Hypoxia-inducible factor 1-dependent induction of intestinal trefoil factor protects barrier function during hypoxia

Interleukin-6 induces keratin expression in intestinal epithelial cells: potential role of keratin-8 in interleukin-6-induced barrier function alterations

Keratin 8 (K8) and keratin-18 (K18) are the major intermediate filament proteins in the intestinal epithelia. The regulation and function of keratin in the intestinal epithelia is largely unknown. In this study we addressed the role and regulation of K8 and K18 expression by interleukin 6 (IL-6). Caco2-BBE cell line and IL-6 null mice were used to study the effect of IL-6 on keratin expression. Keratin expression was studied by Northern blot, Western blot, and confocal microscopy. Paracellular permeability was assessed by apical-to-basal transport of a fluorescein isothiocyanate dextran probe (FD-4). K8 was silenced using the small interfering RNA approach. IL-6 significantly up-regulated mRNA and protein levels of K8 and K18. Confocal microscopy showed a reticular pattern of intracellular keratin localized to the subapical region after IL-6 treatment. IL-6 also induced serine phosphorylation of K8. IL-6 decreased paracellular flux of FD-4 compared with vehicle-treated monolayers. K8 silencing abolished the decrease in paracellular permeability induced by IL-6. Administration of dextran sodium sulfate (DSS) significantly increased intestinal permeability in IL-6-/- mice compared with wild type mice given DSS. Collectively, our data demonstrate that IL-6 regulates the colonic expression of K8 and K18, and K8/K18 mediates barrier protection by IL-6 under conditions where intestinal barrier is compromised. Thus, our data uncover a novel function of these abundant cytoskeletal proteins, which may have implications in intestinal disorders such as inflammatory bowel disease wherein barrier dysfunction underlies the inflammatory response.

Recognition of intestinal epithelial HIF-1alpha activation by Pseudomonas aeruginosa

Human intestinal epithelial cell monolayers (Caco-2) subjected to hypoxia and reoxygenation release soluble factors into the apical medium that activate the virulence of the opportunistic pathogen Pseudomonas aeruginosa to express the potent barrier-dysregulating protein PA-I lectin/adhesin. In this study, we defined the role of hypoxia-inducible factor (HIF)-1alpha in this response. We tested the ability of medium from Caco-2 cells with forced expression of HIF-1alpha to increase PA-I expression in P. aeruginosa and found that medium from Caco-2 cells overexpressing HIF-1alpha increased PA-I expression compared with medium from control cells (P < 0.001, ANOVA). To identify the components responsible for this response, medium was fractionated by molecular weight and subjected to mass spectroscopy, which identified adenosine as the possible mediator. Both adenosine and its immediate downstream metabolite inosine induced PA-I expression in P. aeruginosa in a dose-dependent fashion. Because inosine was not detectable in the medium of Caco-2 cells exposed to hypoxia or overexpressing HIF-1alpha, we hypothesized that P. aeruginosa itself might metabolize adenosine to inosine. Using mutant and parental strains of P. aeruginosa, we demonstrated that P. aeruginosa metabolized adenosine to inosine via adenosine deaminase and that the conditioned medium enhanced the extracellular accumulation of inosine. Together, these results provide evidence that P. aeruginosa can recognize and respond to extracellular end products of intestinal hypoxia that are released after activation of HIF-1alpha. The ability of P. aeruginosa to metabolize adenosine to inosine may represent a subversive microbial virulence strategy that deprives the epithelium of the cytoprotective actions of adenosine.

TFF3-peptide increases transepithelial resistance in epithelial cells by modulating claudin-1 and -2 expression

TFF3 plays an important role in the protection and repair of the gastrointestinal mucosa. The molecular mechanisms of TFF function, however, are still largely unknown. Increasing evidence indicates that apart from stabilizing mucosal mucins TFF3 induces cellular signals that modulate cell-cell junctions of epithelia. In transfected HT29/B6 and MDCK cells stably expressing FLAG-tagged human TFF3 we have recently shown that TFF3 down-regulates E-cadherin, impairs the function of adherens junctions and thus facilitates cell migration in wounded epithelial cell layers. Here we investigate TFF3-induced effects on the composition and function of tight junctions in these cells. TFF3 increased the cellular level of tightening claudin-1 and decreased the amount of claudin-2 known to form cation-selective channels. Expression of ZO-1, ZO-2 and occludin was not altered. The change in claudin-1 and -2 expression in TFF3-expressing HT29/B6 cells was accompanied by an increase in the transepithelial resistance in confluent monolayers of these cells. These data suggest that TFF3 plays a role in the regulation of intestinal barrier function by altering the claudin composition within tight junctions thus decreasing paracellular permeability of the intestinal mucosa.

HIF-dependent induction of adenosine A2B receptor in hypoxia

Adenosine has been widely associated with hypoxia of many origins, including those associated with inflammation and tumorogenesis. A number of recent studies have implicated metabolic control of adenosine generation at sites of tissue hypoxia. Here, we examine adenosine receptor control and amplification of signaling through transcriptional regulation of endothelial and epithelial adenosine receptors. Initial studies confirmed previous findings indicating selective induction of human adenosine A2B receptor (A2BR) by hypoxia. Analysis of the cloned human A2BR promoter identified a functional hypoxia-responsive region, including a functional binding site for hypoxia-inducible factor (HIF) within the A2BR promoter. Further studies examining HIF-1alpha DNA binding and HIF-1alpha gain and loss of function confirmed strong dependence of A2BR induction by HIF-1alpha in vitro and in vivo mouse models. Additional studies in endothelia overexpressing full-length A2BR revealed functional phenotypes of increased barrier function and enhanced angiogenesis. Taken together, these results demonstrate transcriptional coordination of A2BR by HIF-1alpha and amplified adenosine signaling during hypoxia. These findings may provide an important link between hypoxia and metabolic conditions associated with inflammation and angiogenesis.

Selective ablation of matrix metalloproteinase-2 exacerbates experimental colitis: contrasting role of gelatinases in the pathogenesis of colitis

The matrix metalloproteinases (MMPs), MMP-2 and MMP-9, share structural and substrate similarities and are up-regulated during human as well as animal models of inflammatory bowel disease. We recently demonstrated that epithelial-derived MMP-9 is an important mediator of inflammation and tissue damage in colitis. In this study, we examined the role of MMP-2 in acute colitis. Colitis was induced using two models, administration of dextran sodium sulfate (DSS) and Salmonella enterica subsp. serovar Typhimurium (S.T.). Bone marrow chimeras were performed using bone marrow cells from wild-type (WT) and MMP-2(-/-) mice. Colitis was evaluated by clinical symptoms, myeloperoxidase assay, and histology. MMP-2 protein expression and activity were up-regulated in WT mice treated with DSS or S.T. MMP-2(-/-) mice were highly susceptible to the development of colitis induced by DSS (or S.T.) compared with WT. During inflammation, MMP-2 expression was increased in epithelial cells as well as in the infiltrating immune cells. Bone marrow chimera demonstrated that mucosa-derived MMP-2 was required for its protective effects toward colitis. Furthermore, we demonstrate that severe colitis in MMP-2(-/-) is not due to a compensatory increase in MMP-9. Finally, we show that MMP-2 regulates epithelial barrier function. In contrast to MMP-9, mucosa-derived MMP-2 may be a critical host factor that is involved in the prevention or cessation of the host response to luminal pathogens or toxins, an important aspect of healing and tissue resolution. Together, our data suggest that a critical balance between the two gelatinases determines the outcome of inflammatory response during acute colitis.

