HX600, a synthetic agonist for RXR-Nurr1 heterodimer complex, prevents ischemia-induced neuronal damage

Ischemic stroke is amongst the leading causes of death and disabilities. The available treatments are suitable for only a fraction of patients and thus novel therapies are urgently needed. Blockage of one of the cerebral arteries leads to massive and persisting inflammatory reaction contributing to the nearby neuronal damage. Targeting the detrimental pathways of neuroinflammation has been suggested to be beneficial in conditions of ischemic stroke. Nuclear receptor 4A-family (NR4A) member Nurr1 has been shown to be a potent modulator of harmful inflammatory reactions, yet the role of Nurr1 in cerebral stroke remains unknown. Here we show for the first time that an agonist for the dimeric transcription factor Nurr1/retinoid X receptor (RXR), HX600, reduces microglia expressed proinflammatory mediators and prevents inflammation induced neuronal death in in vitro co-culture model of neurons and microglia. Importantly, HX600 was protective in a mouse model of permanent middle cerebral artery occlusion and alleviated the stroke induced motor deficits. Along with the anti-inflammatory capacity of HX600 in vitro, treatment of ischemic mice with HX600 reduced ischemia induced Iba-1, p38 and TREM2 immunoreactivities, protected endogenous microglia from ischemia induced death and prevented leukocyte infiltration. These anti-inflammatory functions were associated with reduced levels of brain lysophosphatidylcholines (lysoPCs) and acylcarnitines, metabolites related to proinflammatory events. These data demonstrate that HX600 driven Nurr1 activation is beneficial in ischemic stroke and propose that targeting Nurr1 is a novel candidate for conditions involving neuroinflammatory component.

Molecular mechanism by which acyclic retinoid induces nuclear localization of transglutaminase 2 in human hepatocellular carcinoma cells

Nuclear accumulation of transglutaminase 2 (TG2) is an important step in TG2-dependent cell death. However, the underlying molecular mechanisms for nuclear translocation of TG2 are still poorly understood. In this study, we demonstrated that acyclic retinoid (ACR) induced nuclear accumulation of TG2 in JHH-7 cells, a hepatocellular carcinoma (HCC) leading to their apoptosis. We further demonstrated molecular mechanism in nuclear-cytoplasmic trafficking of TG2 and an effect of ACR on it. We identified a novel 14-amino acid nuclear localization signal (NLS) (466)AEKEETGMAMRIRV(479) in the 'C' domain and a leucine-rich nuclear export signal (NES) (657)LHMGLHKL(664) in the 'D' domain that allowed TG2 to shuttle between the nuclear and cytosolic milieu. Increased nuclear import of GAPDH myc-HIS fused with the identified NLS was observed, confirming its nuclear import ability. Leptomycin B, an inhibitor of exportin-1 as well as point mutation of all leucine residues to glutamine residues in the NES of TG2 demolished its nuclear export. TG2 formed a trimeric complex with importin-α and importin-β independently from transamidase activity which strongly suggested the involvement of a NLS-based translocation of TG2 to the nucleus. ACR accelerated the formation of the trimeric complex and that may be at least in part responsible for enhanced nuclear localization of TG2 in HCC cells treated with ACR.

Inhibition of RXR and PPARgamma ameliorates diet-induced obesity and type 2 diabetes

PPARgamma is a ligand-activated transcription factor and functions as a heterodimer with a retinoid X receptor (RXR). Supraphysiological activation of PPARgamma by thiazolidinediones can reduce insulin resistance and hyperglycemia in type 2 diabetes, but these drugs can also cause weight gain. Quite unexpectedly, a moderate reduction of PPARgamma activity observed in heterozygous PPARgamma-deficient mice or the Pro12Ala polymorphism in human PPARgamma, has been shown to prevent insulin resistance and obesity induced by a high-fat diet. In this study, we investigated whether functional antagonism toward PPARgamma/RXR could be used to treat obesity and type 2 diabetes. We show herein that an RXR antagonist and a PPARgamma antagonist decrease triglyceride (TG) content in white adipose tissue, skeletal muscle, and liver. These inhibitors potentiated leptin's effects and increased fatty acid combustion and energy dissipation, thereby ameliorating HF diet-induced obesity and insulin resistance. Paradoxically, treatment of heterozygous PPARgamma-deficient mice with an RXR antagonist or a PPARgamma antagonist depletes white adipose tissue and markedly decreases leptin levels and energy dissipation, which increases TG content in skeletal muscle and the liver, thereby leading to the re-emergence of insulin resistance. Our data suggested that appropriate functional antagonism of PPARgamma/RXR may be a logical approach to protection against obesity and related diseases such as type 2 diabetes.

The fat-derived hormone adiponectin reverses insulin resistance associated with both lipoatrophy and obesity

Adiponectin is an adipocyte-derived hormone. Recent genome-wide scans have mapped a susceptibility locus for type 2 diabetes and metabolic syndrome to chromosome 3q27, where the gene encoding adiponectin is located. Here we show that decreased expression of adiponectin correlates with insulin resistance in mouse models of altered insulin sensitivity. Adiponectin decreases insulin resistance by decreasing triglyceride content in muscle and liver in obese mice. This effect results from increased expression of molecules involved in both fatty-acid combustion and energy dissipation in muscle. Moreover, insulin resistance in lipoatrophic mice was completely reversed by the combination of physiological doses of adiponectin and leptin, but only partially by either adiponectin or leptin alone. We conclude that decreased adiponectin is implicated in the development of insulin resistance in mouse models of both obesity and lipoatrophy. These data also indicate that the replenishment of adiponectin might provide a novel treatment modality for insulin resistance and type 2 diabetes.

Alpha-glucosidase inhibitors with a 4,5,6,7-tetrachlorophthalimide skeleton pendanted with a cycloalkyl or dicarba-closo-dodecaborane group

Previous studies of alpha-glucosidase inhibitors derived from thalidomide revealed that 4,5,6,7-tetrachloro-N-alkylphthalimide derivatives are superior lead compounds. Structure-activity relationship studies indicated that a hydrophobic group at the N(2) position is mandatory for potent activity. Accordingly, we have designed and synthesized some 4,5,6,7-tetrachloro-N-cycloalkylphthalimide and 4,5,6,7-tetrachloro-N-dicarba-closo-dodecaborane derivatives. The prepared compounds exhibited potent alpha-glucosidase-inhibitory activity. Among them, 4,5,6,7-tetrachloro-N-cycloheptylphthalimide (9) showed the most potent activity, being approximately 30 times more active than the classical inhibitor, 1-deoxynojirimycin (1).

