Commentary: getting real on addressing health care disparities and other systems problems

Physician membership organizations vary in the extent of their engagement in activities to address health disparities. Increasing engagement of those organizations not already highly active in this critical area is, thus, an opportunity. Studies that provide definitional contours of key issues, like disparities, are necessary and must be iteratively refined. However, parallel activities of intervention with measured outcomes to assess the effects of these interventions are necessary to truly address major problems in the health care system. To date, work in the problem definition category exceeds work toward intervention in and mitigation of these problems with measured outcomes. Many problems in health care, including disparities, are now sufficiently understood that it is time to shift focus toward bold intervention with measured outcomes. Optimal approaches that yield superior outcomes generally require collaboration across the provider-payer spectrum and the private sectors, including physicians, hospitals, insurers, etc. Stakeholders are now free to act in such coordinated fashion; it only requires social capital that permits cooperation and compromise. Interventions for problems such as health care disparities can be developed in the private sector and mirrored by government payers if physicians and organizations can get real about collaborating to implement outcomes-based initiatives to improve the health of all patients.

Evidence-based appointment and promotion of academic faculty at the University of Chicago

The authors report how one academic medical center (AMC) and associated nonclinical departments implemented evidence-based academic criteria and an evidence-based academic vetting process, which may be models for other institutions. In 2004-2005, The University of Chicago Division of the Biological Sciences and Pritzker School of Medicine reconceptualized its appointment, promotion, and tenure criteria to recognize all forms of scholarship as equally legitimate bases for academic tenure. The revised criteria also accommodate differences in academic effort consistent with varying clinical demands. Implementation of these criteria, however, necessitated revised practices in providing objective evidence and analysis of their satisfaction. Three complementary mechanisms now yield excellent evidence and analysis. The first, electronic forms (e-forms) comprise highly specific response items with embedded instructions, advice, and rationale. The e-forms encourage candidates and departments to provide the evidence that subsequent review needs to evaluate appointment or promotion proposals. Unexpectedly, the e-forms have been coopted as effective mechanisms for faculty development. Second, a faculty dean of academic affairs, a regular faculty member, was appointed to provide robust academic authority and perspective to the process. Third, the promotion and tenure advisory committee was restricted to evaluating academic criteria, and from considerations of institutional value. This change interposed a "firewall" between academic and institutional review. These changes have attenuated dissatisfaction with the appointments and promotions process both within and outside the AMC.

Promoting an environment of professionalism: the University of Chicago "Roadmap"

In 2005, the University of Chicago Pritzker School of Medicine unveiled an institution-wide Roadmap to Professionalism designed to both increase awareness about issues of medical professionalism across the institution and gain a better understanding of how medical trainees' professional behaviors' change during their training as a result of the medical learning environment. The authors describe the institution's approach and progress to date. To gain buy-in from all levels of learners at the Pritzker School of Medicine, the initiative uses vertically integrated advisory groups to engage medical trainees in the assessment and development of experiential workshops and evaluation initiatives based on the principles outlined in the American Board of Internal Medicine / American College of Physicians / European Federation of Internal Medicine's Physician Charter for Medical Professionalism. Advisory groups provide targeted assessments and programming at each stage of the medical learner: preclinical students, clinical students, residents, and faculty. Surveys of medical students' perceptions of professionalism have provided an important baseline assessment of the learning environment, from which the professionalism steering committee plans to target future curricular interventions. Efforts to engage residents have focused on experiential workshops addressing interactions with the pharmaceutical industry and patient-care hand-offs, as well as the development of a tool for patients to evaluate their resident physicians. The establishment of a series of medical education grants aimed at supporting professionalism research has helped raise faculty awareness. This institution's approach and experience to date may provide valuable lessons for educators and leaders aiming to assess and improve their learning environments.

Involvement of the vitamin D receptor in the regulation of NF-kappaB activity in fibroblasts

We have used mouse embryonic fibroblasts (MEFs) derived from VDR(+/-) and VDR(-/-) mice to determine whether the nuclear vitamin D receptor (VDR) is directly involved in the regulation of NF-kappaB activation. We found that the basal IkappaBalpha protein level was markedly decreased in VDR(-/-) MEFs compared to VDR(+/-) MEFs; however, degradation of IkappaBalpha and its phosphorylation were not altered in VDR(-/-) cells, neither were the levels of IKKalpha and IKKbeta proteins. Consistently, p65 nuclear translocation was increased in unstimulated VDR(-/-) cells. The physical interaction between VDR and p65 was absent in VDR(-/-) MEFs, which may free p65 and increase its activity. Consequently, these alterations combined led to a marked increase in NF-kappaB transcriptional activity. Consistently, induction of IL-6 by TNFalpha or IL-1beta was much more robust in VDR(-/-) than in VDR(+/-) cells, indicating that VDR(-/-) cells are more susceptible to inflammatory stimulation. Therefore, fibroblasts lacking VDR appear to be more pro-inflammatory due to the intrinsic high NF-kappaB activity. The reduction of IkappaBalpha in VDR(-/-) MEFs may be partially explained by the lack of VDR-mediated stabilization of IkappaBalpha by 1,25(OH)(2)D(3). These data suggest that VDR plays an inhibitory role in the regulation of NF-kappaB activation.

Flagellin-induced tolerance of the Toll-like receptor 5 signaling pathway in polarized intestinal epithelial cells

Salmonella typhimurium is a gram-negative enteric pathogen that invades the mucosal epithelium and is associated with diarrheal illness in humans. Flagellin from S. typhimurium and other gram-negative bacteria has been shown to be the predominant proinflammatory mediator through activation of the basolateral Toll-like receptor 5 (TLR5). Recent evidence has shown that prior exposure can render immune cells tolerant to subsequent challenges by TLR ligands. Accordingly, we examined whether prior exposure to purified flagellin would render human intestinal epithelial cells insensitive to future contact. We found that flagellin-induced tolerance is common to polarized epithelial cells and prevents further activation of proinflammatory signaling cascades by both purified flagellin and Salmonella bacteria but does not affect TNF-alpha stimulation of the same pathways. Flagellin tolerance is a rapid process that does not require protein synthesis, and that occurs within 1 to 2 h of flagellin exposure. Prolonged flagellin exposure blocks activation of the NF-kappaB, MAPK, and phosphoinositol 3-kinase signaling pathways and results in the internalization of a fraction of the basolateral TLR5 without affecting the polarity or total expression of TLR5. After removal of flagellin, cells require more than 24 h to fully recover their ability to mount a normal proinflammatory response. We have found that activation of phosphoinositol 3-kinase and Akt by flagellin has a small damping effect in the early stages of flagellin signaling but is not responsible for tolerance. Our study indicates that inhibition of TLR5-associated IL-1 receptor-associated kinase-4 activity occurs during the development of flagellin tolerance and is likely to be the cause of tolerance.

