Tumor hypoxia reduces the effectiveness of radiation therapy by limiting the biologically effective dose. An acute increase in tumor oxygenation before radiation treatment should therefore significantly improve the tumor cell kill after radiation. Efforts to increase oxygen delivery to the tumor have not shown positive clinical results. Here we show that targeting mitochondrial respiration results in a significant reduction of the tumor cells' demand for oxygen, leading to increased tumor oxygenation and radiation response. We identified an activity of the FDA-approved drug papaverine as an inhibitor of mitochondrial complex I. We also provide genetic evidence that papaverine's complex I inhibition is directly responsible for increased oxygenation and enhanced radiation response. Furthermore, we describe derivatives of papaverine that have the potential to become clinical radiosensitizers with potentially fewer side effects. Importantly, this radiosensitizing strategy will not sensitize well-oxygenated normal tissue, thereby increasing the therapeutic index of radiotherapy.

Tumor microenvironment substantially influences the process of tumorigenesis. In many solid tumors, imbalance between the demand of rapidly proliferating cancer cells and the capabilities of the vascular system generates areas with insufficient oxygen supply. In response to tumor hypoxia, cancer cells modulate their gene expression pattern to match the requirements of the altered microenvironment. One of the most significant adaptations to this milieu is the shift towards anaerobic glycolysis to keep up the energy demands. This oncogenic metabolism is often maintained also in aerobic cells. Lactic acid, its metabolic end-product, accumulates hand-in-hand with carbon dioxide, leading to acidification of the extracellular environment. Carbonic anhydrase IX (CA IX) is the most widely expressed gene in response to hypoxia. Its crucial role in intracellular pH maintenance represents the means by which cancer cells adapt to the toxic conditions of the extracellular milieu. Furthermore, the activity of CA IX stimulates the migratory pathways of cancer cells and is connected with the increase of the aggressive/invasive phenotype of tumors. CA IX expression in many types of tumors indicates its relevance as a general marker of tumor hypoxia. Moreover, its expression is closely related to prognosis of the clinical outcome in several tumor types. All above mentioned facts support the strong position of CA IX as a potential drug therapy target. Here, we summarize the state-of-the-art knowledge on its regulation and role in cancer development.

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 CO 2 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 CO 2 to bicarbonate, and indirectly, by maintaining glycolysis-permissive intracellular milieu.