Multiple domains of the Receptor-Interacting Protein 140 contribute to transcription inhibition

RIP140 regulates transcription factor HES1 oscillatory expression and mitogenic activity in colon cancer cells

The transcription factor receptor-interacting protein 140 (RIP140) regulates intestinal homeostasis and tumorigenesis through Wnt signaling. In this study, we investigated its effect on the Notch/HES1 signaling pathway. In colorectal cancer (CRC) cell lines, RIP140 positively regulated HES1 gene expression at the transcriptional level via a recombining binding protein suppressor of hairless (RBPJ)/neurogenic locus notch homolog protein 1 (NICD)-mediated mechanism. In support of these in vitro data, RIP140 and HES1 expression significantly correlated in mouse intestine and in a cohort of CRC samples, thus supporting the positive regulation of HES1 gene expression by RIP140. Interestingly, when the Notch pathway is fully activated, RIP140 exerted a strong inhibition of HES1 gene transcription controlled by the level of HES1 itself. Moreover, RIP140 directly interacts with HES1 and reversed its mitogenic activity in human CRC cells. In line with this observation, HES1 levels were associated with a better patient survival only when tumors expressed high levels of RIP140. Our data identify RIP140 as a key regulator of the Notch/HES1 signaling pathway, with a dual effect on HES1 gene expression at the transcriptional level and a strong impact on colon cancer cell proliferation.

14-3-3 binding motif phosphorylation disrupts Hdac4-organized condensates to stimulate cardiac reprogramming

Cell fate conversion is associated with extensive post-translational modifications (PTMs) and architectural changes of sub-organelles, yet how these events are interconnected remains unknown. We report here the identification of a phosphorylation code in 14-3-3 binding motifs (PC14-3-3) that greatly stimulates induced cardiomyocyte (iCM) formation from fibroblasts. PC14-3-3 is identified in pivotal functional proteins for iCM reprogramming, including transcription factors and chromatin modifiers. Akt1 kinase and protein phosphatase 2A are the key writer and key eraser of the PC14-3-3 code, respectively. PC14-3-3 activation induces iCM formation with the presence of only Tbx5. In contrast, PC14-3-3 inhibition by mutagenesis or inhibitor-mediated code removal abolishes reprogramming. We discover that key PC14-3-3-embedded factors, such as histone deacetylase 4 (Hdac4), Mef2c, and Foxo1, form Hdac4-organized inhibitory nuclear condensates. PC14-3-3 activation disrupts Hdac4 condensates to promote cardiac gene expression. Our study suggests that sub-organelle dynamics regulated by a PTM code could be a general mechanism for stimulating cell reprogramming.

Histone H3 Acetylation Is Involved in Retinoid Acid-Induced Neural Differentiation through Increasing Mitochondrial Function

Histone acetylation and mitochondrial function contribute importantly to neural differentiation, which is critically associated with neurodevelopmental disorders such as Down Syndrome (DS). However, whether and how histone acetylation regulates mitochondrial function and further affects neural differentiation has not been well described. In this study, when treated with retinoid acid (RA), the human neuroblastoma SH-SY5Y cell line was used as a neural differentiation model. We found that the acetylation of histone H3, especially H3 lysine 14 acetylation (H3K14ac), and mitochondrial function, including biogenesis and electron transport chain, were enhanced during neural differentiation. Specific inhibition of histone acetyltransferases (HATs) induced neural differentiation deficits, accompanied by downregulation of mitochondrial function. Furthermore, RA receptors (RARs) interacting with HATs were involved in the increased H3K14ac and the enhanced mitochondrial function during the neural differentiation process. Finally, receptor-interacting protein 140 (RIP140), a co-repressor of RARs, was also involved in regulating histone acetylation. RIP140 overexpression inhibited histone acetylation and mediated negative feedback on target genes which are involved in RA signaling. These findings evidenced that when interacting with RARs which had been negatively regulated by RIP140, RA promoted neural differentiation by promoting H3K14ac and enhanced mitochondrial function. This provides a molecular foundation for further investigations into abnormal neural development.

14-3-3 binding motif phosphorylation disrupts Hdac4 organized condensates to stimulate cardiac reprogramming

Cell fate conversion is associated with extensive epigenetic and post translational modifications (PTMs) and architectural changes of sub-organelles and organelles, yet how these events are interconnected remains unknown. We report here the identification of a phosphorylation code in 14-3-3 binding motifs (PC14-3-3) that greatly stimulates induced cardiomyocyte (iCM) formation from fibroblasts. PC14-3-3 was identified in pivotal functional proteins for iCM reprogramming, including transcription factors and epigenetic factors. Akt1 kinase and PP2A phosphatase were a key writer and eraser of the PC14-3-3 code, respectively. PC14-3-3 activation induces iCM formation with the presence of only Tbx5. In contrast, PC14-3-3 inhibition by mutagenesis or inhibitor-mediated code removal abolished reprogramming. We discovered that key PC14-3-3 embedded factors, such as Hdac4, Mef2c, Nrip1, and Foxo1, formed Hdac4 organized inhibitory nuclear condensates. Notably, PC14-3-3 activation disrupted Hdac4 condensates to promote cardiac gene expression. Our study suggests that sub-organelle dynamics regulated by a post-translational modification code could be a general mechanism for stimulating cell reprogramming and organ regeneration.

Highlights

A PC14-3-3 (phosphorylation code in 14-3-3 binding motifs) is identified in pivotal functional proteins, such as HDAC4 and Mef2c, that stimulates iCM formation.Akt1 kinase and PP2A phosphatase are a key writer and a key eraser of the PC14-3-3 code, respectively, and PC14-3-3 code activation can replace Mef2c and Gata4 in cardiac reprogramming.PC14-3-3 activation disrupts Hdac4 organized condensates which results in releasing multiple 14-3-3 motif embedded proteins from the condensates to stimulate cardiac reprogramming.Sub-organelle dynamics and function regulated by a post-translational modification code could be a general mechanism in stimulating cell reprogramming and organ regeneration.

Graphic abstract

Fusion gene detection by RNA-sequencing complements diagnostics of acute myeloid leukemia and identifies recurring NRIP1-MIR99AHG rearrangements

Identification of fusion genes in clinical routine is mostly based on cytogenetics and targeted molecular genetics, such as metaphase karyotyping, fluorescence in situ hybridization and reverse-transcriptase polymerase chain reaction. However, sequencing technologies are becoming more important in clinical routine as processing time and costs per sample decrease. To evaluate the performance of fusion gene detection by RNAsequencing compared to standard diagnostic techniques, we analyzed 806 RNA-sequencing samples from patients with acute myeloid leukemia using two state-of-the-art software tools, namely Arriba and FusionCatcher. RNA-sequencing detected 90% of fusion events that were reported by routine with high evidence, while samples in which RNA-sequencing failed to detect fusion genes had overall lower and inhomogeneous sequence coverage. Based on properties of known and unknown fusion events, we developed a workflow with integrated filtering strategies for the identification of robust fusion gene candidates by RNA-sequencing. Thereby, we detected known recurrent fusion events in 26 cases that were not reported by routine and found discrepancies in evidence for known fusion events between routine and RNA-sequencing in three cases. Moreover, we identified 157 fusion genes as novel robust candidates and comparison to entries from ChimerDB or Mitelman Database showed novel recurrence of fusion genes in 14 cases. Finally, we detected the novel recurrent fusion gene NRIP1- MIR99AHG resulting from inv(21)(q11.2;q21.1) in nine patients (1.1%) and LTN1-MX1 resulting from inv(21)(q21.3;q22.3) in two patients (0.25%). We demonstrated that NRIP1-MIR99AHG results in overexpression of the 3' region of MIR99AHG and the disruption of the tricistronic miRNA cluster miR-99a/let-7c/miR-125b-2. Interestingly, upregulation of MIR99AHG and deregulation of the miRNA cluster, residing in the MIR99AHG locus, are known mechanisms of leukemogenesis in acute megakaryoblastic leukemia. Our findings demonstrate that RNA-sequencing has a strong potential to improve the systematic detection of fusion genes in clinical applications and provides a valuable tool for fusion discovery.

