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Query: UMLS:C0011849 (diabetes)
 277,896 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The hexosamine biosynthesis pathway has been hypothesized to mediate some of the regulatory as well as the deleterious effects of glucose. We have stably overexpressed the cDNA for human glutamine:fructose-6-phosphate amidotransferase (GFA), the rate-limiting enzyme in the hexosamine biosynthesis pathway, in rat-1 fibroblasts. Two cell lines expressing the human RNA were selected by Northern analysis, and they exhibited 51-95% increases in GFA activity. Insulin-stimulated glycogen synthase (GS) activity and net glycogen synthesis were assayed, and GFA cells revealed decreased insulin sensitivity for both GS and net glycogen synthesis. The ED50 for insulin stimulation of GS was 2.45 +/- 0.4 nmol/l insulin in controls and 5.29 +/- 1.01 nmol/l in GFA cells (P < 0.005). For insulin-stimulated glycogen synthesis, the ED50 was 3.43 +/- 0.88 nmol/l in controls and 5.54 +/- 0.98 nmol/l in GFA cells (P < 0.005). There were no significant differences in maximally insulin-stimulated or total GS activities, insulin binding or receptor number, or glucose uptake between GFA and control cells. We also examined the effects of glucose on GS activity. GFA cells had a twofold increase in GS activity at low glucose (0.5 mmol/l) when compared with controls (P < 0.025). Both GFA and control cells had an approximately 75-80% decrease in GS activity as glucose concentration was increased from 0.5 to 20 mmol/l. This change in GS activity was not observed until after 12 h in culture. GFA cells were more sensitive to the effects of glucose.(ABSTRACT TRUNCATED AT 250 WORDS)
Diabetes 1995 Mar
PMID:Regulation of glycogen synthase by glucose, glucosamine, and glutamine:fructose-6-phosphate amidotransferase. 788 19

High glucose concentrations such as are seen in diabetes mellitus are known to have deleterious effects on cells, but the pathways by which glucose induces these effects are unknown. One hypothesis is that metabolism of glucose to glucosamine might be involved. For example, it has been shown that glucosamine is more potent than glucose in inducing insulin resistance in cultured adipocytes and in regulating the transcription of the growth factor transforming growth factor alpha in smooth muscle cells. The rate-limiting step in glucosamine synthesis is the conversion of fructose-6-phosphate to glucosamine-6-phosphate by the enzyme glutamine:fructose-6-phosphate amidotransferase. To test the hypothesis that this hexosamine biosynthesis pathway is involved in the induction of insulin resistance, we have overexpressed the enzyme glutamine:fructose-6-phosphate amidotransferase in Rat-1 fibroblasts and investigated its effects on insulin action in those cells. We electroporated Rat-1 fibroblasts with expression plasmids that did and did not contain the gene for glutamine:fructose-6-phosphate amidotransferase and measured glycogen synthase activity at varying insulin concentrations. Insulin stimulation was blunted in the glutamine:fructose-6-phosphate amidotransferase-transfected cells, resulting in decreased insulin sensitivity reflected by a rightward shift in the dose-response curve for activation of synthase (ED50 = 7.5 nM vs. 3.4 nM insulin, in glutamine:fructose-6-phosphate amidotransferase and control cells, respectively). Rat-1 fibroblasts incubated with 5.- mM glucosamine for 3 days exhibited a similar shift in the dose-response curve. The rightward shift in the dose-response curve is seen as early as 2 days after poration.(ABSTRACT TRUNCATED AT 250 WORDS)
Diabetes 1993 Sep
PMID:Regulation of insulin-stimulated glycogen synthase activity by overexpression of glutamine: fructose-6-phosphate amidotransferase in rat-1 fibroblasts. 834 40

