Activating Mutations in Cytokine Receptors: Implications for Receptor Function and Role in Disease

An Integrative multi-lineage model of variation in leukopoiesis and acute myelogenous leukemia

Background

Acute myelogenous leukemia (AML) progresses uniquely in each patient. However, patients are typically treated with the same types of chemotherapy, despite biological differences that lead to differential responses to treatment.

Results

Here we present a multi-lineage multi-compartment model of the hematopoietic system that captures patient-to-patient variation in both the concentration and rates of change of hematopoietic cell populations. By constraining the model against clinical hematopoietic cell recovery data derived from patients who have received induction chemotherapy, we identified trends for parameters that must be met by the model; for example, the mitosis rates and the probability of self-renewal of progenitor cells are inversely related. Within the data-consistent models, we found 22,796 parameter sets that meet chemotherapy response criteria. Simulations of these parameter sets display diverse dynamics in the cell populations. To identify large trends in these model outputs, we clustered the simulated cell population dynamics using k-means clustering and identified thirteen ‘representative patient’ dynamics. In each of these patient clusters, we simulated AML and found that clusters with the greatest mitotic capacity experience clinical cancer outcomes more likely to lead to shorter survival times. Conversely, other parameters, including lower death rates or mobilization rates, did not correlate with survival times.

Conclusions

Using the multi-lineage model of hematopoiesis, we have identified several key features that determine leukocyte homeostasis, including self-renewal probabilities and mitosis rates, but not mobilization rates. Other influential parameters that regulate AML model behavior are responses to cytokines/growth factors produced in peripheral blood that target the probability of self-renewal of neutrophil progenitors. Finally, our model predicts that the mitosis rate of cancer is the most predictive parameter for survival time, followed closely by parameters that affect the self-renewal of cancer stem cells; most current therapies target mitosis rate, but based on our results, we propose that additional therapeutic targeting of self-renewal of cancer stem cells will lead to even higher survival rates.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-017-0469-2) contains supplementary material, which is available to authorized users.

Growth and Differentiation Factors

This review is restricted to neutrophilic granulocytes (granulocytes), monocytes (macrophages), and eosinophils, with only passing reference to cells that are also usually included in the "myeloid" category-megakaryocytes, mast cells, and erythroid cells. Although some dendritic cells are of myeloid origin, they are discussed elsewhere. The validity of the information to be described depends on two assumptions: (a) that in vitro data are applicable to events in vivo and (b) that mouse data reflect events in man. Both assumptions are likely to be broadly correct.

The colony-stimulating factors and cancer

The colony-stimulating factors (CSFs) are the master regulators of granulocyte and macrophage populations. There are four different aspects of the connection between the CSFs and cancer: (a) the CSFs can accelerate the regeneration of protective white cells damaged by chemotherapy; (b) the CSFs can mobilize stem cells to the peripheral blood in convenient numbers for transplantation; (c) the CSFs can enhance anticancer immune responses and (d) the CSFs are potentially involved in the genesis of the myeloid leukemias.

Progress in detecting cell-surface protein receptors: the erythropoietin receptor example

Testing for the presence of specific cell-surface receptors (such as EGFR or HER2) on tumor cells is an integral part of cancer care in terms of treatment decisions and prognosis. Understanding the strengths and limitations of these tests is important because inaccurate results may occur if procedures designed to prevent false-negative or false-positive outcomes are not employed. This review discusses tests commonly used to identify and characterize cell-surface receptors, such as the erythropoietin receptor (EpoR). First, a summary is provided on the biology of the Epo/EpoR system, describing how EpoR is expressed on erythrocytic progenitors and precursors in the bone marrow where it mediates red blood cell production in response to Epo. Second, studies are described that investigated whether erythropoiesis-stimulating agents could stimulate tumor progression in cancer patients and whether EpoR is expressed and functional on tumor cells or on endothelial cells. The methods used in these studies included immunohistochemistry, Northern blotting, Western blotting, and binding assays. This review summarizes the strengths and limitations of these methods. Critically analyzing data from tests for cell-surface receptors such as EpoR requires understanding the techniques utilized and demonstrating that results are consistent with current knowledge about receptor biology.

The colony-stimulating factors and cancer

The four colony-stimulating factors (CSFs) are glycoproteins that regulate the generation and some functions of infection-protective granulocytes and macrophages. Recombinant granulocyte-CSF (G-CSF) and granulocyte-macrophage-CSF (GM-CSF) have now been used to increase dangerously low white blood cell levels in many millions of cancer patients following chemotherapy. These CSFs also release haematopoietic stem cells to the peripheral blood, and these cells have now largely replaced bone marrow as more effective populations for transplantation to cancer patients who have treatment-induced bone marrow damage.

The Asn505 mutation of the c-MPL gene, which causes familial essential thrombocythemia, induces autonomous homodimerization of the c-Mpl protein due to strong amino acid polarity

We previously reported that a dominant-positive activating mutation (Asn505) in the transmembrane domain (TMD) of c-MPL, which encodes the thrombopoietin receptor, caused familial essential thrombocythemia. Here, we show that the Asn505 mutation induces both autonomous dimerization of c-Mpl and signal activation in the absence of its ligand. Signal activation was preserved in a truncated mutant of Asn505 that lacked the extracellular domain of c-MPL. We also found that the substitution of the amino acid (AA) residue at position 505 with others of strong polarity (Glu, Asp, or Gln) also resulted in activated dimerization without ligand stimulation. Overall, these data show that the Asn505 mutation transduced the signal through the autonomous dimerization of the c-MPL protein due to strong AA polarity. This finding provides a new insight into the mechanism of disease causation by mutations in the TMD of cytokine/hematopoietic receptors.

