Distinct retinoid X receptor-retinoic acid receptor heterodimers are differentially involved in the control of expression of retinoid target genes in F9 embryonal carcinoma cells

Single-Cell Transcriptomic Analysis of the Potential Mechanisms of Follicular Development in Stra8-Deficient Mice

Follicle development is a critical process in mammalian reproduction, with significant implications for ovarian reserve and fertility. Stra8 is a known key factor regulating the initiation of meiosis; however, oocyte-like cells still appear in Stra8-deficient mice. Nevertheless, the underlying mechanism remains unclear and requires further investigation. Therefore, we used single-cell RNA sequencing to construct a comprehensive transcriptional atlas of ovarian cells from both wild-type and Stra8-deficient mice at embryonic stages E14.5 and E16.5. With stringent quality control, we obtained a total of 14,755 single cells of six major cell types. A further fine-scale analysis of the germ cell clusters revealed notable heterogeneity between wild-type and Stra8-deficient mice. Compared to the wild-type mice, the deficiency in Stra8 led to the downregulation of meiosis-related genes (e.g., Pigp, Tex12, and Sycp3), and the upregulation of apoptosis-related genes (e.g., Fos, Jun, and Actb), thereby hindering the meiotic process. Notably, we observed that, following Stra8 deficiency, the expression levels of Sub1 and Stk31 remained elevated at this stage. Furthermore, an RNA interference analysis confirmed the potential role of these genes as regulatory factors in the formation of primordial follicle-like cells. Additionally, Stra8 deficiency disrupted the signaling between germ cells and pregranulosa cells that is mediated by Mdk-Sdc1, leading to the abnormal expression of the PI3K/AKT signaling pathway. Together, these results shed light on the molecular processes governing germ cell differentiation and folliculogenesis, emphasizing the complex role of Stra8 in ovarian function.

Cell adhesion signals regulate the nuclear receptor activity

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.

HOXB1 Is a Tumor Suppressor Gene Regulated by miR-3175 in Glioma

The HOXB1 gene plays a critical role as an oncogene in diverse tumors. However, the functional role of HOXB1 and the mechanism regulating HOXB1 expression in glioma are not fully understood. A preliminary bioinformatics analysis showed that HOXB1 is ectopically expressed in glioma, and that HOXB1 is a possible target of miR-3175. In this study, we investigated the function of HOXB1 and the relationship between HOXB1 and miR-3175 in glioma. We show that HOXB1 expression is significantly downregulated in glioma tissues and cell lines, and that its expression may be closely associated with the degree of malignancy. Reduced HOXB1 expression promoted the proliferation and invasion of glioma cells, and inhibited their apoptosis in vitro, and the downregulation of HOXB1 was also associated with worse survival in glioma patients. More importantly, HOXB1 was shown experimentally to be a direct target of miR-3175 in this study. The downregulated expression of miR-3175 inhibited cell proliferation and invasion, and promoted apoptosis in glioma. The oncogenicity induced by low HOXB1 expression was prevented by an miR-3175 inhibitor in glioma cells. Our results suggest that HOXB1 functions as a tumor suppressor, regulated by miR-3175 in glioma. These results clarify the pathogenesis of glioma and offer a potential target for its treatment.

RAR/RXR binding dynamics distinguish pluripotency from differentiation associated cis-regulatory elements

In mouse embryonic cells, ligand-activated retinoic acid receptors (RARs) play a key role in inhibiting pluripotency-maintaining genes and activating some major actors of cell differentiation. To investigate the mechanism underlying this dual regulation, we performed joint RAR/RXR ChIP-seq and mRNA-seq time series during the first 48 h of the RA-induced Primitive Endoderm (PrE) differentiation process in F9 embryonal carcinoma (EC) cells. We show here that this dual regulation is associated with RAR/RXR genomic redistribution during the differentiation process. In-depth analysis of RAR/RXR binding sites occupancy dynamics and composition show that in undifferentiated cells, RAR/RXR interact with genomic regions characterized by binding of pluripotency-associated factors and high prevalence of the non-canonical DR0-containing RA response element. By contrast, in differentiated cells, RAR/RXR bound regions are enriched in functional Sox17 binding sites and are characterized with a higher frequency of the canonical DR5 motif. Our data offer an unprecedentedly detailed view on the action of RA in triggering pluripotent cell differentiation and demonstrate that RAR/RXR action is mediated via two different sets of regulatory regions tightly associated with cell differentiation status.

Enhanced expression of retinoic acid receptor alpha (RARA) induces epithelial-to-mesenchymal transition and disruption of mammary acinar structures

The early steps of mammary tumorigenesis include loss of epithelial cell polarity, escape from anoikis, and acquisition of proliferative capacity. The genes responsible for these processes are predicted to be early diagnostic markers or new therapeutic targets. Here we tested 51 genes coamplified with ERBB2 in the 17q12-21 amplicon for these tumorigenic activities using an MCF10A 3D culture-based screening system. We found that overexpression of retinoic acid receptor α (RARA) disrupted normal acinar structure and induced epithelial-to-mesenchymal transition (EMT). The mRNA levels of known EMT-inducing factors, including SLUG, FOXC2, ZEB1, and ZEB2, were significantly increased upon RARA overexpression. Knockdown of ZEB1 suppressed the RARA-mediated EMT phenotype. These results suggest that overexpression of RARA enhances malignant transformation during mammary tumorigenesis.

Integrative genomics to dissect retinoid functions

Retinoids and rexinoids, as all other ligands of the nuclear receptor (NR) family, act as ligand-regulated trans-acting transcription factors that bind to cis-acting DNA regulatory elements in the promoter regions of target genes (for reviews see [12, 22, 23, 26, 36]). Ligand binding modulates the communication functions of the receptor with the intracellular environment, which essentially entails receptor-protein and receptor-DNA or receptor-chromatin interactions. In this communication network, the receptor simultaneously serves as both intracellular sensor and regulator of cell/organ functions. Receptors are "intelligent" mediators of the information encoded in the chemical structure of a nuclear receptor ligand, as they interpret this information in the context of cellular identity and cell-physiological status and convert it into a dynamic chain of receptor-protein and receptor-DNA interactions. To process input and output information, they are composed of a modular structure with several domains that have evolved to exert particular molecular recognition functions. As detailed in other chapters in this volume, the main functional domains are the DNA-binding (DBD) and ligand-binding (LBD) [5-7, 38, 56, 71]. The LBD serves as a dual input-output information processor. Inputs, such as ligand binding or receptor phosphorylations, induce allosteric changes in receptor surfaces that serve as docking sites for outputs, such as subunits of transcription and epigenetic machineries or enzyme complexes. The complexity of input and output signals and their interdependencies is far from being understood.

