .
Further putative host cell receptors for H. pylori VacA have
been suggested; however, it remains uncertain whether
they function as genuine receptors. Since it is not clear
whether identified VacA receptors function independently
of each other, the identification of such a diverse range of
receptors implies a complex network of interactions
and could explain the pleiotropic functions assigned
to H. pylori VacA. In line with this assumption, purified
and acid-activated VacA affected the transepithelial
electrical resistance (TEER) of polarized epithelial cells ,
which is considered to be a strong indicator for the
integrity of a polarized epithelial barrier. However, it is not
known if this cellular phenotype requires a VacA receptor,
although these reports indicate that VacA can exert very
early effects during the multi-step infection by opening
tight junctions and, consequentially, disrupting the
epithelial barrier function.
It is well established that VacA is internalized and
forms pores in membranes. This leads to an immense
swelling, which consequently results in a vacuole-like
phenotype of those organelles which harbor markers for
both early and late endosomes . In transfection
experiments, the major consequence of VacA intoxication
in gastric epithelial cells is clearly the induction of
apoptosis in a mitochondria-dependent fashion . A
special hydrophobic N-terminal signal in VacAp33 subunit
was identified in biochemical experiments that targets
VacA to the inner mitochondrial membrane, where it
also forms anion channels [96,97]. However, the precise
route of VacA trafficking from endosomes to the inner
membrane of mitochondria is still unknown. A recent
study has suggested an important role for the
proapoptotic multi-domain proteins BAX and BAK (both
members of the Bcl-2 family) in membrane trafficking
after vacuolization . In this study, it was shown that
translocation of H. pylori VacA to mitochondria and the
induction of apoptosis strongly depends on the channel
function of VacA. This leads to recruitment of BAX
and, in turn, close contact of the vacuoles and
mitochondria, and consequently, to co-purification of
otherwise compartment-restricted marker proteins . From
genomic VacA-deletion and re-complementation
analyses, Jain and colleagues concluded that the induction
of apoptosis is preceded by a dynamin-related protein 1
(Drp1)-dependent mitochondrial fission and BAX
recruiting and activation . In conclusion, VacA
intoxication can severely interfere with membrane trafficking and
consequently disintegrate mitochondrial stability, which
finally leads to cytochrome C release and apoptosis . In
previous studies, the anion-channel function of VacA was
suggested to disrupt the inner membrane potential of
isolated mitochondria , yet in the light of these recent
studies, it is questionable whether VacA-induced loss of
membrane potential is key in the apoptosis-inducing
process of cytochrome C release.
High temperature requirement A (HtrA)
In Escherichia coli, HtrA is a well-studied periplasmic
chaperone and serine protease, and it has often been
described as a bacterial factor contributing to the
pathogenesis of a wide range of bacteria by increasing the viability
of microbes under stress conditions . Secretion of
H. pylori HtrA was detected more than 10 years ago in
comprehensive secretome analyses [60,61]. In fact, H. pylori
HtrA is highly stable under extreme acidic stress
conditions, suggesting that it could contribute to the
establishment of persistent infection in vivo . Like HtrA
proteases from other Gram-negative bacteria, H. pylori
HtrA contains an N-terminal signal peptide, a serine
protease domain with a highly conserved catalytic triad, and two
PDZ (postsynaptic density protein [PSD95], Drosophila disc
large tumor suppressor [Dlg1], and zonula occludens-1
protein [ZO-1]) domains. Although its extracellular
localization has been determined, it was unknown for long
time whether HtrA exhibits a functional role in H. pylori
infections. The investigation of H. pylori HtrA function is
limited by the fact that all attempts to create a deletion or a
protease-inactive htra mutant in the genome of H. pylori
have hitherto failed [103,104].
Recently, a completely novel aspect of HtrA function
has been discovered. It has been demonstrated that
H. pylori HtrA is secreted into the extracellular space as an
active serine protease  where it cleaves off the
extracellular domain of the cell adhesion molecule and tumor
suppressor E-cadherin . Whether HtrA-mediated
E-cadherin cleavage has an influence on the integrity and
tumor-suppressive function of the intracellular E-cadherin
signaling complex composed of -catenin and p120 catenin
is not yet known. Together with H. pylori-activated
matrixmetalloproteases (MMPs) of the host cell [104,106], several
modes of shedding and modifying cell surface molecules
are now known. Mechanistically, E-cadherin ectodomain
shedding leads to a local disruption of adherence junctions
of polarized gastric epithelial cells which allows bacterial
entry into the intercellular space . This is supported
by the observation that intercellular H. pylori could
actually be detected in biopsies of gastric cancer patients .
