Mechanisms by which S-albuterol induces human bronchial smooth muscle cell proliferation

Racemic albuterol is a 50:50 mixture of the R-isomer, levalbuterol, and the S-isomer, S-albuterol. S-Albuterol increases airway hyperresponsiveness to spasmogens, exacerbates asthmatic conditions and stimulates cell growth, whereas levalbuterol attenuates cell growth in culture. The mechanisms of S-albuterol-induced cell proliferation are not well understood. We studied the role of albuterol isomers and intracellular cell cycle regulators on proliferation of human bronchial smooth muscle cells.

Serum-starved cells (72 h) were fed test agents for 24 h and cell proliferation was measured. The expression of nuclear factor-kappaB inhibitory protein IkappaBalpha, nuclear factor-kappaB, cyclin-dependent kinases 2 and 4, interleukin (IL)-6, and retinoblastoma and platelet-activating factor (PAF) receptor protein were measured by Western blotting.

S-Albuterol, PAF and platelet-derived growth factor stimulated cell proliferation, but levalbuterol and the racemic mixture inhibited cell proliferation compared with the effect of 5% fetal bovine serum alone. The proliferative effect of platelet-derived growth factor on S-albuterol was not additive, suggesting that the 2 mediators act by different mechanisms. S-Albuterol induced greater expression of all the measured proteins than either levalbuterol, the racemic mixture or 5% fetal bovine serum. S-Albuterol stimulated IL-6 secretion and abolished the ability of levalbuterol to inhibit IL-6 secretion.

Our data show that S-albuterol stimulates cell proliferation by activating expression and phosphorylation of several intracellular mitogenic proteins and may exacerbate asthma by stimulating the release of IL-6. Induction of PAF receptor protein expression by S-albuterol strongly suggests that S-albuterol may exert its adverse effects by binding to a G protein-coupled receptor such as the PAF receptor.