Regulation of angiogenesis by hypoxia-inducible factor 1

Hypoxia is an imbalance between oxygen supply and demand that occurs in cancer and in ischemic cardiovascular disease. Hypoxia-inducible factor 1 (HIF-1) was originally identified as the transcription factor that mediates hypoxia-induced erythropoietin expression. More recently, the delineation of molecular mechanisms of angiogenesis has revealed a critical role for HIF-1 in the regulation of angiogenic growth factors. In this review, we discuss the role of HIF-1 in developmental, adaptive and pathological angiogenesis. In addition, potential therapeutic interventions involving modulation of HIF-1 activity in ischemic cardiovascular disease and cancer will be discussed.

Current understanding of HIF in renal disease

Hypoxia-inducible factors (HIF) are ubiquitous transcription factors regulated by oxygen-dependent proteolysis, and hence rapidly mount an adaptational response to hypoxia. The HIF system is apparently more complex than initially considered in the perspective of the increasing number of HIF target genes, and the inter-relationship with various additional regulatory pathways. Regional hypoxia is believed to play a major role in renal disease. Experimental data confirm a role for HIF in renal pathophysiology. The discovery of HIF prolyl-hydroxylases as key enzymes of oxygen sensing and HIF proteolysis offer new possibilities to therapeutically target HIF. Herein, we review basic concepts of HIF regulation, and existing data on HIF activation in renal disease.

Functional and biochemical characterization of epithelial bactericidal/permeability-increasing protein

Epithelial cells of many mucosal organs have adapted to coexist with microbes and microbial products. In general, most studies suggest that epithelial cells benefit from interactions with commensal microorganisms present at the lumenal surface. However, potentially injurious molecules found in this microenvironment also have the capacity to elicit local inflammatory responses and even systemic disease. We have recently demonstrated that epithelia cells express the anti-infective molecule bactericidal/permeability-increasing protein (BPI). Here, we extend these findings to examine molecular mechanisms of intestinal epithelial cell (IEC) BPI expression and function. Initial experiments revealed a variance of BPI mRNA and protein expression among various IEC lines. Studies of BPI promoter expression in IECs identified regulatory regions of the BPI promoter and revealed a prominent role for CCAAT/enhancer binding protein and especially Sp1/Sp3 in the basal regulation of BPI. To assess the functional significance of this protein, we generated an IEC line stably transfected with full-length BPI. We demonstrated that, whereas epithelia express markedly less BPI protein than neutrophils, epithelial BPI contributes significantly to bacterial killing and attenuating bacterial-elicted proinflammatory signals. Additional studies in murine tissue ex vivo revealed that BPI is diffusely expressed along the crypt-villous axis and that epithelial BPI levels decrease along the length of the intestine. Taken together, these data confirm the transcriptional regulation of BPI in intestinal epithelia and provide insight into the relevance of BPI as an anti-infective molecule at intestinal surfaces.

HIF-dependent induction of apical CD55 coordinates epithelial clearance of neutrophils

Sites of inflammation are associated with dramatic shifts in tissue metabolism. Inflammation can result in significant tissue hypoxia, with resultant induction of hypoxia-responsive genes. Given this association, we hypothesized that neutrophil (PMN) ligands expressed on epithelial cells may be regulated by hypoxia. Initial studies confirmed earlier results that epithelial hypoxia enhances PMN transepithelial migration and promotes apical clearance of PMN from the epithelial surface. A screen of known PMN ligands revealed a surprisingly stable expression pattern in hypoxia. However, this screen identified one gene, CD55, as a highly hypoxia-inducible molecule expressed on the apical membrane of mucosal epithelia. Subsequent studies verified the induction of CD55 mRNA and protein expression by hypoxia. Overexpression of CD55 by transfection in nonhypoxic epithelia resulted in a similar pattern of apical PMN clearance, and peptide mimetics corresponding to the PMN binding site on DAF blocked such apical clearance of PMN. Studies directed at understanding molecular pathways of hypoxia inducibility revealed that a approximately 200 bp region of the CD55 gene conferred hypoxia inducibility for CD55. These studies identified a functional binding site for the transcriptional regulator hypoxia-inducible factor (HIF). Taken together, these results identify HIF-dependent induction of epithelial CD55 in the resolution of ongoing inflammation through clearance of apical PMN.

HIF-1-dependent repression of equilibrative nucleoside transporter (ENT) in hypoxia

Extracellular adenosine (Ado) has been implicated as central signaling molecule during conditions of limited oxygen availability (hypoxia), regulating physiologic outcomes as diverse as vascular leak, leukocyte activation, and accumulation. Presently, the molecular mechanisms that elevate extracellular Ado during hypoxia are unclear. In the present study, we pursued the hypothesis that diminished uptake of Ado effectively enhances extracellular Ado signaling. Initial studies indicated that the half-life of Ado was increased by as much as fivefold after exposure of endothelia to hypoxia. Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia. Examination of the ENT1 promoter identified a hypoxia inducible factor 1 (HIF-1)–dependent repression of ENT1 during hypoxia. Using in vitro and in vivo models of Ado signaling, we revealed that decreased Ado uptake promotes vascular barrier and dampens neutrophil tissue accumulation during hypoxia. Moreover, epithelial Hif1α mutant animals displayed increased epithelial ENT1 expression. Together, these results identify transcriptional repression of ENT as an innate mechanism to elevate extracellular Ado during hypoxia.

Hypoxia induces epithelial amphiregulin gene expression in a CREB-dependent manner

Hypoxia occurs during a number of conditions in which altered epithelial proliferation is critical, including tumor development. Microarray analysis of colon-derived epithelial cells revealed a hypoxia-dependent increase in the expression of amphiregulin, an EGF receptor (EGFR) ligand that activates epithelial proliferation and has been associated with the development of colonic tumors. Amphiregulin expression was also induced in tissues from mice exposed to whole animal hypoxia. The hypoxic upregulation of amphiregulin was independent of the classic transcriptional response mediated via hypoxia-inducible factor (HIF)-1α. Transfection of HeLa cells with truncated amphiregulin promoter reporter constructs revealed that a 37-bp segment upstream from the TATA box retained hypoxic sensitivity. This sequence contains an evolutionarily conserved cAMP response element (CRE) that constitutively binds the CRE binding protein (CREB). Deletion of the CRE abolished sensitivity to hypoxia. Thus hypoxia promotes intestinal epithelial amphiregulin expression in a CRE-dependent manner, an event that may contribute to increased proliferation. These data also further support a role for CREB as an HIF-independent hypoxia-responsive transcription factor in the regulation of intestinal epithelial gene expression.

Interplay between VHL/HIF1α and Wnt/β-catenin pathways during colorectal tumorigenesis

Activation of the Wnt signaling pathway initiates the transformation of colorectal epithelial cells, although the transition to metastatic cancer requires angiogenesis. We have investigated the expression of the von Hippel-Lindau (VHL) tumor suppressor in the intestines from humans and mice. Here, we show that VHL expression is regulated by TCF4 and is restricted to the proliferative compartment at the bottom of intestinal crypts. Accordingly, VHL is completely absent from the proliferative intestinal pockets of Tcf4(-/-) perinatal mice. We observed complementary staining of the hypoxia-inducible factor (HIF) 1alpha to VHL in normal intestinal epithelium as well as in all stages of colorectal cancer (CRC). To the best of our knowledge, this is the first report demonstrating the presence of nuclear HIF1alpha in normoxic healthy adult tissue. Although we observed upregulated levels of VHL in very early CRC lesions from sporadic and familial adenomatous polyposis patients - presumably due to activated Wnt signaling - a clear reduction of VHL expression is observed in later stages of CRC progression, coinciding with stabilization of HIF1alpha. As loss of VHL in later stages of CRC progression results in stabilization of HIF, these data provide evidence that selection for VHL downregulation provides a proangiogenic impulse for CRC progression.

Induction of the von Hippel-Lindau tumor suppressor gene by late hypoxia limits HIF-1 expression

Hypoxia-inducible factor (HIF) remains the central focus of oxygen sensing during hypoxia. HIF is a heterodimeric transcription factor consisting of an oxygen-regulated alpha- and a constitutively expressed beta subunit. The von Hippel-Lindau tumor suppressor (pVHL) is a component of the E3 ubiquitin ligase complex and targets HIF-alpha to proteasomal degradation, but also is known to exert a significant control on HIF transactivation activity. However, the understanding of the full interaction between HIF and pVHL has been hindered by a lack in the understanding of pVHL regulation. Here, we report that pVHL itself is induced in prolonged hypoxia in a kinetic that parallels the observed downregulation of HIF-1alpha protein under such conditions. In addition, we document direct HIF-1alpha binding to the VHL promoter and identify a functional hypoxia response element (HRE) within the VHL promoter. Such induction of pVHL in hypoxia furthermore has functional implications for the HIF dependent hypoxic response, implicating a physiologically relevant feedback mechanism. These results provide an intriguing model, whereby HIF self-regulates expression through VHL and highlight the role of pVHL as a unifying mechanism of HIF regulation.

Expression of trefoil factor family members correlates with patient prognosis and neoangiogenesis

PURPOSE

Trefoil factor family (TFF) peptides are thought to contribute to epithelial protection and restitution by virtue of their protease-resistant nature and their strong affinity for mucins. However, they are often overexpressed in tumors, where they seem to be negative prognostic factors, possibly contributing to tumor spread, although the precise mechanisms have not been defined.

EXPERIMENTAL DESIGN

Tissue sections from 111 patients with curatively resected advanced gastric carcinoma were immunohistochemically stained for TFF2, ITF (TFF3), and CD34. Microvessel density was expressed as number and area of microvessels. Results were correlated with clinicopathological characteristics and patient survival.

RESULTS

Forty-nine (44.1%) and 41 (36.9%) tumors were immunohistochemically positive for TFF3 and TFF2, respectively. Among the various clinicopathologic variables, overexpression of TFF3 had a significant correlation with patient age only. In addition, a significantly higher prevalence of positive TFF2 staining was detected in large, diffuse tumors and in tumors with lymph node metastasis. The number of microvessels had a significant correlation with both TFF3 and TFF2 staining, whereas the area of microvessels had a significant correlation only with TFF3 staining. Both TFF3 and TFF2 were independent predictors of a worse disease-free survival. TFF3 had a gender-specific negative survival advantage, with a 91.3% disease-free survival in female patients with TFF3-negative advanced gastric carcinoma.

CONCLUSIONS

Induction of increased tumor vascularity might be one of the mechanisms by which TFFs confer metastatic phenotype and frequent disease recurrence in gastric carcinomas. Female patients with TFF3-negative advanced gastric carcinoma have comparable survival as that reported for patients with early gastric carcinoma.

Eosinophils alter colonic epithelial barrier function: role for major basic protein

Mucosal eosinophils increase in a number of gastrointestinal diseases that are often associated with altered epithelial barrier function, including food allergic enteropathies and inflammatory bowel diseases. Although eosinophils are known to secrete biologically active mediators including granule proteins, their role in gastrointestinal diseases is uncertain. The aim of this study was to determine the impact of eosinophils on intestinal barrier function. Epithelial barrier function was determined in a coculture of eosinophils and T84 epithelial cells and in a murine model of T helper (Th) type 2-mediated colitis. Coculture conditions resulted in decreased transepithelial resistance (TER) and increased transepithelial flux. Cell-free coculture supernatants contained a > or =5-kDa soluble factor that also diminished TER; these supernatants contained the eosinophil-granule proteins major basic protein (MBP) and eosinophil-derived neurotoxin (EDN). T84 barrier function decreased significantly when basolateral surfaces were exposed to native human MBP but not EDN. Additional studies identified downregulation of the tight junctional molecule occludin as at least one mechanism for MBP action. MBP-null mice were protected from inflammation associated with oxazolone colitis compared with wild-type mice. In conclusion, MBP decreases epithelial barrier function and in this manner contributes to the pathogenesis of inflammatory bowel diseases.

OVEREXPRESSION OF ??B CRYSTALLIN IN THE GASTROINTESTINAL TRACT OF THE NEWBORN PIGLET AFTER HYPOXIA

Newborn animals are particularly sensitive to hypoxic stress. Oxygen is spared for sensitive tissues, including brain and heart. Scarce information is available concerning the molecular effects of hypoxia in the gastrointestinal tract (GIT). Moreover, stress protein expressions, including heat shock proteins (HSP), are still poorly documented in the GIT. Our objective was to determine the possible effect of hypoxia on HSP expression at birth. After western blotting, alphaB crystallin, HSP 27, and HSP 70 expressions were determined in newborn controls and piglets exposed to 1 or 4 h hypoxia (5% O2, 95% N2) allowed to recover from 1 to 68 h. Cytosol and nuclei were also separated and the extracts were tested for HSF1 and alphaB crystallin expressions. Surprisingly, alphaB crystallin was overexpressed in the stomach and colon in animals submitted to hypoxia, whereas HSP 27 and HSP 70 expression remained stable. Increases and return to basal levels in HSF1 and alphaB crystallin were simultaneously observed in the unique nuclear compartment. To our knowledge, the present study is the first to demonstrate the oxygen dependency of an HSP in the GIT, particularly in the colon in newborn piglets. The kinetics of alphaB crystallin overexpression after hypoxia parallels the activation of HSF1. This observation possibly indicates a correlation between this factor and alphaB crystallin after hypoxia. Taken together, the present results open the field of wide investigation about the specific response of this low-molecular-weight HSP and its possible involvement in pathological states in the GIT such as stomach and colon.

Integration of oxygen signaling at the consensus HRE

The hypoxia-inducible factor 1 (HIF-1) was initially identified as a transcription factor that regulated erythropoietin gene expression in response to a decrease in oxygen availability in kidney tissue. Subsequently, a family of oxygen-dependent protein hydroxylases was found to regulate the abundance and activity of three oxygen-sensitive HIFalpha subunits, which, as part of the HIF heterodimer, regulated the transcription of at least 70 different effector genes. In addition to responding to a decrease in tissue oxygenation, HIF is proactively induced, even under normoxic conditions, in response to stimuli that lead to cell growth, ultimately leading to higher oxygen consumption. The growing cell thus profits from an anticipatory increase in HIF-dependent target gene expression. Growth stimuli-activated signaling pathways that influence the abundance and activity of HIFs include pathways in which kinases are activated and pathways in which reactive oxygen species are liberated. These pathways signal to the HIF protein hydroxylases, as well as to HIF itself, by means of covalent or redox modifications and protein-protein interactions. The final point of integration of all of these pathways is the hypoxia-response element (HRE) of effector genes. Here, we provide comprehensive compilations of the known growth stimuli that promote increases in HIF abundance, of protein-protein interactions involving HIF, and of the known HIF effector genes. The consensus HRE derived from a comparison of the HREs of these HIF effectors will be useful for identification of novel HIF target genes, design of oxygen-regulated gene therapy, and prediction of effects of future drugs targeting the HIF system.

Components of intestinal epithelial hypoxia activate the virulence circuitry of Pseudomonas

We have previously shown that a lethal virulence trait in Pseudomonas aeruginosa, the PA-I lectin, is expressed by bacteria within the intestinal lumen of surgically stressed mice. The aim of this study was to determine whether intestinal epithelial hypoxia, a common response to surgical stress, could activate PA-I expression. A fusion construct was generated to express green fluorescent protein downstream of the PA-I gene, serving as a stable reporter strain for PA-I expression in P. aeruginosa. Polarized Caco-2 monolayers were exposed to ambient hypoxia (0.1-0.3% O2) for 1 h, with or without a recovery period of normoxia (21% O2) for 2 h, and then inoculated with P. aeruginosa containing the PA-I reporter construct. Hypoxic Caco-2 monolayers caused a significant increase in PA-I promoter activity relative to normoxic monolayers (165% at 1 h; P < 0.001). Similar activation of PA-I was also induced by cell-free apical, but not basal, media from hypoxic Caco-2 monolayers. PA-I promoter activation was preferentially enhanced in bacterial cells that physically interacted with hypoxic epithelia. We conclude that the virulence circuitry of P. aeruginosa is activated by both soluble and contact-mediated elements of the intestinal epithelium during hypoxia and normoxic recovery.

