Regulation der Adenylatzyklasen und der extrazellulär-signalregulierten Proteinkinasen durch den Delta-Opioidrezeptor in HEK293 Zellen

Regulation of adenylate cyclases and extracellular signal-regulated protein kinases by delta-opioid receptors in HEK293 cells Stimulation of G protein coupled opioid receptors result in both inhibition of adenylate cyclases and stimulation of extracellular signal-regulated protein kinases ERK1/2. As regulation of cellular effectors may be accomplished by various G proteins as well as by the different G protein subunits (G alpha, G betagamma), delta-opioid receptors were thus examined for activating different G proteins underlying different regulation of these cellular effectors. In transfected HEK293 cells, activation of delta-opioid receptors by peptidergic opioids (DADLE, DPDPE) and alkaloids (etorphine, morphine) brought about concentration-dependent inhibition of adenylate cyclases and stimulation of the ERKs, respectively. Since the high-affinity opioids DADLE, DPDPE and etorphine accomplished regulation of respective effector molecules already at nanomolar ligand concentrations, a 1000-fold higher dose of low-affinity agonist morphine was required for both inhibition of adenylate cyclases and ERK activation. However, for all tested opioids, a higher EC50 could have been determined for inhibition of adenylate cyclases than for stimulation of the ERKs. Thus, adenylate cyclases expressed in HEK cells seems to be more sensitive to delta-opioid receptor activation than the ERKs. As previously shown, exposure of HEK-DOR cells to pertussis toxin (PTX) resulted in incomplete inhibition of adenylate cyclases by DADLE and DPDPE, whereas etorphine and morphine totally lost their ability to inhibit the cyclases under these conditions. In contrast, activation of ERKs by all tested opioids was abolished by PTX treatment. However, PTX also blocked ERK activation by Gq-coupled receptors and receptor tyrosine kinases, both regulating ERKs independent from PTX-sensitive Gi proteins. Thus, PTX is suggested to inhibit ERK activation also independent from affecting G protein activation. Since inhibition of G alpha q subunits by the G alpha q-binding protein EBP50 did not affect effector regulation, inhibition of G alpha q and G alpha 12 mediated signaling by neomycin and 1-butanol brought about partial blockade of ERK activation by all tested opioids. Exposure of HEK-DOR cells with neomycin and 1-butanol together even totally blocked ERK activation by respective opioids. In contrast, inhibition of G alpha 11 signaling partially blocked inhibition of adenylate cyclases by DADLE and DPDPE, whereas regulation of the cyclases by the alkaloids was not affected under this condition. Since inhibition of G betagamma signaling by phosducin did not affect regulation of adenylate cyclases and ERKs by opioids, delta-opoioid receptors are supposed to regulate these cellular effectors by G alpha subunits. Although the tested cellular effectors are regulated differently, inhibition of adenylate cyclases seems to support activation of ERKs, since simultaneous stimulation of the cyclases by forskolin impairs ERK activation by DPDPE and etorphin. In contrast, activation of ERKs did not affect regulation of the cyclases by delta-opioid receptors. Together the findings let suppose that different G alpha subunits might be involved in regulation of adenylate cyclases and ERKs by delta-opioid receptors. Since stimulation of delta-receptors might be supposed to bring about inhibition of adenylate cyclases by G alpha i and G alpha 11 subunits, alkaloids seems to regulate cyclases by G alpha i subunits. In contrast, both peptide and alkaloid opioids seem to stimulate ERKs by G alpha 11- and G alpha 12-mediated signaling.