Apolipoprotein E: a cholesterol transport protein with lipid transport-independent cell signaling properties

Apolipoprotein (apo) E is a 34-kDa glycoprotein associated with plasma lipoproteins. In contrast to other apolipoproteins, apoE is expressed ubiquitously in all tissues and appears to have a wide variety of functions in addition to lipid transport. Although the lipid transport properties of apoE are well characterized, the appreciation apoE functions other than lipid transport is still in an infancy stage. Nevertheless, there is increasing evidence that apoE regulates cell functions in a variety of tissues and organs. This review article summarizes briefly the current status of our understanding on the lipid transport-independent properties of this apolipoprotein.