Glucocorticoids also promote activation of scavenger immune cells that help remove cell debris, promote repair and prevent lasting damage such as fibrosis. At the cellular level, glucocorticoids are able to control metabolism, inflammation, adhesion, migration and survival, in a cell-specific and context-specific manner.
Glucocorticoids mediate their cellular effects through binding and activating the glucocorticoid receptor GR , which is expressed in almost every tissue. Glucocorticoids diff use into the cell, and bind and activate the GR, which then moves into the nucleus. The GR can bind DNA directly at specific glucocorticoid response elements GREs , and then recruit coactivator or corepressor proteins to increase or decrease expression of target genes.
The GR can also bind or tether itself to other DNA-bound transcription factors and modulate their ability to change gene expression Figure 2. This adds an additional layer of control, because gene accessibility and transcription factor expression and activity are cell-specific and dynamically regulated.
For example, liver-specific transcription factors such as HNF4 hepatocyte nuclear factor 4 facilitate GR binding to and regulation of metabolic genes, whereas macrophage-specific transcription factors such as PU. Glucocorticoid responses are therefore fine-tuned to consider context. This is how glucocorticoids can control metabolism in liver, activation of macrophages, and promote death of T-cells.
This explains why the GR and glucocorticoids is a potent inhibitor of inflammation only when required: i. Glucocorticoids through the GR are therefore perfectly adapted to integrate incoming signals from other pathways in order to respond appropriately to each specific challenge.
We still have a long way to go in order to fully understand the whole spectrum of glucocorticoid action. We here provide a general overview on the current insights in GC biology with a focus on GC synthesis, regulation and physiology, role in inflammation inhibition, and on GR function and plasticity. Furthermore, novel and selective therapeutic strategies are proposed based on recently recognized distinct molecular mechanisms of the GR. The first steps leading to the discovery of glucocorticoids GCs took place in the 19th century when the physician Thomas Addison described that patients suffering from chronic fatigue, muscular degeneration, weight loss, and a strange darkening of the skin could obtain beneficial effects from adrenal extracts 1.
This disease is now known as Addison's disease, which is a form of adrenal insufficiency. Compound E, would become known as cortisol and was synthesized later that year by Sarett 3. The therapeutic potential was discovered by rheumatologist Philip Hench in a patient suffering from rheumatoid arthritis 4. Hench and Kendall were awarded the Nobel prize for Medicine and Physiology in together with Tadeus Reichstein who succeeded in isolating several steroid hormones from the adrenals, eventually leading to the discovery of cortisol.
Since the discovery of their anti-inflammatory potential GCs were hailed as wonder drugs to treat various inflammatory diseases and became part of the group of most used and cost-effective anti-inflammatory drugs. GCs bind the GC receptor GR , a member of the nuclear receptor NR family of intracellular receptors, which also contains the estrogen receptor ER , progesterone receptor PR , androgen receptor AR , and mineralocorticoid receptor MR as well as several orphan receptors with no known ligand 5 , 6.
In , the GR was identified as the principal receptor responsible for the physiological and pharmacological effects of GCs 7. NRs are involved in many aspects of mammalian biology, including various metabolic functions, cardiac function, reproduction and embryonic development, and the immune system GCs are steroid hormones that are essential for the daily functioning of mammals.
They are involved in several physiological processes, namely in metabolism 12 , water and electrolyte balance 13 , the immune response 14 , 15 , growth 16 , cardiovascular function 17 , 18 , mood and cognitive functions 19 — 21 , reproduction 22 , and development The latter is the precursor of testosterone and estrogen. Aldosterone, GCs, and DHEA are synthesized by different steroidogenic enzymes in the mitochondria of, respectively, the zona glomerulosa, the zona fasciculate, and the zona reticularis of the adrenal cortex.
They are however all synthesized from the same precursor, namely cholesterol Extra-adrenal GC production in the thymus, vasculature, brain, and epithelial barriers has also been observed 25 — These locally produced GCs are thought to predominantly exert local effects and contribute only minimally to the systemically circulating pool of GCs allowing a high spatial specificity of steroid actions, which are also independent of the circadian and stress induced regulation of endogenous GCs.
Under basal, unstressed conditions GCs are released from the adrenal glands in the bloodstream in a circadian and ultradian rhythm characterized by peak levels during the active phase which is in the morning in humans and in the beginning of nighttime in nocturnal animals such as mice. The activity of the HPA axis is further increased upon physiological e.
ACTH will in turn stimulate the adrenal gland to synthetize and secrete GC hormones cortisol in the circulation Figure 1. Hypothalamic-pituitary-adrenal axis. The hypothalamic-pituitary-adrenal HPA axis activity is controlled by the circadian rhythm and can be induced by physiological and emotional stress.
This induces the release of adrenocorticotrophic hormone ACTH from the pituitary gland into the systemic circulation. ACTH will activate cortisol synthesis in the cortex of the adrenal gland. Cortisol negatively regulates the HPA-axis activity, e. The HPA axis is subject to a negative feedback inhibition by GCs, both in a genomic and a non-genomic way. Next to this, GR is also able to physically interact with the Nur77 protein which also binds in the POMC promoter, thereby preventing it from performing its transcription function 35 , Once secreted in the bloodstream GCs are bound to and transported by plasma proteins which keep the GCs inactive.
Several proteases target CBG, such as neutrophil elastase at sites of infection 40 , causing the release of bound GCs. Due to their lipophilic nature, free GCs diffuse through the cell membrane to exert their function. However, the actual bioavailability of GCs in the cytoplasm is regulated by the balance between active and inactive forms of GCs. Two enzymes are responsible for the conversion between inactive cortisone or dehydrocorticosterone in mice on the one hand and the active cortisol or corticosterone, in mice on the other hand.
Biologically active GCs will bind their receptor in the cytoplasm which exerts their physiological effects. This mechanism also confers a tight spatial regulation of GC actions, as the levels of these enzymes may be tissue or even cell specifically regulated and will directly determine the balance between the inactive and active form of GCs and thus the strength of the effect. Figure 2. Conversion of inactive GCs to active GCs.
Under physiological conditions the role of endogenous GCs is not simply anti-inflammatory or immunosuppressive and shows more immunomodulation. It has been shown that GCs can also work pro-inflammatory This occurs mainly in conditions of acute stress and is related to the concentration of GCs present 14 , Next to the endogenous GCs, various synthetic GCs e.
Experiments with structural modifications, mainly replacing side chains, resulted in synthetic GCs with optimized characteristics for medical use pharmacokinetics, bioavailability, cross-reactivity with the MR. These facts are important to keep in mind when giving GC treatment or performing research using synthetic GCs. The GR mediates the actions of GCs in cells. It belongs to the nuclear receptor superfamily of transcription factors TFs and is a 97 kDa protein that is constitutively and ubiquitously expressed throughout the body Nevertheless, GCs exert cellular and tissue-specific effects due to the existence of different GR isoforms on the one hand and cell- and context-specific allosteric signals influencing GR function on the other hand 52 — The GR functions by regulating the expression of GC responsive genes in a positive or negative manner.
The mouse Nr3c1 gene is localized on chromosome Differential use of these promoters, located about 5 kb upstream of the transcription start site, causes varying expression levels of GR protein isoforms between cells and tissues 57 — Furthermore, these exon-1 variants are subject to epigenetic regulation. The presence or absence of such modifications has been related to GR gene expression levels, GC resistance in certain cancers, promotion of cancer development, and mental health 64 — Figure 3.
Glucocorticoid receptor gene and protein. A Genomic structure of the glucocorticoid receptor GR gene. B Alternative splice and translation-initiation variants of the GR protein.