Increased NF-kappaB activity in fibroblasts lacking the vitamin D receptor

1,25-Dihydroxyvitamin D [1,25(OH)2D3] is known to have anti-inflammatory activity; however, the molecular mechanism remains poorly defined. Here we show that the nuclear vitamin D receptor (VDR) is directly involved in the regulation of NF-kappaB activation, a pathway essential for inflammatory response. In mouse embryonic fibroblasts (MEFs) derived from VDR-/- mice, the basal level of kappaB inhibitor (IkappaB) alpha protein was markedly decreased compared with VDR+/- MEFs; however, degradation of IkappaBalpha and its phosphorylation in response to TNF-alpha treatment or Salmonella infection were not altered in VDR-/- cells, neither were the levels of IkappaB kinase-alpha and IkappaB kinase-beta proteins. Consistent with IkappaBalpha reduction, p65 accumulation in the nucleus was markedly increased in unstimulated VDR-/- cells. In addition, the physical interaction between VDR and p65 was absent in VDR-/- MEFs, which may free p65 and increase its activity. Consequently, these alterations combined led to a marked increase in nuclear p65 DNA binding and NF-kappaB transcriptional activity; consistently, induction of IL-6 by TNF-alpha or IL-1beta was much more robust in VDR-/- than in VDR+/- cells, indicating that VDR-/- cells are more susceptible to inflammatory stimulation. Therefore, cells lacking VDR appear to be more proinflammatory due to the intrinsic high NF-kappaB activity. The reduction of IkappaBalpha in VDR-/- MEFs may be partially explained by the lack of VDR-mediated stabilization of IkappaBalpha by 1,25(OH)2D3. This is supported by the observation that IkappaBalpha degradation induced by TNF-alpha was inhibited by 1,25(OH)2D3 in VDR+/- cells, but not in VDR-/- cells. Taken together, these data suggest that VDR plays an inhibitory role in the regulation of NF-kappaB activation.

Acute induction of human IL-8 production by intestinal epithelium triggers neutrophil infiltration without mucosal injury

AIM

Neutrophil migration in the intestine depends on chemotaxis of neutrophils to CXC chemokines produced by epithelial cells. The goal of this project was to determine if acute induction of a CXC chemokine gradient originating from intestinal epithelial cells is sufficient to induce neutrophil influx into intact intestinal tissue.

METHODS AND RESULTS

The authors developed a double transgenic mouse model with doxycycline induced human IL-8 expression restricted to intestinal epithelial cells. Doxycycline treatment of double transgenic mice for three days resulted in a 50-fold increase in the caecal IL-8 concentration and influx of neutrophils into the lamina propria. Although neutrophils entered the paracellular space between epithelial cells, complete transepithelial migration was not observed. Doxycycline treatment also increased the water content of the caecal and colonic stool, indicating dysfunctional water transport. However, the transmural electrical resistance was not decreased. Neutrophils recruited to the intestinal epithelium did not show evidence of degranulation and the epithelium remained intact as judged by histology.

CONCLUSIONS

This conditional transgenic model of chemokine expression provides evidence that acute induction of IL-8 in the intestinal epithelium is sufficient to trigger neutrophil recruitment to the lamina propria, but additional activation signals are needed for full activation and degranulation of neutrophils, mucosal injury, and complete transepithelial migration.

Crosstalk between NF-kappaB and beta-catenin pathways in bacterial-colonized intestinal epithelial cells

Salmonella-epithelial cell interactions are known to activate the proinflammatory NF-kappaB signaling pathway and have recently been found to also influence the beta-catenin signaling pathway, an important regulator of epithelial cell proliferation and differentiation. Here, using polarized epithelial cell models, we demonstrate that these same bacteria-mediated effects also direct the molecular crosstalk between the NF-kappaB and beta-catenin signaling pathways. Convergence of these two pathways is a result of the direct interaction between the NF-kappaB p50 subunit and beta-catenin. We show that PhoP(c), the avirulent derivative of a wild-type Salmonella strain, attenuates NF-kappaB activity by stabilizing the association of beta-catenin with NF-kappaB. In cell lines expressing constitutively active beta-catenin, IkappaBalpha protein was indirectly stabilized and NF-kappaB activity was repressed after wild-type Salmonella colonization. Accordingly, constitutively active beta-catenin was found to inhibit the secretion of IL-8. Thus our findings strongly suggest that the crosstalk between the beta-catenin and NF-kappaB signaling pathways is an important regulator of intestinal inflammation.

Bacterial activation of beta-catenin signaling in human epithelia

The mucosal lining of the human intestine is constantly bathed in a milieu of commensal gut flora, the vast majority of these being nonpathogenic microorganisms. Here, we demonstrate that microbial-epithelial cell interactions not only affect proinflammatory pathways but also influence beta-catenin signaling, a key component in regulating epithelial cell proliferation. The nonpathogenic Salmonella strain PhoP(c) activates the beta-catenin signaling pathway of human epithelia via a blockade of beta-catenin degradation. Normal beta-catenin ubiquitination necessary for constitutive beta-catenin degradation is abolished, allowing the accumulation and translocation of beta-catenin to the nucleus. Transcriptional activation mediated by the beta-catenin/T cell factor complex increases c-myc expression and enhances cell proliferation. We also show that the Salmonella effector protein AvrA is involved in modulating this beta-catenin activation. These data suggest that nonvirulent bacterial-epithelial interactions can influence beta-catenin signaling and cell growth control in a manner previously unsuspected.

Identification of hepoxilin A3 in inflammatory events: a required role in neutrophil migration across intestinal epithelia

The mechanism by which neutrophils [polymorphonuclear leukocyte (PMNs)] are stimulated to move across epithelial barriers at mucosal surfaces has been basically unknown in biology. IL-8 has been shown to stimulate PMNs to leave the bloodstream at a local site of mucosal inflammation, but the chemical gradient used by PMNs to move between adjacent epithelial cells and traverse the tight junction at the apical neck of these mucosal barriers has eluded identification. Our studies not only identify this factor, previously termed pathogen-elicited epithelial chemoattractant, as the eicosanoid hepoxilin A(3) (hepA(3)) but also demonstrate that it is a key factor promoting the final step in PMN recruitment to sites of mucosal inflammation. We show that hepA(3) is synthesized by epithelial cells and secreted from their apical surface in response to conditions that stimulate inflammatory events. Our data further establish that hepA(3) acts to draw PMNs, via the establishment of a gradient across the epithelial tight junction complex. The functional significance of hepA(3) to target PMNs to the lumen of the gut at sites of inflammation was demonstrated by the finding that disruption of the 12-lipoxygenase pathway (required for hepA(3) production) could dramatically reduce PMN-mediated tissue trauma, demonstrating that hepA(3) is a key regulator of mucosal inflammation.