Structure-Activity study of retinoid agonists bearing substituted dicarba-closo-dodecaborane. Relation between retinoidal activity and conformation of two aromatic nuclei

We have investigated the structure activity relationships of the potent retinoid agonist, 4-[4-(2-propyl-1,2-dicarba-closo-dodecaboran-l-yl)phenylamino]benzoic acid (BR403), which we have previously reported. Substitution of a methyl group on the aromatic nucleus or a methyl group on the nitrogen atom, or replacement of the amino group with ether, methylene, carboxyl or 1,1-ethylene greatly decreased the activity. The relatively planar conformation at the phenyl-N-phenyl moiety seems to play a critical role in the appearance of the biological activity.

Novel retinoidal tropolone derivatives. Bioisosteric relationship of tropolone ring with benzoic acid moiety in retinoid structure

Several tropolone derivatives (4-7) were designed as novel retinoids on the assumption that the tropolone ring may mimic the benzoic acid moiety in retinoid structures, such as Am80 (2). Among the synthesized compounds, 5-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)ethynyl]tropolone (7a) showed moderate potency as a differentiation-inducer of HL-60 cells. The activities of the tropolones were greatly enhanced in the presence of HX630, an RXR agonist (retinoid synergist).

Inhibition by retinoids of antigen-induced IL-4 production in rat mast cell line RBL-2H3

The retinoic acid receptor (RAR) agonists, Re80 and Am80, partially inhibited the antigen-induced IL-4 production by rat mast cell line RBL-2H3 in a concentration-dependent manner (0.1 to 1000 nM). Both Re80 and Am80 also reduced the antigen-induced increase in IL-4 mRNA levels. The RAR antagonist LE540 at 4 microM reversed Re80 (100 nM)- and Am80 (100 nM)-induced inhibition of IL-4 production. The retinoid X receptor agonist HX600 (1 microM) by itself did not affect IL-4 production, but enhanced the inhibitory effect of Re80 (10 nM) and of Am80 (10 nM). Cyclosporin A suppressed the antigen-induced IL-4 production almost completely at 0.3 microM. These findings indicated that the antigen-induced IL-4 production by RBL-2H3 cells is partially inhibited by retinoids via RAR-dependent mechanisms.

Synergistic potentiation of thiazolidinedione-induced ST 13 preadipocyte differentiation by RAR synergists

Peroxisome proliferator-activated receptor gamma (PPAR gamma) belongs to a nuclear receptor super family that functions as a master regulator of adipocyte differentiation. PPAR gamma binds its DNA response element together with a partner, retinoid X receptor (RXR), in fat cells. Five RXR ligands (HX600, HX630, DA022, DA124, LGD1069, referred to as retinoid synergists) by themselves exhibit weak transactivation activity on the PPAR gamma response element. However, addition of PPAR gamma-specific ligand in this assay gave rise to a 5- to 13-fold increase, indicating a strong synergy between these ligands. LGD1069 was the most effective activator of the RXR/PPAR gamma heterodimer on the transactivation of the reporter gene. But, in contrast to the other four RXR ligands, LGD1069 did not show synergistic induction of ST 13 preadipocytes to adipocytes. This apparent contradiction may result from the ligand-binding property of LGD1069. In this article we discuss the fact that retinoid synergists also act as PPAR gamma synergists.

Retinoidal pyrimidinecarboxylic acids. Unexpected diaza-substituent effects in retinobenzoic acids

Several pyridine- and pyrimidine-carboxylic acids were synthesized as ligand candidates for retinoid nuclear receptors, retinoic acid receptors (RARs) and retinoic X receptors (RXRs). Although the pyridine derivatives, 6-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)carbamoyl]pyri dine-3-carboxylic acid (2b) and 6-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)carboxamido]py ridine-3-carboxylic acid (5b) are more potent than the corresponding benzoic acid-type retinoids, Am80 (2a) and Am580 (5a), the replacement of the benzene ring of Am580 (5a), Am555 (6a), or Am55 (7a) with a pyrimidine ring caused loss of the retinoidal activity both in HL-60 cell differentiation assay and in RAR transactivation assay using COS-1 cells. On the other hand, pyrimidine analogs (PA series, 10 and 11) of potent RXR agonists (retinoid synergists) with a diphenylamine skeleton (DA series, 8 and 9) exhibited potent retinoid synergistic activity in HL-60 cell differentiation assay and activated RXRs. Among the synthesized compounds, 2-[N-n-propyl-N-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)a mino]pyrimidine-5-carboxylic acid (PA013, 10e) is most active retinoid synergist in HL-60 assay.

Polymethylcarborane as a novel bioactive moiety: derivatives with potent retinoid antagonistic activity

4[(Deca-B-methyl-1,12-dicarba-closo-dodecaboran-1-yl)c arbamoyl]benzoic acid and its congeners showed potent antagonistic activity at concentrations of 10(-7)-10(-8) M on the differentiation-inducing action of retinoids towards human promyelocytic leukemia HL-60 cells. This is the first example of derivatives of polymethylcarborane, which resembles C60 in size, with biological activity.

Retinoid X receptor-antagonistic diazepinylbenzoic acids

Several dibenzodiazepine derivatives were identified as novel retinoid X receptor (RXR) antagonists on the basis of inhibitory activity on retinoid-induced cell differentiation of human promyelocytic leukemia cells HL-60 and transactivation assay using retinoic acid receptors (RARs) and RXRs in COS-1 cells. 4-(5H-2,3-(2,5-Dimethyl-2,5-hexano)-5-n- propyldibenzo[b,e][1,4]diazepin-11-yl)benzoic acid (HX603, 6c) is an N-n-propyl derivative of an RXR pan-agonist HX600 (6a), and exhibited RXR-selective antagonistic activity. Similar RXR-antagonistic activities were observed with 4-(5H-2,3-(2,5-dimethyl-2,5-hexano)-5-methyl- 8-nitrodibenzo[b,e][1,4]diazepin-11-yl)benzoic acid (HX531, 7a) and 4-(5H-10,11-dihydro-5,10-dimethyl-2,3-(2,5-dimethyl- 2,5-hexano)-dibenzo[b,e][1,4]diazepin-11-yl)benzoic acid (HX711, 8b), which also inhibited transactivation of RARs induced by an RAR agonist, Am80. These compounds inhibited HL-60 cell differentiation induced by the combination of a low concentration of the retinoid agonist Am80 with an RXR agonist (a retinoid synergist, HX600). These results indicated that HX603 (6c), and the related RXR antagonists inhibit the activation of RAR-RXR heterodimers as well as RXR homodimers, which is a distinct characteristic different from that of the known RXR antagonist, LG100754 (9).

