Vitamin A regulation of gene expression: molecular mechanism of a prototype gene

Effect of vitamin A deficiency on the immune response in obesity

Obesity has been associated with low-grade systemic inflammation and with micronutrient deficiencies. Obese individuals have been found to have lower vitamin A levels and lower vitamin A intake compared with normal-weight individuals. Vitamin A plays a major role in the immune function, including innate immunity, cell-mediated immunity and humoral antibody immunity. It has also been recognised recently that vitamin A has important regulatory functions. Vitamin A status has an important effect on the chronic inflammatory response. Vitamin A deficiency increases a T-helper type 1 (Th1) response, elevates levels of pro-inflammatory cytokines, increases the expression of leptin, resistin and uncoupling proteins (UCP) and promotes adipogenesis. The effect of vitamin A deficiency on obesity might be increasing the risk of fat deposition and also the risk of chronic inflammation associated with obesity. Supplementation with vitamin A in vitro and in animal models has been found to reduce concentrations of adipocytokines, such as leptin and resistin. In conclusion, vitamin A deficiency increases a Th1 response in the presence of obesity and thus, increases the inflammatory process involved in chronic inflammation and fat deposition. The metabolism of leptin and other adipocytokines may play a critical role in the effect of vitamin A deficiency in the inflammatory response observed in obesity.

Hepatic metabolism of retinoids and disease associations

The liver is the most important tissue site in the body for uptake of postprandial retinoid, as well as for retinoid storage. Within the liver, both hepatocytes and hepatic stellate cells (HSCs) are importantly involved in retinoid metabolism. Hepatocytes play an indispensable role in uptake and processing of dietary retinoid into the liver, and in synthesis and secretion of retinol-binding protein (RBP), which is required for mobilizing hepatic retinoid stores. HSCs are the central cellular site for retinoid storage in the healthy animal, accounting for as much as 50-60% of the total retinoid present in the entire body. The liver is also an important target organ for retinoid actions. Retinoic acid is synthesized in the liver and can interact with retinoid receptors which control expression of a large number of genes involved in hepatic processes. Altered retinoid metabolism and the accompanying dysregulation of retinoid signaling in the liver contribute to hepatic disease. This is related to HSCs, which contribute significantly to the development of hepatic disease when they undergo a process of cellular activation. HSC activation results in the loss of HSC retinoid stores and changes in extracellular matrix deposition leading to the onset of liver fibrosis. An association between hepatic disease progression and decreased hepatic retinoid storage has been demonstrated. In this review article, we summarize the essential role of the liver in retinoid metabolism and consider briefly associations between hepatic retinoid metabolism and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

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.

TRIM32 promotes neural differentiation through retinoic acid receptor-mediated transcription

Retinoic acid (RA), a metabolite of vitamin A, plays versatile roles in development, differentiation, cell cycles and regulation of apoptosis by regulating gene transcription through nuclear receptor activation. Ubiquitinylation, which is one of the post-translational modifications, appears to be involved in the transcriptional activity of intranuclear receptors including retinoic acid receptor α (RARα). Mutations in the tripartite motif-containing protein 32 gene (TRIM32; also known as E3 ubiquitin-protein ligase) have been reported to be responsible for limb-girdle muscular dystrophy type 2H in humans, and its encoded protein has been shown to interact with several other important proteins. In this study, we found that TRIM32 interacts with RARα and enhances its transcriptional activity in the presence of RA. We also found that overexpression of TRIM32 in mouse neuroblastoma cells and embryonal carcinoma cells promoted stability of RARα, resulting in enhancement of neural differentiation. These findings suggest that TRIM32 functions as one of the co-activators for RARα-mediated transcription, and thereby TRIM32 is a potential therapeutic target for developmental disorders and RA-dependent leukemias.

Retinoic acid receptor antagonist LE540 attenuates wakefulness via the dopamine D1 receptor in mice

