Mechanisms of Resistance to JAK2 Inhibitors in Myeloproliferative Neoplasms

Expression of CD4+T Cells in Myeloproliferative Diseases and the Effect of Ruxolitinib Treatment on Prognosis

Myeloproliferative disorders (MPDs) are rare diseases in which the bone marrow produces too many red, white, or platelets. Myeloproliferative disorders are neither acute nor leukaemia. To study ruxolitinib's effect on MPD therapy and CD4+ T cell expression. In total, 66 JAK2V617F-positive MPD patients were admitted to our hospital. The patients were randomly assigned to control and research groups (each 33). Hydroxyurea pills were given to the control group and ruxolitinib to the observation group. The MPN-10 assesses 10 of the most clinically relevant symptoms, including fatigue and generates a Total Symptom Score (TSS). In addition, by comparing myelofibrosis (MF), spleen length, JAK2V617F gene expression, peripheral blood lymphocyte and T cell levels, and prognostic levels, analyze the shortcomings of each group. Post-treatment, MPN-10, MF, and spleen length diameter were reduced in both groups (P < 0.05), with the study group showing a higher reduction than the control group (P < 0.05). Compared to prior treatment, JAK2V617F gene expression was reduced in all groups after 6 months and a year of medication. The study category had a higher decrease in expression than the control group. After therapy, CD4 and CD4/CD8 levels rose, but CD8 and Treg levels decreased. The study group had increased CD4 and CD4/CD8 levels, whereas the control group had lower CD8 and Treg levels . The study group had a greater 1-year survival rate than the control group, but the control group had lower mortality and adverse event rates. In JAK2V617F-positive MPD patients, ruxolitinib reduces JAK2V617F gene expression, myelofibrosis, and therapeutic impact.

Functional and Structural Characterization of Clinical-Stage Janus Kinase 2 Inhibitors Identifies Determinants for Drug Selectivity

Janus kinase 2 (JAK2) plays a critical role in orchestrating hematopoiesis, and its deregulation leads to various blood disorders, most importantly myeloproliferative neoplasms (MPNs). Ruxolitinib, fedratinib, momelotinib, and pacritinib are FDA-/EMA-approved JAK inhibitors effective in relieving symptoms in MPN patients but show variable clinical profiles due to poor JAK selectivity. The development of next-generation JAK2 inhibitors is hampered by the lack of comparative functional analysis and knowledge of the molecular basis of their selectivity. Here, we provide mechanistic profiling of the four approved and six clinical-stage JAK2 inhibitors and connect selectivity data with high-resolution structural and thermodynamic analyses. All of the JAK inhibitors potently inhibited JAK2 activity. Inhibitors differed in their JAK isoform selectivity and potency for erythropoietin signaling, but their general cytokine inhibition signatures in blood cells were comparable. Structural data indicate that high potency and moderate JAK2 selectivity can be obtained by targeting the front pocket of the adenosine 5'-triphosphate-binding site.

Jak2V617F Reversible Activation Shows Its Essential Requirement in Myeloproliferative Neoplasms

Gain-of-function mutations activating JAK/STAT signaling are seen in the majority of patients with myeloproliferative neoplasms (MPN), most commonly JAK2V617F. Although clinically approved JAK inhibitors improve symptoms and outcomes in MPNs, remissions are rare, and mutant allele burden does not substantively change with chronic therapy. We hypothesized this is due to limitations of current JAK inhibitors to potently and specifically abrogate mutant JAK2 signaling. We therefore developed a conditionally inducible mouse model allowing for sequential activation, and then inactivation, of Jak2V617F from its endogenous locus using a combined Dre-rox/Cre-lox dual-recombinase system. Jak2V617F deletion abrogates MPN features, induces depletion of mutant-specific hematopoietic stem/progenitor cells, and extends overall survival to an extent not observed with pharmacologic JAK inhibition, including when cooccurring with somatic Tet2 loss. Our data suggest JAK2V617F represents the best therapeutic target in MPNs and demonstrate the therapeutic relevance of a dual-recombinase system to assess mutant-specific oncogenic dependencies in vivo.

SIGNIFICANCE

Current JAK inhibitors to treat myeloproliferative neoplasms are ineffective at eradicating mutant cells. We developed an endogenously expressed Jak2V617F dual-recombinase knock-in/knock-out model to investigate Jak2V617F oncogenic reversion in vivo. Jak2V617F deletion abrogates MPN features and depletes disease-sustaining MPN stem cells, suggesting improved Jak2V617F targeting offers the potential for greater therapeutic efficacy. See related commentary by Celik and Challen, p. 701. This article is featured in Selected Articles from This Issue, p. 695.

Study on tissue distribution, metabolite profiling, and excretion of [14C]-labeled flonoltinib maleate in rats

Flonoltinib Maleate (FM) is a dual-target inhibitor that selectively suppresses Janus kinase 2/FMS-like tyrosine kinase 3 (JAK2/FLT3), which is currently in phase I/IIa clinical trial in China for the treatment of myeloproliferative neoplasms (MPNs). In this research, we used [14C]-labeled FM (14C-FM) to investigate the distribution, metabolism, and excretion of FM in rats using High-Performance Liquid Chromatography coupled with High-Resolution Mass Spectrometry/Radioactivity Monitoring (HPLC-HRMS/RAM) and liquid scintillation counter. The results revealed that FM displayed widespread distribution in rats. Furthermore, FM demonstrated rapid clearance without any observed risk of organ toxicity attributed to accumulation. Profiling of FM metabolites in rat plasma, feces, urine, and bile identified a total of 17 distinct metabolites, comprising 7 phase I metabolites and 10 phase II metabolites. The major metabolic reactions involved oxygenation, dealkylation, methylation, sulfation, glucuronidation and glutathione conjugation. Based on these findings, a putative metabolic pathway of FM in rats was proposed. The overall recovery rate in the excretion experiment ranged from 93.04 % to 94.74 %. The results indicated that FM undergoes extensive hepatic metabolism in SD rats, with the majority being excreted through bile as metabolites and ultimately eliminated via feces. A minor fraction of FM (<10 %) was excreted through renal excretion in the form of urine. Integration of the current results with previous pharmacokinetic investigations of FM in rats and dogs enables a comprehensive elucidation of the in vivo ADME processes and characteristics of FM, thereby establishing a solid foundation for subsequent clinical investigations of FM.

Development of Resistance to Type II JAK2 Inhibitors in MPN Depends on AXL Kinase and Is Targetable

PURPOSE

Myeloproliferative neoplasms (MPN) dysregulate JAK2 signaling. Because clinical JAK2 inhibitors have limited disease-modifying effects, type II JAK2 inhibitors such as CHZ868 stabilizing inactive JAK2 and reducing MPN clones, gain interest. We studied whether MPN cells escape from type ll inhibition.

EXPERIMENTAL DESIGN

MPN cells were continuously exposed to CHZ868. We used phosphoproteomic analyses and ATAC/RNA sequencing to characterize acquired resistance to type II JAK2 inhibition, and targeted candidate mediators in MPN cells and mice.

RESULTS

MPN cells showed increased IC50 and reduced apoptosis upon CHZ868 reflecting acquired resistance to JAK2 inhibition. Among >2,500 differential phospho-sites, MAPK pathway activation was most prominent, while JAK2-STAT3/5 remained suppressed. Altered histone occupancy promoting AP-1/GATA binding motif exposure associated with upregulated AXL kinase and enriched RAS target gene profiles. AXL knockdown resensitized MPN cells and combined JAK2/AXL inhibition using bemcentinib or gilteritinib reduced IC50 to levels of sensitive cells. While resistant cells induced tumor growth in NOD/SCID gamma mice despite JAK2 inhibition, JAK2/AXL inhibition largely prevented tumor progression. Because inhibitors of MAPK pathway kinases such as MEK are clinically used in other malignancies, we evaluated JAK2/MAPK inhibition with trametinib to interfere with AXL/MAPK-induced resistance. Tumor growth was halted similarly to JAK2/AXL inhibition and in a systemic cell line-derived mouse model, marrow infiltration was decreased supporting dependency on AXL/MAPK.

CONCLUSIONS

We report on a novel mechanism of AXL/MAPK-driven escape from type II JAK2 inhibition, which is targetable at different nodes. This highlights AXL as mediator of acquired resistance warranting inhibition to enhance sustainability of JAK2 inhibition in MPN.

Design, Synthesis, and Antitumor Efficacy of Substituted 2-Amino[1,2,4]triazolopyrimidines and Related Heterocycles as Dual Inhibitors for Microtubule Polymerization and Janus Kinase 2

Preclinical and clinical studies have demonstrated the synergistic effect of microtubule-targeting agents in combination with Janus kinase 2 (JAK2) inhibitors, prompting the development of single agents with enhanced therapeutic efficacy by dually inhibiting tubulin polymerization and JAK2. Herein, we designed and synthesized a series of substituted 2-amino[1,2,4]triazolopyrimidines and related heterocycles as dual inhibitors for tubulin polymerization and JAK2. Most of these compounds exhibited potent antiproliferative activity against the selected cancer cells, with compound 7g being the most active. This compound effectively inhibits both tubulin assembly and JAK2 activity. Furthermore, phosphorylated compound 7g (i.e., compound 7g-P) could efficiently convert to compound 7g in vivo. Compound 7g, whether it was administered directly or in the form of a phosphorylated prodrug (i.e., compound 7g-P), significantly inhibited the growth of A549 xenografts in nude mice. The present findings strongly suggest that compound 7g represents a promising chemotherapeutic agent with high antitumor efficacy.

New scaffolds for type II JAK2 inhibitors overcome the acquired G993A resistance mutation

Recurrent JAK2 alterations are observed in myeloproliferative neoplasms, B-cell acute lymphoblastic leukemia, and other hematologic malignancies. Currently available type I JAK2 inhibitors have limited activity in these diseases. Preclinical data support the improved efficacy of type II JAK2 inhibitors, which lock the kinase in the inactive conformation. By screening small molecule libraries, we identified a lead compound with JAK2 selectivity. We highlight analogs with on-target biochemical and cellular activity and demonstrate in vivo activity using a mouse model of polycythemia vera. We present a co-crystal structure that confirms the type II binding mode of our compounds with the "DFG-out" conformation of the JAK2 activation loop. Finally, we identify a JAK2 G993A mutation that confers resistance to the type II JAK2 inhibitor CHZ868 but not to our analogs. These data provide a template for identifying novel type II kinase inhibitors and inform further development of agents targeting JAK2 that overcome resistance.

