C/EBPα Regulation of the Growth-Arrest-Associated Gene gadd45

CCAAT/enhancer-binding protein alpha (C/EBPα) is expressed in postmitotic, differentiated adipocytes and is required for adipose conversion of 3T3-L1 cells in culture. Temporal misexpression of C/EBPα in undifferentiated adipoblasts leads to mitotic growth arrest. We report here that growth arrest- and DNA damage- inducible gene 45 (gadd45) is preferentially expressed in differentiated 3T3-L1 adipocytes similar to phenotype- associated genes. Furthermore, C/EBPα transactivates a reporter plasmid containing 1.5 kb of the gadd45 promoter region. The proto-oncogene myc, which inhibits adipocyte differentiation, abrogates C/EBPα activation of gadd45. gadd45 is known to be a target of the tumor suppressor p53 in a G 1 checkpoint activated by DNA damage. Immunoprecipitation of radiolabeled proteins with conformation-specific antibodies revealed that wild-type p53 is expressed throughout 3T3-L1 adipocyte development, including the postmitotic period characterized by the accumulation of gadd45 and C/EBPα. A stable 3T3-L1 subline was engineered to express a dominant negative p53, human p53 143ala . The p53 143ala subline differentiated to adipocytes and showed appropriate developmental expression of gadd45. These findings suggest that postmitotic growth arrest is coupled to adipocyte differentiation via C/EBPα stimulation of growth arrest-associated and phenotype-associated genes.