Abstract
Apolipoprotein (apo-) E2 and beta-migrating very low density lipoproteins (beta-VLDL) (which were isolated from type III hyperlipoproteinemic subjects) both demonstrated defective binding to apo-E and apo-B,E receptors on dog liver membranes and to apo-B,E low density lipoproteins (LDL) receptors on fibroblasts. The defective binding activity of the apo-E2 and beta-VLDL varied from very poor to nearly normal. The ability of the beta-VLDL to interact with hepatic apo-E receptors was enhanced by the addition of normal apo-E3 to the beta-VLDL. Furthermore, cysteamine treatment of the apo-E2 in beta-VLDL enhanced binding of the beta-VLDL to both apo-E and apo-B,E receptors. The importance of apo-E in mediating the receptor binding of beta-VLDL to these receptors was confirmed by using monoclonal antibodies. The residual binding activity of beta-VLDL to apo-E and apo-B,E receptors was inhibited by greater than 90% with anti-apo-E, while the addition of anti-apo-B had little effect. The apo-B in the beta-VLDL was capable of binding to apo-B,E receptors after the hydrolysis of the beta-VLDL triglycerides with milk lipoprotein lipase. Lipase treatment yielded, two subfractions of beta-VLDL. One fraction (d = 1.02 to 1.03 g/ml) was enriched with apo-B100; the other fraction (d less than 1.006 g/ml) was enriched with apo-B48 and apo-E2. Significantly increased amounts of the apo-B100-enriched fraction bound to apo-B,E receptors. Inhibition of this binding caused by the addition of anti-apo-B indicated that the binding activity of this subfraction was mediated by apo-B100. The apo-B48-enriched fraction did not show a significant increase in receptor binding, suggesting that apo-B48 does not bind to these receptors. In a control experiment, it was shown that triglyceride-rich VLDL, which contain normal apo-E3 and apo-B100, bind significantly to both liver apo-E receptors and fibroblast apo-B,E receptors. This binding activity was inhibited by greater than 90% with anti-apo-E. Lipase hydrolysis of the VLDL did not further enhance their receptor-binding activity. These results demonstrate that apo-E, and not apo-B, is the major determinant mediating the receptor-binding activity of cholesterol-rich beta-VLDL and triglyceride-rich VLDL.
Highlights
From the Gladstone Foundation Laboratories for Cardiovascular DiseasCea,rdiovascular Research Institute, Departmentsof Pathology and Medicine, University of California, San Francisco, California 94140
Apolipoprotein E2 and &migrating very low cardiovascular disease. Patients with this lipid disorder are densitylipoproteins (8-VLDL) bodthem- triglycerides, which are associated with the appearance of an onstrated defective binding to apo-E and apo-B,E re- abnormal lipoprotein referred to as /3-VLDL.‘ In contrast to ceptors on dog Iiver membranes and to apo-B,E low normal VLDL, which are triglyceride-rich lipoproteins condensitylipoproteins (LDL) receptors on fibroblasts
Three days following the final boost, the Previous results have demonstrated that the apo-E2 isomouse was killed, and the spleen was removed under aseptic condi- lated from different type111 hyperlipoproteinemic patients is tions
Summary
From the Gladstone Foundation Laboratories for Cardiovascular DiseasCea,rdiovascular Research Institute, Departmentsof Pathology and Medicine, University of California, San Francisco, California 94140. Apolipoprotein (apo-) E2 and &migrating very low cardiovascular disease Patients with this lipid disorder are densitylipoproteins (8-VLDL) (which were isolated characterized by elevated levels of plasma cholesterol and from type111hyperlipoproteinemic subjects) bodthem- triglycerides, which are associated with the appearance of an onstrated defective binding to apo-E and apo-B,E re- abnormal lipoprotein referred to as /3-VLDL.‘ In contrast to ceptors on dog Iiver membranes and to apo-B,E low normal VLDL, which are triglyceride-rich lipoproteins condensitylipoproteins (LDL) receptors on fibroblasts. Lipase hydrolysisof the VLDL did notfurther enhance zygous for a different allele (t2*), hasa cysteine substituted their receptor-binding activity These results demon- for an arginine at residue 145, and itdisplayed nearly normal strate that apo-E, and not apo-B, is the major deter- binding to apo-B,E receptors [9]. The defective binding of P-VLDL to these receptors appears t o be a direct consequence of the abnormal apo-E2 present in these lipoproteins
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