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

Daphnanes diterpenes from the latex of Hura crepitans L. and their PKCζ-dependent anti-proliferative activity on colorectal cancer cells

Hura crepitans L. (Euphorbiaceae) is a thorn-covered tree widespread in South America, Africa and Asia which produces an irritating milky latex containing numerous secondary metabolites, notably daphnane-type diterpenes known as Protein Kinase C activators. Fractionation of a dichloromethane extract of the latex led to the isolation of five new daphnane diterpenes (1-5), along with two known analogs (6-7) including huratoxin. Huratoxin (6) and 4',5'-epoxyhuratoxin (4) were found to exhibit significant and selective cell growth inhibition against colorectal cancer cell line Caco-2 and primary colorectal cancer cells cultured as colonoids. The underlying mechanism of 4 and 6 was further investigated revealing the involvement of PKCζ in the cytostatic activity.

Oxygen-dependent regulation of permeability in low resistance intestinal epithelial cells infected with Giardia lamblia

Intestinal epithelial cells (IECs) reside in a highly anaerobic environment that is subject to daily fluctuations in partial oxygen pressure (pO₂), depending on intestinal tissue perfusion. This condition, known as physiological hypoxia, has a major impact on the maintenance of gut homeostasis, such as effects on the integrity and function of the intestinal epithelial barrier. Giardia lamblia is a microaerophilic protozoan parasite that infects and colonizes the small intestine of its host, causing watery diarrhea. The disease, known as giardiasis, is associated with enhanced intestinal permeability and disruption or reorganization of tight junction (TJ) proteins between IECs. Given the central role of oxygen in gut homeostasis, in this study, we aimed to evaluate whether pO₂ affects intestinal permeability (flux of ions and macromolecules) and TJ protein expression in human IECs during G. lamblia infection. Using human cell lines HuTu-80 and Caco-2 as models of "loose" (low resistance) and "tight" (high resistance) intestines, respectively, we elucidated that low pO₂ drives intestinal barrier dysfunction in IECs infected with trophozoites through dephosphorylation of protein kinase C (PKC α/β II). Additionally, we demonstrated that IECs infected with trophozoites in the presence of a pharmacological PKC activator (phorbol 12-myristate 13-acetate) partially restored the barrier function, which was correlated with increased protein expression levels of zonula occludens (ZO)-2 and occludin. Collectively, these results support the emerging theory that molecular oxygen impacts gut homeostasis during Giardia infection via direct host signaling pathways. These findings further our knowledge regarding Giardia-host interactions and the pathophysiological mechanisms of human giardiasis.

Taxifolin ameliorate high-fat-diet feeding plus acute ethanol binge-induced steatohepatitis through inhibiting inflammatory caspase-1-dependent pyroptosis

Excessive alcohol drinking and a high-fat diet (HFD) promote steatohepatitis in the comorbidity of NAFLD and AFLD. Taxifolin (TAX) is a rich dihydroxyflavone compound found in onions, milk thistle and Douglas fir. We aimed to explore the intervention mechanism of TAX on chronic steatohepatitis induced by HFD feeding plus acute ethanol binge. We established an in vivo model by HFD feeding plus a single dose of ethanol binge, and established an in vitro model by oleic acid or palmitic acid on HepG2 cells to induce lipid accumulation. TAX regulated lipid synthesis by inhibiting the expression of SREBP1 and upregulating the PPARγ level. In addition, TAX inhibited the expression of P2X7R, IL-1β, and caspase-1. Moreover, TAX reduced the expression of caspase-1 activation; thereby inhibiting the recruitment of macrophages and neutrophils. TAX also improved the inflammatory response caused by caspase-1 activation in steatotic hepatocytes. TAX exhibited an inhibitory effect on lipid accumulation and caspase-1-related pyroptosis. Collectively, TAX has therapeutic potential as an intervention of steatohepatitis induced by alcohol combined with HFD and for preventing non-alcoholic fatty liver degeneration targeting caspase-1-dependent pyroptosis.

Anatomical and Gene Expression Changes in the Retinal Pigmented Epithelium Atrophy 1 (rpea1) Mouse: A Potential Model of Serous Retinal Detachment

Purpose

The purpose of this study was to examine the rpea1 mouse whose retina spontaneously detaches from the underlying RPE as a potential model for studying the cellular effects of serous retinal detachment (SRD).

Methods

Optical coherence tomography (OCT) was performed immediately prior to euthanasia; retinal tissue was subsequently prepared for Western blotting, microarray analysis, immunocytochemistry, and light and electron microscopy (LM, EM).

Results

By postnatal day (P) 30, OCT, LM, and EM revealed the presence of small shallow detachments that increased in number and size over time. By P60 in regions of detachment, there was a dramatic loss of PNA binding around cones in the interphotoreceptor matrix and a concomitant increase in labeling of the outer nuclear layer and rod synaptic terminals. Retinal pigment epithelium wholemounts revealed a patchy loss in immunolabeling for both ezrin and aquaporin 1. Anti-ezrin labeling was lost from small regions of the RPE apical surface underlying detachments at P30. Labeling for tight-junction proteins provided a regular array of profiles outlining the periphery of RPE cells in wild-type tissue, however, this pattern was disrupted in the mutant as early as P30. Microarray analysis revealed a broad range of changes in genes involved in metabolism, signaling, cell polarity, and tight-junction organization.

Conclusions

These data indicate changes in this mutant mouse that may provide clues to the underlying mechanisms of SRD in humans. Importantly, these changes include the production of multiple spontaneous detachments without the presence of a retinal tear or significant degeneration of outer segments, changes in the expression of proteins involved in adhesion and fluid transport, and a disrupted organization of RPE tight junctions that may contribute to the formation of focal detachments.

The Novel PKCθ from Benchtop to Clinic

The protein kinases C (PKCs) are a family of serine/threonine kinases involved in regulating multiple essential cellular processes such as survival, proliferation, and differentiation. Of particular interest is the novel, calcium-independent PKCθ which plays a central role in immune responses. PKCθ shares structural similarities with other PKC family members, mainly consisting of an N-terminal regulatory domain and a C-terminal catalytic domain tethered by a hinge region. This isozyme, however, is unique in that it translocates to the immunological synapse between a T cell and an antigen-presenting cell (APC) upon T cell receptor-peptide MHC recognition. Thereafter, PKCθ interacts physically and functionally with downstream effectors to mediate T cell activation and differentiation, subsequently leading to inflammation. PKCθ-specific perturbations have been identified in several diseases, most notably autoimmune disorders, and hence the modulation of its activity presents an attractive therapeutic intervention. To that end, many inhibitors of PKCs and PKCθ have been developed and tested in preclinical and clinical studies. And although selectivity remains a challenge, results are promising for the future development of effective PKCθ inhibitors that would greatly advance the treatment of several T-cell mediated diseases.

Emerging multifunctional roles of Claudin tight junction proteins in bone

The imbalance between bone formation and resorption during bone remodeling has been documented to be a major factor in the pathogenesis of osteoporosis. Recent evidence suggests a significant role for the tight junction proteins, Claudins (Cldns), in the regulation of bone remodeling processes. In terms of function, whereas Cldns act "canonically" as key determinants of paracellular permeability, there is considerable recent evidence to suggest that Cldns also participate in cell signaling, ie, a "noncanonical function". To this end, Cldns have been shown to regulate cell proliferation, differentiation, and gene expression in a variety of cell types. The present review will discuss Cldns' structure, their expression profile, regulation of expression, and their canonical and non- canonical functions in general with special emphasis on bone cells. In order to shed light on the noncanonical functions of Cldns in bone, we will highlight the role of Cldn-18 in regulating bone resorption and osteoclast differentiation. Collectively, we hope to provide a framework for guiding future research on understanding how Cldns modulate osteoblast and osteoclast function and overall bone homeostasis. Such studies should provide valuable insights into the pathogenesis of osteoporosis, and may highlight Cldns as novel targets for the diagnosis and therapeutic management of osteoporosis.

Clinicopathologic significance of claudin-6, occludin, and matrix metalloproteinases -2 expression in ovarian carcinoma

Background

Tight junctions (TJs) are mainly composed of claudins, occludin, and tight junction adhesion molecules (JAM). The invasive and metastatic phenotype of highly invasive cancer cells has been related to abnormal structure and function of TJs, and with expression of activated matrix metalloproteinases (MMPs). The relevance of these mechanisms responsible for the invasion and metastasis of ovarian carcinoma is unclear. Similarly, it is not known if the expression of claudin-6, occludin and MMP2 is related with the clinical properties of these tumors.

