Hepatic Very Low Density Lipoprotein-ApoB Overproduction Is Associated with Attenuated Hepatic Insulin Signaling and Overexpression of Protein-tyrosine Phosphatase 1B in a Fructose-fed Hamster Model of Insulin Resistance

Abstract

A fructose-fed hamster model of insulin resistance was previously documented to exhibit marked hepatic very low density lipoprotein (VLDL) overproduction. Here, we investigated whether VLDL overproduction was associated with down-regulation of hepatic insulin signaling and insulin resistance. Hepatocytes isolated from fructose-fed hamsters exhibited significantly reduced tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2. Phosphatidylinositol 3-kinase activity as well as insulin-stimulated Akt-Ser 473 and Akt-Thr 308 phosphorylation were also significantly reduced with fructose feeding. Interestingly, the protein mass and activity of protein-tyrosine phosphatase-1B (PTP-1B) were significantly higher in fructose-fed hamster hepatocytes. Chronic ex vivo exposure of control hamster hepatocytes to high insulin also appeared to attenuate insulin signaling and increase PTP-1B. Elevation in PTP-1B coincided with marked suppression of ER-60, a cysteine protease postulated to play a role in intracellular apoB degradation, and an increase in the synthesis and secretion of apoB. Sodium orthovanadate, a general phosphatase inhibitor, partially restored insulin receptor phosphorylation and significantly reduced apoB secretion. In summary, we hypothesize that fructose feeding induces hepatic insulin resistance at least in part via an increase in expression of PTP-1B. Induction of hepatic insulin resistance may then contribute to reduced apoB degradation and enhanced VLDL particle assembly and secretion.

Proteomic Profiling of Hepatic Endoplasmic Reticulum-associated Proteins in an Animal Model of Insulin Resistance and Metabolic Dyslipidemia

“…Induction of insulin resistance was accompanied by a considerable rise in hepatic VLDL-apoB and triglyceride production (4). Evidence was also obtained indicating impaired hepatic insulin signaling and insulin resistance in the liver of fructose-fed hamsters (5). Interestingly, amelioration of insulin resistance in these hamsters using the insulin-sensitizing agent rosiglitazone significantly reduced hepatic lipoprotein production, suggesting a direct link between insulin sensitivity and lipoprotein abnormalities (6).…”

“…We have identified a number of key molecules, including protein-tyrosine phosphatase 1B, in the livers of fructose-fed hamsters which appear to link hepatic insulin signaling with alterations in the synthesis and secretion of lipids and lipoproteins (5,7). These studies would undoubtedly be enhanced and complemented by employing a proteomics approach to analyze changes in the protein profile of key insulin-sensitive tissues in various dietary models of insulin resistance.…”

Protein Phosphotyrosine Phosphatase 1B (PTP1B) in Calpain-dependent Feedback Regulation of Vascular Endothelial Growth Factor Receptor (VEGFR2) in Endothelial Cells

“…PTP1B Activity Assay-PTP1B activity assay was conducted as previously published with minor modifications (31,32). Briefly, skin samples of wounded area were collected on day 12 from control and diabetic mice in the wound healing experiments.…”

Protein-tyrosine-phosphatase 1B activation is regulated developmentally in muscle of neonatal pigs

“…The in vitro PTP1B activity was assayed according to a procedure previously described by Taghibiglou et al (38). Briefly, samples of frozen skeletal muscle and liver were pulverized in liquid nitrogen and homogenized in ice-cold solubilization buffer (PBS containing 1% NP-40, 1% deoxycholate, 5 mM EDTA, 1 mM EGTA, 2 mM PMSF, and 0.1 mM leupeptin).…”