Pyruvate kinase M2: A simple molecule with complex functions

Abstract

Pyruvate kinase M2 is a critical enzyme that regulates cell metabolism and growth under different physiological conditions. In its metabolic role, pyruvate kinase M2 catalyzes the last glycolytic step which converts phosphoenolpyruvate to pyruvate with the generation of ATP. Beyond this metabolic role in glycolysis, PKM2 regulates gene expression in the nucleus, phosphorylates several essential proteins that regulate major cell signaling pathways, and contribute to the redox homeostasis of cancer cells. The expression of PKM2 has been demonstrated to be significantly elevated in several types of cancer, and the overall inflammatory response. The unusual pattern of PKM2 expression inspired scientists to investigate the unrevealed functions of PKM2 and the therapeutic potential of targeting PKM2 in cancer and other disorders. Therefore, the purpose of this review is to discuss the mechanistic and therapeutic potential of targeting PKM2 with the focus on cancer metabolism, redox homeostasis, inflammation, and metabolic disorders. This review highlights and provides insight into the metabolic and non-metabolic functions of PKM2 and its relevant association with health and disease.

Keywords: Cancer; Glycolysis; Inflammation; Metabolic diseases; Oxidants; Pyruvate kinase M2; Redox homeostasis; Warburg effect.

Figure 1.. Tissue distribution of PK isoforms and the role of PKM2 in health and disease.

Mammalian pyruvate kinase has four isoforms transcribed by two different genes. PKL and PKR are both products of the PKLR gene, while PKM1 and PKM2 are products of the PKM gene. PKL is predominantly expressed in the liver and at lower levels in the pancreas, kidneys, and enterocytes. PKR is expressed in erythrocytes only. PKM1 is known to be expressed in muscle, mature spermatozoa, central nervous system, heart, and kidneys. PKM2 is the predominant isoform expressed during embryogenesis. It is also expressed in proliferative cells, and several healthy differentiated tissues, including pancreatic islets, adipose tissue, brain, kidneys, lungs, and spleen. However, the precise function of PKM2 in these tissues is largely unexplored. In addition, PKM2 is expressed in most tumors and within the liver under abnormal conditions such as cirrhosis, and hepatic steatosis. In activated macrophages, PKM2 expression is upregulated and contributes to sepsis and other inflammatory disorders. In tumors, PKM2 expression enhances aerobic glycolysis that provides cancer cells with growth advantages and has been associated with poor clinical outcomes.

Figure 2.. Post-translational regulation of PKM2 and its role in redox homeostasis.

PKM2 catalyzes the generation of pyruvate and ATP from phosphoenolpyruvate (PEP) and ADP. PKM2 enzymatic function is partially controlled by upstream glycolytic metabolites. Fructose 1.6-bisphosphate (FBP) is considered as an allosteric activator that interacts with PKM2 to enhance its active configuration, while ATP binds to the allosteric site to inhibit its catalytic activity. Post-translational modifications of PKM2 by oxidation, hydroxylation, S-nitrosylation, acetylation, methylation, succinylation/desuccinylation, or phosphorylation/dephosphorylation regulate its activity and subcellular localization. In addition, exposure to nitric oxide (NO) and H₂O₂ reduces its enzymatic activity and drive glycolytic metabolites toward the pentose phosphate pathway (PPP) to generate NADPH and reestablish redox homeostasis. EGFR/ERK2-dependent PKM2 phosphorylation or oxidation of the sulfhydryl group of cysteine-358 (C-358) inhibited PKM2, and it was proposed to promote PKM2 nuclear translocation. Additionally, endothelial NO synthase (eNOS) can directly interact with PKM2 leading to its S-nitrosylation at C-358 and inhibition of its enzymatic activity. Likewise, SIRT5 mediates the desuccinylation of PKM2 at lysine (K-498) leading to a decrease in its enzymatic activity. In contrast, AKAR1A1 inhibits the S-nitrosylation of PKM2 at cysteines-423/424 (C-423/424). The S-nitrosylation of PKM2 at C-423/424 was reported to inhibit PKM2 enzymatic activity. In addition, several kinases were demonstrated to phosphorylate PKM2 at various sites, and differentially regulate its activity and subcellular location. Oncogenic growth factors such as IGF-1, EGFR, and bFGF promote PKM2 phosphorylation in a mechanism mediated by several kinases including AKT1, ERK1/2, and JAK2. Phosphorylation of PKM2 at tyrosine-105 (Y-105) inhibits its catalytic activity by altering the tetramer and the dimer states that promote aerobic glycolysis. Similarly, the phosphorylation of PKM2 at serine-37 (S-37) and threonine-454 (T-454) induces the Warburg effect by enhancing the nuclear accumulation of PKM2. Likewise, acetylation of PKM2 at lysine-433 (K-433) enhances its nuclear localization, while its de-acetylation by SIRT6 causes PKM2 to be exported from the nucleus. In addition, methylation of PKM2 by CARM1 has been reported to be specific to the dimeric form and occurs at several arginine sites including R-445, 447, and 455. Furthermore, PKM2-hydroxylation at prolines-403/408 (P-403/408) by PHD3 was reported to promote its interaction with HIF-1α and its nuclear translocation.

Figure 3.. Direct and indirect regulation of gene expression by PKM2.

In addition to its enzymatic role, PKM2 exhibits a role in the regulation of gene expression. In its dimeric form, PKM2 can translocate to the nucleus where it interacts with HIF-1a, c-MYC, STAT3, STAT5, β-Catenin, and many others, to regulate the expression of numerous proteins involved in complex biological and biochemical processes. In addition, PKM2 can directly bind to P53 and MDM2, which leads to P53 ubiquitination and degradation. Recent studies have shown that PKM2-MDM2 interaction leads to increased MDM2 phosphorylation at serine-166 (S-166) and threonine-351 (T-351) sites and prevents its recruitment to nuclear chromatin [205]. However, these findings contradict previous reports showing that MDM2 phosphorylation at S-166 is essential for its nuclear translocation [98] and its role in regulating the expression of genes involved in serine-glycine metabolism, and redox homeostasis [97].

Figure 4.. PKM2 regulation of cancer redox homeostasis.

At early stages of cancer, increased oxidant levels play a critical role in promoting cancer initiation and development. Once the tumor is formed, cancer cells adapt a mechanism to reduce oxidant levels and to prevent the cells against oxidant-induced cell death. PKM2 enzymatic activity plays an important role in this antioxidant defense. The increase in oxidant levels leads to a decrease in PKM2 enzymatic activity and accumulation of the glycolytic metabolites; glucose-6-phosphate (G6P) and 3-phosphoglycerate (3PG), which serve as precursors for the pentose phosphate and the serine-glycine biosynthetic pathways, respectively. This result in increased production of reduced equivalents such as NADPH, and glutathione (GSH) to amplify the antioxidant defense and promote tumor growth and resistance to chemotherapy. PKM2 activators such as ML-285 and DASA-10 sensitize cancer cells to oxidant-induced cell death, in part, through a decrease in the levels of glycolytic metabolites and the overall antioxidant defense.