Angiopoietin-like Protein 3 Inhibits Lipoprotein Lipase Activity through Enhancing Its Cleavage by Proprotein Convertases

Abstract

Lipoprotein lipase (LPL)-mediated lipolysis of triglycerides is the first and rate-limiting step in chylomicron/very low density lipoprotein clearance at the luminal surface of the capillaries. Angiopoietin-like protein 3 (ANGPTL3) is shown to inhibit LPL activity and plays important roles in modulating lipoprotein metabolism in vivo. However, the mechanism by which it inhibits LPL activity remains poorly understood. Using cell-based analysis of the interaction between ANGPTL3, furin, proprotein convertase subtilisin/kexin type 5 (PCSK5), paired amino acid converting enzyme-4 (PACE4), and LPL, we demonstrated that the cleavage of LPL by proprotein convertases is an inactivation process, similar to that seen for endothelial lipase cleavage. At physiological concentrations and in the presence of cells, ANGPTL3 is a potent inhibitor of LPL. This action is due to the fact that ANGPTL3 can enhance LPL cleavage by endogenous furin and PACE4 but not by PCSK5. This effect is specific to LPL but not endothelial lipase. Both N-and C-terminal domains of LPL are required for ANGPTL3-enhanced cleavage, and the N-terminal domain of ANGPTL3 is sufficient to exert its effect on LPL cleavage. Moreover, ANGPTL3 enhances LPL cleavage in the presence of either heparan sulfate proteoglycans or glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1). By enhancing LPL cleavage, ANGPTL3 dissociates LPL from the cell surface, inhibiting both the catalytic and noncatalytic functions of LPL. Taken together, our data provide a molecular connection between ANGPTL3, LPL, and proprotein convertases, which may represent a rapid signal communication among different metabolically active tissues to maintain energy homeostasis. These novel findings provide a new paradigm of specific protease-substrate interaction and further improve our knowledge of LPL biology.ANGPTL3 is a newly discovered modulator of plasma triglyceride levels, an independent risk factor of atherosclerosis. Genome-wide association studies in humans link single nucleotide polymorphisms near ANGPTL3 to plasma triglyceride levels (1-3). Several loss of function mutations of ANGPTL3 contribute to variation in plasma triglyceride levels in the Dallas Heart Study population as well as in subjects in the Atherosclerosis Risk in Communities study (4). However, two small studies suggested that plasma ANGPTL3 protein concentration does not correlate with plasma triglyceride levels (5, 6). Data from ANGPTL3 knock-out mice, the mutant KK/san mice, neutralization using antibody, and overexpression studies demonstrate that ANGPTL3 raises plasma triglyceride levels by inhibiting LPL, 2 a rate-limiting enzyme in plasma triglyceride metabolism (7). ANGPTL3 is mainly made in liver, whereas most active LPL protein resides in adipose and muscle, yet the nature of its interaction with LPL is not fully understood. Based on several studies from cell-free systems (8), ANGPTL3 is proposed to directly bind and inhibit the LPL catalytic activity, and the inhibitory cons...

Proteolytic Processing of Angiopoietin-like Protein 4 by Proprotein Convertases Modulates Its Inhibitory Effects on Lipoprotein Lipase Activity

“…ANGPTL4 and ANGPTL3 are similar in that both have the ability to enhance LPL cleavage by PCs and that this property is not influenced by the extent to which they themselves are cleaved (26) (Fig. 5).…”

“…5). This effect also seems LPL-specific (26). ANGPTL3 cleavage is reported to be affected by the O-glycosylation state of threonine at amino acid position 226 by polypeptide N-acetylgalactosaminyltransferase 2, adjacent to the proprotein convertase-processing site (27).…”

Proprotein convertase furin/PCSK3 and atherosclerosis: New insights and potential therapeutic targets

“…Since LPL mediates the hydrolysis of triglycerides (TG), the proteolytic processing of ANGPTL4 by PCs including furin can markedly increase plasma TG levels. Moreover, Liu et al [18] observed that ANGPTL3, besides ANGPTL4, can also amplify the cleavage of LPL by enhancing the activity of endogenous furin and PACE4, but not by PCSK5, and this effect is LPL-specific. Furthermore, this process also results in the inactivation of LPL and the dissociation of LPL from the cell surface, a process that raises plasma TG levels.…”

GPIHBP1, an endothelial cell transporter for lipoprotein lipase

“…Interestingly, however, they found that GPIHBP1-bound LPL was not susceptible to inhibition by ANGPTL3 and ANGPTL4. In a subsequent study, Liu et al ( 59 ) reported that ANGPTL3 inactivates LPL by increasing its susceptibility to endoproteolytic cleavage by proprotein convertases ( 59 ). In their hands, soluble GPIHBP1 did not protect LPL from this cleavage event.…”