Targeted disruption of retinoic acid receptor alpha (RAR alpha) and RAR gamma results in receptor-specific alterations in retinoic acid-mediated differentiation and retinoic acid metabolism

Retinoic acid signaling and bladder cancer: Epigenetic deregulation, therapy and beyond

Retinoic acid (RA) signaling is a crucial developmental pathway involved in urothelium development, differentiation and regeneration. Deregulation of the RA signaling is highly implicated in several cancers, including bladder cancer, underlying the need to unravel the complete regulatory aspects of the retinoids in bladder tumorigenesis. Given the fact that RA receptors are transcription factors functioning at the chromatin level and act in close cooperation with chromatin modifiers, it is known that retinoids show their efficacy by changing the epigenome. Bladder cancer can be defined as a "disease of chromatin" with mutations identified in the genes involved in chromatin regulation in 80% of the patients. Therefore, a careful examination of the epigenetic backgrounds and the breakdown of the emerging and highly underexplored field of RA dependent regulation of the epigenome is essential to fully understand the retinoid-dependent effects on bladder cancer. With this motivation, in this review, we evaluate the role of RA signaling in bladder cancer with a focus on the regulatory and mutational aspects, emphasizing the deregulatory characteristics in bladder cancer and highlighting the potential treatment opportunities with the RA and derivatives alone or in combination with epigenetic drugs.

Silencing of tissue factor by antisense deoxyoligonucleotide mitigates thioacetamide-induced liver injury

BACKGROUND

Retinoid receptors (RRs), RAR-α and RXR-α, work as transcription factors that regulate cell growth, differentiation, survival, and death. Hepatic stellate cells (HSCs) store retinoid and release its RRs as lipid droplets upon their activation.

PURPOSE

We test the hypothesis that loss of retinoid receptors RAR-α and RXR-α from HSCs is dependent on tissue factor (TF) during thioacetamide (TAA)-induced liver injury.

METHODS

Liver toxicity markers, TF, fibrin, cleaved caspase-3, and cyclin D1 as well as histopathology were investigated.

RESULTS

Increased TF, fibrin, cleaved caspase-3, and cyclin D1 protein expression is seen in zone of central vein after TAA injection compared with vehicle-treated mice. A strong downregulation of RAR-α and RXR-α is seen in TAA-induced liver injury. In addition, histopathological obliteration and pericentral expression of cleaved caspase 3 and cyclin D1 are observed after TAA injection compared with the normal vehicle-treated mice. No changes have been seen in TAA/TF-sense (SC) in whole parameters compared with TAA-treated animals. TAA/TF-antisense (AS)-treated mice show normal expression of all parameters and normal histopathological features when compared with the control mice. In conclusion, this study declares that the strong downregulation of RAR-α and RXR-α may cause liver injury and particularly activation of HSCs in TAA-induced toxicity. TF-AS treatment not only downregulates TF protein expression but also alleviates loss of liver RAR-α and RXR-α and suppresses the activated apoptosis signals in TAA-induced liver toxicity. Finally, TF and RAR-α/RXR-α are important regulatory molecules in TAA induced acute liver injury.

Direct Conversion of Human Urine Cells to Neurons by Small Molecules

Transdifferentiation of other cell type into human neuronal cells (hNCs) provides a platform for neural disease modeling, drug screening and potential cell-based therapies. Among all of the cell donor sources, human urine cells (hUCs) are convenient to obtain without invasive harvest procedure. Here, we report a novel approach for the transdifferentiation of hUCs into hNCs. Our study demonstrated that a combination of seven small molecules (CAYTFVB) cocktail induced transdifferentiation of hUCs into hNCs. These chemical-induced neuronal cells (CiNCs) exhibited typical neuron-like morphology and expressed mature neuronal markers. The neuronal-like morphology revealed in day 1, and the Tuj1-positive CiNCs reached to about 58% in day 5 and 38.36% Tuj1+/MAP2+ double positive cells in day 12. Partial electrophysiological properties of CiNCs was obtained using patch clamp. Most of the CiNCs generated using our protocol were glutamatergic neuron populations, whereas motor neurons, GABAergic or dopaminergic neurons were merely detected. hUCs derived from different donors were converted into CiNCs in this work. This method may provide a feasible and noninvasive approach for reprogramming hNCs from hUCs for disease models and drug screening.

Stage-specific roles of Ezh2 and Retinoic acid signaling ensure calvarial bone lineage commitment

Development of the skull bones requires the coordination of two stem progenitor populations, the cranial neural crest cells (CNCC) and head paraxial mesoderm (PM), to ensure cell fate selection and morphogenesis. The epigenetic methyltransferase, Ezh2, plays a role in skull bone formation, but the spatiotemporal function of Ezh2 between the CNCC- and PM-derived bone formation in vivo remains undefined. Here, using a temporally-inducible conditional deletion of Ezh2 in both the CNCC- and PM- derived cranial mesenchyme between E8.5 and E9.5, we find a reduction of the CNCC-derived calvarial bones and a near complete loss of the PM-derived calvarial bones due to an arrest in calvarial bone fate commitment. In contrast, deletion of Ezh2 after E9.5 permits PM-derived skull bone development, suggesting that Ezh2 is required early to guide calvarial bone progenitor commitment. Furthermore, exposure to all-trans Retinoic acid at E10.0 can mimic the Ezh2 mutant calvarial phenotype, and administration of the pan retinoic acid receptor (RAR) antagonist, BMS-453, to Ezh2 mutants partially restores the commitment to the calvarial bone lineage and PM-derived bone development in vivo. Exogenous RA signaling activation in the Ezh2 mutants leads to synergistic activation of the anti-osteogenic factors in the cranial mesenchyme in vivo. Thus, RA signaling and EZH2 can function in parallel to guide calvarial bone progenitor commitment by balancing the suppression of anti-osteogenic factors.

Combinatorial knockout of RARα, RARβ, and RARγ completely abrogates transcriptional responses to retinoic acid in murine embryonic stem cells

All-trans-retinoic acid (RA), a potent inducer of cellular differentiation, functions as a ligand for retinoic acid receptors (RARα, β, and γ). RARs are activated by ligand binding, which induces transcription of direct genomic targets. However, whether embryonic stem cells respond to RA through routes that do not involve RARs is unknown. Here, we used CRISPR technology to introduce biallelic frameshift mutations in RARα, RARβ, and RARγ, thereby abrogating all RAR functions in murine embryonic stem cells. We then evaluated RA-responsiveness of the RAR-null cells using RNA-Seq transcriptome analysis. We found that the RAR-null cells display no changes in transcripts in response to RA, demonstrating that the RARs are essential for the regulation of all transcripts in murine embryonic stem cells in response to RA. Our key finding, that in embryonic stem cells the transcriptional effects of RA all depend on RARs, addresses a long-standing topic of discussion in the field of retinoic acid signaling.

Neurogenesis in Response to Synthetic Retinoids at Different Temporal Scales

All-trans retinoic acid (ATRA) plays key roles in neurogenesis mediated by retinoic acid receptors (RARs). RARs are important targets for the therapeutic regulation of neurogenesis but effective drug development depends on modelling-based strategies to design high-specificity ligands in combination with good biological assays to discriminate between target-specificity and off-target effects. Using neuronal differentiation as a model, the aim of this study was to test the hypothesis that responses across different temporal scales and assay platforms can be used as comparable measures of retinoid activity. In biological assays based on cell phenotype or behaviour, two structurally similar synthetic retinoids, differing in RAR affinity and specificity, retained their relative activities across different temporal scales. In contrast, assays based on the transcriptional activation of specific genes in their normal genomic context were less concordant with biological assays. Gene-induction assays for retinoid activity as modulators of neurogenesis require careful interpretation in the light of variation in ligand-receptor affinity, receptor expression and gene function. A better characterization of neuronal phenotypes and their regulation by retinoids is badly needed as a framework for understanding how to regulate neuronal development.

RARβ2-dependent signaling represses neuronal differentiation in mouse ES cells

Embryonic Stem (ES) cells are pluripotent cells that can be induced to differentiate into cells of all three lineages: mesoderm, endoderm, and ectoderm. In culture, ES cells can be differentiated into mature neurons by treatment with Retinoic Acid (RA) and this effect is mediated mainly through the activation of the RA nuclear receptors (RAR α, β, and γ), and their isoforms. However, little is known about the role played by specific RAR types on ES cell differentiation. Here, we found that treatment of ES cells with AC55649, an RARβ2 agonist, increased endodermal marker gene expression. On the other hand, we found that the inhibition of RARβ with 5μM LE135, together with RA treatment, increased the efficiency of mouse ES cell differentiation into neurons by more than 4-fold as compared to cells treated with RA only. Finally, we performed proteomic analyses on ES cells treated with RA vs RA plus AC55649 in order to identify the signaling pathways activated by the RARβ agonist. Our proteomic analyses using antibody microarrays indicated that proteins such as p38 and AKT were upregulated in cells treated with RA plus the agonist, as compared to cells treated with RA alone. Our results indicate that RARβ may function as a repressor of neuronal differentiation through the activation of major cell signaling pathways, and that the pharmacological inhibition of this nuclear receptor may constitute a novel method to increase the efficiency of ES to neuronal differentiation in culture.