Potent homophthalimide-type inhibitors of B16F10/L5 mouse melanoma cell invasion

Recently, we developed a series of novel and potent aminopeptidase inhibitors with a homophthalimide skeleton. Among them, N-(2,6-diethylphenyl)homophthalimide (PIQ-22) possesses a specific aminopeptidase-inhibiting activity more potent than that of bestatin or actinonin, as assayed in terms of hydrolysis of L-alanine 4-methylcoumaryl-7-amide (Ala-AMC) by human acute lymphoblastic leukemia MOLT-4 cells. We show here that PIQ-22 and its 2,6-dimethylphenyl derivative (PIQ-11) are more potent inhibitors of tumor cell invasion than bestatin and actinonin in a Matrigel assay using mouse melanoma B16F10/L5 cells.

Thiazolidinediones with thyroid hormone receptor agonistic activity

Several thiazolidinedione derivatives (3-7) were designed and synthesized as candidate thyromimetic drugs. Among them, the dihydrogenated compounds, such as 5-2-[[4-(3-tert-butyl-4-hydroxyphenyl)oxy-3,5-diiodophenyl] ethyl]-2,4-thiazolidinedione (6b) and its 3-isopropyl analog (7b), exhibited potent thyroid hormone receptor alpha 1 (TR alpha 1) activation activity.

Identification of a novel class of retinoic acid receptor beta-selective retinoid antagonists and their inhibitory effects on AP-1 activity and retinoic acid-induced apoptosis in human breast cancer cells

Four candidate retinoid antagonists (LE135, LE511, LE540, and LE550) were designed on the basis of the ligand superfamily concept and synthesized. Analysis of these related retinoids by transient transfection assay demonstrated that LE135, LE540, and LE550 are effective retinoic acid receptor (RAR) antagonists, whereas LE511 selectively induced RARbeta transcriptional activity. Both LE135 and LE540 inhibited retinoic acid (RA)-induced transcriptional activation of RARbeta, but not RARalpha, RARgamma or retinoid X receptor alpha (RXRalpha), on a variety of RA response elements. The retinoid antagonists also inhibited all-trans-RA-induced transcriptional activation of RARbeta/RXRalpha heterodimers, although they did not show any effect on transactivation activity of RXR/RXR homodimers. In ZR-75-1 human breast cancer cells, cotreatment of LE135 and LE540 with all-trans-RA inhibited all-trans-RA-induced apoptosis of the cells, further demonstrating that RARbeta plays a role in RA-induced apoptosis of breast cancer cells. We also evaluated the effect of these retinoids on AP-1 activity. Our data showed that LE135 and LE540 strongly repressed 12-O-tetradecanoylphorbol-13-acetate-induced AP-1 activity in the presence of RARbeta and RXRalpha. Interestingly, LE550 induced AP-1 activity when RARbeta and RXRalpha were expressed in HeLa cells but not in breast cancer cells. These results demonstrate that LE135 and LE540 were a novel class of RARbeta-selective antagonists and anti-AP-1 retinoids and should be useful tools for studying the role of retinoids and their receptors.

Dicarba-closo-dodecaboranes as a pharmacophore. Novel potent retinoidal agonists

The synthesis and biological evaluation of the dicarba-closo-dodecaborane (carborane) derivatives of retinoids are described. Retinoidal activity was examined in terms of the differentiation-inducing ability toward human promyelocytic leukemia HL-60 cells. High retinoidal activity (agonist or antagonist for the retinoid receptor RAR) requires a carboxylic acid moiety and an appropriate hydrophobic group located at a suitable position on the molecule. 4-[4-(1,2-Dicarba-closo-dodecaboran-1-yl)phenylamino]b enzoic acids and 4-[3-(1,2-dicarba-closo-dodecaboran-1-yl)phenylamino]b enzoic acids showed potent agonistic activity at concentrations of 10(-8)-10(-9) M. The results indicate that carboranes are applicable as the hydrophobic moiety of biologically active molecules.

Dicarba-closo-dodecaboranes as a pharmacophore. Retinoidal antagonists and potential agonists

Synthesis and biological evaluation of the first dicarba-closo-dodecaborane (carborane) derivatives of retinoids are described. Their retinoidal activity were examined in terms of the differentiation-inducing ability toward human promyelocytic leukemia HL-60 cells. High retinoidal activity (agonist or antagonist for retinoic acid receptor (RAR) requires a carboxylic acid moiety and an appropriate hydrophobic group located at a suitable position on the molecule. The 4-carboranyl-substituted compounds (7, 11) showed antagonistic activity but no agonistic activity even in the presence of the potent synergist HX630. On the other hand, the 3-carboranyl-substituted compounds (8, 12) showed potential agonistic activity, but no antagonistic activity. The results indicates that carboranes are applicable as the hydrophobic moiety of biologically active molecules.

Potent retinoid synergists with a diphenylamine skeleton

4-[N-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)amino]benzoic acid (5) exhibited weak retinoidal and retinoid synergistic activities in HL-60 cell differentiation assay. N-Alkylation of 5 caused decrease or loss of differentiation-inducing activity, but enhanced the synergistic activity with a synthetic retinoid Am80 (2), as reflected in the potent synergistic EC50 (SEC50) values of DA023 (11, 1.6 x 10(-10) M) and DA113 (14, 1.4 x 10(-10) M) in the presence of 1.0 x 10(-10) M Am80 (2).

Novel thiazolidinedione derivatives with retinoid synergistic activity

Several arylmethylidene thiazolidinediones were synthesized and their retinoidal activities were examined. TZ181 (7a), having a benzanilide skeleton, exhibited differentiation-inducing activity in HL-60 cell assay, while TZ191 (7b), the N-methylated analog of TZ181 (7a), TZ245 (9) and TZ335 (10) acted as retinoid synergists like the RXR-selective ligand, LGD1069 (5).

