Pyruvate kinase activity and isozyme composition in normal fibrous tissue and fibroblastic proliferations

Extracellular PKM2 facilitates organ-tissue fibrosis progression

Persistent activation of fibroblasts and resistance of myofibroblasts to turnover play important roles in organ-tissue fibrosis development and progression. The mechanism that mediates apoptosis resistance of myofibroblasts is not understood. Here, we report that myofibroblasts express and secrete PKM2. Extracellular PKM2 (EcPKM2) facilitates progression of fibrosis by protecting myofibroblasts from apoptosis. EcPKM2 upregulates arginase-1 expression in myofibroblasts and therefore facilitates proline biosynthesis and subsequent collagen production. EcPKM2 interacts with integrin αvβ3 on myofibroblasts to activate FAK-PI3K signaling axis. Activation of FAK-PI3K by EcPKM2 activates downstream NF-κB survival pathway to prevent myofibroblasts from apoptosis. On the other hand, activation of FAK- PI3K by EcPKM2 suppresses PTEN to subsequently upregulate arginase-1 in myofibroblasts. Our studies uncover an important mechanism for organ fibrosis progression. More importantly, an antibody disrupting the interaction between PKM2 and integrin αvβ3 is effective in reversing fibrosis, suggesting a possible therapeutic strategy and target for treatment of organ fibrosis.

Pyruvate Kinase M2 Coordinates Metabolism Switch between Glycolysis and Glutaminolysis in Cancer Cells

Cancer cells alter their nutrition metabolism to cope the stressful environment. One important metabolism adjustment is that cancer cells activate glutaminolysis in response to the reduced carbon from glucose entering into the TCA cycle due to inactivation of several enzymes in glycolysis. An important question is how the cancer cells coordinate the changes of glycolysis and glutaminolysis. In this report, we demonstrate that the pyruvate kinase inactive dimer PKM2 facilitates activation of glutaminolysis. Our experiments show that growth stimulations promote PKM2 dimer. The dimer PKM2 plays a role in regulation of glutaminolysis by upregulation of mitochondrial glutaminase I (GLS-1). PKM2 dimer regulates the GLS-1 expression by controlling internal ribosome entry site (IRES)-dependent c-myc translation. Growth stimulations promote PKM2 interacting with c-myc IRES-RNA, thus facilitating c-myc IRES-dependent translation. Our study reveals an important linker that coordinates the metabolism adjustment in cancer cells.

Pyruvate kinase M2: A simple molecule with complex functions

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.

A critical review of the role of M2PYK in the Warburg effect

It is becoming generally accepted in recent literature that the Warburg effect in cancer depends on inhibition of M₂PYK, the pyruvate kinase isozyme most commonly expressed in tumors. We remain skeptical. There continues to be a general lack of solid experimental evidence for the underlying idea that a bottle neck in aerobic glycolysis at the level of M₂PYK results in an expanded pool of glycolytic intermediates (which are thought to serve as building blocks necessary for proliferation and growth of cancer cells). If a bottle neck at M₂PYK exists, then the remarkable increase in lactate production by cancer cells is a paradox, particularly since a high percentage of the carbons of lactate originate from glucose. The finding that pyruvate kinase activity is invariantly increased rather than decreased in cancer undermines the logic of the M₂PYK bottle neck, but is consistent with high lactate production. The "inactive" state of M₂PYK in cancer is often described as a dimer (with reduced substrate affinity) that has dissociated from an active tetramer of M₂PYK. Although M₂PYK clearly dissociates easier than other isozymes of pyruvate kinase, it is not clear that dissociation of the tetramer occurs in vivo when ligands are present that promote tetramer formation. Furthermore, it is also not clear whether the dissociated dimer retains any activity at all. A number of non-canonical functions for M₂PYK have been proposed, all of which can be challenged by the finding that not all cancer cell types are dependent on M₂PYK expression. Additional in-depth studies of the Warburg effect and specifically of the possible regulatory role of M₂PYK in the Warburg effect are needed.

