Calmodulin is essential for estrogen receptor interaction with its motif and activation of responsive promoter

Calmodulin-like protein 3 is an estrogen receptor alpha coregulator for gene expression and drug response in a SNP, estrogen, and SERM-dependent fashion

BACKGROUND

We previously performed a case-control genome-wide association study in women treated with selective estrogen receptor modulators (SERMs) for breast cancer prevention and identified single nucleotide polymorphisms (SNPs) in ZNF423 as potential biomarkers for response to SERM therapy. The ZNF423rs9940645 SNP, which is approximately 200 bp away from the estrogen response elements, resulted in the SNP, estrogen, and SERM-dependent regulation of ZNF423 expression and, "downstream", that of BRCA1.

METHODS

Electrophoretic mobility shift assay-mass spectrometry was performed to identify proteins binding to the ZNF423 SNP and coordinating with estrogen receptor alpha (ERα). Clustered, regularly interspaced short palindromic repeats (CRISPR)/Cas9 genome editing was applied to generate ZR75-1 breast cancer cells with different ZNF423 SNP genotypes. Both cultured cells and mouse xenograft models with different ZNF423 SNP genotypes were used to study the cellular responses to SERMs and poly(ADP-ribose) polymerase (PARP) inhibitors.

RESULTS

We identified calmodulin-like protein 3 (CALML3) as a key sensor of this SNP and a coregulator of ERα, which contributes to differential gene transcription regulation in an estrogen and SERM-dependent fashion. Furthermore, using CRISPR/Cas9-engineered ZR75-1 breast cancer cells with different ZNF423 SNP genotypes, striking differences in cellular responses to SERMs and PARP inhibitors, alone or in combination, were observed not only in cells but also in a mouse xenograft model.

CONCLUSIONS

Our results have demonstrated the mechanism by which the ZNF423 rs9940645 SNP might regulate gene expression and drug response as well as its potential role in achieving more highly individualized breast cancer therapy.

Calmodulin Lobes Facilitate Dimerization and Activation of Estrogen Receptor-α

Estrogen receptor α (ER-α) is a nuclear hormone receptor that controls selected genes, thereby regulating proliferation and differentiation of target tissues, such as breast. Gene expression controlled by ER-α is modulated by Ca²⁺ via calmodulin (CaM). Here we present the NMR structure of Ca²⁺-CaM bound to two molecules of ER-α (residues 287-305). The two lobes of CaM bind to the same site on two separate ER-α molecules (residues 292, 296, 299, 302, and 303), which explains why CaM binds two molecules of ER-α in a 1:2 complex and stabilizes ER-α dimerization. Exposed glutamate residues in CaM (Glu-11, Glu-14, Glu-84, and Glu-87) form salt bridges with key lysine residues in ER-α (Lys-299, Lys-302, and Lys-303), which is likely to prevent ubiquitination at these sites and inhibit degradation of ER-α. Transfection of cells with full-length CaM slightly increased the ability of estrogen to enhance transcriptional activation by ER-α of endogenous estrogen-responsive genes. By contrast, expression of either the N- or C-lobe of CaM abrogated estrogen-stimulated transcription of the estrogen responsive genes pS2 and progesterone receptor. These data suggest that CaM-induced dimerization of ER-α is required for estrogen-stimulated transcriptional activation by the receptor. In light of the critical role of ER-α in breast carcinoma, our data suggest that small molecules that selectively disrupt the interaction of ER-α with CaM may be useful in the therapy of breast carcinoma.

Calcium-induced activation of estrogen receptor alpha--New insight

Calcium being an important modulator in multiple regulatory processes, we overviewed reported investigations concerning its potential influence on ERα transcriptional activity in breast cancer cells. Three main activating mechanisms depending on either intra- or extracellular calcium are described. At physiological intracellular concentration (μM), Ca(++) activates calmodulin and promotes its association with ERα; the resulting complex stably interacts with EREs at promoter sites, giving rise to enhanced transcription of estrogen target genes. Hypercalcemic concentrations (mM) produce a similar response through a direct association of the ion with the ligand binding domain of the receptor, this binding of calcium conferring an active conformation to ER. In contrast to these intracellular processes, very high extracellular concentrations of Ca(++) (>10mM) detected in bones at time of tumor metastasis operate via a signal transduction pathway initiated at the cell membrane through a specific activation of the calcium-sensing receptor.

