The biologic basis for the use of retinoids in cancer prevention and treatment

Combination of 13-Cis retinoic acid and lovastatin: marked antitumor potential in vivo in a pheochromocytoma allograft model in female athymic nude mice

Currently, there are no reliably effective therapeutic options for metastatic pheochromocytoma (PCC) and paraganglioma. Moreover, there are no therapies that may prevent the onset or progression of tumors in patients with succinate dehydrogenase type B mutations, which are associated with very aggressive tumors. Therefore, we tested the approved and well-tolerated drugs lovastatin and 13-cis-retinoic acid (13cRA) in vitro in an aggressive PCC mouse cell line, mouse tumor tissue-derived (MTT) cells, and in vivo in a PCC allograft nude mouse model, in therapeutically relevant doses. Treatment was started 24 hours before sc tumor cell injection and continued for 30 more days. Tumor sizes were measured from outside by caliper and sizes of viable tumor mass by bioluminescence imaging. Lovastatin showed antiproliferative effects in vitro and led to significantly smaller tumor sizes in vivo compared with vehicle treatment. 13cRA promoted tumor cell growth in vitro and led to significantly larger viable tumor mass and significantly faster increase of viable tumor mass in vivo over time compared with vehicle, lovastatin, and combination treatment. However, when combined with lovastatin, 13cRA enhanced the antiproliferative effect of lovastatin in vivo. The combination-treated mice showed slowest tumor growth of all groups with significantly slower tumor growth compared with the vehicle-treated mice and significantly smaller tumor sizes. Moreover, the combination-treated group displayed the smallest size of viable tumor mass and the slowest increase in viable tumor mass over time of all groups, with a significant difference compared with the vehicle- and 13cRA-treated group. The combination-treated tumors showed highest extent of necrosis, lowest median microvessel density and highest expression of α-smooth muscle actin. The combination of high microvessel density and low α-smooth muscle actin is a predictor of poor prognosis in other tumor entities. Therefore, this drug combination may be a well-tolerated novel therapeutic or preventive option for malignant PCC.

Anterior pituitary adenomas: inherited syndromes, novel genes and molecular pathways

Pituitary adenomas are common tumors. Although rarely malignant, pituitary adenomas cause significant morbidity due to mass effects and/or hormonal hypo- and/or hyper-secretion. Molecular understanding of pituitary adenoma formation is essential for the development of medical therapies and the treatment of post-operative recurrences. In general, mutations in genes involved in genetic syndromes associated with pituitary tumors are not a common finding in sporadic lesions. By contrast, multiple endocrine neoplasia type 1 (MEN-1) and aryl hydrocarbon receptor-interacting protein (AIP) mutations may be more frequent among specific subgroups of patients, such as children and young adults, with growth hormone-producing adenomas. In this article, we present the most recent data on the molecular pathogenesis of pituitary adenomas and discuss some of the most recent findings from our laboratory. Guidelines for genetic screening and clinical counseling of patients with pituitary tumors are provided.

Vitamin E supplementation does not prevent ethanol-reduced hepatic retinoic acid levels in rats

Chronic, excessive ethanol intake can increase retinoic acid (RA) catabolism by inducing cytochrome P450 2E1 (CYP2E1). Vitamin E (VE) is an antioxidant implicated in CYP2E1 inhibition. In the current study, we hypothesized that VE supplementation inhibits CYP2E1 and decreases RA catabolism, thereby preventing ethanol-induced hepatocyte hyperproliferation. For 1 month, 4 groups of Sprague-Dawley rats were fed a Lieber-DeCarli liquid ethanol (36% of the total energy) diet as follows: either ethanol alone (Alc group) or ethanol in combination with 0.1 mg/kg body weight of all-trans-RA (Alc + RA group), 2 mg/kg body weight of VE (Alc + VE group), or both together (Alc + RA + VE group). Control rats were pair-fed a liquid diet with an isocaloric amount of maltodextrin instead of ethanol. The ethanol-fed groups had 3-fold higher hepatic CYP2E1 levels, 50% lower hepatic RA levels, and significantly increased hepatocyte proliferation when compared with the controls. The ethanol-fed rats given VE had more than 4-fold higher hepatic VE concentrations than the ethanol-fed rats without VE, but this did not prevent ethanol induction of CYP2E1, lower hepatic retinoid levels, or hepatocellular hyperproliferation. Furthermore, VE supplementation could not prevent RA catabolism in liver microsomal fractions of the ethanol-fed rats in vitro. These results show that VE supplementation can neither inhibit ethanol-induced changes in RA catabolism nor prevent ethanol-induced hepatocyte hyperproliferation in the rat liver.

Medical treatment of Cushing's disease: Overview and recent findings

Cushing's disease, due to pituitary adrenocorticotropic hormone (ACTH) hypersecretion, is the most common etiology of spontaneous excess cortisol production. The majority of pituitary tumors causing Cushing's disease measure <1 cm and the excess morbidity associated with these tumors is mostly due to the effects of elevated, nonsuppressible, ACTH levels leading to adrenal steroid hypersecretion. Elevated circulating cortisol levels lead to abnormal fat deposition, hypertension, diabetes, coronary artery disease, osteoporosis, muscle weakness and psychological disturbances. At experienced centers, initial surgical remission rate via transnasal, transphenoidal resection approaches 80% for tumors less than 1 cm, but may be as low as 30% for larger lesions and long-term recurrence in all groups approaches 25%. Residual disease may be managed with more radical surgery, pituitary-directed radiation, bilateral adrenalectomy, or medical therapy. This paper addresses current and novel therapies in various stages of development for Cushing's disease.

Potential of retinoic acid derivatives for the treatment of corticotroph pituitary adenomas

Cushing's disease is a severe clinical condition caused by hypersecretion of corticosteroids due to excessive ACTH secretion from a pituitary adenoma. This complex endocrine disorder still represents a major challenge for the physician in terms of efficient treatment. In the last years there was only little progress in elucidating the molecular mechanisms responsible for the constitutive and autonomous ACTH secretion of pituitary corticotrophinomas. As a consequence, no effective drug therapy is currently available, particularly if surgical excision is not successful. In the present article we examine recent studies that have investigated the therapeutic potential of retinoic acid receptors as nuclear receptor targets for the treatment of Cushing's disease. Retinoic acid is an efficient drug used for the treatment of different types of cancers and it proved to act in animal models of Cushing's disease. The efficiency of this treatment in patients with this disorder still needs to be tested in clinical trials.

Receptor-specific scoring functions derived from quantum chemical models improve affinity estimates for in-silico drug discovery

The adaptation of forcefield-based scoring function to specific receptors remains an important challenge for in-silico drug discovery. Here we compare binding energies of forcefield-based scoring functions with models that are reparameterized on the basis of large-scale quantum calculations of the receptor. We compute binding energies of eleven ligands to the human estrogen receptor subtype alpha (ERalpha) and four ligands to the human retinoic acid receptor of isotype gamma (RARgamma). Using the FlexScreen all-atom receptor-ligand docking approach, we compare docking simulations parameterized by quantum-mechanical calculation of a large protein fragment with purely forcefield-based models. The use of receptor flexibility in the FlexScreen permits the treatment of all ligands in the same receptor model. We find a high correlation between the classical binding energy obtained in the docking simulation and quantum mechanical binding energies and a good correlation with experimental affinities R=0.81 for ERalpha and R=0.95 for RARgamma using the quantum derived scoring functions. A significant part of this improvement is retained, when only the receptor is treated with quantum-based parameters, while the ligands are parameterized with a purely classical model.

Proliferative changes in the bronchial epithelium of former smokers treated with retinoids

BACKGROUND

Retinoids have shown antiproliferative and chemopreventive activity. We analyzed data from a randomized, placebo-controlled chemoprevention trial to determine whether a 3-month treatment with either 9-cis-retinoic acid (RA) or 13-cis-RA and alpha-tocopherol reduced Ki-67, a proliferation biomarker, in the bronchial epithelium.

METHODS

Former smokers (n = 225) were randomly assigned to receive 3 months of daily oral 9-cis-RA (100 mg), 13-cis-RA (1 mg/kg) and alpha-tocopherol (1200 IU), or placebo. Bronchoscopic biopsy specimens obtained before and after treatment were immunohistochemically assessed for changes in the Ki-67 proliferative index (i.e., percentage of cells with Ki-67-positive nuclear staining) in the basal and parabasal layers of the bronchial epithelium. Per-subject and per-biopsy site analyses were conducted. Multicovariable analyses, including a mixed-effects model and a generalized estimating equations model, were used to investigate the treatment effect (Ki-67 labeling index and percentage of bronchial epithelial biopsy sites with a Ki-67 index > or = 5%) with adjustment for multiple covariates, such as smoking history and metaplasia. Coefficient estimates and 95% confidence intervals (CIs) were obtained from the models. All statistical tests were two-sided.

