Overexpression of protein kinase C-delta increases tight junction permeability in LLC-PK1 epithelia

Ophiopogon Saponin C1 Inhibits Lung Tumors by Stabilizing Endothelium Permeability via Inhibition of PKCδ

As the most frequent cause of cancer-related death worldwide, lung cancer is closely related to inflammation. The interaction between tumor cells and inflammatory cells promotes tumor development and metastasis. During tumor development, vascular endothelial cells form the most important barrier to prevent tumor cell migration to the blood and tissue. Increased vascular permeability provides favorable conditions for the migration of tumor cells, and endothelial tight junctions are an important component of the vascular barrier. Protein kinase C δ is involved in the occurrence of non-small cell lung cancer and regulates vascular permeability and tight junction protein expression. Src kinase was reported to play an important role in TNF-α-induced endothelial inflammation. Ophiopogon Saponin C1 is a new chemical compound isolated from Liriope muscari, but its pharmacological activities have not been fully elucidated. Therefore, we tested the protective effects of C1 on endothelial permeability in a model of TNF-α-induced endothelial inflammation by transendothelial electrical resistance and sodium fluorescein assays and verified these results in a nude mouse model of experimental pulmonary adenocarcinoma metastasis. We further elucidated the mechanism of C1, which was based on the PKCδ and Src proteins, by Western blotting. C1 can inhibit lung cancer in vivo, regulate the level of plasma inflammation in tumor-bearing mice, and protect the pulmonary vascular barrier against injury induced by cancer. It was investigated the expression and distribution of the TJ index protein ZO-1 in mouse vascular endothelium and HUVECs and found that C1 could inhibit the degradation and breakage of the ZO-1 protein. Related signaling experiments confirmed that C1 can inhibit TNF-α and activation of PKCδ and Src kinase. This study laid the foundation for further analysis of new drugs with clear mechanisms and independent intellectual property rights of traditional Chinese medicines.

Regulation of the intestinal tight junction by natural polyphenols: A mechanistic perspective

Impairment of the epithelial barrier function is closely linked to the pathogenesis of various gastrointestinal diseases, food allergies, type I diabetes, and other systematic diseases. Plant-derived polyphenols are natural secondary metabolites and exert various physiological benefits, including anti-inflammatory, anti-oxidative, anti-carcinogenic, and anti-aging effects. Recent studies also show the role of plant polyphenols in regulation of the intestinal barrier and prevention of intestinal inflammatory diseases. Here we summarize the regulatory pathways and mediators linking polyphenols to their beneficial effects on tight junction and gut epithelial barrier functions, and provide useful information about using polyphenols as nutraceuticals for intestinal diseases.

Endotoxin induces leukocyte transmigration and changes in permeability of the airway epithelium via protein-kinase C and extracellular regulated kinase activation

Lipopolysaccharide (LPS) endotoxin of Gram-negative bacteria compromises the integrity of the airway epithelial barrier and initiates migration of leukocytes across the epithelium. The goal of the present study was to identify the role of extracellular regulated kinase (ERK1/2) transduction pathways in these processes. The first aim was to determine whether LPS induces ERK1/2 activation and changes in epithelial permeabilityin epithelial cells alone or only in the presence of immune cells. The second aim was to determine whether the changes in the epithelial permeability were diminished by ERK1/2 blockade. The third aim was to investigatethe role of protein kinase C (PKC) activation as an upstream event in activation of ERK1/2. In vitro 20 μg/ml LPS challenge reduced epithelial barrier function, and induced ERK1/2 phosphorylation in primary cultures of bovine tracheal epithelium and in the transformed human airway epithelial cell line, Calu-3. LPS initiated migration of neutrophil-like and monocyte-like transformed HL-60 cell across sheets of Calu-3 cells. The migration rate and the associated changes in the electrical resistance, permeability to albumin, and ERK1/2 phosphorylation were all blocked by calphostin C, the specific blocker of PKC and by PD98059 (2'-amino-3'methoxyflavone), a selective cell-permeable inhibitor of MAP kinase kinase. In rats, in vivo perfusion of the lumen of an isolated segment of trachea with LPS (0.1 mg/ml) initiated migration of neutrophils and increased the permeability to albumin. Again, these effects were markedly inhibited by PD98059 and calphostin C (by > 50%). We conclude that epithelial ERK1/2 is activated by endotoxin via PKC and is an important pathway in regulation of epithelial permeability.

Regulation of Thrombin-Induced Lung Endothelial Cell Barrier Disruption by Protein Kinase C Delta

Protein Kinase C (PKC) plays a significant role in thrombin-induced loss of endothelial cell (EC) barrier integrity; however, the existence of more than 10 isozymes of PKC and tissue-specific isoform expression has limited our understanding of this important second messenger in vascular homeostasis. In this study, we show that PKCδ isoform promotes thrombin-induced loss of human pulmonary artery EC barrier integrity, findings substantiated by PKCδ inhibitory studies (rottlerin), dominant negative PKCδ construct and PKCδ silencing (siRNA). In addition, we identified PKCδ as a signaling mediator upstream of both thrombin-induced MLC phosphorylation and Rho GTPase activation affecting stress fiber formation, cell contraction and loss of EC barrier integrity. Our inhibitor-based studies indicate that thrombin-induced PKCδ activation exerts a positive feedback on Rho GTPase activation and contributes to Rac1 GTPase inhibition. Moreover, PKD (or PKCμ) and CPI-17, two known PKCδ targets, were found to be activated by PKCδ in EC and served as modulators of cytoskeleton rearrangement. These studies clarify the role of PKCδ in EC cytoskeleton regulation, and highlight PKCδ as a therapeutic target in inflammatory lung disorders, characterized by the loss of barrier integrity, such as acute lung injury and sepsis.

Bile duct epithelial tight junctions and barrier function

Bile ducts play a crucial role in the formation and secretion of bile as well as excretion of circulating xenobiotic substances. In addition to its secretory and excretory functions, bile duct epithelium plays an important role in the formation of a barrier to the diffusion of toxic substances from bile into the hepatic interstitial tissue. Disruption of barrier function and toxic injury to liver cells appear to be involved in the pathogenesis of a variety of liver diseases such as primary sclerosing cholangitis, primary biliary cirrhosis and cholangiocarcinoma. Although the investigations into understanding the structure and regulation of tight junctions in gut, renal and endothelial tissues have expanded rapidly, very little is known about the structure and regulation of tight junctions in the bile duct epithelium. In this article we summarize the current understanding of physiology and pathophysiology of bile duct epithelium, the structure and regulation of tight junctions in canaliculi and bile duct epithelia and different mechanisms involved in the regulation of disruption and protection of bile duct epithelial tight junctions. This article will make a case for the need of future investigations toward our understanding of molecular organization and regulation of canalicular and bile duct epithelial tight junctions.

Zinc enhancement of LLC-PK(1) renal epithelial barrier function

BACKGROUND AND AIMS

Earlier work by our group and others has documented improvement of epithelial barrier function in human gastrointestinal models. Here we tested zinc's ability to improve a renal epithelial model. Our aim was to compare the functional and structural effects of zinc on the tight junctional (TJ) complexes of these two very distinct epithelial cell types. Zinc's ability to achieve barrier enhancement in very different epithelial cell types by action upon distinct molecular targets in each epithelial model may suggest a fundamental general role for supplemental zinc in epithelial barrier improvement throughout the body.

METHODS

Cell layers were exposed to 50 or 100 μM zinc on both cell surfaces for 48 h followed by measurement of transepithelial electrical resistance (Rt) and transepithelial (14)C-mannitol flux (Jm). TJ proteins in cell layers were analyzed by Western immunoblot.

RESULTS AND CONCLUSIONS

Zinc supplementation improved the basal TJ barrier function of LLC-PK1 renal cell layers, exemplified by increased Rt and decreased Jm. These zinc-induced changes were also accompanied by decreased NaCl dilution potentials. Of the tight junctional proteins that were tested (occludin, claudins 1, 2, 3, 4, and 5, and tricellulin), we did not observe a zinc-induced change in abundance of any of them, in detergent-soluble fractions of lysates of confluent differentiated cell layers. However, examination of cytosolic fractions showed concentration-dependent increases in the levels of claudins -2 and -4 in this compartment as a result of supplemental zinc. The effects of supplemental zinc on the tight junctional complexes and barrier properties of this renal epithelial model are contrasted with zinc effects on the CACO-2 gastrointestinal model.

Effects of Lactobacillus plantarum on gut barrier function in experimental obstructive jaundice

AIM: To investigate the mechanisms of Lactobacillus plantarum (L. plantarum) action on gut barrier in preoperative and postoperative experimental obstructive jaundice in rats.

METHODS: Forty rats were randomly divided into groups of sham-operation, bile duct ligation (BDL), BDL + L. plantarum, BDL + internal biliary drainage (IBD), and BDL + IBD + L. plantarum. Ten days after L. plantarum administration, blood and ileal samples were collected from the rats for morphological examination, and intestinal barrier function, liver function, intestinal oxidative stress and protein kinase C (PKC) activity measurement. The distribution and expression of the PKC and tight junction (TJ) proteins, such as occludin, zonula occludens-1, claudin-1, claudin-4, junction adhesion molecule-A and F-actin, were examined by confocal laser scanning microscopy, immunohistochemistry, Western blotting, real-time fluorescent quantitative polymerase chain reaction assay.

