Prolactin augments progesterone-dependent uteroglobin gene expression by modulating promoter-binding proteins

The sequence-specific binding of endometrial nuclear proteins to uteroglobin 200 (UG200) (-194/+9), the 203-base pair 5'-flanking region of the rabbit uteroglobin gene, and UG99 (-170/-85), a subfragment of UG200, was compared with gel shift assays. PRL + progesterone treatment of estrous rabbits produced a 6- to 7-fold increase in the primary shift compared to progesterone alone. PRL + progesterone treatment of long-term ovariectomized rabbits increased the primary shift 60% over progesterone alone, which increased the primary shift 30-fold over similarly treated estrous rabbits. The primary shift was eliminated when rabbits were treated with progesterone + estradiol benzoate (E2Bz). Changes in the steady state levels of UG mRNA paralleled changes in the intensity of the primary gel shift. Southwestern blotting revealed four proteins from progesterone-dominated endometrial nuclei that bind UG200. PRL pretreatment produced a 3- to 12-fold increase in each of the proteins. Protein binding was eliminated by E2Bz. A 100-kilodalton (kDa) protein from progesterone-dominated endometrial nuclei was UV cross-linked to UG200 and UG99. An additional protein was detected by each probe with long autoradiographic exposure. PRL pretreatment increased the 100-kDa protein, whereas covalent attachment of the 100-kDa protein to UG promoter DNA was eliminated by E2Bz. UV cross-linking in situ was used to identify the 100-kDa protein as responsible for the primary shift. Collectively, these experiments provide compelling evidence that PRL augments the progesterone-dependent transcription of the UG gene in the uterus by regulating at least one and as many as four proteins that bind to the UG promoter.