Biomedical subjects

M I Dawson

Publications and source records attributed to M I Dawson.

At least 73 records · Page 4Linked to original sources

PubMed1995

Correlation of retinoid binding affinity to retinoic acid receptor alpha with retinoid inhibition of growth of estrogen receptor-positive MCF-7 mammary carcinoma cells.

Both anchorage-dependent growth and anchorage-independent growth of the estrogen receptor-positive mammary carcinoma cell line MCF-7 are inhibited by all-trans-retinoic acid. This cell line has nuclear retinoic acid receptors (RARs) alpha and gamma. The natural retinoids all-trans-retinoic acid and 9-cis-retinoic acid and a series of 12 conformationally restricted retinoids, which showed a range of binding selectivities for these receptors and had either agonist or antagonist activity for gene transcriptional activation by the RARs, were evaluated for their abilities to inhibit anchorage-dependent (adherent) and anchorage-independent (clonal) growth of MCF-7 cells. Correlation analyses were performed to relate growth inhibition by these retinoids with their binding affinity to RAR alpha or RAR gamma. Inhibition of anchorage-dependent growth in culture after 7 days of retinoid treatment correlated with binding to RAR alpha (n = 14; P < or = 0.001) and not to RAR gamma (n = 14; P > 0.1). Both the RAR alpha-selective retinoid agonists and the two RAR antagonists that were evaluated inhibited adherent cell growth. The RAR gamma-selective agonists had very low growth inhibitory activity (< 10%) at concentrations as high as 12.5 microM. These results suggest that RAR alpha is the retinoid receptor involved in the inhibition of adherent cell growth by retinoids and that transcriptional activation by this receptor on a RAR response element does not appear to be required for this process to occur. For this series of retinoids, inhibition of anchorage-independent growth after 21 days of retinoid treatment only correlated (n = 12; P < or = 0.005) with binding affinity to RAR alpha for the retinoid agonists, although the RAR gamma-selective retinoids displayed weak activity. The RAR antagonists were very poor inhibitors of growth. These results suggest that activation of gene transcription by RAR alpha appears to be required for inhibition of anchorage-independent growth by retinoids in this estrogen receptor-positive mammary carcinoma cell line.

Animals↗

PubMed1995

Conformational effects on retinoid receptor selectivity. 2. Effects of retinoid bridging group on retinoid X receptor activity and selectivity.

The natural retinoid 9-cis-retinoic acid is an activating ligand for both the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), which are members of the retinoid/thyroid hormone/steroid hormone family of nuclear receptor proteins that activate gene transcription through specific response elements. The pharmacophoric groups necessary to confer RXR selectivity were established by evaluating the ability of 21 conformationally restricted retinoids to activate the TREpal retinoic acid receptor response element for gene transcription in the presence of one of the three RAR subtypes or RXR alpha. In contrast to those retinoids selective for the RARs, these RXR-selective retinoids have one less atom in the bridge linking the hydrophobic and carboxylic acid termini of the retinoid skeleton. Therefore, a one-carbon bridge replaces the 19-methyl group and 9E-double bond of 9-cis-retinoic acid and is further functionalized by inclusion in an isopropylidene group, a dioxolane ring, or a cyclopropane ring for optimal RXR alpha activity and selectivity. In addition, the beta-geranylidene and 20-methyl-(11E,13E)-dienoic acid groups of 9-cis-retinoic acid are replaced by a 5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl ring and a 4-carboxylphenyl ring, respectively, for optimal activation and selectivity. RXR alpha selectivity is reduced on replacement of the 4-carboxylphenyl group by a 2-carboxyl-5-thienyl group or the 9-cis-retinoic acid methylpentadienoic acid terminus.

Cell Line↗

PubMed1995

p53 independent G0/G1 arrest and apoptosis induced by a novel retinoid in human breast cancer cells.

The biological activity of a novel synthetic retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN) was investigated in human breast carcinoma (HBC) cells. Although capable of selective binding to the RAR gamma nuclear receptor, AHPN inhibited the growth of a number of HBC cell lines via RAR- or RXR-independent pathways. AHPN also inhibited the growth of the human leukemia cell line HL-60R which does not possess functional RARs. RA significantly inhibited AP-1 mediated gene activation in MCF-7 cells while AHPN displayed no such anti-AP-1 activity. Retinoids normally are cytostatic in their inhibition of breast carcinoma growth and permit cell proliferation upon their removal, wher as AHPN induced G0/G1 arrest within 6h followed by apoptosis. In MCF-7 cells that harbor wild type p53, AHPN-induced G0/G1 arrest and apoptosis was accompanied by p53-independent regulation of WAF1/CIP1 as well as bax mRNA levels while bcl-2 mRNA levels were decreased. In MDA-MB-231 cells which possess a mutant p53, AHPN-mediated G0/G1 arrest and apoptosis was also associated with a concomitant up regulation of WAF1/CIP1 mRNA while these cells did not express bax or bcl-2 messages. Thus AHPN represents a novel retinoid that induces G0/G1 arrest and apoptosis via a unique pathway which appears to involve activation of known downstream effectors of p53 in a p53-independent manner.

Apoptosis↗

PubMed1995

Evidence for the involvement of retinoic acid receptor RAR alpha-dependent signaling pathway in the induction of tissue transglutaminase and apoptosis by retinoids.

In this study, we show that all-trans-retinoic acid (RA) is a potent inducer of tissue transglutaminase (TGase II) and apoptosis in the rat tracheobronchial epithelial cell line SPOC-1. We demonstrate that these cells express the retinoid receptors RAR alpha, RAR gamma, and RXR beta. To identify which of these receptors are involved in regulating these processes, we analyzed the effects of several receptor-selective agonists, an antagonist, and a dominant-negative RAR alpha. We show that the RAR-selective retinoid SRI-6751-84 strongly increased TGase II expression at both the protein and mRNA levels, whereas the RXR-selective retinoid SR11217 had little effect. The RAR alpha-selective retinoid Ro40-6055 was also able to induce TGase II, whereas the RAR gamma-selective retinoid CD437 was inactive. The induction of TGase II by the RAR-selective retinoid was completely inhibited by the RAR alpha-antagonist Ro41-5253. Overexpression of a truncated RAR alpha gene with dominant-negative activity also inhibited the induction of TGase II expression. The increase in TGase II is associated with an induction of apoptosis as revealed by DNA fragmentation and the generation of apoptotic cells. We demonstrate that apoptosis is affected by retinoids in a manner similar to TGase II. Our results suggest that the induction of TGase II expression and apoptosis in SPOC-1 cells are mediated through an RAR alpha-dependent signaling pathway.

Animals↗

PubMed1995

Enhanced efficacy of combinations of retinoic acid- and retinoid X receptor-selective retinoids and alpha-interferon in inhibition of cervical carcinoma cell proliferation.

Retinoic acid receptors and retinoid X receptors form heterodimers, bind to retinoic acid response elements, and transactivate the transcription of retinoid-responsive genes. Two synthetic retinoids [4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-anthracenyl)benzoic acid (TTAB) and 6-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-2-naphthale n ecarboxylic acid (TTNN)], which preferentially bind retinoic acid receptors, inhibited the proliferation of cervical carcinoma ME180 cells by 50% at 0.2 nM and 0.2 microM, respectively. In contrast, two other retinoids [2-(4-carboxyphenyl)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)-1,3-dithiane (SR11203) and 4-(2-methyl-1-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)propenyl)benzoic acid (SR11217)], which preferentially bind retinoic X receptors, inhibited growth by only 12 and 18% at 1 microM, respectively. The combination of suboptimal concentrations of TTAB (0.1 nM) or TTNN (10 nM) with each of the retinoic X receptor-selective retinoids at 1 microM showed more than additive effects on cell proliferation, especially with SR11217. Further increases in proliferation inhibition were observed when IFN-alpha (100 units/ml) was added to these retinoid combinations. Activation of transcription of a reporter gene linked 3' to the retinoic acid receptor beta retinoic acid response element in transiently transfected cells also exhibited additive effects when the cells were treated with combinations of TTAB or TTNN with SR11217. This additive activation of transcription may be the reason why the combination of retinoids is more effective than each retinoid alone. The results also suggest that the use of combinations of retinoids and IFN-alpha may lead to enhanced antitumor effects.

Cell Division↗

PubMed1995

Retinoic acid nuclear receptor beta inhibits breast carcinoma anchorage independent growth.

