Apolipoprotein AV Accelerates Plasma Hydrolysis of Triglyceriderich Lipoproteins by Interaction with Proteoglycan-bound Lipoprotein Lipase

Abstract

Apolipoprotein A5 (APOA5) is associated with differences in triglyceride levels and familial combined hyperlipidemia. In genetically engineered mice, apoAV plasma levels are inversely correlated with plasma triglycerides. To elucidate the mechanism by which apoAV influences plasma triglycerides, metabolic studies and in vitro assays resembling physiological conditions were performed. In human APOA5 transgenic mice (hAPOA5tr), catabolism of chylomicrons and very low density lipoprotein (VLDL) was accelerated due to a faster plasma hydrolysis of triglycerides by lipoprotein lipase (LPL). Hepatic VLDL and intestinal chylomicron production were not affected. The functional interplay between apoAV and LPL was further investigated by cross-breeding a human LPL transgene with the apoa5 knock-out and the hAPOA5tr to an lpl-deficient background. Increased LPL activity completely normalized hypertriglyceridemia of apoa5-deficient mice; however, overexpression of human apoAV modulated triglyceride levels only slightly when LPL was reduced. To reflect the physiological situation in which LPL is bound to cell surface proteoglycans, we examined hydrolysis in the presence or absence of proteoglycans. Without proteoglycans, apoAV derived either from triglyceride-rich lipoproteins, hAPOA5tr high density lipoprotein, or a recombinant source did not alter the LPL hydrolysis rate. In the presence of proteoglycans, however, apoAV led to a significant and dose-dependent increase in LPL-mediated hydrolysis of VLDL triglycerides. These results were confirmed in cell culture using a proteoglycan-deficient cell line. A direct interaction between LPL and apoAV was found by ligand blotting. It is proposed, that apoAV reduces triglyceride levels by guiding VLDL and chylomicrons to proteoglycan-bound LPL for lipolysis. Plasma triglyceride (TG)1 levels are an important independent risk factor for cardiovascular disease susceptibility with both genetic and environmental determinants (1). Among the genetic factors, the contribution of variations at the APOA1/ C3/A4/A5 locus to the determination of TG levels has been well defined (2, 3). APOA5, the newest member of the apolipoprotein family, is located at this locus 27 kb distal (3Ј) to APOA4. It was discovered 3 years ago independently by comparative sequencing (4) and by differential display as a liver regeneration protein (5). Single nucleotide polymorphisms in the APOA5 gene have been associated with differences in human plasma TG levels, familial combined hyperlipidemia, and increased risk of cardiovascular disease (4, 6 -8). Mice expressing a human APOA5 transgene had one-third lower plasma TG levels, whereas knock-out mice lacking apoa5 had four times higher plasma TG levels (4). These data indicate that apoAV plays a major role in the regulation of TG metabolism.However, the molecular mechanism underlying the influence of apoAV on plasma TG is currently not understood. Because apoAV is an apolipoprotein with high lipid affinity and low elasticity, it has been proposed that apoAV may impa...

Apolipoprotein A-V-heparin Interactions

“…By contrast, apoC-II had a major stimulatory effect on LPL activity. Importantly, in support of our findings, Merkel et al (16) recently reported that apoA-V did not affect LPL hydrolysis rates in the absence of proteoglycans. Thus, although a clear explanation for the different results is not available at present, it is conceivable that, under certain incubation conditions, apoA-V may alter the substrate emulsion, thereby increasing TG accessibility and LPL activity.…”

“…Subsequent interaction studies with chylomicrons revealed that prior enrichment with apoA-V results in an increased binding. These studies, which provide direct evidence that ternary interactions between LPL, TG-rich lipoprotein, and heparin are enhanced in the presence of apoA-V, are congruent with the recent findings of Merkel et al (16) who showed that proteoglycan-bound LPL activity is modulated by apoA-V. Attempts to demonstrate the release of cell surface heparan sulfate proteoglycan-associated apoA-V upon injection of heparin into human volunteers, however, failed to show an affect.…”

Endocytosis of Apolipoprotein A-V by Members of the Low Density Lipoprotein Receptor and the Vps10p Domain Receptor Families

“…Lookene et al (13) showed that apoA-V binds to heparin and facilitates binding of chylomicrons and VLDL to heparin by bridging the lipoproteins to the glycan chains. In accordance, Merkel et al found that apoA-V stimulates lipolysis when the lipase is bound to heparan sulfate proteoglycans on microtiter plates (9).…”

ApoO, a Novel Apolipoprotein, Is an Original Glycoprotein Up-regulated by Diabetes in Human Heart

“…not incorporated into lipoproteins) could compete with the binding to the arterial wall and prevent arteriosclerosis. In addition, lipoprotein lipase binds to proteoglycans (62). Thus, lipoprotein lipase could be bound to lipoproteins with the involvement of the chondroitin sulfate chain of apoO.…”