Expression of p27Kip1 in osteoblast-like cells during differentiation with parathyroid hormone.
PTH is a major systemic regulator of bone metabolism and plays an important role in both bone formation and resorption. PTH either inhibits or stimulates osteoblastic cell proliferation depending on the model that is studied. We analyzed the cell cycle of the UMR-106 cell line, a relatively differentiated osteoblastic osteogenic sarcoma line in which PTH is known to inhibit proliferation but the mechanism of action is unknown. PTH decreased the proportion of cells in S phase and increased the number of G1 phase cells. We examined the effect of PTH on the regulators of the G1 phase cyclin-dependent kinases and found that PTH increased p27Kip1, but not p21Cip1, levels. This effect was mimicked by 8-bromo-cAMP, but not by phorbol 12-myristate 13-acetate. The protein kinase A inhibitor KT5720 abolished the effect of PTH on the increase in p27Kip1 expression. PTH increased CDK2-associated p27Kip1 without affecting the levels of CDK2. CDK2 activity was down-regulated by both PTH and 8-bromo-cAMP treatment. These data suggest that PTH blocks entry of cells into S phase and inhibits cell proliferation as the consequence of an increase in p27Kip1, which is mediated through the protein kinase A pathway. The inhibition of G1 cyclin-dependent kinases by p27Kip1 could cause a reduction of phosphorylation of key substrates and inactivation of transcription factors essential for entry into S phase. The inhibition of cell cycle progression through PKA-mediated p27Kip1 induction might play an important role in PTH-induced differentiation of osteoblasts.
Onishi
T
T
Renal Division, Barnes-Jewish Hospital, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Hruska
K
K
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D002227:Carbazoles; D018797:Cell Cycle Proteins; D002454:Cell Differentiation / Q000187:drug effects*; D002460:Cell Line; D000242:Cyclic AMP / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016193:G1 Phase / Q000187:drug effects; D015870:Gene Expression; D007211:Indoles / Q000494:pharmacology; D008869:Microtubule-Associated Proteins / Q000235:genetics*; D010006:Osteoblasts / Q000378:metabolism*; D012516:Osteosarcoma; D010281:Parathyroid Hormone / Q000494:pharmacology*; D011758:Pyrroles / Q000494:pharmacology; D051381:Rats; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins
D002227:Carbazoles; C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D004791:Enzyme Inhibitors; D007211:Indoles; D008869:Microtubule-Associated Proteins; D010281:Parathyroid Hormone; D011758:Pyrroles; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D015124:8-Bromo Cyclic Adenosine Monophosphate; C057416:KT 5720; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D018844:Cyclin-Dependent Kinases; D013755:Tetradecanoylphorbol Acetate