The two native estrogen receptor forms of 8S and 4S present in cytosol from human uterine tissues display opposite reactivities with the antiestrogen tamoxifen aziridine and the estrogen responsive element

Expression of estrogen receptor co-regulators SRC-1, RIP140 and NCoR and their interaction with estrogen receptor in rat uterus, under the influence of ormeloxifene

Ormeloxifene binds competitively to ERs and antagonizes estrogen-induced gene expression in the uterus. However its detailed molecular mechanisms are not well understood. Present study was aimed to examine the changes in expression pattern of co-regulatory proteins SRC-1 (co-activator), RIP140 and NCoR (co-repressors) and their interaction with ERalpha in rat uterus under the influence of ormeloxifene (Orm) and tamoxifen (Tam). Adult ovariectomized rats were treated with estradiol (E(2)) (5 microg/100g), or Orm or Tam (200 microg/100g, s.c.) alone or along with E(2), for 3 days. RT-PCR analysis of uterine RNA and immunoblotting of uterine extracts revealed that expression of SRC-1, RIP140 and NCoR was insensitive to E(2) or Orm or Tam treatment. Direct protein-protein interaction experiments using co-immunoprecipitation revealed that E(2)-induced the interaction of ERalpha with co-activator SRC-1. In rats given Orm alone or along with E(2), there was a significant reduction in E(2)-induced effect on ERalpha-SRC-1 interaction. In case of ERbeta and SRC-1, Orm reduced interaction only in the absence of E(2). Interaction of RIP140 or NCoR with ERalpha was found to be more in rats treated with Orm along with E(2) as compared to that in E(2)-treated rats whereas no such recruitment was found in Tam treated rats. Interaction of RIP140 with ERbeta was insensitive to Orm or Tam treatment whereas the interaction of NCoR with ERalpha and ERbeta was increased in Orm treated rats. Ormeloxifene also showed inhibitory effects on uterine ER-ERE binding and estrogen-induced expression of progesterone receptor. Taken together, these findings demonstrate that ormeloxifene antagonizes ERalpha-mediated transcription by inhibiting the recruitment of SRC-1 and inducing the recruitment of RIP140 and NCoR.

Selective estrogen receptor modulators (SERMS)

Hormone receptors and, specifically, estrogen receptors were described about four decades ago. For estrogens, there are two receptors, estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta). The two receptors are coded by different genes and their tissue expression varies across organs. ERalpha is predominantly expressed in reproductive tissues (uterus, breast, ovaries) liver and central nervous system, whereas ERbeta is expressed in other tissues such as bone, endothelium, lungs, urogenital tract, ovaries, central nervous system and prostate. More than seventy molecules that belong to the SERMS class have been described. There are 5 chemical groups: triphenylethylenes, benzotiophenes, tetrahydronaphtylenes, indoles and benzopyrans. All of these non-hormonal compounds are capable of activating the ER, reduce bone turnover rate and, as an antiresorptive, clearly improve bone density. Estrogens reduce bone turnover rate and, as an antiresorptive, clearly improve bone density. They are also beneficial for the relief of menopausal symptoms. An ongoing debate that extends over the decades, relates to to overall benefit/risk profile of estrogen or estrogen-progestin therapy since these therapies can increase the risk of serious health disorders, such as breast cancer. SERMs have increased our understanding of hormone-receptor regulatory mechanisms. Their development has permitted a targeted efficacy profile avoiding some of the side effects of the hormone therapy. Their clinical utility relies today mostly on the effects on breast cancer and bone.

Modulation of estrogen receptor transactivation and estrogen-induced gene expression by ormeloxifene-a triphenylethylene derivative

The study was aimed to investigate the interaction of D,L-ormeloxifene (Orm), a triphenylethylene and its hydroxy derivative with estrogen receptor subtypes alpha and beta, its influence on ERE-driven transcriptional activation and progesterone receptor expression. In competitive binding experiments using human recombinant ERalpha and ERbeta, Orm showed interaction with both ER subtypes, with more selectivity and higher affinity towards ERalpha (8.8%) as compared to ERbeta (3%). In case of 7-hydroxy derivative, the relative binding affinity for both ERs was enhanced several folds. Orm showed lower Ki, i.e. higher affinity for ERalpha (250 nM) than for ERbeta (750 nM). It was observed that Orm promoted the formation of ER-ERE complexes in uterine tissue extract whereas its hydroxy derivative showed inhibitory effects. Transient co-transfection assay in COS-1 cells using ERE-luciferase reporter construct, revealed that Orm showed estrogenic response whereas its hydroxy-derivative was potent antiestrogen at ERalpha at transcription level. In immature rats, Orm (2 mg/kg) was associated with less increase in uterine weight and in luminal epithelial cell height than E2 or Tam. Orm also induced the expression of PR mRNA but the expression level was significantly less than estradiol treated group. These results suggest that ER-ERE complexes formed under the influence of 7-hydroxy Orm appear to be transcriptionally less effective hence antagonizing the E2-regulated gene expression in this target tissue.

Molecular forms of the estrogen receptor in breast cancer

Estrogen receptors (ERs) of which two isoforms have been identified (alpha and beta) are subjected to phosphorylation, glycozylation, ubiquitination and other post-transcriptional conformational changes giving rise to a very high molecular heterogeneity. Partial proteolysis of these receptors, as well as their high tendency to associate within oligomeric structures, reinforces this property. Investigations demonstrated that this heterogeneity is not a biochemical artefact suggesting some biological relevance. Our purpose was to review this topic, especially with regard to ERalpha from breast cancers.

Selectivity of antibodies to estrogen receptors alpha and beta (ERalpha and ERbeta) for detecting DNA-bound ERalpha and ERbeta in vitro

Antibodies are widely used to detect estrogen receptor (ER) in ER-DNA complexes in electrophoretic mobility shift assays (EMSA). We compared the specificity of antibodies raised to different regions of ERalpha or ERbeta for detecting recombinant human ERalpha (rhERalpha) and recombinant rat ERbeta (rrERbeta) when bound to a consensus estrogen response element (ERE). ERalpha-specific antibodies specifically slowed the migration of the ER-ERE complex by 32 to 84% and inhibited rhERalpha-ERE binding by 17 to 75%. None of antibodies to ERbeta supershifted rhERalpha-ERE complex. Some ERalpha-specific antibodies increased whereas some decreased rrERbeta-ERE binding. Anti-ERbeta antibodies supershifted different amounts of the rrERbeta-ERE complex. Our results indicate that supershift and inhibition of ER-ERE interaction with a specific antibody are equally reliable in the detection of rhERalpha and rrERbeta. ERalpha antibody Ab10, antisera G20 and AT3B, and ERbeta-antiserum Y19 offered the best discrimination between ERalpha and ERbeta. Comparison of the peptide sequences against which various antibodies were raised indicate directions for new ERalpha and ERbeta- specific antibody development. We conclude that a cognate ER antibody that retards the migration of the ER-ERE complex by at least 40% or inhibits ER-ERE interaction by at least 8% provides a reliable detection of a specific ER isoform in EMSA.

Protective action of 17beta-estradiol and tamoxifen on glutamate toxicity in glial cells

Estrogens influence differentiation, growth and function of neurons, but less is known of their effects on glia. In our experiments reported here, the ovarian steroid, 17beta-estradiol, and the "designer", non-steroidal estrogen, tamoxifen, effectively protected C-6 glioma 2B clone cells from the cytotoxicity of the excitatory neurotransmitter, glutamate. Exposure of these cells to 10-20 mM glutamate induced 61-78% cell death. Pre-treatment of the cells with 0.01 mM estradiol or with 2 microM tamoxifen significantly reduced the glutamate-induced cell death, estradiol being the most effective in this regard. Estradiol- or tamoxifen-treated cells that had survived glutamate damage appeared more mature than controls. Thus, estrogens often used in therapy (estradiol as replacement after menopause and tamoxifen for treatment/prevention of breast cancer) may significantly protect glial cells against glutamate toxicity and stimulate cell differentiation.

Estrogen receptor of primary breast cancers: evidence for intracellular proteolysis

Iodinated oestradiol-labeled oestrogen receptor (ER) isoforms devoid of amino-terminal ABC domains represent about two-thirds of the whole receptor population detected in cytosol samples from human breast cancers. This high frequency could not be ascribed to the expression of truncated mRNAs, or to the proteolysis of the native ER peptide at the time of homogenization or assay, suggesting an intracellular proteolysis. Free amino-terminal and ligand-binding domains maintained together within oligomeric structure(s); increase of ionic strength separated them. The amino-terminal region was consistently detected in the cell nucleus by specific immunohistochemistry leading to the concept of a potential intranuclear association between ER cleavage products and/or other regulatory proteins.

Tamoxifen aziridine binding to cytosolic proteins from human breast specimens is negatively associated with estrogen receptors, progesterone receptors, pS2, and cathepsin-D

[3H]Tamoxifen Aziridine ([3H]TAZ) is a derivative of the antiestrogen tamoxifen that covalently labels the Estrogen Receptor (ER), and perhaps other uncharacterized proteins. In a previous article we described that [3H]TAZ binds to a cytosolic protein from human uterine tissues that shares some, but not all, the ER properties. Here we have extended these studies to [3H]TAZ binding to cytosol proteins from human breast cancer specimens, and studied its quantitative association with other molecular markers and clinico-pathological variables. Cytosols were obtained in hypotonic buffer containing 20 mM molybdate and protease inhibitors, incubated with [3H]TAZ, and subjected to Sucrose Gradient Analysis (SGA). A [3H]TAZ labeled peak that consistently migrated with the 4S fractions was found in most of the assayed cytosols (range of 0 to 1278 fmol/ mg p.). The 4S peak of [3H]TAZ was partially inhibited by both estrogens and antiestrogens. When [3H]E2 was used instead of [3H]TAZ, only an 8S peak was detected. [3H]TAZ was covalently bound to a protein with an apparent MW of 65 kDa, as determined by SDS-PAGE and fluorography. The mean of [3H]TAZ binding was significantly higher in the subgroups of samples classified as ER-, PR-, pS2- or cathepsin D-, than in the respective positive subgroups (P < 0.01 in all the cases). [3H]TAZ binding was not associated with clinico-pathological variables, except that its mean was significantly larger in tumors larger than 5 cm than in smaller tumors. These results, and those previously reported, suggest that: 1) [3H]TAZ labels a cytosolic protein present in human breast cancers and uterine tissues that does not share all the ER properties, and 2) the [3H]TAZ binding by breast cancer cytosols is negatively associated with markers of estrogenic dependency, and its quantification may provide valuable information on antiestrogen responsiveness of a given tumor.