Oxidant-induced activation of protein kinase C in UC11MG cells

Phosphorylated extracellular signal-regulated protein kinases 1 and 2 phosphorylate Sp1 on serine 59 and regulate cellular senescence via transcription of p21Sdi1/Cip1/Waf1

Expression of p21(Sdi1) downstream of p53 is essential for induction of cellular senescence, although cancer cell senescence can also occur in the p53 null condition. We report herein that senescence-associated phosphorylated extracellular signal-regulated protein kinases 1 and 2 (SA-pErk1/2) enhanced p21(Sdi1) transcription by phosphorylating Sp1 on Ser(59) downstream of protein kinase C (PKC) alpha. Reactive oxygen species (ROS), which was increased in cellular senescence, significantly activated both PKCalpha and PKCbetaI. However, PKCalpha, but not PKCbetaI, regulated ROS generation and cell proliferation in senescent cells along with activation of cdk2, proven by siRNAs. PKCalpha-siRNA also reduced SA-pErk1/2 expression in old human diploid fibroblast cells, accompanied with changes of senescence phenotypes to young cell-like. Regulation of SA-pErk1/2 was also confirmed by using catalytically active PKCalpha and its DN-mutant construct. These findings strongly suggest a new pathway to regulate senescence phenotypes by ROS via Sp1 phosphorylation between PKCalpha and SA-pErk1/2: employing GST-Sp1 mutants and MEK inhibitor analyses, we found that SA-pErk1/2 regulated Sp1 phosphorylation on the Ser(59) residue in vivo, but not threonine, in cellular senescence, which regulated transcription of p21(Sdi1) expression. In summary, PKCalpha, which was activated in senescent cells by ROS strongly activated Erk1/2, and the SA-pErk1/2 in turn phosphorylated Sp1 on Ser(59). Sp1-enhanced transcription of p21(Sdi1) resulted in regulation of cellular senescence in primary human diploid fibroblast cells.

Cyclosporin A up-regulates and activates protein kinase C-zeta in EBV-infected and EBV-transformed human B-cells

BACKGROUND

Protein Kinase C (PKC) is a family of enzymes that plays a key role in cell signaling pathways leading to cellular activation and proliferation. Conventional PKC (cPKC) is dependent on calcium for activation. We have proposed that cyclosporin A (CsA), despite being a calcineurin inhibitor, will activate PKC in B cells, thus promoting Epstein-Barr virus (EBV)-induced transformation. Here we show that CsA promoted atypical PKC isoform PKC-zeta in B cells.

MATERIALS AND METHODS

Western-blot was used to assay PKC-zeta protein level in EBV-B cells. Confocal microscopy was used to assay PKC-zeta translocation from cytosol to cell membrane, a known process of PKC activation.

RESULTS

CsA (500 ng/mL) time dependently increased PKC-zeta from control of 7055 units to 7145, 10,805, 10,914, and 12,705 units, respectively, after 15 min, 1 h, 12 h, and 24 h of incubation in EBV-transformed human B-cell line (LCL). CsA increased PKC-zeta expression was inhibited 50% by Vit.E (40 microM) indicating that this effect may be due to oxidative stress induced by CsA. Indeed, after oxidant H(2)O(2) (0.1 mM) treatment, PKC-zeta protein level in LCL cells increased 124%, 257%, 349%, and 359% after 15 min, 1 h, 12 h, and 24 h of culture compared with control. Addition of Vit.E (40 microM) in H(2)O(2) (0.1 mM) treatment and then with Vit.E in the culture decreased PKC-zeta level in LCL cells 26%, 20%, 41%, and 60% after 15 min, 1 h, 12 h, and 24 h of culture. In confocal microscopy in Jurkat T cell line, phorbol 12-myristate 13-acetate (PMA) activated cPKC isoform PKCalpha after 30 min treatment and activated PKC-zeta after 60 min treatment. CsA inhibited PMA activation of PKC-alpha, but not PKC-zeta. CsA alone did not activate PKC-alpha or PKC-zeta in Jurkat T cells. In LCL and in EBV-infected human B-cells, PMA stimulated PKC-alpha activation after 30 min treatment and stimulated PKC-zeta activation after 60 min treatment. CsA inhibited PMA activation of PKC-alpha, but not PKC-zeta. In addition, CsA activated PKC-zeta in the EBV-transformed and EBV-infected human B cells.

