Molecular effects of inhaled glucocorticoid therapy in asthma

INTRODUCTION — Inhaled glucocorticoids (ie, glucocorticosteroids, corticosteroids, steroids) suppress airway inflammation by activating anti-inflammatory genes, switching off inflammatory gene expression, and inhibiting inflammatory cells. In addition, they enhance beta 2 adrenergic signaling by increasing beta 2-receptor expression and function. The net effect is control of the symptoms and signs of asthma in most patients.

The molecular effects of inhaled glucocorticoids in asthmatic airways are discussed in this topic review. In addition, the molecular determinants of glucocorticoid sensitivity are reviewed. The pharmacology of glucocorticoids, the role of inhaled glucocorticoids in the management of asthma, and the potential adverse effects of inhaled glucocorticoids are presented separately. (See "Pharmacologic use of glucocorticoids" and "Determinants of glucocorticoid dosing" and "An overview of asthma management" and "Asthma in children younger than 12 years: Overview of initiating therapy and monitoring control" and "Mechanisms and clinical implications of glucocorticoid resistance in asthma" and "Major side effects of inhaled glucocorticoids".)

SUPPRESSION OF INFLAMMATION — Discoveries related to gene transcription have improved our understanding of the mechanisms by which inhaled glucocorticoids suppress airway inflammation [1,2]. These mechanisms include anti-inflammatory gene activation and switching off inflammatory gene expression, which alter the expression of inflammatory and anti-inflammatory enzymes, receptors, cytokines, adhesion molecules, and chemokines. The net effect is decreased inflammatory cell recruitment, survival, and accumulation.

Anti-inflammatory gene activation — There are two types of glucocorticoid receptors (GR), GR alpha and GR beta. Glucocorticoid action is facilitated by GR alpha, but potentially inhibited by GR beta.

●GR alpha – Glucocorticoids passively diffuse across the cell membrane and bind to GR alpha in the cytoplasm [2]. The glucocorticoid/GR alpha complex (ie, activated GR) rapidly translocates into the nucleus. There, the activated GR form dimers, which bind to glucocorticoid response elements (GREs) within the promoter of glucocorticoid-responsive genes. There are positive and negative GREs.

Interaction with positive GREs stimulates gene transcription. Such genes usually encode anti-inflammatory proteins [3]. As an example, glucocorticoids stimulate expression of secretory leukoprotease inhibitor and mitogen-activated protein kinase phosphatase-1 (MKP-1), which inhibit mitogen-activated protein (MAP) kinase pathways (figure 1) [4]. Interaction with positive GREs appears to account for most of the side effects of glucocorticoids [5]. As an example, binding the GR to the promoter site on the osteocalcin gene interferes with its transcription, resulting in reduced expression and reduction in bone synthesis.

Interaction with negative GREs suppresses gene transcription, but there are only a few examples of reduced expression of inflammatory genes through interaction with negative GREs, indicating that other molecular mechanisms are involved in the suppression of inflammation.

●GR beta – GR beta is an alternatively spliced form of GR. It binds DNA, but not glucocorticoids. As a result, GR beta may act as an inhibitor of glucocorticoid action by antagonizing the binding of GR alpha dimers to DNA, although the amounts of GR beta are thought to be too small to cause significant inhibition of glucocorticoid effects [6].

Switching off inflammatory genes — In all inflammatory diseases, including asthma, many inflammatory genes are activated. These include genes that encode cytokines, chemokines, adhesion molecules, inflammatory enzymes, and receptors. Such genes are switched on when a coactivator molecule (eg, CREB-binding protein) binds to a proinflammatory transcription factor (eg, nuclear factor-kappa B [NF-kB], activator protein-1 [AP-1]). The coactivator molecule then acetylates core histones, which opens the chromatin structure and allows transcription to proceed [7].

There are several ways in which inhaled glucocorticoids interfere with this process and switch off the inflammatory cascade:

●Glucocorticoids interact with the coactivator molecules and inhibit their binding to proinflammatory transcription factors [1,8].

●Activated GR recruit histone deacetylase-2 (HDAC2), which deacetylates core histones. HDAC2 has broad impact, suppressing all activated inflammatory genes within the nucleus (figure 2).

●GR is normally acetylated and binds to DNA in its acetylated form. It has to be deacetylated by HDAC2 in order to inhibit NF-kB [9].

●GR is translocated into the nucleus via the nuclear import protein importin-alpha [10]. The transcription factor GATA3 in T-helper 2 (Th2) lymphocytes regulates the expression of Th2 cytokines (interleukin [IL]-4, IL-5, and IL-13), which orchestrate allergic inflammation. GATA3 is also imported into the nucleus via importin-alpha, but activated GR takes precedence, preventing the nuclear import of GATA3, and thus rapidly inhibiting the expression of Th2 cytokines and suppressing allergic inflammation [11].

Regardless of the mechanism, the net effect is reduced histone acetylation, failure of the chromatin structure to open, and decreased transcription of inflammatory genes. Inhaled glucocorticoids preferentially impact inflammatory genes and do so primarily on a local level. Thus, they are effective at controlling asthma and are associated with few adverse effects.

There may also be post-transcriptional mechanisms by which glucocorticoids switch off inflammation. As an example, some inflammatory genes (eg, granulocyte-macrophage colony stimulating factor) normally have an unstable messenger RNA that is rapidly degraded by RNAses. Inflammatory mediators stabilize this messenger RNA during inflammation. Glucocorticoids reverse this effect, allowing rapid degradation of mRNA and reduced inflammatory protein secretion [12]. The mechanism of action is uncertain, but may be glucocorticoid-mediated inhibition of the proteins that stabilize mRNAs, such as tristetraprolin [13].