The hexosamine biosynthesis pathway has been hypothesized to be involved in mediating some of the adverse effects of high glucose. We have previously shown that glucose downregulates basal glycogen synthase (GS) activity in Rat-1 cells and that overexpressing the rate-limiting enzyme in the hexosamine biosynthesis pathway (glutamine:fructose-6-phosphate amidotransferase [GFA]) makes the cells more sensitive to these effects of glucose. GFA overexpression also leads to a reduction in insulin sensitivity of GS. Here we examine the effects of glucose and glucosamine on insulin-stimulated GS activity and on protein phosphatase-1 (PP1) activity. These activities were assayed in cytoplasmic extracts from Rat-1 fibroblasts overexpressing human GFA and cultured in varying glucose concentrations. Both maximal insulin-stimulated GS activity and insulin sensitivity decreased with increasing glucose. Overexpression of GFA leads to a further reduction in insulin sensitivity but not in maximal insulin-stimulated GS activity. Because there were no differences in total (glucose-6-phosphate-dependent) GS activity between cell lines or as a function of glucose concentration, these results most likely reflect a change in the phosphorylation state of the synthase. Activity of PP1, a potential mediator of these effects, was responsive to glucose and hexosamines. Control cells showed a 9.3 +/- 4.3% decrease in PP1 activity with increasing glucose. GFA cells showed a greater response to glucose, with PP1 activity decreasing 34.2 +/- 5.5% with increasing glucose. Glucosamine was more potent than glucose in decreasing PP1 activity in control cells. Cells overexpressing the normal human insulin receptor (HIRc-B) were used to facilitate analysis of insulin-stimulated PP1 activity. Stimulation with 1.7 mmol/l insulin led to a 37.6 +/- 9.9% increase in PP1 activity in HIRc-B cells cultured in 1 mmol/l glucose, while cells cultured in 5 mmol/l glucosamine or 20 mmol/l glucose demonstrated only 3.79 +/- 0.60 or 1.6 +/- 0.75% increases, respectively. We conclude that both basal and insulin- stimulable GS and PP1 activity are downregulated by high glucose in fibroblasts and this regulation is mediated by products of the hexosamine biosynthesis pathway.
Diabetes 1996 Mar
PMID:Regulation of glycogen synthase and protein phosphatase-1 by hexosamines. 859 37

We examined the activity of the rate-limiting enzyme for hexosamine biosynthesis, glutamine:fructose-6-phosphate amidotransferase (GFA) in human skeletal muscle cultures (HSMC), from 17 nondiabetic control and 13 subjects with non-insulin-dependent diabetes. GFA activity was assayed from HSMC treated with low (5 mM) or high (20 mM) glucose and low (22 pM) or high (30 microM) concentrations of insulin. In control subjects GFA activity decreased with increasing glucose disposal rate (r = -0.68, P < 0.025). In contrast, a positive correlation existed between GFA and glucose disposal in the diabetics (r = 0.86, P < 0.005). Increased GFA activity was also correlated with body mass index in controls but not diabetics. GFA activity was significantly stimulated by high glucose (22%), high insulin (43%), and their combination (61%). GFA activity and its regulation by glucose and insulin were not significantly different in diabetic HSMC. We conclude that glucose and insulin regulate GFA activity in skeletal muscle. More importantly, our results are consistent with a regulatory role for the hexosamine pathway in human glucose homeostasis. This relationship between hexosamine biosynthesis and the regulation of glucose metabolism is altered in non-insulin-dependent diabetes.
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PMID:Glutamine:fructose-6-phosphate amidotransferase activity in cultured human skeletal muscle cells: relationship to glucose disposal rate in control and non-insulin-dependent diabetes mellitus subjects and regulation by glucose and insulin. 863 35

Glucose is an important regulator of cell growth and metabolism. Thus, it is likely that some of the adverse effects of hyperglycemia are reflections of normal regulation by abnormal concentrations of glucose. How the cell senses glucose, however, is still incompletely understood. Evidence has been presented that the hexosamine biosynthesis pathway serves this function for regulation of aspects of glucose uptake, glycogen synthesis, glycolysis, and synthesis of growth factors. Excess hexosamine flux causes insulin resistance in cultured cells, tissues, and intact animals. Further evidence for the possible role of this pathway in normal glucose homeostasis and disease is that the level of activity of the rate-limiting enzyme in hexosamine synthesis, glutamine:fructose-6-phosphate amidotransferase, is correlated with glucose disposal rates (GDRs) in normal humans and transgenic mice.
Diabetes 1996 Aug
PMID:Hexosamines and insulin resistance. 869 Jan 44

