Regulation of c-Myc Expression through the Depletion of THAP11 and Fbw7 by Bcr-Abl Promotes CML Cell Proliferation

In various signaling pathways activated by Bcr-Abl, c-Myc promotes the Bcr-Abl transformed cell proliferation through activated Akt, Jak2 and NF-κB. We have previously shown that Bcr-Abl represses the expression of THAP11 and continuously expressed c-Myc, resulting in the proliferation of CML cells. However, the mechanism of c-Myc regulation is unclear in CML cell proliferation. In the present study, we investigated the mechanism of c-Myc transcription regulation in the Bcr-Abl+ progenitor cells derived from CML patients.

The cells used in this study were human CML cell lines, K562, Meg01 and SHG3 cells. Primary CML cells (ALDH^hi cells) were obtained from the bone marrow of CML (CP) patients (n=12). Human normal ALDH^hi cells were isolated from bone marrow of healthy volunteers after obtaining informed consents. For analysis of THAP11 mRNA expression, quantitative RT-PCR was performed in all cell lines treated with Abl kinase inhibitors (STI571, AMN107, and BMS354825). For cell survival analysis and the levels of c-Myc, Fbw7 and some CDKIs in CML cells, MTT assays, western blot and cell cycle analysis were performed in all cell lines transfected with THAP11 siRNA or cDNA. For colony analysis, the colonies of CFU-GEMM, CFU-GM, and BFU-E were counted in CML stem/progenitor cells transfected with THAP11 siRNA or cDNA, or treated with Abl kinase inhibitors.

In CML cell lines, the expressions of THAP11 mRNA and protein were significantly increased by treatment with Abl kinase inhibitors or transfection with Bcr-Abl siRNA. In CML cells transfected with the THAP11 cDNA, it is shown that CML cell proliferation was inhibited, and the expression of c-Myc protein was decreased compared to the untransfected cells. In addition, the inhibition of Bcr-Abl by the Abl kinase inhibitors or depletion of Bcr-Abl induced the expression of Fbw7 in CML cells. In CML stem/progenitor cells (ALDH^hi cells) obtained from patients with CML, the expression of THAP11 mRNA was suppressed, and the transfection with Bcr-Abl siRNA or treatment with Abl kinase inhibitors increased the expression of THAP11 and Fbw7. In CFU-GEMM, CFU-GM, and BFU-E, treatment with the Abl kinase inhibitors and depletion of Bcr-Abl induced the THAP11 and Fbw7 expression and reduced the c-Myc expression, and inhibited colony formation of Bcr-Abl+ progenitor cells.

In CML cells, we found that the depletion of THAP11 and Fbw7 expression. We showed that Bcr-Abl induced the expression of c-Myc by depletion of THAP11 and also inhibited the degradation of c-Myc by depletion of Fbw7. This study showed that Bcr-Abl promoted the CML cell proliferation through the THAP11 and Fbw7 mediated-c-Myc aberrant expression.

No relevant conflicts of interest to declare.