C-FMS dependent HL-60 cell differentiation and regulation of RB gene expression

The dependence of induced myelomonocytic cell differentiation, and regulation of the RB tumor suppressor gene during this process, on the c-fms gene product, the CSF-1 lymphokine receptor, was determined in HL-60 promyelocytic leukemia cells. Adding a monoclonal antibody with specificity for the c-fms gene product to cells treated with various inducers of myelomonocytic or macrophage differentiation, including retinoic acid and 1,25-dihydroxy vitamin D3, inhibited the rate of differentiation. During the period of inducer treatment usually preceding onset of differentiation, longer periods of antibody exposure caused greater inhibition of differentiation. In a stable HL-60 transfectant overexpressing the CSF-1 receptor at the cell surface due to a constitutively driven c-fms trans gene, the rate of differentiation was enhanced compared to the wild type cell, consistent with a positive regulatory role for the CSF-1 receptor. The anti-fms antibody caused much less inhibition of differentiation in the transfectants than in wild type cells, consistent with a larger number of receptors causing reduced sensitivity. During the induced metabolic cascade leading to differentiation, the typical early down regulation of RB gene expression was inhibited by the antibody. The antibody itself caused an increase in RB expression per cell, which offset the decrease normally caused by differentiation inducers (1,25-dihydroxy vitamin D3 and retinoic acid). The changes in RB expression preceded changes in the RB protein to the hypophosphorylated state. Most of the RB protein in proliferating cells was phosphorylated and no significant accumulation of hypophosphorylated RB protein occurred until after onset of G0 arrest. Thus the metabolic cascade leading to myelomonocytic differentiation of HL-60 cells appears to be driven by a function of the c-fms protein. Inhibiting that process by attacking this receptor impedes differentiation and also compromises the early down regulation of RB tumor suppressor gene expression which normally precedes differentiation. These findings provide additional support for a potential role for down regulating RB expression in promoting cell differentiation, and suggest the possibility that RB may be either a target or intermediate mediator of CSF-1 actions.