Identified post-translational modifications of the GR are indicated in the black circles. Regions important in GR function are indicated below the protein. It is inherently unstructured, vulnerable to proteases and only becomes structured when the protein binds DNA and forms dimers This AF1 binds cofactors, chromatin modulators, and the transcription machinery 71 — It is characterized by two highly conserved subdomains each containing a Cys4-type zinc finger.
The second subdomain contains the distal box D box which is important for GR dimerization Not a single, but multiple GR protein isoforms are identified. This is the result from alternative splicing and the use of 8 different translation initiation start sites Both isoforms are identical up to AA , but contain non-homologous AA thereafter. Perhaps some synthetic GR agonists could also bind to this isoform.
The AUG start codons are differently selected due to ribosomal leaky scanning and ribosomal shunting mechanisms Because the same AUG start sites are also present in the GR splice-variants, all the translation-initiation isoforms are expected to occur in each of the splice-variants The GR translation variants all have a similar GC and glucocorticoid responsive element GRE -binding affinity, but they differ in the length of their N-termini and their transcriptional activity. They show different subcellular localization, regulate distinct sets of genes and their relative levels vary between and within cells The mechanism of regulation of alternative translation start sites and alternative splicing in response to physiological, pathological, and cell-specific signals is still poorly understood.
In vitro work proved that these isoforms do have the capability to regulate distinct transcriptional programs In the absence of intracellular bioactive GCs, the GR finds itself as a monomer in the cytoplasm where it resides in a multiprotein complex. This chaperone complex is important for GR maturation, ligand binding, nuclear transport, and activation. Recruitment of p23 and FKBP51 to the multiprotein complex leads to maturation of GR-chaperone complex into a conformation that has very high affinity for GR ligands.
These are subsequently bound by nucleoporin and importins that carry the GR through the nuclear pore complex into the nucleus , Initially it was believed that the GR disassociates from the cytoplasmic chaperone complex upon ligand binding.
However, recent research has shown that the chaperone complex is required for efficient nuclear translocation of the receptor Figure 4. Glucocorticoid receptor chaperone complex and maturation. C Hop induces the binding of Hsp The GR has now matured into a high affinity complex. Once inside the nucleus, the activated GR can go on to exert its function or it can be transported back to the cytoplasm, inhibiting the GR's transcriptional activity. The balance between nuclear import and export determines the proportion of GR protein in the nucleus and has a direct influence on the strength of GR's transcriptional activities.
In the nucleus, the GR acts as a TF that can activate trans-activation or inhibit trans-repression genes as well as modulate the function of other TFs tethering. Most of the GR functions are restricted to the nucleus, but some non-nuclear actions of GR are also known.
In the nucleus, the GR is able to transcriptionally activate transactivate TA or transcriptionally repress transrepress TR gene-expression, both as a monomer and as a dimer, and usually via direct contact with DNA.
The importance of this GR tetramer in transcriptional regulation is not well-understood and needs further investigation. Figure 5. Glucocorticoid receptor activation and function. Lipophilic glucocorticoids GCs diffuse through the cell membrane and bind the glucocorticoid receptor GR in the cytoplasm.
The GR can transactivate genes by binding to glucocorticoid responsive elements GRE as a dimer, but also as a monomer by binding to other transcription factors TF through tethering or by binding to composite-elements. GR might also function as a tetramer, but its function is not known. Different modes of genomic GR transcriptional regulation are described Figure 5.
Classically, the GR exerts its transactivation function by binding to GREs, which are 15 bp long sequence motifs of 2 imperfect inverted palindromic repeats of 6 bp separated by a 3 bp spacer.
However, this may be better represented as a sequence logo Figure 6 , which illustrates that some positions are much more variable than others. In this D box a hydrogen bond is formed between Ala of one dimer partner and Ile of the other partner 74 , A second interface important for dimerization Ile has been identified in the LBD 77 , Recent research proposes that the LBD may have other dimerization interfaces related to another dimer structure Figure 6.
