Transcriptional activities of retinoic acid receptors

Nuclear GRP75 binds retinoic acid receptors to promote neuronal differentiation of neuroblastoma

Retinoic acid (RA) has been approved for the differentiation therapy of neuroblastoma (NB). Previous work revealed a correlation between glucose-regulated protein 75 (GRP75) and the RA-elicited neuronal differentiation of NB cells. The present study further demonstrated that GRP75 translocates into the nucleus and physically interacts with retinoid receptors (RARα and RXRα) to augment RA-elicited neuronal differentiation. GRP75 was required for RARα/RXRα-mediated transcriptional regulation and was shown to reduce the proteasome-mediated degradation of RARα/RXRαin a RA-dependent manner. More intriguingly, the level of GRP75/RARα/RXRα tripartite complexes was tightly associated with the RA-induced suppression of tumor growth in animals and the histological grade of differentiation in human NB tumors. The formation of GRP75/RARα/RXRα complexes was intimately correlated with a normal MYCN copy number of NB tumors, possibly implicating a favorable prognosis of NB tumors. The present findings reveal a novel function of nucleus-localized GRP75 in actively promoting neuronal differentiation, delineating the mode of action for the differentiation therapy of NB by RA.

Effects of retinoic acid and butyric acid on the expression of prestin and Gata-3 in organotypic cultures of the organ of corti of newborn rats

Prestin is the motor protein of the outer hair cells of the organ of Corti and a key factor in ensuring a high level of sensitivity of mammalian hearing. The factors that influence prestin expression are still largely unknown. We studied the effects of the application of retinoic acid, a ligand of a nuclear receptor, and of butyric acid, an inhibitor of histone deacetylase activity, on the expression of mRNA of prestin and Gata-3 in the organotypic culture of the organ of Corti of newborn rats using RT-PCR. Application of retinoic acid at concentrations of 1-50 μM results in a dose-dependent expression decrease after two days in culture. Treatment with sodium butyrate (0.5-2 mM) elevated the expression of prestin and Gata-3. Statistically significant correlations between Gata-3 and prestin mRNA levels were observed under all conditions. The data indicate that retinoid nuclear transcription factors, GATA-3 and histone acetylation/deacetylation processes may have a regulatory role to play in prestin expression.

Ascorbic Acid and gene expression: another example of regulation of gene expression by small molecules?

Ascorbic acid (vitamin C, AA) has long been considered a food supplement necessary for life and for preventing scurvy. However, it has been reported that other small molecules such as retinoic acid (vitamin A) and different forms of calciferol (vitamin D) are directly involved in regulating the expression of numerous genes. These molecules bind to receptors that are differentially expressed in the embryo and are therefore crucial signalling molecules in vertebrate development. The question is: is ascorbic acid also a signalling molecule that regulates gene expression?

We therefore present and discuss recent publications that demonstrate that AA regulates the expression of a battery of genes. We offer a clue to understanding the biochemical mechanism by which AA regulates gene expression. Finally we will discuss the question of a receptor for AA and its potential involvement in embryonic development and cell differentiation.

Regulation of keratin expression by retinoids

Vitamin A and its natural and synthetic metabolites (retinoids) affect growth and differentiation of human skin and among the genes affected by retinoids in epidermis are keratin genes. Keratins are intermediate filament proteins that have essential functions in maintaining the structural integrity of epidermis and its appendages. Their expressions are under strict control to produce keratins that are optimally adapted to their environment. In this article, retinoid regulation of keratin expression in cultured human epidermal keratinocytes and in human skin in vivo will be reviewed. The direct and indirect mechanisms involved will be discussed and novel therapeutic strategies will be proposed for utilizing retinoids in skin disorders due to keratin mutations (e.g., epidermolysis bullosa simplex and epidermolytic ichthyosis).

p53-Inducible DHRS3 is an endoplasmic reticulum protein associated with lipid droplet accumulation

The transcription factor p53 plays a critical role in maintaining homeostasis as it relates to cellular growth, proliferation, and metabolism. In an effort to identify novel p53 target genes, a microarray approach was utilized to identify DHRS3 (also known as retSDR1) as a robust candidate gene. DHRS3 is a highly conserved member of the short chain alcohol dehydrogenase/reductase superfamily with a reported role in lipid and retinoid metabolism. Here, we demonstrate that DHRS3 is an endoplasmic reticulum (ER) protein that is shuttled to the ER via an N-terminal endoplasmic reticulum targeting signal. One important function of the ER is synthesis of neutral lipids that are packaged into lipid droplets whose biogenesis occurs from ER-derived membranes. DHRS3 is enriched at focal points of lipid droplet budding where it also localizes to the phospholipid monolayer of ER-derived lipid droplets. p53 promotes lipid droplet accumulation in a manner consistent with DHRS3 enrichment in the ER. As a p53 target gene, the observations of Dhrs3 location and potential function provide novel insight into an unexpected role for p53 in lipid droplet dynamics with implications in cancer cell metabolism and obesity.

Retinoids, retinoic acid receptors, and cancer

Retinoids (i.e., vitamin A, all-trans retinoic acid, and related signaling molecules) induce the differentiation of various types of stem cells. Nuclear retinoic acid receptors mediate most but not all of the effects of retinoids. Retinoid signaling is often compromised early in carcinogenesis, which suggests that a reduction in retinoid signaling may be required for tumor development. Retinoids interact with other signaling pathways, including estrogen signaling in breast cancer. Retinoids are used to treat cancer, in part because of their ability to induce differentiation and arrest proliferation. Delivery of retinoids to patients is challenging because of the rapid metabolism of some retinoids and because epigenetic changes can render cells retinoid resistant. Successful cancer therapy with retinoids is likely to require combination therapy with drugs that regulate the epigenome, such as DNA methyltransferase and histone deacetylase inhibitors, as well as classical chemotherapeutic agents. Thus, retinoid research benefits both cancer prevention and cancer treatment.

