Differential expression of the alpha and beta retinoic acid receptors in tissues of the rat

Expression of retinoic acid, triiodothyronine, and glucocorticoid hormone nuclear receptors is decreased in the liver of rats fed a hypercholesterolemia-inducing diet

Several studies have shown that dietary factors modulate cell signaling pathways. The aim of this study was to determine whether a hypercholesterolemia-inducing diet rich in saturated fat and cholesterol modifies rat liver expression of the nuclear receptors of retinoic acid (RAR), triiodothyronine (TR), and glucocorticoid hormone (GR), which are transcriptional factors. The experimental diet contained coconut oil 25 g/100 g as a source of lipids, cholesterol 1 g/100 g, and cholic acid 0.5 g/100 g, and the control diet contained olive oil 5 g/100 g. After 26 days of feeding the hypercholesterolemia-inducing diet, a lower binding capacity of the nuclear receptors and a smaller amount of their mRNA were observed. Moreover, the activities of malic enzyme (ME) and tyrosine aminotransferase (TAT), whose gene promotors contain a response element to TR and GR, respectively, were significantly decreased. These changes occurred in a cellular environment characterized by a high level of cholesterol and free fatty acids (FFAs). Thus, two nonexclusive hypotheses can be proposed to explain this decreased expression of nuclear receptors, one emphasizing the effect of lipidic components on the cellular amount of receptor ligands (retinoic acid [RA] and triiodothyronine [T3]), the other emphasizing a modification of the balance between nuclear receptors that could impede the upregulation of TR and RAR.

Nuclear retinoic acid receptors in the lacrimal gland

The lacrimal gland secretes and metabolizes retinoids and responds to retinoic acid in culture. Like other retinoid responsive organs it is expected to express the nuclear retinoid receptors. The goal of this study was to identify the retinoic acid receptors (RAR) in the lacrimal glands of rats, rabbits, and humans. Total RNA was prepared from whole lacrimal glands and rat lacrimal gland acinar cells grown in culture. RNA was subjected to Northern blot analysis and probed for the RAR alpha, RAR beta, and RAR gamma mRNAs. Nuclear extracts of rat and rabbit lacrimal glands were incubated with 3H-all-trans retinoic acid and analyzed by gel filtration chromatography. Western blots of the nuclear extracts were probed using monoclonal antibodies to RAR alpha and RAR beta. Rat lacrimal gland expresses RAR alpha mRNA with two transcripts (3.8 and 3.0 kb), a single RAR beta mRNA transcript (3.3 kb), and a single RAR gamma mRNA transcript (3.3 kb). Cultured rat lacrimal acinar cells also expressed the mRNA for all three RAR subtypes. Rabbit lacrimal glands express mRNAs for RAR alpha (3.7 and 2.9 kb) and RAR beta (3.2 kb) but RAR gamma mRNA is not detectable. Human lacrimal glands also express mRNA for RAR alpha (3.5 and 2.3 kb), RAR beta (3.4 kb) and RAR gamma (3.0 kb). Lacrimal gland nuclear extracts contain proteins in the 50 kDa range that specifically bind retinoic acid with Kd = 1.25 nM in rat lacrimal gland and 0.3 nM in rabbit. The monoclonal antibodies identified RAR alpha and RAR beta in both rat and rabbit lacrimal glands. The results of this study support a role for retinoids in maintaining the structure and function of the lacrimal gland. The presence of RARs suggests potential interactions of these receptors with other members of their superfamily, including androgen and thyroid receptors, which also may be involved in lacrimal function.

Retinoic acid receptors and retinoid binding proteins in endometrial adenocarcinoma: differential expression of cellular retinoid binding proteins in endometrioid tumours

Retinoic acid is apparently required for the normal differentiation of reproductive epithelium. Cellular abnormalities in retinoid homeostasis could be a factor in the development of endometrial malignancy. We have thus investigated the expression of nuclear retinoic acid receptors (RARs and RXRs) and cellular binding proteins for retinol (CRBP) and retinoic acid (CRABP) in endometrial adenocarcinoma of the endometrioid histological subtype. Ten grade I, II grade 2 and 10 grade 3 tumour samples, as well as 4 samples of severe atypical precancerous endometrial hyperplasia, were studied. No significant difference in expression of RAR-beta was detected in tumour samples compared with normal epithelial cells. RAR-gamma was significantly elevated in grade 1 and 2 carcinomas, but this may be due to greater stromal cell involvement in these lower grade tumours. There was significant elevation of CRBP I mRNA in tumour samples. Furthermore, although undetectable in normal endometrial epithelium, CRABP I was expressed in 3/II grade 2 and 9/10 grade 3 carcinomas, with expression being significantly higher where the primary tumour had invaded more than 50% of the total myometrial thickness. Analysis of 2 epithelial-like endometrial adenocarcinoma cell lines supported the idea that CRABP I expression is characteristic of poorly differentiated endometrial adenocarcinoma. Our data suggest that alterations in mechanisms of retinoid homeostasis are a feature of endometrial adenocarcinoma and may contribute to the severity of disease.

Expression of cytosolic retinoid-binding protein genes in human skin biopsies and cultured keratinocytes and fibroblasts

Using reverse transcription coupled to polymerase chain reaction we have studied the mRNA expression of serum retinol-binding protein and cytosolic receptors for retinol and retinoic acid in skin biopsies, and in cultured epidermal keratinocytes and dermal fibroblasts. Transcripts for cellular retinol-binding protein (CRBP) I and cellular retinoic-acid-binding protein (CRABP) I were found in normal skin, keratinocytes, and fibroblasts. CRABP II transcripts were detected in skin and keratinocytes. A decreased mRNA expression of CRABP I and an increased mRNA expression of CRABP II were found in lesional psoriatic skin compared with uninvolved skin. mRNA transcripts for serum retinol-binding protein (s-RBP) were detected in all tissues and cells. The biological importance of s-RBP expression in keratinocytes and fibroblasts is not known, but hypothetically this protein may be involved in the intracellular shuttling of retinol and retinoic acid, or in the retransportation of cellular retinoids into the extracellular space.

Expression of retinoic acid receptor genes in fetal and newborn rat lung

Lung differentiation and development are affected by vitamin A and its metabolites. One mechanism through which retinoids might exert their effects is through nuclear retinoic acid receptors (RAR). The gene expression profile of the RAR family (alpha, beta, gamma) has previously been determined in both the developing mouse embryo to 14.5 days gestation, and in the adult lung. The purpose of this study was to determine the expression of the RAR genes during the period of gestation that results in the formation of the saccular lung stage. Total RNA was extracted from fetal lungs of Sprague-Dawley rats at gestational days 17, 19, 20, 21, and 22, and from 12-hour-old newborns for Northern hybridization. Two transcripts of RAR alpha mRNA (3.7 and 2.7 kb) were found at each time point. At day 17, the 2.7 kb RAR alpha mRNA was increased two-fold or more than at any other time studied. At days 19-22 the levels of the 3.7 kb RAR alpha species were also lower than day 17 and newborn levels. One RAR beta mRNA transcript (3.4 kb), present at all time points, was significantly higher in the newborn than on days 17-22. Expression of RAR gamma mRNA could only be demonstrated by reverse transcriptase-polymerase chain reaction. We speculate that the higher RAR alpha species at day 17 indicates a role for RAR alpha in the maintenance of the columnar epithelial cells of the glandular phase of lung development.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoic acid up-regulates nuclear retinoic acid receptor-alpha expression in human neuroblastoma cells

Retinoic acid (RA) nuclear receptors (RARs) are thought to mediate the cellular and molecular effects of RA on a wide variety of tissues. In most cell types, RAR alpha expression remains relatively constant following exposure to RA, while that of RAR beta is rapidly induced. In this study, we show that in human neuroblastoma, a cell type exceptionally sensitive to RA-induced differentiation, RAR alpha as well as RAR beta is markedly up-regulated by RA treatment. This effect was consistent in all 5 neuroblastoma cell lines tested and was reflected in a 2- to 5-fold increase in receptor mRNA levels as assessed by Northern-blot analysis. Using LA-N-5 human neuroblastoma cells, we found that receptor up-regulation occurred in a time- and dose-dependent fashion with increases in both RAR alpha and beta mRNA detectable 1-2 hr after the addition of RA. These inductions were not abrogated by cycloheximide, indicating that protein synthesis was not required for the RA responses. Nuclear run-off experiments combined with Northern-blot analysis of RAR alpha stability directly demonstrated that the up-regulation of RAR alpha mRNA levels reflected an increased rate of transcription without changes in message half-life. These findings, showing direct activation by RA of RAR alpha gene transcription in human neuroblastoma cells, suggest differences in the overall regulation of this receptor from that found in most other RA-inducible tissue.

Immunolocalization of retinoic acid receptors in rat, mouse and human ovary and uterus

We raised an antibody against a synthetic peptide corresponding to amino acids 155-174 of human retinoic acid receptor alpha (RAR-alpha). The sequence is highly homologous in all RARs and their isoforms. When mouse and human RARs (alpha, beta and gamma) expressed in Cos cell were analysed with immunoblot, all receptors gave a specific 51 K signal. Mouse RAR-gamma gave an additional signal corresponding to 58 K. In human teratocarcinoma cells (F9) both 51 and 58K molecule sizes were detected. The RAR expression in F9 cells was slightly down-regulated in charcoal-stripped culture medium and returned to normal level after retinoic acid treatment. The 51 K protein was found in all ovarian and uterine samples, but the quantity of the 58 K protein varied in different species and organs, being highest in the mouse uterus and the rat and human ovary. Using immunohistochemistry the RARs were found in the nuclear compartment. In the rat uterus, positive immunoreaction was found mainly in the nuclei of epithelial, uterine glandular and stromal cells. In the rat ovary, positive reaction was found in the nuclei of germinal epithelial, follicular and stromal cells.

