Cytokine receptors and signal transduction

A common bipotent progenitor generates the erythroid and megakaryocyte lineages in embryonic stem cell-derived primitive hematopoiesis

The megakaryocytic (MK) and erythroid lineages are tightly associated during differentiation and are generated from a bipotent megakaryocyte-erythroid progenitor (MEP). In the mouse, a primitive MEP has been demonstrated in the yolk sac. In human, it is not known whether the primitive MK and erythroid lineages are generated from a common progenitor or independently. Using hematopoietic differentiation of human embryonic stem cells on the OP9 cell line, we identified a primitive MEP in a subset of cells coexpressing glycophorin A (GPA) and CD41 from day 9 to day 12 of coculturing. This MEP differentiates into primitive erythroid (GPA(+)CD41(-)) and MK (GPA(-)CD41(+)) lineages. In contrast to erythropoietin (EPO)-dependent definitive hematopoiesis, KIT was not detected during erythroid differentiation. A molecular signature for the commitment and differentiation toward both the erythroid and MK lineages was detected by assessing expression of transcription factors, thrombopoietin receptor (MPL) and erythropoietin receptor (EPOR). We showed an inverse correlation between FLI1 and both KLF1 and EPOR during primitive erythroid and MK differentiation, similar to definitive hematopoiesis. This novel MEP differentiation system may allow an in-depth exploration of the molecular bases of erythroid and MK commitment and differentiation.

Erythropoietin receptor expression in canine mammary tumor: an immunohistochemical study

Erythropoietin (EPO) is a cytokine primarily involved in the regulation of the erythropoiesis. Recently, it has been demonstrated that EPO and its receptor (EPOR) are expressed in several neoplastic cell lines and solid tumors. Furthermore, in vitro and in vivo studies have shown that EPO could promote human breast carcinoma growth by means of the binding with its receptor, although a clear function for EPO in this setting has not been yet established. While the human medical literature has been accumulating strong evidence on EPO's role in oncogenesis, to date, there are no veterinary reports focusing on such an issue. The aim of the present study was to investigate the immunohistochemical expression of EPOR in canine mammary gland dysplastic and neoplastic lesions. Our results show a weak to moderate EPOR expression in dysplastic glands, being immunoreactivity enhanced as the lesion shows an increasing malignant pattern. On the basis of these findings, this study describes, for the first time, the evidence for EPOR expression in canine mammary gland tumor and suggests a feasible EPO's role for canine mammary tumor progression.

Erythropoietin and Erythropoietin Receptor Expression after Experimental Spinal Cord Injury Encourages Therapy by Exogenous Erythropoietin

OBJECTIVE

Erythropoietin (EPO) is a pleiotropic cytokine originally identified for its role in erythropoiesis. Recent studies have demonstrated that EPO and its receptor (EPO-R) are expressed in the central nervous system, where EPO exerts neuroprotective functions. Because the expression of the EPO and EPO-R network is poorly investigated in the central nervous system, the aim of the present study was to investigate whether the resident EPO and EPO-R network is activated in the injured nervous system.

METHODS

A well-standardized model of compressive spinal cord injury in rats was used. EPO and EPO-R expression was determined by immunohistochemical analysis at 8 hours and at 2, 8, and 14 days in the spinal cord of injured and noninjured rats.

RESULTS

In noninjured spinal cord, weak immunohistochemical expression of EPO and EPO-R was observed in neuronal and glial cells as well as in endothelial and ependymal cells. In injured rats, a marked increase of expression of EPO and EPO-R was observed in neurons, vascular endothelium, and glial cells at 8 hours after injury, peaking at 8 days, after which it gradually decreased. Two weeks after injury, EPO immunoreactivity was scarcely detected in neurons, whereas glial cells and vascular endothelium expressed strong EPO-R immunoreactivity.

CONCLUSION

These observations suggest that the local EPO and EPO-R system is markedly engaged in the early stages after nervous tissue injury. The reduction in EPO immunoexpression and the increase in EPO-R staining strongly support the possible usefulness of a therapeutic approach based on exogenous EPO administration.

