Retinoid X receptors

Nuclear receptors in ovarian cancer: changing paradigms in cancer therapeutics

Ovarian cancer (OVC) is one of the most common causes of cancer-related deaths in women worldwide. Despite advancements in detection and therapy, the prognosis of OVC remains poor due to late diagnosis and the lack of effective therapeutic options at advanced stages. Therefore, a better understanding of the biology underlying OVC is essential for the development of effective strategies for early detection and targeted therapies. Nuclear receptors (NRs) are a superfamily of 48 transcription factors that, upon binding to their specific ligand, play a vital role in regulating various cellular processes such as growth, development, metabolism, and homeostasis. Accumulating evidence from several studies has shown that their aberrant expression is associated with multiple human diseases. Numerous NRs have shown significant effects in the development of various cancers, including OVC. This review summarizes the recent findings on the role of NRs in OVC, as well as their potential as prognostic and therapeutic markers. Further, the basic structure and signaling mechanism of NRs have also been discussed briefly. Moreover, this review highlights their cellular and molecular mechanisms in chemoresistance and chemosensitization. Further, the clinical trials targeting NRs for the treatment of OVC have also been discussed.

PPARs in Clinical Experimental Medicine after 35 Years of Worldwide Scientific Investigations and Medical Experiments

This year marks the 35th anniversary of Professor Walter Wahli's discovery of the PPARs (Peroxisome Proliferator-Activated Receptors) family of nuclear hormone receptors. To mark the occasion, the editors of the scientific periodical Biomolecules decided to publish a special issue in his honor. This paper summarizes what is known about PPARs and shows how trends have changed and how research on PPARs has evolved. The article also highlights the importance of PPARs and what role they play in various diseases and ailments. The paper is in a mixed form; essentially it is a review article, but it has been enriched with the results of our experiments. The selection of works was subjective, as there are more than 200,000 publications in the PubMed database alone. First, all papers done on an animal model were discarded at the outset. What remained was still far too large to describe directly. Therefore, only papers that were outstanding, groundbreaking, or simply interesting were described and briefly commented on.

Comparison of the clinical efficacy of topical tretinoin 0.05% cream and tacrolimus 0.1% ointment plus iontophoresis in the management of palmoplantar psoriasis

BACKGROUND

Palmoplantar psoriasis (PPP) is a localized variant of psoriasis that may be resistant to topical therapy, owing to the poor penetrability of topical agents at this anatomical site. Modalities that enhance localized cutaneous delivery of drugs could help to solve this problem. Iontophoresis is one such procedure that augments transdermal drug delivery, thus enabling better and expeditious therapeutic outcomes.

OBJECTIVE

To compare the therapeutic efficacy and safety of iontophoresis with tretinoin 0.05% cream and tacrolimus 0.1% ointment in treating patients with PPP.

METHODS

Sixty patients with PPP (28 males and 32 females, age range 8-76 years) were enrolled and randomly assigned to one of two groups comprising 30 patients each. One group (12 males and 18 females) received iontophoresis with tretinoin 0.05% cream; the other (16 males and 14 females) received iontophoresis treatment with tacrolimus 0.1% ointment. Both groups received treatment weekly from baseline until 4 weeks and then fortnightly at weeks 6 and 8. Clinical images were taken at each visit and improvement of psoriasis was evaluated using the erythema, scaling, induration and fissuring (ESIF) score. The percentage reduction in ESIF score was also assessed on completion of treatment and the grade of improvement noted for each patient.

RESULTS

Twenty-seven patients in the iontophoresis with tretinoin 0.05% cream group and 29 in the iontophoresis treatment with tacrolimus 0.1% ointment group completed the study. The mean (SD) ESIF score in the former decreased significantly from 8.7 (2) at baseline to 3.2 (1.7) at the study endpoint (P < 0.001). Similarly, in the latter group, there was a substantial reduction in mean (SD) ESIF score from 8.2 (1.9) at baseline to 3.3 (1.1) at the study end (P < 0.001). No significant adverse effects were encountered in either treatment arm.

CONCLUSIONS

Iontophoresis using tretinoin and tacrolimus was found to be effective and safe for the treatment of PPP. Although iontophoresis with tretinoin showed slightly better results than with tacrolimus, these were not statistically significant.

Attempts to Improve Lipophilic Drugs' Solubility and Bioavailability: A Focus on Fenretinide

The development of numerous drugs is often arrested at clinical testing stages, due to their unfavorable biopharmaceutical characteristics. It is the case of fenretinide (4-HPR), a second-generation retinoid, that demonstrated promising in vitro cytotoxic activity against several cancer cell lines. Unfortunately, response rates in early clinical trials with 4-HPR did not confirm the in vitro findings, mainly due to the low bioavailability of the oral capsular formulation that was initially developed. Capsular 4-HPR provided variable and insufficient drug plasma levels attributable to the high hepatic first-pass effect and poor drug water solubility. To improve 4-HPR bioavailability, several approaches have been put forward and tested in preclinical and early-phase clinical trials, demonstrating generally improved plasma levels and minimal systemic toxicities, but also modest antitumor efficacy. The challenge is thus currently still far from being met. To redirect the diminished interest of pharmaceutical companies toward 4-HPR and promote its further clinical development, this manuscript reviewed the attempts made so far by researchers to enhance 4-HPR bioavailability. A comparison of the available data was performed, and future directions were proposed.

All-trans retinoic acid induces durable tumor immunity in IDH-mutant gliomas by rescuing transcriptional repression of the CRBP1-retinoic acid axis

Diffuse gliomas are epigenetically dysregulated, immunologically cold, and fatal tumors characterized by mutations in isocitrate dehydrogenase (IDH). Although IDH mutations yield a uniquely immunosuppressive tumor microenvironment, the regulatory mechanisms that drive the immune landscape of IDH mutant (IDHm) gliomas remain unknown. Here, we reveal that transcriptional repression of retinoic acid (RA) pathway signaling impairs both innate and adaptive immune surveillance in IDHm glioma through epigenetic silencing of retinol binding protein 1 (RBP1) and induces a profound anti-inflammatory landscape marked by loss of inflammatory cell states and infiltration of suppressive myeloid phenotypes. Restorative retinoic acid therapy in murine glioma models promotes clonal CD4 + T cell expansion and induces tumor regression in IDHm, but not IDH wildtype (IDHwt), gliomas. Our findings provide a mechanistic rationale for RA immunotherapy in IDHm glioma and is the basis for an ongoing investigator-initiated, single-center clinical trial investigating all-trans retinoic acid (ATRA) in recurrent IDHm human subjects.

