Apolipoprotein B, a paradigm for proteins regulated by intracellular degradation, does not undergo intracellular degradation in CaCo2 cells

Studies in different liver-derived cells in culture indicate that apolipopr otein (apo) B-100 production is regulated largely by intracellular degradat ion and the ubiquitin-proteasome pathway is a major mechanism for the degra dation. The proteasomal degradation of apoB-100 was postulated to be an int rinsic property of the protein that occurs even in the presence of optimal amounts of lipids supplied to the cell. We examined apoB-100 and apoB-48 bi ogenesis in CaCo2, a human colon carcinoma cell line. To our surprise, apoB -100 and apoB-48 were quantitatively secreted by CaCo2 cells; essentially n one of the newly synthesized apoB was degraded before secretion in a 2-h pe riod whether the cells were cultured on filter or on plastic. Furthermore, although ubiquitin immunoreactivity was readily detected in the intracellul ar apoB isolated from HepG2 cells, little or no ubiquitin was detectable in the intracellular apoB from CaCo2 cells. The amounts of free ubiquitin and total and non-apoB ubiquitinated proteins were comparable in HepG2 and CaC o2 cells, indicating that CaCo2 cells have the necessary machinery for tagg ing ubiquitin chains onto cellular proteins for proteasomal degradation. In cubation in lipoprotein-deficient serum did not induce apoB degradation, bu t the addition of a microsomal triglyceride transfer protein inhibitor led to apoB degradation in CaCo2 cells. Finally, similar proportions of apoB po lypeptide in isolated microsomes from CaCo2 and HepG2 cells were accessible to exogenously added trypsin, indicating that the mere exposure of apoB na scent chains to the cytosolic compartment is insufficient to cause the prot easomal degradation. Therefore, the intracellular degradation of apoB is no t an intrinsic property of the protein, and the phenomenon is neither unive rsal nor inevitable. The unconditional use of apoB as a paradigm for intrac ellular protein degradation is not warranted.