Involvement of the retinoblastoma protein in monocytic and neutrophilic lineage commitment of human bone marrow progenitor cells.

The retinoblastoma gene product (pRb) is involved in both cell cycle regulation and cell differentiation. pRb may have dual functions during cell differentiation: partly by promoting a cell cycle brake at G(1) and also by interacting with tissue-specific transcription factors. We recently showed that pRb mediates differentiation of leukemic cell lines involving mechanisms other than the induction of G(1) arrest. In the present study, we investigated the role of pRb in differentiation of human bone marrow progenitor cells. Human bone marrow cells were cultured in a colony-forming unit-granulocyte-macrophage (CFU-GM) assay. The addition of antisense RB oligonucleotides (alpha-RB), but not the addition of sense orientated oligonucleotides (SO) or scrambled oligonucleotides (SCR), reduced the number of colonies staining for nonspecific esterase without affecting the clonogenic growth. Monocytic differentiation of CD34(+) cells supported by FLT3-ligand and interleukin-3 (IL-3) was correlated to high levels of hypophosphorylated pRb, whereas neutrophilic differentiation, supported by granulocyte colony-stimulating factor (G-CSF) and stem cell factor (SCF), was correlated to low levels. The addition of alpha-RB to liquid cultures of CD34(+) cells, supported with FLT3-ligand and IL-3, inhibited monocytic differentiation. This was judged by morphology, the expression of CD14, and staining for esterase. Moreover, the inhibition of monocytic differentiation of CD34(+) cells mediated by alpha-RB, which is capable of reducing pRb expression, was counterbalanced by an enhanced neutrophilic differentiation response, as judged by morphology and the expression of lactoferrin. CD34(+) cells incubated with oligo buffer, alpha-RB, SO, or SCR showed similar growth rates. Taken together, these data suggest that pRb plays a critical role in the monocytic and neutrophilic lineage commitment of human bone marrow progenitors, probably by mechanisms that are not strictly related to control of cell cycle progression.