(1/832) IL-4 preferentially activates a novel STAT6 isoform in mast cells.
IL-4 is a pleiotropic cytokine that signals through STAT6 to direct the transactivation of multiple gene targets. In this study, we demonstrate that mast cells express a distinct STAT6 isoform. This "mast cell STAT" is a product of the STAT6 gene, but is only 65 kDa in size and appears to lack the defined C-terminal transactivation domain. Despite the presence of the conventional 94-kDa STAT6 molecule, it is the smaller isoform that associates with a consensus STAT6 binding site in extracts from IL-4-treated mast cells. This is the first evidence that STAT6 isoforms can be preferentially activated and bind to DNA in a cell-specific manner. These results imply that an additional level of specificity in the IL-4R signaling mechanism exists and may partially explain the diverse effects that IL-4 exerts on different cell types. (+info)
(2/832) Inhibition of IL-4-inducible gene expression in human monocytes by type I and type II interferons.
The Th2-type cytokines, interleukin-4 (IL-4) and interleukin-13 (IL-13), induce expression of a distinct subset of genes in human monocytes, including FcepsilonRIIb (CD23), 15-lipoxygenase, IL-1 receptor antagonist (IL-1ra), and type I and type II IL-1 receptors (IL-1R). Type I interferons (IFN-alpha and IFN-beta) and type II interferon (IFN-gamma) inhibit induction of these genes by IL-4 and IL-13. However, the mechanism by which IFNs mediate this inhibition has not been defined. In this overview, we discuss the role of the transcription factor, STAT6 (signal transducer and activator of transcription-6) in mediating IL-4- and IL-13-induced gene expression in monocytes. We also discuss our recent findings that type I and type II IFNs suppress IL-4/IL-13-inducible gene expression by inhibiting tyrosine phosphorylation and nuclear translocation of STAT6. The ability of type I and type II IFNs to inhibit IL-4/IL-13-induced STAT6 activity is dose- and time-dependent, and is not unique to monocytes because IFNs induce the same effects in fibroblasts. Inhibition of STAT6 activity is not evident unless cells are preincubated with IFN for at least 1 h before IL-4 stimulation. Furthermore, inhibition can be blocked by actinomycin D, indicating a requirement for de novo transcription. We propose a model in which stimulation of monocytes by IFN activates de novo synthesis of an inhibitory factor, possibly one or more members of the SOCS/ SSI/CIS gene family, capable of suppressing activation of STAT6 by IL-4 and IL-13. Because STAT6 activation plays an essential role in IL-4/IL-13-induced gene expression, the ability of IFN-beta and IFN-gamma to inhibit STAT6 activity provides an explanation for how IFNs can suppress IL-4/IL-13-inducible gene expression. (+info)
(3/832) Cutting edge: SOCS-1 is a potent inhibitor of IL-4 signal transduction.
IL-4 is an important regulator of the activation, proliferation, and differentiation of many hematopoetic cells. Many of these biological effects result from the activation of Janus kinases (JAK)1 and JAK3 and the transcription factor Stat6. Recent data suggest that members of the SOCS (suppressor of cytokine signaling) family of proteins can inhibit JAK-STAT signaling. We have examined the ability of SOCS family members to suppress IL-4 signaling, and we have found that SOCS-1 potently inhibits the activation of JAK1 kinase and Stat6 in response to IL-4. Furthermore, SOCS-1 can inhibit the induction of CD23 expression by IL-4. SOCS-2 does not inhibit induction of signaling by IL-4, while inhibition of IL-4 signaling by SOCS-3 can be detected in transient transfection systems, but not in stable cell lines. These studies implicate SOCS-1 in modulation of IL-4 signaling and suggest that SOCS-1 may play a role in regulating the immune response. (+info)
(4/832) Repression of IL-4-induced gene expression by IFN-gamma requires Stat1 activation.