RESULTS: L. plantarum administration substantially restored gut barrier, decreased enterocyte apoptosis, improved intestinal oxidative stress, promoted the activity and expression of protein kinase (BDL vs BDL + L. plantarum, 0.295 ± 0.007 vs 0.349 ± 0.003, P < 0.05; BDL + IBD vs BDL + IBD + L. plantarum, 0.407 ± 0.046 vs 0.465 ± 0.135, P < 0.05), and particularly enhanced the expression and phosphorylation of TJ proteins in the experimental obstructive jaundice (BDL vs BDL + L. plantarum, 0.266 ± 0.118 vs 0.326 ± 0.009, P < 0.05). The protective effect of L. plantarum was more prominent after internal biliary drainage ( BDL + IBD vs BDL + IBD + L. plantarum, 0.415 ± 0.105 vs 0.494 ± 0.145, P < 0.05).

CONCLUSION: L. plantarum can decrease intestinal epithelial cell apoptosis, reduce oxidative stress, and prevent TJ disruption in biliary obstruction by activating the PKC pathway.

Differential targeting of the E-Cadherin/β-Catenin complex by gram-positive probiotic lactobacilli improves epithelial barrier function

The intestinal ecosystem is balanced by dynamic interactions between resident and incoming microbes, the gastrointestinal barrier, and the mucosal immune system. However, in the context of inflammatory bowel diseases (IBD), where the integrity of the gastrointestinal barrier is compromised, resident microbes contribute to the development and perpetuation of inflammation and disease. Probiotic bacteria have been shown to exert beneficial effects, e.g., enhancing epithelial barrier integrity. However, the mechanisms underlying these beneficial effects are only poorly understood. Here, we comparatively investigated the effects of four probiotic lactobacilli, namely, Lactobacillus acidophilus, L. fermentum, L. gasseri, and L. rhamnosus, in a T84 cell epithelial barrier model. Results of DNA microarray experiments indicating that lactobacilli modulate the regulation of genes encoding in particular adherence junction proteins such as E-cadherin and β-catenin were confirmed by quantitative reverse transcription-PCR (qRT-PCR). Furthermore, we show that epithelial barrier function is modulated by Gram-positive probiotic lactobacilli via their effect on adherence junction protein expression and complex formation. In addition, incubation with lactobacilli differentially influences the phosphorylation of adherence junction proteins and the abundance of protein kinase C (PKC) isoforms such as PKCδ that thereby positively modulates epithelial barrier function. Further insight into the underlying molecular mechanisms triggered by these probiotics might also foster the development of novel strategies for the treatment of gastrointestinal diseases (e.g., IBD).

Protein kinase Cζ phosphorylates occludin and promotes assembly of epithelial tight junctions

Protein kinases play an important role in the regulation of epithelial tight junctions. In the present study, we investigated the role of PKCζ (protein kinase Cζ) in tight junction regulation in Caco-2 and MDCK (Madin-Darby canine kidney) cell monolayers. Inhibition of PKCζ by a specific PKCζ pseudosubstrate peptide results in redistribution of occludin and ZO-1 (zona occludens 1) from the intercellular junctions and disruption of barrier function without affecting cell viability. Reduced expression of PKCζ by antisense oligonucleotide or shRNA (short hairpin RNA) also results in compromised tight junction integrity. Inhibition or knockdown of PKCζ delays calcium-induced assembly of tight junctions. Tight junction disruption by PKCζ pseudosubstrate is associated with the dephosphorylation of occludin and ZO-1 on serine and threonine residues. PKCζ directly binds to the C-terminal domain of occludin and phosphorylates it on threonine residues. Thr403, Thr404, Thr424 and Thr438 in the occludin C-terminal domain are the predominant sites of PKCζ-dependent phosphorylation. A T424A or T438A mutation in full-length occludin delays its assembly into the tight junctions. Inhibition of PKCζ also induces redistribution of occludin and ZO-1 from the tight junctions and dissociates these proteins from the detergent-insoluble fractions in mouse ileum. The present study demonstrates that PKCζ phosphorylates occludin on specific threonine residues and promotes assembly of epithelial tight junctions.

Lactobacillus plantarum inhibits intestinal epithelial barrier dysfunction induced by unconjugated bilirubin

Although a large number of in vitro and in vivo tests have confirmed that taking probiotics can improve the intestinal barrier, few studies have focused on the relationship between probiotics and the intestinal epithelial barrier in hyperbilirubinaemia. To investigate the effects of and mechanisms associated with probiotic bacteria (Lactobacillus plantarum; LP) and unconjugated bilirubin (UCB) on the intestinal epithelial barrier, we measured the viability, apoptotic ratio and protein kinase C (PKC) activity of Caco-2 cells. We also determined the distribution and expression of tight junction proteins such as occludin, zonula occludens (ZO)-1, claudin-1, claudin-4, junctional adhesion molecule (JAM)-1 and F-actin using confocal laser scanning microscopy, immunohistochemistry, Western blotting and real-time quantitative PCR. The present study demonstrated that high concentrations of UCB caused obvious cytotoxicity and decreased the transepithelial electrical resistance (TER) of the Caco-2 cell monolayer. Low concentrations of UCB inhibited the expression of tight junction proteins and PKC but could induce UDP-glucuronosyltransferases 1 family-polypeptide A1 (UGT1A1) expression. UCB alone caused decreased PKC activity, serine phosphorylated occludin and ZO-1 levels. After treatment with LP, the effects of UCB on TER and apoptosis were mitigated; LP also prevented aberrant expression and rearrangement of tight junction proteins. Moreover, PKC activity and serine phosphorylated tight junction protein levels were partially restored after treatment with LP, LP exerted a protective effect against UCB damage to Caco-2 monolayer cells, and it restored the structure and distribution of tight junction proteins by activating the PKC pathway. In addition, UGT1A1 expression induced by UCB in Caco-2 cells could ameliorate the cytotoxicity of UCB.

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.

Sustained pharmacological inhibition of deltaPKC protects against hypertensive encephalopathy through prevention of blood-brain barrier breakdown in rats

Hypertensive encephalopathy is a potentially fatal condition associated with cerebral edema and the breakdown of the blood-brain barrier (BBB). The molecular pathways leading to this condition, however, are unknown. We determined the role of deltaPKC, which is thought to regulate microvascular permeability, in the development of hypertensive encephalopathy using deltaV1-1 - a selective peptide inhibitor of deltaPKC. As a model of hypertensive encephalopathy, Dahl salt-sensitive rats were fed an 8% high-salt diet from 6 weeks of age and then were infused s.c. with saline, control TAT peptide, or deltaV1-1 using osmotic minipumps. The mortality rate and the behavioral symptoms of hypertensive encephalopathy decreased significantly in the deltaV1-1-treated group relative to the control-treated group, and BBB permeability was reduced by more than 60%. Treatment with deltaV1-1 was also associated with decreased deltaPKC accumulation in capillary endothelial cells and in the endfeet of capillary astrocytes, which suggests decreased microvasculature disruption. Treatment with deltaV1-1 prevented hypertension-induced tight junction disruption associated with BBB breakdown, which suggests that deltaPKC may specifically act to dysregulate tight junction components. Together, these results suggest that deltaPKC plays a role in the development of hypertension-induced encephalopathy and may be a therapeutic target for the prevention of BBB disruption.

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.

Adipose-specific effect of rosiglitazone on vascular permeability and protein kinase C activation: novel mechanism for PPARgamma agonist's effects on edema and weight gain

PPARgamma agonists, thiazolidinediones, cause fluid retention and edema due to unknown mechanisms. We characterized the effect of rosiglitazone (RSG), a thiazolidinedione, to induce vascular permeability, vascular endothelial growth factor (VEGF) expression, and protein kinase C (PKC) activation with edema and wt gain. In lean, fatty and diabetic Zucker rats, and endothelial insulin receptor knockout mice, RSG increased wt and vascular permeability, selectively in fat and retina, but not in heart or skeletal muscle. H2O content and wt of epididymal fat were increased by RSG and correlated to increases in capillary permeability in fat and body wt. RSG induced VEGF mRNA expression and PKC activation in fat and retina up to 2.5-fold. Ruboxistaurin, a PKCbeta isoform inhibitor, in the latter 2 wk of a 4-wk study, normalized vascular permeability in fat and decreased total wt gain, H2O content, and wt of fat vs. RSG alone but did not decrease VEGF expression, basal permeability, or food intake. Finally, RSG did not increase wt or vascular permeability in PKCbeta knockout vs. control mice. Thus, thiazolidinedione's effects on edema and wt are partially due to an adipose tissue-selective activation of PKC and vascular permeability that may be prevented by PKCbeta inhibition.

