THE INHIBITION OF GLYCOGEN-SYNTHASE KINASE-3 BY INSULIN OR INSULIN-LIKE GROWTH-FACTOR-1 IN THE RAT SKELETAL-MUSCLE CELL-LINE-L6 IS BLOCKED BY WORTMANNIN, BUT NOT BY RAPAMYCIN - EVIDENCE THAT WORTMANNIN BLOCKS ACTIVATION OF THE MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY IN L6-CELLSBETWEEN RAS AND RAF

Glycogen synthase kinase-3 (GSK3) is inactivated in vitro by p70 S6 ki nase or MAP kinase-activated protein kinase-1 beta (MAPKAP kinase-1 be ta; also known as Rsk-2). Here we show that GSK3 isoforms are inhibite d by 40% within minutes after stimulation of the rat skeletal-muscle c ell line L6 with insulin-like growth factor-1 (IGF-1) or insulin. GSK3 was similarly inhibited in rabbit skeletal muscle after an intravenou s injection of insulin. Inhibition resulted from increased phosphoryla tion of GSK3, probably at a serine/threonine residue(s), because it wa s reversed by incubation with protein phosphatase-2A. Rapamycin blocke d the activation of p70 S6 kinase by IGF-1 in L6 cells, but had no eff ect on the inhibition of GSK3 or the activation of MAPKAP kinase-1 bet a. In contrast, wortmannin, a potent inhibitor of PtdIns 3-kinase, pre vented the inactivation of GSK3 and the activation of MAPKAP kinase-1 beta and p70 S6 kinase by IGF-1 or insulin. Wortmannin also blocked th e activation of p74(raf-1), MAP kinase kinase and p42 MAP kinase, but not the formation of GTP-Ras by IGF-1. The results suggest that the st imulation of glycogen synthase by insulin/IGF-1 in skeletal muscle inv olves the MAPKAP kinase-1-catalysed inhibition of GSK3, as well as the previously described activation of the glycogen-associated form of pr otein phosphatase-1.