Opioid Modulation of Extracellular Signal‐Regulated Protein Kinase Activity Is Ras‐Dependent and Involves G βγ Subunits

Although it is well‐established that G protein‐coupled receptor signaling systems can network with those of tyrosine kinase receptors by several mechanisms, the point(s) of convergence of the two pathways remains largely undelineated, particularly for opioids. Here we demonstrate that opioid agonists modulate the activity of the extracellular signal‐regulated protein kinase (ERK) in African green monkey kidney COS‐7 cells transiently cotransfected with μ‐, δ‐, or κ‐opioid receptors and ERK1‐ or ERK2‐containing plasmids. Recombinant proteins in transfected cells were characterized by binding assay or immunoblotting. On treatment with corresponding μ‐ ([ d ‐Ala 2 ,Me‐Phe 4 ,Gly‐ol 5 ]enkephalin)‐, δ‐ ([ d ‐Pen 2 , d ‐Pen 5 ]enkephalin)‐, or κ‐ (U69593)‐selective opioid agonists, a dose‐dependent, rapid stimulation of ERK1 and ERK2 activity was observed. This activation was inhibited by specific antagonists, suggesting the involvement of opioid receptors. Pretreatment of cells with pertussis toxin abolished ERK1 and ERK2 activation by agonists. Cotransfection of cells with dominant negative mutant N17‐Ras or with a βγ scavenger, CD8‐β‐adrenergic receptor kinase‐C, suppressed opioid stimulation of ERK1 and ERK2. When epidermal growth factor was used to activate ERK1, chronic (>2‐h) opioid agonist treatment resulted in attenuation of the stimulation by the growth factor. This inhibition was blocked by the corresponding antagonists and CD8‐β‐adrenergic receptor kinase‐C cotransfection. These results suggest a mechanism involving Ras and βγ subunits of G i/o proteins in opioid agonist activation of ERK1 and ERK2, as well as opioid modulation of epidermal growth factor‐induced ERK activity.