CCAAT/enhancer-binding protein (C/EBP) isoforms are thought to be important regulators of the hepatocyte phenotype. However, the specific physiological roles of different isoforms are poorly understood because hepatocytes express multiple C/EBPs, and various isoforms have overlapping functions. To identify the functions of C/EBP in mature hepatocytes, replication-defective adenovirus vectors were used to efficiently and homogeneously overexpress the mouse C/EBP gene in a SV40 virus-conditionally transformed rat hepatocyte line that can be induced to express C/EBP and C/EBP but that has little endogenous C/EBP expression. Hepatocytes were infected with a recombinant adenovirus vector carrying the cDNA for C/EBP driven by Rous sarcoma virus promoter elements (AdCEBP) or a similar vector carrying the Escherichia coli lacZ gene (Adgal). Staining for -galactosidase demonstrated an infection efficiency of 100% at a multiplicity of infection of 25 plaque-forming units/cell and persistence of foreign gene expression for at least 9 days. Cultures infected with AdCEBP had 50-fold higher levels of C/EBP mRNA and protein than those infected with Adgal, but similar expression of C/EBP. Infection with AdCEBP inhibited proliferation in cells expressing little C/EBP, even when proliferation was driven by the SV40 transforming antigen, and also blunted mitogenic induction of the c-myc proto-oncogene in nontransformed cells with high levels of C/EBP. Although overexpression of C/EBP consistently increased C/EBP DNA binding activity, it was not sufficient for albumin expression. Infection with AdCEBP only increased albumin mRNA levels in nontransformed cells that also expressed relatively high levels of C/EBP. Thus, in hepatocytes, C/EBP has a dominant antiproliferative function, but must interact with other factors to regulate hepatocyte-specific gene expression.

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