Butyrate suppresses hypoxia-inducible factor-1 activity in intestinal epithelial cells under hypoxic conditions

Interaction between the products of intestinal bacteria and the intestinal epithelial cells is a key event in understanding the biological, physiological, and pathological functions of the intestinal epithelium. Here, we examined the effect of butyrate, one of the major intestinal bacterial products, on hypoxia-inducible factor-1 (HIF-1) activity under hypoxic conditions in intestinal epithelial cells. HIF-1 activity was assessed by luciferase assay using cytoplasmic extracts of intestinal epithelial cells, Caco-2, and IEC-6 cells. These cells were transiently transfected with hypoxia response element (HRE)-luciferase reporter plasmids and cultured under hypoxic conditions in the presence or absence of sodium butyrate (NaB). The effect of NaB on HRE DNA binding activity in Caco-2 cells under hypoxic conditions was assessed by electrophoretic mobility shift assay. Expression of a hypoxia-responsive gene encoding intestinal trefoil factor (ITF) in Caco-2 cells after NaB treatment was assessed using reverse-transcription PCR. The barrier function of Caco-2 cells under hypoxic conditions was also evaluated by transepithelial electrical resistance measurement. NaB suppressed up-regulation of HIF-1 transcriptional activity under hypoxic conditions in Caco-2 and IEC-6 cells. In parallel, NaB reduced HRE DNA binding activity under the same conditions. Furthermore, NaB down-regulated enhanced transcription of ITF gene. Addition of NaB under hypoxic conditions delayed recovery of transepithelial electrical resistance of the monolayers after hypoxia-reoxygenation treatment. These findings indicate that NaB suppresses HIF-1 transcriptional activity on hypoxia-responsive genes by reducing the HRE DNA binding activity under hypoxic conditions in intestinal epithelial cells.

Hypoxia-inducible factor-1alpha is involved in the attenuation of experimentally induced rat glomerulonephritis

BACKGROUND/AIM

Among various kidney disease models, there are few rat glomerulonephritis (GN) models that develop in a short time, and with mainly glomerular lesions. Hypoxia-inducible factor (HIF)-1alpha is a transcriptional factor that induces genes supporting cell survival, but the involvement of HIF-1alpha in attenuating the progression of GN remains to be elucidated. We developed a new model of rat GN by coadministration of angiotensin II (AII) with Habu snake venom (HV) and investigated whether HIF-1alpha is involved in renal protection.

METHODS

Male Wistar rats were unilaterally nephrectomized on day 1, and divided into 4 groups on day 0; N group (no treatment), HV group, A group (AII), and H+A group (HV and AII). To preinduce HIF-1alpha, cobalt chloride (CoCl2) was injected twice before injections of HV and AII in 11 rats.

RESULTS

GN was detected only in the H+A group; observed first on day 2 and aggravated thereafter. HIF-1alpha was expressed in the glomeruli and renal tubules in the A and H+A groups. In the H+A group, GN was remarkably reduced by CoCl2 pretreatment (44.9 to 12.2%, p < 0.01).

CONCLUSION

Both HV and AII were critical for the development of GN, and HIF-1alpha remarkably attenuated the progression of GN.

Hypoxia-inducible factor 1 alpha and vascular endothelial growth factor overexpression in ischemic colitis

AIM: To examine the etiology and pathophysiology in human ischemic colitis from the viewpoint of ischemic factors such as hypoxia-inducible factor 1 alpha (HIF-1 alpha and vascular endothelial growth factor (VEGF).

METHODS: Thirteen patients with ischemic colitis and 21 normal controls underwent colonoscopy. The follow-up colonoscopy was performed in 8 patients at 7 to 10 d after the occurrence of ischemic colitis. Biopsy samples were subjected to real-time RT-PCR and immunohistochemistry to detect the expression of HIF-1 alpha and VEGF.

RESULTS: HIF-1 alpha and VEGF expression were found in the normal colon tissues by RT-PCR and immunohistochemistry. HIF-1 alpha and VEGF were overexpressed in the lesions of ischemic colitis. Overexpressed HIF-1 alpha and VEGF RNA quickly decreased to the normal level in the scar regions at 7 to 10 d after the occurrence of ischemic colitis.

CONCLUSION: Constant expression of HIF-1 alpha and VEGF in normal human colon tissue suggested that HIF-1 alpha and VEGF play an important role in maintaining tissue integrity. We confirmed the ischemic crisis in ischemic colitis at the molecular level, demonstrating overexpression of HIF-1 alpha and VEGF in ischemic lesions. These ischemic factors may play an important role in the pathophysiology of ischemic colitis.

Persistent HIF-1alpha activation in gut ischemia/reperfusion injury: potential role of bacteria and lipopolysaccharide

In both animal models of hemorrhagic shock and clinical settings, shock-induced gut ischemia has been implicated in the development of the systemic inflammatory response syndrome and distant organ injury, yet the factors transducing these events remain to be fully determined. Because hypoxia-inducible factor (HIF-1), a transcription factor composed of oxygen-labile HIF-1alpha and constitutive HIF-1beta subunits, regulates the physiologic/pathophysiologic response to hypoxia and ischemia, we examined the HIF-1 response in two rat models of gut ischemia-reperfusion. We found that ileal nuclear HIF-1alpha protein levels were induced in rats subjected to trauma (laparotomy) plus hemorrhagic shock for 90 min relative to their trauma sham-shock and naïve counterparts and that this trauma hemorrhagic shock-induced mucosal HIF-1alpha protein response persisted after 1 h and 3 h of reperfusion. Likewise, in a model of isolated gut ischemia-reperfusion injury, where the superior mesenteric artery was occluded for 45 min, nuclear HIF-1alpha were induced in the gut mucosa relative to their sham counterparts and persisted after 1 h and 3 h or reperfusion. Similar to the in vivo response, in vitro hypoxia induced HIF-alpha expression in three different enterocyte cell lines (rat IEC-6 and human Caco-2 and HT-29 cell lines). However, in contrast to the in vivo response, HIF-1 expression rapidly disappeared on subsequent reoxygenation. Because in vivo enterocytes are exposed to bacteria, we tested whether the in vitro HIF-1alpha response would persist on reoxygenation if the enterocytes were cocultured with bacteria. P. aeruginosa, an enteric bacterium, markedly induced enterocyte HIF-1alpha protein levels under normoxic conditions. Furthermore, the addition of P. aeruginosa during either the hypoxic or reoxygenation phase prevented the degradation of HIF-1alpha protein levels. Moreover, the observation that lipopolysaccharide induced HIF-1alpha expression in a time-dependent manner in IEC-6 cells indicated that the induction of HIF-1 by exposure to P. aeruginosa is not dependent on bacterial viability. In conclusion, these results suggest that HIF-1alpha activation is an early reperfusion-independent event in models of gut ischemia-reperfusion and that this HIF-1alpha response is potentiated by the presence of P. aeruginosa or lipopolysaccharide.