Disease-causing mutations in genes encoding transcription factors critical for photoreceptor development

Photoreceptor development of the vertebrate visual system is controlled by a complex transcription regulatory network. OTX2 is expressed in the mitotic retinal progenitor cells (RPCs) and controls photoreceptor genesis. CRX that is activated by OTX2 is expressed in photoreceptor precursors after cell cycle exit. NEUROD1 is also present in photoreceptor precursors that are ready to specify into rod and cone photoreceptor subtypes. NRL is required for the rod fate and regulates downstream rod-specific genes including the orphan nuclear receptor NR2E3 which further activates rod-specific genes and simultaneously represses cone-specific genes. Cone subtype specification is also regulated by the interplay of several transcription factors such as THRB and RXRG. Mutations in these key transcription factors are responsible for ocular defects at birth such as microphthalmia and inherited photoreceptor diseases such as Leber congenital amaurosis (LCA), retinitis pigmentosa (RP) and allied dystrophies. In particular, many mutations are inherited in an autosomal dominant fashion, including the majority of missense mutations in CRX and NRL. In this review, we describe the spectrum of photoreceptor defects that are associated with mutations in the above-mentioned transcription factors, and summarize the current knowledge of molecular mechanisms underlying the pathogenic mutations. At last, we deliberate the outstanding gaps in our understanding of the genotype-phenotype correlations and outline avenues for future research of the treatment strategies.

RXR – centralny regulator wielu ścieżek sygnałowych w organizmie

Receptory jądrowe (NRs) tworzą największą nadrodzinę czynników transkrypcyjnych, które odgrywają ważną rolę w regulacji wielu procesów biologicznych. Receptor kwasu 9-cis-retinowego (RXR) wydaje się odgrywać szczególną rolę wśród tej grupy białek, a to ma związek z jego zdolnością do tworzenia dimerów z innymi NRs. Ze względu na kontrolę ekspresji wielu genów, RXR stanowi bardzo dobry cel licznych terapii. Nieprawidłowości w szlakach modulowanych przez RXR są powiązane m.in. z chorobami neurodegeneracyjnymi, otyłością, cukrzycą, a także nowotworami. Istnieje wiele związków mogących regulować aktywność transkrypcyjną RXR. Jednak obecnie dopuszczonych do użytku klinicznego jest tylko kilka z nich. Retinoidy normalizują wzrost i różnicowanie komórek skóry i błon śluzowych, ponadto działają immunomodulująco oraz przeciwzapalnie. Stąd są stosowane przede wszystkim w chorobach skóry i w terapii niektórych chorób nowotworowych. W artykule przedstawiono ogólne wiadomości na temat RXR, jego budowy, ligandów i mechanizmu działania oraz potencjalnej roli w terapii nowotworów i zespołu metabolicznego.

A splicing factor phosphorylated by protein kinase A is increased in HL60 cells treated with retinoic acid

Retinoic acid (RA) induces the differentiation of human promyelocytic leukemia HL60 cells into granulocytic cells and inhibits proliferation. Certain of actions of RA are mediated by RA nuclear receptors that regulate gene expression. However, it is also known that direct protein modification by RA (retinoylation) can occur. One such retinoylated protein in HL60 cells is a regulatory subunit of protein kinase A (PKA), which is increased in the nucleus following RA treatment and which then increases phosphorylation of other nuclear proteins. However, a complete understanding of which nuclear proteins are phosphorylated is lacking. In the current study, we employed mass spectrometry to identify one of the PKA-phosphorylated proteins as a serine/arginine-rich splicing factor 1 (SF2, SRSF1). We found that RA treatment increased the level of PKA-phosphorylated SF2 but decreased the level of SF2. While SF2 regulates myelogenous cell leukemia-1 (Mcl-1, anti-apoptotic factor), RA treatment reduced the level of Mcl-1L (full-length Mcl-1 long) and increased the level of Mcl-1S (Mcl-1 short; a short splicing variant of the Mcl-1). Furthermore, treatment with a PKA inhibitor reversed these effects on Mcl-1 and inhibited RA-induced cell differentiation. In contrast, treatment with a Mcl-1L inhibitor enhanced RA-induced cell differentiation. These results indicate that RA activates PKA in the nucleus, increases phosphorylation of SF2, raises levels of Mcl-1S and lowers levels of Mcl-1L, resulting in the induction of differentiation. RA-modified PKA may play an important role in inducing cell differentiation and suppressing cell proliferation.

Development and validation of bexarote by bioanalytical methods using liquid chromatography-tandem mass spectroscopy (LC-MS/MS)

Background

The aim of this study was to develop and validate accurate and precise UPLC method with tandem mass spectrometry (Waters) for the determination of bexarotene in human plasma using bexarotene D4 as internal standard (IS).

Results

The retention time of bexarotene was 2.75 ± 0.30 min. The method was validated with respect to system suitability, linearity, accuracy, precision, matrix effect, auto sampler carryover test, and recovery. Linearity was found to be 1.04 to 351.93 μg/mL. LOQQC, LQC, INTQC, MQC, and HQC were found to be 1.0550, 2.7800, 25.2700, 131.61, and 263.23 respectively. The mean percentage recovery was found to be 95.72%

Conclusion

The bioanalytical method, a selective and sensitive liquid chromatography-mass spectrometry method to quantitate bexarotene in K2EDTA human plasma over the concentration range 1.0440 to 351.9320 ng/mL, was successfully validated. This method is suitable for sample analysis to support bioequivalence/bioavailability and/or pharmacokinetic studies involving formulations of bexarotene.

A Brief Review of the Effects of Vitamin D on Multiple Sclerosis

Multiple sclerosis (MS) is characterized as an autoimmune disease affecting the central nervous system. It is one of the most common neurological disorders in young adults. Over the past decades, increasing evidence suggested that hypovitaminosis D is a contributing factor to the risk of developing MS. From different risk factors contributing to the development of MS, vitamin D status is of particular interest since it is not only a modifiable risk factor but is also associated with MS disease activity. MS patients with lower serum vitamin D concentrations were shown to have higher disease activity. However, this finding does not demonstrate causality. In this regard, prospective vitamin D supplementation studies missed statistical significance in its primary endpoints but showed promising results in secondary outcome measures or post hoc analyses. An explanation for missed primary endpoints may be underpowered trials. Besides vitamin D supplementation as a potential add-on to long-term immunotherapeutic treatment, a recent laboratory study of our group pointed toward a beneficial effect of vitamin D to improve the efficacy of glucocorticoids in relapse therapy. In the following article, we will briefly review the effects of vitamin D on MS by outlining its effects on the immune and nervous system and by reviewing the association between vitamin D and MS risk as well as MS disease activity. We will also review the effects of vitamin D supplementation on MS risk and MS disease activity.