Structural basis for Ca2+-induced activation and dimerization of estrogen receptor α by calmodulin

The estrogen receptor α (ER-α) regulates expression of target genes implicated in development, metabolism, and breast cancer. Calcium-dependent regulation of ER-α is critical for activating gene expression and is controlled by calmodulin (CaM). Here, we present the NMR structures for the two lobes of CaM each bound to a localized region of ER-α (residues 287-305). A model of the complete CaM·ER-α complex was constructed by combining these two structures with additional data. The two lobes of CaM both compete for binding at the same site on ER-α (residues 292, 296, 299, 302, and 303), which explains why full-length CaM binds two molecules of ER-α in a 1:2 complex and stabilizes ER-α dimerization. Exposed glutamate residues in CaM (Glu(11), Glu(14), Glu(84), and Glu(87)) form salt bridges with key lysine residues in ER-α (Lys(299), Lys(302), and Lys(303)), which are likely to prevent ubiquitination at these sites and inhibit degradation of ER-α. Mutants of ER-α at the CaM-binding site (W292A and K299A) weaken binding to CaM, and I298E/K299D disrupts estrogen-induced transcription. CaM facilitates dimerization of ER-α in the absence of estrogen, and stimulation of ER-α by either Ca(2+) and/or estrogen may serve to regulate transcription in a combinatorial fashion.

Profiling neuroendocrine gene expression changes following fadrozole-induced estrogen decline in the female goldfish

Teleost fish represent unique models to study the role of neuroestrogens because of the extremely high activity of brain aromatase (AroB; the product of cyp19a1b). Aromatase respectively converts androstenedione and testosterone to estrone and 17β-estradiol (E2). Specific inhibition of aromatase activity by fadrozole has been shown to impair estrogen production and influence neuroendocrine and reproductive functions in fish, amphibians, and rodents. However, very few studies have identified the global transcriptomic response to fadrozole-induced decline of estrogens in a physiological context. In our study, sexually mature prespawning female goldfish were exposed to fadrozole (50 μg/l) in March and April when goldfish have the highest AroB activity and maximal gonadal size. Fadrozole treatment significantly decreased serum E2 levels (4.7 times lower; P = 0.027) and depressed AroB mRNA expression threefold in both the telencephalon ( P = 0.021) and the hypothalamus ( P = 0.006). Microarray expression profiling of the telencephalon identified 98 differentially expressed genes after fadrozole treatment ( q value <0.05). Some of these genes have shown previously to be estrogen responsive in either fish or other species, including rat, mouse, and human. Gene ontology analysis together with functional annotations revealed several regulatory themes for physiological estrogen action in fish brain that include the regulation of calcium signaling pathway and autoregulation of estrogen receptor action. Real-time PCR verified microarray data for decreased (activin-βA) or increased (calmodulin, ornithine decarboxylase 1) mRNA expression. These data have implications for our understanding of estrogen actions in the adult vertebrate brain.

Calmodulin, a regulatory partner of the estrogen receptor alpha in breast cancer cells

Although calmodulin (CaM) interaction with estrogen receptor alpha (ERalpha) has been known for more than two decades, it is only recently that the molecular mechanism of CaM-mediated regulation of ERalpha is beginning to emerge. Others and we have identified a putative calmodulin binding site (P(295)LMIKRSKKNSLALSTADQMVS(317)) in ERalpha, at the boundary between the hinge and the ligand binding domain. ERalpha mutations affecting its association with CaM have been reported to generate high basal, estrogen-independent transactivation activity, indicating that the P(295)-T(317) sequence has an inhibitory function. Moreover, we found that a synthetic peptide (ERalpha17p: P(295)-T(311)) containing residues crucial for CaM binding exerts estrogenic effects on breast carcinoma cells. Finally, computer-aided conformational studies revealed that the CaM binding site might associate with a region located downstream in ERalpha (the beta turn/H4 region), this association likely resulting in an auto-inhibitory folding of the receptor. Thus, we propose as a hypothesis that CaM acts as a positive regulator by relieving this ERalpha auto-inhibition.

Regulatory function of the P295-T311 motif of the estrogen receptor alpha - does proteasomal degradation of the receptor induce emergence of peptides implicated in estrogenic responses?

The way in which estrogen receptor α (ERα) mediates gene transcription and hormone-dependent cancer cell proliferation is now being largely reconsidered in view of several recent discoveries. ERα-mediated transcription appears to be a cyclic and transient process where the proteasome - and thus receptor degradation - plays a pivotal role. In view of our recent investigations, which demonstrate the estrogenic activity of a synthetic peptide corresponding to a regulatory motif of the receptor (ERα17p), we propose that ERα proteasomal degradation could induce the emergence of regulatory peptide(s). The latter would function as a signal and contribute to the ERα activation process, amplifying the initial hormonal stimulation and giving rise to sustained estrogenic response.

Calmodulin-independent, agonistic properties of a peptide containing the calmodulin binding site of estrogen receptor alpha

Calmodulin (CaM) contributes to estrogen receptor alpha (ER)-mediated transcription. In order to study the underlying mechanisms, we synthesized a peptide including the CaM binding site: ERalpha17p (P(295)-T(311)). This peptide inhibited ER-CaM association, unlike two analogs in which two amino acids required for CaM binding were substituted. Exposure of MCF-7 cells to ERalpha17p down regulated ER, stimulated ER-dependent transcription and enhanced the proliferation of ER-positive breast cancer cell lines. Interestingly, ERalpha17p analogs unable to bind to CaM induced similar responses, demonstrating that ERalpha17p-mediated effects are mainly relevant to mechanisms independent of ER-CaM dissociation. The P(295)-T(311) motif is indeed a platform for multiple post-translational modifications not necessarily CaM-dependent. The additional finding that deletion of the P(295)-T(311) sequence in ER produced a constitutive transcriptional activity revealed that this platform motif has autorepressive functions. With regard to cell function, association of CaM to ER would counteract this autorepression, leading thereby to enhanced ER-mediated transactivation.

Functional interactions between calmodulin and estrogen receptor-α

The effects of estrogen in breast epithelial cells are mediated primarily via estrogen receptor alpha and estrogen receptor beta (ERalpha and ERbeta). Accumulating evidence implicates a role for Ca2+ and calmodulin in breast carcinoma, as well as in the function of ERalpha. Calmodulin contributes to the regulation of both ERalpha degradation and ERalpha-mediated transcriptional activation. Recent data will be summarized in this review. Models to explain the molecular mechanisms by which calmodulin modulates ERalpha function are proposed.

Calcium/calmodulin-dependent protein kinases as potential targets in cancer therapy

In this review the authors discuss the expression and activation of a family of protein kinases known as the calcium/calmodulin-dependent kinases (CaM-kinase) and the role that these kinases have in the activation of antiapoptotic signalling pathways. In addition, the authors outline a novel mechanism of activation of these kinases by oxidative stress. Founded on this novel mechanism of activation and the role that these kinases have in activating antiapoptotic signalling pathways, the authors propose that the CaM-kinases would make very good targets for sensitising cancer cells to certain therapeutic treatments. Furthermore, the authors discuss the role that these kinases have in cell transformation and in the regulation of the cell cycle. Based on these roles the authors suggest that inhibition of the CaM-kinases not only has the potential to sensitise cancer cells, but also has the potential to induce cytostasis in these cells.

Tuberin nuclear localization can be regulated by phosphorylation of its carboxyl terminus

Tuberin, the tuberous sclerosis 2 (TSC2) gene product, has been identified as a tumor suppressor protein genetically implicated in the pathology of tuberous sclerosis and the female-specific lung disease lymphangioleiomyomatosis. Tuberin and its predominant cytoplasmic binding partner hamartin have been shown to complex with a variety of intracellular signaling regulators and affect the processes of protein translation, cellular proliferation, cellular migration, and cellular transcription. In previous studies, we have presented evidence for tuberin binding to the calcium-dependent intracellular signaling protein calmodulin (CaM), overlap of tuberin CaM binding domain with a binding domain for estrogen receptor alpha, and the phosphorylation-associated nuclear localization of tuberin. In the study presented here, we expand our findings on the mechanism of tuberin nuclear localization to show that the CaM-estrogen receptor-alpha binding domain of tuberin can also serve as a tuberin nuclear localization sequence. Furthermore, we identify an Akt/p90 ribosomal S6 kinase-1 phosphorylation site within the carboxyl terminus of tuberin that can regulate tuberin nuclear localization and significantly affect the ability of tuberin to modulate estrogen genomic signaling events. These findings suggest a link between tuberin nuclear localization and a variety of intracellular signaling events that have direct implications with respect to the role of tuberin in the pathology of tuberous sclerosis and lymphangioleiomyomatosis.

Tissue levels of cadmium and trace elements in patients with myoma and uterine cancer

The aim of this study was to investigate the cadmium (Cd), copper (Cu), zinc (Zn), iron (Fe), magnesium (Mg) and calcium (Ca) concentrations in uterine cancer and uterine myoma. Tissue levels of six elements in 15 uterine cancers and 28 uterine myomas were measured by atomic absorption spectrometry. The samples were collected from women aged 32-79 (uterine myomas, uterine cancer and non-lesion uterine tissues from the same women). The results showed that the tissue Cd concentration was significantly lower in myoma than in non-lesion tissue. In uterine cancer, however, it was statistically significant, but only slightly lower than controls (the non-lesion uterine tissue). In the investigated tissues, the correlation between Cd concentration and age was found, but no effect of menopausal status or smoking habits on Cd level was detected. In uterine cancer tissue, a significant increase in Ca concentration and an insignificant increase in Mg level was observed when compared to normal uterine tissue. In uterine myoma, a significant increase of Mg and Mg/Ca ratio, as well as a decrease in Fe concentration were found. Statistical analysis showed no correlation between smoking habits, age, menopausal status and concentration of Cu, Zn, Fe, Mg and Ca trace metals in myoma or cancer tissue.

Calcium transport and signaling in the mammary gland: targets for breast cancer

The mammary gland is subjected to extensive calcium loads during lactation to support the requirements of milk calcium enrichment. Despite the indispensable nature of calcium homeostasis and signaling in regulating numerous biological functions, the mechanisms by which systemic calcium is transported into milk by the mammary gland are far from completely understood. Furthermore, the implications of calcium signaling in terms of regulating proliferation, differentiation and apoptosis in the breast are currently uncertain. Deregulation of calcium homeostasis and signaling is associated with mammary gland pathophysiology and as such, calcium transporters, channels and binding proteins represent potential drug targets for the treatment of breast cancer.

E6AP and calmodulin reciprocally regulate estrogen receptor stability

Estrogen promotes the proliferation of human breast epithelial cells by interacting with the estrogen receptor (ER). Physiological responses of cells to estrogen are regulated in part by degradation of the ER. Previous studies revealed that calmodulin binds directly to the ER, thereby enhancing its stability. Consistent with these findings, cell-permeable calmodulin antagonists dramatically reduced the number of ER in MCF-7 human breast epithelial cells. Here we investigated the molecular mechanism by which calmodulin attenuates ER degradation. MG132 and lactacystin, inhibitors of the ubiquitin-proteasome pathway, prevented the calmodulin antagonist CGS9343B from reducing the amount of ER in MCF-7 cells. In contrast, protease inhibitors afforded no protection. Moreover, CGS9343B enhanced ER ubiquitination. A point mutant ER construct that is unable to bind calmodulin, termed ERDeltaCaM, is ubiquitinated to a greater extent than wild type ER. The ubiquitin-protein isopeptide ligase E6-associated protein (E6AP) associated with and promoted the degradation of ER. The possible convergence of calmodulin and E6AP on ER degradation was examined. ERDeltaCaM bound E6AP with higher affinity than that of wild type ER. Moreover, calmodulin attenuated the in vitro interaction between ER and E6AP in a Ca(2+)-dependent manner. Collectively, our data reveal that E6AP is a component of ER degradation via the ubiquitin-proteasome pathway and that Ca(2+)/calmodulin modulates this degradation mechanism. These results have potential implications for the development of selectively targeted therapeutic agents for breast cancer.

Cross-talk between tuberin, calmodulin, and estrogen signaling pathways

Lymphangioleiomyomatosis (LAM) is a rare disease that occurs primarily in women and has been linked to both estrogen-mediated signaling events and mutations associated with the tuberous sclerosis complex 2 gene product tuberin. These two observations fostered the hypothesis that tuberin's impact on estrogen-mediated signaling might be through a direct interaction with the intracellular receptor for estrogen, estrogen receptor alpha (ERalpha). In the study presented here, tuberin was shown to co-immunoprecipitate and directly bind ERalpha through a domain localized within the carboxyl 73 amino acids of tuberin. This domain had previously been shown to serve as a binding domain for the intracellular calcium signaling molecule calmodulin (CaM). Competition binding studies identified a potential competitive relationship for binding of tuberin by ERalpha and CaM. Additionally, tuberin-ERalpha interactions were found to be modulated by the presence of tuberin's predominant intracellular binding partner hamartin, suggesting that tuberin-hamartin interactions negatively impact the ability of tuberin to modulate ERalpha-mediated gene transcription events. Cumulatively, data presented here support the hypothesis that interactions between tuberin, ERalpha, and CaM may play a critical role in the pathology of LAM disease.

The transcriptional activity of estrogen receptor-alpha is dependent on Ca2+/calmodulin

Estrogen binds to estrogen receptors in cells, thereby activating the receptors and eliciting biological effects. One of the best characterized effects of estrogen receptor-alpha (ERalpha) is transcriptional activation that regulates selected target genes in the nucleus. Work from several laboratories has documented a Ca2+-dependent interaction between ERalpha and calmodulin. In addition, we previously showed that antagonism of calmodulin function in cells prevented estradiol from inducing ERalpha transcriptional activity, suggesting that association of ERalpha with calmodulin participates in ERalpha function. In this study we adopted a multifaceted approach to directly address this hypothesis. The calmodulin binding domain on ERalpha was identified and several mutant ERalpha constructs unable to bind calmodulin were generated. Elimination of calmodulin binding prevented estradiol from stimulating ERalpha transcriptional activation. Essentially identical results were obtained when intracellular Ca2+ was chelated with the cell-permeable chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester (BAPTA-AM). Moreover, CaM1234, a calmodulin mutant that is unable to bind Ca2+, functioned as a dominant negative construct. Transfection of cells with CaM1234 reduced estradiol-stimulated ERalpha transcriptional activity. These data indicate that binding to calmodulin is required for normal transcriptional function of ERalpha.

