Structure and substrate specificity of the Pim-1 kinase

Synthesis of new thieno[2,3-b]pyridine derivatives as pim-1 inhibitors

Three series of 5-bromo-thieno[2,3-b]pyridines bearing amide or benzoyl groups at position 2 were prepared as pim-1 inhibitors. All the prepared compounds were tested for their pim-1 enzyme inhibitory activity. Two compounds (3c and 5b) showed moderate pim-1 inhibitory activity with IC50 of 35.7 and 12.71 μM, respectively. Three other compounds (3d, 3g and 6d) showed poor pim-1 inhibition. The most active compounds were tested for their cytotoxic activity on five cell lines [MCF7, HEPG2, HCT116, A549 and PC3]. Compound 3g was the most potent cytotoxic agent on almost all the cell lines tested.

PIM1 kinase as a promise of targeted therapy in prostate cancer stem cells

Since the last decade, the PIM family serine/threonine kinases have become a focus in cancer research. Numerous clinical data supports that overexpression of PIM1 is associated with tumor formation in various tissues. However, little is known regarding the function of PIM1 in cancer stem cells. In cancer cells, PIM1 has essential roles in the regulation of the cell cycle, cell proliferation, cell survival and multiple drug resistance. In stem cells, PIM1 kinase exhibits a significant function in stem cell proliferation, self-renewal and expansion. Thus, PIM1 shows a great promise in cancer therapy by targeting stem cells. Furthermore, it is imperative to investigate Pim-1 targeting in cancer stem cells by applicable inhibitors for improving future outcomes. The present review investigated the potential of PIM1 as a therapy target in prostate cancer stem cells.

Unmasking determinants of specificity in the human kinome

Protein kinases control cellular responses to environmental cues by swift and accurate signal processing. Breakdowns in this high-fidelity capability are a driving force in cancer and other diseases. Thus, our limited understanding of which amino acids in the kinase domain encode substrate specificity, the so-called determinants of specificity (DoS), constitutes a major obstacle in cancer signaling. Here, we systematically discover several DoS and experimentally validate three of them, named the αC1, αC3, and APE-7 residues. We demonstrate that DoS form sparse networks of non-conserved residues spanning distant regions. Our results reveal a likely role for inter-residue allostery in specificity and an evolutionary decoupling of kinase activity and specificity, which appear loaded on independent groups of residues. Finally, we uncover similar properties driving SH2 domain specificity and demonstrate how the identification of DoS can be utilized to elucidate a greater understanding of the role of signaling networks in cancer (Creixell et al., 2015 [this issue of Cell]).

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Residues driving specificity in the kinase and SH2 domains are globally identified

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Three new such residues, termed αC1, αC3, and APE-7, are experimentally validated

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Specificity and catalytic activity appear to be encoded in distinct sets of residues

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The global identification of determinants allows the modeling of rewiring mutations

Overcoming ABCG2-mediated drug resistance with imidazo-[1,2-b]-pyridazine-based Pim1 kinase inhibitors

PURPOSE

Multidrug efflux pumps such as ABCG2 confer drug resistance to a number of cancer types, leading to poor prognosis and outcome. To date, the strategy of directly inhibiting multidrug efflux pumps in order to overcome drug resistance in cancer has been unsuccessful. An alternative strategy is to target proteins involved in the regulation of multidrug efflux pump activity or expression. Pim1 kinase has been demonstrated to phosphorylate ABCG2, promote its oligomerisation and contribute to its ability to confer drug resistance.

METHODS

In the present manuscript, imidazo-pyridazine-based inhibitors of Pim1 were examined for their ability to overcome ABCG2-mediated drug resistance. Drug efficacy was measured as a cytotoxic response or an effect on transport by ABCG2. Protein expression patterns were assessed using western immuno-blotting.

RESULTS

The two Pim1 inhibitors increased the potency of flavopiridol, mitoxantrone, topotecan and doxorubicin, specifically in ABCG2-expressing cells. This effect was associated with an increase in the cellular accumulation of [(3)H]-mitoxantrone, suggesting direct impairment of the transporter. However, prolonged pre-incubation with the studied inhibitors greatly enhanced the effect on mitoxantrone accumulation. The inhibitors caused a significant time-dependent reduction in the expression of ABCG2 in the resistant cells, an effect that would improve drug efficacy.

CONCLUSION

Consequently, it appears that the Pim1 inhibitors display a dual-mode effect on ABCG2-expressing cancer cells. This may provide a powerful new strategy in overcoming drug resistance by targeting proteins that regulate expression of efflux pumps.

Targeting the Pim kinases in multiple myeloma

Multiple myeloma (MM) is a plasma cell malignancy that remains incurable. Novel treatment strategies to improve survival are urgently required. The Pims are a small family of serine/threonine kinases with increased expression across the hematological malignancies. Pim-2 shows highest expression in MM and constitutes a promising therapeutic target. It is upregulated by the bone marrow microenvironment to mediate proliferation and promote MM survival. Pim-2 also has a key role in the bone destruction typically seen in MM. Additional putative roles of the Pim kinases in MM include trafficking of malignant cells, promoting oncogenic signaling in the hypoxic bone marrow microenvironment and mediating resistance to therapy. A number of Pim inhibitors are now under development with lead compounds entering the clinic. The ATP-competitive Pim inhibitor LGH447 has recently been reported to have single agent activity in MM. It is anticipated that Pim inhibition will be of clinical benefit in combination with standard treatments and/or with novel drugs targeting other survival pathways in MM.

PIM kinase (and Akt) biology and signaling in tumors

The initiation and progression of human cancer is frequently linked to the uncontrolled activation of survival kinases. Two such pro-survival kinases that are commonly amplified in cancer are PIM and Akt. These oncogenic proteins are serine/threonine kinases that regulate tumorigenesis by phosphorylating substrates that control the cell cycle, cellular metabolism, proliferation, and survival. Growing evidence suggests that cross-talk exists between the PIM and Akt kinases, indicating that they control partially overlapping survival signaling pathways that are critical to the initiation, progression, and metastatic spread of many types of cancer. The PI3K/Akt signaling pathway is activated in many human tumors, and it is well established as a promising anticancer target. Likewise, based on the role of PIM kinases in normal and tumor tissues, it is clear that this family of kinases represents an interesting target for anticancer therapy. Pharmacological inhibition of PIM has the potential to significantly influence the efficacy of standard and targeted therapies. This review focuses on the regulation of PIM kinases, their role in tumorigenesis, and the biological impact of their interaction with the Akt signaling pathway on the efficacy of cancer therapy.

