Differential expression of c-myc and the transferrin receptor in G1 synchronized M1 myeloid leukemia cells

We have studied the transcriptional activity, steady-state mRNA levels, and steady-state protein levels of the c-myc and transferrin receptor (TfR) genes in murine M1 myeloid leukemia cells arrested in G1 phase of the cell cycle by different methods. When cells are growth-arrested by density inhibition, a technique that places the majority of cells in early G1, c-myc protein, as detected by Western analysis, is expressed at 80% of the level seen in proliferating cells. Steady-state mRNA levels and, to a lesser extent, transcriptional activity of the c-myc gene, parallel the protein findings. Under these conditions, TfR gene expression is much lower than in normally cycling cells. We have previously demonstrated that density-inhibited M1 cells, released from density inhibition and treated with the DNA polymerase alpha inhibitor aphidicolin, remain in G1, but at a point temporally closer to S phase. Cells treated in this manner demonstrate reduced transcriptional activity and expression of the c-myc gene, but TfR gene expression approximates the level found in proliferating cells. These data suggest that neither c-myc nor TfR gene expression is constant throughout the G1 phase of the cell cycle in M1 cells. c-myc gene expression is highest in early G1 and falls to low levels by late G1, while the reverse is true for TfR gene expression.