TNF-alpha-induced increase in intestinal epithelial tight junction permeability requires NF-kappa B activation

Crohn's disease (CD) patients have an abnormal increase in intestinal epithelial permeability. The defect in intestinal tight junction (TJ) barrier has been proposed as an important etiologic factor of CD. TNF-alpha increases intestinal TJ permeability. Because TNF-alpha levels are markedly increased in CD, TNF-alpha increase in intestinal TJ permeability could be a contributing factor of intestinal permeability defect in CD. Our purpose was to determine some of the intracellular mechanisms involved in TNF-alpha modulation of intestinal epithelial TJ permeability by using an in vitro intestinal epithelial system consisting of filter-grown Caco-2 monolayers. TNF-alpha produced a concentration- and time-dependent increase in Caco-2 TJ permeability. TNF-alpha-induced increase in Caco-2 TJ permeability correlated with Caco-2 NF-kappa B activation. Inhibition of TNF-alpha-induced NF-kappa B activation by selected NF-kappa B inhibitors, curcumin and triptolide, prevented the increase in Caco-2 TJ permeability, indicating that NF-kappa B activation was required for the TNF-alpha-induced increase in Caco-2 TJ permeability. This increase in Caco-2 TJ permeability was accompanied by down-regulation of zonula occludens (ZO)-1 proteins and alteration in junctional localization of ZO-1 proteins. TNF-alpha modulation of ZO-1 protein expression and junctional localization were also prevented by NF-kappa B inhibitors. TNF-alpha did not induce apoptosis in Caco-2 cells, suggesting that apoptosis was not the mechanism involved in TNF-alpha-induced increase in Caco-2 TJ permeability. These results demonstrate for the first time that TNF-alpha-induced increase in Caco-2 TJ permeability was mediated by NF-kappa B activation. The increase in permeability was associated with NF-kappa B-dependent downregulation of ZO-1 protein expression and alteration in junctional localization.

Lipopolysaccharide causes an increase in intestinal tight junction permeability in vitro and in vivo by inducing enterocyte membrane expression and localization of TLR-4 and CD14

Bacterial-derived lipopolysaccharides (LPS) play an essential role in the inflammatory process of inflammatory bowel disease. A defective intestinal tight junction (TJ) barrier is an important pathogenic factor of inflammatory bowel disease and other inflammatory conditions of the gut. Despite its importance in mediating intestinal inflammation, the physiological effects of LPS on the intestinal epithelial barrier remain unclear. The major aims of this study were to determine the effects of physiologically relevant concentrations of LPS (0 to 1 ng/mL) on intestinal barrier function using an in vitro (filter-grown Caco-2 monolayers) and an in vivo (mouse intestinal perfusion) intestinal epithelial model system. LPS, at physiologically relevant concentrations (0 to 1 ng/mL), in the basolateral compartment produced a time-dependent increase in Caco-2 TJ permeability without inducing cell death. Intraperitoneal injection of LPS (0.1 mg/kg), leading to clinically relevant plasma concentrations, also caused a time-dependent increase in intestinal permeability in vivo. The LPS-induced increase in intestinal TJ permeability was mediated by an increase in enterocyte membrane TLR-4 expression and a TLR-4-dependent increase in membrane colocalization of membrane-associated protein CD14. In conclusion, these studies show for the first time that LPS causes an increase in intestinal permeability via an intracellular mechanism involving TLR-4-dependent up-regulation of CD14 membrane expression.

IL-1beta causes an increase in intestinal epithelial tight junction permeability

IL-1beta is a prototypical proinflammatory cytokine that plays a central role in the intestinal inflammation amplification cascade. Recent studies have indicated that a TNF-alpha- and IFN-gamma-induced increase in intestinal epithelial paracellular permeability may be an important mechanism contributing to intestinal inflammation. Despite its central role in promoting intestinal inflammation, the role of IL-1beta on intestinal epithelial tight junction (TJ) barrier function remains unclear. The major aims of this study were to determine the effect of IL-1beta on intestinal epithelial TJ permeability and to elucidate the mechanisms involved in this process, using a well-established in vitro intestinal epithelial model system consisting of filter-grown Caco-2 intestinal epithelial monolayers. IL-1beta (0-100 ng/ml) produced a concentration- and time-dependent decrease in Caco-2 transepithelial resistance. Conversely, IL-1beta caused a progressive time-dependent increase in transepithelial permeability to paracellular marker inulin. IL-1beta-induced increase in Caco-2 TJ permeability was accompanied by a rapid activation of NF-kappaB. NF-kappaB inhibitors, pyrrolidine dithiocarbamate and curcumin, prevented the IL-1beta-induced increase in Caco-2 TJ permeability. To further confirm the role of NF-kappaB in the IL-1beta-induced increase in Caco-2 TJ permeability, NF-kappaB p65 expression was silenced by small interfering RNA transfection. NF-kappaB p65 depletion completely inhibited the IL-1beta-induced increase in Caco-2 TJ permeability. IL-1beta did not induce apoptosis in the Caco-2 cell. In conclusion, our findings show for the first time that IL-1beta at physiologically relevant concentrations causes an increase in intestinal epithelial TJ permeability. The IL-1beta-induced increase in Caco-2 TJ permeability was mediated in part by the activation of NF-kappaB pathways but not apoptosis.

Mechanism of TNF-{alpha} modulation of Caco-2 intestinal epithelial tight junction barrier: role of myosin light-chain kinase protein expression

TNF-alpha plays a central role in the intestinal inflammation of various inflammatory disorders including Crohn's disease (CD). TNF-alpha-induced increase in intestinal epithelial tight junction (TJ) permeability has been proposed as one of the proinflammatory mechanisms contributing to the intestinal inflammation. The intracellular mechanisms involved in the TNF-alpha-induced increase in intestinal TJ permeability remain unclear. The purpose of this study was to investigate the possibility that the TNF-alpha-induced increase in intestinal epithelial TJ permeability was regulated by myosin light-chain kinase (MLCK) protein expression, using an in vitro intestinal epithelial model system consisting of the filter-grown Caco-2 intestinal epithelial monolayers. TNF-alpha (10 ng/ml) produced a time-dependent increase in Caco-2 MLCK expression. The TNF-alpha increase in MLCK protein expression paralleled the increase in Caco-2 TJ permeability, and the inhibition of the TNF-alpha-induced MLCK expression (by cycloheximide) prevented the increase in Caco-2 TJ permeability, suggesting that MLCK expression may be required for the increase in Caco-2 TJ permeability. The TNF-alpha increase in MLCK protein expression was preceded by an increase in MLCK mRNA expression but not an alteration in MLCK protein degradation. Actinomycin-D prevented the TNF-alpha increase in MLCK mRNA expression and the subsequent increase in MLCK protein expression and Caco-2 TJ permeability, suggesting that the increase in MLCK mRNA transcription led to the increase in MLCK expression. The TNF-alpha increase in MLCK protein expression was also associated with an increase in Caco-2 MLCK activity. The cycloheximide inhibition of MLCK protein expression prevented the TNF-alpha increase in MLCK activity and Caco-2 TJ permeability. Moreover, inhibitors of MLCK, Mg(2+)-myosin ATPase, and metabolic energy prevented the TNF-alpha increase in Caco-2 TJ permeability, suggesting that the increase in MLCK activity was required for the TNF-alpha-induced opening of the Caco-2 TJ barrier. In conclusion, our results indicate for the first time that 1) the TNF-alpha increase in Caco-2 TJ permeability was mediated by an increase in MLCK protein expression, 2) the increase in MLCK protein expression was regulated by an increase in MLCK mRNA transcription, and 3) the increase in Caco-2 TJ permeability required MLCK protein expression-dependent increase in MLCK activity.

Mechanism of IL-1beta-induced increase in intestinal epithelial tight junction permeability

The IL-1beta-induced increase in intestinal epithelial tight junction (TJ) permeability has been postulated to be an important mechanism contributing to intestinal inflammation of Crohn's disease and other inflammatory conditions of the gut. The intracellular and molecular mechanisms that mediate the IL-1beta-induced increase in intestinal TJ permeability remain unclear. The purpose of this study was to elucidate the mechanisms that mediate the IL-1beta-induced increase in intestinal TJ permeability. Specifically, the role of myosin L chain kinase (MLCK) was investigated. IL-1beta caused a progressive increase in MLCK protein expression. The time course of IL-1beta-induced increase in MLCK level correlated linearly with increase in Caco-2 TJ permeability. Inhibition of the IL-1beta-induced increase in MLCK protein expression prevented the increase in Caco-2 TJ permeability. Inhibition of the IL-1beta-induced increase in MLCK activity also prevented the increase in Caco-2 TJ permeability. Additionally, knock-down of MLCK protein expression by small interference RNA prevented the IL-1beta-induced increase in Caco-2 TJ permeability. The IL-1beta-induced increase in MLCK protein expression was preceded by an increase in MLCK mRNA expression. The IL-1beta-induced increase in MLCK mRNA transcription and subsequent increase in MLCK protein expression and Caco-2 TJ permeability was mediated by activation of NF-kappaB. In conclusion, our data indicate that the IL-1beta increase in Caco-2 TJ permeability was mediated by an increase in MLCK expression and activity. Our findings also indicate that the IL-1beta-induced increase in MLCK protein expression and Caco-2 TJ permeability was mediated by an NF-kappaB-dependent increase in MLCK gene transcription.

Molecular mechanism of tumor necrosis factor-alpha modulation of intestinal epithelial tight junction barrier

A TNF-alpha-induced increase in intestinal epithelial tight junction (TJ) permeability has been proposed to be an important proinflammatory mechanism contributing to intestinal inflammation in Crohn's disease and other inflammatory conditions. Previous studies from our laboratory suggested that the TNF-alpha-induced increase in intestinal TJ permeability was mediated by an increase in myosin light chain kinase (MLCK) protein expression. However, the molecular mechanisms that mediate the TNF-alpha increase in intestinal TJ permeability and MLCK protein expression remain unknown. The purpose of this study was to delineate the intracellular and molecular mechanisms that mediate the TNF-alpha-induced increase in intestinal TJ permeability; using an in vitro intestinal epithelial model system consisting of filter-grown Caco-2 intestinal epithelial monolayers. To examine the molecular mechanisms involved in the TNF-alpha regulation of intestinal TJ barrier, we identified and cloned for the first time a functionally active MLCK promoter region. TNF-alpha treatment of filter-grown Caco-2 monolayers transfected with plasmid vector containing the MLCK promoter region produced an increase in MLCK promoter activity and MLCK transcription. The TNF-alpha-induced increase in MLCK transcription corresponded to a sequential increase in MLCK protein expression, MLCK activity, and Caco-2 TJ permeability. The TNF-alpha-induced increase in MLCK promoter activity was mediated by NF-kappaB activation, and the inhibition of NF-kappaB activation prevented the TNF-alpha-induced increase in promoter activity and the subsequent increase in MLCK protein expression and Caco-2 TJ permeability. The TNF-alpha-induced activation of MLCK promoter was mediated by binding of the activated NF-kappaB p50/p65 dimer to the downstream kappaB binding site (-84 to -75) on the MLCK promoter region; deletion of the kappaB binding site prevented the TNF-alpha increase in promoter activity. Additionally, siRNA silencing of NF-kappaB p65 also prevented the TNF-alpha increase in MLCK promoter activity. In conclusion, our findings indicated that the TNF-alpha-induced increase in intestinal epithelial TJ permeability was mediated by NF-kappaB p50/p65 binding and activation of the MLCK promoter. NF-kappaB p50/p65 activation of the MLCK promoter then leads to a stepwise increase in MLCK transcription, expression and activity, and MLCK-mediated opening of the intestinal TJ barrier.

