[Autophagy 4:5, 713-714; 1 July 2008]; ©2008 Landes Bioscience
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Roles of the FOXA transcription factor Fork head in autophagic developmental cell death
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Article Addendum
Michael Lehmann
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Department of Biological Sciences; University of Arkansas; Fayetteville, Arkansas USA
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Animals progress normally through embryonic and postembryonic development only if obsolete cells and tissues are removed in a “programmed” spatial and temporal pattern.1 The two major types of programmed cell death (PCD) through which this is accomplished are apoptosis and autophagic cell death. The genetic program underlying apoptotic PCD has been studied in great detail, revealing evolutionarily conserved genes and mechanisms.2,3 In contrast, autophagic cell death is largely defined by morphological criteria,4 and little is known about the molecular mechanisms that distinguish autophagic death from apoptosis. A hallmark of autophagic death is the formation of autophagosomes that deliver bulk cytoplasm to the cell’s lysosomal compartment for degradation. Autophagy (here used synonymously with macroautophagy) has initially been studied in mammalian cells as a response to the hormone glucagon and in yeast as a process that facilitates cellular survival under starvation conditions. Subsequent studies show that the process is conserved across eukaryotes.5 In view of the function of autophagy as a survival mechanism in response to starvation, a role in cell death may seem surprising, and the causal relationship between autophagy and death has been a subject of intense studies and debate.6,7
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Correspondence to: Michael Lehmann; Department of Biological Sciences (SCEN 601); 1 University of Arkansas; Fayetteville, Arkansas 72701-1201 USA; Tel.: 479.575.3688; Fax: 479.575.4010; Email:
[email protected] Submitted: 05/12/08; Revised: 05/21/08; Accepted: 05/26/08 Previously published online as an Autophagy E-publication: http://www.landesbioscience.com/journals/autophagy/article/6335 Addendum to: Liu Y, Lehmann M. Genes and biological processes controlled by the Drosophila FOXA orthologue Fork head. Insect Mol Biol 2008; 17:91–101.
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Autophagic cell death is observed when groups of cells or entire tissues die. It is the form of PCD used for the massive destruction of larval tissues during metamorphosis of Drosophila and other insect species. Surprisingly, studies on PCD in the larval salivary glands and the larval midgut of Drosophila reveal that key components of the apoptotic machinery, including caspases, are required for the autophagic type of death observed in these tissues.8 Thus, autophagic cell death may not be as different from apoptotic death as morphological studies initially suggested. In fact, larval organs are successfully removed in animals lacking the core autophagy regulator Atg7.9 However, tissue destruction is delayed in Atg7 mutants and mutants of other autophagy genes.9,10 Autophagy thus contributes to the timely disposal of larval cells.
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Autophagy and Cell Death during Development
Autophagic Cell Death Removes Obsolete Tissues during Insect Metamorphosis
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Fork head (Fkh) is the only fruit fly ortholog of the FOXA subgroup of FOX transcription factors. Recent studies on the role of Fkh in autophagic cell death during Drosophila metamorphosis identified Fkh as a competence factor for steroid-induced cell death. Microarray analyses suggest that, in addition to controlling essential apoptosis genes, Fkh controls autophagic and cell survival pathways. The data support a model in which Fkh promotes cell growth and survival by coordinating the cell’s ability to respond to death and autophagy-controlling signals.
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Key words: apoptosis, autophagic cell death, developmental competence, FOX, PI3K/Akt, steroid signaling, survival signaling
Salivary Gland Death Results from a Shifted Balance between Pro-Death and Survival Signals Cell death during development must be restricted to selected cells and the appropriate developmental stage. During Drosophila metamorphosis, two consecutive pulses of the steroid hormone 20-hydroxyecdysone (20E) provide the temporal triggers for tissue destruction (Fig. 1). These pulses are followed by two waves of cell death affecting different groups of larval tissues. Death of the larval salivary glands in response to the second pulse requires the activation of the pro-apoptotic genes hid and reaper11 and the preceding downregulation of survival signaling through the PI3K/Akt pathway.10,12 As in vertebrates, in Drosophila, PI3K/Akt negatively regulates the pro-apoptotic FOX transcription factor FOXO.13,14 However, whereas Drosophila FOXO (dFOXO) has been shown to be required for UV-induced apoptosis in the eye15 and, more recently, for starvation-induced autophagy,16 it is not clear whether dFOXO plays a similar role in normal developmental PCD or autophagy. Salivary gland death occurs independently of dFOXO, despite the fact that PI3K and Akt are potent inhibitors of PCD in this tissue.12 A possible target of Akt in the salivary glands is the autophagy regulator TOR.10 However, as autophagy is not required for salivary gland death, the strong block of death imposed by PI3K/Akt12 is probably mediated by another downstream target of the pathway. It will be interesting to identify this target and to establish whether dFOXO has a conserved role in developmental cell death and autophagy in flies.
