Transcriptional inhibition by Stat5. Differential activities at growth-related versus differentiation-specific promoters

Mitochondrial STAT5A promotes metabolic remodeling and the Warburg effect by inactivating the pyruvate dehydrogenase complex

Signal transducer and activator 5a (STAT5A) is a classical transcription factor that plays pivotal roles in various biological processes, including tumor initiation and progression. A fraction of STAT5A is localized in the mitochondria, but the biological functions of mitochondrial STAT5A remain obscure. Here, we show that STAT5A interacts with pyruvate dehydrogenase complex (PDC), a mitochondrial gatekeeper enzyme connecting two key metabolic pathways, glycolysis and the tricarboxylic acid cycle. Mitochondrial STAT5A disrupts PDC integrity, thereby inhibiting PDC activity and remodeling cellular glycolysis and oxidative phosphorylation. Mitochondrial translocation of STAT5A is increased under hypoxic conditions. This strengthens the Warburg effect in cancer cells and promotes in vitro cell growth under hypoxia and in vivo tumor growth. Our findings indicate distinct pro-oncogenic roles of STAT5A in energy metabolism, which is different from its classical function as a transcription factor.

Interleukin-4 Downregulation of Involucrin Expression in Human Epidermal Keratinocytes Involves Stat6 Sequestration of the Coactivator CREB-Binding Protein

Skin barrier defects play an important role in atopic dermatitis (AD). Involucrin, an important barrier protein suppressed in human AD, is downregulated by interleukin-4 (IL-4). However, the molecular mechanism for IL-4 downregulation of involucrin has not been delineated, and especially how Stat6, a transcriptional activator, represses involucrin expression is unknown. Since Stats usually recruit p300/CBP in the general transcription machinery of their target genes and involucrin expression also involves p300/CBP, we hypothesize that Stat6 activated by IL-4 may sequestrate p300/CBP from the involucrin transcription complex, thus suppressing involucrin expression in keratinocytes. Using IL-4 transgenic mice, an AD mouse model, we find that involucrin expression is similarly downregulated as in human AD. In HaCat cells, the Jak inhibitor and dominant negative studies indicate that the Jaks-Stat6 pathway is involved in IL-4 downregulation of involucrin. Next, we transfected HaCat cells with an involucrin promoter-luciferase construct and then treated them with IL-4. IL-4 greatly suppresses the promoter activity, which is totally abolished by cotransfecting the CREB-binding protein (CBP) expression vector, indicating that IL-4 cannot downregulate involucrin in the presence of excess CBP. Finally, chromatin immunoprecipitation assay demonstrates that IL-4 decreases CBP binding to the involucrin transcription complex. For the first time, we defined a molecular mechanism for IL-4 downregulation of involucrin in keratinocytes, which may play an important role in the pathogenesis of AD.

Interleukin-4 up-regulation of epidermal interleukin-19 expression in keratinocytes involves the binding of signal transducer and activator of transcription 6 (Stat6) to the imperfect Stat6 sites

Interleukin-19 (IL-19) plays an important role in asthma by stimulating T helper type 2 (Th2) cytokine production. Interestingly, IL-4, a key Th2 cytokine, in turn up-regulates IL-19 expression in bronchial epithelial cells, so forming a positive feedback loop. In atopic dermatitis (AD), another Th2 disease closely related to asthma, IL-19 is up-regulated in the skin. We propose to use IL-4 transgenic (Tg) mice and human keratinocyte culture to delineate the molecular mechanisms involved in the up-regulation of IL-19 in AD. IL-19 is similarly up-regulated in the skin of IL-4 Tg mice as in human AD. Next we show that IL-4 up-regulates IL-19 expression in keratinocytes. Interestingly, the up-regulation was suppressed by a pan-Janus kinase (Jak) inhibitor, suggesting that the Jak-signal transducer and activator of transcription (Jak-STAT) pathway may be involved. Dominant negative studies further indicate that STAT6, but not other STATs, mediates the up-regulation. Serial 5' deletion of the IL-19 promoter and mutagenesis studies demonstrate that IL-4 up-regulation of IL-19 in keratinocytes involves two imperfect STAT6 response elements. Finally, chromatin immunoprecipitation assay studies indicate that IL-4 increases the binding of STAT6 to its response elements in the IL-19 promoter. Taken together, we delineate the detailed molecular pathway for IL-4 up-regulation of IL-19 in keratinocytes, which may play an important role in AD pathogenesis.

The transcriptional responsiveness of LKB1 to STAT-mediated signaling is differentially modulated by prolactin in human breast cancer cells

Background

Liver kinase 1 (LKB1) is an important multi-tasking protein linked with metabolic signaling, also controlling polarity and cytoskeletal rearrangements in diverse cell types including cancer cells. Prolactin (PRL) and Signal transducer and activator of transcription (STAT) proteins have been associated with breast cancer progression. The current investigation examines the effect of PRL and STAT-mediated signaling on the transcriptional regulation of LKB1 expression in human breast cancer cells.

Methods

MDA-MB-231, MCF-7, and T47D human breast cancer cells, and CHO-K1 cells transiently expressing the PRL receptor (long form), were treated with 100 ng/ml of PRL for 24 hours. A LKB1 promoter-luciferase construct and its truncations were used to assess transcriptional changes in response to specific siRNAs or inhibitors targeting Janus activated kinase 2 (JAK2), STAT3, and STAT5A. Real-time PCR and Western blotting were applied to quantify changes in mRNA and protein levels. Electrophoretic mobility shift (EMSA) and chromatin immunoprecipitation (ChIP) assays were used to examine STAT3 and STAT5A binding to the LKB1 promoter.

Results

Consistent with increases in mRNA, the LKB1 promoter was up-regulated by PRL in MDA-MB-231 cells, a response that was lost upon distal promoter truncation. A putative GAS element that could provide a STAT binding site mapped to this region, and its mutation decreased PRL-responsiveness. PRL-mediated increases in promoter activity required signaling through STAT3 and STAT5A, also involving JAK2. Both STATs imparted basally repressive effects in MDA-MB-231 cells. PRL increased in vivo binding of STAT3, and more definitively, STAT5A, to the LKB1 promoter region containing the GAS site. In T47D cells, PRL down-regulated LKB1 transcriptional activity, an effect that was reversed upon culture in phenol red-free media. Interleukin 6, a cytokine activating STAT signaling in diverse cell types, also increased LKB1 mRNA levels and promoter activity in MDA-MB-231 cells.

Conclusions

LKB1 is differentially regulated by PRL at the level of transcription in representative human breast cancer cells. Its promoter is targeted by STAT proteins, and the cellular estrogen receptor status may affect PRL-responsiveness. The hormonal and possibly cytokine-mediated control of LKB1 expression is particularly relevant in aggressive breast cancer cells, potentially promoting survival under energetically unfavorable conditions.

Bortezomib inhibits STAT5-dependent degradation of LEF-1, inducing granulocytic differentiation in congenital neutropenia CD34(+) cells

The transcription factor lymphoid enhancer-binding factor 1 (LEF-1), which plays a definitive role in granulocyte colony-stimulating factor (G-CSF) receptor-triggered granulopoiesis, is downregulated in granulocytic progenitors of severe congenital neutropenia (CN) patients. However, the exact mechanism of LEF-1 downregulation is unclear. CN patients are responsive to therapeutically high doses of G-CSF and are at increased risk of developing acute myeloid leukemia. The normal expression of LEF-1 in monocytes and lymphocytes, whose differentiation is unaffected in CN, suggests the presence of a granulopoiesis-specific mechanism downstream of G-CSF receptor signaling that leads to LEF-1 downregulation. Signal transducer and activator of transcription 5 (STAT5) is activated by G-CSF and is hyperactivated in acute myeloid leukemia. Here, we investigated the effects of activated STAT5 on LEF-1 expression and functions in hematopoietic progenitor cells. We demonstrated that constitutively active STAT5a (caSTAT5a) inhibited LEF-1-dependent autoregulation of the LEF-1 gene promoter by binding to the LEF-1 protein, recruiting Nemo-like kinase and the E3 ubiquitin-ligase NARF to LEF-1, leading to LEF-1 ubiquitination and a reduction in LEF-1 protein levels. The proteasome inhibitor bortezomib reversed the defective G-CSF-triggered granulocytic differentiation of CD34(+) cells from CN patients in vitro, an effect that was accompanied by restoration of LEF-1 protein levels and LEF-1 messenger RNA autoregulation. Taken together, our data define a novel mechanism of LEF-1 downregulation in CN patients via enhanced ubiquitination and degradation of LEF-1 protein by hyperactivated STAT5.