β-cryptoxanthin restores nicotine-reduced lung SIRT1 to normal levels and inhibits nicotine-promoted lung tumorigenesis and emphysema in A/J mice

Nicotine, a large constituent of cigarette smoke, is associated with an increased risk of lung cancer, but the data supporting this relationship are inconsistent. Here, we found that nicotine treatment not only induced emphysema but also increased both lung tumor multiplicity and volume in 4-nitrosamino-1-(3-pyridyl)-1-butanone (NNK)-initiated lung cancer in A/J mice. This tumor-promoting effect of nicotine was accompanied by significant reductions in survival probability and lung Sirtuin 1 (SIRT1) expression, which has been proposed as a tumor suppressor. The decreased level of SIRT1 was associated with increased levels of AKT phosphorylation and interleukin (il)-6 mRNA but decreased tumor suppressor p53 and retinoic acid receptor (RAR)-β mRNA levels in the lungs. Using this mouse model, we then determined whether β-cryptoxanthin (BCX), a xanthophyll that is strongly associated with a reduced risk of lung cancer in several cohort studies, can inhibit nicotine-induced emphysema and lung tumorigenesis. We found that BCX supplementation at two different doses was associated with reductions of the nicotine-promoted lung tumor multiplicity and volume, as well as emphysema in mice treated with both NNK and nicotine. Moreover, BCX supplementation restored the nicotine-suppressed expression of lung SIRT1, p53, and RAR-β to that of the control group, increased survival probability, and decreased the levels of lung il-6 mRNA and phosphorylation of AKT. The present study indicates that BCX is a preventive agent against emphysema and lung cancer with SIRT1 as a potential target. In addition, our study establishes a relevant animal lung cancer model for studying tumor growth within emphysematous microenvironments.

Decalpenic acid induces early osteoblastic markers in pluripotent mesenchymal cells via activation of retinoic acid receptor γ

Decalpenic acid is a natural small molecule previously isolated from the fermentation broth of fungi that induces early osteoblastic markers in pluripotent mesenchymal cells. Treatment of mouse pluripotent mesenchymal C3H10T1/2 cells with decalpenic acid gave rise to a morphological change similar to that induced by the treatment with retinoic acid, i.e. the cells adopted a more elongated spindle shape. Using a retinoic acid response element reporter and receptor activity assays, we show that decalpenic acid is a new retinoid with selectivity towards retinoic acid receptors γ and α. The induction of early osteoblastic markers by decalpenic acid was significantly inhibited by treatment with the retinoid antagonist, LE540, or with small interfering RNA-mediated knockdown of retinoic acid receptor γ. These results demonstrated that decalpenic acid induces early osteoblastic markers in pluripotent mesenchymal cells through activation of retinoic acid receptor γ.

Dissecting the retinoid-induced differentiation of F9 embryonal stem cells by integrative genomics

Retinoic acid (RA) triggers physiological processes by activating heterodimeric transcription factors (TFs) comprising retinoic acid receptor (RARα, β, γ) and retinoid X receptor (RXRα, β, γ). How a single signal induces highly complex temporally controlled networks that ultimately orchestrate physiological processes is unclear. Using an RA-inducible differentiation model, we defined the temporal changes in the genome-wide binding patterns of RARγ and RXRα and correlated them with transcription regulation. Unexpectedly, both receptors displayed a highly dynamic binding, with different RXRα heterodimers targeting identical loci. Comparison of RARγ and RXRα co-binding at RA-regulated genes identified putative RXRα-RARγ target genes that were validated with subtype-selective agonists. Gene-regulatory decisions during differentiation were inferred from TF-target gene information and temporal gene expression. This analysis revealed six distinct co-expression paths of which RXRα-RARγ is associated with transcription activation, while Sox2 and Egr1 were predicted to regulate repression. Finally, RXRα-RARγ regulatory networks were reconstructed through integration of functional co-citations. Our analysis provides a dynamic view of RA signalling during cell differentiation, reveals RAR heterodimer dynamics and promiscuity, and predicts decisions that diversify the RA signal into distinct gene-regulatory programs.

Detection of variable levels of RARα and RARγ proteins in pluripotent and differentiating mouse embryonal carcinoma and mouse embryonic stem cells

Pluripotent mouse embryonal carcinoma (mEC) and mouse embryonic stem (mES) cells differentiate into several cell lineages upon retinoic acid (RA) addition. Differentiation is facilitated, in part, by RA activation of nuclear RA receptors (RARs) that bind to DNA response elements located in the promoters of target genes. The purpose of the studies reported here was to immunolocalize RARα and RARγ protein in mEC and mES cells and in their RA-induced differentiated progeny. Fixed cells were reacted with three different RARα antibodies and one RARγ antibody. Pluripotent and differentiated mEC and mES cells showed positive nuclear immunoreactivity with all antibodies tested. Two RARα antibodies also showed positive reactivity in the cytoplasm. Surprisingly, our results revealed variability in immunofluorescence intensity and in RARα and RARγ distribution from one cell to the other, suggesting that RARα and RARγ protein levels were not synchronous throughout the cell population. The results indicate that RARα and RARγ are present in pluripotent and differentiating mEC and mES cells and suggest that the expression of these proteins is dynamic.

DNA recognition by thyroid hormone and retinoic acid receptors: 3,4,5 rule modified

It has been proposed that retinoic acid receptors (RARs) and thyroid hormone receptors (TRs) both bind to AGGTCA "half-site" sequences, but distinguish their different target genes by recognizing different half-site spacings. We report here that artificial DNA binding sites based on these AGGTCA half-sites confer high affinity, but poor specificity, and that spacing alone does not account for the divergent DNA recognition properties of TRs and RARs. Instead, we have determined that the non-consensus half-sites that are present in naturally occurring RAR and TR target genes play a crucial role in defining receptor DNA recognition specificity, and work together with flanking sequences and half-site spacing to produce receptor-specific DNA binding in vitro. We also provide evidence that auxiliary proteins in cells generate an additional layer of receptor-specific target gene recognition, in part by destabilizing the binding of nuclear receptors to the "wrong" response elements.

