Nuclear receptors in health and disease: signaling pathways, biological functions and pharmaceutical interventions

The known sequence of the human genome revealed that forty-eight human NRs detect ligands, such as hormones, and translocate them into the nucleus to function as transcription factors regulating gene expression.⁵⁹ These receptors are classified into three main types on the basis of their ligand types or into eight subgroups on the basis of sequence homology.^11,60 Type I receptors are steroid receptors, including the ER, AR, progesterone receptor (PR), mineralocorticoid receptor (MR) and GR.^61,62 Their ligands are steroid hormones such as sex hormones, glucocorticoids and mineralocorticoids.⁶³ Type II receptors are nonsteroid receptors, such as thyroid hormones (TRα and TRβ), retinoic acid receptors (RARα, β), vitamin D receptors (VDRs) and peroxisome proliferator-activated receptors (PPARα, β and γ).⁶⁴ Type III receptors include orphan receptors whose endogenous ligands are unknown or that do not bind to any ligands, such as the testicular receptor and germ cell nuclear factor.⁶⁵ Alternatively, NRs can be divided into eight subgroups that are similar to classical receptors but differ enough individually, named thyroid hormone receptor-like family, retinoid X receptor-like family, estrogen receptor like family, nerve growth factor IB-like family, steroidogenic factor-like family, germ cell nuclear factor-like family, and those with two DNA-binding domains, as well as recently identified typically structured NR (CgNR8A1) in metazoans such as mollusks, annelids, cnidarians, echinoderms and hemichordates.^66,67

The NTD, which is located at the N-terminus and comprises 25–603 amino acids (AAs), contains the first of two transactivation regions (AF-1) and possesses transcriptional activator functions.^69,70 The A/B domain is intrinsically disordered, which makes obtaining full-length 3D representations of the receptor difficult.^71,72 The second identified subunit is the DBD, with 66--68 AAs, including two zinc fingers, which are typically obscured because of lower DNA affinity when the hormone is absent.^73–75 Once a hormone is bound, the zinc fingers dock the hormone–receptor complex to hexanucleotide response elements located within NR-regulated promoters that determine the receptor’s regulatory specificity. The DBD also serves as an intermolecular interaction site for receptor dimerization.⁷⁶ The LBD is located at the receptor’s C-terminus and contains 220--250 AAs.⁷⁷ The LBD binds to the cognate hormone or ligand through an interior binding pocket.^78,79 It also has an AF-2 site for recruiting various coactivating proteins, such as heat shock proteins (HSPs), to facilitate interactions with chromatin-remodeling proteins and the transactivation domain.^80–82 The LBD plays a crucial role in mediating self-assembly reactions, such as dimerization or tetramerization, which are essential for high-affinity binding to DNA response elements.⁸³ The DBD is connected to the LBD via a flexible domain, the ‘hinge’, which is related primarily to the nuclear localization signal of NRs and may influence the intracellular trafficking and subcellular distribution of the hormone‒receptor complex.^84,85 The hinge region also contains a short NLS, and its phosphorylation is coupled to elevated transactivation.^86,87

However, not all NRs have all four typical domains. Members of the NR0 subfamily, for example, frequently have neither a DBD nor an LBD in their structures.⁸⁸ Among these, NRs with only the LBD and no DBD, such as Dax-1 and SHP, are nonetheless capable of binding to other transcription factors (TFs) and suppressing the expression of genes downstream. Finally, some NRs also possess a short, highly variable C-terminal domain (CTD or F domain) that often has unknown functions.^89,90 However, current research has revealed a critical function of the F domain in the ERα reaction to select ER modulators (SERMs), particularly in SERM-mediated LBD dimerization and AF-1 activity.⁹¹

NRs were thought to have evolved approximately 600 million years ago in the common ancestor of bilaterian animals, enabling them to sense and respond to internal and external signals.⁹² This evolution likely involved genetic amplification and mutation, contributing to the functional diversity of the NR superfamily.⁹³ Most NR ligands are small, hydrophobic molecules that pass through cell membranes, although some, such as thyroid hormones, are actively transported into cells.⁹⁴ While NRs were initially termed ligand-binding proteins, it remains debated whether their common ancestor bound ligands or functioned as constitutively active receptors. Over 50% of NRs are orphan receptors lacking known ligands, although some have been shown to bind metabolites, suggesting a role in sensing metabolic changes.^95,96 The evolution of the LBD enabled interactions with small molecules and coregulators, increasing NR versatility.⁹⁷ Gene duplication during early metazoan evolution expanded and specialized the NR superfamily into subfamilies such as steroid hormone receptors, thyroid hormone receptors, and orphan receptors. The coevolution of NRs and their ligands increases their physiological complexity. For example, vertebrate NRs, such as glucocorticoid and estrogen receptors, have evolved high specificity for steroid hormones, which are crucial for the stress response and reproduction.⁹⁸

In 1992, a comparison of the DNA-binding domains of known NRs led to the construction of a phylogenetic tree, suggesting a common ancestor.⁹⁹ By 1997, an alternative hypothesis emerged, suggesting that the original NR was an orphan receptor that gradually gained ligand-binding ability.¹⁰⁰ The earliest NRs are believed to have evolved as orphan receptors that lack defined ligands and are involved in regulating fundamental processes such as metabolism and cellular signaling.¹⁰¹ Over time, orphan receptors adapt to regulate broader metabolic pathways, including lipid homeostasis and detoxification. A recent discovery of the NR7 subfamily has filled a critical gap in the understanding of the evolution of NR dimerization capabilities. Phylogenetically situated between class I NRs (monomers or homodimers) and class II NRs (heterodimers), the NR7 subfamily acts as a “missing link” in the transition from homodimerizing to heterodimerizing nuclear receptors.¹⁰²

Nuclear receptors can modulate gene expression through direct binding to target genes as well as through interactions with other NRs.^103,104 The interplay between NR actions can be described at progressively more detailed levels, with various regulatory modes involved in upstream events that influence downstream gene transcription and crosstalk events at the DNA level that directly influence gene transcription.¹⁰⁵ The existence or lack of physical connections between the relevant NRs could further alter the crosstalk mechanisms at the DNA level.

Nuclear receptors are of key importance in physiology and pathology because they regulate multiple cellular functions (Fig. 5). For example, NHR-6, the only NR4A-type nuclear receptor in the roundworm Caenorhabditis elegans, has been implicated in the development of the nervous system.²²⁶ During development, the nervous system produces highly specialized neurons with distinct functional characteristics. A prime example is the C. elegans BAG chemosensory neurons.²²⁷ Under certain circumstances, BAG neurons sense the concentration of carbon dioxide in respiratory gas and translate it into reactions of attraction or repulsion; ETS-5 and its target gene NHR-6 are essential for this process.²²⁸ In contrast to ETS-5, which is engaged in both the attraction and avoidance of carbon dioxide, NHR-6 is an NR that is exclusively involved in the attraction process.²²⁹ These findings imply that gene regulatory mechanisms supported by NR subtypes have undergone evolutionary conservation. NHR-6 is necessary for BAGs to exhibit exceptional capacity to adaptively assign positive or negative valences to chemosensory stimuli, and it is a key modulator of the functional plasticity of the neural system.²²⁶ It is clear that NRs are of the greatest importance for the proper functioning of numerous processes in cellular lipid metabolism,^230,231 energy homeostasis²³² and the inflammatory response²³³; cell proliferation and differentiation are critically dependent on nuclear receptors.

