Arylaminobenzoate block of the cardiac cyclic AMP-dependent chloride current

Chloride channel-3 promotes tumor metastasis by regulating membrane ruffling and is associated with poor survival

The chloride channel-3 (ClC-3) protein is known to be a component of Cl⁻ channels involved in cell volume regulation or acidification of intracellular vesicles. Here, we report that ClC-3 was highly expressed in the cytoplasm of metastatic carcinomatous cells and accelerated cell migration in vitro and tumor metastasis in vivo. High-grade expression of cytoplasmic ClC-3 predicted poor survival in cancer patients. We found that independent of its volume-activated Cl⁻ channel properties, ClC-3 was able to promote cell membrane ruffling, required for tumor metastasis. ClC-3 mediated membrane ruffling by regulating keratin 18 phosphorylation to control β1 Integrin recycling. Therefore, cytoplasmic ClC-3 plays an active and key role in tumor metastasis and may be a valuable prognostic biomarker and a therapeutic target to prevent tumor spread.

Catalyst-like modulation of transition states for CFTR channel opening and closing: new stimulation strategy exploits nonequilibrium gating

Two gating transition states determine open probability of CFTR (the chloride channel mutated in cystic fibrosis), defining strategic targets for therapeutic intervention.

Cystic fibrosis transmembrane conductance regulator (CFTR) is the chloride ion channel mutated in cystic fibrosis (CF) patients. It is an ATP-binding cassette protein, and its resulting cyclic nonequilibrium gating mechanism sets it apart from most other ion channels. The most common CF mutation (ΔF508) impairs folding of CFTR but also channel gating, reducing open probability (P_o). This gating defect must be addressed to effectively treat CF. Combining single-channel and macroscopic current measurements in inside-out patches, we show here that the two effects of 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB) on CFTR, pore block and gating stimulation, are independent, suggesting action at distinct sites. Furthermore, detailed kinetic analysis revealed that NPPB potently increases P_o, also of ΔF508 CFTR, by affecting the stability of gating transition states. This finding is unexpected, because for most ion channels, which gate at equilibrium, altering transition-state stabilities has no effect on P_o; rather, agonists usually stimulate by stabilizing open states. Our results highlight how for CFTR, because of its unique cyclic mechanism, gating transition states determine P_o and offer strategic targets for potentiator compounds to achieve maximal efficacy.

Catalyst-Free Microwave-Assisted Amination of 2-Chloro-5-nitrobenzoic Acid

The synthesis of N-substituted 5-nitroanthranilic acid derivatives 3a-w was achieved by a new, mild, microwave-assisted, regioselective amination reaction of 5-nitro-2-chlorobenzoic acid (1a) with a diverse range of aliphatic and aromatic amines 2a-w without added solvent or catalyst. Up to >99% isolated yield was obtained within 5-30 min at 80-120 degrees C. The reaction, which is suitable for upscaling, yielded new compounds that are of considerable interest as useful building blocks and as potential drugs.

Inhibition of CFTR channels by a peptide toxin of scorpion venom

Peptide toxins have been valuable probes in efforts to identify amino acid residues that line the permeation pathway of cation-selective channels. However, no peptide toxins have been identified that interact with known anion-selective channels such as the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR channels are expressed in epithelial cells and are associated with several genetic disorders, including cystic fibrosis and polycystic kidney disease. Several organic inhibitors have been used to investigate the structure of the Cl−permeation pathway in CFTR. However, investigations of the wider cytoplasmic vestibule have been hindered by the lack of a high-affinity blocker that interacts with residues in this area. In this study we show that venom of the scorpion Leiurus quinquestriatus hebraeus reversibly inhibits CFTR, in a voltage-independent manner, by decreasing single-channel mean burst duration and open probability only when applied to the cytoplasmic surface of phosphorylated channels. Venom was able to decrease burst duration and open probability even when CFTR channels were locked open by treatment with either vanadate or adenosine 5′-(β,γ-imido)triphosphate, and block was strengthened on reduction of extracellular Cl−concentration, suggesting inhibition by a pore-block mechanism. Venom had no effect on ATP-dependent macroscopic opening rate in channels studied by inside-out macropatches. Interestingly, the inhibitory activity was abolished by proteinase treatment. We conclude that a peptide toxin contained in the scorpion venom inhibits CFTR channels by a pore-block mechanism; these experiments provide the first step toward isolation of the active component, which would be highly valuable as a probe for CFTR structure and function.

Closure of gap junction channels by arylaminobenzoates

We determined the effect of flufenamic acid (FFA) and related derivatives on gap junction channel currents, applying the dual whole-cell patch-clamp technique to pairs of N2A neuroblastoma cells transfected with various connexins. FFA reduced gap junction channel currents in a reversible and concentration-dependent manner. Half-maximal concentrations for FFA-induced reduction of junctional conductance in cell pairs coupled by different connexins were similar (20 to 60 microM), indicating that FFA does not greatly discriminate between connexin subtypes. Hill coefficients for blockade were approximately 3, indicating a high degree of cooperativity. Analogs of FFA also reduced junctional conductance with similar potencies, whereas other unrelated chloride channel blockers had no effect. Inhibition of gap junction channels by FFA (pKa approximately 3.8) was increased at low external pH, suggesting that the uncharged form of the drug is important for blockade. The effect of FFA did not seem to be mediated by direct binding of the drug to the pore of the gap junction channel. Internal application of high concentrations of FFA by addition to patch pipettes did not cause inhibition of channel currents. The magnitude of inhibition was neither voltage-dependent nor influenced by the nature of permeant ion. Single-channel recordings indicated that FFA reduced the channel-open probability without modifying the current amplitude and induced slow transitions between open and closed states. We propose that FFA inhibits gap junctions by inducing a conformational change in the protein upon binding to a site that is presumably located within the membrane.

Ca2+-activated Cl- channel currents in rat ventral prostate epithelial cells

BACKGROUND

In many epithelial tissues, the Cl(-) efflux via Ca(2+)-activated Cl(-) channels (Cl(Ca)) play a key role for the fluid secretion. To elucidate the mechanism of prostatic fluid secretion, the properties of whole-cell chloride conductance were investigated.

MATERIALS AND METHODS

Rat prostate secretory epithelial cells (RPSECs) were isolated by collagenase treatment, and were used for the whole-cell voltage clamp. Both extra- and intracellular monovalent cations were replaced by N-methyl-D-glucamate to record the Cl(-) current selectively.

RESULTS

A bath application of Ca(2+)-ionophore, ionomycin (0.2 micro M), increased the membrane conductance with outwardly rectifying voltage-dependence. On step-like depolarization from -60 to +80 mV (500 msec), the ionomycin-induced current showed slowly activating kinetics, a known property of Cl(Ca) current (I(Cl(Ca))) of other tissues. The relative permeability of Cl(Ca) to various anions was calculated from the reversal potentials measured under a total replacement of extracellular Cl(-) with various anions, and the relative order of permeability was SCN(-)>I(-)>Br(-)>Cl(-)>>gluconate. The amplitude of I(Cl(Ca)) was decreased by various anion channel blockers: niflumic acid (100 micro M), DPC (100 micro M), DIDS (1 mM), and NPPB (200 micro M).

CONCLUSIONS

RPSECs have Cl(Ca) that may provide Cl(-) efflux pathways for the exocrine secretions of the prostate.

Indazole inhibition of cystic fibrosis transmembrane conductance regulator Cl(-) channels in rat epididymal epithelial cells

