The relationship among retinoid structure, affinity for retinoic acid-binding protein, and ability to respecify pattern in the regenerating axolotl limb

To further our understanding of the action of retinoids on the respecification of pattern in the regenerating axolotl limb we have studied the relative potencies of a range of synthetic and natural retinoids administered locally to the blastema. Alterations in the polar end group of the retinoic acid (RA) molecule to produce esters, the alcohol, or the aldehyde abolish the ability of the molecule to respecify pattern. On the other hand, alterations of the ring or side chain to produce the synthetic retinoids arotinoid and TTNPB considerably increases the potency of the molecule to respecify pattern--TTNPB is at least 100X more potent than retinoic acid. To examine the role of cellular retinoic acid-binding protein (CRABP) in the respecification process we determined the relative binding affinities of these retinoids for CRABP. These data correlated well with the respecification series: retinoids which showed no affinity for CRABP did not respecify pattern and those which did show affinity for CRABP did respecify pattern. Furthermore the most potent retinoid, TTNPB, has a higher affinity for CRABP than RA itself. This suggests that CRABP may be playing an important role in the action of RA on pattern formation in the regenerating limb.     

An in vitro cell model system to study the action of retinoids on Leydig cell steroidogenesis

In this study we examined the effects of retinol and retinoic acid on steroid production in MA-10 mouse Leydig tumor cells. Results showed that both retinol and retinoic acid greatly increased progesterone production in this cloned cell line. The stimulatory effect of retinoids is not inhibited by cycloheximide suggesting that de novo protein synthesis is not required. The presence of the retinoid binding proteins CRBP and CRABP could not be detected in MA-10 Leydig cell cytosol indicating that the stimulatory action of retinoids on progesterone production is not mediated through these cellular binding proteins. Both previous and present findings suggest that retinoids play an important role in the regulation of Leydig cell steroidogenesis and that MA-10 Leydig tumor cells may represent an ideal in vitro cell system to study the mechanism of action of retinoids in Leydig cell steroidogenesis.     

Biologically active aromatic retinoids bearing azido photoaffinity-labeling groups and their binding to cellular retinoic acid-binding protein

Retinoids bearing azido photoaffinity-labeling groups (azidoretinoids) have potential as probes for investigating the molecular mechanisms of action of all-trans-retinoic acid (RA) as mediated by its cellular retinoic acid-binding protein (CRABP) and nuclear receptor proteins. Two new azidoretinoids, 3-azido-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1E- propen-1-yl]-benzonic acid and 4-(4-azido-5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-anthracenyl)be nzoic acid were synthesized, and evaluated for their in vitro biological potency, and binding affinity for CRABP. Like RA, these aromatic azides had significant activity in modulating cell differentiation in retinoid-deficient hamster tracheal organ culture (ED500.02 nM and 0.03 nM, respectively) and in the inhibition of the induction of ornithine decarboxylase in mouse epidermis (ED50 7.0 nmol and 0.5 nmol, respectively). They also possessed high binding affinity for CRABP (ID50 0.9 microM and 0.85 microM, respectively). The tritiated aromatic azides were further evaluated for their ability to bind covalently to CRABP after photolysis. On photolysis at -78 degrees C, the two radiolabeled azidoretinoids formed stable adducts with CRABP. Treatment of the adducts with either RA or p-chloromercuriphenylsulfonic acid (CMPS) and subsequent dialysis did not cause any dissociation, indicating the formation of a covalent bond. In contrast, treatment of the unirradiated complexes with RA or CMPS led to dissociation of the complex. Synthesis of affinity labels and characterization of CRABP-retinoid complexes should provide useful information on the ligand-binding regions and insights into the mechanism of action of RA.     

Effects of retionoids on invasion of organ cultures of chick chorioallantoic membrane by adenovirus transformed cells

Summary Invasion of chick chorioallantoic membrane (CAM) organ cultures by rat 3Y1 cells transformed by the highly oncogenic human adenovirus type 12 (3Y1/12-10 cells) was inhibited by several retinoids tested. The anti-invasive activity of the retinoids was dependent on retinoid concentration and continuous (4d) exposure of the CAM. The 50% retinoid dose (dose effective in achieving a response in half of the organ cultures) that inhibited invasion was 0.85 μg/ml of retinol palmitate, 0.39 μg/ml of retinoic acid, or 0.16 μg/ml of retinol acetate. This dose was of the same order of magnitude as that which induced CAM differentiation, and was three-to fourfold less than the dose that caused cytotoxic damage of CAM. In addition, the retinoids inhibited 3Y1/12-10 cell growth by approximately 40% at levels over 10-fold higher than those needed for anti-invasion activity. The findings suggest that the anti-invasive activity of retinoids was at least partly due to direct induction of cell differentiation of the CAM host tissue. This work was supported by National Cancer Institute Grant CA 13231 and by University of Akron Grant RG 832.     

