Molecular complexes grouped under the names of tight, adherent or gap junction regulate the flow of water, ions and macromolecules through epithelium paracellular spaces. The main constituents of tight junctions are claudins, a family of 26 different proteins whose expression and distribution are tissue specific but varies in tumors. A change in claudin 1, 3, 4, 5, 6, 7, 9 and 18 expression, that contributes to lose epithelial cohesion, has been associated to enhanced cell proliferation, migration, and invasiveness in gastric neoplastic tissue. Chronic inflammation process induced by H. pylori infection, a major risk factor for gastric cancer development, disrupts tight junctions via CagA gene, Cag pathogenicity island, and VacA, but the effect upon the epithelial barrier of H. pylori lipopolysaccharides or H. pylori-induced up-regulation of mTOR and ERK signaling pathways by microRNA-100 establishes new concepts of proof.
- gastric epithelia
- H. pylori
- tight junctions
Disruption of the epithelium apical-junctional complex is an initial step of the process which allows many bacteria and/or its toxins to permeate across an otherwise tight mucosa. Normally, the most likely target are claudins, a family of 27 different molecules , essential for the maintenance of intercellular tight junctions, that viruses and bacteria such as Hepatitis C virus or
3. Epithelial barrier
The epithelial barrier is a fence composed by intercellular structures termed tight junctions, located at the apical border between gastric epithelial cells, formed by four different transmembrane proteins [occludin, claudins, junction-adhesion-molecules, and CAR –Coxsackievirus and Adenovirus Receptor- proteins] anchored to actin filaments and myosin light chains (MLC) by the actin cytoskeleton and linker proteins zonula occludens ZO-1, ZO-2 and ZO-3 which are members of the membrane-associated guanylate kinase cytoplasmic adaptors. Other highly important members of the barrier are the Adherens Junctions, the Desmosome, the Gap junctions and the Hemidesmosomes. Occludin and claudins interact with adjacent cells through their extracellular loops, whereas JAMs and CAR contain extracellular IgG-like domains [15, 16]. Different proteins form the regulatory complex (Rac, Cdc42, Par3, Par6, PKC). Figure 2 shows the structural conformation of tight junctions1. Claudins, a family of 27 different proteins, are essential to establish and maintain the barrier function as they regulate paracellular permeability  whereas occludin is important for epithelial differentiation but not for establishing the barrier . Paracellular transport across the tight junctions is achieved through the leak pathway which is size-dependent and/or the pore pathway which is size and charge-dependent; size-dependance enables transportation of proteins and lipopolysaccharides and it is controlled by MLC kinase and occludin  whereas the pore pathway, controlled by claudins, enables the permeability of cations and anions across different epithelia and exclude molecules larger than 4A .
Claudins are responsible for watertight stability and transit of cations and anions. Claudins expression and regulation is tissue specific and their physiological and regulatory function varies according to the organ where they are being expressed [22, 23]. As an example, claudin-4 in ovarian cancer has a pro-angiogenic function whereas in pancreatic cancer it suppresses invasion [24, 25]. The expression of claudins is dysregulated in various cancers, and in gastric tissue the expression of claudin-1, −4, −6 and − 17 is modified when cancer develops but many other claudins such as −3, −5, −7 and − 18 have also been implicated; the loss or gain of claudins is linked to inflammation and inflammatory cytokines such as IFNy, IL-1, IL-6, IL-10, IL-17, IL-22, EGF, TGFb and TNF , as well as to several malignancies, drugs, antibiotics, toxins, pesticides, chemicals, microbiota imbalance and stress . The integrity or modifications in tight junctions that affect claudin distribution is via the MAPK/ERK1/2 pathway [28, 29, 30]. It has been postulated that in
The effect of the secretory molecules released by of
Amongst the major toxins that