Previous studies have shown that two indazole compounds, lonidamine [1-(2,4-dichlorobenzyl)-indazole-3-carboxylic acid] and its analogue AF2785 [(1-(2,4-dichlorobenzyl)-indazol-3-acrylic acid], suppress fertility in male rats. We also found that these compounds inhibit the cystic fibrosis transmembrane conductance regulator chloride (CFTR-Cl(-)) current in epididymal epithelial cells. To further investigate how lonidamine and AF2785 inhibit the current, we used a spectral analysis protocol to study whole-cell CFTR current variance. Application of lonidamine or AF2785 to the extracellular membrane of rat epididymal epithelial cells introduced a new component to the whole-cell current variance. Spectral analysis of this variance suggested a block at a rate of 3.68 micro mol(-1)/sec(-1) and an off rate of 69.01 sec(-1) for lonidamine, and an on rate of 3.27 micro mol(-1)/sec(-1) and an off rate of 108 sec(-1) for AF2785. Single CFTR-Cl(-) channel activity using excised inside-out membrane patches from rat epididymal epithelial cells revealed that addition of lonidamine to the intracellular solution caused a flickery block (a reduction in channel-open time) at lower concentration (10 micro M) without any effect on open channel probability or single-channel current amplitude. At higher concentrations (50 and 100 micro M), lonidamine showed a flickery block and a decrease in open-channel probability. The flickery block by lonidamine was both voltage-dependent and concentration-dependent. These results suggest that lonidamine and AF2785, which are open-channel blockers of CFTR at low concentrations, also affect CFTR gating at high concentrations. We conclude that these indazole compounds provide new pharmacological tools for the investigation of CFTR. By virtue of their interference with reproductive processes, these drugs have the potential for being developed into novel male contraceptives.

Permeation through the CFTR chloride channel

The cystic fibrosis transmembrane conductance regulator (CFTR) protein forms a Cl(−) channel found in the plasma membranes of many epithelial cells, including those of the kidney, gut and conducting airways. Mutation of the gene encoding CFTR is the primary defect in cystic fibrosis, a disease that affects approximately 30 000 individuals in the United States alone. Alteration of CFTR function also plays an important role in the pathophysiology of secretory diarrhea and polycystic kidney disease. The basic mechanisms of permeation in this channel are not well understood. It is not known which portions of the protein contribute to forming the pore or which amino acid residues in those domains are involved in the biophysical processes of ion permeation. In this review, I will discuss (i) the present understanding of ion transport processes in the wild-type CFTR channel, (ii) the experimental approaches currently being applied to investigate the pore, and (iii) a proposed structure that takes into account the present data on mechanisms of ion selectivity in the CFTR channel and on blockade of the pore by open-channel blockers.

Inhibition of cystic fibrosis transmembrane conductance regulator chloride channel currents by arachidonic acid

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is inhibited by a number of different classes of organic anions which are able to enter and block the channel pore from its cytoplasmic end. Here I show, using patch clamp recording from CFTR-transfected baby hamster kidney cell lines, that the cis-unsaturated fatty acid arachidonic acid also inhibits CFTR Cl- currents when applied to the cytoplasmic face of excised membrane patches. This inhibition was of a relatively high affinity compared with other known CFTR inhibitors, with an apparent Kd of 6.5 ± 0.9 µM. However, in contrast with known CFTR pore blockers, inhibition by arachidonic acid was only very weakly voltage dependent, and was insensitive to the extracellular Cl- concentration. Arachidonic acid-mediated inhibition of CFTR Cl- currents was not abrogated by inhibitors of lipoxygenases, cyclooxygenases or cytochrome P450, suggesting that arachidonic acid itself, rather than some metabolite, directly affects CFTR. Similar inhibition of CFTR Cl- currents was seen with other fatty acids, with the rank order of potency linoleic [Formula: see text] arachidonic [Formula: see text] oleic > elaidic [Formula: see text] palmitic [Formula: see text] myristic. These results identify fatty acids as novel high affinity modulators of the CFTR Cl- channel.Key words: CFTR, chloride channel, fatty acid, channel block, cystic fibrosis.

Properties of Ca(2+) release-activated Ca(2+) channel block by 5-nitro-2-(3-phenylpropylamino)-benzoic acid in Jurkat cells

Ca(2+) release-activated Ca(2+) current (I(crac)) has been previously characterized biophysically in Jurkat lymphocytes and other non-excitable cells, but pharmacology remains poorly developed. The present objective was to delineate with whole cell recording details of the interaction of the chloride channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), with I(crac) in Jurkat cells. NPPB reversibly inhibited I(crac) in a concentration-dependent manner (IC(50)=5 microM). Kinetics for block and unblock of I(crac) by NPPB indicated a bimolecular interaction. Michaelis-Menten analysis indicated that NPPB interacts competitively with extracellular Ca(2+) permeating the I(crac) pathway. Finally, analysis of the pH dependence of I(crac) block by NPPB revealed a reduction in the apparent affinity during extracellular alkalinization that based on the pK(a) for NPPB, suggested that the neutral form of NPPB blocks the Ca(2+) release-activated Ca(2+) (CRAC) channel. Taken together, our results suggest a direct interaction between NPPB and the CRAC channel, and should help guide insights for developing novel and more selective analogues.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Molecular pharmacology of the CFTR Cl- channel

