Estrogen receptor variants

Androgen receptor and estrogen receptor variants in prostate and breast cancers

The androgen receptor (AR) and estrogen receptor alpha (ERα) are steroid receptor transcription factors with critical roles in the development and progression of prostate and breast cancers. Advances in the understanding of mechanisms underlying the ligand-dependent activation of these transcription factors have contributed to the development of small molecule inhibitors that block AR and ERα actions. These inhibitors include competitive antagonists and degraders that directly bind the ligand binding domains of these receptors, luteinizing hormone releasing hormone (LHRH) analogs that suppress gonadal synthesis of testosterone or estrogen, and drugs that block specific enzymes required for biosynthesis of testosterone or estrogen. However, resistance to these therapies is frequent, and is often driven by selection for tumor cells with alterations in the AR or ESR1 genes and/or alternatively spliced AR or ESR1 mRNAs that encode variant forms AR or ERα. While most investigations involving AR have been within the context of prostate cancer, and the majority of investigations involving ERα have been within the context of breast cancer, important roles for AR have been elucidated in breast cancer, and important roles for ERα have been elucidated in prostate cancer. Here, we will discuss the roles of AR and ERα in breast and prostate cancers, outline the effects of gene- and mRNA-level alterations in AR and ESR1 on progression of these diseases, and identify strategies that are being developed to target these alterations therapeutically.

The role of photodynamic therapy on multidrug resistant breast cancer

Breast cancer heterogeneity allows cells with different phenotypes to co-exist, contributing to treatment failure and development of drug resistance. In addition, abnormal signal transduction and dysfunctional DNA repair genes are common features with breast cancer resistance. Chemo-resistance of breast cancer associated with multidrug resistance events utilizes ATP-binding cassette (ABC) efflux transporters to decrease drug intracellular concentration. Photodynamic therapy (PDT) is the treatment that involves a combination of a photosensitizer (PS), light and molecular oxygen to induce cell death. This treatment modality has been considered as a possible approach in combatting multidrug resistance phenomenon although its therapeutic potential towards chemo-resistance is still unclear. Attempts to minimize the impact of efflux transporters on drug resistance suggested concurrent use of chemotherapy agents, nanotechnology, endolysosomal release of drug by photochemical internalization and the use of structurally related compound inhibitors to block the transport function of the multidrug resistant transporters. In this review, we briefly summarize the role of membrane ABC efflux transporters in therapeutic outcomes and highlight research findings related to PDT and its applications on breast cancer with multidrug resistance phenotype. With the development of an ideal PS for photodynamic cancer treatment, it is possible that light activation may be used not only to sensitize the tumour but also to enable release of PS into the cytosol and as such bypass efflux membrane proteins and inhibit escape pathways that may lead to resistance.

Overexpression of the erythropoietin receptor in RAMA 37 breast cancer cells alters cell growth and sensitivity to tamoxifen

Erythropoietin (EPO) is the main regulator of erythropoiesis, and its receptor (EPOR) is expressed in various tissues, including tumors. Expression of EPOR in breast cancer tissue has been shown to correlate with expression of the estrogen receptor (ER). However, EPOR promotes proliferation in an EPO-independent manner. In patients with breast cancer, EPOR is associated with impaired tamoxifen response in ER-positive tumors, but not in ER-negative tumors. Furthermore, a positive correlation between EPOR/ER status and increased local cancer recurrence has been demonstrated, and EPOR expression is associated with G-protein coupled ER (GPER). Herein, we assessed the effects of EPOR on cell physiology and tamoxifen response in the absence of EPO stimulation using two cell lines that differ only in their EPOR expression status: RAMA 37 cells (low EPOR expression) and RAMA 37-28 cells (high EPOR expression). Alterations in cell growth, morphology, response to tamoxifen cytotoxicity, and EPOR-activated signal transduction were observed. RAMA 37 cells showed higher proliferation capacity without tamoxifen treatment, while RAMA 37-28 cells were more resistant to tamoxifen and proliferated more rapidly in the presence of tamoxifen. EPOR overexpression induced cell-morphology changes upon tamoxifen treatment, which resulted in the production of cell protrusions and subsequent cell death. Short-term treatment with tamoxifen (6 h) prompted RAMA 37 cells to acquired longer protrusions than RAMA 37-28 cells, which indicated a pre-apoptotic stage. Furthermore, prolonged treatment with tamoxifen (72 h) caused a greater reduction in RAMA 37 cell numbers, which indicated a higher rate of cell death. RAMA 37-28 cells showed prolonged activation of AKT signaling. We propose sustained AKT phosphorylation in EPOR-overexpressing cells as a mechanism that can lead to EPOR-induced tamoxifen resistance.

Effects of the lifestyle habits in breast cancer transcriptional regulation

Through research carried out in the last 25 years about the breast cancer etiology, it has been possible to estimate that less than 10 % of patients who are diagnosed with the condition are carriers of some germline or somatic mutation. The clinical reports of breast cancer patients with healthy twins and the development of disease in women without high penetrance mutations detected, warn the participation more factors in the transformation process. The high incidence of mammary adenocarcinoma in the modern woman and the urgent need for new methods of prevention and early detection have demanded more information about the role that environment and lifestyle have on the transformation of mammary gland epithelial cells. Obesity, alcoholism and smoking are factors that have shown a close correlation with the risk of developing breast cancer. And although these conditions affect different cell regulation levels, the study of its effects in the mechanisms of transcriptional and epigenetic regulation is considered critical for a better understanding of the loss of identity of epithelial cells during carcinogenesis of this tissue. The main objective of this review was to establish the importance of changes occurring to transcriptional level in the mammary gland as a consequence of acute or chronic exposure to harmful products such as obesity-causing foods, ethanol and cigarette smoke components. At analyze the main studies related to topic, it has concluded that the understanding of effects caused by the lifestyle factors in performance of the transcriptional mechanisms that determine gene expression of the mammary gland epithelial cells, may help explain the development of this disease in women without genetic propensity and different phenotypic manifestations of this cancer type.

Why (multi)targeting of cyclin-dependent kinases is a promising therapeutic option for hormone-positive breast cancer and beyond

Estrogens, via induction of their specific receptors (e.g., ER-α), regulate cell proliferation, differentiation and morphogenesis in mammary epithelium. Cell-cycle progression is driven by activation of complexes consisting of cyclin-dependent kinases (CDKs) and cyclins, which also modulate the activity of ER-α. Loss of control over the cell-cycle results in accelerated cell division and malignant transformation. Thus, a reciprocal relation exists between estrogen signaling and cell proliferation. Based on these findings, a new concept was developed to reduce ER-α activity and bring the cell cycle in transformed cells to heel. Prevention of ER-α activation and control over the deregulated cell cycle was achieved by supplementation with pharmacological CDK inhibitors alone or in combination with selective antiestrogens.