Changes in bone mineral density in Down syndrome individuals: a systematic review and meta-analysis

Data evaluating changes in bone mineral density (BMD) in Down syndrome (DS) individuals remains controversial. Therefore, we conducted a systematic review and meta-analysis to better understand associations between BMD and DS. A systematic literature search of PubMed, EMBASE, Web of Science, and the Cochrane Library up until 1st January 2021 was conducted. We used the keywords "bone mineral density" and "Down Syndrome." Fifteen studies were included. Overall, our results showed a significant decrease in BMD of total body (TB BMD) [MD =  - 0.18; 95% CI (- 0.23 and - 0.12), P < 0.00001, I2 = 89%], total hip (TH BMD) [MD =  - 0.12; 95% CI (- 0.15 and - 0.10), P < 0.00001, I2 = 0%], lumbar spine (LS BMD) [MD =  - 0.12; 95% CI (- 0.14 and - 0.09), P < 0.00001, I2 = 18%], and femoral neck (FN BMD) [MD =  - 0.08; 95% CI (- 0.10 and - 0.06), P < 0.00001, I2 = 0%] in DS individuals when compared with controls. Moreover, the volumetric BMD of lumbar spine (LS vBMD) [MD =  - 0.01; 95% CI (- 0.02 and - 0.01), P = 0.0004, I2 = 19%] also showed a decreasing tendency while the volumetric BMD of the femoral neck (FN vBMD) [MD = 0.01; 95% CI (0.00 and 0.02), P = 0.02, I2 = 0%] was elevated in DS individuals versus controls. These findings demonstrated that individuals with DS had a decreased total and regional (TH, LS, and FN) BMD when compared with the general population. Additionally, when BMD was adjusted for skeletal volume, LS vBMD was also lower, while FN vBMD was elevated in DS individuals versus controls.

A Truncated NRIP1 Mutant Amplifies Microsatellite Instability of Colorectal Cancer by Regulating MSH2/MSH6 Expression, and Is a Prognostic Marker of Stage III Tumors

Simple Summary

The alteration of mismatch repair (MMR) genes leads to microsatellite instability and plays a key role in colorectal cancer (CRC) pathogenesis and prognosis. The transcription factor NRIP1 is involved in intestinal tumorigenesis and is a good prognostic marker in CRC. In this study, we demonstrate that NRIP1 induces MSH2 and MSH6 MMR gene transcription and reduces microsatellite instability. A dominant-negative truncated NRIP1 mutant amplifies the MMR-deficient phenotype and appears as a key player in MSI-driven tumorigenesis since it significantly correlates with a short overall survival of patients with advanced CRC, especially MLH1-deficient ones.

Abstract

Microsatellite instability (MSI) is related to the alteration of mismatch repair (MMR) genes and plays a key role in colorectal cancer (CRC) pathogenesis. We previously reported that the transcription factor Nuclear Receptor Interacting Protein 1 (NRIP1) is involved in sporadic intestinal tumorigenesis. The aim of this study was to decipher its role in MSI CRC. By using different mouse models and engineered cell lines, we demonstrated that NRIP1 increased MSH2 and MSH6 MMR gene transcription and mRNA/protein levels. In human CRC cells, NRIP1 expression was associated with decreased MSI and the hypermutator phenotype, and with resistance to chemotherapy drugs. Using a cohort of 194 CRC patients, we detected in 22% of the cases a MSI-induced frameshift mutation in the NRIP1 coding sequence. This genetic alteration generates a truncated protein with a dominant negative activity that increased human CRC cell proliferation and impaired the regulation of MSH2 and MSH6 gene expression. Moreover, the NRIP1 mutant correlated with a decreased overall survival of patients with advanced CRC, especially when MLH1-deficient. By decreasing the expression of MSH2 and MSH6 gene expression, the NRIP1 variant may amplify MLH1-dependent CRC progression and behave as a new prognostic marker of advanced MSI CRC.

Regulation of LCoR and RIP140 expression in cervical intraepithelial neoplasia and correlation with CIN progression and dedifferentiation

Purpose

Ligand-dependent corepressor (LCoR) and receptor-interacting protein 140 (RIP140/NRIP1) play an important role in the regulation of multiple oncogenic signaling pathways and the development of cancer. LCoR and RIP140 form a nuclear complex in breast cancer cells and are of prognostic value in further prostate and cervical cancer. The purpose of this study was to analyze the regulation of these proteins in the development of cervical intraepithelial neoplasia (CIN I–III).

Methods

Immunohistochemical analysis was obtained to quantify RIP140 and LCoR expression in formalin-fixed paraffin embedded tissue sections of cervical intraepithelial neoplasia samples. Tissue (n = 94) was collected from patients treated in the Department of Gynecology and Obstetrics, Ludwig-Maximilians-University of Munich, Germany, between 2002 and 2014. Correlations of expression levels with clinical outcome were carried out to assess for prognostic relevance in patients with CIN2 progression. Kruskal–Wallis test and Mann–Whitney U test were used for data analysis.

Results

Nuclear LCoR overexpression correlates significantly with CIN II progression. Nuclear RIP140 expression significantly increases and nuclear LCoR expression decreases with higher grading of cervical intraepithelial neoplasia. Cytoplasmic RIP140 expression is significantly higher in CIN III than in CIN I or CIN II.

Conclusion

A decrease of nuclear LCoR expression in line with an increase of dedifferentiation of CIN can be observed. Nuclear LCoR overexpression correlates with CIN II progression indicating a prognostic value of LCoR in cervical intraepithelial neoplasia. Nuclear and cytoplasmic RIP140 expression increases significantly with higher grading of cervical intraepithelial neoplasia underlining its potential role in the development of pre-cancerous lesions. These findings support the relevance of LCoR and RIP140 in the tumorigenesis indicating a possible role of LCoR and RIP140 as targets for novel therapeutic approaches in cervical intraepithelial neoplasia and cervical cancer.

Estrogen receptor alpha (ERα)-mediated coregulator binding and gene expression discriminates the toxic ERα agonist diethylstilbestrol (DES) from the endogenous ERα agonist 17β-estradiol (E2)

Diethylstilbestrol (DES) is a synthetic estrogen and proven human teratogen and carcinogen reported to act via the estrogen receptor α (ERα). Since the endogenous ERα ligand 17β-estradiol (E2) does not show these adverse effects to a similar extent, we hypothesized that DES' interaction with the ERα differs from that of E2. The current study aimed to investigate possible differences between DES and E2 using in vitro assays that detect ERα-mediated effects, including ERα-mediated reporter gene expression, ERα-mediated breast cancer cell (T47D) proliferation and ERα-coregulator interactions and gene expression in T47D cells. Results obtained indicate that DES and E2 activate ERα-mediated reporter gene transcription and T47D cell proliferation in a similar way. However, significant differences between DES- and E2-induced binding of the ERα to 15 coregulator motifs and in transcriptomic signatures obtained in the T47D cells were observed. It is concluded that differences observed in binding of the ERα with several co-repressor motifs, in downregulation of genes involved in histone deacetylation and DNA methylation and in upregulation of CYP26A1 and CYP26B1 contribute to the differential effects reported for DES and E2.

