cancers Review Kinase Regulation of HOX Transcription Factors Monika Primon 1, Keith D. Hunter 2 , Hardev S. Pandha 3 and Richard Morgan 1,* 1 Institute of Cancer Therapeutics, Faculty of Life Sciences, University of Bradford, Bradford BD7 1DP, UK; [email protected] 2 Unit of Oral and Maxillofacial Pathology, School of Clinical Dentistry, University of Sheffield, Sheffield S10 2TN, UK; k.hunter@sheffield.ac.uk 3 Faculty of Health and Medical Sciences, University of Surrey, Guildford GU2 7XH, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-1274-233225; Fax: +44-1274-233234 Received: 12 March 2019; Accepted: 7 April 2019; Published: 10 April 2019 Abstract: The HOX genes are a group of homeodomain-containing transcription factors that play important regulatory roles in early development, including the establishment of cell and tissue identity. HOX expression is generally reduced in adult cells but is frequently re-established as an early event in tumour formation and supports an oncogenic phenotype. HOX transcription factors are also involved in cell cycle regulation and DNA repair, along with normal adult physiological process including stem cell renewal. There have been extensive studies on the mechanism by which HOX proteins regulate transcription, with particular emphasis on their interaction with cofactors such as Pre-B-cell Leukaemia Homeobox (PBX) and Myeloid Ecotropic Viral Integration Site 1 (MEIS). However, significantly less is known of how the activity of HOX proteins is regulated. There is growing evidence that phosphorylation may play an important role in this context, and in this review, we draw together a number of important studies published over the last 20 years, and discuss the relevance of phosphorylation in the regulation and function of HOX proteins in development, evolution, cell cycle regulation, and cancer. Keywords: HOX; phosphorylation; embryonic patterning; cell cycle; cancer 1. Introduction The HOX genes encode a family of homeodomain-containing transcription factors that play important roles in the early embryo, including the establishment of cell and tissue identity, and the regulation of cell proliferation, differentiation, and survival [1]. A characteristic feature of this gene family is their organisation within four clusters, each of which is located on a different chromosome. These clusters, A, B, C, and D, are used in the nomenclature of HOX genes, which are also numbered according to their relative position in the cluster, with, for example, HOXB1 being the most 3’ member of the B cluster [2]. Despite being originally characterised as developmental genes, the HOX transcription factors are known to have additional roles in the adult, including, for example, the proliferation of hematopoietic stem cells (HSCs) [3], and the maintenance of tissue identity during the menstrual cycle [4]. The highly conserved homeodomain of HOX proteins mediates their binding to DNA, although the strength and specificity of this interaction is greatly increased by the binding of co-factors such as Pre-B-cell Leukaemia Homeobox (PBX), which forms heterodimers with HOX proteins in groups 1-10, and Myeloid Ecotropic Viral Integration Site 1 Homolog (MEIS) proteins that dimerize with HOX proteins 9–13 [5]. These cofactors have a role in in the recruitment of RNA polymerase II or III, as well as transcriptional inhibitors such as histone deacetylase (HDAC), resulting in differential gene regulation depending on the sequence and context of the target site in the enhancer or promoter region [6,7]. Cancers 2019, 11, 508; doi:10.3390/cancers11040508 www.mdpi.com/journal/cancers Cancers 2019, 11, 508 2 of 9 Many of the HOX genes are also highly over-expressed in a range of cancers including melanoma [8], and head and neck [9], prostate [10], breast [11], ovarian [12], and pancreatic cancer [13]. In this contextCancers they 2019, 11 usually, x FOR PEER have REVIEW a pro-oncogenic role, supporting a malignant phenotype.2 of 9 The latter includes promotingregulation depending proliferation on the sequence and blocking and context apoptosis of the target [14 ],site the in the induction enhancer ofor promoter angiogenesis [15], and facilitatingregion metastasis[6,7]. [16], drug resistance [11,17,18], and radiation resistance [19]. The key roles that HOX proteinsMany playof the in HOX cancer genes make are also them highly potential over-expressed therapeutic in a range targets, of cancers although including a high level of melanoma [8], and head and neck [9], prostate [10], breast [11], ovarian [12], and pancreatic cancer functional redundancy amongst HOX proteins presents a barrier to this approach. An alternative [13]. In this context they usually have a pro-oncogenic role, supporting a malignant