Hematopoietic growth factor mimetics: from concept to clinic

Hematopoietic growth factor (HGF) mimetics offer a number of attractive advantages as therapeutic agents. Small chemical compounds, in particular, provide reduced cost and oral availability. As many of these mimetics are unrelated in structure to the normal cytokine the immunogenic response is not a significant issue. Isolation of small peptide agonists for erythropoietin (EPO) and thrombopoietin (TPO) receptors has been associated with significant translational challenges and here we summarize approaches used to achieve the potency and stability required for clinical utility. We also compare and contrast the initial screening approaches, and the translational and clinical issues associated with two recently approved TPO mimetics, romiplostim and the orally available eltrombopag. Finally we summarize the development and clinical findings for the EPO mimetic, Hematide, consider alternative approaches, and discuss the future potential for isolation of growth factor (GF) mimetics.

The Shc-binding site of the betac subunit of the GM-CSF/IL-3/IL-5 receptors is a negative regulator of hematopoiesis

Tyrosine and serine phosphorylation of the common beta chain (beta(c)) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is widely viewed as a general mechanism that provides positive inputs by coupling the receptor to signaling pathways that stimulate several cellular functions. We show here that despite the known action of Tyr577 in beta(c) to recruit Shc-PI-3 kinase (PI3K) pathway members, Tyr577 plays, surprisingly, a negative regulatory role in cell function, and that this is mediated, at least in part, through the uncoupling of SH2-containing inositol 5'-phosphatase (SHIP) from beta(c). Fetal liver cells from beta(c)/beta(IL-3)(-/-) mice expressing human GM-CSF receptor alpha chain and beta(c) Tyr577Phe mutant showed enhanced colony formation and expansion of progenitor cells in response to GM-CSF. Dissection of these activities revealed that basal survival was increased, as well as cytokine-stimulated proliferation. As expected, the recruitment and activation of Shc was abolished, but interestingly, Gab-2 and Akt phosphorylation increased. Significantly, the activation of PI3K was enhanced and prolonged, accompanied by loss of SHIP activity. These results reveal a previously unrecognized negative signaling role for Tyr577 in beta(c) and demonstrate that uncoupling Shc from cytokine receptors enhances PI3K signaling as well as survival and proliferation.

Could prolactin receptor gene polymorphism play a role in pathogenesis of breast carcinoma?

OBJECTIVE

Constitutive activation of various hormone and growth factor receptors is newly recognised as a common cause of tumour development. This study investigated the presence of any mutation or polymorphism of prolactin receptor (PRLR) in 38 patients with breast cancer.

RESEARCH METHODS

Genomic DNA was extracted and PCR amplification was carried out for exon 1-10 of PRLR from tumoral and adjacent non-cancerous breast tissue of tumour specimens from 38 breast cancer patients. PCR products were analysed by SSCP and automatic sequencing for mutations.

RESULTS

For the first time, A150C (Leu-->Ile) transversion at exon 6 of PRLR in tumour tissues, in adjacent non-cancerous breast tissues, and in blood samples of two (5.3%) out of 38 patients with breast cancer were detected. In contrast to this finding, no polymorphism of PRLR in blood samples of 100 normal individuals were found.

CONCLUSION

Polymorphism of prolactin receptors might play a role in mammary carcinogenesis as a consequence of intracellular changes of PRLR signalling.

Constitutive mutants of the GM-CSF receptor reveal multiple pathways leading to myeloid cell survival, proliferation, and granulocyte-macrophage differentiation

Activation of the granulocyte-macrophage colony-stimulating factor (GM-CSF) family of receptors promotes the survival, proliferation, and differentiation of cells of the myeloid compartment. Several signaling pathways are activated downstream of the receptor, however it is not clear how these induce specific biologic outcomes. We have previously identified 2 classes of constitutively active mutants of the shared signaling subunit, human (h) betac, of the human GM-CSF/interleukin-3 (IL-3)/IL-5 receptors that exhibit different modes of signaling. In a factor-dependent bipotential myeloid cell line, FDB1, an activated mutant containing a substitution in the transmembrane domain (V449E) induces factor-independent proliferation and survival, while mutants in the extracellular domain induce factor-independent granulocyte-macrophage differentiation. Here we have used further mutational analysis to demonstrate that there are nonredundant functions for several regions of the cytoplasmic domain with regard to mediating proliferation, viability, and differentiation, which have not been revealed by previous studies with the wild-type GM-CSF receptor. This unique lack of redundancy has revealed an association of a conserved membrane-proximal region with viability signaling and a critical but distinct role for tyrosine 577 in the activities of each class of mutant.

Formation of an active form of the interleukin-2/15 receptor beta-chain by insertion of the intracisternal A particle in a radiation-induced mouse thymic lymphoma and its role in tumorigenesis

Although many reports suggest that aberrant regulation of cytokine signaling pathways via the interleukin-2 receptor (IL-2R) induces tumorigenic transformation, constitutively active IL-2R in tumors has not been reported. We searched for genomic alteration of the IL-2/15R beta-subunit gene (IL-2/15R beta) in cytokine-independent cell lines established from radiation-induced mouse thymic lymphomas. In the TL34 cell line and its primary tumor, one of the IL-2/15R beta alleles was rearranged by the insertion of an intracisternal A particle (IAP) retrotransposon. The IAP-IL2/15R beta chimeric gene expressed chimeric mRNA in which IAP-coding Gag-Pol mRNA was fused to IL-2/15R beta mRNA and coded for Gag-Pol-IL-2/15R beta chimeric protein. Forced expression of the Gag-Pol-IL-2/15R beta chimeric cDNA in a mouse cytotoxic T-cell line (CTLL-2) converted IL-2-dependent cell growth to IL-2-independent growth, suggesting that the chimeric protein activates some of the IL-2 signaling pathways necessary for cell proliferation. Downregulation of the expression of the Gag-Pol-IL-2/15R beta chimeric protein in TL34 by antisense RNA inhibited cell growth, and concomitantly reduced the level of c-myc protein. These results suggest that the Gag-Pol-IL-2/15R beta is a constitutively active form that transmits proliferative signals by expressing downstream target genes, including c-myc. Thus, we demonstrated that the chimeric receptor gene produced by the insertion of an IAP functions as an oncogene by providing IL-2-independent autonomous growth potential.

Clinical manifestations associated with the aberrant expression of the soluble granulocyte-macrophage colony-stimulating factor receptor in patients presenting with haematological malignancies

The receptor for granulocyte-macrophage colony-stimulating factor (GM-CSF) can exist as both transmembrane (tmGMRalpha) and soluble (solGMRalpha) isoforms, and the latter, is a normal constituent of human plasma. We investigated if aberrant solGMRalpha expression occurs in haematopoietic malignancies and whether or not solGMRalpha expression levels correlated with clinical presentation. Compared with the normal population, patients with acute lymphoblastic leukaemia (ALL) had low levels of solGMRalpha whereas clonal disorders of the myeloid lineage demonstrated higher levels of solGMRalpha. Patients with acute myelogenous leukaemia (AML) and high levels of solGMRalpha presented with a distinct clinical picture. These patients were older, predominantly belonged to the M4 and M5 French-American-British (FAB) subtypes, and they had higher white blood cell counts at presentation including myeloid precursors and myeloblasts. They often presented with either unexplained lung infiltrates or hypoxia and lower rates of microbiologically defined infections. Elevated solGMRalpha levels were not associated with decreased relapse-free and overall survival in the AML population. On multivariate analysis, the correlation between elevated solGMRalpha levels and age, M4 and M5 FAB subtypes and decreased numbers of infections persisted. Our study is the first to describe that distinct clinical presentations are associated with aberrant solGMRalpha levels in haematological malignancies.

An activating mutation in the transmembrane domain of the granulocyte colony-stimulating factor receptor in patients with acute myeloid leukemia

To date, constitutively activating point mutations reported in hematopoietic growth factor receptors in patients with acute myeloid leukemia (AML) have been restricted to receptors with intrinsic tyrosine kinase activity such as c-kit and FLT3. We describe here a Thr617Asn mutation in the transmembrane domain of the non-tyrosine kinase receptor for granulocyte colony-stimulating factor (G-CSF) in the blast cells of two out of 555 AML patients examined. The mutant receptor conferred growth factor independence on factor-dependent Ba/F3 cells. In the absence of ligand, immunoblotting showed weak phosphorylation of JAK2, STAT3, ERKs 1 and 2 and the receptor itself, and there was approximately 70% of maximal growth in a proliferation assay. All signals were significantly enhanced in the presence of G-CSF. Retroviral transduction of mutant receptor into primary hematopoietic CD34+ cells induced G-CSF independent myeloid differentiation as assessed by the development of neutrophils and surface expression of CD11b and CD14. These results confirm the importance of the transmembrane domain for receptor function and suggest that introduction of an asparagine residue can cause sufficient stabilization of helix-helix interactions in the absence of ligand to activate downstream signaling pathways involved in directing proliferation and differentiation.

Mutational analysis of the PRL receptor gene in human breast tumors with differential PRL receptor protein expression

PRL is a major growth and differentiating hormone in the human breast, with activation of the PRL-PRL receptor complex increasingly recognized as an important mechanism in the induction and progression of mammary tumors. Although constitutive activation of various hormone and growth factor receptors is newly recognized as a common cause of tumor development, the PRL receptor gene has not been analyzed for similar aberrations in breast and other tumors. Therefore, using bacterial artificial chromosomes containing the PRL receptor gene and intron-spanning PCR, we determined the exon-surrounding intron sequences providing primers for the first analysis of the entire coding region of the human PRL receptor gene. We examined the presence of PRL receptor in 41 breast tumors by immunohistochemistry and attempted a correlation of its expression to pathological grading of the disease. Then tumor cells were isolated by laser capture microdissection to examine DNA from 30 patients for PRL receptor mutations. The PRL receptor immunoreactive score did not correlate to the tumor size, histopathological grading, age, or family history of patients. PRL receptor immunoreactivity was predominantly found in steroid hormone receptor-positive tumors, but without overall correlation of immunoreactive score. In both PRL receptor-positive and PRL receptor- negative breast cancer cells, direct sequencing of the coding sequence of the PRL receptor gene did not detect any somatic or hereditary gene aberrations. In conclusion, PRL receptor mutations do not appear to be common in human breast cancer, suggesting that constitutive activation of the PRL receptor can be excluded as a major cause of mammary tumor genesis. The molecular structure of the PRL receptor seems to remain intact in tumor tissue, and systemic and local production of PRL may participate in tumor cell growth and proliferation through functional receptors.

Peptide insertions in domain 4 of hbeta(c), the shared signalling receptor subunit for GM-CSF, IL3 and IL5, induce ligand-independent activation

A mutant form of the common beta-subunit of the GM-CSF, interleukin-3 (IL3) and IL5 receptors is activated by a 37 residue duplicated segment which includes the WSXWS motif and an adjacent, highly conserved, aliphatic/basic element. Haemopoietic expression of this mutant, hbeta(c)FIDelta, in mice leads to myeloproliferative disease. To examine the mechanism of activation of this mutant we targetted the two conserved motifs in each repeat for mutagenesis. Here we show that this mutant exhibits constitutive activity in BaF-B03 cells in the presence of mouse or human GM-CSF receptor alpha-subunit (GMRalpha) and this activity is disrupted by mutations of the conserved motifs in the first repeat. In the presence of these mutations the receptor reverts to an alternative conformation which retains responsiveness to human IL3 in a CTLL cell line co-expressing the human IL3 receptor alpha-subunit (hIL3Ralpha). Remarkably, the activated conformation is maintained in the presence of substitutions, deletions or replacement of the second repeat. This suggests that activation occurs due to insertion of extra sequence after the WSXWS motif and is not dependent on the length or specific sequence of the insertion. Thus hbeta(c) displays an ability to fold into functional receptor conformations given insertion of up to 37 residues in the membrane-proximal region. Constitutive activation most likely results from a specific conformational change which alters a dormant, inactive receptor complex, permitting functional association with GMRalpha and ligand-independent mitogenic signalling.