Autophagy
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Roles of the FOXA transcription factor Fork head in autophagic developmental cell death
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References
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1. Meier P, Finch A, Evan G. Apoptosis in development. Nature 2000; 407:796-801. 2. Yuan J. Divergence from a dedicated cellular suicide mechanism: exploring the evolution of cell death. Mol Cell 2006; 23:1-12. 3. Kornbluth S, White K. Apoptosis in Drosophila: neither fish nor fowl (nor man, nor worm). J Cell Sci 2005; 118:1779-87. 4. Schweichel JU, Merker HJ. The morphology of various types of cell death in prenatal tissues. Teratology 1973; 7:253-66. 5. Klionsky DJ, Emr SD. Autophagy as a regulated pathway of cellular degradation. Science 2000; 290:1717-21. 6. Neufeld TP, Baehrecke EH. Eating on the fly: Function and regulation of autophagy during cell growth, survival and death in Drosophila. Autophagy 2008; 4. 7. Edinger AL, Thompson CB. Death by design: apoptosis, necrosis and autophagy. Curr Opin Cell Biol 2004; 16:663-9. 8. Yin VP, Thummel CS. Mechanisms of steroid-triggered programmed cell death in Drosophila. Semin Cell Dev Biol 2005; 16:237-43. 9. Juhasz G, Erdi B, Sass M, Neufeld TP. Atg7-dependent autophagy promotes neuronal health, stress tolerance and longevity but is dispensable for metamorphosis in Drosophila. Genes Dev 2007; 21:3061-6. 10. Berry DL, Baehrecke EH. Growth arrest and autophagy are required for salivary gland cell degradation in Drosophila. Cell 2007; 131:1137-48. 11. Yin VP, Thummel CS. A balance between the diap1 death inhibitor and reaper and hid death inducers controls steroid-triggered cell death in Drosophila. Proc Natl Acad Sci USA 2004; 101:8022-7. 12. Liu Y, Lehmann M. FOXO-independent suppression of programmed cell death by the PI3K/Akt signaling pathway in Drosophila. Dev Genes Evol 2006; 216:531-5. 13. Jünger MA, Rintelen F, Stocker H, Wasserman JD, Vegh M, Radimerski T, Greenberg ME, Hafen E. The Drosophila Forkhead transcription factor FOXO mediates the reduction in cell number associated with reduced insulin signaling. J Biol 2003; 2:20. 14. Puig O, Marr MT, Ruhf ML, Tjian R. Control of cell number by Drosophila FOXO: downstream and feedback regulation of the insulin receptor pathway. Genes Dev 2003; 17:2006-20. 15. Luo X, Puig O, Hyun J, Bohmann D, Jasper H. Foxo and Fos regulate the decision between cell death and survival in response to UV irradiation. EMBO J 2007; 26:380-90. 16. Juhasz G, Puskas LG, Komonyi O, Erdi B, Maroy P, Neufeld TP, Sass M. Gene expression profiling identifies FKBP39 as an inhibitor of autophagy in larval Drosophila fat body. Cell Death Differ 2007; 14:1181-90. 17. Cao C, Liu Y, Lehmann M. Fork head controls the timing and tissue selectivity of steroidinduced developmental cell death. J Cell Biol 2007; 176:843-52. 18. Liu Y, Lehmann M. Genes and biological processes controlled by the Drosophila FOXA orthologue Fork head. Insect Mol Biol 2008; 17:91-101. 19. Levine B, Klionsky DJ. Development by self-digestion: molecular mechanisms and biological functions of autophagy. Dev Cell 2004; 6:463-77. 20. Carroll JS, Liu XS, Brodsky AS, Li W, Meyer CA, Szary AJ, Eeckhoute J, Shao W, Hestermann EV, Geistlinger TR, Fox EA, Silver PA, Brown M. Chromosome-wide mapping of estrogen receptor binding reveals long-range regulation requiring the forkhead protein FoxA1. Cell 2005; 122:33-43. 21. Friedman JR, Kaestner KH. The Foxa family of transcription factors in development and metabolism. Cell Mol Life Sci 2006; 63:2317-28. 22. Behr R, Sackett SD, Bochkis IM, Le PP, Kaestner KH. Impaired male fertility and atrophy of seminiferous tubules caused by haploinsufficiency for Foxa3. Dev Biol 2007; 306:636-45.
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Microarray Analyses Suggest a Key Role of Fork Head in Coordinating the Response to Death, Survival and Autophagic Signals
Figure 1. Model illustrating how Fkh affects pathways that control cell death and autophagy in the larval salivary glands. The effect of Fkh on the cell death pathway has been experimentally tested and confirmed,17 while the remaining parts of the model depict testable hypotheses based on microarray results.18
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Why do the larval salivary glands not already die in response to earlier hormone signals, and how is an individual tissue such as the salivary glands selected for a death response to a systemic signal? Clearly, tissues must experience a fundamental change that makes them competent to respond with death. One possibility is that they lose a tissue-specific survival factor that confers resistance to earlier hormone pulses. Recent evidence strongly supports such a model for the larval salivary glands. Loss of Fkh, a founding member of the FOX family of transcription factors, is both necessary and sufficient to make the salivary glands sensitive to a death-inducing steroid signal (Fig. 1).17 Interestingly, loss of Fkh is triggered by the 20E pulse that precedes the death-inducing pulse, demonstrating how a hormone can modulate its own effects within a temporal sequence of hormone releases. Loss of Fkh by itself does not cause death. Instead, it renders essential death genes hormone-inducible, including the apoptotic genes hid and reaper.17 Fkh thus plays a permissive role in generating competence for a cell death response in the salivary glands (Fig. 1).
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Fork Head Acts as a Competence Factor for Steroid-Induced Autophagic Death
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Genome-wide expression profiling using Affymetrix gene chips suggests that Fkh not only blocks the steroid activation of death genes, but that it may also promote survival signaling through the PI3K/Akt pathway (by upregulating the Akt activator PDK1).17 This idea is supported by a decline of PI3K activity that coincides with the loss of Fkh from the salivary glands and the observation that experimentally increased activity of PI3K or Akt counteracts steroid-induced salivary gland death (Fig. 1).10,12 Interestingly, the microarray analyses indicate that Fkh also controls a whole cassette of autophagy genes.18 Atg1 and Atg6 are upregulated, whereas Atg2, Atg5, Atg7, Atg9 and Atg18 are downregulated. Atg1 is required for the induction of autophagy and Atg6 for the early step of vesicle nucleation. In contrast, all Atg genes that are downregulated by Fkh are required for downstream steps of autophagy; Atg5 and Atg7 for autophagosomal vesicle expansion and completion; and Atg2 and Atg18 for the recycling of autophagosomal proteins.19 These data suggest that Fkh protects the salivary glands from PCD at multiple levels, coordinately repressing death and autophagy genes, while activating the PI3K/Akt survival pathway. A test of this hypothesis will require direct assays of autophagy and PI3K activity in the absence and presence of Fkh. Finally, the microarray analyses identify many target genes of the vertebrate FOXAs as responsive to Fkh, suggesting a high degree of evolutionary conservation of FOXA function. In support of this conclusion, vertebrate FOXAs have been shown to modulate tissue-specific steroid responses,20,21 and FOXA3 has recently been associated with an anti-apoptotic function.22 Future studies will most likely reveal additional conserved functions of this important group of transcription factors in autophagy and survival signaling. Acknowledgements
Research in the author’s laboratory is supported by NSF grant number IOS-0641347 and NIH grant number 1R15DK079277-01. 714
Autophagy
2008; Vol. 4 Issue 5