Epithelial hypoxia-inducible factor-1 is protective in murine experimental colitis

Mucosal epithelial cells are uniquely equipped to maintain barrier function even under adverse conditions. Previous studies have implicated hypoxia in mucosal tissue damage resulting from both acute and chronic inflammation. Given the importance of the transcriptional regulator hypoxia-inducible factor-1 (HIF-1) for adaptive hypoxia responses, we hypothesized that HIF-1 may serve as a barrier-protective element during mucosal inflammation. Initial studies of hapten-based murine colitis revealed extensive mucosal hypoxia and concomitant HIF-1 activation during colitis. To study this in more detail, we generated 2 mouse lines with intestinal epithelium-targeted expression of either mutant Hif1a (inability to form HIF-1) or mutant von Hippel-Lindau gene (Vhlh; constitutively active HIF-1). Studies of colitis in these mice revealed that decreased HIF-1 expression correlated with more severe clinical symptoms (mortality, weight loss, colon length), while increased HIF levels were protective in these parameters. Furthermore, colons with constitutive activation of HIF displayed increased expression levels of HIF-1-regulated barrier-protective genes (multidrug resistance gene-1, intestinal trefoil factor, CD73), resulting in attenuated loss of barrier during colitis in vivo. Taken together, these studies provide insight into tissue microenvironmental changes during model inflammatory bowel disease and identify HIF-1 as a critical factor for barrier protection during mucosal insult.

Trefoil factor family peptide 3 prevents the development and promotes healing of ischemia-reperfusion injury in weanling rats

BACKGROUND/PURPOSE

Although the pathogenesis of necrotizing enterocolitis (NEC) is not completely defined, ischemia appears to be one of the most important causative factors. Trefoil factor family peptide 3 (TFF3) is a peptide normally expressed in the small bowel and colon and is involved in the maintenance and repair of mucosal integrity. The authors hypothesized that monomeric (TFF3 Ser57) and dimeric (TFF3 Cys57) recombinant TFF3 may prevent the development and accelerate healing of intestinal ischemia-reperfusion injury in weanling rats.

METHODS

Intestinal injury was induced in 18-day-old rats by occlusion of the superior mesenteric vessels for 60 minutes. To examine the protective effect, rats were given 3 microg/g of TFF3 Ser57 or TFF3 Cys57 by subcutaneous or enteral administration 30 minutes before the vascular occlusion. To examine the healing effect, rats were given 3 microg/g of TFF3 Ser57 or TFF3 Cys57 by subcutaneous or enteral administration 60 minutes after the beginning of reperfusion. Samples from small bowel and colon were collected for morphometric analysis after 3 hours of reperfusion. Mucosal damage was assessed by the Chiu score.

RESULTS

Both forms of TFF3 reduced the amount of damage when administered before the ischemia. Administration of TFF3 Ser57 and TFF3 Cys57 after the beginning of reperfusion significantly increased the villous height and decreased the Chiu score in the small intestine and colon.

CONCLUSIONS

TFF3 Ser57 monomer and TFF3 Cys57 dimer prevent the development and promote healing of ischemia-reperfusion injury in weanling rats. There are no differences between the routes of administration of TFF3.

Epithelial hypoxia-inducible factor-1 is protective in murine experimental colitis

Mucosal epithelial cells are uniquely equipped to maintain barrier function even under adverse conditions. Previous studies have implicated hypoxia in mucosal tissue damage resulting from both acute and chronic inflammation. Given the importance of the transcriptional regulator hypoxia-inducible factor-1 (HIF-1) for adaptive hypoxia responses, we hypothesized that HIF-1 may serve as a barrier-protective element during mucosal inflammation. Initial studies of hapten-based murine colitis revealed extensive mucosal hypoxia and concomitant HIF-1 activation during colitis. To study this in more detail, we generated 2 mouse lines with intestinal epithelium-targeted expression of either mutant Hif1a (inability to form HIF-1) or mutant von Hippel-Lindau gene (Vhlh; constitutively active HIF-1). Studies of colitis in these mice revealed that decreased HIF-1 expression correlated with more severe clinical symptoms (mortality, weight loss, colon length), while increased HIF levels were protective in these parameters. Furthermore, colons with constitutive activation of HIF displayed increased expression levels of HIF-1-regulated barrier-protective genes (multidrug resistance gene-1, intestinal trefoil factor, CD73), resulting in attenuated loss of barrier during colitis in vivo. Taken together, these studies provide insight into tissue microenvironmental changes during model inflammatory bowel disease and identify HIF-1 as a critical factor for barrier protection during mucosal insult.

Endogenous adenosine produced during hypoxia attenuates neutrophil accumulation: coordination by extracellular nucleotide metabolism

Hypoxia is a well-documented inflammatory stimulus and results in tissue polymorphonuclear leukocyte (PMN) accumulation. Likewise, increased tissue adenosine levels are commonly associated with hypoxia, and given the anti-inflammatory properties of adenosine, we hypothesized that adenosine production via adenine nucleotide metabolism at the vascular surface triggers an endogenous anti-inflammatory response during hypoxia. Initial in vitro studies indicated that endogenously generated adenosine, through activation of PMN adenosine A(2A) and A(2B) receptors, functions as an antiadhesive signal for PMN binding to microvascular endothelia. Intravascular nucleotides released by inflammatory cells undergo phosphohydrolysis via hypoxia-induced CD39 ectoapyrase (CD39 converts adenosine triphosphate/adenosine diphosphate [ATP/ADP] to adenosine monophosphate [AMP]) and CD73 ecto-5'-nucleotidase (CD73 converts AMP to adenosine). Extensions of our in vitro findings using cd39- and cd73-null animals revealed that extracellular adenosine produced through adenine nucleotide metabolism during hypoxia is a potent anti-inflammatory signal for PMNs in vivo. These findings identify CD39 and CD73 as critical control points for endogenous adenosine generation and implicate this pathway as an innate mechanism to attenuate excessive tissue PMN accumulation.

Regulation of intestinal epithelial gene expression in hypoxia

Mucosal hypoxia is intimately associated with chronic inflammation in the gastrointestinal tract in disease such as Crohn's disease. Under such conditions, intestinal epithelial cells may become a source of proinflammatory cytokines, including tumor necrosis factor alpha (TNFalpha), which actively contribute to ongoing inflammation through autocrine disruption of epithelial barrier function. These events are critically dependent upon alterations in the expression and function of the cAMF response element binding protein (CREB). Here we review our understanding of the molecular mechanisms underlying the regulation of CREB activity in intestinal epithelial cells in hypoxia.

Leukocyte adhesion during hypoxia is mediated by HIF-1-dependent induction of beta2 integrin gene expression

Inflammatory responses are associated with significant changes in tissue metabolism. In particular, metabolic shifts during inflammation can result in significant tissue hypoxia, with resultant induction of hypoxia-responsive genes. Given this association, we hypothesized that leukocyte functional responses are influenced by hypoxia. Initial experiments revealed that exposure of the promonocytic cell line U937 to hypoxia resulted in increased adhesion to activated endothelia. Such increases were transcription-dependent and were blocked by antibodies directed against beta2, but not beta1, integrins. Analysis of beta2 integrin mRNA and protein in U937 cells revealed a 5- to 6-fold increase with hypoxia. Extension of this analysis to hypoxic human whole blood revealed prominent induction of beta2 integrin mRNA and protein ex vivo. Furthermore, murine beta2 integrin mRNA was found to be significantly induced during hypoxia in vivo. Subsequent studies identified a binding site for hypoxia-inducible factor 1 (HIF-1) in the CD18 gene. This gene encodes the subunit common to all four known types of beta2 integrin heterodimer. HIF-1 binding was demonstrated in vivo, and mutational analysis of the HIF-1 site within the CD18 promoter resulted in a loss of hypoxia inducibility. Taken together, these results demonstrate that hypoxia induces leukocyte beta2 integrin expression and function by transcriptional mechanisms dependent upon HIF-1.

Human breast milk suppresses the transcriptional regulation of IL-1beta-induced NF-kappaB signaling in human intestinal cells

Neonatal necrotizing enterocolitis (NEC), which is a disease with a poor prognosis, is considered to be caused by the coincidence of intestinal ischemia-reperfusion injury and systemic inflammation due to the colonization of pathogenic bacteria. Interleukin (IL)-8, a proinflammatory cytokine, plays an important role in the pathophysiology of NEC. It was recently reported that IL-1beta activates the IL-8 gene by regulating the transcriptional nuclear factor kappaB (NF-kappaB) signaling pathways in intestinal cells. The protective role of maternal milk in NEC pathogenesis has been reported in both human and animal studies. In this study, we show that human breast milk dramatically suppressed the IL-1beta-induced activation of the IL-8 gene promoter by inhibiting the activation pathway of NF-kappaB. Moreover, we also show that human breast milk induced the production of IkappaBalpha. These results suggest that human breast milk could be protective and therapeutic in neonates with NEC by inhibiting the activation pathway of NF-kappaB.

