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Regulation of all-trans retinoic acid (ATRA) signaling is crucial to early embryonic development. Embryonic stem (ES) cell-derived gastruloids mimic normal development in response to the Wnt/β-catenin agonist CHIR9901, and this study has examined the importance of the activities of RAR (retinoic acid receptor) α and γ to gastruloid development. Expression of retinoic acid receptor (RAR)γ within developing gastruloids was spatially restricted to primitive cells that co-expressed ES cell and early progenitor cell markers, i.e., Nanog, Sox2, and Oct4. In contrast, RARα expression was ubiquitous. mRNAs for the key enzymes involved in ATRA synthesis (Aldh1a2) and degradation (Cyp26a1) were not seen in cells that expressed RARγ. Treatment of ES cell-derived gastruloids with physiologically relevant (10 nm) levels of ATRA or with a highly selective RARγ agonist blocked normal developmental processes, preventing symmetry-breaking and axial elongation. This was not seen following treatments with an RARα agonist, where there was a tendency for enhanced axial elongation. Brachyury (TBXT) immuno-positive cells localized in the posterior end of elongated gastruloids in control- and RARα agonist-treated cultures, with Sox2 immuno-positive cells seen more widely, whilst both TBXT and Sox2 immuno-positive cells were randomly distributed throughout ATRA- and RARγ agonist-treated gastruloids. Concurrent treatment of gastruloids with 10 nm ATRA and 100 nm of an RARγ antagonist partially abrogated the ATRA-mediated block to axial elongation. Conversely, 10 nm RARγ antagonist treatments were associated with the formation of multi-axis gastruloid elongations, with comparatively little effect seen after treatments with an RARα antagonist. These findings reveal that RARγ plays a crucial role in the development of embryonic tissues.

The authors report a transcriptional profile of the discrete stages of spontaneous remyelination following toxin-induced focal demyelination in rats. They find an enrichment of retinoid X receptor (RXR) signaling pathways over the course of remyelination and show that RXR activation stimulates oligodendrocyte differentiation to enhance remyelination. The molecular basis of CNS myelin regeneration (remyelination) is poorly understood. We generated a comprehensive transcriptional profile of the separate stages of spontaneous remyelination that follow focal demyelination in the rat CNS and found that transcripts that encode the retinoid acid receptor RXR-γ were differentially expressed during remyelination. Cells of the oligodendrocyte lineage expressed RXR-γ in rat tissues that were undergoing remyelination and in active and remyelinated multiple sclerosis lesions. Knockdown of RXR-γ by RNA interference or RXR-specific antagonists severely inhibited oligodendrocyte differentiation in culture. In mice that lacked RXR-γ, adult oligodendrocyte precursor cells efficiently repopulated lesions after demyelination, but showed delayed differentiation into mature oligodendrocytes. Administration of the RXR agonist 9- cis -retinoic acid to demyelinated cerebellar slice cultures and to aged rats after demyelination caused an increase in remyelinated axons. Our results indicate that RXR-γ is a positive regulator of endogenous oligodendrocyte precursor cell differentiation and remyelination and might be a pharmacological target for regenerative therapy in the CNS.

Pregnancy critically depends on the transformation of the human endometrium into a decidual matrix that controls embryo implantation and placenta formation, a process driven foremost by differentiation and polarization of endometrial stromal cells into mature and senescent decidual cells. Perturbations in the decidual process underpin a spectrum of prevalent reproductive disorders, including implantation failure and early pregnancy loss, emphasizing the need for new therapeutic interventions. Resveratrol is a naturally occurring polyphenol, widely used for its antioxidant and anti-inflammatory properties. Using primary human endometrial stromal cell (HESC) cultures, we demonstrate that resveratrol has anti-deciduogenic properties, repressing not only the induction of the decidual marker genes PRL and IGFBP1 but also abrogating decidual senescence. Knockdown of Sirtuin 1, a histone deacetylase activated by resveratrol, restored the expression of IGFBP1 but not the induction of PRL or senescence markers in decidualizing HESCs, suggesting involvement of other pathways. We demonstrate that resveratrol interferes with the reprogramming of the retinoic acid signaling pathway in decidualizing HESCs by accelerating down-regulation of cellular retinoic acid-binding protein 2 (CRABP2) and retinoic acid receptor (RAR). Notably, knockdown of CRABP2 or RAR in HESCs was sufficient to recapitulate the anti-deciduogenic effects of resveratrol. Thus, while resveratrol has been advanced as a potential fertility drug, our results indicate it may have detrimental effects on embryo implantation by interfering with decidual remodeling of the endometrium.

