Metabolic Reprogramming

Definition

“Metabolic reprogramming” refers to the collective changes that occur within multiple metabolic pathways in cancer cells. This essay will focus on changes associated with altered metabolism of glucose, lactate, and glutamine (Gln) and will briefly discuss certain side pathways related to glycolysis and the TCA cycle.

Characteristics

Introduction

Metabolic reprogramming or “deregulating cellular energetics” has been defined as a hallmark of cancer and also contributes to several of the other hallmarks of cancer as defined by Hanahan and Weinberg (Hanahan and Weinberg 2011). Metabolic reprogramming allows for adequate ATP production under stressful conditions but also impacts macromolecular biosynthesis, cellular redox balance and antioxidant defenses, and epigenetic orchestration of gene expression (Boroughs and DeBerardinis 2015; Vander Heiden et al. 2011). The study of metabolic reprogramming in cancer has led to the identification of several differentially utilized...

Glossary

De Novo Lipogenesis

The production of palmitoyl CoA from acetyl CoA is normally performed by the liver as a means of converting excess glucose into triglycerides, which can ultimately be packaged into very low density lipoproteins (VLDL) and transferred to adipocytes for storage.

The first reaction converts cytoplasmic citrate to acetyl CoA and oxaloacetate by the enzyme, ATP/citrate lyase (ACLY).

Cytoplasmic acetyl CoA is converted to malonyl CoA by acetyl CoA carboxylase (ACC1, ACC2).

Malonyl CoA is an inhibitor of the carnitine/palmitoyl transferase 1 (CPT1) enzyme, which transports fatty acyl CoA across the outer mitochondrial membrane on the way to undergoing β-oxidation. This avoids the futile cycle of making and oxidizing fatty acids at the same time. This inhibitory function of malonyl CoA is reduced at least in some cancers.

Malonyl CoA is then converted to palmitoyl CoA through several steps as catalyzed by fatty acyl synthase (or fatty acid synthase).

These three enzymes of DNL (ACLY, ACC, FASN) are upregulated at the transcriptional level by SREBP-1c and ChREBP. ACC activity is inhibited by phosphorylation by AMP kinase (AMPK) under low energy conditions.

Malonyl CoA can be converted back to acetyl CoA by malonyl CoA decarboxylase (MCD). MCD is highly regulated (e.g., by mTORC1/SIRT4), and inhibition of MCD induces apoptosis in MCF7 breast cancer cells, possibly due to elevated consumption of ATP and NADPH by FASN or by inhibition of the TCA cycle and electron transport system by the metabolite, malonate.

GLUT transporters (Solute Carrier 2A Family – SLC2A)

Bidirectional facilitative transporters of glucose (GLUTs 1–14).

GLUT1 and GLUT3 are high affinity transporters.

GLUT2 is a low affinity, high capacity transporter normally expressed in pancreatic β cells, hepatocytes, and the basolateral membranes GI and renal tubular epithelia.

GLUT4 is a high affinity transporter that is normally expressed in skeletal muscle and adipocytes and is dependent on insulin/Akt signaling to move into the cell membrane.

Expression of GLUTs, especially GLUT1 and GLUT3, is frequently upregulated in cancer.

GPR81

Member of the seven-transmembrane, G protein-coupled receptor superfamily.

Receptor for lactate.

Expressed at high levels in adipocytes. Under FED conditions with a high insulin/glucagon ratio, adipocytes actively import glucose and metabolize it through glycolysis (and through the glycerol-3-phosphate side pathway to make the backbone of triglycerides [TG]). Adipocytes produce a significant amount of lactate, which in turn binds to and activates GPR81. GPR81 is coupled to Gi, and thereby inhibits adenylate cyclase. Thus, during a time when the adipocyte is making TGs, it also inhibits cAMP-protein kinase A in order to inhibit the lipolysis of TG via activation of hormone-sensitive lipase.

GPR81 is expressed in several cancer cell lines and shown to be important for the expression of MCT transporters and the ability of cancer cells to adjust to low glucose/high lactate conditions in pancreatic cancer cells.

Hexokinase (HK)

ATP-dependent kinases of six carbon sugars, especially glucose.

Conversion of glucose to glucose-6-phosphate traps it inside the cell, as glucose-6-phosphate is not recognized by GLUTs.

Most isoforms of hexokinases are high affinity, low Vmax enzymes that are product inhibited by glucose-6-phosphate, making their activity closely linked to the need of cells for more glucose-6-phosphate.

Hexosamine Biosynthetic Pathway (HBP)

Glutamine/fructose-6-P transamidase (GFPT 1 and/or 2) converts the glycolytic intermediate, fructose-6-P, to glucosamine-6-phosphate.

An important use of the HBP is glycosylation of proteins to produce glycoproteins. Glycoproteins include cell membrane proteins such as growth factor receptors, etc.

Isocitrate Dehydrogenase (IDH)

Catalyzes reversible conversion of isocitrate and 2-oxoglutarate (2-OG; also called α-ketoglutarate).

2-OG is an essential cofactor for several enzymes, including ones that alter chromatin structure and gene expression.

IDH1 is a cytoplasmic isoform that can generate NADPH from conversion of isocitrate to 2-OG, or consume NADPH from conversion of 2-OG to isocitrate.

IDH2 and 3 are mitochondrial isoforms. IDH2 reversibly catalyzes the reaction and generates NADPH from isocitrate conversion to 2-OG. IDH3 can only catalyze this reaction in the direction of isocitrate to 2-OG, producing NADH.

Gliomas and other cancers are caused by mutations in IDH isoforms. Mutations reduce the amount of 2-OG produced but also make a structurally similar metabolite called 2- hydroxyglutarate (2-HG).

High levels of 2-HG induce neoplastic transformation. Thus, 2-HG is called an oncometabolite.

Mitochondrial Pyruvate Dehydrogenase (PDH)

Catalyzes the decarboxylation of mitochondrial pyruvate to form acetyl CoA.

A multienzyme complex composed of pyruvate decarboxylase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).

PDH requires the following cofactors: thiamine pyrophosphate, lipoic acid, coenzyme A (CoA), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide (NAD+).

PDH is directly allosterically inactivated by NADH and acetyl CoA.

PDH is inactivated by PDH kinase (PDK), which is activated by ATP, acetyl CoA, and NADH but inactivated by pyruvate.

PDH is activated by PDH phosphatase (PDPH), which is activated by Ca²⁺ and insulin signaling via Akt.

Deficiency in PDH E1 is the most common cause of congenital lactic acidosis (overall a rare disorder).

Monocarboxylate Transporters (MCTs; Solute Carrier 16A Family – SLC16A)

Cotransporters of lactate anion and H⁺.

MCT1 and MCT2 are typically involved in the import of lactate in oxidative cells – or in cells in which lactate serves as a signaling molecule (e.g., endothelial cells) – and both can also allow the export of lactate from the cell in certain contexts.

MCT1 expression is induced by c-myc and is upregulated in several cancers.

MCT4 is typically expressed by highly glycolytic cancers and is primarily involved in the export of lactate and H⁺.

Glucokinase (Hexokinase IV) is a low affinity, high capacity enzyme expressed in pancreatic β cells and hepatocytes that only acts on glucose when it is highly abundant (i.e., after a meal).

Hexokinases (especially HK2) are upregulated in several cancers.

Serine/Gycine Biosynthetic Pathway (Ser/Gly BP)

The Ser/Gly BP is composed of four enzymatic steps. The first step converts the glycolytic intermediate, 3-phosphoglycerate, into phosphohydroxypyruvate (PHP). This reaction is catalyzed by the enzyme, phosphoglycerate dehydrogenase (PHGDH), and consumes NAD+. The PHGDH gene is amplified in some cancers.

The second reaction converts PHP into phosphoserine. This reaction is catalyzed by phosphoserine aminotransferase 1 (PSAT1). This reaction converts glutamic acid to α-ketoglutarate, which can enter the mitochondria and the TCA cycle.

The third reaction converts phosphoserine to Ser by the enzyme phosphoserine phosphatase.

Ser is used for the synthesis of other amino acids, proteins, complex lipids, and nucleotides.

Ser is converted to glycine by the enzyme, Serine Hydroxymethyltransferase (SHMT1 or 2). This reaction is linked to single-carbon folate metabolism, generating 5,10-methylene tetratrahydrofolate (5,10-methyleneTHF) from THF. 5,10-methylene THF is used for deoxythymidine triphosphate (dTTP) synthesis or can be converted to other forms of THF for use in pyrimidine synthesis.