Lipopolysaccharide Regulation of Intestinal Tight Junction Permeability Is Mediated by TLR4 Signal Transduction Pathway Activation of FAK and MyD88

Gut-derived bacterial LPS plays an essential role in inducing intestinal and systemic inflammatory responses and have been implicated as a pathogenic factor in necrotizing enterocolitis and inflammatory bowel disease. The defective intestinal tight junction barrier was shown to be an important factor contributing to the development of intestinal inflammation. LPS, at physiological concentrations, causes an increase in intestinal tight junction permeability (TJP) via a TLR4-dependent process; however, the intracellular mechanisms that mediate LPS regulation of intestinal TJP remain unclear. The aim of this study was to investigate the adaptor proteins and the signaling interactions that mediate LPS modulation of intestinal tight junction barrier using in vitro and in vivo model systems. LPS caused a TLR4-dependent activation of membrane-associated adaptor protein focal adhesion kinase (FAK) in Caco-2 monolayers. LPS caused an activation of both MyD88-dependent and -independent pathways. Small interfering RNA silencing of MyD88 prevented an LPS-induced increase in TJP. LPS caused MyD88-dependent activation of IL-1R-associated kinase 4. TLR4, FAK, and MyD88 were colocalized. Small interfering silencing of TLR4 inhibited TLR4-associated FAK activation, and FAK knockdown prevented MyD88 activation. In vivo studies also confirmed that the LPS-induced increase in mouse intestinal permeability was associated with FAK and MyD88 activation; knockdown of intestinal epithelial FAK prevented an LPS-induced increase in intestinal permeability. Additionally, high-dose LPS-induced intestinal inflammation was dependent on the TLR4/FAK/MyD88 signal transduction axis. To our knowledge, our data show for the first time that the LPS-induced increases in intestinal TJP and intestinal inflammation were regulated by TLR4-dependent activation of the FAK/MyD88/IL-1R-associated kinase 4 signaling pathway.

MicroRNA regulation of intestinal epithelial tight junction permeability

BACKGROUND & AIMS

Defects in the intestinal epithelial tight junction (TJ) barrier contribute to intestinal inflammation. A tumor necrosis factor (TNF)-α-induced increase in intestinal TJ permeability contributes to the intestinal TJ barrier defect in inflammatory disorders. We investigated the mechanisms by which TNF-α induces occludin depletion and an increase in intestinal TJ permeability.

METHODS

We assessed intestinal TJ barrier function using intestinal epithelial model systems: filter-grown Caco-2 monolayers and recycling perfusion studies of mouse small intestine.

RESULTS

TNF-α caused a rapid increase in expression of microRNA (miR)-122a in enterocytes, cultured cells, and intestinal tissue. The overexpressed miR-122a bound to a binding motif at the 3'-untranslated region of occludin messenger RNA (mRNA) to induce its degradation; mRNA degradation depleted occludin from enterocytes, resulting in increased intestinal TJ permeability. Transfection of enterocytes with an antisense oligoribonucleotide against miR-122a blocked the TNF-α-induced increase in enterocyte expression of miR-122a, degradation of occludin mRNA, and increase in intestinal permeability. Overexpression of miR-122a in enterocytes using pre-miR-122a was sufficient to induce degradation of occludin mRNA and an increase in intestinal permeability.

CONCLUSIONS

TNF-α regulates intestinal permeability by inducing miR-122a-mediated degradation of occludin mRNA. These studies show the feasibility of therapeutically targeting miR-122a in vivo to preserve the intestinal barrier.

Isolation and functional characterization of the actin binding region in the tight junction protein ZO-1

Zonula occludens (ZO)-1 is a member of the MAGUK (membrane-associated guanylate kinase homologs) family of membrane-associated signaling molecules that binds directly to both cytosolic and transmembrane components of the tight junction and is believed to organize these proteins within the apical junctional complex. It also binds directly to F-actin, although the functional relevance of this interaction is unknown. To address this issue, we have used VSVG-tagged transgenes to dissect ZO-1 and have identified a 220 amino acid region of ZO-1 that is necessary for its association with F-actin in MDCK cell pull-down assays. A GST fusion expressing this region can bind directly to F-actin in vitro, whereas a GFP fusion expressing this domain decorates actin stress fibers when expressed in MDCK cells. These results indicate that this actin-binding region (ABR) is both necessary and sufficient for binding to F-actin in vitro and in vivo. VSVG-tagged transgenes that lack the ABR still accumulate at both early and late cell-cell contacts in MDCK cells, suggesting that the ABR is not required for tight junction localization. However, accumulation of constructs lacking the ABR is markedly reduced at tight junctions in confluent cells, suggesting that the ABR does play an important role in the localization of ZO-1 at junctions. Furthermore, the ABR is required for localization to a novel actin-rich pool of ZO-1 that accumulates in puncta at the free edge of cells before initiation of cell-cell contact. We conclude that direct interactions between ZO-1 and F-actin play a role in several different steps of junction assembly.

IL-1β and the Intestinal Epithelial Tight Junction Barrier

The intestinal epithelial tight junction (TJ) barrier controls the paracellular permeation of contents from the intestinal lumen into the intestinal tissue and systemic circulation. A defective intestinal TJ barrier has been implicated as an important pathogenic factor in inflammatory diseases of the gut including Crohn's disease, ulcerative colitis, necrotizing enterocolitis, and celiac disease. Previous studies have shown that pro-inflammatory cytokines, which are produced during intestinal inflammation, including interleukin-1β (IL-1β), tumor necrosis factor-α, and interferon-γ, have important intestinal TJ barrier-modulating actions. Recent studies have shown that the IL-1β-induced increase in intestinal TJ permeability is an important contributing factor of intestinal inflammation. The IL-1β-induced increase in intestinal TJ permeability is mediated by regulatory signaling pathways and activation of nuclear transcription factor nuclear factor-κB, myosin light chain kinase gene activation, and post-transcriptional occludin gene modulation by microRNA and contributes to the intestinal inflammatory process. In this review, the regulatory role of IL-1β on intestinal TJ barrier, the intracellular mechanisms that mediate the IL-1β modulation of intestinal TJ permeability, and the potential therapeutic targeting of the TJ barrier are discussed.

