Expression of G1 cyclins and cyclin-dependent kinase-2 activity during terminal differentiation of cultured human trophoblast.
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 Cdk2; however, levels of the Cdk inhibitor, p27Kip1, 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 p27Kip1 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 G1-phase Cdk activity may be involved in the terminal differentiation process of cytotrophoblasts.
McKenzie
P P
PP
Graduate School of Medicine, University of Tennessee Medical Center, Knoxville 37920, USA. mcf7@msn.com
Foster
J S
JS
House
S
S
Bukovsky
A
A
Caudle
M R
MR
Wimalasena
J
J
D015152:Blotting, Northern; D015153:Blotting, Western; D042846:CDC2-CDC28 Kinases; D018797:Cell Cycle Proteins; D002454:Cell Differentiation; D002478:Cells, Cultured; D056746:Cyclin G; D056766:Cyclin G1; D051357:Cyclin-Dependent Kinase 2; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000096:biosynthesis; D016213:Cyclins / Q000096:biosynthesis*; D006801:Humans; D007150:Immunohistochemistry; D008869:Microtubule-Associated Proteins / Q000096:biosynthesis; D010920:Placenta / Q000166:cytology; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis; D014327:Trophoblasts / Q000378:metabolism*; D025521:Tumor Suppressor Proteins
C534702:CCNG1 protein, human; D018797:Cell Cycle Proteins; D056746:Cyclin G; D056766:Cyclin G1; D016213:Cyclins; D008869:Microtubule-Associated Proteins; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D011494:Protein Kinases; C102333:histone H1 kinase; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases