title: Taxol normalizes the impaired agonist-induced beta2-adrenoceptor internalization in splenocytes from GRK2+/- mice.
abstract: G protein-coupled receptor kinase 2 (GRK2) is involved in the agonist-induced desensitization of beta2-adrenoceptors. In addition, GRK2 is capable of binding and phosphorylating tubulin. Interestingly, microtubule dynamics profoundly affect agonist-induced internalization of beta2-adrenoceptors. Here, we analyzed agonist-induced beta2-adrenoceptor internalization and signaling in splenocytes from GRK2+/- mice that have a approximately 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 approximately 50% less beta2-adrenoceptors on the cell surface and show impaired agonist-induced beta2-adrenoceptor internalization. Disruption of microtubules using colchicine reduces agonist-induced beta2-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 beta2-adrenoceptor internalization in cells from GRK2+/- animals, without affecting WT cells. Despite lower surface receptor numbers, cells of GRK2+/- mice show normal beta2-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 beta2-adrenoceptors on the cell surface. In addition, the cellular GRK2 level determines the extent of agonist-induced beta2-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.
authors.affiliation: Department of Psychoneuroimmunology, University Medical Center Utrecht, 3584 EA Utrecht, The Netherlands.
authors.forename: Anne
authors.initials: A
authors.lastname: Vroon
authors.forename: Maria Stella
authors.initials: MS
authors.lastname: Lombardi
authors.forename: Annemieke
authors.initials: A
authors.lastname: Kavelaars
authors.forename: Cobi J
authors.initials: CJ
authors.lastname: Heijnen
mesh_terms: D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology*; D003078:Colchicine / Q000494:pharmacology; D000242:Cyclic AMP; D054769:G-Protein-Coupled Receptor Kinase 2; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D008870:Microtubules / Q000378:metabolism; D017239:Paclitaxel / Q000494:pharmacology*; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*; D015398:Signal Transduction; D013154:Spleen / Q000166:cytology; D014404:Tubulin / Q000378:metabolism; D050257:Tubulin Modulators / Q000494:pharmacology; D051552:beta-Adrenergic Receptor Kinases / Q000235:genetics / Q000378:metabolism*
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D018343:Receptors, Adrenergic, beta-2; D014404:Tubulin; D050257:Tubulin Modulators; D000242:Cyclic AMP; C496568:GRK2 protein, mouse; D051552:beta-Adrenergic Receptor Kinases; D054769:G-Protein-Coupled Receptor Kinase 2; D017239:Paclitaxel; D003078:Colchicine