title: Mu opioid transactivation and down-regulation of the epidermal growth factor receptor in astrocytes: implications for mitogen-activated protein kinase signaling.
abstract: Astroglia are a principal target of long-term mu antiproliferative actions. The mitogen-activated protein (MAP) kinase known as extracellular signal-regulated kinase (ERK), is a key mediator of cell proliferation. In studies on the mechanism of short- and long-term mu opioid regulation of the ERK signaling pathway, we show that the mu opioid agonist [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), acting via the endogenous mu opioid receptor (MOR), induced sequential epidermal growth factor receptor (EGF) receptor (EGFR) Tyr phosphorylation, Ser phosphorylation, and down-regulation in immortalized rat cortical astrocytes. The short-term action of DAMGO resulted in the stimulation of ERK phosphorylation. 4(3-Chlorophenylamino)-6,7-dimethoxyquinazoline (Tyrphostin AG1478), a selective inhibitor of EGFR Tyr kinase activity, blocked EGFR and ERK activation by short-term DAMGO administration, implicating EGFR transactivation in its stimulation of ERK activity. Inhibitors of matrix metalloproteinases attenuated MOR-mediated ERK phosphorylation, suggesting that shedding of EGF-like ligands from the plasma membrane may be involved in the EGFR transactivation process. Prolonged DAMGO exposure induced EGFR internalization/down-regulation, did not activate ERK, and inhibited exogenous EGF-stimulated ERK phosphorylation. MOR-mediated EGFR down-regulation seems to be MAP kinase-dependent, because it was inhibited by the ERK kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio) butadiene (U0126), and tyrphostin AG1478. The kappa opioid agonist (5alpha,7alpha,8beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl) benzeneacetamide (U69,593) induced Tyr and Ser phosphorylation of EGFR and activation of ERK. However, long-term application of U69,593 neither down-regulated EGFR nor inhibited EGF-induced ERK activation. Instead, it engendered a sustained activation of ERK. Collectively, our data suggest that long-term application of DAMGO initiates heterologous down-regulation of EGFR via a mechanism involving ERK in astrocytes.
authors.affiliation: Department of Biochemistry and Molecular Biology, St. Louis University School of Medicine, 1402 S. Grand Blvd., St. Louis, MO 63104, USA.
authors.forename: Mariana M
authors.initials: MM
authors.lastname: Belcheva
authors.forename: Yun
authors.initials: Y
authors.lastname: Tan
authors.forename: Virginia M
authors.initials: VM
authors.lastname: Heaton
authors.forename: Amy L
authors.initials: AL
authors.lastname: Clark
authors.forename: Carmine J
authors.initials: CJ
authors.lastname: Coscia
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D015536:Down-Regulation / Q000187:drug effects / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D066246:ErbB Receptors / Q000235:genetics / Q000378:metabolism*; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000502:physiology*; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D017450:Receptors, Opioid, mu / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation / Q000187:drug effects / Q000502:physiology*
chemical_list: D004791:Enzyme Inhibitors; D017450:Receptors, Opioid, mu; D066246:ErbB Receptors; D020928:Mitogen-Activated Protein Kinases