C-RAF-1 serine/threonine kinase is required in BCR/ABL-dependent and normal hematopoiesis

BCR/ABL oncogenic tyrosine kinase is responsible for initiating and maintaining the leukeic phenotype of Philadelphia chromosome-positive cells. c-RAF-1 serine/threonine kinase is known to be activated by receptor and nonreceptor tyrosine kinases. To determine whether c-RAF-1 plays a role in the growth of BCR/ABL-dependent cells, we examined whether c-RAF-1 associates with and/or is regulated by BCR/ABL and, if so, whether this interaction is functionally significant for BCR/ABL-dependent growth of chronic myelogenous leukemia cells and for growth factor-dependent proliferation of normal bone marrow cells. We show that c-RAF-1 enzymatic activity is regulated by BCR/ABL, although the protein does not associate with BCR/ABL. Downregulation of c-RAF-1 expression with antisense oligodeoxynucleotides or cDNA constructs, and inhibition of c-RAF-1 activity by its dominant negative mutants, inhibited both BCR/ABL-dependent growth of chronic myelogenous leukemia cells and growth factor-dependent proliferation of normal hematopoietic progenitors and the MO7 cell line without affecting the BCR/ABL-and growth factor-independent proliferation of HL-60 cells. These results indicate that c-RAF-1 plays an important role in Philadelphia chromosome-positive and normal hematopoiesis.