The ATRA-dependent overexpression of the glutamate transporter EAAC1 requires RARbeta induction

The mechanisms underlying trafficking and membrane targeting of EAAC1, the rodent counterpart of the human EAAT3 carrier for anionic amino acids, are well characterized. In contrast, much less is known on the regulation of Slc1a1, the gene that encodes for the transporter. We have recently found that all-trans retinoic acid (ATRA) stimulates EAAC1 expression and anionic amino acid transport in C6 rat glioma cells. We report here that the ATRA effect on EAAC1 activity was inhibited by the specific RAR antagonist LE540 and mimicked by Am80, a RAR agonist, but not by the RXR agonist HX630. Moreover, the ATRA-dependent induction of Slc1a1 mRNA required the synthesis of a protein intermediate and was not associated with changes in the messenger half-life. ATRA treatment induced the expression of both Rarb mRNA and RARbeta protein several hours before the induction of Slc1a1, while the mRNA for RFX1, a transcription factor recently involved in Slc1a1 transcription, was unchanged. In addition, Rarb silencing markedly inhibited the ATRA-dependent increase of both Rarb and Slc1a1 mRNAs. We conclude that in C6 glioma cells the induction of Slc1a1 by ATRA requires the synthesis of RARbeta, suggesting that the receptor is involved in the regulation of the transporter gene.

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.

Synthetic retinoid AM80 inhibits Th17 cells and ameliorates experimental autoimmune encephalomyelitis

Recent evidence suggests that interleukin-17-producing CD4(+) T cells (Th17 cells) are the dominant pathogenic cellular component in autoimmune inflammatory diseases, including multiple sclerosis. It has recently been demonstrated that all-trans retinoic acid can suppress Th17 differentiation and promote the generation of Foxp3(+) regulatory T cells via retinoic acid receptor signals. Here, we investigated the effects of AM80, a synthetic retinoid with enhanced biological properties to all-trans retinoic acid, on Th17 differentiation and function and evaluated its therapeutic potential in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. AM80 treatment was more effective than all-trans retinoic acid in inhibiting Th17 differentiation in vitro. Oral administration of AM80 was protective for the early development of EAE and the down-modulation of Th17 differentiation and effector functions in vivo. Moreover, AM80 inhibited interleukin-17 production by splenic memory T cells, in vitro-differentiated Th17 cells, and central nervous system-infiltrating effector T cells. Accordingly, AM80 was effective when administered therapeutically after the onset of EAE. Continuous AM80 treatment, however, was ineffective at inhibiting late EAE symptoms despite the maintained suppression of RORgammat and interleukin-17 expression levels by central nervous system-infiltrating T cells. We reveal that continuous AM80 treatment also led to the suppression of interleukin-10 production by a distinct T cell subset that expressed both Foxp3 and RORgammat. These findings suggest that retinoid signaling regulates both inflammatory Th17 cells and Th17-like regulatory cells.

Transcriptional activity of the murine retinol-binding protein gene is regulated by a multiprotein complex containing HMGA1, p54 nrb/NonO, protein-associated splicing factor (PSF) and steroidogenic factor 1 (SF1)/liver receptor homologue 1 (LRH-1)

Retinol-binding protein (RBP4) transports retinol in the circulation from hepatic stores to peripheral tissues. Since little is known regarding the regulation of this gene, we analysed the cis-regulatory sequences of the mouse RBP4 gene. Our data show that transcription of the gene is regulated through a bipartite promoter: a proximal region necessary for basal expression and a distal segment responsible for cAMP-induction. This latter region contains several binding sites for the structural HMGA1 proteins, which are important to promoter regulation. We further demonstrate that HMGA1s play a key role in basal and cAMP-induction of Rbp4 transcription and the RBP4 and HMGA1 genes are coordinately regulated in vitro and in vivo. HMGA1 acts to recruit transcription factors to the RBP4 promoter and we specifically identified p54(nrb)/NonO and protein-associated splicing factor (PSF) as components that interact with this complex. Steroidogenic factor 1 (SF1) or the related liver receptor homologue 1 (LRH-1) are also associated with this complex upon cAMP-induction. Depletion of SF1/LRH-1 by RNA interference resulted in a dramatic loss of cAMP-induction. Collectively, our results demonstrate that basal and cAMP-induced Rbp4 transcription is regulated by a multiprotein complex that is similar to ones that modulate expression of genes of steroid hormone biosynthesis. Since genes related to glucose metabolism are regulated in a similar fashion, this suggests that Rbp4 expression may be regulated as part of a network of pathways relevant to the onset of type 2 diabetes.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

SUG-1 plays proteolytic and non-proteolytic roles in the control of retinoic acid target genes via its interaction with SRC-3

Nuclear retinoic acid receptor alpha (RARalpha) activates gene expression through dynamic interactions with coregulatory protein complexes, the assembly of which is directed by the ligand and the AF-2 domain of RARalpha. Then RARalpha and its coactivator SRC-3 are degraded by the proteasome. Recently it has emerged that the proteasome also plays a key role in RARalpha-mediated transcription. Here we show that SUG-1, one of the six ATPases of the 19 S regulatory complex of the 26 S proteasome, interacts with SRC-3, is recruited at the promoters of retinoic acid (RA) target genes, and thereby participates to their transcription. In addition, SUG-1 also mediates the proteasomal degradation of SRC-3. However, when present in excess amounts, SUG-1 blocks the activation of RARalpha target genes and the degradation of RARalpha that occurs in response to RA, via its ability to interfere with the recruitment of SRC-3 and other coregulators at the AF-2 domain of RARalpha. We propose a model in which the ratio between SUG-1 and SRC-3 is crucial for the control of RARalpha functioning. This study provides new insights into how SUG-1 has a unique role in linking the transcription and degradation processes via its ability to interact with SRC-3.

