Phosphorylation of Ribosomal Protein S6 Is Inhibitory for Autophagy in Isolated Rat Hepatocytes (*)
- Edward F. C. Blommaart(1),
- Joost J. F. P. Luiken(1),
- Pietjan J. E. Blommaart(2),
- George M. van Woerkom(1) and
- Alfred J. Meijer(1)(§)
- From the (1)E.C. Slater Institute, Department of Biochemistry, University of Amsterdam and the
- (2)Department of Anatomy and Embryology, University of Amsterdam, The Netherlands
- ↵§ To whom correspondence should be addressed: Academic Medical Centre, University of Amsterdam, E.C. Slater Inst., Dept. of Biochemistry, P. O. Box 22700, 1100 DE Amsterdam, The Netherlands. Tel.: 31-20-5665144; Fax: 31-20-6915519.
Abstract
In rat hepatocytes, autophagy is known to be inhibited by amino acids. Insulin and cell swelling promote inhibition by amino acids. Each of the conditions leading to inhibition of autophagic proteolysis was found to be associated with phosphorylation of a 31-kDa protein that we identified as ribosomal protein S6. A combination of leucine, tyrosine, and phenylalanine, which efficiently inhibits autophagic proteolysis, was particularly effective in stimulating S6 phosphorylation. The relationship between the percentage inhibition of proteolysis and the degree of S6 phosphorylation was linear. Thus, inhibition of autophagy and phosphorylation of S6 are under the control of the same signal transduction pathway. Stimulation of S6 phosphorylation by the presence of amino acids was due to activation of S6 kinase and not to inhibition of S6 phosphatase.
The inhibition by amino acids of both autophagic proteolysis and autophagic sequestration of electro-injected cytosolic [14C]sucrose was partially prevented by rapamycin, a compound known to inhibit activation of p70 S6 kinase. In addition, rapamycin partially inhibited the rate of protein synthesis.
We conclude that the fluxes through the autophagic and protein synthetic pathways are regulated in an opposite manner by the degree to which S6 is phosphorylated. Possible mechanisms by which S6 phosphorylation can cause inhibition of autophagy are discussed.
Footnotes
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↵* This study was supported in part by a grant (project 900-523-168) from the Netherlands Organization for Scientific Research (NWO) under the auspices of the Netherlands Foundation for Medical Research (GMW). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Received September 19, 1994.
- Revision received November 7, 1994.
- © 1995 by The American Society for Biochemistry and Molecular Biology, Inc.
