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Several reports have suggested an interaction between the erythropoietin receptor (EpoR) and the shared signaling subunit (hβc) of the human granulocyte macrophage-colony stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 receptors, although the functional consequences of this interaction are unclear. We previously showed that in vivo expression of constitutively active extracellular (EC) mutants of hβc induces erythrocytosis and Epo independence of erythroid colony-forming units (CFU-E). This occurs despite an apparent requirement of these mutants for the GM-CSF receptor α-subunit (GMRα), which is not expressed in CFU-E. Here, we show that coexpression of hβc EC mutants and EpoR in BaF-B03 cells, which lack GMRα, results in factor-independent proliferation and JAK2 activation. Mutant receptors that cannot activate JAK2 fail to produce a functional interaction. As there is no detectable phosphorylation of hβc on intracellular tyrosine residues, EpoR displays constitutive tyrosine phosphorylation. These observations suggest that JAK2 activation mediates cross-talk between EC mutants of hβc and EpoR. The implications of these data are discussed as are our findings that activated hβc mutants can functionally interact with certain other cytokine receptors.

Mechanisms controlling the balance between proliferation and self-renewal versus growth suppression and differentiation during normal and leukemic myelopoiesis are not understood. We have used the bi-potent FDB1 myeloid cell line model, which is responsive to myelopoietic cytokines and activated mutants of the granulocyte macrophage-colony stimulating factor (GM-CSF) receptor, having differential signaling and leukemogenic activity. This model is suited to large-scale gene-profiling, and we have used a factorial time-course design to generate a substantial and powerful data set. Linear modeling was used to identify gene-expression changes associated with continued proliferation, differentiation, or leukemic receptor signaling. We focused on the changing transcription factor profile, defined a set of novel genes with potential to regulate myeloid growth and differentiation, and demonstrated that the FDB1 cell line model is responsive to forced expression of oncogenes identified in this study. We also identified gene-expression changes associated specifically with the leukemic GM-CSF receptor mutant, V449E. Signaling from this receptor mutant down-regulates CCAAT/enhancer-binding protein α (C/EBPα) target genes and generates changes characteristic of a specific acute myeloid leukemia signature, defined previously by gene-expression profiling and associated with C/EBPα mutations.

Several reports have suggested an interaction between the erythropoietin receptor (EpoR) and the shared signaling subunit (hbeta(c)) of the human granulocyte macrophage-colony stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 receptors, although the functional consequences of this interaction are unclear. We previously showed that in vivo expression of constitutively active extracellular (EC) mutants of hbeta(c) induces erythrocytosis and Epo independence of erythroid colony-forming units (CFU-E). This occurs despite an apparent requirement of these mutants for the GM-CSF receptor alpha-subunit (GMRalpha), which is not expressed in CFU-E. Here, we show that coexpression of hbeta(c) EC mutants and EpoR in BaF-B03 cells, which lack GMRalpha, results in factor-independent proliferation and JAK2 activation. Mutant receptors that cannot activate JAK2 fail to produce a functional interaction. As there is no detectable phosphorylation of hbeta(c) on intracellular tyrosine residues, EpoR displays constitutive tyrosine phosphorylation. These observations suggest that JAK2 activation mediates cross-talk between EC mutants of hbeta(c) and EpoR. The implications of these data are discussed as are our findings that activated hbeta(c) mutants can functionally interact with certain other cytokine receptors.