Metabolic stress opens K+ channels in hepatoma cells through a Ca2+- and protein kinase calpha-dependent mechanism

Multiple action mechanism and in vivo antimicrobial efficacy of antimicrobial peptide Jelleine-I

With the extensive use of antibiotics in medicine, agriculture and food chemistry, the emergence of multi-drug resistant bacteria become more and more frequent and posed great threats to human health and life. So novel antimicrobial agents were urgently needed to defend the resistant bacteria. Jelleine-I was a small antimicrobial peptide (AMP) with eight amino acids in its sequence. It was believed to be an ideal template for developing antimicrobial agents. In the present study, the possible action mode against both gram-negative bacteria and gram-positive bacteria and in vivo antimicrobial activity was explored. Our results showed that Jelleine-I exhibits its antimicrobial activity mainly by disrupting the integrity of the cell membrane, which would not be affected by the conventional resistant mechanism. It also aims at some intracellular targets such as genomic DNA to inhibit the growth of microbes. In addition, the result of in vivo antimicrobial activity experiment showed that Jelleine-I performed a good therapeutic effect toward the mice with Escherichia coli infected peritonitis. Notably, Jelleine-I has negligible cytotoxicity toward the tested mammalian cells, indicating excellent cell selectivity between prokaryotic cells and eurkayotic cells. In summary, our results showed that Jelleine-I would be a potential candidate to be developed as a novel antimicrobial agent.

Gain-of-Function Mutations in KCNN3 Encoding the Small-Conductance Ca2+-Activated K+ Channel SK3 Cause Zimmermann-Laband Syndrome

Zimmermann-Laband syndrome (ZLS) is characterized by coarse facial features with gingival enlargement, intellectual disability (ID), hypertrichosis, and hypoplasia or aplasia of nails and terminal phalanges. De novo missense mutations in KCNH1 and KCNK4, encoding K⁺ channels, have been identified in subjects with ZLS and ZLS-like phenotype, respectively. We report de novo missense variants in KCNN3 in three individuals with typical clinical features of ZLS. KCNN3 (SK3/KCa2.3) constitutes one of three members of the small-conductance Ca²⁺-activated K⁺ (SK) channels that are part of a multiprotein complex consisting of the pore-forming channel subunits, the constitutively bound Ca²⁺ sensor calmodulin, protein kinase CK2, and protein phosphatase 2A. CK2 modulates Ca²⁺ sensitivity of the channels by phosphorylating SK-bound calmodulin. Patch-clamp whole-cell recordings of KCNN3 channel-expressing CHO cells demonstrated that disease-associated mutations result in gain of function of the mutant channels, characterized by increased Ca²⁺ sensitivity leading to faster and more complete activation of KCNN3 mutant channels. Pretreatment of cells with the CK2 inhibitor 4,5,6,7-tetrabromobenzotriazole revealed basal inhibition of wild-type and mutant KCNN3 channels by CK2. Analogous experiments with the KCNN3 p.Val450Leu mutant previously identified in a family with portal hypertension indicated basal constitutive channel activity and thus a different gain-of-function mechanism compared to the ZLS-associated mutant channels. With the report on de novo KCNK4 mutations in subjects with facial dysmorphism, hypertrichosis, epilepsy, ID, and gingival overgrowth, we propose to combine the phenotypes caused by mutations in KCNH1, KCNK4, and KCNN3 in a group of neurological potassium channelopathies caused by an increase in K⁺ conductance.

PKCα regulates TMEM16A-mediated Cl⁻ secretion in human biliary cells

TMEM16A is a newly identified Ca(2+)-activated Cl(-) channel in biliary epithelial cells (BECs) that is important in biliary secretion. While extracellular ATP stimulates TMEM16A via binding P2 receptors and increasing intracellular Ca(2+) concentration ([Ca(2+)]i), the regulatory pathways have not been elucidated. Protein kinase C (PKC) contributes to ATP-mediated secretion in BECs, although its potential role in TMEM16A regulation is unknown. To determine whether PKCα regulates the TMEM16A-dependent membrane Cl(-) transport in BECs, studies were performed in human biliary Mz-cha-1 cells. Addition of extracellular ATP induced a rapid translocation of PKCα from the cytosol to the plasma membrane and activation of whole cell Ca(2+)-activated Cl(-) currents. Currents demonstrated outward rectification and reversal at 0 mV (properties consistent with TMEM16A) and were inhibited by either molecular (siRNA) or pharmacologic (PMA or Gö6976) inhibition of PKCα. Intracellular dialysis with recombinant PKCα activated Cl(-) currents with biophysical properties identical to TMEM16A in control cells but not in cells after transfection with TMEM16A siRNA. In conclusion, our studies demonstrate that PKCα is coupled to ATP-stimulated TMEM16A activation in BECs. Targeting this ATP-Ca(2+)-PKCα signaling pathway may represent a therapeutic strategy to increase biliary secretion and promote bile formation.

Basolateral potassium (IKCa) channel inhibition prevents increased colonic permeability induced by chemical hypoxia

Major liver resection is associated with impaired intestinal perfusion and intestinal ischemia, resulting in decreased mucosal integrity, increased bacterial translocation, and an increased risk of postoperative sepsis. However, the mechanism by which ischemia impairs intestinal mucosal integrity is unclear. We therefore evaluated the role of Ca(2+)-sensitive, intermediate-conductance (IK(Ca)) basolateral potassium channels in enhanced intestinal permeability secondary to chemical hypoxia. The effects of chemical hypoxia induced by 100 μM dinitrophenol (DNP) and 5 mM deoxyglucose (DG) on basolateral IK(Ca) channel activity and whole cell conductance in intact human colonic crypts, and paracellular permeability (G(S)) in isolated colonic sheets, were determined by patch-clamp recording and transepithelial electrical measurements, respectively. DNP and DG rapidly stimulated IK(Ca) channels in cell-attached basolateral membrane patches and elicited a twofold increase (P = 0.004) in whole cell conductance in amphotericin B-permeabilized membrane patches, changes that were inhibited by the specific IK(Ca) channel blockers TRAM-34 (100 nM) and clotrimazole (CLT; 10 μM). In colonic sheets apically permeabilized with nystatin, DNP elicited a twofold increase (P = 0.005) in G(S), which was largely inhibited by the serosal addition of 50 μM CLT. We conclude that, in intestinal epithelia, chemical hypoxia increases G(S) through a mechanism involving basolateral IK(Ca) channel activation. Basolateral IK(Ca) channel inhibition may prevent or limit increased intestinal permeability during liver surgery.

ATP Depletion Via Mitochondrial F1F0 Complex by Lethal Factor is an Early Event in B. Anthracis-Induced Sudden Cell Death

Bacillus anthracis’ primary virulence factor is a tripartite anthrax toxin consisting of edema factor (EF), lethal factor (LF) and protective antigen (PA). In complex with PA, EF and LF are internalized via receptor-mediated endocytosis. EF is a calmodulin-dependent adenylate cyclase that induces tissue edema. LF is a zinc-metalloprotease that cleaves members of mitogen-activated protein kinase kinases. Lethal toxin (LT: PA plus LF)-induced death of macrophages is primarily attributed to expression of the sensitive Nalp1b allele, inflammasome formation and activation of caspase-1, but early events that initiate these processes are unknown. Here we provide evidence that an early essential event in pyroptosis of alveolar macrophages is LF-mediated depletion of cellular ATP. The underlying mechanism involves interaction of LF with F₁F₀-complex gamma and beta subunits leading to increased ATPase activity in mitochondria. In support, mitochondrial DNA-depleted MH-S cells have decreased F₁F₀ ATPase activity due to the lack of F₀6 and F₀8 polypeptides and show increased resistance to LT. We conclude that ATP depletion is an important early event in LT-induced sudden cell death and its prevention increases survival of toxin-sensitive cells.

Determination of cell electroporation from the release of intracellular potassium ions

When cells are exposed to a strong enough external electric field, transient aqueous pores are formed in the membrane. The fraction of electroporated cells can be determined by measuring the release of intracellular potassium ions. The current work is the first study where such a method was employed successfully not only with cells suspended in the medium with a rather high concentration of potassium (4-5 mM) but also with cells that release some part of intracellular potassium responding, in this way, to the stress caused by manipulation procedures during the preparation of the cell suspension. Experiments were carried out on mouse hepatoma MH-A22 cells exposed to a square-wave electric pulse. The curves showing the dependence of the fraction of the cells that have become permeable to bleomycin, a membrane-impermeable cytotoxic drug, are close to the ones showing the release of intracellular potassium ions.

Modulation of mitochondrial metabolic function by phorbol 12-myristate 13-acetate through increased mitochondrial translocation of protein kinase Calpha in C2C12 myocytes

Protein kinase C (PKC) agonists including phorbol 12-myristate 13-acetate (PMA) not only induce the redistribution of cytosolic PKC to various subcellular compartments but also activate the kinase domain of the protein. In the present study we have investigated the nature of mitochondrial PKC pool and its effects on mitochondrial function in cells treated with PMA. Treatment of C2C12 myoblasts, C6 glioma and COS7 cells with PMA resulted in a dramatic redistribution of intracellular PKCalpha pool, with large fraction of the protein pool sequestered in the mitochondrial compartment. We also observed mitochondrial PKCdelta accumulation in a cell restricted manner. The intramitochondrial localization was ascertained by using a combination of protection against protease treatment of isolated mitochondria and immunofluorescence microscopy. PMA-induced mitochondrial localization of PKCalpha was accompanied by increased mitochondrial PKC activity, altered cell morphology, disruption of mitochondrial membrane potential, decreased complex I and pyruvate dehydrogenase activities, and increased mitochondrial ROS production. All of these changes could be retarded by treatment with PKC inhibitors. These results show a direct role for PMA-mediated PKCalpha translocation to mitochondria in inducing mitochondrial toxicity.

Modulation of hepatocellular swelling-activated K+currents by phosphoinositide pathway-dependent protein kinase C

K+channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K+current ( IKVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K+flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to IKVolactivation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP2) increased IKVol, whereas conditions that decrease cellular PIP2decreased IKVol. GTP and AlF4−stimulated IKVoldevelopment, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased IKVoland inhibited the stimulatory response to PIP2or GTP. Protein kinase C (PKC) is involved, because K+current was enhanced by 1-oleoyl-2-acetyl- sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both IKVoland the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated IKVolactivation involves the cytoskeleton. Under isotonic conditions, a slowly developing K+current similar to IKVolwas activated by PIP2, lipid phosphatase inhibitors to counter PIP2depletion, a PLC-coupled α1-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate IKVolthrough a PIP2/PKC-dependent pathway. The results indicate that PIP2indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.

