Structural Analysis on the Pathologic Mutant Glucocorticoid Receptor Ligand-Binding Domains

UBR5 forms ligand-dependent complexes on chromatin to regulate nuclear hormone receptor stability

Nuclear hormone receptors (NRs) are ligand-binding transcription factors that are widely targeted therapeutically. Agonist binding triggers NR activation and subsequent degradation by unknown ligand-dependent ubiquitin ligase machinery. NR degradation is critical for therapeutic efficacy in malignancies that are driven by retinoic acid and estrogen receptors. Here, we demonstrate the ubiquitin ligase UBR5 drives degradation of multiple agonist-bound NRs, including the retinoic acid receptor alpha (RARA), retinoid x receptor alpha (RXRA), glucocorticoid, estrogen, liver-X, progesterone, and vitamin D receptors. We present the high-resolution cryo-EMstructure of full-length human UBR5 and a negative stain model representing its interaction with RARA/RXRA. Agonist ligands induce sequential, mutually exclusive recruitment of nuclear coactivators (NCOAs) and UBR5 to chromatin to regulate transcriptional networks. Other pharmacological ligands such as selective estrogen receptor degraders (SERDs) degrade their receptors through differential recruitment of UBR5 or RNF111. We establish the UBR5 transcriptional regulatory hub as a common mediator and regulator of NR-induced transcription.

The multivalency of the glucocorticoid receptor ligand-binding domain explains its manifold physiological activities

The glucocorticoid receptor (GR) is a ubiquitously expressed transcription factor that controls metabolic and homeostatic processes essential for life. Although numerous crystal structures of the GR ligand-binding domain (GR-LBD) have been reported, the functional oligomeric state of the full-length receptor, which is essential for its transcriptional activity, remains disputed. Here we present five new crystal structures of agonist-bound GR-LBD, along with a thorough analysis of previous structural work. We identify four distinct homodimerization interfaces on the GR-LBD surface, which can associate into 20 topologically different homodimers. Biologically relevant homodimers were identified by studying a battery of GR point mutants including crosslinking assays in solution, quantitative fluorescence microscopy in living cells, and transcriptomic analyses. Our results highlight the relevance of non-canonical dimerization modes for GR, especially of contacts made by loop L1-3 residues such as Tyr545. Our work illustrates the unique flexibility of GR's LBD and suggests different dimeric conformations within cells. In addition, we unveil pathophysiologically relevant quaternary assemblies of the receptor with important implications for glucocorticoid action and drug design.

Cortisol resistance in the degu (Octodon degus)

Cortisol resistance has also been reported in the degu, Octodon degus, a New World hystricomorph endemic to central Chile. The degu is used as a model for studies of stress and diurnal rhythms, parental behaviour and female masculinization. Another New World hystricomorph, the guinea pig, also exhibits glucocorticoid resistance, a result of amino acid sequences that differ from other mammalian glucocorticoid receptors (GR). Mutations in the ligand-binding domain of the human GR have been identified in familial or sporadic generalised cortisol resistance as have variants in the guinea pig. To address the possibility that the high levels of cortisol observed in the degu are a result of the same or similar sequence variations observed in the guinea pig GR, we have cloned, expressed and characterised the ligand-binding domain (LBD) of the degu GR. Somewhat unexpectedly, neither the amino acids nor the region involved in the resistance observed in the guinea pig GR are relevant in the degu GR. The relative resistance to cortisol observed in the degu GR is conferred by the substitution of two isoleucine residues, which are highly conserved in the GR across species, with a valine doublet. These amino acids lie in the region between helices 5 and 6 of the GR LBD, a region known to be important in determining the affinity of ligand-binding in steroid receptors.

Glucocorticoid Resistance: Interference between the Glucocorticoid Receptor and the MAPK Signalling Pathways

Endogenous glucocorticoids (GCs) are steroid hormones that signal in virtually all cell types to modulate tissue homeostasis throughout life. Also, synthetic GC derivatives (pharmacological GCs) constitute the first-line treatment in many chronic inflammatory conditions with unquestionable therapeutic benefits despite the associated adverse effects. GC actions are principally mediated through the GC receptor (GR), a ligand-dependent transcription factor. Despite the ubiquitous expression of GR, imbalances in GC signalling affect tissues differently, and with variable degrees of severity through mechanisms that are not completely deciphered. Congenital or acquired GC hypersensitivity or resistance syndromes can impact responsiveness to endogenous or pharmacological GCs, causing disease or inadequate therapeutic outcomes, respectively. Acquired GC resistance is defined as loss of efficacy or desensitization over time, and arises as a consequence of chronic inflammation, affecting around 30% of GC-treated patients. It represents an important limitation in the management of chronic inflammatory diseases and cancer, and can be due to impairment of multiple mechanisms along the GC signalling pathway. Among them, activation of the mitogen-activated protein kinases (MAPKs) and/or alterations in expression of their regulators, the dual-specific phosphatases (DUSPs), have been identified as common mechanisms of GC resistance. While many of the anti-inflammatory actions of GCs rely on GR-mediated inhibition of MAPKs and/or induction of DUSPs, the GC anti-inflammatory capacity is decreased or lost in conditions of excessive MAPK activation, contributing to disease susceptibility in tissue- and disease- specific manners. Here, we discuss potential strategies to modulate GC responsiveness, with the dual goal of overcoming GC resistance and minimizing the onset and severity of unwanted adverse effects while maintaining therapeutic potential.

Generalized and tissue specific glucocorticoid resistance

Glucocorticoids (GCs) are steroid hormones that influence several physiologic functions and are among the most frequently prescribed drugs worldwide. Resistance to GCs has been observed in the context of the familial generalized GC resistance (Chrousos' syndrome) or tissue specific GC resistance in chronic inflammatory states. In this review, we have summarized the major factors that influence individual glucocorticoid sensitivity/resistance. The fine-tuning of GC action is determined in a tissue-specific fashion that includes the combination of different GC receptor promoters, translation initiation sites, splice isoforms, interacting proteins, post-translational modifications, and alternative mechanisms of signal transduction.

Stressful Newborn Memories: Pre-Conceptual, In Utero, and Postnatal Events

Early-life stressful experiences are critical for plasticity and development, shaping adult neuroendocrine response and future health. Stress response is mediated by the autonomous nervous system and the hypothalamic-pituitary-adrenal (HPA) axis while various environmental stimuli are encoded via epigenetic marks. The stress response system maintains homeostasis by regulating adaptation to the environmental changes. Pre-conceptual and in utero stressors form the fetal epigenetic profile together with the individual genetic profile, providing the background for individual stress response, vulnerability, or resilience. Postnatal and adult stressful experiences may act as the definitive switch. This review addresses the issue of how preconceptual in utero and postnatal events, together with individual differences, shape future stress responses. Putative markers of early-life adverse effects such as prematurity and low birth weight are emphasized, and the epigenetic, mitochondrial, and genomic architecture regulation of such events are discussed.

The Hypothalamic-Pituitary-Adrenal Axis and the Fetus

Glucocorticoids (GCs), cortisol in humans, influence multiple essential maturational events during gestation. In the human fetus, fetal hypothalamic-pituitary-adrenal (HPA) axis function, fetal adrenal steroidogenesis, placental 11β- hydroxysteroid dehydrogenase type 2 activity, maternal cortisol concentrations, and environmental factors impact fetal cortisol exposure. The beneficial effects of synthetic glucocorticoids (sGCs), such as dexamethasone and betamethasone, on fetal lung maturation have significantly shifted the management of preterm labor and threatened preterm birth. Accumulating evidence suggests that exposure to sGCs in utero at critical developmental stages can alter the function of organ systems and that these effects may have sequelae that extend into adult life. Maternal stress and environmental influences may also impact fetal GC exposure. This article explores the vulnerability of the fetal HPA axis to endogenous GCs and exogenous sGCs.

