Altered localization of retinoid X receptor alpha coincides with loss of retinoid responsiveness in human breast cancer MDA-MB-231 cells

Catching the glucocorticoid receptor in the act: Lessons from fluorescence fluctuation methods

Technological innovation can drive scientific inquiry by allowing researchers to answer questions that were once out of reach. Eukaryotic mRNA synthesis was not so long ago thought of as a deterministic, sequential process in which transcriptional regulators and general transcription factors assemble in an orderly fashion into chromatin to, ultimately, activate RNA polymerase II. Advances in fluorescence microscopy techniques have revealed a much more complex scenario, wherein transcriptional regulators dynamically engage with chromatin in a more stochastic, probabilistic way. In this review, we will concentrate on what fluorescence fluctuation methods have taught us about the journey of transcription factors within live cells. Specifically, we summarized how these techniques have contributed to reshaping our understanding of the mechanism(s) of action of the glucocorticoid receptor, a ligand-regulated transcription factor involved in many physiological and pathological processes. This receptor regulates a variety of gene networks in a context-specific manner and its activity can be quickly and easily controlled by the addition of specific ligands. Thus, it is widely used as a model to study the mechanisms of transcription factors through live-cell imaging.

Discovery of bipyridine amide derivatives targeting pRXRα-PLK1 interaction for anticancer therapy

Retinoid X receptor alpha (RXRα) is an important therapeutic target of cancer. Recently, small molecules (e.g.,XS-060 and its derivatives), which can significantly induce RXRα-dependent mitotic arrest by inhibiting pRXRα-PLK1 interaction, have been demonstrated as excellent anticancer agents. To further obtain novel RXR-targeted antimitotic agents with excellent bioactivity and drug-like properties, we herein synthesized two new series of bipyridine amide derivatives with XS-060 as the lead compound. In the reporter gene assay, most synthesized compounds showed antagonistic activity against RXRα. The most active compound, bipyridine amide B9 (BPA-B9), showed better activity than XS-060, with excellent RXRα-binding affinity (KD = 39.29 ± 1.12 nM) and anti-proliferative activity against MDA-MB-231 (IC50 = 16 nM, SI > 3). Besides, a docking study revealed a proper fitting of BPA-B9 into the coactivator binding site of RXRα, rationalizing its potent antagonistic effect on RXRα transactivation. Further, the mechanism studies revealed that the anticancer activity of BPA-B9 was dependent on its cellular RXRα-targeted mechanism, such as inhibiting pRXRα-PLK1 interaction and inducing RXRα-dependent mitotic arrest. Besides, BPA-B9 displayed better pharmacokinetics than the lead XS-060. Further, animal assays indicated BPA-B9 had significant anticancer efficacy in vivo with no considerable side effects. Together, our study reveals a novel RXRα ligand BPA-B9 targeting the pRXRα-PLK1 interaction, with great potential as a promising anticancer drug candidate for further development.

METTL3 preferentially enhances non-m6A translation of epigenetic factors and promotes tumourigenesis

METTL3 encodes the predominant catalytic enzyme to promote m⁶A methylation in nucleus. Recently, accumulating evidence has shown the expression of METTL3 in cytoplasm, but its function is not fully understood. Here we demonstrated an m⁶A-independent mechanism for METTL3 to promote tumour progression. In gastric cancer, METTL3 could not only facilitate cancer progression via m⁶A modification, but also bind to numerous non-m⁶A-modified mRNAs, suggesting an unexpected role of METTL3. Mechanistically, cytoplasm-anchored METTL3 interacted with PABPC1 to stabilize its association with cap-binding complex eIF4F, which preferentially promoted the translation of epigenetic factors without m⁶A modification. Clinical investigation showed that cytoplasmic distributed METTL3 was highly correlated with gastric cancer progression, and this finding could be expanded to prostate cancer. Therefore, the cytoplasmic METTL3 enhances the translation of epigenetic mRNAs, thus serving as an oncogenic driver in cancer progression, and METTL3 subcellular distribution can assist diagnosis and predict prognosis for patients with cancer.

RXR – centralny regulator wielu ścieżek sygnałowych w organizmie

Receptory jądrowe (NRs) tworzą największą nadrodzinę czynników transkrypcyjnych, które odgrywają ważną rolę w regulacji wielu procesów biologicznych. Receptor kwasu 9-cis-retinowego (RXR) wydaje się odgrywać szczególną rolę wśród tej grupy białek, a to ma związek z jego zdolnością do tworzenia dimerów z innymi NRs. Ze względu na kontrolę ekspresji wielu genów, RXR stanowi bardzo dobry cel licznych terapii. Nieprawidłowości w szlakach modulowanych przez RXR są powiązane m.in. z chorobami neurodegeneracyjnymi, otyłością, cukrzycą, a także nowotworami. Istnieje wiele związków mogących regulować aktywność transkrypcyjną RXR. Jednak obecnie dopuszczonych do użytku klinicznego jest tylko kilka z nich. Retinoidy normalizują wzrost i różnicowanie komórek skóry i błon śluzowych, ponadto działają immunomodulująco oraz przeciwzapalnie. Stąd są stosowane przede wszystkim w chorobach skóry i w terapii niektórych chorób nowotworowych. W artykule przedstawiono ogólne wiadomości na temat RXR, jego budowy, ligandów i mechanizmu działania oraz potencjalnej roli w terapii nowotworów i zespołu metabolicznego.

