L-Serine protects mouse hippocampal neuronal HT22 cells against oxidative stress-mediated mitochondrial damage and apoptotic cell death

The cytoprotective mechanism of L-serine against oxidative stress-mediated neuronal apoptosis was investigated in mouse hippocampal neuronal HT22 cells. Treatment with the reactive oxygen species (ROS) inducer 2,3-dimethoxy-1,4-naphthoquinone (DMNQ) increased cytosolic and mitochondrial ROS and apoptosis, without necrosis, in HT22 cells. ROS-mediated apoptosis was accompanied by the induction of the endoplasmic reticulum (ER) stress-mediated apoptotic pathway, involving CHOP/GADD153 upregulation, JNK and p38 MAPK activation, and caspase-12 and caspase-8 activation, and subsequent induction of the mitochondrial apoptotic pathway through BAK and BAX activation, mitochondrial membrane potential (Delta psi m) loss, caspase-9 and caspase-3 activation, PARP cleavage, and nucleosomal DNA fragmentation. However, the DMNQ-caused ROS elevation and ER stress- and mitochondrial damage-induced apoptotic events were dose-dependently suppressed by co-treatment with L-serine (7.5-20 mM). Although DMNQ reduced both the intracellular glutathione (GSH) level and the ratios of reduced GSH to oxidized GSH (GSSG), the reduction was restored by co-treatment with L-serine. Co-treatment with GSH or N-acetylcysteine also blocked DMNQ-caused ROS elevation and apoptosis; however, co-treatment with the GSH synthesis inhibitor buthionine sulfoximine significantly promoted ROS-mediated apoptosis and counteracted the protection by L-serine. In HT22 cells, DMNQ treatment appeared to tilt the mitochondrial fusion-fission balance toward fission by down-regulating the levels of profusion proteins (MFN1/2 and OPA1) and inhibitory phosphorylation of profission protein DRP1 at Ser-637, resulting in mitochondrial fragmentation. These DMNQ-caused alterations were prevented by L-serine. A comparison of mitochondrial energetic function between DMNQ- and DMNQ/L-serine-treated HT22 cells showed that the DMNQ-caused impairment of the mitochondrial energy generation capacity was restored by L-serine. These results demonstrate that L-serine can protect neuronal cells against oxidative stress-mediated apoptotic cell death by contributing to intracellular antioxidant GSH synthesis and maintaining the mitochondrial fusion-fission balance.