Ligand recognition by RAR and RXR receptors: binding and selectivity

Structure-Activity Relationships and Therapeutic Applications of Retinoids in View of Potential Benefits from Drug Repurposing Process

Vitamin A, an essential micronutrient, is integral to various biological processes crucial for organismal development and maintenance. Dietary sources of vitamin A encompass preformed retinol, retinyl esters, and provitamin A carotenoids. Retinoic acid (RA), a key component, plays pivotal roles in vision, cell proliferation, apoptosis, immune function, and gene regulation. Drug repurposing, an effective strategy for identifying new therapeutic applications for existing drugs, has gained prominence in recent years. This review seeks to provide a comprehensive overview of the current research landscape surrounding retinoids and drug repurposing. The scope of this review encompasses a comprehensive examination of retinoids and their potential for repurposing in various therapeutic contexts. Despite their efficacy in treating dermatological conditions, concerns about toxicity persist, driving the search for safer and more potent retinoids. The molecular mechanisms underlying retinoid activity involve binding to retinoic acid receptors (RARs) and retinoid X receptors (RXRs), leading to transcriptional regulation of target genes. This review seeks to shed light on the possibilities for repurposing retinoids to cover a wider spectrum of therapeutic uses by exploring recent scientific progress. It also aims to offer a more comprehensive understanding of the therapeutic prospects of retinoids and the broader impact of drug repositioning in contemporary medicine.

Direct evaluation of polarity of the ligand binding pocket in retinoid X receptor using a fluorescent solvatochromic agonist

High selectivity of small-molecule drug candidates for their target molecule is important to minimize potential side effects. One factor that contributes to the selectivity is the internal polarity of the ligand-binding pocket (LBP) in the target molecule, but this is difficult to measure. Here, we first confirmed that the retinoid X receptor (RXR) agonist 6-(ethyl(1-isobutyl-2-oxo-4-(trifluoromethyl)-1,2-dihydroquinolin-7-yl)amino)nicotinic acid (NEt-iFQ, 1) exhibits fluorescence solvatochromism, i.e., its Stokes shift depends on the polarity of the solvent, and then we utilized this property to directly measure the internal polarity of the RXRα-LBP. The Stokes shift of 1 when bound to the RXRα-LBP corresponded to that of 1 in chloroform solution. This finding is expected to be helpful for designing RXR-selective ligands. A similar approach should be appliable to evaluate the internal polarity of the LBPs of other receptors.

CRABP1 and CRABP2 Protein Levels Correlate with Each Other but Do Not Correlate with Sensitivity of Breast Cancer Cells to Retinoic Acid

Retinoic acid (RA) binding proteins, CRABP1 and CRABP2, are molecular chaperones that mediate intracellular activity of RA, the key promoter of cell differentiation with tumor suppressor activity. One of the main functions of CRABP2 is delivery and transfer of RA to the nuclear receptors RAR/RXR, which leads to activation of the transcription of a wide range of retinoid-responsive genes. The functions of CRABP1 are less studied but are apparently associated with sequestration of RA in cytoplasm and limitation of its transcriptional activity, suggesting involvement of this protein in the development of RA resistance. The mechanisms regulating activity of CRABP1 are also poorly understood. Comparison of the CRABP1 level in tumor cell lines of various origins, performed for the first time here, showed absence of the CRABP1 protein in the cell lines of tumors considered to be RA-resistant, and pronounced production of this protein in the RA-sensitive cells. However, analysis carried out with a panel of breast cancer cell lines with different levels of RA-sensitivity showed that there was no correlation between the production of CRABP1 protein and the sensitivity of the cells to RA. At the same time, we found strong correlation between the expression of CRABP1 and CRABP2 proteins in all studied cell types, regardless of their origin and RA-sensitivity/resistance. Moreover, suppression of the CRABP1 level in both RA-sensitive and RA-resistant cells was shown in the cells with cells with knockdown of CRABP2 gene. The revealed CRABP2-dependent regulation of CRABP1 production is a new mechanism of the intracellular retinoic signaling system.