Retinoid receptor subtype-selective modulators through synthetic modifications of RARgamma agonists

A series of retinoids designed to interfere with the repositioning of H12 have been synthesized to identify novel RARgamma antagonists based on the structure of known RARgamma agonists. The transcriptional activities of the novel ligands were revealed by cell-based reporting assays, using engineered cells containg RAR subtype-selective fusions of the RAR ligand-binding domains with the yeast GAL4 activator DNA-binding domain and the cognate luciferase reporter gene. Whereas none of the ligands exhibited features of a selective RARgamma antagonist, some of them are endowed with interesting activities. In particular 24a acts as a pan-RAR agonist that induces at high concentration a higher transactivation potential on RARalpha than TTNPB and synergizes at low concentration with TTNPB-bound RARalpha but not RARbeta or RARgamma. Similarly, 24c synergizes with TTNPB-bound RARgamma and exhibits RARalpha,beta antagonist activity. Compounds 24b and 25b are strong RARalpha,beta-selective antagonists without agonist or antagonist activities for RARgamma. Compounds 24b and 24c display weak RXR antagonist activity. In addition several pan-antagonists and partial agonist/antagonists have been defined.

Retinoic acid receptors and GATA transcription factors activate the transcription of the human lecithin:retinol acyltransferase gene

Lecithin:retinol acyltransferase (LRAT) catalyzes the esterification of retinol (vitamin A). Retinyl esters and LRAT protein levels are reduced in many types of cancer cells. We present data that both the LRAT and retinoic acid receptor beta(2) (RARbeta(2)) mRNA levels in the human prostate cancer cell line PC-3 are lower than those in cultured normal human prostate epithelial cells (PrEC). The activity of the human LRAT promoter (2.0 kb) driving a luciferase reporter gene in PC-3 cells is less than 40% of that in PrEC cells. Retinoic acid (RA) treatment increased this LRAT promoter-luciferase activity in PrEC cells, but not in PC-3 cells. Deletion of various regions of the human LRAT promoter demonstrated that a 172-bp proximal promoter region is essential for LRAT transcription and confers RA responsiveness in PrEC cells. This 172-bp region, contained within the 186 bp pLRAT/luciferase construct, has five putative GATA binding sites. Cotransfection of RARbeta(2) or RARgamma and the transcription factor GATA-4 increased LRAT (pLRAT186) promoter activity in both PrEC and PC-3 cells. In addition, we found that both retinoic acid and retinol induced transcripts for the STRA6 gene, which encodes a membrane receptor involved in retinol (vitamin A) uptake, in PrEC cells but not in PC-3 cells. In summary, our data show that the transcriptional regulation of the human LRAT gene is aberrant in human prostate cancer cells and that GATA transcription factors are involved in the transcriptional activation of LRAT in PrEC cells.

Derangement of a factor upstream of RARalpha triggers the repression of a pleiotropic epigenetic network

Background

Chromatin adapts and responds to extrinsic and intrinsic cues. We hypothesize that inheritable aberrant chromatin states in cancer and aging are caused by genetic/environmental factors. In previous studies we demonstrated that either genetic mutations, or loss, of retinoic acid receptor alpha (RARα), can impair the integration of the retinoic acid (RA) signal at the chromatin of RA-responsive genes downstream of RARα, and can lead to aberrant repressive chromatin states marked by epigenetic modifications. In this study we tested whether the mere interference with the availability of RA signal at RARα, in cells with an otherwise functional RARα, can also induce epigenetic repression at RA-responsive genes downstream of RARα.

Methodology/Principal Findings

To hamper the availability of RA at RARα in untransformed human mammary epithelial cells, we targeted the cellular RA-binding protein 2 (CRABP2), which transports RA from the cytoplasm onto the nuclear RARs. Stable ectopic expression of a CRABP2 mutant unable to enter the nucleus, as well as stable knock down of endogenous CRABP2, led to the coordinated transcriptional repression of a few RA-responsive genes downstream of RARα. The chromatin at these genes acquired an exacerbated repressed state, or state “of no return”. This aberrant state is unresponsive to RA, and therefore differs from the physiologically repressed, yet “poised” state, which is responsive to RA. Consistent with development of homozygosis for epigenetically repressed loci, a significant proportion of cells with a defective CRABP2-mediated RA transport developed heritable phenotypes indicative of loss of function.

Conclusion/Significance

Derangement/lack of a critical factor necessary for RARα function induces epigenetic repression of a RA-regulated gene network downstream of RARα, with major pleiotropic biological outcomes.

The effect of retinoic acid and deoxycholic acid on the differentiation of primary human esophageal keratinocytes

The mechanism linking gastroduodenal reflux disease to intestinal metaplasia in the esophagus (Barrett's esophagus) has not been determined. Active conjugate metabolites of retinoic acid, in addition to bile acids, undergo an enterohepatic circulation in bile. Retinoic acid and bile acids are candidate mediators of keratinocyte transdifferentiation in Barrett's esophagus. We studied the effects of retinoic acid on the differentiation of primary human esophageal keratinocytes cultured in vitro. Retinoic acid induces expression of a marker of intestinal differentiation, MUC2, in these cells. However, retinoic acid, alone or in combination with the hydrophobic bile acid, deoxycholic acid, does not affect esophageal keratinocyte squamous differentiation as assessed by involucrin expression and cellular morphology. The ability of retinoic acid to induce MUC2 expression may be relevant to the pathogenesis of Barrett's esophagus. However, this does not result in suppression of squamous differentiation.

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.

Tight junction proteins claudin-2 and -12 are critical for vitamin D-dependent Ca2+ absorption between enterocytes

Ca(2+) is absorbed across intestinal epithelial monolayers via transcellular and paracellular pathways, and an active form of vitamin D(3), 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], is known to promote intestinal Ca(2+) absorption. However, the molecules driving the paracellular Ca(2+) absorption and its vitamin D dependency remain obscure. Because the tight junction proteins claudins are suggested to form paracellular channels for selective ions between neighboring cells, we hypothesized that specific intestinal claudins might facilitate paracellular Ca(2+) transport and that expression of these claudins could be induced by 1alpha,25(OH)(2)D(3). Herein, we show, by using RNA interference and overexpression strategies, that claudin-2 and claudin-12 contribute to Ca(2+) absorption in intestinal epithelial cells. We also provide evidence showing that expression of claudins-2 and -12 is up-regulated in enterocytes in vitro and in vivo by 1alpha,25(OH)(2)D(3) through the vitamin D receptor. These findings strongly suggest that claudin-2- and/or claudin-12-based tight junctions form paracellular Ca(2+) channels in intestinal epithelia, and they highlight a novel mechanism behind vitamin D-dependent calcium homeostasis.

p-Dodecylaminophenol derived from the synthetic retinoid, fenretinide: antitumor efficacy in vitro and in vivo against human prostate cancer and mechanism of action

Fenretinide, N-(4-hydroxyphenyl)retinamide (4-HPR) is an aminophenol-containing synthetic retinoid derivative of all-trans-retinoic acid, which is a potent chemopreventive and antiproliferative agent against various cancers. Clinical studies of 4-HPR have shown side effects consisting of night blindness and ocular toxicity. To maintain potent anticancer activity without side effects, p-dodecylaminophenol (p-DDAP) was designed based on structure-activity relationships of 4-HPR. In our study, we investigate whether p-DDAP shows anticancer activity against human prostate cancer cell line PC-3 when compared with 4-HPR. p-DDAP inhibited PC-3 cell growth progressively from low to high concentration in a dose-dependent manner. p-DDAP was the most potent antiproliferative agent in vitro among 6 p-alkylaminophenols and 3 4-hydroxyphenyl analogs examined including 4-HPR. Cells treated with p-DDAP were shown to undergo apoptosis, based on condensation nuclei, cytofluorimetric analysis, propidium iodide staining and the expression of bcl-2 and caspase 3. p-DDAP arrested the S phase of the cell cycle, while 4-HPR arrested the G(0)/G(1) phase. In addition, both the i.v. and i.p. administration of p-DDAP suppressed tumor growth in PC-3-implanted mice in vivo. p-DDAP showed no effects on blood retinol concentrations, in contrast to reductions after 4-HPR administration. These results indicate that p-DDAP exhibits excellent anticancer efficacy against hormonal independent prostate cancer in vitro and in vivo, and it may have great potential for clinical use in the treatment of prostate cancer with reduced side effects.

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.

Retinoic acid, meiosis and germ cell fate in mammals

Although mammalian sex is determined genetically, the sex-specific development of germ cells as sperm or oocytes is initiated by cues provided by the gonadal environment. During embryogenesis, germ cells in an ovary enter meiosis, thereby committing to oogenesis. By contrast, germ cells in a testicular environment do not enter meiosis until puberty. Recent findings indicate that the key to this sex-specific timing of meiosis entry is the presence or absence of the signaling molecule retinoic acid. Although this knowledge clarifies a long-standing mystery in reproductive biology, it also poses many new questions, which we discuss in this review.

PPARdelta enhances keratinocyte proliferation in psoriasis and induces heparin-binding EGF-like growth factor

Psoriasis is a common skin disease involving keratinocyte proliferation and altered differentiation, as well as T-cell activation. Here, we show that altered gene transcription in psoriatic skin lesions is highly reproducible between independent data sets. Analysis of gene expression confirmed dysregulation in all expected functional categories, such as IFN signaling and keratinocyte differentiation, and allowed molecular fingerprinting of a previously characterized dendritic cell subset associated with psoriasis tumor necrosis factor alpha (TNF-alpha)- and inducible nitric oxide synthase (iNOS)-producing CD11b(INT) DC (Tip-DC). Unexpectedly, a large group of dysregulated transcripts was related to fatty acid signaling and adipocyte differentiation, exhibiting a pattern consistent with the activation of peroxisome proliferator-activated receptor delta (PPARdelta). PPARdelta itself was strongly induced in psoriasis in vivo. In primary keratinocytes, PPARdelta was induced by the transcription factor activator protein 1, in particular by junB, but not by canonical WNT signaling, in contrast to its regulation in colon carcinoma cells. Activation of PPARdelta enhanced proliferation of keratinocytes, while this was inhibited by knockdown of PPARdelta. Finally, heparin-binding EGF-like growth factor (HB-EGF), known to induce epidermal hyperplasia and itself overexpressed in psoriasis, was identified as a direct target gene of PPARdelta. The present data suggest that activation of PPARdelta is a major event in psoriasis, contributing to the hyperproliferative phenotype by induction of HB-EGF.

The nuclear receptor hepatocyte nuclear factor 4alpha acts as a morphogen to induce the formation of microvilli

Microvilli are actin-based organelles found on apical plasma membranes that are involved in nutrient uptake and signal transduction. Numerous components, including ezrin/radixin/moesin (ERM) proteins, have been identified that link filamentous actins to transmembrane proteins, but the signals driving microvillus biogenesis are not known. In this study, we show that the conditional and/or ectopic expression of a nuclear receptor, hepatocyte nuclear factor 4α (HNF4α), triggers microvillus morphogenesis. We also demonstrate that HNF4α expression induces ERM-binding phosphoprotein 50 (EBP50) expression and that attenuation of EBP50 using RNA interference inhibits microvillus development. We conclude that HNF4α acts as a morphogen to trigger microvillus formation.

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.

International Union of Pharmacology. LXIII. Retinoid X receptors

The physiological effects of retinoic acids (RAs) are mediated by members of two families of nuclear receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), which are encoded by three distinct human genes, RXRalpha, RXRbeta, and RXRgamma. RARs bind both all-trans- and 9-cis-RA, whereas only the 9-cis-RA stereoisomer binds to RXRs. As RXR/RAR heterodimers, these receptors control the transcription of RA target genes through binding to RA-response elements. This review is focused on the structure, mode of action, ligands, expression, and pharmacology of RXRs. Given their role as common partners to many other members of the nuclear receptor superfamily, these receptors have been the subject of intense scrutiny. Moreover, and despite numerous studies since their initial discovery, RXRs remain enigmatic nuclear receptors, and there is still no consensus regarding their role. Indeed, multiple questions about the actual biological role of RXRs and the existence of an endogenous ligand have still to be answered.

International Union of Pharmacology. LX. Retinoic acid receptors

Retinoid is a term for compounds that bind to and activate retinoic acid receptors (RARalpha, RARbeta, and RARgamma), members of the nuclear hormone receptor superfamily. The most important endogenous retinoid is all-trans-retinoic acid. Retinoids regulate a wide variety of essential biological processes, such as vertebrate embryonic morphogenesis and organogenesis, cell growth arrest, differentiation and apoptosis, and homeostasis, as well as their disorders. This review summarizes the considerable amount of knowledge generated on these receptors.