The ability of purified HtrA to cleave E-cadherin in vitro
and on gastric epithelial cells has also been demonstrated
for other pathogens of the gastro-intestinal tract, such as
enteropathogenic E. coli (EPEC) , Shigella flexneri
 and Campylobacter pylori [108,109], but not for the
urogenital pathogen Neisseria gonorrhoeae . This
indicates that HtrA-mediated E-cadherin cleavage is not
unique to H. pylori, but might represent a more general
mechanism to promote bacterial pathogenesis via bona
fide virulence factors that requires transmigration across
a polarized epithelium. The finding that HtrA cleaves
E-cadherin supports the hypothesis that bacterial HtrA
does not only indirectly influence microbial
pathogenicity through improvement of bacterial fitness under
stress conditions, but also exhibits direct effects on
infected host cells.
The cagPAI type IV secretion system and effectors
Another group of H. pylori factors is translocated into
the host cell cytoplasm via a type four secretion system
(T4SS). As effectors, cytotoxin-associated gene A (CagA)
and peptidoglycan have been described to alter and/or
trigger host cell signaling. While CagA may primarily
function in the regulation of cell morphology and
polarity [110,111], peptidoglycan has been described as a
possible factor inducing nucleotide-binding oligomerization
domain protein 1 (Nod1)-mediated NF-B signaling
(Figure 2c) [112,113]. However, there are other models
for the role of Nod1 in H. pylori infection .
The T4SS is encoded by the cag pathogenicity island
(cagPAI), which carriesdepending on the clinical
isolateabout 30 genes encoding for proteins that are
necessary for pilus formation and T4SS function. The
known structural and functional aspects of the T4SS
have been summarized in several excellent reviews
[115-117]. The current model of the T4SS involves
structural core components forming a needle-shaped
protrusion, which facilitates interaction with host-cell surface
receptors and is indispensable for effector translocation
into the host cell [115-117]. A comprehensive knockout
study of all individual cagPAI genes by Fischer et al.
defined an essential cagPAI-encoded protein repertoire that
is required for CagA translocation, and in addition, an
overlapping, but different panel of proteins that is required
for IL-8 induction . To date, the detailed mechanism
of CagA transmembrane transport remains unclear;
nevertheless, several host cell interactions with T4SS pilus
proteins have been characterized, as discussed below.
Interaction of the T4SS pilus with the cell membrane
In several in vitro studies, the interaction with 1-integrin
has proven to be essential for CagA translocation [119,120].
The first and best characterized T4SS-dependent host cell
interaction occurs between CagL and the 51-integrin on
gastric epithelial cells . CagL is localized on the
surface of T4SS pili and serves as an adhesin crucial for
CagA translocation, phosphorylation and IL-8 induction
[118,120]. CagL harbors the classical integrin-activating
Arg-Gly-Asp (RGD) motif, which is also found in natural
integrin ligands like fibronectin or vitronectin [120,121].
It has been suggested that CagL binding to 1-integrin
leads to the activation of 1-integrin and, subsequently, to
activation of several host kinases, including FAK, Src,
EGFR and HER3 (heregulin receptor 3)/ErbB3 in an
RGD-dependent manner [50,120]. However, regulation of
these signal transduction cascades might be more
complex, since it has recently been proposed that a
CagL/5integrin/ILK (integrin-linked kinase) stimulates EGFR
Raf MAPK pathways independently of the RGD motif.
In the same study, a weak interaction of CagL with the
integrin 3-subunit was also observed, although no
biological function has so far been described .
CagL binding to 1-integrin is necessary for the
translocation of CagA . In line with this, several other
structural components of the T4SS pilus have been
shown to bind to the 1-integrin subunit in
Yeast-TwoHybrid studies. These include CagI, CagY, and the
translocated CagA itself, which are all thought to localize
preferentially to the pilus surface and tip [119,123].
Considering the in vivo localization of the 51-integrin
at the basal side of the epithelium, which is not accessible
prior to the disruption of the epithelial integrity, the idea
of an omnipresent CagA injection was highly appealing.
