A clinical feature of hyperlipidemia in patients with central diabetes insipidus.

In this study, we analyzed plasma lipid and lipoprotein levels before and after treatment with 1-desamino-8-D-arginine vasopressin (DDAVP) in subjects with partial and complete central diabetes insipidus (DI) in order to determine how a shortage and supplement of this hormone affect plasma lipid metabolism. The subjects consisted of 6 patients with partial and 6 with complete central DI. After treatment with DDAVP through nasal cavity, plasma total cholesterol (TC) level did not decrease either in complete or partial form. Plasma triglyceride (TG) levels decreased from 306+/-175 mg/dl to 198+/-91 (35% decrease, p=0.027) in complete form, while TG did not change significantly in partial form. A detailed investigation of plasma lipoprotein metabolism during treatment with DDAVP was carried out in 3 of the 6 subjects with complete form of DI. Lipoprotein lipase activity and mass in post-heparin plasma from those three subjects tended to increase after treatment with DDAVP, along with the complete disappearance of an unusual lipoprotein between low density lipoprotein (LDL) and very low density lipoprotein (VLDL) as analyzed by polyacrylamide gel electrophoresis. These results suggest that the DDAVP treatment has a favorable effect on lipid and lipoprotein metabolism, especially triglyceride-rich lipoproteins, either directly or through modifying factors contributing to lipid metabolism.

Gab-family adapter proteins act downstream of cytokine and growth factor receptors and T- and B-cell antigen receptors.

We previously found that the adapter protein Gab1 (110 kD) is tyrosine-phosphorylated and forms a complex with SHP-2 and PI-3 kinase upon stimulation through either the interleukin-3 receptor (IL-3R) or gp130, the common receptor subunit of IL-6-family cytokines. In this report, we identified another adapter molecule (100 kD) interacting with SHP-2 and PI-3 kinase in response to various stimuli. The molecule displays striking homology to Gab1 at the amino acid level; thus, we named it Gab2. It contains a PH domain, proline-rich sequences, and tyrosine residues that bind to SH2 domains when they are phosphorylated. Gab1 is phosphorylated on tyrosine upon stimulation through the thrombopoietin receptor (TPOR), stem cell factor receptor (SCFR), and T-cell and B-cell antigen receptors (TCR and BCR, respectively), in addition to IL-3R and gp130. Tyrosine phosphorylation of Gab2 was induced by stimulation through gp130, IL-2R, IL-3R, TPOR, SCFR, and TCR. Gab1 and Gab2 were shown to be substrates for SHP-2 in vitro. Overexpression of Gab2 enhanced the gp130 or Src-related kinases-mediated ERK2 activation as that of Gab1 did. These data indicate that Gab-family molecules act as adapters for transmitting various signals.

Gab1 acts as an adapter molecule linking the cytokine receptor gp130 to ERK mitogen-activated protein kinase.

Gab1 has structural similarities with Drosophila DOS (daughter of sevenless), which is a substrate of the protein tyrosine phosphatase Corkscrew. Both Gab1 and DOS have a pleckstrin homology domain and tyrosine residues, potential binding sites for various SH2 domain-containing adapter molecules when they are phosphorylated. We found that Gab1 was tyrosine phosphorylated in response to various cytokines, such as interleukin-6 (IL-6), IL-3, alpha interferon (IFN-alpha), and IFN-gamma. Upon the stimulation of IL-6 or IL-3, Gab1 was found to form a complex with phosphatidylinositol (PI)-3 kinase and SHP-2, a homolog of Corkscrew. Mutational analysis of gp130, the common subunit of IL-6 family cytokine receptors, revealed that neither tyrosine residues of gp130 nor its carboxy terminus was required for tyrosine phosphorylation of Gab1. Expression of Gab1 enhanced gp130-dependent mitogen-activated protein (MAP) kinase ERK2 activation. A mutation of tyrosine 759, the SHP-2 binding site of gp130, abrogated the interactions of Gab1 with SHP-2 and PI-3 kinase as well as ERK2 activation. Furthermore, ERK2 activation was inhibited by a dominant negative p85 PI-3 kinase, wortmannin, or a dominant negative Ras. These observations suggest that Gab1 acts as an adapter molecule in transmitting signals to ERK MAP kinase for the cytokine receptor gp130 and that SHP-2, PI-3 kinase, and Ras are involved in Gab1-mediated ERK activation.

Tec tyrosine kinase links the cytokine receptors to PI-3 kinase probably through JAK.

Tec/Btk tyrosine kinases are members of a subgroup of Src tyrosine kinase family. They are reported to be activated in response to cytokines, such as IL-3 and IL-6. Janus kinases (JAKs) are known to associate with certain cytokine receptors, e.g. gp130, the signal transducing subunit of IL-6 receptor, and the common beta chain of IL-3 receptor, which can be activated upon receptor dimerization in response to cytokines. Here we show the association between Jak1/Jak2 and Tec or Jak1 and Btk. Furthermore, Jak1 but not Jak2 induces tyrosine phosphorylation of Btk, but not Tec. These observations suggest that upon cytokine stimulation JAKs activate Tec/Btk or induce their dimerization resulting in endogenous tyrosine phosphorylation. Furthermore using a yeast two-hybrid system we have identified the target molecules for Tec, the p85 and p55PIK subunits of PI-3 kinase, and Vav. Tec associated with Vav through its SH2 domain independently of its kinase activity. In contrast the p85 and p55PIK subunits of PI-3 kinase associated with the SH2-kinase domain of Tec, dependent on Tec kinase activity. Consistent with these, IL-6 or IL-3 induced the association between Tec and the p85 subunit of PI-3 kinase in mammalian cells. These findings suggest that Tec tyrosine kinase links cytokine receptors to PI-3 kinase probably through JAKs.

