Regulation of AMP-activated protein kinase by human delta opioid receptors

AMP-activated protein kinase (AMPK) is a multisubstrate serine/threonine protein kinase that is activated under conditions of metabolic stress causing depletion of cellular ATP, such as glucose deprivation and hypoxia/ischemia. In the present study we investigated the regulation of AMPK by human delta opioid receptors stably expressed in Chinese hamster ovary cells. We found that exposure of cells to the delta opioid receptor agonists [D-Pen2,5]enkephalin and SNC 80 caused a rapid phosphorylation of AMPK on Thr172 of the α subunit, which is associated with enhanced enzyme activity. The stimulatory effect was concentration dependent, blocked by the selective delta opioid receptors antagonist naltrindole and cell treatment with pertussis toxin, indicating the involvement of G proteins of the Gi/Go family. The delta opioid receptor-induced AMPK phosphorylation was not prevented by cell treatment with inhibitors of extracellular signal-regulated protein kinases 1 and 2, p38 mitogen-activated protein kinase, c-Jun N-terminal protein kinase and protein kinase C. On the other hand, it was almost completely blocked by STO-609, a selective inhibitor of camodulin-dependent protein kinase kinase (CaMKK), which is a major upstream regulator of AMPK. These results indicate that human delta opioid receptor can up-regulate AMPK activity through a pathway involving increased intracellular Ca2+ and activation of CaMKK. This novel regulatory action may be relevant for the neuroprotective effects of delta opioid receptors under conditions of limited energy supply.