Therapeutic modulators of STAT signalling for human diseases

Nanoparticle-Assisted Transcutaneous Delivery of a Signal Transducer and Activator of Transcription 3-Inhibiting Peptide Ameliorates Psoriasis-like Skin Inflammation

Signal transducer and activator of transcription 3 (STAT3) is constitutively activated in psoriatic skin inflammation and acts as a key player in the pathogenesis and progression of this autoimmune disease. Although numerous inhibitors that intervene in STAT3-associated pathways have been tested, an effective, highly specific inhibitor of STAT3 has yet to be identified. Here, we evaluated the in vitro and in vivo biological activity and therapeutic efficacy of a high-affinity peptide specific for STAT3 (APTstat3) after topical treatment via intradermal and transcutaneous delivery. Using a preclinical model of psoriasis, we show that intradermal injection of APTstat3 tagged with a 9-arginine cell-penetrating peptide (APTstat3-9R) reduced disease progression and modulated psoriasis-related cytokine signaling through inhibition of STAT3 phosphorylation. Furthermore, by complexing APTstat3-9R with specific lipid formulations led to formation of discoidal lipid nanoparticles (DLNPs), we were able to achieve efficient skin penetration of the STAT3-inhibiting peptide after transcutaneous administration, thereby effectively inhibiting psoriatic skin inflammation. Collectively, these findings suggest that DLNP-assisted transcutaneous delivery of a STAT3-inhibiting peptide could be a promising strategy for treating psoriatic skin inflammation without causing adverse systemic events. Moreover, the DLNP system could be used for transdermal delivery of other therapeutic peptides.

Targeting JAK-STAT signal transduction in IBD

An unmet medical need exists for novel targeted therapies for inflammatory bowel disease (IBD) as many patients experience inadequate responses to antibody-based biologics. An oral drug formulation with reduced production costs and redundancy for healthcare staff to administer therapy ideally should result in diminished healthcare expenses and improved patient compliance. A new drug class of small molecules, the Janus kinase (JAK) inhibitors (jakinibs), fulfills these criteria and has recently shown efficacy in IBD. Here we provide an overview of the mode of action of jakinibs and provide a comprehensive overview of existing clinical studies. Convincing clinical data show that a complex cytokine-driven inflammation can efficiently be modulated by therapeutic inhibition of the JAK proteins.

Multi-OMICS analyses unveil STAT1 as a potential modifier gene in mevalonate kinase deficiency

OBJECTIVES

The objective of the present study was to explain why two siblings carrying both the same homozygous pathogenic mutation for the autoinflammatory disease hyper IgD syndrome, show opposite phenotypes, that is, the first being asymptomatic, the second presenting all classical characteristics of the disease.

METHODS

Where single omics (mainly exome) analysis fails to identify culprit genes/mutations in human complex diseases, multiomics analyses may provide solutions, although this has been seldom used in a clinical setting. Here we combine exome, transcriptome and proteome analyses to decipher at a molecular level, the phenotypic differences between the two siblings.

RESULTS

This multiomics approach led to the identification of a single gene-STAT1-which harboured a rare missense variant and showed a significant overexpression of both mRNA and protein in the symptomatic versus the asymptomatic sister. This variant was shown to be of gain of function nature, involved in an increased activation of the Janus kinase/signal transducer and activator of transcription signalling (JAK/STAT) pathway, known to play a critical role in inflammatory diseases and for which specific biotherapies presently exist. Pathway analyses based on information from differentially expressed transcripts and proteins confirmed the central role of STAT1 in the proposed regulatory network leading to an increased inflammatory phenotype in the symptomatic sibling.

CONCLUSIONS

This study demonstrates the power of a multiomics approach to uncover potential clinically actionable targets for a personalised therapy. In more general terms, we provide a proteogenomics analysis pipeline that takes advantage of subject-specific genomic and transcriptomic information to improve protein identification and hence advance individualised medicine.

STAT3 Interactors as Potential Therapeutic Targets for Cancer Treatment

Signal transducers and activators of transcription (STATs) mediate essential signaling pathways in different biological processes, including immune responses, hematopoiesis, and neurogenesis. Among the STAT members, STAT3 plays crucial roles in cell proliferation, survival, and differentiation. While STAT3 activation is transient in physiological conditions, STAT3 becomes persistently activated in a high percentage of solid and hematopoietic malignancies (e.g., melanoma, multiple myeloma, breast, prostate, ovarian, and colon cancers), thus contributing to malignant transformation and progression. This makes STAT3 an attractive therapeutic target for cancers. Initial strategies aimed at inhibiting STAT3 functions have focused on blocking the action of its activating kinases or sequestering its DNA binding ability. More recently, the diffusion of proteomic-based techniques, which have allowed for the identification and characterization of novel STAT3-interacting proteins able to modulate STAT3 activity via its subcellular localization, interact with upstream kinases, and recruit transcriptional machinery, has raised the possibility to target such cofactors to specifically restrain STAT3 oncogenic functions. In this article, we summarize the available data about the function of STAT3 interactors in malignant cells and discuss their role as potential therapeutic targets for cancer treatment.

Interleukin-6: designing specific therapeutics for a complex cytokine

Interleukin-6 (IL-6) is a pivotal cytokine with a diverse repertoire of physiological functions that include regulation of immune cell proliferation and differentiation. Dysregulation of IL-6 signalling is associated with inflammatory and lymphoproliferative disorders such as rheumatoid arthritis and Castleman disease, and several classes of therapeutics have been developed that target components of the IL-6 signalling pathway. So far, monoclonal antibodies against IL-6 or IL-6 receptor (IL-6R) and Janus kinases (JAK) inhibitors have been successfully developed for the treatment of autoimmune diseases such as rheumatoid arthritis. However, clinical trials of agents targeting IL-6 signalling have also raised questions about the diseases and patient populations for which such agents have an appropriate benefit-risk profile. Knowledge from clinical trials and advances in our understanding of the complexities of IL-6 signalling, including the potential to target an IL-6 trans-signalling pathway, are now indicating novel opportunities for therapeutic intervention. In this Review, we overview the roles of IL-6 in health and disease and analyse progress with several approaches of inhibiting IL-6-signalling, with the aim of illuminating when and how to apply IL-6 blockade.