An alternative pathway for STAT activation that is mediated by the direct interaction between JAK and STAT.

JAK is believed to be an essential tyrosine kinase that mediates signals from the cytokine receptor to its downstream events. JAK associates with the cytoplasmic domain of the type I cytokine receptor superfamily and upon the ligand stimulation it can be activated, resulting in the receptor phosphorylation. In signaling from gp130, a common signal transducer for the IL-6 family cytokines, STAT3, a transcription factor that contains an SH2 domain, is recruited by phosphotyrosines on gp130 and is subsequently phosphorylated by gp130-associated JAKs. In this study, we attempted to find a new target for JAK that is directly activated by JAK, independent of gp130 tyrosine phosphorylation, by using a yeast two-hybrid system. In the process we found that the JH2 domain of JAK1, JAK2 or JAK3 could specifically associate with the carboxy-terminal portion of STAT5, but not with STAT3 or STAT1. The interaction was confirmed using both a transient expression system in a cell line and a GST-fusion protein binding assay. Furthermore, we showed that the activation of STAT5 via gp130 did not need any phosphotyrosines on gp130 while that of STAT3 strictly depended on phosphotyrosines on gp130. Mutations of STAT5 that eliminated the interaction with JAK1 reduced the activation of STAT5 upon the gp130 stimulation, although such mutants could be still activated through erythropoietin receptor. These results indicate that STATs are activated through cytokine receptors by two distinct mechanisms, one dependent on receptor tyrosine phosphorylation and the other mediated by the JAK-STAT direct interaction.

Two signals are necessary for cell proliferation induced by a cytokine receptor gp130: involvement of STAT3 in anti-apoptosis.

gp130 is a common signal transducer for the interleukin-6-related cytokines. To delineate the gp130-mediated growth signal, we established a series of pro-B cell lines expressing chimeric receptors composed of the extracellular domain of the granulocyte colony-stimulating factor receptor and the transmembrane and cytoplasmic domains of gp130. The second tyrosine (from the membrane) of gp130, which was required for the tyrosine phosphorylation of SHP-2, its association with GRB2, and activation of a MAP kinase, was essential for mitogenesis, but not for anti-apoptosis. On the other hand, the tyrosine in the YXXQ motifs essential for STAT3 activation was required for bcl-2 induction and anti-apoptosis. Furthermore, dominant-negative STAT3 inhibited anti-apoptosis. These data demonstrate that two distinct signals, mitogenesis and anti-apoptosis, are required for gp130-induced cell growth and that STAT3 is involved in anti-apoptosis.

Activation of Fes tyrosine kinase by gp130, an interleukin-6 family cytokine signal transducer, and their association.

gp130 is a signal-transducing subunit of receptors for the interleukin-6 (IL-6)-related cytokine subfamily including IL-6, leukemia inhibitory factor, oncostatin M, IL-11, and ciliary neurotrophic factor, indicating that gp130-mediated signals are involved in the immune response, hematopoiesis, inflammation, and endocrine and nervous system activity. We previously showed that gp130 stimulation rapidly activates Jak, Btk, and Tec tyrosine kinases, all of which constitutively associate with gp130. To further elucidate intracellular signal transduction through gp130, we examined the possible involvement of another nonreceptor tyrosine kinase, p92c-fes (Fes). We showed that gp130 stimulation rapidly induced tyrosine phosphorylation of Fes and actually activated its kinase activity in hematopoietic lineage cells. Furthermore, Fes associated with gp130 independently of ligand stimulation like Jak, Btk, and Tec tyrosine kinases. These results indicate that multiple nonreceptor tyrosine kinases are involved in the gp130-mediated signal transduction pathway. Because both gp130 and Fes are expressed not only in hematopoietic lineage cells but also in heart and nerve cells, Fes may play a role in signal transduction through gp130 in these tissues.

Association and activation of Btk and Tec tyrosine kinases by gp130, a signal transducer of the interleukin-6 family of cytokines.

Interleukin-6 (IL-6), leukemia inhibitory factor, oncostatin M, IL-11, and ciliary neurotrophic factor constitute the IL-6 family of cytokines and play important roles in hematopoiesis, immune response, and nervous system. The receptors for the IL-6 family of cytokines share gp130 through which signals are generated, although the cytoplasmic region of gp130 does not contain any catalytic domain. In this study we show that in addition to Jak family tyrosine kinase, the stimulation of gp130 by IL-6 plus soluble IL-6 receptor alpha induced the activation of Btk and Tec tyrosine kinases, whereas IL-3 and granulocyte colony-stimulating factor activated Tec but not Btk in a pro-B cell line. Furthermore, both Btk and Tec kinases were associated with gp130 without the ligand stimulation. Because Btk is a critical tyrosine kinase for B lymphopoiesis and Tec is considered to be involved in hematopoiesis, the results suggest the involvement of gp130-Btk-Tec signal pathway in early lymphohematopoiesis.