Carbonic anhydrase inhibitors. Inhibition studies of the human secretory isoform VI with anions

Exploring the Redox and pH Dimension of Carbonic Anhydrases in Cancer: A Focus on Carbonic Anhydrase 3

Significance: Both redox and pH are important regulatory processes that underpin cell physiological functions, in addition to influencing cancer cell development and tumor progression. The thioredoxin (Trx) and glutathione redox systems and the carbonic anhydrase (CA) proteins are considered key regulators of cellular redox and pH, respectively, with components of the Trx system and CAs regarded as cancer therapeutic targets. However, the redox and pH axis in cancer cells is an underexplored topic of research. Recent Advances: Structural studies of a CA family member, CA3, localized two of its five cysteine residues to the protein surface. Redox-regulated modifications to CA3 have been identified, including glutathionylation. CA3 has been shown to bind to other proteins, including B cell lymphoma-2-associated athanogene 3, and squalene epoxidase, which can modulate autophagy and proinflammatory signaling, respectively, in cancer cells. Critical Issues: CA3 has also been associated with epithelial-mesenchymal transition processes, which promote cancer cell metastasis, whereas CA3 overexpression activates the phosphatidylinositol-3 kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway, which upregulates cell growth and inhibits autophagy. It is not yet known if CA3 modulates cancer progression through its reported antioxidant functions. Future Directions: CA3 is one of the least studied CA isozymes. Further studies are required to assess the cellular antioxidant role of CA3 and its impact on cancer progression. Identification of other binding partners is also required, including whether CA3 binds to Trx in human cells. The development of specific CA3 inhibitors will facilitate these functional studies and allow CA3 to be investigated as a cancer therapeutic target.

Bortezomib inhibits mammalian carbonic anhydrases

We investigated the carbonic anhydrase (CA, EC 4.2.1.1) inhibitory activity of the clinically used antitumor agent bortezomib, a marketed proteasome inhibitor, against all the catalytically active mammalian isoforms CA I-VII, IX, XII-XV. Bortezomib effectively inhibited all these CAs in the micromolar range. hCA II, the physiologically dominant cytosolic isoform showed the highest affinity for the drug, with a K_I of 1.16μM. The cytosolic slow isoform hCA I was also effectively inhibited, with a K_I of 1.29μM, whereas the next best affinity was observed for the membrane-anchored form mCA XV, with a K_I of 2.68μM, followed by two transmembrane isoforms, hCA IX and XIV (K_Is of 3.28-3.38μM). The remaining cytosolic (CA III, VII and XIII), membrane-anchored (CA IV), mitochondrial (CA VA, VB), transmembrane (CA XII) and secreted (CA VI) isoforms were slightly less inhibited by bortezomib compared to isoforms discussed above, with K_Is ranging between 4.38 and 8.45μM. These data may shed some light on possible side effects and novel antitumor mechanisms of action of this drug.

Altered gene expression in the lower respiratory tract of Car6 (-/-) mice

From birth, the respiratory tract mucosa is exposed to various chemical, physical, and microbiological stress factors. Efficient defense mechanisms and strictly regulated renewal systems in the mucosa are thus required. Carbonic anhydrase VI (CA VI) is the only secreted isoenzyme of the α-CA gene family. It is transported in high concentrations in saliva and milk into the alimentary tract where it contributes to optimal pH homeostasis. Earlier study of transcriptomic responses of Car6 (-/-) mice has shown changes in the response to oxidative stress and brown fat cell differentiation in the submandibular gland. It has been suggested that CA VI delivered to the mucosal surface of the bronchiolar epithelium is an essential factor in defense and renewal of the lining epithelium. In this study, the transcriptional effects of CA VI deficiency were investigated in both trachea and lung of Car6 (-/-) mice using a cDNA microarray analysis. Functional clustering of the results indicated significant changes of gene transcription in the lower airways. The altered biological processes included antigen transport by M-cells, potassium transport, muscle contraction, and thyroid hormone synthesis. Immunohistochemical staining confirmed the absence of CA VI in the submandibular gland of Car6 (-/-) mice. Immunostaining of the trachea and lung samples revealed no differences between the knockout and wild type groups nor were any morphological changes observed. The present findings can help us to recognize novel functions for CA VI-one of the major protein constituents of saliva and milk.

Cloning, characterization and anion inhibition study of a β-class carbonic anhydrase from the caries producing pathogen Streptococcus mutans

The oral pathogenic bacterium involved in human dental caries formation Streptococcus mutans, encodes for two carbonic anhydrase (CA, EC 4.2.1.1) one belonging to the α- and the other one to the β-class. This last enzyme (SmuCA) has been cloned, characterized and investigated for its inhibition profile with a major class of CA inhibitors, the inorganic anions. Here we show that SmuCA has a good catalytic activity for the CO2 hydration reaction, with kcat 4.2×10(5)s(-1) and kcat/Km of 5.8×10(7)M(-1)×s(-1), being inhibited by cyanate, carbonate, stannate, divannadate and diethyldithiocarbamate in the submillimolar range (KIs of 0.30-0.64mM) and more efficiently by sulfamide, sulfamate, phenylboronic acid and phenylarsonic acid (KIs of 15-46μM). The anion inhibition profile of the S. mutans enzyme is very different from other α- and β-CAs investigated earlier. Identification of effective inhibitors of this new enzyme may lead to pharmacological tools useful for understanding the role of S. mutans CAs in dental caries formation, and eventually the development of pharmacological agents with a new mechanism of antibacterial action.

Sulfonamide inhibition study of the β-class carbonic anhydrase from the caries producing pathogen Streptococcus mutans

Streptococcus mutans, the oral pathogenic bacterium provoking dental caries formation, encodes for a β-class carbonic anhydrase (CA, EC 4.2.1.1), SmuCA. This enzyme was cloned, characterized and investigated for its inhibition profile with the major class of CA inhibitors, the primary sulfonamides. SmuCA has a good catalytic activity for the CO2 hydration reaction, with a kcat of 4.2×10(5) s(-1) and kcat/Km of 5.8×10(7) M(-1)×s(-1), and is efficiently inhibited by most sulfonamides (KIs of 246 nM-13.5 μM). The best SmuCA inhibitors were bromosulfanilamide, deacetylated acetazolamide, 4-hydroxymethylbenzenesulfonamide, a pyrimidine-substituted sulfanilamide derivative, aminobenzolamide and compounds structurally similar to it, as well as acetazolamide, methazolamide, indisulam and valdecoxib. These compounds showed inhibition constants ranging between 246 and 468 nM. Identification of effective inhibitors of this enzyme may lead to pharmacological tools useful for understanding the role of S. mutans CAs in dental caries formation, and eventually the development of pharmacological agents with a new mechanism of antibacterial action.

Local pH domains regulate NHE3-mediated Na⁺ reabsorption in the renal proximal tubule

The proximal tubule Na(+)/H(+) exchanger 3 (NHE3), located in the apical dense microvilli (brush border), plays a major role in the reabsorption of NaCl and water in the renal proximal tubule. In response to a rise in blood pressure NHE3 redistributes in the plane of the plasma membrane to the base of the brush border, where NHE3 activity is reduced. This NHE3 redistribution is assumed to provoke pressure natriuresis; however, it is unclear how NHE3 redistribution per se reduces NHE3 activity. To investigate if the distribution of NHE3 in the brush border can change the reabsorption rate, we constructed a spatiotemporal mathematical model of NHE3-mediated Na(+) reabsorption across a proximal tubule cell and compared the model results with in vivo experiments in rats. The model predicts that when NHE3 is localized exclusively at the base of the brush border, it creates local pH microdomains that reduce NHE3 activity by >30%. We tested the model's prediction experimentally: the rat kidney cortex was loaded with the pH-sensitive fluorescent dye BCECF, and cells of the proximal tubule were imaged in vivo using confocal fluorescence microscopy before and after an increase of blood pressure by ∼50 mmHg. The experimental results supported the model by demonstrating that a rise of blood pressure induces the development of pH microdomains near the bottom of the brush border. These local changes in pH reduce NHE3 activity, which may explain the pressure natriuresis response to NHE3 redistribution.