JAK2 Alterations in Acute Lymphoblastic Leukemia: Molecular Insights for Superior Precision Medicine Strategies

Acute lymphoblastic leukemia (ALL) is the most common pediatric cancer, arising from immature lymphocytes that show uncontrolled proliferation and arrested differentiation. Genomic alterations affecting Janus kinase 2 (JAK2) correlate with some of the poorest outcomes within the Philadelphia-like subtype of ALL. Given the success of kinase inhibitors in the treatment of chronic myeloid leukemia, the discovery of activating JAK2 point mutations and JAK2 fusion genes in ALL, was a breakthrough for potential targeted therapies. However, the molecular mechanisms by which these alterations activate JAK2 and promote downstream signaling is poorly understood. Furthermore, as clinical data regarding the limitations of approved JAK inhibitors in myeloproliferative disorders matures, there is a growing awareness of the need for alternative precision medicine approaches for specific JAK2 lesions. This review focuses on the molecular mechanisms behind ALL-associated JAK2 mutations and JAK2 fusion genes, known and potential causes of JAK-inhibitor resistance, and how JAK2 alterations could be targeted using alternative and novel rationally designed therapies to guide precision medicine approaches for these high-risk subtypes of ALL.

Understanding Aberrant Signaling to Elude Therapy Escape Mechanisms in Myeloproliferative Neoplasms

Aberrant signaling in myeloproliferative neoplasms may arise from alterations in genes coding for signal transduction proteins or epigenetic regulators. Both mutated and normal cells cooperate, altering fragile balances in bone marrow niches and fueling persistent inflammation through paracrine or systemic signals. Despite the hopes placed in targeted therapies, myeloid proliferative neoplasms remain incurable diseases in patients not eligible for stem cell transplantation. Due to the emergence of drug resistance, patient management is often very difficult in the long term. Unexpected connections among signal transduction pathways highlighted in neoplastic cells suggest new strategies to overcome neoplastic cell adaptation.

JAK2 in Myeloproliferative Neoplasms: Still a Protagonist

The discovery of the activating V617F mutation in Janus kinase 2 (JAK2) has been decisive for the understanding of myeloproliferative neoplasms (MPN). Activated JAK2 signaling by JAK2, CALR, and MPL mutations has become a focus for the development of targeted therapies for patients with MPN. JAK2 inhibitors now represent a standard of clinical care for certain forms of MPN and offer important benefits for MPN patients. However, several key aspects remain unsolved regarding the targeted therapy of MPN with JAK2 inhibitors, such as reducing the MPN clone and how to avoid or overcome a loss of response. Here, we summarize the current knowledge on the structure and signaling of JAK2 as central elements of MPN pathogenesis and feature benefits and limitations of therapeutic JAK2 targeting in MPN.

Molecular hybrids: A five-year survey on structures of multiple targeted hybrids of protein kinase inhibitors for cancer therapy

Protein kinases have grown over the past few years as a crucial target for different cancer types. With the multifactorial nature of cancer, and the fast development of drug resistance for conventional chemotherapeutics, a strategy for designing multi-target agents was suggested to potentially increase drug efficacy, minimize side effects and retain the proper pharmacokinetic properties. Kinase inhibitors were used extensively in such strategy. Different kinase inhibitor agents which target EGFR, VEGFR, c-Met, CDK, PDK and other targets were merged into hybrids with conventional chemotherapeutics such as tubulin polymerization and topoisomerase inhibitors. Other hybrids were designed gathering kinase inhibitors with targeted cancer therapy such as HDAC, PARP, HSP 90 inhibitors. Nitric oxide donor molecules were also merged with kinase inhibitors for cancer therapy. The current review presents the hybrids designed in the past five years discussing their design principles, results and highlights their future perspectives.

Blocking the JAK2/STAT3 and ERK pathways suppresses the proliferation of gastrointestinal cancers by inducing apoptosis

Dysregulated crosstalk between different signaling pathways contributes to tumor development, including resistance to cancer therapy. In the present study, we found that the mitogen-activated extracellular signal-regulated kinase (MEK) inhibitor trametinib failed to suppress the proliferation of PANC-1 and MGC803 cells by activating the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway, while the JAK2 inhibitor fedratinib failed to inhibit the growth of the PANC-1 cells upon stimulation of extracellular signal-regulated kinase (ERK) signaling. In particular, the most prominent enhancement of the anti-proliferative effect resulted from the concurrent blockage of the JAK2/STAT3 and ERK signaling pathways. Furthermore, the combination of the two inhibitors resulted in a reduced tumor burden in mice. Our evidence suggests novel crosstalk between JAK2/STAT3 and ERK signaling in gastric cancer (GC) and pancreatic ductal adenocarcinoma (PDAC) cells and provides a therapeutic strategy to overcome potential resistance in gastrointestinal cancer.

RAS/CBL mutations predict resistance to JAK inhibitors in myelofibrosis and are associated with poor prognostic features

The dysregulation of the JAK/STAT pathway drives the pathogenesis of myelofibrosis (MF). Recently, several JAK inhibitors (JAKis) have been developed for treating MF. Select mutations (MTs) have been associated with impaired outcomes and are currently incorporated in molecularly annotated prognostic models. Mutations of RAS/MAPK pathway genes are frequently reported in cancer and at low frequencies in MF. In this study, we investigated the phenotypic, prognostic, and therapeutic implications of NRASMTs, KRASMTs, and CBLMTs (RAS/CBLMTs) in 464 consecutive MF patients. A total of 59 (12.7%) patients had RAS/CBLMTs: NRASMTs, n = 25 (5.4%); KRASMTs, n = 13 (2.8%); and CBLMTs, n = 26 (5.6%). Patients with RAS/CBLMTs were more likely to present with high-risk clinical and molecular features. RAS/CBLMTs were associated with inferior overall survival compared with patients without MTs and retained significance in a multivariate model, including the Mutation-Enhanced International Prognostic Score System (MIPSS70) risk factors and cytogenetics; however, inclusion of RAS/CBLMTs in molecularly annotated prognostic models did not improve the predictive power of the latter. The 5-year cumulative incidence of leukemic transformation was notably higher in the RAS/CBLMT cohort. Among 61 patients treated with JAKis and observed for a median time of 30 months, the rate of symptoms and spleen response at 6 months was significantly lower in the RAS/CBLMT cohort. Logistic regression analysis disclosed a significant inverse correlation between RAS/CBLMTs and the probability of achieving a symptom or spleen response that was retained in multivariate analysis. In summary, our study showed that RAS/CBLMTs are associated with adverse phenotypic features and survival outcomes and, more important, may predict reduced response to JAKis.

Constitutive JAK/STAT signaling is the primary mechanism of resistance to JAKi in TYK2-rearranged acute lymphoblastic leukemia

Activating TYK2-rearrangements have recently been identified and implicated in the leukemogenesis of high-risk acute lymphoblastic leukemia (HR-ALL) cases. Pre-clinical studies indicated the JAK/TYK2 inhibitor (JAKi), cerdulatinib, as a promising therapeutic against TYK2-rearranged ALL, attenuating the constitutive JAK/STAT signaling resulting from the TYK2 fusion protein. However, following a period of clinical efficacy, JAKi resistance often occurs resulting in relapse. In this study, we modeled potential mechanisms of JAKi resistance in TYK2-rearranged ALL cells in vitro in order to recapitulate possible clinical scenarios and provide a rationale for alternative therapies. Cerdulatinib resistant B-cells, driven by the MYB-TYK2 fusion oncogene, were generated by long-term exposure to the drug. Sustained treatment of MYB-TYK2-rearranged ALL cells with cerdulatinib led to enhanced and persistent JAK/STAT signaling, co-occurring with JAK1 overexpression. Hyperactivation of JAK/STAT signaling and JAK1 overexpression was reversible as cerdulatinib withdrawal resulted in re-sensitization to the drug. Importantly, histone deacetylase inhibitor (HDACi) therapies were efficacious against cerdulatinib-resistant cells demonstrating a potential alternative therapy for use in TYK2-rearranged B-ALL patients who have lost response to JAKi treatment regimens.

Structural Insights into JAK2 Inhibition by Ruxolitinib, Fedratinib, and Derivatives Thereof

The discovery that aberrant activity of Janus kinase 2 (JAK2) is a driver of myeloproliferative neoplasms (MPNs) has led to significant efforts to develop small molecule inhibitors for this patient population. Ruxolitinib and fedratinib have been approved for use in MPN patients, while baricitinib, an achiral analogue of ruxolitinib, has been approved for rheumatoid arthritis. However, structural information on the interaction of these therapeutics with JAK2 remains unknown. Here, we describe a new methodology for the large-scale production of JAK2 from mammalian cells, which enabled us to determine the first crystal structures of JAK2 bound to these drugs and derivatives thereof. Along with biochemical and cellular data, the results provide a comprehensive view of the shape complementarity required for chiral and achiral inhibitors to achieve highest activity, which may facilitate the development of more effective JAK2 inhibitors as therapeutics.

Challenges and Perspectives for Therapeutic Targeting of Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are hematopoietic stem cell disorders with dysregulated myeloid blood cell production and propensity for transformation to acute myeloid leukemia, thrombosis, and bleeding. Acquired mutations in JAK2, MPL, and CALR converge on hyperactivation of Janus kinase 2 (JAK2) signaling as a central feature of MPN. Accordingly, JAK2 inhibitors have held promise for therapeutic targeting. After the JAK1/2 inhibitor ruxolitinib, similar JAK2 inhibitors as fedratinib are entering clinical use. While patients benefit with reduced splenomegaly and symptoms, disease-modifying effects on MPN clone size and clonal evolution are modest. Importantly, response to ruxolitinib may be lost upon treatment suggesting the MPN clone acquires resistance. Resistance mutations, as seen with other tyrosine kinase inhibitors, have not been described in MPN patients suggesting that functional processes reactivate JAK2 signaling. Compensatory signaling, which bypasses JAK2 inhibition, and other processes contribute to intrinsic resistance of MPN cells restricting efficacy of JAK2 inhibition overall. Combinations of JAK2 inhibition with pegylated interferon-α, a well-established therapy of MPN, B-cell lymphoma 2 inhibition, and others are in clinical development with the potential to enhance therapeutic efficacy. Novel single-agent approaches targeting other molecules than JAK2 are being investigated clinically. Special focus should be placed on myelofibrosis patients with anemia and thrombocytopenia, a delicate patient population at high need for options. The extending range of new treatment approaches will increase the therapeutic options for MPN patients. This calls for concomitant improvement of our insight into MPN biology to inform tailored therapeutic strategies for individual MPN patients.

