INFLUENCE OF TRIACYLGLYCEROL BIOSYNTHESIS RATE ON THE ASSEMBLY OF APO-B-100-CONTAINING LIPOPROTEINS IN HEP G2 CELLS

Apolipoprotein B-100 (apoB-100) appears in three forms in the endoplas mic reticulum of Hep G2 cells: (1) tightly bound to the membrane, ie, not extractable by sodium carbonate. This form is glycosylated but pro tease sensitive when present in intact microsomes, suggesting that it is only partially translocated to the microsomal lumen; (2) extractabl e by sodium carbonate and present on low-density lipoprotein-very-low- density lipoprotein (LDL-VLDL)-like particles. This form is glycosylat ed and secreted into the medium; and (3) extractable by sodium carbona te but having a higher density than the LDL-VLDL-like particles. This form, referred to as Fraction I, is glycosylated and protected against proteases when present in intact microsomal vesicles, indicating that it is completely translocated to the luminal side of the microsomal m embrane. Fraction I is not secreted into the medium, but it disappears with time from the cell, suggesting that it is degraded. Oleic acid i nduced a 2.7-fold increase in the rate of the biosynthesis of triacylg lycerol but not of phosphatidylcholine in Hep G2 cells. Incubation of the cells with oleic acid had no significant effect on the rate of ini tiation of the apoB-100-containing lipoproteins, nor did it influence the amount of apoB-100 that was associated with the membrane or the tu rnover of apoB-100 in the membrane. Instead, it increased the proporti on of the nascent apoB polypeptides on initiated lipoproteins that was converted into full-length apoB-100 on LDL-VLDL-like particles, givin g rise to an increased amount of these particles in the lumen of the s ecretory pathway. Pulse-chase experiments showed that incubation with oleic acid gave rise to an increased formation of LDL-VLDL-like partic les ion behalf of the formation of Fraction I. This effect of oleic ac id could partially explain the protective effect of the fatty acid on apoB-100, preventing it from undergoing posttranslational degradation.