[Autophagy 1:1, 59-61; April/May/June 2005]; ©2004 Landes Bioscience
Autophagy and p70S6 Kinase Views and Commentaries
Daniel J. Klionsky1,* Alfred J. Meijer2 Patrice Codogno3 Thomas P. Neufeld4 Ryan C. Scott4
4Department of Genetics; Cell Biology and Development; University of Minnesota;
Minneapolis, Minnestoa USA
*Correspondence to: Daniel J. Klionsky; University of Michigan; Life Sciences Institute; Rm. 6036; 210 Washtenaw Ave.; Ann Arbor, Michigan 48109-2216 USA; Tel.: 734.615.6556; Fax:734.763.6492; Email:
[email protected] Received 01/05/05; Accepted 01/05/05
Previously published online as an Autophagy E-publication: http://www.landesbioscience.com/journals/autophagy/abstract.php?id=1515
References
1. Scott RC, Schuldiner O, Neufeld TP. Role and regulation of starvation-induced autophagy in the Drosophila fat body. Dev Cell 2004; 7:167-78. 2. Klionsky DJ. Cell biology: regulated self-cannibalism. Nature 2004; 431:31-2.
COMMENTARY
In issue 7004 of Nature, D.J. Klionsky1 discusses a recent paper by Scott et al.2 concerning the regulation of autophagy in the fat body of Drosophila melanogaster. In their paper Scott et al.2 convincingly show that the protein kinase p70S6 kinase, which is downstream of Tor (target of rapamycin) in the insulin signalling pathway, is essential for autophagy in the fat body of Drosophila. This finding refutes the proposal, based on experiments with rat hepatocytes, that p70S6k suppresses autophagy.3 The basis of this proposal was the linear correlation between the degree of phosphorylation of ribosomal protein S6, reflecting p70S6k activity in situ, and autophagic flux, with high rates of autophagy when S6 phosphorylation was minimal by amino acid depletion or by rapamycin addition.3 Because p70S6k is needed for autophagy in Drosophila, Scott et al.2 conclude that the inhibition of autophagy by Tor and the activation of p70S6k are effected through different branches of the Tor pathway. Since activation of p70S6k is Tor-dependent it is assumed that in starvation, when Tor is switched off, p70S6k needs to be activated for some time.1 p70S6k is not only essential for autophagy2 but the same protein kinase is also required for protein synthesis.4 How to solve this dilemma? Of course, it cannot be excluded that Tor controls the synthesis of proteins required for expansion and maturation of the autophagosome,2 but this remains to be tested. An attractive alternative explanation may be found in the observation that, at least in mammalian cells, p70S6k exerts a negative feedback on signalling upstream of Tor in that it inhibits activation of phosphatidylinositol-3-OH kinase (PI3K) class 1 through inhibition by p70S6k -mediated phosphorylation of insulin receptor substrate.5,6 Because the product of the lipid kinase, PtdIns(3,4,5)P3, is an inhibitor of autophagy,7,8 overactivation of p70S6k would de-inhibit autophagy. This is important because even under nutrient-rich conditions cells must be able to carry out autophagy to some extent, not for production of nutrients, which are plenty, but to eliminate damaged cell structures or structures that are no longer needed by the cell. Conversely, when nutrients become scarce, e.g., in starvation, inactivation of Tor by a fall in amino acid concentration accelerates autophagy, provided sufficient active p70S6k is still present (see above). In long-term starvation, p70S6k activity may become so low that PI3K class 1 is activated again and restrains excessive autophagy in order to prevent cell death, as suggested.2
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of Medical Biochemistry; Academic Medical Center; University of Amsterdam; Amsterdam, The Netherlands
Daniel J. Klionsky University of Michigan Life Sciences Institute Ann Arbor, Michigan USA
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1University of Michigan; Life Sciences Institute; Ann Arbor, Michigan USA
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A paper by Scott et al.,1 suggested that p70S6 kinase (p70S6k) is a positive regulatory factor for autophagy (Fig. 1). This finding is in contrast to previous data suggesting a negative role for this factor. The Scott et al. article was highlighted in Nature News & Views,2 which elicited a commentary by A.J. Meijer and P. Codogno. These authors present an alternate model for the role of p70S6k in autophagic induction, although still as a positive factor. Following the initial commentary is a response by T.P. Neufeld and R.C. Scott.
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insulin receptor, lipid kinase, rapamycin, signaling, starvation, Tor
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Figure 1. Two models for the role of p70S6 kinase in the regulation of autophagy. In the feedback model, low-level homeostatic activation of autophagy (bold purple lines) occurs even under nutrient-rich conditions through inhibition of the PI3K/Akt pathway. When nutrient levels drop, Tor inactivation allows autophagy to proceed at a higher level (bold blue line). During long-term starvation, the drop in p70S6k activity would remove the inhibition of PI3K and reactivate Tor to prevent excessive autophagy. In the direct model, activation of p70S6k stimulates autophagy directly (bold green lines), and independent of PI3K/Akt, although it was proposed that p70S6k may also act indirectly through effects on protein synthesis. In starvation conditions, the inhibition of Tor prevents further activation of p70S6k, which eventually limits autophagy and prevents excessive autophagy. In this case, the activities of Tor and p70S6k are balanced to allow low levels of homeostatic autophagy in nutrient-rich conditions. AH, acid hydrolases; EcR, ecdysone receptor; InR, insulin receptor.
A.J. Meijer Department of Medical Biochemistry Academic Medical Center University of Amsterdam Amsterdam, The Netherlands
6. Shah OJ, Wang Z, Hunter T. Inappropriate activation of the TSC/Rheb/mTOR/S6K cassette induces IRS1/2 depletion, insulin resistance, and cell survival deficiencies. Curr Biol 2004; 14:1650-6. 7. Petiot A, Ogier-Denis E, Blommaart EF, Meijer AJ, Codogno PJ. Distinct classes of phosphatidylinositol 3'-kinases are involved in signaling pathways that control macroautophagy in HT-29 cells. Biol Chem 2000; 275:992-8. 8. Rusten TE, Lindmo K, Juhasz G, Sass M, Seglen PO, Brech A, Stenmark H. Programmed autophagy in the Drosophila fat body is induced by ecdysone through regulation of the PI3K pathway. Dev Cell 2004; 7:179-92.
Patrice Codogno INSERM U504 Villejuif Cedex, France
References 1. Klionsky DJ. Cell biology: regulated self-cannibalism. Nature 2004; 431:31-2. 2. Scott RC, Schuldiner O, Neufeld TP. Role and regulation of starvation-induced autophagy in the Drosophila fat body. Dev Cell 2004; 7:167-78. 3. Blommaart EF, Luiken JJ, Blommaart PJ, van Woerkom GM, Meijer AJ. Phosphorylation of ribosomal protein S6 is inhibitory for autophagy in isolated rat hepatocytes. J Biol Chem 1995; 270:2320-6. 4. Dennis PB, Fumagalli S, Thomas G. Target of rapamycin (TOR): balancing the opposing forces of protein synthesis and degradation. Curr Opin Genet Dev 1999; 9:49-54. 5. Um SH, Frigerio F, Watanabe M, Picard F, Joaquin M, Sticker M, Fumagalli S, Allegrini PR, Kozma SC, Auwerx J, Thomas G. Absence of S6K1 protects against age- and dietinduced obesity while enhancing insulin sensitivity. Nature 2004; 431:111-228.
