Kinase Activity of RTKs Is Dispensable for Factor Independent Growth and Transformation of a Myeloid Cell Line 32D in the Presence of Cbl Mutants.

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 recently identified the first c-Cbl mutation in human disease in an AML patient, named Cbl-R420Q. Co-expression of two different dominant negative mutants of c-Cbl (Cbl-R420Q or Cbl-70Z) with Kit induced cytokine-independent proliferation, survival and clonogenic growth. Importantly, transformation was observed also with kinase-dead forms of Kit and Flt3 in the presence of Cbl-70Z, but not in the absence of Kit or Flt3, suggesting a mechanism dependent on RTKs, but independent of their kinase activity. Instead, transformation appeared to depend on Src family kinases (SFKs), as c-Cbl co-immunoprecipitated with SFKs and SFK inhibition abolished transformation.

Our results indicate that c-Cbl has an important role in c-Kit signal mitigation. They demonstrate that disturbed mechanisms of c-Kit internalization have important implications for its transforming potential, possibly in the development of AML. Furthermore, these findings may explain primary resistance to tyrosine kinase inhibitors targeted at RTKs.

No relevant conflicts of interest to declare.