Fluoxetine-mediated 5-HT2B receptor stimulation in astrocytes causes EGF receptor transactivation and ERK phosphorylation

Fluoxetine has relatively high affinity for Gq/11 protein-coupled 5-HT2 receptors. Part of these receptors in brain are on astrocytes, where fluoxetine causes an increase in free cytosolic calcium concentration ([Ca2+](i)) and phosphorylation of extracellular regulated kinase 1 and 2 (ERK1/2). The objectives of the study are to identify subtype of the 5-HT2 receptor involved, to establish whether ERK1/2 phosphorylation is a result of 5-HT2-mediated transactivation of epidermal growth factor (EGF) receptors (EGFRs), and to determine signaling pathways up- and downstream of ERK1/2. Primary cultures of mouse astrocytes, which express all three subtypes of the 5-HT2 receptor but no 5-HT2 transporter, were used. ERK1/2 phosphorylation and c-Fos and FosB protein expression were determined with Western blotting, and c-fos and fosB mRNA expression with reverse transcription polymerase chain reaction. Receptor subtype was investigated with subtype-specific 5-HT antagonists and 5-HT2B receptor depletion and signaling pathways by EGFR phosphorylation, using immunoprecipitation and Western blotting, inhibition of protein kinase C (PKC), and [Ca2+](i) chelation by BAPTA/AM. ERK1/2 phosphorylation was abolished by SB204741, a universal 5-HT2 receptor antagonist, and in 5-HT2B receptor-depleted cells, but unaffected by 5-HT2A or 5-HT2C receptor antagonists (M100907 and SB242084). Phosphorylation of ERK1/2 and EGFRs was abolished by AG 1478, an inhibitor of EGFR tyrosine kinases, and GM 6001, an inhibitor of Zn-dependent metalloproteinases, suggesting growth factor shedding and transactivation of EGFRs. Chelation of [Ca2+](i) or PKC inhibition with GF 109203X abrogated ERK1/2 phosphorylation. Up-regulated mRNA and protein expression of c-fos and fosB was abolished by SB204741, AG1478, and by U0126, an inhibitor of ERK phosphorylation by MAP kinase/ERK kinase.