Trefoil factor family (TFF) peptides and cancer progression

TFF peptides are involved in mucosal maintenance and repair through motogenic and antiapoptotic activities. These peptides are overexpressed during inflammatory processes and cancer progression. They also function as scatter factors, proinvasive and angiogenic agents. Such a divergence is related to the pathophysiological state of tissues submitted to persistent aggressive situations during digestive processes in the normal gastrointestinal tract, inflammatory and neoplastic diseases. In agreement with this model, TFF peptides are connected with multiple oncogenic pathways. As a consequence, the TFF signaling pathways may serve as potential targets in the control of chronic inflammation and progression of human solid tumors.

Hyperinsulinemia may boost both hematocrit and iron absorption by up-regulating activity of hypoxia-inducible factor-1α

There is growing evidence that increases in both hematocrit and body iron stores are components of the insulin resistance syndrome. The ability of insulin and of IGF-I - whose effective activity is increased in the context of insulin resistance - to boost activity of the transcription factor hypoxia-inducible factor-1alpha (HIF-1alpha), may be at least partially responsible for this association. HIF-1alpha, which functions physiologically as a detector of both hypoxia and iron-deficiency, promotes synthesis of erythropoietin, and may also mediate the up-regulatory impact of hypoxia on intestinal iron absorption. Insulin/IGF-I may also influence erythropoiesis more directly, as they are growth factors for developing reticulocytes. Conversely, the activation of HIF-1alpha associated with iron deficiency may be responsible for the increased glucose tolerance noted in iron-deficient animals; HIF-1alpha promotes efficient glucose uptake and glycolysis - a sensible adaptation to hypoxia - by inducing increased synthesis of glucose transporters and glycolytic enzymes. Recent reports that phlebotomy can increase the efficiency of muscle glucose uptake in lean healthy omnivores are intriguing and require further confirmation. Whether increased iron stores contribute to the elevated vascular risk associated with insulin resistance is doubtful, inasmuch as most prospective studies fail to correlate serum ferritin or transferrin saturation with subsequent vascular events. However, current data are reasonably consistent with the possibility that moderately elevated iron stores are associated with increased overall risk for cancer - and for colorectal cancer in particular; free iron may play a catalytic role in 'spontaneous' mutagenesis. Thus, iron excess may mediate at least some of the increased cancer risk associated with insulin resistance and heme-rich diets. People who are insulin resistant can minimize any health risk associated with iron overload by avoiding heme-rich flesh foods and donating blood regularly.

The role of HIF-1 alpha in transcriptional regulation of the proximal tubular epithelial cell response to hypoxia

Epithelial cells of the kidney represent a primary target for hypoxic injury in ischemic acute renal failure (ARF); however, the underlying transcriptional mechanism(s) remain undefined. In this study, human proximal tubular epithelial cells (HK-2) exposed to hypoxia in vitro demonstrated a non-lethal but dysfunctional phenotype, closely reflective of the epithelial pathobiology of ARF. HK-2 cells exposed to hypoxia demonstrated increased paracellular permeability, decreased proliferation, loss of tight junctional integrity, and significant actin disassembly in the absence of cell death. Microarray analysis of transcriptomic changes underlying this response identified a distinct cohort of 48 genes with a closely shared hypoxia-dependent expression profile. Within this hypoxia-sensitive cluster were genes identified previously as hypoxia-inducible factor-1 (HIF-1)-dependent (e.g. vascular endothelial growth factor and adrenomedullin) as well as genes not previously known to be hypoxia-responsive (e.g. stanniocalcin 2). In hypoxia, HIF-1 bound to evolutionarily conserved hypoxia-response elements (HRE) in the promoters of these genes as well as to the HRE consensus motif. A further subset of these genes, not associated with transcriptional regulation by HIF-1, was also present, suggesting alternative HIF-1-independent pathways. Overexpression of HIF-1 alpha in normoxia induced the expression of a significant number of the hypoxia-dependent genes; however, it did not induce the pathophysiologic epithelial response. In summary, hypoxia-elicited alterations in renal proximal tubular epithelial cells in vitro closely resemble the epithelial pathophysiology of ARF. Our data indicate that although this event may rely heavily on HIF-1-dependent gene transcription, it is likely that separate hypoxia-dependent transcriptional regulators also play a role.

Coordinated adenine nucleotide phosphohydrolysis and nucleoside signaling in posthypoxic endothelium: role of ectonucleotidases and adenosine A2B receptors

Limited oxygen delivery to tissues (hypoxia) is common in a variety of disease states. A number of parallels exist between hypoxia and acute inflammation, including the observation that both influence vascular permeability. As such, we compared the functional influence of activated polymorphonuclear leukocytes (PMN) on normoxic and posthypoxic endothelial cells. Initial studies indicated that activated PMN preferentially promote endothelial barrier function in posthypoxic endothelial cells (>60% increase over normoxia). Extension of these findings identified at least one soluble mediator as extracellular adenosine triphosphate (ATP). Subsequent studies revealed that ATP is coordinately hydrolyzed to adenosine at the endothelial cell surface by hypoxia-induced CD39 and CD73 (>20-and >12-fold increase in mRNA, respectively). Studies in vitro and in cd39-null mice identified these surface ecto-enzymes as critical control points for posthypoxia-associated protection of vascular permeability. Furthermore, insight gained through microarray analysis revealed that the adenosine A2B receptor (AdoRA2B) is selectively up-regulated by hypoxia (>5-fold increase in mRNA), and that AdoRA2B antagonists effectively neutralize ATP-mediated changes in posthypoxic endothelial permeability. Taken together, these results demonstrate transcription coordination of adenine nucleotide and nucleoside signaling at the vascular interface during hypoxia.

Intestinal epithelial hyperpermeability: update on the pathogenesis of gut mucosal barrier dysfunction in critical illness

PURPOSE OF REVIEW

Tight junctions between adjacent epithelial cells are essential for the maintenance of compositionally distinct fluid compartments in various organs, such as the liver, lungs, kidneys, and intestine. These epithelial organs are commonly affected in the condition known as multiple organ dysfunction syndrome, which can complicate the clinical course of patients with sepsis or other conditions associated with poorly controlled systemic inflammation. The gut serves as a useful model for this problem, and studies using reductionist in vitro models and experiments carried out using laboratory animals are starting to clarify the cellular and biochemical mechanisms that are responsible for intestinal epithelial hyperpermeability secondary to critical illness.

RECENT FINDINGS

One key factor that has been identified is excessive production of nitric oxide and related species, although other factors, such as increased expression of the cytokine interleukin 6, appear to be important as well. A newly described, cytokine-like molecule, high-mobility group B1, increases permeability of cultured epithelial monolayers in vitro and murine ileal mucosa in vivo.

SUMMARY

Epithelial dysfunction may be a common final pathway contributing to organ dysfunction in sepsis and other forms of critical illness.

Oxygen-dependent regulation of hypoxia-inducible factors by prolyl and asparaginyl hydroxylation

To sustain life mammals have an absolute and continual requirement for oxygen, which is necessary to produce energy for normal cell survival and growth. Hence, maintaining oxygen homeostasis is a critical requirement and mammals have evolved a wide range of cellular and physiological responses to adapt to changes in oxygen availability. In the past few years it has become evident that the transcriptional protein complex hypoxia-inducible factor (HIF) is a key regulator of these processes. In this review we will focus on the way oxygen availability regulates HIF proteins and in particular we will discuss the way oxygen-dependent hydroxylation of specific amino acid residues has been demonstrated to regulate HIF function at the level of both protein stability and transcriptional potency.