Retinoids modulate cellular proliferation and mediate gene function through a series of nuclear receptors. The retinoic acid nuclear receptor beta (RAR beta) plays an important role in the differentiation of a number of cell types. We now demonstrate that RAR beta expression is confined to normal mammary tissue and is not expressed in either immortalized normal or malignant cell lines. Treatment of RAR beta-transfected MDA-MB-231 cells with 1 microM all-trans-retinoic acid (RA) significantly inhibited monolayer growth of the cells which express recombinant RAR beta. RAR beta-expressing MDA-MB-231 cells formed significantly smaller and fewer colonies in soft agar than the mock-transfected cells. Addition of 1 microM RA stimulated colony size and number in the RAR beta-transfected MDA-MB-231 cells. In contrast to the RAR beta-expressing cells, colony formation by the RAR alpha-expressing cells was similar to the mock-transfected controls and the addition of 1 microM RA to the RAR alpha-transfected cells inhibited colony formation. While demonstrating decreased colony formation in agar, RAR beta-expressing MDA-MB-231 cells failed to exhibit decreased growth in SCID mice. Our results show that RAR beta functions as a negative regulator of growth in breast epithelial cells. In addition, the growth of these cells is differentially regulated by RAR alpha and RAR beta which is most likely the result of the modulation of different genes.

Agar↗

PubMed1995

Thyroid hormone enhancement of estradiol stimulation of breast carcinoma proliferation.

Thyroid hormone (T3) and estradiol (Est) modulate biological processes by binding to nuclear receptor proteins that, through interactions with specific response elements in the regulatory regions of genes, modulate gene transcription. Est stimulation of estrogen receptor (ER)-positive breast carcinoma cell growth occurs through its ability to bind to the ER and activate gene transcription. We now report that physiological concentrations of T3 significantly enhance Est stimulation of growth of a number of human breast carcinoma cell lines. The effect of T3 is specific for Est stimulation of growth and has no effect on insulin-like growth factor-I stimulation of growth. The effect of T3 on enhancing Est-mediated growth was specifically blocked by the addition of ligands inducing retinoid X receptor (RXR) homodimer receptor formation, suggesting that RXR-thyroid nuclear receptor (TR) heterodimer formation is required for the T3-mediated effect on estradiol-stimulated growth. Four thyroid nuclear receptors have been described in tissues, TR alpha 1, alpha 2, beta 1, and beta 2. Breast carcinoma cells were found to express TR beta 1 and TR alpha 2 mRNA and very low levels of TR alpha 1 mRNA. T3 did not increase ER mRNA or protein levels and did not enhance Est-mediated increases in gene transcription of a number of genes, i.e., transforming growth factor-alpha and pS2 which contain estrogen-response elements (EREs) in their regulatory regions. However, T3 enhanced Est-stimulated ERE-TK-CAT activity. Thus significant cross-talk appears to occur between the TRs and ER and T3 appears to enhance Est-mediated gene transcription.

Blotting, Northern↗

PubMed1995

N-(4-hydroxyphenyl)retinamide (4-HPR)-mediated biological actions involve retinoid receptor-independent pathways in human breast carcinoma.

Retinoid response pathways involve retinoic acid receptors (RARs) and retinoid X receptors. N-(4-hydroxyphenyl) retinamide (4-HPR), a derivative of all-trans-retinoic acid (RA) is currently in clinical trials as a chemopreventive agent for breast cancer. The issue whether 4-HPR mediates its biological actions via classical retinoid receptor pathways remains to be investigated. In this study, we provide several lines of evidence that 4-HPR mediates its biological actions via a novel pathway(s) that does not involve the classical retinoid receptor pathways. For example, 4-HPR was more potent than RA as an antiproliferative agent and inhibited growth of otherwise RA-resistant human breast carcinoma cells. Exposure to 4-HPR resulted in the generation of DNA fragmentation with subsequent cell death in both RA-positive estrogen receptor (ER)-positive as well as RA-refractory ER-negative breast carcinoma cell lines. N-(4-Methoxyphenyl)retinamide (4-MPR), which is the major 4-HPR metabolite in circulation, was biologically inert in this system. 4-HPR and 4-MPR bound poorly to the RAR alpha, beta and gamma in vitro and only minimally activated the retinoic acid receptor element (RARE) and retinoid X receptor response elements (RXREs) in human breast carcinoma cells. Neither 4-HPR nor 4-MPR are metabolized to any of the known conventional retinoids. In addition, 4-HPR or 4-MPR transactivation of RAREs or RXREs transfected into MCF-7 and MDA-MB-231 cells was not noted at 48 h. Nevertheless 4-HPR-mediated cell death was observed at 48 h, further suggesting that neither 4-HPR nor 4-MPR are metabolized to retinoids which activate the RAREs or RXREs in breast carcinoma cells. Furthermore, unlike RA, which exhibited anti-AP1 activity, 4-HPR inhibition of growth did not involve anti-AP1 activity. These results suggest that 4-HPR acts by a unique pathway that is not mediated by retinoid receptors.

Anticarcinogenic Agents↗

PubMed1994

A new class of retinoids with selective inhibition of AP-1 inhibits proliferation.

Retinoids regulate many biological processes, including differentiation, morphogenesis and cell proliferation. They are also important therapeutic agents, but their clinical usefulness is limited because of side effects. Retinoid activities are mediated by specific nuclear receptors, the RARs and RXRs, which can induce transcriptional activation through specific DNA sites or by inhibiting the transcription factor AP-1 (refs 12-15), which usually mediates cell proliferation signals. Because the two types of receptor actions are mechanistically distinct, we investigated whether conformationally restricted retinoids, selective for each type of receptor action, could be identified. Here we describe a new class of retinoids that selectively inhibits AP-1 activity but does not activate transcription. These retinoids do not induce differentiation in F9 cells but inhibit effectively the proliferation of several tumour cell lines, and could thus serve as candidates for new retinoid therapeutic agents with reduced side effects.

Animals↗

PubMed1994

Myeloid differentiation mediated through retinoic acid receptor/retinoic X receptor (RXR) not RXR/RXR pathway.

Retinoids, such as all-trans-retinoic acid and 9-cis-retinoic acid, are naturally occurring ligands of the nuclear retinoic acid receptors (RARs). In concert with binding of ligand, these receptors from heterodimers with the retinoic X receptor (RXR) and transactivate RAR/RXR-responsive genes. Retinoids can differentiate leukemic cell lines in vitro and induce clinically complete remissions in patients with acute promyelocytic leukemia. Synthetic ligands to the RAR and RXR receptors have been developed that selectively bind and activate RAR/RXR (TTAB) and RXR/RXR dimers (SR11217). We investigated the affect of these ligands, either alone or in combination, on in vitro growth and differentiation of cells from the HL-60, KG-1, THP-1, and WEHI-3 myeloid cell lines as well as on clonal growth of fresh myeloid leukemic blasts from patients. Clonal inhibition of proliferation of these cells was studied in soft agar cultures. Cells were plated in the presence of either one or a combination of retinoids at concentrations of 10(-5) to 10(-10) mol/L. TTAB inhibited 50% clonal growth at an effective dose (ED50) that was about 1,000-fold lower than the concentration of SR11217 required to achieve an ED50 for the same leukemic cells. Combination of both ligands at a variety of concentrations showed no synergistic effects. Superoxide production (nitroblue tetrazolium reduction) and CD11b expression as parameters of differentiation of HL-60 cells were also examined. Results paralleled those of clonal growth, with SR11217 being markedly less potent than TTAB. These results show that the ligand selective for RXR-homodimers has little effect on either inducing differentiation or inhibiting clonal growth of leukemic cells. The differentiating and antiproliferative effects of retinoids are mainly induced through RAR/RXR heterodimers, and development of therapeutic analogs should focus on this category of retinoids.

Animals↗

PubMed1994

A synthetic retinoid antagonist inhibits the human immunodeficiency virus type 1 promoter.

Retinoids regulate a broad range of biological processes and affect cell growth and differentiation of many cell types, including the immune system. Recently, it was reported that human immunodeficiency virus type 1 (HIV-1) expression in macrophages is enhanced by retinoic acid (RA). Retinoid signals are mediated by the RA receptors (RARs) and retinoid X receptors (RXRs) that bind to specific RA responsive elements (RAREs) in the promoter region of susceptible genes. Here, we report on a RARE in the long terminal repeat (LTR) region that allows activation of the HIV-1 LTR. The RARE is composed of two consensus RARE half-sites (A/GGGTCA) arranged as a palindrome separated by 9 nucleotides and is activated by both RAR/RXR heterodimers and RXR homodimers. We show that the COUP (chicken ovalbumin upstream promoter) orphan receptors also bind to the HIV-1 RARE and repress the retinoid response of the HIV-1 RARE or the HIV-1 LTR. Furthermore, a newly discovered synthetic retinoid is shown to be a potent inhibitor of retinoid-induced activation of the HIV-1 RARE. These observations suggest additional approaches for the inhibition of HIV replication.