Virtual high-throughput screens identifying hPK-M2 inhibitors: Exploration of model extrapolation

Glycolysis with PK-M2 occurs typically in anaerobic conditions and atypically in aerobic conditions, which is known as the Warburg effect. The Warburg effect is found in many oncogenic situations and is believed to provide energy and biomass for oncogenesis to persist. The work presented targets human PK-M2 (hPK-M2) in a virtual high-throughput screen to identify new inhibitors and leads for further study. In the initial screen, one of the 12 candidates selected for experimental validation showed biological activity (hit-rate = 8.13%). In the second screen with retrained models, six of 11 candidates selected for experimental validation showed biological activity (hit-rate: 54.5%). Additionally, four different scaffolds were identified for further analysis when examining the tested candidates and compounds in the training data. Finally, extrapolation was necessary to identify a sufficient number of candidates to test in the second screen. Examination of the results suggested stepwise extrapolation to maximize efficiency.

Mammalian Target of Rapamycin (mTOR) Regulates Transforming Growth Factor-β1 (TGF-β1)-Induced Epithelial-Mesenchymal Transition via Decreased Pyruvate Kinase M2 (PKM2) Expression in Cervical Cancer Cells

BACKGROUND Epithelial-mesenchymal transition (EMT) plays an important role in cancer tumorigenesis. Transforming growth factor β1 (TGF-β1) can induced EMT, which could increase tumor migration and invasion. Moreover, recent studies have been proven that mammalian target of rapamycin (mTOR) is a critical regulator of EMT. We investigated the mechanisms of mTOR in transforming growth factor β1 (TGF-β1)-induced EMT in cervical cancer cells. MATERIAL AND METHODS HeLa and SiHa cells were treated with 10 ng/ml TGF-β1 to induce EMT. Then, they were treated with or without rapamycin. CCK8 assay was performed to determine cell proliferation. Cell migration was detected by wound-healing assay; apoptosis was analyzed by flow cytometry; mTOR inhibitors inhibited mTOR pathway to assess the expression of E-cadherin, Vimentin STAT3, Snail2, p-p70s6k, and PKM2 expression. RESULTS TGF-β1 promoted proliferation and migration, and attenuated apoptosis in cervical carcinoma cells. Rapamycin abolished TGF-β1-induced EMT cell proliferation and migration and reversed TGF-β1-induced EMT. E-cadherin were suppressed, whereas Vimentin and PKM2 were increased in HeLa and SiHa cells after stimulation with TGF-β1. Moreover, mTOR was activated in the process of TGF-β1-induced EMT. Rapamycin inhibited the phosphorylation of p70s6k. Furthermore, inhibition of the mTOR pathway decreased PKM2 expression. CONCLUSIONS Inhibition of the mTOR pathway abolished TGF-β1-induced EMT and reduced mTOR/p70s6k signaling, which downregulated PKM2 expression. Our results provide novel mechanistic insight into the anti-tumor effects of inhibition of mTOR.

Desuccinylation of pyruvate kinase M2 by SIRT5 contributes to antioxidant response and tumor growth

Tumor cells trends to express high level of pyruvate kinase M2 (PKM2). The inhibition of PKM2 activity is needed for antioxidant response by diverting glucose flux into the pentose phosphate pathway and thus generating sufficient reducing potential. Here we report that PKM2 is succinylated at lysine 498 (K498) and succinylation increases its activity. SIRT5 binds to, desuccinylates and inhibits PKM2 activity. Increased level of reactive oxygen species (ROS) decreases both the succinylation and activity of PKM2 by increasing its binding to SIRT5. Substitution of endogenous PKM2 with a succinylation mimetic mutant K498E decreases cellular NADPH production and inhibits cell proliferation and tumor growth. Moreover, inhibition of SIRT5 suppresses tumor cell proliferation through desuccinylation of PKM2 K498. These results reveal a new mechanism of PKM2 modification, a new function of SIRT5 in response to oxidative stress which stimulates cell proliferation and tumor growth, and also a potential target for clinical cancer research.

PKM2 released by neutrophils at wound site facilitates early wound healing by promoting angiogenesis

Neutrophils infiltration/activation following wound induction marks the early inflammatory response in wound repair. However, the role of the infiltrated/activated neutrophils in tissue regeneration/proliferation during wound repair is not well understood. Here, we report that infiltrated/activated neutrophils at wound site release pyruvate kinase M2 (PKM2) by its secretive mechanisms during early stages of wound repair. The released extracellular PKM2 facilitates early wound healing by promoting angiogenesis at wound site. Our studies reveal a new and important molecular linker between the early inflammatory response and proliferation phase in tissue repair process.

