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Specific G protein activation and mu-opioid receptor internalization caused by morphine, DAMGO and endomorphin I
N T Burford1, L M Tolbert, W Sadee
1Department of Biopharmaceutical Sciences and Pharmaceutical Chemistry, University of California, San Francisco 94143-0446, USA. ntburf@itsa.ucsf.edu
Abstract:
Previous studies have shown that the agonist [D-Ala2, N-Me-Phe4, Gly-ol5]enkephalin (DAMGO) but not morphine induces mu-opioid receptor internalization [Arden, J.R., Segredo, V., Wang, Z., Lameh, J., Sadee, W., 1995. J. Neurochem. 65, 1636-1645]. In the present study we investigated the relationship between internalization of the mu-opioid receptor and the specific G proteins activated following treatment with morphine, DAMGO and endomorphin I (Tyr-Pro-Trp-Phe-NH2) (a putative endogenous mu-opioid receptor agonist) in human embryonic kidney (HEK) cells. Endomorphin I and DAMGO, but not morphine, caused mu-opioid receptor internalization. Morphine, DAMGO and endomorphin I each activated Gi1 alpha/Gi2 alpha, Go alpha and Gi3 alpha to a similar extent, but not Gq alpha/G11 alpha or Gs alpha in HEK membranes. Therefore, the three ligands tested differed in their ability to internalize mu-opioid receptors even though they were similar in activating individual G proteins.
Insights
Morphine, DAMGO, and endomorphin I activate G proteins similarly, but only DAMGO and endomorphin I induce mu-opioid receptor internalization. This suggests receptor internalization is independent of G protein activation.
Area of Science:
- Pharmacology
- Molecular Biology
- Cell Biology
Background:
- Mu-opioid receptor (MOR) signaling is complex, involving G protein activation and receptor trafficking.
- Previous research indicated that agonists like DAMGO induce MOR internalization, unlike morphine.
Purpose of the Study:
- To investigate the relationship between MOR internalization and specific G protein activation by morphine, DAMGO, and endomorphin I.
- To elucidate the signaling pathways downstream of MOR activation.
Main Methods:
- Utilized human embryonic kidney (HEK) cells expressing MOR.
- Assessed MOR internalization via ligand treatment.
- Measured G protein activation (Gi1α/Gi2α, Gαo, Gi3α, Gqα/G11α, Gsα) in HEK cell membranes using specific assays.
Main Results:
- Endomorphin I and DAMGO, but not morphine, induced significant mu-opioid receptor internalization.
- Morphine, DAMGO, and endomorphin I activated Gi1α/Gi2α, Gαo, and Gi3α to comparable levels.
- No significant activation of Gqα/G11α or Gsα was observed for any of the tested ligands.
Conclusions:
- Mu-opioid receptor internalization is not directly correlated with the activation of specific G proteins (Gi1α/Gi2α, Gαo, Gi3α) by morphine, DAMGO, and endomorphin I.
- Ligand-specific pathways likely regulate MOR internalization, independent of broad G protein activation patterns.