N-acetyl-L-cysteine inhibits 26S proteasome function: implications for effects on NF-kappaB activation

Nitric oxide inhibits H2O2-induced transferrin receptor-dependent apoptosis in endothelial cells: Role of ubiquitin-proteasome pathway

We investigated here the mechanism of cytoprotection of nitric oxide (*NO) in bovine aortic endothelial cells treated with H2O2. NONOates were used as *NO donors that released *NO slowly at a well defined rate in the extracellular and intracellular milieus. H2O2-mediated intracellular dichlorofluorescein fluorescence and apoptosis were enhanced by the transferrin receptor (TfR)-mediated iron uptake. *NO inhibited the TfR-mediated iron uptake, dichlorofluorescein fluorescence, and apoptosis in H2O2-treated cells. *NO increased the proteasomal activity and degradation of nitrated TfR via ubiquitination. Nomega-nitro-L-arginine methyl ester, a nonspecific inhibitor of endogenous *NO biosynthesis, decreased the trypsin-like activity of 26S proteasome. *NO, by activating proteolysis, mitigates TfR-dependent iron uptake, dichlorodihydrofluorescein oxidation, and apoptosis in H2O2-treated bovine aortic endothelial cells. The relevance of biological nitration on redox signaling is discussed.

Transcription factors activated in mammalian cells after clinically relevant doses of ionizing radiation

Over the past 15 years, a wealth of information has been published on transcripts and proteins 'induced' (requiring new protein synthesis) in mammalian cells after ionizing radiation (IR) exposure. Many of these studies have also attempted to elucidate the transcription factors that are 'activated' (i.e., not requiring de novo synthesis) in specific cells by IR. Unfortunately, all too often this information has been obtained using supralethal doses of IR, with investigators assuming that induction of these proteins, or activation of corresponding transcription factors, can be 'extrapolated' to low-dose IR exposures. This review focuses on what is known at the molecular level about transcription factors induced at clinically relevant (< or =2 Gy) doses of IR. A review of the literature demonstrates that extrapolation from high doses of IR to low doses of IR is inaccurate for most transcription factors and most IR-inducible transcripts/proteins, and that induction of transactivating proteins at low doses must be empirically derived. The signal transduction pathways stimulated after high versus low doses of IR, which act to transactivate certain transcription factors in the cell, will be discussed. To date, only three transcription factors appear to be responsive (i.e. activated) after physiological doses (doses wherein cells survive or recover) of IR. These are p53, nuclear factor kappa B(NF-kappaB), and the SP1-related retinoblastoma control proteins (RCPs). Clearly, more information on transcription factors and proteins induced in mammalian cells at clinically or environmentally relevant doses of IR is needed to understand the role of these stress responses in cancer susceptibility/resistance and radio-sensitivity/resistance mechanisms.

The role of the ubiquitin/proteasome system in cellular responses to radiation

In the last few years, the ubiquitin(Ub)/proteasome system has become increasingly recognized as a controller of numerous physiological processes, including signal transduction, DNA repair, chromosome maintenance, transcriptional activation, cell cycle progression, cell survival, and certain immune cell functions. This is in addition to its more established roles in the removal of misfolded, damaged, and effete proteins. This review examines the role of the Ub/proteasome system in processes underlying the classical effects of irradiation on cells, such as radiation-induced gene expression, DNA repair and chromosome instability, oxidative damage, cell cycle arrest, and cell death. Furthermore, recent evidence suggests that the proteasome is a redox-sensitive target for ionizing radiation and other oxidative stress signals. In other words, the Ub/proteasome system may not simply be a passive player in radiation-induced responses, but may modulate them. The extent of the modulation will be influenced by the functional and structural diversity that is expressed by the system. Cell types vary in the Ub/proteasome structures they possess and the level at which they function, and this changes as they go from the normal to the cancerous condition. Cancer-related functional changes within the Ub/proteasome system may therefore present unique targets for cancer therapy, especially when targeting agents are used in combination with radio- or chemotherapy. The peptide boronic acid compound PS-341, which was designed to inhibit proteasome chymotryptic activity, is in clinical trials for the treatment of solid and hematogenous tumors. It has shown some efficacy on its own and in combination with chemotherapy. Preclinical studies have shown that PS-341 will also potentiate the cytotoxic effects of radiation therapy. In addition, other drugs in common clinical use have been shown to affect proteasome function, and their activities may be valuably reconsidered from this perspective.