CONCLUSION

These experiments show that CsA-induced oxidative stress caused PKC-zeta up-regulation in LCL cells, and show the differential effect of CsA in the PKC signaling pathways in T cells versus B cells. CsA-induced PKC-zeta activation may be an important signaling step in EBV-induced post-transplant lymphoproliferative disorders.

Effect of phenobarbital on hepatic cell proliferation and apoptosis in mice deficient in the p50 subunit of NF-kappaB

Phenobarbital (PB) is a nongenotoxic tumor promoter in the liver. One mechanism by which PB may exert its tumor promoting activity is by inducing oxidative stress. We previously found that PB administration increased hepatic NF-kappaB DNA binding activity. In this study we examined the hypothesis that the effects of PB on cell proliferation and apoptosis are dependent on NF-kappaB. We used a mouse model that is deficient in the p50 subunit of NF-kappaB; previous studies had found that p50-/- mice were less sensitive to the induction of hepatic cell proliferation by PCBs or peroxisome proliferators. Mice (p50-/- and wild-type B6129) were fed a control diet or one containing 0.05% PB for 3, 10 or 34 days. At the end of the experiment, the mice were euthanized and livers removed and processed. PB increased cell proliferation at 3 and 10 days (but not at 34 days), but the deletion of the NF-kappaB p50 subunit did not inhibit these increases. p50-/- Mice had higher cell proliferation at the 3 day (only in mice fed PB) and 34-day timepoints. PB decreased hepatocyte apoptosis after 3 days, slightly decreased it after 10 days, and did not affect it after 34 days. The deletion of the NF-kappaB p50 subunit did not influence PB's effect on apoptosis. In p50-/- mice, apoptosis was increased after 3 or 10 days compared to wild-type mice, but no effect was seen after 34 days. The hepatic expression of the NF-kappaB-regulated gene TNF-alpha correlated more with the hepatic cell proliferation data than with hepatic apoptosis, and was not decreased by the deletion of the p50 subunit. These findings show that the p50 subunit of NF-kappaB is not required for the alteration of hepatocyte proliferation or apoptosis by PB up to 34 days after its administration.

Molecular and signaling mechanisms of atherosclerosis in insulin resistance

Although the prevalence of cardiovascular complications is increased in insulin-resistant individuals, the underlying causes of this link have been elusive. Recent work suggests that several intracellular signal transduction pathways are inappropriately activated by hyperinsulinemia, hyperglycemia, increased free fatty acids, dyslipidemia, various inflammatory cytokines and adipokines--factors that are increased in insulin resistance. Once activated, substantial cross talk occurs between these pathways, especially a self-reinforcing cascade of vascular inflammation and cell dysfunction, greatly increasing the risk and severity of atherosclerosis in the insulin-resistant individual. We review several key cell-signalling pathways, describe how they are activated in they insulin-resistant state and the damage they induce, and discusses possible therapeutic approaches to limit vascular damage.

Reactive oxygen species: Biological stimuli of neuroblastoma cell response

Reactive oxygen species play a critical role in differentiation, proliferation and apoptosis acting as 'second messengers' able to regulate sulphydryl groups in signaling molecules as protein kinase C, a family of isoenzymes involved in many cellular responses and implicated in cell transformation. Neuroblastoma is characterised by the production of oxygen intermediates and L-buthionine-S,R-sulfoximine, a glutathione-depleting agent that has been tested in the clinics, exploits this biological peculiarity to induce cell death. The latter process is mediated by the oxidative activation of PKC delta which might be involved also in the production of reactive oxygen species, thus amplifying the apoptotic cascade.

