Carbonic anhydrase IX and acid transport in cancer

Abstract

Alterations in tumour metabolism and acid/base regulation result in the formation of a hostile environment, which fosters tumour growth and metastasis. Acid/base homoeostasis in cancer cells is governed by the concerted interplay between carbonic anhydrases (CAs) and various transport proteins, which either mediate proton extrusion or the shuttling of acid/base equivalents, such as bicarbonate and lactate, across the cell membrane. Accumulating evidence suggests that some of these transporters interact both directly and functionally with CAIX to form a protein complex coined the 'transport metabolon'. Transport metabolons formed between bicarbonate transporters and CAIX require CA catalytic activity and have a function in cancer cell migration and invasion. Another type of transport metabolon is formed by CAIX and monocarboxylate transporters. In this complex, CAIX functions as a proton antenna for the transporter, which drives the export of lactate and protons from the cell. Since CAIX is almost exclusively expressed in cancer cells, these transport metabolons might serve as promising targets to interfere with tumour pH regulation and energy metabolism. This review provides an overview of the current state of research on the function of CAIX in tumour acid/base transport and discusses how CAIX transport metabolons could be exploited in modern cancer therapy.

Fig. 1

Tumour pH is regulated by the concerted interplay between acid/base transporters and carbonic anhydrase. Metabolic acids are produced by glycolysis and mitochondrial respiration. Anaerobic glycolysis yields lactate and H⁺ that are excreted from the cell by monocarboxylate transporters (MCTs) in a 1:1 stoichiometry. Mitochondrial respiration produces CO₂, which is hydrated in the cell, forming HCO₃⁻ and H⁺. CO₂ can leave the cell by passive diffusion over the plasma membrane or through gas channels (not shown). Efficient pH regulation requires the function of additional transporters and enzymes, which either export protons from the cell or mediate the reimport of HCO₃⁻. Additional export of H⁺ can be mediated by the Na⁺/H⁺ exchanger 1 (NHE) and by vacuolar H⁺-ATPase (V-ATPase). CO₂ venting is further supported by the catalytic function of the extracellular carbonic anhydrase (CA) isoforms CAIX and CAXII (the latter one is omitted from this cartoon for clarity). Extracellular CAs catalyse the hydration of CO₂ to HCO₃⁻ and H⁺ at the membrane. HCO₃⁻ can diffuse away from the cell or can be reimported by Na⁺/HCO₃^– cotransporters (NBC) to support intracellular buffering. The extracellular HCO₃⁻ can either be formed from ‘endogenous' CO₂, which is produced by the cell through mitochondrial respiration or titration of HCO₃⁻ and H⁺, or from extracellular CO₂, produced from distant sources. Cl⁻/HCO₃⁻ exchangers (AEs) have been suggested to function either as HCO₃⁻ importers for pH buffering or HCO₃⁻ exporters that extrude HCO₃⁻ to load cellular compartments with Cl^– during cell migration. Transport activity of many acid/base transporters is facilitated by interaction with intracellular and extracellular CAs. NBC and AE interact with CAII and CAIX that either provide or remove HCO₃⁻ to/from the transporter via their catalytic function. MCTs form a protein complex with CAII and CAIX, in which the CAs function as ‘proton antenna’ for the transporter, which mediates the rapid exchange of H⁺ between transporter pore and the surrounding protonatable residues.

Fig. 2

Bicarbonate transport metabolons with carbonic anhydrase. (a) Cl^–/HCO₃⁻ exchangers (AEs) and (b) Na⁺/HCO₃⁻ cotransporters (NBCs) form bicarbonate transport metabolons with intracellular and extracellular carbonic anhydrases (CAs). Cytosolic CAII binds to the transporter’s C-terminal tail. Extracellular CAs, which are tethered to the plasma membrane by a transmembrane domain (CAIX, CAXIV) or GPI anchor (CAIV), bind to the transporter’s fourth extracellular loop. By catalysing the reversible hydration of CO₂ to HCO₃⁻ and H⁺ in the immediate vicinity of the transporter, intracellular and extracellular CAs either provide or remove HCO₃⁻ to/from the transporter. Through this mechanism, they suppress the depletion of HCO₃⁻ at the cis-side of the transporter and HCO₃⁻ accumulation at the trans-side, which, in turn, drives HCO₃⁻ flux across the cell membrane.

Fig. 3

Carbonic anhydrases function as proton antenna for monocarboxylate transporters. The monocarboxylate transporter isoforms 1 and 4 (MCT1/4) form a non-catalytic transport metabolon with CAII, CAIV and CAIX. This type of interaction is independent from carbonic anhydrase (CA) catalytic activity. Intracellular CAII, which is bound to the MCT1/4 C-terminal tail, functions as proton antenna for the transporter, which mediates the rapid exchange of H⁺ between transporter pore and surrounding protonatable residues (blue–grey circles) at the inner face of the plasma membrane. On the extracellular site, CAIV and CAIX, which are bound to the Ig1 domain of the MCT chaperone CD147, mediate shuttling of protons between the transporter and protonatable residues at the extracellular face of the plasma membrane. In CAIX, proton shuttling is mediated by the CAIX–PG domain. The proton shuttle in CAIV is yet unidentified.