RhoA, Rac1, and Cdc42 exert distinct effects on epithelial barrier via selective structural and biochemical modulation of junctional proteins and F-actin

Epithelial intercellular junctions regulate cell-cell contact and mucosal barrier function. Both tight junctions (TJs) and adherens junctions (AJs) are regulated in part by their affiliation with the F-actin cytoskeleton. The cytoskeleton in turn is influenced by Rho family small GTPases such as RhoA, Rac1, and Cdc42, all of which constitute eukaryotic targets for several pathogenic organisms. With a tetracycline-repressible system to achieve regulated expression in Madin-Darby canine kidney (MDCK) epithelial cells, we used dominant-negative (DN) and constitutively active (CA) forms of RhoA, Rac1, and Cdc42 as tools to evaluate the precise contribution of each GTPase to epithelial structure and barrier function. All mutant GTPases induced time-dependent disruptions in epithelial gate function and distinct morphological alterations in apical and basal F-actin pools. TJ proteins occludin, ZO-1, claudin-1, claudin-2, and junctional adhesion molecule (JAM)-1 were dramatically redistributed in the presence of CA RhoA or CA Cdc42, whereas only claudins-1 and -2 were redistributed in response to CA Rac1. DN Rac1 expression also induced selective redistribution of claudins-1 and -2 in addition to JAM-1, whereas DN Cdc42 influenced only claudin-2 and DN RhoA had no effect. AJ protein localization was unaffected by any mutant GTPase, but DN Rac1 induced a reduction in E-cadherin detergent solubility. All CA GTPases increased the detergent solubility of claudins-1 and -2, but CA RhoA alone reduced claudin-2 and ZO-1 partitioning to detergent-insoluble membrane rafts. We conclude that Rho family GTPases regulate epithelial intercellular junctions via distinct morphological and biochemical mechanisms and that perturbations in barrier function reflect any imbalance in active/resting GTPase levels rather than simply loss or gain of GTPase activity.

Proinflammatory cytokines disrupt epithelial barrier function by apoptosis-independent mechanisms

It is well known that inflammatory conditions of the intestinal mucosa result in compromised barrier function. Inflammation is characterized by an influx into the mucosa of immune cells that influence epithelial function by releasing proinflammatory cytokines such as IFN-gamma and TNF-alpha. Mucosal barrier function is regulated by the epithelial apical junctional complex (AJC) consisting of the tight junction and the adherens junction. Since the AJC regulates barrier function, we analyzed the influence of IFN-gamma and TNF-alpha on its structure/function and determined the contribution of apoptosis to this process using a model intestinal epithelial cell line, T84, and IFN-gamma and TNF-alpha. AJC structure/function was analyzed by confocal microscopy, biochemical analysis, and physiologic measurement of epithelial gate/fence function. Apoptosis was monitored by determining cytokeratin 18 cleavage and caspase-3 activation. IFN-gamma induced time-dependent disruptions in epithelial gate function that were potentiated by coincubation with TNF-alpha. Tight junction fence function was somewhat disrupted. Cytokine treatment was associated with internalization of AJC transmembrane proteins, junction adhesion molecule 1, occludin, and claudin-1/4 with minimal effects on the cytoplasmic plaque protein zonula occludens 1. Detergent solubility profiles of junction adhesion molecule 1 and E-cadherin and their affiliation with "raft-like" membrane microdomains were modified by these cytokines. Inhibition of cytokine-induced apoptosis did not block induced permeability defects; further emphasizing their primary influence on the epithelial AJC structure and barrier function. Our findings for the first time clearly separate the proapoptotic effects of IFN-gamma and TNF-alpha from their abilities to disrupt barrier function.

Paneth cell cryptdins act in vitro as apical paracrine regulators of the innate inflammatory response

Intestinal-specific antimicrobial alpha-defensins, termed cryptdins, are secreted into the intestinal lumen by mouse Paneth cells in response to microbial pathogens. Cryptdins kill microbes by forming pores in their limiting membranes. The cryptdin isoforms 2 and 3 also can form anion-conductive pores in eukaryotic cell membranes, thus affecting cell physiology. Here, we find that when applied to apical membranes of the human intestinal cell line T84, cryptdin 3 (Cr3) induces secretion of the proinflammatory cytokine interleukin 8 (IL-8) in a dose-dependent manner. The induction of IL-8 secretion is specific to the cryptdins that form channels in mammalian cell membranes because cryptdin 4, which does not form pores in T84 cells, does not induce IL-8 secretion. Cr3 induces inflammatory cytokine secretion by activating NF-kappaB and p38 mitogen-activated protein kinase in a Ca2+-dependent manner, but influx by extra-cellular Ca2+ is not involved. Unlike other known inflammatory agonists, signal transduction by Cr3 occurs slowly, suggesting a novel mechanism of action. These results show that selective cryptdins may amplify their roles in innate immunity by acting as novel paracrine agonists to coordinate an inflammatory response with the antimicrobial secretions of Paneth cells.

Flagellin is the major proinflammatory determinant of enteropathogenic Salmonella

The gastroenteritis-causing pathogen Salmonella typhimurium induces profound transcriptional changes in intestinal epithelia resulting in the recruitment of neutrophils whose presence is the histopathologic hallmark of salmonellosis. Here we used cDNA microarray expression profiling to define the molecular determinants that mediate such changes in model intestinal epithelia. Enteropathogenic Salmonella induced a classical proinflammatory gene expression program similar to that activated by the canonical proinflammatory agonist TNF-alpha. Nonproinflammatory bacteria, both commensals (Escherichia coli) and systemic pathogens (S. typhi), did not activate this expression profile. While S. typhimurium strains lacking the SPI-1-encoded type III system were fully proinflammatory, strains lacking the genes for the flagellar structural component flagellin were nearly devoid of proinflammatory signaling. Lastly, the epithelial proinflammatory response could be largely recapitulated by basolateral addition of purified flagellin. Thus, S. typhimurium flagellin is the major molecular trigger by which this pathogen activates gut epithelial proinflammatory gene expression.

Mechanisms of active intestinal inflammation and potential down-regulation via lipoxins

Chronic inflammatory diseases of the intestine (i.e. Crohn's and chronic ulcerative colitis- collectively known as inflammatory bowel disease [IBD]) are a very significant public health problem in the United States and other industrialized nations. Thus, effort has been made toward understanding the biological mechanisms that regulate such inflammation. Largely, these efforts have focused on identifying the mechanisms that mediate activation of inflammation and have succeeded in identifying a variety of signaling pathways by which a wide range of agonists can activate a mucosal immune inflammatory response. Playing a central role in many of these pathways is the intestinal epithelium, which serves as a barrier to, and interfaces with the outside world. However, recent studies have shown that not only can some agonists activate pro-inflammatory signals in intestinal epithelial cells, but other agonists can activate "anti-inflammatory" signals in these cells that dampen the responses to pro-inflammatory agonists. One such anti-inflammatory agonist is the eicosanoid lipoxin A4 (LXA4). Specifically, LXA4, its epimer 15-LXA4, and their analogs potently down-regulate defining and causative events of intestinal inflammation in an in vitro model. These compounds are now being tested for their ability to down-regulate inflammation in mouse models of colitis and may ultimately prove to be of significant benefit to individuals suffering from debilitating chronic inflammatory intestinal disorders.

