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

The myb gene has been shown to be an important regulator of hematopoie tic cell proliferation, differentiation and apoptosis, Activation of t he myb gene into an oncogenic form has involved structural alterations to the coding sequences, Thus, the v-myb gene encoded by the Avian My eloblastosis Virus, is truncated at both the 5' and 3' ends, Additiona lly, tumor cells containing rearrangements in the myb locus, such as t he ABPL tumors or NFS60 tumor cell line have recently been shown to di splay a heterogeneity of structure, In this study, we examined the gro wth and differentiation properties of clonal cell lines derived from 3 2Dcl3 which harbor myb transgenes derived from v-myb, and the ABPL-1, ABPL-2, ABPL-4 and NFS-60 cell Lines, Retroviral vectors containing th e 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 l ine, Expression of c-my6, ABPL1-myb and ABPL-2-myb isoforms in 32D cel ls resulted in a block to their ability to terminally differentiate in to granulocytes at the pro-myelocytic stage, However, expression of AB PL-4-myb or NFS60-myb in these cells failed:to result in a similar eff ect, These cells differentiated into granulocytes in the presence of G -CSF, albeit more slowly than control 32Dcl3 cells, We also examined t he ability of various Myb-isoforms to transactivate transcription of r eporter genes containing Myb-binding elements in their promoter/enhanc er sequences, to determine whether the phenotypic effects produced by these various isoforms correlate with their ability to transactivate t ranscription, Our results show that while v-myb and c-myb transactivat ed transcription equally well, the MFS60-myb exhibited the highest lev els 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 th e 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.