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.

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 ...

We previously reported that the chromatin high-mobility group protein 1 (HMG-1) enhances the sequencespecific DNA binding activity of progesterone receptor (PR) in vitro, thus providing the first evidence that HMG-1 may have a coregulatory role in steroid receptor-mediated gene transcription. Here we show that HMG-1 and the highly related HMG-2 stimulate DNA binding by other steroid receptors, including estrogen, androgen, and glucocorticoid receptors, but have no effect on DNA binding by several nonsteroid nuclear receptors, including retinoid acid receptor (RAR), retinoic X receptor (RXR), and vitamin D receptor (VDR). As highly purified recombinant full-length proteins, all steroid receptors tested exhibited weak binding affinity for their optimal palindromic hormone response elements (HREs), and the addition of purified HMG-1 or -2 substantially increased their affinity for HREs. Purified RAR, RXR, and VDR also exhibited little to no detectable binding to their cognate direct repeat HREs but, in contrast to results with steroid receptors, the addition of HMG-1 or HMG-2 had no stimulatory effect. Instead, the addition of purified RXR enhanced RAR and VDR DNA binding through a heterodimerization mechanism and HMG-1 or HMG-2 had no further effect on DNA binding by RXR-RAR or RXR-VDR heterodimers. HMG-1 and HMG-2 (HMG-1/-2) themselves do not bind to progesterone response elements, but in the presence of PR they were detected as part of an HMG-PR-DNA ternary complex. HMG-1/-2 can also interact transiently in vitro with PR in the absence of DNA; however, no direct protein interaction was detected with VDR. These results, taken together with the fact that PR can bend its target DNA and that HMG-1/-2 are non-sequence-specific DNA binding proteins that recognize DNA structure, suggest that HMG-1/-2 are recruited to the PR-DNA complex by the combined effect of transient protein interaction and DNA bending. In transient-transfection assays, coexpression of HMG-1 or HMG-2 increased PR-mediated transcription in mammalian cells by as much as 7-to 10-fold without altering the basal promoter activity of target reporter genes. This increase in PR-mediated gene activation by coexpression of HMG-1/-2 was observed in different cell types and with different target promoters, suggesting a generality to the functional interaction between HMG-1/-2 and PR in vivo. Cotransfection of HMG-1 also increased reporter gene activation mediated by other steroid receptors, including glucocorticoid and androgen receptors, but it had a minimal influence on VDR-dependent transcription in vivo. These results support the conclusion that HMG-1/-2 are coregulatory proteins that increase the DNA binding and transcriptional activity of the steroid hormone class of receptors but that do not functionally interact with certain nonsteroid classes of nuclear receptors.Steroid hormone receptors are members of a superfamily of ligand-dependent transcriptional activators which direct the expression of specific gene networks involved in regulating the differen...

Human progesterone receptor (PR) contains a motif that interacts with the SH3 domain of Src and mediates rapid activation of Src and downstream MAPK (Erk-1/-2) without relying on the transcriptional activity of the receptor. Here we investigated the role and intracellular location of this nontranscriptional activity of PR. Progestin activation of Src/MAPK occurred outside the nucleus with the B isoform of PR that was distributed between the cytoplasm and nucleus, but not with PR-A that was predominantly nuclear. Breast cancer cells stably expressing wild-type PR-B or PR-B with disrupting point mutations in the SH3 domain binding motif (PR-BDeltaSH3) that do not affect the transcriptional activity of PR, were compared for effects of progestin on endogenous target gene expression and cell proliferation. Progestin induction of the cyclin D1 gene, which lacks a progesterone response element, was dependent on PR activation of the Src/MAPK pathway, whereas induction of the Sgk (serum and glucocorticoid regulated kinase) gene that contains a functional progesterone response element was unaffected by mutations that interfere with PR activation of Src. Progestin induction of cell cycle progression was also abrogated in cells expressing PR-BDeltaSH3, and no effect of progestin on cyclin D1 expression and cell cycle was observed in the presence of PR-A. These results highlight the importance of PR activation of the Src/MAPK signaling pathway for progesterone-induced transcription of select target genes and cell cycle progression.

No abstract