Purification and kinetic analysis of recombinant CA XII, a membrane carbonic anhydrase overexpressed in certain cancers

Design, synthesis, and biological investigation of selective human carbonic anhydrase II, IX, and XII inhibitors using 7-aryl/heteroaryl triazolopyrimidines bearing a sulfanilamide scaffold

A novel library of human carbonic anhydrase (hCA) inhibitors based on the 2-sulfanilamido[1,2,4]triazolo[1,5-a]pyrimidine skeleton modified at its 7-position was prepared by an efficient convergent procedure. These derivatives were evaluated in vitro for their inhibition properties against a representative panel of hCA isoforms (hCA I, II, IV, IX, and XII). The target tumour-associated isoforms hCA IX and XII were potently inhibited with KIs in the low nanomolar range of 5-96 nM and 4-72 nM, respectively. Compounds 1d, 1j, 1v, and 1x were the most potent hCA IX inhibitors with KIs of 5.1, 8.6, 4.7, and 5.1 nM, respectively. Along with derivatives 1d and 1j, compounds 1r and 1ab potently inhibited hCA XII isoform with KIs in a single-digit nanomolar range of 8.8, 5.4, 4.3, and 9.0 nM, respectively. Compounds 1e, 1m, and 1p exhibited the best selectivity against hCA IX and hCA XII isoforms over off-target hCA II, with selectivity indexes ranging from 5 to 14.

Design, synthesis, biological evaluation and crystal structure determination of dual modulators of carbonic anhydrases and estrogen receptors

Multi-target compounds have become increasingly important for the development of safer and more effective drug candidates. In this work, we devised a combined ligand-based and structure-based multi-target repurposing strategy and applied it to a series of hexahydrocyclopenta[c]quinoline compounds synthesized previously. The in silico analyses identified human Carbonic Anhydrases (hCA) and Estrogen Receptors (ER) as top scoring candidates for dual modulation. hCA isoforms IX and XII, and ER subtypes ER⍺ and/or ERβ are co-expressed in various cancer cell types, including breast and prostate cancer cells. ER⍺ is the primary target of anti-estrogen therapy in breast cancer, and the hCA IX isoform is a therapeutic target in triple-negative breast cancer. ER⍺-mediated transcriptional programs and hCA activity in cancer cells promote favorable microenvironments for cell proliferation. Interestingly, several lines of evidence indicate that the combined modulation of these two targets may provide significant therapeutic benefits. Moving from these first results, two additional hexahydrocyclopenta[c]quinoline derivatives bearing a sulfonamide zinc binding group (hCA) and a phenolic hydroxyl (ER) pharmacophoric group placed at the appropriate locations were designed and synthesized. Interestingly, these compounds were able to directly modulate the activities of both hCA and ER targets. In cell-based assays, they inhibited proliferation of breast and prostate cancer cells with micromolar potency and cell type-selective efficacy. The compounds inhibited hCA activity with nanomolar potency and isoform-selectivity. In transactivation assays, they reduced estrogen-driven ER activity with micro-molar potency. Finally, crystal structures of the synthesized ligands in complex with the two targets revealed that the compounds bind directly to the hCA active site, as well as to the ER ligand-binding domain, providing structural explanation to the observed activity and a rationale for optimization of their dual activity. To the best of our knowledge, this work describes the design, synthesis and biological characterization of the first dual modulators of hCA and ER, laying the ground for the structure-based optimization of their multi-target activity.

Synthesis of Sulfonamides Incorporating Piperidinyl-Hydrazidoureido and Piperidinyl-Hydrazidothioureido Moieties and Their Carbonic Anhydrase I, II, IX and XII Inhibitory Activity

Here we report a small library of hydrazinocarbonyl-ureido and thioureido benzenesulfonamide derivatives, designed and synthesized as potent and selective human carbonic anhydrase inhibitors (hCAIs). The synthesized compounds were evaluated against isoforms hCA I, II, IX and XII using acetazolamide (AAZ) as standard inhibitor. Several urea and thiourea derivatives showed inhibitory activity at low nanomolar levels with selectivity against the cytosolic hCA II isoform, as well as the transmembrane, tumor-associated enzymes hCA IX and XII. The thiourea derivatives showed enhanced potency as compared to urea analogues. Additionally, eight compounds 5g, 5m, 5o, 5q, 6l, 6j, 6o and 6u were selected for docking analysis on isoform I, II, IX, XII to illustrate the potential interaction with the enzyme to better understand the activity against the different isoforms.

Post-translational modifications in tumor-associated carbonic anhydrases

Human carbonic anhydrases IX (hCA IX) and XII (hCA XII) are two proteins associated with tumor formation and development. These enzymes have been largely investigated both from a biochemical and a functional point of view. However, limited data are currently available on the characterization of their post-translational modifications (PTMs) and the functional implication of these structural changes in the tumor environment. In this review, we summarize existing literature data on PTMs of hCA IX and hCA XII, such as disulphide bond formation, phosphorylation, O-/N-linked glycosylation, acetylation and ubiquitination, highlighting, when possible, their specific role in cancer pathological processes.

Experimental Approaches to Identify Selective Picomolar Inhibitors for Carbonic Anhydrase IX

BACKGROUND

Carbonic anhydrases (CAs) regulate pH homeostasis via the reversible hydration of CO₂, thereby emerging as essential enzymes for many vital functions. Among 12 catalytically active CA isoforms in humans, CA IX has become a relevant therapeutic target because of its role in cancer progression. Only two CA IX inhibitors have entered clinical trials, mostly due to low affinity and selectivity properties.

OBJECTIVE

The current review presents the design, development, and identification of the selective nano- to picomolar CA IX inhibitors VD11-4-2, VR16-09, and VD12-09.

METHODS AND RESULTS

Compounds were selected from our database, composed of over 400 benzensulfonamides, synthesized at our laboratory, and tested for their binding to 12 human CAs. Here we discuss the CA CO₂ hydratase activity/inhibition assay and several biophysical techniques, such as fluorescent thermal shift assay and isothermal titration calorimetry, highlighting their contribution to the analysis of compound affinity and structure- activity relationships. To obtain sufficient amounts of recombinant CAs for inhibitor screening, several gene cloning and protein purification strategies are presented, including site-directed CA mutants, heterologous CAs from Xenopus oocytes, and native endogenous CAs. The cancer cell-based methods, such as clonogenicity, extracellular acidification, and mass spectrometric gas-analysis are reviewed, confirming nanomolar activities of lead inhibitors in intact cancer cells.