Functional identification of secondary mutations inducing autonomous growth in synergy with a truncated interleukin-3 receptor Implications for multi-step oncogenesis

OBJECTIVE

A truncated common beta chain (Deltabeta(C)) of the interleukin-3 (IL-3) receptor complex was previously identified as a key factor in inducing autonomous growth of IL-3-independent mutants. Expression of Deltabeta(C) in IL-3-dependent hematopoietic cells does not result in immediate factor-independent growth, but increases the frequency of obtaining autonomous mutants by three to four orders of magnitude. This study was designed to delineate the mechanisms by which Deltabeta(C) increases the frequency to autonomous growth.

DESIGN AND METHODS

Retroviral vectors were used to express Deltabeta(C) into IL-3-dependent myeloid cells, which were then tested for factor-independent growth. To determine if secondary genetic events were required for conversion to autonomous growth, elements of the Cre-loxP recombinant system were used to excise Deltabeta(C) in factor-independent clones.

RESULTS

Excision of Deltabeta(C) in factor-independent clones revealed two types of phenotypes: reversion to factor-dependent growth (1/8) or continued IL-3-dependent growth (7/8). Analysis of cells that remained factor independent revealed constitutive activation of STAT5, not observed in factor-dependent revertants. Analysis of revertant cells demonstrated the presence of interacting secondary mutations that synergize with Deltabeta(C)-induced proliferation. A cysteine residue within the truncated extracellular domain of Deltabeta(C) was also found to be required for its oncogenic potential, supporting a model of dimerization for receptor activation.

CONCLUSIONS

The high incidence of obtaining factor-independent mutants from cells expressing Deltabeta(C) results from the selection of mutations that either complement Deltabeta(C) expression to promote proliferation or that singly or in synergy with other secondary mutations negate the requirement of Deltabeta(C) expression for proliferation.

GM-CSF binding to its receptor induces oligomerisation of the common beta-subunit

The stoichiometry of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor complex is still unresolved. We have utilised a sensitive, functional assay for receptor homodimerisation to show that GM-CSF induces dimerisation of the common signalling subunit, hbeta(c). We generated a chimeric cytokine receptor in which the extracellular and transmembrane domains of hbeta(c)are fused to the cytoplasmic domain of erythropoietin receptor (EPO-R). Given that to induce EPO-R activation and mitogenic signalling there is a requirement for formation of a specific homodimeric complex, we reasoned that the cytoplasmic domain of EPO-R could be utilised as a highly sensitive reporter for functional homodimer formation. We show that, in the presence of a cytoplasmically truncated GM-CSF alpha-subunit, the hbetac-EPO receptor chimera transduces a mitogenic signal in BaF-B03 in response to GM-CSF. This is consistent with formation of a hbeta(c)homodimer following GM-CSF binding and implies that ligand stimulation induces formation of a higher order complex that contains the hbeta(c)homodimer.

Mouse model of congenital polycythemia: Homologous replacement of murine gene by mutant human erythropoietin receptor gene

Mutations causing truncations of the cytoplasmic domain of the human erythropoietin receptor (EPOR) result in a dominantly inherited disorder-primary familial congenital polycythemia. This disorder is characterized by increased numbers of erythrocytes (polycythemia) and by in vitro hypersensitivity of erythroid precursors to erythropoietin. The consequences of EPOR truncation in nonerythroid tissues are unknown. We replaced the murine EPOR gene with a wild-type human EPOR gene and a mutant human EPOR gene that we initially identified in a patient with polycythemia. This mutation leads to an EPOR truncated after the first tyrosine residue of the intracellular domain. Mice heterozygous for this mutant allele and a wild-type human EPOR allele mimicked the human disorder. Interestingly, mice that were homozygous for the mutant human allele were severely polycythemic but viable. Our results provide a model for functional studies of EPOR-triggered signaling pathways in erythropoiesis. These animals can now be used to investigate the molecular pathophysiology of this gain-of-function EPOR mutation in erythroid tissue and in those nonerythroid tissues that express EPOR.

Expression of interleukin 3 and granulocyte-macrophage colony-stimulating factor receptor common chain betac, betaIT in normal haematopoiesis: lineage specificity and proliferation-independent induction

Interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 5 (IL-5) exert their biological activities through interaction with cell-surface receptors that consist of two subunits, a specific alpha subunit and a common beta transducing subunit (betac). We have evaluated the expression of betac on purified haematopoietic progenitor cells (HPCs) induced to unilineage differentiation/maturation through the erythroid (E), granulocytic (G), megakaryocytic (Mk) or monocytic (Mo) lineage. HPCs displayed low betac expression, which increased during the initial stages of HPC differentiation along the E, G, Mo or Mk lineages. At later stages of differentiation, betac chain expression increased in both G and Mo lineages, was expressed at low levels in the Mk lineage and declined to undetectable levels in the E lineage. Analysis of the full-length betac and intracytoplasmically truncated betac (betaIT) mRNAs showed that the former was predominant in the G and Mo lineages, whereas the latter was prevalent in the E and Mk lineages. The betac induction takes place even in the absence of cell cycling. Thus, incubation of HPCs with graded amounts of IL-3 showed that the initial induction of betac expression is unrelated to cell proliferation. Furthermore, circulating monocytes and granulocytes exhibit a low level of betac expression that is greatly stimulated following incubation with either IL-3 or GM-CSF.

The solution structure of the cytokine-binding domain of the common beta-chain of the receptors for granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5

The haemopoietic cytokines, granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5 bind to cell-surface receptors comprising ligand-specific alpha-chains and a shared beta-chain. The beta-chain is the critical signalling subunit of the receptor and its fourth domain not only plays a critical role in interactions with ligands, hence in receptor activation, but also contains residues whose mutation can lead to ligand-independent activation of the receptor. We have determined the NMR solution structure of the isolated human fourth domain of the beta-chain. The protein has a fibronectin type III fold with a well-defined hydrophobic core and is stabilised by an extensive network of pi-cation interactions involving Trp and Arg side-chains, including two Trp residues outside the highly conserved Trp-Ser-Xaa-Trp-Ser motif (where Xaa is any amino acid) that is found in many cytokine receptors. Most of the residues implicated in factor-independent mutants localise to the rigid core of the domain or the pi-cation stack. The loops between the B and C, and the F and G strands, that contain residues important for interactions with cytokines, lie adjacent at the membrane-distal end of the domain, consistent with their being involved cooperatively in binding cytokines. The elucidation of the structure of the cytokine-binding domain of the beta-chain provides insight into the cytokine-dependent and factor-independent activation of the receptor.

Expression, purification, and characterization of human recombinant thrombopoietin in Chinese hamster ovary cells

Thrombopoietin (TPO) is a primary regulator of megakaryocytopoiesis, a process through which megakaryocytes proliferate and mature into platelets. Recombinant human TPO (rhTPO) was expressed in Chinese hamster ovary (CHO) cells and purified from the culture medium. The cDNA encoding full-length TPO, including the native signal peptide sequence, was amplified by PCR from a human fetal liver cDNA library. The product was cloned into a mammalian expression vector under the control of the SV40 early promoter and enhancer. Secreted rhTPO was purified in three conventional chromatography steps. It migrates on SDS-PAGE as a broad band, characteristic of a heavily glycosylated protein, with an average molecular mass of 85 kDa. rhTPO expressed in CHO cells is biologically active in vitro as demonstrated by its ability to stimulate the proliferation of a megakaryocytic cell line and to trigger the JAK/STAT signal transduction pathway. rhTPO also shows activity in vivo as judged by the elevation of platelet count in treated mice.

A model for assembly and activation of the GM-CSF, IL-3 and IL-5 receptors: insights from activated mutants of the common beta subunit

Granulocyte-macrophage colony stimulating factor (GM-CSF), Interleukin-3 (IL-3) and Interleukin-5 (IL-5) have overlapping, pleiotropic effects on hematopoietic cells, including neutrophils, eosinophils, monocytes and early progenitor cells. The high-affinity receptors for human GM-CSF, IL-3, and IL-5 share a common beta-subunit (hbeta(c)), which is essential for signalling and plays a major role in recruiting intracellular signalling molecules. While activation of the cytoplasmic tyrosine kinase JAK2 appears to be the initiating event for signalling, the immediate events that trigger this are still unclear. We have isolated a number of activated mutants of hbeta(c), which can be grouped into classes defined by their state of receptor phosphorylation, their requirement for alpha subunit as a cofactor, and their activities in primary cells and cell lines. We discuss these findings with regard to the stoichiometry, activation, and signalling of the normal GM-CSF/IL-3/IL-5 receptor complexes. Specifically, this work has implications for the role of the ligand-specific alpha-subunits in initiating the signalling through the beta-subunit, the role of beta subunit dimerization as a receptor trigger, and the function of receptor tyrosine phosphorylation in generating growth and survival signals. Based on the properties of the activated mutants and the recent structures of erythropoietin receptor (Epo-R) complexes, we propose a model in which (1) activation of hbeta(c) can occur via alternative states that differ with respect to stoichiometry and subunit assembly, but which all mediate proliferative responses, and (2) each of the different classes of activated mutants mimics one of these alternative states.

Importance of the membrane-proximal extracellular domains for activation of the signal transducer glycoprotein 130

The transmembrane glycoprotein gp130 is the common signal transducing receptor subunit of the IL-6-type cytokines. The gp130 extracellular part is predicted to consist of six individual domains. Whereas the role of the three membrane-distal domains (D1-D3) in binding of IL-6 and IL-11 is well established, the function of the membrane-proximal domains (D4-D6) is unclear. Mapping of a neutralizing mAb to the membrane-proximal part of gp130 suggests a functional role of D4-D6 in receptor activation. Individual deletion of these three domains differentially interferes with ligand binding of the soluble and membrane-bound receptors. All deletion mutants do not signal in response to IL-6 and IL-11. The deletion mutants Delta4 and, to a lesser extent, Delta6 are still activated by agonistic monoclonal gp130 Abs, whereas the deletion mutant Delta5 does not respond. Because membrane-bound Delta5 binds IL-6/soluble IL-6R as does wild-type gp130, but does not transduce a signal in response to various stimuli, this domain plays a prominent role in coupling of ligand binding and signal transduction. Replacement of the fifth domain of gp130 by the corresponding domain of the homologous G-CSF receptor leads to constitutive activation of the chimera upon overexpression in COS-7 cells. In HepG2 cells this mutant responds to IL-6 comparable to wild-type gp130. Our findings suggest a functional role of the membrane-proximal domains of gp130 in receptor activation. Thus, within the hematopoietic receptor family the mechanism of receptor activation critically depends on the architecture of the receptor ectodomain.

Myeloproliferative disorder and leukaemia in mice induced by different classes of constitutive mutants of the human IL-3/IL-5/GM-CSF receptor common beta subunit

Several constitutively active mutant forms of the common beta subunit of the human IL-3, IL-5 and GM-CSF receptors (hbetac), which enable it to signal in the absence of ligand, have recently been described. Two of these, V449E and I374N, are amino acid substitutions in the transmembrane and extracellular regions of hbetac, respectively. A third, FIDelta, contains a 37 amino acid duplication in the extracellular domain. We have shown previously that when expressed in primary murine haemopoietic cells, the extracellular mutants confer factor-independence on cells of the neutrophil and monocyte lineages only, whereas V449E does so on all cell types of the myeloid and erythroid compartments. To study the in vivo effects and leukaemic potential of these mutants, we have expressed all three in mice by bone marrow reconstitution using retrovirally infected donor cells. Expression of the extracellular mutants leads to an early onset, chronic myeloproliferative disorder marked by elevations in the neutrophil, monocyte, erythrocyte and platelet lineages. In contrast, expression of V449E leads to an acute leukaemia-like syndrome of anaemia, thrombocytopaenia and blast cell expansion. These data support the possibility that activating mutations in hbetac are involved in haemopoietic disorders in man.

Deletion of the Extracellular Membrane-Distal Cytokine Receptor Homology Module of Mpl Results in Constitutive Cell Growth and Loss of Thrombopoietin Binding

The thrombopoietin receptor, Mpl, is a member of the cytokine receptor superfamily. The extracellular domain of Mpl contains two copies of the cytokine receptor homology module (CRM). Mpl is encoded by c-mpl, the cellular homologue of the oncogene v-mpl.The oncogenic potential of v-mpl may arise from deletion of all but the 43 most membrane-proximal amino acids of the extracellular domain of the wild-type receptor. To test the hypothesis that the extracellular domain of Mpl plays a role in controlling receptor activity, we created mutants of murine Mpl in which the membrane-distal CRM was either deleted or replaced by the membrane-proximal CRM. Introduction of these mutant receptors into factor-dependent BaF3 cells led to constitutive cell growth in the absence of growth factor. Both mutant receptors failed to bind 125I-Tpo. These results suggest that the membrane-distal CRM of Mpl acts as a brake on cell proliferation and that this region is required for ligand binding.

Constitutive activation of the prolactin receptor results in the induction of growth factor-independent proliferation and constitutive activation of signaling molecules

The ability to induce the oncogenic activation of the human prolactin receptor (PRLR) was examined by deleting 178 amino acids of the extracellular ligand-binding domain. Expression of this deletion mutant in the interleukin-3 (IL-3)-dependent murine myeloid cell line 32Dcl3 resulted in the induction of growth factor-independent proliferation. Parental 32Dcl3 cells proliferated only in the presence of exogenous murine IL-3 (mIL-3), while 32Dcl3 cells transfected with the long form of the human PRLR were able to proliferate in response to mIL-3, ovine prolactin, or human PRL. Cells expressing the Delta178 deletion mutant contained numerous phosphotyrosine-containing proteins in the absence of stimulation with either mIL-3 or ovine prolactin. Growth factor stimulation increased the number of proteins phosphorylated and the intensity of phosphorylation. These proteins included constitutively phosphorylated Janus kinase 2, signal transducer and activator of transcription 5, and SHC. Activated extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2) were observed in unstimulated 32Dcl3 cells expressing the Delta178 mutant. Likewise, transfection of Nb2 cells with the Delta178 deletion mutant induced growth factor-independent proliferation and constitutive activation of Janus kinase 2, ERK1, and ERK2. In addition to the induction of a growth factor-independent state, the expression of the Delta178 deletion mutant also suppressed the apoptosis that occurs when 32Dcl3 cells are cultured in the absence of growth factors such as IL-3. These data suggest that the constitutive activation of the PRLR can be achieved by deletion of the ligand binding domain and that this mutation leads to the oncogenic activation of the receptor as determined by the ability of the receptor to induce growth factor-independent proliferation of factor-dependent hematopoietic cells.

A cell type-specific constitutive point mutant of the common beta-subunit of the human granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 receptors requires the GM-CSF receptor alpha-subunit for activation

The high affinity receptor for human granulocyte-macrophage colony-stimulating factor (GM-CSF) consists of a cytokine-specific alpha-subunit (hGMRalpha) and a common signal-transducing beta-subunit (hbetac) that is shared with the interleukin-3 and -5 receptors. We have previously identified a constitutively active extracellular point mutant of hbetac, I374N, that can confer factor independence on murine FDC-P1 cells but not BAF-B03 or CTLL-2 cells (Jenkins, B. J., D'Andrea, R. J., and Gonda, T. J. (1995) EMBO J. 14, 4276-4287). This restricted activity suggested the involvement of cell type-specific signaling molecules in the activation of this mutant. We report here that one such molecule is the mouse GMRalpha (mGMRalpha) subunit, since introduction of mGMRalpha, but not hGMRalpha, into BAF-B03 or CTLL-2 cells expressing the I374N mutant conferred factor independence. Experiments utilizing mouse/human chimeric GMRalpha subunits indicated that the species specificity lies in the extracellular domain of GMRalpha. Importantly, the requirement for mGMRalpha correlated with the ability of I374N (but not wild-type hbetac) to constitutively associate with mGMRalpha. Expression of I374N in human factor-dependent UT7 cells also led to factor-independent proliferation, with concomitant up-regulation of hGMRalpha surface expression. Taken together, these findings suggest a critical role for association with GMRalpha in the constitutive activity of I374N.

A human erythropoietin receptor gene mutant causing familial erythrocytosis is associated with deregulation of the rates of Jak2 and Stat5 inactivation

The erythropoietin receptor (EpoR) has been previously shown to contain a cytoplasmic C-terminal negative regulatory domain, experimental deletion or mutation of which leads to increased sensitivity of expressing cells to the effects erythropoietin (Epo). We have studied a naturally occurring C-terminal truncation mutant of the human EpoR by stably transfecting the growth factor-dependent hematopoietic tissue culture cell line 32D with expression plasmids containing either the wildtype or mutant human EpoR cDNA, thus rendering the cells dependent on Epo for viability and proliferation. In Epo dose-response assays, cells expressing the mutant EpoR displayed hyperresponsiveness to Epo compared with cells expressing comparable numbers of the wild-type EpoR cultured in the presence of fetal bovine serum. We investigated whether enhanced Epo sensitivity of cells expressing the truncated EpoR is associated with alteration in Epo receptor-mediated activation of Stat5, which could have a role in Epo-induced proliferation. Although maximal Stat5 activation in response to a given concentration of Epo was comparable in 32D cells expressing the wild-type or truncated EpoRs, the time course of Epo-induced Stat5 activation was very different. Gel-mobility shift studies revealed the presence of Stat5 DNA-binding activity in nuclear and cytoplasmic extracts of cells expressing the truncated EpoR for a significantly longer time than that observed in similar extracts of cells expressing the wild-type EpoR consistent with decreased rate of inactivation of Stat5 in cells expressing the mutant EpoR. Epo-dependent tyrosine phosphorylation of both Stat5 and Jak2 was also substantially prolonged in cells expressing the truncated EpoR. These results suggest a role for Stat5 in regulation of Epo-mediated cell growth and implicate altered kinetics of Epo-induced Jak2 and Stat5 activation in the pathogenesis of familial erythrocytosis associated with this naturally occurring EpoR gene mutation.

Activating point mutations in the betaC chain of the GM-CSF, IL-3 and IL-5 receptors are not a major contributory factor in the pathogenesis of acute myeloid leukaemia

A number of mutant growth factor receptors have been described which are constitutively activated and confer factor independence on growth factor dependent cells, possibly through constitutive dimerization in the absence of ligand or induction of a conformational change. Mutations in receptor chains may therefore contribute to the pathogenesis of haemopoietic malignancies, for example by causing constitutive receptor activation or uncontrolled downstream signalling. Since most of the activated mutants reported for the betaC chain of the GM-CSF/IL-3/IL-5 receptor involve point mutations or truncations of the extracellular domain, we have analysed the coding sequence of this region using RT-PCR-SSCP of RNA from blast cells of 31 patients with acute myeloid leukaemia (AML). Two point mutations detected were silent, C301-->T (Cys91) and C1306-->T (Ser426). The latter had previously been identified with an allele frequency of 0.13 in the general population. Two further point mutations detected led to amino acid substitutions, G773-->C (Glu249Gln), which is equivalent to the mouse sequences, and G962-->A (Asp312Asn), both of which were found at similar frequencies in normal controls. Activating mutations of the betaC chain which might contribute to the pathogenesis of the disease are therefore rare in AML.

Saturation Mutagenesis of the β Subunit of the Human Granulocyte-Macrophage Colony-Stimulating Factor Receptor Shows Clustering of Constitutive Mutations, Activation of ERK MAP Kinase and STAT Pathways, and Differential β Subunit Tyrosine Phosphorylation

The high-affinity receptors for human granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are heterodimeric complexes consisting of cytokine-specific  subunits and a common signal-transducing β subunit (hβc). We have previously demonstrated the oncogenic potential of this group of receptors by identifying constitutively activating point mutations in the extracellular and transmembrane domains of hβc. We report here a comprehensive screen of the entire hβc molecule that has led to the identification of additional constitutive point mutations by virtue of their ability to confer factor independence on murine FDC-P1 cells. These mutations were clustered exclusively in a central region of hβc that encompasses the extracellular membrane-proximal domain, transmembrane domain, and membrane-proximal region of the cytoplasmic domain. Interestingly, most hβc mutants exhibited cell type-specific constitutive activity, with only two transmembrane domain mutants able to confer factor independence on both murine FDC-P1 and BAF-B03 cells. Examination of the biochemical properties of these mutants in FDC-P1 cells indicated that MAP kinase (ERK1/2), STAT, and JAK2 signaling molecules were constitutively activated. In contrast, only some of the mutant β subunits were constitutively tyrosine phosphorylated. Taken together, these results highlight key regions involved in hβc activation, dissociate hβc tyrosine phosphorylation from MAP kinase and STAT activation, and suggest the involvement of distinct mechanisms by which proliferative signals can be generated by hβc.

© 1998 by The American Society of Hematology.

Saturation Mutagenesis of the β Subunit of the Human Granulocyte-Macrophage Colony-Stimulating Factor Receptor Shows Clustering of Constitutive Mutations, Activation of ERK MAP Kinase and STAT Pathways, and Differential β Subunit Tyrosine Phosphorylation

The high-affinity receptors for human granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are heterodimeric complexes consisting of cytokine-specific  subunits and a common signal-transducing β subunit (hβc). We have previously demonstrated the oncogenic potential of this group of receptors by identifying constitutively activating point mutations in the extracellular and transmembrane domains of hβc. We report here a comprehensive screen of the entire hβc molecule that has led to the identification of additional constitutive point mutations by virtue of their ability to confer factor independence on murine FDC-P1 cells. These mutations were clustered exclusively in a central region of hβc that encompasses the extracellular membrane-proximal domain, transmembrane domain, and membrane-proximal region of the cytoplasmic domain. Interestingly, most hβc mutants exhibited cell type-specific constitutive activity, with only two transmembrane domain mutants able to confer factor independence on both murine FDC-P1 and BAF-B03 cells. Examination of the biochemical properties of these mutants in FDC-P1 cells indicated that MAP kinase (ERK1/2), STAT, and JAK2 signaling molecules were constitutively activated. In contrast, only some of the mutant β subunits were constitutively tyrosine phosphorylated. Taken together, these results highlight key regions involved in hβc activation, dissociate hβc tyrosine phosphorylation from MAP kinase and STAT activation, and suggest the involvement of distinct mechanisms by which proliferative signals can be generated by hβc.

