Inactivation of erythropoietin receptor function by point mutations in a region having homology with other cytokine receptors

Lyn Physically Associates With the Erythropoietin Receptor and May Play a Role in Activation of the Stat5 Pathway

Protein tyrosine phosphorylation plays a crucial role in signaling from the receptor for erythropoietin (Epo), although the Epo receptor (EpoR) lacks the tyrosine kinase domain. We have previously shown that the Jak2 tyrosine kinase couples with the EpoR to transduce a growth signal. In the present study, we demonstrate that Lyn, a Src family tyrosine kinase, physically associates with the EpoR in Epo-dependent hematopoietic cell lines, 32D/EpoR-Wt and F36E. Coexpression experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of the EpoR and that both LynA and LynB, alternatively spliced forms of Lyn, bind with the membrane-proximal 91-amino acid region of the EpoR cytoplasmic domain. In vitro binding studies using GST-Lyn fusion proteins further showed that the Src homology (SH)-2 domain of Lyn specifically binds with the tyrosine-phosphorylated EpoR in lysate from Epo-stimulated cells, whereas the tyrosine kinase domain of Lyn binds with the unphosphorylated EpoR. Far-Western blotting and synthetic phosphopeptide competition assays further indicated that the Lyn SH2 domain directly binds to the tyrosine-phosphorylated EpoR, most likely through its interaction with phosphorylated Y-464 or Y-479 in the carboxy-terminal region of the EpoR. In vitro binding studies also demonstrated that the Lyn SH2 domain directly binds to tyrosine-phosphorylated Jak2. In vitro reconstitution experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of Stat5, mainly on Y-694, and activates the DNA-binding and transcription-activating abilities of Stat5. In agreement with this, Lyn enhanced the Stat5-dependent transcriptional activation when overexpressed in 32D/EpoR-Wt cells. In addition, Lyn was demonstrated to phosphorylate the EpoR and Stat5 on tyrosines in vitro. These results suggest that Lyn may play a role in activation of the Jak2/Stat5 and other signaling pathways by the EpoR.

Lyn Physically Associates With the Erythropoietin Receptor and May Play a Role in Activation of the Stat5 Pathway

Protein tyrosine phosphorylation plays a crucial role in signaling from the receptor for erythropoietin (Epo), although the Epo receptor (EpoR) lacks the tyrosine kinase domain. We have previously shown that the Jak2 tyrosine kinase couples with the EpoR to transduce a growth signal. In the present study, we demonstrate that Lyn, a Src family tyrosine kinase, physically associates with the EpoR in Epo-dependent hematopoietic cell lines, 32D/EpoR-Wt and F36E. Coexpression experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of the EpoR and that both LynA and LynB, alternatively spliced forms of Lyn, bind with the membrane-proximal 91-amino acid region of the EpoR cytoplasmic domain. In vitro binding studies using GST-Lyn fusion proteins further showed that the Src homology (SH)-2 domain of Lyn specifically binds with the tyrosine-phosphorylated EpoR in lysate from Epo-stimulated cells, whereas the tyrosine kinase domain of Lyn binds with the unphosphorylated EpoR. Far-Western blotting and synthetic phosphopeptide competition assays further indicated that the Lyn SH2 domain directly binds to the tyrosine-phosphorylated EpoR, most likely through its interaction with phosphorylated Y-464 or Y-479 in the carboxy-terminal region of the EpoR. In vitro binding studies also demonstrated that the Lyn SH2 domain directly binds to tyrosine-phosphorylated Jak2. In vitro reconstitution experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of Stat5, mainly on Y-694, and activates the DNA-binding and transcription-activating abilities of Stat5. In agreement with this, Lyn enhanced the Stat5-dependent transcriptional activation when overexpressed in 32D/EpoR-Wt cells. In addition, Lyn was demonstrated to phosphorylate the EpoR and Stat5 on tyrosines in vitro. These results suggest that Lyn may play a role in activation of the Jak2/Stat5 and other signaling pathways by the EpoR.

The thrombopoietin receptor can mediate proliferation without activation of the Jak-STAT pathway

Cytokine receptors of the hematopoietic receptor superfamily lack intrinsic tyrosine kinase domains for the intracellular transmission of their signals. Instead all members of this family associate with Jak family nonreceptor tyrosine kinases. Upon ligand stimulation of the receptors, Jaks are activated to phosphorylate target substrates. These include STAT (signal transducers and activators of transcription) proteins, which after phosphorylation translocate to the nucleus and modulate gene expression. The exact role of the Jak-STAT pathway in conveying growth and differentiation signals remains unclear. Here we describe a deletion mutant of the thrombopoietin receptor (c-mpl) that has completely lost the capacity to activate Jaks and STATs but retains its ability to induce proliferation. This mutant still mediates TPO-induced phosphorylation of Shc, Vav, mitogen-activated protein kinase (MAPK) and Raf-1 as well as induction of c-fos and c-myc, although at somewhat reduced levels. Furthermore, we show that both wild-type and mutant receptors activate phosphatidylinositol (PI) 3-kinase upon thrombopoietin stimulation and that thrombopoietin-induced proliferation is inhibited in the presence of the PI 3-kinase inhibitor wortmannin. These results demonstrate that the Jak-STAT pathway is dispensable for the generation of mitogenic signals by a cytokine receptor.

The role of tyrosine phosphorylation in proliferation and maturation of erythroid progenitor cells--signals emanating from the erythropoietin receptor

Red blood cells arise continuously from pluripotent stem cells which mature and become functionally specialized upon commitment to the erythroid lineage. In mammals, the key regulator of this process is the hormone erythropoietin (EPO). Hormone binding to the cognate receptor, the erythropoietin receptor (EPO-R), causes receptor homodimerization and transiently triggers tyrosine phosphorylation within target cells. Although the EPO-R lacks intrinsic enzymatic activity it couples, presumably sequentially, to the protein tyrosine kinase receptor c-KIT and the cytosolic protein tyrosine kinase JAK2. Signaling through the EPO-R is promoted by tyrosine phosphorylation of the cytosolic domain and the recruitment of secondary signaling molecules such as the lipid kinase inositolphospholipid 3-kinase (phosphatidylinositol 3-kinase) and protein tyrosine phosphatase SHP-2 to the activated receptor. Complex formation of the activated EPO-R with the protein tyrosine phosphatase SHP-1 terminates signaling. In primary fetal liver cells redundant signals emanating from phosphotyrosine residues in the EPO-R support formation of erythroid colonies in vitro. However, since the last tyrosine residue in the cytosolic domain of the EPO-R, Y479, uniquely supports in the absence of other tyrosine residues an almost normal level of colony-forming unit-erythroid (CFU-E) colony formation, Y479 represents one of the key residues required in vivo for erythroid proliferation and differentiation. The signal emanating from Y479 involves sequential EPO-induced recruitment of phosphoinositol lipid 3-kinase to the EPO-R and activation of mitogen-activated-protein(MAP)kinase activity. The MAP-kinase signaling cascade could serve as an intracellular switch integrating signals mediated by several phosphotyrosine residues in the cytosolic domain of the EPO-R and provide a possible explanation for partial redundancy in signaling.