RARα and RARγ reciprocally control K5+ progenitor cell expansion in developing salivary glands

Understanding the mechanisms of controlled expansion and differentiation of basal progenitor cell populations during organogenesis is essential for developing targeted regenerative therapies. Since the cytokeratin 5-positive (K5+) basal epithelial cell population in the salivary gland is regulated by retinoic acid signaling, we interrogated how isoform-specific retinoic acid receptor (RAR) signaling impacts the K5+ cell population during salivary gland organogenesis to identify RAR isoform-specific mechanisms that could be exploited in future regenerative therapies. In this study, we utilized RAR isoform-specific inhibitors and agonists with murine submandibular salivary gland organ explants. We determined that RARα and RARγ have opposing effects on K5+ cell cycle progression and cell distribution. RARα negatively regulates K5+ cells in both whole organ explants and in isolated epithelial rudiments. In contrast, RARγ is necessary but not sufficient to positively maintain K5+ cells, as agonism of RARγ alone failed to significantly expand the population. Although retinoids are known to stimulate differentiation, K5 levels were not inversely correlated with differentiated ductal cytokeratins. Instead, RARα agonism and RARγ inhibition, corresponding with reduced K5, resulted in premature lumenization, as marked by prominin-1. With lineage tracing, we demonstrated that K5+ cells have the capacity to become prominin-1+ cells. We conclude that RARα and RARγ reciprocally control K5+ progenitor cells endogenously in the developing submandibular salivary epithelium, in a cell cycle-dependent manner, controlling lumenization independently of keratinizing differentiation. Based on these data, isoform-specific targeting RARα may be more effective than pan-RAR inhibitors for regenerative therapies that seek to expand the K5+ progenitor cell pool.

SUMMARY STATEMENT

RARα and RARγ reciprocally control K5+ progenitor cell proliferation and distribution in the developing submandibular salivary epithelium in a cell cycle-dependent manner while regulating lumenization independently of keratinizing differentiation.

Radiolabeling and initial biological evaluation of [18F]KBM-1 for imaging RAR-α receptors in neuroblastoma

Retinoic acid receptor alpha (RAR-α) plays a significant role in a number of diseases, including neuroblastoma. Children diagnosed with high-risk neuroblastoma are treated13-cis-retinoic acid, which reduces risk of cancer recurrence. Neuroblastoma cell death is mediated via RAR-α, and expression of RAR-α is upregulated after treatment. A molecular imaging probe that binds RAR-α will help clinicians to diagnose and stratify risk for patients with neuroblastoma, who could benefit from retinoid-based therapy. In this study, we report the radiolabeling, and initial in vivo evaluation of [¹⁸F]KBM-1, a novel RAR-α agonist. The radiochemical synthesis of [¹⁸F]KBM-1 was carried out through KHF₂ assisted substitution of [¹⁸F]^- from aryl-substituted pinacolatoesters-based retinoid precursor. In vitro cell uptake assay in human neuroblastoma cell line showed that the uptake of [¹⁸F]KBM-1 was significantly inhibited by all three blocking agents (KBM-1, ATRA, BD4) at all the selected incubation times. Standard biodistribution in mice bearing neuroblastoma tumors demonstrated increased tumor uptake from 5min to 60min post radiotracer injection and the uptake ratios for target to non-target (tumor: muscle) increased 2.2-fold to 3.7-fold from 30min to 60min post injection. Tumor uptake in subset of 30min blocking group was 1.7-fold lower than unblocked. These results demonstrate the potential utility of [¹⁸F]KBM-1 as a RAR-α imaging agent.

Retinoic Acid Synthesis and Degradation

Retinoic acid (RA) was identified as the biologically active form of vitamin A almost 70 years ago and work on its function and mechanism of action is still of major interest both from a scientific and a clinical perspective. The currently accepted model postulates that RA is produced in two sequential oxidative steps: first, retinol is oxidized reversibly to retinaldehyde, and then retinaldehyde is oxidized irreversibly to RA. Excess RA is inactivated by conversion to hydroxylated derivatives. Much is left to learn, especially about retinoid binding proteins and the trafficking of the hydrophobic retinoid substrates between membrane bound and cytosolic enzymes. Here, background on development of the field and an update on recent advances in our understanding of the enzymatic pathways and mechanisms that control the rate of RA production and degradation are presented with a focus on the many questions that remain unanswered.

Signalling Through Retinoic Acid Receptors is Required for Reprogramming of Both Mouse Embryonic Fibroblast Cells and Epiblast Stem Cells to Induced Pluripotent Stem Cells

We previously demonstrated that coexpressing retinoic acid (RA) receptor gamma and liver receptor homolog-1 (LRH1 or NR5A2) with OCT4, MYC, KLF4, and SOX2 (4F) rapidly reprograms mouse embryonic fibroblast cells (MEFs) into induced pluripotent stem cells (iPSCs). Here, we further explore the role of RA in reprogramming and report that the six factors (6F) efficiently and directly reprogram MEFs into integration-free iPSCs in defined medium (N2B27) in the absence of feeder cells. Through genetic and chemical approaches, we find that RA signalling is essential, in a highly dose-sensitive manner, for MEF reprogramming. The removal of exogenous RA from N2B27, the inhibition of endogenous RA synthesis or the expression of a dominant-negative form of RARA severely impedes reprogramming. By contrast, supplementing N2B27 with various retinoids substantially boosts reprogramming. In addition, when coexpressed with LRH1, RA receptors (RARs) can promote reprogramming in the absence of both exogenous and endogenously synthesized RA. Remarkably, the reprogramming of epiblast stem cells into embryonic stem cell-like cells also requires low levels of RA, which can modulate Wnt signalling through physical interactions of RARs with β-catenin. These results highlight the important functions of RA signalling in reprogramming somatic cells and primed stem cells to naïve pluripotency. Stem Cells 2015;33:1390-1404.

Retinoic acid receptor γ (Rarg) and nuclear receptor subfamily 5, group A, member 2 (Nr5a2) promote conversion of fibroblasts to functional neurons

Somatic cells can be reprogrammed to neurons and various other cell types with retrovirus or lentivirus. The limitation of this technology is that these genome-integration viruses may increase the risk of gene mutation and cause insertional mutagenesis. We recently found that non-integration adenovirus carrying neuronal transcription factors can induce fibroblasts to neurons. However, the conversion efficiency by the adenovirus is lower than that of the retrovirus or lentivirus. Therefore, it is crucial to identify other factors or chemical compounds to obtain neurons with high efficiency. In this study we show that the combination of Rarg (retinoic acid receptor γ) and Nr5a2 (nuclear receptor subfamily 5, group A, member 2; also known as Lrh-1 (liver receptor homologue 1)) rapidly promote the iN cell maturation within 1 week and greatly facilitate the conversion with neuronal purities of ∼50% and yields of >130%. They also improve neuronal pattern formation, electrophysiological characteristics, and functional integration in vivo. Moreover, the chemical compound agonists to Rarg and Nr5a2 function effectively as well. This approach may be used for the generation and application of iN cells in regenerative medicine.

Polycomb recruitment attenuates retinoic acid-induced transcription of the bivalent NR2F1 gene

Polycomb proteins play key roles in mediating epigenetic modifications that occur during cell differentiation. The Polycomb repressive complex 2 (PRC2) mediates the tri-methylation of histone H3 lysine 27 (H3K27me3). In this study, we identify a distinguishing feature of two classes of PRC2 target genes, represented by the Nr2F1 (Coup-TF1) and the Hoxa5 gene, respectively. Both genes are transcriptionally activated by all-trans retinoic acid (RA) and display increased levels of the permissive H3K9/K14ac and tri-methylated histone H3 lysine 4 epigenetic marks in response to RA. However, while in response to RA the PRC2 and H3K27me3 marks are greatly decreased at the Hoxa5 promoter, these marks are initially increased at the Nr2F1 promoter. Functional depletion of the essential PRC2 protein Suz12 by short hairpin RNA (shRNA) technology enhanced the RA-associated transcription of Nr2F1, Nr2F2, Meis1, Sox9 and BMP2, but had no effect on the Hoxa5, Hoxa1, Cyp26a1, Cyp26b1 and RARβ2 transcript levels in wild-type embryonic stem cells. We propose that PRC2 recruitment attenuates the RA-associated transcriptional activation of a subset of genes. Such a mechanism would permit the fine-tuning of transcriptional networks during differentiation.

