Signaling pathways of isoproterenol-induced ERK1/2 phosphorylation in primary cultures of astrocytes are concentration-dependent

P>Stimulation of beta-adrenoceptors activates the canonical adenylate cyclase pathway (via G(s) protein) but can also evoke phosphorylation of extracellular-regulated kinases 1 and 2 (ERK1/2) via G(s)/G(i) switching or beta-arrestin-mediated recruitment of Src. In primary cultures of mouse astrocytes, activation of the former of these pathways required micromolar concentrations of the beta(1)/beta(2)-adrenergic agonist isoproterenol, that acted on beta(1)-adrenoceptors, whereas the latter was activated already by nanomolar concentrations, acting on beta(2) receptors. Protein kinase A activity was required for G(s)/G(i) switching, which was followed by Ca2+ release from intracellular stores and G(i alpha)- and metalloproteinase-dependent transactivation of the epidermal growth factor receptor (EGFR; at its Y1173 phophorylation site), via its receptor-tyrosine kinase, beta-arrestin 1/2 recruitment, and MAPK/ERK kinase-dependent ERK1/2 phosphorylation. ERK1/2 phosphorylation by Src activation depended on beta-arrestin 2, but not beta-arrestin 1, was accompanied by Src/EGFR co-precipitation and phosphorylation of the EGFR at the Src-phosphorylated Y845 site and the Y1045 autophosphorylation site; it was independent of transactivation but dependent on MAPK/ERK kinase activity, suggesting EGFR phosphorylation independently of the receptor-tyrosine kinase or activation of Ras or Raf directly from Src. Most astrocytic consequences of activating either pathway (or both) are unknown, but morphological differentiation and increase in glial fibrillary acidic protein in response to dibutyryl cAMP-mediated increase in cAMP depend on G(s)/G(i) switching and transactivation.