Role of pyruvate kinase M2 in transcriptional regulation leading to epithelial-mesenchymal transition

Pyruvate kinase M2 (PKM2) is an alternatively spliced variant of the pyruvate kinase gene that is preferentially expressed during embryonic development and in cancer cells. PKM2 alters the final rate-limiting step of glycolysis, resulting in the cancer-specific Warburg effect (also referred to as aerobic glycolysis). Although previous reports suggest that PKM2 functions in nonmetabolic transcriptional regulation, its significance in cancer biology remains elusive. Here we report that stimulation of epithelial-mesenchymal transition (EMT) results in the nuclear translocation of PKM2 in colon cancer cells, which is pivotal in promoting EMT. Immunoprecipitation and LC-electrospray ionized TOF MS analyses revealed that EMT stimulation causes direct interaction of PKM2 in the nucleus with TGF-β-induced factor homeobox 2 (TGIF2), a transcriptional cofactor repressor of TGF-β signaling. The binding of PKM2 with TGIF2 recruits histone deacetylase 3 to the E-cadherin promoter sequence, with subsequent deacetylation of histone H3 and suppression of E-cadherin transcription. This previously unidentified finding of the molecular interaction of PKM2 in the nucleus sheds light on the significance of PKM2 expression in cancer cells.

Pyruvate kinase M2 in blood circulation facilitates tumor growth by promoting angiogenesis

It is long known that pyruvate kinase isoform M2 (PKM2) is released into the circulation of cancer patients. The PKM2 levels in patients have been suggested as a diagnostic marker for many types of cancers. However, it is not known how PKM2 is released in the blood, and whether the circulating PKM2 has any physiological function(s) in tumor progression. In this report, we demonstrate that PKM2 in the blood facilitates tumor growth by promoting tumor angiogenesis. Our experiments show that PKM2 promotes tumor angiogenesis by increasing endothelial cell proliferation, migration, and cell-ECM adhesion. Only the dimeric PKM2 possess the activity in promoting tumor angiogenesis, which is consistent with the observations that PKM2 in circulation of cancer patients is a dimer form.

Activators of PKM2 in cancer metabolism

Pyruvate kinase converts phosphoenolpyruvate to pyruvate, catalyzing the rate-limiting step of glycolysis. The M1 isoenzyme of pyruvate kinase (PKM1) is found in adult tissues; whereas, PKM2 is a splicesome variant found in embryonic and cancer cells. PKM2 expression in malignant cells is a result of the tumor microenvironment and is responsible for maintaining a glycolytic phenotype. PKM2 has other nonmetabolic functions in malignant cells, including transcriptional coactivation and protein kinase activity. PKM2 activators have antitumor properties by inducing tetramerization of two PKM2 dimers causing PKM2 to function like PKM1. Restoring PKM2 to PKM1-like levels of activity causes reversal of the Warburg effect in cancer cells. PKM2 activators have therapeutic potential in the treatment of cancer and other metabolic diseases.

Reciprocal regulation of protein kinase and pyruvate kinase activities of pyruvate kinase M2 by growth signals

Pyruvate kinase isoform M2 (PKM2) is an enzyme-catalyzing conversion of phosphoenolpyruvate to pyruvate in the glycolysis pathway. It was demonstrated that PKM2 interacts with tyrosine phosphopeptide, and the interaction with the tyrosine phosphopeptide affects the pyruvate kinase activity of PKM2. Our experiments suggest that PKM2 is also an active protein kinase (Gao, X., Wang, H., Yang, J. J., Liu, X., and Liu, Z. R. (2012) Mol. Cell 45, 598-609). We report here that growth signals reciprocally regulate the pyruvate kinase and protein kinase activities of PKM2 by different mechanisms. On the one hand, growth signals induce protein tyrosine phosphorylations. The tyrosine-phosphorylated protein(s) regulates the conversion of pyruvate kinase and protein kinase of PKM2 by directly interacting with PKM2. Binding of the tyrosyl-phosphorylated proteins at the fructose 1,6-bisphosphate-binding site converts the tetrameric PKM2 to a dimer. On the other hand, growth stimulations also lead to PKM2 phosphorylation, which consequently regulates the conversion of protein kinase and pyruvate kinase activities. Growth factor stimulations significantly increase the dimer/tetramer PKM2 ratio in cells and consequently activate the protein kinase activity of PKM2. Our study suggests that the conversion between the pyruvate kinase and protein kinase activities of PKM2 may be an important mechanism mediating the effects of growth signals in promoting cell proliferation.

