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 v ery low density lipoproteins from hepatocytes (Yao, Z. and D. E. Vance . 1988. J. Biol. Chem. 263: 2998-3001). We have now investigated wheth er or not phosphatidylcholine biosynthesis is required for secretion o f carboxyl-truncated apoBs 15, 18, 23, 28, and 37 from transfected McA rdle RH7777 cells. Transfected cells were choline-deprived for 1 day a nd then choline-supplemented or maintained choline-deficient. Pulse-ch ase experiments showed that in the presence of 100 mu M choline, the s ecretion of apoBs 28 and 37 was increased by up to 50 and 45%, respect ively, whereas the secretion of apoBs 15, 18, and 23 was not affected by the addition of choline. Immunoblot analyses also showed that choli ne in the medium increased the secretion of apoBs 25 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% o f 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. 2 69: 2852-2862), we conclude that only apoB-truncations that assemble a neutral lipid core require phosphatidylcholine syn thesis. Other expe riments established that the requirement of phosphatidylcholine for ap oB secretion is related to the presence of neutral lipid associated wi th a truncation, rather than the length of apoB.