Differential expression and subcellular localization of claudin-7, -8, -12, -13, and -15 along the mouse intestine

Among tight-junction proteins, claudins, which play a key role in paracellular transport across epithelia, claudins 1 to 5 are expressed in the intestine, and changes in their abundance and/or distribution are considered to contribute to various gastrointestinal diseases. We investigated, by reverse transcription-PCR, immunoblot, and immunofluorescence analyses, which other claudin species were expressed in the mouse intestine, and whether they showed unique expression profiles. Rabbit polyclonal antibodies against mouse claudin-8, claudin-12, and claudin-15 were generated, and their specificity was verified by immunoblotting using COS-7 cells transfected with individual claudin cDNAs. Claudin-7, -8, -12, -13, and -15 appeared to be expressed in the duodenum, jejunum, ileum, and/or colon with remarkable variations in the expression levels along the intestinal tract, and had distinct subcellular localization in the intestinal epithelium. In addition, claudin-13 and -15 exhibited gradients along the crypt-surface axis of the colon. By contrast, claudin-6, -9, -10, -11, -14, -16, -18, and -19 were not observed in the intestine. Our results indicate that five additional species of claudins have very complex expression patterns along and within the intestine, and that this may reflect differences in paracellular permeable properties, providing valuable resources for studying the significance of these claudins in gastrointestinal disorders. This manuscript contains online supplemental material available at http://www.jhc.org. Please visit this article online to view these materials.

Retinoids activate the RXR/SXR-mediated pathway and induce the endogenous CYP3A4 activity in Huh7 human hepatoma cells

Steroid and xenobiotic receptor (SXR) or human pregnane X receptor (hPXR) dimerizes with retinoid X receptor (RXR) and regulates the transcription of genes encoding xenobiotic-metabolizing enzymes such as CYP3A4. Rifampin, the classical activator of CYP3A4, binds to SXR directly. It is unclear whether various natural and synthetic retinoids can regulate the expression of CYP3A4. To evaluate the effects of retinoids on the RXR/SXR-mediated pathway, transient transfection assays were performed on both CV-1 and human hepatoma Huh7 cells using a reporter construct containing multiple RXR/SXR consensus binding elements (an everted repeat with a 6-nucleotide spacer, ER-6). The results revealed that eight out of 13 retinoids screened significantly induced the RXR/SXR-mediated pathway in Huh7 cells. At an equal molar concentration, the acid forms (9-cis-RA, 13-cis-RA, and all-trans-RA) or aldehyde, the direct precursor of acid (9-cis-retinal and 13-cis-retinal), exhibited a greater or similar potency than rifampin. Depending on the ligands, RXR may serve as a silent or an active partner of SXR. Additionally, retinoids can increase CYP3A4 enzyme activity in Huh7 cells. To further evaluate the potential drug-drug interactions, which may be caused by retinoids, Huh7 cells were pretreated with 9-cis-RA and followed by acetaminophen. We showed that 9-cis-RA enhanced the covalent binding of N-acetyl-p-quinoneimine, a toxic intermediate of acetaminophen produced by phase I enzymes oxidation. This result suggested that drug-drug interaction might occur between 9-cis-RA and acetaminophen in human liver cells. Taken together, retinoids activate the RXR/SXR-mediated pathway and regulate the expression of CYP3A4. Thus, retinoids potentially can cause drug-drug interactions when they are administered with other CYP3A4 substrates.

Chemoprevention of Skin Carcinogenesis by Phenylretinamides: Retinoid Receptor–Independent Tumor Suppression

Fenretinide [N-(4-hydroxyphenyl)retinamide or 4-HPR] is a synthetic retinoid analogue with antitumor and chemopreventive activities. N-(4-Methoxyphenyl)retinamide (4-MPR) is the most abundant metabolite of 4-HPR detected in human serum following 4-HPR therapy. We have shown in in vitro studies that 4-HPR and 4-MPR can act independent of the classic nuclear retinoid receptor pathway and that 4-HPR, but not 4-MPR, can also activate nuclear retinoid receptors. In this study, we have compared the chemopreventive effects of topically applied 4-HPR and 4-MPR with the primary biologically active retinoid, all-trans retinoic acid (ATRA), in vivo in the mouse skin two-stage chemical carcinogenesis model. All three retinoids suppressed tumor formation but the effect of 4-HPR and 4-MPR, and not of ATRA, was sustained after their discontinuation. The tumor-suppressive effects of 4-HPR and 4-MPR were quantitatively and qualitatively similar, suggesting that the two may be acting through the same retinoid receptor-independent mechanism(s). We further explored this effect in vitro by analyzing primary cultures of mouse keratinocytes treated with the same retinoids. All three could induce apoptosis with a 48-hour treatment and only ATRA and 4-HPR induced an accumulation of cells in the G1 phase of the cell cycle. This finding is consistent with our previous results showing that the effects of phenylretinamides on the cell cycle are retinoid receptor dependent whereas apoptosis induction is not. A microarray-based comparison of gene expression profiles for mouse skin treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) alone and TPA + 4-HPR or TPA + 4-MPR reveals a high degree of coincidence between the genes regulated by the two phenylretinamides. We propose that 4-HPR may exert therapeutic and chemopreventive effects by acting primarily through a retinoid receptor-independent mechanism(s) and that 4-MPR may contribute to the therapeutic effect of 4-HPR by acting through the same retinoid receptor-independent mechanism(s).

Impaired retinoic acid (RA) signal leads to RARbeta2 epigenetic silencing and RA resistance

Resistance to the growth-inhibitory action of retinoic acid (RA), the bioactive derivative of vitamin A, is common in human tumors. One form of RA resistance has been associated with silencing and hypermethylation of the retinoic acid receptor beta2 gene (RARbeta2), an RA-regulated tumor suppressor gene. The presence of an epigenetically silent RARbeta2 correlates with lack of the RA receptor alpha (RARalpha). Normally, RARalpha regulates RARbeta2 transcription by mediating dynamic changes of RARbeta2 chromatin in the presence and absence of RA. Here we show that interfering with RA signal through RARalpha (which was achieved by use of a dominant-negative RARalpha, by downregulation of RARalpha by RNA interference, and by use of RARalpha antagonists) induces an exacerbation of the repressed chromatin status of RARbeta2 and leads to RARbeta2 transcriptional silencing. Further, we demonstrate that RARbeta2 silencing causes resistance to the growth-inhibitory effect of RA. Apparently, RARbeta2 silencing can also occur in the absence of DNA methylation. Conversely, we demonstrate that restoration of RA signal at a silent RARbeta2 through RARalpha leads to RARbeta2 reactivation. This report provides proof of principle that RARbeta2 silencing and RA resistance are consequent to an impaired integration of RA signal at RARbeta2 chromatin.