Roles of protein kinase C in regulation of P2X7 receptor-mediated calcium signalling of cultured type-2 astrocyte cell line, RBA-2

The role of protein kinase C (PKC) on regulation of P2X(7) receptor-mediated Ca(2+) signalling was examined on RBA-2 astrocytes. Activation of PKC decreased the receptor-mediated Ca(2+) signalling and the decrease was restored by PKC inhibitors. Down regulation of PKC also caused a decrease in the Ca(2+) signalling. Thus PKC might play a dual role on the P2X(7) receptor signalling. Successive stimulation of the P2X(7) receptor induced a gradual decline of Ca(2+) signalling but PKC inhibitors failed to restore the decline. Nevertheless, PMA stimulated translocation of PKC-alpha, -betaI, -betaII, and -gamma, but only anti-PKC-gamma co-immunoprecipitated the receptors. To examine the role of PKC-gamma, Ca(2+) signalling was measured by Ca(2+) imaging. Our results revealed that the agonist-stimulated Ca(2+) signalling were reduced in the cells that the transfection of either P2X(7) receptor or PKC-gamma morpholino antisense oligo was identified. Thus, we concluded that PKC-gamma interacted with P2X(7) receptor complex and positively regulated the receptor-mediated Ca(2+) signalling.

Molecular modeling and docking simulations of scorpion toxins and related analogs on human SKCa2 and SKCa3 channels

The small-conductance Ca2+-activated K+ (SKCa) channels modulate cytosolic Ca2+ concentration in excitable and non-excitable tissues by regulating the membrane potential and are responsible of slow action potential after hyperpolarization that inhibits cell firing. Among these, human SKCa2 and SKCa3 channels differ in the pore region by only two residues: Ala331 and Asn367 (human small-conductance calcium-activated potassium channel, hSKCa2) instead of Val485 and His521 (hSKCa3). To design highly selective blockers of hSKCa channels, a number of known hSKCa2 and/or hSKCa3-active peptides (i.e. scorpion toxins and analogs thereof) were analyzed for their interactions and selectivities toward these channels. Molecular models of hSKCa2 and hSKCa3 channels (S5-H5-S6 portion) were generated, and scorpion toxins/peptides of unsolved three-dimensional (3D) structures were modeled. Models of toxin-channel complexes were generated by the bimolecular complex generation with global evaluation, and ranking (BiGGER) docking software and selected by using a screening method of the docking solutions. A high degree of correlation was found to exist between docking energies and experimental Kd values of peptides that blocked hSKCa2 and/or hSKCa3 channels, suggesting it could be appropriate to predict Kd values of other bioactive peptides. The best scoring complexes were also used to identify key residues of both interacting partners, indicating that such an approach should help the design of more active and/or selective peptide blockers of targeted ion channels.

Protein kinase C-alpha activity is required for respiratory syncytial virus fusion to human bronchial epithelial cells

Respiratory syncytial virus (RSV) infection activates protein kinase C (PKC), but the precise PKC isoform(s) involved and its role(s) remain to be elucidated. On the basis of the activation kinetics of different signaling pathways and the effect of various PKC inhibitors, it was reasoned that PKC activation is important in the early stages of RSV infection, especially RSV fusion and/or replication. Herein, the role of PKC-alpha during the early stages of RSV infection in normal human bronchial epithelial cells is determined. The results show that the blocking of PKC-alpha activation by classical inhibitors, pseudosubstrate peptides, or the overexpression of dominant-negative mutants of PKC-alpha in these cells leads to significantly decreased RSV infection. RSV induces phosphorylation, activation, and cytoplasm-to-membrane translocation of PKC-alpha. Also, PKC-alpha colocalizes with virus particles and is required for RSV fusion to the cell membrane. Thus, PKC-alpha could provide a new pharmacological target for controlling RSV infection.

Voltage-dependent anion channel-1 (VDAC-1) contributes to ATP release and cell volume regulation in murine cells

Extracellular ATP regulates several elements of the mucus clearance process important for pulmonary host defense. However, the mechanisms mediating ATP release onto airway surfaces remain unknown. Mitochondrial voltage-dependent anion channels (mt-VDACs) translocate a variety of metabolites, including ATP and ADP, across the mitochondrial outer membrane, and a plasmalemmal splice variant (pl-VDAC-1) has been proposed to mediate ATP translocation across the plasma membrane. We tested the involvement of VDAC-1 in ATP release in a series of studies in murine cells. First, the full-length coding sequence was cloned from a mouse airway epithelial cell line (MTE7b⁻) and transfected into NIH 3T3 cells, and pl-VDAC-1-transfected cells exhibited higher rates of ATP release in response to medium change compared with mock-transfected cells. Second, ATP release was compared in cells isolated from VDAC-1 knockout [VDAC-1 (−/−)] and wild-type (WT) mice. Fibroblasts from VDAC-1 (−/−) mice released less ATP than WT mice in response to a medium change. Well-differentiated cultures from nasal and tracheal epithelia of VDAC-1 (−/−) mice exhibited less ATP release in response to luminal hypotonic challenge than WT mice. Confocal microscopy studies revealed that cell volume acutely increased in airway epithelia from both VDAC-1 (−/−) and WT mice after luminal hypotonic challenge, but VDAC-1 (−/−) cells exhibited a slower regulatory volume decrease (RVD) than WT cells. Addition of ATP or apyrase to the luminal surface of VDAC-1 (−/−) or WT cultures with hypotonic challenge produced similar initial cell height responses and RVD kinetics in both cell types, suggesting that involvement of VDAC-1 in RVD is through ATP release. Taken together, these studies suggest that VDAC-1, directly or indirectly, contributes to ATP release from murine cells. However, the observation that VDAC-1 knockout cells released a significant amount of ATP suggests that other molecules also play a role in this function.

