Distinct domains of the human granulocyte-macrophage colony-stimulating factor receptor α subunit mediate activation of Jak/Stat signaling and differentiation

The alpha subunit of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor has several isoforms that result from alternative splicing events. Two forms, alpha -1 and alpha -2, have intracytoplasmic sequences that are identical within a membrane-proximal domain but differ completely distally. Variant and mutated GM-CSF receptor a subunits, along with the beta subunit (beta (c) protein) were expressed in M1 murine leukemia cells. and the ability of the receptors to signal for differentiation events and to activate Jak/Stat signaling pathways was examined. All cell lines expressing both alpha and beta (c) proteins exhibited high-affinity binding of radiolabeled human GM-CSF. Receptor or subunits with intact membrane-proximal intracellular domains could induce expression of the macrophage antigen F4/80 and down-regulate the expression of CDIIb. Addition of recombinant human GM-CSF to cells expressing alpha -1 subunits induced the expression of CD86 and tyrosine phosphorylation of Jak-2 and its putative substrates SHPTP-2, Stat-5, and the GM-CSF receptor beta (c) subunit. Cells containing a subunits that lacked a distal domain (term-3) or had the alternatively spliced alpha -2 distal domain showed markedly decreased ability to support tyrosine phosphorylation of Jak-2 and its substrates or to up-regulate CD86, Ligand binding induced stable association of the alpha -1 subunit and beta (c) protein. In contrast, the alpha -2 subunit did not stably associate with the beta (c) subunit. These data identify potential molecular mechanisms for differential signaling of the alpha -1 and alpha -2 proteins, The association of unique signaling events with the 2 active GMCSF or subunit isoforms offers a model for variable response phenotypes to the same ligand, (Blood, 2001;97:1662-1670) (C) 2001 by The American Society of Hematology.