Lipid acquisition by apolipoprotein A-I in ER and Golgi compartments of primary mouse hepatocytes

Abstract

It was previously unknown what the significance and location of intracellular lipidation of newly synthesized apoA-I in hepatocytes were in plasma HDL formation. By labeling primary mouse hepatocytes with 3H-choline, we showed that phospholipidation of apoA-I is most significant in endoplasmic reticulum (ER) and medial Golgi compartments with minor lipidation upon export from the cell. Intracellular LDL-cholesterol lipidation of apoA-I is absent, with rapid cholesterol accumulation at the plasma membrane. De novo synthesized cholesterol was able to lipidate apoA-I intracellularly to a small but significant level. In hepatocytes lacking ABCA1, phospholipidation and lipidation by de novo cholesterol were both reduced in Golgi, while ER lipidation remained mostly unchanged. Plasma membrane lipidation by LDL-cholesterol was also significantly reduced. This implies that HDL formation begins with apoA-I phospholipidation in the ER, followed by modest cholesterol lipidation in the Golgi, dependent on ABCA1, with the bulk of cholesterol lipidation occurring at the plasma membrane.