Bcr-Abl kinase promotes cell cycle entry of primary myeloid CML cells in the absence of growth factors

Cell cycle control of both immature and differentiated primary myeloid normal and chronic-phase chronic myeloid leukaemia (CML) cells to growth factor deprivation was studied. CD34⁺ cells were cultured in liquid culture. After removal of growth factors for 48 h normal cells were very efficiently arrested with the fraction of cells in S phase reduced by 70.8 ± 6.5% in CD34⁺ and 50.5 ± 4.2% in CD34⁻ cells. In contrast, a significantly higher proportion of leukaemic cells remained in S phase. The fraction of S-phase cells was reduced by only 29.3 ± 5.7% in CD34⁺ CML cells and 21.2 ± 3.8% in CD34⁻ cells. This abnormal negative cell cycle control in leukaemic cells was specific for growth factor deprivation. Reaction to IFN-α and TNF-α treatment was identical both in normal and CML cells. Equal quantities of the cytokines TNF-α, IL-1α, IL-1RA and IL-6 were produced by CML and normal cells. However, production of GM-CSF, with a median of 11 ± 5 pg/ml, was found only in the supernatants of CML cells. But antibodies to GM-CSF did not restore growth factor dependence of the leukaemic cells. The defect was completely corrected by the abl-specific tyrosine kinase inhibitor CGP 57148 without effecting cell cycle control of normal cells. Our results demonstrate a directly Bcr-Abl-dependent defective response of both immature and differentiated primary myeloid CML cells to growth factor deprivation.