Effects of tocotrienol on the intracellular translocation and degradation of apolipoprotein B: possible involvement of a proteasome independent pathway.
gamma-Tocotrienol (gamma-T3), a HMG CoA reductase inhibitor, was previously shown to stimulate the intracellular degradation of apolipoprotein B (apoB) in HepG2 cells. The aim of this study was to explore the effects of gamma-T3 on the proteasome dependent co-translational degradation and the proteasome independent post-translational degradation of apoB. Previous studies have shown that apoB translocation across the endoplasmic reticulum (ER) membrane governs the co-translational degradative pathway of apoB. Therefore, we first examined the effects of gamma-T3 on this pathway using a specific translocation assay derived from HepG2 cells. Our results indicated that gamma-T3 reduced the efficiency of apoB translocation across the ER membrane, suggesting that co-translational degradation may be partially involved. Evidence of an ER associated post-translational degradation was also provided upon pre-treating digitonin-permeabilized HepG2 cells with a proteasome inhibitor, lactacystin. When chased for 2h, ER degradation of apoB was observed and was further enhanced in the presence of gamma-T3 versus untreated control, in spite of proteasome inhibition. Combined with the ability of ALLN, a proteasome and cysteine protease inhibitor, to block the post-translational degradation of apoB, the data suggest that gamma-T3 diverted more apoB to a cytosolic proteasomal dependent and possibly an ER-associated proteasomal independent degradation pathways.
Wang
Q
Q
Division of Medical Technology, University of Hawaii at Manoa, Honolulu 96822, USA.
Theriault
A
A
Gapor
A
A
Adeli
K
K
D000111:Acetylcysteine / Q000031:analogs & derivatives / Q000494:pharmacology; D001055:Apolipoproteins B / Q000378:metabolism*; D001692:Biological Transport; D002463:Cell Membrane Permeability; D002839:Chromans / Q000494:pharmacology*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006801:Humans; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors / Q000494:pharmacology*; D007976:Leupeptins / Q000494:pharmacology; D008099:Liver / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D011499:Protein Processing, Post-Translational; D014407:Tumor Cells, Cultured; D014810:Vitamin E / Q000031:analogs & derivatives* / Q000494:pharmacology
D001055:Apolipoproteins B; D002839:Chromans; D015853:Cysteine Proteinase Inhibitors; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D007976:Leupeptins; D009097:Multienzyme Complexes; C063261:acetylleucyl-leucyl-norleucinal; C067713:lactacystin; D014810:Vitamin E; C013649:plastochromanol 8; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D000111:Acetylcysteine