Effect of GM-CSF deprivation on proliferation and apoptosis of TF-1 cells, GM-CSF–independent TF-1 cells, as well as TF-1 cells transfected with either control vector or MDM2 gene. Parent TF-1 cells (▵) and the GM-CSF–independent TF-1 subclone (◊) were cultured in media with GM-CSF (1 ng/mL) for 2 days, washed three times with PBS, and resuspended in fresh medium without GM-CSF. TF-1 control vector transfectants (□) and TF-1 MDM2 transfectants (○) were cultured in media with G418 (400 μg/mL) and GM-CSF (1 ng/mL) for 2 days, washed three times with PBS, and resuspended in fresh media with G418 (400 μg/mL) without GM-CSF. (A) Proliferation of cells cultured in the absence of GM-CSF were determined by counting number of viable cells with trypan blue. Mean standard deviation was determined for three independent experiments. (B) Apoptosis was evaluated with acridine orange/ethidium bromide staining, and the percentage of apoptotic cells was calculated: apoptotic cells / (apoptotic cells + viable cells). Mean standard deviation was determined for three independent experiments. (C) TF-1 control vector transfectants, GM-CSF–independent TF-1 cells, and TF-1 MDM2 transfectants were cultured for 2 days in the presence of GM-CSF (1 ng/mL), washed three times with PBS, and resuspended in fresh media with varying concentrations of GM-CSF (0.0, 0.1, 0.2, and 1 ng/mL). Cells were harvested at 16 hours and apoptosis was evalutated by DNA fragmentation assay. A