MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN IN CACO-2 CELLS - CHARACTERIZATION AND REGULATION

As microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apoB-containing lipoproteins by intestinal e pithelial cells, the characterization and regulation of MTP in human i ntestinal cells, CaCo-2, was studied. CaCo-2 cells express MTP mRNA of 4.2 kb and a 97 kDa subunit of MTP protein. Similar to the expression of apoB mRNA, MTP mRNA expression was dependent upon cell differentia tion and was directly related to the ability of the cells to assemble and secrete apoB-containing lipoproteins. MTP mRNA expression was high est in fully differentiated cells, with Small but detectable amounts f ound in undifferentiated cells. Under conditions of increased apoB sec retion by oleic acid, phosphatidylcholine, or lysophosphatidylcholine, MTP mass, MTP activity, and MTP gene expressions were not altered. In cells treated with calcium ionophore or phorbol 12-myristate 13-aceta te, no relationship could be established between apoB secretion and MT P mRNA or activity. Similarly, in cells treated with sphingomyelinase, trifluoperazine, verapamil, okadaic acid, vanadate, or monensin, agen ts that decrease apoB secretion, no corresponding decrease in MTP acti vity or mass was observed. The results suggest that the various mediat ors of apoB secretion alter steps in lipoprotein assembly and secretio n that are not dependent on MTP activity. CaCo-2 cells have an abundan t supply of MTP for the assembly of lipoproteins when apoB secretion i s stimulated by dietary lipids.