Animals↗

PubMed1994

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

All-trans-retinoic acid mediates cell growth and differentiation by binding to and then activating nuclear retinoid receptor proteins that regulate gene transcription. Recombinant human retinoic acid receptor beta was cloned and expressed in Escherichia coli as a fusion protein rMBP-RAR beta with maltose-binding protein to facilitate purification. After isolation from bacterial lysates, rMBP-RAR beta was used for binding with selected retinoids. Scatchard analysis with [11,12-3H2]all-trans-retinoic acid gave a Kd of 0.34 nM. Competitive binding studies with a series of conformationally restricted aromatic retinoids indicated that the Ki values for binding to rMBP-RAR beta correlated with the logs of the EC50 values for gene transcriptional activation (p < or = 0.05) and with those for the relative activation compared to that of all-trans-retinoic acid (p < or = 0.01). Inspection of binding-activation correlation diagrams indicates candidate structures for improved retinoid agonists or antagonists.

Amino Acid Sequence↗

PubMed1993

Correlation of the ability of retinoids to inhibit promoter-induced anchorage-independent growth of JB6 mouse epidermal cells with their activation of retinoic acid receptor gamma.

Retinoids inhibit the biological effects induced in mouse epidermal cells by the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Specific nuclear retinoic acid receptors (RARs) have been identified in the epidermis, but the specific receptor that mediates the inhibitory response by retinoids is not established. Retinoic acid and six conformationally restricted retinoids were evaluated in an in vitro bioassay using the JB6 mouse epidermal cell line. These activities were then compared with the ability of these retinoids to activate the RARs in transient transfection assays for transcriptional activation to identify the retinoid receptor involved in inhibiting TPA-induced anchorage-independent growth. The retinoids inhibited TPA-induced colony formation of JB6 cells in semisolid medium at concentrations that were not toxic based on colony formation of attached cells. These concentrations ranged from less than 10(-9)-10(-6) M. 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethylanthracen-2-yl)benzoic acid (TTAB) was the most potent retinoid, with an EC50 of 0.8 nM. Both RAR alpha and RAR gamma were expressed in JB6 cells. Expression of RAR beta was not detected in these cells using a polymerase chain reaction assay, consistent with its extremely low level in mouse skin. Inhibition of the TPA response by these retinoids in JB6 cells correlated only with their transcriptional activation of RAR alpha, but not with that of RAR alpha. These results suggest that RAR gamma is most probably the receptor that mediates the chemopreventive effects of retinoids in mouse epidermis.

Animals↗

PubMed1993

Conformational effects on retinoid receptor selectivity. 1. Effect of 9-double bond geometry on retinoid X receptor activity.

A major challenge is the development of retinoids with selective biological activities. Recently, studies on retinoid response mechanisms indicate that retinoids activate two classes of nuclear receptor proteins, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Here, we analyze the activity of a series of (E)- and (Z)-stilbenecarboxylic acids for gene transcriptional activation of the RARs and RXR-alpha to determine the optimum pharmacophore for receptor activation. The data obtained indicate that RAR and RXR response pathways can be separated by using the appropriate ligand. The conformations of (Z)-4-[2-(5-,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)prop en-1-yl]benzoic acid (Z)-4-[1-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)propen-2-yl]benzoic acid were examined by experimental and theoretical methods to establish the appropriate conformation of the latter that specifically activated the retinoid RXR. A palladium(0)-catalyzed aryl bromide-arylboronic acid coupling under nonanhydrous conditions was used to construct a biaryl bond in the conformationally restricted retinoid 2'- (5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthaleny)biphenyl-4-c arboxylic acid, which had RXR activity.

Binding Sites↗

PubMed1993

9-cis-retinoic acid: effects on normal and leukemic hematopoiesis in vitro.