Cytoplasmic Localization of RXRα Determines Outcome in Breast Cancer

Simple Summary

Considering the immense development of today’s therapeutic approaches in oncology towards customized therapy, this study aimed to assess the prognostic value of nuclear versus cytoplasmic retinoid X receptor α (RXRα) expression in breast cancer. Our results demonstrate that RXRα expression may have different roles in tumorigenesis according to its subcellular localization. This study strengthens the need for further research on the behavior of RXRα, depending on its intracellular localization.

Abstract

The aim of this retrospective study was to assess the prognostic value of cytoplasmic versus nuclear RXRα expression in breast cancer (BC) tissue samples and to correlate the results with clinicopathological parameters. In 319 BC patients, the expression of RXRα was evaluated via immunohistochemistry. Prognosis-determining aspects were calculated through uni- and multivariate analyses. Correlation analysis revealed a trend association with nuclear RXRα expression regarding an improved overall survival (OS) (p = 0.078), whereas cytoplasmic RXRα expression was significantly correlated with a poor outcomes in terms of both OS (p = 0.038) and disease-free survival (DFS) (p = 0.037). Strengthening these results, cytoplasmic RXRα was found to be an independent marker for DFS (p = 0.023), when adjusted to clinicopathological parameters, whereas nuclear RXRα expression was positively associated with lower TNM-staging, i.e., pT (p = 0.01), pN (p = 0.029) and pM (p = 0.001). Additionally, cytoplasmic RXRα expression was positively associated with a higher histopathological tumor grading (p = 0.02). Cytoplasmic RXRα was also found to be a negative prognosticator for Her-2neu-negative and triple-negative patients. Altogether, these findings support the hypothesis that the subcellular localization of RXRα plays an important role in carcinogenesis and the prognosis of BC. The expression of cytoplasmic RXRα is correlated with a more aggressive course of the disease, whereas nuclear RXRα expression appears to be a protective factor. These data may help to identify high-risk BC subgroups in order to find possible specific options in targeted tumor therapy.

Phasing the intranuclear organization of steroid hormone receptors

Steroid receptors (SRs) encompass a family of transcription factors that regulate the expression of thousands of genes upon binding to steroid hormones and include the glucocorticoid, androgen, progesterone, estrogen and mineralocorticoid receptors. SRs control key physiological and pathological processes, thus becoming relevant drug targets. As with many other nuclear proteins, hormone-activated SRs concentrate in multiple discrete foci within the cell nucleus. Even though these foci were first observed ∼25 years ago, their exact structure and function remained elusive. In the last years, new imaging methodologies and theoretical frameworks improved our understanding of the intranuclear organization. These studies led to a new paradigm stating that many membraneless nuclear compartments, including transcription-related foci, form through a liquid–liquid phase separation process. These exciting ideas impacted the SR field by raising the hypothesis of SR foci as liquid condensates involved in transcriptional regulation. In this work, we review the current knowledge about SR foci formation under the light of the condensate model, analyzing how these structures may impact SR function. These new ideas, combined with state-of-the-art techniques, may shed light on the biophysical mechanisms governing the formation of SR foci and the biological function of these structures in normal physiology and disease.

Nuclear Receptors Are Differentially Expressed and Activated in KAIMRC1 Compared to MCF7 and MDA-MB231 Breast Cancer Cells

