Small molecule inhibition of the steroid receptor coactivators, SRC-3 and SRC-1

The AR/NCOA1 axis regulates prostate cancer migration by involvement of PRKD1

Due to the urgent need for new prostate cancer (PCa) therapies, the role of androgen receptor (AR)-interacting proteins should be investigated. In this study we aimed to address whether the AR coactivator nuclear receptor coactivator 1 (NCOA1) is involved in PCa progression. Therefore, we tested the effect of long-term NCOA1 knockdown on processes relevant to metastasis formation. [(3)H]-thymidine incorporation assays revealed a reduced proliferation rate in AR-positive MDA PCa 2b and LNCaP cells upon knockdown of NCOA1, whereas AR-negative PC3 cells were not affected. Furthermore, Boyden chamber assays showed a strong decrease in migration and invasion upon NCOA1 knockdown, independently of the cell line's AR status. In order to understand the mechanistic reasons for these changes, transcriptome analysis using cDNA microarrays was performed. Protein kinase D1 (PRKD1) was found to be prominently up-regulated by NCOA1 knockdown in MDA PCa 2b, but not in PC3 cells. Inhibition of PRKD1 reverted the reduced migratory potential caused by NCOA1 knockdown. Furthermore, PRKD1 was negatively regulated by AR. Immunohistochemical staining of PCa patient samples revealed a strong increase in NCOA1 expression in primary tumors compared with normal prostate tissue, while no final conclusion could be drawn for PRKD1 expression in tumor specimens. Thus, our findings directly associate the AR/NCOA1 complex with PRKD1 regulation and cellular migration and support the concept of therapeutic inhibition of NCOA1 in PCa.

Frequent amplification of AIB1, a critical oncogene modulating major signaling pathways, is associated with poor survival in gastric cancer

Amplified in breast cancer 1 (AIB1) is a member of p160 steroid receptor coactivator (SRC) family that mediates the transcriptional activities of nuclear receptors and other transcription factors. It acts as a major oncogene in diverse cancers, whereas biological function of AIB1 in gastric cancer remains largely unclear. This study was designed to explore the role of AIB1 in gastric tumorigenesis and its potential as a useful prognostic marker and therapeutic target in this cancer. Our data demonstrated that AIB1 was significantly up-regulated in gastric cancer tissues as compared with control subjects. Moreover, AIB1 amplification was found in 47 of 133 (35.3%) gastric cancer cases, but not in control subjects. AIB1 amplification was positively associated with its protein expression, and was significantly correlated with poor patient survival. AIB1 knockdown in gastric cancer cells dramatically inhibited cell proliferation, invasiveness and tumorigenic potential in nude mice, and induced cell cycle arrest and apoptosis. Mechanically, AIB1 promotes gastric cancer cell proliferation, survival and invasiveness through modulating major signaling pathways such as ErbB and Wnt/β-catenin pathways. Collectively, these findings suggest that AIB1 plays an important role in the pathogenesis of gastric cancer and represents a potential prognostic marker and therapeutic target for this cancer.

The Role of Steroid Receptor Coactivators in Hormone Dependent Cancers and Their Potential as Therapeutic Targets

Steroid receptor coactivator (SRC) family members (SRC-1, SRC-2, SRC-3) interact with nuclear receptors (NRs) and many transcription factors to enhance target gene transcription. Deregulation of SRCs is widely implicated in NR mediated diseases, especially hormone dependent cancers. By integrating steroid hormone signaling and growth factor pathways, SRC proteins exert multiple modes of oncogenic regulation in cancers and represent emerging targets for cancer therapeutics. Recent work has identified SRC-targeting agents that show promise in blocking tumor growth in vitro and in vivo, and have the potential to function as powerful and broadly encompassing treatments for different cancers.

Development of potent small-molecule inhibitors to drug the undruggable steroid receptor coactivator-3

Protein-protein interactions (PPIs) play a central role in most biological processes, and therefore represent an important class of targets for therapeutic development. However, disrupting PPIs using small-molecule inhibitors (SMIs) is challenging and often deemed as "undruggable." We developed a cell-based functional assay for high-throughput screening to identify SMIs for steroid receptor coactivator-3 (SRC-3 or AIB1), a large and mostly unstructured nuclear protein. Without any SRC-3 structural information, we identified SI-2 as a highly promising SMI for SRC-3. SI-2 meets all of the criteria of Lipinski's rule [Lipinski et al. (2001) Adv Drug Deliv Rev 46(1-3):3-26] for a drug-like molecule and has a half-life of 1 h in a pharmacokinetics study and a reasonable oral availability in mice. As a SRC-3 SMI, SI-2 can selectively reduce the transcriptional activities and the protein concentrations of SRC-3 in cells through direct physical interactions with SRC-3, and selectively induce breast cancer cell death with IC50 values in the low nanomolar range (3-20 nM), but not affect normal cell viability. Furthermore, SI-2 can significantly inhibit primary tumor growth and reduce SRC-3 protein levels in a breast cancer mouse model. In a toxicology study, SI-2 caused minimal acute cardiotoxicity based on a hERG channel blocking assay and an unappreciable chronic toxicity to major organs based on histological analyses. We believe that this work could significantly improve breast cancer treatment through the development of "first-in-class" drugs that target oncogenic coactivators.

Moving Beyond the Androgen Receptor (AR): Targeting AR-Interacting Proteins to Treat Prostate Cancer

Medical or surgical castration serves as the backbone of systemic therapy for advanced and metastatic prostate cancer, taking advantage of the importance of androgen signaling in this disease. Unfortunately, resistance to castration emerges almost universally. Despite the development and approval of new and more potent androgen synthesis inhibitors and androgen receptor (AR) antagonists, prostate cancers continue to develop resistance to these therapeutics, while often maintaining their dependence on the AR signaling axis. This highlights the need for innovative therapeutic approaches that aim to continue disrupting AR downstream signaling but are orthogonal to directly targeting the AR itself. In this review, we discuss the preclinical research that has been done, as well as clinical trials for prostate cancer, on inhibiting several important families of AR-interacting proteins, including chaperones (such as heat shock protein 90 (HSP90) and FKBP52), pioneer factors (including forkhead box protein A1 (FOXA1) and GATA-2), and AR transcriptional coregulators such as the p160 steroid receptor coactivators (SRCs) SRC-1, SRC-2, SRC-3, as well as lysine deacetylases (KDACs) and lysine acetyltransferases (KATs). Researching the effect of-and developing new therapeutic agents that target-the AR signaling axis is critical to advancing our understanding of prostate cancer biology, to continue to improve treatments for prostate cancer and for overcoming castration resistance.

