Retinoic acid and synthetic analogs differentially activate retinoic acid receptor dependent transcription

Retinoid metabolism: new insights

Vitamin A (retinol) is a critical micronutrient required for the control of stem cell functions, cell differentiation, and cell metabolism in many different cell types, both during embryogenesis and in the adult organism. However, we must obtain vitamin A from food sources. Thus, the uptake and metabolism of vitamin A by intestinal epithelial cells, the storage of vitamin A in the liver, and the metabolism of vitamin A in target cells to more biologically active metabolites, such as retinoic acid (RA) and 4-oxo-RA, must be precisely regulated. Here, I will discuss the enzymes that metabolize vitamin A to RA and the cytochrome P450 Cyp26 family of enzymes that further oxidize RA. Because much progress has been made in understanding the regulation of ALDH1a2 (RALDH2) actions in the intestine, one focus of this review is on the metabolism of vitamin A in intestinal epithelial cells and dendritic cells. Another focus is on recent data that 4-oxo-RA is a ligand required for the maintenance of hematopoietic stem cell dormancy and the important role of RARβ (RARB) in these stem cells. Despite this progress, many questions remain in this research area, which links vitamin A metabolism to nutrition, immune functions, developmental biology, and nuclear receptor pharmacology.

P450 oxidoreductase regulates barrier maturation by mediating retinoic acid metabolism in a model of the human BBB

The blood-brain barrier (BBB) selectively regulates the entry of molecules into the central nervous system (CNS). A crosstalk between brain microvascular endothelial cells (BMECs) and resident CNS cells promotes the acquisition of functional tight junctions (TJs). Retinoic acid (RA), a key signaling molecule during embryonic development, is used to enhance in vitro BBB models’ functional barrier properties. However, its physiological relevance and affected pathways are not fully understood. P450 oxidoreductase (POR) regulates the enzymatic activity of microsomal cytochromes. POR-deficient (PORD) patients display impaired steroid homeostasis and cognitive disabilities. Here, we used both patient-specific POR-deficient and CRISPR-Cas9-mediated POR-depleted induced pluripotent stem cell (iPSC)-derived BMECs (iBMECs) to study the role of POR in the acquisition of functional barrier properties. We demonstrate that POR regulates cellular RA homeostasis and that POR deficiency leads to the accumulation of RA within iBMECs, resulting in the impaired acquisition of TJs and, consequently, to dysfunctional development of barrier properties.

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Retinoic acid (RA) promotes functional barrier properties

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POR-deficient iPS-brain endothelial-like cells display impaired barrier development

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POR mediates CYP26-dependent cellular RA catabolism

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RA accumulation induces a pro-inflammatory response

CYP26C1 Is a Hydroxylase of Multiple Active Retinoids and Interacts with Cellular Retinoic Acid Binding Proteins

The clearance of retinoic acid (RA) and its metabolites is believed to be regulated by the CYP26 enzymes, but the specific roles of CYP26A1, CYP26B1, and CYP26C1 in clearing active vitamin A metabolites have not been defined. The goal of this study was to establish the substrate specificity of CYP26C1, and determine whether CYP26C1 interacts with cellular retinoic acid binding proteins (CRABPs). CYP26C1 was found to effectively metabolize all-trans retinoic acid (atRA), 9-cis-retinoic acid (9-cis-RA), 13-cis-retinoic acid, and 4-oxo-atRA with the highest intrinsic clearance toward 9-cis-RA. In comparison with CYP26A1 and CYP26B1, CYP26C1 resulted in a different metabolite profile for retinoids, suggesting differences in the active-site structure of CYP26C1 compared with other CYP26s. Homology modeling of CYP26C1 suggested that this is attributable to the distinct binding orientation of retinoids within the CYP26C1 active site. In comparison with other CYP26 family members, CYP26C1 was up to 10-fold more efficient in clearing 4-oxo-atRA (intrinsic clearance 153 μl/min/pmol) than CYP26A1 and CYP26B1, suggesting that CYP26C1 may be important in clearing this active retinoid. In support of this, CRABPs delivered 4-oxo-atRA and atRA for metabolism by CYP26C1. Despite the tight binding of 4-oxo-atRA and atRA with CRABPs, the apparent Michaelis-Menten constant in biological matrix (K_m) value of these substrates with CYP26C1 was not increased when the substrates were bound with CRABPs, in contrast to what is predicted by free drug hypothesis. Together these findings suggest that CYP26C1 is a 4-oxo-atRA hydroxylase and may be important in regulating the concentrations of this active retinoid in human tissues.

Retinoic Acid-Signaling Regulates the Proliferative and Neurogenic Capacity of Müller Glia-Derived Progenitor Cells in the Avian Retina

In the retina, Müller glia have the potential to become progenitor cells with the ability to proliferate and regenerate neurons. However, the ability of Müller glia-derived progenitor cells (MGPCs) to proliferate and produce neurons is limited in higher vertebrates. Using the chick model system, we investigate how retinoic acid (RA)-signaling influences the proliferation and the formation of MGPCs. We observed an upregulation of cellular RA binding proteins (CRABP) in the Müller glia of damaged retinas where the formation of MGPCs is known to occur. Activation of RA-signaling was stimulated, whereas inhibition suppressed the proliferation of MGPCs in damaged retinas and in fibroblast growth factor 2-treated undamaged retinas. Furthermore, inhibition of RA-degradation stimulated the proliferation of MGPCs. Levels of Pax6, Klf4, and cFos were upregulated in MGPCs by RA agonists and downregulated in MGPCs by RA antagonists. Activation of RA-signaling following MGPC proliferation increased the percentage of progeny that differentiated as neurons. Similarly, the combination of RA and insulin-like growth factor 1 (IGF1) significantly increased neurogenesis from retinal progenitors in the circumferential marginal zone (CMZ). In summary, RA-signaling stimulates the formation of proliferating MGPCs and enhances the neurogenic potential of MGPCs and stem cells in the CMZ. Stem Cells 2018;36:392-405.

Role of Retinoic Acid-Metabolizing Cytochrome P450s, CYP26, in Inflammation and Cancer

Vitamin A (retinol) and its active metabolite, all-trans-retinoic acid (atRA), play critical roles in regulating the differentiation, growth, and migration of immune cells. Similarly, as critical signaling molecules in the regulation of the cell cycle, retinoids are important in cancers. Concentrations of atRA are tightly regulated in tissues, predominantly by the availability of retinol, synthesis of atRA by ALDH1A enzymes and metabolism and clearance of atRA by CYP26 enzymes. The ALDH1A and CYP26 enzymes are expressed in several cell types in the immune system and in cancer cells. In the immune system, the ALDH1A and CYP26 enzymes appear to modulate RA concentrations. Consequently, alterations in the activity of ALDH1A and CYP26 enzymes are expected to change disease outcomes in inflammation. There is increasing evidence from various disease models of intestinal and skin inflammation that treatment with atRA has a positive effect on disease markers. However, whether aberrant atRA concentrations or atRA synthesis and metabolism play a role in inflammatory disease development and progression is not well understood. In cancers, especially in acute promyelocytic leukemia and neuroblastoma, increasing intracellular concentrations of atRA appears to provide clinical benefit. Inhibition of the CYP26 enzymes to increase atRA concentrations and combat therapy resistance has been pursued as a drug target in these cancers. This chapter covers the current knowledge of how atRA and retinol regulate the immune system and inflammation, how retinol and atRA metabolism is altered in inflammation and cancer, and what roles atRA-metabolizing enzymes have in immune responses and cancers.

Hierarchical differentiation competence in response to retinoic acid ensures stem cell maintenance during mouse spermatogenesis

Stem cells ensure tissue homeostasis through the production of differentiating and self-renewing progeny. In some tissues, this is achieved by the function of a definitive stem cell niche. However, the mechanisms that operate in mouse spermatogenesis are unknown because undifferentiated spermatogonia (Aundiff) are motile and intermingle with differentiating cells in an 'open' niche environment of seminiferous tubules. Aundiff include glial cell line-derived neurotrophic factor receptor α1 (GFRα1)(+) and neurogenin 3 (NGN3)(+) subpopulations, both of which retain the ability to self-renew. However, whereas GFRα1(+) cells comprise the homeostatic stem cell pool, NGN3(+) cells show a higher probability to differentiate into KIT(+) spermatogonia by as yet unknown mechanisms. In the present study, by combining fate analysis of pulse-labeled cells and a model of vitamin A deficiency, we demonstrate that retinoic acid (RA), which may periodically increase in concentration in the tubules during the seminiferous epithelial cycle, induced only NGN3(+) cells to differentiate. Comparison of gene expression revealed that retinoic acid receptor γ (Rarg) was predominantly expressed in NGN3(+) cells, but not in GFRα1(+) cells, whereas the expression levels of many other RA response-related genes were similar in the two populations. Ectopic expression of RARγ was sufficient to induce GFRα1(+) cells to directly differentiate to KIT(+) cells without transiting the NGN3(+) state. Therefore, RARγ plays key roles in the differentiation competence of NGN3(+) cells. We propose a novel mechanism of stem cell fate selection in an open niche environment whereby undifferentiated cells show heterogeneous competence to differentiate in response to ubiquitously distributed differentiation-inducing signals.

Development of yeast reporter assay for screening specific ligands of retinoic acid and retinoid X receptor subtypes

INTRODUCTION

Retinoic acids are essential for embryonic development, tissue organization, and homeostasis and act via retinoic acid receptors (RARs) that form heterodimers with retinoid X receptors (RXRs). Human RARs and RXRs include the three subtypes α, β, and γ, which have varying distributions and physiological functions among human tissues. Recent reports show that subtype-specific binding of several chemicals to RARs or RXRs may lead to endocrine disruption. To evaluate these ligand-like chemicals, convenient assay systems for each receptor subtype are required.

METHODS

We developed reporter assay yeasts to screen ligands for RXR subtype receptor homodimers. To screen RAR ligands, yeasts were engineered to express RAR subtypes with defective RXRα, which fails to bind to coactivators because of its shortened c-terminus.

RESULTS

These assay yeasts were validated using known RXR- and RAR-specific ligands and subtype-specific responses were clearly shown. Subtype-specific ligand activities of the suspected chemical RAR or RXR ligands o-t-butylphenol, triphenyltin chloride, tributyltin chloride, and 4-nonylphenol were determined.

DISCUSSION

The present assay yeasts may be valuable tools for subtype-specific assessments of unidentified environmental ligand chemicals and receptor-specific pharmaceuticals.

Reversal by RARα agonist Am580 of c-Myc-induced imbalance in RARα/RARγ expression during MMTV-Myc tumorigenesis

INTRODUCTION

Retinoic acid signaling plays key roles in embryonic development and in maintaining the differentiated status of adult tissues. Recently, the nuclear retinoic acid receptor (RAR) isotypes α, β and γ were found to play specific functions in the expansion and differentiation of the stem compartments of various tissues. For instance, RARγ appears to be involved in stem cell compartment expansion, while RARα and RARβ are implicated in the subsequent cell differentiation. We found that over-expressing c-Myc in normal mouse mammary epithelium and in a c-Myc-driven transgenic model of mammary cancer, disrupts the balance between RARγ and RARα/β in favor of RARγ.

METHODS

The effects of c-Myc on RAR isotype expression were evaluated in normal mouse mammary epithelium, mammary tumor cells obtained from the MMTV-Myc transgenic mouse model as well as human normal immortalized breast epithelial and breast cancer cell lines. The in vivo effect of the RARα-selective agonist 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carboxamido]benzoic acid (Am580) was examined in the MMTV-Myc mouse model of mammary tumorigenesis.

RESULTS

Modulation of the RARα/β to RARγ expression in mammary glands of normal mice, oncomice, and human mammary cell lines through the alteration of RAR-target gene expression affected cell proliferation, survival and tumor growth. Treatment of MMTV-Myc mice with the RARα-selective agonist Am580 led to significant inhibition of mammary tumor growth (~90%, P<0.001), lung metastasis (P<0.01) and extended tumor latency in 63% of mice. Immunocytochemical analysis showed that in these mice, RARα responsive genes such as Cyp26A1, E-cadherin, cellular retinol-binding protein 1 (CRBP1) and p27, were up-regulated. In contrast, the mammary gland tumors of mice that responded poorly to Am580 treatment (37%) expressed significantly higher levels of RARγ. In vitro experiments indicated that the rise in RARγ was functionally linked to promotion of tumor growth and inhibition of differentiation. Thus, activation of the RARα pathway is linked to tumor growth inhibition, differentiation and cell death.

CONCLUSIONS

The functional consequence of the interplay between c-Myc oncogene expression and the RARγ to RARα/β balance suggests that prevalence of RARγ over-RARα/β expression levels in breast cancer accompanied by c-Myc amplification or over-expression in breast cancer should be predictive of response to treatment with RARα-isotype-specific agonists and warrant monitoring during clinical trials.

Retinoic acid and androgen receptors combine to achieve tissue specific control of human prostatic transglutaminase expression: a novel regulatory network with broader significance

In the human prostate, expression of prostate-specific genes is known to be directly regulated by the androgen-induced stimulation of the androgen receptor (AR). However, less is known about the expression control of the prostate-restricted TGM4 (hTGP) gene. In the present study we demonstrate that the regulation of the hTGP gene depends mainly on retinoic acid (RA). We provide evidence that the retinoic acid receptor gamma (RAR-G) plays a major role in the regulation of the hTGP gene and that presence of the AR, but not its transcriptional transactivation activity, is critical for hTGP transcription. RA and androgen responsive elements (RARE and ARE) were mapped to the hTGP promoter by chromatin immunoprecipitation (ChIP), which also indicated that the active ARE and RARE sites were adjacent, suggesting that the antagonistic effect of androgen and RA is related to the relative position of binding sites. Publicly available AR and RAR ChIP-seq data was used to find gene potentially regulated by AR and RAR. Four of these genes (CDCA7L, CDK6, BTG1 and SAMD3) were tested for RAR and AR binding and two of them (CDCA7L and CDK6) proved to be antagonistically regulated by androgens and RA confirming that this regulation is not particular of hTGP.

Regulation of retinoid receptors by retinoic acid and axonal contact in Schwann cells

BACKGROUND

Schwann cells (SCs) are the cell type responsible for the formation of the myelin sheath in the peripheral nervous system (PNS). As retinoic acid (RA) and other retinoids have a profound effect as regulators of the myelination program, we sought to investigate how their nuclear receptors levels were regulated in this cell type.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, by using Schwann cells primary cultures from neonatal Wistar rat pups, as well as myelinating cocultures of Schwann cells with embryonic rat dorsal root ganglion sensory neurons, we have found that sustained expression of RXR-γ depends on the continuous presence of a labile activator, while axonal contact mimickers produced an increase in RXR-γ mRNA and protein levels, increment that could be prevented by RA. The upregulation by axonal contact mimickers and the transcriptional downregulation by RA were dependent on de novo protein synthesis and did not involve changes in mRNA stability. On the other hand, RAR-β mRNA levels were only slightly modulated by axonal contact mimickers, while RA produced a strong transcriptional upregulation that was independent of de novo protein synthesis without changes in mRNA stability.

CONCLUSIONS/SIGNIFICANCE

All together, our results show that retinoid receptors are regulated in a complex manner in Schwann cells, suggesting that they could have a prominent role as regulators of Schwann cell physiology.

Retinoic acid regulates myelin formation in the peripheral nervous system

Understanding the mechanisms that control myelin formation is essential for the development of demyelinating diseases treatments. All-trans-retinoic acid (RA) plays an essential role during the development of the nervous system as a potent regulator of morphogenesis, cell growth, and differentiation. In this study, we show that RA is also a potent inhibitor of peripheral nervous system (PNS) myelination. RA acts through its binding to RA receptors (RAR) and retinoid X receptors (RXR), two members of the superfamily of nuclear receptors that act as ligand-dependent transcription factors. Schwann cells (SCs) express all retinoid receptors during the relevant stages of myelin formation. Through the activation of RXR, RA produces an upregulation of Krox20, a SC-specific regulatory transcription factor that plays a central role during myelination. Krox20 upregulation translates into Mbp and Mpz overexpression, therefore blocking myelin formation. This increase in myelin protein expression is accompanied by the induction of an adaptive ER stress response. At the same time, through a RAR-dependent mechanism, RA downregulates myelin-associated glycoprotein, which also contributes to the dysmyelinating effect of the retinoid.

{beta}-Apocarotenoids do not significantly activate retinoic acid receptors {alpha} or {beta}

beta-Carotene oxygenase 2 cleaves beta-carotene asymmetrically at non-central double bonds of the polyene chain, yielding apocarotenal molecules. The hypothesis tested was that apocarotenoids are able to stimulate transcription by activating retinoic acid receptors (RARs). The effects of long- and short-chain apocarotenals and apocarotenoic acids on the activation of RARalpha and RARbeta transfected into monkey kidney fibroblast cells (CV-1) were investigated. We synthesized or purified beta-apo-8'-carotenoic acid (apo-8'-CA), beta-apo-14'-carotenoic acid (apo-14'-CA), beta-cyclocitral (BCL), beta-cyclogernanic acid (BCA), beta-ionone (BI), beta-ionylideneacetaldehyde (BIA) beta-ionylideneacetic acid (BIAA) and a C13 ketone, beta-apo-13-carotenone (C13). None of the apocarotenoids tested showed significant transactivation activity for the RARs when compared with all-trans retinoic acid (RA). The results suggest that biological effects of these apocarotenoids are through mechanisms other than activation of RARalpha and beta.

Mechanism of inhibition of MMTV-neu and MMTV-wnt1 induced mammary oncogenesis by RARalpha agonist AM580

We hypothesized that specific activation of a single retinoic acid receptor-alpha (RARalpha), without direct and concurrent activation of RARbeta and gamma, will inhibit mammary tumor oncogenesis in murine models relevant to human cancer. A total of 50 uniparous mouse mammary tumor virus (MMTV)-neu and 50 nuliparous MMTV-wnt1 transgenic mice were treated with RARalpha agonist (retinobenzoic acid, Am580) that was added to the diet for 40 (neu) and 35 weeks (wnt1), respectively. Among the shared antitumor effects was the inhibition of epithelial hyperplasia, a significant increase (P<0.05) in tumor-free survival and a reduction in tumor incidence and in the growth of established tumors. In both models, the mechanisms responsible for these effects involved inhibition of proliferation and survival pathways, and induction of apoptosis. The treatment was more effective in the MMTV-wnt1 model in which Am580 also induced differentiation, in both in vivo and three-dimensional (3D) cultures. In these tumors Am580 inhibited the wnt pathway, measured by loss of nuclear beta-catenin, suggesting partial oncogene dependence of therapy. Am580 treatment increased RARbeta and lowered the level of RARgamma, an isotype whose expression we linked with tumor proliferation. The anticancer effect of RARalpha, together with the newly discovered pro-proliferative role of RARgamma, suggests that specific activation of RARalpha and inhibition of RARgamma might be effective in breast cancer therapy.

Testing potential developmental toxicants with a cytotoxicity assay based on human embryonic stem cells

Since the differentiation of embryonic stem cells mimics early development, these cells could potentially permit the detection of embryotoxicants which interfere with this process. Although reliable tests based on murine embryonic stem cells exist, no such methods are available for human embryonic stem (hES) cells. Nonetheless, to avoid the false classification of substances due to inter-species differences, human-relevant toxicity tests are needed. We therefore developed an assay based on three human cell types, representing different degrees of developmental maturation, namely, human foreskin fibroblasts, hES cell-derived progenitor cells, and pluripotent hES cells. A set of embryotoxicants for which existing in vivo data were available, namely, all-trans retinoic acid (ATRA), 13-cis retinoic acid (13CRA), valproic acid (VPA) and dimethyl sulphoxide (DMSO), were tested. 5-fluorouracil (5-FU) was used as a positive control, and saccharin as a negative control. Two methods were compared for the assessment of cell viability -- the determination of intracellular ATP content and of resazurin reduction. In addition, the protective capacity of basic fibroblast growth factor (bFGF) against retinoid-induced toxicity was investigated. This novel assay system reliably detected the embryotoxic potentials of the test substances, 5-FU, ATRA, 13-CRA (a substance that displays inter-species differences in its effects) and VPA. This was possible due to the apparent differences in the sensitivities of the human cell types used in the assay system. Thus, our results clearly indicate the advantages and relevance of using hES cells in in vitro developmental toxicity testing.

The Retinoic Acid Receptor-alpha mediates human T-cell activation and Th2 cytokine and chemokine production

BACKGROUND

We have recently demonstrated that all-trans-retinoic acid (ATRA) and 9-cis-retinoic acid (9-cis RA) promote IL-4, IL-5 and IL-13 synthesis, while decreasing IFN-gamma and TNF-alpha expression by activated human T cells and reduces the synthesis of IL-12p70 from accessory cells. Here, we have demonstrated that the observed effects using ATRA and 9-cis RA are shared with the clinically useful RAR ligand, 13-cis retinoic acid (13-cis RA), and the retinoic acid receptor-alpha (RAR-alpha)-selective agonist, AM580 but not with the RAR-beta/gamma ligand, 4-hydroxyphenylretinamide (4-HPR).

RESULTS

The increase in type 2 cytokine production by these retinoids correlated with the expression of the T cell activation markers, CD69 and CD38. The RAR-alpha-selective agonist, AM580 recapitulated all of the T cell activation and type 2 cytokine-inducing effects of ATRA and 9-cis-RA, while the RAR-alpha-selective antagonist, RO 41-5253, inhibited these effects.

CONCLUSION

These results strongly support a role for RAR-alpha engagement in the regulation of genes and proteins involved with human T cell activation and type 2 cytokine production.

