LIGAND-DEPENDENT TRANSFORMATION BY THE RECEPTOR FOR HUMAN GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR AND TYROSINE PHOSPHORYLATION OF THE RECEPTOR BETA-SUBUNIT

The receptor for human granulocyte/macrophage colony-stimulating facto r (hGMR) is composed of two subunits, alpha and beta, which are both r equired for high-affinity binding of the ligand. To examine the transf orming potential of hGMR, we have transfected cDNAs encoding the recep tor alpha and beta subunits into NIH 3T3 cells, which normally do not express GMRs. Introduction of the receptor subunits into these cells r esulted in focal transformation, which was dependent on the presence o f human granulocyte/macrophage colony-stimulating factor (hGM-CSF) in the culture medium. No transformation was observed when hGM-CSF was re placed with other growth factors such as human epidermal growth factor or human interleukin 3 or when cells were transfected with the alpha or beta subunit alone. Individual conditional transformants isolated a fter transfection expressed functional hGMRs, were susceptible to tran sformation by picomolar levels of the ligand, and were capable of anch orage-independent growth in soft agar in the presence but not in the a bsence of hGM-CSF. Biochemical analysis showed that treatment of these cells with hGM-CSF caused a rapid phosphorylation of the beta subunit and other cellular proteins on tyrosine residues, recapitulating some of the events that take place during GM-CSF signaling in myeloid cell s. We conclude that coexpression of the alpha and beta subunits of hGM R in established murine fibroblasts is sufficient to reconstitute a fu nctional receptor, which is capable of causing ligand-dependent transf ormation. The oncogenic potential of hGMR lends support to the hypothe sis that its deregulated or abnormal expression may play a role in leu kemogenesis.