Microfluidic Mobility Shift Profiling of Lysine Acetyltransferases Enables Screening and Mechanistic Analysis of Cellular Acetylation Inhibitors

Lysine acetyltransferases (KATs) are critical regulators of signaling in many diseases, including cancer. A major challenge in establishing the targetable functions of KATs in disease is a lack of well-characterized, cell-active KAT inhibitors. To confront this challenge, here we report a microfluidic mobility shift platform for the discovery and characterization of small molecule KAT inhibitors. Novel fluorescent peptide substrates were developed for four well-known KAT enzymes (p300, Crebbp, Morf, and Gcn5). Enzyme-catalyzed acetylation alters the electrophoretic mobility of these peptides in a microfluidic chip, allowing facile and direct monitoring of KAT activity. A pilot screen was used to demonstrate the utility of microfluidic mobility shift profiling to identify known and novel modulators of KAT activity. Real-time kinetic monitoring of KAT activity revealed that garcinol, a natural product KAT inhibitor used in cellular studies, exhibits time-dependent and detergent-sensitive inhibition, consistent with an aggregation-based mechanism. In contrast, the cell-permeable bisubstrate inhibitor Tat-CoA exhibited potent and time-independent KAT inhibition, highlighting its potential utility as a cellular inhibitor of KAT activity. These studies define microfluidic mobility shift profiling as a powerful platform for the discovery and characterization of small molecule inhibitors of KAT activity, and provide mechanistic insights potentially important for the application of KAT inhibitors in cellular contexts.

HER2 Signaling Drives DNA Anabolism and Proliferation through SRC-3 Phosphorylation and E2F1-Regulated Genes

Approximately 20% of early-stage breast cancers display amplification or overexpression of the ErbB2/HER2 oncogene, conferring poor prognosis and resistance to endocrine therapy. Targeting HER2(+) tumors with trastuzumab or the receptor tyrosine kinase (RTK) inhibitor lapatinib significantly improves survival, yet tumor resistance and progression of metastatic disease still develop over time. Although the mechanisms of cytosolic HER2 signaling are well studied, nuclear signaling components and gene regulatory networks that bestow therapeutic resistance and limitless proliferative potential are incompletely understood. Here, we use biochemical and bioinformatic approaches to identify effectors and targets of HER2 transcriptional signaling in human breast cancer. Phosphorylation and activity of the Steroid Receptor Coactivator-3 (SRC-3) is reduced upon HER2 inhibition, and recruitment of SRC-3 to regulatory elements of endogenous genes is impaired. Transcripts regulated by HER2 signaling are highly enriched with E2F1 binding sites and define a gene signature associated with proliferative breast tumor subtypes, cell-cycle progression, and DNA replication. We show that HER2 signaling promotes breast cancer cell proliferation through regulation of E2F1-driven DNA metabolism and replication genes together with phosphorylation and activity of the transcriptional coactivator SRC-3. Furthermore, our analyses identified a cyclin-dependent kinase (CDK) signaling node that, when targeted using the CDK4/6 inhibitor palbociclib, defines overlap and divergence of adjuvant pharmacologic targeting. Importantly, lapatinib and palbociclib strictly block de novo synthesis of DNA, mostly through disruption of E2F1 and its target genes. These results have implications for rational discovery of pharmacologic combinations in preclinical models of adjuvant treatment and therapeutic resistance.

Estrogen Receptor β Modulates Apoptosis Complexes and the Inflammasome to Drive the Pathogenesis of Endometriosis

Alterations in estrogen-mediated cellular signaling play an essential role in the pathogenesis of endometriosis. In addition to higher estrogen receptor (ER) β levels, enhanced ERβ activity was detected in endometriotic tissues, and the inhibition of enhanced ERβ activity by an ERβ-selective antagonist suppressed mouse ectopic lesion growth. Notably, gain of ERβ function stimulated the progression of endometriosis. As a mechanism to evade endogenous immune surveillance for cell survival, ERβ interacts with cellular apoptotic machinery in the cytoplasm to inhibit TNF-α-induced apoptosis. ERβ also interacts with components of the cytoplasmic inflammasome to increase interleukin-1β and thus enhance its cellular adhesion and proliferation properties. Furthermore, this gain of ERβ function enhances epithelial-mesenchymal transition signaling, thereby increasing the invasion activity of endometriotic tissues for establishment of ectopic lesions. Collectively, we reveal how endometrial tissue generated by retrograde menstruation can escape immune surveillance and develop into sustained ectopic lesions via gain of ERβ function.

Regulators of Androgen Action Resource: a one-stop shop for the comprehensive study of androgen receptor action

Androgen receptor (AR) is a ligand-activated transcription factor that is the main target for treatment of non-organ-confined prostate cancer (CaP). Failure of life-prolonging AR-targeting androgen deprivation therapy is due to flexibility in steroidogenic pathways that control intracrine androgen levels and variability in the AR transcriptional output. Androgen biosynthesis enzymes, androgen transporters and AR-associated coregulators are attractive novel CaP treatment targets. These proteins, however, are characterized by multiple transcript variants and isoforms, are subject to genomic alterations, and are differentially expressed among CaPs. Determining their therapeutic potential requires evaluation of extensive, diverse datasets that are dispersed over multiple databases, websites and literature reports. Mining and integrating these datasets are cumbersome, time-consuming tasks and provide only snapshots of relevant information. To overcome this impediment to effective, efficient study of AR and potential drug targets, we developed the Regulators of Androgen Action Resource (RAAR), a non-redundant, curated and user-friendly searchable web interface. RAAR centralizes information on gene function, clinical relevance, and resources for 55 genes that encode proteins involved in biosynthesis, metabolism and transport of androgens and for 274 AR-associated coregulator genes. Data in RAAR are organized in two levels: (i) Information pertaining to production of androgens is contained in a ‘pre-receptor level’ database, and coregulator gene information is provided in a ‘post-receptor level’ database, and (ii) an ‘other resources’ database contains links to additional databases that are complementary to and useful to pursue further the information provided in RAAR. For each of its 329 entries, RAAR provides access to more than 20 well-curated publicly available databases, and thus, access to thousands of data points. Hyperlinks provide direct access to gene-specific entries in the respective database(s). RAAR is a novel, freely available resource that provides fast, reliable and easy access to integrated information that is needed to develop alternative CaP therapies.

Database URL: http://www.lerner.ccf.org/cancerbio/heemers/RAAR/search/

Therapeutic Rationales, Progresses, Failures, and Future Directions for Advanced Prostate Cancer

Patients with localized prostate cancer (PCa) have several therapeutic options with good prognosis. However, survival of patients with high-risk, advanced PCa is significantly less than patients with early-stage, organ-confined disease. Testosterone and other androgens have been directly linked to PCa progression since 1941. In this review, we chronicle the discoveries that led to modern therapeutic strategies for PCa. Specifically highlighted is the biology of androgen receptor (AR), the nuclear receptor transcription factor largely responsible for androgen-stimulated and castrate-recurrent (CR) PCa. Current PCa treatment paradigms can be classified into three distinct but interrelated categories: targeting AR at pre-receptor, receptor, or post-receptor signaling. The continuing challenge of disease relapse as CR and/or metastatic tumors, destined to occur within three years of the initial treatment, is also discussed. We conclude that the success of PCa therapies in the future depends on targeting molecular mechanisms underlying tumor recurrence that still may affect AR at pre-receptor, receptor, and post-receptor levels.