RAR-mediated epigenetic control of the cytochrome P450 Cyp26a1 in embryocarcinoma cells

Retinoic acid (RA) is a signaling molecule that plays a pivotal role in major cellular processes and vertebrate development. RA action is mediated by specialized transcription factors, the nuclear RA receptors (RARs), which regulate the transcription of genes containing a RA-responsive element (RARE). Here we demonstrate that the genes for the RA-receptor RARbeta2 and the cytochrome P450 RA-specific hydrolase Cyp26a1 involved in RA catabolism are coordinately regulated by RA. We found that both RARbeta2 and Cyp26a1 genes are epigenetically silenced in the absence of DNA methylation in RAC65, a P19 embryocarcinoma cell line derivative carrying a dominant-negative RARalpha mutant and resistant to the growth-inhibitory and differentiation effects of RA. In response to RA, RARbeta2 transcription is epigenetically regulated by RARalpha. Similarly, we found that Cyp26a1 transcription is epigenetically regulated by RARbeta2. Knocking down RARbeta2 transcription by RNA interference in wild-type P19 cells, with an intact RARalpha, induced Cyp26a1 transcriptional repression in the absence of DNA methylation. Concomitantly, cells developed RA resistance and did not undergo RA-induced neuron differentiation. Apparently, RARalpha, RARbeta2 and Cyp26a1 are components of a RA-regulated gene network. Factors affecting an upstream gene of the network can trigger repressive chromatin changes -- which are propagated in a domino fashion - at downstream genes of the network. This study also shows that chromatin inactivity, and consequent transcriptional silencing, can be achieved in the absence of DNA methylation.

Behavior of tight-junction, adherens-junction and cell polarity proteins during HNF-4α-induced epithelial polarization

We previously reported that expression of tight-junction molecules occludin, claudin-6 and claudin-7, as well as establishment of epithelial polarity, was triggered in mouse F9 cells expressing hepatocyte nuclear factor (HNF)-4alpha [H. Chiba, T. Gotoh, T. Kojima, S. Satohisa, K. Kikuchi, M. Osanai, N. Sawada. Hepatocyte nuclear factor (HNF)-4alpha triggers formation of functional tight junctions and establishment of polarized epithelial morphology in F9 embryonal carcinoma cells, Exp. Cell Res. 286 (2003) 288-297]. Using these cells, we examined in the present study behavior of tight-junction, adherens-junction and cell polarity proteins and elucidated the molecular mechanism behind HNF-4alpha-initiated junction formation and epithelial polarization. We herein show that not only ZO-1 and ZO-2, but also ZO-3, junctional adhesion molecule (JAM)-B, JAM-C and cell polarity proteins PAR-3, PAR-6 and atypical protein kinase C (aPKC) accumulate at primordial adherens junctions in undifferentiated F9 cells. In contrast, CRB3, Pals1 and PATJ appeared to exhibit distinct subcellular localization in immature cells. Induced expression of HNF-4alpha led to translocation of these tight-junction and cell polarity proteins to beltlike tight junctions, where occludin, claudin-6 and claudin-7 were assembled, in differentiated cells. Interestingly, PAR-6, aPKC, CRB3 and Pals1, but not PAR-3 or PATJ, were also concentrated on the apical membranes in differentiated cells. These findings indicate that HNF-4alpha provokes not only expression of tight-junction adhesion molecules, but also modulation of subcellular distribution of junction and cell polarity proteins, resulting in junction formation and epithelial polarization.

Recent advances in understanding transforming growth factor beta regulation of orofacial development

Members of the transforming growth factor (TGF) family have emerged as critical contributors to the choreography of cellular and tissue interactions underlying morphogenesis of the orofacial region. The TGFs beta, and their downstream effector molecules, the Smads, play a pivotal role in normal as well as abnormal development of first branchial arch structures. Components of the TGFbeta signal transduction machinery are discussed in relation to regulation of transcription, cell division and tissue differentiation in developing orofacial tissue, as evidence for a functional linkage between the TGFbeta and retinoic acid signal transduction pathways during orofacial development.

Maintenance of spermatogenesis requires TAF4b, a gonad-specific subunit of TFIID

The establishment and maintenance of spermatogenesis in mammals requires specialized networks of gene expression programs in the testis. The gonad-specific TAF4b component of TFIID (formerly TAF(II)105) is a transcriptional regulator enriched in the mouse testis. Herein we show that TAF4b is required for maintenance of spermatogenesis in the mouse. While young Taf4b-null males are initially fertile, Taf4b-null males become infertile by 3 mo of age and eventually exhibit seminiferous tubules devoid of germ cells. At birth, testes of Taf4b-null males appear histologically normal; however, at post-natal day 3 gonocyte proliferation is impaired and expression of spermatogonial stem cell markers c-Ret, Plzf, and Stra8 is reduced. Together, these data indicate that TAF4b is required for the precise expression of gene products essential for germ cell proliferation and suggest that TAF4b may be required for the regulation of spermatogonial stem cell specification and proliferation that is obligatory for normal spermatogenic maintenance in the adult.

Activation of p21CIP1/WAF1 gene expression and inhibition of cell proliferation by overexpression of hepatocyte nuclear factor-4α

The F9 murine embryonal carcinoma cell line provides an attractive system for studying epithelial differentiation and antiproliferative processes. We have recently established F9 cells expressing doxycycline-inducible hepatocyte nuclear factor (HNF)-4alpha and shown that HNF-4alpha triggers the gene expression of tight-junction molecules, occludin, claudin-6, and claudin-7, as well as formation of functional tight junctions and polarized epithelial morphology (Exp. Cell Res. 286, [2003] 288). Since these events were very similar to those induced by retinoids, we investigated whether HNF-4alpha, like retinoid receptors, was involved in the control of cell proliferation. We herein show that HNF-4alpha up-regulates expression of the p21 gene, but not the p15, p16, p18, p19, or p27 gene, in a p53-independent manner, and inhibits cell growth in F9 cells. Similar results were observed in rat lung endothelial cells, in which expression of HNF-4alpha is conditionally induced by doxycycline. Furthermore, we demonstrate, by reporter assay, that HNF-4alpha significantly elevates the transcriptional activity of the p21 promoter. Since, HNF-4alpha is expressed not only in the liver but also in organs containing epithelial cells, such as kidney, intestine, pancreas, and stomach, it might also play critical roles in the regulation of epithelial morphogenesis and proliferation in these organs.