Regulation of retinoidal actions by diazepinylbenzoic acids. Retinoid synergists which activate the RXR-RAR heterodimers

In human HL-60 promyelocytic leukemia cells, diazepinylbenzoic acid derivatives can exhibit either antagonistic or synergistic effects on the differentiation-inducing activities of natural or synthetic retinoids, the activity depending largely on the nature of the substituents on the diazepine ring. Thus, a benzolog of the retinoid antagonist LE135 (6), 4-(13H-10,11,12,13-tetrahydro-10, 10,13,13,15-pentamethyldinaphtho[2,3-b][1,2-e]diazepin-7-yl) benzoic acid (LE540, 17), exhibits a 1 order of magnitude higher antagonistic potential than the parental LE135 (6). In contrast, 4-[5H-2,3-(2,5-dimethyl-2,5-hexano)-5-methyldibenzo[b,e] [1,4]diazepin-11-yl]-benzoic acid (HX600, 7), a structural isomer of the antagonistic LE135 (6), enhanced HL-60 cell differentiation induced by RAR agonists, such as Am80 (2). This synergistic effect was further increased for a thiazepine, HX630 (29), and an azepine derivative, HX640 (30); both synergized with Am80 (2) more potently than HX600 (7). Notably, the negative and positive effects of the azepine derivatives on retinoidal actions can be related to their RAR-antagonistic and RXR-agonistic properties, respectively, in the context of the RAR-RXR heterodimer.

Polyenylidene thiazolidinedione derivatives with retinoidal activities

Several polyenylidene thiazolidine or 2-thioxo-4-thiazolidinone derivatives were synthesized and their retinoidal activities were examined in terms of the differentiation-inducing ability towards human promyelocytic leukemia HL-60 cells and inhibitory effect on interleukin (IL)-1 alpha-induced IL-6 production in MC3T3-E1 cells. Compounds containing a trimethylcyclohexenyl ring induced HL-60 cell differentiation with weaker activity than retinoic acid (1a) by one or two orders of magnitude. The thiazolidinedione derivatives (2, 5, 7) showed stronger activity than the corresponding 2-thioxo-4-thiazolidinone derivatives (3, 6, 8). The effects of a retinoid antagonist (LE540) and synergists (retinoid X receptor (RXR) agonists, HX600 or HX630) on the activities of thiazolidine derivatives indicate that these compounds elicit their activities through the nuclear retinoic acid receptors (RARs). All the thiazolidines examined also inhibited IL-1 alpha-induced IL-6 production with IC50 values of 10 nM order. The retinoidal activities of the thiazolidines are significant, considering that replacement of the carboxylic acid in retinoid structures with bioisosteric functional groups is generally ineffective, as seen in the structure-activity relationships of retinoidal benzoic acids.

A mutation mimicking ligand-induced conformational change yields a constitutive RXR that senses allosteric effects in heterodimers

Mutations of a single residue in the retinoid X receptor alpha (RXRalpha) ligand-binding pocket (LBP) generate constitutive, ligand-binding-competent mutants with structural and functional characteristics similar to those of agonist-bound wild-type RXR. Modelling of the mouse RXRalphaF318A LBP suggests that, like agonist binding, the mutation disrupts a cluster of van der Waals interactions that maintains helix H11 in the apo-receptor location, thereby shifting the thermodynamic equilibrium to the holo form. Heterodimerization with some apo-receptors (retinoic acid, thyroid hormone and vitamin D3 receptors) results in 'silencing' of RXRalphaF318A constitutive activity, which, on the other hand, efficiently contributes to synergistic transactivation within NGFI-B-RXR heterodimers. RAR mutants disabled for corepressor binding and/or lacking a functional AF-2 activation domain, do not relieve RXR 'silencing'. Not only RAR agonists, but also the RAR antagonist BMS614 induce conformational changes allowing RXR to exert constitutive (RXRalphaF318A) or agonist-induced (wild-type RXR) activity in heterodimers. Interestingly, the RXRalphaF318A constitutive activity generated within heterodimers in the presence of BMS614 requires the integrity of both RXR and RAR AF-2 domains. These observations suggest that, within RXR-RAR heterodimers, RAR can adopt a structure distinct from that of the active holo-RAR, thus allowing RXR to become transcriptionally responsive to agonists.

Photoaffinity labeling of the ligand-interacting helix of the retinoic acid receptor alpha

Two photoaffinity-labeling probes for retinoic acid receptor (RAR) alpha, 4-[(3-(3-(trifluoromethyl)-3H-diazirin-3-yl)phenyl)carboxamido]ben zoic acid (3DIAM) and its para-isomer (4DIAM), were designed and synthesized. Both compounds possess high affinity for recombinant RAR alpha (MBP-RAR alpha/E) and bind covalently to its cognate ligand-binding site. The labeled site of MBP-RAR alpha/E with 3DIAM was determined, by the endoproteinase combination method, to be located in helix 11 of the ligand-binding domain of RAR alpha, which is the position at which the ligand is considered to bind, on the basis of the reported crystal structure of the retinoic acid/RAR gamma complex.

Action mechanism of retinoid-synergistic dibenzodiazepines

4-[5H-2,3-(2,5-Dimethyl-2,5-hexano)-5-methyldibenzo[b,e][1,4 ]diazepin-11-yl]benzoic acid (HX600), as well as its oxa- (HX620) and thia- (HX630) analogs, enhanced the activity of retinoic acid and a receptor alpha (RAR alpha)-selective agonist Am80 in HL-60 cell differentiation assays. HX600 synergizes with Am80 by binding to, and transactivating through, the RXR subunit of the RXR-RAR heterodimer. HX600 exhibited RXR pan-agonist activity in transient transfections with a DR1-based reporter gene and synergized with RA-bound RAR alpha and RAR beta in inducing transcription from a DR5-based reporter. In addition, all three compounds at high concentrations acted as RAR pan-antagonists in stably transfected RAR "reporter cells." These efficient synergists bind only weakly with RXRs in vitro, suggesting that they are RXR-RAR heterodimer-selective activators. These HX retinoids exhibited dual functionality, since they affected signalling through both retinoid receptor families (RARs and RXRs).

Inhibition of IL-1-induced IL-6 production by synthetic retinoids

The effects of retinoids and retinoid antagonists on IL-6 production in MC3T3-E1 cells were investigated. None of the synthetic retinoids examined stimulated IL-6 production, but all of them strongly inhibited IL-6 production induced by mouse IL-1 alpha. Their inhibitory activities correlated well with their differentiation-inducing activities in HL-60 assay or their binding affinities to nuclear retinoic acid receptors (RARs). Among three retinoid antagonists, two weak antagonists exhibited similar inhibition of mouse IL-1 alpha-induced IL-6 production, whereas a potent retinoid antagonist, 4-(13H-10,11,12,13-tetrahydro-10,10,13,13,15-pentamethyl-dinaph tho[2,3-b] [1,2-e]diazepin-7-yl)benzoic acid (LE540, 14), enhanced IL-6 production under the same conditions.

[Retinoid antagonists]

Retinoids, retinoic acid and its bioisosters, regulate many biological functions such as cell differentiation, proliferation and embryonic development in vertebrates, through binding to and activating their specific nuclear receptors. There are two classes of nuclear receptors for retinoids, retinoic acid receptors (RAR alpha, beta, gamma) and retinoid X receptors (RXR alpha, beta, gamma). Several retinoid antagonists, which bind to but not activate RARs, have been reported. Among them, 4-(5H-7,8,9,10-tetrahydro-5,7,7,10,10-pentamethylbenzo[e]naphtho [2,3-b][1,4]diazepin-13-yl)benzoic acid (LE135, 20) is a RAR beta-selective retinoid antagonist. Structure-activity relationships of LE135 (20) showed that the naphthalenyl analogs [LE540 (21) and LE550 (22)] are more potent retinoid antagonists in HL-60 assay. Contrary to the antagonistic activity of LE135 (20), an isomer of LE135 (20), 4-[5H-2,3-(2,5-dimethyl-2,5-hexano)-5-methyldibenzo- [b,e][1,4]diazepin-11-yl)benzoic acid (HX600, 39) enhanced the activities of retinoids. Although the synergistic activity of HX600 (39) can be explained by the binding to RXRs and the further activation of RAR/RXR heterodimer activated by retinoid (RAR ligand), the significantly different biological character of HX600 (39) from the typical RXR-selective ligand suggested the possibility of the participation of other nuclear receptors or cofactors in the retinoid synergism.

Novel aromatic urea derivatives with DNA-binding ability

Several aromatic urea derivatives were designed and synthesized as DNA-targeting agents. N,N'-Dimethyl-N,N'-bis[(4-amidylphenyl)aminocarbonyl]-2,6-di aminopyridine (1) and 1,3-bis[5-(glycylamino)pyrid-2-yl]urea (3) showed remarkable DNA-binding abilities as determined by ultrafiltration assay using calf thymus DNA, their potencies being equal to and half that of netropsin, respectively. Compound 1 inhibited the proliferation of both L1210 cells and KB cells with similar IC50 values to netropsin.

Evaluation of differentiation-inducing activity of retinoids on human leukemia cell lines HL-60 and NB4

Retinoids, including all-trans-retinoic acid (ATRA), its isomers, and fifty synthetic retinoids (retinobenzoic acids), were tested for differentiation-inducing activity on human leukemia cell lines HL-60 and NB4. Binding activity of typical retinoids to nuclear retinoic acid receptors (RARs) was also investigated. A good linear correlation between the ED50 values of differentiation-inducing activity towards HL-60 cells and those towards NB4 cells was found. Binding activities of retinoids to RAR alpha and RAR beta also correlated well to the differentiation-inducing activities.

Correlation of differentiation-inducing activity of retinoids on human leukemia cell lines HL-60 and NB4

Retinoids, including all-trans-retinoic acid, its isomers, and fifty synthetic retinoids (retinobenzoic acids), were tested for differentiation-inducing activity on human leukemia cell lines HL-60 and NB4. A good linear correlation, with an r value of 0.91, between the ED50 values for the differentiation-inducing activity towards HL-60 cells and that towards NB4 cells was found.

Synergists for retinoid in cellular differentiation of human promyelocytic leukemia cells HL-60

4-[5H-2,3-(2,5-Dimethyl-2,5-hexano)-5-methyldibenzo[b, e]diazepin-11-yl]benzoic acid (4) enhanced the differentiation-inducing activity of retinoic acid (1) and of a synthetic retinoid Am80 (2) toward human promyelocytic leukemia cells HL-60, although 4 alone did not induce differentiation. The synergistic effect of 4 on the activities of retinoids was also seen in suppression of proliferation of HL-60 cells.

Possible involvement of retinoid-like cofactor in serum in hemin/protoporphyrin-IX-induced differentiation of human leukemia K562 cells

Serum is generally added to media used for mammalian cell culture and for investigation of in vitro cell differentiation induction. Erythroid differentiation of human leukemia cell line K562 is induced by hemin or protoporphyrin IX in the presence of fetal bovine serum (FBS). Addition to the medium of retinoids enhanced the differentiation, and addition of retinoid antagonists suppressed the differentiation. Making the added FBS retinoid-free, i.e., treatment of the serum with charcoal, also reduced the efficiency of the cell differentiation induced by hemin or protoporphyrin IX. These facts suggest that retinoids or retinoid-like cofactor(s) in serum are necessary for hemin/protoporphyrin-IX-induced erythroid differentiation of K562 cells.

Synthesis and biological activity of carboxyphenylquinolines and related compounds as new potent retinoids. Retinobenzoic acids. VII

A series of new quinoline, quinolone, and quinazolinedione derivatives was synthesized and tested for retinoid activity in the human promyelocytic cell line HL-60 differentiation assay. All the quinoline compounds exhibited significant activity, depending on the substituent on the heterocycle. However, the quinolone and quinazolinedione derivatives were poor inducers of the differentiation of the HL-60 cells, the activity depending strongly on the polarity of the molecule.

[Novel synthetic retinoid agonists and antagonists]

Retinoic acid acts as a specific modulator of cellular differentiation and proliferation. Its natural and synthetic analogs, classified as retinoids, can be applied to the chemotherapy in the field of dermatology and oncology. Various benzoic acid derivatives exhibited the specific biological responses of retinoic acid and were named retinobenzoic acids. Especially, the aromatic amides such as Am80 and Am580 have better therapeutic effects than retinoic acid. N-Methylation of these highly active aromatic secondary amides caused the disappearance of the activity due to the change of the amide conformation from trans into cis. From such observations, the conformation of the linking group between alkyl-substituted benzene ring and benzoic acid moiety is an important factor for the activity. Some retinobenzoic acids do not bind to the cellular-retinoic acid-binding protein, but bind to nuclear retinoic acid receptors (RARs) with the binding affinity corresponding to the potency of their biological activities. Among them, Am80 can bind to two of the three RAR subtypes (RAR alpha and beta). The selectivity is favorable for the clinical application of retinoid since it has possibility to elicit a part of a number of the biological activities of retinoic acid.

