Pentose Phosphate Pathway

The pentose phosphate pathway (PPP) is a fundamental component of cellular metabolism. The PPP is important to maintain carbon homoeostasis, to provide precursors for nucleotide and amino acid biosynthesis, to provide reducing molecules for anabolism, and to defeat oxidative stress. The PPP shares reactions with the Entner-Doudoroff pathway and Calvin cycle and divides into an oxidative and non-oxidative branch. The oxidative branch is highly active in most eukaryotes and converts glucose 6-phosphate into carbon dioxide, ribulose 5-phosphate and NADPH. The latter function is critical to maintain redox balance under stress situations, when cells proliferate rapidly, in ageing, and for the 'Warburg effect' of cancer cells. The non-oxidative branch instead is virtually ubiquitous, and metabolizes the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate as well as sedoheptulose sugars, yielding ribose 5-phosphate for the synthesis of nucleic acids and sugar phosphate precursors for the synthesis of amino acids. Whereas the oxidative PPP is considered unidirectional, the non-oxidative branch can supply glycolysis with intermediates derived from ribose 5-phosphate and vice versa, depending on the biochemical demand. These functions require dynamic regulation of the PPP pathway that is achieved through hierarchical interactions between transcriptome, proteome and metabolome. Consequently, the biochemistry and regulation of this pathway, while still unresolved in many cases, are archetypal for the dynamics of the metabolic network of the cell. In this comprehensive article we review seminal work that led to the discovery and description of the pathway that date back now for 80 years, and address recent results about genetic and metabolic mechanisms that regulate its activity. These biochemical principles are discussed in the context of PPP deficiencies causing metabolic disease and the role of this pathway in biotechnology, bacterial and parasite infections, neurons, stem cell potency and cancer metabolism.

Energy metabolism is significantly reprogrammed in many human cancers, and these alterations confer many advantages to cancer cells, including the promotion of biosynthesis, ATP generation, detoxification and support of rapid proliferation. The pentose phosphate pathway (PPP) is a major pathway for glucose catabolism. The PPP directs glucose flux to its oxidative branch and produces a reduced form of nicotinamide adenine dinucleotide phosphate (NADPH), an essential reductant in anabolic processes. It has become clear that the PPP plays a critical role in regulating cancer cell growth by supplying cells with not only ribose-5-phosphate but also NADPH for detoxification of intracellular reactive oxygen species, reductive biosynthesis and ribose biogenesis. Thus, alteration of the PPP contributes directly to cell proliferation, survival and senescence. Furthermore, recent studies have shown that the PPP is regulated oncogenically and/or metabolically by numerous factors, including tumor suppressors, oncoproteins and intracellular metabolites. Dysregulation of PPP flux dramatically impacts cancer growth and survival. Therefore, a better understanding of how the PPP is reprogrammed and the mechanism underlying the balance between glycolysis and PPP flux in cancer will be valuable in developing therapeutic strategies targeting this pathway.

The pentose phosphate pathway (PPP) plays a critical role in macromolecule biosynthesis and maintaining cellular redox homoeostasis in rapidly proliferating cells. Upregulation of the PPP has been shown in several types of cancer. However, how the PPP is regulated to confer a selective growth advantage on cancer cells is not well understood. Here we show that glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the PPP, is dynamically modified with an O-linked β-N-acetylglucosamine sugar in response to hypoxia. Glycosylation activates G6PD activity and increases glucose flux through the PPP, thereby providing precursors for nucleotide and lipid biosynthesis, and reducing equivalents for antioxidant defense. Blocking glycosylation of G6PD reduces cancer cell proliferation in vitro and impairs tumor growth in vivo. Importantly, G6PD glycosylation is increased in human lung cancers. Our findings reveal a mechanistic understanding of how O-glycosylation directly regulates the PPP to confer a selective growth advantage to tumours.

BackgroundAlzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline and synaptic dysfunction. Increasing evidence suggests that impaired glucose utilization is a major contributor to AD pathogenesis. Neurons preferentially use glucose through the pentose phosphate pathway (PPP). In AD, the flux through the PPP is significantly reduced; however, the underlying mechanism is still elusive. This study was aimed to elucidate how PPP was affected in AD and its contribution to the AD pathogenesis.MethodsProteomic analyses of temporal cortex synaptosomes from AD patients and controls were conducted to identify dysregulated pathways and significantly affected proteins. Functional analysis was performed by knockdown or restoration of protein expression in primary cultured neurons, as well as in wild-type and 5 × FAD mice. Pseudotargeted metabolomics and biochemical, molecular, electrophysiological and behavioral assessments were performed to evaluate metabolic characteristics, redox status, mitochondrial function, synaptic plasticity and cognition.ResultsProteomic analysis of synaptic compartments identified glucose metabolism as the most significantly dysregulated functional network in AD. Further, transaldolase 1 (TALDO1), a rate-limiting enzyme in the PPP, was identified as a key enzyme affected in AD. TALDO1 was markedly downregulated at the early stage of AD. Downregulation of TALDO1 reduced glucose metabolism by inhibiting the PPP, TCA cycle and oxidative phosphorylation, causing broad metabolic collapse. Further, downregulation of TALDO1 depleted the nicotinamide adenine dinucleotide phosphate and glutathione pools, weakening antioxidant defense, thus resulting in mitochondria impairment and reduced energy supply. These collectively drive synaptic dysfunction and cognitive decline. Conversely, restoring TALDO1 expression in 5 × FAD mice improved glucose uptake, mitigated oxidative stress, restored metabolic homeostasis, and rescued neuronal and cognitive functions.ConclusionThese findings identify TALDO1 as a key regulator of the impaired PPP in AD and may represent a promising therapeutic target for restoring neuronal metabolic homeostasis and function.