Studies of Intracellular and Extracellular Functions of Apolipoprotein A-V on Triglyceride Metabolism
- Author(s): Shu, Xiao
- Advisor(s): Ryan, Robert
- Aponte, Gregory
- et al.
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.
Given the very low concentration of apoA-V in plasma, we hypothesized that apoA-V may influence plasma TG levels by affecting the assembly and/or secretion of apoB-containing lipoproteins. When apoA-V was overexpressed in cultured Hep3B cells, neither the amount of apoB secreted nor the density distribution of apoB-containing lipoproteins was affected. Confocal fluorescence microscopy revealed that apoA-V has a unique association with cellular lipid droplets and may be involved in storage or mobilization of intracellular lipids. It was also confirmed in vivo that intrahepatic apoA-V was associated with lipid droplets isolated from livers of wild type and APOA5 Tg mice.
To define the structural requirements for apoA-V lipid droplet association, hepatoma cells were transfected with a series of C-terminal truncated apoA-V variants. Confocal microscopy analysis revealed that apoA-V-(1-146) did not associate with lipid droplets. Ultracentrifugation of conditioned medium revealed that, unlike full-length apoA-V which associates with lipoproteins, apoA-V-(1-146) was present solely in the lipoprotein deficient fraction. These data suggest that the C-terminus of apoA-V is essential for lipid droplet association in transfected hepatoma cells and lipoprotein association in conditioned medium.
On the other hand, we determined whether parenteral delivery of apoA-V is sufficient to lower plasma TG levels in apoa5-/- mice. Intravenous injection with apoA-V was found to substantially reduce TG in apoa5-/- mice but alterations to the putative heparin binding region of apoA-V attenuated its TG clearing capacity. In addition, parenteral apoA-V had no effect on plasma TG levels in glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) deficient mice. The study suggests a mechanism whereby effective TG clearance requires the interaction of GPIHBP1 and apoA-V.