Chinese hamster ovary cells require the coexpression of microsomal triglyceride transfer protein and cholesterol 7 alpha-hydroxylase for the assemblyand secretion of apolipoprotein B containing lipoproteins

Due to the absence of microsomal triglyceride transfer protein (MTP), Chine se hamster ovary (CHO) cells lack the ability to translocate apoB into the lumen of the endoplasmic reticulum, causing apoB to be rapidly degraded by an N-acetyl-leucyl-leucyl-norleucinal-inhibitable process. The goal of this study was to examine if expression of MTP, whose genetic deletion is respo nsible for the human recessive disorder abetalipoproteinemia, would recapit ulate the lipoprotein assembly pathway in CHO cells. Unexpectedly, expressi on of MTP mRNA and protein in CHO cells did not allow apoB-containing lipop roteins to be assembled and secreted by CHO cells expressing apoB53, Althou gh expression of MTP in cells allowed apoB to completely enter the endoplas mic reticulum, it was degraded by a proteolytic process that was inhibited by dithiothreitol (1 mM) and chloroquine (100 mu M), but resistant to N-ace tyl-leucylleucyl-norleucinal, In marked contrast, coexpression of the liver -specific gene product cholesterol 7 alpha-hydroxylase with MTP resulted in levels of MTP lipid transfer activity that were similar to those in mouse liver and allowed intact apoB53 to be secreted as a lipoprotein particle. T hese data suggest that, although MTP-facilitated lipid transport is not req uired for apoB translocation, it is required for the secretion of apoB-cont aining lipoproteins. We propose that, in CHO cells, MTP plays two roles in the assembly and secretion of apoB-containing-lipoproteins: 1) it acts as a chaperone that facilitates apoB53 translocation, and 2) its lipid transfer activity allows apoB-containing lipoproteins to be assembled and secreted. Our results suggest that the phenotype of the cell (e.g. expression of cho lesterol 7 alpha-hydroxylase by the liver) may profoundly influence the met abolic relationships determining how apoB is processed into lipoproteins an d/or degraded.