ApoO (Apolipoprotein O) is a chrondroitin-sulfate chain containing member of the apolipoprotein family. It is an original glycoprotein up-regulated by diabetes in human heart. Promotes cholesterol efflux from macrophage cells. Detected in HDL, LDL and VLDL. Secreted by a microsomal triglyceride transfer protein (MTTP)-dependent mechanism, probably as a VLDL-associated protein that is subsequently transferred to HDL. May be involved in myocardium-protective mechanisms against lipid accumulation.