[Stereochemistry of benzanilides and N-methylbenzanilides]

Conformations of benzanilide , N-methylbenzanilide and those with a methyl group(s) ortho to the amide bond in solution and in the crystal have been studied. N-Methylbenzanilide exists in cis-amide (E) form in the crystal. In CDCl3 solution, cis-amide form is also predominant (99%), while benzanilide exists in trans-amide (Z) form in the crystal and in solution. In the crystal, all the methyl-substituted benzanilides exist in trans-amide conformation and the introduction of an ortho-methyl group(s) makes the interplanar angles of the aromatic rings and the amide group (Aramide) larger. N-Methylbenzanilides exist in cis form in the crystal except the compound which has four methyl groups ortho to the amide bond. For the N-methylbenzanilides, the effects of introduction of one or two ortho-methyl groups on the dihedral angles of Ar-amide are smaller than that for the secondary benzanilides. In solution, benzanilides exist exclusively in trans conformation except for the compound 12 which has a minor cis conformer (3%) in CDCl3, whereas N-methylbenzanilides exist in equilibrium between the major cis-form and the minor trans-form. The tetramethyl derivative exists in trans conformation in solution as observed in the crystal. For N-methylbenzanilides, an introduction of a methyl group(s) ortho to the amide bond seems to destabilize the cis-amide conformation in solution, resulting in an increased ratio of the trans-amide conformation.

Retinobenzoic acids. 6. Retinoid antagonists with a heterocyclic ring

Several candidate retinoid antagonists were designed on the basis of the ligand superfamily concept and synthesized. Retinoidal activities of these benzimidazole and benzodiazepine derivatives were examined by assay of differentiation-inducing activity on human promyelocytic leukemia cell line HL-60. The parent benzimidazole derivative, 4-(5,6,7,8-tetrahydro-5,5,8,8- tetramethylnaphth-[2,3-d]imidazol-2-yl)benzoic acid (7a), and related compounds with a small alkyl group instead of the hydrogen on the nitrogen (1N) atom of the imidazole ring exhibited retinoidal activity, and the potency strongly depended on the bulkiness of the substituent. The compounds having a phenyl or benzyl group on the nitrogen lacked differentiation-inducing activity on HL-60 cells and acted as antagonists to the potent retinoid 4-[(5,6,7,8-tetrahydro-5,5,8,8- tetramethyl-2-naphthalenyl)carbamoyl]benzoic acid (Am80). Among the compounds possessing a seven-membered heterocyclic ring as a linking group, 4-(5H-7,8,9,10-tetrahydro-5,7,7,10,10- pentamethylbenzo[e]- naphtho[2,3-b][1,4]diazepin-13-yl)benzoic acid (16) also exhibited the antagonistic activity. The binding abilities of these compounds to retinoic acid receptors alpha and beta were consistent with their potency for the inhibition of HL-60 cell differentiation induced by the retinoid Am80.

Expression of the ligand-binding domain-containing region of retinoic acid receptors alpha, beta and gamma in Escherichia coli and evaluation of ligand-binding selectivity

The complete molecule or the ligand-binding domain-containing region of each of the three subtypes of human retinoic acid receptors (hRAR alpha, hRAR beta and hRAR gamma) was expressed in Escherichia coli. The expressed recombinant RARs (rRARs: rRAR alpha/E, rRAR beta/E and rRAR gamma) showed nearly the same magnitude of binding affinity toward [3H]retinoic acid (RA) as hRARs extracted from human cells (Ka values: 6.0 x 10(9) M-1 for rRAR alpha/E and 2.7 x 10(10) M-1 for both rRAR beta/E and rRAR gamma). Therefore, the ligand-binding selectivity of the rRARs toward RA and synthetic retinoids (4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylcarbamoyl )benzoic acid (Am80), (E)-4-[3-(3,5-di-tert-butylphenyl)-3-oxo-1-propenyl]benzoic acid (Ch55)) was examined. Am80 bound rRAR alpha/E preferentially and showed no binding activity toward rRAR gamma, which is consistent with the case of hRAR gamma. Ch55 bound all three subtypes of rRARs, preferentially rRAR beta/E. These results suggest that the intrinsic nature of the binding of each retinoid can be investigated by usage of the rRARs. However, rRARs show quantitatively different ligand-selectivity from that of hRARs: RA showed higher binding affinity toward rRARs than both Am80 and Ch55, but Ch55 binds all three subtypes of hRARs stronger than RA and Am80, which binds hRAR beta stronger than RA.

Differentiation-inducing activity of retinoic acid isomers and their oxidized analogs on human promyelocytic leukemia HL-60 cells

Retinoidal activity of retinoic acid isomers [all-trans-retinoic acid (ATRA), 9-cis-retinoic acid (9CRA) and 13-cis-retinoic acid (13CRA)] and their oxidized derivatives [19-hydroxy and 19-oxo derivatives of ATRA (19-hydroxy-ATRA and 19-oxo-ATRA), 19-oxo derivative of 9CRA (19-oxo-9CRA), and 19-hydroxy derivative of 13CRA (19-hydroxy-13CRA)] was evaluated by means of a human promyelocytic leukemia HL-60 cell differentiation induction assay. All the compounds examined showed this activity with ED50 values of 2-30 nM, which are in accordance with their binding activity to nuclear retinoic acid receptors (RARs).

Base-catalyzed isomerization of retinoic acid. Synthesis and differentiation-inducing activities of 14-alkylated all-trans-, 13-cis-, and 20,14-retro-retinoic acids

Retinoic acid (1) is isomerized regioselectively by excess amounts of lithium diisopropylamide (LDA) to give 20,14-retro-retinoic acid (3). Alkylation of the intermediate dianion of retinoic acid gave 14-alkylated derivatives of 3. By isomerization of the alkylated retro isomers under basic conditions, several 14-alkyl-all-trans- and -13-cis-retinoic acids were synthesized. The retinoidal activities of these derivatives were examined, based on the ability to induce differentiation of human promyelocytic leukemia cell line HL-60. 20,14-retro-Retinoic acid (3) is 1/50 as active as retinoic acid (1). Although 14-methyl-20,14-retro-retinoic acid (4) is as active as 3, the introduction of a 14-methyl group into all-trans- and 13-cis-retinoic acid resulted in decreased activity. Introduction of bulkier alkyl groups at the C-14 position caused the disappearance of the activity.

Fluorescent probes for retinoic acid receptors: molecular measures for the ligand binding pocket

Two fluorescent probes for nuclear retinoic acid receptors (RARs) have been developed, both containing a biologically active retinoid moiety and a fluorescent dansyl moiety, but differing in the length of the spacer arm connecting the two moieties. Both probes bind RARs at their retinoid-binding sites, revealing the usefulness of the compounds as fluorescent RAR probes. By measuring the specific increase of the probes' fluorescence intensity caused by the binding to RARs, the linearized length of the RAR's retinoid-binding pocket could be estimated.