A coordinated phosphorylation cascade initiated by p38MAPK/MSK1 directs RARalpha to target promoters

The nuclear retinoic acid (RA) receptor alpha (RARalpha) is a transcriptional transregulator that controls the expression of specific gene subsets through binding at response elements and dynamic interactions with coregulators, which are coordinated by the ligand. Here, we highlighted a novel paradigm in which the transcription of RARalpha target genes is controlled by phosphorylation cascades initiated by the rapid RA activation of the p38MAPK/MSK1 pathway. We demonstrate that MSK1 phosphorylates RARalpha at S369 located in the ligand-binding domain, allowing the binding of TFIIH and thereby phosphorylation of the N-terminal domain at S77 by cdk7/cyclin H. MSK1 also phosphorylates histone H3 at S10. Finally, the phosphorylation cascade initiated by MSK1 controls the recruitment of RARalpha/TFIIH complexes to response elements and subsequently RARalpha target gene activation. Cancer cells characterized by a deregulated p38MAPK/MSK1 pathway, do not respond to RA, outlining the essential contribution of the RA-triggered phosphorylation cascade in RA signalling.

Anti-adult T-cell leukemia effects of a novel synthetic retinoid, Am80 (Tamibarotene)

Clinical trials for treatment of adult T-cell leukemia (ATL) caused by human T-cell leukemia virus type I (HTLV-I) using all-trans-retinoic acid (ATRA) have shown satisfactory therapeutic responses, although efficacies were limited. Recently, many synthetic retinoids have been developed and among them, a novel synthetic retinoid, Am80 (Tamibarotene) is an RARalpha- and RARbeta-specific retinoid expected to overcome ATRA resistance. The present study examined the inhibitory effects of Am80 on HTLV-I-infected T-cell lines and ATL cells. Am80 had negligible growth inhibition of peripheral blood mononuclear cells but marked growth inhibition of both HTLV-I-infected T-cell lines and ATL cells. Am80 arrested cells in the G1 phase of the cell cycle and induced apoptosis in HTLV-I-infected T-cell lines. It inhibited also the phosphorylation of IkappaBalpha and NF-kappaB-DNA binding, in conjunction with reduction of expression of proteins involved in the G1/S cell cycle transition and apoptosis. Am80 also inhibited the expression of JunD, resulting in suppression of AP-1-DNA binding. Furthermore, severe combined immunodeficient mice with tumors induced by subcutaneous inoculation of HTLV-I-infected T cells, responded to Am80 treatment with partial regression of tumors and no side-effects. These findings demonstrate that Am80 is a potential inhibitor of NF-kappaB and AP-1, and is a potentially useful therapeutic agent against ATL.

Identification of transcriptional regulatory cascades in retinoic acid-induced growth arrest of HepG2 cells

All-trans retinoic acid (ATRA) is a potent inducer of cell differentiation and growth arrest. Here, we investigated ATRA-induced regulatory cascades associated with growth arrest of the human hepatoma cell line HepG2. ATRA induced >2-fold changes in the expression of 402 genes including 55 linked to cell-cycle regulation, cell growth or apoptosis during 48 h treatment. Computational search predicted that 250 transcriptional regulatory factors (TRFs) could recognize the proximal upstream regions of any of the 55 genes. Expression of 61 TRF genes was significantly changed during ATRA incubation, providing many potential regulatory edges. We focused on six TRFs that could regulate many of the 55 genes and found a total of 160 potential edges in which the expression of each of the genes was changed later than the expression change of the corresponding regulator. RNAi knockdown of the selected TRFs caused perturbation of the respective potential targets. The genes showed an opposite regulation pattern by ATRA and specific siRNA treatments were selected as strong candidates for direct TRF targets. Finally, 36 transcriptional regulatory edges were validated by chromatin immunoprecipitation. These analyses enabled us to depict a part of the transcriptional regulatory cascades closely linked to ATRA-induced cell growth arrest.

Retinoic acid- and phorbol ester-induced neuronal differentiation down-regulates caveolin expression in GnRH neurons

GN11 and GT1-7 are immortalized gonadotropin-releasing hormone-positive murine cell lines exhibiting the features of immature olfactory neurons and differentiated hypothalamic neurons, respectively. Using electron microscopy and biochemical assays (RT-PCR and immunoblotting) we determined the presence of numerous caveolae invaginations and of caveolin-1 and -2 mRNAs and proteins in GN11 cells, and their absence in GT1-7 cells. The lack of caveolins in GT1-7 cells might be due to the silencing of gene transcription caused by estrogen receptor alpha whose inhibitory activity in GN11 cells could be counter-balanced by co-expression of caveolin-permissive estrogen receptor beta. To test whether the unique expression of caveolins in GN11 cells is related to their immature state, we treated GN11 cells for 24-72 h with retinoic acid or phorbol ester. Both treatments led to neuronal differentiation of GN11 cells, as shown by emission of long neuritic processes, increased expression of growth cone-associated protein-43 and appearance of voltage-gated K+ and C2+ channel currents. Concurrently, caveolins 1 and 2, and estrogen receptor beta were down-regulated in differentiated GN11, whereas estrogen receptor alpha was unaffected by differentiation. We conclude that caveolin expression in GN11 neurons is down-regulated upon differentiation and up-regulated by estrogen receptor beta.

Retinoid hyposignaling contributes to aging-related decline in hippocampal function in short-term/working memory organization and long-term declarative memory encoding in mice

An increasing body of evidence indicates that the vitamin A metabolite retinoic acid (RA) plays a role in adult brain plasticity by activating gene transcription through nuclear receptors. Our previous studies in mice have shown that a moderate downregulation of retinoid-mediated transcription contributed to aging-related deficits in hippocampal long-term potentiation and long-term declarative memory (LTDM). Here, knock-out, pharmacological, and nutritional approaches were used in a series of radial-arm maze experiments with mice to further assess the hypothesis that retinoid-mediated nuclear events are causally involved in preferential degradation of hippocampal function in aging. Molecular and behavioral findings confirmed our hypothesis. First, a lifelong vitamin A supplementation, like short-term RA administration, was shown to counteract the aging-related hippocampal (but not striatal) hypoexpression of a plasticity-related retinoid target-gene, GAP43 (reverse transcription-PCR analyses, experiment 1), as well as short-term/working memory (STWM) deterioration seen particularly in organization demanding trials (STWM task, experiment 2). Second, using a two-stage paradigm of LTDM, we demonstrated that the vitamin A supplementation normalized memory encoding-induced recruitment of (hippocampo-prefrontal) declarative memory circuits, without affecting (striatal) procedural memory system activity in aged mice (Fos neuroimaging, experiment 3A) and alleviated their LTDM impairment (experiment 3B). Finally, we showed that (knock-out, experiment 4) RA receptor beta and retinoid X receptor gamma, known to be involved in STWM (Wietrzych et al., 2005), are also required for LTDM. Hence, aging-related retinoid signaling hypoexpression disrupts hippocampal cellular properties critically required for STWM organization and LTDM formation, and nutritional vitamin A supplementation represents a preventive strategy. These findings are discussed within current neurobiological perspectives questioning the historical consensus on STWM and LTDM system partition.

The expression of retinoic acid receptors in thymus of young children and the effect of all-transretinoic acid on the development of T cells in thymus

To explore the effect of retinoic acid on the development of T cells in thymus, we examined the expression and distribution of retinoic acid receptors mRNA in thymus of young children (< or =5 years of age) and investigated the affection of all-transretinoic acid on the differentiation and maturation of the T cells in thymus and cultured in vitro. Twenty thymus samples were collected. By in situ hybridization, we found that four retinoic acid receptor genes expressed in thymus, and the levels of mRNA for retinoic acid receptor genes (detected by reverse transcriptase, fluorescent quantitative PCR), were changed with age. In in vitro culture, all-transretinoic acid promoted the maturation from CD4+CD8+ cells to CD4+ cells but inhibited the differentiation from CD4+CD8+ cells to CD8+ cells. Thus, retinoic acid likely plays important roles in T cell development in thymus and perhaps by affecting the relative expression of retinoic acid receptors.

Retinoic acid receptor beta silences human papillomavirus-18 oncogene expression by induction of de novo methylation and heterochromatinization of the viral control region

Retinoic acid receptor beta2 (RAR beta2) is often down-regulated during the multistep process to cervical cancer. In that way, its inhibitory function on the transcription factor AP-1, indispensable to maintain human papillomavirus (HPV) gene expression is relieved. Using HPV-18 positive HeLa cells as a model system, we show that ectopic expression of RAR beta2 is able to down-regulate HPV-18 transcription by selectively abrogating the binding of AP-1 to the viral regulatory region in a ligand-independent manner. This resulted in down-regulation of the viral mRNAs at the level of initiation of transcription. Decreased oncogene expression was accompanied by a re-induction of cell cycle inhibitory proteins such as p53, p21(CIP1), and p27(KIP) as well as by a cessation of cellular growth. Reduced transcriptional activity as a consequence of AP-1 reduction by selective c-Jun degradation apparently targets the HPV-18 regulatory region for epigenetic modification such as de novo methylation and nucleosomal condensation. This mechanism is otherwise counterbalanced by active and abundant viral transcription in malignant cells, because RAR beta2 itself becomes inactivated during cervical carcinogenesis. Hence, our study shows that the temporal co-existence of a potential repressor and viral oncoproteins is mutually exclusive and provides evidence of a cross-talk between a nuclear receptor, AP-1, and the epigenetic machinery.

Rapid, nongenomic actions of retinoic acid on phosphatidylinositol-3-kinase signaling pathway mediated by the retinoic acid receptor

Retinoic acid (RA) treatment of SH-SY5Y neuroblastoma cells results in activation of phosphatidylinositol-3-kinase (PI3K) signaling pathway, and this activation is required for RA-induced differentiation. Here we show that RA activates PI3K and ERK1/2 MAPK signaling pathways through a rapid, nongenomic mechanism that does not require new gene transcription or newly synthesized proteins. Activation of PI3K by RA appears to involve the classical nuclear receptor, retinoic acid receptor (RAR), on the basis of the pharmacological profile of the activation, loss, and gain of function experiments with mouse embryo fibroblast-RAR(alpha beta gamma)(L-/L-) null cells, and the physical association between liganded RAR and PI3K activity. The association of RAR with the two subunits of PI3K was differentially regulated by the ligand. Immunoprecipitation experiments performed in SH-SY5Y cells showed stable association between RARalpha and p85, the regulatory subunit of PI3K, independently of the presence of RA. In contrast, ligand administration increased the association of p110, the catalytic subunit of PI3K, to this complex. The intracellular localization of RAR proved to be relevant for PI3K activation. A chimerical RAR fusing c-Src myristylation domain to the N terminus of RARalpha (Myr-RARalpha) was targeted to plasma membrane. Transfection of Myr-RARalpha to mouse embryo fibroblast-RAR(alpha beta gamma)(L-/L-) null cells and COS-7 cells results in strong activation of the PI3K signaling pathway, although both in the absence as well in the presence of RA. Our results support a mechanism in which ligand binding to RAR would play a major role in the assembly and intracellular location of a signaling complex involving RAR and the subunits of PI3K.

Protein kinases and the proteasome join in the combinatorial control of transcription by nuclear retinoic acid receptors

Nuclear retinoic acid receptors (RARs) are transcriptional transregulators that control the expression of specific subsets of genes in a ligand-dependent manner. The basic mechanism for switching on gene transcription by agonist-liganded RARs involves their binding at specific response elements located in target genes. It also involves 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, several recent studies highlighted a fundamental role for the N-terminal domain in the transcriptional activity of RARs, following phosphorylation by the CDK7 kinase of the general transcription factor TFIIH and by p38MAPK. It has also emerged that the ubiquitin-proteasome system has a key role in RAR-mediated transcription. Here, we review new insights into how N-terminal domain and the proteasome pathway can influence the dynamics of RAR transcriptional activity.