Ecdysone receptor expression in the CNS correlates with stage-specific responses to ecdysteroids during Drosophila and Manduca development

In insects, the ecdysteroids act to transform the CNS from its larval to its adult form. A key gene in this response is the ecdysone receptor (EcR), which has been shown in Drosophila to code for 3 protein isoforms. Two of these isoforms, EcR-A and EcR-B1, are prominently expressed in the CNS and we have used isoform-specific antibodies to examine their fluctuations through postembryonic life. EcR expression at the onset of metamorphosis is extremely diverse but specific patterns of EcR expression correlate with distinct patterns of steroid response. Most larval neurons show high levels of EcR-B1 at the start of metamorphosis, a time when they lose larval features in response to ecdysteroids. Earlier, during the larval molts, the same cells have no detectable receptors and show no response to circulating ecdysteroids; later, during the pupal-adult transformation, they switch to EcR-A expression and respond by maturing to their adult form. During the latter period, a subset of the larval neurons hyperexpress EcR-A and these cells are fated to die after the emergence of the adult. The stem cells for the imaginal neurons show prominent EcR-B1 expression during the last larval stage correlated with their main proliferative period. Most imaginal neurons, by contrast, express only EcR-A when they subsequently initiate maturation at the start of metamorphosis. The imaginal neurons of the mushroom bodies are unusual amongst imaginal neurons in expressing the B1 isoform at the start of metamorphosis but they also show regressive changes at this time as they lose their larval axons. Imaginal neurons of the optic lobe show a delayed expression of EcR-B1 through the period when cell-cell interactions are important for establishing connections within this region of the CNS. Overall, the appearance of the two receptor isoforms in cells correlates with different types of steroid responses: EcR-A predominates when cells are undergoing maturational responses whereas EcR-B1 predominates during proliferative activity or regressive responses. The heterogeneity of EcR expression at the start of metamorphosis presumably reflects the diverse origins and requirements of the neurons that nevertheless are all exposed to a common hormonal signal.

Regulation of human renal adenocarcinoma cell growth by retinoic acid and its interactions with epidermal growth factor

Retinoic acid (RA) is a natural derivative of vitamin A which regulates the growth and differentiation of epithelia. We have previously proposed that RA participates in compensatory kidney growth and reported that RA inhibits rat mesangial cell growth. This paper describes the effects of RA on a human renal adenocarcinoma cell line (PAD) under different growth conditions, and its interactions with epidermal growth factor (EGF). PAD cells were shown to express RA receptors alpha and beta by Northern blot analysis. In serum free cultures, addition of RA (10(-7) M) markedly increased thymidine incorporation by PAD cells (155 +/- 7% mean +/- SE vs. control in 6 separate experiments; P < 0.0001). RA also caused a significant increase in thymidine incorporation by PAD cells under conditions of rapid growth in serum supplemented medium (115 +/- 2% vs. control; P < 0.001). RA by itself was unable to reverse contact inhibition of PAD cell growth (NS vs. control), but it synergistically enhanced the mitogenic effect of EGF on confluent monolayers (110 +/- 0.6% vs. EGF alone; P < 0.05). Northern blot analysis demonstrated that PAD cells express EGF receptor mRNA, and this was not significantly modified by the addition of RA. Growth arrested (serum starved) PAD cells expressed RAR-alpha mRNA which was upregulated eightfold at three hours following the addition of 10% FCS. Thus, our data show that RA is directly mitogenic for serum starved human renal adenocarcinoma cells and that it exerts complex modulation of cell growth in the presence of EGF and serum components.

Retinoic acid receptor gamma 2 gene expression is up-regulated by retinoic acid in 3T3-L1 preadipocytes

Retinoids, especially all-trans retinoic acid (RA), have been shown to inhibit the differentiation of preadipose cells. In the present study, the expression of retinoic acid receptors (RAR alpha, beta and gamma) and retinoid X receptors (RXR alpha, beta and gamma) was examined by Northern blot analysis in rat adipose tissue and mouse 3T3-L1 adipose cells. The adipose tissue and/or 3T3-L1 cells expressed mRNAs for a number of nuclear retinoid receptors, including RAR alpha, beta and gamma, and RXR alpha, beta and gamma. RAR alpha, RAR gamma, RXR alpha and RXR beta mRNAs were abundant in adipose tissue and 3T3-L1 cells. RXR gamma mRNA was detected in adipose tissue but not in 3T3-L1 cells. Treatment of 3T3-L1 cells with 1 microM RA led to a 4-5-fold increase in the RAR gamma mRNA level, but only a trace amount of RAR beta mRNA was detected. RAR gamma mRNA expression was rapidly (within 2 h) induced by physiological concentrations of RA in a dose-dependent manner. The response of RAR gamma mRNA expression to RA was reversible; rapid disappearance of RAR gamma mRNA occurred on RA removal. In addition, the induction of RAR gamma expression did not require de novo protein synthesis, but was completely abolished by an inhibitor of RNA synthesis. Using RAR gamma 1 and gamma 2 isoform-specific probes, the patterns of RAR gamma 1 and gamma 2 mRNA expression in 3T3-L1 cells in the presence and absence of RA were examined. RAR gamma 1 mRNA was detected in 3T3-L1 cells but was not affected by RA treatment; however, RAR gamma 2 mRNA was strongly induced by RA.

