Induction of CABLES1 by the TKI Inhibits the Cell Cycle Progression and Induces Apoptosis In CML Cell.

CABLES1 (cyclin-dependent kinase (CDK)-5 and ABL enzyme 1) is a regulator of cell proliferation, apoptosis, and cell cycle, and it has been reported to be lost in a variety cancers. It has been also reported that knockout of the Cable1 gene has minimal to no effect on hematopoietic stem cells. However, we found that the expression of Cables1 gene and CABLES1 protein was suppressed in CML cells, and its function is little known in CML. In this study, we have investigated the function of CABLES1 in CML cell proliferation.

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 Cables1 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 p53 and some CDKIs in CML cells, MTT assays, western blot and cell cycle analysis were performed in all cell lines transfected with Cables1 shRNA or cDNA. For colony analysis, the colonies of CFU-GEMM, CFU-GM, and BFU-E were counted in CML stem/progenitor cells transfected with Cables1 cDNA or shiRNA, or treated with Abl kinase inhibitors.

In CML cell lines, the expressions of Cables1 mRNA and CABLES1 protein were significantly increased by treatment with Abl kinase inhibitors or transfection with Bcr-Abl shRNA. In CML cells transfected with the Cables1 cDNA, it is shown that CML cell proliferation was inhibited, and the phosphorylation levels of p53, and the expression of BAX and p21 protein were markedly increased compared to the untransfected cells. In addition, the overexpression of CABLES1 induced G1 cell cycle arrest and reduced the DiOC6 fluorescence, indicating breakdown of the mitochondrial membrane potential in CML cells. On the other hand, the changes of p73 and p27 protein expression were not detected. Moreover, in CML cells transfected with Cables1 shRNA, the inhibition of CML cell proliferation by the Abl kinase inhibitors were weakened. In CML stem/progenitor cells (ALDH^hi cells) obtained from patients with CML, the expression of Cables1 mRNA was suppressed, and the transfection with Bcr-Abl shRNA or treatment with Abl kinase inhibitors increased the expression of Cables1 mRNA and CABLES1 protein, and decreased the counts of CFU-GEMM, CFU-GM and BFU-E.

Our results demonstrated that the Bcr-Abl suppressed the expression of CABLES1, and the depletion of CABLES1 promotes cell cycle progression and p53-dependent apoptosis. Moreover, the induction of CABLES1 expression has the potentiality to eradicate CML stem/progenitor cells.

No relevant conflicts of interest to declare.