IFN-gamma antagonizes many physiological responses mediated by IL-4, including the inhibition of IL-4-induced IgE production. This event is largely mediated at the level of transcription. We observed that the IL-4 response element of the germline epsilon promoter is sufficient to confer IFN-gamma-mediated repression onto a reporter construct. The inhibitory effects were observed in both lymphoid and nonlymphoid cell lines. Stat1, which is activated by IFN-gamma, cannot recognize the Stat6-specific IL-4 response element in the epsilon promoter. Hence, competitive DNA binding does not seem to be the underlying mechanism for the inhibitory effect. This is supported by the observation that inhibition is not seen at early time points, but requires prolonged IFN-gamma treatment. IFN-gamma stimulation results in a loss of IL-4-induced Stat6 tyrosine phosphorylation, nuclear translocation, and DNA binding. Using the fibrosarcoma cell line U3A, which lacks Stat1, we demonstrated that the transcription activation function of Stat1 is required for the IFN-gamma-mediated repression. Repression was restored by overexpression of Stat1alpha, but not Stat1beta, in U3A cells. Treatment with IFN-gamma, but not IL-4, specifically up-regulates the expression of SOCS-1 (silencer of cytokine signaling), a recently characterized inhibitor of cytokine signaling pathways, such as IL-6 and IFN-gamma. Overexpression of SOCS-1 effectively blocks IL-4-induced Stat6 phosphorylation and transcription. This suggests that IFN-gamma-mediated repression of IL-4-induced transcription is at least in part mediated by SOCS-1. (+info)
(5/832) Cutting edge: effects of an allergy-associated mutation in the human IL-4R alpha (Q576R) on human IL-4-induced signal transduction.
A mutation in the human (hu) IL-4R alpha, Q576R, has been linked with allergy in humans. Increased sensitivity of patients cells with this mutation to IL-4 suggest that a Q576R change enhances IL-4 signaling. To directly test this hypothesis, we analyzed the ability of huIL-4R alpha cDNA bearing the Q576R and Y575F mutations to signal tyrosine phosphorylation, DNA-binding activity, proliferation, protection from apoptosis, and CD23 induction in response to huIL-4 in murine cells. Responses generated by the Q576R and Y575F mutants were similar to those of the wild-type receptor, using various concentrations of huIL-4 and times of stimulation. These results indicate that neither the Q576R nor the Y575F mutations have a significant direct effect on IL-4 signal transduction, and that hypersensitive induction of CD23 in cells derived from human allergy patients may be due to different and/or additional alterations in the IL-4 signaling pathway. (+info)
(6/832) IL-3 and IL-4 activate cyclic nucleotide phosphodiesterases 3 (PDE3) and 4 (PDE4) by different mechanisms in FDCP2 myeloid cells.
In FDCP2 myeloid cells, IL-4 activated cyclic nucleotide phosphodiesterases PDE3 and PDE4, whereas IL-3, granulocyte-macrophage CSF (GM-CSF), and phorbol ester (PMA) selectively activated PDE4. IL-4 (not IL-3 or GM-CSF) induced tyrosine phosphorylation of insulin-receptor substrate-2 (IRS-2) and its association with phosphatidylinositol 3-kinase (PI3-K). TNF-alpha, AG-490 (Janus kinase inhibitor), and wortmannin (PI3-K inhibitor) inhibited activation of PDE3 and PDE4 by IL-4. TNF-alpha also blocked IL-4-induced tyrosine phosphorylation of IRS-2, but not of STAT6. AG-490 and wortmannin, not TNF-alpha, inhibited activation of PDE4 by IL-3. These results suggested that IL-4-induced activation of PDE3 and PDE4 was downstream of IRS-2/PI3-K, not STAT6, and that inhibition of tyrosine phosphorylation of IRS molecules might be one mechnism whereby TNF-alpha could selectively regulate activities of cytokines that utilized IRS proteins as signal transducers. RO31-7549 (protein kinase C (PKC) inhibitor) inhibited activation of PDE4 by PMA. IL-4, IL-3, and GM-CSF activated mitogen-activated protein (MAP) kinase and protein kinase B via PI3-K signals; PMA activated only MAP kinase via PKC signals. The MAP kinase kinase (MEK-1) inhibitor PD98059 inhibited IL-4-, IL-3-, and PMA-induced activation of MAP kinase and PDE4, but not IL-4-induced activation of PDE3. In FDCP2 cells transfected with constitutively activated MEK, MAP kinase and PDE4, not PDE3, were activated. Thus, in FDCP2 cells, PDE4 can be activated by overlapping MAP kinase-dependent pathways involving PI3-K (IL-4, IL-3, GM-CSF) or PKC (PMA), but selective activation of PDE3 by IL-4 is MAP kinase independent (but perhaps IRS-2/PI3-K dependent). (+info)
(7/832) Insulin-like growth factor I synergizes with interleukin 4 for hematopoietic cell proliferation independent of insulin receptor substrate expression.
