Astrocytes as a regulated source of retinoic acid for the brain.

Retinaldehyde dehydrogenases (RALDH) catalyze the synthesis of the regulatory factor retinoic acid (RA). Cultured astrocytes express several of the RALDH enzyme family, and it has been assumed that this can be extrapolated to astrocytes in vivo. However, this study finds that few astrocytes in the rodent brain express detectable RALDH enzymes, and only when these cells are grown in culture are these enzymes upregulated. Factors controlling the expression of the RALDHs in cultured astrocytes were explored to determine possible reasons for differences between in vitro versus in vivo expression. Retinoids were found to feedback to suppress several of the RALDHs, and physiological levels of retinoids may be one route by which astrocytic RALDHs are maintained at low levels. In the case of RALDH2, in vivo reduction of vitamin A levels in rats resulted in an increase in astrocyte RALDH2 expression in the hippocampus. Other factors though are likely to control RALDH expression. A shift in astrocytic RALDH subcellular localization is a potential mechanism for regulating RA signaling. Under conditions of vitamin A deficiency, RALDH2 protein moved from the cytoplasm to the nucleus where it may synthesize RA at the site of the nuclear RA receptors. Similarly, in conditions of oxidative stress RALDH1 and RALDH2 moved from the cytoplasm to a predominantly nuclear position. Thus, the RALDHs have been revealed to be dynamic in their expression in astrocytes where they may maintain retinoid homeostasis in the brain.

Photoperiodic expression of two RALDH enzymes and the regulation of cell proliferation by retinoic acid in the rat hypothalamus.

Retinoic acid (RA) has been found to regulate hypothalamic function, but precisely where it acts is unknown. This study shows expression of retinaldehyde dehydrogenase (RALDH) enzymes in tanycytes that line the third ventricle in an area overlapping with the site of hypothalamic neural stem cells. The influence of RA was examined on the proliferation of progenitors lining the third ventricle using organotypic slice cultures. As has been shown in other regions of neurogenesis, RA was found to inhibit proliferation. Investigations of the dynamics of RALDH1 expression in the rat hypothalamus have shown that this enzyme is in tanycytes under photoperiodic control with highest levels during long versus short days. In parallel to this shift in RA synthesis, cell proliferation in the third ventricle was found to be lowest during long days when RA was highest, implying that RALDH1 synthesized RA may regulate neural stem cell proliferation. A second RA synthesizing enzyme, RALDH2 was also present in tanycytes lining the third ventricle. In contrast to RALDH1, RALDH2 showed little change with photoperiodicity, but surprisingly the protein was present in the apparent absence of mRNA transcript and it is hypothesized that the endocytic tanycytes may take this enzyme up from the cerebrospinal fluid (CSF).

Photoperiodic regulation of retinoic acid signaling in the hypothalamus.

Both retinoic acid (RA) and thyroid hormone (TH) regulate transcription via specific nuclear receptors. TH regulates hypothalamic homeostasis and active T3 is generated by deiodinase enzymes in tanycytes surrounding the third ventricle. However, RA has not been previously considered in such a role. Data presented here highlights novel parallels between the TH and RA synthetic pathways in the hypothalamus implying that RA also acts to regulate hypothalamic gene expression and function. Key elements of the RA cellular signaling pathway were shown to be regulated in the rodent hypothalamus. Retinoid synthetic enzymes and the retinol transport protein Stra6 were located in the cells lining the third ventricle allowing synthesis of RA from retinol present in the CNS to act via RA receptors and retinoid X receptors in the hypothalamus. Photoperiod manipulation was shown to alter the expression of synthetic enzymes and receptors with lengthening of photoperiod leading to enhanced RA signaling. In vitro RA can regulate the hypothalamic neuroendocrine peptide adrenocorticotrophic hormone. This work presents the new concept of controlled RA synthesis by hypothalamic tanycytes giving rise to possible involvement of this system in endocrine, and possibly vitamin A, homeostasis.

Oestrogen formation and connective tissue growth factor expression in rat granulosa cells.

