Inactivation of ANGPTL3 reduces hepatic VLDL-triglyceride secretion

An elevated plasma level of LDL-cholesterol (LDL-C) is the cardinal risk factor for coronary heart disease (CHD), and reducing plasma LDL-C levels is the cornerstone for CHD prevention (1). Inhibitors of HMG-CoA reductase, an enzyme that catalyzes an early step in cholesterol biosynthesis, effectively lower plasma LDL-C levels and prevent CHD (2). Human genetic studies have provided several new targets for LDL-C lowering. Individuals who lack ApoB (3), or microsomal transfer protein (MTP), the enzyme that transfers TG to TG-rich lipoproteins in the liver and intestine (4), have very low LDL levels. Agents that suppress ApoB expression (5) or inhibit MTP activity (6) are now available for treatment of severe hypercholesterolemia in individuals with homozygous familial hypercholesterolemia. Loss-of-function mutations in PCSK9, a secreted proprotein convertase that promotes degradation of the LDL receptor (LDLR), cause a 30% reduction in LDL-C and substantial protection from CHD (7). Anti-PCSK9 antibodies lower circulating LDL-C levels (8) and clinical trials are now underway to determine whether there is an associated reduction in CHD (9). More recently, several families with inactivating mutations in angiopoietin-like protein 3 (ANGPTL3) were identified (10–12). Family members with two loss-of-function mutations in ANGPTL3 have striking pan-hypolipidemia; plasma levels of TGs, NEFAs, VLDL-cholesterol (VLDL-C), LDL-C, and HDL-cholesterol (HDL-C) are all markedly reduced. The mechanisms by which ANGPTL3 modulates TG metabolism have been extensively investigated (13). ANGPTL3 inhibits the activity of two intravascular lipases: LPL, which catalyzes hydrolysis of TGs in TG-rich lipoproteins, and endothelial lipase (EL), which hydrolyzes lipoprotein phospholipids (14–16). Thus, increased activity of LPL and EL may account for the low plasma levels of TG and HDL-C associated with ANGPTL3 deficiency. The finding that LPL and EL activities are increased in Angptl3−/− mice (14, 15) is consistent with this hypothesis. Corresponding data from humans is limited to a single study of four individuals who were homozygous for a nonsense mutation (S17X) in ANGPTL3 (17). Postheparin plasma LPL activity was higher in the individuals harboring the nonsense mutation than in family members who did not carry mutations in ANGPTL3, but no significant differences in EL activity were observed. In contrast, the mechanism responsible for the lower plasma levels of LDL-C associated with ANGPTL3 deficiency remains enigmatic. Plasma LDL-C levels are determined by the relative rates of production and clearance of LDL (18). LDL is produced in the circulation from VLDL, a TG-rich lipoprotein assembled in the liver. The surface of VLDL is decorated with multiple proteins, including several copies of ApoE as well as a single copy of ApoB. Following lipolysis by LPL, VLDL and its metabolic intermediate, IDL, have two possible fates: clearance by the liver or further metabolism to LDL. Hepatic uptake of VLDL remnants (and intestine-derived chylomicron remnants) is mediated by ApoE, which is bound by LDLRs (19) and by LDLR related protein 1 (LRP1) (20) or by proteoglycans (syndecan 1) (21). The remaining VLDL remnant particles mature into LDL, which is removed from the blood primarily by LDLR-mediated endocytosis in the liver. Inactivation of ANGPTL3 in mice by gene targeting or by anti-ANGPTL3 antibodies reduces plasma cholesterol levels in mice lacking functional ApoE (22) or LDLR (23). These data indicate that ANGPTL3 lowers LDL by a mechanism that is independent of both a major ligand for receptor-mediated clearance of lipoproteins and the major receptor that mediates clearance of circulating LDL. To investigate the mechanism by which ANGPTL3 deficiency lowers plasma LDL-C levels, we used REGN1500 (24), a fully human monoclonal antibody that binds with high affinity to both human and mouse ANGPTL3, to inactivate ANGPTL3 in mice with genetically defined defects in the major pathways through which circulating lipoproteins are cleared. Here we report the effect of ANGPTL3 inactivation on the synthesis and clearance of LDL in mice.