The role of protein kinase C isozymes in TNF-alpha-induced cytotoxicity to a rat intestinal epithelial cell line

Unleashing Spinal Cord Repair: The Role of cAMP-Specific PDE Inhibition in Attenuating Neuroinflammation and Boosting Regeneration after Traumatic Spinal Cord Injury

Traumatic spinal cord injury (SCI) is characterized by severe neuroinflammation and hampered neuroregeneration, which often leads to permanent neurological deficits. Current therapies include decompression surgery, rehabilitation, and in some instances, the use of corticosteroids. However, the golden standard of corticosteroids still achieves minimal improvements in functional outcomes. Therefore, new strategies tackling the initial inflammatory reactions and stimulating endogenous repair in later stages are crucial to achieving functional repair in SCI patients. Cyclic adenosine monophosphate (cAMP) is an important second messenger in the central nervous system (CNS) that modulates these processes. A sustained drop in cAMP levels is observed during SCI, and elevating cAMP is associated with improved functional outcomes in experimental models. cAMP is regulated in a spatiotemporal manner by its hydrolyzing enzyme phosphodiesterase (PDE). Growing evidence suggests that inhibition of cAMP-specific PDEs (PDE4, PDE7, and PDE8) is an important strategy to orchestrate neuroinflammation and regeneration in the CNS. Therefore, this review focuses on the current evidence related to the immunomodulatory and neuroregenerative role of cAMP-specific PDE inhibition in the SCI pathophysiology.

Testicular Lmcd1 regulates phagocytosis by Sertoli cells through modulation of NFAT1/Txlna signaling pathway

Increased oxidative stress is well known to cause testicular dysfunction in aging males, but the detailed relationships between aging, oxidative stress, and testicular function remain to be elucidated. LIM and cysteine‐rich domains 1 (LMCD1) regulates fundamentally cellular process by interacting with transcription factors. A recent study has identified Lmcd1 as one of the most upregulated nuclear proteins associated with Sertoli cell (SC) differentiation, raising the possibility that testicular actions of LMCD1 are likely to take place. Herein, we reported that LMCD1 was exclusively expressed in the nuclei of SCs. This expression was regulated by TNF‐α signaling produced by apoptotic germ cells (GCs) and was suppressed by oxidative stress in a STAT3‐dependent manner. Ablation of endogenous LMCD1 expression caused lipid accumulation and senescence in GC co‐incubated SCs. Using a previously validated in vivo siRNA approach, we showed that LMCD1 depletion significantly impaired male fertility by inducing oligozoospermia and asthenospermia. Mechanistically, LMCD1 upregulation was associated with the nuclear enrichment of the nuclear factor of activated T cells 1 (NFAT1), a core component of Ca²⁺/calmodulin‐dependent pathway. LMCD1 facilitated the dephosphorylation and nuclear translocation of NFAT1, which consequently expedited the transactivation of Txlna, a binding partner of the syntaxin family essential for testicular phagocytosis, and thus promoted the removal of apoptotic GCs by phagocytic SCs. Collectively, LMCD1 may operate as a novel pretranscriptional integrator linking SC phagocytosis, lipid homeostasis, and cell senescence.

MARCKS-related protein regulates cytoskeletal organization at cell-cell and cell-substrate contacts in epithelial cells

Treatment of epithelial cells with interferon-γ and TNF-α (IFN/TNF) results in increased paracellular permeability. To identify relevant proteins mediating barrier disruption, we performed proximity-dependent biotinylation (BioID) of occludin and found that tagging of MARCKS-related protein (MRP; also known as MARCKSL1) increased ∼20-fold following IFN/TNF administration. GFP-MRP was focused at the lateral cell membrane and its overexpression potentiated the physiological response of the tight junction barrier to cytokines. However, deletion of MRP did not abrogate the cytokine responses, suggesting that MRP is not required in the occludin-dependent IFN/TNF response. Instead, our results reveal a key role for MRP in epithelial cells in control of multiple actin-based structures, likely by regulation of integrin signaling. Changes in focal adhesion organization and basal actin stress fibers in MRP-knockout (KO) cells were reminiscent of those seen in FAK-KO cells. In addition, we found alterations in cell-cell interactions in MRP-KO cells associated with increased junctional tension, suggesting that MRP may play a role in focal adhesion-adherens junction cross talk. Together, our results are consistent with a key role for MRP in cytoskeletal organization of cell contacts in epithelial cells.

Vibrio cholerae Cholix Toxin-Induced HepG2 Cell Death is Enhanced by Tumor Necrosis Factor-Alpha Through ROS and Intracellular Signal-Regulated Kinases

Cholix toxin (Cholix) from Vibrio cholerae is a potent virulence factor exhibiting ADP-ribosyltransferase activity on eukaryotic elongation factor 2 (eEF2) of host cells, resulting in the inhibition of protein synthesis. Administration of Cholix or its homologue Pseudomonas exotoxin A (PEA) to mice causes lethal hepatocyte damage. In this study, we demonstrate cytotoxicity of Cholix on human hepatocytes in the presence of tumor necrosis factor α (TNF-α), which has been reported to play a fatal role in PEA administered to mice. Compared with incubating HepG2 cells with Cholix alone, co-treatment with TNF-α and Cholix (TNF-α/Cholix) significantly enhanced the activation of caspases, cytochrome c release from mitochondria into cytoplasm, and poly-ADP-ribose polymerase (PARP) cleavage, while incubation with TNF-α alone or co-treatment with TNF-α/catalytically inactive Cholix did not. In the early stage of cell death, Cholix increased phosphorylation of mitogen-activated protein kinases (e.g., p38, ERK, JNK) and Akt, which was not affected by TNF-α alone. MAPK inhibitors (SP600125, SB20852, and U0126) suppressed PARP cleavage induced by TNF-α/Cholix. Protein kinase inhibitor Go6976 suppressed JNK phosphorylation and PARP cleavage by TNF-α/Cholix. In contrast, PKC activator PMA in the absence of TNF-α promoted Cholix-induced PARP cleavage. Reactive oxygen species (ROS) inhibitor, N-acetyl cysteine (NAC), suppressed TNF-α/Cholix-induced JNK and ERK phosphorylation, resulting in inhibition of PARP cleavage. These data suggest that ROS and JNK pathways are important mediators of TNF-α/Cholix-induced HepG2 cell death.

TNF-α stimulates endothelial palmitic acid transcytosis and promotes insulin resistance

Persistent elevation of plasma TNF-α is a marker of low grade systemic inflammation. Palmitic acid (PA) is the most abundant type of saturated fatty acid in human body. PA is bound with albumin in plasma and could not pass through endothelial barrier freely. Albumin-bound PA has to be transported across monolayer endothelial cells through intracellular transcytosis, but not intercellular diffusion. In the present study, we discovered that TNF-α might stimulate PA transcytosis across cardiac microvascular endothelial cells, which further impaired the insulin-stimulated glucose uptake by cardiomyocytes and promoted insulin resistance. In this process, TNF-α-stimulated endothelial autophagy and NF-κB signaling crosstalk with each other and orchestrate the whole event, ultimately result in increased expression of fatty acid transporter protein 4 (FATP4) in endothelial cells and mediate the increased PA transcytosis across microvascular endothelial cells. Hopefully the present study discovered a novel missing link between low grade systemic inflammation and insulin resistance.

Vasoactive intestinal peptide prevents PKCε-induced intestinal epithelial barrier disruption during EPEC infection