The core component of the mammalian SWI/SNF complex SMARCD3/BAF60c is a coactivator for the nuclear retinoic acid receptor

Retinoic acid receptors (RARs) activate transcription by recruiting coactivator complexes such as histone acetyltransferases (HAT) and the mediator complex, to increase chromatin accessibility by general transcription factors and to promote transcription initiation. Indirect evidences have suggested a role for the ATP-dependent chromatin remodeling complex SWI/SNF in RAR-mediated transcription. Here we demonstrate that two highly related subunits of the core SWI/SNF complex, BAF60c1 and BAF60c2, interact physically with retinoid receptors and are coactivators for RARs. This coactivating property is dependent on SRC1 expression, showing that HATs and SWI/SNF cooperate in this retinoid-controlled transcriptional process.

Vitamin A deficiency does not influence longitudinal growth in mice

OBJECTIVE

Growth hormone (GH) stimulates longitudinal growth, directly and indirectly, through the mediation of circulating and locally produced insulin-like growth factor 1 (IGF-1). The exact role of individual vitamins in modulating GH secretion or action, and somatic growth in general, is still not completely understood. It has been suggested that vitamin A (VA) and its physiologic active metabolite retinoic acid influence longitudinal growth by promoting the differentiation of pituitary cells toward GH-secreting cells and by stimulating secretion of GH. Accordingly, epidemiologic studies have shown that short children have lower VA intake than do children with normal stature.

METHODS

To determine whether VA deficiency causes impairment of GH secretion, we have investigated the effect of a severely VA-deficient diet on growth in mice. Ten male mice born from mothers fed with VA-deficient diet since conception were maintained on a VA-deficient diet until the end of week 8 of life. Ten male mice born from mothers fed with a VA-sufficient diet and receiving a normal diet after weaning served as the control group. At the end of the study, we measured animals' weight and length, body composition, tibia and femur lengths, liver retinol and retinyl esters storages, serum IGF-1, serum thyroxine, serum GH, and pituitary GH mRNA levels.

RESULTS

Despite evidence of significant VA deficiency, we observed no effect on longitudinal growth or changes in pituitary GH mRNA, serum thyroxine, serum GH, or serum IGF-1 levels.

CONCLUSION

VA deficiency does not negatively affect longitudinal growth and pituitary GH content and action in mice.

The regulation of cyclin D1 degradation: roles in cancer development and the potential for therapeutic invention

Cyclin D1 is an important regulator of cell cycle progression and can function as a transcriptionl co-regulator. The overexpression of cyclin D1 has been linked to the development and progression of cancer. Deregulated cyclin D1 degradation appears to be responsible for the increased levels of cyclin D1 in several cancers. Recent findings have identified novel mechanisms involved in the regulation of cyclin D1 stability. A number of therapeutic agents have been shown to induce cyclin D1 degradation. The therapeutic ablation of cyclin D1 may be useful for the prevention and treatment of cancer. In this review, current knowledge on the regulation of cyclin D1 degradation is discussed. Novel insights into cyclin D1 degradation are also discussed in the context of ablative therapy. A number of unresolved questions regarding the regulation of cellular cyclin D1 levels are also addressed.

Vitamin A potentiates CpG-mediated memory B-cell proliferation and differentiation: involvement of early activation of p38MAPK

Foreign CpG-DNA from viruses and bacteria can activate memory B cells through binding to toll-like receptor 9, and this pathway has been hypothesized to be involved in the continuous activation of memory B cells ensuring life-long humoral immunity. In this study, we demonstrate that retinoic acid (RA) is a potent coactivator of this pathway in human B cells. RA enhanced the CpG-mediated proliferation of CD27(+) memory B cells, and the proliferative response was accompanied by increased immunoglobulin (Ig) secretion indicative of plasma-cell formation. The RA-induced proliferation was preceded by enhanced expression of cyclin D3, and both the expression of cyclin D3 and the induced Ig secretion were found to be dependent on IL-10. Of importance, RA increased the CpG-induced phosphorylation of ERK1/2, p38MAPK, and IkappaB as early as 30 minutes after stimulation. By using specific inhibitors, all the RA-mediated events, including proliferation, cyclin D3 expression, IL-10 secretion, and Ig secretion, were shown to be dependent on p38MAPK. Hence, we propose that RA can strengthen humoral immunity by promoting CpG-mediated stimulation of CD27(+) B cells via activation of p38MAPK resulting in increased proliferation and differentiation to Ig-secreting plasma cells.

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

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

Accumulation of retinoid X receptor-alpha in uterine leiomyomas is associated with a delayed ligand-dependent proteasome-mediated degradation and an alteration of its transcriptional activity

An alteration of the retinoid pathway can influence the development of uterine leiomyomas in animal models, and retinoids have shown efficacy in inhibiting the growth of this benign tumor both in vitro and in vivo. However, the underlying mechanisms and biological implications are unclear. The present study was based on the demonstration of an accumulation of full-length retinoid X receptor alpha (RXRalpha) in leiomyomas that was not associated with a modification of its gene expression. This accumulation was shown to increase the transcription of the RXR-responsive gene cellular retinoic acid binding protein II (CRABP-II) and to be linked to the cellular redistribution of the receptor and to its retarded degradation via the ubiquitin/proteasome pathway. Accordingly, treatment with a specific proteasome inhibitor but not with protease inhibitors strongly inhibited the degradation of full-length RXRalpha in cells deriving from both myometrium and leiomyoma, but the formation of RXRalpha/ubiquitin conjugates was differentially regulated between the two cell types. Moreover, full-length RXRalpha accumulated in leiomyomas was abnormally phosphorylated at serine/threonine residues relative to myometrial tissue. The ligand to RXRalpha, 9-cis-retinoic acid, induced the receptor breakdown in smooth muscle cells deriving from both normal and tumor tissue, whereas a MAPK-specific inhibitor was able to reduce RXRalpha levels only in leiomyoma cells. These results suggest that switching of the ubiquitin/proteasome-dependent degradation of RXRalpha by phosphorylation in leiomyomas may be responsible for the accumulation of the receptor and the consequent dysregulation of retinoic acid target genes. The ability of retinoids to modify this molecular alteration may be the rationale for their use in the treatment of leiomyomas.

Role of small bioorganic molecules in stem cell differentiation to insulin-producing cells

The use of small specific molecules has been instrumental in the modulation of stem cell proliferation and differentiation to obtain insulin-containing cells. Examples include nutrients (glucose, nicotinamide and retinoic acid), acids (butyrate), alkaloids (cyclopamine and conophylline) and pharmacological agents (LY294002 and wortmannin). These molecules, alone or in combination with specific growth factors and hormones, will likely provide key information to design specific culture media in order to obtain customized cells for implantation in diabetes. In addition, the study of such molecules will help to understand the mechanisms involved in stem cell biology as well as contribute to the design of specific drugs for islet repair and regeneration in diabetes.

Down-Regulation of the Tumor Suppressor Gene Retinoic Acid Receptor β2 through the Phosphoinositide 3-Kinase/Akt Signaling Pathway

The retinoic acid receptor β2 (RARβ2) is a potent, retinoid-inducible tumor suppressor gene, which is a critical molecular relay for retinoid actions in cells. Its down-regulation, or loss of expression, leads to resistance of cancer cells to retinoid treatment. Up to now, no primary mechanism underlying the repression of the RARβ2 gene expression, hence affecting cellular retinoid sensitivity, has been identified. Here, we demonstrate that the phosphoinositide 3-kinase/Akt signaling pathway affects cellular retinoid sensitivity, by regulating corepressor recruitment to the RARβ2 promoter. Through direct phosphorylation of the corepressor silencing mediator for retinoic and thyroid hormone receptors (SMRT), Akt stabilized RAR/SMRT interaction, leading to an increased tethering of SMRT to the RARβ2 promoter, decreased histone acetylation, down-regulation of the RARβ2 expression, and impaired cellular differentiation in response to retinoid. The phosphoinositide 3-kinase/Akt signaling pathway, an important modulator of cellular survival, has thus a direct impact on cellular retinoid sensitivity, and its deregulation may be the triggering event in retinoid resistance of cancer cells.

Role of all-trans retinoic acid in neurite outgrowth and axonal elongation

The vitamin A metabolite, all-trans retinoic acid (atRA) plays essential roles in nervous system development, including neuronal patterning, survival, and neurite outgrowth. Our understanding of how the vitamin A acid functions in neurite outgrowth comes largely from cultured embryonic neurons and model neuronal cell systems including human neuroblastoma cells. Specifically, atRA has been shown to increase neurite outgrowth from embryonic DRG, sympathetic, spinal cord, and olfactory receptor neurons, as well as dissociated cerebra and retina explants. A role for atRA in axonal elongation is also supported by a limited number of studies in vivo, in which a deficiency in retinoid signaling produced either by dietary or genetic means has been shown to alter neurite outgrowth from the spinal cord and hindbrain regions. Human neuroblastoma cells also show enhanced numbers of neurites and longer processes in response to atRA. The mechanism whereby retinoids regulate neurite outgrowth includes, but is not limited to, the regulation of the transcription of neurotrophin receptors. More recent evidence supports a role for atRA in regulating components of other signaling pathways or candidate neurite-regulating factors. Some of these effects, such as that on neuron navigator 2 (NAV2), may be direct, whereas others may be secondary to other atRA-induced changes in the cell. This review focuses on what is currently known about neurite initiation and growth, with emphasis on the manner in which atRA may influence these events.

Molecular and metabolic retinoid pathways in human amniotic membranes

Vitamin A (retinol) and its active derivatives (the retinoids) are essential for the growth and development of the mammalian fetus and placenta. The amniotic membranes are extra-embryonic structures that are indispensable for normal gestation in mammals. Although placental involvement of retinoids is clearly established, little is known about the roles of retinoids for the associated amniotic membranes. The aim of this study was to define the metabolic and molecular pathways of retinoic signaling in human fetal membranes. The expression of retinoid receptors (RARalpha, beta and RXRalpha, beta) was established at transcript and protein levels. Enzymes involved in retinoic acid generation were also detected. The enzymatic generation of functional retinoids was confirmed using specific inhibitors of retinol metabolism. Finally, the functionality of retinoid pathways was demonstrated by inducing established retinoid target gene expression. Our results clearly demonstrated that the molecular and metabolic actors of retinoic signaling pathways are functional in human fetal membranes.

Distinct Roles of the Steroid Receptor Coactivator 1 and of MED1 in Retinoid-induced Transcription and Cellular Differentiation

Retinoic acid receptors (RARs) are the molecular relays of retinoid action on transcription, cellular differentiation and apoptosis. Transcriptional activation of retinoid-regulated promoters requires the dismissal of corepressors and the recruitment of coactivators to promoter-bound RAR. RARs recruit in vitro a plethora of coactivators whose actual contribution to retinoid-induced transcription is poorly characterized in vivo. Embryonal carcinoma P19 cells, which are highly sensitive to retinoids, were depleted from archetypical coactivators by RNAi. SRC1-deficient P19 cells showed severely compromised retinoid-induced responses, in agreement with the supposed role of SRC1 as a RAR coactivator. Unexpectedly, Med1/TRAP220/DRIP205-depleted cells exhibited an exacerbated response to retinoids, both in terms transcriptional responses and of cellular differentiation. Med1 depletion affected TFIIH and cdk9 detection at the prototypical retinoid-regulated RARbeta2 promoter, and favored a higher RNA polymerase II detection in transcribed regions of the RARbeta2 gene. Furthermore, the nature of the ligand impacted strongly on the ability of RARs to interact with a given coactivator and to activate transcription in intact cells. Thus RAR accomplishes transcriptional activation as a function of the ligand structure, by recruiting regulatory complexes which control distinct molecular events at retinoid-regulated promoters.