Insulin resistance is associated with diabetes. Hyperglycemia per se causes insulin resistance as well as increased flux of glucose through the hexosamine biosynthetic pathway. The rate-limiting enzyme for entry of glucose into this pathway is glutamine:fructose-6-phosphate amidotransferase (GFAT). To directly evaluate the role of GFAT in modulating insulin-stimulated glucose transport, we co-transfected primary cultures of rat adipose cells with expression vectors for human GFAT as well as an epitope-tagged GLUT4 and examined the effect of overexpressed GFAT on insulin-stimulated translocation of GLUT4. When we measured cell surface tagged GLUT4 in response to insulin, cells overexpressing GFAT and tagged GLUT4 had an insulin-dose response curve that was similar to that of control cells expressing only tagged GLUT4. As an alternative means of increasing flux through the hexosamine biosynthetic pathway, we incubated adipose cells with glucosamine (a substrate of the pathway downstream from GFAT) and insulin. Interestingly, for short incubation times (4 h) we observed a decrease in both basal and insulin-stimulated glucose transport without a detectable effect on insulin-stimulated translocation of GLUT4. However, for longer incubation times (16 h), we observed a significant decrease in the amount of GLUT4 in the plasma membrane. Our data suggest that products of the hexosamine biosynthetic pathway may cause insulin resistance, in part, by acutely decreasing intrinsic activity of GLUT4 as well as chronically altering the amount of GLUT4 at the cell surface.
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PMID:Effects of overexpression of glutamine:fructose-6-phosphate amidotransferase (GFAT) and glucosamine treatment on translocation of GLUT4 in rat adipose cells. 945 42

Recent in vitro and in vivo studies suggested that the increased flux of glucose through the hexosamine biosynthetic pathway may contribute to glucose-induced insulin resistance and to the induction of the synthesis of growth factors. Because glutamine:fructose-6-phosphate amidotransferase (GFAT) catalyzes the first and rate-limiting step in the formation of hexosamine products, this enzyme is the key regulator in this pathway and is therefore possibly also involved in the alterations occurring in preclinical or manifest diabetic patients. To study the expression of GFAT in human tissues, we produced and characterized a peptic antiserum specifically recognizing GFAT protein and a riboprobe for the detection of GFAT mRNA. Immunohistochemical and nonradioactive in situ hybridization analysis revealed high levels of expression of GFAT protein and mRNA in adipocytes and skeletal muscle. Furthermore, a marked GFAT expression was found in vascular smooth muscle cells with unexpectedly high variability and lower levels in other cells, e.g., peripheral nerve sheath cells or endocrine-active cells, including the pancreatic islet cell. GFAT protein expression was below detection level in endothelium, osteocytes, lymphocytes, granulocytes, and in most quiescent fibroblasts. In renal tissue, GFAT was expressed in tubular epithelial cells, while glomerular cells remained essentially unstained. Renal sections obtained from patients with diabetic nephropathy showed significant GFAT expression in some glomerular epithelial and mesangial cells, indicating that GFAT expression may be induced by manifest diabetes. Our data indicate that GFAT is expressed in most tissues involved in the development of diabetic late complications. Furthermore, the results suggest that GFAT gene expression is highly regulated.
Diabetes 1998 Feb
PMID:Expression of glutamine:fructose-6-phosphate amidotransferase in human tissues: evidence for high variability and distinct regulation in diabetes. 951 9

We subcloned human and mouse full-length cDNAs of a novel subtype of glutamine:fructose-6-phosphate amidotransferase (GFAT), which was designated GFAT2 (the previously reported GFAT was named GFAT1). Both the human and the mouse GFAT2 proteins deduced from their open reading frame sequences are composed of 682 amino acids of approximately 77.0 kDa. At the amino acid level, homologies between the human GFAT1 and GFAT2, between the mouse GFAT1 and GFAT2, and between the human GFAT2 and the mouse GFAT2 were 75.6, 74.7, and 97. 2%, respectively. Northern blot analysis using probe specific to human GFAT1 or GFAT2 showed that major transcripts were approximately 3.0 kb in both the human GFAT subtypes. The analysis also revealed different tissue distribution between GFAT1 and GFAT2: GFAT1 was more highly expressed in the placenta, pancreas, and testis than GFAT2; GFAT2 was expressed throughout the central nervous system, especially in the spinal cord, but GFAT1 expression was weak. The locus was mapped to human chromosome 5q and mouse chromosome 11, where a synteny between the two species has been known. GFAT2 can provide insights into understanding the roles of the hexosamine pathway in various tissues, particularly with the development of glucose toxicity and diabetes complications.
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PMID:cDNA cloning and mapping of a novel subtype of glutamine:fructose-6-phosphate amidotransferase (GFAT2) in human and mouse. 1019 62

