Regulation of Signal Transducer and Activator of Transcription Signaling by the Tyrosine Phosphatase PTP-BL

Stage-specific GATA3 induction promotes ILC2 development after lineage commitment

Group 2 innate lymphoid cells (ILC2s) are a subset of innate lymphocytes that produce type 2 cytokines, including IL-4, IL-5, and IL-13. GATA3 is a critical transcription factor for ILC2 development at multiple stages. However, when and how GATA3 is induced to the levels required for ILC2 development remains unclear. Herein, we identify ILC2-specific GATA3-related tandem super-enhancers (G3SE) that induce high GATA3 in ILC2-committed precursors. G3SE-deficient mice exhibit ILC2 deficiency in the bone marrow, lung, liver, and small intestine with minimal impact on other ILC lineages or Th2 cells. Single-cell RNA-sequencing and subsequent flow cytometry analysis show that GATA3 induction mechanism, which is required for entering the ILC2 stage, is lost in IL-17RB+PD-1- late ILC2-committed precursor stage in G3SE-deficient mice. Cnot6l, part of the CCR4-NOT deadenylase complex, is a possible GATA3 target during ILC2 development. Our findings implicate a stage-specific regulatory mechanism for GATA3 expression during ILC2 development.

FERM domain-containing proteins are active components of the cell nucleus

The FERM domain is a conserved and widespread protein module that appeared in the common ancestor of amoebae, fungi, and animals, and is therefore now found in a wide variety of species. The primary function of the FERM domain is localizing to the plasma membrane through binding lipids and proteins of the membrane; thus, for a long time, FERM domain-containing proteins (FDCPs) were considered exclusively cytoskeletal. Although their role in the cytoplasm has been extensively studied, the recent discovery of the presence and importance of cytoskeletal proteins in the nucleus suggests that FDCPs might also play an important role in nuclear function. In this review, we collected data on their nuclear localization, transport, and possible functions, which are still scattered throughout the literature, with special regard to the role of the FERM domain in these processes. With this, we would like to draw attention to the exciting, new dimension of the role of FDCPs, their nuclear activity, which could be an interesting novel direction for future research.

The oncogenic mechanisms of the Janus kinase-signal transducer and activator of transcription pathway in digestive tract tumors

Digestive tract tumors are heterogeneous and involve the dysregulation of multiple signaling pathways. The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway plays a notable role in the oncogenesis of digestive tract tumors. Typically activated by pro-inflammatory cytokines, it regulates important biological processes, such as cell growth, differentiation, apoptosis, immune responses, and inflammation. The aberrant activation of this pathway manifests in different forms, including mutations in JAKs, overexpression of cytokine receptors, and sustained STAT activation, and contributes to promoting the malignant characteristics of cancer cells, including uncontrolled proliferation, resistance to apoptosis, enhanced invasion and metastasis, angiogenesis, acquisition of stem-like properties, and drug resistance. Numerous studies have shown that aberrant activation of the JAK-STAT pathway is closely related to the development and progression of digestive tract tumors, contributing to tumor survival, angiogenesis, changes in the tumor microenvironment, and even immune escape processes. In addition, this signaling pathway also affects the sensitivity of digestive tract tumors to chemotherapy and targeted therapy. Therefore, it is crucial to comprehensively understand the oncogenic mechanisms underlying the JAK-STAT pathway in order to develop effective therapeutic strategies against digestive tract tumors. Currently, several JAK-STAT inhibitors are undergoing clinical and preclinical trials as potential treatments for various human diseases. However, further investigation is required to determine the role of this pathway, as well as the effectiveness and safety of its inhibitors, especially in the context of digestive tract tumors. In this review, we provide an overview of the structure, classic activation, and negative regulation of the JAK-STAT pathway. Furthermore, we discuss the pathogenic mechanisms of JAK-STAT signaling in different digestive tract tumors, with the aim of identifying potential novel therapeutic targets. Video Abstract.

Targeting Protein Tyrosine Phosphatases to Improve Cancer Immunotherapies

Advances in immunotherapy have brought significant therapeutic benefits to many cancer patients. Nonetheless, many cancer types are refractory to current immunotherapeutic approaches, meaning that further targets are required to increase the number of patients who benefit from these technologies. Protein tyrosine phosphatases (PTPs) have long been recognised to play a vital role in the regulation of cancer cell biology and the immune response. In this review, we summarize the evidence for both the pro-tumorigenic and tumour-suppressor function of non-receptor PTPs in cancer cells and discuss recent data showing that several of these enzymes act as intracellular immune checkpoints that suppress effective tumour immunity. We highlight new data showing that the deletion of inhibitory PTPs is a rational approach to improve the outcomes of adoptive T cell-based cancer immunotherapies and describe recent progress in the development of PTP inhibitors as anti-cancer drugs.

Decision level integration of unimodal and multimodal single cell data with scTriangulate

Decisively delineating cell identities from uni- and multimodal single-cell datasets is complicated by diverse modalities, clustering methods, and reference atlases. We describe scTriangulate, a computational framework to mix-and-match multiple clustering results, modalities, associated algorithms, and resolutions to achieve an optimal solution. Rather than ensemble approaches which select the "consensus", scTriangulate picks the most stable solution through coalitional iteration. When evaluated on diverse multimodal technologies, scTriangulate outperforms alternative approaches to identify high-confidence cell-populations and modality-specific subtypes. Unlike existing integration strategies that rely on modality-specific joint embedding or geometric graphs, scTriangulate makes no assumption about the distributions of raw underlying values. As a result, this approach can solve unprecedented integration challenges, including the ability to automate reference cell-atlas construction, resolve clonal architecture within molecularly defined cell-populations and subdivide clusters to discover splicing-defined disease subtypes. scTriangulate is a flexible strategy for unified integration of single-cell or multimodal clustering solutions, from nearly unlimited sources.

The druggable transcription factor Fli-1 regulates T cell immunity and tolerance in graft-versus-host disease

Graft-versus-host disease (GVHD), manifesting as either acute (aGVHD) or chronic (cGVHD), presents significant life-threatening complications following allogeneic hematopoietic cell transplantation. Here, we investigated Friend virus leukemia integration 1 (Fli-1) in GVHD pathogenesis and validated Fli-1 as a therapeutic target. Using genetic approaches, we found that Fli-1 dynamically regulated different T cell subsets in allogeneic responses and pathogenicity in the development of aGVHD and cGVHD. Compared with homozygous Fli1-deficient or WT T cells, heterozygous Fli1-deficient T cells induced the mildest GVHD, as evidenced by the lowest Th1 and Th17 cell differentiation. Single-cell RNA-Seq analysis revealed that Fli-1 differentially regulated CD4+ and CD8+ T cell responses. Fli-1 promoted the transcription of Th1/Th17 pathways and T cell receptor-inducible (TCR-inducible) transcription factors in CD4+ T cells, while suppressing activation- and function-related gene pathways in CD8+ T cells. Importantly, a low dose of camptothecin, topotecan, or etoposide acted as a potent Fli-1 inhibitor and significantly attenuated GVHD severity, while preserving the graft-versus-leukemia (GVL) effect. This observation was extended to a xenograft model, in which GVHD was induced by human T cells. In conclusion, we provide evidence that Fli-1 plays a crucial role in alloreactive CD4+ T cell activation and differentiation and that targeting Fli-1 may be an attractive strategy for treating GVHD without compromising the GVL effect.

JAK-STAT Pathway Inhibition and their Implications in COVID-19 Therapy

As the incidence of COVID-19 increases with time, more and more efforts are made to pave a way out for the therapeutic strategies to deal with the disease progression. Inflammation being a significant influencer in COVID-19 patients, it drives our focus onto the signaling cascades of the JAK/STAT pathway. JAK phosphorylation mediated by cytokine receptor activation leads to phosphorylation of STATs that translocate into the nucleus to translate for inflammatory mediators. The SARS-CoV-2 structural proteins like spike, nucleocapsid, membrane and envelope proteins along with the non- structural proteins 1-16 including proteases like 3CL pro and PLpro promote its entry and survival in hosts. The SARS-CoV-2 infection triggers inflammation via the JAK/STAT pathway leading to recruitment of pneumocytes, endothelial cells, macrophages, monocytes, lymphocytes, natural killer cells and dendritic cells progressing towards cytokine storm. This produces various inflammatory markers in the host that determine the disease severity. The JAK/STAT signaling also mediates immune responses via B cell and T cell differentiation.With an attempt to reduce excessive inflammation, JAK/STAT inhibitors like Ruxolitinib, Baricitinib, Tofacitinib have been employed that mediate its actions via suppressors of cytokine signaling, cytokine inducible SH2 containing protein, Protein inhibitor of activated STAT and protein tyrosine phosphatases. Even though they are implicated with multiple adverse effects, the regulatory authorities have supported its use, and numerous clinical trials are in progress to prove their safety and efficacy. On the contrary, the exact mechanism of JAK/STAT inhibition at molecular levels remains speculative for which further investigations are required.

Dual Role of the PTPN13 Tyrosine Phosphatase in Cancer

In this review article, we present the current knowledge on PTPN13, a class I non-receptor protein tyrosine phosphatase identified in 1994. We focus particularly on its role in cancer, where PTPN13 acts as an oncogenic protein and also a tumor suppressor. To try to understand these apparent contradictory functions, we discuss PTPN13 implication in the FAS and oncogenic tyrosine kinase signaling pathways and in the associated biological activities, as well as its post-transcriptional and epigenetic regulation. Then, we describe PTPN13 clinical significance as a prognostic marker in different cancer types and its impact on anti-cancer treatment sensitivity. Finally, we present future research axes following recent findings on its role in cell junction regulation that implicate PTPN13 in cell death and cell migration, two major hallmarks of tumor formation and progression.

Infanticide vs. inherited cardiac arrhythmias

AIMS

In 2003, an Australian woman was convicted by a jury of smothering and killing her four children over a 10-year period. Each child died suddenly and unexpectedly during a sleep period, at ages ranging from 19 days to 18 months. In 2019 we were asked to investigate if a genetic cause could explain the children's deaths as part of an inquiry into the mother's convictions.

METHODS AND RESULTS

Whole genomes or exomes of the mother and her four children were sequenced. Functional analysis of a novel CALM2 variant was performed by measuring Ca2+-binding affinity, interaction with calcium channels and channel function. We found two children had a novel calmodulin variant (CALM2 G114R) that was inherited maternally. Three genes (CALM1-3) encode identical calmodulin proteins. A variant in the corresponding residue of CALM3 (G114W) was recently reported in a child who died suddenly at age 4 and a sibling who suffered a cardiac arrest at age 5. We show that CALM2 G114R impairs calmodulin's ability to bind calcium and regulate two pivotal calcium channels (CaV1.2 and RyR2) involved in cardiac excitation contraction coupling. The deleterious effects of G114R are similar to those produced by G114W and N98S, which are considered arrhythmogenic and cause sudden cardiac death in children.

CONCLUSION

A novel functional calmodulin variant (G114R) predicted to cause idiopathic ventricular fibrillation, catecholaminergic polymorphic ventricular tachycardia, or mild long QT syndrome was present in two children. A fatal arrhythmic event may have been triggered by their intercurrent infections. Thus, calmodulinopathy emerges as a reasonable explanation for a natural cause of their deaths.

STAT4: an immunoregulator contributing to diverse human diseases

Signal transducer and activator of transcription 4 (STAT4) is a member of the STAT family and localizes to the cytoplasm. STAT4 is phosphorylated after a variety of cytokines bind to the membrane, and then dimerized STAT4 translocates to the nucleus to regulate gene expression. We reviewed the essential role played by STAT4 in a wide variety of cells and the pathogenesis of diverse human diseases, especially many kinds of autoimmune and inflammatory diseases, via activation by different cytokines through the Janus kinase (JAK)-STAT signaling pathway.

PTPN13 induces cell junction stabilization and inhibits mammary tumor invasiveness

Clinical data suggest that the protein tyrosine phosphatase PTPN13 exerts an anti-oncogenic effect. Its exact role in tumorigenesis remains, however, unclear due to its negative impact on FAS receptor-induced apoptosis.

Methods: We crossed transgenic mice deleted for PTPN13 phosphatase activity with mice that overexpress human HER2 to assess the exact role of PTPN13 in tumor development and aggressiveness. To determine the molecular mechanism underlying the PTPN13 tumor suppressor activity we developed isogenic clones of the aggressive human breast cancer cell line MDA-MB-231 overexpressing either wild type or a catalytically-inactive mutant PTPN13 and subjected these to phosphoproteomic and gene ontology analyses.

We investigated the PTPN13 consequences on cell aggressiveness using wound healing and Boyden chamber assays, on intercellular adhesion using videomicroscopy, cell aggregation assay and immunofluorescence.

Results: The development, growth and invasiveness of breast tumors were strongly increased by deletion of the PTPN13 phosphatase activity in transgenic mice. We observed that PTPN13 phosphatase activity is required to inhibit cell motility and invasion in the MDA-MB-231 cell line overexpressing PTPN13. In vivo, the negative PTPN13 effect on tumor invasiveness was associated with a mesenchymal-to-epithelial transition phenotype in athymic mice xenografted with PTPN13-overexpressing MDA-MB-231 cells, as well as in HER2-overexpressing mice with wild type PTPN13, compared to HER2-overexpressing mice that lack PTPN13 phosphatase activity. Phosphoproteomic and gene ontology analyses indicated a role of PTPN13 in the regulation of intercellular junction-related proteins. Finally, protein localization studies in MDA-MB-231 cells and HER2-overexpressing mice tumors confirmed that PTPN13 stabilizes intercellular adhesion and promotes desmosome formation.

Conclusions: These data provide the first evidence for the negative role of PTPN13 in breast tumor invasiveness and highlight its involvement in cell junction stabilization.

Protein Tyrosine Phosphatase-N13 Promotes Myofibroblast Resistance to Apoptosis in Idiopathic Pulmonary Fibrosis

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is a progressive, fibrotic interstitial lung disease characterized by (myo)fibroblast accumulation and collagen deposition. Resistance to Fas-induced apoptosis is thought to facilitate (myo)fibroblast persistence in fibrotic lung tissues by poorly understood mechanisms.

OBJECTIVES

To test the hypothesis that PTPN13 (protein tyrosine phosphatase-N13) is expressed by IPF lung (myo)fibroblasts, promotes their resistance to Fas-induced apoptosis, and contributes to the development of pulmonary fibrosis.

METHODS

PTPN13 was localized in lung tissues from patients with IPF and control subjects by immunohistochemical staining. Inhibition of PTPN13 function in primary IPF and normal lung (myo)fibroblasts was accomplished by: 1) downregulation with TNF-α (tumor necrosis factor-α)/IFN-γ, 2) siRNA knockdown, or 3) a cell-permeable Fas/PTPN13 interaction inhibitory peptide. The role of PTPN13 in the development of pulmonary fibrosis was assessed in mice with genetic deficiency of PTP-BL, the murine ortholog of PTPN13.

MEASUREMENTS AND MAIN RESULTS

PTPN13 was constitutively expressed by (myo)fibroblasts in the fibroblastic foci of patients with IPF. Human lung (myo)fibroblasts, which are resistant to Fas-induced apoptosis, basally expressed PTPN13 in vitro. TNF-α/IFN-γ or siRNA-mediated PTPN13 downregulation and peptide-mediated inhibition of the Fas/PTPN13 interaction in human lung (myo)fibroblasts promoted Fas-induced apoptosis. Bleomycin-challenged PTP-BL-/- mice, while developing inflammatory lung injury, exhibited reduced pulmonary fibrosis compared with wild-type mice.

CONCLUSIONS

These findings suggest that PTPN13 mediates the resistance of human lung (myo)fibroblasts to Fas-induced apoptosis and promotes pulmonary fibrosis in mice. Our results suggest that strategies aimed at interfering with PTPN13 expression or function may represent a novel strategy to reduce fibrosis in IPF.

Gene Expression Profiles of Human Phosphotyrosine Phosphatases Consequent to Th1 Polarisation and Effector Function

Phosphotyrosine phosphatases (PTPs) constitute a complex family of enzymes that control the balance of intracellular phosphorylation levels to allow cell responses while avoiding the development of diseases. Despite the relevance of CD4 T cell polarisation and effector function in human autoimmune diseases, the expression profile of PTPs during T helper polarisation and restimulation at inflammatory sites has not been assessed. Here, a systematic analysis of the expression profile of PTPs has been carried out during Th1-polarising conditions and upon PKC activation and intracellular raise of Ca²⁺ in effector cells. Changes in gene expression levels suggest a previously nonnoted regulatory role of several PTPs in Th1 polarisation and effector function. A substantial change in the spatial compartmentalisation of ERK during T cell responses is proposed based on changes in the dose of cytoplasmic and nuclear MAPK phosphatases. Our study also suggests a regulatory role of autoimmune-related PTPs in controlling T helper polarisation in humans. We expect that those PTPs that regulate T helper polarisation will constitute potential targets for intervening CD4 T cell immune responses in order to generate new therapies for the treatment of autoimmune diseases.

PTPN13 and β-Catenin Regulate the Quiescence of Hematopoietic Stem Cells and Their Interaction with the Bone Marrow Niche

The regulation of hematopoietic stem cells (HSCs) depends on the integration of the multiple signals received from the bone marrow niche. We show the relevance of the protein tyrosine phosphatase PTPN13 and β-catenin as intracellular signaling molecules to control HSCs adhesiveness, cell cycling, and quiescence. Lethally irradiated mice transplanted with Lin^– bone marrow cells in which PTPN13 or β-catenin had been silenced showed a significant increase of long-term (LT) and short-term (ST) HSCs. A decrease in cycling cells was also found, together with an increase in quiescence. The decreased expression of PTPN13 or β-catenin was linked to the upregulation of several genes coding for integrins and several cadherins, explaining the higher cell adhesiveness. Our data are consistent with the notion that the levels of PTPN13 and β-catenin must be strictly regulated by extracellular signaling to regulate HSC attachment to the niche and the balance between proliferation and quiescence.

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PTPN13 or β-catenin silencing increases LT-HSCs and ST-HSCs frequency in vivo

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The cell cycling of HSPCs was decreased by PTPN13 or β-catenin downregulation

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LT-HSCs and ST-HSCs quiescence was increased by PTPN13 or β-catenin downregulation

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PTPN13 and β-catenin levels modulate the interaction of HSPCs with the BM niche

Protein tyrosine phosphatases as potential therapeutic targets

Protein tyrosine phosphorylation is a key regulatory process in virtually all aspects of cellular functions. Dysregulation of protein tyrosine phosphorylation is a major cause of human diseases, such as cancers, diabetes, autoimmune disorders, and neurological diseases. Indeed, protein tyrosine phosphorylation-mediated signaling events offer ample therapeutic targets, and drug discovery efforts to date have brought over two dozen kinase inhibitors to the clinic. Accordingly, protein tyrosine phosphatases (PTPs) are considered next-generation drug targets. For instance, PTP1B is a well-known targets of type 2 diabetes and obesity, and recent studies indicate that it is also a promising target for breast cancer. SHP2 is a bona-fide oncoprotein, mutations of which cause juvenile myelomonocytic leukemia, acute myeloid leukemia, and solid tumors. In addition, LYP is strongly associated with type 1 diabetes and many other autoimmune diseases. This review summarizes recent findings on several highly recognized PTP family drug targets, including PTP1B, Src homology phosphotyrosyl phosphatase 2(SHP2), lymphoid-specific tyrosine phosphatase (LYP), CD45, Fas associated phosphatase-1 (FAP-1), striatal enriched tyrosine phosphatases (STEP), mitogen-activated protein kinase/dual-specificity phosphatase 1 (MKP-1), phosphatases of regenerating liver-1 (PRL), low molecular weight PTPs (LMWPTP), and CDC25. Given that there are over 100 family members, we hope this review will serve as a road map for innovative drug discovery targeting PTPs.

PTPN13 regulates cellular signalling and β-catenin function during megakaryocytic differentiation

PTPN13 is a high-molecular weight intracellular phosphatase with several isoforms that exhibits a highly modular structure. Although in recent years different roles have been described for PTPN13, we are still far from understanding its function in cell biology. Here we show that PTPN13 expression is activated during megakaryocytic differentiation at the protein and mRNA level. Our results show that the upregulation of PTPN13 inhibits megakaryocytic differentiation, while PTPN13 silencing triggers differentiation. The ability of PTPN13 to alter megakaryocytic differentiation can be explained by its capacity to regulate ERK and STAT signalling. Interestingly, the silencing of β-catenin produced the same effect as PTPN13 downregulation. We demonstrate that both proteins coimmunoprecipitate and colocalise. Moreover, we provide evidence showing that PTPN13 can regulate β-catenin phosphorylation, stability and transcriptional activity. Therefore, the ability of PTPN13 to control megakaryocytic differentiation must be intimately linked to the regulation of β-catenin function. Moreover, our results show for the first time that PTPN13 is stabilised upon Wnt signalling, which makes PTPN13 an important player in canonical Wnt signalling. Our results show that PTPN13 behaves as an important regulator of megakaryocytic differentiation in cell lines and also in murine haematopoietic progenitors. This importance can be explained by the ability of PTPN13 to regulate cellular signalling, and especially through the regulation of β-catenin stability and function. Our results hold true for different megakaryocytic cell lines and also for haematopoietic progenitors, suggesting that these two proteins may play a relevant role during in vivo megakaryopoiesis.

Protein tyrosine phosphatases as wardens of STAT signaling

Signaling by signal transducers and activators of transcription (STATs) is controlled at many levels of the signaling cascade. Protein tyrosine phosphatases (PTPs) regulate STAT activation at several layers, including direct pSTAT dephosphorylation in both cytoplasm and nucleus. Despite the importance of this regulation mode, many aspects are still incompletely understood, e.g., the identity of PTPs acting on certain members of the STAT family. After a brief introduction into the STAT and PTP families, we discuss here the current knowledge on PTP mediated regulation of STAT activity, focusing on the interaction of individual STATs with specific PTPs. Finally, we highlight open questions and propose important tasks of future research.

The impact of anti-inflammatory cytokines on the pancreatic β-cell

Considerable efforts have been invested to understand the mechanisms by which pro-inflammatory cytokines mediate the demise of β-cells in type 1 diabetes but much less attention has been paid to the role of anti-inflammatory cytokines as potential cytoprotective agents in these cells. Despite this, there is increasing evidence that anti-inflammatory molecules such as interleukin (IL)-4, IL-10 and IL-13 can exert a direct influence of β-cell function and viability and that the circulating levels of these cytokines may be reduced in type 1 diabetes. Thus, it seems possible that targeting of anti-inflammatory pathways might offer therapeutic potential in this disease. In the present review, we consider the evidence implicating IL-4, IL-10 and IL-13 as cytoprotective agents in the β-cell and discuss the receptor components and downstream signaling pathways that mediate these effects.

Qualitatively different T cell phenotypic responses to IL-2 versus IL-15 are unified by identical dependences on receptor signal strength and duration

IL-2 and IL-15 are common γ-chain family cytokines involved in regulation of T cell differentiation and homeostasis. Despite signaling through the same receptors, IL-2 and IL-15 have non-redundant roles in T cell biology, both physiologically and at the cellular level. The mechanisms by which IL-2 and IL-15 trigger distinct phenotypes in T cells remain elusive. To elucidate these mechanisms, we performed a quantitative comparison of the phosphotyrosine signaling network and resulting phenotypes triggered by IL-2 and IL-15. This study revealed that the signaling networks activated by IL-2 or IL-15 are highly similar and that T cell proliferation and metabolism are controlled in a quantitatively distinct manner through IL-2/15R signal strength independent of the cytokine identity. Distinct phenotypes associated with IL-2 or IL-15 stimulation therefore arise through differential regulation of IL-2/15R signal strength and duration because of differences in cytokine-receptor binding affinity, receptor expression levels, physiological cytokine levels, and cytokine-receptor intracellular trafficking kinetics. These results provide important insights into the function of other shared cytokine and growth factor receptors, quantitative regulation of cell proliferation and metabolism through signal transduction, and improved design of cytokine based clinical immunomodulatory therapies for cancer and infectious diseases.

Opposing roles of STAT4 and Dnmt3a in Th1 gene regulation

The STAT transcription factor STAT4 is a critical regulator of Th1 differentiation and inflammatory disease. Yet, how STAT4 regulates gene expression is still unclear. In this report, we define a STAT4-dependent sequence of events including histone H3 lysine 4 methylation, Jmjd3 association with STAT4 target loci, and a Jmjd3-dependent decrease in histone H3 lysine 27 trimethylation and DNA methyltransferase (Dnmt) 3a association with STAT4 target loci. Dnmt3a has an obligate role in repressing Th1 gene expression, and in Th1 cultures deficient in both STAT4 and Dnmt3a, there is recovery in the expression of a subset of Th1 genes that is sufficient to increase IFN-γ production. Moreover, although STAT4-deficient mice are protected from the development of experimental autoimmune encephalomyelitis, mice deficient in STAT4 and conditionally deficient in Dnmt3a in T cells develop paralysis. Th1 genes that are derepressed in the absence of Dnmt3a have greater induction after the ectopic expression of the Th1-associated transcription factors T-bet and Hlx1. Together, these data demonstrate that STAT4 and Dnmt3a play opposing roles in regulating Th1 gene expression, and that one mechanism for STAT4-dependent gene programming is in establishing a derepressed genetic state susceptible to transactivation by additional fate-determining transcription factors.

Protein tyrosine phosphatase variants in human hereditary disorders and disease susceptibilities

Reversible tyrosine phosphorylation of proteins is a key regulatory mechanism to steer normal development and physiological functioning of multicellular organisms. Phosphotyrosine dephosphorylation is exerted by members of the super-family of protein tyrosine phosphatase (PTP) enzymes and many play such essential roles that a wide variety of hereditary disorders and disease susceptibilities in man are caused by PTP alleles. More than two decades of PTP research has resulted in a collection of PTP genetic variants with corresponding consequences at the molecular, cellular and physiological level. Here we present a comprehensive overview of these PTP gene variants that have been linked to disease states in man. Although the findings have direct bearing for disease diagnostics and for research on disease etiology, more work is necessary to translate this into therapies that alleviate the burden of these hereditary disorders and disease susceptibilities in man.

Valosin containing protein (VCP/p97) is a novel substrate for the protein tyrosine phosphatase PTPL1

Identification of Protein Tyrosine Phosphatase (PTP) substrates is critical in understanding cellular role in normal cells as well as cancer cells. We have previously shown that reduction of PTPL1 protein levels in Ewings sarcoma (ES) inhibit cell growth and tumorigenesis. Therefore, we sought to identify novel PTPL1 substrates that may be important for tumorigenesis. In this current work, we demonstrated that mouse embryonic fibroblasts without PTPL1 catalytic activity fail to form foci when transfected with oncogenes. We proved that catalytic activity of PTPL1 is important for ES cell growth. Using a substrate-trapping mutant of PTPL1 we identified putative PTPL1 substrates by mass-spectrometry. One of these putative substrates was characterized as Valosin Containing Protein (VCP/p97). Using multiple biochemical assays we validated VCP as a novel substrate of PTPL1. We also provide evidence that tyrosine phosphorylation of VCP might be important for its midbody localization during cytokinesis. In conclusion, our work identifies VCP as a new substrate for PTPL1, which may be important in cellular transformation. Our investigation link an oncogenic transcription factor EWS-FLI1, with a key transcriptional target protein tyrosine phosphatase PTPL1, and its substrate VCP. Given our observation that PTPL1 catalytic activity is important for cell transformation, our results may also suggest that VCP regulation by PTPL1 might be important for tumorigenesis.

Regulation of TCR signalling by tyrosine phosphatases: from immune homeostasis to autoimmunity

More than half of the known protein tyrosine phosphatases (PTPs) in the human genome are expressed in T cells, and significant progress has been made in elucidating the biology of these enzymes in T-cell development and function. Here we provide a systematic review of the current understanding of the roles of PTPs in T-cell activation, providing insight into their mechanisms of action and regulation in T-cell receptor signalling, the phenotypes of their genetically modified mice, and their possible involvement in T-cell-mediated autoimmune disease. Our projection is that the interest in PTPs as mediators of T-cell homeostasis will continue to rise with further functional analysis of these proteins, and PTPs will be increasingly considered as targets of immunomodulatory therapies.

Twist1 regulates Ifng expression in Th1 cells by interfering with Runx3 function

A transcription factor network that includes STAT4, T-bet, and Runx3 promotes the differentiation of Th1 cells and inflammatory immune responses. How additional transcription factors regulate the function of Th1 cells has not been defined. In this study we show that the negative regulatory factor Twist1 decreases expression of T-bet, Runx3, and IL-12Rβ2 as it inhibits IFN-γ production. Ectopic expression of Runx3, but not T-bet or IL-12Rβ2, compensates for the effects of Twist1 on IFN-γ production, and Twist1 regulation of Ifng depends on complex formation with Runx3. Twist1 decreases Runx3 and T-bet binding at the Ifng locus, and it decreases chromatin looping within the Ifng locus. These data define an IL-12/STAT4-induced negative regulatory loop that impacts multiple components of the Th1 transcriptional network and provide further insight into regulation of Th1 differentiation.

Signal transducers and enzyme cofactors are susceptible to oxidation by nanographite impurities in carbon nanotube materials

Carbon nanotubes (CNTs) are often employed in biofuel cells, artificial photosystems and bioelectronics in order to enhance electron transfer and to efficiently shuttle electrons between redox active molecules and the electrode surface. However, it should be noted that typical CNTs are highly heterogeneous materials, containing large amounts of impurities. Herein, we report the influence of nanographite impurities contained within CNTs upon the redox properties of signal transducers and enzyme cofactors that are vital for the functioning of biofuel cells, artificial leaves and bioelectronics as well as for the survival of living organisms. We investigate the susceptibility of tyrosine and tryptophan, amino acids involved in electron transfer and biorecognition reactions as well in the synthesis of neurotransmitters, in addition we also consider the susceptibility of the principal electron carrier β-nicotinamide adenine dinucleotide. We conclude that nanographite impurities within CNTs are responsible for the "electrocatalytic" oxidation of NADH and two amino acids involved in signal transduction, tyrosine and tryptophan. Our findings are of high importance for both industrial and biomedical applications.

Phosphoinositide 3-kinase-regulated adapters in lymphocyte activation

Signaling via phosphoinositide 3-kinases (PI3Ks) has emerged as a central component of lymphocyte activation via immunoreceptors, costimulatory receptors, cytokine receptors, and chemokine receptors. The discovery of phosphoinositide-binding pleckstrin homology (PH) domains has substantially increased understanding of how PI3Ks activate cellular responses. Accumulating evidence indicates that PH-domain containing adapter molecules provide important links between PI3K and lymphocyte function. Here, we review data on PI3K-regulated adapter proteins of the Grb-associated binder (GAB), Src kinase-associated phosphoprotein (SKAP), and B-lymphocyte adapter molecule of 32 kDa (Bam32)/ dual-adapter for phosphotyrosine and 3-phosphoinositides (DAPP)/TAPP families, with a focus on the latter group. Current data support the model that recruitment of these adapters to the plasma membrane of activated lymphocytes is driven by the phosphoinositides phosphatidylinositol-3,4,5-tris-phosphate and phosphatidylinositol-3,4-bisphosphate, generated through the action of PI3Ks and under the regulatory control of lipid phosphatases Src homology 2 domain-containing inositol phosphatase (SHIP), phosphatase and tensin homolog, and inositol polyphosphate 4-phosphatase. At the plasma membrane, these adapters serve to assemble distinct protein complexes. Bam32/DAPP1 and SKAPs function to promote activation of monomeric guanosine triphosphatases, including Rac and Rap, and promote integrin activation, lymphocyte adhesion to matrix proteins, and cell:cell interactions between B and T lymphocytes. GABs can provide feedforward amplification or feedback inhibition of PI3K signaling. Current work is further defining the molecular interactions driven by these molecules and identifying the functions of TAPP adapters, which also appear to be involved in lymphocyte adhesion and are specific effectors downstream of the SHIP product phosphatidylinositol-3,4-bisphosphate.

PTPL1/PTPN13 regulates breast cancer cell aggressiveness through direct inactivation of Src kinase

The protein tyrosine phosphatase PTPL1/PTPN13, the activity of which is decreased through allelic loss, promoter methylation, or somatic mutations in some tumors, has been proposed as a tumor suppressor gene. Moreover, our recent clinical study identified PTPL1 expression level as an independent prognostic indicator of a favorable outcome for patients with breast cancer. However, how PTPL1 can affect tumor aggressiveness has not been characterized. Here, we first show that PTPL1 expression, assessed by immunohistochemistry, is decreased in breast cancer and metastasis specimens compared with nonmalignant tissues. Second, to evaluate whether PTPL1 plays a critical role in breast cancer progression, RNA interference experiments were performed in poorly tumorigenic MCF-7 breast cancer cells. PTPL1 inhibition drastically increased tumor growth in athymic mice and also enhanced several parameters associated with tumor progression, including cell proliferation on extracellular matrix components and cell invasion. Furthermore, the inhibition of Src kinase expression drastically blocked the effects of PTPL1 silencing on cell growth. In PTPL1 knockdown cells, the phosphorylation of Src on tyrosine 419 is increased, leading to the activation of its downstream substrates Fak and p130cas. Finally, substrate-trapping experiments revealed that Src tyrosine 419 is a direct target of the phosphatase. Thus, by identification of PTPL1 as the first phosphatase able to inhibit Src through direct dephosphorylation in intact cells, we presently describe a new mechanism by which PTPL1 inhibits breast tumor aggressiveness.

Inferring relevant control mechanisms for interleukin-12 signaling in naïve CD4+ T cells

Interleukin-12 (IL-12) is a key cytokine involved in shaping the cell-mediated immunity to intracellular pathogens. IL-12 initiates a cellular response through the IL-12 signaling pathway, a member of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) family of signaling networks. The JAK/STAT pathway includes several regulatory elements; however, the dynamics of these mechanisms are not fully understood. Therefore, the objective of this study was to infer the relative importance of regulatory mechanisms that modulate the activation of STAT4 in naïve CD4(+) T cells. Dynamic changes in protein expression and activity were measured using flow cytometry and these data were used to calibrate a mathematical model of IL-12 signaling. An empirical Bayesian approach was used to infer the relative strengths of the different regulatory mechanisms in the system. The model predicted that IL-12 receptor expression is regulated by a dynamic, autonomous program that was independent of STAT4 activation. In summary, a mathematical model of the canonical IL-12 signaling pathway used in conjunction with a Bayesian framework provided high-confidence predictions of the system-specific control mechanisms from the available experimental observations.

Human and rodent pancreatic β-cells express IL-4 receptors and IL-4 protects against β-cell apoptosis by activation of the PI3K and JAK/STAT pathways

Secretion of pro-inflammatory cytokines is associated with loss of pancreatic β-cell viability and cell death. IL-4 (interleukin-4) has been reported to mediate a protective effect against the loss of pancreatic β-cells, and IL-4 receptors have been found in rat pancreatic β-cells at both the RNA and the protein level. The aim of the present study was to investigate IL-4 receptor expression in human islet cells and to examine the signalling pathways by which IL-4 exerts its effects using the rat β-cell lines, BRIN-BD11 and INS-1E. By means of immunohistochemistry, it was demonstrated that IL-4 receptors are present on human islet cells. Using a flow cytometric method for evaluating cell death, it was confirmed that incubating β-cells with IL-4 attenuated cell death induced by IL-1β and interferon-γ by approx. 65%. This effect was abrogated by the presence of the PI3K (phosphoinositide 3-kinase) inhibitor, wortmannin, suggesting that activation of the PI3K pathway is involved. In support of this, Western blotting revealed that incubation of cells with IL-4 resulted in increased phosphorylation of Akt (also called protein kinase B), a downstream target of PI3K. Increased tyrosine phosphorylation of STAT6 (signal transducer and activator of transcription 6) also occurred in response to IL-4 and a selective JAK3 (Janus kinase 3) inhibitor reduced the cytoprotective response. Both effects were prevented by overexpression of the tyrosine phosphatase, PTP-BL (protein tyrosine phosphatase-BL). We conclude that IL-4 receptors are functionally competent in pancreatic β-cells and that they signal via PI3K and JAK/STAT pathways. These findings may have implications for future therapeutic strategies for the management of diabetes.

Impaired PTPN13 phosphatase activity in spontaneous or HPV-induced squamous cell carcinomas potentiates oncogene signaling through the MAP kinase pathway

Human papillomaviruses (HPVs) are a causative factor in over 90% of cervical and 25% of head and neck squamous cell carcinomas (HNSCCs). The C terminus of the high-risk HPV 16 E6 oncoprotein physically associates with and degrades a non-receptor protein tyrosine phosphatase (PTPN13), and PTPN13 loss synergizes with H-Ras(V12) or ErbB2 for invasive growth in vivo. Oral keratinocytes that have lost PTPN13 and express H-Ras(V12) or ErbB2 show enhanced Ras/RAF/MEK/Erk signaling. In co-transfection studies, wild-type PTPN13 inhibited Ras/RAF/MEK/Erk signaling in HEK 293 cells that overexpress ErbB2, EGFR or H-Ras(V12), whereas an enzymatically inactive PTPN13 did not. Twenty percent of HPV-negative HNSCCs had PTPN13 phosphatase mutations that did not inhibit Ras/RAF/MEK/Erk signaling. Inhibition of Ras/RAF/MEK/Erk signaling using MEK inhibitor U0126 blocked anchorage-independent growth in cells lacking PTPN13. These findings show that PTPN13 phosphatase activity has a physiologically significant role in regulating MAP kinase signaling.

Tumor suppression by phospholipase C-beta3 via SHP-1-mediated dephosphorylation of Stat5

Given its catalytic activity to generate diacylglycerol and inositol 1,4,5-trisphosphate, phospholipase C (PLC) is implicated in promoting cell growth. However, we found that PLC-beta3-deficient mice develop myeloproliferative disease, lymphoma, and other tumors. The mutant mice have increased numbers of hematopoietic stem cells with increased proliferative, survival, and myeloid-differentiative abilities. These properties are dependent on Stat5 and can be antagonized by the protein phosphatase SHP-1. Stat5-dependent cooperative transformation by active c-Myc and PLC-beta3 deficiency was suggested in mouse lymphomas in PLC-beta3(-/-) and in Emicro-myc;PLC-beta3(+/-) mice and human Burkitt's lymphoma cells. The same mechanism for malignant transformation seems to be operative in other human lymphoid and myeloid malignancies. Thus, PLC-beta3 is likely a tumor suppressor.

Identification of the small protein rich in arginine and glycine (SRAG): a newly identified nucleolar protein that can regulate cell proliferation

The characterization of new proteins will aid in our explanation of normal biology and disease. Toward that goal, we describe the initial characterization of the small protein rich in arginine and glycine (SRAG). Human and mouse SRAG are 248/249-amino acid arginine- and glycine-rich proteins that are widely expressed in tissues and cell lines. Two SRAG isoforms, SRAG-5 and SRAG-3, which are truncations of full-length SRAG, were also identified. Although all SRAG proteins reside in the nucleus, they were also found within the nucleolus. Localization within the nucleolus was regulated by the N terminus of the protein. Our initial studies indicated that SRAG can interact with RNA. Full-length SRAG protein levels were highest in resting cells and were reduced in proliferating cells. The reduction in SRAG protein that occurs in proliferating cells was mapped with inhibitors to the G(2)/M phase of the cell cycle. As expected, the overexpression of SRAG reduced the percentage of cells in the G(2)/M phase and increased cell death. In sum, we have identified a new and intriguing member of the nucleolar proteome.

Expression of the putative tumor suppressor gene PTPN13/PTPL1 is an independent prognostic marker for overall survival in breast cancer

Although it is well established that some protein tyrosine kinases have a prognostic value in breast cancer, the involvement of protein tyrosine phosphatases (PTPs) is poorly substantiated for breast tumors. Three of these enzymes (PTP-gamma, LAR, and PTPL1) are already known to be regulated by estrogens or their antagonists in human breast cancer cells. We used a real-time reverse transcriptase polymerase chain reaction method to test the expression levels of PTP-gamma, LAR and its neuronal isoform, and PTPL1 in a training set of RNA from 59 breast tumors. We sought correlations between levels of these molecular markers, current tumor markers, and survival. We then quantified the expression level of the selected phosphatase in 232 additional samples, resulting in a testing set of 291 breast tumor RNAs from patients with a median follow-up of 6.4 years. The Spearman nonparametric test revealed correlations between PTPL1 expression and differentiation markers. Cox univariate analysis of the overall survival studies demonstrated that PTPL1 is a prognostic factor [risk ratio (RR)=0.45], together with the progesterone receptor (PR) (RR=0.52) and node involvement (RR=1.58). In multivariate analyses, PTPL1 and PR retained their prognostic value (RRs of 0.48 and 0.55, respectively). This study demonstrates for the first time that PTPL1 expression level is an independent prognostic indicator of favorable outcome for patients with breast cancer. In conjunction with our mechanistic studies, this finding identifies PTPL1 as an important regulatory element of human breast tumor aggressiveness and sensitivity to treatments such as antiestrogens and antiaromatase.

The FERM and PDZ domain-containing protein tyrosine phosphatases, PTPN4 and PTPN3, are both dispensable for T cell receptor signal transduction

PTPN3 and PTPN4 are two closely-related non-receptor protein tyrosine phosphatases (PTP) that, in addition to a PTP domain, contain FERM (Band 4.1, Ezrin, Radixin, and Moesin) and PDZ (PSD-95, Dlg, ZO-1) domains. Both PTP have been implicated as negative-regulators of early signal transduction through the T cell antigen receptor (TCR), acting to dephosphorylate the TCRzeta chain, a component of the TCR complex. Previously, we reported upon the production and characterization of PTPN3-deficient mice which show normal TCR signal transduction and T cell function. To address if the lack of a T cell phenotype in PTPN3-deficient mice can be explained by functional redundancy of PTPN3 with PTPN4, we generated PTPN4-deficient and PTPN4/PTPN3 double-deficient mice. As in PTPN3 mutants, T cell development and homeostasis and TCR-induced cytokine synthesis and proliferation were found to be normal in PTPN4-deficient and PTPN4/PTPN3 double-deficient mice. PTPN13 is another FERM and PDZ domain-containing non-receptor PTP that is distantly-related to PTPN3 and PTPN4 and which has been shown to function as a negative-regulator of T helper-1 (Th1) and Th2 differentiation. Therefore, to determine if PTPN13 might compensate for the loss of PTPN3 and PTPN4 in T cells, we generated mice that lack functional forms of all three PTP. T cells from triple-mutant mice developed normally and showed normal cytokine secretion and proliferative responses to TCR stimulation. Furthermore, T cell differentiation along the Th1, Th2 and Th17 lineages was largely unaffected in triple-mutants. We conclude that PTPN3 and PTPN4 are dispensable for TCR signal transduction.

PTP1B is a negative regulator of interleukin 4-induced STAT6 signaling

Protein tyrosine phosphatase 1B (PTP1B) is a ubiquitously expressed enzyme shown to negatively regulate multiple tyrosine phosphorylation-dependent signaling pathways. PTP1B can modulate cytokine signaling pathways by dephosphorylating JAK2, TYK2, and STAT5a/b. Herein, we report that phosphorylated STAT6 may serve as a cytoplasmic substrate for PTP1B. Overexpression of PTP1B led to STAT6 dephosphorylation and the suppression of STAT6 transcriptional activity, whereas PTP1B knockdown or deficiency augmented IL-4-induced STAT6 signaling. Pretreatment of these cells with the PTK inhibitor staurosporine led to sustained STAT6 phosphorylation consistent with STAT6 serving as a direct substrate of PTP1B. Furthermore, PTP1B-D181A "substrate-trapping" mutants formed stable complexes with phosphorylated STAT6 in a cellular context and endogenous PTP1B and STAT6 interacted in an interleukin 4 (IL-4)-inducible manner. We delineate a new negative regulatory loop of IL-4-JAK-STAT6 signaling. We demonstrate that IL-4 induces PTP1B mRNA expression in a phosphatidylinositol 3-kinase-dependent manner and enhances PTP1B protein stability to suppress IL-4-induced STAT6 signaling. Finally, we show that PTP1B expression may be preferentially elevated in activated B cell-like diffuse large B-cell lymphomas. These observations identify a novel regulatory loop for the regulation of IL-4-induced STAT6 signaling that may have important implications in both neoplastic and inflammatory processes.

Inteferons pen the JAK-STAT pathway

Characterization of how interferons (IFNs) mediate their biological response led to identification of the JAK-STAT signaling cascade, where JAKs are receptor-associated kinases and STATs the transcription factors they activate. Today, 4 JAKs and 7 STATs are known to transduce pivotal signals for the over 50 members of the four-helix bundle family of cytokines. This review will provide an overview and historical perspective of the JAK-STAT paradigm.

PTPL1: a large phosphatase with a split personality

Protein tyrosine phosphatase, PTPL1, (also known as PTPN13, FAP-1, PTP-BAS, PTP1E) is a non-receptor type PTP and, at 270 kDa, is the largest phosphatase within this group. In addition to the well-conserved PTP domain, PTPL1 contains at least 7 putative macromolecular interaction domains. This structural complexity indicates that PTPL1 may modulate diverse cellular functions, perhaps exerting both positive and negative effects. In accordance with this idea, while certain studies suggest that PTPL1 can act as a tumor-promoting gene other experimental studies have suggested that PTPL1 may function as a tumor suppressor. The role of PTPL1 in the cancer cell is therefore likely to be both complex and context dependent with possible roles including the modulation of growth, stress-response, and cytoskeletal remodeling pathways. Understanding the nature of molecular complexes containing PTPL1, its interaction partners, substrates, regulation and subcellular localization are key to unraveling the complex personality of this protein phosphatase.

The actin cytoskeleton in T cell activation

T cell cytoarchitecture differs dramatically depending on whether the cell is circulating within the bloodstream, migrating through tissues, or interacting with antigen-presenting cells. The transition between these states requires important signaling-dependent changes in actin cytoskeletal dynamics. Recently, analysis of actin-regulatory proteins associated with T cell activation has provided new insights into how T cells control actin dynamics in response to external stimuli and how actin facilitates downstream signaling events and effector functions. Among the actin-regulatory proteins that have been identified are nucleation-promoting factors such as WASp, WAVE2, and HS1; severing proteins such as cofilin; motor proteins such as myosin II; and linker proteins such as ezrin and moesin. We review the current literature on how signaling pathways leading from diverse cell surface receptors regulate the coordinated activity of these and other actin-regulatory proteins and how these proteins control T cell function.

Vaccinia virus blocks Stat1-dependent and Stat1-independent gene expression induced by type I and type II interferons

Blocking the function of Stat (signal transducer and activator of transcription) proteins, which are critical for antiviral responses, has evolved as a common mechanism for pathogen immune evasion. The poxvirus-encoded phosphatase H1 is critical for viral replication, and may play an additional role in the evasion of host defense by dephosphorylating Stat1 and blocking interferon (IFN)-stimulated innate immune responses. Vaccinia virus (VACV) H1 can inhibit the phosphorylation of the transcription factor Stat1 after IFN-gamma stimulation of epithelial cells, greatly attenuating IFN-induced biological functions. In this study, we demonstrate that VACV infection is capable of inhibiting the phosphorylation of Stat1 and Stat2 after stimulation of fibroblasts or bone marrow-derived macrophages with either type I or type II IFNs, but did not inhibit the activation of Stat3 or Stat5 in either cell type. By using recombinant proteins for in vitro assays, we observe that variola virus H1 is more active than VACV H1, although it has similar selectivity for Stat targets. Differential effects of VACV infection were observed on the induction of IFN-stimulated genes, with complete inhibition of some genes by VACV infection, while others were less affected. Despite the IFN-gamma-induced expression of some genes in VACV-infected cells, IFN-gamma was unable to rescue the VACV-mediated inhibition of MHC class II antigen presentation. Moreover, VACV infection can affect the IFN-induced expression of Stat1-dependent and Stat1-independent genes, suggesting that the virus may target additional IFN-activated pathways. Thus, VACV targets multiple signaling pathways in the evasion of antiviral immune responses.

Protein tyrosine phosphatases in the JAK/STAT pathway

The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway is crucial in controlling cellular activities in response to extracellular cytokines. Dysfunctions of the JAK/STAT pathway result in various hematopoietic and immune disorders. The central events in regulating this pathway are tyrosine phosphorylation and dephosphorylation of the signaling components, which are carried out by protein tyrosine kinases and protein tyrosine phosphatases (PTP), respectively. Here, we review recent advances in the regulatory roles of PTPs, in particular, SHP2 phosphatase, in the JAK/STAT signaling pathway.

Modulation of interleukin-1 transcriptional response by the interaction between VRK2 and the JIP1 scaffold protein

Background

Cellular biological responses to specific stimulation are determined by a balance among signaling pathways. Protein interactions are likely to modulate these pathways. Vaccinia-related kinase-2 (VRK2) is a novel human kinase that can modulate different signaling pathways.

Principal Findings

We report that in vivo, the activity of JIP1-JNK complexes is downregulated by VRK2 in response to interleukin-1β. Also the reduction of endogenous VRK2 with shRNA increases the transcriptional response to IL-1β. The JIP1 scaffold protein assembles three consecutive members of a given MAPK pathway forming signaling complexes and their signal can be modulated by interactions with regulatory proteins that remain to be identified. Knocking-down JIP1 with siRNA resulted in elimination of the AP1 transcriptional response to IL-1β. VRK2, a member of novel Ser-Thr kinase family, is able to stably interact with JIP1, TAK1 and MKK7, but not JNK, and can be isolated forming oligomeric complexes with different proportions of TAK1, MKK7β1 and JNK. JIP1 assembles all these proteins in an oligomeric signalosome. VRK2 binding to the JIP1 signalosome prevents the association of JNK and results in a reduction in its phosphorylation and downregulation of AP1-dependent transcription.

Conclusions/Significance

This work suggests that the intracellular level of VRK2 protein can modulate the flow through a signaling pathway and alter the response from a receptor that can be distributed by more than one pathway, and thus contribute to the cellular specificity of the response by forming alternative signaling complexes. Furthermore, the effect might be more general and affect other signaling routes assembled on the JIP1 scaffold protein for which a model is proposed.

The putative tumor suppressor gene PTPN13/PTPL1 induces apoptosis through insulin receptor substrate-1 dephosphorylation

The protein tyrosine phosphatase (PTP) PTPL1/PTPN13 is a candidate tumor suppressor gene. Indeed, PTPL1 activity has been reported recently to be decreased through somatic mutations, allelic loss, or promoter methylation in some tumors. We showed previously that its expression was necessary for inhibition of Akt activation and induction of apoptosis by antiestrogens in breast cancer cells. Implications of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway in cancer progression are now well established, and our study was therefore designed to define whether PTPL1 is sufficient to inhibit this pathway and, if so, to identify a direct substrate of this PTP, which may trigger a proapoptotic effect. We first show by complementary approaches that PTPL1 specifically dephosphorylates insulin receptor substrate-1 (IRS-1) in vitro and in cellulo. Next, our experiments using a dominant-negative mutant and RNA interference confirm the crucial role of PTPL1 in IRS-1 dephosphorylation. Finally, we report that PTPL1 expression is sufficient to block the IRS-1/PI3K/Akt signaling pathway, to inhibit the insulin-like growth factor-I effect on cell survival, and to induce apoptosis. Altogether, these data provide the first evidence for a direct positive role of the putative tumor suppressor gene PTPL1/PTPN13 on apoptosis and identify its target in the IRS-1/PI3K/Akt signaling pathway.