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J Biol Chem, Vol. 274, Issue 41, 28875-28879, October 8, 1999

Inhibition of Osteoblast-specific Transcription Factor Cbfa1 by the cAMP Pathway in Osteoblastic Cells
UBIQUITIN/PROTEASOME-DEPENDENT REGULATION

Yin Tintut, Farhad Parhami, Vien Le, Gerard Karsenty^¶, and Linda L. Demer

From the  Division of Cardiology, Department of Medicine, UCLA School of Medicine, Los Angeles, California 90095, the ^¶ Department of Human and Molecular Genetics, Baylor College of Medicine, Houston, Texas 77030, and the  Department of Physiology, UCLA School of Medicine, Los Angeles, California 90095

The cAMP pathway, a major intracellular pathway mediating parathyroid hormone signal, regulates osteoblastic function. Parathyroid hormone (through activation of protein kinase A) has also been shown to stimulate ubiquitin/proteasome activity in osteoblasts. Since the osteoblast-specific transcription factor Osf2/Cbfa1 is important for differentiation of osteoblastic cells, we examined the roles of the cAMP and ubiquitin/proteasome pathways in regulation of Cbfa1. In the osteoblastic cell line, MC3T3-E1, continuous treatment with cAMP elevating agents inhibited both osteoblastic differentiation based on alkaline phosphatase assay and DNA binding ability of Cbfa1 based on a gel retardation assay. Cbfa1 inhibition was paralleled by an inhibitory effect of forskolin on Cbfa1-regulated genes. Northern and Western blot analyses suggested that the inhibition of Cbfa1 by forskolin was mainly at the protein level. Pretreatment with proteasome inhibitors prior to forskolin treatment reversed the effect of forskolin. Furthermore, addition of proteasome inhibitors to forskolin-pretreated samples resulted in recovery of Cbfa1 protein levels and accumulation of polyubiquitinated forms of Cbfa1, indicating a role for the proteasome pathway in the degradation of Cbfa1. These results suggest that suppression of osteoblastic function by the cAMP pathway is through proteolytic degradation of Cbfa1 involving a ubiquitin/proteasome-dependent mechanism.

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Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.

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