title: Post-proliferative cyclin E-associated kinase activity in differentiated osteoblasts: inhibition by proliferating osteoblasts and osteosarcoma cells.
abstract: Spontaneous differentiation of normal diploid osteoblasts in culture is accompanied by increased cyclin E associated kinase activity on (1) the retinoblastoma susceptibility protein pRB, (2) the p107 RB related protein, and (3) two endogenous cyclin E-associated substrates of 78 and 105 kD. Activity of the differentiation-related cyclin E complexes (diff.ECx) is not recovered in cdc2 or cdk2 immunoprecipitates. Phosphorylation of both the 105 kD endogenous substrate and the p107 exogenous substrate is sensitive to inhibitory activity (diff.ECx-i) present in proliferating osteoblasts. This inhibitory activity is readily recruited by the cyclin E complexes of differentiated osteoblasts but is not found in cyclin E immunoprecipitates of the proliferating cells themselves. Strong inhibitory activity on diff.ECx kinase activity is excerted by proliferating ROS 17/2.8 osteosarcoma cells. However, unlike the normal diploid cells, the diff.ECx-i activity of proliferating ROS 17/2.8 cells is recovered by cyclin E immunoprecipitation. The cyclin-dependent kinase inhibitor p21CIP1/WAF1 inhibits diff.ECx kinase activity. Thus, our results suggest the existence of a unique regulatory system, possibly involving p21CIP1/WAF1, in which inhibitory activity residing in proliferating cells is preferentially targeted towards differentiation-related cyclin E-associated kinase activity.
authors.affiliation: Department of Cell Biology and Cancer Center, University of Massachusetts Medical Center, Worcester 01655, USA.
authors.forename: E
authors.initials: E
authors.lastname: Smith
authors.forename: B
authors.initials: B
authors.lastname: Frenkel
authors.forename: T K
authors.initials: TK
authors.lastname: MacLachlan
authors.forename: A
authors.initials: A
authors.lastname: Giordano
authors.forename: J L
authors.initials: JL
authors.lastname: Stein
authors.forename: J B
authors.initials: JB
authors.lastname: Lian
authors.forename: G S
authors.initials: GS
authors.lastname: Stein
mesh_terms: D000818:Animals; D016203:CDC2 Protein Kinase / Q000378:metabolism; D042846:CDC2-CDC28 Kinases; D002454:Cell Differentiation / Q000187:drug effects; D002455:Cell Division; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000378:metabolism*; D016213:Cyclins / Q000494:pharmacology / Q000502:physiology*; D006358:Hot Temperature; D010006:Osteoblasts / Q000166:cytology / Q000201:enzymology*; D012516:Osteosarcoma / Q000201:enzymology* / Q000473:pathology; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D051381:Rats; D016160:Retinoblastoma Protein / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: C493958:Cdkn1a protein, rat; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D016160:Retinoblastoma Protein; D017346:Protein Serine-Threonine Kinases; D016203:CDC2 Protein Kinase; D042846:CDC2-CDC28 Kinases; C495892:Cdk2 protein, rat; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases