SIRT2, ERK and Nrf2 Mediate NAD+ Treatment-Induced Increase in the Antioxidant Capacity of PC12 Cells Under Basal Conditions.

NAD+ (oxidized form of nicotinamide adenine dinucleotide) administration is highly beneficial in numerous models of diseases and aging. It is becoming increasingly important to determine if NAD+ treatment may directly increase the antioxidant capacity of cells under basal conditions. In the current study, we tested our hypothesis that NAD+ can directly enhance the antioxidant capacity of cells under basal conditions by using PC12 cells as a cellular model. We found that NAD+ treatment can increase the GSH/GSSG ratios in the cells under basal conditions. NAD+ can also increase both the mRNA and protein level of γ-glutamylcysteine ligase (γ-GCL)-a key enzyme for glutathione synthesis, which appears to be mediated by the NAD+-induced increase in Nrf2 activity. These NAD+-induced changes can be prevented by both SIRT2 siRNA and the SIRT2 inhibitor AGK2. The NAD+-induced changes can also be blocked by the ERK signaling inhibitor U0126. Moreover, the NAD+-induced ERK activation can be blocked by both SIRT2 siRNA and AGK2. Collectively, our study has provided the first evidence that NAD+ can enhance directly the antioxidant capacity of the cells under basal conditions, which is mediated by SIRT2, ERK, and Nrf2. These findings have suggested not only the great nutritional potential of NAD+, but also a novel mechanism underlying the protective effects of the NAD+ administration in the disease models: the NAD+ administration can enhance the resistance of the normal cells to oxidative insults by increasing the antioxidant capacity of the cells.

SIRT2 mediates NADH-induced increases in Nrf2, GCL, and glutathione by modulating Akt phosphorylation in PC12 cells.

SIRT2 plays important roles in multiple biological processes. It is unclear whether SIRT2 affects antioxidant capacity by modulating Nrf2, a key transcription factor for multiple antioxidant genes. By studying NADH-treated differentiated PC12 cells, we found that NADH induced a significant increase in the nuclear Nrf2, which was prevented by both SIRT2 siRNA and SIRT2 inhibitor, AGK2. SIRT2 siRNA also blocked the NADH-induced increases in glutamate cysteine ligase (GCL) and glutathione. Moreover, SIRT2 siRNA and AGK2 blocked NADH-induced Akt phosphorylation, and inhibition of Akt phosphorylation prevented NADH-induced increases in the nuclear Nrf2 and glutathione. Collectively, our study shows that SIRT2 regulates nuclear Nrf2 levels by modulating Akt phosphorylation, thus modulating the levels of GCL and total glutathione.

Dose-rate plays a significant role in synchrotron radiation X-ray-induced damage of rodent testes.

Synchrotron radiation (SR) X-ray has significant potential for applications in medical imaging and cancer treatment. However, the mechanisms underlying SR X-ray-induced tissue damage remain unclear. Previous studies on regular X-ray-induced tissue damage have suggested that dose-rate could affect radiation damage. Because SR X-ray has exceedingly high dose-rate compared to regular X-ray, it remains to be determined if dose-rate may affect SR X-ray-induced tissue damage. We used rodent testes as a model to investigate the role of dose-rate in SR X-ray-induced tissue damage. One day after SR X-ray irradiation, we determined the effects of the irradiation of the same dosage at two different dose-rates, 0.11 Gy/s and 1.1 Gy/s, on TUNEL signals, caspase-3 activation and DNA double-strand breaks (DSBs) of the testes. Compared to those produced by the irradiation at 0.11 Gy/s, irradiation at 1.1 Gy/s produced higher levels of DSBs, TUNEL signals, and caspase-3 activation in the testes. Our study has provided the first evidence suggesting that dose-rate could be a significant factor in SR X-ray-induced tissue damage, which may establish a valuable base for utilizing this factor to manipulate the tissue damage in SR X-ray-based medical applications.

Antioxidant protects blood-testis barrier against synchrotron radiation X-ray-induced disruption.

