Steroid hormones have rapid nongenomic effects on cell-signaling pathways, but the receptor mechanisms responsible for this are not understood. We have identified a specific polyproline motif in the amino-terminal domain of conventional progesterone receptor (PR) that mediates direct progestin-dependent interaction of PR with SH3 domains of various cytoplasmic signaling molecules, including c-Src tyrosine kinases. Through this interaction, PR is a potent activator of Src kinases working by an SH3 domain displacement mechanism. By mutagenesis, we also show that rapid progestin-induced activation of Src and downstream MAP kinase in mammalian cells is dependent on PR-SH3 domain interaction, but not on the transcriptional activity of PR. Preliminary evidence for the biological significance of this PR signaling pathway through regulatory SH3 domains was shown with respect to an influence on progestin-induced growth arrest of breast epithelial cells and induction of Xenopus oocyte maturation.

We previously demonstrated that 17beta hydroxysteroid dehydrogenase type 2, the enzyme that inactivates estradiol to estrone, is expressed in luteal eutopic endometrium in response to progesterone but not in simultaneously biopsied peritoneal endometriotic tissue. This molecular evidence of progesterone resistance, together with the clinical observation of resistance of endometriosis to treatment with progestins, led us to determine the levels of progesterone receptor (PR) isoforms PR-A and PR-B in eutopic endometrial and extra-ovarian endometriotic tissues. It was proposed that progesterone action on target genes is mediated primarily by homodimers of PR-B, whereas the truncated variant PR-A acts as a repressor of PR-B function. Immunoprecipitation, followed by Western blot analysis, was performed to detect bands specific for PR-A and PR-B in paired samples of endometriotic and eutopic endometrial tissues simultaneously biopsed from 18 women undergoing laparoscopy during various phases of the menstrual cycle. PR-B was present in 17 of 18 eutopic endometrial samples, and its level increased in the preovulatory phase, as expected, whereas PR-A was detected in all samples (n = 18) with a similar, but less prominent, cyclic variation in its levels. In endometriotic samples, however, no detectable PR-B could be demonstrated, whereas PR-A was detected in all samples (n = 18), albeit in much lower levels and without any cyclic variation in contrast with the eutopic endometrium. Levels of PR-A and PR-B in endometriotic and eutopic endometrial tissues were determined and compared after normalization to total protein and estrogen receptor-alpha levels. Using RNase protection assay, we also demonstrated indirectly that only PR-A transcripts were present in endometriotic tissue samples (n = 8), whereas both PR-A and PR-B transcripts were readily detectable in all eutopic endometrial samples (n = 8). This was indicative that failure to detect PR-B protein in endometriotic tissues is due to the absence of PR-B transcripts. We conclude that progesterone resistance in endometriotic tissue from laboratory and clinical observations may be accounted for by the presence of the inhibitory PR isoform PR-A and the absence of the stimulatory isoform PR-B.

The female sex steroid hormones 17beta-estradiol and progesterone mediate their biological effects on development, differentiation, and maintenance of reproductive tract and other target tissues through gene regulation by nuclear steroid receptors that function as ligand-dependent transcription factors. However, not all effects of 17beta-estradiol and progesterone are mediated by direct control of gene expression. These hormones also have rapid stimulatory effects on the activities of a variety of signal transduction molecules and pathways and, in many cases, these effects appear to be initiated from the plasma cell membrane. There is growing evidence that a subpopulation of the conventional nuclear steroid receptor localized at the cell membrane mediates many of the rapid signaling actions of steroid hormones; however, novel membrane receptors unrelated to conventional steroid receptors have also been implicated. This chapter reviews the nature of the receptors that mediate rapid signaling actions of estrogen and progesterone and describes the signaling molecules and pathways involved, the mechanisms by which receptors couple with components of signaling complexes and trigger responses, and the target tissues and cell functions regulated by this mode of steroid hormone action.

Steroid receptors are ligand-inducible transcription factors, and their association with steroid receptor coactivators (SRCs) upon binding to DNA is necessary for them to achieve full transcriptional potential. To understand the mechanism of SRC-1 action, its ability to interact and enhance the transcriptional activity of steroid receptors was analyzed. First, we show that SRC-1 is a modular coactivator that possesses intrinsic transcriptional activity when tethered to DNA and that it harbors two distinct activation domains, AD1 and AD2, needed for the maximum coactivation function of steroid receptors. We also demonstrate that SRC-1 interacts with both the amino-terminal A/B or AF1-containing domain and the carboxyl-terminal D/E or AF2-containing domain of the steroid receptors. These interactions are carried out by multiple regions of SRC-1, and they are relevant for transactivation. In addition to the inherent histone acetyltransferase activity of SRC-1, the presence of multiple receptor-coactivator interaction sites in SRC-1 and its ability to interact with components of the basic transcriptional machinery appears to be, at least in part, the mechanism by which the individual activation functions of the steroid receptors act cooperatively to achieve full transcriptional activity.Steroid receptors belong to a superfamily of transcription factors that regulate hormone-responsive genes and thereby cellular growth and differentiation. In the absence of hormone, the receptor is maintained in an inactive or repressive state by association with heat shock proteins and/or corepressors. Activation of the aporeceptor by ligand binding involves structural and functional changes in the receptor molecule that promote release from the inactive or repressive state to bind specific DNA hormone response elements. In addition, the ligandbound receptor promotes the recruitment of coactivators to the receptor-DNA complex and thus entitles the receptor to achieve its full transactivation capacity (for review, see Refs. 1-4).Formation of the preinitiation complex at the promoter involves numerous general transcription factors that recruit RNA polymerase II binding to DNA and initiation of transcription (5). Steroid receptors activate target genes by stabilizing this preinitiation complex through direct interactions with components of the transcription machinery, including TFIIB, TATA-binding protein (TBP), TFIID, and TFIIF (6 -16). However, the mechanism by which receptors activate transcription is more complex. The squelching observed between receptor family members and between their various AFs 1 suggests that limiting intracellular coactivators are also needed for mediating receptor function (17, 18). Furthermore, the synergism observed between the two transactivation functions (AF1 and AF2) of a single receptor suggest that the proper assembly of the individual activation functions of the steroid receptor is required to render the steroid receptor-DNA complex transcriptionally productive. Today, several receptor coactivators ...