Transcriptional activation potential of normal and tumor-associated myb isoforms does not correlate with their ability to block GCSF-induced terminal differentiation of murine myeloid precursor cells

Abstract

The myb gene has been shown to be an important regulator of hematopoietic cell proliferation, differentiation and apoptosis. Activation of the myb gene into an oncogenic form has involved structural alterations to the coding sequences. Thus, the v-myb gene encoded by the Avian Myeloblastosis Virus, is truncated at both the 5' and 3' ends. Additionally, tumor cells containing rearrangements in the myb locus, such as the ABPL tumors or NFS60 tumor cell line have recently been shown to display a heterogeneity of structure. In this study, we examined the growth and differentiation properties of clonal cell lines derived from 32Dcl3 which harbor myb transgenes; derived from v-myb, and the ABPL-1, ABPL-2, ABPL-4 and NFS-60 cell lines. Retroviral vectors containing the appropriate myb cDNAs were produced, transfected into packaging cell lines, and the viruses were used to generate the 32D derivative cell clones. Abrogation of IL-3 dependence was never observed in any cell line. Expression of c-myb, ABPL-1-myb and ABPL-2-myb isoforms in 32D cells resulted in a block to their ability to terminally differentiate into granulocytes at the pro-myelocytic stage. However, expression of ABPL-4-myb or NFS60-myb in these cells failed to result in a similar effect. These cells differentiated into granulocytes in the presence of G-CSF, albeit more slowly than control 32Dcl3 cells. We also examined the ability of various Myb-isoforms to transactivate transcription of reporter genes containing Myb-binding elements in their promoter/enhancer sequences, to determine whether the phenotypic effects produced by these various isoforms correlate with their ability to transactivate transcription. Our results show that while v-myb and c-myb transactivated transcription equally well, the NFS60-myb exhibited the highest levels of transcriptional transactivation. The ABPL-1, ABPL-2 and ABPL-4-myb isoforms showed very low levels of transcriptional transactivation potential with the same reporter genes. These results suggest that the ability of various Myb-isoforms to transactivate transcription does not by itself correlate with their ability to induce a block to G-CSF-induced terminal differentiation of myeloid precursor cells.