Glycogen synthase kinase-3 (GSK3) is implicated in the regulation of several physiological processes, including the control of glycogen and protein synthesis by insulin, modulation of the transcription factors AP-1 and CREB, the specification of cell fate in Drosophila and dorsoventral patterning in Xenopus embryos. GSK3 is inhibited by serine phosphorylation in response to insulin or growth factors and in vitro by either MAP kinase-activated protein (MAPKAP) kinase-1 (also known as p90rsk) or p70 ribosomal S6 kinase (p70S6k). Here we show, however, that agents which prevent the activation of both MAPKAP kinase-1 and p70S6k by insulin in vivo do not block the phosphorylation and inhibition of GSK3. Another insulin-stimulated protein kinase inactivates GSK3 under these conditions, and we demonstrate that it is the product of the proto-oncogene protein kinase B (PKB, also known as Akt/RAC). Like the inhibition of GSK3 (refs 10, 14), the activation of PKB is prevented by inhibitors of phosphatidylinositol (PI) 3-kinase.

Inhibition of glycogen synthase kinase-3 by insulin mediated by protein kinase B.

The mitogen-activated protein kinase signal transduction pathway.

SB 203580 is a specific inhibitor of a MAP kinase homologue which is stimulated by cellular stresses and interleukin-1

Glycogen synthase kinase 3 (GSK-3) is a multifunctional serine/threonine kinase found in all eukaryotes. The enzyme is a key regulator of numerous signalling pathways, including cellular responses to Wnt, receptor tyrosine kinases and G-protein-coupled receptors and is involved in a wide range of cellular processes, ranging from glycogen metabolism to cell cycle regulation and proliferation. GSK-3 is unusual in that it is normally active in cells and is primarily regulated through inhibition of its activity. Another peculiarity compared with other protein kinases is its preference for primed substrates, that is, substrates previously phosphorylated by another kinase. Several recent advances have improved our understanding of GSK-3 regulation in multiple pathways. These include the solution of the crystal structure of GSK-3, which has provided insight into GSK-3's penchant for primed substrates and the regulation of GSK-3 by serine phosphorylation, and findings related to the involvement of GSK-3 in the Wnt/beta-catenin and Hedgehog pathways. Finally, since increased GSK-3 activity may be linked to pathology in diseases such as Alzheimer's disease and non-insulin-dependent diabetes mellitus, several new GSK-3 inhibitors, such as the aloisines, the paullones and the maleimides, have been developed. Although they are just starting to be characterized in cell culture experiments, these new inhibitors hold promise as therapeutic agents.

GSK-3: tricks of the trade for a multi-tasking kinase.

ERKs: A family of protein-serine/threonine kinases that are activated and tyrosine phosphorylated in response to insulin and NGF

One of the important metabolic effects of insulin is its ability to stimulate the synthesis of glycogen in skeletal muscle within minutes; much research has been directed toward understanding the molecular basis of this effect. Almost 25 years ago, Joseph Larner and co-workers showed that glycogen synthase, the rate-limiting enzyme in this pathway, could exist in at least two forms. One possessed little activity in the absence of glucose 6-phosphate (G6P), while the other was almost fully active in the absence of G6P.(1) Incubation of rat hemidiaphragms with insulin decreased the proportion of glycogen synthase in the G6P-dependent form which paralleled the increased rate of glycogen production.(1–3) Furthermore, the conversion of glycogen synthase to a G6Pindependent form was still observed when glucose was omitted from the perfusion medium,(2,3) indicating that it was not a consequence of the increased rate of transport of glucose into muscle, which is also stimulated by insulin. Conversely, incubation of the hemidiaphragms with epinephrine increased the proportion of the G6P-dependent form and decreased the rate of glycogen synthesis.(2,3)

The molecular mechanism by which insulin stimulates glycogen synthesis in mammalian skeletal muscle

Argininosuccinate synthetase (ASS, EC 6.3.4.5) catalyses the condensation of citrulline and aspartate to form argininosuccinate, the immediate precursor of arginine. First identified in the liver as the limiting enzyme of the urea cycle, ASS is now recognized as a ubiquitous enzyme in mammalian tissues. Indeed, discovery of the citrulline-NO cycle has increased interest in this enzyme that was found to represent a potential limiting step in NO synthesis. Depending on arginine utilization, location and regulation of ASS are quite different. In the liver, where arginine is hydrolyzed to form urea and ornithine, the ASS gene is highly expressed, and hormones and nutrients constitute the major regulating factors: (a) glucocorticoids, glucagon and insulin, particularly, control the expression of this gene both during development and adult life; (b) dietary protein intake stimulates ASS gene expression, with a particular efficiency of specific amino acids like glutamine. In contrast, in NO-producing cells, where arginine is the direct substrate in the NO synthesis, ASS gene is expressed at a low level and in this way, proinflammatory signals constitute the main factors of regulation of the gene expression. In most cases, regulation of ASS gene expression is exerted at a transcriptional level, but molecular mechanisms are still poorly understood.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1432-1033.2003.03559.x

Argininosuccinate synthetase from the urea cycle to the citrulline-NO cycle

The insulin-stimulated protein kinase (ISPK) was purified over 50,000-fold from extracts of rabbit skeletal muscle by a procedure involving chromatography on phosphocellulose, fractionation with ammonium sulphate, and further chromatography on DEAE-cellulose, phenyl-Superose, Mono S and Mono Q. About 10 micrograms enzyme was isolated from 800 g muscle (one rabbit) in four days with an overall recovery of 5%. The purified enzyme showed a single protein-staining band of apparent molecular mass 91 kDa when analysed by SDS/polyacrylamide gel electrophoresis. The ISPK comigrated during SDS/polyacrylamide gel electrophoresis with the enzyme S6 kinase II from Xenopus eggs, and was recognised in immunoblotting and immunoprecipitation experiments by antibodies raised against S6 kinase II. The substrate specificities of ISPK and S6 kinase II were also very similar and like S6 kinase II, ISPK that had been inactivated by protein phosphatase 2A could be reactivated by incubation with mitogen-activated protein kinase and MgATP. ISPK was distinct from an insulin-stimulated 70-kDa S6 kinase from rat liver in both substrate specificity and immunological cross reactivity. It is concluded that ISPK is closely related in structure to S6 kinase II and may be a mammalian equivalent of this enzyme. The possibility that ISPK is involved in mediating a number of the actions of insulin is discussed.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1991.tb16176.x

Purification and characterisation of the insulin-stimulated protein kinase from rabbit skeletal muscle; close similarity to S6 kinase II

One of the main secondary toxic side effects of antimitotic agents used to treat cancer patients is intestinal mucositis. This one is characterized by compromised digestive and absorptive functions...

https://www.physiology.org/doi/pdf/10.1152/ajpendo.90459.2008

Alain Lavoinne

Papers

The molecular mechanism by which insulin stimulates glycogen synthesis in mammalian skeletal muscle

Argininosuccinate synthetase from the urea cycle to the citrulline-NO cycle

Purification and characterisation of the insulin-stimulated protein kinase from rabbit skeletal muscle; close similarity to S6 kinase II

Methotrexate induces intestinal mucositis and alters gut protein metabolism independently of reduced food intake

Osmotic stress, a proinflammatory signal in Caco-2 cells