Localization of retinaldehyde dehydrogenases and retinoid binding proteins to sustentacular cells, glia, Bowman's gland cells, and stroma: potential sites of retinoic acid synthesis in the postnatal rat olfactory organ

Work from our laboratory suggests that retinoic acid (RA) influences neuron development in the postnatal olfactory epithelium (OE). The studies reported here were carried out to identify and localize retinaldehyde dehydrogenase (RALDH) expression in postnatal rat OE to gain a better understanding of potential in vivo RA synthesis sites in this continuously regenerating tissue. RALDH 1, 2, and 3 mRNAs were detected in postnatal rat olfactory tissue by RT-PCR analysis, but RALDH 1 and 2 transcripts were predominant. RALDH 1 immunoreactivity was localized to sustentacular cells in the OE and to Bowman's gland cells, and GFAP(+)/p75(-) olfactory ensheathing cells (OECs) in the underlying lamina propria (LP). RALDH 2 did not colocalize with RALDH 1, but appeared to be expressed in GFAP(-)/RALDH 1(-) OECs as well as in unidentified structures in the LP. Cellular RA binding protein (CRABP II) colocalized with RALDH 1. Cellular retinol/retinaldehyde binding protein (CRBP I) was localized to RALDH 1(+) sites in the OE and LP and RALDH 2(+) sites, primarily surrounding nerve fiber bundles in the LP. Vitamin A deficiency altered RALDH 1, but not RALDH 2 protein expression. The isozymes and binding proteins exhibited random variability in levels and areas of expression both within and between animals. These findings support the hypothesis that RA is synthesized in the postnatal OE (catalyzed by RALDH 1) and underlying LP (differentially catalyzed by RALDH 1 and RALDH 2) at sites that could influence the development, maturation, targeting, and/or turnover of olfactory receptor neurons throughout the olfactory organ.

Phosphorylation by PKA potentiates retinoic acid receptor alpha activity by means of increasing interaction with and phosphorylation by cyclin H/cdk7

Nuclear retinoic acid receptors (RARs) work as ligand-dependent heterodimeric RAR/retinoid X receptor transcription activators, which are targets for phosphorylations. The N-terminal activation function (AF)-1 domain of RARalpha is phosphorylated by the cyclin-dependent kinase (cdk) 7/cyclin H complex of the general transcription factor TFIIH and the C-terminal AF-2 domain by the cAMP-dependent protein kinase A (PKA). Here, we report the identification of a molecular pathway by which phosphorylation by PKA propagates cAMP signaling from the AF-2 domain to the AF-1 domain. The first step is the phosphorylation of S369, located in loop 9-10 of the AF-2 domain. This signal is transferred to the cyclin H binding domain (at the N terminus of helix 9 and loop 8-9), resulting in enhanced cyclin H interaction and, thereby, greater amounts of RARalpha phosphorylated at S77 located in the AF-1 domain by the cdk7/cyclin H complex. This molecular mechanism relies on the integrity of the ligand-binding domain and the cyclin H binding surface. Finally, it results in higher DNA-binding efficiency, providing an explanation for how cAMP synergizes with retinoic acid for transcription.

NIK-333 inhibits growth of human T-cell leukemia virus type I-infected T-cell lines and adult T-cell leukemia cells in association with blockade of nuclear factor-kappaB signal pathway

Adult T-cell leukemia (ATL) is caused by human T-cell leukemia virus type I (HTLV-I) and remains incurable. NIK-333, a novel synthetic retinoid, prevents the recurrence of human hepatoma after surgical resection of primary tumors. We explored the effects of NIK-333 on HTLV-I-infected T-cell lines and ATL cells. NIK-333 inhibited cell proliferation, induced G1 arrest, and resulted in massive apoptosis in all tested HTLV-I-infected T-cell lines and ATL cells, whereas little effect was observed on normal peripheral blood mononuclear cells. NIK-333 treatment decreases the levels of cyclin D1, cyclin D2, cIAP2, and XIAP proteins. Further analysis showed that NIK-333 inactivated nuclear factor-kappaB in HTLV-I-infected T-cell lines. In animal studies, treatment with NIK-333 (100 mg/kg given orally every other day) produced partial inhibition of growth of tumors of a HTLV-I-infected T-cell line transplanted s.c. in severe combined immunodeficient mice. Our results indicate that NIK-333 is a potentially useful therapeutic agent for patients with ATL.

Vitamin A combined with retinoic acid increases retinol uptake and lung retinyl ester formation in a synergistic manner in neonatal rats

Vitamin A (VA) is stored in tissues predominantly as retinyl esters (REs), which provide substrate for the production of bioactive retinoids. Retinoic acid (RA), a principal metabolite, has been shown to induce postnatal lung development. To better understand lung RE storage, we compared VA (given as retinyl palmitate), RA, and a nutrient-metabolite combination, VARA, given orally on postnatal days 5-7, for their ability to increase lung RE in neonatal rats. VARA increased lung RE significantly [ approximately 14, 2.4, 2.1, and <1 nmol/g for VARA, VA, RA, and control (C), respectively; P < 0.001]; the increase by VARA was more than additive compared with the effects of VA and RA alone. Lung histology and morphometry were unchanged. In a 6 h metabolic study, providing [(3)H]retinol with VARA, compared with VA or C, increased the uptake of newly absorbed (3)H by 3-fold, indicating that VARA stimulated the uptake of [(3)H]retinol and its retention as [(3)H]RE in neonatal lungs. After cessation of VARA, lung RE remained increased for 9 d afterward, through the period of alveolar development. In conclusion, VARA, a 10:1 nutrient-metabolite combination, increased lung RE significantly compared with VA alone and could be a promising therapeutic option for enhancing the delivery of VA to the lungs.

