THE PROXIMAL PROMOTER OF THE BOVINE LUTEINIZING-HORMONE BETA-SUBUNIT GENE CONFERS GONADOTROPE-SPECIFIC EXPRESSION AND REGULATION BY GONADOTROPIN-RELEASING-HORMONE, TESTOSTERONE, AND 17-BETA-ESTRADIOL IN TRANSGENIC MICE

Transient transfection studies have proven useful in unraveling the mo lecular mechanisms underlying gonadotrope-specific expression and horm onal regulation of the gene encoding the alpha-subunit of the glycopro tein hormones. In contrast, similar studies performed with the LH beta gene have been less informative. When assayed by transient transfecti on into alpha T3-1 cells, activity of a 776-basepair bovine LH beta pr omoter-chloramphenicol acetyltransferase fusion gene (bLH beta CAT) wa s no greater than that of a promoterless control. To determine whether limited activity in vitro reflected the absence of critical regulator y elements, we examined activity of bovine LH beta fusion genes after stable integration in transgenic mice. In contrast to transient transf ection studies, the LH beta promoter targeted high levels of CAT expre ssion specifically to the pituitary. In addition, a bLH beta TK fusion gene was active only in gonadotropes. The bLH beta CAT transgene was also evaluated for responsiveness to gonadal steroids and GnRH. Testos terone and 17 beta-estradiol were capable of suppressing activity 70-8 0% in castrated males, despite the absence of high affinity binding si tes for androgen or estrogen receptors. This suggests that feedback in hibition of LH beta CAT transgene expression by gonadal steroids may o ccur through an indirect mechanism, possibly at the level of the hypot halamus. To address whether the bLH beta CAT transgene could be regula ted by GnRH, we treated ovariectomized females with antide, a GnRH ant agonist. Antide suppressed transgene activity by 60%. Thus, the proxim al promoter of the bovine LH beta subunit gene directs appropriate pat terns of cell-specific expression and retains responsiveness to gonada l steroids and GnRH. In light of the robust activity of the LH beta pr omoter in transgenic mice, we suggest that this animal model can be ex ploited further to dissect the complex mechanisms that underlie gonado trope-specific expression and hormonal regulation of the LH beta gene.