MEMBERS OF THE CAAT ENHANCER-BINDING PROTEIN, HEPATOCYTE NUCLEAR FACTOR-I AND NUCLEAR FACTOR-I FAMILIES CAN DIFFERENTIALLY MODULATE THE ACTIVITIES OF THE RAT ALPHA-FETOPROTEIN PROMOTER AND ENHANCER/

The promoter of the rat alpha-fetoprotein (AFP) gene, which makes the expression of the developmentally regulated AFP gene specific to the l iver, is a putative target for transcription factors of the CAAT/enhan cer-binding protein (C/EBP), hepatocyte nuclear factor-1 (HNF-1) and n uclear factor-1 (NF-1) families. We have evaluated the influence of th ese factors on the activity of the AFP promoter by transfection of Hep G2 hepatoma cells with the appropriate expression vector plus a CAT pl asmid under the control of the AFP promoter. A similar plasmid bearing the rat albumin promoter was used as a control. C/EBP alpha, C/EBP be ta and D-binding protein (DBP) acted as trans-activators on the AFP pr omoter, whereas liver inhibitory protein (LIP), a truncated form of C/ EBP beta, was a potent negative regulator of the promoter. C/EBP alpha also bound to and stimulated the activity of the AFP enhancer at -2.5 kb. Interestingly, HNF-1 beta was found to be more potent than HNF-1 alpha in activating the AFP promoter. This effect was specific, as it did not occur with the rat albumin promoter. HNF-1 beta, which is prod uced earlier than HNF-1 alpha during liver development, would thus hav e the greater influence on the AFP promoter in early development. Both HNF-1s allowed expression of the AFP promoter in cells of non-hepatic origin. Overexpression of NF-1 induced a specific decrease in the act ivity of the AFP promoter. This strongly suggests that competition bet ween NF-1 and HNF-1 for binding to their overlapping binding sites on the AFP promoter is critical for modulating its activity. Thus changin g combinations of these trans-acting factors may tightly modulate the AFP promoter activity in the course of liver development and carcinoge nesis.