Adherens junctions and tight junctions are regulated via different pathways by progastrin in epithelial cells

Expression of tight-junction protein claudin-7 is an early event in gastric tumorigenesis

Trefoil factor-1 (Tff1) expression is remarkably down-regulated in nearly all human gastric cancers. Therefore, we used the Tff1 knockout mouse model to detect molecular changes in preneoplastic gastric dysplasia. Oligonucleotide microarray gene expression analysis of gastric dysplasia of Tff1-/- mice was compared to that of normal gastric mucosa of wild-type mice. The genes most overexpressed in Tff1-/- mice included claudin-7 (CLDN7), early growth response-1 (EGR1), and epithelial membrane protein-1 (EMP1). Quantitative real-time reverse transcriptase-polymerase chain reaction and immunohistochemistry showed that Cldn7 was overexpressed in all 10 Tff1-/- gastric dysplasia samples. Comparison with our serial analysis of gene expression database of human gastric cancer revealed similar deregulation in human gastric cancers. Quantitative real-time reverse transcriptase-polymerase chain reaction of human gastric adenocarcinoma samples indicated that, of these three genes, CLDN7 was the most frequently up-regulated gene. Using immunohistochemistry, both mouse and human gastric glands overexpressed Cldn7 in dysplastic but not surrounding normal glands. Cldn7 expression was observed in 30% of metaplasia, 80% of dysplasia, and 70% of gastric adenocarcinomas. Interestingly, 82% of human intestinal-type gastric adenocarcinomas expressed Cldn7 whereas diffuse-type gastric adenocarcinomas did not (P < 0.001). These results suggest that Cldn7 expression is an early event in gastric tumorigenesis that is maintained throughout tumor progression.

Endocytosis of the apical junctional complex: mechanisms and possible roles in regulation of epithelial barriers

Tight junctions (TJ) and adherens junctions (AJ) regulate cell-cell adhesion and barrier function of simple polarized epithelia. These junctions are positioned in the apical end of the lateral plasma membrane and form the so-called apical junctional complex (AJC). Although initially seen as purely structural features, the AJC is now known to play important roles in cell differentiation and proliferation. The AJC is a highly dynamic entity, undergoing rapid remodeling during normal epithelial morphogenesis and under pathologic conditions. There is growing evidence that remodeling of the AJC is mediated by internalization of junctional proteins. This review summarizes what is known about endocytic pathways, intracellular destinations and signaling cascades involved in internalization of AJC proteins. Potential biological roles for AJC endocytosis in maintaining functional apical junctions, reversible opening of epithelial barrier and disruption of intercellular adhesion are also discussed.

Effects of fermentation products of pro- and prebiotics on trans-epithelial electrical resistance in an in vitro model of the colon

Evidence from in vivo and in vitro studies suggests that the consumption of pro- and prebiotics may inhibit colon carcinogenesis; however, the mechanisms involved have, thus far, proved elusive. There are some indications from animal studies that the effects are being exerted during the promotion stage of carcinogenesis. One feature of the promotion stage of colorectal cancer is the disruption of tight junctions, leading to a loss of integrity across the intestinal barrier. We have used the Caco-2 human adenocarcinoma cell line as a model for the intestinal epithelia. Trans-epithelial electrical resistance measurements indicate Caco-2 monolayer integrity, and we recorded changes to this integrity following exposure to the fermentation products of selected probiotics and prebiotics, in the form of nondigestible oligosaccharides (NDOs). Our results indicate that NDOs themselves exert varying, but generally minor, effects upon the strength of the tight junctions, whereas the fermentation products of probiotics and NDOs tend to raise tight junction integrity above that of the controls. This effect was bacterial species and oligosaccharide specific. Bifidobacterium Bb 12 was particularly effective, as were the fermentation products of Raftiline and Raftilose. We further investigated the ability of Raftilose fermentations to protect against the negative effects of deoxycholic acid (DCA) upon tight junction integrity. We found protection to be species dependent and dependent upon the presence of the fermentation products in the media at the same time as or after exposure to the DCA. Results suggest that the Raftilose fermentation products may prevent disruption of the intestinal epithelial barrier function during damage by tumor promoters.

Gastrin and cancer: a review

In 1905, a Cambridge physiologist, John Sydney Edkins, initially identified a hormone responsible of gastric acid secretion, which he called gastric secretin, or gastrin. While gastrin's role in acid secretion is now well defined, more recent studies have implicated the various isoforms of gastrin in cancer. Important advances in the last decade have included the recognition of biological activity for processing intermediates such as progastrin and the glycine-extended gastrin. Here, we give an overview of the roles of these peptides in cancer, highlighted by molecular, cellular and integrated studies on animal models for progastrin-derived peptides and their receptors.

