Dependence of the Spectrum of Shock-Accelerated Ions on the Dynamics at the Shock Crossing

Diffusive shock acceleration (DSA) of ions occurs due to pitch-angle diffusion in the upstream and downstream regions of the shock and multiple crossing of the shock by these ions. The classical DSA theory implies continuity of the distribution at the shock transition and predicts a universal spectrum of accelerated particles, depending only on the ratio of the upstream and downstream fluid speeds. However, the ion dynamics at the shock front occurs within a collision-free region and is gyrophase dependent. The ions fluxes have to be continuous at the shock front. The matching conditions for the gyrophase-averaged distribution functions at the shock transition are formulated in terms of the transition and reflection probabilities. These probabilities depend on the shock angle and the magnetic compression as does the power spectrum of accelerated ions. Their spectral index is expressed in terms of the reflectivity. The spectrum is typically harder than the spectrum predicted by the classical DSA theory.

Dysregulation of plasma amino acid levels in HIV-infection and cancer and its relevance for the immune system

T cells have a weak membrane transport actitivity for cystine but strong transport activity for cysteine. Even moderate variations of the cysteine concentration affect T cell functions in spite of the high concentration of cystine in cultures with physiological amino acid concentrations. The IL-2 dependent DNA synthesis and the activation of cytotoxic T cells are positively regulated by cysteine, while the activity of the transcription factor NFkB and the production of IL-2 are stimulated by active oxygen species and inhibited by cysteine or GSH. Macrophages, in contrast to T cells, take up more cystine than they need and release the excess after intracellular reduction as cysteine into the extracellular space. This "cysteine pumping activity" of macrophages raises intracellular GSH levels and DNA synthesis of T cells in the vicinity. The difference between the cystine transport activities of T cells and macrophages, therefore, enables T cells to switch between prooxidant and antioxidant states. The "cysteine pump" favors selectively the antigen-specific T cells that are about to be stimulated by antigen-presenting macrophages. The capacity of macrophages to take up cystine and to release cysteine is inhibited, however, by elevated extracellular glutamate concentrations. Elevated plasma glutamate levels have been found in several pathological conditions including cancer and HIV-infection. In HIV-infected patients, the hyperglutamataemia is aggravated by hypocystinaemia and hypocysteinaemia. Our studies, therefore, suggest that the cysteine supply is impaired in several pathological conditions with immunodeficiencies including AIDS. N-acetyl-cysteine (NAC) is a safe and well established drug that may be considered for the treatment of patients with HIV-infection.

Aberrant insulin receptor signaling and amino acid homeostasis as a major cause of oxidative stress in aging

The mechanisms leading to the increase in free radical-derived oxidative stress in "normal aging" remains obscure. Here we present our perspective on studies from different fields that reveal a previously unnoticed vicious cycle of oxidative stress. The plasma cysteine concentrations during starvation in the night and early morning hours (the postabsorptive state) decreases with age. This decrease is associated with a decrease in tissue concentrations of the cysteine derivative and quantitatively important antioxidant glutathione. The decrease in cysteine reflects changes in the autophagic protein catabolism that normally ensures free amino acid homeostasis during starvation. Autophagy is negatively regulated by the insulin receptor signaling cascade that is enhanced by oxidative stress in the absence of insulin. This synopsis of seemingly unrelated processes reveals a novel mechanism of progressive oxidative stress in which decreasing antioxidant concentrations and increasing basal (postabsorptive) insulin receptor signaling activity compromise not only the autophagic protein catabolism but also the activity of FOXO transcription factors (i.e., two functions that were found to have an impact on lifespan in several animal models of aging). In addition, the aging-related decrease in glutathione levels is likely to facilitate certain "secondary" disease-related mechanisms of oxidative stress. Studies on cysteine supplementation show therapeutic promise.

Cysteine-rich protein reverses weight loss in lung cancer patients receiving chemotherapy or radiotherapy

Oxidative stress plays a role in the tumor-cytotoxic effect of cancer chemotherapy and radiotherapy and also in certain adverse events. In view of these conflicting aspects, a double-blind trial over a 6-month period was performed to determine whether a cysteine-rich protein (IMN1207) may have a positive or negative effect on the clinical outcome if compared with casein, a widely used protein supplement low in cysteine. Sixty-six patients with stage IIIB-IV non-small cell lung cancer were randomly assigned to IMN1207 or casein. Included were patients with a previous involuntary weight loss of > or =3%, Karnofsky status > or =70, and an estimated survival of >3 months. Thirty-five lung cancer patients remained on study at 6 weeks. Overall compliance was not different between treatment arms (42-44% or 13 g/day). The patients treated with the cysteine-rich protein had a mean increase of 2.5% body weight, whereas casein-treated patients lost 2.6% (p = 0.049). Differences in secondary endpoints included an increase in survival, hand-grip force, and quality of life. Adverse events were mild or moderate. Further studies will have to show whether the positive clinical effects can be confirmed and related to specific parameters of oxidative stress in the host.

Guidelines for the use and interpretation of assays for monitoring autophagy in higher eukaryotes

Research in autophagy continues to accelerate,(1) and as a result many new scientists are entering the field. Accordingly, it is important to establish a standard set of criteria for monitoring macroautophagy in different organisms. Recent reviews have described the range of assays that have been used for this purpose.(2,3) There are many useful and convenient methods that can be used to monitor macroautophagy in yeast, but relatively few in other model systems, and there is much confusion regarding acceptable methods to measure macroautophagy in higher eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers of autophagosomes versus those that measure flux through the autophagy pathway; thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from fully functional autophagy that includes delivery to, and degradation within, lysosomes (in most higher eukaryotes) or the vacuole (in plants and fungi). Here, we present a set of guidelines for the selection and interpretation of the methods that can be used by investigators who are attempting to examine macroautophagy and related processes, as well as by reviewers who need to provide realistic and reasonable critiques of papers that investigate these processes. This set of guidelines is not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to verify an autophagic response.

Oxidative stress and aberrant signaling in aging and cognitive decline

Brain aging is associated with a progressive imbalance between antioxidant defenses and intracellular concentrations of reactive oxygen species (ROS) as exemplified by increases in products of lipid peroxidation, protein oxidation, and DNA oxidation. Oxidative conditions cause not only structural damage but also changes in the set points of redox-sensitive signaling processes including the insulin receptor signaling pathway. In the absence of insulin, the otherwise low insulin receptor signaling is strongly enhanced by oxidative conditions. Autophagic proteolysis and sirtuin activity, in turn, are downregulated by the insulin signaling pathway, and impaired autophagic activity has been associated with neurodegeneration. In genetic studies, impairment of insulin receptor signaling causes spectacular lifespan extension in nematodes, fruit flies, and mice. The predicted effects of age-related oxidative stress on sirtuins and autophagic activity and the corresponding effects of antioxidants remain to be tested experimentally. However, several correlates of aging have been shown to be ameliorated by antioxidants. Oxidative damage to mitochondrial DNA and the electron transport chain, perturbations in brain iron and calcium homeostasis, and changes in plasma cysteine homeostasis may altogether represent causes and consequences of increased oxidative stress. Aging and cognitive decline thus appear to involve changes at multiple nodes within a complex regulatory network.

Low postabsorptive net protein degradation in male cancer patients: lack of sensitivity to regulatory amino acids?

Autophagic (lysosomal) and proteasomic protein degradation are important regulatory mechanisms in the homeostasis of muscle mass, that may be profoundly disturbed in cancer and other wasting syndromes. Due to the inhibiting effect of amino acids and insulin, net proteolysis is restricted to the fasted state, and in autophagy certain amino acids have been identified as 'regulatory' in the rat, including leucine, tyrosine, phenylalanine, methionine, and histidine (i.e. LYFMH). The present cross-sectional study assessed postabsorptive net protein catabolism in male cancer patients as well as in healthy male volunteers, to analyse its relation to such 'regulatory amino acids'. Postabsorptive amino acid exchange rates across the leg were determined in patients with gastrointestinal cancer (GIC, n=47) or renal cell carcinoma (RCC, n=15), age-matched (n=33), and young male control subjects (n=42). Both groups of cancer patients revealed a significantly lower postabsorptive net protein catabolism than control subjects. Furthermore, in the control subjects, the postabsorptive net protein catabolism was found to be inversely and significantly correlated with the arterial concentrations of the 8 amino acids YSHMFGI and L which include 5 of the 'regulatory amino acids'. Cancer patients, in contrast, revealed no such significant correlations. These results may indicate i) that postabsorptive net protein catabolism in skeletal muscle of healthy subjects may be sensitive to amino acids which reportedly regulate autophagy and ii) that such amino acid-sensitive mechanism of protein catabolism may be disturbed in cancer patients.

The deficit in low molecular weight thiols as a target for antiageing therapy

The popular use of antioxidative vitamins illustrates the growing awareness of oxidative stress as an important hazard to our health and as an important factor in the ageing process. Superoxide radicals and superoxide-derived reactive oxygen species (ROS) are constantly formed in most cells and tissues. To ensure that ROS can function as biological signaling molecules without excessive tissue damage, ROS are typically scavenged by antioxidants such as glutathione and the vitamins A, C, and E. "Oxidative stress" occurs if the production of ROS is abnormally increased or antioxidant concentrations are decreased. Genetic studies in mice, Drosophila, and C.elegans suggested that ageing may be mechanistically linked to oxidative stress. Several manifestations of oxidative stress were shown to increase with age, whereas tissue levels of vitamin E, plasma concentrations of vitamin C, and intracellular glutathione concentrations decrease with age. In at least two independent studies, cysteine supplementation on top of the normal protein diet has shown significant beneficial effects on each of several different parameters relevant to ageing, including skeletal muscle functions. As the quality of life in old age is severely compromised by the loss of skeletal muscle function, and as muscle function can be measured with satisfactory precision, loss of muscle function is one of the most attractive surrogate parameters of ageing. The mechanisms by which a deficit in glutathione and its precursor cysteine contributes to various ageing-related degenerative processes appears to be related largely but not exclusively to the dysregulation of redox-regulated biological signaling cascades.

Low intestinal glutamine level and low glutaminase activity in Crohn's disease: a rational for glutamine supplementation?

Intestinal glutamine utilization is integral to mucosal regeneration. We analyzed the systemic and intestinal glutamine status in Crohn's disease (CD) and evaluated the therapeutic effect of glutamine supplementation in an animal model of ileitis. In CD, glutamine concentrations were decreased systemically and in noninflamed and inflamed ileal/colonic mucosa. Mucosal glutaminase activities were depressed in the ileum independent of inflammation but were not different from controls in the colon. In experimental ileitis, oral glutamine feeding prevented macroscopic inflammation, enhanced ileal and colonic glutaminase activities above controls, and normalized the intestinal glutathione redox status. However, glutamine supplementation enhanced myeloperoxidase activity along the gastrointestinal tract and potentiated lipid peroxidation in the colon. In conclusion, glutamine metabolism is impaired in CD. In experimental ileitis, glutamine supplementation prevents inflammatory tissue damage. In the colon, however, which does not use glutamine as its principal energy source, immune enhancement of inflammatory cells by glutamine increases oxidative tissue injury.

Increased gene expression of scavenger receptors and proinflammatory markers in peripheral blood mononuclear cells of hyperlipidemic males

Interactions between peripheral blood mononuclear cells (PBMCs) and those within plaques are suggested to be pathophysiologically relevant to lipid-induced arteriosclerosis. In this study, gene expressions of scavenger receptors (CD36, CD68), LPS receptor (CD14), proinflammatory (tumor necrosis factor alpha [TNFalpha], CD40, interleukin-1 beta [IL-1beta]) and oxidative stress-related (manganese superoxide dismutase [MnSOD]) markers were analyzed in PBMCs of clinically asymptomatic males with classical proatherogenic risk factors such as smoking and/or hyperlipidemia. PBMCs were isolated from venous blood of normolipidemic non-smokers (n = 10) and smokers (n = 8), and hyperlipidemic non-smokers (n = 9) and smokers (n = 8). RNA from PBMCs was used for PCR analyses. Plasma concentrations of oxidized low-density lipoproteins (oxLDL) were measured by ELISA. The gene expressions of CD36, CD68, CD40, TNFalpha, and MnSOD were significantly higher in PBMCs of hyperlipidemics than in normolipidemics, irrespective of whether they were smoking or not. The individual expression of these genes showed significant positive correlations with each other but also with serum cholesterol or plasma oxLDL concentrations. The higher expressions of scavenger receptors, proinflammatory and oxidative stress-related genes of PBMCs are suggested to result mainly from hyperlipidemia and the accompanied increase of oxLDL concentrations.

