Abstract 1019: Injection of Apolipoprotein A-V Reverses the Hypertriglyceridemic Phenotype of apoa5 Knockout Mice
Elevated plasma triglyceride (TG) is a major and independent risk factor for cardiovascular disease. It is generally recognized that apolipoprotein (apo) A-V, a low abundance protein secreted solely by the liver, plays a critical role in TG metabolism. Studies with human APOA5 transgenic (Tg) and gene disrupted mice (apoa5−/−) revealed profound metabolic effects: APOA5 Tg mice exhibited a two-thirds decrease in plasma TG concentration whereas apoa5−/− mice had a four-fold elevation in plasma TG. The present studies were carried out to determine whether exogenous human wild type (WT) apoA-V protein injected into apoa5−/− mice would lower plasma TG. Recombinant apoA-V reconstituted discoidal high density lipoprotein was injected at a concentration of 0.4 μg/gm mouse to reach an initial blood concentration of 12.5 μg/ml. Plasma TG levels decreased 60±3% (Mean±SE, n=25) 4 h post apoA-V injection while PBS injected animals showed no significant change. Over this time period, ~80% of the injected apoA-V was cleared from the circulation. Subsequently, lipoprotein particle size and distribution were determined by fast protein liquid chromatography and ion mobility analysis. VLDL TG and cholesterol decreased more than 50% while no change in LDL or HDL TG or cholesterol was observed. The number of TG-rich particles decreased approximately 50% with no parallel increase in small VLDL or LDL. These findings suggest increased clearance of VLDL following apoA-V injection into apoa5−/− mice. Previous in vitro studies showed that apoA-V can bind to heparan sulfate proteoglycans (HSPG) as well as low density lipoprotein receptor family members through a sequence of positively charged amino acids, and that mutation of these residues (apoA-V heparin mutant) resulted in decreased HSPG binding. When the apoA-V heparin mutant was injected into apoa5−/− mice (n=7), TG lowering over 4 h was significantly attenuated compared to WT apoA-V (43±6% vs. 60±3% by 4 h, p<0.01) suggesting that decreased HSPG binding affected the in vivo TG lowering ability of apoA-V, possibly by reducing VLDL uptake and clearance. Based on its profound TG lowering effect, parenteral administration of apoA-V has the potential to be a therapy for hypertriglyceridemia.
This research has received full or partial funding support from the American Heart Association, Western States Affiliate (California, Nevada & Utah).