Interleukin-6 modulation of intestinal epithelial tight junction permeability is mediated by JNK pathway activation of claudin-2 gene

Defective intestinal epithelial tight junction (TJ) barrier has been shown to be a pathogenic factor in the development of intestinal inflammation. Interleukin-6 (IL-6) is a pleiotropic, pro-inflammatory cytokine which plays an important role in promoting inflammatory response in the gut and in the systemic circulation. Despite its key role in mediating variety inflammatory response, the effect of IL-6 on intestinal epithelial barrier remains unclear. The purpose of this study was to investigate the effect of IL-6 on intestinal epithelial TJ barrier and to delineate the intracellular mechanisms involved using in-vitro (filter-grown Caco-2 monolayers) and in-vivo model (mouse intestinal perfusion) systems. Our results indicated that IL-6 causes a site-selective increase in Caco-2 intestinal epithelia TJ permeability, causing an increase in flux of small-sized molecules having molecular radius <4 Å. The size-selective increase in Caco-2 TJ permeability was regulated by protein-specific increase in claudin-2 expression. The IL-6 increase in TJ permeability required activation of JNK signaling cascade. The JNK pathway activation of AP-1 resulted in AP-1 binding to its binding sequence on the claudin-2 promoter region, leading to promoter activation and subsequent increase in claudin-2 gene transcription and protein synthesis and TJ permeability. Our in-vivo mouse perfusion showed that IL-6 modulation of mouse intestinal permeability was also mediated by AP-1 dependent increase in claudin-2 expression. In conclusion, our studies show for the first time that the IL-6 modulation of intestinal TJ permeability was regulated by JNK activation of AP-1 and AP-1 activation of claudin-2 gene.

Autophagy enhances intestinal epithelial tight junction barrier function by targeting claudin-2 protein degradation

Autophagy is an intracellular degradation pathway and is considered to be an essential cell survival mechanism. Defects in autophagy are implicated in many pathological processes, including inflammatory bowel disease. Among the innate defense mechanisms of intestinal mucosa, a defective tight junction (TJ) barrier has been postulated as a key pathogenic factor in the causation and progression of inflammatory bowel disease by allowing increased antigenic permeation. The cross-talk between autophagy and the TJ barrier has not yet been described. In this study, we present the novel finding that autophagy enhances TJ barrier function in Caco-2 intestinal epithelial cells. Nutrient starvation-induced autophagy significantly increased transepithelial electrical resistance and reduced the ratio of sodium/chloride paracellular permeability. Nutrient starvation reduced the paracellular permeability of small-sized urea but not larger molecules. The role of autophagy in the modulation of paracellular permeability was confirmed by pharmacological induction as well as pharmacological and genetic inhibition of autophagy. Consistent with the autophagy-induced reduction in paracellular permeability, a marked decrease in the level of the cation-selective, pore-forming TJ protein claudin-2 was observed after cell starvation. Starvation reduced the membrane presence of claudin-2 and increased its cytoplasmic, lysosomal localization. Therefore, our data show that autophagy selectively reduces epithelial TJ permeability of ions and small molecules by lysosomal degradation of the TJ protein claudin-2.

Pulmonary arteriovenous malformations: cerebral ischemia and neurologic manifestations

BACKGROUND

There is an increasingly recognized association between pulmonary arteriovenous malformations (PAVM) and cerebral ischemia, frequently attributed to paradoxical embolization. PAVM occur in 20 to 30% of the hereditary hemorrhagic telangiectasia (HHT) population.

OBJECTIVE

To evaluate the risk determinants for cerebral ischemia and neurologic manifestations in patients with PAVM.

METHODS

A retrospective cross-sectional study was performed on consecutive patients admitted between 1988 and 1992 for treatment of PAVM. The number of PAVM, feeding artery (FA) diameters, and aneurysmal sizes were determined by pulmonary angiography. Patients were categorized as having single or multiple PAVM with an FA diameter of > or = 3 mm. History, examination, and cerebral imaging studies were used to determine the prevalence of neurologic manifestations. Patients were defined as having cerebral paradoxical embolization if there was radiologic evidence of cortical infarction.

RESULTS

There were 75 cases: 26 single PAVM and 49 multiple PAVM. Cortical infarction was present in 14% of patients with single PAVM. Patients with multiple PAVM had a greater prevalence of any infarction (OR 3.2; 95% CI, 1.2 to 9.44, p = 0.030), cortical infarctions (OR 2.3; 95% CI, 0.58 to 9.2, p = 0.230), subcortical infarctions (OR 2.1; 95% CI, 0.58 to 7.95, p = 0.249), abscesses (OR 2.3; 95% CI, 0.46 to 11.94; p = 0.295), and seizures (OR 6.4, 95% CI 0.77 to 53.2, p = 0.054). Patients with multiple PAVM had markedly greater odds of having any clinical or radiologic evidence of cerebral ischemic involvement (OR 4.5; 95% CI, 1.47 to 14; p = 0.008).

CONCLUSION

There is a strong association between single PAVM and various neurologic manifestations. The prevalence is greater for patients with multiple PAVM, suggesting increased predisposition for paradoxical embolization with a greater number of malformations.