Localization of specific retinoid-binding sites and expression of cellular retinoic-acid-binding protein (CRABP) in the early mouse embryo

Retinoids (vitamin A derivatives) are important for normal embryogenesis and retinoic acid, an acidic derivative of vitamin A, was recently proposed to be an endogenous morphogen. Several retinoids are also potent teratogens. Using an autoradiographic technique, we have identified tissues and cells in early mouse embryos that are able to specifically accumulate a radiolabelled synthetic derivative of retinoic acid. Strong accumulation of radioactivity was seen in several neural crest derivatives and in specific areas of the CNS. Gel filtration analyses of cytosols from embryos that received the radiolabelled retinoid in utero suggested that cellular retinoic acid-binding protein (CRABP) was involved in the accumulation mechanism. Immunohistochemical localization confirmed that cells accumulating retinoids also expressed CRABP. Strong CRABP immunoreactivity was found in neural crest-derived mesenchyme of the craniofacial area, in visceral arches, in dorsal root ganglia and in cells along the gut and the major vessels of the trunk region. In CNS, CRABP expression and retinoid binding was largely restricted to the hindbrain, to a single layer of cells in the roof of the midbrain and to cells in the mantle layer of the neural tube. Our data suggest that cells in the embryo expressing CRABP are target cells for exogenous retinoids as well as endogenous retinoic acid. Retinoic acid may thus play an essential role in normal development of the CNS and of tissues derived from the neural crest. We propose that the teratogenic effects of exogenous retinoids are due to an interference with mechanisms by which endogenous retinoic acid regulates differentiation and pattern formation in these tissues.     

Localization of retinoid binding proteins, retinoid receptors, and retinaldehyde dehydrogenase in the chick eye

Retinoids have many functions in the eye, including, perhaps, the visual guidance of ocular growth. Therefore, we identified where retinoid receptors, binding proteins, and biosynthetic enzymes are located in the ocular tissues of the chick as a step toward discovering where retinoids are generated and where they act. Using antibodies to interphotoreceptor retinoid binding protein (IRBP), cellular retinol binding protein (CRBP), cellular retinoic acid binding protein (CRABP), cellular retinaldehyde binding protein (CRALBP), retinaldehyde dehydrogenase (RALDH), and retinoic acid receptors (RAR and RXR), we localized these proteins to cells in the retina, retinal pigmented epithelium, choroid and sclera of the chick eye. IRBP was detected in the photoreceptor layer and pigmented epithelium; CRBP was in the pigmented epithelium; CRABP was in amacrine and bipolar cells in the retina; CRALBP was in Müller cells, pigmented epithelium, choroid, and fibrous sclera; RALDH was in retinal amacrine cells, pigmented epithelium, and choroid; RAR was in amacrine cells, choroid, and chondrocytes and fibroblasts in the sclera; and RXR was in amacrine and ganglion cells, bipolar cell nuclei, choroid, and chondrocytes. We also found that the growth-modulating toxins colchicine and quisqualate destroyed selectively different subsets of CRABP-containing amacrine cells. We conclude that the distribution of proteins involved in retinoid metabolism is consistent with a role of retinoids not only in phototransduction, but also in maintenance of cellular phenotype and visual guidance of ocular growth.     

The respecification of limb pattern by new synthetic retinoids and their interaction with cellular retinoic acid-binding protein

We describe here experiments to examine the role of cellular retinoic-acid-binding protein (CRABP) during the induction of limb duplication in the chick limb bud and regenerating axolotl limb by retinoids. A newly synthesised class of retinoic acid analogues have been used because among them, some have been specifically designed with the property of binding to the retinoic acid receptors, but not to CRABP. We can thus test whether binding to CRABP is an obligatory step during limb respecification. The binding of four of these compounds to chick limb bud and axolotl CRABP was tested in sucrose density gradient assays and then their potencies at inducing limb duplications tested. Two of the four compounds do not bind to limb CRABP and yet are able to induce limb duplications, suggesting that an interaction with CRABP is not an obligatory step in the process. However, the two compounds which do bind to CRABP are more potent than the two which do not, suggesting that an interaction with CRABP may, nevertheless, increase the potency of a retinoid.     

