Decreased expression of retinoid X receptor isoforms in human thyroid carcinomas

Nuclear receptor RXRα binds the precursor of miR-103 to inhibit its maturation

BACKGROUND

The maturation of microRNAs (miRNAs) successively undergoes Drosha, Dicer, and Argonaute -mediated processing, however, the intricate regulations of the individual miRNA maturation are largely unknown. Retinoid x receptor alpha (RXRα) belongs to nuclear receptors that regulate gene transcription by binding to DNA elements, however, whether RXRα binds to miRNAs to exert physiological functions is not known.

RESULTS

In this work, we found that RXRα directly binds to the precursor of miR-103 (pre-miR-103a-2) via its DNA-binding domain with a preferred binding sequence of AGGUCA. The binding of RXRα inhibits the processing of miR-103 maturation from pre-miR-103a-2. Mechanistically, RXRα prevents the nuclear export of pre-miR-103a-2 for further processing by inhibiting the association of exportin-5 with pre-miR-103a-2. Pathophysiologically, the negative effect of RXRα on miR-103 maturation correlates to the positive effects of RXRα on the expression of Dicer, a target of miR-103, and on the inhibition of breast cancer.

CONCLUSIONS

Our findings unravel an unexpected role of transcription factor RXRα in specific miRNA maturation at post-transcriptional level through pre-miRNA binding, and present a mechanistic insight regarding RXRα role in breast cancer progression.

A maternal high-fat diet induces fetal origins of NASH-HCC in mice

Maternal overnutrition affects offspring susceptibility to nonalcoholic steatohepatitis (NASH). Male offspring from high-fat diet (HFD)-fed dams developed a severe form of NASH, leading to highly vascular tumor formation. The cancer/testis antigen HORMA domain containing protein 1 (HORMAD1), one of 146 upregulated differentially expressed genes in fetal livers from HFD-fed dams, was overexpressed with hypoxia-inducible factor 1 alpha (HIF-1alpha) in hepatoblasts and in NASH-based hepatocellular carcinoma (HCC) in offspring from HFD-fed dams at 15 weeks old. Hypoxia substantially increased Hormad1 expression in primary mouse hepatocytes. Despite the presence of three putative hypoxia response elements within the mouse Hormad1 gene, the Hif-1alpha siRNA only slightly decreased hypoxia-induced Hormad1 mRNA expression. In contrast, N-acetylcysteine, but not rotenone, inhibited hypoxia-induced Hormad1 expression, indicating its dependency on nonmitochondrial reactive oxygen species production. Synchrotron-based phase-contrast micro-CT of the fetuses from HFD-fed dams showed significant enlargement of the liver accompanied by a consistent size of the umbilical vein, which may cause hypoxia in the fetal liver. Based on these findings, a maternal HFD induces fetal origins of NASH/HCC via hypoxia, and HORMAD1 is a potential therapeutic target for NASH/HCC.

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.

Nuclear receptors: Lipid and hormone sensors with essential roles in the control of cancer development

Nuclear receptors (NRs) are a superfamily of ligand-activated transcription factors that act as biological sensors and use a combination of mechanisms to modulate positively and negatively gene expression in a spatial and temporal manner. The highly orchestrated biological actions of several NRs influence the proliferation, differentiation, and apoptosis of many different cell types. Synthetic ligands for several NRs have been the focus of extensive drug discovery efforts for cancer intervention. This review summarizes the roles in tumour growth and metastasis of several relevant NR family members, namely androgen receptor (AR), estrogen receptor (ER), glucocorticoid receptor (GR), thyroid hormone receptor (TR), retinoic acid receptors (RARs), retinoid X receptors (RXRs), peroxisome proliferator-activated receptors (PPARs), and liver X receptors (LXRs). These studies are key to develop improved therapeutic agents based on novel modes of action with reduced side effects and overcoming resistance.

VHL enhances 9-cis-retinoic acid treatment by down-regulating retinoid X receptor α in renal cell carcinomas

Renal cell carcinoma (RCC) is the most common malignant kidney tumors in adults. Von Hippel-Lindau (VHL) gene is deficient in >50% of RCC cases, but the role of VHL as a potential therapeutic target in RCC has not been well established. In the present study, 9-cis-Retinoic acid, which is a potent natural agonist of retinoid X receptors (RXRs), was found to decrease the viability of VHL-proficient RCC cells, but had little effect on VHL-deficient RCC cells. In addition, it was demonstrated that VHL transcriptionally regulated RXRα in a hypoxia-inducible factor-α independent manner. Moreover, a negative correlation was observed between the expressions of VHL and RXRα in RCC tissues. Collectively, these data indicate that VHL-proficient RCC patients may be more sensitive to treatment with 9-cis-retinoic acid, which acts by regulating RXRα expression, compared with VHL-deficient RCC patients. The findings of the present study demonstrate a novel function of VHL and highlight the potential of VHL expression as a therapeutic modality for the optimized treatment of RCC patients.

Hypoxia-inducible factor-1α is the therapeutic target of the SGLT2 inhibitor for diabetic nephropathy

Previous studies have demonstrated intrarenal hypoxia in patients with diabetes. Hypoxia-inducible factor (HIF)-1 plays an important role in hypoxia-induced tubulointerstitial fibrosis. Recent clinical trials have confirmed the renoprotective action of SGLT2 inhibitors in diabetic nephropathy. We explored the effects of an SGLT2 inhibitor, luseogliflozin on HIF-1α expression in human renal proximal tubular epithelial cells (HRPTECs). Luseogliflozin significantly inhibited hypoxia-induced HIF-1α protein expression in HRPTECs. In addition, luseogliflozin inhibited hypoxia-induced the expression of the HIF-1α target genes PAI-1, VEGF, GLUT1, HK2 and PKM. Although luseogliflozin increased phosphorylated-AMP-activated protein kinase α (p-AMPKα) levels, the AMPK activator AICAR did not changed hypoxia-induced HIF-1α expression. Luseogliflozin suppressed the oxygen consumption rate in HRPTECs, and subsequently decreased hypoxia-sensitive dye, pimonidazole staining under hypoxia, suggesting that luseogliflozin promoted the degradation of HIF-1α protein by redistribution of intracellular oxygen. To confirm the inhibitory effect of luseogliflozin on hypoxia-induced HIF-1α protein in vivo, we treated male diabetic db/db mice with luseogliflozin for 8 to 16 weeks. Luseogliflozin attenuated cortical tubular HIF-1α expression, tubular injury and interstitial fibronectin in db/db mice. Together, luseogliflozin inhibits hypoxia-induced HIF-1α accumulation by suppressing mitochondrial oxygen consumption. The SGLT2 inhibitors may protect diabetic kidneys by therapeutically targeting HIF-1α protein.

Oncogenic potential of truncated RXRα during colitis-associated colorectal tumorigenesis by promoting IL-6-STAT3 signaling

Retinoid X receptor-alpha (RXRα) is a potent regulator of inflammatory responses; however, its therapeutic potential for inflammatory cancer remains to be explored. We previously discovered that RXRα is abnormally cleaved in tumor cells and tissues, producing a truncated RXRα (tRXRα). Here, we show that transgenic expression of tRXRα in mice accelerates the development of colitis-associated colon cancer (CAC). The tumorigenic effect of tRXRα is primarily dependent on its expression in myeloid cells, which results in interleukin-6 (IL-6) induction and STAT3 activation. Mechanistic studies reveal an extensive interaction between tRXRα and TRAF6 in the cytoplasm of macrophages, leading to TRAF6 ubiquitination and subsequent activation of the NF-κB inflammatory pathway. K-80003, a tRXRα modulator derived from nonsteroidal anti-inflammatory drug (NSAID) sulindac, suppresses the growth of tRXRα-mediated colorectal tumor by inhibiting the NF-κB-IL-6-STAT3 signaling cascade. These results provide new insight into tRXRα action and identify a promising tRXRα ligand for treating CAC.

