The retinoblastoma gene product (pRb) is essential for normal embryonic development, Phosphorylation of pRb by cyclin dependent kinases (cdk's) is believed to be crucial for the regulation of its function, In this report we have studied the regulation of pRb and cdk's during in vitro differentiation of P19 embryonal carcinoma (EC) cells, as a model for early developmental processes, During EC cell differentiation, the synthesis of pRb is strongly induced, In addition, the phosphorylation state of induced pRb is modulated, yielding mainly underphosphorylated pRb, Concomitantly, the pRb kinases cdk2 and cdk4 are differentially regulated: cdk2 kinase activity is impaired, whereas cdk4 kinase activity is stimulated, due to an induction of cyclins D1 and D2. Furthermore, the DNA binding activity of E2F transcription factors is strongly impaired during differentiation and the number of cells in G1 is increased. Thus, P19 EC cell differentiation is accompanied by changes in cdk-activities, pRb regulation and E2F DNA-binding, resulting in the generation of cell types with an altered cell cycle profile.
