Cardiomyocyte protein O-GlcNAcylation is regulated by GFAT1 not GFAT2.

In response to cardiac injury, increased activity of the hexosamine biosynthesis pathway (HBP) is linked with cytoprotective as well as adverse effects depending on the type and duration of injury. Glutamine-fructose amidotransferase (GFAT; gene name gfpt) is the rate-limiting enzyme that controls flux through HBP. Two protein isoforms exist in the heart called GFAT1 and GFAT2. There are conflicting data on the relative importance of GFAT1 and GFAT2 during stress-induced HBP responses in the heart. Using neonatal rat cardiac cell preparations, targeted knockdown of GFPT1 and GFPT2 were performed and HBP activity measured. Immunostaining with specific GFAT1 and GFAT2 antibodies was undertaken in neonatal rat cardiac preparations and murine cardiac tissues to characterise cell-specific expression. Publicly available human heart single cell sequencing data was interrogated to determine cell-type expression. Western blots for GFAT isoform protein expression were performed in human cardiomyocytes derived from induced pluripotent stem cells (iPSCs). GFPT1 but not GFPT2 knockdown resulted in a loss of stress-induced protein O-GlcNAcylation in neonatal cardiac cell preparations indicating reduced HBP activity. In rodent cells and tissue, immunostaining for GFAT1 identified expression in both cardiac myocytes and fibroblasts whereas immunostaining for GFAT2 was only identified in fibroblasts. Further corroboration of findings in human heart cells identified an enrichment of GFPT2 gene expression in cardiac fibroblasts but not ventricular myocytes whereas GFPT1 was expressed in both myocytes and fibroblasts. In human iPSC-derived cardiomyocytes, only GFAT1 protein was expressed with an absence of GFAT2. In conclusion, these results indicate that GFAT1 is the primary cardiomyocyte isoform and GFAT2 is only present in cardiac fibroblasts. Cell-specific isoform expression may have differing effects on cell function and should be considered when studying HBP and GFAT functions in the heart.

Mammalian Target of Rapamycin 2 (MTOR2) and C-MYC Modulate Glucosamine-6-Phosphate Synthesis in Glioblastoma (GBM) Cells Through Glutamine: Fructose-6-Phosphate Aminotransferase 1 (GFAT1).

Glucose and glutamine are two essential ingredients for cell growth. Glycolysis and glutaminolysis can be linked by glutamine: fructose-6-phosphate aminotransferase (GFAT, composed of GFAT1 and GFAT2) that catalyzes the synthesis of glucosamine-6-phosphate and glutamate by using fructose-6-phosphate and glutamine as substrates. The role of mammalian target of rapamycin (MTOR, composed of MTOR1 and MTOR2) in regulating glycolysis has been explored in human cancer cells. However, whether MTOR can interact with GFAT to regulate glucosamine-6-phosphate is poorly understood. In this study, we report that GFAT1 is essential to maintain the malignant features of GBM cells. And MTOR2 rather than MTOR1 plays a robust role in promoting GFAT1 protein activity, and accelerating the progression of glucosamine-6-phosphate synthesis, which is not controlled by the PI3K/AKT signaling. Intriguingly, high level of glucose or glutamine supply promotes MTOR2 protein activity. In turn, up-regulating glycolytic and glutaminolytic metabolisms block MTOR dimerization, enhancing the release of MTOR2 from the MTOR complex. As a transcriptional factor, C-MYC, directly targeted by MTOR2, promotes the relative mRNA expression level of GFAT1. Notably, our data reveal that GFAT1 immunoreactivity is positively correlated with the malignant grades of glioma patients. Kaplan-Meier assay reveals the correlations between patients' 5-year survival and high GFAT1 protein expression. Taken together, we propose that the MTOR2/C-MYC/GFAT1 axis is responsible for the modulation on the crosstalk between glycolysis and glutaminolysis in GBM cells. Under the condition of accelerated glycolytic and/or glutaminolytic metabolisms, the MTOR2/C-MYC/GFAT1 axis will be up-regulated in GBM cells.

2-Deoxy-d-glucose increases GFAT1 phosphorylation resulting in endoplasmic reticulum-related apoptosis via disruption of protein N-glycosylation in pancreatic cancer cells.

The glycolytic inhibitor 2-deoxy-d-glucose (2DG) causes energy starvation, affecting cell viability in a wide range of cancer cell lines. To determine the action of 2DG in pancreatic cancer, we performed proteomic analysis of pancreatic cancer cell line after 2DG treatment. Eighty proteins showed differential expression and among these, proteins involved in phosphohexose metabolism were upregulated. Up-regulation of glutamine: fructose 6-phosphate aminotransferase 1 (GFAT1), which belongs to the hexosamine biosynthesis pathway (HBP) that produces uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) to maintain glycoprotein, was validated by evaluation of mRNA and protein levels. Therefore, we assessed the amounts of total N-glycoproteins. Unexpectedly, we found a reduction of total N-glycoproteins and phosphorylation of GFAT1 by AMP-activated protein kinase (AMPK). These data may shed light on HBP dysfunction. Furthermore, we found endoplasmic reticulum (ER) stress accompanied by increased expression of ER stress markers, such as glucose response protein 78 (GRP78) and C/EBP-homologous protein (CHOP), in 2DG-treated cells. Moreover, the additive activation of AMPK by metformin (Met) synergistically enhanced the reduction of protein N-glycosylation and cell growth inhibition in the presence of 2DG. These results suggest that 2DG reduces N-glycosylation of proteins following the increase of phosphorylation of GFAT1 and results in the inhibition of cell growth mediated by ER stress in pancreatic cancer cells.

GFAT1 phosphorylation by AMPK promotes VEGF-induced angiogenesis.

Activation of AMP-activated protein kinase (AMPK) in endothelial cells regulates energy homeostasis, stress protection and angiogenesis, but the underlying mechanisms are incompletely understood. Using a label-free phosphoproteomic analysis, we identified glutamine:fructose-6-phosphate amidotransferase 1 (GFAT1) as an AMPK substrate. GFAT1 is the rate-limiting enzyme in the hexosamine biosynthesis pathway (HBP) and as such controls the modification of proteins by O-linked ß-N-acetylglucosamine (O-GlcNAc). In the present study, we tested the hypothesis that AMPK controls O-GlcNAc levels and function of endothelial cells via GFAT1 phosphorylation using biochemical, pharmacological, genetic and in vitro angiogenesis approaches. Activation of AMPK in primary human endothelial cells by 5-aminoimidazole-4-carboxamide riboside (AICAR) or by vascular endothelial growth factor (VEGF) led to GFAT1 phosphorylation at serine 243. This effect was not seen when AMPK was down-regulated by siRNA. Upon AMPK activation, diminished GFAT activity and reduced O-GlcNAc levels were observed in endothelial cells containing wild-type (WT)-GFAT1 but not in cells expressing non-phosphorylatable S243A-GFAT1. Pharmacological inhibition or siRNA-mediated down-regulation of GFAT1 potentiated VEGF-induced sprouting, indicating that GFAT1 acts as a negative regulator of angiogenesis. In cells expressing S243A-GFAT1, VEGF-induced sprouting was reduced, suggesting that VEGF relieves the inhibitory action of GFAT1/HBP on angiogenesis via AMPK-mediated GFAT1 phosphorylation. Activation of GFAT1/HBP by high glucose led to impairment of vascular sprouting, whereas GFAT1 inhibition improved sprouting even if glucose level was high. Our findings provide novel mechanistic insights into the role of HBP in angiogenesis. They suggest that targeting AMPK in endothelium might help to ameliorate hyperglycaemia-induced vascular dysfunction associated with metabolic disorders.

The sweet side of AMPK signaling: regulation of GFAT1.

Maintaining a steady balance between nutrient supply and energy demand is essential for all living organisms and is achieved through the dynamic control of metabolic processes that produce and consume adenosine-5'-triphosphate (ATP), the universal currency of energy in all cells. A key sensor of cellular energy is the adenosine-5'-monophosphate (AMP)-activated protein kinase (AMPK), which is the core component of a signaling network that regulates energy and nutrient metabolism. AMPK is activated by metabolic stresses that decrease cellular ATP, and functions to restore energy balance by orchestrating a switch in metabolism away from anabolic pathways toward energy-generating catabolic processes. A new study published in a recent issue of Biochemical Journal by Zibrova et al. shows that glutamine:fructose-6-phosphate amidotransferase-1 (GFAT1), the rate-limiting enzyme of the hexosamine biosynthesis pathway (HBP), is a physiological substrate of AMPK. The HBP is an offshoot of the glycolytic pathway that drives the synthesis of uridine-5'-diphospho-N-acetylglucosamine, the requisite donor metabolite needed for dynamic ß-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) of cellular proteins. O-GlcNAcylation is a nutrient-sensitive post-translational modification that, like phosphorylation, regulates numerous intracellular processes. Zibrova et al. show that inhibitory phosphorylation of the GFAT1 residue Ser243 by AMPK in response to physiological or small-molecule activators leads to a reduction in cellular protein O-GlcNAcylation. Further work revealed that AMPK-dependent phosphorylation of GFAT1 promotes angiogenesis in endothelial cells. This elegant study demonstrates that the AMPK-GFAT1 signaling axis serves as an important communication point between two nutrient-sensitive signaling pathways and is likely to play a significant role in controlling physiological processes in many other tissues.

The Hexosamine Biosynthesis Pathway: Regulation and Function.

The hexosamine biosynthesis pathway (HBP) produces uridine diphosphate-N-acetyl glucosamine, UDP-GlcNAc, which is a key metabolite that is used for N- or O-linked glycosylation, a co- or post-translational modification, respectively, that modulates protein activity and expression. The production of hexosamines can occur via de novo or salvage mechanisms that are catalyzed by metabolic enzymes. Nutrients including glutamine, glucose, acetyl-CoA, and UTP are utilized by the HBP. Together with availability of these nutrients, signaling molecules that respond to environmental signals, such as mTOR, AMPK, and stress-regulated transcription factors, modulate the HBP. This review discusses the regulation of GFAT, the key enzyme of the de novo HBP, as well as other metabolic enzymes that catalyze the reactions to produce UDP-GlcNAc. We also examine the contribution of the salvage mechanisms in the HBP and how dietary supplementation of the salvage metabolites glucosamine and N-acetylglucosamine could reprogram metabolism and have therapeutic potential. We elaborate on how UDP-GlcNAc is utilized for N-glycosylation of membrane and secretory proteins and how the HBP is reprogrammed during nutrient fluctuations to maintain proteostasis. We also consider how O-GlcNAcylation is coupled to nutrient availability and how this modification modulates cell signaling. We summarize how deregulation of protein N-glycosylation and O-GlcNAcylation can lead to diseases including cancer, diabetes, immunodeficiencies, and congenital disorders of glycosylation. We review the current pharmacological strategies to inhibit GFAT and other enzymes involved in the HBP or glycosylation and how engineered prodrugs could have better therapeutic efficacy for the treatment of diseases related to HBP deregulation.

Hexosamine pathway activation improves memory but does not extend lifespan in mice.

Glucosamine feeding and genetic activation of the hexosamine biosynthetic pathway (HBP) have been linked to improved protein quality control and lifespan extension. However, as an energy sensor, the HBP has been implicated in tumor progression and diabetes. Given these opposing outcomes, it is imperative to explore the long-term effects of chronic HBP activation in mammals. Thus, we asked if HBP activation affects metabolism, coordination, memory, and survival in mice. N-acetyl-D-glucosamine (GlcNAc) supplementation in the drinking water had no adverse effect on weight in males but increased weight in young females. Glucose or insulin tolerance was not affected up to 20 months of age. Of note, we observed improved memory in young male mice supplemented with GlcNAc. Survival was not changed by GlcNAc treatment. To assess the effects of genetic HBP activation, we overexpressed the pathway's key enzyme GFAT1 and a constitutively activated mutant form in all mouse tissues. We detected elevated levels of the HBP product UDP-GlcNAc in mouse brains, but did not find any effects on behavior, memory, or survival. Together, while dietary GlcNAc supplementation did not extend survival in mice, it positively affected memory and is generally well tolerated.

Characterization of Gfat1 (zeppelin) and Gfat2, Essential Paralogous Genes Which Encode the Enzymes That Catalyze the Rate-Limiting Step in the Hexosamine Biosynthetic Pathway in Drosophila melanogaster.

