Chronic treatment of astrocytes with therapeutically relevant fluoxetine concentrations enhances cPLA2 expression secondary to 5-HT2B-induced, transactivation-mediated ERK1/2 phosphorylation

Abstract

We have recently shown that fluoxetine, a serotonin-specific reuptake inhibitor (SSRI), has low micromolar affinity for the 5-HT(2C) receptor (but not for 5-HT(2A) and 5-HT(2B) receptors) in primary cultures of mouse astrocytes. This was determined as phosphorylation (stimulation) of extracellular-regulated kinase 1 and 2 (ERK(1/2)) by transactivation-mediated phosphorylation of the epidermal growth factor (EGF) receptor, followed by conventional EGF receptor signaling (Li et al., Psychopharmacology 194:333-334, 2007). Paroxetine has an identical effect. The present study shows that chronic fluoxetine treatment with even higher affinity (EC(50) = 0.5-2.0 microM) upregulates Ca(2+)-dependent phospholipase A(2) (cPLA(2)), which releases arachidonic acid from the sn-2 position of membrane-bound phospholipid, without effect on secretory PLA(2) (sPLA(2)) and intracellular PLA(2) (iPLA(2)). This demonstration replicates the fluoxetine-induced cPLA(2) upregulation in rat brain shown by Rao et al. (Pharmacogenomics J 6:413-420, 2006) and provides the new information that upregulation (1) occurs in astrocytes, (2) is evoked by stimulation of 5-HT(2B) receptor, and (3) requires transactivation-mediated ERK(1/2) phosphorylation. Similar upregulation of cPLA(2) in intact brain in response to 5-HT(2)-mediated signaling by elevated serotonin levels and/or an SSRI during antidepressant treatment may explain the repeatedly reported ability of SSRIs to normalize regional decreases which occur in brain metabolism during major depression, since (1) arachidonic acid strongly stimulates glucose metabolism in cultured astrocytes (Yu et al., J Neurosci Res 64:295-303, 1993) and (2) plasma concentrations of arachidonic acid in depressed patients are linearly correlated with regional brain glucose metabolism (Elizabeth Sublette et al., Prostaglandins Leukot Essent Fatty Acids 80:57-64, 2009).