Estrogen-related receptor alpha (ERRalpha) inverse agonist XCT-790 induces cell death in chemotherapeutic resistant cancer cells

Nuclear receptors in ovarian cancer: changing paradigms in cancer therapeutics

Ovarian cancer (OVC) is one of the most common causes of cancer-related deaths in women worldwide. Despite advancements in detection and therapy, the prognosis of OVC remains poor due to late diagnosis and the lack of effective therapeutic options at advanced stages. Therefore, a better understanding of the biology underlying OVC is essential for the development of effective strategies for early detection and targeted therapies. Nuclear receptors (NRs) are a superfamily of 48 transcription factors that, upon binding to their specific ligand, play a vital role in regulating various cellular processes such as growth, development, metabolism, and homeostasis. Accumulating evidence from several studies has shown that their aberrant expression is associated with multiple human diseases. Numerous NRs have shown significant effects in the development of various cancers, including OVC. This review summarizes the recent findings on the role of NRs in OVC, as well as their potential as prognostic and therapeutic markers. Further, the basic structure and signaling mechanism of NRs have also been discussed briefly. Moreover, this review highlights their cellular and molecular mechanisms in chemoresistance and chemosensitization. Further, the clinical trials targeting NRs for the treatment of OVC have also been discussed.

Selenadiazole Inhibited Adenovirus-Induced Apoptosis through the Oxidative-Damage-Mediated Bcl-2/Stat 3/NF-κB Signaling Pathway

Human adenovirus type 7 (HAdV7) infection causes severe pneumonia, yet there are still no breakthroughs in treatment options for adenovirus, and the road to antiviral drug development faces major challenges. We attempted to find new drugs and we stumbled upon one: selenadiazole. Selenadiazole has been shown to have significant anti-tumor effects due to its unique chemical structure and drug activity. However, its effectiveness against viruses has not been evaluated yet. In our study, selenadiazole also showed superior antiviral activity. In vitro experiments, selenadiazole was able to inhibit adenovirus-mediated mitochondrial-oxidative-damage-related apoptosis, and in in vivo experiments, selenadiazole was able to inhibit apoptosis by modulating the apoptotic signaling pathway Bcl-2/Stat3/NF-κB, etc., and was able to largely attenuate adenovirus-infection-induced pneumonia and lung injury in mice. This study aims to describe a new antiviral treatment option from the perspective of anti-adenovirus-mediated oxidative stress and its associated apoptosis and to provide theoretical guidance for the treatment of clinical adenovirus infection to a certain extent.

ERRα contributes to HDAC6-induced chemoresistance of osteosarcoma cells

Chemotherapy resistance is an important problem for clinical therapy of osteosarcoma (OS). The potential effects of histone deacetylases (HDACs) on OS chemoresistance are studied. The expression of HDACs in OS cells resistance to doxorubicin (Dox) and cisplatin (CDDP) is checked. Among 11 members of HDACs, levels of HDAC6 are significantly upregulated in OS cells resistance to Dox and CDDP. Inhibition of HDAC6 via its specific inhibitor ACY1215 restores chemosensitivity of OS-resistant cells. Further, HDAC6 directly binds with estrogen-related receptors alpha (ERRα) to regulate its acetylation and protein stability. Inhibition of ERRα further strengthens ACY1215-increased chemosensitivity of OS-resistant cells. Mechanistically, K129 acetylation is the key residue for HDAC6-regulated protein levels of ERRα. Collectively, we find that ERRα contributes to HDAC6-induced chemoresistance of OS cells. Inhibition of HDAC6/ERRα axis might be a potential approach to overcome chemoresistance and improve therapy efficiency for OS treatment. 1. HDAC6 was significantly upregulated in Dox and CDDP resistant OS cells; 2. Inhibition of HDAC6 can restore chemosensitivity of OS cells; 3. HDAC6 binds with ERRα at K129 to decrease its acetylation and increase protein stability; 4. ERRα contributes to HDAC6-induced chemoresistance of OS cells.

The freezability of Mediterranean buffalo sperm is associated with lysine succinylation and lipid metabolism

Semen cryopreservation is used for the propagation of variety among species and domestic breeding. Mitochondria are implicated in sperm freezability, and their proteins are prone to succinylation, but the relationship between sperm freezability and mitochondrial protein succinylation is unclear. In this study, six bulls were classified as having good or poor freezability ejaculates (GFE or PFE, each 3 bulls). The fresh sperm mitochondrial membrane potential (MMP) and pan succinylation level of the two groups were first detected. Then the lysine succinylome and fatty acid content of the two groups were analyzed using label-free LC-MS/MS and GC-MS/MS in multiple reaction monitoring (MRM) modes, respectively. The results indicated that the GFE sperm had significantly higher MMPs than the PFE group (p < 0.05). A total of 1393 succinylation sites corresponding to 426 proteins were assessed and 5 succinylated peptides of the GFE group were markedly upregulated, while 3 were significantly downregulated (FC > 2.0 - < 0.5 and p-value < 0.05) when compared to the PFE group. Forty-six succinylated proteins were identified to have consistent presence/absence expression. The upregulated succinylated proteins in the GFE sperm were enriched in lipid metabolic processes. A total of 31 fatty acids were further subjected to quantitative analysis of which 23 including arachidic (C20:0), linolenic (C18:3n3), and docosahexaenoic acids (C22:6n3) were decreased in GFE sperm when compared with PFE (p < 0.05). These results suggest that lysine succinylation can potentially influence the sperm freezability of Mediterranean buffaloes through mitochondrial lipid metabolism. This novel study provides our understanding of sperm succinylation and the molecular basis for the mechanism of sperm freezability.

Glaucocalyxin A induces apoptosis of NSCLC cells by inhibiting the PI3K/Akt/GSK3β pathway

Lung cancer is one of the fastest growing malignancies in morbidity and mortality, and current therapies are in general not sufficiently effective for this deadly disease. This study characterizes the anti-cancer effects of Glaucocalyxin A (GLA) and explores the underlying mechanisms using human non-small cell lung carcinoma (NSCLC) cells. First, our data showed that GLA suppressed the viability of cancer cells, while no effect was observed in the normal bronchial epithelial cell Bease 2B cells. Second, GLA inhibited colony formation, induced apoptosis of cancer cells. Third, GLA down-regulated the expression of B-cell lymphoma-2 (Bcl-2) protein, up-regulated the expression of Bcl2-associated X protein (Bax) , and strengthened cleavage of Caspase-3 and poly ADP-ribose polymerase (PARP). Fourth, GLA also diminished mitochondrial membrane potential and inhibited phosphatidylinositol 3-kinase (PI3K)/Akt/ glycogen synthase kinase-3β (GSK3β) pathway. In addition, injection of GLA (20 mg/kg) every two days significantly inhibited A549 xenograft tumor growth, accompanied by increased apoptosis and decreased proliferation. Together, our study provides evidence that the anticancer effect of GLA in NSCLC is mediated by inducing apoptosis through inhibiting PI3K/Akt/GSK3β pathway and suggests that GLA may be used as a promising natural medicine for NSCLC therapy. This article is protected by copyright. All rights reserved.

Estrogen-Related Receptors Gene Expression and Copy Number Alteration Association With the Clinicopathologic Characteristics of Breast Cancer

Purpose:

It has been suggested that dysregulation of transcription factors expression or activity plays significant roles in breast cancer (BC) severity and poor prognosis. Therefore, our study aims to thoroughly evaluate the estrogen-related receptor isoforms (ESRRs) expression and copy number alteration (CNA) status and their association with clinicopathologic characteristics in BC.

Methods:

A METABRIC dataset consist of 2509 BC patients’ samples was obtained from the cBioPortal public domain. The gene expression, putative CNA, and relevant tumor information of ESRRs were retrieved. ESRRs messenger RNA (mRNA) expression in BC cell lines was obtained from the Cancer Cell Line Encyclopedia (CCLE). Association and correlation analysis of ESRRs expression with BC clinicopathologic characteristics and molecular subtype were performed. Kaplan–Meier survival analysis was conducted to evaluate the prognostic value of ESRRs expression on patient survival.

Results:

ESRRα expression correlated negatively with patients’ age and overall survival, whereas positively correlated with tumor size, the number of positive lymph nodes, and Nottingham prognostic index (NPI). Conversely, ESRRγ expression was positively correlated with patients’ age and negatively correlated with NPI. ESRRα and ESRRγ expression were significantly associated with tumor grade, expression of hormone receptors, human epidermal growth factor receptor 2 (HER2), and molecular subtype, whereas ESRRβ was only associated with tumor stage. A significant and distinct association of each of ESRRs CNA with various clinicopathologic and prognostic factors was also observed. Kaplan–Meier survival analysis demonstrated no significant difference for survival curves among BC patients with high or low expression of ESRRα, β, or γ. On stratification, high ESRRα expression significantly reduced survival among premenopausal patients, patients with grade I/II, and early-stage disease. In BC cell lines, only ESRRα expression was significantly higher in HER2-positive cells. No significant association was observed between ESRRβ expression and any of the clinicopathologic characteristics examined.

Conclusions:

In this clinical dataset, ESRRα and ESRRγ mRNA expression and CNA show a significant correlation and association with distinct clinicopathologic and prognostic parameters known to influence treatment outcomes; however, ESRRβ failed to show a robust role in BC pathogenesis. ESRRα and ESRRγ can be employed as therapeutic targets in BC-targeted therapy. However, the role of ESRRβ in BC pathogenesis remains unclear.

miR-137/ERRα axis mediates chemoresistance of nasopharyngeal carcinoma cells

Nasopharyngeal carcinoma (NPC) is the most common malignant tumor of the head and neck region and is characterized by an increased risk of developing chemoresistance after treatment. The present study demonstrated that estrogen-related receptor α (ERRα) was upregulated in cisplatin- and fluorouracil-resistant NPC cells. In addition, ERRα knockdown or treatment of cells with the ERRα inverse agonist XCT-790 attenuated the chemoresistance of NPC cells. Mechanistically, the increased expression of ERRα in chemoresistant cells was associated with enhanced mRNA stability. Bioinformatics analysis for screening microRNAs (miRs) regulating the expression of ERRα revealed that miR-137 was downregulated in chemoresistant NPC cells. Additionally, transfection of cells with miR-137 mimics reduced ERRα mRNA stability and increased the chemosensitivity of NPC cells. Furthermore, ERRα knockdown reduced glucose consumption, and lactate and ATP production rates in chemoresistant cells. The aforementioned findings suggested that the miR-137/ERRα-mediated metabolic programming could be involved in the chemoresistance of NPC cells.

Study on the Reparative Effect of PEGylated Growth Hormone on Ovarian Parameters and Mitochondrial Function of Oocytes From Rats With Premature Ovarian Insufficiency

Premature ovarian insufficiency (POI) is a heterogeneous disorder and lacks effective interventions in clinical applications. This research aimed to elucidate the potential effects of recombinant human PEGylated growth hormone (rhGH) on follicular development and mitochondrial function in oocytes as well as ovarian parameters in POI rats induced by the chemotherapeutic agent. The impacts of rhGH on ovarian function before superovulation on follicles, estrous cycle, and sex hormones were evaluated. Oocytes were retrieved to determine oocyte quality and oxidative stress parameters. Single-cell sequencing was applied to investigate the latent regulatory network. This study provides new evidence that a high dosage of rhGH increased the number of retrieved oocytes even though it did not completely restore the disturbed estrous cycle and sex hormones. rhGH attenuated the apoptosis of granulosa cells and oxidative stress response caused by reactive oxygen species (ROS) and mitochondrial superoxide. Additionally, rhGH modulated the energy metabolism of oocytes concerning the mitochondrial membrane potential and ATP content but not mtDNA copy numbers. Based on single-cell transcriptomic analysis, we found that rhGH directly or indirectly promoted the balance of oxidative stress and cellular oxidant detoxification. Four hub genes, Pxmp4, Ehbp1, Mt-cyb, and Enpp6, were identified to be closely related to the repair process in oocytes as potential targets for POI treatment.

Modulation of estrogen-related receptors subtype selectivity: Conversion of an ERRβ/γ selective agonist to ERRα/β/γ pan agonists

Estrogen Related Receptors (ERRs) are key regulators of energy homeostasis and play important role in the etiology of metabolic disorders, skeletal muscle related disorders, and neurodegenerative diseases. Among the three ERR isoforms, ERRα emerged as a potential drug target for metabolic and neurodegenerative diseases. Although ERRβ/γ selective agonist chemical tools have been identified, there are no chemical tools that effectively target ERRα agonism. We successfully engineered high affinity ERRα agonism into a chemical scaffold that displays selective ERRβ/γ agonist activity (GSK4716), providing novel ERRα/β/γ pan agonists that can be used as tools to probe the physiological roles of these nuclear receptors. We identified the structural requirements to enhance selectivity toward ERRα. Molecular modeling shows that our novel modulators have favorable binding modes in the LBP of ERRα and can induce conformational changes where Phe328 that originally occupies the pocket is dislocated to accommodate the ligands in a rather small cavity. The best agonists up-regulated the expression of target genes PGC-1α and PGC-1β, which are necessary to achieve maximal mitochondrial biogenesis. Moreover, they increased the mRNA levels of PDK4, which play an important role in energy homeostasis.

Targeting the Mitochondrial Metabolic Network: A Promising Strategy in Cancer Treatment

Metabolic reprogramming is a hallmark of cancer, which implements a profound metabolic rewiring in order to support a high proliferation rate and to ensure cell survival in its complex microenvironment. Although initial studies considered glycolysis as a crucial metabolic pathway in tumor metabolism reprogramming (i.e., the Warburg effect), recently, the critical role of mitochondria in oncogenesis, tumor progression, and neoplastic dissemination has emerged. In this report, we examined the main mitochondrial metabolic pathways that are altered in cancer, which play key roles in the different stages of tumor progression. Furthermore, we reviewed the function of important molecules inhibiting the main mitochondrial metabolic processes, which have been proven to be promising anticancer candidates in recent years. In particular, inhibitors of oxidative phosphorylation (OXPHOS), heme flux, the tricarboxylic acid cycle (TCA), glutaminolysis, mitochondrial dynamics, and biogenesis are discussed. The examined mitochondrial metabolic network inhibitors have produced interesting results in both preclinical and clinical studies, advancing cancer research and emphasizing that mitochondrial targeting may represent an effective anticancer strategy.

Estrogen-related receptor participates in regulating glycolysis and influences embryonic development in silkworm Bombyx mori

Estrogen-related receptors (ERRs) play indispensable roles in development, energy metabolism, and cancers and are metabolic switches in Drosophila. However, the mechanism underlying their metabolic role is unknown in insects. This study analysed the expression profiles of Bombyx mori ERR (BmERR), hexokinase (BmHK), pyruvate kinase (BmPK) and phosphofructokinase (BmPFK) during embryonic development. The expression of BmERR tended to be similar to that of the other genes. We observed a regulatory association between BmERR and glycolytic rate-limiting enzymes by BmERR overexpression, RNA interference (RNAi), and ERR inhibitors in B. mori embryo cells. Subsequently, ERR cis-regulation elements (ERREs) were predicted and identified in the BmPFK promoter. Transfection assays, electrophoretic mobility shift assays and chromatin immunoprecipitation showed that BmERR can bind to one of these elements to regulate the expression of BmPFK. ERREs were also predicted in the BmHK and BmPK promoters. In the eggs, the expression of glycolytic rate-limiting enzyme genes was suppressed when the expression of BmERR was interference by double-stranded BmERR, the glucose levels also was increased. Meanwhile, the development of silkworm embryos was delayed by about 1 day. These results indicate that BmERR can bind to the ERREs of glycolytic gene promoters and regulate the expression of glycolytic genes, ultimately affecting embryonic development in silkworms.

Repression of the transcriptional activity of ERRα with sequence-specific DNA-binding polyamides

The orphan nuclear receptors estrogen-related receptors (ERRs) bind to the estrogen-related receptor response element (ERRE) to regulate transcriptional programs in cellular metabolism and cancer cell growth. In this study, we evaluated the potential for a pyrrole-imidazole polyamide to block ERRα binding to ERREs to inhibit gene expression. We demonstrated that the ERRE-targeted polyamide 1 blocked the binding of ERRα to the consensus ERRE and reduced the transcriptional activity of ERRα in cell culture. We further showed that inhibiting ERRα transcriptional activity with polyamide 1 led to reduced mitochondrial oxygen consumption, a primary biological effect regulated by ERRα. Finally, our data demonstrated that polyamide 1 is an inhibitor for cancer cell growth.

The enhanced expression of estrogen-related receptor α in human bladder cancer tissues and the effects of estrogen-related receptor α knockdown on bladder cancer cells

Estrogen-related receptor α (ERRα) belongs to the superfamily of nuclear orphan receptors. However, the role of ERRα in bladder cancer remains unknown. This study examined the expression of ERRα in bladder cancer tissues and explored the molecular mechanisms of ERRα in bladder cancer progression. The expression of ERRα in bladder cancer tissues from 61 patients was determined by immunohistochemistry. We performed quantitative real-time polymerase chain reaction assay to detect the gene expression levels and carried out Western blot assay to measure protein levels. In vitro functional assays, including colony formation, Cell Counting Kit-8, Transwell invasion, and migration assays, were performed to detect bladder cancer cell growth, proliferation, invasion, and migration, respectively. Flow cytometry was used to determine the cell apoptotic rate of bladder cancer cells. Among the 61 detected bladder cancer tissues, 39 bladder cancer tissues showed positive ERRα immunoreactivity. Higher ERRα immunoreactivity score was significantly associated with TNM stage, tumor grade, distant metastasis, and poor survival in patients with bladder cancer. Univariate and multivariate analyses showed that ERRα immunoreactivity was an independent prognostic factor for overall survival in patients with bladder cancer. ERRα was found to be upregulated in bladder cancer cell lines, and knockdown of ERRα suppressed bladder cancer cell growth, proliferation, invasion, and migration; promoted bladder cancer cell apoptosis; and inhibited the epithelial-mesenchymal transition of bladder cancer cells. On the other hand, bladder cancer cell proliferation, invasion, and migration were significantly enhanced after cells were transfected with an ERRα-overexpressing vector. In vivo tumor growth and metastasis assays showed that ERRα knockdown resulted in remarkable inhibition of tumor growth and tumor metastasis in nude mice. Collectively, our results suggest that the enhanced expression of ERRα may play a key role in the development and progression of bladder cancer and ERRα may serve as an important prognostic factor for bladder cancer.

Nuclear receptors, cholesterol homeostasis and the immune system

Cholesterol is essential for maintaining membrane fluidity in eukaryotes. Additionally, the synthetic cascade of cholesterol results in precursor molecules important for cellular function such as lipid raft formation and protein prenylation. As such, cholesterol homeostasis is tightly regulated. Interestingly, it is now known that some cholesterol precursors and many metabolites serve as active signaling molecules, binding to different classes of receptors including the nuclear receptors. Furthermore, many cholesterol metabolites or their nuclear receptors have been implicated in the regulation of the immune system in normal physiology and disease. Therefore, in this focused review, cholesterol homeostasis and nuclear receptors involved in this regulation will be discussed, with particular emphasis on how these cascades influence the immune system.

Antitumor effect of XCT790, an ERRα inverse agonist, on ERα-negative endometrial cancer cells

PURPOSE

The estrogen-related receptor (ERR) α is structurally similar to classical estrogen receptors (ERs), but is considered to be an orphan nuclear receptor. We previously found that ERRα regulates uterine endometrial cancer progression. Here, we investigated the efficacy of XCT790, a selective inverse agonist of ERRα, on endometrial cancer cells in vitro and in vivo.

METHODS

HEC-1A and KLE, ERα-negative endometrial cancer cells exhibiting high ERRα expression levels, and HEC-1A cell-derived xenograft model mice were treated with XCT790. Transcriptional activity and cell proliferation were examined using luciferase, WST-8 and colony formation assays, respectively. Cell cycle progression was evaluated using flow cytometry, immunofluorescence cytochemistry and Western blotting. Apoptosis was evaluated using a caspase-3/7 activity assay.

