Phospholipid transfer protein plays a major role in the initiation of apolipoprotein B-containing lipoprotein assembly in mouse primary hepatocytes [Metabolism]

January 31st, 2015 by Manchekar, M., Liu, Y., Sun, Z., Richardson, P. E., Dashti, N.

In this study, we tested the hypothesis that phospholipid transfer protein (PLTP) is a plausible mediator of PL transfer to the N-terminal 1000 residues of apoB (apoB:1000) leading to the initiation of apoB-containing lipoprotein assembly. To this end, primary cultures of hepatocytes from wild type (WT) and PLTP-knockout (KO) mice were infected with adenovirus (Adv)-apoB:1000 with or without co-infection with Adv-PLTP and the assembly and secretion of apoB:1000-containing lipoprotein were assessed. PLTP deficiency resulted in a 65% and 72% reduction in the protein and lipid content, respectively, of secreted apoB:1000-containg lipoproteins. Particles secreted by WT hepatocytes contained 69% PL, 9% diacylglycerol (DAG) and 23% triacylglycerols (TAG), with the stoichiometry of 46 PL, 6 DAG, and 15 TAG molecules per apoB:1000. PLTP absence drastically altered the lipid composition of apoB:1000 lipoproteins; these particles contained 46% PL, 13% DAG, and 41% TAG, with stoichiometry of 27 PL, 10 DAG, and 23 TAG molecules per apoB:1000. Re-introduction of Pltp gene into PLTP-KO hepatocytes stimulated the lipidation and secretion of apoB:1000-containing lipoproteins by ~3-fold; the lipid composition and stoichiometry of these particles were identical to those secreted by WT hepatocytes. In contrast to the WT, apoB:1000 in PLTP-KO hepatocytes was susceptible to intracellular degradation, predominantly in the post-ER, pre-secretory compartment. Re-introduction of Pltp gene into PLTP-KO hepatocytes restored the stability of apoB:1000. These results provide compelling evidence that, in hepatocytes, initial recruitment of PL by apoB:1000 leading to the formation of the PL-rich apoB-containing initiation complex is mediated, to a large extent, by PLTP.
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