The MMTV LTR promoter is induced by progesterone and dihydrotestosterone but not by estrogen

Induction of the mouse mammary tumor virus (MMTV) promoter by steroid hormones was examined in chick oviduct primary cell cultures transfected with MMTV-chloramphenicol acetyl transferase fusion constructs. Our results demonstrate that in this system glucocorticoids, progesterone and dihydrotestosterone are all able to stimulate MMTV transcription; induction by progesterone and dihydrotestosterone is not mediated by glucocorticoid receptors, since the specific glucocorticoid antagonist RU486 did not inhibit the response. In contrast, estrogen does not stimulate MMTV transcription, although estrogen does induce the endogenous ovalbumin gene in the same cells. While progesterone effects are mediated by the same response elements within the MMTV long terminal repeat that were originally characterized for the glucocorticoid receptor, an androgen response element has not yet been defined. Our data indicate that the MMTV long terminal repeat does not contain an estrogen response element.