Carbonic Anhydrase Activity Monitored In Vivo by Hyperpolarized 13C-Magnetic Resonance Spectroscopy Demonstrates Its Importance for pH Regulation in Tumors
Carbonic Anhydrase Activity Monitored In Vivo by Hyperpolarized 13C-Magnetic Resonance Spectroscopy Demonstrates Its Importance for pH Regulation in Tumors
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DOI：
10.1158/0008-5472.can-15-0857
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发表时间：
2015-10-01
期刊：
影响因子：
11.2
通讯作者：
Brindle, Kevin M.
中科院分区：
文献类型：
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作者：
Gallagher, Ferdia A.;Sladen, Helen;Brindle, Kevin M.
碳酸酐酶通过催化二氧化碳（CO2）和碳酸氢盐（HCO 3-）的快速相互转化来缓冲组织pH。我们通过在注射超极化（HCO 3-）-C-13后测量碳酸氢盐（（HCO 3-）-C-13）和二氧化碳（（CO2）-C-13）之间超极化C-13标记的交换速率，使用C-13-磁共振波谱（C-13-MRS）磁化转移测量。pH探针3-氨丙基膦酸盐的化学位移的P-31-MRS测量和（HCO 3-）-C-13/（CO2）-C-13峰强度比的C-13-MRS测量显示CAIX过表达降低了这些肿瘤中的细胞外pH。然而，由于（HCO 3-）-C-13和（CO2）-C-13合并液之间超极化C-13标记物的不完全平衡，C-13测量值高估了pH值。奇怪的是，使用磁化转移测量，过表达CAIX的肿瘤显示出较低的酶活性，这可以通过这些肿瘤中更酸性的细胞外pH和在低pH下酶活性降低来解释。这种解释通过在饮用水中施用碳酸氢盐来证实，其升高了肿瘤细胞外pH并将酶活性恢复到对照水平。这些结果表明，CAIX表达在缺氧中增加，以补偿由低细胞外pH产生的其活性降低，并支持CAIX的主要功能是降低细胞外pH的假设。
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 C-13 label between bicarbonate ((HCO3-)-C-13) and carbon dioxide ((CO2)-C-13), following injection of hyperpolarized (HCO3-)-C-13, using C-13-magnetic resonance spectroscopy (C-13-MRS) magnetization transfer measurements. P-31-MRS measurements of the chemical shift of the pH probe, 3-aminopropylphosphonate, and C-13-MRS measurements of the (HCO3-)-C-13/(CO2)-C-13 peak intensity ratio showed that CAIX overexpression lowered extracellular pH in these tumors. However, the C-13 measurements overestimated pH due to incomplete equilibration of the hyperpolarized C-13 label between the (HCO3-)-C-13 and (CO2)-C-13 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. (C) 2015 AACR.