STAT1 ASSOCIATES WITH C-KIT AND IS ACTIVATED IN RESPONSE TO STEM-CELLFACTOR

Interaction of stem cell factor (SCF), a haematopoietic growth factor, with the receptor tyrosine kinase c-kit leads to autophosphorylation of c-kit as well as tyrosine phosphorylation of various substrates. Li ttle is known about the role of the JAK/STAT pathway in signal transdu ction via receptor tyrosine kinases, although this pathway has been we ll characterized in cytokine receptor signal transduction. We recently found that the Janus kinase Jak2 associates with c-kit and that SCF i nduces rapid and transient phosphorylation of Jak2. Here we present ev idence that SCF activates the transcription factor Stat1. Phosphorylat ed c-kit co-immunoprecipitates with Stat1 within 1 min of SCF stimulat ion of the human cell line MO7e. Coprecipitation experiments using glu tathione S-transferase fusion proteins indicate that association with c-kit is mediated by the Stat1 SH2 domain. Stat1 is rapidly tyrosine-p hosphorylated in response to SCF in MO7e cells, the murine cell line F DCP-1 and normal progenitor cells. SCF-induced phosphorylation of Jak2 and Stat1 was also observed in murine 3T3 fibroblasts stably transfec ted with full-length human c-kit receptor. Furthermore c-kit directly phosphorylates Stat1 fusion proteins in in vitro kinase assays. Electr ophoretic mobility-shift assays with nuclear extracts from SCF-stimula ted cell lines and normal progenitor cells indicate that activated Sta t1 binds the m67 oligonucleotide, a high-affinity SIE promoter sequenc e. These results demonstrate that Stat1 is activated in response to SC F, and suggest that Stat1 is a component of the SCF signal-transductio n pathway.