Methods

Expression of claudin-6, occludin, and MMP2 was detected in samples of human ovarian cancer tissues by immunohistochemistry and correlated with the clinical properties of the tumors.

Results

The positive expression rates of claudin-6 and MMP-2 were higher in ovarian papillary serous carcinomas than n ovarian serous adenomas (P < 0.05). There were no differences in the expression of occludin (P > 0.05). The expression of claudin-6 and occludin in ovarian cancer was not correlated with patient age, pathological grade, clinical stage, and metastasis (P > 0.05). MMP-2 expression was enhanced with increased clinical stage and metastasis (P < 0.05), but was unrelated to patient age or tumor grade (P > 0.05). There were no apparent correlations between expression of claudin-6, occludin and MMP-2 in ovarian cancer tissue (P > 0.05).

Conclusions

Our data suggest, for the first time, that the claudin-6 and MMP-2 are up-regulated in ovarian papillary serous carcinomas, MMP-2 expression was enhanced with increased clinical stage and metastasis. Claudin-6 and MMP-2 may play a positive role in the invasion and metastasis of ovarian cancer.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1775628454106511.

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.

Oxidative stress and innate immunity status in chickens exposed to high dose of ascorbic acid

The effects of high dose ascorbic acid (10 000 mg·kg(-1) in the diet) and the transition metal on the presence of oxidative stress in the internal organs of growing chicks, as well as on the innate immune system status, were investigated. Supplementation with a high dose of ascorbic acid had pro-inflammatory effects on the intestinal mucosa, and lysozyme levels were decreased significantly in all organs studied. High-dose ascorbic acid caused an imbalance between prooxidative and antioxidative activities and was associated with the generation of semiquinone radicals. We observed that ascorbic acid increased iron and cadmium absorption. When a high dose of ascorbic acid was applied, elevated kidney and intestinal mucosa iron concentrations were observed. The amount of free malondialdehyde in the above organs has increased as well. These data have important implications for the mechanism of the oxidative stress development under the influence of high dose of ascorbic acid, indicating the importance of the side reactions of the mitochondrial electron transport chain with the formation of semiquinone radicals and the role of transition metals in this process.

Disruption of spectrin-like cytoskeleton in differentiating keratinocytes by PKCδ activation is associated with phosphorylated adducin

Spectrin is a central component of the cytoskeletal protein network in a variety of erythroid and non-erythroid cells. In keratinocytes, this protein has been shown to be pericytoplasmic and plasma membrane associated, but its characteristics and function have not been established in these cells. Here we demonstrate that spectrin increases dramatically in amount and is assembled into the cytoskeleton during differentiation in mouse and human keratinocytes. The spectrin-like cytoskeleton was predominantly organized in the granular and cornified layers of the epidermis and disrupted by actin filament inhibitors, but not by anti-mitotic drugs. When the cytoskeleton was disrupted PKCδ was activated by phosphorylation on Thr505. Specific inhibition of PKCδ^(Thr505) activation with rottlerin prevented disruption of the spectrin-like cytoskeleton and the associated morphological changes that accompany differentiation. Rottlerin also inhibited specific phosphorylation of the PKCδ substrate adducin, a cytoskeletal protein. Furthermore, knock-down of endogenous adducin affected not only expression of adducin, but also spectrin and PKCδ, and severely disrupted organization of the spectrin-like cytoskeleton and cytoskeletal distribution of both adducin and PKCδ. These results demonstrate that organization of a spectrin-like cytoskeleton is associated with keratinocytes differentiation, and disruption of this cytoskeleton is mediated by either PKCδ^(Thr505) phosphorylation associated with phosphorylated adducin or due to reduction of endogenous adducin, which normally connects and stabilizes the spectrin-actin complex.

