Cbl Mutants Interact with c-Kit and Lead to Factor Independent Growth and Transformation of a Myeloid Cell Line 32D.

The Cbl proto-oncogene products have emerged as important components of the signal transduction cascades downstream of both non-receptor and receptor tyrosine kinases (RTKs). By regulation of receptor trafficking and degradation, they have been shown to tightly regulate the intensity and amplitude of RTK activation. c-Kit belongs to the family of the class-III RTKs and plays an important role in the pathogenesis of acute myeloid leukemia (AML). So far, very little is known about the role of c-Cbl mutants in the role of c-Kit signaling. We analyzed the interaction of c-Cbl with c-Kit and the functional relevance of this interaction in the IL-3-dependent murine myeloid progenitor cell line 32Dcl3. We analyzed the effect of c-Cbl and two different dominant negative mutants of c-Cbl (Cbl-70Z and Cbl-R420Q) on c-Kit-ligand-activated internalization. The transfection of c-Cbl mutants, but not of wildtype c-Cbl, significantly inhibited receptor internalization, as analyzed by FACS analysis. Expression of Cbl-70Z in 32Dcl3 cells severely inhibited apoptosis induced by growth factor deprivation, as has been described before. However, when coexpressed with wildtype c-Kit, 32Dcl3 cells also rapidly proliferated in the absence of any exogenously added growth factors. We furthermore analyzed SCF induced c-Kit ubiquitination in the presence and absence of c-Cbl mutants. SCF induced rapid ubiquitination of c-Kit that was strongly reduced in the presence of Cbl-70Z and abolished by Cbl-R420Q. Also, the Cbl mutants altered the amplitude and changed the quality of c-Kit dependent signaling events. In colony assays we were able to show ligand independent colony growths in methyl cellulose only in the presence of wildtype c-Kit together with a c-Cbl mutant. Our results indicate that c-Cbl has an important role in c-Kit signal mitigation. Furthermore, they demonstrate that disturbed mechanisms of c-Kit internalization have important implications for its transforming potential, possibly in the development of AML.