Hormonal regulation of lactate dehydrogenase-A through activation of protein kinase C pathways in MCF-7 breast cancer cells

Lactate dehydrogenase A (LDH-A) is hormonally regulated in rodents, and increased expression of LDH-A is observed during mammary gland tumorigenesis. The mechanisms of hormonal regulation of LDH-A were investigated using a series of deletion and mutant constructs derived from the rat LDH-A gene promoter. Results of these studies show that constructs containing the -92 to -37 region of the LDH-A promoter are important for basal and E2-induced transactivation, and mutation of the consensus CRE motif within this region results in significant loss of basal activity and hormone-responsiveness. Gel mobility shift assays using nuclear extracts from MCF-7 cells show that both CREB and ATF-1 interact with the CRE. Studies with kinase inhibitors show that E2-induced activation of this CRE is dependent on protein kinase C, and these data indicate that LDH-A is induced through a non-genomic pathway of estrogen action.

Physical and functional interaction of androgen receptor with calmodulin in prostate cancer cells

Ca(2+) and calmodulin (CaM) play a critical role in proliferation and viability of a wide variety of cells, including prostate cancer cells. We examined two prostate cancer cell lines, androgen-sensitive LNCaP and androgen-independent PC-3. Proliferation of LNCaP cells was six to eight times more sensitive to the inhibitory effect of the CaM antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) than were PC-3 cells. Because LNCaP cell proliferation is sensitive to stimulation by androgen, we assessed the physical and functional interaction between androgen receptor (AR) and CaM. We observed tight binding of AR to CaM when LNCaP cell extracts were subjected to CaM-affinity column chromatography. AR binding to CaM was Ca(2+)-dependent and was inhibited by pretreatment of the cell extracts with W-7. Using immunofluorescence staining and confocal microscopy, we demonstrated colocalization of AR and CaM in the nucleus of LNCaP cells. Furthermore, the functional relevance of AR-CaM interactions in intact cells was revealed by the observation that W-7 was as effective as Casodex, an antiandrogen, in blocking AR-regulated expression of prostate-specific antigen in LNCaP cells. AR seems to interact with CaM directly because purified human AR could bind to CaM-agarose, and CaM could be detected in AR-immunoprecipitate prepared from purified soluble proteins. These studies provide direct evidence for physical and functional interaction between AR and CaM and suggest the potential usefulness of CaM antagonists in blocking AR activity in prostate cancer.

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.

Characterization of calmodulin binding to the orphan nuclear receptor Errgamma

The estrogen receptor-related receptor gamma (ERRgamma/ ERR3/NR3B3), a member of the nuclear receptor superfamily, activates transcription in the absence of ligands. In order to identify ligand-independent mechanisms of activation, we tested whether calmodulin (CaM), a key regulator of numerous cellular processes and a predominant intracellular receptor for Ca2+-signals, interacts with ERRgamma. In vitro pull-down experiments with calmodulin-Sepharose demonstrated a Ca2+-dependent interaction with cellularly expressed ERRgamma. As shown by truncation analysis, the CaM binding site is highly unusual in that it is composed of two discontinuous elements. Moreover, by surface plasmon resonance (SPR) biosensor technology, we detected a direct interaction of immobilized bacterially expressed ERR-gamma fusion protein with Ca2+-calmodulin. This is best described by a model which assumes a conformational change of the initially formed complex to a more stable form. Whereas in vitro DNA binding was calmodulin-independent, transient transfection analysis revealed a Ca2+-influx-dependent ERRgamma-mediated transcriptional activation of a luciferase reporter gene. Thus, we propose that CaM acts as a mediator in the Ca2+-dependent modulation of ERRgamma.

