Bryostatin-1 enhances barrier function in T84 epithelia through PKC-dependent regulation of tight junction proteins

MUC13 negatively regulates tight junction proteins and intestinal epithelial barrier integrity via protein kinase C

Glycosylated mucin proteins contribute to the essential barrier function of the intestinal epithelium. The transmembrane mucin MUC13 is an abundant intestinal glycoprotein with important functions for mucosal maintenance that are not yet completely understood. We demonstrate that in human intestinal epithelial monolayers, MUC13 localized to both the apical surface and the tight junction (TJ) region on the lateral membrane. MUC13 deletion resulted in increased transepithelial resistance (TEER) and reduced translocation of small solutes. TEER buildup in ΔMUC13 cells could be prevented by addition of MLCK, ROCK or protein kinase C (PKC) inhibitors. The levels of TJ proteins including claudins and occludin were highly increased in membrane fractions of MUC13 knockout cells. Removal of the MUC13 cytoplasmic tail (CT) also altered TJ composition but did not affect TEER. The increased buildup of TJ complexes in ΔMUC13 and MUC13-ΔCT cells was dependent on PKC. The responsible PKC member might be PKCδ (or PRKCD) based on elevated protein levels in the absence of full-length MUC13. Our results demonstrate for the first time that a mucin protein can negatively regulate TJ function and stimulate intestinal barrier permeability.

Direct Action of Non-Digestible Oligosaccharides against a Leaky Gut

The epithelial monolayer is the primary determinant of mucosal barrier function, and tight junction (TJ) complexes seal the paracellular space between the adjacent epithelial cells and represent the main "gate-keepers" of the paracellular route. Impaired TJ functionality results in increased permeation of the "pro-inflammatory" luminal contents to the circulation that induces local and systemic inflammatory and immune responses, ultimately triggering and/or perpetuating (chronic) systemic inflammatory disorders. Increased gut leakiness is associated with intestinal and systemic disease states such as inflammatory bowel disease and neurodegenerative diseases such as Parkinson's disease. Modulation of TJ dynamics is an appealing strategy aiming at inflammatory conditions associated with compromised intestinal epithelial function. Recently there has been a growing interest in nutraceuticals, particularly in non-digestible oligosaccharides (NDOs). NDOs confer innumerable health benefits via microbiome-shaping and gut microbiota-related immune responses, including enhancement of epithelial barrier integrity. Emerging evidence supports that NDOs also exert health-beneficial effects on microbiota independently via direct interactions with intestinal epithelial and immune cells. Among these valuable features, NDOs promote barrier function by directly regulating TJs via AMPK-, PKC-, MAPK-, and TLR-associated pathways. This review provides a comprehensive overview of the epithelial barrier-protective effects of different NDOs with a special focus on their microbiota-independent modulation of TJs.

Putative role of natural products as Protein Kinase C modulator in different disease conditions

INTRODUCTION

Protein kinase C (PKC) is a promising drug target for various therapeutic areas. Natural products derived from plants, animals, microorganisms, and marine organisms have been used by humans as medicine from prehistoric times. Recently, several compounds derived from plants have been found to modulate PKC activities through competitive binding with ATP binding site, and other allosteric regions of PKC. As a result fresh race has been started in academia and pharmaceutical companies to develop an effective naturally derived small-molecule inhibitor to target PKC activities. Herein, in this review, we have discussed several natural products and their derivatives, which are reported to have an impact on PKC signaling cascade.

METHODS

All information presented in this review article regarding the regulation of PKC by natural products has been acquired by a systematic search of various electronic databases, including ScienceDirect, Scopus, Google Scholar, Web of science, ResearchGate, and PubMed. The keywords PKC, natural products, curcumin, rottlerin, quercetin, ellagic acid, epigallocatechin-3 gallate, ingenol 3 angelate, resveratrol, protocatechuic acid, tannic acid, PKC modulators from marine organism, bryostatin, staurosporine, midostaurin, sangivamycin, and other relevant key words were explored.

RESULTS

The natural products and their derivatives including curcumin, rottlerin, quercetin, ellagic acid, epigallocatechin-3 gallate, ingenol 3 angelate, resveratrol, bryostatin, staurosporine, and midostaurin play a major role in the management of PKC activity during various disease progression.

CONCLUSION

Based on the comprehensive literature survey, it could be concluded that various natural products can regulate PKC activity during disease progression. However, extensive research is needed to circumvent the challenge of isoform specific regulation of PKC by natural products.

Activation of the Ca2+ sensing receptor and the PKC/WNK4 downstream signaling cascade induces incorporation of ZO-2 to tight junctions and its separation from 14-3-3

Zonula occludens-2 (ZO-2) is a tight junction (TJ) cytoplasmic protein, whose localization varies according to cell density and Ca²⁺ in the media. In cells cultured in low calcium (LC), ZO-2 displays a diffuse cytoplasmic distribution, but activation of the Ca²⁺ sensing receptor (CaSR) with Gd³⁺ triggers the appearance of ZO-2 at the cell borders. CaSR downstream signaling involves activation of protein kinase C, which phosphorylates and activates with no lysine kinase-4 that phosphorylates ZO-2 inducing its concentration at TJs. In LC, ZO-2 is protected from degradation by association to 14-3-3 proteins. When monolayers are transferred to normal calcium, the complexes ZO-2/14-3-3ζ and ZO-2/14-3-3σ move to the cell borders and dissociate. The 14-3-3 proteins are then degraded in proteosomes, whereas ZO-2 integrates to TJs. From the plasma membrane residual ZO-2 is endocyted and degradaded in lysosomes. The unique region 2 of ZO-2, and S261 located within a nuclear localization signal, are critical for the interaction with 14-3-3 ζ and σ and for the efficient nuclear importation of ZO-2. These results explain the molecular mechanism through which extracellular Ca²⁺ triggers the appearance of ZO-2 at TJs in epithelial cells and reveal the novel interaction between ZO-2 and 14-3-3 proteins, which is critical for ZO-2 protection and intracellular traffic.

Intracerebral GM-CSF contributes to transendothelial monocyte migration in APP/PS1 Alzheimer's disease mice

Although tight junctions between human brain microvascular endothelial cells in the blood-brain barrier prevent molecules or cells in the bloodstream from entering the brain, in Alzheimer's disease, peripheral blood monocytes can "open" these tight junctions and trigger subsequent transendothelial migration. However, the mechanism underlying this migration is unclear. Here, we found that the CSF2RB, but not CSF2RA, subunit of the granulocyte-macrophage colony-stimulating factor receptor was overexpressed on monocytes from Alzheimer's disease patients. CSF2RB contributes to granulocyte-macrophage colony-stimulating factor-induced transendothelial monocyte migration. Granulocyte-macrophage colony-stimulating factor triggers human brain microvascular endothelial cells monolayer tight junction disassembly by downregulating ZO-1 expression via transcription modulation and claudin-5 expression via the ubiquitination pathway. Interestingly, intracerebral granulocyte-macrophage colony-stimulating factor blockade abolished the increased monocyte infiltration in the brains of APP/PS1 Alzheimer's disease model mice. Our results suggest that in Alzheimer's disease patients, high granulocyte-macrophage colony-stimulating factor levels in the brain parenchyma and cerebrospinal fluid induced blood-brain barrier opening, facilitating the infiltration of CSF2RB-expressing peripheral monocytes across blood-brain barrier and into the brain. CSF2RB might be useful as an Alzheimer's disease biomarker. Thus, our findings will help to understand the mechanism of monocyte infiltration in Alzheimer's disease pathogenesis.

Bryostatin-1 Restores Blood Brain Barrier Integrity following Blast-Induced Traumatic Brain Injury

Recent wars in Iraq and Afghanistan have accounted for an estimated 270,000 blast exposures among military personnel. Blast traumatic brain injury (TBI) is the 'signature injury' of modern warfare. Blood brain barrier (BBB) disruption following blast TBI can lead to long-term and diffuse neuroinflammation. In this study, we investigate for the first time the role of bryostatin-1, a specific protein kinase C (PKC) modulator, in ameliorating BBB breakdown. Thirty seven Sprague-Dawley rats were used for this study. We utilized a clinically relevant and validated blast model to expose animals to moderate blast exposure. Groups included: control, single blast exposure, and single blast exposure + bryostatin-1. Bryostatin-1 was administered i.p. 2.5 mg/kg after blast exposure. Evan's blue, immunohistochemistry, and western blot analysis were performed to assess injury. Evan's blue binds to albumin and is a marker for BBB disruption. The single blast exposure caused an increase in permeability compared to control (t = 4.808, p < 0.05), and a reduction back toward control levels when bryostatin-1 was administered (t = 5.113, p < 0.01). Three important PKC isozymes, PKCα, PKCδ, and PKCε, were co-localized primarily with endothelial cells but not astrocytes. Bryostatin-1 administration reduced toxic PKCα levels back toward control levels (t = 4.559, p < 0.01) and increased the neuroprotective isozyme PKCε (t = 6.102, p < 0.01). Bryostatin-1 caused a significant increase in the tight junction proteins VE-cadherin, ZO-1, and occludin through modulation of PKC activity. Bryostatin-1 ultimately decreased BBB breakdown potentially due to modulation of PKC isozymes. Future work will examine the role of bryostatin-1 in preventing chronic neurodegeneration following repetitive neurotrauma.

Hyperosmolarity-Induced Down-Regulation of Claudin-2 Mediated by Decrease in PKCβ-Dependent GATA-2 in MDCK Cells

Hyperosmolarity decreases claudin-2 expression in renal tubular epithelial cells, but the molecular mechanism remains undefined. Here, we found that the hyperosmolarity-induced decrease in claudin-2 expression is inhibited by Go6983, a non-selective protein kinase C (PKC) inhibitor, and PKCβ specific inhibitor in Madin-Darby canine kidney II cells. Hyperosmolarity increased intracellular free Ca(2+) concentration and phosphorylated PKCβ level, which were inhibited by RN-1734, an antagonist of transient receptor potential vanilloid 4 channel. Phorbol 12-myristate 13-acetate, a PKC activator, decreased claudin-2 expression. These results indicate hyperosmolarity decreases claudin-2 expression mediated by the activation of RN-1734-sensitive channel and PKCβ. Hyperosmolarity decreased promoter activity of claudin-2, which was inhibited by Go6983 and PKCβ inhibitor similar to those in real-time PCR and Western blotting. The effect of hyperosmolarity on promoter activity was not observed in the construct of -469/-6, a deletion mutant. Claudin-2 has hyperosmolarity-sensitive region in its promoter, which includes GATA binding site. Hyperosmolarity decreased the nuclear level of GATA-2, which was inhibited by Go6983 and PKCβ inhibitor. Mutation of GATA binding site decreased the basal promoter activity and inhibited the effect of hyperosmolarity. In contrast, the hyperosmolarity-induced decrease in reporter activity and claudin-2 expression were rescued by over-expression of wild type GATA-2. Chromatin immunoprecipitation assay showed that GATA-2 bound to promoter region of claudin-2. These results suggest that hyperosmolarity decreases the expression level of claudin-2 via a decrease in PKCβ-dependent GATA-2 transcriptional activity in renal tubular epithelial cells.

Regulation of paracellular permeability: factors and mechanisms

Epithelial permeability is composed of transcellular permeability and paracellular permeability. Paracellular permeability is controlled by tight junctions (TJs). Claudins and occludin are two major transmembrane proteins in TJs, which directly determine the paracellular permeability to different ions or large molecules. Intracellular signaling pathways including Rho/Rho-associated protein kinase, protein kinase Cs, and mitogen-activated protein kinase, modulate the TJ proteins to affect paracellular permeability in response for diverse stimuli. Cytokines, growth factors and hormones in organism can regulate the paracellular permeability via signaling pathway. The transcellular transporters such as Na-K-ATPase, Na(+)-coupled transporters and chloride channels, can interact with paracellular transport and regulate the TJs. In this review, we summarized the factors affecting paracellular permeability and new progressions of the related mechanism in recent studies, and pointed out further research areas.

Ischemic preconditioning enhances integrity of coronary endothelial tight junctions

Ischemic preconditioning (IPC) is one of the most effective procedures known to protect hearts against ischemia/reperfusion (IR) injury. Tight junction (TJ) barriers occur between coronary endothelial cells. TJs provide barrier function to maintain the homeostasis of the inner environment of tissues. However, the effect of IPC on the structure and function of cardiac TJs remains unknown. We tested the hypothesis that myocardial IR injury ruptures the structure of TJs and impairs endothelial permeability whereas IPC preserves the structural and functional integrity of TJs in the blood-heart barrier. Langendorff hearts from C57BL/6J mice were prepared and perfused with Krebs-Henseleit buffer. Cardiac function, creatine kinase release, and myocardial edema were measured. Cardiac TJ function was evaluated by measuring Evans blue-conjugated albumin (EBA) content in the extravascular compartment of hearts. Expression and translocation of zonula occludens (ZO)-2 in IR and IPC hearts were detected with Western blot. A subset of hearts was processed for the observation of ultra-structure of cardiac TJs with transmission electron microscopy. There were clear TJs between coronary endothelial cells of mouse hearts. IR caused the collapse of TJs whereas IPC sustained the structure of TJs. IR increased extravascular EBA content in the heart and myocardial edema but decreased the expression of ZO-2 in the cytoskeleton. IPC maintained the structure of TJs. Cardiac EBA content and edema were reduced in IPC hearts. IPC enhanced the translocation of ZO-2 from cytosol to cytoskeleton. In conclusion, TJs occur in normal mouse heart. IPC preserves the integrity of TJ structure and function that are vulnerable to IR injury.

Bile salts disrupt human esophageal squamous epithelial barrier function by modulating tight junction proteins

Reflux of acid and bile acids contributes to epithelial tissue injury in gastro-esophageal reflux disease. However, the influence of refluxed material on human esophageal stratified epithelial barrier function and tight junction (TJ) proteins has not been fully elucidated. Here, we investigated the influence of acid and bile acids on barrier function and TJ protein distribution using a newly developed air-liquid interface (ALI) in vitro culture model of stratified squamous epithelium based on primary human esophageal epithelial cells (HEECs). Under ALI conditions, HEECs formed distinct epithelial layers on Transwell inserts after 7 days of culture. The epithelial layers formed TJ, and the presence of claudin-1, claudin-4, and occludin were detected by immunofluorescent staining. The NP-40-insoluble fraction of these TJ proteins was significantly higher by day 7 of ALI culture. Exposure of HEECs to pH 2, and taurocholic acid (TCA) and glycocholic acid (GCA) at pH 3, but not pH 4, for 1 h decreased transepithelial electrical resistance (TEER) and increased paracellular permeability. Exposure of cell layers to GCA (pH 3) and TCA (pH 3) for 1 h also markedly reduced the insoluble fractions of claudin-1 and -4. We found that deoxycholic acid (pH 7.4 or 6, 1 h) and pepsin (pH 3, 24 h) significantly decreased TEER and increased permeability. Based on these findings, ALI-cultured HEECs represent a new in vitro model of human esophageal stratified epithelium and are suitable for studying esophageal epithelial barrier functions. Using this model, we demonstrated that acid, bile acids, and pepsin disrupt squamous epithelial barrier function partly by modulating TJ proteins. These results provide new insights into understanding the role of TJ proteins in esophagitis.

Acid modulates the squamous epithelial barrier function by modulating the localization of claudins in the superficial layers

Acid is a major cause of gastro-esophageal reflux disease. However, the influence of acid on the esophageal stratified epithelial barrier function and tight junction (TJ) proteins is not fully understood. Here, we explore the influence of acid on barrier function and TJ proteins using a newly developed model of the esophageal-like squamous epithelial cell layers that employs an air-liquid interface (ALI) system. Barrier function was determined by measuring trans-epithelial electrical resistance (TEER) and diffusion of paracellular tracers. TJ-related protein (claudin-1, claudin-4, occludin and ZO-1) expression and localization was examined by immunofluorescent staining, and by western blotting of 1% NP-40 soluble and insoluble fractions. We also examined the influence of acid (pH 2-4) on the barrier created by these cells. The in vitro ALI culture system showed a tight barrier (1500-2500 Ω·cm(2)) with the expression of claudin-1, claudin-4, occludin and ZO-1 in the superficial layers. Claudin-1, claudin-4, occludin and ZO-1 were detected as dots and whisker-like lines in the superficial layers, and as a broad line in the suprabasal layers. These localization patterns are similar to those in the human esophagus. On day 7 under ALI culture, TJ proteins were detected in the superficial layers with functional properties, including decreased permeability and increased TEER. Dilated intercellular spaces were detected at the suprabasal cell layers even under the control conditions of ALI cells. pH 2 acid on the apical side significantly reduced the TEER in ALI-cultured cells. This decrease in TEER by the acid was in parallel with the decreased amount of detergent-insoluble claudin-4. Claudin-4 delocalization was confirmed by immunofluorescent staining. In conclusion, TJs are located in the superficial layers of the esophagus, and acid stimulation disrupts barrier function, at least in part by modulating the amount and localization of claudin-4 in the superficial layers.

TGF-β1 mediates sirolimus and cyclosporine A-induced alteration of barrier function in renal epithelial cells via a noncanonical ERK1/2 signaling pathway

The immunosuppressant drugs cyclosporine A (CsA) and sirolimus (SRL) used in combination demonstrated beneficial effects in organ transplantation, but this combination can also result in increased adverse effects. We previously showed that not only CsA treatment but also its combination with SRL decreased paracellular permeability in renal proximal tubular cells by modification of the tight junction proteins, claudins, through ERK1/2 signaling pathway. In this present study, evidence is presented that not only CsA but also the combination of CsA/SRL may have adverse effects on the barrier function of renal proximal cells, at least in part, through the expression of the cytokine transforming growth factor (TGF)-β1. CsA treatment upregulated TGF-β1 gene expression and this upregulation was enhanced when CsA and SRL were applied together. Addition of TGF-β1 (5 ng/ml) altered the barrier function with increased transepithelial electrical resistance (TER) and claudin-1 expression. Use of a TGF-β1-blocking antibody or blockage of TGF-β1 receptor kinase activity with SD208 prevented the CsA- and CsA/SRL-induced increase in TER. No evidence was found in the present studies to indicate that CsA or CsA/SRL treatment activated the TGF-β1 Smad canonical signaling pathway, whereas addition of TGF-β1 (5 ng/ml) did activate the Smad pathway. Addition of the ERK1/2 signaling inhibitor U0126 was able to prevent the TGF-β1-mediated increase in TER and claudin expression. It is most likely that the CsA- and CsA/SRL-induced increases in TGF-β1 expression may not be sufficient to trigger the Smad pathway but however may trigger other TGF-β1 receptor-mediated signaling including the ERK1/2 signaling pathway.

Transcriptional regulation of claudin-18 via specific protein kinase C signaling pathways and modification of DNA methylation in human pancreatic cancer cells

Since claudin-18 (Cldn18) is overexpressed in precursor lesion PanIN and pancreatic duct carcinoma, it serves as a diagnostic marker and a target of immunotherapy. The stomach isoform of Cldn18, Cldn18a2 is regulated via a PKC/MAPK/AP-1-dependent pathway in PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA)-stimulated gastric cancer cells. However, little is known about how Cldn18 is regulated, not only in pancreatic duct carcinoma but also in normal human pancreatic duct epithelial cells (HPDE cells). In the present study, four pancreatic cancer cell lines, HPAF-II, HPAC, PANC-1 and BXPC3, and hTERT-HPDE cells in which the hTERT gene was introduced into HPDE cells in primary culture, were treated with TPA. In all human pancreatic cancer cell lines and hTERT-HPDE cells, Cldn18 mRNA indicated as Cldn18a2 was markedly induced by TPA and in well- or moderately differentiated human pancreatic cancer cells HPAF-II and HPAC and hTERT-HPDE cells, the protein was also strongly increased. The upregulation of Cldn18 by TPA in human pancreatic cancer cell lines was prevented by inhibitors of PKCδ, PKCε, and PKCα, whereas the upregulation of Cldn18 by TPA in hTERT-HPDE cells was prevented by inhibitors of PKCδ, PKCθ, and PKCα. Furthermore, a CpG island was identified within the coding sequence of the Cldn18 gene and treatment with the demethylating agent 5-azadeoxycytidine enhanced upregulation of Cldn18 by TPA in HPAF-II and HPAC, but not hTERT-HPDE cells. Our findings suggest that in human pancreatic cancer cells, Cldn18 is primarily regulated at the transcriptional level via specific PKC signaling pathways and modified by DNA methylation.

Acidic bile salts modulate the squamous epithelial barrier function by modulating tight junction proteins

Experimental models for esophageal epithelium in vitro either suffer from poor differentiation or complicated culture systems. An air-liquid interface system with normal human bronchial epithelial cells can serve as a model of esophageal-like squamous epithelial cell layers. Here, we explore the influence of bile acids on barrier function and tight junction (TJ) proteins. The cells were treated with taurocholic acid (TCA), glycocholic acid (GCA), or deoxycholic acid (DCA) at different pH values, or with pepsin. Barrier function was measured by transepithelial electrical resistance (TEER) and the diffusion of paracellular tracers (permeability). The expression of TJ proteins, including claudin-1 and claudin-4, was examined by Western blotting of 1% Nonidet P-40-soluble and -insoluble fractions. TCA and GCA dose-dependently decreased TEER and increased paracellular permeability at pH 3 after 1 h. TCA (4 mM) or GCA (4 mM) did not change TEER and permeability at pH 7.4 or pH 4. The combination of TCA and GCA at pH 3 significantly decreased TEER and increased permeability at lower concentrations (2 mM). Pepsin (4 mg/ml, pH 3) did not have any effect on barrier function. DCA significantly decreased the TEER and increased permeability at pH 6, a weakly acidic condition. TCA (4 mM) and GCA (4 mM) significantly decreased the insoluble fractions of claudin-1 and claudin-4 at pH 3. In conclusion, acidic bile salts disrupted the squamous epithelial barrier function partly by modulating the amounts of claudin-1 and claudin-4. These results provide new insights for understanding the role of TJ proteins in esophagitis.

ClC-2 regulates mucosal barrier function associated with structural changes to the villus and epithelial tight junction

We have previously shown an important role of the chloride channel ClC-2 in orchestrating repair of tight junctions in ischemia-injured mucosa. In this study, we examined the role of ClC-2 in regulating barrier function of normal murine intestinal mucosa. Ex vivo, ClC-2-/- ileal mucosa mounted in Ussing chambers had significantly higher transepithelial electrical resistance (TER) and reduced [(3)H]mannitol mucosal-to-serosal flux compared with wild-type (WT) mouse mucosa. We also noted that ileum from ClC-2-/- mice had a significantly reduced in vivo [(3)H]mannitol blood-to-lumen clearance compared with WT animals. By scanning electron microscopy, flat leaflike villi were found to have tapering, rounded apical tips in ClC-2-/- mucosa. By transmission electron microscopy, the apical intercellular tight junctions in ClC-2-/- intestine revealed lateral membranes that were less well defined but closely aligned compared with electron-dense and closely apposed tight junctions in WT mucosa. The width of apical tight junctions was significantly reduced in ClC-2-/- intestine. Such an alteration in tight junction ultrastructure was also noted in the testicular tissue from ClC-2-/- mice. The ClC-2-/- intestinal mucosa had reduced expression of phospho-myosin light chain (MLC), and inhibition of myosin light chain kinase (MLCK) in WT mucosa partially increased TER toward the TER in ClC-2-/- intestine. Contrary to our prior work on the reparative role of ClC-2 in injured mucosa, this study indicates that ClC-2 reduces barrier function in normal mucosa. The mechanisms underlying these differing roles are not entirely clear, although ultrastructural morphology of tight junctions and MLCK appear to be important to the function of ClC-2 in normal mucosa.

Transcriptional control of tight junction proteins via a protein kinase C signal pathway in human telomerase reverse transcriptase-transfected human pancreatic duct epithelial cells

In human pancreatic cancer, integral membrane proteins of tight junction claudins are abnormally regulated, making these proteins promising molecular diagnostic and therapeutic targets. However, the regulation of claudin-based tight junctions remains unknown not only in the pancreatic cancer cells but also in normal human pancreatic duct epithelial (HPDE) cells. To investigate the regulation of tight junction molecules including claudins in normal HPDE cells, we introduced the human telomerase reverse transcriptase (hTERT) gene into HPDE cells in primary culture. The hTERT-transfected HPDE (hTERT-HPDE) cells were positive for the pancreatic duct epithelial markers such as CK7, CK19, and carbonic anhydrase isozyme 2 and expressed epithelial tight junction molecules claudin-1, -4, -7 and, -18, occludin, JAM-A, ZO-1, ZO-2, and tricellulin. By treatment with fetal bovine serum or 12-O-tetradecanoylphorbol 13-acetate (TPA), the tight junction molecules were up-regulated at the transcriptional level via a protein kinase C (PKC) signal pathway. A PKC-alpha inhibitor, Gö6976, prevented up-regulation of claudin-4 by TPA. Furthermore, a PKC-delta inhibitor, rottlerin, prevented up-regulation of claudin-7, occludin, ZO-1, and ZO-2 by TPA. By GeneChip analysis, up-regulation of the transcription factor ELF3 was observed in both fetal bovine serum- and TPA-treated cells. Treatment with small interfering RNAs of ELF3 prevented up-regulation of claudin-7 by TPA. These data suggest that tight junctions of normal HPDE cells were at least in part regulated via a PKC signal pathway by transcriptional control.

Sirolimus and cyclosporine A alter barrier function in renal proximal tubular cells through stimulation of ERK1/2 signaling and claudin-1 expression

Alteration of the tight junction complex in renal epithelial cells can affect renal barrier function and perturb normal kidney homeostasis. The immunosuppressant drugs cyclosporine A (CsA) and sirolimus (SRL) used in combination demonstrated beneficial effects in organ transplantation but this combination can also result in increased adverse effects. We previously showed that CsA treatment alone caused an alteration of the tight junction complex, resulting in changes in transepithelial permeability in Madin-Darby canine kidney distal tubular/collecting duct cells. The potential effect of SRL on transepithelial permeability in kidney cells is unknown. In this study, subcytotoxic doses of SRL or CsA were found to decrease the paracellular permeability of the porcine proximal tubular epithelial cells, LLC-PK1 cell monolayers, which was detected as an increase in transepithelial electrical resistance (TER). The cotreatment with SRL and CsA was found to increase TER in a synergistic manner. CsA treatment increased total cellular expression and membrane localization of the tight junction protein claudin-1 and this further increased with the combination of SRL/CsA. SRL and CsA treatment alone or in combination stimulated the phosphorylation of ERK1/2. The MEK-ERK1/2 pathway inhibitor, U0126, reduced the SRL, CsA, and CsA/SRL-induced increase in TER. U0126 also reduced the CsA and CsA/SRL-induced increase in the membrane localization of claudin-1. Alterations in claudin-2 and claudin-4 were also detected. However, the results suggest that the modulation in expression and localization of claudin-1 appears to be pivotal in the SRL- and CsA-induced modulation of the epithelial barrier function and that modulation is regulated by ERK1/2 signaling pathway.

Disruptions of occludin and claudin-5 in brain endothelial cells in vitro and in brains of mice with acute liver failure

Brain edema in acute liver failure (ALF) remains lethal. The role of vasogenic mechanisms of brain edema has not been explored. We previously demonstrated that matrix metalloproteinase-9 (MMP-9) contributes to the pathogenesis of brain edema. Here, we show that MMP-9 mediates disruptions in tight junction (TJ) proteins in vitro and in brains of mice with ALF. We transfected murine brain endothelial cells (ECs) with MMP-9 complementary DNA (cDNA) using pc DNA3.1 (+)/Myc-His A expression vector. Tissue inhibitor of matrix metalloproteinases (TIMP-1) cDNA transfection or GM6001 was used to inhibit MMP-9. ALF was induced in mice with azoxymethane. Endogenous overexpression of MMP-9 in brain ECs resulted in significant degradation of the TJ proteins occludin and claudin-5. The alterations in TJ proteins correlated with increased permeability to fluorescein isothiocyanate-dextran molecules. The degradation of TJ proteins and the increased permeability were reversed by TIMP-1 and GM6001. Similar results were found when MMP-9 was exogenously added to brain ECs. We also found that TJ protein degradation was reversed with GM6001 in the brains of mice with ALF.

CONCLUSION

TJ proteins are significantly perturbed in brains of mice with ALF. These data corroborate the important role of MMP-9 in the vasogenic mechanism of brain edema in ALF.

Interleukin-25 expressed by brain capillary endothelial cells maintains blood-brain barrier function in a protein kinase Cepsilon-dependent manner

Interleukin (IL)-25, a member of the IL-17 family of cytokines, is expressed in the brains of normal mice. However, the cellular source of IL-25 and its function in the brain remain to be elucidated. Here, we show that IL-25 plays an important role in preventing infiltration of the inflammatory cells into the central nervous system. Brain capillary endothelial cells (BCECs) express IL-25. However, it is down-regulated by inflammatory cytokines, including tumor necrosis factor (TNF)-alpha, IL-17, interferon-gamma, IL-1beta, and IL-6 in vitro, and is also reduced in active multiple sclerosis (MS) lesions and in the inflamed spinal cord of experimental autoimmune encephalomyelitis, an animal model of MS. Furthermore, IL-25 restores the reduced expression of tight junction proteins, occludin, junction adhesion molecule, and claudin-5, induced by TNF-alpha in BCECs and consequently repairs TNF-alpha-induced blood-brain barrier (BBB) permeability. IL-25 induces protein kinase Cepsilon (PKCepsilon) phosphorylation, and up-regulation of claudin-5 is suppressed by PKCepsilon inhibitor peptide in the IL-25-stimulated BCECs. These results suggest that IL-25 is produced by BCECs and protects against inflammatory cytokine-induced excessive BBB collapse through a PKCepsilon-dependent pathway. These novel functions of IL-25 in maintaining BBB integrity may help us understand the pathophysiology of inflammatory brain diseases such as MS.

Protein kinase C activation disrupts epithelial apical junctions via ROCK-II dependent stimulation of actomyosin contractility

BACKGROUND

Disruption of epithelial cell-cell adhesions represents an early and important stage in tumor metastasis. This process can be modeled in vitro by exposing cells to chemical tumor promoters, phorbol esters and octylindolactam-V (OI-V), known to activate protein kinase C (PKC). However, molecular events mediating PKC-dependent disruption of epithelial cell-cell contact remain poorly understood. In the present study we investigate mechanisms by which PKC activation induces disassembly of tight junctions (TJs) and adherens junctions (AJs) in a model pancreatic epithelium.

RESULTS

Exposure of HPAF-II human pancreatic adenocarcinoma cell monolayers to either OI-V or 12-O-tetradecanoylphorbol-13-acetate caused rapid disruption and internalization of AJs and TJs. Activity of classical PKC isoenzymes was responsible for the loss of cell-cell contacts which was accompanied by cell rounding, phosphorylation and relocalization of the F-actin motor nonmuscle myosin (NM) II. The OI-V-induced disruption of AJs and TJs was prevented by either pharmacological inhibition of NM II with blebbistatin or by siRNA-mediated downregulation of NM IIA. Furthermore, AJ/TJ disassembly was attenuated by inhibition of Rho-associated kinase (ROCK) II, but was insensitive to blockage of MLCK, calmodulin, ERK1/2, caspases and RhoA GTPase.

CONCLUSION

Our data suggest that stimulation of PKC disrupts epithelial apical junctions via ROCK-II dependent activation of NM II, which increases contractility of perijunctional actin filaments. This mechanism is likely to be important for cancer cell dissociation and tumor metastasis.

Experimental oesophagitis in the rat is associated with decreased voluntary movement

Growing interest has arisen regarding the mechanism of dyspeptic symptom generation. However, no evaluation system of these symptoms in animals has been developed. In this study, we examined whether voluntary movement of rats could be a measure to assess visceral symptoms of reflux oesophagitis. A chronic acid reflux oesophagitis model was made using rats, and the size of erosions was measured. Omeprazole was administered to the oesophagitis rats for 10 days. The amount of voluntary movement was measured by an infrared sensor. Intracellular spaces in oesophageal epithelium were also measured using a emission electron microscope. NP-40 soluble and insoluble fractions of claudins were examined by Western blot. Voluntary movement was significantly lower in the oesophagitis model rats than in the sham-operated rats (P < 0.01). Although omeprazole reduced the size of erosions, it did not significantly affect the total amount of voluntary movement (r = -0.033, P = 0.916). Intracellular spaces were significantly dilated in the oesophagitis model rats and claudin-3 showed a significantly lower relative quantity in the NP-40 insoluble fraction. Omeprazole significantly increased voluntary movement of oesophagitis model rats and the relative quantity of claudin-3 in the insoluble fraction (P < 0.05). Dilated intercellular spaces and the lower level of claudin-3 may relate to the voluntary movement of oesophagitis model rats. Decreases in voluntary movement of oesophagitis model rats may reflect visceral symptoms and be able to serve as an index of chronic abdominal symptoms.

PKC and PKA phosphorylation affect the subcellular localization of claudin-1 in melanoma cells

Cytoplasmic expression of claudin-1 in metastatic melanoma cells correlates to increased migration, and increased secretion of MMP-2 in a PKC dependent manner, whereas claudin-1 nuclear expression is found in benign nevi. Melanoma cells were transfected with a vector expressing CLDN-1 fused to a nuclear localization signal (NLS). Despite significant nuclear localization of claudin-1, there was still transport of claudin-1 to the cytoplasm. Phorbol ester treatment of cells transfected with NLS-claudin-1 resulted in an exclusion of claudin-1 from the nucleus, despite the NLS. To ascertain whether PKC or PKA were involved in this translocation, we mutated the putative phosphorylation sites within the protein. We found that mutating the PKC phosphorylation sites to mimic a non-phosphorylated state did not cause a shift of claudin-1 to the nucleus of the cells, but mutating the PKA sites did. Mutations of either site to mimic constitutive phosphorylation resulted in cytoplasmic claudin-1 expression. Stable claudin-1 transfectants containing non-phosphorylatable PKA sites exhibited decreased motility. These data imply that subcellular localization of claudin-1 can be controlled by phosphorylation, dicating effects on metastatic capacity.

ClC-2 is required for rapid restoration of epithelial tight junctions in ischemic-injured murine jejunum

BACKGROUND AND AIMS

Involvement of the epithelial chloride channel ClC-2 has been implicated in barrier recovery following ischemic injury, possibly via a mechanism involving ClC-2 localization to the tight junction. The present study investigated mechanisms of intestinal barrier repair following ischemic injury in ClC-2(-/-) mice.

METHODS

Wild type, ClC-2 heterozygous and ClC-2(-/-) murine jejunal mucosa was subjected to complete ischemia, after which recovery of barrier function was monitored by measuring in vivo blood-to-lumen clearance of (3)H-mannitol. Tissues were examined by light and electron microscopy. The role of ClC-2 in re-assembly of the tight junction during barrier recovery was studied by immunoblotting, immunolocalization and immunoprecipitation.

RESULTS

Following ischemic injury, ClC-2(-/-) mice had impaired barrier recovery compared to wild type mice, defined by increases in epithelial paracellular permeability independent of epithelial restitution. The recovering ClC-2(-/-) mucosa also had evidence of ultrastructural paracellular defects. The tight junction proteins occludin and claudin-1 shifted significantly to the detergent soluble membrane fraction during post-ischemic recovery in ClC-2(-/-) mice whereas wild type mice had a greater proportion of junctional proteins in the detergent insoluble fraction. Occludin was co-immunoprecipitated with ClC-2 in uninjured wild type mucosa, and the association between occludin and ClC-2 was re-established during ischemic recovery. Based on immunofluorescence studies, re-localization of occludin from diffuse sub-apical areas to apical tight junctions was impaired in ClC-2(-/-) mice.

CONCLUSIONS

These data demonstrate a pivotal role of ClC-2 in recovery of the intestinal epithelium barrier by anchoring assembly of tight junctions following ischemic injury.