Targeting Pim kinases for cancer treatment: opportunities and challenges

Pim oncogenes are highly expressed in many types of hematological and solid cancers. Pim kinases regulate the network of signaling pathways that are critical for tumorigenesis and development, making Pim kinases the attractive drug targets. Currently, two approaches have been employed in designing Pim kinase inhibitors: ATP-mimetics and non-ATP mimetics; but all target the ATP-binding pocket and are ATP-competitive. In this review, we summarize the current progress in understanding the Pim-related structure and biology, and provide insights into the binding modes of some prototypical Pim-1 inhibitors. The challenges as well as opportunities are highlighted for development of Pim kinase inhibitors as potential anticancer agents.

Structural basis and functions of abscisic acid receptors PYLs

Abscisic acid (ABA) plays a key role in many developmental processes and responses to adaptive stresses in plants. Recently, a new family of nucleocytoplasmic PYR/PYL/RCAR (PYLs) has been identified as bona fide ABA receptors. PYLs together with protein phosphatases type-2C (PP2Cs), Snf1 (Sucrose-non-fermentation 1)-related kinases subfamily 2 (SnRK2s) and downstream substrates constitute the core ABA signaling network. Generally, PP2Cs inactivate SnRK2s kinases by physical interaction and direct dephosphorylation. Upon ABA binding, PYLs change their conformations and then contact and inhibit PP2Cs, thus activating SnRK2s. Here, we reviewed the recent progress in research regarding the structures of the core signaling pathways of ABA, including the (+)-ABA, (-)-ABA and ABA analogs pyrabactin as well as 6AS perception by PYLs, SnRK2s mimicking PYLs in binding PP2Cs. PYLs inhibited PP2Cs in both the presence and absence of ABA and activated SnRK2s. The present review elucidates multiple ABA signal perception and transduction by PYLs, which might shed light on how to design small chemical compounds for improving plant performance in the future.

Label-free fluorescence detection of kinase activity using a gold nanoparticle based indicator displacement assay

A straightforward indicator-displacement assay (IDA) has been developed for the quantitative analysis of ATP→ADP conversion. The IDA relies on the use of gold nanoparticles passivated with a monolayer of thiols terminating with a 1,4,7-triazacyclononane (TACN)·Zn(2+) head group. The analytes ATP and ADP compete to a different extent with a fluorescent probe for binding to the monolayer surface. In the presence of ATP the fluorescent probe is free in solution, whereas in the presence of ADP the fluorescent probe is captured by the nanoparticles and its fluorescence is quenched. The linear response of the fluorescence signal towards different ratios of ATP : ADP permitted the detection of protein kinase activity simply by adding aliquots of the enzyme solution to the assay solution followed by measurement of the fluorescent intensity. The assay poses no restrictions on the target kinase nor does it require labeling of the kinase substrate. The assay was tested on the protein kinases PIM-1 and Src and validated through a direct comparison with the classical radiometric assay using the [γ-(32)P]-labeled ATP.

Global analysis of human nonreceptor tyrosine kinase specificity using high-density peptide microarrays

Protein kinases phosphorylate substrates in the context of specific phosphorylation site sequence motifs. The knowledge of the specific sequences that are recognized by kinases is useful for mapping sites of phosphorylation in protein substrates and facilitates the generation of model substrates to monitor kinase activity. Here, we have adapted a positional scanning peptide library method to a microarray format that is suitable for the rapid determination of phosphorylation site motifs for tyrosine kinases. Peptide mixtures were immobilized on glass slides through a layer of a tyrosine-free Y33F mutant avidin to facilitate the analysis of phosphorylation by radiolabel assay. A microarray analysis provided qualitatively similar results in comparison with the solution phase peptide library “macroarray” method. However, much smaller quantities of kinases were required to phosphorylate peptides on the microarrays, which thus enabled a proteome scale analysis of kinase specificity. We illustrated this capability by microarray profiling more than 80% of the human nonreceptor tyrosine kinases (NRTKs). Microarray results were used to generate a universal NRTK substrate set of 11 consensus peptides for in vitro kinase assays. Several substrates were highly specific for their cognate kinases, which should facilitate their incorporation into kinase-selective biosensors.

EBNA3C augments Pim-1 mediated phosphorylation and degradation of p21 to promote B-cell proliferation

Epstein–Barr virus (EBV), a ubiquitous human herpesvirus, can latently infect the human population. EBV is associated with several types of malignancies originating from lymphoid and epithelial cell types. EBV latent antigen 3C (EBNA3C) is essential for EBV-induced immortalization of B-cells. The Moloney murine leukemia provirus integration site (PIM-1), which encodes an oncogenic serine/threonine kinase, is linked to several cellular functions involving cell survival, proliferation, differentiation, and apoptosis. Notably, enhanced expression of Pim-1 kinase is associated with numerous hematological and non-hematological malignancies. A higher expression level of Pim-1 kinase is associated with EBV infection, suggesting a crucial role for Pim-1 in EBV-induced tumorigenesis. We now demonstrate a molecular mechanism which reveals a direct role for EBNA3C in enhancing Pim-1 expression in EBV-infected primary B-cells. We also showed that EBNA3C is physically associated with Pim-1 through its amino-terminal domain, and also forms a molecular complex in B-cells. EBNA3C can stabilize Pim-1 through abrogation of the proteasome/Ubiquitin pathway. Our results demonstrate that EBNA3C enhances Pim-1 mediated phosphorylation of p21 at the Thr¹⁴⁵ residue. EBNA3C also facilitated the nuclear localization of Pim-1, and promoted EBV transformed cell proliferation by altering Pim-1 mediated regulation of the activity of the cell-cycle inhibitor p21/WAF1. Our study demonstrated that EBNA3C significantly induces Pim-1 mediated proteosomal degradation of p21. A significant reduction in cell proliferation of EBV-transformed LCLs was observed upon stable knockdown of Pim-1. This study describes a critical role for the oncoprotein Pim-1 in EBV-mediated oncogenesis, as well as provides novel insights into oncogenic kinase-targeted therapeutic intervention of EBV-associated cancers.

The oncogenic serine/threonine kinase Pim-1 is upregulated in a number of human cancers including lymphomas, gastric, colorectal and prostate carcinomas. EBV nuclear antigen 3C (EBNA3C) is essential for EBV-induced transformation of human primary B-lymphocytes. Our current study revealed that EBNA3C significantly enhances Pim-1 kinase expression at both the transcript and protein levels. EBNA3C also interacts with Pim-1 and can form a complex in EBV-transformed cells. Moreover, EBNA3C increases nuclear localization of Pim-1 and stabilizes Pim-1 protein levels by inhibiting its poly-ubiquitination. Additionally, EBNA3C augments Pim-1 mediated phosphorylation of p21 and its proteosomal degradation. Stable knockdown of Pim-1 using si-RNA showed a significant decrease in proliferation of EBV transformed lymphoblastoid cell lines and subsequent induction of apoptosis by triggering the intrinsic apoptotic pathway. Therefore, our study demonstrated a new mechanism by which the oncogenic Pim-1 kinase targeted by EBV latent antigen 3C can inhibit p21 function, and is therefore a potential therapeutic target for the treatment of EBV-associated malignancies.