We recently established a KAIMRC1 cell line that has unique features compared to the known breast cancer cell lines, MCF7 and MDA-MB231. To characterize it further, we investigated the expression profile of nuclear receptors and their respective co-factors in these cell lines. We confirm that in contrast to the triple negative cell line MDA-MB231, the MCF7 and KAIMRC1 are estrogen receptor alpha (ERa) and progesterone receptor alpha (PRa) positive, with significant lower expression of these receptors in KAIMRC1. KAIMRC1 cell is a vitamin D receptor (VDR) negative and V-ErbA-Related Protein 2 (EAR2) positive in contrast to MCF7 and MDA-MB231. Remarkably, the histone deacetylases (HDACs) are highly expressed in KAIRMC1 with HDAC6 and HDAC 7 are exclusively expressed in KAIMRC1 while thyroid hormone receptor-associated protein 80 (TRAP80), telomeric DNA binding protein 1 (TBP1) and TGF-beta receptor interacting protein (TRIP1) are absent in KAIMRC1 but present in MCF7 and MDA-MB231. In a luciferase reporter assay, the ERa coexpression is needed for estrogen receptor element (ERE)-luciferase activation by estradiol in KAIMRC1 but not in MCF7. The co-expression of exogenous Liver X receptor alpha (LXRa)/retinoid X receptor alpha (RXRa) are necessary for LXR responsive element (LXRE) activation by the GW3696 in the three cell lines. However, the activity of peroxisome proliferator-activated receptor response element (PPARE)-tk-luciferase reporter increased when peroxisome proliferator-activated receptors alpha (PPARa)/RXRa were coexpressed but the addition of PPARa agonist (GW7647) did not stimulate further the reporter. The signal of the PPARE reporter increased in a dose-dependent manner with rosiglitazone (PPARg agonist) in KAIMRC1, MCF7, and MDA-MB231 when the proliferator-activated receptors gamma (PPARg)/RXRa receptors were cotransfected. Retinoic acid-induced activation of retinoic acid receptor response element (RARE)-tk-luciferase is dependent on exogenous expression of retinoic acid receptor alpha (RARa)/RXRa heterodimer in MDA-MB 231 but not in MCF7 and KAIMRC1 cell lines. In the three cell lines, Bexarotene-induced retinoid X receptor response element (RXRE)-luciferase reporter activation was induced only if the RXRa/LXRa heterodimer were co-expressed. The vitamin D receptor response element (VDRE)-luciferase reporter activity showed another distinct feature of KAIMRC1, where only co-expression of exogenous vitamin D receptor (VDR)/RXRa heterodimer was sufficient to reach the maximum rate of activation of VDRE reporter. In the proliferation assay, nuclear receptors ligands showed a distinct effect on KAIMRC1 compared to MCF7 and MDA-MB231. Growth inhibition effects of used ligands suggest that KAIMRC1 correlate more closely to MDA-MB231 than MCF7. Vitamin D3, rosiglitazone, novel RXR compound (RXRc) and PPARa compound (GW6471) have the most profound effects. In conclusion, we showed that nuclear receptors are differentially expressed, activated and also their ligand produced distinct effects in KAIMRC1 compared to MCF7 and MDA-MB231. This finding gives us confidence that KAIMRC1 has a unique biological phenotype.

RXRα provokes tumor suppression through p53/p21/p16 and PI3K-AKT signaling pathways during stem cell differentiation and in cancer cells

The retinoid X receptor alpha (RXRα) is an important therapeutic target impacting diverse biological processes. Activation of RXRα is known to suppress cancer cell growth. However, the cellular mechanism has been elusive. In the present study, we addressed its role during stem cell differentiation and the underlying connections with carcinogenesis. RXRα was significantly upregulated following the differentiation of human mesenchymal stem cell (hMSC) toward the formation of endothelial cell (EC). However, overexpression of RXRα in hMSC provoked a senescence-like phenotype accompanied by the elevation of tumor suppressor p53, p21, and p16. Consistently, RXRα level was suppressed in cancer cells (~five times lower compared to differentiated hMSC), and its elevation could inhibit the proliferation, migration, and angiogenesis of cancer cells. We further demonstrated that these inhibitory effects were related to RXRα’s interaction with estrogen receptor α (ERα) as well as EGF and ANGPTL3 through modulating PI3K/AKT signaling pathway by AKT and FAK phosphorylation. Moreover, RXRα inhibited glycolytic metabolism in cancer cells, which might be underlying its inhibition of differentiation and carcinogenic features. These data suggest that RXRα acts as a suppressor rather than a driving force during stem cell differentiation, and unbalanced RXRα can trigger multiple yet connected signaling pathways in preventing carcinogenesis.

Expression and clinical significance of retinoid X receptor α in esophageal carcinoma

PURPOSE

Esophageal carcinoma (EC) is one of the most aggressive type cancers and dysregulation of retinoid X receptor α (RXRα) involves various tumors. However, the relationship of RXRα with the clinicopathological factors of EC, particularly prognostic characteristics, remains unclear. This present study was to evaluate the effect of RXRα expression in the development of EC.

METHODS

The mRNA and protein expression level of RXRα in EC and normal esophageal tissues using reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, respectively. The subcellular localization was detected by immunohistochemistry (IHC) analysis. The clinicopathological parameters were included age, sex, tumor size, differentiation, TNM stages and lymph node metastasis. Kaplan-Meier method and Cox's regression analyses were performed to evaluate the prognosis of 60 patients with EC.

RESULTS

RXRα was elevated in EC tissues comparing with normal esophageal tissues at both mRNA and protein levels. The overexpression level of RXRα was closely associated to the tumor differentiation, TNM stage and lymph node metastasis of patients with EC. In addition, EC patients with RXRα high expression had significantly lower disease-free survival (DFS) and overall survival (OS). Multivariate analysis showed RXRα expression as an independent predictor for the DFS and OS rate of patients with EC.

CONCLUSIONS

Our results showed that overexpression of RXRα was correlated with unfavorable prognosis, suggesting that RXRα may serve as a potential targeted therapeutic marker in the treatment of EC.

Targeting truncated RXRα for cancer therapy

Retinoid X receptor-alpha (RXRα), a unique member of the nuclear receptor superfamily, is a well-established drug target, representing one of the most important targets for pharmacologic interventions and therapeutic applications for cancer. However, how RXRα regulates cancer cell growth and how RXRα modulators suppress tumorigenesis are poorly understood. Altered expression and aberrant function of RXRα are implicated in the development of cancer. Previously, several studies had demonstrated the presence of N-terminally truncated RXRα (tRXRα) proteins resulted from limited proteolysis of RXRα in tumor cells. Recently, we discovered that overexpression of tRXRα can promote tumor growth by interacting with tumor necrosis factor-alpha-induced phosphoinositide 3-kinase and NF-κB signal transduction pathways. We also identified nonsteroidal anti-inflammatory drug Sulindac and analogs as effective inhibitors of tRXRα activities via a unique binding mechanism. This review discusses the emerging roles of tRXRα and modulators in the regulation of cancer cell survival and death as well as inflammation and our recent understanding of tRXRα regulation by targeting the alternate binding sites on its surface.

Retinoids and rexinoids in cancer prevention: from laboratory to clinic

Early in the age of modern medicine the consequences of vitamin A deficiency drew attention to the fundamental link between retinoid-dependent homeostatic regulation and malignant hyperproliferative diseases. The term "retinoid" includes a handful of endogenous and a large group of synthetic derivatives of vitamin A. These multifunctional lipid-soluble compounds directly regulate target genes of specific biological functions and critical signaling pathways to orchestrate complex functions from vision to development, metabolism, and inflammation. Many of the retinoid activities on the cellular level have been well characterized and translated to the regulation of processes like differentiation and cell death, which play critical roles in the outcome of malignant transformation of tissues. In fact, retinoid-based differentiation therapy of acute promyelocytic leukemia was one of the first successful examples of molecularly targeted treatment strategies. The selectivity, high receptor binding affinity and the ability of retinoids to directly modulate gene expression programs present a distinct pharmacological opportunity for cancer treatment and prevention. However, to fully exploit their potential, the adverse effects of retinoids must be averted. In this review we provide an overview of the biology of retinoid (activated by nuclear retinoic acid receptors [RARs]) and rexinoid (engaged by nuclear retinoid X receptors [RXRs]) action concluded from a long line of preclinical studies, in relation to normal and transformed states of cells. We will also discuss the past and current uses of retinoids in the treatment of malignancies, the potential of rexinoids in the cancer prevention setting, both as single agents and in combinations.

Regulation of the nongenomic actions of retinoid X receptor-α by targeting the coregulator-binding sites

Retinoid X receptor-α (RXRα), a unique member of the nuclear receptor superfamily, represents an intriguing and unusual target for pharmacologic interventions and therapeutic applications in cancer, metabolic disorders and neurodegenerative diseases. Despite the fact that the RXR-based drug Targretin (bexarotene) is currently used for treating human cutaneous T-cell lymphoma and the fact that RXRα ligands (rexinoids) show beneficial effects in the treatment of cancer and diseases, the therapeutic potential of RXRα remains unexplored. In addition to its conventional transcription regulation activity in the nucleus, RXRα can act in the cytoplasm to modulate important biological processes, such as mitochondria-dependent apoptosis, inflammation, and phosphatidylinositol 3-kinase (PI3K)/AKT-mediated cell survival. Recently, new small-molecule-binding sites on the surface of RXRα have been identified, which mediate the regulation of the nongenomic actions of RXRα by a class of small molecules derived from the nonsteroidal anti-inflammatory drug (NSAID) Sulindac. This review discusses the emerging roles of the nongenomic actions of RXRα in the RXRα signaling network, and their possible implications in cancer, metabolic and neurodegenerative disorders, as well as our current understanding of RXRα regulation by targeting alternate binding sites on its surface.

AEG-1 regulates retinoid X receptor and inhibits retinoid signaling

Retinoid X receptor (RXR) regulates key cellular responses such as cell growth and development, and this regulation is frequently perturbed in various malignancies, including hepatocellular carcinoma (HCC). However, the molecule(s) that physically govern this deregulation are mostly unknown. Here, we identified RXR as an interacting partner of astrocyte-elevated gene-1 (AEG-1)/metadherin (MTDH), an oncogene upregulated in all cancers. Upon interaction, AEG-1 profoundly inhibited RXR/retinoic acid receptor (RAR)-mediated transcriptional activation. Consequently, AEG-1 markedly protected HCC and acute myelogenous leukemia (AML) cells from retinoid- and rexinoid-induced cell death. In nontumorigenic cells and primary hepatocytes, AEG-1/RXR colocalizes in the nucleus in which AEG-1 interferes with recruitment of transcriptional coactivators to RXR, preventing transcription of target genes. In tumor cells and AEG-1 transgenic hepatocytes, overexpressed AEG-1 entraps RXR in cytoplasm, precluding its nuclear translocation. In addition, ERK, activated by AEG-1, phosphorylates RXR that leads to its functional inactivation and attenuation of ligand-dependent transactivation. In nude mice models, combination of all-trans retinoic acid (ATRA) and AEG-1 knockdown synergistically inhibited growth of human HCC xenografts. The present study establishes AEG-1 as a novel homeostatic regulator of RXR and RXR/RAR that might contribute to hepatocarcinogenesis. Targeting AEG-1 could sensitize patients with HCC and AML to retinoid- and rexinoid-based therapeutics.

Sequential multi-molecule delivery using vortex-assisted electroporation

We developed an on-chip microscale electroporation system that enables sequential delivery of multiple molecules with precise and independent dosage controllability into pre-selected identical populations of target cells. The ability to trap cells with uniform size distribution contributed to enhanced molecular delivery efficiency and cell viability. Additionally, the system provides real-time monitoring ability of the entire delivery process, allowing timely and independent modification of cell- and molecule-specific electroporation parameters. The precisely controlled amount of inherently membrane-impermeant molecules was transferred into human cancer cells by varying electric field strengths and molecule injection durations. The proposed microfluidic electroporation system's improved viability and comparable gene transfection efficiency to that of commercial systems suggest that the current system has great potential to expand the research fields that on-chip electroporation techniques can be used in.

Correlation of thyroid hormone, retinoid X, peroxisome proliferator-activated, vitamin D and oestrogen/progesterone receptors in breast carcinoma

Non-steroidal nuclear receptors play a major role in breast cancer development. A correlation among, and possible prognostic function of, the members of the nuclear receptor superfamily has been discussed controversially over the years. Hence, we conducted a quantification of the different expression levels of the thyroid receptor (TR), retinoid X receptor (RXR), peroxisome proliferator-activated receptor (PPAR) and vitamin D receptor (VDR) in malignant breast tumour tissue samples. Patients diagnosed and treated for breast cancer between 1990 and 2000 were included. Receptor expression was detected by immunohistochemical staining. Correlation analyses for the expression of the receptors were performed for the clinical and histopathological data. The paraffin-embedded tissue from 82 breast cancer patients was available. The different steroid receptors showed varying results when correlated with known histopathological markers. TRα2 demonstrated the most significant correlations with steroid hormone receptors. Significant correlations between the major isoforms of TR, and between RXR, PPAR and VDR, were demonstrated in the patient sample. The immunohistochemical association of these receptors may provide the first proof of an interaction on the molecular level. This assumption awaits confirmation in studies with larger cohorts.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Retinoic acid-induced CCR9 expression requires transient TCR stimulation and cooperativity between NFATc2 and the retinoic acid receptor/retinoid X receptor complex

Retinoic acid (RA) imprints gut-homing specificity on T cells upon activation by inducing the expression of chemokine receptor CCR9 and integrin α4β7. CCR9 expression seemed to be more highly dependent on RA than was the α4β7 expression, but its molecular mechanism remained unclear. In this article, we show that NFAT isoforms NFATc1 and NFATc2 directly interact with RA receptor (RAR) and retinoid X receptor (RXR) but play differential roles in RA-induced CCR9 expression on murine naive CD4(+) T cells. TCR stimulation for 6-24 h was required for the acquisition of responsiveness to RA and induced activation of NFATc1 and NFATc2. However, RA failed to induce CCR9 expression as long as TCR stimulation continued. After terminating TCR stimulation or adding cyclosporin A to the culture, Ccr9 gene transcription was induced, accompanied by inactivation of NFATc1 and sustained activation of NFATc2. Reporter and DNA-affinity precipitation assays demonstrated that the binding of NFATc2 to two NFAT-binding sites and that of the RAR/RXR complex to an RA response element half-site in the 5'-flanking region of the mouse Ccr9 gene were critical for RA-induced promoter activity. NFATc2 directly bound to RARα and RXRα, and it enhanced the binding of RARα to the RA response element half-site. NFATc1 also bound to the NFAT-binding sites and directly to RARα and RXRα, but it inhibited the NFATc2-dependent promoter activity. These results suggest that the cooperativity between NFATc2 and the RAR/RXR complex is essential for CCR9 expression on T cells and that NFATc1 interferes with the action of NFATc2.

Retinoid X receptors: common heterodimerization partners with distinct functions

Retinoid X receptors (RXRs) have been implicated in a diversity of cellular processes ranging from cellular proliferation to lipid metabolism. These pleiotropic effects stem not only from the ability of RXRs to dimerize with diverse nuclear receptors, which exert transcriptional control on specific aspects of cell biology, but also because binding of RXR ligands to heterodimers can stimulate transcriptional activation by RXR partner receptors. This signaling network is rendered more complex by the existence of different RXR isotypes (RXRα, RXRβ, RXRγ) with distinct properties that thereby modulate the transcriptional activity of RXR-containing heterodimers. This review discusses the emerging roles of RXR isotypes in the RXR signaling network and possible implications for our understanding of nuclear receptor biology and pharmacology.

Epigenetic modulation of the retinoid X receptor alpha by green tea in the azoxymethane-Apc Min/+ mouse model of intestinal cancer

We investigated the possible mechanisms of inhibition of colorectal carcinogenesis by green tea (GT) in azoxymethane-treated (AOM) Apc(Min/+) mice. Mice received water or a 0.6% (w/v) solution of GT as the only source of beverage. GT treatment commenced at the 8th week of age and lasted for 8 wk. The treatment caused a statistically significant reduction in the number of newly formed tumors (28%, P < 0.05). Immunohistochemical analysis showed that GT decreased the levels of beta-catenin and its downstream target cyclin D1. To probe a mechanism, we further investigated the expression of retinoic X receptor alpha (RXR alpha) in AOM/Apc(Min/+) tumors. Our results show that RXR alpha is selectively downregulated in AOM/Apc(Min/+) mouse intestinal tumors. In contrast, other retinoic receptors including retinoic acid receptor alpha (RAR alpha), RAR beta, RXR beta, and RXR gamma were all expressed in Apc(Min/+) adenomas. Furthermore, our results show that RXR alpha downregulation is an early event in colorectal carcinogenesis and is independent of beta-catenin expression. GT significantly increased the protein levels of RXR alpha. In addition, RT-PCR analysis showed that GT induced a similar increase in the levels of RXR alpha mRNA. Genomic bisulfite treatment of colonic DNA followed by pyrosequencing of 24 CpG sites in the promoter region of RXR alpha gene showed a significant decrease in CpG methylation with GT treatment. The results suggest that a low concentration of GT is sufficient to desilence RXR alpha and inhibit intestinal tumorigenesis in the Apc(Min/+) mouse.

Therapeutic potential of "rexinoids" in cancer prevention and treatment

Retinoid X receptor (RXR) is a combinatorial partner for one third of the 48 human nuclear receptor superfamily members and acts as a master coordinator of nuclear receptor signaling pathways involved in the control of cell growth and differentiation. Thus, ligand-dependent simultaneous activation of multiple pathways is an attractive strategy for molecular-targeted therapy of neoplastic disease. However, clinical trials in RXR-targeted molecular therapy with the RXR ligand (rexinoid) have yielded disappointing outcomes. In this review, we discuss a possible mechanism underlying the loss of sensitivity to rexinoid therapy.

Sensitization by docosahexaenoic acid (DHA) of breast cancer cells to anthracyclines through loss of glutathione peroxidase (GPx1) response

Docosahexaenoic acid (DHA, a lipid of marine origin) has been found to enhance the activity of several anticancer drugs through an oxidative mechanism. To examine the relation between chemosensitization by DHA and tumor cells antioxidant status, we used two breast cancer cell lines: MDA-MB-231, in which DHA increases sensitivity to doxorubicin, and MCF-7, which does not respond to DHA. Under these conditions, reactive oxygen species (ROS) level increased on anthracycline treatment only in MDA-MB-231. This was concomitant with a decreased cytosolic glutathione peroxidase (GPx1) activity, a crucial enzyme for protection against hydrogen and lipid peroxides, while major antioxidant enzyme activities increased in both cell lines in response to ROS. GPx-decreased activity was accompanied by an accumulation of glutathione, the GPx cosubstrate, and resulted from a decreased amount of GPx protein. In rat mammary tumors, when a DHA dietary supplementation led to an increased tumor sensitivity to anthracyclines, GPx1 activity was similarly decreased. Furthermore, vitamin E abolished both DHA effects on chemotherapy efficacy enhancement and on GPx1 inhibition. Thus, loss of GPx response to an oxidative stress in transformed cells may account for the ability of peroxidizable targets such as DHA to enhance tumor sensitivity to ROS-generating anticancer drugs.

The effect of body weight on altered expression of nuclear receptors and cyclooxygenase-2 in human colorectal cancers

Background

Epidemiological studies on risk factors for colorectal cancer (CRC) have mainly focused on diet, and being overweight is now recognized to contribute significantly to CRC risk. Overweight and obesity are defined as an excess of adipose tissue mass and are associated with disorders in lipid metabolism. Peroxisome proliferator-activated receptors (PPARs) and retinoid-activated receptors (RARs and RXRs) are important modulators of lipid metabolism and cellular homeostasis. Alterations in expression and activity of these ligand-activated transcription factors might be involved in obesity-associated diseases, which include CRC. Cyclooxygenase-2 (COX-2) also plays a critical role in lipid metabolism and alterations in COX-2 expression have already been associated with unfavourable clinical outcomes in epithelial tumors. The objective of this study is to examine the hypothesis questioning the relationship between alterations in the expression of nuclear receptors and COX-2 and the weight status among male subjects with CRC.

Method

The mRNA expression of the different nuclear receptor subtypes and of COX-2 was measured in 20 resected samples of CRC and paired non-tumor tissues. The association between expression patterns and weight status defined as a body mass index (BMI) was statistically analyzed.

Results

No changes were observed in PPARγ mRNA expression while the expression of PPARδ, retinoid-activated receptors and COX-2 were significantly increased in cancer tissues compared to normal colon mucosa (P ≤ 0.001). The weight status appeared to be an independent factor, although we detected an increased level of COX-2 expression in the normal mucosa from overweight patients (BMI ≥ 25) compared to subjects with healthy BMI (P = 0.002).

