Transcription of the mouse mammary tumor virus DNA is known to be induced by several steroid hormones. Using chimeric MMTV plasmids containing mutations within the hormone regulatory element, we have previously studied the regions required for the glucocorticoid response in mouse fibroblasts. Here we report the characterization of elements essential for the stimulation by progestins and androgens as compared with glucocorticoids. The same set of mutant plasmids was transfected into the human mammary tumor cell line T47D, and the specific transcripts were analyzed by an Si nuclease protection assay. Androgen-mediated stimulation, although weak, showed an extended sensitivity to mutations, with a slight preference for the proximal region. The results with progestin suggest that sequences within all the described sites protected by the receptor in vitro are required and that the promoter-proximal region (-128 to -78 from the RNA start site) is more important than the distal one (-190 to -160). Moreover, a binding site for nuclear factor I was not required for the progestin response, whereas it was required for glucocorticoids. Thus, the various steroid receptors play a role in the differential regulation of mouse mammary tumor virus transcription by recognizing distinct sequence differences in the hormone regulatory element and interacting with different factors bound to the promoter.The mouse mammary tumor virus (MMTV) has been successfully used as a model for studying the control of gene expression by steroid hormones (reviewed in references 41 and 54). In particular, glucocorticoids have been shown to stimulate MMTV transcription in infected cells (42,55) through the action of the glucocorticoid receptor. Mainly because of the widespread occurrence of this receptor in cultured cell lines, the DNA sequences required for glucocorticoid regulation have been investigated in more detail by using cells transfected with chimeric DNA molecules (25,28) and have subsequently been mapped to the 200 base pairs (bp) upstream of the MMTV RNA start site in the viral long terminal repeat (LTR) (4,26,31). Using a series of mutant plasmids containing clustered point mutations, small deletions, or additions, we demonstrated that maximal stimulation by glucocorticoids requires the cooperative action of multiple, distinct sequence elements (5, 27). The strongest, distal element maps at -181 to -172, inside an area that binds the purified glucocorticoid receptor (39, 44). Further elements map around -120, in one of the three proximal receptor-binding sites (43), and in the symmetric sequence around -70 recognized by nuclear factor I (NF-I [35,38] described previously (5, 27). The results provide evidence that in the natural context of the MMTV promoter, the proximal upstream region is more important than the distal one for the progestin and possibly the androgen responses and that the NF-I binding site is not required for the progestin response, in contrast to the situation observed with glucocorticoids. It is suggested that subtle ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.