Functional interaction of STAT5 and nuclear receptor co-repressor SMRT: implications in negative regulation of STAT5-dependent transcription

Signal transducers and activators of transcription (STATs) play a central role in cytokine signaling. Activating and repressing gene transcription is a dynamic process involving chromatin remodeling by histone acetylases and deacetylases, yet the role of this process in STAT-dependent transcription remains largely unknown. In a search for STAT5-interacting proteins by yeast two-hybrid screening, we identified the nuclear receptor co-repressor SMRT (silencing mediator for retinoic acid receptor and thyroid hormone receptor) as a potential STAT5-binding partner. SMRT binds to both STAT5A and 5B, and strongly repressed STAT5-dependent transcription in vitro. SMRT binds to the N-terminal coiled-coil domain of STAT5 and a mutation within this region previously found to render STAT5 hyperactive in response to cytokines abolished the interaction with SMRT. Overexpression of SMRT suppressed the induction of STAT5 target genes by interleukin-3, whereas the histone deacetylase inhibitor trichostatin A effectively enhanced and prolonged their expression. Together, these findings illuminate the potential role of SMRT in down-regulating STAT5 activity, with a consequent reduction of STAT5 target gene expression.

Defective c-Mpl signaling in the syndrome of thrombocytopenia with absent radii

Thrombocytopenia with absent radii (TAR) syndrome is a rare congenital defect with severe hypomegakaryocytic thrombocytopenia and bilateral radial aplasia. To elucidate a possible relationship between thrombocytopenia in TAR and defects in the thrombopoietin (TPO)/c-Mpl system, we examined TPO activity in sera from six patients and in vitro reactivity of the patients' platelets to recombinant human TPO. We found elevated TPO serum levels in all patients, excluding a TPO production defect as a pathomechanism for the thrombocytopenia. In contrast to healthy controls, however, platelets of TAR patients failed to respond to recombinant TPO as measured by testing TPO synergism to suboptimal concentration of platelet activators. Most interestingly, TPO-induced tyrosine phosphorylation of platelet proteins was completely absent (four out of five) or markedly decreased (one out of five). More detailed investigations of the signal cascades of c-Mpl demonstrated the absence of Jak2 phosphorylation after TPO stimulation in a TAR patient's platelets. A defect in the early events of c-Mpl signal transduction might be the reason for impaired megakaryocytopoiesis in TAR syndrome.

The human homologue of the yeast proteins Skb1 and Hsl7p interacts with Jak kinases and contains protein methyltransferase activity

To expand our understanding of the role of Jak2 in cellular signaling, we used the yeast two-hybrid system to identify Jak2-interacting proteins. One of the clones identified represents a human homologue of the Schizosaccaromyces pombe Shk1 kinase-binding protein 1, Skb1, and the protein encoded by the Saccharomyces cerevisiae HSL7 (histone synthetic lethal 7) gene. Since no functional motifs or biochemical activities for this protein or its homologues had been reported, we sought to determine a biochemical function for this human protein. We demonstrate that this protein is a protein methyltransferase. This protein, designated JBP1 (Jak-binding protein 1), and its homologues contain motifs conserved among protein methyltransferases. JBP1 can be cross-linked to radiolabeled S-adenosylmethionine (AdoMet) and methylates histones (H2A and H4) and myelin basic protein. Mutants containing substitutions within a conserved region likely to be involved in AdoMet binding exhibit little or no activity. We mapped the JBP1 gene to chromosome 14q11.2-21. In addition, JBP1 co-immunoprecipitates with several other proteins, which serve as methyl group acceptors and which may represent physiological targets of this methyltransferase. Messenger RNA for JBP1 is widely expressed in human tissues. We have also identified and sequenced a homologue of JBP1 in Drosophila melanogaster. This report provides a clue to the biochemical function for this conserved protein and suggests that protein methyltransferases may have a role in cellular signaling.

