Multiple hepatic trans-acting factors are required for in vitro transcription of the human alpha-1-antitrypsin gene

Abstract

The human alpha-1-antitrypsin (AAT) gene is expressed in the liver, and its deficiency causes pulmonary emphysema. We have demonstrated that its 5'-flanking region contains cis-acting elements capable of directing proper transcription in the presence of rat liver nuclear extract. The in vitro transcription system is tissue-specific in that the AAT promoter is functional in nuclear extracts prepared from the liver but not from HeLa cells. Experiments in which rat liver and HeLa nuclear extracts were mixed suggested the presence of a specific activator(s) in hepatocytes rather than a repressor(s) in nonproducing cells. Two protected regions were detected in the promoter by DNase I footprinting analysis with rat liver nuclear extracts. Region one spanned -78 to -52 and region two spanned -125 to -100 in the 5'-flanking sequence of the gene. By gel retardation assays with synthetic oligonucleotides, at least two distinct liver nuclear factors were identified, HNF-1 and HNF-2 (hepatocyte nuclear factors), which bound specifically to the first and second region, respectively. We present evidence that HNF-1 and HNF-2 are positively acting, tissue-specific transcription factors that regulate hepatic expression of the human AAT gene.