Distinct roles for cellular retinoic acid-binding proteins I and II in regulating signaling by retinoic acid

Involvement of Fatty Acid Binding Protein 5 and PPARβ/δ in Prostate Cancer Cell Growth

Fatty acid binding protein 5 (FABP5) delivers ligands from the cytosol directly to the nuclear receptor PPARβ/δ and thus facilitates the ligation and enhances the transcriptional activity of the receptor. We show here that expression levels of both FABP5 and PPARβ/δ are correlated with the tumorigenic potential of prostate cancer cell lines. We show further that FABP5 comprises a direct target gene for PPARβ/δ and thus the binding protein and its cognate receptor are engaged in a positive feedback loop. The observations demonstrate that, similarly to effects observed in mammary carcinomas, activation of the FABP5/PPARβ/δ pathway induces PPARβ/δ target genes involved in cell survival and growth and enhances cell proliferation and anchorage-independent growth in prostate cancer cells. Furthermore, the data show that downregulation of either FABP5 or PPARβ/δ inhibits the growth of the highly malignant prostate cancer PC3M cells. These studies suggest that the FABP5/PPARβ/δ pathway may play a general role in facilitating tumor progression and that inhibition of the pathway may comprise a novel strategy in treatment of cancer.

Alteration in protein expression in estrogen receptor alpha-negative human breast cancer tissues indicates a malignant and metastatic phenotype

Ductal carcinoma in situ (DCIS) represents the earliest identifiable breast cancer lesion. Disruption of the myoepithelial cell layer and basement membrane is a prerequisite for DCIS to initiate invasion into the stroma. The majority of epithelial cells overlying a focally-disrupted myoepithelial cell layer are estrogen receptor-alpha negative (ER(-)); however, adjacent cells within the same duct confined by an intact myoepithelial cell layer express high levels of ER. These ER (+) and ER (-) cells were microdissected from the same ducts of breast cancer patients. Differential proteins expressed by ER(+) and ER(-) cells were identified using two-dimensional gel electrophoresis followed by mass spectrometry and Western blot analysis. ER(-) cells express lower levels of superoxide dismutase, RalA binding protein, galectin-1, uridine phosphorylase 2, cellular retinoic acid-binding protein 1, S100 calcium binding protein A11, and nucleoside diphosphate kinase A or non-metastasis protein 23-H1 (nm23-H1). The upregulated protein, Rho GDP-dissociation inhibitor 1 alpha, may induce chemotherapy resistance. The significant findings are that the microdissected ER(-) cells express 12.6 times less cellular retinoic acid-binding protein 1, a protein involved in cellular differentiation, and 4.1 times less nucleoside diphosphate kinase A or nm23-H1, a metastasis suppressor, and express fewer proteins than adjacent ER(+) cells. The collective role of the alterations of protein expression in ER(-) cells may be to promote a more malignant phenotype than adjacent ER(+) cells, including a decreased ability to undergo apoptosis and differentiation, and an increased potential to damage DNA, metastasize, and resist to chemotherapy.

Altered retinoic acid metabolism in diabetic mouse kidney identified by O isotopic labeling and 2D mass spectrometry

BACKGROUND

Numerous metabolic pathways have been implicated in diabetes-induced renal injury, yet few studies have utilized unbiased systems biology approaches for mapping the interconnectivity of diabetes-dysregulated proteins that are involved. We utilized a global, quantitative, differential proteomic approach to identify a novel retinoic acid hub in renal cortical protein networks dysregulated by type 2 diabetes.

METHODOLOGY/PRINCIPAL FINDINGS

Total proteins were extracted from renal cortex of control and db/db mice at 20 weeks of age (after 12 weeks of hyperglycemia in the diabetic mice). Following trypsinization, (18)O- and (16)O-labeled control and diabetic peptides, respectively, were pooled and separated by two dimensional liquid chromatography (strong cation exchange creating 60 fractions further separated by nano-HPLC), followed by peptide identification and quantification using mass spectrometry. Proteomic analysis identified 53 proteins with fold change >or=1.5 and p<or=0.05 after Benjamini-Hochberg adjustment (out of 1,806 proteins identified), including alcohol dehydrogenase (ADH) and retinaldehyde dehydrogenase (RALDH1/ALDH1A1). Ingenuity Pathway Analysis identified altered retinoic acid as a key signaling hub that was altered in the diabetic renal cortical proteome. Western blotting and real-time PCR confirmed diabetes-induced upregulation of RALDH1, which was localized by immunofluorescence predominantly to the proximal tubule in the diabetic renal cortex, while PCR confirmed the downregulation of ADH identified with mass spectrometry. Despite increased renal cortical tissue levels of retinol and RALDH1 in db/db versus control mice, all-trans-retinoic acid was significantly decreased in association with a significant decrease in PPARbeta/delta mRNA.

CONCLUSIONS/SIGNIFICANCE

Our results indicate that retinoic acid metabolism is significantly dysregulated in diabetic kidneys, and suggest that a shift in all-trans-retinoic acid metabolism is a novel feature in type 2 diabetic renal disease. Our observations provide novel insights into potential links between altered lipid metabolism and other gene networks controlled by retinoic acid in the diabetic kidney, and demonstrate the utility of using systems biology to gain new insights into diabetic nephropathy.

Genetic analysis of expression profile involved in retinoid metabolism in non-alcoholic fatty liver disease

AIM

The patients with non-alcoholic fatty liver disease (NAFLD) have been reported to be at greater risk for progression to chronic liver disease including liver cirrhosis (LC). To examine the mechanisms for the progression of NAFLD, a genetic analysis of hepatic expression profile in retinoid metabolism in NAFLD was performed since the loss of retinoid signaling is associated with the progression of liver disease via reactive oxygen species (ROS) generation.

METHODS

Fifty-one genes, which are associated with retinoid metabolism and action, were examined in thirty six subjects including 17 patients with simple steatosis, 11 with non-alcoholic steatohepatitis (NASH) and eight controls were examined by real-time reverse transcriptase polymerase chain reaction. Immunohistochemical study was also done by 3 kinds of antibodies.

RESULTS

Higher expression of CRBP1 LRAT, DGT1/2 and CES1 in NAFLD suggests that mutual conversion between retinyl ester and retinal occurs actively. Expression of ADH1/2/3, RDH5/10/11, DHRS3 and RALDH1/3 was increased in NAFLD, suggesting that oxidation process from retinol to all-trans retinoic acid (ATRA) was enhanced. Importantly, greater expression of CYP26A1 indicated that degradation of ATRA was enhanced in NAFLD. Further, expression of SOD1/2, catalase, thioredoxin and uncoupling protein 2 was also enhanced.

CONCLUSION

Hyperdynamic state of retinoid metabolism is present in the liver tissues with NAFLD, which may be a putative mechanism by which NAFLD progresses to chronic liver disease including LC.

Newborn serum retinoic acid level is associated with variants of genes in the retinol metabolism pathway

Retinoic acid (RA) is a critical regulator of gene expression during embryonic development. In rodents, moderate maternal vitamin A deficiency leads to subtle morphogenetic defects and inactivation of RA pathway genes causes major disturbances of embryogenesis. In this study, we quantified RA in umbilical cord blood of 145 healthy full-term Caucasian infants from Montreal. Sixty seven percent of values were <10 nmol/L (84 were <0.07 nmol/L) and 33% had moderate or high levels. Variation in RA could not be explained by parallel variation in its precursor, retinol (ROL). However, we found that the (A) allele of the rs12591551 single nucleotide polymorphism (SNP) in the ALDH1A2 gene (ALDH1A2rs12591551(A)), occurring in 19% of newborns, was associated with 2.5-fold higher serum RA levels. ALDH1A2 encodes retinaldehyde dehydrogenase (RALDH) 2, which synthesizes RA in fetal tissues. We also found that homozygosity for the (A) allele of the rs12724719 SNP in the CRABP2 gene (CRABP2rs12724719(A/A)) was associated with 4.4-fold increase in umbilical cord serum RA. CRABP2 facilitates RA binding to its cognate receptor complex and transfer to the nucleus. We hypothesize that individual variation in RA pathway genes may account for subtle variations in RA-dependent human embryogenesis.

From carrot to clinic: an overview of the retinoic acid signaling pathway

Vitamin A is essential for the formation and maintenance of many body tissues. It is also important for embryonic growth and development and can act as a teratogen at critical periods of development. Retinoic acid (RA) is the biologically active form of vitamin A and its signaling is mediated by the RA and retinoid X receptors. In addition to its role as an important molecule during development, RA has also been implicated in clinical applications, both as a potential anti-tumor agent as well as for the treatment of skin diseases. This review presents an overview of how dietary retinoids are converted to RA, hence presenting the major players in RA metabolism and signaling, and highlights examples of treatment applications of retinoids. Moreover, we discuss the origin and diversification of the retinoid pathway, which are important factors for understanding the evolution of ligand-specificity among retinoid receptors.

Fatty acid-binding protein 5 and PPARbeta/delta are critical mediators of epidermal growth factor receptor-induced carcinoma cell growth

Epidermal growth factors and their receptors (EGFRs) promote breast cancer cell proliferation and can drive tumorigenesis. However, the molecular mechanisms that mediate these effects are incompletely understood. We previously showed that mammary tumor development in the mouse model of breast cancer MMTV-neu, a model characterized by amplification of the EGFR ErbB2 in mammary tissue, correlates with a marked up-regulation of fatty acid-binding protein 5 (FABP5). FABP5 functions to deliver ligands to and enhance the transcriptional activity of the nuclear receptor peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta), a receptor whose target genes include genes involved in cell growth and survival. We show here that in MCF-7 mammary carcinoma cells, EGFR signaling directly up-regulates the expression of FABP5. The data demonstrate that treatment of these cells with the EGFR ligand heregulin-beta1 signals through the ERK and the phophatidylinositol-3-kinase cascades, resulting in activation of the transcription factor NF-kappaB. In turn, NF-kappaB induces the expression of FABP5 through two cognate response elements in the promoter of this gene. The observations further demonstrate that FABP5 and PPARbeta/delta are critical mediators of the ability of EGFR to enhance cell proliferation, indicating that this transcriptional pathway plays a key role in EGFR-induced tumorigenesis. Additional observations indicate that the expression of FABP5 is down-regulated by the Krüppel-like factor KLF2, suggesting a tumor suppressor activity for this factor.

Repression of cellular retinoic acid-binding protein II during adipocyte differentiation

In preadipocytes, retinoic acid (RA) regulates gene expression by activating the nuclear RA receptor (RAR) and its cognate intracellular lipid-binding protein CRABP-II. It was previously reported that RA inhibits adipocyte differentiation but only when administered early during the differentiation program. The data presented here indicate that the diminished ability of RA to activate RAR following induction of differentiation stems from down-regulation of CRABP-II. The observations show that expression of CRABP-II in preadipocytes is repressed by all three components of the classical hormonal mixture that induces adipocyte differentiation, i.e. isobutylmethylxanthine, insulin, and dexamethasone. Isobutylmethylxanthine-dependent activation of protein kinase A triggered the phosphorylation of the transcription factor cAMP-response element-binding protein, which induced the expression of the cAMP-response element-binding protein family repressor cAMP-response element modulator. In turn, cAMP-response element modulator was found to associate with a cognate response element in the CRABP-II promoter and to repress CRABP-II expression. The data further show that CRABP-II is a direct target gene for the glucocorticoid receptor and that it is subjected to dexamethasone-induced glucocorticoid receptor-mediated repression during adipogenesis. Finally, the observations demonstrate that permanent repression of CRABP-II in mature adipocytes is exerted by the master regulator of adipocyte differentiation CCAAT/enhancer-binding protein alpha and is directly mediated through CCAAT/enhancer-binding protein alpha-response elements in the CRABP-II promoter. Taken together, the observations emphasize the important role of CRABP-II in regulating the transcriptional activity of RA through RAR, and they demonstrate that repression of this gene is critical for allowing adipogenesis to proceed.

Dissection of the critical binding determinants of cellular retinoic acid binding protein II by mutagenesis and fluorescence binding assay

The binding of retinoic acid to mutants of Cellular Retinoic Acid Binding Protein II (CRABPII) was evaluated to better understand the importance of the direct protein/ligand interactions. The important role of Arg111 for the correct structure and function of the protein was verified and other residues that directly affect retinoic acid binding have been identified. Furthermore, retinoic acid binding to CRABPII mutants that lack all previously identified interacting amino acids was rescued by providing a carboxylic acid dimer partner in the form of a Glu residue.

Synthetic retinoids: structure-activity relationships

Retinoid signalling pathways are involved in numerous processes in cells, particularly those mediating differentiation and apoptosis. The endogenous ligands that bind to the retinoid receptors, namely all-trans-retinoic acid (ATRA) and 9-cis-retinoic acid, are prone to double-bond isomerisation and to oxidation by metabolic enzymes, which can have significant and deleterious effects on their activities and selectivities. Many of these problems can be overcome through the use of synthetic retinoids, which are often much more stable, as well as being more active. Modification of their molecular structures can result in retinoids that act as antagonists, rather than agonists, or exhibit a large degree of selectivity for particular retinoid-receptor isotypes. Several such selective retinoids are likely to be of value as pharmaceutical agents with reduced toxicities, particularly in cancer therapy, as reagents for controlling cell differentiation, and as tools for elucidating the precise roles that specific retinoid signalling pathways play within cells.

Fluorescence-based technique for analyzing retinoic acid

Retinoic acid (RA) is a potent transcriptional activator whose actions are mediated by members of the nuclear hormone receptor family. In addition to playing key roles in embryonic development and in tissue maintenance in the adult, RA is a potent anticarcinogenic agent currently in clinical use for treatment of various cancers. Here, we describe an optical method for measuring the concentrations of RA in biological samples. This method uses cellular retinoic acid-binding protein I (CRABP-I), a protein that binds RA with high affinity and specificity, as a "read-out" for its ligand. Replacing (28)Leu of CRABP-I with a Cys residue allows for covalently attaching an environmentally sensitive fluorescent probe to the protein at a region that undergoes a significant conformational change upon ligand binding. Association of RA with the modified protein thus results in changes in the fluorescence of the probe, enabling reliable measurements of RA concentrations as low as 50 nM. We show that the method can be effectively used to measure RA concentrations in serum and to monitor the biosynthesis and the degradation of RA in cultured mammalian cells.

Retinoic Acid-mediated Nuclear Receptor Activation and Hepatocyte Proliferation

Due to their well-known differentiation and apoptosis-inducing abilities, retinoic acid (RA) and its analogs have strong anti-cancer efficacy in human cancers. However, in vivo RA is a liver mitogen. While speculation has persisted that RA-mediated signaling is likely involved in hepatocyte proliferation during liver regeneration, direct evidence is still required. Findings in support of this proposition include observations that a release of retinyl palmitate (the precursor of RA) occurs in liver stellate cells following liver injury. Nevertheless, the biological action of this released vitamin A is virtually unknown. More likely is that the released vitamin A is converted to RA, the biological form, and then bound to a specific receptor (retinoid x receptor; RXRα), which is most abundantly expressed in the liver. Considering the mitogenic effects of RA, the RA-activated RXRα would likely then influence hepatocyte proliferation and liver tissue repair. At present, the mechanism by which RA stimulates hepatocyte proliferation is largely unknown. This review summarizes the activation of nuclear receptors (peroxisome proliferator activated receptor-α, pregnane x receptor, constitutive androstane receptor, and farnesoid x receptor) in an RXRα dependent manner to induce hepatocyte proliferation, providing a link between RA and its proliferative role.

A microarray screen for direct targets of Zic1 identifies an aquaporin gene, aqp-3b, expressed in the neural folds

The Zic1 transcription factor plays multiple roles during early development, for example, in patterning the early neural plate and formation of the neural crest, somites, and cerebellum. To identify direct downstream target genes of Zic1, a microarray screen was conducted in Xenopus laevis that identified 85 genes upregulated twofold or more. These include transcription factors, receptors, enzymes, proteins involved in retinoic acid signaling, and an aquaglyceroporin (aqp-3b), but surprisingly no genes known to be involved in cell proliferation. We show that both aqp-3 and aqp-3b were expressed in adult tissues, while during early embryonic development, only aqp-3b was transcribed. During neurula stages, aqp-3b was expressed specifically in the neural folds. This pattern of aqp-3b expression closely resembled that of NF-protocadherin (NFPC), which is involved in cell adhesion and neural tube closure. Aqp-3b may also be involved in neural tube closure, since mammalian Aqp-3 promotes cell migration and proliferation.

Serum retinol, retinol-binding protein, and transthyretin in children receiving dialysis

OBJECTIVE

We investigated the relationships of retinol (ROH), retinol-binding protein (RBP), and transthyretin (TTR) in children with end-stage renal disease (ESRD). Our hypothesis was that levels of ROH and RBP would be elevated in children with ESRD.

METHODS AND PATIENTS

We measured ROH, RBP, and TTR serum concentrations in a group of pediatric ESRD patients biannually. Children were grouped according to age and method of dialysis, i.e., hemodialysis (HD) or peritoneal dialysis (PD): HD1, aged <12 years (n = 8); PD1, aged <12 years (n = 19); HD2, aged >or=12 years (n =19); and PD2, aged >or=12 years (n = 29).

RESULTS

No differences in ROH, RBP, TTR, or their ratios were found as a function of type of dialysis in groups PD2 and HD2. The ROH and TTR were significantly higher in PD1 than HD1 (P = .01 and P = .003, respectively). No correlations were evident between ROH and RBP or TTR with length of time on dialysis, serum calcium, or serum creatinine, except for group PD2, in which ROH was positively correlated with RBP (P = .025). There were no significant differences among any of the ratios in terms of age or method of dialysis.

CONCLUSIONS

The data indicate that children with ESRD exhibit elevated levels of serum ROH, RBP, and TTR, in proportions similar to those reported in the adult ESRD literature. Further study is needed to clarify the consequences of increased ROH in uremic children.

Understanding abnormal retinoid signaling as a causative mechanism in congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is a frequently occurring source of severe neonatal respiratory distress. It has been hypothesized that abnormal retinoid signaling contributes to the etiology of this developmental anomaly. Here, we use rodent models toward specifically understanding the role of retinoid signaling in the developing diaphragm and how its perturbation is a common mechanism in drug-induced CDH. This includes monitoring of retinoic acid (RA) response element (RARE) activation with RARE-lacZ mice, RA supplementation studies, systematic analyses of the expression profile of key elements in the RA signaling pathway within the developing diaphragm, and the in utero delivery of a RA receptor (RAR) antagonist. These data demonstrate the timing of RARE perturbation by CDH-inducing teratogens and the efficacy of RA supplementation. Furthermore, a detailed profile of retinoid binding proteins, synthetic enzymes, and retinoid receptors within primordial diaphragm cells was obtained. The expression profile of RAR-alpha was particularly striking in regard to its overlap with the regions of primordial diaphragm affected in multiple CDH models. Blocking of RAR signaling with the pan-RAR antagonist BMS493 induced a very high degree of CDH, with a marked left-right sidedness that depended on the timing of drug delivery. Collectively, these data demonstrate that retinoid signaling is essential for normal diaphragm development, providing further support to the hypothesis that abnormalities related to the retinoid signaling pathway cause diaphragmatic defects. This study also yielded a novel experimental model that should prove particularly useful for further studies of CDH.

Dynamic and combinatorial control of gene expression by nuclear retinoic acid receptors (RARs)

Nuclear retinoic acid receptors (RARs) are transcriptional regulators controlling the expression of specific subsets of genes in a ligand-dependent manner. The basic mechanism for switching on transcription of cognate target genes involves RAR binding at specific response elements and a network of interactions with coregulatory protein complexes, the assembly of which is directed by the C-terminal ligand-binding domain of RARs. In addition to this scenario, new roles for the N-terminal domain and the ubiquitin-proteasome system recently emerged. Moreover, the functions of RARs are not limited to the regulation of cognate target genes, as they can transrepress other gene pathways. Finally, RARs are also involved in nongenomic biological activities such as the activation of translation and of kinase cascades. Here we will review these mechanisms, focusing on how kinase signaling and the proteasome pathway cooperate to influence the dynamics of RAR transcriptional activity.

All-trans-retinoic acid represses obesity and insulin resistance by activating both peroxisome proliferation-activated receptor beta/delta and retinoic acid receptor

Many biological activities of all-trans-retinoic acid (RA) are mediated by the ligand-activated transcription factors termed retinoic acid receptors (RARs), but this hormone can also activate the nuclear receptor peroxisome proliferation-activated receptor beta/delta (PPARbeta/delta). We show here that adipocyte differentiation is accompanied by a shift in RA signaling which, in mature adipocytes, allows RA to activate both RARs and PPARbeta/delta, thereby enhancing lipolysis and depleting lipid stores. In vivo studies using a dietary-induced mouse model of obesity indicated that onset of obesity is accompanied by downregulation of adipose PPARbeta/delta expression and activity. RA treatment of obese mice induced expression of PPARbeta/delta and RAR target genes involved in regulation of lipid homeostasis, leading to weight loss and improved insulin responsiveness. RA treatment also restored adipose PPARbeta/delta expression. The data indicate that suppression of obesity and insulin resistance by RA is largely mediated by PPARbeta/delta and is further enhanced by activation of RARs. By targeting two nuclear receptors, RA may be a uniquely efficacious agent in the therapy and prevention of the metabolic syndrome.

