Corticosteroids Systemic (Oral and Injectable)Corticosteroids are the most effective anti-inflammatory therapy for many chronic inflammatory diseases, such as asthma but are relatively ineffective corticosteroid anti inflammatory action other diseases such as chronic obstructive pulmonary disease COPD. Chronic corticosteroid anti inflammatory action is characterised by the increased expression of multiple inflammatory genes that are regulated corticosteroid anti inflammatory action proinflammatory transcription factors, such as nuclear factor-kappaB and activator protein-1, that bind to and activate coactivator molecules, which then acetylate core histones to switch on gene transcription. Corticosteroids suppress the multiple inflammatory genes that are activated in chronic inflammatory diseases, such as asthma, mainly by reversing histone acetylation of activated inflammatory genes through binding of liganded denkall anavar side effects receptors GR to coactivators and recruitment of histone deacetylase-2 HDAC2 to the activated transcription complex. At higher concentrations of corticosteroids GR homodimers also interact with DNA recognition sites to active transcription of anti-inflammatory genes and to inhibit transcription of several genes linked to indlammatory side effects. Corticozteroid, by activating HDAC, may reverse this corticosteroid resistance.
How corticosteroids control inflammation: Quintiles Prize Lecture
Corticosteroids are the most effective anti-inflammatory therapy for many chronic inflammatory diseases, such as asthma but are relatively ineffective in other diseases such as chronic obstructive pulmonary disease COPD.
Chronic inflammation is characterised by the increased expression of multiple inflammatory genes that are regulated by proinflammatory transcription factors, such as nuclear factor-kappaB and activator protein-1, that bind to and activate coactivator molecules, which then acetylate core histones to switch on gene transcription.
Corticosteroids suppress the multiple inflammatory genes that are activated in chronic inflammatory diseases, such as asthma, mainly by reversing histone acetylation of activated inflammatory genes through binding of liganded glucocorticoid receptors GR to coactivators and recruitment of histone deacetylase-2 HDAC2 to the activated transcription complex.
At higher concentrations of corticosteroids GR homodimers also interact with DNA recognition sites to active transcription of anti-inflammatory genes and to inhibit transcription of several genes linked to corticosteroid side effects. Theophylline, by activating HDAC, may reverse this corticosteroid resistance. This research may lead to the development of novel anti-inflammatory approaches to manage severe inflammatory diseases. Corticosteroids also known as glucocorticosteroids, glucocorticoids or just steroids are among the most widely used drugs in the world and are effective in many inflammatory and immune diseases.
The most common use of corticosteroids is in the treatment of asthma, where inhaled corticosteroids have become first-line therapy and by far the most effective anti-inflammatory treatment. This new understanding of these new molecular mechanisms also helps to explain how corticosteroids are able to switch off multiple inflammatory pathways, yet remain a safe treatment. It also provides insights into why corticosteroids fail to work in patients with inflammatory diseases such as chronic obstructive pulmonary disease COPD and cystic fibrosis Barnes et al.
The Nobel Prize for Medicine and Physiology in was awarded to Kendall and Reichstein, who had independently isolated and synthesised cortisol and then adrenocorticotropic hormone ACTH , and Philip Hench, a rheumatologist working at the Mayo Clinic, who had described the dramatic efficacy of ACTH in patients with rheumatoid arthritis.
Oral corticosteroids were subsequently shown to be as effective but their use was limited by systemic side effects that are well known today. The breakthrough that revolutionised asthma therapy was the introduction of inhaled corticosteroids that had topical activity in Brown et al. The predominant effect of corticosteroids is to switch off multiple inflammatory genes encoding cytokines, chemokines, adhesion molecules, inflammatory enzymes, receptors and proteins that have been activated during the chronic inflammatory process.
In higher concentrations they have additional effects on the synthesis of anti-inflammatory proteins and postgenomic effects.
This review discusses how corticosteroids so effectively switch off multiple inflammatory genes in steroid-sensitive inflammatory diseases, such as asthma, whereas fail to control inflammation in other inflammatory diseases, such as COPD. Chronic inflammatory diseases, such as asthma, COPD rheumatoid arthritis and inflammatory bowel disease, involve the infiltration and activation of many inflammatory and immune cells, which release multiple inflammatory mediators that interact and activate structural cells at the site of inflammation.
The pattern of inflammation clearly differs between these diseases, with the involvement of many different cells and mediators Barnes et al. These proinflammatory transcription factors are activated in all inflammatory diseases and play a critical role in amplifying and perpetuating the inflammatory process. The molecular pathways involved in regulating inflammatory gene expression are now being delineated and it is now clear that chromatin remodelling plays a critical role in the transcriptional control of genes.
Stimuli that switch on inflammatory genes do so by changing the chromatin structure of the gene, whereas corticosteroids reverse this process. Chromatin is composed of DNA and histones, which are basic proteins that provide the structural backbone of the chromosome. It has long been recognised that histones play a critical role in regulating the expression of genes and determines which genes are transcriptionally active and which ones are suppressed silenced.
Chromatin is made up of nucleosomes which are particles consisting of base pairs of DNA wound almost twice around an octomer of two molecules each of the core histone proteins H2A, H2B, H3 and H4.
In the last decade it has been shown that expression and repression of genes is associated with remodelling of this chromatin structure by enzymatic modification of the core histone proteins, particularly through acetylation of lysine residues. Each core histone has a long N-terminal tail that is rich in lysine residues, which may become acetylated, thus changing the electrical charge of the core histone. This conformation of the chromatin structure is described as closed and is associated with suppression of gene expression.
Gene transcription only occurs when the chromatin structure is opened up, with unwinding of DNA so that RNA polymerase II and basal transcription complexes can now bind to DNA to initiate transcription. This results in acetylation of core histones, thereby reducing their charge which allows the chromatin structure to transform from the resting closed conformation to an activated open form Roth et al. This molecular mechanism is common to all genes, including those involved in differentiation, proliferation and activation of cells.
This process is reversible and deacetylation of acetylated histones is associated with gene silencing. This is mediated by histone deacetylases HDACs which act as corepressors, together with other corepressor proteins which are subsequently recruited.
Gene regulation by histone acetylation. This results in acetylation Ac of core histones, opening up the chromatin structure to allow binding on RNA polymerase II, which initiates gene transcription. These fundamental gene regulatory mechanisms have now been applied to understand the regulation of inflammatory genes in diseases, such as asthma and COPD Barnes, a. The acetylation of histone that is associated with increased expression of inflammatory genes is counteracted by the activity of HDACs, of which 11 that deacetylate histones are now identified in mammalian cells de Ruijter et al.
There is now evidence that the different HDACs target different patterns of acetylation and therefore regulate different types of gene Peterson, HDACs act as corepressors in consort with other corepressor proteins, such as nuclear receptor corepressor NCoR and silencing mediator of retinoid and thyroid hormone receptors SMRT , forming a corepressor complex that silences gene expression Privalsky, In biopsies from patients with asthma there is an increase in HAT and a reduction in HDAC activity, thereby favouring increased inflammatory gene expression Ito et al.
Understanding the molecular basis for inflammatory gene expression provides the background for understanding how corticosteroids are so effective in suppressing complex inflammatory diseases that involve the increased expression of multiple inflammatory proteins. Corticosteroids diffuse readily across cell membranes and bind to glucocorticoid receptors GR in the cytoplasm. Cytoplasmic GR are normally bound to proteins, known as molecular chaperones, such as heat shock protein hsp90 and FK-binding protein, that protect the receptor and prevent its nuclear localisation by covering the sites on the receptor that are needed for transport across the nuclear membrane into the nucleus Wu et al.
GR may also be modified by phosphorylation and other modifications, which may alter the response to corticosteroids by affecting ligand binding, translocation to the nucleus, trans -activating efficacy, protein—protein interactions or recruitment of cofactors Bodwell et al.