PKCθ activation in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters and growth factors is needed for stimulation of numerous important cellular signaling cascades

The novel PKCθ isoform is highly expressed in T-cells, brain and skeletal muscle and originally thought to have a restricted distribution. It has been extensively studied in T-cells and shown to be important for apoptosis, T-cell activation and proliferation. Recent studies showed its presence in other tissues and importance in insulin signaling, lung surfactant secretion, intestinal barrier permeability, platelet and mast-cell functions. However, little information is available for PKCθ activation by gastrointestinal (GI) hormones/neurotransmitters and growth factors. In the present study we used rat pancreatic acinar cells to explore their ability to activate PKCθ and the possible interactions with important cellular mediators of their actions. Particular attention was paid to cholecystokinin (CCK), a physiological regulator of pancreatic function and important in pathological processes affecting acinar function, like pancreatitis. PKCθ-protein/mRNA was present in the pancreatic acini, and T538-PKCθ phosphorylation/activation was stimulated only by hormones/neurotransmitters activating phospholipase C. PKCθ was activated in time- and dose-related manner by CCK, mediated 30% by high-affinity CCK(A)-receptor activation. CCK stimulated PKCθ translocation from cytosol to membrane. PKCθ inhibition (by pseudostrate-inhibitor or dominant negative) inhibited CCK- and TPA-stimulation of PKD, Src, RafC, PYK2, p125(FAK) and IKKα/β, but not basal/stimulated enzyme secretion. Also CCK- and TPA-induced PKCθ activation produced an increment in PKCθ's direct association with AKT, RafA, RafC and Lyn. These results show for the first time the PKCθ presence in pancreatic acinar cells, its activation by some GI hormones/neurotransmitters and involvement in important cell signaling pathways mediating physiological responses (enzyme secretion, proliferation, apoptosis, cytokine expression, and pathological responses like pancreatitis and cancer growth).

Protein kinase C activation modulates reversible increase in cortical blood-brain barrier permeability and tight junction protein expression during hypoxia and posthypoxic reoxygenation

Hypoxia (Hx) is a component of many disease states including stroke. Ischemic stroke occurs when there is a restriction of cerebral blood flow and oxygen to part of the brain. During the ischemic, and subsequent reperfusion phase of stroke, blood-brain barrier (BBB) integrity is lost with tight junction (TJ) protein disruption. However, the mechanisms of Hx and reoxygenation (HR)-induced loss of BBB integrity are not fully understood. We examined the role of protein kinase C (PKC) isozymes in modifying TJ protein expression in a rat model of global Hx. The Hx (6% O(2)) induced increased hippocampal and cortical vascular permeability to 4 and 10 kDa dextran fluorescein isothiocyanate (FITC) and endogenous rat-IgG. Cortical microvessels revealed morphologic changes in nPKC-θ distribution, increased nPKC-θ and aPKC-ζ protein expression, and activation by phosphorylation of nPKC-θ (Thr538) and aPKC-ζ (Thr410) residues after Hx treatment. Claudin-5, occludin, and ZO-1 showed disrupted organization at endothelial cell margins, whereas Western blot analysis showed increased TJ protein expression after Hx. The PKC inhibition with chelerythrine chloride (5 mg/kg intraperitoneally) attenuated Hx-induced hippocampal vascular permeability and claudin-5, PKC (θ and ζ) expression, and phosphorylation. This study supports the hypothesis that nPKC-θ and aPKC-ζ signaling mediates TJ protein disruption resulting in increased BBB permeability.

Phosphorylation of alpha6-tubulin by protein kinase Calpha activates motility of human breast cells

Engineered overexpression of protein kinase Calpha (PKCalpha) was previously shown to endow nonmotile MCF-10A human breast cells with aggressive motility. A traceable mutant of PKCalpha (Abeyweera, T. P., and Rotenberg, S. A. (2007) Biochemistry 46, 2364-2370) revealed that alpha6-tubulin is phosphorylated in cells expressing traceable PKCalpha and in vitro by wild type PKCalpha. Gain-of-function, single site mutations (Ser-->Asp) were constructed at each PKC consensus site in alpha6-tubulin (Ser158, Ser165, Ser241, and Thr337) to simulate phosphorylation. Following expression of each construct in MCF-10A cells, motility assays identified Ser165 as the only site in alpha6-tubulin whose pseudophosphorylation reproduced the motile behavior engendered by PKCalpha. Expression of a phosphorylation-resistant mutant (S165N-alpha6-tubulin) resulted in suppression of MCF-10A cell motility stimulated either by expression of PKCalpha or by treatment with PKCalpha-selective activator diacylglycerol-lactone. MCF-10A cells treated with diacylglycerol-lactone showed strong phosphorylation of endogenous alpha-tubulin that could be blocked when S165N-alpha6-tubulin was expressed. The S165N mutant also inhibited intrinsically motile human breast tumor cells that express high endogenous PKCalpha levels (MDA-MB-231 cells) or lack PKCalpha and other conventional isoforms (MDA-MB-468 cells). Comparison of Myc-tagged wild type alpha6-tubulin and S165N-alpha6-tubulin expressed in MDA-MB-468 cells demonstrated that Ser165 is also a major site of phosphorylation for endogenously active, nonconventional PKC isoforms. PKC-stimulated motility of MCF-10A cells was nocodazole-sensitive, thereby implicating microtubule elongation in the mechanism. These findings support a model in which PKC phosphorylates alpha-tubulin at Ser165, leading to microtubule elongation and motility.

