Opioid control of MAP kinase cascade.

Activation of G protein-coupled receptors (GPCRs) may result in phosphorylation of extracellular signal-regulated kinases 1/2 (ERK 1/2). The signaling pathway involves ectodomain shedding, generating epidermal growth factor (EGF)-like ligands, which in turn stimulate the mitogen-activated protein kinase (MAPK) via EGF receptors. The present study investigates into the control of MAPKs by opioidergic GPCRs in human embryonic kidney cells (HEK 293). Experiments were conducted with cells expressing opioid receptors, G protein-coupled receptor kinases, and ERKs. The outcome of our studies let us suggest that EGF-like ligands released by opioid receptor stimulation utilize different EGF receptors to phosphorylate ERKs, while EGF utilizes type 1 receptors. Differences between multiple opioid receptors are apparent with respect to the activation of ERKs. EGF rapidly triggers internalization of the fluorescent EGF receptor type 1, but we failed to observe any sequestration of this receptor type upon exposure of cells to an opioid, since opioids most likely trigger stimulation of a different EGF receptor type. In conclusion, G protein-coupled opioid receptors control the MAPK cascade in a similar fashion as described for non-opioid GPCRs, although distinct differences exist between mu-, delta- and kappa-receptors. EGF-induced ERK activation is mediated by EGF receptor type 1 while opioid receptor activation seems to brings about stimulation via EGF receptor type.