Once corticosteroids have bound to GR, changes in the receptor structure result in dissociation of molecular chaperone proteins, thereby exposing nuclear localisation signals on GR.
This results in rapid transport of the activated GR—corticosteroid complex into the nucleus, where it binds to DNA at specific sequences in the promoter region of corticosteroid-responsive genes known as glucocorticoid response elements GRE. Two GR molecules bind together as a homodimer and bind to GRE, leading to changes in gene transcription. There are few well-documented examples of negative GREs, but some are relevant to corticosteroid side effects, including genes that regulate the hypothalamic—pituitary axis pro-opiomelanocortin and corticotrophin releasing factor , bone metabolism osteocalcin and skin structure keratins.
Corticosteroids may regulate gene expression in several ways. Corticosteroids enter the cell to bind to GR in the cytoplasm that translocate to the nucleus. GR homodimers bind to GRE in the promoter region of steroid-sensitive genes, which may encode anti-inflammatory proteins. Less commonly, GR homodimers interact with negative GREs to suppress genes, particularly those linked to side effects of corticosteroids.
Corticosteroids produce their effect on responsive cells by activating GR to directly or indirectly regulate the transcription of target genes. The number of genes per cell directly regulated by corticosteroids is estimated to be between 10 and , but many genes are indirectly regulated through an interaction with other transcription factors and coactivators.
GR homodimers bind to GRE sites in the promoter region of corticosteroid-responsive genes. GR may increase transcription by interacting with coactivator molecules, such as CBP and pCAF, thus inducing histone acetylation and gene transcription.
For example, relatively high concentrations of corticosteroids increase the secretion of the antiprotease secretory leukoprotease inhibitor SLPI from epithelial cells Ito et al. The activation of genes by corticosteroids is associated with a selective acetylation of lysine residues 5 and 16 on histone H4, resulting in increased gene transcription Ito et al.
Corticosteroids activation of anti-inflammatory gene expression. However, therapeutic doses of inhaled corticosteroids have not been shown to increase annexin-1 concentrations in bronchoalveolar lavage fluid Hall et al. However, it seems unlikely that the widespread anti-inflammatory actions of corticosteroids could be entirely explained by increased transcription of small numbers of anti-inflammatory genes, particularly as high concentrations of corticosteroids are usually required for this effect, whereas in clinical practice corticosteroids are able to suppress inflammation at low concentrations.
Relatively little is known about the molecular mechanisms of corticosteroid side effects, such as osteoporosis, growth retardation in children, skin fragility and metabolic effects. These actions of corticosteroids are related to their endocrine effects. The systemic side effects of corticosteroids may be due to gene activation.
Some insight into this has been provided by mutant GR, which do not dimerise and therefore cannot bind to GRE to switch on genes. In controlling inflammation, the major effect of corticosteroids is to inhibit the synthesis of multiple inflammatory proteins through suppression of the genes that encode them. Activated GRs may interact functionally with other activated transcription factors, without the necessity of binding to DNA nongenomic effects.
Most of the inflammatory genes that are activated in asthma do not have recognisable GRE sites in their promoter regions, yet are potently repressed by corticosteroids. Activated GR can interact directly with other activated transcription factors by protein—protein binding, but this may be a particular feature of cells in which these genes are artificially overexpressed, rather than a property of normal cells.
This suggests that corticosteroids are more likely to be acting downstream of the binding of proinflammatory transcription factors to DNA and attention has now focused on their effects on chromatin structure and histone acetylation. More importantly, particularly at low concentrations that are likely to be relevant therapeutically in asthma treatment, activated GR recruits HDAC2 to the activated transcriptional complex, resulting in deacetylation of hyperacetylated histones, and thus a decrease in inflammatory gene transcription Ito et al.
An important issue that is not yet resolved is why corticosteroids selectively switch off inflammatory genes, while having no effect on genes that regulate proliferation, metabolism and cell survival.
Corticosteroids suppression of activated inflammatory genes. This results in acetylation of core histone H4, resulting in increased expression of genes encoding multiple inflammatory proteins.
