RESUMO
Based on their bronchodilatory effect, ß2-adrenoceptor agonists constitute essential elements in the treatment of bronchial asthma and COPD. As treatment with ß2-adrenoceptor agonists has been associated with worsening of airway hyper-reactivity, possibly because of loss of ß-adrenoceptor function, molecular mechanism of the regulation of ß2-adrenoceptor expression were studied. MRC-5 human lung fibroblasts were cultured in absence or presence of test substances followed by ß2-adrenoceptor messenger RNA (mRNA) determination by qPCR. After inhibition of mRNA synthesis by actinomycin D, ß2-adrenoceptor mRNA decreased with a half-life of 23 min, whereas inhibition of protein synthesis by cycloheximide caused an about 5- and 6-fold increase within 1.5 and 4 h, respectively. ß2-Adrenoceptor mRNA was increased by about 100 % after 1 h exposure to formoterol or olodaterol but decreased by about 60 % after 4 h agonist exposure. Both effects of ß2-adrenoceptor agonists were mimicked by forskolin, a direct activator of adenylyl cyclase and cholera toxin, which stimulates adenylyl cyclase by permanent activation of Gs. ß2-Adrenoceptor agonist-induced upregulation of ß2-adrenoceptor mRNA was blocked by the ß2-adrenoceptor antagonist ICI 118551 and prevented by actinomycin D, but not by cycloheximide. Moreover, in presence of cycloheximide, ß2-adrenoceptor agonist-induced reduction in ß2-adrenoceptor mRNA was converted into stimulation, resulting in a more than 10-fold increase. In conclusion, expression of ß2-adrenoceptors in human lung fibroblasts is highly regulated at transcriptional level. The ß2-adrenoceptor gene is under strong inhibitory control of short-living suppressor proteins. ß2-Adrenoceptor activation induces via adenylyl cyclase - cyclic adenosine monophosphate (cAMP) signaling a rapid in onset direct stimulation of the ß2-adrenoceptor gene transcription, an effect opposed by a delayed upregulation of inhibitory factors.