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A more recent version of this article appeared on June 1, 2006
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Submitted on July 21, 2005
Revised on March 22, 2006
Accepted on March 24, 2006
*Department of Anatomy and Molecular Cell Biology, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Japan; Department of Biology and Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan
Monitoring Editor: Jeffrey Brodsky
Lipid esters stored in cytoplasmic lipid droplets (CLDs) of hepatocytes are utilized to synthesize very low-density lipoproteins (VLDL), into which apolipoprotein B (ApoB) is integrated cotranslationally. In the present study, by using Huh7 cells, derived from human hepatoma and competent for VLDL secretion, we found that ApoB is highly concentrated around CLDs to make "ApoB-crescents." ApoB-crescents were seen in <10% of Huh7 cells under normal conditions, but the ratio increased to nearly 50% after 12 h of proteasomal inhibition by ALLN. Electron microscopy showed ApoB to be localized to a cluster of electron-lucent particles 50-100 nm in diameter adhering to CLDs. ApoB, proteasome subunits, and ubiquitinated proteins were detected in the CLD fraction, and this ApoB was ubiquitinated. Interestingly, proteasome inhibition also caused increases in autophagic vacuoles and ApoB in lysosomes. ApoB-crescents began to decrease after 12-24 h of proteasomal inhibition, but the decrease was blocked by an autophagy inhibitor, 3-methyladenine. Inhibition of autophagy alone caused an increase in ApoB-crescents. These observations indicate that both proteasomal and autophagy/lysosomal degradation of ApoB occur around CLDs, and that the CLD surface functions as a unique platform for convergence of the two pathways.
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