Structure, function and applications of carbonic anhydrase isozymes

The carbonic anhydrases enzymes (CAs, EC 4.2.1.1) are zinc containing metalloproteins, which efficiently catalyse the reversible conversion of carbon dioxide to bicarbonate and release proton. These enzymes are essentially important for biological system and play several important physiological and patho-physiological functions. There are 16 different alpha-carbonic anhydrase isoforms studied, differing widely in their cellular localization and biophysical properties. The catalytic domains of all CAs possess a conserved tertiary structure fold, with predominately β-strands. We performed an extensive analysis of all 16 mammalian CAs for its structure and function in order to establish a structure-function relationship. CAs have been a potential therapeutic target for many diseases. Sulfonamides are considered as a strong and specific inhibitor of CA, and are being used as diuretics, anti-glaucoma, anti-epileptic, anti-ulcer agents. Currently CA inhibitors are widely used as a drug for the treatment of neurological disorders, anti-glaucoma drugs, anti-cancer, or anti-obesity agents. Here we tried to emphasize how CAs can be used for drug discovery, design and screening. Furthermore, we discussed the role of CA in carbon capture, carbon sensor and metabolon. We hope this review provide many useful information on structure, function, mechanism, and applications of CAs in various discipline.

(In)organic anions as carbonic anhydrase inhibitors

Carbonic anhydrases (CAs, EC 4.2.1.1) are widespread enzymes in all life kingdoms with five distinct genetic families known to date, the α-, β-, γ-, δ- and ζ-CAs. With the exception of the δ-class, which is less investigated to date, enzymes from the remaining classes found in vertebrates, corals, fungi, bacteria and archaea have been studied for their inhibition with simple inorganic anions as well as more complex inorganic and organic ones. In this paper we review the available data for the inhibition of these enzymes with all anions except sulfonamides and their bioisosteres (sulfamates, sulfamides) which have been reviewed earlier. Anion inhibitors are important both for understanding the inhibition/catalytic mechanisms of these enzymes and for designing novel types of inhibitors which may have clinical applications for the management of a variety of disorders in which CAs are involved. Environmental aspects of CO(2) fixation by CAs present in plants, corals, algae or diatoms and how this may be affected by inhibitors are also discussed.

Inhibition of the R1 fragment of the cadmium-containing zeta-class carbonic anhydrase from the diatom Thalassiosira weissflogii with anions

We investigated the catalytic activity and inhibition of both the zinc and cadmium-containing R1 fragment of the zeta-class carbonic anhydrase (CA, EC 4.2.1.1) from the marine diatom Thalassiosira weissflogii. Our data prove that these enzymes are not only very efficient catalysts for the physiological reaction, but also sensitive to sulfonamide and anion inhibitors, with inhibition constants from the nanomolar to millimolar range. Acetazolamide inhibited the two enzymes with K(I)s in the range of 58-92 nM. The best anion inhibitors of Cd-R1 were thiocyanate, sulfamate and sulfamide, with K(I)s of 10-89 microM, whereas the best Zn-R1 anion inhibitors were sulfamate and sulfamide with K(I)s of 60-72 microM. These enzymes were only weakly inhibited by chloride, bromide or sulfate, main anion components of sea water, with inhibition constants in the range of 0.24-0.85 mM. Thus, similarly to CAs belonging to other classes, the zeta-class CA (with either cadmium or zinc ions at the active site) was inhibited by both anions and sulfonamides.

Correlation analyses on binding affinity of substituted benzenesulfonamides with carbonic anhydrase using ab initio MO calculations on their complex structures

Quantitative structure-activity relationship analyses on the free energy change during complex formation between substituted benzenesulfonamides (BSAs) and bovine carbonic anhydrase II (bCA II) were performed using generilized Born/surface area (GB/SA) and ab initio fragment molecular orbital (FMO) calculations for the whole complex structures. The result shows that the overall free energy change is governed by the contribution from solvation and dissociation free energy changes accompanying by complex formation. The FMO-IFIE (interfragment interaction energy) analysis quantitatively revealed that the intrinsic interaction energy of bCA II with BSAs is mostly from interactions with amino acid residues in the active site of bCA II. The "Zn block" (Zn(2+) and three histidine residues coordinated to Zn(2+)) in the active site shows the lowest interaction energy and the greatest variance of interaction energy with BSAs through their coordination interaction. The proposed procedure was demonstrated to provide a quantitative basis for understanding a ligand-protein interaction at electronic and atomic levels.