Pathological changes in nuclear receptors, such as mutations, altered expression levels, or dysregulated activity, are linked to a wide range of diseases, including cancer, metabolic disorders, inflammation, and cardiovascular conditions. For example, the overexpression of ER in breast cancer drives tumor growth,²³⁴ whereas aberrant AR can cause therapy resistance in prostate cancer.²³⁵ Impaired PPAR function is associated with metabolic disorders such as obesity and type 2 diabetes,²³⁶ whereas dysregulated liver X receptors (LXRs) disrupt cholesterol homeostasis, exacerbating cardiovascular disease.²³⁷ Aberrant GR activity exacerbates inflammatory conditions and may induce corticosteroid resistance. Altered retinoid X receptor (RXR) function has been linked to neurodegenerative diseases such as Alzheimer’s disease. These pathological changes not only hold diagnostic and prognostic value but also serve as therapeutic targets. For example, ER, PR, and HER2 expression levels guide breast cancer classification and treatment, with SERMs such as tamoxifen proving effective in ER-positive patients. AR inhibitors are crucial in prostate cancer, where AR expression levels and mutations influence therapeutic outcomes. Additionally, radiolabeled ligands such as [F-18]-estradiol enable imaging of ER-positive tumors via PET scans,²³⁸ aiding in diagnosis and treatment monitoring. In summary, nuclear receptor alterations play a dual role in clinical practice as both therapeutic targets and biomarkers, underscoring their critical contribution to precision medicine and improved patient outcomes.

The expression of nuclear receptors is not only related to tissue, sex, and disease but also varies across different age groups within the same individual.³³⁶ These variations reflect developmental needs, metabolic changes, hormonal shifts, and adaptations to aging-related physiological demands. Generally, developmental NRs such as RARs, PPARs and RXRs are highly expressed during embryogenesis and early development to support rapid growth, differentiation, and development.³³⁷ During adolescence, ARs, ERα and ERβ are elevated to correlate with puberty and the onset of secondary sexual characteristics, whereas GRs act to moderate stress responses and energy metabolism during periods of rapid physical and emotional development. PPARs, LXRs, and FXRs live in adult individuals to maintain lipid and glucose homeostasis, supporting metabolic stability during peak years of physical activity and reproductive capability. Diet, exercise, and stress significantly modulate NR expression, affecting metabolic and endocrine balance during this period. The expression of these NRs decreases with increasing age, which causes impaired lipid metabolism and insulin sensitivity, cholesterol and bile acid metabolism, and calcium homeostasis and immune function, leading to an increased risk of metabolic syndrome, compromised organ function and chronic inflammation. For example, Nr4a1 is reduced in peripheral blood mononuclear cells and CA1 pyramidal neurons with aging, which impairs cognition and excitatory synaptic function.³³⁸ Monitoring age-specific NR expression patterns could guide personalized therapies to optimize metabolic health and prevent age-related diseases.

The metabolic cycling process of NRs, including their synthesis, activation, degradation, and recycling, is integral to maintaining homeostasis and ensuring that NRs can adapt to changing physiological demands. Since the activation and transcriptional regulation of nuclear receptors have been discussed above, we mainly emphasize their degradation and recycling. The degradation of nuclear receptors is a highly regulated process that involves proteasomal and lysosomal pathways, ensuring the dynamic control of receptor levels in response to physiological and environmental cues. Ligand binding can either stabilize or destabilize NRs by inducing conformational changes that influence ubiquitination.^339,340 In addition, posttranslational modifications (PTMs), such as phosphorylation,³⁴¹ SUMOylation³⁴² and acetylation,³⁴³ modify NR degradation rates by altering their interaction with ubiquitin ligases or stabilizing proteins. Moreover, many NRs, such as REV-ERBs and PPARs, exhibit circadian cycling, with their expression and activity oscillating in a 24-hour rhythm to align with metabolic needs.^344,345 These rhythms are coregulated by core circadian clock genes (e.g., CLOCK and BMAL1), which synchronize metabolic functions with environmental cycles such as light and feeding. Altogether, cycling of NRs ensures appropriate receptor levels, preventing excessive or prolonged signaling that could lead to cellular dysfunction.

With aging, the efficiency of key regulatory pathways declines, leading to altered NR turnover, disrupted signaling, and increased susceptibility to age-related diseases.³⁴⁶ Proteasomal dysfunction, a hallmark of aging, impairs the degradation of damaged or misfolded proteins, including NRs. This results in the accumulation of inactive or malfunctioning receptors, which disrupt signaling and contribute to metabolic and endocrine dysregulation, key features of aging. These disruptions play a central role in the development of diseases such as obesity, diabetes, cardiovascular conditions, and neurodegenerative disorders.^347,348 Additionally, aging reduces autophagic flux, hindering the clearance of damaged proteins, including aggregated NRs. As autophagy becomes less efficient with age, NR aggregates persist, disrupting receptor signaling and promoting chronic inflammation, mitochondrial dysfunction, and cellular stress.^349,350 These factors are implicated in the progression of age-related diseases like Alzheimer’s, diabetes, and frailty. Oxidative stress also contributes to aging-related cellular damage by directly modifying NRs. ROS cause conformational changes in NRs, preventing their proper ubiquitination and degradation. This oxidation makes NRs resistant to degradation, leading to the accumulation of damaged receptors and further dysfunction in signaling pathways.³⁵¹ Understanding these age-related changes offers valuable insights for developing targeted therapies to restore NR homeostasis and mitigate aging-associated disorders.

NRs, especially those that bind to three steroid hormones, namely, estrogen, progesterone and androgen, play a significant role in the development of human malignancies, particularly breast cancer,^355,356 ovarian cancer³⁵⁷ and prostate cancer.³⁵⁸ In recent years, NR-targeted drugs have attracted widespread attention as novel therapeutic strategies^359–361 that play a significant role in inhibiting the growth and progression of many types of cancer.

