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Developmentally regulated inhibition of cell cycle progression by glucocorticoids through repression of cyclin a transcription in primary osteoblast cultures

Authors

  • Yankel Gabet,

    1. Department of Biochemistry & Molecular Biology, Keck School of Medicine at the University of Southern California, Los Angeles, California
    2. Department of Institute for Genetic Medicine, Keck School of Medicine at the University of Southern California, Los Angeles, California
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  • Tommy Noh,

    1. Department of Biochemistry & Molecular Biology, Keck School of Medicine at the University of Southern California, Los Angeles, California
    2. Department of Institute for Genetic Medicine, Keck School of Medicine at the University of Southern California, Los Angeles, California
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  • Christopher Lee,

    1. Department of Institute for Genetic Medicine, Keck School of Medicine at the University of Southern California, Los Angeles, California
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  • Baruch Frenkel

    Corresponding author
    1. Department of Biochemistry & Molecular Biology, Keck School of Medicine at the University of Southern California, Los Angeles, California
    2. Department of Institute for Genetic Medicine, Keck School of Medicine at the University of Southern California, Los Angeles, California
    3. Department of Orthopaedic Surgery, Keck School of Medicine at the University of Southern California, Los Angeles, California
    • Institute for Genetic Medicine, University of Southern California Keck School of Medicine, 2250 Alcazar Street, CSC/IGM240, Los Angeles, CA 90033.
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  • Yankel Gabet and Tommy Noh contributed equally to this work.

Abstract

Synthetic glucocorticoids (GCs) like dexamethasone (DEX) are effective immunosuppressants indicated for autoimmune and inflammatory diseases. However, they often promote osteoporosis and bone fractures. Here, we investigated the anti-mitogenic effect of GCs in primary osteoblast cultures. DEX did not affect cell cycle progression in confluent (day 2), or early post-confluent cultures. Starting on day 5, however, DEX strongly inhibited the G1/S cell cycle transition. Day 5 also marked the beginning of a ∼2-day commitment stage, during which cultures acquired resistance to the inhibitory effect of DEX on mineralization. Considering the importance of Wnt signaling in osteoblast development, we analyzed the effect of DEX on the Wnt pathway. DEX treatment did not inhibit the TOPGAL Wnt reporter before, but only during and after the commitment stage. However, this inhibition was not associated with repression of neither cyclin D1 nor c-Myc mRNA, well-established Wnt targets regulating cell cycle progression. On the other hand, acute (24-h) and chronic (7 days) DEX treatment significantly reduced both the mRNA and protein levels of cyclin A, another cell cycle regulator. Moreover, cyclin A repression by DEX was not observed before, but only during and after the commitment stage. Using gel shift and reporter assays, we identified an ATF/CREB-binding site critical for the DEX-mediated repression of cyclin A transcription. Furthermore, and similar to cyclin A, Atf4 expression was repressed by DEX only during and after commitment. Our data suggest that GCs attenuate cell cycle progression in osteoblasts in a developmental stage-specific manner by repressing Atf4-dependent cyclin A gene expression. J. Cell. Physiol. 226: 991–998, 2011. © 2010 Wiley-Liss, Inc.

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