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is associated with a wide spectrum of disease. In the search for modulators of CFTR, pharmacological agents that interact directly with the CFTR Cl- channel have been identified. Some agents stimulate CFTR by interacting with the nucleotide-binding domains that control channel gating, whereas others inhibit CFTR by binding within the channel pore and preventing Cl- permeation. Knowledge of the molecular pharmacology of CFTR might lead to new treatments for diseases caused by the dysfunction of CFTR.

Structural and ionic determinants of 5-nitro-2-(3-phenylprophyl-amino)-benzoic acid block of the CFTR chloride channel

1. The goals of this study were to identify the structural components required for arylaminobenzoate block of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel and to determine the involvement of two positively charged amino acid residues, found within the channel, in drug binding. 2. Wild-type and mutant CFTR chloride channels were expressed in Xenopus oocytes and CFTR currents measured using the two microelectrode voltage clamp. Block of the wild-type CFTR current by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) occurred in a voltage-dependent manner with preferential inhibition of the inward currents (Kd = 166 microM at -90 mV). 3. Removal of the phenyl ring from the aliphatic chain of NPPB, with the compound 2-butylamino-5-nitrobenzoic acid, caused only a small change in CFTR inhibition (Kd = 243 microM), while addition of an extra phenyl ring at this position (5-nitro-2-(3,3-diphenylpropylamino)-benzoic acid) increased drug potency (Kd = 58 microM). In contrast, removal of the benzoate ring (2-amino-4-phenylbutyric acid) or the 5-nitro group (2-(3-phenylpropylamino)-benzoic acid) of NPPB severely limited drug block of the wild-type channel. 4. NPPB inhibition of CFTR currents in oocytes expressing the mutants K335E and R347E also occurred in a voltage-dependent manner. However, the Kds for NPPB block were increased to 371 and 1573 microM, for the K335E and R347E mutants, respectively. 5. NPPB block of the inward wild-type CFTR current was reduced in the presence of 10 mM of the permeant anion SCN-. 6. These studies present the first step in the development of high affinity probes to the CFTR channel.

Substrates of multidrug resistance-associated proteins block the cystic fibrosis transmembrane conductance regulator chloride channel

1. The effects of physiological substrates of multidrug resistance-associated proteins (MRPs) on cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel currents were examined using patch clamp recording from CFTR-transfected mammalian cell lines. 2. Two MRP substrates, taurolithocholate-3-sulphate (TLCS) and beta-estradiol 17-(beta-D-glucuronide) (E217betaG) caused a voltage-dependent block of macroscopic CFTR Cl- currents when applied to the intracellular face of excised membrane patches, with mean apparent dissociation constants (KDs) of 96+/-10 and 563+/-103 microM (at 0 mV) respectively. The unconjugated bile salts taurocholate and cholate were also effective CFTR channel blockers under these conditions, with KDs of 453+/-44 and 3760+/-710 microM (at 0 mV) respectively. 3. Reducing the extracellular Cl- concentration from 154 to 20 mM decreased the KD for block intracellular TLCS to 54+/-1 microM, and also significantly reduced the voltage dependence of block, by suggesting that TLCS blocks Cl- permeation through CFTR by binding within the channel pore. 4. Intracellular TLCS reduced the apparent amplitude of CFTR single channel currents, suggesting that the duration of block is very fast compared to the gating of the channel. 5. The apparent affinity of block by TLCs is comparable to that of other well-known CFTR channel blockers, suggesting that MRP substrates may comprise a novel class of probes of the CFTR channel pore. 6. These results also suggest that the related proteins CFTR and MRP may share a structurally similar anion binding site at the cytoplasmic face of the membrane.