On the compartmentalization of catalase, fatty acyl-CoA oxidase and urate oxidase in mammalian livers, and the influence of clofibrate treatment on this microlocalization

Reactive Oxygen Species and Oncoprotein Signaling-A Dangerous Liaison

SIGNIFICANCE

There is evidence to implicate reactive oxygen species (ROS) in tumorigenesis and its progression. This has been associated with the interplay between ROS and oncoproteins, resulting in enhanced cellular proliferation and survival. Recent Advances: To date, studies have investigated specific contributions of the crosstalk between ROS and signaling networks in cancer initiation and progression. These investigations have challenged the established dogma of ROS as agents of cell death by demonstrating a secondary function that fuels cell proliferation and survival. Studies have thus identified (onco)proteins (Bcl-2, STAT3/5, RAS, Rac1, and Myc) in manipulating ROS level as well as exploiting an altered redox environment to create a milieu conducive for cancer formation and progression.

CRITICAL ISSUES

Despite these advances, drug resistance and its association with an altered redox metabolism continue to pose a challenge at the mechanistic and clinical levels. Therefore, identifying specific signatures, altered protein expressions, and modifications as well as protein-protein interplay/function could not only enhance our understanding of the redox networks during cancer initiation and progression but will also provide novel targets for designing specific therapeutic strategies.

FUTURE DIRECTIONS

Not only a heightened realization is required to unravel various gene/protein networks associated with cancer formation and progression, particularly from the redox standpoint, but there is also a need for developing more sensitive tools for assessing cancer redox metabolism in clinical settings. This review attempts to summarize our current knowledge of the crosstalk between oncoproteins and ROS in promoting cancer cell survival and proliferation and treatment strategies employed against these oncoproteins. Antioxid. Redox Signal.

Dynamics of H2O2 availability to ARPE-19 cultures in models of oxidative stress

Oxidative injury to cells such as the retinal pigment epithelium (RPE) is often modeled using H(2)O(2)-treated cultures, but H(2)O(2) concentrations are not sustained in culture medium. Here medium levels of H(2)O(2) and cytotoxicity were analyzed in ARPE-19 cultures after H(2)O(2) delivery as a single pulse or with continuous generation using glucose oxidase (GOx). When added as a pulse, H(2)O(2) is rapidly depleted (within 2 h); cytotoxicity at 24 h, determined by the MTT assay for mitochondrial function, is unaffected by medium replacement at 2 h. Continuous generation of H(2)O(2) produces complex outcomes. At low GOx concentrations, H(2)O(2) levels are sustained by conditions under which generation matches depletion, but when GOx concentrations produce cytotoxic levels of H(2)O(2), oxidant depletion accelerates. Acceleration results partly from the release of contents from oxidant-damaged cells as indicated by testing depletion after controlled membrane disruption with detergents. Cytotoxicity analyses show that cells can tolerate short exposure to high H(2)O(2) doses delivered as a pulse but are susceptible to lower chronic doses. The results provide broadly applicable guidance for using GOx to produce sustained H(2)O(2) levels in cultured cells. This approach will be specifically useful for modeling chronic stress relevant to RPE aging and have a wider value for studying cellular effects of sublethal oxidant injury and for evaluating antioxidants that may protect significantly against mild but not lethal stress.

Redox activation of mitochondrial intermembrane space Cu,Zn-superoxide dismutase

The localization of Cu,Zn-superoxide dismutase in the mitochondrial intermembrane space suggests a functional relationship with superoxide anion (O2*-) released into this compartment. The present study was aimed at examining the functionality of Cu,Zn-superoxide dismutase and elucidating the molecular basis for its activation in the intermembrane space. Intact rat liver mitochondria neither scavenged nor dismutated externally generated O2*-, unless the mitochondrial outer membrane was disrupted selectively by digitonin. The activation of the intermembrane space Cu,Zn-superoxide dismutase following the disruption of mitochondrial outer membrane was largely inhibited by bacitracin, an inhibitor of protein disulphide-isomerase. Thiol alkylating agents, such as N-methylmaleimide or iodoacetamide, decreased intermembrane space Cu,Zn-superoxide dismutase activation during, but not after, disruption of the outer membrane. This inhibitory effect was overcome by exposing mitochondria to low micromolar concentrations of H2O2 before disruption of the outer membrane in the presence of the alkylating agents. Moreover, H2O2 treatment alone enabled intact mitochondria to scavenge externally generated O2*-. These findings suggest that intermembrane space Cu,Zn-superoxide dismutase is inactive in intact mitochondria and that an oxidative modification of its critical thiol groups is necessary for its activation.