Intracellular Transport of Fat‐Soluble Vitamins A and E

Vitamins are compounds that are essential for the normal growth, reproduction and functioning of the human body. Of the 13 known vitamins, vitamins A, D, E and K are lipophilic compounds and are therefore called fat‐soluble vitamins. Because of their lipophilicity, fat‐soluble vitamins are solubilized and transported by intracellular carrier proteins to exert their actions and to be metabolized properly. Vitamin A and its derivatives, collectively called retinoids, are solubilized by intracellular retinoid‐binding proteins such as cellular retinol‐binding protein (CRBP), cellular retinoic acid‐binding protein (CRABP) and cellular retinal‐binding protein (CRALBP). These proteins act as chaperones that regulate the metabolism, signaling and transport of retinoids. CRALBP‐mediated intracellular retinoid transport is essential for vision in human. α‐Tocopherol, the main form of vitamin E found in the body, is transported by α‐tocopherol transfer protein (α‐TTP) in hepatic cells. Defects of α‐TTP cause vitamin E deficiency and neurological disorders in humans. Recently, it has been shown that the interaction of α‐TTP with phosphoinositides plays a critical role in the intracellular transport of α‐tocopherol and is associated with familial vitamin E deficiency. In this review, we summarize the mechanisms and biological significance of the intracellular transport of vitamins A and E.      

Retinoids, retinoid-binding proteins, and retinyl palmitate hydrolase distributions in different types of rat liver cells

A study was conducted to determine the levels and distributions of retinoids, retinol-binding protein (RBP), retinyl palmitate hydrolase (RPH), cellular retinol-binding protein (CRBP), and cellular retinoic acid-binding protein (CRABP) in different types of isolated liver cells. Highly purified fractions of parenchymal, fat-storing (stellate), endothelial, and Kupffer cells were isolated in high yield from rat livers. The retinoid content of each fraction was measured by HPLC analysis. RBP, CRBP, and CRABP were measured by sensitive and specific radioimmunoassays, and RPH activity was measured by a sensitive microassay. The concentrations of each parameter expressed per 10(6) parenchymal or fat-storing cells were, respectively: retinoids, 1.5 and 83.9 micrograms of retinol equivalents; RBP, 138 and 7.4 ng; RPH, 826 and 1152 pmol FFA formed hr-1; CRBP, 470 and 236 ng; and CRABP, 5.6 and 8.7 ng. When these data were expressed on the basis of per unit mass of cellular protein, the concentrations of RPH, CRBP, and CRABP in the fat-storing cells, which contain 10-fold less protein than the large parenchymal cells, were seen to be greatly enriched over parenchymal cells. The parenchymal cells contained approximately 9% of the total retinoids, 98% of the total RBP, 90% of the total RPH activity, 91% of the total CRBP, and 71% of the total CRABP found in the liver. The fat-storing cells accounted for approximately 88% of the total retinoids, 0.7% of the total RBP, 10% of the RPH activity, 8% of the total CRBP, and 21% of the CRABP in the liver. The endothelial and Kupffer cell fractions contained very low levels of all of these parameters. Thus, the large and abundant parenchymal cells account for greater than 70% of the liver's RBP, RPH, CRBP, and CRABP; but the much smaller and less abundant fat-storing cells contain the majority of hepatic retinoids and greatly enriched concentrations of RPH, CRBP, and CRABP.     

Application of photoaffinity labeling with [(3)H] all trans- and 9-cis-retinoic acids for characterization of cellular retinoic acid--binding proteins I and II

