Only late, nonmitotic stages of granulocyte differentiation in 32Dcl3 cells are blocked by ectopic expression of murine c-myb and its truncated forms.

In murine leukemia virus-induced myeloid leukemias, insertional mutagenesis of the c-myb locus has been shown to occur frequently. Proto-oncogene activation is achieved in most leukemias by integration of murine leukemia virus upstream of exons 3 or 4 or by integration into exon 9 with consequent truncation of the protein. The present study investigates the effect of ectopic expression of full-length c-myb or c-myb containing amino- or carboxyl-terminal truncations (minus 47 and 248 amino acids, respectively) on granulocyte differentiation in vitro. Recombinant myb retroviruses were used to infect an interleukin 3-dependent progenitor cell line, 32Dcl3, which undergoes terminal differentiation to mature neutrophilic granulocytes in the presence of granulocyte colony-stimulating factor. Overexpression of c-myb did not abrogate the interleukin 3 dependency of the parental cell line. However, cells expressing all forms of c-myb were blocked at an intermediate stage of granulocyte differentiation and continued to proliferate in the presence of granulocyte colony-stimulating factor. After 14 days in medium with granulocyte colony-stimulating factor, myb-expressing cultures predominantly consisted of promyelocytes with some myelocytes and almost undetectable numbers of neutrophilic granulocytes. This suggested that early stages of granulocyte differentiation were not inhibited, a finding that was further supported by the induction of myeloperoxidase, a biochemical marker of promyelocytes. Interestingly, the expression of lactoferrin, known to be a marker of late stages of granulocyte differentiation, was completely inhibited in the cells infected with myb viruses. It was concluded that c-myb expression blocked granulocyte differentiation to the terminal mitotic stages and that deletion of the NH2-terminal 47 amino acids and/or the COOH-terminal 248 amino acids of c-myb neither enhanced nor diminished this effect.