DIFFERENTIATION OF P19 EC CELLS LEADS TO DIFFERENTIAL MODULATION OF CYCLIN-DEPENDENT KINASE-ACTIVITIES AND TO CHANGES IN THE CELL-CYCLE PROFILE

The retinoblastoma gene product (pRb) is essential for normal embryoni c development, Phosphorylation of pRb by cyclin dependent kinases (cdk 's) is believed to be crucial for the regulation of its function, In t his report we have studied the regulation of pRb and cdk's during in v itro differentiation of P19 embryonal carcinoma (EC) cells, as a model for early developmental processes, During EC cell differentiation, th e synthesis of pRb is strongly induced, In addition, the phosphorylati on state of induced pRb is modulated, yielding mainly underphosphoryla ted pRb, Concomitantly, the pRb kinases cdk2 and cdk4 are differential ly regulated: cdk2 kinase activity is impaired, whereas cdk4 kinase ac tivity is stimulated, due to an induction of cyclins D1 and D2. Furthe rmore, the DNA binding activity of E2F transcription factors is strong ly impaired during differentiation and the number of cells in G1 is in creased. Thus, P19 EC cell differentiation is accompanied by changes i n cdk-activities, pRb regulation and E2F DNA-binding, resulting in the generation of cell types with an altered cell cycle profile.