Epigallocatechin-3-gallate inhibited cancer stem cell-like properties by targeting hsa-mir-485-5p/RXRα in lung cancer

Non-small-cell lung cancer (NSCLC) appears to be a significant threat to public health worldwide. MicroRNAs have been identified as significant regulators for the development of NSCLC. Previous reports have suggested that hsa-mir-485-5p is dysregulated in various cancers. RXRα, as a kind of nuclear receptor, is an effective target of cancer treatment. Cancer stem cells (CSCs) are recognized as the main cause for tumor metastasis, recurrence, and chemotherapy resistance. However, the mechanism by which hsa-mir-485-5p and RXRα modulate CSCs in NSCLC remains unknown. Here, we found that hsa-mir-485-5p was decreased in serum samples from patients with NSCLC and NSCLC cells. Meanwhile, epigallocatechin-3-gallate (EGCG), an effective anticancer compound extracted from green tea, can enhance hsa-mir-485-5p expression. Hsa-mir-485-5p mimics markedly inhibited NSCLC cell growth and induced cell apoptosis. However, inhibition of hsa-mir-485-5p significantly enriched CSC-like traits. Moreover, bioinformatics analysis predicted the binding correlation between hsa-mir-485-5p and RXRα, which was confirmed by a dual-luciferase reporter assay. We observed that RXRα was increased in NSCLC and EGCG could inhibit RXRα levels dose dependently. In addition, RXRα upregulation or activation expanded the CSC-like properties of NSCLC cells, whereas RXRα inhibition or inactivation could exert a reverse phenomenon. Consistently, in vivo experiments also validated that EGCG could repress the CSC-like characteristics by modulating the hsa-mir-485-5p/RXRα axis. Our findings may reveal a novel molecular mechanism for the treatment of NSCLC.

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.

Modulation of nongenomic activation of PI3K signalling by tetramerization of N-terminally-cleaved RXRα

Retinoid X receptor-alpha (RXRα) binds to DNA either as homodimers or heterodimers, but it also forms homotetramers whose function is poorly defined. We previously discovered that an N-terminally-cleaved form of RXRα (tRXRα), produced in tumour cells, activates phosphoinositide 3-kinase (PI3K) signalling by binding to the p85α subunit of PI3K and that K-80003, an anti-cancer agent, inhibits this process. Here, we report through crystallographic and biochemical studies that K-80003 binds to and stabilizes tRXRα tetramers via a ‘three-pronged’ combination of canonical and non-canonical mechanisms. K-80003 binding has no effect on tetramerization of RXRα, owing to the head–tail interaction that is absent in tRXRα. We also identify an LxxLL motif in p85α, which binds to the coactivator-binding groove on tRXRα and dissociates from tRXRα upon tRXRα tetramerization. These results identify conformational selection as the mechanism for inhibiting the nongenomic action of tRXRα and provide molecular insights into the development of RXRα cancer therapeutics.

The transcription factor retinoid X receptor-alpha (RXRα) can also form homotetramers. Here the authors show that the anti-cancer agent K-80003 selectively inhibits the nongenomic action of N-terminally-cleaved RXRα in tumour cells by stabilizing its tetramerization but not that of full-length RXRα.

TRC4, an improved triptolide derivative, specifically targets to truncated form of retinoid X receptor-alpha in cancer cells

The nuclear retinoid X receptor-α (RXRα) plays critical roles in cell homeostasis and in many physiological processes mainly through its transcriptional function. However, an N-terminal truncated form of RXRα, tRXRα, was frequently described in various cancer cells and tumor tissues, thus representing a new promising drug target. We recently demonstrated that triptolide (TR01) could target to the oncogenic activity of tRXRα. To improve its tumor selectivity, we developed several TR01 derivatives by introducing different amine ester groups on C-14-hydroxyl site. Interestingly, C-14 modification could differently affect the expression of tRXRα without interfering the level of its full length RXRα. Among the derivatives, TRC4 could strongly reduce tRXRα expression, while TRC5-7 increased it. The capability of inhibiting tRXRα expression was shown to be closely associated with its inactivation of AKT and induction of apoptosis in various cancer cells. Conversely, treatment of cancer cells with the tRXRα-stabilizing compounds TRC5-7 resulted in enhanced AKT activity and apoptosis-resistance. However, although TR01 could strongly reduce tRXRα expression and AKT activity, it also strongly inhibited the expression and transcriptional activity of RXRα in normal cells. Importantly, the tRXRα-selective TRC4 that did not significantly inhibit RXRα transcriptional function retained the most potency of the anticancer effect of TR01 and had no significant effect on the viability of normal cells. In conclusion, our results demonstrated that tRXRα-selective TRC4 will have potential clinical application in terms of drug target and side effects. Our findings will offer new strategies to develop improved triptolide analogs for cancer therapy.