HIV-1 Tat protein-induced alterations of ZO-1 expression are mediated by redox-regulated ERK 1/2 activation

HIV-1 Tat protein plays an important role in inducing monocyte infiltration into the brain and may alter the structure and functions of the blood-brain barrier (BBB). The BBB serves as a frontline defense system, protecting the central nervous system from infected monocytes entering the brain. Therefore, the aim of the present study was to examine the mechanisms of Tat effect on the integrity of the BBB in the mouse brain. Tat was injected into the right hippocampi of C57BL/6 mice and expression of tight junction protein zonula occludens-1 (ZO-1) was determined in control and treated mice. Tat administration resulted in decreased mRNA levels of ZO-1 and marked disruption of ZO-1 continuity. These changes were associated with accumulation of inflammatory cells in brain tissue of Tat-treated mice. Further experiments indicated that Tat-mediated alterations of redox-related signaling may be responsible for decreased ZO-1 expression. Specifically, injections with Tat resulted in activation of extracellular signal-regulated kinase 1/2 (ERK 1/2) and pretreatment with U 0126, a specific inhibitor of ERK kinase, effectively ameliorated the Tat-induced diminished ZO-1 levels. In addition, administration of N-acetylcysteine (NAC), a precursor of glutathione and a potent antioxidant, attenuated both Tat-induced ERK 1/2 activation and alterations in ZO-1 expression. These results indicate that Tat-induced oxidative stress can play an important role in affecting the integrity of the BBB through the ERK 1/2 pathway.

Ras mutation impairs epithelial barrier function to a wide range of nonelectrolytes

Although ras mutations have been shown to affect epithelial architecture and polarity, their role in altering tight junctions remains unclear. Transfection of a valine-12 mutated ras construct into LLC-PK1 renal epithelia produces leakiness of tight junctions to certain types of solutes. Transepithelial permeability of D-mannitol increases sixfold but transepithelial electrical resistance increases >40%. This indicates decreased paracellular permeability to NaCl but increased permeability to nonelectrolytes. Permeability increases to D-mannitol (Mr 182), polyethylene glycol (Mr 4000), and 10,000-Mr methylated dextran but not to 2,000,000-Mr methylated dextran. This implies a "ceiling" on the size of solutes that can cross a ras-mutated epithelial barrier and therefore that the increased permeability is not due to loss of cells or junctions. Although the abundance of claudin-2 declined to undetectable levels in the ras-overexpressing cells compared with vector controls, levels of occludin and claudins 1, 4, and 7 increased. The abundance of claudins-3 and -5 remained unchanged. An increase in extracellular signal-regulated kinase-2 phosphorylation suggests that the downstream effects on the tight junction may be due to changes in the mitogen-activated protein kinase signaling pathway. These selective changes in permeability may influence tumorigenesis by the types of solutes now able to cross the epithelial barrier.

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.

Involvement of protein kinase C in chitosan glutamate-mediated tight junction disruption

Chitosan has been successfully used as an excipient for trans-epithelial drug delivery systems. It is known to transiently open intercellular tight junctions thus increasing the permeability of an epithelium. In order to investigate the possible role of protein kinases in trans-epithelial delivery, changes in trans-epithelial electrical resistance ('TEER') of epithelial (Caco-2) cell monolayers were assessed in response to chitosan glutamate treatment, in the presence and absence of specific protein kinase inhibitors. Changes in subcellular localisation of the tight junction protein ZO-1 observed by immunofluorescence and western blotting of cellular fractions were also assessed. Inhibition of protein kinase C (PKC), but not mitogen activated protein kinase (MAPK) was found to prevent the chitosan-mediated decrease in TEER, and changes in localisation of ZO-1. In order to determine which PKC isozymes were responsible for the chitosan-mediated tight junction disruption, the activation of the PKC isozymes alpha, beta and delta was investigated. A chitosan-mediated translocation of PKC alpha but not PKC beta or delta from the cytosol to the membrane fraction, indicative of PKC alpha activation was observed. Thus, treatment of Caco-2 cells with chitosan may result in the activation of PKC-dependent signal transduction pathways which affect tight junction integrity.

Regulation of tight junctions and loss of barrier function in pathophysiology

The mechanism by which epithelial and endothelial cells interact to form polarized tissue is of fundamental importance to multicellular organisms. Dysregulation of these barriers occurs in a variety of diseases, destroying the normal cellular environments and leading to organ failure. Increased levels of growth factors are a common characteristic of diseases exhibiting tissue permeability, suggesting that growth factors play a direct role in elevating permeability. Of particular concern for this laboratory, increased expression of vascular endothelial growth factor may enhance vascular permeability in diabetic retinopathy, leading to vision impairment and blindness. However, the mechanism by which growth factors increase permeability is unclear. Polarized cells form strong barriers through the development of tight junctions, which are specialized regions of the junctional complex. Tight junctions are composed of three types of transmembrane proteins, a number of peripheral membrane structural proteins, and are associated with a variety of regulatory proteins. Recent data suggest that growth factor-stimulated alterations in tight junctions contribute to permeability in a variety of disease states. The goal of this review was to elucidate potential mechanisms by which elevated growth factors elicit deregulated paracellular permeability via altered regulation of tight junctions, with particular emphasis on the tight junction proteins occludin and ZO-1, protein kinase C signaling, and endocytosis of junctional proteins. Understanding the molecular mechanisms underlying growth factor-mediated regulation of tight junctions will facilitate the development of novel treatments for diseases such as brain tumors, diabetic retinopathy and other diseases with compromised tight junction barriers.

Critical role of the atypical {lambda} isoform of protein kinase C (PKC-{lambda}) in oxidant-induced disruption of the microtubule cytoskeleton and barrier function of intestinal epithelium

Oxidant injury to epithelial cells and gut barrier disruption are key factors in the pathogenesis of inflammatory bowel disease. Studying monolayers of intestinal (Caco-2) cells, we reported that oxidants disrupt the cytoskeleton and cause barrier dysfunction (hyperpermeability). Because the lambda isoform of protein kinase C (PKC-lambda), an atypical diacylglycerol-independent isozyme, is abundant in parental (wild type) Caco-2 cells and is translocated to the particulate fractions upon oxidant exposure, we hypothesized that PKC-lambda is critical to oxidative injury to the assembly and architecture of cytoskeleton and the intestinal barrier function. To this end, Caco-2 cells were transfected with an inducible plasmid, a tetracycline-responsive system, to create novel clones stably overexpressing native PKC-lambda. Other cells were transfected with a dominant-negative plasmid to stably inhibit the activity of native PKC-lambda. Cells were exposed to oxidant (H(2)O(2)) +/- modulators. Parental Caco-2 cells were treated similarly. We then monitored barrier function (fluorescein sulfonic acid clearance), microtubule cytoskeletal stability (confocal microscopy, immunoblotting), subcellular distribution of PKC-lambda (immunofluorescence, immunoblotting, immunoprecipitation), and PKC-lambda isoform activity (in vitro kinase assay). Monolayers were also processed to assess alterations in tubulin assembly, polymerized tubulin (S2, an index of cytoskeletal integrity), and monomeric tubulin (S1, an index of cytoskeletal disassembly) (polyacrylamide gel electrophoresis fractionation and immunoblotting. In parental cells, oxidant caused: 1) translocation of PKC-lambda from the cytosol to the particulate (membrane + cytoskeletal) fractions, 2) activation of native PKC-lambda, 3) tubulin pool instability (increased monomeric S1 and decreased polymerized S2), 4) disruption of cytoskeletal architecture, and 5) barrier dysfunction (hyperpermeability). In transfected clones, overexpression of the atypical (74 kDa) PKC-lambda isoform by itself ( approximately 3.2-fold increase) led to oxidant-like disruptive effects, including cytoskeletal and barrier hyperpermeability. Overexpressed PKC-lambda was mostly found in particulate cell fractions (with a smaller cytosolic distribution) indicating its activation. Disruption by PKC-lambda overexpression was also potentiated by oxidant challenge. Stable inactivation of endogenous PKC-lambda ( approximately 99.6%) by a dominant-negative protected against all measures of oxidant-induced disruption. We conclude that: 1) oxidant induces disruption of epithelial barrier integrity by disassembling the cytoskeleton, in large part, through the activation of PKC-lambda isoform; and 2) activation of PKC-lambda by itself appears to be sufficient for disruption of cellular cytoskeleton and monolayer barrier permeability. The unique ability to mediate an oxidant-like injury and cytoskeletal depolymerization and instability is a novel mechanism not previously attributed to the atypical subfamily of PKC isoforms.

Circumvention of Nuclear Factor κB-Induced Chemoresistance by Cytoplasmic-Targeted Anthracyclines

Nuclear factor kappaB (NF-kappaB) has been implicated in inducible chemoresistance against anthracyclines. In an effort to improve the cytotoxicity of anthracyclines while reducing their cardiotoxic effects, we have developed a novel class of extranuclear-localizing 14-O-acylanthracyclines that bind to the phorbol ester/diacylglycerol-binding C1b domain of conventional and novel protein kinase C (PKC) isoforms, thereby promoting an apoptotic response. Because PKCs have been shown to be involved in NF-kappaB activation, in this report, we determined the mechanism of NF-kappaB activation by N-benzyladriamycin-14-valerate (AD 198) and N-benzyladriamycin-14-pivalate (AD 445), two novel 14-O-acylanthracylines. We show that the induction of NF-kappaB activity in response to drug treatment relies on the activation of PKC-delta and NF-kappaB-activating kinase (NAK), independent of ataxia telengectasia mutated and p53 activities. In turn, NAK activates the IKK complex through phosphorylation of the IKK-2 subunit. We find that neither NF-kappaB activation nor ectopic expression of Bcl-X(L) confers protection from AD 198-induced cell killing. Overall, our data indicate that activation of novel PKC isoforms by cytoplasmic-targeted 14-O-acylanthracyclines promotes an apoptotic response independent of DNA damage, which is unimpeded by inducible activation of NF-kappaB.