Polymorphisms in the LPL and CETP Genes and Haplotype in the ESR1 Gene Are Associated with Metabolic Syndrome in Women from Southwestern Mexico

Metabolic syndrome (MetS) is a combination of metabolic disorders associated with an increased risk for cardiovascular disease (CVD). Studies in women reported associations between polymorphisms in ESR1, LPL and CETP genes and MetS. Our aim was to evaluate the association between variants in ESR1, LPL and CETP genes with MetS and its components. Four hundred and eighty women were analyzed, anthropometric features and biochemical profiles were evaluated, and genotyping was performed by real-time PCR. We found an association with elevated glucose levels (odds ratio (OR) = 2.9; p = 0.013) in carrying the AA genotype of rs1884051 in the ESR1 gene compared with the GG genotype, and the CC genotype of rs328 in the LPL gene was associated with MetS compared to the CG or GG genotype (OR = 2.8; p = 0.04). Moreover, the GA genotype of rs708272 in the CETP gene is associated with MetS compared to the GG or AA genotype (OR = 1.8; p = 0.006). In addition the ACTCCG haplotype in the ESR1 gene is associated with a decrease in the risk of MetS (OR = 0.02; p < 0.001). In conclusion, our results show the involvement of the variants of ESR1, LPL and CETP genes in metabolic events related to MetS or some of its features.

The impact of multi-targeted cyclin-dependent kinase inhibition in breast cancer cells: clinical implications

INTRODUCTION

The progression of the mammalian cell cycle is driven by the transient activation of complexes consisting of cyclins and cyclin-dependent kinases (CDKs). Loss of control over the cell cycle results in accelerated cell division and malignant transformation and can be caused by the upregulation of cyclins, the aberrant activation of CDKs or the inactivation of cellular CDK inhibitors. For these reasons, cell cycle regulators are regarded as very promising therapeutic targets for the treatment of human malignancies.

AREAS COVERED

This review covers the structures and anti-breast cancer activity of selected pharmacological pan-specific CDK inhibitors. Multi-targeted CDK inhibitors affect CDKs involved in the regulation of both cell cycle progression and transcriptional control. The inhibition of CDK7/CDK9 has a serious impact on the activity of RNA polymerase II; when its carboxy-terminal domain is unphosphorylated, it is unable to recruit the cofactors required for transcriptional elongation, resulting in a global transcriptional block. Multi-targeted inhibition of CDKs represses anti-apoptotic proteins and thus promotes the induction of apoptosis. Moreover, the inhibition of CDK7 in estrogen receptor (ER)-positive breast cancer cells prevents activating phosphorylation of ER-α.

EXPERT OPINION

These diverse modes of action make multi-targeted CDK inhibitors promising drugs for the treatment of breast cancers.

Differential expression of estrogen receptors in two hippocampal regions during the estrous cycle of the rat

In the hippocampus, estrogens increase dendritic arborization, long-term potentiation, neuroprotection, and participate in many functions related with learning, memory, and affective behaviors. The presence of both estrogen receptors alpha (ERα) and beta (ERβ) isoforms has been described in the hippocampus where they play different physiological roles. The aim of this study was to investigate, by using both techniques immunohistochemistry and Western Blot, the expression pattern of ERα and ERβ in the hippocampus of the rat along the estrous cycle. Western blot analysis was used to confirm the specificity of the antibodies used against ERα and ERβ and its relative content in the hippocampus. Results from immunohistochemical studies indicate that ERβ expression increased more than the ERα in CA1 and CA3 regions during all phases of the estrous cycle. ERβ immunoreactivity was mainly located in the nucleus and predominantly expressed in CA1 during estrous and metestrus, and in CA3 during diestrus. ERα was more abundant during estrous in comparison to other phases of the cycle in CA1 region, while it was more abundant during metestrus in CA3. Interestingly, the immunolocalization of ERα subtype was both cytoplasmic and nuclear. The overall results indicate that there is a differential expression, cellular localization, and distribution of both ER subtypes in CA1 and CA3 regions, suggesting different roles for these two receptors in the hippocampus along the estrous cycle.

Interference with ER-α enhances the therapeutic efficacy of the selective CDK inhibitor roscovitine towards ER-positive breast cancer cells

In recent years many risk factors for the development of breast cancer that are linked to estrogens have been identified, and roscovitine (ROSC), a selective cyclin-dependent kinase (CDK) inhibitor, has been shown to be an efficient inhibitor of the proliferation of human breast cancer cells. Therefore, we have examined the possibility that interference with estrogen signaling pathways, using tamoxifen (TAM), a selective estrogen receptor modulator (SERM), could modulate the efficacy of treatment with ROSC. In conjunction with TAM, ROSC exhibited enhanced anti-proliferative activity and CDK inhibition, particularly in estrogen-dependent MCF-7 cells. The interaction between both drugs was synergistic. However, in ER-α-negative cells the interaction was antagonistic. Exposure of MCF-7 cells to ROSC abolished the activating phosphorylation of CDK2 and CDK7 at Ser(164/170). This in turn prevented the phosphorylation of the carboxyl-terminal repeat domain of RNA Polymerase II and ER-α at Ser(118), resulting in the down-regulation of the latter. Concomitantly, wt p53 was strongly activated by phosphorylation at Ser(46). Our results demonstrate that ROSC negatively affects the functional status of ER-α, making it potentially useful in the treatment of estrogen-dependent breast cancer cells.

Roscovitine, a selective CDK inhibitor, reduces the basal and estrogen-induced phosphorylation of ER-α in human ER-positive breast cancer cells

Roscovitine (ROSC), a selective cyclin-dependent kinase (CDK) inhibitor, arrests human estrogen receptor-α (ER-α) positive MCF-7 breast cancer cells in the G(2) phase of the cell cycle and concomitantly induces apoptosis via a p53-dependent pathway. The effect of ROSC is markedly diminished in MCF-7 cells maintained in the presence of estrogen-mimicking compounds. Therefore, we decided to examine whether ROSC has any effect on the functional status of the ER-α transcription factor. Exposure of MCF-7 cells to ROSC abolished the activating phosphorylation of CDK2 and CDK7 in a concentration and time-dependent manner. This inhibition of site-specific modification of CDK7 at Ser164/170 prevented phosphorylation of RNA polymerase II and reduced basal phosphorylation of ER-α at Ser118 in non-stimulated MCF-7 cells (resulting in its down-regulation). In MCF-7 cells, estrogen induced strong phosphorylation of ER-α at Ser118 but not at Ser104/Ser106. ROSC prevented this estrogen-promoted activating modification of ER-α. Furthermore, we sought to determine whether the activity of ROSC could be enhanced by combining it with an anti-estrogen. Tamoxifen (TAM), a selective estrogen receptor modulator (SERM), affected breast cancer cell lines irrespective of their ER status. In combination with ROSC, however, it had a different impact, enhancing G(1) or G(2) arrest. Our results indicate that ROSC prevents the activating phosphorylation of ER-α and that its mode of action is strongly dependent on the cellular context. Furthermore, our data show that ROSC can be combined with anti-estrogen therapy. The inhibitory effect of TAM on ER-negative cancer cells indicates that SERMs crosstalk with other steroid hormone receptors.

Oestrogen-receptor-mediated transcription and the influence of co-factors and chromatin state

Oestrogen receptor-alpha (ERalpha)-regulated transcription in breast cancer cells involves protein co-factors that contribute to the regulation of chromatin structure. These include co-factors with the potential to regulate histone modifications such as acetylation or methylation, and therefore the transcriptional state of target genes. Although much of the information regarding the interaction of specific co-factors with ER has been generated by studying specific promoter regions, we now have an improved understanding of the nature of these interactions and are better placed to relate these with ER activity and potentially with the activity of breast cancer drugs, including tamoxifen.

Phylogenetic conservation of the androgen receptor AR45 variant form in placental mammals

A cDNA coding for a tissue-specific AR45 variant form of the androgen receptor (AR) has recently been identified in humans, with highest expression levels found in heart. The deduced protein comprises the DNA-binding domain, hinge region and ligand-binding domain of the AR, but not the N-terminal domain which is replaced by a unique, short, seven amino-acid-long stretch. This sequence is encoded by the mutually exclusive exon 1B, located between exons 1 and 2 of the human AR gene. As transcript variants of the steroid receptor family have been shown to have important implications for hormone function, we set out to analyse the genomes of different organisms for potential AR45 expression. We found exon 1B to be conserved in the syntenic chromosomal region of non-human primates such as the chimpanzee Pan troglodytes, the orang-utan Pongo pygmaeus, the macaque Macaca mulatta and the marmoset Callithrix jacchus, and of the elephant Loxondonta africana, the pig Sus scrofa and the dog Canis familiaris. Quantification of AR45 transcript levels in heart, skeletal muscle and lung of Macaca fascicularis showed the heart to be the main organ of expression. A complete AR45 cDNA was furthermore isolated from the heart of this species. Comparative analysis of the identified AR45 exon 1B regions and of the deduced amino acids revealed a high conservation among species. The four N-terminal residues were identical in all eight species, whereas a few changes were seen in the other three residues in the marmoset, elephant and pig. In contrast, we observed more divergence in the mouse Mus musculus and rat Rattus norvegicus syntenic regions. Here a stop codon was found downstream of the potential start codon in the putatively deduced protein sequence and it can be inferred that no protein corresponding to AR45 exists in these two species. The existence of AR45 in different placental mammals with the exception of mouse and rat suggests a disappearance in rodents late in evolution, before the separation of the mouse and rat lineages, about 16 million years ago. In view of the potential function of AR45 as a regulator of AR function, and considering the multiple roles of androgens in normal physiology and in several diseases, these findings have important implications with regard to subtle differences in the action of the male sexual hormone in various organisms.

Overview of nomenclature of nuclear receptors

Nuclear receptor pharmacology has, to a certain extent, led the way, compared with other receptor systems, in the appreciation that ligands may exert very diverse pharmacology, based on their individual chemical structure and the allosteric changes induced in the receptor/accessory protein complex. This can lead to very selective pharmacological effects, which may not necessarily be predicted from the experience with other agonists/partial agonists/antagonists. If this is the case, then drug discovery may be back to drug-specific pharmacology (where each drug may have an original profile), rather than specific-drug pharmacology (where agents specific for a receptor have a distinct profile). As functional selectivity is indeed a crucial mechanism to be considered when going through the drug discovery development process, then initial screens using reconstituted systems may not show the appropriate pharmacology, simply because the required stoichiometry of corepressors and coactivators may not be present to select the best compounds; therefore, multiple effector systems are necessary to screen for differential activation, and, even then, screening with in vivo pathophysiological models may ultimately be required for the selection process-a massive but necessary task for pharmacologists. Thus, the characterization of nuclear receptors and their associated proteins and the ligands that interact with them will remain a challenge to pharmacologists.

Choline phospholipid metabolism in cancer: consequences for molecular pharmaceutical interventions

Over the past decade, our program has focused on understanding the role of the physiological environment, tumor vasculature, and metabolism in several of the aggressive phenotypic traits of cancer, such as invasion and metastasis. These studies have been performed primarily with magnetic resonance (MR) imaging (MRI) and spectroscopy (MRS) on human breast and prostate cancer models. During the course of these studies, we observed specific changes in choline phospholipid metabolism associated with a more aggressive phenotype. Molecular or pharmacologic interventions that reduced this aggressiveness were also consistent with a reversal of these alterations. In this contextual review, we have outlined the insight we have gained from these studies and have discussed some of the enzymes and pathways that may present novel targets for pharmaceutical interventions in cancer.

Full-length sequence and expression analysis of estrogen receptor alpha mRNA in feline mammary tumors

We report here on the isolation and sequencing of feline estrogen receptor alpha (ER-alpha) mRNA. Feline ER-alpha showed three alternative 5' untranslated exons and a common transcript of 6183 base pairs (bp). The putative coding sequence (1788 bp) and the unusually long 3' untranslated (3'-UTR) region (4305 bp) displayed high sequence similarity with human ER-alpha. A highly conserved sequence block was found in the 3'-UTR corresponding to a putative regulatory element for mRNA stability. In addition to wild-type ER-alpha mRNA, several exon-deleted splicing variants (ERDeltaE2, ERDeltaE3, ERDeltaE4, ERDeltaE5, ERDeltaE6, ERDeltaE7) were found in various feline tissues. This is similar to what observed in human tissues and opposite to the near absence of exon-deleted isoforms in rodents. Expression analysis of exon-deleted variants was also conducted on 24 samples of feline mammary carcinoma (FMC) and 15 normal mammary gland (NMG) controls, using a "splice targeted approach". The prevalence of some variants was similar for human and feline ER-alpha, while other isoforms were expressed at different frequencies in the two species. Two of the most frequent isoforms (ERDeltaE4, ERDeltaE7) were significantly less frequent in FMCs than in NMGs, likely as a consequence of decreased expression of ER-alpha in FMCs.

Impact of metastatic estrogen receptor and progesterone receptor status on survival

Hormone responsive breast cancer is usually determined by the presence of estrogen receptors (ER) or progesterone receptors (PR) on primary invasive breast cancers. Adjuvant and metastatic hormone therapy are recommended based on primary ER and PR determination. Little information is available to determine if primary hormone receptors correlate with metastatic disease and if survival is influenced by metastatic receptor status. We retrospectively compared primary to metastatic tumor ER and PR content from 200 metastatic breast cancer patients. ER and PR analyses were available in both primary and metastatic disease in 200 and 173 patients, respectively. There was a correlation between both the ER and PR in the primary and metastatic lesion (p < 0.001). However, in 60 of 200 (30%) patients, discordance between primary and metastatic ER was noted. Tumors from 68 of 173 (39.3%) showed discordance for PR. In 39 (19.5%) patients, the ER primary status was positive and metastatic status was negative and in 21 (10.5%) patients, the primary status was negative and metastatic status was positive. Survival from the time of metastatic diagnosis was calculated. Those patients with ER positive primary and metastatic tumors (Positive/Positive) or only the metastatic lesion (Negative/Positive) had similar median survival (1131 and 1111 days, respectively). However, patients with tumors that changed from positive primary to negative metastasis (Positive/Negative) experienced significantly shorter median survival (669 days, p < 0.05). Likewise, median survival (580 days) was significantly shorter for patients with primary and metastasis ER negative (Negative/Negative, p < 0.001) compared to Positive/Positive (p < 0.001) or compared to Negative/Positive (p < 0.02). The changes in PR status were not associated with a change in survival. We found a significant discordance between hormone receptor content of primary versus metastatic breast cancer. The ER status of the metastatic lesion was a better predictor of survival. Therefore, optimal metastatic treatment cannot be determined solely on primary ER and PR analysis.

Relationship between the expression of various markers and prognostic factors in breast cancer

The immunohistochemical detection of six markers of breast cancer has been compared in the present study with known prognostic factors of the disease to establish locally a standard panel of markers for the management of breast cancer. Sections of tissue of 114 consecutive breast cancer cases were studied immunohistochemically, using antibodies against oestrogen receptor (ER), progesterone receptor (PR), androgen receptor, c-erbB2, cathepsin D, and cyclin D. Marker labelling was graded as recommended in the literature. Using the chi(2)-test, relationships were determined between marker labelling and histological type of cancer, tumour grade, tumour size, axillary lymph node status and age of patient. A p value below 0.05 was considered significant. A positive relationship was found between ER and PR and lower grades of cancer, and a negative relationship was found with medullary and atypical medullary carcinoma. The four other markers showed no relationship with grade or type of cancer. All markers showed no significant relationship with size of tumour, presence of axillary node metastasis or age of patient. There was positive correlation between c-erbB2 and cathepsin D. Our study confirms the association between ER and PR and histological type and grade of breast cancer, both known parameters of good prognosis. We found no consistent relationship between the other four markers and prognostic factors studied, other than the suggestion that c-erbB2 and cathepsin D may be useful markers for poor prognosis and can be usefully applied locally, especially in the light of the current availability of trastuzumab (Herceptin) for management of c-erbB2-positive cases. We found no relationship between the markers and tumour size, axillary lymph node status or age.

Hsp90: an emerging target for breast cancer therapy

Rapidly evolving insights into the specific molecular genetic abnormalities that drive the growth and metastasis of breast cancer have led to the development of targeted therapeutics that do not rely on the generalized disruption of DNA metabolism and cell division for activity. Of particular interest are inhibitors of cellular signal transduction pathways involving tyrosine kinases as well as selective modulators of steroid hormone signaling, histone acetylation, angiogenesis and tumor cell apoptosis. Unique within this array of promising new agents, however, are compounds that target heat shock protein 90 (Hsp90). This molecular chaperone associates with a distinct, but surprisingly diverse, set of proteins that are referred to as Hsp90 client proteins. Hsp90 binds to these clients, and plays a key role in regulating their stability and function. Many of the proteins chaperoned by Hsp90 are involved in breast cancer progression and resistance to therapy, including the estrogen receptor, receptor tyrosine kinases of the erbB family, Akt, and mutant p53. Several small molecule inhibitors of Hsp90 have been identified that can deplete cellular levels of multiple oncogenic client proteins simultaneously by enhancing their ubiquitination and proteasome-mediated degradation. The activity of Hsp90 inhibitors has been well validated in preclinical breast cancer models, both in single-agent studies and in combination with conventional chemotherapy. One of these inhibitors, 17-allylamino, 17-demethoxygeldanamycin (17-AAG, NSC 330507) has recently completed phase I testing. The agent was well tolerated at drug exposures that were shown to cause modulation of Hsp90 client protein levels. Given the redundancy and complexity of the molecular abnormalities present in most breast cancers, the ability of Hsp90 inhibitors to alter the activity of multiple oncogenic targets may prove of unique therapeutic benefit.

Molecular identification of ERalpha-positive breast cancer cells by the expression profile of an intrinsic set of estrogen regulated genes

Estrogens exert a key biological role in mammary gland epithelial cells and promote breast carcinogenesis and tumor progression. We recently identified a new large set of estrogen responsive genes from breast cancer (BC) cells by DNA microarray analysis of the gene expression profiles induced by 17beta-estradiol in ZR-75.1 and MCF-7 cells. The purpose of the present study was to test whether the expression pattern of hormone regulated genes from this set identifies estrogen receptor (ERalpha) positive, hormone responsive BC cells. To this aim, we carried out in silico metanalysis of ERalpha positive and ERalpha negative human BC cell line transcriptomes, focusing on two sets of 171 and 218 estrogen responsive genes, respectively. Results show that estrogen dependent gene activity in hormone responsive BC cells is significantly different from that of non-responsive cells and, alone, allows to discriminate these two cellular phenotypes. Indeed, we have identified 61 genes whose expression profile specifically marks ERalpha positive BC cells, suggesting that this gene set may be exploited for phenotypic characterization of breast tumors. This possibility was tested with data obtained by gene expression profiling of BC surgical samples, where the ERalpha positive phenotypes were highlighted by the expression profile of a subset of 27 such hormone responsive genes and four additional BC marker genes, not including ERs. These results provide direct evidence that the expression pattern of a limited number of estrogen responsive genes can be exploited to assess the estrogen signaling status of BC cells both in vitro and ex-vivo.

