Glucocorticoids inhibit developmental stage-specific osteoblast cell cycle. Dissociation of cyclin A-cyclin-dependent kinase 2 from E2F4-p130 complexes.
Unique cell cycle control is instituted in confluent osteoblast cultures, driving growth to high density. The postconfluent dividing cells share features with cells that normally exit the cell cycle; p27(kip1) is increased, p21(waf1/cip1) is decreased, free E2F DNA binding activity is reduced, and E2F4 is primarily nuclear. E2F4-p130 becomes the predominant E2F-pocket complex formed on E2F sites, but, unlike the complex that typifies resting cells, cyclin A and CDK2 are also present. Administration of dexamethasone at this, but not earlier stages, results in reduction of cyclin A and CDK2 levels with a parallel decrease in the associated kinase activity, dissociation of cyclin A-CDK2 from the E2F4-p130 complexes, and inhibition of G(1)/S transition. The glucocorticoid-mediated cell cycle attenuation is also accompanied by, but not attributable to, increased p27(kip1) and decreased p21(waf1/cip1) levels. The attenuation of osteoblast growth to high density by dexamethasone is associated with severe impairment of mineralized extracellular matrix formation, unless treatment commences in cultures that have already grown to high density. Both the antimitotic and the antiphenotypic effects are reversible, and both are antagonized by RU486. Thus, glucocorticoids induce premature attenuation of the osteoblast cell cycle, possibly contributing to the osteoporosis induced by these drugs in vivo.
Smith
E
E
Department of Orthopaedic Surgery, Institute for Genetic Medicine, University of Southern California Keck School of Medicine, Los Angeles, California 90033, USA.
Redman
R A
RA
Logg
C R
CR
Coetzee
G A
GA
Kasahara
N
N
Frenkel
B
B
D016475:3T3 Cells; D000818:Animals; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle / Q000187:drug effects*; D018797:Cell Cycle Proteins; D002455:Cell Division; D002467:Cell Nucleus / Q000378:metabolism; D019925:Cyclin A / Q000378:metabolism*; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology; D050704:E2F4 Transcription Factor; D005109:Extracellular Matrix / Q000378:metabolism; D005434:Flow Cytometry; D005938:Glucocorticoids / Q000494:pharmacology*; D006727:Hormone Antagonists / Q000494:pharmacology; D008156:Luciferases / Q000378:metabolism; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D015735:Mifepristone / Q000494:pharmacology; D010006:Osteoblasts / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010770:Phosphotransferases / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011506:Proteins; D050717:Retinoblastoma-Like Protein p130; D013997:Time Factors; D014157:Transcription Factors / Q000378:metabolism*; D014162:Transfection; D025521:Tumor Suppressor Proteins
C493957:Cdkn1a protein, mouse; C493961:Cdkn1b protein, mouse; D018797:Cell Cycle Proteins; D019925:Cyclin A; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004268:DNA-Binding Proteins; D050704:E2F4 Transcription Factor; D005938:Glucocorticoids; D006727:Hormone Antagonists; D008869:Microtubule-Associated Proteins; D010750:Phosphoproteins; D011506:Proteins; C493838:Rbl2 protein, mouse; D050717:Retinoblastoma-Like Protein p130; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D015735:Mifepristone; D003907:Dexamethasone; D008156:Luciferases; D010770:Phosphotransferases; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495893:Cdk2 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases