Forkhead box M1B transcriptional activity requires binding of Cdk-cyclin complexes for phosphorylation-dependent recruitment of p300/CBP coactivators

The same, only different - DNA damage checkpoints and their reversal throughout the cell cycle

Cell cycle checkpoints activated by DNA double-strand breaks (DSBs) are essential for the maintenance of the genomic integrity of proliferating cells. Following DNA damage, cells must detect the break and either transiently block cell cycle progression, to allow time for repair, or exit the cell cycle. Reversal of a DNA-damage-induced checkpoint not only requires the repair of these lesions, but a cell must also prevent permanent exit from the cell cycle and actively terminate checkpoint signalling to allow cell cycle progression to resume. It is becoming increasingly clear that despite the shared mechanisms of DNA damage detection throughout the cell cycle, the checkpoint and its reversal are precisely tuned to each cell cycle phase. Furthermore, recent findings challenge the dogmatic view that complete repair is a precondition for cell cycle resumption. In this Commentary, we highlight cell-cycle-dependent differences in checkpoint signalling and recovery after a DNA DSB, and summarise the molecular mechanisms that underlie the reversal of DNA damage checkpoints, before discussing when and how cell fate decisions after a DSB are made.

Kinome-wide functional screen identifies role of PLK1 in hormone-independent, ER-positive breast cancer

Estrogen receptor (ER) α-positive breast cancers initially respond to antiestrogens but eventually become estrogen independent and recur. ER(+) breast cancer cells resistant to long-term estrogen deprivation (LTED) exhibit hormone-independent ER transcriptional activity and growth. A kinome-wide siRNA screen using a library targeting 720 kinases identified Polo-like kinase 1 (PLK1) as one of the top genes whose downregulation resulted in inhibition of estrogen-independent ER transcriptional activity and growth of LTED cells. High PLK1 mRNA and protein correlated with a high Ki-67 score in primary ER(+) breast cancers after treatment with the aromatase inhibitor letrozole. RNAi-mediated knockdown of PLK1 inhibited ER expression, estrogen-independent growth, and ER transcription in MCF7 and HCC1428 LTED cells. Pharmacologic inhibition of PLK1 with volasertib, a small-molecule ATP-competitive PLK1 inhibitor, decreased LTED cell growth, ER transcriptional activity, and ER expression. Volasertib in combination with the ER antagonist, fulvestrant, decreased MCF7 xenograft growth in ovariectomized mice more potently than each drug alone. JUNB, a component of the AP-1 complex, was expressed 16-fold higher in MCF7/LTED compared with parental MCF7 cells. Furthermore, JUNB and BCL2L1 (which encodes antiapoptotic BCL-xL) mRNA levels were markedly reduced upon volasertib treatment in MCF7/LTED cells, while they were increased in parental MCF7 cells. Finally, JUNB knockdown decreased ER expression and transcriptional activity in MCF7/LTED cells, suggesting that PLK1 drives ER expression and estrogen-independent growth via JUNB. These data support a critical role of PLK1 in acquired hormone-independent growth of ER(+) human breast cancer and is therefore a promising target in tumors that have escaped estrogen deprivation therapy.

Concise review: forkhead pathway in the control of adult neurogenesis

New cells are continuously generated from immature proliferating cells in the adult brain in two neurogenic niches known as the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus and the sub-ventricular zone (SVZ) of the lateral ventricles. However, the molecular mechanisms regulating their proliferation, differentiation, migration and functional integration of newborn neurons in pre-existing neural network remain largely unknown. Forkhead box (Fox) proteins belong to a large family of transcription factors implicated in a wide variety of biological processes. Recently, there has been accumulating evidence that several members of this family of proteins play important roles in adult neurogenesis. Here, we describe recent advances in our understanding of regulation provided by Fox factors in adult neurogenesis, and evaluate the potential role of Fox proteins as targets for therapeutic intervention in neurodegenerative diseases.

Targeting FoxM1 transcription factor in T-cell acute lymphoblastic leukemia cell line

The Forkhead box protein M1 (FoxM1) is an important transcription factor having significant roles in various cellular events. FoxM1 overexpression has been reported to be related with many types of cancer. However, it is not known whether it contributes to oncogenesis of acute lymphoblastic leukemia. Siomycin A, a thiazol antibiotic, is known to inhibit FoxM1 transcriptional activity. In this study, we aimed to determine gene expression levels of FoxM1 in Jurkat cells (T-cell acute lymphoblastic leukemia cell line) and therapeutic potential of targeting FoxM1 by siomycin A alone and in combination with dexamethasone which improves the survival of children with T-cell acute lymphoblastic leukemia (ALL). We also examined the molecular mechanisms of siomycin A and dexamethasone-induced cell death in Jurkat cells. We demonstrated that FoxM1 mRNA is highly expressed in Jurkat cells. Dexamethasone and siomycin A caused a significant reduction in gene expression levels of FoxM1 in Jurkat cells. Targeting FoxM1 by siomycin A and dexamethasone caused a significant decrease in T-ALL cell line proliferation through induction of G1 cell cycle arrest. All these findings suggest a possible role of FoxM1 in T-cell ALL pathogenesis and represent FoxM1 as an attractive target for T-cell ALL therapy.

Different expression of FoxM1 in human benign and malignant pleural effusion

The aims of this study were as follows: to analyze the forkhead box M1 (FoxM1) expression in benign and malignant pleural effusion by reverse transcription-polymerase chain reaction assay (RT-PCR); to explore the role of FoxM1 in formation and progress in malignant pleural effusion, and whether there is significant difference in expression level of FoxM1 between benign and malignant pleural effusion; to seek a gene marker diagnostically useful to identify benign and malignant pleural effusion in diagnosis and treatment of pleural effusion; and to collect expression level data of FoxM1 in 23 malignant pleural effusion samples (17 adenocarcinoma samples, four squamous carcinoma samples and two small cell lung carcinoma samples) and 15 benign pleural effusion samples (11 inflammatory pleural effusions, two transudates, two tuberculous pleural effusions) by RT-PCR. Among all 38 samples, average FoxM1 expression level of benign pleural effusions is (235.09 ± 59.99), while malignant pleural effusions (828.77 ± 109.76). Among 23 malignant samples, average FoxM1 expression level is (529.27 ± 75.85) in samples without cytological diagnostic evidence, while (1,218.12 ± 167.21) in samples with cytological diagnostic evidence. Differences of FoxM1 expression level between benign pleural effusions and malignant ones have statistical significance. There is an area of 0.881 under the receiver-operating characteristic curve, which verifies the accuracy of using FoxM1 expression level as diagnostic index to identify benign and malignant pleural effusions. According to our study, diagnostic sensitivity and specificity for FoxM1 expression level at 418.1 were 82.6 and 86.7 %, respectively, while 47.8 and 100 %, respectively, at 768.7. FoxM1 expression level in malignant pleural effusions is significantly higher than in benign ones. This study provides a new approach in clinical diagnosis, with FoxM1 as a specific molecule marker to identify benign and malignant pleural effusions. FoxM1 expression level could provide evidence for diagnosis and treatment of malignant pleural effusions and lung cancer.

Transforming growth factor β-mediated site-specific Smad linker region phosphorylation in vascular endothelial cells

Objectives

Transforming growth factor (TGF)-β regulates the function of vascular endothelial cells and may be involved in endothelial dysfunction. The canonical TGF-β pathway involves TGF-β receptor-mediated carboxy-terminal phosphorylation of Smad2; however, TGF-β signalling also activates numerous serine/threonine kinases that phosphorylate Smad2 in its linker region. The expression of phosphorylated Smad linker proteins were determined following TGF-β stimulation in the absence and presence of different serine/threonine kinase inhibitors in vascular endothelial cells.

Methods

Proteins were quantified by Western blotting using specific antibodies to individual phosphorylated Smad2 linker region residues.

Key findings

TGF-β mediated the phosphorylation of all four Smad2 linker region residues of interest. Erk and Jnk specifically phosphorylate Ser245 while all mitogen-activated protein kinases phosphorylate Ser250 and Ser255. Thr220 and Ser245 are phosphorylated by phosphoinositide 3 kinase (PI3K), while Ser255 was phosphorylated by the PI3K/Akt pathway. CDK and GSK-3 were shown to phosphorylate Thr220 and Ser245. TGF-β also mediated plasminogen activator inhibitor-1 gene expression that was attenuated by p38 and CDK inhibitors.

Conclusions

TGF-β-mediated phosphorylation of individual serine/threonine sites in the linker region of Smad2 occurs in a highly specific manner by kinases. These phosphorylations provide an opportunity to further understand a therapeutically targeted and very specific signalling pathway in vascular endothelial cells.

Inferring protein modulation from gene expression data using conditional mutual information

Systematic, high-throughput dissection of causal post-translational regulatory dependencies, on a genome wide basis, is still one of the great challenges of biology. Due to its complexity, however, only a handful of computational algorithms have been developed for this task. Here we present CINDy (Conditional Inference of Network Dynamics), a novel algorithm for the genome-wide, context specific inference of regulatory dependencies between signaling protein and transcription factor activity, from gene expression data. The algorithm uses a novel adaptive partitioning methodology to accurately estimate the full Condition Mutual Information (CMI) between a transcription factor and its targets, given the expression of a signaling protein. We show that CMI analysis is optimally suited to dissecting post-translational dependencies. Indeed, when tested against a gold standard dataset of experimentally validated protein-protein interactions in signal transduction networks, CINDy significantly outperforms previous methods, both in terms of sensitivity and precision.

Pregnancy-associated plasma protein A regulates mitosis and is epigenetically silenced in breast cancer

Aberrant mitosis is a common feature of cancer, yet little is known about the altered genes causing mitotic defects. We screened human tumours for cells with morphological signatures of highly specific mitotic defects previously assigned to candidate genes in a genome-wide RNA interference screen carried out in HeLa cells (www.mitocheck.org). We discovered a striking enrichment of early mitotic configurations indicative of prophase/prometaphase delay in breast cancer. Promoter methylation analysis of MitoCheck candidate genes assigned to the corresponding 'mitotic delay' class linked this defect to epigenetic silencing of the gene encoding pregnancy-associated plasma protein-A (PAPPA), a secreted protease. PAPPA silencing was highly prevalent in precursor lesions and invasive breast cancer. Experimental manipulation of PAPPA protein levels in human mammary epithelial cells and in breast cancer cell lines demonstrates that progression through early mitosis is dependent on PAPPA function, and that breast cancer cells become more invasive after down-regulation of this protease. PAPPA regulates mitotic progression through modulating the IGF-1 signalling pathway resulting in activation of the forkhead transcription factor FoxM1, which drives a transcriptional cluster of essential mitotic genes. Our results show that PAPPA has a critical function in normal cell division and is targeted early in breast cancer development.

SPDEF inhibits prostate carcinogenesis by disrupting a positive feedback loop in regulation of the Foxm1 oncogene

SAM-pointed domain-containing ETS transcription factor (SPDEF) is expressed in normal prostate epithelium. While its expression changes during prostate carcinogenesis (PCa), the role of SPDEF in prostate cancer remains controversial due to the lack of genetic mouse models. In present study, we generated transgenic mice with the loss- or gain-of-function of SPDEF in prostate epithelium to demonstrate that SPDEF functions as tumor suppressor in prostate cancer. Loss of SPDEF increased cancer progression and tumor cell proliferation, whereas over-expression of SPDEF in prostate epithelium inhibited carcinogenesis and reduced tumor cell proliferation in vivo and in vitro. Transgenic over-expression of SPDEF inhibited mRNA and protein levels of Foxm1, a transcription factor critical for tumor cell proliferation, and reduced expression of Foxm1 target genes, including Cdc25b, Cyclin B1, Cyclin A2, Plk-1, AuroraB, CKS1 and Topo2alpha. Deletion of SPDEF in transgenic mice and cultures prostate tumor cells increased expression of Foxm1 and its target genes. Furthermore, an inverse correlation between SPDEF and Foxm1 levels was found in human prostate cancers. The two-gene signature of low SPDEF and high FoxM1 predicted poor survival in prostate cancer patients. Mechanistically, SPDEF bound to, and inhibited transcriptional activity of Foxm1 promoter by interfering with the ability of Foxm1 to activate its own promoter through auto-regulatory site located in the −745/−660 bp Foxm1 promoter region. Re-expression of Foxm1 restored cellular proliferation in the SPDEF-positive cancer cells and rescued progression of SPDEF-positive tumors in mouse prostates. Altogether, SPDEF inhibits prostate carcinogenesis by preventing Foxm1-regulated proliferation of prostate tumor cells. The present study identified novel crosstalk between SPDEF tumor suppressor and Foxm1 oncogene and demonstrated that this crosstalk is required for tumor cell proliferation during progression of prostate cancer in vivo.

Development of prostate cancer is a multistep process that involves the loss of tumor suppressor functions and activation of oncogenes. SPDEF transcription factor is expressed in normal prostate epithelium and its expression changes during prostate carcinogenesis (PCa). Since the role of SPDEF in PCa remains controversial, we generated transgenic mice with loss- and gain-of-function of SPDEF to demonstrate that SPDEF functions as a tumor suppressor in PCa. In animal models, the loss of SPDEF promoted PCa and increased the levels of Foxm1, a well-known oncogenic protein. Overexpression of SPDEF in prostate epithelium decreased PCa and reduced Foxm1 levels. Proliferation defects in SPDEF-containing tumor cells were corrected by re-expression of Foxm1, providing direct evidence that SPDEF inhibits tumor cell proliferation through Foxm1. We further showed that SPDEF directly bound to Foxm1 promoter and prevented its auto-regulatory activation. In prostate cancer patients, the low SPDEF and high Foxm1 were found in most aggressive prostate tumors that were associated with poor prognosis. The combined two-gene signature of low SPDEF and high Foxm1 was a strong predictor of survival in prostate cancer patients. The present study identified novel molecular mechanism of prostate cancer progression, providing a crosstalk between SPDEF tumor suppressor and Foxm1 oncogene.

Activated FoxM1 attenuates streptozotocin-mediated β-cell death

The forkhead box transcription factor FoxM1, a positive regulator of the cell cycle, is required for β-cell mass expansion postnatally, during pregnancy, and after partial pancreatectomy. Up-regulation of full-length FoxM1, however, is unable to stimulate increases in β-cell mass in unstressed mice or after partial pancreatectomy, probably due to the lack of posttranslational activation. We hypothesized that expression of an activated form of FoxM1 could aid in recovery after β-cell injury. We therefore derived transgenic mice that inducibly express an activated version of FoxM1 in β-cells (RIP-rtTA;TetO-hemagglutinin (HA)-Foxm1(Δ)(NRD) mice). This N-terminally truncated form of FoxM1 bypasses 2 posttranslational controls: exposure of the forkhead DNA binding domain and targeted proteasomal degradation. Transgenic mice were subjected to streptozotocin (STZ)-induced β-cell ablation to test whether activated FoxM1 can promote β-cell regeneration. Mice expressing HA-FoxM1(ΔNRD) displayed decreased ad libitum-fed blood glucose and increased β-cell mass. β-Cell proliferation was actually decreased in RIP-rtTA:TetO-HA-Foxm1(NRD) mice compared with that in RIP-rtTA mice 7 days after STZ treatment. Unexpectedly, β-cell death was decreased 2 days after STZ treatment. RNA sequencing analysis indicated that activated FoxM1 alters the expression of extracellular matrix and immune cell gene profiles, which may protect against STZ-mediated death. These studies highlight a previously underappreciated role for FoxM1 in promoting β-cell survival.

Targeting forkhead box transcription factors FOXM1 and FOXO in leukemia (Review)

Deregulation of forkhead box (FOX) proteins has been found in many genetic diseases and malignancies including leukemia. Leukemia is a common neoplastic disease of the blood or bone marrow characterized by the presence of immature leukocytes and is one of the leading causes of death due to cancer. Forkhead transcription factors, FOXM1 and FOXO family members (FOXOs), are important mediators in leukemia development. Aberrant expression of FOXM1 and FOXOs results in leukemogenesis. Usually the expression of FOXM1 is upregulated, whereas the expression of FOXOs is downregulated due to phosphorylation, nuclear exclusion and degradation in leukemia. On the one hand, FOXOs are bona fide tumor suppressors, on the other hand, active FOXOs maintain leukemia stem cells and stimulate drug resistance genes, contributing to leukemogenesis. FOXM1 and FOXOs have been proven to be potential targets for the development of leukemia therapeutics. They are also valuable diagnostic and prognostic markers in leukemia for clinical applications. This review summarizes the present knowledge concerning the molecular mechanisms by which FOXM1 and FOXOs modulate leukemogenesis and leukemia development, the clinical relevance of these FOX proteins in leukemia and related areas that warrant further investigation.

USP22 promotes the G1/S phase transition by upregulating FoxM1 expression via β-catenin nuclear localization and is associated with poor prognosis in stage II pancreatic ductal adenocarcinoma

Ubiquitin-specific protease 22 (USP22), a newly discovered member of ubiquitin hydrolase family, exhibits a critical function in cell cycle progression and tumorigenesis. The forkhead box M1 (FoxM1) transcription factor plays a crucial role in cell proliferation, differentiation and transformation. However, the expression and functions of USP22 in pancreatic ductal adenocarcinoma (PDA) and whether FoxM1 is involved in USP22-mediated cell cycle regulation have not been studied. We examined the expression of USP22 and FoxM1 in 136 stage II PDA tissues by immunohistochemistry. Clinical significance was analyzed by multivariate Cox regression analysis, Kaplan-Meier curves and log-rank test. RT-PCR, western blot analysis, luciferase and immunofluorescence assays were used to investigate the molecular function of USP22 and FoxM1 in PDA fresh tissues and cell lines. USP22 and FoxM1 were significantly upregulated in PDA tissues compared with the paired normal carcinoma-adjacent tissues. A statistical correlation was observed between USP22 and FoxM1 expression. The expression of USP/FoxM1 and co-expression of both factors correlated with tumor size, lymph node metastasis and overall survival. Multivariate Cox regression analysis revealed that the expression of USP22/FoxM1, especially the co-expression of both factors, is an independent, unfavorable prognostic factor. USP22 overexpression is accompanied by an increase in FoxM1 expression and USP22 increases FoxM1 expression to promote G1/S transition and cell proliferation through promoting β-catenin nuclear translocation in PDA cell lines. USP22 promotes the G1/S phase transition by upregulating FoxM1 expression via promoting β-catenin nuclear localization. USP22 and FoxM1 may act as prognostic markers and potential targets for PDA.

SUMOylation of FOXM1B alters its transcriptional activity on regulation of MiR-200 family and JNK1 in MCF7 human breast cancer cells

Transcription factor Forkhead Box Protein M1 (FOXM1) is a well-known master regulator in controlling cell-cycle pathways essential for DNA replication and mitosis, as well as cell proliferation. Among the three major isoforms of FOXM1, FOXM1B is highly associated with tumor growth and metastasis. The activities of FOXM1B are modulated by post-translational modifications (PTMs), such as phosphorylation, but whether it is modified by small ubiquitin-related modifier (SUMO) remains unknown. The aim of the current study was to determine whether FOXM1B is post-translationally modified by SUMO proteins and also to identify SUMOylation of FOXM1B on its target gene transcription activity. Here we report that FOXM1B is clearly defined as a SUMO target protein at the cellular levels. Moreover, a SUMOylation protease, SENP2, significantly decreased SUMOylation of FOXM1B. Notably, FOXM1B is selectively SUMOylated at lysine residue 463. While SUMOylation of FOXM1B is required for full repression of its target genes MiR-200b/c and p21, SUMOylation of FOXM1B is essential for full activation of JNK1 gene. Overall, we provide evidence that FOXM1B is post-translationally modified by SUMO and SUMOylation of FOXM1B plays a functional role in regulation of its target gene activities.

Regulation of a transcription factor network by Cdk1 coordinates late cell cycle gene expression

To maintain genome stability, regulators of chromosome segregation must be expressed in coordination with mitotic events. Expression of these late cell cycle genes is regulated by cyclin-dependent kinase (Cdk1), which phosphorylates a network of conserved transcription factors (TFs). However, the effects of Cdk1 phosphorylation on many key TFs are not known. We find that elimination of Cdk1-mediated phosphorylation of four S-phase TFs decreases expression of many late cell cycle genes, delays mitotic progression, and reduces fitness in budding yeast. Blocking phosphorylation impairs degradation of all four TFs. Consequently, phosphorylation-deficient mutants of the repressors Yox1 and Yhp1 exhibit increased promoter occupancy and decreased expression of their target genes. Interestingly, although phosphorylation of the transcriptional activator Hcm1 on its N-terminus promotes its degradation, phosphorylation on its C-terminus is required for its activity, indicating that Cdk1 both activates and inhibits a single TF. We conclude that Cdk1 promotes gene expression by both activating transcriptional activators and inactivating transcriptional repressors. Furthermore, our data suggest that coordinated regulation of the TF network by Cdk1 is necessary for faithful cell division.

FOXM1 and its oncogenic signaling in pancreatic cancer pathogenesis

Pancreatic cancer is a devastating disease with an overall 5-year survival rate less than 5%. Multiple signaling pathways are implicated in the pathogenesis of pancreatic cancer, such as Wnt/β-catenin, Notch, Hedgehog, hypoxia-inducible factor, signal transducer and activator of transcription, specificity proteins/Krüppel-like factors, and Forkhead box (FOX). Recently, increasing evidence has demonstrated that the transcription factor FOXM1 plays important roles in the initiation, progression, and metastasis of a variety of human tumors, including pancreatic cancer. In this review, we focus on the current understanding of the molecular pathogenesis of pancreatic cancer with a special focus on the function and regulation of FOXM1 and rationale for FOXM1 as a novel molecular target for pancreatic cancer prevention and treatment.

IL-12 and type I interferon prolong the division of activated CD8 T cells by maintaining high-affinity IL-2 signaling in vivo

The signal 3 cytokines interleukin-12 and type I interferon sustain CD8 T cell division by prolonging expression of CD25 in vivo.

TCR ligation and co-stimulation induce cellular division; however, optimal accumulation of effector CD8 T cells requires direct inflammatory signaling by signal 3 cytokines, such as IL-12 or type I IFNs. Although in vitro studies suggest that IL-12/type I IFN may enhance T cell survival or early proliferation, the mechanisms underlying optimal accumulation of CD8 T cells in vivo are unknown. In particular, it is unclear if disparate signal 3 cytokines optimize effector CD8 T cell accumulation by the same mechanism and how these inflammatory cytokines, which are transiently produced early after infection, affect T cell accumulation many days later at the peak of the immune response. Here, we show that transient exposure of CD8 T cells to IL-12 or type I IFN does not promote survival or confer an early proliferative advantage in vivo, but rather sustains surface expression of CD25, the high-affinity IL-2 receptor. This prolongs division of CD8 T cells in response to basal IL-2, through activation of the PI3K pathway and expression of FoxM1, a positive regulator of cell cycle progression genes. Thus, signal 3 cytokines use a common pathway to optimize effector CD8 T cell accumulation through a temporally orchestrated sequence of cytokine signals that sustain division rather than survival.

Foxm1 transcription factor is required for the initiation of lung tumorigenesis by oncogenic Kras(G12D.)

Lung cancer is the leading cause of deaths in cancer patients in the United States. Identification of new molecular targets is clearly needed to improve therapeutic outcomes of this devastating human disease. Activating mutations in K-Ras oncogene and increased expression of FOXM1 protein are associated with poor prognosis in patients with non-small-cell lung cancer. Transgenic expression of activated Kras(G12D) in mouse respiratory epithelium is sufficient to induce lung adenocarcinomas; however, transcriptional mechanisms regulated by K-Ras during the initiation of lung cancer remain poorly understood. Foxm1 transcription factor, a downstream target of K-Ras, stimulates cellular proliferation during embryogenesis, organ repair and tumor growth, but its role in tumor initiation is unknown. In the present study, we used transgenic mice expressing Kras(G12D) under control of Sftpc promoter to demonstrate that Foxm1 was induced in type II epithelial cells before the formation of lung tumors. Conditional deletion of Foxm1 from Kras(G12D)-expressing respiratory epithelium prevented the initiation of lung tumors in vivo. The loss of Foxm1 inhibited expression of K-Ras target genes critical for the nuclear factor-κB (NF-κB) and c-Jun N-terminal kinase (JNK) pathways, including Ikbkb, Nfkb1, Nfkb2, Rela, Jnk1, N-Myc, Pttg1 and Cdkn2a. Transgenic overexpression of activated FOXM1 mutant was sufficient to induce expression of these genes in alveolar type II cells. FOXM1 directly bound to promoter regions of Ikbkb, Nfkb2, N-Myc, Pttg1 and Cdkn2a, indicating that these genes are direct FOXM1 targets. FOXM1 is required for K-Ras-mediated lung tumorigenesis by activating genes critical for the NF-κB and JNK pathways.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

Identification of cell cycle-regulated genes periodically expressed in U2OS cells and their regulation by FOXM1 and E2F transcription factors

Characterization of the cell cycle–regulated transcripts in U2OS cells yielded 1871 unique genes. FOXM1 targets were identified via ChIP-seq, and novel targets in G2/M and S phases were verified using a real-time luciferase assay. ChIP-seq data were used to map cell cycle transcriptional regulators of cell cycle–regulated gene expression in U2OS cells.

We identify the cell cycle–regulated mRNA transcripts genome-wide in the osteosarcoma-derived U2OS cell line. This results in 2140 transcripts mapping to 1871 unique cell cycle–regulated genes that show periodic oscillations across multiple synchronous cell cycles. We identify genomic loci bound by the G2/M transcription factor FOXM1 by chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq) and associate these with cell cycle–regulated genes. FOXM1 is bound to cell cycle–regulated genes with peak expression in both S phase and G2/M phases. We show that ChIP-seq genomic loci are responsive to FOXM1 using a real-time luciferase assay in live cells, showing that FOXM1 strongly activates promoters of G2/M phase genes and weakly activates those induced in S phase. Analysis of ChIP-seq data from a panel of cell cycle transcription factors (E2F1, E2F4, E2F6, and GABPA) from the Encyclopedia of DNA Elements and ChIP-seq data for the DREAM complex finds that a set of core cell cycle genes regulated in both U2OS and HeLa cells are bound by multiple cell cycle transcription factors. These data identify the cell cycle–regulated genes in a second cancer-derived cell line and provide a comprehensive picture of the transcriptional regulatory systems controlling periodic gene expression in the human cell division cycle.

Cdks, cyclins and CKIs: roles beyond cell cycle regulation

Cyclin-dependent kinases (Cdks) are serine/threonine kinases and their catalytic activities are modulated by interactions with cyclins and Cdk inhibitors (CKIs). Close cooperation between this trio is necessary for ensuring orderly progression through the cell cycle. In addition to their well-established function in cell cycle control, it is becoming increasingly apparent that mammalian Cdks, cyclins and CKIs play indispensable roles in processes such as transcription, epigenetic regulation, metabolism, stem cell self-renewal, neuronal functions and spermatogenesis. Even more remarkably, they can accomplish some of these tasks individually, without the need for Cdk/cyclin complex formation or kinase activity. In this Review, we discuss the latest revelations about Cdks, cyclins and CKIs with the goal of showcasing their functional diversity beyond cell cycle regulation and their impact on development and disease in mammals.

ROS inhibitor N-acetyl-L-cysteine antagonizes the activity of proteasome inhibitors

NAC (N-acetyl-L-cysteine) is commonly used to identify and test ROS (reactive oxygen species) inducers, and to inhibit ROS. In the present study, we identified inhibition of proteasome inhibitors as a novel activity of NAC. Both NAC and catalase, another known scavenger of ROS, similarly inhibited ROS levels and apoptosis associated with H₂O₂. However, only NAC, and not catalase or another ROS scavenger Trolox, was able to prevent effects linked to proteasome inhibition, such as protein stabilization, apoptosis and accumulation of ubiquitin conjugates. These observations suggest that NAC has a dual activity as an inhibitor of ROS and proteasome inhibitors. Recently, NAC was used as a ROS inhibitor to functionally characterize a novel anticancer compound, piperlongumine, leading to its description as a ROS inducer. In contrast, our own experiments showed that this compound depicts features of proteasome inhibitors including suppression of FOXM1 (Forkhead box protein M1), stabilization of cellular proteins, induction of ROS-independent apoptosis and enhanced accumulation of ubiquitin conjugates. In addition, NAC, but not catalase or Trolox, interfered with the activity of piperlongumine, further supporting that piperlongumine is a proteasome inhibitor. Most importantly, we showed that NAC, but not other ROS scavengers, directly binds to proteasome inhibitors. To our knowledge, NAC is the first known compound that directly interacts with and antagonizes the activity of proteasome inhibitors. Taken together, the findings of the present study suggest that, as a result of the dual nature of NAC, data interpretation might not be straightforward when NAC is utilized as an antioxidant to demonstrate ROS involvement in drug-induced apoptosis.

Forkhead box proteins: tuning forks for transcriptional harmony

Forkhead box (FOX) proteins are multifaceted transcription factors that are responsible for fine-tuning the spatial and temporal expression of a broad range of genes both during development and in adult tissues. This function is engrained in their ability to integrate a multitude of cellular and environmental signals and to act with remarkable fidelity. Several key members of the FOXA, FOXC, FOXM, FOXO and FOXP subfamilies are strongly implicated in cancer, driving initiation, maintenance, progression and drug resistance. The functional complexities of FOX proteins are coming to light and have established these transcription factors as possible therapeutic targets and putative biomarkers for specific cancers.