Erythropoietin and IL-3 induce tyrosine phosphorylation of CrkL and its association with Shc, SHP-2, and Cbl in hematopoietic cells

The present study demonstrates that erythropoietin (Epo) and IL-3 induce tyrosine phosphorylation of the SH2/SH3-containing adapter protein CrkL and its transient association with tyrosine-phosphorylated SHP-2, Shc, and Cbl in a murine IL-3-dependent cell line, 32D, expressing the Epo receptor (EpoR). In these cells, CrkL was constitutively complexed with the guanine nucleotide exchange factor C3G, which was found to coimmunoprecipitate with Shc from Epo- or IL-3-stimulated cells. Studies using cells expressing mutant EpoRs showed that the Epo-induced tyrosine phosphorylation of CrkL is dependent on the membrane-proximal EpoR cytoplasmic region involved in the activation of Jak2 as well as the C-terminal 145 amino acid region which is required for tyrosine phosphorylation of SHP-2 and Shc. It was further revealed that CrkL is recruited to the tyrosine-phosphorylated EpoR, most likely through its interaction with tyrosine-phosphorylated Shc and SHP-2. These results suggest that CrkL is involved in the signaling pathways from the receptors for Epo and IL-3, most likely by modulating the activity of the Ras family GTPases through its interaction with C3G.

A region of the beta subunit of the interferon alpha receptor different from box 1 interacts with Jak1 and is sufficient to activate the Jak-Stat pathway and induce an antiviral state

Coexpression of the alpha and betaL subunits of the human interferon alpha (IFNalpha) receptor is required for the induction of an antiviral state by human IFNalpha. To explore the role of the different domains of the betaL subunit in IFNalpha signaling, we coexpressed wild-type alpha subunit and truncated forms of the betaL chain in L-929 cells. Our results demonstrated that the first 82 amino acids (AAs) (AAs 265-346) of the cytoplasmic domain of the betaL chain are sufficient to activate the Jak-Stat pathway and trigger an antiviral state after IFNalpha2 binding to the receptor. This region of the betaL chain, required for Jak1 binding and activation, contains the Box 1 motif that is important for the interaction of some cytokine receptors with Jak kinases. However, using glutathione S-transferase fusion proteins containing amino- and carboxyl-terminal deletions of the betaL cytoplasmic domain, we demonstrate that the main Jak1-binding region (corresponding to AAs 300-346 on the beta subunit) is distinct from the Box 1 domain (AAs 287-295).

Granulocyte colony-stimulating factor receptor mutations in severe chronic neutropenia and acute myeloid leukaemia: biological and clinical significance

Blood cell formation is governed by the haemopoietic growth factors that control the proliferation, maturation and survival of the haemopoietic progenitor cells via activation of receptors expressed on the cell membrane. Most of these receptors share structural features and have been grouped in the haemopoietin or class I receptor superfamily. Recently considerable progress has been made in elucidating the regions critical for the function of these receptors and the signal transduction mechanisms that they activate. Moreover, it has become clear that certain clinical haematological conditions can be linked to specific defects in these receptors. The significance of defects in the receptor for granulocyte colony-stimulating factor (G-CSF) in the pathogenesis of severe congenital neutropenia and acute myeloid leukaemias is discussed.

Critical cytoplasmic domains of human interleukin-9 receptor alpha chain in interleukin-9-mediated cell proliferation and signal transduction

Interleukin-9 receptor (IL-9R) complex consists of a ligand-specific alpha chain and IL-2R gamma chain. In this study, two regions in the cytoplasmic domain of human IL-9Ralpha were found to be important for IL-9-mediated cell growth. A membrane-proximal region that contains the BOX1 consensus sequence is required for IL-9-induced cell proliferation and tyrosine phosphorylation of Janus kinases (JAKs). Deletion of this region or internal deletion of the BOX1 motif abrogated IL-9-induced cell proliferation and signal transduction. However, substitution of the Pro-X-Pro in the BOX1 motif with Ala-X-Ala failed to abolish IL-9-induced cell proliferation but decreased IL-9-mediated tyrosine phosphorylation of JAK kinases, insulin receptor substrate-2, and signal transducer and activator of transcription 3 (STAT3) and expression of c-myc and junB. Another important region is downstream of the BOX1 motif and contains a STAT3 binding motif YLPQ. Deletion of this region significantly impaired IL-9-induced cell growth, activation of JAK kinases, insulin receptor substrate-2, and STAT3 and expression of early response genes. A point mutation changing YLPQ into YLPA greatly reduced IL-9-induced activation of STAT3 and expression of c-myc but did not affect cell proliferation. These results suggest that cooperation or cross-talk of signaling molecules associated with different domains of IL-9Ralpha other than STAT3 is essential for IL-9-mediated cell growth.

