Overexpression of the cellular retinoic acid binding protein-I (CRABP-I) results in a reduction in differentiation-specific gene expression in F9 teratocarcinoma cells

Hypothalamic GABAergic Neurons Expressing Cellular Retinoic Acid Binding Protein 1 (CRABP1) Are Sensitive to Metabolic Status and Liraglutide in Male Mice

INTRODUCTION

Owing to their privileged anatomical location, neurons of the arcuate nucleus of the hypothalamus (ARC) play critical roles in sensing and responding to metabolic signals such as leptin and glucagon-like peptide 1 (GLP-1). In addition to the well-known proopiomelanocortin (POMC)- and agouti-related peptide (AgRP)-expressing neurons, subpopulations of GABAergic neurons are emerging as key regulators of energy balance. However, the precise identity of these metabolic neurons is still elusive. Here, we identified and characterized the molecular signature of a novel population of GABAergic neurons of the ARC expressing Cellular retinoic acid binding protein 1 (Crabp1).

METHODS

Using a combination of immunohistochemistry and in situ hybridization techniques, we investigated the expression of Crabp1 across the mouse brain and characterized the molecular identity of Crabp1ARC neurons. We also determined whether Crabp1ARC neurons are sensitive to fasting, leptin, and GLP1R agonism by assessing cFOS immunoreactivity as a marker of neuronal activity.

RESULTS

Crabp1ARC neurons represent a novel GABAergic neuronal population robustly enriched in the ARC and are distinct from the prototypical melanocortin neurons. Crabp1ARC neurons overlap with three subpopulations of yet uncharacterized ARC neurons expressing Htr3b, Tbx19, and Tmem215. Notably, Crabp1ARC neurons express receptors for metabolic hormones and their activity is modulated by the nutritional state and GLP1R agonism.

CONCLUSION

Crabp1ARC neurons represent a novel heterogeneous population of GABAergic neurons sensitive to metabolic status.

Opposite Effects of CRABP1 and CRABP2 Homologs on Proliferation of Breast Cancer Cells and Their Sensitivity to Retinoic Acid

Resistance of tumor cells to retinoic acid (RA), a promising therapeutic agent, is the major factor limiting the use of RA in clinical practice. The mechanisms of resistance to RA are still poorly understood. Cellular Retinoic Acid Binding Proteins, CRABP1 and CRABP2, are essential mediators of RA signaling, but role of the two CRABP homologs in regulating cellular sensitivity to RA has not been well studied. In addition, the effects of CRABP1 and CRABP2 on cell proliferation have not been compared. Here, using a broad panel of breast cancer cell lines with different levels of RA sensitivity/resistance, we show for the first time that in the RA-sensitive cells, CRABP1 expression is restricted by methylation, and protein levels are highly variable. In the moderately-RA-resistant cell lines, high level of CRABP1 is observed both at the mRNA and protein levels, unchanged by inhibition of DNA methylation. The cell lines with maximum resistance to RA are characterized by complete repression of CRABP1 expression realized at transcriptional and posttranscriptional levels, and exogenous expression of each of the CRABP homologs has no effect on the studied characteristics. CRABP1 and CRABP2 proteins have opposing effects on proliferation and sensitivity to RA. In particular, CRABP1 stimulates and CRABP2 reduces proliferation and resistance to RA in the initially RA-sensitive cells, while in the more resistant cells the role of each homolog in both of these parameters is reversed. Overall, we have shown for the first time that CRABP proteins exert different effects on the growth and sensitivity to RA of breast cancer cells (stimulation, suppression, or no effect) depending on the baseline level of RA-sensitivity, with the effects of CRABP1 and CRABP2 homologs on the studied properties always being opposite.

CRABP-I Expression Patterns in the Developing Chick Inner Ear

The vertebrate inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions, regarded as an excellent system for analyzing events that occur during development, such as patterning, morphogenesis, and cell specification. Retinoic acid (RA) is involved in all these development processes. Cellular retinoic acid-binding proteins (CRABPs) bind RA with high affinity, buffering cellular free RA concentrations and consequently regulating the activation of precise specification programs mediated by particular regulatory genes. In the otic vesicle, strong CRABP-I expression was detected in the otic wall's dorsomedial aspect, where the endolymphatic apparatus develops, whereas this expression was lower in the ventrolateral aspect, where part of the auditory system forms. Thus, CRABP-I proteins may play a role in the specification of the dorsal-to-ventral and lateral-to-medial axe of the otic anlagen. Regarding the developing sensory patches, a process partly involving the subdivision of a ventromedial pro-sensory domain, the CRABP-I gene displayed different levels of expression in the presumptive territory of each sensory patch, which was maintained throughout development. CRABP-I was also relevant in the acoustic-vestibular ganglion and in the periotic mesenchyme. Therefore, CRABP-I could protect RA-sensitive cells in accordance with its dissimilar concentration in specific areas of the developing chick inner ear.

Interspecies transcriptomics identify genes that underlie disproportionate foot growth in jerboas

Despite the great diversity of vertebrate limb proportion and our deep understanding of the genetic mechanisms that drive skeletal elongation, little is known about how individual bones reach different lengths in any species. Here, we directly compare the transcriptomes of homologous growth cartilages of the mouse (Mus musculus) and bipedal jerboa (Jaculus jaculus), the latter of which has "mouse-like" arms but extremely long metatarsals of the feet. Intersecting gene-expression differences in metatarsals and forearms of the two species revealed that about 10% of orthologous genes are associated with the disproportionately rapid elongation of neonatal jerboa feet. These include genes and enriched pathways not previously associated with endochondral elongation as well as those that might diversify skeletal proportion in addition to their known requirements for bone growth throughout the skeleton. We also identified transcription regulators that might act as "nodes" for sweeping differences in genome expression between species. Among these, Shox2, which is necessary for proximal limb elongation, has gained expression in jerboa metatarsals where it has not been detected in other vertebrates. We show that Shox2 is sufficient to increase mouse distal limb length, and a nearby putative cis-regulatory region is preferentially accessible in jerboa metatarsals. In addition to mechanisms that might directly promote growth, we found evidence that jerboa foot elongation may occur in part by de-repressing latent growth potential. The genes and pathways that we identified here provide a framework to understand the modular genetic control of skeletal growth and the remarkable malleability of vertebrate limb proportion.

