Involvement of proteasomes in regulating Jak-STAT pathways upon interleukin-2 stimulation

Proteasomal regulation of betac signaling reveals a novel mechanism for cytokine receptor heterotypic desensitization

IL-5, IL-3, and GM-CSF are hematopoietic cytokines that are key mediators of the allergic inflammatory response. The receptors for these three cytokines consist of a cytokine-specific alpha (Ralpha) chain and a shared common beta (betac) chain. Herein, we demonstrate that agonistic ligation of these receptor subunits rapidly induces proteasomal degradation of the betac, but not the Ralpha, cytoplasmic domain, resulting in termination of signal transduction and yielding a truncated betac isoform ligated to the Ralpha subunit. Proteasomal degradation of the betac cytoplasmic domain was also a prerequisite for endocytosis and lysosomal degradation of the ligated receptor subunits. Moreover, proteasome-dependent termination of signaling induced by one betac-engaging cytokine resulted in cellular desensitization to signal transduction by subsequent stimulation with another betac-engaging cytokine. These data provide the first evidence for ligand-dependent proteasomal degradation of the betac cytoplasmic domain, and they establish a novel mechanism for heterotypic desensitization of shared cytokine receptor signaling.

HTLV-1 p12(I) protein enhances STAT5 activation and decreases the interleukin-2 requirement for proliferation of primary human peripheral blood mononuclear cells

The p12(I) protein, encoded by the pX open reading frame I of the human T-lymphotropic virus type 1 (HTLV-1), is a hydrophobic protein that localizes to the endoplasmic reticulum and the Golgi. Although p12(I) contains 4 minimal proline-rich, src homology 3-binding motifs (PXXP), a characteristic commonly found in proteins involved in signaling pathways, it has not been known whether p12(I) has a role in modulating intracellular signaling pathways. This study demonstrated that p12(I) binds to the cytoplasmic domain of the interleukin-2 receptor (IL-2R) beta chain that is involved in the recruitment of the Jak1 and Jak3 kinases. As a result of this interaction, p12(I) increases signal transducers and activators of transcription 5 (STAT5) DNA binding and transcriptional activity and this effect depends on the presence of both IL-2R beta and gamma(c) chains and Jak3. Transduction of primary human peripheral blood mononuclear cells (PBMCs) with a human immunodeficiency virus type 1-based retroviral vector expressing p12(I) also resulted in increased STAT5 phosphorylation and DNA binding. However, p12(I) could increase proliferation of human PBMCs only after stimulation of T-cell receptors by treatment of cells with low concentrations of alphaCD3 and alphaCD28 antibodies. In addition, the proliferative advantage of p12(I)-transduced PBMCs was evident mainly at low concentrations of IL-2. Together, these data indicate that p12(I) may confer a proliferative advantage on HTLV-1-infected cells in the presence of suboptimal antigen stimulation and that this event may account for the clonal proliferation of infected T cells in vivo. (Blood. 2001;98:823-829)

DUB-2A, a new member of the DUB subfamily of hematopoietic deubiquitinating enzymes

Protein ubiquitination is an important regulator of cytokine-activated signal transduction pathways and hematopoietic cell growth. Protein ubiquitination is controlled by the coordinate action of ubiquitin-conjugating enzymes and deubiquitinating enzymes. Recently a novel family of genes encoding growth-regulatory deubiquitinating enzymes (DUB-1 and DUB-2) has been identified. DUBs are immediate-early genes and are induced rapidly and transiently in response to cytokine stimuli. By means of polymerase chain reaction amplification with degenerate primers for the DUB-2 complementary DNA, 3 murine bacterial artificial chromosome (BAC) clones that contain DUB gene sequences were isolated. One BAC contained a novel DUB gene (DUB-2A) with extensive homology to DUB-2. Like DUB-1 and DUB-2, the DUB-2A gene consists of 2 exons. The predicted DUB-2A protein is highly related to other DUBs throughout the primary amino acid sequence, with a hypervariable region at its C-terminus. In vitro, DUB-2A had functional deubiquitinating activity; mutation of its conserved amino acid residues abolished this activity. The 5' flanking sequence of the DUB-2A gene has a hematopoietic-specific functional enhancer sequence. It is proposed that there are at least 3 members of the DUB subfamily (DUB-1, DUB-2, and DUB-2A) and that different hematopoietic cytokines induce specific DUB genes, thereby initiating a cytokine-specific growth response. (Blood. 2001;98:636-642)

Identification of distinct roles for a dileucine and a tyrosine internalization motif in the interleukin (IL)-13 binding component IL-13 receptor alpha 2 chain

Interleukin (IL)-13 receptor alpha2 (IL-13Ralpha2) chain is an essential binding component for IL-13-mediated ligand binding. Recently, we have demonstrated that this receptor chain also plays an important role in the internalization of IL-13. To study the mechanism of IL-13 internalization, we generated mutated IL-13Ralpha2 chains that targeted trileucine residues (Leu(335), Leu(336), and Leu(337)) in the transmembrane domain and a tyrosine motif (Tyr(343)) in the intracellular domain and transfected these cDNAs in COS-7 cells. Cells that expressed a C-terminally truncated IL-13Ralpha2 chain (Delta335) did not bind IL-13, suggesting that the trileucine region modulates IL-13 binding. Truncation of IL-13Ralpha2 chain with a mutation in the trileucine region resulted in significantly decreased internalization compared with wild type IL-13Ralpha2 chain transfected cells. COS-7 cells transfected with tyrosine motif mutants exhibited a similar internalization level compared with wild type IL-13Ralpha2 chain transfected cells; however, dissociation of cell surface IL-13 was faster compared with wild type IL-13Ralpha2 transfectants. These results were further confirmed by determining the cytotoxicity of a chimeric protein composed of IL-13 and a mutated form of Pseudomonas exotoxin (IL13-PE38QQR) to cells that expressed IL-13Ralpha2 chain mutants. We further demonstrate that the IL-13Ralpha2 chain is not ubiquitinated and that internalization of IL-13Ralpha2 did not depend on ubiquitination. Together, our findings suggest that the dileucine motif in the trileucine region and tyrosine motif participate in IL-13Ralpha2 internalization in distinct manners.

Specific down-regulation of interleukin-12 signaling through induction of phospho-STAT4 protein degradation

Interleukin-12 (IL-12) plays a critical role in modulating the function of T lymphocytes and natural killer cells. IL-12 has potent antitumor effects in animal models, mediated primarily by its ability to enhance cytolytic activity and secretion of interferon-gamma (IFN-gamma). Unfortunately, the antitumor effect of IL-12 has not been demonstrated in clinical trials. Repeated administration of IL-12 in humans results in decreasing levels of IFN-gamma secretion. To understand the mechanism underlying this loss of responsiveness, the effect of IL-12 on its own signaling in activated human T cells was examined. These experiments demonstrate that the level of the signal transducer and activator of transcription 4 (STAT4) protein, a critical IL-12 signaling component, is dramatically decreased 24 hours after IL-12 stimulation, whereas levels of STAT4 messenger RNA are not affected. The decrease of STAT4 protein appears to be due to specific degradation of phospho-STAT4, possibly through the proteasome degradation pathway. Decreased levels of STAT4 protein lead to decreased STAT4 DNA-binding activity and reduced proliferation and secretion of IFN-gamma. This down-regulation of STAT4 is specific for IL-12 signaling, presumably owing to the prolonged activation of STAT4 induced by IL-12. IFN-alpha stimulation, which leads to transient phosphorylation of STAT4, does not reduce the level of STAT4 protein. These findings provide new insights into the regulation of IL-12 signaling in human T cells, where IL-12 promotes T(H)1 responses, but persistent IL-12 stimulation may also limit this response. The cellular depletion of STAT4 following prolonged IL-12 stimulation may also explain the loss of responsiveness following the repeated administration of IL-12 in clinical trials. (Blood. 2001;97:3860-3866)

Involvement of proteasomes in gene induction by interferon and double-stranded RNA

Cytokine induced gene expression is mediated through the ligand-dependent activation of the janus kinase (jak)/signal transducer and activator of transcription (STAT) signal transduction pathway. The ubiquitin proteasome pathway functions in the controlled degradation of cellular proteins, and regulates cytokine signal transduction through the degradation of specific signaling components. Interferon (IFN) treatment induces genes that function in ubiquitin conjugation, suggesting a reciprocal regulation of proteasome activity and IFN action; however, a role for the proteasome in IFN-alpha-induced gene expression has not been examined. In this report, we find that proteasome inhibitors markedly reduce the induction of interferon-stimulated-gene 15 (ISG15), ISG43, and STAT1 by IFN-alpha and double-stranded RNA (dsRNA). The reduction in gene expression by proteasome inhibitors was dose-dependent, and was specific for ISGs. Neither STAT1 phosphorylation nor ISGF-3 activation was affected by proteasome inhibition at early times post-IFN treatment. Cycloheximide treatment diminished the effect of proteasome inhibitors on ISG induction, implicating an IFN/dsRNA-induced protein in this activity. These findings demonstrate that a functional proteasome is required for optimal ISG induction, and are consistent with a model in which IFN and dsRNA induce a proteasome-sensitive repressor of ISG expression.

Induction of JAB/SOCS-1/SSI-1 and CIS3/SOCS-3/SSI-3 is involved in gp130 resistance in cardiovascular system in rat treated with cardiotrophin-1 in vivo

CIS (cytokine-inducible SH2 protein), SOCS (suppressor of cytokine signaling), or SSI (signal transducers and activators of transcription [STAT]-induced STAT inhibitor) proteins are a family of cytokine-inducible negative regulators of cytokine signaling via Janus kinase (JAK)-STAT pathways. Given the evidence that the JAK-STAT pathway plays a critical role in the cardiovascular system, the primary objective of this study was to assess the effects of the CIS family on JAK-STAT signaling in the cardiovascular system in rats treated with cardiotrophin-1 (CT-1), an interleukin-6 family of cytokines. Intravenous injection of 20 microgram/kg body weight of CT-1 induced a transient, marked increase in STAT3 activation in various tissues, including heart and lung, and subsequent upregulation of 2 members of the CIS family, JAK-binding protein (JAB)/SOCS-1/SSI-1 and CIS3/SOCS-3/SSI-3, in the same tissues. It was also observed that CIS3 was directly associated with JAK2 in vivo. Pretreatment with the same dose of CT-1 60 minutes before significantly attenuated the STAT3 activation induced by a second injection of CT-1. We previously reported that intravenous injection of CT-1 results in the nitric oxide (NO)-dependent hypotension accompanied by the induction of inducible NO synthase mRNA. In rats pretreated with CT-1, the induction of inducible NO synthase mRNA or hypotension by subsequent CT-1 injection was not observed. Forced expression of JAB or CIS3, but not other CISs, directly blocked CT-1-induced STAT3 activation in 293 cells. These results suggest that JAB and CIS3 serve as endogenous inhibitors of CT-1-mediated JAK-STAT signaling in the cardiovascular system in vivo.

The signal transduction of the growth hormone receptor is regulated by the ubiquitin/proteasome system and continues after endocytosis

The growth hormone receptor (GHR) intracellular domain contains all of the information required for signal transduction as well as for endocytosis. Previously, we showed that the proteasome mediates the clathrin-mediated endocytosis of the GHR. Here, we present evidence that the proteasomal inhibitor MG132 prolongs the GH-induced activity of both GHR and JAK2, presumably through stabilization of GHR and JAK2 tyrosine phosphorylation. If proteasomal inhibitor was combined with ligand in an endocytosis-deficient GHR mutant, the same phenomenon occurred indicating that proteasomal action on tyrosine dephosphorylation is independent of endocytosis. Experiments with a GHR-truncated tail mutant (GHR-(1-369)) led to a prolonged JAK2 phosphorylation caused by the loss of a phosphatase-binding site. This raised the question of what happens to the signal transduction of the GHR after its internalization. Co-immunoprecipitation of GH.GHR complexes before and after endocytosis showed that JAK2 as well as other activated proteins are bound to the GHR not only at the cell surface but also intracellularly, suggesting that the GHR signal transduction continues in endosomes. Additionally, these results provide evidence that GHR is present in endosomes both in its full-length and truncated form, indicating that the receptor is down-regulated by the proteasome.

Negative regulators of cytokine signaling

The interaction of a cytokine with its specific cell surface receptor triggers the activation of intracellular signaling pathways that ultimately program the cellular response. Although the specific components and actions of the pathways driving these responses, such as the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway, are relatively well defined, it is becoming clear that important mechanisms exist to restrain these signaling cascades. This review discusses the key biochemical actions and biological roles of the phosphatase SHP-1, the protein inhibitors of activated STATs (PIAS) and the suppressor of cytokine signaling (SOCS) protein family in the negative regulation of cytokine signal transduction.

Effect of phosphorylation of MAPK and Stat3 and expression of c-fos and c-jun proteins on hepatocarcinogenesis and their clinical significance

AIM: To study the effect of phosphorylation of MAPK and Stat3 and the expression of c-fos and c-jun proteins on hepatocellular carcinogenesis and their clinical significance.

METHODS: SP immunohistochemistry was used to detect the expression of p42/44^MAPK, p-Stat3, ca2fos and c-jun proteins in 55 hepatocellular carcinomas (HCC) and their surrounding liver tissues.

RESULTS: The positive rates and expression levels of p42/44^MAPK, p-Stat3, c-fos and c-jun proteins in HCCs were significantly higher than those in pericarcinomatous liver tissues (PCLT). A positive correlation was observed between the expression of p42/44^MAPK and c-fos proteins, and between pa2Stat3 and c-jun, but there was no significant correlation between p42/44^MAPK and p-Stat3 in HCCs and their surrounding liver tissues.

CONCLUSION: The abnormalities of Ras/Raf/MAPK and JAKs/Stat3 cascade reaction may contribute to malignant transformation of hepatocytes.Hepatocytes which are positive for p42/44^MAPK, c-fos or c-jun proteins may be potential malignant pre-cancerous cells. Activation of MAPK and Stat3 proteins may be an early event in hepatocellular carcinogenesis.

Prolactin activation of IRF-1 transcription involves changes in histone acetylation

In response to prolactin (PRL) signaling, transcription of the interferon regulatory factor-1 gene (IRF-1) is rapidly induced during early G(1), declines in mid G(1), and rises again over the G(1)/S transition phase of the cell cycle in Nb2 T cells. Using chromatin immunoprecipitation assays, we show that histone H4 acetylation increases over a 1.7 kb region of the IRF-1 promoter in early G(1) and again at the G(1)/S transition in response to PRL stimulation. These results demonstrate a correlation between histone H4 hyperacetylation at the IRF-1 promoter and biphasic transcription of IRF-1 in response to PRL signaling in vivo.

The proteasome regulates receptor-mediated endocytosis of interleukin-2

Recent studies have increasingly implicated the proteasome in the regulation of cell surface receptors. In the present study, we investigated the role of the proteasome for ligand-dependent endocytosis and degradation of the interleukin-2 (IL-2)-interleukin-2 receptor (IL-2R) complex. Proteasome inhibitors impaired internalization of IL-2.IL-2R and prevented the lysosomal degradation of this cytokine. Based on time-course studies, proteasome activity is primarily required after initial endocytosis of the IL-2.IL-2R. Proteasome function was also necessary for the lysosomal degradation of IL-2 internalized by IL-2R that were comprised of cytoplasmic tailless beta- or gamma c-subunits, suggesting that the target protein for the proteasome is independent of either the cytoplasmic tail of the IL-2R beta- or gamma c-subunits and their associated signaling components. Therefore, a functional proteasome is required for optimal endocytosis of the IL-2R/ligand complex and is essential for the subsequent lysosomal degradation of IL-2, possibly by regulating trafficking to the lysosome.

A cytosolic protein-tyrosine phosphatase PTP1B specifically dephosphorylates and deactivates prolactin-activated STAT5a and STAT5b

Prolactin (PRL) plays a central and crucial role in the regulation of milk protein gene expression in mammary epithelial cells. PRL binding to its cognate receptor leads to receptor dimerization and activation of the tyrosine kinase Janus kinase 2 (JAK2), associated with the membrane-proximal, intracellular domain of the receptor. In turn, JAK2 phosphorylates and activates STAT5, a member of the signal transducers and activators of transcription (STAT) family. We have recently reported that 16 different protein-tyrosine phosphatases (PTP) were expressed in lactating mouse mammary gland and mammary epithelial cells (Aoki, N., Kawamura, M., Yamaguchi-Aoki, Y., Ohira, S., and Matsuda, T. (1999) J. Biochem. (Tokyo) 125, 669-675). We investigated the involvement of each PTP in PRL signaling. Among the 12 phosphatases including SHP-2 examined, a cytosolic phosphatase PTP1B was found to specifically dephosphorylate STAT5a and STAT5b in transfected COS7 and in vitro. Nuclear translocation of STAT5a and STAT5b was largely inhibited upon overexpression of PTP1B. The PRL-dependent transcriptional activation of the beta-casein gene promoter was also inhibited by PTP1B. Furthermore, retrovirus-mediated overexpression of PTP1B resulted in dephosphorylation of endogenous STAT5 and down-regulation of beta-casein gene expression in mammary epithelial COMMA-1D cells when the cells were treated with lactogenic hormones. Endogenous tyrosine-phosphorylated STAT5 proteins in mammary epithelial COMMA-1D cells as well as tyrosine-phosphorylated STAT5a and STAT5b expressed in COS7 cells were co-precipitated by substrate-trapping mutants of recombinant PTP1B. These results strongly suggest that PTP1B dephosphorylates PRL-activated STAT5a and STAT5b, thereby negatively regulating PRL-mediated signaling pathway.

Janus kinases: components of multiple signaling pathways

Cytoplasmic Janus protein tyrosine kinases (JAKs) are crucial components of diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Evidence to date, indicates that JAK kinase function may integrate components of diverse signaling cascades. While it is likely that activation of STAT proteins may be an important function attributed to the JAK kinases, it is certainly not the only function performed by this key family of cytoplasmic tyrosine kinases. Emerging evidence indicates that phosphorylation of cytokine and growth factor receptors may be the primary functional attribute of JAK kinases. The JAK-triggered receptor phosphorylation can potentially be a rate-limiting event for a successful culmination of downstream signaling events. In support of this hypothesis, it has been found that JAK kinase function is required for optimal activation of the Src-kinase cascade, the Ras-MAP kinase pathway, the PI3K-AKT pathway and STAT signaling following the interaction of cytokine/interferon receptors with their ligands. Aberrations in JAK kinase activity, that may lead to derailment of one or more of the above mentioned pathways could disrupt normal cellular responses and result in disease states. Thus, over-activation of JAK kinases has been implicated in tumorigenesis. In contrast, loss of JAK kinase function has been found to result in disease states such as severe-combined immunodeficiency. In summary, optimal JAK kinase activity is a critical determinant of normal transmission of cytokine and growth factor signals.

Regulation of expression of the rat SOCS-3 gene in hepatocytes by growth hormone, interleukin-6 and glucocorticoids mRNA analysis and promoter characterization

Suppressors of cytokine signalling (SOCS) represent a newly discovered family of molecules that seem to play an important role in the shutting off of cytokine and possibly peptide hormone action. Thus, understanding the mechanisms controlling their expression is of cardinal importance. In the present study, we have cloned the rat SOCS-3 gene and analyzed its expression and the functioning of its promoter in hepatocytes. Expression of SOCS-3 mRNA, which is very weak in freshly isolated cells, tended to increase when hepatocytes were incubated without hormones. Growth hormone (GH) and, to a much larger extent, interleukin-6 (IL-6) rapidly activated mRNA synthesis whereas glucocorticoids (GC) strongly inhibited both basal and hormone-dependent expressions. A short promoter fragment (-137/+35) responded maximally to GH and IL-6 (a threefold stimulation for each effector) and to GC (a 70-80% inhibition), whereas longer promoter sequences supported higher basal activity and lower positive hormonal responses. Deletion and mutation analyses indicated that all hormonal responses were dependent on two cis-acting sequences termed the G-rich and the A/T-rich elements. Only the A/T-rich element was active in a heterologous context, thus behaving as a typical enhancer. Unexpectedly, the two signal transducer and activator of transcription (STAT) binding sites found immediately upstream of the G-rich motif didn't seem to participate in either GH or IL-6 effect, despite the fact that one of them strongly responded to IL-6 when placed in front of a heterologous promoter. Finally, the negative regulation of SOCS-3 promoter by GC that may contribute to gene silencing in vivo, appeared to involve interactions of the GC receptor with other transcription factors and not direct binding to DNA, as no GC-response element was found in the sequence.

IL-1 beta attenuates IFN-alpha beta-induced antiviral activity and STAT1 activation in the liver: involvement of proteasome-dependent pathway

IFN-alphabeta is the only established treatment for viral hepatitis; however, more than 60% of patients are poorly responsive. Because viral hepatitis is associated with inflammation, we hypothesized that inflammation may attenuate the efficacy of IFN therapy. To test this hypothesis, the effect of IL-1beta, one of the major proinflammatory cytokines, on IFN signaling pathway in the liver was examined. Administration of IL-1beta in vivo attenuated IFN-alphabeta-induced STAT1 tyrosine phosphorylation in the liver but not in the spleen. The inhibitory action of IL-1beta in vivo was not affected by depleting hepatic Kupffer cells, suggesting that IL-1beta may directly target IFN-alphabeta signaling in hepatocytes. Indeed, pretreatment of human hepatocellular carcinoma HepG2 cells with IL-1beta suppressed IFN-alphabeta-induced antiviral activity and antiviral protein MxA mRNA expression. Furthermore, IL-1beta attenuated IFN-alphabeta-induced STAT1 binding and tyrosine phosphorylation without affecting the level of STAT1 protein. This inhibitory effect can be reversed by pretreatment with either proteasome inhibitors or transfection of dominant negative NF-kappaB inducing kinase mutants. Taken together, these findings suggest that IL-1beta attenuates IFN-alphabeta-induced STAT1 activation by a proteasome-dependent mechanism. In view of high levels of IL-1beta in the serum or within the liver of patients with chronic liver diseases, attenuation of IFN-alphabeta signaling in the liver by IL-1beta could be one of the mechanisms underlying the resistance to IFN therapy in chronic hepatitis C, and IL-1beta could be a potential therapeutic target for improving the efficacy of IFN therapy.

Nucleocytoplasmic translocation of Stat1 is regulated by a leucine-rich export signal in the coiled-coil domain

Signal transducer and activator of transcription (Stat) proteins are latent transcription factors that reside in the cytoplasm before activation. On cytokine-induced tyrosine phosphorylation, these molecules dimerize and accumulate transiently in the nucleus. No specific signals mediating these processes have been identified to date. In this report, we examine the nuclear export of Stat1. We find that treatment of cells with the export inhibitor leptomycin B does not affect steady-state localization of Stat1 but impedes nuclear export after IFNgamma-induced nuclear accumulation. We identify a conserved leucine-rich helical segment in the coiled-coil domain of Stat1, which is responsible for the efficient nuclear export of this protein. Mutation of two hallmark leucines within this segment greatly attenuate the back transport of Stat1 in the cytoplasm. When fused to a carrier protein, the Stat1 export sequence can mediate nuclear export after intranuclear microinjection. We show that prolonging the nuclear presence of Stat1 by inhibiting nuclear export reduces the transcriptional response to stimulation with IFNgamma. These data suggest that Stats are actively exported from the nucleus via several separate pathways and link this activity to transcriptional activation.

IL-10 attenuates IFN-alpha-activated STAT1 in the liver: involvement of SOCS2 and SOCS3

Interleukin-10 (IL-10) has been used in the treatment of viral hepatitis in interferon-alpha (IFN-alpha) non-responders while patients who have high levels of IL-10 are poorly responsive to IFN-alpha. The mechanism underlying such controversial functions of IL-10 remains unknown. Here we demonstrated that injection of IL-10 into mice attenuated IFN-alpha-induced signal transducer and activator transcription factor (STAT)1 tyrosine phosphorylation in the liver. Reverse transcriptase-polymerase chain reaction assay demonstrated that mouse liver expressed high levels of IL-10 receptor 2 (IL-10R2) but low levels of IL-10R1. Injection of IL-10 into mice activated STAT3 but not STAT1 tyrosine phosphorylation and induced suppressor of cytokine signal 2 (SOCS2), SOCS3, and cytokine-inducible SH2 protein (CIS) mRNA expression in the liver. Furthermore, overexpression of SOCS2 or SOCS3 inhibited IFN-alpha-induced reporter activity in hepatic cells. These findings suggest that IL-10 inhibits IFN-alpha-activated STAT1 in the liver, at least in part, by inducing SOCS2, SOCS3, and CIS expression, which may be responsible for the resistance of IFN-alpha therapy in patients who have high levels of IL-10 and recommends that IL-10 treatment for viral hepatitis should be cautious.

SOCS: physiological suppressors of cytokine signaling

Cytokines regulate cellular behavior by interacting with receptors on the plasma membrane of target cells and activating intracellular signal transduction cascades such as the JAK-STAT pathway. Suppressors of cytokine signaling (SOCS) proteins negatively regulate cytokine signaling. The SOCS family consists of eight proteins: SOCS1-SOCS7 and CIS, each of which contains a central Src-homology 2 (SH2) domain and a C-terminal SOCS box. The expression of CIS, SOCS1, SOCS2 and SOCS3 is induced in response to stimulation by a wide variety of cytokines, and overexpression of these proteins in cell lines results in inhibition of cytokine signaling. Thus, SOCS proteins appear to form part of a classical negative feedback loop. The analysis of mice lacking SOCS1 has revealed that it is critical in the negative regulation of IFN(gamma) signaling and in the differentiation of T cells. Additionally, the analysis of mouse embryos lacking SOCS3 suggests that SOCS3 negatively regulates fetal liver erythropoiesis, probably through its ability to modulate erythropoietin (Epo) signaling. Thus, the use of gene targeting has confirmed that SOCS proteins regulate cytokine signaling in a physiological setting.

STAT-signalling through the cytoplasmic compartment: consideration of a new paradigm

The binding of a large number of cytokines and growth factors to their cognate receptors on the surface of mammalian-cell plasma membrane activates a signalling cascade involving the cytoplasmic STAT-family proteins, which is characterized by the nuclear translocation of a cytokine- or growth factor-specific subset of the cytoplasmic pool of the respective tyrosine- and serine-phosphorylated STAT proteins and the consequent transcriptional activation of specific target genes. In the standard model of cytokine-induced STAT signalling such as that elicited by various interferons and interleukins, it is thought that STAT proteins are recruited to the cytoplasmic side of the cell-surface receptor complex from within a monomeric cytosolic pool, and upon tyrosine-phosphorylation by respective Janus kinase family members, dimerize and translocate to the nucleus. The mechanisms which determine and regulate the recruitment of cytosolic STAT proteins to the plasma membrane-receptor complex, the transit of "activated" STATs through the expanse of the cytoplasmic compartment from the plasma membrane to the nuclear pore region, and the transit of STATs through the nuclear pore complex into the nuclear compartment, remain largely unknown. New data from different laboratories suggests consideration of a model for STAT signalling in which STAT proteins function in the cytoplasm not only as free monomers and dimers but as part of heteromeric complexes ("statosomes"), with accessory proteins which may serve to present specific STATs to the plasma membrane-receptor complex, and to chaperone "activated" STATs through the cytoplasmic compartment toward the nucleus and then into the nuclear compartment.

Proteasomes regulate the duration of erythropoietin receptor activation by controlling down-regulation of cell surface receptors

The binding of erythropoietin (Epo) to its receptor leads to the transient phosphorylation of the Epo receptor (EpoR) and the activation of intracellular signaling pathways. Inactivation mechanisms are simultaneously turned on, and Epo-induced signaling pathways return to nearly basal levels after 30-60 min of stimulation. We show that proteasomes control these inactivation mechanisms. In cells treated with the proteasome inhibitors N-Ac-Leu-Leu-norleucinal (LLnL) or lactacystin, EpoR tyrosine phosphorylation and activation of intracellular signaling pathways (Jak2, STAT5, phosphatidylinositol 3-kinase) were sustained for at least 2 h. We show that this effect was due to the continuous replenishment of the cell surface pool of EpoRs in cells treated with proteasome inhibitors. Proteasome inhibitors did not modify the internalization and degradation of Epo.EpoR complexes, but they allowed the continuous replacement of the internalized receptors by newly synthesized receptors. Proteasome inhibitors did not modify the synthesis of EpoRs, but they allowed their transport to the cell surface. N-Ac-Leu-Leu-norleucinal, but not lactacystin, also inhibited the degradation of internalized Epo.EpoR complexes, most probably through cathepsin inhibition. The internalized EpoRs were not tyrosine-phosphorylated, and they did not activate intracellular signaling pathways. Our results show that the proteasome controls the down-regulation of EpoRs in Epo-stimulated cells by inhibiting the cell surface replacement of internalized EpoRs.

The role of Stat5a and Stat5b in signaling by IL-2 family cytokines

The activation of Stat5 proteins (Stat5a and Stat5b) is one of the earliest signaling events mediated by IL-2 family cytokines, allowing the rapid delivery of signals from the membrane to the nucleus. Among STAT family proteins, Stat5a and Stat5b are the two most closely related STAT proteins. Together with other transcription factors and co-factors, they regulate the expression of the target genes in a cytokine-specific fashion. In addition to their activation by cytokines, activities of Stat5a and Stat5b, as well as other STAT proteins, are negatively controlled by CIS/SOCS/SSI family proteins. The outcome of Stat5 activation in regulating expression of target genes varies, depending upon the complexity of the promoter region of target genes and the other signaling pathways that are activated by each cytokine as well. Here, we mainly focus on the IL2-/IL-2 receptor system, as it is one of the best-studied systems that depend on Stat5-mediated signals. We will summarize what we have learned about the molecular mechanisms of how Stat5 is activated by IL-2 family cytokines from in vitro biochemical studies as well as the role that is played by Stat5 in each of the cytokine signaling pathways from in vivo gene-targeting analyses. Oncogene (2000).

