Discovery of Highly Potent Liver X Receptor β Agonists

A mutation in LXRα uncovers a role for cholesterol sensing in limiting metabolic dysfunction-associated steatohepatitis

Liver x receptor alpha (LXRα) functions as an intracellular cholesterol sensor that regulates lipid metabolism at the transcriptional level in response to the direct binding of cholesterol derivatives. We have generated mice with a mutation in LXRα that reduces activity in response to endogenous cholesterol derived LXR ligands while still allowing transcriptional activation by synthetic agonists. The mutant LXRα functions as a dominant negative that shuts down cholesterol sensing. When fed a high fat, high cholesterol diet LXRα mutant mice rapidly develop pathologies associated with Metabolic Dysfunction-Associated Steatohepatitis (MASH) including ballooning hepatocytes, liver inflammation, and fibrosis. Strikingly LXRα mutant mice have decreased liver triglycerides but increased liver cholesterol. Therefore, elevated cholesterol in the liver may play a critical role in the development of MASH. Reengaging LXR signaling by treatment with synthetic agonist reverses MASH in LXRα mutant mice suggesting that LXRα normally functions to impede the development of liver disease.

The Liver X Receptor Promotes Immune Homeostasis via Controlled Activation of the Innate Immune System in the Liver

The liver is an indispensable metabolic organ, responsible for accumulating and transporting various nutritional compounds in hepatocytes. However, the transport of these materials from the liver is an energetically intensive task because they contain a considerable number of hydrophobic components, including free cholesterol, and require specialized transfer proteins to shuttle these substances through an aqueous phase. Liver X receptors (LXRs) induce the expression of cholesterol transporters in macrophages to transport free cholesterol derived from apoptotic cells into extracellular space via high-density lipoproteins. Additionally, LXRs control innate immune cells through two major mechanisms: upregulating the phagocytic activity of macrophages and suppressing inflammatory reactions to prevent aggressive activation of immune cells. Therefore, the primary role of LXRs is to accelerate efferocytosis without provoking inflammation and facilitate the transfer of free cholesterol from the intracellular space. This mechanism makes the innate immune system a substantial contributor to systemic metabolic control. Concomitantly, LXRs are important factors in regulating systemic defense mechanisms through the efficient regulation of immune cells. LXR activation, therefore, has great potential for clinical applications in the treatment of metabolic, infectious, and autoimmune diseases. In this review, we discuss the current understanding of the link between LXRs and innate immune cells in the liver, along with prospects for clinical applications of LXR agonists.

Nonlipogenic ABCA1 Inducers (NLAI) for Alzheimer's Disease Validated in a Mouse Model Expressing Human APOE3/APOE4

Therapeutics enhancing apolipoprotein (APOE) positive function are a priority, because APOE4 is the major genetic risk factor for Alzheimer's disease (AD). The function of APOE, the key constituent of lipoprotein particles that transport cholesterol and lipids in the brain, is dependent on lipidation by ABCA1, a cell-membrane cholesterol transporter. ABCA1 transcription is regulated by liver X receptors (LXR): agonists have been shown to increase ABCA1, often accompanied by unwanted lipogenesis and elevated triglycerides (TG). Therefore, nonlipogenic ABCA1-inducers (NLAI) are needed. Two rounds of optimization of an HTS hit, derived from a phenotypic screen, gave lead compound 39 that was validated and tested in E3/4FAD mice that express human APOE3/4 and five mutant APP and PSEN1 human transgenes. Treatment with 39 increased ABCA1 expression, enhanced APOE lipidation, and reversed multiple AD phenotypes, without increasing TG. This NLAI/LXR-agonist study is the first in a human APOE-expressing model with hallmark amyloid-β pathology.

Antibody drug conjugates beyond cytotoxic payloads

For many years, antibody drug conjugates (ADC) have teased with the promise of targeted payload delivery to diseased cells, embracing the targeting of the antibody to which a cytotoxic payload is conjugated. During the past decade this promise has started to be realised with the approval of more than a dozen ADCs for the treatment of various cancers. Of these ADCs, brentuximab vedotin really laid the foundations of a template for a successful ADC with lysosomal payload release from a cleavable dipeptide linker, measured DAR by conjugation to the Cys-Cys interchain bonds of the antibody and a cytotoxic payload. Using this ADC design model oncology has now expanded their repertoire of payloads to include non-cytotoxic compounds. These new payload classes have their origins in prior medicinal chemistry programmes aiming to design selective oral small molecule drugs. While this may not have been achieved, the resulting compounds provide excellent starting points for ADC programmes with some compounds amenable to immediate linker attachment while for others extensive SAR and structural information offer invaluable design insights. Many of these new oncology payload classes are of interest to other therapeutic areas facilitating rapid access to drug-linkers for exploration as non-oncology ADCs. Other therapeutic areas have also pursued unique payload classes with glucocorticoid receptor modulators (GRM) being the most clinically advanced in immunology. Here, ADC payloads come full circle, as oncology is now investigating GRM payloads for the treatment of cancer. This chapter aims to cover all these new ADC approaches while describing the medicinal chemistry origins of the new non-cytotoxic payloads.

A structural perspective of liver X receptors

Liver X receptors α and β are members of the nuclear receptor family, which comprise a flexible N-terminal domain, a DNA binding domain, a hinge linker, and a ligand binding domain. Liver X receptors are important regulators of cholesterol and lipid homeostasis by controlling the transcription of numerous genes. Key to their transcriptional role is synergetic interaction among the domains. DNA binding domain binds on DNA; ligand binding domain is a crucial switch to control the transcription activity through conformational change caused by ligand binding. The Liver X receptors form heterodimers with retinoid X receptor and then the liganded heterodimer may recruit other necessary transcription components to form an active transcription complex.

In silico molecular modeling and simulations of black tea theaflavins revealed theaflavin-3'-gallate as putative liver X receptor-beta agonist

The low constitutive activation of Liver X receptor, an endogenous nuclear receptor with two subtypes (α and β), is a condition lying at the crossroad of cancer and cardiovascular disease. Both natural and synthetic Liver X receptor agonists have reportedly shown remarkable antiproliferative and atheroprotective effects but the repeated doses of its synthetic ones are also paradoxically associated with hyperlipidaemic effects and neurotoxicity, though attributed to the alpha subtype. This highlights the need for novel, safe, and potent LXR-beta-selective agonists. Hypocholesterolaemic effects of black theaflavins have been widely reported, but data on the exact theaflavin compound (s) responsible for these effects is currently lacking. Neither is information on the possible modulatory effects of the compound (s) on LXR-beta nor its possible implications in the context of drug development for cardiovascular diseases and cancers is explored. On this account, we investigated the potential interaction of four main theaflavin monomers (TF1, TF2A, TF2B & TF3) with human LXR-beta through robust computational modelling that entails molecular docking, free energy calculations and molecular dynamics simulations. The ligands were further profiled (in silico) for absorption, distribution, metabolism, excretion, and toxicological properties. Our result revealed theaflavin TF2B as a putative LXR-beta agonist, possibly responsible for the widely observed hypocholesterolaemic effect in black tea. This finding, while encouraging, needs to be experimentally verified in wet studies.Communicated by Ramaswamy H. Sarma.

Liver X Receptor: a potential target in the treatment of atherosclerosis

INTRODUCTION

Liver X receptors (LXRs) are master regulators of atherogenesis. Their anti-atherogenic potential has been attributed to their role in the inhibition of macrophage-mediated inflammation and promotion of reverse cholesterol transport. Owing to the significance of their anti-atherogenic potential, it is essential to develop and test new generation LXR agonists, both synthetic and natural, to identify potential LXR-targeted therapeutics for the future.

AREAS COVERED

This review describes the role of LXRs in atherosclerotic development, provides a summary of LXR agonists and future directions for atherosclerosis research. We searched PubMed, Scopus and Google Scholar for relevant reports, from last 10 years, using atherosclerosis, liver X receptor, and LXR agonist as keywords.

EXPERT OPINION

LXRα has gained widespread recognition as a regulator of cholesterol homeostasis and expression of inflammatory genes. Further research using models of cell type-specific knockout and specific agonist-targeted LXR isoforms is warranted. Enthusiasm for therapeutic value of LXR agonists has been tempered due to LXRα-mediated induction of hepatic lipogenesis. LXRα agonism and LXRβ targeting, gut-specific inverse LXR agonists, investigations combining LXR agonists with other lipogenesis mitigating agents, like IDOL antagonists and synthetic HDL, and targeting ABCA1, M2 macrophages and LXRα phosphorylation, remain as promising possibilities.

PROTACs: great opportunities for academia and industry (an update from 2020 to 2021)

PROteolysis TArgeting Chimeras (PROTACs) technology is a new protein-degradation strategy that has emerged in recent years. It uses bifunctional small molecules to induce the ubiquitination and degradation of target proteins through the ubiquitin-proteasome system. PROTACs can not only be used as potential clinical treatments for diseases such as cancer, immune disorders, viral infections, and neurodegenerative diseases, but also provide unique chemical knockdown tools for biological research in a catalytic, reversible, and rapid manner. In 2019, our group published a review article "PROTACs: great opportunities for academia and industry" in the journal, summarizing the representative compounds of PROTACs reported before the end of 2019. In the past 2 years, the entire field of protein degradation has experienced rapid development, including not only a large increase in the number of research papers on protein-degradation technology but also a rapid increase in the number of small-molecule degraders that have entered the clinical and will enter the clinical stage. In addition to PROTAC and molecular glue technology, other new degradation technologies are also developing rapidly. In this article, we mainly summarize and review the representative PROTACs of related targets published in 2020-2021 to present to researchers the exciting developments in the field of protein degradation. The problems that need to be solved in this field will also be briefly introduced.

Remembering your A, B, C's: Alzheimer's disease and ABCA1

The function of ATP binding cassette protein A1 (ABCA1) is central to cholesterol mobilization. Reduced ABCA1 expression or activity is implicated in Alzheimer's disease (AD) and other disorders. Therapeutic approaches to boost ABCA1 activity have yet to be translated successfully to the clinic. The risk factors for AD development and progression, including comorbid disorders such as type 2 diabetes and cardiovascular disease, highlight the intersection of cholesterol transport and inflammation. Upregulation of ABCA1 can positively impact APOE lipidation, insulin sensitivity, peripheral vascular and blood–brain barrier integrity, and anti-inflammatory signaling. Various strategies towards ABCA1-boosting compounds have been described, with a bias toward nuclear hormone receptor (NHR) agonists. These agonists display beneficial preclinical effects; however, important side effects have limited development. In particular, ligands that bind liver X receptor (LXR), the primary NHR that controls ABCA1 expression, have shown positive effects in AD mouse models; however, lipogenesis and unwanted increases in triglyceride production are often observed. The longstanding approach, focusing on LXRβ vs. LXRα selectivity, is over-simplistic and has failed. Novel approaches such as phenotypic screening may lead to small molecule NHR modulators that elevate ABCA1 function without inducing lipogenesis and are clinically translatable.

Development of Agonist-Based PROTACs Targeting Liver X Receptor

Liver X receptors (LXRs) belong to the nuclear hormone receptor superfamily and function as ligand-dependent transcription factors that regulate cholesterol homeostasis, lipid homeostasis, and immune responses. LXR antagonists are promising treatments for hypercholesterolemia and diabetes. However, effective LXR antagonists and inhibitors are yet to be developed. Thus, we aimed to develop LXR degraders (proteolysis targeting chimeras PROTACs against LXR) as a complementary strategy to provide a similar effect to LXR inhibition. In this study, we report the development of GW3965-PEG5-VH032 (3), a PROTAC capable of effectively degrading LXRβ protein. Compound 3 induced the ubiquitin-proteasome system-dependent degradation of the LXRβ protein, which requires VHL E3 ligase. We hope that PROTACs targeting LXR proteins will become novel therapeutic agents for LXR-related diseases.

Liver X receptors and skeleton: Current state-of-knowledge

The liver X receptors (LXR) is a nuclear receptor that acts as a prominent regulator of lipid homeostasis and inflammatory response. Its therapeutic effectiveness against various diseases like Alzheimer's disease and atherosclerosis has been investigated in detail. Emerging pieces of evidence now reveal that LXR is also a crucial modulator of bone remodeling. However, the molecular mechanisms underlying the pharmacological actions of LXR on the skeleton and its role in osteoporosis are poorly understood. Therefore, in the current review, we highlight LXR and its actions through different molecular pathways modulating skeletal homeostasis. The studies described in this review propound that LXR in association with estrogen, PTH, PPARγ, RXR hedgehog, and canonical Wnt signaling regulates osteoclastogenesis and bone resorption. It regulates RANKL-induced expression of c-Fos, NFATc1, and NF-κB involved in osteoclast differentiation. Additionally, several studies suggest suppression of RANKL-induced osteoclast differentiation by synthetic LXR ligands. Given the significance of modulation of LXR in various physiological and pathological settings, our findings indicate that therapeutic targeting of LXR might potentially prevent or treat osteoporosis and improve bone quality.

