title: Carbonic Anhydrase Activity Monitored In Vivo by Hyperpolarized 13C-Magnetic Resonance Spectroscopy Demonstrates Its Importance for pH Regulation in Tumors.
abstract: Carbonic anhydrase buffers tissue pH by catalyzing the rapid interconversion of carbon dioxide (CO2) and bicarbonate (HCO3 (-)). We assessed the functional activity of CAIX in two colorectal tumor models, expressing different levels of the enzyme, by measuring the rate of exchange of hyperpolarized (13)C label between bicarbonate (H(13)CO3(-)) and carbon dioxide ((13)CO2), following injection of hyperpolarized H(13)CO3(-), using (13)C-magnetic resonance spectroscopy ((13)C-MRS) magnetization transfer measurements. (31)P-MRS measurements of the chemical shift of the pH probe, 3-aminopropylphosphonate, and (13)C-MRS measurements of the H(13)CO3(-)/(13)CO2 peak intensity ratio showed that CAIX overexpression lowered extracellular pH in these tumors. However, the (13)C measurements overestimated pH due to incomplete equilibration of the hyperpolarized (13)C label between the H(13)CO3(-) and (13)CO2 pools. Paradoxically, tumors overexpressing CAIX showed lower enzyme activity using magnetization transfer measurements, which can be explained by the more acidic extracellular pH in these tumors and the decreased activity of the enzyme at low pH. This explanation was confirmed by administration of bicarbonate in the drinking water, which elevated tumor extracellular pH and restored enzyme activity to control levels. These results suggest that CAIX expression is increased in hypoxia to compensate for the decrease in its activity produced by a low extracellular pH and supports the hypothesis that a major function of CAIX is to lower the extracellular pH.
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom. Department of Radiology, University of Cambridge, Addenbrooke's Hospital, Cambridge, United Kingdom. fag1000@cam.ac.uk.
authors.forename: Ferdia A
authors.initials: FA
authors.lastname: Gallagher
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Helen
authors.initials: H
authors.lastname: Sladen
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Mikko I
authors.initials: MI
authors.lastname: Kettunen
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Eva M
authors.initials: EM
authors.lastname: Serrao
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Tiago B
authors.initials: TB
authors.lastname: Rodrigues
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Alan
authors.initials: A
authors.lastname: Wright
authors.affiliation: Department of Radiology, University of Cambridge, Addenbrooke's Hospital, Cambridge, United Kingdom.
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Gill
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Sarah
authors.initials: S
authors.lastname: McGuire
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Booth
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Joan
authors.initials: J
authors.lastname: Boren
authors.affiliation: Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Headington, Oxford, United Kingdom.
authors.forename: Alan
authors.initials: A
authors.lastname: McIntyre
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Jodi L
authors.initials: JL
authors.lastname: Miller
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Shen-Han
authors.initials: SH
authors.lastname: Lee
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Davina
authors.initials: D
authors.lastname: Honess
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Sam E
authors.initials: SE
authors.lastname: Day
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: De-En
authors.initials: DE
authors.lastname: Hu
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: William J
authors.initials: WJ
authors.lastname: Howat
authors.affiliation: Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Headington, Oxford, United Kingdom.
authors.forename: Adrian L
authors.initials: AL
authors.lastname: Harris
authors.affiliation: Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom.
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Brindle
mesh_terms: D000818:Animals; D000951:Antigens, Neoplasm / Q000032:analysis / Q000235:genetics / Q000502:physiology*; D001639:Bicarbonates / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D002247:Carbon Isotopes / Q000032:analysis; D000071231:Carbonic Anhydrase IX; D002256:Carbonic Anhydrases / Q000032:analysis / Q000235:genetics / Q000502:physiology*; D045744:Cell Line, Tumor; D015179:Colorectal Neoplasms / Q000378:metabolism* / Q000473:pathology; D064593:Heterografts; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D051379:Mice; D016688:Mice, Inbred NOD; D016513:Mice, SCID; D009363:Neoplasm Proteins / Q000032:analysis / Q000502:physiology*; D011993:Recombinant Fusion Proteins / Q000032:analysis; D059016:Tumor Microenvironment
chemical_list: D000951:Antigens, Neoplasm; D001639:Bicarbonates; D002247:Carbon Isotopes; D009363:Neoplasm Proteins; D011993:Recombinant Fusion Proteins; D002245:Carbon Dioxide; C089226:CA9 protein, human; D000071231:Carbonic Anhydrase IX; D002256:Carbonic Anhydrases