Figure 7.

Induction of glycolysis through HIF-1α is anti-proliferative. (A) Growth factors stimulate cellular glucose uptake and the expression of HIF-1α. Under conditions of normoxia, the accumulation of HIF-1α is repressed by O₂-dependent hydroxylation and degradation of the HIF-1α protein. When HIF-1α is repressed by O₂-dependent degradation and/or HIF-1α shRNA, glycolytic pyruvate is diverted into mitochondrial-dependent lipid synthesis. Pyruvate is catabolized by mitochondrial PDH into acetyl-CoA, which is used in the TCA cycle to produce citrate that is transported into the cytosol, where it acts as a further regulator of glucose-6-P metabolism. Citrate is metabolized to produce acetyl-CoA by the enzyme ATP-citrate lyase (ACL) to supply the cell with a source of acetyl-CoA for lipid synthesis. When HIF-1α is stabilized by mitochondrial ROS or oxygen deprivation, the increased HIF-1α-dependent transcription can result in a metabolic reprogramming of intracellular glucose fate. The HIF-1α-dependent increases in glycolytic genes including Ldh-A lead to an enhanced rate of anaerobic glycolysis, and the induction of mitochondrial regulatory enzymes such as PDK-1 decrease pyruvate metabolism in the mitochondria, resulting in decreased mitochondrial TCA cycle activity and cytosolic citrate levels. These effects would support non-oxygen-dependent ATP production by degradation of glucose to lactate, and suppress mitochondrial activity and cytosolic lipid synthesis. (B) Glycolytic rate of cells cultured in the presence of IL-3 following 24 h incubation under 21% O₂ or 1.5% O₂. In the absence of IL-3 (not shown), glycolysis was reduced <6.58 nmol of glucose per hour in both the vector controls and HIF-1α shRNA cells. Data are the mean of three independent experiments ± SD performed in triplicate. (C) Cells cultured in the presence of IL-3 were subjected to 4 h of 21% O₂ or 1.5% O₂ followed by an additional 20–24 h incubation with ¹⁴C-labeled pyruvate. Lipids were extracted from cell lysates and total incorporation of ¹⁴C-labeled lipid was measured by scintillation counting. Data are a representative experiment ± SD for triplicate samples. (D) Accumulation of lactate in cell culture supernatants. Cells were cultured in 1 mL of medium for 5 d in the presence of IL-3 and subjected to 1.5% O₂ for an additional 24 h. Lactate levels in supernatants were measured as described in Materials and Methods. Data represent three independent experiments ± SD.