INTERMEDIARY METABOLISM IN CANCER

MOLECULAR ONCOLOGY2015

Michael Lea

Intermediary Metabolism - Lecture Outline

• Glycolysis and respiration in cancer cells• Convergence and deletions • Correlation of biochemical parameters with tumor

growth• Polyamines

GLYCOLYSIS AND RESPIRATION IN CANCER CELLS

The first metabolic pathways to be studied in cancer cells were those of glycolysis and cell respiration. Otto Warburg studied these parameters using tissue slices incubated in a bicarbonate buffer in flasks attached to a manometer. By incubating in media gassed with either 95% oxygen/5% CO2 or 95% nitrogen/5% CO2 it was possible to measure glycolysis under aerobic or anaerobic conditions. The production of lactic or pyruvic acids causes the release of CO2 from the bicarbonate buffer. Quotients were measured for aerobic glycolysis (QL O2), anaerobic glycolysis (QL N2) and respiratory activity (QO2).

The data indicated that, in general, glycolysis was greater in malignant than in non-malignant tissues. This was more marked under aerobic than anaerobic conditions. This difference suggested that the Pasteur effect was greater in normal tissues. It should be noted that there is an overlap of values in Warburg’s data.

For debate see Science, 124: 267-272, 1956

CONVERGENCE AND DELETIONS

• Warburg concluded that cancer originated from an irreversible injury of respiration

• Greenstein noted that many tumors showed a convergence in their metabolic patterns

• In 1947 the Millers suggested that carcinogenesis results from “a permanent alteration or loss of proteins essential for the control of growth.”

• Studies by Weber on the Morris series of chemically induced hepatomas in rats led to the Molecular Correlation Concept in which some biochemical parameters are viewed as correlating with tumor growth. (Reference; G. Weber, New England J. Med. 296: 486 and 541, 1977)

UPREGULATION OF GLYCOLYSIS LEADS TO MICROENVIRONMENTAL ACIDOSIS

Clinical use of 18fluorodeoxyglucose positron-emission tomography (FdG PET) has demonstrated that increased glucose uptake is observed in most human cancer.

Increased FdG uptake occurs because of upregulation of glucose transporters, notably GLUT1 and GLUT3, and results in increased glycolysis.

Increased glycolysis results in microenvironmental acidosis and requires further adaptation through somatic evolution to phenotypes resistant to acid-induced toxicity.

Reference: R.A. Gatenby and R.J. Gillies. Why do cancers have high aerobic glycolysis? Nature Reviews Cancer 4: 891-899, 2004.

INHIBITING GLYCOLYSIS• A lack of tumor-specific inhibitors of glycolysis has

historically prevented glycolysis being used as a chemotherapeutic target.

• Glycolysis can be activated by an increase in the concentration of fructose 2,6-bisphosphate which activates the rate-limiting enzyme phosphofructokinase 1.

• Fructose 2,6-bisphosphate is produced by the bifunctional enzyme phosphofructokinase 2/ fructose 2,6-bisphosphatase (PFKFB).

• The inducible PFKFB3 isozyme is constitutively expressed by many tumor cells.

• A small molecule inhibitor of PFKFB3 has been reported to inhibit the growth of tumors in mice.

• Reference: Clem et al., Mol. Cancer Ther. 7: 110-120, 2008

Levine and Puzio-KuterScience 330:1340-1344.2010

TIGAR: TP53 induced glycolysis and apoptosis regulator

ERK1/2-dependent phosphorylation and nuclear translocation of PKM2 promotes the Warburg effect

Pyruvate kinase M2 (PKM2) is upregulated in multiple cancer types and contributes to the Warburg effect by unclear mechanisms. EGFR-activated ERK2 binds directly to PKM2 Ile 4291Leu 431 through the ERK2 docking groove and phosphorylates PKM2 at Ser 37, but does not phosphorylate PKM1. Phosphorylated PKM2 Ser 37 recruits PIN1 for cis-trans isomerization of PKM2, which promotes PKM2 binding to importin a5 and translocating to the nucleus. Nuclear PKM2 acts as a coactivator of beta-catenin to induce c-Myc expression, resulting in the upregulation of GLUT1, LDHA and, in a positive feedback loop, PTB-dependent PKM2 expression. Replacement of wild-type PKM2 with a nuclear translocation-deficient mutant (S37A) blocks the EGFR-promoted Warburg effect and brain tumour development in mice. In addition, levels of PKM2 Ser 37 phosphorylation correlate with EGFR and ERK1/2 activity in human glioblastoma specimens. These findings suggest the importance of nuclear functions of PKM2 in the Warburg effect and tumorigenesis.

Reference: Yang, W et al., Nature Cell Biol. 14: 1295 (2012)

IDH mutations and cancer

  Mutations in isocitrate dehydrogenase 1 and 2 result in the formation of 2-hydroxyglutarate (2HG) instead of alpha-ketoglutarate. 2HG is a competitive inhibitor of alpha-ketoglutarate-dependent dioxygenases. Dioxygenases have an important role in demethylation reactions for histones and DNA causing hypermethylation in glioma and AML.

• Reference: Yen KE and Schenkein DP: Cancer-associated isocitrate dehydrogenase mutations. The Oncologist 17: 5-5, 2012

Fumarate hydratase

  Low activities of fumarate hydratase (fumarase) drives a metabolic shift to aerobic glycolysis in some kidney tumors and thereby enhances the Warburg effect in which aerobic glycolysis tends to be increased in cancer cells

POLYAMINES

Polyamines are organic cations formed by the enzymatic decarboxylation of ornithine to yield putrescine and by further additions from decarboxylated S-adenosyl methionine to form spermidine and spermine.

Ornithine decarboxylase and polyamine content are increased in many carcinomas including skin and colon cancer.

DFMO (difluoromethylornithine) is an inhibitor of ornithine decarboxylase and has some antitumor action.

Polyamines work at least in part by regulating specific gene expression

Reference: E.W. Gerner and F.L. Meyskens. Polyamines and cancer: old molecules, new understanding. Nature reviews Cancer 4: 781-792, 2004.

INTERMEDIARY METABOLISM - SUGGESTED READING

• R.W. Ruddon and R.W. Kufe, In Holland-Frei Cancer Medicine - 8th Ed, Part II, Section 1, 9. Biochemistry of Cancer (2010)

• A.J. Levine and A.M. Puzio-Kuter. The control of the metabolic switch in cancers by oncogenes and tumor suppressor genes. Science 330: 1340-1344, 2010.