Maturation of the Regulation of GLUT4 Activity by p38 MAPK during L6 Cell Myogenesis

Abstract

Insulin stimulates glucose uptake in skeletal muscle cells and fat cells by promoting the rapid translocation of GLUT4 glucose transporters to the plasma membrane. Recent work from our laboratory supports the concept that insulin also stimulates the intrinsic activity of GLUT4 through a signaling pathway that includes p38 MAPK. Here we show that regulation of GLUT4 activity by insulin develops during maturation of skeletal muscle cells into myotubes in concert with the ability of insulin to stimulate p38 MAPK. In L6 myotubes expressing GLUT4 that carries an exofacial myc-epitope (L6-GLUT4myc), insulin-stimulated GLUT4myc translocation equals in magnitude the glucose uptake response. Inhibition of p38 MAPK with SB203580 reduces insulinstimulated glucose uptake without affecting GLUT4myc translocation. In contrast, in myoblasts, the magnitude of insulin-stimulated glucose uptake is significantly lower than that of GLUT4myc translocation and is insensitive to SB203580. Activation of p38 MAPK by insulin is considerably higher in myotubes than in myoblasts, as is the activation of upstream kinases MKK3/ MKK6. In contrast, the activation of all three Akt isoforms and GLUT4 translocation are similar in myoblasts and myotubes. Furthermore, GLUT4myc translocation and phosphorylation of regulatory sites on Akt in L6-GLUT4myc myotubes are equally sensitive to insulin, whereas glucose uptake and phosphorylation of regulatory sites on p38 MAPK show lower sensitivity to the hormone. These observations draw additional parallels between Akt and GLUT4 translocation and between p38 MAPK and GLUT4 activation. Regulation of GLUT4 activity by insulin develops upon muscle cell differentiation and correlates with p38 MAPK activation by insulin.

The Role of Phosphoinositide 3-Kinase C2α in Insulin Signaling

“…In agreement with previous reports (50,51), the insulin-induced glucose uptake in these cells was small but reproducible (Fig. 5, A and B).…”

Lysophosphatidylcholine as an effector of fatty acid-induced insulin resistance

“…For differentiation, 4 × 10 4 /ml L6 myoblasts were plated in 60 mm dishes and cultured in a -MEM 10% FBS for 24 h. The medium was replaced with a -MEM 2% FBS to induce fusion of myoblasts to myotubes ( 10 ). Formation of myotubes was monitored by phase contrast microscopy.…”

Bcl2, a transcriptional target of p38α, is critical for neuronal commitment of mouse embryonic stem cells

“…17,18 P38 activity is increased in various neurodegenerative diseases, including Alzheimer's and Prion-related syndromes, and proper neurons integrity relies on its tight regulation. [19][20][21] Many reports, based mainly on the use of p38 inhibitors, have concluded on the involvement of p38 kinases in glucose uptake, 22 nucleotide metabolism, 23 stimulation of the Mnk kinases 24 and chromatin remodelling, through the MSK1 kinase. 25 However, the fact that p38 inhibitors have different outcomes on ES-derived differentiated cell apoptosis 8 indicates that data obtained with these inhibitors could be due to secondary effects of these chemicals along with a modulation in p38 kinase's activity.…”