The Role of Pyruvate Kinase M2 in Cancer Metabolism

Abstract

The M2 isoform of pyruvate kinase is expressed preferentially in cancer cells over other pyruvate kinase isoforms. PKM2 is unique in its ability to be regulated allosterically by nutrients and growth signaling pathways, allowing cells to adapt their metabolic program to match physiological needs in different environments. Here, we discuss the role of pyruvate kinase M2 in glioma and in cancer metabolism.

Introduction

Cancer cells proliferate at a much faster rate than normal cells and, as a result, have altered metabolism to meet the biosynthetic demands of rapid proliferation. The idea that cancer metabolism differs from most normal tissue metabolism has been appreciated since 1924, with the discovery of the Warburg effect: the observation that cancer cells have a phenotype of increased lactic acid formation even when there is sufficient oxygen 36. This was initially interpreted as a defect in mitochondrial function 35; however, more recent data have suggested that oxidative metabolism and mitochondrial function remain important for many cancers 21, 22, 37, 38. A definitive explanation for the Warburg effect remains controversial, but it has been suggested that this and other metabolic alterations associated with cancer cells reflect a program to support production of biomass necessary to produce daughter cells 23, 31.

The M2 isoform of pyruvate kinase (PKM2), the enzyme that catalyzes the final step of glycolysis, is preferentially expressed in cancer cells over the three other isoforms of pyruvate kinase and is unique in its ability to be allosterically regulated by multiple signaling inputs 11, 23. PKM2 can bind to proteins phosphorylated on tyrosine residues in response to growth factor signaling, resulting in enzyme inhibition 5. It is hypothesized that expressing the PKM2 isoform is advantageous for cancer cells because pyruvate kinase inhibition allows the channeling of glycolytic intermediates into the pentose phosphate pathway for nucleotide biosynthesis 9 and redox regulation 1. This review focuses on the different pyruvate kinase isoforms and discusses the importance of PKM2 in cancer metabolism.

Pyruvate Kinase: Isoforms, Differential Expression and Kinetics

Pyruvate kinase is an important enzyme for glucose metabolism, catalyzing the final step of glycolysis by transferring the phosphoryl group of phosphoenolpyruvate (PEP) onto ADP to form ATP and pyruvate. There are four different mammalian isozymes of pyruvate kinase: PKL, PKR, PKM1 and PKM2 14. PKL and PKR are produced by the PKLR gene and are expressed in the liver and red blood cells, respectively 14. All other tissues express either PKM1 or PKM2, which are alternative splice products of the PKM gene 28. PKM1 is predominantly expressed by adult tissues that require high levels of ATP, such as muscle and brain, whereas PKM2 is expressed during fetal development and in various tissues in the adult such as the lungs, spleen and in proliferating cells 14, 23. PKM2 is also expressed in most cancers 15, 23. There are a total of 12 exons in the human, rat and mouse PKM genes 28, 30. Exons 9 and 10 determine the isoform that is expressed through alternative mRNA splicing to either include exon 9 to generate PKM1, or include exon 10 to generate PKM2 28.

Pyruvate kinase enzymes function as tetramers in most species, including mammals, with the tetramers composed of identical subunits configured as a dimer of dimers. The pyruvate kinase monomer is composed of three main domains and a small N‐terminal domain 26, 29. The enzyme is most active in the tetramer form, and PKM1 is a constitutive tetramer and thus also exhibits high activity 8, 25. The PKM2 tetramer, on the contrary, is reversibly associated and dissociated depending on the presence of fructose‐1‐6‐bisphosphate (FBP) 3. FBP is an upstream glycolytic intermediate that activates PKM2 by stabilizing the active tetramer form 2. FBP binding is inhibited by the interaction of PKM2 with tyrosine phosphorylated proteins, providing a mechanism for growth signaling to regulate PKM2 activity 5, 12. PKM2 activity is regulated by a number of other allosteric regulators that can either enhance or inhibit its activity, such as serine, phenylalanine and SAICAR 4, 11, 17, 25.