Cellular retinoic acid-binding proteins (CRABPs) are carrier proteins thought to play a crucial role in the transport and metabolism of all-trans-retinoic acid (atRA) and its derivatives within the cell. This report describes a novel photoaffinity-based binding assay involving competition between potential ligands of CRABP and [(3)H]atRA or [(3)H]-9-cis-RA for binding to the atRA-binding sites of CRABP I and II. Photoaffinity labeling of purified CRABPs with [(3)H]atRA was light- and concentration-dependent, saturable, and protected by several retinoids in a concentration-dependent manner, indicating that binding occurred in the CRABP atRA-binding site. Structure-function relationship studies demonstrated that oxidative changes to the atRA beta-ionone ring did not affect ligand potency. However, derivatives lacking a terminal carboxyl group and some cis isomers did not bind to CRABPs. These studies also identified two novel ligands for CRABPs: 5,6-epoxy-RA and retinoyl-beta-D-glucuronide (RAG). The labeling of both CRABPs with 9-cis-RA occurred with much lower affinity. Experimental evidence excluded nonspecific binding of RAG to CRABPs and UDP-glucuronosyltransferases, the enzymes responsible for RAG synthesis. These results established that RAG is an effective ligand of CRABPs. Therefore, photoaffinity labeling with [(3)H]atRA can be used to identify new ligands for CRABP and retinoid nuclear receptors and also provide information concerning the identity of amino acid(s) localized in the atRA-binding site of these proteins.     

Enzymes and binding proteins affecting retinoic acid concentrations

Free retinoids suffer promiscuous metabolism in vitro. Diverse enzymes are expressed in several subcellular fractions that are capable of converting free retinol (retinol not sequestered with specific binding proteins) into retinal or retinoic acid. If this were to occur in vivo, regulating the temporal-spatial concentrations of functionally-active retinoids, such as RA (retinoic acid), would be enigmatic. In vivo, however, retinoids occur bound to high-affinity, high-specificity binding proteins, including cellular retinol-binding protein, type I (CRBP) and cellular retinoic acid-binding protein, type I (CRABP). These binding proteins, members of the superfamily of lipid binding proteins, are expressed in concentrations that exceed those of their ligands. Considerable data favor a model pathway of RA biosynthesis and metabolism consisting of enzymes that recognize CRBP (apo and holo) and holo-CRABP as substrates and/or affecters of activity. This would restrict retinoid access to enzymes that recognize the appropriate binding protein, imparting specificity to RA homeostasis; preventing, e.g. opportunistic RA synthesis by alcohol dehydrogenases with broad substrate tolerances. An NADP-dependent microsomal retinol dehydrogenase (RDH) catalyzes the first reaction in this pathway. RDH recognizes CRBP as substrate by the dual criteria of enzyme kinetics and chemical crosslinking. A cDNA of RDH has been cloned, expressed and characterized as a short-chain alchol dehydrogenase. Retinal generated in microsomes from holo-CRBP by RDH supports cytosolic RA synthesis by an NAD-dependent retinal dehydrogenase (RalDH). RalDH has been purified, characterized with respect to substrate specificity, and its cDNA has been cloned. CRABP is also important to modulating the steady-state concentrations of RA, through sequestering RA and facilitating its metabolism, because the complex CRABP/RA acts as a low K_m substrate.     

Functional and ultrastructural effects of nontypeable haemophilus influenzae in a hamster trachea organ culture system

Summary A hamster trachea organ culture system was utilized to evaluate quantitatively the effects of a strain of nontypeableHaemophilus influenzae (NTHI) and culture supernatants of the same strain on ciliary activity. Tracheal explants were maintained in organ culture for 96 to 144 h and ciliary activity was observed daily with an inverted microscope. Explants continuously exposed to a strain of NTHI had a progressive decline in ciliary activity which was significantly lower than uninfected controls evaluated concomitantly by 48 h of exposure and thereafter. Histologic studies revealed a progressive degeneration of mucosal cells and exfoliation of ciliated cells. Scanning electron microscopy showed little adherence of NTHI to the mucosal surface. Sterile broth cultures of NTHI and supernatants of organ cultures infected with the same NTHI strain had no adverse effect on ciliary activity. Infected tracheal explants treated with ampicillin 24, 48, or 72 h after continuous bacterial challenge had no significant decline in ciliary activity compared to controls. The lack of adherence and the histologic changes observed when hamster trachea cultures were infected with NTHI suggested a toxin might mediate the damage observed. Broth and organ culture supernatants, however, produced no damage. Therefore, further studies are needed to determine the role, if any, of a toxin in the production of damage to hamster tracheal explants by NTHI. This work was supported by a Merit Review grant from the Veterans Administration and by Grant AI-19641 from the National Institute of Allergy and Infectious Diseases.     

