Forkhead box class O (FOXO) proteins are transcription factors that function downstream of the PTEN tumor suppressor and directly control the expression of genes involved in apoptosis, cell cycle progression, and stress responses. In the present study, we show that FOXO1 interacts with four and a half LIM 2 (FHL2) in prostate cancer cells. This interaction occurred in the nucleus and was enhanced by lysophosphatic acid. FHL2 decreased the transcriptional activity of FOXO1 and the expression of known FOXO target genes and inhibited FOXO1-induced apoptosis. Interestingly, SIRT1, a mammalian homolog of yeast Sir2, bound to and deacetylated FOXO1 and inhibited its transcriptional activity. FHL2 enhanced the interaction of FOXO1 and SIRT1 and the deacetylation of FOXO1 by Sirtuin-1 (SIRT1). Overall, our data show that FHL2 inhibits FOXO1 activity in prostate cancer cells by promoting the deacetylation of FOXO1 by SIRT1.

SIRT1 is the closest mammalian homologue of yeast SIR2, an important ageing regulator that prolongs lifespan in response to caloric restriction. Despite its importance, the mechanisms that regulate SIRT1 activity are unclear. Our study identifies a novel post-translational modification of SIRT1, namely sumoylation at Lys 734. In vitro sumoylation of SIRT1 increased its deacetylase activity. Conversely, mutation of SIRT1 at Lys 734 or desumoylation by SENP1, a nuclear desumoylase, reduced its deacetylase activity. Stress-inducing agents promoted the association of SIRT1 with SENP1 and cells depleted of SENP1 (but not of SENP1 and SIRT1) were more resistant to stress-induced apoptosis than control cells. We suggest that stress-inducing agents counteract the anti-apoptotic activity of SIRT1 by recruiting SENP1 to SIRT1, which results in the desumoylation and inactivation of SIRT1 and the consequent acetylation and activation of apoptotic proteins.Sirtuins (SIRTs) are mammalian NAD + -dependent histone deacetylases (HDACs) 1 . Of the seven SIRTs, SIRT1 is the closest homologue of yeast SIR2 (ref. 2,3), which has important roles in diverse cellular processes, including transcriptional silencing 4 , rDNA recombination 5 , glucose metabolism and energy homeostasis 6 , DNA repair and cell survival [7][8][9] . Because of its dependency on NAD + , the activity of SIRT1 is regulated by the NAD + :NADH ratio and, is therefore sensitive to the status of redox and cellular metabolism. Author Contributions: Y.Y. designed (with W.B.) and performed (with W.F.) the included studies. J.C., X.Z. and K.B. contributed scientifically to the revision. The manuscript was written by W.B., edited by N.O. and S.V.N. and read by all authors. The senior author (W.B.) designed the project, helped with the analyses and the organization of the data, and provided the financial support. All authors have discussed the data and had scientific input into the manuscript.Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions/ NIH Public Access Similar to SIR2 in lower organisms, SIRT1 is potentially a nutrient sensor that regulates the lifespan of mammals in response to caloric restriction or nutrient starvation [9][10][11][12][13] . By deacetylating and inactivating apoptotic proteins (such as the p53 tumour suppressor 14-17 ), SIRT1 also protects cancer cells from apoptosis induced by DNA damage. The expression of SIRT1 is known to be controlled at both transcriptional and post-transcriptional levels, but post-translational mechanisms that regulate SIRT1 activity have yet to be defined.Sumoylation is a reversible post-translational modification in which proteins termed small ubiquitin-related modifiers (SUMOs) are covalently linked to lysine residues of target proteins. Similar to ubiquitination, sumoylation is catalysed by a three-step enzymatic reaction involving an E1 activating enzyme, an E2 conjugating enzyme and E3 ligases. The reverse reaction is catalysed by SENP desumoylases, a famil...