Abstract 30: ApoE and ApoCIII Interact to Modulate the Metabolism of HDL ApoA-I in Humans

ApoE has potential roles in HDL metabolism by promoting enlargement and clearance, and apoCIII could delay apoE-mediated clearance by the liver as it does for VLDL metabolism. To determine whether apoE and apoCIII modulate the kinetics of apoA-I HDL, we compared the metabolism of apoA-I in HDL subspecies that have apoE, apoCIII, both, or neither. We recruited 10 participants (4M, 6F) with low HDL-C (range 24-54 mg/dl) and BMI between 25-35 kg/m 2 . They were given an IV bolus of d3-leucine and blood collected up to 46hr. HDL was isolated from plasma by anti-apoA-I immunoaffinity chromatography, separated by sequential anti-apoE and anti-apoCIII chromatography, and size-separated using NDPAGE into alpha-1, alpha-2, alpha-3, and prebeta-1 HDL. ApoA-I was purified from HDL subspecies on SDS-PAGE, and pool size of apoA-I was determined from the protein bands, adjusted to plasma total apoA-I. D3-leucine enrichment was measured by GC-MS. We used SAAM-II modeling software to compute apoA-I fractional catabolic rates (FCR) and fluxes for each HDL subspecies using a published multicompartmental model. The main findings from our preliminary model investigation are: - The liver secretes a range of HDL sizes for each of these HDL subspecies. About 2-6% of plasma HDL apoA-I is associated with apoE and/or apoCIII. Regardless of size, apoE- and apoCIII-containing HDL are detectable in the circulation slightly earlier after tracer administration than HDL containing neither apoE nor apoCIII. - HDL that contains apoE but not apoCIII is especially active in size conversions, such as generating prebeta-1 HDL. Prebeta-1 HDL types are not a universal precursor of larger size HDL. - HDL that contains apoE but not apoCIII has about a 4-fold increased FCR (range 1.3-8.8) across all sizes of HDL compared to other HDL subspecies, consistent with the role of apoE as a liver receptor ligand. When coexisting with apoE, apoCIII abolished the apoE-accelerated clearance, making the FCR similar to that of HDL that does not have apoE. But, when apoCIII is present on HDL that does not have apoE, there is no reduction in clearance compared to HDL containing neither apolipoprotein. In conclusion, these results suggest that apoE accelerates the metabolism of HDL apoA-I, whereas apoCIII impedes this process.