Foxm1 expression in prostate epithelial cells is essential for prostate carcinogenesis

The treatment of advanced prostate cancer (PCa) remains a challenge. Identification of new molecular mechanisms that regulate PCa initiation and progression would provide targets for the development of new cancer treatments. The Foxm1 transcription factor is highly up-regulated in tumor cells, inflammatory cells, and cells of tumor microenvironment. However, its functions in different cell populations of PCa lesions are unknown. To determine the role of Foxm1 in tumor cells during PCa development, we generated two novel transgenic mouse models, one exhibiting Foxm1 gain-of-function and one exhibiting Foxm1 loss-of-function under control of the prostate epithelial-specific Probasin promoter. In the transgenic adenocarcinoma mouse prostate (TRAMP) model of PCa that uses SV40 large T antigen to induce PCa, loss of Foxm1 decreased tumor growth and metastasis. Decreased prostate tumorigenesis was associated with a decrease in tumor cell proliferation and the down-regulation of genes critical for cell proliferation and tumor metastasis, including Cdc25b, Cyclin B1, Plk-1, Lox, and Versican. In addition, tumor-associated angiogenesis was decreased, coinciding with reduced Vegf-A expression. The mRNA and protein levels of 11β-Hsd2, an enzyme playing an important role in tumor cell proliferation, were down-regulated in Foxm1-deficient PCa tumors in vivo and in Foxm1-depleted TRAMP C2 cells in vitro. Foxm1 bound to, and increased transcriptional activity of, the mouse 11β-Hsd2 promoter through the -892/-879 region, indicating that 11β-Hsd2 was a direct transcriptional target of Foxm1. Without TRAMP, overexpression of Foxm1 either alone or in combination with inhibition of a p19(ARF) tumor suppressor caused a robust epithelial hyperplasia, but was insufficient to induce progression from hyperplasia to PCa. Foxm1 expression in prostate epithelial cells is critical for prostate carcinogenesis, suggesting that inhibition of Foxm1 is a promising therapeutic approach for prostate cancer chemotherapy.

Combination of oxidative stress and FOXM1 inhibitors induces apoptosis in cancer cells and inhibits xenograft tumor growth

Tumor cells accumulate high level of reactive oxygen species (ROS) because they are metabolically more active than normal cells. Elevated ROS levels increase tumorigenecity but also render cancer cells more vulnerable to oxidative stress than normal cells. The oncogenic transcription factor Forkhead Box M1 (FOXM1), which is overexpressed in a wide range of human cancers, was reported to protect cancer cells from the adverse effects of oxidative stress by up regulating the expression of scavenger enzymes. We therefore hypothesized that the combination of FOXM1 ablation and ROS inducers could selectively eradicate cancer cells. We show that RNA interference-mediated knockdown of FOXM1 further elevates intracellular ROS levels and increases sensitivity of cancer cells to ROS-mediated cell death after treatment with ROS inducers. We also demonstrate that the combination of ROS inducers with FOXM1/proteasome inhibitors induces robust apoptosis in different human cancer cells. In addition, we show evidence that FOXM1/proteasome inhibitor bortezomib in combination with the ROS inducer β-phenylethyl isothiocyanate efficiently inhibits the growth of breast tumor xenografts in nude mice. We conclude that the combination of ROS inducers and FOXM1 inhibitors could be used as a therapeutic strategy to selectively eliminate cancer cells.

Transcription factor interactions mediate EGF-dependent COX-2 expression

Cyclooxygenase-2 (COX-2) is linked to poor prognosis in patients with malignant gliomas. Amplification/overexpression of epidermal growth factor receptor (EGFR) is commonly seen in these tumors. EGFR signaling, through activation of the p38-MAPK/PKC-δ/Sp1 cascade, plays an essential role in the regulation of COX-2 expression in glioma cells. Here, we report that Src kinase contributes upstream to this signaling cascade. In addition, more detailed analysis revealed the involvement of FOXM1, a member of the forkhead box family of transcriptional activators, in EGF-dependent COX-2 induction. FOXM1 protein increased after stimulation with EGF, although its role in modulating COX-2 expression does not depend on this increase. While a conventional FOXM1 responsive element resides in a distal region (-2872/-2539 relative to the transcriptional start site) of the COX-2 promoter, this is not required for EGF-dependent induction of COX-2. Instead, FOXM1 forms a cooperative interaction with Sp1 at the Sp1-binding site (-245/-240 relative to the start site) of the COX-2 promoter to mediate EGF-induced COX-2 expression. Definition of this novel interaction provides a clearer understanding of the mechanistic basis for EGF induction of COX-2.

IMPLICATIONS

These data provide a guide for the evaluation of potential newer therapeutic targets that have relevance in this disease.

Mechanisms of activin-stimulated FSH synthesis: the story of a pig and a FOX

Activins were discovered and, in fact, named more than a quarter century ago based on their abilities to stimulate pituitary follicle-stimulating hormone (FSH) synthesis and secretion. However, it is only in the last decade that we have finally come to understand their underlying mechanisms of action in gonadotroph cells. In this minireview, we chronicle the research that led to the recent discovery of forkhead box L2 (FOXL2) as an essential mediator of activin-regulated FSH beta subunit (Fshb) transcription in vitro and in vivo.

Correlation profiling for determining kinase-substrate relationships

Arguably the most important issue in phosphorylation studies is the determination of kinase-substrate relationships. Kinase Activity-Abundance Correlation (KAAC) profiling is a technique that can be used to suggest the kinase responsible for a particular phosphorylation event. The method involves the separation of a lysate at the protein level, after which fractions are analyzed to produce an activity profile for each substrate peptide. This activity profile is correlated with the kinase abundance profiles (obtained using shotgun proteomics) to obtain candidate kinases responsible for phosphorylation of the substrate peptide. We demonstrate this approach by determining potential kinase-substrate pairs for six peptides, selected from the literature, that were shown to be upregulated during mitosis. Finally, for a subset of fractions we evaluated the use of stable isotope labeling to improve quantification.

Genome-wide mapping of FOXM1 binding reveals co-binding with estrogen receptor alpha in breast cancer cells

BACKGROUND

The forkhead transcription factor FOXM1 is a key regulator of the cell cycle. It is frequently over-expressed in cancer and is emerging as an important therapeutic target. In breast cancer FOXM1 expression is linked with estrogen receptor (ERα) activity and resistance to endocrine therapies, with high levels correlated with poor prognosis. However, the precise role of FOXM1 in ER positive breast cancer is not yet fully understood.

RESULTS

The study utilizes chromatin immunoprecipitation followed by high-throughput sequencing to map FOXM1 binding in both ERα-positive and -negative breast cancer cell lines. The comparison between binding site distributions in the two cell lines uncovered a previously undescribed relationship between binding of FOXM1 and ERα. Further molecular analyses demonstrated that these two factors can bind simultaneously at genomic sites and furthermore that FOXM1 regulates the transcriptional activity of ERα via interaction with the coactivator CARM1. Inhibition of FOXM1 activity using the natural product thiostrepton revealed down-regulation of a set of FOXM1-regulated genes that are correlated with patient outcome in clinical breast cancer samples.

CONCLUSIONS

These findings reveal a novel role for FOXM1 in ERα transcriptional activity in breast cancer and uncover a FOXM1-regulated gene signature associated with ER-positive breast cancer patient prognosis.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

Novel interactions between FOXM1 and CDC25A regulate the cell cycle

FOXM1 is a critical regulator of the G1/S and G2/M cell cycle transitions, as well as of the mitotic spindle assembly. Previous studies have suggested that FOXM1 regulates CDC25A gene transcription, but the mechanism remains unknown. Here, we provide evidence that FOXM1 directly regulates CDC25A gene transcription via direct promoter binding and indirect activation of E2F-dependent pathways. Prior literature reported that CDC25B and CDC25C activate CDK1/cyclinB complexes in order to enable phosphorylation of FOXM1. It was unknown if CDC25A functions in a similar manner. We report that FOXM1 transcriptional activity is synergistically enhanced when co-expressed with CDC25A. The increase is dependent upon CDK1 phosphorylation of FOXM1 at T600, T611 and T620 residues. We also report a novel protein interaction between FOXM1 and CDC25A via the C-terminus of FOXM1. We demonstrate that the phosphorylation of Thr 600 and Thr 611 residues of FOXM1 enhanced this interaction, and that the interaction is dependent upon CDC25A phosphatase activity. Our work provides novel insight into the underlying mechanisms by which FOXM1 controls the cell cycle through its association with CDC25A.

Transgenic expression of FoxM1 promotes endothelial repair following lung injury induced by polymicrobial sepsis in mice

Enhancing endothelial barrier integrity for the treatment of acute lung injury (ALI) is an emerging novel therapeutic strategy. Our previous studies have demonstrated the essential role of FoxM1 in mediating endothelial regeneration and barrier repair following lipopolysaccharide-induced lung injury. However, it remains unclear whether FoxM1 expression is sufficient to promote endothelial repair in experimental models of sepsis. Here, employing the FoxM1 transgenic (FoxM1 Tg) mice, we showed that transgenic expression of FoxM1 promoted rapid recovery of endothelial barrier function and survival in a clinically relevant model of sepsis induced by cecal ligation and puncture (CLP). We observed lung vascular permeability was rapidly recovered and returned to levels similar to baseline at 48 h post-CLP challenge in FoxM1 Tg mice whereas it remained markedly elevated in WT mice. Lung edema and inflammation were resolved only in FoxM1 Tg mice at 24 h post-CLP. 5-bromo-2-deoxyuridine incorporation assay revealed a drastic induction of endothelial proliferation in FoxM1 Tg lungs at 24h post-CLP, correlating with early induction of expression of FoxM1 target genes essential for cell cycle progression. Additionally, deletion of FoxM1 in endothelial cells, employing the mouse model with endothelial cell-restricted disruption of FoxM1 (FoxM1 CKO) resulted in impaired endothelial repair following CLP challenge. Together, these data suggest FoxM1 expression in endothelial cells is necessary and sufficient to mediate endothelial repair and thereby promote survival following sepsis challenge.

FOXM1 promotes allergen-induced goblet cell metaplasia and pulmonary inflammation

Chronic airway disorders, including chronic obstructive pulmonary disease (COPD), cystic fibrosis, and asthma, are associated with persistent pulmonary inflammation and goblet cell metaplasia and contribute to significant morbidity and mortality worldwide. While the molecular pathogenesis of these disorders is actively studied, little is known regarding the transcriptional control of goblet cell differentiation and mucus hyperproduction. Herein, we demonstrated that pulmonary allergen sensitization induces expression of FOXM1 transcription factor in airway epithelial and inflammatory cells. Conditional deletion of the Foxm1 gene from either airway epithelium or myeloid inflammatory cells decreased goblet cell metaplasia, reduced lung inflammation, and decreased airway resistance in response to house dust mite allergen (HDM). FOXM1 induced goblet cell metaplasia and Muc5AC expression through the transcriptional activation of Spdef. FOXM1 deletion reduced expression of CCL11, CCL24, and the chemokine receptors CCR2 and CX3CR1, resulting in decreased recruitment of eosinophils and macrophages to the lung. Deletion of FOXM1 from dendritic cells impaired the uptake of HDM antigens and decreased cell surface expression of major histocompatibility complex II (MHC II) and costimulatory molecule CD86, decreasing production of Th2 cytokines by activated T cells. Finally, pharmacological inhibition of FOXM1 by ARF peptide prevented HDM-mediated pulmonary responses. FOXM1 regulates genes critical for allergen-induced lung inflammation and goblet cell metaplasia.

The FOXM1-PLK1 axis is commonly upregulated in oesophageal adenocarcinoma

BACKGROUND

The transcription factor FOXM1 is an important regulator of the cell cycle through controlling periodic gene expression during the G2 and M phases. One key target for FOXM1 is the gene encoding the protein kinase PLK1 and PLK1 itself acts in a positive feedback loop to phosphorylate and activate FOXM1. Both FOXM1 and PLK1 have been shown to be overexpressed in a variety of different tumour types.

METHODS

We have used a combination of RT-PCR, western blotting, tissue microarrays and metadata analysis of microarray data to study whether the FOXM1-PLK1 regulatory axis is upregulated and operational in oesophageal adenocarcinoma.

RESULTS

FOXM1 and PLK1 are expressed in oesophageal adenocarcinoma-derived cell lines and demonstrate cross-regulatory interactions. Importantly, we also demonstrate the concomitant overexpression of FOXM1 and PLK1 in a large proportion of oesophageal adenocarcinoma samples. This co-association was extended to the additional FOXM1 target genes CCNB1, AURKB and CKS1. In a cohort of patients who subsequently underwent surgery, the expression of several FOXM1 target genes was prognostic for overall survival.

CONCLUSIONS

FOXM1 and its target gene PLK1 are commonly overexpressed in oesophageal adenocarcinomas and this association can be extended to other FOXM1 target genes, providing potentially important biomarkers for predicting post-surgery disease survival.

The forkhead transcription factor FOXM1 controls cell cycle-dependent gene expression through an atypical chromatin binding mechanism

There are nearly 50 forkhead (FOX) transcription factors encoded in the human genome and, due to sharing a common DNA binding domain, they are all thought to bind to similar DNA sequences. It is therefore unclear how these transcription factors are targeted to specific chromatin regions to elicit specific biological effects. Here, we used chromatin immunoprecipitation followed by sequencing (ChIP-seq) to investigate the genome-wide chromatin binding mechanisms used by the forkhead transcription factor FOXM1. In keeping with its previous association with cell cycle control, we demonstrate that FOXM1 binds and regulates a group of genes which are mainly involved in controlling late cell cycle events in the G(2) and M phases. However, rather than being recruited through canonical RYAAAYA forkhead binding motifs, FOXM1 binding is directed via CHR (cell cycle genes homology region) elements. FOXM1 binds these elements through protein-protein interactions with the MMB transcriptional activator complex. Thus, we have uncovered a novel and unexpected mode of chromatin binding of a FOX transcription factor that allows it to specifically control cell cycle-dependent gene expression.

Binding of FoxM1 to G2/M gene promoters is dependent upon B-Myb

The promoters of genes which regulate entry into and progress through mitosis are typically induced maximally in G2 by transcription factors that include B-Myb and FoxM1. As FoxM1 gene transcription is a target of B-Myb, we investigated in this study how these transcription factors functionally interact to regulate these G2/M genes. Using a 3T3 cell line containing floxed B-myb alleles (B-myb(F/F)) that could be conditionally deleted by Cre recombinase, we confirmed that B-myb knockout caused both decreased mRNA expression of several G2/M genes, including FoxM1, and delayed entry into mitosis. Although FoxM1 protein expression was actually unaffected by B-myb knockout when quiescent B-myb(F/F) 3T3 cells re-entered the cell cycle upon serum-stimulation, chromatin immunoprecipitation revealed that FoxM1 binding to G2/M promoters was substantially reduced. FoxM1 transcriptional activity requires sequential phosphorylation by Cyclin-dependent kinases and Plk1, which are B-Myb target genes, and we found that phosphorylation at Plk1-specific sites was somewhat reduced upon B-myb knockout. Neither this effect nor nuclear accumulation of FoxM1, which was unaffected by B-myb knockout, was sufficient to account for the dependence on B-Myb for FoxM1 promoter binding, however. More significantly, assays using paired Birc5 (survivin) promoter-luciferase reporters with either wild-type or mutated Myb binding sites showed that FoxM1 was unable to bind and activate the promoter in the absence of B-Myb binding. Our data suggest that B-Myb is required as a pioneer factor to enable FoxM1 binding to G2/M gene promoters and explains how these transcription factors may collaborate to induce mitosis.

Foxm1 mediates cross talk between Kras/mitogen-activated protein kinase and canonical Wnt pathways during development of respiratory epithelium

While Kras/mitogen-activated protein kinase (MAPK) and canonical Wnt/β-catenin are critical for lung morphogenesis, mechanisms integrating these important signaling pathways during lung development are unknown. Herein, we demonstrate that the Foxm1 transcription factor is a key downstream target of activated Kras(G12D). Deletion of Foxm1 from respiratory epithelial cells during lung formation prevented structural abnormalities caused by activated Kras(G12D). Kras/Foxm1 signaling inhibited the activity of canonical Wnt signaling in the developing lung in vivo. Foxm1 decreased T-cell factor (TCF) transcriptional activity induced by activated β-catenin in vitro. Depletion of Foxm1 by short interfering RNA (siRNA) increased nuclear localization of β-catenin, increased expression of β-catenin target genes, and decreased mRNA and protein levels of the β-catenin inhibitor Axin2. Axin2 mRNA was reduced in distal lung epithelium of Foxm1-deficient mice. Foxm1 directly bound to and increased transcriptional activity of the Axin2 promoter region. Foxm1 is required for Kras signaling in distal lung epithelium and provides a mechanism integrating Kras and canonical Wnt/β-catenin signaling during lung development.

The p38 MAPK-MK2 axis regulates E2F1 and FOXM1 expression after epirubicin treatment

E2F1 is responsible for the regulation of FOXM1 expression, which plays a key role in epirubicin resistance. Here, we examined the role and regulation of E2F1 in response to epirubicin in cancer cells. We first showed that E2F1 plays a key role in promoting FOXM1 expression, cell survival, and epirubicin resistance as its depletion by siRNA attenuated FOXM1 induction and cell viability in response to epirubicin. We also found that the p38-MAPK activity mirrors the expression patterns of E2F1 and FOXM1 in both epirubicin-sensitive and -resistant MCF-7 breast cancer cells, suggesting that p38 has a role in regulating E2F1 expression and epirubicin resistance. Consistently, studies using pharmacologic inhibitors, siRNA knockdown, and knockout mouse embryonic fibroblasts (MEF) revealed that p38 mediates the E2F1 induction by epirubicin and that the induction of E2F1 by p38 is, in turn, mediated through its downstream kinase MK2 [mitogen-activated protein kinase (MAPK)-activated protein kinase 2; MAPKAPK2]. In agreement, in vitro phosphorylation assays showed that MK2 can directly phosphorylate E2F1 at Ser-364. Transfection assays also showed that E2F1 phosphorylation at Ser-364 participates in its induction by epirubicin but also suggests that other phosphorylation events are also involved. In addition, the p38-MK2 axis can also limit c-jun-NH(2)-kinase (JNK) induction by epirubicin and, notably, JNK represses FOXM1 expression. Collectively, these findings underscore the importance of p38-MK2 signaling in the control of E2F1 and FOXM1 expression as well as epirubicin sensitivity.

FoxM1 regulates mammary luminal cell fate

Elevated expression of FoxM1 in breast cancer correlates with an undifferentiated tumor phenotype and a negative clinical outcome. However, a role for FoxM1 in regulating mammary differentiation was not known. Here, we identify another function of FoxM1, the ability to act as a transcriptional repressor, which plays an important role in regulating the differentiation of luminal epithelial progenitors. Regeneration of mammary glands with elevated levels of FoxM1 leads to aberrant ductal morphology and expansion of the luminal progenitor pool. Conversely, knockdown of FoxM1 results in a shift toward the differentiated state. FoxM1 mediates these effects by repressing the key regulator of luminal differentiation, GATA-3. Through association with DNMT3b, FoxM1 promotes methylation of the GATA-3 promoter in an Rb-dependent manner. This study identifies FoxM1 as a critical regulator of mammary differentiation with significant implications for the development of aggressive breast cancers.

Peroxiredoxin 3 is a redox-dependent target of thiostrepton in malignant mesothelioma cells

Thiostrepton (TS) is a thiazole antibiotic that inhibits expression of FOXM1, an oncogenic transcription factor required for cell cycle progression and resistance to oncogene-induced oxidative stress. The mechanism of action of TS is unclear and strategies that enhance TS activity will improve its therapeutic potential. Analysis of human tumor specimens showed FOXM1 is broadly expressed in malignant mesothelioma (MM), an intractable tumor associated with asbestos exposure. The mechanism of action of TS was investigated in a cell culture model of human MM. As for other tumor cell types, TS inhibited expression of FOXM1 in MM cells in a dose-dependent manner. Suppression of FOXM1 expression and coincidental activation of ERK1/2 by TS were abrogated by pre-incubation of cells with the antioxidant N-acetyl-L-cysteine (NAC), indicating its mechanism of action in MM cells is redox-dependent. Examination of the mitochondrial thioredoxin reductase 2 (TR2)-thioredoxin 2 (TRX2)-peroxiredoxin 3 (PRX3) antioxidant network revealed that TS modifies the electrophoretic mobility of PRX3. Incubation of recombinant human PRX3 with TS in vitro also resulted in PRX3 with altered electrophoretic mobility. The cellular and recombinant species of modified PRX3 were resistant to dithiothreitol and SDS and suppressed by NAC, indicating that TS covalently adducts cysteine residues in PRX3. Reduction of endogenous mitochondrial TRX2 levels by the cationic triphenylmethane gentian violet (GV) promoted modification of PRX3 by TS and significantly enhanced its cytotoxic activity. Our results indicate TS covalently adducts PRX3, thereby disabling a major mitochondrial antioxidant network that counters chronic mitochondrial oxidative stress. Redox-active compounds like GV that modify the TR2/TRX2 network may significantly enhance the efficacy of TS, thereby providing a combinatorial approach for exploiting redox-dependent perturbations in mitochondrial function as a therapeutic approach in mesothelioma.

Multiple modes of chromatin remodeling by Forkhead box proteins

Forkhead box (FOX) proteins represent a large family of transcriptional regulators unified by their DNA binding domain (DBD) known as a 'forkhead' or 'winged helix' domain. Over 40 FOX genes have been identified in the mammalian genome. FOX proteins share significant sequence similarities in the DBD which allow them to bind to a consensus DNA response element. However, their modes of action are quite diverse as they regulate gene expression by acting as pioneer factors, transcription factors, or both. This review focuses on the mechanisms of chromatin remodeling with an emphasis on three sub-classes-FOXA, FOXO, and FOXP members. FOXA proteins serve as pioneer factors to open up local chromatin structure and thereby increase accessibility of chromatin to factors regulating transcription. FOXP proteins, in contrast, function as classic transcription factors to recruit a variety of chromatin modifying enzymes to regulate gene expression. FOXO proteins represent a hybrid subclass having dual roles as pioneering factors and transcription factors. A subset of FOX proteins interacts with condensed mitotic chromatin and may function as 'bookmarking' agents to maintain transcriptional competence at specific genomic sites. The overall diversity in chromatin remodeling function by FOX proteins is related to unique structural motifs present within the DBD flanking regions that govern selective interactions with core histones and/or chromatin coregulatory proteins. This article is part of a Special Issue entitled: Chromatin in time and space.

