Background: Chimeric oncogenes encoding constitutively active protein tyrosine kinases are associated with chronic myeloid neoplasms. TEL-PDGFRbeta (TPbeta, also called ETV6-PDGFRB) is a hybrid protein produced by the t(5;12) translocation, FIP1L1-PDGFRalpha (FPalpha) results from a deletion on chromosome 4q12 and ZNF198-FGFR1 is created by the t(8;13) translocation. These fusion proteins are found in patients with myeloid neoplasms associated with eosinophilia. Wild-type receptor tyrosine kinases are efficiently targeted for degradation upon activation, in a process that requires Cbl-mediated monoubiquitination of receptor lysines. Since protein degradation pathways have been identified as useful targets for cancer therapy, the aim of this study was to compare the degradation of hybrid and wild-type receptor tyrosine kinases.
Design and methods: We used Ba/F3 as a model cell line, as well as leukocytes from two patients, to analyze hybrid protein degradation.
Results: In contrast to the corresponding wild-type receptors, which are quickly degraded upon activation, we observed that TPbeta, FPalpha and the ZNF198-FGFR1 hybrids escaped down-regulation in Ba/F3 cells. The high stability of TPbeta and FPalpha hybrid proteins was confirmed in leukocytes from leukemia patients. Ubiquitination of TPbeta and FPalpha was much reduced compared to that of wild-type receptors, despite marked Cbl phosphorylation in cells expressing hybrid receptors. The fusion of a destabilizing domain to TPbeta induced protein degradation. Instability was reverted by adding the destabilizing domain ligand, Shield1. The destabilization of this modified TPbeta reduced cell transformation and STAT5 activation.
Conclusions: We have shown that chimeric receptor tyrosine kinases escape ubiquitination and down-regulation and that their stabilization is critical to efficient stimulation of cell proliferation.