Is HIF-1? Required for V-ATPase-Dependent Regulation of Glycolytic Enzymes?
Vacuolar-ATPase (V-ATPase) is frequently up-regulated in tumors and tumor cell lines where it contributes to tumorigenic phenotypes, including the Warburg Effect (e.g. the phenomenon where glycolysis is preferred over oxidative phosphorylation (Vander Heiden et al. 2009)). The preferential use of glycolytic metabolism favored by tumor cells may be caused by at least two factors (Vander Heiden et al. 2009). First, the metabolic intermediates generated by the citric acid (TCA) cycle are used in a variety of anabolic pathways in the cell, and may supply a ready source of intermediate buildingblocks for biosynthetic processes. Second, hypoxia observed in some tumors may limit the amount of oxygen for oxidative phosphorylation. V-ATPase up-regulation in tumors and tumor cell lines could possibly be occurring in order to address the increased concentration of H+ caused by the increase in glycolysis (Sennoune and Martinez 2012). Given the preference of tumor cells for glycolysis they are ideal for studying the effects of V-ATPase inhibition on glycolytic metabolism. V-ATPase is an ATP-dependent proton pump that generates and sustains proton gradients throughout the endomembrane system (Forgac 2007). V-ATPase is composed of two domains: V0 and V1 (Forgac 2007). The V0 domain is embedded in the membranes of the endomembrane system, and is responsible for the transportation of H+ (Forgac 2007). The V1 domain is located in the cytoplasm and is responsible for the catalytic hydrolysis of ATP (Forgac 2007). Enzymes such as the regulator of ATPase of vacuoles and endosomes (RAVE) in yeast, phosphofructokinase (PFK-1), glycolytic enzymes, and glucose levels regulate V-ATPase activity by promoting assembly of V0V1 (Parra et al. 2014). Exposure to V-ATPase-specific inhibitors, Concanamycin A (CCA) or Bafilomycin A (Baf), increases expression of several glycolytic enzymes in breast tumor cell lines. Studies using the transcription-inhibitor Actinomycin D suggest that the induction of glycolytic genes observed when V-ATPase is inhibited is dependent upon increased transcription (Kane and Parra 2000). Hypoxia-inducible factor (HIF-1?) is a transcriptional inducer of many glycolytic genes and increases transport of glucose into cells (Lee et al. 2004). Preliminary results from this study suggested HIF-1? is induced by V-ATPase inhibition and is sufficient to induce transcription of glycolytic genes. I postulate that the induction of HIF-1? observed after V-ATPase inhibition will increase glucose uptake and induce several glycolytic enzymes, thus completing a circle of biochemical regulation whereby V-ATPase inhibits glycolysis.