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Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of circulating lipoprotein triglyceride molecules, is synthesized in myocytes and adipocytes but functions while bound to heparan sulfate proteoglycans (HSPGs) on the luminal surface of vascular endothelial cells. This requires transfer of LPL from the abluminal side to the luminal side of endothelial cells. Studies were performed to investigate the mechanisms of LPL transcytosis using cultured monolayers of bovine aortic endothelial cells. We tested whether HSPGs and members of the low density lipoprotein (LDL) receptor superfamily were involved in transfer of LPL from the basolateral to the apical side of cultured endothelial cells. Heparinase/heparinitase treatment of the basolateral cell surface or addition of heparin to the basolateral medium decreased the movement of LPL. This suggested a requirement for HSPGs. To assess the role of receptors, we used either receptor-associated protein, the 39-kDa inhibitor of ligand binding to the LDL receptor-related protein and the very low density lipoprotein (VLDL) receptor, or specific receptor antibodies. Receptor-associated protein reduced (125)I-LPL and LPL activity transfer across the monolayers. When the basolateral surface of the cells was treated with antibodies, only anti-VLDL receptor antibodies inhibited transcytosis. Moreover, overexpression of the VLDL receptor using adenoviral-mediated gene transfer increased LPL transcytosis. Thus, movement of active LPL across endothelial cells involves both HSPGs and VLDL receptor.

The effects of diabetes and lipoprotein lipase (LpL) on plasma lipids were studied in mice expressing hu- man apolipoprotein B (HuBTg). Our overall objective was to produce a diabetic mouse model in which the sole effects of blood glucose elevation on atherosclerosis could be as- sessed. Mice were made diabetic by intraperitoneal injec- tion of streptozotocin, which led to a 2- to 2.5-fold increase in plasma glucose. Lipids were assessed in mice on chow and on an atherogenic Western type diet (WTD), consisting of 21% (wt/wt) fat and 0.15% (wt/wt) cholesterol. Plasma tri- glyceride and cholesterol were the same in diabetic and non- diabetic mice on the chow diet. On the WTD, male diabetic HuBTg mice had a . 50% increase in plasma cholesterol and more very low density lipoprotein (VLDL) cholesterol and triglyceride as assessed by FPLC analysis. A Triton study showed no increase in triglyceride or apolipoprotein B pro- duction, suggesting that the accumulation of VLDL was due to a decrease in lipoprotein clearance. Surprisingly, the VLDL increase in these mice was not due to a decrease in LpL activity in postheparin plasma. To test whether LpL overexpression would alter these diabetes-induced lipopro- tein changes, HuBTg mice were crossed with mice express- ing human LpL in muscle. LpL overexpression reduced plasma triglyceride, but not cholesterol, in male mice on WTD. Aortic root atherosclerosis assessed in 32-week-old mice on the WTD was not greater in diabetic mice. In sum- mary, diabetes primarily increased plasma VLDL in HuBTg mice. LpL activity was not decreased in these animals. How- ever, additional LpL expression eliminated the diabetic lipoprotein changes. These mice did not have more athero- sclerosis with diabetes. —Kako, Y., L-S. Huang, J. Yang, T. Katopodis, R. Ramakrishnan, and I. J. Goldberg. Streptozo- tocin-induced diabetes in human apolipoprotein B trans- genic mice: effects on lipoproteins and atherosclerosis. J. Lipid Res. 1999. 40: 2185-2194.