Bcr-Abl–expressing leukemic cells are highly resistant to apoptosis induced by chemotherapeutic drugs. Although a number of signaling molecules have been shown to be activated by the Bcr-Abl kinase, the antiapoptotic pathway triggered by this oncogene has not been elucidated. Here, we show that the interleukin 3-independent expression of the antiapoptotic protein, Bcl-xL, is induced by Bcr-Abl through activation of signal transducer and activator of transcription (Stat)5. Inhibition of the Bcr-Abl kinase activity in Bcr-Abl–expressing cell lines and CD34+ cells from chronic myelogenous leukemia (CML) patients induces apoptosis by suppressing the capacity of Stat5 to interact with the bcl-x promoter. Interestingly, after inhibition of the Bcr-Abl kinase, the expression of Bcl-xL is downregulated more rapidly in chronic phase than in blast crisis CML cells, suggesting an involvement of this protein in disease progression. Overall, we describe a novel antiapoptotic pathway triggered by Bcr-Abl that may contribute to the resistance of CML cells to undergo apoptosis.

This study was registered with ClinicalTrials.gov (NCT02144987).

Chronic myelogenous leukemia (CML) is characterized by the expression of the BCR-ABL tyrosine kinase, which results in increased cell proliferation and inhibition of apoptosis. In this study, we show in both BCR-ABL cells (Mo7e-p210 and BaF/3-p210) and primary CML CD34+ cells that STI571 inhibition of BCR-ABL tyrosine kinase activity results in a G 1 cell cycle arrest mediated by the PI3K pathway. This arrest is associated with a nuclear accumulation of p27Kip1 and down-regulation of cyclins D and E. As a result, there is a reduction of the cyclin E/Cdk2 kinase activity and of the retinoblastoma protein phosphorylation. By quantitative reverse transcription-PCR we show that BCR-ABL/PI3K regulates the expression of p27Kip1 at the level of transcription. We further show that BCR-ABL also regulates p27Kip1 protein levels by increasing its degradation by the proteasome. This degradation depends on the ubiquitinylation of p27Kip1 by Skp2-containing SFC complexes: silencing the expression of Skp2 with a small interfering RNA results in the accumulation of p27Kip1 . We also demonstrate that BCR-ABL cells show transcriptional up-regulation of Skp2. Finally, expression of a p27Kip1 mutant unable of being recognized by Skp2 results in inhibition of proliferation of BCR-ABL cells, indicating that the degradation of p27Kip1 contributes to the pathogenesis of CML. In conclusion, these results suggest that BCR-ABL regulates cell cycle in CML cells at least in part by inducing proteasome-mediated degradation of the cell cycle inhibitor p27Kip1 and provide a rationale for the use of inhibitors of the proteasome in patients with BCR-ABL leukemias. (Cancer Res 2005; 65(8): 3264-72)

Summary:Despite the wide use of G-CSF for mobilization of PBPC the best dose and schedule of G-CSF has not been definitively established. In this study we have compared three different schedules of G-CSF for mobilization of PBPC in normal donors including a single daily dose of 10 g/kg/day for 5 days (21 donors) and doses of 6 (21 donors) or 8 g/kg/12 h (6 donors) for 5 days. We demonstrate that G-CSF at doses of 6 and 8 g/kg/12 h mobilizes significantly more CD34 ؉ cells/ml of blood (83.3 ؎ 6.7 and 121 ؎ 6.9, respectively) than 10 g/kg/day (71.6 ؎ 6.5). Mobilization with 6 or 8 g/kg/12 h of G-CSF was also associated with collection of significantly more CD34؉ cells in comparison with 10 g/kg/24 h (2.24 ؎ 1.2 and 2.46 ؎ 1.22 vs 1.15 ؎ 0.8 CD34 ؉ cells/kg of donor/blood volume). PBPC collection was associated with a significant decrease in platelet count which was not significantly different between the three groups. Ten days after the last PBPC collection platelet counts were within normal limits while there was a decrease in WBC and ANC. We conclude that G-CSF administered every 12 h at doses of 6 g/kg provides better CD34 ؉ cell yield than 10 g/kg once a day in normal donors which may translate into a decrease in the number of aphereses required to obtain enough numbers of CD34 ؉ cells for allogeneic PBPC transplant.