Oxidative DNA modifications

Oxidative DNA modifications are frequent in mammalian DNA and have been suggested an important mechanism in carcinogenesis, diabetes and ageing. The foundations for this suggestion are: Evidence for the importance of oxidative DNA modifications in cancer development is: high levels of oxidative lesions in cancer tissue; highly conserved and specific DNA repair systems targeting oxidative lesions; high levels of oxidative DNA lesions in oxidative DNA repair knock-out animals; defective repair of oxidative lesions in cancer-prone progeria syndromes; reduced cancer incidence in populations with high dietary antioxidant intake; and increased oxidative stress to DNA in tobacco smokers. Conflicting evidence for a relation between oxidative stress to DNA and cancer is: disagreement about the true levels and occurrence of the oxidative lesions in vivo; failure to identify the localization of oxidative lesions in important genes, e.g. tumor suppressor and oncogenes; lack of evidence that the oxidative lesions induce mutations in vivo; no cancer development in animals knocked-out for specific DNA repair enzymes in spite of high tissue levels of oxidative lesions; and unchanged cancer rates after antioxidant interventions in large clinical controlled and randomized trials. The rate of DNA oxidation has been estimated from urinary excretion of repair products and it is evident that if these lesions were not repaired, a large part of DNA would be oxidized to a degree not compatible with living. The methodologies by which oxidative DNA modifications are measured cover a wide and different range, advantages and disadvantages will be presented. One particular problem is artificial oxidation, and methods to prevent such artifacts will be presented together with results from a large interlaboratory standardization program. The methodology by which the lesions can be measured is complicated and prone to artifacts during DNA isolation, digestion, derivatization and maybe even during the separation procedure proper prior to detection. A large effort from 20+ laboratories supported by a grant from the EU has reduced artifacts considerably and work towards interlaboratory standardization of the methodology is in progress. The presently agreed "normal" levels of the most frequent known lesion 8-oxodG is about 5 per million dG's in DNA. A comprehensive evaluation of the evidence, from chemistry to clinical and epidemiological trials, linking oxidative modifications to cancer will be given. Finally, an estimate of the quantitative role oxidative DNA modifications play among the multiplicity of other insults is given. While there is no question that all of these oxidative mechanisms do exist, quantitative data on their importance for the human situation do not exist. Prospective human studies that can provide such quantitative data on different mechanisms are underway.

Differential subcellular targeting of PKC-epsilon in response to pharmacological or ischemic stimuli in intestinal epithelia

Ischemia is the central pathogenic factor underlying a spectrum of intestinal disorders. The study of the cellular signaling responses to ischemic stress in nonepithelial cells has progressed substantially in the previous several years, but little is known about the response in epithelial cells. Unique features of the epithelial response to ischemic stress suggest differential regulation with regards to signaling. The PKC family of proteins has been implicated in ischemic stress in nonepithelial systems. The role of PKC isoforms in chemical ischemia in intestinal epithelial cells is evaluated in this study. Additionally, the phosphorylation of the F-actin cross-linking protein myristoylated alanine-rich C kinase substrate (MARCKS) is also studied. Chemical ischemia resulted in the transient activation of only the isoform PKC-epsilon as detected by translocation employing the subcellular fractionation technique. The pharmacological agonists phorbol 12-myristate 13-acetate and carbachol also led to the translocation of PKC-epsilon. By immunofluoresence, MARCKS is noted to be located at the lateral membrane under control conditions. In response to carbachol, MARCKS translocates to the cytosol, indicating its phosphorylation, which is additionally confirmed biochemically. Consistent with this observation, carbachol induces the translocation of PKC-epsilon to proximity with MARCKS at the lateral membrane. In response to chemical ischemia, MARCKS fails to translocate and phosphorylation does not increase. Additionally, the translocation of PKC-epsilon is not to the lateral membrane but rather basally. The data suggest that the differential translocation of PKC-epsilon in response to pharmacological agonists versus ischemic stress may lead to different effects on downstream targets.

Protective role of Apigenin on the status of lipid peroxidation and antioxidant defense against hepatocarcinogenesis in Wistar albino rats

Apigenin, a dietary plant derived flavone subclass of flavonoid is expected to play a role in cancer chemoprevention and cancer chemotherapy. Here we designed our experiment to establish whether treatment of apigenin (25 mg/kg body weight) for 14 consecutive days to (N-nitrosodiethylamine) DEN induced (200 mg/kg body weight; by single ip. injection) and phenobarbital promoted (0.05% through drinking water for 14 successive weeks) rats provide protection against the oxidative stress caused by the carcinogen. The level of lipid peroxidation (LPO) markedly increased in carcinogen administered animals, which was brought back to near normal by apigenin treatment. In contrast the activities/levels of the antioxidant status both in liver and kidney were decreased in carcinogen administered animals, which was recouped back to near normal upon apigenin administration. From our findings we concluded that apigenin prevents LPO and protects antioxidant system in DEN induced and phenobarbital promoted hepatocellular carcinogenesis.

