Activation of the Hexosamine Signaling Pathway in Adipose Tissue Results in Decreased Serum Adiponectin and Skeletal Muscle Insulin Resistance

Abstract

Overexpression of the rate-limiting enzyme for hexosamine synthesis (glutamine:fructose-6-phosphate amidotransferase) in muscle and adipose tissue of transgenic mice was previously shown to result in insulin resistance and hyperleptinemia. Explanted muscle from transgenic mice was not insulin resistant in vitro, suggesting that muscle insulin resistance could be mediated by soluble factors from fat tissue. To dissect the relative contributions of muscle and fat to hexosamine-induced insulin resistance, we overexpressed glutamine:fructose-6-phosphate amidotransferase 2.5-fold, specifically in fat under control of the aP2 promoter. Fasting glucose, insulin, and triglycerides were unchanged in the transgenic mice; leptin and beta-hydroxybutyrate levels were 91% and 29% higher, respectively. Fasted transgenic mice have mild glucose intolerance and skeletal muscle insulin resistance in vivo. In fasting transgenic mice, glucose disposal rates with hyperinsulinemia were decreased 27% in females and 10% in males. Uptake of 2-deoxy-D-glucose into muscle was diminished by 45% in female and 21% in male transgenics. Serum adiponectin was also lower in the fasted transgenics, by 37% in females and 22% in males. TNF alpha and resistin mRNA levels in adipose tissue were not altered in the fasted transgenics; levels of mRNA for leptin were increased and peroxisome proliferator-activated receptor gamma decreased. To further explore the relationship between adiponectin and insulin sensitivity, we examined mice that have been refed for 6 h after a 24-h fast. Refeeding wild-type mice resulted in decreased serum adiponectin and increased leptin. In transgenic mice, however, the regulation of these hormones by refeeding was lost for adiponectin and diminished for leptin. Refed transgenic female and male mice no longer exhibited decreased serum adiponectin in the refed state, and they were no longer insulin resistant as by lower or unchanged insulin and glucose levels. We conclude that increased hexosamine levels in fat, mimicking excess nutrient delivery, are sufficient to cause insulin resistance in skeletal muscle. Changes in serum adiponectin correlate with the insulin resistance of the transgenic animals.

Effects of exercise on the hexosamine biosynthetic pathway and glycosylation

“…In fact, transgenic mice overexpressing OGT in adipose tissue and striated muscle show decreased insulin-stimulated wholebody glucose disposal, and hyperinsulinemia and hyperleptinemia 40) , which are consistent with the phenotypes observed in GFAT transgenic mice 32,35,36) . Furthermore, insulin stimulation recruits OGT from the nucleus to the plasma membrane, wherein OGT catalyzes O-GlcNAcylation on some insulin signaling proteins and attenuates its signal transduction by altering their phosphorylation status, and hepatic overexpression of OGT impairs the transcription of insulin-responsive genes and causes the perturbation of insulin-induced inhibition of gluconeotransfer of a single GlcNAc from UDP-GlcNAc to serine/threonine residues of substrate proteins 25,26) ; whereas OGA catalyzes the removal of the GlcNAc from such residues 27,28) .…”

“…GLUT, glucose transporter; Glc, glucose; P, phosphate; Fruc, fructose; GlcN, glucosamine; GlcNAc, N-acetylglucosamine; UDP, uridine-5'-diphosphate; UTP, uridine-5'-triphosphate; GFAT, glutamine:Fruc-6-P amidotransferase; GNA, GlcN-6-P N-acetyltransferase; UAP, UDP-GlcNAc pyrophosphorylase; GnT, GlcNAc transferase; OGT, O-linked GnT. levels, indicating that cross-talk between adipocytes and skeletal muscle cells contributes to the development of whole body insulin resistance 32) . In contrast, transgenic mice overexpressing GLUT1 in skeletal muscle show impaired insulin-stimulated glucose transport activity in skeletal muscle 37) .…”

“…Since the affinity of OGT for substrate proteins is highly sensitive to the intracellular UDP-GlcNAc level 29) , glucose flux via the hexosamine biosynthetic pathway directly influences O-GlcNAc levels. Indeed, hyperglycemia 30) , glucosamine infusion 30) , GLUT overexpression 31) , and GFAT overexpression 32) , all of which increase flux via the hexosamine biosynthetic pathway, result in increased intracellular UDP-GlcNAc and OGlcNAc levels.…”

Cellular Content of UDP-N-acetylhexosamines Controls Hyaluronan Synthase 2 Expression and Correlates with O-Linked N-Acetylglucosamine Modification of Transcription Factors YY1 and SP1

“…The rate-limiting step in hexosamine synthesis from glucose to UDP-GlcNAc is considered to be the GFAT1 (glutamine:fructose-6-phosphate amidotransferase 1) and GFAT2 isoenzymes (20). The flux of glucose through the hexosamine pathway serves as a cellular sensor of glucose availability, and it regulates the expression of a number of genes probably through the cellular content of UDP-GlcNAc (19,21). Cytosolic UDP-GlcNAc is a substrate for UDP-GlcNAc:peptide ␤GlcNAc-transferase, an enzyme that adds a single GlcNAc sugar unit to -OH groups of selected Thr and Ser residues of cytosolic and nuclear proteins (22).…”

Importance of Dietary Fatty Acid Profile and Experimental Conditions in the Obese Insulin-Resistant Rodent Model of Metabolic Syndrome

“…In the presence of high circulating levels of FFAs, a decrease in the rate of carbohydrate oxidation is seen, and vice versa when high circulating levels of glucose prevail. It is in this context that the HBP serves as a nutrientsensing mechanism and both excess FFA and glucose can lead to augmented flux through this pathway, which ultimately plays a role in causing insulin resistance in the adipocyte, liver, muscle and pancreatic β-cell via reduced recruitment and translocation of GLUT4 to the plasma membrane [97,98]. In the presence of excess FFA, glycolysis is inhibited distal to fructose-6-phosphate by increased levels of acetyl-CoA, leading to inhibition of pyruvatedehydrogenase, which in turn increases fructose-6-phosphate availability and thus increased flux via catalyzation of glutamine:fructose 6-phosphate amido transferase (GFAT).…”