CDK1 interacts with RARγ and plays an important role in treatment response of acute myeloid leukemia

Alterations in cell cycle pathways and retinoic acid signaling are implicated in leukemogenesis. However, little is known about the roles of cyclin-dependent kinases (CDKs) in treatment response of leukemia. In this study, we observed that CDK1 expression was significantly higher in bone marrow from 42 patients with acute myeloid leukemia (AML) at recurrence than that at first diagnosis (p = 0.04). AML patients had higher level of nuclear CDK1 in their leukemic blasts tended to have poorer clinical outcome compared with those with lower levels. We showed that CDK1 function is required for all-trans retinoic acid (ATRA) to achieve the optimal effect in U-937 human leukemic cells. CDK1 modulates the levels of P27(kip) and AKT phosphorylation in response to ATRA treatment. Further, we show, for the first time, that RARγ in concert with ATRA regulates protein levels of CDK1 and its subcellular localization. The regulation of the subcellular content of CDK1 and RARγ by ATRA is an important process for achieving an effective response in treatment of leukemia. RARγ and CDK1 form a reciprocal regulatory circuit in the nucleus and influence the function and protein stability of each other and the level of P27(kip) protein. In addition, expression of wee1 kinase and Cdc25A/C phosphatases also coincide with CDK1 expression and its subcellular localization in response to ATRA treatment. Our study reveals a novel mechanism by which CDK1 and RARγ coordinate with ATRA to influence cell cycle progression and cellular differentiation.

RARγ is essential for retinoic acid induced chromatin remodeling and transcriptional activation in embryonic stem cells

We have utilized retinoic acid receptor γ (gamma) knockout (RARγ(-/-)) embryonic stem (ES) cells as a model system to analyze RARγ mediated transcriptional regulation of stem cell differentiation. Most of the transcripts regulated by all-trans retinoic acid (RA) in ES cells are dependent upon functional RARγ signaling. Notably, many of these RA-RARγ target genes are implicated in retinoid uptake and metabolism. For instance, Lrat (lecithin:retinol acyltransferase), Stra6 (stimulated by retinoic acid 6), Crabp2 (cellular retinoic acid binding protein 2), and Cyp26a1 (cytochrome p450 26a1) transcripts are induced in wild type (WT), but not in RARγ(-/-) cells. Transcripts for the transcription factors Pbx1 (pre-B cell leukemia homeobox-1), Wt1 (Wilm's tumor gene-1), and Meis1 (myeloid ecotropic viral integration site-1) increase upon RA treatment of WT, but not RARγ(-/-) cells. In contrast, Stra8, Dleu7, Leftb, Pitx2, and Cdx1 mRNAs are induced by RA even in the absence of RARγ. Mapping of the epigenetic signature of Meis1 revealed that RA induces a rapid increase in the H3K9/K14ac epigenetic mark at the proximal promoter and at two sites downstream of the transcription start site in WT, but not in RARγ(-/-) cells. Thus, RA-associated increases in H3K9/K14ac epigenetic marks require RARγ and are associated with increased Meis1 transcript levels, whereas H3K4me3 is present at the Meis1 proximal promoter even in the absence of RARγ. In contrast, at the Lrat proximal promoter primarily the H3K4me3 mark, and not the H3K9/K14ac mark, increases in response to RA, independently of the presence of RARγ. Our data show major epigenetic changes associated with addition of the RARγ agonist RA in ES cells.

ATRA and the specific RARα agonist, NRX195183, have opposing effects on the clonogenicity of pre-leukemic murine AML1-ETO bone marrow cells

All-trans retinoic acid (ATRA) is used successfully in the treatment of acute promyelocytic leukemia (APL). ATRA enhances hematopoietic stem cell self-renewal through retinoic acid receptor (RAR)γ activation while promoting differentiation of committed myeloid progenitors through RARα activation. Its lack of success in the treatment of non-APL acute myeloid leukemia (AML) may be related to ATRA's non-selectivity for the RARα and RARγ isotypes, and specific RARα activation may be more beneficial in promoting myeloid differentiation. To investigate this hypothesis, the effects of ATRA and the specific RARα agonist NRX195183 was assessed in AML1-ETO (AE)-expressing murine bone marrow (BM) progenitors. ATRA potentiated the in vitro clonogenicity of these cells while NRX195183 had the opposite effect. Morphological and flow cytometric analysis confirmed a predominantly immature myeloid population in the ATRA-treated AE cells while the NRX195183-treated cells demonstrated an increase in the mature myeloid population. Similarly, NRX195183 treatment promoted myeloid differentiation in an AE9a in vivo murine model. In the ATRA-treated AE cells, gene expression analyses revealed functional networks involving SERPINE1 and bone morphogenetic protein 2; AKT phosphorylation was upregulated. Collectively, these findings confirm the contrasting roles of specific RARα and RARγ activation in the clonogenicity and differentiation of AE cells with potential significant implications in the treatment of non-APL AML using a specific RARα agonist.

Epigenetic regulation by RARα maintains ligand-independent transcriptional activity

Retinoic acid receptors (RARs) α, β and γ are key regulators of embryonic development. Hematopoietic differentiation is regulated by RARα, and several types of leukemia show aberrant RARα activity. Through microarray expression analysis, we identified transcripts differentially expressed between F9 wild-type (Wt) and RARα knockout cells cultured in the absence or presence of the RAR-specific ligand all trans retinoic acid (RA). We validated the decreased Mest, Tex13, Gab1, Bcl11a, Tcfap2a and HMGcs1 transcript levels, and increased Slc38a4, Stmn2, RpL39l, Ref2L, Mobp and Rlf1 transcript levels in the RARa knockout cells. The decreased Mest and Tex13 transcript levels were associated with increased promoter CpG-island methylation and increased repressive histone modifications (H3K9me3) in RARα knockout cells. Increased Slc38a4 and Stmn2 transcript levels were associated with decreased promoter CpG-island methylation and increased permissive histone modifications (H3K9/K14ac, H3K4me3) in RARα knockout cells. We demonstrated specific association of RARα and RXRα with the Mest promoter. Importantly, stable expression of a dominant negative, oncogenic PML–RARα fusion protein in F9 Wt cells recapitulated the decreased Mest transcript levels observed in RARα knockout cells. We propose that RARα plays an important role in cellular memory and imprinting by regulating the CpG methylation status of specific promoter regions.

Identification of a novel non-retinoid pan inverse agonist of the retinoic acid receptors

Retinoids are potent forms of vitamin A and are involved in a broad range of physiological processes and the pharmacological effects of retinoids are primarily mediated by the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Several natural and synthetic RAR modulators have proven to be clinically useful for a number of therapeutic indications including cancer, psoriasis, and diabetes. Unfortunately, these agents lead to a number of significant side effects. Most synthetic retinoid ligands are based on the retinoid scaffold and thus have similarities to the natural ligand with all previously disclosed RAR ligands having a carboxylic acid that makes a critical ionic bridge within the ligand binding domain of the receptors. The potential therapeutic value offered from RAR modulation provides the impetus to identify novel ligands based on unique scaffolds that may offer improved toxicity and pharmacokinetic profiles. Here we describe the identification of an atypical RAR inverse agonist that represents the first non-acid, non-retinoid direct modulator of RAR receptor subfamily. SR-0065 functions as a pan-RAR inverse agonist suppressing the basal activity of RARα, RARβ, and RARγ, as well as inhibiting agonist-induced RAR activity. SR-0065 treatment enhanced receptor interaction with a peptide representative of the corepressor SMRT, and in cells SR-0065 enhances recruitment of SMRT to the promoter of the RARγ dependent gene, Cyp26A1. The acid form of SR-0065, SR-1758, was inactive in all assays. Thus, SR-0065 represents a new class of non-acid, non-retinoid RAR modulator that may be used as a point to initiate development of improved RAR-targeted drugs.

Coordinated gene expression during gilthead sea bream skeletogenesis and its disruption by nutritional hypervitaminosis A

BACKGROUND

Vitamin A (VA) has a key role in vertebrate morphogenesis, determining body patterning and growth through the control of cell proliferation and differentiation processes. VA regulates primary molecular pathways of those processes by the binding of its active metabolite (retinoic acid) to two types of specific nuclear receptors: retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which promote transcription of downstream target genes. This process is well known in most of higher vertebrates; however, scarce information is available regarding fishes. Therefore, in order to gain further knowledge of fish larval development and its disruption by nutritional VA imbalance, the relative expression of some RARs and RXRs, as well as several genes involved in morpho- and skeletogenesis such as peroxisome proliferator-activated receptors (PPARA, PPARB and PPARG); retinol-binding protein (RBP); insulin-like growth factors I and II (IGF1 and IGF2, respectively); bone morphogenetic protein 2 (Bmp2); transforming growth factor β-1 (TGFB1); and genes encoding different extracellular matrix (ECM) proteins such as matrix Gla protein (mgp), osteocalcin (bglap), osteopontin (SPP1), secreted protein acidic and rich in cysteine (SPARC) and type I collagen α1 chain (COL1A1) have been studied in gilthead sea bream.