Bexarotene Impairs Cognition and Produces Hypothyroidism in a Mouse Model of Down Syndrome and Alzheimer's Disease

All individuals with Down syndrome (DS) eventually develop Alzheimer's disease (AD) neuropathology, including neurodegeneration, increases in β-amyloid (Aβ) expression, and aggregation and neurofibrillary tangles, between the third and fourth decade of their lives. There is currently no effective treatment to prevent AD neuropathology and the associated cognitive degeneration in DS patients. Due to evidence that the accumulation of Aβ aggregates in the brain produces the neurodegenerative cascade characteristic of AD, many strategies which promote the clearance of Aβ peptides have been assessed as potential therapeutics for this disease. Bexarotene, a member of a subclass of retinoids that selectively activates retinoid receptors, modulates several pathways essential for cognitive performance and Aβ clearance. Consequently, bexarotene might be a good candidate to treat AD-associated neuropathology. However, the effects of bexarotene treatment in AD remain controversial. In the present study, we aimed to elucidate whether chronic bexarotene treatment administered to the most commonly used murine model of DS, the Ts65Dn (TS) mouse could reduce Aβ expression in their brains and improve their cognitive abilities. Chronic administration of bexarotene to aged TS mice and their CO littermates for 9 weeks diminished the reference, working, and spatial learning and memory of TS mice, and the spatial memory of CO mice in the Morris water maze. This treatment also produced marked hypoactivity in the plus maze, open field, and hole board tests in TS mice, and in the open field and hole board tests in CO mice. Administration of bexarotene reduced the expression of Aβ1-40, but not of Aβ1-42, in the hippocampi of TS mice. Finally, bexarotene increased Thyroid-stimulating hormone levels in TS mice and reduced Thyroid-stimulating hormone levels in CO mice, while animals of both karyotypes displayed reduced thyroxine levels after bexarotene administration. The bexarotene-induced hypothyroidism could be responsible for the hypoactivity of TS and CO mice and their diminished performance in the Morris water maze. Together, these results do not provide support for the use of bexarotene as a potential treatment of AD neuropathology in the DS population.

Highly Chemoselective [2+1] Annulation of α-Alkylidene Pyrazolones with α-Bromonitroalkenes: Synthesis of Pyrazolone-Based Vinylcyclopropanes and Computational Studies

A highly chemoselective [2+1] annulation of α-alkylidene pyrazolones with α-bromonitroalkenes has been achieved under mild conditions. α-Alkylidene pyrazolones were unprecedentedly used as a C1 synthon to participate in annulation reactions, providing access to diverse vinylcyclopropane-based pyrazolone products. In addition, a spectrum of pharmaceutically interesting pyrazole-fused pyranone oximes could be rapidly obtained through a [2+1] annulation/rearrangement sequential process. Computational studies disclosed the origin of the observed chemoselectivity of the [2+1] cycloaddition.

CRABP2 and FABP5 expression levels in diseased and normal pancreas

Recently, stromal targeting, by agents such as All trans retinoic acid (ATRA), has been regarded as a promising avenue for the treatment of pancreatic ductal adenocarcinoma (PDAC). The intra-cellular transportation of ATRA to the nuclear receptors is performed by either: fatty acid binding protein 5 (FABP5) or cellular retinoic acid binding protein 2 (CRABP2), dictating the transcription of downstream genes and, thus, eventual cell phenotype. Here, we explored the levels of each protein, in pancreatic tissues of patients presenting with a range of pancreatic diseases (pancreatic ductal adenocarcinoma (PDAC), chronic pancreatitis (CP), cholangiocarcinoma (CC)). We demonstrate that there is a significantly lower CRABP2 and FABP5 expression in activated fibroblasts or pancreatic stellate cells (PSC) in PDAC, as well as other diseased pancreas as in CC and CP, versus quiescent fibroblasts. The quiescent fibroblasts consistently show a pattern of high FABP5:CRABP2 ratio, whereas PSC in all non-PDAC tissues showed a low FABP5:CRABP2 ratio. PSC in PDAC patients had a range of FABP5:CRABP2 ratios (high, even and low). There was a lower CRABP2 expression in cancerous epithelial cells (PDAC) versus normal epithelial cells. This is also present in other disease states (CP, CC). Contrasting to the patterns seen for fibroblasts, the FABP5 expression in PDAC epithelial cells matched that of the normal epithelial cells. However, the normal epithelial cells had a high FABP5:CRABP2 ratio, compared to the PDAC epithelial cells. These ratios may have correlation with tumor progression, and overall survival. These findings could be confirmed in in vitro cell lysates. CRABP2 and FABP5 levels and ratios could serve as valuable biomarkers.

