Accelerated cell cycle progression in osteoblasts overexpressing the c-fos proto-oncogene - Induction of cyclin A and enhanced CDK2 activity

A Sunters, D P Thomas, W A Yeudall, A E Grigoriadis

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66 Citations (Scopus)


Transgenic mice overexpressing the c-Fos oncoprotein develop osteosarcomas that are associated with deregulated expression of cell cycle genes. Here we have generated osteoblast cell lines expressing c-fos under the control of a tetracycline-regulatable promoter to investigate the role of c-Fos in osteoblast cell cycle control in vitro. Three stable subclones, AT9.2, AT9.3, and AT9.7, derived from MC3T3-E1 mouse osteoblasts, expressed high levels of exogenous c-fos mRNA and protein in the absence of tetracycline. Functional contribution of ectopic c-Fos to AP-1 complexes was confirmed by electromobility shift assays and transactivation of AP-1 reporter constructs. Induction of exogenous c-Fos in quiescent AT9.2 cells caused accelerated S-phase entry following serum stimulation, resulting in enhanced growth rate. Ectopic c-Fos resulted in increased expression of cyclins A and E protein levels, and premature activation of cyclin A-, cyclin E-, and cyclin-dependent kinase (CDK) 2-associated kinase activities, although cyclin D levels and CDK4 activity were not affected significantly in these cell lines. The enhanced CDK2 kinase activity was associated with a rapid, concomitant dissociation of p27 from CDK2-containing complexes. Deregulated cyclin A expression and CDK2 activity was also observed in primary mouse osteoblasts overexpressing c-Fos, but not in fibroblasts, and c-Fos transgenic tumor-derived osteosarcoma cells constitutively expressed high levels of cyclin A protein. These data suggest that overexpression of c-Fos in osteoblasts results in accelerated S phase entry as a result of deregulated cyclin A/E-CDK2 activity. This represents a novel role for c-Fos in osteoblast growth control and may provide c-Fos-overexpressing osteoblasts with a growth advantage during tumorigenesis.
Original languageEnglish
Pages (from-to)9882 - 9891
Number of pages10
JournalJournal of Biological Chemistry
Issue number11
Publication statusPublished - 12 Mar 2004


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