Calmodulin regulates the transcriptional activity of estrogen receptors. Selective inhibition of calmodulin function in subcellular compartments

The steroid hormone estrogen elicits biological effects in cells by binding to and activating the estrogen receptor (ER). Estrogen binding induces a conformational change in the receptor, inducing nuclear translocation and transcriptional activation of ER. The ubiquitous Ca(2+)-binding protein calmodulin has been shown to interact directly with ER and enhance its stability. To further elucidate the functional sequelae of the association between calmodulin and ER, we examined the effect on ER transcriptional activation of specifically inhibiting calmodulin. The cell-permeable calmodulin antagonist CGS9343B prevented estrogen-induced transcriptional activation by ER, without altering basal transcription. The inhibition was dose-dependent and independent of the time of estrogen stimulation. To validate these findings, calmodulin function was also neutralized by targeted expression of a specific inhibitor peptide. By inserting localization signals, the inhibitor peptide was selectively targeted to different subcellular domains. Inactivation of calmodulin function in the nucleus virtually eliminated estrogen-stimulated ER transcriptional activation. By contrast, when membrane calmodulin was specifically neutralized, estrogen-stimulated transcriptional activation by ER was only slightly attenuated. Importantly, the inhibitor peptides did not significantly reduce the amount of ER in the cells. Together, these data demonstrate that calmodulin is a fundamental component of ER transcriptional activation.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Estrogen regulation of c-fos gene expression through phosphatidylinositol-3-kinase-dependent activation of serum response factor in MCF-7 breast cancer cells

17Beta-estradiol (E2) induces proliferation and c-fos gene expression in MCF-7 cells and both responses are partially blocked by wortmannin and LY294002 which are inhibitors of phosphatidylinositol-3-kinase (PI3-K). Analysis of the c-fos gene promoter shows that the effects of wortmannin and LY294002 are associated with inhibition of E2-induced activation through the serum response factor (SRF) motif within the proximal serum response element at -325 and -296. E2 activates constructs containing multiple copies of the SRF (pSRF) and a GAL4-SRF fusion protein; these responses are accompanied by PI3-K-dependent phosphorylation of Akt and inhibited by wortmannin/LY294002, the antiestrogen ICI 182780, but not by the mitogen-activated protein kinase kinase (MAPKK) inhibitor PD98059. Using a series of kinase inhibitors and dominant negative kinase expression plasmids, it was shown that the non-genomic activation of SRF by E2 was associated with src-ras-PI3-K pathway, thus, demonstrating hormonal activation of the SRE through src-ras activation of both PI3-K- and MAPK-dependent signaling pathways.

Calmodulin is a selective modulator of estrogen receptors

In the search for differences between ERalpha and ERbeta, we analyzed the interaction of both receptors with calmodulin (CaM) and demonstrated that ERalpha but not ERbeta directly interacts with CaM. Using transiently transfected HeLa cells, we examined the effect of the CaM antagonist N-(6-aminohexyl)-5-chloro-naphthalene sulfonilamide hydrochloride (W7) on the transactivation properties of ERalpha and ERbeta in promoters containing either estrogen response elements or activator protein 1 elements. Transactivation by ERalpha was dose-dependently inhibited by W7, whereas that of ERbeta was not inhibited or even activated at low W7 concentrations. In agreement with these results, transactivation of an estrogen response element containing promoter in MCF-7 cells (which express a high ERalpha/ERbeta ratio) was also inhibited by W7. In contrast, transactivation in T47D cells (which express a low ERalpha/ERbeta ratio) was not affected by this CaM antagonist. The sensitivity of MCF-7 cells to W7 was abolished when cells were transfected with increasing amounts of ERbeta, indicating that the sensitivity to CaM antagonists of estrogen-responsive tissues correlates with a high ERalpha/ERbeta ratio. Finally, substitution of lysine residues 302 and 303 of ERalpha for glycine rendered a mutant ERalpha unable to interact with CaM whose transactivation activity became insensitive to W7. Our results indicate that CaM antagonists are selective modulators of ER able to inhibit ERalpha-mediated activity, whereas ERbeta actions were not affected or even potentiated by W7.

A calmodulin binding site in the tuberous sclerosis 2 gene product is essential for regulation of transcription events and is altered by mutations linked to tuberous sclerosis and lymphangioleiomyomatosis

Mutations in the tuberous sclerosis 2 (TSC2) gene product have been genetically linked to the pathology of both tuberous sclerosis (TSC) and the gender-specific lung disease, lymphangioleiomyomatosis (LAM). Both diseases are classified as disorders of cellular migration, proliferation, and differentiation. Earlier studies from our laboratory (1) linked TSC2 with steroid/nuclear receptor signaling. Studies presented here provide evidence for calmodulin (CaM) signaling in the propagation of this TSC2 activity. Far Western screening of a lambda phage human brain cDNA library to identify interacting proteins for the TSC2 gene product (tuberin) yielded multiple clones encoding human CaM. Direct binding with 32P-labeled tuberin demonstrated Ca2+-dependent binding to CaM-Sepharose which was lost upon deletion of the C-terminal 72 residues. The sequence (1740)WIARLRHIKRLRQRIC(1755) was identified as one capable of forming a basic amphipathic helix indicative of CaM binding domains in known calmodulin binding proteins. Studies with a synthetic peptide of this sequence demonstrated very tight Ca2+-dependent binding to CaM as judged by tryptophan fluorescence perturbation studies and phosphodiesterase activation by CaM. Deletion mutagenesis studies further suggested that this CaM binding domain is required for tuberin modulation of steroid receptor function and that mutations in this region may be involved in the pathology of TSC and LAM.