Low-dose isotretinoin therapy and blood lipid abnormality: A case series with sixty patients

Background:

prolonged isotretinoin therapy for various skin diseases causes change in various parameters of lipid profile. Aim: to find out the effect of low dose isotretinoin on various parameters of lipid profile.

Methods:

A clinic based observational study with 60 patients of various skin diseases carried out in a skin outpatient department of a tertiary care hospital in eastern India. Patients were prescribed isotretinoin for relevant indications. Baseline lipid profile was checked and repeated after three months. The results were compared with the baseline lipid levels.

Result:

Out of 60 patients (male-32, female-28) hyperlipidemia was present in 25% (15 out of 60) patients at the end of three month's therapy. Among the hyperlipidemia, hyper triglyceridemia was the commonest (16.67%, 10 out of 60 patients) followed by elevation of VLDL (11.67%, 7 out of 60 patients), elevation of LDL (10%, 6 out of 60), hypercholesterolemia (5%, 3 out of 60). Combination of hyperlipidemia was present in 11.67% patients. Among the male patients 28.12%, while in females 21.43% had hyperlipidemia at the end of the study. Among the hyperlipidemic females, hypertriglyceridemia was present in 83.3% (5 out of 6) of patients, while in male it was 55.5% (5 out of 9 patients).

Conclusion:

Low dose Isotretinoin therapy causes variable rise in various parameters of lipid profile. It should be used cautiously in patient with risk factors of metabolic syndrome and frequent monitoring of serum lipid profile is needed.

Perspectives of differentiation therapies of acute myeloid leukemia: the search for the molecular basis of patients' variable responses to 1,25-dihydroxyvitamin d and vitamin d analogs

The concept of differentiation therapy of cancer is ~40 years old. Despite many encouraging results obtained in laboratories, both in vitro and in vivo studies, the only really successful clinical application of differentiation therapy was all-trans-retinoic acid (ATRA)-based therapy of acute promyelocytic leukemia (APL). ATRA, which induces granulocytic differentiation of APL leukemic blasts, has revolutionized the therapy of this disease by converting it from a fatal to a curable one. However, ATRA does not work for other acute myeloid leukemias (AMLs). Since 1,25-dihydroxyvitamin D₃ (1,25D) is capable of inducing monocytic differentiation of leukemic cells, the idea of treating other AMLs with vitamin D analogs (VDAs) was widely accepted. Also, some types of solid cancers responded to in vitro applied VDAs, and hence it was postulated that VDAs can be used in many clinical applications. However, early clinical trials in which cancer patients were treated either with 1,25D or with VDAs, did not lead to conclusive results. In order to search for a molecular basis of such unpredictable responses of AML patients toward VDAs, we performed ex vivo experiments using patient’s blast cells. Experiments were also performed using 1,25D-responsive and 1,25D-non-responsive cell lines, to study their mechanisms of resistance toward 1,25D-induced differentiation. We found that one of the possible reasons might be due to a very low expression level of vitamin D receptor (VDR) mRNA in resistant cells, which can be increased by exposing the cells to ATRA. Our considerations concerning the molecular mechanism behind the low VDR expression and its regulation by ATRA are reported in this paper.

Developmental origins of a novel gut morphology in frogs

Phenotypic variation is a prerequisite for evolution by natural selection, yet the processes that give rise to the novel morphologies upon which selection acts are poorly understood. We employed a chemical genetic screen to identify developmental changes capable of generating ecologically relevant morphological variation as observed among extant species. Specifically, we assayed for exogenously applied small molecules capable of transforming the ancestral larval foregut of the herbivorous Xenopus laevis to resemble the derived larval foregut of the carnivorous Lepidobatrachus laevis. Appropriately, the small molecules that demonstrate this capacity modulate conserved morphogenetic pathways involved in gut development, including downregulation of retinoic acid (RA) signaling. Identical manipulation of RA signaling in a species that is more closely related to Lepidobatrachus, Ceratophrys cranwelli, yielded even more similar transformations, corroborating the relevance of RA signaling variation in interspecific morphological change. Finally, we were able to recover the ancestral gut phenotype in Lepidobatrachus by performing a reverse chemical manipulation to upregulate RA signaling, providing strong evidence that modifications to this specific pathway promoted the emergence of a lineage-specific phenotypic novelty. Interestingly, our screen also revealed pathways that have not yet been implicated in early gut morphogenesis, such as thyroid hormone signaling. In general, the chemical genetic screen may be a valuable tool for identifying developmental mechanisms that underlie ecologically and evolutionarily relevant phenotypic variation.

Regulation of intestinal epithelial permeability by tight junctions

The gastrointestinal epithelium forms the boundary between the body and external environment. It effectively provides a selective permeable barrier that limits the permeation of luminal noxious molecules, such as pathogens, toxins, and antigens, while allowing the appropriate absorption of nutrients and water. This selective permeable barrier is achieved by intercellular tight junction (TJ) structures, which regulate paracellular permeability. Disruption of the intestinal TJ barrier, followed by permeation of luminal noxious molecules, induces a perturbation of the mucosal immune system and inflammation, and can act as a trigger for the development of intestinal and systemic diseases. In this context, much effort has been taken to understand the roles of extracellular factors, including cytokines, pathogens, and food factors, for the regulation of the intestinal TJ barrier. Here, I discuss the regulation of the intestinal TJ barrier together with its implications for the pathogenesis of diseases.

Intake of vitamins D and A and calcium and risk of non-Hodgkin lymphoma: San Francisco Bay Area population-based case-control study

Several nutrients identified as potentially cancer protective have been inconsistently associated with non-Hodgkin lymphoma (NHL) risk. Dietary history data, including use of vitamin supplements, were collected using a semiquantitative food frequency questionnaire administered during in-person interviews with 4,133 participants (2,052 cases, 2,081 controls) in a San Francisco Bay Area population-based case-control study. Data were used to determine the association of intake levels of vitamins D and A and calcium with risk of NHL and NHL subtypes. Odds ratios (OR) and 95% confidence intervals (CI) were computed as estimates of relative risk using adjusted unconditional logistic regression. Increasing vitamin D intake from food and supplements was positively associated with NHL risk in men (5th quintile: OR = 1.6, 95% CI = 1.0-2.4, P(trend) = 0.07) and with diffuse large B-cell lymphoma (DLBCL) in women and men (5th quintile: OR = 1.6, 95% CI = 1.0-2.5, P(trend) = 0.02); that was largely due to the effect in men (P(trend) = 0.03). These results do not support a strong role for vitamin D intake with NHL risk, with the exception of a potential association for DLBCL risk in men. Our results should be interpreted conservatively until further investigation in larger pooled studies can be conducted to better assess the role of vitamin D intake in lymphomagenesis.

