Cyclin-dependent kinases (Cdks) and their cyclin partners regulate mammalian cell proliferation and withdrawal from the cell cycle and, as such, control differentiation in many tissues. Studies were undertaken to examine the roles of cell cycle proteins in differentiating cytotrophoblasts. Cyclin E gene and protein expression was down-regulated after 24 h in cultured trophoblasts. Cdk2-associated kinase activity was decreased after 96 h in culture as was the amount of cyclin E in complexes with

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... Cdk2; however, levels of the Cdk inhibitor, p27Kipl, were significantly increased. In freshly isolated trophoblasts and in 24-h cultures, the retinoblastoma gene product (pRb) was found in both the active and inactive forms, yet only hypophosphorylated, active pRb was present in syncytiotrophoblast. Thus, inactivation of Cdk2 through cyclin E down-regulation and increased p27KP expression leads to an accumulation of active pRb in syncytiotrophoblast. Prevention of entry into S phase by hypophosphorylated pRb may allow trophoblasts to respond to signals that potentiate differentiation. Our studies suggest that regulation of Gt-phase Cdk activity may be involved in the terminal differentiation process of cytotrophoblasts.