Differences in BCL-X_L expression and STAT5 phosphorylation in chronic myeloid leukaemia patients

Chronic myelogenous leukaemia (CML) cells show expression of BCL-X_L, an anti-apoptotic oncogene. This expression is induced by BCR-ABL protein kinase through activation of the signal transducer and activator of transcription-5 protein (STAT5). To date, however, the contribution of BCL-X_L and STAT5 to the transforming phenotype in CML is still unclear. This study was aimed at defining the status of activated STAT5 and BCL-X_L expression and their relation to BCR-ABL rearrangement in CML cells derived from patients at different clinical stages. Twenty-seven consecutive patients with CML were enrolled in the study. Peripheral blood mononuclear cells were lysed and subjected to immunoprecipitation and Western blotting to analyse phosphorylated STAT5. The p210 BCR-ABL rearrangements were determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and BCL-X_L expression by semi-quantitative RT-PCR. We found that increased transcription of BCL-X_L gene was associated with phosphorylated STAT5 in the majority of blast crisis patients and in a few accelerated and chronic phase patients. Moreover, BCL-X_L expression levels were found to be decreased in chronic phase, contrary to a marked increase in blast crisis. We found no difference in expression of BCL-X_L and phosphorylated STAT5 when related with b3a2 and b2a2 BCR-ABL rearrangements. These results suggest that STAT5 activity and BCL-X_L overexpression may reflect a stage of differentiation among CML phases, and this could contribute to BCR-ABL-dependent transformation.