Differential CRABP-II and FABP5 expression patterns and implications for medulloblastoma retinoic acid sensitivity

Medulloblastoma (MB) cells exhibit different responses to retinoid acid (RA) for reasons that are poorly understood. RA signaling can be transduced by two approaches that are mediated by cellular retinoic acid-binding protein 2 (CRABP-II) as a tumor-suppressive pathway, and by fatty acid-binding protein 5 (FABP5) as a tumor-promoting pathway. The biological effects of RA on cancer cells are largely determined by the patterns of CRABP-II and FABP5 expression. This study aims to profile the statuses of CRABP-II and FABP5 expression in MB and to evaluate their correlation with RA sensitivities using RA-sensitive (Med-3) and RA-insensitive (UW228-2, UW228-3) MB cells. Our results show that CRABP-II is distinctly expressed and the level of FABP5 is extremely low in Med-3 cells, while the patterns of CRABP-II and FABP5 expression are reversed in UW228-2 and UW228-3 cells. RA up-regulates CRABP-II expression in Med-3 cells, whereas it up-regulates FABP5 expression in the other two cell lines. The FABP5-specific inhibitor BMS309403 increases the RA sensitivity of UW228-2 cells (p < 0.01). Tissue microarray-based immunohistochemical staining showed CRABP-II/FABP5 expression patterns in MB that were variable (CRABP-II-/FABP5-, CRABP-II-/FABP5+, CRABP-II+/FABP5- and CRABP-II+/FABP5+) and imbalanced (CRABP-II↑/FABP5↓ and CRABP-II↓/FABP5↑). MB cases exhibited patterns ofCRABP-II-/FABP5- (12.24%, 6/49), CRABP-II-/FABP5+ (30.61%, 15/49) or CRABP-II↓/FABP5↑ (12.24%, 6/49), implicating unresponsiveness or insensitivity to RA. In conclusion, the ratios of CRABP-II/FABP5 levels are closely related to the RA sensitivities of MB cells. The differential CRABP-II and FABP5 expression patterns are prospective parameters, and of potential value in personalized RA therapy for MB.

Molecular dynamics simulations of apo and holo forms of fatty acid binding protein 5 and cellular retinoic acid binding protein II reveal highly mobile protein, retinoic acid ligand, and water molecules

Structural and dynamic properties from a series of 300 ns molecular dynamics, MD, simulations of two intracellular lipid binding proteins, iLBPs, (Fatty Acid Binding Protein 5, FABP5, and Cellular Retinoic Acid Binding Protein II, CRABP-II) in both the apo form and when bound with retinoic acid reveal a high degree of protein and ligand flexibility. The ratio of FABP5 to CRABP-II in a cell may determine whether it undergoes natural apoptosis or unrestricted cell growth in the presence of retinoic acid. As a result, FABP5 is a promising target for cancer therapy. The MD simulations presented here reveal distinct differences in the two proteins and provide insight into the binding mechanism. CRABP-II is a much larger, more flexible protein that closes upon ligand binding, where FABP5 transitions to an open state in the holo form. The traditional understanding obtained from crystal structures of the gap between two β-sheets of the β-barrel common to iLBPs and the α-helix cap that forms the portal to the binding pocket is insufficient for describing protein conformation (open vs. closed) or ligand entry and exit. When the high degree of mobility between multiple conformations of both the ligand and protein are examined via MD simulation, a new mode of ligand motion that improves understanding of binding dynamics is revealed.