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.

Retinoid X receptor α enhances human cholangiocarcinoma growth through simultaneous activation of Wnt/β-catenin and nuclear factor-κB pathways

Retinoid X receptor α (RXRα) plays important roles in the malignancy of several cancers such as human prostate tumor, breast cancer, and thyroid tumor. However, its exact functions and molecular mechanisms in cholangiocarcinoma (CCA), a chemoresistant carcinoma with poor prognosis, remain unclear. In this study we found that RXRα was frequently overexpressed in human CCA tissues and CCA cell lines. Downregulation of RXRα led to decreased expression of mitosis‐promoting factors including cyclin D1and cyclin E, and the proliferating cell nuclear antigen, as well as increased expression of cell cycle inhibitor p21, resulting in inhibition of CCA cell proliferation. Furthermore, RXRα knockdown attenuated the expression of cyclin D1 through suppression of Wnt/β‐catenin signaling. Retinoid X receptor α upregulated proliferating cell nuclear antigen expression through nuclear factor‐κB (NF‐κB) pathways, paralleled with downregulation of p21. Thus, the Wnt/β‐catenin and NF‐κB pathways account for the inhibition of CCA cell growth induced by RXRα downregulation. Retinoid X receptor α plays an important role in proliferation of CCA through simultaneous activation of Wnt/β‐catenin and NF‐κB pathways, indicating that RXRα might serve as a potential molecular target for CCA treatment.

NSC-640358 acts as RXRα ligand to promote TNFα-mediated apoptosis of cancer cell

Retinoid X receptor α (RXRα) and its N-terminally truncated version tRXRα play important roles in tumorigenesis, while some RXRα ligands possess potent anti-cancer activities by targeting and modulating the tumorigenic effects of RXRα and tRXRα. Here we describe NSC-640358 (N-6), a thiazolyl-pyrazole derived compound, acts as a selective RXRα ligand to promote TNFα-mediated apoptosis of cancer cell. N-6 binds to RXRα and inhibits the transactivation of RXRα homodimer and RXRα/TR3 heterodimer. Using mutational analysis and computational study, we determine that Arg316 in RXRα, essential for 9-cis-retinoic acid binding and activating RXRα transactivation, is not required for antagonist effects of N-6, whereas Trp305 and Phe313 are crucial for N-6 binding to RXRα by forming extra π–π stacking interactions with N-6, indicating a distinct RXRα binding mode of N-6. N-6 inhibits TR3-stimulated transactivation of Gal4-DBD-RXRα-LBD by binding to the ligand binding pocket of RXRα-LBD, suggesting a strategy to regulate TR3 activity indirectly by using small molecules to target its interacting partner RXRα. For its physiological activities, we show that N-6 strongly inhibits tumor necrosis factor α (TNFα)-induced AKT activation and stimulates TNFα-mediated apoptosis in cancer cells in an RXRα/tRXRα dependent manner. The inhibition of TNFα-induced tRXRα/p85α complex formation by N-6 implies that N-6 targets tRXRα to inhibit TNFα-induced AKT activation and to induce cancer cell apoptosis. Together, our data illustrate a new RXRα ligand with a unique RXRα binding mode and the abilities to regulate TR3 activity indirectly and to induce TNFα-mediated cancer cell apoptosis by targeting RXRα/tRXRα.

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.

Nuclear receptor expression in human differentiated thyroid tumors

BACKGROUND

Nuclear receptors (NRs) play a key role in endocrine signaling and metabolism and are important therapeutic targets in a number of hormone-dependent malignancies. Studies on the role of NRs in thyroid cancer are limited.

OBJECTIVE

The objective of the study was to examine systematically the expression of the 48 human NRs in a series of benign and malignant thyroid tissues. Within the papillary carcinoma cohort, we sought to determine if NR expression differed significantly by BRAF mutation status.