IL-1beta-induced increase in intestinal epithelial tight junction permeability is mediated by MEKK-1 activation of canonical NF-kappaB pathway

IL-1β is a proinflammatory cytokine that plays a central role in the inflammatory process of the gut. IL-1β causes an increase in intestinal epithelial tight junction (TJ) permeability, but the intracellular pathways that mediate intestinal TJ permeability remain unclear. The major aims of this study were to delineate the protein kinases that regulate the IL-1β modulation of intestinal TJ barrier function and to determine the intracellular mechanisms involved, using filter-grown Caco-2 monolayers as the in vitro model system. Our results showed that IL-1β caused a rapid activation of MEKK-1 and NIK. The knockdown of MEKK-1, but not NIK, inhibited the IL-1β increase in Caco-2 TJ permeability. IL-1β caused an activation of both canonical and noncanonical NF-κB pathways; MEKK-1 regulated the activation of the canonical pathway, while NIK regulated the activation of the noncanonical pathway. Inhibition of MEKK-1 activation of the canonical pathway prevented the IL-1β increase in TJ permeability. Our data also indicated that inhibitory κB kinase was the catalytic subunit primarily involved in canonical pathway activation and TJ barrier opening. MEKK-1 also played an essential role in myosin light chain kinase gene activation. In conclusion, our data show for the first time that MEKK-1 plays an integral role in IL-1β modulation of Caco-2 TJ barrier function by regulating the activation of the canonical NF-κB pathway and the MLCK gene.

Physiologically relevant increase in temperature causes an increase in intestinal epithelial tight junction permeability

The effects of physiologically relevant increase in temperature (37-41 degrees C) on intestinal epithelial tight junction (TJ) barrier have not been previously studied. Additionally, the role of heat-shock proteins (HSPs) in the regulation of intestinal TJ barrier during heat stress remains unknown. Because heat-induced disturbance of intestinal TJ barrier could lead to endotoxemia and bacterial translocation during physiological thermal stress, the purpose of this study was to investigate the effects of modest, physiologically relevant increases in temperature (37-41 degrees C) on intestinal epithelial TJ barrier and to examine the protective role of HSPs on intestinal TJ barrier. Filter-grown Caco-2 intestinal epithelial cells were used as an in vitro intestinal epithelial model system to assess the effects of heat exposure on intestinal TJ barrier. Exposure of filter-grown Caco-2 monolayers to modest increases in temperatures (37-41 degrees C) resulted in a significant time- and temperature-dependent increases in Caco-2 TJ permeability. Exposure to modest heat (39 or 41 degrees C) resulted in rapid and sustained increases in HSP expression; and inhibition of HSP expression produced a marked increase in heat-induced increase in Caco-2 TJ permeability (P < 0.001). Heat exposure (41 degrees C) resulted in a compensatory increase in Caco-2 occludin protein expression and an increase in junctional localization. Inhibition of HSP expression prevented the compensatory upregulation of occludin protein expression and produced a marked disruption in junctional localization of occludin protein during heat stress. In conclusion, our findings demonstrate for the first time that a modest, physiologically relevant increase in temperature causes an increase in intestinal epithelial TJ permeability. Our data also show that HSPs play an important protective role in preventing the heat-induced disruption of intestinal TJ barrier and suggest that HSP mediated upregulation of occludin expression may be an important mechanism involved in the maintenance of intestinal epithelial TJ barrier function during heat stress.

Lipopolysaccharide-Induced Increase in Intestinal Epithelial Tight Permeability Is Mediated by Toll-Like Receptor 4/Myeloid Differentiation Primary Response 88 (MyD88) Activation of Myosin Light Chain Kinase Expression

Lipopolysaccharides (LPSs) are a major component of the Gram-negative bacterial cell wall and play an important role in mediating intestinal inflammatory responses in inflammatory bowel disease. Although recent studies suggested that physiologically relevant concentrations of LPS (0 to 1 ng/mL) cause an increase in intestinal epithelial tight junction (TJ) permeability, the mechanisms that mediate an LPS-induced increase in intestinal TJ permeability remain unclear. Herein, we show that myosin light chain kinase (MLCK) plays a central role in the LPS-induced increase in TJ permeability. Filter-grown Caco-2 intestinal epithelial monolayers and C57BL/6 mice were used as an in vitro and in vivo intestinal epithelial model system, respectively. LPS caused a dose- and time-dependent increase in MLCK expression and kinase activity in Caco-2 monolayers. The pharmacologic MLCK inhibition and siRNA-induced knock-down of MLCK inhibited the LPS-induced increase in Caco-2 TJ permeability. The LPS increase in TJ permeability was mediated by toll-like receptor 4 (TLR-4)/MyD88 signal-transduction pathway up-regulation of MLCK expression. The LPS-induced increase in mouse intestinal permeability also required an increase in MLCK expression. The LPS-induced increase in intestinal permeability was inhibited in MLCK^-/- and TLR-4^-/- mice. These data show, for the first time, that the LPS-induced increase in intestinal permeability was mediated by TLR-4/MyD88 signal-transduction pathway up-regulation of MLCK. Therapeutic targeting of these pathways can prevent an LPS-induced increase in intestinal permeability.

IL1B Increases Intestinal Tight Junction Permeability by Up-regulation of MIR200C-3p, Which Degrades Occludin mRNA

BACKGROUND & AIMS

Defects in the epithelial tight junction (TJ) barrier contribute to development of intestinal inflammation associated with diseases. Interleukin 1 beta (IL1B) increases intestinal permeability in mice. We investigated microRNAs that are regulated by IL1B and their effects on expression of TJ proteins and intestinal permeability.

