title: Insulin signalling in hepatocytes of humans with type 2 diabetes: excessive production and activity of protein kinase C-ι (PKC-ι) and dependent processes and reversal by PKC-ι inhibitors.
abstract: OBJECTIVE
  We examined the role of protein kinase C-ι (PKC-ι) in mediating alterations in the abundance of enzymes in hepatocytes of type 2 diabetic humans that contribute importantly to the development of lipid and carbohydrate abnormalities in type 2 diabetes.
  
  
  METHODS
  We examined (1) insulin signalling in isolated hepatocytes of non-diabetic and type 2 diabetic humans and (2) the effects of two newly developed small molecule PKC-ι inhibitors on aberrant signalling and downstream processes.
  
  
  RESULTS
  In contrast with PKC-ι deficiency in diabetic muscle, which diminishes glucose transport, PKC-ι in diabetic hepatocytes was overproduced and overactive, basally and after insulin treatment, and, moreover, was accompanied by increased abundance of PKC-ι-dependent lipogenic, proinflammatory and gluconeogenic enzymes. Heightened PKC-ι activity most likely reflected heightened activity of IRS-2-dependent phosphatidylinositol 3-kinase (PI3K), as IRS-1 levels and IRS-1/PI3K activity were markedly diminished. Importantly, insulin-stimulated PKC-ι abundance and its overabundance in diabetic hepatocytes was reversed in vitro by both insulin deprivation and PKC-ι inhibitors; this suggested operation of an insulin-driven, feed-forward/positive-feedback mechanism. In contrast with PKC-ι, protein kinase B (Akt2) activity and activation by insulin was diminished, apparently reflecting IRS-1 deficiency. Treatment of diabetic hepatocytes with PKC-ι/λ inhibitors diminished abundance of lipogenic, proinflammatory and gluconeogenic enzymes.
  
  
  CONCLUSIONS
  Our findings suggest that a vicious cycle of PKC-ι overactivity and overproduction exists in hepatocytes of humans with type 2 diabetes and contributes importantly to maintaining overactivity of lipogenic, proinflammatory and gluconeogenic pathways, which underlies the lipid and carbohydrate abnormalities in type 2 diabetes.
authors.affiliation: Medical and Research Services, James A Haley Veterans Medical Center, 13000 Bruce B Downs Blvd, Tampa, FL 33612, USA.
authors.forename: M P
authors.initials: MP
authors.lastname: Sajan
authors.forename: R V
authors.initials: RV
authors.lastname: Farese
mesh_terms: D000328:Adult; D000368:Aged; D002478:Cells, Cultured; D003924:Diabetes Mellitus, Type 2 / Q000378:metabolism*; D005260:Female; D022781:Hepatocytes / Q000187:drug effects* / Q000378:metabolism*; D006801:Humans; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins / Q000378:metabolism; D007527:Isoenzymes / Q000037:antagonists & inhibitors* / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D058539:Phosphatidylinositol 3-Kinase / Q000378:metabolism; D011493:Protein Kinase C / Q000037:antagonists & inhibitors* / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D000099293:Protein Kinase C-lambda
chemical_list: D007004:Hypoglycemic Agents; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D007527:Isoenzymes; D058539:Phosphatidylinositol 3-Kinase; D011493:Protein Kinase C; D047428:Protein Kinase Inhibitors; D051057:Proto-Oncogene Proteins c-akt; D000099293:Protein Kinase C-lambda; C526292:IRS1 protein, human; C494921:AKT2 protein, human