Background Acute myeloid leukemia (AML) remains a challenging disease with a poor prognosis, necessitating more personalized therapeutic strategies. Retinoic acid receptor (RAR) activation is crucial for myeloid differentiation, but non-APL AML cells resist differentiation induced by the pan-RAR agonist all-trans retinoic acid (ATRA). Am80 (tamibarotene), a specific RARA agonist, has been reported to partially overcome this resistance in AML with high RARA expression. However, its effects on myeloid differentiation, especially in comparison to ATRA, remain understudied. Methods In this study we compared the effects of Am80 and ATRA in non-APL AML samples, focusing on subsets with high RARA and/or RARG expression, using molecular and phenotypic differentiation assessments. Results In contrast to previous findings, Am80 showed no advantage over ATRA in inducing differentiation in AML cell lines or primary samples, regardless of RARA and RARG expression levels. Cotreatment with inhibitors of the epigenetic modifiers LSD1 and GCN5, which facilitates retinoid-induced myeloid differentiation, enhanced the effects of both Am80 and ATRA to the same extent, with more pronounced responses observed in RARA-high samples. Gene expression analysis revealed identical molecular responses to Am80 and ATRA. Conclusions The study provides evidence that ATRA can be substituted by the more stable Am80 in retinoid-based AML therapies. It also identifies elevated RARA expression as a potential marker for sensitivity to combination therapy with retinoids and epigenetic inhibitors in AML.

Pre-clinical models have shown that targeting pancreatic stellate cells with all-trans-retinoic-acid (ATRA) reprograms pancreatic stroma to suppress pancreatic ductal adenocarcinoma (PDAC) growth. Here, in a phase Ib, dose escalation and expansion, trial for patients with advanced, unresectable PDAC (n = 27), ATRA is re-purposed as a stromal-targeting agent in combination with gemcitabine-nab-paclitaxel chemotherapy using a two-step adaptive continual re-assessment method trial design. The maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D, primary outcome) is the FDA/EMEA approved dose of gemcitabine-nab-paclitaxel along-with ATRA (45 mg/m 2 orally, days 1–15/cycle). Dose limiting toxicity (DLT) is grade 4 thrombocytopenia (n = 2). Secondary outcomes show no detriment to ATRA pharmacokinetics.. Median overall survival for RP2D treated evaluable population, is 11.7 months (95%CI 8.6–15.7 m, n = 15, locally advanced (2) and metastatic (13)). Exploratory pharmacodynamics studies including changes in diffusion-weighted (DW)-MRI measured apparent diffusion coefficient after one cycle, and, modulation of cycle-specific serum pentraxin 3 levels over various cycles indicate stromal modulation. Baseline stromal-specific retinoid transport protein (FABP5, CRABP2) expression may be predicitve of response. Re-purposing ATRA as a stromal-targeting agent with gemcitabine-nab-paclitaxel is safe and tolerable. This combination will be evaluated in a phase II randomized controlled trial for locally advanced PDAC. Clinical trial numbers: EudraCT: 2015-002662-23; NCT03307148. Trial acronym: STARPAC. All-trans retinoic acid - ATRA- is known to remodulate the stroma of pancreatic cancer in mice. Here, the authors carried out a Phase Ib trial in pancreatic patients and show that ATRA in combination with chemotherapy is a safe potential treatment for patients with advanced pancreatic cancer, and demonstrate a stromal modulatory effect.

