EXPRESSION OF G(1) CYCLINS AND CYCLIN-DEPENDENT KINASE-2 ACTIVITY DURING TERMINAL DIFFERENTIATION OF CULTURED HUMAN TROPHOBLAST

Cyclin-dependent kinases (Cdks) and their cyclin partners regulate mam malian cell proliferation and withdrawal from the cell cycle and, as s uch, control differentiation in many tissues. Studies were undertaken to examine the roles of cell cycle proteins in differentiating cytotro phoblasts. Cyclin E gene and protein expression was down-regulated aft er 24 h in cultured trophoblasts. Cdk2-associated kinase activity was decreased after 96 h in culture as was the amount of cyclin E in compl exes with Cdk2; however, levels of the Cdk inhibitor, p27(Kip1), 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 cy clin E down-regulation and increased p27(Kip1) expression leads to an accumulation of active pRb in syncytiotrophoblast. Prevention of entry into S phase by hypophosphorylated pRb may allow trophoblasts to resp ond to signals that potentiate differentiation. Our studies suggest th at regulation of G(1)-phase Cdk activity may be involved in the termin al differentiation process of cytotrophoblasts.