The molecular aspects of absorption and metabolism of carotenoids and retinoids in vertebrates

Vitamin A is an essential nutrient necessary for numerous basic physiological functions, including reproduction and development, immune cell differentiation and communication, as well as the perception of light. To evade the dire consequences of vitamin A deficiency, vertebrates have evolved specialized metabolic pathways that enable the absorption, transport, and storage of vitamin A acquired from dietary sources as preformed retinoids or provitamin A carotenoids. This evolutionary advantage requires a complex interplay between numerous specialized retinoid-transport proteins, receptors, and enzymes. Recent advances in molecular and structural biology resulted in a rapid expansion of our understanding of these processes at the molecular level. This progress opened new avenues for the therapeutic manipulation of retinoid homeostasis. In this review, we summarize current research related to the biochemistry of carotenoid and retinoid-processing proteins with special emphasis on the structural aspects of their physiological actions. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Cellular Retinoic-Acid Binding Protein 2 in Solid Tumor

The retinoic acid (RA) signaling pathway is crucial for many biological processes. The RA transporter, Cellular Retinoic-Acid Binding Protein 2 (CRABP2), is abnormally expressed in various tumor types. CRABP2 presents significant effects on tumorous behaviors and functions, including cell proliferation, apoptosis, invasion, migration, metastasis, and angiogenesis. The tumorigenesis mechanism of CRABP2, as both suppressor and promotor, is complicated, therefore, there remains the need for further investigation. Elucidating the regulating mechanisms in a specific stage of the tumor could facilitate CRABP2 to be a biomarker in cancer diagnosis and prognosis. Besides, clarifying the pathways of CRABP2 in cancer development will contribute to the gene-targeted therapy. In this review, we summarized the expression, distribution, and mechanism of CRABP2 in solid tumors. Illuminating the CRABP2 signaling pathway may benefit understanding the retinoid signaling pathway, providing a useful biomarker for future clinical trials.

Identification of influential proteins in the classical retinoic acid signaling pathway

Background

In the classical pathway of retinoic acid (RA) mediated gene transcription, RA binds to a nuclear hormone receptor dimer composed of retinoic acid receptor (RAR) and retinoid X receptor (RXR), to induce the expression of its downstream target genes. In addition to nuclear receptors, there are other intracellular RA binding proteins such as cellular retinoic acid binding proteins (CRABP1 and CRABP2) and cytochrome P450 (CYP) enzymes, whose contributions to the RA signaling pathway have not been fully understood. The objective of this study was to compare the significance of various RA binding receptors, i.e. CRABP1, CRABP2, CYP and RAR in the RA signaling pathway. In this regard, we developed a mathematical model of the RA pathway, which is one of the few models, if not the only one, that includes all main intracellular RA binding receptors. We then performed a global sensitivity analysis (GSA) to investigate the contribution of the RA receptors to RA-induced mRNA production, when the cells were treated with a wide range of RA levels, from physiological to pharmacological concentrations.

Results

Our results show that CRABP2 and RAR are the most and the least important proteins, respectively, in controlling the model performance at physiological concentrations of RA (1–10 nM). However, at higher concentrations of RA, CYP and RAR are the most sensitive parameters of the system. Furthermore, we found that depending on the concentrations of all RA binding proteins, the rate of metabolism of RA can either change or remain constant following RA therapy. The cellular levels of CRABP1 are more important than that of CRABP2 in controlling RA metabolite formation at pharmacological conditions (RA = 0.1–1 μM). Finally, our results indicate a significant negative correlation between total mRNA production and total RA metabolite formation at pharmacological levels of RA.

Conclusions

Our simulations indicate that the significance of the RA binding proteins in the RA pathway of gene expression strongly depends on intracellular concentration of RA. This study not only can explain why various cell types respond to RA therapy differently, but also can potentially help develop pharmacological methods to increase the efficacy of the drug.

Electronic supplementary material

The online version of this article (10.1186/s12976-018-0088-7) contains supplementary material, which is available to authorized users.

Cellular retinoid binding-proteins, CRBP, CRABP, FABP5: Effects on retinoid metabolism, function and related diseases

Cellular binding-proteins (BP), including CRBP1, CRBP2, CRABP1, CRABP2, and FABP5, shepherd the poorly aqueous soluble retinoids during uptake, metabolism and function. Holo-BP promote efficient use of retinol, a scarce but essential nutrient throughout evolution, by sheltering it and its major metabolite all-trans-retinoic acid from adventitious interactions with the cellular milieu, and by imposing specificity of delivery to enzymes, nuclear receptors and other partners. Apo-BP reflect cellular retinoid status and modify activities of retinoid metabolon enzymes, or exert non-canonical actions. High ligand binding affinities and the nature of ligand sequestration necessitate external factors to prompt retinoid release from holo-BP. One or more of cross-linking, kinetics, and colocalization have identified these factors as RDH, RALDH, CYP26, LRAT, RAR and PPARβ/δ. Michaelis-Menten and other kinetic approaches verify that BP channel retinoids to select enzymes and receptors by protein-protein interactions. Function of the BP and enzymes that constitute the retinoid metabolon depends in part on retinoid exchanges unique to specific pairings. The complexity of these exchanges configure retinol metabolism to meet the diverse functions of all-trans-retinoic acid and its ability to foster contrary outcomes in different cell types, such as inducing apoptosis, differentiation or proliferation. Altered BP expression affects retinoid function, for example, by impairing pancreas development resulting in abnormal glucose and energy metabolism, promoting predisposition to breast cancer, and fostering more severe outcomes in prostate cancer, ovarian adenocarcinoma, and glioblastoma. Yet, the extent of BP interactions with retinoid metabolon enzymes and their impact on retinoid physiology remains incompletely understood.