Splicing factor YBX1 mediates persistence of JAK2-mutated neoplasms

Janus kinases (JAKs) mediate responses to cytokines, hormones and growth factors in haematopoietic cells^1,2. The JAK gene JAK2 is frequently mutated in the ageing haematopoietic system^3,4 and in haematopoietic cancers⁵. JAK2 mutations constitutively activate downstream signalling and are drivers of myeloproliferative neoplasm (MPN). In clinical use, JAK inhibitors have mixed effects on the overall disease burden of JAK2-mutated clones^6,7, prompting us to investigate the mechanism underlying disease persistence. Here, by in-depth phosphoproteome profiling, we identify proteins involved in mRNA processing as targets of mutant JAK2. We found that inactivation of YBX1, a post-translationally modified target of JAK2, sensitizes cells that persist despite treatment with JAK inhibitors to apoptosis and results in RNA mis-splicing, enrichment for retained introns and disruption of the transcriptional control of extracellular signal-regulated kinase (ERK) signalling. In combination with pharmacological JAK inhibition, YBX1 inactivation induces apoptosis in JAK2-dependent mouse and primary human cells, causing regression of the malignant clones in vivo, and inducing molecular remission. This identifies and validates a cell-intrinsic mechanism whereby differential protein phosphorylation causes splicing-dependent alterations of JAK2-ERK signalling and the maintenance of JAK2^V617F malignant clones. Therapeutic targeting of YBX1-dependent ERK signalling in combination with JAK2 inhibition could thus eradicate cells harbouring mutations in JAK2.

Heat Shock Proteins and PD-1/PD-L1 as Potential Therapeutic Targets in Myeloproliferative Neoplasms

Simple Summary

Myeloproliferative neoplasms (MPN), which are a heterogeneous group of rare disorders that affect blood cell production in bone marrow, present many significant challenges for clinicians. Though considerable progress has been made, in particular with the JAK1/2 inhibitor ruxolitinib, more effective alternative therapeutic approaches are needed. In the search for new and more efficient therapies, heat shock proteins, also known as stress proteins, and the programmed cell death 1 (PD-1)/programmed death ligand 1 (PD-L1) immune checkpoint axis have been found to be of great interest in hematologic malignancies. Here, we review the therapeutic potential of stress protein inhibitors in the management of patients diagnosed with MPN and summarize the accumulating evidence of the role of the PD-1/PD-L1 axis in MPN in order to provide perspectives on future therapeutic opportunities relative to the inhibition of these targets.

Abstract

Myeloproliferative neoplasms (MPN) are a group of clonal disorders that affect hematopoietic stem/progenitor cells. These disorders are often caused by oncogenic driver mutations associated with persistent Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling. While JAK inhibitors, such as ruxolitinib, reduce MPN-related symptoms in myelofibrosis, they do not influence the underlying cause of the disease and are not curative. Due to these limitations, there is a need for alternative therapeutic strategies and targets. Heat shock proteins (HSPs) are cytoprotective stress-response chaperones involved in protein homeostasis and in many critical pathways, including inflammation. Over the last decade, several research teams have unraveled the mechanistic connection between STAT signaling and several HSPs, showing that HSPs are potential therapeutic targets for MPN. These HSPs include HSP70, HSP90 (chaperoning JAK2) and both HSP110 and HSP27, which are key factors modulating STAT3 phosphorylation status. Like the HSPs, the PD-1/PD-L1 signaling pathway has been widely studied in cancer, but the importance of PD-L1-mediated immune escape in MPN was only recently reported. In this review, we summarize the role of HSPs and PD-1/PD-L1 signaling, the modalities of their experimental blockade, and the effect in MPN. Finally, we discuss the potential of these emerging targeted approaches in MPN therapy.

PD-L1 expression increased by IFN-γ via JAK2-STAT1 signaling and predicts a poor survival in colorectal cancer

PD-L1 inhibitors are widely used in tumor immunotherapy, but their mechanism in colorectal cancer remains unclear. The present study aimed to investigate the mechanisms underlying programmed death ligand 1 (PD-L1) regulation via the interferon-γ (IFN-γ)/janus kinase (JAK)/STAT signaling pathway, and its prognostic value in patients with colorectal cancer (CRC). A cohort of 181 patients were recruited to determine the association between PD-L1 expression and CRC prognosis; the patients were newly diagnosed with colorectal adenocarcinoma and had also undergone a physical tumorectomy. Immunohistochemical staining and survival analysis were used to evaluate the predictive value of PD-L1 protein expression in CRC. Gene set enrichment analysis, RT-qPCR and western blotting, etc were performed to confirm that PD-L1 is regulated by the IFN-γ/JAK/STAT signaling pathway. PD-L1 up-regulation was more frequently observed in patients with larger tumors, positive vascular or lymphatic infiltration and a poorly differentiated stage in addition to being associated with a poor survival in patients with CRC. Following the stimulation with IFN-γ, PD-L1 expression levels were revealed to be increased via the JAK2/STAT1 signaling pathway. In conclusion, the findings of the present study indicated that the expression levels of PD-L1 may be associated with a poor prognosis in patients with CRC. In addition, the results suggested that the IFN-γ-mediated overexpression of PD-L1 in CRC cells may be regulated by the JAK2/STAT1 signaling pathway.