Importance of RIP140 and LCoR Sub-Cellular Localization for Their Association With Breast Cancer Aggressiveness and Patient Survival

New markers are needed to improve diagnosis and to personalize treatments for patients with breast cancer (BC). Receptor-interacting protein of 140 kDa (RIP140) and ligand-dependent corepressor (LCoR), two transcriptional co-regulators of estrogen receptors, strongly interact in BC cells. Although their role in cancer progression has been outlined in the last few years, their function in BC has not been elucidated yet. In this study, we investigated RIP140 and LCoR localization (cytoplasm vs nucleus) in BC samples from a well-characterized cohort of patients (n = 320). RIP140 and LCoR were expressed in more than 80% of tumors, (predominantly in the cytoplasm), and the two markers were highly correlated. Expression of RIP140 and LCoR in the nucleus was negatively correlated with tumor size. Conversely, RIP140 and LCoR cytoplasmic expression strongly correlated with expression of two tumor aggressiveness markers: N-cadherin and CD133 (epithelial mesenchymal transition and cancer stem cell markers, respectively). Finally, high RIP140 nuclear expression was significantly correlated with longer overall survival, whereas high total or cytoplasmic expression of RIP140 was associated with shorter disease-free survival. Our study strongly suggests that the role of RIP140 and LCoR in BC progression could vary according to their prevalent sub-cellular localization, with opposite prognostic values for nuclear and cytoplasmic expression. The involvement in BC progression/invasiveness of cytoplasmic RIP140 could be balanced by the anti-tumor action of nuclear RIP140, thus explaining the previous contradictory findings about its role in BC.

Receptor-interacting protein 140 as a co-repressor of Heat Shock Factor 1 regulates neuronal stress response

Heat shock response (HSR) is a highly conserved transcriptional program that protects organisms against various stressful conditions. However, the molecular mechanisms modulating HSR, especially the suppression of HSR, is poorly understood. Here, we found that RIP140, a wide-spectrum cofactor of nuclear hormone receptors, acts as a co-repressor of heat shock factor 1 (HSF1) to suppress HSR in healthy neurons. When neurons are stressed such as by heat shock or sodium arsenite (As), cells engage specific proteosome-mediated degradation to reduce RIP140 level, thereby relieving the suppression and activating HSR. RIP140 degradation requires specific Tyr-phosphorylation by Syk that is activated in stressful conditions. Lowering RIP140 level protects hippocampal neurons from As stress, significantly it increases neuron survival and improves spine density. Reducing hippocampal RIP140 in the mouse rescues chronic As-induced spatial learning deficits. This is the first study elucidating RIP140-mediated suppression of HSF1-activated HSR in neurons and brain. Importantly, degradation of RIP140 in stressed neurons relieves this suppression, allowing neurons to efficiently and timely engage HSR programs and recover. Therefore, stimulating RIP140 degradation to activate anti-stress program provides a potential preventive or therapeutic strategy for neurodegeneration diseases.

RIP140 and LCoR expression in gastrointestinal cancers

The transcription coregulators RIP140 and LCoR are part of a same complex which controls the activity of various transcription factors and cancer cell proliferation. In this study, we have investigated the expression of these two genes in human colorectal and gastric cancers by immunohistochemistry. In both types of tumors, the levels of RIP140 and LCoR appeared highly correlated. Their expression tended to decrease in colorectal cancer as compared to adjacent normal tissues but was found higher in gastric cancer as compared to normal stomach. RIP140 and LCoR expression correlated with TNM and tumor differentiation. Significant correlations were observed with expression levels of key proteins involved in tumor progression and invasion namely E-cadherin and Cyclooxygenase-2. Survival analysis showed that patients with LCoR^low/RIP140^high colorectal tumors have a significant prolonged overall and disease-free survival. In gastric cancer, high LCoR expression was identified as an independent marker of poor prognosis suggesting a key role in this malignancy. Altogether, these results demonstrate that RIP140 and LCoR have a prognostic relevance in gastrointestinal cancers and could represent new potential biomarkers in these tumors.

Investigation of RIP140 and LCoR as independent markers for poor prognosis in cervical cancer

Introduction

RIP140 (Receptor Interacting Protein) is involved in the regulation of oncogenic signaling pathways and in the development of breast and colon cancers. The aim of the study was to analyze the expression of RIP140 and its partner LCoR in cervical cancers, to decipher their relationship with histone protein modifications and to identify a potential link with patient survival.

Methods

Immunohistochemical analyses were carried out to quantify RIP140 and LCoR expression in formalin-fixed paraffin-embedded tissue sections cervical cancer samples. Correlations of RIP140 and LCoR expression with histopathological variables were determined by correlation analyses. Survival rates of patients expressing low or high levels of RIP140 and LCoR were compared by Kaplan-Meier curves.

Results

RIP140 overexpression was associated with a significantly shorter overall survival of cervical cancer patients. This effect was significant in the squamous cell carcinoma subtype but not in adenocarcinomas. RIP140 is no longer a significant negative prognosticator for cervical cancer when LCoR expression is low.

Discussion

RIP140 is an independent predictor of poor survival of patients with cervical cancer. Patients with tumors expressing low levels of both RIP140 and LCoR showed a better survival compared to patients expressing high levels of RIP140. Modulation of RIP140 and LCoR may represent a novel targeting strategy for cervical cancer prevention and therapy.

Expression and role of nuclear receptor coregulators in colorectal cancer

Colorectal cancer (CRC) is one of the most common human cancers and the cause of about 700000 deaths per year worldwide. Deregulation of the WNT/β-catenin pathway is a key event in CRC initiation. This pathway interacts with other nuclear signaling pathways, including members of the nuclear receptor superfamily and their transcription coregulators. In this review, we provide an overview of the literature dealing with the main coactivators (NCoA-1 to 3, NCoA-6, PGC1-α, p300, CREBBP and MED1) and corepressors (N-CoR1 and 2, NRIP1 and MTA1) of nuclear receptors and summarize their links with the WNT/β-catenin signaling cascade, their expression in CRC and their role in intestinal physiopathology.

Complex regulation of LCoR signaling in breast cancer cells

Ligand-dependent corepressor (LCoR) is a transcriptional repressor of ligand-activated estrogen receptors (ERs) and other transcription factors that acts both by recruiting histone deacetylases and C-terminal binding proteins. Here, we first studied LCOR gene expression in breast cancer cell lines and tissues. We detected two mRNAs variants, LCoR and LCoR2 (which encodes a truncated LCoR protein). Their expression was highly correlated and localized in discrete nuclear foci. LCoR and LCoR2 strongly repressed transcription, inhibited estrogen-induced target gene expression and decreased breast cancer cell proliferation. By mutagenesis analysis, we showed that the helix-turn-helix domain of LCoR is required for these effects. Using in vitro interaction, coimmunoprecipitation, proximity ligation assay and confocal microscopy experiments, we found that receptor-interacting protein of 140 kDa (RIP140) is a LCoR and LCoR2 partner and that this interaction requires the HTH domain of LCoR and RIP140 N- and C-terminal regions. By increasing or silencing LCoR and RIP140 expression in human breast cancer cells, we then showed that RIP140 is necessary for LCoR inhibition of gene expression and cell proliferation. Moreover, LCoR and RIP140 mRNA levels were strongly correlated in breast cancer cell lines and biopsies. In addition, RIP140 positively regulated LCoR expression in human breast cancer cells and in transgenic mouse models. Finally, their expression correlated with overall survival of patients with breast cancer. Taken together, our results provide new insights into the mechanism of action of LCoR and RIP140 and highlight their strong interplay for the control of gene expression and cell proliferation in breast cancer cells.

A Dominant Mutation in Nuclear Receptor Interacting Protein 1 Causes Urinary Tract Malformations via Dysregulation of Retinoic Acid Signaling

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common cause of CKD in the first three decades of life. However, for most patients with CAKUT, the causative mutation remains unknown. We identified a kindred with an autosomal dominant form of CAKUT. By whole-exome sequencing, we identified a heterozygous truncating mutation (c.279delG, p.Trp93fs*) of the nuclear receptor interacting protein 1 gene (NRIP1) in all seven affected members. NRIP1 encodes a nuclear receptor transcriptional cofactor that directly interacts with the retinoic acid receptors (RARs) to modulate retinoic acid transcriptional activity. Unlike wild-type NRIP1, the altered NRIP1 protein did not translocate to the nucleus, did not interact with RARα, and failed to inhibit retinoic acid-dependent transcriptional activity upon expression in HEK293 cells. Notably, we also showed that treatment with retinoic acid enhanced NRIP1 binding to RARα RNA in situ hybridization confirmed Nrip1 expression in the developing urogenital system of the mouse. In explant cultures of embryonic kidney rudiments, retinoic acid stimulated Nrip1 expression, whereas a pan-RAR antagonist strongly reduced it. Furthermore, mice heterozygous for a null allele of Nrip1 showed a CAKUT-spectrum phenotype. Finally, expression and knockdown experiments in Xenopus laevis confirmed an evolutionarily conserved role for NRIP1 in renal development. These data indicate that dominant NRIP1 mutations can cause CAKUT by interference with retinoic acid transcriptional signaling, shedding light on the well documented association between abnormal vitamin A levels and renal malformations in humans, and suggest a possible gene-environment pathomechanism in this disease.

Transcriptional coregulator RIP140: an essential regulator of physiology

Transcriptional coregulators drive gene regulatory decisions in the transcriptional space. Although transcription factors including all nuclear receptors provide a docking platform for coregulators to bind, these proteins bring enzymatic capabilities to the gene regulatory sites. RIP140 is a transcriptional coregulator essential for several physiological processes, and aberrations in its function may lead to diseased states. Unlike several other coregulators that are known either for their coactivating or corepressing roles, in gene regulation, RIP140 is capable of acting both as a coactivator and a corepressor. The role of RIP140 in female reproductive axis and recent findings of its role in carcinogenesis and adipose biology have been summarised.

Minireview: The Link Between ERα Corepressors and Histone Deacetylases in Tamoxifen Resistance in Breast Cancer

Approximately 70% of breast cancers express the estrogen receptor (ER)α and are treated with the ERα antagonist, tamoxifen. However, resistance to tamoxifen frequently develops in advanced breast cancer, in part due to a down-regulation of ERα corepressors. Nuclear receptor corepressors function by attenuating hormone responses and have been shown to potentiate tamoxifen action in various biological systems. Recent genomic data on breast cancers has revealed that genetic and/or genomic events target ERα corepressors in the majority of breast tumors, suggesting that the loss of nuclear receptor corepressor activity may represent an important mechanism that contributes to intrinsic and acquired tamoxifen resistance. Here, the biological functions of ERα corepressors are critically reviewed to elucidate their role in modifying endocrine sensitivity in breast cancer. We highlight a mechanism of gene repression common to corepressors previously shown to enhance the antitumorigenic effects of tamoxifen, which involves the recruitment of histone deacetylases (HDACs) to DNA. As an indicator of epigenetic disequilibrium, the loss of ERα corepressors may predispose cancer cells to the cytotoxic effects of HDAC inhibitors, a class of drug that has been shown to effectively reverse tamoxifen resistance in numerous studies. HDAC inhibition thus appears as a promising therapeutic approach that deserves to be further explored as an avenue to restore drug sensitivity in corepressor-deficient and tamoxifen-resistant breast cancers.

LRH-1 drives colon cancer cell growth by repressing the expression of the CDKN1A gene in a p53-dependent manner

Liver receptor homologue 1 (LRH-1) is an orphan nuclear receptor that has been implicated in the progression of breast, pancreatic and colorectal cancer (CRC). To determine mechanisms underlying growth promotion by LRH-1 in CRC, we undertook global expression profiling following siRNA-mediated LRH-1 knockdown in HCT116 cells, which require LRH-1 for growth and in HT29 cells, in which LRH-1 does not regulate growth. Interestingly, expression of the cell cycle inhibitor p21 (CDKN1A) was regulated by LRH-1 in HCT116 cells. p21 regulation was not observed in HT29 cells, where p53 is mutated. p53 dependence for the regulation of p21 by LRH-1 was confirmed by p53 knockdown with siRNA, while LRH-1-regulation of p21 was not evident in HCT116 cells where p53 had been deleted. We demonstrate that LRH-1-mediated p21 regulation in HCT116 cells does not involve altered p53 protein or phosphorylation, and we show that LRH-1 inhibits p53 recruitment to the p21 promoter, likely through a mechanism involving chromatin remodelling. Our study suggests an important role for LRH-1 in the growth of CRC cells that retain wild-type p53.

KDM1 class flavin-dependent protein lysine demethylases

Flavin-dependent, lysine-specific protein demethylases (KDM1s) are a subfamily of amine oxidases that catalyze the selective posttranslational oxidative demethylation of methyllysine side chains within protein and peptide substrates. KDM1s participate in the widespread epigenetic regulation of both normal and disease state transcriptional programs. Their activities are central to various cellular functions, such as hematopoietic and neuronal differentiation, cancer proliferation and metastasis, and viral lytic replication and establishment of latency. Interestingly, KDM1s function as catalytic subunits within complexes with coregulatory molecules that modulate enzymatic activity of the demethylases and coordinate their access to specific substrates at distinct sites within the cell and chromatin. Although several classes of KDM1-selective small molecule inhibitors have been recently developed, these pan-active site inhibition strategies lack the ability to selectively discriminate between KDM1 activity in specific, and occasionally opposing, functional contexts within these complexes. Here we review the discovery of this class of demethylases, their structures, chemical mechanisms, and specificity. Additionally, we review inhibition of this class of enzymes as well as emerging interactions with coregulatory molecules that regulate demethylase activity in highly specific functional contexts of biological and potential therapeutic importance.

The emerging role of the transcriptional coregulator RIP140 in solid tumors

RIP140 is a transcriptional coregulator (also known as NRIP1) which plays very important physiological roles by finely tuning the activity of a large number of transcription factors. Noticeably, the RIP140 gene has been shown to be involved in the regulation of energy expenditure, in mammary gland development and intestinal homeostasis as well as in behavior and cognition. RIP140 is also involved in the regulation of various oncogenic signaling pathways and participates in the development and progression of solid tumors. This short review aims to summarize the role of this transcription factor on nuclear estrogen receptors, E2F and Wnt signaling pathways based on recent observations focusing on breast, ovary, liver and colon tumors.

