Hepatic apolipoprotein J is secreted as a lipoprotein

Apolipoprotein J (apoJ) is a unique glycoprotein thought to be involved in a variety of physiological processes, including lipid transport, regulation of complement function, sperm maturation, programmed cell death, and membrane recycling. In the plasma, apoJ is associated with apoA-I in high and very high density lipoproteins. In this report we demonstrate that HepG2 human hepatocellular carcinoma cells secrete apoJ in association with a significant amount of lipid, providing unequivocal evidence that apoJ can transport lipids. The HepG2 cell line has provided important clues about the structural organization of nascent lipoprotein particles. HepG2 cell apoJ-containing lipoproteins are dense and heterogenous in size, ranging from 100 to 910 kDa. Plasma and HepG2 cell apoJ-lipoproteins differ in size distribution. Both have alpha 2 electrophoretic mobility, although their average mobilities differ within the alpha 2 region. In contrast to plasma apoJ-HDL which contain little triglyceride and which can associate with apoA-I, HepG2 cell apoJ-lipoproteins are rich in triglyceride and lack apoA-I. By implication, nascent apoJ-lipoproteins undergo plasma remodeling that results in triglyceride depletion and apoA-I association. We propose that the metabolic consequences of this remodeling play an important role in lipid homeostasis in localized tissue environments, particularly where organs are isolated from the blood by cellular barriers such as in testis and brain. In such tissues, apoJ is expressed constitutively in high level compared to other lipid transport proteins.