Calcineurin and cytoskeleton in low-frequency depression

Transmitter release at high probability phasic synapses of crayfish neuromuscular junctions depresses by over 50% in 60 min when stimulated at 0.2 Hz. Inhibition of the protein phosphatase calcineurin by intracellular pre-synaptic injection of autoinhibitory peptide inhibited low-frequency depression (LFD) and resulted in facilitation of transmitter release. Since this inhibitor had no major effects when injected into the post-synaptic cell, only pre-synaptic calcineurin activity is necessary for LFD. To examine changes in phosphoproteins during LFD we performed a phosphoproteomic screen on proteins extracted from motor axons and nerve terminals after LFD induction or treatment with various drugs that affect kinase and phosphatase activity. Proteins separated by PAGE were stained with phospho-specific/total protein ratio stains (Pro-Q Diamond/SYPRO Ruby) to identify protein bands for analysis by mass spectrometry. Phosphorylation of actin and tubulin decreased during LFD, but increased when calcineurin was blocked. Tubulin and phosphoactin immunoreactivity in pre-synaptic terminals were also reduced after LFD. The actin depolymerizing drugs cytochalasin and latrunculin and the microtubule stabilizer taxol inhibited LFD. Therefore, dephosphorylation of pre-synaptic actin and tubulin and consequent changes in the cytoskeleton may regulate LFD. LFD is unlike long-term depression found in mammalian synapses because the latter requires in most instances post-synaptic calcineurin activity.Thus, this simpler invertebrate synapse discloses a novel pre-synaptic depression mechanism.

Protein kinase C-theta regulates KIT expression and proliferation in gastrointestinal stromal tumors

Oncogenic KIT or PDGFRA receptor tyrosine kinase mutations are compelling therapeutic targets in gastrointestinal stromal tumors (GISTs), and the KIT/PDGFRA kinase inhibitor, imatinib, is standard of care for patients with metastatic GIST. However, most of these patients eventually develop clinical resistance to imatinib and other KIT/PDGFRA kinase inhibitors and there is an urgent need to identify novel therapeutic strategies. We reported previously that protein kinase C-theta (PKCtheta) is activated in GIST, irrespective of KIT or PDGFRA mutational status, and is expressed at levels unprecedented in other mesenchymal tumors, therefore serving as a diagnostic marker of GIST. Herein, we characterize biological functions of PKCtheta in imatinib-sensitive and imatinib-resistant GISTs, showing that lentivirus-mediated PKCtheta knockdown is accompanied by inhibition of KIT expression in three KIT+/PKCtheta+ GIST cell lines, but not in a comparator KIT+/PKCtheta- Ewing's sarcoma cell line. PKCtheta knockdown in the KIT+ GISTs was associated with inhibition of the phosphatidylinositol-3-kinase/AKT signaling pathway, upregulation of the cyclin-dependent kinase inhibitors p21 and p27, antiproliferative effects due to G(1) arrest and induction of apoptosis, comparable to the effects seen after direct knockdown of KIT expression by KIT short-hairpin RNA. These novel findings highlight that PKCtheta warrants clinical evaluation as a potential therapeutic target in GISTs, including those cases containing mutations that confer resistance to KIT/PDGFRA kinase inhibitors.

