O-GlcNAc signaling in the cardiovascular system

Abstract

Cardiovascular function is regulated at multiple levels. Some of the most important aspects of such regulation involve alterations in an ever-growing list of posttranslational modifications. One such modification orchestrates input from numerous metabolic cues to modify proteins and alter their localization and/or function. Known as the beta-O-linkage of N-acetylglucosamine (ie, O-GlcNAc) to cellular proteins, this unique monosaccharide is involved in a diverse array of physiological and pathological functions. This review introduces readers to the general concepts related to O-GlcNAc, the regulation of this modification, and its role in primary pathophysiology. Much of the existing literature regarding the role of O-GlcNAcylation in disease addresses the protracted elevations in O-GlcNAcylation observed during diabetes. In this review, we focus on the emerging evidence of its involvement in the cardiovascular system. In particular, we highlight evidence of protein O-GlcNAcylation as an autoprotective alarm or stress response. We discuss recent literature supporting the idea that promoting O-GlcNAcylation improves cell survival during acute stress (eg, hypoxia, ischemia, oxidative stress), whereas limiting O-GlcNAcylation exacerbates cell damage in similar models. In addition to addressing the potential mechanisms of O-GlcNAc-mediated cardioprotection, we discuss technical issues related to studying protein O-GlcNAcylation in biological systems. The reader should gain an understanding of what protein O-GlcNAcylation is and that its roles in the acute and chronic disease settings appear distinct.

Figure 1. Hexosamine Biosynthetic Pathway

Phosphorylated glucose enters either the glycogen synthetic pathway or is further converted to fructose-6-phosphate by glucose-6-phosphate isomerase. The majority of fructose-6-phosphate is channeled to glycolysis. Less than 5% of glucose uptake is ultimately channeled to a unique accessory pathway for glucose metabolism, the hexosamine biosynthetic pathway (HBP). This pathway begins with the rate-limiting enzyme, L-glutamine:D-fructose-6-phosphate amidotransferase (GFAT), followed by acetylation of gluocsamine-6-phosphate by glucosamine-6-phosphate acetyltransferase (Emeg32) to N-acetylglucosamine-6-phosphate (GlcNAc-6-P). Next are two reversible reactions: the conversion of GlcNAc-6-P to GlcNAc-1-P by phosphate-acetylglucosamine mutase and then the formation uridine diphosphate-GlcNAc (UDP-GlcNAc) by UDP-GlcNAc pyrophosphorylase. This high-energy molecule serves as the monosaccharide donor for the post-translational modification by O-GlcNAc transferase (OGT). O-GlcNAcase (OGA) removes O-GlcNAc modification from proteins.

Figure 2. O-GlcNAc Affects Insulin Signaling

Insulin binds to tyrosine kinase receptor and rapidly activates intracellular signaling. PI3K gives rise to PIP3, which serves to anchor PDK1 and AKT on the membrane. According to Yang et al, PIP3 also recruits OGT to the plasma membrane where OGT attenuates insulin signaling by O-glycosylation of Thr 308, consequently inhibiting phosphorylation of Akt at the same residue. Excessive activation of OGT could disturb insulin signaling. Hyperglycemia may also stimulate the O-GlcNAcylation of CRTC2 and its migration into the nucleus, where this coactivator binds to CREB:CBP and stimulates transcription of gluconeogenic genes (PEPCK and G6Pase). The transcription factor FOXO1 is also O-GlcNAcylated under similar conditions and stimulates gluconeogenic gene transcription, .

Figure 3. Mitochondria and O-GlcNAc Signaling

Myocardial ischemia induces mitochondrial Ca²⁺ overload and ROS generation, with subsequent mitochondrial permeability transition pore formation (mPTP). Formation of mPTP causes loss of mitochondrial trans-inner membrane potential, mitochondrial swelling, rupture of mitochondrial membrane, and cytochrome c release. Augmentation of O-GlcNAc levels prior to myocardial ischemia attenuates ischemia-induced Ca²⁺ overload, ROS generation, and subsequent mPTP formation. Augmented O-GlcNAc signaling also mitigates mPTP formation by possibly augmenting O-GlcNAcylation of VDAC and/or BCl-2 interaction with VDAC. This is an example of one potential mechanism; there are likely several. Moreover, augmented O-GlcNAc levels diminish mPTP-mediated mitochondrial swelling, loss of mitochondrial membrane potential, and cytochrome c release.

Figure 4. Common Control Measures for O-GlcNAc Immunoblots

O-GlcNAc immunoblots show multiple bands because it is a post-translational modification of numerous proteins. The blot on the left is the result of a CTD110.6 antibody co-incubated with N-acetylglucosamine, which competes for binding with the antibody. The blot on the right is the same membrane and primary antibody (CTD110.6), without GlcNAc. In addition, the lysate loaded in lane 6 is the result of a parallel aliquot of lane 5 incubated with O-GlcNAcase (in vitro), showing the loss of immunopositivity and validating the signal is O-GlcNAc. Such simple measures can confirm the fidelity of the O-GlcNAc signal via westerns.