Stable isotope-resolved metabolomics and applications for drug development

Advances in analytical methodologies, principally nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), during the last decade have made large-scale analysis of the human metabolome a reality. This is leading to the reawakening of the importance of metabolism in human diseases, particularly cancer. The metabolome is the functional readout of the genome, functional genome, and proteome; it is also an integral partner in molecular regulations for homeostasis. The interrogation of the metabolome, or metabolomics, is now being applied to numerous diseases, largely by metabolite profiling for biomarker discovery, but also in pharmacology and therapeutics. Recent advances in stable isotope tracer-based metabolomic approaches enable unambiguous tracking of individual atoms through compartmentalized metabolic networks directly in human subjects, which promises to decipher the complexity of the human metabolome at an unprecedented pace. This knowledge will revolutionize our understanding of complex human diseases, clinical diagnostics, as well as individualized therapeutics and drug response. In this review, we focus on the use of stable isotope tracers with metabolomics technologies for understanding metabolic network dynamics in both model systems and in clinical applications. Atom-resolved isotope tracing via the two major analytical platforms, NMR and MS, has the power to determine novel metabolic reprogramming in diseases, discover new drug targets, and facilitates ADME studies. We also illustrate new metabolic tracer-based imaging technologies, which enable direct visualization of metabolic processes in vivo. We further outline current practices and future requirements for biochemoinformatics development, which is an integral part of translating stable isotope-resolved metabolomics into clinical reality.

Pyruvate kinase M2 regulates gene transcription by acting as a protein kinase

Pyruvate kinase isoform M2 (PKM2) is a glycolysis enzyme catalyzing conversion of phosphoenolpyruvate (PEP) to pyruvate by transferring a phosphate from PEP to ADP. We report here that PKM2 localizes to the cell nucleus. The levels of nuclear PKM2 correlate with cell proliferation. PKM2 activates transcription of MEK5 by phosphorylating stat3 at Y705. In vitro phosphorylation assays show that PKM2 is a protein kinase using PEP as a phosphate donor. ADP competes with the protein substrate binding, indicating that the substrate may bind to the ADP site of PKM2. Our experiments suggest that PKM2 dimer is an active protein kinase, while the tetramer is an active pyruvate kinase. Expression of a PKM2 mutant that exists as a dimer promotes cell proliferation, indicating that protein kinase activity of PKM2 plays a role in promoting cell proliferation. Our study reveals an important link between metabolism alteration and gene expression during tumor transformation and progression.

Rhabdomyosarcoma cells show an energy producing anabolic metabolic phenotype compared with primary myocytes

BACKGROUND

The functional status of a cell is expressed in its metabolic activity. We have applied stable isotope tracing methods to determine the differences in metabolic pathways in proliferating Rhabdomysarcoma cells (Rh30) and human primary myocytes in culture. Uniformly 13C-labeled glucose was used as a source molecule to follow the incorporation of 13C into more than 40 marker metabolites using NMR and GC-MS. These include metabolites that report on the activity of glycolysis, Krebs' cycle, pentose phosphate pathway and pyrimidine biosynthesis.

RESULTS

The Rh30 cells proliferated faster than the myocytes. Major differences in flux through glycolysis were evident from incorporation of label into secreted lactate, which accounts for a substantial fraction of the glucose carbon utilized by the cells. Krebs' cycle activity as determined by 13C isotopomer distributions in glutamate, aspartate, malate and pyrimidine rings was considerably higher in the cancer cells than in the primary myocytes. Large differences were also evident in de novo biosynthesis of riboses in the free nucleotide pools, as well as entry of glucose carbon into the pyrimidine rings in the free nucleotide pool. Specific labeling patterns in these metabolites show the increased importance of anaplerotic reactions in the cancer cells to maintain the high demand for anabolic and energy metabolism compared with the slower growing primary myocytes. Serum-stimulated Rh30 cells showed higher degrees of labeling than serum starved cells, but they retained their characteristic anabolic metabolism profile. The myocytes showed evidence of de novo synthesis of glycogen, which was absent in the Rh30 cells.

CONCLUSION

The specific 13C isotopomer patterns showed that the major difference between the transformed and the primary cells is the shift from energy and maintenance metabolism in the myocytes toward increased energy and anabolic metabolism for proliferation in the Rh30 cells. The data further show that the mitochondria remain functional in Krebs' cycle activity and respiratory electron transfer that enables continued accelerated glycolysis. This may be a common adaptive strategy in cancer cells.

