Abstract
Apolipoprotein (APO) A5 is a recently discovered member of the APOA1/C3/A4 gene cluster. Studies with APOA5 transgenic and gene disrupted mice showed that apoA-V is a key regulator of plasma triglyceride (TG) levels. However, the molecular basis whereby apoA-V functions to modulate plasma TG is still unclear. Considering the very low plasma concentration of apoA-V (~1ug/ml), its expression only in liver and its highly hydrophobic nature, we hypothesized that apoA-V may regulate plasma TG level by affecting the secretion of apoB containing lipoproteins. Hep3B cells were transfected with a vector containing APOA5 cDNA with a C-terminal FLAG tag. Expression of apoA-V in transfected cells increased >10 fold compared to non transfected cells. ApoA-I levels were not affected. Compared to apoA-I, which was efficiently secreted into the media, >80% apoA-V was recovered inside the cell. Cells transfected with ApoA-V showed decreased apoB secretion compared to controls in a dose-dependent manner. When cells were supplemented with oleic acid, apoB secretion increased but transfected cells still showed less apoB secretion compared to non transfected cells. To determine the subcellular localization of apoA-V, FLAG tagged apoA-V in transfected Hep3B cells was visualized by immunocytochemistry using an anti-FLAG monoclonal antibody. Fluorescence microscope images showed that apoA-V was distributed mainly in ER/Golgi, the subcellular compartment in which apoB lipoprotein particles are assembled and lipidated. Together, these data suggest that apoA-V modulates apoB lipoprotein particle assembly and secretion from cultured hepatoma cells.
Published Version
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