Targeted perturbation of signaling-driven condensates.

Biomolecular condensates have emerged as a major organizational principle in the cell. However, the formation, maintenance, and dissolution of condensates are still poorly understood. Transcriptional machinery partitions into biomolecular condensates at key cell identity genes to activate these. Here, we report a specific perturbation of WNT-activated ß-catenin condensates that disrupts oncogenic signaling. We use a live-cell condensate imaging method in human cancer cells to discover FOXO and TCF-derived peptides that specifically inhibit ß-catenin condensate formation on DNA, perturb nuclear ß-catenin condensates in cells, and inhibit ß-catenin-driven transcriptional activation and colorectal cancer cell growth. We show that these peptides compete with homotypic intermolecular interactions that normally drive condensate formation. Using this framework, we derive short peptides that specifically perturb condensates and transcriptional activation of YAP and TAZ in the Hippo pathway. We propose a "monomer saturation" model in which short interacting peptides can be used to specifically inhibit condensate-associated transcription in disease.

A FOXO-dependent replication checkpoint restricts proliferation of damaged cells.

DNA replication is challenged by numerous exogenous and endogenous factors that can interfere with the progression of replication forks. Substantial accumulation of single-stranded DNA during DNA replication activates the DNA replication stress checkpoint response that slows progression from S/G2 to M phase to protect genomic integrity. Whether and how mild replication stress restricts proliferation remains controversial. Here, we identify a cell cycle exit mechanism that prevents S/G2 phase arrested cells from undergoing mitosis after exposure to mild replication stress through premature activation of the anaphase promoting complex/cyclosome (APC/CCDH1). We find that replication stress causes a gradual decrease of the levels of the APC/CCDH1 inhibitor EMI1/FBXO5 through Forkhead box O (FOXO)-mediated inhibition of its transcription factor E2F1. By doing so, FOXOs limit the time during which the replication stress checkpoint is reversible and thereby play an important role in maintaining genomic stability.

FOXO Transcription Factors Both Suppress and Support Breast Cancer Progression.

FOXO transcription factors are regulators of cellular homeostasis and putative tumor suppressors, yet the role of FOXO in cancer progression remains to be determined. The data on FOXO function, particularly for epithelial cancers, are fragmentary and come from studies that focused on isolated aspects of cancer. To clarify the role of FOXO in epithelial cancer progression, we characterized the effects of inducible FOXO activation and loss in a mouse model of metastatic invasive lobular carcinoma. Strikingly, either activation or loss of FOXO function suppressed tumor growth and metastasis. We show that the multitude of cellular processes critically affected by FOXO function include proliferation, survival, redox homeostasis, and PI3K signaling, all of which must be carefully balanced for tumor cells to thrive.Significance: FOXO proteins are not solely tumor suppressors, but also support tumor growth and metastasis by regulating a multitude of cellular processes essential for tumorigenesis. Cancer Res; 78(9); 2356-69. ©2018 AACR.

FOXO target gene CTDSP2 regulates cell cycle progression through Ras and p21(Cip1/Waf1).

Activity of FOXO (forkhead box O) transcription factors is inhibited by growth factor-PI3K (phosphoinositide 3-kinase)-PKB (protein kinase B)/Akt signalling to control a variety of cellular processes including cell cycle progression. Through comparative analysis of a number of microarray datasets we identified a set of genes commonly regulated by FOXO proteins and PI3K-PKB/Akt, which includes CTDSP2 (C-terminal domain small phosphatase 2). We validated CTDSP2 as a genuine FOXO target gene and show that ectopic CTDSP2 can induce cell cycle arrest. We analysed transcriptional regulation after CTDSP2 expression and identified extensive regulation of genes involved in cell cycle progression, which depends on the phosphatase activity of CTDSP2. The most notably regulated gene is the CDK (cyclin-dependent kinase) inhibitor p21(Cip1/Waf1) and in the present study we show that p21(Cip1/Waf1) is partially responsible for the cell cycle arrest through decreasing cyclin-CDK activity. Our data suggest that CTDSP2 induces p21(Cip1/Waf1) through increasing the activity of Ras. As has been described previously, Ras induces p21(Cip1/Waf1) through p53-dependent and p53-independent pathways and indeed both p53 and MEK inhibition can mitigate the CTDSP2-induced p21(Cip1/Waf1) mRNA up-regulation. In support of Ras activation by CTDSP2, depletion of endogenous CTDSP2 results in reduced Ras activity and thus CTDSP2 seems to be part of a larger set of genes regulated by FOXO proteins, which increase growth factor signalling upon FOXO activation.

FOXO3 selectively amplifies enhancer activity to establish target gene regulation.

Forkhead box O (FOXO) transcription factors regulate diverse cellular processes, affecting tumorigenesis, metabolism, stem cell maintenance, and lifespan. We show that FOXO3 transcription regulation mainly proceeds through the most active subset of enhancers. In addition to the general distinction between "open" and "closed" chromatin, we show that the level of activity marks (H3K27ac, RNAPII, enhancer RNAs) of these open chromatin regions prior to FOXO3 activation largely determines FOXO3 DNA binding. Consequently, FOXO3 amplifies the levels of these activity marks and their absolute rather than relative changes associate best with FOXO3 target gene regulation. The importance of preexisting chromatin state in directing FOXO3 gene regulation, as shown here, provides a mechanism whereby FOXO3 can regulate cell-specific homeostasis. Genetic variation is reported to affect these chromatin signatures in a quantitative manner, and, in agreement, we observe a correlation between cancer-associated genetic variations and the amplitude of FOXO3 enhancer binding.

Genome-wide analysis of FOXO3 mediated transcription regulation through RNA polymerase II profiling.

Forkhead box O (FOXO) transcription factors are key players in diverse cellular processes affecting tumorigenesis, stem cell maintenance and lifespan. To gain insight into the mechanisms of FOXO-regulated target gene expression, we studied genome-wide effects of FOXO3 activation. Profiling RNA polymerase II changes shows that FOXO3 regulates gene expression through transcription initiation. Correlative analysis of FOXO3 and RNA polymerase II ChIP-seq profiles demonstrates FOXO3 to act as a transcriptional activator. Furthermore, this analysis reveals a significant part of FOXO3 gene regulation proceeds through enhancer regions. FOXO3 binds to pre-existing enhancers and further activates these enhancers as shown by changes in histone acetylation and RNA polymerase II recruitment. In addition, FOXO3-mediated enhancer activation correlates with regulation of adjacent genes and pre-existence of chromatin loops between FOXO3 bound enhancers and target genes. Combined, our data elucidate how FOXOs regulate gene transcription and provide insight into mechanisms by which FOXOs can induce different gene expression programs depending on chromatin architecture.

Oxidative stress-dependent regulation of Forkhead box O4 activity by nemo-like kinase.

Forkhead box O (FOXO) transcription factors are involved in various cellular processes, including cell proliferation, stress resistance, metabolism, and longevity. Regulation of FOXO transcriptional activity occurs mainly through a variety of post-translational modifications, including phosphorylation, acetylation, and ubiquitination. Here we describe nemo-like kinase (NLK) as a novel regulator of FOXOs. NLK binds to and phosphorylates FOXO1, FOXO3a, and FOXO4 on multiple residues. NLK acts as a negative regulator of FOXO transcriptional activity. For FOXO4 we show that NLK-mediated loss of FOXO4 activity co-occurs with inhibition of FOXO4 monoubiquitination. Previously, we have shown that oxidative stress-induced monoubiquitination of FOXO4 stimulates its transactivation, which leads to activation of an antioxidant defensive program. Conversely, NLK-dependent inhibition of FOXO4 activity can provide a means to downregulate this defensive program, when oxidative stress reaches a level beyond which repair is no longer feasible and cells need to undergo apoptosis.

Redox-sensitive cysteines bridge p300/CBP-mediated acetylation and FoxO4 activity.

