CINCINNATI—ß-Agonists, a mainstay of asthma therapy, appear to lose their effectiveness in some patients when used on a regular basis. The result is that instead of acting to relieve bronchoconstriction, these drugs increase bronchial hyperresponsiveness. In the past, researchers have suggested that desensitization to ß-agonists is responsible for this untoward effect.

Recently, however, a team from the University of Cincinnati concluded that prolonged activation of the ß-adrenergic receptor (ß2AR) leads to increased expression of phospholipase C (PLC) in airway smooth muscle. The increase in PLC expression causes “crosstalk” between the ß2AR signaling pathways, leading to airway hyperresponsiveness.[1] If these results are confirmed, new approaches to the pharmacotherapy of asthma may open up.

UNEXPECTED RESPONSES

The researchers used mice that were bred to either lack or overexpress ß2AR. Both groups were compared with wild-type mice matched for age and strain.

Airway responsiveness to methacholine was measured noninvasively, and then invasive assessment of airway reactivity was performed. Tracheal ring contractility was measured using excised tracheas from which primary cultures of airway smooth muscle were taken.

The wild-type mice showed a normal bronchoconstrictor response to methacholine, with a maximum enhanced pause of 4.9. Because they were missing an important protective pathway, the ß2AR-knockout mice were expected to have an increased bronchoconstrictor response. Instead, they had a maximum enhanced pause of only 1.4. The mice were then exposed to a different bronchoconstrictor (U46619), with the same result: a diminished response in the knockout group.

In the tracheal rings of wild-type mice, acetylcholine-mediated maximal active force was 13.3 mN/mm2, whereas the active force attained by the knockout mice was only 5.8 mN/mm2. The contractile response of tracheal rings to U46619 was approximately sevenfold lower in the knockout mice than in the wild-type mice. However, contractility was present—and even slightly greater than that of the wild-type mice—in response to ß-escin. Thus, the intrinsic ability of airway smooth muscle to contract was unaffected by removing the ß2AR gene.

In mice overexpressing ß2AR, levels of gene expression were about 10 times higher than those in wild-type mice, indicating persistent activation of the signaling pathway in the smooth muscle cells of the airway. The physiologic response to contractile agents was similarly increased. In the tracheal rings, the maximal active force generated by the mice overexpressing ß2AR was 33% greater than that generated by the wild-type mice.

PHOSPHOLIPASE C

Western blot analysis of airway smooth muscle cells showed that PLC content in the cells of knockout mice was decreased by more than 60% compared with that in wild-type derived cells. In contrast, PLC content in cells from mice overexpressing ß2AR was twice as high as that in cells from the wild-type mice.

OBSERVATIONS AND THEORIES

The phenomenon of increased airway responsiveness with prolonged and regular use of ß-agonists has been considered a physiologically relevant adverse effect, and an explanation for it has long been sought by researchers.

Considerable evidence suggests that genetic factors contribute to the increased risk. For example, in a study by Israel et al,[2] patients with asthma were randomized to use ß-agonists either regularly (two puffs, four times a day) or as needed, for 16 weeks. Some patients in the regular-use group experienced a steady decline in morning peak flow; the sharpest decreases occurred in the four-week period after ß-agonists had been discontinued.

In this study, the decline in morning peak flow was only apparent in patients with a particular ß2AR polymorphism. Interestingly, patients with this polymorphism who were in the as-needed group experienced no adverse effects.

“If it were simply a case of desensitization,” observed Stephanie A. Shore, PhD, a Senior Lecturer in Physiology at the Harvard School of Public Health, “one would have expected patients to be back to normal within a few days of stopping ß-agonists. Instead, their lung function continued to decline for weeks afterward.”

Thus, additional explanations for the increased airway responsiveness are needed. Although desensitization of airway ß2AR may contribute to less effective bronchodilation, it does not account for increased ß-agonist– induced hyperreactivity. The new mouse study provides another piece in this puzzle, demonstrating that ß2AR activation can trigger a chain of events leading to enhanced bronchoconstriction.

CONFIRMATION IN HUMANS STILL NEEDED

In an editorial commentary,[3] Dr. Shore and Jeffrey Drazen, MD, pointed out that the observations in mice must be confirmed in human airway smooth muscle stimulated with ß-agonists. They also suggested that other explanations for the ß-agonist paradox might exist in addition to the one observed in the present study.

For instance, said Dr. Shore in an interview, “ß-agonists can induce the expression of proinflammatory genes, such as interleukin 6 or vascular endothelial growth factor, which could ultimately result in airway remodeling. If ß-agonists are indeed remodeling the airways, it would take a long time to recover from their effects.”

As a potential asthma-modifier gene, PLC may become a target for new therapies. A PLC inhibitor, said Dr. Shore, would be expected to decrease G-protein receptor–mediated bronchoconstriction and minimize the ß-agonist–induced increase in airway hyperresponsiveness.

Thus, the ß-agonist paradox may lead to “paradoxic pharmacology” in asthma patients who need to use ß-agonists regularly. As for the full mechanism behind the ß-agonist paradox? “The jury is still out,” said Dr. Shore.

—Gale Jurasek