Zinc finger proteins RUSH in where others fear to tread

The uteroglobin (UG) gene family encodes a fascinating group of secreted proteins that includes rat prostatic steroid-binding protein subunit C3, human Clara cell 10-kDa protein, human mammaglobin, and rabbit UG. In the rabbit, UG is a progesterone-dependent preimplantation uterine protein with peak availability on Day 5 of pregnancy. Progesterone stimulates UG synthesis in estrous rabbits and in short-term (3 day to 4 wk) ovariectomized rabbits. However, with increased time after ovariectomy, the uterus becomes increasingly refractory to progesterone challenge, such that after 12 weeks, normal levels of UG synthesis could not be measured. The treatment of these long-term ovariectomized rabbits with either prolactin or progesterone resulted in a dramatic increase in the receptor for the other hormone. Sequential treatment with prolactin plus progesterone increased the endometrial UG mRNA content and stimulated UG production to a concentration equal to that found on the fifth day of pregnancy. Because the increase in UG mRNA could result from an increased rate of transcription, gel shift assays, Southwestern blots, and UV cross-linking were used to show that prolactin augments the binding of four progesterone-dependent proteins to an 85-base pair (bp) 5'-flanking region (-170/-85) of the UG gene. The cDNAs for two of these UG promoter-binding proteins, RUSH-1alpha (113 kDa) and -1beta (95 kDa), were cloned by recognition site screening and identified as new members of the SWI/SNF superfamily of nuclear receptor coactivators. RUSH-1alpha and -1beta result from alternative splicing of a 57-bp exon, and each phosphoprotein has the novel RING-finger motif near its C terminus. Competitive reverse transcription-polymerase chain reaction and HPLC analysis showed that RUSH-1alpha is the progesterone-dependent splice variant. When an endometrial cell line (HRE-H9) was transfected with either the full-length UG construct, pUG3.1-LUC, or the deletion mutant, pUG3.1 deltaRUSH-LUC, progesterone increased the transcriptional activity of pUG3.1. Transcription of pUG3.1-LUC was further increased when cells were treated with prolactin plus progesterone. Prolactin alone had no effect. Progesterone also increased the transcriptional activity of pUG3.1 deltaRUSH-LUC. However, deletion of the proximal promoter region (pUG3.1 deltaRUSH-LUC) eliminated the prolactin effect and implicated RUSH proteins as prolactin signal transducers.