Lack of evidence for a role of TRB3/NIPK as an inhibitor of PKB-mediated insulin signalling in primary hepatocytes.
The protein TRB3 (tribbles 3), also called NIPK (neuronal cell death-inducible protein kinase), was recently identified as a protein-protein interaction partner and an inhibitor of PKB (protein kinase B). To explore the hypothesis that TRB3/NIPK might act as a negative regulator of insulin signalling in the liver, this protein was overexpressed by adenoviral transduction of primary cultures of rat hepatocytes, and various aspects of insulin action were investigated. The insulin-induced phosphorylation of Ser-473 and Thr-308 of PKB was found to be undiminished in transduced hepatocytes with a molar excess of TRB3/NIPK over PKB of more than 25-fold. Consistent with unimpaired insulin activation of PKB, the stimulation of Ser-21 and Ser-9 phosphorylation of glycogen synthase kinase 3-alpha and -beta, and the apparent phosphorylation level of 4E-BP1 (eukaryotic initiation factor 4-binding protein 1), were similar in transduced and control hepatocytes. The induction by insulin of the mRNAs encoding glucokinase and SREBF1 (sterol-regulatory-element-binding factor 1) were also normal in TRB3/NIPK hepatocytes. In contrast, the insulin-dependent induction of these two genes, as well as the activation of PKB, were shown to be suppressed in hepatocytes treated with the lipid ether compound PIA6 (phosphatidylinositol ether lipid analogue 6), a recently discovered specific inhibitor of PKB. Since TRB3/NIPK was reported to be increased in the liver of fasting mice, the effects of glucagon, glucocorticoids and insulin on the level of endogenous TRB3/NIPK mRNA in primary hepatocytes were investigated. No significant change in mRNA level occurred under any of the hormonal treatments. The present study does not support the hypothesis that the physiological role of TRB3/NIPK might be to put a brake on insulin signalling in hepatocytes.
Department of Cell Physiology and Metabolism, University of Geneva School of Medicine, CH-1211 Geneva, Switzerland. Patrick.Iynedjian@medecine.unige.ch
Patrick B
PB
Iynedjian
D000818:Animals; D002478:Cells, Cultured / Q000187:drug effects / Q000378:metabolism; D000242:Cyclic AMP / Q000494:pharmacology; D003907:Dexamethasone / Q000494:pharmacology; D004790:Enzyme Induction; D005215:Fasting; D005934:Glucagon / Q000494:pharmacology; D005941:Glucokinase / Q000096:biosynthesis / Q000235:genetics; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology / Q000502:physiology*; D008297:Male; D018129:Phosphatidylinositol Phosphates / Q000494:pharmacology; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011494:Protein Kinases / Q000235:genetics / Q000502:physiology*; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D017208:Rats, Wistar; D011993:Recombinant Fusion Proteins / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D014161:Transduction, Genetic
D007328:Insulin; D018129:Phosphatidylinositol Phosphates; D047428:Protein Kinase Inhibitors; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; C502443:Trib3 protein, rat; C502444:phosphatidylinositol ether lipid analog 6; D003907:Dexamethasone; D005934:Glucagon; D000242:Cyclic AMP; D011494:Protein Kinases; C119271:neuronal cell death inducible putative kinase; D005941:Glucokinase; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt