Cytokines that associate with the signal transducer gp130 activate the interferon-induced transcription factor p91 by tyrosine phosphorylation

Expression of STAT3 and Bcl-6 oncoprotein in sodium arsenite-treated SV-40 immortalized human uroepithelial cells

Arsenic is widely distributed in the environment, and it is a proven toxic and carcinogenic agent. On the southwest coast of Taiwan, an endemic occurrence of chronic arsenical poisoning due to a high concentration of arsenic in artesian-well water has been reported. However, the mechanisms of its carcinogenic action are still unclear. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway is an essential cascade for mediating normal functions of different cytokines in the development of the hematopoietic and immune systems. In this study, the substantial morphological changes observed in SV-40 immortalized human uroepithelial cells (SV-HUC-1) after treatment of various concentrations of arsenite were examined, and the expression of Bcl-6, Jak-2 and p-STAT3 (Tyr 705) were evaluated by immunocytochemistry and Western blotting. Our results showed that the expression of Bcl-6 increased dose-dependently in arsenite-treated urothelial cells. Sodium arsenite treatment reduced Jak-2 protein expression in a dose-dependent manner. However, treatment of SV-HUC-1 cells with arsenite at concentration ranges from 2 and 4microM for 48h dramatically increased p-STAT3 (Tyr 705), but the levels decreased at 8-40microM of arsenite. Our data suggest that arsenic-mediated inactivation of the JAK-STAT signaling pathway might be caused by Bcl-6 interaction with JAK tyrosine kinase or STAT. In conclusion, our findings indicate that arsenic inhibits JAK tyrosine kinase protein expression and suggest the interference in the JAK-STAT pathway might be through Bcl-6 playing an important role in arsenic-associated carcinogenesis.

Inhibition of STAT3 signaling induces apoptosis and decreases survivin expression in primary effusion lymphoma

Despite some exciting new leads in molecular pathogenesis, AIDS-defining primary effusion lymphoma (PEL) remains a fatal malignancy. The lack of substantial progress in the management of PEL demands innovative treatment approaches. Targeting intracellular molecules critical to cell survival is one unexplored strategy for treating PEL. Here we show that inhibition of signal transducer and activator of transcription-3 (STAT3) leads to apoptosis in PEL cells. STAT3 is constitutively phosphorylated in PEL cell lines BC-1, BCBL-1, and VG-1. Transduction of dominant-negative STAT3 and pharmacological STAT3 inhibition caused caspase-dependent cell death. Although STAT3 activation is known to induce expression of Bcl-2 family proteins, PEL cell apoptosis was independent of Bcl-2, Bcl-X(L), or Mcl-1 protein expression. Instead, STAT3 inhibition induced transcriptional repression of survivin, a recently identified inhibitor of apoptosis. Forced overexpression of survivin rescued VG-1 cells from apoptosis induced by STAT3 inhibition. Our findings suggest that activated STAT3 signaling directly contributes to malignant progression of PEL by preventing apoptosis, acting through the prosurvival protein survivin. Since constitutive STAT3 activation and survivin expression have been widely documented in different types of cancers, their linkage may extend to many malignancies and be critical to their pathogenesis.

The role of IL-6 type cytokines and their receptors in bone

Substantial evidence indicates that the IL-6 type of cytokines have profound effects on bone metabolism by regulating osteoclast and osteoblast development and function. In addition, there is evidence that the gp130 signal transduction pathway may be a critical site for the regulation of the rate of bone remodeling, and probably the coupling of bone resorption to bone formation. Sex steroids inhibit the expression of the genes encoding IL-6, gp80, and gp130, most likely by repressing the activity of transcription factors such as NF kappa B and NF-IL-6. Considering this and the evidence that IL-6 autoregulates its own production and can upregulate the components of its receptor, removal of the direct inhibitory effects of sex steroids on IL-6, gp80, and gp130 could unleash a self-amplifying cascade of events responsible for increasing not only the production of IL-6, but also the responsiveness of osteoclast progenitors, osteoblast progenitors, and stromal/osteoblastic cells that support osteoclastogenesis, or combinations of these cells, to IL-6 type cytokines. Such a scenario could explain both the increased osteoclastogenesis and osteoblastogenesis that follows loss of gonadal function and thereby the effect of such loss on the rate of bone remodeling and skeletal homeostasis. Manipulation of the effects of IL-6 type cytokines, by selectively targeting to specific bone cell precursors, may allow means of altering the balance between bone resorption and formation in favor of the latter.

Leukemia inhibitory factor: part of a large ingathering family

Leukemia Inhibitory Factor (LIF) has a wide variety of biological activities. It regulates the differentiation of embryonic stem cells, neural cells, osteoblasts, adipocytes, hepatocytes and kidney epithelial cells. It also triggers the proliferation of myoblasts, primordial germ cells and some endothelial cells. Many of these biological functions parallel those of interleukin-6, Oncostatin M, ciliary neurotrophic factor, interleukin-11 and cardiotrophin-1. These structurally related cytokines also share subunits of their receptors which could partially explain the redundancy in this system of soluble mediators. In vivo LIF proves important in regulating the inflammatory response by fine tuning of the delicate balance of at least four systems in the body, namely the immune, the hematopoietic, the nervous and the endocrine systems. Although we are far from its therapeutic applications, the fast increasing knowledge in this field may bring new insights for the understanding of the cytokine biology in general.

The amino acid residues immediately carboxyl-terminal to the tyrosine phosphorylation site contribute to interleukin 6-specific activation of signal transducer and activator of transcription 3

Signal transducers and activators of transcription (Stat) proteins play an important role in signaling through a variety of cytokine and growth factor receptors. Each of the Stat proteins is activated in a ligand-specific manner. Only the Src homology 2 (SH2) domains of Stat1 and Stat2 are critical for the ligand-specific activation of interferon signaling. In this study we determined the domains in Stat3 protein that contribute to interleukin 6 (IL-6)-specific phosphorylation. Based on evidence that Stat3, but not Stat1, is activated in the presence of low levels of IL-6 and soluble IL-6 receptor, we constructed various swap mutants between Stat3 and Stat1 and examined their response to IL-6 after their transient expression into COS7 cells. The region upstream of the SH2 domain was exchangeable between Stat1 and Stat3, whereas the region carboxyl-terminal to the SH2 domain of Stat3 was critical to phosphorylation by IL-6. However, unlike Stat1 and Stat2 in interferon signaling, the swap mutant in which 5 amino acid residues just carboxyl-terminal to the tyrosine phosphorylation site (Tyr705) in Stat3 was replaced by the corresponding region derived from Stat1 was not phosphorylated in response to IL-6. Substituting 1 amino acid (Lys709) at position +4 relative to Tyr705 abolished the tyrosine phosporylation of Stat3 in response to IL-6. Co-immunoprecipitation experiments demonstrated that these mutants were associated with gp130 at an extent similar to wild-type Stat3. Taken together, these results show that the amino acid residues immediately carboxyl-terminal to the tyrosine phosphorylation site are involved in IL-6-specific activation of Stat3.

Bone biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. Bone modelling is the process associated with growth and re-shaping of bones in childhood and adolescence. This is distinguished from bone remodelling, which describes the lifelong process whereby skeletal tissue is continually being resorbed and replaced in order to maintain skeletal integrity, shape and mass. Bone remodelling is controlled by systemic hormones and cytokines and is an integral part of the calcium homeostatic system. The maintenance of a normal, healthy skeletal mass depends on interactions between osteoblasts, osteoclasts and constituents of the bone matrix to keep the process of bone resorption and formation in balance. The factors, local and systemic, which regulate these processes are discussed.

Interferon gamma-dependent induction of human intercellular adhesion molecule-1 gene expression involves activation of a distinct STAT protein complex

In response to interferon gamma (IFNgamma), intercellular adhesion molecule-1 (ICAM-1) is expressed on human keratinocytes, a cell type that is critically involved in cutaneous inflammation. An ICAM-1 5' regulatory region palindromic response element, pIgammaRE, has been shown to confer IFNgamma-dependent transcription enhancement. By electrophoretic mobility shift assays (EMSA), pIgammaRE forms a distinct complex with proteins from IFNgamma-treated human keratinocytes, termed gamma response factor (GRF). Binding of GRF is tyrosine phosphorylation-dependent, and mutations of pIgammaRE that disrupt the palindromic sequence or alter its spatial relationship abrogate GRF binding. Supershift EMSAs using antibodies to characterized STAT proteins suggest that GRF contains a Stat1alpha-like protein; however, non-ICAM-1 IFNgamma-responsive elements (REs) known to bind Stat1alpha homodimers fail to compete for GRF binding in EMSA, and pIgammaRE does not cross-compete with these REs that complex with homodimeric stat1alpha. The pIgammaRE x GRF complex also displays a distinctly different electrophoretic mobility compared to that of IFNgammaREs complexed to homodimeric Stat1alpha. These findings indicate that a distinct complex containing a Stat1alpha-like protein mediates IFNgamma-induced ICAM-1 gene transcription and identifies a subset of IFNgamma-responsive genes that appear to be regulated by this complex.

The SH2 domain-containing tyrosine phosphatase PTP1D is required for interferon alpha/beta-induced gene expression

Interferons (IFNs) induce early response genes by stimulating Janus family (Jak) tyrosine kinases, leading to tyrosine phosphorylation of Stat (signal transducer and activator of transcription) proteins. Previous studies demonstrated that a protein-tyrosine phosphatase (PTP) is required for activation of the ISGF3 transcription complex by IFNalpha/beta, but the specific PTP responsible remained unidentified. We now show that the SH2 domain containing tyrosine phosphatase PTP1D (also designated as SHPTP2, SHPTP3, PTP2C, or Syp) is constitutively associated with the IFNalpha/beta receptor and becomes tyrosine-phosphorylated in response to ligand. Furthermore, transient expression of a phosphatase-inactive mutant or the COOH-terminal SH2 domain of PTP1D causes a dominant negative effect on IFNalpha/beta-induced early response gene expression. These results provide strong evidence that PTP1D functions as a positive regulator of the IFNalpha/beta-induced Jak/Stat signal transduction pathway.

Leukemia inhibitory factor as a mediator of JAK/STAT activation in murine osteoblasts

A number of cytokines have been shown to exert their effects via a recently discovered signaling cascade. One step in this pathway is mediated by a family of nonreceptor protein tyrosine kinases, the Janus kinases or JAK kinases, which become phosphorylated upon ligand-receptor binding and receptor phosphorylation. This in turn is followed by phosphorylation of certain members of a family of latent transcription factors, called signal transducers and activators of transcription (STATs), which subsequently enter the nucleus, bind to DNA in a sequence-specific fashion, and modulate transcription. In view of the apparent role of leukemia inhibitory factor (LIF) in bone remodeling, we sought to determine which, if any, of the JAK/STAT family members are involved in mediating the actions of LIF using the MC3T3-E1 cell line (a spontaneously immortalized osteoblast) and normal murine calvarial osteoblasts. We report here rapid and transient phosphorylation of the LIF receptor, and similarly, we detect phosphorylation of predominantly JAK1 and to a minor extent JAK2 in response to LIF treatment in MC3T3-E1 cells. In these experiments we also detect phosphorylation of STAT1 and to a much lesser degree STAT3 upon addition of LIF. Phosphorylation of the STAT1 proteins correlates directly with their ability to bind DNA in a gel mobility shift assay in MC3T3-E1 and in normal calvarial osteoblasts. These studies suggest that LIF action in these cells, as in other cell types, is mediated in part via specific members of the JAK/STAT pathway.

Protein tyrosine phosphorylation as a mechanism which regulates cytokine activation of early response genes

Two well-defined rapid responses which occur as a consequence of growth factors binding to their cell surface receptors involve tyrosine phosphorylation of cellular proteins and the induction of the transcription of cellular genes. Recent advances have been made in purification and cloning of Src homology 2 and 3 (SH2/SH3) domain-containing transcription factors which are required for the activation of early response genes by interferons. These transcription factors are covalently modified by tyrosine phosphorylation such that they interact with enhancers needed for interferon-stimulated gene expression. The Jak family of tyrosine kinases are also an integral component in these signalling cascades. The information gained concerning interferon signalling has now been extended to include a broad network of cytokine-regulated signalling systems which use tyrosine phosphorylation of a family of structurally related proteins to activate transcription of early response genes.

Activation of acute phase response factor (APRF)/Stat3 transcription factor by growth hormone

The mechanism by which the binding of growth hormone (GH) to its cell surface receptor elicits changes in gene transcription are largely unknown. The transcription factor Stat1/p91 has been shown to be activated by GH. Here we show that acute phase response factor or Stat3 f1p4an antigenically related protein), is also activated by GH. Stat3 has been implicated in the interleukin-6-dependent induction of acute phase response genes. GH promotes in 3T3-F442A fibroblasts the tyrosyl phosphorylation of a protein immunoprecipitated by antibodies to Stat3. This protein co-migrates with a tyrosyl phosphorylated protein from cells treated with leukemia inhibitory factor, a cytokine known to activate Stat3. Tyrosyl phosphorylated Stat3 is also observed in response to interferon-gamma. Stat3 is present in GH-inducible DNA-binding complexes that bind the sis-inducible element in the c-fos promoter and the acute phase response element in the alpha 2-macroglobulin promoter. The ability of GH to activate both Stat1 and Stat3 (i.e. increase their tyrosyl phosphorylation and ability to bind to DNA) suggests that gene regulation by GH involves multiple Stat proteins. Shared transcription factors among hormones and cytokines that activate JAK kinases provide an explanation for shared responses, while the ability of the different ligands to differentially recruit various Stat family members suggests mechanisms by which specificity in gene regulation could be achieved.

Signaling mechanisms through cytokine receptors that share signal transducing receptor components

Most of the receptors for soluble factors functioning in immune and hematopoietic systems belong to the cytokine receptor family. These receptors often share common signal transducing receptor components with other members of the same family. Such receptors and signal transducers possess no intrinsic tyrosine kinase domain but have recently been found to be associated with members of a JAK family of cytoplasmic tyrosine kinases. The JAK kinases become activated after ligand-induced dimerization of the receptor components. This activation appears to link the cell surface receptors to the nuclear genes through tyrosine phosphorylation and activation of latent cytoplasmic transcription factors called signal transducers and activators of transcription (STATs).