How Do Corticosteroids Work in Asthma?

Clinical Principles Asthma is the most common chronic disease in westernized countries. Patients with asthma have an underlying chronic inflammation of the airways characterized by activated mast cells, eosinophils, and T-helper 2 lymphocytes. This results in increased responsiveness of the airways to such triggers as exercise, allergens, and air pollutants. This chronic inflammation underlies the typical symptoms of asthma, which include intermittent wheezing, coughing, shortness of breath, and chest tightness. Corticosteroids are the most effective treatment for asthma, and inhaled corticosteroids have become first-line treatment for children and adults with persistent symptoms. Corticosteroids suppress the chronic airway inflammation in patients with asthma, and the molecular mechanisms involved are now being elucidated. Physiologic Principles Inflammation in asthma is characterized by the increased expression of multiple inflammatory genes, including those encoding for cytokines, chemokines, adhesion molecules, and inflammatory enzymes and receptors. Increased expression of inflammatory genes is regulated by proinflammatory transcription factors, such as nuclear factor-B and activator protein-1. These bind to and activate coactivator molecules, which then acetylate core histones and switch on gene transcription. Corticosteroids suppress the multiple inflammatory genes that are activated in asthmatic airways by reversing histone acetylation of the activated inflammatory genes. This mechanism acts by binding of the activated glucocorticoid receptors to coactivators and recruitment of histone deacetylases to the activated transcription complex. Understanding how corticosteroids work in patients with asthma may help in designing novel corticosteroids with less systemic effects, as well as novel anti-inflammatory approaches. These molecular mechanisms of action of corticosteroids may also help elucidate the molecular basis of chronic inflammation and why corticosteroids are ineffective in patients with steroid-resistant asthma and with chronic obstructive pulmonary disease. Corticosteroids (or glucocorticosteroids) are widely used to treat various inflammatory and immune diseases. The most common use of corticosteroids today is in the treatment of asthma, and inhaled corticosteroids have become established as first-line treatment in adults and children with persistent asthma, the most common chronic inflammatory disease. Recent developments in understanding the fundamental mechanisms of gene transcription (see Glossary) have led to major advances in understanding the molecular mechanisms by which corticosteroids suppress inflammation. This may have important clinical implications, as it will lead to a better understanding of the inflammatory mechanisms of many diseases and may signal the future development of new anti-inflammatory treatments. The new understanding of these new molecular mechanisms also helps explain how corticosteroids switch off multiple inflammatory pathways; in addition, it provides insights into why corticosteroids fail to work in patients with steroid-resistant asthma and in patients with chronic obstructive pulmonary disease (COPD). The Molecular Basis of Inflammation in Asthma All patients with asthma have a specific pattern of inflammation in the airways that is characterized by degranulated mast cells, an infiltration of eosinophils, and an increased number of activated T-helper 2 cells (see Glossary) (1). It is believed that this specific pattern of inflammation underlies the clinical features of asthma, including intermittent wheezing, dyspnea, cough, and chest tightness. Suppression of this inflammation by corticosteroids controls and prevents these symptoms in most patients. Multiple mediators are produced in asthma, and the approximately 100 known inflammatory mediators that are increased in patients with asthma include lipid mediators, inflammatory peptides, chemokines, cytokines, and growth factors (2). Increasing evidence suggests that structural cells of the airways, such as epithelial cells, airway smooth-muscle cells, endothelial cells, and fibroblasts, are a major source of inflammatory mediators in asthma. Epithelial cells may play a particularly important role because they may be activated by environmental signals and may release multiple inflammatory proteins, including cytokines, chemokines, lipid mediators, and growth factors. Inflammation is mediated by the increased expression of multiple inflammatory proteins, including cytokines, chemokines, adhesion molecules, and inflammatory enzymes and receptors. Most of these inflammatory proteins are regulated by increased gene transcription, which is controlled by proinflammatory transcription factors, such as nuclear factor-B (NF-B) and activator protein-1 (AP-1), that are activated in asthmatic airways (see Glossary) (3). For example, NF-B is markedly activated in epithelial cells of asthmatic patients (4), and this transcription factor regulates many of the inflammatory genes that are abnormally expressed in asthma (5). Nuclear factor-B may be activated by rhinovirus infection and allergen exposure, both of which exacerbate asthmatic inflammation (6). Chromatin Remodeling The molecular mechanisms by which inflammatory genes are switched on by transcription factors are now much better understood. Alteration in the structure of chromatin (see Glossary) is critical to the regulation of gene expression. Chromatin is made up of nucleosomes, which are particles consisting of DNA associated with an octomer of two molecules each of the core histone proteins (see Glossary) (H2A, H2B, H3, and H4) (Figure 1). Expression and repression of genes are associated with remodeling of this chromatic structure by enzymatic modification of core histones. Each core histone has a long terminal that is rich in lysine residues that may be acetylated, thus changing the electrical charge of the core histone. In the resting cell, DNA is wound tightly around these basic core histones, excluding the binding of the enzyme RNA polymerase II (see Glossary), which activates the formation of messenger RNA (mRNA) (see Glossary). This conformation of the chromatin structure is described as closed and is associated with suppression of gene expression. Gene transcription occurs only 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. When proinflammatory transcription factors, such as NF-B, are activated, they bind to specific recognition sequences in DNA and subsequently interact with large coactivator molecules, such as p300/CREB (cyclic adenosine monophosphate response elementbinding protein)-binding protein (CBP) and p300/CBP-associated factor (PCAF) (see Glossary). These coactivator molecules act as the molecular switches that control gene transcription. All have intrinsic histone acetyltransferase (HAT) (see Glossary) activity (7, 8), which results in acetylation of core histones, thereby reducing their charge. Acetylation allows the chromatin structure to transform from the resting closed conformation to an activated open form (8). This results in unwinding of DNA, binding of TATA boxbinding protein (TBP) (see Glossary), TBP-associated factors, and RNA polymerase II, which initiates gene transcription. This molecular mechanism is common to all genes, including those involved in differentiation, proliferation, and activation of cells. An important step forward has been the discovery of the enzymes that regulate histone acetylation. Core histones are characterized by long N-terminal tails rich in lysine residues that are the target for acetylation. In general, HATs act as coactivators that switch genes on; histone deacetylases (HDACs), which act as co-repressors (see Glossary), switch genes off (Figure 2). Figure 1. Structure of chromatin. K Figure 2. Gene activation and repression are regulated by acetylation of core histones. HDAC Recently, these fundamental mechanisms have been applied to understanding the regulation of inflammatory genes that become activated in inflammatory diseases. In humans, epithelial cell line activation of NF-B (by exposing the cell to inflammatory signals, such as interleukin-1, tumor necrosis factor-, or endotoxin) results in acetylation of specific lysine residues on histone-4 (the other histones do not seem to be so markedly acetylated), and this is correlated with increased expression of inflammatory genes, such as granulocyte-macrophage colony-stimulating factor (GM-CSF) (9). The acetylation of histone that is associated with increased expression of inflammatory genes is counteracted by the activity of HDACs (> 12 that are associated with gene suppression have been characterized [10]). In biopsy samples from patients with asthma, HAT activity is increased and HDAC activity is decreased, thus favoring increased inflammatory gene expression (11). Improved understanding of the molecular basis of asthma has helped to explain how corticosteroids are so effective in suppressing this complex inflammation that involves many cells, mediators, and inflammatory effects. Cellular Effects of Corticosteroids Corticosteroids are the only therapy that suppresses the inflammation in asthmatic airways; this action underlies the clinical improvement in asthma symptoms and prevention of exacerbations (12, 13). At a cellular level, corticosteroids reduce the number of inflammatory cells in the airways, including eosinophils, T lymphocytes, mast cells, and dendritic cells (Figure 3). These remarkable effects of corticosteroids are produced through inhibiting the recruitment of inflammatory cells into the airway by suppressing the production of chemotactic mediators and adhesion molecules and by inhibiting the survival in the airways of inflammatory cells, such as eosinophils, T lymphocytes, and mast cells. Epithelial cells m