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J Biol Chem, Vol. 275, Issue 6, 3950-3956, February 11, 2000
From the Departments of Molecular and Cellular Biology and
Medicine, Baylor College of Medicine, Houston, Texas 77030-3498
Studies in different liver-derived cells in
culture indicate that apolipoprotein (apo) B-100 production is
regulated largely by intracellular degradation and the
ubiquitin-proteasome pathway is a major mechanism for the degradation.
The proteasomal degradation of apoB-100 was postulated to be an
intrinsic 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 biogenesis in CaCo2, a human colon carcinoma cell line. To
our surprise, apoB-100 and apoB-48 were quantitatively secreted by CaCo2 cells; essentially none of the newly synthesized apoB was degraded before secretion in a 2-h period whether the cells were cultured on filter or on plastic. Furthermore, although ubiquitin immunoreactivity was readily detected in the intracellular 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
CaCo2 cells, indicating that CaCo2 cells have the necessary machinery
for tagging ubiquitin chains onto cellular proteins for proteasomal
degradation. Incubation in lipoprotein-deficient serum did not induce
apoB degradation, but the addition of a microsomal triglyceride
transfer protein inhibitor led to apoB degradation in CaCo2 cells.
Finally, similar proportions of apoB polypeptide in isolated microsomes
from CaCo2 and HepG2 cells were accessible to exogenously added
trypsin, indicating that the mere exposure of apoB nascent chains to
the cytosolic compartment is insufficient to cause the proteasomal
degradation. Therefore, the intracellular degradation of apoB is not an
intrinsic property of the protein, and the phenomenon is neither
universal nor inevitable. The unconditional use of apoB as a paradigm
for intracellular protein degradation is not warranted.
Apolipoprotein B, a Paradigm for Proteins Regulated by
Intracellular Degradation, Does Not Undergo Intracellular Degradation
in CaCo2 Cells*
and
*
This work was supported by National Institutes of Health
Grants HL-56668 and HL-16512 (to L. C.) and by American Heart
Association, Texas affiliate Grant 9960083Y (to W. L.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
An American Liver Foundation Clarence A. Kruse Memorial Liver
Scholar, supported by the Karolinska Institute/Baylor College of
Medicine Exchange Program and by the Henning and Johan Throne-Holsts Foundation.
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