Potent inhibition of the CFTR chloride channel by suramin

Comprehensive Analysis of Chemical Structures That Have Been Tested as CFTR Activating Substances in a Publicly Available Database CandActCFTR

Background: Cystic fibrosis (CF) is a genetic disease caused by mutations in CFTR, which encodes a chloride and bicarbonate transporter expressed in exocrine epithelia throughout the body. Recently, some therapeutics became available that directly target dysfunctional CFTR, yet research for more effective substances is ongoing. The database CandActCFTR aims to provide detailed and comprehensive information on candidate therapeutics for the activation of CFTR-mediated ion conductance aiding systems-biology approaches to identify substances that will synergistically activate CFTR-mediated ion conductance based on published data. Results: Until 10/2020, we derived data from 108 publications on 3,109 CFTR-relevant substances via the literature database PubMed and further 666 substances via ChEMBL; only 19 substances were shared between these sources. One hundred and forty-five molecules do not have a corresponding entry in PubChem or ChemSpider, which indicates that there currently is no single comprehensive database on chemical substances in the public domain. Apart from basic data on all compounds, we have visualized the chemical space derived from their chemical descriptors via a principal component analysis annotated for CFTR-relevant biological categories. Our online query tools enable the search for most similar compounds and provide the relevant annotations in a structured way. The integration of the KNIME software environment in the back-end facilitates a fast and user-friendly maintenance of the provided data sets and a quick extension with new functionalities, e.g., new analysis routines. CandActBase automatically integrates information from other online sources, such as synonyms from PubChem and provides links to other resources like ChEMBL or the source publications. Conclusion: CandActCFTR aims to establish a database model of candidate cystic fibrosis therapeutics for the activation of CFTR-mediated ion conductance to merge data from publicly available sources. Using CandActBase, our strategy to represent data from several internet resources in a merged and organized form can also be applied to other use cases. For substances tested as CFTR activating compounds, the search function allows users to check if a specific compound or a closely related substance was already tested in the CF field. The acquired information on tested substances will assist in the identification of the most promising candidates for future therapeutics.

100 Years of Suramin

Suramin is 100 years old and is still being used to treat the first stage of acute human sleeping sickness, caused by Trypanosoma brucei rhodesiense Suramin is a multifunctional molecule with a wide array of potential applications, from parasitic and viral diseases to cancer, snakebite, and autism. Suramin is also an enigmatic molecule: What are its targets? How does it get into cells in the first place? Here, we provide an overview of the many different candidate targets of suramin and discuss its modes of action and routes of cellular uptake. We reason that, once the polypharmacology of suramin is understood at the molecular level, new, more specific, and less toxic molecules can be identified for the numerous potential applications of suramin.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Suppression of cell membrane permeability by suramin: involvement of its inhibitory actions on connexin 43 hemichannels

BACKGROUND AND PURPOSE

Suramin is a clinically prescribed drug for treatment of human African trypanosomiasis, cancer and infection. It is also a well-known pharmacological antagonist of P2 purinoceptors. Despite its clinical use and use in research, the biological actions of this molecule are still incompletely understood. Here, we investigated the effects of suramin on membrane channels, as exemplified by its actions on non-junctional connexin43 (Cx43) hemichannels, pore-forming α-haemolysin and channels involved in ATP release under hypotonic conditions.

EXPERIMENTAL APPROACH

Hemichannels were activated by removing extracellular Ca(2+) . The influences of suramin on hemichannel activities were evaluated by its effects on influx of fluorescent dyes and efflux of ATP. The membrane permeability and integrity were assessed through cellular retention of preloaded calcein and LDH release.

KEY RESULTS

Suramin blocked Cx43 hemichannel permeability induced by removal of extracellular Ca(2+) without much effect on Cx43 expression and gap junctional intercellular communication. This action of suramin was mimicked by its analogue NF023 and NF449 but not by another P2 purinoceptor antagonist PPADS. Besides hemichannels, suramin also significantly blocked intracellular and extracellular exchanges of small molecules caused by α-haemolysin from Staphylococcus aureus and by exposure of cells to hypotonic solution. Furthermore, it prevented α-haemolysin- and hypotonic stress-elicited cell injury.

CONCLUSION AND IMPLICATIONS

Suramin blocked membrane channels and protected cells against toxin- and hypotonic stress-elicited injury. Our finding provides novel mechanistic insights into the pharmacological actions of suramin. Suramin might be therapeutically exploited to protect membrane integrity under certain pathological situations.

Loop diuretics are open-channel blockers of the cystic fibrosis transmembrane conductance regulator with distinct kinetics

BACKGROUND AND PURPOSE

Loop diuretics are widely used to inhibit the Na(+), K(+), 2Cl(-) co-transporter, but they also inhibit the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. Here, we investigated the mechanism of CFTR inhibition by loop diuretics and explored the effects of chemical structure on channel blockade.

EXPERIMENTAL APPROACH

Using the patch-clamp technique, we tested the effects of bumetanide, furosemide, piretanide and xipamide on recombinant wild-type human CFTR.

KEY RESULTS

When added to the intracellular solution, loop diuretics inhibited CFTR Cl(-) currents with potency approaching that of glibenclamide, a widely used CFTR blocker with some structural similarity to loop diuretics. To begin to study the kinetics of channel blockade, we examined the time dependence of macroscopic current inhibition following a hyperpolarizing voltage step. Like glibenclamide, piretanide blockade of CFTR was time and voltage dependent. By contrast, furosemide blockade was voltage dependent, but time independent. Consistent with these data, furosemide blocked individual CFTR Cl(-) channels with 'very fast' speed and drug-induced blocking events overlapped brief channel closures, whereas piretanide inhibited individual channels with 'intermediate' speed and drug-induced blocking events were distinct from channel closures.

CONCLUSIONS AND IMPLICATIONS

Structure-activity analysis of the loop diuretics suggests that the phenoxy group present in bumetanide and piretanide, but absent in furosemide and xipamide, might account for the different kinetics of channel block by locking loop diuretics within the intracellular vestibule of the CFTR pore. We conclude that loop diuretics are open-channel blockers of CFTR with distinct kinetics, affected by molecular dimensions and lipophilicity.

Cystic fibrosis transmembrane conductance regulator chloride channel blockers: Pharmacological, biophysical and physiological relevance

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel causes cystic fibrosis, while inappropriate activity of this channel occurs in secretory diarrhea and polycystic kidney disease. Drugs that interact directly with CFTR are therefore of interest in the treatment of a number of disease states. This review focuses on one class of small molecules that interacts directly with CFTR, namely inhibitors that act by directly blocking chloride movement through the open channel pore. In theory such compounds could be of use in the treatment of diarrhea and polycystic kidney disease, however in practice all known substances acting by this mechanism to inhibit CFTR function lack either the potency or specificity for in vivo use. Nevertheless, this theoretical pharmacological usefulness set the scene for the development of more potent, specific CFTR inhibitors. Biophysically, open channel blockers have proven most useful as experimental probes of the structure and function of the CFTR chloride channel pore. Most importantly, the use of these blockers has been fundamental in developing a functional model of the pore that includes a wide inner vestibule that uses positively charged amino acid side chains to attract both permeant and blocking anions from the cell cytoplasm. CFTR channels are also subject to this kind of blocking action by endogenous anions present in the cell cytoplasm, and recently this blocking effect has been suggested to play a role in the physiological control of CFTR channel function, in particular as a novel mechanism linking CFTR function dynamically to the composition of epithelial cell secretions. It has also been suggested that future drugs could target this same pathway as a way of pharmacologically increasing CFTR activity in cystic fibrosis. Studying open channel blockers and their mechanisms of action has resulted in significant advances in our understanding of CFTR as a pharmacological target in disease states, of CFTR channel structure and function, and of how CFTR activity is controlled by its local environment.

Glycolipid-Dependent, Protease Sensitive Internalization of Pseudomonas aeruginosa Into Cultured Human Respiratory Epithelial Cells

Internalization of PAK strain Pseudomonas aeruginosa into human respiratory epithelial cell lines and HeLa cervical cancer cells in vitro was readily demonstrable via a gentamycin protection assay. Depletion of target cell glycosphingolipids (GSLs) using a glucosyl ceramide synthase inhibitor, P4, completely prevented P. aeruginosa internalization. In contrast, P4 treatment had no effect on the internalization of Salmonella typhimurium into HeLa cells. Internalized P. aeruginosa were within membrane vacuoles, often containing microvesicles, between the bacterium and the limiting membrane. P. aeruginosa internalization was markedly enhanced by target cell pretreatment with the exogenous GSL, deacetyl gangliotetraosyl ceramide (Gg₄). Gg₄ binds the lipid raft marker, GM1 ganglioside. Target cell pretreatment with TLCK, but not other (serine) protease inhibitors, prevented both P. aeruginosa host cell binding and internalization. NFkB inhibition also prevented internalization. A GSL-containing lipid-raft model of P. aeruginosa host cell binding/internalization is proposed

Cyclic AMP regulates bicarbonate secretion in cholangiocytes through release of ATP into bile

BACKGROUND & AIMS

Bicarbonate secretion is a primary function of cholangiocytes. Either adenosine 3',5'-cyclic monophosphate (cAMP) or cytosolic Ca(2+) can mediate bicarbonate secretion, but these are thought to act through separate pathways. We examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R) in mediating bicarbonate secretion because this is the only intracellular Ca(2+) release channel in cholangiocytes.

METHODS

Intrahepatic bile duct units (IBDUs) were microdissected from rat liver then luminal pH was examined by confocal microscopy during IBDU microperfusion. Cyclic AMP was increased using forskolin or secretin, and Ca(2+) was increased using acetylcholine (ACh) or adenosine triphosphate (ATP). Apyrase was used to hydrolyze extracellular ATP, and suramin was used to block apical P2Y ATP receptors. In selected experiments, IBDUs were pretreated with short interfering RNA (siRNA) to silence expression of specific InsP3R isoforms.

RESULTS

Both cAMP and Ca(2+) agonists increased luminal pH. The effect of ACh on luminal pH was reduced by siRNA for basolateral (types I and II) but not apical (type III) InsP3R isoforms. The effect of forskolin on luminal pH was reduced by a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor and by siRNA for the type III InsP3R. Luminal apyrase or suramin blocked the effects of forskolin but not ACh on luminal pH.

CONCLUSIONS

Cyclic AMP-induced ductular bicarbonate secretion depends on an autocrine signaling pathway that involves CFTR, apical release of ATP, stimulation of apical nucleotide receptors, and then activation of apical, type III InsP3Rs. The primary role of CFTR in bile duct secretion may be to regulate secretion of ATP rather than to secrete chloride and/or bicarbonate.

Identification of a second blocker binding site at the cytoplasmic mouth of the cystic fibrosis transmembrane conductance regulator chloride channel pore

Chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is inhibited by a broad range of substances that bind within a wide inner vestibule in the pore and physically occlude Cl(-) permeation. Binding of many of these so-called open-channel blockers involves electrostatic interactions with a positively charged lysine residue (Lys95) located in the pore. Here, we use site-directed mutagenesis to identify a second blocker binding site located at the cytoplasmic mouth of the pore. Mutagenesis of a positively charged arginine at the cytoplasmic mouth of the pore, Arg303, leads to significant weakening of the blocking effects of suramin, a large negatively charged organic molecule. Apparent suramin affinity is correlated with the side chain charge at this position, consistent with an electrostatic interaction. In contrast, block by suramin is unaffected by mutagenesis of Lys95, suggesting that it does not approach close to this important pore-forming lysine residue. We propose that the CFTR pore inner vestibule contains two distinct blocker binding sites. Relatively small organic anions enter deeply into the pore to interact with Lys95, causing an open-channel block that is sensitive to both the membrane potential and the extracellular Cl(-) concentration. Larger anionic molecules can become lodged in the cytoplasmic mouth of the pore where they interact with Arg303, causing a distinct type of open-channel block that is insensitive to membrane potential or extracellular Cl(-) ions. The pore may narrow significantly between the locations of these two blocker binding sites.

Biochemical Characterization of Suramin as a Selective Inhibitor for the PKA-Mediated Phosphorylation of HBV Core Protein in Vitro

The inhibitory effect of suramin on the phosphorylation of GST-HBV core fusion protein (GST-Hcore) and two GST-Hcore fusion polypeptides (Hcore157B and Hcore164B) by two alpha-type cAMP-dependent protein kinases (PKAIalpha and PKAIIalpha) was biochemically investigated in vitro. It was found that (i) this phosphorylation was inhibited by suramin at a low concentration (IC(50)=approx. 10 nM); (ii) a relative high dose of suramin was required to inhibit an autophosphorylation of PKAIIalpha (IC(50)=approx. 0.7 muM) and the PKAIIalpha-mediated phosphorylation of histone H2B (IC(50)=approx. 0.4 muM); (iii) the PKAIIalpha-mediated phosphorylation of Hcore157B was more sensitive to suramin than the phosphorylation of Hcore157B by Ca(2+)-dependent protein kinase (PKC); and (iv) suramin had a high binding affinity for Hcore157B, but not for histone H2B in vitro. These results suggest that suramin selectively inhibits the PKA-mediated phosphorylation of HBV-CP through the direct binding in vitro of suramin to the Arg-rich C-terminal region (containing three potential phosphorylation sites for PKA) on HBV-CP.