Expression and role of RIP140/NRIP1 in chronic lymphocytic leukemia

RIP140 is a transcriptional coregulator, (also known as NRIP1), which finely tunes the activity of various transcription factors and plays very important physiological roles. Noticeably, the RIP140 gene has been implicated in the control of energy expenditure, behavior, cognition, mammary gland development and intestinal homeostasis. RIP140 is also involved in the regulation of various oncogenic signaling pathways and participates in the development and progression of solid tumors. During the past years, several papers have reported evidences linking RIP140 to hematologic malignancies. Among them, two recent studies with correlative data suggested that gene expression signatures including RIP140 can predict survival in chronic lymphocytic leukemia (CLL). This review aims to summarize the literature dealing with the expression of RIP140 in CLL and to explore the potential impact of this factor on transcription pathways which play key roles in this pathology.

Estrogen receptor coregulators and pioneer factors: the orchestrators of mammary gland cell fate and development

The steroid hormone, 17β-estradiol (E2), plays critical role in various cellular processes such as cell proliferation, differentiation, migration and apoptosis, and is essential for reproduction and mammary gland development. E2 actions are mediated by two classical nuclear hormone receptors, estrogen receptor α and β (ERs). The activity of ERs depends on the coordinated activity of ligand binding, post-translational modifications (PTMs), and importantly the interaction with their partner proteins called “coregulators.” Because coregulators are proved to be crucial for ER transcriptional activity, and majority of breast cancers are ERα positive, an increased interest in the field has led to the identification of a large number of coregulators. In the last decade, gene knockout studies using mouse models provided impetus to our further understanding of the role of these coregulators in mammary gland development. Several coregulators appear to be critical for terminal end bud (TEB) formation, ductal branching and alveologenesis during mammary gland development. The emerging studies support that, coregulators along with the other ER partner proteins called “pioneer factors” together contribute significantly to E2 signaling and mammary cell fate. This review discusses emerging themes in coregulator and pioneer factor mediated action on ER functions, in particular their role in mammary gland cell fate and development.

RIP140 increases APC expression and controls intestinal homeostasis and tumorigenesis

Deregulation of the Wnt/APC/β-catenin signaling pathway is an important consequence of tumor suppressor APC dysfunction. Genetic and molecular data have established that disruption of this pathway contributes to the development of colorectal cancer. Here, we demonstrate that the transcriptional coregulator RIP140 regulates intestinal homeostasis and tumorigenesis. Using Rip140-null mice and mice overexpressing human RIP140, we found that RIP140 inhibited intestinal epithelial cell proliferation and apoptosis. Interestingly, following whole-body irradiation, mice lacking RIP140 exhibited improved regenerative capacity in the intestine, while mice overexpressing RIP140 displayed reduced recovery. Enhanced RIP140 expression strongly repressed human colon cancer cell proliferation in vitro and after grafting onto nude mice. Moreover, in murine tissues and human cancer cells, RIP140 stimulated APC transcription and inhibited β-catenin activation and target gene expression. Finally, RIP140 mRNA and RIP140 protein levels were decreased in human colon cancers compared with those in normal mucosal tissue, and low levels of RIP140 expression in adenocarcinomas from patients correlated with poor prognosis. Together, these results support a tumor suppressor role for RIP140 in colon cancer.

Coordinated repressive chromatin-remodeling of Oct4 and Nanog genes in RA-induced differentiation of embryonic stem cells involves RIP140

Maintaining pluripotency and indefinite self-renewal of embryonic stem cells requires a tight control of the expression of several key stemness factors, particularly Nanog and Oct4 transcription factors. The mammalian SWItch/Sucrose NonFermentable (SWI/SNF) complex contains Brg1 or Brm as its core subunit, along with Brg1-associated factors. Our previous studies have addressed chromatin-remodeling of the Oct4 gene locus in retinoic acid (RA)-treated embryonal carcinoma cell line P19, which involves receptor-interacting protein 140 (RIP140) for heterochromatinization on the proximal promoter region of this gene locus. However, the mechanism of RIP140 action in RA-triggered repressive chromatin-remodeling is unclear. The current study examines RA repression of the Nanog gene and compares the results with RA repression of the Oct4 gene on the chromatin level. The results show a loose nucleosome array on the Nanog gene promoter in undifferentiated embryonic stem cells. On RA treatment, the Nanog gene locus remodels specifically in the CR1 region of its proximal promoter, with the insertion of a nucleosome and compaction of this region. Further, RA induces coordinated chromatin-remodeling of both Nanog and Oct4 gene loci, which requires RA receptor-α, RIP140 and Brm. Finally, in these RA-triggered repressive chromatin-remodeling processes, lysine acetylation of RIP140 is critical for its recruiting Brm.

Distinct functions for RIP140 in development, inflammation, and metabolism

Nuclear receptors (NRs) regulate tissue development and function by controlling transcription from distinct sets of genes in response to fluctuating levels of hormones or cues that modulate receptor activity. Such target gene activation or repression depends on the recruitment of coactivators or corepressors that lead to chromatin remodelling in the vicinity of target genes. Similarly to receptors, coactivators and corepressors often serve pleiotropic functions, and Nrip1 (RIP140) is no exception, playing roles in animal development and physiology. At first sight, however, RIP140 is unusual in its ability to function either as a coactivator or as a corepressor, and also serve a cytoplasmic role. The functions of RIP140 in different tissues will be summarised together with its potential contribution to disease.

Negative regulation of estrogen signaling by ERβ and RIP140 in ovarian cancer cells

In hormone-dependent tissues such as breast and ovary, tumorigenesis is associated with an altered expression ratio between the two estrogen receptor (ER) subtypes. In this study, we investigated the effects of ERβ ectopic expression on 17β-estradiol (E2)-induced transactivation and cell proliferation in ERα-positive BG1 ovarian cancer cells. As expected, ERβ expression strongly decreased the mitogenic effect of E2, significantly reduced E2-dependent transcriptional responses (both on a stably integrated estrogen response element [ERE] reporter gene and on E2-induced mRNAs), and strongly enhanced the formation of ER heterodimers as evidenced by chromatin immunoprecipitation analysis. Inhibition by the ERα-selective ligand propyl pyrazole triol was less marked than with the pan-agonist (E2) or the ERβ-selective (8β-vinyl-estradiol) ligands, indicating that ERβ activation reinforced the inhibitory effects of ERβ. Interestingly, in E2-stimulated BG1 cells, ERβ was more efficient than ERα to regulate the expression of receptor-interacting protein 140 (RIP140), a major ERα transcriptional corepressor. In addition, we found that the RIP140 protein interacted better with ERβ than with ERα (both in vitro and in intact cells by fluorescence cross-correlation spectroscopy). Moreover, RIP140 recruitment on the stably integrated reporter ERE was increased upon ERβ overexpression, and ERβ activity was more sensitive to repression by RIP140. Finally, small interfering RNA-mediated knockdown of RIP140 expression abolished the repressive effect exerted by activated ERβ on the regulation of ERE-controlled transcription by estrogens. Altogether, these data demonstrate the inhibitory effects of ERβ on estrogen signaling in ovarian cancer cells and the key role that RIP140 plays in this phenomenon.