Retinoid status and responsiveness to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice lacking retinoid binding protein or retinoid receptor forms

We have investigated the role of Vitamin A (retinoid) proteins in hepatic retinoid processing under normal conditions and during chemical stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a chemical known to interfere with retinoid turnover and metabolism. Three separate studies were performed in wildtype control mice and transgenic mice that lack one or more isoforms of retinoic acid receptors (RAR), retinoid X receptors (RXR), or intracellular retinoid-binding proteins (CRABP I, CRABP II, CRBP I). Body and organ weight development was monitored from 2 weeks of age to adult, and hepatic levels of retinyl esters, retinol, and retinoic acid were investigated. In addition, hepatic concentrations of 9-cis-4-oxo-13,14-dihydro-retinoic acid, a recently discovered retinoid metabolite that has proven sensitive to both TCDD exposure and Vitamin A status, were also determined. Mice absent in the three proteins CRBP I, CRABP I, and CRABP II (CI/CAI/CAII-/-) displayed significantly lower hepatic retinyl ester, retinol, and all-trans-retinoic acid levels compared to wildtype mice, whereas the liver concentrations of 9-cis-4-oxo-13,14-dihydro-retinoic acid was considerably higher. After treatment with TCDD, hepatic total retinoids were almost entirely depleted in the CI/CAI/CAII-/- mice, whereas wildtype mice and mice lacking CRABP I, and CRABP II (CAI/CAII-/-) retained approximately 60-70% of their Vitamin A content compared to controls at 28 days. RAR and RXR knockout mice responded similarly to wildtype mice with respect to TCDD-induced retinoid disruption, with the exception of RXRbeta-/- mice which showed no decrease in hepatic Vitamin A concentration, suggesting that the role of RXRbeta in TCDD-induced retinoid disruption should be further investigated. Overall, the abnormal retinoid profile in the triple knockout mice (CI/CAI/CAII-/-), but not double knockout (CAI/CAII-/-) mice, suggests that a loss of CRBP I may account for the difference in retinoid profile in CI/CAI/CAII-/- mice, and is likely to result in an increased susceptibility to hepatic retinoid depletion following dioxin exposure.     

Effects of cyclic adenosine 3′:5′-monophosphate-elevating agents and retinoic acid on differentiation in retinoid-deficient tracheal cultures

Summary The ability of cyclic AMP-elevating agents to induce normal differentiation has been investigated in retinoid-deficient hamster tracheal epithelium in organ culture. Dibutyryl cAMP (dbcAMP) and other cAMP-regulating agents alone caused disappearance of keratin and regeneration of normal mucociliary epithelium in retinoid-deficient cultures. Incubation of retinoid-deficient cultures with dbcAMP, isoproterenol, and cholera toxin (CT) (without addition of exogenous retinoid) reversed keratinization in a dose-dependent manner. The ED₅₀ of cultures treated with dbcAMP was 4×10⁻⁶ M; ED₅₀ of isoproterenol was 7×10⁻⁵ M; and CT, 0.6 μg/ml. Phosphodiesterase inhibitors and other cAMP analogs were inactive. Dibutyryl cAMP in combination with theophylline enhanced normal differentiation. Retinoid-deficient tracheas pretreated for 20 h with 10⁻⁹ M all-trans-retinoic acid (RA) responded to 10⁻⁶ M dbcAMP by potentiating normal differentiation; this concentration of dbcAMP alone was inactive. Isoproterenol showed a similar response but to a lesser degree. These cAMP-elevating agents applied in combination with theophylline did not increase activity. This investigation was supported by National Cancer Institute Contract NO1-CP-31012.     

Transfer of retinoic acid from its complex with cellular retinoic acid-binding protein to the nucleus

Cellular retinoic acid-binding protein (CRABP), a potential mediator of retinoic acid action, enables retinoic acid to bind in a specific manner to nuclei and chromatin isolated from testes of control and vitamin A-deficient rats. The binding of retinoic acid was followed after complexing [3H]retinoic acid with CRABP purified from rat testes. The binding was specific, saturable, and temperature dependent. If CRABP charged with nonlabeled retinoic acid was included in the incubation, binding of radioactivity was diminished, whereas inclusion of free retinoic acid, or the complex of retinol with cellular retinol binding protein (CRBP) or serum retinol binding protein had no effect. Approximately 4.0 X 10(4) specific binding sites for retinoic acid were detected per nucleus from deficient animals. The number of binding sites observed was influenced by vitamin A status. Refeeding vitamin A-deficient rats (4 h) with retinoic acid lowered the amount of detectable binding sites in the nucleus. CRABP itself did not remain bound to these sites, indicating a transfer of retinoic acid from its complex with CRABP to the nuclear sites. Further, CRBP, the putative mediator of retinol action, was found to enable retinol to be bound to testicular nuclei, in an interaction similar to the binding of retinol to liver nuclei described previously.     

