Inhibition of cell proliferation by C/EBPα occurs in many cell types, does not require the presence of p53 or Rb, and is not affected by large T-antigen

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Abstract
The transcription factor CCAAT/enhancer binding protein (C/EBPoc) is expressed predominantly in differentiated tissues and is able to induce growth arrest and differentiation in preadipocytes. C/EBPα expression is high in non-dividing hepatocytes, but decreases during liver regeneration. These observations suggest that C/EBPα is inversely related to cell proliferation. To investigate the mechanism of growth inhibition by C/EBPα, the response of immortal human cells to cotransfection of a C/EBPα expression vector (CMVα) and a CMVβ-galactosidase expression vector was examined. Hep3B2, a hepatoma; Saos2, an osteosarcoma deficient for p53 and Rb; and 639, a fibroblast expressing SV40 T-antigen, were examined. Transiently transfected cells were stained for p-gal activity to monitor their ability to undergo division. The ability of stable transformants to form colonies was also assessed for each cell line. Cells transfected with CMVα remained as non-dividing cells while control cells divided to form colonies. Mutations of the C/EBPα sequence demonstrated that only a small, previously uncharacterized activation domain was required for antimitotic activity. Our results suggest that C/EBPa may play a role In maintaining the quiescent state of hepatocytes and other cells. Furthermore, it appears that the effects of C/EBPα are not mediated through p53 or Rb and are not altered by T-antigen.