We previously showed that vasoactive intestinal peptide (VIP) protects against bacterial pathogen-induced epithelial barrier disruption and colitis, although the mechanisms remain poorly defined. The aim of the current study was to identify cellular pathways of VIP-mediated protection with use of pharmacological inhibitors during enteropathogenic Escherichia coli (EPEC) infection of Caco-2 cell monolayers and during Citrobacter rodentium-induced colitis. EPEC-induced epithelial barrier disruption involved the PKC pathway but was independent of functional cAMP, Rho, and NF-κB pathways. VIP mediated its protective effects by inhibiting EPEC-induced PKC activity and increasing expression of the junctional protein claudin-4. Short-term treatment with TPA, which is known to activate PKC, was inhibited by VIP pretreatment, while PKC degradation via long-term treatment with TPA mimicked the protective actions of VIP. Immunostaining for specific PKC isotypes showed upregulated expression of PKCθ and PKCε during EPEC infection. Treatment with specific inhibitors revealed a critical role for PKCε in EPEC-induced barrier disruption. Furthermore, activation of PKCε and loss of barrier integrity correlated with claudin-4 degradation. In contrast, inhibition of PKCε by VIP pretreatment or the PKCε inhibitor maintained membrane-bound claudin-4 levels, along with barrier function. Finally, in vivo treatment with the PKCε inhibitor protected mice from C. rodentium-induced colitis. In conclusion, EPEC infection increases intracellular PKCε levels, leading to decreased claudin-4 levels and compromising epithelial barrier integrity. VIP inhibits PKCε activation, thereby attenuating EPEC-induced barrier disruption.

GREEN TEA POLYPHENOLS MEDIATED APOPTOSIS IN INTESTINAL EPITHELIAL CELLS BY A FADD-DEPENDENT PATHWAY

Colorectal cancer is the most common malignant complication in patients with chronic inflammatory bowel disease (IBD). In addition, these patients are at risk for developing painful complications during chemotherapy due to cytotoxic effects of drugs currently in use. Past studies have suggested a protective effect of tea consumption on gastrointestinal (GI) malignancies. Green tea polyphenols (GrTP) inhibited carcinogen-induced GI tumors in rodents and induced apoptosis in various carcinoma cell lines. We hypothesized that GrTP and its polyphenolic compounds regulate apoptosis in the intestinal epithelia. In this study, the effects of GrTP and its polyphenolics on apoptosis was evaluated in intestinal epithelial, IEC-6, cells grown to 85% confluency. GrTP (400-800 mg/ml) induced DNA fragmentation in a dose dependent fashion. Higher concentrations (>800 mg/ml) induced a mixed apoptosis and cytolysis. Epithelial cells exposed to GrTP and a major polyphenol, EGCG, but not EGC or EC, increased caspase activities in a time and dose dependent manner. The caspase inhibitors rescued cells from GrTP and EGCG-induced cell death. Concomitantly, GrTP resulted in activation of fatty acid synthase (Fas)-associated protein with death domain (FADD) and recruitment to Fas/CD95 domain 30 minutes following treatment. While GrTP also blocked NF-κB activation, an NFκ-B inhibitor (MG132) only promoted cytolysis. In conclusion, these data demonstrated GrTP and EGCG induced apoptosis in intestinal epithelia mediated by caspase-8 through a FADD dependent pathway. Future investigation may warrant preventive as well as therapeutic strategies for GrTP in GI malignancy.

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.

Inhibition of apoptosis and caspase-3 in vascular smooth muscle cells by plasminogen activator inhibitor type-1

Increased expression of plasminogen activator inhibitor type 1 (PAI-1) is associated with decreased apoptosis of neoplastic cells. We sought to determine whether PAI-1 alters apoptosis in vascular smooth muscle cells (VSMC) and, if so, by what mechanisms. A twofold increase in the expression of PAI-1 was induced in VSMC from transgenic mice with the use of the SM-22alpha gene promoter (SM22-PAI+). Cultured VSMC from SM22-PAI+ mice were more resistant to apoptosis induced by tumor necrosis factor plus phorbol myristate acetate or palmitic acid compared with VSMC from negative control littermates. Both wild type (WT) and a stable active mutant form of PAI-1 (Active) inhibited caspase-3 amidolytic activity in cell lysates while a serpin-defective mutant (Mut) PAI-1 did not. Similarly, both WT and Active PAI-1 decreased amidolytic activity of purified caspase-3, whereas Mut PAI-1 did not. WT but not Mut PAI-1 decreased the cleavage of poly-[ADP-ribose]-polymerase (PARP), the physiological substrate of caspase-3. Noncovalent physical interaction between caspase-3 and PAI-1 was demonstrable with the use of both qualitative and quantitative in vitro binding assays. High affinity binding was eliminated by mutations that block PAI-1 serpin activity. Accordingly, attenuated apoptosis resulting from elevated expression of PAI-1 by VSMC may be attributable, at least in part, to reversible inhibition of caspase-3 by active PAI-1.

