Skeletal muscle O-GlcNAc transferase is important for muscle energy homeostasis and whole-body insulin sensitivity

Abstract

ObjectiveGiven that cellular O-GlcNAcylation levels are thought to be real-time measures of cellular nutrient status and dysregulated O-GlcNAc signaling is associated with insulin resistance, we evaluated the role of O-GlcNAc transferase (OGT), the enzyme that mediates O-GlcNAcylation, in skeletal muscle.MethodsWe assessed O-GlcNAcylation levels in skeletal muscle from obese, type 2 diabetic people, and we characterized muscle-specific OGT knockout (mKO) mice in metabolic cages and measured energy expenditure and substrate utilization pattern using indirect calorimetry. Whole body insulin sensitivity was assessed using the hyperinsulinemic euglycemic clamp technique and tissue-specific glucose uptake was subsequently evaluated. Tissues were used for histology, qPCR, Western blot, co-immunoprecipitation, and chromatin immunoprecipitation analyses.ResultsWe found elevated levels of O-GlcNAc-modified proteins in obese, type 2 diabetic people compared with well-matched obese and lean controls. Muscle-specific OGT knockout mice were lean, and whole body energy expenditure and insulin sensitivity were increased in these mice, consistent with enhanced glucose uptake and elevated glycolytic enzyme activities in skeletal muscle. Moreover, enhanced glucose uptake was also observed in white adipose tissue that was browner than that of WT mice. Interestingly, mKO mice had elevated mRNA levels of Il15 in skeletal muscle and increased circulating IL-15 levels. We found that OGT in muscle mediates transcriptional repression of Il15 by O-GlcNAcylating Enhancer of Zeste Homolog 2 (EZH2).ConclusionsElevated muscle O-GlcNAc levels paralleled insulin resistance and type 2 diabetes in humans. Moreover, OGT-mediated signaling is necessary for proper skeletal muscle metabolism and whole-body energy homeostasis, and our data highlight O-GlcNAcylation as a potential target for ameliorating metabolic disorders.

Cross-Dysregulation of O-GlcNAcylation and PI3K/AKT/mTOR Axis in Human Chronic Diseases

“…These mutations may lead to reduced OGA expression and increased GFPT2 expression respectively ( 56 , 57 ), and an up-regulation of cellular O -GlcNAcylation levels. O -GlcNAcylation levels are significantly increased in skeletal muscle, liver, heart, colon-rectum, erythrocytes, and leukocytes of diabetic animals and humans ( 35 , 58 – 62 ). Consistent with these epidemiologic data, db/db mice overexpressing Oga showed improved hepatic insulin sensitivity ( 63 ), whereas Ogt overexpression and subsequent elevation of global O -GlcNAcylation level inhibits insulin signaling pathway, both in vitro in 3T3-L1 adipocyte and Fao hepatic cell lines, and in vivo in skeletal muscle and adipose tissue in mice ( 15 , 64 , 65 ).…”

Real Talk: The Inter-play Between the mTOR, AMPK, and Hexosamine Biosynthetic Pathways in Cell Signaling

“…HBP flux regulates multiple steps of the insulin signaling pathway ( 18 ). Skeletal muscle of mice containing an OGT-KO showed heightened glucose uptake in response to insulin as opposed to wild type counterparts, suggesting a link between insulin sensitivity and O-GlcNAc levels ( 19 ). When looking into the molecular mechanism behind this phenomenon, insulin receptor substrate 1 (IRS-1), a protein phosphorylated by the insulin receptor (IR) tyrosine kinase after it binds extracellular insulin, has multiple O-GlcNAcylation sites ( 20 ).…”

Critical observations that shaped our understanding of the function(s) of intracellular glycosylation (O‐GlcNAc)

“…Notably, these mice also display enhanced energy expenditure and reduced basal insulin levels. Consistent with the work of Ruan et al ., deletion of OGT protected from high‐fat–induced insulin resistance . In contrast to these data, and speaking to the complex role of O‐GlcNAc in regulating metabolism, deletion of OGT from the αCamKII neurons of the paraventricular nucleus (PVN) leads to enhanced body weight, a result of increased food intake .…”