Biological effects induced by variable levels of BCR-ABL protein in the pluripotent hematopoietic cell line UT-7

Abstract

There is currently no satisfactory model allowing analysis of dose-effect relationships of BCR-ABL proteins in human hematopoietic cells. To study comparatively the proliferative, differentiative and anti-apoptotic actions of different levels of BCR-ABL proteins in the context of the same cellular background, we have introduced the BCR-ABL gene into the GM-CSF-dependent pluripotent human cell line UT-7. Individual clones expressing BCR-ABL were analyzed by Western blots. After normalization to equivalent levels of endogenous ABL protein, 14 clones always grown in GM-CSF were found to express low but variable levels of BCR-ABL whereas two clones selected in the absence of GM-CSF expressed very high levels of BCR-ABL. All low-level BCR-ABL expressing clones exhibited a behavior similar to that of the GM-CSF-dependent parental cells as they ceased to proliferate upon growth factor deprivation and showed a strong proliferative response upon GM-CSF addition. One out of 14 clones showed progressive GM-CSF independence during culture over several weeks and was found to have a significant increase of BCR-ABL expression at that time. The resistance of this clone (E8-2) to different apoptotic stimuli was found to be increased as compared to its low BCR-ABL-expressing counterpart (E8-1) and similar to that observed in clones with very high levels of BCR-ABL (UT-7/9 and UT-7/11) which were totally resistant to apoptotic stimuli. When injected into nude mice, parental UT-7 cells and clones with low-level of BCR-ABL were not tumorigenic over 10 weeks of observation whereas UT-7 clones with high levels of BCR-ABL (UT-7/9, UT-7/11 and UT-7/E8-2) induced aggressive tumors in 2-4 weeks with a significant correlation between the amount of BCR-ABL protein and the rate of tumor growth. In conclusion, the establishment of an in vitro and in vivo CML model using UT-7 cells suggests for the first time in human cells, that the fully transformed phenotype induced by BCR-ABL requires high levels of BCR-ABL expression. These findings suggest that variable levels of BCR-ABL in primary patient cells could also be responsible for the different phenotypic features seen in chronic and acute phases of CML, such as the differentiation ability induced by growth factors. Leukemia (2000) 14, 662-670.

Expression ofBCR/ABLandBCL-2in myeloid progenitors leads to myeloid leukemias

“…Low disease incidence and long latency in our model are likely to result from lower expression of the p210 BCR/ABL transgene when compared with models using the robust but nonspecific retroviral and metallothionein promoters. Although an increase in the dosage of p210 BCR/ABL has been shown to be crucial for both antiapoptotic effects (28,29) and resistance of the disease to treatment (30), perhaps the expression levels found in our transgenic mice more closely mimic those found in preleukemic and chronic phase patients (10,31,32). The strain-specific decrease in CML-like disease also suggests that heritable factors, in particular genetic backgrounds, can either enhance or suppress the physiologic effects of BCR͞ABL.…”

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

“…High levels of Bcr-Abl were required to protect the cell lines from apoptotic stimuli. In contrast, Issaad et al (2000) have shown that low-level expression of Bcr-Abl in the pluripotent human cell line, UT-7, was insufficient to produce a growth factor independent phenotype. Only when cells expressed high levels of Bcr-Abl did they become independent of the need for growth factors.…”

Fusion tyrosine kinases: a result and cause of genomic instability

“…Drug resistance depends also on the high expression levels of BCR/ABL suggesting amplification/hyperactivation of signaling pathways and/or DNA repair mechanisms relevant for response to genotoxic stress (Cambier et al, 1998;Issaad et al, 2000;Keeshan et al, 2001). Since drug resistance depended on the BCR/ABL kinase activity, it was most likely not owing to clonal evolution arising from the accumulation of additional genomic aberrations.…”