Estrogen receptor profiles: changes in mouse and rat mammary tumors by treatment with selective estrogen receptor modifiers

OBJECTIVE

The aim of this work was to analyze the effect of estradiol (E(2)), medroxyprogesterone and the two selective estrogen receptor modulators (SERMs) (tamoxifen (Tam) and raloxifene (Ral)) on the estrogen receptor (ER) conformers profile performed by size exclusion HPLC in relation to hormone dependence of mammary tumors.

MATERIALS AND METHODS

Two types of mammary tumors were studied: tumors transplanted in BALB/c mice that are medroxyprogesterone acetate (MPA)-dependent for growth, and tumors induced in Sprague-Dawley rats by intraperitoneal injection of N-nitroso-N-methylurea (NMU). Tumors from mice treated with MPA, E(2), Tam or Ral and NMU-treated rats were analyzed and compared to that of control.

RESULTS

The tumor conformer profiles were as follows: control and MPA-treated mice showed only one peak (oligomeric form); E(2)-treated mice also showed only one peak (dimer); Tam-treated mice showed one peak corresponding to a possible proteolytic fragment, and Ral-treated mice showed two peaks (oligomeric and a possible proteolytic fragment). On the other hand, NMU-induced mammary tumors from rats showed three peaks (oligomeric, monomeric and proteolytic).

CONCLUSION

Our findings may indicate that SERMs affect the aggregation state of ER and thereby its ability to modulate genomic transcription mechanisms related to growth rate.

Tamoxifen resistance by a conformational arrest of the estrogen receptor alpha after PKA activation in breast cancer

Using a novel approach that detects changes in the conformation of ERalpha, we studied the efficacy of anti-estrogens to inactivate ERalpha under different experimental conditions. We show that phosphorylation of serine-305 in the hinge region of ERalpha by protein kinase A (PKA) induced resistance to tamoxifen. Tamoxifen bound but then failed to induce the inactive conformation, invoking ERalpha-dependent transactivation instead. PKA activity thus induces a switch from antagonistic to agonistic effects of tamoxifen on ERalpha. In clinical samples, we found that downregulation of a negative regulator of PKA, PKA-RIalpha, was associated with tamoxifen resistance prior to treatment. Activation of PKA by downregulation of PKA-RIalpha converts tamoxifen from an ERalpha inhibitor into a growth stimulator, without any effect on ICI 182780 (Fulvestrant).

Antiestrogen resistance in breast cancer and the role of estrogen receptor signaling

Antiestrogens include agents such as tamoxifen, toremifene, raloxifene, and fulvestrant. Currently, tamoxifen is the only drug approved for use in breast cancer chemoprevention, and it remains the treatment of choice for most women with hormone receptor positive, invasive breast carcinoma. While antiestrogens have been available since the early 1970s, we still do not fully understand their mechanisms of action and resistance. Essentially, two forms of antiestrogen resistance occur: de novo resistance and acquired resistance. Absence of estrogen receptor (ER) expression is the most common de novo resistance mechanism, whereas a complete loss of ER expression is not common in acquired resistance. Antiestrogen unresponsiveness appears to be the major acquired resistance phenotype, with a switch to an antiestrogen-stimulated growth being a minor phenotype. Since antiestrogens compete with estrogens for binding to ER, clinical response to antiestrogens may be affected by exogenous estrogenic exposures. Such exposures include estrogenic hormone replacement therapies and dietary and environmental exposures that directly or indirectly increase a tumor's estrogenic environment. Whether antiestrogen resistance can be conferred by a switch from predominantly ERalpha to ERbeta expression remains unanswered, but predicting response to antiestrogen therapy requires only measurement of ERalpha expression. The role of altered receptor coactivator or corepressor expression in antiestrogen resistance also is unclear, and understanding their roles may be confounded by their ubiquitous expression and functional redundancy. We have proposed a gene network approach to exploring the mechanistic aspects of antiestrogen resistance. Using transcriptome and proteome analyses, we have begun to identify candidate genes that comprise one component of a larger, putative gene network. These candidate genes include NFkappaB, interferon regulatory factor-1, nucleophosmin, and the X-box binding protein-1. The network also may involve signaling through ras and MAPK, implicating crosstalk with growth factors and cytokines. Ultimately, signaling affects the expression/function of the proliferation and/or apoptotic machineries.

Regulation of the intronic promoter of rat estrogen receptor alpha gene, responsible for truncated estrogen receptor product-1 expression

We have characterized the intronic promoter of the rat estrogen receptor (ER) alpha gene, responsible for the lactotrope-specific truncated ER product (TERP)-1 isoform expression. Transcriptional regulation was investigated by transient transfections using 5'-deletion constructs. TERP promoter constructs were highly active in MMQ cells, a pure lactotrope cell line, whereas a low basal activity was detected in alphaT3-1 gonadotrope cells or in COS-7 monkey kidney cells. Serial deletion analysis revealed that 1) a minimal -693-bp region encompassing the TATA box is sufficient to allow lactotrope-specific expression; 2) the promoter contains strong positive cis-acting elements both in the distal and proximal regions, and 3) the region spanning the -1698/-1194 region includes repressor elements. Transient transfection studies, EMSAs, and gel shifts demonstrated that estrogen activates the TERP promoter via an estrogen-responsive element (ERE1) located within the proximal region. Mutation of ERE1 site completely abolishes the estradiol-dependent transcription, indicating that ERE1 site is sufficient to confer estrogen responsiveness to TERP promoter. In addition, ERalpha action was synergized by transfection of the pituitary-specific factor Pit-1. EMSAs showed that a single Pit-1 DNA binding element in the vicinity of the TATA box is sufficient to confer response by the TERP promoter. In conclusion, we demonstrated, for the first time, that TERP promoter regulation involves ERE and Pit-1 cis-elements and corresponding trans-acting factors, which could play a role in the physiological changes that occur in TERP-1 transcription in lactotrope cells.