Calcium-dependent regulation of secretion in biliary epithelial cells: the role of apamin-sensitive SK channels

BACKGROUND & AIMS

Increases in intracellular Ca 2+ are thought to complement cAMP in stimulating Cl - secretion in cholangiocytes, although the site(s) of action and channels involved are unknown. We have identified a Ca 2+ -activated K + channel (SK2) in biliary epithelium that is inhibited by apamin. The purpose of the present studies was to define the role of SK channels in Ca 2+ -dependent cholangiocyte secretion.

METHODS

Studies were performed in human Mz-Cha-1 cells and normal rat cholangiocytes (NRC). Currents were measured by whole-cell patch clamp technique and transepithelial secretion by Ussing chamber.

RESULTS

Ca 2+ -dependent stimuli, including purinergic receptor stimulation, ionomycin, and increases in cell volume, each activated K + -selective currents with a linear IV relation and time-dependent inactivation. Currents were Ca 2+ dependent and were inhibited by apamin and by Ba 2+. In intact liver, immunoflourescence with an antibody to SK2 showed a prominent signal in cholangiocyte plasma membrane. To evaluate the functional significance, NRC monolayers were mounted in a Ussing chamber, and the short-circuit current ( I sc ) was measured. Exposure to ionomycin caused an increase in I sc 2-fold greater than that induced by cAMP. Both the basal and ionomycin-induced I sc were inhibited by basolateral Ba 2+, and approximately 58% of the basolateral K + current was apamin sensitive.

CONCLUSIONS

These studies demonstrate that cholangiocytes exhibit robust Ca 2+ -stimulated secretion significantly greater in magnitude than that stimulated by cAMP. SK2 plays an important role in mediating the increase in transepithelial secretion due to increases in intracellular Ca 2+. SK2 channels, therefore, may represent a target for pharmacologic modulation of bile flow.

Monitoring of water content and water distribution in ischemic hearts

We determined water content and water distribution by fitting dielectric spectra of ischemic canine hearts between 5 MHz and 3 GHz with a newly developed model which describes heart cells and subcellular organelles as rotational ellipsoids filled with electrolyte enclosed by an isolating membrane. The fraction of dry material is modelled by spherical particles with a small dielectric permittivity. Free model parameters were water content, cell volume fraction, and the conductivity of the electrolytes. Resulting model parameters were compared to data from tissue desiccation and to conductivity changes produced by protons and lactate ions. We investigated hearts in two states: during ischemia after interruption of blood flow (pure ischemia, PI, n=5) and during ischemia after resuscitation with Tyrode's solution (IAR, n=14). The difference between water content determined by tissue desiccation and by dielectric spectroscopy was less than 0.5%. During 360 min of ischemia, water content in IAR decreased from 85+/-1.6% to 83+/-2.2% and in PI from 80+/-0.8% to 78+/-1.5%. Cellular volume fraction in IAR increased from 0.47+/-0.045 to 0.63+/-0.031 and in PI from 0.62+/-0.014 to 0.73+/-0.013, which is consistent with published morphometric data. After 180 min of ischemia, the increase of the cytosolic conductivity was 0.14+/-0.02 S/m as calculated from the dielectric spectrum and was similar to the conductivity increase which was roughly estimated on the basis of tissue lactate concentration. In conclusion, dielectric spectroscopy combined with our model analysis facilitates the monitoring of water content and distribution by means of nondestructive surface probes.

Volumetric and ionic responses of goldfish hepatocytes to anisotonic exposure and energetic limitation

The relationship between cell volume and K(+) transmembrane fluxes of goldfish (Carassius auratus) hepatocytes exposed to anisotonic conditions or energetic limitation was studied and compared with the response of hepatocytes from trout (Oncorhynchus mykiss) and rat (Rattus rattus). Cell volume was studied by video- and fluorescence microscopy, while K(+) fluxes were assessed by measuring unidirectional (86)Rb(+) fluxes. In trout and rat hepatocytes, hyposmotic (180 mosmoll(-1)) exposure at pH 7.45 caused cell swelling followed by a regulatory volume decrease (RVD), a response reported to be mediated by net efflux of KCl and osmotically obliged water. By contrast, goldfish hepatocytes swelled but showed no RVD under these conditions. Although in goldfish hepatocytes a net ((86)Rb(+))K(+) efflux could be activated by N-ethylmaleimide, this flux was not, or only partially, activated by hyposmotic swelling (120-180 mosmoll(-1)). Blockage of glycolysis by iodoacetic acid (IAA) did not alter cell volume in goldfish hepatocytes, whereas in the presence of cyanide (CN(-)), an inhibitor of oxidative phosphorylation, or CN(-) plus IAA (CN(-)+IAA), cell volume decreased by 3-7%. Although in goldfish hepatocytes, energetic limitation had no effect on ((86)Rb(+))K(+) efflux, ((86)Rb(+))K(+) influx decreased by 57-66% in the presence of CN(-) and CN(-)+IAA but was not significantly altered by IAA alone. Intracellular K(+) loss after 20 min of exposure to CN(-) and CN(-)+IAA amounted to only 3% of the total intracellular K(+). Collectively, these observations suggest that goldfish hepatocytes, unlike hepatocytes of anoxia-intolerant species, avoid a decoupling of transmembrane K(+) fluxes in response to an osmotic challenge. This may underlie both the inability of swollen cells to undergo RVD but also the capability of anoxic cells to maintain intracellular K(+) concentrations that are almost unaltered, thereby prolonging cell survival.