Steroid receptor coactivators as therapeutic targets in the female reproductive system

The steroid receptor coactivators (SRCs/p160/NCOA) are a family of three transcriptional coregulators initially discovered to transactivate the transcriptional potency of steroid hormone receptors. Even though SRCs were also found to modulate the activity of multiple other transcription factors, their function is still strongly associated with regulation of steroid hormone action and many studies have found that they are critical for the regulation of reproductive biology. In the case of the female reproductive tract, SRCs have been found to play crucial roles in its physiology, ranging from ovulation, implantation, to parturition. Not surprisingly, SRCs' action has been linked to numerous abnormalities and debilitating disorders of female reproductive tissues, including infertility, cancer, and endometriosis. Many of these pathologies are still in critical need of therapeutic intervention and "proof-of-principle" studies have found that SRCs are excellent targets in pathological states. Therefore, small molecule modulators of SRCs' activity could be applied in the future in the treatment of many diseases of the female reproductive system.

Steroid Receptor Coactivator-3 (SRC-3/AIB1) as a Novel Therapeutic Target in Triple Negative Breast Cancer and Its Inhibition with a Phospho-Bufalin Prodrug

Triple negative breast cancer (TNBC) has the poorest prognosis of all types of breast cancer and currently lacks efficient targeted therapy. Chemotherapy is the traditional standard-of-care for TNBC, but is frequently accompanied by severe side effects. Despite the fact that high expression of steroid receptor coactivator 3 (SRC-3) is correlated with poor survival in estrogen receptor positive breast cancer patients, its role in TNBC has not been extensively investigated. Here, we show that high expression of SRC-3 correlates with both poor overall survival and post progression survival in TNBC patients, suggesting that SRC-3 can serve as a prognostic marker for TNBC. Furthermore, we demonstrated that bufalin, a SRC-3 small molecule inhibitor, when introduced even at nM concentrations, can significantly reduce TNBC cell viability and motility. However, because bufalin has minimal water solubility, its in vivo application is limited. Therefore, we developed a water soluble prodrug, 3-phospho-bufalin, to facilitate its in vivo administration. In addition, we demonstrated that 3-phospho-bufalin can effectively inhibit tumor growth in an orthotopic TNBC mouse model, suggesting its potential application as a targeted therapy for TNBC treatment.

Reprint of "Steroid receptor coactivators as therapeutic targets in the female reproductive system"

The steroid receptor coactivators (SRCs/p160/NCOA) are a family of three transcriptional coregulators initially discovered to transactivate the transcriptional potency of steroid hormone receptors. Even though SRCs were also found to modulate the activity of multiple other transcription factors, their function is still strongly associated with regulation of steroid hormone action and many studies have found that they are critical for the regulation of reproductive biology. In the case of the female reproductive tract, SRCs have been found to play crucial roles in its physiology, ranging from ovulation, implantation, to parturition. Not surprisingly, SRCs' action has been linked to numerous abnormalities and debilitating disorders of female reproductive tissues, including infertility, cancer, and endometriosis. Many of these pathologies are still in critical need of therapeutic intervention and "proof-of-principle" studies have found that SRCs are excellent targets in pathological states. Therefore, small molecule modulators of SRCs' activity could be applied in the future in the treatment of many diseases of the female reproductive system.

Characterization of a Steroid Receptor Coactivator Small Molecule Stimulator that Overstimulates Cancer Cells and Leads to Cell Stress and Death

By integrating growth pathways on which cancer cells rely, steroid receptor coactivators (SRC-1, SRC-2, and SRC-3) represent emerging targets in cancer therapeutics. High-throughput screening for SRC small molecule inhibitors (SMIs) uncovered MCB-613 as a potent SRC small molecule "stimulator" (SMS). We demonstrate that MCB-613 can super-stimulate SRCs' transcriptional activity. Further investigation revealed that MCB-613 increases SRCs' interactions with other coactivators and markedly induces ER stress coupled to the generation of reactive oxygen species (ROS). Because cancer cells overexpress SRCs and rely on them for growth, we show that we can exploit MCB-613 to selectively induce excessive stress in cancer cells. This suggests that over-stimulating the SRC oncogenic program can be an effective strategy to kill cancer cells.

Steroid Receptor Coactivator 1 Promotes Human Hepatocellular Carcinoma Progression by Enhancing Wnt/β-Catenin Signaling

Steroid receptor coactivator 1 (SRC-1) is a transcriptional coactivator not only for steroid receptors, such as androgen receptor and estrogen receptor, but also for other transcription factors. SRC-1 has been shown to play an important role in the progression of breast cancer and prostate cancer. However, its role in liver cancer progression remains unknown. In this study, we report that SRC-1 was overexpressed in 25 (62.5%) of 40 human hepatocellular carcinoma (HCC) specimens. Down-regulation of SRC-1 decreased HCC cell proliferation and impaired tumor maintenance in HCC xenografts. Knockdown of SRC-1 reduced protein levels of the proliferation marker proliferating cell nuclear antigen (PCNA) and the oncogene c-Myc. Knockout of SRC-1 in mice reduced diethylnitrosamine/CCl4-induced tumor formation in the liver and the expression of c-Myc and PCNA in liver tumors. SRC-1 promoted c-Myc expression, at least in part, by directly interacting with β-catenin to enhance Wnt/β-catenin signaling. Consistent with these results, the expression of SRC-1 was positively correlated with PCNA expression in human HCC specimens, and the expression levels of c-Myc in SRC-1-positive HCC specimens were higher than in SRC-1-negative HCC specimens. In addition, SRC-1 and SRC-3 were co-overexpressed in 47.5% of HCC specimens, and they cooperated to promote HCC cell proliferation. Simultaneous down-regulation of SRC-1 and SRC-3 dramatically inhibited HCC cell proliferation. Our results demonstrate that SRC-1 promotes HCC progression by enhancing Wnt/β-catenin signaling and suggest that SRC-1 is a potential therapeutic molecular target for HCC.

miR-137 Targets p160 Steroid Receptor Coactivators SRC1, SRC2, and SRC3 and Inhibits Cell Proliferation

The p160 family of steroid receptor coactivators (SRCs) are pleiotropic transcription factor coactivators and "master regulators" of gene expression that promote cancer cell proliferation, survival, metabolism, migration, invasion, and metastasis. Cancers with high p160 SRC expression exhibit poor clinical outcomes and resistance to therapy, highlighting the SRCs as critical oncogenic drivers and, thus, therapeutic targets. microRNAs are important epigenetic regulators of protein expression. To examine the regulation of p160 SRCs by microRNAs, we used and combined 4 prediction algorithms to identify microRNAs that could target SRC1, SRC2, and SRC3 expression. For validation of these predictions, we assessed p160 SRC protein expression and cell viability after transfection of corresponding microRNA mimetics in breast cancer, uveal melanoma, and prostate cancer (PC) cell lines. Transfection of selected microRNA mimetics into breast cancer, uveal melanoma, and PC cells depleted SRC protein expression levels and exerted potent antiproliferative activity in these cell types. In particular, microRNA-137 (miR-137) depleted expression of SRC1, SRC2, and very potently, SRC3. The latter effect can be attributed to the presence of 3 miR-137 recognition sequences within the SRC3 3'-untranslated region. Using reverse phase protein array analysis, we identified a network of proteins, in addition to SRC3, that were modulated by miR-137 in PC cells. We also found that miR-137 and its host gene are epigenetically silenced in human cancer specimens and cell lines. These results support the development and testing of microRNA-based therapies (in particular based on restoring miR-137 levels) for targeting the oncogenic family of p160 SRCs in cancer.

Tamoxifen inhibits ER-negative breast cancer cell invasion and metastasis by accelerating Twist1 degradation

Twist1 is a transcription factor driving epithelial-mesenchymal transition, invasion and metastasis of breast cancer cells. Mice with germ-line Twist1 knockout are embryonic lethal, while adult mice with inducible Twist1 knockout have no obvious health problems, suggesting that Twist1 is a viable therapeutic target for the inhibition of invasion and metastasis of breast cancer in adult patients. In this study, we expressed a luciferase protein or a Twist1-luciferase fusion protein in HeLa cells as part of a high throughput system to screen 1280 compounds in the Library of Pharmacologically Active Compounds (LOPAC) from Sigma-Aldrich for their effects on Twist1 protein expression. One of the most interesting compounds identified is tamoxifen, a selective estrogen receptor (ER) modulator used to treat ER-positive breast cancer. Tamoxifen treatment significantly accelerated Twist1 degradation in multiple cell lines including HEK293 human kidney cells, 4T1 and 168FARN mouse mammary tumor cells with either ectopically or endogenously expressed Twist1. Tamoxifen-induced Twist1 degradation could be blocked by the MG132 proteasome inhibitor, suggesting that tamoxifen induces Twist1 degradation through the ubiquitination-proteasome pathway. However, tamoxifen-induced Twist1 degradation was independent of Twist1 mRNA expression, estrogen signaling and MAPK-mediated Twist1 phosphorylation in these cells. Importantly, tamoxifen also significantly inhibited invasive behavior in Matrigel and lung metastasis in SCID-bg mice of ER-negative 4T1 mammary tumor cells, which depend on endogenous Twist1 to invade and metastasize. These results indicate that tamoxifen can significantly accelerate Twist1 degradation to suppress cancer cell invasion and metastasis, suggesting that tamoxifen can be used not only to treat ER-positive breast cancers but also to reduce Twist1-mediated invasion and metastasis in ER-negative breast cancers.

Androgen receptor signaling in prostate cancer

The androgen receptor (AR), ligand-induced transcription factor, is expressed in primary prostate cancer and in metastases. AR regulates multiple cellular events, proliferation, apoptosis, migration, invasion, and differentiation. Its expression in prostate cancer cells is regulated by steroid and peptide hormones. AR downregulation by various compounds which are contained in fruits and vegetables is considered a chemopreventive strategy for prostate cancer. There is a bidirectional interaction between the AR and micro-RNA (miRNA) in prostate cancer; androgens may upregulate or downregulate the selected miRNA, whereas the AR itself is a target of miRNA. AR mutations have been discovered in prostate cancer, and their incidence may increase with tumor progression. AR mutations and increased expression of selected coactivators contribute to the acquisition of agonistic properties of anti-androgens. Expression of some of the coactivators is enhanced during androgen ablation. AR activity is regulated by peptides such as cytokines or growth factors which reduce the concentration of androgen required for maximal stimulation of the receptor. In prostate cancer, variant ARs which exhibit constitutive activity were detected. Novel therapies which interfere with intracrine synthesis of androgens or inhibit nuclear translocation of the AR have been introduced in the clinic.

RXR agonist modulates TR: corepressor dissociation upon 9-cis retinoic acid treatment

Transcriptional regulation controlled by thyroid hormone receptor (TR) drives events such as development, differentiation, and metabolism. TRs may act either as homodimers or as heterodimers with retinoid X receptor (RXR). Thyroid hormone T3 preferentially binds TR-RXR heterodimers, which activate transcription through coactivator recruitment. However, it is unclear whether TR-RXR heterodimers may also be responsive to the canonical RXR agonist 9-cis retinoic acid (9C) in the context of physiological gene regulation. New structural studies suggest that 9C promotes the displacement of bound coactivators from the heterodimer, modifying TR-RXR activity. To shed light on the molecular mechanisms that control TR-RXR function, we used biophysical approaches to characterize coregulator recruitment to TR-TR or to TR-RXR in the presence of T3 and/or 9C as well as cell-based assays to establish the functional significance of biophysical findings. Using cell-based and fluorescence assays with mutant and wild-type TR, we show that 9C does indeed have a function in the TR-RXR heterodimer context, in which it induces the release of corepressors. Furthermore, we show that 9C does not promote detectable conformational changes in the structure of the TR-RXR heterodimer and does not affect coactivator recruitment. Finally, our data support the view that DNA binding domain and Hinge regions are important to set up NR-coactivator binding interfaces. In summary, we showed that the RXR agonist 9C can regulate TR function through its modulation of corepressor dissociation.

NCOA3-mediated upregulation of mucin expression via transcriptional and post-translational changes during the development of pancreatic cancer

Pancreatic cancer (PC) is characterized by aberrant overexpression of mucins that contribute to its pathogenesis. Although the inflammatory cytokines contribute to mucin overexpression, the mucin profile of PC is markedly distinct from that of normal or inflamed pancreas. We postulated that de novo expression of various mucins in PC involves chromatin modifications. Analysis of chromatin modifying enzymes by PCR array identified differential expression of NCOA3 in MUC4-expressing PC cell lines. Immunohistochemistry analysis in tumor tissues from patients and spontaneous mouse models, and microarray analysis following the knockdown of NCOA3 were performed to elucidate its role in mucin regulation and overall impact on PC. Silencing of NCOA3 in PC cell lines resulted in significant downregulation of two most differentially expressed mucins in PC, MUC4 and MUC1 (P<0.01). Immunohistochemistry analysis in PC tissues and metastatic lesions established an association between NCOA3 and mucin (MUC1 and MUC4) expression. Spontaneous mouse model of PC (K-ras(G12D); Pdx-1cre) showed early expression of Ncoa3 during pre-neoplastic lesions. Mechanistically, NCOA3 knockdown abrogated retinoic acid-mediated MUC4 upregulation by restricting MUC4 promoter accessibility as demonstrated by micrococcus nuclease digestion (P<0.05) and chromatin immuno-precipitation analysis. NCOA3 also created pro-inflammatory conditions by upregulating chemokines like CXCL1, 2, 5 and CCL20 (P<0.001). AKT, ubiquitin C, ERK1/2 and NF-κB occupied dominant nodes in the networks significantly modulated after NCOA3 silencing. In addition, NCOA3 stabilized mucins post translationally through fucosylation by FUT8, as the knockdown of FUT8 resulted in the downregulation of MUC4 and MUC1 at protein levels.

