A potent enhancer made of clustered liver-specific elements in the transcription control sequences of human alpha 1-microglobulin/bikunin gene

Abstract

alpha 1-Microglobulin (A1M) and bikunin are plasma proteins which are present both as free molecules and as complexes with either IgA heavy chains for A1M or the H1, H2, and H3 heavy chains of the inter-alpha-inhibitor family for bikunin. Mature A1M and bikunin originate from the cleavage of an A1M/bikunin precursor (ABP) synthesized from a single gene with liver-specific expression. Five kilobases of the 5'-flanking region of the human ABP gene were sequenced. Deletion mutants of this region subcloned upstream of a CAT reporter gene were transfected into HepG2 hepatoma cells. A segment covering the -2.7- to -2.8-kb area is required for full activity of the ABP gene. This segment contains a cluster of six elements (boxes 1-6, 5' to 3') which are potential binding sites for the liver-enriched trans-acting factors HNF-1, HNF-4, HNF-3, HNF-1, HNF-3, and HNF-4, respectively. This cluster enhances the activity of heterologous minimal promoters in a position- and distance-independent fashion in HepG2 cells. This enhancer activity is restricted to liver cells as the cluster is unable to activate promoters in Chinese hamster ovary (CHO) or HeLa cells. By band-shift experiments we have shown that the liver-enriched transcription factors HNF-1, or HNF-3, do bind to boxes 1 and 4, or 3, respectively. The combination of a weak promoter and a strong distant and liver-specific enhancer distinguishes the ABP gene from most other plasma protein genes expressed in hepatocytes.