Biomedical subjects

A C Larner

Publications and source records attributed to A C Larner.

At least 19 recordsLinked to original sources

PubMed2001

The carboxyl terminus of the granulocyte colony-stimulating factor receptor, truncated in patients with severe congenital neutropenia/acute myeloid leukemia, is required for SH2-containing phosphatase-1 suppression of Stat activation.

The G-CSF receptor transduces signals that regulate the proliferation, differentiation, and survival of myeloid cells. A subgroup of patients with severe congenital neutropenia (SCN) has been shown to harbor mutations in the G-CSF receptor gene that resulted in the truncation of the receptor's carboxyl-terminal region. SCN patients with mutations in the G-CSF receptor gene are predisposed to acute myeloid leukemia. The truncated receptors from SCN/acute myeloid leukemia patients mediate augmented and sustained activation of Stat transcription factors and are accordingly hyperactive in inducing cell proliferation and survival but are defective in inducing differentiation. Little is known about the molecular mechanisms underlying the negative role of the receptor's carboxyl terminus in the regulation of Stat activation and cell proliferation/survival. In this study, we provide evidence that SH2-containing phosphatase-1 (SHP-1) plays a negative regulatory role in G-CSF-induced Stat activation. We also demonstrate that the carboxyl terminus of the G-CSF receptor is required for SHP-1 down-regulation of Stat activation induced by G-CSF. Our results indicate further that this regulation is highly specific because SHP-1 has no effect on the activation of Akt and extracellular signal-related kinase1/2 by G-CSF. The data together strongly suggest that SHP-1 may represent an important mechanism by which the carboxyl terminus of the G-CSF receptor down-regulates G-CSF-induced Stat activation and thereby inhibits cell proliferation and survival in response to G-CSF.

Animals↗

PubMed2001

Vanadate facilitates interferon alpha-mediated apoptosis that is dependent on the Jak/Stat pathway.

Type I interferon (IFN)-dependent inhibition of cell growth can occur either in the absence or presence of apoptosis. The mechanisms that determine whether or not cells undergo apoptosis after exposure to IFN-alpha are not clear. This study shows that a variety of cell lines that display growth inhibition but not apoptosis in response to IFN-alpha will undergo programmed cell death when low concentrations of the protein-tyrosine phosphatase inhibitor vanadate are added with IFN-alpha. In contrast, the combination of tumor necrosis factor-alpha with vanadate did not trigger apoptosis in these cells. Caspase-3 activity was detected only in cells exposed to IFN-alpha and vanadate but not to IFN-alpha or vanadate alone. The ability of IFN-alpha and vanadate to induce apoptosis did not require expression of p53 and was blocked by N-acetyl-l-cysteine. Activation of the Jak/Stat pathway and expression of IFN-inducible genes was not altered by incubation of cells with IFN-alpha and vanadate compared with IFN-alpha alone. However, mutant cells lacking Stat1, Stat2, Jak1, or Tyk2, or cells expressing kinase inactive Jak1 or Tyk2 did not undergo apoptosis in the presence of IFN-alpha and vanadate. These results suggest that IFN-alpha stimulation of Stat-dependent genes is necessary, but not sufficient, for this cytokine to induce apoptosis. Another signaling cascade that involves the activity of a protein-tyrosine phosphatase and/or the generation of reactive oxygen species may play an important role in promoting IFN-alpha-induced apoptosis.

Acetylcysteine↗

PubMed2001

Granulocyte colony-stimulating factor induces ERK5 activation, which is differentially regulated by protein-tyrosine kinases and protein kinase C. Regulation of cell proliferation and survival.

Granulocyte colony-stimulating factor (G-CSF) plays a major role in the regulation of granulopoiesis. Treatment of cells with G-CSF has been shown to activate multiple signal transduction pathways. We show here that Erk5, a novel member of the MAPK family, and its specific upstream activator MEK5 were activated in response to incubation of cells with G-CSF. Different from other members of the MAPK family including Erk1/2, JNK, and p38, maximal activation of Erk5 by G-CSF required the C-terminal region of the G-CSF receptor. Genistein, a specific inhibitor of protein-tyrosine kinases, blocked G-CSF-induced Erk5 activation. In contrast, inhibition of protein kinase C activity increased G-CSF-mediated activation of Erk5 and MEK5, whereas stimulation of protein kinase C activity inhibited activation of the two kinases by G-CSF. The proliferation of BAF3 cells in response to G-CSF was inhibited by expression of a dominant-negative MEK5 but potentiated by expression of a constitutively active MEK5. Expression of the constitutively active MEK5 also increased the survival of BAF3 cells cultured in the absence of or in low concentrations of G-CSF. Together, these data implicate Erk5 as an important signaling component in the biological actions of G-CSF.

Animals↗

PubMed2000

Signaling via the T cell antigen receptor induces phosphorylation of Stat1 on serine 727.

The Stat1 transcription factor plays a pivotal role in both, the antiviral and antigrowth actions of interferons. Stat1 acquires the ability to bind DNA by becoming phosphorylated on Tyr(701). However, to effectively stimulate gene transcription, it must also be phosphorylated on Ser(727). We show that engagement of T cell antigen receptor (TCR)/CD3 complex in either Jurkat cells or peripheral blood lymphocytes stimulates phosphorylation of Ser(727) but not Tyr(701) of Stat1. This process does not require the expression of tyrosine kinases Lck and Zap-70. Interestingly, pretreatment of T cells with the Src kinase inhibitor PP1 completely abrogated CD3-mediated serine phosphorylation of Stat1, whereas inhibitors to MEK1 and phosphatidylinositol 3-kinase had no effect. Phosphorylation of Ser(727) of Stat1 in T cells is not restricted to TCR/CD3 but also results when cells are stimulated via the costimulatory molecule CD28. The combination of CD3 and CD28 did not augment phosphorylation of Stat1 Ser(727). Surprisingly, Stat1-mediated transcriptional activity in response to IFN-alpha was enhanced with CD3 stimulation, whereas CD3 alone had little effect. These findings suggest that Stat1 is a signaling molecule in TCR signaling and may play a role in T cell function.

CD28 Antigens↗

PubMed2000

Overlapping and specific functions of Braf and Craf-1 proto-oncogenes during mouse embryogenesis.

The three mammalian Raf serine/threonine protein kinases mediate the transduction of proliferative and differentiative signals from cell surface receptors to the nucleus. In vertebrates, Raf signaling has been implicated in the progression of mouse embryos through the two-cell stage and in the induction of posterior mesoderm. However, mouse embryos mutant for each of the Raf genes exhibit no developmental defects before mid-gestation. Here we describe the phenotype of mouse mutants with different combinations of mutant Craf-1 and Braf alleles. Our results show that Raf signaling is indeed indispensable for normal development beyond the blastocyst stage. However, due to a significant redundancy between Craf-1 and Braf, either gene is sufficient for normal development until mid-gestation. The molecular and developmental mechanisms for this redundancy were investigated by monitoring the expression of Raf genes throughout embryogenesis and by biochemical studies in mutant cell lines.

Animals↗

PubMed2000

Activation of Akt kinase by granulocyte colony-stimulating factor (G-CSF): evidence for the role of a tyrosine kinase activity distinct from the Janus kinases.

Activation of the serine/threonine kinase Akt has been shown to be a critical component for growth factor and cytokine stimulation of cell survival. Although some of the immediate upstream activators of Akt have been defined, the roles of tyrosine kinases in the activation of Akt are not well delineated. Granulocyte colony-stimulating factor (G-CSF) regulates the proliferation, differentiation, and survival of neutrophilic granulocytes. G-CSF exerts its actions by stimulating several signaling cascades after binding its cell surface receptor. Both Jak (Janus) and Src families of tyrosine kinases are stimulated by incubation of cells with G-CSF. In this report, we show that G-CSF stimulation of cells leads to activation of Akt. The membrane-proximal 55 amino acids of the G-CSF receptor cytoplasmic domain are sufficient for mediating Akt activation. However, activation of Akt appears to be downregulated by the receptor's carboxy-terminal region of 98 amino acids, a region that has been shown to be truncated in some patients with acute myeloid leukemia associated with severe congenital neutropenia. Furthermore, we demonstrate that G-CSF-induced activation of Akt requires the activities of Src family kinases but can be clearly dissociated from G-CSF-stimulated activation of Stats (signal transducers and activators of transcription) by the Jak kinases. Thus, cytokine activation of the Jak/Stat and other signaling cascades can be functionally separated. (Blood. 2000;95:1656-1662)

Androstadienes↗

PubMed1999

A lymphoma growth inhibitor blocks some but not all prolactin-stimulated signaling pathways.

Cytokines and hormones activate a network of intracellular signaling pathways to regulate cell division, survival and differentiation. In parallel, a series of growth inhibitory mechanisms critically restrict cell population sizes. For example, mitogens can be opposed in crowded cell cultures through contact-inhibition or by autocrine release of antiproliferative substances. Here, we characterize a small, heat-stable growth inhibitor secreted by a rat T lymphoma line when cultured at high cell density. Short term incubation (<60 min) of prolactin-responsive Nb2 lymphoma cells at high density selectively blocked prolactin stimulation of p42/p44 mitogen-activated protein kinases and transcription factors Stat1 and Stat3 but not prolactin activation of Stat5 or the tyrosine kinase Jak2. The selective effects of cell density on prolactin signaling were reversible. Furthermore, exposure of cells at low density to conditioned media from cells incubated at high density had the same inhibitory effects on prolactin signaling. This selective inhibition of discrete prolactin signals was mimicked by short term preincubation of cells at low density with staurosporine or genistein but not with bis-indoleyl maleimide, cyclic nucleotide analogs, calcium ionophore A23187, or phorbol 12-myristate 13-acetate. A heat-stable, proteinase K-resistant, low molecular weight factor with these characteristics was recovered from high density culture medium. The partially purified inhibitor suppressed Nb2 cell growth with a sigmoidal concentration response consistent with a saturable, receptor-mediated process.

Animals↗

PubMed1999

Interleukin-10 inhibits expression of both interferon alpha- and interferon gamma- induced genes by suppressing tyrosine phosphorylation of STAT1.

Interleukin-10 (IL-10) helps maintain polarized T-helper cells in a T-helper lymphocyte 2 (Th2) phenotype. Part of this process involves the prevention of the development of Th1 cells, which are a primary source of interferon gamma (IFNgamma), a potent activator of monocytes and an inhibitor of Th2 proliferation. Because monocytes and macrophages are important mediators of Th1-type responses, such as delayed-type hypersensitivity, we sought to determine if IL-10 could directly mediate inhibition of IFNgamma- and IFNalpha-induced gene expression in these cells. Highly purified monocytes were incubated with IL-10 for 60 to 90 minutes before the addition of IFNgamma or IFNalpha. IL-10 preincubation resulted in the inhibition of gene expression for several IFN-induced genes, such as IP-10, ISG54, and intercellular adhesion molecule-1. The reduction in gene expression resulted from the ability of IL-10 to suppress IFN-induced assembly of signal transducer and activator of transcription (STAT) factors to specific promoter motifs on IFNalpha- and IFNgamma-inducible genes. This was accomplished by preventing the IFN-induced tyrosine phosphorylation of STAT1, a component of both IFNalpha- and IFNgamma-induced DNA binding complexes. Therefore, IL-10 can directly inhibit STAT-dependent early response gene expression induced by both IFNalpha and IFNgamma in monocytes by suppressing the tyrosine phosphorylation of STAT1. This may occur through the ability of IL-10 to induce expression of the gene, suppressor of cytokine signaling 3 (SOCS3).

Cells, Cultured↗

PubMed1998

Activation of Raf-1 by interferon gamma and oncostatin M requires expression of the Stat1 transcription factor.

A primary signaling cascade responsible for the expression of cytokine-stimulated immediate early genes involves the activation of the Jak/Stat pathway. In addition to being tyrosine-phosphorylated, several signal transducers and activators of transcription (Stats), including Stat1alpha, Stat3, and Stat4, are phosphorylated on a conserved serine residue, which is a consensus phosphorylation site for mitogen-activated protein kinases (MAPKs). Serine phosphorylation of Stat1alpha is required for maximal transcriptional activation of early response genes by interferon gamma (IFNgamma) as well as the antiviral and antigrowth actions of this cytokine. Incubation of cells with either IFNgamma or oncostatin M (OSM) activates Raf-1, a serine/threonine kinase responsible for the ultimate activation of p42 MAPK. To examine whether any of the signaling components that are required for activation of the Jak/Stat pathway are also necessary for activation of Raf-1 by IFNs and OSM, we examined activation of Raf-1 in cell lines that are deficient in either Stat1alpha or Stat2. Unexpectedly, incubation of Stat1-deficient, but not Stat2-deficient cells with IFNgamma or OSM for 5 min displayed no increase in Raf-1 activity. In peripheral blood lymphocytes Raf-1 was associated with Stat1, and this interaction was disrupted after incubation of cells with IFNgamma. Stat1-negative cells reconstituted with either Stat1alpha or Stat1alpha with a point mutation in the site where it is serine-phosphorylated displayed normal activation of Raf-1 by IFNgamma and OSM. However, activation of Raf-1 was not observed in lines that expressed Stat1alpha containing a mutation in its tyrosine phosphorylation site or in its SH2 domain. These results provide the first example of a novel role of Stat1alpha not as a transcription factor, but as a protein which may function to scaffold signaling components required for activation of the distinct Raf/MEK/MAPK signaling cascade.

