Pyruvate Kinase M2
PKM2 (pyruvate kinase muscle isoform 2) is an isoform of pyruvate kinase (PK; ATP, pyruvate 2-O-phosphotranferase; EC 22.214.171.124), a terminal glycolytic enzyme that catalyzes an irreversible, rate-limiting transphosphorylation reaction between phosphoenolpyruvate (PEP) and adenosine diphosphate (ADP) to generate pyruvate and ATP, accounting for net glycolytic energy (ATP) generation (Mazurek 2011). Consumption of pyruvate in a number of pathways places this enzyme at a crucial metabolic intersection. PK is ubiquitously present in simple to complex organisms. In organisms where PK is absent, its function is fulfilled by another homologue, pyruvate phosphate dikinase (PPDK) (Saavedra-Lira et al. 1998). In most bacteria and lower eukaryotes, only one form of PK is found, although many bacteria have two isozymes. In plants, PK exists in the form of cytoplasmic and plastid isoforms, and in vertebrate tissues, four different isozymes are distinguished. In the hierarchy of evolution, PKM1 and PKM2 are recognized as discrete isoforms, subsequent to the evolution of fish (Mazurek 2011).
PK Gene Isoforms, Their Expression Regulation, and Subcellular Localization
The exprssion of PKM gene is regulated by hormones (insulin, triiodothyronine-T3), cytokines (interleukin-2), mitogens, nutrient status, and hypoxia. PKM gene possesses the putative consensus DNA-binding sites of numerous transcription factors including Sp1, Sp2, HIF-1alpha, and Myc. Sp1/Sp2 and HIF1-alpha transcription factors are experimentally validated to regulate PKM2 expression (Iqbal et al. 2014a). c-Myc controlled expression of hnRNPs (heterogeneous nuclear ribonucleoproteins) has been shown to regulate the alternative splicing of PKM transcripts by repressing the presence of exon 9 and supporting the inclusion of exon 10 in PKM primary transcript to yield the PKM2 spliced isoform (Fig. 1) (David et al. 2010). Nuclear localization of PKM2 has been associated with various non-glycolytic functions, and the nuclear presence of PKM2 is regulated by epidermal growth factor (EGF), interleukin-3 (IL-3), lipopolysaccharide, and hypoxia (non-glycolytic features are discussed below).
Structure and Function of PKM2
Molecular and biochemical properties of human pyruvate kinase enzyme
Pyruvate kinase muscle isoforms
Name of gene
Size of gene
No. of amino acids
Molecular mass of subunit (Da)
Cytoplasmic and nuclear
Brain, heart, skeletal muscle, etc.
Embryonic cells, cancer cells, and proliferating cells
Monomer and tetramers
Monomer dimer and tetramer
Enzyme commission no. (EC)
EC = 126.96.36.199
FBP, serine, SAICAR
Role of PKM2 in Cancer
PKM2 in Cancer Metabolism
Non-metabolic Attributes of PKM2 in Cancer
PKM2 as a Therapeutic Target in Cancer
Considering the variety of benefits that PKM2 confers to cancer, it could be a promising therapeutic target. Several reports have suggested strategies to target PKM2 and retard cancer progression. Natural compounds like resveratrol, gemcitabine, and shikonin have been shown to inhibit PKM2 (Chen et al. 2011; Iqbal and Bamezai 2012; Pandita et al. 2014). Small interfering RNA (siRNA) against PKM2 has been shown to significantly abrogate the tumor growth and induce caspase-dependent apoptosis in cancer cells, both in vitro and in vivo. It is also reported that silencing of PKM2 may have synergistic effect with anticancer drugs (Kumar and Bamezai 2015). Synthetic small molecule inhibitors ((N-(3-carboxy-4-hydroxy)phenyl 1–2, 5-dimethylpyrole) and Arava), identified through high-throughput screen, have been shown to selectively target PKM2 activity. In addition, cell permeable structural analogs of FBP, TEPP-46 (ML265; thieno-[3,2-b]pyrrole[3,2-d]pyridazinone), and DASA-58 (ML203; substituted N,N′-diarylsulfonamide) that drive PKM2 tetramer formation in cancer cells have been shown to inhibit human lung carcinoma progression in xenograft mouse models (reviewed in Gupta et al. 2014; Iqbal et al. 2014a).
Role of PKM2 Beyond Cancer
Multifunctional Role of PKM2 and Cellular Physiology
The understanding of PKM2, in the recent past, has been overemphasized in the context of cancer pathophysiology, neglecting its significance in governing physiology at cellular and whole-body level. In the past decades, a few studies proposing the involvement of PKM2 in diverse aspects of organismal physiology, such as regulating immune metabolism and supporting immune inflammatory response against harmful stimuli and maintaining homeostasis, should have earned a principal position for PKM2 in the several cellular signaling networks. Such multifunctional roles (Gupta and Bamezai 2010) in cellular physiology are now getting unraveled for PKM2. A study by Palsson et al. has demonstrated that lipopolysaccharide (LPS)-activated macrophages tend to accumulate less active dimeric PKM2 which translocate to the nucleus and form complex with HIF1α to enhance the transactivation of a pro-inflammatory cytokine IL-1β. Further, in their in vivo studies, they demonstrated that abolishment of PKM2 or induction of PKM2 tetramer formation in LPS- or S. typhimurium-induced macrophage abrogated PKM2 nuclear translocation and HIF1α-dependent transactivation of IL-1 β expression. Thus, the study established an important role for the glycolytic enzyme PKM2 in inflammatory responses (Palsson-McDermott et al. 2015). PKM2-HIF1α axis, in addition, has been involved in the release of a potent pro-inflammatory cytokine high-mobility group box-1 (HMGB1) in activated macrophages (Yang et al. 2014). Further, PKM2-dependent transactivation of STAT3 transcription factor has been shown in colorectal carcinoma cells stimulated with LPS with an increased production of IL-1β and TNF-α. Also, PKM2 secreted by the activated neutrophils at the site of wound has recently been proposed to be a key step in early inflammatory response in wound repair. PKM2 released by infiltrated neutrophils facilitates the early wound healing processes by promoting angiogenesis at the wound site (Zhang et al. 2016).