Retinoic acid exhibits effects on the proliferation and differentiation of many hematopoietic cells. Cellular responsiveness to retinoic acid (RA) is conferred through two distinct classes of nuclear receptors, the RA receptors (RARs) and the retinoid X receptors (RXRs). The RARs bind to both 9-cis- and all-trans-RAs, but 9-cis-RA alone directly binds and activates RXR. This suggested that 9-cis-RA could have expanded hematopoietic activities as compared with all-trans-RA. We compared the abilities of 9-cis- and all-trans-RAs to induce differentiation and inhibit proliferation of three acute myelogenous leukemia (AML) cell lines and fresh leukemic cells from 28 patients and found that: (1) 9-cis-RA in general was more potent than all-trans-RA in suppressing the clonal growth of two AML cell lines and 17 AML samples from patients, including four from individuals with acute promyelocytic leukemia (APL). Eleven leukemic samples, including three from patients with chronic myelogenous or chronic myelomonocytic leukemia, were relatively refractory to both retinoids. (2) The range of activities of both retinoids was similar except that the clonal growth of samples from three AML patients were inhibited by 9-cis-RA, but not by all-trans-RA. (3) Both retinoids inhibited the clonal proliferation of leukemia cells without necessarily inducing their differentiation; in fact, the only fresh AML cells that were able to undergo differentiation were from patients with APL and one individual with M2 AML. (4) Both retinoids enhanced myeloid and erythroid clonal growth from normal individuals, and 9-cis-RA showed slightly more stimulation of the myeloid clonal growth than did the all-trans-RA. Our study suggests that 9-cis-RA is worthy of further study for the treatment of selected individuals with AML.

Cell Differentiation↗

PubMed1992

Retinoids selective for retinoid X receptor response pathways.

Retinoids have a broad spectrum of biological activities and are useful therapeutic agents. Their physiological activities are mediated by two types of receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). RARs, as well as several related receptors, require heterodimerization with RXRs for effective DNA binding and function. However, in the presence of 9-cis-retinoic acid, a ligand for both RARs and RXRs, RXRs can also form homodimers. A series of retinoids is reported that selectively activates RXR homodimers but does not affect RAR-RXR heterodimers and thus demonstrates that both retinoid response pathways can be independently activated.

Animals↗

PubMed1992

Homodimer formation of retinoid X receptor induced by 9-cis retinoic acid.

Retinoid response pathways are mediated by two classes of receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). A central question is whether distinct response pathways are regulated by these two classes of receptors. The observation that the stereoisomer 9-cis-retinoic acid binds with high affinity to RXRs suggested that this retinoid has a distinct role in controlling RXR activity, but it was almost simultaneously discovered that RXRs function as auxiliary receptors for RARs and related receptors, and are essential for DNA binding and function of those receptors. Hence, although RARs seem to operate effectively only as heterodimeric RAR/RXR complexes, RXRs themselves apparently function predominantly, if not exclusively, as auxiliary receptors. Here we report that 9-cis-retinoic acid induces RXR homodimer formation. Our results demonstrate a new mechanism for retinoid action by which a ligand-induced homodimer mediates a distinct retinoid response pathway.

Animals↗

PubMed1992

Structure-affinity relationships of retinoids with embryonic cellular retinoic acid-binding protein.

Separation and quantitation of cellular retinoic acid-binding protein (CRABP) in embryonic and fetal hamster tissues was accomplished with high-performance size-exclusion chromatography. Binding affinity of 26 retinoids was established by in vitro displacement of high specific activity all-trans-[3H2]retinoic acid from fetal CRABP. The CRABP concentration in presomite-to-early somite (Day 8) hamster embryos was 1.9 pmol/mg cytosolic protein and increased to 7.5 pmol/mg protein in Day 13 fetuses; CRABP concentrations subsequently declined as gestation progressed. CRABP was located primarily in fetal brain and skin (5.8 +/- 0.3 and 2.2 +/- 0.1 pmol/mg protein, respectively), whereas only trace concentrations were found in fetal liver, placenta, and maternal uterus. Retinoids that could displace all-trans-retinoic acid from CRABP had a free acid at the polar terminus (or were carboxylate esters that were readily hydrolyzed to the corresponding free acid) and had a hydrophobic ring at the distal position. The ligand specificity of the CRABP studied here suggests that this protein was analogous to the CRABP I isoform. The in vitro binding affinities of teratogenic retinoids that competed for embryonic CRABP failed to correlate directly with relative teratogenic potency. In some instances, the latter observation can be related to extensive in vivo biotransformation of retinoids to multiple teratogenic metabolites and to retinoid persistence in the embryo. Three analogs containing a free carboxy terminus, SRI 5898-21, SRI 7323-78, and SRI 6153-40, were identified with high teratogenic potency but failed to bind fetal hamster CRABP. The structure-activity and binding data of the analogs studied here indicate that many, if not most, teratogenic retinoids (or their acidic metabolites) bind with embryonic/fetal CRABP, but the present data question the role for CRABP in their teratogenic mechanism of action.

Animals↗