Three levels of functional interaction determine the activity of CCAAT/enhancer binding protein-alpha on the serum albumin promoter.
We have studied the activation of the serum albumin promoter by transcription factor CCAAT/enhancer binding protein-alpha (C/EBP alpha) in the HepG2 hepatoma cell line. We find that three distinct mechanisms determine the ability of C/EBP alpha to activate this promoter in a cell-type-specific and cooperative manner. First, the trans-activating function of C/EBP alpha 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/EBP alpha 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/EBP alpha was bound in the environment of the albumin promoter. This formed the basis for cooperative activation of this promoter by C/EBP alpha. Finally, we demonstrate that the leucine zipper of C/EBP alpha 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/EBP alpha and show a novel function for leucine zipper in cell-type-specific gene expression.
Nerlov
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Howard Hughes Medical Institute, Kaplan Cancer Center, New York University Medical Center, New York 10016.
Ziff
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D000595:Amino Acid Sequence; D001483:Base Sequence; D022763:CCAAT-Enhancer-Binding Protein-alpha; D022762:CCAAT-Enhancer-Binding Proteins; D002454:Cell Differentiation; D004252:DNA Mutational Analysis; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D016350:Leucine Zippers / Q000235:genetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D017384:Sequence Deletion; D012709:Serum Albumin / Q000096:biosynthesis / Q000235:genetics*; D015534:Trans-Activators / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation; D014407:Tumor Cells, Cultured
C085848:C-EBP nuclear protein trans-activation element II; D022763:CCAAT-Enhancer-Binding Protein-alpha; D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D012709:Serum Albumin; D015534:Trans-Activators