Bcr-Abl-induced tyrosine phosphorylation of Emi1 to stabilize Skp2 protein via inhibition of ubiquitination in chronic myeloid leukemia cells.

Our previous study demonstrates that Bcr-Abl fusion oncogene frequently found in chronic myeloid leukemia (CML) cells can up-regulate Skp2 expression via transcriptional activation. However, Bcr-Abl also modulates Skp2 protein stability in these cells. Treatment of Bcr-Abl kinase inhibitor imatinib led to G1 growth arrest accompanied with reduced Skp2 expression. Interestingly, reduction of Skp2 protein occurred prior to down-regulation of Skp2 mRNA suggesting a post-translational control. The half-life of Skp2 protein was significantly attenuated in imatinib-treated cells. These effects are not cell line specific because similar results were also found in CML cells obtained from patients. Knockdown of Bcr-Abl similarly caused Skp2 protein instability. The decrease of Skp2 was induced by increased protein degradation through the ubiquitin/proteasome pathway. Imatinib treatment or Bcr-Abl knockdown reduced Emi1, an endogenous inhibitor of the E3 ligase APC/Cdh1 which mediated Skp2 degradation. We found that Emi1 stability was regulated by phosphorylation and mutation of tyrosine 142 reduced the stability. Our data suggested Bcr-Abl-induced Emi1 phosphorylation might be mediated by Src kinase. Firstly, Src inhibitor SU6656 inhibited Emi1 tyrosine phosphorylation in K562 cells. Secondly, transfection of v-Src rescued the reduction of Emi1 by imatinib. Thirdly, mutation of tyrosine 142 to phenylalanine (Y142F) abolished the phosphorylation of Emi1 by recombinant Src kinase. In addition, ectopic expression of wild type but not Y142F mutant Emi1 counteracted imatinib-caused growth arrest. Collectively, our results suggest that Bcr-Abl increases Emi1 phosphorylation and stability to prevent Skp2 protein degradation via APC/Cdh1-induced ubiquitination and to enhance proliferation of CML cells.
(© 2010 Wiley-Liss, Inc.)