RESULTS

In per-subject analyses, Ki-67 labeling in the basal layer was not changed by any treatment; the percentage of subjects with a high Ki-67 labeling in the parabasal layer dropped statistically significantly after treatment with 13-cis-RA and alpha-tocopherol treatment (P = .04) compared with placebo, but the drop was not statistically significant after 9-cis-RA treatment (P = .17). A similar effect was observed in the parabasal layer in a per-site analysis; the percentage of sites with high Ki-67 labeling dropped statistically significantly after 9-cis-RA treatment (coefficient estimate = -0.72, 95% CI = -1.24 to -0.20; P = .007) compared with placebo, and after 13-cis-RA and alpha-tocopherol treatment (coefficient estimate = -0.66, 95% CI = -1.15 to -0.17; P = .008).

CONCLUSIONS

In per-subject analyses, treatment with 13-cis-RA and alpha-tocopherol, compared with placebo, was statistically significantly associated with reduced bronchial epithelial cell proliferation; treatment with 9-cis-RA was not. In per-site analyses, statistically significant associations were obtained with both treatments.

Bone morphogenetic protein-4 inhibits corticotroph tumor cells: involvement in the retinoic acid inhibitory action

The molecular mechanisms governing the pathogenesis of ACTH-secreting pituitary adenomas are still obscure. Furthermore, the pharmacological treatment of these tumors is limited. In this study, we report that bone morphogenetic protein-4 (BMP-4) is expressed in the corticotrophs of human normal adenohypophysis and its expression is reduced in corticotrophinomas obtained from Cushing's patients compared with the normal pituitary. BMP-4 treatment of AtT-20 mouse corticotrophinoma cells has an inhibitory effect on ACTH secretion and cell proliferation. AtT-20 cells stably transfected with a dominant-negative form of the BMP-4 signal cotransducer Smad-4 or the BMP-4 inhibitor noggin have increased tumorigenicity in nude mice, showing that BMP-4 has an inhibitory role on corticotroph tumorigenesis in vivo. Because the activation of the retinoic acid receptor has an inhibitory action on Cushing's disease progression, we analyzed the putative interaction of these two pathways. Indeed, retinoic acid induces both BMP-4 transcription and expression and its antiproliferative action is blocked in Smad-4dn- and noggin-transfected Att-20 cells that do not respond to BMP-4. Therefore, retinoic acid induces BMP-4, which participates in the antiproliferative effects of retinoic acid. This new mechanism is a potential target for therapeutic approaches for Cushing's disease.

Fenretinide: a prototype cancer prevention drug

Fenretinide (N-4-hydroxyphenylretinamide [4-HPR]) is a synthetic retinoid that has been examined in in vitro assays, preclinical animal models and clinical trials as a cancer chemopreventive agent. Its pharmacology, toxicity and mechanisms of action initially suggested an increased therapeutic index relative to native retinoids for the control of tumours of the breast, prostate, bladder, colon, cervix and head and neck. Although fenretinide at the doses and schedules used in several pivotal Phase II and III clinical trials has not been proven to be efficacious in reducing the incidence of cancer or in retarding the development of preneoplastic lesions, encouraging observations regarding unanticipated preventative activity, such as for ovarian cancer control, have arisen from these studies. Research in cancer therapy and the elucidation of molecular pathways activated by fenretinide have also yielded clues about how this agent might be better used in a prevention setting. Current trials are underway to re-examine both dose and schedule of fenretinide administration as well as the target tissues of interest. Investigations of potential synergism between fenretinide and other candidate chemopreventative molecules with complementary mechanisms of action may support future assessments of this prototype cancer prevention drug or its newer analogues.

Rhabdomyosarcoma

Rhabdomyosarcoma, the most common soft tissue sarcoma of children, carried a 10-15% survival rate in the late 1960s. Since then, better understanding of biology and pathology of the tumor and the judicious use of chemotherapy and radiation has improved the prognosis drastically. In addition instead of extirpative surgery, organ salvage is now feasible in many of these children. In this article the authors review current information regarding pathology, diagnosis and treatment of rhabdomyosarcoma.