Erythropoietin Induces Tyrosine Phosphorylation of the Interleukin-3 Receptor β Subunit (βIL3 ) and Recruitment of Stat5 to Possible Stat5-Docking Sites in βIL3

The receptors for erythropoietin (Epo) and interleukin-3 (IL-3) both induce the ligand-dependent activation of the Jak2 tyrosine kinase. Activated Jak2 then phosphorylates these receptors and thereby recruits various signaling molecules containing the Src homology (SH)-2 domain, including Stat5, to the tyrosine phosphorylated receptors. In the present study, we demonstrate that Epo stimulation induces unidirectional cross-phosphorylation of the IL-3 receptor β subunit (βIL3) on tyrosines and its rapid and transient association with Stat5 in murine IL-3–dependent cell lines engineered to express the Epo receptor (EpoR). Using cell lines expressing various EpoR mutants, it was demonstrated that the Epo-induced tyrosine phosphorylation of βIL3 is dependent on the membrane-proximal EpoR cytoplasmic region involved in the activation of Jak2, but not on the extracellular and transmembrane regions or on the carboxy-terminal 145 amino acid region containing all the intracellular tyrosine residues. It was also shown that IL-3 induces rapid and dose-dependent association of Jak2 with βIL3. However, Epo failed to induce any detectable association of βIL3 with Jak2 or the EpoR. The present study also demonstrates that in IL-3–stimulated cells, an ovine Stat5 mutant harboring a substitution of Tyr694 to Phe, which abolishes the tyrosine phosphorylation required for activation, fails to dimerize with endogenous Stat5, shows sustained binding with tyrosine-phosphorylated βIL3, and inhibits the tyrosine phosphorylation of endogenous Stat5. These results suggest that βIL3 may have Stat5 docking sites, similar to those found in the EpoR, that facilitate the activation of Stat5 by Jak2 and raise the possibility that Epo may cross-activate or transmodulate the IL-3 receptor signaling pathways.

The last proline of Box 1 is essential for association with JAK2 and functional activation of the prolactin receptor

The interaction of prolactin (PRL) with its receptor leads to activation of the tyrosine kinase, Janus kinase 2 (JAK2). In the cytoplasmic juxtamembrane region, a short segment (Box 1) which is conserved in other receptors of the PRL/growth hormone (GH)/cytokine receptor family, is required for signal transduction. To assess the contribution of the different amino acids of Box 1, individual alanine substitutions of all residues, grouped substitution of four prolines (4PA mutant) and individual leucine replacement of the two last prolines (P248L and P250L mutants) were introduced. Here we show that P250L and 4PA (i) inhibit PRL-induced transactivation of a luciferase reporter governed by a beta-caseine gene promoter; (ii) decrease in JAK2 tyrosine kinase activity in biotinylated-PRL precipitates; (iii) impair the interaction between PRLR and JAK2, as evidenced by lack of co-immunoprecipitation, (iv) and prevent the activation of signal transducer and activator of transcription (Stat) as determined by absence of tyrosine phosphorylation of Stat5. Our data suggest that the Box 1 region of the PRL receptor and particularly the last proline is critical for JAK2 association and subsequent activation. These results support the notion that the tyrosine kinase JAK2 is implicated in activation of downstream protein effectors such as Stat5, which are involved in transcription of PRL-responsive genes.

Identification of genes induced by interleukin-3 and erythropoietin via the Jak-Stat5 pathway using enhanced differential display-reverse southern

Cytokines mediate their effects on growth and maturation of hematopoietic cells by binding to their cognate receptors and activating target genes. Interleukin-3 (IL-3) and erythropoietin (Epo) induce signal transduction via the Jak-Stat pathway. We report here on the identification of several known and novel genes induced by IL-3 and Epo, using a modified version of the PCR-based technique, enhanced differential display (EDD). We modified the technique to facilitate the screening and verification of the differential expression of the genes by using reverse Southern blotting (RS) and PCR-Southern blotting, and we called it EDD-RS. From the initial 110 genetags that were identified as differential expressed genes, 14 contained more than one gene. Among the differentially expressed genes, 24 are known genes and 39 are novel genes. Several of the known genes, such as IRF-1 and P21waf, were previously observed by others to be induced by IL-3 and Epo, but their dependence on Stat5 activation in cytokine-dependent cells was unknown. Other known genes, such as crp and Mssp2/1, were not described previously as target genes for cytokine induction. The results demonstrate that EDD-RS is an efficient method to identify cytokine-induced genes and can be productive in delineating the signal required for their induction.

Mitogenic signaling and inhibition of apoptosis via the erythropoietin receptor Box-1 domain

Studies of proliferative signaling via type 1 cytokine receptors have revealed a three-step activation mechanism. Cytokine-induced receptor dimerization mediates the trans-phosphorylation of Jak kinases, Jaks phosphorylate receptors at tyrosine sites, and SH2 domain-encoding effectors then are recruited to these sites. Signaling factors that associate with activated erythropoietin (Epo) receptor complexes include phospholipase C-gamma, phosphatidylinositol 3-kinase, SHIP, Shc, Grb2, Cbl, Crk-l, HCP, Syp, and STAT5. While at least certain of these factors modulate proliferative signaling, mutated Epo receptor forms lacking Tyr(P) sites retain substantial mitogenic activity. Presently we show that a highly truncated Epo receptor form that retains box-1, yet lacks the conserved box-2 domain (and all Tyr(P) sites) nonetheless effectively promotes mitogenesis, survival, and Myc and Pim-1 expression. In addition, mitogenesis and Myc expression are shown to be supported by a direct Epo receptor-Jak2 kinase domain chimera. Thus, Epo-dependent mitogenesis and inhibition of apoptosis each depend critically upon only the Epo receptor box-1 domain, with no essential role exerted in these response pathways by the box-2 domain.

THE IFNγ RECEPTOR:A Paradigm for Cytokine Receptor Signaling

▪ Abstract  During the last several years, the mechanism of IFNγ-dependent signal transduction has been the focus of intense investigation. This research has recently culminated in the elucidation of a comprehensive molecular understanding of the events that underlie IFNγ-induced cellular responses. The structure and function of the IFNγ receptor have been defined. The mechanism of IFNγ signal transduction has been largely elucidated, and the physiologic relevance of this process validated. Most recently, the molecular events that link receptor ligation to signal transduction have been established. Together these insights have produced a model of IFNγ signaling that is nearly complete and that serves as a paradigm for signaling by other members of the cytokine receptor superfamily.

Erythropoietin-Induced Activation of STAT5 Is Impaired in the Myelodysplastic Syndrome

Patients with myelodysplastic syndrome (MDS) have ineffective in vivo and in vitro erythropoiesis, characterized by an impaired response to erythropoietin (Epo). We examined proliferation and maturation of MDS marrow cells in response to Epo in more detail. Epo-dependent DNA synthesis as well as induction of GATA-1 binding activity in marrow cells from 15 MDS cases were severely reduced as compared with normal bone marrow (NBM). Additionally, the appearance of morphologically identifiable erythroid cells was decreased in MDS cell cultures. These data indicate that both the Epo-dependent proliferation as well as the differentiation induction by Epo is suppressed. To study more upstream events of the Epo signal transduction route we investigated activation of the signal transducer and activator of transcription (STAT) 5. In all 15 MDS samples tested, STAT5 activation was absent or greatly suppressed in response to Epo. In contrast, interleukin-3 induced a normal STAT5 response in MDS cells. Further, in MDS the subset of CD71+ BM cells that is phenotypically similar to Epo-responsive cells in normal marrow, was present. We conclude that the Epo response in MDS is disturbed at an early point in the Epo receptor (EpoR) signal transduction pathway.

Erythropoietin-Induced Activation of STAT5 Is Impaired in the Myelodysplastic Syndrome

Patients with myelodysplastic syndrome (MDS) have ineffective in vivo and in vitro erythropoiesis, characterized by an impaired response to erythropoietin (Epo). We examined proliferation and maturation of MDS marrow cells in response to Epo in more detail. Epo-dependent DNA synthesis as well as induction of GATA-1 binding activity in marrow cells from 15 MDS cases were severely reduced as compared with normal bone marrow (NBM). Additionally, the appearance of morphologically identifiable erythroid cells was decreased in MDS cell cultures. These data indicate that both the Epo-dependent proliferation as well as the differentiation induction by Epo is suppressed. To study more upstream events of the Epo signal transduction route we investigated activation of the signal transducer and activator of transcription (STAT) 5. In all 15 MDS samples tested, STAT5 activation was absent or greatly suppressed in response to Epo. In contrast, interleukin-3 induced a normal STAT5 response in MDS cells. Further, in MDS the subset of CD71+ BM cells that is phenotypically similar to Epo-responsive cells in normal marrow, was present. We conclude that the Epo response in MDS is disturbed at an early point in the Epo receptor (EpoR) signal transduction pathway.

Ectopic Expression of the Erythropoietin Receptor in a Murine Interleukin-6–Dependent Plasmacytoma Cell Line (TEPC-2027) Confers Proliferative Responsiveness to Erythropoietin

To compare the signal transduction pathways used by erythropoietin (Epo) and interleukin-6 (IL-6), the cDNA for the murine Epo receptor (Epo-R) was introduced into an IL-6–responsive plasmacytoma cell line (TEPC-2027) by retrovirally mediated gene transfer. G418-resistant clones were amplified in IL-6 and studied for their ability to grow and differentiate in response to Epo. Epo-R synthesized from the viral gene showed the same affinity for Epo as did the receptor on erythroid cells; however, the numbers of Epo receptors expressed on the cell membrane varied among clones. After a delay of 3 to 5 days in the presence of Epo, all the clones studied proliferated as well in response to Epo as in response to IL-6. In response to IL-6, Stat3 was activated and JunB mRNA was accumulated, whereas in response to Epo, Jak2 and Stat5 were activated and JunB mRNA was not accumulated in Epo-R–expressing TEPC (Epo-R/TEPC) cells. These results suggest that Epo and IL-6 transduced their proliferative signals through different pathways. Further studies showed that, in Epo-R/TEPC cells, Epo neither induces the synthesis of erythroid-specific mRNA nor modifies the synthesis of γ1 Ig heavy chain, suggesting that ectopic expression of the Epo-R in plasmacytoma cells does not modify their differentiative potential. The data show that Epo induces a proliferative response without differentiation providing a new cellular model for evaluating molecular events specific for proliferation.

The IFN gamma receptor: a paradigm for cytokine receptor signaling

During the last several years, the mechanism of IFN gamma-dependent signal transduction has been the focus of intense investigation. This research has recently culminated in the elucidation of a comprehensive molecular understanding of the events that underlie IFN gamma-induced cellular responses. The structure and function of the IFN gamma receptor have been defined. The mechanism of IFN gamma signal transduction has been largely elucidated, and the physiologic relevance of this process validated. Most recently, the molecular events that link receptor ligation to signal transduction have been established. Together these insights have produced a model of IFN gamma signaling that is nearly complete and that serves as a paradigm for signaling by other members of the cytokine receptor superfamily.

Erythropoietin Activates Raf1 by an Shc-Independent Pathway in CTLL-EPO-R Cells

Stimulation of the erythropoietin receptor (EPO-R) or the interleukin-2 receptor (IL-2-R) by their respective ligands has been reported to activate tyrosine phosphorylation of the cytoplasmic protein, Shc. We have recently characterized a cell line, CTLL-EPO-R, that contains functional cell-surface receptors for both EPO and IL-2. Although stimulation with IL-2 or IL-15 resulted in the rapid, dose-dependent tyrosine phosphorylation of Shc, stimulation with EPO failed to activate Shc. EPO, IL-2, and IL-15 activated the tyrosine phosphorylation of the adaptor protein, Shp2, and the association of Shp2/Grb2/cytokine receptor complexes. In addition, EPO, IL-2, and IL-15 activated Raf1 and ERK2, demonstrating that the Raf1/MEK/MAP kinase pathway was activated. These results indicate that multiple biochemical pathways are capable of conferring a mitogenic signal in CTLL-EPO-R. EPO can activate the Raf1/MEK/ MAP kinase pathway via Shc-dependent or Shc-independent pathways, and Shc activation is not required for EPO-dependent cell growth in CTLL-EPO-R.

Erythropoietin Activates Raf1 by an Shc-Independent Pathway in CTLL-EPO-R Cells

Stimulation of the erythropoietin receptor (EPO-R) or the interleukin-2 receptor (IL-2-R) by their respective ligands has been reported to activate tyrosine phosphorylation of the cytoplasmic protein, Shc. We have recently characterized a cell line, CTLL-EPO-R, that contains functional cell-surface receptors for both EPO and IL-2. Although stimulation with IL-2 or IL-15 resulted in the rapid, dose-dependent tyrosine phosphorylation of Shc, stimulation with EPO failed to activate Shc. EPO, IL-2, and IL-15 activated the tyrosine phosphorylation of the adaptor protein, Shp2, and the association of Shp2/Grb2/cytokine receptor complexes. In addition, EPO, IL-2, and IL-15 activated Raf1 and ERK2, demonstrating that the Raf1/MEK/MAP kinase pathway was activated. These results indicate that multiple biochemical pathways are capable of conferring a mitogenic signal in CTLL-EPO-R. EPO can activate the Raf1/MEK/ MAP kinase pathway via Shc-dependent or Shc-independent pathways, and Shc activation is not required for EPO-dependent cell growth in CTLL-EPO-R.

Interleukin-3 (IL-3) inhibits erythropoietin-induced differentiation in Ba/F3 cells via the IL-3 receptor alpha subunit

Introduction of erythropoietin receptors (EpoRs) into the interleukin-3 (IL-3)-dependent murine hemopoietic cell line, Ba/F3, enables these cells to not only proliferate, after an initial lag in G1, but also to increase beta-globin mRNA levels in response to erythropoietin (Epo). With IL-3 and Epo costimulation, IL-3-induced signaling appears to be dominant since no increase in beta-globin mRNA occurs. Differentiation and proliferation signals may be uncoupled since EpoRs lacking all eight intracellular tyrosines were compromised in proliferative signaling but retained erythroid differentiation ability. Intriguingly, a chimeric receptor of the extracellular domain of the EpoR and the transmembrane and intracellular domains of IL-3RbetaIL-3 chain (EpoR/IL-3RbetaIL-3) was capable of Epo-induced proliferative and differentiating signaling, suggesting either the existence of a second EpoR subunit responsible for differentiation or that the alpha subunit of the IL-3 receptor (IL-3R) prevents it. Arguing against the former, a truncated EpoR lacking an intracellular domain was incapable of promoting proliferation or differentiation. An EpoR/IL-3Ralpha chimera, in contrast, was capable of transmitting a weak Epo-induced proliferative signal but failed to stimulate accumulation of beta-globin mRNA. Most significantly, coexpression of the EpoR/IL-3Ralpha chimera with either EpoR/IL-3Rbeta or wild-type EpoRs suppressed Epo-induced beta-globin mRNA accumulation. Taken together, these results suggest an active role for the IL-3Ralpha subunit in inhibiting EpoR-specific differentiating signals.

Cytokine receptor signal transduction and the control of hematopoietic cell development

The cytokine receptor superfamily is characterized by structural motifs in the exoplasmic domain and by the absence of catalytic activity in the cytosolic segment. Activated by ligand-triggered multimerization, these receptors in turn activate a number of cytosolic signal transduction proteins, including protein tyrosine kinases and phosphatases, and affect an array of cellular functions that include proliferation and differentiation. Molecular study of these receptors is revealing the roles they play in the control of normal hematopoiesis and in the development of disease.

An epidermal growth factor receptor/Jak2 tyrosine kinase domain chimera induces tyrosine phosphorylation of Stat5 and transduces a growth signal in hematopoietic cells

The Jak family of tyrosine kinases and the Stat family of transcription factors have been implicated in transducing signals from the hematopoietic growth factor receptors. To explore the role played by a member of the Jak family, Jak2, in hematopoietic cell growth signaling, we constructed a chimeric cDNA coding for the Jak2 tyrosine kinase domain linked to the extracellular and transmembrane regions of the epidermal growth factor (EGF) receptor (EGFR) and expressed the chimera in an interleukin (IL)-3-dependent cell line, 32D. When deprived of IL-3, EGF prevented apoptosis of the transfected cells, induced dose-dependent proliferation, and supported long-term growth. EGF stimulation of the transfectants induced dose-dependent tyrosine phosphorylation of the EGFR/Jak2 chimera and Stat5, which correlated with the EGF dose dependence of cell proliferation. On the other hand, EGF did not induce tyrosine phosphorylation of other factors implicated in cytokine receptor signaling, including the IL-3 receptor beta subunit, Jak kinases, Stat proteins other than Stat5, Shc, Syp, and mitogen-activated protein kinases. These results suggest that the activation of Jak2 may be sufficient for transducing a growth signal in hematopoietic cells by activating the Stat5 pathway or previously unidentified signaling pathways. In addition, because EGF induces homodimerization of the EGFR to activate its tyrosine kinase activity, the present study, which shows EGF-dependent activation of the EGFR/Jak2 chimera, implies that Jak2 may also become activated by homodimerization.

Signal transduction of erythropoietin in endothelial cells

Erythropoietin (EPO) induces endothelin expression in endothelial cells (EC) and has angiogenic effects. We investigated the intracellular signal transduction of EPO in EC and tested the hypothesis that the proliferative effects of EPO may be mediated by cytosolic calcium, changes in intracellular pH, or tyrosine phosphorylation. Cytosolic calcium and pH were measured with fura-2 and BCECF. Protein phosphorylation was assessed with 32P-labeled EC and two-dimensional (2D) gel chromatography. Tyrosine phosphorylation was measured using specific antityrosine antibodies and confocal microscopy. Proliferation was measured by thymidine incorporation and cell count. No effects of EPO on cytosolic calcium and pH were observed. In contrast, erythropoietin increased phosphorylation of 94, 70, 42, 40, 29 and 25 kDa proteins at five minutes and 60 minutes. Most of the early proteins were tyrosine phosphorylated. Confocal microscopy showed cytosolic as well as membrane-bound tyrosine phosphorylation in resting cells and an EPO-induced translocation of immunoreactivity to the nucleus. Immunostaining for the transcription factor STAT-5 showed that EPO induced a nuclear translocation of STAT-5. EPO 0.5, 2, and 4 U/ml increased proliferation, an effect that was prevented by incubation with the tyrosine kinase inhibitor genistein. We conclude that EPO induces proliferation in EC initially via tyrosine phosphorylation of six distinct proteins, and that the phosphorylation and nuclear translocation of the transcription factor STAT-5 is important for the effects of EPO on EC.

The box1 domain of the erythropoietin receptor specifies Janus kinase 2 activation and functions mitogenically within an interleukin 2 beta-receptor chimera

Several distinct classes of cytokine receptors engage Jak kinases as primary effectors. Among type 1 receptors, Janus-activated kinase (Jak) recruitment is mediated by membrane-proximal cytoplasmic domains, which typically contain conserved box motifs. In the erythropoietin receptor (Epo-R), two such motifs (box1 and box2) have been suggested to be essential for the activation of Jak2 and mitogenesis. Presently, an Epo-R chimera containing the extracellular and box1 domains of the Epo-R (Jak2-associated receptor) and the box2 and carboxyl-terminal domains of the interleukin 2 beta-receptor (IL2beta-R; a Jak1-associated subunit) is shown to activate Jak2. Interestingly, Jak2 also was activated in FDC-P1 cells by a control Epo-R chimera containing the complete IL2beta-R cytoplasmic domain, and mitogenesis was supported by each of these above chimeras. By comparison, in BaF3 cells expressing IL2 receptor alpha and gamma subunits, an ectopically expressed IL2beta-R chimera containing the box1 domain of the Epo-R, activated Jak2 and Jak3 and was as mitogenically active as the wild-type IL2beta-R (Jak1 and Jak3 activation). Thus, the box1 domain of the Epo-R specifies Jak2 activation and functions efficiently within a heterologous IL2 receptor complex that normally activates Jak1 and Jak3.

