Retinoic acid inhibits pRB phosphorylation. (A) Human B cells (1.5 × 10⁶ cells/mL) were stimulated with μ (37 μg/mL) and SAC (0.005%) for 32 hours in the absence (control) or presence of retinoic acid at the indicated concentrations. Total cell lysates were prepared and 30 μg of total protein was analyzed for pRB protein by Western blotting. The lane indicated as 0 hrs refers to a sample harvested before mitogenic stimulation of the cells. (B) Left panel: Cells (1.5 × 10⁶ /mL) were stimulated with μ (37 μg/mL) and SAC (0.005%) and harvested at the indicated times after stimulation. Right panel: Cells were stimulated with μ (37 μg/mL) and SAC (0.005%). Retinoic acid (1000 nmol/L) was then added to the cell cultures at each indicated time before harvesting the cells at 32 hours. Total cell lysates were prepared and subjected to Western blot analysis for pRB. μ, F(ab′)₂ fragment of rabbit antihuman IgM; SAC, Staphylococcus aureus crude cell suspension; RA, retinoic acid.