The nuclear receptors belong to a superfamily of proteins, many of which are ligand-regulated, that bind to specific DNA sequences and control specific gene transcription. Recent data show that, in addition to contacting the basal transcription machinery directly, nuclear receptors inhibit or enhance transcription by recruiting an array of coactivator or corepressor proteins to the transcription complex. In this review we define the properties of these putative coregulatory factors; we describe the basal and coregulatory factors that are currently known to interact with nuclear receptors; we suggest various mechanisms by which coactivators and corepressors act; we discuss issues that are raised by the presence of multiple, perhaps competing, coregulatory factors; and we speculate how these additional regulatory layers may explain the heterogeneity of hormone responses that are observed in normal and malignant tissues.

Objectives Cell lines derived from human ovarian and endometrial cancers, and their immortalized non-malignant counterparts, are critical tools to investigate and characterize molecular mechanisms underlying gynecologic tumorigenesis, and facilitate development of novel therapeutics. To determine the extent of misidentification, contamination and redundancy, with evident consequences for the validity of research based upon these models, we undertook a systematic analysis and cataloging of endometrial and ovarian cell lines. Methods Profiling of cell lines by analysis of DNA microsatellite short tandem repeats (STR), p53 nucleotide polymorphisms and microsatellite instability. Results Fifty-one ovarian cancer lines were profiled with ten found to be redundant and five (A2008, OV2008, C13, SK-OV-4 and SK-OV-6) identified as cervical cancer cells. Ten endometrial cell lines were analyzed, with RL-92, HEC-1A, HEC-1B, HEC-50, KLE, and AN3CA all exhibiting unique, uncontaminated STR profiles. Multiple variants of Ishikawa and ECC-1 endometrial cancer cell lines were genotyped and analyzed by sequencing of mutations in the p53 gene. The profile of ECC-1 cells did not match the EnCa-101 tumor, from which it was reportedly derived, and all ECC-1 isolates genotyped as Ishikawa cells, MCF-7 breast cancer cells, or a combination thereof. Two normal, immortalized endometrial epithelial cell lines, HES cells and the hTERT-EEC line, were identified as HeLa cervical carcinoma and MCF-7 breast cancer cells, respectively. Conclusions Results demonstrate significant misidentification, duplication, and loss of integrity of endometrial and ovarian cancer cell lines. Authentication by STR DNA profiling is a simple and economical method to verify and validate studies undertaken with these models.

Steroid receptor antagonists, such as the antiestrogen tamoxifen or the antiprogestin RU486, can have inappropriate agonist-like effects in tissues and tumors. To explain this paradox we postulated that coactivators are inadvertently brought to the promoters of DNA-bound, antagonist-occupied receptors. The human (h) progesterone receptor (PR) hinge-hormone binding domain (H-HBD) was used as bait in a two-hybrid screen of a HeLa cDNA library, in which the yeast cells were treated with RU486. We have isolated and characterized two interesting steroid receptor-interacting proteins that regulate transcription in opposite directions. The first is L7/SPA, a previously described 27-kDa protein containing a basic region leucine zipper domain, having no known nuclear function. When coexpressed with tamoxifen-occupied estrogen receptors (hER) or RU486-occupied hPR or glucocorticoid receptors (hGR), L7/SPA increases the partial agonist activity of the antagonists by 3- to 10-fold, but it has no effect on agonist-mediated transcription. The interaction of L7/SPA with hPR maps to the hinge region, and indeed, the hPR hinge region squelches L7/SPA-dependent induction of antagonist-mediated transcription. Interestingly, pure antagonists that lack partial agonist effects, such as the antiestrogen ICI164,384 or the antiprogestin ZK98299, cannot be up-regulated by L7/SPA. We also isolated, cloned, and sequenced the human homolog (hN-CoR) of the 270-kDa mouse (m) thyroid/retinoic acid receptor corepressor. Binding of hN-CoR maps to the hPR-HBD. mN-CoR, and a related human corepressor, SMRT, suppress RU486 or tamoxifen-mediated partial agonist activity by more than 90%. This suppression is completely squelched by overexpression of the hPR H-HBD. Additionally, both corepressors reverse the antagonist-dependent transcriptional up-regulation produced by L7/SPA. Our data suggest that the direction of transcription by antagonist-occupied steroid receptors can be controlled by the ratio of coactivators to corepressors recruited to the transcription complex by promoter-bound receptors. In normal tissues and in hormone-resistant breast cancers in which the agonist activity of mixed antagonists predominates, steroid receptors may be preferentially bound by coactivators. This suggests a strategy by which such partial agonist activity can be eliminated and by which candidate receptor ligands can be screened for this activity.

During the maturation of the cardiac myocyte, a transition occurs from hyperplastic to hypertrophic growth. The factors that control this transition in the developing heart are unknown. Proto-oncogenes such as c-myc have been implicated in the regulation of cellular proliferation and differentiation, and in the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc can influence myocyte proliferation or differentiation, we examined the in vivo effect of increasing c-myc expression during embryogenesis and of preventing the decrease in c-myc mRNA expression that normally occurs during cardiac development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. In these transgenic mice, increased c-myc mRNA expression was found to be associated with both atrial and ventricular enlargement. This increase in cardiac mass was secondary to myocyte hyperplasia, with the transgenic hearts containing more than twice as many myocytes as did nontransgenic hearts. The results suggest that in the transgenic animals there is additional hyperplastic growth during fetal development. However, this additional proliferative growth is not reflected in abnormal myocyte maturation, as assessed by the expression of the cardiac and skeletal isoforms of a-actin. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth and suggest a regulatory role for this proto-oncogene in cardiac myogenesis.Development of the tissue-specific cells of the heart, the cardiac myocytes, has been extensively studied in vivo. Myocytes proliferate throughout fetal and early postnatal development, followed by a transition whereby proliferation ceases and further cardiac growth occurs through an increase in myocyte size rather than number (5, 9). The factors that control myocyte proliferation and the transition from hyperplastic to hypertrophic growth are unknown.Recent interest has centered on the role of proto-oncogenes in cellular development. In particular, c-myc has been implicated in controlling both proliferation and differentiation in various cell types (29). Increased expression of c-myc in chicken embryo fibroblasts results in an increase in the rate of proliferation of these cells (34). In hematopoietic cells, the expression of c-myc decreases concomitant with differentiation, and if these cells are made to constitutively express c-myc, differentiation is prevented (10,13,25,33 genes) encode proteins that induce myogenic determination in skeletal muscle. These genes all share regions of similarity with c-myc, suggesting a potential interaction of these gene products with c-myc, or with a common intracellular target, in regulating skeletal muscle differentiation. Although expression of these particular myogenic determination genes has not been observed in heart (4, 11, 43...