Within the nuclear receptor superfamily, ERs were the first nuclear receptors to be described. Breast cancer is the most common cancer in women, with ER⁺ breast cancer accounting for approximately 75% of all cases. Numerous studies have confirmed that ERα plays a causative role in carcinogenesis. As a result, an increasing number of ERα-based targeted medicines have entered preclinical and clinical trials (Table 1), and some have even received FDA approval. This includes directly antagonizing the estrogen receptor via SERMs and selective estrogen receptor degraders (SERDs). These drugs include ER antagonists, selective ER modulators and aromatase inhibitors.³⁶² ER antagonists competitively inhibit the ER, whereas selective ER modulators function as partial agonists or antagonists on the basis of their tissue affinity. Aromatase inhibitors decrease estrogen production by blocking aromatase in tissues such as the ovaries and adipose tissue.^363,364 Tamoxifen is the most widely used antiestrogen for hormone-dependent breast cancer and is indicated for various treatment settings. Evidence shows that patients with estrogen receptor-positive tumors are more likely to benefit from tamoxifen. FDA-approved treatments include treating breast cancer in both females and males, providing adjuvant therapy after surgery and radiation, treating female patients with ductal carcinoma in situ (DCIS) postsurgery and radiation to lower the risk of invasive breast cancer and reduce the risk of breast cancer in certain high-risk patients.^365,366 In addition to tamoxifen, many other SERMs, including raloxifene, lasofoxifene, bazedoxifene and fulvestrant, have been developed and used for breast cancer treatment.³⁶³ These drugs have also been used to treat other tumors with high estrogen expression. In addition to hormone-dependent breast cancer, antiestrogen medications such as tamoxifen and aromatase inhibitors have shown efficacy in managing conditions such as endometrial cancer and certain types of ovarian cancer.^367,368 Although estrogen levels are low in prostate tissue, ER expression has been detected in some prostate cancer patients.³⁶⁹ ER activation is believed to be associated with the proliferation and metastasis of prostate cancer cells.³⁷⁰ Thus, antiestrogen therapy targeting the ER has also been proposed as a new treatment strategy for prostate cancer. For example, in males with advanced prostate cancer, an ERα agonist, GTx-758, has been shown to lower testosterone with fewer side effects associated with androgen-deprivation therapy.³⁷¹

Bidedoxifene is the most recent generation of SERMs. Fanning and colleagues demonstrated that bazedoxifene works in concert with palbociclib, a CDK4/6b inhibitor, and is more effective than tamoxifen against specific ERα mutants, such as Y537S and D538G. A phase II clinical trial in patients with ductal carcinoma in situ [NCT02694809↗] and a phase I/II clinical trial on bazedoxifene in combination with palbociclib in hormone receptor-positive breast cancer [NCT02448771↗] are two examples of the numerous clinical trials that have examined bazedoxifene across a variety of cancer types. Degradation of this receptor by SERDs is another tactic to target ERs. Fulvestrant is the most researched SERD, and its clinical application has been growing. Clinical trials have combined fulvestrant with CDK4/6 inhibitors, such as ribociclib in men and postmenopausal women with advanced breast cancer [NCT02422615↗] and palbociclib in hormone receptor+HER2-metastasized breast cancer after endocrine failure [NCT01942135↗], as well as phosphatidylinositol 3-kinasitol kinase (PI3K)/AKT/mTOR pathway inhibitors, such as pictilisib, in advanced or metastatic breast cancer in participants resistant to aromatase inhibitor therapy [NCT01437566↗].

The abnormal expression of AR in prostate cancer cells makes anti-AR drugs a logical treatment strategy.³⁷² These drugs inhibit AR function through various mechanisms, reducing androgen stimulation of tumor cells. Among the most common anti-AR agents are AR antagonists such as abiraterone and enzalutamide. These drugs competitively bind to the AR, preventing androgen attachment and inhibiting receptor activity. Abiraterone has been shown to prolong survival in patients with metastatic castration-resistant prostate cancer (mCRPC).³⁷³ In the context of metastatic castration-sensitive prostate cancer (mCSPC), a combination treatment involving abiraterone, prednisone and docetaxel has favorable effects.³⁷⁴ Furthermore, for metastasis-free survival, combining enzalutamide with leuprolide has shown superior outcomes compared with leuprolide alone in high-risk patients with biochemical recurrence. Additionally, enzalutamide monotherapy has outperformed leuprolide monotherapy.³⁶ The prostate cancer community has recently focused much attention on a number of newly created AR proteolysis-targeting chimeras (PROTACs), which were created as SARDs. ARV-110 [NCT03888612↗], the first AR PROTAC to reach clinical trials, is now being used in patients with metastatic CRPC and is undergoing a phase I/II clinical trial. In contrast to clinically licensed AR antagonists, ARD-61, a novel AR PROTAC, has been shown to efficiently promote on-target AR degradation, elicit more potent antiproliferative and proapoptotic effects, and reduce downstream AR target gene expression in prostate cancer cells. The most effective AR PROTACs may eventually be utilized extensively in cancer patients, and we expect more advancements to the clinical trial stage.

Additionally, RARs and their heterodimer partners, such as PPARs and RXR, which are involved in vitamin A metabolism, cell differentiation and immune regulation, also have therapeutic potential.^375,376 Aberrant RXR signaling can promote cancer development by disrupting normal cellular homeostasis and enhancing oncogenic processes. RXR agonists have shown potential in treating various advanced cancers in clinical trials. IRX4204, in combination with erlotinib, is under investigation for previously treated advanced non-small cell lung cancer (NSCLC) [NCT02991651↗]. NRX194204 is being studied for its efficacy in castration- and taxane-resistant prostate cancer [NCT01540071↗] and advanced NSCLC [NCT00964132↗]. Additionally, bexarotene is being evaluated for the treatment of metastatic breast cancer and relapsed or refractory cutaneous T-cell lymphoma in patients with stage III or IV disease [NCT00003752↗, NCT00514293↗, NCT01134341↗, NCT00316030↗]. These studies underscore the therapeutic potential of RXR agonists in managing resistant and advanced malignancies. PPAR agonists have shown promising antitumor effects across a range of cancers, either as standalone treatments or in combination therapies. Troglitazone has shown efficacy in prostate cancer and liposarcoma [NCT00003058↗], whereas pioglitazone is being studied for advanced solid tumors [NCT02133625↗], pancreatic cancer [NCT01838317↗], and thyroid cancer [NCT01655719↗], often with chemotherapy. Inolitazone combined with paclitaxel is under investigation for advanced anaplastic thyroid cancer [NCT02152137↗]. Rosiglitazone is being explored for the treatment of liposarcoma [NCT00004180↗], metastatic thyroid cancer [NCT00098852↗], and androgen-dependent prostate cancer [NCT00182052↗]. These trials demonstrate the therapeutic potential of PPAR agonists in treating challenging malignancies. Retinoic acid, a natural RAR agonist, regulates the expression of genes related to cell growth. Studies suggest that RAR/RXR agonists may have antitumor effects on prostate cancer. For example, the RAR agonist adapalene has been shown to induce a tumor-suppressive senescence-associated secretory phenotype (SASP) in prostate cancer cells. In combination with docetaxel, adapalene enhanced tumor suppression and improved natural killer (NK) cell-mediated clearance in preclinical models. This approach also increased the efficacy of NK cell infusion in mice with human prostate cancer, leading to greater tumor suppression.³⁷⁷

REV-ERBs, including REV-ERBα (NR1D1) and REV-ERBβ (NR1D2), are key regulators of the circadian clock. Notably, many tumor types exhibit elevated expression of REV-ERBβ, regardless of ERBB2 or ER status, whereas nonmalignant mammary epithelial cells primarily express REV-ERBα. High levels of REV-ERBβ in cancer cells have been linked to reduced sensitivity to chloroquine via the modulation of autophagy, suggesting a role in therapy resistance. REV-ERB agonists, such as SR9009 and SR9011, selectively target cancer cells and oncogene-induced senescent cells, such as melanocytic naevi, without impacting normal cell viability. These agonists have shown promising anticancer effects, including reduced glioblastoma growth and improved survival in mice, without causing significant toxicity.³⁷⁸

Metabolic diseases such as nonalcoholic fatty liver disease (NAFLD), obesity, type 2 diabetes and atherosclerosis represent a growing global health burden.^379,380 These conditions are frequently associated with dysregulated lipid or glucose metabolism.^381,382 NRs play crucial roles in maintaining metabolic homeostasis by controlling the expression of genes involved in lipid, glucose and energy metabolism. Among NRs, PPARs, farnesoid X receptor (FXR), liver X receptor (LXR) and pregnane X receptor (PXR) have been identified as promising therapeutic targets. Drugs that modulate these receptors, such as PPAR agonists and FXR agonists, have demonstrated promising activity in clinical trials for the treatment of metabolic disorders, particularly NAFLD. Here, we explore current and emerging therapies that target these receptors.

One of the most well-known nuclear receptors is the PPAR family. PPAR agonists, such as pioglitazone and rosiglitazone, can increase insulin sensitivity in individuals with T2D. By activating PPARγ, these drugs enhance glucose uptake and lipid storage, which helps to lower blood sugar levels and improve metabolic control. However, these drugs are associated with certain risks, such as weight gain and fluid retention, necessitating careful patient management. PPARα agonists, such as fibrates, have been used clinically for 35 years to regulate blood lipids and prevent atherosclerosis.³⁸³ Pemafibrate was introduced in Japan in June 2018 and has since been demonstrated to be effective in inhibiting VLDL secretion and enhancing triglyceride (TG) clearance by activating LPL.³⁸⁴ In mice fed a high-fat diet, it suppressed postprandial hyperlipidemia by downregulating the mRNA expression of the intestinal cholesterol transporter NPC1L1 and upregulating genes involved in β-oxidation to inhibit VLDL secretion from the liver. In a 4-week trial with dyslipidemic patients (0.4 mg/day), pemafibrate significantly reduced fasting and nonfasting serum TG, RemL-C, and apo B-48 levels, with a marked reduction in the postprandial TG AUC, indicating improved postprandial hypertriglyceridemia.³⁸⁵ Similar results were observed in diabetic patients treated with pemafibrate.³⁸⁶ PPARγ and PPARδ regulate energy utilization and insulin sensitivity in skeletal muscle and adipose tissue. High insulin levels accelerate the development of NAFLD through a PPARγ-dependent pathway.³⁸⁷ PPARγ agonists, thiazolidinediones, have been used as insulin sensitizers in diabetic patients.³⁸⁸ PPARδ agonists improved the pathological manifestations of nonalcoholic steatohepatitis (NASH) without affecting body weight. The dual PPARα/δ agonist elafibranor (GFT505) improved glycemic and lipid metabolism in NASH patients in a phase II clinical trial, reduced liver inflammation and did not exacerbate fibrosis, demonstrating good tolerability. Phase III clinical trials targeting NASH patients [NCT02704403↗] are ongoing. A phase II clinical trial of the dual PPARα/γ agonist Saroglitazar [NCT03061721↗] in NASH patients was completed in April 2020. Midterm reports revealed not only regression of NASH tissue lesions but also decreased serum ALT levels, indicating improved liver function. The Indian pharmaceutical company Zydus Cadila has submitted a marketing application for this drug.

Another good NR drug target is FXR, which regulates bile acid metabolism and lipid homeostasis. FXR activation improves lipid metabolism and reduces liver inflammation, offering a new avenue for managing metabolic liver disorders.^389–391 Obeticholic acid, an FXR agonist also known as INT-747, is approved for treating NAFLD and NASH³⁹² [NCT02548351↗]. Owing to its ability to lower ALP levels and combat cholestasis and fibrosis, obeticholic acid (Ocaliva) was initially approved as a treatment for primary biliary cholangitis. In a phase III clinical trial for NASH [NCT02548351↗], obeticholic acid improved liver function and reduced tissue pathology, although it exhibited some side effects, such as pruritus and increased low-density lipoprotein levels. Given that pruritus can be managed symptomatically and that dyslipidemia can be addressed by the concomitant use of statins, the United States Food and Drug Administration has approved obeticholic acid as the primary treatment for NASH. Several other FXR agonists with diverse physicochemical characteristics, including bile acid derivatives, nonbile acid-derived steroidal FXR agonists, nonsteroidal FXR agonists and partial FXR agonists, are also progressing through advanced stages of clinical development. The noncarboxylic acid steroidal FXR agonist EDP-305 has been shown to modulate many pathways that trigger antilipogenic, anti-inflammatory and antifibrotic gene signatures. This resulted in hepatoprotective and antisteatotic effects in a mouse model of NASH.³⁹³ Patients with intestinal failure require more complicated care when they have parenteral nutrition (PN)-associated cholestasis (PNAC). By restoring hepatic FXR signaling, GW4064 protected animals against PNAC by increasing the expression of sterol and phospholipid transporters and canalicular bile and suppressing the recruitment and activation of macrophages. These findings suggest that increasing FXR activity could be used as a therapeutic approach to treat or prevent PNAC.³⁹⁴

In addition to PPAR and FXR, LXR is involved in cholesterol metabolism and can be targeted for atherosclerosis treatment. LXR agonists have been shown to increase cholesterol efflux and reduce the development of atherosclerotic plaques in preclinical models.³⁹⁵ However, treating patients with medications that increase cholesterol is not a practical option for metabolic illnesses; instead, methods for separating the advantageous effects of LXR ligands from unfavorable side effects must be investigated. Taken together, these examples highlight the growing potential of NR-targeted drugs in addressing the underlying causes of metabolic disorders, offering more precise and effective treatments than traditional approaches do.

The targeting of NRs for the treatment of cardiovascular diseases has gained significant attention because of the pivotal role these receptors play in regulating the expression of genes involved in metabolic, inflammatory and lipid processes.^{361,396–398} Nuclear receptors, such as PPARs, LXR and FXR, are key regulators of lipid metabolism, glucose homeostasis and inflammation, all of which are critical factors in the development of cardiovascular diseases. By modulating these receptors, it is possible to control pathological processes such as atherosclerosis, hypertension and other cardiovascular disorders.

The NRs RORα and REV-ERBα are key components of the circadian system and regulate biological rhythms, glucose metabolism, lipid metabolism, vascular inflammation and fibrinolysis.³⁹⁹ RORα is instrumental in endothelial progenitor cell (EPC) differentiation and promotes angiogenesis, tissue repair and vascular stability by inducing erythropoietin production and activating AMPK.⁴⁰⁰ RORα mediates the protective effects of melatonin, including its anti-inflammatory, antioxidant and antiapoptotic effects, making it a potential therapeutic target for atherosclerosis and endothelial dysfunction.^401,402 In hypertensive rats, melatonin altered clock gene expression patterns, maintaining high levels of REV-ERBα mRNA, which regulated blood pressure rhythms. This effect is linked to melatonin receptor activity and may involve posttranslational mechanisms.⁴⁰³ Increased RORα expression, which is stimulated by melatonin and the sympathetic nervous system, also protects against programmed hypertension. A number of small-molecule agonists for RORα and REV-ERBα, such as the pharmacological agents SR9009 and GSK4112 for REV-ERBα, have been developed⁴⁰⁴ and are available for use in cellular and animal experiments, but prospective studies on the impact of stable circadian rhythms on CHD and heart failure are still limited.^405,406 When considered collectively, the bulk of current research points to considerable clinical value and therapeutic potential for medications that target RORα and REV-ERBα to treat hypertension and atherosclerosis.