Curcumin interacts directly with the Cysteine 259 residue of STAT3 and induces apoptosis in H-Ras transformed human mammary epithelial cells

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is latent but constitutively activated in many types of cancers. It is well known that STAT3 plays a key role in inflammation-associated tumorigenesis. Curcumin is an anti-inflammatory natural compound isolated from the turmeric (Curcuma longa L., Zingiberaceae) that has been extensively used in a traditional medicine over the centuries. In the present study, we have found that curcumin inhibits STAT3 signaling that is persistently overactivated in H-Ras transformed breast epithelial cells (H-Ras MCF10A). Specific cysteine residues present in STAT3 appear to be critical for the activity as well as conformation of this transcription factor. We identified the cysteine residue 259 of STAT3 as a putative site for curcumin binding. Site-directed mutation of this cysteine residue abolished curcumin-induced inactivation of STAT3 and apoptosis in H-Ras MCF10A cells. The α,β-unsaturated carbonyl moiety of curcumin appears to be essential in its binding to STAT3 in H-Ras MCF10A cells. Tetrahydrocurcumin that lacks such electrophilic moiety failed to interact with STAT3 and to induce apoptosis in the same cell line. Taken together, our findings suggest that curcumin can abrogate aberrant activation of STAT3 through direct interaction, thereby inhibiting STAT3-mediated mammary carcinogenesis.

Silibinin is a direct inhibitor of STAT3

We herein combined experimental and computational efforts to delineate the mechanism of action through which the flavonolignan silibinin targets STAT3. Silibinin reduced IL-6 inducible, constitutive, and acquired feedback activation of STAT3 at tyrosine 705 (Y705). Silibinin attenuated the inducible phospho-activation of Y705 in GFP-STAT3 genetic fusions without drastically altering the kinase activity of the STAT3 upstream kinases JAK1 and JAK2. A comparative computational study based on docking and molecular dynamics simulation over 14 different STAT3 inhibitors (STAT3i) predicted that silibinin could directly bind with high affinity to both the Src homology-2 (SH2) domain and the DNA-binding domain (DBD) of STAT3. Silibinin partially overlapped with the cavity occupied by other STAT3i in the SH2 domain to indirectly prevent Y705 phosphorylation, yet showing a unique binding mode. Moreover, silibinin was the only STAT3i predicted to establish direct interactions with DNA in its targeting to the STAT3 DBD. The prevention of STAT3 nuclear translocation, the blockade of the binding of activated STAT3 to its consensus DNA sequence, and the suppression of STAT3-directed transcriptional activity confirmed silibinin as a direct STAT3i. The unique characteristics of silibinin as a bimodal SH2- and DBD-targeting STAT3i make silibinin a promising lead for designing new, more effective STAT3i.

STAT3: a multifaceted oncoprotein

Signal transducer and activator of transcription (STAT) 3 is a key signalling protein engaged by a multitude of growth factors and cytokines to elicit diverse biological outcomes including cellular growth, differentiation, and survival. The complete loss of STAT3 is not compatible with life and even partial loss of function mutations lead to debilitating pathologies like hyper IgE syndrome. Conversely, augmented STAT3 activity has been reported in as many as 50% of all human tumours. The dogma of STAT3 activity posits that it is a tyrosine phosphorylated transcription factor which modulates the expression of hundreds of genes. However, the regulation and biological consequences of STAT3 activation are far more complex. In addition to tyrosine phosphorylation, STAT3 is decorated with a plethora of post-translational modifications which regulate STAT3's nuclear function in addition to its non-genomic activities. In addition to these emerging complexities in the biochemical regulation of STAT3 activity, recent studies reveal that STAT3 is either oncogenic or a tumour suppressor. This review will explore these complexities.

Viral manipulation of STAT3: Evade, exploit, and injure

Signal transducer and activator of transcription 3 (STAT3) is a key regulator of numerous physiological functions, including the immune response. As pathogens elicit an acute phase response with concerted activation of STAT3, they are confronted with two evolutionary options: either curtail it or employ it. This has important consequences for the host, since abnormal STAT3 function is associated with cancer development and other diseases. This review provides a comprehensive outline of how human viruses cope with STAT3-mediated inflammation and how this affects the host. Finally, we discuss STAT3 as a potential target for antiviral therapy.

Linker Variation and Structure-Activity Relationship Analyses of Carboxylic Acid-based Small Molecule STAT3 Inhibitors

The molecular determinants for the activities of the reported benzoic acid (SH4-54), salicylic acid (BP-1-102), and benzohydroxamic acid (SH5-07)-based STAT3 inhibitors were investigated to design optimized analogues. All three leads are based on an N-methylglycinamide scaffold, with its two amine groups condensed with three different functionalities. The three functionalities and the CH₂ group of the glycinamide scaffold were separately modified. The replacement of the pentafluorobenzene or cyclohexylbenzene, or replacing the benzene ring of the aromatic carboxylic or hydroxamic acid motif with heterocyclic components (containing nitrogen and oxygen elements) all decreased potency. Notably, the Ala-linker analogues, 1a and 2v, and the Pro-based derivative 5d, all with (R)-configuration at the chiral center, had improved inhibitory activity and selectivity against STAT3 DNA-binding activity in vitro, with IC₅₀ of 3.0 ± 0.9, 1.80 ± 0.94, and 2.4 ± 0.2 μM, respectively. Compounds 1a, 2v, 5d, and other analogues inhibited constitutive STAT3 phosphorylation and activation in human breast cancer and melanoma lines, and blocked tumor cell viability, growth, colony formation, and migration in vitro. Pro-based analogue, 5h, with a relatively polar tetrahydropyranyl (THP) ring, instead of the cyclohexyl, showed improved permeability. In general, the (R)-configuration Pro-based analogs showed the overall best profile, including physicochemical properties (e.g., microsomal metabolic stability, Caco-2 permeability), and in particular, 5d showed improved tumor-cell specificity.

A First-in-Human Phase I Study of OPB-111077, a Small-Molecule STAT3 and Oxidative Phosphorylation Inhibitor, in Patients with Advanced Cancers

Lessons Learned.

OPB‐111077 is a novel inhibitor of STAT3 and mitochondrial oxidative phosphorylation that exhibited promising anticancer activity in preclinical models.

In this first‐in‐human phase I study of OPB‐111077 in unselected advanced cancers, treatment‐emergent adverse events, most frequently nausea, fatigue, and vomiting, were generally mild to moderate in intensity and could be medically managed.

Overall, only modest clinical activity was observed after OPB‐111077 given as monotherapy. Notable antitumor activity was seen in a subject with diffuse large B‐cell lymphoma.

Background.

OPB‐111077 is a novel inhibitor of STAT3 and mitochondrial oxidative phosphorylation with promising anticancer activity in preclinical models.

Methods.

Open‐label, phase I trial of OPB‐111077 in advanced cancers with no available therapy of documented benefit. Initial dose escalation in unselected subjects was followed by dose expansion. Patients received oral OPB‐111077 daily in 28‐day cycles until loss of clinical benefit.

Results.

Eighteen subjects enrolled in dose escalation, and 127 in dose expansion. Dose‐limiting toxicities were observed at 300 mg and 400 mg QD; maximum tolerated dose was defined as 250 mg QD. Frequently reported treatment‐emergent adverse events (TEAEs) included nausea, fatigue, and vomiting. TEAEs were generally mild to moderate and could be medically managed. OPB‐111077 reached micromolar drug concentrations, had an elimination half‐life of approximately 1 day, and reached steady‐state by day 8. A durable partial response was observed in one subject with diffuse large B‐cell lymphoma. Seven subjects with diverse tumor types had stable disease or minor responses for at least eight treatment cycles (224 days).

Conclusion.

OPB‐111077 is generally well tolerated, and its pharmacokinetic profile is sufficient for further clinical development. Notable clinical activity was observed in a subject with diffuse large B‐cell lymphoma. Overall, modest efficacy was observed against unselected tumors.

Cytokine Signaling Protein 3 Deficiency in Myeloid Cells Promotes Retinal Degeneration and Angiogenesis through Arginase-1 Up-Regulation in Experimental Autoimmune Uveoretinitis

The suppressor of cytokine signaling protein 3 (SOCS3) critically controls immune cell activation, although its role in macrophage polarization and function remains controversial. Using experimental autoimmune uveoretinitis (EAU) as a model, we show that inflammation-mediated retinal degeneration is exaggerated and retinal angiogenesis is accelerated in mice with SOCS3 deficiency in myeloid cells (LysM^Cre/+SOCS3^fl/fl). At the acute stage of EAU, the population of infiltrating neutrophils was increased and the population of macrophages decreased in LysM^Cre/+SOCS3^fl/fl mice compared with that in wild-type (WT) mice. Real-time RT-PCR showed that the expression of tumor necrosis factor-α, IL-1β, interferon-γ, granulocyte-macrophage colony-stimulating factor, and arginase-1 was significantly higher in the LysM^Cre/+SOCS3^fl/fl EAU retina in contrast to the WT EAU retina. The percentage of arginase-1⁺ infiltrating cells was significantly higher in the LysM^Cre/+SOCS3^fl/fl EAU retina than that in the WT EAU retina. In addition, bone marrow-derived macrophages and neutrophils from the LysM^Cre/+SOCS3^fl/fl mice express significantly higher levels of chemokine (C-C motif) ligand 2 and arginase-1 compared with those from WT mice. Inhibition of arginase using an l-arginine analog amino-2-borono-6-hexanoic suppressed inflammation-induced retinal angiogenesis without affecting the severity of inflammation. Our results suggest that SOCS3 critically controls the phenotype and function of macrophages and neutrophils under inflammatory conditions and loss of SOCS3 promotes the angiogenic phenotype of the cells through up-regulation of arginase-1.

Pharmacologic inhibition of STAT5 in acute myeloid leukemia

The transcription factor STAT5 is an essential downstream mediator of many tyrosine kinases (TKs), particularly in hematopoietic cancers. STAT5 is activated by FLT3-ITD, which is a constitutively active TK driving the pathogenesis of acute myeloid leukemia (AML). Since STAT5 is a critical mediator of diverse malignant properties of AML cells, direct targeting of STAT5 is of significant clinical value. Here, we describe the development and preclinical evaluation of a novel, potent STAT5 SH2 domain inhibitor, AC-4–130, which can efficiently block pathological levels of STAT5 activity in AML. AC-4–130 directly binds to STAT5 and disrupts STAT5 activation, dimerization, nuclear translocation, and STAT5-dependent gene transcription. Notably, AC-4–130 substantially impaired the proliferation and clonogenic growth of human AML cell lines and primary FLT3-ITD⁺ AML patient cells in vitro and in vivo. Furthermore, AC-4–130 synergistically increased the cytotoxicity of the JAK1/2 inhibitor Ruxolitinib and the p300/pCAF inhibitor Garcinol. Overall, the synergistic effects of AC-4–130 with TK inhibitors (TKIs) as well as emerging treatment strategies provide new therapeutic opportunities for leukemia and potentially other cancers.

Loss of SOCS3 in myeloid cells prolongs survival in a syngeneic model of glioma

In glioma, microglia and macrophages are the largest population of tumor-infiltrating cells, referred to as glioma associated macrophages (GAMs). Herein, we sought to determine the role of Suppressor of Cytokine Signaling 3 (SOCS3), a negative regulator of Signal Transducer and Activator of Transcription 3 (STAT3), in GAM functionality in glioma. We utilized a conditional model in which SOCS3 deletion is restricted to the myeloid cell population. We found that SOCS3-deficient bone marrow-derived macrophages display enhanced and prolonged expression of pro-inflammatory M1 cytokines when exposed to glioma tumor cell conditioned medium in vitro. Moreover, we found that deletion of SOCS3 in the myeloid cell population delays intracranial tumor growth and increases survival of mice bearing orthotopic glioma tumors in vivo. Although intracranial tumors from mice with SOCS3-deficient myeloid cells appear histologically similar to control mice, we observed that loss of SOCS3 in myeloid cells results in decreased M2 polarized macrophage infiltration in the tumors. Furthermore, loss of SOCS3 in myeloid cells results in increased CD8+ T-cell and decreased regulatory T-cell infiltration in the tumors. These findings demonstrate a beneficial effect of M1 polarized macrophages on suppressing glioma tumor growth, and highlight the importance of immune cells in the tumor microenvironment.

Clinical Utility of a STAT3-Regulated miRNA-200 Family Signature with Prognostic Potential in Early Gastric Cancer

Purpose: The majority of gastric cancer patients are diagnosed with late-stage disease, for which distinct molecular subtypes have been identified that are potentially amenable to targeted therapies. However, there exists no molecular classification system with prognostic power for early-stage gastric cancer (EGC) because the molecular events promoting gastric cancer initiation remain ill-defined.Experimental Design: miRNA microarrays were performed on gastric tissue from the gp130^F/F preclinical EGC mouse model, prior to tumor initiation. Computation prediction algorithms were performed on multiple data sets and independent gastric cancer patient cohorts. Quantitative real-time PCR expression profiling was undertaken in gp130^F/F-based mouse strains and human gastric cancer cells genetically engineered for suppressed activation of the oncogenic latent transcription factor STAT3. Human gastric cancer cells with modulated expression of the miR-200 family member miR-429 were also assessed for their proliferative response.Results: Increased expression of miR-200 family members is associated with both tumor initiation in a STAT3-dependent manner in gp130^F/F mice and EGC (i.e., stage IA) in patient cohorts. Overexpression of miR-429 also elicited contrasting pro- and antiproliferative responses in human gastric cancer cells depending on their cellular histologic subtype. We also identified a miR-200 family-regulated 15-gene signature that integrates multiple key current indicators of EGC, namely tumor invasion depth, differentiation, histology, and stage, and provides superior predictive power for overall survival compared with each EGC indicator alone.Conclusions: Collectively, our discovery of a STAT3-regulated, miR-200 family-associated gene signature specific for EGC, with predictive power, provides a molecular rationale to classify and stratify EGC patients for endoscopic treatment. Clin Cancer Res; 24(6); 1459-72. ©2018 AACR.

Targeted inhibition of STATs and IRFs as a potential treatment strategy in cardiovascular disease

Key factors contributing to early stages of atherosclerosis and plaque development include the pro-inflammatory cytokines Interferon (IFN)α, IFNγ and Interleukin (IL)-6 and Toll-like receptor 4 (TLR4) stimuli. Together, they trigger activation of Signal Transducer and Activator of Transcription (STAT) and Interferon Regulatory Factor (IRF) families. In particular, STAT1, 2 and 3; IRF1 and 8 have recently been recognized as prominent modulators of inflammation, especially in immune and vascular cells during atherosclerosis. Moreover, inflammation-mediated activation of these STATs and IRFs coordinates a platform for synergistic amplification leading to pro-atherogenic responses.

Searches for STAT3-targeting compounds, exploring the pTyr-SH2 interaction area of STAT3, yielded many small molecules including natural products. Only a few inhibitors for other STATs, but none for IRFs, are described. Promising results for several STAT3 inhibitors in recent clinical trials predicts STAT3-inhibiting strategies may find their way to the clinic. However, many of these inhibitors do not seem STAT-specific, display toxicity and are not very potent. This illustrates the need for better models, and screening and validation tools for novel STAT and IRF inhibitors.

This review presents a summary of these findings. It postulates STAT1, STAT2 and STAT3 and IRF1 and IRF8 as interesting therapeutic targets and targeted inhibition could be a potential treatment strategy in CVDs. In addition, it proposes a pipeline approach that combines comparative in silico docking of STAT-SH2 and IRF-DBD models with in vitro STAT and IRF activation inhibition validation, as a novel tool to screen multi-million compound libraries and identify specific inhibitors for STATs and IRFs.

Evidence of microglial activation following exposure to serum from first-onset drug-naïve schizophrenia patients

Abnormal activation of brain microglial cells is widely implicated in the pathogenesis of schizophrenia. Previously the pathophysiology of microglial activation was considered to be intrinsic to the central nervous system. We hypothesised that due to their perivascular localization, microglia can also be activated by factors present in circulating blood. Through application of high-content functional screening, we show that peripheral blood serum from first-onset drug-naïve schizophrenia patients is sufficient to provoke microglial cell signalling network responses in vitro which are indicative of proinflammatory activation. We further explore the composition of the serum for the presence of analytes, with the potential to activate microglia, and the utility of the resultant microglial cellular phenotype for novel drug discovery.

Coptis Chinensis affects the function of glioma cells through the down-regulation of phosphorylation of STAT3 by reducing HDAC3

Background

Glioma remains the most common cause of brain cancer-related mortality. Glioma accounts for 50–60% of brain cancer. Due to their low toxicity and infrequent side effects, traditional herbs have been increasingly popular. Coptis Chinensis is commonly used in cancer treatment in combination with other Chinese Medicine herbs. However, little is known about its biological functions and mechanisms in glioma cells.

Methods

In this study, the anti-glioma cell effect of Coptis Chinensis was determined using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) method, plate clone test, scratch tests, flow cytometry, western blotting and a glioma xenograft tumor model.

Results

The results showed that Coptis Chinensis significantly suppressed glioma cell proliferation, tumor formation, migration and tumor growth, and prolonged the survival time of glioma cell-bearing mice. The flow cytometry result showed that Coptis Chinensis induced cell cycle arrest and apoptosis in glioma cells. Western blotting showed that Coptis Chinensis down-regulated the Signal transducer and activator of transcription 3 (STAT3) phosphorylation levels and reduced the expression of Histone deacetylase 3 (HDAC3) and caspase 3.

Conclusions

Coptis Chinensis can inhibit various aspects of glioma cell functions. This study provides favorable scientific evidence for the potential use of natural products such as Coptis Chinensis in the clinical treatment of patients with glioma.

Signal transducer and activator of transcription gain-of-function primary immunodeficiency/immunodysregulation disorders

PURPOSE OF REVIEW

To describe primary immunodeficiencies caused by gain-of-function (GOF) mutations of signal transducer and activator of transcription (STAT) genes, a group of genetically determined disorders characterized by susceptibility to infections and, in many cases, autoimmune manifestations.

RECENT FINDINGS

GOF mutations affecting STAT1 result in increased STAT tyrosine phosphorylation and secondarily increased response to STAT1-signaling cytokines, such as interferons. In contrast, STAT3 hyperactivity is not usually related to hyperphosphorylation but rather to increased STAT3-mediated transcriptional activity. In both cases, heterozygous STAT1 and STAT3 GOF mutations trigger a distinct set of genes in target cells that lead to abnormal functioning of antimicrobial response and/or autoimmunity and result in autosomal dominant diseases.

SUMMARY

Clinical manifestations of patients with STAT1 GOF are characterized by mucocutaneous candidiasis and recurrent lower tract respiratory infections. In addition, many patients have thyroiditis, type 1 diabetes mellitus, autoimmune cytopenias, cancer or aneurysms. Patients with germline STAT3 GOF mutations have an increased frequency of early-onset multiorgan autoimmunity (i.e. autoimmune enteropathy, type 1 diabetes mellitus, autoimmune interstitial lung disease and autoimmune cytopenias), lymphoproliferation, short stature and, less frequently, severe recurrent infections. Treatment options range from antimicrobial therapy, intravenous or subcutaneous immunoglobulin and immunosuppressive drugs. Some patients with STAT1 GOF disorder have undergone hematopoietic stem cell transplantation, although these have been difficult because of the underlying proinflammatory milieu from the mutation.

New sulfurated derivatives of cinnamic acids and rosmaricine as inhibitors of STAT3 and NF-κB transcription factors

A set of new sulfurated drug hybrids, mainly derived from caffeic and ferulic acids and rosmaricine, has been synthesized and their ability to inhibit both STAT3 and NF-κB transcription factors have been evaluated. Results showed that most of the new hybrid compounds were able to strongly and selectively bind to STAT3, whereas the parent drugs were devoid of this ability at the tested concentrations. Some of them were also able to inhibit the NF-κB transcriptional activity in HCT-116 cell line and inhibited HCT-116 cell proliferation in vitro with IC50 in micromolar range, thus suggesting a potential anticancer activity. Taken together, our study described the identification of new derivatives with dual STAT3/NF-κB inhibitory activity, which may represent hit compounds for developing multi-target anticancer agents.

Benzotriazoles Reactivate Latent HIV-1 through Inactivation of STAT5 SUMOylation

The presence of latent HIV-1 in infected individuals represents a major barrier preventingviral eradication. For that reason, reactivation of latent viruses in the presence of antiretroviral regimens has been proposed as a therapeutic strategy to achieve remission. We screened for small molecules and identified several benzotriazole derivatives with the ability to reactivate latent HIV-1. In the presence of IL-2, benzotriazoles reactivated and reduced the latent reservoir in primary cells, and, remarkably, viral reactivation was achieved without inducing cell proliferation, T cell activation, or cytokine release. Mechanistic studies showed that benzotriazoles block SUMOylation of phosphorylated STAT5, increasing STAT5’s activity and occupancy of the HIV-1 LTR. Our results identify benzotriazoles as latency reversing agents and STAT5 signaling and SUMOylation as targets for HIV-1 eradication strategies. These compounds represent a different direction in the search for “shock and kill” therapies.

Naphthoquinones: A continuing source for discovery of therapeutic antineoplastic agents

Naturally occurring naphthoquinones, usually in forms of botanical extracts, have been implicated with human life since ancient time, far earlier than their isolation and identification in modern era. The long use history of naphthoquinones has witnessed their functional shift from the original purposes as dyes and ornaments toward medicinal benefits. Hitherto, numerous studies have been carried out to elucidate the pharmacological profile of both natural and artificial naphthoquinones. A number of entities have been identified with promising therapeutic potential. Apart from the traditional effects of wound healing, anti-inflammatory, hemostatic, antifertility, insecticidal and antimicrobial, etc., the anticancer potential of naphthoquinones either in combination with other treatment approaches or on their own is being more and more realized. The molecular mechanisms of naphthoquinones in cells mainly fall into two categories as inducing oxidant stress by ROS (reactive oxygen species) generation and directly interacting with traditional therapeutic targets in a non-oxidant mechanism. Based on this knowledge, optimized agents with naphthoquinones scaffold have been acquired and further tested. Hereby, we summarize the explored biological mechanisms of naphthoquinones in cells and review the application perspective of promising naphthoquinones in cancer therapies.

Integrated genomic analysis identifies deregulated JAK/STAT-MYC-biosynthesis axis in aggressive NK-cell leukemia

Aggressive NK-cell leukemia (ANKL) is a rare form of NK cell neoplasm that is more prevalent among people from Asia and Central and South America. Patients usually die within days to months, even after receiving prompt therapeutic management. Here we performed the first comprehensive study of ANKL by integrating whole genome, transcriptome and targeted sequencing, cytokine array as well as functional assays. Mutations in the JAK-STAT pathway were identified in 48% (14/29) of ANKL patients, while the extracellular STAT3 stimulator IL10 was elevated by an average of 56-fold (P < 0.0001) in the plasma of all patients examined. Additional frequently mutated genes included TP53 (34%), TET2 (28%), CREBBP (21%) and MLL2 (21%). Patient NK leukemia cells showed prominent activation of STAT3 phosphorylation, MYC expression and transcriptional activities in multiple metabolic pathways. Functionally, STAT3 activation and MYC expression were critical for the proliferation and survival of ANKL cells. STAT signaling regulated the MYC transcription program, and both STAT signaling and MYC transcription were required to maintain the activation of nucleotide synthesis and glycolysis. Collectively, the JAK-STAT pathway represents a major target for genomic alterations and IL10 stimulation in ANKL. This newly discovered JAK/STAT-MYC-biosynthesis axis may provide opportunities for the development of novel therapeutic strategies in treating this subtype of leukemia.

Targeting Oncogenic Transcription Factors: Therapeutic Implications of Endogenous STAT Inhibitors

Misregulation of transcription factors, including signal transducer and activator of transcription (STAT) proteins, leads to inappropriate gene expression patterns that can promote tumor initiation and progression. Under physiologic conditions, STAT signaling is stimulus dependent and tightly regulated by endogenous inhibitors, namely, suppressor of cytokine signaling (SOCS) proteins, phosphatases, and protein inhibitor of activated STAT (PIAS) proteins. However, in tumorigenesis, STAT proteins become constitutively active and promote the expression of progrowth and prosurvival genes. Although STAT activation has been widely implicated in cancer, therapeutic STAT inhibitors are still largely absent from the clinic. This review dissects the mechanisms of action of two families of endogenous STAT inhibitors, the SOCS and PIAS families, to potentially inform the development of novel therapeutic inhibitors.

High expression of IL-4R enhances proliferation and invasion of hepatocellular carcinoma cells

OBJECTIVE

In this study, we aimed to investigate the expression and function of interleukin-4 receptor (IL-4R) in hepatocellular carcinoma (HCC).

METHODS

We collected 40 pairs of human HCC and adjacent normal tissue specimens and examined the expression levels of IL-4R. After IL-4R knockdown in HCC cell lines, cell proliferation and invasion ability were examined. Cell cycle and apoptosis were analyzed by flow cytometry. The activity of multiple signaling pathways was examined by Western blot.

RESULTS

IL-4R was overexpressed in HCC tumors compared with adjacent normal control tissues and was associated with tumor differentiation status. IL-4R knockdown resulted in enhanced apoptosis, impaired proliferation and reduced invasion of HCC cells. Furthermore, IL-4R knockdown abolished IL-4-induced activation of the Janus Kinase 1 (JAK1)/signal transducer and activator of transcription 6 (STAT6) and JUN N-terminal kinase (JNK)/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathways.

CONCLUSIONS

IL-4R plays an important role in regulating HCC cell survival and metastasis, and regulates the activity of the JAK1/STAT6 and JNK/ERK1/2 signaling pathways. We therefore suggest that IL-4/IL-4R may be a new therapeutic target for HCC.

Tumor-related interleukins: old validated targets for new anti-cancer drug development

In-depth knowledge of cancer molecular and cellular mechanisms have revealed a strong regulation of cancer development and progression by the inflammation which orchestrates the tumor microenvironment. Immune cells, residents or recruited, in the inflammation milieu can have rather contrasting effects during cancer development. Accumulated clinical and experimental data support the notion that acute inflammation could exert an immunoprotective effect leading to tumor eradication. However, chronic immune response promotes tumor growth and invasion. These reactions are mediated by soluble mediators or cytokines produced by either host immune cells or tumor cells themselves. Herein, we provide an overview of the current understanding of the role of the best-validated cytokines involved in tumor progression, IL-1, IL-4 and IL-6; in addition to IL-2 cytokines family, which is known to promote tumor eradication by immune cells. Furthermore, we summarize the clinical attempts to block or bolster the effect of these tumor-related interleukins in anti-cancer therapy development.

Berkeleyacetal C, a meroterpenoid isolated from the fungus Penicillium purpurogenum MHZ 111, exerts anti-inflammatory effects via inhibiting NF-κB, ERK1/2 and IRF3 signaling pathways

Berkeleyacetal C (BAC), a meroterpenoid compound, was isolated from the fungus Penicillium purpurogenum MHZ 111 and showed favorable activity of inhibiting nitrogen oxide (NO) production of macrophages stimulated by lipopolysaccharide (LPS) in our preliminary screening. In order to develop novel therapeutic drug for acute and chronic inflammatory diseases, the anti-inflammatory activity and underlying mechanisms of BAC were investigated in macrophages and neutrophils. The results showed that BAC significantly inhibited the expression of inducible nitric oxide synthase (iNOS) and the following NO production by macrophages. The expression and secretion of key pro-inflammatory factors and chemokines, including tumor necrosis factor-α (TNF-α)，interleukin-6 (IL-6), interleukin-1β (IL-1β), macrophage inflammatory protein-1α (MIP-1α), and monocyte chemotactic protein-1 (MCP-1) were also intensively suppressed by BAC. Furthermore, BAC also markedly inhibited activation of neutrophils and reactive oxygen species production. In mechanism study, BAC selectively suppressed phosphorylation of nuclear factor-κB (NF-κB), extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), and interferon regulatory transcription factor 3 (IRF3) during the activation of NF-κB, mitogen-activated protein kinase (MAPK), signal transducer and activator of transcription 1 and 3 (STAT1/3), and IRF3 signaling pathways induced by LPS. In summary, BAC exerts strong anti-inflammatory effects by inhibiting NF-κB, ERK1/2 and IRF3 signaling pathways and thereby shows great potential to be developed into therapeutic agent for inflammatory disorders.

Novel STAT binding elements mediate IL-6 regulation of MMP-1 and MMP-3

Dynamic remodelling of the extracellular matrix (ECM) is a key feature of cancer progression. Enzymes that modify the ECM, such as matrix metalloproteinases (MMPs), have long been recognised as important targets of anticancer therapy. Inflammatory cytokines are known to play a key role in regulating protease expression in cancer. Here we describe the identification of gamma-activated site (GAS)-like, signal transducer and activator of transcription (STAT) binding elements (SBEs) within the proximal promoters of the MMP-1 and MMP-3 genes, which in association with AP-1 components (c-Fos or Jun), bind STAT-1 in a homodimer like complex (HDLC). We further demonstrate that MMP expression and binding of this complex to SBEs can either be enhanced by interleukin (IL)-6, or reduced by interferon gamma (IFN-γ), and that IL-6 regulation of MMPs is not STAT-3 dependent. Collectively, this data adds to existing understanding of the mechanism underlying cytokine regulation of MMP expression via STAT-1, and increases our understanding of the links between inflammation and malignancy in colon cancer.

Cellular and Molecular Mechanisms of Diabetic Atherosclerosis: Herbal Medicines as a Potential Therapeutic Approach

An increasing number of patients diagnosed with diabetes mellitus eventually develop severe coronary atherosclerosis disease. Both type 1 and type 2 diabetes mellitus increase the risk of cardiovascular disease associated with atherosclerosis. The cellular and molecular mechanisms affecting the incidence of diabetic atherosclerosis are still unclear, as are appropriate strategies for the prevention and treatment of diabetic atherosclerosis. In this review, we discuss progress in the study of herbs as potential therapeutic agents for diabetic atherosclerosis.

Role of CPS1 in Cell Growth, Metabolism and Prognosis in LKB1-Inactivated Lung Adenocarcinoma

Background

Liver kinase B1 ( LKB1 ) is a tumor suppressor in lung adenocarcinoma (LADC). We investigated the proteomic profiles of 45 LADC cell lines with and without LKB1 inactivation. Carbamoyl phosphate synthetase 1 (CPS1), the first rate-limiting mitochondrial enzyme in the urea cycle, was distinctively overexpressed in LKB1-inactivated LADC cell lines. We therefore assessed the role of CPS1 and its clinical relevance in LKB1-inactivated LADC.

Methods

Mass spectrometric profiling of proteome and metabolome and function of CPS1 were analyzed in LADC cell lines. CPS1 and LKB1 expression in tumors from 305 LADC and 160 lung squamous cell carcinoma patients was evaluated by immunohistochemistry. Kaplan-Meier and Cox regression analyses were applied to assess the association between overall survival and CPS1 and LKB1 expression. All statistical tests were two-sided.

Results

CPS1 knockdown reduced cell growth, decreased metabolite levels associated with nucleic acid biosynthesis pathway, and contributed an additive effect when combined with gemcitabine, pemetrexed, or CHK1 inhibitor AZD7762. Tissue microarray analysis revealed that CPS1 was expressed in 65.7% of LKB1-negative LADC, and only 5.0% of LKB1-positive LADC. CPS1 expression showed statistically significant association with poor overall survival in LADC (hazard ratio = 3.03, 95% confidence interval = 1.74 to 5.25, P < .001).

