O-Glycosylation Regulates Ubiquitination and Degradation of the Anti-Inflammatory Protein A20 to Accelerate Atherosclerosis in Diabetic ApoE-Null Mice

Abstract

BackgroundAccelerated atherosclerosis is the leading cause of morbidity and mortality in diabetic patients. Hyperglycemia is a recognized independent risk factor for heightened atherogenesis in diabetes mellitus (DM). However, our understanding of the mechanisms underlying glucose damage to the vasculature remains incomplete.Methodology/Principal FindingsHigh glucose and hyperglycemia reduced upregulation of the NF-κB inhibitory and atheroprotective protein A20 in human coronary endothelial (EC) and smooth muscle cell (SMC) cultures challenged with Tumor Necrosis Factor alpha (TNF), aortae of diabetic mice following Lipopolysaccharide (LPS) injection used as an inflammatory insult and in failed vein-grafts of diabetic patients. Decreased vascular expression of A20 did not relate to defective transcription, as A20 mRNA levels were similar or even higher in EC/SMC cultured in high glucose, in vessels of diabetic C57BL/6 and FBV/N mice, and in failed vein grafts of diabetic patients, when compared to controls. Rather, decreased A20 expression correlated with post-translational O-Glucosamine-N-Acetylation (O-GlcNAcylation) and ubiquitination of A20, targeting it for proteasomal degradation. Restoring A20 levels by inhibiting O-GlcNAcylation, blocking proteasome activity, or overexpressing A20, blocked upregulation of the receptor for advanced glycation end-products (RAGE) and phosphorylation of PKCβII, two prime atherogenic signals triggered by high glucose in EC/SMC. A20 gene transfer to the aortic arch of diabetic ApoE null mice that develop accelerated atherosclerosis, attenuated vascular expression of RAGE and phospho-PKCβII, significantly reducing atherosclerosis.ConclusionsHigh glucose/hyperglycemia regulate vascular A20 expression via O-GlcNAcylation-dependent ubiquitination and proteasomal degradation. This could be key to the pathogenesis of accelerated atherosclerosis in diabetes.

Hyperglycemic Conditions Prime Cells for RIP1-dependent Necroptosis

“…RIP1 is subject to proteasome degradation via K48-linked polyubiquitylation promoted by A20 (19). As hyperglycemia induces degradation of A20 (20) this may account for elevated RIP1 levels. The increased RIP1 may result in increased recruitment of RIP3 and MLKL (2).…”

Regulation of Protein Degradation by O-GlcNAcylation: Crosstalk with Ubiquitination

“…Shrikhande et al reported that hyperglycemia promotes the O-GlcNAcylation, ubiquitination, and degradation of A20, which accelerate atherosclerosis in diabetic mice (54). Interestingly, A20 has both ubiquitin ligase and deubiquitinase activities, suggesting that regulation of the A20 protein level by O-GlcNAcylation is a control point for the ubiquitination of A20 target proteins.…”

Dynamic O-GlcNAcylation and its roles in the cellular stress response and homeostasis

“…It was also speculated that an E1 ubiquitin-activating enzyme interacts with Hsp70 only in its O-GlcNAcylated form (Guinez et al 2008). An agonistic relationship between these two modifications is suggested such that O-GlcNAc may alter the activity of E1 enzymes to modulate stressinduced ubiquitination (Shrikhande et al 2010;Shimura et al 2001;Fujiki et al 2011). It has also been shown that OGlcNAcylation of a protein may facilitate its subsequent ubiquitination.…”