Dual Control of Low-Density Lipoprotein Cholesterol Levels by ANGPTL3 and ANGPTL8.
TL;DR
A8 acts in concert with A3 to differentially modulate lipoprotein lipase- and endothelial lipase-mediated lipolysis, which antagonizes hepatic clearance of newly secreted atherogenic ApoB-containing lipoproteins.
Abstract
Background
Inactivation of ANGPTL3 (angiopoietin-like protein 3; A3) is a proven therapeutic strategy for lowering plasma lipid levels independently of the low-density lipoprotein receptor, yet the optimal approach to inactivate A3 remains unclear. A3 is proteolytically cleaved and circulates as full-length, N-terminal, and C-terminal fragments. The specific contribution of each form of A3, and of its paralog, ANGPTL8 (angiopoietin-like protein 8; A8), in modulating circulating levels of ApoB (apolipoprotein B)-containing lipoproteins remain poorly defined. Clarifying these relationships will inform next-generation A3-directed therapies.
Methods
We performed liver perfusion studies to directly compare the number and composition of very-low-density lipoprotein particles secreted from mice with and without A3. To amplify effects on cholesterol metabolism, we generated Ldlr-/- mice expressing wild-type A3, full-length A3, or N-terminal A3 fragments, with or without coexpression of A8, and analyzed plasma lipids, circulating A3 and A8 complexes, and intravascular lipase activities. Complementary in vitro assays and structural modeling were used to assess relative endothelial lipase inhibition by A3 alone or in complex with A8.
Results
Liver perfusion studies revealed that A3 inactivation does not alter the rates of hepatic secretion of very-low-density lipoprotein in wild-type or Ldlr-/- mice. Inactivation of A8 alone lowered plasma low-density lipoprotein cholesterol levels by ≈20%, an effect dependent upon the expression of both endothelial lipase and A3. Maximal inhibition of lipoprotein lipase required coexpression of A8 plus both full-length A3 and N-terminal A3 fragments, indicating that A3 cleavage, in addition to A8 expression, is essential for maximal lipoprotein lipase inhibition. In contrast, A8 expression, but not A3 cleavage, was required for optimal endothelial lipase inhibition.
Conclusions
A8 acts in concert with A3 to differentially modulate lipoprotein lipase- and endothelial lipase-mediated lipolysis, which antagonizes hepatic clearance of newly secreted atherogenic ApoB-containing lipoproteins. This mechanistic framework refines our understanding of A3-targeted lipid lowering and highlights the therapeutic potential of dual A3- plus A8-directed strategies to treat dyslipidemia and prevent atherosclerotic cardiovascular disease.