Reactive oxygen species are downstream products of TRAF-mediated signal transduction

Radiation-induced activation of nuclear factor-kappaB involves selective degradation of plasma membrane-associated I(kappa)B(alpha)

In contrast to nuclear factor-kappaB (NF-kappaB) activation by tumor necrosis factor-alpha (TNF-alpha), the specific processes involved in the activation of this transcription factor by ionizing radiation (IR) have not been completely defined. According to the classical paradigm, a critical event in NF-kappaB activation is the degradation of I(kappa)B(alpha). Data presented herein show that, in contrast to treatment with TNF-alpha, IR-induced NF-kappaB activation was not accompanied by degradation of I(kappa)B(alpha) in the U251 glioblastoma cell line as determined in whole cell lysates. However, treatment with the proteosome inhibitor MG-132 inhibited NF-kappaB activation induced by IR, suggesting that I(kappa)B(alpha) degradation was a critical event in this process. To reconcile these results, U251 cell lysates were separated into soluble and insoluble fractions and I(kappa)B(alpha) levels evaluated. Although I(kappa)B(alpha) was found in both subcellular fractions, treatment with IR resulted in the degradation of I(kappa)B(alpha) only in the insoluble fraction. Further subcellular fractionation suggested that the IR-sensitive, insoluble pool of I(kappa)B(alpha) was associated with the plasma membrane. These data suggest that the subcellular location of I(kappa)B(alpha) is a critical determinant in IR-induced NF-kappaB activation.

Tumor necrosis factor-alpha inhibits myogenesis through redox-dependent and -independent pathways

Muscle wasting accompanies diseases that are associated with chronic elevated levels of circulating inflammatory cytokines and oxidative stress. We previously demonstrated that tumor necrosis factor-alpha (TNF-alpha) inhibits myogenic differentiation via the activation of nuclear factor-kappaB (NF-kappaB). The goal of the present study was to determine whether this process depends on the induction of oxidative stress. We demonstrate here that TNF-alpha causes a decrease in reduced glutathione (GSH) during myogenic differentiation of C(2)C(12) cells, which coincides with an elevated generation of reactive oxygen species. Supplementation of cellular GSH with N-acetyl-l-cysteine (NAC) did not reverse the inhibitory effects of TNF-alpha on troponin I promoter activation and only partially restored creatine kinase activity in TNF-alpha-treated cells. In contrast, the administration of NAC before treatment with TNF-alpha almost completely restored the formation of multinucleated myotubes. NAC decreased TNF-alpha-induced activation of NF-kappaB only marginally, indicating that the redox-sensitive component of the inhibition of myogenic differentiation by TNF-alpha occurred independently, or downstream of NF-kappaB. Our observations suggest that the inhibitory effects of TNF-alpha on myogenesis can be uncoupled in a redox-sensitive component affecting myotube formation and a redox independent component affecting myogenic protein expression.

The involvement of oxidative stress in tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in HeLa cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) serves as an extracellular signal triggering apoptosis in tumor cells. However, the molecular mechanisms leading to the apoptosis are largely unknown. To characterize the molecular events involved in TRAIL-induced apoptosis, we examined the association of reactive oxygen species (ROS) in human adenocarcinoma HeLa cells. In this study, we show strong ROS accumulation upon TRAIL induction, with activation of caspases, followed by apoptosis. The pre-treatment with gamma-glutamylcysteinylglycine or estrogen, both effective antioxidants, significantly attenuated TRAIL-induced apoptosis through the reduction of ROS accumulation and diminished caspases activity. Furthermore, zVAD-fmk, an inhibitor of pan-caspase, effectively inhibited the activation of caspases and prevented apoptosis by TRAIL, although TRAIL-induced ROS generation was not attenuated. These data indicate that ROS may play a role as an upstream mediator of caspases. Taken together, our results suggest that oxidative stress mediates TRAIL-induced apoptosis in HeLa cells.

Temporal patterns of cytokine and apoptosis-related gene expression in spinal cords of the G93A-SOD1 mouse model of amyotrophic lateral sclerosis

Familial amyotrophic lateral sclerosis (FALS) is often caused by gain-of-function mutations in Cu,Zn-superoxide dismutase (SOD1). Multiprobe ribonuclease protection assays (RPAs) were used to investigate expression of 36 different cytokines and apoptosis-related genes in spinal cords of mice that ubiquitously express human SOD1 bearing a glycine (r) alanine substitution at residue 93 (G93A-SOD1). Mice were studied at late presymptomatic stage (80 days), and at 120 days when the animals experience severe hindlimb paralysis and accumulation of oxidatively modified proteins. Spinal cord tissue from G93A-SOD1 mice expressed a selective subset of macrophage-typical cytokines (monokines) including interleukin (IL)1alpha, IL1beta and IL1RA at 80 days increasing by 120 days. Contrastingly, T-cell derived cytokines (lymphokines) including IL2, IL3 and IL4 were detected at low levels in non-transgenic mice but these were not elevated in G93A-SOD1 mice even at 120 days. Apoptosis-related genes were generally unaffected at 80 days but multiple caspases and death receptor components were up-regulated at 120 days; the only exceptions being FADD and the tumor necrosis factor (TNF)alpha receptor p55 which was up-regulated at 80 days and increased further at 120 days. These data indicate that in the G93A-SOD1 mouse: (i) cytokine expression changes precede bulk protein oxidation and apoptosis gene expression; (ii) lymphocyte contributions to cytokine expression in FALS are likely minor; and (iii) TNFalpha and its receptors may link inflammation to apoptosis in ALS.

