Three levels of functional interaction determine the activity of CCAAT/enhancer binding protein-α on the serum albumin promoter

Abstract

We have studied the activation of the serum albumin promoter by transcription factor CCAAT/enhancer binding protein-α (C/EBPα) in the HepG2 hepatoma cell line. We find that three distinct mechanisms determine the ability of C/EBPα to activate this promoter in a cell-type-specific and cooperative manner. First, the trans-activating function of C/EBPα is generated through cooperation between three separate domains of the protein that we have named trans-activation elements (TE-I through TE-III). The TEs have little or no ability to activate transcription by themselves, but any two can cooperate to do so, both in the C/EBPα protein and when linked to the GAL4 DNA-binding domain. Second, TE-III was found to contain a negative regulatory subdomain, the function of which was alleviated when C/EBPα was bound in the environment of the albumin promoter. This formed the basis for cooperative activation of this promoter by C/EBPα. Finally, we demonstrate that the leucine zipper of C/EBPα participates in determining the cell type specificity of albumin promoter activation, as it exerts a strong negative effect on albumin promoter activation in the nonhepatic HeLa cell line but not in HepG2 cells. These findings shed new light on the mode of action of C/EBPα and show a novel function for a leucine zipper in cell-type-specific gene expression.