Carbonic anhydrase inhibitors. Inhibition studies of a coral secretory isoform by sulfonamides

The inhibition of a newly cloned coral carbonic anhydrase (CA, EC 4.2.1.1) has been investigated with a series of sulfonamides, including some clinically used derivatives (acetazolamide, methazolamide, ethoxzolamide, dichlorophenamide, dorzolamide, brinzolamide, benzolamide, and sulpiride, or indisulam, a compound in clinical development as antitumor drug), as well as the sulfamate antiepileptic topiramate. Some simple amino-/hydrazine-/hydroxy-substituted aromatic/heterocyclic sulfonamides have also been included in the study. All types of activity have been detected, with low potency inhibitors (K(I)s in the range of 163-770nM), or with medium potency inhibitors (K(I)s in the range of 75.1-105nM), whereas ethoxzolamide, several clinically used sulfonamides and heterocyclic compounds showed stronger potency, with K(I)s in the range of 16-48.2nM. These inhibitors may be useful to better understand the physiological role of the Stylophora pistillata CA (STPCA) in corals and its involvement in biomineralisation in this era of global warming.

Carbonic anhydrase inhibitors: inhibition studies of a coral secretory isoform with inorganic anions

The inhibition of a coral carbonic anhydrase (CA, EC 4.2.1.1) has been investigated with a series of inorganic anions such as halogenides, pseudohalogenides, bicarbonate, carbonate, nitrate, nitrite, hydrogen sulfide, bisulfite, perchlorate, sulfate. The full-length scleractinian coral Stylophora pistillata CA, STPCA, has a significant catalytic activity for the physiological reaction of CO(2) hydration to bicarbonate, similarly to the ubiquitous human isoforms hCA I (cytosolic) and hCA VI (secreted). The best STPCA anion inhibitors were bromide, iodide, carbonate, and sulfamate, with inhibition constants of 9.0-10.0 microM.

Biochemical characterization of CA IX, one of the most active carbonic anhydrase isozymes

Carbonic anhydrase IX (CA IX) is an exceptional member of the CA protein family; in addition to its classical role in pH regulation, it has also been proposed to participate in cell proliferation, cell adhesion, and tumorigenic processes. To characterize the biochemical properties of this membrane protein, two soluble recombinant forms were produced using the baculovirus-insect cell expression system. The recombinant proteins consisted of either the CA IX catalytic domain only (CA form) or the extracellular domain, which included both the proteoglycan and catalytic domains (PG + CA form). The produced proteins lacked the small transmembrane and intracytoplasmic regions of CA IX. Stopped-flow spectrophotometry experiments on both proteins demonstrated that in the excess of certain metal ions the PG + CA form exhibited the highest catalytic activity ever measured for any CA isozyme. Investigations on the oligomerization and stability of the enzymes revealed that both recombinant proteins form dimers that are stabilized by intermolecular disulfide bond(s). Mass spectrometry experiments showed that CA IX contains an intramolecular disulfide bridge (Cys(119)-Cys(299)) and a unique N-linked glycosylation site (Asn(309)) that bears high mannose-type glycan structures. Parallel experiments on a recombinant protein obtained by a mammalian cell expression system demonstrated the occurrence of an additional O-linked glycosylation site (Thr(78)) and characterized the nature of the oligosaccharide structures. This study provides novel information on the biochemical properties of CA IX and may help characterize the various cellular and pathophysiological processes in which this unique enzyme is involved.

Carbonic anhydrase inhibitors: The inhibition profiles of the human mitochondrial isoforms VA and VB with anions are very different

The first anion inhibition study of the mitochondrial human carbonic anhydrase (hCA, EC 4.2.1.1) isoform hCA VB is reported. Fluoride, chloride, bromide, iodide, cyanate, thiocyanate, cyanide, azide, bicarbonate, carbonate, nitrate, nitrite, hydrogen sulfide, bisulfite, sulfate, sulfamide, sulfamic acid, phenylboronic acid and phenylarsonic acid were compared as inhibitors of the two mitochondrial isozymes hCA VA and hCA VB. These enzymes are involved in biosynthetic reactions leading to fatty acid and Krebs cycle intermediates biosynthesis in addition to acting as catalysts for the interconversion of carbon dioxide and bicarbonate. The anion inhibition profiles of the two isoforms are dramatically different. The best hCA VB inhibitors were cyanate, thiocyanate, cyanide and hydrogensulfide (K(I)s of 80-76 microM) whereas the least effective ones were the halides (K(I)s of 11-72 mM), with the best inhibitor being fluoride and the least effective ones bromide and iodide. Whereas hCA VA is not sensitive to bicarbonate inhibition (K(I) of 82 mM) similarly to the cytosolic isoform hCA II, hCA VB is well inhibited by this anion, with a K(I) of 0.71 mM. Overall, hCA VB is more sensitive to anion inhibitors as compared to hCA VA. Such data support prior suggestions that the two mitochondrial isozymes play different physiological functions.