Cellular damage invoked by reactive oxygen species plays a key role in the pathobiology of cancer and aging. Forkhead box class O (FoxO) transcription factors are involved in various cellular processes including cell cycle regulation, apoptosis and resistance to reactive oxygen species, and studies in animal models have shown that these transcription factors are of vital importance in tumor suppression, stem cell maintenance and lifespan extension. Here we report that the activity of FoxO in human cells is directly regulated by the cellular redox state through a unique mechanism in signal transduction. We show that reactive oxygen species induce the formation of cysteine-thiol disulfide-dependent complexes of FoxO and the p300/CBP acetyltransferase, and that modulation of FoxO biological activity by p300/CBP-mediated acetylation is fully dependent on the formation of this redox-dependent complex. These findings directly link cellular redox status to the activity of the longevity protein FoxO.

Interaction of FOXO with beta-catenin inhibits beta-catenin/T cell factor activity.

Wingless (Wnt) signaling regulates many aspects of development and tissue homeostasis, and aberrant Wnt signaling can lead to cancer. Upon a Wnt signal beta-catenin degradation is halted and consequently the level of beta-catenin in the cytoplasm increases. This allows entry of beta-catenin into the nucleus where it can regulate gene transcription by direct binding to members of the lymphoid enhancer factor/T cell factor (TCF) family of transcription factors. Recently, we identified Forkhead box-O (FOXO) transcription factors as novel interaction partners of beta-catenin (Essers, M. A., de Vries-Smits, L. M., Barker, N., Polderman, P. E., Burgering, B. M., and Korswagen, H. C. (2005) Science 308, 1181-1184). Here we show that the beta-catenin binding to FOXO serves a dual effect. beta-catenin, through binding, enhances FOXO transcriptional activity. In addition, FOXO competes with TCF for interaction with beta-catenin, thereby inhibiting TCF transcriptional activity. Reduced binding between TCF and beta-catenin is observed after FOXO overexpression and cellular oxidative stress, which simultaneously increases binding between beta-catenin and FOXO. Furthermore, small interfering RNA-mediated knock down of FOXO reverts loss of beta-catenin binding to TCF after cellular oxidative stress. Taken together, these results provide evidence for a cross-talk mechanism between FOXO and TCF signaling in which beta-catenin plays a central regulatory role.

Functional interaction between beta-catenin and FOXO in oxidative stress signaling.

beta-Catenin is a multifunctional protein that mediates Wnt signaling by binding to members of the T cell factor (TCF) family of transcription factors. Here, we report an evolutionarily conserved interaction of beta-catenin with FOXO transcription factors, which are regulated by insulin and oxidative stress signaling. beta-Catenin binds directly to FOXO and enhances FOXO transcriptional activity in mammalian cells. In Caenorhabditis elegans, loss of the beta-catenin BAR-1 reduces the activity of the FOXO ortholog DAF-16 in dauer formation and life span. Association of beta-catenin with FOXO was enhanced in cells exposed to oxidative stress. Furthermore, BAR-1 was required for the oxidative stress-induced expression of the DAF-16 target gene sod-3 and for resistance to oxidative damage. These results demonstrate a role for beta-catenin in regulating FOXO function that is particularly important under conditions of oxidative stress.

FOXO4 is acetylated upon peroxide stress and deacetylated by the longevity protein hSir2(SIRT1).

FOXO transcription factors have important roles in metabolism, cellular proliferation, stress tolerance, and aging. FOXOs are negatively regulated by protein kinase B/c-Akt-mediated phosphorylation. Here we show that FOXO factors are also subject to regulation by reversible acetylation. We provide evidence that the acetyltransferase CREB-binding protein (CBP) binds FOXO resulting in acetylation of FOXO. This acetylation inhibits FOXO transcriptional activity. Binding of CBP and acetylation are induced after treatment of cells with peroxide stress. Deacetylation of FOXOs involves binding of the NAD-dependent deacetylase hSir2(SIRT1). Accordingly, hSir2(SIRT1)-mediated deacetylation precludes FOXO inhibition through acetylation and thereby prolongs FOXO-dependent transcription of stress-regulating genes. These data demonstrate that acetylation functions in a second pathway of negative control for FOXO factors and provides a novel mechanism whereby hSir2(SIRT1) can promote cellular survival and increase lifespan.