Stimulation of endothelin B receptors in astrocytes induces cAMP response element-binding protein phosphorylation and c-fos expression via multiple mitogen-activated protein kinase signaling pathways

The vasoconstrictor peptide endothelin (ET-1) exerts its physiological and pathological effects via activation of ETA and ETB receptor (ET-R) subtypes . In this study, we demonstrate that both ET-R subtypes are highly expresse d in rat astrocytes in vivo, indicating that these cells are potential targ ets of the biological effects of ET-1 in the brain. In cultured cortical as trocytes, both ET-R subtypes are expressed, and selective stimulation of ET B-R with ET-1 induces phosphorylation of cAMP response element-binding prot ein (CREB). The signal transduction pathway activated by ET-1 includes the Rap1/B-Raf and the Ras/Raf-1 complexes, protein kinase C (PKC) together wit h extracellular signal-regulated kinases (ERK), and the ribosomal S6 kinase (RSK) isoforms RSK2 and RSK3, two kinases that lie immediately downstream of ERK and are able to phosphorylate CREB. Moreover, ET-1 activates the p38 mitogen-activated protein kinase (MAPK)-dependent, but not the c-jun N-ter minal kinase (JNK)-dependent pathway. By using selective protein kinase inh ibitors and expression of dominant-negative Rap1 protein, we also found tha t the Rap1/PKC/ERK-dependent pathway induces the phosphorylation of activat ing transcription factor-1, CREB, and Elk-1, whereas the p38MAPK-dependent pathway only causes CREB phosphorylation. ET-1-induced transcription of the immediate early gene c-fos requires the concomitant activation of both the PKC/ERK- and p38MAPK-dependent pathways, because inhibitors of either path way block the ET-1-induced increase of c-fos mRNA. Our findings indicate th at changes in the expression of cAMP response element-dependent immediate a nd delayed response genes could play a pivotal role in the physiological ef fects elicited by ET-1 in astrocytes.