Opportunities for studying cancer by etabolomics: preliminary observations on tumors deficient in hypoxia-inducible factor 1

摘要

The study of metabolism, like other biological sciences, is being revolutionized by the flood of information and methods produced from studies of the genome. After the genome came the transcriptome, followed by the protcome, the totality of proteins expressed in an organism during its lifetime. Beyond the proteome lies the metabolome, which is usually defined as the totality of small molecules in an organism. Metabolomics has already been studied in the context of plants and microorganisms, and thisapproach will doubtless have a major impact on research in mammalian and human metabolism. Traditionally, metabolism has mainly been studied by focusing closely on individual metabolic pathways. Metabolomics, on the other hand, will enable us to look formulti-faceted alterations in metabolism, by analogy with gene-array studies. Indeed, metabolic control analysis has shown that the concentration changes observed in pathway metabolites can be much higher than the changes in enzyme expression that bringthem about, whereas large changes in enzyme activity may have negligible effect on pathway flux. Nuclear magnetic resonance is a simple method for performing melabolomic studies on cell or tissue extracts. A large number of metabolites can be quantifiedin a single examination, without any pre-conceptions as to the pathways that are likely to be affected. We illustrate this with a recent study (Griffiths et al., 2002) on tumors deficient in the HIF-lbeta submit. We had hypothesized that they would havedeficient angiogcnesis (because of reduced VEGF formation) and/or deficient anaerobic capacity (because of reduced expression of glucose transporters and glycolytic enzymes) but it turned out that the main effect was on ATP synthesis: their ATP content was 20% of normal. NMR spectroscopy of tumor extracts showed that the mutant cells had low levels of glycine. betaine and various cholines. This suggested that the primary effect of failure to upregulate glucose transporter and glycolytic enzyme synthesiswas to reduce provision of intermediates for anabolic synthesis of the purine rings required to make ATP. Since tumors in vivo consume glucose almost exclusively we calculate that about 0.003% is bled off from the glycolytic pathway to form carbons 4 and 5 of the purine ring of newly synthesized ATP molecules.