PIM1 kinase phosphorylates the human transcription factor FOXP3 at serine 422 to negatively regulate its activity under inflammation

Previous reports have suggested that human CD4(+) CD25(hi)FOXP3(+) T regulatory cells (Tregs) have functional plasticity and may differentiate into effector T cells under inflammation. The molecular mechanisms underlying these findings remain unclear. Here we identified the residue serine 422 of human FOXP3 as a phosphorylation site that regulates its function, which is not present in murine Foxp3. PIM1 kinase, which is highly expressed in human Tregs, was found to be able to interact with and to phosphorylate human FOXP3 at serine 422. T cell receptor (TCR) signaling inhibits PIM1 induction, whereas IL-6 promotes PIM1 expression in in vitro expanded human Tregs. PIM1 negatively regulates FOXP3 chromatin binding activity by specifically phosphorylating FOXP3 at Ser(422). Our data also suggest that phosphorylation of FOXP3 at the Ser(418) site could prevent FOXP3 phosphorylation at Ser(422) mediated by PIM1. Knockdown of PIM1 in in vitro expanded human Tregs promoted FOXP3-induced target gene expression, including CD25, CTLA4, and glucocorticoid-induced tumor necrosis factor receptor (GITR), or weakened FOXP3-suppressed IL-2 gene expression and enhanced the immunosuppressive activity of Tregs. Furthermore, PIM1-specific inhibitor boosted FOXP3 DNA binding activity in in vitro expanded primary Tregs and also enhanced their suppressive activity toward the proliferation of T effector cells. Taken together, our findings suggest that PIM1 could be a new potential therapeutic target in the prevention and treatment of human-specific autoimmune diseases because of its ability to modulate the immunosuppressive activity of human Tregs.

Pathophysiological roles of Pim-3 kinase in pancreatic cancer development and progression

Pim-3 is a member of the provirus integration site for Moloney murine leukemia virus (Pim) family proteins that exhibit serine/threonine kinase activity. Similar to the other Pim kinases (Pim-1 and Pim-2), Pim-3 is involved in many cellular processes, including cell proliferation, survival, and protein synthesis. Although Pim-3 is expressed in normal vital organs, it is overexpressed particularly in tumor tissues of endoderm-derived organs, including the liver, pancreas, and colon. Silencing of Pim-3 expression can retard in vitro cell proliferation of hepatocellular, pancreatic, and colon carcinoma cell lines by promoting cell apoptosis. Pim-3 lacks the regulatory domains similarly as Pim-1 and Pim-2 lack, and therefore, Pim-3 can exhibit its kinase activity once it is expressed. Pim-3 expression is regulated at transcriptional and post-transcriptional levels by transcription factors (e.g., Ets-1) and post-translational modifiers (e.g., translationally-controlled tumor protein), respectively. Pim-3 could promote growth and angiogenesis of human pancreatic cancer cells in vivo in an orthotopic nude mouse model. Furthermore, a Pim-3 kinase inhibitor inhibited cell proliferation when human pancreatic cancer cells were injected into nude mice, without inducing any major adverse effects. Thus, Pim-3 kinase may serve as a novel molecular target for developing targeting drugs against pancreatic and other types of cancer.

A small-molecule inhibitor of PIM kinases as a potential treatment for urothelial carcinomas

The proto-oncogene proviral integration site for moloney murine leukemia virus (PIM) kinases (PIM-1, PIM-2, and PIM-3) are serine/threonine kinases that are involved in a number of signaling pathways important to cancer cells. PIM kinases act in downstream effector functions as inhibitors of apoptosis and as positive regulators of G₁-S phase progression through the cell cycle. PIM kinases are upregulated in multiple cancer indications, including lymphoma, leukemia, multiple myeloma, and prostate, gastric, and head and neck cancers. Overexpression of one or more PIM family members in patient tumors frequently correlates with poor prognosis. The aim of this investigation was to evaluate PIM expression in low- and high-grade urothelial carcinoma and to assess the role PIM function in disease progression and their potential to serve as molecular targets for therapy. One hundred thirty-seven cases of urothelial carcinoma were included in this study of surgical biopsy and resection specimens. High levels of expression of all three PIM family members were observed in both noninvasive and invasive urothelial carcinomas. The second-generation PIM inhibitor, TP-3654, displays submicromolar activity in pharmacodynamic biomarker modulation, cell proliferation studies, and colony formation assays using the UM-UC-3 bladder cancer cell line. TP-3654 displays favorable human ether-à-go-go-related gene and cytochrome P450 inhibition profiles compared with the first-generation PIM inhibitor, SGI-1776, and exhibits oral bioavailability. In vivo xenograft studies using a bladder cancer cell line show that PIM kinase inhibition can reduce tumor growth, suggesting that PIM kinase inhibitors may be active in human urothelial carcinomas.

Pim kinase inhibitory and antiproliferative activity of a novel series of meridianin C derivatives

A novel series of meridianin C derivatives substituted at C-5 position were prepared. These derivatives were tested for their kinase inhibitory potencies against all three family members of the pim kinases (Pim-1, Pim-2 and Pim-3). In addition, their antiproliferative activity towards three human leukemia cell lines as MV4-11, Jurkat clone E6-1 and K562 has been evaluated. Structure activity relationships at C-3 and C-5 positions of indole were performed to better understand the mechanism behind the enhanced potency. Compound 7f, the most active compound of the series showed a single-digit nanomolar IC50 with selectivity towards Pim-1 kinase.

Use of regulators and inhibitors of Pim-1, a serine/threonine kinase, for tumour therapy (review)

Pim-1 is a proto-oncogene that encodes a serine/threonine kinase that is overexpressed in a range of haematopoietic malignancies and solid cancers. Pim-1 expression is tightly regulated by multiple biomolecules at different levels. Several lines of evidence have indicated that dysregulation of Pim-1 can interfere with the cell cycle and apoptosis to promote malignant transformation of a number of types of tumour. Thus, investigation of Pim-1 regulation may provide important theoretical guidance for the development of molecular targeting therapies and drug treatments for Pim-1‑associated diseases. Regulators of Pim-1 expression, include microRNAs, oestrogen, inecalcitol, adenosine triphosphate (ATP) mimetic inhibitors and ATP competitive inhibitors of Pim-1. Combinations of inhibitors of Pim-1 expression and Pim-1‑specific inhibitors may provide novel therapies for cancer patients and directions for cancer treatment.

REPERTOIRE OF PROTEIN KINASES ENCODED IN THE GENOME OF ZEBRAFISH SHOWS REMARKABLY LARGE POPULATION OF PIM KINASES

In recent times, zebrafish has garnered lot of popularity as model organism to study human cancers. Despite high evolutionary divergence from humans, zebrafish develops almost all types of human tumors when induced. However, mechanistic details of tumor formation have remained largely unknown. Present study is aimed at analysis of repertoire of kinases in zebrafish proteome to provide insights into various cellular components. Annotation using highly sensitive remote homology detection methods revealed "substantial expansion" of Ser/Thr/Tyr kinase family in zebrafish compared to humans, constituting over 3% of proteome. Subsequent classification of kinases into subfamilies revealed presence of large number of CAMK group of kinases, with massive representation of PIM kinases, important for cell cycle regulation and growth. Extensive sequence comparison between human and zebrafish PIM kinases revealed high conservation of functionally important residues with a few organism specific variations. There are about 300 PIM kinases in zebrafish kinome, while human genome codes for only about 500 kinases altogether. PIM kinases have been implicated in various human cancers and are currently being targeted to explore their therapeutic potentials. Hence, in depth analysis of PIM kinases in zebrafish has opened up new avenues of research to verify the model organism status of zebrafish.

Functional divergence and evolutionary turnover in mammalian phosphoproteomes

Protein phosphorylation is a key mechanism to regulate protein functions. However, the contribution of this protein modification to species divergence is still largely unknown. Here, we studied the evolution of mammalian phosphoregulation by comparing the human and mouse phosphoproteomes. We found that 84% of the positions that are phosphorylated in one species or the other are conserved at the residue level. Twenty percent of these conserved sites are phosphorylated in both species. This proportion is 2.5 times more than expected by chance alone, suggesting that purifying selection is preserving phosphoregulation. However, we show that the majority of the sites that are conserved at the residue level are differentially phosphorylated between species. These sites likely result from false-negative identifications due to incomplete experimental coverage, false-positive identifications and non-functional sites. In addition, our results suggest that at least 5% of them are likely to be true differentially phosphorylated sites and may thus contribute to the divergence in phosphorylation networks between mouse and humans and this, despite residue conservation between orthologous proteins. We also showed that evolutionary turnover of phosphosites at adjacent positions (in a distance range of up to 40 amino acids) in human or mouse leads to an over estimation of the divergence in phosphoregulation between these two species. These sites tend to be phosphorylated by the same kinases, supporting the hypothesis that they are functionally redundant. Our results support the hypothesis that the evolutionary turnover of phosphorylation sites contributes to the divergence in phosphorylation profiles while preserving phosphoregulation. Overall, our study provides advanced analyses of mammalian phosphoproteomes and a framework for the study of their contribution to phenotypic evolution.

Understanding how differences in cellular regulation lead to phenotypic differences between species remains an open challenge in evolutionary genetics. The extensive phosphorylation data currently available allows to compare the human and mouse phosphoproteomes and to measure changes in their phosphoregulation. We found a general conservation of phosphorylation sites between these two species. However, a fraction of sites are conserved at the sequence level (the same amino acid is present in both species) but differ in their phosphorylation status. These sites represent candidate sites that have the potential to explain differences between human and mouse signalling networks that do not depend on the divergence of orthologous residues. Furthermore, we identified several sites where to a phosphorylation site in one species corresponds a non-phosphorylatable residue in the other one. These cases represent clear differences in protein regulation. Recent studies suggest that phosphorylation sites can shift position during evolution, leading to configurations in which pairs of divergent phosphorylation sites are functionally redundant. We identified more than 100 putative such cases, suggesting that divergence in amino acid does not necessarily imply functional divergence when comparing phosphoproteomes. Overall, our study provides new key concepts and data for the study of how regulatory differences may be linked to phenotypic ones at the network level.

Benzoselenadiazole-based responsive long-lifetime photoluminescent probes for protein kinases

Benzoselenadiazole was used as a novel scaffold for construction of microsecond scale responsive photoluminescent probes for protein kinases.

Identification of a major determinant for serine-threonine kinase phosphoacceptor specificity

Eukaryotic protein kinases are generally classified as being either tyrosine or serine-threonine specific. Though not evident from inspection of their primary sequences, many serine-threonine kinases display a significant preference for serine or threonine as the phosphoacceptor residue. Here we show that a residue located in the kinase activation segment, which we term the “DFG+1” residue, acts as a major determinant for serine-threonine phosphorylation site specificity. Mutation of this residue was sufficient to switch the phosphorylation site preference for multiple kinases, including the serine-specific kinase PAK4 and the threonine-specific kinase MST4. Kinetic analysis of peptide substrate phosphorylation and crystal structures of PAK4-peptide complexes suggested that phosphoacceptor residue preference is not mediated by stronger binding of the favored substrate. Rather, favored kinase-phosphoacceptor combinations likely promote a conformation optimal for catalysis. Understanding the rules governing kinase phosphoacceptor preference allows kinases to be classified as serine or threonine specific based on their sequence.

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A single active site residue can determine kinase phosphoacceptor specificity

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Favored and disfavored substrates promote distinct kinase-bound conformations

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A simple rule predicts kinase phosphoacceptor preference from its DFG+1 residue

Stochastic detection of Pim protein kinases reveals electrostatically enhanced association of a peptide substrate

In stochastic sensing, the association and dissociation of analyte molecules is observed as the modulation of an ionic current flowing through a single engineered protein pore, enabling the label-free determination of rate and equilibrium constants with respect to a specific binding site. We engineered sensors based on the staphylococcal α-hemolysin pore to allow the single-molecule detection and characterization of protein kinase-peptide interactions. We enhanced this approach by using site-specific proteolysis to generate pores bearing a single peptide sensor element attached by an N-terminal peptide bond to the trans mouth of the pore. Kinetics and affinities for the Pim protein kinases (Pim-1, Pim-2, and Pim-3) and cAMP-dependent protein kinase were measured and found to be independent of membrane potential and in good agreement with previously reported data. Kinase binding exhibited a distinct current noise behavior that forms a basis for analyte discrimination. Finally, we observed unusually high association rate constants for the interaction of Pim kinases with their consensus substrate Pimtide (~10(7) to 10(8) M(-1) · s(-1)), the result of electrostatic enhancement, and propose a cellular role for this phenomenon.

Small molecule inhibitors of PIM1 kinase: July 2009 to February 2013 patent update

INTRODUCTION

The proviral insertion in murine (PIM) lymphoma proteins for which three isoforms, PIM1, PIM2 and PIM3 have been identified, belonging to the family of serine/threonine kinases has emerged recently as an important therapeutic target for the development of selective inhibitors as the new drugs for treating hematological malignancies and solid tumors. The small molecules developed by academia and the pharmaceutical industry have steadily increased in the last few years. Several drug discovery groups focus on treating disorders, such as cancer mediated by PIM kinase, have provided preclinical evidence suggesting that PIM inhibitor provides anti-apoptotic activity, inhibit cell survival and cell proliferation.

AREAS COVERED

This article discloses recent reviews on research and advances published in the patent literature and scientific publications from July 2009 to February 2013, highlighting discoveries on PIM1 kinase.

EXPERT OPINION

Several PIM1 kinase small molecule inhibitors are now at the pre-clinical research stage, development and testing. Though nearly 40 patents emerged in the last 3 years, greater efforts towards additional designs and medicinal chemistry continues for developing clinically efficacious PIM1 inhibitors, due to the significance of the target for cancer and the potential for novel and diverse inhibitors as drug candidates.

Discovery and optimization of pyrrolo[1,2-a]pyrazinones leads to novel and selective inhibitors of PIM kinases

A novel series of PIM inhibitors was derived from a combined effort in natural product-inspired library generation and screening. The novel pyrrolo[1,2-a]pyrazinones initial hits are inhibitors of PIM isoforms with IC50 values in the low micromolar range. The application of a rational optimization strategy, guided by the determination of the crystal structure of the complex in the kinase domain of PIM1 with compound 1, led to the discovery of compound 15a, which is a potent PIM kinases inhibitor exhibiting excellent selectivity against a large panel of kinases, representative of each family. The synthesis, structure-activity relationship studies, and pharmacokinetic data of compounds from this inhibitor class are presented herein. Furthermore, the cellular activities including inhibition of cell growth and modulation of downstream targets are also described.

Pim-1 kinase phosphorylates and stabilizes 130 kDa FLT3 and promotes aberrant STAT5 signaling in acute myeloid leukemia with FLT3 internal tandem duplication

The type III receptor tyrosine kinase fms-like tyrosine kinase 3 (FLT3) is expressed on both normal hematopoietic stem cells and acute myeloid leukemia (AML) cells and regulates their proliferation. Internal tandem duplication (ITD) mutation of FLT3 is present in a third of AML cases, results in constitutive activation and aberrant signaling of FLT3, and is associated with adverse treatment outcomes. While wild-type (WT) FLT3 is predominantly a 150 kDa complex glycosylated cell surface protein, FLT3-ITD is partially retained in the endoplasmic reticulum as a 130 kDa underglycosylated species associated with the chaperones calnexin and heat shock protein (HSP) 90, and mediates aberrant STAT5 signaling, which upregulates the oncogenic serine/threonine kinase Pim-1. FLT3 contains a Pim-1 substrate consensus serine phosphorylation site, and we hypothesized that it might be a Pim-1 substrate. Pim-1 was indeed found to directly interact with and serine-phosphorylate FLT3. Pim-1 inhibition decreased the expression and half-life of 130 kDa FLT3, with partial abrogation by proteasome inhibition, in association with decreased FLT3 binding to calnexin and HSP90, and increased 150 kDa FLT3 expression and half-life, with abrogation by inhibition of glycosylation. These findings were consistent with Pim-1 stabilizing FLT3-ITD as a 130 kDa species associated with calnexin and HSP90 and inhibiting its glycosylation to form the 150 kDa species. Pim-1 knockdown effects were similar. Pim-1 inhibition also decreased phosphorylation of FLT3 at tyrosine 591 and of STAT5, and expression of Pim-1 itself, consistent with inhibition of the FLT3-ITD-STAT5 signaling pathway. Finally, Pim-1 inhibition synergized with FLT3 inhibition in inducing apoptosis of FLT3-ITD cells. This is, to our knowledge, the first demonstration of a role of Pim-1 in a positive feedback loop promoting aberrant signaling in malignant cells.

Pim2 is required for maintaining multiple myeloma cell growth through modulating TSC2 phosphorylation

Multiple myeloma (MM) is the second most common hematologic malignancy. Despite recent treatment advances, it remains incurable. Here, we report that Pim2 kinase expression is highly elevated in MM cells and demonstrate that it is required for MM cell proliferation. Functional interference of Pim2 activity either by short hairpin RNAs or by a potent and selective small-molecule inhibitor leads to significant inhibition of MM cell proliferation. Pim inhibition results in a significant decrease of mammalian target of rapamycin C1 (mTOR-C1) activity, which is critical for cell proliferation. We identify TSC2, a negative regulator of mTOR-C1, as a novel Pim2 substrate and show that Pim2 directly phosphorylates TSC2 on Ser-1798 and relieves the suppression of TSC2 on mTOR-C1. These findings support Pim2 as a promising therapeutic target for MM and define a novel Pim2-TSC2-mTOR-C1 pathway that drives MM proliferation.

Crystal structure of pim1 kinase in complex with a pyrido[4,3-d]pyrimidine derivative suggests a unique binding mode

Human Pim1 kinase is a serine/threonine protein kinase that plays important biological roles in cell survival, apoptosis, proliferation, and differentiation. Moreover, Pim1 is up-regulated in various hematopoietic malignancies and solid tumors. Thus, Pim1 is an attractive target for cancer therapeutics, and there has been growing interest in developing small molecule inhibitors for Pim1. Here, we describe the crystal structure of Pim1 in complex with a newly developed pyrido[4,3-d]pyrimidine-derivative inhibitor (SKI-O-068). Our inhibitor exhibits a half maximum inhibitory concentration (IC₅₀) of 123 (±14) nM and has an unusual binding mode in complex with Pim1 kinase. The interactions between SKI-O-068 and the Pim1 active site pocket residue are different from those of other scaffold inhibitor-bound structures. The binding mode analysis suggests that the SKI-O-068 inhibitor can be improved by introducing functional groups that facilitate direct interaction with Lys67, which aid in the design of an optimized inhibitor.

Profiling the substrate specificity of protein kinases by on-bead screening of peptide libraries

A robust, high-throughput method has been developed to screen one-bead-one-compound peptide libraries to systematically profile the sequence specificity of protein kinases. Its ability to provide individual sequences of the preferred substrates permits the identification of sequence contextual effects and nonpermissive residues. Application of the library method to kinases Pim1, MKK6, and Csk revealed that Pim1 and Csk are highly active toward peptide substrates and recognize specific sequence motifs, whereas MKK6 has little activity or sequence selectivity against peptide substrates. Pim1 recognizes peptide substrates of the consensus RXR(H/R)X(S/T); it accepts essentially any amino acid at the S/T-2 and S/T+1 positions, but strongly disfavors acidic residues (Asp or Glu) at the S/T-2 position and a proline residue at the S/T+1 position. The selected Csk substrates show strong sequence covariance and fall into two classes with the consensus sequences of (D/E)EPIYϕXϕ and (D/E)(E/D)S(E/D/I)YϕXϕ (where X is any amino acid and ϕ is a hydrophobic amino acid). Database searches and in vitro kinase assays identified phosphatase PTP-PEST as a Pim1 substrate and phosphatase SHP-1 as a potential Csk substrate. Our results demonstrate that the sequence specificity of protein kinases is defined not only by favorable interactions between permissive residue(s) on the substrate and their cognate binding site(s) on the kinase but also by repulsive interactions between the kinase and nonpermissive residue(s).

eIF4B phosphorylation by pim kinases plays a critical role in cellular transformation by Abl oncogenes

Alterations in translation occur in cancer cells, but the precise pathogenic processes and mechanistic underpinnings are not well understood. In this study, we report that interactions between Pim family kinases and the translation initiation factor eIF4B are critical for Abl oncogenicity. Pim kinases, Pim-1 and Pim-2, both directly phosphorylated eIF4B on Ser406 and Ser422. Phosphorylation of eIF4B on Ser422 was highly sensitive to pharmacologic or RNA interference-mediated inhibition of Pim kinases. Expression and phosphorylation of eIF4B relied upon Abl kinase activity in both v-Abl- and Bcr-Abl-expressing leukemic cells based on their blockade by the Abl kinase inhibitor imatinib. Ectopic expression of phosphomimetic mutants of eIF4B conferred resistance to apoptosis by the Pim kinase inhibitor SMI-4a in Abl-transformed cells. In contrast, silencing eIF4B sensitized Abl-transformed cells to imatinib-induced apoptosis and also inhibited their growth as engrafted tumors in nude mice. Extending these observations, we found that primary bone marrow cells derived from eIF4B-knockdown transgenic mice were less susceptible to Abl transformation, relative to cells from wild-type mice. Taken together, our results identify eIF4B as a critical substrate of Pim kinases in mediating the activity of Abl oncogenes, and they highlight eIF4B as a candidate therapeutic target for treatment of Abl-induced cancers.

Effect of single amino acid substitution observed in cancer on Pim-1 kinase thermodynamic stability and structure

Pim-1 kinase, a serine/threonine protein kinase encoded by the pim proto-oncogene, is involved in several signalling pathways such as the regulation of cell cycle progression and apoptosis. Many cancer types show high expression levels of Pim kinases and particularly Pim-1 has been linked to the initiation and progression of the malignant phenotype. In several cancer tissues somatic Pim-1 mutants have been identified. These natural variants are nonsynonymous single nucleotide polymorphisms, variations of a single nucleotide occurring in the coding region and leading to amino acid substitutions. In this study we investigated the effect of amino acid substitution on the structural stability and on the activity of Pim-1 kinase. We expressed and purified some of the mutants of Pim-1 kinase that are expressed in cancer tissues and reported in the single nucleotide polymorphisms database. The point mutations in the variants significantly affect the conformation of the native state of Pim-1. All the mutants, expressed as soluble recombinant proteins, show a decreased thermal and thermodynamic stability and a lower activation energy values for kinase activity. The decreased stability accompanied by an increased flexibility suggests that Pim-1 variants may be involved in a wider network of protein interactions. All mutants bound ATP and ATP mimetic inhibitors with comparable IC50 values suggesting that the studied Pim-1 kinase mutants can be efficiently targeted with inhibitors developed for the wild type protein.

Selective bisubstrate inhibitors with sub-nanomolar affinity for protein kinase Pim-1

Potent and selective: The unique nature of the ATP binding pocket structure of Pim family protein kinases (PKs) was used for the development of bisubstrate inhibitors and a fluorescent probe with sub-nanomolar affinity. Conjugates of arginine-rich peptides with two ATP mimetic scaffolds were synthesized and tested as inhibitors of Pim-1. Against a panel of 124 protein kinases, a novel ARC-PIM conjugate selectively inhibited PKs of the Pim family.

The PIM family of serine/threonine kinases in cancer

The proviral insertion site in Moloney murine leukemia virus, or PIM proteins, are a family of serine/threonine kinases composed of three different isoforms (PIM1, PIM2, and PIM3) that are highly evolutionarily conserved. These proteins are regulated primarily by transcription and stability through pathways that are controlled by Janus kinase/Signal transducer and activator of transcription, JAK/STAT, transcription factors. The PIM family proteins have been found to be overexpressed in hematological malignancies and solid tumors, and their roles in these tumors were confirmed in mouse tumor models. Furthermore, the PIM family proteins have been implicated in the regulation of apoptosis, metabolism, cell cycle, and homing and migration, which has led to the postulation of these proteins as interesting targets for anticancer drug discovery. In the present work, we review the importance of PIM kinases in tumor growth and as drug targets.

Conditional transgenic expression of PIM1 kinase in prostate induces inflammation-dependent neoplasia

The Pim proteins are a family of highly homologous protein serine/threonine kinases that have been found to be overexpressed in cancer. Elevated levels of Pim1 kinase were first discovered in human leukemia and lymphomas. However, more recently Pim1 was found to be increased in solid tumors, including pancreatic and prostate cancers, and has been proposed as a prognostic marker. Although the Pim kinases have been identified as oncogenes in transgenic models, they have weak transforming abilities on their own. However, they have been shown to greatly enhance the ability of other genes or chemical carcinogens to induce tumors. To explore the role of Pim1 in prostate cancer, we generated conditional Pim1 transgenic mice, expressed Pim1 in prostate epithelium, and analyzed the contribution of PIM1 to neoplastic initiation and progression. Accordingly, we explored the effect of PIM1 overexpression in 3 different settings: upon hormone treatment, during aging, and in combination with the absence of one Pten allele. We have found that Pim1 overexpression increased the severity of mouse prostate intraepithelial neoplasias (mPIN) moderately in all three settings. Furthermore, Pim1 overexpression, in combination with the hormone treatment, increased inflammation surrounding target tissues leading to pyelonephritis in transgenic animals. Analysis of senescence induced in these prostatic lesions showed that the lesions induced in the presence of inflammation exhibited different behavior than those induced in the absence of inflammation. While high grade prostate preneoplastic lesions, mPIN grades III and IV, in the presence of inflammation did not show any senescence markers and demonstrated high levels of Ki67 staining, untreated animals without inflammation showed senescence markers and had low levels of Ki67 staining in similar high grade lesions. Our data suggest that Pim1 might contribute to progression rather than initiation in prostate neoplasia.

