ATRA treatment of U-937 cells results in decreased CDK activity and a reduction in hyperphosphorylated pRb.

(A) CDK2, 4, and 6 kinase activity was measured in an in vitro kinase assay using pRb as a substrate. Cells were grown in the presence or absence of ATRA for 72 hours, whole cell lysates were prepared, and the activities of CDK2, CDK4, and CDK6 were determined as described in “Materials and methods.” The graph indicates quantification of the bands correlated to control. Mean ± SD (n = 4). The asterisk indicates significant differences from the corresponding controls (P < .05). (B-D) The levels of total pRb or Ser780, Ser795, or Ser807/811 phosphorylated pRb (B), p130 (C), or p107 (D) during ATRA-induced differentiation were determined by Western blot of whole cell lysates prepared at the indicated times using specific antibodies.