Several studies have highlighted the function of RXRs in cardiovascular disease. Treatment with 9cRA or the synthetic RXR agonist LGD1069 (bexarotene) has been shown to reduce oxidative stress-induced cell damage and preserve the mitochondrial membrane potential.⁴⁰⁷ In heart failure, oxidative stress activates angiotensin-II (AngII), leading to harmful cardiac remodeling and dysfunction. Bexarotene treatment of cultured rat aortic smooth muscle cells inhibited AngII-driven inflammation and p38 MAPK activation, suggesting that RXR activation has protective effects on cardiac disease.⁴⁰⁸ ERRα and ERRγ are essential for normal postnatal cardiovascular function, whereas ERRβ supports maximal ATP generation in contracting cardiomyocytes. ERR agonists help maintain oxidative metabolism, which protects the heart against pressure overload-induced heart failure (HF) in vivo. Using a structure-based design, Xu and colleagues synthesized two pan-ERR agonists, SLU-PP-332 and SLU-PP-915, each of which significantly improved the ejection fraction, reduced fibrosis and enhanced survival in models of pressure overload-induced HF without affecting cardiac hypertrophy.⁴⁰⁹ Targeting other nuclear receptors, such as RARs, PPARs, and LXRs, has also shown promise for ameliorating cardiovascular disease.^410,411 These findings provide strong pharmacological evidence supporting the potential of targeting NRs for cardiovascular disease management.

Alzheimer’s disease (AD), PD, Huntington’s disease (HD), multiple sclerosis (MS), cerebral ischemia, stroke and other conditions are common central nervous system disorders that currently affect approximately one hundred million people worldwide. Compelling evidence suggests that nuclear receptors are important in these disorders and that targeting these receptors has significant therapeutic potential.³⁵³

Liver X receptor LXRs are key regulators of cholesterol trafficking in the central nervous system (CNS), working with ApoE to facilitate cholesterol recycling and efflux, which are essential for maintaining CNS homeostasis. Dysregulation of LXR signaling is linked to neurodegenerative diseases such as AD, PD and Niemann‒Pick type C disease. LXR activation offers therapeutic potential by modulating neuroinflammation in CNS disorders. In AD models, LXR agonists such as T0901317 reduce inflammation associated with amyloid-beta (Aβ) deposition and tau tangles by suppressing microglial activation and the expression of inflammatory markers such as iNOS, COX-2, and NF-κB. Similar effects are observed in PD models, where T0901317 decreases iNOS and COX-2 levels. In Niemann–Pick type C disease, LXR activation promotes cholesterol clearance, reduces cerebellar inflammation, and extends lifespan. LXRs also enhance blood‒brain barrier (BBB) integrity by increasing the expression of ABC transporters such as ABCB1, which prevents toxin entry into the brain. Natural LXR agonists (e.g., oxysterols) and RXR activation (e.g., with bexarotene) improve BBB function in coculture models, whereas LXR agonists support tissue regeneration in ischemic stroke by stabilizing the BBB and reducing inflammatory cytokines. Although preclinical results are promising, the clinical translation of LXR agonists is hindered by side effects, including elevated triglycerides and LDL cholesterol.²⁸⁴ Efforts to mitigate these issues include the development of selective LXRβ agonists, regional drug delivery, and the targeting of specific downstream pathways, making LXRs promising therapeutic targets for CNS diseases.

PPARs, particularly PPARγ, play pivotal roles in neuroprotection by regulating neuroinflammation, mitochondrial function, and antioxidant defenses. It is expressed in various CNS cell types, including neurons, astrocytes, oligodendrocytes, and microglia. Dysregulation of PPARs is associated with neurodegenerative diseases characterized by inflammation, such as AD and amyotrophic lateral sclerosis (ALS). PPARγ agonists, including endogenous ligands such as OA and synthetic compounds such as rosiglitazone and pioglitazone, have shown promise in preclinical models because they reduce inflammatory cytokines, increase IL-10 levels, and enhance mitochondrial function in conditions such as stroke, ischemia, and traumatic brain injury. Moreover, PPARγ activation has exhibited potential benefits in neuropsychiatric disorders (e.g., depression, anxiety, and mood disorders) and neurodegenerative diseases, with ongoing clinical trials exploring its applications in AD, PD, MS, and bipolar disorder⁴¹² [NCT00982202↗, NCT00736996↗, NCT00265148↗, NCT00733785↗, NCT00688207↗, NCT01280123↗]. However, mixed clinical outcomes highlight gaps in understanding the mechanisms of PPARγ within the CNS.⁴¹³ Further research aimed at optimizing PPARγ activity may unlock its full therapeutic potential for treating neurodegenerative and neuropsychiatric conditions.

The GR also plays a critical role in neuroinflammation, neuroprotection, and brain homeostasis. Its anti-inflammatory effects in the brain are mediated through interactions with AP-1 and NF-κB, limiting their transcriptional activity.⁴¹⁴ GR also regulates brain-derived neurotrophic factor (BDNF), promotes neurogenesis, neuronal survival, and synaptic plasticity, and contributes to memory, aging, and behavior. Additionally, GR supports BBB integrity by upregulating tight junction proteins and establishing metabolic barriers to xenobiotics.^415,416 GR dysfunction has been implicated in various neuropathologies, including AD, HD and epilepsy. Abnormal GR expression in AD is correlated with hippocampal overexpression, impaired cortisol feedback, and memory deficits.⁴¹⁷ Similarly, GR overexpression in epilepsy has been associated with reactive gliosis and coregulation with PXR in epileptic brain endothelial cells. GR dysregulation also affects HD-related changes, such as increased cortisol secretion and muscular atrophy, alongside reduced GR expression in the hypothalamic and pituitary regions. GR abnormalities also underlie mood disorders such as depression and anxiety, as dysfunction in GR and MR signaling leads to maladaptive stress responses, dendritic retraction, and impaired serotonin activity.^418,419 GR dysregulation in the prefrontal cortex and hippocampus has been linked to anxiety and depression-like behaviors in animal models. Pharmacological modulation of the GR provides therapeutic opportunities. GR agonists, such as dexamethasone and methylprednisolone, are widely used to treat neurological conditions such as gliomas, MS, and traumatic brain injuries. Dexamethasone reduces edema, prevents cerebrovascular leakage, and upregulates tight junction proteins at the BBB.^420,421 Methylprednisolone improves memory and long-term potentiation at low doses but has dose-dependent effects, with high doses alleviating motor deficits in MS models but not cognition.^422,423 GR antagonists, such as mifepristone and relacorilant, have shown efficacy in treating mood disorders, neurodegenerative conditions, and PTSD. Mifepristone restores serotonin signaling and alleviates depression-like behaviors, whereas relacorilant is undergoing phase II and III trials for conditions such as Cushing syndrome and prostate cancer^424,425 [NCT036748142019, NCT036971092019]. These findings underscore the potential of targeting GRs in a range of CNS disorders.

