Activation of human histidine decarboxylase gene promoter activity by gastrin is mediated by two distinct nuclear factors

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Gastrin stimulates expression of plasminogen activator inhibitor-1 in gastric epithelial cells

Plasminogen activator inhibitor (PAI)-1 is associated with cancer progression, fibrosis and thrombosis. It is expressed in the stomach but the mechanisms controlling its expression there, and its biological role, are uncertain. We sought to define the role of gastrin in regulating PAI-1 expression and to determine the relevance for gastrin-stimulated cell migration and invasion. In gastric biopsies from subjects with elevated plasma gastrin, the abundances of PAI-1, urokinase plasminogen activator (uPA), and uPA receptor (uPAR) mRNAs measured by quantitative PCR were increased compared with subjects with plasma concentrations in the reference range. In patients with hypergastrinemia due to autoimmune chronic atrophic gastritis, there was increased abundance of PAI-1, uPA, and uPAR mRNAs that was reduced by octreotide or antrectomy. Immunohistochemistry revealed localization of PAI-1 to parietal cells and enterochromaffin-like cells in micronodular neuroendocrine tumors in hypergastrinemic subjects. Transcriptional mechanisms were studied by using a PAI-1-luciferase promoter-reporter construct transfected into AGS-G(R) cells. There was time- and concentration-dependent increase of PAI-1-luciferase expression in response to gastrin that was reversed by inhibitors of the PKC and MAPK pathways. In Boyden chamber assays, recombinant PAI-1 inhibited gastrin-stimulated AGS-G(R) cell migration and invasion, and small interfering RNA treatment increased responses to gastrin. We conclude that elevated plasma gastrin concentrations are associated with increased expression of gastric PAI-1, which may act to restrain gastrin-stimulated cell migration and invasion.

Histamine pharmacogenomics

Genetic polymorphisms for histamine-metabolizing enzymes are responsible for interindividual variation in histamine metabolism and are associated with diverse diseases. Initial reports on polymorphisms of histamine-related genes including those coding for the enzymes histidine decarboxylase (HDC), diamine oxidase (ABP1) and histamine N-methyltransferase (HNMT), as well as histamine receptor genes, often have pointed to polymorphisms that occur with extremely low frequencies or that could not be verified by later studies. In contrast, common and functionally significant polymorphisms recently described have been omitted in many association studies. In this review we analyze allele frequencies, functional and clinical impact and interethnic variability on histamine-related polymorphisms. The most relevant nonsynonymous polymorphisms for the HDC gene are rs17740607 Met31Thr, rs16963486 Leu553Phe and rs2073440 Asp644Glu. For ABP1 the most relevant polymorphisms are rs10156191 Thr16Met, rs1049742 Ser332Phe, and particularly because of its functional effect, rs1049793 His645Asp. In addition the ABP1 polymorphisms rs45558339 Ile479Met and rs35070995 His659Asn are relevant to Asian and African subjects, respectively. For HNMT the only nonsynonymous polymorphism present with a relevant frequency is rs1801105 Thr105Ile. For HRH1 the polymorphism rs7651620 Glu270Gly is relevant to African subjects only. The HRH2 rs2067474 polymorphism, located in an enhancer element of the gene promoter, is common in all populations. No common nonsynonymous SNPs were observed in the HRH3 gene and two SNPs were observed with a significant frequency in the HRH4 gene: rs11665084 Ala138Val and rs11662595 His206Arg. This review summarizes relevant polymorphisms, discusses controversial findings on association of histamine-related polymorphisms and allergies and other diseases, and identifies topics requiring further investigation.

C314T polymorphism in histamine N-methyltransferase gene and susceptibility to duodenal ulcer in Chinese population

BACKGROUND

Histamine is a regulator of gastric acid secretion, which is involved in the development of duodenal ulcer (DU). Histamine is metabolized by both histamine N-methyltransferase (HNMT) and diamine oxidase, and its local action is terminated primarily by methylation which is catalyzed by HNMT.

METHODS

Polymerase chain reaction-restriction fragment length polymorphism assay was used to identify the polymorphism of the point mutation C314T of HNMT gene of 498 Chinese patients with DU and 151 healthy individuals.

RESULTS

In normal controls, the allele frequency of HNMT T314 was 3.3%, which was significantly lower than American Caucasians. The HNMT T314 allele was detected in 3.5% of the DU patients. In cases and controls, the frequency of C/C genotypes were 93.0% and 93.4%, respectively. The HNMT T/T genotype was not found in this population. No significant differences were seen in both genotype frequencies and allele frequencies between DU groups and controls. After stratified by H. pylori infection, they also could not reach significant differences in our current study.

CONCLUSION

The HNMT T314 allele frequency is lower in Chinese population than in American Caucasians. No association can be found in the involvement of HNMT C314T polymorphism in the susceptibility to duodenal ulcer.

