Expression of cyclins A and E in PMA-treated U937 cells. (A) Treatment of U937 cells downregulates cyclin A. Cells were treated with PMA for the indicated times and cell extracts (300 μg total protein) were subjected to Western blotting with an anticyclin A antibody (1:333 dilution). (B) Cyclin E levels are sustained in PMA-treated U937 cells. Cell extracts (250 μg total protein) were subjected to Western blotting with anticyclin E antibody (1:1,000 dilution).