Apolipoprotein C-II: New findings related to genetics, biochemistry, and role in triglyceride metabolism

Abstract

Apolipoprotein C-II (apoC-II) is a small exchangeable apolipoprotein found on triglyceride-rich lipoproteins (TRL), such as chylomicrons (CM) and very low-density lipoproteins (VLDL), and on high-density lipoproteins (HDL), particularly during fasting. ApoC-II plays a critical role in TRL metabolism by acting as a cofactor of lipoprotein lipase (LPL), the main enzyme that hydrolyses plasma triglycerides (TG) on TRL. Here, we present an overview of the role of apoC-II in TG metabolism, emphasizing recent novel findings regarding its transcriptional regulation and biochemistry. We also review the 24 genetic mutations in the APOC2 gene reported to date that cause hypertriglyceridemia (HTG). Finally, we describe the clinical presentation of apoC-II deficiency and assess the current therapeutic approaches, as well as potential novel emerging therapies.

Keywords: ApoC-II; ApoC-II deficiency; ApoC-II mutations; Hypertriglyceridemia; Lipoprotein lipase; Triglyceride-rich lipoproteins; Triglycerides.

Fig. 1. Pathways for TRL metabolism

(A) The exogenous pathway. Dietary lipids are absorbed in the small intestine, packaged into CM and secreted into lymph just 1 hour after dietary fat consumption. CM contain apoB-48 as the main structural protein and acquire apoC-II and apoE (shown) as well as additional apolipo-proteins such as apoC-I, apoC-III, and apoA-V (not shown) in the perimesenteric lymphatics. Once in circulation, CM are hydrolysed by LPL bound to endothelial cells to FFA and mono-glycerides (MG) and are converted to CM remnants. CM remnants are removed from circulation by the liver via binding to the low-density lipoprotein receptor (LDLR) or the LDL receptor-related protein (LRP). Free fatty acids (FFA) and glycerol generated during lipolysis are utilized by parenchymal tissues. (B) The endogenous pathway. Hepatically derived VLDL are present in the plasma in the both the fasting and postprandial states. VLDL, which contain apoB-100 as their main structural protein, are secreted into the systemic circulation, and like CM acquire many of the same apolipo-proteins in plasma. The lipolysis of VLDL bound to LPL on endothelial cells generates smaller particles deplete of TG, which are also called remnants or intermediate density lipoproteins (IDL). A fraction of VLDL particles undergoes further lipolysis and is converted to LDL particles without apoE that are taken up by the liver or peripheral tissues via the LDLR. Excess LDL can be deposited into the vessel wall, where it can cause atherosclerosis.

Fig. 1. Pathways for TRL metabolism

(A) The exogenous pathway. Dietary lipids are absorbed in the small intestine, packaged into CM and secreted into lymph just 1 hour after dietary fat consumption. CM contain apoB-48 as the main structural protein and acquire apoC-II and apoE (shown) as well as additional apolipo-proteins such as apoC-I, apoC-III, and apoA-V (not shown) in the perimesenteric lymphatics. Once in circulation, CM are hydrolysed by LPL bound to endothelial cells to FFA and mono-glycerides (MG) and are converted to CM remnants. CM remnants are removed from circulation by the liver via binding to the low-density lipoprotein receptor (LDLR) or the LDL receptor-related protein (LRP). Free fatty acids (FFA) and glycerol generated during lipolysis are utilized by parenchymal tissues. (B) The endogenous pathway. Hepatically derived VLDL are present in the plasma in the both the fasting and postprandial states. VLDL, which contain apoB-100 as their main structural protein, are secreted into the systemic circulation, and like CM acquire many of the same apolipo-proteins in plasma. The lipolysis of VLDL bound to LPL on endothelial cells generates smaller particles deplete of TG, which are also called remnants or intermediate density lipoproteins (IDL). A fraction of VLDL particles undergoes further lipolysis and is converted to LDL particles without apoE that are taken up by the liver or peripheral tissues via the LDLR. Excess LDL can be deposited into the vessel wall, where it can cause atherosclerosis.

Fig. 2. Lipolytic complex

LPL action is dependent on multiple regulators, both positive (green) and negative (red). LMF1 is responsible for proper folding and assembly of LPL, whereas Sel1L stabilizes the LPL-LMF1 complex. LPL is transported from parenchymal cells to the endothelial cell surface of the capillary lumen, where it binds to GPIHBP1. ApoC-II is an essential cofactor for LPL activation, whereas apoC-I and apoC-III may inhibit lipolysis. ApoA-V stabilizes the LPL-apoC-II complex by helping TRL to bind to the endothelial cell surface via HSPG. ANGPTL 3, 4, and 8 all inhibit LPL but in different tissue beds. Generated FFA and MG during TG hydrolysis are taken up by cells for energy metabolism or storage.

Fig. 3. Transcriptional control of the human APOC2 gene

(A) Relative gene position and location of major regulatory elements that control APOC2 gene and nearby genes in E/C-I/C-IV/C-II gene cluster on Chromosome 19. The liver-specific elements HCR.1 and HCR.2 are 319 bp elements approximately 22 kb and 11 kb, respectively, upstream of the APOC2 promoter. Both contain FXR elements required for bile-acid – dependent activation of the APOC2 gene. The HCR.1 FXRE (nt 214/226 of HCR.1) also binds HNF-4 and ARP1. The proximal APOC2 gene promoter is 545 bp long and contains binding sites for HNF-4 (−102 − −81), RXRα/T3Rβ (−140 − −155), cAMP (between −36 and −170) and STAT1 (−500 − −493). Long-range interactions between FXR/RXRα in HCR.1 and RXRα at the RXR/T3Rβ binding site have been shown to mediate hepatocyte-specific bile-acid regulation of the APOC2 gene (shown by upper bracket and green + at top of Fig. 3A). The RXRα/T3Rβ binding site also binds ARP1 and EAR1, which may affect transcriptional activation by RXR. The ME.1 and ME.2 are 620-bp multienhancer elements that contain LXREs, both located at nt 442 − 466 of each ME. The LXRE in ME.2 regulates the entire gene cluster. In macrophages, two STAT1 binding elements, one in ME.2 (between nt 174 − 182 of the 620 bp ME.2 element) and one between nucleotides -500 and -493 of the proximal APOC2 promoter, upregulate APOC2 gene expression. Long-range interactions between STAT1 bound to ME.2 and RXRα at the RXR/T3Rβ binding site have been shown to mediate macrophage-specific STAT1 regulation of the APOC2 gene (shown by lower bracket and purple + at top of Fig. 3A). Fibrates, statins and ezetimibe decrease APOC2 gene expression but the promoter elements involved have not been defined. Figure is not to scale. (B) Regulation of APOC2 gene by long noncoding RNA. lncLSTR indirectly inhibits APOC2 gene expression by a bile-acid-dependent mechanism, involving CYP8B1 and its transcriptional repressor TDP-43. Figure is not to scale.

Fig. 4. ApoC-II structure

(A) Sequence and the helical wheel plot of apoC-II helices. Charged residues are colored light blue, hydrophobic residues are green, neutral polar residues orange and Q and N are colored red. Position of hydrophobic moment and score is shown in center of helix. (B) Human APOC2 gene mutations. Position of known mutations are listed below exon-intron diagram of APOC2 gene with position 1 after the signal peptide cleavage site. * or X: STOP codon. The four mutations shown in bold account for more than half of all APOC2 mutations.