Effect of hypoxia on gene expression by human hepatocytes (HepG2)

The full extent to which hypoxia produces gene expression changes in human cells is unknown. We used late-generation oligonucleotide arrays to catalog hypoxia-induced changes in gene expression in HepG2 cells. Five paired sets of cultures were subjected to either control (room air-5% CO(2)) or hypoxic (1% O(2)-5% CO(2)) conditions for 24 h, and RNA was analyzed on an Affymetrix cDNA array containing approximately 12,600 sequences. A statistically significant change in expression was shown by 2,908 sequences (1,255 increased and 1,653 decreased). The observed changes were highly concordant with published literature on hypoxic stress but showed relatively little overlap (12-22%) with changes in gene expression that have been reported to occur after heat stress in other systems. Of note, of these 2,908 sequences, only 387 (213 increased and 174 decreased) both exhibited changes in expression of twofold or greater and were highly expressed in at least three of the five experiments. We conclude that the effect of hypoxia on gene expression by HepG2 cells is broad, has a significant component of downregulation, and includes a relatively small number of genes whose response is truly independent of cell and stress type.

Trefoil peptide expression and goblet cell number in rat intestine: effects of KGF and fasting-refeeding

The trefoil factor family peptides TFF1, TFF2, and TFF3 are important for gut mucosal protection and restitution. Keratinocyte growth factor (KGF) stimulates proliferation and differentiation of epithelial cells with potent effects on goblet cells. To investigate interactions between food intake and KGF, rats were fed ad libitum (control), fasted for 72 h, or fasted for 72 h and then refed for 72 h with or without KGF (3 mg. kg(-1). day(-1)). With fasting, goblet cell number in duodenum increased, TFF3 mRNA in duodenum and jejunum decreased, and TFF3 protein did not change or increased. KGF during fasting stimulated colonic growth, normalized TFF3 mRNA in duodenum and jejunum, and broadly upregulated gut goblet cell number and TFF3 protein expression. With fasting-refeeding, KGF increased small bowel and colonic mucosal growth, goblet cell number, and TFF3 protein but had variable effects on TFF3 mRNA. KGF induced TFF2 mRNA and protein in duodenum and jejunum with both nutritional regimens. We conclude that nutrient availability modifies rat intestinal goblet cell number, TFF3 mRNA, and the gut-trophic effects of KGF in a region-specific manner. KGF enhances TFF2 expression in proximal small bowel and increases goblet cell number and TFF3 protein content throughout the intestine independent of food intake.

Trefoil peptides as proangiogenic factors in vivo and in vitro: implication of cyclooxygenase-2 and EGF receptor signaling

We previously established that the trefoil peptides (TFFs) pS2, spasmolytic polypeptide, and intestinal trefoil factor are involved in cellular scattering and invasion in kidney and colonic cancer cells. Using the chorioallantoic membrane (CAM) assay and the formation of tube-like structures by human umbilical vein endothelial cells (HUVEC) plated on the Matrigel matrix substratum, we report here that TFFs are proangiogenic factors. Angiogenic activity of TFFs is comparable to that induced by vascular endothelial growth factor, leptin, and transforming growth factor-alpha. Stimulation of angiogenesis by pS2 in the CAM assay is blocked by pharmacological inhibitors of cyclooxygenase COX-2 (NS-398) and epidermal growth factor receptor (EGF-R) tyrosine kinase (ZD1839), but is independent of KDR/Flk-1 and thromboxane A2 receptors. In contrast, the morphogenic switch induced by pS2 in HUVEC cells could be inhibited by the specific KDR heptapeptide antagonist ATWLPPR and by inhibitors of COX-2 and EGF-R signaling. These results implicate TFFs in the formation of new blood vessels during normal and pathophysiological processes linked to wound healing, inflammation, and cancer progression in the digestive mucosa and other human solid tumors associated with aberrant expression of TFFs.

Ecto-5'-nucleotidase (CD73) regulation by hypoxia-inducible factor-1 mediates permeability changes in intestinal epithelia

Under conditions of limited oxygen availability (hypoxia), multiple cell types release adenine nucleotides in the form of ATP, ADP, and AMP. Extracellular AMP is metabolized to adenosine by surface-expressed ecto-5'-nucleotidase (CD73) and subsequently activates surface adenosine receptors regulating endothelial and epithelial barrier function. Therefore, we hypothesized that hypoxia transcriptionally regulates CD73 expression. Microarray RNA analysis revealed an increase in CD73 and ecto-apyrase CD39 in hypoxic epithelial cells. Metabolic studies of CD39/CD73 function in intact epithelia revealed that hypoxia enhances CD39/CD73 function as much as 6 +/- 0.5-fold over normoxia. Examination of the CD73 gene promoter identified at least one binding site for hypoxia-inducible factor-1 (HIF-1) and inhibition of HIF-1alpha expression by antisense oligonucleotides resulted in significant inhibition of hypoxia-inducible CD73 expression. Studies using luciferase reporter constructs revealed a significant increase in activity in cells subjected to hypoxia, which was lost in truncated constructs lacking the HIF-1 site. Mutagenesis of the HIF-1alpha binding site resulted in a nearly complete loss of hypoxia-inducibility. In vivo studies in a murine hypoxia model revealed that hypoxia-induced CD73 may serve to protect the epithelial barrier, since the CD73 inhibitor alpha,beta-methylene ADP promotes increased intestinal permeability. These results identify an HIF-1-dependent regulatory pathway for CD73 and indicate the likelihood that CD39/CD73 protects the epithelial barrier during hypoxia.

Intestinal barrier function

Intestinal barrier function regulates transport and host defense mechanisms at the mucosal interface with the outside world. Transcellular and paracellular fluxes are tightly controlled by membrane pumps, ion channels and tight junctions, adapting permeability to physiological needs. Food and microbial antigens are under constant surveillance of the mucosal immune system. Tolerance against commensals and immunity against pathogens require intact antigen uptake, recognition, processing and response mechanisms. Disturbance at any level, but particularly bacterial translocation due to increased permeability and breakdown of oral tolerance due to compromised epithelial and T cell interaction, can result in inflammation and tissue damage. New therapeutic approaches including probiotics and peptides to restore disrupted barrier function are evolving.

Ecto-5'-nucleotidase (CD73) regulation by hypoxia-inducible factor-1 mediates permeability changes in intestinal epithelia

Under conditions of limited oxygen availability (hypoxia), multiple cell types release adenine nucleotides in the form of ATP, ADP, and AMP. Extracellular AMP is metabolized to adenosine by surface-expressed ecto-5'-nucleotidase (CD73) and subsequently activates surface adenosine receptors regulating endothelial and epithelial barrier function. Therefore, we hypothesized that hypoxia transcriptionally regulates CD73 expression. Microarray RNA analysis revealed an increase in CD73 and ecto-apyrase CD39 in hypoxic epithelial cells. Metabolic studies of CD39/CD73 function in intact epithelia revealed that hypoxia enhances CD39/CD73 function as much as 6 +/- 0.5-fold over normoxia. Examination of the CD73 gene promoter identified at least one binding site for hypoxia-inducible factor-1 (HIF-1) and inhibition of HIF-1alpha expression by antisense oligonucleotides resulted in significant inhibition of hypoxia-inducible CD73 expression. Studies using luciferase reporter constructs revealed a significant increase in activity in cells subjected to hypoxia, which was lost in truncated constructs lacking the HIF-1 site. Mutagenesis of the HIF-1alpha binding site resulted in a nearly complete loss of hypoxia-inducibility. In vivo studies in a murine hypoxia model revealed that hypoxia-induced CD73 may serve to protect the epithelial barrier, since the CD73 inhibitor alpha,beta-methylene ADP promotes increased intestinal permeability. These results identify an HIF-1-dependent regulatory pathway for CD73 and indicate the likelihood that CD39/CD73 protects the epithelial barrier during hypoxia.