Estrogens and their receptors play critical roles in neuroprotection and the regulation of inflammatory and neurodegenerative processes in the CNS. Owing to their antiapoptotic, antioxidative, and anti-inflammatory properties, estrogens provide early protection against ischemic injury. ERα and ERβ agonists, such as diarylpropionitrile (DPN), have demonstrated neuroprotective effects by mitigating damage in ischemic models, including the caudate nucleus and CA1 region of ovariectomized mice and the striatum of male mice. The membrane receptor GPER-1 also contributes to ischemic protection, with its agonist G1 offering estrogen-like neuroprotection, particularly in males, as suggested by sex-dependent differences in GPER-1 expression. In addition to estrogens, NRs such as Nr4a2 and bile acid receptors play vital roles in alleviating neuroinflammation and neuropathic pain. Nr4a2 helps regulate depressive-like behaviors by preventing structural and functional damage to anterior cingulate cortex (ACC) microglia and excitatory neurons during chronic neuroinflammation.⁴²⁶ Its upregulation through the dopaminergic agonist pramipexole has demonstrated neuroprotective effects on dopaminergic neurons in Parkinson’s disease.⁴²⁷ Similarly, bile acids activate TGR5 and FXR, alleviating mechanical hyperalgesia by reducing glial activation and neuronal sensitization in the spinal dorsal horn. The presence of bile acid synthesis enzymes in spinal astrocytes suggests an endogenous mechanism for decreasing nociceptive neuron transmission and mechanical hypersensitivity, emphasizing bile acid receptors as promising targets for neuropathic pain relief.⁴²⁸ In MS, Th17-driven inflammation is aggravated by RORγt overexpression, which promotes Th17 cell differentiation. By suppressing RORγt, vitamin A has shown therapeutic potential for MS⁴²⁹[NCT01225289↗]. Collectively, these findings highlight the diverse and pivotal roles of estrogens and nuclear receptors in neuroprotection, inflammation regulation, and the development of innovative therapeutic strategies.

In recent decades, global spending on infectious diseases has increased significantly, with treatment costs rising faster than those for other diseases. Infectious diseases caused by bacteria, viruses, fungi, and parasites include common conditions such as pneumonia, influenza, gastroenteritis, hepatitis, malaria, and dengue fever, as well as diseases such as HIV/AIDS, tuberculosis, and COVID-19. NRs play a key role in regulating immune responses by modulating genes involved in inflammation, pathogen defense, and immune cell function, thus influencing the outcome of both bacterial and viral infections. Given the increasingly appreciated roles of NRs in the context of infections, targeting NRs may provide a new, largely unexplored area for infectious disease treatment.⁴³⁰

The role of FXR extends to hepatitis C virus (HCV) infection, where it, along with other NRs, such as HNF4α, PPARα, and PXR, reprograms hepatocyte metabolism, influencing glycolysis, ketogenesis, and drug metabolism.⁴³¹ The targeting of FXR or PPARα in HCV infection has shown complex antiviral effects, with receptor inhibition reducing viral replication by reversing HCV-induced metabolic shifts. Furthermore, FXR agonists such as GW4064 have demonstrated potential in alleviating inflammation and preserving intestinal barrier function in experimental colitis, primarily through the modulation of NF-κB signaling.^432,433 In addition, GW4064 has been reported to downregulate scavenger receptors via FXR and indirectly inhibits HCV entry into cells.⁴³⁴

In addition to their roles in viral infections, NRs are essential in bacterial infection. For example, PPARγ signaling is crucial for resolving Staphylococcus aureus (MRSA) infections and improving outcomes in sepsis models by modulating inflammation.⁴³⁵ PPARγ agonists such as pioglitazone have been shown to restore mitochondrial function and reduce oxidative stress in macrophages, highlighting their potential in treating inflammatory and infectious diseases.⁴³⁶ LXRs are involved in macrophage phagocytic responses, particularly during Salmonella Typhimurium infections.⁴³⁷ In a mouse model, LXR agonist treatment ameliorated clinical signs of Salmonella infection in a CD38-dependent manner.⁴³⁸ Recent transcriptomic studies investigating susceptibility to dengue virus in a Cuban cohort of African ancestry revealed increased gene expression of RXR in individuals who were more resistant to infection and suggested a protective role for LXR/RXR against dengue virus.⁴³⁹ In the case of tuberculosis (TB), NRs such as Nur77 and Nor1 are upregulated in response to Mycobacterium tuberculosis infection. Interestingly, the first-line TB drug rifampicin activates PXR, which can impair its effectiveness in infected macrophages, underscoring the complex interactions between NRs and drug efficacy.^440,441 Rifampicin, a first-line anti-tuberculosis (TB) drug, is a potent activator of PXRs, which in turn compromises the effect of rifampicin in M. tuberculosis-infected human macrophages.⁴⁴²

NRs also influence fungal infections, with PPARγ playing a central role in defending against Candida albicans in the gastrointestinal tract. PPARγ ligands reduce fungal colonization through a dectin-1-dependent mechanism. Sepsis models have revealed a critical role for PPARγ in controlling inflammation and improving overall disease outcomes. PPARγ gene expression was downregulated in freshly isolated peripheral blood mononuclear cells (PBMCs) from human sepsis patients as well as in the lungs in a murine sepsis model.⁴⁴³ Treatment with PPARγ agonists improved sepsis outcomes, decreased proinflammatory cytokine levels, increased IL-10 levels, and improved mouse survival.⁴⁴⁴ Other PPAR isoforms have also been linked to decreased severity of sepsis through the downregulation of inflammation. In a cecal ligation and puncture rat sepsis model, treatment with a PPARβ/δ agonist inhibited signal transducer and activator of transcription 3 (STAT3) activation in AMs, reduced the inflammatory response, and prolonged rat survival.^445,446

Parasitic worms are among the global health threats to billions of humans worldwide.^447,448 Although several anthelmintic drugs, including benzimidazole, levamisole/morantel and ivermectin, have been used to treat these parasites, resistance frequently occurs. Therefore, identifying novel therapeutic targets in parasitic diseases is urgently needed. Nuclear receptors, which are well-known therapeutic targets in humans, are currently being investigated in parasitic worms because of their widespread distribution and important functions in controlling transcriptional networks involved in metabolism and development.⁴⁴⁹ Among them, DAF-12 is a nuclear receptor that is necessary for the typical development of nematodes, including the crucial infectious stage. In free-living nematodes such as C. elegans, a lack of DAF-12 ligands (such as dafachronic acid (DA)) under stress conditions promotes entry into the L3d (dauer diapause) state, whereas supplementation with DAF-12 ligands in prosperous environments leads to their exit from L3d and the beginning of reproductive development.⁴⁵⁰ In contrast, in parasitic nematodes such as S. stercoralis, the absence of a DAF-12 ligand leads to an L3d ortholog stage, termed the infectious L3 (iL3) stage, during preinfection, whereas the presence of a DAF-12 ligand in the definitive host can resume reproductive development.⁴⁵¹

The mechanism underlying DAF-12-mediated cell fate decisions is similar to that of other nuclear receptors. Briefly, under favorable conditions or in a host-specific environment, DAF-12 binds with its ligand and is thus activated, which induces the transcription of target genes that facilitate metabolism and development. In contrast, under stress conditions, DAF-12 is inactive without ligand binding, resulting in transcriptional repression and subsequent development arrest and infection. Thus, strategies targeting DAF-12 or its ligands have potential for the treatment of parasitic diseases. In addition to exogenous DA, endogenous Δ7-DA was recently reported as a novel ligand of DAF-12 in S. stercoralis. The abundance of Δ7-DA is regulated by several enzymes, including DAF-36, DHS-16, and DAF-9, which govern the on/off switch of the reproduction, infection and development processes in this parasitic nematode.⁴⁵² In addition to receptors and ligands, other components, such as cofactors, can also be potential targets for the manipulation of the life cycle of parasitic nematodes. For example, DAF-12 interacting protein-1 (DIP-1) was recently identified as a coactivator of DAF-12, which is indispensable for the ligand-activated transcriptional activity of DAF-12 in Strongyloides spp.⁴⁵³ Together, this evidence suggests that the nuclear receptor DAF-12 is a promising target for the treatment of parasitic diseases.

Polycystic ovary syndrome (PCOS) is a type of reproductive disease that affects 5–18% of women worldwide who are of reproductive age.⁴⁵⁴ Interestingly, patients with PCOS are frequently associated with metabolic syndrome, such as obesity, dyslipidemia, impaired glucose tolerance or increased fasting insulin,⁴⁵⁵ which is closely linked to an abnormal gut microbiome. A recent study revealed that PCOS can be induced by alterations in the gut microbiome via the activation of the nuclear receptor FXR.⁴⁵⁶ Mechanistically, agmatine produced by B. vulgatus can activate FXR, which in turn prevents L cells from secreting glucagon-like peptide-1 (GLP-1), suggesting that FXR is a possible therapeutic target for the treatment of PCOS.⁴⁵⁶ Moreover, PPAR is a large nuclear receptor superfamily related to energy metabolism and is thus involved in the progression of PCOS. In fact, all three PPAR family members, including PPARα, PPARβ/δ and PPARγ, play critical roles in PCOS via multiple mechanisms, such as the regulation of lipogenesis; glucose homeostasis; and fatty acid oxidation, synthesis and storage.⁴⁵⁷ Given the important function of PPARs in PCOS pathogenesis, modulating the activity of PPARs profoundly affects the development of PCOS. For example, the activation of PPARγ by a polyphenolic compound, sinapic acid, was shown to ameliorate metabolic dysfunction and attenuate PCOS phenotypes in rats.⁴⁵⁸ Another study focusing on the downstream mechanism underlying PPAR-mediated alleviation of PCOS indicated that PPARα can bind to the promoter of FADS2, a ferroptosis-related gene, thus increasing FADS2 transcription. This event inhibits ferroptosis of granulosa cells in a dehydroepiandrosterone (DHEA)-induced PCOS mouse model, suggesting that PPARα alleviates PCOS by protecting granulosa cells from ferroptosis.⁴⁵⁹

Uterine leiomyomas, another name for uterine fibroids, are benign tumors of the fibrotic smooth muscle in the reproductive system. The ovarian steroid hormone estrogen, as well as its receptor ER, are involved in this disease.⁴⁶⁰ In addition, other nuclear receptors, such as the NR4A protein family, may also participate in the progression of uterine fibroids.⁴⁶¹ NR4A restricts the proliferation of primary leiomyoma smooth muscle cells by modeling the extracellular matrix, whereas low expression of NR4A contributes to the pathogenesis of uterine fibroids. Moreover, leuprolide acetate, a GnRH analog used to treat uterine fibroids, can restore the expression of NR4A1 and NR4A3 in clinical samples, suggesting that these two NR4A family members might be additional targets of leuprolide acetate in addition to sex steroid receptors.

While NRs are intimately involved in proper kidney function, they are also implicated in a variety of renal diseases, such as diabetes, acute kidney injury, and other conditions, such as aging. In the last 10 years, our understanding of renal disease etiology and progression has been greatly shaped by knowledge regarding how NRs are dysregulated in these conditions. Importantly, NRs have also become attractive therapeutic targets for the attenuation of renal diseases, and their modulation for this purpose has been the subject of intense investigation. Here, we review the roles of six key renal NRs, including PPAR, ERR, FXR, ER, LXR, and VDR, in health and disease, with an emphasis on recent findings over the last decade.

PPARs are involved in various renal diseases, both acute and chronic. PPARα-null mice treated with streptozotocin (STZ) exhibit accelerated diabetic nephropathy, including worsened albuminuria, glomerular sclerosis, mesangial matrix expansion, collagen deposition, and macrophage infiltration. PPARα agonists have been studied as potential treatments for diabetic nephropathy (DN) in rodents, with evidence supporting their renoprotective effects. Recent research by Chung et al.demonstrated that impairment of PPARα and fatty acid oxidation exacerbates kidney fibrosis during aging, with reduced PPARα expression and lipid accumulation in tubular cells.⁴⁶² PPARα-null mice also exhibit more severe renal aging and fibrosis. Jao et al.reported that ATF6 downregulates PPARα transcription, leading to decreased fatty acid oxidation and increased lipotoxicity in proximal tubule cells, whereas ATF6-deficient mice exhibit increased PPARα expression and decreased lipid accumulation.⁴⁶³ In addition to PPARα, PPARγ has also been studied in DN, with evidence suggesting that its activity is suppressed in this disease. Unlike other PPAR family members, PPARβ/δ is ubiquitously expressed in both cortical and medullary cells. Recent studies have highlighted a potential role for PPARβ/δ in renal homeostasis. For example, Kirkby et al. demonstrated that activation of renal PPARβ/δ by endogenous prostacyclin induces vasodilation and improves blood flow, a process regulated by cyclooxygenase-2 (COX-2).⁴⁶⁴ This COX-2/prostacyclin/PPARβ/δ axis offers new insights into kidney vascular physiology and therapeutic possibilities. Additionally, activation of PPARβ/δ has been shown to improve renal dysfunction in a lupus mouse model by lowering blood pressure, reducing renal hypertrophy, decreasing albuminuria, and preventing renal injury.⁴⁶⁵

