The Degradation of Apolipoprotein B100 Is Mediated by the Ubiquitin-proteasome Pathway and Involves Heat Shock Protein 70*
- Edward A. Fisherद,
- Mingyue Zhou‖,
- Deborah M. Mitchell‡,
- Xujun Wu‖,
- Satoshi Omura**,
- Hongxing Wang‡,
- Alfred L. Goldberg‡ and
- Henry N. Ginsberg§‖§§
- From the ‡Laboratory of Lipoprotein Research, Cardiovascular Institute, Mount Sinai School of Medicine, New York, New York 10029, the ‖Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, New York 10032, **The Kitasato Institute, Tokyo 108, Japan, and the‡Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115
Abstract
Apolipoprotein B (apoB) is the major protein component of atherogenic lipoproteins of hepatic origin. In HepG2 cells, the standard cell culture model of human hepatic lipoprotein metabolism, there is a limited availability of core lipids in the endoplasmic reticulum for association with nascent apoB. Under these conditions, apoB is partially translocated, interacts with cytosolic Hsp70, and undergoes rapid degradation. We show that increasing the expression of Hsp70 in HepG2 cells promotes apoB degradation. In addition, apoB is polyubiquitinated and its degradation both normally and after Hsp70 induction is blocked by inhibitors of the proteasome. The apoB that accumulates after proteasome inhibition is endoplasmic reticulum-associated and can be assembled into lipoproteins and secreted if new lipid synthesis is stimulated. Thus, apoB is the first example of a wild-type mammalian protein whose secretion is regulated by degradation in the cytosol via the ubiquitin-proteasome pathway. Furthermore, targeting of this secretory protein to the proteasome is regulated by the molecular chaperone Hsp70 and the availability of apoB’s lipid-ligands.
Footnotes
-
↵* This work was supported by grants from the American Heart Association (NYC-96-GA21), the National Institutes of Health (HL07824, HL36000, HL55638,GM51923), the Human Frontiers Science Program, as well as funds provided by the Mount Sinai Cardiovascular Institute.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.
-
↵§ The laboratories of each contributed equally to this report.
-
↵¶ To whom correspondence may be addressed: Cardiovascular Institute, Mount Sinai Medical Center, 1 Gustave Levy Place, New York, NY 10029. Tel.: 212-241-7784; Fax: 212-860-7032.
-
↵§§ To whom correspondence may be addressed: Dept. of Medicine, Columbia University College of Physicians & Surgeons, 630 W. 168th St., New York, NY 10032. Tel.: 212-305-9562; Fax: 212-305-3213.
-
↵1 The abbreviations used are: apoB, apolipoprotein B-100; apoA-I, apolipoprotein A-I; ALLN, acetyl-leucyl-leucyl-norleucinal; ER, endoplasmic reticulum; HA, herbimycin A; HSP, heat shock protein; MTP, microsomal triglyceride transfer protein; OA, oleic acid; BSA, bovine serum albumin; PAGE, polyacrylamide gel electrophoresis.
-
↵2 H. Wang, E. A. Fisher, X. Wu, J. L. Dixon, and H. N. Ginsberg, unpublished data.
-
↵3 N. Sakata and H. N. Ginsberg, unpublished data.
-
- Received April 4, 1997.
