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Intestinal Insulin Resistance and Aberrant Production of Apolipoprotein B48 Lipoproteins in an Animal Model of Insulin Resistance and Metabolic Dyslipidemia

Evidence for Activation of Protein Tyrosine Phosphatase-1B, Extracellular Signal–Related Kinase, and Sterol Regulatory Element–Binding Protein-1c in the Fructose-Fed Hamster Intestine

From the Clinical Biochemistry Division, Department of Laboratory Medicine and Pathobiology, Hospital for Sick Children, University of Toronto, Toronto, Ontario

Address correspondence and reprint requests to Khosrow Adeli, Division of Clinical Biochemistry DPLM, Hospital for Sick Children, 555 University Ave., Toronto, Ontario, Canada M5G 1X8. E-mail: k.adeli{at}utoronto.ca

Abbreviations: apo, apolipoprotein; CVD, cardiovascular disease; ERK, extracellular signal–related kinase; IRS, insulin receptor substrate; MAP, mitogen-activated protein; MEK, mitogen-activated protein/ERK kinase; PI, phosphatidylinositol; PTP, protein tyrosine phosphatase; SREBP, sterol regulatory element–binding protein; TNF, tumor necrosis factor

Postprandial dyslipidemia is recognized as an important complication of insulin-resistant states, and recent evidence implicates intestinal lipoprotein overproduction as a causative factor. The mechanisms linking intestinal lipoprotein overproduction and aberrant insulin signaling in intestinal enterocytes are currently unknown. Intestinal insulin sensitivity and lipid metabolism were studied in a fructose-fed hamster model of insulin resistance and metabolic dyslipidemia. Intestinal lipoprotein production in chow-fed hamsters was responsive to the inhibitory effects of insulin, and a decrease in circulating levels of triglyceride-rich apolipoprotein (apo)B48-containing lipoproteins occurred 60 min after insulin administration. However, fructose-fed hamster intestine was not responsive to the insulin-induced downregulation of apoB48-lipoprotein production, suggesting insulin insensitivity at the level of the intestine. Enterocytes from the fructose-fed hamster exhibited normal activity of the insulin receptor but reduced levels of insulin receptor substrate-1 phosphorylation and mass and Akt protein mass. Conversely, the protein mass of the p110 subunit of phosphatidylinositol 3-kinase, protein tyrosine phosphatase-1B, and basal levels of phosphorylated extracellular signal–related kinase (ERK) were significantly increased in the fructose-fed hamster intestine. Modulating the ERK pathway through in vivo inhibition of mitogen-activated protein/ERK kinase 1/2, the upstream activator of ERK1/2, we observed a significant decrease in intestinal apoB48 synthesis and secretion. Interestingly, enhanced basal ERK activity in the fructose-fed hamster intestine was accompanied by an increased activation of sterol regulatory element–binding protein. In summary, these data suggest that insulin insensitivity at the level of the intestine and aberrant insulin signaling are important underlying factors in intestinal overproduction of highly atherogenic apoB48-containing lipoproteins in the insulin-resistant state. Basal activation of the ERK pathway may be an important contributor to the aberrant insulin signaling and lipoprotein overproduction in this model.

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