PATIENTS AND METHODS

RNA was isolated from multinodular goiter (MNG; n=6), papillary carcinoma (PTC, n=14), follicular carcinoma (FC; n=5), and Hürthle cell carcinoma (HCC; n=7). The 48 human NRs were profiled in this panel by quantitative real time polymerase chain reaction. Protein expression for selected NRs (Rev-erbα and LXR-β) was examined by immunohistochemistry (IHC) on tissue microarrays comprising benign and malignant thyroid tissues.

RESULTS

Across all groups of benign and malignant thyroid tissue, there was prominent expression of LXR-β and ROR-γ. Key findings in PTC were marked overexpression of RXR-γ and Rev-erbα compared to MNG. Within the PTC cohort, when BRAF(V600E) tumors were compared with wild type BRAF, there was relative upregulation of RXR-γ and Rev-erbα and downregulation of AR, ERR-γ, and ROR-γ. In FC, EAR-2 was overexpressed, while PPAR-α and PPAR-δ were underexpressed compared to MNG. The NR expression profile of HCC was distinct, characterized by significant downregulation of a wide range of NRs. IHC for Rev-erbα and LXR-β localized protein expression to the tumor cells. Moderate to strong Rev-erbα immunostaining was seen in 22 out of 23 PTC, and, overall, staining was stronger than in the benign group.

CONCLUSIONS

These results represent the first systematic examination of NR expression in thyroid cancer. Our finding of tumor-specific patterns of NR expression, as well as significant differences in NR expression between BRAF(V600E) and wild type BRAF PTC, provides a basis for further mechanistic studies and highlights potential novel therapeutic targets for this malignancy.

RXRs: collegial partners

Retinoid X Receptors (RXR) were initially identified as nuclear receptors binding with stereo-selectivity the vitamin A derivative 9-cis retinoic acid, although the relevance of this molecule as endogenous activator of RXRs is still elusive. Importantly, within the nuclear receptor superfamily, RXRs occupy a peculiar place, as they are obligatory partners for a number of other nuclear receptors, thus integrating the corresponding signaling pathways. In this chapter, we describe the structural features allowing RXR to form homo- and heterodimers, and the functional consequences of this unique ability. Furthermore, we discuss the importance of studying RXR activity at a genome-wide level in order to comprehensively address the biological implications of their action that is fundamental to understand to what extent RXRs could be exploited as new therapeutic targets.

Targeting truncated retinoid X receptor-α by CF31 induces TNF-α-dependent apoptosis

A truncated version of retinoid X receptor-α, tRXR-α, promotes cancer cell survival by activating the phosphoinositide 3-kinase (PI3K)/AKT pathway. However, targeting the tRXR-α-mediated survival pathway for cancer treatment remains to be explored. We report here our identification of a new natural product molecule, CF31, a xanthone isolated from Cratoxylum formosum ssp. pruniflorum, and the biologic evaluation of its regulation of the tRXR-α-mediated PI3K/AKT pathway. CF31 binds RXR-α and its binding results in inhibition of RXR-α transactivation. Through RXR-α mutational analysis and computational studies, we show that Arg316 of RXR-α, known to form salt bridges with certain RXR-α ligands, such as 9-cis-retinoic acid (9-cis-RA), is not required for the antagonist effect of CF31, showing a distinct binding mode. Evaluation of several CF31 analogs suggests that the antagonist effect is mainly attributed to an interference with Leu451 of helix H12 in RXR-α. CF31 is a potent inhibitor of AKT activation in various cancer cell lines. When combined with TNF-α, it suppresses TNF-α activation of AKT by inhibiting TNF-α-induced tRXR-α interaction with the p85α regulatory subunit of PI3K. CF31 inhibition of TNF-α activation of AKT also results in TNF-α-dependent activation of caspase-8 and apoptosis. Together, our results show that CF31 is an effective converter of TNF-α signaling from survival to death by targeting tRXR-α in a unique mode and suggest that identification of a natural product that targets an RXR-mediated cell survival pathway that regulates PI3K/AKT may offer a new therapeutic strategy to kill cancer cells.