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.

Association of the transcriptional corepressor TIF1beta with heterochromatin protein 1 (HP1): an essential role for progression through differentiation

The transcriptional intermediary factor 1beta (TIF1beta) is a corepressor for KRAB-domain-containing zinc finger proteins and is believed to play essential roles in cell physiology by regulating chromatin organization at specific loci through association with chromatin remodeling and histone-modifying activities and recruitment of heterochromatin protein 1 (HP1) proteins. In this study, we have engineered a modified embryonal carcinoma F9 cell line (TIF1beta(HP1box/-)) expressing a mutated TIF1beta protein (TIF1beta(HP1box)) unable to interact with HP1 proteins. Phenotypic analysis of TIF1beta(HP1box/-) and TIF1beta(+/-) cells shows that TIF1beta-HP1 interaction is not required for differentiation of F9 cells into primitive endoderm-like (PrE) cells on retinoic acid (RA) treatment but is essential for further differentiation into parietal endoderm-like (PE) cells on addition of cAMP and for differentiation into visceral endoderm-like cells on treatment of vesicles with RA. Complementation experiments reveal that TIF1beta-HP1 interaction is essential only during a short window of time within early differentiating PrE cells to establish a selective transmittable competence to terminally differentiate on further cAMP inducing signal. Moreover, the expression of three endoderm-specific genes, GATA6, HNF4, and Dab2, is down-regulated in TIF1beta(HP1box/-) cells compared with wild-type cells during PrE differentiation. Collectively, these data demonstrate that the interaction between TIF1beta and HP1 proteins is essential for progression through differentiation by regulating the expression of endoderm differentiation master players.

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.

Hyperinsulinemic diseases of civilization: more than just Syndrome X

Compensatory hyperinsulinemia stemming from peripheral insulin resistance is a well-recognized metabolic disturbance that is at the root cause of diseases and maladies of Syndrome X (hypertension, type 2 diabetes, dyslipidemia, coronary artery disease, obesity, abnormal glucose tolerance). Abnormalities of fibrinolysis and hyperuricemia also appear to be members of the cluster of illnesses comprising Syndrome X. Insulin is a well-established growth-promoting hormone, and recent evidence indicates that hyperinsulinemia causes a shift in a number of endocrine pathways that may favor unregulated tissue growth leading to additional illnesses. Specifically, hyperinsulinemia elevates serum concentrations of free insulin-like growth factor-1 (IGF-1) and androgens, while simultaneously reducing insulin-like growth factor-binding protein 3 (IGFBP-3) and sex hormone-binding globulin (SHBG). Since IGFBP-3 is a ligand for the nuclear retinoid X receptor alpha, insulin-mediated reductions in IGFBP-3 may also influence transcription of anti-proliferative genes normally activated by the body's endogenous retinoids. These endocrine shifts alter cellular proliferation and growth in a variety of tissues, the clinical course of which may promote acne, early menarche, certain epithelial cell carcinomas, increased stature, myopia, cutaneous papillomas (skin tags), acanthosis nigricans, polycystic ovary syndrome (PCOS) and male vertex balding. Consequently, these illnesses and conditions may, in part, have hyperinsulinemia at their root cause and therefore should be classified among the diseases of Syndrome X.

Cyclic AMP induces phosphorylation of claudin-5 immunoprecipitates and expression of claudin-5 gene in blood-brain-barrier endothelial cells via protein kinase A-dependent and -independent pathways

Cyclic AMP (cAMP) promotes functions of tight junctions in endothelial cells, although its target remains unknown. We showed here that cAMP increased gene expression of claudin-5 and decreased that of claudin-1 in porcine blood-brain-barrier endothelial cells via protein kinase A (PKA)-independent and -dependent pathways, respectively. cAMP also enhanced immunoreactivity of claudin-5 along cell borders and in the cytoplasm, reorganized actin filaments, and altered signals of claudin-5, occludin, ZO-1, and ZO-2 along cell boundaries from zipperlike to linear patterns. In contrast, claudin-1 was detected only in the cytoplasm in a dotlike pattern, and its immunolabeling was reduced by cAMP. Interestingly, 31- and 62-kDa claudin-5 immunoprecipitates in the NP-40-soluble and -insoluble fractions, respectively, were highly phosphorylated on threonine residue(s) upon cAMP treatment. All these changes induced by cAMP, except for claudin-5 expression and its signals in the cytoplasm, were reversed by an inhibitor of PKA, H-89. We also demonstrated that cAMP elevated the barrier function of tight junctions in porcine blood-brain-barrier endothelial cells in PKA-dependent and -independent manners. These findings indicate that both PKA-induced phosphorylation of claudin-5 immunoprecipitates and cAMP-dependent but PKA-independent induction of claudin-5 expression could be involved in promotion of tight-junction function in endothelial cells.

Induction of multidrug resistance protein 3 (mrp3) in vivo is independent of constitutive androstane receptor

We previously demonstrated that multidrug resistance protein 3 (Mrp3/ABCC3) is induced in rat liver by phenobarbital (PB) and several other microsomal enzyme inducers that induce cytochrome P450 2B (CYP2B). CYP2B is induced by constitutive androstane receptor (CAR)-retinoid X receptor (RXR) heterodimer binding to a phenobarbital-responsive promoter element in the CYP2B promoter. Hepatic mRNA levels of CYP2B and Mrp3 were measured in three models of altered CAR activity to determine whether CAR is also involved in the induction of Mrp3. In Wistar Kyoto rats, where males express higher CAR protein levels than females, the induction of CYP2B1/2 was significantly higher in males than in females by PB, diallyl sulfide, and trans-stilbene oxide but not oltipraz. Mrp3 was induced by each of these treatments, but in contrast to CYP2B1/2, to a similar magnitude in males and females. In male hepatocyte-specific RXRalpha-/- mice, CYP2B10 was not induced by diallyl sulfide or oltipraz but remained inducible by PB and trans-stilbene oxide after considering the decrease in basal CYP2B10 expression. Mrp3, however, was induced by PB, diallyl sulfide, trans-stilbene oxide and oltipraz in both wild-type and RXRalpha-/- mice. Additionally, constitutive expression of Mrp3 was significantly reduced in RXRalpha-/- mice. In CAR-/- mice, the robust induction of CYP2B10 by PB was completely absent. However, Mrp3 was equally induced both in wild-type and CAR-/- mice by PB. These data clearly demonstrate that induction of hepatic Mrp3 by PB and other microsomal enzyme inducers is CAR-independent and implies a role for RXRalpha in the constitutive expression of Mrp3.

