Intracellular Carbonic Anhydrase Activity Sensitizes Cancer Cell pH-signaling to Dynamic Changes in CO2 Partial Pressure [Signal Transduction]

July 24th, 2014 by Hulikova, A., Aveyard, N., Harris, A., Vaughan-Jones, R., Swietach, P.

Carbonic anhydrase (CA) enzymes catalyze chemical equilibration between CO2, HCO3- and H+. Intracellular CAs (CAi) are present in certain types of cancer and some evidence suggests that low levels correlate with disease severity. However, their physiological role remains unclear. Cancer cell CAi activity, measured as cytoplasmic CO2 hydration rate (kh), ranged from high in colorectal HCT116 (≈2 s-1), bladder RT112, colorectal HT29 and fibrosarcoma HT1080, to negligible (i.e. spontaneous kh=0.18 s-1) in cervical HeLa and breast MDA-MB-468 cells. CAi activity in intact cells correlated with CAII immunoreactivity and enzymic activity in membrane-free lysates, suggesting that soluble CAII is a major intracellular isoform. CAi catalysis was not obligatory for supporting acid-extrusion by H+-efflux or HCO3--influx, nor for maintaining intracellular pH (pHi) uniformity. However, in the absence of CAi activity, acid-loading from a highly alkaline pHi was rate-limited by HCO3- supply from CO2 hydration. In solid tumors, time-dependence of capillary flow can result in CO2 partial pressure (pCO2) fluctuations, which continuously disturb cytoplasmic CO2-HCO3--H+ equilibrium. In cancer cells with high CAi activity, extracellular pCO2 fluctuations evoked faster and larger pHi-oscillations. Functionally, these resulted in larger pH-dependent intracellular [Ca2+] oscillations and stronger inhibition of the mTORC1 pathway reported by p70-S6 kinase phosphorylation. In contrast, the pHi of cells with low CAi activity was less sensitive to pCO2 fluctuations and such low-pass filtering would "buffer" cancer cell pHi from non-steady-state interstitial pCO2. Thus, CAi activity determines the coupling between pCO2 (a function of tumor perfusion) and pHi (a potent modulator of cancer cell physiology).
  • Posted in Journal of Biological Chemistry, Publications
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