Signaling pathways associated with colonic mucosa hyperproliferation in mice overexpressing gastrin precursors

MTI/G-Gly mice and hGAS mice, overexpressing glycine-extended gastrin (G-Gly) and progastrin, respectively, display colonic mucosa hyperplasia, hyperproliferation, and an increased susceptibility to intestinal neoplasia. Here, we have used these transgenic mice to analyze in vivo the modulation of intracellular signaling pathways that may be responsible for the proliferative effects of gastrin precursors. The expression, activation, and localization of signaling and cell-to-cell adhesion molecules were studied using immunofluorescence and Western blot techniques on colonic tissues derived from MTI/G-Gly, hGAS, or wild-type FVB/N mice. These analyses revealed an up-regulation of Src tyrosine kinase and related signaling pathways [phosphatidyl inositol 3'-kinase (PI3K)/Akt, Janus-activated kinase (JAK) 2, signal transducer and activator of transcription (STAT) 3, and extracellular-signal regulated kinases (ERK)] in both MTI/G-Gly and hGAS mice compared with the wild-type control animals as well as an overexpression of transforming growth factor-alpha (TGF-alpha). In contrast, overexpression of the gastrin precursors did not affect the activation status of STAT1 nor the expression and the distribution of adhesion proteins (focal adhesion kinase, cadherins, and catenins). We report for the first time that the transition from a normal colonic epithelium to a hyperproliferative epithelium in MTI/G-Gly and hGAS mice may be a consequence of the up-regulation of Src, PI3K/Akt, JAK2, STAT3, ERKs, and TGF-alpha. Deregulation of cell adhesion, a late event in tumor progression, does not occur in these transgenic models.

Signaling the junctions in gut epithelium

This Perspective summarizes recent developments in our understanding of the signaling pathways involved in the regulation of epithelial cell adhesion in the gut. The role of phosphatidylinositol 3-kinase signaling in the modulation of adherens junctions, and the connections between tight junctions and nuclear transcription factors, are discussed. The effect of gastrins on adherens and tight junctions is presented as an example of the regulation of adhesion by growth factors. The consequences of dysregulation of adherens junctions and tight junctions for human pathology are also considered.

Stimulation of proliferation and migration of a colorectal cancer cell line by amidated and glycine-extended gastrin-releasing peptide via the same receptor

Although amidated forms of gastrin-releasing peptide (GRP) have been identified as autocrine growth factors in small cell lung cancer, their role in the development and progression of colorectal carcinoma is less clear. In addition, the biological activity of non-amidated gastrin-releasing peptide has not been investigated in colorectal carcinoma cells. We therefore investigated the effect of bombesin (a homologue of gastrin-releasing peptide) on proliferation, migration and inositol phosphate production in the human colorectal carcinoma cell line DLD-1, and determined the ability of gastrin-releasing peptide receptor antagonists to inhibit these effects. We also compared the biological activities of amidated and non-amidated GRP in the same assays. Treatment with either bombesin, or amidated or non-amidated GRP resulted in significant increase in proliferation, and in migration in a wound-healing assay. Both the mitogenic and migratory effects of amidated and non-amidated forms were inhibited by the GRP receptor antagonist [D-Phe(6), Leu-NHet(13), des-Met(14)]-bombesin(6-13). The presence of GRP receptor mRNA and GRP binding sites in three colorectal carcinoma cell lines was demonstrated by RT-PCR and by binding of radiolabelled bombesin, respectively. Transfection of DLD-1 cells with a dominant negative phosphatidylinositol 3-kinase did not affect bombesin-stimulated cell proliferation, but inhibited bombesin-stimulated cell migration. Bombesin and GRPgly activated phospholipase C, mitogen-activated protein kinase and focal adhesion kinase. We conclude that both amidated and non-amidated forms of gastrin-releasing peptide accelerate proliferation and migration of DLD-1 human colorectal carcinoma cells via the gastrin-releasing peptide receptor, but that phosphatidylinositol 3-kinase is only involved in the cell migration signalling pathway. Our results suggest a potential role for gastrin-releasing peptide receptor antagonists in the management of colorectal carcinoma.

