Expression of Constitutively Activated Human c-Kit in Myb Transformed Early Myeloid Cells Leads to Factor Independence, Histiocytic Differentiation, and Tumorigenicity

STAT3 activation is required for Asp(816) mutant c-Kit induced tumorigenicity

Activating mutations of c-kit at codon 816 (Asp(816)) have been identified in variety of malignancies, including acute myeloid leukemia (AML), mastocytosis and germ cell tumors. The mutant c-Kit receptor confers cytokine independence and induces tumorigenicity. However, the molecular mechanisms, particularly the changes in the signal transduction pathways, responsible for these biological effects induced by mutant c-Kit are largely undefined. Using the human embryonic kidney cell line, 293, we show in the current report that constitutive activation of STAT3 and STAT1 is associated with D816H mutant c-Kit. Transfection of dominant negative STAT3, but not STAT1 inhibits mutant c-Kit mediated anchorage-independent growth in vitro and tumor formation in vivo. Expression of constitutively activated STAT3 restores the mutant c-Kit receptor's transforming ability in 293 cells. These results demonstrate that activation of STAT3 by Asp(816) mutant c-Kit is required for the anchorage-independent growth and tumorigenicity induced by Asp(816) mutant c-Kit.

Activating Mutation in the Catalytic Domain of c-kit Elicits Hematopoietic Transformation by Receptor Self-Association Not at the Ligand-Induced Dimerization Site

The c-kit receptor tyrosine kinase (KIT) is constitutively activated by naturally occurring mutations in either the juxtamembrane domain or the kinase domain. Although the juxtamembrane domain mutations led to ligand-independent KIT dimerization, the kinase domain mutations (Asp814 → Val or Tyr) did not. In an effort to determine if the kinase domain mutant could transfer oncogenic signaling without receptor dimerization, we have constructed the truncated types of c-kitWild and c-kitTyr814 cDNAs (c-kitDel-Wild and c-kitDel-Tyr814 cDNAs, respectively), in which ligand-binding and ligand-induced dimerization domains were deleted. When c-kitDel-Wild and c-kitDel-Tyr814 genes were introduced into a murine interleukin-3 (IL-3)–dependent cell line Ba/F3, KITDel-Tyr814 was constitutively phosphorylated on tyrosine and activated, whereas KITDel-Wild was not. In addition, Ba/F3 cells expressing KITDel-Tyr814(Ba/F3Del-Tyr814) grew in suspension culture without the addition of exogenous growth factor, whereas Ba/F3 cells expressing KITDel-Wild (Ba/F3Del-Wild) required IL-3 for growth. The factor-independent growth of Ba/F3Del-Tyr814 cells was virtually abrogated by coexpression of KITW42 that is a dominant-negative form of KIT, but not by that of KITWild, suggesting that KITDel-Tyr814 may not function as a monomer but may require receptor dimerization for inducing factor-independent growth. Furthermore, KITDel-Tyr814 was found to be coimmunoprecipitated with KITWild or KITW42 by an ACK2 monoclonal antibody directed against the extracellular domain of KIT. Moreover, KITW42 was constitutively associated with a chimeric FMS/KITTyr814 receptor containing the ligand-binding and receptor dimerization domain of c-fmsreceptor (FMS) fused to the transmembrane and cytoplasmic domain of KITTyr814, but not with a chimeric FMS/KITWildreceptor even after stimulation with FMS-ligand. These results suggest that constitutively activating mutation of c-kit at the Asp814 codon may cause a conformation change that leads to receptor self-association not in the extracellular domain and that the receptor self-association of the Asp814 mutant may be important for activation of downstream effectors that are required for factor-independent growth and tumorigenicity.