Normal development and growth of mice carrying a targeted disruption of the alpha 1 retinoic acid receptor gene

Three unlinked genes encode receptors for retinoic acid (RAR alpha, -beta, and -gamma). Each gene expresses two major protein isoforms differing in the amino terminal A domain by alternative promoter use, fused to common exons encoding most of the receptor protein. The two RAR alpha transcripts (RAR alpha 1 and -alpha 2) are differentially expressed and evolutionarily conserved, as are the RAR beta and -gamma transcripts, suggesting that each isoform may have specific functions in the development of animals. To address the biological function of the alpha 1 receptor, we have disrupted the portion of the RAR alpha gene encoding this isoform by homologous recombination in mouse embryonic stem cells. Surprisingly, offspring homozygous for this mutation were viable and showed no apparently altered phenotype. RNA analysis confirmed that the RAR alpha 1 transcript was absent in homozygous tissues, and no evidence for a compensatory increase of RAR alpha 2 or of another RAR gene was obtained to account for the vitality of the mutant animals. These results clearly demonstrate that loss of RAR alpha 1 function does not disrupt embryonic development and argue for combinatorial or overlapping functions among the RAR isoforms.

Retinoid-dependent transcriptional suppression of cytokeratin gene expression in human epidermal squamous cell carcinoma cells

We have previously demonstrated that cytokeratin levels are coordinately regulated in normal cultured human keratinocytes. In the present study we examine the mechanism of this regulation using human squamous cell carcinoma (SCC) cells. Treatment of SCC-13 cells with 20 or 200 nM trans-retinoic acid results in nearly complete suppression of cytokeratin K5 and K6 expression. This change is accompanied by a simultaneous reduction (> 20-fold) in the level of the mRNAs encoding K5 and K6. Transcriptional analysis indicates that the transcription rate of the K5 and K6 genes drops by approximately four to fivefold in retinoid treated nuclei. Retinol (2000 nM) also promotes this change. In contrast, cytokeratin K19 does not increase in the presence of retinoic acid, thus the normal coordinate regulation of keratin gene expression by retinoids appears to be uncoupled in SCC-13 cells. However, this does not represent a general defect in positive regulation of gene expression by retinoids, since in a transient transfection assay trans-retinoic acid positively regulates a reporter plasmid containing the retinoid response element from the retinoic acid receptor-beta gene. The synthetic retinoids Ro 13-6298 (ethyl ester) and its metabolic derivative Ro 13-7410 (free acid) are both active in modulating the differentiation of normal keratinocytes. In contrast, only Ro 13-7410 is active in SCC-13 cells. As Ro 13-6298 binds poorly to the retinoic acid receptors, this suggests that SCC-13 cells, unlike normal keratinocytes, lack the ability to convert Ro 13-6298 to the active Ro 13-7410.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoic acid and interferon combination studies in human cancer

Retinoic acid and interferon-alpha have limited single-agent activity in advanced cancer. Cell culture data indicate that in combination these agents have enhanced activity (modulating growth and differentiation) in a number of malignant cell types. Recent clinical work in advanced squamous cell carcinoma reports major activity with this regimen. This paper reviews the preclinical and clinical data testing retinoic acid in combination with interferons and presents recent work integrating these agents with radiotherapy in locally advanced cervical cancer.

Effect of retinoid status on alpha, beta and gamma retinoic acid receptor mRNA levels in various rat tissues

We have investigated the effects of retinoids, vitamin D and thyroid hormone on the levels of retinoic acid receptor (RAR)alpha, RAR beta and RAR gamma mRNAs in intact animals. Although vitamin A deficiency caused no significant changes in the levels of RAR alpha and RAR gamma mRNAs, the level of RAR beta transcripts was greatly decreased in various tissues of vitamin A-deficient rats, but was restored rapidly to a normal level after administration of retinoic acid. Retinol also restored the RAR beta mRNA level, but the magnitude and kinetics of the induction differed from those by retinoic acid. The use of specific inhibitors demonstrated that this autoregulation of RAR beta gene expression in vivo occurred at the transcriptional level. In addition, from these results it was postulated that the maintenance of the normal RAR beta mRNA levels seemed to require a threshold serum retinol concentration (about 25 micrograms/dl). Moreover, we found that administration of retinol and retinoic acid to normal rats caused the overexpression of RAR beta transcripts (2-15-fold) when compared with the control levels of RAR beta mRNA, although the levels of RAR alpha and RAR gamma mRNAs were not affected. Vitamin D and thyroid hormone did not modulate the levels of RAR transcripts. These findings clearly indicate the specific ligand regulation of RAR beta gene expression in intact animals. The altered levels of RAR beta according to retinoid status may affect retinoid-inducible gene expression.

