MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN, THE ABETALIPOPROTEINEMIA GENE-PRODUCT, MEDIATES THE SECRETION OF APOLIPOPROTEIN B-CONTAINING LIPOPROTEINS FROM HETEROLOGOUS CELLS

Apolipoprotein (apo) B is an obligatory component of triglyceride-rich lipoproteins. In the rare autosomal recessive disorder abetalipoprote inemia (ABL), no triglyceride-rich lipoproteins are secreted. Mutation s in the gene encoding the 97-kDa subunit of a microsomal triglyceride transfer protein (MTP) cause ABL (Sharp, D., Blinderman, L., Combs, K . A., Klenzle, B., Ricci, B., Wager-Smith, K., Gil, C. M., Turck, C. W , Bouma, M. E., Rader, D. J., Aggerbeck, L. P., Gregg, R. E., Gordon, D. A., and Wetterau, J. R. (1993) Nature 365, 65-69; Shoulders, C. C., Brett, D. J., Bayliss, J. D., Narcisi, T. M., Jarmuz, k, Grantham, T. T., Leoni, P. R. D., Bhattacharya, S., Pease, R. J., Cullen, P. M., L evi, S., Byfield, P. G. H., Purkiss, P., and Scott, J. (1993) Hum. Mol . Genet. 2, 2109-2116). Here we have examined whether MTP is both nece ssary and sufficient to mediate the secretion of apoB-containing lipop roteins from cells that do not normally express either of these protei ns. Carboxyl-terminal truncated forms of apoB, apoB17, and apoB41 on t he centile system were expressed in COS-1 cells. ApoB17 was secreted w hereas apoB41 was unable to traverse the secretory pathway. Cotransfec tion of apoB41 and MTP promoted the secretion of apoB41 as a buoyant l ipoprotein particle with a modal density of 1.15 g/ml. When cotransfec ted COS-1 cells were cultured under conditions that increase the secre tion of apoB100 from HepG2 cells, secretion of apoB41 was similarly in creased. N-Acetyl-leucyl-leucyl-norleucinal (ALLN), a calpain I inhibi tor, abolished intracellular degradation of apoB41 and increased secre tion 2.5-fold. Oleate, a substrate for triglyceride synthesis, reduced degradation from 50 to 19% and increased secretion by 2.5 fold. The e ffects of ALLN and oleate were additive. We conclude that the secretio n of apoB from COS-1 cells cotransfected with apoB and MTP is determin ed by the competitive processes of lipoprotein assembly and intracellu lar degradation in the endoplasmic reticulum and that MTP is the only tissue-specific component, other than apoB, required for the secretion of apoB-containing lipoproteins.