Apolipoprotein E (Apo E) is a class of apolipoprotein found in the chylomicron and Intermediate-density lipoprotein that is essential for the normal catabolism of triglyceride-rich lipoprotein constituents. In peripheral tissues, Apo E is primarily produced by the liver and macrophages, and mediates cholesterol metabolism in an isoform-dependent manner. In the central nervous system, Apo E is mainly produced by astrocytes, and transports cholesterol to neurons via Apo E receptors.
Apo E transports lipoproteins, fat-soluble vitamins, and cholesterol into the lymph system and then into the blood. In the field of immune regulation, a growing number of studies point to Apo E’s interaction with many immunological processes, including suppressing T cell proliferation, macrophage functioning regulation, lipid antigen presentation facilitation to natural killer T cell as well as modulation of inflammation and oxidation. The secretion of Apo E by monocytes is down regulated by inflammatory cytokines and upregulated by TGF-β. This protein is involved in Alzheimer’s disease and cardiovascular disease.