P38MAPK-dependent phosphorylation and degradation of SRC-3/AIB1 and RARalpha-mediated transcription

Nuclear retinoic acid (RA) receptors (RARs) activate gene expression through dynamic interactions with coregulators in coordination with the ligand and phosphorylation processes. Here we show that during RA-dependent activation of the RARalpha isotype, the p160 coactivator pCIP/ACTR/AIB-1/RAC-3/TRAM-1/SRC-3 is phosphorylated by p38MAPK. SRC-3 phosphorylation has been correlated to an initial facilitation of RARalpha-target genes activation, via the control of the dynamics of the interactions of the coactivator with RARalpha. Then, phosphorylation inhibits transcription via promoting the degradation of SRC-3. In line with this, inhibition of p38MAPK markedly enhances RARalpha-mediated transcription and RA-dependent induction of cell differentiation. SRC-3 phosphorylation and degradation occur only within the context of RARalpha complexes, suggesting that the RAR isotype defines a phosphorylation code through dictating the accessibility of the coactivator to p38MAPK. We propose a model in which RARalpha transcriptional activity is regulated by SRC-3 through coordinated events that are fine-tuned by RA and p38MAPK.

Cyclin H binding to the RARalpha activation function (AF)-2 domain directs phosphorylation of the AF-1 domain by cyclin-dependent kinase 7

The transcriptional activity of nuclear retinoic acid receptors (RARs), which act as RAR/retinoid X receptor (RXR) heterodimers, depends on two activation functions, AF-1 and AF-2, which are targets for phosphorylations and synergize for the activation of retinoic acid target genes. The N-terminal AF-1 domain of RARalpha is phosphorylated at S77 by the cyclin-dependent kinase (cdk)-activating kinase (CAK) subcomplex (cdk7/cyclin H/MAT1) of the general transcription factor TFIIH. Here, we show that phosphorylation of S77 governing the transcriptional activity of RARalpha depends on cyclin H binding at a RARalpha region that encompasses loop 8-9 and the N-terminal tip of helix 9 of the AF-2 domain. We propose a model in which the structural constraints of this region control the architecture of the RAR/RXR/TFIIH complex and therefore the efficiency of RARalpha phosphorylation by cdk7. To our knowledge, this study provides the first example of a cooperation between the AF-2 and AF-1 domains of RARs through a kinase complex.

Antimetastatic gene expression profiles mediated by retinoic acid receptor beta 2 in MDA-MB-435 breast cancer cells

Background

The retinoic acid receptor beta 2 (RARβ2) gene modulates proliferation and survival of cultured human breast cancer cells. Previously we showed that ectopic expression of RARβ2 in a mouse xenograft model prevented metastasis, even in the absence of the ligand, all-trans retinoic acid. We investigated both cultured cells and xenograft tumors in order to delineate the gene expression profiles responsible for an antimetastatic phenotype.

Methods

RNA from MDA-MB-435 human breast cancer cells transduced with RARβ2 or empty retroviral vector (LXSN) was analyzed using Agilent Human 1A Oligo microarrays. The one hundred probes with the greatest differential intensity (p < 0.004, jointly) were determined by selecting the top median log ratios from eight-paired microarrays. Validation of differences in expression was done using Northern blot analysis and quantitative RT-PCR (qRT-PCR). We determined expression of selected genes in xenograft tumors.

Results

RARβ2 cells exhibit gene profiles with overrepresentation of genes from Xq28 (p = 2 × 10^-8), a cytogenetic region that contains a large portion of the cancer/testis antigen gene family. Other functions or factors impacted by the presence of exogenous RARβ2 include mediators of the immune response and transcriptional regulatory mechanisms. Thirteen of fifteen (87%) of the genes evaluated in xenograft tumors were consistent with differences we found in the cell cultures (p = 0.007).

Conclusion

Antimetastatic RARβ2 signalling, direct or indirect, results in an elevation of expression for genes such as tumor-cell antigens (CTAG1 and CTAG2), those involved in innate immune response (e.g., RIG-I/DDX58), and tumor suppressor functions (e.g., TYRP1). Genes whose expression is diminished by RARβ2 signalling include cell adhesion functions (e.g, CD164) nutritional or metabolic processes (e.g., FABP6), and the transcription factor, JUN.

β-arrestin 2 modulates the activity of nuclear receptor RAR β2 through activation of ERK2 kinase

The activity of retinoid receptors activity can be regulated by various extracellular stimuli. In an effort to understand the molecular basis for this phenomenon, the role of beta-arrestins was investigated. Beta-Arrestins constitute a class of proteins involved in the internalization of agonist-activated receptors. They have also been linked to MAPK activation suggesting a direct involvement in signaling cascades. Here, we report that beta-arrestin 2 stimulates the transcriptional activation of the retinoid RAR and RXR receptors. Of all the retinoid receptors, the RAR beta2 subtype showed the strongest sensitivity to beta-arrestin 2 action. Interestingly, this event requires the presence of the MAP kinase ERK2, but not that of JNK or P38. Site-directed mutagenesis showed that Ser 22 and Leu 217 are critical residues of the RAR beta2 receptor through which beta-arrestin 2 effects are mediated. More importantly, we demonstrate that the induction of PC12 growth inhibition by Nerve Growth Factor is indeed dependent upon RAR beta2 transcriptional activation in a beta-arrestin 2- and ERK2-dependent manner.

The proliferating cell nuclear antigen regulates retinoic acid receptor transcriptional activity through direct protein-protein interaction

Retinoic acid receptors (RARs) interact, in a ligand-dependent fashion, with many coregulators that participate in a wide spectrum of biological responses, ranging from embryonic development to cellular growth control. The transactivating function of these ligand-inducible transcription factors reside mainly, but not exclusively, in their ligand-binding domain (AF2), which recruits or dismiss coregulators in a ligand-dependent fashion. However, little is known about AF2-independent function(s) of RARs. We have isolated the proliferating cell nuclear antigen (PCNA) as a repressor of RAR transcriptional activity, able to interact with an AF2-crippled RAR. The N-terminus of PCNA interacts directly with the DNA-binding domain of RAR, and PCNA is recruited to a retinoid-regulated promoter in intact cells. This interaction affects the transcriptional response to retinoic acid in a promoter-specific manner, conferring an unanticipated role to PCNA in transcriptional regulation. Our findings also suggest a role for RAR as a factor coordinating DNA transcription and repair.