The molecular mechanism by which adrenalin inhibits glycogen synthesis

Abstract

Improved methodology was used to establish that the phosphorylation of a serine located 10 residues from the N-terminus of glycogen synthase (N10) increases from 0.12 mol . mol-' to 0.54 mol . mol-' in vivo in response to adrenalin. The only "10 kinase' detected in muscle extracts was casein kinase-1 (CKl), although its activity was unaffected by injection of adrenalin in vivo or by incubation with cyclic-AMP-dependent protein kinase and MgATP in vitro. Prior phosphorylation of the serine residue N7 by phosphorylase kinase increased sixfold the rate of phosphorylation of glycogen synthase by CK1, and altered the specificity of CK1 so that it phosphorylated the serine residue N10 specifically. Stoichiometric phosphorylation of N7 decreased the activity ratio (T glucose 6-phosphate) of glycogen synthase from 0.80 to 0.45, and subsequent phosphorylation of N10 to 0.8 mol . mol-' produced a further decrease to 0.17, demonstrating that N10 phosphorylation inhibits glycogen synthase. The major "10 phosphatase' in skeletal muscle extracts was identified as the glycogen-associated form of protein phosphatase-I (PPlG), accounting for approximately 75% of the Nl0 phosphatase activity in the extracts and about 90% of the activity in isolated glycogen particles. Phosphorylation of N10, after prior phosphorylation of N7, decreased the rate of dephosphorylation of N7. These results, in conjunction with previous findings, establish that adrenalin inhibits glycogen synthase by increasing the phosphorylation of N7, N10 and three further serines located 30, 34 and 38 residues from the start of the C-terminal CNBr peptide (termed the region C30-C38). They also indicate that increased phosphorylation of N10, the region C30 -C38, and perhaps N7, is initiated through the inhibition of PPlG by adrenalin, which results from phosphorylation of its glycogen-targetting subunit by cyclic-AMP-dependent protein kinase [Hubbard, M. J. & Cohen, P. (1989) Eur. J. Biochem. 186,711 -7161. The conclusion that direct phosphorylation of glycogen synthase by cyclic-AMP-dependent protein kinase makes little contribution to inhibition by adrenalin, is at variance with the teachings of the major textbooks of biochemistry.Glycogen synthase, the rate-limiting enzyme in glycogen synthesis, is under hormonal regulation in mammalian skeletal muscle. Adrenalin inhibits the enzyme by increasing phosphorylation, while insulin stimulates activity by decreasing phosphorylation (reviewed in [l]). Nine serine residues are phosphorylated in vivo, two located in the N-terminal and seven within the C-terminal cyanogen bromide peptide (Fig. 1) [2]. In normally fed rabbits injected with propranolol, an adrenalin antagonist, an average of approximately 3 mol Correspondence to S. Nakielny,

Inhibition of muscle glycogen synthase activity and non-oxidative glucose disposal during hypoglycaemia in normal man

“…In contrast, the cyclic AMP-dependent kinase (activated by e.g. catecholamines) phosphorylates both sites-1 and -2 of the Gsubunit, and activates the specific inhibitor of PP-1, thereby inactivating PP-1 [42,43]. It is in agreement with this observation to hypothesize that insulin stimulation of muscle glycogen synthase may be blocked by elevated plasma epinephrine levels during hypoglycaemia.…”

Caffeine-Induced Impairment of Insulin Action but Not Insulin Signaling in Human Skeletal Muscle Is Reduced by Exercise

“…The recent finding that insulin sensitivity to glucose uptake is increased in stimulatory guanine nucleotide-binding protein (G s ) ␣-knockout mice provide further evidence that G s ␣-coupled pathways, possibly including epinephrine stimulation of G s protein, negatively regulate insulin signaling (39). Similarly, epinephrine is known to potently counteract insulin activation of GS (40,41). Thus, it is likely that the inhibitory actions of caffeine on glucose uptake and GS activity in the present study were secondary to epinephrine.…”

Glycogen synthase sensitivity to insulin and glucose-6-phosphate is mediated by both NH2- and COOH-terminal phosphorylation sites.

“…Of the 7 COOH-terminal phosphorylation sites, Ser-640 (site 3a) and Ser-644 (site 3b) are the most important for regulation of glycogen synthase (4). At the NH 2 -terminus, both Ser-7 (site 2) and Ser-10 (site 2a) influence glycogen synthase activity (5,6). Insulin administration causes dephosphorylation of glycogen synthase and, consequently, activation of the enzyme.…”