Antagonist effect of triptolide on AKT activation by truncated retinoid X receptor-alpha

Background

Retinoid X receptor-alpha (RXRα) is a key member of the nuclear receptor superfamily. We recently demonstrated that proteolytic cleavage of RXRα resulted in production of a truncated product, tRXRα, which promotes cancer cell survival by activating phosphatidylinositol-3-OH kinase (PI3K)/AKT pathway. However, how the tRXRα-mediated signaling pathway in cancer cells is regulated remains elusive.

Methodology/Principal Findings

We screened a natural product library for tRXRα targeting leads and identified that triptolide, an active component isolated from traditional Chinese herb Trypterygium wilfordii Hook F, could modulate tRXRα-mediated cancer cell survival pathway in vitro and in animals. Our results reveal that triptolide strongly induces cancer cell apoptosis dependent on intracellular tRXRα expression levels, demonstrating that tRXRα serves as an important intracellular target of triptolide. We show that triptolide selectively induces tRXRα degradation and inhibits tRXRα-dependent AKT activity without affecting the full-length RXRα. Interestingly, such effects of triptolide are due to its activation of p38. Although triptolide also activates Erk1/2 and MAPK pathways, the effects of triptolide on tRXRα degradation and AKT activity are only reversed by p38 siRNA and p38 inhibitor. In addition, the p38 inhibitor potently inhibits tRXRα interaction with p85α leading to AKT inactivation. Our results demonstrate an interesting novel signaling interplay between p38 and AKT through tRXRα mediation. We finally show that targeting tRXRα by triptolide strongly activates TNFα death signaling and enhances the anticancer activity of other chemotherapies

Conclusions/Significance

Our results identify triptolide as a new xenobiotic regulator of the tRXRα-dependent survival pathway and provide new insight into the mechanism by which triptolide acts to induce apoptosis of cancer cells. Triptolide represents one of the most promising therapeutic leads of natural products of traditional Chinese medicine with unfortunate side-effects. Our findings will offer new strategies to develop improved triptolide analogs for cancer therapy.

Bexarotene via CBP/p300 induces suppression of NF-κB-dependent cell growth and invasion in thyroid cancer

PURPOSE

Retinoic acid (RA) treatment has been used for redifferentiation of metastatic thyroid cancer with loss of radioiodine uptake. The aim of this study was to improve the understanding of RA resistance and investigate the role of bexarotene in thyroid cancer cells.

EXPERIMENTAL DESIGN

A model of thyroid cancer cell lines with differential response to RA was used to evaluate the biological effects of retinoid and rexinoid and to correlate this with RA receptor levels. Subsequently, thyroid cancer patients were treated with 13-cis RA and bexarotene and response evaluated on radioiodine uptake reinduction on posttherapy scan and conventional imaging.

RESULTS

In thyroid cancer patients, 13-cis RA resistance can be bypassed in some tumors by bexarotene. A decreased tumor growth without differentiation was observed confirming our in vitro data. Indeed, we show that ligands of RARs or RXRs exert different effects in thyroid cancer cell lines through either differentiation or inhibition of cell growth and invasion. These effects are associated with restoration of RARβ and RXRγ levels and downregulation of NF-κB targets genes. We show that bexarotene inhibits the transactivation potential of NF-κB in an RXR-dependent manner through decreased promoter permissiveness without interfering with NF-κB nuclear translocation and binding to its responsive elements. Inhibition of transcription results from the release of p300 coactivator from NF-κB target gene promoters and subsequent histone deacetylation.

CONCLUSION

This study highlights dual mechanisms by which retinoids and rexinoids may target cell tumorigenicity, not only via RARs and RXRs, as expected, but also via NF-κB pathway.

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).

NSAID sulindac and its analog bind RXRalpha and inhibit RXRalpha-dependent AKT signaling

Nonsteroidal anti-inflammatory drugs (NSAIDs) exert their anticancer effects through cyclooxygenase-2 (COX-2)-dependent and independent mechanisms. Here, we report that Sulindac, an NSAID, induces apoptosis by binding to retinoid X receptor-alpha (RXRalpha). We identified an N-terminally truncated RXRalpha (tRXRalpha) in several cancer cell lines and primary tumors, which interacted with the p85alpha subunit of phosphatidylinositol-3-OH kinase (PI3K). Tumor necrosis factor-alpha (TNFalpha) promoted tRXRalpha interaction with the p85alpha, activating PI3K/AKT signaling. When combined with TNFalpha, Sulindac inhibited TNFalpha-induced tRXRalpha/p85alpha interaction, leading to activation of the death receptor-mediated apoptotic pathway. We designed and synthesized a Sulindac analog K-80003, which has increased affinity to RXRalpha but lacks COX inhibitory activity. K-80003 displayed enhanced efficacy in inhibiting tRXRalpha-dependent AKT activation and tRXRalpha tumor growth in animals.

A unique cytoplasmic localization of retinoic acid receptor-gamma and its regulations

Recent evidence suggests that extranuclear action of retinoid receptors is involved in mediating the pleiotropic effects of retinoids. However, whether they reside in the cytoplasm remains elusive. Here, we showed that retinoic acid receptor-gamma (RARgamma) was cytoplasmic in confluent cells, or when cells were released from serum depletion or treated with growth factors. In studying the regulation of RARgamma subcellular localization, we observed that ectopically overexpressed RARgamma was mainly cytoplasmic irrespective of serum concentration and cell density. The cytoplasmic retention of RARgamma was inhibited by ligand retinoic acid (RA). In addition, coexpression of retinoid X receptor-alpha (RXRalpha) resulted in nuclear localization of RARgamma through their heterodimerization. Mutagenesis studies revealed that a C-terminal fragment of RXRalpha potently prevents RA-induced RARgamma nuclear localization and transcriptional function. Furthermore, our results showed that the cytoplasmic retention of RARgamma was due to the presence of its unique N-terminal A/B domain, which was subject to regulation by p38 MAPK-mediated phosphorylation. Deletion or mutation of the N-terminal A/B domain largely impaired its cytoplasmic localization. Together, our data demonstrate that the subcellular localization of RARgamma is regulated by complex interactions among ligand binding, receptor phosphorylation, and receptor dimerizations.

Retinoic acid receptor and retinoid X receptor subtype expression for the differential diagnosis of thyroid neoplasms

BACKGROUND

Although differential expression of retinoic acid receptor (RAR) subtypes between benign and malignant thyroid tissues has been described, their diagnostic value has not been reported.

AIM

To investigate the diagnostic accuracy of RAR and retinoid X receptor (RXR) subtype protein expression for the differential diagnosis of thyroid neoplasms.

METHODS

We used a tissue array containing 93 benign thyroid tissues (normal thyroid, multinodular goiter, and follicular adenoma (FA)) and 77 thyroid carcinomas (papillary thyroid carcinoma (PTC), follicular thyroid carcinoma, and follicular variant of PTC (FVPTC)). Immunostaining was done for RAR and RXR subtypes. Staining was analyzed semiquantitatively based on receiver operating curve analyses and using hierarchical cluster analysis.

RESULTS

We found increased expression of cytoplasmic (c) RARA, cRARG, cRXRB and decreased expression of nuclear (n) RARB, nRARG, and nRXRA in thyroid carcinomas compared with benign tissues. We found three proteins differently expressed between FA and FTC and five proteins differentially expressed between FA and FVPTC, with high diagnostic accuracies. Using cluster analysis, the combination of negative staining of membranous RXRB and positive staining for cRXRB had a high positive predictive value (98%) for malignant thyroid disease, whereas the combination of positive nRXRA and negative cRXRB staining had a high predictive value (91%) for benign thyroid lesions.

CONCLUSION

We conclude that differences in RAR and RXR subtype protein expression may be valuable for the differential diagnosis of thyroid neoplasms. The results of this study and especially the value of cluster analysis have to be confirmed in subsequent studies.

In vivo and microarray analysis of rexinoid-responsive anaplastic thyroid carcinoma

PURPOSE

Anaplastic thyroid carcinoma is rare, yet lethal despite aggressive therapy. Molecular targeting may be beneficial using the rexinoid LGD1069, a retinoid X receptor-selective agonist, as a novel treatment. In this report, we describe the efficacy of LGD1069 in anaplastic thyroid carcinoma in vitro and assess the in vivo treatment effects on a responsive cancer. Additionally, we explore potential mediators of the rexinoid effect on a responsive anaplastic thyroid cancer using comparative microarray analysis.