Synergetic Effect of rHDL and LXR Agonist on Reduction of Atherosclerosis in Mice

High-density lipoproteins (HDLs) are unique in that they play an important role in the reverse cholesterol transport process. However, reconstituted HDL (rHDL) infusions have demonstrated limited beneficial effect in clinical practice. This is perhaps a consequence of the limited cholesterol efflux abilities of atheroma macrophages due to decreased expression of cholesterol transporters in advanced atheromas and following rHDL infusion treatment. Thus, we propose that a combination therapy of rHDL and a liver X receptor (LXR) agonist could maximize the therapeutic benefit of rHDL by upregulating ATP-binding cassette transporters A-1 (ABCA1) and ATP-binding cassette transporter G-1 (ABCG1), and enhancing cholesterol efflux to rHDL. In macrophages, rHDL downregulated the expression of ABCA1/G1 in a dose- and rHDL composition-dependent manner. Although LXR agonist, T0901317 (T1317), upregulated the expression of ABCA1 and ABCG1, the drug itself did not have any effect on cholesterol efflux (6.6 ± 0.5%) while the combination of rHDL and T1317 exhibited enhanced cholesterol efflux from [³H]-cholesterol loaded J774A.1 macrophages (23.3 ± 1.3%). Treatment with rHDL + T1317 significantly reduced the area of aortic plaque in ApoE^-/- mice compared to PBS treated control animals (24.16 ± 1.42% vs. 31.59 ± 1.93%, p < 0.001), while neither rHDL nor T1317 treatment alone had a significant effect. Together, we show that rHDL paired with an LXR agonist can induce a synergetic effect in reducing atheroma burden. This synergy could lead to lower overall effective dose for both drugs, potentially overcoming the existing barriers in clinical development and renewing pharmaceutical interest in these two drug classes.

Discovery of new LXRβ agonists as glioblastoma inhibitors

Discovery and optimization of selective liver X receptor β (LXRβ) agonists are challenging due to the high homology of LXRα and LXRβ in the ligand-binding domain. There is only one different residue (Val versus Ile) at the ligand-binding pocket of LXRs. With machine learning methods, we identified pan LXR agonists with a novel scaffold (spiro[pyrrolidine-3,3'-oxindole]). Then, we figured out the mechanism of LXR isoform selectivity from co-crystal structures. Based on the mechanism and the new scaffold, LXRβ selective agonists were designed and synthesized. This led to the discovery of LXRβ agonists 4-7rr, 4-13 and 4-13rr with IC₅₀ values ranging from 1.78 to 6.36 μM against glioblastoma in vitro. Treatment with 50 mg/kg/day of 4-13 for 15 days significantly reduced tumor growth using an in vivo xenograft glioblastoma model.

Synthesis and In Vitro Evaluation of Novel Liver X Receptor Agonists Based on Naphthoquinone Derivatives

We aimed to synthesize novel liver X receptor (LXR) agonists with potent agonist activity and subtype selectivity. Our synthetic scheme started with naphthoquinone derivatives, such as menadione and 2,3-dichloro-1,4-naphthoquinone. We introduced different substituents into the naphthoquinone structures, including aniline, piperidine, pyrrolidine, and morpholine, in one or two steps, and thus, we produced 14 target compounds. All 14 synthetic ligands were tested to determine whether they mediated LXR-mediated transcriptional activity. We investigated the transcriptional activity of each compound with two types of receptors, LXRα and LXRβ. Among all 14 compounds, two showed weak LXRβ-agonist activity, and two others exhibited potent LXRα-agonist activity. We also performed docking studies to obtain a better understanding of the modes of compound binding to LXR at the atomic level. In conclusion, we successfully synthesized naphthoquinone derivatives that act as LXRα/β agonists and selective LXRα agonists.

Structural analysis identifies an escape route from the adverse lipogenic effects of liver X receptor ligands

Liver X receptors (LXRs) are attractive drug targets for cardiovascular disease treatment due to their role in regulating cholesterol homeostasis and immunity. The anti-atherogenic properties of LXRs have prompted development of synthetic ligands, but these cause major adverse effects-such as increased lipogenesis-which are challenging to dissect from their beneficial activities. Here we show that LXR compounds displaying diverse functional responses in animal models induce distinct receptor conformations. Combination of hydrogen/deuterium exchange mass spectrometry and multivariate analysis allowed identification of LXR regions differentially correlating with anti-atherogenic and lipogenic activities of ligands. We show that lipogenic compounds stabilize active states of LXRα and LXRβ while the anti-atherogenic expression of the cholesterol transporter ABCA1 is associated with the ligand-induced stabilization of LXRα helix 3. Our data indicates that avoiding ligand interaction with the activation helix 12 while engaging helix 3 may provide directions for development of ligands with improved therapeutic profiles.

Multiparametric rapid screening of neuronal process pathology for drug target identification in HSP patient-specific neurons

Axonal degeneration is a key pathology of neurodegenerative diseases, including hereditary spastic paraplegia (HSP), a disorder characterized by spasticity in the lower limbs. Treatments for HSP and other neurodegenerative diseases are mainly symptomatic. While iPSC-derived neurons are valuable for drug discovery and target identification, these applications require robust differentiation paradigms and rapid phenotypic read-outs ranging between hours and a few days. Using spastic paraplegia type 4 (SPG4, the most frequent HSP subtype) as an exemplar, we here present three rapid phenotypic assays for uncovering neuronal process pathologies in iPSC-derived glutamatergic cortical neurons. Specifically, these assays detected a 51% reduction in neurite outgrowth and a 60% increase in growth cone area already 24 hours after plating; axonal swellings, a hallmark of HSP pathology, was discernible after only 5 days. Remarkably, the identified phenotypes were neuron subtype-specific and not detectable in SPG4-derived GABAergic forebrain neurons. We transferred all three phenotypic assays to a 96-well setup, applied small molecules and found that a liver X receptor (LXR) agonist rescued all three phenotypes in HSP neurons, providing a potential drug target for HSP treatment. We expect this multiparametric and rapid phenotyping approach to accelerate development of therapeutic compounds for HSP and other neurodegenerative diseases.

Common and Differential Transcriptional Actions of Nuclear Receptors Liver X Receptors α and β in Macrophages

The liver X receptors α and β (LXRα and LXRβ) are oxysterol-activated transcription factors that coordinately regulate gene expression that is important for cholesterol and fatty acid metabolism. In addition to their roles in lipid metabolism, LXRs participate in the transcriptional regulation of macrophage activation and are considered potent regulators of inflammation. LXRs are highly similar, and despite notable exceptions, most of their reported functions are substantially overlapping. However, their individual genomic distribution and transcriptional capacities have not been characterized. Here, we report a macrophage cellular model expressing equivalent levels of tagged LXRs. Analysis of data from chromatin immunoprecipitation coupled with deep sequencing revealed that LXRα and LXRβ occupy both overlapping and exclusive genomic regulatory sites of target genes and also control the transcription of a receptor-exclusive set of genes. Analysis of genomic H3K27 acetylation and mRNA transcriptional changes in response to synthetic agonist or antagonist treatments revealed a putative mode of pharmacologically independent regulation of transcription. Integration of microarray and sequencing data enabled the description of three possible mechanisms of LXR transcriptional activation. Together, these results contribute to our understanding of the common and differential genomic actions of LXRs and their impact on biological processes in macrophages.

The challenges and promise of targeting the Liver X Receptors for treatment of inflammatory disease

The Liver X Receptors (LXRs) are oxysterol-activated transcription factors that upregulate a suite of genes that together promote coordinated mobilization of excess cholesterol from cells and from the body. The LXRs, like other nuclear receptors, are anti-inflammatory, inhibiting signal-dependent induction of pro-inflammatory genes by nuclear factor-κB, activating protein-1, and other transcription factors. Synthetic LXR agonists have been shown to ameliorate atherosclerosis and a wide range of inflammatory disorders in preclinical animal models. Although this has suggested potential for application to human disease, systemic LXR activation is complicated by hepatic steatosis and hypertriglyceridemia, consequences of lipogenic gene induction in the liver by LXRα. The past several years have seen the development of multiple advanced LXR therapeutics aiming to avoid hepatic lipogenesis, including LXRβ-selective agonists, tissue-selective agonists, and transrepression-selective agonists. Although several synthetic LXR agonists have made it to phase I clinical trials, none have progressed due to unforeseen adverse reactions or undisclosed reasons. Nonetheless, several sophisticated pharmacologic strategies, including structure-guided drug design, cell-specific drug targeting, as well as non-systemic drug routes have been initiated and remain to be comprehensively explored. In addition, recent studies have identified potential utility for targeting the LXRs during therapy with other agents, such as glucocorticoids and rexinoids. Despite the pitfalls encountered to date in translation of LXR agonists to human disease, it appears likely that this accelerating field will ultimately yield effective and safe applications for LXR targeting in humans.

Liver X receptors link lipid metabolism and inflammation

The response of immune cells to pathogens is often associated with changes in the flux through basic metabolic pathways. Indeed, in many cases changes in metabolism appear to be necessary for a robust immune response. The Liver X receptors (LXRs) are members of the nuclear hormone receptor superfamily that regulate gene networks controlling cholesterol and lipid metabolism. In immune cells, particularly in macrophages, LXRs also inhibit proinflammatory gene expression. This Review will highlight recent studies that connect LXR-dependent control of lipid metabolism to regulation of the immune response.