Enforced expression of full length c-Myb leads to density-dependent transformation of murine haemopoietic cells

The oncogenic activation of c-myb has been associated with structural alterations to the Myb protein. Although such alterations can increase the ability of Myb to transform haemopoietic cells, it has been unresolved whether over-expression of wild type (WT) c-Myb can lead to transformation. We show here that infection with a retrovirus that expresses WT i.e. full length c-Myb leads to transformation of primary haemopoietic cells (as indicated by clonogenic assays). The transformed cells are similar to those obtained with carboxyl-truncated (CT) c-Myb in that they show phenotypic and morphological characteristics of early myeloid cells and remain dependent on exogenous growth factors. Cells expressing WTMyb form lower numbers of colonies on average and have a greater tendency to spontaneously differentiate than those expressing truncated c-Myb. Additionally, our results show that transformation by both forms of Myb is dependent on the density at which the infected cells are cultured, and that low levels of transformation can be increased by addition of conditioned medium from myb transformed cells grown at high density. This implies that transformation can be enhanced by the effects of an autocrine growth factor. Moreover, the production of, or sensitivity to, such a factor may be influenced by Myb itself, since CT Myb-infected cells cultured at low densities show higher levels of transformation than WT Myb-infected cells.