Fluorescent and photoaffinity labeling probes for retinoic acid receptors

A fluorescent probe for retinoid receptors (RARs) was designed and prepared. The probe consists of a retinoid moiety and a dansyl moiety, i.e., 2-[3-(5-dimethylaminonaphthalene-1-sulfonyl)- aminopropyl-1-oxy]-4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)carboxamido]benzoic acid: DAM-3. DAM-3 specifically bound RARs. Additionally, a photoreactive RAR fluorescent probe was designed and prepared, i.e., 2-[3-(5-azidonaphthalene- 1-sulfonyl)aminopropyl-1-oxy]-4-[(5,6,7,8-tetrahydro-5,5,8,8- tetramethyl-2-naphthalenyl)carboxamido]benzoic acid (ADAM-3). ADAM-3 irreversibly and specifically bound RARs using ultraviolet irradiation.

Synthetic retinoids, retinobenzoic acids, Am80, Am580 and Ch55 regulate morphogenesis in chick limb bud

The retinobenzoic acids Am80, Am580 and Ch55 are synthetic stable analogs of retinoic acid (RA), and show very strong differentiation-inducing activity in human myelogeneous leukemia cell line HL-60. To examine the effects of these synthetic retinoids on limb pattern formation, AG1-X2 beads containing these retinoids were applied to the anterior margin of stage 19-20 chick wing buds. By implanting the beads with 1 microgram/ml retinoids, normal wings were formed and extra digits 2 or 32 were rarely formed. As the retinoid concentrations increased from 10 micrograms/ml to 100 micrograms/ml duplicated limbs 3234, 43234, 432234, 4334 were progressively produced. At higher concentrations, 1 mg/ml, the wings often truncated, although duplication occurred in some embryos. These synthetic analogs seem to have the same degree of morphogenetic potential as RA, since the activity index of these retinoids was similar to that of RA. Since these synthetic retinoids hardly bind to CRABP (cellular retinoic acid-binding protein), it may be possible that the retinoids and RA may affect limb-pattern formation without the interaction with CRABP. It is known that limb buds cannot develop distal structures when the posterior region including all ZPA (zone of polarizing activity) is removed. When beads containing the above mentioned retinoids were implanted to the anterior margin of wing buds from which the posterior one third region including all ZPA had been removed, distal growth of the wing buds and the formation of digit elements were observed. Some of the wing buds produced a completely reverse digit pattern 432. From these results, we discussed the roles of RA in limb development and pattern formation.

Retinobenzoic acids. 5. Retinoidal activities of compounds having a trimethylsilyl or trimethylgermyl group(s) in human promyelocytic leukemia cells HL-60

The retinoidal activities of trimethylsilyl or trimethylgermyl-containing retinobenzoic acids are discussed on the basis of differentiation-inducing activity on human promyelocytic leukemia cells HL-60. Compounds with a trimethylsilyl or trimethylgermyl group at the meta position of the generic formula 2 have more potent activities than the corresponding retinobenzoic acids with a m-tert-butyl group. Compounds having two m-trimethylsilyl or -trimethylgermyl groups also have strong activities, and (E)-4-[3-[3,5-bis(trimethylsilyl)phenyl]-3-oxo-1-propenyl]benzoic acid (22, Ch55S) and (E)-4-[3-[3,5-bis(trimethylgermyl)phenyl]-3-oxo-1- propenyl]benzoic acid (35, Ch55G) are more active than retinoic acid by 1 order of magnitude. However, in the para-substituted chalcone derivatives, the replacement of a tert-butyl group (49, Ch40) with a trimethylsilyl (27, Ch40S) or a trimethylgermyl (30, Ch40G) group caused the disappearance of the activity.

Expression of retinoic acid receptor genes and the ligand-binding selectivity of retinoic acid receptors (RAR's)

Retinoids of a structurally new type, 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylcarbamoyl)benzoic acid (Am80) and (E)-4-[3-(3,5-di-tert-butylphenyl)-3-oxo-1-propenyl]-benzoic acid (Ch55), were used for the investigation of retinoid receptors in some human cell lines. Previously, using these retinoids, we have established that HL-60 cells possess two retinoid receptors, RAR-alpha and RAR-beta. In this paper, we report coexpression of RAR-alpha- and RAR-beta-genes established by Northern analysis. Further, the binding selectivity of the RAR products toward all-trans-retinoic acid (RA), Am80 and Ch55 was studied by filter binding assay. The affinity of the ligands toward RAR-alpha decreased in the order of Ch55 greater than RA greater than Am80. The affinity of the ligands toward RAR-beta decreased in the order of Ch55 greater than Am80 greater than RA.

Retinobenzoic acids. 4. Conformation of aromatic amides with retinoidal activity. Importance of trans-amide structure for the activity

N-Methylation of two retinoidal amide compounds, 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)carbamoyl]benz oic acid (3, Am80) and 4-[[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2- naphthalenyl)carbonyl]amino]benzoic acid (5, Am580), resulted in the disappearance of their potent differentiation-inducing activity on human promyelocytic leukemia cell line HL-60. Studies with 1H NMR and UV spectroscopy indicated that large conformational differences exist between the active secondary amides and the inactive N-methyl amides. From a comparison of the spectroscopic results of these amides with those of stilbene derivatives, the conformations of the active amides are expected to resemble that of (E)-stilbene, whereas the inactive amides resemble the Z isomer: 3 (Am80) and 5 (Am580) have a trans-amide bond and their whole structures are elongated, while the N-methylated compounds [4 (Am90) and 6 (Am590)] have a cis-amide bond, resulting in the folding of the two benzene rings. These structures in the crystals were related to those in solution by 13C NMR spectroscopic comparison between the two phases (solid and solution).

The retinoic acid receptors alpha and beta are expressed in the human promyelocytic leukemia cell line HL-60

The human promyelocytic leukemia cell line HL-60 can be induced to differentiate into granulocytes upon exposure to retinoids. Previously we have shown that extracts of undifferentiated HL-60 cells possess a specific retinoid-binding activity (RSBP-1) corresponding to an approximate 95 kilodalton (kDa) protein as determined by size-exclusion chromatography. We now extend these observations to reveal a second approximate 95 kDa retinoic acid-binding component (RSBP-2), which is separable from RSBP-1 using anion exchange chromatography. We further show that the chromatographic properties of RSBP-1 and RSBP-2 are identical to those found for the retinoid-binding activities present in extracts of HeLa cells transfected with the human retinoic acid receptor (RAR) expression vectors RAR-beta phi and RAR-alpha phi, respectively. Moreover, an antiserum preparation directed against RAR-beta selectively immunoprecipitated both the retinoid-binding activity in extracts of HeLa cells transfected with RAR-beta phi and that corresponding to RSBP-1 in HL-60 cell extracts. Similarly, an antiserum preparation directed against RAR-alpha immunoprecipitated the retinoid-binding activity in extracts from RAR-alpha phi transfected HeLa cell as well as that corresponding to RSBP-2 in HL-60 cell extracts. Using these antisera, Western blot analyses of extracts from HL-60 cells, and from HeLa cells transfected with either RAR-alpha phi or RAR-beta phi, confirmed that RSBP-2 and RSBP-1 are identical to RAR-alpha and RAR-beta, respectively. However, RAR-alpha, RAR-beta, RSBP-1, and RSBP-2 appeared as an approximate 51 kDa species in sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis in contrast with an apparent approximate 95 k mol wt as estimated from size-exclusion chromatography in the presence of 0.6 M KCl.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinobenzoic acids. 3. Structure-activity relationships of retinoidal azobenzene-4-carboxylic acids and stilbene-4-carboxylic acids

Alkyl-substituted azobenzene-4-carboxylic acids are potent differentiation inducers of human promyelocytic leukemia cell line HL-60 to mature granulocytes. Their structure-activity relationships are very similar to those of other retinoidal benzoic acids which are generally represented by 4 and named retinobenzoic acids. The structure-activity relationships of azobenzenecarboxylic acids can also be applied to the known retinoid TTNPB (3). Thus, (E)-4-[2-(3,4-diisopropylphenyl)-1-propenyl]benzoic acid (St30 (28] and (E)-4-[2-(3-tert-butylphenyl)ethenyl]benzoic acid (St40 (29], the acyclic alkyl analogues of TTNPB, are nearly as active as retinoic acid. Among the oxidatively derived compounds (Az90, Ep series and Ox series) of azobenzene- or stilbenecarboxylic acids, Az90 (71) and Ep80 (61) have strong activities. However, all the bishydroxylated derivatives of TTNPB are inactive, while a diketo analogue Ox580 (69) has only weak potency. The activities of conformationally restricted compounds of TTNPB offer some information on the stereochemistry of the active form of these retinoidal compounds.

Retinobenzoic acids. 2. Structure-activity relationships of chalcone-4-carboxylic acids and flavone-4'-carboxylic acids

The structure-activity relationships of (E)-chalcone-4-carboxylic acids, which are retinoidal benzoic acids represented by R-Ph-X-Ph-COOH (4, X = -COCH = CH-), are discussed on the basis of differentiation-inducing activity on human promyelocytic leukemia cells HL-60. The activity was increased by the substitution of a bulky alkyl group(s) (R), and among such compounds, (E)-4-[3-(3,5-di-tert-butylphenyl)-3-oxo-1-propenyl]benzoic acid (Ch55) and (E)-4-[3-oxo-3-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1 -propenyl]benzoic acid (Ch80) are several times more active than retinoic acid. Though the stable conformer of chalcone derivatives is linear (s-cis form), the conformationally restricted analogue 4-(6,7,8,9-tetrahydro-6,6,9,9-tetramethyl-4H-4-oxonaphtho[2,3-b]py ran-2-yl)benzoic acid (Fv80) is more active than Ch80. While the effect of introduction of an oxygen atom varied, 4-[1-hydroxy-3-oxo-3-(5,6,7,8-tetrahydro-3-hydroxy-5,5,8,8-tetramethyl-2 - naphthalenyl)-1-propenyl]benzoic acid (Re80), regarded as a derivative of Ch80 with two additional hydroxyl groups, has very strong activity.

Retinobenzoic acids. 1. Structure-activity relationships of aromatic amides with retinoidal activity

Two types of aromatic amides, terephthalic monoanilides and (arylcarboxamido)benzoic acids, have been shown to possess potent retinoidal activities and can be classified as retinoids. The structure-activity relationships of these amides are discussed on the basis of differentiation-inducing activity on human promyelocytic leukemia cells HL-60. In generic formula 4 (X = NHCO or CONH), the necessary factors to elicit the retinoidal activities are a medium-sized alkyl group (isopropyl, tert-butyl, etc.) at the meta position and a carboxyl group at the para position of the other benzene ring. The bonding of the amide structure can be reversed, this moiety apparently having the role of locating the two benzene rings at suitable positions with respect to each other. Substitution at the ring position ortho to the amide group or N-methylation of the amide group caused loss of activity, presumably owing to the resultant change of conformation. It is clear that the mutual orientation of the benzylic methyl group(s) and the carboxyl group and their distance apart are essential factors determining the retinoidal activity. Among the synthesized compounds, 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)carbamoyl]benz oic acid (Am80) and 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)carboxamido] benzoic acid (Am580) were several times more active than retinoic acid in the assay. They are structurally related to retinoic acid, as is clear from the biological activity of the hybrid compounds (M2 and R2).

[Recent advances in retinoids studies]

Retinoids are defined as the compounds which elicit the specific biological responses through binding or activating the specific receptor(s). Retinoids modulate the cellular differentiation and proliferation in many types of cells. A series of retinoidal benzoic acids, named retinobenzoic acids, have potent activities on human promyelocytic leukemia cells, HL-60, and other assay systems. Among them, Am80, AM580 and Ch55 are more active than retinoic acid in most cases. As these compounds possess quite different structures or physicochemical properties from retinoic acid or conventional retinoids, they are expected to be applied clinically for the treatments of the diseases in oncology and dermatology. Recently, progresses on the mechanistic studies on retinoidal actions have been reported. One of them is isolation and cloning of human retinoic acid-receptors and the mechanism of retinoidal action was concluded that the retinoid-receptor complex interact directly with DNA to regulate the expression of the gene, like steroid hormone. The other is the establishment of the presence of the retinoid-specific-binding protein (RSBP) by the use of our retinobenzoic acids as the probe.

Affinity gels for purification of retinoid-specific binding protein (RSBP)

Retinoids are defined as compounds which elicit specific biological effects such as control of cell growth and cell differentiation by binding to a specific receptor. Recently, we demonstrated the presence of a protein (RSBP) which satisfies the criteria for the retinoid receptor. For purification of RSBP, we prepared two types of affinity gels with retinoidal ligands (Gel-Am and Gel-Ch) based on synthetic retinobenzoic acids which possess very potent retinoidal activities. RSBP in the crude fraction extracted from cultured cells could be purified about 300-fold by affinity column chromatography using these affinity gels.