Canine H(+)-K(+)-ATPase alpha-subunit gene promoter: studies with canine parietal cells in primary culture

How Helicobacter pylori infection controls gastric acid secretion

Infection of the human stomach mucosa by Helicobacter pylori induces strong inflammatory responses and a transitory hypochlorhydria which can progress in ~2 % of patients to atrophic gastritis, dysplasia, or gastric adenocarcinoma. H. pylori infection of gastric biopsies or cultured gastric epithelial cells in vitro represses the activity of endogenous or transfected promoter of the alpha-subunit (HKα) of gastric H,K-adenosine triphosphatase (H,K-ATPase), the parietal cell enzyme mediating acid secretion. Some mechanistic details of H. pylori-mediated repression of HKα and ensuing hypochlorhydria have been recently elucidated. H. pylori strains expressing a type IV secretion system (T4SS) encoded by the cag pathogenicity island are known to upregulate the transcription factor nuclear factor (NF)-κB. The NF-κB-binding regions in the HKα promoter were identified and shown to repress its transcriptional activity. Interaction studies have indicated that although active phosphorylated NF-κB p65 is present in infected cells, an NF-κB p50/p65 heterodimeric complex fails to bind to the HKα promoter. Point mutations at -159 and -161 bp in the HKα promoter NF-κB binding sequence prevent the binding of NF-κB p50 and prevent H. pylori repression of point-mutated HKα promoter activity. The T4SS factors CagL, CagE, CagM, and possibly CagA and the lytic transglycosylase Slt, are mechanistically involved in NF-κB activation and repression of HKα transcription. CagL, a T4SS pilus component, binds to the integrin α(5)β(1) to mediate translocation of virulence factors into the host cell and initiate signaling. During acute H. pylori infection, CagL dissociates ADAM 17 (a disintegrin and a metalloprotease 17) from the integrin α(5)β(1) complex and stimulates ADAM17-dependent release of heparin-binding epidermal growth factor (HB-EGF), EGF receptor (EGFR) stimulation, ERK1/2 kinase activation, and NF-κB-mediated repression of HKα. These studies suggest that H. pylori inhibits HKα gene expression by an integrin α(5)β(1) → ADAM17 → HB-EGF → EGFR → ERK1/2 → NF-κB pathway mediating NF-κB p50 homodimer binding to the HKα promoter. Here we review the molecular basis and recent progress of this novel pathogen-dependent mechanism of H,K-ATPase inhibition, which contributes significantly to our current understanding of H. pylori pathophysiology.

The role of Sp1 in IL-1beta and H. pylori-mediated regulation of H,K-ATPase gene transcription

Helicobacter pylori infection of the gastric body induces transient hypochlorhydria and contributes to mucosal progression toward gastric carcinoma. Acid secretion is mediated by parietal cell H,K-ATPase, in which the catalytic alpha-subunit (HKalpha) promoter activity in transfected gastric epithelial [gastric adenocarcinoma (AGS)] cells is repressed by H. pylori through NF-kappaB p50 homodimer binding to the promoter. IL-1beta, an acid secretory inhibitor whose mucosal level is increased by H. pylori, upregulates HKalpha promoter activity in AGS cells. Because IL-1beta also activates NF-kappaB signaling, we investigated disparate HKalpha regulation by H. pylori and IL-1beta, testing the hypothesis that IL-1beta-induced HKalpha promoter activation is mediated by the transcription factor Sp1. DNase I footprinting revealed Sp1 binding to the HKalpha promoter at -56 to -39 bp. IL-1beta stimulated the activity of three HKalpha promoter constructs containing NF-kappaB and Sp1 sites transfected into AGS cells and also stimulated a construct containing only an Sp1 site. This stimulation was abrogated by mutating the HKalpha promoter Sp1 binding site. Gelshift assays showed that IL-1beta increased Sp1 but not p50 binding to cognate HKalpha probes and that Sp1 also interacts with an HKalpha NF-kappaB site when bound to its cognate HKalpha cis-response element. H. pylori did not augment Sp1 binding to an HKalpha Sp1 probe, and small interfering RNA-mediated knockdown of Sp1 expression abrogated IL-1beta-induced HKalpha promoter stimulation. We conclude that IL-1beta upregulates HKalpha gene transcription by inducing Sp1 binding to HKalpha Sp1 and NF-kappaB sites and that the H. pylori perturbation of HKalpha gene expression is independent of Sp1-mediated basal HKalpha transcription.