Atypical protein kinase C (iota) activates ezrin in the apical domain of intestinal epithelial cells

Atypical protein kinase iota (PKCiota) is a key organizer of the apical domain in epithelial cells. Ezrin is a cytosolic protein that, upon activation by phosphorylation of T567, is localized under the apical membrane where it connects actin filaments to membrane proteins and recruits protein kinase A (PKA). To identify the kinase that phosphorylates ezrin T567 in simple epithelia, we analyzed the expression of active PKC and the appearance of T567-P during enterocyte differentiation in vivo. PKCiota phosphorylated ezrin on T567 in vitro, and in Sf9 cells that do not activate human ezrin. In CACO-2 human intestinal cells in culture, PKCiota co-immunoprecipitated with ezrin and was knocked down by shRNA expression. The resulting phenotype showed a modest decrease in total ezrin, but a steep decrease in T567 phosphorylation. The PKCiota-depleted cells showed fewer and shorter microvilli and redistribution of the PKA regulatory subunit. Expression of a dominant-negative form of PKCiota also decreased T567-P signal, and expression of a constitutively active PKCiota mutant showed depolarized distribution of T567-P. We conclude that, although other molecular mechanisms contribute to ezrin activation, apically localized phosphorylation by PKCiota is essential for the activation and normal distribution of ezrin at the early stages of intestinal epithelial cell differentiation.

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.

Hepatic PCSK9 expression is regulated by nutritional status via insulin and sterol regulatory element-binding protein 1c

Familial autosomal dominant hypercholesterolemia is associated with high risk for cardiovascular accidents and is related to mutations in the low density lipoprotein receptor or its ligand apolipoprotein B (apoB). Mutations in a third gene, proprotein convertase subtilisin kexin 9 (PCSK9), were recently associated to this disease. PCSK9 acts as a natural inhibitor of the low density lipoprotein receptor pathway, and both genes are regulated by depletion of cholesterol cell content and statins, via sterol regulatory element-binding protein (SREBP). Here we investigated the regulation of PCSK9 gene expression during nutritional changes. We showed that PCSK9 mRNA quantity is decreased by 73% in mice after 24 h of fasting, leading to a 2-fold decrease in protein level. In contrast PCSK9 expression was restored upon high carbohydrate refeeding. PCSK9 mRNA increased by 4-5-fold in presence of insulin in rodent primary hepatocytes, whereas glucose had no effect. Moreover, insulin up-regulated hepatic PCSK9 expression in vivo during a hyperinsulinemic-euglycemic clamp in mice. Adenoviral mediated overexpression of a dominant or negative form of SREBP-1c confirmed the implication of this transcription factor in insulin-mediated stimulation of PCSK9 expression. Liver X receptor agonist T0901317 also regulated PCSK9 expression via this same pathway (a 2-fold increase in PCSK9 mRNA of primary hepatocytes cultured for 24 h in presence of 1 microm T0901317). As our last investigation, we isolated PCSK9 proximal promoter and verified the functionality of a SREBP-1c responsive element located from 335 bp to 355 bp upstream of the ATG. Together, these results show that PCSK9 expression is regulated by nutritional status and insulinemia.

Activation of PKC modulates blood-brain barrier endothelial cell permeability changes induced by hypoxia and posthypoxic reoxygenation

The blood-brain barrier (BBB) is a metabolic and physiological barrier important for maintaining brain homeostasis. The aim of this study was to determine the role of PKC activation in BBB paracellular permeability changes induced by hypoxia and posthypoxic reoxygenation using in vitro and in vivo BBB models. In rat brain microvessel endothelial cells (RMECs) exposed to hypoxia (1% O2-99% N2; 24 h), a significant increase in total PKC activity was observed, and this was reduced by posthypoxic reoxygenation (95% room air-5% CO2) for 2 h. The expression of PKC-βII, PKC-γ, PKC-η, PKC-μ, and PKC-λ also increased following hypoxia (1% O2-99% N2; 24 h), and these protein levels remained elevated following posthypoxic reoxygenation (95% room air-5% CO2; 2 h). Increases in the expression of PKC-ε and PKC-ζ were also observed following posthypoxic reoxygenation (95% room air-5% CO2; 2 h). Moreover, inhibition of PKC with chelerythrine chloride (10 μM) attenuated the hypoxia-induced increases in [14C]sucrose permeability. Similar to what was observed in RMECs, total PKC activity was also stimulated in cerebral microvessels isolated from rats exposed to hypoxia (6% O2-94% N2; 1 h) and posthypoxic reoxygenation (room air; 10 min). In contrast, hypoxia (6% O2-94% N2; 1 h) and posthypoxic reoxygenation (room air; 10 min) significantly increased the expression levels of only PKC-γ and PKC-θ in the in vivo hypoxia model. These data demonstrate that hypoxia-induced BBB paracellular permeability changes occur via a PKC-dependent mechanism, possibly by differentially regulating the protein expression of the 11 PKC isozymes.