Further research is needed to determine which mechanisms are relevant in patients with asthma. There have been past examples of glucocorticoid-mediated effects that were detected experimentally, but could not subsequently be confirmed in patients. Specifically, the activated GR directly interacts with transcription factors and inhibits their activity in experimental systems, but not in patients with asthma [14].

Additionally, some effects of glucocorticoids may be mediated through non-genomic effects that are GR-independent and not blocked by the GR antagonist RU486 [15]. These responses include rapid effects on calcium signaling, apoptosis of inflammatory cells, and endothelial nitric oxide synthase, but whether these are relevant to the action of glucocorticoids in airway disease is uncertain.

Inflammatory cell inhibition — By activating anti-inflammatory genes and switching off inflammatory gene expression, inhaled glucocorticoids inhibit inflammatory cell survival and suppress production of chemotactic mediators and adhesion molecules. The net effect is decreased mucosal inflammation, which is characterized by fewer inflammatory cells, including T lymphocytes, eosinophils, mast cells, and dendritic cells (figure 3).

Suppression of mucosal inflammation begins within hours of administration [16,17]. As an example, in a double-blind crossover trial, 41 adults with stable asthma and at least 7 percent sputum eosinophilia were randomly assigned to receive a single dose of inhaled budesonide or placebo on two consecutive days [16]. Budesonide decreased sputum eosinophilia within six hours.

Decreased airway inflammation is associated with decreased airway responsiveness, which also begins within hours of inhaled glucocorticoid administration [16-18]. As an example, in three randomized, controlled crossover trials, fluticasone decreased airway responsiveness two- to three-fold within two hours [17]. This effect may gradually increase over several months before reaching a plateau [18].

BETA 2-RECEPTOR EFFECTS — Suppression of airway inflammation is not the only way in which inhaled glucocorticoids reduce the symptoms and signs of asthma. They also enhance beta 2 adrenergic signaling by increasing beta 2-receptor expression and function.

Inhaled beta 2-agonists and glucocorticoids are frequently used together in the management of asthma. It is now recognized that there are important molecular interactions between these two classes of medication [19,20]. Specifically, glucocorticoids increase transcription of the gene encoding beta 2-receptors, inducing increased expression of beta 2-receptors on the cell surface [21,22]. They also enhance the coupling of beta 2-receptors to G-proteins. Taken together, these actions have the following impact:

●Increased beta 2-agonist effects (eg, mast cell stabilization)

●Protection from the down-regulation of beta 2-receptors that is associated with long-term beta 2-agonist administration [23]

●Reversal or prevention of the uncoupling of beta 2-receptors from G proteins, which is induced by some inflammatory mediators (eg, interleukin-1 beta) [24]

In a reciprocal manner, beta 2-agonists influence the activated glucocorticoid receptors (GR). Specifically, beta 2-agonists increase translocation of the activated GR from the cytoplasm to the nucleus, which enhances the anti-inflammatory effects of glucocorticoids [25]. This has been demonstrated in the sputum macrophages of patients with asthma following administration of an inhaled glucocorticoid plus an inhaled long-acting beta 2-agonist [26].

GLUCOCORTICOID SENSITIVITY — Some patients (usually with severe asthma) fail to respond to glucocorticoids, both inhaled and systemic. Such "steroid resistant" asthma may be due to reduced anti-inflammatory actions of glucocorticoids, rather than impaired absorption, increased metabolism, or poor compliance. (See "Mechanisms and clinical implications of glucocorticoid resistance in asthma".)

The ratio of GR alpha (which facilitates glucocorticoid action) to GR beta (which inhibits glucocorticoid action) can be altered by changing the expression GR alpha, GR beta, or both. In glucocorticoid sensitive cells the ratio tends to be higher, whereas in glucocorticoid resistant cells the ratio tends to be lower [6]. However, there is little evidence that decreases in the ratio of GR alpha to GR beta reduce glucocorticoid responsiveness because the amount of GR beta is too small to influence GR action. Other molecular mechanisms are more likely to account for glucocorticoid resistance in asthma [27,28].

SUMMARY AND RECOMMENDATIONS

●Clinical use – Inhaled glucocorticoids are potent suppressors of inflammation in the airways. They effectively control the symptoms and signs of asthma. (See 'Introduction' above.)

●Molecular effects in the airways – The success of inhaled glucocorticoids in controlling the symptoms and signs of asthma is the net result of many favorable effects in the airways. These effects include:

•Anti-inflammatory gene activation and switching off inflammatory gene expression, which combine to inhibit inflammatory cells infiltration and survival in the airways. (See 'Suppression of inflammation' above.)

•Increased expression of beta 2-receptors and enhanced coupling of beta 2-receptors to G-proteins, which combine to increase beta 2 adrenergic signaling. (See 'Beta 2-receptor effects' above.)

●Glucocorticoid sensitivity – Glucocorticoid resistant asthma is most commonly due to reduced anti-inflammatory actions of glucocorticoids, rather than impaired absorption, increased metabolism, or poor compliance. (See 'Glucocorticoid sensitivity' above and "Mechanisms and clinical implications of glucocorticoid resistance in asthma".)