Murata-Kamiya and co-workers observed that CagA
binding to phosphatidylserine is a prerequisite for CagA
translocation across the apical membrane . In addition,
cholesterol also appears to be a crucial membrane
component for CagA transport. Several studies indicate that
H. pylori targets cholesterol-rich lipid rafts , and
cholesterol depletion impairs CagA translocation . Of
note, lipid rafts also harbor the V5 integrin complex
. However, no study has yet investigated the interplay
of these putative entry mechanisms. Hence, it is
conceivable that the above-mentioned molecules act in a
Another idea is that CagA is mainly translocated across
the basolateral membrane of polarized cells, which is
supported by the detection of tyrosine-phosphorylated CagA
(CagApY) in basolaterally expressed 1-integrin-based focal
adhesions . These represent hotspots of tyrosine
phosphorylation events in cultured cells, which are
important for CagApY-dependent processes. In this context,
the finding that the soluble H. pylori factors urease, VacA
and finally HtrA can open tight junctions and adherence
junctions supports this hypothesis, because H. pylori
thereby directly disintegrates the polarized epithelium
allowing direct contact between CagL and 1-integrin at
the basolateral membrane of epithelial cells (Figure 2c).
Role of intracellular CagA in eukaryotic signaling
CagA is one of the most abundant H. pylori proteins and
has been found to be translocated into several gastric and
non-gastric cell lines upon infection (listed in: ).
Once inside the cell, CagA becomes rapidly tyrosine
phosphorylated in its C-terminally located Glu-Pro-Ile-Tyr-Ala
(EPIYA) motifs by host cell kinases [128-131].
CagL-1integrin interaction is required for CagA translocation;
hence, tyrosine-phosphorylated CagApY is mainly localized
in focal adhesions of cultured gastric epithelial cells along
with CagA-phosphorylating kinases [120,130]. CagApY
exhibits pronounced effects on the cell morphology of
gastric epithelial cells [132,133], which putatively contribute
to the disruption of the epithelial barrier in vivo.
Depending on their surrounding sequence, the EPIYA motifs can
be classified as EPIYA-A, EPIYA-B, EPIYA-C and
EPIYAD motifs. In western H. pylori strains, EPIYA-A, EPIYA-B,
and varying numbers of EPIYA-C motifs have been found,
whereas the combination of EPIYA-A and EPIYA-B with
EPIYA-D motifs has been predominantly identified in
East-Asian H. pylori isolates . All types of EPIYA
motifs can be phosphorylated, but not more than two
simultaneously. Phosphorylation of EPIYA-C or EPIYA-D
clearly primes phosphorylation of EPIYA-A or EPIYA-B,
indicating a strict regulation of EPIYA motif
phosphorylation, similar to what we know of tyrosine
phosphorylation of mammalian factors . Among the Src family
kinases (SFKs), c-Src, Fyn, Lyn and Yes have been shown
to phosphorylate CagA [128,129]. Recently, it was found
that SFKs target the EPIYA-C/D motif, but not EPIYA-A
or EPIYA-B .
SFKs and FAK become rapidly inactivated via a
negative feed-back loop, which comprises binding of CagApY
to SHP-2 and/or Csk (C-terminal Src kinase) [136-138].
The inactivation of SFKs then leads to the tyrosine
dephosphorylation of ezrin, vinculin and cortactin, which
are all important structural proteins in the regulation of
the actin cytoskeleton [136,139,140]. Cortactin is also a
substrate for Src, ERK, and PAK1, leading to a
controlled phosphorylation pattern allowing regulated
binding to FAK . Although SFKs are inactive upon
H. pylori infection, phosphorylation of CagA is maintained
by c-Abl, which is obviously necessary for the functional
activity of CagA in the cell morphological changes of
cultured gastric epithelial cells [130,131]. In contrast to
SFKs, c-Abl can target EPIYA-A, EPIYA-B and
EPIYAC motifs .
The way in which translocated CagA and/or CagApY
interfere with host cell functions has not been fully
investigated. The idea that bacterial CagA might function
as a eukaryotic signaling adaptor upon translocation has
arisen from observations of a transgenic Drosophila
model. In the absence of the Drosophila Grb2-associated
binder (Gab) homolog Daughter of Sevenless (DOS),
CagA restored photoreceptor development, supporting
the hypothesis that CagA can mimic the function of Gab
. To date, more than 25 proteins have been
identified as possible interaction partners of CagA (Table 1),
although it remains unclear which of them bind directly
or indirectly (listed in ). CagA binds to a subset of
proteins (Par proteins, c-Met, E-cadherin, p120 catenin,
ZO-1, etc.) that are well known regulators of cellular
polarity and adhesion independently of its tyrosine
phosphorylation . Accordingly, CagA might directly
target intercellular adhesions by disrupting tight 
and adherence junctions .
