Multifaceted roles of glycolytic enzymes

Although glycolysis is a biochemical pathway that evolved under ancient anaerobic terrestrial conditions, recent studies have provided evidence that some glycolytic enzymes are more complicated, multifaceted proteins rather than simple components of the glycolytic pathway. These glycolytic enzymes have acquired additional non-glycolytic functions in transcriptional regulation [hexokinase (HK)-2, lactate dehydrogenase A, glyceraldehyde-3-phosphate dehydrogenase (GAPD) and enolase 1], stimulation of cell motility (glucose-6-phosphate isomerase) and the regulation of apoptosis (glucokinase, HK and GAPD). The existence of multifaceted roles of glycolytic proteins suggests that links between metabolic sensors and transcription are established directly through enzymes that participate in metabolism. These roles further underscore the need to consider the non-enzymatic functions of enzymes in proteomic studies of cells and tissues.

Introduction

Before the existence of atmospheric oxygen, anaerobic glucose metabolism involving glycolysis evolved to produce energy for ancient anaerobic life forms. The glycolytic pathway and its enzymes, which convert glucose to pyruvic acid using the oxidative potential of NAD⁺, are among the most ancient molecular metabolic networks [1] (Box 1). In the absence of oxygen, NAD⁺ is regenerated from reduced NADH by the conversion of pyruvate to lactic acid. Glycolysis has been highly conserved among species, even after the emergence of atmospheric oxygen, which provided a means to further oxidize pyruvate by oxidative phosphorylation, resulting in a high yield of energy. Although ambient expression of glycolytic enzymes is necessary for steady-state glucose metabolism, the coordinated increased expression of genes encoding glycolytic enzymes is particularly important for adaptation to hypoxia. The hypoxia-inducible transcription factor HIF-1, which is stabilized by hypoxia, induces almost all the genes encoding glycolytic enzymes 2, 3, 4, 5, 6. With oxygen depletion, the unmodified, stable HIF-1α in a heterodimer binds cis-regulatory elements to activate hypoxia-responsive genes including those encoding many glycolytic enzymes 7, 8.

In contrast to normal cells that increase glycolysis in response to hypoxia, cancer cells undergo glycolysis with increased lactate production even in the presence of oxygen. This phenomenon in cancer cells is termed the Warburg effect. In 1929, it was hypothesized that the reliance of cancer cells on glycolysis results from defective tumor mitochondria that could not mediate normal oxidative metabolism [9]. Although the exact mechanisms contributing to the Warburg effect are still under scrutiny, it is known that micro-environmental tumor hypoxia and oncogenic activation of Akt and Myc can enhance glycolysis by non-transcriptional and transcriptional mechanisms 10, 11, 12. Furthermore, HIF-1α might be stabilized independently of hypoxia via loss of vHL (von Hippel–Lindau protein), a tumor suppressor and a component of the proteasomal degradation pathway, or via non-hypoxic signal-transduction pathways 5, 13. These observations indicate that both cell autonomous, oncogenic signals and micro-environmental hypoxia contribute to the high rates of tumor glycolysis.

Most early structural and functional investigations of glycolytic enzymes have focused on their glycolytic functions. However, with the precedence of aconitase – a tricarboxylic-acid-cycle enzyme – having a role in the regulation of iron metabolism by binding RNA hairpin loops, it is perhaps not surprising that glycolytic enzymes might participate in other cellular processes 14, 15. In fact, recent studies have provided evidence that some glycolytic enzymes are more complicated, multi-functional proteins rather than simple components of the glycolytic pathway. As shown in Table 1, unexpected functional roles of several glycolytic enzymes have been identified. These new functions include transcriptional regulation [hexokinase (HK)-2, lactate dehydrogenase (LDH)-A, glyceraldehyde-3-phosphate dehydrogenase (GAPD) and enolase (ENO) 1], apoptosis (HK and GAPD) and cell motility [glucose-6-phosphate isomerase (GPI)]. Furthermore, in yeast, the mitochondrial metabolic enzyme Arg5,6 binds both nuclear and mitochondrial DNA, and seems to regulate nuclear and mitochondrial target genes [16]. Thus, it is important to reevaluate and recognize the roles for glycolytic enzymes in other biological processes, in particular, because glycolytic enzymes are usually regarded as boring ‘housekeeping’ proteins that exist only to fuel more important, intriguing biochemical processes. Here, the evidence supporting the unexpected multi-functional roles for the glycolytic proteins are reviewed with the hope that interpretation of data pertaining to the expression of glycolytic enzymes also considers their non-glycolytic functions.