Synchrotron radiation (SR) X-ray has wide biomedical applications including high resolution imaging and brain tumor therapy due to its special properties of high coherence, monochromaticity and high intensity. However, its interaction with biological tissues remains poorly understood. In this study, we used the rat testis as a model to investigate how SR X-ray would induce tissue responses, especially the blood-testis barrier (BTB) because BTB dynamics are critical for spermatogenesis. We irradiated the male gonad with increasing doses of SR X-ray and obtained the testicles 1, 10 and 20 d after the exposures. The testicle weight and seminiferous tubule diameter reduced in a dose- and time-dependent manner. Cryosections of testes were stained with tight junction (TJ) component proteins such as occludin, claudin-11, JAM-A and ZO-1. Morphologically, increasing doses of SR X-ray consistently induced developing germ cell sloughing from the seminiferous tubules, accompanied by shrinkage of the tubules. Interestingly, TJ constituent proteins appeared to be induced by the increasing doses of SR X-ray. Up to 20 d after SR X-ray irradiation, there also appeared to be time-dependent changes on the steady-state level of these protein exhibiting differential patterns at 20-day after exposure, with JAM-A/claudin-11 still being up-regulated whereas occludin/ZO-1 being down-regulated. More importantly, the BTB damage induced by 40 Gy of SR X-ray could be significantly attenuated by antioxidant N-Acetyl-L-Cysteine (NAC) at a dose of 125 mg/kg. Taken together, our studies characterized the changes of TJ component proteins after SR X-ray irradiation, illustrating the possible protective effects of antioxidant NAC to BTB integrity.

NAD+ treatment can prevent rotenone-induced increases in DNA damage, Bax levels and nuclear translocation of apoptosis-inducing factor in differentiated PC12 cells.

Nicotinamide adenine dinucleotide (NAD(+)) plays critical roles in energy metabolism, mitochondrial functions, calcium homeostasis and immunological functions. Our previous studies have found that NAD(+) administration can profoundly decrease ischemic brain injury and traumatic brain injury. Our recent study has also provided first direct evidence indicating that NAD(+) treatment can decrease cellular apoptosis, while the mechanisms underlying this protective effect remain unclear. In our current study, we determined the effects of NAD(+) treatment on several major factors in apoptosis and necrosis, including levels of Bax and nuclear translocation of apoptosis-inducing factor (AIF), as well as levels of DNA double-strand breaks (DSBs) and intracellular ATP in rotenone-treated differentiated PC12 cells. We found that NAD(+) treatment can markedly attenuate the rotenone-induced increases in the levels of Bax and nuclear translocation of AIF in the cells. We further found that NAD(+) treatment can significantly attenuate the rotenone-induced increase in the levels of DSBs and decrease in the intracellular ATP levels. Collectively, our study has suggested mechanisms underlying the preventive effects of NAD(+) on apoptosis, which has highlighted the therapeutic potential of NAD(+) for decreasing apoptotic changes in multiple major diseases.

NAD⁺/NADH metabolism and NAD⁺-dependent enzymes in cell death and ischemic brain injury: current advances and therapeutic implications.

NAD(+) and NADH play crucial roles in a variety of biological processes including energy metabolism, mitochondrial functions, and gene expression. Multiple studies have indicated that NAD(+) administration can profoundly decrease oxidative cell death as well as ischemic and traumatic brain injury, suggesting NAD(+) metabolism as a promising therapeutic target for cerebral ischemia and head injury. Cumulating evidence has suggested that NAD(+) can produce its protective effects by multiple mechanisms, including preventing mitochondrial alterations, enhancing energy metabolism, preventing virtually all forms of cell death including apoptosis, necrosis and autophagy, inhibiting inflammation, directly increasing antioxidation capacity of cells and tissues, and activating SIRT1. Increasing evidence has also suggested that NADH metabolism is a potential therapeutic target for treating several neurological disorders. A number of studies have further indicated that multiple NAD(+)-dependent enzymes such as sirtuins, polymerase(ADP-ribose) polymerases (PARPs) and CD38 mediate cell death and multiple biological processes. In this article, an overview of the recent findings regarding the roles of NAD(+)/NADH and NAD(+)-dependent enzymes in cell death and ischemic brain injury is provided. These findings have collectively indicated that NAD(+)/NADH and NAD(+)-dependent enzymes play fundamental roles in oxidative stress-induced cell death and ischemic brain injury, which may become promising therapeutic targets for brain ischemia and multiple other neurological disorders.

