C/EBPα Inhibits Cell Growth via Direct Repression of E2F-DP-Mediated Transcription

Abstract

Using an inducible transcription system which allows the regulated expression of C/EBP isoforms in tissue culture cells, we have found that the ectopic expression of C/EBP␣, at a level comparable to that found in normal liver tissue, has a pronounced antimitogenic effect in mouse L cells and NIH 3T3 cells. The inhibition of cell division by C/EBP␣ in mouse cells cannot be reversed by simian virus 40 T antigen, by oncogenic ras, or by adenovirus E1a protein. When expressed in thymidine kinase-deficient L cells or 3T3 cells, C/EBP␣ is detected in a protein complex which binds to the E2F binding sites found in the promoters of the genes for E2F-1 and dihydrofolate reductase (DHFR). Bacterially expressed C/EBP␣ has no affinity for these E2F sites, but when recombinant C/EBP␣ is added to nuclear extracts from mouse fibroblasts, a new E2F binding activity appears, which contains the C/EBP␣ protein. Using an E2F-DP1-responsive promoter linked to a reporter gene, it can be shown that C/EBP␣ directly inhibits the induction of this promoter by E2F-DP1 in transienttransfection assays. Furthermore, C/EBP␣ can be shown to inhibit the S-phase induction of the E2F and DHFR promoters in permanent cell lines. These findings delineate a straightforward mechanism for C/EBP␣-mediated cell growth arrest through repression of E2F-DP-mediated S-phase transcription.

CCAAT/enhancer-binding proteins are required for granulopoiesis independent of their induction of the granulocyte colony-stimulating factor receptor

“…C/EBP␣ may inhibit the G 1 /S transition via interaction with E2F or with cdk2. [57][58][59][60] Interaction with cdk2 occurs via the C/EBP␣ basic region, and this segment is retained in K␣ER. 60 Expression of p21 WAF1/CIP1 or p27 Kip1 in U937 cells induces monocytic markers.…”

C/EBPαp30 plays transcriptional regulatory roles distinct from C/EBPαp42

“…It initiates growth arrest by stabilizing p21, and by disrupting E2F transcriptional complexes during the G1 phase of the cell cycle [7][8][9][10][11][12]. It was suggested that C/EBPα might be a tumor suppressor gene.…”

Transcriptional regulation of granulocyte and monocyte development

“…Perhaps C/ EBPa dimers are required for interaction with E2F or with another protein associated with the E2F complexes, or perhaps C/EBPa monomers interact with one of these proteins via its leucine zipper. In fact, E2F contains a leucine zipper, as noted (Slomiany et al, 2000). C/EBPe also slows 32D cl3 proliferation (Nakajima and Ihle, 2001).…”

“…Both C/EBPaWT-ER and KaER inhibit progression from G1 to S phase in 32D cl3 cells (Wang et al, 1999;Friedman and Wang, 2000). These proteins may inhibit the G1/S transition via interaction with E2F (Timchenko et al, 1999;Slomiany et al, 2000;Johansen et al, 2001). A KaER variant with a defective leucine zipper domain does not slow 32D cl3 proliferation (Wang and Friedman, 2002).…”