The molecular biology of retinoic acid receptors: orphan from good family seeks home

A variety of c-DNAs coding for nuclear retinoic acid receptors (RARs) have recently been cloned. These receptors are members of the steroid/thyroid receptor superfamily and are believed to act as ligand-inducible transactivating factors; retinoic acid induces changes in receptor configuration that allows DNA binding and increased gene transcription from specific genes to occur. The retinoic acid receptor family itself may consist of up to 20 separate receptors each with a specific distribution and ligand binding characteristics. The RAR-gamma in the adult is found almost exclusively in the skin but other receptors which are found in a variety of other tissues are also present in skin. Associations of cutaneous disease states with receptor mutants have not yet been reported although some cases of leukaemia may be secondary to retinoic acid receptor gene rearrangements. A variety of approaches to identify the biological function of these receptors based on recombinant DNA technology are already underway.

Up-regulation of the integrin alpha 1/beta 1 in human neuroblastoma cells differentiated by retinoic acid: correlation with increased neurite outgrowth response to laminin

Retinoic acid (RA) is known to induce differentiation of neuroblastoma cells in vitro. Here we show that treatment of two human neuroblastoma cell lines, SY5Y and IMR32, with RA resulted in a fivefold increase of the integrin alpha 1/beta 1 expression. The effect was selective because expression of the alpha 3/beta 1 integrin, also present in these cells, was not increased. The up-regulation of the alpha 1/beta 1 differentiated SY5Y cells correlated with increased neurite response to laminin. In fact, RA-treated SY5Y cells elongated neurites on laminin-coated substratum more efficiently compared with untreated cells or cells treated with nerve growth factor, insulin, or phorbol 12-myristate 13-acetate. These three agents induced partial morphological differentiation but did not increase alpha 1 integrin expression. Neurite extension in RA-treated cells was more efficient on laminin than on fibronectin or collagen type I and was inhibited with beta 1 integrin antibodies on all three substrates. Affinity chromatography experiments showed that alpha 1/beta 1 is the major laminin receptor in both untreated and RA-treated SY5Y cells. These data show that RA, a naturally occurring morphogen implicated in embryonic development, can selectively regulate the expression of integrin complexes in neuronal cells and suggest an important role of the alpha 1/beta 1 laminin receptor in the morphological differentiation of nerve cells.

Differential effects of all-trans and 13-cis-retinoic acid on mRNA levels of nuclear retinoic acid receptors in rat lung and liver

The effects of three retinoids, all-trans-retinoic acid (all-trans-RA), 13-cis-RA, and etretin were examined on mRNA abundance of nuclear retinoic acid receptors (RAR-alpha, beta, and gamma) in lung and liver of retinol deficient and chow fed rats. All-trans-RA increased lung RAR-beta mRNA levels 5 or 11-fold in chow fed and retinol deficient rats, respectively. Similarly to lung, liver RAR-beta mRNA levels were 3-fold higher in retinol deficient rats fed all-trans-RA than the rats fed cottonseed oil. Lung RAR-gamma mRNA levels were also induced 2-fold by all-trans-RA. In contrast to this, 13-cis-RA and etretin at equimolar doses failed to enhance lung or liver RAR-beta or lung RAR-gamma mRNA levels in retinol deficient rats. These data for the first time show that all-trans-RA is more effective than its 13-cis-isomer in regulating the expression of RAR-beta and gamma transcripts in adult animal.

Immunosuppressive activity of 13-cis-retinoic acid and prevention of experimental autoimmune encephalomyelitis in rats

Some activities of retinoids on cellular and humoral immunity have been described, but the available data are conflicting or obtained at concentrations that are toxic in vivo. In this study, we demonstrate that 13-cis-retinoic acid (13-cRA), a retinoid well tolerated in human therapy, can suppress T cell-mediated immunity in rats. Treatment with pharmacological concentrations of 13-cRA prevented active as well as passive transfer experimental autoimmune encephalomyelitis (EAE) and suppressed lymphocyte responsiveness to T cell mitogens, suggesting that the drug activity included suppression of an effector T cell response. In addition, mitogen- and antigen-induced lymphocyte proliferation was inhibited in vitro in the presence of concentrations of 13-cRA equivalent to or less than those achieved in vivo, further suggesting that the prevention of EAE was due to a suppressive activity on T cell-mediated immunity. The immunosuppressive activity of 13-cRA included suppression of interleukin 2, whose production was inhibited in splenocytes. These data indicate that, in an in vivo mammalian system, 13-cRA exerts a suppressive activity on T cell-mediated immunity intensive enough to suppress an ongoing immune response, and that this effect can be achieved at nontoxic concentrations that may also be attained in human therapy.

