Tunicamycin induces ubiquitination and degradation of apolipoprotein B in HepG2 cells

Apolipoprotein (apo) B-100 is an essential component of atherogenic plasma lipoproteins. Previous studies have demonstrated that the production of apo B-100 is regulated largely by intracellular degradation at both the co-tran slational and posttranslational levels and that proteasome-mediated and non -proteasome-mediated pathways are involved in this process. ApoB-100 is a g lycoprotein. The present study was undertaken to address the question of wh ether the inhibition of N-linked glycosylation with tunicamycin would inter fere with apoB-100 production. We demonstrated that the treatment of HepG2 cells with tunicamycin decreased the net production of apoB-100 by enhancin g co-translational degradation of the protein. This effect of tunicamycin w as partly prevented by lactacystin, a specific proteasome inhibitor, Becaus e lactacystin only partly reversed the effects of tunicamycin on apoB bioge nesis, tunicamycin seemed also to induce apoB co-translational degradation in HepG2 cells by one or more non-proteasomal pathways, Furthermore, tunica mycin increased apoB ubiquitination approx. 4-fold, The proportion of the n ewly synthesized apoB-100 that was secreted and incorporated into the nasce nt lipoprotein particles was unaffected by tunicamycin. Thus the tunicamyci n-mediated inhibition of N-linked glycosylation interferes with the product ion of apoB-100 that is mediated by both proteasomal and non-proteasomal pa thways.