Cytodifferentiation enhances Erk activation induced by endothelin-1 in primary cultured astrocytes

We have previously demonstrated that endothelin-1 (ET-1)-induced extracellular signal-regulated kinase (Erk) activity via the ETB receptor (EDNRB) is mediated through two independent pathways, a protein kinase C-dependent pathway and a pertussis toxin (PTX)-sensitive pathway, in astrocytes. In this study, we showed that the molar potency of ET-1 to induce Erk activation was two orders of magnitude higher in dibutyryl cAMP (DBcAMP)-treated astrocytes than in quiescent astrocytes. This DBcAMP-enhanced molar potency of ET-1 in Erk activation was selectively inhibited by pretreatment of astrocytes with PTX. The expression level of EDNRB in astrocytes was markedly upregulated by DBcAMP-induced cytodifferentiation. However, this up-regulation was simply attributed to the high expression of low-affinity sites. The molar potency of ET-1 to induce both stimulation of inositol trisphosphate production and activation of protein kinase C in DBcAMP-treated astrocytes was similar to that in quiescent astrocytes. On the contrary, the molar potency of ET-1 to induce accumulation of Ras-GTP was two orders of magnitude higher in DBcAMP-treated astrocytes than in quiescent astrocytes, which was consistent with the case of ET-1-induced Erk activation. Moreover, the ET-1-induced Ras activation was PTX sensitive. These results suggest that cytodifferentiation selectively enhances the PTXsensitive Ras/Erk pathway induced by ET-1 in astrocytes, and that cytodifferentiation-induced EDNRB up-regulation might not contribute to this selective potentiation of ET-1 signaling.