Cyclic AMP-dependent protein kinase A negatively modulates adherens junction integrity and differentiation of intestinal epithelial cells

Intestinal epithelial cell differentiation is a complex process in which many different signaling pathways are likely involved. An increase in the intracellular levels of cyclic AMP (cAMP) has been shown to inhibit enterocyte differentiation; however, the mechanisms through which cAMP/PKA signaling modulates differentiation of human intestinal epithelial cells are still not well understood. Herein, we report that: (1) treatment of Caco-2/15 cells with 8Br-cAMP repressed sucrase-isomaltase and villin protein expression and strongly attenuated morphological differentiation of enterocyte-like features in Caco-2/15 such as epithelial cell polarity and brush border formation; (2) treatment of confluent Caco-2/15 cells with 8Br-cAMP led to a strong decrease in F-actin localized at cell-cell contact sites along with a reduced amount of E-cadherin and catenins, but not of ZO-1, at cell-cell interfaces concomitant with a decreased association of these proteins with the actin cytoskeleton; (3) inhibition of PKA by H89 prevented disruption of adherens junctions by extracellular calcium depletion; (4) treatment of Caco-2/15 cells with 8Br-cAMP prevented the recruitment and activation of p85/PI-3K to E-cadherin-mediated cell-cell contacts, an important event in the assembly of adherens junctions and differentiation of these cells; (5) E-cadherin appears to be phosphorylated on serine in vivo in a PKA-dependent mechanism.

CONCLUSION

Our studies show that cAMP/PKA signaling negatively regulates adherens junction integrity as well as morphological and functional differentiation of intestinal epithelial cells.

Expression of cholecystokinin-2/gastrin receptor in the murine pancreas modulates cell adhesion and cell differentiation in vivo

The presence of gastrin and cholecystokinin-2 (CCK2) receptors in human preneoplastic and neoplastic gastrointestinal lesions suggests a role in cancer development. In addition to the growth-promoting action of gastrin, recently a role of the cholecystokinin-2/gastrin receptor (CCK2-R) modulating cellular morphology in cultured epithelial cells has been shown. Here, we have investigated in transgenic (ElasCCK2) mice whether ectopic expression of human CCK2-R in the exocrine pancreas affected epithelial differentiation. Cellular localization of cell adhesion molecules, differentiation markers, and transcription factors was determined using immunofluorescence techniques. Before tumor formation, expression and subcellular localization of proteins of the adherens junction complex, differentiation markers, and transcription factors were altered in ElasCCK2 exocrine pancreas, indicating an evolution from an acinar to a ductal phenotype. Loss of cell polarity, defective secretion, and loss of intercellular adhesion in acini of ElasCCK2 mice was confirmed by ultrastructural analysis. Finally, expression of the transgene in mice treated with the carcinogen azaserine resulted in enhanced size of preneoplastic lesions as well as an increased degree of acinar-ductal transdifferentiation. Thus, these data represent the first evidence for the CCK2-R modulating intercellular adhesion and cell fate in vivo and show that these alterations may contribute to enhanced sensitivity of ElasCCK2 pancreas to chemical carcinogens.

Rac and Rho play opposing roles in the regulation of hypoxia/reoxygenation-induced permeability changes in pulmonary artery endothelial cells

Hypoxia/reoxygenation-induced changes in endothelial permeability are accompanied by endothelial actin cytoskeletal and adherens junction remodeling, but the mechanisms involved are uncertain. We therefore measured the activities of the Rho GTPases Rac1, RhoA, and Cdc42 during hypoxia/reoxygenation and correlated them with changes in endothelial permeability, remodeling of the actin cytoskeleton and adherens junctions, and production of ROS. Dominant negative forms of Rho GTPases were introduced into cells by adenoviral gene transfer and transfection, and inhibitors of NADPH oxidase, PI3 kinase, and Rho kinase were used to characterize the signaling pathways involved. In some experiments constitutively activated forms of RhoA and Rac1 were also used. We show for the first time that hypoxia/reoxygenation-induced changes in endothelial permeability result from coordinated actions of the Rho GTPases Rac1 and RhoA. Rac1 and RhoA rapidly respond to changes in oxygen tension, and their activity depends on NADPH oxidase- and PI3 kinase-dependent production of ROS. Rac1 acts upstream of RhoA, and its transient inhibition by acute hypoxia leads to activation of RhoA followed by stress fiber formation, dispersion of adherens junctions, and increased endothelial permeability. Reoxygenation strongly activates Rac1 and restores cortical localization of F-actin and VE-cadherin. This effect is a result of Rac1-mediated inhibition of RhoA and can be prevented by activators of RhoA, L63RhoA, and lysophosphatidic acid. Cdc42 activation follows the RhoA pattern of activation but has no effect on actin remodeling, junctional integrity, or endothelial permeability. Our results show that Rho GTPases act as mediators coupling cellular redox state to endothelial function.