Sequence logo of the human glucocorticoid responsive element to which the GR binds to. See text for more details. GREs contain relatively few highly conserved residues and because GREs are rather short, they are abundantly present in the genome. Why this is the case is still a topic of research, but it has been shown that the chromatin structure plays a big role in determining which sites are accessible to GR under certain conditions , If a binding site for another TF is nearby the GRE-half site, both elements may act as a composite site where there is an interaction positive or negative between the GR monomer and the other TF Figure 5.
An analysis in mouse liver showed that under endogenous corticosterone levels i. In response to exogenous GCs i. Binding to such an element leads to inhibition of gene expression. Indeed, recently GR half-sites were even found embedded in AP-1 response elements The GR operates in a cell- and context-specific manner. This is not only due to a different expression of GR protein isoforms but is also the cause of different signals that modulate the GR's activity at specific GBSs.
Four signals are described to influence the GR's function. GRE sequences differing by only one single base pair were namely shown to affect GR conformation and regulatory activity Moreover, allosteric changes provoked by one half site can be transduced via the GR lever arm located between the P and D box, see Figure 3 and the receptor's D box to the dimer partner, affecting the GR's transcriptional activity , A second signal influencing the GR transcriptional output obviously comes from the ligand that binds to the LBD.
Depending on the ligand, the LBD will adopt another conformation and attract other cofactors thereby influencing the GR's transcriptional outcome , Several phosphorylation — , ubiquitination , sumoylation , acetylation 76 , and nitrosylation sites as depicted in Figure 3 have been identified influencing GR-localization, stability, DNA binding, ligand response, and regulatory activity.
Last, the GR's transcriptional output is influenced by its interaction partners. These include other TFs that bind direct or indirect to GR and cofactors which are recruited to GR and are involved in functions such as chromatin regulation and regulation of the transcriptional machinery function 53 , The composition of the cofactor complex recruited to the GR depends on the cell specific expression of cofactors, the cell context and the integration of the previous described signals DNA, ligand, and PTMs that influence the GR's conformation This cofactor complex eventually determines the transcriptional output of the GR.
The GR is not only able to function by genomic actions, but also through non-genomic actions. These include GC-mediated effects on membrane lipids, changing their physicochemical properties Further, GCs have also been seen to act on a membrane-bound GR which is related to the classical GR and probably the result from differential splicing, alternative transcription initiation and PTMs , Another membrane receptor, unrelated to the classical GR, probably also binds GCs.
This protein is probably a G-coupled receptor that signals through cAMP and that binds endogenous GCs with high affinity. However, it does not bind most GC analogs such as dexamethasone Other non-genomic actions, e. A final type of non-genomic action of the GR is its effect on mitochondrial function. It was show that the GR can translocate to and reside in mitochondria , This mitochondrial GR is capable of regulating gene transcription from the mitochondrial chromosome by binding to GRE like elements alone or in complex with other factors.
This was demonstrated in vitro , using a hepatoma cell line and in brain cell of mice and rats — A recent study showed that a GR isoform, GR? GCs are therapeutically mainly used for their anti-inflammatory and immunosuppressive effects. These are a. GCs are used to treat inflammatory disorders such as asthma , skin rashes , rheumatoid arthritis RA , multiple sclerosis , and systemic lupus erythematosus SLE In most cases, synthetic glucocorticoids are used but hydrocortisone is also a popular option.
Despite its strong anti-inflammatory capacity, GC therapy is limited by two major drawbacks. First, GCs are well-known to be associated with adverse effects, particularly when given in high doses for long time periods.
