Having outlined the role of O-GlcNAc in metabolism, the following section will discuss its roles specifically in the liver. The liver is a central player in carbohydrate, lipid, and amino acid metabolism, storing, releasing, and interconverting carbohydrates and lipids to respond to systemic nutrient availability. Dysregulation of liver carbohydrate and lipid metabolism is also a key feature in metabolic diseases such as T2DM, metabolic syndrome, and nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis. Under normal circumstances, the liver takes up glucose from the blood to make glycogen in the fed state which it slowly breaks down and releases
to maintain blood sugar in the fasted state. As this glycogen reserve is exhausted, i.e. with prolonged fasting, the liver will produce glucose from other sources, such as those derived from lipid metabolism or amino acids. Similarly, when there is a prolonged excess of carbohydrate and amino acid levels, the liver converts these molecules into fatty acids and triglycerides, which are then exported to and stored in adipose tissue throughout the body. The liver typically undertakes these functions in response to hormonal signals from the pancreas (and to a lesser extent, other organs). Briefly, in response to rising glucose levels, the pancreas excretes insulin which promotes uptake of glucose and glycogen and fatty acid synthesis by the liver (conversely inhibiting gluconeogenesis). Conversely, in response to falling glucose levels, the pancreas produces glucagon, which stimulates gluconeogenesis and glycogenolysis. Thus, the liver is the major organ responsible for maintaining systemic glucose homeostasis.
As mentioned in the previous sections, altered O-GlcNAcylation plays a central role in regulating insulin signaling, and this is no different in the liver.1 However, as the liver is the major site of gluconeogenesis for the organism, insulin resistance in the liver can be particularly problematic due to the continued synthesis and export of glucose in the fed state (a major cause of diabetic hyperglycemia).84 Therefore, understanding the molecular underpinnings of insulin- resistant gluconeogenesis in the liver would undoubtedly have important implications for therapeutic intervention in T2DM. Given the role of O-GlcNAcylation in insulin resistance generally, it is perhaps not surprising then that it also plays a major role in regulating hepatic gluconeogenesis. For instance, increased O-GlcNAcylation of cyclic AMP-responsive element- binding protein (CREB)-regulated transcription coactivator 2 (CRTC2) in response to hyperglycemia results in its nuclear localization, enhanced promoter binding at gluconeogenic genes, and increased expression of gluconeogenic enzymes.85 Interestingly, CRTC2 is O-
GlcNAcylated at two protein kinase A (PKA) phosphorylation sites, sites that are known to sequester CRTC2 in the cytoplasm by engaging 14-3-3 proteins.86 Thus, in addition to its role in stimulating aberrant gluconeogenesis, CRTC2 O-GlcNAcylation also highlights an important general function of O-GlcNAc in blocking phosphorylation (as they often share the same residues).
In addition to CRTC2, a master regulator of gluconeogenic gene expression, peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC-1α) is also O-GlcNAcylated in response to hyperglycemia.87 Here, O-GlcNAcylation facilitates the binding of ubiquitin carboxyl- terminal hydrolase BAP1 (BAP1), which stabilizes PGC-1α through its potent deubiquitinase activity. The modification of PGC-1α is also interesting in that it is mediated through host cell factor 1 (HCFC1), which targets OGT to O-GlcNAcylate PGC-1α.87 This is perhaps one of the most striking examples of the adaptor protein hypothesis (discussed in more detail in Chapters 4, 5, and 7). In addition to the aforementioned examples, there are also numerous others showing that increased O-GlcNAcylation of transcription factors, e.g. forkhead box protein O1 (FOXO1) whose O-GlcNAcylation is dependent on the PGC-1α-OGT interaction,88 receptors, e.g. IR-β,1 and proteins across multiple signaling cascades paradoxically lead to increased liver gluconeogenesis in hyperglycemic conditions.50 Finally, at the organismal level, multiple studies have shown that decreasing liver O-GlcNAcylation through OGT knockdown87 or OGA overexpression85 can lead to increased insulin sensitivity and an improved metabolic profile in diabetic mice. Interestingly, hepatic OGT KO also decreases glucagon sensitivity, preventing autophagy and the subsequent production of glucose and ketone bodies in response to starvation.89 Altogether, the O- GlcNAcylation of hepatic proteins is a critical PTM for regulating the homeostatic functions of the liver in both the fed and fasting states.
In addition to its role in hepatic gluconeogenesis, O-GlcNAcylation events have also been demonstrated to regulate hepatic lipid metabolism. For example, multiple lipogenic transcription factors are also O-GlcNAcylated in the liver, including two central regulators of lipogenesis enzyme expression carbohydrate responsive element binding protein (ChREBP) and liver x receptor α (LXR-α).50 Like with CRTC2, ChREBP O-GlcNAcylation facilitates its nuclear localization and subsequent expression of lipogenic genes.90 LXR-α O-GlcNAcylation also increases its promoter activity and subsequent expression of sterol regulatory element-binding protein 1c (SREBP1c).91 SREBP1c directly promotes fatty acid synthase (FAS) expression and de novo lipogenesis.92 Interestingly, this positive regulation of lipogenesis by O-GlcNAcylation may represent a resolution to a long standing paradox in T2DM, metabolic syndrome, and NAFLD, namely, that the increased insulin resistance seen in these diseases should result in decreased lipogenesis (however, in all cases, lipogenesis in the liver is elevated which is, at the very least, partially responsible for the fatty liver pathology seen in these diseases).93 Despite marked insulin resistance, lipogenesis proceeds unhindered in the liver due to the hyperglycemia-induced upregulation of O-GlcNAcylation on lipogenic transcription factors, receptors, and other signaling proteins.50 This hypothesis also bears out at organismal level where OGT overexpression (presumably leading to markedly upregulated O-GlcNAcylation of proteins promoting lipogenesis) results in increased circulating lipids in mice.1
Overall, the regulation of hepatic metabolism and systemic energy homeostasis depends intimately on O-GlcNAc. In many cases, O-GlcNAc serves as a nutrient sensor to fine tune hepatic responses to extracellular glucose and lipids. However, in metabolic disease, aberrantly increased O-GlcNAcylation can lead to a host of deleterious effects likely including the paradoxical gluconeogenesis and lipogenesis seen in T2DM, metabolic syndrome, and NAFLD. Future
investigations into the role of O-GlcNAc in these and other disorders, including elucidating the full landscape of O-GlcNAcylated proteins and how it changes in response to disease state, are thus likely to reveal new avenues for therapeutic intervention.