Amplified in breast cancer 1 promotes colorectal cancer progression through enhancing notch signaling

Aberrant activation of Notch signaling has an essential role in colorectal cancer (CRC) progression. Amplified in breast cancer 1 (AIB1), also known as steroid receptor coactivator 3 or NCOA3, is a transcriptional coactivator that promotes cancer cell proliferation and invasiveness. However, AIB1 implication in CRC progression through enhancing Notch signaling is unknown. In this study, we found that several CRC cell lines expressed high levels of AIB1, and knockdown of AIB1 decreased cell proliferation, colony formation and tumorigenesis of these CRC cells. Specifically, knockdown of AIB1 inhibited cell cycle progression at G1 phase by decreasing the mRNA levels of cyclin A2, cyclin B1, cyclin E2 and hairy and enhancer of split (Hes) 1. Furthermore, AIB1 interacted with Notch intracellular domain and Mastermind-like 1 and was recruited to the Hes1 promoter to enhance Notch signaling. Downregulation of AIB1 also decreased CRC cell invasiveness in vitro and lung metastasis in vivo. Besides that, knockout of AIB1 in mice inhibited colon carcinogenesis induced by azoxymethane/dextran sodium sulfate treatment. The mRNA levels of cyclin B1 and Hes5 were downregulated, but p27, ATOH1 and MUC2 were upregulated in the colon tumors from AIB1-deficient mice compared with those from wild-type mice. Thus, our results signify the importance of AIB1 in CRC and demonstrate that AIB1 promotes CRC progression at least in part through enhancing Notch signaling, suggesting that AIB1 is a potential molecular target for CRC treatment.

Steroid receptor coactivators: servants and masters for control of systems metabolism

Coregulator recruitment to nuclear receptors (NRs) and other transcription factors is essential for proper metabolic gene regulation, with coactivators enhancing and corepressors attenuating gene transcription. The steroid receptor coactivator (SRC) family is composed of three homologous members (SRC-1, SRC-2, and SRC-3), which are uniquely important for mediating steroid hormone and mitogenic actions. An accumulating body of work highlights the diverse array of metabolic functions regulated by the SRCs, including systemic metabolite homeostasis, inflammation, and energy regulation. We discuss here the cooperative and unique functions among the SRCs to provide a comprehensive atlas of systemic SRC metabolic regulation. Deciphering the fractional and synergistic contributions of the SRCs to metabolic homeostasis is crucial to understanding fully the networks underlying metabolic transcriptional regulation.

Transcriptional coregulators: fine-tuning metabolism

Metabolic homeostasis requires that cellular energy levels are adapted to environmental cues. This adaptation is largely regulated at the transcriptional level, through the interaction between transcription factors, coregulators, and the basal transcriptional machinery. Coregulators, which function as both metabolic sensors and transcriptional effectors, are ideally positioned to synchronize metabolic pathways to environmental stimuli. The balance between inhibitory actions of corepressors and stimulatory effects of coactivators enables the fine-tuning of metabolic processes. This tight regulation opens therapeutic opportunities to manage metabolic dysfunction by directing the activity of cofactors toward specific transcription factors, pathways, or cells/tissues, thereby restoring whole-body metabolic homeostasis.

Targeting the androgen receptor in prostate cancer

INTRODUCTION

The androgen receptor (AR) is a ligand-activated transcription factor that is expressed in primary and metastatic prostate cancers. There are advances in endocrine therapy for prostate cancer that are based on improved understanding of AR function.

AREAS COVERED

PubMed has been used to include most important publications on targeting the AR in prostate cancer. AR expression may be downregulated by agents used for chemoprevention of prostate cancer or, in models of advanced prostate cancer, by antisense oligonucleotides. New drugs that inhibit the steroidogenic enzyme CYP17A1 (abiraterone acetate) or diminish nuclear translocation of the AR (enzalutamide) have been shown to improve patients' survival in prostate cancer. However, it is clear that there is a development of resistance to these novel therapies. They may include increased expression of truncated, constitutively active AR or activation of the signaling pathway of signal transducers and activators of transcription.

EXPERT OPINION

Although introduction of novel drugs have improved patients' survival, there is a need to investigate the mechanisms of resistance further. The role of truncated AR and compensatory activation of signaling pathways as well as the development of scientifically justified combination therapies seems to be issues of a high priority.

BNIP3 regulates AT101 [(-)-gossypol] induced death in malignant peripheral nerve sheath tumor cells

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive Schwann cell-derived sarcomas and are the leading cause of mortality in patients with neurofibromatosis type 1 (NF1). Current treatment modalities have been largely ineffective, resulting in a high rate of MPNST recurrence and poor five-year patient survival. This necessitates the exploration of alternative chemotherapeutic options for MPNST patients. This study sought to assess the cytotoxic effect of the BH3-mimetic AT101 [(-)-gossypol] on MPNST cells in vitro and to identify key regulators of AT101-induced MPNST cell death. We found that AT101 caused caspase-independent, non-apoptotic MPNST cell death, which was accompanied by autophagy and was mediated through HIF-1α induced expression of the atypical BH3-only protein BNIP3. These effects were mediated by intracellular iron chelation, a previously unreported mechanism of AT101 cytotoxicity.

Identification of a novel potential antitumor activity of gossypol as an APE1/Ref-1 inhibitor

The human apurinic/apyrimidinic endonuclease 1/redox enhancing factor-1 (APE1/Ref-1), an essential multifunctional protein involved in the repair of oxidative deoxyribonucleic acid (DNA) damage and transcriptional regulation, is often overexpressed in tumor tissues and cancer cells. Moreover, APE1/Ref-1 (APE1) overexpression has been linked to chemoresistance in human tumors. Thus, inhibiting APE1 function in cancer cells is considered a promising strategy to overcome resistance to therapeutic agents. Gossypol is a Bcl-2 homology 3 (BH3)-mimetic agent and is able to bind to the BH3 domain of B-cell lymphoma 2 (Bcl-2) family members. Other studies demonstrated that Bcl-2 directly interacted with APE1 via its BH domains. Using apurinic/apyrimidinic (AP) endonuclease assays, we found that gossypol inhibits the repair activity of APE1. Electrophoretic mobility shift assays and dual luciferase assays showed that gossypol could also inhibit the redox function of APE1. Using dual polarization interferometry technology, we show that gossypol can directly interact with APE1. Furthermore, addition of gossypol, in conjunction with APE1 overexpression, leads to cancer cell death. The addition of gossypol also enhances the cell killing effect of the laboratory alkylating agent methyl methanesulfonate and the clinical agent cisplatin (DDP). Administration of gossypol significantly inhibited the growth of xenografts. Furthermore, the combined treatment of gossypol and DDP resulted in a statistically higher antitumor activity compared with DDP alone in vivo. In conclusion, we have demonstrated that gossypol effectively inhibits the repair and redox activity of APE1 through a direct interaction.

