The Possible Roles of Glucosamine-6-Phosphate Deaminases in Ammonium Metabolism in Cancer

Abstract

Nearly 5% of the glucose-6-phosphate (Glc6P) in cells is diverted into the hexosamine biosynthetic pathway (HBP) to synthesize glucosamine-6-phosphate (GlcN6P) and uridine diphosphate N-acetyl-glucosamine-6-phosphate (UDP-GlcN6P). Fructose-6-phosphate (Fru6P) is a common intermediary between glycolysis and the HBP. Changes in HBP regulation cause abnormal protein N-glycosylation and O-linked-N-acetylglucosamine modification (O-GlcNAcylation), affecting protein function and modifying cellular responses to signals. The HBP enzymes glucosamine-6-phosphate deaminases 1 and 2 (GNPDA1 and 2) turn GlcN6P back into Fru6P and ammonium, and have been implicated in cancer and metabolic diseases. Despite the plentiful literature on this topic, the mechanisms involved are just beginning to be studied. In this review, we summarize, for the first time, the current knowledge regarding the possible roles of the isoenzymes of both GNPDAs in the pathogenesis and development of metabolic diseases and cancer from a molecular point of view, highlighting their importance not only in supplying carbon from glycolysis, but also in ammonia metabolism.

Keywords: ammonium; cancer; glucosamine-6-phosphate deaminases.

Figure 1

Graphic description of the HBP showing the position of GNPDAs. Glucose, a key player in the metabolic pathways, is transported into the cell and then phosphorylated by hexokinase (1). It is then isomerized into Fru6P and follows the glycolytic pathway until pyruvate. GlcN6P, a significant metabolite, is synthesized from Fru6P and glutamine (Gln) by GFAT (Glutamine-fructose-6-phosphate transaminase or glucosamine-6-phosphate synthase) (2) or from NH₄⁺ instead of Gln by the reverse reaction of GNPDA (3). GNPDA activity is a crucial part of the HBP, and the catabolic direction of this reaction is favored. GlcN6P is acetylated from Acetyl-CoA to form GlcNAc6P, the allosteric activator of GNPDA, by the enzyme Glucosamine-Phosphate N-Acetyltransferase 1 (4). Deacetylation can occur according to a reverse reaction or be driven by the enzyme N-acetylglucosamine-6-phosphate deacetylase (5). Phosphoacetylglucosamine mutase isomerizes GlcNAc6P into GlcNAc1P (6). Next, the UDP-N-acetyl hexosamine pyrophosphorylase produces UDP-GlcNAc from UTP and GlcNAc-1P (7). O-GlcNAc transferase (OGT) transfers the O-GlcNAc moiety to specific serine/threonine residues of diverse proteins (8). Finally, the enzyme Protein-O-GlcNAc hydrolase (OGA) removes GlcNAc from the modified protein (9). GlcNAc can be phosphorylated from ATP by a kinase to regenerate GlcNAc-1P. The fate of this metabolite depends on several conditions, such as the requirements of hyaluronan synthesis [1,7]. It can be recycled back into UDP-GlcNAc or to GlcN6P; then, GNPDA (3) can use the carbon in glycolysis and the ammonium in other biosynthetic pathways.

Figure 2

Graphic representation of GNPDAs’ role in producing energy and lipid synthesis. GNPDAs provide Fru6P for glycolytic pathways and ATP production. Fru6P is phosphorylated by Phosphofructokinase 1 (FFK1), yielding Fru1,6 biphosphate (Fru1, 6 bi P). Aldolase produces glyceraldehyde three phosphate (GAP) and dihydroxyacetone phosphate (DHAP). The glycolytic pathway generates ATP and pyruvate, which can enter the tricarboxylic cycle. Acetyl-coenzyme A (AcCoA) is the precursor of fatty acid synthase. The glycerol required for triacylglycerols is obtained from oxidation of DHAP into glycerol 3-phosphate (G3P). Increased GlcNAc6P activates GNPDAs.

Figure 3

Graphic description of GNPDAs in adipocyte proliferation. GNPDA2, a key player in the metabolic process, significantly increases the levels of STAT5 and PPAR-γ, thereby stimulating the proliferation of adipose tissue cells. This pivotal role of GNPDA2 sheds light on the association of GNPDA2 with higher body mass index and obesity. The activation of both deaminases also contributes to the activation of fatty acid and cholesterol synthesis by increasing the amount of NH₄⁺ and SRBP1 activation.

Figure 4

Graphic illustration of ammonium metabolism in a tumor cell, emphasizing the role of GNPDA1.Tumor cells are known for surviving in reduced blood supply conditions and hypoxic environments. The concentration of NH₄⁺ can reach significantly high levels of 0.14–5 mM, compared to the 0.027–0.05 mM found in normal tissues [90]. The GNPDA1 catabolic reaction releases NH₄⁺, as well as glutaminase 1, and contributes to increasing the intracellular concentration of NH₄⁺. On the contrary, the fixing NH₄⁺ reactions include (a) carbamoyl phosphate synthase II (CPS-II) and (b) GlcN6P synthase (GFAT). CPS-II and GFAT use Gln to synthesize carbamoyl-phosphate (CP) and GlcN6P, respectively. CP is used in the pyrimidine nucleotides biosynthetic pathway, yielding uridine triphosphate (UTP). Finally, UTP is utilized in the synthesis of UDP-GlcNAc. In addition, amino acids, glutathione, DNA, and RNA purine nucleotides can also be synthesized from Gln. They are necessary for cell proliferation and survival. These mechanisms decrease the potential toxic effects of NH₄⁺.