TY - JOUR T1 - A novel non-canonical signaling pathway for μ-Opioid receptor JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.113.088278 SP - mol.113.088278 AU - Lei Zhang AU - Horace H. Loh AU - Ping-yee Law Y1 - 2013/01/01 UR - http://molpharm.aspetjournals.org/content/early/2013/09/23/mol.113.088278.abstract N2 - μ-Opioid receptor (OPRM1) signals as a classical G protein-coupled receptor (GPCR) by activating heterotrimeric Gi/Go proteins resulting in adenylyl cyclase (AC) inhibition. Such AC inhibition is desensitized after prolonged agonist treatment. However, after receptor desensitization, intracellular cAMP level remains regulated by OPRM1 as demonstrated by the intracellular cAMP level increase or AC superactivation upon removal of agonist or addition of antagonist. We now demonstrate that such intracellular cAMP regulation is mediated by a novel non-canonical signaling pathway resulted from OPRM1 being converted to a receptor tyrosine kinase (RTK)-like entity. This non-canonical OPRM1 signaling is initiated by the receptor recruiting and activating Src kinase within receptor complex leading to phosphorylation of the OPRM1 Tyr336 residue. Phospho-Tyr336 serves as the docking site for Grb/SOS leading to the recruitment and activation of the Ras/Raf-1, and subsequent phosphorylation and activation of AC5/6 by Raf-1. Such sequence of events was established by the absence of Ras/Raf1 recruitment and activation by the OPRM1-Y336F mutant, by the presence of Src kinase inhibitor PP2 or the absence of Src activity, by the presence of specific Raf-1 inhibitor GW5074 or the absence of Raf-1, or by the dominant negative RasN17 mutant. Src together with Ras activate Raf1 was established by the inability of the Raf1-Tyr340/341 mutant to activate AC. Hence, the phosphorylation of OPRM1 at Tyr336 by Src serves as the trigger for the conversion of a classical Gi/Go-coupled receptor into an RTK-like entity resulting in a non-canonical pathway even after the original Gi/Go signals are blunted. ER -