Cellular Retinoic Acid Binding Proteins: Genomic and Non-genomic Functions and their Regulation

Cellular retinoic acid binding proteins (CRABPs) are high-affinity retinoic acid (RA) binding proteins that mainly reside in the cytoplasm. In mammals, this family has two members, CRABPI and II, both highly conserved during evolution. The two proteins share a very similar structure that is characteristic of a "β-clam" motif built up from10-strands. The proteins are encoded by two different genes that share a very similar genomic structure. CRABPI is widely distributed and CRABPII has restricted expression in only certain tissues. The CrabpI gene is driven by a housekeeping promoter, but can be regulated by numerous factors, including thyroid hormones and RA, which engage a specific chromatin-remodeling complex containing either TRAP220 or RIP140 as coactivator and corepressor, respectively. The chromatin-remodeling complex binds the DR4 element in the CrabpI gene promoter to activate or repress this gene in different cellular backgrounds. The CrabpII gene promoter contains a TATA-box and is rapidly activated by RA through an RA response element. Biochemical and cell culture studies carried out in vitro show the two proteins have distinct biological functions. CRABPII mainly functions to deliver RA to the nuclear RA receptors for gene regulation, although recent studies suggest that CRABPII may also be involved in other cellular events, such as RNA stability. In contrast, biochemical and cell culture studies suggest that CRABPI functions mainly in the cytoplasm to modulate intracellular RA availability/concentration and to engage other signaling components such as ERK activity. However, these functional studies remain inconclusive because knocking out one or both genes in mice does not produce definitive phenotypes. Further studies are needed to unambiguously decipher the exact physiological activities of these two proteins.

CRABP1 is associated with a poor prognosis in breast cancer: adding to the complexity of breast cancer cell response to retinoic acid

Background

Clinical trials designed to test the efficacy of retinoic acid (RA) as an adjuvant for the treatment of solid cancers have been disappointing, primarily due to RA resistance. Estrogen receptor (ER)-negative breast cancer cells are more resistant to RA than ER-positive cells. The expression and subcellular distribution of two RA-binding proteins, FABP5 and CRABP2, has already been shown to play critical roles in breast cancer cell response to RA. CRABP1, a third member of the RA-binding protein family, has not previously been investigated as a possible mediator of RA action in breast cancer.

Methods

CRABP1 and CRABP2 expression in primary breast tumor tissues was analyzed using gene expression and tissue microarrays. CRABP1 levels were manipulated using siRNAs and by transient overexpression. RA-induced subcellular translocation of CRABPs was examined by immunofluorescence microscopy and immunoblotting. RA-induced transactivation of RAR was analyzed using a RA response element (RARE)-driven luciferase reporter system. Effects of CRABP1 expression and RA treatment on downstream gene expression were investigated by semi-quantitative RT-PCR analysis.

Results

Compared to normal mammary tissues, CRABP1 expression is significantly down-regulated in ER+ breast tumors, but maintained in triple-negative breast cancers. Elevated CRABP1 levels are associated with poor patient prognosis, high Ki67 immunoreactivity and high tumor grade in breast cancer. The prognostic significance of CRABP1 is attributed to its cytoplasmic localization. We demonstrate that CRABP1 expression attenuates RA-induced cell growth arrest and inhibits RA signalling in breast cancer cells by sequestering RA in the cytoplasm. We also show that CRABP1 affects the expression of genes involved in RA biosynthesis, trafficking and metabolism.

Conclusions

CRABP1 is an adverse factor for clinical outcome in triple-negative breast cancer and a potent inhibitor of RA signalling in breast cancer cells. Our data indicate that CRABP1, in conjunction with previously identified CRABP2 and FABP5, plays a key role in breast cancer cell response to RA. We propose that these three RA-binding proteins can serve as biomarkers for predicting triple-negative breast cancer response to RA, with elevated levels of either cytoplasmic CRABP1 or FABP5 associated with RA resistance, and elevated levels of nuclear CRABP2 associated with sensitivity to RA.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0380-7) contains supplementary material, which is available to authorized users.

Intracellular transport of fat-soluble vitamins A and E

Vitamins are compounds that are essential for the normal growth, reproduction and functioning of the human body. Of the 13 known vitamins, vitamins A, D, E and K are lipophilic compounds and are therefore called fat-soluble vitamins. Because of their lipophilicity, fat-soluble vitamins are solubilized and transported by intracellular carrier proteins to exert their actions and to be metabolized properly. Vitamin A and its derivatives, collectively called retinoids, are solubilized by intracellular retinoid-binding proteins such as cellular retinol-binding protein (CRBP), cellular retinoic acid-binding protein (CRABP) and cellular retinal-binding protein (CRALBP). These proteins act as chaperones that regulate the metabolism, signaling and transport of retinoids. CRALBP-mediated intracellular retinoid transport is essential for vision in human. α-Tocopherol, the main form of vitamin E found in the body, is transported by α-tocopherol transfer protein (α-TTP) in hepatic cells. Defects of α-TTP cause vitamin E deficiency and neurological disorders in humans. Recently, it has been shown that the interaction of α-TTP with phosphoinositides plays a critical role in the intracellular transport of α-tocopherol and is associated with familial vitamin E deficiency. In this review, we summarize the mechanisms and biological significance of the intracellular transport of vitamins A and E.

CRABP1 provides high malignancy of transformed mesenchymal cells and contributes to the pathogenesis of mesenchymal and neuroendocrine tumors

CRABP1 (cellular retinoic acid binding protein 1) belongs to the family of fatty acid binding proteins. Retinoic acid binding is the only known functional activity of this protein. The role of CRABP1 in human carcinogenesis remains poorly understood. Here, for the first time we demonstrated pro-metastatic and pro-tumorigenic activity of CRABP1 in mesenchymal tumors. Further functional analysis revealed that the pro-tumorigenic effect of CRABP1 does not depend on retinoic acid binding activity. These results suggest that CRABP1 could have an alternative intracellular functional activity that contributes to the high malignancy of transformed mesenchymal cells. Microarray analysis detected CRABP1-mediated alterations in the expression of about 100 genes, including those encoding key regulatory proteins. CRABP1 is ubiquitously expressed in monophasic synovial sarcomas, while in biphasic synovial sarcomas it is expressed uniquely by the spindle cells of the aggressive mesenchymal component. High level of CRABP1 expression is associated with lymph node metastasis and poor differentiation/high grade of pancreatic neuroendocrine tumors (pNETs). Presented data suggest CRABP1 as a promising biomarker of pNETs' clinical behavior. Our results give the first evidence of pro-tumorigenic and pro-metastatic activity of CRABP1 in mesenchymal and neuroendocrine tumors.

A mid-life vitamin A supplementation prevents age-related spatial memory deficits and hippocampal neurogenesis alterations through CRABP-I

Age-related memory decline including spatial reference memory is considered to begin at middle-age and coincides with reduced adult hippocampal neurogenesis. Moreover, a dysfunction of vitamin A hippocampal signalling pathway has been involved in the appearance of age-related memory deficits but also in adult hippocampal neurogenesis alterations. The present study aims at testing the hypothesis that a mid-life vitamin A supplementation would be a successful strategy to prevent age-related memory deficits. Thus, middle-aged Wistar rats were submitted to a vitamin A enriched diet and were tested 4 months later in a spatial memory task. In order to better understand the potential mechanisms mediating the effects of vitamin A supplementation on hippocampal functions, we studied different aspects of hippocampal adult neurogenesis and evaluated hippocampal CRABP-I expression, known to modulate differentiation processes. Here, we show that vitamin A supplementation from middle-age enhances spatial memory and improves the dendritic arborisation of newborn immature neurons probably resulting in a better survival and neuronal differentiation in aged rats. Moreover, our results suggest that hippocampal CRABP-I expression which controls the intracellular availability of retinoic acid (RA), may be an important regulator of neuronal differentiation processes in the aged hippocampus. Thus, vitamin A supplementation from middle-age could be a good strategy to maintain hippocampal plasticity and functions.

Enzymology of retinoic acid biosynthesis and degradation

All-trans-retinoic acid is a biologically active derivative of vitamin A that regulates numerous physiological processes. The concentration of retinoic acid in the cells is tightly regulated, but the exact mechanisms responsible for this regulation are not completely understood, largely because the enzymes involved in the biosynthesis of retinoic acid have not been fully defined. Recent studies using in vitro and in vivo models suggest that several members of the short-chain dehydrogenase/reductase superfamily of proteins are essential for retinoic acid biosynthesis and the maintenance of retinoic acid homeostasis. However, the exact roles of some of these recently identified enzymes are yet to be characterized. The properties of the known contributors to retinoid metabolism have now been better defined and allow for more detailed understanding of their interactions with retinoid-binding proteins and other retinoid enzymes. At the same time, further studies are needed to clarify the interactions between the cytoplasmic and membrane-bound proteins involved in the processing of hydrophobic retinoid metabolites. This review summarizes current knowledge about the roles of various biosynthetic and catabolic enzymes in the regulation of retinoic acid homeostasis and outlines the remaining questions in the field.

Non-canonical activity of retinoic acid in epigenetic control of embryonic stem cell

Non-canonical cytoplasmic activities and signal transduction of retinoic acid (RA) expand RA's pleiotropic effects in coordinating the epigenome in embryonic stem cell (ESC). Examples include RA-bound cellular retinoic acid binding protein I, which activates ERK2. By engaging both cytosolic and nuclear mediators, RA can efficiently augment ESC's epigenome.

Retinoic acid induces neurogenesis by activating both retinoic acid receptors (RARs) and peroxisome proliferator-activated receptor β/δ (PPARβ/δ)

Retinoic acid (RA) regulates gene transcription by activating the nuclear receptors retinoic acid receptor (RAR) and peroxisome proliferator-activated receptor (PPAR) β/δ and their respective cognate lipid-binding proteins CRABP-II and FABP5. RA induces neuronal differentiation, but the contributions of the two transcriptional pathways of the hormone to the process are unknown. Here, we show that the RA-induced commitment of P19 stem cells to neuronal progenitors is mediated by the CRABP-II/RAR path and that the FABP5/PPARβ/δ path can inhibit the process through induction of the RAR repressors SIRT1 and Ajuba. In contrast with its inhibitory activity in the early steps of neurogenesis, the FABP5/PPARβ/δ path promotes differentiation of neuronal progenitors to mature neurons, an activity mediated in part by the PPARβ/δ target gene PDK1. Hence, RA-induced neuronal differentiation is mediated through RAR in the early stages and through PPARβ/δ in the late stages of the process. The switch in RA signaling is accomplished by a transient up-regulation of RARβ concomitantly with a transient increase in the CRABP-II/FABP5 ratio at early stages of differentiation. In accordance with these conclusions, hippocampi of FABP5-null mice display excess accumulation of neuronal progenitor cells and a deficit in mature neurons versus wild-type animals.

Cellular retinoic acid-binding proteins are essential for hindbrain patterning and signal robustness in zebrafish

The vitamin A derivative retinoic acid (RA) is a morphogen that patterns the anterior-posterior axis of the vertebrate hindbrain. Cellular retinoic acid-binding proteins (Crabps) transport RA within cells to both its nuclear receptors (RARs) and degrading enzymes (Cyp26s). However, mice lacking Crabps are viable, suggesting that Crabp functions are redundant with those of other fatty acid-binding proteins. Here we show that Crabps in zebrafish are essential for posterior patterning of the hindbrain and that they provide a key feedback mechanism that makes signaling robust as they are able to compensate for changes in RA production. Of the four zebrafish Crabps, Crabp2a is uniquely RA inducible and depletion or overexpression of Crabp2a makes embryos hypersensitive to exogenous RA. Computational models confirm that Crabp2a improves robustness within a narrow concentration range that optimizes a 'robustness index', integrating spatial information along the RA morphogen gradient. Exploration of signaling parameters in our models suggests that the ability of Crabp2a to transport RA to Cyp26 enzymes for degradation is a major factor in promoting robustness. These results demonstrate a previously unrecognized requirement for Crabps in RA signaling and hindbrain development, as well as a novel mechanism for stabilizing morphogen gradients despite genetic or environmental fluctuations in morphogen availability.