Retinoic acid affects the expression of nuclear retinoic acid receptors in tissues of retinol-deficient rats

The multitude of biological effects of the vitamin A metabolite, retinoic acid, are mediated by nuclear retinoic acid receptors (RARs), which are members of the steroid/thyroid hormone receptor superfamily. RAR-alpha, -beta, and -gamma are encoded by three genes from which multiple isoforms can be generated. Recent studies suggest that the expression of at least some RAR isoforms can be regulated by retinoic acid in certain cell lines. Here we examined regulation of RAR expression in the adult animal. RARs were analyzed by Northern blots from lung, liver, and testes of retinol-deficient rats. Retinol deficiency caused a 65-70% decrease in the mRNA levels of lung and liver RAR-beta, whereas no change was observed in RAR-alpha and -gamma mRNA levels in these organs. In the testes of retinol-deficient animals, two transcripts, RAR-alpha 1 (3.7 kb) and RAR-alpha 2 (2.8 kb), were detected as compared with one RAR-alpha 1 (3.7 kb) transcript in retinol-sufficient testes. When retinol-deficient rats were orally administered 1 dose of retinoic acid (100 micrograms per rat), lung RAR-beta mRNA levels started to increase after 1 hr and reached a 16-fold higher level after 4 hr; after 4 hr these retinoic acid-fed rats also showed a 7-fold increase in liver RAR-beta mRNA levels as compared with levels in the retinol-deficient rats. In contrast, liver, lung, and testes RAR-alpha transcripts remained either unchanged or showed only a slight increase in response to retinoic acid. RAR-gamma was constitutively expressed in lung, and its mRNA levels were induced 2-fold by retinoic acid. These results show tissue diversity in the rapid induction of RAR-beta and RAR-gamma by retinoic acid in the adult animal and suggest distinct roles for the various receptor isoforms in the control of the retinoid response.

Retinoic acid stimulates the differentiation of PC12 cells that are deficient in cAMP-dependent protein kinase

Retinoic acid (RA) induced neuronal differentiation in A126-1B2 cells and 123.7 cells, two mutant lines of PC12 that are deficient in cAMP-dependent protein kinase, but not in the parental PC12 cell line. A single exposure to RA was sufficient to cause neurite formation and inhibit cell division for a period of greater than 3 wk, suggesting that RA may cause a long-term, stable change in the state of these cells. In A126-1B2 cells, RA also induced the expression of other markers of differentiation including acetylcholinesterase and the mRNAs for neurofilament (NF-M) and GAP-43 as effectively as nerve growth factor (NGF). Neither NGF nor RA stimulated an increase in the expression of smg-25A in A126-1B2 cells, suggesting that the cAMP-dependent protein kinases may be required for an increase in the expression of this marker. RA also caused a rapid increase in the expression of the early response gene, c-fos, but did not effect the expression of egr-1. RA equivalently inhibited the division of A126-1B2 cells, 123.7 cells and parental PC12 cells, so RA induced differentiation is not an indirect response to growth arrest. In contrast, the levels of retinoic acid receptors (RAR alpha and RAR beta), and retinoic acid binding protein (CRABP) mRNA were strikingly higher in both A126-1B2 cells and 123.7 cells than in the parental PC12 cells. The deficiencies in cAMP-dependent protein kinase may increase the expression of CRABP and the RARs; and, thus, cAMP may indirectly regulate the ability of RA to control neurite formation and neural differentiation. Thus, RA appears to regulate division and differentiation of PC12 cells by a biochemical mechanism that is quite distinct from those used by peptide growth factors.

Expression of nuclear retinoic acid receptors in rat adipose tissue

Expression of nuclear retinoic acid receptors (RAR-alpha, beta and gamma) was examined by Northern blots in rat lung, liver, and adipose tissue. Three transcripts of approximate sizes 7.3, 3.7, and 2.8 Kb were detected in adipose tissue. In contrast to adipose tissue only 3.7 and 2.8 Kb mRNA species were detected in liver and lung. Two RAR-beta gene transcripts (3.3 and 3.0 Kb) were expressed in adipose tissue, liver, and lung. RAR-gamma mRNA (3.36 Kb) was detected in adipose tissue and lung. The distribution of three RARs in adipose tissue suggests the physiological role of retinoic acid in the regulation of specific genes via RARs in adipocytes.

Physiological occurrence, biosynthesis and metabolism of retinoic acid: evidence for roles of cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) in the pathway of retinoic acid homeostasis