Cognitive impairments in adult mice with constitutive inactivation of RIP140 gene expression

Receptor-interacting protein 140 (RIP140) is a negative transcriptional coregulator of nuclear receptors such as estrogen, retinoic acid or glucocorticoid receptors. Recruitment of RIP140 results in an inhibition of target gene expression through different repressive domains interacting with histone deacetylases or C-terminal binding proteins. In this study, we analyzed the role of RIP140 activity in memory processes using RIP140-deficient transgenic mice. Although the RIP140 protein was clearly expressed in the brain (cortical and hippocampus areas), the morphological examination of RIP140(-/-) mouse brain failed to show grossly observable alterations. Using male 2-month-old RIP140(-/-) , RIP140(+/-) or RIP140(+/+) mice, we did not observe any significant differences in the open-field test, rotarod test and in terms of spontaneous alternation in the Y-maze. By contrast, RIP140(-/-) mice showed long-term memory deficits, with an absence of decrease in escape latencies when animals were tested using a fixed platform position procedure in the water maze and in the passive avoidance test. Noteworthy, RIP140(-/-) mice showed decreased swimming speed, suggesting swimming alterations that may in part account for the marked alterations measured in the water maze. Moreover, RIP140(+/-) and RIP140(-/-) mice showed a significant increase in immobility time in the forced swimming test as compared with wild-type animals. These observations showed that RIP140 gene depletion results in learning and memory deficits as well as stress response, bringing to light a major role for this transcriptional coregulator in the neurophysiological developmental mechanisms underlying cognitive functions.

Manipulating protein acetylation in breast cancer: a promising approach in combination with hormonal therapies?

Estrogens play an essential role in the normal physiology of the breast as well as in mammary tumorigenesis. Their effects are mediated by two nuclear estrogen receptors, ERα and β, which regulate transcription of specific genes by interacting with multiprotein complexes, including histone deacetylases (HDACs). During the past few years, HDACs have raised great interest as therapeutic targets in the field of cancer therapy. In breast cancer, several experimental arguments suggest that HDACs are involved at multiple levels in mammary tumorigenesis: their expression is deregulated in breast tumors; they interfere with ER signaling in intricate ways, restoring hormone sensitivity in models of estrogen resistance, and they clinically represent new potential targets for HDACs inhibitors (HDIs) in combination with hormonal therapies. In this paper, we will describe these different aspects and underline the clinical interest of HDIs in the context of breast cancer resistance to hormone therapies (HTs).

Transcription factor co-repressors in cancer biology: roles and targeting

Normal transcription displays a high degree of flexibility over the choice, timing and magnitude of mRNA expression levels that tend to oscillate and cycle. These processes allow for combinatorial actions, feedback control and fine-tuning. A central role has emerged for the transcriptional co-repressor proteins such as NCOR1, NCOR2/SMRT, CoREST and CTBPs, to control the actions of many transcriptional factors, in large part, by recruitment and activation of a range of chromatin remodeling enzymes. Thus, co-repressors and chromatin remodeling factors are recruited to transcription factors at specific promoter/enhancer regions and execute changes in the chromatin structure. The specificity of this recruitment is controlled in a spatial-temporal manner. By playing a central role in transcriptional control, as they move and target transcription factors, co-repressors act as a key driver in the epigenetic economy of the nucleus. Co-repressor functions are selectively distorted in malignancy, by both loss and gain of function and contribute to the generation of transcriptional rigidity. Features of transcriptional rigidity apparent in cancer cells include the distorted signaling of nuclear receptors and the WNTs/beta-catenin axis. Understanding and predicting the consequences of altered co-repressor expression patterns in cancer cells has diagnostic and prognostic significance, and also have the capacity to be targeted through selective epigenetic therapies.

The transcriptional coregulator RIP140 represses E2F1 activity and discriminates breast cancer subtypes

PURPOSE

Receptor-interacting protein of 140 kDa (RIP140) is a transcriptional cofactor for nuclear receptors involved in reproduction and energy homeostasis. Our aim was to investigate its role in the regulation of E2F1 activity and target genes both in breast cancer cell lines and in tumor biopsies.

EXPERIMENTAL DESIGN

Glutathione S-transferase pull-down assays, coimmunoprecipitation experiments, and chromatin immunoprecipitation analysis were used to evidence interaction between RIP140 and E2F1. The effects of RIP140 expression on E2F1 activity were determined using transient transfection and quantification of E2F target mRNAs by quantitative real-time PCR. The effect on cell cycle was assessed by fluorescence-activated cell sorting analysis on cells overexpressing green fluorescent protein-tagged RIP140. A tumor microarray data set was used to investigate the expression of RIP140 and E2F1 target genes in 170 breast cancer patients.

RESULTS

We first evidenced the complex interaction between RIP140 and E2F1 and showed that RIP140 represses E2F1 transactivation on various transiently transfected E2F target promoters and inhibits the expression of several E2F1 target genes (such as CCNE1 and CCNB2). In agreement with a role for RIP140 in the control of E2F activity, we show that increasing RIP140 levels results in a reduction in the proportion of cells in S phase in various human cell lines. Finally, analysis of human breast cancers shows that low RIP140 mRNA expression was associated with high E2F1 target gene levels and basal-like tumors.

CONCLUSION

This study shows that RIP140 is a regulator of the E2F pathway, which discriminates luminal- and basal-like tumors, emphasizing the importance of these regulations for a clinical cancer phenotype.

The steroid receptor RNA activator protein is recruited to promoter regions and acts as a transcriptional repressor

Products of the steroid receptor RNA activator (SRA1) gene have the unusual property to function both at the RNA and the protein levels. SRA-RNA has long been known to increase the activity of multiple nuclear receptors. It has more recently been proposed than steroid receptor RNA activator protein (SRAP) also modulates steroid receptors activity. Herein, we show for the first time that SRAP physically interacts with multiple transcription factors and is recruited to specific promoter regions. Artificially recruiting SRAP to the promoter of a luciferase reporter gene under the control of the strong transcriptional activator VP16 leads to a decrease in transcription. Altogether we propose that SRAP could be a new transcriptional regulator, able to function as a repressor through direct association with promoters.

Estrogen receptor interactions and dynamics monitored in live cells by fluorescence cross-correlation spectroscopy

Quantitative characterization of protein interactions in live cells remains one of the most important challenges in modern biology. In the present work we have used two-photon, two-color, fluorescence cross-correlation spectroscopy (FCCS) in transiently transfected COS-7 cells to measure the concentrations and interactions of estrogen receptor (ER) subtypes alpha and beta with one of their transcriptional coactivator proteins, TIF2, as well as heterodimerization between the two ER subtypes. Using this approach in a systematic fashion, we observed a strong ligand-dependent modulation of receptor-coactivator complexation, as well as strong protein concentration dependence for complex formation in the absence of ligand. These quantitative values for protein and complex concentrations provide the first estimates for the ER-TIF2 K(d) for the full-length proteins and in a cellular context (agonist, < approximately 6 nM; antagonist, > approximately 3 microM; unliganded, approximately 200 nM). Coexpression of the two ER subtypes revealed substantial receptor heterodimer formation. They also provide, for the first time, estimated homo- and heterodimerization constants found to be similar and in the low nanomolar range. These results underscore the importance of receptor and coregulator expression levels and stability in the tissue-dependent modulation of receptor function under normal and pathological conditions.

