|Topic:||Glucose Metabolism and Cancer .|
|Details:|| It has been known for over 75 years that cancer cells metabolize glucose differently than differentiated cells. In 1924, Otto Warburg made an observation that cancer cells undertook glucose metabolism in a manner that was distinct from the glycolytic process of cells in normal tissues. Warburg discovered that, unlike most normal tissues, cancer cells tended to "ferment" glucose into lactate even in the presence of sufficient oxygen to support mitochondrial oxidative phosphorylation. This observation became known as the Warburg Effect.
In the presence of oxygen, most differentiated cells primarily metabolize glucose to CO2 and H2O by oxidation of glycolytic pyruvate in the mitochondrial TCA cycle. Originally postulated to be the result of mitochondrial dysfunction in cancer cells, it was subsequently shown that this was not the mitigating reason for the increased conversion of glucose lactate in cancer cells. It might seem counterintuitive that highly proliferative cells, such as is characteristic of cancers, would bypass oxidation of pyruvate (form glucose) in mitochondria since this is the organelle where the vast majority of the ATP from glycolysis is generated. However, it has been shown that oncogenic mutations can result in the uptake of nutrients, particularly glucose and glutamine, that meet or exceed the bioenergetic demands of cell growth and proliferation. Proliferating cells, including cancer cells, require altered metabolism to efficiently incorporate nutrients such as glucose into biomass. The ultimate fate of glucose depends not only on the proliferative state of the cell but also on the activities of the specific glycolytic enzymes that are expressed. This is particularly true for pyruvate kinase, the terminal enzyme in glycolysis.
In mammals, two genes encode a total of four pyruvate kinase (PK) isoforms. The PKLR gene encodes the liver (L-PK or PKL) and the erythrocyte (R-PK or PKR) isoforms of pyruvate kinase via a process of alternative promoter usage. The PKM gene, so called originally due to initial characterization in muscle tissues, encodes the PKM1 and PKM2 isoforms. PKM1 and PKM2 are derived via alternative splicing of the PKM gene encoded mRNA. This results in mutual exclusion of a single conserved exon encoding 56 amino acids. Most tissues express either the PKM1 or PKM2. PKM1 is found in many normal differentiated tissues, whereas PKM2 is expressed in most proliferating cells, including in all cancer cell lines and tumors tested to date. Although PKM1 and PKM2 are highly similar in amino acid sequence they have different catalytic and regulatory properties. PKM1 has high constitutive enzymatic activity. In contrast, PKM2 is much less active but is allosterically activated by the upstream glycolytic metabolite fructose 1,6-bisphosphate (FBP). PKM2 is also unique in that, unlike other PK isoforms, it can interact with phosphotyrosine in tyrosine phosphorylated proteins such as those resulting from growth factor stimulation of cells. The interaction of PKM2 with tyrosine phosphorylated proteins results in the release of FBP leading to reduced activity of the enzyme. Low PKM2 activity, in conjunction with increased glucose uptake, facilitates the diversion of glucose carbons into the anabolic pathways that are derived from glycolysis. PKM2 is also inhibited by direct oxidation of a cysteine residue (Cys358) as an adaptive response to increased intracellular reactive oxygen species (ROS). This inhibition does not occur in PKM1. In cells in culture, the replacement of PKM2 with PKM1 (the constitutively active isoform) results in reduced lactate production and enhanced oxygen consumption. An additional observation has been made that in cells expressing PKM2 there is increased phosphorylation of an active site histidine (His11) in the upstream glycolytic enzyme phosphoglycerate mutase (PGAM1). His11 phosphorylation of PGAM1 increases its mutase activity. The phosphorylation of PGAM1 is not seen in PKM1 expressing cells. It turns out that the phosphate donor for His11 phosphorylation of PGAM1 is phosphoenolpyruvate (PEP) which is the substrate for pyruvate kinases. Phosphate transfer from PEP to PGAM1 yields pyruvate without concomitant generation of ATP. This reaction occurs at physiological concentrations of PEP and produces pyruvate in the absence of the normal glycolytic pyruvate kinase activity. Thus, the PEP-dependent histidine phosphorylation of PGAM1 may provide an alternate glycolytic pathway that decouples ATP production from normal pyruvate kinase-mediated phosphotransfer from PEP. This alternate pathway allows for a high rate of glycolysis that is needed to support the anabolic metabolism observed in many proliferating cells. In addition to altered pyruvate kinase activity, cancer cells also have increased levels of expression of the kinases that phosphorylate the pyruvate dehydrogenase complex (PDHc), further limiting pyruvate oxidation, and increased epxression of lactate dehydrogenase which drives pyruvate into lactate production
PGAM1 is unique with respect to glycolytic enzymes because its rate of transcription is regulated by the tumor suppressor p53. In addition, increased expression of PGAM1 has been shown to immortalize primary cells, although the mechanism of this immortalization remains unknown. When PKM2 activity is down-regulated, as a consequence of growth factor-mediated tyrosine phosphorylations, PGAM1 mutase activity is enhanced due to the consequent increase in His11 phosphorylation from PEP. Thus, a positive feedback loop is activated, whereby the production of PEP increases the enzymatic activity of PGAM1. Activation of this feedback loop between PEP and His11 modified PGAM1 may be the mechanism that promotes the redistribution of glycolytic carbons, upstream of PGAM1, into biosynthetic pathways that branch from glycolysis. In order for this alternative pathway to continue, the phosphate on His11 of PGAM1 must be removed so that it can serve as a continual acceptor of PEP phosphate. When PGAM1 converts 3-phosphoglycerate (3-PG) to 2-phosphoglycerate (2-PG) there is spontaneous hydrolysis of the phosphohistidine. In addition, it has been observed that 2,3-bisphosphoglycerate (2,3-BPG) can be formed from either 3-PG or 2-PG via phosphate transfer from His11 of PGAM1.
So, for proliferating cells such as cancers, even though it may seem counterproductive to prevent complete oxidation of glucose solely for ATP production, the demands for carbon incorporation into biomass clearly supersedes the needs for ATP production from glucose. Also, cancer cells bypass the hormonal signals required of normal cells for nutrient uptake so there is no limit to the sources of carbon atoms for ATP production (e.g. fatty acids and amino acids). Of metabolic significance to proliferating cells is that they must avoid ATP production in excess of demand to avoid allosteric inhibition of PFK-1 and other rate-limiting steps in glycolysis that are inhibited by a high ATP/ADP ratio. Therefore, the inhibition of PKM2 by binding to tyrosine phosphorylated proteins, following growth factor stimulation, may serve to uncouple the ability of cells to divert the carbons from nutrients (such as glucose) into biosynthetic pathways from the production of ATP. This may, in fact, be the underlying reason why PKM2 activity has evolved to be decreased in rapidly dividing cells.
Targeting PKM2 for the treatment of cancers is a distinct possibility. Recent work has demonstrated that small molecule PKM2-specific activators are functional in tumor growth models in mice. These new drugs have been shown to constitutively activate PKM2 and the activated enzyme is resistant to inhibition by tyrosine phosphorylated proteins. PKM2-specific activators reduce the incorporation of glucose into lactate and lipids. In addition, PKM2 activation results in decreased pools of nucleotide, amino acid, and lipid precursors and these effects may account for the suppression of tumorigenesis observed with these drugs.
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