Diversity of Cl(-) channels

Cl(-) channels are widely found anion pores that are regulated by a variety of signals and that play various roles. On the basis of molecular biologic findings, ligand-gated Cl(-) channels in synapses, cystic fibrosis transmembrane conductors (CFTRs) and ClC channel types have been established, followed by bestrophin and possibly by tweety, which encode Ca(2+)-activated Cl(-) channels. The ClC family has been shown to possess a variety of functions, including stabilization of membrane potential, excitation, cell-volume regulation, fluid transport, protein degradation in endosomal vesicles and possibly cell growth. The molecular structure of Cl(-) channel types varies from 1 to 12 transmembrane segments. By means of computer-based prediction, functional Cl(-) channels have been synthesized artificially, revealing that many possible ion pores are hidden in channel, transporter or unidentified hydrophobic membrane proteins. Thus, novel Cl(-)-conducting pores may be occasionally discovered, and evidence from molecular biologic studies will clarify their physiologic and pathophysiologic roles.

Location of a common inhibitor binding site in the cytoplasmic vestibule of the cystic fibrosis transmembrane conductance regulator chloride channel pore

Chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is inhibited by a broad range of organic anions that enter the channel pore from its cytoplasmic end, physically occluding the Cl- permeation pathway. These open channel blocker molecules are presumed to bind within a relatively wide pore inner vestibule that shows little discrimination between different large anions. The present study uses patch clamp recording to identify a pore-lining lysine residue, Lys-95, that acts to attract large blocker molecules into this inner vestibule. Mutations that remove the fixed positive charge associated with this amino acid residue dramatically weaken the blocking effects of five structurally unrelated open channel blockers (glibenclamide, 4,4'-dinitrostilbene-2,2'-disulfonic acid, lonidamine, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and taurolithocholate-3-sulfate) when applied to the cytoplasmic face of the membrane. Mutagenesis of Lys-95 also induced amino acid side chain charge-dependent rectification of the macroscopic current-voltage relationship, consistent with the fixed positive charge on this residue normally acting to attract Cl- ions from the intracellular solution into the pore. These results identify Lys-95 as playing an important role in attracting permeant anions into the channel pore inner vestibule, probably by an electrostatic mechanism. This same electrostatic attraction mechanism also acts to attract larger anionic molecules into the relatively wide inner vestibule, where these substances bind to block Cl- permeation. Thus, structurally diverse open channel blockers of CFTR appear to share a common molecular mechanism of action that involves interaction with a positively charged amino acid side chain located in the inner vestibule of the pore.

Strategies to investigate the mechanism of action of CFTR modulators

The malfunction 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, pharmaceutical agents have been identified that (i) act indirectly by regulating the protein kinases and phosphatases, which control CFTR, and (ii) interact directly with CFTR. Some agents modulate CFTR by altering the function of the nucleotide-binding domains (NBDs) that control channel gating, whereas others inhibit CFTR by preventing Cl- flow through the channel pore. Knowledge of CFTR modulators might lead to new understanding of the CFTR Cl- channel, its physiological role and malfunction in disease.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Potent stimulation and inhibition of the CFTR Cl(-) current by phloxine B

The effects of the fluoresceine derivative, phloxine B, on the Cl(-) current through the cystic fibrosis transmembrane conductance regulator (CFTR) were examined in Xenopus oocytes expressing human CFTR. In whole oocytes, the CFTR Cl(-) current (I(CFTR)) was activated by superfusion with isobutylmethylxanthine and forskolin. I(CFTR) was stable during activation and deactivated rapidly upon washout of the activation solution. Phloxine B slowed deactivation and, at high concentrations, inhibited I(CFTR) weakly. In excised inside-out macropatches, I(CFTR) was activated by the catalytic subunit of protein kinase A (cPKA) and MgATP. Phloxine B (0.01 - 3 microM), applied after activation, increased I(CFTR) within 30 s followed by a slow decrease which became dominant at high concentrations. Slowing of deactivation of the CFTR was observed at all concentrations. The effect of phloxine B after 30 s had a bell-shaped concentration-dependence with midpoints at 45 and 1600 nM for the stimulatory and the inhibitory limb, respectively; maximum stimulation was about 1.8 times. The slow inhibitory component, measured after 6 min, occurred with an IC(50) value of approximately 1 microM. In the absence of cPKA, phloxine B did not stimulate I(CFTR). In the presence of cPKA and MgATP, the effects of phloxine B were more prominent at low (0.02 mM) than at high ATP (2 mM). The data show that phloxine B modulates I(CFTR) by increasing channel activity and slowing channel deactivation; at high concentrations inhibition dominates. The effects may be mediated by direct interactions with CFTR from the inside of the cell.