Taxol normalizes the impaired agonist-induced β 2-adrenoceptor internalization in splenocytes from GRK2+/− mice

Abstract

G protein-coupled receptor kinase 2 (GRK2) is involved in the agonist-induced desensitization of β 2-adrenoceptors. In addition, GRK2 is capable of binding and phosphorylating tubulin. Interestingly, microtubule dynamics profoundly affect agonist-induced internalization of β 2-adrenoceptors. Here, we analyzed agonist-induced β 2-adrenoceptor internalization and signaling in splenocytes from GRK2+/− mice that have a ∼ 50% lower level of GRK2 protein compared to wild type (WT) mice. In addition, we investigated the role of microtubule stability in these processes. Splenocytes from GRK2+/− mice express ∼ 50% less β 2-adrenoceptors on the cell surface and show impaired agonist-induced β 2-adrenoceptor internalization. Disruption of microtubules using colchicine reduces agonist-induced β 2-adrenoceptor internalization in cells from WT, but not in cells from GRK2+/− mice. Importantly, increasing tubulin stability by taxol almost completely restores the defective agonist-induced β 2-adrenoceptor internalization in cells from GRK2+/− animals, without affecting WT cells. Despite lower surface receptor numbers, cells of GRK2+/− mice show normal β 2-adrenoceptor agonist-induced cAMP responses. Although interfering with microtubule stability has major effects on agonist-induced receptor internalization in GRK2+/− cells, microtubule dynamics do not influence cAMP responses. Our data suggest that cells with low GRK2 adapt to the lower GRK2 level by decreasing the number of β 2-adrenoceptors on the cell surface. In addition, the cellular GRK2 level determines the extent of agonist-induced β 2-adrenoceptor internalization via a mechanism involving microtubule stability. Importantly, however, normalization of agonist-induced receptor internalization by taxol is not sufficient to alter receptor signaling.