Animals↗

PubMed1998

Control of lytic function by mitogen-activated protein kinase/extracellular regulatory kinase 2 (ERK2) in a human natural killer cell line: identification of perforin and granzyme B mobilization by functional ERK2.

The signal pathways that control effector function in human natural killer (NK) cells are little known. In this study, we have identified the critical role of the mitogen-activated protein kinase (MAPK) pathway in NK lysis of tumor cells, and this pathway may involve the mobilization of granule components in NK cells upon interaction with sensitive tumor target cells. Evidence was provided by biological, biochemical, and gene transfection methods. NK cell binding to tumor cells for 5 min was sufficient to maximally activate MAPK/extracellular signal-regulatory kinase 2 (ERK2), demonstrated by its tyrosine phosphorylation and by its ability to function as an efficient kinase for myelin basic protein. MAPK activation was achieved in NK cells only after contact with NK-sensitive but not NK-resistant target cells. In immunocytochemical studies, cytoplasmic perforin and granzyme B were both maximally redirected towards the tumor contact zone within 5 min of NK cell contact with tumor cells. A specific MAPK pathway inhibitor, PD098059, could block not only MAPK activation but also redistribution of perforin/granzyme B in NK cells, which occur upon target ligation. PD098059 also interfered with NK lysis of tumor cells in a 5-h 51Cr-release assay, but had no ability to block NK cell proliferation. Transient transfection studies with wild-type and dominant-negative MAPK/ERK2 genes confirmed the importance of MAPK in NK cell lysis. These results document a pivotal role of MAPK in NK effector function, possibly by its control of movement of lytic granules, and clearly define MAPK involvement in a functional pathway unlinked to cell growth or differentiation.

Calcium-Calmodulin-Dependent Protein Kinases↗

PubMed1998

Involvement of SHP-2 in multiple aspects of IL-2 signaling: evidence for a positive regulatory role.

Binding of IL-2 to its receptor activates several biochemical pathways, but precisely how these pathways are linked is incompletely understood. Here, we report that SHP-2, an SH2-domain containing tyrosine phosphatase, associates with different molecules of the IL-2 signaling cascade. Upon IL-2 stimulation, SHP-2 was coimmunoprecipitated with Grb2 and the p85 subunit of phosphatidylinositol 3-kinase. In contrast, SHP-2 was constitutively associated with JAK1 and JAK3. Finally, SHP-2 expression amplified STAT-dependent transcriptional activation whereas a dominant negative allele inhibited transactivation and the IL-2-induced activation of MAPK (mitogen-activated protein kinase). These results demonstrate the involvement of SHP-2 in multiple pathways of the IL-2 signaling cascade and provide evidence for its positive regulatory role.

Adaptor Proteins, Signal Transducing↗

PubMed1998

Interferon gamma activation of Raf-1 is Jak1-dependent and p21ras-independent.

Signal transduction through the interferongamma (IFNgamma) receptor involves the formation of a ligand-dependent multimolecular association of receptor chains (alpha and beta), Janus tyrosine kinases (Jak1 and Jak2), and the transcription factor (signal transducers and activators of transcription 1alpha (STAT1alpha)) in addition to activation of mitogen-activated protein kinases (MAPK). Interactions between components of the Jak/STAT cascade and the p21(ras)/Raf-1/MAPK cascade are unexplored. Treatment of HeLa cells with IFNgamma resulted in the rapid and transient activation of Raf-1 and MAPK. Parallel activation of cells resulted in essentially no enhancement of p21(ras) activation despite marked enhancement after treatment with epidermal growth factor. In HeLa (E1C3) and fibrosarcoma (U4A) cell lines, both of which are deficient in Jak1 kinase, Raf-1 activation by IFNgamma was absent. Reconstitution of Raf-1 activity was observed only with kinase active Jak1 in both cell lines. In COS cells, transient expression of wild type or kinase-inactive Jak1 coimmunoprecipitated with Raf-1, but activation of Raf-1 activity was only observed in cells expressing kinase-active Jak1. These observations suggest that a kinase-active Jak1 is required for IFNgamma activation of Raf-1 that is p21(ras)-independent.