Signal transduction to hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator that functions as a master regulator of O2 homeostasis. HIF-1 target genes encode proteins that increase O2 delivery and mediate adaptive responses to O2 deprivation. HIF-1 activity is regulated by the cellular O2 concentration and by the major growth factor-stimulated signal transduction pathways. In human cancer cells, both intratumoral hypoxia and genetic alterations affecting signal transduction pathways lead to increased HIF-1 activity, which promotes angiogenesis, metabolic adaptation, and other critical aspects of tumor progression.

Cell type specific expression of secretory TFF peptides: colocalization with mucins and synthesis in the brain

The "TFF domain" is an ancient cysteine-rich shuffled module forming the basic unit for the family of secretory TFF peptides (formerly P-domain peptides and trefoil factors). It is also an integral component of mosaic proteins associated with mucous surfaces. Three mammalian TFF peptides are known (i.e., TFF1-TFF3); however, in Xenopus laevis the pattern is more complex (xP1, xP4.1, xP4.2, and xP2). TFF peptides are typical secretory products of a variety of mucin-producing epithelial cells (e.g., the conjunctiva, the salivary glands, the gastrointestinal tract, the respiratory tract, and the uterus). Each TFF peptide shows an unique expression pattern and different mucin-producing cells are characterized by their specific TFF peptide/secretory mucin combinations. TFF peptides have a pivotal role in maintaining the surface integrity of mucous epithelia in vivo. They are typical constituents of mucus gels, they modulate rapid mucosal repair ("restitution") by their motogenic and their cell scattering activity, they have antiapoptotic effects, and they probably modulate inflammatory processes. Pathological expression of TFF peptides occurs as a result of chronic inflammatory diseases or certain tumors. TFF peptides are also found in the central nervous system, at least in mammals. In particular, TFF3 is synthesized from oxytocinergic neurons of the hypothalamus and is released from the posterior pituitary into the bloodstream.

Regulation of hypoxia-inducible factor 1 in enterocytic cells

BACKGROUND

Mucosal hypoxia due to intestinal hypoperfusion is characteristic of a number of clinical disorders. An early event in the adaptive response to cellular hypoxia is the binding of hypoxia-inducible factor 1 (HIF-1) to cis-acting regulatory sites in target genes.

METHODS

We characterized the expression of HIF-1 in transformed (Caco-2(BBe) and T84) and nontransformed human (FHs 74 Int) and rat (IEC-6) intestinal epithelial cell lines.

RESULTS

The electrophoretic mobility shift assay detected increased HIF-1 DNA-binding activity in each cell line within 2 h of hypoxia (1% O2). HIF-1 binding was maximal within 4 h and remained stable for 24 h. HIF-1 DNA-binding activity was maximal in the established IEC-6 cell line below 2% oxygen, but HIF-1 DNA-binding activity was not detectable above 0.5% O2 in the primary human FHs 74 Int cell line. The nonspecific protein kinase inhibitor genistein (200 microM) inhibited HIF-1 binding at 4 h. Transfection of Caco-2 cells with a wild-type, but not a mutant, HIF-1-dependent luciferase expression vector resulted in a fourfold induction of reporter gene expression during hypoxia.

CONCLUSIONS

In conclusion, HIF-1 regulates gene expression in enterocytes and an undefined phosphorylation event is important for O2 sensing.

Neutrophil-derived glutamate regulates vascular endothelial barrier function

Endothelial barrier function is altered by the release of soluble polymorphonuclear leukocyte (PMN)-derived mediators during inflammatory states. However, endogenous pathways to describe such changes are only recently appreciated. Using an in vitro endothelial paracellular permeability model, cell-free supernatants from formylmethionylleucylphenylalanine-stimulated PMNs were observed to significantly alter endothelial permeability. Biophysical and biochemical analysis of PMN supernatants identified PMN-derived glutamate in modulating endothelial permeability. Furthermore, novel expression of metabotropic glutamate receptor 1 (mGluR1), mGluR4, and mGluR5 by human brain and dermal microvascular endothelial cells was demonstrated by reverse transcription-PCR, in situ hybridization, immunofluorescence, and Western blot analysis. Treatment of human brain endothelia with glutamate or selective, mGluR group I or III agonists resulted in a time-dependent loss of phosphorylated vasodilator-stimulated phosphoprotein (VASP) and significantly increased endothelial permeability. Glutamate-induced decreases in brain endothelial barrier function and phosphorylated VASP were significantly attenuated by pretreatment of human brain endothelia with selective mGluR antagonists. These observations were extended to an in vivo hypoxic mouse model in which pretreatment with mGluR antagonists significantly decreased fluorescein isothiocyanate-dextran flux across the blood-brain barrier. We conclude that activated human PMNs release glutamate and that endothelial expression of group I or III mGluRs function to decrease human brain endothelial VASP phosphorylation and barrier function. These results identify a novel pathway by which PMN-derived glutamate may regulate human endothelial barrier function.

Lactase gene transcription is activated in response to hypoxia in intestinal epithelial cells

Lactase-phlorizin hydrolase, a brush-border membrane disaccharidase, is a marker of intestinal epithelial cell differentiation and digestive function. The intestine is susceptible to conditions of hypoxia resulting from vascular perfusion deficits. We hypothesized that lactase gene induction may provide a mechanism to efficiently increase nutrient energy substrates during gut hypoxia. These studies sought to characterize expression of the lactase gene in response to hypoxia and to characterize a role for hypoxia-inducible factor (HIF-1) in mediating the hypoxic response. Microarray analysis and confirmatory RT-PCR identified a 4-fold induction of lactase mRNA abundance in intestinal epithelial Caco-2 cells exposed to hypoxia. Lactase promoter activity was similarly induced by hypoxia in cells stably transfected with a 2.0-kb 5' flanking region of the rat lactase gene linked to a reporter gene. Transient cotransfection with HIF-1alpha and beta stimulated lactase promoter activity 2.4- and 3.5-fold under conditions of normoxia and hypoxia, respectively. We conclude that HIF-1 can activate the lactase promoter in intestinal epithelial cells exposed to hypoxia. Induction of lactase transcription may represent an adaptive response to gut hypoxia.

Hypoxia-inducible factor 1-mediated inhibition of peroxisome proliferator-activated receptor alpha expression during hypoxia

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone-binding proteins that regulate transcriptional responses to peroxisome proliferators and structurally diverse fatty acids. PPARs have been implicated in a wide variety of functions, including lipid homeostasis and inflammatory responses. In this study, we examined the expression of PPAR-alpha in response to ambient hypoxia. Initial studies using microarray analysis of intestinal epithelial mRNA revealed that hypoxia rapidly down-regulates PPAR-alpha mRNA and protein in epithelial cells in vitro and in vivo. Subsequent studies revealed that the PPAR-alpha gene bears a DNA consensus motif for the transcription factor hypoxia-inducible factor 1 (HIF-1). EMSA analysis revealed that ambient hypoxia induces HIF-1alpha binding to the HIF-1 consensus domain of PPAR-alpha in parallel to HIF-1 nuclear accumulation, and antisense depletion of HIF-1alpha resulted in a loss of PPAR-alpha down-regulation. The PPAR-alpha ligand pirinixic acid (WY14643) functionally promoted IFN-gamma-induced ICAM-1 expression in normoxic epithelia, and this response was lost in cells pre-exposed to ambient hypoxia. Such results indicate that HIF-1-dependent down-regulation of PPAR-alpha may provide an adaptive response to proinflammatory stimuli during cellular hypoxia. These studies provide unique insight into the regulation of PPAR-alpha expression and, importantly, provide an example of a down-regulatory pathway mediated by HIF-1.