Recombinant human retinoic acid receptor beta. Binding of synthetic retinoids and transcriptional activation

Effects of receptor class- and subtype-selective retinoids and an apoptosis-inducing retinoid on the adherent growth of the NIH:OVCAR-3 ovarian cancer cell line in culture

Comparison of the adherent growth inhibition of NIH:OVCAR-3 ovarian cancer cells by retinoid receptor class-selective and subtype-selective compounds with their receptor binding affinities and transcriptional activation activities indicated no correlation for RAR alpha and RAR gamma although both receptors are present. Retinoids that activated RXR alpha inhibited cell growth in the range as all-trans-retinoic acid and 9-cis-retinoic acid. The most potent inhibitor was 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN), which has been found to inhibit breast and lung cancer and leukemia cell growth and induce cancer cell apoptosis through a pathway independent of the retinoid receptors.

High activity, soluble, bacterially expressed human vitamin D receptor and its ligand binding domain

The effects of 1 alpha, 25(OH)2 vitamin D3 on cell growth and differentiation are primarily mediated by the nuclear vitamin D receptor (VDR). In order to study aspects of receptor function and ultimately the structural basis of the VDR-ligand interaction, it is necessary to produce large quantities of purified VDR. To achieve this, we have expressed the human VDR and its ligand binding domain in E. coli as fusion proteins with the maltose binding protein using the expression vector pMal-c2. In this system high level expression of both fusion proteins in a soluble form was achieved, whereas previous attempts to express the VDR in E. coli have resulted in an insoluble product. After affinity purification on amylose resin, the fusion proteins were isolated with yields of 10-20 mg/l of culture. Both forms of the recombinant receptor bound 1 alpha, 25(OH)2 vitamin D3 with high affinity; estimated Kd values from Scatchard analysis for the purified full-length receptor and the ligand binding domain were 0.16 +/- 0.07 nM and 0.04 +/- 0.02 nM, respectively. The nonhypercalcemic analogs of vitamin D, MC903 and delta 22-1, 25S, 26 (OH)3 vitamin D3, bound the recombinant fusion proteins with a similar affinity to the native ligand, 1 alpha, 25(OH)2 vitamin D3. In addition, the full-length VDR fusion protein was shown by gel shift analysis to bind weakly to the human osteocalcin gene vitamin D response element, an interaction greatly facilitated by addition of RXR alpha. These results show that the bacterial expression system detailed here is readily able to produce soluble and functional VDR and its ligand binding domain in high yield. These proteins are easily purified and should be suitable for further structural and functional analysis.

Specific teratogenic effects of different retinoic acid isomers and analogs in the developing anterior central nervous system of zebrafish

Vertebrate embryos are sensitive to retinoic acid, either in deficiency or in excess. Although all-trans retinoic acid (RA) and 9-cis RA are known to have distinct but overlapping activities in higher organisms, only the all-trans isomer has been investigated in detail as a teratogen in zebrafish. We have identified profound and specific effects of 9-cis RA when administered to zebrafish embryos, and have confirmed the results of prior studies on the teratogenic effects of exogenous all-trans RA. Moreover, we have identified a 1-hr period during gastrulation in which embryos are particularly sensitive to the teratogenic effects of RA. In the course of these investigations, we have also studied the effects of two synthetic retinoids-a 9-cis RA analog, SR11217, and an all-trans RA analog, TTAB. An application of all-trans RA to the early zebrafish gastrula leads to defects that are limited to the caudal midbrain and rostral hindbrain. Our experiments show that an application of exogenous 9-cis RA for a period as short as 1 hr and at a concentration as low as 0.1 mu M can block differentiation of the rostral CNS. We have observed abnormal phenotypes using DIC optics, and have demonstrated further abnormalities using whole-mount immunocytochemical staining with antibodies to HNK-1 and acetylated alpha-tubulin. Major axon tract formation in the anterior CNS is unambiguously disrupted by the administration of 9-cis RA but not all-trans RA. Furthermore, exogenous 9-cis RA produces a qualitative alteration in the multiple-site expression pattern of the hlx-1 gene within the rostral CNS, while treatment with all-trans RA leads only to a weakened expression signal. The administration of TTAB and SR11217 result in distinctive inhibitions of hlx-1 expression. Unlike all-trans RA, which causes premature par-2 expression in the posterior midbrains of a majority of embryos, 9-cis RA leads to a complete deletion of this domain throughout development. These results suggest that 9-cis RA is a more active teratogen than all-trans RA in rostral CNS structures of the zebrafish embryo.