STATs in oncogenesis

Since their discovery as key mediators of cytokine signaling, considerable progress has been made in defining the structure-function relationships of Signal Transducers and Activators of Transcription (STATs). In addition to their central roles in normal cell signaling, recent studies have demonstrated that diverse oncoproteins can activate specific STATs (particularly Stat3 and Stat5) and that constitutively-activated STAT signaling directly contributes to oncogenesis. Furthermore, extensive surveys of primary tumors and cell lines derived from tumors indicate that inappropriate activation of specific STATs occurs with surprisingly high frequency in a wide variety of human cancers. Together, these findings provide compelling evidence that aberrant STAT activation associated with oncogenesis is not merely adventitious but instead contributes to the process of malignant transformation. These studies are beginning to reveal the molecular mechanisms leading to STAT activation in the context of oncogenesis, and candidate genes regulated by STATs that may contribute to oncogenesis are being identified. Recent studies suggest that activated STAT signaling participates in oncogenesis by stimulating cell proliferation and preventing apoptosis. This review presents the evidence for critical roles of STATs in oncogenesis and discusses the potential for development of novel cancer therapies based on mechanistic understanding of STAT signaling. Oncogene (2000).

Induction of apoptosis by extracellular ubiquitin in human hematopoietic cells: possible involvement of STAT3 degradation by proteasome pathway in interleukin 6-dependent hematopoietic cells

The ubiquitin–proteasome pathway is responsible for selective degradation of short-lived cellular proteins and is critical for the regulation of many cellular processes. We previously showed that ubiquitin (Ub) secreted from hairy cell leukemia cells had inhibitory effects on clonogenic growth of normal hematopoietic progenitor cells. In this study, we examined the effects of exogenous Ub on the growth and survival of a series of human hematopoietic cells, including myeloid cell lines (HL-60 and U937), a B-cell line (Daudi), and T-cell lines (KT-3, MT-4, YTC-3, and MOLT-4). Exogenous Ub inhibited the growth of various hematopoietic cell lines tested, especially of KT-3 and HL-60 cells. The growth-suppressive effects of Ub on KT-3 and HL-60 cells were almost completely abrogated by the proteasome inhibitor PSI or MG132, suggesting the involvement of the proteasome pathway in this process. Furthermore, exogenous Ub evoked severe apoptosis of KT-3 and HL-60 cells through the activation of caspase-3. In interleukin-6 (IL-6)-dependent KT-3 cells, STAT3 was found to be conjugated by exogenous biotinylated Ub and to be degraded in a proteasome-dependent manner, whereas expression levels of STAT1, STAT5, or mitogen-activated protein kinase were not affected. Moreover, IL-6-induced the up-regulation of Bcl-2 and c-myc, and JunB was impaired in Ub-treated KT-3 cells, suggesting that the anti-apoptotic and mitogenic effects of IL-6 were disrupted by Ub. These results suggest that extracellular Ub was incorporated into hematopoietic cells and mediated their growth suppression and apoptosis through proteasome-dependent degradation of selective cellular proteins such as STAT3.

Induction of apoptosis by extracellular ubiquitin in human hematopoietic cells: possible involvement of STAT3 degradation by proteasome pathway in interleukin 6-dependent hematopoietic cells

The ubiquitin–proteasome pathway is responsible for selective degradation of short-lived cellular proteins and is critical for the regulation of many cellular processes. We previously showed that ubiquitin (Ub) secreted from hairy cell leukemia cells had inhibitory effects on clonogenic growth of normal hematopoietic progenitor cells. In this study, we examined the effects of exogenous Ub on the growth and survival of a series of human hematopoietic cells, including myeloid cell lines (HL-60 and U937), a B-cell line (Daudi), and T-cell lines (KT-3, MT-4, YTC-3, and MOLT-4). Exogenous Ub inhibited the growth of various hematopoietic cell lines tested, especially of KT-3 and HL-60 cells. The growth-suppressive effects of Ub on KT-3 and HL-60 cells were almost completely abrogated by the proteasome inhibitor PSI or MG132, suggesting the involvement of the proteasome pathway in this process. Furthermore, exogenous Ub evoked severe apoptosis of KT-3 and HL-60 cells through the activation of caspase-3. In interleukin-6 (IL-6)-dependent KT-3 cells, STAT3 was found to be conjugated by exogenous biotinylated Ub and to be degraded in a proteasome-dependent manner, whereas expression levels of STAT1, STAT5, or mitogen-activated protein kinase were not affected. Moreover, IL-6-induced the up-regulation of Bcl-2 and c-myc, and JunB was impaired in Ub-treated KT-3 cells, suggesting that the anti-apoptotic and mitogenic effects of IL-6 were disrupted by Ub. These results suggest that extracellular Ub was incorporated into hematopoietic cells and mediated their growth suppression and apoptosis through proteasome-dependent degradation of selective cellular proteins such as STAT3.

Cytosolic tyrosine dephosphorylation of STAT5. Potential role of SHP-2 in STAT5 regulation

STAT5, a member of the signal transducers and activators of transcription (STATs), is important in modulating T cell functions through interleukin-2 (IL-2) receptors. Like other STAT proteins, STAT5 undergoes a rapid activation and inactivation cycle upon cytokine stimulation. Tyrosine phosphorylation and dephosphorylation are critical in regulating STAT5 activity. A number of protein tyrosine kinases have been shown to phosphorylate STAT5; however, the phosphatases responsible for STAT5 dephosphorylation remain unidentified. Using CTLL-20 as a model system, we provide evidence that tyrosine dephosphorylation of STAT5 subsequent to IL-2-induced phosphorylation occurs in the absence of STAT5 nuclear translocation and new protein synthesis. Nevertheless, down-regulation of the upstream Janus kinase activity during the deactivation cycle of IL-2-induced signaling does involve new protein synthesis. These findings point to the constitutive presence of STAT5 tyrosine phosphatase activity in the cytosolic compartment. We further demonstrate that SHP-2, but not SHP-1, directly dephosphorylates STAT5 in an in vitro tyrosine phosphatase assay with purified proteins. Furthermore, tyrosine-phosphorylated STAT5 associates with the substrate-trapping mutant (Cys --> Ser) of SHP-2 but not SHP-1. These results suggest a potential role for cytoplasmic protein-tyrosine phosphatases in directly dephosphorylating STAT proteins and in maintaining a basal steady state level of STAT activity.

Cross-talk between alpha(1B)-adrenergic receptor (alpha(1B)AR) and interleukin-6 (IL-6) signaling pathways. Activation of alpha(1b)AR inhibits il-6-activated STAT3 in hepatic cells by a p42/44 mitogen-activated protein kinase-dependent mechanism

Treatment of primary rat hepatocytes or tranfected HepG2 cells with the alpha(1B)-adrenergic receptor (alpha(1B)AR) agonist phenylephrine (PE) significantly inhibited interleukin 6 (IL-6)-induced STAT3 binding, tyrosine phosphorylation, and IL-6-induced serum amyloid A mRNA expression. Western analyses and in vitro kinase assays indicate that this inhibition is not due to either down-regulation of STAT3 protein expression nor inactivation of upstream-located JAK1 and JAK2. Blocking the new RNA and protein syntheses antagonized the inhibitory effect of PE on IL-6-activated STAT3, suggesting synthesis of an inhibitory factor(s) is involved. The inhibitory effect of PE on IL-6 activation of STAT3 was also abolished by the tyrosine phosphatase inhibitor sodium vanadate, indicating involvement of protein tyrosine phosphatases. Furthermore, preincubation of the cells with the specific MEK1 inhibitor PD98059 or a dominant negative MEK1 reversed the inhibitory effect of PE, and expression of constitutively activated MEK1 alone abolished IL-6-activated STAT3. Taken together, these data indicate that PE inhibits IL-6 activation of STAT3 in hepatic cells by a p42/44 mitogen-activated protein kinase-dependent mechanism, and tyrosine phosphatases are involved. This inhibitory cross-talk between the alpha(1B)AR and IL-6 signaling pathways implicates the alpha(1B)AR involvement in regulating the IL-6-mediated inflammatory responses.