Interleukin-6 inhibition of peroxisome proliferator-activated receptor alpha expression is mediated by JAK2- and PI3K-induced STAT1/3 in HepG2 hepatocyte cells

Interleukin-6 (IL-6) is the major activator of the acute phase response (APR). One important regulator of IL-6-activated APR is peroxisome proliferator-activated receptor alpha (PPARα). Currently, there is a growing interest in determining the role of PPARα in regulating APR; however, studies on the molecular mechanisms and signaling pathways implicated in mediating the effects of IL-6 on the expression of PPARα are limited. We previously revealed that IL-6 inhibits PPARα gene expression through CAAT/enhancer-binding protein transcription factors in hepatocytes. In this study, we determined that STAT1/3 was the direct downstream molecules that mediated the Janus kinase 2 (JAK2) and phosphatidylinositol-3 kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathways in IL-6-induced repression of PPARα. Treatment of cells with pharmacological inhibitors of JAK2, PI3K, AKT, and mTOR attenuated the inhibitory effect of IL-6 on PPARα protein in a dose-dependent manner. These inhibitors also decreased the IL-6-induced repression of PPARα mRNA expression and promoter activity. Overexpression of STAT1 and STAT3 in HepG2 cells cotransfected with a reporter vector containing this PPARα promoter region revealed that both the expression plasmids inhibited the IL-6-induced repression of PPARα promoter activity. In the presence of inhibitors of JAK2 and mTOR (AG490 and rapamycin, respectively), IL-6-regulated protein expression and DNA binding of STAT1 and STAT3 were either completely or partially inhibited simultaneously, and the IL-6-induced repression of PPARα protein and mRNA was also inhibited. This study has unraveled novel pathways by which IL-6 inhibits PPARα gene transcription, involving the modulation of JAK2/STAT1-3 and PI3K/AKT/mTOR by inducing the binding of STAT1 and STAT3 to STAT-binding sites on the PPARα promoter. Together, these findings represent a new model of IL-6-induced suppression of PPARα expression by inducing STAT1 and STAT3 phosphorylation and subsequent down-regulation of PPARα mRNA expression.

Rac1 signalling modulates a STAT5/BCL-6 transcriptional switch on cell-cycle-associated target gene promoters

Gene expression depends on binding of transcriptional regulators to gene promoters, a process controlled by signalling pathways. The transcriptional repressor B-cell lymphoma (BCL)-6 downregulates genes involved in cell-cycle progression and becomes inactivated following phosphorylation by the Rac1 GTPase-activated protein kinase PAK1. Interestingly, the DNA motifs recognized by BCL-6 and signal transducers and activators of transcription 5 (STAT5) are similar. Because STAT5 stimulation in epithelial cells can also be triggered by Rac1 signalling, we asked whether both factors have opposing roles in transcriptional regulation and whether Rac1 signalling may coordinate a transcription factor switch. We used chromatin immunoprecipitation to show that active Rac1 promotes release of the repressor BCL-6 while increasing binding of STAT5A to a BCL-6-regulated reporter gene. We further show in colorectal cell lines that the endogenous activation status of the Rac1/PAK1 pathway correlated with the phosphorylation status of BCL-6 and STAT5A. Three cellular genes (cyclin D2, p15^INK4B, small ubiquitin-like modifier 1) were identified to be inversely regulated by BCL-6 and STAT5A and responded to Rac1 signalling with increased expression and corresponding changes in promoter occupancy. Together, our data show that Rac1 signalling controls a group of target genes that are repressed by BCL-6 and activated by STAT5A, providing novel insights into the modulation of gene transcription by GTPase signalling.

The role of Stat5 transcription factors as tumor suppressors or oncogenes

Stat5 is constitutively activated in many human cancers affecting the expression of cell proliferation and cell survival controlling genes. These oncogenic functions of Stat5 have been elegantly reproduced in mouse models. Aberrant Stat5 activity induces also mitochondrial dysfunction and reactive oxygen species leading to DNA damage. Although DNA damage can stimulate tumorigenesis, it can also prevent it. Stat5 can inhibit tumor progression like in the liver and it is a tumor suppressor in fibroblasts. Stat5 proteins are able to regulate cell differentiation and senescence activating the tumor suppressors SOCS1, p53 and PML. Understanding the context dependent regulation of tumorigenesis through Stat5 function will be central to understand proliferation, survival, differentiation or senescence of cancer cells.

Tyrosines 868, 966, and 972 in the kinase domain of JAK2 are autophosphorylated and required for maximal JAK2 kinase activity

Janus kinase 2 (JAK2) is activated by a majority of cytokine family receptors including receptors for GH, leptin, and erythropoietin. To identify novel JAK2-regulatory and/or -binding sites, we set out to identify autophosphorylation sites in the kinase domain of JAK2. Two-dimensional phosphopeptide mapping of in vitro autophosphorylated JAK2 identified tyrosines 868, 966, and 972 as sites of autophosphorylation. Phosphorylated tyrosines 868 and 972 were also identified by mass spectrometry analysis of JAK2 activated by an erythropoietin-bound chimeric erythropoietin receptor/leptin receptor. Phosphospecific antibodies suggest that the phosphorylation of all three tyrosines increases in response to GH. Compared with wild-type JAK2, which is constitutively active when overexpressed, JAK2 lacking tyrosine 868, 966, or 972 has substantially reduced activity. Coexpression with GH receptor and protein tyrosine phosphatase1B allowed us to investigate GH-dependent activation of these mutated JAK2s in human embryonic kidney 293T cells. All three mutated JAK2s are activated by GH, although to a lesser extent than wild-type JAK2. The three mutated JAK2s also mediate GH activation of signal transducer and activator of transcription 3 (Stat3), signal transducer and activator of transcription 5b (Stat5b) and ERK1, but at reduced levels. Coexpression with Src-homology 2B1beta (SH2B1beta), like coexpression with GH-bound GH receptor, partially restores the activity of all three JAK2 mutants. Based on these results and the crystal structure of the JAK2 kinase domain, we hypothesize that small changes in the conformation of the regions of JAK2 surrounding tyrosines 868, 966, and 972 due to e.g. phosphorylation, binding to a ligand-bound cytokine receptor, and/or binding to Src-homology 2B1, may be essential for JAK2 to assume a maximally active conformation.

STAT5 activity in pancreatic β-cells

Signal transducer and activator of transcription (STAT)5A and -5B are latent transcription factors activated by cytokines and hormones of the cytokine family. In pancreatic insulin-secreting β-cells, STAT5A and -5B are activated primarily by prolactin and growth hormone stimulation and are important mediators of the potent stimulation of proliferation and insulin production caused by these hormones. STAT5A and -5B are both expressed in β-cells and control the expression of a number of mRNAs implicated in cell replication control, insulin biosynthesis and secretion. In addition to STAT5A and -5B being transcriptional activators, they may also repress gene transcription. By these means, STAT5 proteins increase the levels of anti-apoptotic transcripts in β-cells and repress expression of pro-apoptotic genes. This review focuses on the anti-apoptotic role of STAT5 signaling, providing a mechanism for β-cell resistance to pro-apoptotic cytokines, Type 1 diabetes mellitus and obesity-associated β-cell stress. It is clear from studies of STAT5 signaling in pancreatic β-cells that STAT5 is important for postnatal β-cell compensatory growth (as in pregnancy or obesity) and in the defense against β-cell stress factors.