CONCLUSIONS

Novel CA IX inhibitors are promising derivatives for in vivo explorations. Furthermore, the simultaneous targeting of several proteins involved in proton flux upon tumor acidosis and the disruption of transport metabolons might improve cancer management.

Switching the Inhibitor-Enzyme Recognition Profile via Chimeric Carbonic Anhydrase XII

A key part of the optimization of small molecules in pharmaceutical inhibitor development is to vary the molecular design to enhance complementarity of chemical features of the compound with the positioning of amino acids in the active site of a target enzyme. Typically this involves iterations of synthesis, to modify the compound, and biophysical assay, to assess the outcomes. Selective targeting of the anti-cancer carbonic anhydrase isoform XII (CA XII), this process is challenging because the overall fold is very similar across the twelve CA isoforms. To enhance drug development for CA XII we used a reverse engineering approach where mutation of the key six amino acids in the active site of human CA XII into the CA II isoform was performed to provide a protein chimera (chCA XII) which is amenable to structure-based compound optimization. Through determination of structural detail and affinity measurement of the interaction with over 60 compounds we observed that the compounds that bound CA XII more strongly than CA II, switched their preference and bound more strongly to the engineered chimera, chCA XII, based on CA II, but containing the 6 key amino acids from CA XII, behaved as CA XII in its compound recognition profile. The structures of the compounds in the chimeric active site also resembled those determined for complexes with CA XII, hence validating this protein engineering approach in the development of new inhibitors.

Prediction of activity and selectivity profiles of human Carbonic Anhydrase inhibitors using machine learning classification models

The development of selective inhibitors of the clinically relevant human Carbonic Anhydrase (hCA) isoforms IX and XII has become a major topic in drug research, due to their deregulation in several types of cancer. Indeed, the selective inhibition of these two isoforms, especially with respect to the homeostatic isoform II, holds great promise to develop anticancer drugs with limited side effects. Therefore, the development of in silico models able to predict the activity and selectivity against the desired isoform(s) is of central interest. In this work, we have developed a series of machine learning classification models, trained on high confidence data extracted from ChEMBL, able to predict the activity and selectivity profiles of ligands for human Carbonic Anhydrase isoforms II, IX and XII. The training datasets were built with a procedure that made use of flexible bioactivity thresholds to obtain well-balanced active and inactive classes. We used multiple algorithms and sampling sizes to finally select activity models able to classify active or inactive molecules with excellent performances. Remarkably, the results herein reported turned out to be better than those obtained by models built with the classic approach of selecting an a priori activity threshold. The sequential application of such validated models enables virtual screening to be performed in a fast and more reliable way to predict the activity and selectivity profiles against the investigated isoforms.

Pyridinium derivatives of 3-aminobenzenesulfonamide are nanomolar-potent inhibitors of tumor-expressed carbonic anhydrase isozymes CA IX and CA XII

Building on the conclusions of previous inhibition studies with pyridinium-benzenesulfonamides from our team and on the X-ray crystal structure of the lead compound identified, a series of 24 pyridinium derivatives of 3-aminobenzenesulfonamide was synthesized and investigated for carbonic anhydrase inhibition. The new pyridinium-sulfonamides were evaluated as inhibitors of four human carbonic anhydrase (CA, EC 4.2.1.1) isoforms, namely CA I, CA II (cytosolic), CA IX and XII (transmembrane, tumor-associated forms). Excellent inhibitory activity in the nanomolar range was observed against CA IX with most of these sulfonamides, and against CA XII (nanomolar/sub-nanomolar) with some of the new compounds. These sulfonamides were generally potent inhibitors of CA II and CA I too. Docking studies revealed a preference of these compounds to bind the P1 hydrophobic site of CAs, supporting the observed inhibition profile. The salt-like nature of these positively charged sulfonamides can further focus the inhibitory ability on membrane-bound CA IX and CA XII and could efficiently decrease the viability of three human carcinomas under hypoxic conditions where these isozymes are over-expressed, thus recommending the new compounds as potential diagnostic tools or therapeutic agents.

Thermodynamic, kinetic, and structural parameterization of human carbonic anhydrase interactions toward enhanced inhibitor design

The aim of rational drug design is to develop small molecules using a quantitative approach to optimize affinity. This should enhance the development of chemical compounds that would specifically, selectively, reversibly, and with high affinity interact with a target protein. It is not yet possible to develop such compounds using computational (i.e., in silico) approach and instead the lead molecules are discovered in high-throughput screening searches of large compound libraries. The main reason why in silico methods are not capable to deliver is our poor understanding of the compound structure-thermodynamics and structure-kinetics correlations. There is a need for databases of intrinsic binding parameters (e.g., the change upon binding in standard Gibbs energy (ΔGint), enthalpy (ΔHint), entropy (ΔSint), volume (ΔVintr), heat capacity (ΔCp,int), association rate (ka,int), and dissociation rate (kd,int)) between a series of closely related proteins and a chemically diverse, but pharmacophoric group-guided library of compounds together with the co-crystal structures that could help explain the structure-energetics correlations and rationally design novel compounds. Assembly of these data will facilitate attempts to provide correlations and train data for modeling of compound binding. Here, we report large datasets of the intrinsic thermodynamic and kinetic data including over 400 primary sulfonamide compound binding to a family of 12 catalytically active human carbonic anhydrases (CA). Thermodynamic parameters have been determined by the fluorescent thermal shift assay, isothermal titration calorimetry, and by the stopped-flow assay of the inhibition of enzymatic activity. Kinetic measurements were performed using surface plasmon resonance. Intrinsic thermodynamic and kinetic parameters of binding were determined by dissecting the binding-linked protonation reactions of the protein and sulfonamide. The compound structure-thermodynamics and kinetics correlations reported here helped to discover compounds that exhibited picomolar affinities, hour-long residence times, and million-fold selectivities over non-target CA isoforms. Drug-lead compounds are suggested for anticancer target CA IX and CA XII, antiglaucoma CA IV, antiobesity CA VA and CA VB, and other isoforms. Together with 85 X-ray crystallographic structures of 60 compounds bound to six CA isoforms, the database should be of help to continue developing the principles of rational target-based drug design.

Biochemical and molecular characterization of N66 from the shell of Pinctada mazatlanica

Mollusk shell mineralization is a tightly controlled process made by shell matrix proteins (SMPs). However, the study of SMPs has been limited to a few model species. In this study, the N66 mRNA of the pearl oyster Pinctada mazatlanica was cloned and functionally characterized. The full sequence of the N66 mRNA comprises 1,766 base pairs, and encodes one N66 protein. A sequence analysis revealed that N66 contained two carbonic anhydrase (CA) domains, a NG domain and several glycosylation sites. The sequence showed similarity to the CA VII but also with its homolog protein nacrein. The native N66 protein was isolated from the shell and identified by mass spectrometry, the peptide sequence matched to the nucleotide sequence obtained. Native N66 is a glycoprotein with a molecular mass of 60–66 kDa which displays CA activity and calcium carbonate precipitation ability in presence of different salts. Also, a recombinant form of N66 was produced in Escherichia coli, and functionally characterized. The recombinant N66 displayed higher CA activity and crystallization capability than the native N66, suggesting that the lack of posttranslational modifications in the recombinant N66 might modulate its activity.

Carbonic anhydrases as disease markers

INTRODUCTION

The physiologic importance of fast CO₂/HCO₃^- interconversion in various tissues requires the presence of carbonic anhydrase (CA, EC 4.2.1.1). Fourteen CA isozymes are present in humans, all of them being used as biomarkers.

AREAS COVERED

A great number of patents and articles were focused on the use of CA isozymes as biomarkers for various diseases and syndromes in the recent years, in an ascending trend over the last decade. The review highlights the most important studies related with each isozyme and covers the most recent patent literature.

EXPERT OPINION

The CAs biomarker research area expanded significantly in recent years, shifting from the predominant use of CA IX and CA XII in cancer diagnostic, staging, and prognosis towards a wider use of CA isozymes as disease biomarkers. CA isozymes are currently used either alone, in tandem with other CA isozymes and/or in combination with other proteins for the detection, staging, and prognosis of a huge repertoire of human dysfunctions and diseases, ranging from mild transformation of the normal tissues to extreme shifts in tissue organization and function. The techniques used for their detection/quantitation and the state-of-the-art in each clinical application are presented through relevant clinical examples and corresponding statistical data.

Cancer Drug Development of Carbonic Anhydrase Inhibitors beyond the Active Site

Carbonic anhydrases (CAs) catalyze the reversible hydration of carbon dioxide to produce bicarbonate and a proton. Multiple CA isoforms are implicated in a range of diseases, including cancer. In solid tumors, continuously dividing cells create hypoxic conditions that eventually lead to an acidic microenvironment. Hypoxic tumor cells have different mechanisms in place to regulate and adjust the surrounding microenvironment for survival. These mechanisms include expression of CA isoform IX (CA IX) and XII (CA XII). These enzymes help maintain a physiological intracellular pH while simultaneously contributing to an acidic extracellular pH, leading to tumor cell survival. Expression of CA IX and CA XII has also been shown to promote tumor cell invasion and metastasis. This review discusses the characteristics of CA IX and CA XII, their mechanism of action, and validates their prospective use as anticancer targets. We discuss the current status of small inhibitors that target these isoforms, both classical and non-classical, and their future design in order to obtain isoform-specificity for CA IX and CA XII. Biologics, such as monoclonal antibodies, monoclonal-radionuclide conjugated chimeric antibodies, and antibody-small molecule conjugates are also discussed.

Carbonic anhydrase XII functions in health and disease

Human CAXII was initially identified as a cancer marker in different cancers and tumors. Expression of CAXII is regulated by hypoxia and estrogen receptors. CAXII expression has been also detected in several tissues, whereas in cancer and tumor tissues its expression is several fold higher. In brain tumors, an alternatively spliced form of CAXII is expressed. Higher expression of CAXII in breast cancer is indicative of lower grade disease. CAXII plays a key role in several physiological functions. Mutation in the CAXII gene causes cystic fibrosis-like syndrome and salt wasting disease. CAXII is also seen in nuclear pulposus cells of the vertebrae. Aging dependent stiffness or degeneration of backbone correlates with CAXII expression level. This finding suggests a possible implication of CAXII as a biomarker for chronic back pain and a pharmacological target for possible treatment of chronic back pain.

Picomolar inhibitors of carbonic anhydrase: Importance of inhibition and binding assays

The K_i of carbonic anhydrase (CA) inhibitors is often determined by the stopped- flow CO₂ hydration assay, the method that directly follows the inhibition of CA enzymatic activity. However, the assay has limitations, such as largely unknown concentration of CO₂ and the inability to determine the K_i below several nM. The widely used direct binding assay, isothermal titration calorimetry, also does not determine the K_d below several nM. In contrast, the thermal shift assay can accurately determine picomolar affinities. New equations estimating CO₂ concentration were developed for the determination of k_cat and K_M of CA I and CA II. The inhibitor dose-response curves were analyzed using Hill and Morrison equations demonstrating that only the Morrison model is applicable for the determination of tight-binding inhibitor K_i. The measurements of interactions between ten inhibitors and seven CA isoforms showed the limitations and advantages of all three techniques. Inhibitor 6 exhibited the K_d of 50 pM and was highly selective towards human CA IX, an isoform which is nearly absent in healthy human, but highly overexpressed in numerous cancers. Combination of inhibition and binding techniques was necessary for precise determination of CA-high-affinity inhibitor interactions and future drug design.

The Structure of Carbonic Anhydrase IX Is Adapted for Low-pH Catalysis

Human carbonic anhydrase IX (hCA IX) expression in many cancers is associated with hypoxic tumors and poor patient outcome. Inhibitors of hCA IX have been used as anticancer agents with some entering Phase I clinical trials. hCA IX is transmembrane protein whose catalytic domain faces the extracellular tumor milieu, which is typically associated with an acidic microenvironment. Here, we show that the catalytic domain of hCA IX (hCA IX-c) exhibits the necessary biochemical and biophysical properties that allow for low pH stability and activity. Furthermore, the unfolding process of hCA IX-c appears to be reversible, and its catalytic efficiency is thought to be correlated directly with its stability between pH 3.0 and 8.0 but not above pH 8.0. To rationalize this, we determined the X-ray crystal structure of hCA IX-c to 1.6 Å resolution. Insights from this study suggest an understanding of hCA IX-c stability and activity in low-pH tumor microenvironments and may be applicable to determining pH-related effects on enzymes.

Carbonic anhydrase XII is a new therapeutic target to overcome chemoresistance in cancer cells

Multidrug resistance (MDR) in cancer cells is a challenging phenomenon often associated with P-glycoprotein (Pgp) surface expression. Finding new ways to bypass Pgp-mediated MDR still remains a daunting challenge towards the successful treatment of malignant neoplasms such as colorectal cancer.

We applied the Cell Surface Capture technology to chemosensitive and chemoresistant human colon cancer to explore the cell surface proteome of Pgp-expressing cells in a discovery-driven fashion. Comparative quantitative analysis of identified cell surface glycoproteins revealed carbonic anhydrase type XII (CAXII) to be up-regulated on the surface of chemoresistant cells, similarly to Pgp. In cellular models showing an acquired MDR phenotype due to the selective pressure of chemotherapy, the progressive increase of the transcription factor hypoxia-inducible factor-1 alpha was paralleled by the simultaneous up-regulation of Pgp and CAXII. CAXII and Pgp physically interacted at the cell surface. CAXII silencing or pharmacological inhibition with acetazolamide decreased the ATPase activity of Pgp by altering the optimal pH at which Pgp operated and promoted chemosensitization to Pgp substrates in MDR cells.

We propose CAXII as a new secondary marker of the MDR phenotype that influences Pgp activity directly and can be used as a pharmacological target for MDR research and potential treatment.

Hit Recycling: Discovery of a Potent Carbonic Anhydrase Inhibitor by in Silico Target Fishing

In silico target fishing is an emerging tool in drug discovery, which is mostly used for primary target or off-target prediction and drug repositioning. In this work, we developed an in silico target fishing protocol to identify the primary target of GV2-20, a false-positive hit highlighted in a cell-based screen for 14-3-3 modulators. Although GV2-20 does not bind to 14-3-3 proteins, it showed remarkable antiproliferative effects in CML cells, thus raising interest toward the identification of its primary target. Six potential targets of GV2-20 were prioritized in silico and tested in vitro. Our results show that the molecule is a potent inhibitor of carbonic anhydrase 2 (CA2), thus confirming the predictive capability of our protocol. Most notably, GV2-20 experienced a remarkable selectivity for CA2, CA7, CA9, and CA12, and its scaffold was never explored before as a chemotype for CA inhibition, thus becoming an interesting lead candidate for further development.

Physiological functions of the alpha class of carbonic anhydrases

Carbonic anhydrases are ubiquitous enzymes that catalyze the reversible hydration of carbon dioxide. These enzymes are of ancient origin as they are found in the deepest of branches of the evolutionary tree. Of the five different classes of carbonic anhydrases, the alpha class has perhaps received the most attention because of its role in human pathology. This review focuses on the physiological function of this class of carbonic anhydrases organized by their cellular location.

Antitumor efficacy of a monoclonal antibody that inhibits the activity of cancer-associated carbonic anhydrase XII

Carbonic anhydrase XII (CA XII) is a membrane-tethered cell surface enzyme that is highly expressed on many human tumor cells. Carbonic anhydrase members in this class of exofacial molecules facilitate tumor metabolism by facilitating CO2 venting and intracellular pH regulation. Accordingly, inhibition of exofacial CAs has been proposed as a general therapeutic strategy to target cancer. The recent characterization of 6A10, the first CA XII-specific inhibitory monoclonal antibody, offered an opportunity to evaluate this strategy with regard to CA XII-mediated catalysis. Using functional assays, we showed that 6A10 inhibited exofacial CA activity in CA XII-expressing cancer cells. 6A10 reduced spheroid growth in vitro under culture conditions where CA XII was active (i.e., alkaline pH) and where its catalytic activity was likely rate-limiting (i.e., restricted extracellular HCO3-supply). These in vitro results argued that the antibody exerted its growth-retarding effect by acting on the catalytic process, rather than on antigen binding per se. Notably, when administered in a mouse xenograft model of human cancer, 6A10 exerted a significant delay on tumor outgrowth. These results corroborate the notion that exofacial CA is critical for cancer cell physiology and they establish the immunotherapeutic efficacy of targeting CA XII using an inhibitory antibody.

Insights towards sulfonamide drug specificity in α-carbonic anhydrases

Carbonic anhydrases (CAs, EC 4.2.1.1) are a group of metalloenzymes that play important roles in carbon metabolism, pH regulation, CO2 fixation in plants, ion transport etc., and are found in all eukaryotic and many microbial organisms. This family of enzymes catalyzes the interconversion of CO2 and HCO3(-). There are at least 16 different CA isoforms in the alpha structural class (α-CAs) that have been isolated in higher vertebrates, with CA isoform II (CA II) being ubiquitously abundant in all human cell types. CA inhibition has been exploited clinically for decades for various classes of diuretics and anti-glaucoma treatment. The characterization of the overexpression of CA isoform IX (CA IX) in certain tumors has raised interest in CA IX as a diagnostic marker and drug target for aggressive cancers and therefore the development of CA IX specific inhibitors. An important goal in the field of CA is to identify, rationalize, and design potential compounds that will preferentially inhibit CA IX over all other isoforms of CA. The variations in the active sites between isoforms of CA are subtle and this causes non-specific CA inhibition which leads to various side effects. In the case of CA IX inhibition, CA II along with other isoforms of CA provide off-target binding sites which is undesirable for cancer treatment. The focus of this article is on CA IX inhibition and two different structural approaches to CA isoform specific drug designing: tail approach and fragment addition approach.

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.

Tumor-associated carbonic anhydrase XII is linked to the growth of primary oral squamous cell carcinoma and its poor prognosis

The pattern of protein expression in tumors is under the influence of nutrient stress, hypoxia, and low pH, which determines the survival of neoplastic cells and the development of tumors. Carbonic anhydrase (CA) XII is a transmembrane enzyme that catalyzes the reversible hydration of cell-generated carbon dioxide into protons and bicarbonate. Hypoxic conditions activate its transcription and translation, and enhanced expression is often present in several types of tumors. However, CA XII expression in oral squamous cell carcinoma (OSCC) and its correlation with patients' prognosis have not been investigated so far. In this study, we detected the expression of CA XII in 264 patients with OSCC using tissue microarrays (TMAs), and evaluated its correlation with clinicopathologic factors and disease prognosis. CA XII expression was present in 185/264 (70%) cases and was associated with more-advanced clinical stages (p=0.003), a larger tumor size (p<0.001), and postoperative recurrence (p=0.047), but was not associated with positive lymph node metastasis or distal metastasis. Importantly, CA XII expression was correlated with a poorer patient prognosis in a univariate (p=0.034, log-rank test) survival analysis. According to our results, the expression of CA XII in OSCC samples can predict the progression of OSCC and survival of OSCC patients.

Generation and characterization of the first inhibitory antibody targeting tumour-associated carbonic anhydrase XII

The carbonic anhydrases (CAs) constitute a family of almost ubiquitous enzymes of significant importance for many physiological and pathological processes. CAs reversely catalyse the conversion of CO(2) + H(2)O to HCO(3) (-) and H(+), thereby contributing to the regulation of intracellular pH. Above all, CAs are of key importance for cells that perform glycolysis that inevitably leads to the intracellular accumulation of lactate. CA XII is a plasma membrane-associated isoform of the enzyme, which is induced by hypoxia and oestrogen and, consequently, expressed at high levels on various types of cancer and, intriguingly, on cancer stem cells. The enzyme is directly involved in tumour progression, and its inhibition has an anti-tumour effect. Apart from its role in carcinogenesis, the enzyme contributes to various other diseases like glaucoma and arteriosclerotic plaques, among others. CA XII is therefore regarded as promising target for specific therapies. We have now generated the first monoclonal antibody (6A10) that binds to the catalytic domain of CA XII on vital tumour cells and inhibits CA XII enzyme activity at nanomolar concentrations and thus much more effective than acetazolamide. In vitro results demonstrate that inhibition of CA XII by 6A10 inhibits the growth of tumour cells in 3-dimensional structures. In conclusion, we generated the first specific and efficient biological inhibitor of tumour-associated CA XII. This antibody may serve as a valuable tool for in vivo diagnosis and adjuvant treatment of different types of cancer.

Carbonic anhydrase XII promotes invasion and migration ability of MDA-MB-231 breast cancer cells through the p38 MAPK signaling pathway

Carbonic anhydrase (CA) XII, an extracellular enzyme involved in the regulation of the microenvironment acidity and tumor malignant phenotype, was originally identified as a protein overexpressed in some types of cancers, including breast cancer. However, the cellular function and mechanism of CAXII remained unclear. In this study, the effects of CAXII expression on invasion and migration of breast cancer cells was investigated. Gene knockdown of CAXII in the human breast cancer cell line MDA-MB-231 resulted in decreased invasion and migration by interfering with the p38 MAPK pathway. CAXII knockdown also decreased the expression of matrix metalloproteinase (MMP)-2, MMP-9, and urokinase-type plasminogen activator (u-PA), but increased tissue inhibitor of metalloproteinases (TIMP)-2 and plasminogen activator inhibitor (PAI)-1 expression. Furthermore, decreased invasive and migration ability of CAXII-knockdown cells were restored by an overexpression of CAXII. Results also showed that CAXII knockdown may decrease anchorage-independent growth and cell growth by inhibiting CDK6 and cyclin D1 expression. Furthermore, the impact of CAXII knockdown on invasion, migration and cell growth was further evidenced by effects on tumor size and metastasis of MDA-MB-231 cells in vivo. Taken together, these data suggested that CAXII may affect the capability of invasion and migration of MDA-MB-231 cells, which may be mediated through the p38 MAPK pathway.

Overexpression of carbonic anhydrase XII in tissues from resectable non-small cell lung cancers is a biomarker of good prognosis

The pattern of protein expression in tumors is under the influence of nutrient stress, hypoxia and low pH, which determines the survival of neoplastic cells and the development of tumors. Carbonic anhydrase XII (CAXII) is a transmembrane enzyme that catalyzes the reversible hydration of cell-generated carbon dioxide into protons and bicarbonate. Hypoxic conditions activate its transcription and translation and enhanced expression is often present in several types of tumors. The aim of our study was to assess the prognostic significance of CAXII tumor tissues expression in patients with NSCLC. Five hundred fifty-five tumors were immunostained for CAXII on tissue microarrays (TMA) and the results were correlated with clinicopathological parameters and outcome of patients. CAXII overexpression was present in 105/555 (19%) cases and was associated with tumors of lower grade (p = 0.015) and histological type (p < 0.001), being significantly higher in squamous cell carcinoma. High CAXII expression correlated with better overall and disease-specific survival of patients with resectable NSCLC in univariate (p < 0.001) and multivariate survival analyses (p < 0.001). In conclusion, this is the first study demonstrating that a high CAXII tumor tissue expression evaluated on TMAs is related to a better outcome in a large series of patients with resectable NSCLC.

Infused Fc-tagged beta-glucuronidase crosses the placenta and produces clearance of storage in utero in mucopolysaccharidosis VII mice

Glycosaminoglycan storage begins in prenatal life in patients with mucopolysaccharidosis (MPS). In fact, prenatal hydrops is a common manifestation of MPS VII because of beta-glucuronidase (GUS) deficiency. One way to address prenatal storage might be to deliver the missing enzyme across the placenta into the fetal circulation. Maternal IgG is transported across the placenta by the neonatal Fc receptor (FcRn), which recognizes the Fc domain of IgG and mediates transcytosis from maternal to fetal circulation. We hypothesized that we could exploit this process to deliver corrective enzyme to the fetus. To test this hypothesis, the C-terminal fusion protein, GUS-Fc, was compared with native, untagged, recombinant GUS for clearance from the maternal circulation, delivery to the fetus, and reduction of lysosomal storage in offspring of MPS VII mice. We observed that GUS-Fc, infused into pregnant mothers on embryonic days 17 and 18, was transported across the placenta. Similarly infused untagged GUS was not delivered to the fetus. GUS-Fc plasma enzyme activity in newborn MPS VII mice was 1,000 times that seen after administration of untagged GUS and approximately 100 times that of untreated WT newborns. Reduced lysosomal storage in heart valves, liver, and spleen provided evidence that in utero enzyme replacement therapy with GUS-Fc targeted sites of storage in the MPS VII fetus. We hypothesize that this noninvasive approach could deliver the missing lysosomal enzyme to a fetus with any lysosomal storage disease. It might also provide a method for inducing immune tolerance to the missing enzyme or another foreign protein.

Acidic amino acid tag enhances response to enzyme replacement in mucopolysaccharidosis type VII mice

We have tested an acidic oligopeptide-based targeting system for delivery of enzymes to tissues, especially bone and brain, in a murine mucopolysaccharidosis type VII (MPS VII) model. This strategy is based upon tagging a short peptide consisting of acidic amino acids (AAA) to N terminus of human beta-glucuronidase (GUS). The pharmacokinetics, biodistribution, and the pathological effect on MPS VII mouse after 12 weekly infusions were determined for recombinant human untagged and tagged GUS. The tagged GUS was taken up by MPS VII fibroblasts in a mannose 6-phosphate receptor-dependent manner. Intravenously injected AAA-tagged enzyme had five times more prolonged blood clearance compared with the untagged enzyme. The tagged enzyme was delivered effectively to bone, bone marrow, and brain in MPS VII mice and was effective in reversing the storage pathology. The storage in osteoblasts was cleared similarly with both enzyme types. However, cartilage showed a little response to any of the enzymes. The tagged enzyme reduced storage in cortical neurons, hippocampus, and glia cells. A highly sensitive method of tandem mass spectrometry on serum indicated that the concentration of serum dermatan sulfate and heparan sulfate in mice treated with the tagged enzyme decreased more than the untagged enzyme. These preclinical studies suggest that this AAA-based targeting system may enhance enzyme-replacement therapy.

Chemically modified beta-glucuronidase crosses blood-brain barrier and clears neuronal storage in murine mucopolysaccharidosis VII

Enzyme replacement therapy has been used successfully in many lysosomal storage diseases. However, correction of brain storage has been limited by the inability of infused enzyme to cross the blood-brain barrier. The newborn mouse is an exception because recombinant enzyme is delivered to neonatal brain after mannose 6-phosphate receptor-mediated transcytosis. Access to this route is very limited after 2 weeks of age. Recently, several studies showed that multiple infusions of high doses of enzyme partially cleared storage in adult brain. These results raised the question of whether correction of brain storage by repeated high doses of enzyme depends on mannose 6-phosphate receptor-mediated uptake or whether enzyme gains access to brain storage by another route when brain capillaries are exposed to prolonged, high levels of circulating enzyme. To address this question, we used an enzyme whose carbohydrate-dependent receptor-mediated uptake was inactivated by chemical modification. Treatment of human beta-glucuronidase (GUS) with sodium metaperiodate followed by sodium borohydride reduction (PerT-GUS) eliminated uptake by mannose 6-phosphate and mannose receptors in cultured cells and dramatically slowed its plasma clearance from a t(1/2) of <10 min to 18 h. Surprisingly, PerT-GUS infused weekly for 12 weeks was more effective in clearing central nervous system storage than native GUS at the same dose. In fact, PerT-GUS resulted in almost complete reversal of storage in neocortical and hippocampal neurons. This enhanced correction of neuronal storage by long-circulating enzyme, which targets no known receptor, suggests a delivery system across the blood-brain barrier that might be exploited therapeutically.

Carbonic anhydrases IV and IX: subcellular localization and functional role in mouse skeletal muscle

The subcellular localization of carbonic anhydrase (CA) IV and CA IX in mouse skeletal muscle fibers has been studied immunohistochemically by confocal laser scanning microscopy. CA IV has been found to be located on the plasma membrane as well as on the sarcoplasmic reticulum (SR) membrane. CA IX is not localized in the plasma membrane but in the region of the t-tubular (TT)/terminal SR membrane. CA IV contributes 20% and CA IX 60% to the total CA activity of SR membrane vesicles isolated from mouse skeletal muscles. Our aim was to examine whether SR CA IV and TT/SR CA IX affect muscle contraction. Isolated fiber bundles of fast-twitch extensor digitorum longus and slow-twitch soleus muscle from mouse were investigated for isometric twitch and tetanic contractions and by a fatigue test. The muscle functions of CA IV knockout (KO) fibers and of CA IX KO fibers do not differ from the function of wild-type (WT) fibers. Muscle function of CA IV/XIV double KO mice unexpectedly shows a decrease in rise and relaxation time and in force of single twitches. In contrast, the CA inhibitor dorzolamide, whether applied to WT or to double KO muscle fibers, leads to a significant increase in rise time and force of twitches. It is concluded that the function of mouse skeletal muscle fibers expressing three membrane-associated CAs, IV, IX, and XIV, is not affected by the lack of one isoform but is possibly affected by the lack of all three CAs, as indicated by the inhibition studies.

Tumor-associated carbonic anhydrases and their clinical significance

Carbonic anhydrases (CAs) are physiologically important enzymes that catalyze a reversible conversion of carbon dioxide to bicarbonate and participate in ion transport and pH control. Two human isoenzymes, CA IX and CA XII, are overexpressed in cancer and contribute to tumor physiology. Particularly CA IX is confined to only few normal tissues but is ectopically induced in many tumor types mainly due to its strong transcriptional activation by hypoxia accomplished via HIF-1 transcription factor. Therefore, CA IX can serve as a surrogate marker of hypoxia and a prognostic indicator. CA IX appears implicated in cell adhesion and in balance of pH disturbances caused by tumor metabolism. Both tumor-related expression pattern and functional involvement in tumor progression make it a suitable target for anticancer treatment. Here we summarize a current knowledge on CA IX and CA XII, and discuss possibilities of their exploitation for cancer detection, diagnostics, and therapy.

The role of carbonic anhydrases in renal physiology

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO(2). CA is expressed in most segments of the kidney. CAII and CAIV predominate in human and rabbit kidneys; in rodent kidneys, CAXII, and CAXIV are also present. CAIX is expressed by renal cell carcinoma (RCC). Most of these isoforms, except for rodent CAIV, have high turnover rates. CAII is a cytoplasmic enzyme, whereas the others are membrane-associated; CAIV is anchored by glycosylphosphatidylinositol linkage. Membrane polarity is apical for CAXIV, basolateral for CAXII, and apical and basolateral for CAIV. Luminal membrane CAs facilitate the dehydration of carbonic acid (H(2)CO(3)) that is formed when secreted protons combine with filtered bicarbonate. Basolateral CA enhances the efflux of bicarbonate via dehydration of H(2)CO(3). CAII and CAIV can associate with bicarbonate transporters (e.g., AE1, kNBC1, NBC3, and SCL26A6), and proton antiporter, NHE1 in a membrane protein complex called a transport metabolon. CAXII and CAXIV may also be associated with transporters in normal kidney and CAIX in RCCs. The multiplicity of CAs implicates their importance in acid-base and other solute transport along the nephron. For example, CAII on the cytoplasmic face and CAIV on the extracellular surface provide the 'push' and 'pull' for bicarbonate transport by supplying and dissipating substrate respectively.