Role of PKM2 in Cancer

The preferential expression of PKM2 by cancer cells suggests that this isoform provides a selective advantage over the other isoforms in the context of malignancy. Different levels of pyruvate kinase activity can impact cancer cell proliferation, and reduced PKM2 activity resulting from either phosphotyrosine signaling or acetylation favors proliferation in some contexts 5, 12, 20. It has also been shown that switching pyruvate kinase expression in cancer cells from the M2 isoform to the constitutively active M1 isoform leads to reduced primary cell proliferation and a decreased ability to form tumors 6, 19. In addition, PKM1 expression or chemical activation of PKM2 with drugs leads to suppression of tumor growth 2, 15. Heterogeneous point mutations have also been reported for PKM2 in human cancers that result in decreased pyruvate kinase activity, and genetic loss of PKM2 can favor tumor progression in some tissues 15. Taken together, these data support a model where PKM2 is advantageous for cancer cells because downregulation of pyruvate kinase activity favors cell proliferation.

This hypothesis, however, does not address the reasons why cancer cells preferentially express PKM2 rather than inactivate pyruvate kinase through mechanisms such as gene mutation, deletion or epigenetic silencing. One possible explanation is that PKM2 has some noncanonical function that is important in cancer cells. Several studies have suggested noncanonical PKM2 activities, such that the enzyme acts as a transcriptional coactivator or as a protein kinase with the ability to transfer the phosphate of PEP directly to protein substrates to promote mitotic progression 10, 16, 39; however, the ability of PKM2 to act directly as a protein kinase has been questioned 13. Decreased pyruvate kinase activity appears to be important for nucleotide synthesis 19, and activation of PKM2 can synergize with serine deprivation to slow down proliferation of some cancer cells 18. While the effect of decreased pyruvate kinase activity appears to be more complicated than the accumulation of upstream intermediates in glycolysis 19, these data support the notion that regulation of pyruvate kinase activity is necessary to promote a metabolic program that is conducive to cell proliferation. They are also consistent with the evidence that PKM2 is not required for tumor growth or for cancer proliferation in vivo 7, 15, 34.

When PKM2 is selectively deleted in a breast cancer model, some tumor cells switch to expressing PKM1 instead. The finding that this switch occurs only in nonproliferating tumor cells suggests that while cancer cells require low pyruvate kinase activity during cell proliferation, they may require high levels of activity in other points of their life cycle. PKM2 would then be selected for expression because of its ability to be regulated into either high‐activity or low‐activity states to allow for adaptation to different metabolic needs. A tumor cell is likely to experience periods of low oxygen or lack of exogenous growth signals, which can limit proliferation 32, 33. Cell survival in these conditions requires a metabolic program focused more on ATP production rather than building biomass, and therefore, nonproliferating tumor cells require more pyruvate kinase activity, whereas proliferating cells do not. The existence of loss‐of‐function heterozygous mutations of PKM2 in human cancers supports this idea: reducing pyruvate kinase activity is beneficial during proliferation, but retention of a functional copy allows cells to upregulate pyruvate kinase activity during periods of stress.

PKM2 and Glioma

The pyruvate kinase isoform expressed in the brain is PKM1; however, like most cancers, brain malignancies express PKM2 23. Specifically in human glioma, PKM2 expression is upregulated in a grade‐specific manner, and all gliomas have lowered levels of PKM1 expression as well as decreased pyruvate kinase activity compared with normal brain tissue 27. Aggressive grade IV glioblastoma multiforme shows the greatest increase in PKM2 RNA and protein expression, whereas lower grade gliomas have modestly increased levels relative to normal brain 27. Loss of PKM2 results in glioma cell death in spheroid culture models 24, and a role for non‐glycolytic PKM2 functions has been reported specifically for glioma models 16, 39. Whether PKM2 plays a unique role in primary brain cancers or is regulated to allow proliferative metabolism in this tissue remains an open question.

Conclusion

The M2 isoform of pyruvate kinase differs from the M1 isoform because it can be allosterically regulated to assume a high‐ or low‐activity state. Cancer cells preferentially express PKM2 because the ability to downregulate pyruvate kinase activity favors a metabolic program that is conducive to cell proliferation. However, nonproliferating cancer cells appear to require high pyruvate kinase activity; therefore, PKM2 expression provides the flexibility of adjusting enzyme activity to adapt to both proliferative and nonproliferative conditions.