TNF-alpha has been proposed as a link between insulin resistance and obesity, however the molecular mechanism whereby TNF-alpha attenuates insulin signaling is not well understood. The aim of this thesis was to map the in-vivo mechanism by which TNF-alpha contributes to the pathogenesis of impaired insulin signaling, using obese Zucker rats in which TNF-alpha activity was inhibited through adenovirus (AdV)-mediated expression of a TNF inhibitor (TNFi). Control animals consisted of obese Zucker rats infected with the same titer of AdV carrying the lac-z cDNA. Both peripheral and hepatic insulin sensitivity were improved in obese Zucker rats following neutralization of TNF-alpha. Additionally, TNF-alpha neutralization led to a 2.5 fold increase in tyrosine phosphorylation (pY) of IR in skeletal muscle while this was unchanged in liver, suggesting that TNF-alpha is a mediator of insulin resistance in obesity and may modulate IR signaling in skeletal muscle and liver through different pathways. Further examination showed that pY of pp125FAK was significantly higher in TNF-alpha neutralized animals, indicating that TNF-alpha may induce insulin resistance in the liver by preventing insulin-stimulated pY of pp125FAK. Thus, TNF-alpha may induce insulin resistance in the liver by dephosphorylating pp125FAK or preventing its pY response to insulin treatment To explore the cellular mechanism whereby TNF-alpha prevents insulin-stimulated pY of pp125FAK in the liver, we measured c-Src kinase activity and the abundance of 3 major protein tyrosine phosphatases (PTPs)---PTP-1B, LAR and SH-PTP2---in liver homogenates from obese Zucker rats following TNF-alpha blockade. TNF-alpha neutralization did not alter hepatic c-Src kinase activity, but led to a 75% reduction in LAR protein levels while PTP-1B and SH-PTP2 levels remained unchanged. To further verify that TNF-alpha exerts direct effects on LAR expression, we treated HepG2 with TNF-alpha and found that LAR protein levels were elevated in a TNF-alpha dose-dependent manner while PTP-1B and SH-PTP2 abundance remained constant. Furthermore, treatment of HepG2 cells with TNF-alpha (0.8 mM, 72 hr) attenuated insulin stimulation of pp125FAK pY. Taken together, our data suggest that TNF-alpha may attenuate insulin action in the liver by preventing insulin-mediated pY of pp125FAK through increased LAR expression and that this probably represents the critical interface between TNF-alpha and insulin signaling / acase@tulane.edu
| Identifer | oai:union.ndltd.org:TULANE/oai:http://digitallibrary.tulane.edu/:tulane_27474 |
| Date | January 1999 |
| Contributors | Cheung, Anthony Tzeyew (Author), Walters, Marian R (Thesis advisor) |
| Publisher | Tulane University |
| Source Sets | Tulane University |
| Language | English |
| Detected Language | English |
| Rights | Access requires a license to the Dissertations and Theses (ProQuest) database., Copyright is in accordance with U.S. Copyright law |
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