Examination of two independent kinetic assays for determining the inhibition of carbonic anhydrases I and II: structure-activity comparison of sulfamates and sulfamides

Enzyme inhibition assays often require deviations from physiological conditions. For carbonic anhydrases, procedures involving native CO(2) and non-native substrates have been used. We compared a native and a non-native substrate in the context of inhibition of human carbonic anhydrases I and II by examining various sulfamate and sulfamide compounds in two kinetic assays: hydration of CO(2) and hydrolysis of 4-nitrophenylacetate. For carbonic anhydrase II, the two assays consistently generated similar K(i) values, with the relative difference between the assays never exceeding 2.5-fold. However, for carbonic anhydrase I there was more variability between the two assays, with K(i) values for three compounds differing by more than 2.5-fold, up to eightfold. In the CO(2) hydration assay, some sulfamates and sulfamides exhibited mixed kinetics or partial inhibition. Our results indicate that K(i) or K(d) values from carbonic anhydrase assays involving non-native substrates should be confirmed by assays that use CO(2) (or HCO), to establish pharmacological relevance. From structure-activity comparisons, the sulfamate is more effective than the sulfamide in inhibiting carbonic anhydrase I and II, but the sulfamate does not confer selectivity. In contrast, the sulfonamide confers selectivity for carbonic anhydrase I (10- to 30-fold). Selectivity for carbonic anhydrase II occurred with the substituted fructose moiety, especially the d-enantiomer (>100-fold).

Expression of hypoxia-inducible carbonic anhydrases in brain tumors

Malignant brain tumors exhibit distinct metabolic characteristics. Despite high levels of lactate, the intracellular pH of brain tumors is more alkaline than normal brain. Additionally, with increasing malignancy, brain tumors display intratumoral hypoxia. Carbonic anhydrase (CA) IX and XII are transmembrane isoenzymes that are induced by tissue hypoxia. They participate in regulation of pH homeostasis by catalyzing the reversible hydration of carbon dioxide. The aim of our study was to investigate whether brain tumors of different histology and grade of malignancy express elevated levels of CA IX and XII as compared to normal brain. We analyzed 120 tissue specimens from brain tumors (primary and metastatic) and normal brain for CA IX and XII expression by immunohistochemistry, Western blot, and in situ hybridization. Whereas normal brain tissue showed minimal levels of CA IX and XII expression, expression in tumors was found to be upregulated with increased level of malignancy. Hemangioblastomas, from patients with von Hippel-Lindau disease, also displayed high levels of CA IX and XII expression. Comparison of CA IX and XII staining with HIF-1alpha staining revealed a similar microanatomical distribution, indicating hypoxia as a major, but not the only, induction factor. The extent of CA IX and XII staining correlated with cell proliferation, as indicated by Ki67 labeling. The results demonstrate that CA IX and XII are upregulated in intrinsic and metastatic brain tumors as compared to normal brain tissue. This may contribute to the management of tumor-specific acid load and provide a therapeutic target.

Expression of membrane-associated carbonic anhydrase isoforms IV, IX, XII, and XIV in the rabbit: induction of CA IV and IX during maturation

Several carbonic anhydrase (CA) isoforms are associated with plasma membranes. It is probable that these enzymes interact with anion transporters to facilitate the movement of HCO3- into or out of the cell. A better knowledge of CA isoform expression in a given tissue would facilitate a systematic examination of any associations with such transporters. We examined the expression of CAs IV, IX, XII, and XIV mRNAs in rabbit tissues, including kidney, heart, lung, skeletal muscle, liver, pancreas, gall bladder, stomach, small intestine, colon, and spleen, using quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). CA IV mRNA was mainly in kidney, heart, lung, colon, and gall bladder. CA IX mRNA was restricted to stomach, gall bladder, duodenum, and early jejunum. CA XII mRNA was found in kidney and colon. CA XIV mRNA was localized to heart, lung, skeletal muscle, and liver. The data indicate that there are different patterns of CA expression in various tissues: CA IX was expressed in the proximal gastrointestinal tract, whereas CA XII and CA IV were more distal. CA IV and CA XII are important kidney isoforms. CA XIV was abundant in metabolically active tissues such as liver, heart, lung, and skeletal muscle. Some significant species differences were noted in the expression of some of these isoforms; for example, CA XIV is not expressed in rabbit kidney, despite being abundant in mouse kidney. Maturational studies showed that the expression of CA IX mRNA and protein increased markedly with weaning ( approximately 3-4 postnatal wk) and was well correlated with the maturational expression of the alpha-subunit of the gastric H+,K+-ATPase, suggesting that function of CA IX and the gastric H+ pump might be linked in the digestion of adult foodstuffs. The unique pattern of membrane-bound CA isoforms suggests different functional associations with transporters, depending on the physiological demands on the tissue.

Carbonic anhydrase inhibitors. Inhibition of the transmembrane isozyme XII with sulfonamides—a new target for the design of antitumor and antiglaucoma drugs?

The inhibition of a newly cloned human carbonic anhydrase (CA, EC 4.2.1.1), isozyme XII (hCA XII), 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 drug 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 several medium potency inhibitors (K(I)s in the range of 34-220 nM), whereas ethoxzolamide and several halogenated sulfanilamides showed stronger potency, with K(I)s in the range of 11-22 nM. The antiglaucoma sulfonamides used clinically, except dichlorophenamide, which is a moderate inhibitor (K(I) of 50 nM), as well as topiramate, indisulam, and sulpiride behave as very potent hCA XII inhibitors, with K(I)s in the range of 3.0-5.7 nM. Several subnanomolar inhibitors (K(I)s in the range of 0.30-0.85 nM) have also been detected. Compounds with excellent selectivity against hCA XII over hCA II have been found, showing selectivity ratios in the range of 177.7-566.7. Apparently, hCA XII is a target of the antiglaucoma sulfonamides, and potent hCA XII inhibitors may be developed/used for the management of hypoxic tumors, together with inhibitors of the other tumor-associated isozyme, CA IX.

Carbonic anhydrases in normal gastrointestinal tract and gastrointestinal tumours

Carbonic anhydrases (CAs) catalyse the hydration of CO₂ to bicarbonate at physiological pH. This chemical interconversion is crucial since HCO₃^- is the substrate for several biosynthetic reactions. This review is focused on the distribution and role of CA isoenzymes in both normal and pathological gastrointestinal (GI) tract tissues. It has been known for many years that CAs are widely present in the GI tract and play important roles in several physiological functions such as production of saliva, gastric acid, bile, and pancreatic juice as well as in absorption of salt and water in intestine. New information suggests that these enzymes participate in several processes that were not envisioned earlier. Especially, the recent reports on plasma membrane-bound isoenzymes IX and XII have raised considerable interest since they were reported to participate in cancer invasion and spread. They are induced by tumour hypoxia and may also play a role in von Hippel-Lindau (VHL)-mediated carcinogenesis.

