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

Abstract

CCAAT/enhancer-binding protein alpha (C/EBP␣) is expressed in postmitotic, differentiated adipocytes and is required for adipose conversion of 3T3-L1 cells in culture. Temporal misexpression of C/EBP␣ in undifferentiated adipoblasts leads to mitotic growth arrest. We report here that growth arrest-and DNA damageinducible gene 45 (gadd45) is preferentially expressed in differentiated 3T3-L1 adipocytes similar to phenotypeassociated genes. Furthermore, C/EBP␣ transactivates a reporter plasmid containing 1.5 kb of the gadd45 promoter region. The proto-oncogene myc, which inhibits adipocyte differentiation, abrogates C/EBP␣ 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/EBP␣. 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/EBP␣ stimulation of growth arrest-associated and phenotype-associated genes.

Myc represses the growth arrest gene gadd45

“…The CAT sites are of particular interest as earlier studies suggest Myc can repress gene transcription by inhibiting the activities of CAT-binding transcription factors belonging to the CTF/NF1 family and the C/EBP family of trans-acting factors (Martin, 1991;Wedel and Ziegler-Heitbrock, 1995 (Yang et al, 1991(Yang et al, , 1993. Recently, transient transfection reporter assays demonstrate that in 3T3-L1 cells, ectopic Myc expression can abrogate the C/EBPa mediated activation of gadd45 transcription associated with adipocyte di erentiation (Constance et al, 1996). In addition, Mink et al show ectopic v-Myc can suppress the expression of myelomonocytic speci®c genes by inhibiting both the expression and the function of C/ EBPa transcription factors (Mink et al, 1996) C/EBPa is predominantly expressed in di erentiated cells (Birkenmeier et al, 1989), hence the role of C/EBPa in the G0/G1-associated up-regulation, or Mycmediated repression of gadd45 in cycling cells, remains unclear.…”

“…Elevated gadd45 expression has also been linked to growth arrest triggered either by developmental signals (Constance et al, 1996) or to nutrientdeprivation associated with G0 phase of the cell cycle (Fornace et al, 1989;Smith and Fornace, 1996;Zhan et al, 1994b). Upon exposure to ionizing radiation gadd45 is induced (Hollander et al, 1993;, and this induction is dependent upon the p53 transcription factor (Zhan et al, 1993).…”

Repression of Gadd45α by activated FLT3 and GM-CSF receptor mutants contributes to growth, survival and blocked differentiation

“…This is also associated with an increased replating ability of CFU-GM from Gadd45aÀ/À bone marrow, suggesting a potential role for Gadd45a as an AML tumor suppressor gene. 50 Taken together with reports that Gadd45a is a target of C/ EBPa, 51,52 Gadd45a may have an important role in promoting granulocyte differentiation. Gadd45a is also a direct target of C/EBPa in adipocytes 34,35 and thus may have a more general role in coupling growth arrest to induction of differentiation.…”

BRCA1 and cell signaling

“…Notably, previous works have demonstrated that cMyc can repress GADD45 expression by inhibiting C/ EBP activity at the promoter (Amundson et al, 1998;Constance et al, 1996;Li et al, 1994;Marhin et al, 1997). In this regard, suppression of c-Myc by BRCA1 may be important for activation of GADD45, since BRCA1 may thus de-repress the GADD45 promoter.…”