Role of Pentose Phosphate Pathway in Tumor

The pentose phosphate pathway (PPP) is a mode of oxidative glucose catabolism. The main physiological function of PPP is to produce pentose phosphate for nucleic acid metabolism and NADPH for reductive biosynthesis, which are involved in various metabolic reactions as a raw material for DNA synthesis and a hydrogen donor, respectively. Some intermediates are involved in amino acid synthesis and fatty acid synthesis, etc.

Steps of Pentose Phosphate Pathway

The pentose phosphate pathway takes place in the cytosol and can be divided into two phases:

• Oxidation reaction:

Dehydrogenation of glucose-6-phosphate to produce glucose-6-phospholactone. Catalyzed by glucose-6-phosphate dehydrogenase, and the coenzyme NADP+.

Hydrolysis of glucose-6-phosphate lactone to produce glucose-6-phosphate. Catalyzed by glucose-6-phosphate lactonase, the reaction is reversible.

Oxidative decarboxylation and dehydrogenation of glucose-6-phosphate to produce ribulose-5-phosphate. Catalyzed by glucose-6-phosphate dehydrogenase, NADP+ again acts as an acceptor for hydrogen, converts to NADPH, and produces a molecule of CO₂.

• Non-oxidative reactions:

Ribulose 5-phosphate undergoes a series of trans-keto- and trans-aldol reactions, and finally produces glyceraldehyde 3-phosphate and fructose 6-phosphate through intermediate metabolites such as butyl phosphate, pentose phosphate and heptose phosphate. The latter two can also re-enter the glycolytic pathway and be metabolized.

Schematic drawing of the pentose phosphate pathway (Masi et al., 2021)

Pentose Phosphate Pathway in Tumors

For tumor cells, PPP is very important. Tumorigenesis is a dynamic and complex process. The alteration of energy and metabolism is one of the basic features of tumors.

The pentose phosphate pathway (PPP) is associated with several cancer- and cell-proliferation-related signalling cascades (Stincone et al., 2015).

In normal cells, glucose generates pyruvate through glycolysis, which is converted to acetyl coenzyme A and then enters the mitochondria to participate in the tricarboxylic acid cycle (TCA). Eventually, adenosine triphosphate (ATP) is generated through oxidative phosphorylation, which also provides intermediate metabolites for lipid and non-essential amino acid synthesis.

However, the tumor cells reprogrammed the above metabolic pathways. The reprogramming of metabolism leads to a change in the primary mode of energy acquisition by tumor cells to aerobic glycolysis (Warburg effect). Glucose is phosphorylated to glucose-6-phosphate (G-6-P) by hexokinase upon entry into the cell and undergoes a series of reactions to produce pyruvate, which is ultimately available for energy via glycolysis. A large number of intermediates of glycolytic metabolism also promote PPP.

G6PD, as the rate-limiting enzyme of the oxidative branch of PPP, is involved in regulating the synthesis of R-5-P and NADPH. r-5-P is not only an important component of nucleotide synthesis, but also a precursor for the synthesis of many intracellular biomolecules, including ATP, ADP, AMP, c-AMP, coenzyme A, FAD, NAD(P) and NAD(P)H. Another metabolite of PPP, NADPH, promotes the production of tetrahydrofolate (thymidylate synthase cofactor) and participates in the synthesis of nucleotide reductase.

Tumor cells are not only active in DNA synthesis, which requires a lot of reducing power, but also higher ROS levels require more NADPH to maintain redox homeostasis. Therefore, tumor cells have higher flux of PPP and more expression of its rate-limiting enzyme G6PDH. There are many studies trying to treat tumors by inhibiting PPP, and G6PDH is the main target.

NADPH and NADH can be interconverted by the mitochondrial transhydrogenase (nicotinamide nucleotide transhydrogenase, NNT). It has been suggested that NNT also has an important role in some cancers.

The analysis of pentose phosphate pathway metabolites will help in tumor mechanism research, oncology therapy development, and drug development. Small molecules are evaluated by liquid chromatography and high resolution mass spectrometry in combination with targeted and untargeted metabolomics strategies. Targeted metabolomics enables precise analysis of specific metabolite levels. Untargeted metabolomics enables the identification of novel biomarkers.

Creative Proteomics can provide a metabolomics solution to analyze pentose phosphate pathway metabolites.

References

1. Masi, A., et al. (2021). The pentose phosphate pathway in industrially relevant fungi: crucial insights for bioprocessing. Applied Microbiology and Biotechnology, 105(10), 4017-4031.
2. Stincone, Anna & Prigione, et al. (2015). Stincone et al-2014-Biological Reviews.