EXPERIMENTAL DESIGN

Anaplastic thyroid cancer cell lines DRO, ARO, and FRO were treated with LGD1069 in vitro. Responsive DRO xenograft tumors were treated with control chow or chow containing a low dose (30 mg/kg/d) or a high dose (100 mg/kg/d) of LGD1069. Comparative microarray analysis of DRO cells treated with LGD1069 compared with volume-equivalent control was assessed after 24 h of treatment to evaluate early gene expression changes.

RESULTS

DRO xenograft tumor growth was inhibited by LGD1069 treatment in a dose-dependent manner. Comparative microarray analysis showed that 80 genes had a significant increase in expression and 29 genes had a decrease in expression after 24 h of treatment with LGD1069. Expression of angiopoietin-like 4 (ANGPTL4) mRNA was increased 6.5-fold. A trend towards an increase in ANGPTL4 mRNA (not statistically significant) was seen in treated tumors in vivo and this correlated with decreased tumor vascularity and increased necrosis.

CONCLUSIONS

LGD1069 therapy decreases proliferation in an anaplastic thyroid cancer cell line that expresses retinoid X receptor-gamma, and this effect is confirmed with decreased tumor size in vivo in a nude mouse model. ANGPTL4 is increased in DRO in response to LGD1069 and may be a potential mediator of the effects of rexinoid treatment.

Decreased Expression of Retinoid Receptors in Melanoma: Entailment in Tumorigenesis and Prognosis

PURPOSE

Retinoids inhibit proliferation and induce differentiation in melanoma cells. Retinoic acid receptors (RAR) and retinoid X receptors (RXR) mediate the various modulatory effects of retinoids in cells. We have studied the in situ expression of each RAR and RXR protein (alpha, beta, gamma) in a large series of melanocytic lesions and correlated the expression with clinicopathologic features and prognosis of the patients.

EXPERIMENTAL DESIGN

Tissue microarray blocks of 226 melanocytic lesions were semiquantitatively evaluated by immunohistochemistry for the cytoplasmic and nuclear expression of RAR and RXR protein (alpha, beta, gamma).

RESULTS

A significant decrease of RARbeta protein (P < 0.0001), nuclear expression of RARgamma (P < 0.0001), and RXRalpha (P < 0.0001) was found in primary and metastatic melanomas as compared with nevi. Loss of nuclear immunoreactivity for RARgamma (P = 0.048) and RXRalpha (P = 0.001) was observed in the lesions showing vertical growth pattern. In addition, in patients with concomitant loss of cytoplasmic staining for RARalpha and RXRalpha, the probability of overall survival (log-rank test, P = 0.002) and disease-specific survival (log-rank test, P = 0.014) was significantly lower.

CONCLUSIONS

Aberrant expression of retinoid receptors seems to be a frequent event in melanoma and suggests an impairment of the retinoid pathway in this cancer. Our data indicate the loss of retinoid receptor expression with melanoma progression and suggest a possible prognostic significance of the analysis of retinoid receptors in melanoma.

Macrophages and neurons are targets of retinoic acid signaling after spinal cord contusion injury

The physiological reactions after spinal cord injury are accompanied by local synthesis of the transcriptional activator retinoic acid (RA). RA exerts its effects by binding to retinoic acid receptors (RAR) which heterodimerize with retinoid X receptors (RXR) and then act as ligand-activated transcription factors. To identify possible cellular targets of RA we investigated protein levels and cellular distribution of retinoid receptors in the rat spinal cord at 4, 7, 14 and 21 days after a contusion injury. In the nonlesioned spinal cord, immunoreactivity for RARalpha, RXRalpha, RXRbeta and RXRgamma was localized in the cytosol of neurons, that of RXRalpha and RXRbeta in astrocytes and that of RARalpha, RXRalpha and RXRgamma in some oligodendrocytes. After contusion injury RARalpha and all RXRs appeared in the cell nuclei of reactive microglia and macrophages. This nuclear staining began at 4 days, was most prominent at 7 and 14 days and had decreased at 21 days after injury. A similar nuclear translocation was also observed for the RARalpha, RXRalpha and RXRbeta staining in neurons situated around the border of the contusion. These observations suggest that RA participates as a signal for the physiological responses of microglia and neurons after CNS injury.