Actin filaments facilitate insulin activation of the src and collagen homologous/mitogen-activated protein kinase pathway leading to DNA synthesis and c-fos expression.
The exact mechanism of the spatial organization of the insulin signaling pathway leading to nuclear events remains unknown. Here, we investigated the involvement of the actin cytoskeleton in propagation of insulin signaling events leading to DNA synthesis and expression of the immediate early genes c-fos and c-jun in L6 muscle cells. Insulin reorganized the cellular actin network and increased the rate of DNA synthesis and the levels of c-fos mRNA, but not those of c-jun mRNA, in undifferentiated L6 myoblasts. Similarly, insulin markedly elevated the levels of c-fos mRNA but not of c-jun mRNA in differentiated L6 myotubes. Disassembly of the actin filaments by cytochalasin D, latrunculin B, or botulinum C2 toxin significantly inhibited insulin-mediated DNA synthesis in myoblasts and abolished stimulation of c-fos expression by the hormone in myoblasts and myotubes. Actin disassembly abolished insulin-induced phosphorylation and activation of extracellulor signal-regulated kinases, activation of a 65-kda member of the p21-activated kinases, and phosphorylation of p38 mitogen-activated protein kinases but did not prevent activation of phosphatidylinositol 3-kinase and p70(S6k). Under these conditions, insulin-induced Ras activation was also abolished, and Grb2 association with the Src and collogen homologous (Shc) molecule was inhibited without inhibition of the tyrosine phosphorylation of Shc. We conclude that the actin filament network plays an essential role in insulin regulation of Shc-dependent signaling events governing gene expression by facilitating the interaction of Shc with Grb2.
Department of Medicine, Division of Respirology, University of Toronto, Toronto M5S 1A8, Ontario, Canada.
T
T
Tsakiridis
A
A
Bergman
R
R
Somwar
C
C
Taha
K
K
Aktories
T F
TF
Cruz
A
A
Klip
G P
GP
Downey
D008841:Actin Cytoskeleton / Q000378:metabolism*; D000199:Actins / Q000378:metabolism*; D048868:Adaptor Proteins, Signal Transducing; D033942:Adaptor Proteins, Vesicular Transport; D001905:Botulinum Toxins / Q000494:pharmacology; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000494:pharmacology; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002454:Cell Differentiation; D002478:Cells, Cultured; D015638:Cytochalasin D / Q000494:pharmacology; D004247:DNA / Q000096:biosynthesis; D004789:Enzyme Activation; D051380:GRB2 Adaptor Protein; D007328:Insulin / Q000494:pharmacology*; D008954:Models, Biological; D009132:Muscles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation; D011485:Protein Binding; D011506:Proteins / Q000378:metabolism; D016760:Proto-Oncogene Proteins c-fos / Q000096:biosynthesis*; D016755:Proto-Oncogene Proteins c-jun / Q000096:biosynthesis; D055765:Shc Signaling Adaptor Proteins; D015398:Signal Transduction; D013844:Thiazoles / Q000494:pharmacology; D053778:Thiazolidines; D018631:ras Proteins / Q000378:metabolism; D019061:src-Family Kinases / Q000378:metabolism
D000199:Actins; D048868:Adaptor Proteins, Signal Transducing; D033942:Adaptor Proteins, Vesicular Transport; D019086:Bridged Bicyclo Compounds, Heterocyclic; D051380:GRB2 Adaptor Protein; D007328:Insulin; D011506:Proteins; D016760:Proto-Oncogene Proteins c-fos; D016755:Proto-Oncogene Proteins c-jun; D055765:Shc Signaling Adaptor Proteins; D013844:Thiazoles; D053778:Thiazolidines; D015638:Cytochalasin D; D004247:DNA; D019061:src-Family Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D001905:Botulinum Toxins; D018631:ras Proteins; C096324:botulinum toxin type C; C037068:latrunculin B