© 1998 by The American Society of Hematology.

Immunodeficiencies and haematological disorders--direct connections to cellular signalling pathways

Extracellular signals, such as cytokines and foreign antigens, regulate the development and activation of haematopoietic cells through receptor-mediated transcriptional responses. Investigation of the biochemical events that transduce receptor activation to intracellular signal transduction have revealed a general mechanism for both the antigen receptor and the cytokine receptor nuclear signalling. Receptor stimulation is coupled to down-stream signalling cascades through activation of cytoplasmic tyrosine kinases but both receptor types utilize distinct signalling proteins. Antigen receptors employ protein tyrosine kinases of the Src, Syk, and Tec families which phosphorylate and activate cytoplasmic signalling proteins leading to activation of NF-AT, AP-1 and NF-kappaB transcription factors, while the principal signal transducers for cytokine receptors are the Jak tyrosine kinases and Stat transcription factors. The molecular basis for several immune and haematological disorders has been characterized during the last few years and they have been found to be caused by defects in various signalling proteins. In this review the aetiology of immune and haematological disorders is discussed in the light of the recent developments in signal transduction.

The Human Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF ) Receptor Exists as a Preformed Receptor Complex That Can Be Activated by GM-CSF, Interleukin-3, or Interleukin-5

The granulocyte-macrophage colony-stimulating factor (GM-CSF ) receptor is expressed on normal and malignant hematopoietic cells as well as on cells from other organs in which it transduces a variety of functions. Despite the widespread expression and pleiotropic nature of the GM-CSF receptor, little is known about its assembly and activation mechanism. Using a combination of biochemical and functional approaches, we have found that the human GM-CSF receptor exists as an inducible complex, analogous to the interleukin-3 (IL-3) receptor, and also as a preformed complex, unlike the IL-3 receptor or indeed other members of the cytokine receptor superfamily. We found that monoclonal antibodies to the GM-CSF receptor α chain (GMRα) and to the common β chain of the GM-CSF, IL-3, and IL-5 receptors (βc ) immunoprecipitated both GMRα and βc from the surface of primary myeloid cells, myeloid cell lines, and transfected cells in the absence of GM-CSF. Further association of the two chains could be induced by the addition of GM-CSF. The preformed complex required only the extracellular regions of GMRα and βc , as shown by the ability of soluble βc to associate with membrane-anchored GMRα or soluble GMRα. Kinetic experiments on eosinophils and monocytes with radiolabeled GM-CSF, IL-3, and IL-5 showed association characteristics unique to GM-CSF. Significantly, receptor phosphorylation experiments showed that not only GM-CSF but also IL-3 and IL-5 stimulated the phosphorylation of GMRα-associated βc . These results indicate a pattern of assembly of the heterodimeric GM-CSF receptor that is unique among receptors of the cytokine receptor superfamily. These results also suggest that the preformed GM-CSF receptor complex mediates the instantaneous binding of GM-CSF and is a target of phosphorylation by IL-3 and IL-5, raising the possibility that some of the biologic activities of IL-3 and IL-5 are mediated through the GM-CSF receptor complex.

Receptors of the cytokine superfamily: mechanisms of activation and involvement in disease

Cytokine receptors are members of a diverse family of proteins that serve the dual function of recognizing their cognate ligands among a plethora of other factors and of initiating a series of cellular signals that ultimately lead to multiple cellular functions. Although cytokine receptors are only activated by their specific cytokines, some functional overlap occurs as a result of receptor subunit promiscuity, kinase recruitment and the activation of coincident signalling pathways. Knock-out experiments are extremely useful in helping to elucidate functionally relevant interactions between cytokine receptor activation, signalling molecules and cellular function. Defects in cytokine receptors or activation, signalling molecules continue to be identified as the underlying cause of clinical conditions. We discuss newly recognized clinical syndromes and recent research into the molecular basis of cytokine receptor activation that provides new insights into the role of cytokine receptors in normal physiology and disease.

Familial Erythrocytosis Associated With a Short Deletion in the Erythropoietin Receptor Gene

Familial erythrocytosis (familial polycythemia) inherited as an autosomal dominant trait has recently been reported to be associated with mutations in the gene encoding the erythropoietin receptor (EpoR) in a small number of families. We studied a new kindred with dominantly inherited familial erythrocytosis associated with heterozygosity for a deletion of seven nucleotides between positions 5985 and 5991 in exon 8 of the EpoR gene, resulting in an EpoR peptide that is truncated by 59 amino acids at its C-terminus. A 7-bp direct repeat is present in the normal EpoR gene at the site of this mutation, consistent with the slipped mispairing model for the generation of short deletions during DNA replication. Hypersensitivity to Epo of erythroid progenitors from an affected individual was observed in in vitro methylcellulose cultures, as indicated by more numerous and larger colonies compared with those of a control subject. To study mutant EpoR function, the cDNA encoding the mutant EpoR was synthesized by reverse transcription-polymerase chain reaction of peripheral blood RNA from the proband and stably tranfected into murine interleukin-3–dependent 32D cells. Epo dose-response assays showed that cells expressing the mutant EpoR displayed fivefold to 10-fold increased sensitivity to Epo compared with cells expressing similar numbers of the wild-type EpoR.