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Over the last decade, the CMGC kinase DYRK2 has been reported as a tumor suppressor across various cancers triggering major antitumor and proapoptotic signals in breast, colon, liver, ovary, brain, and lung cancers, with lower DYRK2 expression correlated with poorer prognosis in patients. Contrary to this, various medicinal chemistry studies reported robust antiproliferative properties of DYRK2 inhibitors, whereas unbiased ‘omics’ and genome-wide association study-based studies identified DYRK2 as a highly overexpressed kinase in various patient tumor samples. A major paradigm shift occurred in the last 4 years when DYRK2 was found to regulate proteostasis in cancer via a two-pronged mechanism. DYRK2 phosphorylated and activated the 26S proteasome to enhance degradation of misfolded/tumor-suppressor proteins while also promoting the nuclear stability and transcriptional activity of its substrate, heat-shock factor 1 triggering protein folding. Together, DYRK2 regulates proteostasis and promotes protumorigenic survival for specific cancers. Indeed, potent and selective small-molecule inhibitors of DYRK2 exhibit in vitro and in vivo anti-tumor activity in triple-negative breast cancer and myeloma models. However, with conflicting and contradictory reports across different cancers, the overarching role of DYRK2 remains enigmatic. Specific cancer (sub)types coupled to spatiotemporal interactions with substrates could decide the procancer or anticancer role of DYRK2. The current review aims to provide a balanced and critical appreciation of the literature to date, highlighting top substrates such as p53, c-Myc, c-Jun, heat-shock factor 1, proteasome, or NOTCH1, to discuss DYRK2 inhibitors available to the scientific community and to shed light on this duality of protumorigenic and antitumorigenic roles of DYRK2.
Protein kinase DYRK2 is a member of the Dual-specificity tYrosine phosphorylation–Regulated Kinase (DYRK) family, which in turn belongs to the Cyclin-dependent kinases, Mitogen-activated protein kinases, Glycogen synthase kinases, and CDC-like kinases (CMGC) superfamily within the kinase complement of the human genome (
). The DYRK family consists of 5 members divided into two classes: Class I is comprised of DYRK1A and DYRK1B, whereas class II is comprised of DYRK2, DYRK3, and DYRK4 (Fig. 1A). DYRK2 is a class II DYRK that exhibits various structural features such as the NAPA or N-terminal autophosphorylation accessory domains (yellow/orange), DYRK-homology domain (green), activation loop segment (purple), nuclear localization sequence (red), the CMGC family–specific insert domain (gray) (Fig. 1B) most of which are conserved across the DYRK family (
). In DYRK2, specific loss-of-function mutations have been reported in cancer (Fig. 1B), which affect either the activity of the kinase or impede its ability to form functional complexes with interactors (
). Class I DYRKs exhibit two distinct nuclear localization sequences and a stretch of polyserine and polyproline (PEST, Pro-Glu-Ser-Thr) domain with no distinct NAPA domains as in class II paralogues (Fig. 1C). Despite subtle structural differences between class I and II members, all DYRK isoforms exhibit a highly conserved autophosphorylation-mediated activation mechanism (
). During translation, hydroxylation of a highly conserved proline residue (proline441 for hDYRK2) on the inert/nascent kinase domain of DYRKs triggers a tyrosine autophosphorylation event within the activation loop (tyrosine382 for hDYRK2), which leads to conversion of the inactive to the active conformation of the kinase (
). The CMGC-specific insert is conserved across the CMGC kinase superfamily and is proposed to play important roles in stabilization of the tertiary structure of the kinase and promoting complex formation with interactors/substrates (
Like most CMGC kinases, DYRKs have an amino acid motif of preference on their substrates. DYRKs prefer an arginine (R) at the −3 position of the phosphoserine/threonine residue along with a strong preference for a proline (P) at +1: Rxx(pS/T)P motif (
The kinase DYRK phosphorylates protein-synthesis initiation factor eIF2Bepsilon at Ser539 and the microtubule-associated protein tau at Thr212: Potential role for DYRK as a glycogen synthase kinase 3-priming kinase.
