Oxygen-sensitive δ-Opioid Receptor-regulated Survival and Death Signals

Abstract

The detrimental effect of severe hypoxia (SH) on neurons can be mitigated by hypoxic preconditioning (HPC), but the molecular mechanisms involved remain unclear, and an understanding of these may provide novel solutions for hypoxic/ischemic disorders (e.g. stroke). Here, we show that the ␦-opioid receptor (DOR), an oxygen-sensitive membrane protein, mediates the HPC protection through specific signaling pathways. Although SH caused a decrease in DOR expression and neuronal injury, HPC induced an increase in DOR mRNA and protein levels and reversed the reduction in levels of the endogenous DOR peptide, leucine enkephalin, normally seen during SH, thus protecting the neurons from SH insult. The HPC-induced protection could be blocked by DOR antagonists. The DOR-mediated HPC protection depended on an increase in ERK and Bcl 2 activity, which counteracted the SH-induced increase in p38 MAPK activities and cytochrome c release. The cross-talk between ERK and p38 MAPKs displays a "yinyang" antagonism under the control of the DOR-G protein-protein kinase C pathway. Our findings demonstrate a novel mechanism of HPC neuroprotection (i.e. the intracellular up-regulation of DOR-regulated survival signals).Neuronal death as a result of neuronal injury following hypoxic/ischemic insults, such as stroke, is an irreversible process that leads to long term neurological deficit. The prevention of neuronal injury is therefore critical in rescuing the brain from neurological disaster. However, clinical strategies that may help mitigate the effects of hypoxic/ischemic injury are still very limited.One strategy that has been shown to provide effective protection from harmful stress is known as preconditioning. This involves transient, but sublethal, exposure to a stress, resulting in enhanced cellular resistance to subsequent severe stress. It was initially demonstrated in the heart (1) and subsequently found to work in other organ beds (2). The effects of preconditioning have been widely studied in the whole brain (3, 4), as well as in vitro in brain slices (5, 6) and neuronal cultures (7-9). Most studies to date have shown the beneficial effects of preconditioning on rescuing neurons from cell injury in response to subsequent severe insults. Hypoxic preconditioning (HPC), 1 for example, caused by lowering the oxygen content or by combined oxygen and glucose deprivation, protects against subsequent hypoxic injury (4,7,8,10). However, a recent study failed to show neuronal protection with HPC treatment (11), suggesting that the neuronal response to preconditioning may vary depending on neuronal situation and involve complex mechanisms. These molecular mechanisms remain unclear, especially with regard to intracellular signal transduction. In this study, we demonstrated that, in neurons in culture, HPCinduced neuroprotection is dependent on specific factors and that the effect is mediated by intracellular up-regulation of ␦-opioid receptor (DOR)-regulated survival signals, which suppress the increased activity of intracellular death s...

Characteristic MicroRNA Expression Induced by δ-Opioid Receptor Activation in the Rat Liver Under Prolonged Hypoxia

“…DOR activation inhibits serum deprivation-induced apoptosis via the activation of PKC and the mitochondrial pathway in human liver cells [63]. DOR activation inhibited mitochondria-mediated apoptosis in liver cancer cells through the PKC/ERK signaling pathway [64], which is consistent with the observation made in neuronal cells [60, 62]. Apparently, DOR actively participates in the regulation of hepatic cell survival.…”

“…DOR has been established as an effective regulator against hypoxia-induced injury [20, 60-62] via multiple mechanisms including the regulation of survival/death signals [60, 62]. DOR activation inhibits serum deprivation-induced apoptosis via the activation of PKC and the mitochondrial pathway in human liver cells [63].…”

Major Differences in Hypoxia Tolerance and P38 Regulation Among Different Renal Cells

“…Interestingly, our recent work has shown that in normal rat kidney epithelial cells (NRK-52E cells), both ERK1/2 and p38 phosphorylation (P-ERK1/2 and P-p38) remarkably increased after prolonged hypoxia _ ENREF_6[14, 15]. This notion is very different from what we have seen in the neurons [16]. …”

Neuroprotection Against Hypoxic/Ischemic Injury: δ-Opioid Receptors and BDNF-TrkB Pathway

“…These opioid receptors are members of the rhodopsin subfamily in the superfamily of seven-transmembrane nucleotide binding regulatory G-protein coupled receptors (GPCRs) [1, 37, 38]. Although homology is present, they recognize structurally diverse exogenous/endogenous peptide and non-peptide ligands [39-41].…”

“…Therefore, it is important to better understand the mechanisms of neuroprotection and explore new strategies for the prevention and treatment of hypoxic/ischemic brain injury. As a result of this goal, we have recently found that the delta-opioid receptor (DOR) is an important neuroprotector against hypoxic/ischemic injury in the brain [1-4]…”