In vitro metabolism and biological activity of all-trans-retinoic acid and its metabolites in hamster trachea

The in vitro metabolism of all-trans-[11,12-3h]retinoic acid to several more polar compounds has been demonstrated in a hamster tracheal organ culture system. The production of these metabolites is dependent on the presence of tissue. The physiological significance of these compounds is shown by the cochromatography of several of the in vitro formed metabolites synthesized from [carboxy-14C]retinoic acid with metabolites isolated from the intestine and urine of hamsters previously injected with 0.1 to 1.5 microgram of [3H]retinoic acid. One of the metabolites shows about one-tenth the biological activity of all-trans-retinoic acid when tested in a hamster tracheal organ culture assay. This biologically active metabolite is converted by the hamster trachea in vitro to a biologically inactive metabolite.     

Metabolism of the chemopreventive retinoid N-(4-hydroxyphenyl)retinamide by mammary gland in organ culture.

N-(4-Hydroxyphenyl)retinamide (4-HPR) is considered to be the most effective chemopreventive retinoid for chemically induced mammary carcinogenesis in rats. However, the mechanism of 4-HPR action in mammary cells is poorly understood. In the present study we examined the metabolism of 4-HPR in the mouse mammary gland in organ culture. Mammary glands excised from BALB/c mice were incubated with 4-HPR in the presence of insulin, prolactin and steroid hormones for 6 days. The glands were extracted with chloroform/methanol (2:1, v/v), and the metabolites were separated on a reversed-phase h.p.l.c. column. Three metabolites were separated in addition to 4-HPR; one of the metabolites, M2, was co-eluted with 13-cis-4-HPR, M3 was co-eluted with N-(4-methoxyphenyl)retinamide (4-MPR) and M1 remains unidentified. There appeared to be some hormonal regulation in the distribution of metabolites in the glands. Increased levels of 4-MPR and M1 were observed in insulin-plus-prolactin-treated glands as compared with the glands incubated with steroid hormones. Furthermore, it was observed that M1 isolated from the livers of 4-HPR-treated rats competed for the cellular retinoic acid-binding protein (CRABP) sites; however, 4-HPR did not bind to CRABP. These results indicate that mouse mammary gland can metabolize 4-HPR and that the metabolites which compete for CRABP sites may have physiological significance in the retinoid inhibition of mammary carcinogenesis.     

Cellular Retinoic Acid Binding Protein 2 Is Strikingly Downregulated in Human Esophageal Squamous Cell Carcinoma and Functions as a Tumor Suppressor

Esophageal squamous cell carcinoma (ESCC) is the predominant pathotype of esophageal carcinoma (EC) in China, especially in Henan province, with poor prognosis and limited 5-year survival rate. Cellular retinoic acid binding protein 2 (CRABP2) is a member of the retinoic acid (RA) and lipocalin/cytosolic fatty-acid binding protein family and plays a completely contrary role in tumorigenesis through the retinoid signaling pathway, depending on the nuclear RA receptors (RAR) and PPARbeta/delta receptors. Presently, the biological role of CRABP2 in the development of ESCC has never been reported. Here, we firstly evaluated the expression of CRABP2 at both mRNA and protein levels and showed that it was remarkably downregulated in clinical ESCC tissues and closely correlated with the occurrence position, pathology, TNM stage, size, infiltration depth and cell differentiation of the tumor. Additionally, the biological function assays demonstrated that CRABP2 acted as a tumor suppressor in esophageal squamous carcinogenesis by significantly inhibiting cell growth, inducing cell apoptosis and blocking cell metastasis both in vitro and in vivo. All in all, our finding simplicate that CRABP2 is possibly an efficient molecular marker for diagnosing and predicting the development of ESCC.     

Synthesis and biological activity of retinoic acid receptor-alpha specific amides

Retinoids are analogues of all-trans-retinoic acid, a powerful hormone that mediates many fundamental biological processes. Cancer and other serious hyperproliferative diseases are attractive therapeutic targets for retinoids, but the therapeutic use of retinoids is limited due to severe toxicity. We report here the design of retinoid receptor-alpha specific ligands with growth inhibitory activity in breast cancer cell lines, and which do not cause the cutaneous toxicity associated with the currently available nonselective retinoid agonists.