Negative regulation of cytokine signaling: STAT-induced STAT inhibitor

The growth and differentiation of cells that make up multicellular entities such as the blood and immune systems are under the control of glycoprotein mediators known as cytokines. These cytokines bind to membrane receptors on the cell surface and initiate a signaling cascade that ends with the transcription of specific sets of genes within the cell nucleus. Although knowledge is accumulating concerning the intracellular signal pathways that are activated by cytokines, little is known about inhibition of cytokine signals. This review will focus on the negative regulation of the Janus tyrosine kinase (JAK)/signal transducers and activators of transcription (STAT) signaling pathway by proteins related to STAT-induced STAT inhibitor-1 (SSI-1).

Aryl hydrocarbon receptor imported into the nucleus following ligand binding is rapidly degraded via the cytosplasmic proteasome following nuclear export

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that dimerizes with the AHR nuclear translocator protein to mediate gene regulation. However, the AHR protein is rapidly depleted in vitro and in vivo following exposure to ligands. The purpose of the studies in this report was to characterize the mechanism of AHR degradation and determine the consequence of blocking the degradation process. Western blot and immunological analysis of rat smooth muscle (A7), murine Hepa-1, and human HepG2 cells show that ligand-induced degradation of AHR is blocked when the proteasome is inhibited by MG-132. AHR degradation is also blocked in Hepa-1 and HepG2 cells when nuclear export is inhibited with leptomycin B. Mutation of a putative nuclear export signal present in the AHR results in the accumulation of AHR in the nucleus and reduced levels of degradation following ligand exposure. In addition, inhibition of AHR degradation results in an increase in the concentration of AHR.AHR nuclear translocator complexes associated with DNA and extends the duration that the complex resides in the nucleus. These findings show that nuclear export and degradation of the AHR protein are two additional steps in the AHR-mediated signal transduction pathway and suggest novel areas for regulatory control.

A nuclear protein tyrosine phosphatase is required for the inactivation of Stat1

The Stat1 activation-inactivation cycle involves phosphorylation of Stat1 in the cytoplasm, translocation to the nucleus, and then a return of the protein to the cytoplasm in a dephosphorylated state. However, the intracellular site of Stat1 dephosphorylation has not been determined. As receptor signaling declines, the flow of activated Stat1 molecules should be to the site of their dephosphorylation. We found that upon receptor-Janus kinase inactivation, either gradual or abruptly induced by staurosporine treatment, the flow of Stat1 was from cytoplasm to the nucleus and the nucleus was the final compartment in which phosphorylated Stat1 was detected. N-terminal mutants of Stat1, previously shown to remain phosphorylated for a longer time than wild-type Stat1, were able to enter the nucleus and were not inactivated in the presence of staurosporine, directly demonstrating that these mutations affect phosphatase access and/or activity during the normal dephosphorylation of Stat1. In the presence of sodium vanadate, a phosphatase inhibitor, phosphorylated Stat1 accumulated in the nucleus as the total amount of Stat1 in the cytoplasm declined to low levels. We conclude that the nucleus is the site of Stat1 inactivation and that dephosphorylation is required for the rapid nuclear export of Stat1.

Ethanol rapidly inhibits IL-6-activated STAT3 and C/EBP mRNA expression in freshly isolated rat hepatocytes

The ability of ethanol to inhibit regenerative processes in the liver is thought to play a key role in the development of alcoholic liver disease. To understand the underlying mechanisms, we investigated the effects of ethanol on the Janus kinasesignal transducer and activator transcription factor (JAK-STAT) signaling pathways in hepatocytes. Treatment of freshly isolated adult rat hepatocytes with 10-100 mM ethanol rapidly (< 3 min) inhibits interleukin-6 (IL-6)-induced STAT3 activation, tyrosine and serine phosphorylation and IL-6-induced CCAAT enhancer binding protein (C/EBP) alpha and beta mRNA expression. Western analyses, in vitro kinase assays and in vivo cell labelling assays indicate that this inhibitory effect is not due to blocking the upstream-located JAK1, JAK2 or Tyk2 activation. On the contrary, acute ethanol exposure significantly potentiates IL-6-induced JAK1 autophosphorylation in vitro and in vivo. Pretreatment with sodium vanadate, a non-selective tyrosine phosphatase inhibitor, or with MG132 and lactacystin, proteasome inhibitors, does not abolish the ethanol inhibition of IL-6-induced STAT3 activation, suggesting that activation of protein tyrosine phosphatases or the ubiquitin-proteasome pathway is not involved. In view of the critical role of IL-6 signaling in liver regeneration, these findings suggest that the ability of biologically relevant concentrations of ethanol to markedly inhibit IL-6-induced STAT3 phosphorylation is one of the cellular mechanisms involved in the pathogenesis and progression of alcoholic liver diseases.

Ineffective erythropoiesis in myelodysplastic syndromes: correlation with Fas expression but not with lack of erythropoietin receptor signal transduction

Ineffective erythropoiesis in myelodysplasia is characterized by a defect in erythroid progenitor growth and by abnormal erythroid differentiation. Increased apoptosis of erythroid, granulocytic and megakaryocytic lineages is thought to account for cytopenias. Erythropoietin (Epo)-induced BFU-E and CFU-E growth was studied in 25 myelodysplastic syndrome (MDS) marrow specimens and found to be drastically diminished. To investigate the functionality of Epo-R in MDS marrow, we focused on Epo-induced STAT5 activation. Epo was able to stimulate STAT5 DNA binding activity in all normal and 12/24 MDS marrows tested, with no correlation between the level of STAT5 activation and the development of erythroid colonies in response to Epo. In contrast, impaired proliferation of erythroid progenitors was related to an increased expression of the transmembrane mediator of apoptotic cell death Fas/CD95 on the glycophorin A+ subpopulation. Therefore we conclude that the stimulation of pro-apoptotic signals rather than the defect of anti-apoptotic pathways resulting from Epo-stimulated Jak2-STAT5 pathway, predominantly accounts for ineffective erythropoiesis in myelodysplasia.

SOCS-3 is tyrosine phosphorylated in response to interleukin-2 and suppresses STAT5 phosphorylation and lymphocyte proliferation

Members of the recently discovered SOCS/CIS/SSI family have been proposed as regulators of cytokine signaling, and while targets and mechanisms have been suggested for some family members, the precise role of these proteins remains to be defined. To date no SOCS proteins have been specifically implicated in interleukin-2 (IL-2) signaling in T cells. Here we report SOCS-3 expression in response to IL-2 in both T-cell lines and human peripheral blood lymphocytes. SOCS-3 protein was detectable as early as 30 min following IL-2 stimulation, while CIS was seen only at low levels after 2 h. Unlike CIS, SOCS-3 was rapidly tyrosine phosphorylated in response to IL-2. Tyrosine phosphorylation of SOCS-3 was observed upon coexpression with Jak1 and Jak2 but only weakly with Jak3. In these experiments, SOCS-3 associated with Jak1 and inhibited Jak1 phosphorylation, and this inhibition was markedly enhanced by the presence of IL-2 receptor beta chain (IL-2Rbeta). Moreover, following IL-2 stimulation of T cells, SOCS-3 was able to interact with the IL-2 receptor complex, and in particular tyrosine phosphorylated Jak1 and IL-2Rbeta. Additionally, in lymphocytes expressing SOCS-3 but not CIS, IL-2-induced tyrosine phosphorylation of STAT5b was markedly reduced, while there was only a weak effect on IL-3-mediated STAT5b tyrosine phosphorylation. Finally, proliferation induced by both IL-2- and IL-3 was significantly inhibited in the presence of SOCS-3. The findings suggest that when SOCS-3 is rapidly induced by IL-2 in T cells, it acts to inhibit IL-2 responses in a classical negative feedback loop.

The conserved SOCS box motif in suppressors of cytokine signaling binds to elongins B and C and may couple bound proteins to proteasomal degradation

The suppressors of cytokine signaling (SOCS) family of proteins act as intracellular inhibitors of several cytokine signal transduction pathways. Their expression is induced by cytokine activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway and they act as a negative feedback loop by subsequently inhibiting the JAK/STAT pathway either by direct interaction with activated JAKs or with the receptors. These interactions are mediated at least in part by the SH2 domain of SOCS proteins but these proteins also contain a highly conserved C-terminal homology domain termed the SOCS box. Here we show that the SOCS box mediates interactions with elongins B and C, which in turn may couple SOCS proteins and their substrates to the proteasomal protein degradation pathway. Analogous to the family of F-box-containing proteins, it appears that the SOCS proteins may act as adaptor molecules that target activated cell signaling proteins to the protein degradation pathway.

Negative regulation of the JAK/STAT pathway

Cytokines induce a variety of biological responses by binding to specific cell surface receptors and activating cytoplasmic signal transduction pathways, such as the JAK/STAT pathway. Although these responses are generally transient, few molecules have been characterised that switch the signal off. Several different steps of the signal transduction pathway appear to be targeted by negative regulators, including the receptor/ligand complex, JAK kinases, and STAT transcription factors. Negative regulation is achieved by dephosphorylation of signalling intermediates by protein tyrosine phosphatases such as SHP-1, and by proteolytic degradation. Recent studies have identified two new families of negative regulatory molecules, SOCS and PIAS, which function in novel ways to suppress signal transduction pathways. The duration and intensity of a cell's response to cytokine therefore appear to be determined by the net effect of several regulatory mechanisms.

Innovative treatment programs against cancer: II. Nuclear factor-kappaB (NF-kappaB) as a molecular target

Nuclear factor-kappaB (NF-kappaB) activity affects cell survival and determines the sensitivity of cancer cells to cytotoxic agents as well as to ionizing radiation. Preventing the protective function of NF-kappaB may result in chemo- and radio-sensitization of cancer cells. Therefore, NF-kappaB has emerged as one of the most promising molecular targets in rational drug design efforts of translational cancer research programs.

Stimulation of Stat5 by Granulocyte Colony-Stimulating Factor (G-CSF) Is Modulated by Two Distinct Cytoplasmic Regions of the G-CSF Receptor

In a manner similar to many other cytokines, treatment of cells with granulocyte CSF (G-CSF) has been shown to induce the tyrosine phosphorylation of the STAT proteins. Activation of Stat1 and Stat5 by G-CSF requires the membrane-proximal cytoplasmic domain of the receptor, including box1 and box2, while G-CSF-stimulated tyrosine phosphorylation of Stat3 also requires a region distal to box 2. In this study, we show that although the membrane-proximal 55 amino acids of the G-CSF receptor are sufficient for activation of Stat5, the maximal rate of Stat5 activation requires an additional 30 amino acids of the cytoplasmic domain. In contrast, the distal carboxyl-terminal region of the receptor appears to down-regulate Stat5 activation in that deletion of this carboxyl terminus results in increased amplitude and prolonged duration of Stat5 activation by G-CSF. Significantly, expression of a truncated dominant-negative Stat5 protein in hemopoietic cells not only inhibits G-CSF-dependent cell proliferation, but also suppresses cell survival upon G-CSF withdrawal. We further show that a potential protein tyrosine phosphatase may play a critical role in the down-regulation of G-CSF-stimulated Stat5 activation. These results demonstrate that two distinct cytoplasmic regions of the G-CSF receptor are involved in the regulation of the intensity and duration of Stat5 activation, and that Stat5 may be an important player in G-CSF-mediated cell proliferation and survival.

Activation of the Jak-STAT-signaling pathway in embryonic lens cells

Previous studies showed that lens epithelial cells proliferate rapidly in the embryo and that a lens mitogen, most likely derived from the blood, is present in the anterior chamber of the embryonic eye (Hyatt, G. A., and Beebe, D. C., Development 117, 701-709, 1993). Messenger RNAs for several growth factor receptors have been identified in embryonic lens epithelial cells. We tested several growth factors that are ligands for these receptors for their ability to maintain lens cell proliferation. Embryo serum, PDGF, GM-CSF, and G-CSF maintained lens cell proliferation, but NGF, VEGF, and HGF did not. This and a previous study (Potts, J. D., Harocopos, G. J., and Beebe, D. C., Curr. Eye Res. 12, 759-763, 1993) detected members of the Janus kinase family (Jaks) in the developing lens. Because Jaks are central players in the Jak-STAT-signaling pathway, we identified STAT proteins in the lens and tested whether they were phosphorylated in response to mitogens. STAT1 and STAT3, but not STAT 5 were detected in chicken embryo lens epithelial cells. Only STAT3 was found in terminally differentiated lens fiber cells. STAT1 and STAT3 were phosphorylated in lens cells analyzed immediately after removal from the embryo and when lens epithelial explants were treated with embryo serum, PDGF, or GM-CSF, but not with NGF. Chicken embryo vitreous humor or IGF-1, factors that stimulate lens cell differentiation, but not proliferation, did not cause STAT phosphorylation. When lens epithelial cells were cultured for 4 h in unsupplemented medium, STAT1 and STAT3 declined to nearly undetectable levels. Treatment with PDGF or embryo serum for an additional 15 min restored STAT1 and -3 levels. This recovery was blocked by cycloheximide, but not actinomycin D, suggesting that STAT levels are regulated at the level of translation. STAT levels were maintained in epithelial explants by lens mitogens, but not by factors that stimulated lens fiber differentiation. Both factors that stimulated lens cell proliferation and those that caused fiber differentiation protected cultured lens epithelial cells from apoptosis. These data suggest that the factor(s) responsible for lens cell proliferation in vivo activates the Jak-STAT-signaling pathway. They also indicate that growth factors maintain STAT protein levels in lens epithelial cells by promoting the translation of STAT mRNA, an aspect of STAT regulation that has not been described previously. Signaling by most of the growth factors and cytokines known to activate the Jak-STAT pathway has been disrupted in mice by mutation or targeted deletion. Consideration of the phenotypes of these mice suggests that the factor responsible for lens cell proliferation in vivo may be a growth factor or cytokine that has not yet been described.

Proteasomes regulate erythropoietin receptor and signal transducer and activator of transcription 5 (STAT5) activation. Possible involvement of the ubiquitinated Cis protein

Cis is an Src homology 2 domain-containing protein, which binds to the erythropoietin receptor and decreases erythropoietin-stimulated cell proliferation. We show that Cis associates with the second tyrosine residue of the intracellular domain of the erythropoietin receptor (Tyr401). Two forms of Cis with molecular masses of 32 and 37 kDa were detected, and we demonstrate that the 37-kDa protein resulted from post-translational modifications of the 32-kDa form. Anti-ubiquitin antibodies recognized the 37-kDa form of Cis and the proteasome inhibitors N-acetyl-leucyl-leucyl-norleucinal and lactacystin inhibited its degradation, showing that the 37-kDa form of Cis is a ubiquitinated protein, which seems to be rapidly degraded by the proteasome. In erythropoietin-stimulated UT-7 cells, the activation of the erythropoietin receptor and signal transducer and activator of transcription 5 (STAT5) was transient and returned to basal levels after 30-60 min of erythropoietin stimulation. In contrast, these proteins remained strongly phosphorylated, and STAT5 remained activated for at least 120 min in the presence of proteasome inhibitors. These experiments demonstrate that the proteasomes are involved in the down-regulation of the erythropoietin receptor activation signals. Because the proteasome inhibitors induced the accumulation of both the ubiquitinated form of Cis and the Cis-erythropoietin receptor complexes, our results suggest that the ubiquitinated form of Cis could be involved in the proteasome-mediated inactivation of the erythropoietin receptor.