The zeppelin (zep) locus is known for its essential role in the development of the embryonic cuticle of Drosophila melanogaster. We show here that zep encodes Gfat1 (Glutamine: Fructose-6-Phosphate Aminotransferase 1; CG12449), the enzyme that catalyzes the rate-limiting step in the hexosamine biosynthesis pathway (HBP). This conserved pathway diverts 2%-5% of cellular glucose from glycolysis and is a nexus of sugar (fructose-6-phosphate), amino acid (glutamine), fatty acid [acetyl-coenzymeA (CoA)], and nucleotide/energy (UDP) metabolism. We also describe the isolation and characterization of lethal mutants in the euchromatic paralog, Gfat2 (CG1345), and demonstrate that ubiquitous expression of Gfat1+ or Gfat2+ transgenes can rescue lethal mutations in either gene. Gfat1 and Gfat2 show differences in mRNA and protein expression during embryogenesis and in essential tissue-specific requirements for Gfat1 and Gfat2, suggesting a degree of functional evolutionary divergence. An evolutionary, cytogenetic analysis of the two genes in six Drosophila species revealed Gfat2 to be located within euchromatin in all six species. Gfat1 localizes to heterochromatin in three melanogaster-group species, and to euchromatin in the more distantly related species. We have also found that the pattern of flanking-gene microsynteny is highly conserved for Gfat1 and somewhat less conserved for Gfat2.

GFAT1 is highly expressed in cancer stem cells of pancreatic cancer.

Background: Cancer stem cells (CSCs) play pivotal roles in the growth, invasion, metastasis, and chemoresistance of pancreatic cancer (PC). The current characterization of CSCs in PC is not complete. Glutamine-fructose-6-phosphate transaminase 1 (GFAT1) is a key enzyme that regulates the hexosamine pathway (HP), in which it not only controls glucose influx but also catalyzes the reaction to form glucosamine 6-phosphate. Recently, it was reported that GFAT1 is highly expressed in PC. However, the relevance of this high expression of GFAT1, especially its association with cancer stemness, has not been well defined and is thus addressed in the current study. Methods: GFAT1 levels were determined in PC from a public database and assessed by bioinformatics tools. GFAT1 expression in CD133+ and CD44+ CSCs in PC was analyzed by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and immunostaining on cytospun cells after flow cytometry-based cell sorting. GFAT1+ cells were separated from GFAT1- cells by transfection of PC cell lines with a designed plasmid that expressed red fluorescent protein (RFP) under a GFAT1 promoter. Tumor sphere formation, tumor growth, tumor cell invasion, cell migration, and the resistance to gemcitabine-induced apoptosis were determined in GFAT1+ vs. GFAT1- PC cells. Results: GFAT1 levels were significantly upregulated in PC compared to the adjacent non-PC tissue. There were significantly more GFAT1+ cells in the CD133+ population than in the CD133- population, and similarly, there were significantly more GFAT1+ cells in the CD44+ population than in the CD44- population. Compared to GFAT1- PC cells, GFAT1+ PC cells generated significantly more tumor spheres in culture, appeared to be more invasive and migratory, and were significantly more resistant to gemcitabine-induced cell apoptosis. Conclusions: GFAT1 is highly expressed in CSCs among PC cells and may be crucial for PC growth, metastasis, and chemoresistance.

Exploring the antidiabetic potential of compounds isolated from Anacardium occidentale using computational aproach: ligand-based virtual screening.

Diabetes mellitus is becoming an important public health challenge worldwide and especially in developing nations. About 8.8 percent of the world adult population has been reported to have diabetes. Glutamine-fructose-6-phosphate amidotransferase 1 (GFAT1) catalyses the first committed step in the pathway for biosynthesis of hexosamines in mammals, and its inhibition has been thought to prevent hyperglycaemia. Dipeptidyl peptidase-4 (DPP-4), on the other hand, degrades hormone glucagon-like peptide-1 (GLP-1), an enzyme that plays a major role in the enhancement of glucose-dependent insulin secretion, making these two proteins candidate targets for diabetes. To find potential inhibitors of DPP-4 and GFAT1 from Anacardium occidentale using a computational approach, glide XP (extra precision) docking, Induced Fit Docking (IFD), Binding free energy of the compounds were determined against prepared crystal structure of DPP-4 and GFAT1 using the Maestro molecular interface of Schrödinger suites. The Lipinski's rule of five (RO5) and ADME properties of the compounds were assessed. Predictive models for both protein targets were built using AutoQSAR. This study identified 8 hit compounds. Most of these compounds passed the RO5 and were within the recommended range for defined ADME parameters. In addition, the predicted pIC50 for the hit compounds were promising. The results obtained from the present study can be used to design an antidiabetic drug. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s40203-021-00084-z.

