Studies On Protective Effects And Mechanisms Of Pyrroloquinoline Quinone On Methylmercury Induced Cytotoxicity In PC12 Cells

Mercury is a worldwide environmental contaminant. Methylmercury (MeHg) is an organic form of mercury that can cause central nervous system (CNS) damage in both the adult and developing brain. The general population is exposed to MeHg primarily through excessive ingestion from a diet that can lead to a dangerous toxicant burden with clinical or subclinical implications. Though the mechanisms of action for MeHg are not well understood, extensive research has been conducted to elucidate the cellular events associated with MeHg-induced neurotoxicity. Investigators have presented work on induction of apoptosis by MeHg in multiple cell types. On the other hand, previous studies on the mechanism of MeHg neurotoxicity have implicated the oxidative injury. Moreover, the fact that a variety of antioxidants can protect cells against MeHg-induced toxicity has also confirmed the above inference.Pyrroloquinoline quinone (PQQ) is a low molecular weight compound that synthesized by some Gram-negative bacteria. This ortho-quinone compound serves as a noncovalent cofactor for bacterial dehydrogenases. In present, PQQ has been evaluated as the third coenzyme following nicotinamide adenine dinucleotide (NAD⁺) or nicotinamide adenine dinucleotide phosphate (NADP⁺) , and flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) in biological oxidoreduction. Rather soon after the discovery of PQQ, it has been identified in various microorganisms, plants and animal tissues. Because animals or humans do not appear to synthesize PQQ, the diet is assumed to be the major source. Due to its inherent antioxidative properties and redox regulative functions, PQQ has been drawing attention from both the nutritional and the pharmacological viewpoint.In this study, we investigated the protective effects of PQQ on MeHg-induced neurotoxicity in PC12 cells model by MTT and LDH assay. The results showed that MeHg decreased the PC12 cell viability in a dose-dependent and time- dependent manner. After pretreatment of PC12 cells with PQQ (3～300 nM) prior to MeHg exposure, the MeHg-induced cytotoxicity was significantly attenuated. Further, PQQ can also maintain the plasma membrane integrity. But with the change in treatment method and increase in treatment concentrations, the effect of PQQ on MeHg-induced cytotoxicity is different.MeHg exposure results in apoptotic cell death, increased the proportion of cells in S-phase, decreased the mitochondrial membrane potential and the level of Bcl-2 protein, and then increased the activity of caspase-3. Further, the overproduction of ROS was also oberved in PC12 cells exposed to MeHg. The addition of PQQ evidently reduced the number of apoptotic cells, ameliorated S-phase arrest, recovered the mitochondrial membrane potential, regulated the level of Bcl-2 , and then inhibited the activation of caspase-3. Moreover, PQQ also decreased the production in PC12 cells exposed to MeHg. Thus, these results indicate that PQQ can protect PC12 cells against MeHg-induced apoptosis via ameliorating the mitochondrial dysfunction.Exposure to MeHg increased the production of superoxide anion (O₂·^-), hydrogen peroxide (H₂O₂) and hydroxyl free radical (·OH), and further resulted in lipid peroxidation in PC12 cells. Pretreatment with PQQ significantly suppressed these ROS production and attenuated lipid peroxidation in a dose-dependent manner. Moreover, PQQ increased the activity of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) in PC12 cells exposed to MeHg.In conclusion, altogether our fidings demonstrated that PQQ could protect PC12 cells from MeHg-induced apoptosis by ameliorating the mitochondrial dysfunction. Moreover, the underlying mechanism of the protective effects of PQQ may be associated with reducing the MeHg-caused oxidative injury and modulating the antioxidative defense system. To our knowledge, this is the first report describing the in vitro protective effect of PQQ on MeHg-induced neurotoxicity. Data from this study provided not only a useful strategy for the treatment of neuronal toxicity induced by mercury toxins, but a novel insight into the physiological functions and mechanisms of PQQ.