Thrombopoietin acts synergistically on Ca(2+) mobilization in platelets caused by ADP or thrombin receptor agonist peptide

Thrombopoietin (TPO) is the main regulator of megakaryopoiesis and influences also the function of mature platelets. TPO has been shown to synergize in multiple platelet activation processes induced by various agonists. Our aim was to elucidate whether TPO affects calcium signaling during platelet activation processes. TPO demonstrated a synergistic effect on the exocytosis induced by suboptimal doses of adenosine diphosphate (ADP) and the thrombin receptor agonist peptide (TRAP). We detected synergistic effects of TPO on the ADP or TRAP induced Ca(2+) mobilization in a small range of very low agonist concentrations. The TPO synergism on Ca(2+) mobilization and CD62P expression was measurable in different, nonoverlapping ranges of ADP or TRAP concentrations. Sustaining the agonist-induced calcium signal with thapsigargin led to a detectable TPO synergism in CD62P expression even in agonist concentrations in which the synergism only occurs in Ca(2+) signaling without thapsigargin.

Thrombopoietin in Patients With Congenital Thrombocytopenia and Absent Radii: Elevated Serum Levels, Normal Receptor Expression, But Defective Reactivity to Thrombopoietin

The pathophysiology of thrombocytopenia in the syndrome of thrombocytopenia with absent radii (TAR) is not yet understood. We examined thrombopoietin (TPO) serum levels and the in vitro reactivity of platelets to TPO in five patients affected with TAR syndrome. We found elevated TPO serum levels in all patients tested, excluding a TPO production defect as cause for thrombocytopenia in TAR syndrome. In addition, we found similar expression of the TPO receptor c-Mpl on the surface of platelets from TAR patients (5 of 5) and a similar molecular weight of the receptor as compared with healthy controls (4 of 4). Platelet response to adenosine diphosphate or thrombin receptor agonist peptide SFLLRN (TRAP) was normal in TAR patients. However, in contrast to results with healthy controls we could show absence of in vitro reactivity of platelets from TAR patients to recombinant TPO as measured by testing TPO synergism to adenine diphosphate and TRAP in platelet activation. TPO induced tyrosine phosphorylation of platelet proteins was completely absent (3 of 4) or markedly decreased (1 of 4). Our results indicate that defective megakaryocytopoiesis/thrombocytopoiesis in TAR syndrome is not caused by a defect in TPO production but a lack of response to TPO in the signal transduction pathway of c-Mpl.

Thrombopoietin in Patients With Congenital Thrombocytopenia and Absent Radii: Elevated Serum Levels, Normal Receptor Expression, But Defective Reactivity to Thrombopoietin

The pathophysiology of thrombocytopenia in the syndrome of thrombocytopenia with absent radii (TAR) is not yet understood. We examined thrombopoietin (TPO) serum levels and the in vitro reactivity of platelets to TPO in five patients affected with TAR syndrome. We found elevated TPO serum levels in all patients tested, excluding a TPO production defect as cause for thrombocytopenia in TAR syndrome. In addition, we found similar expression of the TPO receptor c-Mpl on the surface of platelets from TAR patients (5 of 5) and a similar molecular weight of the receptor as compared with healthy controls (4 of 4). Platelet response to adenosine diphosphate or thrombin receptor agonist peptide SFLLRN (TRAP) was normal in TAR patients. However, in contrast to results with healthy controls we could show absence of in vitro reactivity of platelets from TAR patients to recombinant TPO as measured by testing TPO synergism to adenine diphosphate and TRAP in platelet activation. TPO induced tyrosine phosphorylation of platelet proteins was completely absent (3 of 4) or markedly decreased (1 of 4). Our results indicate that defective megakaryocytopoiesis/thrombocytopoiesis in TAR syndrome is not caused by a defect in TPO production but a lack of response to TPO in the signal transduction pathway of c-Mpl.

The prolactin receptor and severely truncated erythropoietin receptors support differentiation of erythroid progenitors

Activation of the erythropoietin receptor is essential for the survival, proliferation, and differentiation of erythroid progenitors. To understand the role of erythropoietin receptor (EpoR) activation in erythroid differentiation, we infected primary erythroid progenitors with high-titer retrovirus encoding the non-hematopoietic prolactin receptor. The infected progenitors responded to prolactin in the absence of Epo by generating fully differentiated erythroid colonies. Therefore, differentiation of erythroid progenitors does not require an intracellular signal generated uniquely by the EpoR; the EpoR does not have an instructive role in erythroid differentiation. We also infected primary erythroid progenitors with retrovirus encoding chimeric receptors containing the extracellular domain of PrlR and the intracellular domain of either the wild-type or truncated EpoRs. A chimeric receptor containing only the membrane-proximal 136 amino acids of the EpoR cytoplasmic domain efficiently supported prolactin-dependent differentiation of erythroid progenitors. Substitution of the single cytoplasmic domain tyrosine in this receptor with phenylalanine (Y343F) eliminated its ability to support differentiation. The minimal EpoR cytoplasmic domain required for erythroid differentiation is therefore the same as that previously reported to be sufficient to support cell proliferation (D'Andrea, A. D., Yoshimura, A., Youssoufian, H., Zon, L. I., Koo, J. W., and Lodish, H. F. (1991) Mol. Cell. Biol. 11, 1980-1987; Miura, O., D'Andrea, A. D., Kabat, D., and Ihle, J. N. (1991) Mol. Cell. Biol. 11, 4895-4902; He, T.-C., Jiang, N., Zhuang, H., Quelle, D. E., and Wojchowski, D. M. (1994) J. Biol. Chem. 269, 18291-18294).

Activation of Stat 5b in erythroid progenitors correlates with the ability of ErbB to induce sustained cell proliferation

Self renewal of normal erythroid progenitors is induced by the receptor tyrosine kinase c-ErbB, whereas other receptors (c-Kit/Epo-R) regulate erythroid differentiation. To address possible mechanisms that could explain this selective activity of c-ErbB, we analyzed the ability of these receptors to activate the different members of the Stat transcription factor family. Ligand activation of c-ErbB induced the tyrosine phosphorylation, DNA-binding, and reporter gene transcription of Stat 5b in erythroblasts. In contrast, ligand activation of c-Kit was unable to induce any of these effects in the same cells. Activation of the erythropoietin receptor caused specific DNA-binding of Stat 5b, but failed to induce reporter gene transcription. These biochemical findings correlate perfectly with the selective ability of c-ErbB to cause sustained self renewal in erythroid progenitors.

Nuclear accumulation of multiple protein kinases during prolactin-induced proliferation of Nb2 rat lymphoma cells

Intracellular kinases play important roles in signal transduction and are involved in the surface receptor-mediated regulation of cellular functions, including mitogenesis. In the present study, we examined the possible involvement of various protein kinases in the passage of a mitogenic signal from the cell surface to the nucleus of Nb2 cells, a rat nodal lymphoma cell line in which prolactin is a mitogen. Following a prolactin challenge, various kinase activities were monitored at short intervals in different cellular fractions over a 60 min period. Protein kinase C (PKC) activity in the cytosolic fraction rapidly declined to 50% of its original activity within the first 30 min, while PKC activity in the nuclear fractions increased sharply, reaching its highest level by 30 min following a prolactin challenge. There were also increases in both casein kinase and protein tyrosine kinase (PTK) activities in the nuclear fractions during the first 30 min following a prolactin challenge that paralleled PKC activity. The activities of all three kinases declined thereafter, reaching levels close to their respective basal values by 60 min following initiation of prolactin treatment. These observations suggest the possibility that multiple protein kinases may be involved in mitogenic signal transduction for prolactin in Nb2 cells.