RESULTS

We found that XCT790 significantly inhibited ERRα-induced in vitro transcriptional activity, including that of the vascular endothelial growth factor (VEGF) gene, in a concentration-dependent manner (p < 0.05). We also found that XCT790 suppressed colony formation and cell proliferation in a concentration and time-dependent manner (p < 0.01) without cytotoxicity, and induced apoptosis (p < 0.01). XCT790 was found to cause cell cycle arrest at the mitotic phase. Akt and mTOR phosphorylation was found to be inhibited by XCT790, but PI3K levels were not found to be significantly affected. Combination therapy of XCT790 with paclitaxel elicited a synergistic inhibitory effect. Additionally, we found that XCT790 significantly inhibited in vivo tumor growth and angiogenesis, and induced apoptosis without a reduction in body weight, in xenograft models (p < 0.01).

CONCLUSIONS

From our data we conclude that XCT790 has an anti-tumor effect on endometrial cancer cells in vitro and in vivo. As such, it may serve as a novel therapeutic agent for endometrial cancer.

Oestrogen-related receptor alpha mediates chemotherapy resistance of osteosarcoma cells via regulation of ABCB1

Chemotherapy resistance is one of the major challenges for the treatment of osteosarcoma (OS). The potential roles of oestrogenic signals in the chemoresistance of OS cells were investigated. As compare to the parental cells, the doxorubicin and cisplatin (CDDP) resistant OS cells had greater levels of oestrogen-related receptors alpha (ERRα). Targeted inhibition of ERRα by its specific siRNAs or inverse agonist XCT-790 can restore the sensitivity of OS resistant cells to chemotherapy. This might be due to that si-ERRα can decrease the expression of P-glycoprotein (P-gp, encoded by ABCB1), one important ABC membrane transporter for drug efflux. XCT-790 can decrease the transcription and mRNA stability of ABCB1, while had no effect on protein stability of P-gp. ERRα can bind to the transcription factor of SP3 to increase the transcription of ABCB1. Furthermore, XCT-790 treatment decreased the expression of miR-9, which can bind to the 3'UTR of ABCB1 and trigger its decay. Collectively, we found that ERRα can regulate the chemoresistance of OS cells via regulating the transcription and mRNA stability of ABCB1. Targeted inhibition of ERRα might be a potential approach for OS therapy.

Cholesterol and Its Metabolites in Tumor Growth: Therapeutic Potential of Statins in Cancer Treatment

Cholesterol is essential for cell function and viability. It is a component of the plasma membrane and lipid rafts and is a precursor for bile acids, steroid hormones, and Vitamin D. As a ligand for estrogen-related receptor alpha (ESRRA), cholesterol becomes a signaling molecule. Furthermore, cholesterol-derived oxysterols activate liver X receptors (LXRs) or estrogen receptors (ERs). Several studies performed in cancer cells reveal that cholesterol synthesis is enhanced compared to normal cells. Additionally, high serum cholesterol levels are associated with increased risk for many cancers, but thus far, clinical trials with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) have had mixed results. Statins inhibit cholesterol synthesis within cells through the inhibition of HMG-CoA reductase, the rate-limiting enzyme in the mevalonate and cholesterol synthetic pathway. Many downstream products of mevalonate have a role in cell proliferation, since they are required for maintenance of membrane integrity; signaling, as some proteins to be active must undergo prenylation; protein synthesis, as isopentenyladenine is an essential substrate for the modification of certain tRNAs; and cell-cycle progression. In this review starting from recent acquired findings on the role that cholesterol and its metabolites fulfill in the contest of cancer cells, we discuss the results of studies focused to investigate the use of statins in order to prevent cancer growth and metastasis.

Alterations in PGC1α expression levels are involved in colorectal cancer risk: a qualitative systematic review

Background

Colorectal cancer (CRC) is a major global public health problem and the second leading cause of cancer-related death. Mitochondrial dysfunction has long been suspected to be involved in this type of tumorigenesis, as supported by an accumulating body of research evidence. However, little is known about how mitochondrial alterations contribute to tumorigenesis. Mitochondrial biogenesis is a fundamental cellular process required to maintain functional mitochondria and as an adaptive mechanism in response to changing energy requirements. Mitochondrial biogenesis is regulated by peroxisome proliferator-activated receptor gamma coactivator 1-α (PPARGC1A or PGC1α). In this paper, we report a systematic review to summarize current evidence on the role of PGC1α in the initiation and progression of CRC. The aim is to provide a basis for more comprehensive research.

Methods

The literature search, data extraction and quality assessment were performed according to the document Guidance on the Conduct of Narrative Synthesis in Systematic Reviews and the PRISMA declaration.

Results

The studies included in this review aimed to evaluate whether increased or decreased PGC1α expression affects the development of CRC. Each article proposes a possible molecular mechanism of action and we create two concept maps.

Conclusion

Our systematic review indicates that altered expression of PGC1α modifies CRC risk. Most studies showed that overexpression of this gene increases CRC risk, while some studies indicated that lower than normal expression levels could increase CRC risk. Thus, various authors propose PGC1α as a good candidate molecular target for cancer therapy. Reducing expression of this gene could help to reduce risk or progression of CRC.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3725-3) contains supplementary material, which is available to authorized users.

Curcumin Promotes Osteosarcoma Cell Death by Activating miR-125a/ERRα Signal Pathway

Curcumin has demonstrated valuable therapeutic potential against a variety of human cancers including osteosarcoma. However, the molecular mechanisms underlying its anti-tumor effect remain to be poorly understood. By RNA sequence profiling, we found that curcumin significantly down-regulates the expression of estrogen-related receptor alpha (ERRα) in osteosarcoma cells. Overexpression of ERRα diminished curcumin-activated apoptotic cell death and scavenged curcumin-induced reactive oxygen species (ROS), while ERRα silencing sensitized osteosarcoma cells to curcumin, resulting in increased inhibition of cell proliferation. In addition, we found that curcumin suppressed the ERRα gene expression through upregulation of miR-125a. Data from this study revealed a novel mechanism for curcumin-mediated apoptotic cell death, which involves tumor cell killing via activating miR-125a/ERRα pathway. Our studies also provide further support for osteosarcoma therapy by targeting ERRα alone or in combination with curcumin. J. Cell. Biochem. 118: 74-81, 2017. © 2016 Wiley Periodicals, Inc.

MARCH5 RNA promotes autophagy, migration, and invasion of ovarian cancer cells

MARCH5 is a crucial regulator of mitochondrial fission. However, the expression and function of MARCH5 in ovarian cancer have not been determined. This study investigated the expression and function of MARCH5 in ovarian cancer with respect to its potential role in the tumorigenesis of the disease as well as its usefulness as an early diagnostic marker. We found that the expression of MARCH5 was substantially upregulated in ovarian cancer tissue in comparison with the normal control. Silencing MARCH5 in SKOV3 cells decreased TGFB1-induced cell macroautophagy/autophagy, migration, and invasion in vitro and in vivo, whereas the ectopic expression of MARCH5 in A2780 cells had the opposite effect. Mechanistic investigations revealed that MARCH5 RNA may function as a competing endogenous RNA (ceRNA) to regulate the expression of SMAD2 and ATG5 by competing for MIR30A. Knocking down SMAD2 or ATG5 can block the effect of MARCH5 in A2780 cells. Also, silencing the expression of MARCH5 in SKOV3 cells can inhibit the TGFB1-SMAD2/3 pathway. In contrast, the ectopic expression of MARCH5 in A2780 cells can activate the TGFB1-SMAD2/3 pathway. In turn, the TGFB1-SMAD2/3 pathway can regulate MARCH5 and ATG5 through MIR30A. Overall, the results of this study identified MARCH5 as a candidate oncogene in ovarian cancer and a potential target for ovarian cancer therapy.

Research progress on the reproductive and non-reproductive endocrine tumors by estrogen-related receptors

Oncologists have traditionally considered that tumorigenesis are closely related to classical nuclear estrogen receptors (ERs), such as estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ), through the ligands binding and target gene transcription induction. Estrogen-related receptors (ERRs) have similar structures with ERs, which are also gradually thought to be relevant to reproductive endocrine tumor diseases, even non-reproductive endocrine tumors. In this review, different subtypes of ERRs and their structures firstly will be introduced, then the expression patterns in gynecological oncology (i.e., breast cancer, endometrial cancer, and ovarian cancer), male genitourinary system malignancy especially prostatic cancer along with other non-reproductive endocrine tumors (i.e., lung cancer, colorectal cancer, and liver cancer) will be described, and simultaneously the role of tumorigenesis related to ERRs will be discussed. Therefore, the review is benefit to explore the way of tumor prevention and treatment.

Ligand Activation of ERRα by Cholesterol Mediates Statin and Bisphosphonate Effects

Nuclear receptors (NRs) are key regulators of gene expression and physiology. Nearly half of all human NRs lack endogenous ligands including estrogen-related receptor α (ERRα). ERRα has important roles in cancer, metabolism, and skeletal homeostasis. Affinity chromatography of tissue lipidomes with the ERRα ligand-binding domain (LBD) and subsequent transcriptional assays identified cholesterol as an endogenous ERRα agonist. Perturbation of cholesterol biosynthesis or inhibition of ERRα revealed the interdependence of cholesterol and ERRα. In bone, the effects of cholesterol, statin, and bisphosphonate on osteoclastogenesis require ERRα; and consequently, cholesterol-induced bone loss or bisphosphonate osteoprotection is lost in ERRα knockout mice. Furthermore, statin induction of muscle toxicity and cholesterol suppression of macrophage cytokine secretion are impaired by loss or inhibition of ERRα. These findings reveal a key step in ERRα regulation and explain the actions of two highly prescribed drugs, statins and bisphosphonates.

Computational investigation of the interaction mechanism between the estrogen related receptor α and its agonists

The agonists may promote the binding of coactivator PGC-1α to ERRα by stabilizing the conformation and the site of H12.

Mitochondrial biogenesis is required for the anchorage-independent survival and propagation of stem-like cancer cells

Here, we show that new mitochondrial biogenesis is required for the anchorage independent survival and propagation of cancer stem-like cells (CSCs). More specifically, we used the drug XCT790 as an investigational tool, as it functions as a specific inhibitor of the ERRα-PGC1 signaling pathway, which governs mitochondrial biogenesis. Interestingly, our results directly demonstrate that XCT790 efficiently blocks both the survival and propagation of tumor initiating stem-like cells (TICs), using the MCF7 cell line as a model system. Mechanistically, we show that XCT790 suppresses the activity of several independent signaling pathways that are normally required for the survival of CSCs, such as Sonic hedgehog, TGFβ-SMAD, STAT3, and Wnt signaling. We also show that XCT790 markedly reduces oxidative mitochondrial metabolism (OXPHOS) and that XCT790-mediated inhibition of CSC propagation can be prevented or reversed by Acetyl-L-Carnitine (ALCAR), a mitochondrial fuel. Consistent with our findings, over-expression of ERRα significantly enhances the efficiency of mammosphere formation, which can be blocked by treatment with mitochondrial inhibitors. Similarly, mammosphere formation augmented by FOXM1, a downstream target of Wnt/β-catenin signaling, can also be blocked by treatment with three different classes of mitochondrial inhibitors (XCT790, oligomycin A, or doxycycline). In this context, our unbiased proteomics analysis reveals that FOXM1 drives the expression of >90 protein targets associated with mitochondrial biogenesis, glycolysis, the EMT and protein synthesis in MCF7 cells, processes which are characteristic of an anabolic CSC phenotype. Finally, doxycycline is an FDA-approved antibiotic, which is very well-tolerated in patients. As such, doxycycline could be re-purposed clinically as a 'safe' mitochondrial inhibitor, to target FOXM1 and mitochondrial biogenesis in CSCs, to prevent tumor recurrence and distant metastasis, thereby avoiding patient relapse.

MicroRNA-125a reduces proliferation and invasion of oral squamous cell carcinoma cells by targeting estrogen-related receptor α: implications for cancer therapeutics

Estrogen-related receptor α (ESRRA) functions as a transcription factor and regulates the expression of several genes, such as WNT11 and OPN. Up-regulation of ESRRA has been reported in several cancers. However, the mechanism underlying its up-regulation is unclear. Furthermore, the reports regarding the role and regulation of ESRRA in oral squamous cell carcinoma (OSCC) are completely lacking. Here, we show that tumor suppressor miR-125a directly binds to the 3'UTR of ESRRA and represses its expression. Overexpression of miR-125a in OSCC cells drastically reduced the level of ESRRA, decreased cell proliferation, and increased apoptosis. Conversely, the delivery of an miR-125a inhibitor to these cells drastically increased the level of ESRRA, increased cell proliferation, and decreased apoptosis. miR-125a-mediated down-regulation of ESRRA impaired anchorage-independent colony formation and invasion of OSCC cells. Reduced cell proliferation and increased apoptosis of OSCC cells were dependent on the presence of the 3'UTR in ESRRA. The delivery of an miR-125a mimic to OSCC cells resulted in marked regression of xenografts in nude mice, whereas the delivery of an miR-125a inhibitor to OSCC cells resulted in a significant increase of xenografts and abrogated the tumor suppressor function of miR-125a. We observed an inverse correlation between the expression levels of miR-125a and ESRRA in OSCC samples. In summary, up-regulation of ESRRA due to down-regulation of miR-125a is not only a novel mechanism for its up-regulation in OSCC, but decreasing the level of ESRRA by using a synthetic miR-125a mimic may have an important role in therapeutic intervention of OSCC and other cancers.