Designed Spiroketal Protein Modulation

Spiroketals are structural motifs found in many biologically active natural products, which has stimulated considerable efforts toward their synthesis and interest in their use as drug lead compounds. Despite this, the use of spiroketals, and especially bisbenzanulated spiroketals, in a structure-based drug discovery setting has not been convincingly demonstrated. Herein, we report the rational design of a bisbenzannulated spiroketal that potently binds to the retinoid X receptor (RXR) thereby inducing partial co-activator recruitment. We solved the crystal structure of the spiroketal-hRXRα-TIF2 ternary complex, and identified a canonical allosteric mechanism as a possible explanation for the partial agonist behavior of our spiroketal. Our co-crystal structure, the first of a designed spiroketal-protein complex, suggests that spiroketals can be designed to selectively target other nuclear receptor subtypes.

Apolipoprotein E as a Therapeutic Target in Alzheimer's Disease: A Review of Basic Research and Clinical Evidence

Alzheimer's disease (AD) is a devastating neurodegenerative disorder that causes progressive cognitive decline. The majority of AD cases are sporadic and late-onset (>65 years old) making it the leading cause of dementia in the elderly. While both genetic and environmental factors contribute to the development of late-onset AD (LOAD), APOE polymorphism is a major genetic risk determinant for LOAD. In humans, the APOE gene has three major allelic variants: ε2, ε3, and ε4, of which APOE ε4 is the strongest genetic risk factor for LOAD, whereas APOE ε2 is protective. Mounting evidence suggests that APOE ε4 contributes to AD pathogenesis through multiple pathways including facilitated amyloid-β deposition, increased tangle formation, synaptic dysfunction, exacerbated neuroinflammation, and cerebrovascular defects. Since APOE modulates multiple biological processes through its corresponding protein apolipoprotein E (apoE), APOE gene and apoE properties have been a promising target for therapy and drug development against AD. In this review, we summarize the current evidence regarding how the APOE ε4 allele contributes to the pathogenesis of AD and how relevant therapeutic approaches can be developed to target apoE-mediated pathways in AD.

Rescuing effects of RXR agonist bexarotene on aging-related synapse loss depend on neuronal LRP1

Apolipoprotein E (apoE) plays a critical role in maintaining synaptic integrity by transporting cholesterol to neurons through the low-density lipoprotein receptor related protein-1 (LRP1). Bexarotene, a retinoid X receptor (RXR) agonist, has been reported to have potential beneficial effects on cognition by increasing brain apoE levels and lipidation. To investigate the effects of bexarotene on aging-related synapse loss and the contribution of neuronal LRP1 to the pathway, forebrain neuron-specific LRP1 knockout (nLrp1(-/-)) and littermate control mice were administered with bexarotene-formulated diet (100mg/kg/day) or control diet at the age of 20-24 months for 8 weeks. Upon bexarotene treatment, levels of brain apoE and ATP-binding cassette sub-family A member 1 (ABCA1) were significantly increased in both mice. While levels of PSD95, glutamate receptor 1 (GluR1), and N-methyl-d-aspartate receptor NR1 subunit (NR1), which are key postsynaptic proteins that regulate synaptic plasticity, were decreased with aging, they were restored by bexarotene treatment in the brains of control but not nLrp1(-/-) mice. These results indicate that the beneficial effects of bexarotene on synaptic integrity depend on the presence of neuronal LRP1. However, we also found that bexarotene treatment led to the activation of glial cells, weight loss and hepatomegaly, which are likely due to hepatic failure. Taken together, our results demonstrate that apoE-targeted treatment through the RXR pathway has a potential beneficial effect on synapses during aging; however, the therapeutic application of bexarotene requires extreme caution due to its toxic side effects.