GR after activation by corticosteroids translocate to the nucleus and bind to coactivators to inhibit HAT activity directly and recruiting HDAC2, which reverses histone acetylation leading in suppression of these activated inflammatory genes.
Methylation of histones, particularly histone H3, by histone methyltransferases, usually results in gene suppression Bannister et al. Nonhistone proteins are also acetylated by HATs and deacetylated by HDACs and this may be an important mechanism of regulating their function Glozak et al. Several nuclear receptors, including the oestrogen and androgen receptors, may be acetylated and this affects binding of their hormones Fu et al.
The site of acetylation of GR is the lysine rich region —— with the sequence KKTK, which is analogous to the acetylation sites identified on other nuclear hormone receptors. Although most of the actions of corticosteroids are mediated by changes in transcription through chromatin remodelling, it is increasingly recognised that they may also affect protein synthesis by reducing the stability of mRNA so that less protein is synthesised.
It is increasingly recognised that several inflammatory proteins are regulated post-transcriptionally at the level of mRNA stability Anderson et al. This may be an important anti-inflammatory mechanism as it allows corticosteroids to switch off the ongoing production of inflammatory proteins after the inflammatory gene has been activated.
Corticosteroids may have inhibitory effects on the proteins that stabilise mRNA, leading to more rapid breakdown and thus a reduction in inflammatory protein expression Newton et al.
Although inhaled corticosteroids are highly effective in asthma, they provide relatively little therapeutic benefit in COPD, despite the fact that active airway and lung inflammation is present. This may reflect that the inflammation in COPD is not suppressed by corticosteroids, with no reduction in inflammatory cells, cytokines or proteases in induced sputum even with high doses of inhaled and oral corticosteroids Keatings et al.
Furthermore, histological analysis of peripheral airways of patients with severe COPD shows an intense inflammatory response, despite treatment with high doses of inhaled corticosteroids Hogg et al. In vitro studies show that cytokine release from alveolar macrophages is markedly resistant to the anti-inflammatory effects of corticosteroids, compared to cells from normal smokers and these in turn are more resistant than alveolar macrophages from nonsmokers Lim et al.
This lack of response to corticosteroids may be explained, at least in part, by an inhibitory effect of cigarette smoking and oxidative stress on HDAC function, thus interfering with the critical anti-inflammatory action of corticosteroids Ito et al.
Indeed, there is a correlation between HDAC activity and the suppressive effects of a corticosteroid on cytokine release. The reduced HDAC2 expression in alveolar macrophages in COPD patients can be restored by inducing overexpression of HDAC2 using a viral vector and this is associated with restoration of corticosteroid responsiveness in these cells Ito et al.
Oxidative stress in the presence of increased nitric oxide production results in the formation of peroxynitrite, which may then nitrate certain tyrosine residues on proteins. Peroxynitrite markedly reduces the anti-inflammatory effect of corticosteroids Ito et al. We have also demonstrated that nitration of HDAC2 targets it for ubiquitination and destruction by the proteasome, resulting in the low protein levels found in COPD patients.
A proposed mechanism of corticosteroid resistance in COPD, severe asthma and smoking asthma. In COPD and smoking asthmatic patients cigarette smoke generates oxidative stress acting through the formation of peroxynitrite to impair the activity of HDAC2. A similar mechanism may operate in severe asthma where increased oxidative stress is generated by airway inflammation. Peroxynitrite nitrates certain tyrosine residues Tyr and this may inactive the catalytic activity of HDAC2 and also mark the enzyme for ubiquitination Ub , resulting in destruction by the proteasome.
This loss of HDAC2 results in increased histone acetylation, leading to amplification of inflammation and blocking the anti-inflammatory effects of corticosteroids. Moreover, this results in excessive acetylated GR that may then bind to GRE sites to induce side effect genes. This means that in COPD patients, where there is a reduction in acetylated GR unpublished observations , not only is the anti-inflammatory action of corticosteroids lost, but side effects more be seen more frequently.