Carbonic anhydrase IX is a pH-stat that sets an acidic tumour extracellular pH in vivo

Shen-Han Lee,John R Griffiths,Adrian L Harris,Alzbeta Hulikova,Dominick Mcintyre,Jesús Pacheco-Torres,Pawel Swietach,Davina Honess,Sebastián Cerdán

doi:10.1038/s41416-018-0216-5

Abstract

BackgroundTumour carbonic anhydrase IX (CAIX), a hypoxia-inducible tumour-associated cell surface enzyme, is thought to acidify the tumour microenvironment by hydrating CO2 to form protons and bicarbonate, but there is no definitive evidence for this in solid tumours in vivo.MethodsWe used 1H magnetic resonance spectroscopic imaging (MRSI) of the extracellular pH probe imidazolyl succinic acid (ISUCA) to measure and spatially map extracellular pH in HCT116 tumours transfected to express CAIX and empty vector controls in SCID mice. We also measured intracellular pH in situ with 31P MRS and measured lactate in freeze-clamped tumours.ResultsCAIX-expressing tumours had 0.15 pH-unit lower median extracellular pH than control tumours (pH 6.71 tumour vs pH 6.86 control, P = 0.01). Importantly, CAIX expression imposed an upper limit for tumour extracellular pH at 6.93. Despite the increased lactate concentration in CAIX-expressing tumours, 31P MRS showed no difference in intracellular pH, suggesting that CAIX acidifies only the tumour extracellular space.ConclusionsCAIX acidifies the tumour microenvironment, and also provides an extracellular pH control mechanism. We propose that CAIX thus acts as an extracellular pH-stat, maintaining an acidic tumour extracellular pH that is tolerated by cancer cells and favours invasion and metastasis.

Highlights

Tumour carbonic anhydrase IX (CAIX), a hypoxia-inducible tumour-associated cell surface enzyme, is thought to acidify the tumour microenvironment by hydrating CO2 to form protons and bicarbonate, but there is no definitive evidence for this in solid tumours in vivo
Discussions about tumour acidification focussed on the lactic acid formed from glucose by aerobic glycolysis, a mechanism known as the “Warburg Effect”[5,6] that is widely regarded as a tumour characteristic.[7]
The HCT116 CA9 colorectal tumours uniformly expressing high levels of CAIX protein, along with their empty vector (EV) counterparts in which CAIX is induced only in hypoxic regions, offer a tractable experimental platform to study the effect of CAIX expression on tumour pHe in vivo, independent of changes in tumour vascularity, oxygenation and metabolism

Summary

Introduction

Tumour carbonic anhydrase IX (CAIX), a hypoxia-inducible tumour-associated cell surface enzyme, is thought to acidify the tumour microenvironment by hydrating CO2 to form protons and bicarbonate, but there is no definitive evidence for this in solid tumours in vivo. We propose that CAIX acts as an extracellular pH-stat, maintaining an acidic tumour extracellular pH that is tolerated by cancer cells and favours invasion and metastasis. We report the first direct in vivo observation of the isoenzyme carbonic anhydrase IX (CAIX) acting as a pH-stat that sets the extracellular pH (pHe) of solid tumours at an acidic level. This is important because the acidic pHe can be tolerated by tumour cells but it inhibits growth of normal host cells, facilitating invasion and metastasis.[1,2]. Human xenografts in nude rats produced 588–850 nmol/g/min CO2 vs 527 nmol/g/min lactate (calculated from data in Kallinowski et al.9), whereas colonic tumours in human patients produced 1296 nmol/g/min CO2 vs 220 nmol/g/

Methods

Results

Conclusion