This article will address recent work on the physiological occurrence, biogenesis and metabolism of retinoic acid and summarize the data that retinoic acid is synthesized in situ in multiple tissues and cell types via enzymes or enzyme complexes that are distinct from the alcohol dehydrogenases. There is now considerable evidence that retinoic acid is an activated metabolite of retinol that supports the systemic functions of vitamin A in vivo. Many studies in vitro, for example, have shown that retinoic acid is the most potent naturally-occurring retinoid with an ED-50 in the range of 1 pM to 10 nM, depending on the assay system. This is below the tissue concentrations of retinoic acid which range from approximately 20-600 nM. Retinoic acid synthesis from retinol in the dog kidney cell line MDCK maintained in serum-free medium is inhibited by the prostanoid, PGE, and the phorbol ester, TPA. In tissues, one pathway of retinoic acid synthesis begins with apo-CRBP stimulating retinyl ester hydrolysis by a microsomal, cholate-independent retinyl ester hydrolase to form holo-CRBP. The holo-CRBP itself is used as substrate by an NADP-dependent, microsomal retinol dehydrogenase to generate retinal, which is converted into retinoic acid by a cytosolic NAD-dependent retinal dehydrogenase. Therefore, cellular retinol-binding protein (CRBP) apparently has at least 2 functions in retinoic acid synthesis: the apo form stimulates retinol mobilization from retinyl ester stores; the holo form delivers the retinol via direct transfer to dehydrogenase(s). Retinoic acid is converted into a mixture of at least 4 metabolites by testes microsomes which migrate closely on reverse-phase HPLC with 4-hydroxyretinoic acid, and may be mistaken for either 4-hydroxy or 4-oxo-retinoic acid. More rigorous analysis, however, shows that only one of them is 4-hydroxyretinoic acid, and another is 18-hydroxyretinoic acid. Two others remain unidentified. These metabolites are also formed in the presence of excess cellular retinoic acid-binding protein (CRABP), which increases the elimination half-life of retinoic acid, but does not prevent retinoic acid catabolism, suggesting that holo-CRABP may be a substrate for retinoic acid catabolism that modulates the steady-state concentrations of retinoic acid. Thus, both retinoid binding proteins, CRBP and CRABP, may each have direct roles as substrate in the biosynthesis and metabolism of retinoic acid, respectively.

Nuclear receptors for retinoids: mediators of retinoid effects on normal and malignant cells

Retinoids are a group of natural and synthetic vitamin A analogues that possess the unique ability to modulate the growth and differentiation of normal, premalignant and malignant epithelial and mesenchymal cells in vitro and in vivo. Naturally occurring retinoids act as physiologic regulators of embryonal development and maintain the proper differentiation of many epithelial tissues in the adult. Retinoids also act pharmacologically to restore regulation of differentiation and growth in certain premalignant and malignant cells in vitro and in vivo. Consequently, these compounds are being evaluated clinically for cancer prevention and therapy. There are strong indications that retinoids exert their diverse effects by regulating the expression of specific genes. The mechanism of this action is beginning to be unraveled following the discovery that nuclear receptors for retinoic acid are present in many cells and tissues. These receptors belong to the large family comprising steroid, thyroid hormone, and vitamin D3 receptors, which are DNA-binding proteins that function as trans-acting transcription modulating factors. We review here some of the properties of these receptors, their interactions with responsive elements in the promoter region of specific genes, and the recent demonstration of their involvement as mediators of the effects of retinoids on growth and differentiation.

Induction of beta-retinoic acid receptor mRNA by teratogenic doses of retinoids in murine fetuses

Retinoic acid is required for normal growth and development, however excessive doses are teratogenic. Recently several nuclear retinoic acid receptors (RAR) have been identified and postulated to mediate the response of retinoic acid at the gene level. We wished to determine if alpha-RAR mRNA or beta-RAR mRNA levels are modulated by teratogenic doses of retinoic acid in vivo. We have found that beta-RAR mRNA levels in 9-day-gestation mouse conceptuses are increased as early as 3 h after administration of a completely teratogenic dose of retinoic acid (100 mg/kg body weight; b.w.) and reach a maximum of approximately sixfold after 6 h of treatment. Maternal liver and maternal kidney demonstrated a similar pattern of increase in beta-RAR mRNA, however this was only approximately threefold. Retinoic acid dose-response experiments demonstrated a reduced increase of beta-RAR mRNA levels with 10 mg/kg b.w. (minimally teratogenic dose), and no increase with a more-physiological dose of 1 mg/kg b.w. in the conceptuses. beta-RAR mRNA levels were elevated in 18-day-gestation fetuses to a similar extent to that observed in the 9-day-gestation conceptuses. Therefore, the twofold difference in the extent to which beta-RAR mRNA levels increase does not occur because the fetuses are at a developmental stage that is sensitive to the teratogenic effects of retinoic acid. Finally, treatment with another teratogenic retinoid, etretinate, and a nonteratogenic retinoid, retinoyl beta-glucuronide, both resulted in increase in the level of beta-RAR mRNA in the conceptuses and the maternal tissues. Therefore, an increase in beta-RAR mRNA levels caused by treatment with retinoids does not necessarily commit a fetus to undergo an abnormal pattern of development characteristic of teratogenic retinoids.

The biological activity of retinoids in melanoma cells. Induction of expression of retinoic acid receptor-beta by retinoic acid in S91 melanoma cells

The expression of mRNA for retinoic acid receptor beta (RAR-beta) was induced by all trans-retinoic acid in murine S91 melanoma cells. The induction of RAR-beta was dose-dependent, rapid and insensitive to cycloheximide. Both 13-cis-retinoic acid and 3,4-didehydro-all trans-retinoic acid also induced expression of RAR-beta but were only effective at concentrations 100-fold greater than all trans-retinoic acid. The expression of RAR-alpha and RAR-gamma was unaffected by retinoic acid.