Involvement of retinoic acid signaling in goldfish optic nerve regeneration

Recently, we identified a retina-specific retinol-binding protein, purpurin, as a trigger molecule in the early stage of goldfish optic nerve regeneration. Purpurin protein was secreted by photoreceptors to injured ganglion cells, at 2-5 days after optic nerve injury. Purpurin bound to retinol induced neurite outgrowth in retinal explant cultures and retinoic acid (RA) had a comparable effect on neurite outgrowth. These results indicate that purpurin acts as a retinol transporter and facilitates conversion of retinol to RA. Intracellularly, RA is transported into the nucleus with cellular retinoic acid-binding protein IIb (CRABPIIb) and binds with retinoic acid receptor alpha (RARalpha) as a transcriptional regulator of target genes. Here, we investigated the RA signaling through RA synthesis to RARalpha in the goldfish retina during optic nerve regeneration by RT-PCR. Retinaldehyde dehydrogenase 2 (RALDH2; an RA synthetic enzyme) mRNA was increased by 2.7-fold in the retina at 7-10 days and then gradually decreased until 40 days after nerve injury. In contrast, cytochrome P450 26a1 (CYP26a1; an RA degradative enzyme) mRNA was decreased to less than half in the retina at 5-20 days and then gradually returned to the control level by 40 days after nerve injury. CRABPIIb mRNA was increased by 1.5-fold in the retina at 10 days after axotomy, RARalphaa mRNA was increased by 1.8-fold in the retina at 10 days after axotomy. The cellular changes in the RA signaling molecules after optic nerve injury were almost all located in the ganglion cells, as evaluated by in situ hybridization. The present data described for the first time that RA signaling through RALDH2 and CRABPIIb to RARalpha was serially upregulated in the ganglion cells at 7-10 days just after the purpurin induction. Therefore, we conclude that the triggering action of purpurin on optic nerve regeneration is mediated by RA signaling pathway.

Retinoic acid in the immune system

On occasion, emerging scientific fields intersect and great discoveries result. In the last decade, the discovery of regulatory T cells (T(reg)) in immunity has revolutionized our understanding of how the immune system is controlled. Intersecting the rapidly emerging field of T(reg) function, has been the discovery that retinoic acid (RA) controls both the homing and differentiation of T(reg). Instantly, the wealth and breadth of knowledge of the molecular basis for RA action, its receptors, and how it controls cellular differentiation can and will be exploited to understand its profound effects on T(reg). Historically, vitamin A deprivation and repletion and RA agonists have been shown to profoundly affect immunity. Now these findings can be interpreted in light of the revelations that RA controls leukocyte homing and T(reg) function.

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.

Structural analysis of site-directed mutants of cellular retinoic acid-binding protein II addresses the relationship between structural integrity and ligand binding

The structural integrity of cellular retinoic acid-binding protein II (CRABPII) has been investigated using the crystal structures of CRABPII mutants. The overall fold was well maintained by these CRABPII mutants, each of which carried multiple different mutations. A water-mediated network is found to be present across the large binding cavity, extending from Arg111 deep inside the cavity to the alpha2 helix at its entrance. This chain of interactions acts as a ;pillar' that maintains the integrity of the protein. The disruption of the water network upon loss of Arg111 leads to decreased structural integrity of the protein. A water-mediated network can be re-established by introducing the hydrophilic Glu121 inside the cavity, which results in a rigid protein with the alpha2 helix adopting an altered conformation compared with wild-type CRABPII.

How degrading: Cyp26s in hindbrain development

The vitamin A derivative retinoic acid performs many functions in vertebrate development and is thought to act as a diffusible morphogen that patterns the anterior-posterior axis of the hindbrain. Recent work in several systems has led to insights into how the spatial distribution of retinoic acid is regulated. These have shown local control of synthesis and degradation, and computational models suggest that degradation by the Cyp26 enzymes plays a critical role in the formation of a morphogen gradient as well as its ability to compensate for fluctuations in RA levels.

Expression and functional characterization of cytochrome P450 26A1, a retinoic acid hydroxylase

Retinoic acid (RA) is a critical signaling molecule that performs multiple functions required to maintain cellular viability. It is also used in the treatment of some cancers. Enzymes in the CYP26 family are thought to be responsible for the elimination of RA, and CYP26A1 appears to serve the most critical functions in this family. In spite of its importance, CYP26A1 has neither been heterologously expressed nor characterized kinetically. We expressed the rCYP26A1 in baculovirus-infected insect cells and purified the hexahistidine tagged protein to homogeneity. Heme incorporation was determined by carbon monoxide difference spectrum and a type 1 spectrum was observed with RA binding to CYP26A1. We found that RA is a tight binding ligand of CYP26A1 with low nM binding affinity. CYP26A1 oxidized RA efficiently (depletion K(m) 9.4+/-3.3nM and V(max) 11.3+/-4.3pmolesmin(-1)pmoleP450(-1)) when supplemented with P450 oxidoreductase and NADPH but was independent of cytochrome b5. 4-Hydroxy-RA (4-OH-RA) was the major metabolite produced by rCYP26A1 but two other primary products were also formed. 4-OH-RA was further metabolized by CYP26A1 to more polar metabolites and this sequential metabolism of RA occurred in part without 4-OH-RA leaving the active site of CYP26A1. The high efficiency of CYP26A1 in eliminating both RA and its potentially active metabolites supports the major role of this enzyme in regulating RA clearance in vivo. These results provide a biochemical framework for CYP26A1 function and offer insight into the role of CYP26A1 as a drug target as well as in fetal development and cell cycle regulation.

Dynamic regulation of retinoic acid-binding proteins in developing, adult and neoplastic skin reveals roles for beta-catenin and Notch signalling

Retinoic acid (RA) signalling is essential for epidermal differentiation; however, the mechanisms by which it acts are largely unexplored. Partitioning of RA between different nuclear receptors is regulated by RA-binding proteins. We show that cellular RA-binding proteins CRABP1 and CRABP2 and the fatty acid-binding protein FABP5 are dynamically expressed during skin development and in adult tissue. CRABP1 is expressed in embryonic dermis and in the stroma of skin tumours, but confined to the hair follicle dermal papilla in normal postnatal skin. CRABP2 and FABP5 are expressed in the differentiating cells of sebaceous gland, interfollicular epidermis and hair follicles, with FABP5 being a prominent marker of sebaceous glands and anagen follicle bulbs. All three proteins are upregulated in response to RA treatment or Notch activation and are negatively regulated by Wnt/beta-catenin signalling. Ectopic follicles induced by beta-catenin arise from areas of the sebaceous gland that have lost CRABP2 and FABP5; conversely, inhibition of hair follicle formation by N-terminally truncated Lef1 results in upregulation of CRABP2 and FABP5. Our findings demonstrate that there is dynamic regulation of RA signalling in different regions of the skin and provide evidence for interactions between the RA, beta-catenin and Notch pathways.

Overcoming retinoic acid-resistance of mammary carcinomas by diverting retinoic acid from PPARbeta/delta to RAR

Retinoic acid (RA) displays potent anticarcinogenic activities that are mediated by the nuclear retinoic acid receptors (RARs). However, use of RA in oncology is limited by RA resistance acquired during carcinogenesis. Moreover, in some cancers, RA facilitates rather than inhibits growth. A clue to this paradoxical behavior was recently suggested by the findings that RA also activates PPARbeta/delta, a receptor involved in mitogenic and anti-apoptotic activities. The observations that partitioning of RA between its two receptors is regulated by two intracellular lipid-binding proteins-CRABP-II, which targets RA to RAR, and FABP5, which delivers it to PPARbeta/delta-further suggest that RA resistance may stem from the deregulation of the binding proteins, resulting in activation of PPARbeta/delta rather than RAR. Here, we show that, in the RA-resistant mouse model of breast cancer MMTV-neu, RA indeed activates the nonclassical RA receptor PPARbeta/delta. This behavior was traced to an aberrantly high intratumor FABP5/CRABP-II ratio. Decreasing this ratio in mammary tissue diverted RA from PPARbeta/delta to RAR and suppressed tumor growth. The data demonstrate the existence of a mechanism that underlies RA resistance in tumors, indicate that CRABP-II functions as a tumor suppressor, and suggest that the inhibition of FABP5 may comprise a therapeutic strategy for overcoming RA resistance in some tumors.