LXA4 stimulates ZO-1 expression and transepithelial electrical resistance in human airway epithelial (16HBE14o-) cells

Lipoxin A(4) (LXA(4)) is a biologically active eicosanoid produced in human airways that displays anti-inflammatory properties. In cystic fibrosis and severe asthma, LXA(4) production has been reported to be decreased, and, in such diseases, one of the consequences of airway inflammation is disruption of the tight junctions. In the present study, we investigated the possible role of LXA(4) on tight junction formation, using transepithelial electrical resistance (TER) measurements, Western blotting, and immunofluorescence. We observed that exposure to LXA(4) (100 nM) for 2 days significantly increased zonula occludens-1 (ZO-1), claudin-1, and occludin expression at the plasma membrane of confluent human bronchial epithelial 16HBE14o- cells. LXA(4) (100 nM) stimulated the daily increase of the 16HBE14o- cell monolayer TER, and this effect was inhibited by boc-2 (LXA(4) receptor antagonist). LXA(4) also had a rapid effect on ZO-1 immunofluorescence at the plasma membrane and increased TER within 10 min. In conclusion, our experiments provide evidence that LXA(4) plays certainly a new role for the regulation of tight junction formation and stimulation of the localization and expression of ZO-1 at the plasma membrane through a mechanism involving the LXA(4) receptor.

Mice lacking the Na+/H+ exchanger 2 have impaired recovery of intestinal barrier function

Ischemic injury induces breakdown of the intestinal barrier. Recent studies in porcine postischemic tissues indicate that inhibition of NHE2 results in enhanced recovery of barrier function in vitro via a process involving interepithelial tight junctions. To further study this process, recovery of barrier function was assessed in wild-type (NHE2(+/+)) and NHE2(-/-) mice in vivo and wild-type mice in vitro. Mice were subjected to complete mesenteric ischemia in vivo, after which barrier function was measured by blood-to-lumen mannitol clearance over a 3-h recovery period or measurement of transepithelial electrical resistance (TER) in Ussing chambers immediately following ischemia. Tissues were assessed for expression of select junctional proteins. Compared with NHE2(+/+) mice, NHE2(-/-) mice had greater intestinal permeability during the postischemic recovery process. In contrast to prior porcine studies, pharmacological inhibition of NHE2 in postischemic tissues from wild-type mice also resulted in significant reductions in TER. Mucosa from NHE2(-/-) mice displayed a shift of occludin and claudin-1 expression to the Triton-X-soluble membrane fractions and showed disruption of occludin and claudin-1 localization patterns following injury. This was qualitatively and quantitatively recovered in NHE2(+/+) mice compared with NHE2(-/-) mice by the end of the 3-h recovery period. Serine phosphorylation of occludin and claudin-1 was downregulated in NHE2(-/-) postischemia compared with wild-type mice. These data indicate an important role for NHE2 in recovery of barrier function in mice via a mechanism involving tight junctions.

Crosstalk of tight junction components with signaling pathways

Tight junctions (TJs) regulate the passage of ions and molecules through the paracellular pathway in epithelial and endothelial cells. TJs are highly dynamic structures whose degree of sealing varies according to external stimuli, physiological and pathological conditions. In this review we analyze how the crosstalk of protein kinase C, protein kinase A, myosin light chain kinase, mitogen-activated protein kinases, phosphoinositide 3-kinase and Rho signaling pathways is involved in TJ regulation triggered by diverse stimuli. We also report how the phosphorylation of the main TJ components, claudins, occludin and ZO proteins, impacts epithelial and endothelial cell function.

Role for TGF-β in Cyclosporine-Induced Modulation of Renal Epithelial Barrier Function

It was previously shown that cyclosporine A (CsA) increases transepithelial resistance in MDCK cells. Activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) mitogen-activated protein kinase (MAPK) cascade seems to be pivotal to the CsA-induced increase in transepithelial electrical resistance (TER). This study examined the role played by TGF-beta in mediating the CsA-induced activation of ERK1/2 and the resulting increase in TER in MDCK cells. Paracellular permeability across MDCK monolayers after various treatments was assessed by measurement of TER. TGF-beta secretion was measured by Western blot and ELISA. Activation of the ERK1/2 pathway and tight junction protein expression were also assessed by Western blot analysis. CsA increased production and secretion of TGF-beta and expression of the TGF-beta receptor II. Exogenous addition of TGF-beta1 activated ERK1/2 and increased TER across MDCK monolayers, both of which were attenuated by the MEK inhibitor U0126. Neutralizing antibodies against TGF-beta1 and the TGF-beta receptor II significantly reduced the CsA-induced increase in TER. Both CsA and TGF-beta1 increased expression of tight junction proteins claudin-1 and zonula occludens 2. Inhibition of the p38 MAPK pathway also attenuated the TGF-beta1-induced increase in TER. The results presented here suggest that the CsA-induced modulation of paracellular permeability may be mediated, at least in part, by an increase in TGF-beta production.

Gastrointestinal development and meeting the nutritional needs of premature infants

The fear of necrotizing enterocolitis and feeding intolerance are major factors inhibiting the use of the enteral route as the primary means of nourishing premature infants. Parenteral nutrition may help to meet many of the nutritional needs of these infants, but has significant detrimental side effects that include intestinal atrophy, sepsis, and increased susceptibility to inflammatory stimuli and systemic inflammatory responses. Being able to minimize the use of the parenteral route and still maintain appropriate nutrition safely would be a major advance in neonatology. At the basis of our inability to use the enteral route is a poorly understood immature gastrointestinal tract. Approaches such as minimal enteral nutrition or trophic feedings may partially alleviate these problems. However, if we are to progress in greater utilization of the gastrointestinal tract, other factors need to be considered. These include the macronutrient composition of minimal enteral or trophic feedings and the microecology of the intestinal lumen. Some of the developmental aspects of the intestine, which include intestinal growth, motor activity, barrier and other innate immune functions, and the microecology of the developing intestine, are briefly reviewed here. The purpose of this review is to suggest important areas of future research in neonatal and developmental gastroenterology that could affect several conditions that are related to immaturity of the gastrointestinal tract.

Chronic PKC-beta2 activation in HT-29 Cl.19a colonocytes prevents cAMP-mediated ion secretion by inhibiting apical membrane CFTR targeting

We investigated the effects of chronically applied PKC-stimulating phorbol esters on subcellular CFTR expression and localization in polarized HT-29 Cl.19A monolayers. Modulation of PKC activity with the PKC-beta-specific agonist 12-deoxyphorbol 13-phenylacetate 20-acetate (DOPPA) or nonisoform-selective PMA altered monolayer CFTR immunofluorescence. A decrease in the CFTR signal within the luminal cellular pole was noted with both phorbol esters. Volumetric analysis of the intracellular CFTR signal revealed that both compounds promoted CFTR accumulation into punctate vesicle-like structures found adjacent to the cellular tight junction [labeled with zona occludens (ZO)-1 antibody], extending basally (DOPPA) into the cell. Puncta were more frequent with DOPPA and larger in size with PMA. DOPPA also promoted ZO-1 accumulation at tricellular corners associated with enhanced CFTR puncta number. The observed loss of CFTR immunofluorescence signal induced by low-dose PMA was related to CFTR sequestration into fewer cytoplasmic puncta and correlated with larger increases in PKC substrate phosphorylation. Both phorbol esters downregulated steady-state cellular CFTR mRNA levels by 70%. However, the effects of DOPPA and PMA were largely independent of CFTR biosynthesis: expression levels were 80-85% of control, and the glycosylation status of immunoprecipitated protein remained largely unchanged. Thus changes in cellular CFTR localization correlated with our companion study showing that PMA-induced inhibition of transcellular cAMP-dependent short-circuit current (ISC) was accompanied by cytoplasmic PKC-beta2 accumulation and modest activation of PKC-beta1 and PKC-epsilon. The inhibitory effect of DOPPA on ISC was related solely to increased cytoplasmic PKC-beta2 levels. Thus PKC-beta2 is hypothesized to participate in the regulation of CFTR apical plasma membrane targeting within the constitutive cellular biosynthetic pathway.

