International Journal of Molecular Sciences Review Targeting Casein Kinase 1 (CK1) in Hematological Cancers Pavlína Janovská 1, Emmanuel Normant 2 , Hari Miskin 2 and VítˇezslavBryja 1,3,* 1 Department of Experimental Biology, Faculty of Science, Masaryk University, 62500 Brno, Czech Republic; [email protected] 2 TG Therapeutics, New York, NY 10014, USA; [email protected] (E.N.); [email protected] (H.M.) 3 Department of Cytokinetics, Institute of Biophysics, Academy of Sciences of the Czech Republic, 61265 Brno, Czech Republic * Correspondence: [email protected]; Tel.: +420-54949-3291 Received: 24 October 2020; Accepted: 21 November 2020; Published: 27 November 2020 Abstract: The casein kinase 1 enzymes (CK1) form a family of serine/threonine kinases with seven CK1 isoforms identified in humans. The most important substrates of CK1 kinases are proteins that act in the regulatory nodes essential for tumorigenesis of hematological malignancies. Among those, the most important are the functions of CK1s in the regulation of Wnt pathways, cell proliferation, apoptosis and autophagy. In this review we summarize the recent developments in the understanding of biology and therapeutic potential of the inhibition of CK1 isoforms in the pathogenesis of chronic lymphocytic leukemia (CLL), other non-Hodgkin lymphomas (NHL), myelodysplastic syndrome (MDS), acute myeloid leukemia (AML) and multiple myeloma (MM). CK1δ/" inhibitors block CLL development in preclinical models via inhibition of WNT-5A/ROR1-driven non-canonical Wnt pathway. While no selective CK1 inhibitors have reached clinical stage to date, one dual PI3Kδ and CK1" inhibitor, umbralisib, is currently in clinical trials for CLL and NHL patients. In MDS, AML and MM, inhibition of CK1α, acting via activation of p53 pathway, showed promising preclinical activities and the first CK1α inhibitor has now entered the clinical trials. Keywords: casein kinase 1; CK1α; CK1"; leukemia; CLL; AML; MM; inhibitors; umbralisib; WNT pathway 1. Introduction Phosphorylation, mediated by protein kinases, is a common posttranslational modification with 500,000 potential phosphorylation sites in the human proteome and 25,000 phosphorylation events described for 7000 human proteins [1]. Since protein kinases play dominant roles in the regulation of a wide range of cellular functions including initiation of cancer, tumor progression and the development of metastatic diseases, many of them represent attractive targets for modern medicine. Some of these kinases have been targeted by novel therapeutics (protein kinase inhibitors) for treatment of cancer [2]. A vast array of kinase inhibitors are under clinical investigation and 52 small molecule kinase inhibitors have been already approved by the U.S. Food and Drug Administration (FDA) [3]. Whereas some protein kinases have been long recognized as attractive targets, others have come to the forefront of biomedical research only recently. These include the family of casein kinase 1 (CK1). CK1 proteins are important regulators of signal transduction, involved in diverse signaling pathways and acting in most cell types. In human, the CK1 protein family includes seven isoforms, α, γ1, γ2, γ3, δ, " and less well described α-like, with CK1α, δ and " being the most studied to date [4]. All CK1 proteins are serine/threonine, monomeric protein kinases. All CK1 enzymes share high homology in their kinase domains (53–98%) [5]. However, each isoform has a different length of N-terminal Int. J. Mol. Sci. 2020, 21, 9026; doi:10.3390/ijms21239026 www.mdpi.com/journal/ijms Int.Int. J. J.Mol. Mol. Sci. Sci. 20202020, 21, 21, x, 9026FOR PEER REVIEW 22 of of 19 19 homology in their kinase domains (53–98%) [5]. However, each isoform has a different length of N- terminal(8–45 amino (8–45 acidsamino (aa)) acids and (aa)) C-terminal and C-terminal (51–138 (51–138 aa) regions. aa) regions. The The CK1 CK1δ andδ and" isoforms ε isoforms share share the thehighest highest homology homology in thein the primary primary structure structure and and have have a long a long and and similar similar regulatory regulatory C-terminus C-terminus [6]. [6].The The crystal crystal structure structure was was solved solved for for all all human human CK1 CK1 isoformsisoforms alonealone or in complex with with specific specific inhibitors:inhibitors: CK1 CK1αα –-Protein Protein Data Data Bank Bank (PDB) (PDB) structure structure 5FQD 5FQD [7 ],[7], 6GZD 6GZD [8 ],[8], CK1 CK1γ1-3-2CMW,γ1–3 – 2CMW, 2C47, 2C47, 2IZU, 2IZU,CK1δ CK1-3UYTδ – [3UYT9], CK1 [9],"-4HNI CK1ε [-10 4HNI] (Figure [10]1 (Figure). 1). " FigureFigure 1. 