cancers Review Regulation of Wnt Signaling by FOX Transcription Factors in Cancer Stefan Koch 1,2 1 Wallenberg Centre for Molecular Medicine (WCMM), Linköping University, 58185 Linköping, Sweden; [email protected]; Tel.: +46-132-829-69 2 Department of Biomedical and Clinical Sciences (BKV), Linköping University, 58185 Linköping, Sweden Simple Summary: Cancer is caused by a breakdown of cell-to-cell communication, which results in the unrestricted expansion of cells within a tissue. In many cases, tumor growth is maintained by the continuous activation of cell signaling programs that normally drive embryonic development and wound repair. In this review article, I discuss how one of the largest human protein families, namely FOX proteins, controls the activity of the Wnt pathway, a major regulatory signaling cascade in developing organisms and adult stem cells. Evidence suggests that there is considerable crosstalk between FOX proteins and the Wnt pathway, which contributes to cancer initiation and progression. A better understanding of FOX biology may therefore lead to the development of new targeted treatments for many types of cancer. Abstract: Aberrant activation of the oncogenic Wnt signaling pathway is a hallmark of numerous types of cancer. However, in many cases, it is unclear how a chronically high Wnt signaling tone is maintained in the absence of activating pathway mutations. Forkhead box (FOX) family transcription factors are key regulators of embryonic development and tissue homeostasis, and there is mounting evidence that they act in part by fine-tuning the Wnt signaling output in a tissue-specific and context- dependent manner. Here, I review the diverse ways in which FOX transcription factors interact with Citation: Koch, S. Regulation of Wnt the Wnt pathway, and how the ectopic reactivation of FOX proteins may affect Wnt signaling activity Signaling by FOX Transcription in various types of cancer. Many FOX transcription factors are partially functionally redundant Factors in Cancer. Cancers 2021, 13, and exhibit a highly restricted expression pattern, especially in adults. Thus, precision targeting of 3446. https://doi.org/10.3390/ individual FOX proteins may lead to safe treatment options for Wnt-dependent cancers. cancers13143446 Keywords: Wnt; forkhead; FOX; beta-catenin; transcription factors Academic Editor: Masaru Katoh Received: 16 June 2021 Accepted: 7 July 2021 1. Wnt Signaling in Cancer Published: 9 July 2021 Wnt signaling is one of the fundamental developmental signaling pathways in the an- Publisher’s Note: MDPI stays neutral imal kingdom. In adult organisms, it functions as a key regulator of stem cell maintenance with regard to jurisdictional claims in and proliferation. Consequently, Wnt signaling is indispensable for numerous biological published maps and institutional affil- processes ranging from embryogenesis to tissue homeostasis and regeneration. iations. The basic principles of Wnt signaling are well-understood [1,2] (Figure1). In the central Wnt pathway, referred to as Wnt/β-catenin or canonical Wnt signaling, pathway activity is determined by the abundance of the transcription co-factor β-catenin. In unstim- ulated cells, cytosolic β-catenin is constitutively marked for proteasomal degradation by a multi-protein destruction complex. Upon activation of Frizzled (FZD) family core receptors Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. and Low-density lipoprotein receptor-related protein (LRP) 5/6 co-receptors by secreted β This article is an open access article Wnt ligands, the destruction complex is inhibited, which allows -catenin accumulation. distributed under the terms and β-catenin then translocates to the nucleus, where it drives the expression of target genes of conditions of the Creative Commons Transcription factor/Lymphoid enhancer-binding factor (TCF/LEF) family transcription Attribution (CC BY) license (https:// factors. This gene expression signature, which includes numerous shared as well as tissue- creativecommons.org/licenses/by/ specific gene targets [3], promotes stem cell maintenance, cell cycle progression, and cell 4.0/). proliferation, and is thus essential for organogenesis and tissue homeostasis. Besides Cancers 2021, 13, 3446. https://doi.org/10.3390/cancers13143446 https://www.mdpi.com/journal/cancers Cancers 2021, 13, 2 of 21 expression of target genes of Transcription factor/Lymphoid enhancer-binding factor Cancers 2021, 13, 3446 (TCF/LEF) family transcription factors. This gene expression signature, which includes2 of 20 numerous shared as well as tissue-specific gene targets [3], promotes stem cell maintenance, cell cycle progression, and cell proliferation, and is thus essential for organogenesis and tissue homeostasis. Besides