Nicotinamide mononucleotide improves energy activity and survival rate in an in vitro model of Parkinson's disease.

Nicotinamide adenine dinucleotide (NAD+) repletion has been shown to provide marked neuroprotection from genotoxic agent-induced neuronal and astrocyte cell death. One of the key precursors of NAD+ is nicotinamide mononucleotide (NMN). Therefore, it was hypothesized that NMN may attenuate apoptosis and improve energy metabolism in Parkinson's disease (PD)-like behavioral and neuropathological changes, and produce significant beneficial effects. In this study, a cellular model of PD, using rotenone-treated PC12 cells, was established to test the hypothesis that NMN may decrease PD-like pathological changes. Experiments were carried out to investigate cell survival, including an intracellular lactate dehydrogenase (LDH) assay. Apoptotic and necrotic cell death, NAD+ levels and ATP levels were also evaluated. It was observed that NMN was able to significantly attenuate the rotenone-induced reduction in the survival rate of PC12 cells, as assessed by MTT and LDH assays. NMN treatment also significantly reduced the rotenone-induced apoptosis of the cells, as assessed by flow cytometry-based Annexin V/7-aminoactinomycin D staining. Furthermore, NMN restored intracellular levels of NAD+ and ATP in the rotenone-treated cells, thus demonstrating the capacity of NMN to ameliorate mitochondrial inhibitor-induced impairments of energy metabolism. The present study indicates that NMN produces significant beneficial effects by attenuating apoptosis and improving energy metabolism in a cellular model of PD. These results suggest that NMN may become a promising therapeutic drug for PD.

Malate-aspartate shuttle mediates the intracellular ATP levels, antioxidation capacity and survival of differentiated PC12 cells.

NAD(+) and NADH play pivotal roles in numerous redox reactions in cells. While increasing evidence has indicated important roles of NAD(+) in cell survival and cellular functions, there has been distinct deficiency in the studies regarding the biological functions of NADH. NADH shuttles mediate the transfer of the reducing equivalents of the cytosolic NADH into mitochondria. Cumulating evidence has suggested that malate-aspartate shuttle (MAS), one of the two types of NADH shuttles, plays significant roles in multiple biological processes such as glutamate synthesis in neurons. Because there has been no information regarding the roles of NADH shuttle in the energy metabolism, antioxidation capacity, and survival of any type of neural cells, in this study we used differentiated PC12 cells as a cellular model to investigate the roles of MAS in the energy metabolism, antioxidation capacity and survival of cells. We found that MAS inhibition led to a significant decrease in the levels of GSH - a major antioxidation molecule in cells, suggesting an important role of MAS in maintaining the antioxidation capacity of cells. Our study has also suggested that MAS could play critical roles in maintaining the intracellular ATP levels of the cells. Moreover, MAS inhibition was shown to significantly decrease the survival of differentiated PC12 cells. Collectively, our study has provided first evidence suggesting important roles of NADH shuttles in maintaining antioxidation capacity of cells. Our study has also suggested important roles of MAS in maintaining the intracellular ATP levels and survival of differentiated PC12 cells.

SIRT2 mediates oxidative stress-induced apoptosis of differentiated PC12 cells.

Sirtuin 2 (SIRT2) is a member of the sirtuin family. Previous studies have suggested that SIRT2 mediates the cell death in models of Parkinson's disease and Huntington's disease. However, the role of SIRT2 in oxidative stress-induced cell death has remained unclear. In this study, we investigated the roles of SIRT2 in oxidative stress-induced cell death using differentiated PC12 cells as a cell model. We found that H2O2 induced a significant increase in the SIRT2 level in the cells. Both SIRT2 silencing and the SIRT2 inhibitor AGK2 significantly decreased H2O2-induced apoptosis, partially by inhibiting caspase-3 activation. We further found that silencing of SIRT2 led to decreased reactive oxygen species levels in the H2O2-treated cells. Collectively, our observations have suggested that SIRT2 plays a significant role in oxidative stress-induced cell death.

NAD(+) treatment prevents rotenone-induced apoptosis and necrosis of differentiated PC12 cells.

