Discrete roles of apoA-I and apoE in the biogenesis of HDL species: lessons learned from gene transfer studies in different mouse models.

Using adenovirus-mediated gene transfer in apolipoprotein A-I (apoA-I)-deficient mice, we have established that apoA-I mutations inhibit discrete steps in a pathway that leads to the biogenesis and remodeling of high-density lipoprotein (HDL). To this point, five discrete categories of apoA-I mutants have been characterized that may affect the interactions of apoA-I with ATP-binding cassette superfamily A, member 1 (ABCA1) or lecithin:cholesterol acyl transferase (LCAT) or may influence the plasma phospholipid transfer protein activity or may cause various forms of dyslipidemia. Biogenesis of HDL is not a unique property of apoA-I. Using adenovirus-mediated gene transfer of apoE in apoA-I- or ABCA1-deficient mice, we have established that apolipoprotein E (apoE) also participates in a novel pathway of biogenesis of apoE-containing HDL particles. This process requires the functions of the ABCA1 lipid transporter and LCAT, and it is promoted by substitution of hydrophobic residues in the 261 to 269 region of apoE by Ala. The apoE-containing HDL particles formed in the circulation may have atheroprotective properties. ApoE-containing HDL may also have important biological functions in the brain that confer protection from Alzheimer's disease.

Probing the pathways of chylomicron and HDL metabolism using adenovirus-mediated gene transfer.

PURPOSE OF THE REVIEW: This review clarifies the functions of key proteins of the chylomicron and the HDL pathways. RECENT FINDINGS: Adenovirus-mediated gene transfer of several apolipoprotein (apo)E forms in mice showed that the amino-terminal 1-185 domain of apoE can direct receptor-mediated lipoprotein clearance in vivo. Clearance is mediated mainly by the LDL receptor. The carboxyl-terminal 261-299 domain of apoE induces hypertriglyceridemia, because of increased VLDL secretion, diminished lipolysis and inefficient VLDL clearance. Truncated apoE forms, including apoE2-202, have a dominant effect in remnant clearance and may have future therapeutic applications for the correction of remnant removal disorders. Permanent expression of apoE and apoA-I following adenoviral gene transfer protected mice from atherosclerosis. Functional assays, protein cross-linking, and adenovirus-mediated gene transfer of apoA-I mutants in apoA-I deficient mice showed that residues 220-231, as well as the central helices of apoA-I, participate in ATP-binding cassette transporter A1-mediated lipid efflux and HDL biogenesis. Following apoA-I gene transfer, an amino-terminal deletion mutant formed spherical alpha-HDL, a double amino- and carboxyl-terminal deletion mutant formed discoidal HDL, and a carboxyl-terminal deletion mutant formed only pre-beta-HDL. The findings support a model of cholesterol efflux that requires direct physical interactions between apoA-I and ATP-binding cassette transporter A1, and can explain Tangier disease and other HDL deficiencies. SUMMARY: New insights are provided into the role of apoE in cholesterol and triglyceride homeostasis, and of apoA-I in the biogenesis of HDL. Clearance of the lipoprotein remnants and increase in HDL synthesis are obvious targets for therapeutic interventions.

Domains of apoE required for binding to apoE receptor 2 and to phospholipids: implications for the functions of apoE in the brain.

We have studied the contribution of the carboxy terminal domains of lipid-free apoE isolated from apoE-expressing cell cultures in binding to phospholipids and have determined the affinities of reconstituted POPC-apoE particles for the apoER2. It was found that the initial rate of association of apoE2, apoE3, apoE4, and a mutant form apoE4R158M to multilamellar DMPC vesicles was similar and was reduced and eventually diminished by gradual deletion of the carboxy terminal segments. The truncated apoE forms retained their ability to associate with plasma lipoproteins. Receptor binding studies were performed using the ldlA-7 cells expressing apoER2 and transiently transfected COS-M6 and the appropriate control untransfected cells. Specific binding to apoER2 was obtained by subtracting from the total binding to the receptor-expressing cells the nonspecific binding values of the untransfected cells. POPC-apoE particles generated using apoE3, apoE4, the truncated apoE4-259, apoE4-229, apoE4-202, and apoE-165, and the mutant apoE4R158M all bound tightly to the apoER2 (K(d) range of 12 +/- 3 to 19 +/- 4 microg/mL). POPC-apoE2 bound with reduced affinity (K(d) = 31 +/- 5.3 microg/mL). The findings establish that the apoER2 binding domain of apoE is in the 1-165 amino terminal region, whereas the carboxy terminal 230-299 region of apoE is required for efficient initial association with phospholipids.