Translocational status of ApoB in the presence of an inhibitor of microsomal triglyceride transfer protein.
Despite numerous studies demonstrating that microsomal triglyceride transfer protein (MTP) activity is critical to apoB secretion, there is still controversy as to whether MTP directly facilitates the translocation of apoB across the membrane of the endoplasmic reticulum (ER) through either the recruitment of lipids and/or chaperone activity. In the present study, a specific inhibitor of MTP (BMS 197636) was utilized in HepG2 cells to investigate whether a direct relationship exists between the translocation of apoB across the ER membrane and the lipid-transferring activity of MTP. Inhibition of MTP (with 10 and 50 nmol/L of the inhibitor) did not significantly affect the translocation of newly synthesized apoB (P = 0.77) or the translocational efficiency of the steady-state apoB mass (P = 0.45), despite a 49% decrease in apoB secretion and increased proteosomal degradation. These results compared well with subcellular fractionation experiments which showed no significant change in the fraction of apoB accumulated in the lumen of isolated microsomes in MTP-treated cells (P = 0.35). In summary, MTP lipid transfer activity does not appear to influence translocational status of apoB, but its inhibition is associated with an increased susceptibility to proteasome-mediated degradation and reduced assembly and secretion of apoB lipoprotein particles.
Macri
J
J
Department of Laboratory Medicine and Pathobiology, Hospital for Sick Children, Toronto, Ontario, M5G 1X8, Canada.
Kazemian
P
P
Kulinski
A
A
Rudy
D
D
Aiton
A
A
Thibert
R J
RJ
Adeli
K
K
D001055:Apolipoproteins B / Q000096:biosynthesis / Q000378:metabolism*; D002352:Carrier Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D021381:Protein Transport / Q000187:drug effects; D012709:Serum Albumin / Q000378:metabolism; D013379:Substrate Specificity; D013816:Thermodynamics; D013997:Time Factors; D014280:Triglycerides / Q000378:metabolism; D014357:Trypsin / Q000378:metabolism; D014407:Tumor Cells, Cultured
D001055:Apolipoproteins B; D002352:Carrier Proteins; D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D009097:Multienzyme Complexes; D012709:Serum Albumin; D014280:Triglycerides; C106092:microsomal triglyceride transfer protein; D014357:Trypsin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde