Stat3 cleavage by caspases: impact on full-length Stat3 expression, fragment formation, and transcriptional activity

Signaling pathways in rheumatoid arthritis: implications for targeted therapy

Rheumatoid arthritis (RA) is an incurable systemic autoimmune disease. Disease progression leads to joint deformity and associated loss of function, which significantly impacts the quality of life for sufferers and adds to losses in the labor force. In the past few decades, RA has attracted increased attention from researchers, the abnormal signaling pathways in RA are a very important research field in the diagnosis and treatment of RA, which provides important evidence for understanding this complex disease and developing novel RA-linked intervention targets. The current review intends to provide a comprehensive overview of RA, including a general introduction to the disease, historical events, epidemiology, risk factors, and pathological process, highlight the primary research progress of the disease and various signaling pathways and molecular mechanisms, including genetic factors, epigenetic factors, summarize the most recent developments in identifying novel signaling pathways in RA and new inhibitors for treating RA. therapeutic interventions including approved drugs, clinical drugs, pre-clinical drugs, and cutting-edge therapeutic technologies. These developments will hopefully drive progress in new strategically targeted therapies and hope to provide novel ideas for RA treatment options in the future.

Chronic Proinflammatory Signaling Accelerates the Rate of Degeneration in a Spontaneous Polygenic Model of Inherited Retinal Dystrophy

Collectively, retinal neurodegenerative diseases are comprised of numerous subtypes of disorders which result in loss of a varying cell types in the retina. These diseases can range from glaucoma, which results in retinal ganglion cell death, to age-related macular degeneration and retinitis pigmentosa, which result in cell death of the retinal pigment epithelium, photoreceptors, or both. Regardless of the disease, it’s been recently found that increased release of proinflammatory cytokines and proliferation of active microglia result in a remarkably proinflammatory microenvironment that assists in the pathogenesis of the disease; however, many of the details of these inflammatory events have yet to be elucidated. In an ongoing study, we have used systems genetics to identify possible models of spontaneous polygenic age-related macular degeneration by mining the BXD family of mice using single nucleotide polymorphism analyses of known genes associated with the human retinal disease. One BXD strain (BXD32) was removed from the study as the rate of degeneration observed in these animals was markedly increased with a resultant loss of most all photoreceptors by 6 months of age. Using functional and anatomical exams including optokinetic nystamography, funduscopy, fluorescein angiography, and optical coherence tomography, along with immunohistochemical analyses, we show that the BXD32 mouse strain exhibits a severe neurodegenerative phenotype accompanied by adverse effects on the retinal vasculature. We also expose the concurrent establishment of a chronic proinflammatory microenvironment including the TNFα secretion and activation of the NF-κB and JAK/STAT pathways with an associated increase in activated macrophages and phagoptosis. We conclude that the induced neuronal death and proinflammatory pathways work synergistically in the disease pathogenesis to enhance the rate of degeneration in this spontaneous polygenic model of inherited retinal dystrophy.

The role of understudied post-translational modifications for the behavior and function of Signal Transducer and Activator of Transcription 3

The Signal Transducer and Activator of Transcription (STAT) family of transcription factors is involved in inflammation, immunity, development, cancer, and response to injury, among other biological phenomena. Canonical STAT signaling is often represented as a 3-step pathway involving the sequential activation of a membrane receptor, an intermediate kinase, and a STAT transcription factor. The rate-limiting phosphorylation at a highly conserved C-terminal tyrosine residue determines the nuclear translocation and transcriptional activity of STATs. This apparent simplicity is actually misleading and can hardly explain the pleiotropic nature of STATs, the existence of various noncanonical STAT pathways, or the key role of the N-terminal domain in STAT functions. More than 80 post-translational modifications (PTMs) have been identified for STAT3, but their functions remain barely understood. Here, we provide a brief but comprehensive overview of these underexplored PTMs and their role on STAT3 canonical and noncanonical functions. A less tyrosine-centric point of view may be required to advance our understanding of STAT signaling.

Myristoylation-mediated phase separation of EZH2 compartmentalizes STAT3 to promote lung cancer growth

N-myristoylation is a crucial signaling and pathogenic modification process that confers hydrophobicity to cytosolic proteins. Although different large-scale approaches have been applied, a large proportion of myristoylated proteins remain to be identified. EZH2 is overexpressed in lung cancer cells and exerts oncogenic effects via its intrinsic methyltransferase activity. Using a well-established click chemistry approach, we found that EZH2 can be modified by myristoylation at its N-terminal glycine in lung cancer cells. Hydrophobic interaction is one of the main forces driving or stabilizing liquid-liquid phase separation (LLPS), raising the possibility that myristoylation can modulate LLPS by mediating hydrophobic interactions. Indeed, myristoylation facilitates EZH2 to form phase-separated liquid droplets in lung cancer cells and in vitro. Furthermore, we provide evidence that myristoylation-mediated LLPS of EZH2 compartmentalizes its non-canonical substrate, STAT3, and activates STAT3 signaling, ultimately resulting in accelerated lung cancer cell growth. Thus, targeting EZH2 myristoylation may have significant therapeutic efficacy in the treatment of lung cancer. Altogether, these observations not only extend the list of myristoylated proteins, but also indicate that hydrophobic lipidation may serve as a novel incentive to induce or maintain LLPS.

Overexpression of Stat3 increases circulating cfDNA in breast cancer

PURPOSE

Current studies on circulating cell-free DNA (cfDNA) have been focusing on its potential as biomarkers in liquid biopsy by detecting its content or genetic and epigenetic changes for the evaluation of tumor burden and therapeutic efficacy. However, the regulatory mechanism of cfDNA release remains unclear. Stat3 has been documented as an oncogene for the development and metastasis of breast cancer cells. In this study, we investigated whether Stat3 affects the release of cfDNA into blood and its association with the number of circulating tumor cells (CTCs).

METHODS

The cfDNA level in plasma of patients with breast cancer and healthy volunteers were determined by quantitative real-time PCR. Three mouse breast cancer models with different Stat3 expression were generated and used to established three breast cancer orthotopic animal models to examine the effect of Stat3 on cfDNA release in vivo. Stat3 mediated Epithelial-mesenchymal phenotype transition of CTCs was determined by immunofluorescence assay and Western blot assay.

RESULTS

The data showed that Stat3 increased circulating cfDNA, which is correlated with the increased volume of primary tumors and number of CTCs, accompanied with the dynamic EMT changes regulated by Snail induction. Furthermore, the high level of total circulating cfDNA and Stat3-cfDNA in patients with breast cancer were detected by quantitative real-time PCR using GAPDH and Stat3 primers.

CONCLUSION

Our results suggested that Stat3 increases the circulating cfDNA and CTCs in breast cancer.

The resistant starch from sorghum regulates lipid metabolism in menopausal rats via equol

Equol is a metabolite of daidzein and has a higher biological activity than daidzein. High levels of non-starch polysaccharides can stimulate fermentation in the intestine leading to rapid conversion of daidzein into equol that has great potential to reduce obesity in postmenopausal women. In the present study, female Sprague-Dawley rats were used to establish a menopausal model by oral administration of formestane and to compare the protective effect of resistant starch on lipid metabolism, with or without soybean feed. The resistant starch was found to effectively control body weight and adipose tissue quality, while increasing the high-density lipoprotein cholesterol (HDL-C) concentration and lowering the glycerol, triacylglycerols (TG), total cholesterol (TC), and low density lipoprotein cholesterol (LDL-C) concentrations with soybean feed. Equol inhibited the expression of SREBPC1 gene by inhibiting SHP in the liver via transcription factor FXR, thereby inhibiting the synthesis of triglyceride and fatty acid in the liver. PRACTICAL APPLICATIONS: Intake of a certain amount of resistant starch while eating the soy product can better regulate lipid metabolism in menopausal obese rats compared to consumption of resistant starch alone. Studies have shown that resistant starch converts daidzein to Equol by regulating the structure of the intestinal flora and acts as an estrogen in menopausal rats. This research will further expand the health applications of resistant starch and provide useful information for the food industry.

Mechanistic Study of Triazole Based Aminodiol Derivatives in Leukemic Cells-Crosstalk between Mitochondrial Stress-Involved Apoptosis and Autophagy

Various derivatives that mimic ceramide structures by introducing a triazole to connect the aminodiol moiety and long alkyl chain have been synthesized and screened for their anti-leukemia activity. SPS8 stood out among the derivatives, showing cytotoxic selectivity between leukemic cell lines and human peripheral blood mononuclear cells (about ten times). DAPI nuclear staining and H&E staining revealed DNA fragmentation under the action of SPS8. SPS8 induced an increase in intracellular Ca²⁺ levels and mitochondrial stress in HL-60 cells identified by the loss of mitochondrial membrane potential, transmission electron microscopy (TEM) examination, and altered expressions of Bcl-2 family proteins. SPS8 also induced autophagy through the detection of Atg5, beclin-1, and LC3 II protein expression, as well as TEM examination. Chloroquine, an autophagy inhibitor, promoted SPS8-induced apoptosis, suggesting the cytoprotective role of autophagy in hindering SPS8 from apoptosis. Furthermore, SPS8 was shown to alter the expressions of a variety of genes using a microarray analysis and volcano plot filtering. A further cellular signaling pathways analysis suggested that SPS8 induced several cellular processes in HL-60, including the sterol biosynthesis process and cholesterol biosynthesis process, and inhibited some cellular pathways, in which STAT3 was the most critical nuclear factor. Further identification revealed that SPS8 inhibited the phosphorylation of STAT3, representing the loss of cytoprotective activity. In conclusion, the data suggest that SPS8 induces both apoptosis and autophagy in leukemic cells, in which autophagy plays a cytoprotective role in impeding apoptosis. Moreover, the inhibition of STAT3 phosphorylation may support SPS8-induced anti-leukemic activity.

CREBZF as a Key Regulator of STAT3 Pathway in the Control of Liver Regeneration in Mice

BACKGROUND AND AIMS

STAT3, a member of the signal transducer and activator of transcription (STAT) family, is strongly associated with liver injury, inflammation, regeneration, and hepatocellular carcinoma development. However, the signals that regulate STAT3 activity are not completely understood.

APPROACH AND RESULTS

Here we characterize CREB/ATF bZIP transcription factor CREBZF as a critical regulator of STAT3 in the hepatocyte to repress liver regeneration. We show that CREBZF deficiency stimulates the expression of the cyclin gene family and enhances liver regeneration after partial hepatectomy. Flow cytometry analysis reveals that CREBZF regulates cell cycle progression during liver regeneration in a hepatocyte-autonomous manner. Similar results were observed in another model of liver regeneration induced by intraperitoneal injection of carbon tetrachloride (CCl4 ). Mechanistically, CREBZF potently associates with the linker domain of STAT3 and represses its dimerization and transcriptional activity in vivo and in vitro. Importantly, hepatectomy-induced hyperactivation of cyclin D1 and liver regeneration in CREBZF liver-specific knockout mice was reversed by selective STAT3 inhibitor cucurbitacin I. In contrast, adeno-associated virus-mediated overexpression of CREBZF in the liver inhibits the expression of the cyclin gene family and attenuates liver regeneration in CCl4 -treated mice.

CONCLUSIONS

These results characterize CREBZF as a coregulator of STAT3 to inhibit regenerative capacity, which may represent an essential cellular signal to maintain liver mass homeostasis. Therapeutic approaches to inhibit CREBZF may benefit the compromised liver during liver transplantation.

Targeting Janus Kinases and Signal Transducer and Activator of Transcription 3 to Treat Inflammation, Fibrosis, and Cancer: Rationale, Progress, and Caution

Before it was molecularly cloned in 1994, acute-phase response factor or signal transducer and activator of transcription (STAT)3 was the focus of intense research into understanding the mammalian response to injury, particularly the acute-phase response. Although known to be essential for liver production of acute-phase reactant proteins, many of which augment innate immune responses, molecular cloning of acute-phase response factor or STAT3 and the research this enabled helped establish the central function of Janus kinase (JAK) family members in cytokine signaling and identified a multitude of cytokines and peptide hormones, beyond interleukin-6 and its family members, that activate JAKs and STAT3, as well as numerous new programs that their activation drives. Many, like the acute-phase response, are adaptive, whereas several are maladaptive and lead to chronic inflammation and adverse consequences, such as cachexia, fibrosis, organ dysfunction, and cancer. Molecular cloning of STAT3 also enabled the identification of other noncanonical roles for STAT3 in normal physiology, including its contribution to the function of the electron transport chain and oxidative phosphorylation, its basal and stress-related adaptive functions in mitochondria, its function as a scaffold in inflammation-enhanced platelet activation, and its contributions to endothelial permeability and calcium efflux from endoplasmic reticulum. In this review, we will summarize the molecular and cellular biology of JAK/STAT3 signaling and its functions under basal and stress conditions, which are adaptive, and then review maladaptive JAK/STAT3 signaling in animals and humans that lead to disease, as well as recent attempts to modulate them to treat these diseases. In addition, we will discuss how consideration of the noncanonical and stress-related functions of STAT3 cannot be ignored in efforts to target the canonical functions of STAT3, if the goal is to develop drugs that are not only effective but safe. SIGNIFICANCE STATEMENT: Key biological functions of Janus kinase (JAK)/signal transducer and activator of transcription (STAT)3 signaling can be delineated into two broad categories: those essential for normal cell and organ development and those activated in response to stress that are adaptive. Persistent or dysregulated JAK/STAT3 signaling, however, is maladaptive and contributes to many diseases, including diseases characterized by chronic inflammation and fibrosis, and cancer. A comprehensive understanding of JAK/STAT3 signaling in normal development, and in adaptive and maladaptive responses to stress, is essential for the continued development of safe and effective therapies that target this signaling pathway.

Antiparkinson Drug Benztropine Suppresses Tumor Growth, Circulating Tumor Cells, and Metastasis by Acting on SLC6A3/DAT and Reducing STAT3

Tumor growth, progression, and therapy resistance are crucial factors in the prognosis of cancer. The properties of three-dimensional (3D) tumor-like organoids (tumoroids) more closely resemble in vivo tumors compared to two-dimensionally cultured cells and are therefore effectively used for assays and drug screening. We here established a repurposed drug for novel anticancer research and therapeutics using a 3D tumoroid-based screening system. We screened six pharmacologically active compounds by using an original tumoroid-based multiplex phenotypic screening system with a matrix metalloproteinase 9 (MMP9) promoter-driven fluorescence reporter for the evaluation of both tumoroid formation and progression. The antiparkinson drug benztropine was the most effective compound uncovered by the screen. Benztropine significantly inhibited in vitro tumoroid formation, cancer cell survival, and MMP9 promoter activity. Benztropine also reduced the activity of oncogenic signaling transducers and trans-activators for MMP9, including STAT3, NF-κB, and β-catenin, and the properties of cancer stem cells/cancer-initiating cells. Benztropine and GBR-12935 directly targeted the dopamine transporter DAT/SLC6A3, whose genetic alterations such as amplification were correlated with poor prognosis for cancer patients. Benztropine also inhibited the tumor growth, circulating tumor cell (CTC) number, and rate of metastasis in a tumor allograft model in mice. In conclusion, we propose the repurposing of benztropine for anticancer research and therapeutics that can suppress tumor progression, CTC, and metastasis of aggressive cancers by reducing key pro-tumorigenic factors.

Targeting STAT-3 signaling pathway in cancer for development of novel drugs: Advancements and challenges

Signal transducers and activators of transcription 3 (STAT-3) is a transcription factor that regulates the gene expression of several target genes. These factors are activated by the binding of cytokines and growth factors with STAT-3 specific receptors on cell membrane. Few years ago, STAT-3 was considered an acute phase response element having several cellular functions such as inflammation, cell survival, invasion, metastasis and proliferation, genetic alteration, and angiogenesis. STAT-3 is activated by several types of inflammatory cytokines, carcinogens, viruses, growth factors, and oncogenes. Thus, the STAT3 pathway is a potential target for cancer therapeutics. Abnormal STAT-3 activity in tumor development and cellular transformation can be targeted by several genomic and pharmacological methodologies. An extensive review of the literature has been conducted to emphasize the role of STAT-3 as a unique cancer drug target. This review article discusses in detail the wide range of STAT-3 inhibitors that show antitumor effects both in vitro and in vivo. Thus, targeting constitutive STAT-3 signaling is a remarkable therapeutic methodology for tumor progression. Finally, current limitations, trials and future perspectives of STAT-3 inhibitors are also critically discussed.

A cell type-specific transcriptomic approach to map B cell and monocyte type I interferon-linked pathogenic signatures in Multiple Sclerosis

Alteration in endogenous Interferon (IFN) system may profoundly impact immune cell function in autoimmune diseases. Here, we provide evidence that dysregulation in IFN-regulated genes and pathways are involved in B cell- and monocyte-driven pathogenic contribution to Multiple Sclerosis (MS) development and maintenance. In particular, by using an Interferome-based cell type-specific approach, we characterized an increased susceptibility to an IFN-linked caspase-3 dependent apoptotic cell death in both B cells and monocytes of MS patients that may arise from their chronic activation and persistent stimulation by activated T cells. Ongoing caspase-3 activation functionally impacts on MS monocyte properties influencing the STAT-3/IL-16 axis, thus, driving increased expression and massive release of the bio-active IL-16 triggering and perpetuating CD4⁺ T cell migration. Importantly, our analysis also identified a previously unknown multi-component defect in type I IFN-mediated signaling and response to virus pathways specific of MS B cells, impacting on induction of anti-viral responses and Epstein-barr virus infection control in patients. Taking advantage of cell type-specific transcriptomics and in-depth functional validation, this study revealed pathogenic contribution of endogenous IFN signaling and IFN-regulated cell processes to MS pathogenesis with implications on fate and functions of B cells and monocytes that may hold therapeutic potential.

Differential Modulation of Transcription Factors and Cytoskeletal Proteins in Prostate Carcinoma Cells by a Bacterial Lactone

The present study tested the effect of a bacterial lactone N-(3-oxododecanoyl)-homoserine lactone (C12-HSL) on the cytoskeletal and transcriptional genes and proteins in prostate adenocarcinoma (PA) cells (DU145 and LNCaP) and prostate small cell neuroendocrine carcinoma (SCNC) PC3 cells including their cellular viability and apoptosis. Our data indicate that cell migration and colony formation were affected in the presence of C12-HSL. C12-HSL induced apoptosis and altered viability of both PA and SCNC cells in a concentration dependent manner as measured by fluorescence and chemiluminescence assays. Compared to PCa cells, noncancerous prostate epithelial cells (RWPE1) were resistant to modification by C12-HSL. Further, the viability of PC3 cells in 3D matrix was suppressed by C12-HSL treatment as detected using calcein AM fluorescence in situ. C12-HSL treatment induced cytoskeletal associated protein expression of vinculin and RhoC, which may have implications in cancer cell motility, adhesion, and metastasis. IQGAP protein expression was reduced in DU145 and RWPE1 cells in the presence of C12-HSL. C12-HSL decreased STAT3 phosphorylation in DU145 cells but increased STAT1 protein phosphorylation in PC3 and LNCaP cells. Overall, these studies indicate that C12-HSL can trigger changes in transcription factors and cytoskeletal proteins and thereby modulate growth and migration properties of PCa cells.

Vascular smooth muscle cell proliferation as a therapeutic target. Part 1: molecular targets and pathways

Cardiovascular diseases are a major cause of human death worldwide. Excessive proliferation of vascular smooth muscle cells contributes to the etiology of such diseases, including atherosclerosis, restenosis, and pulmonary hypertension. The control of vascular cell proliferation is complex and encompasses interactions of many regulatory molecules and signaling pathways. Herein, we recapitulated the importance of signaling cascades relevant for the regulation of vascular cell proliferation. Detailed understanding of the mechanism underlying this process is essential for the identification of new lead compounds (e.g., natural products) for vascular therapies.

STAT3: The art of multi-tasking of metabolic and immune functions in obesity

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that has recently attracted increased attention. Accumulating studies have demonstrated that STAT3 plays an important role in various diseases, such as cancer and ischemic injury. In light of the distinctive effects of STAT3 on the regulation of metabolism and immune responses, we present the elaborate network of STAT3 in obesity. In this review, we first introduce the general background of STAT3, including a discussion regarding the STAT family and the characterization and regulation of STAT3. Then, we describe the STAT3 signaling network and its pathophysiological roles in lipid and glucose metabolism and immune function. Finally, we highlight the research progress regarding STAT3 in obesity. The information presented here may be useful for the design of future studies and may highlight the potential of STAT3 as a future therapeutic target for obesity.

The roles of signal transducer and activator of transcription factor 3 in tumor angiogenesis

Angiogenesis is the development of new blood vessels, which is required for tumor growth and metastasis. Signal transducer and activator of transcription factor 3 (STAT3) is a transcription factor that regulates a variety of cellular events including proliferation, differentiation and apoptosis. Previous studies revealed that activation of STAT3 promotes tumor angiogenesis. In this review, we described the activities of STAT3 signaling in different cell types involved in angiogenesis. Particularly, we elucidated the molecular mechanisms of STAT3-mediated gene regulation in angiogenic endothelial cells in response to external stimulations such as hypoxia and inflammation. The potential for STAT3 as a therapeutic target was also discussed. Overall, this review provides mechanistic insights for the roles of STAT3 signaling in tumor angiogenesis.

Stress-induced dynamic regulation of mitochondrial STAT3 and its association with cyclophilin D reduce mitochondrial ROS production

Signal transducer and activator of transcription 3 (STAT3) is associated with various physiological and pathological functions, mainly as a transcription factor that translocates to the nucleus upon tyrosine phosphorylation induced by cytokine stimulation. In addition, a small pool of STAT3 resides in the mitochondria, where it serves as a sensor for various metabolic stressors including reactive oxygen species (ROS). Mitochondrially localized STAT3 largely exerts its effects through direct or indirect regulation of the activity of the electron transport chain (ETC). It has been assumed that the amounts of STAT3 in the mitochondria are static. We showed that various stimuli, including oxidative stress and cytokines, triggered a signaling cascade that resulted in a rapid loss of mitochondrially localized STAT3. Recovery of the mitochondrial pool of STAT3 over time depended on phosphorylation of Ser727 in STAT3 and new protein synthesis. Under these conditions, mitochondrially localized STAT3 also became competent to bind to cyclophilin D (CypD). Binding of STAT3 to CypD was mediated by the amino terminus of STAT3, which was also important for reducing mitochondrial ROS production after oxidative stress. These results outline a role for mitochondrially localized STAT3 in sensing and responding to external stimuli.

Notch signalling drives bone marrow stromal cell-mediated chemoresistance in acute myeloid leukemia

Both preclinical and clinical investigations suggest that Notch signalling is critical for the development of many cancers and for their response to chemotherapy. We previously showed that Notch inhibition abrogates stromal-induced chemoresistance in lymphoid neoplasms. However, the role of Notch in acute myeloid leukemia (AML) and its contribution to the crosstalk between leukemia cells and bone marrow stromal cells remain controversial. Thus, we evaluated the role of the Notch pathway in the proliferation, survival and chemoresistance of AML cells in co-culture with bone marrow mesenchymal stromal cells expanded from both healthy donors (hBM-MSCs) and AML patients (hBM-MSCs*). As compared to hBM-MSCs, hBM-MSCs* showed higher level of Notch1, Jagged1 as well as the main Notch target gene HES1. Notably, hBM-MSCs* induced expression and activation of Notch signalling in AML cells, supporting AML proliferation and being more efficientin inducing AML chemoresistance than hBM-MSCs*. Pharmacological inhibition of Notch using combinations of Notch receptor-blocking antibodies or gamma-secretase inhibitors (GSIs), in presence of chemotherapeutic agents, significant lowered the supportive effect of hBM-MSCs and hBM-MSCs* towards AML cells, by activating apoptotic cascade and reducing protein level of STAT3, AKT and NF-κB.These results suggest that Notch signalling inhibition, by overcoming the stromal-mediated promotion of chemoresistance,may represent a potential therapeutic targetnot only for lymphoid neoplasms, but also for AML.

The STAT3 HIES mutation is a gain-of-function mutation that activates genes via AGG-element carrying promoters

Cytokine or growth factor activated STAT3 undergoes multiple post-translational modifications, dimerization and translocation into nuclei, where it binds to serum-inducible element (SIE, ‘TTC(N3)GAA’)-bearing promoters to activate transcription. The STAT3 DNA binding domain (DBD, 320–494) mutation in hyper immunoglobulin E syndrome (HIES), called the HIES mutation (R382Q, R382W or V463Δ), which elevates IgE synthesis, inhibits SIE binding activity and sensitizes genes such as TNF-α for expression. However, the mechanism by which the HIES mutation sensitizes STAT3 in gene induction remains elusive. Here, we report that STAT3 binds directly to the AGG-element with the consensus sequence ‘AGG(N3)AGG’. Surprisingly, the helical N-terminal region (1–355), rather than the canonical STAT3 DBD, is responsible for AGG-element binding. The HIES mutation markedly enhances STAT3 AGG-element binding and AGG-promoter activation activity. Thus, STAT3 is a dual specificity transcription factor that promotes gene expression not only via SIE- but also AGG-promoter activity.

Characterization of STAT3 expression, signaling and inhibition in feline oral squamous cell carcinoma

Background

Signal transducer and activator of transcription 3 (STAT3) plays a critical role in tumor development by regulating signaling pathways involved in cell proliferation, survival, metastasis and angiogenesis. STAT3 is activated in many cancers, including head and neck squamous cell carcinoma (HNSCC) in people. Feline oral squamous cell carcinoma (OSCC) is similar to advanced or recurrent HNSCC as it is poorly responsive to traditional therapies and carries a poor long-term prognosis. The purpose of this study was to characterize expression and activation of STAT3 in feline OSCC cell lines and tumor samples and to investigate the biologic activity of a novel, allosteric STAT3 inhibitor, LLL12, in feline OSCC cell lines.

Results

We evaluated 3 feline OSCC cell lines and one of these (SCCF2) exhibited high levels of constitutive STAT3 phosphorylation and high sensitivity to LLL12 treatment. Exposure of SCCF2 cells to LLL12 resulted in decreased expression of pSTAT3 and total STAT3, apoptosis as assessed by caspase 3/7 activation, inhibition of colony formation and reduced expression of the STAT3 transcriptional target survivin. In contrast, the STAT3 transcriptional targets VEGF and MCL-1 increased after LLL12 treatment. This was, in part, likely due to LLL12 mediated upregulation of HIF-1α, which is known to drive VEGF and MCL-1 expression. The OSCC cell lines with low basal STAT3 phosphorylation did not exhibit these effects, suggesting that STAT3 inhibition was responsible for the observed results. Lastly, immunohistochemistry for pSTAT3 was performed using a feline OSCC tissue microarray, demonstrating expression in 48 % of samples tested.

Conclusions

These data demonstrate that LLL12 has biologic activity against a feline OSCC cell line expressing pSTAT3 and that STAT3 represents a target for therapeutic intervention in this disease. However, given the up-regulation of several STAT3 transcriptional targets following treatment, further investigation of STAT3 and its related signaling pathways in OSCC is warranted.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0505-7) contains supplementary material, which is available to authorized users.

Lung Tumor Suppressor GPRC5A Binds EGFR and Restrains Its Effector Signaling

GPRC5A is a G-protein-coupled receptor expressed in lung tissue but repressed in most human lung cancers. Studies in Gprc5a(-/-) mice have established its role as a tumor-suppressor function in this setting, but the basis for its role has been obscure. Here, we report that GPRC5A functions as a negative modulator of EGFR signaling. Mouse tracheal epithelial cells (MTEC) from Gprc5a(-/-) mice exhibited a relative increase in EGFR and downstream STAT3 signaling, whereas GPRC5A expression inhibited EGFR and STAT3 signaling. GPRC5A physically interacted with EGFR through its transmembrane domain, which was required for its EGFR inhibitory activity. Gprc5a(-/-) MTEC were much more susceptible to EGFR inhibitors than wild-type MTEC, suggesting their dependence on EGFR signaling for proliferation and survival. Dysregulated EGFR and STAT3 were identified in the normal epithelia of small and terminal bronchioles as well as tumors of Gprc5a(-/-) mouse lungs. Moreover, in these lungs EGFR inhibitor treatment inhibited EGFR and STAT3 activation along with cell proliferation. Finally, overexpression of ectopic GPRC5A in human non-small cell lung carcinoma cells inhibited both EGF-induced and constitutively activated EGFR signaling. Taken together, our results show how GPRC5A deficiency leads to dysregulated EGFR and STAT3 signaling and lung tumorigenesis. Cancer Res; 75(9); 1801-14. ©2015 AACR.

Caspase-3 and caspase-6 cleave STAT1 in leukemic cells

Signal Transducer and Activator of Transcription-1 (STAT1) is phosphorylated upon interferon (IFN) stimulation, which can restrict cell proliferation and survival. Nevertheless, in some cancers STAT1 can act in an anti-apoptotic manner. Moreover, certain malignancies are characterized by the overexpression and constitutive activation of STAT1. Here, we demonstrate that the treatment of transformed hematopoietic cells with epigenetic drugs belonging to the class of histone deacetylase inhibitors (HDACi) leads to the cleavage of STAT1 at multiple sites by caspase-3 and caspase-6. This process does not occur in solid tumor cells, normal hematopoietic cells, and leukemic cells that underwent granulocytic or monocytic differentiation. STAT1 cleavage was studied under cell free conditions with purified STAT1 and a set of candidate caspases as well as with mass spectrometry. These assays indicate that unmodified STAT1 is cleaved at multiple sites by caspase-3 and caspase-6. Our study shows that STAT1 is targeted by caspases in malignant undifferentiated hematopoietic cells. This observation may provide an explanation for the selective toxicity of HDACi against rapidly proliferating leukemic cells.

Raf kinase inhibitor protein (RKIP) blocks signal transducer and activator of transcription 3 (STAT3) activation in breast and prostate cancer

Raf kinase inhibitor protein (RKIP) is a member of the phosphatidylethanolamine-binding-protein (PEBP) family that modulates the action of many kinases involved in cellular growth, apoptosis, epithelial to mesenchymal transition, motility, invasion and metastasis. Previously, we described an inverse association between RKIP and signal transducers and activators of transcription 3 (STAT3) expression in gastric adenocarcinoma patients. In this study, we elucidated the mechanism by which RKIP regulates STAT3 activity in breast and prostate cancer cell lines. RKIP over expression inhibited c-Src auto-phosphorylation and activation, as well as IL-6-, JAK1 and 2-, and activated Raf-mediated STAT3 tyrosine and serine phosphorylation and subsequent activation. In MDA-231 breast cancer cells that stably over express RKIP, IL-6 treatment blocked STAT3 phosphorylation and transcriptional activation. Conversely, in RKIP knockdown MDA-231 cells: STAT3 phosphorylation and activation increased in comparison to parental MDA-231 cells. RKIP over expression resulted in constitutive physical interaction with STAT3 and blocked c-Src and STAT3 association. The treatment of DU145 prostate, but not PC3 prostate or MDA-231 breast, cancer cell lines with ENMD-1198 or MKC-1 dramatically increased expression of RKIP. Overexpression of RKIP sensitized PC3 and MDA-231 cells to MTI-induced apoptosis. Moreover, MTI treatment resulted in a decrease in Src-mediated STAT3 tyrosine phosphorylation and activation, an effect that was significantly enhanced by RKIP over expression. In stable RKIP over expressing MDA-231 cells, tumor xenograft growth induced by activated STAT3 is inhibited. RKIP synergizes with MTIs to induce apoptosis and inhibit STAT3 activation of breast and prostate cancer cells. RKIP plays a critical role in opposing the effects of pro-oncogenic STAT3 activation.

XPO1 (CRM1) inhibition represses STAT3 activation to drive a survivin-dependent oncogenic switch in triple-negative breast cancer

Inhibition of XPO1 (CRM1)-mediated nuclear export of multiple tumor suppressor proteins has been proposed as a novel cancer therapeutic strategy to turn off oncogenic signals and enhance tumor suppression. Survivin is a multifunctional protein with oncogenic properties when expressed in the cytoplasm that requires the XPO1-RanGTP complex for its nuclear export. We investigated the antitumor mechanisms of the drug-like selective inhibitors of nuclear export (SINE) XPO1 antagonists KPT-185, KPT-251 KPT-276, and KPT-330 in estrogen receptor-positive and triple-negative breast cancer (TNBC) cell lines and xenograft models of human breast tumors. KPT compounds significantly inhibited breast cancer cell growth and induced tumor cell death, both in vitro and in vivo. These drugs initially promoted survivin accumulation within tumor cell nuclei. However, their major in vitro effect was to decrease survivin cytoplasmic protein levels, correlating with the onset of apoptosis. XPO1 inhibition repressed Survivin transcription by inhibiting CREB-binding protein-mediated STAT3 acetylation, and blocking STAT3 binding to the Survivin promoter. In addition, caspase-3 was activated to cleave survivin, rendering it unavailable to bind X-linked inhibitor of apoptosis protein and block the caspase cascade. Collectively, these data demonstrate that XPO1 inhibition by SINE compounds represses STAT3 transactivation to block the selective oncogenic properties of survivin and supports their clinical use in TNBC.

Cleavage of the JunB transcription factor by caspases generates a carboxyl-terminal fragment that inhibits activator protein-1 transcriptional activity

The activator protein-1 (AP-1) family transcription factor, JunB, is an important regulator of proliferation, apoptosis, differentiation, and the immune response. In this report, we show that JunB is cleaved in a caspase-dependent manner in apoptotic anaplastic lymphoma kinase-positive, anaplastic large cell lymphoma cell lines and that ectopically expressed JunB is cleaved in murine RAW 264.7 macrophage cells treated with the NALP1b inflammasome activator, anthrax lethal toxin. In both cases, we identify aspartic acid 137 as the caspase cleavage site and demonstrate that JunB can be directly cleaved in vitro by multiple caspases at this site. Cleavage of JunB at aspartic acid 137 separates the N-terminal transactivation domain from the C-terminal DNA binding and dimerization domains, and we show that the C-terminal cleavage fragment retains both DNA binding activity and the ability to interact with AP-1 family transcription factors. Furthermore, this fragment interferes with the binding of full-length JunB to AP-1 sites and inhibits AP-1-dependent transcription. In summary, we have identified and characterized a novel mechanism of JunB post-translational modification and demonstrate that the C-terminal JunB caspase cleavage product functions as a potent inhibitor of AP-1-dependent transcription.

Mechanisms of unphosphorylated STAT3 transcription factor binding to DNA

Phosphorylation of signal transducer and activator of transcription 3 (STAT3) on a single tyrosine residue in response to growth factors, cytokines, interferons, and oncogenes activates its dimerization, translocation to the nucleus, binding to the interferon γ (gamma)-activated sequence (GAS) DNA-binding site and activation of transcription of target genes. STAT3 is constitutively phosphorylated in various cancers and drives gene expression from GAS-containing promoters to promote tumorigenesis. Recently, roles for unphosphorylated STAT3 (U-STAT3) have been described in response to cytokine stimulation, in cancers, and in maintenance of heterochromatin stability. However, the mechanisms underlying U-STAT3 binding to DNA has not been fully investigated. Here, we explore STAT3-DNA interactions by atomic force microscopy (AFM) imaging. We observed that U-STAT3 molecules bind to the GAS DNA-binding site as dimers and monomers. In addition, we observed that U-STAT3 binds to AT-rich DNA sequence sites and recognizes specific DNA structures, such as 4-way junctions and DNA nodes, within negatively supercoiled plasmid DNA. These structures are important for chromatin organization and our data suggest a role for U-STAT3 as a chromatin/genome organizer. Unexpectedly, we found that a C-terminal truncated 67.5-kDa STAT3 isoform recognizes single-stranded spacers within cruciform structures that also have a role in chromatin organization and gene expression. This isoform appears to be abundant in the nuclei of cancer cells and, therefore, may have a role in regulation of gene expression. Taken together, our data highlight novel mechanisms by which U-STAT3 binds to DNA and supports U-STAT3 function as a transcriptional activator and a chromatin/genomic organizer.

MicroRNA miR-885-5p targets CDK2 and MCM5, activates p53 and inhibits proliferation and survival

Several microRNA (miRNA) loci are found within genomic regions frequently deleted in primary neuroblastoma, including miR-885-5p at 3p25.3. In this study, we demonstrate that miR-885-5p is downregulated on loss of 3p25.3 region in neuroblastoma. Experimentally enforced miR-885-5p expression in neuroblastoma cell lines inhibits proliferation triggering cell cycle arrest, senescence and/or apoptosis. miR-885-5p leads to the accumulation of p53 protein and activates the p53 pathway, resulting in upregulation of p53 targets. Enforced miR-885-5p expression consistently leads to downregulation of cyclin-dependent kinase (CDK2) and mini-chromosome maintenance protein (MCM5). Both genes are targeted by miR-885-5p via predicted binding sites within the 3'-untranslated regions (UTRs) of CDK2 and MCM5. Transcript profiling after miR-885-5p introduction in neuroblastoma cells reveals alterations in expression of multiple genes, including several p53 target genes and a number of factors involved in p53 pathway activity. Taken together, these data provide evidence that miR-885-5p has a tumor suppressive role in neuroblastoma interfering with cell cycle progression and cell survival.

The novel curcumin analog FLLL32 decreases STAT3 DNA binding activity and expression, and induces apoptosis in osteosarcoma cell lines

Background

Curcumin is a naturally occurring phenolic compound shown to have a wide variety of antitumor activities; however, it does not attain sufficient blood levels to do so when ingested. Using structure-based design, a novel compound, FLLL32, was generated from curcumin. FLLL32 possesses superior biochemical properties and more specifically targets STAT3, a transcription factor important in tumor cell survival, proliferation, metastasis, and chemotherapy resistance. In our previous work, we found that several canine and human osteosarcoma (OSA) cell lines, but not normal osteoblasts, exhibit constitutive phosphorylation of STAT3. Compared to curcumin, we hypothesized that FLLL32 would be more efficient at inhibiting STAT3 function in OSA cells and that this would result in enhanced downregulation of STAT3 transcriptional targets and subsequent death of OSA cells.

Methods

Human and canine OSA cells were treated with vehicle, curcumin, or FLLL32 and the effects on proliferation (CyQUANT^®), apoptosis (SensoLyte^® Homogeneous AMC Caspase- 3/7 Assay kit, western blotting), STAT3 DNA binding (EMSA), and vascular endothelial growth factor (VEGF), survivin, and matrix metalloproteinase-2 (MMP2) expression (RT-PCR, western blotting) were measured. STAT3 expression was measured by RT-PCR, qRT- PCR, and western blotting.

Results

Our data showed that FLLL32 decreased STAT3 DNA binding by EMSA. FLLL32 promoted loss of cell proliferation at lower concentrations than curcumin leading to caspase-3- dependent apoptosis, as evidenced by PARP cleavage and increased caspase 3/7 activity; this could be inhibited by treatment with the pan-caspase inhibitor Z-VAD-FMK. Treatment of OSA cells with FLLL32 decreased expression of survivin, VEGF, and MMP2 at both mRNA and protein levels with concurrent decreases in phosphorylated and total STAT3; this loss of total STAT3 occurred, in part, via the ubiquitin-proteasome pathway.

Conclusions

These data demonstrate that the novel curcumin analog FLLL32 has biologic activity against OSA cell lines through inhibition of STAT3 function and expression. Future work with FLLL32 will define the therapeutic potential of this compound in vivo.

Signal transducer and activator of transcription-3, inflammation, and cancer: how intimate is the relationship?

Signal transducer and activator of transcription-3 (STAT-3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute-phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT-3. STAT-3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT-3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT-3 at serine 727 may regulate its activity negatively or positively. STAT-3 regulates the expression of genes that mediate survival (survivin, bcl-xl, mcl-1, cellular FLICE-like inhibitory protein), proliferation (c-fos, c-myc, cyclin D1), invasion (matrix metalloproteinase-2), and angiogenesis (vascular endothelial growth factor). STAT-3 activation has also been associated with both chemoresistance and radioresistance. STAT-3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor-kappaB, hypoxia-inducible factor-1, and peroxisome proliferator activated receptor-gamma. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT-3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT-3, inflammation, and cancer in more detail.

Signal transducer and activator of transcription-3, inflammation, and cancer: how intimate is the relationship?

Signal transducer and activator of transcription-3 (STAT-3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute-phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT-3. STAT-3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT-3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT-3 at serine 727 may regulate its activity negatively or positively. STAT-3 regulates the expression of genes that mediate survival (survivin, bcl-xl, mcl-1, cellular FLICE-like inhibitory protein), proliferation (c-fos, c-myc, cyclin D1), invasion (matrix metalloproteinase-2), and angiogenesis (vascular endothelial growth factor). STAT-3 activation has also been associated with both chemoresistance and radioresistance. STAT-3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor-kappaB, hypoxia-inducible factor-1, and peroxisome proliferator activated receptor-gamma. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT-3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT-3, inflammation, and cancer in more detail.

Caspase 6 regulates B cell activation and differentiation into plasma cells

Caspase (Casp) family proteases regulate not only lymphocyte apoptosis but also lymphocyte activation and development. In this study, we show that Casp6 regulates B cell activation and differentiation into plasma cells by modifying cell cycle entry. B cells from Casp6 knockout (Casp6 KO) mice examined ex vivo have more cells in G(1) than wild-type B cells, and mitogen-induced G(1) entry of Casp6 KO B cells is much faster than that of wild-type B cells. Even so, S phase entry and proliferation are not increased in Casp6 KO B cells. Rather than proliferating, activated Casp6 KO B cells preferentially differentiate into syndecan-1(+) plasma cells and produce Abs. In Casp6 KO mice compared with WT mice, serum levels of IgG1, IgG2a, and IgG2b are increased and Ag-specific Ab responses are also enhanced along with increased percentages of syndecan-1(+) plasma cells. Casp6 may regulate both B cell activation and differentiation by modifying requirements for G(0) B cells to enter G(1).

Inverse association between Raf Kinase Inhibitory Protein and signal transducers and activators of transcription 3 expression in gastric adenocarcinoma patients: implications for clinical outcome

PURPOSE

Raf Kinase Inhibitory Protein (RKIP) plays a pivotal role in cancer by regulating apoptosis induced by chemotherapeutic agents, or immune-mediated stimuli and is a metastasis suppressor protein. The signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is frequently activated in gastric adenocarcinomas, thereby promoting tumor growth. We examined the expression patterns of RKIP and STAT3 with regard to human gastric cancer, predicting that elevated RKIP status may favor clinical outcome.

EXPERIMENTAL DESIGN

Tissue microarrays were created from samples from 143 patients with gastric adenocarcinomas. The microarrays were immunohistochemically stained for RKIP and STAT3, and the intensity and extent of the staining was semiquantitatively scored.

RESULTS

In intestinal-type gastric adenocarcinomas, RKIP and STAT3, expression were inversely associated. Cytoplasmic RKIP expression directly correlated with patient survival. Nuclear STAT3 expression inversely correlated with survival. In the diffuse tumor type, no significant correlation was found between RKIP and patient outcome. In the intestinal-type gastric adenocarcinoma, multivariate analysis adjusted for treatment types revealed RKIP and tumor stage to be significant independent predictors of survival. In the diffuse tumor type, stage was the only significant predictor of survival.

CONCLUSION

These results indicate the predictive and protective role of cytoplasmic RKIP expression in gastric adenocarcinoma of the intestinal subtype. In contrast, nuclear STAT3 expression is associated with poor patient prognosis in the intestinal subtype. Significantly, we show an inverse association between RKIP and STAT3 and a positive correlation between RKIP and patient survival.

Analysis of changes in the proteome of HL-60 promyeloid leukemia cells induced by the proteasome inhibitor PSI

Proteasome inhibitors display potent anti-neoplastic and anti-angiogenic properties both in vitro and in vivo. The mechanisms, however, by which proteasome inhibitors kill tumor cells are still fairly elusive as is the molecular basis of resistance to treatment. To address these questions, we employed a high-throughput Western blotting procedure to analyze changes in a subproteome of approximately 800 proteins in the promyelocytic leukemia cell line HL-60 upon treatment with the proteasome inhibitor PSI (Z-Ile-Glu(OtBu)-Ala-Leu-aldehyde) and correlated the changes of selected target proteins with the changes in two multidrug-resistant HL-60 variants. In total, 105 proteins were upregulated more than 1.5-fold after PSI treatment, while 79 proteins were downregulated. Activation of caspases-3 and -8, modulation of members of the Bcl-2 family as well as stimulation of stress signaling pathways was prominent during HL-60 apoptosis. We also identified changes in the abundance of proteins previously not known to be affected by proteasome inhibitors. In contrast, two multidrug-resistant HL-60 cell lines, overexpressing either MRP1 or P-glycoprotein were largely resistant to PSI-induced apoptosis and could not be resensitized by the pharmacological inhibitors of the drug efflux pumps MK571 or PSC833. Drug resistance was also independent of the upregulation of Bad. Overexpression of multidrug resistance proteins, P-glycoprotein and MRP-1 is thus not sufficient to explain resistance of HL-60 cells to treatment with proteasome inhibitor PSI, which remains more closely related to a low level of Bax expression and to the inability to activate JNK. Alternative routes to the acquisition of resistance to PSI have therefore to be considered.

