AMP-activated protein kinase phosphorylates and inactivates liver glycogen synthase

Abstract

Recombinant muscle GYS1 (glycogen synthase 1) and recombinant liver GYS2 were phosphorylated by recombinant AMPK (AMP-activated protein kinase) in a time-dependent manner and to a similar stoichiometry. The phosphorylation site in GYS2 was identified as Ser7, which lies in a favourable consensus for phosphorylation by AMPK. Phosphorylation of GYS1 or GYS2 by AMPK led to enzyme inactivation by decreasing the affinity for both UDP-Glc (UDP-glucose) [assayed in the absence of Glc-6-P (glucose-6-phosphate)] and Glc-6-P (assayed at low UDP-Glc concentrations). Incubation of freshly isolated rat hepatocytes with the pharmacological AMPK activators AICA riboside (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside) or A769662 led to persistent GYS inactivation and Ser7 phosphorylation, whereas inactivation by glucagon treatment was transient. In hepatocytes from mice harbouring a liver-specific deletion of the AMPK catalytic α1/α2 subunits, GYS2 inactivation by AICA riboside and A769662 was blunted, whereas inactivation by glucagon was unaffected. The results suggest that GYS inactivation by AMPK activators in hepatocytes is due to GYS2 Ser7 phosphorylation.

Effects of Whey Proteins on Glucose Metabolism in Normal Wistar Rats and Zucker Diabetic Fatty (ZDF) Rats

“…The finding that liver GYS2 levels were reduced in the milk protein-fed ZDF rats is consistent with the lower circulating glucose levels, presumably resulting in lower liver glycogen synthesis [22]. Furthermore, one may speculate that the net result of lowered GYS2 and lower glucagon levels can lead to reduced hepatic glucose production.…”

Chromatin recruitment of activated AMPK drives fasting response genes co-controlled by GR and PPARα

“…Using the mammalian two-hybrid assay, termed MAPPIT (27,43) (Figure 6C) we also identified a statistically significant interaction between two regulatory subunits of AMPK (PRKAG2/AMPK-γ2 and PRKAG3/AMPK-γ3) and full length PPARα but not full length GR (Figure 6C). GST-pull down analysis using GST-PPARα combined with a recombinant LKB1-activated pAMPK (α1β2γ1) heterotrimeric complex (44), and in vitro transcribed and translated GRα in reticulocyte lysates shows that, at least in vitro , a signal for GRα can be detected when PPARα is pulled down along with recombinant active pAMPK (Figure 6D and Supplementary Figure S8) suggesting a physical complex harboring these three proteins is possible. The interaction data suggest that a contact interface involving PPARα and the γ-subunits of AMPK may support promoter recruitment of the pAMPK complex, in a constellation that may additionally accommodate GRα.…”

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 .…”