RESULTS

During gilthead sea bream larval development, specific expression profiles for each gene were tightly regulated during fish morphogenesis and correlated with specific morphogenetic events and tissue development. Dietary hypervitaminosis A during early larval development disrupted the normal gene expression profile for genes involved in RA signalling (RARA), VA homeostasis (RBP) and several genes encoding ECM proteins that are linked to skeletogenesis, such as bglap and mgp.

CONCLUSIONS

Present data reflects the specific gene expression patterns of several genes involved in larval fish RA signalling and skeletogenesis; and how specific gene disruption induced by a nutritional VA imbalance underlie the skeletal deformities. Our results are of basic interest for fish VA signalling and point out some of the potential molecular players involved in fish skeletogenesis. Increased incidences of skeletal deformities in gilthead sea bream fed with hypervitaminosis A were the likely ultimate consequence of specific gene expression disruption at critical development stages.

Retinoic acids potentiate BMP9-induced osteogenic differentiation of mesenchymal progenitor cells

Background

As one of the least studied bone morphogenetic proteins (BMPs), BMP9 is one of the most osteogenic BMPs. Retinoic acid (RA) signaling is known to play an important role in development, differentiation and bone metabolism. In this study, we investigate the effect of RA signaling on BMP9-induced osteogenic differentiation of mesenchymal progenitor cells (MPCs).

Methodology/Principal Findings

Both primary MPCs and MPC line are used for BMP9 and RA stimulation. Recombinant adenoviruses are used to deliver BMP9, RARα and RXRα into MPCs. The in vitro osteogenic differentiation is monitored by determining the early and late osteogenic markers and matrix mineralization. Mouse perinatal limb explants and in vivo MPC implantation experiments are carried out to assess bone formation. We find that both 9CRA and ATRA effectively induce early osteogenic marker, such as alkaline phosphatase (ALP), and late osteogenic markers, such as osteopontin (OPN) and osteocalcin (OC). BMP9-induced osteogenic differentiation and mineralization is synergistically enhanced by 9CRA and ATRA in vitro. 9CRA and ATRA are shown to induce BMP9 expression and activate BMPR Smad-mediated transcription activity. Using mouse perinatal limb explants, we find that BMP9 and RAs act together to promote the expansion of hypertrophic chondrocyte zone at growth plate. Progenitor cell implantation studies reveal that co-expression of BMP9 and RXRα or RARα significantly increases trabecular bone and osteoid matrix formation.

Conclusion/Significance

Our results strongly suggest that retinoid signaling may synergize with BMP9 activity in promoting osteogenic differentiation of MPCs. This knowledge should expand our understanding about how BMP9 cross-talks with other signaling pathways. Furthermore, a combination of BMP9 and retinoic acid (or its agonists) may be explored as effective bone regeneration therapeutics to treat large segmental bony defects, non-union fracture, and/or osteoporotic fracture.

All-trans retinoic acid directs urothelial specification of murine embryonic stem cells via GATA4/6 signaling mechanisms

The urinary bladder and associated tract are lined by the urothelium, a transitional epithelium that acts as a specialized permeability barrier that protects the underlying tissue from urine via expression of a highly specific group of proteins known as the uroplakins (UP). To date, our understanding of the developmental processes responsible for urothelial differentiation has been hampered due to the lack of suitable models. In this study, we describe a novel in vitro cell culture system for derivation of urothelial cells from murine embryonic stem cells (ESCs) following cultivation on collagen matrices in the presence all trans retinoic acid (RA). Upon stimulation with micromolar concentrations of RA, ESCs significantly downregulated the pluripotency factor OCT-4 but markedly upregulated UP1A, UP1B, UP2, UP3A, and UP3B mRNA levels in comparison to naïve ESCs and spontaneously differentiating controls. Pan-UP protein expression was associated with both p63- and cytokeratin 20-positive cells in discrete aggregating populations of ESCs following 9 and 14 days of RA stimulation. Analysis of endodermal transcription factors such as GATA4 and GATA6 revealed significant upregulation and nuclear enrichment in RA-treated UP2-GFP+ populations. GATA4-/- and GATA6-/- transgenic ESC lines revealed substantial attenuation of RA-mediated UP expression in comparison to wild type controls. In addition, EMSA analysis revealed that RA treatment induced formation of transcriptional complexes containing GATA4/6 on both UP1B and UP2 promoter fragments containing putative GATA factor binding sites. Collectively, these data suggest that RA mediates ESC specification toward a urothelial lineage via GATA4/6-dependent processes.

Mechanism of regulation and suppression of melanoma invasiveness by novel retinoic acid receptor-gamma target gene carbohydrate sulfotransferase 10

Retinoic acid (RA) induces growth arrest and differentiation of S91 murine melanoma cells and serves as a valuable model for this disease. RA acts through activation of RA receptors (RAR), which are members of the nuclear receptor superfamily of ligand-inducible transcription factors. Interestingly, differentiation is mediated by RARgamma, but not by RARalpha or RARbeta, suggesting that RARgamma possesses unique and uncharacterized molecular properties. To address this question, DNA microarrays in combination with RAR isoform-specific agonists were employed to identify novel RARgamma target genes that may play a role in this process. Here, we identified and validated carbohydrate sulfotransferase 10 (CHST10) as a novel RARgamma target gene in S91 cells. The RARgamma-inducible CHST10 promoter was obtained, and two atypical, independently functioning RA response elements were identified in a 425 bp region. Surprisingly, this fragment is bound by RARgamma, but not by RARalpha or RARbeta, thus providing a mechanism for the observed RARgamma-specific regulation. CHST10 is a sulfotransferase that forms HNK-1 glycan on neural cell adhesion proteins and glycolipids, and HNK-1 is thought to modulate cell adhesion and possibly metastasis. We show that CHST10 is also regulated by RARgamma in a significant subset of human melanoma cells, and three-dimensional cell culture migration assays suggest that CHST10 functions as a suppressor of invasiveness, but not proliferation, in these cells. Induction of CHST10 by RARgamma-activating retinoids may present a novel therapeutic strategy to inhibit invasiveness in a subset of melanoma patients.

Retinoic acid receptor gamma activates receptor tyrosine kinase Tie1 gene transcription through transcription factor GATA4 in F9 stem cells

OBJECTIVE

The retinoic acid receptors (RARs) alpha, beta2, and gamma regulate specific subsets of target genes during all-trans retinoic acid (RA) induced differentiation of F9 teratocarcinoma stem cells. The Tie1 gene exhibited reduced expression in RA-treated F9 RARgamma-/- cells as compared to wild-type (WT) by microarray analysis. Our goal was to analyze the Tie1 gene, which encodes a surface receptor tyrosine kinase expressed in the hematovascular system.

MATERIALS AND METHODS

We assessed Tie1, Tie2, Flk1, Runx1, Peg/Mest2, and angiopoietin-1 and 2 mRNA levels and Tie1 promoter activity.

RESULTS

We showed that RARgamma, but not RARalpha or RARbeta2, is required for Tie1 promoter activation by RA. Treatment with a RARgamma selective agonist plus a retinoid X receptor agonist (LGD1069) increased Tie1 mRNA levels by 11- +/- 2.5-fold 48 hours after RA addition in F9 WT, but not in F9 RARgamma-/- cells, by quantitative reverse transcription polymerase chain reaction. Multiple putative GATA elements were identified in the Tie1 proximal promoter. RA increased GATA4 transcripts by 12- +/- 1-fold in F9 WT at 48 hours, but not in F9 RARgamma-/- cells. In addition, transfection of a GATA4 expression vector increased Tie1 promoter/luciferase activity in both RA-treated F9 WT and RARgamma-/- cells. Tie1 promoter deletion analyses indicated that a region of the promoter that possessed multiple GATA sites mediated the RA-associated Tie1 transcriptional increase.

CONCLUSIONS

Our results indicate that GATA4 plays a role in the RA/RARgamma-associated transcriptional activation of the Tie1 promoter. An understanding of RAR specificity in RA signaling should result in insights into hematopoietic stem cell signaling and potentially in improved therapies for several human diseases.

Gene expression profiling elucidates a specific role for RARgamma in the retinoic acid-induced differentiation of F9 teratocarcinoma stem cells

The biological effects of all-trans-retinoic acid (RA), a major active metabolite of retinol, are mainly mediated through its interactions with retinoic acid receptor (RARs alpha, beta, gamma) and retinoid X receptor (RXRs alpha, beta, gamma) heterodimers. RAR/RXR heterodimers activate transcription by binding to RA-response elements (RAREs or RXREs) in the promoters of primary target genes. Murine F9 teratocarcinoma stem cells have been widely used as a model for cellular differentiation and RA signaling during embryonic development. We identified and characterized genes that are differentially expressed in F9 wild type (Wt) and F9 RARgamma-/- cells, with and without RA treatment, through the use of oligonucleotide-based microarrays. Our data indicate that RARgamma, in the absence of exogenous RA, modulates gene expression. Genes such as Sfrp2, Tie1, Fbp2, Emp1, and Emp3 exhibited higher transcript levels in RA-treated Wt, RARalpha-/- and RARbeta2-/- lines than in RA-treated RARgamma-/- cells, and represent specific RARgamma targets. Other genes, such as Runx1, were expressed at lower levels in both F9 RARbeta2-/- and RARgamma-/- cell lines than in F9 Wt and RARalpha-/-. Genes specifically induced by RA at 6h with the protein synthesis inhibitor cycloheximide in F9 Wt, but not in RARgamma-/- cells, included Hoxa3, Hoxa5, Gas1, Cyp26a1, Sfrp2, Fbp2, and Emp1. These genes represent specific primary RARgamma targets in F9 cells. Several genes in the Wnt signaling pathway were regulated by RARgamma. Delineation of the receptor-specific actions of RA with respect to cell proliferation and differentiation should result in more effective therapies with this drug.

