BIRC6 Is Up‐Regulated Via Transcriptional and Post‐Transcriptional Mechanisms in Imatinib Resistant Chronic Myeloid Leukemia

Abstract

BIRC6 is a recently discovered anti‐apoptotic protein and increased expression is correlated with adverse outcomes in childhood leukemia and other cancers. The expression of BIRC6 was examined in cell models of imatinib sensitive and resistant CML (MYL/MYL‐R). A significant increase in BIRC6 mRNA and protein expression was observed in the MYL‐R cells compared to the drug sensitive MYL cells. Knockdown of BIRC6 in MYL‐R cells increased imatinib sensitivity confirming the importance of this protein in drug resistance. In this study we investigated the mechanisms responsible for the up‐regulation of BIRC6 in MYL‐R cells. Using a novel CDK9 inhibitor (HY‐16462), we were able to identify CDK9 as a required kinase for BIRC6 mRNA transcription. In comparison with MCL‐1, BIRC6 protein levels remained stable despite inhibition of BIRC6 mRNA transcription by the CDK9 inhibitor. This data showed that the regulation of BIRC6 protein levels was post‐transcriptionally determined. A time course treatment with cyclohexamide showed a difference in BIRC6 degradation between MYL and MYL‐R cells. The half‐life of BIRC6 protein in MYL cells was around 3.5 hours compared to 6 hours, nearly double, in MYL‐R cells. Mass spectrometry experiments demonstrated that BIRC6 was differentially phosphorylated in MYL‐R cells. In summary, these studies identify BIRC6 as a key component of cell survival in drug‐resistant leukemia and show that its regulation lies with both mRNA transcription and protein stability. These studies suggest that compounds targeting BIRC6 or BIRC6 stability could be developed to treat cancers resistant to traditional therapies.