Effect of marginal vitamin A deficiency during pregnancy on retinoic acid receptors and N-methyl-D-aspartate receptor expression in the offspring of rats

This study examined whether pregnancy-related marginal vitamin A deficiency (MVAD) influences postnatal development of retinoic acid receptors (RARs) and N-methyl-D-aspartate (NMDA) receptor subunit 1 (NR1) in hippocampus of rat pups. Sixteen female rats were randomized equally into control and MVAD groups. Dams and pups were fed with either a normal control diet or one deficient in vitamin A. Eight female pups in each group were killed at 1 day, 2 weeks, 4 weeks and 8 weeks after birth, respectively. Serum retinol levels were monitored. The messenger RNA (mRNA) and protein expressions and subcellular localization of RARα, RARβ and NR1 in postnatal hippocampus were detected. At 1 day, 2 weeks and 8 weeks after birth, serum retinol levels in the MVAD group were significantly lower than those in the control group. Results of Morris water maze test at 7 weeks of age showed that spatial learning and memory in the MVAD group were affected. Vitamin A deficiency resulted in decreased mRNA levels of RARα, RARβ and NR1 (P<.05). The protein level of RARα and NR1 in the MVAD group was lower than that of the control group (P<.05). There was no significant difference in RARβ between the groups (P>.05). A mass of RARα and NR1 colocalized in hippocampal cell cytoplasm on postnatal day 1. Our data suggested that vitamin A deficiency in pregnancy may affect the postnatal expression of RARα and NR1, affecting learning and memory function in the hippocampus and synaptic plasticity of the calcium signaling pathway.

A review of acitretin for the treatment of psoriasis

BACKGROUND

Acitretin is an oral retinoid that is approved for the treatment of psoriasis. It is unique compared to other systemic therapies for psoriasis such as methotrexate and cyclosporine in that it is not immunosuppressive. It is, therefore, safe for use in psoriasis patients with a history of chronic infection such as HIV, hepatitis B, hepatitis C or malignancy who have a contraindication to systemic immunosuppressive therapy and require systemic therapy because topical therapy is inadequate and they are unable to commit to phototherapy. Acitretin is one of the treatments of choice for pustular psoriasis. Even though acitretin is less effective as a monotherapy for chronic plaque psoriasis, combination therapy with other agents, especially UVB or psoralen plus UVA phototherapy, can enhance efficacy.

OBJECTIVE

To provide an updated review of the safety and efficacy of acitretin in the treatment for psoriasis.

METHODS

Literature review of journal articles from 2008 to 2009 since the last review of acitretin evaluated medical literature from 2005 to 2008.

RESULTS/CONCLUSION

Acitretin is an effective systemic therapy for psoriasis and is generally well tolerated at low doses for long-term use. If monotherapy with acitretin is inadequate, it can be used in combination with other treatments, particularly UVB phototherapy, to increase efficacy.

Demonstration of basic proteins that bind retinoic acid in the human myeloid leukemia cell line HL60

Retinoic acid (RA) has a variety of biological effects in mammalian cells and tissues. It is well known that RA is a potent anticancer agent that induces differentiation of leukemia cells as well as inhibiting cell growth. The current study examined HL60 proteins using anti-RA monoclonal antibodies (ARMAs) and found that some RA-binding proteins may be histones. These proteins eluted in the void volume fractions following Mono Q anion exchange chromatography and immunostained with ARMAs. These ARMAs showed specific binding to the proteins in a saturable manner that depended on antibody concentration. Certain of these proteins co-migrated with histones on one-dimensional polyacrylamide gel electrophoresis. It was also found that histones isolated from HL60 cells treated with RA also immunostained with ARMAs. These results indicate that basic proteins, including histones, may be bound to RA covalently in HL60 cells and that RA-binding histones may play significant roles in the transcriptional regulation of genes by RA.

Cytokeratins 16 and 10 bind to retinoic acid covalently in skin tissue of mice

BACKGROUND

Retinoic acid (RA) has various biological effects in mammalian cells and tissues. In epidermal cells, RA is an inhibitor of differentiation to the squamous phenotype. The molecular mechanisms underlying the effects of RA on epidermal cells and other cell types are mediated by RA nuclear receptors and retinoylation (acylation by RA) of proteins.

OBJECTIVES

To understand the components responsible for RA effects via RA nuclear receptors and retinoylation.

METHODS

We examined for the first time RA-binding proteins in mouse skin in vivo by immunoblotting using anti-RA monoclonal antibodies and identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry.

RESULTS

We identified eight RA-binding proteins in the skin of hairless mice that were increased by topical RA treatment. Three of these proteins were identified as cytokeratin 10, cytokeratin 16 and serum albumin.

CONCLUSION

These results raise the possibility that RA binding to cytokeratins in vivo may be involved in the effect of RA on keratinocytes in mouse skin.

Association between the peroxisome proliferator-activated receptor gamma Pro12Ala variant and haplotype and pancreatic cancer in a high-risk cohort of smokers: a pilot study

OBJECTIVES

The Pro12Ala variant in the peroxisome proliferator-activated receptor gamma (PPARG) gene has been associated with diabetes and several cancers. This pilot study tested for the association between Pro12Ala and pancreatic cancer risk in a high-risk sample of smokers.

METHODS

A nested case-control study was conducted in 83 incident cases of pancreatic cancer and 166 matched controls originally recruited into a cohort chemoprevention study of lung cancer. Associations between Pro12Ala and pancreatic cancer risk were measured using conditional logistic regression.

RESULTS

Carriers of the G allele (Ala) of the Pro12Ala variant had a borderline increased relative risk of pancreatic cancer compared with homozygous carriers of the C allele (Pro), with an odds ratio of 1.79 (95% confidence interval [CI], 0.96-3.33; P=0.06). Among subjects randomized to high-dose vitamin A, the odds ratio was 2.80 (95% CI, 1.16-6.74; P=0.02) versus 1.20 (95% CI, 0.45-3.23; P=0.71) in the placebo group. A haplotype including Pro12Ala was also significantly associated with pancreatic cancer risk in all subjects and in subjects randomized to vitamin A.

CONCLUSIONS

This analysis presents the first evidence that PPARG may be associated with pancreatic cancer risk, and this candidate gene should be investigated in future, larger studies.

Association between the peroxisome proliferator-activated receptor gamma Pro12Ala variant and haplotype and pancreatic cancer in a high-risk cohort of smokers: a pilot study

OBJECTIVES

The Pro12Ala variant in the peroxisome proliferator-activated receptor gamma (PPARG) gene has been associated with diabetes and several cancers. This pilot study tested for the association between Pro12Ala and pancreatic cancer risk in a high-risk sample of smokers.

METHODS

A nested case-control study was conducted in 83 incident cases of pancreatic cancer and 166 matched controls originally recruited into a cohort chemoprevention study of lung cancer. Associations between Pro12Ala and pancreatic cancer risk were measured using conditional logistic regression.

RESULTS

Carriers of the G allele (Ala) of the Pro12Ala variant had a borderline increased relative risk of pancreatic cancer compared with homozygous carriers of the C allele (Pro), with an odds ratio of 1.79 (95% confidence interval [CI], 0.96-3.33; P=0.06). Among subjects randomized to high-dose vitamin A, the odds ratio was 2.80 (95% CI, 1.16-6.74; P=0.02) versus 1.20 (95% CI, 0.45-3.23; P=0.71) in the placebo group. A haplotype including Pro12Ala was also significantly associated with pancreatic cancer risk in all subjects and in subjects randomized to vitamin A.

CONCLUSIONS

This analysis presents the first evidence that PPARG may be associated with pancreatic cancer risk, and this candidate gene should be investigated in future, larger studies.

Association of leptin, 25-hydroxyvitamin D, and parathyroid hormone in women

Vitamin D deficiency and adipocytokines have been implicated in the etiology of aging-related diseases such as cancer, osteoporosis, and diseases of the cardiovascular system. The association between elevated parathyroid hormone (PTH) and low 25-hydroxyvitamin D (25-OH-VitD) in plasma is used to define vitamin D deficiency, yet their associated mechanistic pathways are unclear. Utilizing plasma samples from women in a previous intervention study, we measured plasma 25-OH-VitD, leptin, adiponectin, PTH, and lipid levels. We observed strong positive associations for leptin with PTH, gamma -tocopherol, and body mass index (BMI) and inverse associations with 25-OH-VitD and adiponectin. Although commonly accepted that vitamin D deficiency causes hyperparathyroidism, we observed this association primarily in individuals with elevated leptin levels, suggesting that leptin may be an important modifier of this effect consistent with 25-OH-VitD-mediated inhibition of leptin. Leptin was highly correlated with the BMI/25-OH-VitD ratio (r = 0.80; P < 0.0001), consistent with a model in which BMI (adiposity) and 25-OH-VitD are the primary determinants of circulating leptin and PTH levels. This model may explain the failure of some studies to observe elevated PTH in vitamin D deficient adolescents and provides important insight into epidemiological studies exploring the associations of these individual biomarkers with chronic disease risk and mortality.

Effect of nutritional vitamin A deficiency on lipid metabolism in the rat heart: Its relation to PPAR gene expression

OBJECTIVE

We studied the effect of dietary vitamin A deprivation on lipid composition and mRNA expression of regulatory enzymes involved in rat heart energetic lipid metabolism and its relation to the expression of peroxisome proliferator-activated receptor (PPAR) and retinoid X receptor (RXR) genes.

METHODS

Male Wistar 21-d-old rats were fed for 3 mo with a vitamin A-free diet (vitamin A-deficient group) and the same diet plus 8 mg of retinol palmitate per kilogram of diet (control group). One group of deficient animals received the control diet 15 d before sacrifice (vitamin A-refed group). Heart ventricular and mitochondrial lipid contents were determined. Lipid synthesis was measured using radioactive precursors and acetyl-coenzyme A carboxylase and mitochondrial carnitine palmitoyltransferase-I (CPT-I) activities using radioactive substrates. Fatty acid composition of mitochondrial phospholipids was analyzed by gas-liquid chromatography. Heart expression of acetyl-coenzyme A carboxylase, CPT-I, PPAR-alpha, PPAR-beta, RXR-alpha, and RXR-beta was assessed by reverse transcriptase polymerase chain reaction, and CPT-I expression was also measured by real-time polymerase chain reaction.

RESULTS

Vitamin A deficiency induced changes in heart ventricular lipid content and synthesis. Mitochondrial cardiolipin decreased and the proportion of phospholipids/saturated fatty acids increased. Heart activity and mRNA levels of CPT-I and expression of PPAR-alpha and PPAR-beta genes were enhanced, whereas acetyl-coenzyme A carboxylase activity diminished. Furthermore, vitamin A deficiency decreased heart mRNA levels of RXRs. Vitamin A refeeding reverted most of the observed changes.

CONCLUSION

Lipid metabolism is significantly modified in hearts of vitamin A-deficient rats. Alteration of mitochondrial energetic processes by modifying the activity and gene expressions of the regulatory enzymes is associated with a high PPAR expression induced by vitamin A deprivation.

An updated review of acitretin--a systemic retinoid for the treatment of psoriasis

BACKGROUND

Acitretin is a systemic retinoid used for psoriasis. It normalizes cellular differentiation and maturation and is also used as a chemopreventive agent against cutaneous malignancies. However, it is not used frequently because of its side-effect profile.

OBJECTIVE

Safety and efficacy of acitretin was evaluated as monotherapy, as well as in combination with other systemic agents.

METHODS

Medical literature from 2005 to 2008 was reviewed. The most scientifically rigorous clinical trials were selected for Psoriasis Area and Severity Index. Articles were limited to case reports or clinical trials, human subjects and English language journals.

RESULTS/CONCLUSION

Acitretin is effective as monotherapy for pustular and erythrodermic psoriasis and for plaque psoriasis (with other systemic agents). Side effects of acitretin use occur more commonly with high doses. Hence, acitretin is safe and effective for psoriasis.