The role of interleukin-11 in pregnancy involves up-regulation of alpha2-macroglobulin gene through janus kinase 2-signal transducer and activator of transcription 3 pathway in the decidua

IL-11 expressed by endometrial stromal cells is crucial for normal pregnancy. IL-11 receptor alpha (IL-11Ralpha) null mice are infertile due to abnormal development of the placenta. In these mice, the mesometrial decidual tissue, which is the site of trophoblast invasion, thins and disappears at mid-pregnancy. Degeneration of the decidua is accompanied by uncontrolled trophoblast invasion. In this report, we show, using IL-11Ralpha null mice, that a defect in IL-11 signaling in the decidua leads to severe down-regulation of alpha(2)-macroglobulin (alpha(2)-MG), a metalloproteinase inhibitor crucial for limiting trophoblast invasion. We also present evidence, using uterine stromal cells that decidualize in culture, that IL-11 robustly stimulates the endogenous alpha(2)-MG expression and enhances alpha(2)-MG promoter activity. Serial 5' deletion and internal deletion of the promoter reveal two important signal transducer and activator of transcription (Stat) binding sites. Mutation of either one of these motifs decreases IL-11 stimulation, whereas double mutation prevents IL-11 action. We also found that IL-11 activates Janus kinase 2 (Jak2) and induces rapid phosphorylation, nuclear translocation, and promoter binding activity of Stat3 in decidual cells, whereas Jak1, Tyk2, and Stat5 activities are not affected. In addition, Jak2 inhibitor totally prevents alpha(2)-MG expression in decidual cells. Taken together, results of this investigation provide, at least in part, an explanation for the overinvasiveness of the trophoblast in IL-11Ralpha null mice and reveal, for the first time, that IL-11 signals through the Jak2/Stat3 pathway in decidual cells to stimulate the expression of alpha(2)-MG, a protease inhibitor essential for normal placentation in pregnancy.

STAT5 represses BCL6 expression by binding to a regulatory region frequently mutated in lymphomas

Deregulated expression of BCL6 is a pathogenic event in many lymphomas. BCL6 blocks cellular differentiation by repressing transcription of its target genes, and this may promote tumorigenesis. Conversely, the transcription factor signal transducers and activators of transcription (STAT)5 promotes differentiation in many systems. STAT5 upregulates a number of genes repressed by BCL6, raising the possibility that STAT5 and BCL6 have opposing roles in transcriptional regulation. Therefore, we sought to determine the effects of STAT5 activation on BCL6 expression and function. We found that activation of STAT5 downregulates BCL6 expression in B-lymphoma cells and other hematopoietic cell lines. We identified two potential STAT-binding regions in the first exon and first intron of BCL6 that fell within regions of high inter-species homology, suggesting conservation of regulatory function. STAT5 can bind inducibly and regulate transcription at one of these regions, identifying BCL6 as a STAT5 target gene. Additionally, STAT5-mediated downregulation of BCL6 results in loss of BCL6 repression of its target genes, confirming that STAT5 is a negative regulator of BCL6 function. The STAT5 responsive region of the BCL6 gene is mutated frequently in B-cell lymphomas, suggesting that loss of the repressive effects of STAT5 on BCL6 might contribute to the pathogenesis of these cancers.

Phosphorylation of JAK2 at serine 523: a negative regulator of JAK2 that is stimulated by growth hormone and epidermal growth factor

The tyrosine kinase JAK2 is a key signaling protein for at least 20 receptors in the cytokine/hematopoietin receptor superfamily and is a component of signaling for multiple receptor tyrosine kinases and several G-protein-coupled receptors. In this study, phosphopeptide affinity enrichment and mass spectrometry identified serine 523 (Ser523) in JAK2 as a site of phosphorylation. A phosphoserine 523 antibody revealed that Ser523 is rapidly but transiently phosphorylated in response to growth hormone (GH). MEK1 inhibitor UO126 suppresses GH-dependent phosphorylation of Ser523, suggesting that extracellular signal-regulated kinases (ERKs) 1 and/or 2 or another kinase downstream of MEK1 phosphorylate Ser523 in response to GH. Other ERK activators, phorbol 12-myristate 13-acetate and epidermal growth factor, also stimulate phosphorylation of Ser523. When Ser523 in JAK2 was mutated, JAK2 kinase activity as well as GH-dependent tyrosyl phosphorylation of JAK2 and Stat5 was enhanced, suggesting that phosphorylation of Ser523 inhibits JAK2 kinase activity. We hypothesize that phosphorylation of Ser523 in JAK2 by ERKs 1 and/or 2 or other as-yet-unidentified kinases acts in a negative feedback manner to dampen activation of JAK2 in response to GH and provides a mechanism by which prior exposure to environmental factors that regulate Ser523 phosphorylation might modulate the cell's response to GH.

Prolactin signals via Stat5 and Oct-1 to the proximal cyclin D1 promoter

Prolactin (PRL) modulates proliferation in the mammary gland and other tissues, in part through inducing transcription of cyclin D1, a key regulator of G(1) phase cell cycle progression. We showed previously that PRL, via Jak2, induces binding of Stat5 to a distal GAS site (GAS1) in the cyclin D1 promoter. However, full promoter activity requires additional regions, and in this paper we explored PRL-induced activity at sites other than GAS1. We defined a second PRL-responsive region spanning -254 to -180 that contains a second GAS site (GAS2) and an Oct-1 binding site. Although mutational analysis indicated independence from GAS2, proximal promoter activity remained Stat5-dependent, suggesting alternative mechanisms. EMSA showed that Oct-1 binds the -254 to -180 region and that PRL decreased Oct-1 binding, leading to increased PRL-responsiveness of the proximal cyclin D1 promoter in multiple cell lines. This suggests a role for Oct-1 in PRL-dependent control of cyclin D1 transcription.

Functional relevance of the conserved DNA-binding domain of STAT2

Several distinct type I interferon (IFN)-inducible STAT2-containing complexes have been identified. For the IFN-stimulated gene factor 3 (ISGF3), STAT1 and IRF-9 mediate IFN-stimulated response element (ISRE) binding, whereas STAT2 provides a potent transactivational domain. ISGF3-independent STAT2-containing complexes, specifically STAT2:1 and STAT2:3, bind a gamma-activated sequence (GAS)-like element, yet the contribution of each STAT to DNA binding is unknown. Moreover, the contribution of these ISGF3-independent STAT2-containing complexes to IFN-inducible responses is not defined. Accordingly, we generated mutant cDNAs, targeting the DNA-binding domain in STAT2. These cDNAs were introduced by transfection into U6A cells lacking STAT2, resulting in a panel of cell lines expressing mutant STAT2 proteins. Studies assessed the sensitivity of U6A cells reconstituted with intact STAT2 (U6A-2) and cells expressing mutant STAT2s (U6A-2E426A,E427A (EE-AA), U6A-2V453I, U6A-2V454I, U6A-2V454A, U6A-2V453I,V454I(VV-II), U6A-2N458A) to IFN-inducible responses. Our data reveal that none of the mutations in the STAT2 DNA-binding domain affected IFN-inducible ISGF3 activation, and only the VV-II mutation restricted antiviral and growth inhibitory responses to IFN. Indeed, U6A-2VV-II cells are refractory to these IFN-inducible biological activities and also exhibit impaired IFN-inducible GAS-driven transcriptional activation and subsequent gene expression. Chromatin immunoprecipitation assays revealed that the VV-II mutation in STAT2 does not abrogate, but reduces the DNA binding activity of STAT2:1 heterodimers. Taken together, these data suggest a role for the conserved DNA-binding domain of STAT2 specific to the activity of ISGF3-independent STAT2-containing complexes.

