Dose-dependent effects of Bcr-Abl in cell line models of different stages of chronic myeloid leukemia.

Chronic myeloid leukemia (CML) is caused by Bcr-Abl, an activated tyrosine kinase. The amounts of Bcr-Abl mRNA and protein in cells from patients in blast crisis (BC) are higher than in those chronic phase (CP), indicating that their expression rises with disease progression. In order to study this phenomenon on cells with the same genetic background, we transfected the 32D cell line with the BCR-ABL transgene and selected clones with graded expression of Bcr-Abl within the range found in cells from CP to BC. In vitro, we found that Bcr-Abl exerted dose-dependent effects upon growth factor dependence, clonogenicity and migration. However, the relationship between Bcr-Abl expression and cellular adhesion to fibronectin was more complex: rather than a direct positive correlation, clones that expressed low, but not high, levels of Bcr-Abl were less adhesive than growth factor stimulated, parental BCR-ABL-negative 32D cells. This finding parallels the situation with normal and CML-CP progenitors where the latter exhibit defective adhesion to fibronectin. Treatment with the tyrosine kinase inhibitor imatinib mesylate reversed the adhesion deficient phenotype of clones expressing low levels of Bcr-Abl. When injected subcutaneously into syngeneic mice, cell lines expressing high levels of Bcr-Abl rapidly induced tumors, whereas low-expressing clones led to tumor formation only after a prolonged latency. These findings suggest that the level of Bcr-Abl may be essential in determining the phenotype of the leukemic clone at different stages of the disease.