Ovarian follicular development involves continual remodelling of the extracellular matrix (ECM) forming the basement membrane and intercellular framework that support granulosa cell (GC) growth and differentiation. Insight into the molecular regulation of ovarian ECM remodelling is potentially translatable to tissue remodelling elsewhere in the body. We therefore studied the link between a gene marker of ECM remodelling (connective tissue growth factor (CTGF)) and oestrogen biosynthesis (cytochrome P450(aromatase) (P450(arom))) in rat granulosa cells. To determine if a cause-effect interaction exists, we used semi-quantitative in situ hybridisation to analyse patterns of CTGF and P450(arom) mRNA expression and immunohistochemistry to detect CTGF protein localisation throughout follicular development, and tested the actions of CTGF on oestrogen biosynthesis and oestradiol on CTGF mRNA expression in isolated GC in vitro. CTGF mRNA levels in GC rose gradually through small preantral (SP) and small antral (SA) stages of development to a maximum (fivefold higher) in large antral (LA) follicles. In preovulatory (PO) follicles, the CTGF mRNA level fell to 30% of that in SP follicles. P450(arom) mRNA also increased (threefold in LA relative to SP) throughout antral development follicles, but in contrast to CTGF continued to increase (12-fold) in PO follicles. In the cumulus oophorus of PO follicles, the residual GC CTGF mRNA expression increased with proximity to the oocyte, being inversely related to P450(arom). Elsewhere in the follicle wall, there was a mural-to-antral gradient of CTGF mRNA expression, again inversely related to P450(arom). Immunohistochemistry showed CTGF protein localisation broadly followed mRNA expression during follicular development, although the protein was also present in the theca interna and ovarian surface epithelium. Gradients in CTGF expression across the cumulus oophorus and follicle wall were similar to those observed for mRNA with CTGF protein expression being greatest in proximity to the oocyte. Treatment of isolated GC from preantral and SA follicles with recombinant human CTGF (1-100 ng/ml) did not affect basal or FSH-stimulated GC aromatase activity. However, in the absence of FSH, oestradiol (10(-7)-10(-5) M) stimulated CTGF mRNA expression up to twofold. Conversely, FSH (10 ng/ml) alone reduced CTGF mRNA expression by 40% and combined treatment with FSH and oestradiol further suppressed CTGF to 10% of the control value. The oestrogen receptor (ER) antagonist, ICI 182 780 blocked the stimulatory and inhibitory effects of oestradiol, suggesting a specific ER-mediated mode of action on CTGF. Therefore, CTGF gene expression in GC is under local control by oestrogen whose effect (positive or negative) is modulated by FSH. This helps explain why gene expression of CTGF and P450(arom) diverge in FSH-induced PO follicles and has implications for oestrogenic regulation of CTGF formation elsewhere in the body.

Elevated Fibroblast growth factor 21 (FGF21) in obese, insulin resistant states is normalised by the synthetic retinoid Fenretinide in mice.

Fibroblast growth factor 21 (FGF21) has emerged as an important beneficial regulator of glucose and lipid homeostasis but its levels are also abnormally increased in insulin-resistant states in rodents and humans. The synthetic retinoid Fenretinide inhibits obesity and improves glucose homeostasis in mice and has pleotropic effects on cellular pathways. To identify Fenretinide target genes, we performed unbiased RNA-seq analysis in liver from mice fed high-fat diet ± Fenretinide. Strikingly, Fgf21 was the most downregulated hepatic gene. Fenretinide normalised elevated levels of FGF21 in both high-fat diet-induced obese mice and in genetically obese-diabetic Leprdbmice. Moreover, Fenretinide-mediated suppression of FGF21 was independent of body weight loss or improved hepatic insulin sensitivity and importantly does not induce unhealthy metabolic complications. In mice which have substantially decreased endogenous retinoic acid biosynthesis, Fgf21 expression was increased, whereas acute pharmacological retinoid treatment decreased FGF21 levels. The repression of FGF21 levels by Fenretinide occurs by reduced binding of RARα and Pol-II at the Fgf21 promoter. We therefore establish Fgf21 as a novel gene target of Fenretinide signalling via a retinoid-dependent mechanism. These results may be of nutritional and therapeutic importance for the treatment of obesity and type-2 diabetes.

Fenretinide prevents obesity in aged female mice in association with increased retinoid and estrogen signaling.

OBJECTIVE: The synthetic retinoid fenretinide (FEN) inhibits adiposity in male mice fed a high-fat diet (HFD) in association with alterations in retinoic acid (RA) signaling. Young female mice are protected from obesity via estrogen signaling. We, therefore, investigated whether FEN also influences adiposity in aged female mice differing in parity and whether such effects are mediated by retinoid and estrogen signaling. METHODS: Aged nulliparous and parous female mice were maintained on HFD ± FEN, and adiposity was assessed. Quantitative polymerase chain reaction was performed on white adipose tissue (WAT), liver, and 3T3-L1 adipocytes treated with RA or FEN ± estrogen. RESULTS: Parous females were more obese than nulliparous mice independent of age. FEN-HFD prevented the HFD-induced increase in adiposity and leptin levels independently of parity. FEN-HFD induced retinoid-responsive genes in WAT and liver. Parous females had reduced expression of hepatic estrogen-responsive genes, but FEN-HFD up-regulated WAT Cyp19a1 and Esr2 in parous mice. Estrogen and RA acted synergistically to increase RA receptor-mediated gene expression in 3T3-L1 adipocytes. FEN increased Cyp19a1 and Esr2, similar to our findings in vivo. CONCLUSIONS: The prevention of adiposity by FEN in response to HFD in female mice seems to involve increased retinoid signaling in association with induction of local estrogen production and estrogen signaling in WAT.