Inhibition of endothelin- and phorbol ester-stimulated tyrosine kinase activity by corticotrophin in the rat adrenal zona glomerulosa

1. The experiments described in this study were carried out to investigate the role of tyrosine kinase in the acute adrenal response to peptide hormone stimulation, and to determine whether the activity of this kinase may be subject to regulation by other intracellular signalling mechanisms in the adrenal zona glomerulosa. 2. Previous studies from this laboratory have shown that angiotensin II stimulates tyrosine kinase activity in the rat adrenal cortex. This study has shown, for the first time, that endothelin-1 also stimulates tyrosine kinase activity in this tissue. 3. Using the specific inhibitor of protein kinase C (PKC) activity, Ro 31-8220, we have shown that stimulation of tyrosine kinase activity, in response to endothelin-1, angiotensin II or the phorbol ester phorbol 12-myristate 13-acetate, is at least partly dependent on increased PKC activity. 4. The data presented also provide further evidence of cross-talk between signalling systems in the adrenal cortex. Corticotrophin and its intracellular second messenger, cyclic AMP, significantly attenuate the increment in tyrosine kinase activity seen in response to each of the effectors used. 5. The results of this study provide important new evidence for the regulation of protein kinases by other intracellular second messenger systems.

Megakaryocyte growth and development factor (MGDF) moderately enhances in-vitro platelet aggregation

Megakaryocyte growth and development factor (MGDF) is a novel cytokine which promotes the development of immature megakaryocytes into platelets. We tested the hypothesis that MGDF would alter the sensitivity of platelets to aggregating agents as assessed by in-vitro platelet aggregometry. Platelet aggregation in the presence or absence of MGDF was tested with single doses of clinically relevant aggregating agents. A dose-dependent enhancement of the aggregation response to epinephrine was noted in MGDF treated platelets. When a range of concentrations of ADP were used to generate an aggregation dose-response curve, the addition of MGDF to platelet rich plasma shifted the dose response curve to the left. The effect of MGDF on platelet aggregation was partially prevented by the coincubation of platelets with a soluble form of the receptor for MGDF, the extracellular domain of c-mpl. In addition, we demonstrate that exogenous MGDF is able to induce tyrosine phosphorylation of platelet proteins with apparent molecular weights of 85 kDa and 130 kDa. From these data we conclude that exogenously added MGDF moderately increases the sensitivity of platelets to aggregating agents through a mechanism which appears to involve tyrosine phosphorylation of platelet proteins.

Identification and characterization of an interleukin-3 receptor-associated 110-kDa serine/threonine kinase

We recently reported that interleukin-3 (IL-3) stimulation of the murine IL-3-responsive cell line, B6SUtA1, results in the rapid phosphorylation of the beta subunit of the IL-3 receptor (IL-3R), not only on tyrosine residues but on serine/threonine (Ser/Thr) residues as well. Since this occurred even at 4 degrees C, it suggested that a Ser/Thr-specific kinase might be closely associated with the IL-3R. To test this possibility, IL-3R complexes were isolated with anti-IL-3R (alpha IL-3R) antibodies, and in vitro phosphorylation studies were undertaken. These revealed the presence of a 110-kDa protein that was heavily phosphorylated in vitro on serine and threonine residues and that bound selectively to gamma-ATP-Sepharose beads. Moreover, this protein, which was not the 110-kDa subunit of phosphatidylinositol 3-kinase, was tyrosine phosphorylated in response to IL-3 and was specifically labeled in vitro with azido-[32P]ATP. These data, together with in vitro kinase inhibitor studies, suggest that an as yet uncharacterized H7- and staurosporine-sensitive 110-kDa Ser/Thr kinase may be constitutively associated with the IL-3R and activated following IL-3 stimulation. A comparison of IL-3R and erythropoietin receptor complexes suggests that this 110-kDa protein may be preferentially associated with the IL-3R.

Cloning of murine Stat6 and human Stat6, Stat proteins that are tyrosine phosphorylated in responses to IL-4 and IL-3 but are not required for mitogenesis

By searching a database of expressed sequences, we identified a member of the signal transducers and activators of transcription (Stat) family of proteins. Human and murine full-length cDNA clones were obtained and sequenced. The sequence of the human cDNA was identical to the recently published sequence for interleukin-4 (IL-4)-Stat (J. Hou, U. Schindler, W.J. Henzel, T.C. Ho, M. Brasseur, and S. L. McKnight, Science 265:1701-1706, 1994), while the murine Stat6 amino acid and nucleotide sequences were 83 and 84% identical to the human sequences, respectively. Using Stat6-specific antiserum, we demonstrated that Stat6 is rapidly tyrosine phosphorylated following stimulation of appropriate cell lines with IL-4 or IL-3 but is not detectably phosphorylated following stimulation with IL-2, IL-12, or erythropoietin. In contrast, IL-2, IL-3, and erythropoietin induce the tyrosine phosphorylation of Stat5 while IL-12 uniquely induces the tyrosine phosphorylation of Stat4. Inducible tyrosine phosphorylation of Stat6 requires the membrane-distal region of the IL-4 receptor alpha chain. This region of the receptor is not required for cell growth, demonstrating that Stat6 tyrosine phosphorylation does not contribute to mitogenesis.

The c-Mpl ligand (thrombopoietin) stimulates tyrosine phosphorylation of Jak2, Shc, and c-Mpl

c-Mpl is a member of the cytokine receptor superfamily, expressed primarily on hematopoietic cells. Recently, the c-Mpl ligand was cloned and found to have thrombopoietic activity. In this paper we report that ligand binding induced tyrosine phosphorylation in BaF3 cells engineered to express the murine Mpl receptor (BaF3/mMpl). Phosphorylation occurred within 1 min at cytokine concentrations sufficient for proliferation of receptor-bearing cells. Using specific antibodies for immunoprecipitation and Western blotting, several of these phosphorylated proteins were identified. Shc and Jak2, known cytokine signaling molecules, and the c-Mpl receptor were shown to be major substrates for tyrosine phosphorylation. In contrast, phospholipase C-gamma and phosphatidylinositol 3-kinase displayed little and no tyrosine phosphorylation, respectively, after thrombopoietin stimulation. Co-immunoprecipitation studies demonstrated that Jak2 became physically associated with c-Mpl relatively late in the observed time course (20-60 min), significantly later than tyrosine phosphorylation of Jak2 (1-5 min). These results suggest that c-Mpl induces signal transduction pathways similar to those of other known cytokines. Additionally, in light of its late physical association with c-Mpl following ligand binding, Jak2 may not be the initiating tyrosine kinase in the thrombopoietin-induced signaling cascade.

Tec kinase associates with c-kit and is tyrosine phosphorylated and activated following stem cell factor binding

Stem cell factor (SCF) plays a crucial role in hematopoiesis through its interaction with the receptor tyrosine kinase c-kit. However, the signaling events that are activated by this interaction and involved in the control of growth or differentiation are not completely understood. We demonstrate here that Tec, a cytoplasmic, src-related kinase, physically associates with c-kit through a region that contains a proline-rich motif, amino terminal of the SH3 domain. Following SCF binding, Tec is tyrosine phosphorylated and its in vitro kinase activity is increased. Tyrosine phosphorylation of Tec is not detected in the response to other cytokines controlling hematopoiesis, including colony-stimulating factor-1 (CSF-1), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3). Conversely, the cytoplasmic kinase JAK2 is activated by IL-3 but not by SCF stimulation. The activation of distinct cytoplasmic kinases may account for the synergy seen in the actions of SCF and IL-3 on hematopoietic stem cells.

Tec kinase associates with c-kit and is tyrosine phosphorylated and activated following stem cell factor binding

Stem cell factor (SCF) plays a crucial role in hematopoiesis through its interaction with the receptor tyrosine kinase c-kit. However, the signaling events that are activated by this interaction and involved in the control of growth or differentiation are not completely understood. We demonstrate here that Tec, a cytoplasmic, src-related kinase, physically associates with c-kit through a region that contains a proline-rich motif, amino terminal of the SH3 domain. Following SCF binding, Tec is tyrosine phosphorylated and its in vitro kinase activity is increased. Tyrosine phosphorylation of Tec is not detected in the response to other cytokines controlling hematopoiesis, including colony-stimulating factor-1 (CSF-1), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3). Conversely, the cytoplasmic kinase JAK2 is activated by IL-3 but not by SCF stimulation. The activation of distinct cytoplasmic kinases may account for the synergy seen in the actions of SCF and IL-3 on hematopoietic stem cells.