Feed-back regulation of disabled-2 (Dab2) p96 isoform for GATA-4 during differentiation of F9 cells

F9 embryonic carcinoma (EC) cells undergo extra-embryonic endodermal (ExE) differentiation in response to retinoic acid (RA) treatment, which induces the expression of two isoforms (p96 and p67) of the adaptor protein, Disabled-2 (Dab2). In the current study, constitutive and ectopic expression of the p96 isoform induced ExE differentiation in F9 EC cells in the absence of RA treatment via the activation of GATA-4 by p96. During the RA-induced differentiation process, Dab2 expression is induced by the GATA factors in a coherent feed-forward loop; on the other hand, we showed that p96 regulates GATA-4 in a positive feed-back manner in this study. Our results indicate that p96 Dab2 plays a key role in the ExE differentiation process.

Photoperiod regulates vitamin A and Wnt/β-catenin signaling in F344 rats

In seasonal mammals, growth, energy balance, and reproductive status are regulated by the neuroendocrine effects of photoperiod. Thyroid hormone (TH) is a key player in this response in a number of species. A neuroendocrine role for the nutritional factor vitamin A has not been considered, although its metabolic product retinoic acid (RA) regulates transcription via the same nuclear receptor family as TH. We hypothesized that vitamin A/RA plays a role in the neuroendocrine hypothalamus alongside TH signaling. Using a reporter assay to measure RA activity, we demonstrate that RA activity levels in the hypothalamus of photoperiod-sensitive F344 rats are reduced in short-day relative to long-day conditions. These lower RA activity levels can be explained by reduced expression of a whole network of RA signaling genes in the ependymal cells around the third ventricle and in the arcuate nucleus of the hypothalamus. These include genes required for uptake (Ttr, Stra6, and Crbp1), synthesis (Raldh1), receptor response (RAR), and ligand clearance (Crapb1 and Cyp26B1). Using melatonin injections into long-day rats, we show that the probable trigger of the fall in RA is melatonin. Surprisingly we also found RPE65 expression in the mammalian hypothalamus for the first time. Similar to RA signaling genes, members of the Wnt/β-catenin pathway and NMU and its receptor NMUR2 are also under photoperiodic control. Our data provide strong evidence for a novel endocrine axis, involving the nutrient vitamin A regulated by photoperiod and melatonin and suggest a role for several new players in the photoperiodic neuroendocrine response.

Expression of CRABP1, GRP, and RERG mRNA in clinically non-functioning and functioning pituitary adenomas

BACKGROUND

Pituitary tumors account for approximately 10-15% of intracranial neoplasms.

AIM

Using the cDNA microarray method, we have previously compared expression under two distinct conditions: a pool of 4 clinically non-functioning pituitary adenomas (NFPA) and a spinal cord metastasis of a non-functioning pituitary carcinoma, in order to gain biological insights into genomic changes of pituitary neoplasias. In the present study, we further investigated the mRNA expression of 3 selected genes previously described as being involved in other neoplasias based on a series of 60 pituitary adenomas: CRABP1 (cellular retinoic acid binding protein 1), GRP (gastrin-releasing peptide), and RERG (Ras-related, estrogen- regulated, growth inhibitor).

MATERIAL AND METHODS

The expression of CRABP1, GRP, and RERG was determined by quantitative RT-PCR.

RESULTS

A significantly higher content of CRABP1 mRNA was observed in NFPA compared to functioning adenomas, and PRL-secreting adenomas showed a lower expression of this gene compared to normal pituitary. A lower expression of GRP mRNA was detected in NFPA compared to normal pituitary and also to functioning adenomas. RERG mRNA was overexpressed in NFPA in comparison to functioning adenomas and to normal pituitary. Among the functioning adenomas, only the ACTH-secreting adenomas presented a higher expression of RERG mRNA compared to normal pituitary.

CONCLUSIONS

The findings of differential expression of CRABP1 in prolactinomas and of RERG in NFPA compared to normal pituitary suggests that retinoic acid and estrogen receptor, respectively, could be involved in the tumorigenesis of these adenomas subtypes. Additional studies are required to further confirm this hypothesis.

Directed neural differentiation of mouse embryonic stem cells is a sensitive system for the identification of novel Hox gene effectors

The evolutionarily conserved Hox family of homeodomain transcription factors plays fundamental roles in regulating cell specification along the anterior posterior axis during development of all bilaterian animals by controlling cell fate choices in a highly localized, extracellular signal and cell context dependent manner. Some studies have established downstream target genes in specific systems but their identification is insufficient to explain either the ability of Hox genes to direct homeotic transformations or the breadth of their patterning potential. To begin delineating Hox gene function in neural development we used a mouse ES cell based system that combines efficient neural differentiation with inducible Hoxb1 expression. Gene expression profiling suggested that Hoxb1 acted as both activator and repressor in the short term but predominantly as a repressor in the long run. Activated and repressed genes segregated in distinct processes suggesting that, in the context examined, Hoxb1 blocked differentiation while activating genes related to early developmental processes, wnt and cell surface receptor linked signal transduction and cell-to-cell communication. To further elucidate aspects of Hoxb1 function we used loss and gain of function approaches in the mouse and chick embryos. We show that Hoxb1 acts as an activator to establish the full expression domain of CRABPI and II in rhombomere 4 and as a repressor to restrict expression of Lhx5 and Lhx9. Thus the Hoxb1 patterning activity includes the regulation of the cellular response to retinoic acid and the delay of the expression of genes that commit cells to neural differentiation. The results of this study show that ES neural differentiation and inducible Hox gene expression can be used as a sensitive model system to systematically identify Hox novel target genes, delineate their interactions with signaling pathways in dictating cell fate and define the extent of functional overlap among different Hox genes.