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.

Substrate cycling between de novo lipogenesis and lipid oxidation: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity

Life is a combustion, but how the major fuel substrates that sustain human life compete and interact with each other for combustion has been at the epicenter of research into the pathogenesis of insulin resistance ever since Randle proposed a 'glucose-fatty acid cycle' in 1963. Since then, several features of a mutual interaction that is characterized by both reciprocality and dependency between glucose and lipid metabolism have been unravelled, namely: the inhibitory effects of elevated concentrations of fatty acids on glucose oxidation (via inactivation of mitochondrial pyruvate dehydrogenase or via desensitization of insulin-mediated glucose transport),the inhibitory effects of elevated concentrations of glucose on fatty acid oxidation (via malonyl-CoA regulation of fatty acid entry into the mitochondria), and more recentlythe stimulatory effects of elevated concentrations of glucose on de novo lipogenesis, that is, synthesis of lipids from glucose (via SREBP1c regulation of glycolytic and lipogenic enzymes). This paper first revisits the physiological significance of these mutual interactions between glucose and lipids in skeletal muscle pertaining to both blood glucose and intramyocellular lipid homeostasis. It then concentrates upon emerging evidence, from calorimetric studies investigating the direct effect of leptin on thermogenesis in intact skeletal muscle, of yet another feature of the mutual interaction between glucose and lipid oxidation: that of substrate cycling between de novo lipogenesis and lipid oxidation. It is proposed that this energy-dissipating substrate cycling that links glucose and lipid metabolism to thermogenesis could function as a 'fine-tuning' mechanism that regulates intramyocellular lipid homeostasis, and hence contributes to the protection of skeletal muscle against lipotoxicity.

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.

Novel effect of NF-kappaB activation: carbonylation and nitration injury to cytoskeleton and disruption of monolayer barrier in intestinal epithelium

Using monolayers of intestinal cells, we reported that upregulation of inducible nitric oxide synthase (iNOS) is required for oxidative injury and that activation of NF-kappaB is key to cytoskeletal instability. In the present study, we hypothesized that NF-kappaB activation is crucial to oxidant-induced iNOS upregulation and its injurious consequences: cytoskeletal oxidation and nitration and monolayer dysfunction. Wild-type (WT) cells were pretreated with inhibitors of NF-kappaB, with or without exposure to oxidant (H(2)O(2)). Other cells were transfected with an IkappaBalpha mutant (an inhibitor of NF-kappaB). Relative to WT cells exposed to vehicle, oxidant exposure caused increases in IkappaBalpha instability, NF-kappaB subunit activation, iNOS-related activity (NO, oxidative stress, tubulin nitration), microtubule disassembly and instability (increased monomeric and decreased polymeric tubulin), and monolayer disruption. Monolayers pretreated with NF-kappaB inhibitors (MG-132, lactacystin) were protected against oxidation, showing decreases in all measures of the NF-kappaB --> iNOS --> NO pathway. Dominant mutant stabilization of IkappaBalpha to inactivate NF-kappaB suppressed all measures of the iNOS/NO upregulation while protecting monolayers against oxidant insult. In these mutants, we found prevention of tubulin nitration and oxidation and enhancement of cytoskeletal and monolayer stability. We concluded that 1) NF-kappaB is required for oxidant-induced iNOS upregulation and for the consequent nitration and oxidation of cytoskeleton; 2) NF-kappaB activation causes cytoskeletal injury following upregulation of NO-driven processes; and 3) the molecular event underlying the destabilizing effects of NF-kappaB appears to be increases in carbonylation and nitrotyrosination of the subunit components of cytoskeleton. The ability to promote NO overproduction and cytoskeletal nitration/oxidation is a novel mechanism not previously attributed to NF-kappaB in cells.