Calcium-Calmodulin-Dependent Protein Kinases↗

PubMed1998

Craf-1 protein kinase is essential for mouse development.

The three mammalian Raf serine/threonine protein kinases mediate the transduction of proliferative and differentiative signals from a variety of cell surface receptors to the nucleus. We report here that Craf-1 is essential for mouse development, as its mutation results in embryonic lethality. Developmental defects are found in mutant placentas as well as in the skin and in the lungs of mutant embryos. Craf-1 mutants also display a generalized growth retardation which is consistent with the ubiquitous expression of Craf-1 and which could be due to the reduced proliferation of mutant cells. Interestingly, the time-point of embryonal death varies depending on the genetic background. This suggests that Craf-1-mediated signaling is affected by genetic background-specific alleles of other genes.

Abnormalities, Multiple↗

PubMed1997

Antiproliferative action of interferon-alpha requires components of T-cell-receptor signalling.

Signal transduction through both cytokine and lymphocyte antigen receptors shares some common pathways by which they initiate cellular responses, such as activation of mitogen-activated protein kinase(s). However, other signalling components appear to be uniquely coupled to each receptor. For example, the interferon receptors transduce regulatory signals through the JAK/STAT pathway, resulting in an inhibition of growth and of antiviral effects, whereas this pathway apparently plays no role in T-cell-receptor (TCR)-dependent gene expression. Conversely, signal transduction through the TCR requires the tyrosine kinases Lck and ZAP-70 and the tyrosine phosphatase CD45. Here we show that, unexpectedly, transmission of growth-inhibitory signals by interferon-alpha (IFN-alpha) in T cells requires the expression and association of CD45, Lck and ZAP-70 with the IFN-alpha-receptor signalling complex.

Animals↗

PubMed1997

Prolactin stimulates serine/tyrosine phosphorylation and formation of heterocomplexes of multiple Stat5 isoforms in Nb2 lymphocytes.

Transcription factors of the Stat gene family are selectively activated by many hormones and cytokines. Stat5 originally was cloned as a prolactin-stimulated DNA-binding protein, but is also activated by non-lactogenic cytokines in many cell types. The recent identification of two distinct Stat5 genes, which encode a 94-kDa Stat5a and a 92-kDa Stat5b as well as several lower molecular weight isoforms, suggests additional complexity and combinatorial possibilities for transcriptional regulation. We now report a biochemical analysis of prolactin activation of Stat proteins in Nb2 lymphocytes, which was associated with: 1) rapid tyrosine phosphorylation of Stat5a, Stat5b, a COOH-terminally truncated 80-kDa Stat5 form, Stat1alpha, and Stat3; 2) rapid and selective formation of Stat5a/b heterodimers, without involvement of Stat1alpha or Stat3; 3) marked serine, but not threonine phosphorylation of Stat5a and Stat5b; and 4) the appearance of two qualitatively distinct Stat5 protein complexes, which discriminated between oligonucleotides corresponding to the prolactin response elements of the beta-casein and interferon regulatory factor-1 gene promoters. Collectively, our analyses showed that Stat5a and Stat5b respond similarly to prolactin receptor activation, but also suggested that the two genes have evolved unique properties that may contribute to the specificity of receptors that utilize Stat5 signaling proteins.

Cell Line↗

PubMed1997

Mapping of a cytoplasmic domain of the human growth hormone receptor that regulates rates of inactivation of Jak2 and Stat proteins.