Expression in the human brain of retinoic acid induced 1, a protein associated with neurobehavioural disorders.

Retinoic acid induced 1 (RAI1) is a protein of uncertain mechanism of action which nevertheless has been the focus of attention because it is a major contributing factor in several human developmental disorders including Smith-Magenis and Potocki-Lupski syndromes. Further, RAI1 may be linked to adult neural disorders with developmental origins such as schizophrenia and autism. The protein has been extensively examined in the rodent but very little is known about its distribution in the human central nervous system. This study demonstrated the presence of RAI1 transcript in multiple regions of the human brain. The cellular expression of RAI1 protein in the human brain was found to be similar to that described in the mouse, with high levels in neurons, but not glia, of the dentate gyrus and cornus ammonis of the hippocampus. In the cerebellum, a second region of high expression, RAI1 was present in Purkinje cells, but not granule cells. RAI1 was also found in neurons of the occipital cortex. The expression of this retinoic acid-induced protein matched well in the hippocampus with expression of the retinoic acid receptors. The subcellular distribution of human neuronal RAI1 indicated its presence in both cytoplasm and nucleus. Overall, human RAI1 protein was found to be a highly expressed neuronal protein whose distribution matches well with its role in cognitive and motor skills.

Myeloid-cell protein tyrosine phosphatase-1B deficiency in mice protects against high-fat diet and lipopolysaccharide-induced inflammation, hyperinsulinemia, and endotoxemia through an IL-10 STAT3-dependent mechanism.

Protein tyrosine phosphatase-1B (PTP1B) negatively regulates insulin and leptin signaling, rendering it an attractive drug target for treatment of obesity-induced insulin resistance. However, some studies suggest caution when targeting macrophage PTP1B, due to its potential anti-inflammatory role. We assessed the role of macrophage PTP1B in inflammation and whole-body metabolism using myeloid-cell (LysM) PTP1B knockout mice (LysM PTP1B). LysM PTP1B mice were protected against lipopolysaccharide (LPS)-induced endotoxemia and hepatic damage associated with decreased proinflammatory cytokine secretion in vivo. In vitro, LPS-treated LysM PTP1B bone marrow-derived macrophages (BMDMs) displayed increased interleukin (IL)-10 mRNA expression, with a concomitant decrease in TNF-α mRNA levels. These anti-inflammatory effects were associated with increased LPS- and IL-10-induced STAT3 phosphorylation in LysM PTP1B BMDMs. Chronic inflammation induced by high-fat (HF) feeding led to equally beneficial effects of macrophage PTP1B deficiency; LysM PTP1B mice exhibited improved glucose and insulin tolerance, protection against LPS-induced hyperinsulinemia, decreased macrophage infiltration into adipose tissue, and decreased liver damage. HF-fed LysM PTP1B mice had increased basal and LPS-induced IL-10 levels, associated with elevated STAT3 phosphorylation in splenic cells, IL-10 mRNA expression, and expansion of cells expressing myeloid markers. These increased IL-10 levels negatively correlated with circulating insulin and alanine transferase levels. Our studies implicate myeloid PTP1B in negative regulation of STAT3/IL-10-mediated signaling, highlighting its inhibition as a potential anti-inflammatory and antidiabetic target in obesity.

Fenretinide treatment prevents diet-induced obesity in association with major alterations in retinoid homeostatic gene expression in adipose, liver, and hypothalamus.