Conclusions

Our findings suggest functional relevance of CPS1 in LKB1-inactivated LADC and association with worse outcome of LADC. CPS1 is a promising therapeutic target in combination with other chemotherapy agents, as well as a prognostic biomarker, enabling a personalized approach to treatment of LADC.

Bioactive polyprenylated benzophenone derivatives from the fruits extracts of Garcinia xanthochymus

Two new polycyclic prenylated xanthones (1 and 2) and a new phenylpropanoid glycoside (3), along with seven known compounds (4-10) were isolated from the fruits of Garcinia xanthochymus. The structures were elucidated by 1D- and 2D-NMR, and HRMS experiments. The isolates were evaluated for their inhibitory effects against the viability of U251MG glioblastoma and MDA-MB-231 breast cancer cells that harbor an aberrantly active signal transducer and exhibit activation of transcription 3 (STAT3), and compared to normal NIH3T3 mouse fibroblasts. Among the isolates, compounds 1, 2, 5, and 6-9 inhibited the viability of glioma cancer cells with IC₅₀ values in the range of 1.6-6.5μM. Furthermore, treatment of U251MG with 6 and 7 inhibited intracellular STAT3 tyrosine phosphorylation and glioma cell migration in vitro, respectively.

Identification of galiellalactone-based novel STAT3-selective inhibitors with cytotoxic activities against triple-negative breast cancer cell lines

Signal transducer and activator of transcription 3 (STAT3) is phosphorylated in breast cancer cells, particularly triple-negative breast cancers (TNBCs). Therefore, the inhibition of constitutive phosphorylated STAT3 is a promising therapeutic for TNBC treatment. Recently, a series of novel STAT3 inhibitors based on natural (-)-galiellalactone have been identified to inhibit STAT3 phosphorylation at the Tyr705 residue. Interestingly, the truncation of the cyclohexene moiety of (-)-galiellalactone to [3.3] bicyclic lactone as a pharmacophoric core produced improved cytotoxic effects against TNBCs. The potent analogues 16 and 17, identified from a STAT3-mediated luciferase reporter assay, selectively inhibited the STAT3 signaling pathway without affecting STAT1 or STAT5.

Mechanisms and consequences of Jak-STAT signaling in the immune system

Kinases of the Jak ('Janus kinase') family and transcription factors (TFs) of the STAT ('signal transducer and activator of transcription') family constitute a rapid membrane-to-nucleus signaling module that affects every aspect of the mammalian immune system. Research on this paradigmatic pathway has experienced breakneck growth in the quarter century since its discovery and has yielded a stream of basic and clinical insights that have profoundly influenced modern understanding of human health and disease, exemplified by the bench-to-bedside success of Jak inhibitors ('jakinibs') and pathway-targeting drugs. Here we review recent advances in Jak-STAT biology, focusing on immune cell function, disease etiology and therapeutic intervention, as well as broader principles of gene regulation and signal-dependent TFs.

STAT3 Induces Immunosuppression by Upregulating PD-1/PD-L1 in HNSCC

Head and neck cancer is one of the most prevalent cancers around the world. Head and neck squamous cell carcinoma (HNSCC) accounts for nearly 90% of head and neck cancer. In recent years, significant advances have been made in immunotherapy for HNSCC. Although some clinical trials targeting immune checkpoints have shown success, the molecular mechanism for regulation of programmed death 1 (PD-1) and its ligand (PD-L1) is partially understood. In an effort to explore the effect of activation of signal transducers and activators of transcriptions (STAT3) on PD-1/PD-L1, the expression and correlation between phosphorylation of STAT3 and PD-1/PD-L1 were determined with immunostaining of human and mouse HNSCC tissue sections. PD-1/PD-L1 overexpression was found to be significantly associated with p-STAT3 in human and mouse HNSCC. Targeting STAT3 by a small molecule effectively inhibited the expression of PD-L1 in the CAL27 cell line. Furthermore, we found that blockade of STAT3 signaling downregulated PD-1/PD-L1 in a Tgfbr1/Pten 2cKO HNSCC mouse model. These findings suggest that STAT3 signaling plays an important role in PD-1/PD-L1 regulation and the antitumor immune response of HNSCC.

Identification of New Shikonin Derivatives as Antitumor Agents Targeting STAT3 SH2 Domain

Signal transducer and activator of transcription 3 (STAT3) is hyper-activated in diversiform human tumors and has been validated as an attractive therapeutic target. Current research showed that a natural product, shikonin, along with its synthetic analogues, is able to inhibit the activity of STAT3 potently. The potential space of shikonin in developing novel anti-cancer agents encouraged us to carry out the investigation of the probable binding mode with STAT3. From this foundation, we have designed new types of STAT3 SH2 inhibitors. Combined simulations were performed to filter for the lead compound, which was then substituted, synthesized and evaluated by a variety of bioassays. Among the entities, PMM-172 exhibited the best anti-proliferative activity against MDA-MB-231 cells with IC₅₀ value 1.98 ± 0.49 μM. Besides, it was identified to decrease luciferase activity, induce cell apoptosis and reduce mitochondrial transmembrane potential in MDA-MB-231 cells. Also, PMM-172 inhibited constitutive/inducible STAT3 activation without affecting STAT1 and STAT5 in MDA-MB-231 cells, and had no effect in non-tumorigenic MCF-10A cells. Moreover, PMM-172 suppressed STAT3 nuclear localization and STAT3 downstream target genes expression. Overall, these results indicate that the antitumor activity of PMM-172 is at least partially due to inhibition of STAT3 in breast cancer cells.

Mitochondrial dysfunction induced by a SH2 domain-targeting STAT3 inhibitor leads to metabolic synthetic lethality in cancer cells

In addition to its canonical role in nuclear transcription, signal transducer and activator of transcription 3 (STAT3) is emerging as an important regulator of mitochondrial function. Here, we demonstrate that a novel inhibitor that binds with high affinity to the STAT3 SH2 domain triggers a complex cascade of events initiated by interference with mitochondrial STAT3 (mSTAT3). The mSTAT3-drug interaction leads to mitochondrial dysfunction, accumulation of proteotoxic STAT3 aggregates, and cell death. The cytotoxic effects depend directly on the drug's ability to interfere with mSTAT3 and mitochondrial function, as demonstrated by site-directed mutagenesis and use of STAT3 knockout and mitochondria-depleted cells. Importantly, the lethal consequences of mSTAT3 inhibition are enhanced by glucose starvation and by increased reliance of cancer cells and tumor-initiating cells on mitochondria, resulting in potent activity in cell cultures and tumor xenografts in mice. These findings can be exploited for eliciting synthetic lethality in metabolically stressed cancer cells using high-affinity STAT3 inhibitors. Thus, this study provides insights on the role of mSTAT3 in cancer cells and a conceptual framework for developing more effective cancer therapies.

GYF-21, an Epoxide 2-(2-Phenethyl)-Chromone Derivative, Suppresses Innate and Adaptive Immunity via Inhibiting STAT1/3 and NF-κB Signaling Pathways

Multiple sclerosis is a chronic inflammatory autoimmune disease of the central nervous system characterized by demyelinating plaques and axonal loss. Inhibition on over activation of innate and adaptive immunity provides a rationale strategy for treatment of multiple sclerosis. In the present study, we investigated the inhibitory effects of GYF-21, an epoxide 2-(2-phenethyl)-chromone derivative isolated from Chinese agarwood, on innate and adaptive immunity for revealing its potential to treat multiple sclerosis. The results showed that GYF-21 markedly inhibited the activation of microglia, and dendritic cells as well as neutrophils, all of which play important roles in innate immunity. Furthermore, GYF-21 significantly suppressed adaptive immunity via inhibiting the differentiation of naive CD4⁺ T cells into T helper 1 (Th1) and T helper 17 (Th17) cells, and suppressing the activation, proliferation, and IFN-γ secretion of CD8⁺ T cells. The mechanism study showed that GYF-21 evidently inhibited the activation of STAT1/3 and NF-κB signaling pathways in microglia. In conclusion, we demonstrated that GYF-21 can significantly inhibit innate and adaptive immunity via suppressing STAT1/3 and NF-κB signaling pathways, and has potential to be developed into therapeutic drug for multiple sclerosis.

Nifuroxazide prompts antitumor immune response of TCL-loaded DC in mice with orthotopically-implanted hepatocarcinoma

Hepatocellular carcinoma (HCC) is a highly aggressive malignancy with a poor prognosis and high mortality. At present, vaccination with tumor cell lysate (TCL) loaded dendritic cells (DC) has been shown to be an effective therapy against HCC. However, the ability of promoting the specific T cell immune response is rather weak, influencing the antitumor response. Thus, it is necessary to find a strategy to improve the antitumor effect of TCL-loaded DC. Activation of signal transducer and activator of transcription 3 (STAT3) significantly inhibits antitumor immune response and DC maturity. Nifuroxazide, an antidiarrheal agent, has been proved to directly inhibit STAT3 activation. Thus, we investigated whether nifuroxazide could improve the antitumor immune response in mice vaccinated with TCL-loaded DC. The study provides the theoretical and experimental basis for developing an effective adjuvant for DC vaccine to treat HCC. Our results showed that the administration of nifuroxazide and DC-loaded TCL could significantly improve the survival rate, inhibit the tumor growth, and prompt the antitumor immune responses in mice with orthotopically implanted hepatocarcinomas, thus, possibly providing a new combination strategy to treat HCC.

Genetics of eosinophilic esophagitis

Eosinophilic esophagitis (EoE) is a chronic, allergic disease associated with marked mucosal eosinophil accumulation. EoE disease risk is multifactorial and includes environmental and genetic factors. This review will focus on the contribution of genetic variation to EoE risk, as well as the experimental tools and statistical methodology used to identify EoE risk loci. Specific disease-risk loci that are shared between EoE and other allergic diseases (TSLP, LRRC32) or unique to EoE (CAPN14), as well as Mendellian Disorders associated with EoE, will be reviewed in the context of the insight that they provide into the molecular pathoetiology of EoE. We will also discuss the clinical opportunities that genetic analyses provide in the form of decision support tools, molecular diagnostics, and novel therapeutic approaches.

Reduction-oxidation pathways involved in cancer development: a systematic review of literature reviews

Oxidative stress results from an imbalance of the reactive oxygen species/reactive nitrogen species (ROS/RNS) production and the oxidants defense system. Extensive research during the last decades has revealed that oxidative stress can mediate cancer initiation and development by leading not only to molecular damage but also to a disruption of reduction-oxidation (redox) signaling. In order to provide a global overview of the redox signaling pathways, which play a role in cancer formation, we conducted a systematic literature search in PubMed and ISI Web of Science and identified 185 relevant reviews published in the last 10 years. The 20 most frequently described pathways were selected to be presented in this systematic review and could be categorized into 3 groups: Intracellular ROS/RNS generating organelles and enzymes, signal transduction cascades kinases/phosphatases and transcription factors. Intracellular ROS/RNS generation organelles are mitochondria, endoplasmic reticulum and peroxisomes. Enzymes, including NOX, COX, LOX and NOS, are the most prominent enzymes generating ROS/RNS. ROS/RNS act as redox messengers of transmembrane receptors and trigger the activation or inhibition of signal transduction kinases/phosphatases, such as the family members of protein tyrosine kinases and protein tyrosine phosphatases. Furthermore, these reactions activate downstream signaling pathways including protein kinase of the MAPK cascade, PI3K and PKC. The kinases and phosphatases regulate the phosphorylation status of transcription factors including APE1/Ref-1, HIF-1α, AP-1, Nrf2, NF-κB, p53, FOXO, STAT, and β-catenin. Finally, we briefly discuss cancer prevention and treatment opportunities, which address redox pathways and further research needs.

Rational development of Stafib-2: a selective, nanomolar inhibitor of the transcription factor STAT5b

The transcription factor STAT5b is a target for tumour therapy. We recently reported catechol bisphosphate and derivatives such as Stafib-1 as the first selective inhibitors of the STAT5b SH2 domain. Here, we demonstrate STAT5b binding of catechol bisphosphate by solid-state nuclear magnetic resonance, and report on rational optimization of Stafib-1 (K_i = 44 nM) to Stafib-2 (K_i = 9 nM). The binding site of Stafib-2 was validated using combined isothermal titration calorimetry (ITC) and protein point mutant analysis, representing the first time that functional comparison of wild-type versus mutant protein by ITC has been used to characterize the binding site of a small-molecule ligand of a STAT protein with amino acid resolution. The prodrug Pomstafib-2 selectively inhibits tyrosine phosphorylation of STAT5b in human leukaemia cells and induces apoptosis in a STAT5-dependent manner. We propose Pomstafib-2, which currently represents the most active, selective inhibitor of STAT5b activation available, as a chemical tool for addressing the fundamental question of which roles the different STAT5 proteins play in various cell processes.