Modulation of growth factor receptor function by isoform heterodimerization

Activation of prolactin (PRL)-dependent signaling occurs as the result of ligand-induced dimerization of receptor (PRLr). Although three PRLr isoforms (short, intermediate, and long) have been characterized and are variably coexpressed in PRL-responsive tissues, the functional effects of ligand-induced PRLr isoform heterodimerization have not been examined. To determine whether heterodimeric PRLr complexes were capable of ligand-induced signaling and cellular proliferation, chimeras consisting of the extracellular domain of either the alpha or beta subunit of human granulocyte-macrophage colony-stimulating factor receptor (GM-CSFr) and the intracellular domain of the rat intermediate or short PRLr isoforms (PRLr-I or PRLr-S) were synthesized. Because high affinity binding of GM-CSF is mediated by the extracellular domain of one alpha and beta GM-CSFr pair, use of GM-CSFr/PRLr chimera specifically directed the dimerization of the PRLr intracellular domains within ligand-receptor complexes. Stable transfection of these constructs into the Ba/F3 line was demonstrated by Northern blot and immunoprecipitation analyses. Flow cytometry revealed specific binding of a phycoerythrin-conjugated human GM-CSF to the transfectants, confirming cell surface expression of the chimeric receptors. When tested for their ability to proliferate in response to GM-CSF, only chimeric transfectants expressing GM-CSFr/PRLr-I homodimers demonstrated significant [3H]thymidine incorporation. GM-CSF stimulation of transfectants expressing either GM-CSFr/PRLr-S homodimers or GM-CSFr/PRLr-S+1 heterodimers failed to induce proliferation. Consistent with these data, the GM-CSF-induced activation of two phosphotyrosine kinases, Jak2 and Fyn, was observed only in homodimeric GM-CSFr/PRLr-I transfectants. These results show that the PRLr-S functions as a dominant negative isoform, down-regulating both signaling and proliferation mediated by the receptor complex. Thus, structural motifs necessary for Jak2 and Fyn activation within the carboxy terminus of the PRLr-I, absent in the PRLr-S, are required in each member of the dimeric PRLr complex.

Granulocyte-macrophage colony-stimulating factor provokes RAS activation and transcription of c-fos through different modes of signaling

Granulocyte-macrophage colony-stimulating factor (GM-CSF) provokes a proliferative response and induction of early-response genes such as c-fos in target cells. It also induces rapid tyrosine phosphorylation of cellular proteins, including the beta subunit (betac) of its functional receptor. However, locations and functions of phosphorylated tyrosine residues within the betac are unclear. To elucidate the mechanism of the human GM-CSF receptor signal transduction, mutational analyses were made of the cytoplasmic domain of the beta-c, using murine BA/F3 cells. Deletion of the conserved box 1 motif resulted in loss of tyrosine phosphorylation of the betac, thereby indicating an essential role for this motif in activating the tyrosine kinase which phosphorylates betac. A C-terminal truncated mutant at position 589 activated the c-fos promoter, and this activation was diminished by a substitution at tyrosine 577 (Tyr577). However, the same substitution in the full-length betac did not completely abrogate the c-fos promoter activation, hence, redundant signaling pathways probably exist. When we analyzed signaling molecules functioning downstream of the beta-c we found that Tyr577 is essential for Shc phosphorylation, while tyrosine phosphorylation of PTP1D was mediated through Tyr577 as well as through other site(s). We suggest that GM-CSF stimulates at least two modes of signals leading to Ras activation, an event which ultimately gives rise to promoter activation of c-fos.

Truncated erythropoietin receptor in a murine erythroleukemia cell line

The Friend spleen focus forming virus produces a 55 kDa envelope glycoprotein which associates with the erythropoietin receptor. We compared the erythropoietin receptor in Friend virus transformed murine erythroleukemic F4N and 707 cell lines with the J2E erythroid line generated by the J2 retrovirus. Reverse transcriptase PCR was used to determine transcript size. Erythropoietin receptor cDNAs were then sequenced and protein products analysed by Western blotting and immunoprecipitation. We show here that the F4N murine erythroleukemic cell line had an enlarged erythropoietin receptor mRNA. In contrast, the 707 and J2E cell line had normal sized transcripts for the receptor. Sequence analysis of the receptor in F4N cells revealed that introns which separate the exons coding for the cytoplasmic domain of the receptor were retained in these transcripts. As a consequence, a premature stop codon had been introduced, leaving only four amino acids in the intracellular portion of the receptor molecule. The normal erythropoietin receptor is approx. 66-70 kDa, but immunoprecipitation of [35S]methionine/cysteine labelled cell lysates with an antibody to the amino-terminus of the erythropoietin receptor identified a truncated 37 kDa protein in F4N cells. Despite the severe carboxy-terminal truncation of the erythropoietin receptor, F4N cells continued to proliferate like the other murine erythroleukemia cell lines. This study shows that failure to remove introns from the erythropoietin receptor mRNA in F4N cells has resulted in the production of a smaller protein with virtually no cytoplasmic domain.

Identification of an interferon-gamma receptor alpha chain sequence required for JAK-1 binding

We have shown previously that a four-amino acid block residing at positions 266-269 (LPKS) in the intracellular domain of the human interferon-gamma (IFN-gamma) receptor alpha chain is critical for IFN-gamma-dependent tyrosine kinase activation and biologic response induction. Herein we show that this sequence is required for the constitutive attachment of the tyrosine kinase JAK-1. Using a vaccinia expression system, a receptor alpha chain-specific monoclonal antibody coprecipitated JAK-1 from cells coexpressing JAK-1 and either (a) wild type IFN-gamma receptor alpha chain, (b) a receptor alpha chain truncation mutant containing only the first 59 intracellular domain amino acids, or (c) a receptor mutant containing alanine substitutions for the functionally irrelevant residues 272-275. In contrast, JAK-1 was not coprecipitated when coexpressed with a receptor alpha chain mutant containing alanine substitutions for the functionally critical residues 266-269 (LPKS). Mutagenesis of the LPKS sequence revealed that Pro-267 is the only residue obligatorily required for receptor function. In addition, Pro-267 is required for JAK-1 binding. These results thus identify a site in the IFN-gamma receptor alpha chain required for constitutive JAK-1 association and establish that this association is critical for IFN-gamma signal transduction.

