Apolipoprotein E

Apolipoprotein E (APOE) is a 317-amino-acid apolipoprotein that functions primarily in lipoprotein-mediated lipid transport between organs via plasma and interstitial fluids (UniProt: P02649). It is a core component of multiple lipoprotein classes—including chylomicrons, VLDL, IDL, and HDL—and mediates their cellular uptake through binding to receptors such as LDLR, LRP1, LRP2, LRP8, and VLDLR. APOE also plays critical roles in reverse cholesterol transport, hepatic VLDL biosynthesis, and lipid homeostasis, and is involved in immune responses and lipid transport in the central nervous system.

APOE is widely expressed and functions in plasma lipoprotein metabolism and, notably, in neuronal survival and neurite sprouting within the CNS (UniProt: P02649). It is associated with multiple disorders including hyperlipoproteinemia 3, sea-blue histiocyte disease, and lipoprotein glomerulopathy. The protein also interacts with the amyloid precursor protein (APP) through a receptor-dependent mechanism that activates MAP3K12 signaling and enhances AP-1-mediated APP transcription.

APOE is significantly upregulated in Alzheimer disease. In post-mortem AD brain tissue compared to age-matched controls, APOE shows a mean log2 fold-change of +1.05 (Chaparral AD proteomics), indicating elevated abundance in the disease state. This upregulation aligns with APOE's established role in AD pathogenesis, as the ε4 allele is a major genetic risk factor for late-onset Alzheimer disease (AD2, MIM 104310), and APOE is implicated in amyloid-beta metabolism and neurodegeneration (UniProt: P02649).