Data from Carbonic Anhydrase Activity Monitored In Vivo by Hyperpolarized ¹³C-Magnetic Resonance Spectroscopy Demonstrates Its Importance for pH Regulation in Tumors

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 ¹³C label between bicarbonate (H¹³CO₃⁻) and carbon dioxide (¹³CO₂), following injection of hyperpolarized H¹³CO₃⁻, using ¹³C-magnetic resonance spectroscopy (¹³C-MRS) magnetization transfer measurements. ³¹P-MRS measurements of the chemical shift of the pH probe, 3-aminopropylphosphonate, and ¹³C-MRS measurements of the H¹³CO₃⁻/¹³CO₂ peak intensity ratio showed that CAIX overexpression lowered extracellular pH in these tumors. However, the ¹³C measurements overestimated pH due to incomplete equilibration of the hyperpolarized ¹³C label between the H¹³CO₃⁻ and ¹³CO₂ 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. Cancer Res; 75(19); 4109–18. ©2015 AACR.