Hydrogen peroxide inhibits activity of the IGF-1 receptor kinase

IGF-1 receptor (IGF1R) is a transmembrane tyrosine kinase, which is indispensable for cellular growth and differentiation. Using a recombinant GST-tagged cytosolic fragment of IGF1R (GST-IGFK), we now show that oxidation by low doses (50 muM) of hydrogen peroxide markedly inhibits maximum phosphate incorporation in autophosphorylation and substrate phosphorylation assays. A similar inhibition was observed on the activity of intact IGF1R after treatment of T-47D cells. These results are in sharp contrast to the positive influence of hydrogen peroxide on the highly homologous insulin receptor kinase, which was assayed for comparison. This reciprocal influence of physiologically relevant doses of hydrogen peroxide may have important implications on signal transduction of the closely related receptors for insulin and IGF-1.

Oxidative stress and ageing: is ageing a cysteine deficiency syndrome?

Reactive oxygen species (ROS) are constantly produced in biological tissues and play a role in various signalling pathways. Abnormally high ROS concentrations cause oxidative stress associated with tissue damage and dysregulation of physiological signals. There is growing evidence that oxidative stress increases with age. It has also been shown that the life span of worms, flies and mice can be significantly increased by mutations which impede the insulin receptor signalling cascade. Molecular studies revealed that the insulin-independent basal activity of the insulin receptor is increased by ROS and downregulated by certain antioxidants. Complementary clinical studies confirmed that supplementation of the glutathione precursor cysteine decreases insulin responsiveness in the fasted state. In several clinical trials, cysteine supplementation improved skeletal muscle functions, decreased the body fat/lean body mass ratio, decreased plasma levels of the inflammatory cytokine tumour necrosis factor alpha (TNF-alpha), improved immune functions, and increased plasma albumin levels. As all these parameters degenerate with age, these findings suggest: (i) that loss of youth, health and quality of life may be partly explained by a deficit in cysteine and (ii) that the dietary consumption of cysteine is generally suboptimal and everybody is likely to have a cysteine deficiency sooner or later.

Redox regulation in anabolic and catabolic processes

PURPOSE OF REVIEW

Muscle wasting as it typically occurs in old age and in certain diseases is poorly understood. This review summarizes recent findings suggesting a role for redox-sensitive signaling cascades in catabolic processes.

RECENT FINDINGS

The redox-sensitive transcription factors nuclear factor kappaB and activator protein 1 facilitate ubiquitin-proteasome-dependent proteolysis. Nuclear factor kappaB also plays a role in induced expression of tumor necrosis factor alpha and other inflammatory cytokines that have been implicated in catabolic processes. The activities of nuclear factor kappaB and activator protein 1 are stimulated not only by hydrogen peroxide, which is produced in tissues by regulated enzymatic processes, but also by an oxidative shift in thiol-disulfide redox status. The oxidative shift that is typically seen in old age and certain catabolic conditions may thus play a causative role in catabolic processes. Another prominent case in point is insulin-independent 'basal' insulin receptor kinase activity, which is strongly enhanced by hydrogen peroxide or by an oxidative shift in redox status. The insulin receptor signaling cascade induces anabolic and anticatabolic effects, but its abnormal upregulation under starving conditions potentially compromises glucose and amino acid homeostasis. In genetic animal studies, impairment of insulin receptor signaling was shown to increase life span.

SUMMARY

These findings may provide a rationale for cysteine supplementation in catabolic conditions.

Oxidative Aging and Insulin Receptor Signaling

The life span of nematodes, fruit flies, and mice can be significantly increased (and aging-related changes decreased) by mutations affecting insulin receptor signaling. This effect involves several cellular functions which are negatively regulated by the insulin receptor and thus typically expressed under fasting conditions. This involvement raises the question of whether the insulin-independent basal receptor kinase activity in the postabsorptive state can be decreased without compromising the physiologically important response to insulin in the postprandial state. Recent studies have shown that (a) the basal human insulin receptor kinase activity is increased under oxidative conditions in the absence of insulin and (b) insulin signaling in the fasted state can be decreased by cysteine supplementation. Cysteine supplementation has also been shown to improve certain aging-related parameters, suggesting that the average dietary cysteine consumption in Western countries may be suboptimal. These findings provide a conceptual framework that extends the "free radical theory of aging."

Autophagy and aging: the importance of maintaining "clean" cells

A decrease in the turnover of cellular components and the intracellular accumulation of altered macromolecules and organelles are features common to all aged cells. Diminished autophagic activity plays a major role in these age-related manifestations. In this work we review the molecular defects responsible for the malfunctioning of two forms of autophagy, macroautophagy and chaperone-mediated autophagy, in old mammals, and highlight general and cell-type specific consequences of dysfunction of the autophagic system with age. Dietary caloric restriction and antilipolytic agents have been proven to efficiently stimulate autophagy in old rodents. These and other possible experimental restorative efforts are discussed.

Oxidative enhancement of insulin receptor signaling: experimental findings and clinical implications

Signaling through the insulin receptor and several other receptor tyrosine kinases is subject to redox regulation. Prolonged exposure to hydrogen peroxide impairs the action of insulin, and may account to some extent for the decreased insulin responsiveness in hyperglycemic diabetic patients. However, insulin receptor kinase (IRK) autophosphorylation and/or kinase activity were found to be markedly enhanced by a more limited exposure to hydrogen peroxide or by an oxidative shift in the thiol/disulfide redox status. Oxidative enhancement of IRK function may be mediated by two different mechanisms with similar effects, i.e., by direct oxidative activation of IRK activity or by oxidative inactivation of a protein tyrosine phosphatase, which otherwise down-regulates IRK-mediated signaling. As both mechanisms enhance IRK activity in the absence of insulin, there is a strong possibility that the background IRK activity in the postabsorptive period may be abnormally increased in certain oxidative conditions and thereby disturb the metabolism of glucose and other energy substrates. This remains to be tested. In line with the oxidative enhancement of IRK activity, clinical studies have shown that treatment with a thiol-containing antioxidant increases the postabsorptive glucose and/or insulin concentrations (i.e., the HOMA-R index) at least under certain conditions. This effect may have therapeutic implications.

Increased cyclooxygenase-2 expression in peripheral blood mononuclear cells of smokers and hyperlipidemic subjects

Cyclooxygenase (COX)-2 is expressed in macrophages of arteriosclerotic lesions and promotes inflammation. We investigated whether COX-2 is already expressed in peripheral blood mononuclear cells (PBMCs) of subjects possessing risk-related factors, such as in smokers and hyperlipidemics. PBMCs were isolated from the venous blood of normolipidemic nonsmokers (NL-NSM; n = 15), normolipidemic smokers (NL-SM; n = 12), hyperlipidemic nonsmokers (HL-NSM; n = 10), and hyperlipidemic smokers (HL-SM; n = 10). RNA from PBMCs was used for RT-PCR. Plasma concentrations of oxidized low-density lipoproteins (oxLDL) were measured by ELISA, those of glutamate and cystine by HPLC. The results show that COX-2 expression in PBMCs was significantly increased in the groups with cardiovascular risk factors (NL-SM, HL-SM, HL-NSM) compared with NL-NSM. COX-2 expression in PBMCs was positively correlated with concentrations of total serum cholesterol, oxLDL, glutamate, or cystine. We suggest that the elevated COX-2 expression indicates a priming of PBMCs as a response to a systemic pro-oxidative and proinflammatory shift in subjects with cardiovascular risk factors, which might also contribute to growth and instability of arteriosclerotic lesions.

Interdependent regulation of insulin receptor kinase activity by ADP and hydrogen peroxide

Insulin signaling requires autophosphorylation of the insulin receptor kinase (IRK) domain. Using purified recombinant IRK fragments and the isolated intact insulin receptor, we show here that autophosphorylation is inhibited by ADP and that this effect is essentially reversed by hydrogen peroxide. Autophosphorylation was inhibited by hydrogen peroxide (60 microM) in the absence of ADP but enhanced in the presence of inhibitory concentrations of ADP (67 microM). Enhancement by hydrogen peroxide required direct interaction of hydrogen peroxide with the kinase domain and was not seen in insulin receptor mutants C1245A and C1308A. A similar enhancement was obtained in intact cells in the absence of insulin upon treatment with 1-(2-chloroethyl)-3-(2-hydroxyethyl)-1-nitrosourea, indicating that IRK activity can be alternatively enhanced by a shift in the thiol/disulfide redox status. Molecular modeling of the IRK domain indicated that the ATP-binding site becomes distorted after releasing the nucleotide unless the IRK domain is oxidatively derivatized at Cys1245. Recent clinical studies suggest that these effects may play a role in obesity due to the fact that cytoplasmic creatine kinase in combination with phosphocreatine normally ensures rapid removal of ADP in muscle cells but not in fat cells.

Effect of thiol antioxidant on body fat and insulin reactivity

Insulin signaling is enhanced by moderate concentrations of reactive oxygen species (ROS) and suppressed by persistent exposure to ROS. Diabetic patients show abnormally high ROS levels and a decrease in insulin reactivity which is ameliorated by antioxidants, such as N-acetylcysteine (NAC). A similar effect of NAC has not been reported for non-diabetic subjects. We now show that the insulin receptor (IR) kinase is inhibited in cell culture by physiologic concentrations of cysteine. In two double-blind trials involving a total of 140 non-diabetic subjects we found furthermore that NAC increased the HOMA-R index (derived from the fasting insulin and glucose concentrations) in smokers and obese patients, but not in nonobese non-smokers. In obese patients NAC also caused a decrease in glucose tolerance and body fat mass. Simultaneous treatment with creatine, a metabolite utilized by skeletal muscle and brain for the interconversion of ADP and ATP, reversed the NAC-mediated increase in HOMA-R index and the decrease in glucose tolerance without preventing the decrease in body fat. As the obese and hyperlipidemic patients had lower plasma thiol concentrations than the normolipidemic subjects, our results suggest that low thiol levels facilitate the development of obesity. Supplementation of thiols plus creatine may reduce body fat without compromising glucose tolerance.

Autophagy and aging—importance of amino acid levels

Melendez et al. [Science 301 (2003) 1387] have recently shown that the increased longevity of Caenorhabditis elegans mutants with defective Daf-2 protein, i.e. an insulin receptor analog, involves increased autophagy. Autophagy increases the free amino acid pool and is in certain cells essential for survival at times of limited amino acid availability. In addition, autophagy plays an important role in the turnover of proteins and organelles including mitochondria. The autophagic activity is sensitive to changes in physiological conditions, i.e. it is strongly inhibited by an increase in amino acid concentrations or in insulin receptor signaling. In line with this fact, clinical studies indicate that autophagy mainly occurs at times of low plasma amino acid and insulin concentrations in the post-absorptive (fasted) state, and that the post-absorptive amino acid-sensitive protein catabolism may be taken as a bona fide indicator of autophagic activity. The increased longevity of insulin receptor mutants or of organisms subjected to calorie restriction may, therefore, be attributed to an increase in autophagic activity. Importantly, the autophagic activity decreases with age. Recent studies suggest that this decrease may result from an age-related increase in post-absorptive amino acid levels and/or from an increase in baseline insulin receptor signaling. If so, it is potentially reversible.

Oxidative stress and aging

Free radical-derived reactive oxygen species (ROS) are constantly generated in most living tissue and can potentially damage DNA, proteins and lipids. "Oxidative stress" occurs if ROS reach abnormally high concentrations. Harman was the first to propose that the damaging effects of ROS may play a key role in the mechanism of aging. Genetic studies of such distantly related species as C. elegans, Drosophila melanogaster, and mice support this hypothesis. However, ROS are not only a cause of structural damage, but also physiologically important mediators in biological signaling processes. Abnormally high levels of ROS may therefore lead to dysregulation of redox-sensitive signaling pathways. The redox-sensitive targets in these pathways are often signaling proteins with redox-sensitive cysteine residues which are oxidized to sulfenic acid moieties and mixed disulfides, thereby altering the signaling function of the protein. Because the formation of these mixed disulfides can also occur through a prooxidative shift in the intracellular thiol/disulfide redox status (REDST), the respective signaling pathways respond not only to ROS but also to changes in REDST. Information about the concentration of ROS in living tissue is scarce, but aging-related changes in REDST are well documented. Several studies with cell cultures or experimental animals have shown that the oxidative shift in the intracellular glutathione REDST is typically associated with cellular dysfunction. Complementary studies in humans have shown that oxidative changes in the plasma (i.e., extracellular) REDST are correlated with aging-related pathophysiological processes. The available evidence suggests that these changes play a key role in various conditions which limit the human life span. Several attempts have been made to ameliorate the consequences of aging by thiol-containing antioxidants, but this approach requires a detailed knowledge of the effects of thiol-containing antioxidants on cysteine homeostasis, REDST, and redox-sensitive signaling pathways of the host.

Sp100 is important for the stimulatory effect of homeodomain-interacting protein kinase-2 on p53-dependent gene expression

HIPK2 shows overlapping localization with p53 in promyelocytic leukemia (PML) nuclear bodies (PML-NBs) and functionally interacts with p53 to increase gene expression. Here we demonstrate that HIPK2 and the PML-NB resident protein Sp100 synergize for the activation of p53-dependent gene expression. Sp100 and HIPK2 interact and partially colocalize in PML-NBs. The cooperation of HIPK2 and Sp100 for the induction of p21(Waf1) is completely dependent on the presence of p53 and the kinase function of HIPK2. Downregulation of Sp100 levels by expression of siRNA does not interfere with p53-mediated transcription, but obviates the enhancing effect of HIPK2. In summary, these experiments reveal a novel function for Sp100 as a coactivator for HIPK2-mediated p53 activation.

Cholesterol levels linked to abnormal plasma thiol concentrations and thiol/disulfide redox status in hyperlipidemic subjects

Treatment of hyperlipidemic patients with the thiol compound N-acetylcysteine (NAC) was previously shown to cause a significant dose-related increase in the high-density lipoprotein (HDL)-cholesterol serum level, suggesting the possibility that its disease-related decrease may result from a diminished thiol concentration and/or thiol/disulfide redox status (REDST) in the plasma. We therefore investigated plasma thiol levels and REDST in normo-/hyperlipidemic subjects with and without coronary heart disease (CHD). The thiol level, REDST, and amino acid concentrations in the plasma and intracellular REDST of peripheral blood mononuclear cells (PBMC) have been determined in 62 normo- and hyperlipidemic subjects. Thirty-three of these subjects underwent coronary angiography, because of clinical symptoms of CHD. All groups of hyperlipidemic patients under test and those normolipidemic individuals with documented coronary stenoses showed a marked decrease in plasma thiol concentrations, plasma and intracellular REDST of PBMCs, and a marked increase in plasma taurine levels. Individual plasma thiol concentrations and plasma REDST were strongly negatively correlated with the serum LDL-cholesterol and positively correlated with the serum HDL-cholesterol level. Together with the earlier report about the effect of NAC on the HDL-cholesterol serum level, our findings suggest strongly that lower HDL-cholesterol serum levels may result from a decrease in plasma thiol level and/or REDST possibly through an excessive cysteine catabolism into taurine.

PML is required for homeodomain-interacting protein kinase 2 (HIPK2)-mediated p53 phosphorylation and cell cycle arrest but is dispensable for the formation of HIPK domains

Here we demonstrate that endogenous human homeodomain-interacting protein kinase (HIPK) 2 and the highly homologous kinase HIPK3 are found in a novel subnuclear domain, the HIPK domains. These are distinct from other subnuclear structures such as Cajal bodies and nucleoli and show only a partial colocalization with promyelocytic leukemia (PML) nuclear bodies (PML-NBs). A kinase inactive HIPK2 point mutant is localized in the nucleoplasm. The occurrence of HIPK domains in PML-/- fibroblasts reveals their independence from the PML protein. HIPK2 can be almost completely recruited to PML-NBs by the PML isoform PML IV, but not by PML-III. PML IV-mediated recruitment of HIPK2 does not rely on its kinase function and also occurs in PML-/- fibroblasts, showing that this PML isoform is sufficient for recruitment of HIPK2. Whereas the architecture of HIPK domains is PML independent, HIPK2-mediated enhancement of p53-dependent transcription, p53 serine 46 phosphorylation and the antiproliferative function of HIPK2 strictly rely on the presence of PML.

Improvement in muscular performance and decrease in tumor necrosis factor level in old age after antioxidant treatment

Aging-related loss of muscle function is a predictor of mortality and a surrogate parameter of the aging process. Its consequences include a high risk for falls, hip fractures, and loss of autonomy. Aging is associated with changes in the oxidant/antioxidant balance including a decrease in plasma thiol (cysteine) concentration. To assess the importance of cysteine, we determined in a double-blind study the effects of N-acetylcysteine on the functional capacity of frail geriatric patients and their response to physical exercise. The subjects on placebo showed only a relatively weak response, and 31% showed even a decrease in more than one parameter during the observation period. Low plasma arginine levels were correlated with a weak overall performance before exercise and a poor response to exercise. N-Acetylcysteine strongly enhanced the increase in knee extensor strength and significantly increased the sum of all strength parameters if adjusted for baseline arginine level as a confounding parameter. N-acetylcysteine had no significant effect on growth hormone and IGF-1 levels but caused a significant decrease in plasma TNF-alpha. These findings may provide a basis for therapeutic intervention and suggest that the loss of function involves limitations in cysteine and one or more other amino acids which may compromise muscular protein synthesis.

Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa and phospholipase C gamma 1 are required for NF-kappa B activation and lipid raft recruitment of protein kinase C theta induced by T cell costimulation

The NF-kappaB activation pathway induced by T cell costimulation uses various molecules including Vav1 and protein kinase C (PKC)theta. Because Vav1 inducibly associates with further proteins including phospholipase C (PLC)gamma1 and Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76), we investigated their role for NF-kappaB activation in Jurkat leukemia T cell lines deficient for expression of these two proteins. Cells lacking SLP-76 or PLCgamma1 failed to activate NF-kappaB in response to T cell costimulation. In contrast, replenishment of SLP-76 or PLCgamma1 expression restored CD3/CD28-induced IkappaB kinase (IKK) activity as well as NF-kappaB DNA binding and transactivation. PKCtheta activated NF-kappaB in SLP-76- and PLCgamma1-deficient cells, showing that PKCtheta is acting further downstream. In contrast, Vav1-induced NF-kappaB activation was normal in SLP-76(-) cells, but absent in PLCgamma1(-) cells. CD3/CD28-stimulated recruitment of PKCtheta and IKKgamma to lipid rafts was lost in SLP-76- or PLCgamma1-negative cells, while translocation of Vav1 remained unaffected. Accordingly, recruitment of PKCtheta to the immunological synapse strictly relied on the presence of SLP-76 and PLCgamma1, but synapse translocation of Vav1 identified in this study was independent from both proteins. These results show the importance of SLP-76 and PLCgamma1 for NF-kappaB activation and raft translocation of PKCtheta and IKKgamma.

Aging-related changes in the thiol/disulfide redox state: implications for the use of thiol antioxidants

Genetic and biochemical studies suggest that free radical-derived reactive oxygen species play a key role in a common mechanism of aging in many or all animal species. This led to the hypothesis that the quality of life in old age may be improved by pharmacological or dietary thiol antioxidants. This review describes important details about how the organism deals with its own thiol antioxidants. Aging was found to be associated with an oxidative shift in the thiol/disulfide redox state (REDST) of the intracellular glutathione pool and of the plasma cyst(e)ine and albumin pools. There is also a decrease in plasma thiol (mainly cysteine) concentration. The oxidative shift in intracellular REDST was found to be typically associated with cellular dysfunctions. Studies in humans related to plasma REDST revealed correlations with aging-related pathophysiological processes, suggesting that oxidative changes in REDST play a key role in processes and diseases which limit the human life span. The age-related shift in plasma REDST is mediated, at least partly, by the decreasing capacity to remove dietary cysteine from the oxidative environment of the blood. Thiol antioxidants were found to ameliorate various aging-related processes but obviously ought to be used with caution in consideration of the oxidative environment of the blood.

Plasma cystine concentration and redox state in aging and physical exercise

Because redox-regulated signalling pathways are often modulated by the thiol/disulfide redox state (REDST), changes in plasma REDST may possibly account for pathological processes. We, therefore, investigated the mechanisms that account for changes in the plasma REDST as derived in first approximation from the cystine and acid soluble thiol (mainly cysteine) concentrations. Elderly subjects (studies A) and younger subjects after intensive physical exercise (IPE) (study B) i.e. subjects in conditions typically associated with decreased insulin responsiveness, showed, on the average, an increase in the plasma total free amino acid (TAA) concentration to approximately 3000 microM, including an increase in cystine but no increase in the thiol concentration if compared with controls. The REDST was decreased accordingly. A study on the postabsorptive amino acid exchange rates across the lower extremities (study C) indicated that a TAA level > or =2800 microM supports a balanced net protein synthesis even under conditions of weak insulin stimulation, suggesting that high TAA levels may prevent the release of cysteine into the blood in the postabsorptive state. Collectively, these studies indicate that the age-related oxidative shift in plasma REDST may result from the decrease in amino acid clearance capacity and may be aggravated by excessive physical exercise.

The plasma redox state and ageing

One of the hallmarks of ageing is the increase in probability to die within a given period of time. A substantial body of evidence suggests that several major causes of death including cardiovascular diseases, cancer, and ageing-related wasting share a conspicuous common risk factor, i.e. the oxidative shift in the plasma thiol/disulfide redox state (REDST). This review deals with the questions how the shift in REDST is generated and how it contributes to the major age-related diseases and conditions, which ultimately limit the maximum and/or average human lifespan. Methods to correct the plasma REDST in the elderly are now being developed to determine whether such methods may improve the quality of life in old age and to test the hypothesis that the shift in REDST may account for important aspects of the ageing process.

Effect of N-acetyl-cysteine on the hypoxic ventilatory response and erythropoietin production: linkage between plasma thiol redox state and O(2) chemosensitivity

Oxygen-sensing chemoreceptors contribute significantly to the regulation of the respiratory drive and arterial PO(2) levels. The hypoxic ventilatory response (HVR) decreases strongly with age and is modulated by prolonged hypoxia and physical exercise. Several earlier studies indicated that the regulation of the ventilatory response and erythropoietin (EPO) production by the respective oxygen sensors involves redox-sensitive signaling pathways, which are triggered by the O(2)-dependent production of reactive oxygen species. The hypothesis that HVR and EPO production are modulated by thiol compounds or changes in the plasma thiol-disulfide redox state (REDST) was investigated. It was demonstrated that both responses are enhanced by oral treatment with N-acetyl-cysteine (NAC) and that HVR is correlated with plasma thiol level and REDST. Results suggest the possibility that age-related changes in plasma REDST may account for the age-related changes in HVR.

Free radicals in the physiological control of cell function

At high concentrations, free radicals and radical-derived, nonradical reactive species are hazardous for living organisms and damage all major cellular constituents. At moderate concentrations, however, nitric oxide (NO), superoxide anion, and related reactive oxygen species (ROS) play an important role as regulatory mediators in signaling processes. Many of the ROS-mediated responses actually protect the cells against oxidative stress and reestablish "redox homeostasis." Higher organisms, however, have evolved the use of NO and ROS also as signaling molecules for other physiological functions. These include regulation of vascular tone, monitoring of oxygen tension in the control of ventilation and erythropoietin production, and signal transduction from membrane receptors in various physiological processes. NO and ROS are typically generated in these cases by tightly regulated enzymes such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. In a given signaling protein, oxidative attack induces either a loss of function, a gain of function, or a switch to a different function. Excessive amounts of ROS may arise either from excessive stimulation of NAD(P)H oxidases or from less well-regulated sources such as the mitochondrial electron-transport chain. In mitochondria, ROS are generated as undesirable side products of the oxidative energy metabolism. An excessive and/or sustained increase in ROS production has been implicated in the pathogenesis of cancer, diabetes mellitus, atherosclerosis, neurodegenerative diseases, rheumatoid arthritis, ischemia/reperfusion injury, obstructive sleep apnea, and other diseases. In addition, free radicals have been implicated in the mechanism of senescence. That the process of aging may result, at least in part, from radical-mediated oxidative damage was proposed more than 40 years ago by Harman (J Gerontol 11: 298-300, 1956). There is growing evidence that aging involves, in addition, progressive changes in free radical-mediated regulatory processes that result in altered gene expression.

Regulation of p53 activity by its interaction with homeodomain-interacting protein kinase-2

Transcriptional activity of p53, a central regulatory switch in a network controlling cell proliferation and apoptosis, is modulated by protein stability and post-translational modifications including phosphorylation and acetylation. Here we demonstrate that the human serine/threonine kinase homeodomain-interacting protein kinase-2 (HIPK2) colocalizes and interacts with p53 and CREB-binding protein (CBP) within promyelocytic leukaemia (PML) nuclear bodies. HIPK2 is activated by ultraviolet (UV) radiation and selectively phosphorylates p53 at Ser 46, thus facilitating the CBP-mediated acetylation of p53 at Lys 382, and promoting p53-dependent gene expression. Accordingly, the kinase function of HIPK2 mediates the increased expression of p53 target genes, which results in growth arrest and the enhancement of UV-induced apoptosis. Interference with HIPK2 expression by antisense oligonucleotides impairs UV-induced apoptosis. Our results imply that HIPK2 is a novel regulator of p53 effector functions involved in cell growth, proliferation and apoptosis.

Flux of amino acids and energy substrates across the leg in weight-stable HIV-infected patients with acute opportunistic infections: indication of a slow protein wasting process

Increased whole-body proteolysis with muscle protein net degradation has been suggested as one of the causes of weight loss in patients infected with human immunodeficiency virus (HIV). We studied the exchange rates of amino acids and energy substrates across the lower extremity in 16 HIV patients and 16 age-matched controls with similar body cell mass. The patients had either opportunistic infections or chronic diarrhea but no signs of clinical malnutrition. The following findings were obtained in the HIV patients: an augmented peripheral net release of arginine and lysine; an increase in both the negative arterial-venous difference and the efflux of the nitrogen contained in nonmetabolized amino acids; diminished export of 3-methylhistidine; lowered plasma and erythrocyte amino acid concentrations; reduced output of glycerol and furthermore; and neither a net release nor a net uptake of free fatty acids. The findings concerning nitrogen metabolism support the hypothesis that, in the presence of a reduction in protein breakdown, peripheral protein synthesis is severely depressed, making a slow protein wasting process likely to occur. The balances of glycerol and free fatty acids are due not only to the leg tissues but perhaps also in part to increased net retention of these substrates by skeletal muscle.

Influence of N-acetylcysteine on hepatic amino acid metabolism in patients undergoing orthotopic liver transplantation

Experimental treatment with the antioxidant and glutathione precursor N-acetylcysteine (NAC) has been performed in orthotopic liver transplantation (OLT) to reduce reperfusion injury. To investigate the effect of NAC on the hepatic and intestinal amino acid metabolism, intraoperative amino acid exchange rates were studied in liver transplant recipients with high dose NAC treatment (n = 10) and in control patients (n = 9). Treatment with NAC was found to cause a loss of amino acids and increased urea nitrogen release from the liver graft. The net balance of most amino acids was shifted to increased hepatic release or decreased hepatic uptake. The initial cumulative splanchnic release of all proteinogenic amino acids in the NAC treated group was significantly higher than in the control group. These findings are tentatively explained by an increased net protein catabolism in the liver. The increased hepatic urea and glutamine production rate of the NAC treated patients is expected to increase the energy and oxygen demand of the liver in this critical situation. Thus, NAC may have caused marked metabolic disturbances in the freshly implanted graft. The dosage of NAC should therefore be modified to avoid these disadvantages.

CD95-induced JNK activation signals are transmitted by the death-inducing signaling complex (DISC), but not by Daxx

Here we investigated CD95-mediated JNK activation pathways and their physiological relevance by employing a variety of cell lines with deficiencies in individual signal transmitting proteins. JNK activation was completely dependent on the activation of caspases in type I and type II cells, as revealed by the inhibitory effects of the caspase inhibitors zVAD-fmk or the cowpoxvirus-encoded CrmA protein. Jurkat cells deficient in caspase-8 or expressing a dominant negative (DN) form of FADD were unable to induce JNK in response to CD95 ligation, indicating that these death-inducing signaling complex (DISC) proteins are required for signal transmission. Activation of caspases, JNK and apoptosis occurred with a markedly slower kinetics in cells expressing a DN version of ASK1, revealing an important contribution of ASK1 for these processes. A C-terminally truncated version of Daxx impaired CD95-mediated apoptosis without affecting the JNK signal. DN forms of FADD, MKK4 and MKK7 completely inhibited CD95-mediated JNK activation but remained without impact on cell killing, indicating that JNK activation is not required for the execution process of CD95-mediated cell killing.

Protein kinase C theta cooperates with Vav1 to induce JNK activity in T-cells

Here we show that in human T-cell leukemia cells Vav1 and protein kinase C theta (PKCtheta) synergize for the activation of c-Jun N-terminal kinase (JNK) but not p38 MAP kinase. Vav1 and PKCtheta also cooperated to induce transcription of reporter genes controlled either by AP-1 binding sites or the CD28RE/AP composite element contained in the IL-2 promoter by stimulating the binding of transcription factors to these two elements. Dominant negative versions of Vav1 and PKCtheta inhibited CD3/CD28-induced activation of JNK, revealing their relative importance for this activation pathway. Gel filtration experiments revealed the existence of constitutively associated Vav1/PKCtheta heterodimers in extracts from unstimulated T-cells, whereas T-cell costimulation induced the recruitment of Vav1 into high molecular weight complexes. Several experimental approaches showed that Vav1 is located upstream from PKCtheta in the control of the pathway leading to synergistic JNK activation. Vav1-derived signals lead to the activation of JNK by at least two different pathways. The major contribution of Vav1 for the activation of JNK relies on the PKCtheta-mediated Ca(2+)-independent synergistic activation pathway, whereas JNK is also activated by a separate Ca(2+)-dependent signaling route.

The Drosophila proteins Pelle and Tube induce JNK/AP-1 activity in mammalian cells

The mammalian interleukin-1 (IL-1) signal transduction pathways display remarkable homology to the Toll signaling cascade in Drosophila. To address the question whether members of the Drosophila Toll pathway are functional in mammalian cells, inactive and constitutively active versions of the protein kinase Pelle and its regulator Tube were expressed in HeLa cells and tested for their impact on IL-1-dependent signaling events. The Drosophila proteins failed to induce the IL-1-responsive transcription factor, nuclear factor-kappaB, but selectively activated the IL-1-regulated kinase, c-Jun N-terminal kinase (JNK), thus resulting in elevated AP-1 activity. Activation of JNK/AP-1 activity was seen upon expression of a Pelle mutant lacking its C-terminal half or by a membrane-bound and multimerised Tube protein, showing the functionality of the Drosophila proteins in mammalian cells.

Human homeodomain-interacting protein kinase-2 (HIPK2) is a member of the DYRK family of protein kinases and maps to chromosome 7q32-q34

Here we identified the human serine/threonine kinase HIPK2 as a novel member of the DYRK kinase subfamily. Alignment of several DYRK family proteins including the kinases minibrain, MJAK, PKY, the Dictyostelium kinase YakA and Saccharomyces YAK1 allowed the identification of several evolutionary conserved DYRK consensus motifs within the kinase domain. A lysine residue conserved between all DYRK kinase family members was found to be essential for the kinase function of HIPK2. Human HIPK2 was mapped to chromosome 7q32-q34 and murine HIPK2 to chromosome 6B, the homologue to human chromosome 7.