Physiological insights into all-trans-retinoic acid biosynthesis

All-trans-retinoic acid (atRA) provides essential support to diverse biological systems and physiological processes. Epithelial differentiation and its relationship to cancer, and embryogenesis have typified intense areas of interest into atRA function. Recently, however, interest in atRA action in the nervous system, the immune system, energy balance and obesity has increased considerably, especially concerning postnatal function. atRA action depends on atRA biosynthesis: defects in retinoid-dependent processes increasingly relate to defects in atRA biogenesis. Considerable evidence indicates that physiological atRA biosynthesis occurs via a regulated process, consisting of a complex interaction of retinoid binding-proteins and retinoid recognizing enzymes. An accrual of biochemical, physiological and genetic data have identified specific functional outcomes for the retinol dehydrogenases, RDH1, RDH10, and DHRS9, as physiological catalysts of the first step in atRA biosynthesis, and for the retinal dehydrogenases RALDH1, RALDH2, and RALDH3, as catalysts of the second and irreversible step. Each of these enzymes associates with explicit biological processes mediated by atRA. Redundancy occurs, but seems limited. Cumulative data support a model of interactions among these enzymes with retinoid binding-proteins, with feedback regulation and/or control by atRA via modulating gene expression of multiple participants. The ratio apo-CRBP1/holo-CRBP1 participates by influencing retinol flux into and out of storage as retinyl esters, thereby modulating substrate to support atRA biosynthesis. atRA biosynthesis requires the presence of both an RDH and an RALDH: conversely, absence of one isozyme of either step does not indicate lack of atRA biosynthesis at the site. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

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.

Retinoic acid and its binding protein modulate apoptotic signals in hypoxic hepatocellular carcinoma cells

Hypoxia induces survival signals in hepatocellular carcinoma (HCC). We attempted to find a hypoxia-induced signal that could be used for modulating HCC cell death. Cellular retinoic acid binding protein-II (CRABP-II) expression was significantly increased in hypoxic HCC cells. Treatment with retinoic acid (RA), a ligand for CRABP-II, induced HCC cell apoptosis more effectively in hypoxia than in normoxia, whereas hypoxia-induced CRABP-II expression attenuated RA-induced apoptosis. Inhibition of CRABP-II enhanced RA-induced apoptosis and sensitized RA-resistant HCC cells to RA cytotoxicity by attenuating p42/44 MAPK and Akt activation. Therefore, RA/CRABP-II signal modulation is therapeutically implicated in infiltrative HCCs exposed to hypoxia.

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.

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.

Loss of CAK phosphorylation of RAR{alpha} mediates transcriptional control of retinoid-induced cancer cell differentiation

Although the role of the classic retinoic acid (RA)-induced genomic pathway in cancer cell differentiation is well recognized, the underlying mechanisms remain to be dissected. Retinoic acid receptor alpha (RARalpha) is a transcription factor activated by RA, and its serine 77 (RARalphaS77) is the main residue phosphorylated by the cyclin-dependent kinase (CDK)-activating kinase (CAK) complex. We report here that in both human myeloid leukemia and mouse embryonic teratocarcinoma stem cells, either RA-suppressed CAK phosphorylation of RARalpha or mutation of RARalphaS77 to alanine (RARalphaS77A) coordinates CAK-dependent G(1) arrest with cancer cell differentiation by transactivating RA-target genes. Both hypophosphorylated RARalpha and RARalphaS77A reduce binding to retinoic acid-responsive elements (RARE) in the promoters of RA-target genes while stimulating gene transcription. The enhanced transactivation and reduced RARalpha-chromatin interaction are accompanied by RARalpha dissociation from the transcriptional repressor N-CoR and are association with the coactivator NCoA-3. Such effects of decreased CAK phosphorylation of RARalphaS77 on mediating RA-dependent transcriptional control of cancer cell differentiation are examined correspondingly in both RA-resistant myeloid leukemia and embryonic teratocarcinoma stem RARalpha(-/-) cells. These studies demonstrate, for the first time, that RA couples G(1) arrest to transcriptional control of cancer cell differentiation by suppressing CAK phosphorylation of RARalpha to release transcriptional repression.-Wang, A., Alimova, I. N., Luo, P. Jong, A., Triche, T. J., Wu, L. Loss of CAK phosphorylation of RARalpha mediates transcriptional control of retinoid-induced cancer cell differentiation.

The retinoic acid binding protein CRABP2 is increased in murine models of degenerative joint disease

Introduction

Osteoarthritis (OA) is a debilitating disease with poorly defined aetiology. Multiple signals are involved in directing the formation of cartilage during development and the vitamin A derivatives, the retinoids, figure prominently in embryonic cartilage formation. In the present study, we examined the expression of a retinoid-regulated gene in murine models of OA.

Methods

Mild and moderate forms of an OA-like degenerative disease were created in the mouse stifle joint by meniscotibial transection (MTX) and partial meniscectomy (PMX), respectively. Joint histopathology was scored using an Osteoarthritis Research Society International (OARSI) system and gene expression (Col1a1, Col10a1, Sox9 and Crabp2) in individual joints was determined using TaqMan quantitative PCR on RNA from microdissected articular knee cartilage.