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.

Hepatocyte nuclear factor (HNF)-4alpha triggers formation of functional tight junctions and establishment of polarized epithelial morphology in F9 embryonal carcinoma cells

F9 murine embryonal carcinoma cells provide an attractive system for facilitating molecular mechanisms for epithelial morphogenesis, since they have the capability of differentiating into polarized epithelial cells bearing an apical junctional complexes. We previously showed that a specific retinoid X receptor-retinoic acid receptor heterodimer transduced retinoid signals for biogenesis of functional tight junctions in F9 cells (Exp. Cell Res. 263, (2001) 163). In the present study we generated F9 cells expressing doxycycline-inducible hepatocyte nuclear factor (HNF)-4alpha, a nuclear receptor. We herein show that induction of HNF-4alpha initiates differentiation of F9 cells to polarized epithelial cells, in which tight-junction proteins occludin, claudin-6, claudin-7, and ZO-1 are concentrated at the apical-most regions of lateral membranes. Expression of occludin, claudin-6, and claudin-7 was induced in the cells by doxycycline treatment in a dose- and time-dependent manner, in terms of the amount of HNF-4alpha. In contrast, expression levels of ZO-1, ZO-2, E-cadherin, and beta-catenin were not altered by HNF-4alpha. We also demonstrate, by analysis of diffusion of labeled sphingomyelin, that the fence function of tight junctions is achieved by induction of HNF-4alpha. These findings indicate that HNF-4alpha triggers de novo formation of functional tight junctions and establishment of epithelial cell polarity.

Retinoic acid-induced developmental defects are mediated by RARbeta/RXR heterodimers in the pharyngeal endoderm

Fusion and hypoplasia of the first two branchial arches, a defect typically observed in retinoic acid (RA) embryopathy, is generated in cultured mouse embryos upon treatment with BMS453, a synthetic compound that exhibits retinoic acid receptor beta (RARbeta) agonistic properties in transfected cells. By contrast, no branchial arch defects are observed following treatment with synthetic retinoids that exhibit RARalpha or RARgamma agonistic properties. The BMS453-induced branchial arch defects are mediated through RAR activation, as they are similar to those generated by a selective pan-RAR agonist, are prevented by a selective pan-RAR antagonist and cannot be mimicked by exposure to a pan-RXR agonist alone. They are enhanced in the presence of a pan-RXR agonist, and cannot be generated in Rarb-null embryos. Furthermore, they are accompanied, in the morphologically altered region, by ectopic expression of Rarb and of several other direct RA target genes. Therefore, craniofacial abnormalities characteristic of the RA embryopathy are mediated through ectopic activation of RARbeta/RXR heterodimers, in which the ligand-dependent activity of RXR is subordinated to that of RARbeta. Endodermal cells lining the first two branchial arches respond to treatment with the RARbeta agonist, in contrast to neural crest cells and ectoderm, which suggests that a faulty endodermal regionalization is directly responsible for RA-induced branchial arch dysmorphologies. Additionally, we provide the first in vivo evidence that the synthetic RARbeta agonist BMS453 exhibits an antagonistic activity on the two other RAR isotypes.

Enhancement of ATRA-induced cell differentiation by inhibition of calcium accumulation into the endoplasmic reticulum: cross-talk between RAR alpha and calcium-dependent signaling

Sarco-endoplasmic reticulum calcium ATPase (SERCA) enzymes control calcium-induced cellular activation by accumulating calcium from the cytosol into the endoplasmic reticulum (ER). To better understand the role of SERCA proteins and cellular calcium homeostasis in all-trans retinoic acid (ATRA)-induced differentiation, we investigated the effect of pharmacologic inhibition of SERCA-dependent calcium uptake into the ER on ATRA-induced differentiation of the HL-60 myelogenous and the NB4 promyelocytic cell lines. SERCA inhibitors di-tert-butyl-benzohydroquinone (tBHQ), thapsigargin, and cyclopiazonic acid significantly enhanced the induction of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and CD11b marker expression induced by suboptimal concentrations of ATRA (50 nM) in both cell lines. Analysis of cellular calcium homeostasis revealed that a 60% mobilization of the total SERCA-dependent intracellular calcium pool was necessary to obtain enhancement of ATRA-dependent differentiation by tBHQ. Moreover, after 3 days of ATRA treatment in combination with tBHQ, NB4 cells showed a significantly decreased calcium mobilization compared with treatments with tBHQ or ATRA alone, suggesting that enhanced differentiation and calcium mobilization are causally related. Interestingly, several ATRA-resistant NB4-derived cell lines were partially responsive to the differentiation-inducing effect of the combination of the 2 drugs. In addition, we found that retinoic acid receptor alpha (RAR alpha) and PML-RAR alpha proteins are protected from ATRA-induced proteolytic degradation by SERCA inhibition, indicating that cellular calcium homeostasis may interact with signaling systems involved in the control of ATRA-dependent transcriptional activity. By linking calcium to ATRA-dependent signaling, our data open new avenues in the understanding of the mechanisms of differentiation-induction therapy of leukemia.

The role of a retinoic acid response element in establishing the anterior neural expression border of Hoxd4 transgenes

The zebrafish hoxd4a locus was compared to its murine ortholog, Hoxd4. The sequence of regulatory elements, including a DR5 type retinoic acid response element (RARE) required for Hoxd4 neural enhancer activity, are highly conserved. Additionally, zebrafish and mouse neural enhancers function identically in transgenic mouse embryos. We tested whether sequence conservation reflects functional importance by altering the spacing and sequence of the RARE in the Hoxd4 neural enhancer. Stabilizing receptor-DNA interactions did not anteriorize transgene expression. By contrast, conversion of the RARE from a DR5 to a DR2 type element decreased receptor-DNA stability and posteriorized expression. Hence, the setting of the Hox anterior expression border is not a simple function of the affinity of retinoid receptors for their cognate element.