Retinoic acid, dibutyryl-cAMP, and differentiation affect the expression of retinoic acid receptors in F9 cells

Expression of the retinoic acid receptors alpha and beta (RAR-alpha and RAR-beta) was examined in F9 cells, an embryonal carcinoma cell model established for the study of retinoid metabolism and function. Addition of retinoic acid to F9 cell medium caused a dose-dependent increase in RAR-beta mRNA within 3 hr that reached 5- to 30-fold greater than the constitutively expressed mRNA by 24 hr. The elevation in mRNA resulted from increased transcription, as demonstrated by nuclear run-on transcription, did not require protein synthesis, and required the constant presence of retinoic acid. N6,O2'-Dibutyryl-cAMP attenuated the retinoic acid-induced increase in RAR-beta mRNA by a post-transcriptional mechanism. In contrast, RAR-alpha mRNA in F9 stem cells was affected less (1.2- to 1.4-fold increase) by retinoic acid and decreased 3-fold transiently when fresh serum was added to the medium. Differentiation of F9 cells resulted in increased steady-state levels of RAR-beta mRNA in primitive (4-fold), parietal (3-fold), and visceral (8-fold) endoderm but decreased steady-state levels of RAR-alpha mRNA in primitive (2-fold), parietal (3-fold), and visceral (1.4-fold) endoderm. These data demonstrate that RAR-beta is a primary target gene for retinoic acid in a characterized model of retinoid function, indicate that constitutive expression of both RAR-beta and RAR-alpha is dependent upon the differentiation state, and suggest hormonal modulation of RAR-beta by cAMP and modulation of RAR-alpha by serum factors. These results distinguish the effects of serum, cAMP, and retinoic acid on the expression of RAR from the effects mediated by differentiation.

Retinoids, sex steroids and glucocorticoids regulate ectocervical cell envelope formation but not the level of the envelope precursor, involucrin

In spite of extensive study of the reproductive tract, little knowledge is available regarding the function of ectocervical epithelial (ECE) cells. In the present study we utilized a feeder layer of 3T3 cells to grow homogeneous cultures of human ectocervical epithelial cells and demonstrated the presence of the cornified envelope precursor, involucrin. Treatment of these cultures with 1 nM Ro 13-6298, a synthetic analogue of trans-retinoic acid, suppresses envelope formation 6-fold with half-maximal suppression at 0.005-0.01 nM. Treatment with 1 microM hydrocortisone elevates envelope production 2.5-fold. Sex steroids also regulate desquamation: 10 nM diethylstilbestrol, a synthetic estrogen, increases envelope levels 2- to 3-fold, while 300 nM progesterone reduces envelope production 2- to 3-fold. In spite of the retinoid-, glucocorticoid- and sex-steroid-stimulated changes in envelope production, the level of the envelope precursor, involucrin, remains constant. Our results suggest: (1) that, in vivo, ectocervical cell squame formation is regulated by the combined direct action of estrogens, progestins, glucocorticoids and retinoids; and (2) that envelope formation is not regulated by changes in the cellular content of the envelope precursor, involucrin. We present a model summarizing the estrogen, progestin, glucocorticoid and retinoid effects on ectocervical epithelial cell function.

Expression of the alpha and beta retinoic acid receptors in skin

Retinoic acid receptor (RAR) -alpha and -beta transcripts are expressed in rat and human skin, and in rat and human dermal fibroblasts and keratinocytes in vitro. RAR-alpha transcripts (ca. 2.8 and 3.6 kb) were expressed in all tissues but were more abundant in dermis and dermal fibroblasts than in epidermis or keratinocytes. RAR-beta mRNA was expressed in skin, but patterns of expression differed between human and neonatal rat samples. In human dermal fibroblasts, keratinocytes and whole skin, two RAR-beta transcripts (ca. 3.1 and 3.4 kb) were expressed. Conversely, in neonatal rat skin, dermal fibroblasts, and keratinocytes only the smaller transcript was detectable and was more abundant in cultured cells than in whole tissue. These results suggest that retinoic acid may have complex, as yet undefined, RAR-mediated regulatory functions in both dermis and epidermis.

Spatial and temporal expression pattern of retinoic acid receptor genes during mouse bone development

Spatial and temporal expression pattern of retinoic acid receptor (RAR) genes was investigated in mouse finger bones during development by an in situ hybridization method with riboprobes synthesized from a human cDNA of the RAR-alpha. We found that the RAR genes are expressed intensively and specifically in calcifying fronts of the mouse finger bones, whereas the expression pattern is rather uniform in the limb buds and cartilage matrices of the embryonic fingers. Our findings are consistent with the fact that vitamin A is essential for normal mammalian bone development.