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Regulation of CCAAT/enhancer-binding protein (C/EBP) activator proteins by heterodimerization with C/EBP gamma (Ig/EBP)

  1. Author:
    Parkin, S. E.
    Baer, M.
    Copeland, T. D.
    Schwartz, R. C.
    Johnson, P. F.

  2. Author Address

    NCI Frederick, Basic Res Lab, Regulat Cell Growth Lab, Frederick, MD 21702 USA NCI Frederick, Basic Res Lab, Regulat Cell Growth Lab, Frederick, MD 21702 USA NCI Frederick, Basic Res Lab, Eukaryot Transcript Regulat Sect, Frederick, MD 21702 USA Michigan State Univ, Dept Microbiol & Mol Genet, E Lansing, MI 48824 USA Johnson PF NCI Frederick, Basic Res Lab, Regulat Cell Growth Lab, Frederick, MD 21702 USA
    1. Year: 2002
  2. Journal: Journal of Biological Chemistry
    1. 277
    2. 26
    3. Pages: 23563-23572
  4. Type of Article: Article
  1. Abstract:

    The CCAAT/enhancer-binding proteins (C/EBPs) are basic leucine zipper transcription factors that play important roles in regulating cell growth and differentiation. C/EBP proteins form leucine zipper-mediated homodimers but are also capable of heterodimerizing with other C/EBPs in vitro. Here we show that C/EBPbeta occurs predominantly as a heterodimer that displays rapid mobility in gel shift assays. Biochemical fractionation and antibody supershift assays demonstrate that the C/EBPbeta heterodimeric partner is C/EBPgamma (Ig/EBP), a C/EBP protein that has been implicated as an inhibitor of other family members. Although most cell types express C/EBPbeta.C/EBPgamma heterodimers, macrophages contain a C/EBPbeta partner that is serologically distinct from C/EBPgamma. We found that C/EBPgamma blocked the ability of C/EBPbeta and C/EBPgamma to activate a reporter gene in L cell fibroblasts but did not inhibit a chimeric C/EBPbeta protein containing the GCN4 leucine zipper. Repression by C/EBPgamma occurs at the level of transactivation and requires heterodimerization with the C/EBP partner. C/EBPgamma was an ineffective repressor in HepG2 hepatoma cells despite forming C/EBP heterodimers, and C/EBPalpha was not effectively inhibited in either L or HepG2 cells. Our findings demonstrate that C/EBPgamma modulates C/EBP activity in a cell- and isoform-specific manner.

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