Kruppel-like factor 2 suppresses mammary carcinoma growth by regulating retinoic acid signaling

The transcription factor Kruppel-like factor 2 (KLF2) displays anticarcinogenic activities but the mechanism that underlies this activity is unknown. We show here that KLF2 is markedly downregulated in human breast cancers and that its expression positively correlates with breast cancer patient survival. We show further that KLF2 suppresses tumor development by controlling the transcriptional activity of the vitamin A metabolite retinoic acid (RA). RA regulates gene transcription by activating two types of nuclear receptors: RA receptors (RARs), which inhibit tumor development, and peroxisome proliferator-activated receptor β/δ (PPARβ/δ), which promotes tumorigenesis. The partitioning of RA between these receptors is regulated by two carrier proteins: cellular retinoic acid-binding protein 2 (CRABP2), which delivers RA to RARs, and fatty acid-binding protein 5 (FABP5), which shuttles ligands to PPARβ/δ. We show that KLF2 induces the expression of CRABP2 and RARγ and inhibits the expression FABP5 and PPARβ/δ thereby shifting RA signaling from the pro-carcinogenic FABP5/PPARβ/δ to the growth-suppressing CRABP2/RAR path. The data thus reveal that KLF2 suppresses tumor growth by controlling the transcriptional activities of RA.

A temporal gate for viral enhancers to co-opt Toll-like-receptor transcriptional activation pathways upon acute infection

Viral engagement with macrophages activates Toll-Like-Receptors (TLRs) and viruses must contend with the ensuing inflammatory responses to successfully complete their replication cycle. To date, known counter-strategies involve the use of viral-encoded proteins that often employ mimicry mechanisms to block or redirect the host response to benefit the virus. Whether viral regulatory DNA sequences provide an opportunistic strategy by which viral enhancer elements functionally mimic innate immune enhancers is unknown. Here we find that host innate immune genes and the prototypical viral enhancer of cytomegalovirus (CMV) have comparable expression kinetics, and positively respond to common TLR agonists. In macrophages but not fibroblasts we show that activation of NFκB at immediate-early times of infection is independent of virion-associated protein, M45. We find upon virus infection or transfection of viral genomic DNA the TLR-agonist treatment results in significant enhancement of the virus transcription-replication cycle. In macrophage time-course infection experiments we demonstrate that TLR-agonist stimulation of the viral enhancer and replication cycle is strictly delimited by a temporal gate with a determined half-maximal time for enhancer-activation of 6 h; after which TLR-activation blocks the viral transcription-replication cycle. By performing a systematic siRNA screen of 149 innate immune regulatory factors we identify not only anticipated anti-viral and pro-viral contributions but also new factors involved in the CMV transcription-replication cycle. We identify a central convergent NFκB-SP1-RXR-IRF axis downstream of TLR-signalling. Activation of the RXR component potentiated direct and indirect TLR-induced activation of CMV transcription-replication cycle; whereas chromatin binding experiments using wild-type and enhancer-deletion virus revealed IRF3 and 5 as new pro-viral host transcription factor interactions with the CMV enhancer in macrophages. In a series of pharmacologic, siRNA and genetic loss-of-function experiments we determined that signalling mediated by the TLR-adaptor protein MyD88 plays a vital role for governing the inflammatory activation of the CMV enhancer in macrophages. Downstream TLR-regulated transcription factor binding motif disruption for NFκB, AP1 and CREB/ATF in the CMV enhancer demonstrated the requirement of these inflammatory signal-regulated elements in driving viral gene expression and growth in cells as well as in primary infection of neonatal mice. Thus, this study shows that the prototypical CMV enhancer, in a restricted time-gated manner, co-opts through DNA regulatory mimicry elements, innate-immune transcription factors to drive viral expression and replication in the face of on-going pro-inflammatory antiviral responses in vitro and in vivo and; suggests an unexpected role for inflammation in promoting acute infection and has important future implications for regulating latency.