Helicobacter pylori-induced H,K-ATPase alpha-subunit gene repression is mediated by NF-kappaB p50 homodimer promoter binding

Infection of human gastric body mucosa by the gram-negative, microaerophilic bacterium Helicobacter pylori induces an inflammatory response and a transitory hypochlorhydria that progresses in approximately 2% of patients to atrophic gastritis, dysplasia, and gastric adenocarcinoma. We have previously shown that H. pylori infection of cultured gastric epithelial cells (AGS) represses the activity of the transfected alpha-subunit (HKalpha) promoter of H,K-ATPase, the parietal cell enzyme mediating acid secretion. However, the mechanistic details of H. pylori-mediated repression of HKalpha and ensuing hypochlorhydria are unknown. H. pylori is known to upregulate the transcription factor NF-kappaB through the ERK1/2 MAPK pathway. We identified NF-kappaB-binding regions in the HKalpha promoter and found that H. pylori inoculation of AGS cells increased NF-kappaB p50 binding to the transfected HKalpha promoter and repressed its transcriptional activity. Immunoblot and DNA-protein interaction studies showed that although active phosphorylated NF-kappaB p65 is present in H. pylori-infected AGS cells, an NF-kappaB p50/p65 heterodimeric complex fails to bind to the HKalpha promoter. Point mutations at -159 and -161 bp in the HKalpha promoter NF-kappaB binding sequence prevented binding of NF-kappaB p50 and prevented H. pylori repression of point-mutated HKalpha promoter activity in transfected AGS cells. Small interfering RNA-mediated knockdown of NF-kappaB p50 in H. pylori-infected AGS cells also abrogated H. pylori-induced HKalpha repression, whereas NF-kappaB p65 knockdown did not. We conclude that H. pylori inhibits HKalpha gene expression by ERK1/2-mediated NF-kappaB p50 homodimer binding to the HKalpha promoter. This study identifies a novel pathogen-dependent mechanism of H,K-ATPase inhibition and contributes to understanding of H. pylori pathophysiology.

Functional role of bone morphogenetic protein-4 in isolated canine parietal cells

Bone morphogenetic protein (BMP)-4 is an important regulator of cellular growth and differentiation. Expression of BMP-4 has been documented in the gastric mucosa. We reported that incubation of canine parietal cells with EGF for 72 h induced both parietal cell morphological transformation and inhibition of H(+)/K(+)-ATPase gene expression through MAPK-dependent mechanisms. We explored the role of BMP-4 in parietal cell maturation and differentiation. Moreover, we investigated if BMP-4 modulates the actions of EGF in parietal cells. H(+)/K(+)-ATPase gene expression was examined by Northern blots and quantitative RT-PCR. Acid production was assessed by measuring the uptake of [(14)C]aminopyrine. Parietal cell apoptosis was quantitated by Western blots with anti-cleaved caspase 3 antibodies and by counting the numbers of fragmented, propidium iodide-stained nuclei. MAPK activation and Smad1 phosphorylation were measured by Western blots with anti-phospho-MAPK and anti-phospho-Smad1 antibodies. Parietal cell morphology was examined by immunohistochemical staining of cells with anti-H(+)/K(+)-ATPase alpha-subunit antibodies. BMP-4 stimulated Smad1 phosphorylation and induced H(+)/K(+)-ATPase gene expression. BMP-4 attenuated EGF-mediated inhibition of H(+)/K(+)-ATPase gene expression and blocked EGF induction of both parietal cell morphological transformation and MAPK activation. Incubation of cells with BMP-4 enhanced histamine-stimulated [(14)C]aminopyrine uptake. BMP-4 had no effect on parietal cell apoptosis, whereas TGF-beta stimulated caspase-3 activation and nuclear fragmentation. In conclusion, BMP-4 promotes the induction and maintenance of a differentiated parietal cell phenotype. These findings may provide new clues for a better understanding of the mechanisms that regulate gastric epithelial cell growth and differentiation.

