Abstract 17023: Endoplasmic Reticulum Stress Regulates Protein-Tyrosine Phosphatase 1B Expression and Cellular Glucose Uptake in Obesity

Abstract

Obesity is associated with an increased risk of developing insulin resistance and type-2 diabetes. Sustained obesity overwhelms the capacity of endoplasmic reticulum (ER) for the folding of proteins leading to insulin resistance. Protein tyrosine-phosphatase 1B (PTP1B) is a negative regulator of insulin signaling. Because PTP1B is localized in the ER, this study was undertaken to test the hypothesis that ER stress upregulates PTP1B leading to insulin resistance. Leptin deficient obese mice and PTP1B knockout mice were used for in vivo studies. For in vitro studies cultured, differentiated C2C12 myotubes were treated with the ER-stressor tunicamycin and insulin-stimulated deoxyglucose uptake was assessed. Obese mice exhibited higher expression levels of ER stress markers and PTP1B in the liver and gastrocnemius muscle compared to age and sex-matched lean control mice. Oral administration of ER-chaperone tauroursodeoxycholic acid (TUDCA, 50 mg/kg/d) attenuated the PTP1B expression in obese mice. No differences were observed in the expression levels of the ER stress marker Bip between C57Bl/6 and PTP1B knockout mice challenged with the tunicamycin. In contrast, induction of other ER stress markers - phosphorylated eIF2α and JNK2, was significantly attenuated PTP1B knockout mice. Western blot and immunohistochemical analysis revealed upregulation of ER stress markers Bip and CHOP in tunicamycin-stimulated myoblasts at the 6h timepoint. In contrast, increase in PTP1B protein expression occurred only after 24h. TUDCA lowered protein levels of PTP1B and Bip, whereas PTP1B siRNA failed to alter tunicamycin induction of ER stress markers Bip and CHOP. Overexpression of wild type recombinant PTP1B failed to induce expression of Bip or CHOP in the myotubes. Additionally, insulin-stimulated glucose uptake in C2C12 cells was abrogated by pre-treating the cells with tunicamycin. Silencing of PTP1B and treatment with TUDCA rescued myotubes from tunicamycin-induced blunting of glucose uptake. Taken together, our data show that ER stress induces PTP1B expression which contributes to muscle insulin resistance associated with obesity. In turn, PTP1B necessary for ER stress response and there exist a positive feedback loop between PTP1B and ER stress.