phenotype. The strategy islatter to inhibitincludes thepromoting interaction proliferation between and blocking HOX apoptosis and PBX, [14], which the indu isction mediated of angiogenesis by a conserved hexapeptide[15], sequence and facilitating in HOX metastasis proteins. [16], drug This resistance interaction [11,17,18], can beand inhibited radiation resistance using HXR9, [19]. The a key small peptide mimic of theroles hexapeptide that HOX proteins sequence play in cancer that causes make them apoptosis potential intherapeutic a range targets, of cancers although [20 a]. high level of functional redundancy amongst HOX proteins presents a barrier to this approach. An alternative Givenstrategy the profound is to inhibit homeotic the interaction activity between of HOX HOX genesand PBX, and which their is abilitymediated to by influence a conserved processes in the adult, includinghexapeptide DNAsequence repair in HOX and proteins. cell cycle This interaction regulation, can therebe inhibited is relatively using HXR9, little a small known peptide of how HOX activity is itselfmimic regulated.of the hexapeptide Regulation sequence occurs that caus ates the apoptosis level ofin a transcription range of cancers in [20]. a process that is influenced by enhancer sharingGiven the asprofound well ashomeotic epigenetic activity changesof HOX genes and and signalling their abilitythrough to influence a processes range of in pathways, the adult, including DNA repair and cell cycle regulation, there is relatively little known of how HOX most notablyactivity retinoic is itself acid regulated. [1]. There Regulation have occurs also at been the level many of transcription studies of in the a process role played that is influenced by HOX co-factors such as PBXby andenhancer MEIS sharing [20]. as However, well as epigenetic relatively changes little and issignalling known through of how a range HOX of transcription pathways, most factors are regulated atnotably the post-translationalretinoic acid [1]. There level,have also despite been ma HOXny studies proteins of the containing role played by multiple HOX co-factors consensus sites for a rangesuch of as kinases PBX and (Figure MEIS [20].1). Howeve Therer, wasrelatively a recent, little is known excellent of how review HOX transcription on HOX factors post-translational are regulated at the post-translational level, despite HOX proteins containing multiple consensus sites modifications,for a range which of includedkinases (Figure phosphorylation 1). There was a rece [21nt,]. Here,excellent however, review on weHOX focus post-translational on what is currently known aboutmodifications, HOX protein which phosphorylationincluded phosphorylation with [21] special. Here, emphasishowever, we on focus its functionalon what is currently consequences in development,known cancer, about HOX and protein cell-cycle phosphorylation regulation. with special emphasis on its functional consequences in development, cancer, and cell-cycle regulation. Figure 1. ConsensusFigure 1. Consensus kinase kinase sites sites in HOXB4in HOXB4 andand HOXB7. The The relative relative positions positions of the conserved of the conserved hexapeptide domain (“HEX”) that mediates Pre-B-cell Leukaemia Homeobox (PBX) binding and the hexapeptide domain (“HEX”) that mediates Pre-B-cell Leukaemia Homeobox (PBX) binding and the homeodomain (“HD”) that mediates DNA binding are shown. ATM, kinase mutated in ataxia telangiectasia; ATR, ataxia telangiectasia and Rad3-related protein; CK2, casein kinase 2; CDK, cyclin-dependent kinase; CLK, CDC2-like kinase; MAP3K19, mitogen-activated protein kinase kinase kinase 19 (also known as RCK and YSK); PKA, protein kinase A; PKC, protein kinase C; SLK, STE-20 like serine/threonine protein kinase. Cancers 2019, 11, x FOR PEER REVIEW 3 of 9 homeodomain (“HD”) that mediates DNA binding are shown. ATM, kinase mutated in ataxia telangiectasia; ATR, ataxia telangiectasia and Rad3-related protein; CK2, casein kinase 2; CDK, cyclin- dependent kinase; CLK, CDC2-like kinase; MAP3K19, mitogen-activated protein kinase kinase kinase Cancers192019 (also, 11 ,known 508 as RCK and YSK); PKA, protein kinase A; PKC, protein kinase C; SLK, STE-20 like 3 of 9 serine/threonine protein kinase. 2. Kinase Regulation of HOX ProteinsProteins in Development and EvolutionEvolution As discussed above, the HOX genes play a key role in the patterning of both the vertebrate and invertebrateinvertebrate bodybody plan.plan. In the latter, there is growing evidence that variations in HOX proteins might have helpedhelped drive drive evolutionary evolutionary changes changes in the in morphology the morphology and number and ofnumber appendages. of appendages. A particularly A well-studiedparticularly