Vitamin A is a common lipophilic vitamin, and its function is mainly mediated by the binding of its metabolite retinoic acid to retinoic acid receptors (RARs) and retinoid X receptors. Recently, it was reported that the expression of the RARb (an RAR subtype) gene determines the contribution of the delta oscillation in the sleep electroencephalogram (EEG) patterns in mice. We also reported that 4-week dietary deficiency of vitamin A (VAD) causes the attenuation of delta power in sleep and spontaneous activity in mice. However, our previous study could not clarify whether the attenuation of delta power by VAD is attributed to the suppression of RARs. To address this problem, we investigated whether the chronic administration of LE540 (30mg/kg/day), an antagonist of RARs, for 1 or 4weeks attenuated EEG delta power during sleep in mice. Consequently, 4-week LE540 administration induced a significant attenuation of wakefulness and delta power in non-rapid eye movement sleep. Western blot analysis revealed a significant decrease in the expression of dopamine D1 receptor (D1DR) in the striatum and tyrosine hydroxylase in the midbrain of mice that were administered LE540 for 4weeks. High-performance liquid chromatography analysis of striatal tissue revealed a significant decrease in the homovanillic acid/dopamine ratio. Meanwhile, dopamine levels did not change in these mice. Our results suggest that the 4-week antagonism of RARs induces the attenuation of delta power. However, the attenuation of delta power may be elicited indirectly by the decrease of wakefulness followed by the hypo-expression of dopamine receptors especially D1DR.

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.

Genomics of long-range regulatory elements

Transcriptional regulation of gene expression plays a significant role in establishing the diversity of human cell types and biological functions from a common set of genes. The components of regulatory control in the human genome include cis-acting elements that act across immense genomic distances to influence the spatial and temporal distribution of gene expression. Here we review the established categories of distant-acting regulatory elements, discussing the classical and contemporary evidence of their regulatory potential and clinical importance. Current efforts to identify regulatory sequences throughout the genome and elucidate their biological significance depend heavily on advances in sequence conservation-based analyses and on increasingly large-scale efforts applying transgenic technologies in model organisms. We discuss the advantages and limitations of sequence conservation as a predictor of regulatory function and present complementary emerging technologies now being applied to annotate regulatory elements in vertebrate genomes.

[Vitamin A deficiency and nutritional status in patients with Down's syndrome]

INTRODUCTION

Vitamin A deficiency (VAD) is a worldwide public health problem. Epidemiological studies of VAD prevalence have been conducted in individuals with chromosome load and genetic potential compared with the general population; however, there are few studies in patients with Down's syndrome (DS). The objective of this study was to determine the prevalence of VAD and analyse nutritional status in patients with DS.

METHODS

A prospective and cross-sectional study was performed, with 50 karyotypically normal (KN) individuals (10.4+/-3.7 years old) and 38 randomly selected patients with DS (8.2+/-4.1 years old). Serum retinol was determined by HPLC using the Bieri method, with an international reference standard to define VAD (serum retinol <20 microg/dL). The data were analysed using the SAS/STAT statistical program.

RESULTS

The prevalence of VAD was 18.4% in individuals with DS and 4% in KN individuals (OR: 5.42; 95% CI=0.93-40.64; p=0.02). Children with DS between two and six years old shown a significativily lower serum retinol (p=<0.05).The patients with DS also showed a significant decrease in height and weight compared to KN (p=<0.001).

CONCLUSIONS

The prevalence of VAD detected in patients with DS could be considered a public health problem. Also, the chromosome 21 trisomy represent a risk factor associated with VAD.

Molecular biology of pancreatic ductal adenocarcinoma progression: aberrant activation of developmental pathways

Embryonic development marks a period of peak tissue growth and morphogenesis in the mammalian lifecycle. Many of the pathways that underlie cell proliferation and movement are relatively quiescent in adult animals but become reactivated during carcinogenesis. This phenomenon has been particularly well documented in pancreatic cancer, where detailed genetic studies and a robust mouse model have permitted investigators to test the role of various developmental signals in cancer progression. In this chapter, we review current knowledge regarding the signaling pathways that act during pancreatic development and the evidence that the reactivation of developmentally important signals is critical for the pathogenesis of this treatment-refractory malignancy.

Zinc finger transcription factor ecotropic viral integration site 1 is induced by all-trans retinoic acid (ATRA) and acts as a dual modulator of the ATRA response

Ecotropic viral integration site 1 (EVI1) plays important roles in leukaemia and development, and its expression is temporally and spatially highly restricted during the latter process. Nevertheless, the only physiological agent that to date has been shown to regulate transcription of this gene in mammalian cells is all-trans retinoic acid. Here we describe the identification of a retinoic acid response element that was located in the most distal of several alternative first exons of the human EVI1 gene and was constitutively bound by canonical retinoid receptors in NTERA-2 teratocarcinoma cells. Furthermore, it was the target of negative feedback by EVI1 on the induction of its own promoter by retinoic acid. This process required a previously described transcription repression domain of EVI1. Extending its role as a modulator of the retinoic acid response, EVI1 had the opposite effect on the RARβ retinoic acid response element, whose induction by all-trans retinoic acid it enhanced through a mechanism that involved almost all of its known functional domains. Augmentation of the retinoic acid response by EVI1 was also observed for the endogenous RARβ gene. Thus, we have established EVI1 as a novel type of modulator of the retinoic acid response, which can both enhance and repress induction by this agent in a promoter-specific manner.