Apoptotic effect of PP2 a Src tyrosine kinase inhibitor, in murine B cell leukemia

Src is a non-receptor protein tyrosine kinase that transduces signals regulating cell growth and differentiation. We report here that activation of signaling pathway after blockade of tyrosine phosphorylation by PP2 (4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), a potent and selective inhibitor of the Src-family tyrosine kinase, can lead to cell death in murine B cell leukemia, 70Z/3. Death from PP2 occurred by apoptosis as indicated by the induction of caspase activation and annexin V/propidium iodide staining. Interestingly, PP2 was found to be able to enhance the DNA binding activity of nuclear factor kappaB (NF-kappaB) before induction of apoptosis without accompanying by increased phosphorylation of inhibitor of NF-kappaB-alpha (IkappaB-alpha). Additionally, immunoblotting analysis with PP2-treated cell extract demonstrated that, compared to other protein kinase C (PKC) isotypes, the translocation of novel PKC isotypes from the cytosol to membrane fraction was sustained for a longer time. These data suggest that the inhibition of Src-mediated tyrosine phosphorylation by PP2 may tilt the balance between each PKC isotypes, which in turn, activate NF-kappaB transcription factor, leading to apoptosis.

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.

Long-term effect of heat shock protein 60 from Actinobacillus actinomycetemcomitans on epithelial cell viability and mitogen-activated protein kinases

Our previous studies showed that bacterial heat shock protein 60 (hsp60) induces cultured epithelial cell proliferation within 24 h. Here we investigated the long-term effects of heat shock protein 60 isolated from Actinobacillus actinomycetemcomitans on skin keratinocyte (HaCaT cell line) viability and the cell signaling involved. Prolonged incubation in the presence of hsp60 increased the rate of epithelial cell death. The number of viable cells in hsp60-treated culture was 37% higher than the number in the control at 24 h but 27% lower at 144 h. A kinetics study of the effect of hsp60 on the phosphorylation of mitogen-activated protein kinases (MAPKs) involving Western blotting with phospho-specific antibodies showed that in addition to a transient early increase in p38 levels, a second peak appeared in keratinocytes 24 h after the addition of hsp60. In contrast, prolonged incubation with hsp60 caused a decrease in the level of phosphorylated extracellular signal-regulated kinase 1/2 (ERK1/2) compared with that in the controls, possibly as a result of protein phosphatase activity. We found that hsp60 increased the levels of several phosphatases, including MAP-2, which strongly dephosphorylates ERK1/2. Moreover, hsp60 increased the level of tumor necrosis factor alpha (TNF-alpha) in culture medium in a dose-dependent manner. TNF-alpha added to culture showed a cytotoxic effect on epithelial cells, particularly with longer incubation periods. TNF-alpha also induced the phosphorylation of p38. Finally, our results show that bacterial hsp60 inhibited stress-induced synthesis of cellular hsp60. Therefore, several cell behavior changes caused by long-term exposure to bacterial hsp60 may lead to impaired epithelial cell viability.

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.

