ISSN:
1471-4159
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Medicine
Notes:
Abstract : Phosphorylation of specific amino acid residues isbelieved to be crucial for the agonist-induced regulation of several Gprotein-coupled receptors. This is especially true for the three types ofopioid receptors (μ, δ, and α), which contain consensus sitesfor phosphorylation by numerous protein kinases. Protein kinase C (PKC) hasbeen shown to catalyze the in vitro phosphorylation of μ- andδ-opioid receptors and to potentiate agonist-induced receptordesensitization. In this series of experiments, we continue our investigationof how opioid-activated PKC contributes to homologous receptor down-regulationand then expand our focus to include the exploration of the mechanism(s) bywhich μ-opioids produce PKC translocation in SH-SY5Y neuroblastoma cells.[dAla2,N-Me-Phe4,Gly-ol]enkephalin(DAMGO)-induced PKC translocation follows a time-dependent and biphasicpattern beginning 2 h after opioid addition, when a pronounced translocationof PKC to the plasma membrane occurs. When opioid exposure is lengthened to〉12 h, both cytosolic and particulate PKC levels drop significantly belowthose of control-treated cells in a process we termed “reversetranslocation.” The opioid receptor antagonist naloxone, the PKCinhibitor chelerythrine, and the L-type calcium channel antagonist nimodipineattenuated opioid-mediated effects on PKC and μ-receptor down-regulation,suggesting that this is a process partially regulated byCa2+-dependent PKC isoforms. However, chronic exposure to phorbolester, which depletes the cells of diacylglycerol (DAG) andCa2+-sensitive PKC isoforms, before DAMGO exposure, had no effecton opioid receptor down-regulation. In addition to expressing conventional(PKC-α) and novel (PKC-ε) isoforms, SH-SY5Y cells also contain aDAG-and Ca2+-independent, atypical PKC isozyme (PKC-ξ), which does not decrease in expression after prolonged DAMGO or phorbol ester treatment. This led us to investigate whether PKC-ξ is similarly sensitive to activation by μ-opioids. PKC-ξ translocates from the cytosol to the membrane with kinetics similar to those of PKC-α and ε in response to DAMGO but does not undergo reverse translocation after longer exposure times. Our evidence suggests that direct PKC activation by μ-opioid agonists is involved in the processes that result in μ-receptor down-regulation in human neuroblastoma cells and that conventional, novel, and atypical PKC isozymes are involved.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1046/j.1471-4159.1999.0720594.x
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