IL-1beta modulation of H,K-ATPase alpha-subunit gene transcription in Helicobacter pylori infection

Helicobacter pylori infection of the human gastric body induces hypochlorhydria by perturbing acid secretion. H. pylori inhibits parietal cell H,K-ATPase alpha-subunit (HKalpha) gene and protein expression, providing a mechanistic basis for clinical hypochlorhydria. Given that H. pylori infection increases gastric mucosal IL-1beta, an acid secretory inhibitor, we investigated the role of IL-1beta in H. pylori-mediated inhibition of HKalpha transcription. Human gastric adenocarcinoma (AGS) cells were transfected with promoter-reporter constructs containing human HKalpha 5'-flanking sequence deletions. IL-1beta (10 ng/ml) had no effect on the transcriptional activity of six progressively shorter deletion constructs of the HKalpha promoter (HKalpha2179-HKalpha340) and significantly stimulated the activity of HKalpha206, HKalpha177, HKalpha165, and HKalpha102 deletion constructs (80%, 100%, 46%, and 35%, respectively). H. pylori inhibited the transcriptional activity of HKalpha2179, HKalpha206, HKalpha177, and HKalpha165; IL-1beta relieved the H. pylori inhibition of HKalpha2179 and HKalpha206 activity but not HKalpha177 and HKalpha165 activity. AGS cell pretreatment with a MEK1/2 inhibitor prevented the IL-1beta-mediated stimulation, but p38 and JNK pathway inhibitors did not. IL-1beta mRNA levels in AGS cells were low and unaffected by H. pylori, and ELISAs of H. pylori-conditioned AGS culture media showed no measurable IL-1beta secretion. These data indicate that an IL-1beta-dependent cis-response element lies downstream of -206 nt in the HKalpha promoter and that IL-1beta-mediated upregulation of HKalpha transcription is affected by an ERK1/2 kinase signal pathway. We conclude that an IL-1beta-responsive HKalpha cis element positively regulates HKalpha gene transcription in shortened deletion constructs and that H. pylori-induced inhibition of HKalpha transcription is not mediated by IL-1beta.

Regulation and function of COX-2 gene expression in isolated gastric parietal cells

We examined expression, function, and regulation of the cyclooxygenase (COX)-2 gene in gastric parietal cells. COX-2-specific mRNA was isolated from purified (>95%) canine gastric parietal cells in primary culture and measured by Northern blots using a human COX-2 cDNA probe. Carbachol was the most potent inducer of COX-2 gene expression. Gastrin and histamine exhibited minor stimulatory effects. Carbachol-stimulated expression was inhibited by intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (90%), protein kinase C (PKC) inhibitor GF-109203X (48%), and p38 kinase inhibitor SB-203580 (48%). Nuclear factor (NF)-kappaB inhibitor 1-pyrrolidinecarbodithioic acid inhibited carbachol-stimulated expression by 80%. Similar results were observed in the presence of adenoviral vector Ad.dom.neg.IkappaB, which expresses a repressor of NF-kappaB. Addition of SB-203580 with Ad.dom.neg.IkappaB almost completely blocked carbachol stimulation of COX-2 gene expression. We examined the effect of carbachol on PGE(2) release by enzyme-linked immunoassay. Carbachol induced PGE(2) release. Ad.dom.neg.IkappaB, alone or with SB-203580, produced, respectively, partial (70%) and almost complete (>80%) inhibition of carbachol-stimulated PGE(2) production. Selective COX-2 inhibitor NS-398 blocked carbachol-stimulated PGE(2) release without affecting basal PGE(2) production. In contrast, indomethacin inhibited both basal and carbachol-stimulated PGE(2) release. Carbachol induces COX-2 gene expression in the parietal cells through signaling pathways that involve intracellular Ca(2+), PKC, p38 kinase, and activation of NF-kappaB. The functional significance of these effects seems to be stimulation of PGE(2) release.

