Abstract
Obese mice without leptin (ob/ob) or the leptin receptor (db/db) have increased plasma HDL levels and accumulate a unique lipoprotein referred to as LDL/HDL1. To determine the role of apolipoprotein A-I (apoA-I) in the formation and accumulation of LDL/HDL1, both ob/ob and db/db mice were crossed onto an apoA-I-deficient (apoA-I(-/-)) background. Even though the obese apoA-I(-/-) mice had an expected dramatic decrease in HDL levels, the LDL/HDL1 particle persisted. The cholesterol in this lipoprotein range was associated with both alpha- and beta-migrating particles, confirming the presence of small LDLs and large HDLs. Moreover, in the obese apoA-I(-/-) mice, LDL particles were smaller and HDLs were more negatively charged and enriched in apoE compared with controls. This LDL/HDL1 particle was rapidly remodeled to the size of normal HDL after injection into C57BL/6 mice, but it was not catabolized in obese apoA-I(-/-) mice even though plasma hepatic lipase (HL) activity was increased significantly. The finding of decreased hepatic scavenger receptor class B type I (SR-BI) protein levels may explain the persistence of LDL/HDL1 in obese apoA-I(-/-) mice. Our studies suggest that the maturation and removal of large HDLs depends on the integrity of a functional axis of apoA-I, HL, and SR-BI. Moreover, the presence of large HDLs without apoA-I provides evidence for an apoA-I-independent pathway of cholesterol efflux, possibly sustained by apoE.
Highlights
Obese mice without leptin or the leptin receptor have increased plasma HDL levels and accumulate a unique lipoprotein referred to as LDL/HDL1
Both ob/ob;apoA-IϪ/Ϫ and db/db;apoA-IϪ/Ϫ mice had higher total cholesterol (TC) levels compared with lean apoA-IϪ/Ϫ mice (P Ͻ 0.01). The cholesterol in these animals was carried on VLDL and on lipoprotein particles in the size range of small LDLs/large HDLs (Fig. 1A)
Levels of low density lipoprotein receptor (LDLR), low density lipoprotein receptor-related protein (LRP), apoA-II, and apoE were not different between groups as assessed by real-time RT-PCR analysis (Table 2), but scavenger receptor class B type I (SR-BI) mRNA was reduced by 50% in db/db and db/db;apoA-IϪ/Ϫ mice
Summary
The primary anti-mouse apoE antibody was produced in the Protein and Immunology Core of the Clinical Research Unit at Vanderbilt University Medical Center. Primary antibody for mouse apoA-I was purchased from Biodesign International. C57BL/6, apoA-IϪ/Ϫ, db/ϩ, and ob/ϩ mice were originally purchased from Jackson Laboratories. The resulting pups heterozygous for the deletion of both apoA-I and leptin or leptin receptor were further crossed to generate mice of all of the genotypes used in the studies described here. Insulin measurements were performed by the Hormone Assay Core Laboratory of the Diabetes Research and Training Center at Vanderbilt University Medical Center. Plasma lipoproteins were electrophoresed on 0.7% agarose gels for the first separation by charge. For Western blotting, agarose gels were loaded with 2 l of a 1:10 dilution of plasma, transferred to nitrocellulose membranes, and probed with antibodies for apoA-I and apoB
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