RIP140 in thyroid hormone-repression and chromatin remodeling of Crabp1 gene during adipocyte differentiation

Cellular retinoic acid binding protein 1 (Crabp1) gene is biphasically (proliferation versus differentiation) regulated by thyroid hormone (T3) in 3T3-L1 cells. This study examines T3-repression of Crabp1 gene during adipocyte differentiation. T3 repression of Crabp1 requires receptor interacting protein 140 (RIP140). During differentiation, the juxtaposed chromatin configuration of Crabp1 promoter with its upstream region is maintained, but the 6-nucleosomes spanning thyroid hormone response element to transcription initiation site slide bi-directionally, with the third nucleosome remaining at the same position throughout differentiation. On the basal promoter, RIP140 replaces coactivators GRIP1 and PCAF and forms a repressive complex with CtBP1, HDAC3 and G9a. Initially active chromatin marks on this promoter, histone modifications H3-Ac and H3K4-me3, are weakened whereas repressive chromatin marks, H3K9-me3 and H3K27-me3 modification and recruitment of G9a, HP1α, HP1γ and H1, are intensified. This is the first study to examine chromatin remodeling, during the phase of hormone repression, of a bi-directionally regulated hormone target gene, and provides evidence for a functional role of RIP140 in chromatin remodeling to repress hormone target gene expression.

Molecular characterization of SMILE as a novel corepressor of nuclear receptors

SMILE (small heterodimer partner interacting leucine zipper protein) has been identified as a coregulator in ER signaling. In this study, we have examined the effects of SMILE on other NRs (nuclear receptors). SMILE inhibits GR, CAR and HNF4α-mediated transactivation. Knockdown of SMILE gene expression increases the transactivation of the NRs. SMILE interacts with GR, CAR and HNF4α in vitro and in vivo. SMILE and these NRs colocalize in the nucleus. SMILE binds to the ligand-binding domain or AF2 domain of the NRs. Competitions between SMILE and the coactivators GRIP1 or PGC-1α have been demonstrated in vitro and in vivo. Furthermore, an intrinsic repressive activity of SMILE is observed in Gal4-fusion system, and the intrinsic repressive domain is mapped to the C-terminus of SMILE, spanning residues 203–354. Moreover, SMILE interacts with specific HDACs (histone deacetylases) and SMILE-mediated repression is released by HDAC inhibitor trichostatin A, in a NR-specific manner. Finally, ChIP (chromatin immunoprecipitation) assays reveal that SMILE associates with the NRs on the target gene promoters. Adenoviral overexpression of SMILE represses GR-, CAR- and HNF4α-mediated target gene expression. Overall, these results suggest that SMILE functions as a novel corepressor of NRs via competition with coactivators and the recruitment of HDACs.

Coactivator function of RIP140 for NFkappaB/RelA-dependent cytokine gene expression

Inflammatory responses represent a hallmark of numerous pathologies including sepsis, bacterial infection, insulin resistance, and malign obesity. Here we describe an unexpected coactivator function for the nuclear receptor interacting protein 140 (RIP140) for nuclear factor kappaB (NFkappaB), a master transcriptional regulator of inflammation in multiple tissues. Previous work has shown that RIP140 suppresses the expression of metabolic gene networks, but we have found that genetic as well as acute deficiency of RIP140 leads to the inhibition of the proinflammatory program in macrophages. The ability of RIP140 to function as a coactivator for cytokine gene promoter activity relies on direct protein-protein interactions with the NFkappaB subunit RelA and histone acetylase cAMP-responsive element binding protein (CREB)-binding protein (CBP). RIP140-dependent control of proinflammatory gene expression via RelA/CBP may, therefore, represent a molecular rational for the cellular integration of metabolic and inflammatory pathways.

SUMOylation modulates the transcription repressor function of RIP140

RIP140/NRIP1 (receptor-interacting protein 140) functions as a corepressor of nuclear receptors. It plays an important role in the transcriptional control of energy metabolism and female fertility. RIP140 contains four distinct repression domains (RD1-RD4), and the repressive activity of RIP140 involves complex mechanisms. The function of both RD1 and RD2 is linked to recruitment of histone deacetylases and C-terminal binding protein, respectively, but the mechanism of repression for RD3 and RD4 has remained elusive. Because covalent modification by small ubiquitin-like modifiers (SUMO-1, -2, and -3; SUMOylation) is often associated with transcriptional repression, we studied whether SUMOylation is involved in the repressive activity of RIP140. We show that two conserved lysines, Lys(756) and Lys(1154), located in RD3 and RD4, respectively, are subject to reversible SUMOylation, with SUMO-1 being more efficiently conjugated than SUMO-2. Interestingly, mutations of the RIP140 SUMOylation sites compromised the transcription repressor function of RIP140 and blunted its capacity to repress estrogen receptor alpha-dependent transcription. Conjugation of SUMO-1 also influenced the subnuclear distribution pattern of RIP140. In sum, our demonstration that the function of RIP140 repression domains 3 and 4 can be modulated by reversible SUMO modification thus adds a novel level to the regulation of RIP140 activity, which may have ramifications in the control of gene networks exerted by RIP140.

Phosphorylation of CtBP1 by cAMP-dependent protein kinase modulates induction of CYP17 by stimulating partnering of CtBP1 and 2

In the human adrenal cortex, the peptide hormone adrenocorticotropin (ACTH) directs cortisol and adrenal androgen biosynthesis by activating a cAMP/cAMP-dependent protein kinase (PKA) pathway. Carboxyl-terminal binding protein 1 (CtBP1) is a corepressor that regulates transcription of the CYP17 gene by periodically interacting with steroidogenic factor-1 in response to ACTH signaling. Given that CtBP1 function is regulated by NADH binding, we hypothesized that ACTH-stimulated changes in cellular pyridine nucleotide concentrations modulate the ability of CtBP1 to repress CYP17 transcription. Further, we postulated that PKA evokes changes in the phosphorylation status of CtBP1 that control the ability of the protein to bind to steroidogenic factor-1 and the coactivator GCN5 (general control nonderepressed 5) and repress CYP17 gene expression. We show that ACTH alters pyridine nucleotide redox state and identify amino acid residues in CtBP1 that are targeted by PKA and PAK6. Both ACTH/cAMP signaling and NADH/NAD+ ratio stimulate nuclear-cytoplasmic oscillation of both CtBP proteins. We provide evidence that PKA 1) induces metabolic changes in the adrenal cortex and 2) phosphorylates CtBP proteins, particularly CtBP1 at T144, resulting in CtBP protein partnering and ACTH-dependent CYP17 transcription.

Regulation of hormone signaling by nuclear receptor interacting proteins

Nuclear receptors are ligand-activated transcription factors which regulate the expression of genes critical for the growth of hormone-dependent cancers. Their expression and activity are controlled by various cofactors which are important players in hormone-dependent carcinogenesis. RIP140 is a negative transcriptional regulator which is recruited by agonist-liganded receptors. Its strong repressive activity involves four silencing domains which interact with histone deacetylases (HDACs), carboxyl-terminal binding proteins (CtBPs) and additional partners. RIP140 positively regulates transactivation when nuclear receptors are recruited to target promoters through interaction with the Sp1 transcription factor. In human breast cancer cells, RIP140 expression is upregulated at the transcriptional level by various ligands of nuclear receptors revealing the existence of regulatory loops. The Mdm2 oncogenic ubiquitin-ligase is another protein which directly interacts with nuclear receptors. It is involved in a ternary complex with ERα and p53 and regulates ERα turn-over. In MCF-7 human breast cancer cells, various p53-inducing agents (such as UV irradiation) abolished E2-dependent turn-over of ERα without affecting its transactivation potential. Altogether, our results show that RIP140 and Mdm2 are two important regulators of ERα expression and activity and could therefore play major roles in hormone-dependent breast carcinogenesis.

