Loss of surface beta-2 adrenoceptors accounts for the insensitivity of cultured human monocytes to beta-2 adrenoceptor agonists

The short-acting beta-2 adrenoceptor agonists (β ₂-agonists) , such as salbutamol, are effective bronchodilators used to treat asthma. They lack significant anti-inflammatory effect in vivo as well as on isolated alveolar macrophages even though they exhibit this effect on freshly isolated monocytes. The purpose of this study was to determine if this observation is related to a change in the expression and/or function of surface β ₂-receptors during the differentiation of these cells to macrophages. Purified monocytes, cultured for 1-48 h were pre-treated with the β ₂-agonists (salbutamol or procaterol) or PGE ₂ before being stimulated with bacterial lipopolysaccharide (LPS). Subsequently, the amount of TNF-α (a typical inflammatory mediator) released over 24 h, as well as agonist-stimulated cAMP, were determined by enzyme immunoassays. Western blotting techniques were used to study the expression of the membrane β ₂-receptor protein. Results showed that in freshly isolated human monocytes, both the β ₂-agonists and PGE ₂ significantly inhibited LPS-induced TNF-α release as well as increased intracellular cAMP. After culturing adherent monocytes for 24-48 h, the ability of the β ₂-agonists to produce both effects was completely lost, whereas that of PGE ₂ was essentially intact. Western blotting data showed a near complete loss of surface expression of β ₂- receptors in cells cultured for ≥ 24 h. These results show that as human monocytes adhere to surfaces to begin differentiation into macrophages, they selectively lose their surface β ₂-receptors and hence become insensitive to the anti-inflammatory effect of β ₂-agonists. This may explain why β ₂-agonists lack significant anti-inflammatory effect on alveolar macrophages or in clinical asthma.