A decreased expression of angiopoietin-like 3 is protective against atherosclerosis in apoE-deficient mice

Abstract

KK/Snk mice (previously KK/San) possessing a recessive mutation ( hypl ) of the angiopoietin-like 3 ( Angptl3 ) gene homozygously exhibit a marked reduction of VLDL due to the decreased Angptl3 expression. Recently, we proposed that Angptl3 is a new class of lipid metabolism modulator regulating VLDL triglyceride (TG) levels through the inhibition of lipoprotein lipase (LPL) activity. In this study, to elucidate the role of Angptl3 in atherogenesis, we investigated the effects of hypl mutation against hyperlipidemia and atherosclerosis in apolipoprotein E knockout (apoEKO) mice. ApoEKO mice with hypl mutation (apoEKO-hypl ) exhibited a significant reduction of VLDL TG, VLDL cholesterol, and plasma apoB levels compared with apoEKO mice. Hepatic VLDL TG secretion was comparable between both apoE-deficient mice. Turnover studies revealed that the clearance of both [ 3 H]TG-labeled and 125 I-labeled VLDL was significantly enhanced in apoEKO-hypl mice. Postprandial plasma TG levels also decreased in apoEKO-hypl mice. Both LPL and hepatic lipase activities in the postheparin plasma increased significantly in apoEKO-hypl mice, explaining the enhanced lipid metabolism. Furthermore, apoEKO-hypl mice developed 3-fold smaller atherogenic lesions in the aortic sinus compared with apoEKO mice. Taken together, the reduction of Angptl3 expression is protective against hyperlipidemia and atherosclerosis, even in the absence of apoE, owing to the enhanced catabolism and clearance of TG-rich lipoproteins. The accrued evidence that lipid-lowering therapy limits the progression of atherosclerosis and reduces the events of coronary artery diseases is overwhelming (1, 2). The focus has been on the reduction of LDL cholesterol (3, 4). Recent studies have also pointed out the importance of reducing triglyceride (TG)-rich lipoproteins such as chylomicrons, VLDL, and their remnants, and of raising HDL cholesterol (5, 6). In addition, postprandial hypertriglyceridemia is mentioned as an independent risk factor for atherogenesis (7,8).In a colony of KK mice with mild obesity, hyperlipidemia, and diabetes, we found mutant mice (KK/Snk, previously KK/San) that were characterized by a significant decrease in plasma lipid levels, mainly due to the reduction of TG-rich lipoproteins, despite their obesity and diabetes. Genetic studies for the mutation, named hypolipidemia ( hypl ), in KK/Snk mice identified a 4 bp nucleotide insertion in exon 6 of a gene encoding angiopoietin-like 3 ( Angptl3 ), which causes a premature stop codon after a frameshift (9). Angptl3 is a secretory protein of 70 kDa expressed predominantly in the liver, and has a signal sequence, coiled-coil domain, and fibrinogen-like domain similar to those of other angiopoietin families (10, 11). In KK/Snk mice with the homozygous hypl mutation, Angptl3 expression was markedly decreased, probably due to the instability of mutant mRNA, resulting in a hypolipidemic trait (9). In contrast, adenovirus-mediated overexpression of Angptl3 or intravenous injection of the purified...

Inactivation of ANGPTL3 reduces hepatic VLDL-triglyceride secretion

“…Inactivation of ANGPTL3 lowered plasma cholesterol levels in Apoe Ϫ / Ϫ mice ( Fig. 2A ), as was reported previously ( 22,23 ). Similar reductions were observed in Apoe…”

“…The nature of the pathway by which ANGPTL3 inactivation promotes the clearance of ApoB is not known. ANG-PTL3 defi ciency is associated with lower cholesterol levels in mice lacking either ApoE or LDLR ( 22,23 ), and the present study extends these fi ndings to LRP1 and syndecan 1. Thus, the increased clearance of ApoB-containing lipoproteins presumably involves a noncanonical pathway that does not require ApoE, the LDLR, LRP1, or syndecan 1.…”

“…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.…”

Clinical characteristics and plasma lipids in subjects with familial combined hypolipidemia: a pooled analysis

“…In fact, Minicocci et al ( 11 ) reported lower fasting glucose and the lack of diabetes mellitus among homozygous carriers of inactivating mutation in the ANGPTL3, but the consistency of these fi ndings remain to be confi rmed. Finally, it has been suggested that the defi ciency of ANGPTL3 may be protective against atherosclerosis ( 22 ). In a previous report, homozygous FHBL2 subjects did not show evidence of history of cardiovascular disease, but the low number of cases investigated precluded any defi nitive conclusion ( 11 ).…”

Atypical angiopoietin-like protein that regulates ANGPTL3

“…However, the concentrations of ANGPTL3 used in these studies were supraphysiological. Moreover in some (14,17), although not all (8) studies, the increase in LPL activity in Angptl3 −/− mice has been modest. ANGPTL3 also inhibits the activity of endothelial lipase (18,19), which catalyzes the hydrolysis of phospholipids in circulating lipoproteins.…”