The synthetic retinoid, Fenretinide (FEN), inhibits obesity and insulin resistance in mice and is in early clinical trials for treatment of insulin resistance in obese humans. We aimed to determine whether alterations in retinoic acid (RA)-responsive genes contribute to the beneficial effects of FEN. We examined the effect of FEN on 3T3-L1 adipocyte differentiation and alterations in gene expression in C57Bl/6 and retinaldehyde dehydrogenase (RALDH) 1 knockout (KO) mice fed a high-fat (HF) diet. FEN completely inhibited adipocyte differentiation by blocking CCAAT/enhancer-binding protein (C/EBP) α/peroxisome proliferator-activated receptor (PPAR) γ-mediated induction of downstream genes and upregulating RA-responsive genes like cellular retinol-binding protein-1. In mice fed an HF diet, RA-responsive genes were markedly increased in adipose, liver, and hypothalamus, with short-term and long-term FEN treatment. In adipose, FEN inhibited the downregulation of PPARγ and improved insulin sensitivity and the levels of adiponectin, resistin, and serum RBP (RBP4). FEN inhibited hyperleptinemia in vivo and leptin expression in adipocytes. Surprisingly, hypothalamic neuropeptide Y expression was completely suppressed, suggesting a central effect of FEN to normalize hyperglycemia. Moreover, FEN induced RA-responsive genes in RALDH1 KO mice, demonstrating that FEN can augment RA signaling when RA synthesis is impaired. We show that FEN-mediated beneficial effects are through alterations in retinoid homeostasis genes, and these are strong candidates as therapeutic targets for the treatment of obesity and insulin resistance.

Retinoic acid and affective disorders: the evidence for an association.

OBJECTIVE: Isotretinoin (13-cis-retinoic acid), approved by the US Food and Drug Administration for the treatment of acne, carries a black box warning related to the risk of depression, suicide, and psychosis. Retinoic acid, the active form of vitamin A, regulates gene expression in the brain, and isotretinoin is its 13-cis isomer. Retinoids represent a group of compounds derived from vitamin A that perform a large variety of functions in many systems, in particular the central nervous system, and abnormal retinoid levels can have neurologic effects. Although infrequent, proper recognition and treatment of psychiatric side effects in acne patients is critical given the risk of death and disability. This article reviews the evidence for isotretinoin's relationships with depression and suicidality. DATA SOURCES: The PsycINFO, MEDLINE, and PubMed searchable database indexes were searched for articles published in the English language from 1960 to June 2010 using the key words isotretinoin, retinoids, retinoic acid, depression, depressive disorders, and vitamin A. Evidence examined includes (1) case reports; (2) temporal association between onset of depression and exposure to the drug; (3) challenge-rechallenge cases; (4) class effect (other compounds in the same class, like vitamin A, having similar neuropsychiatric effects); (5) dose response; and (6) biologically plausible mechanisms. STUDY SELECTION: All articles in the literature related to isotretinoin, depression, and suicide were reviewed, as well as articles related to class effect, dose response, and biologic plausibility. DATA EXTRACTION: Information from individual articles in the literature was extracted, including number of episodes of depression, suicidality, suicide, psychosis, violence and aggression, past psychiatric history, time of onset in relation to isotretinoin usage, medication dosage, duration of treatment, and dechallenge and challenge history. RESULTS: The literature reviewed is consistent with associations of isotretinoin administration with depression and with suicide in a subgroup of vulnerable individuals. CONCLUSIONS: The relationship between isotretinoin and depression may have implications for a greater understanding of the neurobiology of affective disorders.

A vitamin for the brain.

In the central nervous system (CNS) the function of retinoic acid, the active metabolite of vitamin A, is best understood from its action in guiding embryonic development; as development comes to completion, retinoic acid signaling declines. However, it is increasingly recognized that this signaling mechanism does not disappear in the adult brain but becomes more regionally focused and takes on new roles. These functions are often tied to processes of neural plasticity whether in the hippocampus, through homeostatic neural plasticity, the olfactory bulb or the hypothalamus. The role of retinoic acid in the control of plastic processes has led to suggestions of its involvement in neural disorders, both degenerative and psychiatric. This review presents a snapshot of developments in these areas over recent years.

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

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

High expression of retinoic acid receptors and synthetic enzymes in the human hippocampus.

Retinoic acid, the active form of the nutrient vitamin A, regulates several facets of neuronal plasticity in the hippocampus, including neurogenesis and synaptic strength, acting via specific retinoic acid receptors (RARs). Essential for conversion of vitamin A to retinoic acid is the enzyme retinaldehyde dehydrogenase (RALDH) and in the rodent hippocampus this is only present in the adjacent meninges where it must act as a locally released paracrine hormone. Little is known though about the expression of RALDHs and RARs in the human hippocampus. This study confirms that RALDH levels are very low in mouse neurons but, surprisingly, strong expression of RALDH protein is detected by immunohistochemistry in hippocampal neurons. The receptors RARα, ß and γ were also detected, each receptor exhibiting differing subcellular locations implying their potential regulation of both transcription and non-genomic actions. These results imply an essential function of retinoic acid in the human hippocampus likely to include regulation of neuronal plasticity.