Purification of recombinant human carbonic anhydrase-II by metal affinity chromatography without incorporating histidine tags

Due to their involvement in diverse pathological conditions, carbonic anhydrases have been the targets of drug developments for the treatments of glaucoma, epilepsy, high altitude sickness, as well as cancer. Of about 14 isozymes of carbonic anhydrases, carbonic anhydrase-II (hCA-II) has been most extensively investigated from the structural, functional, and inhibitor design point of view. We discovered that hCA-II preferentially binds to the Sepharose-iminodiacetate (IDA)-Zn(2+) column, and such binding does not require incorporation of either N- or C-terminal histidine tags in the protein structure. By using the Sepharose-IDA-Zn(2+) affinity column, we purified the Escherichia coli expressed hCA-II with an overall recovery of 76%. The purified enzyme showed a single band on the SDS-PAGE. Due to ease in preparing the Sepharose-IDA-Zn(2+) column, and purifying hCA-II just in one step, the overall protocol will be ideal for producing bulk quantities of the enzyme for high throughput screening of inhibitors.

Glycosylation-independent targeting enhances enzyme delivery to lysosomes and decreases storage in mucopolysaccharidosis type VII mice

Enzyme-replacement therapy is an established means of treating lysosomal storage diseases. Infused therapeutic enzymes are targeted to lysosomes of affected cells by interactions with cell-surface receptors that recognize carbohydrate moieties, such as mannose and mannose 6-phosphate, on the enzymes. We have tested an alternative, peptide-based targeting system for delivery of enzymes to lysosomes in a murine mucopolysaccharidosis type VII (MPS VII) model. This strategy depends on the interaction of a fragment of insulin-like growth factor II (IGF-II), with the IGF-II binding site on the bifunctional, IGF-II cation-independent mannose 6-phosphate receptor. A chimeric protein containing a portion of mature human IGF-II fused to the C terminus of human beta-glucuronidase was taken up by MPS VII fibroblasts in a mannose 6-phosphate-independent manner, and its uptake was inhibited by the addition of IGF-II. Furthermore, the tagged enzyme was delivered effectively to clinically significant tissues in MPS VII mice and was effective in reversing the storage pathology. The tagged enzyme was able to reduce storage in glomerular podocytes and osteoblasts at a dose at which untagged enzyme was much less effective. This peptide-based, glycosylation-independent lysosomal targeting system may enhance enzyme-replacement therapy for certain human lysosomal storage diseases.

Expression, Assay, and Structure of the Extracellular Domain of Murine Carbonic Anhydrase XIV

Carbonic anhydrase (CA) XIV is the most recently identified mammalian carbonic anhydrase isozyme, and its presence has been demonstrated in a number of tissues. Full-length CA XIV is a transmembrane protein composed of an extracellular catalytic domain, a single transmembrane helix, and a short intracellular polypeptide segment. The amino acid sequence identity of human CA XIV relative to the other membrane-associated isozymes (CA IV, CA IX, and CA XII) is 34-46%. We report here the expression and purification of both the full-length enzyme and a truncated, secretory form of murine CA XIV. Both forms of this isozyme are highly active, and both show an abrogation of activity in the presence of 0.2% SDS, in contrast to the behavior of murine CA IV. We also report the crystal structure of the extracellular domain of murine CA XIV at 2.8 A resolution and of an enzyme-acetazolamide complex at 2.9 A resolution. The structure shows a monomeric glycoprotein with a topology similar to that of other mammalian CA isozymes. Based on the x-ray crystallographic results, we compare and contrast known structures of membrane-associated CA isozymes to rationalize the structural elements responsible for the SDS resistance of CA IV and to discuss prospects for the design of selective inhibitors of membrane-associated CA isozymes.

Cell biology of laryngeal epithelial defenses in health and disease: preliminary studies

Esophageal epithelium has intrinsic antireflux defenses, including carbonic anhydrases (CAs I to IV) that appear to be protective against gastric reflux. This study aimed to investigate the expression and distribution of CA isoenzymes in laryngeal epithelium. Laryngeal biopsy specimens collected from the vocal fold and interarytenoid regions were analyzed by Western blotting and immunofluorescence. Carbonic anhydrases I and II were expressed by the majority of samples analyzed. In contrast, CA III was differentially expressed in the interarytenoid samples and was not detected in any vocal fold samples. The expression of CA III was increased in esophagitis as compared to normal esophageal tissue. Carbonic anhydrase I and III isoenzymes were distributed cytoplasmically in the basal and lower prickle cell layers. The laryngeal epithelium expresses some CA isoenzymes and has the potential to protect itself against laryngopharyngeal reflux. Laryngeal tissue may be more sensitive to injury due to reflux damage than the esophageal mucosa because of different responses of CA isoenzymes.

Crystal structure of the dimeric extracellular domain of human carbonic anhydrase XII, a bitopic membrane protein overexpressed in certain cancer tumor cells

Overexpression of the zinc enzyme carbonic anhydrase (CA; EC ) XII is observed in certain human cancers. This bitopic membrane protein contains an N-terminal extracellular catalytic domain, a membrane-spanning alpha-helix, and a small intracellular C-terminal domain. We have determined the three-dimensional structure of the extracellular catalytic domain of human CA XII by x-ray crystallographic methods at 1.55-A resolution. The structure reveals a prototypical CA fold; however, two CA XII domains associate to form an isologous dimer, an observation that is confirmed by studies of the enzyme in solution. The identification of signature GXXXG and GXXXS motifs in the transmembrane sequence that facilitate helix-helix association is additionally consistent with dimeric architecture. The dimer interface is situated so that the active site clefts of each monomer are clearly exposed on one face of the dimer, and the C termini are located together on the opposite face of the dimer to facilitate membrane interaction. The amino acid composition of the active-site cleft closely resembles that of the other CA isozymes in the immediate vicinity of the catalytic zinc ion, but differs in the region of the nearby alpha-helical "130's segment." The structure of the CA XII-acetazolamide complex is also reported at 1.50-A resolution, and prospects for the design of CA XII-specific inhibitors of possible chemotherapeutic value are discussed.