Phenotypic differentiation of human breast cancer cells by 1,3 cyclic propanediol phosphate

Breast cancer cells in their virulent undifferentiated state are characterized by lack of functional estrogen receptors (ER) and/or progesterone receptors (PR) as well as relatively low levels of other normal differentiation markers such as milk proteins and lipid droplets. To date, no method for in situ elevation of the state of differentiation of breast cancer cells has yet been proven effective in patients. We have recently shown that 1,3 cyclic propanediol phosphate (1,3 cPP), an analog of 1,3 cyclic glycerophosphate (1,3 cGP), can promote morphological, neuronal-like differentiation in pheochromocytoma-12 cells in vitro. In view of this observation, we tested the potential of 1,3 cPP to elevate the state of cellular differentiation of the human breast cancer cell lines MCF-7 (ER(+)) and HCC1954 (ER(-)), as characterized by the expression of steroid receptors, casein kinase, lipid droplet histology and signal-transduction gene profiles. In the range of 5-100 microM 1,3 cPP the in vitro expression of ER-alpha, PR and casein kinase increased by approximately 2-fold while the mRNA transcription increased by 2-6-fold. Moreover, following 9-12 days of incubation with 1,3 cPP, HCC1954 cells exhibited a significant increase in the production of lipid droplets as observed by Oil Red O staining. The in vivo effect of 1,3 PP on MCF-7 xenografted into nude mice was also determined. After 4 biweekly i.p. injections of 0.5 mg 1,3 cPP per mouse, tumors in the 1,3 cPP treated virtually did not grow at all while the tumors in the control group grew rapidly. Based on these findings, we propose that this novel differentiating compound has the potential to transform the malignant tumor phenotype into a near-normal phenotype, as well as to sensitize the tumor cells to anti-estrogen therapy via upgrading the status of steroid hormone receptors.

Estrogen receptor phosphorylation

Estrogen receptor alpha (ERalpha) is phosphorylated on multiple amino acid residues. For example, in response to estradiol binding, human ERalpha is predominately phosphorylated on Ser-118 and to a lesser extent on Ser-104 and Ser-106. In response to activation of the mitogen-activated protein kinase pathway, phosphorylation occurs on Ser-118 and Ser-167. These serine residues are all located within the activation function 1 region of the N-terminal domain of ERalpha. In contrast, activation of protein kinase A increases the phosphorylation of Ser-236, which is located in the DNA-binding domain. The in vivo phosphorylation status of Tyr-537, located in the ligand-binding domain, remains controversial. In this review, I present evidence that these phosphorylations occur, and identify the kinases thought to be responsible. Additionally, the functional importance of ERalpha phosphorylation is discussed.

Nuclear receptor superfamily: Principles of signaling

Nuclear receptors (NRs) comprise a family of 49 members that share a common structural organization and act as ligand-inducible transcription factors with major (patho)physiological impact. For some NRs (“orphan receptors”), cognate ligands have not yet been identified or may not exist. The principles of DNA recognition and ligand binding are well understood from both biochemical and crystal structure analyses. The 3D structures of several DNA-binding domains (DBDs),in complexes with a variety of cognate response elements, and multiple ligand-binding domains (LBDs), in the absence (apoLBD)and presence (holoLBD) of agonist, have been established and reveal canonical structural organization. Agonist binding induces a structural transition in the LBD whose most striking feature is the relocation of helix H12, which is required for establishing a coactivator complex, through interaction with members of the p160 family (SRC1, TIF2, AIB1) and/or the TRAP/DRIP complex. The p160-dependent coactivator complex is a multiprotein complex that comprises histone acetyltransferases (HATs), such as CBP,methyltransferases, such as CARM1, and other enzymes (SUMO ligase,etc.). The agonist-dependent recruitment of the HAT complex results in chromatin modification in the environment of the target gene promoters, which is requisite to, or may in some cases be sufficient for, transcription activation. In the absence of ligands, or in the presence of some antagonists, certain NRs are bound to distinct multiprotein complexes through the interaction with corepressors, such as NCoR and SMRT. Corepressor complexes comprise histone deacetylases (HDACs) that have the capacity to condense chromatin over target gene promoters. Ligands have been designed that selectively modulate the interaction between NRs and their coregulators. Both HATs and HDACs can also modify the acetylation status of nonhistone proteins, but the significance in the context of NR signaling is unclear. NRs communicate with other intracellular signaling pathways on a mutual basis, and their functionality may be altered, positively or negatively, by post-translational modification. The majority of NRs act as retinoid X receptor (RXR) heterodimers in which RXR cannot a priori respond autonomously to its cognate ligand to activate target gene transcription. This RXR subordination allows signaling pathway identity for the RXR partner. The corresponding mechanism is understood and reveals cell and NR selectivity, indicating that RXR can, under certain conditions, act autonomously. NRs are regulators of cell life and death,and NR malfunction can be at the basis of both disease and therapy, as is impressively documented in the case of acute promyelocytic leukemia. Recently, several pathways have been uncovered that link NR action with cell proliferation and apoptosis.

Molecular markers in the diagnosis and staging of breast cancer

Considerable advances have been made in understanding the molecular events that accompany the development of breast cancer. Although our knowledge of these genetic alterations has greatly outpaced clinical applications, many new advances are beginning to have an impact on the diagnosis and staging of breast cancer. Clinical evaluation of estrogen and progesterone receptors and HER2/neu status has become routine. The increasing use of these and other molecular markers promises to help refine diagnoses, define disease subsets, and provide more accurate information about the probable biologic outcome of a given tumor. Studies of molecular markers are also likely to lead to the identification and development of new therapeutic targets. I review the molecular markers currently used in the clinical diagnosis and staging of breast cancer, and discuss other potentially useful markers and assays.

Natural trans-spliced mRNAs are generated from the human estrogen receptor-alpha (hER alpha) gene

The human estrogen receptor-alpha (hER alpha) gene is a complex genomic unit exhibiting alternative splicing and promoter usage in a tissue-specific manner. During the investigation of new hER alpha mRNA variants by rapid amplification of 5' cDNA ends, we identified a cDNA in which the acceptor site of exon 1A, into which the different leader exons are normally alternatively spliced, was spliced accurately the 3' extremity of exon 1A (scrambled 1A-->1A hER alpha cDNA). Reverse transcription-PCR and S1 nuclease mapping analysis revealed that 1A-->1A hER alpha transcripts were not circular RNAs constituted by exon 1A only but corresponded to linear polyadenylated hER alpha RNAs composed of the eight coding exons of the hER alpha gene and characterized by a duplication of exon 1A. Genomic Southern blot experiments excluded the hypothesis of duplication of hER alpha exon 1A in the human genome. Therefore, these data suggested that 1A-->1A hER alpha transcripts were likely generated by trans-splicing. The production of such transcripts by trans-splicing of pre-mRNAs generated from a chimeric gene formed by a single hER alpha exon 1A, exon 2, and their flanking intronic regions was demonstrated in transient transfection experiments. Therefore, in addition to the alternative cis-splicing, the hER alpha gene is also subject to natural trans-splicing.

