Rb is found in C/EBPϵ binding complexes in myeloid cells. (A) EMSA was performed using 10 μg of nuclear extract proteins from either untreated U937 cells (U937) or U937 cells treated with ATRA for 4 days (U937A). Extracts were incubated with 32P-labeled oligonucleotides containing the C/EBP site from the mim-1 promoter. C/EBPϵ antibody (anti-ϵ), Rb antibody (anti-Rb), and unlabeled competitor (competitor) were added to the reactions as indicated. (B) Nuclear extract proteins (10 μg) from COS-1 cells transfected with an Rb expression vector were incubated with the mim-1 probe. In vitro–translated C/EBPϵ protein, C/EBPϵ antibody (anti-ϵ), and Rb antibody (anti-Rb) were added to the reactions as indicated. SS C/EBPϵ indicates the complex supershifted by the C/EBPϵ and the Rb antibodies.