Results

For MTX, there was a significant increase in the joint score at 10 weeks (n = 4, p < 0.001) in comparison to sham surgeries. PMX surgery was slightly more severe and produced significant changes in joint score at six (n = 4, p < 0.01), eight (n = 4, p < 0.001) and 10 (n = 4, p < 0.001) weeks. The expression of Col1a1 was increased in both surgical models at two, four and six weeks post-surgery. In contrast, Col10a1 and Sox9 for the most part showed no significant difference in expression from two to six weeks post-surgery. Crabp2 expression is induced upon activation of the retinoid signalling pathway. At two weeks after surgery in the MTX and PMX animals, Crabp2 expression was increased about 18-fold and about 10-fold over the sham control, respectively. By 10 weeks, Crabp2 expression was increased about three-fold (n = 7, not significant) in the MTX animals and about five-fold (n = 7, p < 0.05) in the PMX animals in comparison to the contralateral control joint.

Conclusions

Together, these findings suggest that the retinoid signalling pathway is activated early in the osteoarthritic process and is sustained during the course of the disease.

Hoxa1 is required for the retinoic acid-induced differentiation of embryonic stem cells into neurons

The ability of embryonic stem (ES) cells to differentiate into different cell fates has been extensively evaluated, and several protocols exist for the generation of various types of cells from mouse and human ES cells. We used a differentiation protocol that involves embryoid body formation and all-trans-retinoic acid (RA, 5 microM) treatment (EB/5 microM RA) to test the ability of Hoxa1 null ES cells to adopt a neuronal fate. Hoxa1(-/-) ES cells, when treated in this EB/5 microM RA protocol, failed to differentiate along a neural lineage; Hoxa1(-/-) ES cells express severalfold lower levels of many neuronal differentiation markers, including nestin, beta-tubulin III, and MAP2, and conversely, higher levels of endodermal differentiation markers (i.e., Sox17, Col4a1) than wild type (Wt) cells. Reintroduction of exogenous Hoxa1, under the control of the metallothionein I promoter, into Hoxa1(-/-) ES cells restored their capacity to generate neurons. Moreover, overexpression of Sox17, a gene that regulates endodermal differentiation, in Wt ES cells resulted in endodermal differentiation and in a complete abolition of beta-tubulin III expression. Thus, Hoxa1 activity is essential for the neuronal differentiation of ES cells in the presence of all-trans-RA, and Hoxa1 may promote neural differentiation by inhibiting Sox17 expression. Pharmacological manipulation of Hoxa1 levels may provide a method for promoting neuronal differentiation for therapeutic uses. Furthermore, because mutations in the Hoxa1 gene can cause autism spectrum disorder in humans, these data also provide important mechanistic insights into the early developmental processes that may result in this disorder.

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.

Disruption of the interaction between transcriptional intermediary factor 1{beta} and heterochromatin protein 1 leads to a switch from DNA hyper- to hypomethylation and H3K9 to H3K27 trimethylation on the MEST promoter correlating with gene reactivation

Here, we identified the imprinted mesoderm-specific transcript (MEST) gene as an endogenous TIF1beta primary target gene and demonstrated that transcriptional intermediary factor (TIF) 1beta, through its interaction with heterochromatin protein (HP) 1, is essential in establishing and maintaining a local heterochromatin-like structure on MEST promoter region characterized by H3K9 trimethylation and hypoacetylation, H4K20 trimethylation, DNA hypermethylation, and enrichment in HP1 that correlates with preferential association to foci of pericentromeric heterochromatin and transcriptional repression. On disruption of the interaction between TIF1beta and HP1, TIF1beta is released from the promoter region, and there is a switch from DNA hypermethylation and histone H3K9 trimethylation to DNA hypomethylation and histone H3K27 trimethylation correlating with rapid reactivation of MEST expression. Interestingly, we provide evidence that the imprinted MEST allele DNA methylation is insensitive to TIF1beta loss of function, whereas the nonimprinted allele is regulated through a distinct TIF1beta-DNA methylation mechanism.

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.

Retinoic acid is a potential dorsalising signal in the late embryonic chick hindbrain

Background

Human retinoic acid teratogenesis results in malformations of dorsally derived hindbrain structures such as the cerebellum, noradrenergic hindbrain neurons and the precerebellar system. These structures originate from the rhombic lip and adjacent dorsal precursor pools that border the fourth ventricle roofplate. While retinoic acid synthesis is known to occur in the meninges that blanket the hindbrain, the particular sensitivity of only dorsal structures to disruptions in retinoid signalling is puzzling. We therefore looked for evidence within the neural tube for more spatiotemporally specific signalling pathways using an in situ hybridisation screen of known retinoic acid pathway transcripts.

Results

We find that there are highly restricted domains of retinoic acid synthesis and breakdown within specific hindbrain nuclei as well as the ventricular layer and roofplate. Intriguingly, transcripts of cellular retinoic acid binding protein 1 are always found at the interface between dividing and post-mitotic cells. By contrast to earlier stages of development, domains of synthesis and breakdown in post-mitotic neurons are co-localised. At the rhombic lip, expression of the mRNA for retinoic acid synthesising and catabolising enzymes is spatially highly organised with respect to the Cath1-positive precursors of migratory precerebellar neurons.

Conclusion

The late developing hindbrain shows patterns of retinoic acid synthesis and use that are distinct from the well characterised phase of rostrocaudal patterning. Selected post-mitotic populations, such as the locus coeruleus, appear to both make and break down retinoic acid suggesting that a requirement for an autocrine, or at least a highly localised paracrine signalling network, might explain its acute sensitivity to retinoic acid disruption. At the rhombic lip, retinoic acid is likely to act as a dorsalising factor in parallel with other roofplate signalling pathways. While its precise role is unclear, retinoic acid is potentially well placed to regulate temporally determined cell fate decisions within the rhombic lip precursor pool.

