title: Involvement of the actin network in insulin signalling.
abstract: The purpose of the studies included in this chapter was to examine the role of the actin network in the propagation of insulin action leading to stimulation of glucose transport and activation of the mitogen-activated protein kinase cascade. The active insulin receptor phosphorylates tyrosine residues of intracellular proteins such as the insulin receptor substrate-1 (IRS-1) which acts as docking sites for molecules containing Src homology 2 (SH2) domains. One such molecule is phosphatidylinositol 3-kinase (PI 3-kinase) which becomes activated by binding to IRS-1. PI 3-kinase activity is required for the insulin-stimulation of glucose transport and glycogen synthesis. Grb2, a small adaptor molecule, can bind IRS-1 and, through the guanine nucleotide exchange factor Sos, leads to the activation of the small GTP binding protein Ras. Through a cascade of protein kinases, activation of Ras results in activation of the Erk 1 and 2 mitogen-activated protein kinases (MAPKs) which appear to control important nuclear and metabolic events. To investigate the role of the actin network in the propagation of insulin action leading to stimulation of glucose transport and the activation of the Erk MAPKs, we used the fungal metabolite cytochalasin D which disassembles the actin network. Actin disassembly abolished almost completely the ability of insulin to increase the rate of glucose transport into L6 muscle cells (myotubes) through prevention of the insulin-induced recruitment of glucose transporters to the plasma membrane which is the event that mediates the increase in the rate of transport. Actin disassembly did not affect either the insulin-mediated phosphorylation of IRS-1, the association of PI 3-kinase with this molecule, or the activation of IRS-1-associated PI 3-kinase. These results were also verified in another insulin responsive cell line, the 3T3-L1 adipocytes. In these cells, actin disassembly inhibited the insulin-induced recruitment of PI 3-kinase to intracellular membranes containing glucose transporters. Moreover, actin disassembly abolished the insulin-mediated phosphorylation of the Erk MAPKs. We conclude that the cellular actin network of insulin responsive cells is not required for the activation of PI 3-kinase but prevents its cellular redistribution. In contrast, intact actin filaments are essential for the propagation of insulin signals leading to the the activation of the MAPKs.
authors.affiliation: Division of Cell Biology, Hospital for Sick Children, Toronto, Ontario, Canada.
authors.forename: T
authors.initials: T
authors.lastname: Tsakiridis
authors.forename: Q
authors.initials: Q
authors.lastname: Wang
authors.forename: C
authors.initials: C
authors.lastname: Taha
authors.forename: S
authors.initials: S
authors.lastname: Grinstein
authors.forename: G
authors.initials: G
authors.lastname: Downey
authors.forename: A
authors.initials: A
authors.lastname: Klip
mesh_terms: D016475:3T3 Cells / Q000737:chemistry / Q000378:metabolism; D000199:Actins / Q000378:metabolism*; D017667:Adipocytes / Q000737:chemistry / Q000187:drug effects / Q000201:enzymology; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D015638:Cytochalasin D / Q000494:pharmacology; D003599:Cytoskeleton / Q000737:chemistry / Q000187:drug effects / Q000502:physiology*; D004815:Epidermal Growth Factor / Q000502:physiology; D005786:Gene Expression Regulation / Q000502:physiology; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D015151:Immunoblotting; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D016023:Integrins / Q000502:physiology; D007425:Intracellular Membranes / Q000737:chemistry / Q000187:drug effects / Q000201:enzymology; D051379:Mice; D020929:Mitogen-Activated Protein Kinase Kinases; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009004:Monosaccharide Transport Proteins / Q000032:analysis; D018485:Muscle Fibers, Skeletal / Q000737:chemistry / Q000187:drug effects / Q000201:enzymology; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins / Q000032:analysis; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D010982:Platelet-Derived Growth Factor / Q000502:physiology; D011494:Protein Kinases / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects* / Q000502:physiology; D018631:ras Proteins / Q000378:metabolism
chemical_list: D000199:Actins; D051275:Glucose Transporter Type 4; D007004:Hypoglycemic Agents; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D016023:Integrins; C526295:Irs1 protein, mouse; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D010750:Phosphoproteins; D010982:Platelet-Derived Growth Factor; C495693:Slc2a4 protein, mouse; D015638:Cytochalasin D; D004815:Epidermal Growth Factor; D011494:Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D020928:Mitogen-Activated Protein Kinases; D020929:Mitogen-Activated Protein Kinase Kinases; D018631:ras Proteins; D005947:Glucose