Defects of the LDL receptor in WHHL transgenic rabbits lead to a marked accumulation of plasma lipoprotein[a].

In this study, we created LDL receptor (LDLr) defective (WHHL) transgenic rabbits expressing human apo[a] to examine whether LDLr mediates the Lp[a] clearance from the plasma. By crossbreeding WHHL rabbits with human apo[a] transgenic rabbits, we obtained two groups of human apo[a] transgenic rabbits with defective LDLr functions: apo[a] 1 /0 WHHL heterozygous (LDLr 1 / 2 ) and apo[a] 1 /0 WHHL homozygous (LDLr 2 / 2 ) rabbits. The lipid and lipoprotein levels of human apo[a] WHHL rabbits were compared to those of human apo[a] transgenic rabbits with normal LDLr functions (LDLr 1 / 1 ). The apo[a] production rate was evaluated by analyzing apo[a] mRNA expression in the liver, the major site for apo[a] synthesis in transgenic rabbits. We found that preb lipoproteins were markedly increased accompanied by a 2-fold increase in the plasma Lp[a] in apo[a] 1 /0 /LDLr 1 / 2 rabbits and a 4.2-fold increase in apo[a] 1 /0 /LDLr 2 / 2 rabbits compared with that in apo[a] 1 /0 rabbits with normal LDLr function. In apo[a] 1 /0 /LDLr 2 / 2 rabbits, there was a marked increase in plasma total cholesterol and triglycerides, as was found in their counterpart non-transgenic WHHL rabbits. Northern blot analysis revealed that hepatic apo[a] expression in WHHL transgenic rabbits was similar to that in LDLr 1 / 1 transgenic rabbits, suggesting the accumulation of plasma Lp[a] in WHHL transgenic rabbits was not due to increased apo[a] synthesis. In conclusion, absence of a functional LDLr leads to a marked accumulation of plasma Lp[a] in human apo[a] transgenic WHHL rabbits and LDLr may participate in the catabolism of Lp[a] in rabbits.— Fan, J., M. Challah, H. Shimoyamada, M. Shiomi, S. Marcovina, and T. Watanabe. Defects of the LDL receptor in WHHL transgenic rabbits lead to a marked accumulation of plasma lipoprotein[a]. J. Lipid Res. 2000. 41: 1004–1012. Supplementary key words lipoprotein[a] • WHHL rabbit • apolipoprotein[a] • LDL receptor • transgenic rabbit • metabolism • catabolism • hypercholesterolemia High lipoprotein[a] (Lp[a]) levels constitute an independent risk factor for the development of atherosclerosis. In many human studies, elevated levels of plasma Lp[a] have been found to be associated with an increased risk of atherosclerotic coronary heart disease, stroke, and restenosis (1–3), although some studies have not detected this association (4–6). The risk of elevated Lp[a] concentrations is significantly increased in patients who also have high levels of LDL cholesterol (7, 8). The Lp[a] particle closely resembles low density lipoprotein (LDL) in both lipid composition and the presence of apolipoprotein (apo) B-100 (apoB-100). Lp[a] is distinguished from LDL by the presence of an additional protein component designated as apolipoprotein[a] (apo[a]), which is complexed to apoB-100 by disulfide linkage (1). Apo[a] is a large plasma glycoprotein (28% carbohydrate by weight) synthesized primarily in the liver (9). Apo[a] displays genetically determined size heterogeneity, and as many as 34 different isoforms of apo[a] have been identified in human plasma, varying in mass from , 300 to . 800 kDa (1, 10, 11). Lp[a] concentrations are strongly genetically determined (12) with at least 90% of the variation determined by variation within the gene for apo[a] (13). Because Lp[a] contains apoB-100 and bears a structural similarity to LDL, it has been proposed that Lp[a] may be removed from the plasma by the LDL receptor (LDLr). Some studies using cultured cells revealed that Lp[a] can bind specifically to the LDLr (14–16) while other studies failed to reproduce those results (17, 18). Utermann et al. (19) first reported that patients with familial hypercholesterolemia (FH) have plasma Lp[a] levels higher than expected for their respective apo[a] phenotypes. This observation was supported by two other clinical studies (20, 21). Furthermore, this notion is further strengthened by a study of transgenic mice overexpressing human LDLr in the Abbreviations: WHHL, Watanabe heritable hyperlipidemic; apo[a], apolipoprotein[a]; Lp[a], lipoprotein[a]; LDL, low density lipoproteins; LDLr, LDL receptor; VLDL, very low density lipoproteins; IDL, intermediate density lipoproteins; HDL, high density lipoproteins; FH, familial hypercholesterolemia; PCR, polymerase chain reaction; ELISA, enzyme-linked immunosorbent assay; mAb, monoclonal antibody. 1 To whom correspondence should be addressed.