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RESPONSE Like many important biological systems, the PI3K/TOR signaling network has evolved mechanisms of negative feedback control, as Drs. Meijer and Codogno point out. In mammalian cells, this has been shown to involve p70S6k-mediated downregulation of the insulin receptor substrates (IRSs), adapter molecules critical for activation of PI3K. This feedback mechanism likely has a strong influence on many aspects of biology regulated by PI3K and TOR
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signaling, including insulin resistance1 and tumor malignancy.2 A related negative feedback loop between p70S6k and PI3K has been described in Drosophila.3 The role of p70S6k in starvation-induced autophagy provides another example of the complex interactions amongst PI3K/TOR signaling components. In the Scott et al. study,4 we found that whereas PI3K and TOR functioned to suppress autophagy in the fat body of Drosophila, p70S6k was required for induction of autophagy. Because p70S6k is dependent on TOR for its activity, these observations imply that p70S6k may become limiting for autophagy in response to TOR inactivation. Indeed, whereas overexpression of an activated form of p70S6k had no effect in a wild type background, it caused a marked increase in autophagy in TOR mutant animals. In addition, we found that the level of autophagy in chronically starved or TOR mutant animals was lower than the peak level elicited by short term starvation. These data are consistent with the idea that a gradual loss of p70S6k activity upon starvation or TOR inactivation could allow for a rapid, intense autophagic response followed by lower sustained levels of autophagy. Could the requirement for p70S6k in autophagy reflect a negative feedback mechanism of p70S6k on PI3K activity, perhaps through IRS proteins? In this scenario inactivation of p70S6k, through mutation, starvation, or downregulation of TOR signaling, would lead to increased PI3K activation, thereby reducing autophagic activity. As suggested by Meijer and Codogno, this is an attractive model, and we agree that it is likely to contribute to the overall effect of this pathway on autophagy. However, some of the data in our study would appear to argue against this being the only such mechanism. For example, we found that mutation of p70S6k inhibited autophagy in response to TOR inactivation (Fig. 4C in Scott et al.4), whereas activation of PI3K did not (Fig. 3I in Scott et al.4), indicating that the requirement for p70S6k must be largely PI3K-independent under these conditions. Likewise, the ability of p70S6k activation to stimulate autophagy in TOR mutants (Fig. 4F in Scott et al.4), which are refractory to PI3K signaling, would also seem to be inconsistent with a PI3K-dependent mechanism. Finally, the failure of activated p70S6k to induce autophagy in wild type animals (Fig. 4D in Scott et al.4) is consistent with a passive requirement: p70S6k activity appears to be necessary but not sufficient to induce autophagy. A more definitive test of the feedback model will be to measure the effects of p70S6k on autophagy in cells lacking PI3K or IRS1 (known as chico in Drosophila). It will also be of interest to examine the role of p70S6k in autophagy in mammalian cells, perhaps using p70S6k1-p70S6k2 double knockout mice. Regardless of mechanism, it is clear that cells have devised clever ways to finely tune the signaling pathways regulating autophagy.
References 1. Tamemoto H, Kadowaki T, Tobe K, Yagi T, Sakura H, Hayakawa T, Terauchi Y, Ueki K, Kaburagi Y, Satoh S. Insulin resistance and growth retardation in mice lacking insulin receptor substrate-1. Nature 1994; 372:182-6. 2. Harrington LS, Findlay GM, Gray A, Tolkacheva T, Wigfield S, Rebholz H, Barnett J, Leslie NR, Cheng S, Shepherd PR, Gout I, Downes CP, Lamb RF. The TSC1-2 tumor suppressor controls insulin-PI3K signaling via regulation of IRS proteins. J Cell Biol 2004; 166:213-23. 3. Radimerski T, Montagne J, Hemmings-Mieszczak M, Thomas G. Lethality of Drosophila lacking TSC tumor suppressor function rescued by reducing dS6K signaling. Genes Dev 2002; 16:2627-32. 4. Scott RC, Schuldiner O, Neufeld TP. Role and regulation of starvation-induced autophagy in the Drosophila fat body. Dev Cell 2004; 7:148-150.
T.P. Neufeld R.C. Scott Department of Genetics, Cell Biology & Development University of Minnesota Minneapolis, MN USA
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