Class IIa histone deacetylases: regulating the regulators

In the last decade, the identification of enzymes that regulate acetylation of histones and nonhistone proteins has revealed the key role of dynamic acetylation and deacetylation in various cellular processes. Mammalian histone deacetylases (HDACs), which catalyse the removal of acetyl groups from lysine residues, are grouped into three classes, on the basis of similarity to yeast counterparts. An abundance of experimental evidence has established class IIa HDACs as crucial transcriptional regulators of various developmental and differentiation processes. In the past 5 years, a tremendous effort has been dedicated to characterizing the regulation of these enzymes. In this review, we summarize the latest discoveries in the field and discuss the molecular and structural determinants of class IIa HDACs regulation. Finally, we emphasize that comprehension of the mechanisms underlying class IIa HDAC functions is essential for potential therapeutic applications.

Molecular basis for repression of liver X receptor-mediated gene transcription by receptor-interacting protein 140

Similarities in physiological roles of LXR (liver X receptors) and co-repressor RIP140 (receptor-interacting protein 140) in regulating energy homoeostasis and lipid and glucose metabolism suggest that the effects of LXR could at least partly be mediated by recruitment of the co-repressor RIP140. In the present study, we have elucidated the molecular basis for regulation of LXR transcriptional activity by RIP140. LXR is evenly localized in the nucleus and neither the N-terminal domain nor the LBD (ligand-binding domain) is necessary for nuclear localization. Both LXR subtypes, LXRalpha and LXRbeta, interact with RIP140 and co-localize in diffuse large nuclear domains. Interaction and co-localization are dependent on the LBD of the receptor. The C-terminal domain of RIP140 is sufficient for full repressive effect. None of the C-terminal NR (nuclear receptor)-boxes is required for the co-repressor activity, whereas the NR-box-like motif as well as additional elements in the C-terminal region are required for full repressive function. The C-terminal NR-box-like motif is necessary for interaction with LXRbeta, whereas additional elements are needed for strong interaction with LXRalpha. In conclusion, our results suggest that co-repression of LXR activity by RIP140 involves an atypical binding mode of RIP140 and a repression element in the RIP140 C-terminus.

Differential Regulation of Estrogen Receptor α Turnover and Transactivation by Mdm2 and Stress-Inducing Agents

In mammalian cells, the level of estrogen receptor alpha (ERalpha) is rapidly decreased upon estrogen treatment, and this regulation involves proteasome degradation. Using different approaches, we showed that the Mdm2 oncogenic ubiquitin-ligase directly interacts with ERalpha in a ternary complex with p53 and is involved in the regulation of ERalpha turnover (both in the absence or presence of estrogens). Several lines of evidence indicated that this effect of Mdm2 required its ubiquitin-ligase activity and involved the ubiquitin/proteasome pathway. Moreover, in MCF-7 human breast cancer cells, various p53-inducing agents (such as UV irradiation) or treatment with RITA (which inhibits the interaction of p53 with Mdm2) stabilized ERalpha and abolished its 17beta-estradiol-dependent turnover. Interestingly, our data indicated that ligand-dependent receptor turnover was not required for efficient transactivation. Altogether, our results indicate that the Mdm2 oncoprotein and stress-inducing agents complexly and differentially regulate ERalpha stability and transcriptional activity in human cancer cells.

Negative regulation of hormone signaling by RIP140

Receptor interacting protein (RIP) 140 is a negative transcriptional regulator of nuclear hormone receptors which is required for the maintenance of energy homeostasis and ovulation. Despite its recruitment by agonist-liganded receptors, this protein exhibits a strong repressive activity which was initially attributed to competition with coactivator binding on nuclear receptors. However, RIP140 also exerts active repression implicating the Carboxyl-terminal binding proteins (CtBPs) and histone deacetylases (HDACs). We recently demonstrated that the Carboxyl-terminal region of the molecule contains two additional silencing domains which require post-translational modifications to be fully active. In human breast cancer cells, RIP140 expression is up-regulated at the transcriptional level by various ligands of nuclear receptors. We have recently cloned the human RIP140 gene and defined the mechanism of its regulation by estrogens. In order to better characterize the role of RIP140 in hormone signaling, we have studied its interaction with the androgen receptor and demonstrated its ability to repress transcriptional regulation by androgens. RIP140 also inhibits transactivation by estrogen receptor-related receptors (ERRalpha, beta and gamma) on natural or artificial reporter genes containing different types of response elements. Surprisingly, RIP140 positively regulates ERR transactivation when the receptors are recruited to target promoters through interaction with the Sp1 transcription factor and this effect could involve titration of histone deacetylases. Altogether, these results underline that transcriptional regulation of hormone signaling by the cofactor RIP140 involves complex mechanisms relying on multiple domains and partners.

Coregulator exchange and sphingosine-sensitive cooperativity of steroidogenic factor-1, general control nonderepressed 5, p54, and p160 coactivators regulate cyclic adenosine 3',5'-monophosphate-dependent cytochrome P450c17 transcription rate

Transcription of the cytochrome P450 17 (CYP17) gene is regulated by cAMP-dependent binding of steroidogenic factor-1 (SF-1) to its promoter in the adrenal cortex. Using temporal chromatin immunoprecipitation and mammalian two-hybrid experiments, we establish the reciprocal presence of coactivators [general control nonderepressed (GCN5), cAMP response element-binding protein-binding protein, p300, p300/cAMP response element-binding protein-binding protein CBP associated factor, p160s, polypyrimidine tract associated splicing factor, and p54(nrb)], corepressors (class I histone deacetylases, receptor interacting protein, nuclear receptor corepressor, and Sin3A), and SWI/SNF (human homolog of yeast mating type switching/sucrose nonfermenting) and imitation SWI chromatin remodeling ATPases on the CYP17 promoter during transcription cycles in the H295R adrenocortical cell line. A ternary GCN5/SRC-1/SF-1 complex forms on the CYP17 promoter with cAMP-dependence within 30 min of cAMP stimulation, and corresponds with SWI/SNF chromatin remodeling. This complex is sensitive to the SF-1 antagonist sphingosine and results in decreased transcription of CYP17. GCN5 acetyltransferase activity and carboxy terminus binding proteins alternatively mediate disassembly of the complex. This work establishes the temporal order of cAMP-induced events on the promoter of a key steroidogenic gene during SF-1-mediated transcription.

The nuclear receptor transcriptional coregulator RIP140

The nuclear receptor superfamily comprises ligand-regulated transcription factors that control various developmental and physiological pathways. These receptors share a common modular structure and regulate gene expression through the recruitment of a large set of coregulatory proteins. These transcription cofactors regulate, either positively or negatively, chromatin structure and transcription initiation. One of the first proteins to be identified as a hormone-recruited cofactor was RIP140. Despite its recruitment by agonist-liganded receptors, RIP140 exhibits a strong transcriptional repressive activity which involves several inhibitory domains and different effectors. Interestingly, the RIP140 gene, located on chromosome 21 in humans, is finely regulated at the transcriptional level by various nuclear receptors. In addition, the protein undergoes several post-translational modifications which control its repressive activity. Finally, experiments performed in mice devoid of the RIP140 gene indicate that this transcriptional cofactor is essential for female fertility and energy homeostasis. RIP140 therefore appears to be an important modulator of nuclear receptor activity which could play major roles in physiological processes and hormone-dependent diseases.