Persistent STAT5-mediated ROS production and involvement of aberrant p53 apoptotic signaling in the resistance of chronic myeloid leukemia to imatinib.

The persistent activation of signal transducer and activator of transcription 5 (STAT5) may principally be attributed to breakpoint cluster region (BCR)-Abelson murine leukemia viral oncogene homolog 1 (ABL1), and have multi-faceted effects in the development of chronic myeloid leukemia (CML). The p53 protein network regulates important mechanisms in DNA damage repair, cell cycle regulation/checkpoints, and cell senescence and apoptosis, as demonstrated by its ability to positively regulate the expression of various pro-apoptotic genes, including B-cell lymphoma-2 (Bcl-2) and Bcl-2-associated X protein (Bax). In the present study, it was observed that the mRNA levels of STAT5A and STAT5B were upregulated in patients with imatinib-resistant CML and in the imatinib-resistant K562/G CML cell line. In addition, increased expression of STAT5 was observed in the BCR-ABL1 mutation group, compared with that in the non-BCR-ABL1 mutation group, regardless of patient imatinib resistance state. Elevated levels of reactive oxygen species (ROS) and DNA double-strand breaks were identified in K562/G cells using flow cytometric and phosphorylated H2AX (γ-H2AX) foci immunofluorescence assays, respectively, compared with the imatinib-sensitive K562 cells. The levels of intracellular ROS and γ-H2AX were decreased by the ROS scavenger (N-acetylcysteine), and ROS levels were also markedly reduced by STAT5 inhibitor (SH-4-54). In addition, imatinib significantly inhibited the proliferation of K562 and K562/G cells, with half maximal inhibitory concentration values of 0.17±0.07 and 14.78±0.43 µM, respectively, and the levels of apoptosis were significantly different between K562 and K562/G cells following treatment with imatinib. The mRNA and protein levels of STAT5 and mouse double minute 2 homolog (MDM2) were upregulated, whereas those of Bax were downregulated in K562/G cells, as determined using western blot analysis. Additionally, although the two cell lines exhibited relatively low protein expression levels of p53, lower levels of p53 and TPp53BP1 transcripts were detected in the K562/G cells. Taken together, these findings suggest that the resistance of CML to the tyrosine kinase inhibitor, imatinib, may be associated with persistent STAT5-mediated ROS production, and the abnormality of the p53 pathway.