A new target for an old drug: identifying mitoxantrone as a nanomolar inhibitor of PIM1 kinase via kinome-wide selectivity modeling

The rational design of selective kinase inhibitors remains a great challenge. Here we describe a physics-based approach to computationally modeling the kinase inhibitor selectivity profile. We retrospectively assessed this protocol by computing the binding profiles of 17 well-known kinase inhibitors against 143 kinases. Next, we predicted the binding profile of the chemotherapy drug mitoxantrone, and chose the predicted top five kinase targets for in vitro kinase assays. Remarkably, mitoxantrone was shown to possess low nanomolar inhibitory activity against PIM1 kinase and to inhibit the PIM1-mediated phosphorylation in cancer cells. We further determined the crystal complex structure of PIM1 bound with mitoxantrone, which reveals the structural and mechanistic basis for a novel mode of PIM1 inhibition. Although mitoxantrone's mechanism of action had been originally thought to act through DNA intercalation and type II topoisomerase inhibition, we hypothesize that PIM1 kinase inhibition might also contribute to mitoxantrone's therapeutic efficacy and specificity.

PIM1 kinase inhibitors induce radiosensitization in non-small cell lung cancer cells

Radiotherapy plays a critical role in the treatment of non-small cell lung cancer (NSCLC). However, radioresistance is a major barrier against increasing the efficiency of radiotherapy for NSCLC. To understand the mechanisms underlying NSCLC radioresistance, we previously focused on the potential involvement of PIM1, PRAS40, FOXO3a, 14-3-3, and protein phosphatases. Among these proteins, PIM1 functioned as an oncogene and was found to act as a crucial mediator in radioresistant NSCLC cells. Therefore, we investigated the use of PIM1-specific inhibitors as novel therapeutic drugs to regulate radiosensitivity in NSCLC. After structure-based drug selection, SGI-1776, ETP-45299, and tryptanthrin were selected as candidates of PIM1 inhibitors that act as radiosensitizers. With irradiation, these drugs inhibited only PIM1 kinase activity without affecting PIM1 mRNA/protein levels or cellular localization. When PIM1 kinase activity was suppressed by these inhibitors, PRAS40 was not phosphorylated. Consequently, unphosphorylated PRAS40 did not form trimeric complexes with 14-3-3 and FOXO3a, leading to increased nuclear localization of FOXO3a. Nuclear FOXO3a promoted the expression of pro-apoptotic proteins such as Bim and FasL, resulting in a radiosensitizing effect on radioresistant NSCLC cells. Moreover, an in vivo xenograft mouse model confirmed this radiosensitizing effect induced by PIM1 inhibitors. In these model systems, tumor volume was significantly reduced by a combinational treatment with irradiation and PIM1 inhibitors compared to irradiation alone. Taken together, our findings provided evidence that PIM1-specific inhibitors, SGI-1776, ETP-45299, and tryptanthrin, can act as novel radiosensitizers to enhance the efficacy of radiotherapy by inhibiting irradiation-induced signaling pathway associated with radioresistance.

T-18, a stemonamide synthetic intermediate inhibits Pim kinase activity and induces cell apoptosis, acting as a potent anticancer drug

Pim-3 kinase has been shown to be aberrantly expressed in premalignant and malignant lesions of endoderm-derived organs such as the liver, pancreas, colon and stomach. Pim-3 kinase inactivates the Bad protein, a proapoptotic molecule, and improves the expression of Bcl-xL, an antiapoptotic molecule, to promote cell proliferation. Thus, blocking Pim-3 kinase activity may be a new strategy for the treatment of pancreatic cancer. In this study, we screened low molecular compounds and observed that the stemonamide synthetic intermediate, T-18, potently inhibited Pim kinase activity. Moreover, T-18 inhibited the proliferation of human pancreatic, as well as that of hepatocellular and colon cancer cells in vitro. It also induced the apoptosis of human pancreatic carcinoma cells in vitro by decreasing the levels of phospho-Ser112-Bad; the levels of Pim-3 kinase and total Bad protein were not altered. Furthermore, T-18 inhibited the growth of human pancreatic cancer cells in nude mice without apparent adverse effects when the tumor was palpable. These observations indicate that stemonamide synthetic intermediates may be novel drugs for the treatment of gastrointestinal cancers, particularly pancreatic cancer.

Selectivity, cocrystal structures, and neuroprotective properties of leucettines, a family of protein kinase inhibitors derived from the marine sponge alkaloid leucettamine B

DYRKs (dual specificity, tyrosine phosphorylation regulated kinases) and CLKs (cdc2-like kinases) are implicated in the onset and development of Alzheimer's disease and Down syndrome. The marine sponge alkaloid leucettamine B was recently identified as an inhibitor of DYRKs/CLKs. Synthesis of analogues (leucettines) led to an optimized product, leucettine L41. Leucettines were cocrystallized with DYRK1A, DYRK2, CLK3, PIM1, and GSK-3β. The selectivity of L41 was studied by activity and interaction assays of recombinant kinases and affinity chromatography and competition affinity assays. These approaches revealed unexpected potential secondary targets such as CK2, SLK, and the lipid kinase PIKfyve/Vac14/Fig4. L41 displayed neuroprotective effects on glutamate-induced HT22 cell death. L41 also reduced amyloid precursor protein-induced cell death in cultured rat brain slices. The unusual multitarget selectivity of leucettines may account for their neuroprotective effects. This family of kinase inhibitors deserves further optimization as potential therapeutics against neurodegenerative diseases such as Alzheimer's disease.

Pim kinases in cancer: diagnostic, prognostic and treatment opportunities

PIM proteins belong to a family of ser/thr kinases composed of 3 members, PIM1, PIM2 and PIM3, with greatly overlapping functions. PIM kinases are mainly responsible for cell cycle regulation, antiapoptotic activity and the homing and migration of receptor tyrosine kinases mediated via the JAK/STAT pathway. PIM kinases have been found to be upregulated in many hematological malignancies and solid tumors. Although these kinases have been described as weak oncogenes, they are heavily targeted for anticancer drug discovery. The present review summarizes the discoveries made to date regarding PIM kinases as driving oncogenes in the process of tumorigenesis and their validation as drug targets.

IL-6 stimulates STAT3 and Pim-1 kinase in pancreatic cancer cell lines

OBJECTIVES

We investigated the signaling pathways activated in response to interleukin 6 (IL-6) in pancreatic cell lines, with a focus on signal transducer and activator of transcription 3 (STAT3) and proto-oncogene serine/threonine-protein (Pim-1) kinase.

METHODS

Interleukin 6 receptor (IL-6R) expression and IL-6-induced cell signaling was measured by Western blotting in human pancreatic cell lines. Cucurbitacin I was used as a pharmacological tool to investigate the role of STAT3 in Pim-1 activation. Stably overexpressing Pim-1 kinase cell lines were characterized for their response to IL-6 in vitro and for their growth rate as flank tumors in scid mice.