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.

Using mouse models to understand normal and abnormal urogenital tract development

Removal of toxic substances from the blood depends on patent connections between the kidneys, ureters and bladder that are established when the ureter is transposed from its original insertion site in the Wolffian duct, to the bladder, its final insertion site. The Ureteral Bud Theory of Mackie and Stephens suggests that repositioning of the ureter orifice occurs as the trigone forms from the common nephric duct (CND), the caudal-most Wolffian duct segment. According to this model, insertion of the CND into the bladder and its expansion into the trigone both repositions the ureter in the bladder and enables it to separate from the Wolffian duct. The availability of new mouse models has enabled to re-examine this hypothesis using morphological analysis and lineage studies to follow the fate of the ureter and CND during the maturation process. We find that in contrast to what has been previously thought, the CND does not differentiate into the trigone but instead, undergoes apoptosis, a step that enables the ureter to separate from the Wolffian duct. Apoptosis occurs as the CND and ureter merge with the urogenital sinus positioning the ureter orifice at a site close to the Wolffian duct. Finally, expansion of the bladder moves the ureter orifice which is now fused with epithelium to its final position which is at the bladder neck. Interestingly, CND apoptosis appears to depend on close proximity to the bladder, suggesting that the bladder may be a source of signals that induce cell death. Together, these studies provide new insights into the normal process of ureter maturation, and shed light on possible causes of obstruction and reflux, ureteral abnormalities that affect 1-2% of the human population.

Retinoids regulate TGFbeta signaling at the level of Smad2 phosphorylation and nuclear accumulation

Indirect regulation of transforming growth factor (TGF)-beta signaling by retinoids occurs on a long-term timescale, secondary to transcriptional events. Studies by our group show loss of retinoid X receptor (RXR) alpha results in increased TGFbeta2 in the midgestational heart, which may play a role in the cardiac defects seen in this model [S.W. Kubalak, D.R. Hutson, K.K. Scott and R.A. Shannon, Elevated transforming growth factor beta2 enhances apoptosis and contributes to abnormal outflow tract and aortic sac development in retinoic X receptor alpha knockout embryos, Development 129 (2002) 733-746.]. Acute and direct interactions between retinoid and TGFbeta signaling, however, are not clearly understood. Treatment of dispersed hearts and NIH3T3 cells for 1 h with TGFbeta and retinoids (dual treatment) resulted in increased phosphorylated Smad2 and Smad3 when compared to treatment with TGFbeta alone. Of all dual treatments, those with the RXR agonist Bexarotene, resulted in the highest level of phosphorylated Smad2, a 7-fold increase over TGFbeta2 alone. Additionally, during dual treatment phosphorylation of Smad2 occurs via the TGFbeta type I receptor but not by increased activation of the receptor. As loss of RXRalpha results in increased levels of Smad2 phosphorylation in response to TGFbeta treatment and since nuclear accumulation of phosphorylated Smad2 is decreased during dual treatment, we propose that RXRalpha directly regulates the activities of Smad2. These data show retinoid signaling influences the TGFbeta pathway in an acute and direct manner that has been unappreciated until now.

An RBP4 promoter polymorphism increases risk of type 2 diabetes

AIMS/HYPOTHESIS

Retinol-binding protein 4 (RBP4), originally known for retinol transport, was recently identified as an adipokine affecting insulin resistance. The RBP4 -803GA promoter polymorphism influences binding of hepatic nuclear factor 1alpha and is associated with type 2 diabetes in case-control studies. We hypothesised that the RBP4 -803GA polymorphism increases type 2 diabetes risk at a population-based level. In addition, information on retinol intake and plasma vitamin A levels enabled us to explore the possible underlying mechanism.

METHODS

In the Rotterdam Study, a prospective, population-based, follow-up study, the -803GA polymorphism was genotyped. In Cox proportional hazards models, associations of the -803GA polymorphism and retinol intake with type 2 diabetes risk were examined. Moreover, the interaction of the polymorphism with retinol intake on type 2 diabetes risk was assessed. In a subgroup of participants the association of the polymorphism and vitamin A plasma levels was investigated.

RESULTS

Homozygous carriers of the -803A allele had increased risk of type 2 diabetes (HR 1.83; 95% CI 1.26-2.66). Retinol intake was not associated with type 2 diabetes risk and showed no interaction with the RBP4 -803GA polymorphism. Furthermore, there was no significant association of the polymorphism with plasma vitamin A levels.

CONCLUSIONS/INTERPRETATION

Our results provide evidence that homozygosity for the RBP4 -803A allele is associated with increased risk of type 2 diabetes in the Rotterdam population. This relationship was not clearly explained by retinol intake and vitamin A plasma levels. Therefore, we cannot differentiate between a retinol-dependent or -independent mechanism of this RBP4 variant.

Disruption of retinoid transport, metabolism and signaling by environmental pollutants

Although the assessment of circulatory levels of retinoids has become a widely used biomarker of exposure to environmental pollutants, the adverse effects caused by imbalance of the retinoid metabolism and signaling in wildlife are not known in detail. Retinoids play an important role in controlling such vital processes as morphogenesis, development, reproduction or apoptosis. Unlike other signaling molecules, retinoids are not strictly endogenous but they are derived from dietary sources of vitamin A or its precursors and thus they are sometimes referred to as 'dietary' hormones. Some environmental pollutants that affect embryogenesis, immunity or epithelial functions were also shown to interfere with retinoid metabolism and signaling in animals. This suggests that at least some of their toxic effects may be related to interaction with the retinoid metabolism, transport or signal transduction. This review summarizes in vivo and in vitro studies on interaction of environmental complex samples, pesticides, polychlorinated dioxins, polychlorinated biphenyls, polycyclic aromatic compounds and other organic pollutants with physiology of retinoids. It sums up contemporary knowledge about levels of interaction and mechanisms of action of the environmental contaminants.

Pharmacologic retinoid signaling and physiologic retinoic acid receptor signaling inhibit basal cell carcinoma tumorigenesis

Basal cell carcinoma (BCC) is the most common human cancer. Patients with basal cell nevus syndrome (Gorlin syndrome) are highly susceptible to developing many BCCs as a result of a constitutive inactivating mutation in one allele of PATCHED 1, which encodes a tumor suppressor that is a major inhibitor of Hedgehog signaling. Dysregulated Hedgehog signaling is a common feature of both hereditary and sporadic BCCs. Recently, we showed remarkable anti-BCC chemopreventive efficacy of tazarotene, a retinoid with retinoic acid receptor (RAR) beta/gamma specificity, in Ptch1+/- mice when treatment was commenced before carcinogenic insults. In this study, we assessed whether the effect of tazarotene against BCC carcinogenesis is sustained after its withdrawal and whether tazarotene is effective against preexisting microscopic BCC lesions. We found that BCCs did not reappear for at least 5 months after topical drug treatment was stopped and that already developed, microscopic BCCs were susceptible to tazarotene inhibition. In vitro, tazarotene inhibited a murine BCC keratinocyte cell line, ASZ001, suggesting that its effect in vivo is by direct action on the actual tumor cells. Down-regulation of Gli1, a target gene of Hedgehog signaling and up-regulation of CRABPII, a target gene of retinoid signaling, were observed with tazarotene treatment. Finally, we investigated the effects of topical applications of other retinoid-related compounds on BCC tumorigenesis in vivo. Tazarotene was the most effective of the preparations studied, and its effect most likely was mediated by RARgamma activation. Furthermore, inhibition of basal RAR signaling in the skin promoted BCC carcinogenesis, suggesting that endogenous RAR signaling restrains BCC growth.

Regulation of FoxP3 regulatory T cells and Th17 cells by retinoids

Vitamin A has both positive and negative regulatory functions in the immune system. While vitamin A is required for normal formation of immune cells and epithelial cell barriers, vitamin A deficiency can lead to increased inflammatory responses and tissue damage. The mechanism with which vitamin A and its metabolites such as retinoids negatively regulate inflammatory responses has not been clearly defined. Recently, it has been established that retinoids promote the generation of immune-suppressive FoxP3⁺ regulatory T cells while they suppress the T cell differentiation into inflammatory Th17 cells in the periphery such as intestine. These novel functions of retinoids provide a potentially important immune regulatory mechanism. In this review, we discuss the functions of retinoids in the development of the FoxP3⁺ cells and Th17 cells, the phenotype and functions of retinoid-induced FoxP3⁺ T cells, and the impact of retinoid-induced FoxP3⁺ T cells on the immune tolerance.