Due to the absence of
microsomal triglyceride transfer protein (MTP), Chinese 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 responsible for the human recessive disorder
abetalipoproteinemia, would recapitulate the
lipoprotein assembly pathway in CHO cells. Unexpectedly, expression of MTP
mRNA and
protein in CHO cells did not allow
apoB-containing
lipoproteins to be assembled and secreted by CHO cells expressing apoB53. Although expression of MTP in cells allowed
apoB to completely enter the endoplasmic reticulum, it was degraded by a proteolytic process that was inhibited by
dithiothreitol (1 mM) and
chloroquine (100 microM), but resistant to
N-acetyl-leucyl-leucyl-norleucinal. In marked contrast, coexpression of the liver-specific gene product
cholesterol 7alpha-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. These data suggest that, although MTP-facilitated
lipid transport is not required for
apoB translocation, it is required for the secretion of
apoB-containing
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
cholesterol 7alpha-hydroxylase by the liver) may profoundly influence the metabolic relationships determining how
apoB is processed into
lipoproteins and/or degraded.