Cell adhesion is essential for proper tissue architecture and function in multicellular organisms. Cell adhesion molecules not only maintain tissue integrity but also possess signaling properties that contribute to diverse cellular events such as cell growth, survival, differentiation, polarity, and migration; however, the underlying molecular basis remains poorly defined. Here we identify that the cell adhesion signal initiated by the tight-junction protein claudin-6 (CLDN6) regulates nuclear receptor activity. We show that CLDN6 recruits and activates Src-family kinases (SFKs) in second extracellular domain-dependent and Y196/200-dependent manners, and SFKs in turn phosphorylate CLDN6 at Y196/200. We demonstrate that the CLDN6/SFK/PI3K/AKT axis targets the AKT phosphorylation sites in the retinoic acid receptor γ (RARγ) and the estrogen receptor α (ERα) and stimulates their activities. Interestingly, these phosphorylation motifs are conserved in 14 of 48 members of human nuclear receptors. We propose that a similar link between diverse cell adhesion and nuclear receptor signalings coordinates a wide variety of physiological and pathological processes.

The identification of neurological symptoms caused by vitamin A deficiency pointed to a critical, early developmental role of vitamin A and its metabolite, retinoic acid (RA). The ability of RA to induce post-mitotic, neural phenotypes in various stem cells, in vitro, served as early evidence that RA is involved in the switch between proliferation and differentiation. In vivo studies have expanded this “opposing signal” model, and the number of primary neurons an embryo develops is now known to depend critically on the levels and spatial distribution of RA. The proneural and neurogenic transcription factors that control the exit of neural progenitors from the cell cycle and allow primary neurons to develop are partly elucidated, but the downstream effectors of RA receptor (RAR) signaling (many of which are putative cell cycle regulators) remain largely unidentified. The molecular mechanisms underlying RA-induced primary neurogenesis in anamniote embryos are starting to be revealed; however, these data have been not been extended to amniote embryos. There is growing evidence that bona fide RARs are found in some mollusks and other invertebrates, but little is known about their necessity or functions in neurogenesis. One normal function of RA is to regulate the cell cycle to halt proliferation, and loss of RA signaling is associated with dedifferentiation and the development of cancer. Identifying the genes and pathways that mediate cell cycle exit downstream of RA will be critical for our understanding of how to target tumor differentiation. Overall, elucidating the molecular details of RAR-regulated neurogenesis will be decisive for developing and understanding neural proliferation–differentiation switches throughout development.

ObjectiveSeveral studies have reported that retinoic acid (RA) might be used to treat malignancies. The effects of RA are mediated by the RA receptor (RAR), and RARα/RARβ especially acts as a tumor suppressor. However, little is known about its role in human endometrial cancer.Materials and MethodsIn this study, we examined the effects of all-trans RA (ATRA) on progression of human endometrial cancer cell line, RL95-2 and Hec1A. We then examined the expression of RARα and RARβ in 50 endometrial cancer tissues by using immunohistochemistry.ResultsWe found inhibitory effects of ATRA on cell proliferation, apoptosis, and migration in RL95-2 cells, but not in Hec1A cells. RARα or RARβ knockdown individually could not cancel out the inhibition of cell proliferation by ATRA in RL95-2 cells, but simultaneous knockdown of RARα and RARβ could block its effect on proliferation. RARα and RARβ knockdown dose dependently reduced the inhibition of migration by ATRA, but the effect was more pronounced with RARβ knockdown than with RARα knockdown. We confirmed that RARβ gene was directly regulated by ATRA in microarray and real-time reverse transcription polymerase chain reaction. Furthermore, the RARβ agonist (BMS453) significantly suppressed proliferation of RL95-2 cells. In immunohistochemical analysis, RARα expression was positively correlated with tumor grade, and RARβ showed the opposite tendency in endometrial cancer.ConclusionsRetinoic acid might have multiple antitumor effects, and RARβ may be a potent therapeutic target in RA treatment for endometrial cancers.

