Tumor necrosis factor-alpha induces hepatic insulin resistance in obese Zucker (fa/fa) rats via interaction of leukocyte antigen-related tyrosine phosphatase with focal adhesion kinase.
The molecular mechanism whereby tumor necrosis factor-alpha (TNF-alpha) induces insulin resistance in obesity is not well understood. Previously, we have shown that inhibition of TNF-alpha improved hepatic insulin sensitivity in obese Zucker rats without altering the tyrosine phosphorylation of liver insulin receptors (IRs), which indicates that the TNF-alpha and insulin-signaling cascades interact distally to the IR. To assess the effects of TNF-alpha on signaling molecules downstream from the IR, we analyzed the tyrosine phosphorylation patterns of liver homogenate proteins from TNF-alpha-neutralized fa/fa rats and showed that focal adhesion kinase (FAK) was consistently hyperphosphorylated (4.5-fold). Moreover, intravenous insulin increased hepatic FAK phosphorylation in a time-dependent manner in Sprague-Dawley rats, which suggests that TNF-alpha may induce hepatic insulin resistance by preventing FAK phosphorylation in response to insulin treatment. To explore the cellular mechanism whereby TNF-alpha regulates phosphorylation of FAK in the liver, we measured c-Src kinase activity and the abundance of 3 major protein tyrosine phosphatases (PTPs) (PTP-1B, leukocyte antigen-related tyrosine phosphatase [LAR], and src homology 2 domain-containing protein-tyrosine phosphatase [SHPTP-2]) in liver homogenates from obese Zucker rats after TNF-alpha blockade. Hepatic c-Src kinase activity was unaltered, but LAR protein was reduced by 75%. In addition, TNF-alpha blockade reduced hepatic PTP activity toward tyrosine phosphorylated FAK by 70%, and this was accounted for by immunodepletion of LAR. Incubation of HepG2 cells with TNF-alpha increased LAR protein levels in a dose-dependent manner. Additionally, pretreatment with TNF-alpha abolished insulin-stimulated tyrosine phosphorylation of FAK in HepG2 cells but had no effect on IR tyrosine phosphorylation or expression. These data suggest that TNF-alpha promotes LAR expression and thus prevents insulin-mediated tyrosine phosphorylation of FAK. This probably represents the interface between TNF-alpha and insulin signaling in the liver.
Cheung
A T
AT
Department of Physiology, Tulane University School of Medicine, New Orleans, Louisiana, USA.
Wang
J
J
Ree
D
D
Kolls
J K
JK
Bryer-Ash
M
M
D000818:Animals; D000081247:CSK Tyrosine-Protein Kinase; D015536:Down-Regulation; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; D007328:Insulin / Q000502:physiology; D007333:Insulin Resistance; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D009765:Obesity / Q000378:metabolism*; D010766:Phosphorylation; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism / Q000502:physiology*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism / Q000502:physiology*; D051381:Rats; D011924:Rats, Zucker; D011972:Receptor, Insulin / Q000378:metabolism; D054630:Receptor-Like Protein Tyrosine Phosphatases, Class 4; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins / Q000494:pharmacology; D014407:Tumor Cells, Cultured; D014409:Tumor Necrosis Factor-alpha / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D014443:Tyrosine / Q000378:metabolism; D019061:src-Family Kinases
D007328:Insulin; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins; D014409:Tumor Necrosis Factor-alpha; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; D011972:Receptor, Insulin; D000081247:CSK Tyrosine-Protein Kinase; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; C496052:Ptk2 protein, rat; D019061:src-Family Kinases; D017027:Protein Tyrosine Phosphatases; C516845:Ptpra protein, rat; D054630:Receptor-Like Protein Tyrosine Phosphatases, Class 4