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

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

Theta-isoform of PKC is required for alterations in cytoskeletal dynamics and barrier permeability in intestinal epithelium: a novel function for PKC-theta

Using intestinal Caco-2 cells, we previously showed that assembly of cytoskeleton is required for monolayer barrier function, but the underlying mechanisms remain poorly understood. Because the theta-isoform of PKC is present in wild-type (WT) intestinal cells, we hypothesized that PKC-theta is crucial for changes in cytoskeletal and barrier dynamics. We have created the first multiple sets of gastrointestinal cell clones transfected with varying levels of cDNA to stably inhibit native PKC-theta (antisense, AS; dominant negative, DN) or to express its activity (sense). We studied transfected and WT Caco-2 cells. First, relative to WT cells, AS clones underexpressing PKC-theta showed monolayer injury as indicated by decreased native PKC-theta activity, reduced tubulin phosphorylation, increased tubulin disassembly (decreased polymerized and increased monomeric pools), reduced architectural integrity of microtubules, reduced stability of occludin, and increased barrier hyperpermeability. In these AS clones, PKC-theta was substantially reduced in the particulate fractions, indicating its inactivation. In WT cells, 82-kDa PKC-theta was constitutively active and coassociated with 50-kDa tubulin, forming an endogenous PKC-theta/tubulin complex. Second, DN transfection to inhibit the endogenous PKC-theta led to similar destabilizing effects on monolayers, including cytoskeletal hypophosphorylation, depolymerization, and instability as well as barrier disruption. Third, stable overexpression of PKC-theta led to a mostly cytosolic distribution of theta-isoform (<10% in particulate fractions), indicating its inactivation. In these sense clones, we also found disruption of occludin and microtubule assembly and increased barrier dysfunction. In conclusion, 1). PKC-theta isoform is required for changes in the cytoskeletal assembly and barrier permeability in intestinal monolayers, and 2). the molecular event underlying this novel biological effect of PKC-theta involves changes in phosphorylation and/or assembly of the subunit components of the cytoskeleton. The ability to alter the cytoskeletal and barrier dynamics is a unique function not previously attributed to PKC-theta.

Reversal of charge selectivity in cation or anion-selective epithelial lines by expression of different claudins

Tight junctions (TJ) regulate paracellular ionic charge selectivity and conductance across epithelial tissues and cell lines. These properties vary among epithelia, and recent evidence implicates the claudins, a family of TJ transmembrane proteins, as important determinants of both characteristics. To test the hypothesis that each claudin contributes a characteristic charge discrimination to the TJ, we expressed claudins-2, -4, -11, and -15 in both cation-selective Madin-Darby canine kidney (MDCK) II cells and in anion-selective LLC-PK1 cells and examined changes in transepithelial electrical resistance (TER) and paracellular charge selectivity. Regulated expression and localization were verified by immunoblot analysis and immunofluorescence microscopy, respectively. Expression of claudin-4 increased TER in both cell lines, whereas effects of the others on TER were variable. Claudin-4 and -11 decreased paracellular permeability for Na+ in MDCK II cells, whereas neither claudin-2 nor -15 had an effect. Conversely, in LLC-PK1 cells, claudin-2 and -15 increased the permeability for Na+, whereas claudin-4 and -11 were without effect. We conclude that the contribution of each claudin is most easily detectable when it reverses the direction of monolayer charge selectivity. These results are consistent with a model in which exogenous claudins add new charge-selective pores, leading to a physiological phenotype that combines endogenous and exogenous contributions. Additionally, it is possible to rationalize the direction of charge selectivity conferred by the individual claudins on the basis of electrostatic effects of the charged amino acids in their first extracellular loops.

PKC-beta1 isoform activation is required for EGF-induced NF-kappaB inactivation and IkappaBalpha stabilization and protection of F-actin assembly and barrier function in enterocyte monolayers

Using monolayers of intestinal Caco-2 cells, we reported that activation of NF-kappaB is required for oxidative disruption and that EGF protects against this injury but the mechanism remains unclear. Activation of the PKC-beta1 isoform is key to monolayer barrier integrity. We hypothesized that EGF-induced activation of PKC-beta1 prevents oxidant-induced activation of NF-kappaB and the consequences of NF-kappaB activation, F-actin, and barrier dysfunction. We used wild-type (WT) and transfected cells. The latter were transfected with varying levels of cDNA to overexpress or underexpress PKC-beta1. Cells were pretreated with EGF or PKC modulators +/- oxidant. Pretreatment with EGF protected monolayers by increasing native PKC-beta1 activity, decreasing IkappaBalpha phosphorylation/degradation, suppressing NF-kappaB activation (p50/p65 subunit nuclear translocation/activity), enhancing stable actin (increased F-actin-to-G-actin ratio), increasing stability of actin cytoskeleton, and reducing barrier hyperpermeability. Cells stably overexpressing PKC-beta1 were protected by low, previously nonprotective doses of EGF or modulators. In these clones, we found enhanced IkappaBalpha stabilization, NF-kappaB inactivation, actin stability, and barrier function. Low doses of the modulators led to increases in PKC-beta1 in the particulate fractions, indicating activation. Stably inhibiting endogenous PKC-beta1 substantially prevented all measures of EGF's protection against NF-kappaB activation. We conclude that EGF-mediated protection against oxidant disruption of the intestinal barrier function requires PKC-beta1 activation and NF-kappaB suppression. The molecular event underlying this unique effect of PKC-beta1 involves inhibition of phosphorylation and increases in stabilization of IkappaBalpha. The ability to inhibit the dynamics of NF-kappaB/IkappaBalpha and F-actin disassembly is a novel mechanism not previously attributed to the classic subfamily of PKC isoforms.

Galpha12 regulates epithelial cell junctions through Src tyrosine kinases

Regulation and assembly of the epithelial cell junctional complex involve multiple signaling mechanisms, including heterotrimeric G proteins. Recently, we demonstrated that Galpha12 binds to the tight junction scaffolding protein ZO-1 through the SH3 domain and that activated Galpha12 increases paracellular permeability in Madin-Darby canine kidney (MDCK) cells (Meyer et al. J Biol Chem 277: 24855-24858, 2002). In the present studies, we explore the effects of Galpha12 expression on tight and adherens junction proteins and examine downstream signaling pathways. By confocal microscopy, we detect disrupted tight and adherens junction proteins with increased actin stress fibers in constitutively active Galpha12 (QLalpha12)-expressing MDCK cells. The normal distribution of ZO-1 and Na-K-ATPase was altered in QLalpha12-expressing MDCK cells, consistent with loss of polarity. We found that the tyrosine kinase inhibitor genistein and the Src-specific inhibitor PP-2 reversibly abrogated the QLalpha12 phenotype on the junctional complex. Junctional protein localization was preserved in PP-2- or genistein-treated QLalpha12-expressing cells, and the increase in paracellular permeability as measured by transepithelial resistance and [3H]mannitol flux was prevented by the inhibitors. Src activity was increased in QLalpha12-expressing MDCK cells as assessed by Src autophosphorylation, and beta-catenin tyrosine phosphorylation was also increased, although there was no detectable increase in Rho activity. Taken together, these results indicate that Galpha12 regulates MDCK cell junctions, in part through Src tyrosine kinase pathways.

Zeta isoform of protein kinase C prevents oxidant-induced nuclear factor-kappaB activation and I-kappaBalpha degradation: a fundamental mechanism for epidermal growth factor protection of the microtubule cytoskeleton and intestinal barrier integrity

Oxidant damage and gut barrier disruption contribute to the pathogenesis of a variety of inflammatory gastrointestinal disorders, including inflammatory bowel disease (IBD). In our studies using a model of the gastrointestinal (GI) epithelial barrier, monolayers of intestinal (Caco-2) cells, we investigated damage to and protection of the monolayer barrier. We reported that activation of nuclear factor-kappaB (NF-kappaB) via degradation of its endogenous inhibitor I-kappaBalpha is key to oxidant-induced disruption of barrier integrity and that growth factor (epidermal growth factor, EGF) protects against this injury by stabilizing the cytoskeletal filaments. Protein kinase C (PKC) activation seems to be required for monolayer maintenance, especially activation of the atypical zeta isoform of PKC. In an attempt to investigate, at the molecular level, the fundamental events underlying EGF protection against oxidant disruption, we tested the intriguing hypothesis that EGF-induced activation of PKC-zeta prevents oxidant-induced activation of NF-kappaB and the consequences of NF-kappaB activation, namely, cytoskeletal and barrier disruption. Monolayers of wild-type (WT) Caco-2 cells were incubated with oxidant (H2O2) with or without EGF or modulators. In other studies, we used the first gastrointestinal cell clones created by stable transfection of varying levels (1-5 microg) of cDNA to either overexpress PKC-zeta or to inhibit its expression. Transfected cell clones were then pretreated with EGF or a PKC activator (diacylglycerol analog 1-oleoyl-2-acetyl-glycerol, OAG) before oxidant. We monitored the following endpoints: monolayer barrier integrity, stability of the microtubule cytoskeleton, subcellular distribution and activity of the PKC-zeta isoform, intracellular levels and phosphorylation of the NF-kappaB inhibitor I-kappaBalpha, and nuclear translocation and activity of NF-kappaB subunits p65 and p50. Monolayers were also fractionated and processed to assess alterations in the structural protein of the microtubules, polymerized tubulin (S2), and monomeric tubulin (S1). Our data indicated that relative to WT monolayers exposed only to oxidant, pretreatment with EGF protected cell monolayers by 1) increasing native PKC-zeta activity; 2) decreasing several variables related to NF-kappaB activation [NF-kappaB (both p50 and p65 subunits) nuclear translocation, NF-kappaB subunits activity, I-kappaBalpha degradation, and phosphorylation]; 3) increasing stable tubulin (increased polymerized S2 tubulin and decreased monomeric S1 tubulin); 4) maintaining the cytoarchitectural integrity of microtubules; and 5) preventing hyperpermeability (barrier disruption). In addition, relative to WT cells exposed to oxidant, monolayers of transfected cells stably overexpressing PKC-zeta (approximately 3.0-fold increase) were protected as indicated by decreases in all measures of NF-kappaB activation as well as enhanced stability of microtubule cytoarchitecture and barrier function. Overexpression induced stabilization of I-kappaBalpha and inactivation of NF-kappaB was OAG-independent, although EGF potentiated this protection. Approximately 90% of the overexpressed PKC-zeta resided in particulate (membrane + cytoskeletal) fractions (with less than 10% in cytosolic fractions), indicating constitutive activation of the zeta isoform of PKC. Furthermore, antisense transfection to stably inhibit native PKC-zeta expression (-95%) and activation (-99%) prevented all measures of EGF-induced protection against NF-kappaB activation and monolayer disruption. We conclude the following: 1) EGF protects against oxidant disruption of the intestinal barrier integrity, in large part, through the activation of PKC-zeta and inactivation of NF-kappaB (an inflammatory mediator); 2) activation of PKC-zeta is by itself required for monolayer protection against oxidant stress of NF-kappaB activation; 3) the mechanism underlying this novel biological effect of the atypical PKC isoform zeta seems to involve suppression of phosphorylation and enhancement of stabilization of I-kappaBalpha; and 4) development of agents that can mimic or enhance PKC-zeta-induced suppression of NF-kappaB activation may be a useful therapeutic strategy for preventing oxidant damage to GI mucosal epithelium in disorders such as IBD. To our knowledge, this is the first report that PKC-zeta can inhibit the dynamics of NF-kappaB and cytoskeletal disassembly in cells.

Involvement of PKCdelta and PKD in pulmonary microvascular endothelial cell hyperpermeability

The involvement of PKC, the isoforms of which are categorized into three subtypes: conventional (alpha, betaI, betaII, and gamma), novel [delta, epsilon, eta, and mu (also known as PKD), theta], and atypical (zeta and iota/lambda), in the regulation of endothelial monolayer integrity is well documented. However, isoform activity varies among different cell types. Our goal was to reveal isoform-specific PKC activity in the microvascular endothelium in response to phorbol 12-myristate 13-acetate (PMA) and diacylglycerol (DAG). Isoform activity was demonstrated by cytosol-to-membrane translocation after PMA treatment and phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) protein after PMA and DAG treatment. Specific isoforms were inhibited by using both antisense oligonucleotides and pharmacological agents. The data showed partial cytosol-to-membrane translocation of isoforms alpha, betaI, and epsilon and complete translocation of PKCdelta and PKD in response to PMA. Furthermore, antisense treatment and pharmacological studies indicated that the novel isoform PKCdelta and PKD are both required for PMA- and DAG-induced MARCKS phosphorylation and hyperpermeability in pulmonary microvascular endothelial cells, whereas isoforms alpha, betaI, and epsilon were dispensable with regard to these same phenomena.

The delta-isoform of protein kinase C causes inducible nitric-oxide synthase and nitric oxide up-regulation: key mechanism for oxidant-induced carbonylation, nitration, and disassembly of the microtubule cytoskeleton and hyperpermeability of barrier of intestinal epithelia

Using intestinal (Caco-2) cells, we found that oxidant-induced disruption of barrier integrity requires microtubule disassembly. Protein kinase C (PKC)-delta isoform seems to be essential for disruption, but the mechanism is unknown. Because inducible nitric-oxide synthase (iNOS) is key to oxidant stress, we hypothesized that PKC-delta activation is essential in oxidant-induced iNOS up-regulation and the consequent cytoskeletal oxidation and disarray and monolayer barrier dysfunction. Cells were transfected with an inducible plasmid to overexpress native PKC-delta or with a dominant-negative to inhibit the activity of native PKC-delta. Clones were then incubated with oxidant (H(2)O(2)) +/- modulators. Parental cells were treated similarly. Exposure to oxidant-disrupted monolayers by increasing native PKC-delta activity, increasing six iNOS-related variables (iNOS activity and protein, nitric oxide, oxidative stress, tubulin oxidation and nitration), decreasing polymerized tubulin, disrupting the cytoarchitecture of microtubules, and causing monolayer dysfunction. Induction of PKC-delta overexpression by itself (3.5-fold) led to oxidant-like disruptive effects, including activation of the iNOS-driven pathway. Overexpression-induced up-regulation of iNOS was potentiated by oxidants. iNOS inhibitors or oxidant scavengers were protective. Dominant inhibition of native PKC-delta activity (99.5%) prevented all measures of oxidant-induced iNOS up-regulation and protected the monolayer barrier. The conclusions are as follows. 1) Oxidants induce loss of epithelial barrier integrity by oxidizing and disassembling the cytoskeleton, in part, through the activation of PKC-delta and up-regulation of iNOS. 2) Overexpression and activation of PKC-delta are by themselves key for cellular injury by oxidative stress of iNOS. 3) We thus report a pathophysiological mechanism, activation of iNOS pathway and its injurious consequences to the cytoskeleton, including oxidation and nitration, among the "novel" subfamily of PKC isoforms.

Role of protein kinase C isoforms in rat epididymal microvascular endothelial barrier function

Endothelial barrier dysfunction is involved in a variety of diseased states. We investigated the role of protein kinase C (PKC) in monolayer permeability using endothelial cells (EC) overexpressing PKC alpha (PKC alpha EC), PKC delta (PKC delta EC) or vector (vector control EC) cDNAs. Thrombin induced permeability changes in all EC, and induced significantly elevated rates of monolayer permeability in PKC alpha EC. Conversely, the basal level of permeability was significantly blunted in PKC delta EC, resulting in diminished thrombin-induced changes in permeability. PKC inhibitors, Gö6976 and rottlerin, reversed the effects of PKC alpha and PKC delta overexpression on permeability, respectively. Immunoblot analyses demonstrated significantly less beta-catenin associated with the cytoskeletal subcellular fraction in thrombin-treated PKC alpha EC, an effect blocked by pretreatment with Gö6976. PKC delta EC contained significantly greater numbers of focal contacts per cell. Thrombin enhanced RhoA GTPase activity in all EC; with a 3-fold greater level of activity in PKC delta EC. Rottlerin significantly blunted RhoA GTPase activity in all EC. Overexpression of RhoA dominant-negative cDNA diminished the size and number of focal contacts in EC, and significantly enhanced the basal rate of PKC delta EC monolayer permeability. These findings demonstrate that monolayer permeability changes are differentially regulated by PKC isoenzymes, suggesting that PKC alpha promotes endothelial barrier dysfunction and PKC delta enhances basal endothelial barrier function.

Role of phospholipase C-beta in the modulation of epithelial tight junction permeability

The results presented in this study establish an association between phospholipase C-beta (PLC-beta) and tight junction permeability across Madin-Darby canine kidney (MDCK) cell monolayers, an in vitro model for epithelial tissue. These results further show that PLC-beta modulates tight junction permeability by affecting actin filament organization. Hexadecylphosphocholine (HPC) inhibited PLC-beta and increased tight junction permeability in MDCK cells. Interestingly, the analogs of HPC, a series of alkylphosphocholines containing various lengths of linear alkyl chains, inhibited PLC-beta and increased tight junction permeability with a wide range of potency. The potency of alkylphosphocholines as enhancers of tight junction permeability significantly correlated (p < 0.05) with their potency as PLC-beta inhibitors. U73122, a steroid derivative that is structurally unrelated to alkylphosphocholines, inhibited PLC-beta and increased tight junction permeability with potencies that fit into the correlation observed for the alkylphosphocholine series. U73122 and HPC induced disorganization of actin filaments in MDCK cell monolayers. The potencies to cause disorganization of actin filaments were consistent with the potencies of these agents as inhibitors of PLC-beta and enhancers of tight junction permeability. Furthermore, ATP, an activator of PLC-beta, attenuated U73122-induced increase in tight junction permeability as well as disorganization of actin filaments. These results provide strong evidence that PLC-beta inhibition leads to increased tight junction permeability across MDCK cell monolayers through disorganization of actin filaments.

Activation of delta-isoform of protein kinase C is required for oxidant-induced disruption of both the microtubule cytoskeleton and permeability barrier of intestinal epithelia

Using monolayers of intestinal (Caco-2) cells, we showed that oxidants disassemble the microtubule cytoskeleton and disrupt barrier integrity (permeability) (Banan et al., 2000a). Because exposure of our parental cells to oxidants causes protein kinase C (PKC)-delta to be translocated to particulate fractions, we hypothesized that PKC-delta activation is required for these oxidant effects. Monolayers of parental Caco-2 cells were incubated with oxidant (H(2)O(2)) +/- modulators. Other cells were transfected with an inducible plasmid to stably overexpress PKC-delta or with a dominant negative plasmid to stably inhibit the activity of native PKC-delta. In parental cells, oxidants caused translocation of PKC-delta to the particulate (membrane + cytoskeletal) fractions, activation of PKC-delta isoform, increases in monomeric (S1) tubulin and decreases in polymerized (S2) tubulin, disruption of the microtubule cytoarchitecture, and loss of barrier integrity (hyperpermeability). In transfected cells, induction of PKC-delta overexpression by itself (3.5-fold over its basal level) led to oxidant-like disruptive effects. Disruption induced by PKC-delta overexpression was potentiated by oxidants. Overexpressed PKC-delta resided in particulate fractions, indicating its activation. Stable inhibition of native PKC-delta activity (98%) by dominant negative transfection substantially protected against all measures of oxidative disruption. We conclude that 1) oxidants induce loss of intestinal epithelial barrier integrity by disassembling the microtubules in large part through the activation of the PKC-delta isoform; and 2) overexpression and activation of PKC-delta is by itself a sufficient condition for disruption of these cytoskeleton and permeation pathways. Thus, PKC-delta activation may play a key role in intestinal dysfunction in oxidant-induced diseases such as inflammatory bowel disease.

PKC-zeta prevents oxidant-induced iNOS upregulation and protects the microtubules and gut barrier integrity

Using intestinal (Caco-2) monolayers, we reported that inducible nitric oxide synthase (iNOS) activation is key to oxidant-induced barrier disruption and that EGF protects against this injury. PKC-zeta was required for protection. We thus hypothesized that PKC-zeta activation and iNOS inactivation are key in EGF protection. Wild-type (WT) Caco-2 cells were exposed to H(2)O(2) (0.5 mM) +/- EGF or PKC modulators. Other cells were transfected to overexpress PKC-zeta or to inhibit it and then pretreated with EGF or a PKC activator (OAG) before oxidant. Relative to WT cells exposed to oxidant, pretreatment with EGF protected monolayers by 1) increasing PKC-zeta activity; 2) decreasing iNOS activity and protein, NO levels, oxidative stress, tubulin oxidation, and nitration); 3) increasing polymerized tubulin; 4) maintaining the cytoarchitecture of microtubules; and 5) enhancing barrier integrity. Relative to WT cells exposed to oxidant, transfected cells overexpressing PKC-zeta (+2.9-fold) were protected as indicated by decreases in all measures of iNOS-driven pathways and enhanced stability of microtubules and barrier function. Overexpression-induced inhibition of iNOS was OAG independent, but EGF potentiated this protection. Antisense inhibition of PKC-zeta (-95%) prevented all measures of EGF protection against iNOS upregulation. Thus EGF protects against oxidative disruption of the intestinal barrier by stabilizing the cytoskeleton in large part through the activation of PKC-zeta and downregulation of iNOS. Activation of PKC-zeta is by itself required for cellular protection against oxidative stress of iNOS. We have thus discovered novel biologic functions, suppression of the iNOS-driven reactions and cytoskeletal oxidation, among the atypical PKC isoforms.

Phospholipase C-gamma modulates epithelial tight junction permeability through hyperphosphorylation of tight junction proteins

Phospholipase C-gamma (PLC-gamma) is stimulated by epidermal growth factor via activation of the epidermal growth factor receptors. The PLC inhibitor, 3-nitrocoumarin (3-NC), selectively inhibited PLC-gamma in Madin-Darby canine kidney cells without affecting the activity of PLC-beta. In contrast, inhibitors of PLC-beta, hexadecylphosphocholine and, had no effect on the activity of PLC-gamma. Inhibition of PLC-gamma by 3-NC was associated with an increase in tight junction permeability across Madin-Darby canine kidney cell monolayers, as evidenced by 3-NC-induced decrease in transepithelial electrical resistance and increase in mannitol flux over a concentration range that was inhibitory to PLC-gamma. An analog of 3-NC, 7-hydroxy-3-NC (7-OH-3-NC), which was inactive as an inhibitor of PLC-gamma, also had no effect on tight junction permeability. Treatment with 3-NC caused punctate disruption in the cortical actin filaments. The PLC-gamma inhibitor, 3-NC, but not the inactive analog, 7-OH-3-NC, caused hyperphosphorylation of the tight junction proteins, occludin, ZO-1, and ZO-2. The serine/threonine kinase inhibitor, staurosporine (50-200 nm), significantly attenuated 3-NC-induced hyperphosphorylation of ZO-2. This corresponded with attenuation by staurosporine of 3-NC-induced increase in tight junction permeability, suggesting a relationship between ZO-2 phosphorylation and tight junction permeability.

Zonula occludens-1 is a scaffolding protein for signaling molecules. Galpha(12) directly binds to the Src homology 3 domain and regulates paracellular permeability in epithelial cells

Zonula occludens proteins are multidomain proteins usually localized at sites of intercellular junctions, yet little is known about their role in regulating junctional properties. Multiple signaling proteins regulate the junctional complex, and several (including G proteins) have been co-localized with zonula occludens-1 (ZO-1) in the tight junction of epithelial cells. However, evidence for direct interactions between signaling proteins and tight junction proteins has been lacking. In these studies, we constructed Galpha-glutathione S-transferase (GST) fusion proteins and tested for interactions with [(35)S]methionine-labeled in vitro translated ZO-1 and ZO-2. Only Galpha(12) directly interacted with in vitro translated ZO-1 and ZO-2. Using a series of ZO-1 domains expressed as GST fusion proteins and in vitro translated [(35)S]methionine-labeled Galpha(12), we found that Galpha(12) and constitutively active (Q229L) alpha(12) (QLalpha(12)) bind to the Src homology 3 (SH3) domain of ZO-1. This binding was not detected with SH3 domains from other proteins. Inducible expression of wild-type alpha(12) and QLalpha(12) in Madin-Darby canine kidney (MDCK) cells was established using the Tet-Off system. In Galpha(12)-expressing cells, we found that ZO-1 and Galpha(12) co-localize by confocal microscopy and co-immunoprecipitate. Galpha(12) from MDCK cell lysates bound to the GST-ZO-1-SH3 domain, and expression of QLalpha(12) in MDCK cells reversibly increased paracellular permeability. These studies indicated that ZO-1 directly interacts with Galpha(12) and that Galpha(12) regulates barrier function of MDCK cells.

PKC-zeta is required in EGF protection of microtubules and intestinal barrier integrity against oxidant injury

Using monolayers of human intestinal (Caco-2) cells, we showed that epidermal growth factor (EGF) protects intestinal barrier integrity against oxidant injury by protecting the microtubules and that protein kinase C (PKC) is required. Because atypical PKC-zeta isoform is abundant in wild-type (WT) Caco-2 cells, we hypothesized that PKC-zeta mediates, at least in part, EGF protection. Intestinal cells (Caco-2 or HT-29) were transfected to stably over- or underexpress PKC-zeta. These clones were preincubated with low or high doses of EGF or a PKC activator [1-oleoyl-2-acetyl-sn-glycerol (OAG)] before oxidant (0.5 mM H(2)O(2)). Relative to WT cells exposed to oxidant, only monolayers of transfected cells overexpressing PKC-zeta (2.9-fold) were protected against oxidant injury as indicated by increases in polymerized tubulin and decreases in monomeric tubulin, enhancement of architectural stability of the microtubule cytoskeleton, and increases in monolayer barrier integrity toward control levels (62% less leakiness). Overexpression-induced protection was OAG independent and even EGF independent, but EGF significantly potentiated PKC-zeta protection. Most overexpressed PKC-zeta (92%) resided in membrane and cytoskeletal fractions, indicating constitutive activation of PKC-zeta. Stably inhibiting PKC-zeta expression (95%) with antisense transfection substantially attenuated EGF protection as demonstrated by reduced tubulin assembly and increased microtubule disassembly, disruption of the microtubule cytoskeleton, and loss of monolayer barrier integrity. We conclude that 1) activation of PKC-zeta is necessary for EGF-induced protection, 2) PKC-zeta appears to be an endogenous stabilizer of the microtubule cytoskeleton and of intestinal barrier function against oxidative injury, and 3) we have identified a novel biological function (protection) among the atypical isoforms of PKC.