Evaluation of potential implication of membrane estrogen binding sites on ERE-dependent transcriptional activity and intracellular estrogen receptor-alpha regulation in MCF-7 breast cancer cells

The potential involvement of membrane estrogen binding sites in the induction of ERE-dependent transcriptional activity as well as in the regulation of intracellular estrogen receptor alpha (ER-alpha) level under estradiol (E2) stimulation was investigated. Our approach relied upon the use of two DCC-treated E2-BSA (bovine serum albumin) solutions (E2-6-BSA and E2-17-BSA). The absence of detectable free E2 in these solutions was established. Both E2-BSA conjugates led to a transient dose-dependent stimulation of the expression of ERE-luciferase (LUC) reporter gene in MVLN cells (MCF-7 cells stably transfected with a pVit-tk-LUC reporter plasmid), a property not recorded with free E2, which maintained enhanced transcriptional activity during the whole experiment. A very low concentration of E2 (10 pM) synergistically acted with E2-BSA conjugates. Hence, ERE-dependent transcriptional activity induced by these conjugates appeared to result from their known interactions with membrane estrogen binding sites. Anti-estrogens (AEs: 4-OH-TAM and RU 58,668), which antagonize genomic ER responses, abrogated the luciferase activity induced by E2-BSA conjugates, confirming a potential relationship between membrane-related signals and intracellular ER. Moreover, induction of luciferase was recorded when the cells were exposed to IBMX (3-isobutyl-1-methylxanthine) and cyclic nucleotides (cAMP/cGMP), suggesting the implication of the latter in the signal transduction pathway leading to the expression of the reporter gene. Growth factors (IGF-I, EGF and TGF-alpha) also slightly stimulated luciferase and synergistically acted with 10 pM E2, or 1 microM E2-BSA conjugates, in agreement with the concept of a cross-talk between steroids and peptides acting on the cell membrane. Remarkably, E2-BSA conjugates, IBMX and all investigated growth factors failed to down-regulate intracellular ER in MCF-7 cells, indicating the need for a direct intracellular interaction of the ligand with the receptor to regulate its level. ER elimination was, however, found in the presence of conditioned media (CMs) prepared from cells pre-exposed to E2-BSA conjugates, suggesting that they may produce (a) modulator(s) that may enhance receptor down-regulation when released within the medium.

Association of steroid receptor coactivator AIB1 with estrogen receptor-alpha in breast cancer cells

The steroid receptor coactivator AIB1 (amplified in breast cancer-1) is a transcriptional coactivator which has been found to be amplified in breast cancer. We have now investigated the role of the AIB1 protein in breast cancer cell lines. Although detectable levels of AIB1 were present in most cell lines, high levels of AIB1 expression were observed only in the ER-positive cell lines MCF-7 and BT-474 by western blot analysis. Newly developed monoclonal antibodies (mAbs) were used in several assays to show an association between AIBI and estrogen receptor-alpha (ER). AIB1 and ER co-localized to the nucleus of ER positive cell lines as shown by immunofluorescence microscopy, and a functional association of native AIB1 and ER in MCF-7 nuclear extracts was shown by EMSA. Recombinant ER also recruited AIB1 protein from nuclear extracts, shown by EMSA and by precipitation of ER-complex proteins bound to a biotinylated-ERE DNA target. Additionally, anti-AIB1 mAbs were able to immunoprecipitate ER from nuclear extracts of chemically cross-linked cells but not from uncross-linked cells. Both immunoprecipitation and oligonucleotide precipitation studies demonstrated the presence of p300 and CBP as part of the ER transcriptional complex. These results suggest that AIB1 and ER do associate physically in ER-positive breast cancer cell lines. We propose that in AIB1 amplified breast cancers, a heightened AIB1/ER association may play a crucial role in the progression of these tumors.

