Biomedical subjects

B A Witthuhn

Publications and source records attributed to B A Witthuhn.

29 records · Page 2Linked to original sources

PubMed1994

Activation of JAK2 kinase mediated by the interleukin 6 signal transducer gp130.

The interleukin 6 receptor-associated signal transducer, gp130, is shared by receptor complexes for leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and interleukin 11. We show here that JAK2 kinase is rapidly tyrosine phosphorylated in mouse embryonic stem cells whose pluripotentiality is maintained only by gp130-sharing cytokines after stimulation that is known to induce gp130 homodimerization. JAK1 is also tyrosine phosphorylated, but to a lesser extent, under the same conditions. Comparable results are obtained with hemopoietic lineage cells such as myeloid leukemic M1 cells and pro-B-cell line-derived transfectants expressing gp130, the former of which differentiate into macrophages after stimulation of gp130 and the latter of which initiate DNA synthesis. gp130-dimerizing stimulus upregulates kinase activity of JAK2 as revealed by immunocomplex kinase assay. Deletion or point mutation in the membrane-proximal cytoplasmic motifs in gp130 that are conserved in the hemopoietic cytokine receptor family results in the loss of tyrosine phosphorylation of JAK2, which coincides with the lack of signal transducing capability of gp130 mutants.

Animals↗

PubMed1994

Association and activation of Jak-Tyk kinases by CNTF-LIF-OSM-IL-6 beta receptor components.

A recently defined family of cytokines, consisting of ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), oncostatin M (OSM), and interleukin-6 (IL-6), utilize the Jak-Tyk family of cytoplasmic tyrosine kinases. The beta receptor components for this cytokine family, gp130 and LIF receptor beta, constitutively associate with Jak-Tyk kinases. Activation of these kinases occurs as a result of ligand-induced dimerization of the receptor beta components. Unlike other cytokine receptors studied to date, the receptors for the CNTF cytokine family utilize all known members of the Jak-Tyk family, but induce distinct patterns of Jak-Tyk phosphorylation in different cell lines.

Animals↗

PubMed1994

Signaling by the cytokine receptor superfamily: JAKs and STATs.

A variety of cytokines, lymphokines and growth factors function by interacting with receptors that are members of the cytokine receptor superfamily. These receptors share extracellular motifs and have limited similarity in their cytoplasmic domains. Although lacking catalytic domains, this family of receptors couples ligand binding with the induction of tyrosine phosphorylation. Recent studies have shown that this is mediated by members of the Janus kinase (JAK) family of cytoplasmic protein tyrosine kinases. JAKs physically associate with the membrane-proximal region of the ligand-bound receptor, leading to their tyrosine phosphorylation and activation. The activated JAKs phosphorylate the receptors as well as cytoplasmic proteins belonging to a family of transcription factors called the signal transducers and activators of transcription (STATs), providing a novel signaling pathway that is shared by all members of the cytokine receptor superfamily.

Amino Acid Sequence↗

PubMed1994

JAK2 associates with the beta c chain of the receptor for granulocyte-macrophage colony-stimulating factor, and its activation requires the membrane-proximal region.

The high-affinity receptor for granulocyte-macrophage colony-stimulating factor (GM-CSF) consists of a unique alpha chain and a beta c subunit that is shared with the receptors for interleukin-3 (IL-3) and IL-5. Two regions of the beta c chain have been defined; these include a membrane-proximal region of the cytoplasmic domain that is required for mitogenesis and a membrane-distal region that is required for activation of Ras, Raf-1, mitogen-activated protein kinase, and S6 kinase. Recent studies have implicated the cytoplasmic protein tyrosine kinase JAK2 in signalling through a number of the cytokine receptors, including the IL-3 and erythropoietin receptors. In the studies described here, we demonstrate that GM-CSF stimulation of cells induces the tyrosine phosphorylation of JAK2 and activates its in vitro kinase activity. Mutational analysis of the beta c chain demonstrates that only the membrane-proximal 62 amino acids of the cytosolic domain are required for JAK2 activation. Thus, JAK2 activation is correlated with induction of mitogenesis but does not, alone, activate the Ras pathway. Carboxyl truncations of the alpha chain, which inactivate the receptor for mitogenesis, are unable to mediate GM-CSF-induced JAK2 activation. Using baculovirus-expressed proteins, we further demonstrate that JAK2 physically associates with the beta c chain but not with the alpha chain. Together, the results further support the hypothesis that the JAK family of kinase are critical to coupling cytokine binding to tyrosine phosphorylation and ultimately mitogenesis.

