TGF-β1 modulates β-adrenergic receptor number and function in cultured human tracheal smooth muscle cells

Pretreatment of cultured human tracheal smooth muscle cells with transforming growth factor-β1 (TGF-β1) decreased adenosine 3',5'-cyclic monophosphate (cAMP) accumulation by intact cells stimulated with the β- adrenergic agonist isoproterenol. The maximal inhibition of isoproterenol- stimulated cAMP accumulation by TGF-β1 was 31 ± 3%, and the mean effective concentration (EC₅₀) of TGF-β1 was ~1.5 pM. TGF-β1 decreased the maximal response to isoproterenol but did not change the EC₅₀ value of isoproterenol. TGF-β1 did not change cAMP accumulation stimulated by forskolin. TGF-β1 pretreatment decreased isoproterenol-stimulated adenylyl cyclase activity measured in broken cell preparations, but did not change the fluoride-stimulated adenylyl cyclase activity. Together these results suggest that the TGF-β1 effect is not by direct inhibition of adenylyl cyclase or by decreased activity of the stimulatory GTP-binding protein. Saturation binding experiments with the β-adrenergic receptor radioligand [¹²⁵I]iodopindolol showed that TGF-β1 pretreatment decreased the β-adrenergic receptor number. The protein synthesis inhibitor cycloheximide abolished the effect of TGF- β1 on both cAMP accumulation and on β-adrenergic receptor number, indicating that protein synthesis is involved. These results suggest that TGF-β1 in the lung could play a role in changing the responsiveness of airway smooth muscle cells to endogenous catecholamines and to β-adrenergic agonists used in therapy.