RARalpha-PLZF overcomes PLZF-mediated repression of CRABPI, contributing to retinoid resistance in t(11;17) acute promyelocytic leukemia

Leukemia-associated chimeric oncoproteins often act as transcriptional repressors, targeting promoters of master genes involved in hematopoiesis. We show that CRABPI (encoding cellular retinoic acid binding protein I) is a target of PLZF, which is fused to RARalpha by the t(11;17)(q23;q21) translocation associated with retinoic acid (RA)-resistant acute promyelocytic leukemia (APL). PLZF represses the CRABPI locus through propagation of chromatin condensation from a remote intronic binding element culminating in silencing of the promoter. Although the canonical, PLZF-RARalpha oncoprotein has no impact on PLZF-mediated repression, the reciprocal translocation product RARalpha-PLZF binds to this remote binding site, recruiting p300, inducing promoter hypomethylation and CRABPI gene up-regulation. In line with these observations, RA-resistant murine PLZF/RARalpha+RARalpha/PLZF APL blasts express much higher levels of CRABPI than standard RA-sensitive PML/RARalpha APL. RARalpha-PLZF confers RA resistance to a retinoid-sensitive acute myeloid leukemia (AML) cell line in a CRABPI-dependent fashion. This study supports an active role for PLZF and RARalpha-PLZF in leukemogenesis, identifies up-regulation of CRABPI as a mechanism contributing to retinoid resistance, and reveals the ability of the reciprocal fusion gene products to mediate distinct epigenetic effects contributing to the leukemic phenotype.

Dynamic and selective interactions of the transcriptional corepressor TIF1β with the heterochromatin protein HP1 isotypes during cell differentiation

Cell differentiation is a multi-step process marked by progressive silencing of gene expression through mechanisms believed to involve heterochromatin. We have previously shown that interaction between the Krüppel associated box-containing zinc finger proteins (KRAB-ZFP) corepressor TIF1beta and the heterochromatin proteins HP1 is essential for progression through differentiation of embryonal carcinoma F9 cells. This analysis clearly demonstrated the link between gene specific repressors, components of heterochromatin and cell differentiation. In mammals, there are three HP1 isotypes, HP1alpha, beta, and gamma, that appear to be involved in both eu- and heterochromatin, but whose individual functions are still poorly defined. Therefore, the aim of the present study was to determine in vivo (i) which HP1 isotypes interact with TIF1beta, (ii) in which sub-nuclear compartments these interactions occur and (iii) whether these interactions are regulated during cell differentiation. To address these questions, we established stable F9 cell lines co-expressing TIF1beta fused to the ECFP fluorophore and HP1alpha, beta, or gamma fused to the EYFP fluorophore. Using the Föster resonance energy transfer (FRET) technology, we map the physical interaction between TIF1beta-CFP and the different HP1-YFP isotypes in living F9 cells. We demonstrate that in non-differentiated cells, TIF1beta-CFP/HP1-YFP interaction occurs only within euchromatin and involves selectively HP1beta-YFP and HP1gamma-YFP, but not HP1alpha-YFP. Furthermore, in differentiated cells, TIF1beta-CFP selectively associates with HP1beta-YFP within heterochromatin, while TIF1beta-CFP/HP1gamma-YFP is exclusively present within euchromatin. No physical TIF1beta-CFP/HP1alpha-YFP interaction is detected in neither non differentiated nor differentiated cells. These results support the notion that, in vivo, HP1 isotypes have specific nonredundant functions and provide evidence for HP1beta playing an essential role in the shuttling of TIF1beta from eu- to heterochromatin during cell differentiation.

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

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

Dynamic changes in gene expression profiles of 22q11 and related orthologous genes during mouse development

22q11 deletion syndrome (22q11DS) is a developmental anomaly caused by a microdeletion on human chromosome 22q11. Although mouse models indicate that Tbx1 is the gene responsible for the syndrome, the phenotypic spectrum of del22q11 patients is complex suggesting that gene-gene and gene-environment interactions are operative in delineating the pathogenesis of 22q11DS. In order to study the regulatory effects of 22q11 haploinsufficiency during development, the expression pattern of the orthologous MM16 genes was analysed in total embryos at different stages (from 4.5 dpc to 14.5 dpc; corresponding to pharyngeal development) by using a low-density oligonucleotide microarray (the "22q11DS-chip"). This microarray consists of 39 mouse genes orthologous to the 22q11 human ones and 29 mouse target genes selected on the basis of their potential involvement in biological pathways regarding 22q11 gene products. Expression level filtering and statistical analysis identified a set of genes that was consistently differentially expressed (FC>+/-2) during specific developmental stages. These genes show a similar profile in expression (overexpression or underexpression). Quantitative real-time PCR analyses showed an identical expression pattern to that found by microarrays. A bioinformatic screening of regulative sequence elements in the promoter region of these genes, revealed the existence of conserved transcription factor binding sites (TFBSs) in co-regulated genes which are functionally active at 4.5, 8.5 and 14.5 dpc. These data are likely to be helpful in studying developmental anomalies detected in del22q11 patients.

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.

Proteomic signature of human embryonic stem cells

Human embryonic stem cells (hESC) represent a population of undifferentiated pluripotent cells with both self-renewal and multilineage differentiation characteristics. Proteomics provides a powerful approach for studying the characteristics of hESC and discovering molecular markers. We have analyzed proteome profiles of three hESC lines using 2-DE and MALDI TOF-TOF. Out of 844 spots analyzed with MALDI TOF-TOF, 685 proteins were identified of which 60 proteins were classified as the most abundant proteins on 2-D gels. A large number of proteins particularly high abundant ones were identified as chaperones, heat shock proteins, ubiquitin/proteasome, and oxidative stress responsive proteins underscoring the ability of these cells to resist oxidative stress and increase the life span. Several proteins involved in cell proliferation and differentiation were also among the highly expressed proteins. Although overall expression pattern of three hESC were similar, 54 spots changed quantitatively and 14 spots changed qualitatively among the hESC cell lines. Most of these proteins were identified as proteins involved in cell growth, metabolism and signal transduction, which may affect the self-renewal and pluripotency. To our knowledge, this study represents the first proteomic dataset for hESC and provides a better insight into the biology of hESC. Proteome maps of hESC are accessible at http://www.RoyanProteomics.ir.