Animals↗

PubMed1994

Protein tyrosine phosphorylation in the regulation of hematopoiesis by receptors of the cytokine-receptor superfamily.

Cytokines regulate the growth and differentiation of hematopoietic cells through their interaction with receptors of the cytokine receptor superfamily. This family of receptors has conserved motifs in the extracellular domain but share only limited similarity in the cytoplasmic domains. Although lacking catalytic domains, a variety of studies demonstrate that the cytokine receptors function by coupling ligand binding to induction of tyrosine phosphorylation. Recent studies have shown that the JAK family of kinases associate with cytokine receptors and are activated by ligand binding. Interaction occurs with the membrane proximal region of the cytoplasmic domain, a region that has been found to be essential for mitogenesis. One of the substrates of tyrosine phosphorylation is the receptor and, in the case of the receptor for Epo, the membrane distal region of the cytoplasmic domain is phosphorylated. Once phosphorylated, this site becomes a binding site for the amino-terminal SH2 domain of hematopoietic cell phosphatase (HCP). HCP is an important negative regulator of hematopoietic cell growth and its recruitment to the receptor complex is speculated to be important for this effect. The role of HCP is best indicated by the observation that the murine mutation, motheaten, is due to a mutation that results in the inability to make HCP. Motheaten mice die soon after birth due to the overproliferation of a variety of hematopoietic lineages. Together the results demonstrate an essential role in both protein tyrosine phosphorylation and de-phosphorylation in the growth regulation of hematopoiesis.

Animals↗

PubMed1993

The protein tyrosine kinase JAK1 complements defects in interferon-alpha/beta and -gamma signal transduction.

We have produced a cell line which lacks the protein tyrosine kinase JAK1 and is completely defective in interferon response. Complementation of this mutant with JAK1 restored the response, establishing the requirement for JAK1 in both the interferon-alpha/beta and -gamma signal transduction pathways. The reciprocal interdependence between JAK1 and Tyk2 activities in the interferon-alpha pathway, and between JAK1 and JAK2 in the interferon-gamma pathway, may reflect a requirement for these kinases in the correct assembly of interferon receptor complexes.

Animals↗

PubMed1993

Complementation by the protein tyrosine kinase JAK2 of a mutant cell line defective in the interferon-gamma signal transduction pathway.

Interferons (IFNs) alpha/beta (type I) and gamma (type II) bind to distinct cell surface receptors, inducing transcription of overlapping sets of genes by intracellular pathways that have recently attracted much attention. Previous studies using cell lines selected for their inability to respond to IFN-alpha (ref. 4) have shown that the protein kinase Tyk2 plays a central role in the IFN alpha/beta response. Here we report the isolation of the cell line gamma 1A, selected for its inability to express IFN-gamma-inducible cell-surface markers, that is deficient in all aspects of the IFN-gamma response tested, but responds normally to IFNs alpha and beta. The mutant cells can be complemented by the expression of another member of the JAK family of protein tyrosine kinases, JAK2 (refs 6-9). Unlike IFNs alpha and beta, IFN-gamma induces rapid tyrosine phosphorylation of JAK2 in wild-type cells, and JAK2 immunoprecipitates from these cells show tyrosine kinase activity. These responses are absent in gamma 1A cells. JAK2 is therefore required for the response to IFN-gamma but not to IFNs alpha and beta.

Animals↗

PubMed1993

Structure of the murine Jak2 protein-tyrosine kinase and its role in interleukin 3 signal transduction.

Interleukin 3 (IL-3) regulates the proliferation and differentiation of hematopoietic cells. Although the IL-3 receptor chains lack kinase catalytic domains, IL-3 induces tyrosine phosphorylation of cellular proteins. To investigate the potential role of the JAK family of protein-tyrosine kinases in IL-3 signal transduction, we have obtained full-length cDNA clones for murine Jak1 and Jak2 protein-tyrosine kinases and prepared antiserum against the predicted proteins. Using antisera against Jak2, we demonstrate that IL-3 stimulation results in the rapid and specific tyrosine phosphorylation of Jak2 and activates its in vitro kinase activity.

Amino Acid Sequence↗

PubMed1993

JAK2 associates with the erythropoietin receptor and is tyrosine phosphorylated and activated following stimulation with erythropoietin.

Erythropoietin (EPO) regulates the proliferation and differentiation of erythroid cells through interaction with its receptor (EPOR). Although EPOR is a member of the cytokine receptor superfamily and lacks a kinase domain, EPO induces tyrosine phosphorylation, which is correlated with gene transcription and mitogenesis. Here we demonstrate that EPO induces tyrosine phosphorylation of JAK2 kinase and activates its in vitro autophosphorylation. Using EPOR mutants, phosphorylation and activation of kinase activity correlate with the induction of mitogenesis. Furthermore, JAK2 physically associates with a membrane-proximal region of the EPOR cytoplasmic domain that is required for biological activity. The results support the hypothesis that JAK2 is the kinase that couples EPO binding to tyrosine phosphorylation and mitogenesis.

3T3 Cells↗

PubMed1993

Identification of JAK2 as a growth hormone receptor-associated tyrosine kinase.

Growth hormone receptor (GHR) forms a complex with a tyrosine kinase, suggesting involvement of a ligand-activated tyrosine kinase in intracellular signaling by growth hormone (GH). Here we identify JAK2, a nonreceptor tyrosine kinase, as a GHR-associated tyrosine kinase. Immunological approaches were used to establish GH-dependent complex formation between JAK2 and GHR, activation of JAK2 tyrosine kinase activity, and tyrosyl phosphorylation of both JAK2 and GHR. The JAK2-GHR and JAK2-erythropoietin receptor interactions described here and in the accompanying paper provide a molecular basis for involvement of tyrosyl phosphorylation in physiological responses to these ligands and suggest a shared signaling mechanism among members of the cytokine/hematopoietin receptor family.

3T3 Cells↗

PubMedDate not supplied

Tissue-specific expression in mouse P815 mastocytoma cells of the cloned rat alpha-subunit gene of the high-affinity receptor for immunoglobulin E.

The high-affinity multisubunit receptor for IgE, Fc epsilon RI, is expressed in vivo in a tissue-specific manner, being found only on mast cells, basophils and epidermal Langerhans cells. The expression of the rat Fc epsilon RI alpha-subunit transcript has been examined here in the mouse mastocytoma cell line, P815, which does not express the endogenous mouse Fc epsilon RI alpha- or beta-subunit transcripts. These studies indicate that an exogenously introduced rat alpha-subunit gene can be faithfully expressed in P815 cells, while no transcripts are produced in the mouse or rat B-lymphoid cell lines, SP2/0 and IR162, respectively. Therefore, in contrast to both B-cell lines, factors necessary for the tissue-specific expression of the alpha-subunit mRNA are present in the P815 cell line, yielding a correctly processed mRNA transcript; nevertheless, this tissue-specific expression is not rigorously species specific. Although the mechanism responsible for these observations is unknown, these results do imply that a specific cis-trans element interaction occurs between this transcriptional unit and factors in the P815 cells that control fidelity of tissue-specific mRNA synthesis and processing from the Fc epsilon RI alpha-subunit gene. Consequently, this reconstructed rat Fc epsilon RI alpha-subunit gene contains minimally sufficient cis elements for tissue-specific expression of mRNA, and so, the P815 cell line should be useful to define these requisite DNA cis elements of the gene, as well as the trans factors from P815 necessary for this process.

Animals↗