Src family kinases phosphorylate protein kinase C delta on tyrosine residues and modify the neoplastic phenotype of skin keratinocytes

Protein kinase C delta (PKC delta) is tyrosine-phosphorylated and catalytically inactive in mouse keratinocytes transformed by a ras oncogene. In several other model systems, Src kinases are upstream regulators of PKC delta. To examine this relationship in epidermal carcinogenesis, v-ras transformed mouse keratinocytes were treated with a selective Src kinase inhibitor (PD 173958). PD 173958 decreased autophosphorylation of Src, Fyn, and Lyn kinases and prevented tyrosine phosphorylation of the Src kinase substrate p120. PD 173958 also prevented PKC delta tyrosine phosphorylation and activated PKC delta as detected by membrane translocation. Expression of keratinocyte differentiation markers increased in PD 173958-treated v-ras-keratinocytes, and fluid-filled domes emerged, indicative of tight junction formation. Antisense PKC delta or bryostatin 1 inhibited dome formation, while overexpression of PKC delta in the presence of PD 173958 enhanced the formation of domes. Plasmids encoding phenylalanine mutants of PKC delta tyrosine residues 64 and 565 induced domes in the absence of PD 173958, while phenylalanine mutants of tyrosine residues 52, 155, and 187 were inactive. Thus, Src kinase mediated post-translational modification of PKC delta on specific tyrosine residues in ras-transformed mouse keratinocytes inactivates PKC delta and contributes to alterations in the differentiated phenotype and tight junction formation associated with neoplasia.

Nonreceptor tyrosine kinase c-Yes interacts with occludin during tight junction formation in canine kidney epithelial cells

Occludin is an integral membrane protein that is tyrosine phosphorylated when localized at tight junctions. When Ca(2+) was depleted from the culture medium, occludin tyrosine phosphorylation was diminished from Madin-Darby canine kidney epithelial cells in 2 min. This dephosphorylation was correlated with a significant reduction in transepithelial electrical resistance (TER), indicating a global loss of the tight junction barrier function. Reconstitution of Ca(2+) resulted in a robust tyrosine rephosphorylation of occludin that was temporally associated with an increase in TER. Moreover, we demonstrate in this study that occludin was colocalized with the nonreceptor tyrosine kinase c-Yes at cell junction areas and formed an immunoprecipitable complex with c-Yes in vivo. This complex dissociated when the cells were incubated in medium without Ca(2+) or treated with a c-Yes inhibitor, CGP77675. In the presence of CGP77675 after Ca(2+) repletion, occludin tyrosine phosphorylation was completely abolished and both tight junction formation and the increase of the TER were inhibited. Our study thus provides strong evidence that occludin tyrosine phosphorylation is tightly linked to tight junction formation in epithelial cells, and that the nonreceptor tyrosine kinase c-Yes is involved in the regulation of this process.

Protein kinase C signaling regulates ZO-1 translocation and increased paracellular flux of T84 colonocytes exposed to Clostridium difficile toxin A

Clostridium difficile toxin A increases paracellular permeability in colonic epithelial T84 cells by mechanisms involving RhoA glucosylation and actin depolymerization. However, we previously observed that toxin A-mediated decline in transepithelial electrical resistance preceded changes in cell morphology and tight junction ultrastructure (Hecht, G., Pothoulakis, C., LaMont, J. T., and Madara, J. L. (1988) J. Clin. Invest. 82, 1516-1524). Recent studies also showed that C. difficile toxins induce early cellular responses, including activation of mitogen-activated protein kinases, generation of reactive oxygen metabolites, and calcium influx. The aim of this study was to investigate whether toxin A-induced early cellular responses contribute to the permeability changes. We found that toxin A stimulated the activities of membrane and cytosolic protein kinase Calpha (PKCalpha) and cytosolic PKCbeta. A specific PKCalpha/beta antagonist (myristoylated PKCalpha/beta peptide) blocked toxin A-mediated RhoA glucosylation. Furthermore, decreased transepithelial electrical resistance and increased translocation of ZO-1 from tight junction occurred within 2-3 h of toxin A exposure and were also inhibited by PKCalpha/beta antagonist. During this time period, toxin exposure did not induce translocation of ZO-2, dephosphorylation or translocation of occludin, or cell rounding. Our data indicate that PKC signaling regulates toxin A-mediated paracellular permeability changes and ZO-1 translocation.

PKC-beta1 mediates EGF protection of microtubules and barrier of intestinal monolayers against oxidants

Using monolayers of human intestinal (Caco-2) cells, we found that oxidants and ethanol damage the cytoskeleton and disrupt barrier integrity; epidermal growth factor (EGF) prevents damage by enhancement of protein kinase C (PKC) activity and translocation of the PKC-beta1 isoform. To see if PKC-beta1 mediates EGF protection, cells were transfected to stably over- or underexpress PKC-beta1. Transfected monolayers were preincubated with low or high doses of EGF (1 or 10 ng/ml) or 1-oleoyl-2-acetyl-sn-glycerol [OAG; a PKC activator (0.01 or 50 microM)] before treatment with oxidant (0.5 mM H(2)O(2)). Only in monolayers overexpressing PKC-beta1 (3.1-fold) did low doses of EGF or OAG initiate protection, increase tubulin polymerization (assessed by quantitative immunoblotting) and microtubule architectural integrity (laser scanning confocal microscopy), maintain normal barrier permeability (fluorescein sulfonic acid clearance), and cause redistribution of PKC-beta1 from cytosolic pools into membrane and/or cytoskeletal fractions (assessed by immunoblotting), thus indicating PKC-beta1 activation. Antisense inhibition of PKC-beta1 expression (-90%) prevented these changes and abolished EGF protection. We conclude that EGF protection against oxidants requires PKC-beta1 isoform activation. This mechanism may be useful for development of novel therapies for the treatment of inflammatory gastrointestinal disorders including inflammatory bowel disease.

Regulation of epithelial transport and barrier function by distinct protein kinase C isoforms

The phorbol ester phorbol 12-myristate 13-acetate (PMA) inhibits Cl(-) secretion (short-circuit current, I(sc)) and decreases barrier function (transepithelial resistance, TER) in T84 epithelia. To elucidate the role of specific protein kinase C (PKC) isoenzymes in this response, we compared PMA with two non-phorbol activators of PKC (bryostatin-1 and carbachol) and utilized three PKC inhibitors (Gö-6850, Gö-6976, and rottlerin) with different isozyme selectivity profiles. PMA sequentially inhibited cAMP-stimulated I(sc) and decreased TER, as measured by voltage-current clamp. By subcellular fractionation and Western blot, PMA (100 nM) induced sequential membrane translocation of the novel PKC epsilon followed by the conventional PKC alpha and activated both isozymes by in vitro kinase assay. PKC delta was activated by PMA but did not translocate. By immunofluorescence, PKC epsilon redistributed to the basolateral domain in response to PMA, whereas PKC alpha moved apically. Inhibition of I(sc) by PMA was prevented by the conventional and novel PKC inhibitor Gö-6850 (5 microM) but not the conventional isoform inhibitor Gö-6976 (5 microM) or the PKC delta inhibitor rottlerin (10 microM), implicating PKC epsilon in inhibition of Cl(-) secretion. In contrast, both Gö-6976 and Gö-6850 prevented the decline of TER, suggesting involvement of PKC alpha. Bryostatin-1 (100 nM) translocated PKC epsilon and PKC alpha and inhibited cAMP-elicited I(sc). However, unlike PMA, bryostatin-1 downregulated PKC alpha protein, and the decrease in TER was only transient. Carbachol (100 microM) translocated only PKC epsilon and inhibited I(sc) with no effect on TER. Gö-6850 but not Gö-6976 or rottlerin blocked bryostatin-1 and carbachol inhibition of I(sc). We conclude that basolateral translocation of PKC epsilon inhibits Cl(-) secretion, while apical translocation of PKC alpha decreases TER. These data suggest that epithelial transport and barrier function can be modulated by distinct PKC isoforms.

Functional modeling of tight junctions in intestinal cell monolayers using polyethylene glycol oligomers

Despite significant advances in the characterization of tight junction (TJ) proteins, little is known about how molecular changes relate to function due primarily to the limitations of conventional paracellular probes. To address this, the paracellular pathway in Caco-2 and T84 cell lines was profiled by measuring the permeabilities of 24 polyethylene glycols (PEG) of increasing molecular radius (3.5--7.4 A) analyzed by mass spectrometry. When combined with a paracellular sieving model, these data provided quantitative descriptors of the pathway under control conditions and after exposure to TJ modulators. PEG profiles in both cell lines conformed to a biphasic process involving a restrictive pore (radius 4.3--4.5 A) and a nonrestrictive component responsible for permeability of larger molecules. PEG profiling revealed significant differences between the effects of EGTA and sodium caprate (C10). The restrictive component of EGTA-treated cells lost all size discrimination due to an increase in pore radius. Sodium caprate had no effect on pore radius but increased permeability via a different mechanism possibly involving increased numbers of functional pores. PEG profiling provides a useful tool for probing the functional regulation of the paracellular route.

