The rabbit sex hormone-binding globulin gene: Structural organization and characterization of its 5 '-flanking region

Sex hormone-binding globulin (SHBG) transports sex steroids in the blood. I n humans and rabbits, the gene encoding SHBG (shbg) is expressed primarily in the liver and testis, whereas the testis is the major site of shbg expre ssion in rodents postnatally. Sequence analysis has revealed that rabbit sh bg (rbshbg) spans 2.5 kb and comprises eight exons with consensus splice si tes at all exon-intron junctions. The major transcription start site of rbs hbg is located 52 bp upstream from the translation initiation codon for the rabbit SHBG precursor. Unlike the situation in humans and rats, rbshbg tra nscripts contain no alternative exon 1 sequences in the Liver or testis, an d this suggests that the rbshbg 5'-flanking region plays an equally importa nt role in controlling transcription of this gene in these tissues. Like th e human and rat shbg promoter sequences, the rbshbg proximal promoter lacks a typical TATA box. It also contains several transcription factor-binding sites, but deoxyribonuclease I footprinting experiments indicated that the human and rabbit shbg proximal promoters interact quite differently with pr oteins extracted from rabbit liver nuclei. However, the predominant footpri nt on the rbshbg promoter is conserved at the same position within the huma n shbg (hshbg) promoter and includes consensus binding sites far the transc ription factor nuclear factor-1. Transient transfection studies of the rbsh bg 5'-flanking sequence (893 bp) revealed regions that actively enhance and repress its activity in human hepatoblastoma and mouse Sertoli cells, but not in Chinese hamster ovary cells. Like the rat shbg proximal promoter, th e rbshbg 5'-flanking sequence lacks a region that corresponds to a cis-elem ent, designated footprinted region 4 in the hshbg proximal promoter. Furthe rmore, the hshbg promoter footprinted region 3 sequence is poorly conserved in rbshbg, and when mutated to resemble the corresponding human sequence i t increased the transcriptional activity of the rbshbg promoter by 7-fold i n hepatoblastoma cells. Thus, the rabbit and hshbg promoters appear to be c ontrolled by a different set of transcriptional regulators. Further compari sons of their functional activities may shed light on species-specific diff erences in the spatial and temporal expression of this gene, the products o f which play important roles in regulating sex steroid access to target cel ls.