Identification of a Gastrin Response Element in the Vesicular Monoamine Transporter Type 2 Promoter and Requirement of 20 S Proteasome Subunits for Transcriptional Activity

Vesicular monoamine transporter type 2 (VMAT2) is crucial for accumulation of monoamine neurotranmitters into neuronal secretory vesicles and histamine into secretory granules of the enterochromaffin-like cell in the acid-secreting gastric mucosa. Gastric VMAT2 expression is regulated by the antral hormone gastrin acting at the CCK(2) receptor. We demonstrate a gastrin response element (-56)ccgccccctc(-47) in the proximal VMAT2 promoter that binds in a gastrin-sensitive manner to nuclear proteins from gastric epithelial cell lines. Mutations within this sequence prevented nuclear protein binding and significantly reduced gastrin-stimulated expression of VMAT2 promoter-reporter constructs in gastric epithelial cells. In a yeast one-hybrid screen of an AR42J cell cDNA library, using the gastrin response element as bait, we identified a beta subunit of the 20 S proteasome, PSMB1, as a potential binding partner. In supershift assays, antibodies to PSMB1 and other proteasome beta subunits disrupted gastrin sensitive nuclear protein binding to the VMAT2 promoter. Moreover, RNA interference of PSMB1 significantly inhibited gastrin-mediated VMAT2 transcription. These data suggest that elements of the 20 S proteasome interact with the VMAT2 promoter to enhance G-protein-coupled receptor-mediated transcription.

Yin yang 1 (YY1) represses histidine decarboxylase gene expression with SREBP-1a in part through an upstream Sp1 site

Histidine decarboxylase (HDC) is the enzyme that converts histidine to histamine, a bioamine that plays an important role in many physiological aspects including allergic responses, inflammation, neurotransmission, and gastric acid secretion. In previous studies, we demonstrated that Kruppel-like factor 4 represses HDC promoter activity in a gastric cell line through both an upstream Sp1-binding GC box (GGGCGG sequence) and downstream gastrin-responsive elements. In the current study, Yin Yang 1 (YY1), a pleiotropic transcriptional factor, was also shown in cotransfection assays to repress HDC promoter activity through the upstream GC box. DNA affinity purification assay demonstrated that YY1 was pulled down specifically by the upstream GC box. In addition, sterol-responsive element-binding protein 1a (SREBP-1a), a transcriptional factor that binds YY1, represses the HDC promoter. Interestingly, deletion analysis and cotransfection assays indicated that mutation of the upstream GC box or truncation of downstream gastrin-responsive elements in the HDC promoter disrupted the inhibitory effect of YY1 and SREBP-1a in an identical fashion. Furthermore, quantitative real-time PCR analysis indicated that gastrin treatment downregulated SREBP-1a gene expression and reduced the DNA binding activity of SREBP in EMSAs. Taken together, these results suggest that YY1 and SREBP-1a form a complex to inhibit HDC gene expression through both the upstream GC box and downstream gastrin-responsive elements and gastrin-induced activation of HDC gene expression is mediated at least partly through downregulation of transcriptional repressors such as SREBPs.

Role of macrophage-derived histamine in atherosclerosis-- chronic participation in the inflammatory response --

The atherosclerotic intimal lesion contains endothelial cells, smooth muscle cells, monocytes/macrophages and T lymphocytes, which constitute a histamine-cytokine network that participates in chronic inflammatory responses. Monocytes/macrophages and T lymphocytes express the histamine-producing enzyme histidine decarboxylase (HDC), and specific histamine receptors (HHR), which are switched from HH2R to HHR1 during macrophage differentiation. Endothelial and smooth muscle cells also express HHR in response to histamine. The effects of histamine on these cells include a regulation of atherosclerosis-related events such as cell proliferation, expression of matrix metalloproteinase, adhesion molecules and cytokines. Furthermore, recent studies have indicated that histamine and the activation of its specific receptors modulate the Th1/Th2 balance in inflammatory lesions through the regulation of cytokine production from inflammatory cells. The histamine-cytokine network in the atherosclerotic intima could regulate inflammatory and immune responses, including Th1/Th2 balance, and contribute to atherogenesis.

Molecular mechanisms of gastrin-dependent gene regulation

The peptide hormone gastrin is the key regulator of gastric acid secretion. Gastrin exerts its effects as acid secretagogue through functional activation of gastric enterochromaffin-like (ECL) cells, which control acid secretion through biosynthesis and release of histamine. In ECL cells, concerted activation of histidine decarboxylase (HDC), vesicular monoamine transporter 2 (VMAT2), and chromogranin A (CgA) genes by gastrin is a prerequisite for proper acid control. To elucidate the molecular pathways underlying gastrin-dependent control of ECL cell genes, we recently analyzed the signaling cascades, regulatory promoter elements, and transcription factors mediating the transcriptional effects of gastrin. Our studies identified the Raf>MEK1>ERK 1/-2 kinase module as the common signaling pathway mediating gastrin-dependent ECL cell gene transcription. In contrast to this uniform signaling cascade, pronounced heterogeneity was detected between cis- and trans-activating regulatory factors conferring gastrin responsiveness. The molecular diversity of transcription factors and regulatory enhancer elements transmitting gastrin-triggered gene transcription offers the molecular basis for synergistic, but differential, regulation of HDC, VMAT2, and CgA genes during a secretory challenge of ECL cells by gastrin and possibly other acid secretagogues.

Gastrin regulates the heparin-binding epidermal-like growth factor promoter via a PKC/EGFR-dependent mechanism

Gastrin is a known growth/differentiation factor for the gastric mucosa. Its effects are likely mediated by the induction of heparin-binding epidermal-like growth factor (HB-EGF), a member of the EGF family of growth factors that is expressed by gastric parietal cells. In this study, we investigated the regulation of the HB-EGF promoter by gastrin in a human gastric cancer cell line. Serial human HB-EGF promoter-luciferase reporter deletion constructs and heterologous promoter constructs were transfected into AGS-E cells and stimulated with gastrin (10(-7) M) with or without various signal transduction inhibitors. EMSA were also performed. Gastrin stimulation resulted in a fivefold increase in HB-EGF-luciferase activity. The cis-acting element mediating gastrin responsiveness was mapped to the -69 to -58 region of the HB-EGF promoter. Gastrin stimulation was PKC dependent and at least partially mediated by activation of the EGF receptor.

Transforming growth factor-alpha directly augments histidine decarboxylase and vesicular monoamine transporter 2 production in rat enterochromaffin-like cells

For the production and vesicle storage of histamine, Enterochromaffin-like (ECL) cells express histidine decarboxylase (HDC) and vesicular monoamine transporter 2 (VMAT2). Although HDC and VMAT2 show dynamic changes during gastric ulcer healing, the control system of their expression has not been fully investigated. In the present study, we investigated the effect of transforming growth factor-alpha (TGF-alpha) and proinflammatory cytokines on HDC and VMAT2 expression in rat ECL cells. Time course changes in the expression of TGF-alpha during the healing of acetic acid-induced ulcers were studied. EGF receptor (EGFR) expression was also examined in ECL cells, whereas the direct effects of TGF-alpha and proinflammatory cytokines on HDC and VMAT2 expression in ECL cells were investigated using in vivo and in vitro models. During the process of ulcer healing, expression of TGF-alpha mRNA was markedly augmented. Furthermore, EGFR was identified in isolated ECL cells. TGF-alpha stimulated HDC and VMAT2 mRNA expression and protein production and also increased histamine release from ECL cells. Selective EGFR tyrosine kinase inhibitor tyrphostin AG1478 almost completely inhibited HDC and VMAT2 gene expression induced by TGF-alpha in vivo and in vitro. During gastric mucosal injury, TGF-alpha was found to stimulate ECL cell functions by increasing HDC and VMAT2 expression.

Kruppel-like factor 4 (KLF4) represses histidine decarboxylase gene expression through an upstream Sp1 site and downstream gastrin responsive elements

Histidine decarboxylase (HDC) is the enzyme that catalyzes the conversion of histidine to histamine, a bioamine that plays an important role in allergic responses, inflammation, neurotransmission, and gastric acid secretion. Previously, we demonstrated that gastrin activates HDC promoter activity in a gastric cancer (AGS-E) cell line through three overlapping downstream promoter elements. In the current study, we used the yeast one-hybrid strategy to identify nuclear factors that bind to these three elements. Among eight positives from the one-hybrid screen, we identified Kruppel-like factor 4 (KLF4) (previously known as gut-enriched Kruppel-like factor (GKLF)) as one factor that binds to the gastrin responsive elements in the HDC promoter. Electrophoretic mobility shift assays confirmed that KLF4 is able to bind all three gastrin responsive elements. In addition, transient cotransfection experiments showed that overexpression of KLF4 dose dependently and specifically inhibited HDC promoter activity. Regulation of HDC transcription by KLF4 was confirmed by changes in the endogenous HDC messenger RNA by KLF4 small interfering RNA and KLF4 overexpression. We further showed that KLF4 inhibits HDC promoter activity by competing with Sp1 at the upstream GC box and also independently by binding the three downstream gastrin responsive elements. Taken together, these results indicate that KLF4 can act to repress HDC gene expression by Sp1-dependent and -independent mechanisms.

Transcriptional regulation of the human trefoil factor, TFF1, by gastrin

BACKGROUND & AIMS

This study aimed to identify gastrin-sensitive genes that may mediate the effects of this hormone on gastric epithelial architecture.

METHODS

Gastrin-sensitive genes were identified by messenger RNA (mRNA) differential display of the gastric fundus from gastrin-deficient (GAS-KO) or wild-type mice. Gastrin-stimulated expression of the trefoil peptide TFF1 in mouse fundus and in the gastric cancer cell line AGS-G(R) was determined by Northern blot and real-time polymerase chain reaction. Transcriptional regulation of TFF1 in AGS-G(R) cells was studied using promoter-reporter assays and electrophoretic mobility shift assay. Expression of TFF1 and the cholecystokinin(B) receptor in response to gastric mucosal injury was determined by immunohistochemistry.

RESULTS

mRNA differential display identified TFF1 as a gastrin-regulated gene. TFF1 mRNA was reversibly reduced in GAS-KO mice and increased in a hypergastrinemic transgenic strain versus respective background strains. TFF1 mRNA expression was rapidly and potently induced by gastrin in a gastric cancer cell line that expresses the gastrin/cholecystokinin(B) receptor. Gastrin responsiveness of the human TFF1 promoter mapped to a G-C rich region 300 base pairs upstream of the transcriptional start site. This region bound the transcription factors SP3 and MAZ. Gastrin activated transcription through a Raf-, Mek- and Erk-dependent but Ras-independent pathway. TFF1 expression was induced both directly and by transactivation between neighboring cells. Neither direct nor indirect gastrin-induced TFF1 expression required activation of the epidermal growth factor receptor.

CONCLUSIONS

Gastrin exerts tonic control of TFF1 expression but also has the potential for rapid up-regulation of this trefoil factor. TFF1 is a potential candidate to counterbalance the proliferative effects of gastrin.

Interaction of early growth response protein 1 (Egr-1), specificity protein 1 (Sp1), and cyclic adenosine 3'5'-monophosphate response element binding protein (CREB) at a proximal response element is critical for gastrin-dependent activation of the chromogranin A promoter

Recently, binding of specific protein 1 (Sp1) and cAMP response element binding protein (CREB) to a GC-rich element at -92/-62 has been identified as a critical step in gastrin-dependent regulation of the chromogranin A (CgA) gene in gastric epithelial cells. Here we demonstrate that binding of early growth response protein 1 (Egr-1) to the distal part of the -92/-62 site is also required for gastrin-dependent CgA transactivation. Gastrin elevated cellular and nuclear Egr-1 levels in a time-dependent manner and also increased Egr-1 binding to the CgA -92/-73 region. Disruption of this site reduced gastrin responsiveness without influencing basal promoter activity, while loss of Sp1 and/or CREB binding sites diminished basal and gastrin-stimulated CgA promoter activity. Ectopic Egr-1 overexpression potently stimulated the CgA promoter, whereas coexpression of Egr-1 with Sp1 and/or CREB resulted in additive effects. Functional analysis of Sp1-, Egr-1-, or CREB-specific promoter mutations in transfection studies confirmed the tripartite organization of the CgA -92/-62 element. Signaling studies revealed that MAPK kinase 1 (MEK1)/ERK1/2 cascades are critical for gastrin-dependent Egr-1 protein accumulation as well as Egr-1 binding to the CgA promoter. Our studies for the first time identify Egr-1 as a nuclear target of gastrin and show that functional interplay of Egr-1, Sp1, and CREB is indispensable for gastrin-dependent CgA transactivation in gastric epithelial cells.

Gastric functions in gastrin gene knock-out mice

The gastric hormone gastrin was the 2nd hormone to be identified and was initially recognized for its ability to induce acid secretion. Following the purification and subsequent development of specific radioimmunoassays for gastrin, it was also shown to be a regulator of oxyntic mucosal growth. To explore the importance of gastrin or its receptors for gastric development and gastric physiology both have been knocked out. The knock-out mice are viable, develop with any gross abnormalities, and are fertile. Even though gastrin acts as a growth factor during hypergastrinaemia there was no general atrophy of the gastrin mucosa in the knock-out mice. But the maturation of both parietal and ECL cells were disturbed and the number of parietal cells reduced. Furthermore, lack of gastrin impaired basal acid secretion and rendered the parietal cells unresponsive to histamine and acetylcholine, the other two major stimulators of gastric acid secretion. Despite the major changes in gastric physiology, the number of genes down-regulated in the gastrin knock-out mice is modest. The achlorhydria due to lack of gastrin also leads to bacterial overgrowth of the stomachs of the gastrin knockout mice, which could facilitate the development of gastric ulcer disease and cancer.

Identification and characterization of a third gastrin response element (GAS-RE3) in the human histidine decarboxylase gene promoter

In human gastric cancer cells the human histidine decarboxylase gene is regulated by gastrin through two overlapping cis-acting elements known as gastrin response elements 1&2 (GAS-RE1, GAS-RE2) [J. Biol. Chem. 274 (1999) 20961]. Here, we report the identification and characterization of a third element GAS-RE3 that was localized to a region +28 to +48 downstream of the transcriptional start site (+1). Gastrin stimulation induced a rapid increase in binding to the element of a novel nuclear factor named gastrin response element-binding protein 3 (GAS-REBP3). Block mutations in the GAS-RE3 sequence (+38GTGCG(+42) to +38TAAGT(+42)) led to reduced promoter activity and decreased binding in EMSA. UV cross-linking studies and Southwestern blot analysis with wildtype and mutant GAS-RE3 showed that GAS-REBP3 was a approximately 110kDa protein. Thus, gastrin-mediated regulation of HDC gene expression appears to be mediated by a complex cis-acting element, which binds at least three distinct nuclear factors.

GATA proteins are potential negative regulators of HDC gene expression in the gastric epithelium

In this study, we used the gastric epithelial cell line AGS-G(R) to investigate the role of GATA transcription factors in the regulation of both basal and gastrin-stimulated L-histidine decarboxylase (HDC) gene transcription. Using reporter gene technology, we compared the transcriptional activity of a construct, hHDC503, which contained the 5'-flanking region of the human HDC gene with that of similar constructs lacking selected GATA consensus sequences. We demonstrated the expression of GATA-4 and GATA-6 proteins within the AGS-G(R) cells and found evidence that these transcription factors can negatively regulate HDC gene expression.

Lessons from the gastrin and gastrin receptor knockout mice

The gastric hormone gastrin was first recognized for its ability to induce acid secretion. Following the purification and subsequent development of specific radioimmunoassays for gastrin, it was also shown to be a regulator of oxyntic mucosal growth. To examine the importance of gastrin or its receptors during development in general and for gastric physiology specifically both have been knocked out. Gastrin and gastrin receptor knockout mice are viable, develop without any gross abnormalities, and are fertile. Even though gastrin acts as a growth factor during hypergastrinemia there was no general atrophy of the gastric mucosa in the knockout mice. However, the maturation of both parietal and ECL cells was disturbed and the number of parietal cells was reduced. Basal acid secretion was impaired and rendered the parietal cells unresponsive to secretagogues. Outside the stomach the mice had no apparent phenotype. However, studies have suggested that progastrin and glycine-extended proforms of gastrin may have biological importance, but these results are still circumstantial and identification of the implicated receptors will be crucial for further studies.

B-Raf/Rap1 signaling, but not c-Raf-1/Ras, induces the histidine decarboxylase promoter in Helicobacter pylori infection

Histidine decarboxylase (HDC) is the key enzyme for gastric histamine synthesis, and enhanced HDC expression is critically involved in the pathogenesis of gastric disorders, including gastroduodenal ulcer disease. We characterized the pathogenicity mechanism underlying activation of the HDC promoter in H. pylori-infected gastric epithelial cells and performed a detailed analysis of the participating signaling elements. We found that H. pylori infection of gastric epithelial cells activated the MEK1-2/ERK1-2 cascade through cAMP-dependent stimulation of Rap1 and B-Raf, but not Ras/c-Raf-1, leading to potent transactivation of the human HDC promoter. H. pylori-triggered elevation of adenylate cyclase activity was directed by GalphaS-subunits of heterotrimeric G proteins. Stimulation of this signaling cascade was triggered independent of bacterial-cell contact by a small molecular- weight component(s) (approximately 1 kDa) released by H. pylori and did not require a functional type IV secretion system. Thus, our studies demonstrate for the first time to our knowledge that the GalphaS-->cAMP-->Rap1--->B-Raf-->MEK1/2-->ERK1/2 pathway is critical for H. pylori-dependent epithelial gene regulation, which can be induced via a bioactive component(s) apart from the site of bacterial colonization. These results further elucidate the molecular mechanisms underlying interaction of H. pylori with gastric epithelial cells and help to define potential molecular targets for therapeutic interventions in the context of H. pylori-related gastric diseases.

L-histidine decarboxylase decreases its own transcription through downregulation of ERK activity

A poorly defined negative feedback loop decreases transcription of the L-histidine decarboxylase (HDC) gene. To help understand this regulation, we have studied the effect of HDC protein expression on HDC gene transcription in transfected AGS-B cells. Expression of the rat HDC protein inhibited HDC promoter activity in a dose-dependent fashion. The region of the HDC promoter mediating this inhibitory effect corresponded to a previously defined gastrin and extracellular signal-related kinase (ERK)-1 response element. Overexpression of the HDC protein reduced nuclear factor binding in this region. Experiments employing specific histamine receptor agonists indicated that the inhibitory effect was not dependent on histamine production, and studies with the HDC inhibitor alpha-fluoromethylhistidine revealed that inhibition was unrelated to enzyme activity. Instead, an enzymatically inactive region at the amino terminal of the HDC enzyme (residues 1-271) was shown to mediate inhibition. Fluorescent chimeras containing this domain were not targeted to the nucleus, arguing against specific inhibition of the HDC transcription machinery. Instead, we found that overexpression of HDC protein decreased ERK protein levels and ERK activity and that the inhibitory effect of HDC protein could be overcome by overexpression of ERK1. These data suggest a novel feedback-inhibitory role for amino terminal sequences of the HDC protein.

Effects of histamine and interleukin-4 synthesized in arterial intima on phagocytosis by monocytes/macrophages in relation to atherosclerosis

We investigated the localization of histidine decarboxylase (HDC), which is the rate-limiting enzyme that generates histamine from histidine, in human aorta/coronary artery. RT-PCR and immunohistochemical staining revealed that the HDC gene was expressed in monocytes/macrophages and T cells in the arterial intima but not in smooth muscle cells in either the arterial intima or the media. A luciferase promoter assay with U937 and Jurkat cells demonstrated that interleukin-4 (IL-4) inhibited the expression of the HDC gene. In contrast, among a scavenger receptor family, IL-4 as well as histamine up-regulated U937 cells to express the LOX-1 gene but not the SR-A gene, which genes encode receptors that scavenge oxidized lipids. These findings suggest that histamine synthesized in the arterial wall participates in the initiation and progression of atherosclerosis and that IL-4 can act as an important inhibitory and/or stimulatory factor in the function of monocytes/macrophages modulated by histamine in relation to the process of atherosclerosis.

Gastrin induces expression and promoter activity of the vesicular monoamine transporter subtype 2

Gastric enterochromaffin-like cells produce histamine in response to the antral hormone gastrin and accumulate the biogenic amine in secretory organelles via vesicular monoamine transporter subtype 2. The putative effects of gastrin on vesicular monoamine transporter subtype 2 expression and promoter activity are poorly understood. In the present study we used highly enriched rat enterochromaffin-like cells (purity, >90%) and rat pheochromocytoma cells stably transfected with a gastrin/cholecystokinin B receptor to investigate the expression and transcriptional regulation of vesicular monoamine transporter subtype 2. Stimulation of vesicular monoamine transporter subtype 2 mRNA and protein expression was observed in isolated enterochromaffin-like cells after 3- to 7-h incubation with gastrin (10(-7) M), forskolin (10(-5) M), or ionomycin (10(-5) M). Deletion analysis of the rat vesicular monoamine transporter subtype 2 promoter defined the minimal promoter sequence necessary for full basal activity as a -121 bp segment upstream of exon 1 containing two Sp1 sites (-97 to -88 bp and -68 to -59 bp) and a cAMP-responsive element (-44 to -35 bp). Gastrin (10(-7) M) stimulated extracellular signal related kinase1/2 phosphorylation, activated Sp1 and cAMP-responsive element-binding protein, and further induced activity of the complete rat vesicular monoamine transporter subtype 2 promoter (-800 bp) in gastrin/cholecystokinin B receptor cells. The -121-bp fragment was able to confer full gastrin responsiveness, and site-directed mutagenesis of the Sp1 and cAMP-responsive element motifs demonstrated their crucial importance for basal and inducible activities. Comparison of promoter activity of histidine decarboxylase, chromogranin A, or vesicular monoamine transporter subtype 2 in transfected cell lines revealed significant differences in basal and gastrin-stimulated activities. Our current study provides the first evidence that gastrin directly stimulates the expression and promoter activity of vesicular monoamine transporter subtype 2. Sp1 and cAMP-responsive element-binding protein recognition motifs located within 121 bp upstream of exon 1 appear to be indispensable for full basal and inducible promoter activities. Diverging effects of gastrin on histidine decarboxylase, chromogranin A, and vesicular monoamine transporter subtype 2 promoter may account for the coordinated synthesis and storage of histamine in this neuroendocrine cell type.

The gastrins: their production and biological activities

Gastric epithelial organization and function are controlled and maintained by a variety of endocrine and paracrine mediators. Peptides encoded by the gastrin gene are an important part of this system because targeted deletion of the gene, or of the gastrin-CCKB receptor gene, leads to decreased numbers of parietal cells and decreased gastric acid secretion. Recent studies indicate that the gastrin precursor, preprogastrin, gives rise to a variety of products, each with a distinctive spectrum of biological activity. The conversion of progastrin to smaller peptides is regulated by multiple mechanisms including prohormone phosphorylation and secretory vesicle pH. Progastrin itself stimulates colonic epithelial proliferation; biosynthetic intermediates (Gly-gastrins) stimulate colonic epithelial proliferation and gastric epithelial differentiation; and C-terminally amidated gastrins stimulate colonic proliferation, gastric epithelial proliferation and differentiation, and acid secretion. The effects of progastrin-derived peptides on gastric epithelial function are mediated in part by release of paracrine factors that include histamine, epidermal growth factor (EGF)-receptor ligands, and Reg. The importance of the appropriate regulation of this system is shown by the observation that prolonged moderate hypergastrinemia in transgenic mice leads to remodelling of the gastric epithelium, and in the presence of Helicobacter, to gastric cancer.

Transcriptional activation of the rat vesicular monoamine transporter 2 promoter in gastric epithelial cells: regulation by gastrin

Vesicular monoamine transporter 2 is important for the accumulation of monoamine neurotransmitters into synaptic vesicles and histamine transport into secretory vesicles of the enterochromaffin-like cell of the gastric corpus. In this study we have investigated the mechanisms regulating the transcriptional activation of the rat vesicular monoamine transporter 2 (VMAT2) promoter in gastric epithelial cells. Maintenance of basal levels of transcription was dependent on the presence of SP1, cAMP-response element (CRE), and overlapping AP2/SP1 consensus sequences within the region of promoter from -86 to +1 base pairs (bp). Gastrin stimulation increased transcriptional activity, and responsiveness was shown to be dependent on the CRE (-33 to -26 bp) and AP2/SP1 (-61 to -48 bp) consensus sites but independent of the SP1 site at -86 to -81 bp. Gastrin-induced transcription was dependent on the cooperative interaction of an uncharacterized nuclear factor of approximately 23.3 kDa that bound to the putative AP2/SP1 site, CRE-binding protein (CREB), and CREB-binding protein/p300. Gastrin stimulation resulted in the increased binding of phosphorylated CREB to the promoter, but it did not result in the increased binding of the AP2/SP1-binding protein. The gastrin responsiveness of the promoter was shown to be dependent on both the protein kinase C and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-signaling pathways, which may converge on the AP2/SP1-binding protein.

Transcription factors recognizing overlapping C1-A2 binding sites positively regulate insulin gene expression

Transcription factors binding the insulin enhancer region, RIPE3b, mediate beta-cell type-specific and glucose-responsive expression of the insulin gene. Earlier studies demonstrate that activator present in the beta-cell-specific RIPE3b1-binding complex is critical for these actions. The DNA binding activity of the RIPE3b1 activator is induced in response to glucose stimulation and is inhibited under glucotoxic conditions. The C1 element within the RIPE3b region has been implicated as the binding site for RIPE3b1 activator. The RIPE3b region also contains an additional element, A2, which shares homology with the A elements in the insulin enhancer. Transcription factors (PDX-1 and HNF-1 alpha) binding to A elements are critical regulators of insulin gene expression and/or pancreatic development. Hence, to understand the roles of C1 and A2 elements in regulating insulin gene expression, we have systematically mutated the RIPE3b region and analyzed the effect of these mutations on gene expression. Our results demonstrate that both C1 and A2 elements together constitute the binding site for the RIPE3b1 activator. In addition to C1-A2 (RIPE3b) binding complexes, three binding complexes that specifically recognize A2 elements are found in nuclear extracts from insulinoma cell lines; the A2.2 complex is detected only in insulin-producing cell lines. Furthermore, two base pairs in the A2 element were critical for binding of both RIPE3b1 and A2.2 activators. Transient transfection results indicate that both C1-A2 and A2-specific binding activators cooperatively activate insulin gene expression. In addition, RIPE3b1- and A2-specific activators respond differently to glucose, suggesting that their overlapping binding specificity and functional cooperation may play an important role in regulating insulin gene expression.

Gastric secretion

This article summarizes data published during the past year that improve our understanding of the mechanisms by which various neurotransmitters, paracrine agents, and hormones regulate gastric acid secretion and are themselves regulated. The main stimulants of acid secretion are histamine, gastrin, and acetylcholine. The main inhibitor is somatostatin, which exerts a tonic restraint on parietal, enterochromaffin-like (ECL), and gastrin cells. Histamine, released from ECL cells, stimulates the parietal cell directly via H(2) receptors and indirectly via H(3) receptors coupled to inhibition of somatostatin secretion. Gastrin, acting via gastrin/cholecystokinin-B (CCK-B), now termed CCK(2), receptors on ECL cells activates histidine decarboxylase, releases histamine, and induces ECL hypertrophy and hyperplasia. The latter might be responsible for the rebound hyperacidity observed after withdrawal of long-term antisecretory therapy. The neurotransmitter pituitary adenylate cyclase-activating polypeptide stimulates histamine secretion from isolated ECL cells, but its physiologic role, if any, is not known. Acetylcholine, released from gastric postganglionic intramural neurons, stimulates the parietal cell directly via muscarinic M(3) receptors and indirectly by inhibiting somatostatin secretion. Although infection with H. pylori is associated with increased basal and stimulated acid outputs in patients with duodenal ulcer, most people infected with the organism are asymptomatic and have pangastritis with decreased acid output. In the latter, eradication of the bacterium leads to an increase in gastric acidity and is associated with a two-to threefold increase in gastroesophageal reflux.

Control of CCK gene transcription by PACAP in STC-1 cells

The mechanisms by which neuroendocrine stimulants regulate CCK gene transcription are unclear. We examined promoter activation by pituitary adenylate cyclase-activating polypeptide (PACAP), a known CCK secretagogue, in the enteroendocrine cell line STC-1. The promoter region from -70 to -87 bp, relative to the transcriptional start site, contains a composite calcium/cyclic AMP response element (CRE)/activator protein 1 (AP1) site that may bind CRE binding protein (CREB) and AP1. PACAP (with IBMX) stimulated expression of an 87-bp construct 3.35+/-0.36-fold but had no effect on a -70 construct. The effect was blocked by the protein kinase A inhibitor H-89 and by a dominant-negative CREB plasmid. Mutation of the CRE/AP1 site to a canonical CRE site did not affect the response to PACAP, but mutation to a canonical AP1 site prevented it. CREB phosphorylation was increased after PACAP treatment. Electrophoretic mobility shift assay and supershift analysis revealed that CREB and not AP1 bound to the CRE/AP1 site and that PACAP increased the proportion of phosphorylated CREB that was bound. We conclude that PACAP increases CCK gene expression via a cAMP-mediated pathway involving CREB phosphorylation by protein kinase A and activation of a composite CRE/AP1 site.

Amino- and carboxy-terminal PEST domains mediate gastrin stabilization of rat L-histidine decarboxylase isoforms

Control of enzymatic function by peptide hormones can occur at a number of different levels and can involve diverse pathways that regulate cleavage, intracellular trafficking, and protein degradation. Gastrin is a peptide hormone that binds to the cholecystokinin B-gastrin receptor and regulates the activity of L-histidine decarboxylase (HDC), the enzyme that produces histamine. Here we show that gastrin can increase the steady-state levels of at least six HDC isoforms without affecting HDC mRNA levels. Pulse-chase experiments indicated that HDC isoforms are rapidly degraded and that gastrin-dependent increases are due to enhanced isoform stability. Deletion analysis identified two PEST domains (PEST1 and PEST2) and an intracellular targeting domain (ER2) which regulate HDC protein expression levels. Experiments with PEST domain fusion proteins demonstrated that PEST1 and PEST2 are strong and portable degradation-promoting elements which are positively regulated by both gastrin stimulation and proteasome inhibition. A chimeric protein containing the PEST domain of ornithine decarboxylase was similarly affected, indicating that gastrin can regulate the stability of other PEST domain-containing proteins and does so independently of antizyme/antizyme inhibitor regulation. At the same time, endoplasmic reticulum localization of a fluorescent chimera containing the ER2 domain of HDC was unaltered by gastrin stimulation. We conclude that gastrin stabilization of HDC isoforms is dependent upon two transferable and sequentially unrelated PEST domains that regulate degradation. These experiments revealed a novel regulatory mechanism by which a peptide hormone such as gastrin can disrupt the degradation function of multiple PEST-domain-containing proteins.

Helicobacter pylori activates the histidine decarboxylase promoter through a mitogen-activated protein kinase pathway independent of pathogenicity island-encoded virulence factors

Helicobacter pylori infection of the gastric mucosa is accompanied by an activated histamine metabolism. Histamine plays a central role in the regulation of gastric acid secretion and is involved in the pathogenesis of gastroduodenal ulcerations. Histidine decarboxylase (HDC) is the rate-limiting enzyme for histamine production, and its activity is regulated through transcriptional mechanisms. The present study investigated the effect of H. pylori infection on the transcriptional activity of the human HDC (hHDC) promoter in a gastric epithelial cell line (AGS) and analyzed the underlying molecular mechanisms. Our studies demonstrate that H. pylori infection potently transactivated the hHDC promoter. The H. pylori-responsive element of the hHDC gene was mapped to the sequence +1 to +27 base pairs, which shows no homology to known cis-acting elements and also functions as a gastrin-responsive element. H. pylori regulates the activity of this element via a Raf-1/MEK/ERK pathway, which was activated in a Ras-independent manner. Furthermore, we found that H. pylori-induced transactivation of the hHDC promoter was independent of the cag pathogenicity island and the vacuolating cytotoxin A gene and therefore may be exerted through (a) new virulence factor(s). A better understanding of H. pylori-directed hHDC transcription can provide novel insights into the molecular mechanisms of H. pylori-dependent gene regulation in gastric epithelial cells and may lead to new therapeutic approaches.