METHODS

We used Targetscan to identify microRNAs that would bind the 3' untranslated region (3'UTR) of occludin mRNA; regions that interacted with microRNAs were predicted using the V-fold server and Assemble2, and 3-dimensional models were created using UCSF Chimera linked with Assemble2. Caco-2 cells were transfected with vectors that express microRNAs, analyzed by immunoblots and real-time polymerase chain reaction (PCR), and grown as monolayers; permeability in response to IL1B was assessed with the marker inulin. Male C57BL/6 mice were given intraperitoneal injections of IL1B and intestinal recycling perfusion was measured; some mice were given dextran sodium sulfate to induce colitis and/or gavage with an antagonist to MIR200C-3p (antagomiR-200C) or the nonspecific antagomiR (control). Intestinal tissues were collected from mice and analyzed by histology and real-time PCR; enterocytes were isolated by laser capture microdissection. We also analyzed colon tissues and organoids from patients with and without ulcerative colitis.

RESULTS

Incubation of Caco-2 monolayers with IL1B increased TJ permeability and reduced levels of occludin protein and mRNA without affecting the expression of other transmembrane TJ proteins. Targetscan identified MIR122, MIR200B-3p, and MIR200C-3p, as miRNAs that might bind to the occludin 3'UTR. MIR200C-3p was rapidly increased in Caco-2 cells incubated with IL1B; the antagomiR-200c prevented the IL1B-induced decrease in occludin mRNA and protein and reduced TJ permeability. Administration of IL1B to mice increased small intestinal TJ permeability, compared with mice given vehicle; enterocytes isolated from mice given IL1B had increased expression of MIR200C-3p and decreased levels of occludin messenger RNA (mRNA) and protein. Intestinal tissues from mice with colitis had increased levels of IL1B mRNA and MIR200C-3p and decreased levels of occludin mRNA; gavage of mice with antagomiR-200C reduced levels of MIR200C-3p and prevented the decrease in occludin mRNA and the increase in colonic permeability. Colon tissues and organoids from patients with ulcerative colitis had increased levels of IL1B mRNA and MIR200C-3p compared with healthy controls. Using 3-dimensional molecular modeling and mutational analyses, we identified the nucleotide bases in the occluding mRNA 3'UTR that interact with MIR200C-3p.

CONCLUSIONS

Intestine tissues from patients with ulcerative colitis and mice with colitis have increased levels of IL1B mRNA and MIR200C-3p, which reduces expression of occludin by enterocytes and thereby increases TJ permeability. Three-dimensional modeling of the interaction between MIR200C-3p and the occludin mRNA 3'UTR identified sites of interaction. The antagomiR-200C prevents the decrease in occludin in enterocytes and intestine tissues of mice with colitis, maintaining the TJ barrier.

TNF-α Modulation of Intestinal Tight Junction Permeability Is Mediated by NIK/IKK-α Axis Activation of the Canonical NF-κB Pathway

Tumor necrosis factor (TNF)-α, a key mediator of intestinal inflammation, causes an increase in intestinal epithelial tight junction (TJ) permeability by activating myosin light chain kinase (MLCK; official name MYLK3) gene. However, the precise signaling cascades that mediate the TNF-α-induced activation of MLCK gene and increase in TJ permeability remain unclear. Our aims were to delineate the upstream signaling mechanisms that regulate the TNF-α modulation of intestinal TJ barrier function with the use of in vitro and in vivo intestinal epithelial model systems. TNF-α caused a rapid activation of both canonical and noncanonical NF-κB pathway. NF-κB-inducing kinase (NIK) and mitogen-activated protein kinase kinase-1 (MEKK-1) were activated in response to TNF-α. NIK mediated the TNF-α activation of inhibitory κB kinase (IKK)-α, and MEKK1 mediated the activation of IKK complex, including IKK-β. NIK/IKK-α axis regulated the activation of both NF-κB p50/p65 and RelB/p52 pathways. Surprisingly, the siRNA induced knockdown of NIK, but not MEKK-1, prevented the TNF-α activation of both NF-κB p50/p65 and RelB/p52 and the increase in intestinal TJ permeability. Moreover, NIK/IKK-α/NF-κB p50/p65 axis mediated the TNF-α-induced MLCK gene activation and the subsequent MLCK increase in intestinal TJ permeability. In conclusion, our data show that NIK/IKK-α/regulates the activation of NF-κB p50/p65 and plays an integral role in the TNF-α-induced activation of MLCK gene and increase in intestinal TJ permeability.

Ethanol modulation of intestinal epithelial tight junction barrier

Previous studies have shown that high concentrations of ethanol (>/=40%) cause functional damage of the gastrointestinal epithelial barrier by direct cytotoxic effect on the epithelial cells. The effects of lower noncytotoxic doses of ethanol on epithelial barrier function are unknown. A major function of gastrointestinal epithelial cells is to provide a barrier against the hostile substances in the gastrointestinal lumen. The apicolaterally located tight junctions (TJs) form a paracellular seal between the lateral membranes of adjacent cells and act as a paracellular barrier. In this study, we investigated the effects of lower doses of ethanol on intestinal epithelial TJ barrier function using filter-grown Caco-2 intestinal epithelial monolayers. The Caco-2 TJ barrier function was assessed by measuring epithelial resistance or paracellular permeability of the filter-grown monolayers. Ethanol (0, 1, 2.5, 5, 7.5, and 10%) produced a dose-related drop in Caco-2 epithelial resistance and increase in paracellular permeability. Ethanol also produced a progressive disruption of TJ protein (ZO-1) with separation of ZO-1 proteins from the cellular junctions and formation of large gaps between the adjacent cells. Ethanol, at the doses used (</=10%), did not cause cytotoxicity (lactate dehydrogenase release) to the Caco-2 cells. Ethanol produced a disassembly and displacement of perijunctional actin and myosin filaments from the perijunctional areas. On ethanol removal, actin and myosin filaments rapidly reassembled at the cellular borders. Ethanol stimulated the Caco-2 myosin light chain kinase (MLCK) activity but did not affect the MLCK protein levels. Specific MLCK inhibitor ML-7 inhibited both ethanol increases in MLCK activity and TJ permeability without affecting the MLCK protein levels. Consistent with these findings, metabolic inhibitors sodium azide and 2,4-dinitrophenol significantly prevented ethanol-induced increase in Caco-2 TJ permeability, whereas cycloheximide or actinomycin D had no effect. The results of this study indicate that ethanol at low noncytotoxic doses causes a functional and structural opening of the Caco-2 intestinal epithelial TJ barrier by activating MLCK.

