Please keep me 2uned to PKM2

Mechanistic and Structural Insights into Cysteine-Mediated Inhibition of Pyruvate Kinase Muscle Isoform 2

Cancer cells regulate key enzymes in the glycolytic pathway to control the glycolytic flux, which is necessary for their growth and proliferation. One of the enzymes is pyruvate kinase muscle isoform 2 (PKM2), which is allosterically regulated by various small molecules. Using detailed biochemical and kinetic studies, we demonstrate that cysteine inhibits wild-type (wt) PKM2 by shifting from an active tetramer to a mixture of a tetramer and a less active dimer/monomer equilibrium and that the inhibition is dependent on cysteine concentration. The cysteine-mediated PKM2 inhibition is reversed by fructose 1,6-bisphosphate, an allosteric activator of PKM2. Furthermore, kinetic studies using two dimeric PKM2 variants, S437Y PKM2 and G415R PKM2, show that the reversal is caused by the tetramerization of wtPKM2. The crystal structure of the wtPKM2-Cys complex was determined at 2.25 Å, which showed that cysteine is held to the amino acid binding site via its main chain groups, similar to that observed for phenylalanine, alanine, serine, and tryptophan. Notably, ligand binding studies using fluorescence and isothermal titration calorimetry show that the presence of phosphoenolpyruvate alters the binding affinities of amino acids for wtPKM2 and vice versa, thereby unravelling the existence of a functionally bidirectional coupling between the amino acid binding site and the active site of wtPKM2.

PKM2 is not required for colon cancer initiated by APC loss

BACKGROUND

Cancer cells express the M2 isoform of the glycolytic enzyme pyruvate kinase (PKM2). PKM2 expression is not required for some cancers, and PKM2 loss can promote cancer progression; however, PKM2 has been reported to be essential in other tumor contexts, including a proposed non-metabolic role in β-catenin nuclear translocation. PKM2 is expressed in colon cancers where loss of the Apc tumor suppressor results in β-catenin nuclear translocation and aberrant activation of the canonical Wnt signaling pathway. Whether PKM2 is required in this colon cancer context has not been investigated.

RESULTS

Colon tumorigenesis was induced in mice harboring conditional Apc and Pkm2 alleles, and tumor progression was monitored by serial colonoscopy. PKM2 deletion had no effect on overall survival, the number of mice that developed tumors, or the number of tumors that developed per animal. Immunohistochemical analysis demonstrated PKM2 expression in wild-type tumors and the expected loss of PKM2 expression in tumors from Pkm2 conditional mice. Loss of PKM2 resulted in pyruvate kinase M1 expression but had no effect on nuclear β-catenin staining. These findings are consistent with tumor growth and activated Wnt signaling despite PKM2 loss in this model. We also found a large fraction of human colon cancers had very low or undetectable levels of PKM2 expression.

CONCLUSIONS

PKM2 is not required for Apc-deficient colon cancer or for nuclear translocation of β-catenin in Apc-null tumor cells. These findings suggest that PKM2 expression is not required for colon tumor formation or progression.

Lack of Evidence for PKM2 Protein Kinase Activity

The role of pyruvate kinase M2 (PKM2) in cell proliferation is controversial. A unique function of PKM2 proposed to be important for the proliferation of some cancer cells involves the direct activity of this enzyme as a protein kinase; however, a detailed biochemical characterization of this activity is lacking. Using [(32)P]-phosphoenolpyruvate (PEP) we examine the direct substrates of PKM2 using recombinant enzyme and in vitro systems where PKM2 is genetically deleted. Labeling of some protein species from [(32)P]-PEP can be observed; however, most were dependent on the presence of ADP, and none were dependent on the presence of PKM2. In addition, we also failed to observe PKM2-dependent transfer of phosphate from ATP directly to protein. These findings argue against a role for PKM2 as a protein kinase.

Complex role of HIF in cancer: the known, the unknown, and the unexpected

Tumor hypoxia has long been recognized as a driving force of malignant progression and therapeutic resistance. The discovery of hypoxia-inducible transcription factors (HIFs) has greatly advanced our understanding of how cancer cells cope with hypoxic stress by maintaining bioenergetics through the stimulation of glycolysis. Until recently, however, it remained perplexing why proliferative cancer cells opt for aerobic glycolysis, an energy-inefficient process of glucose metabolism. Furthermore, the role of HIF in cancer has also become complex. In this review, we highlight recent groundbreaking findings in cancer metabolism, put forward plausible explanations to the complex role of HIF, and underscore remaining issues in cancer biology.