The nuclear factor kappa B (NF-kappa B): a potential therapeutic target for estrogen receptor negative breast cancers

The effect of a kinase inhibitor Go6796 on growth of epidermal growth factor (EGF)-stimulated estrogen receptor negative (ER-) breast cancer cells in vivo and role of nuclear factor kappa B (NF-kappaB) on tumorogenesis have been investigated. This was studied in an animal model by implanting ER- mouse mammary epithelial tumor cells (CSMLO) in syngeneic A-J mice. (i) Local administration of Go6976 an inhibitor of protein kinases C alpha and beta inhibited growth of tumors and caused extensive necrotic degeneration and regression of the tumors without causing any microscopically detectable damage to the vital organs liver and lung. (ii) Stable expression of dominant-negative mutants of the beta subunit (dnIkkbeta) of the inhibitory kappa B (IkappaB) kinase (dnIkk) that selectively blocked activation of NF-kappaB caused loss of tumorigenic potential of CSMLO cells. Stable expression of dnIkkbeta also blocked phorbol 12-myristate 13-acetate (PMA)-induced activation of NF-kappaB and overexpression of cyclin D1, concomitantly with the loss or reduced tumorigenic potential of these cells. Thus, results from in vivo and in vitro experiments strongly suggest the involvement of NF-kappaB in ER- mammary epithelial cell-mediated tumorigenesis. We propose that blocking NF-kappaB activation not only inhibits cell proliferation, but also antagonizes the antiapoptotic role of this transcription factor in ER- breast cancer cells. Thus, NF-kappaB is a potential target for therapy of EGFR family receptor-overexpressing ER- breast cancers.

Calmodulin enhances the stability of the estrogen receptor

The estrogen receptor mediates breast cell proliferation and is the principal target for chemotherapy of breast carcinoma. Previous studies have demonstrated that the estrogen receptor binds to calmodulin-Sepharose in vitro. However, the association of endogenous calmodulin with endogenous estrogen receptors in intact cells has not been reported, and the function of the interaction is obscure. Here we demonstrate by co-immunoprecipitation from MCF-7 human breast epithelial cells that endogenous estrogen receptors bind to endogenous calmodulin. Estradiol treatment of the cells had no significant effect on the interaction. However, incubation of the cells with tamoxifen enhanced by 5-10-fold the association of calmodulin with the estrogen receptor and increased the total cellular content of estrogen receptors by 1.5-2-fold. In contrast, the structurally distinct calmodulin antagonists trifluoperazine and CGS9343B attenuated the interaction between calmodulin and the estrogen receptor and dramatically reduced the number of estrogen receptors in the cell. Neither of these agents altered the amount of estrogen receptor mRNA, suggesting that calmodulin stabilizes the protein. This hypothesis is supported by the observation that, in the presence of Ca2+, calmodulin protected estrogen receptors from in vitro proteolysis by trypsin. Furthermore, overexpression of wild type calmodulin, but not a mutant calmodulin incapable of binding Ca2+, increased the concentration of estrogen receptors in MCF-7 cells, whereas transient expression of a calmodulin inhibitor peptide reduced the estrogen receptor concentration. These data demonstrate that calmodulin binds to the estrogen receptor in intact cells in a Ca2+-dependent, but estradiol-independent, manner, thereby modulating the stability and the steady state level of estrogen receptors.

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.

Epidermal growth factor-induced nuclear factor kappa B activation: A major pathway of cell-cycle progression in estrogen-receptor negative breast cancer cells

The epidermal growth factor (EGF) family of receptors (EGFR) is overproduced in estrogen receptor (ER) negative (-) breast cancer cells. An inverse correlation of the level of EGFR and ER is observed between ER- and ER positive (+) breast cancer cells. A comparative study with EGFR-overproducing ER- and low-level producing ER+ breast cancer cells suggests that EGF is a major growth-stimulating factor for ER- cells. An outline of the pathway for the EGF-induced enhanced proliferation of ER- human breast cancer cells is proposed. The transmission of mitogenic signal induced by EGF-EGFR interaction is mediated via activation of nuclear factor kappaB (NF-kappaB). The basal level of active NF-kappaB in ER- cells is elevated by EGF and inhibited by anti-EGFR antibody (EGFR-Ab), thus qualifying EGF as a NF-kappaB activation factor. NF-kappaB transactivates the cell-cycle regulatory protein, cyclin D1, which causes increased phosphorylation of retinoblastoma protein, more strongly in ER- cells. An inhibitor of phosphatidylinositol 3 kinase, Ly294-002, blocked this event, suggesting a role of the former in the activation of NF-kappaB by EGF. Go6976, a well-characterized NF-kappaB inhibitor, blocked EGF-induced NF-kappaB activation and up-regulation of cell-cycle regulatory proteins. This low molecular weight compound also caused apoptotic death, predominantly more in ER- cells. Thus Go6976 and similar NF-kappaB inhibitors are potentially novel low molecular weight therapeutic agents for treatment of ER- breast cancer patients.