Phosphatidylcholine biosynthesis is required for secretion of truncated apolipoprotein Bs from McArdle RH7777 cells only when a neutral lipid core is formed.

Decreased phosphatidylcholine biosynthesis inhibits the secretion of very low density lipoproteins from hepatocytes (Yao, Z. and D. E. Vance. 1988. J. Biol. Chem. 263: 2998-3004). We have now investigated whether or not phosphatidylcholine biosynthesis is required for secretion of carboxyl-truncated apoBs 15, 18, 23, 28, and 37 from transfected McArdle RH7777 cells. Transfected cells were choline-deprived for 1 day and then choline-supplemented or maintained choline-deficient. Pulse-chase experiments showed that in the presence of 100 microM choline, the secretion of apoBs 28 and 37 was increased by up to 50 and 45%, respectively, whereas the secretion of apoBs 15, 18, and 23 was not affected by the addition of choline. Immunoblot analyses also showed that choline in the medium increased the secretion of apoBs 28 and 37 by 30-50% whereas the secretion of apoBs 15, 18, and 23 was unaffected over 24 h. As only carboxyl-terminal truncations with a size greater than 23% of apoB-100 are able to assemble a neutral lipid core (McLeod, R. S., Y. Zhao, S. L. Selby, J. Westerlund, and Z. Yao. 1994. J. Biol. Chem. 269: 2852-2862), we conclude that only apoB-truncations that assemble a neutral lipid core require phosphatidylcholine synthesis. Other experiments established that the requirement of phosphatidylcholine for apoB secretion is related to the presence of neutral lipid associated with a truncation, rather than the length of apoB.