A phenotypic small-molecule screen identifies an orphan ligand-receptor pair that regulates neural stem cell differentiation

High-throughput identification of small molecules that selectively modulate molecular, cellular, or systems-level properties of the mammalian brain is a significant challenge. Here we report the chemical genetic identification of the orphan ligand phosphoserine (P-Ser) as an enhancer of neurogenesis. P-Ser inhibits neural stem cell/progenitor proliferation and self-renewal, enhances neurogenic fate commitment, and improves neuronal survival. We further demonstrate that the effects of P-Ser are mediated by the group III metabotropic glutamate receptor 4 (mGluR4). siRNA-mediated knockdown of mGluR4 abolished the effects of P-Ser and increased neurosphere proliferation, at least in part through upregulation of mTOR pathway activity. We also found that P-Ser increases neurogenesis in human embryonic stem cell-derived neural progenitors. This work highlights the tremendous potential of developing effective small-molecule drugs for use in regenerative medicine or transplantation therapy.

Leptin-mediated cell survival signaling in hippocampal neurons mediated by JAK STAT3 and mitochondrial stabilization

Leptin plays a pivotal role in the regulation of energy homeostasis and metabolism, primarily by acting on neurons in the hypothalamus that control food intake. However, leptin receptors are more widely expressed in the brain suggesting additional, as yet unknown, functions of leptin. Here we show that both embryonic and adult hippocampal neurons express leptin receptors coupled to activation of STAT3 and phosphatidylinositol 3-kinase-Akt signaling pathways. Leptin protects hippocampal neurons against cell death induced by neurotrophic factor withdrawal and excitotoxic and oxidative insults. The neuroprotective effect of leptin is antagonized by the JAK2-STAT3 inhibitor AG-490, STAT3 decoy DNA, and phosphatidylinositol 3-kinase/Akt inhibitors but not by an inhibitor of MAPK. Leptin induces the production of manganese superoxide dismutase and the anti-apoptotic protein Bcl-xL, and stabilizes mitochondrial membrane potential and lessens mitochondrial oxidative stress. Leptin receptor-deficient mice (db/db mice) are more vulnerable to seizure-induced hippocampal damage, and intraventricular administration of leptin protects neurons against seizures. By enhancing mitochondrial resistance to apoptosis and excitotoxicity, our findings suggest that leptin signaling serves a neurotrophic function in the developing and adult hippocampus.

Lestaurtinib (CEP701) is a JAK2 inhibitor that suppresses JAK2/STAT5 signaling and the proliferation of primary erythroid cells from patients with myeloproliferative disorders

Recent studies have demonstrated that patients with myeloproliferative disorders (MPDs) frequently have acquired activating mutations in the JAK2 tyrosine kinase. A multikinase screen determined that lestaurtinib (formerly known as CEP-701) inhibits wild type JAK2 kinase activity with a concentration that inhibits response by 50% (IC(50)) of 1 nM in vitro. We hypothesized that lestaurtinib would inhibit mutant JAK2 kinase activity and suppress the growth of cells from patients with MPDs. We found that lestaurtinib inhibits the growth of HEL92.1.7 cells, which are dependent on mutant JAK2 activity for growth in vitro and in xenograft models. Erythroid cells expanded from primary CD34(+) cells from patients with MPDs were inhibited by lestaurtinib at concentrations of 100 nM or more in 15 of 18 subjects, with concomitant inhibition of phosphorylation of STAT5 and other downstream effectors of JAK2. By contrast, growth of erythroid cells derived from 3 healthy controls was not significantly inhibited. These results demonstrate that lestaurtinib, in clinically achievable concentrations, inhibits proliferation and JAK2/STAT5 signaling in cells from patients with MPDs, and therefore holds promise as a therapeutic agent for patients with these disorders.

Caspase-dependent proteolytic cleavage of STAT3alpha in ES cells, in mammary glands undergoing forced involution and in breast cancer cell lines

Background

The STAT (Signal Transducers and Activators of Transcription) transcription factor family mediates cellular responses to a wide range of cytokines. Activated STATs (particularly STAT3) are found in a range of cancers. Further, STAT3 has anti-apoptotic functions in a range of tumour cell lines. After observing a proteolytic cleavage in STAT3α close to a potential apoptotic caspase protease cleavage site we investigated whether STAT3α might be a caspase substrate.

Methods

STAT3α status was investigated in vitro in several cell systems:- HM-1 murine embryonic stem (ES) cells following various interventions; IOUD2 murine ES cells following induction to differentiate along neural or adipocyte lineages; and in a number of breast cancer cell lines. STAT3α status was also analysed in vivo in wild type murine mammary glands undergoing controlled, forced involution.

Results

Immunoblotting for STAT3α in HM-1 ES cell extracts detected amino and carboxy terminal species of approximately 48 kDa and 43 kDa respectively – which could be diminished dose-dependently by cell treatment with the nitric oxide (NO) donor drug sodium nitroprusside (SNP). UV irradiation of HM-1 ES cells triggered the STAT3α cleavage (close to a potential caspase protease cleavage site). Interestingly, the pan-caspase inhibitor z-Val-Ala-DL-Asp-fluoromethylketone (z-VAD-FMK) and the JAK2 tyrosine kinase inhibitor AG490 both inhibited cleavage dose-dependently, and cleavage was significantly lower in a heterozygous JAK2 knockout ES cell clone. STAT3α cleavage also occurred in vivo in normal murine mammary glands undergoing forced involution, coinciding with a pulse of phosphorylation of residue Y705 on full-length STAT3α. Cleavage also occurred during IOUD2 ES cell differentiation (most strikingly along the neural lineage) and in several human breast cancer cell lines, correlating strongly with Y705 phosphorylation.

Conclusion

This study documents a proteolytic cleavage of STAT3α into 48 kDa amino and 43 kDa carboxyl terminal fragments in a range of cell types. STAT3α cleavage occurs close to a potential caspase site, and can be inhibited dose-dependently by SNP, AG490 and z-VAD-FMK. The cleavage seems to be caspase-dependent and requires the phosphorylation of STAT3α at the Y705 residue. This highly regulated STAT3α cleavage may play an important role in modulating STAT3 transcriptional activity.

Structure, function, and regulation of STAT proteins

The Signal Transducer and Activator of Transcription (STAT) family of proteins was first discovered in the 1990's as key proteins in cytokine signaling. Since then, the field has greatly advanced in the past 15 years, providing significant insight into the structure, function, and regulation of STATs. STATs are latent cytoplasmic transcription factors consisting of seven mammalian members. They are Tyr phosphorylated upon activation, a post-translational modification critical for dimerization, nuclear import, DNA binding, and transcriptional activation. In recent years, unphosphorylated STATs have also been observed to dimerize and drive transcription, albeit by yet an obscure mechanism. In addition, the function of cytoplasmic STATs is beginning to emerge. Here, we describe the structure, function, and regulation of both unphosphorylated and phosphorylated STATs. STAT isoforms from alternative splicing or proteolytic processing, and post-translational modifications affecting STAT activities are also discussed.