Gastrin: old hormone, new functions

It is exactly a century since the gastric hormone gastrin was first described as a blood-borne regulator of gastric acid secretion. The identities of the main active forms of the hormone (the "classical gastrins") and their cellular and molecular sites of action in regulating acid secretion have all attracted sustained attention. However, recent work on peptides derived from the gastrin precursor that do not stimulate acid secretion ("non-classical gastrins"), together with studies on mice over-expressing the gene, or in which the gastrin gene has been deleted, suggest hitherto unsuspected roles in regulating cell proliferation, migration, and differentiation. Moreover, microarray and proteomic studies have identified previously unsuspected target genes of the classical gastrins. Some of the newer actions have implications for our understanding of the progression to cancer in oesophagus, stomach, pancreas and colon, all of which have recently been linked in one way or another to dysfunctional signalling involving products of the gastrin gene. The present review focuses on recent progress in understanding the biology of both classical and non-classical gastrins.

Glycine-extended gastrin induces matrix metalloproteinase-1- and -3-mediated invasion of human colon cancer cells through type I collagen gel and Matrigel

The effects of glycine-extended gastrin (G-Gly) on the invasion by colon cancer cells through stromal extracellular matrix and the role of metalloproteinases (MMPs) in this invasion were investigated. We found that 10(-9)-10(-6) M G-Gly significantly increased the invasiveness of 2 human colon cancer cell lines, LoVo and HT-29, both expressing the G-Gly-specific binding site but little gastrin/CCK-B receptor (gastrin receptor). LoVo cells expressed MMP-1, -2, -3 and -9. An amount of 10(-7) M G-Gly enhanced collagenase MMP-1 expression. Overexpression of enhanced green fluorescent protein (EGFP)-fused MMP-1 in LoVo cells, by cDNA transfection, enhanced invasiveness through type I collagen gel. Immunofluorescence study revealed that G-Gly increased the number of cytoplasmic vesicles containing MMP-1, some vesicles being released from the cells. The MMP-1 vesicles contained one of the ubiquitous coat proteins, Golgi-localized, gamma-adaptin ear-containing, ARF-binding proteins-2 (GGA-2). MMP-1 also colocalized with CD147 (EMMPRIN, an extracellular matrix metalloproteinase inducer in adjacent stromal cells). It was suggested that G-Gly increased the number of vesicles containing MMP-1 and that MMP-1 interacted with CD147 to increase invasion. G-Gly significantly enhanced the production of MMP-3, an activator of MMP-1 and -9, as well as gelatinase MMP-9 activity. The G-Gly-mediated MMP-9 increase was inhibited by treatment with anti-MMP-3 IgG and MMP-3 siRNA. Furthermore, G-Gly increased the proMMP-2 level, although no activated MMP-2 was found in conditioned medium in either the presence or the absence of G-Gly. By contrast, gastrin (10(-7) M) had no effect on the levels of these MMPs or the invasiveness of colon cancer cells in type I collagen gel and Matrigel. These effects of G-Gly on the activity and expression of MMPs and the invasiveness of colon cancer cells were inhibited by treating the cells with a broad-spectrum metalloproteinase inhibitor (CGS27023A) and nonselective gastrin/CCK receptor antagonists (proglumide and benzotript). But a gastrin/CCK-B receptor antagonist (YM022) did not inhibit the increased invasion by G-Gly. Together, these results demonstrate that G-Gly renders colon cancer cells more invasive by increasing MMP-1 and MMP-3 expressions via the putative G-Gly receptor and would thus be a good molecular target in a clinical setting.

Endocytosis of epithelial apical junctional proteins by a clathrin-mediated pathway into a unique storage compartment

The adherens junction (AJ) and tight junction (TJ) are key regulators of epithelial polarity and barrier function. Loss of epithelial phenotype is accompanied by endocytosis of AJs and TJs via unknown mechanisms. Using a model of calcium depletion, we defined the pathway of internalization of AJ and TJ proteins (E-cadherin, p120 and beta-catenins, occludin, JAM-1, claudins 1 and 4, and ZO-1) in T84 epithelial cells. Proteinase protection assay and immunocytochemistry revealed orchestrated internalization of AJs and TJs into a subapical cytoplasmic compartment. Disruption of caveolae/lipid rafts did not prevent endocytosis, nor did caveolin-1 colocalize with internalized junctional proteins. Furthermore, AJ and TJ proteins did not colocalize with the macropinocytosis marker dextran. Inhibitors of clathrin-mediated endocytosis blocked internalization of AJs and TJs, and junctional proteins colocalized with clathrin and alpha-adaptin. AJ and TJ proteins were observed to enter early endosomes followed by movement to organelles that stained with syntaxin-4 but not with markers of late and recycling endosomes, lysosomes, or Golgi. These results indicate that endocytosis of junctional proteins is a clathrin-mediated process leading into a unique storage compartment. Such mechanisms may mediate the disruption of intercellular contacts during normal tissue remodeling and in pathology.