The kinase DYRK phosphorylates protein-synthesis initiation factor eIF2Bepsilon at Ser539 and the microtubule-associated protein tau at Thr212: Potential role for DYRK as a glycogen synthase kinase 3-priming kinase.
). DYRK2 is an integral part of the EDVP (EDD [ubiquitin protein ligase] + DDB1 [damage-specific DNA-binding protein] + VPRBP [HIV-1 Vpr-binding protein]) E3 ubiquitin ligase complex that carries out phosphorylation-mediated degradation of various cell cycle components to ensure smooth transition of G2/M stages of cell cycle (
). Thus, over the past few decades, many groups have identified various molecular mechanism and substrates for DYRK2 playing diverse roles in cellular growth, proliferation, and developmental processes with a focal point being its role in cancer (
Besides DYRK2, the other DYRK isoforms, especially the class I's, have a long history in the field of cancer. Although DYRK1B has an overall protumorigenic role specifically in pancreatic and ovarian cancers, DYRK1A exhibits a more controversial role with reports of both protumorigenic and antitumorigenic mechanism in different cancers (reviewed in Boni et al. [
), have brought the kinase to the forefront of oncology research. For the past 2 decades, multiple studies have reported an overarching tumor suppressor role of DYRK2 across various cancers (reviewed in Yoshida and Yoshida [
). Furthermore, mRNA expression analyses from The Cancer Genome Atlas (TCGA) tumors along with matched normal controls reveal that the majority of cancers have higher median expression of DYRK2 than adjacent normal tissues (
). All of these data suggest that DYRK2 might be an excellent potential drug target. With such high profile, oncology-related substrates, could the function of DYRK2 differ based on cancer type or cell type? To shed some light onto this question, this review will re-examine the current literature on the role of DYRK2 in cancer and follow up with existing knowledge of small-molecule inhibitors developed to target DYRK2.
DYRK2 regulates proteostasis: an oncogenic role
DYRK2 maintains proteostasis of cancer cells by regulating two major players of the proteotoxic response pathway, which promotes the proper folding and/or degradation of proteins (Fig. 2). More than 90% of all solid human tumors carry numerous aberrations in chromosomes, referred to as aneuploidy (
). To survive proteotoxic stress, cancer cells can either increase protein folding capacity (controlled by the transcription factor HSF1) or increase the degradation of the misfolded/aggregated proteins (via the 26S proteasome and/or autophagy). DYRK2 phosphorylates and activates both HSF1 and the 26S proteasome and thereby activates the proteotoxic stress pathway promoting tumorigenesis in cancers such as triple-negative breast cancer (TNBC) and multiple myeloma (MM).
DYRK2 regulates 26S proteasome function
In 2016, an RNAi kinase screen identified DYRK2 as a kinase-regulating 26S proteasome activity (
). The mature 26S proteasome is a complex of more than 30 distinct subunits that catalyzes 80% of eukaryotic protein degradation and harbors three distinct peptidase activities in the core subunit (chymotryptic, tryptic, and caspase-like) (
). Besides the core of the proteasome, the complex also consists of the 19S regulatory subunit that binds to ubiquitylated proteins, whereas a six-membered ATPase ring hydrolyzes the protein into a polypeptide chain for entry into the peptidase core for degradation (
), but the function of the phosphorylation was not known. A phospho-specific antibody generated against pT25 Rpt3 showed that the site is dynamically upregulated during G2/M stage of the cell cycle and that serum starvation leads to loss of Thr25 phosphorylation (
). Furthermore, CRISPR/Cas9 knock-in of a phospho-deficient Thr25Ala on Rpt3 mimics the DYRK2 KO phenotypes in cells wherein there is a delay in mitotic progression, slower cell proliferation rates, and inhibition of all three peptidase activities of the 26S proteasome (
). This study further established the DYRK2-proteasome axis as potentially tumor promoting because higher expression of DYRK2 significantly correlated with higher mortality and poorer relapse-free survival in patients with breast cancer (
), suggesting that DYRK2 might play a role in driving drug resistance in some myeloma cases. The potent and selective DYRK2 inhibitor, LDN192960, induces cytotoxicity in myeloma cells both in vitro and in vivo with minimal off-target effects (
). The fact that the DYRK2 inhibitor alleviates myeloma burden in vivo suggests DYRK2 could indeed be a viable in vivo target for myeloma therapeutics. Resistance to proteasome inhibitors have been reported in patients, and this is either brought about by cancer mutations in the proteasome core or via upregulation of HSF1-mediated proteotoxic response pathway.