RESULTS

Interleukin 6 receptor was expressed across multiple cancer cell lines. In Panc-1 cells, IL-6 treatment increased expression of phosphorylation of signal transducer and activator of transcription 3 and Pim-1 kinase. Cucurbitacin I treatment alone increased pErk1/2 expression in wild-type and Pim-1-overexpressing cell lines and resulted in exaggerated Pim-1 kinase protein levels in control and IL-6-stimulated cells, suggesting that up-regulation of Pim-1 may be partially STAT3 independent. Pim-1 overexpression did not significantly affect growth rate in vitro or in vivo in Panc-1 or MiaPaCa2 cell lines.

CONCLUSIONS

Interleukin 6 activates STAT3 and stimulates Pim-1 kinase in pancreatic cell line models. The regulation and consequence of Pim-1 expression seems to be highly context dependent.

Rapid determination of multiple linear kinase substrate motifs by mass spectrometry

Kinase-substrate recognition depends on the chemical properties of the phosphorylatable residue as well as the surrounding linear sequence motif. Detailed knowledge of these characteristics increases the confidence of linking identified phosphorylation sites to kinases, predicting phosphorylation sites, and designing optimal peptide substrates. Here, we present a mass spectrometry-based approach for determining linear kinase substrate motifs by elaborating the positional and chemical preference of the kinase for a phosphorylatable residue using libraries of naturally-occurring peptides that are amenable to peptide identification by commonly used proteomics platforms. We applied this approach to a structurally and functionally diverse set of purified kinases, which recapitulated their previously described substrate motifs and discovered additional ones, including preferences of certain kinases for phosphorylatable residues adjacent to peptide termini. Furthermore, we identify specific and distinguishable motif elements for the four members of the polo-like kinase (Plk) family and verify members of these motif elements for Plk1 in vivo.

Differential regulation of androgen receptor by PIM-1 kinases via phosphorylation-dependent recruitment of distinct ubiquitin E3 ligases

Androgen receptor (AR) plays a pivotal role in prostate cancer. Regulation of AR transcriptional activity by post-translational modifications, such as phosphorylation by multiple kinases, is well documented. Here, we report that two PIM-1 kinase isoforms which are up-regulated during prostate cancer progression, namely PIM-1S and PIM-1L, modulate AR stability and transcriptional activity through differentially phosphorylating AR at serine 213 (Ser-213) and threonine 850 (Thr-850). Although both kinases are capable of interacting with and phosphorylating AR at Ser-213, only PIM-1L could phosphorylate Thr-850. We also showed that PIM-1S induced Ser-213 phosphorylation destabilizes AR by recruiting the ubiquitin E3 ligase Mdm2 and promotes AR degradation in a cell cycle-dependent manner, while PIM-1L-induced Thr-850 phosphorylation stabilizes AR by recruiting the ubiquitin E3 ligase RNF6 and promotes AR-mediated transcription under low-androgen conditions. Furthermore, both PIM-1 isoforms could promote prostate cancer cell growth under low-androgen conditions. Our data suggest that these kinases regulate AR stability and transcriptional activity through recruitment of different functional partners in a phosphorylation-dependent manner. As AR turnover has been previously shown to be critical for cell cycle progression in prostate cancer cells, PIM-1 kinase isoforms may promote prostate cancer cell growth, at least in part, through modulating AR activity via distinct mechanisms.

Targeting diverse signaling interaction sites allows the rapid generation of bivalent kinase inhibitors

The identification of potent and selective modulators of protein kinase function remains a challenge, and new strategies are needed for generating these useful ligands. Here, we describe the generation of bivalent inhibitors of three unrelated protein kinases: the CAMK family kinase Pim1, the mitogen-activated protein kinase (MAPK) p38α, and the receptor tyrosine kinase (RTK) epidermal growth factor receptor (EGFR). These bivalent inhibitors consist of an ATP-competitive inhibitor that is covalently tethered to an engineered form of the self-labeling protein O(6)-alkylguanine-DNA alkyltransferase (SNAP-tag). In each example, SNAP-tag is fused to a peptide ligand that binds to a signaling interaction site of the kinase being targeted. These interactions increase the overall selectivity and potency of the bivalent inhibitors that were generated. The ability to exploit disparate binding sites in diverse kinases points to the generality of the method described. Finally, we demonstrate that ATP-competitive inhibitors that are conjugated to the bio-orthogonal tag O(4)-benzyl-2-chloro-6-aminopyrimidine (CLP) are cell-permeable. The selective labeling of SNAP-tag with CLP conjugates allows the rapid assembly of bivalent inhibitors in living cells.

PIM1 kinase as a target for cancer therapy

INTRODUCTION

Inhibition of protein kinases has become a standard of modern clinical oncology. PIM1 belongs to a novel class of serine/threonine kinases with distinct molecular and biochemical features regulating various oncogenic pathways, for example hypoxia response, cell cycle progression and apoptosis resistance. PIM1 is overexpressed in human cancer diseases and has been associated with metastasis and overall treatment response; in experimental models, inhibition of PIM1 suppressed cell proliferation and migration, induced apoptotic cell death and synergized with other chemotherapeutic agents.

AREAS COVERED

A PubMed literature search was performed to review the currently available data on PIM1 expression, regulation and targets; its implication in different types of cancer and its impact on prognosis are described. We present ATP-competitive PIM1 inhibitors and the state of the art of PIM1 inhibitor design. Finally, we highlight the development of the unusual class of highly selective and potent organometallic PIM1 inhibitors.

EXPERT OPINION

As PIM1 possesses oncogenic functions and is overexpressed in various kinds of cancer diseases, its inhibition provides a new option in cancer therapy. Based on the ability of highly selective organometallic PIM1 inhibitors, promising in vivo applicability is expected.

The PIM kinases in hematological cancers

The PIM genes represent a family of proto-oncogenes that encode three different serine/threonine protein kinases (PIM1, PIM2 and PIM3) with essential roles in the regulation of signal transduction cascades, which promote cell survival, proliferation and drug resistance. PIM kinases are overexpressed in several hematopoietic tumors and support in vitro and in vivo malignant cell growth and survival, through cell cycle regulation and inhibition of apoptosis. PIM kinases do not have an identified regulatory domain, which means that these proteins are constitutively active once transcribed. They appear to be critical downstream effectors of important oncoproteins and, when overexpressed, can mediate drug resistance to available agents, such as rapamycin. Recent crystallography studies reveal that, unlike other kinases, they possess a hinge region, which creates a unique binding pocket for ATP, offering a target for an increasing number of potent small-molecule PIM kinase inhibitors. Preclinical studies in models of various hematologic cancers indicate that these novel agents show promising activity and some of them are currently being evaluated in a clinical setting. In this review, we profile the PIM kinases as targets for therapeutics in hematologic malignancies.