Molecular characterization of volume-sensitive SK(Ca) channels in human liver cell lines

In human liver, Ca(2+)-dependent changes in membrane K(+) permeability play a central role in coordinating functional interactions between membrane transport, metabolism, and cell volume. On the basis of the observation that K(+) conductance is partially sensitive to the bee venom toxin apamin, we aimed to assess whether small-conductance Ca(2+)-sensitive K(+) (SK(Ca)) channels are expressed endogenously and contribute to volume-sensitive K(+) efflux and cell volume regulation. We isolated a full-length 2,140-bp cDNA (hSK2) highly homologous to rat brain rSK2 cDNA, including the putative apamin-sensitive pore domain, from a human liver cDNA library. Identical cDNAs were isolated from primary human hepatocytes, human HuH-7 hepatoma cells, and human Mz-ChA-1 cholangiocarcinoma cells. Transduction of Chinese hamster ovary cells with a recombinant adenovirus encoding the hSK2-green fluorescent protein fusion construct resulted in expression of functional apamin-sensitive K(+) channels. In Mz-ChA-1 cells, hypotonic (15% less sodium glutamate) exposure increased K(+) current density (1.9 +/- 0.2 to 37.5 +/- 7.1 pA/pF; P < 0.001). Apamin (10-100 nM) inhibited K(+) current activation and cell volume recovery from swelling. Apamin-sensitive SK(Ca) channels are functionally expressed in liver and biliary epithelia and likely contribute to volume-sensitive changes in membrane K(+) permeability. Accordingly, the hSK2 protein is a potential target for pharmacological modulation of liver transport and metabolism through effects on membrane K(+) permeability.

Protein kinase epsilon dampens the secretory response of model intestinal epithelia during ischemia

BACKGROUND

Luminal fluid sequestration and diarrhea are early manifestations of mesenteric ischemia. This can be modeled in vitro with the use of T84 intestinal epithelia, where ischemia induces Cl(-) secretion with adenosine-mediated autocrine feedback. Protein kinase C (PKC) regulates epithelial transport and, in some organ systems, is involved in the response to ischemic stress. The purpose of this study was to define the role of PKC on epithelial transport during ischemia.

METHODS

By voltage-current clamp, short-circuit current (Isc) equals Cl(-) secretion. Ischemic conditions were simulated with the use of a well-established chemical hypoxia protocol.

RESULTS

Chemical hypoxia briskly activated Isc. Gö6850, an antagonist of novel and conventional PKC isoforms, markedly enhanced the ischemia-induced Isc response, although Gö6976 (which inhibits only conventional isoforms) had no effect. Rottlerin, a specific inhibitor of PKC delta, did not attenuate ischemic Isc. Both phorbol 12-myristate, 13-acetate and bryostatin-1, which selectively activate PKC epsilon in T84 cells, markedly attenuated the Isc response to ischemia. Both agents also inhibited the Isc response to exogenous adenosine.

CONCLUSIONS

PKC (likely the novel epsilon isoform) in intestinal epithelia modulates ischemia-induced alterations in ion transport. Inhibition of PKC epsilon exaggerates the secretory response that is induced by ischemia and by authentic adenosine; conversely, augmented activation of PKC epsilon inhibits secretion. Manipulation of PKC epsilon could limit luminal fluid sequestration during mesenteric ischemia.

Cloning and functional expression of a liver isoform of the small conductance Ca2+-activated K+ channel SK3

Small conductance Ca2+-activated K+ (SK) channels have been cloned from mammalian brain, but little is known about the molecular characteristics of SK channels in nonexcitable tissues. Here, we report the isolation from rat liver of an isoform of SK3. The sequence of the rat liver isoform differs from rat brain SK3 in five amino acid residues in the NH3 terminus, where it more closely resembles human brain SK3. SK3 immunoreactivity was detectable in hepatocytes in rat liver and in HTC rat hepatoma cells. Human embryonic kidney (HEK-293) cells transfected with liver SK3 expressed 10 pS K+ channels that were Ca2+ dependent (EC(50) 630 nM) and were blocked by the SK channel inhibitor apamin (IC(50) 0.6 nM); whole cell SK3 currents inactivated at membrane potentials more positive than -40 mV. Notably, the Ca2+ dependence, apamin sensitivity, and voltage-dependent inactivation of SK3 are strikingly similar to the properties of hepatocellular and biliary epithelial SK channels evoked by metabolic stress. These observations raise the possibility that SK3 channels influence membrane K+ permeability in hepatobiliary cells during liver injury.

Activation of potassium and chloride channels by tumor necrosis factor alpha. Role in liver cell death

Despite abundant evidence for changes in mitochondrial membrane permeability in tumor necrosis factor (TNF)-mediated cell death, the role of plasma membrane ion channels in this process remains unclear. These studies examine the influence of TNF on ion channel opening and death in a model rat liver cell line (HTC). TNF (25 ng/ml) elicited a 2- and 5-fold increase in K(+) and Cl(-) currents, respectively, in HTC cells. These increases occurred within 5-10 min after TNF exposure and were inhibited either by K(+) or Cl(-) substitution or by K(+) channel blockers (Ba(2+), quinine, 0.1 mm each) or Cl(-) channel blockers (10 microm 5-nitro-2-(3-phenylpropylamino)benzoic acid and 0.1 mm N-phenylanthranilic acid), respectively. TNF-mediated increases in K(+) and Cl(-) currents were each inhibited by intracellular Ca(2+) chelation (5 mm EGTA), ATP depletion (4 units/ml apyrase), and the protein kinase C (PKC) inhibitors chelerythrine (10 micrometer) or PKC 19-36 peptide (1 micrometer). In contrast, currents were not attenuated by the calmodulin kinase II 281-309 peptide (10 micrometer), an inhibitor of calmodulin kinase II. In the presence of actinomycin D (1 micrometer), each of the above ion channel blockers significantly delayed the progression to TNF-mediated cell death. Collectively, these data suggest that activation of K(+) and Cl(-) channels is an early response to TNF signaling and that channel opening is Ca(2+)- and PKC-dependent. Our findings further suggest that K(+) and Cl(-) channels participate in pathways leading to TNF-mediated cell death and thus represent potential therapeutic targets to attenuate liver injury from TNF.