Agonist-induced cytoplasmic volume changes in cultured rabbit parietal cells

Concomitant Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange activation occurs during stimulation of acid secretion in cultured rabbit parietal cells, possibly related to a necessity for volume regulation during the secretory process. We investigated whether cytoplasmic volume changes occur during secretagogue stimulation of cultured rabbit parietal cells. Cells were loaded with the fluorescent dye calcein, and the calcein concentration within a defined cytoplasmic volume was recorded by confocal microscopy. Forskolin at 10(-5) M, carbachol at 10(-4) M, and hyperosmolarity (400 mosmol) resulted in a rapid increase in the cytoplasmic dye concentration by 21 +/- 6, 9 +/- 4, and 23 +/- 5%, respectively, indicative of cell shrinkage, followed by recovery to baseline within several minutes, indicative of regulatory volume increase (RVI). Depolarization by 5 mM barium resulted in a decrease of the cytoplasmic dye concentration by 10 +/- 2%, indicative of cell swelling, with recovery within 15 min, and completely prevented forskolin- or carbachol-induced cytoplasmic shrinkage. Na(+)/H(+) exchange inhibitors slightly reduced the initial cell shrinkage and significantly slowed the RVI, whereas 100 microM bumetanide had no significant effect on either parameter. We conclude that acid secretagoguges induce a rapid loss of parietal cell cytoplasmic volume, followed by RVI, which is predominantly mediated by Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange.

Inhibition of human gastric H(+)-K(+)-ATPase alpha-subunit gene expression by Helicobacter pylori

Clinical studies and in vitro data from isolated parietal cells suggest that acute Helicobacter pylori infection inhibits acid secretion. Gastric acidification is mediated by H(+)-K(+)-ATPase, an integral protein of parietal cell apical membranes. To test the hypothesis that H. pylori downregulates H(+)-K(+)-ATPase alpha-subunit (HKalpha) gene expression and to identify potential intracellular signaling pathways mediating such regulation, we transfected human gastric adenocarcinoma (AGS) cells with human and rat HKalpha 5'-flanking DNA fused to a luciferase reporter plasmid. Histamine caused dose-dependent, cimetidine-sensitive (10(-4) M) increases in cAMP, free intracellular Ca(2+), and HKalpha promoter activation in AGS cells. H. pylori infection of transfected AGS cells dose dependently inhibited basal and histamine-stimulated HKalpha promoter activity by 80% and 66%, respectively. H. pylori dose dependently inhibited phorbol myristate acetate-induced (10(-7) M) and staurosporine- (10(-7) M) and calphostin C-sensitive (5 x 10(-8) M) activation of HKalpha promoter. Also, H. pylori inhibited epidermal growth factor (EGF) (10(-8) M), genistein-sensitive (5 x 10(-5) M) activation of HKalpha promoter, reducing activity to 60% of basal level. These data suggest that H. pylori inhibits HKalpha gene expression via intracellular pathways involving protein kinases A and C and protein tyrosine kinase, AGS cells have functional histamine H(2) and EGF receptors, and transiently transfected AGS cells are a useful model for studying regulation of HKalpha gene expression.

Regulation and function of p38 protein kinase in isolated canine gastric parietal cells

We examined the regulation and functional role of p38 kinase in gastric acid secretion. p38 kinase was immunoprecipitated from cell lysates of highly purified gastric parietal cells in primary culture, and its activity was quantitated by in vitro kinase assay. Carbachol effects were dose- and time-dependent, with a maximal 10-fold stimulatory effect detected after 30 min of incubation. SB-203580, a highly selective inhibitor of p38 kinase, blocked carbachol induction of p38 kinase activity, with maximal inhibition at 10 microM. Stimulation by carbachol was unaffected by preincubation of parietal cells with the intracellular Ca(2+) chelator BAPTA-AM, but incubation of cells in Ca(2+)-free medium led to a 50% inhibition of carbachol induction of p38 kinase activity. Because some of the effects of carbachol are mediated by the small GTP-binding protein Rho, we examined the role of Rho in carbachol induction of p38 kinase activity. We tested the effect of exoenzyme C3 from Clostridium botulinum (C3), a toxin known to ADP-ribosylate and specifically inactivate Rho. C3 led to complete ADP-ribosylation of Rho, and it inhibited carbachol induction of p38 kinase by 50%. We then tested the effect of SB-203580 and C3 on carbachol-stimulated uptake of [(14)C]aminopyrine (AP). Inhibition of p38 kinase by SB-203580 led to a dose-dependent increase in AP uptake induced by carbachol, with maximal (threefold) effect at 10 microM SB-203580. Similarly, preincubation of parietal cells with C3 led to a twofold increase in AP uptake induced by carbachol. Thus carbachol induces a cascade of events in parietal cells that results in activation of p38 kinase through signaling pathways that are at least in part dependent on Rho activation and on the presence of extracellular Ca(2+). p38 kinase appears to inhibit gastric acid secretion.