How can the products of a single gene be localized to more than one intracellular compartment?

Protein-targeting sequences are specific for each intracellular compartment, so that most proteins are found at only one location within the eukaryotic cell. Increasingly, however, examples are being found of proteins that occur and function in more than one cellular compartment. In some cases, the multicompartmentalized isoforms are encoded by the same gene. Several mechanisms have evolved to enable such genes to encode and differentially express multiple types of topogenic information. These mechanisms include alternative forms of transcription initiation, translation initiation, splicing and post-translational modification.

Effects of peroxisome proliferators and/or hypothyroidism on xenobiotic-metabolizing enzymes in rat testis

The objectives of the present work were to study the effects of certain peroxisome proliferators on xenobiotic-metabolizing enzyme activities in the testes of normal and hypothyroid rats, i.e. phenol sulfotransferases (pST), phenol UDP-glucuronosyl transferases (pUDPGT), glutathione transferases (GST), catalase, epoxide hydrolase (EH), glutathione peroxidase (GPX) and NAD(P)H quinone oxidoreductase (QR). Adult male rats (normal and hypothyroid) were treated for 10 days with clofibrate (0.5%), perfluorooctanoic acid (0.5%, PFOA), acetylsalisylic acid (1%, ASA) and di(2-ethylhexyl)phthalate (2%, DEHP) in their diet. The results show that treatment of normal rats with peroxisome proliferators dramatically affects the activities of xenobiotic-metabolizing enzymes (40-60% reduction). The highest effects are seen in catalase activity (50-60% with PFOA and ASA), pUDPGT (55% with PFOA), pST (55% with PFOA) and QR (50% with DEHP). These effects are not seen or are weaker after induction of hypothyroidism. Taken together, it is concluded that different classes of peroxisome proliferators have different effects on rat testicular xenobiotic-metabolizing enzymes.

Catalase inactivation following photosensitization with tetrasulfonated metallophthalocyanines

Catalase (CAT) in solution or incorporated in erythrocytes and K562 leukemic cells is inactivated during photosensitization with tetrasulfonated metallophthalocyanines (MePcS4). The effect of added scavengers and D2O showed that both singlet oxygen and free radical species are involved in this process. Evidence was found that direct interactions of ground or excited-stated photosensitizer with CAT are not responsible for CAT inactivation. Specific techniques to probe early damage to the CAT structure involved optical and EPR spectroscopy, HPLC and polyacrylamide gel electrophoresis analyses. Different primary events of photosensitized protein damage included oxidation of cysteine residues as well as other amino acids, as demonstrated by the formation of carbon-centered free radicals and the loss of absorbance at lambda = 275 nm. In parallel, we detected degradation of the CAT heme groups, accompanied by release of Fe(II) ions in solution. These combined phenomena initiate cross-linkages between CAT subunits and subsequent degradation of the protein with formation of irreversible aggregates in solution. Phthalocyanine-mediated photoinactivation of cell-bound CAT results in loss of protection against accumulating H2O2, providing an additional pathway of phototoxicity.

Perfluorooctane sulfonic acid is a potent inducer of peroxisomal fatty acid beta-oxidation and other activities known to be affected by peroxisome proliferators in mouse liver

Perfluorooctane sulfonic acid is almost as potent as perfluorooctanoic acid in causing increases in peroxisomal fatty acid beta-oxidation, peroxisomal catalase activity, omega-hydroxylation of lauric acid, cytosolic epoxide hydrolase activity and cytosolic DT-diaphorase activity. Octane sulfonic acid was ineffective at doses used for perfluorooctane sulfonic acid and perfluorooctanoic acid. The results support the theory of co-regulation of these parameters and peroxisome proliferation. The fact that perfluorooctane sulfonic acid causes peroxisome proliferation challenges the hypothesis that the first step in this process is formation of a thioester between the proliferator (the carboxylic group) and coenzyme A.

Is the cytosolic catalase induced by peroxisome proliferators in mouse liver on its way to the peroxisomes?

Dietary treatment of male C57B1/6 mice with clofibrate, nafenopin or WY-14.643 resulted in a modest (at most 2-fold) increase in the total catalase activity in the whole homogenate and mitochondrial fraction prepared from the livers of these animals. On the other hand, the catalase activity recovered in the cytosolic fraction was increased 12- to 18-fold, i.e. 30-35% of the total catalase activity in the hepatic homogenate was present in the high-speed supernatant fraction after treatment with these peroxisome proliferators. A study of the time course of the changes in peroxisomal and cytosolic catalase activities demonstrated that the peroxisomal activity both increased upon initiation of exposure and decreased after termination of treatment several days after the increase and decrease, respectively, in the corresponding cytosolic activity. This finding suggests that the cytosolic catalase may be on its way to incorporation into peroxisomes.

Changes in catalase activity and concentration during ovarian development and differentiation

The ovaries of immature rats were used to prepare a peroxisome-enriched fraction by differential centrifugation. Following gonadotropin stimulation, which caused large numbers of follicles to develop into corpora lutea, the specific activity of catalase in the peroxisome-enriched fraction increased 5-fold, while catalase recovered in the post-30,000 x g supernatant did not increase in activity. The increase in catalase specific activity in the peroxisome enriched fraction was shown to be due to an increased concentration of the enzyme as determined by Western blotting. Catalase in pig granulosa cells also increased in specific activity as the follicles aged and luteinized. This increase appeared to parallel increases in the concentration of cytochrome P-450scc. We conclude there is a differential regulation of the peroxisomal and cytosolic pools of rat ovarian catalase.

Studies on the intracellular distributions of soluble epoxide hydrolase and of catalase by digitonin-permeabilization of hepatocytes isolated from control and clofibrate-treated mice

Digitonin permeabilization of hepatocytes from control and clofibrate-treated (0.5% by mass, 10 days) male C57bl/6 mice was used to study the intracellular distributions of soluble ('cytosolic') epoxide hydrolase and of catalase. The following conclusions were drawn. (1) About 60% of the total soluble epoxide hydrolase activity in control mouse hepatocytes is situated in the cytosol. (2) The rest is not mitochondrial, but probably peroxisomal. (3) Of the total catalase activity in control mouse hepatocytes, 5-10% is found in the cytosol. (4) Treatment of mice with clofibrate increases the total hepatocyte activity of soluble epoxide hydrolase 4-fold, but does not influence the relative distribution of this enzyme between cytosol and peroxisomes. (5) The total catalase activity is increased 3.5-fold by clofibrate treatment and 15-35% of this activity is shifted from the peroxisomes to the cytosol.

Alterations in the integrity of peroxisomal membranes in livers of mice treated with peroxisome proliferators

Catalase leakage from its particulate compartment within the light mitochondrial fraction of liver was used as an index of the integrity of peroxisomes in untreated mice and in mice treated with the peroxisome proliferators clofibrate(ethyl-p-chlorophenoxyisobutyrate), Wy-14,643(4-chloro-6[2,3-xylidino)-2-pyrimidinylthio]acetic acid) and DEHP(di-(2-ethylhexyl)phthalate). Catalase leakage represented about 2% of the total catalase activity when fractions from untreated mice were incubated at 4 degrees C, increasing to about 5% during 60 min incubation at 37 degrees C. In fractions from livers of mice treated with peroxisome proliferators, catalase leakage was significantly higher, being 7-11% at 4 degrees C and increasing to approximately 20% after 60 min incubation at 37 degrees C. The pattern of release was similar for all proliferators. Parallel data were obtained for catalase latency in these fractions, i.e. following 60 min incubation at 37 degrees C, free (non-latent) catalase activity was 18% in control mice and 65, 67, and 83% in fractions from clofibrate-, Wy-14,643- and DEHP-treated mice, respectively. Differences in catalase leakage from peroxisomes in fractions from untreated mice and clofibrate-treated mice were also apparent following treatments designed to effect membrane permeabilization, as in freeze-thawing, osmotic rupture, and extraction with Triton X-100 and lysophosphatidylcholine. These data are consistent with a significant alteration in the integrity of the membranes of peroxisomes in livers of mice which have been treated with peroxisome proliferators, and furthermore indicate a commonality of effect of these agents.