Phospho-proteomic discovery of novel signal transducers including thioredoxin-interacting protein as mediators of erythropoietin-dependent human erythropoiesis

Erythroid cell formation critically depends on signals transduced via erythropoietin (EPO)/EPO receptor (EPOR)/JAK2 complexes. This includes not only core response modules (e.g., JAK2/STAT5, RAS/MEK/ERK), but also specialized effectors (e.g., erythroferrone, ASCT2 glutamine transport, Spi2A). By using phospho-proteomics and a human erythroblastic cell model, we identify 121 new EPO target proteins, together with their EPO-modulated domains and phosphosites. Gene ontology (GO) enrichment for "Molecular Function" identified adaptor proteins as one top EPO target category. This includes a novel EPOR/JAK2-coupled network of actin assemblage modifiers, with adaptors DLG-1, DLG-3, WAS, WASL, and CD2AP as prime components. "Cellular Component" GO analysis further identified 19 new EPO-modulated cytoskeletal targets including the erythroid cytoskeletal targets spectrin A, spectrin B, adducin 2, and glycophorin C. In each, EPO-induced phosphorylation occurred at pY sites and subdomains, which suggests coordinated regulation by EPO of the erythroid cytoskeleton. GO analysis of "Biological Processes" further revealed metabolic regulators as a likewise unexpected EPO target set. Targets included aldolase A, pyruvate dehydrogenase α1, and thioredoxin-interacting protein (TXNIP), with EPO-modulated p-Y sites in each occurring within functional subdomains. In TXNIP, EPO-induced phosphorylation occurred at novel p-T349 and p-S358 sites, and was paralleled by rapid increases in TXNIP levels. In UT7epo-E and primary human stem cell (HSC)-derived erythroid progenitor cells, lentivirus-mediated short hairpin RNA knockdown studies revealed novel pro-erythropoietic roles for TXNIP. Specifically, TXNIP's knockdown sharply inhibited c-KIT expression; compromised EPO dose-dependent erythroblast proliferation and survival; and delayed late-stage erythroblast formation. Overall, new insight is provided into EPO's diverse action mechanisms and TXNIP's contributions to EPO-dependent human erythropoiesis.

Construction of Quantitative Structure Activity Relationship (QSAR) Models to Predict Potency of Structurally Diversed Janus Kinase 2 Inhibitors

Janus kinase 2 (JAK2) inhibitors represent a promising therapeutic class of anticancer agents against many myeloproliferative disorders. Bioactivity data on pIC50 of 2229 JAK2 inhibitors were employed in the construction of quantitative structure-activity relationship (QSAR) models. The models were built from 100 data splits using decision tree (DT), support vector machine (SVM), deep neural network (DNN) and random forest (RF). The predictive power of RF models were assessed via 10-fold cross validation, which afforded excellent predictive performance with R2 and RMSE of 0.74 ± 0.05 and 0.63 ± 0.05, respectively. Moreover, test set has excellent performance of R2 (0.75 ± 0.03) and RMSE (0.62 ± 0.04). In addition, Y-scrambling was utilized to evaluate the possibility of chance correlation of the predictive model. A thorough analysis of the substructure fingerprint count was conducted to provide insights on the inhibitory properties of JAK2 inhibitors. Molecular cluster analysis revealed that pyrazine scaffolds have nanomolar potency against JAK2.

Modeling hematopoietic disorders in zebrafish

Zebrafish offer a powerful vertebrate model for studies of development and disease. The major advantages of this model include the possibilities of conducting reverse and forward genetic screens and of observing cellular processes by in vivo imaging of single cells. Moreover, pathways regulating blood development are highly conserved between zebrafish and mammals, and several discoveries made in fish were later translated to murine and human models. This review and accompanying poster provide an overview of zebrafish hematopoiesis and discuss the existing zebrafish models of blood disorders, such as myeloid and lymphoid malignancies, bone marrow failure syndromes and immunodeficiencies, with a focus on how these models were generated and how they can be applied for translational research.

Summary: This At A Glance article and poster summarize the last 20 years of research in zebrafish models for hematopoietic disorders, highlighting how these models were created and are being applied for translational research.

What sustains the multidrug resistance phenotype beyond ABC efflux transporters? Looking beyond the tip of the iceberg

Identification of multidrug (MDR) efflux transporters that belong to the ATP-Binding Cassette (ABC) superfamily, represented an important breakthrough for understanding cancer multidrug resistance (MDR) and its possible overcoming. However, recent data indicate that drug resistant cells have a complex intracellular physiology that involves constant changes in energetic and oxidative-reductive metabolic pathways, as well as in the molecular circuitries connecting mitochondria, endoplasmic reticulum (ER) and lysosomes. The aim of this review is to discuss the key molecular mechanisms of cellular reprogramming that induce and maintain MDR, beyond the presence of MDR efflux transporters. We specifically highlight how cancer cells characterized by high metabolic plasticity - i.e. cells able to shift the energy metabolism between glycolysis and oxidative phosphorylation, to survive both the normoxic and hypoxic conditions, to modify the cytosolic and mitochondrial oxidative-reductive metabolism, are more prone to adapt to exogenous stressors such as anti-cancer drugs and acquire a MDR phenotype. Similarly, we discuss how changes in mitochondria dynamics and mitophagy rates, changes in proteome stability ensuring non-oncogenic proteostatic mechanisms, changes in ubiquitin/proteasome- and autophagy/lysosome-related pathways, promote the cellular survival under stress conditions, along with the acquisition or maintenance of MDR. After dissecting the complex intracellular crosstalk that takes place during the development of MDR, we suggest that mapping the specific adaptation pathways underlying cell survival in response to stress and targeting these pathways with potent pharmacologic agents may be a new approach to enhance therapeutic efficacy against MDR tumors.

Targeting compensatory MEK/ERK activation increases JAK inhibitor efficacy in myeloproliferative neoplasms

Constitutive JAK2 signaling is central to myeloproliferative neoplasm (MPN) pathogenesis and results in activation of STAT, PI3K/AKT, and MEK/ERK signaling. However, the therapeutic efficacy of current JAK2 inhibitors is limited. We investigated the role of MEK/ERK signaling in MPN cell survival in the setting of JAK inhibition. Type I and II JAK2 inhibition suppressed MEK/ERK activation in MPN cell lines in vitro, but not in Jak2V617F and MPLW515L mouse models in vivo. JAK2 inhibition ex vivo inhibited MEK/ERK signaling, suggesting that cell-extrinsic factors maintain ERK activation in vivo. We identified PDGFRα as an activated kinase that remains activated upon JAK2 inhibition in vivo, and PDGF-AA/PDGF-BB production persisted in the setting of JAK inhibition. PDGF-BB maintained ERK activation in the presence of ruxolitinib, consistent with its function as a ligand-induced bypass for ERK activation. Combined JAK/MEK inhibition suppressed MEK/ERK activation in Jak2V617F and MPLW515L mice with increased efficacy and reversal of fibrosis to an extent not seen with JAK inhibitors. This demonstrates that compensatory ERK activation limits the efficacy of JAK2 inhibition and dual JAK/MEK inhibition provides an opportunity for improved therapeutic efficacy in MPNs and in other malignancies driven by aberrant JAK-STAT signaling.

JAK/Stat5-mediated subtype-specific lymphocyte antigen 6 complex, locus G6D (LY6G6D) expression drives mismatch repair proficient colorectal cancer

BACKGROUND

Human microsatellite-stable (MSS) colorectal cancers (CRCs) are immunologically "cold" tumour subtypes characterized by reduced immune cytotoxicity. The molecular linkages between immune-resistance and human MSS CRC is not clear.

METHODS

We used transcriptome profiling, in silico analysis, immunohistochemistry, western blot, RT-qPCR and immunofluorescence staining to characterize novel CRC immune biomarkers. The effects of selective antagonists were tested by in vitro assays of long term viability and analysis of kinase active forms using anti-phospho antibodies.

RESULTS

We identified the lymphocyte antigen 6 complex, locus G6D (LY6G6D) as significantly overexpressed (around 15-fold) in CRC when compared with its relatively low expression in other human solid tumours. LY6G6D up-regulation was predominant in MSS CRCs characterized by an enrichment of immune suppressive regulatory T-cells and a limited repertoire of PD-1/PD-L1 immune checkpoint receptors. Coexpression of LY6G6D and CD15 increases the risk of metastatic relapse in response to therapy. Both JAK-STAT5 and RAS-MEK-ERK cascades act in concert as key regulators of LY6G6D and Fucosyltransferase 4 (FUT4), which direct CD15-mediated immune-resistance. Momelotinib, an inhibitor of JAK1/JAK2, consistently abrogated the STAT5/LY6G6D axis in vitro, sensitizing MSS cancer cells with an intact JAK-STAT signaling, to efficiently respond to trametinib, a MEK inhibitor used in clinical setting. Notably, colon cancer cells can evade JAK2/JAK1-targeted therapy by a reversible shift of the RAS-MEK-ERK pathway activity, which explains the treatment failure of JAK1/2 inhibitors in refractory CRC.

CONCLUSIONS

Combined targeting of STAT5 and MAPK pathways has superior therapeutic effects on immune resistance. In addition, the new identified LY6G6D antigen is a promising molecular target for human MSS CRC.

The ruxolitinib effect: understanding how molecular pathogenesis and epigenetic dysregulation impact therapeutic efficacy in myeloproliferative neoplasms

The myeloproliferative neoplasms (MPN), polycythaemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF) are linked by a propensity to thrombosis formation and a risk of leukaemic transformation. Activation of cytokine independent signalling through the JAK/STAT cascade is a feature of these disorders. A point mutation in exon 14 of the JAK2 gene resulting in the formation of the JAK2 V617F transcript occurs in 95% of PV patients and around 50% of ET and PMF patients driving constitutive activation of the JAK/STAT pathway. Mutations in CALR or MPL are present as driving mutations in the majority of remaining ET and PMF patients. Ruxolitinib is a tyrosine kinase inhibitor which inhibits JAK1 and JAK2. It is approved for use in intermediate and high risk PMF, and in PV patients who are resistant or intolerant to hydroxycarbamide. In randomised controlled trials it has demonstrated efficacy in spleen volume reduction and symptom burden reduction with a moderate improvement in overall survival in PMF. In PV, there is demonstrated benefit in haematocrit control and spleen volume. Despite these benefits, there is limited impact to induce complete haematological remission with normalisation of blood counts, reduce the mutant allele burden or reverse bone marrow fibrosis. Clonal evolution has been observed on ruxolitinib therapy and transformation to acute leukaemia can still occur. This review will concentrate on understanding the clinical and molecular effects of ruxolitinib in MPN. We will focus on understanding the limitations of JAK inhibition and the challenges to improving therapeutic efficacy in these disorders. We will explore the demonstrated benefits and disadvantages of ruxolitinib in the clinic, the role of genomic and clonal variability in pathogenesis and response to JAK inhibition, epigenetic changes which impact on response to therapy, the role of DNA damage and the role of inflammation in these disorders. Finally, we will summarise the future prospects for improving therapy in MPN in the JAK inhibition era.

Mutation landscape in patients with myelofibrosis receiving ruxolitinib or hydroxyurea

Refractoriness to ruxolitinib in patients with myelofibrosis (MF) was associated with clonal evolution; however, whether genetic instability is promoted by ruxolitinib remains unsettled. We evaluated the mutation landscape in 71 MF patients receiving ruxolitinib (n = 46) and hydroxyurea (n = 25) and correlated with response. A spleen volume response (SVR) was obtained in 57% and 12%, respectively. Highly heterogenous patterns of mutation acquisition/loss and/or changes of variant allele frequency (VAF) were observed in the 2 patient groups without remarkable differences. In patients receiving ruxolitinib, driver mutation type and high-molecular risk profile (HMR) at baseline did not impact on response rate, while HMR and sole ASXL1 mutations predicted for SVR loss at 3 years. In patients with SVR, a decrease of ≥ 20% of JAK2V617F VAF predicted for SVR duration. VAF increase of non-driver mutations and clonal progression at follow-up correlated with SVR loss and treatment discontinuation, and clonal progression also predicted for shorter survival. These data indicate that (i) ruxolitinib does not appreciably promote clonal evolution compared with hydroxyurea, (ii) VAF increase of pre-existing and/or (ii) acquisition of new mutations while on treatment correlated with higher rate of discontinuation and/or death, and (iv) reduction of JAK2V617F VAF associated with SVR duration.

Adapting to stress - chaperome networks in cancer

In this Opinion article, we aim to address how cells adapt to stress and the repercussions chronic stress has on cellular function. We consider acute and chronic stress-induced changes at the cellular level, with a focus on a regulator of cellular stress, the chaperome, which is a protein assembly that encompasses molecular chaperones, co-chaperones and other co-factors. We discuss how the chaperome takes on distinct functions under conditions of stress that are executed in ways that differ from the one-on-one cyclic, dynamic functions exhibited by distinct molecular chaperones. We argue that through the formation of multimeric stable chaperome complexes, a state of chaperome hyperconnectivity, or networking, is gained. The role of these chaperome networks is to act as multimolecular scaffolds, a particularly important function in cancer, where they increase the efficacy and functional diversity of several cellular processes. We predict that these concepts will change how we develop and implement drugs targeting the chaperome to treat cancer.

Discovery of Janus Kinase 2 (JAK2) and Histone Deacetylase (HDAC) Dual Inhibitors as a Novel Strategy for the Combinational Treatment of Leukemia and Invasive Fungal Infections

Clinically, leukemia patients often suffer from the limited efficacy of chemotherapy and high risks of infection by invasive fungal pathogens. Herein, a novel therapeutic strategy was developed in which a small molecule can simultaneously treat leukemia and invasive fungal infections (IFIs). Novel Janus kinase 2 (JAK2) and histone deacetylase (HDAC) dual inhibitors were identified to possess potent anti-proliferative activity toward hematological cell lines and excellent synergistic effects with fluconazole to treat resistant Candida albicans infections. In particular, compound 20a, a highly active and selective JAK2/HDAC6 dual inhibitor, showed excellent in vivo antitumor efficacy in several acute myeloid leukemia (AML) models and synergized with fluconazole for the treatment of resistant C. albicans infections. This study highlights the therapeutic potential of JAK2/HDAC dual inhibitors in treating AML and IFIs and provides an efficient strategy for multitargeting drug discovery.

The role of JAK2 inhibitors in MPNs 7 years after approval

Myeloproliferative neoplasms (MPNs) include essential thrombocythemia, polycythemia vera (PV), and primary myelofibrosis (MF). Phenotype-driver mutations of JAK2, CALR, and MPL genes are present in MPNs and can be variably combined with additional mutations. Driver mutations entail a constitutive activation of the JAK2/STAT pathway, the key signaling cascade in MPNs. Among JAK2 inhibitors (JAKis), ruxolitinib (RUX) has been approved for the treatment of intermediate and high-risk MF and for PV inadequately controlled by or intolerant of hydroxyurea. Other JAKis, such as fedratinib and pacritinib, proved to be useful in MF. The primary end points in MF trials were spleen volume response (SVR) and symptom response, whereas in PV trials they were hematocrit control with or without spleen response. In advanced MF, RUX achieved a long lasting SVR of >35% in ∼60% of patients, establishing a new benchmark for MF treatment. RUX efficacy in early MF is also remarkable and toxicity is mild. In PV, RUX achieved hematocrit control in ∼60% of cases and SVR in 40%. Symptom relief was evident in both conditions. In the long-term, however, many MF patients lose their SVR. Indeed, the definition of RUX failure and the design of new trials in this setting are unmet needs. Decrease of hemoglobin/platelet levels and increased infection rates are the most common side effects of RUX, and nonmelanoma skin tumors need to be monitored while on treatment. In conclusion, the introduction of JAKis raises the bar of treatment goals in MF and PV.