ApoJ regulates endothelial lipase activity and stability

Endothelial lipase (EL) is a key regulator of high-density lipoprotein (HDL) metabolism. Many aspects of EL function remain incompletely understood due to challenges in purifying active EL. This study identifies apolipoprotein J (ApoJ) as a novel chaperone for EL, crucial for its solubility and activity. Using an optimized purification protocol that yields active EL, we discovered that ApoJ consistently co-purifies with EL, maintaining its activity. We further show that knocking down ApoJ decreases the activity of EL. We demonstrate that ApoJ interacts with EL via its hydrophobic lid and tryptophan loop regions, and that mutating these regions abolishes the effect of ApoJ on the solubility and activity of EL. We show that ApoJ, EL, and ApoA1 (the defining lipoprotein of HDL particles) colocalize in HDL particles in mouse plasma. However, we find that ApoJ is not a direct carrier for EL to HDL particles. Instead, our data suggest that ApoJ primarily serves to enhance EL activity through its role as a chaperone, even when incorporated into lipid substrates. Our findings suggest a model in which ApoJ protects EL in plasma and enhances its hydrolysis of lipoprotein substrates. We propose that ApoJ is an accessory protein for EL, analogous to GPIHBP1 for LPL and co-lipase for PL. Further study of the interaction between EL and ApoJ will promote a better understanding of HDL metabolism.