Evaluation of action mechanisms of toxic chemicals using JFCR39, a panel of human cancer cell lines

We previously established a panel of human cancer cell lines, JFCR39, coupled to an anticancer drug activity database; this panel is comparable with the NCI60 panel developed by the National Cancer Institute. The JFCR39 system can be used to predict the molecular targets or evaluate the action mechanisms of the test compounds by comparing their cell growth inhibition profiles (i.e., fingerprints) with those of the standard anticancer drugs using the COMPARE program. In this study, we used this drug activity database-coupled JFCR39 system to evaluate the action mechanisms of various chemical compounds, including toxic chemicals, agricultural chemicals, drugs, and synthetic intermediates. Fingerprints of 130 chemicals were determined and stored in the database. Sixty-nine of 130 chemicals ( approximately 60%) satisfied our criteria for the further analysis and were classified by cluster analysis of the fingerprints of these chemicals and several standard anticancer drugs into the following three clusters: 1) anticancer drugs, 2) chemicals that shared similar action mechanisms (for example, ouabain and digoxin), and 3) chemicals whose action mechanisms were unknown. These results suggested that chemicals belonging to a cluster (i.e., a cluster of toxic chemicals, a cluster of anticancer drugs, etc.) shared similar action mechanism. In summary, the JFCR39 system can classify chemicals based on their fingerprints, even when their action mechanisms are unknown, and it is highly probable that the chemicals within a cluster share common action mechanisms.

Tributyltin and triphenyltin inhibit osteoclast differentiation through a retinoic acid receptor-dependent signaling pathway

Organotin compounds, such as tributyltin (TBT) and triphenyltin (TPT), have been widely used in agriculture and industry. Although these compounds are known to have many toxic effects, including endocrine-disrupting effects, their effects on bone resorption are unknown. In this study, we investigated the effects of organotin compounds, such as monobutyltin (MBT), dibutyltin (DBT), TBT, and TPT, on osteoclast differentiation using mouse monocytic RAW264.7 cells. MBT and DBT had no effects, whereas TBT and TPT dose-dependently inhibited osteoclast differentiation at concentrations of 3-30 nM. Treatment with a retinoic acid receptor (RAR)-specific antagonist, Ro41-5253, restored the inhibition of osteoclastogenesis by TBT and TPT. TBT and TPT reduced receptor activator of nuclear factor-kappaB ligand (RANKL) induced nuclear factor of activated T cells (NFAT) c1 expression, and the reduction in NFATc1 expression was recovered by Ro41-5253. Our results suggest that TBT and TPT suppress osteoclastogenesis by inhibiting RANKL-induced NFATc1 expression via an RAR-dependent signaling pathway.

Modulation of hepatocyte growth factor induction in human skin fibroblasts by retinoic acid

Topical treatment of skin with all-trans-retinoic acid (ATRA), the major biologically active form of vitamin A, results in hyperproliferation of basal keratinocytes, leading to an accelerated turnover of epidermis cells and thickening of the epidermis, probably via induction of production of paracrine growth factors for keratinocytes in epidermal suprabasal keratinocytes and/or dermal fibroblasts. Since hepatocyte growth factor (HGF) is a factor mitogenic to epidermal keratinocytes secreted from dermal fibroblasts, the effect of ATRA on basal and induced HGF production in human dermal fibroblasts in culture was examined. ATRA alone did not induce HGF production, but it significantly enhanced HGF production induced by the cAMP-elevating agent cholera toxin or the membrane-permeable cAMP analog 8-bromo-cAMP. Cholera toxin-induced activation of cAMP responsive element (CRE)-binding protein (CREB) was enhanced by pretreating cells with ATRA for 24 h. In contrast, HGF production induced by epidermal growth factor (EGF) or phorbol 12-myristate 13-acetate (PMA) was potently inhibited by ATRA. These modulatory effects of ATRA were different from the effects of transforming growth factor-beta1 (TGF-beta) and dexamethasone, both of which inhibited HGF production induced by all of the four inducers. Up-regulation of HGF gene expression by cholera toxin and EGF was also enhanced and inhibited, respectively, by ATRA. Both 9-cis-retinoic acid (9-cis-RA) and 13-cis-retinoic acid (13-cis-RA), which are stereo-isomers of ATRA, showed a modulatory effect on HGF induction similar to that of ATRA. These results suggest that ATRA augments the induction of HGF production caused by increased intracellular cAMP.

Increasing the intracellular availability of all-trans retinoic acid in neuroblastoma cells

Recent data indicate that isomerisation to all-trans retinoic acid (ATRA) is the key mechanism underlying the favourable clinical properties of 13-cis retinoic acid (13cisRA) in the treatment of neuroblastoma. Retinoic acid (RA) metabolism is thought to contribute to resistance, and strategies to modulate this may increase the clinical efficacy of 13cisRA. The aim of this study was to test the hypothesis that retinoids, such as acitretin, which bind preferentially to cellular retinoic acid binding proteins (CRABPs), or specific inhibitors of the RA hydroxylase CYP26, such as R116010, can increase the intracellular availability of ATRA. Incubation of SH-SY5Y cells with acitretin (50 μM) or R116010 (1 or 10 μM) in combination with either 10 μM ATRA or 13cisRA induced a selective increase in intracellular levels of ATRA, while 13cisRA levels were unaffected. CRABP was induced in SH-SY5Y cells in response to RA. In contrast, acitretin had no significant effect on intracellular retinoid concentrations in those neuroblastoma cell lines that showed little or no induction of CRABP after RA treatment. Both ATRA and 13cisRA dramatically induced the expression of CYP26A1 in SH-SY5Y cells, and treatment with R116010, but not acitretin, potentiated the RA-induced expression of a reporter gene and CYP26A1. The response of neuroblastoma cells to R116010 was consistent with inhibition of CYP26, indicating that inhibition of RA metabolism may further optimise retinoid treatment in neuroblastoma.

Retinoids increase alpha-1 acid glycoprotein expression at the transcriptional level through two distinct DR1 retinoic acid responsive elements

In the present study, we analyzed the influence of retinoic acids on the expression of alpha-1 acid glycoprotein (AGP). We show that in rat primary hepatocytes, 9-cis retinoic acid and all-trans retinoic acid increase AGP gene expression at the transcriptional level. Transient transfections of rat primary hepatocytes with a reporter construct driven by the rat AGP gene promoter indicated that retinoids regulate AGP gene expression via the -763/-138 region of the AGP promoter. Furthermore, cotransfection experiments with retinoic acid receptor alpha (RARalpha) and retinoid X receptor alpha (RXRalpha) expression vectors in NIH3T3 cells demonstrated that both RXRalpha/RXRalpha homodimer and RXRalpha/RARalpha heterodimer are competent for ligand-induced transactivation of the AGP promoter. Unilateral deletion and site-directed mutagenesis identified two retinoic-acid responsive elements (RARE), RARE-I and RARE-II, which interestingly correspond to a direct repeat of two TGACCT-related hexanucleotides separated by a single bp only (DR1-type response element). Cotransfection assays showed that RXRalpha and RARalpha activate AGP gene transcription through these two elements either as a homodimer (RXRalpha/RXRalpha) or as a heterodimer (RXRalpha/RARalpha). The RXRalpha/RXRalpha homodimer acts most efficiently through the RARE-I response element to promote AGP transactivation, whereas the RXRalpha/RARalpha heterodimer mediates transactivation better via the RARE-II responsive element.