Suppression of FOXM1 sensitizes human cancer cells to cell death induced by DNA-damage

Irradiation and DNA-damaging chemotherapeutic agents are commonly used in anticancer treatments. Following DNA damage FOXM1 protein levels are often elevated. In this study, we sought to investigate the potential role of FOXM1 in programmed cell death induced by DNA-damage. Human cancer cells after FOXM1 suppression were subjected to doxorubicin or γ-irradiation treatment. Our findings indicate that FOXM1 downregulation by stable or transient knockdown using RNAi or by treatment with proteasome inhibitors that target FOXM1 strongly sensitized human cancer cells of different origin to DNA-damage-induced apoptosis. We showed that FOXM1 suppresses the activation of pro-apoptotic JNK and positively regulates anti-apoptotic Bcl-2, suggesting that JNK activation and Bcl-2 down-regulation could mediate sensitivity to DNA-damaging agent-induced apoptosis after targeting FOXM1. Since FOXM1 is widely expressed in human cancers, our data further support the fact that it is a valid target for combinatorial anticancer therapy.

Transcription factor STOX1A promotes mitotic entry by binding to the CCNB1 promotor

Background

In this study we investigated the involvement of the transcription factor STOX1A in the regulation of the cell cycle.

Methodology/Principal Findings

We found that several major cell cycle regulatory genes were differentially expressed upon STOX1A stimulation and knockdown in the neuroblastoma cell line SH-SY5Y. This includes STOX1A dependent differential regulation of cyclin B1 expression, a cyclin which is known to regulate mitotic entry during the cell cycle. The differential regulation of cyclin B1 expression by STOX1A is direct as shown with chromatin immunoprecipitation. Results furthermore suggest that mitotic entry is enhanced through the direct upregulation of cyclin B1 expression effectuated by STOX1A.

Conclusions

In conclusion we hereby show that STOX1A is directly involved in the regulation of the cell cycle.

Nucleophosmin interacts with FOXM1 and modulates the level and localization of FOXM1 in human cancer cells

Using mass spectrometric analysis we found that oncogenic transcription factor FOXM1 that is overexpressed in a majority of human cancers interacts with multifunctional protein NPM, which is also overexpressed in a variety of human tumors. Coimmunoprecipitation and glutathione S-transferase pull-down experiments demonstrated that NPM forms a complex with FOXM1 and also identified the regions responsible for their interaction. Immunofluorescence microscopy confirmed the interaction between FOXM1 and NPM in cancer and immortal cells. Furthermore, knockdown of NPM in immortal and cancer cells led to significant down-regulation of FOXM1 similar to its levels in normal cells, suggesting that NPM might modulate FOXM1 level. In addition, in OCI/AML3 leukemia cells where mutant NPM is localized in the cytoplasm we found that typically nuclear FOXM1 was predominantly co-localized with NPM in the cytoplasm, while NPM knockdown led to the disappearance of FOXM1 from the cytoplasm, suggesting that NPM may also determine intracellular localization of FOXM1. Knockdown of FOXM1 or NPM in MIA PaCa-2 pancreatic cancer cells inhibited anchorage-dependent and independent growth in cell culture, and tumor growth in nude mice. In addition, over-expression of FOXM1 reversed the effect of NPM knockdown in vitro. Our data suggest that in cancer cells NPM interacts with FOXM1 and their interaction is required for sustaining the level and localization of FOXM1. Targeting the interaction between FOXM1 and NPM by peptides or small molecules may represent a novel therapeutic strategy against cancer.

A systematic screen for CDK4/6 substrates links FOXM1 phosphorylation to senescence suppression in cancer cells

Cyclin D-dependent kinases (CDK4 and CDK6) are positive regulators of cell cycle entry and they are overactive in the majority of human cancers. However, it is currently not completely understood by which cellular mechanisms CDK4/6 promote tumorigenesis, largely due to the limited number of identified substrates. Here we performed a systematic screen for substrates of cyclin D1-CDK4 and cyclin D3-CDK6. We identified the Forkhead Box M1 (FOXM1) transcription factor as a common critical phosphorylation target. CDK4/6 stabilize and activate FOXM1, thereby maintain expression of G1/S phase genes, suppress the levels of reactive oxygen species (ROS), and protect cancer cells from senescence. Melanoma cells, unlike melanocytes, are highly reliant on CDK4/6-mediated senescence suppression, which makes them particularly susceptible to CDK4/6 inhibition.

FOXM1: From cancer initiation to progression and treatment

The Forkhead box protein M1 (FOXM1) transcription factor is a regulator of myriad biological processes, including cell proliferation, cell cycle progression, cell differentiation, DNA damage repair, tissue homeostasis, angiogenesis and apoptosis. Elevated FOXM1 expression is found in cancers of the liver, prostate, brain, breast, lung, colon, pancreas, skin, cervix, ovary, mouth, blood and nervous system, suggesting it has an integral role in tumorigenesis. Recent research findings also place FOXM1 at the centre of cancer progression and drug sensitivity. In this review the involvement of FOXM1 in various aspects of cancer, in particular its role and regulation within the context of cancer initiation, progression, and cancer drug response, will be summarised and discussed.

Protein phosphatase 2A (B55α) prevents premature activation of forkhead transcription factor FoxM1 by antagonizing cyclin A/cyclin-dependent kinase-mediated phosphorylation

The forkhead transcription factor FoxM1 controls expression of a large number of genes that are specifically expressed during the G(2) phase of the cell cycle. Throughout most of the cell cycle, FoxM1 activity is restrained by an autoinhibitory mechanism, involving a repressor domain present in the N-terminal part of the protein. Activation of FoxM1 in G(2) is achieved by Cyclin A/Cyclin-dependent kinase (Cdk)-mediated phosphorylation, which alleviates autoinhibition by the N-terminal repressor domain. Here, we show that FoxM1 interacts with B55α, a regulatory subunit of protein phosphatase 2A (PP2A). B55α binds the catalytic subunit of PP2A, and this promotes dephosphorylation and inactivation of FoxM1. Indeed, we find that overexpression of B55α results in decreased FoxM1 activity. Inversely, depletion of B55α results in premature activation of FoxM1. The activation of FoxM1 that is observed upon depletion of B55α is fully dependent on Cyclin A/Cdk-mediated phosphorylation of FoxM1. Taken together, these data demonstrate that B55α acts to antagonize Cyclin A/Cdk-dependent activation of FoxM1, to ensure that FoxM1 activity is restricted to the G(2) phase of the cell cycle.

Down-regulation of Notch-1 is associated with Akt and FoxM1 in inducing cell growth inhibition and apoptosis in prostate cancer cells

Although many studies have been done to uncover the mechanisms by which down-regulation of Notch-1 exerts its anti-tumor activity against a variety of human malignancies, the precise molecular mechanisms remain unclear. In the present study, we investigated the cellular consequence of Notch-1 down-regulation and also assessed the molecular consequence of Notch-1-mediated alterations of its downstream targets on cell viability and apoptosis in prostate cancer (PCa) cells. We found that the down-regulation of Notch-1 led to the inhibition of cell growth and induction of apoptosis, which was mechanistically linked with down-regulation of Akt and FoxM1, suggesting for the first time that Akt and FoxM1 are downstream targets of Notch-1 signaling. Moreover, we found that a "natural agent" (genistein) originally discovered from soybean could cause significant reduction in cell viability and induced apoptosis of PCa cells, which was consistent with down-regulation of Notch-1, Akt, and FoxM1. These results suggest that down-regulation of Notch-1 by novel agents could become a newer approach for the prevention of tumor progression and/or treatment, which is likely to be mediated via inactivation of Akt and FoxM1 signaling pathways in PCa.