ApoJ regulates endothelial lipase activity and stability

Area of Science:

• Lipid metabolism
• Protein biochemistry
• Molecular biology

Background:

• Endothelial lipase (EL) is crucial for high-density lipoprotein (HDL) metabolism.
• Understanding EL function is limited by difficulties in purifying active EL.

Purpose of the Study:

• Identify novel factors influencing EL solubility and activity.
• Elucidate the role of apolipoprotein J (ApoJ) in EL function.

Main Methods:

• Developed an optimized protocol for active EL purification.
• Investigated ApoJ-EL interaction using protein co-purification and mutagenesis.
• Assessed EL activity in vitro and in vivo (mouse plasma).
• Utilized knockdown experiments to evaluate ApoJ's effect on EL activity.

Main Results:

• Apolipoprotein J (ApoJ) was identified as a novel chaperone for endothelial lipase (EL).
• ApoJ consistently co-purifies with active EL, maintaining its solubility and enzymatic activity.
• Mutagenesis of specific ApoJ regions (hydrophobic lid, tryptophan loop) abolished its effect on EL.
• ApoJ enhances EL activity by acting as a chaperone, not a direct carrier to HDL particles.

Conclusions:

• ApoJ is essential for EL solubility and activity, functioning as a chaperone.
• ApoJ protects EL in plasma and enhances its hydrolysis of lipoprotein substrates.
• This interaction provides new insights into HDL metabolism and suggests ApoJ as an accessory protein for EL.