FOXK2 Transcription Factor and Its Emerging Roles in Cancer

Abstract

Forkhead box (FOX) transcription factors compose a large family of regulators of key biological processes within a cell. FOXK2 is a member of FOX family, whose biological functions remain relatively unexplored, despite its description in the early nineties. More recently, growing evidence has been pointing towards a role of FOXK2 in cancer, which is likely to be context-dependent and tumour-specific. Here, we provide an overview of important aspects concerning the mechanisms of regulation of FOXK2 expression and function, as well as its complex interactions at the chromatin level, which orchestrate how it differentially regulates the expression of gene targets in pathophysiology. Particularly, we explore the emerging functions of FOXK2 as a regulator of a broad range of cancer features, such as cell proliferation and survival, DNA damage, metabolism, migration, invasion and metastasis. Finally, we discuss the prognostic value of assessing FOXK2 expression in cancer patients and how it can be potentially targeted for future anticancer interventions.

Keywords: FOXK2 transcription factor; cancer; forkhead box; transcriptional activity.

Figure 1

The domain structure of FOXK2 protein. FOXK2 is composed of a forkhead-associated domain (FHA) localized towards the amino terminus and a highly conserved forkhead DNA-binding domain (FOX) localized towards the carboxy-terminal end of the protein. The FOX domain displays a nuclear localization signal (NLS) and mediates FOXK2 binding to consensus sequences with a GTAAACA core motif. The post-translational modifications (PTM) for FOXK2 are shown, where serines (S) are targets for phosphorylation by cyclin-dependent kinases (CDK) and lysines (K), for SUMOylation. Amino-acids sites which are targets for PTM are depicted above the scheme. FHRE, forkhead responsive elements.

Figure 2

FOXK2 transcription factor and chromatin-associated events. FOXK2 recruits AP-1 to the chromatin, regulating genes involved in cell-cell contact, motility, cell adhesion, metabolism, apoptosis and cancer. Besides that, FOXK2 interacts with BAP1, a component of PR-DUB complex, leading to local histone deubiquitination and to repression and/or activation of transcription of target genes. In addition to these chromatin-associated events, FOXK2 binds to methylated DNA as well as interacts with MBD6 and PR-DUB complex, recruiting transcriptional complexes. Furthermore, FOXK2 is potentially implicated in DNA repair functions, where it binds to G/T-mismatch regions and interacts with BAP1 deubiquitinase. BAP1, BRCA1-associated protein 1; AP-1, activator protein-1; PR-DUB, polycomb repressive deubiquitinase; MBD6, methyl binding domain protein 6; Me, DNA methylation sites.

Figure 3

FOXK1/2 transcription factors in the control of metabolic processes. FOXK1/2 transcription factors are phosphorylated by mTOR, which translocate them to the nucleus where they interact with the Sin3A complex to suppress genes involved in atrophy and autophagy programs. Moreover, mTOR induces nuclear translocation of FOXK transcription factors through GSK regulation. In this context, FOXK1/2 induces HIF1α gene expression and glucose consumption, contributing to cell proliferation. Additionally, FOXK1/2 regulate aerobic glycolysis while suppressing aerobic oxidation through the upregulation of glycolytic target genes. mTOR, mammalian target of rapamycin; Gsk3, glycogen synthase kinase 3; HIF1α, hypoxia-inducing factor 1 alpha; SIN3A complex, SIN3 Transcription Regulator Family Member A complex; HK2, hexokinase-2; PFKM, phosphofructokinase muscle isoform; ALDOA, aldolase A; PKM, pyruvate kinase M1/2; MCT1, monocarboxylate transporter 1; PDK1/4, pyruvate dehydrogenase kinases 1/4; PDP1, Pyruvate dehydrogenase phosphatase 1; GLUD1, glutamate dehydrogenase 1; P, phosphorylation site.

Figure 4

FOXK2 role as an oncogene in cancer. FOXK2 is transcriptionally regulated by the Sox9 oncogene and promotes proliferation of colorectal cancer cell lines. In this context, FOXK2 interacts with DVL, translocating it to the nucleus, and then promoting Wnt/β-catenin signaling pathway. In agreement, FOXK2 overexpression promotes cell migration and proliferation in hepatocellular carcinoma, which is associated with high levels of Survivin, c-Myc, p27, cyclin D1 and phosphorylated AKT protein expression. A role in cell cycle progression has also been attributed to FOXK2, which is phosphorylated by CDK-cyclin complexes in a cell cycle dependent manner, in a process that induces its degradation and impairs its transcriptional activity following cell division. Box represents associations between FOXK2 and other molecules in studies involving overexpression and inhibition of FOXK2. PI3K, phosphoinositide 3-kinase; AKT, protein kinase B; Cyclin D1, regulator of cell cycle progression; p27, a cell cycle inhibitor; Survivin, a member of the inhibitor of apoptosis proteins (IAPs) family; c-myc, proto-oncogene transcription factor; DVL, Dishevelled; Sox9, the sex-determining region Y box 9; TCF, T-Cell Factor; CDK, cyclin-dependent kinase; CDKN1A, gene encoding p21, a cell cycle inhibitor; P, phosphorylation site.

Figure 5

FOXK2 role as a tumor suppressor in cancer. FOXK transcription factors interact with E1A adenovirus proteins and E6 proteins from human papillomaviruses 21/14 suppressing malignant transformation. In breast cancer, FOXK2 acts as a scaffold protein between ERα and BARD1 leading to ubiquitin-mediated degradation of ERα and suppression of cell proliferation. Besides that, FOXK2 interacts with REST/CoREST, NuRD, SIN3A and NCoR/SMRT corepressor complexes to specifically repress different set of genes, inhibiting breast cancer tumorigenesis. Additionally, FOXK2 mediates the cytotoxic effects of chemotherapeutic agents in breast cancer cells through the induction of FOXO3a expression, a process stimulated by SUMOylation of specific lysine residues in FOXK2 sequence. Consistently with the descriptions of FOXK2 as a tumor suppressor, it represses CDH2, SNAIL, CCND1 and CDK4 genes to inhibit cell migration, invasion and proliferation in lung cancer, in which it has been described as a direct target for the miR-1271. Besides that, FOXK2 also suppresses tumorigenesis in clear-cell renal cell carcinoma, which is associated with decreased protein and mRNA levels of EGFR gene. Accordingly, FOXK2 overexpression is accompanied by an increase in epithelial markers and decrease in mesenchymal markers in some tumor models. Box represents associations between FOXK2 and other molecules in studies involving overexpression and inhibition of FOXK2. EGFR, epidermal growth factor receptor; γ-catenin and E-cadherin, epithelial markers; fibronectin and vimentin, mesenchymal markers; ERα, estrogen receptor-α; BARD1, BRCA1-associated RING domain protein 1; BRCA1, breast cancer type 1 susceptibility protein; CDH2, gene encoding N-cadherin; SNAIL (or SNAI1), snail family transcriptional repressor 1; CCND1, gene encoding Cyclin D1; CDK4, gene encoding Cyclin Dependent Kinase 4; REST/CoREST complex, RE1 Silencing Transcription Factor (REST)/REST corepressor 1(CoREST) complex; NuRD complex, nucleosome remodeling and deacetylase complex; SIN3A complex, SIN3 Transcription Regulator Family Member A complex; NCoR/SMRT complex, Nuclear receptor corepressor 1 (NCoR)/ Nuclear Receptor Corepressor 2 (SMRT) complex; HSP90AA1, heat shock protein 90 alpha family class A member 1; BCAS3, microtubule associated cell migration factor; EZH2, enhancer of zeste 2 polycomb repressive complex 2 subunit; FOXC2, forkhead box C2; CD44, cluster of differentiation 44; CREBBP, CREB binding protein; Survivin, a member of the inhibitor of apoptosis proteins (IAPs) family; HIF1β, hypoxia inducible factor 1 subunit beta; CUL4B, cullin 4B; VEGF, vascular endothelial growth factor; HIG2 (or HILPDA), hypoxia inducible lipid droplet associated; FOXO3a, forkhead box O3; EMT, epithelial-mesenchymal transition; Ub, ubiquitination; SUMO, SUMOylation site.