The transcriptional corepressor TPA-inducible sequence 7 regulates adult axon growth through cellular retinoic acid binding protein II expression

TPA-inducible sequence 7 (TIS7) expression is regulated in epithelial cells and acts as a transcriptional corepressor. Using a TIS7 knock-out mouse we demonstrated that TIS7 is involved in the process of muscle regeneration. In this study, we analysed the role of TIS7 in axon regeneration, applying primary neurone cultures derived from adult dorsal root ganglia (DRGs) of TIS7+/+ and TIS7-/- mice. TIS7-/- DRG neurones exhibited a significant decrease in axon initiation and maximal axon extension. In contrast, nerve growth factor-induced axon initiation and branching were significantly enhanced in cultures obtained from TIS7-/- DRGs when compared with wildtype ganglia, suggesting an inhibitory effect of TIS7 on nerve growth factor-stimulated axon growth. TIS7 overexpression in TIS7-/- DRG neurones caused their morphological appearance to revert back to the wildtype phenotype. Furthermore, the expression of cellular retinoic acid binding protein II (CRABP II), previously identified by us as a TIS7 target gene, was up-regulated in adult DRG sensory neurones from TIS7-/- mice. Overexpression of CRABP II in TIS7+/+ neurones strongly increased the number of branch points, making them morphologically similar to TIS7-/- neurones. Based on these results we propose that TIS7 inhibits CRABP II expression during axonal regeneration, thereby modulating retinoic acid signalling. Hence, neurite initiation and branching are regulated by a negative feedback mechanism involving TIS7 and CRABP II.

Retinoic acid receptor isotype specificity in F9 teratocarcinoma stem cells results from the differential recruitment of coregulators to retinoic response elements

The retinoic acid receptor (RAR) alpha, beta(2), and gamma isotypes each regulate specific subsets of target genes in F9 teratocarcinoma stem cells. We used chromatin immunoprecipitation assays to monitor the association of RARgamma, retinoic X receptor (RXR) alpha, and coregulators with the RARbeta(2), Hoxa1, and Cyp26A1 retinoic acid response elements (RAREs) in F9 wild type and RARalpha, -beta(2), and -gamma null cells. Additionally we quantitatively monitored expression of the corresponding mRNAs. We demonstrated that the association of RARgamma and/or RXRalpha with a RARE was not sufficient for retinoic acid (RA)-mediated transcription of the corresponding target gene. However, the ability of RARgamma and/or RXRalpha to recruit pCIP (AIB1/ACTR/RAC-3/TRAM-1/SRC-3) and p300 to a RARE did correlate with RA-associated transcription of target mRNAs. Therefore, the specific functions of the RAR isotypes do not manifest at the level of their DNA binding but rather from a differential ability to recruit specific components of the transcriptional machinery. We also demonstrated that RA-mediated displacement of the polycomb group protein SUZ12 from a RARE was inhibited in the absence of RARgamma. Thus, transcriptional components of the RAR signaling pathway are specifically required for displacement of SUZ12 from RAREs during RA-mediated differentiation of F9 cells.

Overexpression of CRABPI in suprabasal keratinocytes enhances the proliferation of epidermal basal keratinocytes in mouse skin topically treated with all-trans retinoic acid

We investigated whether ectopic expression of CRABPI, a cellular retinoic acid binding protein, influenced the actions of all-trans retinoic acid (ATRA) in transgenic (TG) mice. We targeted CRABPI to the basal vs. suprabasal layers of mouse epidermis by using the keratin 14 (K14) and keratin 10 (K10) promoters, respectively. Greater CRABPI protein levels were detected in the epidermis of adult transgenic(+) mice than in transgenic(-) mice for both transgenes. In adult mouse skin CRABPI overexpression in the basal or suprabasal keratinocytes did not cause morphological abnormalities, but did result in decreased CRABPII mRNA levels. Ectopically overexpressed CRABPI in suprabasal keratinocytes, but not in basal keratinocytes, enhanced the thickening of the epidermis induced by topical ATRA treatments (10 microM, 400 microl for 4 days) by 1.59+/-0.2-fold (p<0.05). ATRA treatment (10 microM) resulted in a 59.9+/-9.8% increase (p<0.05) in the BrdU labeling index in K10/FLAG-CRABPI TG(+) mice vs. TG(-) mice. Retinoid topical treatments reduced p27 and CYP26A1 mRNA levels in TG(+) and TG(-) mouse skin in K14 and K10/FLAG-CRABPI transgenic mice. As epidermal basal keratinocyte proliferation is stimulated by paracrine growth factors secreted by ATRA activated suprabasal keratinocytes, our results indicate that CRABPI overexpression in suprabasal keratinocytes enhances the physiological functions of ATRA.

Retinoid regulated association of transcriptional co-regulators and the polycomb group protein SUZ12 with the retinoic acid response elements of Hoxa1, RARbeta(2), and Cyp26A1 in F9 embryonal carcinoma cells

Hox gene expression is activated by all-trans retinoic acid (RA), through binding to retinoic acid receptor-retinoid X receptor (RAR-RXR) heterodimers bound at RA response elements (RAREs) of target genes. The RARs and RXRs each have three isotypes (alpha, beta, and gamma), which are encoded by distinct genes. Hox genes are also repressed by polycomb group proteins (PcG), though how these proteins are targeted is unclear. We used chromatin immunoprecipitation assays to investigate the association of RXRalpha, RARgamma, cofactors, and the PcG protein SUZ12 with the Hoxa1, RARbeta2, and Cyp26A1 RAREs in F9 embryonal carcinoma cells (teratocarcinoma stem cells) during RA treatment. We demonstrate that RARgamma and RXRalpha are associated with RAREs prior to and during RA treatment. pCIP, p300, and RNA polymerase II levels increased at target RAREs upon exposure to RA. Conversely, SUZ12 was found associated with all RAREs studied and these associations were attenuated by treatment with RA. Upon RA removal, SUZ12 re-associated with RAREs. H3ac, H3K4me2, and H3K27me3 marks were simultaneously detected at target loci, indicative of a bivalent domain chromatin structure. During RA mediated differentiation, H3K27me3 levels decreased at target RAREs whereas H3ac and H3K4me2 levels remained constant. These studies provide insight into the dynamics of association of co-regulators with RAREs and demonstrate a novel link between RA signaling and PcG repression.

Cytodifferentiation by retinoids, a novel therapeutic option in oncology: rational combinations with other therapeutic agents

Retinoic acid (RA) and derivatives are promising antineoplastic agents endowed with both therapeutic and chemopreventive potential. Although the treatment of acute promyelocytic leukemia with all-trans retinoic acid is an outstanding example, the full potential of retinoids in oncology has not yet been explored and a more generalized use of these compounds is not yet a reality. One way to enhance the therapeutic and chemopreventive activity of RA and derivatives is to identify rational combinations between these compounds and other pharmacological agents. This is now possible given the information available on the biochemical and molecular mechanisms underlying the biological activity of retinoids. At the cellular level, the antileukemia and anticancer activity of retinoids is the result of three main actions, cytodifferentiation, growth inhibition, and apoptosis. Cytodifferentiation is a particularly attractive modality of treatment and differentiating agents promise to be less toxic and more specific than conventional chemotherapy. This is the result of the fact that cytotoxicity is not the primary aim of differentiation therapy. At the molecular level, retinoids act through the activation of nuclear retinoic acid receptor-dependent and -independent pathways. The cellular pathways and molecular networks relevant for retinoid activity are modulated by a panoply of other intracellular and extracellular pathways that may be targeted by known drugs and other experimental therapeutics. This chapter aims to summarize and critically discuss the available knowledge in the field.

Role of retinoic acid in the differentiation of embryonal carcinoma and embryonic stem cells

Retinoic acid (RA), the most potent natural form of vitamin A, plays an important role in many diverse biological processes such as embryogenesis and cellular differentiation. This chapter is a review of the mechanism of action of RA and the role of specific RA-regulated genes during the cellular differentiation of embryonal carcinoma (EC) and embryonic stem (ES) cells. RA acts by binding to its nuclear receptors and inducing transcription of specific target genes. The most studied mouse EC cell lines include F9 cells, which can be induced by RA to differentiate into primitive, parietal, and visceral endodermal cells; and P19 cells, which can differentiate to endodermal and neuronal cells upon RA treatment. ES cells can be induced to differentiate into a number of different cell types; many of which require RA treatment. Over the years, many RA-regulated genes have been discovered in EC and ES cells using a diverse set of techniques. Current research focuses on the elucidation how these genes affect differentiation in EC and ES cells using a variety of molecular biology approaches. However, the exact molecule events that lead from a pluripotent stem cell to a fully differentiated cell following RA treatment are yet to be determined.