Modulation by protease-activated receptors of the rat duodenal motility in vitro: possible mechanisms underlying the evoked contraction and relaxation

1 The present study examined effects of agonist enzymes and receptor-activating peptides for protease-activated receptors (PARs) on duodenal motility in the rat, and also investigated possible mechanisms underlying the evoked responses. 2 Thrombin at 0.03-0.1 microM and the PAR-1-activating peptide SFLLR-NH2 at 3-100 microM or TFLLR-NH2 at 10-50 microM produced a dual action, relaxation followed by contraction of the duodenal longitudinal muscle. The PAR-2-activating peptide SLIGRL-NH2 at 10-100 microM elicited only small contraction. Trypsin at 0.08 microM induced small contraction, or relaxation followed by contraction, depending on preparations. The PAR-4-activating peptide GYPGKF-NH2 at 1000 microM exhibited no effect. 3 The contractile responses of the duodenal strips to TFLLR-NH2 and to SLIGRL-NH2 were partially attenuated by the L-type calcium channel blocker nifedipine (1 microM), the protein kinase C inhibitor GF109203X (1 microM) and the tyrosine kinase inhibitor genistein (15 microM), but were resistant to indomethacin (3 microM) and tetrodotoxin (1-10 microM). 4 The relaxation of the preparations exerted by TFLLR-NH2 was unaffected by indomethacin (3 microM), propranolol (5 microM), NG-nitro-L-arginine methyl ester (100 microM) and tetrodotoxin (1-10 microM). This relaxation was resistant to either GF109203X (1 microM) or genistein (15 microM), but was, remarkably, attenuated by combined application of these two kinase inhibitors. 5 Apamin (0.1 microM), an inhibitor of calcium-activated, small-conductance potassium channels, but not charybdotoxin (0.1 microM), completely abolished the PAR-1-mediated duodenal relaxation, and significantly enhanced the PAR-1-mediated contraction. 6 These findings demonstrate that PAR-1 plays a dual role, suppression and facilitation of smooth muscle motility in the rat duodenum, while PAR-2 plays a minor excitatory role in the muscle, and that PAR-4 is not involved in the duodenal tension modulation. The results also suggest that the contractile responses to PAR-1 and PAR-2 activation are mediated, in part, by activation of L-type calcium channels, protein kinase C and tyrosine kinase, and that the relaxation response to PAR-1 activation occurs via activation of apamin-sensitive, but charybdotoxin-insensitive, potassium channels, in which both protein kinase C and tyrosine kinase might be involved synergistically.

Insulin signaling is inhibited by micromolar concentrations of H(2)O(2). Evidence for a role of H(2)O(2) in tumor necrosis factor alpha-mediated insulin resistance

Both hyperglycemia and tumor necrosis factor alpha (TNFalpha) were found to induce insulin resistance at the level of the insulin receptor (IR). How this effect is mediated is, however, not understood. We investigated whether oxidative stress and production of hydrogen peroxide could be a common mediator of the inhibitory effect. We report here that micromolar concentrations of H(2)O(2) dramatically inhibit insulin-induced IR tyrosine phosphorylation (pretreatment with 500 microM H(2)O(2) for 5 min inhibits insulin-induced IR tyrosine phosphorylation to 8%), insulin receptor substrate 1 phosphorylation, as well as insulin downstream signaling such as activation of phosphatidylinositol 3-kinase (inhibited to 57%), glucose transport (inhibited to 36%), and mitogen-activated protein kinase activation (inhibited to 7.2%). Both sodium orthovanadate, a selective inhibitor of tyrosine-specific phosphatases, as well as the protein kinase C inhibitor Gö6976 reduced the inhibitory effect of hydrogen peroxide on IR tyrosine phosphorylation. To investigate whether H(2)O(2) is involved in hyperglycemia- and/or TNFalpha-induced insulin resistance, we preincubated the cells with the H(2)O(2) scavenger catalase prior to incubation with 25 mM glucose, 25 mM 2-deoxyglucose, 5.7 nM TNFalpha, or 500 microM H(2)O(2), respectively, and subsequent insulin stimulation. Whereas catalase treatment completely abolished the inhibitory effect of H(2)O(2) and TNFalpha on insulin receptor autophosphorylation, it did not reverse the inhibitory effect of hyperglycemia. In conclusion, these results demonstrate that hydrogen peroxide at low concentrations is a potent inhibitor of insulin signaling and may be involved in the development of insulin resistance in response to TNFalpha.