Retinoid receptor-activating ligands are produced within the mouse thymus during postnatal development

Vitamin A deficiency is known to be accompanied with immune deficiency and susceptibility to a wide range of infectious diseases. Experimental evidence suggests that the active metabolites of vitamin A that mediate its effects on the immune system are the retinoic acids (RA), which are ligands for the nuclear RA receptor (RAR) family. RA were previously shown both to promote proliferation and to regulate apoptosis of thymocytes. In this study we detected the age-dependent mRNA expression of retinaldehyde dehydrogenases (RALDH1 and 2), cellular RA binding protein-II and CYP26A, proteins responsible for the synthesis, nuclear transport and degradation of RA in the postnatally developing thymus. RALDH1 was located in thymic epithelial cells. However, the amount of all-trans RA in thymic homogenates was close to the detection limit, suggesting that in this tissue all-trans RA is not the main RAR-regulating product of retinol metabolism. At the same time, by measuring the induction of a RAR-responsive transgene in two independent transgenic mouse strains, we demonstrated the production of an RAR-activating ligand, which was age and RALDH dependent. Our data provide evidence for the existence of endogenous retinoid synthesis in the thymus and suggest that retinoids similar to glucocorticoids might indeed be involved in the regulation of thymic proliferation and selection processes by being present in the thymus in functionally effective amounts.

Expression and functional analysis of the cellular retinoic acid binding protein from silkworm pupae (Bombyx mori)

Cellular retinoic acid binding protein (CRABP) is a member of intracellular lipid-binding protein (iLBP), and closely associated with retinoic acid (RA) activity. We have cloned the CRABP gene from silkworm pupae and studied the interaction between Bombyx mori CRABP (BmCRABP) and all-trans retinoic acid (atRA). The MTT assay data indicated that when BmCRABP is overexpressed in Bm5 cells, the cells dramatically resisted to atRA-induced growth inhibition. Conversely, the cells were sensitive to atRA treatment upon knocking down the BmCRABP expression. Subcellular localization revealed that BmCRABP is a cytoplasm protein, even when treated with atRA, the CRABP still remained in the cytoplasm. These data demonstrated that the function of BmCRABP have an effect on the physiological function of atRA.

LIF removal increases CRABPI and CRABPII transcripts in embryonic stem cells cultured in retinol or 4-oxoretinol

Murine embryonic stem (ES) cells cultured without leukemia inhibitory factor (LIF) or with retinoids differentiate and concomitantly metabolize retinol (vitamin A) to 4-oxoretinol. Our objective was to examine the effects of retinol or 4-oxoretinol on cellular retinoic acid binding protein (CRABP) I and II mRNA levels and retinol metabolism. ES cells were cultured with or without LIF, and with various doses of all-trans-retinol, all-trans-4-oxoretinol, or all-trans-retinoic acid (RA). In ES cells treated with retinol or 4-oxoretinol in the absence of LIF the CRABP-I (Crabp1, NM_013496; GI:7304974) and CRABP-II (Crabp2, NM_007759; GI:33469074) mRNA levels at 72h were 66+/-4 and 413+/-6 fold higher, respectively, than the levels in control ES cells cultured without retinoids and in the presence of LIF. The increase in CRABPI mRNA occurred through an increase in CRABPI gene transcription. CRABPI protein was also increased by >50-fold in cells treated with retinol in the absence of LIF. However [(3)H]4-oxoretinol does not bind to murine CRABPI or CRABPII. CYP26A1 mRNA levels and [(3)H]4-oxoretinol production from [(3)H]retinol increased in cells cultured without LIF and with exogenous retinoids. The enormous increases in CRABPI and II transcripts ( approximately 60 and 400-fold, respectively) in the absence of LIF may regulate aspects of the ES cell differentiation program in response to retinol.

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.

Transporter-to-trap conversion: a disulfide bond formation in cellular retinoic acid binding protein I mutant triggered by retinoic acid binding irreversibly locks the ligand inside the protein

Transport proteins must bind their ligands reversibly to enable release at the point of delivery, while irreversible binding is usually associated with the extreme cases of ligand sequestration. Protein conformational dynamics is an important modulator of binding kinetics, as increased flexibility in the regions adjacent to the binding site may facilitate both association and dissociation processes. Ligand entry to, and exit from, the internal binding site of the cellular retinoic acid binding protein I (CRABP I) occurs via a flexible portal region, which functions as a dynamic aperture. We designed and expressed a CRABP I mutant (A35C/T57C), in which a small-scale conformational switch caused by the ligand binding event triggers formation of a disulfide bond in the portal region, thereby arresting structural fluctuations and effectively locking the ligand inside the binding cavity. At the same time, no formation of the disulfide bond is observed in the apo form of the mutant, and most characteristics of the mutant, including protein stability, are very similar to those of the wild-type protein in the absence of retinoic acid. The mutation does not alter the kinetics of retinoic acid binding to the protein, although the disulfide formation makes the binding effectively irreversible, as suggested by the absence of retinoic acid transfer from the holo form of the mutant to lipid vesicles in the absence of a reducing agent. Taken together, these data suggest that the disulfide bond formation in the portal region arrests large-scale structural fluctuations, which are required for retinoic acid release from the protein. The unique properties of the CRABP I mutant described in this work can be used to inspire and guide a design of nanodevices for multiple tasks ranging from sequestering small-molecule toxins in both tissue and circulation to nutrient deprivation of pathogens.

Association of a polymorphism in the promoter of the cellular retinoic acid-binding protein II gene (CRABP2) with increased circulating low-density lipoprotein cholesterol

BACKGROUND

The cellular retinoic acid-binding protein II (CRABP-II), together with nuclear receptors such as the retinoid X receptor (RXR) and retinoic acid receptor (RAR), is involved in the transcriptional regulation of genes that control lipid metabolism via the retinoid signaling pathway and, as such, may be associated with disorders of lipid metabolism. Interestingly, the gene for CRABP-II is located on chromosome 1q21-23, which is a region that has been linked with disorders such as familial combined hyperlipidemia (FCHL), type 2 diabetes mellitus, and partial lipodystrophy, all of which are characterized by dyslipidemia.

METHODS

We investigated the hypothesis that the CRABP2 gene is involved in the regulation of lipid metabolism. Using the promoter -394T>C polymorphism of the CRABP2 gene, we performed association studies in three different cohorts: 299 healthy males, 182 HIV-infected patients and 151 patients with familial hypercholesterolemia (FH). All cholesterol measurements were performed in the absence of any lipid-lowering agents. ANOVA was performed on data adjusted for age, body mass index (BMI), gender, and use of protease inhibitors.

RESULTS

The frequency of the C allele was 0.03 in the three groups. Among healthy males, carriers of the C allele had 9% higher total plasma cholesterol (p=0.027) and 13% higher low-density lipoprotein cholesterol (LDL-C) concentrations (p=0.020). In HIV-infected patients, multivariate analysis of four measures over a 1-year period showed that carriers of the C allele had significantly higher LDL-C of between 10% and 31% (p=0.001) compared with non-carriers of the allele. FH patients who were carriers of the C allele had 16% higher LDL-C (p=0.038). The C allele was significantly over-represented among hypercholesterolemic patients (p=0.001).

CONCLUSIONS

Our results show that the CRABP2 gene, a member of the retinoid signaling pathway, is associated with increased plasma LDL-C concentrations.