1. ((AA)) Visualization Visualization of human CK1CK1ε crystalcrystalstructure structure in in complex complex with with PF-4800567 PF-4800567 bound bound in thein theadenosine adenosine triphosphate triphosphate (ATP)-binding (ATP)-binding pocket. pocket. The The structure structure screenshot screenshot is based is based on theon the PDB PDB 4HNI 4HNI [10 ] B [10]entry entry from from the the RCSB RCSB PDB PDB (rcsb.org) (rcsb.org) [11 ].[11]. ( ) ( ConservedB) Conserved regions regions visualization. visualization. Multiple Multiple alignment alignment was produced by ClustalOmega, the conserved regions present in the kinase domain were visualized by was produced by ClustalOmega, the conserved regions present in the kinase domain were visualized Jalview 2.11.0. * Canonical variant; aa—amino acid. by Jalview 2.11.0. * Canonical variant; aa—amino acid. 2. Biological Functions of the CK1 Family 2. Biological Functions of the CK1 Family CK1 family members are involved in various cellular processes such as mitotic checkpoint signaling,CK1 family DNA repair,members apoptosis are involved and p53 in pathway, various protein cellular translation, processes circadiansuch as rhythm,mitotic checkpoint endocytosis signaling,and autophagy, DNA immunerepair, apoptosis response andand inflammation, p53 pathway, centrosome-associated protein translation, processes, circadian and rhythm, play a endocytosiskey regulatory and roleautophagy, in several immune developmental response pathwaysand inflammation, such as Wnt, centrosome-associated Hedgehog, NF-κB processes, or Yap/Taz andsignaling play a (forkey reviewsregulatory see role [4,12 in– several14]). The devel CK1opmental kinases commonlypathways such act as as bothWnt, positive Hedgehog, and NF negativeкB or Yap/Tazregulators signaling of these (for processes, reviews with see [4,12–14]). redundant, The distinct CK1 orkinases opposite commonly roles of individualact as both CK1positive isoforms. and negativeDysregulation regulators of widespread of these processes, regulatory with network redund controlledant, distinct by or CK1 opposite kinases roles may of resultindividual in various CK1 isoforms.pathological Dysregulation conditions, of including widespread cancer regulatory development, network metastasis controlled spreading by CK1 kinases or neurodegenerative may result in variousdiseases [pathological12,15]. Most ofconditions, these aspects including of CK1 biology cancer are development, well covered inmetastasis the abovementioned spreading recent or neurodegenerativereview articles. Herein, diseases we will [12,15]. thus brieflyMost of summarize these aspects CK1 functionsof CK1 biology only in thoseare well biological covered processes in the abovementionedthat are relevant recent for the review topic of articles. this review, Herein, i.e., we the will role thus of CK1 briefly in hematological summarize CK1 malignancies. functions only in thoseCK1 biological" and redundantly processes functioning that are relevant CK1δ are for well-established the topic of this positive review, regulators i.e., the ofrole Wnt of signalingCK1 in hematologicalpathways [16– malignancies.20]. The Wnt signaling pathways can be classified as either Wnt/β-catenin (canonical) or non-canonical.CK1ε and Whenredundantly a specific functioning WNT ligand CK1 (thereδ are are 19well-establishedWNT genes) binds positive to a specific regulators Frizzled of (FZD)Wnt signalingreceptor (therepathways are 10 [16–20].FZD genes The [Wnt21]), asignaling co-receptor pathways associates can with be classified the FZD receptor.as either According Wnt/β-catenin to the (canonical)current understanding, or non-canonical. if the When co-receptor a specific is the WNT low-density ligand lipoprotein(there are 19 receptor WNT genes) related binds protein to 5a/6 specific(LRP5/6), Frizzled the newly (FZD) formed receptor hetero-trimer (there are WNT-FZD-LRP10 FZD genes [21]), recruits a co-receptor a group of associates proteins described with the asFZD the receptor.“signalosome” According that activates to the current the Wnt understanding,/β-catenin pathway, if the in co-receptor what is described is the low-density as the canonical lipoprotein pathway receptor(Figure2 related). If the protein co-receptor, 5/6 (LRP5/6), however, the is receptor newly formed tyrosine hetero-trimer kinase-like orphan WNT-FZD-LRP receptor (ROR)recruits 1 ora groupROR2 of or proteins RYK, the described newly formed as the trimer“signalosome” WNT-FZD-ROR that activates (or RYK) the recruitsWnt/β-catenin another pathway, set of protein in what that isleads described to the activationas the canonical of the Wnt pathway/Ca2+ pathway (Figure 2). or theIf the Wnt co-receptor,/planar cell polarityhowever, (PCP) is receptor pathway, tyrosine in what kinase-likeis called the orphan non-canonical receptor (ROR) pathway. 1 or The ROR2 same or RYK, Frizzled the cannewly bind formed either trimer a ligand