IL-6 cytokine family and signal transduction: a model of the cytokine system

The interleukin 6 (IL-6) cytokine family, which includes IL-6, leukemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF), IL-11 and cardiotrophin-1 (CT-1), exhibits pleiotropy and redundancy in biological activities. The IL-6 family cytokines exhibit a helical structure. Their receptors belong to the type 1 cytokine receptor family. The receptors of the IL-6 family cytokines share a receptor subunit, which explains one of the mechanisms of functional redundancy. In this review, we describe the general features of the IL-6 cytokine family and its signal transduction mechanisms. Many functional properties of the IL-6 family of cytokines and their receptors are general features of the cytokine system.

The action of interleukin-2 receptor subunits defines a new type of signaling mechanism for hematopoietin receptors in hepatic cells and fibroblasts

The gene regulatory functions of the human IL-2 receptor (IL-2R) were reconstituted in transiently transfected hepatoma cells. The combination of IL-2R beta and -gamma mediated a strong stimulation via the cytokine response element of the alpha 1-acid glycoprotein gene and the hematopoietin receptor response element, but none via the IL-6 response element or the sis-inducible element. IL-2R alpha enhanced 10-fold the sensitivity of the IL-2R beta.gamma complex to respond to IL-2 or IL-15, but did not modify the specificity or the magnitude of maximal gene regulation. A homodimerizing chimeric receptor G-CSFR-IL-2R beta could mimic the IL-2R action. The IL-2R-mediated gene regulation was similar to that seen with receptors for IL-4 and IL-7, but differed from that for IL-6 type cytokines, thrombopoietin, erythropoietin, and growth hormone. The activation of STAT proteins by the IL-2R was assessed in transfected L-cells and COS-1 cells. Although IL-2R subunits were highly expressed in these cells, no STAT protein activation was detectable. Transient overexpression of JAK3 was unable to change the signaling specificity of the hematopoietin receptors in rat hepatoma, L-, and COS cells, but established a prominent activation of the IL-6 response elements by the IL-2R and IL-4R in HepG2 cells. The data support the model that the IL-2R and related hematopoietin receptors produce at least two separate signals which control gene expression.

The conserved box 1 motif of cytokine receptors is required for association with JAK kinases

The JAK2 tyrosine kinase is known to associate with the receptors for growth hormone (GH) and erythropoietin (EPO) and with the interleukin-6 receptor signal transducing protein, gp130. Here we demonstrate that chimeric cytokine receptors which contain the cytoplasmic domain of the receptors for GH and EPO or for gp130 can form complexes with JAK2 when transiently co-expressed in HeLa cells. Mutational analyses of chimeras for the the GH and EPO receptors and gp130 demonstrated that box 1, a motif critical for cytokine receptor signal transduction, was required for the association of JAK2. Although JAK2 was capable of associating with all three of the chimeras, JAK1 co-precipitated only with the gp130 chimera. Association of JAK1 and JAK2 with cytokine receptor proteins, therefore, requires the highly conserved box 1 domain, but other sequences within the receptor proteins may influence the specificity of JAK binding. Mutational analysis of JAK2 revealed that multiple or complex protein sequences within JAK2 are required for association with cytokine receptors.

Involvement of SH2-containing phosphotyrosine phosphatase Syp in erythropoietin receptor signal transduction pathways

Erythropoietin (Epo) regulates the proliferation and differentiation of erythroid precursors. The phosphorylation of proteins at tyrosine residues is critical in the growth signaling induced by Epo. This mechanism is regulated by the activities of both protein-tyrosine kinases and protein tyrosine phosphatases. The discovery of phosphotyrosine phosphatases that contain SH2 domains suggests roles for these molecules in growth factor signaling pathways. We found that Syp, a phosphotyrosine phosphatase, widely expressed in all tissues in mammals became phosphorylated on tyrosine after stimulation with Epo in M07ER cells engineered to express high levels of human EpoR. Syp was complexed with Grb2 in Epo-stimulated M07ER cells. Direct binding between Syp and Grb2 was also observed in vitro. Furthermore, Syp appeared to bind directly to tyrosine-phosphorylated EpoR in M07ER cells. Both NH2-terminal and COOH-terminal SH2 domains of Syp, made as glutathione S-transferase fusion proteins, were able to bind to the tyrosine-phosphorylated EpoR in vitro. These results suggest that Syp may be an important signaling component downstream of the EpoR and may regulate the proliferation and differentiation of hematopoietic cells.

Erythroleukaemia induction by the Friend spleen focus-forming virus

The Friend spleen focus-forming virus has been a valuable tool for understanding the molecular events involved in the multiple stages of leukaemia. As summarized in Figure 3, the primary effect of SFFV, which occurs within days, is to cause a polyclonal proliferation of erythroid precursor cells that can proliferate in the absence of their normal regulator erythropoietin. This is the direct result of the unique envelope glycoprotein encoded by SFFV, which is transported to the cell surface and apparently interacts with the EpoR or another component of the multimeric EpoR complex, resulting in the constitutive activation of the Epo signal transduction pathway. Within this proliferating population of erythroid cells is a rare cell that has undergone several genetic changes due to the integration of the viral genome in specific sites in the mouse DNA. This leads to the activation of a gene encoding the PU.1 transcription factor, whose high expression in erythroid cells may be the cause of the block in differentiation that is characteristic of SFFV-transformed erythroid cells. SFFV integration can also lead to the inactivation of the p53 tumour supressor gene, giving these cells a growth advantage in the mouse. The disease induced by SFFV in mice is very similar to polycythaemia vera in humans (Golde et al, 1981). The major clinical feature of polycythaemia vera is the continuous expansion of the number of mature red blood cells in the presence of low serum Epo levels. Also, BFU-E and CFU-E from these patients can form in the absence of Epo like the analogous cells from SFFV-infected mice (Casadevall et al, 1982). It is possible that haematopoietic cells from individuals suffering from this disease express a protein similar to the envelope glycoprotein of SFFV that can interact with the EpoR and lead to its constitutive activation. Alternatively, these patients may contain a mutant EpoR gene that is constitutively activated like the mutant EpoR described earlier. As we understand more fully how the SFFV envelope protein constitutively activates te EpoR complex, we can begin to design therapies to counteract its action that can then be applied to treating patients with polycythaemia vera or other human diseases associated with uncontrolled erythropoiesis.

