cis-regulatory elements within the proximal promoter of the rat gene encoding corticosteroid-binding globulin

Corticosteroid-binding globulin (CBG) transports and modulates the bioavailability of glucocorticoids in blood plasma. It is produced predominantly by the liver, but is also produced in a complex spatial and temporal pattern during development and is regulated hormonally. The rat Cbg promoter (pCbg) has therefore been cloned to allow identification of cis-acting sequence elements that could contribute to its regulation. Five protein-binding sites (P1 to P5) were identified within 236 bp immediately 5' of the transcription start point by DNase I footprinting with rat liver nuclear extracts. These P1-P5 sites are highly conserved in the human pCbg, and resemble recognition sequences for HNF-1, CP-2, DBP, HNF-3 and C/EBP or NF-1L6, respectively. Electrophoretic mobility-shift assays indicted that the P1 element most likely binds HNF-1, and transient transfection assays with luciferase reporter plasmids demonstrated that P1-P5 represent a positive component of rat pCbg activity, whereas additional 5' sequences repressed promoter activity 2-4-fold in H4IIEC3 rat hepatoblastoma cells.