Hormonal genomics

The availability of the human genomic sequence is changing the way in which biological questions are addressed. Based on the prediction of genes from nucleotide sequences, homologies among their encoded amino acids can be analyzed and used to place them in distinct families. This serves as a first step in building hypotheses for testing the structural and functional properties of previously uncharacterized paralogous genes. As genomic information from more organisms becomes available, these hypotheses can be refined through comparative genomics and phylogenetic studies. Instead of the traditional single-gene approach in endocrine research, we are beginning to gain an understanding of entire mammalian genomes, thus providing the basis to reveal subfamilies and pathways for genes involved in ligand signaling. The present review provides selective examples of postgenomic approaches in the analysis of novel genes involved in hormonal signaling and their chromosomal locations, polymorphisms, splicing variants, differential expression, and physiological function. In the postgenomic era, scientists will be able to move from a gene-by-gene approach to a reconstructionistic one by reading the encyclopedia of life from a global perspective. Eventually, a community-based approach will yield new insights into the complexity of intercellular communications, thereby offering us an understanding of hormonal physiology and pathophysiology.

Endocrine-responsive breast cancer and strategies for combating resistance

Deaths from breast cancer have fallen markedly over the past decade due, in part, to the use of endocrine agents that reduce the levels of circulating oestrogens or compete with oestrogen for binding to its receptor. However, many breast tumours either fail to respond or become resistant to endocrine therapies. By understanding the mechanisms that underlie this resistance, we might be able to develop strategies for overcoming or bypassing it.

The role of estrogen receptors in breast cancer metastasis

It has long been appreciated that the estrogen receptor (ER) plays an important role in the biology of breast cancer. It is an accepted factor predicting favorable disease outcome and treatment response, and as such is generally considered to represent a "good" prognostic marker in breast cancer. In this review we present data suggesting that the ER may also play a pivotal role in the metastatic behavior of breast cancer, and present an argument that the up-regulation of ER and/or the selection of specific ER mutations are early events important for facilitating tumor progression. Thus, ER could serve dual roles in breast cancer, acting as a "bad" prognostic marker later in the disease process.

Estrogen receptor and breast cancer

Breast cancer, the most common malignancy in women, was already known to be associated with the steroid hormone estrogen more than a century ago. The discovery of the estrogen receptor (ER) provided us not only with a powerful predictive and prognostic marker, but also an efficient target for the treatment of hormone-dependent breast cancer with antiestrogens. In this paper we will sketch the important role of ER in the development, progression, and treatment of the disease, which is complicated by the receptor's interaction with co-regulatory proteins, its cross-talk with other signal transduction pathways, and its involvement in the development of antiestrogen resistance.

Novel intronic promoter in the rat ER alpha gene responsible for the transient transcription of a variant receptor

To analyze the molecular origin of an ER variant, the truncated ER product-1, transiently expressed at the proestrus in lactotrope cells, we generated a 2.5-kb sequence of a genomic region upstream and downstream the specific sequence truncated ER product-1. Genomic Southern blot analysis showed that truncated ER product-1 is spliced from a noncoding leader exon localized within the intron 4 of the ER alpha gene. Analysis of the promoter sequence revealed the presence of a major transcriptional start site, a canonical TATA box and putative cis regulatory elements for pituitary specific expression as well as an E-responsive element. In transient transfection, the truncated ER product-1 promoter was transcriptionally the most active in the lactotrope cell lines (MMQ). Analysis of truncated ER product-1 functionality showed that: 1) the protein inhibited ER alpha binding to the E-responsive element in electromobility shift assays, 2) inhibited the E2 binding to ER alpha in binding assays, 3) the truncated ER product-1/ER alpha complex antagonized the transcriptional activity elicited by E2, 4) nuclear localization of green fluorescent protein-ER alpha was altered in Chinese hamster ovary cell lines stably expressing truncated ER product-1. Collectively, these data demonstrated that the protein exerts full dominant negative activity against ER alpha. Moreover, truncated ER product-1/ER alpha complex also repressed the activity of all promoters tested to date, suggesting a general inhibitory effect toward transcription. In conclusion, the data suggest that truncated ER product-1 could regulate estrogen signaling via a specific promoter in lactotrope cells.

Human estrogen receptor beta-specific monoclonal antibodies: characterization and use in studies of estrogen receptor beta protein expression in reproductive tissues

Investigation of the role of the second, more recently described estrogen receptor, denoted ERbeta, will be critical in understanding the molecular mechanisms underlying tissue-specific gene regulation by estrogens. Expression of ERbeta in a variety of tissues has been examined predominantly at the mRNA level, and there is little information regarding the cellular localization and size of the endogenous ERbeta protein, due, in part, to the limited availability of human ERbeta-specific antibodies. Thus, our aim was to generate specific antibodies to human ERbeta and use them to determine the tissue-specific distribution and size(s) of the ERbeta protein. To this end, we have cloned three different hybridoma cell lines that produce monoclonal antibodies specific for the hormone-binding domain of human ERbeta. The antibodies, made in mice against human ERbeta amino acids 256-505 (hormone binding domain lacking the F domain), are designated CFK-E12 (E12), CMK-A9 (A9) and CWK-F12 (F12) and were determined to be the IgG gamma1 isotype for E12, and IgG gamma2b for A9 and F12. All three monoclonal antibodies could be used to detect in vitro translated, baculovirus expressed, and cell transfected and expressed ERbeta protein by Western blot analyses, and all failed to detect ERalpha. A9 and F12 were able to immunoprecipitate efficiently the native form of ERbeta protein in the presence and absence of estradiol. Epitope mapping studies indicate that the E12 and F12 antibodies recognize overlapping peptide sequences in the N-terminal region of the hormone-binding domain, a region that is highly conserved among species. Immunocytochemical studies with these antibodies reveal nuclear-specific localization of the ERbeta protein in granulosa cells of the rat ovary. Nuclear ERbeta is also specifically localized in epithelial and some stromal cells of mouse and rat epididymis. Western blot analysis with protein extracts from ovarian granulosa cells of human, rat, mouse, and pig showed a ca. 52 kDa and an additional ca. 62-64 kDa band in these species. These results indicate the presence of two predominant molecular size forms of the ERbeta protein in ovarian granulosa cells and demonstrate the utility of these antibodies for detection of ERbeta in the human and in several other mammalian species.

Cellular and molecular mechanisms of estrogen regulation of memory function and neuroprotection against Alzheimer's disease: recent insights and remaining challenges

This review focuses on recent advances in our knowledge of estrogen action in the brain. The greatest amount of attention was devoted to those studies that impact our understanding of estrogen regulation of memory function and prevention of degenerative diseases associated with memory systems, such as Alzheimer's disease. A review of recent advances in our understanding of estrogen receptors, both nuclear and membrane, is also presented. Finally, these data are considered in regard to their relevancy to the use of estrogen replacement therapy for cognitive health throughout menopause and the development of an estrogen replacement therapy designed for the unique requirements of the brain.

