Unfolding tyrosine kinase inhibitor sensitivity in chronic myeloid leukemia

Targeting UPR signaling pathway by dasatinib as a promising therapeutic approach in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative disease that mediated by BCR/ABL oncogenic signaling. CML can be targeted with the imatinib, dasatinib, and nilotinib TKI inhibitors, the latter two of them have been approved for imatinib-resistant or -intolerant CML patients. The TKIs resistance occurs by different molecular mechanisms, including overexpression of BCR-ABL, mutations in the TKI binding site of BCR/ABL, and ER-stress. Unfolded protein responses (UPR) is a cytoprotective mechanism which is activated by ER-stress. The IRE1, PERK, and ATF6 are three main arms of the UPR mechanism and are activated by a common mechanism involving the dissociation of the ER-chaperone BiP/GP78. There is a correlation between ER-stress, CML progression, and response to TKI treatment. In the present study, we aimed to determine alterations of the expression levels of genes related to UPR pathway signaling after treatment with dasatinib in K562 chronic myeloid leukemia cell line by quantitative RT-PCR relatively. The array-data revealed that treatment with dasatinib significantly decreased the UPR mechanism-related genes (including HSPA1B, HSPA2, HSPA4L, ATF6, ATF6B, CEBPB, PERK, TRIB3, DNAJB, ERN1, and UHRF1) in K562 cells. In conclusion, the results showed that dasatinib regulates the UPR mechanism that plays a significant role in cancer progression and therapy resistance in CML. Thus, dasatinib-induced dysfunction of the UPR mechanism may promise encouraging therapy for CML.

Downregulation of BRCA1 protein in BCR-ABL1 leukemia cells depends on stress-triggered TIAR-mediated suppression of translation

BRCA1 tumor suppressor regulates crucial cellular processes involved in DNA damage repair and cell cycle control. We showed that expression of BCR-ABL1 correlates with decreased level of BRCA1 protein, which promoted aberrant mitoses and aneuploidy as well as altered DNA damage response. Using polysome profiling and luciferase-BRCA1 3'UTR reporter system here we demonstrate that downregulation of BRCA1 protein in CML is caused by inhibition of BRCA1 mRNA translation, but not by increased protein degradation or reduction of mRNA level and half-life. We investigated 2 mRNA-binding proteins - HuR and TIAR showing specificity to AU-Rich Element (ARE) sites in 3'UTR of mRNA. BCR-ABL1 promoted cytosolic localization of TIAR and HuR, their binding to BRCA1 mRNA and formation of the TIAR-HuR complex. HuR protein positively regulated BRCA1 mRNA stability and translation, conversely TIAR negatively regulated BRCA1 translation and was found localized predominantly in the cytosolic stress granules in CML cells. TIAR-dependent downregulation of BRCA1 protein level was a result of ER stress, which is activated in BCR-ABL1 expressing cells, as we previously shown. Silencing of TIAR in CML cells strongly elevated BRCA1 level. Altogether, we determined that TIAR-mediated repression of BRCA1 mRNA translation is responsible for downregulation of BRCA1 protein level in BCR-ABL1 -positive leukemia cells. This mechanism may contribute to genomic instability and provide justification for targeting PARP1 and/or RAD52 to induce synthetic lethality in "BRCAness" CML and BCR-ABL1 -positive ALL cells.