Mechanism of extracellular calcium regulation of intestinal epithelial tight junction permeability: role of cytoskeletal involvement

Recent studies suggest that an abnormal increase in intestinal tight junction (TJ) permeability may be an important etiologic factor in number of diseases including Crohn's disease, NSAID-associated enteritis, and various infectious diarrheal syndromes. The intracellular processes involved in regulation of intestinal epithelial TJ permeability, however, remain poorly understood. In this study, we used cultured Caco-2 intestinal epithelial cells to examine the intracellular processes involved in extracellular Ca(++) modulation of intestinal epithelial monolayer TJ barrier. Incubation of the filter-grown Caco-2 intestinal monolayers in Ca(++)-free solution (CFS), consisting of modified Krebs-buffer solution containing 0 mM Ca(++) and 1 mM EGTA, resulted in a rapid drop in Caco-2 epithelial resistance and increase in epithelial permeability to paracellular markers mannitol and inulin, indicating an increase in TJ permeability. The increase in Caco-2 TJ permeability was rapidly reversed by the re-introduction of Ca(++) (1.8 mM) into the incubation medium. The CFS-induced increase in Caco-2 TJ permeability was associated with separation of the cytoplasmic and transmembrane TJ proteins, ZO-1 and occludin, and formation of large intercellular openings between the adjoining cells. The CFS-induced modulation of TJ barrier was associated with activation of myosin light chain kinase (MLCK) activity and centripetal retraction of peri-junctional actin and myosin filaments. The inhibition of CFS-induced activation of Caco-2 MLCK with MLCK inhibitor (ML-7) prevented the CFS-induced retraction of actin and myosin filaments and the subsequent alteration of TJ barrier function and structure. Our results suggested that the CFS-induced alteration of TJ proteins and functional increase in TJ permeability was mediated by Caco-2 MLCK activation and the resultant contraction of the peri-junctionally located actin-myosin filaments. Consistent with the role of MLCK in this process, selected inhibitors of Mg(++)-myosin ATPase and metabolic energy, but not protein synthesis inhibitors, also prevented the CFS-induced retraction of actin and myosin filaments and the subsequent increase in TJ permeability. In conclusion, our results indicate that extracellular Ca(++) is crucial for the maintenance of intestinal epithelial TJ barrier function. The removal of extracellular Ca(++) from the incubation medium causes activation of Caco-2 MLCK, which in turn leads to an increase in intestinal monolayer TJ permeability.

Mechanism of IL-1β modulation of intestinal epithelial barrier involves p38 kinase and activating transcription factor-2 activation

The defective intestinal epithelial tight junction (TJ) barrier has been postulated to be an important pathogenic factor contributing to intestinal inflammation. It has been shown that the proinflammatory cytokine IL-1β causes an increase in intestinal permeability; however, the signaling pathways and the molecular mechanisms involved remain unclear. The major purpose of this study was to investigate the role of the p38 kinase pathway and the molecular processes involved. In these studies, the in vitro intestinal epithelial model system (Caco-2 monolayers) was used to delineate the cellular and molecular mechanisms, and a complementary in vivo mouse model system (intestinal perfusion) was used to assess the in vivo relevance of the in vitro findings. Our data indicated that the IL-1β increase in Caco-2 TJ permeability correlated with an activation of p38 kinase. The activation of p38 kinase caused phosphorylation and activation of p38 kinase substrate, activating transcription factor (ATF)-2. The activated ATF-2 translocated to the nucleus where it attached to its binding motif on the myosin L chain kinase (MLCK) promoter region, leading to the activation of MLCK promoter activity and gene transcription. Small interfering RNA induced silencing of ATF-2, or mutation of the ATF-2 binding motif prevented the activation of MLCK promoter and MLCK mRNA transcription. Additionally, in vivo intestinal perfusion studies also indicated that the IL-1β increase in mouse intestinal permeability required p38 kinase-dependent activation of ATF-2. In conclusion, these studies show that the IL-1β-induced increase in intestinal TJ permeability in vitro and in vivo was regulated by p38 kinase activation of ATF-2 and by ATF-2 regulation of MLCK gene activity.

Mechanism of glucocorticoid regulation of the intestinal tight junction barrier

A defective intestinal epithelial tight junction (TJ) barrier has been proposed as an important pathogenic factor contributing to the intestinal inflammation of Crohn's disease. Glucocorticoids are first-line therapeutic agents for the treatment of moderate to severe Crohn's disease. Glucocorticoid treatment has been shown to induce retightening of the intestinal TJ barrier defect in Crohn's disease patients. However, the mechanisms that mediate the glucocorticoid therapeutic action on intestinal TJ barrier function remain unknown. The aim of this study was to elucidate the mechanism of glucocorticoid modulation of the intestinal epithelial TJ barrier using an in vitro model system. Filter-grown Caco-2 intestinal epithelial cells were used as an in vitro model to examine the effects of glucocorticoids on basal intestinal epithelial TJ barrier function and on TNF-alpha-induced disruption of the TJ barrier. Glucocorticoids (prednisolone and dexamethasone) did not have a significant effect on baseline Caco-2 TJ barrier function but prevented the TNF-alpha-induced increase in Caco-2 TJ permeability. The glucocorticoid protective effect against the TNF-alpha-induced increase in Caco-2 TJ permeability required activation of the glucocorticoid receptor (GR) complex. The activation of the GR complex resulted in GR complex binding to the glucocorticoid response element (GRE) site on DNA and activation of a GR-responsive promoter. Glucocorticoids inhibited the TNF-alpha-induced increase in myosin light chain kinase (MLCK) protein expression, a key process mediating the TNF-alpha increase in intestinal TJ permeability. The glucocorticoid inhibition of the TNF-alpha-induced increase in MLCK protein expression was due to the binding of the GR complex to a GRE binding site on the MLCK promoter region suppressing the TNF-alpha-induced activation. Glucocorticoids inhibit the TNF-alpha-induced increase in Caco-2 TJ permeability. The prednisolone protective action was mediated by binding of activated GR complex to the GRE site on the MLCK promoter, suppressing the TNF-alpha-induced increase in MLCK gene activity, protein expression, and subsequent opening of the intestinal TJ barrier.

