mRNA and protein of c-myc during the induction of proliferation by GM-CSF and IL-3 in MO7e and F36P cells. (A) Cell cycle analysis of MO7e (left panel) and F36P (right panel) cells with or without GM-CSF. Propidium iodide staining and fluorescence analysis of cells were performed at the indicated culture condition (control culture and GM-CSF–deprived culture) to determine cell cycle distribution in each condition. The G₁ cells indicated by the open box were separated by centrifugal elutriation for the following experiments. (B) After GM-CSF deprivation, MO7e (left panel) and F36P (right panel) cells were incubated with 10 ng/mL and 1 ng/mL GM-CSF or IL-3 for indicated periods at 37°C. Total cellular RNA was prepared and analyzed by RNase protection assay to determine intracellular level of c-myc mRNA. mRNA of GAPDH was used as control. The ratio was determined by densitometric analysis of the band and was calculated as compared with c-myc mRNA level of control cells (indicated as “C”). Lane-to-lane variation was corrected based upon mRNA level of GAPDH. (C) After GM-CSF deprivation, MO7e (left panel) and F36P (right panel) cells were incubated with 10 ng/mL and 1 ng/mL GM-CSF or IL-3 for the indicated periods at 37°C. Total cell lysates (3 × 10⁵ cells per lane) were separated by SDS-PAGE and were then immunoblotted using specific antibodies for c-myc and β-tubulin. These show the representative results of a minimum of 3 independent experiments.