Lateral membrane LXA4 receptors mediate LXA4's anti-inflammatory actions on intestinal epithelium

Lipoxin A(4) (LXA(4)) and its stable analogs downregulate chemokine secretion in polarized epithelia. This anti-inflammatory effect has been suggested to be mediated by the LXA(4) receptor (LXA(4)R), a G protein-coupled receptor. To determine whether LXA(4)R is expressed on the apical, basolateral, or both poles of intestinal epithelia, an NH(2)-terminal c-myc epitope tag was added to the human LXA(4)R cDNA and recombinant retroviruses were used to transduce polarized epithelial cells. In polarized T84 intestinal epithelial cells, c-myc-LXA(4)R was preferentially expressed on the basolateral surface as indicated by cell surface-selective biotinylation and confocal microscopy. Furthermore, expression of c-myc-LXA(4)R and a truncation mutant lacking the cytoplasmic terminus was primarily confined to the lateral subdomain. We also observed that the expression of myc-LXA(4) conferred enhanced downregulation of IL-8 expression in response to LXA(4) analog and that blockade of the CysLT1 receptor by montelukast did not prevent this response to LXA(4) analog. Thus LXA(4) generated in or near the paracellular space via neutrophil-epithelial interactions can rapidly act on epithelial LXA(4)R to downregulate epithelial promotion of intestinal inflammation.

Cdc42 and Rac1 regulate late events in Salmonella typhimurium-induced interleukin-8 secretion from polarized epithelial cells

Salmonella typhimurium colonization of the intestinal epithelium initiates biochemical cross-talk between pathogen and host that results in the secretion of chemokines, such as interleukin (IL)-8, that direct neutrophil migration to the site of infection. In nonpolarized cells, Rac1 and Cdc42 have been shown to regulate both bacterial invasion and signaling events leading to nuclear responses and IL-8 secretion. However, because the underlying actin cytoskeleton and the associated signaling machinery are distributed much differently in polarized epithelial cells, we used polarized Madin-Darby canine kidney monolayers to investigate the role of Rac1 and Cdc42 in S. typhimurium-induced pro-inflammatory responses in the more physiologically relevant polarized state. In Madin-Darby canine kidney monolayers expressing dominant-negative Rac1 or Cdc42, both Salmonella- and tumor necrosis factor alpha-induced activation of NFkappaB and mitogen-activated protein kinase signaling cascades proceeded normally, but IL-8 secretion was inhibited. We found that Rac1 and Cdc42 were not involved in early pro-inflammatory signaling events, as in nonpolarized cells, but rather regulated the basolateral exocytosis and secretion of IL-8. In contrast, dominant-negative Rac1 inhibited apical actin pedestal formation, indicating that pedestal formation and nuclear signaling for pro-inflammatory activation are not linked. These findings indicate that there are significant differences in the requirements of pathogen-induced host cell signaling pathways in polarized and nonpolarized cells.

Intestinal epithelial pathobiology: past, present and future

The intestinal epithelium serves as one of man's primary interfaces with the outside world. Its importance is illustrated by the fact that the proper functioning of this interface is absolutely essential for human health, and even modest perturbations in its function may lead to diarrhoea, constipation, malnutrition, dehydration, infectious disease or chronic intestinal inflammatory diseases such as Crohn's disease and ulcerative colitis, collectively referred to as inflammatory bowel disease. Both pathogen-induced intestinal inflammation and the active flares of inflammatory bowel disease are histopathologically defined, their sequellae being mediated by neutrophils that migrate across the intestinal epithelium, forming a crypt abscess. Classically, the intestinal epithelium has been thought of primarily as a barrier, and indeed this is a very important aspect of its function, but the intestinal epithelium is also a highly interactive barrier. This chapter will summarize some of the basic research conducted over the past 15 years that has revealed basic insights into how the epithelium participates in the formation of a crypt abscess and how it plays a role in causing the characteristic clinical manifestations that ensue. In addition, the chapter will discuss how this research has resurrected the 'old', yet newly emerging, concept that physiological malfunction of the intestinal epithelium can be the primary defect that leads to the innate and adaptive immune dysregulation mediating inflammatory bowel disease.

Cutting edge: Salmonella AvrA effector inhibits the key proinflammatory, anti-apoptotic NF-kappa B pathway

Secreted prokaryotic effector proteins have evolved to modulate the cellular functions of specific eukaryotic hosts. Generally, these proteins are considered virulence factors that facilitate parasitism. However, in certain plant and insect eukaryotic/prokaryotic relationships, effector proteins are involved in the establishment of commensal or symbiotic interactions. In this study, we report that the AvrA protein from Salmonella typhimurium, a common enteropathogen of humans, is an effector molecule that inhibits activation of the key proinflammatory NF-kappaB transcription factor and augments apoptosis in human epithelial cells. This activity is similar but mechanistically distinct from that described for YopJ, an AvrA homolog expressed by the bacterial pathogen Yersinia. We suggest that AvrA may limit virulence in vertebrates in a manner analogous to avirulence factors in plants, and as such, is the first bacterial effector from a mammalian pathogen that has been ascribed such a function.

The adenosine 2b receptor is recruited to the plasma membrane and associates with E3KARP and Ezrin upon agonist stimulation

We have previously shown that adenosine is formed in the intestinal lumen during active inflammation from neutrophil-derived 5'-AMP. Acting through the adenosine A2b receptor (A2bR), the luminally derived adenosine induces vectorial chloride secretion and a polarized secretion of interleukin-6 to the intestinal lumen. Although some G protein-coupled receptors interact with anchoring or signaling molecules, not much is known in this critical area for the A2bR. We used the model intestinal epithelial cell line, T84, and Caco2-BBE cells stably transfected with GFP-A2b receptor to study the intestinal A2bR. The A2bR is present in both the apical and basolateral membranes of intestinal epithelia. Apical or basolateral stimulation of the A2bR induces recruitment of the receptor to the plasma membrane and caveolar fractions. The A2bR co-immunoprecipitates with E3KARP and ezrin upon agonist stimulation. Ezrin interacts with E3KARP and PKA and the interaction between ezrin and E3KARP is enhanced by agonist stimulation. Our data suggest that the A2bR is recruited to the plasma membrane upon apical or basolateral agonist stimulation and interacts with E3KARP and ezrin. We speculate that such an interaction may not only anchor the A2bR to the plasma membrane but may also function to stabilize the receptor in a signaling complex in the plasma membrane.

Lipoxin a4 analogs attenuate induction of intestinal epithelial proinflammatory gene expression and reduce the severity of dextran sodium sulfate-induced colitis

The anti-inflammatory eicosanoid lipoxin A(4) (LXA(4)), aspirin-triggered 15-epi-LXA(4), and their stable analogs down-regulate IL-8 secretion and subsequent recruitment of neutrophils by intestinal epithelia. In an effort to elucidate the mechanism by which these lipid mediators modulate cellular proinflammatory programs, we surveyed global epithelial gene expression using cDNA microarrays. LXA(4) analog alone did not significantly affect expression of any of the >7000 genes analyzed. However, LXA(4) analog pretreatment attenuated induction of approximately 50% of the 125 genes up-regulated in response to the gastroenteritis-causing pathogen Salmonella typhimurium. A major subset of genes whose induction was reduced by LXA(4) analog pretreatment is regulated by NF-kappaB, suggesting that LXA(4) analog was influencing the activity of this transcription factor. Nanomolar concentrations of LXA(4) analog reduced NF-kappaB-mediated transcriptional activation in a LXA(4) receptor-dependent manner and inhibited induced degradation of IkappaBalpha. LXA(4) analog did not affect earlier stimulus-induced signaling events that lead to IkappaBalpha degradation, such as S. typhimurium-induced epithelial Ca(2+) mobilization or TNF-alpha-induced phosphorylation of IkappaBalpha. To establish the in vivo relevance of these findings, we examined whether LXA(4) analogs could affect intestinal inflammation in vivo using the mouse model of DSS-induced inflammatory colitis. Oral administration of LXA(4) analog (15-epi-16-para-fluoro-phenoxy-LXA(4), 10 microg/day) significantly reduced the weight loss, hematochezia, and mortality that characterize DSS colitis. Thus, LXA(4) analog-mediated down-regulation of proinflammatory gene expression via inhibition of the NF-kappaB pathway can be therapeutic for diseases characterized by mucosal inflammation.

Cryptdin 3 forms anion selective channels in cytoplasmic membranes of human embryonic kidney cells

Cryptdins are antimicrobial peptides secreted by Paneth cells located at the base of intestinal crypts. In addition to their antimicrobial function, cryptdins may also regulate salt and water secretion by intestinal epithelial cells. Recent work with short-circuit current measurements indicated that at least one cryptdin peptide, cryptdin 3, induces apical conductance(s) in Cl(-) secretory, including cystic fibrosis, epithelia. In the present study, we characterized the cryptdin 3-induced anion channel activity in human embryonic kidney (HEK) cells with single-channel patch-clamp techniques. The patch pipette was filled with solution containing different concentrations of cryptdin 3, and, after gigaseal formation, the channel activity was recorded with either cell-attached or inside-out patch modes. We found an anion selective channel with a conductance of 15 pS and open probability of 0.19, regardless of cryptdin 3 concentration. The mean open and closed times varied with the cryptdin 3 concentration. For cryptdin 3 concentrations of 10, 4, 1, and 0.5 microg/ml in the pipette, the corresponding mean open times were 1.2, 7.0, 9.0, and 17.4 ms and the corresponding mean closed times were 1.1, 1.6, 4.2, and 12.5 ms. These results suggest that cryptdin 3 forms anion-selective channels on the cytoplasmic membrane of HEK cells and that the kinetics of one such channel are affected by its interaction with other such channels.

The Salmonella typhimurium flagellar basal body protein FliE is required for flagellin production and to induce a proinflammatory response in epithelial cells

During apical colonization by Salmonella typhimurium, intestinal epithelial cells orchestrate a proinflammatory response that involves secretion of chemoattractants, predominantly interleukin-8, which coordinate neutrophil trans-epithelial migration at the site of infection. This host-pathogen interaction requires several S. typhimurium genes. To identify novel genes that participate in this pathogen-induced proinflammatory response, we created S. typhimurium Tn-10 transposon mutants and identified a single mutant with Tn-10 insertional inactivation within the fliE flagellar locus that was able to adhere to and invade intestinal epithelial cells normally but was unable to induce interleukin-8 secretion in host cells. The fliE-deficient mutant failed to secrete flagellin and lacked any surface assembly of flagellae. Unlike wild-type S. typhimurium, the fliE-deficient mutant did not activate the IkappaBalpha/NF-kappaB signaling pathway or induce the coordinated trans-epithelial migration of isolated human neutrophils. Transcomplementation of the fliE-deficient mutant with a wild-type fliE-harboring plasmid restored all defects and produced a wild-type S. typhimurium phenotype. Furthermore, functional down-regulation of basolateral TLR5 completely inhibited the monolayers' ability to respond to both wild-type S. typhimurium and purified flagellin but had no affect on tumor necrosis factor alpha-induced responses. We therefore conclude that S. typhimurium fliE is essential for flagellin secretion, flagellar assembly, and S. typhimurium-induced proinflammatory responses through basolateral TLR5 and is consistent with the emerging model of S. typhimurium flagellin-induced inflammation.

The role of CD47 in neutrophil transmigration. Increased rate of migration correlates with increased cell surface expression of CD47

CD47, a cell surface glycoprotein, plays an important role in modulating neutrophil (PMN) migration across endothelial and epithelial monolayers. Here we show that anti-CD47 monoclonal antibodies (mAbs) delay PMN migration across collagen-coated filters or T84 epithelial monolayers toward the chemoattractant formylmethionylleucylphenylalanine (fMLP). Despite delayed transmigration by anti-CD47 mAbs, the numbers of PMN migrating across in either condition were the same as in the presence of control non-inhibitory mAbs. Cell surface labeling and immunoprecipitation demonstrated upregulation of CD47 to the PMN cell surface with kinetics similar to those of the transmigration response. Subcellular fractionation studies revealed redistribution of CD47 from intracellular compartments that co-sediment with secondary granules to plasma membrane-containing fractions after fMLP stimulation. Experiments performed to investigate potential signaling pathways revealed that inhibition of tyrosine phosphorylation with genistein reversed the anti-CD47-mediated PMN migration delay, whereas inhibition of phosphatidylinositol 3-kinase only partially reversed anti-CD47 effects that correlated with a rapid increase in PMN cell surface CD47. Analysis of the contribution of epithelial-expressed CD47 to PMN transmigration revealed that PMN migration across CD47-deficient epithelial monolayers (CaCO2) was significantly increased after stable transfection with CD47. These results suggest that cell surface CD47 and downstream tyrosine phosphorylation signaling events regulate, in part, the rate of PMN migration during the inflammatory response.

CD98-mediated links between amino acid transport and beta 1 integrin distribution in polarized columnar epithelia

In non-polarized cells, CD98 has been shown to both influence beta(1) integrins and heterodimerize with LAT-2, which confers amino acid transport capability on the LAT-2/CD98 heterodimer. Since LAT-2 is most heavily expressed in intestine and CD98 associates with the beta(1) integrin splice form selectively found in such epithelia, we investigated the relationship and polarity of these proteins using the intestinal epithelial model Caco2-BBE. CD98 was found to selectively coimmunoprecipitate with both LAT-2 and beta(1) integrin, and, logically, all three proteins were polarized to the same (basolateral) domain. Furthermore, expression of CD98 in polarized epithelia lacking human CD98 (MDCK cells) disrupted beta(1) integrin surface distribution and cytoskeletal architecture, suggesting that CD98 can influence integrin function. Expression of a CD98 mutant lacking the specific residues conferring LAT-2 binding similarly affected cells, confirming that the latter effect was not due to LAT-2 sequestration. Use of CD98 truncation mutants suggest that a 10-amino acid domain located at the putative cytoplasmic tail/transmembrane domain interface was necessary and sufficient to induce the phenotype change. We conclude that the CD98/LAT-2 amino acid transporter is polarized to the same domain on which beta(1) integrin resides. CD98 appears to associate with beta(1) integrin and, in doing so, may influence its function as revealed by disruption of the outside-in signaling that confers cytoskeletal organization. Furthermore, such findings suggest a link between classic transport events and a critical element of barrier function: integrin-mediated influences on cytoskeletal organization.

Cutting edge: bacterial flagellin activates basolaterally expressed TLR5 to induce epithelial proinflammatory gene expression

Flagellin, the structural component of bacterial flagella, is secreted by pathogenic and commensal bacteria. Flagellin activates proinflammatory gene expression in intestinal epithelia. However, only flagellin that contacts basolateral epithelial surfaces is proinflammatory; apical flagellin has no effect. Pathogenic Salmonella, but not commensal Escherichia coli, translocate flagellin across epithelia, thus activating epithelial proinflammatory gene expression. Investigating how epithelia detect flagellin revealed that cell surface expression of Toll-like receptor 5 (TLR5) conferred NF-kappaB gene expression in response to flagellin. The response depended on both extracellular leucine-rich repeats and intracellular Toll/IL-1R homology region of TLR5 as well as the adaptor protein MyD88. Furthermore, immunolocalization and cell surface-selective biotinylation revealed that TLR5 is expressed exclusively on the basolateral surface of intestinal epithelia, thus providing a molecular basis for the polarity of this innate immune response. Thus, detection of flagellin by basolateral TLR5 mediates epithelial-driven inflammatory responses to Salmonella.

Colonic epithelial hPepT1 expression occurs in inflammatory bowel disease: transport of bacterial peptides influences expression of MHC class 1 molecules

BACKGROUND & AIMS

hPepT1 is an intestinal epithelial apical membrane transporter responsible for uptake of di/tripeptides (including bacterial derived proinflammatory n-formyl peptides). hPepT1 expression normally has a strict axial gradient-highest in the proximal small intestine with no expression in the colon.

METHODS

Small intestinal-like cells (Caco2-BBE), and colonic-like cells (HT29-Cl.19A), and colonic mucosa from diseased and control patients were used in the present study.

RESULTS

hPepT1 expression occurs aberrantly in the colon with chronic ulcerative colitis (6 patients) and Crohn's disease (4 patients), but not in normal colon (4 patients) or colon with microscopic colitis (4 patients). To model expression of hPepT1 by colonic-like cells in inflamed states, we stably transfected HT29-Cl.19A cells with a modified hPepT1 tagged on the N-terminus with green fluorescence protein. Analysis of transfected cells revealed that: GFP-hPepT1 protein, like the natural protein, is targeted to the apical plasma membrane. In addition, the tagged protein retains the capability of di/tripeptide absorption, and the expression of the tagged protein by HT29-Cl.19A cells permits absorption of N-formyl-methionyl-leucyl-phenylalanine (fMLP), as occurs in hPepT1 expressing Caco2-BBE cells. fMLP uptake by colonic cells expressing GFP-hPepT1 specifically enhances major histocompatibility complex class I surface expression.

CONCLUSIONS

These data collectively indicate that, in some states of chronic inflammation, hPepT1 may be anomolously expressed in the colon. Further, transport of fMLP by hPepT1 potentially stimulates expression of key accessory immune molecule, MHC-1.

Neutrophil-epithelial crosstalk at the intestinal lumenal surface mediated by reciprocal secretion of adenosine and IL-6

Adenosine is formed in the intestinal lumen during active inflammation from neutrophil-derived 5' AMP. Using intestinal epithelial cell line T84, we studied the effect of adenosine on the secretion of IL-6, a proinflammatory cytokine involved in neutrophil degranulation and lymphocyte differentiation. Stimulation of T84 monolayers with either apical or basolateral adenosine induces A2b receptor-mediated increase in IL-6 secretion, which is polarized to the apical (luminal) compartment. In addition, Salmonella typhimurium, TNF-alpha, and forskolin, known inducers of IL-6 secretion in intestinal epithelial cells, also stimulate IL-6 secretion into the apical compartment. We show that IL6 promoter induction by adenosine occurs through cAMP-mediated activation of nuclear cAMP-responsive element-binding protein (CREB). We also show that IL-6 released in the luminal (apical) compartment achieves a sufficient concentration to activate neutrophils (from which the adenosine signal originates), since such IL-6 is found to induce an intracellular [Ca(++)] flux in neutrophils. We conclude that adenosine released in the intestinal lumen during active inflammation may induce IL-6 secretion, which is mediated by cAMP/CREB activation and occurs in an apically polarized fashion. This would allow sequential activation of neutrophil degranulation in the lumen -- a flow of events that would, in an epithelium-dependent fashion, enhance microbicidal activity of neutrophils as they arrive in the intestinal lumen.

Neutrophil transepithelial migration: regulation at the apical epithelial surface by Fc-mediated events

Neutrophil (PMN) transepithelial migration is a major effector of epithelial defense in inflammatory diseases involving mucosal surfaces. However, major receptor-ligand interactions between epithelial cells and PMN remain incompletely characterized. To better define the molecular events involved in PMN interactions with epithelial cells, we produced a monoclonal antibody called g82 that inhibited PMN transepithelial migration in the physiological basolateral-to-apical direction. The g82 antigen localized to the apical surface of human colonic epithelium and was significantly upregulated under inflammatory conditions. Immunoprecipitation revealed two polypeptides of M(r) 207 and 32 kDa. F(ab')(2) fragments from g82 IgG had no effect on transmigration, suggesting Fc dependence. Further experiments confirmed dependence on the PMN Fc receptor CD32A and that the observed effects were secondary to a failure of PMN to detach from the apical epithelial surface. These Fc-mediated events were epitope specific since binding, isotype-matched antibodies did not affect detachment. These results identify a new mechanism for retention of PMN at the apical epithelial surface following transepithelial migration. This pathway may be important in pathogen clearance and mucosal pathophysiology associated with autoimmunity.

Clostridium difficile toxins disrupt epithelial barrier function by altering membrane microdomain localization of tight junction proteins

The anaerobic bacterium Clostridium difficile is the etiologic agent of pseudomembranous colitis. C. difficile toxins TcdA and TcdB are UDP-glucosyltransferases that monoglucosylate and thereby inactivate the Rho family of GTPases (W. P. Ciesla, Jr., and D. A. Bobak, J. Biol. Chem. 273:16021-16026, 1998). We utilized purified reference toxins of C. difficile, TcdA-10463 (TcdA) and TcdB-10463 (TcdB), and a model intestinal epithelial cell line to characterize their influence on tight-junction (TJ) organization and hence to analyze the mechanisms by which they contribute to the enhanced paracellular permeability and disease pathophysiology of pseudomembranous colitis. The increase in paracellular permeability induced by TcdA and TcdB was associated with disorganization of apical and basal F-actin. F-actin restructuring was paralleled by dissociation of occludin, ZO-1, and ZO-2 from the lateral TJ membrane without influencing the subjacent adherens junction protein, E-cadherin. In addition, we observed decreased association of actin with the TJ cytoplasmic plaque protein ZO-1. Differential detergent extraction and fractionation in sucrose density gradients revealed TcdB-induced redistribution of occludin and ZO-1 from detergent-insoluble fractions constituting "raft-like" membrane microdomains, suggesting an important role of Rho proteins in maintaining the association of TJ proteins with such microdomains. These toxin-mediated effects on actin and TJ structure provide a mechanism for early events in the pathophysiology of pseudomembranous colitis.

Negative regulation of epithelium-neutrophil interactions via activation of CD44

Polymorphonuclear neutrophil (PMN) migration across epithelia is a common feature of active inflammation. Given the suggested role of carbohydrates in this process, we examined the receptor CD44. The standard CD44 isoform was expressed at the cell surface of PMN. PMN migration across model polarized intestinal epithelia was reduced (by 60%) if the CD44 receptor was activated by either a specific antibody (clone IM7) or the natural soluble ligand, hyaluronic acid. This inhibitory effect following receptor activation occurred with both basolateral-to-apical- and apical-to-basolateral-directed migration. The anti-CD44 antibody similarly reduced PMN migration through filters in the absence of epithelia, while preincubation of the antibody with the epithelium did not alter subsequent PMN transepithelial migration. These data suggest that PMN, rather than epithelial, CD44 is responsible for these effects. A similar inhibitory effect of anti-CD44 antibody was also observed on migration of intraepithelial lymphocytes. The molecular mechanism involved in such negative signaling following CD44 activation may include modulation of outside-in cell signaling. While neither the anti-CD44 antibody nor CD44 ligand affected PMN mobilization of intracellular Ca(2+), both led to increased adenylate cyclase activity, an inhibitory signal for PMN migration. Together, these results suggest that CD44 of PMN may potentially serve as a negative regulator of leukocyte migration across biological surfaces such as columnar epithelia.

Cryptdin-3 induces novel apical conductance(s) in Cl- secretory, including cystic fibrosis, epithelia

Opening of anion-conductive pathways in apical membranes of secretory cells lining mucosal surfaces is a critical step in salt and water secretion and, thus, hydration of sites including airway and intestine. In intestine, Paneth cells are positioned at the base of the secretory gland (crypt) and release defensin peptide, in mice termed cryptdins, into the crypt lumen. Because at least some defensins have been shown to form anion-conductive channels in phospholipid bilayers, we tested whether these endogenous antimicrobial peptides could act as soluble inducers of channel-like activity when applied to apical membranes. To directly evaluate the possibility of cryptdin-3-mediated apical anion conductance (G(ap)), we have utilized amphotericin B to selectively permeabilize basolateral membranes of electrically tight monolayers of polarized human intestinal secretory epithelia (T84 cells), thus isolating the apical membrane for study. Cryptdin-3 induces G(ap) that is voltage independent (deltaG(ap) = 1.90 +/- 0.60 mS/cm2) and exhibits ion selectivity contrasting to that elicited by forskolin or thapsigargin (for cryptdin-3, Cl- = gluconate; for forskolin and thapsigargin, Cl- >> gluconate). We cannot exclude the possibility that the macroscopic current induced by cryptdin could be the sum of cation and Cl- currents. Cryptdin-3 induces a current in basolaterally permeabilized epithelial monolayers derived from airway cells harboring the deltaF508 mutation of cystic fibrosis (CF; deltaG(ap) = 0.80 +/- 0.06 mS/cm2), demonstrating that cryptdin-3 restores anion secretion in CF cells; this occurs independently of the CF transmembrane conductance regulator channel. These results support the idea that cryptdin-3 may associate with apical membranes of Cl--secreting epithelia and self-assemble into conducting channels capable of mediating a physiological response.

Characterization of epithelial chemoattractants for human intestinal intraepithelial lymphocytes

BACKGROUND & AIMS

Although homing of intraepithelial lymphocytes (IEL) into intestinal epithelia seems to be guided by signals from epithelia, little is known concerning functional epithelial-derived chemoattractants for IEL.

METHODS

Epithelial chemoattractants for IEL were analyzed using chemotaxis chamber system, enzyme-linked immunosorbent assay, and in situ hybridization using human epithelial lines and IEL lines.

RESULTS

Epithelial-conditioned media induced IEL chemotaxis, and this activity was markedly enhanced by prestimulation of epithelia with interferon-(IFN)-gamma. This chemotaxis (stimulation +) was significantly inhibited by neutralizing antibodies to IFN-gamma inducible protein-10 (IP-10) or to monokine induced by IFN-gamma (MIG). Furthermore, while high amounts of IP-10 and MIG were detected in epithelial-conditioned media after IFN-gamma stimulation, equivalent concentrations of recombinant IP-10 and MIG reproduced IEL chemotaxis. Production of IP-10 and MIG in fresh epithelial cells was supported by in situ hybridization and enzyme-linked immunosorbent assay. Lastly, fresh human IEL constitutively expressed CXCR-3 (the common receptor for IP-10 and MIG), and fresh IEL also exhibited chemotaxis to by rIP-10, rMIG, and epithelial-conditioned media.

CONCLUSIONS

Epithelial cells produce chemoattractants for IEL, and such chemokine production is regulated by proinflammatory cytokines such as IFN-gamma. IP-10 and MIG may serve as potentially important epithelial chemokines for IEL, especially under inflammatory conditions.

Salmonella typhimurium translocates flagellin across intestinal epithelia, inducing a proinflammatory response

This study investigated whether soluble paracrine factors mediated Salmonella-induced IL-8 expression in polarized model intestinal epithelia. We found that the basolateral media of model epithelia that had been apically infected with Salmonella typhimurium for a short period (10 minutes) could activate IL-8 secretion in virgin model epithelia, demonstrating that a proinflammatory factor (PIF) was indeed present. Initial characterization found that PIF was a heat-stable protein with a molecular mass of about 50 kDa that acts on the basolateral, but not apical, surface of model intestinal epithelia to elicit IL-8 secretion. PIF was not present in the media of model epithelia stimulated with other inducers of IL-8 secretion (TNF-alpha or carbachol) but was present in S. typhimurium supernatants, indicating PIF is of bacterial origin. PIF was purified from bacterial culture supernatants by anion/cation exchange chromatography and SDS-PAGE and found by using microsequencing to be the protein flagellin. In support of this finding, flagellin-deficient S. typhimurium mutants did not secrete detectable levels of PIF (i.e., a bioactivity that induced IL-8 secretion when placed basolaterally on model epithelia). Furthermore, viable flagellin-deficient mutant organisms (fliC/fljB and flhD) failed to elicit IL-8 secretion when added apically to model intestinal epithelia. These findings indicate that translocation of flagellin across epithelia, subsequent to apical epithelial-S. typhimurium interaction, is likely a major means of activating a mucosal inflammatory response.