Salivary histatin 5 induces non-lytic release of ATP from Candida albicans leading to cell death

Salivary histatins are potent in vitro antifungal proteins and have promise as therapeutic agents against oral candidiasis. We performed pharmacological studies directed at understanding the biochemical basis of Hst 5 candidacidal activity. Three inhibitors of mitochondrial metabolism: carbonyl cyanide p-chlorophenylhydrazone, dinitrophenol, and azide inhibited Hst 5 killing of Candida albicans, while not inhibiting cellular ATP production. In contrast, Hst 5 caused a drastic reduction of C. albicans intracellular ATP content, which was a result of an efflux of ATP. Carbonyl cyanide p-chlorophenylhydrazone, dinitrophenol, and azide inhibited Hst 5-induced ATP efflux, thus establishing a correlation between ATP release and cell killing. Furthermore, C. albicans cells were respiring and had polarized membranes at least 80 min after ATP release, thus implying a non-lytic exit of cellular ATP in response to Hst 5. Based on evidence that transmembrane ATP efflux can occur in the absence of cytolysis through a channel-like pathway and that released ATP can act as a cytotoxic mediator by binding to membrane purinergic receptors, we evaluated whether extracellular ATP released by Hst 5 may have further functional role in cell killing. Consistent with this hypothesis, purinergic agonists BzATP and adenosine 5'O-(thiotriphosphate) induced loss of C. albicans cell viability and purinergic antagonists prevented Hst 5 killing.

Calcium antagonists ameliorate ischemia-induced endothelial cell permeability by inhibiting protein kinase C

BACKGROUND

Dihydropyridines block calcium channels; however, they also influence endothelial cells, which do not express calcium channels. We tested the hypothesis that nifedipine can prevent ischemia-induced endothelial permeability increases by inhibiting protein kinase C (PKC) in cultured porcine endothelial cells.

METHODS AND RESULTS

Ischemia was induced by potassium cyanide/deoxyglucose, and permeability was measured by albumin flux. Ion channels were characterized by patch clamp. [Ca2+]i was measured by fura 2. PKC activity was measured by substrate phosphorylation after cell fractionation. PKC isoforms were assessed by Western blot and confocal microscopy. Nifedipine prevented the ischemia-induced increase in permeability in a dose-dependent manner. Ischemia increased [Ca2+]i, which was not affected by nifedipine. Instead, ischemia-induced PKC translocation was prevented by nifedipine. Phorbol ester also increased endothelial cell permeability, which was dose dependently inhibited by nifedipine. The effects of non-calcium-channel-binding dihydropyridine derivatives were similar. Analysis of the PKC isoforms showed that nifedipine prevented ischemia-induced translocation of PKC-alpha and PKC-zeta. Specific inhibition of PKC isoforms with antisense oligodeoxynucleotides demonstrated a major role for PKC-alpha.

CONCLUSIONS

Nifedipine exerts a direct effect on endothelial cell permeability that is independent of calcium channels. The inhibition of ischemia-induced permeability by nifedipine seems to be mediated primarily by PKC-alpha inhibition. Anti-ischemic effects of dihydropyridine calcium antagonists could be due in part to their effects on endothelial cell permeability.

Induction of apoptosis in human T-cells by methyl mercury: temporal relationship between mitochondrial dysfunction and loss of reductive reserve

The objective of our study was to define the mechanism by which MeHgCl induces human T-cell apoptosis. We asked the question: does mercury disrupt the Deltapsim and induce a mitochondrial permeability transition state? Using two fluorescent reagents, JC-1 and DiOC6(3), we demonstrated that MeHgCl exposure resulted in a decrease in the Deltapsim. Since a decline in Deltapsim can disturb the pHi, we employed SNARF-1 to assess pHi; results indicate that mercury treatment reduced the pHi from 7.0 to 6.5. Consistent with these observations, we noted that uncoupled electron transfer reactions generated ROS, while cardiolipin, a mitochondrial phospholipid, was oxidized. In concert with the biochemical changes, there was a decrease in overall dimension of the mitochondria of mercury-treated cells and a loss in cristae architecture. The toxicant also depleted the thiol reserves of the cell and promoted translocation of cytochrome c from the mitochondria to the cytosol. Furthermore, when T cells were thiol-depleted, there was increased susceptibility to MeHgCl-induced apoptosis. Finally, we established a temporal relationship between the decline in Deltapsim, generation of ROS, and depletion of thiol reserves. The earliest detectable event was at the level of the mitochondrion; in the presence of MeHgCl there was a profound reduction in mitochondrial Deltapsim and a decline in GSH levels within 1 h. Subsequently, a further decrease in thiol reserves was linked to the generation of ROS. We propose that the target organelle for MeHgCl is the mitochondrion and that induction of oxidative stress leads to activation of death-signaling pathways.

Activation of protein kinase Calpha couples cell volume to membrane Cl- permeability in HTC hepatoma and Mz-ChA-1 cholangiocarcinoma cells

Physiological increases in liver cell volume lead to an adaptive response that includes opening of membrane Cl- channels, which is critical for volume recovery. The purpose of these studies was to assess the potential role for protein kinase C (PKC) as a signal involved in cell volume homeostasis. Studies were performed in HTC rat hepatoma and Mz-ChA-1 human cholangiocarcinoma cells, which were used as model hepatocytes and cholangiocytes, respectively. In each cell type, cell volume increases were followed by: 1) translocation of PKC from cytosolic to particulate (membrane) fractions; 2) a 10- to 40-fold increase in whole-cell membrane Cl- current density; and 3) partial recovery of cell volume. In HTC cells, the volume-dependent Cl- current response (-46 +/- 5 pA/pF) was inhibited by down-regulation of PKC (100 nmol/L phorbol 12-myristate 13-acetate for 18 hours [PMA]; -1.97 +/- 1.5 pA/pF), chelation of cytosolic Ca2+ (2 mmol/L EGTA; -5.3 +/- 4.0 pA/pF), depletion of cytosolic adenosine triphosphate (ATP) (3 U/mL apyrase; -12.58 +/- 1. 45 pA/pF), and by the putative PKC inhibitor, chelerythrine (25 micromol/L; -7 +/- 3 pA/pF). In addition, PKC inhibition by chelerythrine and calphostin C (500 nmol/L) prevented cell volume recovery from swelling. Similar results were obtained in Mz-ChA-1 biliary cells. These findings indicate that swelling-induced activation of PKC represents an important signal coupling cell volume to membrane Cl- permeability in both hepatic and biliary cell models.