Treatment of former smokers with 9-cis-retinoic acid reverses loss of retinoic acid receptor-beta expression in the bronchial epithelium: results from a randomized placebo-controlled trial

BACKGROUND

Loss of retinoic acid receptor beta (RAR-beta) expression in the bronchial epithelium is considered a biomarker of preneoplasia. Retinoids can restore expression of this receptor and, presumably, halt the progression of carcinogenesis. This study was designed to investigate whether either of two retinoid-based regimens, 9-cis-retinoic acid (RA) or 13-cis-RA plus alpha-tocopherol (AT), could reverse RAR-beta expression loss in former smokers after 3 months of treatment.

METHODS

Individuals (n = 226) who had smoked at least 20 pack-years and had ceased smoking for at least 12 months were randomly assigned to receive 3 months of daily oral 9-cis-RA (100 mg), 13-cis-RA (1 mg/kg) + AT (1200 IU), or placebo. Bronchoscopy and biopsy at six predetermined sites of the bronchial tree were performed before treatment and at 3 and 6 months thereafter. Specimens were evaluated for squamous metaplasia, dysplasia, and RAR-beta expression. McNemar's test was used to test changes in RAR-beta expression and squamous metaplasia within each treatment group, and a generalized estimating equations model was applied to model the treatment effect, adjusting for covariates. All statistical tests were two-sided.

RESULTS

A total of 177 assessable subjects completed at least 3 months of therapy and underwent at least the baseline and 3-month bronchoscopic evaluations with biopsies. RAR-beta was detected in 69.7% of all baseline biopsy samples, and metaplasia was evident in 6.9% of all baseline samples from 240 subjects. Restoration of RAR-beta expression (P =.03) and reduction of metaplasia (P =.01) were found in the 9-cis-RA group. After adjustment for years of smoking, packs/day smoked, and metaplasia, treatment with 9-cis-RA, but not with 13-cis-RA + AT, led to a statistically significant increase in RAR-beta expression compared with placebo (P =.03).

CONCLUSION

9-cis-RA treatment can restore RAR-beta expression in the bronchial epithelium of former smokers, raising the possibility that this retinoid has potential chemopreventive properties in former smokers.

Retinoids and alcohol-related carcinogenesis

Chronic and excessive alcohol intake is associated with an increased incidence of a variety of cancers (e.g., liver, oral cavity, esophagus, colorectal and breast). Long-term alcohol intake results in impaired nutritional status of retinoic acid (RA), the most active derivative of vitamin A, which may provide a promoting environment for tumor formation. Recent studies demonstrate that chronic alcohol-induced hepatocellular proliferation, which may convert hepatocytes from a state of resistance to a carcinogen to a state of high susceptibility, is due to alcohol-impaired RA metabolism and signaling and crosstalk with the Jun N-terminal kinases-dependent signaling pathway. Further, the restoration of hepatic RA homeostasis by treatment with either RA supplementation or inhibitors of RA catabolism can suppress alcohol-induced hepatocyte hyperproliferation and restore alcohol-deregulated apoptosis, thereby reducing the risk of alcohol-promoted hepatocellular carcinogenesis. These studies indicate the importance of RA actions in the prevention and/or treatment of alcohol-related carcinogenic process in the liver and other organs.

Human papilloma virus 16 E6 oncoprotein inhibits retinoic X receptor-mediated transactivation by targeting human ADA3 coactivator

The expression of human papillomavirus (HPV) E6 oncoprotein is causally linked to high-risk HPV-associated human cancers. We have recently isolated hADA3, the human homologue of yeast transcriptional co-activator yADA3, as a novel E6 target. Human ADA3 binds to the high-risk (cancer-associated) but not the low-risk HPV E6 proteins and to immortalization-competent but not to immortalization-defective HPV16 E6 mutants, suggesting a role for the perturbation of hADA3 function in E6-mediated oncogenesis. We demonstrate here that hADA3 directly binds to the retinoic X receptor (RXR)alpha in vitro and in vivo. Using chromatin immunoprecipitation, we show that hADA3 is part of activator complexes bound to the native RXR response elements within the promoter of the cyclin-dependent kinase inhibitor gene p21. We show that hADA3 enhances the RXR(alpha)-mediated sequence-specific transactivation of retinoid target genes, cellular retinoic acid-binding protein II and p21. Significantly, we demonstrate that E6 inhibits the RXR(alpha)-mediated transactivation of target genes, implying that perturbation of RXR-mediated transactivation by E6 could contribute to HPV oncogenesis.

