Characterization of c-fos promoter binding proteins in normal and neoplastic liver cells.

Abstract

The ability of the c-fos gene to respond to a variety of stimuli reflects the complexity of its regulation. Indeed, aberrant regulation can lead to cellular transformation. The fine regulation of the gene relies on the interaction of protein factors interacting with a set of regulatory elements present within the promoter of the gene in a cooperative fashion. In these studies, multiprotein complexes capable of binding to the serum response element, the cAMP response element, and sis-conditioned medium response element, were identified in liver cell nuclei. A subset of these proteins was able to interact with multiple regulatory elements and might be able to direct functional cooperation between them. These multiple factors could be targeted by different signalling pathways. The behavior of these multiprotein complexes was analyzed under physiological conditions of transient c-fos expression in regenerating rat liver and under pathological conditions of chemically induced hepatocarcinogenesis and in solid 5123tc Morris hepatomas whereby constitutive expression of the gene occurs. Elevated expression of the c-fos gene in both normal and in transformed cells correlated with the loss and/or complete absence of the 47 kDa CRE-binding activity. The 47 kDa CRE-binding protein was characterized as highly CRE-specific and distinct from the other members of the CREB/CREM/ATF family of proteins. The 47 kDa CREB factor displayed DNA-binding activity in the dephosphorylated form. The functional importance of the c-fos promoter binding proteins was assessed by in vitro transcription which suggested the presence of a transcriptional block in quiescent liver cells which was relieved in proliferating hepatocytes. The results are consistent with the identification of the 47 kDa CRE-specific DNA-binding protein as a transcriptional repressor in quiescent liver cells.