Inhibition of mitochondrial respiration by endogenous nitric oxide: a critical step in Fas signaling

We have found that activation of human adult T cell leukemia (Jurkat) cells with anti-Fas Ab leads, in a concentration-dependent manner, to an early burst of production of nitric oxide (NO), which inhibits cell respiration. This results in mitochondrial hyperpolarization, dependent on the hydrolysis of glycolytic ATP by the F1F(o)-ATPase acting in reverse mode. During this early phase of activation, there is a transient release of superoxide anion. All these processes can be prevented by an inhibitor of NO synthase. Approximately 2 h after stimulation with anti-Fas Ab, a distinct second phase can be detected. This comprises a concentration-dependent collapse in mitochondrial membrane potential, a second wave of free radical production, and activation of caspase-8 leading to apoptosis. This second phase is abolished by an inhibitor of caspase activation. In contrast, inhibition of NO synthesis leads to an enhancement and acceleration of these latter processes, suggesting that the early NO-dependent phase represents a protective mechanism. The significance of the two phases in relation to cell survival and death remains to be studied.

The molecular mechanism in activation-induced cell death of an Ag-reactive B cell clone

TPA-1 is a subclone of B cell hybridomas established by somatic hybridization using B cells of A/J mice immunized with TNP-LPS, and expresses a receptor for TNP on the cell membrane. The present study showed that TPA-1 was induced to apoptotic cell death upon treatment with TNP-BSA. Therefore, TPA-1 is considered to provide a good model for the study on activation-induced cell death of mature B cells induced by soluble antigen. TNP-BSA treatment caused the generation of a large amount of intracellular reactive oxygen species (ROS) of TPA-1, and the addition of the monovalent thiol-reactive compound: monochlorobimane (MCB) rescued it from apoptosis as well as the antioxidant reagent: N-acetyl-L-cysteine. Furthermore, MCB markedly inhibited the generation of ROS and prevented the disruption of mitochondrial membrane potential that was induced by TNP-BSA treatment. In addition, it counteracted the effect of TNP-BSA on the expression of the Bcl-2 family, resulting in down-regulation of Bax and Bad and up-regulation of Bcl-XL. Taken together, these results suggest strongly that oxidative stress of mitochondria may be involved directly in apoptotic cell death by engagement of antigen receptors on mature B cells with soluble antigen.

Pro-inflammatory cytokines tumor necrosis factor alpha and interleukin-6 and survival factor epidermal growth factor positively regulate the murine GSTA4 enzyme in hepatocytes

We hypothesized that glutathione transferases could be induced and may participate to cellular defenses against the oxidative stress occurring during liver regeneration. Here, we evidenced that murine GSTA1 (mGSTA1), A4, Pi, and Mu are up-regulated during mouse liver regeneration, exhibiting a biphasic pattern of induction correlating early G(1) phase and G(1)/S transition of the cell cycle. Using confocal microscopy immunolocalization and subcellular fractionation, mGSTA4 was demonstrated in both mitochondria and cytosol and found preferentially increased in cytosol during liver regeneration. In addition, mGSTA4 was induced in vivo and in cultured hepatocytes by tumor necrosis factor alpha (TNFalpha), interleukin-6 (IL-6), and epidermal growth factor (EGF), factors that play crucial roles in hepatocyte survival and proliferation during liver regeneration. However, the mitogenic effect of EGF was not responsible for the induction of mGSTA4. In transient transfections, IL-6 and EGF, but not TNFalpha, transactivated the human GSTA4 (hGSTA4) promoter cloned upstream of the luciferase reporter gene suggesting that IL-6 and EGF up-regulated hGSTA4 at a transcriptional level, whereas TNFalpha could rather act at a post-transcriptional level. The inhibition of phosphoinositide 3-kinase, p38 MAPK, and MEK/ERK signaling pathways, using specific inhibitors, prevented EGF-dependent induction of mGSTA4 and transactivation of hGSTA4 promoter. Altogether, these data favor the conclusion that, in regenerating hepatocytes, several GST isoforms are induced and that cytokines TNFalpha and IL-6 and survival factor EGF positively regulate mGSTA4 via survival signaling pathways.