Abstract 257: Apolipoprotein M Is Not Present in Prebeta-1 HDL Particles
BACKGROUND: Apolipoprotein M (apoM) is a 25 kD plasma protein present mainly in HDL. It has a hydrophobic pocket for carrying ligands variously reported as retinol, all-trans-retinoic acid, 9-cis-retinoic acid, sphingosine-1-phosphate (S1P) and oxidized phospholipids. In addition to mediating the effects of S1P and modulating oxidative stress, apoM has been reported to enhance cholesterol efflux and to increase plasma levels of small, preβ1 HDL, a particle that efficiently accepts cholesterol effluxed from cholesterol-loaded cells and plays a key role in reverse cholesterol transport. ApoM is present in α-migrating HDL particles but whether it is also present in small preβ1 HDL particles is disputed. Establishing the absence or presence of apoM in preβ1 HDL particles is essential for understanding its role in reverse cholesterol transport.
METHODS: We performed native-native 2D gel electrophoresis on healthy volunteer plasma to separate native HDL particles by size and charge. Particles were blotted onto a membrane and probed with antibodies to apoM or apoA-I to identify specific HDL particles associated with apoM. Similar experiments were performed with plasma from patients with apoE-deficiency or from patients with low plasma levels of apoA-I and HDL. We also performed native 1D electrophoresis to visualize lipoprotein particles containing apoM. Finally, native-native 2D gels of purified HDL were used for proteomics of preβ1 particles.
RESULTS: apoM was present in two large α-migrating HDL particles and in LDL-sized particles, as well as in one small particle that did not contain apoA-I. apoA-I but not apoE was required for formation of the large apoM-containing HDL particles. The small apoM particle was unaffected in apoA-I- and apoE-deficient patients. apoM was not present in preβ1 particles.
CONCLUSION: apoM is not a stable component of small preβ1 apoA-I-containing HDL particles but instead resides mainly in two large HDL molecules and in LDL-sized particles, as well as a small particle that does not contain apoA-I. The role of apoM in cholesterol efflux requires further evaluation.
- © 2012 by American Heart Association, Inc.