Abstract
The hypoxia-responsiveness of the glycolytic machinery may allow pretreatment identification of hypoxic tumors from HIF-1 targets (e.g., Glut-1) or [18F]-fluorodeoxyglucose positron emission tomography but results have been mixed. We hypothesized that this discrepancy is an inevitable consequence of elevated aerobic glycolysis in tumors (Warburg effect) as energetics in predominantly glycolytic cells is little affected by hypoxia. Accordingly, we characterized glycolytic and mitochondrial ATP generation in normoxic and anoxic cell lines. Measurements demonstrated that most cancer cells rely largely on aerobic glycolysis as it accounts for 56-63% of their ATP budget, but in the cervical carcinoma SiHa, ATP synthesis was mainly mitochondrial. Moreover, the stimulatory effect of anoxia on glycolytic flux was inversely correlated to the relative reliance on aerobic glycolysis. Next, tumor cells representing a Warburg or a nonglycolytic phenotype were grown in mice and spatial patterns of hypoxia (pimonidazole-stained), Glut-1 expression and (18)F-FDG uptake were analysed on sectioned tumors. Only in SiHa tumors did foci of elevated glucose metabolism consistently colocalize with regions of hypoxia and elevated Glut-1 expression. In contrast, spatial patterns of Glut-1 and pimonidazole staining correlated reasonably well in all tumors. In conclusion, Glut-1's value as a hypoxia marker is not severely restricted by aerobic glycolysis. In contrast, the specificity of (18)F-FDG uptake and Glut-1 expression as markers of regional hypoxia and glucose metabolism, respectively, scales inversely with the intensity of the Warburg effect. This linkage suggests that multi-tracer imaging combining FDG and hypoxia-specific markers may provide therapeutically relevant information on tumor energetic phenotypes.