The neurobiology of retinoic acid in affective disorders

Current models of affective disorders implicate alterations in norepinephrine, serotonin, dopamine, and CRF/cortisol; however treatments targeted at these neurotransmitters or hormones have led to imperfect resolution of symptoms, suggesting that the neurobiology of affective disorders is incompletely understood. Until now retinoids have not been considered as possible contributors to affective disorders. Retinoids represent a family of compounds derived from vitamin A that perform a large number of functions, many via the vitamin A product, retinoic acid. This signaling molecule binds to specific retinoic acid receptors in the brain which, like the glucocorticoid and thyroid hormone receptors, are part of the nuclear receptor superfamily and regulate gene transcription. Research in the field of retinoic acid in the CNS has focused on the developing brain, in part stimulated by the observation that isotretinoin (13-cis retinoic acid), an isomer of retinoic acid used in the treatment of acne, is highly teratogenic for the CNS. More recent work has suggested that retinoic acid may influence the adult brain; animal studies indicated that the administration of isotretinoin is associated with alterations in behavior as well as inhibition of neurogenesis in the hippocampus. Clinical evidence for an association between retinoids and depression includes case reports in the literature, studies of health care databases, and other sources. A preliminary PET study in human subjects showed that isotretinoin was associated with a decrease in orbitofrontal metabolism. Several studies have shown that the molecular components required for retinoic acid signaling are expressed in the adult brain; the overlap of brain areas implicated in retinoic acid function and stress and depression suggest that retinoids could play a role in affective disorders. This report reviews the evidence in this area and describes several systems that may be targets of retinoic acid and which contribute to the pathophysiology of depression.

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.

Interference of endocrine disrupting chemicals with aromatase CYP19 expression or activity, and consequences for reproduction of teleost fish

Many natural and synthetic compounds present in the environment exert a number of adverse effects on the exposed organisms, leading to endocrine disruption, for which they were termed endocrine disrupting chemicals (EDCs). A decrease in reproduction success is one of the most well-documented signs of endocrine disruption in fish. Estrogens are steroid hormones involved in the control of important reproduction-related processes, including sexual differentiation, maturation and a variety of others. Careful spatial and temporal balance of estrogens in the body is crucial for proper functioning. At the final step of estrogen biosynthesis, cytochrome P450 aromatase, encoded by the cyp19 gene, converts androgens into estrogens. Modulation of aromatase CYP19 expression and function can dramatically alter the rate of estrogen production, disturbing the local and systemic levels of estrogens. In the present review, the current progress in CYP19 characterization in teleost fish is summarized and the potential of several classes of EDCs to interfere with CYP19 expression and activity is discussed. Two cyp19 genes are present in most teleosts, cyp19a and cyp19b, primarily expressed in the ovary and brain, respectively. Both aromatase CYP19 isoforms are involved in the sexual differentiation and regulation of the reproductive cycle and male reproductive behavior in diverse teleost species. Alteration of aromatase CYP19 expression and/or activity, be it upregulation or downregulation, may lead to diverse disturbances of the above mentioned processes. Prediction of multiple transcriptional regulatory elements in the promoters of teleost cyp19 genes suggests the possibility for several EDC classes to affect cyp19 expression on the transcriptional level. These sites include cAMP responsive elements, a steroidogenic factor 1/adrenal 4 binding protein site, an estrogen-responsive element (ERE), half-EREs, dioxin-responsive elements, and elements related to diverse other nuclear receptors (peroxisome proliferator activated receptor, retinoid X receptor, retinoic acid receptor). Certain compounds including phytoestrogens, xenoestrogens, fungicides and organotins may modulate aromatase CYP19 activity on the post-transcriptional level. As is shown in this review, diverse EDCs may affect the expression and/or activity of aromatase cyp19 genes through a variety of mechanisms, many of which need further characterization in order to improve the prediction of risks posed by a contaminated environment to teleost fish population.

Psoriasis of the nail: anatomy, pathology, clinical presentation, and a review of the literature on therapy

Psoriasis is a chronic skin disease that affects millions of people throughout the world. Even though cutaneous signs and symptoms are the most common clinical manifestations, the nails can be involved in up to 50% of cases, and their involvement remains an important yet often overlooked aspect of the disease. There is a broad spectrum of nail dystrophies associated with psoriasis, ranging from the common pitting and loosening of the nail plate to the less frequent discoloration and splinter hemorrhages seen in the nail bed. This article discusses the normal anatomy and embryology of the nail unit as well as the current understanding of the pathogenesis of the disease. It also provides an extensive review of the existing literature with respect to psoriatic nail therapy. Although there have been many recent advances in the treatment of the cutaneous form of the disease-most notably in the field of immunotherapies-the options for nail psoriasis are far more limited. While a number of treatment alternatives currently exist for nail disease, the general paucity of clear evidence regarding these choices often makes it difficult to select the most efficient, safe, and optimal treatment for the patient. Even though the current literature has shown some support for the use of topical, intralesional, radiation, systemic, and combination therapies for nail psoriasis, the available studies lack sufficient power to extrapolate a standardized therapeutic regimen. Therefore, until better-documented evidence validating the treatment options emerges within the literature, clinicians and patients are left with a vague and relatively unproven approach to psoriatic nail disease.

Spatial and temporal expression of glucocorticoid, retinoid, and thyroid hormone receptors is not altered in lungs of congenital diaphragmatic hernia

The degree of associated pulmonary hypoplasia and persistent pulmonary hypertension are major determination factors for survival in congenital diaphragmatic hernia (CDH) patients. Glucocorticoids, thyroid hormone, and vitamin A have been shown to be involved in human lung development. To determine their therapeutic potential in hypoplastic lungs of CDH patients, the temporal and spatial expression of glucocorticoid receptor, thyroid hormone receptors, retinoic acid receptors, and retinoid X receptors were evaluated in lungs of CDH patients, hypoplastic lungs from other causes, and normal lungs. As a series of supportive experiments, the expressions of these receptors were analyzed in lungs of nitrofen-induced CDH rats. Immunohistochemistry (human and rat) and in situ hybridization (rat) demonstrated no overt difference between CDH, hypoplastic, and control lungs, either in the localization nor the timing of the first expression of all analyzed receptors. The mRNA expression of each receptor was detected in all human CDH lungs by quantitative PCR. Our results suggest that, as far as receptors are concerned, hypoplastic lungs of fetuses and newborns with CDH are potentially as responsive to glucocorticoids, thyroid hormone, and retinoic acid as the lungs of normal children.

The synthesis of tethered ligand dimers for PPARgamma-RXR protein heterodimers

Heterodimeric compounds based on tethering the PPARgamma agonist Rosiglitazone to the RXR ligand Targretin have been prepared.

A review of acitretin, a systemic retinoid for the treatment of psoriasis

Acitretin is a second-generation, systemic retinoid that has been approved for the treatment of psoriasis since 1997. It can be considered one of the treatments of choice for pustular and erythrodermic psoriasis. However, the efficacy of acitretin as a monotherapy for plaque psoriasis is less, although it is often used in combination therapy with other systemic psoriasis therapies, especially ultraviolet B or psoralen plus ultraviolet A phototherapy, to increase efficacy. Such combination treatments may potentially minimise toxicity by using lower doses of each of the two agents. All systemic retinoids are potent teratogens. The most common side effects are mucocutanous effects such as cheilitis and hair loss, which are dose-dependent. Acitretin is not immunosuppressive, is generally safe for long-term use and has no time limit restrictions, which makes it useful in combination therapy and for maintenance therapy.

