Insulin signalling: search for the missing links

Ethanol inhibits insulin receptor substrate-1 tyrosine phosphorylation and insulin-stimulated neuronal thread protein gene expression

Neuronal thread proteins (NTPs) are molecules that accumulate in the brains of patients with Alzheimer's disease, and may play a key role in both normal and neurodegenerative neuritic sprouting. In this investigation we determined whether NTP expression is up-regulated by insulin, an important neurotrophic factor that stimulates differentiation-associated neurite outgrowth, and studied the effects of ethanol, a known inhibitor of growth factor receptor tyrosine phosphorylation, on NTP expression and insulin-mediated signal transduction cascade in neuronal [primitive neuroectodermal tumour cell line 2; (PNET2)] cells. PNET2 cells were treated with 50 m-units/ml insulin in the presence or absence of 100 mM ethanol for 0.2-96 h, and cell proliferation and expression of NTP molecules were investigated by metabolic labelling, immunoprecipitation and immunohistochemical staining. Insulin stimulation resulted in an immediate increase in the levels of three (38, 18 and 15 kDa) of five NTP species (the others were of 26 and 21 kDa), followed by a decline in expression within 120 min; however, studies performed up to 96 h of culture demonstrated up-regulation by insulin of all five NTP species. Ethanol either abolished or severely muted the short- and long-term insulin-mediated upregulation of NTP expression, and substantially reduced insulin-mediated neuronal differentiation. The effects of ethanol on NTP gene expression were associated with impaired insulin-mediated tyrosine phosphorylation of both the insulin receptor beta subunit and the insulin receptor substrate-1 (IRS-1), resulting in decreased association of phosphatidylinositol 3-kinase with IRS-1. The findings suggest that ethanol may inhibit NTP expression associated with central nervous system neuronal differentiation by uncoupling the IRS-1-mediated insulin signal transduction pathway.

Insulin-induced differentiation and modulation of neuronal thread protein expression in primitive neuroectodermal tumor cells is linked to phosphorylation of insulin receptor substrate-1

Neuronal thread proteins (NTPs) are a family of developmentally regulated molecules expressed in central nervous system (CNS) neurons and primitive neuroectodermal tumor (PNET) cell lines. NTP gene expression is modulated with DNA synthesis, neuritic sprouting, and neuronal differentiation. The present study explores the mechanism of insulin modulation of NTP gene expression during neuronal differentiation using PNET cell lines of CNS origin. PNET2 cells underwent neuronal differentiation with neurite outgrowth coupled with transient up-regulation of several species of NTP. In contrast, PNET1 cells failed to differentiate in response to insulin stimulation, although insulin receptors were more abundant than in PNET2 cells. Analysis of the insulin-mediated signal transduction pathway demonstrated that the lack of insulin responsiveness in PNET1 cells was primarily caused by impaired insulin-mediated tyrosyl phosphorylation of the insulin receptor substrate-1 (IRS-1). Correspondingly, the association between phosphatidyl-inositol 3 (PI3) kinase and phosphorylated IRS-1 was reduced in PNET1 compared with PNET2 cells. In contrast, the levels of IRS-1 protein were similar in PNET1 and PNET2 cells, and expression of the insulin receptor beta subunit (Ir beta) and insulin-mediated tyrosyl phosphorylation of the Ir beta were greater in PNET1 than PNET2 cells. The findings suggest that insulin effected neuronal differentiation and modulation of NTP gene expression in PNET cells utilizes a signal transduction cascade that requires tyrosyl phosphorylation of IRS-1.

Tyrosine phosphorylation of phosphatase inhibitor 2

Inhibitor 2 is a heat-stable protein that complexes with the catalytic subunit of type-1 protein phosphatase. The reversible phosphorylation of Thr 72 of the inhibitor in this complex has been shown to regulate phosphatase activity. Here we show that inhibitor 2 can also be phosphorylated on tyrosine residues. Inhibitor 2 was 32P-labeled by the insulin receptor kinase in vitro, in the presence of polylysine. Phosphorylation of inhibitor 2 was accompanied by decreased electrophoretic mobility. Dephosphorylation of inhibitor 2 by tyrosine phosphatase 1B, restored normal electrophoretic mobility. Phosphotyrosine in inhibitor 2 was detected by immunoblotting with antiphosphotyrosine antibodies and phosphoamino acid analysis. In addition, following tryptic digestion, one predominant phosphopeptide was recovered at the anode. The ability of inhibitor 2 to inhibit type-1 phosphatase activity was diminished with increasing phosphorylation up to a stoichiometry of 1 mole phosphate incorporated/mole of inhibitor 2, where inhibitory activity was completely lost. These data demonstrate that inhibitor 2 can be phosphorylated on tyrosine residues by the insulin receptor kinase, resulting in a molecule with decreased ability to inhibit type-1 phosphatase activity.

Insulin and IGF-I receptor signaling in cultured neurons

The data presented in this chapter are summarized in the schematic shown in Figure 9. Insulin binds to and stimulates autophosphorylation of neuronal insulin receptors, whereas, IGF-I and IGF-II binds to and stimulate autophosphorylation of neuronal IGF-I receptors. IGF-II is also capable of binding to the insulin receptor. Whether or not it activates the insulin receptor kinase remains to be clarified. Activated insulin and IGF-I receptor kinases phosphorylate a 70-kDa protein at early times in culture. This protein may mediate some actions of insulin, but we speculate that there are other intermediary proteins involved in the transduction pathway resulting in the activation of S6 kinase and PKC epsilon. The stimulation of S6 kinase by insulin and IGF-I may be associated with the translational activation of protein synthesis by these peptides. The stimulation of PKC epsilon appears to be a necessary step in the transcriptional regulation of the c-fos gene by insulin and IGF-I. The regulation of neuronal protein synthesis at a translational step and the initiation of transcriptional programs regulated by AP-1 represent two mechanisms by which insulin and IGFs alter neuronal growth and differentiation.

Induction of c-jun independent of PKC, pertussis toxin-sensitive G protein, and polyamines in quiescent SV40-transformed 3T3 T cells

CSV3 clones of simian virus 40 large T antigen-transformed murine 3T3 T cells can be made quiescent as part of a differentiation process. In these quiescent cells, insulin- and vanadate-induced mitogenesis are both associated with the induction of the c-jun proto-oncogene (Wang and Scott 1991 J. Cell. Physiol. 147, 102-110; Wang et al. 1991 Cell Growth Differ. 2, 645-652). The current studies were therefore designed to compare the early signal transduction pathways employed by insulin and vanadate to regulate c-jun expression. In quiescent CSV3-1 cells, down-regulation of protein kinase C by prolonged exposure to 12-O-tetra-decanoylphorbol-13-acetate or inhibition of protein kinase C activity by treatment with the protein kinase C antagonist staurosporine is shown not to affect c-jun induction by insulin or vanadate. This suggests that both insulin and vanadate act in a protein kinase C-independent manner. Insulin's effect on c-jun induction does, however, involve a G protein because insulin's effect can be inhibited by pertussis toxin. In contrast, vanadate induction of c-jun is not affected by pertussis toxin. Genistein, a general tyrosine kinase inhibitor, can inhibit the ability of vanadate to induce c-jun but it does not inhibit insulin's effect. Finally, the depletion of polyamines, particularly spermidine, by DL-alpha-difluoromethylornithine treatment also prevents c-jun induction by insulin but DL-alpha-difluoromethylornithine treatment has no effect on c-jun induction by vanadate. These observations indicate that the c-jun induction by insulin and vanadate in CSV3-1 cells is mediated by different signal transduction mechanisms. Together with our previously published data, these results suggest that c-jun can be induced independent of protein kinase C activation, without involvement of pertussis toxin-sensitive G protein, independent of induction of c-fos, and without expression of high levels of intracellular polyamines.

Effects of protein phosphatase inhibitors on the regulation of insulin-sensitive enzymes within rat epididymal fat-pads and cells

1. The effects of the protein phosphatase inhibitors okadaic acid and microcystin LR on the regulation by insulin of pyruvate dehydrogenase and acetyl-CoA carboxylase have been studied in rat epididymal fat-pads and isolated cells. These inhibitors both completely blocked the phosphatase activity (against phosphorylase a) present in extracts of epididymal fat-pads, with half-maximal effects in the nanomolar range. 2. Okadaic acid treatment of pads and cells lowered the activity of acetyl-CoA carboxylase assayed in tissue extracts, both before and after treatment of the extracts with the activator, citrate. Further, okadaic acid treatment abolished the 2-3-fold difference in activity observed between extracts from control and insulin-treated tissues, assayed without prior treatment with citrate. 3. Incubation of pads with [32P]Pi, sufficient to label the intracellular pool of ATP, demonstrated that okadaic acid increased the overall phosphorylation of acetyl-CoA carboxylase on a number of distinct sites, as judged by two-dimensional mapping of tryptic peptides. These included the 'I-peptide' [Brownsey & Denton (1982) Biochem. J. 202, 77-86], the phosphorylation of which may be associated with the stimulation of the activity of the enzyme by insulin, as well as inhibitory phosphorylation sites. 4. Incubation with 1 microM-okadaic acid had no effect on the basal level of active pyruvate dehydrogenase apparent after tissue extraction, but abolished the 2-3-fold increase in this parameter which was elicited by insulin in the absence of okadaic acid. However, okadaic acid treatment did not affect the persistent increase in active pyruvate dehydrogenase levels which was apparent in mitochondria subsequently isolated from insulin-treated pads and re-incubated with an oxidizable substrate. It is concluded that the effects of okadaic acid are exerted through changes in metabolite concentrations rather than some direct action on the signalling pathway whereby insulin stimulates pyruvate dehydrogenase. 5. Microcystin LR did not mimic the effects of okadaic acid on intact cells and pads described above.