Cell proliferation inhibition and alterations in retinol esterification induced by phytanic acid and docosahexaenoic acid

We investigated the effects of two natural dietary retinoid X receptor (RXR) ligands, phytanic acid (PA) and docosahexaenoic acid (DHA), on proliferation and on the metabolism of retinol (vitamin A) in both cultured normal human prostate epithelial cells (PrECs) and PC-3 prostate carcinoma cells. PA and DHA inhibited the proliferation of the parental PC-3 cells and PC-3 cells engineered to overexpress human lecithin:retinol acyltransferase (LRAT) in both the absence and presence of retinol. A synthetic RXR-specific ligand also inhibited PC-3 cell proliferation, whereas all-trans retinoic acid (ATRA) did not. PA and DHA treatment increased the levels of retinyl esters (REs) in both PrECs and PC-3 cells and generated novel REs that eluted on reverse-phase HPLC at 54.0 and 50.5 min, respectively. Mass spectrometric analyses demonstrated that these novel REs were retinyl phytanate (54.0 min) and retinyl docosahexaenoate (50.5 min). Neither PA nor DHA increased LRAT mRNA levels in these cells. In addition, we demonstrate that retinyl phytanate was generated by LRAT in the presence of PA and retinol; however, retinyl docosahexaenoate was produced by another enzyme in the presence of DHA and retinol.

Pro-inflammatory Cytokine Tumor Necrosis Factor-α Induces Bone Morphogenetic Protein-2 in Chondrocytes via mRNA Stabilization and Transcriptional Up-regulation

In articular chondrocytes, the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) induces the expression of bone morphogenetic protein-2 (BMP-2), a growth factor known to be involved in the induction of cartilage and bone. A study was performed to clarify the mechanism(s) underlying the induction of BMP-2 in chondrogenic ATDC5 cells and primary cultured adult human articular chondrocytes. In ATDC5 cells, the endogenous BMP-2 expression was consistently low throughout the process of chondrogenic differentiation, and TNF-alpha induced BMP-2 expression only after the cells acquired the chondrogenic phenotype. The results of nuclear run-off assay and cycloheximide treatment consistently indicated that ATDC5 cells acquire the capacity to synthesize BMP-2 mRNA in the nuclei during the differentiation process. In an attempt to explain the discrepancy between the active nuclear mRNA synthesis and the observed low expression level in differentiated ATDC5 cells, the stability of BMP-2 mRNA was evaluated, and the cells were found to regulate the expression of BMP-2 at the post-transcriptional level. Human chondrocytes were confirmed to have a similar post-transcriptional regulation. The result of 3'-rapid amplification of cDNA end revealed that both human and mouse BMP-2 mRNAs contain multiple pentameric AUUUA motifs in a conserved manner in the 3'-untranslated regions, and transient transfection experiments demonstrated that TNF-alpha increases the stability of BMP-2 mRNA through the pentameric motifs. Further experiments revealed that TNF-alpha modulates mRNA stability via p38 signal transduction pathway, whereas the cytokine also augmented the expression of BMP-2 through transcriptional up-regulation via the transcriptional factor NF-kappaB.

PML-RARα and AML1–ETO translocations are rarely associated with methylation of the RARβ2 promoter

The acute promyelocytic leukemia-specific PML-RARalpha fusion protein is a dominant-negative transcriptional repressor of retinoic acid receptor (RAR) target genes, which recruits HDAC and corepressor proteins and inhibits coactivators. Another oncogenic transcription factor, AML1-ETO, was proposed to cause an HDAC-dependent repression of RAR target genes. The RAR target RARbeta2 gene has been reported to be frequently silenced by hypermethylation in many types of cancer cells. We examined the methylation status of the RARbeta2 and asked if demethylation could reverse ATRA resistance in ATRA-resistant PML-RARalpha and AML1-ETO-positive cells. PML-RARalpha positive NB4 and its ATRA-resistant subvariant MR2 and AML1-ETO expressing Kasumi-1 cells had heterozygous methylation of RARbeta2. Although DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine partially reversed RARbeta2 CpG methylation in these cells, it did not significantly enhance ATRA-induced RARbeta2 mRNA expression and induction of maturation. However, the histone acetylase inhibitor SAHA combined with ATRA significantly reactivated RARbeta2 mRNA both in NB4 and MR2 cells with degradation of PML-RARalpha, which was associated with maturation. In contrast, SAHA did not affect AML1-ETO levels and failed to induce RARbeta2 expression and maturation in Kasumi-1 cells. In primary AML samples, RARbeta2 expression was uniformly low; however, no specific correlation was observed between the methylation of the RARbeta2 gene and expression of the fusion proteins, PML-RARalpha, and AML1-ETO. These results demonstrate that oncogenic PML-RARalpha and AML1-ETO translocations are rarely associated with RARbeta2 promoter methylation in primary AML samples.

Endocrine regulation of HOX genes

Hox genes have a well-characterized role in embryonic development, where they determine identity along the anteroposterior body axis. Hox genes are expressed not only during embryogenesis but also in the adult, where they are necessary for functional differentiation. Despite the known function of these genes as transcription factors, few regulatory mechanisms that drive Hox expression are known. Recently, several hormones and their cognate receptors have been shown to regulate Hox gene expression and thereby mediate development in the embryo as well as functional differentiation in the adult organism. Estradiol, progesterone, testosterone, retinoic acid, and vitamin D have been shown to regulate Hox gene expression. In the embryo, the endocrine system directs axial Hox gene expression; aberrant Hox gene expression due to exposure to endocrine disruptors contributes to the teratogenicity of these compounds. In the adult, endocrine regulation of Hox genes is necessary to enable such diverse functions as hematopoiesis and reproduction; endocrinopathies can result in dysregulated HOX gene expression affecting physiology. By regulating HOX genes, hormonal signals utilize a conserved mechanism that allows generation of structural and functional diversity in both developing and adult tissues. This review discusses endocrine Hox regulation and its impact on physiology and human pathology.

Role of retinoic acid receptors alpha1 and gamma in the response of murine limbs to retinol in vitro

BACKGROUND

Derivatives of retinol (vitamin A), commonly referred to as retinoids, signal through retinoic acid and retinoid X receptors (RARs/RXRs) and are essential for normal limb formation. Retinoid imbalances or perturbations in receptor function result in aberrant limb development. To examine the mechanisms underlying retinol-induced limb defects, we determined the responsiveness of limbs from RARalpha1-/-gamma mice to excess retinol in vitro.

METHODS

RARalpha1-/-gamma+/- mice were bred and their embryos were recovered at gestational day (GD) 12.5. The forelimbs were excised and cultured in vitro in the presence of all-trans retinol acetate (0, 1.25, 12.5, or 62.5 microM) for 6 days. The expression profiles of genes known to affect chondrogenesis (sox9 and col2a1) and limb outgrowth (meis1, meis2, and pbx1a) were examined by real-time qRT-PCR following retinol exposure for 3 hr.

RESULTS

Whereas RARalpha1-/-gamma+/+ and RARalpha1-/-gamma+/- limbs exhibited deleterious effects on limb outgrowth and chondrogenesis in the presence of exogenous retinol, this outcome was significantly attenuated in RARalpha1-/-gamma-/- limbs. The expressions of sox9 and col2a1 were significantly decreased in retinol-exposed RARalpha1-/-gamma+/+ limbs. In contrast, expression was not altered in limbs from their RARalpha1-/-gamma+/- or RARalpha1-/-gamma-/- littermates. Retinol exposure upregulated the expression of meis1 and meis2 in RARalpha1-/-gamma+/+ limbs; however, in RARalpha1-/-gamma-/- limbs the expression of both genes was unresponsive to retinol. Pbx1a remained unresponsive to retinol treatment in all genotypes.

CONCLUSION

In the absence of RARalpha1, RARgamma is a functionally important mediator of retinoid-induced limb dysmorphogenesis.

Cancer Stem Cells and Differentiation Therapy

Cancers arise from stem cells in adult tissues and the cells that make up a cancer reflect the same stem cell --> progeny --> differentiation progression observed in normal tissues. All adult tissues are made up of lineages of cells consisting of tissue stem cells and their progeny (transit-amplifying cells and terminally differentiated cells); the number of new cells produced in normal tissue lineages roughly equals the number of old cells that die. Cancers result from maturation arrest of this process, resulting in continued proliferation of cells and a failure to differentiate and die. The biological behavior, morphological appearance, and clinical course of a cancer depend on the stage of maturation at which the genetic lesion is activated. This review makes a comparison of cancer cells to embryonic stem cells and to adult tis sue stem cells while addressing two basic questions: (1) Where do cancers come from?, and (2) How do cancers grow? The answers to these questions are critical to the development of approaches to the detection, prevention, and treatment of cancer.