Induction of mammary gland differentiation in transgenic mice by the fatty acid-binding protein MRG

A mammary-derived growth inhibitor-related gene (MRG) was previously identified and characterized. MRG induces differentiation of mammary epithelial cells in vitro and its expression is associated with mammary differentiation. To further define the role of MRG on mammary gland differentiation, a MRG transgenic mice model under the control of mouse mammary tumor virus promoter was established and the effect of MRG on mammary gland differentiation was investigated at histological and molecular levels. Expression of endogenous mouse MRG gene was significantly increased from the non-differentiated gland of control virgin mice to the functionally differentiated gland of pregnant control mice. Whole mount analyses demonstrated that ductal development was not affected by MRG transgene expression. While there was no lobuloalveolar structure in control virgin mice, expression of MRG transgene in the mammary gland resulted in the development of lobuloalveolar-like structure, which mimics the gland from early pregnancy. Consistent with the morphological change, expression of MRG also increased milk protein beta-casein expression in the gland. To study the mechanism of MRG-induced mammary differentiation, we investigated the Stat5 activation in the glands from the transgenic mouse versus virgin control mouse. While activated Stat5 was expressed at the minimal level in the non-differentiated control virgin gland, a significant Stat5 phosphorylation was observed in the virgin transgenic gland. Our data indicate that MRG is a mediator of the differentiating effects of pregnancy on breast epithelium, and overexpression of MRG in young nulliparous mice can induce differentiation.

Cytokine induction of prolactin receptors mediates prolactin inhibition of nitric oxide synthesis in pulmonary fibroblasts

Prolactin (PRL) has been implicated as a modulator of immune function, and some of its actions may be linked to NO synthesis. Because NO acts as a mediator of inflammation, we speculated that an inflammatory milieu could unmask pathways by which PRL could affect NO synthesis. Here, we show that pro-inflammatory cytokines induce the expression of PRL receptors in pulmonary fibroblasts, allowing PRL to inhibit cytokine-induced NO production and the expression of the inducible nitric oxide synthase (iNOS). Inhibition of iNOS expression by PRL correlates with the phosphorylation of STAT-5b (signal transducer and activator of transcription 5b) and the suppression of expression of IRF-1 (interferon regulatory factor 1), a transcription factor for iNOS. These results reveal previously unrecognized mechanisms by which PRL and PRL receptors may play significant modulatory roles during immune-inflammatory processes.

Signal transduction pathways regulated by prolactin and Src result in different conformations of activated Stat5b

Stat5 is activated by a broad spectrum of cytokines, as well as non-receptor tyrosine kinases, such as Src. In this study, the DNA binding properties of the two closely related Stat5 proteins, Stat5a and Stat5b, induced either by prolactin (Prl) or by Src were analyzed by electrophoretic mobility shift assays using several different Stat5 binding sites. Src-induced Stat5b-DNA binding complexes consistently displayed a slightly faster mobility than those induced by Prl, as well as differences in their ability to be supershifted by anti-Stat5 antibodies. IP-Westerns performed using specific antibodies directed at the N and C termini of Stat5b suggested that depending on the activating stimulus, Stat5b exhibited different conformations, which influenced antibody accessibility at its C terminus. These conformational differences may in part be due to differential effects of Prl and Src on Stat5b tyrosine phosphorylation, since Src induced several additional sites of tyrosine phosphorylation of Stat5b at residues other than Tyr-699, including Tyr-724 and Tyr-679. The latter Tyr-679 is conserved in all mammalian Stat5bs, but is not present in Stat5a. A Stat 5bY679F mutant induced by Src kinase exhibited an altered pattern of nuclear localization as compared with wild-type Stat5b. Furthermore, this mutation inhibited v-Src-induced cyclin D1-luciferase reporter activity in transient transfection assays performed in Stat5a/b-deficient MEFs, suggesting that Tyr-679 phosphorylation may play a role in v-Src induced proliferation. Thus, depending on the signal transduction pathway responsible for activation, different conformations of activated Stat5 may result in selective biological responses.

Ileal cytokine dysregulation in experimental necrotizing enterocolitis is reduced by epidermal growth factor

BACKGROUND

Necrotizing enterocolitis (NEC) is the most common gastrointestinal disease of premature infants. We have shown in previous studies that proinflammatory interleukin-18 and interleukin-12 are up-regulated in the ileum of rats with experimental NEC and that epidermal growth factor (EGF) reduces the development of disease. Here we investigated whether the protective effects of EGF are a result of changes in ileal interleukin-18, interleukin-12 and/or antiinflammatory interleukin-10.

METHODS

Newborn rats were artificially fed with either growth-factor-free rat milk substitute (RMS) or RMS supplemented with 500 ng/mL EGF (RMS + EGF) and NEC was induced via exposure to asphyxia and cold stress. Cytokine expression and localization were assessed using reverse-transcription real-time polymerase chain reaction and immunohistology/confocal microscopy.

RESULTS

Enteral administration of EGF (RMS + EGF) decreased overproduction of interleukin-18 and increased interleukin-10 production in the ileum. Furthermore, increased interleukin-10 production was associated with up-regulation of the transcription factor Sp1 in RMS + EGF rats.

CONCLUSIONS

These data suggest that EGF may reduce NEC via increased interleukin-10 and decreased interleukin-18 and that EGF-mediated up-regulation of Sp1 may account for the increased interleukin-10.

Genomic structure, expression and characterization of a STAT5 homologue from pufferfish (Tetraodon fluviatilis)

The STAT5 (signal transducer and activator of transcription 5) gene was isolated and characterized from a round-spotted pufferfish genomic library. This gene is composed of 19 exons spanning 11 kb. The full-length cDNA of Tetraodon fluviatilis STAT5 (TfSTAT5) contains 2461 bp and encodes a protein of 785 amino acid residues. From the amino acid sequence comparison, TfSTAT5 is most similar to mouse STAT5a and STAT5b with an overall identity of 76% and 78%, respectively, and has < 35% identity with other mammalian STATs. The exon/intron junctions of the TfSTAT5 gene were almost identical to those of mouse STAT5a and STAT5b genes, indicating that these genes are highly conserved at the levels of amino acid sequence and genomic structure. To understand better the biochemical properties of TfSTAT5, a chimeric STAT5 was generated by fusion of the kinase-catalytic domain of carp Janus kinase 1 (JAK1) to the C-terminal end of TfSTAT5. The fusion protein was expressed and tyrosine-phosphorylated by its kinase domain. The fusion protein exhibits specific DNA-binding and transactivation potential toward an artificial fish promoter as well as authentic mammalian promoters such as the beta-casein promoter and cytokine inducible SH2 containing protein (CIS) promoter when expressed in both fish and mammalian cells. However, TfSTAT5 could not induce the transcription of beta-casein promoter via rat prolactin and Nb2 prolactin receptor. To our knowledge, this is the first report describing detailed biochemical characterization of a STAT protein from fish.

Autocrine human growth hormone inhibits placental transforming growth factor-beta gene transcription to prevent apoptosis and allow cell cycle progression of human mammary carcinoma cells

Multiple cellular effects of human growth hormone (hGH) are mediated by an indirect mechanism requiring transcriptional activation of genes encoding protein effector molecules such as insulin-like growth factor-1. Such protein effector molecules then act directly to mediate the cellular functions of hGH. We report here that autocrine hGH production by mammary carcinoma cells specifically results in the transcriptional repression of the p53-regulated placental transforming growth factor-beta (PTGF-beta) gene. Transcriptional repression of the PTGF-beta gene does not require the p53-binding sites in the PTGF-beta promoter, and autocrine hGH also desensitized the response of the PTGF-beta promoter to p53 overexpression. Transcriptional repression of the PTGF-beta gene is accompanied by consequent decreases in its protein product, Smad-mediated transcription, and its cellular effects that include cell cycle arrest and apoptosis. PTGF-beta specifically inhibited the autocrine hGH-stimulated expression of cyclin D1 required for autocrine hGH-stimulated mammary carcinoma cell cycle progression. Thus, one mechanism by which autocrine hGH promotes an increase in mammary carcinoma cell number is by transcriptional repression of protein effector molecules that promote cell cycle arrest and apoptosis. Such transcriptional repression of negative regulatory factors, such as PTGF-beta, may also be requisite for direct stimulation of mammary carcinoma cell mitogenesis by hGH.