Molecular and pharmacological aspects of antiestrogen resistance

Endocrine therapy is effective in approximately one-third of all breast cancers and up to 80% of tumors that express both estrogen and progesterone receptors. Despite the low toxicity, good overall response rates, and additional benefits associated with its partial agonist activity, most Tamoxifen-responsive breast cancers acquire resistance. The development of new antiestrogens, both steroidal and non-steroidal, provides the opportunity for the development of non-cross-resistant therapies and the identification of additional mechanisms of action and resistance. Drug-specific pharmacologic mechanisms may confer a resistance phenotype, reflecting the complexities of both tumor biology/pharmacology and the molecular endocrinology of steroid hormone action. However, since all antiestrogens will be effective only in cells that express estrogen receptors (ER), many mechanisms will likely be directly related to ER expression and signaling. For example, loss of ER expression/function is likely to confer a cross-resistance phenotype across all structural classes of antiestrogens. Altered expression of ERalpha and ERbeta, and/or signaling from transcription complexes driven by these receptors, may produce drug-specific resistance phenotypes. We have begun to study the possible changes in gene expression that may occur as cells acquire resistance to steroidal and non-steroidal antiestrogens. Our preliminary studies implicate the altered expression of several estrogen-regulated genes. However, resistance to antiestrogens is likely to be a multigene phenomenon, involving a network of interrelated signaling pathways. The way in which this network is adapted by cells may vary among tumors, consistent with the existence of a highly plastic and adaptable genotype within breast cancer cells.

Variant estrogen receptor alpha mRNAs in human breast cancer specimens

A panel of human breast cancer specimens was examined for single base change mutations by DNA sequencing and for larger deletions using a PCR-based assay. In the cancer specimens examined, no sequencing variants were detected other than a previously characterized polymorphism. Although most of the specimens contained estrogen receptor (ER) variants at a low level, 2 of 118 specimens exhibited variants which, after amplification, constituted most of the amplified ER cDNA. One specimen contained a single variant form, and there was little evidence of the wild-type ER mRNA by PCR, Northern blotting or immunocytochemistry. The second specimen, despite the presence of a normal-sized mRNA by Northern blotting and normal immunocytochemical staining for ER, contained at least 5 different variant forms as well as the wild-type ER. All but 1 of the variant forms were processing variants, and 3 of these processing variants have not been described before. One variant, although lacking exons 2-4, has break points in exons 1 and 5 that do not correspond to intron-exon boundaries. This variant might reflect more widespread damage to the genome in this breast cancer specimen. The low level of occurrence of variants suggests that ER variant forms, at least in the coding region, do not contribute generally to the progression of breast cancer.

Estrogen receptor alpha in human breast cancer: occurrence and significance

Estrogens have long been recognized as being important for stimulating the growth of a large proportion of breast cancers. Now it is recognized that estrogen action is mediated by two receptors, and the presence of estrogen receptor alpha (ER alpha) correlates with better prognosis and the likelihood of response to hormonal therapy. Over half of all breast cancers overexpress ER alpha and around 70% of these respond to anti-estrogen (for example tamoxifen) therapy. In addition, the presence of elevated levels of ER alpha in benign breast epithelium appears to indicate an increased risk of breast cancer, suggesting a role for ER alpha in breast cancer initiation, as well as progression. However, a proportion of ER alpha-positive tumors does not respond to endocrine therapy and the majority of those that do respond eventually become resistant. Most resistant tumors remain ER alpha-positive and frequently respond to alternative endocrine treatment, indicative of a continued role for ER alpha in breast cancer cell proliferation. The problem of resistance has resulted in the search for and the development of diverse hormonal therapies designed to inhibit ER alpha action, while research on the mechanisms which underlie resistance has shed light on the cellular mechanisms, other than ligand binding, which control ER alpha function.

Is there a growing role for endocrine therapy in the treatment of breast cancer?

Novel biochemical findings on the molecular mechanisms of estrogen actions may help us to understand some of the unexplained observations seen in breast cancer treatment and suggest new therapeutic opportunities. Thus, apart from the challenge of improving the clinical treatment of patients with advanced disease, results from trials in this setting may reveal new therapeutic principles that may be evaluated in the adjuvant setting. The role of endocrine therapy in metastatic as well as early breast cancer is increasing, and the possibility of improving cure rates for breast cancer by implementing therapy with novel aromatase inhibitors in the adjuvant setting is exciting. While the results from prevention trials are most interesting, suggesting the possibility of reducing breast cancer incidence in high-risk groups, more data are needed before we can decide whether such interventions are warranted in women at high risk of developing breast cancer.

Selective oestrogen receptor modifiers (SERMs) and breast cancer therapy

Antioestrogen therapy is currently receiving renewed interest for several reasons. Tamoxifen was introduced in the treatment of metastatic breast cancer more than three decades ago. The drug significantly reduces long term mortality and also reduces the risk of contralateral tumours when administered in early breast cancer. Five years of tamoxifen is now standard in adjuvant endocrine therapy, and the drug is currently being evaluated for breast cancer prevention. Despite this, several aspects regarding the pharmacology of the drug are still unclear, and the scientific rationale for dose selection has recently been challenged. Several novel antioestrogen compounds, called selective oestrogen receptor modifiers (SERMs), express selective oestrogen agonistic or antagonistic properties depending on the organ or test system evaluated. Some of these drugs, like raloxifene, do not seem to promote the development of endometrial cancer, although they still have selected oestrogen-like beneficial effects. This paper reviews the pharmacologic and the pharmacokinetic aspects of the different SERMs with particular emphasis on their potential use in therapy and prevention of breast cancer.

Cloning and characterization of human estrogen receptor beta isoforms

Multiple transcripts which arise from the human estrogen receptor beta (ER beta) gene have been characterized. Three full length isoforms of the hER beta gene, designated hER beta 1-3, were identified in a testis cDNA library. An additional two isoforms, designated hER beta 4 and hER beta 5, were identified by PCR amplification from testis cDNA and from the MDA-MB 435 cell line. hER beta 1 corresponds to the previously described hER beta. All five isoforms diverge at a common position within the predicted helix 10 of the ligand binding domain of hER beta, with nucleotide sequences consistent with differential exon usage. The hER beta isoform mRNAs displayed a differential pattern of expression in human tissues and in tumor cell lines when analyzed by RT-PCR. Further characterization of the three full length isoforms, hER beta 1-3, by in vitro band shift studies indicated that the isoforms were able to form DNA-binding homodimers and heterodimers with each other and with the ER alpha subtype.