Reciprocal regulation of gadd45 by C/EBP alpha and c-Myc.

The CCAAT/enhancer-binding protein-alpha (C/EBP alpha) drives the differentiation of murine 3T3-L1 cells to adipocytes through transcriptional activation of phenotype-associated genes via proximal promoter elements. In addition, C/EBP alpha suppresses mitotic growth. We report here that C/EBP alpha directly regulates gadd45 through a C/EBP-binding site in the proximal promoter. A 3 basepair substitution, directed at the most conserved residues of the sequence, reduced C/EBP alpha-mediated transactivation and impaired binding of C/EBP alpha in adipocyte nuclear extracts. We also found that c-Myc antagonized C/EBP alpha-mediated transactivation of gadd45. Analysis of systematically altered forms of C/EBP alpha revealed that c-Myc antagonism targeted the antimitotic, transcriptional activation domain of C/EBP alpha. In addition, we localized the regulatory sequences in the gadd45 promoter that are required for c-Myc antagonism of C/EBP alpha transactivation. Our findings reveal that C/EBP alpha coordinates cellular differentiation and mitotic growth arrest through direct, coordinate regulation of phenotype-associated and growth-arrest-associated genes. In addition, our findings reveal that the reciprocal relation between C/EBP alpha and c-Myc in 3T3-L1 cells includes antagonistic transcriptional control of a growth-arrest-associated gene.