O-GlcNAcylation in cellular functions and human diseases

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Abstract

O-GlcNAcylation is dynamic and a ubiquitous post-translational modification. O-GlcNAcylated proteins influence fundamental functions of proteins such as protein–protein interactions, altering protein stability, and changing protein activity. Thus, aberrant regulation of O-GlcNAcylation contributes to the etiology of chronic diseases of aging, including cancer, cardiovascular disease, metabolic disorders, and Alzheimer's disease. Diverse cellular signaling systems are involved in pathogenesis of these diseases. O-GlcNAcylated proteins occur in many different tissues and cellular compartments and affect specific cell signaling. This review focuses on the O-GlcNAcylation in basic cellular functions and human diseases.

Introduction

The post-translational attachment of UDP-GlcNAc residue via a β-O-glycosidic linkage to serine and threonine residues of nucleocytoplasmic proteins is termed the O-GlcNAc modification (O-GlcNAcylation). O-GlcNAc addition and removal from the Ser and Thr residues of proteins is catalyzed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively (Fig. 1). O-GlcNAcylation is an end point of the hexosamine biosynthetic pathway, which culminates in the production of UDP-N-acetylglucosamine (UDP-GlcNAc) (Marshall et al., 1991). UDP-GlcNAc is the unique donor for the O-linkage of an O-GlcNAc to many cytoplasmic and nuclear proteins (Holt and Hart, 1986). O-GlcNAcylation influences fundamental functions of proteins by regulating protein–protein interactions, altering protein stability, and changing protein activity (Hart et al., 2007, Hart et al., 2010, Torres and Hart, 1984) (Fig. 2). O-GlcNAcylation is in many ways distinct from ‘classical’ protein glycosylation in that it is found mostly within the cytoplasm or nucleoplasm and O-GlcNAc is not elongated or further modified. Interestingly, O-GlcNAc cycle is similar to phosphorylation.

Many nuclear proteins are modified by O-linked GlcNAc. In fact, over 25% of the O-GlcNAc modified proteins are involved in transcriptional regulation. O-GlcNAc modification of transcription factors is critical for transcriptional regulation in various tissues (Comer and Hart, 1999). Many transcription factors are O-GlcNAcylated in response to physiological stimuli and this modification can modulate their function in different ways. O-GlcNAcylated proteins also directly regulate the activities of various transcription factors.

It has been observed that the cell rapidly changes its OGlcNAcylation levels in response to stress, and O-GlcNAcylation plays a protective role against cellular stress and cell death (Comer and Hart, 1999, Zachara et al., 2004). Specifically, upon heat shock treatment, O-GlcNAc is rapidly increased on cellular proteins followed by elevation of heat shock protein 70 (Hsp70). Experimentally decreasing OGT and/or O-GlcNAc levels results in cells that are less stress tolerant, whereas increasing the levels of OGT and/or O-GlcNAc results in cells that are more viable after severe stress. Thus, many report have suggested that short-term increase in total O-GlcNAcylation might be an important survival mechanism (Zachara and Hart, 2004, Zachara and Hart, 2006).

It has studied the important roles of O-GlcNAc cycling during cell cycle. Many mitotic regulators of O-GlcNAcylation have been identified. OGA and OGT localizes in a transient complex with vimentin (a cytoskeletal protein), Aurora kinase B (AKB), and protein phosphatase 1 (PP1) at the midbody during cytokinesis, which cooperatively regulates the posttranslational status of proteins (Slawson et al., 2008). In addition, nuclear mitotic proteins play an important role through O-GlcNAc modification. Correlatively, OGA deletion causes genomic instability (Yang et al., 2012). Many evidences have suggested that the precise control of O-GlcNAcylation on mitotic regulators is critical for mitosis.

Dysregulation of O-GlcNAcylation is closely associated with chronic diseases of aging, including cancer, cardiovascular disease, metabolic disorders, and Alzheimer's disease (Fig. 3). In particular, the roles of O-GlcNAcylation in metabolic signaling firstly suggested. OGT allows the termination of the insulin transduction cascade by interacting with PIP3 at the plasma membrane. OGT then O-GlcNAcylates many components of the insulin signaling pathway, which consequently attenuates insulin signaling (Yang et al., 2008b). However, several studies suggest that increased O-GlcNAcylation by potent and selective OGA inhibitor doesn't affect insulin signaling (Macauley et al., 2010a, Macauley et al., 2010b). Strikingly, O-GlcNAcylation of Carbohydrate-responsive element–binding protein (ChREBP) increases its stability and increases transcription of glycolytic and lipogenic genes in liver. This suggests that O-GlcNAcylation is important for ChREBP activity in the liver metabolism (Guinez et al., 2011). In addition to ChREBP, liver X receptors (LXRs) is also O-GlcNAcylated in liver. O-GlcNAcylated LXRs regulates sterol regulatory element binding protein 1c (SREBP-1c), a key regulator of lipogenesis (Anthonisen et al., 2010). Notably, elevated O-GlcNAcylation stimulates fatty acid oxidation through AMP-activated protein kinase (AMPK) activation in adipocytes (Luo et al., 2007). Interestingly, OGT/Host cell factor C1 (HCF-1) act as a glucose sensor and important for gluconeogenesis. HCF-1 recruits OGT to modify PGC1-α with O-GlcNAc, O-GlcNAcylated PGC1-α upregulates gluconeogenesis (Ruan et al., 2012). These data indicates that O-GlcNAcylation is critical for regulation of metabolism.

O-GlcNAc and O-GlcNAc cycling enzymes are highly detected in brain. O-GlcNAcylation is involved in neuronal development and synaptic plasticity (O'Donnell et al., 2004, Tallent et al., 2009). OGT and OGA genes are located in chromosomal regions related to the neurodegenerative disease. In particular, aberrant O-GlcNAcylation of τ is associated with Alzheimer's disease. Interestingly, O-GlcNAcylation of human τ negatively regulates its O-phosphorylation in a site-specific manner (Liu et al., 2004). Recent study demonstrated that increased O-GlcNAcylation stabilizes τ and inhibits aggregation of unrelated proteins, which hinder progression of Alzheimer's disease (Yuzwa et al., 2012). In addition to Alzheimer's disease, O-GlcNAcylation of many other proteins plays a role in brain function and diseases (Lefebvre et al., 2003).

O-GlcNAcylation is abnormally regulated in tumors and in cancer cell lines. O-GlcNAcylation plays important roles in cell division, metabolism, and cytoskeletal regulation in tumor growth. Therefore, aberrant regulations of O-GlcNAc cycling affect tumorigenesis and tumor metastasis. In breast tissue or thyroid cancers, decreased O-GlcNAc levels and increased OGA enzymatic activity was observed (Krzeslak et al., 2010, Slawson et al., 2001). In contrast, however, many studies have shown elevated O-GlcNAcylation level and O-GlcNAc cycling enzymes in lung tumor, colon tumor, and chronic lymphocytic leukemia (Gu et al., 2010, Shi et al., 2010, Yu et al., 2011). OGT knockdown suppresses tumor growth both in vitro and in an in vivo mouse model (Caldwell et al., 2010). Correlatively, major oncogenic factors are O-GlcNAcylated; p53, MYC, NF-κB, Snail, HCF-1, and β-catenin (Slawson and Hart, 2011). Thus, abnormally elevated O-GlcNAcylation perturbs cell signaling, transcriptional regulation, and cell cycle control, which contribute to tumorigenesis and metastasis.

O-GlcNAcylation is acutely elevated in response to cellular stresses. In cardiac myocytes, O-GlcNAc cycling is essential for protection against heart damage such as oxidative, hypoxic, ER stress (Yang et al., 2006, Zou et al., 2007, Zou et al., 2009b). Increased O-GlcNAcylation induced by glucosamine and PUGNAc (OGA inhibitor) administration improves recovery from trauma hemorrhage in rats (Yang et al., 2006, Zou et al., 2007). Similarly, glucosamine and PUGNAc inhibits acute inflammatory and in balloon-injured rat carotid arteries (Xing et al., 2008). Moreover, glucosamine significantly blocked sever calcium overload in the isolated perfused rat heart following the calcium paradox. In addition, glucosamine suppresses angiotensin II-induced calcium increase, indicating that O-GlcNAcylation regulates calcium influx in heart (Nagy et al., 2006). Interestingly, OGT regulates mitochondrial level by altering sensitivity to loss of mitochondrial membrane potential and mPTP formation. This mechanism explains that O-GlcNAcylation protect post-hypoxic cardiac myocytes (Ngoh et al., 2008). These results suggest that hexosamine biosynthetic pathway is important for cardiac health and diseases.

Arterial hypertension is a major factor for cardiovascular disease. Several evidences suggest that O-GlcNAcylation is involved in vascular functions. Hyperglycemia stimulates hexosamine biosynthetic pathway, which consequently elevate O-GlcNAcylation. Thus, many studies have suggested that hyperglycemia-induced diabetic vascular defect is closely associated with O-GlcNAcylation. Significantly, Sp-1, endothelial nitric oxide synthase (eNOS), and Akt are O-GlcNAcylated and involved in vascular functions (Du et al., 2000, Lima et al., 2009). Hyperglycemia-induced mitochondrial superoxide overproduction elevates Sp-1 O-GlcNAcylation, which activates expression of genes that contribute to the pathogenesis of diabetic complications (Du et al., 2000). Hyperglycemia in diabetic rats increases O-GlcNAcylation of eNOS in the penis and inhibits eNOS phosphorylation, which impairs erectile response (Musicki et al., 2005). In addition, hyperglycemia inhibits eNOS activity through activating the hexosamine pathway via mitochondrial overproduction of superoxide in cultured Bovine aortic endothelial cells (Du et al., 2001). Such elevated O-GlcNAcylation is also associated with impaired vasodilator activity in deoxycorticosterone acetate-salt hypertension (Lima et al., 2009). Vascular inflammation contributes to pathogenesis of cardiovascular defects. Several reports have suggested that elevated O-GlcNAcylation attenuates NF-κB signaling activation in rat aortic smooth muscle cells and primary cultured cardiomyocytes (Xing et al., 2011, Zou et al., 2009a). This effect is caused O-GlcNAcylation of key regulators (p65, IKKβ and TAK1) of NF-κB signaling (Kawauchi et al., 2009, Pathak et al., 2012, Xing et al., 2011, Yang et al., 2008a). These data indicate that O-GlcNAc signaling play key roles in the pathophysiology of cardiovascular diseases.

Many aspects of O-GlcNAcylation in diverse pathophysiology have been suggested. However, no clear pattern has emerged in diverse diseases. It is dependent on the experimental design and conditions. Because O-GlcNAc cycling enzymes are single genes, unlike kinases and phosphatases, OGT can simultaneously catalyze O-GlcNAcylation of many target proteins. Thus, careful dissection of the effect of different signaling systems on O-GlcNAcylation is needed. To understand more about the O-GlcNAc signaling in human diseases, advances in targeted OGT or OGA knockouts in tissues-specific manner are necessary in pathophysiological conditions.

Section snippets

Acknowledgments

This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (KRF-2007-341-C00027).

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