Of major relevance for this review is the effector protein CagA, one of the most important virulence factors [44, 45]. The cytotoxin-associated gene pathogenicity island (cagPAI) comprises 30 genes . The cytotoxin-associated gene A is a 125-140 kDa protein encoded by the cag pathogenicity island , a chromosomal region that simultaneously encodes a type IV secretion system specialized in transferring peptidoglycan and CagA to the cytosol of the target cell in an ATP-dependent manner [45, 48]; once translocated, it interacts with numerous proteins in a phosphorylation dependent and independent manner within the epithelial cells, stimulating inflammatory responses, perturbing intracellular actin trafficking, and disrupting cellular tight junctions probably via the ERK1/2 signaling pathway [49, 50, 51]. Phosphorylated CagA interacts with Shp2, a host protein that binds to CagA, this complex dephosphorylates the focal adhesion kinase and in turn activates a signal pathway that involves ERK proteins [52, 53]. The transferred peptidoglycan promotes the activation of the pattern-recognition molecule Nod1 within the cytosol of the host cell  and subsequently induces the expression of IL-6 and IL-8 as well as MAPK phosphorylation [55, 56, 57]. The phosphorylation independent activity of CagA disrupts E-cadherin and ZO-1 and consequently cell-to-cell junctions in polarized epithelial cells [10, 49, 58, 59]. CagA modifies the polarity of the infected cells by interacting with Par1b/MARK-2 [60, 61]. CagA also stimulates the expression of NfkB, which subsequently activates the IL-8 promoter and stimulates the release of the chemokine IL-8 into the gastric lumen , which disrupts epithelial tight junctions organization .
CagA is known to affect intercellular junctions and disrupt junction-mediated functions  as it causes an ectopic assembly of tight-junction components by recruiting ZO-1 and JAM to sites of bacterial attachment (Amieva 2003), and disrupts the epithelial barrier function . CagA colocalizes with ZO-1 and JAM proteins, binds Par1b and, by inhibiting atypical PKC-mediated phosphorylation of Par1b, disrupts cell polarity and consequently tight junctions. CagA also targets Cdx2 and therefore claudin-2 expression thus suggesting a novel mechanism for gastric epithelial cells dedifferentiation . Another pathophysiological mechanism by which
One recently recognized mechanism by which CagA disrupts the barrier is mediated by a HtrA (high-temperature requirement A) serine protease . This enzyme is part of a four proteases specific family identified in
HtrA are bacterial proteins that provide tolerance to oxidative and heat stress; they undergo oligomerization when denatured proteins are encountered (Figure 4) . HtrA can be expressed at the bacterial cell surface, or transported into the extracellular space, or shed in outer membrane vesicles. It favors bacterial paracellular transmigration by cleaving cell-to-cell junction factors such as components of tight junctions that leads to disruption of the epithelial barrier . It has been shown that HtrA1 expression in gastric cancers correlates with better response to cisplatin-based chemotherapy .
Gut bacterial lipopolysaccharides (LPS) are known to affect intracellular signaling as well as tight junctions of the blood brain barrier  and the intestinal barrier . LPS, an important structural component of bacterial walls´ outer membrane, is recognized by the membrane toll-like receptor 4, and alterations in permeability induced by LPS are via a TLR-4 dependent process associated to the adaptor protein focal adhesion kinase, which has been shown to co-localize with claudin-1 , and the activation of the MyD88-dependent pathway .
9. N-nitroso compounds
Exposure to N-nitroso compounds (NOCs) is clearly related to development and increased mortality of gastric cancer (Figure 6) [99, 100]. It has been established that nitrogenous constituents of gastric juice can be reduced and lead to the
Modulation of polarized gastric epithelial cells tight junctions by
This work was supported by Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica (PAPIIT) grants IN218019 and IN221519, UNAM, México.
- A profound review of the gastric epithelal barrier can be found at Tegtmeyer and Backert .
- For a complete list of NOCs compounds go to http://ntp.niehs.nih.gov/pubhealth/roc/roc13