DYRK2 phosphorylates HSF1 and modulates proteotoxic response
The transcription factor HSF1 is the master regulator of proteotoxic stress responses and supports oncogenesis by helping cancer cells cope with the proteotoxic stress associated with both aneuploidy and oncogenic mutations. This has been demonstrated by the reduced susceptibility of Hsf1-KO mice to tumor formation driven either by Ras/p53 mutations or by chemical carcinogens (
). Indeed, DYRK2-depleted TNBC cells were far more sensitive to heat shock–mediated proteotoxic stress than parental cells, thus corroborating that DYRK2 plays a major role in maintaining proteostasis in TNBC cells. This link between DYRK2 and HSF1 is also observed in TNBC tumor samples, wherein a marked correlation was observed between high DYRK2 levels and high nuclear HSF1 levels.
The HSF1 pathway and the proteasome are not just two of the main pathways maintaining cell proteostasis, but they are interconnected and can compensate for each other. As mentioned before, proteasome inhibitors lead to the activation of HSF1 (Fig. 2) in an effort to protect the cell against the accumulation of toxic proteins (
), and thus, HSF1 inhibition might be effective to overcome proteasome inhibitor resistance in cancer cells. In that sense, a DYRK2 inhibitor induced cytotoxicity even in MM cells resistant to proteasome inhibitors (
), suggesting that in fact DYRK2 inhibition might be targeting different complementary pathways. This observation was further echoed by a recent study showing that MM cells were extremely sensitive to increased temperatures and heat shock (
). Because cancer cells harbor significantly higher misfolded proteins than normal cells, targeting DYRK2 could indeed tip the scales for proteostasis in malignant cells and provide a significant therapeutic window for targeting specific cancers. This is indeed the case because normal/noncancerous cells were far more resistant to DYRK2 inhibitors (
). Thus, targeting DYRK2 can significantly affect proteostasis (Fig. 2) via perturbation of both HSF1 and 26S proteasome activity leading to cancer cell death.
Hence, in the context of TNBC and MM, DYRK2 plays an overarching role as an oncogenic kinase and a potential therapeutic target.
DYRK2-p53 tumor suppressor link
A major molecular mechanism by which DYRK2 has been reported to exhibit the antitumorigenic role is via phosphorylation of tumor suppressor p53 on serine46 (Ser46). Upon genotoxic stress, energy stress, or heat shock, multiple CMGC kinases such as homeodomain-interacting protein kinase (HIPK2), mitogen-activated protein kinase p38α, and DYRK2 can phosphorylate p53 on Ser46, which triggers transcription of proapoptotic genes leading to cell death or cell senescence (reviewed in Liebl and Hofmann [
]). Upon DNA damage, DYRK2 is phosphorylated by ataxia-telangiectasia mutated kinase, which protects DYRK2 from proteasomal degradation leading to its nuclear accumulation where it phosphorylates p53 on Ser46 and promotes its transcriptional tumor suppressor activity (
). Although phosphorylated Ser46 on p53 is indeed a marker for its tumor suppressor role, DYRK2 by no means is the exclusive kinase here. With multiple kinases including PKCδ, HIPK2, ataxia-telangiectasia mutated kinase, and p38α phosphorylating Ser46 upon genotoxic stress (
), it is hard to decipher to what extent DYRK2 contributes to this tumor suppressor role. Furthermore, p53 is mutated or truncated in a vast number of solid tumors and cancer patients with altered p53 exhibit significantly poorer survival (
). Thus, it is unclear to what extent DYRK2's phosphorylation of p53 could play as a tumor-suppressive role in these solid tumors exhibiting p53 mutation or loss. Furthermore, in endothelial cells, the pan-DYRK inhibitor, harmine (albeit with possible off-target effects), promotes p53 phosphorylation on Ser15, Ser20, and Ser37 (
). Seemingly, in this case, DYRK2 inhibition led to tumor suppression. Hence, it is also important to decipher the molecular functions of DYRK2 in noncancer models or as a potential cancer driver. A recent unbiased deep multiomics study looking at the proteome, phosphoproteome, and transcriptome of murine high-grade brain cancer glioma model reported 41 kinases including DYRK2 exhibiting higher activity and rewired substrate signaling (
). Furthermore, the glioma murine model was generated by intracranial implantation of genetically engineered p53 null astrocytes, thus making the tumor-suppressor role of DYRK2-p53 axis highly untenable in this model.
Besides solid tumors, chronic myeloid leukemia (CML) cell lines exhibit significantly lower protein levels of DYRK2 than other hematological cancer cell lines (
), and hence, DYRK2 could indeed be a tumor suppressor in specific subtypes such as CML. On a similar note, silencing DYRK2 has been reported to increase cell proliferation and reverse cell adhesion–mediated drug resistance in non-Hodgkin's lymphoma cell lines (
). This suggests that in myeloma, the role of DYRK2 as an oncogenic driver probably plays a far greater role than its tumor-suppressor function potentiated by p53 phosphorylation. Hence, stratification of cancer subtypes before assigning molecular functions to DYRK2 is important. However, DYRK2 has tumor-suppressor mechanisms beyond p53 involvement, and it is important to investigate the diverse mechanisms at play to derive a larger perspective (Fig. 3).
Other molecular mechanisms linking DYRK2 and cancer
Various p53-independent tumor-suppressor mechanisms have been reported for DYRK2, while other substrates point to an oncogenic role. Each mechanism focuses on specific cancer types and subtypes (Fig. 3 and Table 1). The main substrates and mechanisms are critically presented below.
Table 1DYRK2 molecular mechanism and substrates/partners listed along with reported phosphorylation sites, overarching role, and the unanswered questions raised by each study
). Post-translational modifications of c-Myc have been a topic of much debate over the past 30 years in which sequential phosphorylation of Ser62 and Threonine58 (Thr58) seems to play major roles in c-Myc transactivation (
). The consensus in the field is that Thr58 is a GSK3-phosphorylation site while Ser62 seems to be the priming site for GSK3 activity, and similar to phosphorylation of p53 at Ser46, various CMGC kinases have been proposed (
), including DYRK2 to phosphorylate c-Myc. Dual phosphorylation of c-Myc on Thr58 and Ser62 triggers binding to an E3 ubiquitin ligase SCF-Fbxw7 (Skp-Cullen-F-box) and consequent proteasomal degradation of c-Myc (
). This indicates a similar conundrum as observed with the p53 Ser46 site wherein multiple kinases and diverse cancer subtypes exhibit altered mechanisms of action of major cancer-associated genes such as p53 and c-Myc.
A similar story is observed in case of c-Jun wherein two phosphorylation sites serine249 (Ser249: a bona fide GSK3 site) and Ser243 (reported to be phosphorylated by DYRK2) have been reported (
), which is not surprising because the site is a +1P. In fact, dephosphorylation of Ser243 enhances c-Jun transcriptional activity in patients with cervical cancer exhibiting lower phosphoSer243 c-Jun in their tumors (
). Although an interesting molecular mechanism, in both studies, DYRK2 ectopic overexpression has been carried out to justify the phosphorylation. Overexpression of CMGC kinases often leads to nonphysiological false-positive subcellular localizations and substrate identifications because of redundancy and high affinity for +1 P sites and hence further tools need to be used to confirm the DYRK2–SNAIL mechanism.
Seven In Absentia Homolog 2 or SIAH2 is an E3 ubiquitin ligase that plays a major role in targeted degradation of various proteins playing essential roles in regulating hypoxia (
); however, the DYRK2–SIAH2 link points to an interesting interplay between a kinase and a ubiquitin ligase regulating each other and thereby balancing protumorigenic and antitumorigenic roles.
EDVP E3 ubiquitin ligase
As stated previously, DYRK2 forms a kinase-independent scaffold for the EDVP E3 ligase complex and a recent study has reported loss-of-function point mutations of DYRK2 in cancer, which largely alters the interactome and substrate specificity of DYRK2 (
). Phosphorylation-mediated degradation of these substrates are required for proper cell cycle transitions especially the G2/M stage. Cancer mutations could result in incomplete EDVP complex formation, and incomplete EDVP can exhibit oncogenic prosurvival role because DYRK2+EDD alone degrades the proapoptotic factor modulator of apoptosis protein 1 independently of DDB1 and VPRBP in ovarian cancer (
). The substrates of DYRK2–EDVP exhibit both protumorigenic and antitumorigenic roles in various cancers thus adding further complexity. Ovarian cancer patients with higher levels of KATNA1 exhibit better overall survival (
]). STAT3 is phosphorylated on various residues upon interleukin/cytokine stimulation, and the phosphorylation on serine727 (Ser727) is thought to be an oncogenic biomarker in some subtypes of breast cancer (
). Thus, DYRK2-mediated downregulation of IFN signaling could play a major oncogenic role in triggering immunotherapy resistance in various cancers. However, the study reporting DYRK2 as the upstream kinase of TBK1 relies on ectopic overexpression of DYRK2 to demonstrate direct phosphorylation of a canonical +1P motif (
). There could be redundancies with other CMGC kinases at that site which needs to be addressed more thoroughly.
In response to chemotherapeutic agents, DYRK2 facilitates phosphorylation-mediated degradation of neurogenic locus notch homolog protein 1 (NOTCH1), which acts as an antiproliferative mechanism in breast cancer cells (
). Interestingly, Thr2512 lies in the intracellular carboxy-terminal region of NOTCH1 that exhibits a PEST domain. The PEST region is the target of multiple CMGC kinases such as DYRK1A, HIPK2, CDKs, and GSK3, which triggers hyperphosphorylation and proteasomal degradation of NOTCH1 (reviewed in Lee et al. [
]). Thus, the redundancy conundrum remains to be solved to understand the function of NOTCH1's phosphorylation by DYRK2.
Besides modulating substrate phosphorylations, transcriptional and epigenetic mechanisms of DYRK2 regulation have also been proposed for some cancers. Specifically, the downregulation of DYRK2's gene expression has been linked to increased stemness in breast cancer (
). The DYRK2 promoter region exhibited a higher level of methylation in cancer tissues than healthy tissues while treatment of cells with hypomethylating drug 5-azacytidine increased DYRK2 mRNA and protein levels (
To reiterate, multiple molecular mechanisms have been proposed for DYRK2, and each mechanism is cancer-type or subtype specific (Fig. 3 and Table 1). The controversial role of DYRK2 is best highlighted in breast and lung cancers.
DYRK2 and breast cancer: a major controversy
Various studies have focused on the role of DYRK2 in TNBC (
). Complementing this information, a recent study with 715 samples of patients with breast cancer have shown that high protein levels of nuclear DYRK2 were associated with significantly reduced cancer survival and a shorter time to recurrence specifically within the TNBC subtype cohort (
). Crispr/Cas9-mediated DYRK2 deletion in MDA-MB-231 or MDA-MB-468 cells showed that tumors derived from TNBC–DYRK2–deficient cells had significantly slower growth rates and lower tumor burden than those derived from their parental cells. Importantly, two studies have shown that treatment with the DYRK2 inhibitors, curcumin and LDN192960, impaired growth of established TNBC tumors (
), thereby functioning as a tumor suppressor. Both the studies used a DYRK2 overexpression system to show that higher DYRK2 decreased tumor formation. One study reported that mice xenografted with DYRK2-overexpressing MDA-MB-231 cells showed few metastatic lesions and a prolonged survival compared with those injected with control cells (
). Some of these discrepancies might be due to the differential approaches used (DYRK2 knockdown/KO versus overexpression systems) or due to underpowered sample sizes. Furthermore, a phosphotyrosine proteomics study in TNBC cells reported that DYRK2 was among the top 5 phosphorylated proteins observed in aggressive basal-like TNBC cells (
). Because there is no evidence of the activation loop tyrosine exhibiting altered stoichiometric phosphorylation, the high levels of phosphorylation observed could be due to higher DYRK2 protein levels. In fact, siRNA knockdown of DYRK2 in basal-like TNBC MDA-MB-231 and HCC1395 cells lead to reduced proliferation, invasion, and colony formation potential of the cells (
Multiple studies looking at the role of DYRK2 in breast cancer have used the hormone receptor–positive and HER2-negative MCF7 cell line for xenograft studies. In the main study that supports the tumor-suppressor role of DYRK2 in breast cancer, the group identified DYRK2 as a priming kinase for c-Jun and c-Myc (
). In this study, the authors used a sample size of n = 3 mice per condition and carried out an orthotopic mammary-fat-pad breast cancer xenograft comparing MCF7 control cells and stable DYRK2 knockdown cells to investigate their ability to produce tumors (
). They found that DYRK2 knockdown cells clearly produce bigger tumors. Furthermore, DYRK2 knockdown cells showed higher invasion potential in vivo in an intracardiac injection model (n = 6 mice per condition). The same shRNA DYRK2 depleted cells were used in other studies as well to report the various tumor-suppressor roles of DYRK2 (
). However, TCGA data suggest that mRNA expression of DYRK2 is higher in breast invasive carcinoma and that higher DYRK2 expression correlates with poor survival in overall patients with breast cancer (
). Because mRNA and protein levels sometimes do not correlate, larger analysis looking at DYRK2 protein levels are needed to reach a finite conclusion.
The best way forward is to generate a conditional lox-cre mouse model for DYRK2 and generate hemizygous/homozygous deletion of DYRK2 in different subtypes of breast cancer genetically engineered mouse models (GEMMs) (
). Comparative tumor growth in the DYRK2 null versus parental GEMM over different subtypes would be a good way of addressing the pending questions on role of DYRK2 in breast cancer.
DYRK2 in lung cancer: unresolved issues
In 2003, the chromosome 12 region 12q13-14 was found to be amplified in adenocarcinomas of the lung and esophagus, and one of the resident genes, DYRK2, was significantly overexpressed in tumor samples as compared with normal tissues (
). In fact, DYRK2 exhibited the highest mRNA overexpression and highest copy numbers in tumors compared with normal tissue and other genes located in the 12q13-14 chromosomal region, suggesting that the overexpression of DYRK2 is the driving force behind the amplicon (
). In fact, pulmonary adenocarcinoma patients with higher DYRK2 expression exhibited a substantially higher 5-year survival than the group with lower DYRK2 expression. The higher DYRK2 levels associating with negative lymphatic invasion (
). Although the response rates to chemotherapy between the DYRK2-positive and DYRK2-negative patients were not different, patients with DYRK2+ tumors in recurrent NSCLC were suggested to have better outcome with chemotherapy (
). Overall, the exact role of DYRK2 in lung neoplasia is still up for debate. Hence, using a similar strategy as suggested previously to generate conditional DYRK2 depletion in genetically engineered lung cancer mouse models for NSCLC, squamous-cell lung cancer, and other subtypes (
). On a similar note, a study using integrated high-resolution microarray analysis of gene copy number and expression in head and neck squamous-cell carcinoma cells reported that DYRK2 had the highest copy number and clear overexpression when compared with other genes in the 12q chromosomal amplicon (
). Thus, unbiased identification of DYRK2 as a protein/kinase involved in potential protumorigenic role along with its substrates such as p53, c-Myc, and c-Jun further fuels the need to stratify cancers into subtypes before embarking on DYRK2 molecular studies. This duality of protumorigenic and antitumorigenic roles has been reported for the paralogue DYRK1A as well (
) (Fig. 3 and Table 1), and hence, there is a clear precedence for such controversial roles in the DYRK family. One way of deconvoluting cancer-type and cell-type functions of a controversial kinase is by generating further tools such as potent and specific small-molecule kinase inhibitors.
Small-molecule inhibitors of DYRK2
Over the past three decades, various studies have been carried out to identify small-molecule inhibitors of kinases leading to the development of worldwide clinical trials and highly successful therapeutic targets and treatment options (
). For the DYRKs, more than 60 reported small-molecule inhibitors have been published or are available in the public domain. ChEMBL (https://www.ebi.ac.uk/chembl) predicts that there are >1500 potential small molecules that can bind and possibly inhibit DYRK2, including established anticancer drugs sunitinib, erlotinib, afatinib, ruxolitinib, and crizotinib. A significant effort has been focused on development of DYRK1A small-molecule inhibitors because DYRK1A has established roles in neurodegenerative disorders. Consequently, early on the only available DYRK2 inhibitors were those targeting DYRK1A with off-target activity on DYRK2. DYRKs are canonical CMGC kinases and broad-spectrum ATP-competitive kinase inhibitors such as staurosporine and its derivatives inhibit DYRK2 at low nanomolar concentrations (https://www.kinase-screen.mrc.ac.uk/kinase-inhibitors). Although there is a high degree of conservation between the kinase domains of class I and class II DYRKs, structural studies indicated that subtle amino acid substitutions in the hydrophobic inhibitor–docking pocket between DYRK1A and DYRK2 could confer significant degrees of inhibitor specificity (
). Interestingly, these amino acid substitutions contributed to the development and identification of various class-specific and often isoform-specific inhibitors for the DYRKs. Indeed, compound 5j that exhibited more than 100-fold sensitivity for DYRK1A over DYRK1B has no activity for class 2 DYRKs (
). Cocrystallization studies revealed that specific isoleucine to valine replacements in the docking site of curcumin resulted in a larger pocket in the class I DYRKs and thus reduced the shape complementarity to the inhibitor (
). Similarly, β-carboline derivatives such as harmine or AnnH75 exhibit more in vivo and in vitro potency for class I than class II DYRKs (Table 2). However, the benzimidazole derivatives such as INDY, TG003, and DYR219 exhibit a pan-DYRK activity in vitro and in vivo (Table 2) and have been reported to trigger degradation of DYRK proteins when treated in cells (
). This might explain some of the pronounced in vivo efficacy compared with in vitro observations for DYRK inhibitors wherein prolonged treatment leads to inhibition + degradation of the DYRK target, leading to a significant phenotype. Some promiscuous casein kinase inhibitors derived from benzimidazole potently inhibited DYRK1A and DYRK2 in vitro (
). Because the kinase domains of DYRK2 and DYRK3 are >90% similar at the amino acid level, there is a good chance that silmitasertib could indeed be a potent DYRK2 inhibitor as well. Silmitasertib exhibits blood–brain barrier penetrance similar to brain-penetrant DYR219 (
Leucettines, a class of potent inhibitors of cdc2-like kinases and dual specificity, tyrosine phosphorylation regulated kinases derived from the marine sponge leucettamine B: Modulation of alternative pre-RNA splicing.
An unusual binding model of the methyl 9-Anilinothiazolo[5,4-f] quinazoline-2-carbimidates (EHT 1610 and EHT 5372) confers high selectivity for dual-specificity tyrosine phosphorylation-regulated kinases.