Expression of estrogen receptor alpha, retinoic acid receptor alpha and cellular retinoic acid binding protein II genes is coordinately regulated in human breast cancer cells

Human breast cancer cell lines expressing the estrogen receptor alpha (ERalpha), all-trans-retinoic acid (ATRA) receptor alpha (RARalpha) and cellular retinoic acid binding protein II (CRABPII) genes are sensitive to ATRA-mediated growth inhibition. To study the relationship among ERalpha, RARalpha and CRABPII expression, the protein levels of each member were compared in five breast cancer cell lines (T47D, MCF-7, ZR-75-1, Hs587 T and MDA-MB-231 cells) and two immortalized nontumorigenic breast epithelial cell lines (MTSV1.7 and MCF-10A). ERalpha, RARalpha and CRABPII proteins were detected in T47D, MCF-7 and ZR-75-1 cells but not in other tested cell lines. RARalpha and CRABPII proteins were either reduced or undetectable in T47D/C4:2W and MCF-7/ADR cells with lost expression of ERalpha. Estradiol increased and anti-estrogens (tamoxifen and ICI 164,384) downregulated the expression of both RARalpha and CRABPII proteins in T47D and MCF-7 cells. RARalpha antagonist Ro-41-5253 inhibited CRABPII expression, but not RARalpha expression in estradiol-treated T47D and MCF-7 cells. Suppression of ERalpha by small interfering RNA (siRNA) reduced RARalpha and CRABPII gene expression and siRNA suppression of RARalpha reduced CRABPII expression while having no effect on ERalpha in T47D cells. Transient transfection of either RARalpha or ERalpha expression vectors increased CRABPII expression in MDA-MB-231 cells but only RARalpha, not ERalpha, activated hCRABPII promoter reporter. These results indicate that there is a gene activation pathway in which ERalpha drives RARalpha transcription and RARalpha drives CRABPII transcription in ERalpha-positive human breast cancer cells.

Differences in gene expression between wild type and Hoxa1 knockout embryonic stem cells after retinoic acid treatment or leukemia inhibitory factor (LIF) removal

Homeobox (Hox) genes encode a family of transcription factors that regulate embryonic patterning and organogenesis. In embryos, alterations of the normal pattern of Hox gene expression result in homeotic transformations and malformations. Disruption of the Hoxa1 gene, the most 3' member of the Hoxa cluster and a retinoic acid (RA) direct target gene, results in abnormal ossification of the skull, hindbrain, and inner ear deficiencies, and neonatal death. We have generated Hoxa1(-/-) embryonic stem (ES) cells (named Hoxa1-15) from Hoxa1(-/-) mutant blastocysts to study the Hoxa1 signaling pathway. We have characterized in detail these Hoxa1(-/-) ES cells by performing microarray analyses, and by this technique we have identified a number of putative Hoxa-1 target genes, including genes involved in bone development (e.g. Col1a1, Postn/Osf2, and the bone sialoprotein gene or BSP), genes that are expressed in the developing brain (e.g. Nnat, Wnt3a, BDNF, RhoB, and Gbx2), and genes involved in various cellular processes (e.g. M-RAS, Sox17, Cdkn2b, LamA1, Col4a1, Foxa2, Foxq1, Klf5, and Igf2). Cell proliferation assays and Northern blot analyses of a number of ES cell markers (e.g. Rex1, Oct3/4, Fgf4, and Bmp4) suggest that the Hoxa1 protein plays a role in the inhibition of cell proliferation by RA in ES cells. Additionally, Hoxa1(-/-) ES cells express high levels of various endodermal markers, including Gata4 and Dab2, and express much less Fgf5 after leukemia inhibitory factor (LIF) withdrawal. Finally, we propose a model in which the Hoxa1 protein mediates repression of endodermal differentiation while promoting expression of ectodermal and mesodermal characteristics.

Stem cell origin of cancer and differentiation therapy

Our forefathers in pathology, on observing cancer tissue under the microscope in the mid-19th century, noticed the similarity between embryonic tissue and cancer, and suggested that tumors arise from embryo-like cells [Recherches dur le Traitement du Cancer, etc. Paris. (1829); Editoral Archiv fuer pathologische Anatomie und Physiologie und fuer klinische Medizin 8 (1855) 23]. The concept that adult tissues contain embryonic remnants that generally lie dormant, but that could be activated to become cancer was later formalized by Cohnheim [Path. Anat. Physiol. Klin. Med. 40 (1867) 1-79; Virchows Arch. 65 (1875) 64] and Durante [Arch. Memori ed Osservazioni di Chirugia Practica 11 (1874) 217-226], as the "embryonal rest" theory of cancer. An updated version of the embryonal rest theory of cancer is that cancers arise from tissue stem cells in adults. Analysis of the cellular origin of carcinomas of different organs indicates that there is, in each instance, a determined stem cell required for normal tissue renewal that is the most likely cell of origin of carcinomas [Lab. Investig. 70 (1994) 6-22]. In the present review, the nature of normal stem cells (embryonal, germinal and somatic) is presented and their relationships to cancer are further expanded. Cell signaling pathways shared by embryonic cells and cancer cells suggest a possible link between embryonic cells and cancer cells. Wilm's tumors (nephroblastomas) and neuroblastomas are presented as possible tumors of embryonic rests in children. Teratocarcinoma is used as the classic example of the totipotent cancer stem cell which can be influenced by its environment to differentiate into a mature adult cell. The observation that "promotion" of an epidermal cancer may be accomplished months or even years after the initial exposure to carcinogen ("initiation"), implies that the original carcinogenic event occurs in a long-lived epithelial stem cell population. The cellular events during hepatocarcinogenesis illustrate that cancers may arise from cells at various stages of differentiation in the hepatocyte lineage. Examples of genetic mutations in epithelial and hematopoietic cancers show how specific alterations in gene expression may be manifested as maturation arrest of a cell lineage at a specific stage of differentiation. Understanding the signals that control normal development may eventually lead us to insights in treating cancer by inducing its differentiation (differentiation therapy). Retinoid acid (RA) induced differentiation therapy has acquired a therapeutic niche in treatment of acute promyelocytic leukemia and the ability of RA to prevent cancer is currently under examination.

Nonclassical Retinoids and Lung Carcinogenesis

The retinoids are natural and synthetic derivatives of vitamin A. These cancer therapeutic and chemopreventive agents exert antiproliferative, differentiation-inducing, proapoptotic, and other biologic effects. The retinoids act through nuclear retinoid receptors to activate target genes that signal biologic effects. Agents that specifically activate the nuclear retinoid X receptors (RXRs) are known as rexinoids. Rexinoid growth suppression of human bronchial epithelial cells was linked to triggering of G1 cell cycle arrest, concomitant growth suppression, and a decrease in expression of G1 cyclins through activation of a proteasome-dependent degradation pathway. Clinical studies have demonstrated prolonged survival of subsets of patients with non-small-cell lung cancer (NSCLC) treated with rexinoids as single agents or as part of combination regimens. The critical role of RXR in downstream signaling makes rexinoids especially attractive agents to consider in combination therapy. There is encouraging evidence for therapeutic benefit of combination regimens of rexinoids with other targeted agents, such as epidermal growth factor receptor inhibitors, and with chemotherapy. Results from randomized phase III clinical trials in NSCLC will ultimately determine the impact for rexinoid-based therapy or chemoprevention for lung cancer.

Conservation of Bmp2 post-transcriptional regulatory mechanisms

Bone morphogenetic protein (BMP) orthologs from diverse species like flies and humans are functionally interchangeable and play key roles in fundamental processes such as dorso-ventral axis formation in metazoans. Because both transcriptional and post-transcriptional mechanisms play central roles in modulating developmental protein levels, we have analyzed the 3'-untranslated region (3'UTR) of the Bmp 2 gene. This 3'UTR is unusually long and is alternatively polyadenylated. Mouse, human, and dog mRNAs are 83-87% identical within this region. A 265-nucleotide sequence, conserved between mammals, birds, frogs, and fish, is present in Bmp2 but not Bmp4. The ability of AmphiBMP2/4, a chordate ortholog to Bmp2 and Bmp4, to align with this sequence suggests that its function may have been lost in Bmp4. Activation of reporter genes by the conserved region acts by a post-transcriptional mechanism. Mouse, human, chick, and zebrafish Bmp2 synthetic RNAs decay rapidly in extracts from cells not expressing Bmp2. In contrast, these RNAs are relatively stable in extracts from Bmp2-expressing cells. Thus, Bmp2 RNA half-lives in vitro correlate with natural Bmp2 mRNA levels. The fact that non-murine RNAs interact appropriately with the mouse decay machinery suggests that the function of these cis-regulatory regions has been conserved for 450 million years since the fish and tetrapod lineages diverged. Overall, our results suggest that the Bmp2 3'UTR contains essential regulatory elements that act post-transcriptionally.

