Neurodegenerative disorders are associated with oxidative stress. Low density lipoprotein (LDL) exists in the brain and is especially sensitive to oxidative damage. Oxidative modification of LDL has been implicated in the pathogenesis of neurodegenerative diseases. Therefore, protecting LDL from oxidation may be essential in the brain. The antioxidative effects of endomorphin 1 (EM1) and endomorphin 2 (EM2), endogenous opioid peptides in the brain, on LDL oxidation has been investigated in vitro. The peroxidation was initiated by either copper ions or a water‐soluble initiator 2,2′‐azobis(2‐amidinopropane hydrochloride) (AAPH). Oxidation of the LDL lipid moiety was monitored by measuring conjugated dienes, thiobarbituric acid reactive substances, and the relative electrophoretic mobility. Low density lipoprotein oxidative modifications were assessed by evaluating apoB carbonylation and fragmentation. Endomorphins markedly and in a concentration‐dependent manner inhibited Cu2+ and AAPH induced the oxidation of LDL, due to the free radical scavenging effects of endomorphins. In all assay systems, EM1 was more potent than EM2 and l‐glutathione, a major intracellular water‐soluble antioxidant. We propose that endomorphins provide protection against free radical‐induced neurodegenerative disorders.
Opioids play a role in the apoptosis machinery. We studied the induction of apoptosis in endomorphin 1 (EM1) and endomorphin 2 (EM2), 2 newly isolated endogenous mu-opioid receptor agonists. These endomorphins were able to reduce the viability of cultured HL-60 cells. The antiproliferative properties of endomorphins appeared to be attributable to their induction of apoptotic cell death as determined by ultrastructural change, internucleosomal DNA fragmentation, and increased proportion of the subdiploid cell population. To elucidate molecular events in the apoptosis, protein expressions of Bcl-2, Bax, Fas, and FasL were measured by western blotting using specific antibodies in HL-60 cells. The level of Bcl-2 indicated down-regulation, but the Bax, Fas, and FasL expression showed up-regulation as compared with the untreated control cells. These data support the idea that endomorphins induce apoptosis in HL-60 cells through the activation of the Bcl-2-Bax and the Fas-FasL pathway. We suggest that endomorphins may play an important role in the regulation of tumor cell death.
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