Additionally, other nuclear receptors play key roles in kidney health. ERRs are crucial regulators in the kidneys, where they help maintain metabolic balance, regulate monovalent cation homeostasis, and control blood pressure.⁴⁶⁶ The importance of ERRs is further highlighted by the fact that the deletion of ERRγ leads to significant mitochondrial dysfunction and progressive renal failure.⁴⁶⁷ For example, ERRα has been shown to protect against AKI in tubular epithelial cells, particularly in cases of cisplatin-induced AKI in mice.⁴⁶⁸ Compared with those in wild-type mice, in ERRα-null mice subjected to cisplatin treatment, tubular injury, a reduced mitochondrial content, abnormal mitochondrial morphology, and increased oxidative stress were observed. A recent study demonstrated that Nr4a1 was upregulated in the kidneys of mice with unilateral ureteral obstruction (UUO), and its expression was positively correlated with the degree of interstitial kidney injury and the level of fibrotic proteins. Nr4a1 may contribute to renal fibrosis by activating the p38 MAPK pathway, as treatment with the p38 MAPK inhibitor SB203580 successfully inhibited fibrotic protein expression and p38 MAPK phosphorylation.¹⁸⁴ Furthermore, VDR has been shown to exert a renoprotective effect by inhibiting fibrosis. VDR translocates into the nucleus to form the VDR:RXR heterodimer, which inhibits fibrogenic gene expression in HK-2 cells exposed to high glucose and reduces fibrogenic gene expression in proximal renal tubules.⁴⁶⁹ In the aging kidney, the expression of FXR and TGR5 is decreased. However, a dual agonist of FXR and TGR5, INT-767, has been found to reverse age-related kidney disease in mice, demonstrating the therapeutic potential of targeting these receptors in renal aging.⁴⁷⁰ In conclusion, nuclear receptors play vital roles in regulating kidney function and protecting against kidney injury and fibrosis. Understanding their mechanisms and interactions could open new avenues for therapeutic strategies to treat kidney diseases, particularly those related to aging and fibrosis.

Several challenges remain in targeting NRs therapeutically due to the complexity of NR signaling, tissue specificity, and the need for precise drug design. However, recent advancements in genomics, proteomics, imaging, and computational modeling have enabled more precise exploration of NR function, regulation, and their involvement in various physiological and pathological processes.

Technologies such as ChIP-chip, ChIP-Seq, and GRO-Seq have enabled unbiased, genome-wide profiling of NR binding sites, revealing new insights into NR mechanisms and their regulatory networks in breast cancer. These genomic methods have highlighted the dynamic interactions between NRs and the chromatin landscape. It is increasingly clear that chromatin, in its varying states of condensation, both regulates and is regulated by NR activity, with NR-chromatin interactions playing a major role in cell type-specific transcriptional regulation.⁴⁷¹ Single-cell RNA sequencing (scRNA-seq) has further advanced our understanding of NR function by enabling gene expression analysis at the single-cell level. This technology is invaluable for investigating tissue-specific NR roles and identifying distinct signaling pathways in various cell populations, which is crucial for understanding their contributions to tissue homeostasis, development, and diseases like cancer and metabolic disorders.⁴⁷² For instance, single-cell transcriptomics revealed the involvement of a previously uncharacterized retinoic acid receptor family member, SmRAR, in reproductive development in Schistosoma mansoni.⁴⁷³ CRISPR-Cas9 technology has revolutionized NR research by allowing precise manipulation of genes in vitro and in vivo. This has facilitated studies of NR roles in diseases such as type 2 diabetes, neurodegeneration, and cancer, and provide reversible and tunable control of NR expression, enabling dynamic investigation of NR function.^474,475

Recent advances in proteomics, particularly mass spectrometry (MS), have enabled the identification and quantification of NR interactomes, providing insights into the molecular mechanisms regulating NR activity. MS also helps identify post-translational modifications like phosphorylation, acetylation, and SUMOylation, which are crucial for NR activation and signaling.³⁰⁶ Cryo-EM has emerged as a powerful tool for visualizing the 3D structure of proteins, including NRs, at near-atomic resolution. This technique has provided key insights into the conformational changes NRs undergo upon ligand binding and co-factor recruitment, essential for drug discovery, as many NRs are targeted by small molecules in diseases like cancer, diabetes, and inflammation.⁴⁷⁶ For example, cryo-EM has elucidated the assembly of the AR:coactivator complex and its role in coactivator assembly and transcriptional regulation.⁷⁰ Additionally, the cryo-EM structure of the NCOA4-FTH1 interface revealed key amino acids crucial for interaction, paving the way for developing ferritinophagy modulators.⁴⁷⁷ These technologies, including cryo-EM and omics, have advanced our understanding of NR activation, NR-chromatin interactions, transcriptional regulation, and non-genomic effects, offering new avenues for therapeutic development.

Artificial intelligence (AI) and machine learning (ML) are increasingly applied in NR research to predict receptor-ligand interactions, identify novel signaling pathways, and design drugs targeting specific NR subtypes. These technologies analyze large datasets to uncover hidden patterns within complex biological systems, enhancing our understanding of NRs in health and disease. For example, chemical language models combined with beam search algorithms have been used for automated molecule design, leading to the development of novel inverse agonists for retinoic acid receptor-related orphan receptors (RORs) with low micromolar to nanomolar potency.⁴⁷⁸ AI and deep docking methodologies have also facilitated the development of VPC-260724, a novel ER-AF2 binder that inhibits ER activity and exhibits antiproliferative effects in ER-positive breast cancer models.⁴⁷⁹ Additionally, AI has been applied to create an adverse outcome pathway (AOP) network for chemical-induced cholestasis, facilitating automated data collection and the identification of key molecular events.⁴⁸⁰ Moreover, Bhimsaria and colleagues identified previously hidden binding sites of human NRs via high-throughput SELEX and a novel MinSeq Find algorithm, revealing new biological roles of NRs and their involvement in disease-associated genetic variations while providing a valuable resource for drug repurposing and precision medicine.⁴⁸¹ With AI and ML, high-throughput screening (HTS) has become more efficient, enabling better prediction of drug candidates and optimization of lead compounds for clinical applications.

In vivo imaging techniques like positron emission tomography (PET) and magnetic resonance imaging (MRI) are increasingly used to study nuclear receptor (NR) activity in living organisms. These methods allow real-time tracking of NR signaling and the effects of ligands or drugs on receptor function in tissues. Live-cell imaging with fluorescent biosensors also reveals dynamic interactions between NRs and their co-regulatory proteins at the cellular level.⁴⁸² This approach offers a quantitative, in vivo equivalent to classic radioligand binding assays.⁴⁸³ In vivo imaging helps identify when and where NRs are active during development, aging, and disease, providing insights into how ligands or receptor modulators influence these processes.⁴⁸⁴ Other technologies, such as organoids, 3D cell cultures, nanotechnology, and systems biology, have also advanced NR research. Notably, PROTACs (proteolysis-targeting chimeras), which degrade target proteins via the 26S proteasome, offer new possibilities for targeting challenging NRs, including orphan receptors or those without enzymatic activity. In cases of drug resistance due to NR mutations (e.g., in hormone-dependent cancers), PROTACs can bypass resistance by degrading the mutated receptors instead of inhibiting their activity.

Emerging technologies are significantly enhancing the field of nuclear receptor research by providing deeper insights into their structure, function, and regulation. The combination of high-resolution imaging, gene editing tools, and advanced computational techniques has opened new avenues for understanding how NRs control physiological processes and contribute to disease. As these technologies continue to evolve, they hold promise for the development of more precise and effective therapeutic strategies targeting nuclear receptors in a variety of disorders.