On the other hand, CagApY interacts with many SH2
domain-containing signaling molecules (c-Abl, Src, Crk
proteins, Grb proteins, Shp proteins, etc.), which are
important for the regulation of proliferation, cell scattering
and morphology. Remarkably, a selectivity of the
EPIYAA, EPIYA-B and EPIYA-C/D motifs in binding of
downstream targets has been detected . The in vivo
importance of CagA phosphorylation is highlighted in
transgenic mice studies demonstrating that CagA has
oncogenic potential and can lead to the development
of gastrointestinal and hematological malignancies. The
occurrence of these phenotypes was dependent upon
intact EPIYA motifs, as phosphorylation-resistant mutants
failed to develop disease in the same experimental
settings . Hence, it is tempting to speculate whether it
might be possible to employ selective SH2-containing
peptides as selective inhibitors of distinct signal
transduction pathways. In summary, CagApY and the
regulated activities of SFKs and c-Abl control a network of
downstream signal transduction pathways leading to
morphological changes and motility of cultured gastric
epithelial cells [111,147].
Interestingly, CagA and VacA functions antagonize each
other in some experiments. VacA-induced apoptosis could
be counteracted by both a phosphorylation-dependent
and a phosphorylation-independent mechanism of
injected CagA . On the other hand, CagA-dependent cell
elongation was decreased by VacA through inactivation of
EGFR and HER2/Neu . These studies underline the
complex network of cellular effects which are induced by
distinct bacterial factors.
In addition to their important functions in forming
H. pyloris cell shape and promoting colonization ,
peptidoglycans have also been described as H. pylori
factors translocating into the cytoplasm of infected host cells
where they bind to Nod1 in a T4SS-dependent manner
. Since it is well established that NF-B activity is
strictly T4SS-dependent, but CagA-independent ,
the finding of a T4SS-dependent intracellular
peptidoglycan might add a piece to the puzzle of NF-B
Table 1 Overview of H. pylori factors that interfere with host cell functions
Receptor / interaction partner
Described cellular responses / proposed protein functions
Lewis B ; Lewis A ; Globo H hexaglycosylceramide 
Adhesion to host cells [14-16]
Sialyl Lewis X, sialyl Lewis A 
Collagen IV, laminin [35,37]
Disruption of adherence junctions 
Apoptosis , cell cycle arrest 
Not known (for intracellular actions see below)
regulation and could help to explain one possible
upstream signal transduction pathway induced by H. pylori
. Nod1 might also influence the activity of AP-1
and MAPKs . However, whether peptidoglycan
prefers a T4SS-mediated translocation or transport across
the membrane via outer membrane vesicles (OMVs)
prior to NF-B activation needs to be investigated in
future studies .
H. pylori expresses a large number of bacterial factors
allowing interaction and interference with its host in
multiple ways. This is reflected by the diversity of
molecules that are either presented on the bacterial
surface, shed/secreted or internalized into host cells.
However, less is known about the local and/or time-phased
interplay of these factors, which might act
simultaneously or at different times in different cellular localities.
Furthermore, factors have been studied that obviously have
an impact on this multi-step pathogenesis, while their
cellular function is not yet understood. Duodenal ulcer
promoting gene A (DupA), for instance, represents a very interesting
factor, since expression of DupA is considered as a marker for
developing duodenal ulcer and a reduced risk for gastric
atrophy and cancer . It induces proinflammatory cytokine
secretion by mononuclear cells , but the molecular
mechanism is completely unclear. This is just one example
indicating the strong interest in unraveling the molecular
and cellular mechanisms through which pathogens
modulate host cell functions, since they represent attractive
targets for novel compounds in the selective fight against
We apologize to all colleagues whose important works could not be cited
here owing to space restrictions. We thank Catherine Haynes for critical
reading of the manuscript. The work was supported by a grant from the
Austrian Science Fund (FWF): P_24315 to SW.
Received: 5 August 2013 Accepted: 1 October 2013
Published: 7 October 2013
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