Nicotinamide adenine dinucleotide (NAD(+)) plays critical roles in not only energy metabolism and mitochondrial functions, but also calcium homeostasis and immunological functions. It has been reported that NAD(+) administration can reduce ischemic brain damage. However, the mechanisms underlying the protective effects remain unclear. Because mitochondrial impairments play a key role in the cell death in cerebral ischemia, in this study we tested our hypothesis that NAD(+) can decrease mitochondrial damage-induced cell death using differentiated PC12 cells as a cellular model. We found that NAD(+) can decrease both early-stage and late-stage apoptosis, as well as necrosis of rotenone-treated PC12 cells, as assessed by FACS-based Annexin V/AAD assay. We also found that NAD(+) treatment can restore the intracellular NAD(+) levels of the rotenone-treated cells. Moreover, NAD(+) treatment can prevent rotenone-induced mitochondria depolarization. In summary, our study has provided first direct evidence that NAD(+) treatment can prevent rotenone-induced apoptosis and necrosis. Our study has also indicated that NAD(+) treatment can prevent mitochondrial damage-induced cell death, which may at least partially result from its protective effects on rotenone-induced mitochondrial depolarization. Because both mitochondrial damage and apoptosis play key roles in multiple neurological disorders, our study has highlighted the therapeutic potential of NAD(+) for brain ischemia and other neurological diseases.

NAD(+) administration significantly attenuates synchrotron radiation X-ray-induced DNA damage and structural alterations of rodent testes.

Synchrotron radiation (SR) X-ray has great potential for its applications in medical imaging and cancer treatment. In order to apply SR X-ray in clinical settings, it is necessary to elucidate the mechanisms underlying the damaging effects of SR X-ray on normal tissues, and to search for the strategies to reduce the detrimental effects of SR X-ray on normal tissues. However, so far there has been little information on these topics. In this study we used the testes of rats as a model to characterize SR X-ray-induced tissue damage, and to test our hypothesis that NAD(+) administration can prevent SR X-ray-induced injury of the testes. We first determined the effects of SR X-ray at the doses of 0, 0.5, 1.3, 4 and 40 Gy on the biochemical and structural properties of the testes one day after SR X-ray exposures. We found that 40 Gy of SR X-ray induced a massive increase in double-strand DNA damage, as assessed by both immunostaining and Western blot of phosphorylated H2AX levels, which was significantly decreased by intraperitoneally (i.p.) administered NAD(+) at doses of 125 and 625 mg/kg. Forty Gy of SR X-ray can also induce marked increases in abnormal cell nuclei as well as significant decreases in the cell layers of the seminiferous tubules one day after SR X-ray exposures, which were also ameliorated by the NAD(+) administration. In summary, our study has shown that SR X-ray can produce both molecular and structural alterations of the testes, which can be significantly attenuated by NAD(+) administration. These results have provided not only the first evidence that SR X-ray-induced tissue damage can be ameliorated by certain approaches, but also a valuable basis for elucidating the mechanisms underlying SR X-ray-induced tissue injury.

NAD+ metabolism and NAD(+)-dependent enzymes: promising therapeutic targets for neurological diseases.

Numerous studies have indicated that four interacting factors, including oxidative stress, mitochondrial alterations, calcium dyshomeostasis and inflammation, play crucial pathological roles in multiple major neurological diseases, including stroke, Alzheimer's disease (AD) and Parkinson's disease (PD). Increasing evidence has also indicated that NAD(+) plays important roles in not only mitochondrial functions and energy metabolism, but also calcium homeostasis and inflammation. The key NAD(+)-consuming enzyme--poly(ADP-ribose) polymerase-1 (PARP-1) and sirtuins--have also been shown to play important roles in cell death and aging, which are two key factors in the pathology of multiple major age-dependent neurological diseases: PARP-1 plays critical roles in both inflammation and oxidative stress-induced cell death; and sirtuins also mediate the process of aging, cell death and inflammation. Thus, it is conceivable that increasing evidence has suggested that NAD(+) metabolism and NAD(+)-dependent enzymes are promising targets for treating a number of neurological illnesses. For examples, the key NAD(+)-dependent enzymes SIRT1 and SIRT2 have been indicated to strongly affect the pathological changes of PD and AD; PARP-1 inhibition can profoundly reduce the brain injury in the animal models of multiple neurological diseases; and administration of either NAD(+) or nicotinamide can also decrease ischemic brain damage. Future studies are necessary to further investigate the roles of NAD+ metabolism and NAD⁺-dependent enzymes in neurological diseases, which may expose novel targets for treating the debilitating illnesses.

