title: Catalysis and pH control by membrane-associated carbonic anhydrase IX in MDA-MB-231 breast cancer cells.
abstract: Carbonic anhydrase IX (CAIX) is a membrane-bound, tumor-related enzyme whose expression is often considered a marker for hypoxia, an indicator of poor prognosis in the majority of cancer patients, and is associated with acidification of the tumor microenvironment. Here, we describe for the first time the catalytic properties of native CAIX in MDA-MB-231 breast cancer cells that exhibit hypoxia-inducible CAIX expression. Using (18)O exchange measured by membrane inlet mass spectrometry, we determined catalytic activity in membrane ghosts and intact cells. Exofacial carbonic anhydrase activity increases with exposure to hypoxia, an activity which is suppressed by impermeant sulfonamide CA inhibitors. Inhibition by sulfonamide inhibitors is not sensitive to reoxygenation. CAIX activity in intact cells increases in response to reduced pH. Data from membrane ghosts show that the increase in activity at reduced pH is largely due to an increase in the dehydration reaction. In addition, the kinetic constants of CAIX in membrane ghosts are very similar to our previous measurements for purified, recombinant, truncated forms. Hence, the activity of CAIX is not affected by the proteoglycan extension or membrane environment. These activities were measured at a total concentration for all CO(2) species at 25 mm and close to chemical equilibrium, conditions which approximate the physiological extracellular environment. Our data suggest that CAIX is particularly well suited to maintain the extracellular pH at a value that favors the survival fitness of tumor cells.
authors: Department of Biochemistry and Molecular Biology, University of Florida, Gainesville, Florida 32610, USA.
authors: Ying
authors: Y
authors: Li
authors: Chingkuang
authors: C
authors: Tu
authors: Hai
authors: H
authors: Wang
authors: David N
authors: DN
authors: Silverman
authors: Susan C
authors: SC
authors: Frost
mesh_terms: D000951:Antigens, Neoplasm / Q000737:chemistry / Q000378:metabolism*; D001943:Breast Neoplasms / Q000201:enzymology*; D002245:Carbon Dioxide / Q000378:metabolism; D000071231:Carbonic Anhydrase IX; D002256:Carbonic Anhydrases / Q000737:chemistry / Q000378:metabolism*; D002384:Catalysis; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000201:enzymology*; D002470:Cell Survival; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009363:Neoplasm Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism
chemical_list: D000951:Antigens, Neoplasm; D004791:Enzyme Inhibitors; D009363:Neoplasm Proteins; D002245:Carbon Dioxide; C089226:CA9 protein, human; D000071231:Carbonic Anhydrase IX; D002256:Carbonic Anhydrases; D010100:Oxygen