Induction of lysosomal phospholipase A2 through the retinoid X receptor in THP-1 cells

An acidic phospholipase A(2) (LPLA(2)) was recently purified and cloned. THP-1 cells were used to characterize the gene induction of LPLA(2). THP-1 cells were stimulated with several differentiation agents. The LPLA(2) mRNA and activity increased in cells treated with phorbol ester but not with vitamin D3, interferon-gamma, or granulocyte macrophage colony-stimulating factor. All-trans-retinoic acid enhanced mRNA expression and enzyme activity in a dose- and time-dependent manner. The natural 9-cis and 13-cis isomers of retinoic acid enhanced transcription and activity. Two classes of nuclear receptors, the retinoic acid receptor (RAR) and the retinoid X receptor (RXR), mediate retinoic acid signaling. Specific RAR and RXR agonists were used to identify the nuclear receptor responsible for LPLA(2) induction by retinoic acid. Treatment with the RAR agonist 4-[E-2-tetrahydro-5,5,8,8-tetra-methyl-2-naphthalenyl]1-propenyl benzoic acid (TTNPB) resulted in a small and statistically significant increase of the mRNA expression and activity of LPLA(2). The RXR agonist methoprene acid worked as well as all-trans-retinoic acid at increasing both mRNA and enzyme activity. The methoprene acid and TTNPB effects were not synergistic. The peroxisome proliferator-activated receptor gamma agonists 15-deoxy-Delta(12,14)-prostaglandin J(2) and troglitazone failed to induce LPLA(2) activity and mRNA. Thus, an RXR-dependent pathway controls LPLA(2) gene activation by retinoic acid in THP-1 cells.

Combined RAR alpha- and RXR-specific ligands overcome N-myc-associated retinoid resistance in neuroblastoma cells

Retinoids induce human neuroblastoma cells to undergo growth inhibition and neuritic differentiation in vitro, through interactions with nuclear retinoid receptor proteins. In this study, we found that three different neuroblastoma cell lines exhibited wide variation in their responsiveness to the growth inhibitory effects of the retinoic acid receptor (RAR) agonist, all-trans-retinoic acid (aRA). Resistance to the growth inhibitory effect of aRA correlated with the presence of N-myc gene amplification and not aRA-induced RAR beta levels. Over-expression of N-myc in a neuroblastoma cell line with no endogenous N-myc expression caused a marked reduction in retinoid-induced growth inhibition. Combination of receptor-specific retinoid agonists for RXR and RAR alpha significantly enhanced the sensitivity of N-myc-amplified neuroblastoma cells to the growth inhibitory effects of aRA. Our results indicate that combination receptor-specific retinoid therapy can overcome N-myc-mediated retinoid resistance and may be a more effective chemo-preventive strategy in the disease.

Activation of retinoic acid receptor-dependent transcription by all-trans-retinoic acid metabolites and isomers

We have shown that four metabolites of all-trans-retinoic acid (ATRA) (4-oxo-, 4-OH-, 18-OH-, and 5,6-epoxy-RA) can induce maturation of NB4 promyelocytic leukemia cells (Idres, N., Benoit, G., Flexor, M. A., Lanotte, M., and Chabot, G. G. (2001) Cancer Res. 61, 700-705). To better understand the mechanism of action of ATRA metabolites and isomers, we assessed their binding to retinoic acid receptors (RARs) and activation of RAR-mediated transcription via a retinoic acid response element (RARE). Competition binding experiments with tritiated ATRA showed that all metabolites could bind to RARs with variable affinity. For transactivation studies, COS-7 cells were cotransfected with RAR alpha, beta, or gamma expression vectors and the reporter plasmid RARE-tk-Luc, and the retinoid concentrations for half-maximal luciferase activity (EC(50)) were determined. All retinoids tested could activate the three RAR isotypes. The lowest EC(50) value for RAR alpha was with 9-cis-RA (13 nM), followed by 4-oxo-RA (33 nM), 5,6-epoxy-RA (77 nM), 13-cis-RA (124 nM), 18-OH-RA (162 nM), ATRA (169 nM), and 4-OH-RA (791 nM). For RAR beta, the EC(50) values increased as follows: 4-oxo-RA (8 nM), ATRA (9 nM), 18-OH-RA (14 nM), 5,6-epoxy-RA (35 nM), 13-cis-RA (47 nM), 4-OH-RA (64 nM), and 9-cis-RA (173 nM). For RAR gamma the EC(50) values were: ATRA (2 nM), 5,6-epoxy-RA (4 nM), 18-OH-RA (14 nM), 13-cis-RA (36 nM), 9-cis-RA (58 nM), 4-oxo-RA (89 nM), and 4-OH-RA (94 nM). By comparing the -fold induction of luciferase activity, all retinoids tested were equipotent at transactivating RARE-tk-Luc whatever the RAR considered. However, the best induction of the transcription was obtained for RAR alpha, which was 5-fold higher than for RAR beta and 10-fold higher than for RAR gamma. In conclusion, these data show that ATRA metabolites can bind to and activate the three RARs with variable relative affinity but with similar efficacy. These results suggest that ATRA metabolites may activate several signaling pathways and probably play an important role in cellular physiology and cancer therapy.

Metabolism and growth inhibition of four retinoids in head and neck squamous normal and malignant cells

Isotretinoin (13-cis-retinoic acid, 13cRA) has proven to be active in chemoprevention of head and neck squamous cell carcinoma (HNSCC). Moreover, both all-trans-retinoic acid (ATRA) and 13cRA induce objective responses in oral premalignant lesions. After binding of retinoids to retinoic acid receptors (RARs and RXRs) dimers are formed that are able to regulate the expression of genes involved in growth and differentiation. We compared the metabolism and level of growth inhibition of 13cRA with that of ATRA, 9cRA and retinol in four HNSCC cell lines and normal oral keratinocyte cultures (OKC). These retinoid compounds are known to bind with different affinities to the retinoic acid receptors. We observed that all retinoids were similar with respect to their capacity to induce growth inhibition. One HNSCC line could be ranked as sensitive, one as moderately sensitive and the remaining two were totally insensitive; OKC were moderately sensitive. The rate at which the cells were able to catabolize the retinoid was similar for all compounds. Retinoid metabolism in HNSCC cells resulted in a profile of metabolites that was unique for each retinoid. These metabolic profiles were different in OKC. Our findings indicate that differences in retinoid receptor selectivity of these retinoids do not influence the level of growth inhibition and rate of metabolism.

Both retinoic-acid-receptor- and retinoid-X-receptor-dependent signalling pathways mediate the induction of the brown-adipose-tissue-uncoupling-protein-1 gene by retinoids

The intracellular pathways and receptors mediating the effects of retinoic acid (RA) on the brown-fat-uncoupling-protein-1 gene (ucp-1) have been analysed. RA activates transcription of ucp-1 and the RA receptor (RAR) is known to be involved in this effect. However, co-transfection of an expression vector for retinoid-X receptor (RXR) increases the action of 9-cis RA but not the effects of all-trans RA on the ucp-1 promoter in brown adipocytes. Either RAR-specific ¿p-[(E)-2-(5,6,7,8,-tetrahydro-5,5,8, 8-tetramethyl-2-naphthalenyl)-1-propenyl]benzoic acid¿ or RXR-specific [isopropyl-(E,E)-(R,S)-11-methoxy-3,7, 11-trimethyldodeca-2,4-dienoate, or methoprene] synthetic compounds increase the expression of UCP-1 mRNA and the activity of chloramphenicol acetyltransferase expression vectors driven by the ucp-1 promoter. The RXR-mediated action of 9-cis RA requires the upstream enhancer region at -2469/-2318 in ucp-1. During brown-adipocyte differentiation RXRalpha and RXRgamma mRNA expression is induced in parallel with UCP-1 mRNA, whereas the mRNA for the three RAR subtypes, alpha, beta and gamma, decreases. Co-transfection of murine expression vectors for the different RAR and RXR subtypes indicates that RARalpha and RARbeta as well as RXRalpha are the major retinoid-receptor subtypes capable of mediating the responsiveness of ucp-1 to retinoids. It is concluded that the effects of retinoids on ucp-1 transcription involve both RAR- and RXR-dependent signalling pathways. The responsiveness of brown adipose tissue to retinoids in vivo relies on a complex combination of the capacity of RAR and RXR subtypes to mediate ucp-1 induction and their distinct expression in the differentiated brown adipocyte.