Effects of PKC alpha activation on Ca2+ pump and K(Ca) channel in deoxygenated sickle cells

We have previously shown that a pretreatment with phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), reduced deoxygenation-induced K+ loss and Ca2+ uptake and prevented cell dehydration in sickle anemia red blood cells (SS cells) (H. Fathallah, E. Coezy, R.-S. De Neef, M.-D. Hardy-Dessources, and F. Giraud. Blood 86: 1999-2007, 1995). The present study explores the detailed mechanism of this PMA-induced inhibition. The main findings are, first, the detection of PKC alpha and PKC zeta in normal red blood cells and the demonstration that both isoforms are expressed at higher levels in SS cells. The alpha-isoform only is translocated to the membrane and activated by PMA and by elevation of cytosolic Ca2+. Second, PMA is demonstrated to activate Ca2+ efflux in deoxygenated SS cells by a direct stimulation of the Ca2+ pump. PMA, moreover, inhibits deoxygenation-induced, charybdotoxin-sensitive K+ efflux in SS cells. This inhibition is partly indirect and explained by the reduced deoxygenation-induced rise in cytosolic Ca2+ resulting from Ca2+ pump stimulation. However, a significant inhibition of the Ca2+-activated K+ channels (K(Ca) channels) by PMA can also be demonstrated when the channels are activated by Ca2+ plus ionophore, under conditions in which the Ca2+ pump is operating near its maximal extrusion rate, but swamped by Ca2+ plus ionophore. The data thus suggest a PKC alpha-mediated phosphorylation both of the Ca2+ pump and of the K(Ca) channel or an auxiliary protein.

Hepatocellular ATP-binding cassette protein expression enhances ATP release and autocrine regulation of cell volume

In a model liver cell line, recovery from swelling is mediated by a sensitive autocrine pathway involving conductive release of ATP, P2 receptor stimulation, and opening of membrane Cl- channels (Wang, Y., Roman, R. M., Lidofsky, S. D., and Fitz, J. G. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 12020-12025). However, the mechanisms coupling changes in cell volume to ATP release are not known. Based on evidence that certain ATP-binding cassette (ABC) proteins may function as ATP channels or channel regulators, we evaluated the potential role of ABC proteins by comparing ATP release and volume regulation in rat HTC and HTC-R hepatoma cells, the latter of which overexpress Mdr proteins. In both cell types, Cl- current activation (ICl-swell) and volume recovery following swelling were dependent on conductive ATP efflux. The rate of volume recovery was approximately 6-fold faster in HTC-R cells compared with HTC cells. This effect is likely due to enhanced ABC protein-dependent ATP release since (i) ICl-swell and cell volume recovery were eliminated by inhibition of P-glycoprotein transport (20 microM verapamil and 15 microM cyclosporin A); (ii) swelling-induced Cl- current density was similar in both cell types (approximately -50 pA/pF; not significant); and (iii) ATP conductance measured by whole-cell techniques was increased approximately 3-fold in HTC-R cells compared with HTC cells. Moreover, HTC-R cells exhibited enhanced survival during hypotonic stress. By modulating ATP release, hepatic ABC proteins may play a key role in the cellular pathways coupling changes in cell volume to ion permeability and secretion.

Cytosolic Ca2+ and protein kinase Calpha couple cellular metabolism to membrane K+ permeability in a human biliary cell line

Cholangiocytes represent an important target of injury during the ischemia and metabolic stress that accompanies liver preservation. Since K+ efflux serves to minimize injury during ATP depletion in certain other cell types, the purpose of these studies was to evaluate the effects of ATP depletion on plasma membrane K+ permeability of Mz-ChA-1 cells, a model human biliary cell line. Cells were exposed to dinitrophenol (50 microM) and 2-deoxyglucose (10 mM) as the standard model of metabolic injury. Whole-cell and single K+ channel currents were measured using patch clamp techniques; and intracellular [Ca2+] ([Ca2+]i) was estimated by calcium green-1 fluorescence. Metabolic stress increased [Ca2+]i, and stimulated translocation of the alpha isoform of protein kinase C (PKCalpha) from cytosolic to particulate cell fractions. The same maneuver increased membrane K+ permeability 40-70-fold as detected by (a) activation of K+selective whole cell currents of 2,176+/-218 pA (n = 34), and (b) opening of apamin-sensitive K+ channels with a unitary conductance of 17.0+/-0.2 pS. PKCalpha translocation and channel opening appear to be related since stress-induced K+ efflux is inhibited by chelation of cytosolic Ca2+, exposure to the PKC inhibitor chelerythrine (25 microM) and downregulation of PKC by phorbol esters. Moreover, K+ currents were activated by intracellular perfusion with recombinant PKCalpha in the absence of metabolic inhibitors. These findings indicate that in biliary cells apamin-sensitive K+ channels are functionally coupled to cell metabolism and suggest that cytosolic Ca2+ and PKCalpha are selectively involved in the response.