Abstract
Signaling by Kit has been extensively studied in hematopoietic cells and is essential for the survival, proliferation and maintenance of hematopoietic stem and progenitor cells. In addition to the activation of intrinsic signaling pathways, Kit has been shown to interact with lineage-restricted type I cytokine receptors and produce cross signals, e.g. erythropoietin receptor, interleukin-7 receptor (IL-7R), IL-3R. Based on the earlier studies, we hypothesize that Kit activate other type I cytokine receptors in a cell-specific manner and execute cell-specific function. To investigate other Kit-activated receptors, we tested Kit and IL-4R cross-receptor activation in murine bone-marrow-derived mast cells, which express both Kit and IL-4R at the surface level. Kit upon activation by Kit ligand (KL), activated IL-4Rα, γ(C) , and signal transducer and activator of transcription 6 independent of its cognate ligand IL-4. Though KL and IL-4 are individually mitogenic, combinations of KL and IL-4 synergistically promoted mast cell proliferation. Furthermore, inhibition of lipid raft formation by methyl-β-cyclodextrin resulted in loss of synergistic proliferation. Together the data suggest IL-4R as a novel Kit-activated receptor. Such cross-receptor activations are likely to be a universal mechanism of Kit signaling in hematopoiesis.