Figure 7 graphically presents GC-associated side effects, with osteoporosis, hyperglycemia, cardiovascular diseases, and infections as the four most worrisome adverse effects for clinicians These side effect may be severe enough to affect the therapy or cause an increased risk to other negative effects. A recent study in RA patients showed a clearly increased risk of bone fractures correlated with the administration of GCs osteoporosis The latter can be caused by ligand induced homologous downregulation of the GR, caused chronical GC treatment , , or by pathophysiological processes accompanying the inflammatory disease states [e.
The pathophysiological processes provoking GCR are very heterogeneous, e. To achieve a positive benefit-to-risk ratio when using GCs, guideline recommendations regarding optimal dosing must be followed and potential adverse effects must be monitored, prevented and managed — Next to this, much research effort is put in developing innovative GCs or GR ligands that improve the therapeutic balance — Currently available GCs in the clinic activate all GR activities.
This D-loop is one of the primary dimerization interfaces, consequently this mutant shows impaired homodimerization and reduced functionality. Initial observation on the GR Dim mutant showed a strongly impaired transactivation and retained capability to transcriptionally repress genes, particularly as a monomer Follow-up work on the GR Dim found that there was still transactivation of certain genes possible by these mutant receptors , Current evidence seems to suggest that the DNA binding capacity of the mutant is at least partially preserved.
This mutation is believed to disrupt a secondary dimerization interface present in the LBD, leading to even poorer dimerization and function than the GR Dim mutant In addition, under normal physiological conditions, GR Dim mice are healthy and show no obvious phenotypes, except that they express interferon genes in their intestinal epithelium , It has been shown that under physiological conditions, GR binds to the DNA as a monomer, exerting transcriptional functions related to cell-type-specific functions, and that only after acute stress or injection of GCs, GR dimers are formed leading to binding to full GRE elements Also, elegant, NMR-based work by Watson et al.
In addition, the aforementioned functional PPI interfaces, recent structural biology work shows that the knowledge on GR dimerization and structural conformation may be incomplete based on structural homology and residue conservation between the NR transcription factor family, and new dimer interfaces that remain unexplored so far.
In one study researchers have postulated that the conformation of the GR that is generally accepted as the dimeric conformation might not be correct and they propose different configurations The fact most of the structural work so far was done on subdomains of the GR, as the whole protein is very hard to crystalize, may contribute to this limited knowledge of GR structure.
Many studies have investigated steroidal and non-steroidal SEGRAM in the hope to be able to dissociate the GC-induced anti-inflammatory effects from the GC-induced side effects — So far, only Fosdagrocorat for RA — and Mapracorat for ocular inflammatory diseases and skin inflammation have reached clinical trials. To prevent GC-induced side effects, strategies other than shifting the balance between the monomeric and the dimeric GR are also followed — Some aim at cell-specific targeting of GCs via antibody- or peptide-GC conjugates or via liposomes , thereby preventing systemic GC-effects.
Other studies investigate the therapeutic use of GC-induced proteins e. By this, steps are undertaken to develop therapies that stimulate only the wanted anti-inflammatory GC-functions without inducing the broad and also the unwanted GC-effects Further, studies also invest in the therapeutic potential of combination therapies, such as the combination of GR and PPAR agonists , During the recent years, it has become clear that the old idea in GC-research, that claims that GC anti-inflammatory effects can be separated from GC-induced side effects by simply dissociating GR TR from GR TA, because the former would be mainly monomeric-driven GR functions and the latter GR homodimeric-driven functions.
To date it is known that this separation cannot be made that strictly. In addition, GR Dim mice studies showed that not all GC-induced side effects are GR dimer-driven and that thus also monomeric GR is involved in at least some side effects.
Reviewed: October 20, Medically Reviewed. These drugs mimic the way your body controls inflammation. Both naturally produced and synthetic glucocorticoids have two main functions: Regulation of sugar metabolism Anti-inflammatory effects Prescription glucocorticoids are used to treat inflammation caused by a wide variety of illnesses, including asthma, inflammatory bowel disease IBD , rheumatoid arthritis, autoimmune diseases such as lupus , and cancer.
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