Direct evidence that Bcr-Abl tyrosine kinase activity disrupts normal synergistic interactions between Kit ligand and cytokines in primary primitive progenitor cells.

We previously reported that chronic myelogenous leukemia (CML) primitive granulocyte-monocyte (GM) progenitors have a greatly reduced requirement for kit ligand (KL) to achieve optimal growth with granulocyte colony-stimulating factor (G-CSF) + granulocyte-monocyte colony-stimulating factor (GM-CSF). Conversely, others have demonstrated that unlike normal, CML CD34+ progenitors can proliferate in response to KL as a sole stimulus. To address these seemingly paradoxical findings, we examined the growth responses of CML CD34+ GM progenitors to various cytokines with and without a potent inhibitor of Bcr-Abl tyrosine kinase activity, PD173955. The heightened growth responses of CML GM progenitors to KL alone and to G-CSF + GM-CSF were abrogated by 10 nM PD173955 while having no effect on normal GM progenitors. While normal GM progenitors exhibited the expected synergistic response when KL was added to G-CSF + GM-CSF, CML GM progenitors had a minimal response; however, some synergism was restored by 10 nM PD173955. Normal erythroid progenitors require the synergistic interaction between KL and a saturating amount of erythropoietin (EPO, 1 unit) for optimal growth. In contrast, CML erythroid progenitors had up to 50% of optimal growth in KL alone, and, only a subthreshold amount of EPO (0.1 unit) was needed with KL to achieve 85% of the optimal response; these heightened growth responses were largely abrogated by 10 nM PD173955. Thus, direct evidence is provided that constitutively activated Bcr-Abl kinase pathways in primitive CML progenitors cooperate with single growth factors producing a heightened growth response, and, in so doing, disrupt the normally required synergistic interactions between KL and other cytokines to achieve activation and optimal growth of primitive progenitors. Coupled with our previous findings that a larger than normal proportion of CML primitive progenitors are at a later stage of maturation, we propose that this disruption of normal synergistic responses leads to increased progenitor recruitment into a committed pool by a process of accelerated maturation.