Editors' ChoiceAUTOPHAGY

Out-FoxO1ing Cancer from the Cytoplasm

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Science Signaling  13 Jul 2010:
Vol. 3, Issue 130, pp. ec212
DOI: 10.1126/scisignal.3130ec212

Members of the Forkhead box O (FoxO) family—a group of transcription factors implicated in cellular responses to growth factors, nutrient availability, and stress—act as tumor suppressors, a function thought to be related to the transcriptional regulation of genes that control the cell cycle, DNA repair, and apoptosis (see Medema and Jäättelä). Noting that FoxO proteins may also play a role in inducing autophagy, Zhao et al. explored the regulation of autophagy by FoxO1 and its role in FoxO1’s antineoplastic activity. Analyses of three human cancer cell lines revealed abundant FoxO1 in HeLa (cervical cancer) and HCT116 (colon cancer) cells, which showed an autophagic response to serum withdrawal or H2O2 exposure, but not in H1299 (lung cancer) cells, which did not. The autophagic response was lost in HCT116 cells in which FoxO1 was stably knocked down and was rescued by an RNAi-resistant FoxO1 mutant. Experiments with FoxO1 mutants localized to either the nucleus or cytoplasm revealed that autophagy depended on cytoplasmic (rather than nuclear) FoxO1. Moreover, real-time polymerase chain reaction analysis of H1299 cells transfected with FoxO1 mutants and of HCT116 cells after serum withdrawal indicated that FoxO1-mediated autophagy did not depend on increased expression of autophagy-associated genes. Although serum withdrawal or H2O2 exposure failed to elicit phosphorylation or ubiquitylation of cytosolic FoxO1, immunoblot analysis revealed an increase in its acetylation. Serum withdrawal led to decreased association of cytosolic FoxO1 with the NAD+-dependent histone deacetylase sirtuin 2 (SIRT2), and treatment with a SIRT2 inhibitor or short hairpin RNA directed against SIRT2 increased FoxO1 acetylation. FoxO1 coimmunoprecipitated with Atg7 (autophagy-related gene 7), an interaction enhanced by serum starvation and inhibited by SIRT2 but not by a SIRT2 mutant lacking deacetylase activity. SIRT2 knockdown or inhibition promoted autophagy in wild-type but not FoxO1-knockdown HCT116 cells, whereas SIRT2 overexpression blocked FoxO1-dependent autophagy. Furthermore, mutation of three lysines predicted as acetylation sites disrupted the interaction between FoxO1 and Atg7 and blocked the ability of FoxO1 to induce autophagy. Both cytoplasmic and nuclear forms of FoxO1 promoted the death of H1299 cells; the former, however, acted through autophagy, whereas the latter acted through apoptosis. Notably, tumors formed in nude mice by H1299 cells expressing cytoplasmic FoxO1 were smaller than those formed by cells expressing nuclear FoxO1 (or not transfected with FoxO1), an effect that depended on Atg7. The authors thus propose that promotion of autophagy by cytosolic FoxO1 contributes to its antineoplastic activity.

Y. Zhao, J. Yang, W. Liao, X. Liu, H. Zhang, S. Wang, D. Wang, J. Feng, L. Yu, W.-G. Zhu, Cytosolic FoxO1 is essential for the induction of autophagy and tumour suppressor activity. Nat. Cell Biol. 12, 665–675 (2010). [PubMed]

R. H. Medema, M. Jäättelä, Cytosolic FoxO1: Alive and killing. Nat. Cell Biol. 12, 642–643 (2010). [PubMed]

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