Wnt/β-catenin signaling, there are several Wnt/non-canonicalβ-catenin Wnt signaling, pathways there that are severalbranch non-canonicaloff at the receptor Wnt level pathways and at that the branchlevel of off the atdestruction the receptor complex level and [4,5]. at In the Wnt/Planar level of the cell destruction polarity (PCP) complex as well [4,5 ].as In Wnt/Ca Wnt/Planar2+ signaling, cell 2+ polarityWnt ligands (PCP) engage as well FZDs as Wnt/Ca and alternativesignaling, co-receptors Wnt ligands to engageelicit a β FZDs-catenin-independent and alternative co-receptorsresponse. The to signaling elicit a β-catenin-independent outcome of these path response.ways is primarily The signaling non-transcriptional, outcome of these and pathwaysis best known is primarily for regulating non-transcriptional, cell migration and isand best polarity. known forFinally, regulating Wnt/Stabilization cell migration of andproteins polarity. (STOP) Finally, and Wnt/Stabilization Wnt/Target of rapamycin of proteins (TOR) (STOP) signaling and Wnt/Target promote the of rapamycin stability of β (TOR)proteins signaling other than promote β-catenin the stability via the of Wnt-induced proteins other inhibition than -catenin of the viadestruction the Wnt-induced complex, inhibitionand support of the proteome destruction maintenance complex, and and support cell growth proteome in maintenancea transcription-independent and cell growth in a transcription-independent manner [4]. manner [4]. FigureFigure 1. 1.Overview Overview of of the the canonical canonical Wnt/ Wnt/ββ-catenin-catenin signalingsignaling pathway. pathway. Hallmark Hallmark alterations alterations in in Wnt Wnt pathway activity in Wnt-dependent cancer cells are indicated on the right. pathway activity in Wnt-dependent cancer cells are indicated on the right. GivenGiven the the prominent prominent roles roles of Wnt of signalingWnt signaling in embryogenesis in embryogenesis and stem celland regulation, stem cell itregulation, is unsurprising it is unsurprising that dysregulated that dysregul Wnt pathwayated Wnt activity pathway is associated activity is with associated numerous with seriousnumerous disorders, serious especially disorders, cancer especially [1,6,7 ].ca Indeed,ncer [1,6,7]. the term Indeed, Wnt the itself term references Wnt itself the identificationreferences the of identification the first mammalian of the first pathway mammalian component pathway (Wnt 1,component then known (Wnt1, as int then-1) asknown an oncogene as int-1) activatedas an oncogene by viral activated integration by viral [8]. integration In the more [8]. than In the 30 more years than following 30 years thisfollowing discovery, this Wnt/discovery,β-catenin Wnt/β signaling-catenin signaling in particular in particular has taken has center taken stage center as stage a major as a oncogenicmajor oncogenic pathway pathway in various in various types of types cancer. of Acancer. prime A example prime example for Wnt-dependent for Wnt-dependent cancer iscancer sporadic is sporadic colorectal colorectal cancer (CRC).cancer The(CRC). vast The majority vast majority of tumors of tumors in CRC in patients CRC patients have activatinghave activating mutations mutations in Wnt in pathway Wnt pathway components, components, of which of most which result most in loss-of-functionresult in loss-of- offunction the destruction of the destruction complex or thecomplex aberrant or stabilizationthe aberrant ofstabilizationβ-catenin [ 9of]. Additionally,β-catenin [9]. elevatedAdditionally, Wnt ligand elevated levels Wnt increase ligand levels pathway increase activity pathway in CRC activity irrespective in CRC of irrespective underlying of mutations [10]. Chronic Wnt pathway activation in colonic epithelial cells leads to the formation of hyper-proliferative adenomas, which are hotspots for mutations in other genes involved in cancer initiation and progression. Evidence from mouse models of CRC suggests that restoration of normal Wnt activity is sufficient to induce complete tumor regression even in fully transformed colon cancer [11], which highlights the importance Cancers 2021, 13, 3446 3 of 20 of sustained Wnt signaling not only in tumor development, but also cancer stem cell maintenance. Wnt pathway mutations are common in other types of cancer as well, although they occur at a lower frequency compared to CRC. For example, one in four hepatocellular carcinomas and uterine corpus endometrial carcinomas exhibit stabilizing β-catenin mutations, while destruction complex mutations are frequently observed in cutaneous melanomas and gastric adenocarcinomas [6]. Wnt pathway mutations are relatively rare in other cancers, including pancreatic and prostate cancer. Nonetheless, Wnt signaling activity is increased