title: Agonist-dependent mu-opioid receptor signaling can lead to heterologous desensitization.
abstract: Desensitization of the micro-opioid receptor (MOR) has been implicated as an important regulatory process in the development of tolerance to opiates. Monitoring the release of intracellular Ca(2+) ([Ca(2+)](i)), we reported that [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO)-induced receptor desensitization requires receptor phosphorylation and recruitment of beta-arrestins (betaArrs), while morphine-induced receptor desensitization does not. In current studies, we established that morphine-induced MOR desensitization is protein kinase C (PKC)-dependent. By using RNA interference techniques and subtype specific inhibitors, PKCepsilon was shown to be the PKC subtype activated by morphine and the subtype responsible for morphine-induced desensitization. In contrast, DAMGO did not increase PKCepsilon activity and DAMGO-induced MOR desensitization was not affected by modulating PKCepsilon activity. Among the various proteins within the receptor signaling complex, Galphai2 was phosphorylated by morphine-activated PKCepsilon. Moreover, mutating three putative PKC phosphorylation sites, Ser(44), Ser(144) and Ser(302) on Galphai2 to Ala attenuated morphine-induced, but not DAMGO-induced desensitization. In addition, pretreatment with morphine desensitized cannabinoid receptor CB1 agonist WIN 55212-2-induced [Ca(2+)](i) release, and this desensitization could be reversed by pretreating the cells with PKCepsilon inhibitor or overexpressing Galphai2 with the putative PKC phosphorylation sites mutated. Thus, depending on the agonist, activation of MOR could lead to heterologous desensitization and probable crosstalk between MOR and other Galphai-coupled receptors, such as the CB1.
authors.affiliation: Department of Pharmacology, University of Minnesota, 6-120 Jackson Hall, 321 Church St. S.E., Minneapolis, Minnesota 55455-0217, USA. chux0086@umn.edu
authors.forename: Ji
authors.initials: J
authors.lastname: Chu
authors.forename: Hui
authors.initials: H
authors.lastname: Zheng
authors.forename: Yuhan
authors.initials: Y
authors.lastname: Zhang
authors.forename: Horace H
authors.initials: HH
authors.lastname: Loh
authors.forename: Ping-Yee
authors.initials: PY
authors.lastname: Law
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D048588:Benzoxazines / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002460:Cell Line; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D051960:GTP-Binding Protein alpha Subunit, Gi2 / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D009020:Morphine / Q000494:pharmacology*; D009025:Morpholines / Q000494:pharmacology; D009281:Naphthalenes / Q000494:pharmacology; D017354:Point Mutation; D011493:Protein Kinase C / Q000235:genetics / Q000378:metabolism*; D051744:Protein Kinase C-epsilon / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D034622:RNA Interference; D043884:Receptor, Cannabinoid, CB1 / Q000819:agonists; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects*
chemical_list: D000701:Analgesics, Opioid; D048588:Benzoxazines; D002121:Calcium Channel Blockers; D009025:Morpholines; D009281:Naphthalenes; D043884:Receptor, Cannabinoid, CB1; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; C070417:(3R)-((2,3-dihydro-5-methyl-3-((4-morpholinyl)methyl)pyrrolo-(1,2,3-de)-1,4-benzoxazin-6-yl)(1-naphthalenyl))methanone; D009020:Morphine; D011493:Protein Kinase C; D051744:Protein Kinase C-epsilon; D051960:GTP-Binding Protein alpha Subunit, Gi2; D002118:Calcium