Glutamine deprivation triggers NAGK-dependent hexosamine salvage

Tumors frequently exhibit aberrant glycosylation, which can impact cancer progression and therapeutic responses. The hexosamine biosynthesis pathway (HBP) produces uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), a major substrate for glycosylation in the cell. Prior studies have identified the HBP as a promising therapeutic target in pancreatic ductal adenocarcinoma (PDA). The HBP requires both glucose and glutamine for its initiation. The PDA tumor microenvironment is nutrient poor, however, prompting us to investigate how nutrient limitation impacts hexosamine synthesis. Here, we identify that glutamine limitation in PDA cells suppresses de novo hexosamine synthesis but results in increased free GlcNAc abundance. GlcNAc salvage via N-acetylglucosamine kinase (NAGK) is engaged to feed UDP-GlcNAc pools. NAGK expression is elevated in human PDA, and NAGK deletion from PDA cells impairs tumor growth in mice. Together, these data identify an important role for NAGK-dependent hexosamine salvage in supporting PDA tumor growth.

Glutamine deprivation triggers NAGK-dependent hexosamine salvage

Tumors of many types exhibit aberrant glycosylation, which can impact cancer progression and therapeutic responses. The hexosamine biosynthesis pathway (HBP) branches from glycolysis at fructose-6-phosphate to synthesize uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), a major substrate for glycosylation in the cell. HBP enzyme gene expression is elevated in pancreatic ductal adenocarcinoma (PDA), and studies have pointed to the potential significance of the HBP as a therapeutic target. Yet, the PDA tumor microenvironment is nutrient poor, and adaptive nutrient acquisition strategies support tumorigenesis. Here, we identify that pancreatic cancer cells salvage GlcNAc via N-acetylglucosamine kinase (NAGK), particularly under glutamine limitation. Glutamine deprivation suppresses de novo HBP flux and triggers upregulation of NAGK. NAGK expression is elevated in human PDA. NAGK deletion forces PDA cells to rely on de novo UDP-GlcNAc synthesis and impairs tumor growth in mice. Together, these data identify an important role for NAGK-dependent hexosamine salvage in supporting PDA tumor growth.

Global Transcriptional and Physiological Responses of Saccharomyces cerevisiae to Ammonium, l-Alanine, or l-Glutamine Limitation

ABSTRACT The yeast Saccharomyces cerevisiae encounters a range of nitrogen sources at various concentrations in its environment. The impact of these two parameters on transcription and metabolism was studied by growing S. cerevisiae in chemostat cultures with l-glutamine, l-alanine, or l-ammonium in limitation and by growing cells in an excess of ammonium. Cells grown in l-alanine-limited cultures had higher biomass yield per nitrogen mole (19%) than those from ammonium-limited cultures. Whole-genome transcript profiles were analyzed with a genome-scale metabolic model that suggested increased anabolic activity in l-alanine-limited cells. The changes in these cells were found to be focused around pyruvate, acetyl coenzyme A, glyoxylate, and α-ketoglutarate via increased levels of ALT1, DAL7, PYC1, GDH2, and ADH5 and decreased levels of GDH3, CIT2, and ACS1 transcripts. The transcript profiles were then clustered. Approximately 1,400 transcripts showed altered levels when amino acid-grown cells were compared to those from ammonium. Another 400 genes had low transcript levels when ammonium was in excess. Overrepresentation of the GATAAG element in their promoters suggests that nitrogen catabolite repression (NCR) may be responsible for this regulation. Ninety-one genes had transcript levels on both l-glutamine and ammonium that were decreased compared to those on l-alanine, independent of the concentration. The GATAAG element in these genes suggests two groups of NCR-responsive genes, those that respond to high levels of nitrogen and those that respond to levels below 30 μM. In conclusion, our results reveal that the nitrogen source has substantial influence on the transcriptome of yeasts and that transcriptional changes may be correlated to physiology via a metabolic model.

Regulation of glutamine-repressible gene products by the GLN3 function in Saccharomyces cerevisiae

Mutants of the yeast Saccharomyces cerevisiae have been isolated which fail to derepress glutamine synthetase upon glutamine limitation. The mutations define a single nuclear gene, GLN3, which is located on chromosome 5 near HOM3 and HIS1 and is unlinked to the structural gene for glutamine synthetase, GLN1. The three gln3 mutations are recessive, and one is amber suppressible, indicating that the GLN3 product is a positive regulator of glutamine synthetase expression. Four polypeptides, in addition to the glutamine synthetase subunit are synthesized at elevated rates when GLN3+ cultures are shifted from glutamine to glutamate media as determined by pulse-labeling and one- and two-dimensional gel electrophoresis. The response of all four proteins is blocked by gln3 mutations. In addition, the elevated NAD-dependent glutamate dehydrogenase activity normally found in glutamate-grown cells is not found in gln3 mutants. Glutamine limitation of gln1 structural mutants has the opposite effect, causing elevated levels of NAD-dependent glutamate dehydrogenase even in the presence of ammonia. We suggest that there is a regulatory circuit that responds to glutamine availability through the GLN3 product.

Regulation of glutamine-repressible gene products by the GLN3 function in Saccharomyces cerevisiae.

Mutants of the yeast Saccharomyces cerevisiae have been isolated which fail to derepress glutamine synthetase upon glutamine limitation. The mutations define a single nuclear gene, GLN3, which is located on chromosome 5 near HOM3 and HIS1 and is unlinked to the structural gene for glutamine synthetase, GLN1. The three gln3 mutations are recessive, and one is amber suppressible, indicating that the GLN3 product is a positive regulator of glutamine synthetase expression. Four polypeptides, in addition to the glutamine synthetase subunit are synthesized at elevated rates when GLN3+ cultures are shifted from glutamine to glutamate media as determined by pulse-labeling and one- and two-dimensional gel electrophoresis. The response of all four proteins is blocked by gln3 mutations. In addition, the elevated NAD-dependent glutamate dehydrogenase activity normally found in glutamate-grown cells is not found in gln3 mutants. Glutamine limitation of gln1 structural mutants has the opposite effect, causing elevated levels of NAD-dependent glutamate dehydrogenase even in the presence of ammonia. We suggest that there is a regulatory circuit that responds to glutamine availability through the GLN3 product.

Three regulatory systems control production of glutamine synthetase in Saccharomyces cerevisiae

Production of glutamine synthetase in Saccharomyces cerevisiae is controlled by three regulatory systems. One system responds to glutamine levels and depends on the positively acting GLN3 product. This system mediates derepression of glutamine synthetase in response to pyrimidine limitation as well, but genetic evidence argues that this is an indirect effect of depletion of the glutamine pool. The second system is general amino acid control, which couples derepression of a variety of biosynthetic enzymes to starvation for many single amino acids. This system operates through the positive regulatory element GCN4. Expression of histidinol dehydrogenase, which is under general control, is not stimulated by glutamine limitation. A third system responds to purine limitation. No specific regulatory element has been identified, but depression of glutamine synthetase is observed during purine starvation in gln3 gcn4 double mutants. This demonstrates that a separate purine regulatory element must exist. Pulse-labeling and immunoprecipitation experiments indicate that all three systems control glutamine synthetase at the level of subunit synthesis.