RARgamma acts as a tumor suppressor in mouse keratinocytes

All-trans retinoic acid (RA), the principle biologically active form of vitamin A, is essential for many developmental process as well as homeostasis in the adult. Many lines of evidence also suggest that RA, acting through the RA receptors (RARs), can also suppress growth of tumors of diverse origin. To assess directly the role of the RARs in a model of epidermal tumorigenesis, we investigated the incidence of tumor formation using keratinocytes lacking specific RAR types. Our data suggest that loss of RARgamma, but not RARalpha, predisposed keratinocytes to v-Ha-Ras-induced squamous cell carcinoma. We also found that ablation of RARgamma, but not RARalpha, abolished RA-induced cell cycle arrest and apoptosis in these keratinocytes. Reconstitution of receptor expression into RAR-null cells restored sensitivity to RA, and reversed the tumorigenic potential of receptor-deficient keratinocytes. These data strongly support a tumor suppressor effect for the RARs, in particular endogenous RARgamma, in murine keratinocytes.

Retinoic acid causes cell growth arrest and an increase in p27 in F9 wild type but not in F9 retinoic acid receptor β2 knockout cells

We have previously shown that an F9 teratocarcinoma retinoic acid receptor beta(2) (RARbeta(2)) knockout cell line exhibits no growth arrest in response to all-trans-retinoic acid (RA), whereas F9 wild type (Wt), F9 RARalpha(-/-), and F9 RARgamma(-/-) cell lines do growth arrest in response to RA. To examine the role of RARbeta(2) in growth inhibition, we analyzed the cell cycle regulatory proteins affected by RA in F9 Wt and F9 RARbeta(2)(-/-) cells. Flow microfluorimetry analyses revealed that RA treatment of F9 Wt cells greatly increased the percentage of cells in the G1/G0 phase of the cell cycle. In contrast, RA did not alter the cell cycle distribution profile of RARbeta(2)(-/-) cells. In F9 Wt cells, cyclin D1, D3, and cyclin E protein levels decreased, while cyclin D2 and p27 levels increased after RA treatment. Compared to the F9 Wt cells, the F9 RARbeta(2)(-/-) cells exhibited lower levels of cyclins D1, D2, D3, and E in the absence of RA, but did not exhibit further changes in the levels of these cell cycle regulators after RA addition. Since RA significantly increased the level of p27 protein (approximately 24-fold) in F9 Wt as compared to the F9 RARbeta(2)(-/-) cells, we chose to study p27 in greater detail. The p27 mRNA level and the rate of p27 protein synthesis were increased in RA-treated F9 Wt cells, but not in F9 RARbeta(2)(-/-) cells. Moreover, RA increased the half-life of p27 protein in F9 Wt cells. Reduced expression of RARbeta(2) is associated with the process of carcinogenesis and RARbeta(2) can mediate the growth arrest induced by RA in a variety of cancer cells. Using both genetic and molecular approaches, we have identified some of the molecular mechanisms, such as the large elevation of p27, through which RARbeta(2) mediates these growth inhibitory effects of RA in F9 cells.

An evolutionary and molecular analysis of Bmp2 expression

The coding regions of many metazoan genes are highly similar. For example, homologs to the key developmental factor bone morphogenetic protein (BMP) 2 have been cloned by sequence identity from arthropods, mollusks, cnidarians, and nematodes. Wide conservation of protein sequences suggests that differential gene expression explains many of the vast morphological differences between species. To test the hypothesis that the regulatory mechanisms controlling this evolutionarily ancient and critical gene are conserved, we compared sequences flanking Bmp2 genes of several species. We identified numerous conserved noncoding sequences including some retained because the fish lineage separated 450 million years ago. We tested the function of some of these sequences in the F9 cell model system of Bmp2 expression. We demonstrated that both mouse and primate Bmp2 promoters drive a reporter gene in an expression pattern resembling that of the endogenous transcript in F9 cells. A conserved Sp1 site contributes to the retinoic acid responsiveness of the Bmp2 promoter, which lacks a classical retinoic acid response element. We have also discovered a sequence downstream of the stop codon whose conservation between humans, rodents, deer, chickens, frogs, and fish is striking. A fragment containing this region influences reporter gene expression in F9 cells. The conserved region contains elements that may mediate the half-life of the Bmp2 transcript. Together, our molecular and evolutionary analysis has identified new regulatory elements controlling Bmp2 expression.

A blockade in Wnt signaling is activated following the differentiation of F9 teratocarcinoma cells

Aberrant activation of the Wnt signaling pathway is a common event in human tumor progression. Wnt signaling has also been implicated in maintaining a variety of adult and embryonic stem cells by imposing a restraint to differentiation. To understand the effect of Wnt signaling on the differentiation of epithelial cells, we used mouse teratocarcinoma F9 cells as a model. The F9 cells can be differentiated into visceral endoderm (VE) resembling absorptive columnar epithelial cells. We performed comparative gene expression analysis on retinoic acid-differentiated and undifferentiated F9 cells and confirmed that markers of VE and intestinal epithelium were induced upon differentiation. The induction of these markers by retinoic acid was reduced in the presence of Wnt, although Wnt alone did not change their expression. This suggests that Wnt signaling inhibited the differentiation of F9 cells by altering gene expression. This inhibition was also reflected in the morphology of the F9 cells as their apical-basal polarity was disrupted by inclusion of Wnt during differentiation. These results support a model in which Wnt modulates the expression of genes required for normal terminal differentiation of the stem cells. However, it follows that progenitor cells must escape from Wnt signaling to attain the differentiated state. Accordingly, we found that differentiated F9 cells no longer responded to Wnt and that a blockade in Wnt signaling occurred upstream of Axin. Consistent with this, Wnt negative regulators, such as Dickkopf-1 and Disabled-2, were induced upon the differentiation of F9 cells. We propose that a similar system to produce Wnt inhibitors regulates homeostasis of certain stem cell compartments in vivo.

Differential modulation of rat hepatic stellate phenotype by natural and synthetic retinoids

Activation of hepatic stellate cells (HSC) is a central event in the pathogenesis of liver fibrosis during chronic liver injury. We examined the expression of retinoic acid (RAR) and retinoid X receptors (RXR) during HSC activation and evaluated the influence of natural and synthetic retinoic acids (RA) on the phenotype of culture-activated HSC. The expression of the major RAR/RXR subtypes and isoforms was analyzed by Northern hybridization. Presence of functional receptor proteins was established by gel shift analysis. Retinoic acids, RAR, and RXR selective agonists and an RAR antagonist were used to evaluate the effects of retinoid signalling on matrix synthesis by Northern blotting and immunoprecipitation, and on cell proliferation by BrdU incorporation. The 9-cisRA and synthetic RXR agonists reduced HSC proliferation and synthesis of collagen I and fibronectin. All-trans RA and RAR agonists both reduced the synthesis of collagen I, collagen III, and fibronectin, but showed a different effect on cell proliferation. Synthetic RAR agonists did not affect HSC proliferation, indicating that ATRA inhibits cell growth independent of its interaction with RARs. In contrast, RAR specific antagonists enhance HSC proliferation and demonstrate that RARs control proliferation in a negative way. In conclusion, natural RAs and synthetic RAR or RXR specific ligands exert differential effects on activated HSC. Our observations may explain prior divergent results obtained following retinoid administration to cultured stellate cells or to animals subjected to fibrogenic stimuli.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Transient modulation of cytoplasmic and nuclear retinoid receptors expression in differentiating human teratocarcinoma NT2 cells

Human embryonal carcinoma Ntera2/D1 (NT2) cells treated with retinoic acid (RA) differentiate into several cell types including post-mitotic neurons. In this study we asked if RA-induced differentiation alters the expression of RA and retinol (ROL) binding proteins. The regulation of the intracellular carrier proteins for ROL and RA, cellular retinol binding protein I (CRBP-I), and cellular retinoic acid binding protein I and II (CRABP-I, CRABP-II) were studied along with the nuclear RA receptors RARalpha, RARbeta and RARgamma2. PCR analysis of total mRNA from RA-treated cells showed a biphasic early induction of CRBP-I, CRABP-II, and RARgamma2 genes. The immediate early gene Krox-24, a zinc finger transcription factor which is up-regulated during neuronal differentiation, was also induced, but after 1 week of treatment. The induction of CRBP-I protein synthesis in differentiating NT2 cells was confirmed by western blotting and immunofluorescence experiments. Conversely, the synthetic retinoid N-(4-hydroxyphenyl)retinamide, which induces cell death, but not differentiation in different tumour cell types, did not produce the same modulation on gene expression in NT2 cells. These data suggest that the RA-specific induction of CRBP-I and CRABP-II could be an early event in the process leading to neuronal differentiation of NT2 cells.