CIS-REGULATORY ELEMENTS WITHIN THE PROXIMAL PROMOTER OF THE RAT GENE ENCODING CORTICOSTEROID-BINDING GLOBULIN

Corticosteroid-binding globulin (CBG) transports and modulates the bio availability of glucocorticoids in blood plasma, It is produced predom inantly by the liver, but is also produced in a complex spatial and te mporal 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 regulati on, Five protein-binding sites (PI to P5) were identified within 236 b p immediately 5' of the transcription start point by DNase I footprint ing with rat liver nuclear extracts. These P1-P5 sites are highly cons erved in the human pCbg, and resemble recognition sequences for HNF-1, CP-2, DBP, HNF-3 and C/EBP or NF-1L6, respectively. Electrophoretic m obility-shift assays indicted that the PI element most likely binds HN F-1, and transient transfection assays with luciferase reporter plasmi ds 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.