Multiple factors contribute to the toxicity of the aromatic retinoid TTNPB (Ro 13-7410): interactions with the retinoic acid receptors

The aromatic retinoid, (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1 -propenyl] benzoic acid (TTNPB) is 1000-fold more teratogenic than all trans-retinoic acid (tRA) in several species. Factors that partially explain the potency of this retinoid include binding affinities to retinoid nuclear receptors (RARs) in the nanomolar range, reduced affinities for the cytosolic binding proteins (CRABPs), and slow rate of metabolism (M. A. Pignatello, F. C. Kauffman, and A. A. Levin, Toxicol. Appl. Pharmacol. 142, 319-327, 1997). The present work investigates the possible involvement of longer receptor occupancy and increased transcriptional activity of the ligand receptor complex in the greater toxicity of TTNPB. Ligand off-rates from nuclear receptors were determined in nucleosol fractions prepared from COS-1 cells transfected with cDNA encoding the appropriate RAR subtype. When assayed at 10 degrees C, [3H]TTNPB was displaced from the RARs at a significantly faster rate than that of [3H]tRA. The difference in displacement was reduced at 4 degrees C. These observations are consistent with the 10-fold lower affinity of TTNPB vs tRA for RARs and, therefore, do not explain the greater potency of TTNPB. The ability of TTNPB and tRA to activate the RARs was determined using a luciferase reporter gene transfected into JEG-3 cells with the appropriate RAR subtype. The expression of the reporter was driven by a retinoic acid response element (RARE) from the RAR beta gene, which was incorporated into the reporter plasmid. Dose-response for gene activation indicated that the potency of TTNPB and tRA in activating mRAR alpha, beta, and gamma was similar after 24 h with comparable EC50s in the nanomolar range. However, after 72 h, activation by TTNPB was greater than that of tRA as indicated by EC50s and threshold for activation. This study indicates that the higher potency of TTNPB in activating the RARs may be due to slower disappearance of the retinoid and, therefore, is a contributing factor to its greater toxicity.

Teratology of retinoids

Either an excess or a deficiency of vitamin A and related compounds (retinoids) causes abnormal morphological development (teratogenesis). Potential retinoid sources come from dietary intake, nutritional supplements, and some therapeutic drugs. Therefore, understanding the mechanisms of retinoid teratogenesis is important. This review first gives an overview of the principles of teratology as they apply to retinoid-induced malformations. It then describes relevant aspects of the biochemical pathway and signal transduction of retinoids. The teratogenic activity of various retinoid compounds, the role of the retinoid receptors, and important toxicokinetic parameters in teratogenesis are reviewed.

Allosteric regulation of the discriminative responsiveness of retinoic acid receptor to natural and synthetic ligands by retinoid X receptor and DNA

Transcriptional activation by retinoids is mediated through two families of nuclear receptors, all-trans-retinoic acid (RARs) and 9-cis retinoic acid receptors (RXRs). Conformationally restricted retinoids are used to achieve selective activation of RAR isotype alpha, beta or gamma, which reduces side effects in therapeutical applications. Synthetic retinoids mimic some of all-trans retinoic acid biological effects in vivo but interact differently with the ligand binding domain of RARalpha and induce distinct structural transitions of the receptor. In this report, we demonstrate that RAR-selective ligands have distinct quantitative activation properties which are reflected by their abilities to promote interaction of DNA-bound human RXRalpha (hRXRalpha)-hRARalpha heterodimers with the nuclear receptor coactivator (NCoA) SRC-1 in vitro. The hormone response element core motifs spacing defined the relative affinity of liganded heterodimers for two NCoAs, SRC-1 and RIP140. hRXRalpha activating function 2 was critical to confer hRARalpha full responsiveness but not differential sensitivity of hRARalpha to natural or synthetic retinoids. We also provide evidence showing that lysines located in helices 3 and 4, which define part of hRARalpha NCoA binding surface, contribute differently to (i) the transcriptional activity and (ii) the interaction of RXR-RAR heterodimers with SRC-1, when challenged by either natural or RAR-selective retinoids. Thus, ligand structure, DNA, and RXR exert allosteric regulations on hRARalpha conformation organized as a DNA-bound heterodimer. We suggest that the use of physically distinct NCoA binding interfaces may be important in controlling specific genes by conformationally restricted ligands.

9-cis retinoic acid induces the expression of the uncoupling protein-2 gene in brown adipocytes

The expression of uncoupling protein-2 (UCP2) mRNA is up-regulated during the differentiation of brown adipocytes in primary culture. When differentiation of brown adipocytes is impaired, UCP2 mRNA expression is down-regulated. 9-cis Retinoic acid causes a dose-dependent induction of UCP2 mRNA levels in brown adipocytes, whereas all-trans retinoic acid has no effect. Specific agonists of retinoid X receptors (RXR) induce UCP2 mRNA expression, whereas specific activators of retinoic acid receptors do not. 9-cis Retinoic acid, acting through RXR receptors, is identified as a major regulator of the expression of the UCP2 gene in the brown fat cell.

Retinoid X receptor-selective ligands produce malformations in Xenopus embryos

Retinoids exert pleiotropic effects on the development of vertebrates through the action of retinoic acid receptors (RAR) and retinoid X receptors (RXR). We have investigated the effect of synthetic retinoids selective for RXR and RAR on the development of Xenopus and zebrafish embryos. In Xenopus, both ligands selective for RAR and RXR caused striking malformations along the anterior-posterior axis, whereas in zebrafish only ligands specific for RAR caused embryonic malformations. In Xenopus, RAR- and RXR-selective ligands regulated the expression of the Xlim-1, gsc, and HoxA1 genes similarly as all-trans-retinoic acid. Nevertheless, RXR-selective ligands activated only an RXR responsive reporter but not an RAR responsive reporter introduced by microinjection into the Xenopus embryo, consistent with our failure to detect conversion of an RXR-selective ligand to different derivatives in the embryo. These results suggest that Xenopus embryos possess a unique response pathway in which liganded RXR can control gene expression. Our observations further illustrate the divergence in retinoid responsiveness between different vertebrate species.

Retinoid actions and implications for prevention and therapy of oral cancer

Squamous cell carcinomas of the oral cavity can be preceded by clinically obvious premalignant changes, and they have a high rate of incidence of development of second primary tumors. Recent studies suggest retinoids not only for the treatment of oral eukoplakia, but also for the prevention of second primaries. Although retinoids are promising therapeutic agents, their therapeutic potential has been limited by their undesirable side-effects. A complete network of nuclear receptors has now been identified that mediate the action of retinoids and can interfere directly with cell proliferation signals by interacting with transcription factors. It has recently been shown that retinoids with receptor-selective activities can be obtained that are likely to have fewer side-effects because of their restricted biologic activities.