Single Nucleotide Variations of the Human GR Gene Manifested as Pathologic Mutations or Polymorphisms

The human genome contains numerous single nucleotide variations, and the human glucocorticoid receptor (GR) gene harbors ∼450 of these genetic changes. Among them, extremely rare, nonsynonymous variants, known as pathologic GR gene mutations, develop a characteristic pathologic condition, familial/sporadic generalized glucocorticoid resistance syndrome, by replacing the amino acids critical for GR protein structure and functions, whereas others, known as pathologic polymorphisms, develop mild manifestations recognized mainly at population bases by changing the GR activities slightly. Recent progress on the structural analysis to the GR protein and subsequent computer-based structural simulation revealed details of the molecular defects caused by such pathologic GR gene mutations, including their impact on the receptor interaction to ligands, nuclear receptor coactivators (NCoAs) or DNA glucocorticoid response elements (GREs). Indeed, those found in the GR ligand-binding domain significantly damage protein structure of the ligand-binding pocket and/or the activation function-2 transactivation domain and change their molecular interaction to glucocorticoids or the LxxLL signature motif of NCoAs. Two mutations found in GR DNA-binding domain also affect interaction of the mutant receptors to GRE DNA by affecting the critical amino acid for the interaction or changing local hydrophobic circumstance. In this review, I discuss recent findings on the structural simulation of the pathologic GR mutants in connection to their functional and clinical impacts, along with a brief explanation to recent research achievement on the GR polymorphisms.

GR-regulating Serine/Threonine Kinases: New Physiologic and Pathologic Implications

Glucocorticoid hormones, end products of the hypothalamic-pituitary-adrenal axis, virtually influence all human functions both in a basal homeostatic condition and under stress. The glucocorticoid receptor (GR), a nuclear hormone receptor superfamily protein, mediates these actions of glucocorticoids by acting as a ligand-dependent transcription factor. Because glucocorticoid actions are diverse and strong, many biological pathways adjust them in local tissues by targeting the GR signaling pathway as part of the regulatory loop coordinating complex human functions. Phosphorylation of GR protein by serine/threonine kinases is one of the major regulatory mechanisms for this communication. In this review, recent progress in research investigating GR phosphorylation by these kinases is discussed, along with the possible physiologic and pathophysiologic implications.

Single-Nucleotide Variations of the Human Nuclear Hormone Receptor Genes in 60,000 Individuals

Nuclear hormone receptors (NRs) mediate biologic actions of lipophilic molecules to gene transcription and are phylogenetically and functionally categorized into seven subfamilies and three groups, respectively. Single-nucleotide variations (SNVs) or polymorphisms are genetic changes influencing individual response to environmental factors and susceptibility to various disorders, and are part of the genetic diversification and basis for evolution. We sorted out SNVs of the human NR genes from 60,706 individuals, calculated three parameters (percentage of all variants, percentage of loss-of-function variants, and ratio of nonsynonymous/synonymous variants in their full protein-coding or major domain-coding sequences), and compared them with several valuables. Comparison of these parameters between NRs and control groups identified that NRs form a highly conserved gene family. The three parameters for the full coding sequence are positively correlated with each other, whereas four NR genes are distinct from the others with much higher tolerance to protein sequence-changing variants. DNA-binding domain and N-terminal domain are respectively those bearing the least and the most variation. NR subfamilies based on their phylogenetic proximity or functionality as well as diversity of tissue distribution and numbers of partner molecules are all not correlated with the variation parameters, whereas their gene age demonstrates an association. Our results suggest that the natural selection driving the NR family evolution still operates in humans. Gene age and probably the potential to adapt to various new ligands, but not current functional diversity, are major determinants for SNVs of the human NR genes.

Hsp90 regulation affects the treatment of glucocorticoid for pancreatitis-induced lung injury

Glucocorticoids are commonly used for the treatment of pancreatitis and complicated acute lung injury and help to reduce the mortality rates of both. The effect of gene variants in heat shock protein 90 (Hsp90), a key chaperone molecule of the glucocorticoid receptor (GR), on the therapeutic effect of glucocorticoids is unclear. Our study aims to investigate the different susceptibility to glucocorticoid treatment in BALB/c and C57BL/6 mice carrying different Hsp90 genotypes in an animal model of pancreatitis-induced lung injury. Compared with BALB/c mice, C57BL/6 mice have lower mortality rates, decreased water content in their lungs, and a lower level of IL-1 beta in an animal model of acute pancreatitis. C57BL/6 mice show a greater therapeutic effect and increased GR binding activities with glucocorticoid responsive element compared to BALB/c mice after a 0.4 mg/kg dexamethasone (DEX) treatment. Treatment with a higher dose of DEX (4 mg/kg) significantly reduced mortality rates and increased GR-GRE binding activity in both strains of mice, and there was no significant difference between the two strains. DEX did not exert a protective role after geldanamycin, a specific inhibitor of Hsp90, was administered in both strains of mice. Our study revealed that Hsp90 gene variants are responsible for the greater therapeutic effect of DEX in C57BL/6 mice compared to BALB/c mice, which implies that combining DEX treatment with Hsp90 regulation would promote the efficiency of DEX and would be an effective way to alleviate the side effects of hormone therapy.

Genome-wide Regulatory Roles of the C2H2-type Zinc Finger Protein ZNF764 on the Glucocorticoid Receptor

The C2H2-type zinc finger protein ZNF764 acts as an enhancer for several steroid hormone receptors, and haploinsufficiency of this gene may be responsible for tissue resistance to multiple steroid hormones including glucocorticoids observed in a patient with 16p11.2 microdeletion. We examined genome-wide regulatory actions of ZNF764 on the glucocorticoid receptor (GR) in HeLa cells as a model system. ZNF764- and GR-binding sites demonstrated similar distribution in various genomic features. They positioned predominantly around 50–500 kbs from the transcription start sites of their nearby genes, and were closely localized with each other, overlapping in ~37% of them. ZNF764 demonstrated differential on/off effects on GR-binding and subsequent mRNA expression: some genes were highly dependent on the presence/absence of ZNF764, but others were not. Pathway analysis revealed that these 3 gene groups were involved in distinct cellular activities. ZNF764 physically interacted with GR at ligand-binding domain through its KRAB domain, and both its physical interaction to GR and zinc finger domain appear to be required for ZNF764 to regulate GR transcriptional activity. Thus, ZNF764 is a cofactor directing GR transcriptional activity toward specific biologic pathways by changing GR binding and transcriptional activity on the glucocorticoid-responsive genes.

AKT1 has dual actions on the glucocorticoid receptor by cooperating with 14-3-3

Glucocorticoids are important therapeutic compounds for acute lymphoblastic leukemia (ALL). AKT1 or the protein kinase B is frequently activated in ALL, and contributes to the development of glucocorticoid resistance. We examined impact of AKT1 on glucocorticoid receptor (GR)-induced transcriptional activity in cooperation with phospho-serine/threonine-binding protein 14-3-3. AKT1 has two distinct actions on GR transcriptional activity, one through segregation of GR in the cytoplasm by phosphorylating GR at Ser-134 and subsequent association of 14-3-3, and the other through direct modulation of GR transcriptional activity in the nucleus. For the latter, AKT1 and 14-3-3 are attracted to DNA-bound GR, accompanied by AKT1-dependent p300 phosphorylation, H3S10 phosphorylation and H3K14 acetylation at the DNA site. These two actions of AKT1 regulate distinct sets of glucocorticoid-responsive genes. Our results suggest that specific inhibition of the AKT1/14-3-3 activity on the cytoplasmic retention of GR may be a promising target for treating glucocorticoid resistance observed in ALL.