Characterization of synthetic retinoids with selectivity for retinoic acid or retinoid X nuclear receptors

The broad spectrum of physiological activities of retinoids is mediate d by two types of receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Though they have 9-cis retinoic acid as a common ligand, the amino acid sequence of their ligand binding domains is only distantly related (27%). This fact makes it probable that the ligand binding pockets of RARs and RXRs differ significantly with respect to their three dimensional structure. Therefore, one can expect that selective ligands for these receptor subclasses do exit. A clear example of a naturally existing RAR-selective retinoid is all-trans retinoic acid. Here we report on two synthetic retinoids which are very closely related to retinoic acid in structure yet show good receptor subclass selectivity. These compounds have a saturated double bond in the polyene side chain between either the 7, 8 or 9, 10 carbon atoms and are highly RAR or RXR selective, respectively (as shown by receptor binding, transactivation activity and the ability to induce RXR homodimer formation). In addition, we present compounds of the synthetic arotinoid class which are highly RAR selective. Interestingly the corresponding '9-cis analogs' are not able to bind or activate RXR alpha and show greatly reduced activity on the RARs.

Differential teratogenic response of mouse embryos to receptor selective analogs of retinoic acid

Early events that initiate teratogenesis by Accutane or other retinoids in mammalian embryos remain unknown. It would be helpful for mechanistic considerations to know whether or not retinoids act through retinoid receptor-dependent pathways, and if they do, which of the two families of receptors (retinoic acid receptors - RARalpha, beta, gamma or retinoid X receptors - RXRalpha, beta, gamma) are more likely involved. We previously used an in vitro bioassay to demonstrate that those retinoid analogs with binding affinity and transactivational activity limited only to the RXRs have a low potential as teratogens. Here, we have extended the study to examine teratogenicity, in pregnant mice, of a number of synthetic retinoids with varying degrees of receptor selectivity. The ability of each compound to induce fetal limb and craniofacial defects after a single exposure on day 11 of gestation was assessed and compared to that of all-trans retinoic acid (RA). The highest dose selected was 100 mg/kg maternal body weight since such a regimen of all-trans RA affects virtually every exposed embryo without any indication of maternal toxicity. We found that although all RAR agonists were strong teratogens, their potencies varied over a wide magnitude. The teratogenic potencies and receptor transactivation profiles of RAR agonists were not directly correlated since compounds with similar receptor activities presented major differences in potencies. Three compounds were exclusively RXR agonists, and these were not teratogenic under our experimental conditions. Two additional compounds which turned out to be non-teratogenic were distinguished by the fact that they activated neither RARs nor RXRs. These data indicate that although RAR-dependent mechanisms are likely involved in retinoid-induced teratogenesis, there are additional factors which determine teratogenic potency. The absence of teratogenic response in the case of RXR agonists suggests that risk-benefit analyses of such receptor-selective compounds may be fruitful in further studies.

Diminished teratogenicity of retinoid X receptor-selective synthetic retinoids

One feature that contraindicates the wide therapeutic use of retinoids is their teratogenicity. Synthetic retinoids are distinguishable from each other on the basis of their partial or exclusive preference in binding and activation of all-trans retinoic acid receptors (RARs) or retinoid X receptors (RXRs). Using mouse embryo limb bud cells in micromass cultures as a bioassay, we examined the inhibitory activities of a number of standard and novel retinoids on chondrogenic cell differentiation. Transient cotransfection of HeLa cells was used to measure the ability of each retinoid to induce transcription of a reporter gene by activating RAR alpha, RAR beta, RAR gamma, or RXR alpha chimeric constructs. All retinoids in this study that activated RARs to any degree in the cotransfection assay also inhibited chondrogenesis in vitro, whereas retinoids that were either specific for RXR or inactive in the cotransfection assay did not. The activity of RAR-selective agonists and the inactivity of RXR-specific agonists in the cotransfection assay correlated well with the relative teratogenicity of six of the representative retinoids studied when orally administered at day 11 to pregnant ICR mice.