Transforming growth factor-beta regulation of epithelial tight junction proteins enhances barrier function and blocks enterohemorrhagic Escherichia coli O157:H7-induced increased permeability

Enterohemorrhagic Escherichia coli O157:H7 (EHEC) is an enteric pathogen that causes potentially fatal symptoms after intimate adhesion, modulation of intestinal epithelial signal transduction, and alteration of epithelial function (eg, barrier disruption). Although the epithelial barrier is critical to gut homeostasis, only a few agents, such as transforming growth factor (TGF)-beta, can enhance or protect epithelial barrier function. Our aims were to delineate the mechanism(s) behind TGF-beta-induced barrier enhancement and to determine whether TGF-beta could prevent EHEC-induced barrier disruption. Using monolayers of the human T84 colonic epithelial cell line, we found that TGF-beta induced a significant increase in transepithelial electrical resistance (a measure of paracellular permeability) through activation of ERK MAPK and SMAD signaling pathways and up-regulation of the tight junction protein claudin-1. Additionally, TGF-beta pretreatment of epithelia blocked the decrease in transepithelial electrical resistance and the increase in transepithelial passage of [(3)H]-mannitol caused by EHEC infection. EHEC infection was associated with reduced expression of zonula occludens-1, occludin, and claudin-2 (but not claudin-1 or claudin-4); TGF-beta pretreatment prevented these changes. These studies provide insight into EHEC pathogenesis by illustrating the mechanisms underlying TGF-beta-induced epithelial barrier enhancement and identifying TGF-beta as an agent capable of blocking EHEC-induced increases in epithelial permeability via maintenance of claudin-2, occludin, and zonula occludens-1 levels.

Changes in tight junctional resistance of the cervical epithelium are associated with modulation of content and phosphorylation of occludin 65-kilodalton and 50-kilodalton forms

Treatment of human cervical epithelial CaSki cells with ATP or with the diacylglyceride sn-1,2-dioctanoyl diglyceride (diC8) induced a staurosporine-sensitive transient increase, followed by a late decrease, in tight-junctional resistance (R(TJ)). CaSki cells express two immunoreactive forms of occludin, 65 and 50 kDa. Treatments with ATP and diC8 decreased the density of the 65-kDa form and increased the density of the 50-kDa form. ATP also decreased threonine phosphorylation of the 65-kDa form and increased threonine phosphorylation of the 50-kDa form and tyrosine phosphorylation of the 65- and 50-kDa forms. Staurosporine decreased acutely threonine and tyrosine phosphorylation of the two isoforms and in cells pretreated with staurosporine ATP increased acutely the density of the 65-kDa form and threonine phosphorylation of the 65-kDa form. Treatment with N-acetyl-leucinyl-leucinyl-norleucinal increased the densities of the 65- and 50-kDa forms. Pretreatment with N-acetyl-leucinyl-leucinyl-norleucinal attenuated the late decreases in R(TJ) induced by ATP and diC8 and the decrease in the 65-kDa and increase in the 50-kDa forms induced by ATP. Correlation analyses showed that high levels of R(TJ) correlated with the 65-kDa form, whereas low levels of R(TJ) correlated negatively with the 65-kDa form and positively with the 50-kDa form. The results suggest that in CaSki cells 1) occludin determines gating of the tight junctions, 2) changes in occludin phosphorylation status and composition regulate the R(TJ), 3) protein kinase-C-mediated, threonine dephosphorylation of the 65-kDa occludin form increases the resistance of assembled tight junctions, 4) the early stage of tight junction disassembly involves calpain-mediated breakdown of occludin 65-kDa form to the 50-kDa form, and 5) increased levels of the 50-kDa form interfere with occludin gating of the tight junctions.

theta Isoform of protein kinase C alters barrier function in intestinal epithelium through modulation of distinct claudin isotypes: a novel mechanism for regulation of permeability

Using monolayers of intestinal Caco-2 cells, we discovered that the isoform of protein kinase C (PKC), a member of the "novel" subfamily of PKC isoforms, is required for monolayer barrier function. However, the mechanisms underlying this novel effect remain largely unknown. Here, we sought to determine whether the mechanism by which PKC- disrupts monolayer permeability and dynamics in intestinal epithelium involves PKC--induced alterations in claudin isotypes. We used cell clones that we recently developed, clones that were transfected with varying levels of plasmid to either stably suppress endogenous PKC- activity (antisense, dominant-negative constructs) or to ectopically express PKC- activity (sense constructs). We then determined barrier function, claudin isotype integrity, PKC- subcellular activity, claudin isotype subcellular pools, and claudin phosphorylation. Antisense transfection to underexpress the PKC- led to monolayer instability as shown by reduced 1) endogenous PKC- activity, 2) claudin isotypes in the membrane and cytoskeletal pools ( downward arrowclaud-1, downward arrowclaud-4 assembly), 3) claudin isotype phosphorylation ( downward arrow phospho-serine, downward arrow phospho-threonine), 4) architectural stability of the claudin-1 and claudin-4 rings, and 5) monolayer barrier function. In these antisense clones, PKC- activity was also substantially reduced in the membrane and cytoskeletal cell fractions. In wild-type (WT) cells, PKC- (82 kDa) was both constitutively active and coassociated with claudin-1 (22 kDa) and claudin-4 (25 kDa), forming endogenous PKC-/claudin complexes. In a second series of studies, dominant-negative inhibition of the endogenous PKC- caused similar destabilizing effects on monolayer barrier dynamics, including claudin-1 and -4 hypophosphorylation, disassembly, and architectural instability as well as monolayer disruption. In a third series of studies, sense overexpression of the PKC- caused not only a mostly cytosolic distribution of this isoform (i.e., <12% in the membrane + cytoskeletal fractions, indicating PKC- inactivity) but also led to disruption of claudin assembly and barrier function of the monolayer. The conclusions of this study are that PKC- activity is required for normal claudin assembly and the integrity of the intestinal epithelial barrier. These effects of PKC- are mediated at the molecular level by changes in phosphorylation, membrane assembly, and/or organization of the subunit components of two barrier function proteins: claudin-1 and claudin-4 isotypes. The ability of PKC- to alter the dynamics of permeability protein claudins is a new function not previously ascribed to the novel subfamily of PKC isoforms.

The role of protein kinase C isoforms in modulating injury and repair of the intestinal barrier

Gastrointestinal cells express a diverse group of protein kinase C (PKC) isoforms that play critical roles in a number of cell functions, including intracellular signaling and barrier integrity. PKC isoforms expressed by gastrointestinal epithelial cells consist of three major PKC subfamilies: conventional isoforms (alpha, beta1, beta2, and gamma), novel isoforms (delta, epsilon, theta, eta, and mu), and atypical isoforms (lambda, tau, and zeta). This review highlights recent discoveries, including our own, that some PKC isoforms in gastrointestinal epithelia monolayer cell culture are involved in injury to, whereas others are involved in protection of, intestinal barrier integrity. For example, certain PKC isoforms aggravate oxidative damage, whereas others protect against it. These findings suggest that the development of agents that selectively activate or inhibit specific PKC isoforms may lead to new therapeutic modalities for important gastrointestinal disorders such as cancer and inflammatory bowel disease.

Role of protein kinase C in intestinal ischemic preconditioning

INTRODUCTION

Tissue protection by ischemic preconditioning (IPC) has been previously characterized in organs such as the heart and involves at least in part PKC activation. It is not yet clear whether such preconditioning against ischemia/reperfusion (I/R) injury operates in the intestine, and, if so, whether IPC involves protein kinase C (PKC).

MATERIALS AND METHODS

IPC of the small intestine in male Sprague Dawley rats was induced by 10-min superior mesenteric artery (SMA) clamp followed by 120-min reperfusion. Sham-operated control or IPC rats were then rechallenged with 20-min SMA clamp. Histological injury to jejunal mucosa was assessed by microscopic examination and Parks' injury score (Grade 0-4; 0 = no damage). PKC activity was determined by immunoprecipitation of specific isoforms followed by in vitro kinase assay using mucosal scrapings of the harvested jejunum. Data were expressed as mean +/- SEM and analyzed by one-way ANOVA with multiple comparison tests.

RESULTS

Ten-minute SMA clamp led to epithelial damage that was fully reversed by 120-min reperfusion. Activity of several PKC isoforms (PKCalpha, -delta, -epsilon) increased after 10-min ischemia. Epithelial injury associated with 20-min SMA clamp was attenuated by prior IPC. The protective effect of IPC on intestinal mucosa was prevented when animals were pretreated with the conventional (c) and novel (n) PKC inhibitor Go6850, but not with Go6976 (selective cPKC inhibitor), rottlerin (selective PKCdelta inhibitor), or saline control.

CONCLUSIONS

Brief mesenteric ischemia induces a reversible epithelial injury in rats associated with activation of several PKC isoforms. Injury induced by mesenteric ischemia is reduced by brief ischemic preconditioning, an effect that is abolished by nonselective PKC inhibition but not by a selective inhibitor of cPKC or PKCdelta. The results suggest that activation of nPKC isoform(s), especially PKCepsilon during and following ischemic insults (IPC), may play an important role in protection against I/R injury in the intestine.

Barriers built on claudins

The fundamental functions of epithelia and endothelia in multicellular organisms are to separate compositionally distinct compartments and regulate the exchange of small solutes and other substances between them. Tight junctions (TJs) between adjacent cells constitute the barrier to the passage of ions and molecules through the paracellular pathway and function as a 'fence' within the plasma membrane to create and maintain apical and basolateral membrane domains. How TJs achieve this is only beginning to be understood. Recently identified components of TJs include the claudins, a family of four-transmembrane-span proteins that are prime candidates for molecules that function in TJ permeability. Their identification and characterization have provided new insight into the diversity of different TJs and heterogeneity of barrier functions in different epithelia and endothelia.

Glutamine regulates Caco-2 cell tight junction proteins

Intestinal epithelial tight junction (TJ) barrier dysfunction may lead to inflammation and mucosal injury. Glutamine (GLN) plays a role in maintenance of intestinal barrier function in various animal models and critically ill humans. Recent evidence from intestinal cell monolayers indicates that GLN maintains transepithelial resistance and decreases permeability. The mechanisms of these effects remain undefined. We hypothesized that GLN affects proteins involved in the intercellular junctional complex. GLN availability was controlled in Caco-2 monolayers by addition to the medium and treatment with methionine sulfoximine (MSO) to inhibit glutamine synthetase (GS). Expression of TJ proteins, claudin-1, occludin, and zonula occluden (ZO)-1 was measured by immunoblotting. Localization of TJ proteins was evaluated by immunofluorescence light microscopy. Structure of TJ was determined by transmission electron microscopy (TEM). Deprivation of GLN decreased claudin-1, occludin, and ZO-1 protein expression and caused a disappearance of perijunctional claudin-1 and a reduction of occludin but had no effect on ZO-1. TEM revealed that MSO-treated cells in the absence of GLN formed irregular junctional complexes between the apical lateral margins of adjoining cells. These findings indicate that TJ protein expression and cellular localization in Caco-2 cell monolayers rely on GLN. This mechanism may similarly relate to GLN-mediated modulation of intestinal barrier function in stressed animals and humans.

Disruption of hepatocellular tight junctions by vascular endothelial growth factor (VEGF): a novel mechanism for tumor invasion

BACKGROUND/AIMS

Vascular endothelial growth factor (VEGF) is expressed by many tumors, including hepatocellular carcinoma (HCC) and is involved in tumor angiogenesis. Little is known about its role for HCC infiltration into normal liver parenchyma.

METHODS

The effects of VEGF on the integrity of tight junctions were studied in HepG2 cells and human HCC by means of confocal laser scanning microscopy.

RESULTS

VEGF induced within 45 min a marked loss of pseudocanaliculi and disruption of occludin-delineated tight junctions. This effect of VEGF was mimicked by phorbol-12-myristate-13-acetate (PMA) and was sensitive to protein kinase C (PKC) inhibition by Gö6850. VEGF induced within 15 min the translocation of the PKC alpha-isoform to the plasma-membrane, but had no effect on the activity of Erks and p38(MAPK). Sections from surgically removed HCC showed expression of VEGF in the tumor and occludin disassembly in normal liver parenchyma next to the tumor.

CONCLUSIONS

VEGF induces disruption of tight junctions in a PKC-alpha dependent manner. In addition to its known angioneogenic properties, VEGF may promote HCC spreading into normal liver parenchyma. The data may provide another rationale for the use of VEGF antagonists for tumor therapy.

Epidermal growth factor receptor activation differentially regulates claudin expression and enhances transepithelial resistance in Madin-Darby canine kidney cells

Tight junctions (TJs) are the most apical cell-cell junctions, and claudins, the recently identified TJ proteins, are critical for maintaining cell-cell adhesion in epithelial cell sheets. Based on their in vivo distribution and the results of overexpression studies, certain claudins, including claudin-1 and -4, are postulated to increase, whereas other claudins, especially claudin-2, are postulated to decrease the overall transcellular resistance. The overall ratio among claudins expressed in a cell/tissue has been hypothesized to define the complexity of TJs. Disruption of the TJs contributes to various human diseases, and a correlation between reduction of TJ function and tumor dedifferentiation has been postulated. The epidermal growth factor (EGF) receptor (EGFR) is overexpressed in a wide spectrum of epithelial cancers, and its expression correlates with a more metastatic cancer phenotype. However, normal functioning of EGFR is essential for normal epithelial cell proliferation and differentiation. The role of EGFR-dependent signaling in the development and maintenance of epithelial TJ integrity has not been studied in detail. This study demonstrates that, in polarized Madin-Darby canine kidney II cells, EGF-induced EGFR activation significantly inhibited claudin-2 expression while simultaneously inducing cellular redistribution and increased expression of claudin-1, -3, and -4. Accompanying these EGF-induced changes in claudin expression was a 3-fold increase in transepithelial resistance, a functional measure of TJs. In contrast, there were no alterations in protein expression and/or intracellular localization of other TJ-related proteins (ZO-1 and occludin) or adherens junction-associated proteins (E-cadherin and beta-catenin), suggesting that EGF regulates TJ function through selective and differential regulation of claudins.