Role of fatty acid binding proteins and long chain fatty acids in modulating nuclear receptors and gene transcription

Abnormal energy regulation may significantly contribute to the pathogenesis of obesity, diabetes mellitus, cardiovascular disease, and cancer. For rapid control of energy homeostasis, allosteric and posttranslational events activate or alter activity of key metabolic enzymes. For longer impact, transcriptional regulation is more effective, especially in response to nutrients such as long chain fatty acids (LCFA). Recent advances provide insights into how poorly water-soluble lipid nutrients [LCFA; retinoic acid (RA)] and their metabolites (long chain fatty acyl Coenzyme A, LCFA-CoA) reach nuclei, bind their cognate ligand-activated receptors, and regulate transcription for signaling lipid and glucose catabolism or storage: (i) while serum and cytoplasmic LCFA levels are in the 200 mircroM-mM range, real-time imaging recently revealed that LCFA and LCFA-CoA are also located within nuclei (nM range); (ii) sensitive fluorescence binding assays show that LCFA-activated nuclear receptors [peroxisome proliferator-activated receptor-alpha (PPARalpha) and hepatocyte nuclear factor 4alpha (HNF4alpha)] exhibit high affinity (low nM KdS) for LCFA (PPARalpha) and/or LCFA-CoA (PPARalpha, HNF4alpha)-in the same range as nuclear levels of these ligands; (iii) live and fixed cell immunolabeling and imaging revealed that some cytoplasmic lipid binding proteins [liver fatty acid binding protein (L-FABP), acyl CoA binding protein (ACBP), cellular retinoic acid binding protein-2 (CRABP-2)] enter nuclei, bind nuclear receptors (PPARalpha, HNF4alpha, CRABP-2), and activate transcription of genes in fatty acid and glucose metabolism; and (iv) studies with gene ablated mice provided physiological relevance of LCFA and LCFA-CoA binding proteins in nuclear signaling. This led to the hypothesis that cytoplasmic lipid binding proteins transfer and channel lipidic ligands into nuclei for initiating nuclear receptor transcriptional activity to provide new lipid nutrient signaling pathways that affect lipid and glucose catabolism and storage.

Structural basis for activation of fatty acid-binding protein 4

Fatty acid-binding protein 4 (FABP4) delivers ligands from the cytosol to the nuclear receptor PPARgamma in the nucleus, thereby enhancing the transcriptional activity of the receptor. Notably, FABP4 binds multiple ligands with a similar affinity but its nuclear translocation is activated only by specific compounds. To gain insight into the structural features that underlie the ligand-specificity in activation of the nuclear import of FABP4, we solved the crystal structures of the protein complexed with two compounds that induce its nuclear translocation, and compared these to the apo-protein and to FABP4 structures bound to non-activating ligands. Examination of these structures indicates that activation coincides with closure of a portal loop phenylalanine side-chain, contraction of the binding pocket, a subtle shift in a helical domain containing the nuclear localization signal of the protein, and a resultant change in oligomeric state that exposes the nuclear localization signal to the solution. Comparisons of backbone displacements induced by activating ligands with a measure of mobility derived from translation, libration, screw (TLS) refinement, and with a composite of slowest normal modes of the apo state suggest that the helical motion associated with the activation of the protein is part of the repertoire of the equilibrium motions of the apo-protein, i.e. that ligand binding does not induce the activated configuration but serves to stabilize it. Nuclear import of FABP4 can thus be understood in terms of the pre-existing equilibrium hypothesis of ligand binding.

Opposing effects of retinoic acid on cell growth result from alternate activation of two different nuclear receptors

Transcriptional activation of the nuclear receptor RAR by retinoic acid (RA) often leads to inhibition of cell growth. However, in some tissues, RA promotes cell survival and hyperplasia, activities that are unlikely to be mediated by RAR. Here, we show that, in addition to functioning through RAR, RA activates the "orphan" nuclear receptor PPARbeta/delta, which, in turn, induces the expression of prosurvival genes. Partitioning of RA between the two receptors is regulated by the intracellular lipid binding proteins CRABP-II and FABP5. These proteins specifically deliver RA from the cytosol to nuclear RAR and PPARbeta/delta, respectively, thereby selectively enhancing the transcriptional activity of their cognate receptors. Consequently, RA functions through RAR and is a proapoptotic agent in cells with high CRABP-II/FABP5 ratio, but it signals through PPARbeta/delta and promotes survival in cells that highly express FABP5. Opposing effects of RA on cell growth thus emanate from alternate activation of two different nuclear receptors.

Guiding ligands to nuclear receptors

Retinoic acid-the active metabolite of vitamin A-influences biological processes by activating the retinoic acid receptor (RAR). In this issue, Schug et al. (2007) demonstrate that retinoic acid also activates the peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta). Remarkably, retinoic acid signaling through RAR or PPARbeta/delta-which depends on cytoplasmic retinoic acid transporters-commits the cell to opposite fates, apoptosis or survival, respectively.

Immunolocalization of enzymes, binding proteins, and receptors sufficient for retinoic acid synthesis and signaling during the hair cycle

Retinoic acid (RA) is essential for maintenance of most epithelial tissues. One RA biosynthesis pathway consists of cellular retinol-binding protein (Crbp), retinol dehydrogenase (Dhrs9/eRoldh), retinal dehydrogenase 1-3 (Aldh1a1-3), and cellular RA-binding protein 2 (Crabp2). Previously, we localized Aldh1a2 and Aldh1a3 to both epithelial and mesenchymal cells within the hair follicle throughout the hair cycle. This study expands that observation by examining the complete pathway of RA biosynthesis and signaling via RA receptors alpha, beta, and gamma by immunohistochemistry in C57BL/6J mice wax-stripped to initiate and synchronize the cycle. This pathway of RA biosynthesis and signaling localized to the majority of layers of the hair follicle, sebaceous gland, and interfollicular epidermis in a hair cycle-dependent manner, suggesting that RA biosynthesis within the hair follicle is regulated in both a spatial and temporal manner. This localization pattern also revealed insights into epithelial-mesenchymal interactions and differentiation state differences within the RA biosynthesis and signaling pathway, as well as novel observations on nuclear versus cytoplasmic localization of Crabp2 and RA receptors. This complex pattern of RA biosynthesis and signaling identified by immunolocalization suggests that endogenous RA regulates specific aspects of hair follicle growth, differentiation, and cycling.

Expression of cellular retinoic acid-binding protein I is specific to neurons in adult transgenic mouse brain

Cellular retinoic acid binding protein I (CRABP-I) plays a role in retinoic acid (RA) metabolism or transport. This report shows specific neuronal expression of CRABP-I in adult transgenic mouse brain using CRABP-I promotor-driven lac-Z and neuron- and astrocyte-markers. Double staining indicates that CRABP-I is expressed in neurons and large cells (>12 microm) but to much lesser degree the astrocytes. CRABP-I-lac-Z(+) neurons were distributed throughout the brain, but in a very discreet pattern in each brain region. CRABP-I expression in specific populations of brain neurons suggests that RA is extensively metabolized in mature brains, mostly in neurons. Additionally, the genetic basis of its specific expression in these brain areas is located in the 5' regulatory region of this gene.

Retinoic acid pathway genes show significantly altered expression in uterine fibroids when compared with normal myometrium

Fibroids are benign neoplasms of myometrial smooth muscle cells (SMC). Despite being the most common tumor in humans, their etiology is poorly understood. Recent microarray studies have demonstrated that multiple members of the retinoid pathway are differentially expressed between myometrium and fibroids. The aim of this present study was to investigate gene expression of members of the retinoid pathway in matched myometrium and fibroids. We have demonstrated differential gene expression of two binding proteins [cellular retinol-binding proteins (CRBP) 1 and 2], three enzymes [alcohol dehydrogenase 1 (ADH1), aldehyde dehydrogenase (ALDH1) and retinol dehydrogenase (RODH)] and two receptors [retinoid X receptors (RXR) alpha and gamma] involved in the retinoid pathway by real-time PCR. There were no differences in gene expression for retinoid receptors RARalpha, beta, gamma and RXRbeta, and for the metabolizing enzyme cytochrome P450, family 26 subfamily A. We confirmed results for ADH1, ALDH1, CRBP1 and CRABP2 at the protein level by western blot. Using immunohistochemistry these proteins were mostly localized to myometrial and fibroid SMC. An exception to this was ALDH1 protein, which displayed strong staining localized to cells of the connective tissue, presumably fibroblasts, with a striking differential expression pattern between myometrium and fibroids. These results demonstrate that the retinoid pathway is altered in fibroids when compared with normal myometrium and specifically identify ALDH1 in fibroid fibroblasts. These alterations can lead to aberrant retinoic acid (RA) production and signaling, and alter the expression of RA target genes, which may be an important step in fibroid development.

Uterine leiomyomas express a molecular pattern that lowers retinoic acid exposure

OBJECTIVE

To analyze expression of the retinoic acid signaling pathway genes that are involved in retinol metabolism, transport, transcriptional activation, and transcriptional products in spontaneous human leiomyomas.

DESIGN

Laboratory study of human leiomyoma and patient-matched myometrial tissue.

PATIENT(S)

Eight women undergoing hysterectomy for symptomatic leiomyomas.

INTERVENTION(S)

Confirmation of an altered retinoic acid pathway analyzed by microarray, real time reverse transcription-polymerase chain reaction, Western blot, immunohistochemistry, and high-performance liquid chromatography (HPLC).

MAIN OUTCOME MEASURE(S)

Gene and protein expression.

RESULT(S)

Regardless of patient demographics and leiomyoma location and size, we found decreased expression of the major genes involved in retinoic acid pathway including alcohol dehydrogenase-1 (-3.97- +/- 0.03-fold), aldehyde dehydrogenase-1 (-3.1- +/- 0.07-fold), cellular retinol binding protein-1 (-2.62- +/- 0.04-fold), and cellular retinoic acid binding protein-1 (-2.42- +/- 0.20-fold). Cytochrome P450 (CYP 26A1), which is responsible for retinoic acid metabolism, was highly up-regulated in leiomyomas (+5.4- +/- 0.53-fold). Nuclear receptors demonstrated a complex pattern of under-expression (RARalpha, RARbeta, RXRalpha, RXRgamma) and over-expression (RARgamma, RXRbeta) at both the mRNA and protein level. Differences in protein amounts were confirmed by Western blot. Finally, a reduced amount of cellular ATRA and 9-cis retinoic acid was confirmed by HPLC in leiomyomas compared with myometrial tissues.

CONCLUSION(S)

Molecular alterations in the retinoic acid pathway of leiomyomata result in a decrease in retinoic acid exposure.

Frequent methylation-associated silencing of a candidate tumor-suppressor, CRABP1, in esophageal squamous-cell carcinoma

Epigenetic alterations and the resulting inactivation of tumor suppressor genes often contribute to the development of various cancers. To identify novel candidates that may be silenced by aberrant methylation in esophageal squamous-cell carcinoma (ESCC), we analysed ESCC cell lines by a recently developed method known as bacterial artificial chromosome array-based methylated CpG island amplification (BAMCA), and selected candidates through BAMCA-assisted strategy. In the course of this program, we identified frequent CpG methylation-dependent silencing of the gene encoding cellular retinoic acid binding protein 1 (CRABP1) in our panel of ESCC cell lines. Expression of CRABP1 mRNA was restored in gene-silenced ESCC cells after treatment with 5-aza 2'-deoxycytidine. The DNA methylation status of the CRABP1 CpG island with clear promoter activity correlated inversely with expression of this gene. CpG methylation of CRABP1 was frequently observed in primary ESCC tissues as well. Restoration of CRABP1 expression in ESCC cells lacking the protein reduced cell growth by inducing arrest at G(0)-G(1), whereas knockdown of the gene in cells expressing CRABP1 promoted cell growth. Among 113 primary ESCC tumors, the absence of immunoreactive CRABP1 was significantly associated with de-differentiation of cancer cells and with distant lymph-node metastases in the patients. These results indicate that CRABP1 appears to have a tumor-suppressor function in esophageal epithelium, and its epigenetic silencing may play a pivotal role during esophageal carcinogenesis. Its expression status in biopsies or resected tumors might serve as an index for identifying ESCC patients for whom combined therapeutic modalities would be recommended.

Suppression of mammary carcinoma cell growth by retinoic acid: the cell cycle control gene Btg2 is a direct target for retinoic acid receptor signaling

The anticarcinogenic activities of retinoic acid (RA) are believed to be mediated by the nuclear RA receptor (RAR) and by the RA-binding protein cellular RA-binding protein-II (CRABP-II). In MCF-7 mammary carcinoma cells, growth inhibition by RA entails an early cell cycle arrest followed by induction of apoptosis. Here, we aimed to obtain insights into the initial cell cycle response. We show that a 3- to 5-h RA pulse is sufficient for inducing a robust growth arrest 2 to 4 days later, demonstrating inhibition of the G1-S transition by RA is triggered by immediate-early RAR targets and does not require the continuous presence of the hormone throughout the arrest program. Expression array analyses revealed that RA induces the expression of several genes involved in cell cycle regulation, including the p53-controlled antiproliferative gene B-cell translocation gene, member 2 (Btg2) and the BTG family member Tob1. We show that induction of Btg2 by RA does not require de novo protein synthesis and is augmented by overexpression of CRABP-II. Additionally, we identify a RA response element in the Btg2 promoter and show that the element binds retinoid X receptor/RAR heterodimers in vitro, is occupied by the heterodimers in cells, and can drive RA-induced activation of a reporter gene. Hence, Btg2 is a novel direct target for RA signaling. In concert with the reports that Btg2 inhibits cell cycle progression by down-regulating cyclin D1, induction of Btg2 by RA was accompanied by a marked decrease in cyclin D1 expression. The observations thus show that the antiproliferative activity of RA in MCF-7 cells is mediated, at least in part, by Btg2.

AP2 transcription factors regulate expression of CRABPII in hormone responsive breast carcinoma

BACKGROUND

The AP2 transcription factor family is a set of developmentally regulated, retinoic acid (RA) inducible genes, which regulate expression of estrogen receptor-alpha (ERalpha) in breast carcinoma. We hypothesized that AP2 factors regulate a set of genes characteristic of the hormone responsive breast cancer phenotype. To better understand the role of AP2 factors in hormone responsive breast cancer, we sought to identify AP2-target genes in breast epithelial cells.

MATERIALS AND METHODS

Overexpression of AP2 factors was achieved in human mammary epithelial cells (HMECs) using adenoviral vectors. AP2 target genes were identified by comparative hybridization to cDNA microarrays containing 30,000 human genes. Expression patterns were confirmed by Northern and Western blot and by elimination of AP2 using siRNA. Potential regulatory elements in promoters of target genes were identified by DNase I hypersensitive site mapping.

RESULTS

Comparative cDNA microarray hybridization identified a set of genes induced by overexpression of AP2alpha and AP2gamma in HMECs. The up-regulation of cellular retinoic acid-binding protein 2 (CRABPII), EST-1, and ECM1 was induced by overexpression of AP2alpha, AP2gamma, or a chimeric AP2 factor in which the activation domain of AP2alpha was replaced by the activation domain of herpesvirus VP16. Interestingly, hormone unresponsive MDA-MB-231 cells were resistant to CRABPII induction by any of the AP2 factors. Elimination of AP2gamma in MCF7 cells resulted in a significant reduction in CRABPII expression. AP2alpha induced DNase I hypersensitive sites in the promoter of the CRABPII gene at -5000 bp, which corresponds to the site of action of RAR/RXR factors.

CONCLUSIONS

AP2 factors regulate CRABPII expression in HMECs and breast cancer cells and accounts for the associated expression of ERalpha and CRABPII in hormone responsive breast cancer. Because CRABPII mediates growth suppressive effects of RA in breast cancer, the data suggest that AP2 factors have the ability to mediate RA responsiveness through the regulation of CRABP II expression.

The Structure of Apo-wild-type Cellular Retinoic Acid Binding Protein II at 1.4 Å and its Relationship to Ligand Binding and Nuclear Translocation

CRABPII is a small, cytosolic protein that solubilizes and transfers retinoic acid (RA) to the nucleus while also enhancing its transcriptional activity. We have determined the first high-resolution structure of apo-wild type (WT) CRABPII at 1.35 A. Using three different data sets collected on apo-WT CRABPII we have shown that apo- and holo-CRABPII share very similar structures. Binding of RA appears to increase the overall rigidity of the structure, although the induced structural changes are not as pronounced as previously thought. The enhanced structural rigidity may be an important determinant for the enhanced nuclear localization of the RA-bound protein. Comparison of our apo-WT with a mutant apo-CRABPII structure shows that mutation of Arg111, a conserved residue of CRABPII and a key residue in RA binding, causes structural changes in the molecule. We further investigated the structural importance of conserved residues by determining the structure of the F15W mutant CRABPII (F15W-CRABPII). Our structures also demonstrate structural changes induced by crystal packing and show that a crystal can harbor demonstrative structural differences in the asymmetric unit.

Characterization of cellular retinoid-binding proteins in human myometrium during pregnancy

Many complementary or competing signalling pathways bear an influence on the myometrium at any one time, and because the retinoic acid signalling pathway influences differentiation of a wide array of human tissues, this may be one of the determinants of myometrial differentiation during pregnancy. We have explored the novel hypothesis that the retinoids may act as important regulators in controlling the differentiated state of the human myometrium during pregnancy by characterizing the expression profiles for cellular retinoid-binding proteins CRBPI, CRABPI and CRABPII in non-pregnant, pregnant (non-labouring) and labouring human myometrium taken from the functionally distinct upper and lower uterine segments. In addition, we have investigated the effect of all-trans retinoic acid (ATRA) on the expression of several retinoic acid response genes including cyclooxygenase-2 (COX-2) and connexin-43 (Cx-43). Different spatial and temporal patterns of expression were observed for CRBPI, CRABPI and CRABPII within the upper and lower uterine segments through the three trimesters of pregnancy and in labour. Furthermore, the expression of COX-2, Cx-43, CRABPI, the transcription factor c-Jun and the retinoic acid receptor RARbeta altered in response to different concentrations of ATRA, suggesting that the differential expression of cellular retinoid-binding proteins may lead to different levels of retinoic acid being delivered to its nuclear targets, leading to the differential expression of specific target genes within the myometrium during pregnancy.

Overview of retinoid metabolism and function

Retinoids (vitamin A) are crucial for most forms of life. In chordates, they have important roles in the developing nervous system and notochord and many other embryonic structures, as well as in maintenance of epithelial surfaces, immune competence, and reproduction. The ability of all-trans retinoic acid to regulate expression of several hundred genes through binding to nuclear transcription factors is believed to mediate most of these functions. The role of all-trans retinoic may extend beyond the regulation of gene transcription because a large number of noncoding RNAs also are regulated by retinoic acid. Additionally, extra-nuclear mechanisms of action of retinoids are also being identified. In organisms ranging from prokaryotes to humans, retinal is covalently linked to G protein-coupled transmembrane receptors called opsins. These receptors function as light-driven ion pumps, mediators of phototaxis, or photosensory pigments. In vertebrates phototransduction is initiated by a photochemical reaction where opsin-bound 11-cis-retinal is isomerized to all-trans-retinal. The photosensitive receptor is restored via the retinoid visual cycle. Multiple genes encoding components of this cycle have been identified and linked to many human retinal diseases. Central aspects of vitamin A absorption, enzymatic oxidation of all-trans retinol to all-trans retinal and all-trans retinoic acid, and esterification of all-trans retinol have been clarified. Furthermore, specific binding proteins are involved in several of these enzymatic processes as well as in delivery of all-trans retinoic acid to nuclear receptors. Thus, substantial progress has been made in our understanding of retinoid metabolism and function. This insight has improved our view of retinoids as critical molecules in vision, normal embryonic development, and in control of cellular growth, differentiation, and death throughout life.

A fluorescence-based method for analyzing retinoic acid in biological samples

Retinoic acid (RA) modulates the rates of transcription of numerous genes and thus plays key roles in multiple biological processes and is used in therapy of a number of diseases. However, RA therapy is often confounded by toxicity, raising the need for methodologies for its ready quantitation in biological samples. We describe a fluorescence-based method for quantitating RA that takes advantage of the high affinity and selectivity of the intracellular lipid-binding protein termed CRABP-I and CRABP-II and that uses them as RA sensors. L28C CRABP mutants were generated, and the inserted cysteine was covalently labeled with an environmentally sensitive fluorescent probe. The label was introduced into a region of the protein that undergoes a conformational shift on ligation. Consequently, RA binding resulted in distinct changes in the fluorescence of the protein-bound probe, allowing direct quantitation of RA. We show that the method can be used to monitor the biosynthesis of RA from its precursor retinal in cultured mammalian cells as well as the detection of exogenous RA in serum. The assay provides ease of use and sensitivity that enable quantitation of RA in biological samples of limited size, and it should prove to be useful in a variety of research and clinical applications.

A retinoic acid-Hox hierarchy controls both anterior/posterior patterning and neuronal specification in the developing central nervous system of the cephalochordate amphioxus

Retinoic acid (RA) mediates both anterior/posterior patterning and neuronal specification in the vertebrate central nervous system (CNS). However, the molecular mechanisms downstream of RA are not well understood. To investigate these mechanisms, we used the invertebrate chordate amphioxus, in which the CNS, although containing only about 20,000 neurons in adults, like the vertebrate CNS, has a forebrain, midbrain, hindbrain, and spinal cord and is regionalized by RA-signaling. Here we show, first, that domains of genes with expression normally limited to diencephalon and midbrain are generally not affected by altered RA-signaling, second, that contrary to previous reports, not only Hox1, 3, and 4, but also Hox2 and Hox6 are collinearly expressed in the amphioxus CNS, and third, that collinear expression of all these Hox genes is controlled by RA-signaling. Finally, we show that Hox1 is involved in mediating both the role of RA-signaling in regionalization of the hindbrain and in specification of hindbrain motor neurons. Thus, morpholino knock-down of the single amphioxus Hox1 mimics the effects of treatments with an RA-antagonist. This analysis establishes RA-dependent regulation of collinear Hox expression as a feature common to the chordate CNS and indicates that the RA-Hox hierarchy functions both in proper anterior/posterior patterning of the developing CNS and in specification of neuronal identity.