Altered lipoprotein metabolism in transgenic mice

Transgenic mouse lines were produced that expressed low levels of a receptor-binding-defective variant of human apolipoprotein (apo) E, apoE(Argll2, Cys142). In transgenic mice, the human apoE was produced only by the kidney, whereas endogenous mouse apoE was produced mainly by the liver. The plasma concentration of the transgenic protein was about half that of endogenous apoE. The expression of transgenic apoE did not affect total plasma cholesterol and triglyceride levels, but the distribution of the human variant differed from that of endogenous apoE in the intermediate size and density range, where the transgenic protein accumulated selectively. Immunoblots of agarose gels of lipoprotein fractions showed that the transgenic protein occurred primarily on large a-migrating particles (HDLI). This phenomenon was not observed in transgenic mice expressing normal human apoE-3, which distributed like endogenous apoE, suggesting that the defective apoE variant perturbed HDLI metabolism. In mice fed a high-fat, highcholesterol diet, the transgenic apoE associated primarily with the apoB-containing lipoproteins. A significantly higher increase in very low density lipoprotein cholesterol was observed in fat-fed transgenics compared to fat-fed nontransgenic mice, suggesting a metabolic perturbation of apoB-containing lipoproteins. I Thus, the receptor-binding-defective variant, apoE(Argll2, Cys142), expressed at low levels by the kidney, alters lipoprotein metabolism in transgenic mice, presumably by interfering with apoE-mediated removal of the lipoproteins from circulation.Fazio, S., Y. Horie, W. S. Simonet, K. H. Weisgraber, J. M. Taylor, and S. C. Rall, Jr. Altered lipoprotein metabolism in transgenic mice expressing low levels of a human receptor-binding-defective apolipoprotein E variant. J Lipid Res. 1994. 35: 408-416. Supplementary key words teinemia dysbetalipoproteinemia lipoprotein metabolism hyperlipoproApolipoprotein (apo) E is a constituent of several classes of lipoproteins, including liver-derived very low density lipoproteins (VLDL) and their remnants, intestinederived chylomicron remnants, and certain subclasses of high density lipoproteins (HDL) (1). T h e major role of apoE in plasma of humans and other animal species is to mediate uptake of lipoproteins by specific lipoprotein receptors on cell surfaces (2). ApoE is a ligand for both the low density lipoprotein (LDL) receptor (3) and the LDL receptor-related protein (LRP) (4). The critical role of apoE in catabolism of lipoproteins has been demonstrated in a variety of ways. Accelerated catabolism of apoB-containing remnants occurs when animals (normolipidemic o r hypercholesterolemic) are injected o r infused with apoE (5, 6). These types of studies suggest that apoE availability is a rate-limiting step in efficient lipoprotein remnant removal from plasma. I m paired catabolism of remnants occurs in humans with the lipid disorder type 111 hyperlipoproteinemia (HLP) , which can arise either because of the expression of any of about a dozen receptor-binding-defective apoE mutants or because of apoE deficiency (7, 8). ApoE also appears to be important in HDL metabolism, where it plays a role in the process of reverse cholesterol transport (9). I n this process, excess peripheral cell cholesterol becomes associated with HDL, and through the action of 1ecithin:cholesterol acyltransferase (LCAT), the resulting cholesteryl esters become part of the neutral lipid core of HDL. ApoE appears to facilitate the expansion of this lipid core (10, ll), creating apoEenriched particles of larger size and lower density that a re termed HDLl (especially in humans and rodents) or HDL, (cholesterol-induced HDL in fat-fed animals) (12). ApoE can mediate the catabolism of these cholesteryl ester-enriched HDL species by the liver (13, 14), completAbbreviations: apo, apolipoprotein; CETP, cholesteryl ester transfer protein; HDL, high density lipoprotein(s); HLP, hyperlipoproteinemia; LCAT, 1ecithin:cholesterol acyltransferase; LDL, low density lipoprotein(s); LRP, LDL receptor-related protein; VLDL, very low density lipoprotein(s). IT0 whom correspondence should be addressed. 408 Journal of Lipid Research Volume 35, 1994 by gest, on O cber 8, 2017 w w w .j.org D ow nladed fom ing the reverse cholesterol transport process. Presumably, impaired catabolism of these HDLl particles could occur in the presence of a dysfunctional apoE. The availability of small animal models affords an opportunity to investigate the above processes in more detail. Mice are an attractive model for the study of HDL metabolism because, unlike humans and some other animal models, they carry a large portion of their cholesterol in HDL rather than in the apoB-containing lipoproteins (15). This is due in large part to the absence of cholesteryl ester transfer protein (CETP) activity in their plasma (16). In CETP-deficient animals, apoE-mediated catabolism of HDLl can be a major route for cholesterol metabolism (17, 18). Because LDL is not a major lipoprotein in mice, the most important ligand for cholesterol metabolism in this species is apoE, rather than apoB (19). The extremely efficient action of apoE in mice has been shown in several ways and is probably at least partly responsible for the resistance to atherosclerosis of most mouse strains (19). Overexpression of either rat apoE (20, 21) or normal human apoE3 (22) in transgenic mice leads to lower levels of VLDL, more efficient clearance of apoB-containing lipoproteins, and appearance of larger, less dense HDLl species compared to normal mice. Conversely, recently created null apoE mice (23, 24) and transgenic mice overexpressing dysfunctional apoE (25) have severe hyperlipidemia on normal diets due to accumulation of apoBcontaining lipoproteins in the plasma. We have created transgenic mice that express a receptorbinding-defective human apoE variant, apoE(Argll2, Cys142). This variant apoE has been described in a single family in which all seven subjects in four generations who are heterozygous for this apoE variant have type I11 HLP (26-28). When expressed in transgenic mice at high levels, this dysfunctional apoE variant yields a phenotype very similar to human type I11 HLP (25). However, as demonstrated in this report, even expression of only modest levels of this receptor-binding-defective apoE variant in transgenic mice was sufficient to perturb lipoprotein metabolism, presumably by interference with apoEmediated removal of lipoproteins from circulation. MATERIALS AND METHODS