Conserved region of the cytoplasmic domain is not essential for erythropoietin-dependent growth

Recent studies have suggested that the membrane proximal region of the cytoplasmic domain of the erythropoietin receptor and other members of the cytokine receptor superfamily may be required for signal transduction. Expression of several deletion mutants of the erythropoietin receptor in Ba/F3 cells showed that a region with homology to the interleukin-2 receptor beta-chain which includes Box 2 is not essential for erythropoietin-dependent cell proliferation. However, a region between Box 1 and Box 2 contains essential residues for proliferative response. Expression of mutant receptors was confirmed by reverse transcriptase-PCR analysis and by Western blotting, which also showed no evidence for expression of endogenous wild-type receptor. These findings are in direct conflict with previously reported mutagenesis studies of the erythropoietin receptor suggesting that mitogenesis may be channelled through more than one pathway depending on the complement of signaling molecules expressed in the cell.

The Janus protein tyrosine kinase family and its role in cytokine signaling

During the past 2 years, research from quite divergent areas has converged to provide the first insights into the mechanisms by which cytokines that utilize receptors of the cytokine receptor superfamily function. On the one hand, the obscure Jak family of cytoplasmic protein tyrosine kinases was independently implicated in IFN and hematopoietic growth factor signaling. Recent studies have expanded these initial observations to demonstrate that Jaks are critical to the functioning of all the receptors of the cytokine receptor superfamily. A variety of questions remain to be explored regarding the structure and function of Jaks and their interaction with receptors. It will also be important to pursue additional approaches to determine if the Jaks are necessary for various biological responses, particularly for mitogenic responses. The second major area of convergence has been the demonstration that members of the Stat family of transcription factors, initially identified in IFN-regulated gene expression, are generally involved in cytokine signaling. Clearly, a number of Stat-like activities remain to be cloned and it can be anticipated that the family contains additional members. Although a variety of genes are known to be regulated by the Stats association with IFN responses, much less is known concerning the genes regulated by the new Stats in cytokine signaling. Of particular importance is information relating to their potential contribution to mitogenic responses. From a biochemical standpoint, the Stats represent a remarkable family of proteins with regard to the ability of the modification of a single tyrosine residue to so dramatically affect cellular localization and DNA binding activity. Studies to identify the domains involved, and associated proteins that might contribute to either property, will be of considerable interest. More generally, it can hypothesized that Jaks and Stats, if important for proliferation and differentiation, may be the targets for malignant transformation. Although none of the genes map to chromosomal breakpoints that have been implicated in transformation, gain of function mutations is a likely mechanism that needs to be explored. Similarly, the Jak-Stat pathway would appear to be an excellent target for the development of drugs that affect a variety of cytokine functions.

JAK protein tyrosine kinases: their role in cytokine signalling

Protein tyrosine kinases (PTKs) are integral components of the cellular machinery that mediates the transduction and/or processing of many extra- and intracellular signals. Members of the JAK family of intracellular PTKs (JAK1, JAK2 and TYK2) are characterized by the possession of a PTK-related domain and five additional homology domains, in addition to a classical PTK domain. An important breakthrough in the understanding of JAK kinases function(s) has come from the recent observations that many cytokine receptors compensate for their lack of a PTK domain by utilizing members of the JAK family for signal transduction.

Signal transduction through gp130 that is shared among the receptors for the interleukin 6 related cytokine subfamily

Interleukin 6 (IL-6) and related cytokines, such as leukemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF) and IL-11 exhibit multiple functions and redundancy in biological activities and play important roles in the immune response, hematopoiesis, the nervous system and acute phase reactions. These IL-6 family cytokines exhibit a similar helical structure, and their receptors are structurally similar and constitute a cytokine receptor super family. In addition, a receptor subunit is shared among these IL-6 related cytokine subfamily receptors, contributing to one of the mechanisms of functional redundancy of cytokine activities and suggesting the presence of a common signal transduction pathway among these receptors. In this review, we describe the structure of the receptors for IL-6 and its related cytokine subfamily members. Furthermore, we propose a novel mechanism for the generation of cytokine diversity, i.e. the complex of a cytokine and one of its receptor subunits act as a novel cytokine on the cells that express the other receptor subunit(s) capable of acting as a receptor for the complex. Finally, we describe a Ras-independent novel signal transduction pathway that utilizes Jak tyrosine kinase family, Stat protein family and yet unidentified H-7-sensitive pathway. This signal transduction pathway is commonly generated through the receptors for a wide range of cytokines and growth factors.

Signaling by the cytokine receptor superfamily just another kinase story

Many cytokines initiate cellular responses through their interaction with members of the cytokine receptor superfamily. This family of receptors contains no catalytic domains in the cytoplasmic domain, but all couple ligand binding to tyrosine phosphorylation, and this activity requires a membrane-proximal region that contains some similarity among the receptors. Recent studies have shown that members of the JAK family of protein tyrosine kinases associate with the membraneproximal region, are rapidly tyrosine-phosphorylated following ligand binding, and their in vitro kinase activity is activated. The JAK family of kinases is characterized by two kinase domains, only one of which contains all of the hallmarks of active kinases. This family of 130-kD kinases lacks SH2 or SH3 domains, but family members contain extensive homology in the large amino terminal region. Individual receptors associate with, or require, one or more of the three known family members including JAKI, JAK2, and tyk2. Putative substrates of the JAK family of kinases include the 91-kD and 113-kD proteins of the interferon-stimulated transcription complex ISGF3 that, when tyrosinephosphorylated, migrate to the nucleus and participate in the activation of gene transcription. Recent evidence suggests that the 91- and 113-kD proteins are members of a large family of genes that are potential substrates o f JAK family members and may regulate a variety o f genes involved in cell growth, differentiation, or function. Together the data provide a new, generalized model for the mechanisms by which cytokines that utilize receptors of the cytokine receptor superfamily regulate cellular activity.