👤 William E Alborn

The role of apolipoprotein A5 (ApoA5) in modulating triglyceride levels in humans is incompletely understood. Some researchers have reported modest positive correlations of ApoA5 with triglycerides while others have reported negative correlations. A recent report suggested that ApoA5 gene expression may be influenced by insulin. In type 2 diabetes, some groups have reported higher levels of ApoA5 compared to normals while others have reported lower levels. To better understand the relationships between ApoA5, apolipoprotein C3 (ApoC3), and triglycerides in type 2 diabetes, ApoA5 levels were measured and correlated with triglyceride, insulin, and HbA1c levels. ApoC3 levels were measured and correlated with triglycerides. In patients with type 2 diabetes, ApoA5 levels were elevated compared to normals, with several patients having markedly increased levels confirmed by Western blotting. ApoA5 levels were positively correlated with triglycerides (r=0.60) but were not correlated with either HbA1c or serum insulin levels. ApoC3 levels were highly positively correlated with triglycerides (r=0.88). These data indicate that in patients with type 2 diabetes ApoA5 levels are positively correlated with triglycerides but are not correlated with HbA1c or insulin levels. ApoC3 levels are strongly positively correlated with triglycerides in these patients. Show less

Apolipoprotein A5 (ApoA5) originally gained attention as a regulator of serum triglyceride concentrations through transgenic mouse studies. Our group recently developed the first assay to quantify serum ApoA5 protein concentrations and demonstrated that they are increased by administration of a potent peroxisome proliferator-activated receptor-alpha agonist. To better characterize the circulating ApoA5, the protein was purified from human serum, and a definitive N-terminal protein sequence was obtained. In light of previous observations that ApoA5 was present in VLDL and not LDL, plasma infranatant and intermediate-density lipoprotein (IDL) were analyzed for ApoA5. Because the mature protein contains a single unpaired cysteine, ApoA5 in human serum was immunoprecipitated, and its migration pattern was examined via Western blotting under reducing and nonreducing conditions to determine whether the protein circulates as a disulfide-linked homodimer or heterodimer. Definitive N-terminal protein sequences obtained from ApoA5 purified from human serum indicated that cleavage of the signal peptide occurs in vivo at the predicted site. We found ApoA5 in VLDL, HDL, and chylomicrons but not in LDL, IDL, or plasma infranatant. Under both reducing and nonreducing conditions, ApoA5 migrated mainly as a single band with a relative molecular mass (Mr) of approximately 39,000, indicating that the protein exists in serum as a monomer and not as a disulfide-linked homodimer or heterodimer. Our data help characterize ApoA5 by defining its lipoprotein particle distribution, by determining its N-terminal protein sequence, and by demonstrating that the mature protein circulates mainly as a monomer and not as a disulfide-linked homodimer or heterodimer. Show less

The recently discovered apolipoprotein A5 (ApoA5) is fast gaining attention as a key regulator of serum triglyceride concentrations. An ApoA5 mouse knock-out model produced an approximately fourfold increase in serum triglycerides, whereas a knock-in model with human ApoA5 produced 50-70% lower concentrations of mouse serum triglycerides. In addition, peroxisome proliferator-activated receptor-alpha agonists, which are used clinically to lower serum triglyceride concentrations, cause increased ApoA5 mRNA expression. Despite these compelling molecular biology data, relatively little is known about ApoA5 protein in human serum. To better understand circulating concentrations and lipoprotein particle distribution of ApoA5, we expressed the recombinant human ApoA5 protein and raised antibodies against both the NH(2) and COOH termini. Using the above reagents, we demonstrate for the first time that ApoA5 is present in human serum, although at much lower concentrations than other apolipoproteins such as ApoA1. Using a dual-antibody sandwich ELISA that we developed, we observed ApoA5 concentrations in human serum ranging from 24 to 406 microg/L compared with approximately 1 g/L for ApoA1. We also examined the lipoprotein particle distribution of ApoA5 and found that ApoA5 was detectable in VLDL, HDL, and chylomicrons, but not LDL. These data demonstrate for the first time that ApoA5 is a secreted protein present in human serum and is associated with specific lipoprotein particles. In addition, our data indicate that the circulating concentration of human ApoA5 is very low compared with other apolipoproteins. Show less