Effect of 1,25(OH)₂D₃ on the levels of CKIs (A), cyclins (B), and CDKs (C) in U937 cells. Whole cell extracts isolated from cells grown in the absence or presence of 1,25(OH)₂D₃ (1 × 10⁻⁷ mol/L) for 0 to 72 hours were analyzed by Western blot. Twenty micrograms of whole cell extract was used in each lane and probed with antibodies to the indicated proteins. As a control, cells were grown for 72 hours in the absence of ligand (ethanol alone), and a representative Western blot probed with p21 antibody is shown in (A). Note that for p15 protein, samples were assayed up to 120 hours of treatment with ligand.