In the present study, we investigated the potential role of insulin-like growth factor I (IGF-I) receptor (IGF-IR) in cell proliferation by overexpressing it in 32D myeloid progenitor cells. The overexpression of IGF-IR caused the transfectants to proliferate in response to IGF-I in the absence of insulin receptor substrate (IRS) expression. The activation of overexpressed wild-type IGF-IR, but not that of an ATP-binding mutant of IGF-IR, resulted in the increased tyrosine phosphorylation of several intracellular proteins, including SHC, Src homology 2-containing inositol-5-phosphatase, protein kinase C-delta, and Erk2. Grb2 association with SHC and mitogen-activated protein kinase (MAPK) activity was also enhanced in response to IGF-I stimulation. Interestingly, the stimulation of the IGF-IR transfectants with interleukin 4 (IL-4) also resulted in strong mitogenesis independent of IRS expression. Moreover, IGF-I and/or IL-4 induced long-term cell growth of the IGF-IR transfectants. IL-4 was able to synergize with IGF-I for DNA synthesis, even in the parental 32D cells and a pro-B-cell line, Baf3, indicating the physiological importance of the two growth factors in hematopoietic cell proliferation. IL-4 stimulation of the IGF-IR transfectants resulted in enhanced tyrosine phosphorylation of SHC, Erk2, and signal transducer and activator of transcription 6 (STAT6) proteins. Both IL-4 and IGF-I were able to induce c-myc early response gene expression, and this expression was maximal in the presence of both factors. Finally, we demonstrated that a MAPK kinase inhibitor was able to suppress mitogenesis of the IGF-IR transfectants in response to IGF-I and/or IL-4. Together, our results suggest that IL-4 synergizes with IGF-I for hematopoietic cell proliferation, likely through cross talk between SHC/Grb2/MAPK and STAT6 pathways and through c-myc gene up-regulation. (+info)
(8/832) Orally induced peripheral nonresponsiveness is maintained in the absence of functional Th1 or Th2 cells.
Intragastric administration of soluble protein Ags results in peripheral tolerance to the fed Ag. To examine the role of cytokine regulation in the induction of oral tolerance, we fed OVA to mice deficient in Th1 (Stat 4-/-) and Th2 (Stat 6-/-) cells and compared their response to that of normal BALB/c controls. We found that, in spite of these deficiencies, OVA-specific peripheral cell-mediated and humoral nonresponsiveness was maintained in both Stat 4-/- and Stat 6-/- mice. In the mucosa, both Peyer's patch T cell proliferative responses and OVA-specific fecal IgA were reduced in Stat 4-/- and Stat 6-/- mice fed OVA but not in normal BALB/c controls. Mucosal, but not peripheral, nonresponsiveness was abrogated by the inclusion of a neutralizing Ab to TGF-beta in the culture medium. Our results show that, in the periphery, tolerance to oral Ag can be induced in both a Th1- or Th2-deficient environment. In the mucosa, however, the absence of Th1 and Th2 cytokines can markedly affect this response, perhaps by regulation of TGF-beta-secreting cells. (+info)