LPS-induced cytokine levels are repressed by elevated expression of HSP70 in rats: possible role of NF-kappaB

Heat shock protein (HSP)70 provides a spectrum of protection against any of a variety of stresses, preventing damage measured at the level of molecules, cells, as well as whole organism. We have previously reported that lipopolysaccharide (LPS)-induced lethality in rats is prevented by a previous exposure to a mild thermal stress and that a thermal stress sufficient to induce HSP70 expression in the liver is accompanied by an inhibition of endotoxin-mediated cytokines and modulation of febrile response. However, the effect of HSP70 upregulation on cytokine expression in animals is unknown. The aim of the present study was to demonstrate the effect of HSP70 overexpression with adenovirus administration on LPS-induced increase in cytokines levels in animals. In the present study, Sprague-Dawley rats were infected with either the control AdTrack or Ad70 virus that directs the expression of human HSP70. After a 5-day incubation, animals were injected with either saline alone or LPS (50 microg/kg). Four hours later, blood samples were drawn and plasma levels of interleukin (IL)-6 or tumor necrosis factor (TNF)-alpha were measured by enzyme-linked immunosorbent assay. Our data demonstrate for the first time that HSP70 overexpression with adenovirus injection prevented the LPS-induced increase in TNF-alpha and IL-6 levels in rats. Repression of LPS-induced cytokines expressions by HSP70 upregulation was associated with inhibited IkappaBalpha degradation and nuclear factor kappa-B (NF-kappaB) p65 nuclear translocation in liver, suggesting that HSP70 overexpression may regulate LPS-induced cytokines expression through NF-kappaB pathway. We conclude that the effects of heat stress-induced increase in HSP70 protein expression on LPS-induced cytokine elaboration in whole animals can be reproduced by the actions of a single gene product.

Cellular and molecular mechanisms that mediate basal and tumour necrosis factor-alpha-induced regulation of myosin light chain kinase gene activity

The patients with Crohn's disease (CD) have a ‘leaky gut’ manifested by an increase in intestinal epithelial tight junction (TJ) permeability. Tumour necrosis factor-α (TNF-α) is a proto-typical pro-inflammatory cytokine that plays a central role in intestinal inflammation of CD. An important pro-inflammatory action of TNF-α is to cause a functional opening of intestinal TJ barrier. Previous studies have shown that TNF-α increase in TJ permeability was regulated by an increase in myosin light chain kinase (MLCK) gene activity and protein expression. The major aim of this study was to elucidate the cellular and molecular mechanisms that mediate basal and TNF-α-induced increase in MLCK gene activity. By progressive 5′ deletion, minimal MLCK promoter was localized between −313 to +118 on MLCK promoter. A p53 binding site located within minimal promoter region was identified as an essential determinant for basal promoter activity. A 4 bp start site and a 5 bp downstream promoter element were required for MLCK gene activity. TNF-α-induced increase in MLCK promoter activity was mediated by NF-κB activation. There were eight κB binding sites on MLCK promoter. The NF-κB1 site at +48 to +57 mediated TNF-α-induced increase in MLCK promoter activity. The NF-κB2 site at −325 to −316 had a repressive role on promoter activity. The opposite effects on promoter activity were due to differences in the NF-κB dimer type binding to the κB sites. p50/p65 dimer preferentially binds to the NF-κB1 site and up-regulates promoter activity; while p50/p50 dimer preferentially binds to the NF-κB2 site and down-regulates promoter activity. In conclusion, we have identified the minimal MLCK promoter region, essential molecular determinants and molecular mechanisms that mediate basal and TNF-α-induced modulation of MLCK promoter activity in Caco-2 intestinal epithelial cells. These studies provide novel insight into the cellular and molecular mechanisms that regulate basal and TNF-α-induced modulation of MLCK gene activity.

Cytochalasin B modulation of Caco-2 tight junction barrier: role of myosin light chain kinase

The intracellular mechanisms that mediate cytochalasin-induced increase in intestinal epithelial tight junction (TJ) permeability are unclear. In this study, we examined the involvement of myosin light chain kinase (MLCK) in this process, using the filter-grown Caco-2 intestinal epithelial monolayers. Cytochalasin B (Cyto B) (5 microg/ml) produced an increase in Caco-2 MLCK activity, which correlated with the increase in Caco-2 TJ permeability. The inhibition of Cyto B-induced MLCK activation prevented the increase in Caco-2 TJ permeability. Additionally, myosin-Mg(2+)-ATPase inhibitor and metabolic inhibitors (which inhibit MLCK induced actin-myosin contraction) also prevented the Cyto B-induced increase in Caco-2 TJ permeability. Cyto B caused a late-phase (15-30 min) aggregation of actin fragments into large actin clumps, which was also inhibited by MLCK inhibitors. Cyto B produced a morphological disturbance of the ZO-1 TJ proteins, visually correlating with the functional increase in Caco-2 TJ permeability. The MLCK and myosin-Mg(2+)-ATPase inhibitors prevented both the functional increase in TJ permeability and disruption of ZO-1 proteins. These findings suggested that Cyto B-induced increase in Caco-2 TJ permeability is regulated by MLCK activation.