Immunohistochemical localization of steroid receptor coactivators in chondrosarcoma: an in vivo tissue microarray study

Chondrosarcoma is the second most common type of primary bone malignancy following up osteosarcoma, characterized by resistance to conventional chemotherapeutic agents and radiation regimens. The p160 family members steroid receptor coactivator-1 and -3 (SRC-1 and SRC-3) have been implied in the regulation of cancer growth, migration, invasion, metastasis and chemotherapeutic resistance; but we still lack detailed information about the levels of SRCs in chondrosarcoma. In this study, expression of SRC-1 and SRC-3 in chondrosarcoma was examined by immunohistochemistry with tissue microarrays; the four score system (0, 1, 2 and 3) was used to evaluate the staining. The results showed that there were no gender-, site- or age-differences regarding the expression of SRC-1 or SRC-3 (p>0.05); organ (bone or cartilage) -differences were only detected for SRC-1 but not SRC-3 (p<0.05). Significant higher levels of SRC-1 and SRC-3 were detected in MDC and PDC when compared to WDC. Our study clearly demonstrated differentiation-dependant expression of SRC-1 and SRC-3 in chondrosarcoma, may be novel targets for the prognosis and/or treatment of chondrosarcoma, would have opened a new avenue and established foundation for studying chondrosarcoma.

Increased radiosensitivity and radiation-induced apoptosis in SRC-3 knockout mice

Steroid receptor coactivator-3 (SRC-3), a multifunctional transcriptional coactivator, plays an important role in regulation of cell apoptosis in chemoresistant cancer cells. However, its role in radiation-induced apoptosis in hematopoietic cells is still unclear. In this study, we used SRC-3 knockout (SRC-3(-/-)) mice to assess the role of SRC-3 in radiation-induced hematopoietic injury in vivo. After a range of doses of irradiation, SRC-3(-/-) mice exhibited lower counts of peripheral blood cells and bone marrow (BM) mononuclear cells and excessive BM depression, which resulted in a significantly higher mortality compared with wildtype mice. Moreover, BM mononuclear cells obtained from SRC-3(-/-) mice showed a remarkable increase in radiation-induced apoptosis. Collectively, our data demonstrate that SRC-3 plays a role in radiation-induced apoptosis of BM hematopoietic cells. Regulation of SRC-3 might influence the radiosensitivity of hematopoietic cells, which highlights a potential therapeutic target for radiation-induced hematopoietic injury.

Identification of verrucarin a as a potent and selective steroid receptor coactivator-3 small molecule inhibitor

Members of the steroid receptor coactivator (SRC) family are overexpressed in numerous types of cancers. In particular, steroid receptor coactivator 3 (SRC-3) has been recognized as a critical coactivator associated with tumor initiation, progression, recurrence, metastasis, and chemoresistance where it interacts with multiple nuclear receptors and other transcription factors to enhance their transcriptional activities and facilitate cross-talk between pathways that stimulate cancer progression. Because of its central role as an integrator of growth signaling pathways, development of small molecule inhibitors (SMIs) against SRCs have the potential to simultaneously disrupt multiple signal transduction networks and transcription factors involved in tumor progression. Here, high-throughput screening was performed to identify compounds able to inhibit the intrinsic transcriptional activities of the three members of the SRC family. Verrucarin A was identified as a SMI that can selectively promote the degradation of the SRC-3 protein, while affecting SRC-1 and SRC-2 to a lesser extent and having no impact on CARM-1 and p300 protein levels. Verrucarin A was cytotoxic toward multiple types of cancer cells at low nanomolar concentrations, but not toward normal liver cells. Moreover, verrucarin A was able to inhibit expression of the SRC-3 target genes MMP2 and MMP13 and attenuated cancer cell migration. We found that verrucarin A effectively sensitized cancer cells to treatment with other anti-cancer drugs. Binding studies revealed that verrucarin A does not bind directly to SRC-3, suggesting that it inhibits SRC-3 through its interaction with an upstream effector. In conclusion, unlike other SRC SMIs characterized by our laboratory that directly bind to SRCs, verrucarin A is a potent and selective SMI that blocks SRC-3 function through an indirect mechanism.

The dynamics of nuclear receptors and nuclear receptor coregulators in the pathogenesis of endometriosis

BACKGROUND

Endometriosis is defined as the colonization and growth of endometrial tissue at anatomic sites outside the uterine cavity. Up to 15% of reproductive-aged women in the USA suffer from painful symptoms of endometriosis, such as infertility, pelvic pain, menstrual cycle abnormalities and increased risk of certain cancers. However, many of the current clinical treatments for endometriosis are not sufficiently effective and yield unacceptable side effects. There is clearly an urgent need to identify new molecular mechanisms that critically underpin the initiation and progression of endometriosis in order to develop more specific and effective therapeutics which lack the side effects of current therapies. The aim of this review is to discuss how nuclear receptors (NRs) and their coregulators promote the progression of endometriosis. Understanding the pathogenic molecular mechanisms for the genesis and maintenance of endometriosis as modulated by NRs and coregulators can reveal new therapeutic targets for alternative endometriosis treatments.

METHODS

This review was prepared using published gene expression microarray data sets obtained from patients with endometriosis and published literature on NRs and their coregulators that deal with endometriosis progression. Using the above observations, our current understanding of how NRs and NR coregulators are involved in the progression of endometriosis is summarized.

RESULTS

Aberrant levels of NRs and NR coregulators in ectopic endometriosis lesions are associated with the progression of endometriosis. As an example, endometriotic cell-specific alterations in gene expression are correlated with a differential methylation status of the genome compared with the normal endometrium. These differential epigenetic regulations can generate favorable cell-specific NR and coregulator milieus for endometriosis progression. Genetic alterations, such as single nucleotide polymorphisms and insertion/deletion polymorphisms of NR and coregulator genes, are frequently detected in ectopic lesions compared with the normal endometrium. These genetic variations impart new molecular properties to NRs and coregulators to increase their capacity to stimulate progression of endometriosis. Finally, post-translational modifications of NR coregulators, such as proteolytic processing, generate endometriosis-specific isoforms. Compared with the unmodified coregulators, these coregulator isoforms have unique functions that enhance the pathogenesis of endometriosis.

CONCLUSIONS

Epigenetic/genetic variations and posttranslational modifications of NRs and coregulators alter their original function so that they become potent 'drivers' of endometriosis progression.