Hexosamines have been shown to mediate effects of hyperglycemia and so-called "glucose toxicity" in insulin-sensitive tissues. To determine the effects of hexosamines on insulin synthesis and secretion, transgenic mice were created to overexpress the rate-limiting enzyme for hexosamine synthesis, glutamine:fructose-6-phosphate amidotransferase (GFA), specifically in beta-cells. GFA activity in islets of heterozygous transgenic mice was elevated 76% compared with littermate controls. The increased GFA activity led to 1.4- and 2.1-fold increased pancreatic insulin content in 2- and 10-month-old transgenic mice, respectively (P < 0.005). Fasting insulin levels were 1.6-fold higher than in littermate controls (P < 0.05). Hyperinsulinemia was evident despite a 28% reduction in insulin mRNA levels. The fasting glucose levels in the transgenic mice equaled that of controls aged 2-4 months but exceeded that of the controls aged 6-10 months (means +/- SE 6.9 +/- 0.2 vs. 5.9 +/- 0.2 mmol/l, P < 0.001). By 8 months, the males were overweight and mildly diabetic (fasting glucose 8.8 +/- 0.5 mmol/l) despite persistent hyperinsulinemia. Insulin resistance was confirmed in both males and females using the euglycemic-hyperinsulinemic clamp technique; glucose disposal rates decreased by 48% in transgenic mice (P < 0.01). Triglyceride levels did not differ, and free fatty acid levels were lower in the transgenic animals. ATP levels were unchanged in the transgenic islets. We conclude that hexosamine biosynthesis is involved in the regulation of insulin content in beta-cells by glucose. Increased hexosamine flux in the beta-cell results in hyperinsulinemia, insulin resistance, and (in males) mild type 2 diabetes.
Diabetes 2000 Sep
PMID:Transgenic mice with increased hexosamine flux specifically targeted to beta-cells exhibit hyperinsulinemia and peripheral insulin resistance. 1096 33

To examine the effect of increased hexosamine flux in liver, the rate-limiting enzyme in hexosamine biosynthesis (glutamine:fructose-6-phosphate amidotransferase [GFA]) was overexpressed in transgenic mice using the PEPCK promoter. Liver from random-fed transgenic mice had 1.6-fold higher GFA activity compared with nontransgenic control littermates (276 +/- 24 pmol x mg(-1) x min(-1) in transgenic mice vs. 176 +/- 18 pmol x mg(-1) x min(-1) in controls, P < 0.05) and higher levels of the hexosamine end product UDP-N-acetyl glucosamine (288 +/- 11 pmol/g in transgenic mice vs. 233 +/- 10 pmol/g in controls, P < 0.001). Younger transgenic mice compared with control mice had lower fasting serum glucose (4.8 +/- 0.5 mmol/l in transgenic mice vs. 6.5 +/- 0.8 mmol/l in controls, P < 0.05) without higher insulin levels (48.0 +/- 7.8 pmol/l in transgenic mice vs. 56.4 +/- 5.4 pmol/l in controls, P = NS); insulin levels were significantly lower in transgenic males (P < 0.05). At 6 months of age, transgenic animals had normal insulin sensitivity by the hyperinsulinemic clamp technique. Hepatic glycogen content was higher in the transgenic mice (108.6 +/- 5.2 pmol/g in transgenic mice vs. 32.8 +/- 1.3 micromol/g in controls, P < 0.01), associated with an inappropriate activation of glycogen synthase. Serum levels of free fatty acids (FFAs) and triglycerides were also elevated (FFAs, 0.67 +/- 0.03 mmol/l in transgenic mice vs. 0.14 +/- 0.01 in controls; triglycerides, 1.34 +/- 0.15 mmol/l in transgenic mice vs. 0.38 +/- 0.01 in controls, P < 0.01). Older transgenic mice became heavier than control mice and exhibited relative glucose intolerance and insulin resistance. The glucose disposal rate at 8 months of age was 154 +/- 5 mg x kg(-1) x min(-1) in transgenic mice vs. 191 +/- 6 mg x kg(-1) x min(-1) in controls (P < 0.05). We conclude that hexosamines are mediators of glucose sensing for the regulation of hepatic glycogen and lipid metabolism. Increased hexosamine flux in the liver signals a shift toward fuel storage, resulting ultimately in obesity and insulin resistance.
Diabetes 2000 Dec
PMID:Overexpression of glutamine: fructose-6-phosphate amidotransferase in the liver of transgenic mice results in enhanced glycogen storage, hyperlipidemia, obesity, and impaired glucose tolerance. 1111 9


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