Glucagon-like peptide-1 (GLP-1), an incretin with glucose-dependent insulinotropic and insulin-independent antidiabetic properties, has insulin-like effects on glucose metabolism in extrapancreatic tissues participating in overall glucose homeostasis. These effects are exerted through specific receptors not associated with cAMP, an inositol phosphoglycan being a possible second messenger. In rat hepatocytes, activation of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB), protein kinase C (PKC) and protein phosphatase 1 (PP-1) has been shown to be involved in the GLP-1-induced stimulation of glycogen synthase. We have investigated the role of enzymes known or suggested to mediate the actions of insulin in the GLP-1-induced increase in glycogen synthase a activity in rat skeletal muscle strips. We first explored the effect of GLP-1, compared with that of insulin, on the activation of PI3K, PKB, p70s6 kinase (p70s6k) and p44/42 mitogen-activated protein kinases (MAPKs) and the action of specific inhibitors of these kinases on the insulin- and GLP-1-induced increment in glycogen synthase a activity. The study showed that GLP-1, like insulin, activated PI3K/PKB, p70s6k and p44/42. Wortmannin (a PI3K inhibitor) reduced the stimulatory action of insulin on glycogen synthase a activity and blocked that of GLP-1, rapamycin (a 70s6k inhibitor) did not affect the action of GLP-1 but abolished that of insulin, PD98059 (MAPK inhibitor) was ineffective on insulin but blocked the action of GLP-1, okadaic acid (a PP-2A inhibitor) and tumour necrosis factor-alpha (a PP-1 inhibitor) were both ineffective on GLP-1 but abolished the action of insulin, and Ro 31-8220 (an inhibitor of some PKC isoforms) reduced the effect of GLP-1 while completely preventing that of insulin. It was concluded that activation of PI3K/PKB and MAPKs is required for the GLP-1-induced increment in glycogen synthase a activity, while PKC, although apparently participating, does not seem to play an essential role; unlike in insulin signaling, p70s6k, PP-1 and PP-2A do not seem to be needed in the action of GLP-1 upon glycogen synthase a activity in rat muscle.
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