EXPRESSION OF CONSTITUTIVELY ACTIVATED HUMAN C-KIT IN MYB TRANSFORMEDEARLY MYELOID CELLS LEADS TO FACTOR INDEPENDENCE, HISTIOCYTIC DIFFERENTIATION, AND TUMORIGENICITY

The cDNAs encoding wild type (WT) human receptor tyrosine kinase c-Kit and a constitutively activated mutant, V816Kit, were introduced into granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent ea rly murine hemopoietic cells, which had been transformed with activate d Myb, WTKit cells were able to grow in the presence of the human liga nd for Kit, stem cell factor (SCF), but displayed reduced growth and c lonogenic potential in either SCF or GM-CSF compared with the parental cells in GM-CSF. In contrast, V816Kit cells grew without factor at a higher rate than the parental cells in GM-CSF and displayed increased clonogenicity. Dissection of the growth characteristics in liquid cult ure showed that in the presence of appropriate factors, the different populations had similar proliferation rates, but that V816Kit profound ly increased cell survival compared with WTKit or parental cells, This suggests that the signals transduced by WTKit activated with SCF, and by V816Kit, were not identical. Also, WTKit and V816Kit-expressing ce lls both varied from the early myeloid progenitor phenotype of the par ental cells and gave rise to a small number of large to giant adherent cells that expressed macrophage (alpha-naphthyl acetate) esterase and neutrophil (naphtol-AS-D-chloroacetate) esterase, were highly phagocy tic and phenotypically resembled histiocytes. Thus, WTKit activated by SCF and V816Kit were able to induce differentiation in a proportion o f Myb-transformed myeloid cells. The factor independent V816Kit cells, unlike the parental and WTKit expressing cells, were shown to produce tumors of highly mitotic, invasive cells at various stages of differe ntiation in syngeneic mice. These results imply that constitutively ac tivated Kit can promote the development of differentiated myeloid tumo rs and that its oncogenic effects are not restricted to lineages (mast cell and B-cell acute lymphoblastic leukemia), which have been report ed previously. Furthermore, the mixed populations of cells in culture and in the tumors phenotypically resembled the leukemic cells from pat ients with monocytic leukemia with histiocytic differentiation (acute myeloid leukemia-M5c), a newly proposed subtype of myeloid leukemia. ( C) 1997 by The American Society of Hematology.