Transepithelial resistance can be regulated by the intestinal brush-border Na(+)/H(+) exchanger NHE3

Initiation of intestinal Na(+)-glucose cotransport results in transient cell swelling and sustained increases in tight junction permeability. Since Na(+)/H(+) exchange has been implicated in volume regulation after physiological cell swelling, we hypothesized that Na(+)/H(+) exchange might also be required for Na(+)-glucose cotransport-dependent tight junction regulation. In Caco-2 monolayers with active Na(+)-glucose cotransport, inhibition of Na(+)/H(+) exchange with 200 microM 5-(N,N-dimethyl)- amiloride induced 36 +/- 2% increases in transepithelial resistance (TER). Evaluation using multiple Na(+)/H(+) exchange inhibitors showed that inhibition of the Na(+)/H(+) exchanger 3 (NHE3) isoform was most closely related to TER increases. TER increases due to NHE3 inhibition were related to cytoplasmic acidification because cytoplasmic alkalinization with 5 mM NH(4)Cl prevented both cytoplasmic acidification and TER increases. However, NHE3 inhibition did not affect TER when Na(+)-glucose cotransport was inhibited. Myosin II regulatory light chain (MLC) phosphorylation decreased up to 43 +/- 5% after inhibition of Na(+)/H(+) exchange, similar to previous studies that associate decreased MLC phosphorylation with increased TER after inhibition of Na(+)-glucose cotransport. However, NHE3 inhibitors did not diminish Na(+)-glucose cotransport. These data demonstrate that inhibition of NHE3 results in decreased MLC phosphorylation and increased TER and suggest that NHE3 may participate in the signaling pathway of Na(+)-glucose cotransport-dependent tight junction regulation.

Noninvasive in vivo analysis of human small intestinal paracellular absorption: regulation by Na+-glucose cotransport

Activation of intestinal Na+-glucose cotransport increases paracellular movement of inert tracers in cultured monolayers, isolated rodent intestinal mucosae, and in rodents in vivo. However, not all studies have demonstrated comparable effects on human intestinal paracellular absorption. We sought to assess the effects of Na+-glucose cotransport on paracellular absorption in human beings using a simple noninvasive assay. Study subjects drank six 200-ml doses of test solution, composed of 0.8% w/v creatinine (sufficient to overwhelm endogenous creatinine) in 277 mM glucose or mannitol and urine was collected. Intestinal creatinine absorption is paracellular. Once absorbed, creatinine is cleared into the urine. Therefore, urinary creatinine recovery reflects intestinal paracellular creatinine absorption. Total urinary creatinine recovery was 55% +/- 4% of creatinine ingested with glucose and 38% +/- 9% of creatinine ingested with mannitol (p < 0.001). Thus, intestinal paracellular absorption of creatinine is increased by the presence of luminal glucose. Our results are consistent with in vivo human regulation of mucosal permeability by Na+-glucose cotransport.

Molecular physiology and pathophysiology of tight junctions. IV. Regulation of tight junctions by extracellular stimuli: nutrients, cytokines, and immune cells

The epithelial lining of the gastrointestinal tract forms a regulated, selectively permeable barrier between luminal contents and the underlying tissue compartments. Permeability across the epithelium is, in part, determined by the rate-limiting barrier of the paracellular pathway-the most apical intercellular junction referred to as the tight junction (TJ). The TJ is composed of a multiprotein complex that affiliates with the underlying apical actomyosin ring. TJ structure and function, and therefore epithelial permeability, are influenced by diverse physiological and pathological stimuli; here we review examples of such stimuli that are detected at the cell surface. For example, luminal glucose induces an increase in paracellular permeability to small molecules. Similarly, but by other means, cytokines and leukocytes in the vicinity of the epithelium also regulate TJ structure and paracellular permeability by influencing the TJ protein complex and/or its association with the underlying actin cytoskeleton.

Prokaryotic regulation of epithelial responses by inhibition of IkappaB-alpha ubiquitination

Epithelia of the vertebrate intestinal tract characteristically maintain an inflammatory hyporesponsiveness toward the lumenal prokaryotic microflora. We report the identification of enteric organisms (nonvirulent Salmonella strains) whose direct interaction with model human epithelia attenuate synthesis of inflammatory effector molecules elicited by diverse proinflammatory stimuli. This immunosuppressive effect involves inhibition of the inhibitor kappaB/nuclear factor kappaB (IkappaB/NF-kappaB) pathway by blockade of IkappaB-alpha degradation, which prevents subsequent nuclear translocation of active NF-kappaB dimer. Although phosphorylation of IkappaB-alpha occurs, subsequent polyubiquitination necessary for regulated IkappaB-alpha degradation is completely abrogated. These data suggest that prokaryotic determinants could be responsible for the unique tolerance of the gastrointestinal mucosa to proinflammatory stimuli.

Constitutive and regulated secretion of secretory leukocyte proteinase inhibitor by human intestinal epithelial cells

BACKGROUND & AIMS

Epithelial cells participate in immune regulation and mucosal integrity by generating a range of biologically active mediators. In the intestine, little is known about the potential endogenous anti-inflammatory molecules. Secretory leukocyte proteinase inhibitor (SLPI) is a major serine proteinase inhibitor, a potent antibiotic, and thus a potential anti-inflammatory molecule, although it is not known if it is secreted by intestinal epithelial cells.

METHODS

We show, by reverse-transcription polymerase chain reaction, the presence of SLPI messenger RNA in human model intestinal epithelial cell lines (Caco2-BBE, T84, and HT29-Cl.19A) and human jejunum and colon biopsy specimens. The polymerase chain reaction product was cloned and sequenced and is identical to that of SLPI isolated previously from the human parotid gland.

RESULTS

As analyzed by enzyme-linked immunosorbent assay, the constitutive secretion of SLPI occurs in a markedly polarized manner toward the apical surface and is enhanced by inflammatory mediators including tumor necrosis factor alpha and interleukin 1beta (approximately 3.5-fold increase over control value). SLPI release is also stimulated by activation of protein kinase C isoenzymes, but not by activation of adenosine 3',5'-cyclic monophosphate- or Ca(2+)-regulated signaling molecules. SLPI protein is detectable in intestinal lavage fluids collected from normal adult humans. Recombinant SLPI attenuates digestive enzyme (trypsin)- or leukocyte proteinase (elastase)-induced permeability alteration of a model epithelia in a dose-dependent manner. Moreover, SLPI exhibits an antibacterial activity against at least one major intestinal pathogen, Salmonella typhimurium. In contrast, SLPI does not influence epithelial barrier integrity as assessed by transepithelial conductance measurements or electrogenic ion transport.

CONCLUSIONS

These results establish that human intestinal epithelium expresses and apically secretes SLPI, a molecule that may significantly contribute to the protection against attack from inflammatory cells and digestive enzymes, as well as against microbial infection.