Multiple Cis-Acting DNA Regulatory Elements Mediate Hepatic Angiotensinogen Gene Expression

Abstract

Angiotensinogen is the glycoprotein precursor of angiotensin II, an octapeptide hormone important for the regulation of blood pressure and volume homeostasis. The gene encoding angiotensinogen is expressed in liver and several other tissues, providing a model gene for understanding the role of cis-acting DNA control elements and trans-acting factors in tissue-type specific gene expression. To identify DNA control elements in the rat angiotensinogen gene we prepared an array of fusion genes consisting of either 5' or 3'-deleted sequences of the 5'-flanking region of the gene linked to a firefly luciferase reporter gene and analyzed the relative cellular specificity of expression of these genes after their introduction into hepato-carcinoma cells (Hep G2) that do express and placental cells (JEG-3) that do not express the endogenous angiotensinogen gene. Six transcriptionally active elements were found within 688 base pairs of 5'-flanking DNA. The interactions of DNA binding proteins with these elements was demonstrated by their specific protection to digestion with DNase I in the presence of liver cell extracts. The orientation and spatial requirements for transcription of two of the elements were analyzed further by the construction and expression of synthetic oligonucleotide cassettes incorporating the sequences of these elements when linked to a homologous (angiotensinogen) or a heterologous Simian virus 40 promoter and enhancer. One element located between 60 and 108 base pairs from the start of gene transcription functioned either as a silencer or an enhancer of transcription (SOAP box element), depending upon the distance from the angiotensinogen and viral gene promoters. Moreover, the SOAP box element demonstrated enhancer activity in JEG-3 cells when linked to the Simian virus 40 early promoter. An oligonucleotide mutation of the SOAP box element interfered with protein binding in a gel mobility shift assay and this mutant was transcriptionally inactive in both homologous and heterologous promoters. These observations indicate that expression of the angiotensinogen gene in liver cells is coordinately regulated by multiple cis-acting elements that interact with DNA binding proteins.