We have demonstrated that cytokine thymic stromal lymphopoietin (TSLP), whose expression is rapidly induced upon keratinocyteselective ablation of retinoid X receptors (RXRs) -α and -β in the mouse ($RXR\\alpha\\beta^{ep-/-} mice$), plays a key role in initiating a skin and systemic atopic dermatitis-like phenotype. We show here that topical application of the physiologically active ligand [lα,25(OH)₂D₃; calcitriol] of the vitamin D receptor, or of its low-calcemic analog MC903 (calcipotriol; Dovonex), induces TSLP expression in epidermal keratinocytes, which results in an atopic dermatitis-like syndrome mimicking that seen in $RXR\\alpha\\beta^{ep-/-}$ mutants and transgenic mice overexpressing TSLP in keratinocytes. Furthermore, topical application of retinoic acid receptor RARγ-selective agonist BMS961 also induces TSLP expression either on its own or synergistically with lα,25-(OH)₂D₃. Our data demonstrate that RXR/ vitamin D receptor and RXR/retinoic acid receptor-γ heterodimers and their ligands cell-autonomously control the expression of TSLP in epidermal keratinocytes of the mouse. We propose molecular mechanisms through which vitamin D3 and retinoic acid signalings could be involved in the pathogenesis of atopic diseases.

Key Points The nuclear receptors retinoic acid receptor-related orphan receptor-α (RORα), RORβ, RORγ, REV-ERBα and REV-ERBβ were originally identified as orphan receptors. RORα and RORβ constitutively activate transcription, whereas REV-ERBα and REV-ERBβ constitutively silence transcription. RORα and RORγ are now known to bind to sterols, with certain oxysterols having a very high affinity for these receptors. REV-ERBs have been found to bind to haem. REV-ERBs function as ligand-dependent (that is, haem-dependent) silencers of transcription. The role of the endogenous ligands for the RORs is less clear, as several sterols and oxysterols have been suggested to function as agonists or inverse agonists. The RORs and REV-ERBs have substantially overlapping functions as they usually recognize similar DNA response elements. These receptors have important roles in many physiological functions, including development, circadian rhythm, metabolism and immune function. Over the past several years, synthetic ligands have been designed that target RORs and REV-ERBs. Many of these have high potency and have been used to examine the utility of targeting RORs and REV-ERBs in animal models of human disease. Synthetic REV-ERB agonists alter the circadian rhythm and have beneficial effects on the metabolic profile in obese mice. REV-ERB agonists increase oxidative metabolism in the skeletal muscle and improve exercise endurance in mice. Synthetic inverse agonists of ROR (that either target RORγ alone or both RORα and RORγ) are effective in treating and preventing autoimmunity in mouse models. Additionally, they have beneficial effects on glucose and lipid metabolism. The continued refinement and development of synthetic ligands that target these former orphan nuclear receptors may yield novel therapeutics to treat a range of diseases in the future. This Review highlights recent progress in the development of ligands to target two classes of nuclear receptors — the REV-ERBs and retinoic acid receptor-related orphan receptors (RORs) — and describes how such ligands might be useful for treating disorders related to metabolism, immune function and the circadian rhythm. The nuclear receptors REV-ERB (consisting of REV-ERBα and REV-ERBβ) and retinoic acid receptor-related orphan receptors (RORs; consisting of RORα, RORβ and RORγ) are involved in many physiological processes, including regulation of metabolism, development and immunity as well as the circadian rhythm. The recent characterization of endogenous ligands for these former orphan nuclear receptors has stimulated the development of synthetic ligands and opened up the possibility of targeting these receptors to treat several diseases, including diabetes, atherosclerosis, autoimmunity and cancer. This Review focuses on the latest developments in ROR and REV-ERB pharmacology indicating that these nuclear receptors are druggable targets and that ligands targeting these receptors may be useful in the treatment of several disorders.