Abnormal activation of lipoprotein lipase by non-equilibrating apoC-ll: further evidence for the presence of non-equilibrating pools of apolipoproteins C-ll and C-Ill in plasma lipoproteins Laszlo Tornoci,’ Catherine A. Scheraldi,l Xianzhou Li,’ Hajime Ide,’ Ira J. Goldberg,f and Ngoc-Anh Le1.* Laboratory of Lipoprotein Physiology,’ Medlantic Research Foundation, Washington, DC, and Department of Medicine, t Columbia University College of Physicians and Surgeons, New York, NY Abstract Using artificial triglyceride emulsions, we have dem- onstrated the presence of non-equilibrating pools of apolipopro- teins C-I1 and C-111 in human plasma lipoproteins. As the con- centrations of acceptor triglycerides were increased, a greater fraction of both apoC-I1 and apoC-111 shifted away from the native plasma lipoproteins to the artificial lipid emulsions. All of the apoC-I1 and apoC-111 in very low density and high density lipoproteins (VLDL and HDL), however, could not be removed from native plasma lipoproteins. The percent of total plasma apoC-I1 and apoC-111 that could be recovered in the VLDL and HDL density fractions vaned when plasma from different in- dividuals was used. When plasma samples from normotri- glyceridemic subjects were used, HDL was the primary donor of apoCs. The percent of total plasma apoCs associated with HDL decreased from 60% to 25% for apoC-I1 and from 65% to 15% for apoC-111. When plasma samples from hypertri- glyceridemic subjects were incubated with artificial lipid emul- sions, VLDL was the primary donor of apoCs. HDL from hypertriglyceridemic subjects only accounted for 5-10% of total fasting plasma apoCs and did not contribute significantly to the final apoC contents of the artificial triglyceride emulsions. To evaluate the significance of the depletion of exchangeable apoCs from plasma HDL, we also examined the ability of control and apoC-depleted HDL to serve as activator for bovine milk lipo- protein lipase (LPL) in vitro. When HDL depleted of exchange- able apoCs were used as the source of plasma apolipoproteins for the activation of LPL in vitro, only 5-10% of the maximal ac- tivity obtained with native HDL was demonstrated. In fact, in the presence of comparable concentrations of HDL apoC-11, ac- tivation of LPL was the least with HDL which lacked exchange- able apoCs. Our data thus indicated that the presence of ex- changeable apoC-I1 on HDL is necessary for the activation of LPL in vitro. This finding is consistent with our data that sug- gest that HDL from hypertriglyceridemic subjects do not stimu- late LPL as well as HDL from normolipidemic subjects. -Tornoci, L., C. A. Scheraldi, X. Li, H. Ide, I. J. Goldberg, and N-A. Le. Abnormal activation of lipoprotein lipase by non- equilibrating apoC-11: further evidence for the presence of non- equilibrating pools of apolipoproteins C-I1 and C-111 in plasma lipoproteins. J. Lipid Res. 1993. 34: 1793-1803. Supplementary key words Intralipid enzyme-linked immunoassays The C apolipoproteins of human plasma are small polypeptides, ranging in molecular weight from 6000 to 9000, that play an important role in modulating the metabolism of several classes of plasma lipoproteins. ApoC-I1 is a required activator for lipoprotein lipase (LPL) (1, 2). Defects or deficiencies in apoC-I1 have been associated with hypertriglyceridemia in humans (3). In- travenous infusion of a synthetic polypeptide based on apoC-I1 amino acid composition has been reported to significantly reduce hypertriglyceridemia in these patients (4,5). ApoC-I11 which is present both as non-glycosylated (apoC-111,) and glycosylated isoforms containing either one (apoC-1111)or two (apoC-1112) moles of sialic acid (2, 6) inhibits LPL (7) and hepatic triglyceride lipase (HTGL, 8) activities in vitro. More direct evidence of the inhibi- tory effect of apoC-I11 was demonstrated in the kindred with familial. apoC-IIUapoA-I deficiency (9) who have very rapid fractional catabolic rates of VLDL triglyceride (TRIG) in vivo. The recent report of marked hypertri- glyceridemia (IO) and delayed TRIG clearance (10) in transgenic mice that overproduce human apoC-I11 is also consistent with the hypothesis that apoC-I11 may have an inhibitory effect on lipolytic activity. Inhibition of LPL has also been recently demonstrated using synthetic pep- tides of apoC-I11 (11). ApoCs as a group have also been shown to interfere with the receptor-mediated uptake of Abbreviations: CHYLO, chylomicrons, S,> 400; VLDL, very low density lipoproteins, S, 20-400, d < 1.006 g/ml; [IDL+ LDL], mixture of intermediate and low density lipoproteins, S, 0-20, d 1.006-1.063 g/ml; HDL, high density lipoproteins, d 1.063-1.21 g/ml; LPL, lipo- protein lipase; HTGL, hepatic triglyceride lipase; apo, apolipoprotein; IL, Intralipid; TRIG, triglyceride; CHOL, cholesterol. ’To whom correspondence should be addressed at: P.O. Drawer AG, Division of Arteriosclerosis and Lipid Metabolism, Department of Medicine, Emory University School of Medicine, Atlanta, GA 30322. Journal of Lipid Research Volume 34, 1993 1793 by guest, on December 18, 2013 www.jlr.org Downloaded from