Gene expression regulation by retinoic acid

Over the last quarter century, more than 532 genes have been put forward as regulatory targets of retinoic acid. In some cases this control is direct, driven by a liganded heterodimer of retinoid receptors bound to a DNA response element; in others, it is indirect, reflecting the actions of intermediate transcription factors, non-classical associations of receptors with other proteins, or even more distant mechanisms. Given the broad range of scientific questions continually under investigation, researchers do not always have occasion to classify target genes along these lines. However, our understanding of the genetic role of retinoids will be enhanced if such a distinction can be made for each regulated gene. We have therefore evaluated published data from 1,191 papers covering 532 genes and have classified these genes into four categories according to the degree to which an hypothesis of direct versus indirect control is supported overall. We found 27 genes that are unquestionably direct targets of the classical pathway in permissive cellular contexts (Category 3 genes), plus 105 genes that appear to be candidates, pending the results of specific additional experiments (Category 2). Data on another 267 targets are not evocative of direct or indirect regulation either way, although control by retinoic acid through some mechanism is clear (Category 1). Most of the remaining 133 targets seem to be regulated indirectly, usually through a transcriptional intermediary, in the contexts studied so far (Category 0).

Retinoic acid inhibits hepatic Jun N-terminal kinase-dependent signaling pathway in ethanol-fed rats

Retinoic acid (RA) supplementation suppresses ethanol-enhanced hepatocyte hyperproliferation in rats; however, little is known about the mechanism(s). Here, we investigated whether RA affects the protein kinase signaling pathways in the liver tissues of rats fed with a high dose of ethanol for a prolonged period of time (6 months). Results show that there were greater levels of phosphorylated Jun N-terminal kinase (JNK) and phosphorylated c-Jun protein, but not total JNK protein, in livers of ethanol-fed rats vs those of controls. Moreover, ethanol feeding to rats increased the levels of phosphorylated mitogen-activated protein kinase kinase-4 (MKK-4) and decreased the levels of mitogen-activated kinase phosphatase-1 (MKP-1) in liver tissue. However, hepatic levels of phosphorylated-p38 protein and total-p38 protein were not altered by the ethanol treatment. In contrast, all-trans-RA supplementation at two doses in ethanol-fed rats greatly attenuated the ethanol-induced hepatic phosphorylation of MKK-4, phosphorylated-JNK and c-Jun proteins. The level of MKP-1 was increased in ethanol-fed rats supplemented with all-trans-RA. Further, ethanol-induced hepatocyte hyperproliferation, measured by immunostaining for proliferating cell nuclear antigen, were markedly decreased by all-trans-RA supplementation. Interestingly, hepatic apoptosis in the liver of ethanol-fed rats after 6 months of treatment decreased significantly. This decrease of hepatic apoptosis in ethanol-fed rats was prevented by all-trans-RA supplementation in a dose-dependent manner. The results from these studies indicate that restoration of RA homeostasis is critical for the regulation of JNK-dependent signaling pathway and apoptosis in the liver of ethanol-fed rats.

Retinoic acid prevents experimental Cushing syndrome

Cushing syndrome is caused by an excess of adrenocorticotropic hormone (ACTH) production by neuroendocrine tumors, which subsequently results in chronic glucocorticoid excess. We found that retinoic acid inhibits the transcriptional activity of AP-1 and the orphan receptors Nur77 and Nurr1 in ACTH-secreting tumor cells. Retinoic acid treatment resulted in reduced pro-opiomelanocortin transcription and ACTH production. ACTH inhibition was also observed in human pituitary ACTH-secreting tumor cells and a small-cell lung cancer cell line, but not in normal cells. This correlated with the expression of the orphan receptor COUP-TFI, which was found in normal corticotrophs but not in pituitary Cushing tumors. COUP-TFI expression in ACTH-secreting tumor cells blocked retinoic acid action. Retinoic acid also inhibited cell proliferation and, after prolonged treatment, increased caspase-3 activity and induced cell death in ACTH-secreting cells. In adrenal cortex cells, retinoic acid inhibited corticosterone production and cell proliferation. The antiproliferative action and the inhibition of ACTH and corticosterone produced by retinoic acid were confirmed in vivo in experimental ACTH-secreting tumors in nude mice. Thus, we conclude that the effects of retinoic acid combine in vivo to reverse the endocrine alterations and symptoms observed in experimental Cushing syndrome.

Treatment prospects for autosomal-dominant polycystic kidney disease

An increased understanding of the molecular genetic and cellular pathophysiologic mechanisms responsible for the development of autosomal-dominant polycystic kidney disease (ADPKD), made possible by the advances in molecular biology and genetics of the last three decades, has laid the foundation for the development of effective therapies. As the concept that a polycystic kidney is a neoplasm in disguise is becoming increasingly accepted, the development of therapies for ADPKD may benefit greatly from the expanding body of information on cancer chemoprevention and chemosuppression. This review summarizes the observations that already have been made and discusses therapies for PKD that deserve investigation.

Genomic organization of the JEM-1 (BLZF1) gene on human chromosome 1q24: molecular cloning and analysis of its promoter region

The Jem-1 (JEM-1, HGMW-approved symbol BLZF1) gene mapping to human chromosome 1q24 codes for a ubiquitously expressed 3-kb mRNA, translated in a 45-kDa nuclear protein. Recent studies have shown a deficient expression of this gene in acute promyelocytic leukemia (APL). However, treatment with retinoids was able to upregulate JEM-1 mRNA in maturing NB4 leukemia cells. Here, we report the characterization of the structural organization of JEM-1. By hybridization screening of a human genomic library derived from blood mononuclear cells, five overlapping genomic DNA clones were isolated. These clones extend over 34 kb of the human genome and comprise the complete JEM-1 gene and a 4-kb 5'flanking region. Determination of the exon-intron structure of Jem-1 revealed seven exons whose junctions with introns exhibited typical splice sequences. A shorter transcript (Jem-1s, 1.3 kb) generated by exon 3 extension and polyadenylation was identified. Its translation generated a 23-kDa protein that exhibited a cytoplasmic localization. 5'RACE-PCR identified a major transcription start site (TSS) located at 403 nt upstream of the ATG. Computer analysis of the 1. 8-kb 5'flanking region showed that it lacks a TATA box, Inr motifs or DPE motifs, but it contains a typical CCAAT box located 95 bp upstream of the TSS. Sequencing also revealed potential cis-acting elements for multiple transcription regulators including Sp1, GATA, C/EBP, AP-1, and Pu1. No retinoic acid receptor elements or retinoic X receptor elements were detected. This 1.8-kb DNA sequence showed a strong constitutive promoter activity determined by a luciferase-reporter gene assay in transiently transfected HeLa cells. Retinoids further increased luciferase expression 2.7-fold. We demonstrated that the 1-kb distal sequence contains yet unidentified elements reducing constitutive transcription. Thus, the maximal constitutive promoter activity was assigned to a -432 + 101 region overlapping the TSS. These data support the idea of a constitutive expression of JEM-1, but a negative regulation in APL released by retinoids.

Structural basis for isotype selectivity of the human retinoic acid nuclear receptor

The human retinoic acid receptor (hRAR) belongs to the family of nuclear receptors that regulate transcription in a ligand-dependent way. The isotypes RARalpha,beta and gamma are distinct pharmacological targets for retinoids that are involved in the treatment of various skin diseases and cancers, in particular breast cancer and acute promyelocytic leukemia. Therefore, synthetic retinoids have been developed aiming at isotype selectivity and reduced side-effects. We report the crystal structures of three complexes of the hRARgamma ligand-binding domain (LBD) bound to agonist retinoids that possess selectivity either for RARgamma (BMS184394) or for RARbeta/gamma (CD564), or that are potent for all RAR-isotypes (panagonist BMS181156). The high resolution data (1.3-1. 5 A) provide a description at the atomic level of the ligand pocket revealing the molecular determinants for the different degrees of ligand selectivity. The comparison of the complexes of the chemically closely related retinoids BMS184394 and CD564 shows that the side-chain of Met272 adopts different conformations depending on the presence of a hydrogen bond between its sulfur atom and the ligand. This accounts for their different isotype selectivity. On the other hand, the difference between the pan- and the RARbeta, gamma-selective agonist is probably due to a steric discrimination at the level of the 2-naphthoic acid moiety of CD564. Based on this study, we propose a model for a complex with the RARgamma-specific agonist CD666 that shows the possible applications for structure-based drug design of RAR isotype-selective retinoids.

Ligand-protein interactions in nuclear receptors of hormones

Nuclear hormone receptors are transcription factors regulated by lipophilic ligands. These hormones bind to their nuclear receptor targets using an induced fit mechanism that triggers a large conformational change and generates the proper surface for the binding of protein coactivators. The molecular details of the various steps of this activation process or its inhibition by antagonists are now understood for several nuclear receptors.