SIRT2 activity is required for the survival of C6 glioma cells.

SIRT2 is a tubulin deacetylase, which can play either detrimental or beneficial roles in cell survival under different conditions. While it has been suggested that reduced SIRT2 expression in human gliomas may contribute to development of gliomas, there has been no study that directly determines the effects of decreased SIRT2 activity on the survival of glioma cells. In this study we applied both pharmacological and molecular approaches to determine the roles of SIRT2 in the survival of glioma cells. Our studies, by conducting such assays as flow cytometry-based Annexin V assay and caspase-3 immunostaining, have indicated that decreased SIRT2 activity leads to apoptosis of C6 glioma cells by caspase-3-dependent pathway. Our experiments have further shown that reduced SIRT2 activity produces necrosis of C6 glioma cells. Moreover, our study applying SIRT2 siRNA has also shown that decreased SIRT2 leads to both necrosis and apoptotic changes of C6 glioma cells. Collectively, our study has provided novel evidence indicating that SIRT2 activity plays a key role in maintaining the survival of glioma cells, and that reduced SIRT2 activity can induce both necrosis and caspase-3-dependent apoptosis of C6 glioma cells. These results have also suggested that inhibition of SIRT2 might become a novel therapeutic strategy for gliomas.

Silencing of SIRT2 induces cell death and a decrease in the intracellular ATP level of PC12 cells.

Sirtuin 2 (SIRT2), a tubulin deacetylase, is a sirtuin family protein. SIRT2 inhibitors have been shown to decrease the cell death in cellular and Drosophila models of Parkinson's disease. However, SIRT2 decreases may also compromise cellular antioxidation capacity. Our current study found that silencing of SIRT2 led to a decrease in the intracellular ATP level of PC12 cells. We also found that AGK2, a selective SIRT2 inhibitor, can exacerbate H2O2-induced decreases in the intracellular ATP level of these cells. Our study further indicated that the reduction in SIRT2 level significantly increased necrosis of PC12 cells without affecting autophagy of the cells. These results suggest that SIRT2 is a key mediator of energy metabolism and basal survival of PC12 cells.

NAD+ treatment decreases tumor cell survival by inducing oxidative stress.

NAD+ plays important roles in various biological processes. It has been shown that NAD+ treatment can decrease genotoxic agent-induced death of primary neuronal and astrocyte cultures, and NAD+ administration can reduce ischemic brain damage. However, the effects of NAD+ treatment on tumor cell survival are unknown. In this study we found that treatment of NAD+ at concentrations from 10 micromolar to 1 mM can significantly decrease the survival of various types of tumor cells such as C6 glioma cells. In contrast, NAD+ treatment did not impair the survival of primary astrocyte cultures. Our study has also indicated that oxidative stress mediates the effects of NAD+ on the survival of tumor cells, and P2X7 receptors and altered calcium homeostasis are involved in the effects of NAD+ on the cell survival. Collectively, our study has provided the first evidence that NAD+ treatment can decrease the survival of tumor cells by such mechanisms as inducing oxidative stress. Because NAD+ treatment can selectively decrease the survival of tumor cells, NAD+ may become a novel agent for treating cancer.

Roles of NAD(+) / NADH and NADP(+) / NADPH in cell death.

A rapidly growing body of information has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could be new fundamental factors in cell death: Many studies have indicated key roles of poly (ADP-ribose) polymerases and sirtuins--two families of NAD-dependent enzymes--in cell death; and NAD may also affect cell survival by influencing mitochondrial permeability transition, apoptosis-inducing factor and GAPDH. NAD may further influence cell survival by its effects on calcium homeostasis, gene expression and immunological functions. Due to the crucial roles of oxidative stress in cell death, NADPH may mediate cell death by its major effects on oxidative stress: NADPH is a key factor in cellular antioxidation systems; and NADPH oxidase is also a major generator of oxidative stress. With growing information about the novel biological properties of NAD and NADP, it is likely that new roles of NAD and NADP in cell death and various diseases will be elucidated. The elucidation may not only improve our understanding about the fundamental mechanisms of cell death, but also suggest new therapeutic targets for a variety of diseases.