Conclusion

Our findings show that alterations in the pattern of nuclear receptor expression observed in CRC do not appear to be correlated with patient weight status. However, the analysis of COX-2 expression in normal colon mucosa from subjects with a high BMI suggests that COX-2 deregulation might be driven by excess weight during the colon carcinogenesis process.

p21WAF1/CIP1 is a common transcriptional target of retinoid receptors: pleiotropic regulatory mechanism through retinoic acid receptor (RAR)/retinoid X receptor (RXR) heterodimer and RXR/RXR homodimer

The divergent response and the molecular mechanisms underlying the anti-cancer effects of retinoid X receptor (RXR) ligand (rexinoid) therapy are poorly understood. This study demonstrates that ligand-activated RXR homodimer facilitated G(1) arrest by up-regulation of p21 in vitro and in vivo but failed to induce G(1) arrest when p21 expression was blocked by p21 small interfering RNA. RXR ligand-dependent p21 up-regulation was transcriptionally controlled through the direct binding of RXR homodimers to two consecutive retinoid X response elements in the p21 promoter. Structural overlap of a retinoic acid response element with these retinoid X response elements led to a high affinity binding of retinoic acid receptor/RXR heterodimer to the retinoic acid response element, resulting in the prevention of RXR ligand-mediated p21 transactivation. These data show that p21 is a potential and novel molecular target for RXR ligand-mediated anti-cancer therapy and that the expression level of retinoic acid receptor and RXR in tumors may be crucial to induce p21-mediated cell growth arrest in RXR ligand therapy.

Novel retinoic acid metabolism blocking agents have potent inhibitory activities on human breast cancer cells and tumour growth

Antitumour effects of retinoids are attributed to their influence on cell proliferation, differentiation, apoptosis and angiogenesis. In our effort to develop useful agents for breast cancer therapy, we evaluated the effects of four representative retinoic acid metabolism blocking agents (RAMBAs, VN/14-1, VN/50-1, VN/66-1 and VN/69-1) on growth inhibition of oestrogen receptor positive (ER +ve, MCF-7 and T-47D) and oestrogen receptor negative (ER −ve, MDA-MB-231) human breast cancer cells. Additionally, we investigated the biological effects/molecular mechanism(s) underlying their growth inhibitory properties as well as their antitumour efficacies against MCF-7 and MCF-7Ca tumour xenografts in nude mice. We also assessed the effect of combining VN/14-1 and all-trans-retinoic acid (ATRA) on MCF-7 tumuor xenografts. The ER +ve cell lines were more sensitive (IC₅₀ values between 3.0 and 609 nM) to the RAMBAs than the ER −ve MDA-MB-231 cell line (IC₅₀=5.6–24.0 μM). Retinoic acid metabolism blocking agents induced cell differentiation as determined by increased expression of cytokeratin 8/18 and oestrogen receptor-α (ER-α). Similar to ATRA, they also induced apoptosis via activation of caspase 9. Cell cycle analysis indicated that RAMBAs arrested cells in the G1 and G2/M phases and caused significant downregulation (>80%) of cyclin D1 protein. In vivo, the growth of MCF-7 mammary tumours was dose-dependently and significantly inhibited (92.6%, P<0.0005) by VN/14-1. The combination of VN/14-1 and ATRA also inhibited MCF-7 breast tumour growth in vivo (up to 120%) as compared with single agents (P<0.025). VN/14-1 was also very effective in preventing the formation of MCF-7Ca tumours and it significantly inhibited the growth of established MCF-7Ca tumours, being as effective as the clinically used aromatase inhibitors, anastrozole and letrozole. Decrease in cyclin D1 and upregulation of cytokeratins, Bad and Bax with VN/14-1 may be responsible for the efficacy of this compound in inhibiting breast cancer cell growth in vitro and in vivo. Our results suggest that our RAMBAs, especially VN/14-1 may be useful novel therapy for breast cancer.

9-cis-retinoic acid up-regulates expression of transcriptional coregulator PELP1, a novel coactivator of the retinoid X receptor alpha pathway

Retinoid X receptor alpha (RXRalpha), functioning as either a homodimer or a heterodimer with peroxisome proliferator receptors, is known to be involved in manifesting antiproliferative effects in cells. Consequently, studies of RXRalpha functions and its coregulators have been in the focus for therapeutic approaches against cancer. Here we have discovered that 9-cis-retinoic acid (9-cis-RA), a RXRalpha-specific ligand, up-regulated the expression of transcriptional coregulatory protein PELP1 (proline-, glutamic acid-, and leucine-rich protein 1). PELP1 functioned as a coactivator of RXRalpha, increasing its transactivation function in response to 9-cis-RA as evident by the retinoid X receptor response element-luciferase assays. PELP1 was found to be a binding partner of RXRalpha, and the binding interactions were confirmed both in vitro and in vivo. An electrophoretic mobility shift assay showed greater formation and stability of RXRalpha homodimers on consensus oligonucleotides in PELP1-overexpressing clones in comparison to the pcDNA clones. The presence of PELP1 in these oligonucleotide-bound RXRalpha homodimers was proved by the supershift of the complex when incubated with PELP1-specific antibody. PELP1-overexpressing stable MCF-7 cells exhibited a significantly higher extent of 9-cis-RA-induced apoptosis than the control pcDNA clones. Silencing of PELP1 expression in parental MCF-7 cells and PELP1-overexpressing clones using PELP1-specific RNA-mediated interference compromised the susceptibility to 9-cis-RA-induced apoptosis. PELP1 could also function as a coactivator of the RXRalpha-peroxisome proliferator-activated receptor (PPARgamma) heterodimer as evident by the peroxisome proliferator-activated receptor response element-luciferase assay in response to both 9-cis-RA and PPARgamma-specific ligands. This was reinforced by the higher propensity of PELP1-overexpressing clones to undergo differentiation in response to PPARgamma-specific ligands. This study has revealed a novel facet of PELP1 functions and identified it to be an important potentiator of the antiproliferative effects of 9-cis-RA and PPARgamma-specific ligands.

