The delta opioid peptide [D-Ala(2),D-Leu(5)]enkephalin (DADLE) has been shown to promote organ survival and to protect against methamphetamine-induced neurodegeneration. However, the cellular mechanisms of these actions of DADLE are not totally clear. We examined the action of DADLE in serum-deprived pheochromocytoma cells (PC12) and found that DADLE protected against cell death in those cells. However, the dose-response curves of the protective effects of DADLE are U-shaped as judged by three biochemical or morphological assays: the LDH release, the DNA laddering, and the apoptotic nuclei. It was found that femtomolar to picomolar concentrations of DADLE are antiapoptotic, whereas micormolar concentrations of DADLE are cytotoxic in PC12 cells. The protective effect of DADLE could be attenuated by a selective delta2 opioid antagonist and the cytotoxic action of DADLE was reduced by a selective mu opioid receptor antagonist. The treatment of cells with PD98059, a selective inhibitor of ERK kinase (MEK), or the transfection of cells with a dominant interfering form of MEK (MEK-KA97) blocked both the protective effect of DADLE and the ERK phosphorylation induced by DADLE. Cytotoxic concentrations of DADLE, on the other hand, caused an increase of Fas-ligand (FasL) in PC12 cells that was attenuated by a selective mu antagonist. Our results suggest, therefore, that endogenous opioid peptides may, at low concentrations, promote cell survival via the MEK-ERK pathway perhaps through delta2 opioid receptors, whereas they may kill cells at high concentrations via the activation of FasL through an as-yet unknown mechanism involving mu opioid receptors.
Copyright 2001 Wiley-Liss, Inc.