A Holy Grail of asthma management: toward understanding how long-acting beta(2)-adrenoceptor agonists enhance the clinical efficacy of inhaled corticosteroids

Abstract

There is unequivocal evidence that the combination of an inhaled corticosteroid (ICS) -- i.e. glucocorticoid -- and an inhaled long-acting beta(2)-adrenoceptor agonist (LABA) is superior to each component administered as a monotherapy alone in the clinical management of asthma. Moreover, Calverley and colleagues (Lancet 2003, 361: 449-456; N Engl J Med 2007, 356: 775-789) reporting for the 'TRial of Inhaled STeroids ANd long-acting beta(2)-agonists (TRISTAN)' and 'TOwards a Revolution in COPD Health (TORCH)' international study groups also demonstrated the superior efficacy of LABA/ICS combination therapies over ICS alone in the clinical management of chronic obstructive pulmonary disease. This finding has been independently confirmed indicating that the therapeutic benefit of LABA/ICS combination therapies is not restricted to asthma and may be extended to other chronic inflammatory diseases of the airways. Despite the unquestionable benefit of LABA/ICS combination therapies, there is a vast gap in our understanding of how these two drugs given together deliver superior clinical efficacy. In this article, we review the history of LABA/ICS combination therapies and critically evaluate how these two classes of drugs might interact at the biochemical level to suppress pro-inflammatory responses. Understanding the molecular basis of this fundamental clinical observation is a Holy Grail of current respiratory diseases research as it could permit the rational exploitation of this effect with the development of new 'optimized' LABA/ICS combination therapies.

Figure 1

Classical β₂-adrenoceptor (β₂-AR) signalling in which the β₂-AR couples via Gsα to adenylyl cyclase (AC) increases the formation of cAMP and activates PKA to elicit responses, such as the enhancement of GRE-dependent transcription. GRE, glucocorticoid response element; PKA, cAMP-dependent protein kinase.

Figure 2

Enhanced expression of MKP-1, but not GILZ, may explain additive effects of LABA/corticosteroid combination therapies in the repression of inflammatory gene expression. A schematic representation of the signalling cascades leading to inflammatory gene expression is depicted with possible targets and sites of action for putative anti-inflammatory glucocorticoid-inducible genes. Activation of a pro-inflammatory cascade, following binding of cytokine to its cognate receptor in the plasma membrane (pm), is shown occurring via a number of kinases (K1–3). The signal crosses the nuclear membrane (nm) and leads to transcription factor (TF) activation and the production of inflammatory gene mRNAs. Under the influence of further kinase cascades (here K1–3), the mRNA is stabilized and translated into protein. Finally, many proteins (for example cytokines, chemokines) are exported into the extracellular space for biological function. Sites of action of the two glucocorticoid-inducible genes, MKP-1 and GILZ, are indicated. MKP-1 is an inhibitor of the MAP kinase family of protein kinases and may, therefore, impact on numerous cellular mechanisms, including the activation of transcription, mRNA stability and translation. GILZ inhibits key inflammatory transcription factors (NF-κB and AP-1). AP-1, activator protein-1; GILZ, glucocorticoid-inducible leucine zipper; LABA, long-acting β₂-adrenoceptor agonist; MAP, mitogen-activated protein; MKP, mitogen-activated protein kinase phosphatase; NF-κB, nuclear factor κB.

Figure 3

LABA/ICS combinations are both steroid-sparing and enhance GRE-dependent transcription. BEAS-2B cells stably expressing a GRE-reporter construct were treated concurrently with salmeterol (100 nM; a) or formoterol (10 nM; b) in the absence and presence of fluticasone and budesonide, respectively (10 pM–100 nM). After 6 h, cells were harvested for luciferase assay. The data show that the effect of formoterol/budesonide and salmeterol/fluticasone in combination promotes GRE-dependent transcription, above what can be achieved by the glucocorticoid alone, and is steroid sparing. Thus, in this simple system, neither LABA activated the GRE reporter construct (not shown), but markedly potentiated glucocorticoid-induced transcription (2.6- and 2.1-fold for salmeterol and formoterol respectively at the E_max). In addition, salmeterol and formoterol were glucocorticoid sparing in this model. Thus, both glucocorticoids at concentrations that evoked 90% of the maximum response produced a 12- to 15-fold induction of the luciferase gene. However, in the presence of salmeterol (100 nM) or formoterol (10 nM), which were inactive, the same degree of gene induction was achieved at a concentration of glucocorticoid that was significantly (∼10-fold) lower. Note: this measurement was made at the EC₉₀ concentration of glucocorticoid (as the upper asymptote, by definition, is never reached) and so the degree to which the LABAs are steroid sparing is underestimated. See text and Kaur et al. (2007) for further details. GRE, glucocorticoid response element; ICS, inhaled corticosteroid; LABA, long-acting β₂-adrenoceptor agonist.

Figure 4

Effect of salmeterol on the induction by dexamethasone of p57KIP2, GILZ and MKP-1. BEAS-2B cells were treated with dexamethasone (1 μM) salmeterol (100 nM), a combination of the two drugs or vehicle for 1–18 h and harvested for real-time PCR analysis of p57KIP2, GILZ, MKP-1 and GAPDH mRNA using SYBR GreenER technology. Data are expressed as a ratio to the housekeeping gene, GAPDH. See text for further details. GILZ, glucocorticoid-inducible leucine zipper; MKP, mitogen-activated protein kinase phosphatase.