Comparison of pyruvate kinase variants from breast tumor and normal breast

BACKGROUND

Pyruvate kinase isozymes in human breast tumor tissue were compared in this study with normal human breast tissue. Two forms of pyruvate kinase present in normal and tumor human breast were purified by ammonium sulfate precipitation, dialysis, gel filtration, ion exchange, and affinity chromatography. Molecular weight of the native enzyme was determined.

METHODS

Presence of pyruvate kinase activity was examined in normal and tumor breast tissues. Pyruvate kinase was purified with Sephadex DEAE-50, Sepharyl S-200, and Blue Sepharose CL-6B chromatography. Spectrophotometric methods were used to determine activities of pyruvate kinase.

RESULTS

Molecular weights of fractions I and II as determined by gel filtration on Sepharyl S-200 were 135,000 Da, 260,000 Da in normal breast tissue, and 72,000 Da, 250,000 Da in tumor breast tissue, respectively. Fractions I and II of pyruvate kinase may be purified approximately 1,591-fold, 636.4-fold in normal breast tissue and 219-fold, 318-fold in tumor breast tissue, respectively. Pyruvate kinase activity in tumor tissue was found higher than in normal tissue. Only tumor fraction II showed tumor-specific sensitivity to L-cysteine. L-phenylalanine inhibited both fractions I and II of normal breast and fraction I of tumor breast, but not fraction II of pyruvate from tumor. ATP inhibited normal and tumor fraction I of pyruvate kinase. The influence of ATP on enzyme activity from normal and tumor fraction II depended upon its concentration.

CONCLUSIONS

It was thought that isozymes of pyruvate kinase from human breast tissue might be M1 and M2 isozymes when compared with those of other tissue pyruvate kinase isoenzymes. Fraction II from breast tumor represented different sensitivity to L-cysteine, L-phenylalanine, and specific activity in comparison with fraction II from normal breast. Different kinetic behavior of fractions in the human breast tumors may support the concept of an isozyme shift.

Aggressive fibromatosis (non-familial desmoid tumour): therapeutic problems and the role of adjuvant radiotherapy

Aggressive fibromatosis is a rare soft tissue tumour with a high tendency to local recurrence, even after apparently adequate resection. Wide local excision with a margin of at least 3 cm, depending on the anatomical location, should be performed to improve rates of recurrent disease. There is no consensus concerning the role of radiotherapy in the treatment of these lesions. The clinical findings of 39 cases diagnosed between 1972 and 1991 were reviewed retrospectively. Local control was effected in 19 of 32 patients treated with surgery alone after a median (range) follow-up of 72 (18-236) months. There were 40 cases of recurrent fibromatosis in 15 patients. Local control was obtained in 13 of 14 patients who received radiotherapy using a wide-field technique and doses of more than 50 Gy over a period of 5 weeks after marginal or incomplete resection of primary or recurrent lesions (P < 0.001). The results suggest that in a selected group of patients with aggressive fibromatosis radiotherapy may effectively achieve control of residual disease after surgery without marked disfigurement and loss of function.

Expression of glucose transporter isoforms (GLUT1, GLUT2) and activities of hexokinase, pyruvate kinase, and malic enzyme in preneoplastic and neoplastic rat renal basophilic cell lesions

Sequential changes in the expression of two glucose transporter isoforms (GLUT1, GLUT2), and in the activities of hexokinase, pyruvate kinase and malic enzyme during the development of rat renal basophilic cell tumors were studied using histochemical techniques. Early basophilic cell tubules are similar to proximal convoluted tubules (PCT) in their overall histochemical pattern, particularly in the expression of glucose transporters, suggesting that basophilic cell tubules and tumors derived from them arise from PCT. In comparison with PCT, basophilic cell tubules show slightly increased activities of all the enzymes studied. In basophilic cell tumors, markedly elevated hexokinase and pyruvate kinase activities are accompanied by a considerable reduction in the expression of GLUT2. GLUT1 expression is not found in basophilic cell tubules or PCT. Small basophilic cell tumors also do not express GLUT1, but GLUT1 is regularly expressed in several cell layers surrounding necrotic areas within large basophilic cell tumors. Our results indicate that increased glycolytic activity and reduced GLUT2 expression take place during the development of renal basophilic cell tumors.