Tumor hypoxia represents a severe microenvironmental stress that is frequently associated with
acidosis.
Cancer cells respond to these stresses with changes in gene expression that promote survival at least in part through pH regulation and metabolic reprogramming.
Hypoxia-induced
carbonic anhydrase IX (CA IX) plays a critical adaptive role in response to hypoxic and acidic environments by catalytically hydrating extracellular CO2 to produce
bicarbonate for buffering intracellular pH (pHi). We used
proteome-wide profiling to study the cellular response to transient CA IX knockdown in
hypoxia and found a decrease in the levels of key glycolytic
enzymes and
lactate dehydrogenase A (LDHA). Interestingly, the activity of LDH was also decreased as demonstrated by native in-gel activity assay. These changes led to a significant reduction in glycolytic flux and extracellular
lactate levels in
cancer cells in vitro, contributing to a decrease in proliferation. Interestingly, addition of the alternative LDH substrate alpha-ketobutyrate restored LDHA activity, extracellular acidification, pHi, and cellular proliferation. These results indicate that in the absence of CA IX, reduction of pHi disrupts LDHA activity and hinders the cellular capacity to regenerate NAD+ and secrete
protons to the extracellular space.
Hypoxia-induced CA IX therefore mediates adaptation to microenvironmental
hypoxia and
acidosis directly, by enzymatically converting extracellular CO2 to
bicarbonate, and indirectly, by maintaining glycolysis-permissive intracellular milieu.