Missense Mutation in APOC3 within the C-terminal Lipid Binding Domain of Human ApoC-III Results in Impaired Assembly and Secretion of Triacylglycerol-rich Very Low Density Lipoproteins

Abstract

Hepatic assembly of triacylglycerol (TAG)-rich very low density lipoproteins (VLDL) is achieved through recruitment of bulk TAG (presumably in the form of lipid droplets within the microsomal lumen) into VLDL precursor containing apolipoprotein (apo)BHepatic assembly of very low density lipoproteins (VLDL) rich in triacylglycerol (TAG) 5 is initiated during and after translation and translocation of apolipoprotein (apo) B-100 across the endoplasmic reticulum membrane. The nascent VLDL particle is further enlarged in TAG content through a "second step" lipidation process, where a bulk TAG (presumably present within the microsomal lumen in the form of lipid droplet) is incorporated (1). Although much has been learned about the co-and post-translational lipidation of apoB-100, little is known about the nature or the dynamics of these lumenal lipid droplets (LLD) that are utilized as lipid precursor for VLDL assembly. Early studies with the abetalipoproteinemia gene MTTP suggested that the microsomal triglyceride transfer protein (MTP) is required for the partitioning of VLDL lipid precursor, mainly TAG, into the microsomal lumen in mouse liver (2), cultured murine primary hepatocytes (3), and the rat hepatoma McA-RH7777 cells (4). Thus, inactivation of MTP in the liver cells was associated with lack of LLD and invariably resulted in diminished assembly and secretion of VLDL. Recent proteomic and lipid characterization of LLD isolated from murine liver microsomes has shown that these lipid-rich entities are devoid of apoB but contain proteins such as TAG hydrolase, carboxylesterase 1, MTP, and apoE (5). Working with McA-RH7777 cells stably expressing recombinant human apoC-III, we have obtained evidence for the presence of a metabolically active TAG-rich entity within the microsomal lumen, which has a buoyant density resembling that of intermediate density lipoproteins (IDL) and low density lipoproteins (LDL)

Hepatic trans-Golgi action coordinated by the GTPase ARFRP1 is crucial for lipoprotein lipidation and assembly

“…However, elevated plasma levels of ApoB100 and reduced triglycerides in the VLDL fraction of Arfrp1 liv Ϫ / Ϫ mice could indicate a premature release of VLDL2. ApoC3 was described to directly interact with lipids via its helix 5, thereby facilitating triglyceride incorporation into VLDLs independently of MTP activity ( 31 ). This fi nding is supported by the fact that the human ApoC3-K58E mutation located in the lipid binding domain of ApoC3 results in a marked reduction of triglycerides and VLDLs in plasma ( 32 ).…”

Apolipoproteins C-I and C-III Inhibit Lipoprotein Lipase Activity by Displacement of the Enzyme from Lipid Droplets

“…In two following studies, the same group showed that two naturally occurring human mutations of apoC-III, A23T and K58E, were unable to promote triglyceride incorporation during VLDL assembly (73,74). Plasma concentrations of triglycerides and apoC-III are lower in carriers of the A23T or K58E mutations (62,63).…”

Changes in helical content or net charge of apolipoprotein C-I alter its affinity for lipid/water interfaces

“…However, several point mutations in the amphipathic ␣ -helices of apoC-II (W26R, L72P) and apoC-III (A23T, Q23K, K58E) alter protein expression, lipid surface-binding, or function ( 20,(55)(56)(57). Since apoC-II and C-III are structurally similar to apoC-I, we speculate that altered helical content or charge in apoC-II and apoC-III point mutations may alter protein-lipid, protein-protein, or protein-enzyme interactions, thereby infl uencing the ability of these proteins to penetrate and bind to, be retained on, and regulate the fate of TAG-rich lipoproteins.…”