Effect of selective PKC isoform activation and inhibition on TNF-alpha-induced injury and apoptosis in human intestinal epithelial cells

(1) We have investigated the effects of specific PKC isoforms in TNF-alpha mediated cellular damage using a human intestinal cell line (SCBN). (2) TNF-alpha treatment induced a decrease in the extent of intestinal cellular viability as determined by a formazan-based assay and an increase in the apoptotic index as assessed by immunohistology. These changes in cellular integrity were found to be related to the degradation of I-kappaBalpha, mobilization of NF-kappaB and release of mitochondrial cytochrome c. (3) TNF-alpha treatment also induced the activation of selective PKC isoforms which were associated with the decrease in cellular viability and an increase of cellular apoptosis. (4) Nonselective PKC antagonists, such as GF109203X and Gö6976 as well as isoform-selective PKC-inhibiting peptides would reverse the cellular injury as well as reduce the degradation of I-kappaBalpha and mitochondrial cytochrome c release. These effects were most highly correlated with changes in PKCdelta and epsilon primarily. (5) Intestinal cellular injury could be induced by treating cells with agonists selective for PKCdelta and epsilon mainly. (6) In conclusion, this study has shown that TNF-alpha treatment can induce the activation of PKCdelta and epsilon in the human intestinal cell line, SCBN, and this response is closely associated with an increase in cellular damage and apoptosis. PKCdelta and epsilon primarily mediate the release of mitochondrial cytochrome c and degradation of I-kappaBalpha and hence mobilization of NF-kappaB, which are responsible for the pathway leading to cell injury.

Effects of protein kinase C on type I inositol 1, 4, 5-triphosphate receptor expression in smooth muscle cells of rat glomerular afferent arterioles

AIM

To investigate the effects of protein kinase C (PKC) on type I inositol 1, 4, 5-triphosphate receptor (IP₃ R) expression in rat glomerular afferent arterioles smooth muscle cells (RASMC) treated with TNF-α.

METHODS

RASMC were isolated and cultured from rats, type I IP₃ R mRNA in RASMC treated with TNF-α and PKC activator or TNF-α and PKC inhibitor or PKC activator or PKC inhibitor were detected by Northern blot.

RESULTS

TNF-α enhanced the expression of type I IP3 R mRNA in RASMC; PKC inhibitor significantly inhibited the expression of type I IP₃ R mRNA induced by TNF-α(14 814.0±2 029.9, 11 334.0±1 104.9, P<0.05). PKC activator significantly enhanced the expression of type I IP₃ R mRNA in RASMC treated without TNF-α(22 554.5±2 625.2, 28 128.0±3 698.6, P<0.05). PKA inhibitor could not inhibit the expression of type I IP₃ R mRNA induced by TNF-α.

CONCLUSION

TNF-α plays a role in signal transduction in RASMC. TNF-αmay act as the promoter of type I IP₃ R mRNA in RASMC or activates PKC that results in the expression of type I IP₃ R protein. PKC and IP₃ promote releasing of intracellular Ca²⁺ in RASMC, inducing RASMC constrict. The renal blood flow diminution is involved in the development of renal dysfunction.

Bryostatin-1 attenuates TNF-induced epithelial barrier dysfunction: role of novel PKC isozymes

Tumor necrosis factor (TNF) increases epithelial permeability in many model systems. Protein kinase C (PKC) isozymes regulate epithelial barrier function and alter ligand-receptor interactions. We sought to define the impact of PKC on TNF-induced barrier dysfunction in T84 intestinal epithelia. TNF induced a dose- and time-dependent fall in transepithelial electrical resistance (TER) and an increase in [(3)H]mannitol flux. The TNF-induced fall in TER was not PKC mediated but was prevented by pretreatment with bryostatin-1, a PKC agonist. As demonstrated by a pattern of sensitivity to pharmacological inhibitors of PKC, this epithelial barrier preservation was mediated by novel PKC isozymes. Bryostatin-1 reduced TNF receptor (TNF-R1) surface availability, as demonstrated by radiolabeled TNF binding and cell surface biotinylation assays, and increased TNF-R1 receptor shedding. The pattern of sensitivity to isozyme-selective PKC inhibitors suggested that these effects were mediated by activation of PKC-epsilon. In addition, after bryostatin-1 treatment, PKC-delta and TNF-R1 became associated, as determined by mutual coimmunoprecipitation assay, which has been shown to lead to receptor desensitization in neutrophils. TNF-induced barrier dysfunction occurs independently of PKC, but selective modulation of novel PKC isozymes may regulate TNF-R1 signaling.

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.

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.