Restoration of blood flow to ischemic myocardial tissue results in an increase in the production of oxygen radicals. Highly reactive, free radical species have the potential to damage cellular components. Clearly, maintenance of cellular viability is dependent, in part, on the removal of altered protein. The proteasome is a major intracellular proteolytic system which degrades oxidized and ubiquitinated forms of protein. Utilizing an in vivo rat model, we demonstrate that coronary occlusion/ reperfusion resulted in declines in chymotrypsin-like, peptidylglutamyl-peptide hydrolase, and trypsin-like activities of the proteasome as assayed in cytosolic extracts. Analysis of purified 20 S proteasome revealed that declines in peptidase activities were accompanied by oxidative modification of the protein. We provide conclusive evidence that, upon coronary occlusion/ reperfusion, the lipid peroxidation product 4-hydroxy-2-nonenal selectively modifies 20 S proteasome ␣-like subunits iota, C3, and an isoform of XAPC7. Occlusion/ reperfusion-induced declines in trypsin-like activity were largely preserved upon proteasome purification. In contrast, loss in chymotrypsin-like and peptidylglutamyl-peptide hydrolase activities observed in cytosolic extracts were not evident upon purification. Thus, decreases in proteasome activity are likely due to both direct oxidative modification of the enzyme and inhibition of fluorogenic peptide hydrolysis by endogenous cytosolic inhibitory protein(s) and/or substrate(s). Along with inhibition of the proteasome, increases in cytosolic levels of oxidized and ubiquitinated protein(s) were observed. Taken together, our findings provide insight into potential mechanisms of coronary occlusion/reperfusion-induced proteasome inactivation and cellular consequences of these events.Restoration of coronary blood flow to previously ischemic cardiac tissue is often associated with declines in cardiac function, including myocardial stunning, ventricular arrhythmias, hemodynamic abnormalities, and, in the long term, development of heart failure (1, 2). This paradoxical phenomenon, broadly termed ischemia/reperfusion injury, is accompanied by dramatic increases in tissue levels of free radicals as well as byproducts of lipid peroxidation (1-9). Due to the high reactivity of these species (10 -12), it has been proposed that free radical events play a key role in myocardial ischemia/reperfusion injury. Nevertheless, mechanisms by which free radicals alter cardiac function have not been fully elucidated. Free radical modification of protein can alter and, in many cases, inhibit, protein activity (10 -15). In addition, oxidatively crosslinked protein(s) are resistant to proteolytic degradation (16 -20). Thus, the presence of oxidized protein could impact cellular function by changing the catalytic and/or regulatory properties of specific proteins and by placing abnormal demands on finite cellular volume. Clearly, the level of oxidatively modified protein reflects the balance between free radical damage and p...

The proinflammatory cytokines IL-1β and IL-18 are inactive until cleaved by the enzyme caspase-1. Stimulation of the P2X7 receptor (P2X7R), an ATP-gated ion channel, triggers rapid activation of caspase-1. In this study we demonstrate that pretreatment of primary and Bac1 murine macrophages with TLR agonists is required for caspase-1 activation by P2X7R but it is not required for activation of the receptor itself. Caspase-1 activation by nigericin, a K+/H+ ionophore, similarly requires LPS priming. This priming by LPS is dependent on protein synthesis, given that cyclohexamide blocks the ability of LPS to prime macrophages for activation of caspase-1 by the P2X7R. This protein synthesis is likely mediated by NF-κB, as pretreatment of cells with the proteasome inhibitor MG132, or the IκB kinase inhibitor Bay 11-7085 before LPS stimulation blocks the ability of LPS to potentiate the activation of caspase-1 by the P2X7R. Thus, caspase-1 regulation in macrophages requires inflammatory stimuli that signal through the TLRs to up-regulate gene products required for activation of the caspase-1 processing machinery in response to K+-releasing stimuli such as ATP.

