Insulin resistance is the major metabolic defect in obesity and Type 2 diabetes. Increased lipid accumulation is strongly associated with insulin resistance. A significant component of insulin resistance is thought to be a reduced ability of insulin to activate the cascade of phosphorylation events that lead to the metabolic effects of this hormone. The broad aims of this thesis were to examine the effect of high-fat diets containing different fat subtypes on in vivo insulin signalling, under conditions normally used to detect whole body insulin resistance, and to compare the effects of acute and chronic lipid oversupply on insulin signalling in vivo. Time-course and dose-response effects of insulin stimulation on site-specific phosphorylation of key signalling proteins were studied in rat tissues in vivo, to establish an appropriate experimental system to examine the onset of activation of the insulin signalling pathway. It was determined that short insulin infusions with concurrent glucose infusion, similar to the beginning of a euglycaemic-hyperinsulinaemic clamp, significantly increased the phosphorylation of major intermediates of the insulin signalling pathway in important tissues of insulin action (skeletal muscle [RQ], liver [LIV] and white adipose tissue [EPI]). These experiments provided a platform to study insulin signalling under the same conditions used to study lipid-induced insulin resistance. The provision of diets enriched in polyunsaturated or saturated fatty acids (FA) resulted in the corresponding enrichment of these fat subtypes in rat plasma and tissues. However, the effects on insulin signalling were essentially the same. Both fat diets induced defects in sitespecific phosphorylation of insulin receptor substrate (IRS)-1 and protein kinase B (PKB) in RQ and LIV, but not EPI. This suggests that the amount of fat in the diet, rather than enrichment in a particular fat subtype, had a greater impact on the development of signalling defects and that the response to high-fat feeding was tissue-specific. A 3hr elevation of circulating FA (using a lipid/heparin infusion), to a level that is relevant in clinical Type 2 diabetes, impaired insulin-stimulated PKB phosphorylation with no significant effect on IRS-1 phosphorylation. This suggests that there may be differences in the way acute and chronic exposure to increased FA impair insulin signalling. The phosphorylation defects observed in both chronic and acute studies did not seem to be associated with activation of major stress signalling pathways (JNK and NFkB), which have been suggested to have a role in lipidinduced insulin resistance. In conclusion, these studies demonstrate that impaired IRS-1 and PKB phosphorylation do have a role in the reduced insulin action observed with lipid oversupply in vivo, because the changes were detected under similar conditions as those used to determine whole body insulin resistance.
| Identifer | oai:union.ndltd.org:ADTP/187263 |
| Date | January 2004 |
| Creators | Frangioudakis, Georgia, St Vincent's Clinical School, UNSW |
| Publisher | Awarded by:University of New South Wales. St Vincent's Clinical School |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Georgia Frangioudakis, http://unsworks.unsw.edu.au/copyright |
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