The overall objective is to understand how apolipoprotein (apo) E, hepatic lipase (HL), apoC-III apoA-II, and lipoprotein lipase (LPL) interact to control the clearance of chylomicron and very low density lipoprotein (VLDL) remnants and to regulate the catabolism of high density lipoproteins (HDL). We will identify which of these protein components is rate-limiting in these pathways in transgenic rabbits and determine how they modulate the development of atherosclerosis in the cholesterol-fed rabbit. In particular, we will test the hypothesis that the content of apoE on remnant lipoproteins is the key determinant in remnant clearance, and that its action is modulated by apoC-III, HL, and LPL. The rabbit is used as the experimental model because of its exaggerated response to dietary cholesterol, its sensitivity to apoE levels, and its distinct phenotypes upon transgene expression. We have established transgenic rabbit lines that express human apoE (E3), HL, apoC-III and apoB, and we will generate transgenic rabbits with appropriate tissue expression of LPL. Since the E3 and E4 variants of apoE have different preferential associations with lipoproteins, we will generate apoE4 transgenic rabbits and compare their lipid metabolism and susceptibility to atherosclerosis with that of our existing apoE3 lines. To help evaluate the functions of HL, we will generate transgenic rabbits that overexpress apoA-II to investigate this potential role as a negative regulator of HL. Cross-breeding various lines will yield specific lipoprotein backgrounds that emphasize particular interactions to help identify rate-limiting steps. In cholesterol-fed transgenic rabbits, increased expression of either HL or apoE results in dramatically different but complementary lipoprotein distributions; abundant thin fatty streak lesions are found in the apoE transgenic rabbits, whereas thick raised lesions appear in the ascending aorta but not the descending aorta of HL transgenic rabbits. These distinctive phenotypes will be characterized further to determine how these remodeled lipoproteins influence lesion development. Our transgenic rabbits are unique new models for the study of specific populations of plasma lipoproteins and result in vascular wall lesions, permitting an evaluation of the atherogenic environment to allow the design of effective and specific therapeutic interventions.
