The overall aim of this project will be to examine, in detail, the hormonal regulation of adipose tissue lipoprotein lipase (ATLPL). Not only is ATLPL important in the physiologic removal of triglyceride from plasma lipoproteins, but alterations in ATLPL occur in common metabolic diseases (uncontrolled diabetes, obesity). In vitro, a cultured isolated adipocyte system which sustains, viability of fat cells 2-3 days has been developed. ATLPL can be measured in cultures as enzyme secreted into culture medium and as activity in cells. ATLPL is measured in cells as enzyme releasable from cell suspensions by heparin (which correlates best with in vivo activity) and as enzyme extractable in deoxycholate-detergent (total cell activity). Experiments will be performed to determine the roles of glucose, insulin, gastric inhibitory polypeptide (GIP) and potential local regulators such as adenbsine on ATLPL synthesis and secretion. To determine the mechanism of abnormal ATLPL regulation in vivo, similar studies will be done in cultured adipocytes from diabetic, preobese and obese rats. This culture system also has been and will be further developed for cultured human adipocytes. In addition, because the function of ATLPL in vivo occurs on the endothelium, cultured human microvascular endothelium will be used to study the binding and release of purified enzyme in vitro. In vivo studies will employ a technique where glycemia is maintained during an insulin infusion. Physiologic studies will examine the effect of oral fat on insulin-stimulated ATLPL, the role of ATLPL in HDL2 (antiatherogenic HDL) formation, and the potential regulation of ATLPL by GIP. The importance of increased ATLPL in the pathogenesis of obesity will be tested before and after weight loss. Increases in basal or insulin-reponsive ATLPL after weight reduction would support such a role. Studies in diabetics will determine at what level of the diabetic state decreases in basal or insulin-stimulated ATLPL occur. It is this abnormality which results in the hypertriglceridemia seen in the uncontrolled diabetics. Recovery of enzyme activity after insulin therapy will also be examined. These studies, through a combined in vitro and in vivo approach, should add consiserable insight into the pathogenesis and potential correction of alterations in ATLPL which contribute to the morbidity and perhaps indirectly to the mortality of patients with metabolic diseases.
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