μ-Opioid Receptor Activates Signaling Pathways Implicated in Cell Survival and Translational Control*

Abstract

The μ-opioid receptor mediates the analgesic and addictive properties of morphine. Despite the clinical importance of this G-protein-coupled receptor and many years of pharmacological research, few intracellular signaling mechanisms triggered by morphine and other μ-opioid agonists have been described. We report that μ-opioid agonists stimulate three different effectors of a phosphoinositide 3-kinase (PI3K)-dependent signaling cascade. By using a cell line stably transfected with the μ-opioid receptor cDNA, we show that the specific agonist [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) stimulates the activity of Akt, a serine/threonine protein kinase implicated in protecting neurons from apoptosis. Activation of Akt by DAMGO correlates with its phosphorylation at serine 473. The selective PI3K inhibitors wortmannin and LY294002 blocked phosphorylation of this site, previously shown to be necessary for Akt enzymatic activity. DAMGO also stimulates the phosphorylation of two other downstream effectors of PI3K, the p70 S6 kinase and the repressors of mRNA translation, 4E-BP1 and 4E-BP2. Upon μ-opioid receptor stimulation, p70 S6 kinase is activated and phosphorylated at threonine 389 and at threonine 421/serine 424. Phosphorylation of p70 S6 kinase and 4E-BP1 is also repressed by PI3K inhibitors as well as by rapamycin, the selective inhibitor of FRAP/mTOR. Consistent with these findings, DAMGO-stimulated phosphorylation of 4E-BP1 impairs its ability to bind the translation initiation factor eIF-4E. These results demonstrate that the μ-opioid receptor activates signaling pathways associated with neuronal survival and translational control, two processes implicated in neuronal development and synaptic plasticity.

Footnotes

  • * This work was supported in part by Grant DA05706 from the National Institutes of Health (to M. J. C.) and by the National Cancer Institute of Canada (to N. S.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

  • § To whom correspondence should be addressed: Cell Signaling Laboratory, New England Biolabs, 32 Tozer Rd., Beverly, MA 01915. Tel.: 978-927-5054; Fax: 978-922-7069; E-mail: roby{at}neb.com.

  • Supported by a Natural Science and Engineering of Canada 1967 Studentship.

  • ** Medical Research Council of Canada Distinguished Scientist and a Howard Hughes Institute International Scholar.

  • Abbreviations:
    PI3K
    phosphoinositide 3-kinase
    DAMGO
    [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin
    p70S6k
    p70 S6 kinase
    4E-BP1
    4E-binding protein 1
    4E-BP2
    4E-binding protein 2
    GSK3-β
    glycogen synthase kinase-3 β
    PTX
    pertussis toxin
    HEK
    human embryonic kidney
    HA
    hemagglutinin
    MAPK
    mitogen-activated protein kinase
    PAGE
    polyacrylamide gel electrophoresis
    GSK3-β
    glycogen synthase kinase 3-β
    eIF-4E
    eukaryotic initiation factor 4E
    MEK
    MAPK/extracellular signal-regulated kinase kinase.
    • Received March 9, 1998.
    • Revision received June 10, 1998.
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