The estrogen-related receptor α inverse agonist XCT 790 is a nanomolar mitochondrial uncoupler

XCT 790 is widely used to inhibit estrogen-related receptor α (ERRα) activity as an inverse agonist. Here, we report that XCT 790 potently activates AMP kinase (AMPK) in a dose-dependent and ERRα-independent manner, with active concentrations more than 25-fold below those typically used to perturb ERRα. AMPK activation is secondary to inhibition of energy production as XCT 790 rapidly depletes the pool of cellular ATP. A concomitant increase in oxygen consumption rates suggests uncoupling of the mitochondrial electron transport chain. Consistent with this, XCT 790 decreased mitochondrial membrane potential without affecting mitochondrial mass. Therefore, XCT 790 is a potent, fast-acting, mitochondrial uncoupler independent of its inhibition of ERRα. The biological activity together with structural features in common with the chemical uncouplers FCCP and CCCP indicates likely mode of action as a proton ionophore.

Novel drugs that target the estrogen-related receptor alpha: their therapeutic potential in breast cancer

THE INCIDENCE OF BREAST CANCER CONTINUES TO RISE

1.7 million women were diagnosed with and 521,000 women died from breast cancer in 2012. This review considers first current treatment options: surgery; radiotherapy; and systemic endocrine, anti-biological, and cytotoxic therapies. Clinical management includes prevention, early detection by screening, treatment with curative intent, management of chronic disease, and palliative control of advanced breast cancer. Next, the potential of novel drugs that target DNA repair, growth factor dependence, intracellular and intercellular signal transduction, and cell cycle are considered. Estrogen-related receptor alpha has attracted attention as a therapeutic target in triple-negative breast cancers with de novo resistance to, and in breast cancers with acquired resistance to, endocrine therapies such as antiestrogens and aromatase inhibitors. Estrogen-related receptor alpha is an orphan receptor and transcription factor. Its activity is regulated by coregulator proteins and posttranslational modification. It is an energy sensor that controls adaptation to energy demand and may facilitate glycolytic metabolism and mitochondrial oxidative respiration in breast cancer cells. Estrogen-related receptor alpha increases breast cancer cell migration, proliferation, and tumor development. It is expressed at high levels in estrogen receptor-negative tumors, and is proposed to activate estrogen-responsive genes in endocrine-resistant tumors. The structures and functions of the ligand-binding domains of estrogen receptor alpha and estrogen-related receptor alpha, their ability to bind estrogens, phytoestrogens, and synthetic ligands, and the effects of ligand agonists, antagonists, and inverse agonists on biological activity, are evaluated. Synthetic ligands of estrogen-related receptor alpha have activity in preclinical models of metabolic disorders, diabetes, osteoporosis, and oncology. The clinical settings in which these novel drugs might have utility in the management of advanced breast cancer, and biomarkers for stratification of patients likely to benefit, are discussed. Finally, the potential side effects of the novel drugs on metabolism, osteoporosis, osteo-metastasis, and cachexia are considered.

Estrogen-related receptor alpha confers methotrexate resistance via attenuation of reactive oxygen species production and P53 mediated apoptosis in osteosarcoma cells

Osteosarcoma (OS) is a malignant tumor mainly occurring in children and adolescents. Methotrexate (MTX), a chemotherapy agent, is widely used in treating OS. However, treatment failures are common due to acquired chemoresistance, for which the underlying molecular mechanisms are still unclear. In this study, we report that overexpression of estrogen-related receptor alpha (ERR α ), an orphan nuclear receptor, promoted cell survival and blocked MTX-induced cell death in U2OS cells. We showed that MTX induced ROS production in MTX-sensitive U2OS cells while ERR α effectively blocked the ROS production and ROS associated cell apoptosis. Our further studies demonstrated that ERR α suppressed ROS induction of tumor suppressor P53 and its target genes NOXA and XAF1 which are mediators of P53-dependent apoptosis. In conclusion, this study demonstrated that ERR α plays an important role in the development of MTX resistance through blocking MTX-induced ROS production and attenuating the activation of p53 mediated apoptosis signaling pathway, and points to ERR α as a novel target for improving osteosarcoma therapy.

Kaempferol inhibits cancer cell growth by antagonizing estrogen-related receptor α and γ activities

Kaempferol is a dietary flavonoid that can function as a selective estrogen receptor modulator (SERM). Estrogen-related receptors alpha and gamma (ERRα and ERRγ) are orphan nuclear receptors that play important roles in mitochondrial biogenesis and cancer development. We have shown that kaempferol can functionally antagonize the activities of ERRs based on both response element reporter systems and target gene analysis. Kaempferol modulation of mitochondrial function and suppression cancer cell growth has been confirmed. These findings suggest that kaempferol may exert their anti-cancer activities through antagonizing ERRs activities.

Estrogen-related receptor alpha interacts cooperatively with peroxisome proliferator-activated receptor-gamma coactivator-1alpha to regulate osteocalcin gene expression

Osteocalcin is one of the most abundant non-collagenous proteins in bone that is commonly used as a preliminary biomarker in bone formation. Osteocalcin also regulates energy metabolism as a hormone. Estrogen-related receptor alpha (ERRα) is primarily thought to regulate energy homeostasis through interacting with peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α). Intriguingly, ERRα may play a functional role in bone formation. We have found there are 3 ERRα response elements (ERR response element, ERRE) in the osteocalcin promoter, and ERRα interacted cooperatively with PGC-1α could improve the osteocalcin promoter activity, whereas ERRα specific antagonist, XCT-790, inhibits this enhancement. XCT-790 inhibits osteocalcin and ERRα target gene PDKs expression in osteoblast cells. Thus ERRs might have physiological roles in bone formation and energy metabolism through modulating osteocalcin gene expression.

ERRα metabolic nuclear receptor controls growth of colon cancer cells

The estrogen-related receptor alpha (ERRα) is a nuclear receptor that acts primarily as a regulator of metabolic processes, particularly in tissues subjected to high-energy demand. In addition to its control of energy metabolism and mitochondrial biogenesis, ERRα has recently been associated with cancer progression. Notably, increased expression of ERRα has been shown in several cancerous tissues, including breast, ovary and colon. However, additional studies are required to gain insight into the action of ERRα in cancer biology, particularly in non-endocrine-related cancers. Therefore, using a short hairpin RNA-mediated approach, we investigated whether ERRα is required for the rapid growth of colon cancer cells and to maintain their neoplastic metabolic state. Results show that silencing ERRα significantly impaired colon cancer cell proliferation and colony formation in vitro as well as their in vivo tumorigenic capacity. A pronounced delay in G1-to-S cell cycle phase transition was observed in ERRα-depleted cells in association with reduced cyclin-dependent kinase 2 activity and hyperphosphorylated state of the retinoblastoma protein along with disturbed expression of several cell cycle regulators, including p15 and p27. Interestingly, ERRα-depleted HCT116 cells also displayed significant reduction in expression of a large set of key genes to glycolysis, tricarboxylic acid cycle and lipid synthesis. Furthermore, using (14)C isotope tracer analysis, ERRα depletion in colon cancer cells resulted in reduced glucose incorporation and glucose-mediated lipogenesis in these cells. These findings suggest that ERRα coordinates colon cancer cell proliferation and tumorigenic capacity with energy metabolism. Thus, ERRα could represent a promising therapeutic target in colon cancer.

Estrogen related receptor α-induced adipogenesis is PGC-1β-dependent

Previous report showed that Estrogen related receptor α (ERRα) knockout mice had a significant reduction in adipose tissue deposition. Although it was reported that ERRα could promote adipogenesis in several immortalized preadipocytes cell lines, the mechanism behind which is still unclear to date. Besides, the expression pattern of ERRα in white adipose tissue is rarely examined. Here, we show that the expression of ERRα in primary cultured adipocytes is closely associated with adipogenesis. Besides, we found that peroxisome proliferator-activated receptor-γ coactivator 1β (PGC-1β) play an important role in regulating ERRα-induced adipogenesis. ERRα-induced adipogenesis was greatly attenuated when knocking down PGC-1β expression, while rescued by overexpression of PGC-1β. However, PGC-1β could still promote adipogenesis when suppressing ERRα expression. Furthermore, we demonstrated that ERRα could transcriptionally active PGC-1β expression and then enhance the formation of sterol-regulatory-element-binding protein-1c (SREBP-1c)/PGC-1β complex and peroxisome proliferator-activated receptor-γ (PPARγ)/PGC-1β complex. Taken together, these results indicate that ERRα-induced adipogenesis is triggered by modulating the expression of PGC-1β.

Protein kinase C epsilon affects mitochondrial function through estrogen-related receptor alpha

Members of the protein kinase C (PKC) family have been implicated in controlling cell proliferation, differentiation, and motility. Many of these processes are energy demanding. How PKCs affect mitochondrial function to regulate energy production is not well defined. Using an inhibitor Gö6983 with broad specificity, we found that inhibiting PKCs reduced mitochondrial mass and altered mitochondrial function characterized by elevations in mitochondrial membrane potential (Δψm) and reactive oxygen species (ROS) levels. These alterations indicated that Gö6983 suppressed the activities of mitochondrial regulators such as estrogen-related receptor α (ERRα). Indeed, Gö6983 dose-dependently suppressed the expression levels of ERRα-target genes peroxisome proliferator-activated receptor α (PPARα) and medium-chain acyl-CoA dehydrogenase (MCAD). Conversely, PKC activator phorbol ester (PMA) enhanced the expression level of another ERRα-target gene pyruvate dehydrogenase kinase 4 (PDK4). This PMA-mediated induction of PDK4 was blunted by an ERRα inverse agonist XCT-790, suggesting that ERRα plays a role in mediating the effects of PKCs on mitochondrial function. By over-expressing constitutively active forms of PKCs, we found that PKCε preferentially stimulated the transcription activity of ERRα. Through mutating residues on ERRα, we established that this PKCε-induced ERRα activity involves threonine 106, serine 110, and threonine 124 of ERRα. Collectively, these pieces of evidence suggest that ERRα plays an important role down-stream of PKCε to regulate mitochondrial homeostasis.

Mitogen-activated protein kinase kinases promote mitochondrial biogenesis in part through inducing peroxisome proliferator-activated receptor γ coactivator-1β expression

Growth factor activates mitogen-activated protein kinase kinases to promote cell growth. Mitochondrial biogenesis is an integral part of cell growth. How growth factor regulates mitochondrial biogenesis is not fully understood. In this study, we found that mitochondrial mass was specifically reduced upon serum starvation and induced upon re-feeding with serum. Using mitogen-activated protein kinase kinases inhibitor U0126, we found that the mRNA expression levels of ATP synthase, cytochrome-C, mitochondrial transcription factor A, and mitofusin 2 were reduced. Since the transcriptional levels of these genes are under the control of peroxisome proliferator-activated receptor γ coactivator-1α and -1β (PGC-1α and PGC-1β), we examined and found that only the mRNA and protein levels of PGC-1β were suppressed. Importantly, over-expression of PGC-1β partially reversed the reduction of mitochondrial mass upon U0126 treatment. Thus, we conclude that mitogen-activated protein kinase kinases direct mitochondrial biogenesis through selectively inducing PGC-1β expression.

Kinase suppressor of ras 1 (KSR1) regulates PGC1α and estrogen-related receptor α to promote oncogenic Ras-dependent anchorage-independent growth

Kinase suppressor of ras 1 (KSR1) is a molecular scaffold of the Raf/MEK/extracellular signal-regulated kinase (ERK) cascade that enhances oncogenic Ras signaling. Here we show KSR1-dependent, but ERK-independent, regulation of metabolic capacity is mediated through the expression of peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α) and estrogen-related receptor α (ERRα). This KSR1-regulated pathway is essential for the transformation of cells by oncogenic Ras. In mouse embryo fibroblasts (MEFs) expressing H-Ras(V12), ectopic PGC1α was sufficient to rescue ERRα expression, metabolic capacity, and anchorage-independent growth in the absence of KSR1. The ability of PGC1α to promote anchorage-independent growth required interaction with ERRα, and treatment with an inhibitor of ERRα impeded anchorage-independent growth. In contrast to PGC1α, the expression of constitutively active ERRα (CA-ERRα) was sufficient to enhance metabolic capacity but not anchorage-independent growth in the absence of KSR1. These data reveal KSR1-dependent control of PGC1α- and ERRα-dependent pathways that are necessary and sufficient for signaling by oncogenic H-Ras(V12) to regulate metabolism and anchorage-independent growth, providing novel targets for therapeutic intervention.

Phosphatidylinositol 3-kinase affects mitochondrial function in part through inducing peroxisome proliferator-activated receptor γ coactivator-1β expression

BACKGROUND AND PURPOSE

Hyperactivation of phosphatidylinositol 3-kinase (PI3K), commonly observed in cancer, is believed to promote cancer cell growth and survival. Appropriate mitochondrial function is an integral part of cellular function. How PI3K affects mitochondrial homeostasis is not fully understood.

EXPERIMENTAL APPROACH

Mitochondrial mass, membrane potential and reactive oxygen species (ROS) were quantified by three different fluorogenic probes. Gene expression at the levels of mRNA and protein were measured by quantitative RT-PCR and Western analysis.

KEY RESULTS

Using the PI3K inhibitors LY294002 and PI103, we found that suppressing PI3K activity altered mitochondrial function. Specifically, LY294002 and PI103 suppressed the mRNA expression levels of mitochondrial regulators nuclear respiratory factors 1 and 2 (NRF1 and NRF2). As NRF1 and NRF2 are under the transcriptional control of peroxisome proliferator-activated receptor γ coactivators-1α and -1β (PGC-1α and PGC-1β), we found that suppressing PI3K activity selectively reduced both the mRNA and protein levels of PGC-1β but not PGC-1α. Reducing PGC-1β expression also led to reduced mRNA expression levels of uncoupling protein 1, 2 (UCP1 and UCP2) and superoxide dismutase 2. Correspondingly, mitochondrial membrane potential (Δψ(m)) and ROS levels were increased. Finally, we partially blunted the LY294002-mediated growth suppression by using an antioxidant or over-expressing PGC-1β.

CONCLUSIONS AND IMPLICATIONS

PI3K regulates mitochondrial homeostasis in part through PGC-1β and blocking this pathway induces ROS to arrest cell growth at the G₁ phase.

Estrogen-related receptor gamma promotes mesenchymal-to-epithelial transition and suppresses breast tumor growth

Estrogen-related receptors (ERR), ERR alpha (ERRα) and ERR gamma (ERRγ), are orphan nuclear receptors implicated in breast cancer that function similarly in the regulation of oxidative metabolism genes. Paradoxically, in clinical studies, high levels of ERRα are associated with poor outcomes whereas high levels of ERRγ are associated with a favorable course. Recent studies suggest that ERRα may indeed promote breast tumor growth. The roles of ERRγ in breast cancer progression and how ERRα and ERRγ may differentially affect cancer growth are unclear. In mammary carcinoma cells that do not express endogenous ERRγ, we found that ectopic expression of ERRγ enhanced oxidative metabolism in vitro and inhibited the growth of tumor xenografts in vivo. In contrast, ectopic expression of the ERRα coactivator PGC-1α enhanced oxidative metabolism but did not affect tumor growth. Notably, ERRγ activated expression of a genetic program characteristic of mesenchymal-to-epithelial transition (MET). This program was apparent by changes in cellular morphology, upregulation of epithelial cell markers, downregulation of mesenchymal markers, and decreased cellular invasiveness. We determined that this program was also associated with upregulation of E-cadherin, which is activated directly by ERRγ. In contrast, PGC-1α activated only a subset of genes characteristic of the MET program and, unlike ERRγ, did not upregulate E-cadherin. In conclusion, these results show that ERRγ induces E-cadherin, promotes MET, and suppresses breast cancer growth. Our findings suggest that ERRγ agonists may have applications in the treatment of breast cancer.

Troglitazone induces cytotoxicity in part by promoting the degradation of peroxisome proliferator-activated receptor γ co-activator-1α protein

BACKGROUND AND PURPOSE

Troglitazone (Tro), rosiglitazone (Rosi) and pioglitazone (Pio) are anti-diabetic thiazolidinediones that function as ligands for peroxisome proliferator-activated receptor γ (PPARγ); however, Tro has been withdrawn from the market due to liver toxicity issues. Mitochondrial dysfunction induced by Tro has been suggested to be an important mechanism behind its cytotoxicity. Constitutively active nuclear hormone receptors, oestrogen-related receptor α and γ are thought to regulate mitochondrial mass and oxidative phosphorylation together with their co-activators PPARγ co-activator-1α and -1β (PGC-1α and PGC-1β). Hence, in this study, we investigated whether Tro affects the expression and activity levels of these regulators.

EXPERIMENTAL APPROACH

Cellular viability was measured by an ATP-based assay. Mitochondrial mass and reactive oxygen species (ROS) were quantified by two different fluorogenic probes. Apoptosis was measured by an Annexin-V-based kit. Gene expression at the levels of mRNA and protein was measured by quantitative RT-PCR and Western analysis. Over-expression of PGC-1α was mediated by an adenovirus.

KEY RESULTS

Tro, but not Rosi or Pio, selectively stimulated PGC-1α protein degradation. As a result, Tro reduced mitochondrial mass, and superoxide dismutases 1 and 2 expressions, but induced ROS to initiate apoptosis. Using a ubiquitin-proteasome inhibitor MG132, it was established that blocking PGC-1α degradation partially suppressed the reduction of mitochondrial mass. Importantly, over-expressing PGC-1α partially restored the Tro-suppressed mitochondrial mass and attenuated the cytotoxic effects of Tro.

CONCLUSIONS AND IMPLICATIONS

Collectively, these results suggest that PGC-1α degradation is an important mechanism behind the cytotoxic effects of Tro in the liver.

Orphan nuclear receptors in breast cancer pathogenesis and therapeutic response

Nuclear receptors comprise a large family of highly conserved transcription factors that regulate many key processes in normal and neoplastic tissues. Most nuclear receptors share a common, highly conserved domain structure that includes a carboxy-terminal ligand-binding domain. However, a subgroup of this gene family is known as the orphan nuclear receptors because to date there are no known natural ligands that regulate their activity. Many of the 25 nuclear receptors classified as orphan play critical roles in embryonic development, metabolism, and the regulation of circadian rhythm. Here, we review the emerging role(s) of orphan nuclear receptors in breast cancer, with a particular focus on two of the estrogen-related receptors (ERRalpha and ERRgamma) and several others implicated in clinical outcome and response or resistance to cytotoxic or endocrine therapies, including the chicken ovalbumin upstream promoter transcription factors, nerve growth factor-induced B, DAX-1, liver receptor homolog-1, and retinoic acid-related orphan receptor alpha. We also propose that a clearer understanding of the function of orphan nuclear receptors in mammary gland development and normal mammary tissues could significantly improve our ability to diagnose, treat, and prevent breast cancer.

Troglitazone is an estrogen-related receptor alpha and gamma inverse agonist

As a ligand for peroxisome proliferators-activated receptor gamma (PPAR gamma), troglitazone inhibits cell growth by mechanisms besides activating PPAR gamma. In this study, we found that troglitazone interfered with the interactions between estrogen-related receptor alpha and gamma (ERR alpha and ERR gamma) and their coactivator PPAR gamma coactivator-1 alpha (PGC-1 alpha) functioning as an inverse agonist. Additionally, troglitazone suppressed the expressions of PGC-1 alpha and its related member PGC-1 beta which are key regulators of mitochondrial function. Consequently, troglitazone reduced mitochondrial mass and suppressed the expressions of superoxide dismutases to elevate reactive oxygen species (ROS) production. The increase in ROS in turn induced the expression of cell cycle inhibitor p21(WAF1). We therefore propose that ERR alpha and ERR gamma are alternative targets of troglitazone important for mediating its growth suppressive effect.

Modulating estrogen receptor-related receptor-alpha activity inhibits cell proliferation

High expression of the estrogen receptor-related receptor (ERR)-alpha in human tumors is correlated to a poor prognosis, suggesting an involvement of the receptor in cell proliferation. In this study, we show that a synthetic compound (XCT790) that modulates the activity of ERRalpha reduces the proliferation of various cell lines and blocks the G(1)/S transition of the cell cycle in an ERRalpha-dependent manner. XCT790 induces, in a p53-independent manner, the expression of the cell cycle inhibitor p21(waf/cip)(1) at the protein, mRNA, and promoter level, leading to an accumulation of hypophosphorylated Rb. Finally, XCT790 reduces cell tumorigenicity in Nude mice.