Drosophila GFAT1 and GFAT2 enzymes encode obligate developmental functions.

Glutamine: fructose-6-phosphate amidotransferase (GFAT) enzymes catalyse the first committed step of the hexosamine biosynthesis pathway (HBP) using glutamine and fructose-6-phosphate to form glucosamine-6-phosphate (GlcN6P). Numerous species (e.g. mouse, rat, zebrafish, chicken) including humans and Drosophila encode two broadly expressed copies of this enzyme but whether these perform redundant, partially overlapping or distinct functions is not known. To address this question, we produced single gene null mutations in the fly counterparts of gfat1 and gfat2. Deletions for either enzyme were fully lethal and homozygotes lacking either GFAT1 or GFAT2 died at or prior to the first instar larval stage. Therefore, when genetically eliminated, neither isoform was able to compensate for the other. Importantly, dietary supplementation with D-glucosamine-6-phosphate rescued GFAT2 deficiency and restored viability to gfat2-/- mutants. In contrast, glucosamine-6-phosphate did not rescue gfat1-/- animals.

High expression of GFAT1 predicts unfavorable prognosis in patients with hepatocellular carcinoma.

Hepatocellular carcinoma (HCC) is the second leading cause of cancer-related deaths worldwide. As a branch of glucose metabolism, hexosamine biosynthesis pathway (HBP) has been reported to play a critical role in the insulin resistance and progression of cancer. Glutamine:fructose-6-phosphate amidotransferase (GFAT) is the rate-limiting enzyme of the HBP; nevertheless, the prognostic value of GFAT1 in HCC remains elusive. In this study, we found that high expression of GFAT1 was significantly associated with serum alpha-fetoprotein (AFP), serum alanine aminotransferase (ALT), tumor size, tumor encapsulation, T stage and TNM stage. High GFAT1 expression was identified as an independent prognostic factor which predicted poor overall survival (OS) and recurrence-free survival (RFS) in HCC patients. Incorporation of GFAT1 expression could improve the prognostic accuracy of traditional TNM stage system. Integration of GFAT1 expression with other independent prognosticators generated a predictive nomogram, which showed better prognostic efficiency for OS and RFS in HCC patients. In vitro studies also revealed that GFAT1 promoted the proliferation, cell cycle progression, migration and invasion of HCC cells. In conclusion, GFAT1 is a potential prognostic biomarker for overall survival and recurrence-free survival of HCC patients after surgery.

Loss of GFAT1 promotes epithelial-to-mesenchymal transition and predicts unfavorable prognosis in gastric cancer.

Gastric cancer remains the third leading cause of cancer-related mortality worldwide, and invasion and metastasis of gastric cancer represent the major reason for its poor prognosis. Glutamine: fructose-6-phosphate amidotransferase 1 (GFAT1) is the first and rate-limiting enzyme of hexosamine biosynthesis pathway (HBP). Nevertheless, the role of GFAT1 in gastric cancer is little investigated. In this study, we found that the expression of GFAT1 was decreased in gastric cancer. Low expression of GFAT1 was positively associated with vessel invasion, late T stage, lymph node metastasis, distant metastasis, advanced TNM stage and poor prognosis in patients with gastric cancer. Furthermore, in vitro and in vivo studies revealed that down-regulation of GFAT1 promoted epithelial-to-mesenchymal transition (EMT) and invasive activities in gastric cancer cells through inducing the expression of TGF-ß1. The GFAT1 expression also significantly correlated with EMT-related factors in gastric cancer patients. Together, these findings indicate that GFAT1 functions as a novel suppressor of EMT and tumor metastasis in gastric cancer.