CHARACTERIZATION OF THE EQUINE GLYCOPROTEIN HORMONE ALPHA-SUBUNIT GENE REVEALS DIVERGENCE IN THE MECHANISM OF PITUITARY AND PLACENTAL EXPRESSION

The equine glycoprotein hormone alpha-subunit gene is expressed in bot h pituitary and placenta, unlike that of all other nonprimate mammals studied, in which expression is limited to pituitary. Previous studies of the 5'-flanking region of the equine alpha-subunit promoter have r evealed unique characteristics as well as similarities with the human alpha-subunit promoter, which demonstrates a similar pattern of tissue -specific expression. We have cloned and sequenced the equine alpha-su bunit gene and have used tissue culture systems and transgenic mice to characterize its expression. Unlike the human promoter, the cloned eq uine ol-subunit promoter failed to direct trophoblast-specific express ion in either tissue culture or transgenic mouse models, suggesting an entirely different mechanism for expression. In contrast, the equine cr-subunit promoter was able to direct gonadotroph expression in both tissue culture and transgenic mouse models. In alpha T3-1 cells, 550 b ase pair (bp) was sufficient for expression. This expression involves promoter elements identified in other species as playing a role in gon adotroph expression, but mutation of these elements reveals difference s in their relative contributions to promoter activity. In mice, 2800 bp of 5'-flanking sequence allowed specific expression in gonadotrophs but not in thyrotrophs or placenta. The pattern of estrogen regulatio n observed in transgenic mice matched neither the repression that has been observed with human and bovine alpha-subunit promoters in transge nic mice nor the stimulation in mRNA levels reported in mares, suggest ing a unique mechanism that is not recapitulated in the transgenic mod el. Thus the equine alpha-subunit promoter uses a combination of conse rved and unique features of gene regulation to direct its pattern of t issue-specific expression.