Abstract
C127, a murine mammary tumor-derived cell line, is capable of lipidating and secreting apolipoprotein B-41 (apoB-41) in the apparent absence of microsomal triglyceride transfer protein (MTP). Using a semiquantitative reverse transcriptase-coupled polymerase chain reaction, mouse MTP mRNA was detected in C127 cells at ~10–20% of the relative abundance of human MTP in HepG2 cells. Radiolabeling of C127 cells with [35S]methionine and [35S]-cysteine followed by immunoprecipitation with anti-MTP antibodies identified a band with an electrophoretic mobility identical to that of authentic mouse MTP. Cotransfection of apoB-41 and the MTP 97-kDa subunit in C127 cells enhanced apoB secretion by ~5-fold relative to apoB-41 transfection alone, suggesting that MTP is limiting in these cells. To establish that MTP expression is responsible for apoB-containing lipoprotein assembly in C127 cells, the effects of the MTP inhibitor BMS-200150 were examined. Secretion of apoB-41 by C127 cells was inhibited to the same extent observed in COS-1 cells cotransfected with apoB-41 and MTP. These results suggest that low MTP expression, and not the expression or overexpression of another known or novel factor(s), is responsible for apoB assembly and secretion in C127 cells and further supports the essential nature of MTP in the biogenesis of apoB-containing lipoproteins.—Sellers, J. A., and G. S. Shelness. Lipoprotein assembly capacity of the mammary tumor-derived cell line C127 is due to the expression of functional microsomal triglyceride transfer protein.
Highlights
C127, a murine mammary tumor-derived cell line, is capable of lipidating and secreting apolipoprotein B-41 in the apparent absence of microsomal triglyceride transfer protein (MTP)
C127 cells transfected with apolipoprotein B (apoB)-28F displayed a level of secretion similar to that observed in MTP-transfected COS-1 cells (Fig. 1A, compare lanes 4 and 6)
The underlying basis for this phenomenon was solved when the gene for the 97-kDa subunit of MTP was cloned and shown to be defective in patients with the apoB assembly and secretion disorder abetalipoproteinemia [10, 34]. When nonhepatic cells such as COS-1 or HeLa were transfected with the 97-kDa subunit of MTP, their capacity to assemble and secrete transfected forms of apoB as VLDL precursor particles was reconstituted [35]
Summary
C127, a murine mammary tumor-derived cell line, is capable of lipidating and secreting apolipoprotein B-41 (apoB-41) in the apparent absence of microsomal triglyceride transfer protein (MTP). These cells were reported to have no detectable MTP protein or neutral lipid transfer activity and yet were capable of assembling apoB-41 into lipoprotein particles containing a triglyceride-rich core [14, 15].
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