Formation of retinoylated proteins from retinoyl-CoA in rat tissues

Retinoylation (acylation of proteins by retinoic acid) is considered as one mechanism of retinoic acid (RA) action occurring in cells in vitro and in vivo. Previously, our studies showed that in rat tissues the formation of retinoyl-CoA from RA, the first step of retinoylation, required ATP, CoA and MgCl(2). In the current study, we examined whether the transfer of retinoyl-CoA into proteins, the second step of retinoylation, occurs in rat tissues. [(3)H]-Labeled-retinoyl-CoA bound covalently to proteins in rat liver, kidney, testis, and brain. The levels of incorporation of retinoyl-CoA into proteins were higher in vitamin A-deficient rats than in normal ones. The formation of retinoylated proteins depended on the incubation time, and the concentrations of retinoyl-CoA and homogenate. The reaction was suppressed by fatty acyl-CoAs and palmitic acid, but not by arachidonic acid. The Vmax and Km values for retinoyl-CoA in the formation of retinoylated proteins using a crude liver extract were estimated to be 2,597.3 pmol/min/mg protein and 9.5 x 10(-5) M, respectively. Retinoylated proteins formed from retinoyl-CoA, including a 17 kDa protein exhibiting high radioactivity, disappeared in the presence of 2-mercaptoethanol, indicating that RA was linked to the proteins through a thioester bond. These results demonstrate that retinoylation in rat tissues occurs via retinoyl-CoA formed from RA. This process may play a significant physiological role in cells.

Retinoic acid signaling and function in the adult hippocampus

Retinoic acid (RA) is an essential growth factor, derived from vitamin A, which controls growth by activating specific receptors that are members of the nuclear receptor family of transcriptional regulators. Its function in control of growth and differentiation in the embryonic CNS has been extensively investigated, but a role for RA in the mature brain has only recently become apparent. Although the adult CNS has much less capacity for change compared to the embryonic CNS, a limited amount of flexibility, referred to as neural plasticity, still exists. It is these processes that RA influences in the adult brain, including long-term potentiation and neurogenesis. The hippocampus is a brain region dependent upon neural plasticity for its function in learning and memory, and this review focuses on the roles that RA may play in regulating these processes in the adult.

Polychlorinated biphenyls disturb differentiation of normal human neural progenitor cells: clue for involvement of thyroid hormone receptors

Polychlorinated biphenyls (PCBs) are ubiquitous environmental chemicals that accumulate in adipose tissues over the food chain. Epidemiologic studies have indicated that PCBs influence brain development. Children who are exposed to PCBs during development suffer from neuropsychologic deficits such as a lower full-scale IQ (intelligence quotient), reduced visual recognition memory, and attention and motor deficits. The mechanisms leading to these effects are not fully understood. It has been speculated that PCBs may affect brain development by interfering with thyroid hormone (TH) signaling. Because most of the data are from animal studies, we established a model using primary normal human neural progenitor (NHNP) cells to determine if PCBs interfere with TH-dependent neural differentiation. NHNP cells differentiate into neurons, astrocytes, and oligodendrocytes in culture, and they express a variety of drug metabolism enzymes and nuclear receptors. Like triiodothyronine (T3), treatment with the mono-ortho-substituted PCB-118 (2,3',4,4 ,5-pentachlorobiphenyl; 0.01-1 microM) leads to a dose-dependent increase of oligodendrocyte formation. This effect was congener specific, because the coplanar PCB-126 (3,3',4,4 ,5-pentachlorobiphenyl) had no effect. Similar to the T3 response, the PCB-mediated effect on oligodendrocyte formation was blocked by retinoic acid and the thyroid hormone receptor antagonist NH-3. These results suggest that PCB-118 mimics T3 action via the TH pathway.

Redifferentiation therapy for thyroid cancer

Thyroid tumorigenesis and carcinogenesis accompany progressive loss of thyroid-specific differentiated functions. Some thyroid cancers are or become dedifferentiated, and they become refractory to efficacy-proven conventional therapies such as radioiodine ablation therapy and thyrotropin (TSH)-suppressive therapy. Redifferentiation therapy by either redifferentiating agents or gene transfer of differentiation-related genes may retard tumor growth and make tumors respond to conventional therapies.

Retinoic acid-mediated phospholipase A2 signaling in the nucleus

Retinoic acid modulates a wide variety of biological processes including proliferation, differentiation, and apoptosis. It interacts with specific receptors in the nucleus, the retinoic acid receptors (RARs). The molecular mechanism by which retinoic acid mediates cellular differentiation and growth suppression in neural cells remains unknown. However, retinoic acid-induced release of arachidonic acid and its metabolites may play an important role in cell proliferation, differentiation, and apoptosis. In brain tissue, arachidonic acid is mainly released by the action of phospholipase A2 (PLA2) and phospholipase C (PLC)/diacylglycerol lipase pathways. We have used the model of differentiation in LA-N-1 cells induced by retinoic acid. The treatment of LA-N-1 cells with retinoic acid produces an increase in phospholipase A2 activity in the nuclear fraction. The pan retinoic acid receptor antagonist, BMS493, can prevent this increase in phospholipase A2 activity. This suggests that retinoic acid-induced stimulation of phospholipase A2 activity is a retinoic acid receptor-mediated process. LA-N-1 cell nuclei also have phospholipase C and phospholipase D (PLD) activities that are stimulated by retinoic acid. Selective phospholipase C and phospholipase D inhibitors block the stimulation of phospholipase C and phospholipase D activities. Thus, both direct and indirect mechanisms of arachidonic acid release exist in LA-N-1 cell nuclei. Arachidonic acid and its metabolites markedly affect the neurite outgrowth and neurotransmitter release in cells of neuronal and glial origin. We propose that retinoic acid receptors coupled with phospholipases A2, C and D in the nuclear membrane play an important role in the redistribution of arachidonic acid in neuronal and non-nuclear neuronal membranes during differentiation and growth suppression. Abnormal retinoid metabolism may be involved in the downstream transcriptional regulation of phospholipase A2-mediated signal transduction in schizophrenia and Alzheimer disease (AD). The development of new retinoid analogs with diminished toxicity that can cross the blood-brain barrier without harm and can normalize phospholipase A2-mediated signaling will be important in developing pharmacological interventions for these neurological disorders.