Synergistic action of prolactin (PRL) and androgen on PRL-inducible protein gene expression in human breast cancer cells: a unique model for functional cooperation between signal transducer and activator of transcription-5 and androgen receptor

The signal transducer and activator of transcription 5 (Stat5) has been shown to cooperate with some nuclear receptors. However, an interaction has never been demonstrated with the androgen receptor (AR). Given that the PRL-inducible protein/gross cystic disease fluid-15 (PIP/GCDFP-15) is both a PRL-controlled and an androgen-controlled protein, we used its promoter region to investigate the potential interaction between Stat5 and androgen receptor. Dihydrotestosterone or PRL alone slightly modulated or did not modulate the luciferase activity of all reporter gene constructs. In contrast, a maximal increase was observed using the -1477+42 reporter gene construct after exposure to both dihydrotestosterone and PRL. The requirement of half-site androgen-responsive elements and two consensus Stat5-binding elements, Stat5#1 and Stat5#2, was determined by site-directed mutagenesis. Activated Stat5B binds with a higher affinity to Stat5#2 than to Stat5#1. Stat5ADelta749 and Stat5BDelta754 mutants demonstrated that the Stat5 trans-activation domain is involved in the hormonal cooperation. The cooperation depends on the PRL-induced phosphorylation on Tyr(694) in Stat5A and Tyr(699) in Stat5B, as demonstrated using the Stat5AY694F and Stat5BY699F proteins. The use of AR Q798E, C619Y, and C784Y mutants showed that trans-activation, DNA-binding, and ligand-binding domains of AR are essential. Our study thus suggests a functional cooperation between AR and Stat5.

Regulated expression of signal transducer and activator of transcription, Stat5, and its enhancement of PRL expression in human endometrial stromal cells in vitro

Differentiation of human endometrium during the secretory phase of the menstrual cycle is characterized by expression of a variety of genes implicated in the establishment and maintenance of pregnancy. An increased abundance of signal transducers and activators of transcription (Stats) in the secretory phase suggests Stat5 as a component of the differentiation of endometrium in response to ovarian hormone stimulation in vivo. Decidualization is initiated in a subset of endometrial stromal cells (ESC) in vivo during the secretory phase, but it is unclear whether regulated expression of Stat5 is a feature of these cells. Here, therefore, the abundance and subcellular distribution of Stat5 in ESC after a decidualization stimulus of cAMP plus medroxyprogesterone acetate (MPA) has been investigated in vitro. Western blotting revealed an increase in the apparent abundance of Stat5a and Stat5b, in the cytosolic and nuclear fractions, at 2, 3, and 4 d after stimulation. The potential functional relevance of this increase in Stat5 is suggested by the ability of transiently transfected Stat5a or Stat5b to significantly enhance the response of the decidual PRL promoter to cAMP/MPA and attenuation of the response to cAMP/MPA by dominant negative Stat5. Recent evidence suggests endometrial differentiation, including PRL production, as a possible target of antiphospholipid antibodies (aPL) prevalent in recurrent miscarriage. Monoclonal antibody, ID2, which has similar reactivity as human aPL, significantly decreased the apparent abundance of nuclear Stat5b in response to cAMP/MPA and was associated with decreased decidual PRL promoter activation and PRL secretion. Regulated expression of Stat5 is therefore a component of decidual differentiation of human ESC and contributes significantly to activation of the decidual PRL promoter. Alteration of this process by an aPL component suggests decidual differentiation as a potential clinical target in recurrent early miscarriages.

The Stat5-RARalpha fusion protein represses transcription and differentiation through interaction with a corepressor complex

The transcription factor Stat5 mediates the cellular response to activation of multiple cytokine receptors involved in the regulation of proliferation and differentiation of hematopoietic cells. Recently, the human Stat5 gene was found to be translocated to the RARalpha gene in a patient with acute promyelocytic leukemia indicating that Stat5 might also play a role in cellular transformation. We investigated the mechanism by which Stat5 might exert this function and studied the biochemical and cellular functions of fusion proteins comprising Stat5 and RARalpha. The expression of Stat5-RARalpha causes the transcriptional repression of gene transcription, a process that requires the coiled-coil domain of Stat5 (amino acid positions 133-333). Oligomerization of this domain in the Stat5-RARalpha fusion protein leads to stable binding of the corepressor SMRT independent of all-trans retinoic acid (ATRA) stimulation and is accompanied by an impaired response to differentiation signals in hematopoietic cells. This inhibitory effect on myeloid differentiation cannot be overcome by simultaneous coexpression of RARalpha. We conclude that Stat5 is capable of interacting with a corepressor complex that alters the pattern of corepressor binding to RARalpha and its dissociation in response to ATRA stimulation, leading to enhanced repressor activity and a block of hematopoietic differentiation.

Identification of novel targets of immunosuppressive agents by cDNA-based microarray analysis

The immunosuppressive agents cyclosporin A (CsA) and tacrolimus (FK506) bind to unrelated intracellular immunophilin receptors, cyclophilin (CyP) and FK506-binding protein (FKBP), respectively. The complexes of CsA-CyP and of FK506-FKBP both bind to and inhibit the activity of the calcium/calmodulin-dependent serine/threonine phosphatase calcineurin. We used cDNA microarray analysis to characterize early human peripheral blood T cell transcriptional responses following antigen receptor stimulation in the absence or presence of CsA or FK506, hoping to identify novel targets dependent upon calcineurin or immunophilins or, perhaps, specific targets of either CyP or FKBP inhibitable by one drug alone. The array data failed to identify genes uniquely sensitive to only one drug, suggesting that transcriptionally regulated, immunophilin-dependent but calcineurin-independent targets fell below the limits of detection in this system. In contrast, transcript profiling identified and mRNA and protein analysis confirmed novel as well as known genes reproducibly induced or inhibited by both immunosuppressive agents. In this context, we show that transcriptional activation of Stat5a and repression of the cytokine interleukin-16 are regulated by T cell receptor engagement and dependent upon drug-immunophilin complexes and, presumably, calcineurin activity.

The Stat1 binding motif of the interferon-gamma receptor is sufficient to mediate Stat5 activation and its repression by SOCS3

Signal transduction via the interferon-gamma (IFN-gamma) receptor requires the tyrosine phosphorylation of signal transducers and activators of transcription (Stats). Whereas tyrosine phosphorylation of Stat1 occurs in all cells, activation of Stat5 by IFN-gamma is cell type-restricted. Here we investigated the mechanism of Stat5 activation by the IFN-gamma receptor. In transfection assays both Stat5 isoforms, Stat5a and Stat5b, were phosphorylated on tyrosine in response to IFN-gamma. Stat5 activation required the presence of tyrosine 420 (Tyr-420) in the murine IFNGR1 receptor chain, which also serves as the Stat1 binding site. Moreover, a peptide including Tyr-440, the Stat1 binding site of the human IFNGR1 chain, conferred the ability upon a synthetic receptor to activate Stat5. Suppressor of cytokine signaling 3 (SOCS3) inhibited the activation of Stat5 by the IFN-gamma receptor, and the Tyr-440-containing peptide stretch was sufficient for repression. SOCS3 expression had little effect on the activity of Jak kinases not associated with cytokine receptors. In IFN-gamma-treated, Stat1-deficient fibroblasts Stat5 was inefficient in inducing transcription of a Stat-dependent reporter gene, suggesting it does not per se make a major contribution to the expression of IFN-gamma-responsive genes.

Cloning of the ovine estrogen receptor-alpha promoter and functional regulation by ovine interferon-tau

Interferon-tau (IFNtau), the ruminant pregnancy recognition signal, inhibits transcription of the estrogen receptor alpha (ERalpha) gene in the endometrial lumenal epithelium of the sheep uterus, thereby abrogating production of luteolytic PGF(2alpha) pulses. The effects of IFNtau are mediated in part by IFN-stimulated response elements (ISREs) and IFN regulatory factor elements (IRFEs). The promoter/enhancer region of the ovine ERalpha gene was cloned, sequenced, and predicted to contain four IRFEs and one ISRE. Electrophoretic mobility shift assays indicated that the -2110 IRFE bound only IRF-1, whereas the -1877 IRFE and the -1284 ISRE were functional in binding IRF-1 and IRF-2. IFNtau inhibited transcriptional activity of the 2.7-kb ovine ERalpha promoter in transfection assays using ovine lumenal epithelium cells. Analyses of sequential 5'-deletion mutants of the ovine ERalpha promoter indicated that the effects of IFNtau may be mediated by IRFEs as well as other elements. Overexpression of ovine IRF-2, but not IRF-1, inhibited transcriptional activity of several regions of the ovine ERalpha promoter containing an IRFE or an ISRE as well as some, but not all, regions lacking these elements.

Interferon-gamma-induced regulation of peroxisome proliferator-activated receptor gamma and STATs in adipocytes

Interferon-gamma (IFN-gamma) is known primarily for its roles in immunological responses but also has been shown to affect fat metabolism and adipocyte gene expression. To further investigate the effects of IFN-gamma on fat cells, we examined the effects of this cytokine on the expression of adipocyte transcription factors in 3T3-L1 adipocytes. Although IFN-gamma regulated the expression of several adipocyte transcription factors, IFN-gamma treatment resulted in a rapid reduction of both peroxisome proliferator-activated receptor (PPAR) protein and mRNA. A 48-h exposure to IFN-gamma also resulted in a decrease of both CCAAT/enhancer-binding alpha and sterol regulatory element binding protein (SREBP-1) expression. The short half-life of both the PPARgamma mRNA and protein likely contributed to the rapid decline of both cytosolic and nuclear PPARgamma in the presence of IFN-gamma. Our studies clearly demonstrated that the IFN-gamma-induced loss of PPARgamma protein is partially inhibited in the presence of two distinct proteasome inhibitors. Moreover, IFN-gamma also inhibited the transcription of PPARgamma, which was accompanied by a decrease in PPARgamma mRNA accumulation. In addition, exposure to IFN-gamma resulted in a substantial increase in STAT 1 expression and a small increase in STAT 3 expression. IFN-gamma treatment of 3T3-L1 adipocytes (48-96 h) resulted in a substantial inhibition of insulin-sensitive glucose uptake. These data clearly demonstrate that IFN-gamma treatment results in the development of insulin resistance, which is accompanied by the regulation of various adipocyte transcription factors, in particular the synthesis and degradation of PPARgamma.

Interferon-tau activates multiple signal transducer and activator of transcription proteins and has complex effects on interferon-responsive gene transcription in ovine endometrial epithelial cells

Interferon-tau (IFNtau), a type I IFN produced by sheep conceptus trophectoderm, is the signal for maternal recognition of pregnancy. Although it is clear that IFNtau suppresses transcription of the estrogen receptor alpha and oxytocin receptor genes and induces expression of various IFN-stimulated genes within the endometrial epithelium, little is known of the signal transduction pathway activated by the hormone. This study determined the effects of IFNtau on signal transducer and activator of transcription (STAT) activation, expression, DNA binding, and transcriptional activation using an ovine endometrial epithelial cell line. IFNtau induced persistent tyrosine phosphorylation and nuclear translocation of STAT1 and -2 (10 min to 48 h), but transient phosphorylation and nuclear translocation of STAT3, -5a/b, and -6 (10 to <60 min). IFNtau increased expression of STAT1 and -2, but not STAT3, -5a/b, and -6. IFN-stimulated gene factor-3 and STAT1 homodimers formed and bound an IFN-stimulated response element (ISRE) and gamma-activated sequence (GAS) element, respectively. IFNtau increased transcription of GAS-driven promoters at 3 h, but suppressed their activity at 24 h. In contrast, the activity of an ISRE-driven promoter was increased at 3 and 24 h. These results indicate that IFNtau activates multiple STATs and has differential effects on ISRE- and GAS-driven gene transcription.

Inhibition of growth hormone action in models of inflammation

Growth hormone (GH) action is attenuated during the hepatic acute-phase response (APR). To understand this attenuation, we asked whether GH and cytokine-signaling pathways intersect during an APR. In hypophysectomized rats treated with lipopolysaccharide (LPS), accumulation of activated signal transducer and transcription activator 5 (Stat5) in hepatic nuclei in response to GH and its binding to a GH response element (GHRE) from the serine protease inhibitor (Spi) 2.1 promoter are diminished in a time-dependent manner. Similarly, accumulation of activated Stat3 in hepatic nuclei in response to LPS and its binding to a high-affinity sis-inducible element (SIE) are also diminished by the simultaneous administration of GH. In functional assays with primary hepatocytes, LPS-stimulated monocyte-conditioned medium (MoCM) inhibits the GH response of Stat5-dependent Spi 2.1 reporter activity but induces Stat3-dependent Spi 2.2 reporter activity, as in an APR. Similar results are obtained when hepatocytes are treated with either tumor necrosis factor-alpha (TNF-alpha) or interleukin (IL)-1beta. TNF-alpha, IL-1beta, and IL-6 also inhibit GH-induced Spi 2.1 mRNA expression in hepatocytes. Thus inhibition of the GH signaling pathway during an APR results in reduced expression of GH-responsive genes.

Synergistic action of hepatocyte nuclear factors 3 and 6 on CYP2C12 gene expression and suppression by growth hormone-activated STAT5b. Proposed model for female specific expression of CYP2C12 in adult rat liver

Growth hormone (GH) exerts sexually dimorphic effects on liver gene transcription through its sex-dependent temporal pattern of pituitary hormone secretion. CYP2C12 encodes a female-specific rat liver P450 steroid hydroxylase whose expression is activated by continuous GH stimulation of hepatocytes. Presently, we investigated the role of liver-enriched and GH-regulated transcription factors in the activation of CYP2C12 gene expression in GH-stimulated liver cells. Transcription of a CYP2C12 promoter-luciferase reporter gene in transfected HepG2 cells was activated 15-40-fold by the liver-enriched hepatocyte nuclear factor (HNF) 3 alpha, HNF3 beta, and HNF6. Synergistic interactions leading to an approximately 300-fold activation of the promoter by HNF3 beta in combination with HNF6 were observed. 5'-Deletion analysis localized the HNF6 response to a single 5'-proximal 96-nucleotide segment. By contrast, the stimulatory effects of HNF3 alpha and HNF3 beta were attributable to five distinct regions within the 1.6-kilobase CYP2C12 proximal promoter. GH activation of the signal transducer and transcriptional activator STAT5b, which proceeds efficiently in male but not female rat liver, inhibited CYP2C12 promoter activation by HNF3 beta and HNF6, despite the absence of a classical STAT5-binding site. The female-specific pattern of CYP2C12 expression is thus proposed to reflect the positive synergistic action in female liver of liver-enriched and GH-regulated transcription factors, such as HNF3 beta and HNF6, coupled with a dominant inhibitory effect of GH-activated STAT5b that is manifest in males.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Negative regulation of growth hormone receptor/JAK2 signaling by signal regulatory protein alpha

Signal regulatory proteins (SIRPs) are receptor-like transmembrane proteins, the majority of which contain a cytoplasmic proline-rich region and four cytoplasmic tyrosines that, when phosphorylated, bind SH2 domain-containing protein tyrosine phosphatases (SHP). We demonstrated previously that growth hormone (GH) induces tyrosyl phosphorylation of SIRPalpha and association of SIRPalpha with SHP-2. The GH-activated tyrosine kinase JAK2 associates with and tyrosyl-phosphorylates SIRPalpha1. Here we show that JAK2-SIRPalpha1 association does not require phosphotyrosines in SIRPalpha1 or JAK2 or the proline-rich region of SIRPalpha1. However, when the C-terminal 30 amino acids of SIRPalpha1 containing the proline-rich region and tyrosine 495 are deleted, tyrosyl phosphorylation of SIRPalpha1 by JAK2 and association of SHP-2 with SIRPalpha1 are reduced. GH-dependent tyrosyl phosphorylation of JAK2 is reduced when wild-type SIRPalpha1 compared with SIRPalpha1 lacking the four cytoplasmic tyrosines (SIRP 4YF) is expressed in cells, suggesting that SIRPalpha1 negatively regulates GHR/JAK2 signaling. Consistent with reduced JAK2 activity, overexpression of wild-type SIRPalpha1 but not SIRP 4YF reduces GH-induced phosphorylation of ERKs 1 and 2, STAT3, and STAT5B. These results suggest that SIRPalpha1 is a negative regulator of GH signaling and that the ability of SIRPalpha1 mutants to negatively regulate GHR-JAK2 signaling correlates with their ability to bind SHP-2.

Endotoxin-induced renal inflammatory response. Oncostatin M as a major mediator of suppressed renin expression

The systemic response to endotoxin is characterized by hypotension and severe reductions in blood pressure, leading to cardiovascular collapse that can accompany septicemia. The renin/angiotensin system would normally be expected to respond to hypotensive challenge; however, inflammation appears to modify this response. This study identifies a strong acute phase response of the kidney that is characterized by enhanced expression of serum amyloid A, haptoglobin and tissue inhibitor for metalloproteinase-1 and a reduced expression of renin. Equivalent regulatory effects were observed for the immortalized As4.1 kidney cell line that models certain features of juxtaglomerular cells. Oncostatin M, a known endotoxin-responsive proinflammatory cytokine, proved to be an effective inhibitor of renin gene expression. Suppression by oncostatin M involves activated STAT5 and requires an inhibitory element in the renin promoter that functions separately from cell type-specific enhancer elements. The renal acute phase reaction, unlike the liver acute phase reaction, is more strongly dependent on locally produced inflammatory factors.

Involvement of SOCS-1, the suppressor of cytokine signaling, in the prevention of prolactin-responsive gene expression in decidual cells

The cells forming the rat decidua produce PRL and PRL-related proteins and express both the long and short forms of the PRL receptor. Yet, only a defined subpopulation, the mesometrial cells, express the PRL-dependent alpha2-macroglobulin gene. This gene is silenced in vivo in the antimesometrial cells and in the GG-AD cell line, derived from antimesometrial cells. To examine whether the lack of alpha2-macroglobulin expression is due to defective components in the PRL signaling pathway, we compared the relative expression of Janus kinase 2 (Jak2), signal transducer and activator of transcription 5 a and b (Stat5 a and b), suppressor of cytokine signaling-1 (SOCS-1), and the tyrosine phosphatase SHP-2 mRNA in mesometrial and antimesometrial decidua on days 12 and 13 of pseudopregnancy, the time of maximal alpha2-macroglobulin expression. We found no significant differences in the relative expression of either Jak2, Stat5 (a and b), or SHP-2 in the two cell populations. However, we discovered a profound difference in the expression of SOCS-1, an inhibitor of the Jak/Stat pathway. This gene was highly expressed in the antimesometrial cells and in the GG-AD cells, which do not produce alpha2-macroglobulin. Immunoprecipitation experiments with GG-AD cells revealed that although Jak2 and Stat5 coprecipitate in response to PRL stimulation, no phosphorylation of Jak2 and Stat5 could be observed. To examine whether SOCS-1 plays a role in silencing the alpha2-macroglobulin gene, we cultured GG-AD cells in the presence of either a SOCS-1 antisense oligonucleotide or an irrelevant oligonucleotide for 4, 12, and 28 h. Cells were also treated with PRL. Within 4 h of SOCS-1 antisense treatment, alpha2-macroglobulin mRNA expression was initiated. After 28 h, only cells treated with PRL and SOCS-1 antisense oligonucleotide retained the ability to express the alpha2-macroglobulin gene. In summary, results of this study reveal that constitutive expression of SOCS-1 can prevent PRL signaling and that the lack of PRL-induced expression of alpha2-macroglobulin in a defined decidual cell population is largely due to SOCS-1 expression in these cells.

Resection of solid tumors reverses T cell defects and restores protective immunity

We have previously reported that CTL were demonstrable early after inoculation of CMS5 fibrosarcoma cells, but that they disappeared within 3 wk. These mice were unable to reject a challenge with CMS5 tumor cells. Other studies demonstrated cell surface phenotype and signaling abnormalities of cells within the spleen. Since we assumed that such an environment would make it more difficult to elicit antitumor immune responses via immunotherapy, we asked whether resection of the tumor could reverse these abnormalities. Although early after tumor cell inoculation tumor resection leads to the development of immunity, the effect at late time points has not been studied critically. To test this, mice were inoculated s.c. with CMS5 cells and after 28 days the tumors were resected. We observed a gradual normalization of the cellular phenotype of the spleen. In particular, there was a decrease in the number of Mac1+/Gr1(high) cells and an increase in the number of CD3+ cells in the spleen within 24-48 h of tumor resection. By day 10, these values were normal. Levels of p56lck increased as well. The functional implications of these changes were illustrated by the reduced growth rate or the complete rejection of a challenge of tumor cells in the resected mice. Both CD4+ and CD8+ cells were involved in the restoration of tumor immunity. Our results suggested that tumor resection not only led to the reversal of immune suppression, but also unmasked a population of primed T cells able to mediate protective immunity.

Association of 2',5'-oligoadenylate synthetase with the prolactin (PRL) receptor: alteration in PRL-inducible stat1 (signal transducer and activator of transcription 1) signaling to the IRF-1 (interferon-regulatory factor 1) promoter

The PRL receptor (PRL-R) signals through the Janus tyrosine kinases (JAK) and other non-JAK tyrosine kinases, some of which are preassociated with the PRL-R. To clone PRL-R interacting proteins, the intracellular domain (ICD) of the long form of the PRL-R was used in a yeast two-hybrid screen of a human B cell cDNA library. One PRL-R interacting protein was identified as the 42-kDa form of the enzyme 2',5'-oligoadenylate synthetase (OAS). The in vivo interactions in yeast were further confirmed by an in vitro interaction assay and by coimmunoprecipitation in transfected mammalian cells. Functionally, OAS reduced the basal activity of two types of promoters in transiently transfected COS-1 cells. In the presence of PRL, OAS inhibited PRL induction of the immediate early IRF-1 (interferon-regulatory factor 1) promoter, but not PRL induction of the differentiation-specific beta-casein promoter, suggesting that OAS exerts specific effects on immediate early gene promoters. The inhibitory effects of OAS were accompanied by a reduction in PRL-inducible Stat1 (signal transducer and activator of transcription 1) DNA binding activity at the IRF-1 GAS (interferon-gamma-activated sequence) element. These results demonstrate a novel interaction of OAS with the PRL-R and suggest a role for OAS in modulating Stat1-mediated signaling to an immediate early gene promoter. Although previously characterized as a regulator of ribonuclease (RNase) L antiviral responses, OAS may have additional effects on cytokine receptor signal transduction pathways.

Stat5b inhibits NFkappaB-mediated signaling

Signal transducers and activators of transcription (Stat) are latent transcription factors that participate in cytokine signaling by regulating the expression of early response genes. Our previous studies showed that Stat5 functions not only as a transcriptional activator but also as a transcriptional inhibitor, depending on the target promoter. This report further investigates the mechanism of Stat5b-mediated inhibition and demonstrates that PRL-inducible Stat5b inhibits nuclear factorkappaB (NFkappaB) signaling to both the interferon regulatory factor-1 promoter and to the thymidine kinase promoter containing multimerized NFkappaB elements (NFkappaB-TK). Further, PRL-inducible Stat5b inhibits tumor necrosis factor-alpha signaling presumably by inhibiting endogenous NFkappaB. This Stat5b-mediated inhibitory effect on NFkappaB signaling is independent of Stat5b-DNA interactions but requires the carboxyl terminus of Stat5b as well as Stat5b nuclear translocation and/or accumulation, suggesting that Stat5b is competing for a nuclear factor(s) necessary for NFkappaB-mediated activation of target promoters. Increasing concentrations of the coactivator p300/CBP reverses Stat5b inhibition at both the interferon-regulatory factor-1 and NFkappaB-TK promoters, suggesting that Stat5b may be squelching limiting coactivators via protein-protein interactions as one mechanism of promoter inhibition. These results further substantiate our observation that Stat factors can function as transcriptional inhibitors. Our studies reveal cross-talk between the Stat5b and NFkappaB signal transduction pathways and suggest that Stat5b-mediated inhibition of target promoters occurs at the level of protein-protein interactions and involves competition for limiting coactivators.

The adipoinsular axis: effects of leptin on pancreatic beta-cells

The prevalence of obesity and related diabetes mellitus is increasing worldwide. Here we review evidence for the existence of an adipoinsular axis, a dual hormonal feedback loop involving the hormones insulin and leptin produced by pancreatic beta-cells and adipose tissue, respectively. Insulin is adipogenic, increases body fat mass, and stimulates the production and secretion of leptin, the satiety hormone that acts centrally to reduce food intake and increase energy expenditure. Leptin in turn suppresses insulin secretion by both central actions and direct actions on beta-cells. Because plasma levels of leptin are directly proportional to body fat mass, an increase of adiposity increases plasma leptin, thereby curtailing insulin production and further increasing fat mass. We propose that the adipoinsular axis is designed to maintain nutrient balance and that dysregulation of this axis may contribute to obesity and the development of hyperinsulinemia associated with diabetes.

Paired Stat6 C-Terminal Transcription Activation Domains Required Both for Inhibition of an IFN-Responsive Promoter and Trans-Activation

The cytokines IL-4 and IFN-γ exert biologically antagonistic effects that in part reflect opposing influences on gene transcription. While the molecular mechanisms for IL-4-mediated transcription activation have been extensively studied, little is known about molecular mechanisms required for IL-4 inhibition of IFN-γ signaling. We have investigated IL-4 inhibition of the IFN-γ-inducible promoter for IFN regulatory factor-1 (IRF-1). In a cell line with low endogenous Stat6, increasing levels of activated Stat6 at constant doses of IFN-γ and IL-4 leads to inhibition of the IRF-1 promoter. The Stat1-dependent IFN-γ activation sequence element of the IRF-1 promoter is a target for Stat6-mediated inhibition despite apparently normal Stat1 DNA binding. However, our data are inconsistent with competition between Stat1 and Stat6 for access to the IRF-1 IFN-γ activation sequence or for an essential coactivator as a mechanism for this Stat6-mediated inhibition. Instead, the data demonstrate that a threshold of Stat6 transcription activation domains is required for IL-4-dependent inhibition. The findings provide evidence of a novel mechanism in which the Stat6 transcription activation domains play a critical role in the IL-4-mediated inhibition of an IFN-γ-inducible promoter.

Growth hormone, but not prolactin, maintains, low-level activation of STAT5a and STAT5b in female rat liver

STAT5b, a member of the signal transducers and activators of transcription family, is activated in rat liver in response to the intermittent (pulsatile) plasma pattern of GH that is characteristic of adult males. Previous studies have shown that the near-continuous plasma GH pattern of adult female rats is associated with a dramatic down-regulation of the STAT5 activation pathway. The present study demonstrates the presence of a low-level STAT5 DNA-binding activity in adult female rat liver and investigates the hormonal factors required for its maintenance. PRL is not responsible for this low-level STAT5 activity, as demonstrated in experiments involving estrus cycle monitoring (to investigate a possible role of the proestrus PRL surge), implantation of bromocriptine pellets (to eliminate PRL release by the pituitary), and direct injection of purified PRL. Rather, the low-level STAT5 activity is shown to result from chronic plasma GH stimulation, as demonstrated by GH infusion studies carried out in hypophysectomized rats. Furthermore, gel mobility supershift experiments demonstrate that the same STAT5-containing DNA-binding complexes are formed by both male and female adult rat liver extracts, albeit at approximately 10- to 20-fold lower levels by the female extracts. This DNA-binding activity is primarily comprised of STAT5b but also contains STAT5a, which is shown to be preferentially activated by the male plasma GH pattern in a manner similar to STAT5b. Thus, the dominance of activated STAT5b, compared with STAT5a, in the strong DNA-binding complexes formed in adult male rat liver nuclear extracts, is a reflection of the relative abundance in liver of the two STAT5 forms and is not attributable to an intrinsic, preferential activation of STAT5b by plasma GH pulses. The physiological significance of the low-level activated STAT5a and STAT5b seen in female rat liver and its effects on liver gene expression are uncertain but may involve the activation of female-expressed cytochromes P450 and other liver genes.

Differential effects of prolactin and src/abl kinases on the nuclear translocation of STAT5B and STAT5A

In this study, DNA binding and tyrosine phosphorylation of STAT5A and STAT5B were compared with their subcellular localization determined using indirect immunofluorescence microscopy. Following prolactin activation, both STAT5A and STAT5B were rapidly translocated into the nucleus and displayed a detergent-resistant, punctate nuclear staining pattern. Similar to prolactin induction, src activation resulted in tyrosine phosphorylation and DNA binding of both STAT5A and STAT5B. However, nuclear translocation of only STAT5B but not STAT5A was observed. This selective nuclear translocation appears to be mediated via the carboxyl-terminal sequences in STAT5B. Furthermore, overexpression of a dominant negative kinase-inactive mutant of JAK2 prevented prolactin-induced tyrosine phosphorylation and nuclear translocation of STAT5A and STAT5B but did not block src kinase activation and nuclear translocation of STAT5B. In co-transfection assays, prolactin-mediated activation but not src kinase-mediated activation of STAT5B resulted in the induction of a beta-casein promoter-driven reporter construct. These results suggest that STAT5 activation by src may occur by a mechanism distinct from that employed in cytokine activation of the JAK/STAT pathway, resulting in the selective nuclear translocation of STAT5B.

Stat5a and Stat5b: fraternal twins of signal transduction and transcriptional activation

Stat5a and Stat5b are discretely encoded transcription factors that mediate signals for a broad spectrum of cytokines. Their activation is often an integral component of redundant cytokine signal cascades involving complex cross-talk and pleiotropic gene regulation by Stat5 has been implicated in cellular functions of proliferation, differentiation and apoptosis with relevance to processes of hematopoiesis and immunoregulation, reproduction, and lipid metabolism. Although Stat5a and Stat5b show peptide sequence similarities of > 90%, targeted gene disruptions in mice yield distinctive phenotypes. Prolactin-directed mammary gland maturation fails without functional Stat5a, while disruption of Stat5b in males mitigates growth hormone effects on hepatic function and body mass. The molecular basis for this biologic dichotomy is probably multifaceted. Limited structural dissimilarities between the Stat5a and Stat5b transactivation domains, or subtle differences in the DNA-binding affinities of Stat5 dimer pairs undoubtedly influence gene regulation, but cell-dependent asymmetries in availability of phosphorylated Stat5 can be an underlying factor. Differences in serine phosphorylation(s) of Stat5a and Stat5b, or Stat5 associations with adaptor proteins or co-transcription factors are other potential sources of functional disparity and the signal amplitude, frequency or duration also can be significant. In addition to Stat5 signal attenuation by phosphatase actions or classical feedback inhibition, truncated forms of Stat5 lacking in transactivation capacity may compete upstream for activation and diminish access of full length molecules to DNA binding sites.