It has been previously demonstrated that growth hormone (GH)-stimulated tyrosine phosphorylation of Jak2 and Stat5a and Stat5b occurs in FDP-C1 cells expressing either the entire GH receptor or truncations of the cytoplasmic domain expressing only the membrane-proximal 80 amino acids. However, other receptor domains that might modulate rates of GH activation and inactivation of this cascade have not been examined. Here we have defined a region in the human GH receptor between amino acids 520 and 540 in the cytoplasmic domain that is required for attenuation of GH-activated Jak/Stat signaling. Immunoprecipitations with antibodies to Jak2 indicate that the protein tyrosine phosphatase SHP-1 is associated with this kinase in cells exposed to GH. To address the possibility that SHP-1 could function as a negative regulator of GH signaling, liver extracts from motheaten mice deficient in SHP-1 or unaffected littermates were analyzed for activation of Stats and Jak2. Extracts from motheaten mice displayed prolonged activation of the Stat proteins as measured by their ability to interact with DNA and prolonged tyrosine phosphorylation of Jak2. These results delineate a novel domain in the GH receptor that regulates the inactivation of the Jak/Stat pathway and appears to be modulated by SHP-1.

Animals↗

PubMed1997

Phosphorylation events modulate the ability of interferon consensus sequence binding protein to interact with interferon regulatory factors and to bind DNA.

Two families of transcription factors mediate interferon (IFN) signaling. The first family, signal transducers and activators of transcription (STATs), is activated within minutes of IFN treatment. Specific phosphorylation events lead to their translocation to the nucleus, formation of transcriptional complexes, and the induction of the second family of transcription factors termed interferon regulatory factors (IRFs). Interferon consensus sequence binding protein (ICSBP) is a member of IRF family that is expressed only in cells of the immune system and acts as a transcriptional repressor. ICSBP binds DNA through the association with other transcription factors such as IRF-1 or IRF-2. In this communication, the domain that is involved in protein-protein interactions was mapped to the carboxyl terminus of ICSBP. This domain is also important for mediating ICSBP-repressing activity. In vitro studies demonstrated that direct binding of ICSBP to DNA is prevented by tyrosine (Tyr) phosphorylation. Yet, Tyr-phosphorylated ICSBP can bind target DNA only through the association with IRF-2 and IRF-1. This type of phosphorylation is essential for the formation of heterocomplexes. Tyr-phosphorylated ICSBP and IRF-2 are detected in expressing cells constitutively, and Tyr-phosphorylated IRF-1 is induced by IFN-gamma. These results strongly suggest that like the STATs, the IRFs are also modulated by Tyr phosphorylation that affects their biological activities.

Amino Acid Sequence↗

PubMed1997

Beta interferon and oncostatin M activate Raf-1 and mitogen-activated protein kinase through a JAK1-dependent pathway.

Activation of early response genes by interferons (IFNs) and other cytokines requires tyrosine phosphorylation of a family of transcription factors termed signal transducers and activators of transcription (Stats). The Janus family of tyrosine kinases (Jak1, Jak2, Jak3, and Tyk2) is required for cytokine-induced tyrosine phosphorylation and dimerization of the Stat proteins. In order for IFNs to stimulate maximal expression of Stat1alpha-regulated genes, phosphorylation of a serine residue in the carboxy terminus by mitogen-activated protein kinase (MAPK) is also required. In HeLa cells, both IFN-beta and oncostatin M (OSM) stimulated MAPK and Raf-1 enzyme activity, in addition to Stat1 and Stat3 tyrosine phosphorylation. OSM stimulation of Raf-1 correlated with GTP loading of Ras, whereas IFN-beta activation of Raf-1 was Ras independent. IFN-beta- and OSM-induced Raf-1 activity could be coimmunoprecipitated with either Jak1 or Tyk2. Furthermore, HeLa cells lacking Jak1 displayed no activation of STAT1alpha, STAT3, and Raf-1 by IFN-beta or OSM and also demonstrated no increase in the relative level of GTP-bound p21ras in response to OSM. The requirement for Jak1 for IFN-beta- and OSM-induced activation of Raf-1 was also seen in Jak1-deficient U4A fibrosarcoma cells. Interestingly, basal MAPK, but not Raf-1, activity was constitutively enhanced in Jak1-deficient HeLa cells. Transient expression of Jak1 in both Jak-deficient HeLa cells and U4A cells reconstituted the ability of IFN-beta and OSM to activate Raf-1 and decreased the basal activity of MAPK, while expression of a kinase-inactive form of the protein showed no effect. Moreover, U4A cells selected for stable expression of Jak1, or COS cells transiently expressing Jak1 or Tyk2 but not Jak3, exhibited enhanced Raf-1 activity. Therefore, it appears that Jak1 is required for Raf-1 activation by both IFN-beta and OSM. These results provide evidence for a link between the Jaks and the Raf/MAPK signaling pathways.

Animals↗