A recent work by Zhang et al. has shown an overexpression and nuclear translocation of PKM2 in astrocytes following spinal cord injury (SCI), which was further correlated with expression of β-catenin, cyclin D1, and PCNA. Authors suggested that PKM2 overexpression and its nuclear translocation may have a significant role in inducing astrocyte proliferation following SCI, thus uncovering the relevance of PKM2 in CNS injury and repair (Zhang et al. 2015). Further, Parkin (an E3 ubiquitin ligase, mutated in most cases of autosomal recessive early onset of Parkinson’s disease) has also been shown to regulate the energy metabolism by the ubiquitination and reduction in activity of PKM1 and PKM2 in vitro and in vivo, highlighting the novel importance of Parkin in energy metabolism and providing an insight in Parkinson’s disease and its possible therapeutic intervention (Liu et al. 2016).
PKM2 in Non-cancer Human Pathologies
Several studies have unraveled a potential link of PKM2 and associated metabolic phenotypes with various pathologies. For instance, studies carried out by Bamezai and associates have indicated that heterozygous mutations in PKM2 (observed in some Bloom syndrome, BS, patients) and/or downregulation of PKM2 activity could be responsible for the inherent susceptibility of BS patients to cancer (Anitha et al. 2004; Akhtar et al. 2009; Gupta and Bamezai 2010; Gupta et al. 2010; Iqbal et al. 2014b). Although aberrations in BLM gene are the prime cause for BS, yet these fail to explain all the clinical symptoms of BS (Bamezai 1996). In addition, about 7% BS patients do not show any mutation in BLM gene (German et al. 2007), thus leaving a scope to search for other genes, like PKM2, which could play a role in explaining the clinical features of BS, such as type 2 diabetes, compromised immune response and inflammation, premature aging, male infertility, and growth retardation, besides cancer occurrence, evaluated by in vitro and in vivo mouse xenograft studies (Akhtar et al. 2009; Gupta et al. 2010; Iqbal et al. 2014b).
The deregulated expression of PKM2 and resultant aerobic glycolysis has been shown to confer defects in energy metabolism of type I myofibers with a possibility of causing myotonic dystrophy (Gao and Cooper 2013). Similar studies conducted by Shirai T et al. have attempted to connect PKM2-driven aerobic glycolysis with atherosclerotic coronary artery disease (CAD). The study shows that the overexpression and nuclear translocation of PKM2 in monocytes and macrophages of CAD patients phosphorylate and activate STAT3 transcription factor for the excessive synthesis of interleukins (IL-6 and IL-1β), which in turn drives systemic and tissue inflammation (Shirai et al. 2016). In yet another study, deregulated expression of PKM2 in activated myeloid dendritic cells (mDCs) was linked with the hematological condition known as severe aplastic anemia (SAA). Notably, work from Bearss, J. Arroyo team highlighted the association of PKM2 with placental pathological condition named preeclampsia (PE) that accounts for premature deliveries and complications for the mother and neonate (Bahr et al. 2014). Further, high level of PKM2 expression has been reported in the intestine of patients with Crohn’s disease and in the synovial tissue of patients with rheumatoid arthritis (Alves-Filho and Palsson-McDermott 2016). Altogether, these reports have strongly suggested the role of deregulated PKM2 expression in a variety of inflammatory disorders.
PKM2 is an evolutionarily conserved, prototype enzyme of glycolytic pathway. Apart from glycolysis, fine-tuning of this dynamic macromolecule, in accordance with the complex intrinsic and extrinsic cues, enables the cells to accomplish key physiological tasks that preserve cellular and whole-body homeostasis. On the other hand, deregulation of PKM2 expression and its resultant awry metabolic pathways have been shown to associate with several pathologies (Fig. 4). The recent burst of studies in the field of cancer metabolism affirms the pivotal role of PKM2 in tumor. During the process of differentiation, PKM2 is replaced by PKM1 or other PK isoform depending upon the differentiated cell type. Remarkably, however, cancer cells revert to PKM2 expression owing to their reliance on this multifaceted enzyme which benefits cancer. The metabolic and non-metabolic support that PKM2 status provides to cancer reiterates the therapeutic value of this enzyme. In addition, the deregulation of PKM2 has also been reported in association with several pathologies, e.g., coronary artery disease, inflammatory bowel disease, myotonic dystrophy, etc., providing a wide scope in designing the therapeutic strategies based on targeting of PKM2 in cancer and treating individuals with the discussed pathologies.
- Iqbal MA, Siddiqui FA, Chaman N, Gupta V, Kumar B, Gopinath P, et al. Missense mutations in pyruvate kinase M2 promote cancer metabolism, oxidative endurance, anchorage independence, and tumor growth in a dominant negative manner. J Biol Chem. 2014b;289(12):8098–105.PubMedPubMedCentralCrossRefGoogle Scholar
- Palsson-McDermott EM, Curtis AM, Goel G, Lauterbach MA, Sheedy FJ, Gleeson LE, et al. Pyruvate kinase M2 regulates Hif-1alpha activity and IL-1beta induction and is a critical determinant of the warburg effect in LPS-activated macrophages. Cell Metab. 2015;21(1):65–80.PubMedPubMedCentralCrossRefGoogle Scholar