Background

Forkhead box M1 (FOXM1), previously named HNF-3, HFH-11 or Trident, is a transcription factor of the Forkhead box (Fox) protein superfamily which is defined by a conserved winged helix DNA-binding domain1 [1]. The human FOXM1 gene consists of 10 exons, of which exons Va and VIIa can be alternatively spliced [2]. In the past, VIIa was treated as a repressor until a novel isoform (FOXM1d) that could promote the epithelial–mesenchymal transition (EMT) and metastasis by activating ROCKs in colorectal cancer was identified [3, 4]. Accordingly, there are four isoforms of human FOXM1 identified to date (Fig. 1a). FOXM1a contains both exons Va and VIIa and lacks transactivation activity, while the rest of the three, FOXM1b (which contains neither exon Va nor VIIa), FOXM1c (no VIIa) and FOXM1d (no Va) are transcriptionally active. The FOXM1 protein contains a conserved forkhead DNA-binding domain (DBD), an N-terminal repressor domain (NRD), and a C-terminal transactivation domain (TAD). The transactivation activity of TAD can be suppressed by direct interaction with the NRD [5, 6]. In addition, murine FOXM1 splice variants display the same DNA-binding specificity as human FOXM1 and bind to DNA-binding sites with the consensus sequence 5′-A-C/T-AAA-C/ T-AA-3’ [7]. The study of murine FOXM1 may also be applied to human FOXM1. For example, it has been demonstrated that Gli1 regulates FOXM1 in murine stem cells [8]. In human basal cell carcinomas and colorectal cancer cells, FOXM1 is also a direct target of Gli1 [9, 10]. This may be due to the evolutionary conservation between the DNA binding domain of both human and murine FOXM1, suggesting the FOXM1 of the two species may share target genes. As such, investigating the regulation of murine FOXM1 may provide significant implications into the dysregulation of human FOXM1. Furthermore, a mouse model is a suitable experimental model for the development of novel FOXM1 inhibitors.

Fig. 1
figure 1

Genomic structure and coding isoforms in the FOXM1 gene and transcription factor binding sites in FOXM1 promoter region. a. Genomic structure and coding isoforms of the FOXM1 gene. b. Schematic diagram of transcription factor binding sites during FOXM1 promoter region. Green: activator, red: repressor, purple: cis-acting element could be bound by both activator and repressor

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FOXM1 is detected primarily in progenitor and regenerating tissues, as well as tumor cells, which are all highly proliferative [11]. As a classic proliferation-associated transcription factor, FOXM1 directly or indirectly activates the expression of target genes at the transcriptional level and exhibits a spatiotemporal pattern whose dysregulation is involved in almost all hallmarks of tumor cells [3, 12]. Increased expression of FOXM1 is observed in a variety of human cancers, such as ovarian cancer, breast cancer, prostate cancer, hepatoma, angiosarcoma, colorectal cancer, melanoma, lung cancer, and gastric cancer [13,14,15,16,17,18,19,20,21], which is consistent with the results obtained from the TCGA database (Fig. 2). Inhibition of FOXM1 in cancer cells leads to decreased cell proliferation and migration, metastasis, angiogenesis, EMT, and drug resistance [22,23,24,25,26]. Furthermore, a recent meta-analysis revealed that elevated FOXM1 expression is related to poor prognosis in most solid tumors [27], which is also further confirmed by the TCGA database (Fig. 3) [28]. These results clearly showed the important role of FOXM1 in tumorigenesis and cancer development. Therefore, FOXM1 has been identified as a potential therapeutic target for the treatment of cancers. Although a few drugs and inhibitors have been shown to be effective at inhibiting the activity of FOXM1 in vitro, they’ve yet to pass successfully into clinical use [29]. This is likely due in major part to the poor current understanding of the regulation of FOXM1. Hence, a comprehensive review of FOXM1 regulation will thus contribute to the extensive effort and research into the gene as a therapeutic target for a number of FOXM1-dependent conditions, such as the cancers mentioned previously.

Fig. 2
figure 2

FOXM1 expression profile from the TCGA database. The FOXM1 transcript per million are presented in differernt cancers and corresponding normal tissues, including ulterine corpus endometrial carcinoma (a), thyroid carcinoma (b), stomach adenocarcinoma (c), rectum adenocarcinoma (d), prostate adenocarcinoma (e), pheochromocytoma and paraganglioma (f), lung squamous cell carcinoma (g), lung adenocarcinoma (h), kidney renal clear cell carcinoma (i),kidney renal papillary cell carcinoma (j), kidney chromophobe (k),  head and neck squamous cell carcinoma (l), glioblastoma multiforme (m), esophageal carcinoma (n), colon carcinoma (o), cholangiocarcinoma (p), cervical squamous carcinoma (q), breast invasive carcinoma (r), liver hepatocellular carcinoma (s), bladder urothelial carcinoma (t)

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Fig. 3
figure 3

Kaplan–Meier analyses of overall survival according to FOXM1 expression levels in different cancer suffers, including adrenocortical carcinoma (a), glioma (b), colon adencarcinoma (c), kidney chromophobe (d),kidney renal clear cell carcinoma (e),kidney renal papillary cell carcinoma (f),liver hepatocellular carcinoma (g),lung adenocarcinoma (h),ulterine corpus endometrial carcinoma (m),uveal melanoma (n)

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In this review, we provide an overview of how FOXM1 is regulated and focus on the transcriptional, post-transcriptional, post-translational, and protein-protein/RNA interaction levels. Though many biomolecules regulate the expression of FOXM1, we emphasize the biomolecules that directly interact with or modify the promoter, RNA, or protein of FOXM1. At the same time, we will discuss briefly the pharmacological inhibition of FOXM1.

Transcriptional regulation of FOXM1

The core promoter regions of the FOXM1 gene contain several classic regulatory elements, such as E-boxes, and other cis-acting elements that can function as responsive elements to other transcription factors. Here, we mainly discuss the transcription factors that directly bind to the promoter regions (which are verified by electrophoretic mobility shift assays and/or chromatin immunoprecipitation assays) (Fig. 1b).

Most responsive elements are located adjacent to FOXM1’s transcriptional start sites, though the furthest element is found approximately 2000 bp away. More than 75% of these binding sites act as cis-activation elements, but they do not all function through the same basic mechanisms. For example, glioma-associated oncogene homolog 1 (Gli1), CCCTC-binding factor (CTCF), cAMP responsive element-binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), and E2F interact directly with their binding sites and upregulate FOXM1 expression [9, 30,31,32,74].

FOXM1 ubiquitination is also linked to SUMOylation. For example, RNF168 can modulate DNA-damage response (DDR) by promoting protein ubiquitination. In breast cancer treated with epirubicin, FOXM1 is modified through SUMOylation, which leads to its ubiquitination and degradation by RNF168 E3 ubiquitin ligase [75].

In contrast, deubiquitinating enzymes (DUBs) can remove the poly-ubiquitin chains on FOXM1 protein. For instance, in epirubicin-resistant breast cancer and in ovarian cancer, OTUB1 catalyzes the cleavage of the K48-specific ubiquitin linkage from FOXM1, which promotes cancer progression via facilitating cell proliferation and drug resistance [76, 77].

SUMOylation

Small Ubiquitin-like Modifier (SUMO) proteins are small proteins that are covalently attached to other proteins and modify their function. In mammals, there are four SUMO isoforms, SUMO-1, SUMO-2, SUMO-3, and SUMO-4. SUMOylation is an important PTM of FOXM1 that regulates its activity, stability, and other PTMs. The functions of FOXM1 SUMOylation are diverse and may regulate its activity in an isoform-specific manner.

Multiple sites of FOXM1 are modified by SUMO-2, and this modification peaks in the M phase, which is consistent with its phosphorylation. This modification blocks the dimerization and relieves the auto-repression of FOXM1, thereby increasing its transcriptional activity [78]. Another SUMOylation of FOXM1 at K463 by SUMO-1 is also required for its transcriptional activity [79].

In contrast, SUMOlyation of FOXM1 at different sites may inhibit its activity. For instance, FOXM1c is modified by SUMO-1 at multiple sites, which promotes the cytoplasmic translocation of FOXM1c and enhances APC/Cdh1-dependent ubiquitin-mediated degradation. This modification subsequently attenuates the transcriptional activity of FOXM1c [80]. This phenomenon has been confirmed with another FOXM1 isoform-FOXM1b [81].

Other PTMs of FOXM1

FOXM1 is also regulated by acetyltransferases and methyltransferases. For instance, FOXM1 can be acetylated by p300/CBP at lysines K63, K422, K440, K603 and K614, which enhances its transcriptional activity by promoting its DNA binding affinity, protein stability, and phosphorylation sensitivity [103]. Furthermore, latest reports revealed that FOXM1 modulates atherosclerosis by inducing macrophage proliferation [104]. However, little is known about the possible role of FOXM1 in tumor microenvironment. Interestingly, FOXM1 (362–370) (YLVPIQFPV), FOXM1(373–382) (SLVLQPSVKV), and FOXM1(640–649) (GLMDLSTTPL) peptides primed HLA-A2-restricted cytotoxic T lymphocytes (CTLs) in the HLA-A2 transgenic mice, suggesting that FOXM1 may be a suitable target for immunotherapy against cancers. However, the HLAA2-restricted epitopes of FOXM1 identified need to be further clinically tested [105]. As the important role of FOXM1 in cell proliferation and determination of cell fate, more study is needed to reveal the possible role of FOXM1 in the tumor infiltrating immune cells.

Conclusions and future perspectives

FOXM1 is a crucial regulator of many biological processes and tissues, and dysregulation of FOXM1 can significantly contribute to tumorigenesis and cancer progression. For its potential as a target for cancer therapies, FOXM1 was named the Molecule of the Year in 2010 [106]. Over the past few decades, understanding of the regulation and function of FOXM1 has rapidly increased, providing new insights into the roles of this transcription factor in cancer and other diseases. At the same time, some small molecule inhibitors that target FOXM1 have promising potential as drugs for cancer treatment [107, 108]. However, there are important challenges that limit the translation of promising drugs into clinical practice. Before the entry of FOXM1 inhibitors into clinical trials, more thorough preclinical studies on their anti-tumor efficacy are still needed. In addition, the toxicity of the above drugs should also be fully evaluated. It is not quite clear how the interaction and isoforms switch between FOXM1a and FOXM1b or FOXM1c or FOXM1d. It also remains unclear how the isoforms of FOXM1 interact and what role they may play in the regulation of FOXM1, in disease progression, or in response to relevant therapeutic strategies. Importantly, although the crystal structure of the FOXM1 DNA-recognition domain has been fully identified [7], it is vital that the complete structure of the FOXM1 protein be elucidated. This will be of utmost importance for the discovery of novel FOXM1 inhibitors.

In this review, we have summarized many of the activators and repressors that directly interact with or modify FOXM1 at multiple levels and drew the FOXM1-interaction network diagram (Fig. 4). The comprehensive understanding of the regulation of FOXM1 will provide a basis for further investigation, which may provide new potential therapeutic strategies.

Fig. 4
figure 4

Schematic diagram of FOXM1 regulation network. This schematic diagram attempts to address the regulation of FOXM1 in a spherical way. Green: promotion of FXOM1 expression or transcriptional activity; Red: repression of FOXM1 expression or transcriptional activity. ①.Transcriptional regulation of FOXM1: iTF (inhibitory transcription factor) binds to the promoter region of FOXM1 and inhibits its transcription. aTF (activating transcription factor) binds to the promoter region of FOXM1 and enhances its transcription. ②.Post-transcriptional regulation of FOXM1: m6A methylation of FOXM1 pre-mRNA and the miRNAs binding to 3′UTRs of FOXM1 mRNA and guiding FOXM1 are listed. lncRNAs can act as ceRNAs and block the suppression of FOXM1 by miRNAs. ③. Protein/RNA directly interacts with FOXM1 protein: The interaction of protein/RNA with FOXM1 protein alter the cellular localization, stabilization, or transcriptional activity of FOXM1. ④. Post-translational modifications of FOXM1 protein

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