Modulation of PKCδ tyrosine phosphorylation and activity in salivary and PC-12 cells by Src kinases

Protein kinase C (PKC) δ becomes tyrosine phosphorylated in rat parotid acinar cells exposed to muscarinic and substance P receptor agonists, which initiate fluid secretion in this salivary cell. Here we examine the signaling components of PKCδ tyrosine phosphorylation and effects of phosphorylation on PKCδ activity. Carbachol- and substance P-promoted increases in PKCδ tyrosine phosphorylation were blocked by inhibiting phospholipase C (PLC) but not by blocking intracellular Ca2+ concentration elevation, suggesting that diacylglycerol, rather than d- myo-inositol 1,4,5-trisphosphate production, positively modulated this phosphorylation. Stimuli-dependent increases in PKCδ activity in parotid and PC-12 cells were blocked in vivo by inhibitors of Src tyrosine kinases. Dephosphorylation of tyrosine residues by PTP1B, a protein tyrosine phosphatase, reduced the enhanced PKCδ activity. Lipid cofactors modified the tyrosine phosphorylation-dependent PKCδ activation. Two PKCδ regulatory sites (Thr-505 and Ser-662) were constitutively phosphorylated in unstimulated parotid cells, and these phosphorylations were not altered by stimuli that increased PKCδ tyrosine phosphorylation. These results demonstrate that PKCδ activity is positively modulated by tyrosine phosphorylation in parotid and PC-12 cells and suggest that PLC-dependent effects of secretagogues on salivary cells involve Src-related kinases.

Inflammatory pain alters blood-brain barrier permeability and tight junctional protein expression

Effects of inflammatory pain states on functional and molecular properties of the rat blood-brain barrier (BBB) were investigated. Inflammation was produced by subcutaneous injection of formalin, lambda-carrageenan, or complete Freund's adjuvant (CFA) into the right hind paw. In situ perfusion and Western blot analyses were performed to assess BBB integrity after inflammatory insult. In situ brain perfusion determined that peripheral inflammation significantly increased the uptake of sucrose into the cerebral hemispheres. Capillary depletion and cerebral blood flow analyses indicated the perturbations were due to increased paracellular permeability rather than vascular volume changes. Western blot analyses showed altered tight junctional protein expression during peripheral inflammation. Occludin significantly decreased in the lambda-carrageenan- and CFA-treated groups. Zonula occluden-1 expression was significantly increased in all pain models. Claudin-1 protein expression was present at the BBB and remained unchanged during inflammation. Actin expression was significantly increased in the lambda-carrageenan- and CFA-treated groups. We have shown that inflammatory-mediated pain alters both the functional and molecular properties of the BBB. Inflammatory-induced changes may significantly alter delivery of therapeutic agents to the brain, thus affecting dosing regimens during chronic pain.

Molecular physiology and pathophysiology of tight junctions III. Tight junction regulation by intracellular messengers: differences in response within and between epithelia

Tight junction permeability differs with the type of permeants, their size, and their charge. Selective changes in permeability do occur, and they illustrate the diversity in functional reactions of tight junctions. This suggests that special structures in the tight junctions are involved. More and more structural components of the tight junctions are becoming known. The divergence in behavior of native tissue and filter-grown epithelial monolayers with respect to the effects of intracellular messengers offers the possibility to relate structure and function. In addition to the tools for conventional permeability studies, probes have become available to detect changes in activation of intracellular effector proteins such as the protein kinase C isotypes, and with in situ imaging techniques the way is open for a functional approach in the study of tight junctions.

Enhancing paracellular permeability by modulating epithelial tight junctions

The intestinal epithelium is a major barrier to the absorption of hydrophilic drugs. The presence of intercellular junctional complexes, particularly the tight junctions (zona occludens), renders the epithelium impervious to hydrophilic drugs, which cannot diffuse across the cells through the lipid bilayer of the cell membranes. There have been significant advances in understanding the structure and cellular regulation of tight junctions over the past decade. This article reviews current knowledge regarding the physiological regulation of tight junctions and paracellular permeability, and recent progress towards the rational design of agents that can effectively and safely increase paracellular permeability via modulation of tight junctions.

Regulation of protein kinase C-delta and -epsilon isoforms by phorbol ester treatment of LLC-PK1 renal epithelia

BACKGROUND

LLC-PK1 renal epithelia are a widely used model for proximal tubular physiology and differentiation. Protein kinase C (PKC) has been observed to play a role in both processes. This study examines the subcellular distribution and down-regulation of PKC-delta and PKC-epsilon isoforms in phorbol ester-treated LLC-PK1 epithelia.

METHODS

Cells were treated with 10-7 mol/L 12-O-tetradecanoyl phorbol 13-acetate (TPA) for up to seven days and were extracted as total cell lysates as well as cytosolic, membrane-associated (Triton-X soluble) and a third (Triton-X insoluble) fraction. The expression and cellular localization of PKC-delta and PKC-epsilon isoforms were then detected using Western immunoblot and immunofluorescence.

RESULTS

Based on the use of an anti-PKC-delta monoclonal antibody, TPA was observed to cause a rapid decrease in total PKC-delta content, which then returned to near control levels by seven days of treatment. Immunofluorescence indicated that PKC-delta had a cytoskeletal localization within the cells, and a subtle cytoskeletal rearrangement occurred upon exposure to TPA. Western immunoblots showed that PKC-delta did not undergo the expected membrane translocation upon activation by TPA, but simply disappeared immediately from the cytosolic compartment. Conventional cell fractionation procedures such as homogenization and Triton extraction prior to Western immunoblot will, however, fail to evaluate completely PKC-delta in LLC-PK1 epithelia because of the highly stringent measures necessary to extract PKC-delta from the cytoskeletal compartment of these cells. Furthermore, we observed that a second (polyclonal) PKC-delta antibody may recognize phosphorylated forms of PKC-delta, which went unrecognized by the other antibody. PKC-epsilon was present in the cytosol, membrane, and Triton-X-insoluble fractions of the cells. TPA treatment resulted in a partial translocation of PKC-epsilon to both the membrane and Triton-X-insoluble fractions of the cell, but total PKC-epsilon remained essentially unchanged.

CONCLUSIONS

The present data indicate that the localization of PKC-delta and subsequent redistribution within the LLC-PK1 cells in response to TPA treatment is highly unique and distinct from that of PKC-epsilon and PKC-alpha. An important methodological finding is that one given antibody may not recognize all phosphoproteins of a given PKC isoform.

MAGUK proteins: structure and role in the tight junction

ZO-1, ZO-2 and ZO-3 are tight junction (TJ)-associated proteins that belong to the MAGUK family. In addition to the presence of the characteristic MAGUK modules (PDZ, SH3 and GK), ZOs have a distinctive carboxyl terminal with splicing domains, acidic- and proline-rich regions. The modular organization of these proteins allows them to function as scaffolds, which associate to transmembrane TJ proteins, the cytoskeleton and signal transduction molecules. ZOs shuttle between the TJ and the nucleus, where they may regulate gene expression.

Show me the pathway! Regulation of paracellular permeability by Na(+)-glucose cotransport

The physiological impact of Na(+)-nutrient cotransport-dependent regulation of intestinal tight junction permeability has been controversial. Nonetheless, increased permeability of small intestinal mucosae and enterocyte tight junctions as a consequence of Na(+)-nutrient cotransport has been documented by a significant number of in vivo and in vitro studies. Some details of the intracellular signaling events that regulate this process have been described recently. The aims of this article are to: (i) review studies of tight junction regulation and paracellular nutrient absorption in mammalian intestine, (ii) identify potential applications of tight junction regulation, and (iii) summarize recent progress in defining molecular mechanisms that lead to altered tight junction permeability.

Restoration of tight junction structure and barrier function by down-regulation of the mitogen-activated protein kinase pathway in ras-transformed Madin-Darby canine kidney cells

In the Madin-Darby canine kidney epithelial cell line, the proteins occludin and ZO-1 are structural components of the tight junctions that seal the paracellular spaces between the cells and contribute to the epithelial barrier function. In Ras-transformed Madin-Darby canine kidney cells, occludin, claudin-1, and ZO-1 were absent from cell-cell contacts but were present in the cytoplasm, and the adherens junction protein E-cadherin was weakly expressed. After treatment of the Ras-transformed cells with the mitogen-activated protein kinase kinase (MEK1) inhibitor PD98059, which blocks the activation of mitogen-activated protein kinase (MAPK), occludin, claudin-1, and ZO-1 were recruited to the cell membrane, tight junctions were assembled, and E-cadherin protein expression was induced. Although it is generally believed that E-cadherin-mediated cell-cell adhesion is required for tight junction assembly, the recruitment of occludin to the cell-cell contact area and the restoration of epithelial cell morphology preceded the appearance of E-cadherin at cell-cell contacts. Both electron microscopy and a fourfold increase in the transepithelial electrical resistance indicated the formation of functional tight junctions after MEK1 inhibition. Moreover, inhibition of MAPK activity stabilized occludin and ZO-1 by differentially increasing their half-lives. We also found that during the process of tight junction assembly after MEK1 inhibition, tyrosine phosphorylation of occludin and ZO-1, but not claudin-1, increased significantly. Our study demonstrates that down-regulation of the MAPK signaling pathway causes the restoration of epithelial cell morphology and the assembly of tight junctions in Ras-transformed epithelial cells and that tyrosine phosphorylation of occludin and ZO-1 may play a role in some aspects of tight junction formation.