Glutathione and immune function

The immune system works best if the lymphoid cells have a delicately balanced intermediate level of glutathione. Even moderate changes in the intracellular glutathione level have profound effects on lymphocyte functions. Certain functions, such as the DNA synthetic response, are exquisitely sensitive to reactive oxygen intermediates and, therefore, are favoured by high levels of the antioxidant glutathione. Certain signal pathways, in contrast, are enhanced by oxidative conditions and favoured by low intracellular glutathione levels. The available evidence suggests that the lymphocytes from healthy human subjects have, on average, an optimal glutathione level. There is no indication that immunological functions such as resistance to infection or the response to vaccination may be enhanced in healthy human subjects by administration of glutathione or its precursor amino acid cysteine. However, immunological functions in diseases that are associated with a cysteine and glutathione deficiency may be significantly enhanced and potentially restored by cysteine supplementation. This factor has been studied most extensively in the case of human immunodeficiency virus (HIV)-infected patients who were found to experience, on average, a massive loss of S equivalent to a net loss of approximately 4 g cysteine/d. Two randomized placebo-controlled trials have shown that treatment of HIV-infected patients with N-acetyl-cysteine caused in both cases a significant increase in all immunological functions under test, including an almost complete restoration of natural killer cell activity. It remains to be tested whether cysteine supplementation may be useful also in other diseases and conditions that are associated with a low mean plasma cystine level and impaired immunological functions.

Enhancement of T cell receptor signaling by a mild oxidative shift in the intracellular thiol pool

Exposure of T cells to the macrophage products hydrogen peroxide (HP) or L-lactate (LAC) was previously shown to enhance IL-2 production and to modulate glutathione (GSH) status. We now found that 50 microM HP and 30 mM LAC enhanced strongly the transcription from the IL-2 promoter in Jurkat T cells after stimulation with anti-CD28 together with or without anti-CD3 but not with anti-CD3 Abs alone. Therefore, we used anti-CD3 plus anti-CD28-stimulated cells to investigate the effect of the GSH reductase inhibitor 1, 3-bis(2-chloroethyl)-1-nitrosourea (BCNU) on the signal cascade. BCNU enhanced the transcription to a similar extent as HP or LAC. Lowering the intracellular GSH/GSH disulfide ratio by BCNU, HP, or NO resulted in all cases in the fulminant enhancement of Jun-N-terminal kinase and p38 mitogen-activated protein kinase but not extracellular signal-regulated kinase 1/2. Jun-N-terminal kinase and NF-kappaB activation was enhanced through pathways involving Rac, Vav1, PKCTheta, p56(lck), p59(fyn), and IkappaB kinases. In a cell-free system, the autophosphorylation of rFyn was stimulated by GSH disulfide but not by HP. These findings suggest that the oxidation of the cellular thiol pool may play a role as an amplifying mechanism for TCR/CD3 signals in immune responses.

Glutathione is a factor of resistance of Jurkat leukemia cells to nitric oxide-mediated apoptosis

We have previously reported that nitric oxide (NO) stimulates apoptosis in different human neoplastic lymphoid cell lines through mitochondrial damage (including degradation of cardiolipin, a major mitochondrial lipid) followed by activation of caspases. Here we demonstrate that Jurkat human leukemia cells which survive after 24 h treatment with NO form subpopulations with higher and lower cardiolipin content (designated as NAO(high) and NAO(low), respectively). Sorted NAO(high) cells were found to survive in culture whereas sorted NAO(low) cells died. Moreover, NAO(high) cells acquired an increased resistance to the exposure to NO donors which remained unchanged during long-term culture. These cells showed a similar cardiolipin content and expressed the same level of anti-apoptotic proteins Bcl-2 and Bcl-x(L) as APO-S unsorted cells but contained significantly higher concentration of the antioxidant glutathione. Depletion of glutathione in these cells with buthionine-sulfoximine (BSO) correlated with a significant stimulation of NO-mediated apoptosis whereas the exposure of NO-sensitive APO-S cells to the glutathione precursor N-acetylcysteine (NAC) resulted in a substantial suppression of this effect. Our data suggest a complex mechanism of the resistence to NO-induced apoptosis in Jurkat human leukemia cells in which glutathione plays an important role.

Improvement of immune functions in HIV infection by sulfur supplementation: two randomized trials

To determine the therapeutic effect of sulfur amino acid supplementation in HIV infection we randomized 40 patients with antiretroviral therapy (ART; study 1) and 29 patients without ART (study 2) to treatment for 7 months with N-acetyl-cysteine or placebo at an individually adjusted dose according to a defined scheme. The main outcome measures were the change in immunological parameters including natural killer (NK) cell and T cell functions and the viral load. Both studies showed consistently that N-acetyl-cysteine causes a marked increase in immunological functions and plasma albumin concentrations. The effect of N-acetyl-cysteine on the viral load, in contrast, was not consistent and may warrant further studies. Our findings suggest that the impairment of immunological functions in HIV+ patients results at least partly from cysteine deficiency. Because immune reconstitution is a widely accepted aim of HIV treatment, N-acetyl-cysteine treatment may be recommended for patients with and without ART. Our previous report on the massive loss of sulfur in HIV-infected subjects and the present demonstration of the immunoreconstituting effect of cysteine supplementation indicate that the HIV-induced cysteine depletion is a novel mechanism by which a virus destroys the immune defense of the host and escapes immune elimination.

The pro- or anti-apoptotic function of NF-kappaB is determined by the nature of the apoptotic stimulus

To test whether the behaviour of transcription factor NF-kappaB as a promoter or antagonist of apoptosis depends on the apoptotic stimulus, we determined the influence of NF-kappaB on cell killing elicited by a variety of inducers within a given cell type. Inhibition of NF-kappaB by genetic and pharmacological approaches rendered HeLa cells more susceptible to TNF-alpha-induced cell killing, but protected them almost completely from H2O2- and pervanadate-induced apoptosis. TNF-alpha was unable to protect HeLa from H2O2- and pervanadate-induced apoptosis and further enhanced the cytotoxicity induced by these two adverse agents. Supernatants from HeLa cells stably overexpressing a transdominant negative form of IkappaB-alpha selectively increased the cytotoxicity of TNF-alpha for HeLa cells, suggesting that the enhanced susceptibility of these cells can be attributed to one or more secretable factors. Supershift experiments showed that the various apoptotic stimuli induced the same subset of DNA-binding subunits. Therefore, the nature of the signals elicited by the respective death inducers determines whether NF-kappaB induction leads to apoptosis or survival, suggesting that the manipulation of NF-kappaB activity may provide a new approach to adjuvant therapy in cancer treatment.