Here we discover how inflammatory signalling may unintentionally promote infection, as a result of viruses evolving DNA sequences, known as enhancers, which act as a bait to prey on the infected cell transcription factors induced by inflammation. The major inflammatory transcription factors activated are part of the TLR-signalling pathway. We find the prototypical viral enhancer of cytomegalovirus can be paradoxically boosted by activation of inflammatory “anti-viral” TLR-signalling independent of viral structural proteins. This leads to an increase in viral gene expression and replication in cell-culture and upon infection of mice. We identify an axis of inflammatory transcription factors, acting downstream of TLR-signalling but upstream of interferon inhibition. Mechanistically, the central TLR-adapter protein MyD88 is shown to play a critical role in promoting viral enhancer activity in the first 6h of infection. The co-option of TLR-signalling exceeds the usage of NFκB, and we identify IRF3 and 5 as newly found viral-enhancer interacting inflammatory transcription factors. Taken together this study reveals how virus enhancers, employ a path of least resistance by directly harnessing within a short temporal window, the activation of anti-viral signalling in macrophages to drive viral gene expression and replication to an extent that has not been recognised before.

Physiological functions of peroxisome proliferator-activated receptor β

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.

Design, synthesis, and biological evaluation of indenoisoquinoline rexinoids with chemopreventive potential

Nuclear receptors, such as the retinoid X receptor (RXR), are proteins that regulate a myriad of cellular processes. Molecules that function as RXR agonists are of special interest for the prevention and control of carcinogenesis. The majority of these ligands possess an acidic moiety that is believed to be key for RXR activation. This communication presents the design, synthesis, and biological evaluation of both acidic and nonacidic indenoisoquinolines as new RXR ligands. In addition, a comprehensive structure-activity relationship study is presented that identifies the important features of the indenoisoquinoline rexinoids. The ease of modification of the indenoisoquinoline core and the lack of the necessity of a carboxyl group for activity make them an attractive and unusual family of RXR agonists. This work establishes a structural foundation for the design of new and novel rexinoid cancer chemopreventive agents.

Effects on gene expression in rat liver after administration of RXR agonists: UAB30, 4-methyl-UAB30, and Targretin (Bexarotene)

Examination of three retinoid X receptor (RXR) agonists [Targretin (TRG), UAB30, and 4-methyl-UAB30 (4-Me-UAB30)] showed that all inhibited mammary cancer in rodents and two (TRG and 4-Me-UAB30) strikingly increased serum triglyceride levels. Agents were administered in diets to female Sprague-Dawley rats. Liver RNA was isolated and microarrayed on the Affymetrix GeneChip Rat Exon 1.0 ST array. Statistical tests identified genes that exhibited differential expression and fell into groups, or modules, with differential expression among agonists. Genes in specific modules were changed by one, two, or all three agonists. An interactome analysis assessed the effects on genes that heterodimerize with known nuclear receptors. For proliferator-activated receptor α/RXR-activated genes, the strongest response was TRG > 4-Me-UAB30 > UAB30. Many liver X receptor/RXR-related genes (e.g., Scd-1 and Srebf1, which are associated with increased triglycerides) were highly expressed in TRG and 4-Me-UAB30- but not UAB30-treated livers. Minimal expression changes were associated with retinoic acid receptor or vitamin D receptor heterodimers by any of the agonists. UAB30 unexpectedly and uniquely activated genes associated with the aryl hydrocarbon hydroxylase (Ah) receptor (Cyp1a1, Cyp1a2, Cyp1b1, and Nqo1). Based on the Ah receptor activation, UAB30 was tested for its ability to prevent dimethylbenzanthracene (DMBA)-induced mammary cancers, presumably by inhibiting DMBA activation, and was highly effective. Gene expression changes were determined by reverse transcriptase-polymerase chain reaction in rat livers treated with Targretin for 2.3, 7, and 21 days. These showed similar gene expression changes at all three time points, arguing some steady-state effect. Different patterns of gene expression among the agonists provided insight into molecular differences and allowed one to predict certain physiologic consequences of agonist treatment.

Molecular determinants of retinoic acid sensitivity in pancreatic cancer

PURPOSE

To identify a predictive molecular "signature" for sensitivity to retinoic acid in pancreatic cancer.

EXPERIMENTAL DESIGN

Fourteen patient-derived, low-passage pancreatic ductal adenocarcinoma (PDAC) lines with varied expression of fatty acid-binding protein 5 (FABP5) and cellular retinoic acid-binding protein 2 (CRABP2) were used to evaluate the response to all-trans retinoic acid (ATRA). Cell proliferation, apoptosis, and migration/invasion assays were used to measure the in vitro response. Tumor growth was monitored in subcutaneous xenografts in athymic nude mice for 4 weeks.

RESULTS

Response to ATRA was observed to be dependent upon differential expression of FABP5 versus CRABP2. Thus, elevated FABP5 expression was associated with minimal cytotoxicity and tumor growth inhibition and a paradoxical increase in migration and invasion. Conversely, CRABP2 expression in the absence of FABP5 was associated with significant tumor growth inhibition with ATRA, even in gemcitabine-resistant tumors. The ATRA-resistant phenotype of FABP5(high)CRABP2(null) cells could be circumvented by ectopic expression of CRABP2. Alternatively, reexpression of endogenous CRABP2 could be enabled in FABP5(high)CRABP2(null) PDAC lines by exposure to decitabine and trichostatin A, thereby relieving epigenetic silencing of the CRABP2 gene promoter. Immunohistochemical staining for FABP5 in archival human tissue microarrays identifies a subset of cases (13 of 63, ~20%) which are negative for FABP5 expression and might be candidates for ATRA therapy.

CONCLUSIONS

The widely used agent ATRA deserves a "second look" in PDAC, but needs to be targeted to patient subsets with biopsy-proven FABP5-negative tumors, or be combined with a chromatin-modifying agent to reexpress endogenous CRABP2.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Is PPARbeta/delta a Retinoid Receptor?

The broad ligand-binding characteristic of PPARβ/δ has long hampered identification of physiologically-meaningful ligands for the receptor. The observations that the activity of PPARβ/δ is supported by fatty acid binding protein 5 (FABP5), which directly delivers ligands from the cytosol to the receptor, suggest that bona fide PPARβ/δ ligands both activate the receptor, and trigger the nuclear translocation of FABP5. Using these criteria, it was recently demonstrated that all-trans-retinoic acid (RA), the activator of the classical retinoic acid receptor RAR, also serves as a ligand for PPARβ/δ. Partitioning of RA between its two receptors was found to be regulated by FABP5, which delivers it to PPARβ/δ, and cellular RA binding protein II (CRABP-II), which targets it to RAR. Consequently, RA activates PPARβ/δ in cells that display a high FABP5/CRABP-II expression ratio. It remains to be clarified whether compounds other than RA may also serve as endogenous activators for this highly promiscuous protein.

Association of FABP5 expression with poor survival in triple-negative breast cancer: implication for retinoic acid therapy

Recent studies using animal models suggest that expression of FABP5 drives the stimulation of cell growth observed in estrogen receptor (ER)-negative breast cancer cells on exposure to retinoic acid (RA). The purpose of this study was to investigate the clinicopathological significance of FABP5 in breast cancer and to evaluate FABP5 as a prognostic marker and a possible novel therapeutic target in breast cancer. Gene expression microarray analysis revealed a significant correlation between elevated FABP5 RNA levels and ER/progesterone receptor (PR)-negative status, high tumor grade, and poor prognosis. Tissue microarray analysis demonstrated similar correlations with cytoplasmic FABP5 protein. Based on multivariate proportional regression analysis, cytoplasmic FABP5 is a significant and independent prognostic marker of overall survival and recurrence-free survival in breast cancer. The effects of FABP5 on tumor growth appear to be mediated primarily through cytoplasmic FABP, because no correlation was found between nuclear FABP5 and ER/PR-negative status, recurrence, and survival. FABP5 knockdown in breast cancer cell lines demonstrates a correlation between FABP5 levels and growth response to RA. We propose a model whereby growth-promoting FABP5 competes with growth-inhibiting CRABP2 for RA, with retention of RA in the cytoplasm by FABP5 preventing the inhibition of tumor growth.

Synthetic retinoids: structure-activity relationships

Retinoid signalling pathways are involved in numerous processes in cells, particularly those mediating differentiation and apoptosis. The endogenous ligands that bind to the retinoid receptors, namely all-trans-retinoic acid (ATRA) and 9-cis-retinoic acid, are prone to double-bond isomerisation and to oxidation by metabolic enzymes, which can have significant and deleterious effects on their activities and selectivities. Many of these problems can be overcome through the use of synthetic retinoids, which are often much more stable, as well as being more active. Modification of their molecular structures can result in retinoids that act as antagonists, rather than agonists, or exhibit a large degree of selectivity for particular retinoid-receptor isotypes. Several such selective retinoids are likely to be of value as pharmaceutical agents with reduced toxicities, particularly in cancer therapy, as reagents for controlling cell differentiation, and as tools for elucidating the precise roles that specific retinoid signalling pathways play within cells.

Structure-based virtual ligand screening with LigandFit: pose prediction and enrichment of compound collections

Virtual ligand screening methods based on the structure of the receptor are extensively used to facilitate the discovery of lead compounds. In the present study, we investigated the LigandFit package on four different proteins (coagulation factor VIIa, estrogen receptor, thymidine kinase, and neuraminidase), a relatively large compound collection of 65,560 unique "drug-like" molecules and four focused libraries (1950 molecules each). We performed virtual screening experiments with the large database and evaluated six scoring functions available in the package (DockScore, LigScore1, LigScore2, PLP1, PLP2, and PMF). The results showed that LigandFit is an efficient program, especially when used with LigScore1. Similar computations were carried out using focused libraries. In this situation the LigScore1 scoring function outperformed the other ones on three out of the four proteins tested. Even for the difficult neuraminidase case, the LigandFit/LigScore1 combination was still reasonably successful. Assessment of docking accuracy was also performed and again, we found that LigandFit (with DockScore and the CFF parameters) was performing well. On the basis of these results and observed increased enrichments after LigandFit/Ligscore1 screening on focused libraries, we suggest that using this program as a final step of a hierarchical protocol can be very beneficial to assist lead finding.

Opposing effects of retinoic acid on cell growth result from alternate activation of two different nuclear receptors

Transcriptional activation of the nuclear receptor RAR by retinoic acid (RA) often leads to inhibition of cell growth. However, in some tissues, RA promotes cell survival and hyperplasia, activities that are unlikely to be mediated by RAR. Here, we show that, in addition to functioning through RAR, RA activates the "orphan" nuclear receptor PPARbeta/delta, which, in turn, induces the expression of prosurvival genes. Partitioning of RA between the two receptors is regulated by the intracellular lipid binding proteins CRABP-II and FABP5. These proteins specifically deliver RA from the cytosol to nuclear RAR and PPARbeta/delta, respectively, thereby selectively enhancing the transcriptional activity of their cognate receptors. Consequently, RA functions through RAR and is a proapoptotic agent in cells with high CRABP-II/FABP5 ratio, but it signals through PPARbeta/delta and promotes survival in cells that highly express FABP5. Opposing effects of RA on cell growth thus emanate from alternate activation of two different nuclear receptors.

RXR–LXR heterodimer modulators for the potential treatment of dyslipidemia

A number of RXR agonists were synthesized and screened in functional assays. The synthesis and the structure-activity relationship (SAR) within the series of compounds will be presented. Some in vivo data in rodent models for dyslipidemia and diabetes will also be presented.