Regulation of retinoid X receptor responsive element-dependent transcription in T lymphocytes by Ser/Thr phosphatases: functional divergence of protein kinase C (PKC)theta; and PKC alpha in mediating calcineurin-induced transactivation

T lymphocyte activation signals regulate the expression and transactivation function of retinoid X receptor (RXR) alpha through an interplay of complex signaling cascades that are not yet fully understood. We show that cellular Ser/Thr protein phosphatases (PPs) play an important role in mediating these processes. Inhibitors specific for PP1 and PP2A decreased basal expression of RXR alpha RNA and protein in T lymphocyte leukemia Jurkat cells and prevented activation-induced RXR alpha accumulation in these cells. In addition, these inhibitors attenuated the RXR responsive element (RXRE)-dependent transcriptional activation in transient transfection assays. Inhibitors of calcineurin (CN), by contrast, did not have any effect on the basal RXR alpha expression and even augmented activation-induced RXR alpha expression. Expression of a dominant-active (DA) mutant of CN together with a DA mutant of protein kinase C (PKC)theta;, a novel PKC isoform, significantly increased RXRE-dependent transcription. Expression of catalytically inactive PKC theta; or a dominant-negative mutant of PKC theta; failed to synergize with CN and did not increase RXRE-dependent transcription. Expression of a DA mutant of PKC alpha or treatment with PMA was found to attenuate PKC theta; and CN synergism. We conclude that PP1, PP2A, and CN regulate levels and transcriptional activation function of RXR alpha in T cells. In addition, CN synergizes with PKC theta; to induce RXRE-dependent activation, a cooperative function that is antagonized by the activation of the conventional PKC alpha isoform. Thus, PKC theta; and PKC alpha may function as positive and negative modulators, respectively, of CN-regulated RXRE-dependent transcription during T cell activation.

A functional retinoic acid response element (RARE) is present within the distal promoter of the rat gonadotropin-releasing hormone (GnRH) gene

We previously demonstrated that all-trans-retinoic acid (all-trans-RA) regulates gonadotropin-releasing hormone (GnRH) release and gene expression in rat hypothalamic fragments and GT1-1 neuronal cells. Promoter analysis of rat GnRH gene revealed that the enhancing effect of all-trans-RA on GnRH transcription is mediated by cis-elements localized within --1640/--1438 of the rat GnRH promoter. In the present study, we attempted to localize functional retinoic acid response elements (RAREs) within the all-trans-RA-responsive region of the rat GnRH gene. Sequence analysis showed that there exist three putative repeats of AGGTCA-related sequences (--1637/--1617, --1579/--1562, and --1494/--1470) within this promoter sequence. Among them, only the --1494/--1470 sequence could compete the specific binding of GT1-1 nuclear extracts to the consensus RARE (direct repeat of AGGTCA with a 5-bp spacer, DR-5) and vice versa in electrophoretic mobility shift assays. In addition, like consensus RARE, the --1494/--1470 sequence could confer all-trans-RA responsiveness when inserted into the upstream region of SV40 promoter. Treatment of GT1-1 cells with all-trans- or 9-cis-RA increased the specific bindings of GT1-1 nuclear extracts to the consensus RARE and to the --1494/--1470 sequence while not affecting the specific binding to the cAMP response element (CRE). Both retinoids induced RARbeta gene expression in GT1-1 cells. The --1494/--1470 sequence (5'-TCTTAGGACTCTGTGTGACCTAAGA) is similar to the direct repeat of TGACCT (complementary sequence of AGGTCA) with a spacer of 5 bp (i.e. DR-5 in the reverse orientation). A mutation of the second core recognition motif of the --1494/--1470 sequence to a more divergent one from consensus RARE (from TGACCT to TTACAT) abolished the responsiveness to all-trans-RA, whereas a mutation of first core recognition motif to a more TGACCT-like sequence (from AGGACT to TGAACT) increased the responsiveness to all-trans-RA. These results indicate that the --1494/--1470 sequence is indeed a weak but functional RARE of the modified DR-5 type. Taken together, these data indicate that all-trans-RA enhances GnRH transcription via functional RARE present in the distal region of the GnRH promoter.

9-cis-Retinoic acid represses transcription of the gonadotropin-releasing hormone (GnRH) gene via proximal promoter region that is distinct from all-trans-retinoic acid response element

We previously reported an enhancing effect of all-trans-retinoic acid (all-trans-RA) on gonadotropin-releasing hormone (GnRH) gene transcription via distal promoter elements of the rat GnRH gene. The present study examined the effects of another biologically active retinoid, 9-cis-retinoic acid (9-cis-RA), on GnRH transcription in GT1-1 cells. Similar to the action of all-trans-RA, 9-cis-RA significantly induced the luciferase activity of the strong retinoic acid response element (RARE) reporter construct, 3X beta RARE-Luc, by about 60-fold, indicating that GT1-1 cells are also responsive to 9-cis-RA. In contrast to the stimulatory effect of all-trans-RA on GnRH transcription, 9-cis-RA inhibited the GnRH promoter activity in a dose- and time-dependent manner. Significant inhibition by 9-cis-RA required at least an 18 h treatment and a further decrease of GnRH promoter-driven luciferase activity was observed up to 48 h of incubation. Accordingly, GnRH mRNA levels were decreased by 9-cis-RA treatment in a similar dose- and time-related manner, indicating that mouse GnRH expression is also negatively regulated by 9-cis-RA. Transient transfections of serial deletion constructs of the rat GnRH promoter revealed that the --230/--110 sequence of the rat GnRH promoter is responsible for 9-cis-RA-induced inhibition of GnRH transcription. Within this region, however, no consensus retinoid X receptor response element was found. To gain insights into the role of retinoid X receptors (RXRs) in GnRH expression, we examined the effects of RXR overexpression on GnRH transcriptional activity. Interestingly, co-transfection of RXR overexpression vectors significantly increased the GnRH promoter-driven luciferase activity, while treatment with 9-cis-RA not only nullified the enhancing effect of RXR overexpression but also decreased the basal GnRH promoter-driven luciferase activity by 50% compared to vehicle-treated controls. This implies that RXRs in the absence of its cognate ligand 9-cis-RA contribute to the maintenance of basal GnRH gene transcription. Northern blot analysis revealed that 9-cis-RA, but not all-trans-RA, down-regulated RXR beta expression in GT1-1 cells, suggesting that one possible mechanism of 9-cis-RA-induced repression involves down-regulation of RXR expression. In conclusion, the present study clearly demonstrates that 9-cis-RA is a negative regulator of GnRH gene expression in immortalized GnRH neurons.