Targeted disruption of specific steps of the ubiquitin-proteasome pathway by oxidation in lens epithelial cells

Several steps in the ubiquitin-proteasome pathway have been shown to be inhibited in models of oxidative stress and aging. We have designed similar models of aging and oxidation in the HLE B-3 human lens epithelial cell line. Following hydrogen peroxide (H2O2) treatment, B-3 cells exhibited an expected activation of c-fos. The effect of these same and similar treatments on the lens proteasome system was unexpected. The 2D gel pattern and the chymotrypsin-like activity of the 20S core were unaffected by this H2O2 treatment, contrary to previous experience in other culture systems. The critical role of proteolysis in the aging lens, and the strong tie between oxidation and proteasome changes, urged us to further model lens oxidation and investigate several steps of the ubiquitin-proteasome pathway with an alternative agent: the thiol-specific oxidant, diamide. The 20S core proteasome, de-ubiquitinating, and ATP-dependent 26S proteasome activities all showed decreases 10 min after diamide was applied, and recovered to near normal within 1h. The higher, 300 microM dose inhibited the 20S by 43%, the de-ubiquitinating activity by 17% and the 26S by 31%. The comparable susceptibility of the 20S activity and the 26S activity differs from several previously published models. Such differences may be the result of tissue or cell line-specific variants in either the components of the ubiquitin-proteasome pathway or in their modification by intracellular oxidants or reductants.

Role of nuclear factor-kappa B activation in acute ischaemia-reperfusion injury in myocardium

(1) Our aims were to characterize activation of the transcription factor, nuclear factor kappa-B(NF-kappaB), during myocardial ischaemia-reperfusion and to assess its functional role in the evolution of acute ischaemia-reperfusion injury in intact myocardium in vivo. (2) Under pentobarbitone anaesthesia, rabbits underwent sham operation, 30 min left coronary artery occlusion followed by 0, 10 or 180 min reperfusion. Saline or NF-kappaB inhibitor diethyldithiocarbamic acid (DDTC, 50, 100 or 200 mg kg(-1)) was given intravenously 5 min prior to reperfusion. (3) Electromobility shift assay revealed that 30 min ischaemia alone did not activate NF-kappaB compared to time-matched sham-operated controls (85+/-13% vs 100+/-28%, respectively). However, ischaemia plus 10 min reperfusion markedly increased activation of NF-kappaB (295+/-77%). DDTC 50 mg kg(-1) did not inhibit NF-kappaB activation (278+/-67%) but at the higher doses complete inhibition was observed (54+/-20%, 31+/-16%, respectively). (4) Infarct to risk ratio was determined by triphenyltetrazolium chloride staining after 30 min ischaemia and 180 min reperfusion. DDTC 50 or 100 mg kg(-1) significantly reduced infarct size compared to the saline-treated control group (34.9+/-5.2%, 37.1+/-5.9%, vs 51.3+/-3.6%, P<0.05, respectively), whereas there was no protection with 200 mg kg(-1) (45.6+/-5.3%). (5) We conclude that ischaemia alone does not activate NF-kappaB, but post-ischaemic reperfusion robustly activates NF-kappaB in the myocardium. DDTC limited irreversible injury at low doses, but this effect appears to be dissociated from inhibition of NF-kappaB. Thus, activation of NF-kappaB during reperfusion does not appear to play a role in the evolution of myocardial infarction during the early phase of reperfusion.

Accumulation of glutathione disulfide mediates NF-kappaB activation during immune stimulation with CpG DNA

Innate immune cells recognize pathogens by detecting molecular patterns that are distinct from those of the host. One such pattern is unmethylated CpG dinucleotides, which are common in bacterial DNA but not in vertebrate genomes. Macrophages respond to such CpG motifs in bacterial DNA or synthetic oligodeoxynucleotides (ODN) by inducing NF-kappaB and secreting proinflammatory cytokines, such as interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha), but the mechanisms regulating this have been unclear. CpG ODN-stimulated cells produce reactive oxygen species (ROS) and have a decreased ratio of intracellular glutathione/glutathione disulfide (GSH/GSSG), indicating a shift to a more oxidized intracellular redox state. To determine whether this may play a role in mediating the CpG-induced macrophage activation, the GSH/GSSG redox state was manipulated in the murine macrophagelike cell line RAW264.7. Treatment of cells with BCNU to inhibit glutathione reductase (GR) enhanced the CpG-induced intracellular oxidation and decreased the GSH/GSSG, with increased activation of NF-kappaB and a doubling in the CpG-induced production of IL-6 and TNF-alpha. Experimental manipulation of the intracellular GSSG concentration during inhibition of cellular prooxidant production demonstrated that increased intracellular GSSG is a primary signal that is directly or indirectly required for CpG-induced NF-kappaB activation but is not in itself sufficient to trigger this in the absence of CpG ODN. These data suggest the existence of a second CpG-induced intracellular signal, independent of GSSG, mediating the activation of innate immunity by bacterial DNA.