Retinoid Receptor Subtypes in Sebaceous Cell Carcinoma of the Eyelid

PURPOSE

To study retinoid receptor expression in sebaceous cell carcinoma of the eyelid.

METHODS

Expression of retinoid receptors (RAR alpha, beta, and gamma and RXR alpha, beta, and gamma) in tumor specimens from 10 patients with sebaceous cell carcinoma of the eyelid and in 3 normal incidental tarsus specimens from healthy adults without cancer was studied immunohistochemically by using antiretinoid receptor antibodies.

RESULTS

In all 3 specimens of normal tarsus, all 6 retinoid receptors were expressed. RARalpha expression was absent in 3 tumors and was decreased in 3 tumors compared with expression in the control tissues. RARbeta expression in carcinomas was primarily perinuclear; 6 tumors showed increased RARbeta expression compared with controls. RARgamma expression was absent in 4 tumors and was decreased in 2 tumors compared with controls. RXRalpha nuclear expression was decreased compared with controls in 5 tumors. RXRbeta expression was low in 7 tumors. RXRgamma expression was absent in 6 tumors.

CONCLUSIONS

Aberrant expression of retinoid receptors in sebaceous cell carcinoma of the eyelid might play a role in the pathogenesis and progression of this carcinoma.

Retinoid X receptors: X-ploring their (patho)physiological functions

Retinoid X receptor (RXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. A class of nuclear receptors requires RXR as heterodimerization partner for their function. This places RXR in the crossroad of multiple distinct biological pathways. This and the fact that the debate on the endogenous ligand requirement for RXR is not yet settled make RXR still an enigmatic transcription factor. Here, we review some of the biology of RXR. We place RXR into the evolution of nuclear receptors, review structural details and ligands of the receptor. Then processes regulated by RXR are discussed focusing on the developmental roles deduced from studies on knockout animals and metabolic roles in diseases such as diabetes and atherosclerosis deduced from pharmacological studies. Finally, aspects of RXR's involvement in myeloid differentiation and apoptosis are summarized along with issues on RXR's suitability as a therapeutic target.

The casein kinase 1 family: participation in multiple cellular processes in eukaryotes

Phosphorylation of serine, threonine and tyrosine residues by cellular protein kinases plays an important role in the regulation of various cellular processes. The serine/threonine specific casein kinase 1 and 2 protein kinase families--(CK1 and CK2)--were among the first protein kinases that had been described. In recent years our knowledge of the regulation and function of mammalian CK1 kinase family members has rapidly increased. Extracellular stimuli, the subcellular localization of CK1 isoforms, their interaction with various cellular structures and proteins, as well as autophosphorylation and proteolytic cleavage of their C-terminal regulatory domains influence CK1 kinase activity. Mammalian CK1 isoforms phosphorylate many different substrates among them key regulatory proteins involved in the control of cell differentiation, proliferation, chromosome segregation and circadian rhythms. Deregulation and/or the incidence of mutations in the coding sequence of CK1 isoforms have been linked to neurodegenerative diseases and cancer. This review will summarize our current knowledge about the function and regulation of mammalian CK1 isoforms.

Casein Kinase 1α Interacts with Retinoid X Receptor and Interferes with Agonist-induced Apoptosis

Agonists of retinoid X receptors (RXRs), which include the natural 9-cis-retinoic acid and synthetic analogs, are potent inducers of growth arrest and apoptosis in some cancer cells. As such, they are being used in clinical trials for the treatment and prevention of solid tumors and are used to treat cutaneous T cell lymphoma. However, the molecular mechanisms that underlie the anti-cancer effects of RXR agonists remain unclear. Here, we show that a novel pro-apoptotic pathway that is induced by RXR agonist is negatively regulated by casein kinase 1alpha (CK1alpha). CK1alpha associates with RXR in an agonist-dependent manner and phosphorylates RXR. The ability of an RXR agonist to recruit CK1alpha to a complex with RXR in cells correlates inversely with its ability to inhibit growth. Remarkably, depletion of CK1alpha in resistant cells renders them susceptible to RXR agonist-induced growth inhibition and apoptosis. Our study shows that CK1alpha can promote cell survival by interfering with RXR agonist-induced apoptosis. Inhibition of CK1alpha may enhance the anti-cancer effects of RXR agonists.