Regulation of CCAAT/Enhancer-binding Protein (C/EBP) Activator Proteins by Heterodimerization with C/EBPγ (Ig/EBP)*

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/EBPβ occurs predominantly as a heterodimer that displays rapid mobility in gel shift assays. Biochemical fractionation and antibody supershift assays demonstrate that the C/EBPβ heterodimeric partner is C/EBPγ (Ig/EBP), a C/EBP protein that has been implicated as an inhibitor of other family members. Although most cell types express C/EBPβ·C/EBPγ heterodimers, macrophages contain a C/EBPβ partner that is serologically distinct from C/EBPγ. We found that C/EBPγ blocked the ability of C/EBPβ and C/EBPγ to activate a reporter gene in L cell fibroblasts but did not inhibit a chimeric C/EBPβ protein containing the GCN4 leucine zipper. Repression by C/EBPγ occurs at the level of transactivation and requires heterodimerization with the C/EBP partner. C/EBPγ was an ineffective repressor in HepG2 hepatoma cells despite forming C/EBP heterodimers, and C/EBPα was not effectively inhibited in either L or HepG2 cells. Our findings demonstrate that C/EBPγ modulates C/EBP activity in a cell- and isoform-specific manner.

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