Hepatic Overexpression of a Constitutively Active Form of Liver Glycogen Synthase Improves Glucose Homeostasis

Abstract

In this study, we tested the efficacy of increasing liver glycogen synthase to improve blood glucose homeostasis. The overexpression of wild-type liver glycogen synthase in rats had no effect on blood glucose homeostasis in either the fed or the fasted state. In contrast, the expression of a constitutively active mutant form of the enzyme caused a significant lowering of blood glucose in the former but not the latter state. Moreover, it markedly enhanced the clearance of blood glucose when fasted rats were challenged with a glucose load. Hepatic glycogen stores in rats overexpressing the activated mutant form of liver glycogen synthase were enhanced in the fed state and in response to an oral glucose load but showed a net decline during fasting. In order to test whether these effects were maintained during long term activation of liver glycogen synthase, we generated liver-specific transgenic mice expressing the constitutively active LGS form. These mice also showed an enhanced capacity to store glycogen in the fed state and an improved glucose tolerance when challenged with a glucose load. Thus, we conclude that the activation of liver glycogen synthase improves glucose tolerance in the fed state without compromising glycogenolysis in the postabsorptive state. On the basis of these findings, we propose that the activation of liver glycogen synthase may provide a potential strategy for improvement of glucose tolerance in the postprandial state.The liver responds to an increase in blood glucose concentration in the postprandial state by net uptake of glucose and conversion to glycogen, which is subsequently mobilized in the postabsorptive state to maintain blood glucose homeostasis. Various attempts have been made to improve blood glucose homeostasis through the modulation of the expression or activity of proteins involved in the control of liver glycogen metabolism. Glucokinase (GK) 3 catalyzes the first step in hepatic glucose metabolism and exerts high control on liver glycogen synthesis (1). Previous studies using either transgenic models (2, 3) or adenoviral vectors targeting the liver (4) demonstrated improved glucose tolerance and/or a lowering of blood glucose in basal conditions. However, GK overexpression also increases flux through glycolysis, and in some circumstances this leads to hypertriglyceridemia (4). An alternative approach to modulating liver glycogen metabolism without stimulating glycolysis and triglyceride formation is through modifying the enzymes that are involved exclusively in glycogen synthesis and degradation or regulatory proteins that may affect the activity of the former, such as protein targeting to glycogen (PTG) (5). Overexpression of PTG in the liver by means of an adenoviral vector increases glucose tolerance without perturbing lipid homeostasis (6). However, a limitation of this experimental model is that it leads to the progressive accumulation of glycogen because PTG promotes the inactivation of glycogen phosphorylase (GP) also during fasting, and consequently there is ne...

Regulation of hepatic glucose metabolism in health and disease

“…This mode of regulation of GYS2 is complex, as it can be phosphorylated by PKA, PKC isoforms, AMPK, glycogen synthase kinase 3 (GSK3) and possibly other kinases 151 . However, site-directed mutagenesis studies indicate that the most important phosphorylation site for GYS2 activity is Ser7, which is a substrate of AMPK and PKA 152,153 ; mice with adenoviral overexpression of GYS2 with a Ser7Ala mutation displayed increased levels of liver glycogen under both fasting and fed conditions 154 . These mice had markedly improved glucose tolerance and lower plasma levels of glucose in the fed state but not in the fasted state, which highlights the crucial role of hepatic glycogen synthesis in postprandial glucose disposal 154 .…”

“…However, site-directed mutagenesis studies indicate that the most important phosphorylation site for GYS2 activity is Ser7, which is a substrate of AMPK and PKA 152,153 ; mice with adenoviral overexpression of GYS2 with a Ser7Ala mutation displayed increased levels of liver glycogen under both fasting and fed conditions 154 . These mice had markedly improved glucose tolerance and lower plasma levels of glucose in the fed state but not in the fasted state, which highlights the crucial role of hepatic glycogen synthesis in postprandial glucose disposal 154 . Insulin, which is essential for hepatic glycogen synthase flux in vivo 10 , was long thought to act through the AKT-dependent inhibition of GSK3, which would favour the dephosphorylation of GYS2 (REF.…”

Liver Glycogen Reduces Food Intake and Attenuates Obesity in a High-Fat Diet–Fed Mouse Model

“…Tissue preparation, RNA extraction, RT-PCR, and quantitative real-time PCR analyses were performed as described (23). The following TaqMan primer/probe sets (Applied Biosystems, Madrid, Spain) were used for quantitative RT-PCR: PTG (Mm01204084_m1), Hprt (Mm00446968_m1), pyruvate kinase (Pklr) (Mm00443090_m1), fatty acid synthase (Fasn) (Mm00662319_m1), acetyl CoA carboxylase (Acc1a) (Mm01304257_m1), SREBP1 (Mm00550338_m1), peroxisome proliferator-activated receptor (…”

Melatonin attenuates smoking‐induced hyperglycemia via preserving insulin secretion and hepatic glycogen synthesis in rats

“…In our study, melatonin administration recovered the expression of LGS in smoking rats (Figure ). Previous studies also showed that the overproduction of LGS in rats was an effective method for improving glucose tolerance, owing to its capacity to enhance glucose storage . Furthermore, the increased activity of LGS potentially enhances insulin sensitivity in the liver, which may explain the insulin‐sensitizing effect of melatonin in smoking rats.…”

“…As a key enzyme in glycogenesis, liver glycogen synthase (LGS), converts glucose into glycogen. The enzyme combines excess glucose into a polymeric chain for storage as glycogen . Therefore, we want to analyze which effects of hepatic glycogen accumulation and the expression of LGS play in the smoking.…”