Steroid receptor coactivator 1 is an integrator of glucose and NAD+/NADH homeostasis

Steroid receptor coactivator 1 (SRC-1) drives diverse gene expression programs necessary for the dynamic regulation of cancer metastasis, inflammation and gluconeogenesis, pointing to its overlapping roles as an oncoprotein and integrator of cell metabolic programs. Nutrient utilization has been intensely studied with regard to cellular adaptation in both cancer and noncancerous cells. Nonproliferating cells consume glucose through the citric acid cycle to generate NADH to fuel ATP generation via mitochondrial oxidative phosphorylation. In contrast, cancer cells undergo metabolic reprogramming to support rapid proliferation. To generate lipids, nucleotides, and proteins necessary for cell division, most tumors switch from oxidative phosphorylation to glycolysis, a phenomenon known as the Warburg Effect. Because SRC-1 is a key coactivator responsible for driving a hepatic gluconeogenic program under fasting conditions, we asked whether SRC-1 responds to alterations in nutrient availability to allow for adaptive metabolism. Here we show SRC-1 is stabilized by the 26S proteasome in the absence of glucose. RNA profiling was used to examine the effects of SRC-1 perturbation on gene expression in the absence or presence of glucose, revealing that SRC-1 affects the expression of complex I of the mitochondrial electron transport chain, a set of enzymes responsible for the conversion of NADH to NAD(+). NAD(+) and NADH were subsequently identified as metabolites that underlie SRC-1's response to glucose deprivation. Knockdown of SRC-1 in glycolytic cancer cells abrogated their ability to grow in the absence of glucose consistent with SRC-1's role in promoting cellular adaptation to reduced glucose availability.

Bufalin is a potent small-molecule inhibitor of the steroid receptor coactivators SRC-3 and SRC-1

Virtually all transcription factors partner with coactivators that recruit chromatin remodeling factors and interact with the basal transcription machinery. Coactivators have been implicated in cancer cell proliferation, invasion, and metastasis, including the p160 steroid receptor coactivator (SRC) family composed of SRC-1 (NCOA1), SRC-2 (TIF2/GRIP1/NCOA2), and SRC-3 (AIB1/ACTR/NCOA3). Given their broad involvement in many cancers, they represent candidate molecular targets for new chemotherapeutics. Here, we report on the results of a high-throughput screening effort that identified the cardiac glycoside bufalin as a potent small-molecule inhibitor for SRC-3 and SRC-1. Bufalin strongly promoted SRC-3 protein degradation and was able to block cancer cell growth at nanomolar concentrations. When incorporated into a nanoparticle delivery system, bufalin was able to reduce tumor growth in a mouse xenograft model of breast cancer. Our work identifies bufalin as a potentially broad-spectrum small-molecule inhibitor for cancer.

Steroid receptor coactivator-1: a versatile regulator and promising therapeutic target for breast cancer

Breast cancer is the leading cause of cancer death for women worldwide. Various therapeutic approaches have been proposed, among which endocrine therapy has recently become popular due to the high sensitivity of breast tissues to steroids such as estrogens and progesterone. The underlying mechanisms of steroid regulation in breast cancer cell proliferation, invasiveness, metastasis and endocrine resistance, however, remain largely unknown. Steroid receptor coactivator-1 (SRC-1) has attracted much attention because it is an important co-regulator and plays a pivotal role in modulating the transcriptional activities of steroid nuclear receptors. Accumulated research has established a strong correlation between SRC-1 and the pathological progression or disease-related features of breast cancer, which supports its potential as a target for specific therapeutic intervention in the clinical management of breast cancer. In addition, a diverse group of downstream molecules have also been shown to participate in various functional pathways related to SRC-1-associated regulation of breast cancer. These downstream molecules are also considered promising therapeutic targets, providing additional options for targeted treatments. In this review, the expression of SRC-1 in breast cancer and the close relationships between SRC-1 and the cell proliferation, invasiveness, metastasis and endocrine resistance of breast cancer will be discussed, followed by a brief summary of its putative functional mechanisms with an emphasis on the potential therapeutic role of SRC-1.

Nuclear receptor coactivators: master regulators of human health and disease

Transcriptional coregulators (coactivators and corepressors) have emerged as the principal modulators of the functions of nuclear receptors and other transcription factors. During the decade since the discovery of steroid receptor coactivator-1 (SRC-1), the first authentic coregulator, more than 400 coregulators have been identified and characterized, and deciphering their function has contributed significantly to our understanding of their role in human physiology. Deregulated expression of coregulators has been implicated in diverse disease states and related pathologies. The advancement of molecular technologies has enabled us to better characterize the molecular associations of the SRC family of coactivators with other protein complexes in the context of gene regulation. These continuing discoveries not only expand our knowledge of the roles of coactivators in various human diseases but allow us to discover novel coactivator-targeting strategies for therapeutic intervention in these diseases.

3D structures and ligand specificities of nuclear xenobiotic receptors CAR, PXR and VDR

The nuclear receptors constitutive androstane receptor (CAR), pregnane X receptor (PXR) and vitamin D receptor (VDR) control a large array of genes that code for important proteins in humans including metabolic enzymes and transporters. 3D structures for the ligand-binding domain (LBD) of these receptors are abundantly available, providing valuable insights into the ligand-binding specificity as well as the activation mechanisms. The ligand-binding site of PXR is large and flexible, whereas those of CAR and VDR are compact and rigid, respectively. In general, the ligand profiles of the receptors are in agreement with the LBD structures. The crystal structures have greatly helped us to understand the promiscuity and/or specificity of CAR, PXR and VDR.

Beyond the ligand-binding pocket: targeting alternate sites in nuclear receptors

Nuclear receptors (NRs) are a family of ligand-modulated transcription factors with significant therapeutic relevance from metabolic disorders and inflammation to cancer, neurodegenerative, and psychiatric disorders. Drug discovery efforts are typically concentrated on modulating the natural ligand action within the ligand-binding pocket (LBP) in the C-terminal ligand-binding domain (LBD). Drawbacks of LBP-based strategies include physiological alterations due to disruption of ligand binding and difficulties in achieving tissue specificity. Furthermore, the lack of a "pure" and predictable mechanism of action predisposes such intervention toward drug resistance. Recent outstanding progress in our understanding of NR biology has shifted the focus of drug discovery efforts from inside to outside the LBP, affording consideration to the interaction between NRs and coactivator proteins, the interaction between NRs and DNA and the NRs' ligand-independent functions. This review encompasses such currently available NR non-LBP-based interventions and their potential application in therapy or as specific tools to probe NR biology.

Noncanonical mechanisms to regulate nuclear receptor signaling

Nuclear receptor (NR)-targeted therapies comprise a large class of clinically employed drugs. A number of drugs currently being used against this protein class were designed as structural analogs of the endogenous ligand of these receptors. In recent years, there has been significant interest in developing newer strategies to target NRs, especially those that rely on mechanistic pathways of NR function. Prominent among these are noncanonical means of targeting NRs, which include selective NR modulation, NR coactivator interaction inhibition, inhibition of NR DNA binding, modulation of NR cellular localization, modulation of NR ligand biosynthesis and downregulation of NR levels in target tissues. This article reviews each of these promising emerging strategies for NR drug development and highlights some of most significant successes achieved in using them.

Long-range transcriptional regulation of breast cancer genes

Breast cancer is a major health problem and understanding the genetic basis of this disease is crucial for predicting risk and developing effective targeted therapeutics. Several breast cancer predisposing genes have been identified, but mutations in the coding regions of these genes only accounts for a small proportion of risk. Research now suggests that combinations of multiple non-coding changes in breast cancer susceptibility genes, which cause moderate alterations in gene expression, will be responsible for the remaining inherited risk. These non-coding changes will include variants in proximal and distal transcriptional and post-transcriptional regulatory elements and may affect the levels and function of trans-acting factors, including proteins and RNAs, which act on these elements. Somatic changes in such elements and factors have also been associated with breast cancer progression. With the recent advent of techniques allowing the detection of long-range DNA interactions spanning the human genome, it has become increasingly clear that long-range regulatory elements constitute an important mechanism for gene regulation. Recent studies have identified several such elements that are important for regulating genes involved in breast cancer, raising the possibility that defects in these sequences may contribute to breast cancer predisposition and progression. In this review, we discuss the emerging functions of cis-regulatory elements and a subset of trans-acting factors in breast tumorigenesis. We also discuss some recent progress in our understanding of how dysregulation in these transcriptional components may contribute to breast cancer, and the potential implications for molecular diagnosis, prognosis prediction, and the treatment of this disease.

Estradiol receptors in breast cancer cells: associated co-factors as targets for new therapeutic approaches

Estrogen receptors α (ERα) and β (ERβ) are nuclear receptors which transduce estradiol (E2) response in many tissues including the mammary gland and breast cancers (BC). They activate or inhibit specific genes involved in cell cycle progression and cell survival through multiple enzyme activities leading to malignant transformation. Hormone therapy (antiestrogens (AEs) and aromatase inhibitors (AIs) have been widely used to block the mitogenic action of E2 in patients with ER-positive BC. ERs act in concert with numerous other proteins outside and inside the nucleus where co-activators such as histone modifying enzymes help reaching optimum gene activation. Moreover, E2-mediated gene regulation can occur through ERs located at the plasma membrane or G protein-coupled estrogen receptor (GPER), triggering protein kinase signaling cascades. Classical AEs as well as AIs are inefficient to block the cascades of events emanating from the membrane and from E2 binding to GPER, leading patients to escape anti-hormone treatments and hormone therapy resistance. Many pathways are involved in resistance, mostly resulting from over-expression of growth factor membrane receptors, in particular the HER2/ErbB2 which can be inhibited by specific antibodies or tyrosine kinases inhibitors. Together with the Hsp90 molecular chaperone machinery, a complex interplay between ERs, co-activators, co-repressors and growth factor-activated membrane pathways represents potent targets which warrant to be manipulated alone and in combination to designing novel therapies. The discovery of new potential targets arising from micro array studies gives the opportunity to activate or inhibit different new ER-modulating effectors for innovative therapeutic interventions.

Nuclear receptor coregulators: modulators of pathology and therapeutic targets

The nuclear receptor superfamily includes transcription factors that transduce steroid, thyroid and retinoid hormones and other ligands in conjunction with coregulators. To date, over 350 coregulators have been reported in the literature, and advances in proteomic analyses of coregulator protein complexes have revealed that a far greater number of coregulator-interacting proteins also exist. Coregulator dysfunction has been implicated in diverse pathological states, genetic syndromes and cancer. A hallmark of disease related to the disruption of normal coregulator function is the pleiotropic effect on animal physiology, which is frequently manifested as the dysregulation of metabolic and neurological systems. Coregulators have broad physiological and pathological functions that make them promising new drug targets for diseases such as hormone-dependent cancers. Advances in proteomics, genomics and transcriptomics have provided novel insights into the biology of coregulators at a system-wide level and will lead the way to a new understanding of how coregulators can be evaluated in the context of complex and multifaceted genetic factors, hormones, diet, the environment and stress. Ultimately, better knowledge of the associations that exist between coregulator function and human diseases is expected to expand the indications for the use of future coregulator-targeted drugs.

Steroid receptor coactivator-3 as a potential molecular target for cancer therapy

INTRODUCTION

Steroid receptor coactivator-3 (SRC-3), also called amplified-in-breast cancer-1 (AIB1), is an oncogenic coactivator in endocrine and non-endocrine cancers. Functional studies demonstrate SRC-3 promotes numerous aspects of cancer, through its capacity as a coactivator for nuclear hormone receptors and other transcription factors, and via its ability to control multiple growth pathways simultaneously. Targeting SRC-3 with specific inhibitors therefore holds future promise for clinical cancer therapy.

AREAS COVERED

We discuss critical advances in understanding SRC-3 as a cancer mediator and prospective drug target. We review SRC-3 structure and function and its role in distinct aspects of cancer. In addition, we discuss SRC-3 regulation and degradation. Finally, we comment on a recently discovered SRC-3 small molecular inhibitor.

EXPERT OPINION

Most targeted chemotherapeutic drugs block only a single cellular pathway. In response, cancers frequently acquire resistance by upregulating alternative pathways. SRC-3 coordinates multiple signaling networks, suggesting SRC-3 inhibition offers a promising therapeutic strategy. Development of an effective SRC-3 inhibitor faces critical challenges. Better understanding of SRC-3 function and interacting partners, in both the nucleus and cytosol, is required for optimized inhibitor development. Ultimately, blockade of SRC-3 oncogenic function may inhibit multiple cancer-related signaling pathways.

The function of steroid receptor coactivator-1 in normal tissues and cancer

In 1995, the steroid receptor coactivator-1 (SRC-1) was identified as the first authentic steroid receptor coactivator. Since then, the SRC proteins have remained at the epicenter of coregulator biology, molecular endocrinology and endocrine-related cancer. Cumulative works on SRC-1 have shown that it is primarily a nuclear receptor coregulator and functions to construct highly specific enzymatic protein complexes which can execute efficient and successful transcriptional activation of designated target genes. The versatile nature of SRC-1 enables it to respond to steroid dependent and steroid independent stimulation, allowing it to bind across many families of transcription factors to orchestrate and regulate complex physiological reactions. This review highlights the multiple functions of SRC-1 in the development and maintenance of normal tissue functions as well as its major role in mediating hormone receptor responsiveness. Insights from genetically manipulated mouse models and clinical data suggest SRC-1 is significantly overexpressed in many cancers, in particular, cancers of the reproductive tissues. SRC-1 has been associated with cellular proliferation and tumor growth but its major tumorigenic contributions are promotion and execution of breast cancer metastasis and mediation of resistance to endocrine therapies. The ability of SRC-1 to coordinate multiple signaling pathways makes it an important player in tumor cells' escape of targeted therapy.