Erythropoietin-stimulated Raf-1 tyrosine phosphorylation is associated with the tyrosine kinase Lyn in J2E erythroleukemic cells

The serine/threonine kinase Raf-1 is crucial for transducing intracellular signals emanating from numerous growth factors. Here we used the J2E erythr oid cell line transformed by the v-raf/v-myc oncogenes to examine the effec ts of erythropoietin on endogenous Raf-1 activity. Despite the presence of constitutively active v-raf in these cells, Raf-1 exokinase activity increa sed after erythropoietin stimulation. This increase in enzymatic activity c oincided with tyrosine phosphorylation of Raf-1 on residue Y341. Significan tly, the tyrosine kinase Lyn coimmunoprecipitated with Raf-1, and Raf-1 was not tyrosine-phosphorylated in a J2E subclone lacking Lyn. Therefore, it w as concluded that Lyn may be the kinase responsible for tyrosine phosphoryl ating Raf-1 and increasing its exokinase activity in response to erythropoi etin. (C) 2001 Elsevier Science.