β‐Adrenergic receptor trafficking by exercise in rat adipocytes: roles of G‐protein‐coupled receptor kinase‐2, β‐arrestin‐2, and the ubiquitin‐proteasome pathway

The effect of exercise on β‐adrenergic receptor (β‐AR) trafficking was investigated in rat adipocytes. The binding sites of a hydrophilic ligand, [ 3 H]CGP12177, increased immediately (0 h) and at 3 h after exercise (3 h) but decreased at 24 h after exercise (24 h). The data of immunoblotting revealed that the alterations in the binding sites mainly paralleled the alterations in the β 2 ‐AR proteins in membrane fractions. The protein expressions of both G‐protein‐coupled receptor kinase (GRK)‐2 and β‐arrestin‐2 were reduced, with a decline in β 2 ‐AR ubiquitination at 0 h and 3 h. The protein expressions of β 2 ‐AR, GRK‐2, β‐arrestin‐2, the β 2 ‐AR/β‐arrestin‐2 complex, and β 2 ‐AR ubiquitination returned to their respective control levels at 24 h, whereas the β 2 ‐AR mRNA level was reduced. Administration of either lactacystin or propranolol did not alter GRK‐2 and β 2 ‐AR protein expressions after exercise. Thus, the mechanism underlying the increased density of β 2 ‐AR up to at least 3 h may involve alterations in a multistep event involving the coordinate interaction among proteins mediating β 2 ‐AR trafficking, in which both the receptor‐agonist interactions and ubiquitin‐proteasome pathway have a key role. However, the decreased protein expression of β 2 ‐AR at 24 h might be due to some change occurring at the translational levels.