O-GlcNAcylation and oxidation of proteins: is signalling in the cardiovascular system becoming sweeter?

Abstract

O-GlcNAcylation is an unusual form of protein glycosylation, where a single-sugar [GlcNAc (N-acetylglucosamine)] is added (via β-attachment) to the hydroxyl moiety of serine and threonine residues of nuclear and cytoplasmic proteins. A complex and extensive interplay exists between O-GlcNAcylation and phosphorylation. Many phosphorylation sites are also known glycosylation sites, and this reciprocal occupancy may produce different activities or alter the stability in a target protein. The interplay between these two post-translational modifications is not always reciprocal, as some proteins can be concomitantly phosphorylated and O-GlcNAcylated, and the adjacent phosphorylation or O-GlcNAcylation can regulate the addition of either moiety. Increased cardiovascular production of ROS (reactive oxygen species), termed oxidative stress, has been consistently reported in various chronic diseases and in conditions where O-GlcNAcylation has been implicated as a contributing mechanism for the associated organ injury/protection (for example, diabetes, Alzheimer's disease, arterial hypertension, aging and ischaemia). In the present review, we will briefly comment on general aspects of O-GlcNAcylation and provide an overview of what has been reported for this post-translational modification in the cardiovascular system. We will then specifically address whether signalling molecules involved in redox signalling can be modified by O-GlcNAc (O-linked GlcNAc) and will discuss the critical interplay between O-GlcNAcylation and ROS generation. Experimental evidence indicates that the interactions between O-GlcNAcylation and oxidation of proteins are important not only for cell regulation in physiological conditions, but also under pathological states where the interplay may become dysfunctional and thereby exacerbate cellular injury.

Figure 1. The HBP

After entering the cell via a glucose transporter and being converted into glucose 6-phosphate (glucose-6P) by a hexokinase and into fructose 6-phosphate (fructose-6P), glucose can either be used in the glycolytic pathway or the HBP. The HBP uses fructose 6-phosphate to form glucosamine 6-phosphate (glucosamine-6P), with glutamine serving as the donor of the amino group. The reaction is catalysed by the rate-limiting enzyme GFAT. Glucosamine 6-phosphate is rapidly acetylated through the action of acetyl-CoA:D-glucosamine-6-phosphate N-acetyltransferase (GAT) and isomerized to N-acetylglucosamine-1-phosphate (GlcNAc-1-P) and activated, via the action of UDP-GlcNAc pyrophosphorylase (AGX), to UDP-GlcNAc that serves as the donor of O-GlcNAc for OGT activity. Glucosamine can also enter the cell through the glucose transporter and is rapidly phosphorylated by hexokinase yielding glucosamine 6-phosphate, thereby bypassing the rate-limiting first step of the HBP. S, serine; T, threonine;

Figure 2. Interplay between O-GlcNAcylation and phosphorylation of proteins

Both phosphorylation and O-GlcNAcylation occur on serine/threonine (Ser/Thr) residues of proteins. In specific proteins, there is a competitive relationship between O-GlcNAc and O-phosphate for the same serine/threonine residues, although there can be adjacent or multiple occupancy for phosphorylation and O-GlcNAcylation on the same protein. The interplay between phosphorylation and O-GlcNAcylation creates molecular diversity by altering specific protein sites that regulate protein functions and signalling events. Tyr, tyrosine.

Figure 3. O-GlcNAc formation in the cardiovascular system produces both protective and harmful effects

The effects of O-GlcNAcylation in the cardiovascular system may depend on whether they are acute or chronic effects on the initial cellular metabolic/energetic/redox status of the cell and on the specific proteins that are O-GlcNAc-modified in each cell type/tissue/organ.

Figure 4. Interplay between ROS, phosphorylation and O-GlcNAcylation

Evidence indicates that ROS stimulates not only phosphorylation, but also O-GlcNAcylation of proteins. On the other hand, augmented O-GlcNAc induces ROS production and/or up-regulation of stress-related proteins. Increased production of ROS has been consistently reported in various chronic diseases and conditions, including those where O-GlcNAcylation has been implicated as a mechanistic contributor or protective agent. The O-GlcNAc symbol indicates proteins that have already been identified as O-GlcNAcylated, according to the dbOGAP. Numbers in parenthesis indicate the dbOGAP ID. Catalase (OG00108–OG00119); glutathione transferase (OG00341, OG00342, OG01049, OG01062 and OG003437); SOD (OG00848 and OG01057); thioredoxin (OG01067, OG010679, OG00648 and OG006487); NO synthase (OG00587, OG005878, OG00589, OG00590, OG005891 and OG005892); NADH dehydrogenase [ubiquinone] 1α subcomplex subunit 9 (OG00557–OG00561). FAK, focal adhesion protein.