Taxol normalizes the impaired agonist-induced β2-adrenoceptor internalization in splenocytes from GRK2± mice

G protein-coupled receptor kinase 2 (GRK2) is involved in the agonist-induced desensitization of beta(2)-adrenoceptors. In addition, GRK2 is capable of binding and phosphorylating tubulin. Interestingly, microtubule dynamics profoundly affect agonist-induced internalization of beta(2)-adrenoceptors. Here, we analyzed agonist-induced beta(2)-adrenoceptor internalization and signaling in splenocytes from GRK2+/- mice that have a similar to 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 similar to 50% less of beta(2)-adrenoceptors on the cell surface and show impaired agonist-induced of beta(2)-adrenoceptor internalization. Disruption of microtubules using colchicine reduces agonist-induced beta(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 beta(2)-adrenoceptor internalization in cells from GRK2+/- animals, without affecting WT cells. Despite lower surface receptor numbers, cells of GRK2+/- mice show normal beta(2)-adrenoceptor agonist-induced cAMP responses. Although interfering with microtubule stability has major effects on agonist-induced receptor internalization in GRK2+/- cells, micrombule 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 beta(2)-adrenoceptors on the cell surface. In addition, the cellular GRK2 level determines the extent of agonist-induced beta(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. (c) 2007 Elsevier B.V. All rights reserved.