Prooxidents can induce reversible inhibition or irreversible inactivation and degradation of the mitochondrial enzyme aconitase. Cardiac ischemia͞reperfusion is associated with an increase in mitochondrial free radical production. In the current study, the effects of reperfusion-induced production of prooxidants on mitochondrial aconitase and proteolytic activity were determined to assess whether alterations represented a regulated response to changes in redox status or oxidative damage. Evidence is provided that ATP-dependent proteolytic activity increased during early reperfusion followed by a time-dependent reduction in activity to control levels. These alterations in proteolytic activity paralleled an increase and subsequent decrease in the level of oxidatively modified protein. In vitro data supports a role for prooxidants in the activation of ATP-dependent proteolytic activity. Despite inhibition during early periods of reperfusion, aconitase was not degraded under the conditions of these experiments. Aconitase activity exhibited a decline in activity followed by reactivation during cardiac reperfusion. Loss and regain in activity involved reversible sulfhydryl modification. Aconitase was found to associate with the iron binding protein frataxin exclusively during reperfusion. In vitro, frataxin has been shown to protect aconitase from [4Fe-4S] 2؉ cluster disassembly, irreversible inactivation, and, potentially, degradation. Thus, the response of mitochondrial aconitase and ATP-dependent proteolytic activity to reperfusioninduced prooxidant production appears to be a regulated event that would be expected to reduce irreparable damage to the mitochondria.H ighly reactive oxygen derived free radicals, such as superoxide anion (O 2•Ϫ ) and the prooxidant hydrogen peroxide (H 2 O 2 ), can interact with a variety of cellular components, altering both structure and function (1). Although evidence for these reactions has long been sought as indication of free radical involvement in degenerative disorders, recent evidence indicates that these processes also participate in the regulation of cellular function (1, 2). This finding is exemplified by the discovery of enzymatic systems, such as thioredoxin reductase͞thioredoxin (3), glutaredoxin (4), methione sulfoxide reductase (5), and sulfiredoxin (6), which catalyze reversal of oxidative modifications to protein and restoration of protein function. Additionally, receptor-and enzyme-mediated systems exist that catalyze the production of free radicals in response to changes in extracellular and intracellular factors (1, 2). It is therefore important that free radicals produced during physiological and pathophysiological conditions be investigated not simply for their potential to carry out damaging processes but also to induce appropriate alterations in response to changes in cellular homeostasis.Reduction and͞or cessation of blood f low to myocardial tissue, termed ischemia, occurs primarily as a result of formation of atherosclerotic lesions in the coronary arteries...

The P2X7 receptor (P2X7R) is an ATP-gated cation channel that activates caspase-1 leading to the maturation and secretion of IL-1β. Because previous studies indicated that extracellular Cl− exerts a negative allosteric effect on ATP-gating of P2X7R channels, we tested whether Cl− attenuates the P2X7R→caspase-1→IL-1β signaling cascade in murine and human macrophages. In Bac1 murine macrophages, substitution of extracellular Cl− with gluconate produced a 10-fold increase in the rate and extent of ATP-induced IL-1β processing and secretion, while reducing the EC50 for ATP by 5-fold. Replacement of Cl− with gluconate also increased the potency of ATP as an inducer of mature IL-1β secretion in primary mouse bone marrow-derived macrophages and in THP-1 human monocytes/macrophages. Our observations were consistent with actions of Cl− at three levels: 1) a negative allosteric effect of Cl−, which limits the ability of ATP to gate the P2X7R-mediated cation fluxes that trigger caspase-1 activation; 2) an intracellular accumulation of Cl− via nonselective pores induced by P2X7R with consequential repression of caspase-1-mediated processing of IL-1β; and 3) a facilitative effect of Cl− substitution on the cytolytic release of unprocessed pro-IL-1β that occurs with sustained activation of P2X7R. This cytolysis was repressed by the cytoprotectant glycine, permitting dissociation of P2X7R-regulated secretion of mature IL-1β from the lytic release of pro-IL-1β. These results suggest that under physiological conditions P2X7R are maintained in a conformationally restrained state that limits channel gating and coupling of the receptor to signaling pathways that regulate caspase-1.

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