13-cis-Retinoic acid affect sheath-shaft interaction of equine hair follicles in vitro

A major challenge to the study of hair follicle growth is an appropriate assay system. Because equine mane follicles are large and noncurved, enabling easy dissection; are readily accessible from a single defined source; and possess a long anagen growth phase, we initiated a study of them in culture. As in our previous studies of human and sheep follicles (Dev Biol 165:469, 1994), we found in this system that transection level dictates the pattern of follicle growth in vitro: follicles transected below the sebaceous gland show a type 1 growth pattern (the shaft grows out with an adherent sheath), while nontransected follicles show a type 2 growth pattern (a naked shaft grows out lacking a sheath). In the present study, we searched for compounds that might influence type 1 or type 2 patterns of growth. We found that 13-cis retinoic acid induced, in a concentration-dependent fashion, a type 1-like pattern of growth under conditions for which a type 2 pattern was expected. All-trans-retinoic acid, SR11237 (a synthetic retinoid X receptor-specific ligand), and meta-carboxy-TTNPB (and inactive synthetic retinoid) did not have these properties. We hypothesize that sheath growth¿processing is mediated by the follicle at the level of the sebaceous gland, or by the sebaceous gland itself, and that persistence of the follicle sheath about the outgrowing shaft in vitro (i) in the physical absence of the sebaceous portion of the follicle, or (ii) in the presence of 13-cis-retinoic acid, is due to the reduced expression of a factor that regulates important shaft sheath interactions.

Receptors as tools for understanding the toxicity of retinoids

Retinoids are derivatives of vitamin A that have numerous biologic activities including induction of epithelial differentiation, pattern formation in embryos, and maintenance of spermatogenesis. Retinoids are used to treat various dermatologic maladies and specific forms of cancer but their use is limited by toxic liabilities: most notably teratogenesis. Retinoids interact with 2 families of receptors, the retinoic acid receptors (RARs) and retinoid X receptors (RXRs). The RARs and RXRs bind and transactivate distinct response elements of numerous genes. This multiplicity of receptors and gene products provides us with multiple targets for developing novel receptor-selective agonists. We are exploiting our knowledge of ligand receptor interactions to design better, more selective drugs and to understand the toxicity of retinoids and their metabolic products.

Evaluation of topical retinoids for cutaneous pharmacological activity in Yucatan microswine

The pharmacological effects of retinoids on skin have been studied primarily in test systems using small animals, such as mice and rabbits. Because of potentially significant differences in skin permeation and metabolism between small animals and humans, we have used Yucatan microswine as an alternative model for testing topical retinoids. Microswine skin resembles human skin, functionally and anatomically, more closely than most other species. In these studies, microswine skin was treated topically with retinoids for 5 consecutive days per week for 5 weeks. We found microswine epidermis to be functionally responsive to retinoids in that it undergoes hyperplasia and shows an increase in transepidermal water loss (TEWL). All-trans-retinoic acid, and its analogs, 13-cis-retinoic acid, 4-hydroxy-retinoic acid and 4-oxo-retinoic acid all caused epidermal thickening and increased TEWL. The three analogs were less potent than all-trans-retinoic acid. A synthetic retinoid, TTNPB, potently induced epidermal hyperplasia and increased TEWL, but a close structural analog, m-carboxy-TTNPB, which is also inactive on nuclear retinoic acid receptors, was without effects on microswine epidermis. These findings show that microswine are useful for evaluating the cutaneous effects of topical retinoids. This model could be of value in identifying retinoids with potential clinical activity.

Human in vivo pharmacology of topical retinoids

All-trans retinoic acid is used topically for treating a variety of dermatologic conditions ranging from acne to photoaged skin. Although the clinical effects of retinoic acid treatment are often considerable, relatively little is known about the basic mechanisms underlying such effects. With the development of an in vivo human assay we have investigated the pleiotypic effects of topical retinoids from the histologic to the molecular. Histologically, retinoic acid induces epidermal proliferation and differentiation coupled with dermal fibroblast production of collagen. Immunologic effects include stimulation of the antigen-presenting capacity of Langerhans cells and induction of keratinocyte ICAM-1 expression. At the biochemical level, retinoic acid regulates transglutaminase and tyrosinase activities and activates protein kinase C. Both polar metabolites and stereoisomers of all-trans retinoic acid are also biologically active. Molecular biologic techniques have revealed that elevation of mRNA for cellular retinoic acid binding protein II is a retinoid-related event and that nuclear receptors such as retinoic acid receptors and retinoid X-receptors may transduce the retinoid response.

Effect of retinoic acid and interferon alpha on granulocyte-macrophage colony forming cells in chronic myeloid leukemia: increased inhibition by all-trans- and 13-cis-retinoic acids in advanced stage disease

Granulocyte-macrophage colony forming units (CFU-GM) from patients with advanced stage chronic myelogenous leukemia (CML), i.e. in blastic crisis (BC) or accelerated phase (AP), were inhibited by all-trans-retinoic acid (tRA) approximately 1000-fold more potently than those from chronic phase (CP) CML patients (median IC50 = 10(-9) M tRA for six CML-AP/BC cases vs > 10(-6) M tRA for seven CML-CP cases). A similar activity pattern was observed for the stereoisomer 13-cis-RA (cRA). There was no apparent correlation of CFU-GM retinoid sensitivity with cloning efficiency or other colony characteristics. Interferon alpha-2a (INF alpha) alone strongly inhibited CFU-GM growth in all four CML-AP/BC cases (IC50 < or = 250 IU/ml) and three out of seven CML-CP cases (IC50 < or = 500 IU/ml), but there was little or no interactive effect between various concentrations of tRA and INF alpha (50 IU/ml) on CFU-GM from either CML-AP/BC or CML-CP cases. These results suggest that CML-AP/BC CFU-GM have some intrinsic molecular alteration(s) which markedly enhances their responsiveness to tRA and cRA, which may be clinically exploitable.

Maternal toxicokinetics, metabolism, and embryo exposure following a teratogenic dosing regimen with 13-cis-retinoic acid (isotretinoin) in the cynomolgus monkey

The maternal pharmacokinetics, metabolism, and placental transfer of 13-cis-retinoic acid (isotretinoin) have been determined in the cynomolgus monkey using a dosing regimen which had been previously shown to result in retinoid-specific teratogenic effects [Hummler et al. (1990) Teratology 42:263-272]. The drug (2.5 mg/kg body weight) was administered by nasogastric intubation once a day between gestational days (GD) 16-26, and twice a day between GD 27-31. Maternal plasma kinetics were determined following dosing on GD 26 and GD 31, and placental transfer was studied following the last dose on GD 31. The plasma half life of 13-cis-retinoic acid in the monkey (13.2 h) was comparable to that in the human. The main plasma metabolite in the monkey was the 13-cis-4-oxo-retinoic acid which occurred at levels lower or comparable to those of the administered drug. During multiple dosing, this metabolite accumulated to the same degree as the parent drug. All-trans-retinoic acid was present in maternal plasma in very low concentrations (2% of 13-cis-retinoic acid). The beta-glucuronides of all-trans- and 13-cis-retinoic acid were further minor plasma metabolites. 13-cis-retinoic acid and its 4-oxo-metabolite reached the monkey embryo slowly but extensively during organogenesis and reached 24 h-AUC values of 956 and 590 ng.h/g embryo wet weight, resulting in embryo/maternal plasma concentration ratios of 0.41 and 0.33, respectively. The AUC value of all-trans-retinoic acid (316 ng.h/g) was only raised approximately 40% above the endogenous AUC level (225 ng.h/g); only at two time periods examined were the embryonic all-trans-retinoic acid concentrations above endogenous levels (at 4 h and 8 h; P < 0.01 and < 0.05, respectively; Student's t-test). The beta-glucuronides of all-trans- and 13-cis-retinoic acid were not detected in the embryo. Accumulation of 13-cis-retinoic acid and the 4-oxo-metabolite during the twice-per-day dosing regimen was apparent both in maternal plasma and embryo. An interspecies comparison suggests that the half life as well as the metabolic pattern of 13-cis-retinoic acid in plasma were similar in monkey and human: 13-cis-4-oxo-retinoic acid was the main metabolite in both species and the beta-glucoronides as well as all-trans-retinoic acid were minor metabolites. However, the plasma AUC values of the administered drug and particularly the 4-oxo-metabolite were found to be lower in the monkey as compared to the human.(ABSTRACT TRUNCATED AT 400 WORDS)

A pleiotropic response is induced in F9 embryonal carcinoma cells and rhino mouse skin by All-trans-retinoic acid, a RAR agonist but not by SR11237, a RXR-selective agonist

We evaluated SR11237, a retinoid X receptor (RXR)-specific compound, for its pharmacologic effects on cell differentiation in F9 embryonal carcinoma cells and rhino mouse epidermis. SR11237 can cause RXR/RXR homodimers to form and transactivate a reporter gene containing a RXR-response element. We confirmed, using nuclear receptor co-transfection assays in COS-1 cells, that SR11237 is effective at transactivating a chloramphenicol acetyltransferase reporter gene through RXRs but not retinoic acid receptors. When SR11237 was tested for its ability to modulate cell differentiation, it was inactive on F9 embryonal carcinoma cells and rhino mouse skin. Because differentiation in these systems is known to be regulated by RAR-specific compounds, such as all-trans-retinoic acid and (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-prope nyl benzoic acid], our results with SR11237 are compatible with the concept that classical retinoid pleiotropic responses are mediated by RXR/RAR heterodimeric nuclear receptors rather than through RXR/RXR homodimers.

Biologic activities of retinoic acid and 3,4-didehydroretinoic acid in human keratinocytes are similar and correlate with receptor affinities and transactivation properties

The biologic activities of retinoic acid and 3,4-didehydroretinoic acid, two endogenous vitamin A derivatives in various tissues, were compared to their affinities for the nuclear retinoic acid receptors and their ability to induce transcriptional activation. Both retinoids were equipotent inducers of differentiation of F9 teratocarcinoma cells. In a morphologic assay, using reconstructed skin, retinoic acid and 3,4-didehydroretinoic acid inhibited keratinization at a concentration of 100 nM. In cultured keratinocytes, a 50% inhibition of the production of the keratinocyte transglutaminase enzyme was achieved with about 20 nM for both retinoids. The in vitro binding to the nuclear retinoic acid receptors alpha, beta, and gamma showed that retinoic acid and 3,4-didehydroretinoic acid had almost equal affinities for the receptors with Kds ranging from 3 to 47 nM. The transcriptional activation resulting from the addition of the two retinoids to cells co-transfected with alpha, beta, or gamma retinoic acid receptor expression vectors and a retinoic acid responsive element linked to the chloramphenicol acetyltransferase reporter gene was similar. Finally, it was demonstrated that retinoic acid did not metabolize to 3,4-didehydroretinoic acid, and a slow conversion of 3,4-didehydroretinoic acid into retinoic acid was not sufficient to explain the biologic effects produced by the former compound. In conclusion, the present study demonstrates that retinoic acid and 3,4-didehydroretinoic acid have the same activity in several different test systems, but their metabolism differs depending on the cell type used.

Retinoic acid receptors in retinoid responsive ovarian cancer cell lines detected by polymerase chain reaction following reverse transcription

The growth inhibitory effects of all-trans and 13-cis retinoic acid (RA) and of the synthetic retinoids TTNPB, TTNPB-ethylester and TTNN were studied on seven human epithelial ovarian cancer cell lines and one ovarian teratocarcinoma cell line. Six of seven ovarian adenocarcinoma cell lines were inhibited in their growth by RA and by synthetic retinoids in a dose dependent manner. No response to these substances was observed for the ovarian teratocarcinoma cell line. The knowledge that RA and retinoids exert their action on the cells via nuclear receptors led us to examine the expression of RAR-alpha, -beta and -gamma mRNA by these cell lines by polymerase chain reaction following reverse transcription. All cell lines expressed RAR-alpha and -gamma mRNA and six of the eight cell lines were found to express additionally RAR-beta mRNA, among them the ovarian teratocarcinoma cell line. Our data indicate that there was no direct association between the presence of RAR subtype transcripts and the response to retinoids in ovarian cancer cell lines.

Retinoic acid induces adult muscle cell differentiation mediated by the retinoic acid receptor-alpha

Retinoic acid (RA) is known to induce differentiation in many cell systems. This induction is mediated by nuclear RA receptors (RARs), which act as transactivating factors belonging to the superfamily of steroid/thyroid nuclear receptors. In this study, effects of RA were studied in a mouse myogenic C2 cell line and in primary chicken satellite cells, the myogenic precursor cells in adult muscle. Addition of RA decreased the rate of DNA synthesis in a dose-dependent manner in both cultures. Differentiation was more rapid in cells treated with RA, and these cells exhibited large myotubes after 48 hr of incubation as compared to untreated cells, which were still proliferating. Addition of 10(-8) M RA to C2 cells elevated mRNA levels of myogenin, a skeletal muscle-specific transcription factor. In parallel, activity levels of the muscle-specific creatine kinase were enhanced in the RA-treated cells nearly twofold as compared to the untreated cells. RA treatment of both mouse C2 and chicken satellite cells caused rapid induction of the RAR-alpha mRNA levels. Maximal mRNA levels were observed after 2 to 5 hr followed by a sharp reduction to nearly zero levels at 9 hr. The RAR-alpha mRNA levels augmented in a dose-dependent manner between concentrations of 10(-10) M and 10(-8) M RA, whereas higher concentrations caused mRNA levels to decrease. These results indicate that RA induces differentiation in both adult skeletal muscle primary satellite cells and a myogenic cell line. The rapid and specific induction of RAR-alpha mRNA in these cells upon exposure to RA may suggest that this receptor is the primary target and a mediator of RA.

Delta retinoic acid receptor isoform delta 1 is distinguished by its exceptional N-terminal sequence and abundance in the limb regeneration blastema

In amphibian limb regeneration memory for position in the proximal-distal axis can be respecified by retinoic acid. The favoured candidates to mediate this effect are the retinoic acid receptors (RARs) and of the RARs identified in the regeneration blastema, the delta receptor is the most abundant. The presence in blastemal mesenchyme of at least two delta receptor isoforms, delta 1 and delta 2, alternatively spliced at the A-B junction, was demonstrated in expression studies and by PCR cloning. The delta 1 receptor is abundant in regenerative structures such as the limb and tail, whereas the delta 2 and alpha receptors show a more uniform pattern of expression across adult newt tissues. Full-length cloning of the delta 1 receptor established the presence of an unusually long open reading frame and N-terminal sequence that appears unique among vertebrate retinoic acid receptors. Transient transfection of expression constructs into COS cells followed by Western blotting confirmed the existence of at least three potential initiation sites for delta 1 translation. The possibility that delta 1 RAR expression may specify positional memory directly was tested in RNase protection experiments. delta 1 receptor message is increased on amputation, but does not exhibit a pronounced differential distribution along the proximal-distal axis in normal and regenerating limbs, nor does it show a persistent alteration in expression levels following a dose of retinoic acid sufficient to respecify position. The possibility that the morphogenetic effects of RA may be mediated through receptor interactions is raised by the finding that single mesenchymal blastemal cells in culture can express multiple RAR subtypes (delta 1 and alpha) and isoforms (delta 1 and delta 2).