Lipoprotein lipase (LPL) is an adipocyte enzyme that hydrolyzes the triglyceride core of chylomicrons and VLDL, and human fat obtains essentially all its lipid through the action of LPL. Since weight loss leads to an increase in adipocyte LPL expression, LPL contributes to weight loss recidivism by increasingly """"""""stuffing"""""""" fat cells. LPL also clears triglyceride-rich lipoproteins from plasma, and therefore is important in the prevention of atherosclerosis. Recent studies, by us and others, indicated that LPL regulation is complex; under certain circumstances LPL regulation occurs transcriptionally, yet under other conditions occurs post- transcriptionally. We recently identified a novel mechanism of LPL regulation which occurs at the level of translation. Thus, these studies will investigate the mechanisms of LPL translational regulation in adipocytes from rats and humans.
The specific aims are as follows: 1. Determine the mechanism of translational regulation of LPL. These studies will first determine whether LPL translational regulation occurs due to post-transcriptional changes in LPL mRNA structure. Subsequently, we will determine whether LPL translational regulation occurs at the level of initiation or elongation, and search for a trans-acting RNA binding protein. Using LPL mRNA constructs deficient or defective in the 5' or 3' untranslated region (UTR), we will determine what structural features of the LPL mRNA are essential for regulation of translation. These constructs will be tested by an in vitro assay for translation regulation, as well as by in vivo analysis of expression, after transfection into 3T3-L1 cells. We will use these approaches to study the mechanism of translational regulation by thyroid hormone, catecholamines, and glucose. 2. Post-translational regulation of LPL. We have already completed most of this specific aim, which examined the role of post-translational oligosaccharide processing on LPL catalytic activity, secretion, and degradation. 3. Characterization of the role of translation in LPL physiologic regulation in humans. Our recent studies have observed translational regulation of LPL when diabetics are treated with either insulin or sulfonylurea drugs. To examine the mechanism of translational regulation of LPL in humans, we will use the approaches described above to study LPL expression in diabetics both before and after improved control. These studies will provide significant information on a novel mechanism of regulation of LPL, which is central to an understanding of the metabolic defects of obesity and atherosclerosis.
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