Regulation of IL-6 Signaling by p53: STAT3- and STAT5-Masking in p53-Val135-Containing Human Hepatoma Hep3B Cell Lines

The influence of p53 on cytokine-triggered Janus kinase-STAT signaling was investigated in human hepatoma Hep3B cell lines engineered to constitutively express the temperature-sensitive Val135 mutant of p53. In comparison to the parental p53-free Hep3B cells, these p53-Val135-containing Hep3B cell lines displayed a reduced response to IL-6 at the wild-type-like p53 temperature (32.5°C). In these cells, IL-6 induced a marked reduction in the immunologic accessibility of cytoplasmic and nuclear STAT3 and STAT5 within 20 to 30 min that lasted 2 to 4 h (STAT-masking) provided that the cells had been previously cultured at 32.5°C for at least 18 to 20 h. The onset of IL-6-induced STAT-masking required protein tyrosine kinase, protein tyrosine phosphatase, proteasomal, phospholipase C, and mitogen-activated protein kinase kinase 1 activities. The maintenance of IL-6-induced STAT-masking was dependent on continued signaling through the phosphatidylinositol-dependent phospholipase C pathway. Despite a reduction in IL-6-induced STAT3 DNA binding activity in the nuclear compartment during STAT-masking, there was increased and prolonged accumulation of tyrosine-phosphorylated STAT3 in both the cytoplasmic and nuclear compartments, indicating that the capacity of tyrosine-phosphorylated STAT3 to bind DNA was reduced during STAT-masking. Thus, IL-6-induced STAT-masking, as dramatically evident on immunomicroscopy, is a visible consequence of a novel cellular process by which a p53-Val135-induced gene product(s) regulates the association of masking protein(s) with and the DNA-binding capacity of STAT3.

Interleukin-3–Induced Activation of the JAK/STAT Pathway Is Prolonged by Proteasome Inhibitors

One facet of cytokine receptor signaling involves the activation of signal transducers and activators of transcription (STATs). STATs are rapidly activated via tyrosine phosphorylation by Janus kinase (JAK) family members and subsequently inactivated within a short period. We investigated the effect of proteasome inhibition on interleukin-3 (IL-3) activation of the JAK/STAT pathway following stimulation of Ba/F3 cells. Treatment of Ba/F3 cells with the proteasome inhibitor,N-acetyl-l-leucinyl-l-leucinyl-norleucinal (LLnL), led to stable tyrosine phosphorylation of the IL-3 receptor, beta common (βc), and STAT5 following stimulation. The effects of LLnL were not restricted to the JAK/STAT pathway, as Shc and mitogen-activated protein kinase (MAPK) phosphorylation were also prolonged in LLnL-treated cells. Further investigation showed these stable phosphorylation events were the result of prolonged activation of JAK2 and JAK1. These observations were confirmed using pharmacologic inhibitors. In the presence of LLnL, stable phosphorylation of STAT5 and βc was abrogated if the tyrosine kinase inhibitor, staurosporine, was added. The effect of staurosporine on STAT5 phosphorylation could be overcome if the phosphatase inhibitor, vanadate, was also added, suggesting phosphorylated STAT5 could be stabilized by phosphatase, but not by proteasome inhibition per se. These observations are consistent with the hypothesis that proteasome-mediated protein degradation can modulate the activity of the JAK/STAT pathway by regulating the deactivation of JAK.

Interleukin-3–Induced Activation of the JAK/STAT Pathway Is Prolonged by Proteasome Inhibitors

One facet of cytokine receptor signaling involves the activation of signal transducers and activators of transcription (STATs). STATs are rapidly activated via tyrosine phosphorylation by Janus kinase (JAK) family members and subsequently inactivated within a short period. We investigated the effect of proteasome inhibition on interleukin-3 (IL-3) activation of the JAK/STAT pathway following stimulation of Ba/F3 cells. Treatment of Ba/F3 cells with the proteasome inhibitor,N-acetyl-l-leucinyl-l-leucinyl-norleucinal (LLnL), led to stable tyrosine phosphorylation of the IL-3 receptor, beta common (βc), and STAT5 following stimulation. The effects of LLnL were not restricted to the JAK/STAT pathway, as Shc and mitogen-activated protein kinase (MAPK) phosphorylation were also prolonged in LLnL-treated cells. Further investigation showed these stable phosphorylation events were the result of prolonged activation of JAK2 and JAK1. These observations were confirmed using pharmacologic inhibitors. In the presence of LLnL, stable phosphorylation of STAT5 and βc was abrogated if the tyrosine kinase inhibitor, staurosporine, was added. The effect of staurosporine on STAT5 phosphorylation could be overcome if the phosphatase inhibitor, vanadate, was also added, suggesting phosphorylated STAT5 could be stabilized by phosphatase, but not by proteasome inhibition per se. These observations are consistent with the hypothesis that proteasome-mediated protein degradation can modulate the activity of the JAK/STAT pathway by regulating the deactivation of JAK.

A Proteasome Inhibitor, an Antioxidant, or a Salicylate, but not a Glucocorticoid, Blocks Constitutive and Cytokine-Inducible Expression of P-Selectin in Human Endothelial Cells

Proteasome inhibitors, antioxidants, salicylates, or glucocorticoids block the cytokine-induced expression of the endothelial cell adhesion molecules E-selectin, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1. These pharmacological agents have been assumed to inhibit the expression of adhesion molecules primarily by blocking activation of the transcription factor NF-κB. We found that the proteasome inhibitor ALLN, the antioxidant PDTC, or sodium salicylate, but not the glucocorticoid dexamethasone, inhibited both the constitutive and the interleukin-4– or oncostatin M–induced expression of the adhesion molecule P-selectin in human endothelial cells. ALLN, PDTC, or sodium salicylate decreased P-selectin expression without a detectable requirement for inhibition of NF-κB activation or for an intact κB element in the P-selectin gene. These results extend the potential anti-inflammatory utility of such drugs to inhibition of P-selectin expression and suggest that they have important actions that do not involve the NF-κB system.

A Proteasome Inhibitor, an Antioxidant, or a Salicylate, but not a Glucocorticoid, Blocks Constitutive and Cytokine-Inducible Expression of P-Selectin in Human Endothelial Cells

Proteasome inhibitors, antioxidants, salicylates, or glucocorticoids block the cytokine-induced expression of the endothelial cell adhesion molecules E-selectin, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1. These pharmacological agents have been assumed to inhibit the expression of adhesion molecules primarily by blocking activation of the transcription factor NF-κB. We found that the proteasome inhibitor ALLN, the antioxidant PDTC, or sodium salicylate, but not the glucocorticoid dexamethasone, inhibited both the constitutive and the interleukin-4– or oncostatin M–induced expression of the adhesion molecule P-selectin in human endothelial cells. ALLN, PDTC, or sodium salicylate decreased P-selectin expression without a detectable requirement for inhibition of NF-κB activation or for an intact κB element in the P-selectin gene. These results extend the potential anti-inflammatory utility of such drugs to inhibition of P-selectin expression and suggest that they have important actions that do not involve the NF-κB system.