Delivery systems for antioxidant nutrients

The novel role of oxidants and antioxidants as part of cell signaling cascades has opened new areas of research in several disease states and their therapeutic strategies. For successful therapeutic manipulation of reactive oxygen species (ROS)-mediated cellular signaling pathways, it would necessitate control of the critical balance of oxidants/antioxidants in the target site by the antioxidant. Another way of controlling the critical balance is to avoid excessive generation of ROS from nutrients and/or drugs. From the viewpoint of controlling the balance between the oxidant and antioxidant status, this review focuses on the prodrug approach for delivery systems of vitamin E, a major antioxidant nutrient in the membrane, and on the reductive activation-independent delivery system of vitamin K hydroquinone by a prodrug approach, which can avoid excessive generation of ROS synchronized with the activation process of vitamin K.

CTP:phosphocholine cytidylyltransferase and protein kinase C recognize different physical features of membranes: differential responses to an oxidized phosphatidylcholine

Protein kinase C (PKC) and CTP:phosphocholine cytidylyltransferase (CT) are two examples of enzymes that are regulated by reversible binding to membranes, and this binding is influenced by membrane physical properties. CT activation by oxidized phosphatidylcholines was recently demonstrated and was linked to the acyl chain disordering effect of the oxidized species (Biochemistry 38, 15606). In this paper, we compare the responses of PKC and CT to an oxidized PC, and investigate the physical properties of lipid bilayers that modulate the activity of these enzymes. We show that 1-palmitoyl, 2-(11,15 dihydroxy) eicosatrienoyl PC (diOH-PAPC) caused less of an increase in the temperature of the lamellar to hexagonal II transition (T(H)) of an unsaturated PE, compared to its parent, PAPC. Using a polarity-sensitive interfacial probe, we also found evidence to suggest that this oxidized PC increases interfacial packing pressure. We found that whereas diOH-PAPC activates CT, it inhibits PKC relative to the parent PAPC. The activities of both CT and PKC are known to increase in the presence of non-lamellar forming lipids. The greater activating effect of diOH-PAPC compared with PAPC, is consistent with a stimulation of the activity of CT by negative curvature strain. However, this is not the case with PKC, for which we suggest that surface packing pressure is of prime importance.

Humic acid induces apoptosis in human endothelial cells

Humic acid (HA) has been implicated as an etiologic factor in the vasculopathy of Blackfoot disease. In this study, the ability of HA to induce apoptosis was studied in cultured human umbilical vein endothelial cells. Treatment of endothelial cells with a variety of concentrations of HA (50-200 microg/ml) resulted in dose- and time-dependent sequences of events marked by apoptosis as shown by loss of cell viability, chromatin condensation, and internucleosomal DNA fragmentation. Antioxidants (superoxide dismutase, vitamin C, and vitamin E) and Ca(2+) chelator (BAPTA) effectively suppressed HA-induced DNA fragmentation (apoptosis). Further studies have shown that HA induced dramatic Ca(2+)-dependent caspase activation (2, 3, 6, 8, and 9). In contrast, negligible caspase-1 activation was observed. The increase in HA-induced apoptosis correlated with a reduction in Bcl-2, a potent cell death inhibitor, and an increase in Bax protein levels, which heterodimerizes with and thereby inhibits Bcl-2. Both of the antioxidants vitamin C and vitamin E prevented the dysregulation of Bcl-2 and Bax in HA-treated endothelial cells. Furthermore, the increase in p53 protein levels correlated with an increase in HA-induced apoptosis. We concluded that both Ca(2+) and oxidative stress were mediators of apoptosis caused by HA and the induction of apoptotic cell death on endothelial cells may be important to the etiology of HA-induced vascular disorder of Blackfoot disease.

Formation of 8-hydroxydeoxyguanosine and cell-cycle arrest in the rat liver via generation of oxidative stress by phenobarbital: association with expression profiles of p21(WAF1/Cip1), cyclin D1 and Ogg1

To evaluate the risk of exposure to so-called non-genotoxic chemicals and elucidate mechanisms underlying their promoting activity on rat liver carcinogenesis the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG), cytochrome P-450 (P-450) and hydroxyl radicals induction, DNA repair and alteration to cellular proliferation and apoptosis in the rat liver were investigated during 2 weeks of phenobarbital (PB) administration at a dose of 0.05%. Significant increase of hydroxyl radical levels by day 4 of PB exposure accompanied the accumulation of 8-OHdG in the nucleus and P-450 isoenzymes CYP2B1/2 and CYP3A2 in the cytoplasm of hepatocytes. Conspicuous elevation of 8-OHdG and apoptosis in the liver tissue were associated with reduction of the proliferating cell nuclear antigen (PCNA) index after 8 days of PB application. Thereafter, 8-OHdG levels decreased with an increase in mRNA expression for the 8-OHdG repair enzyme, DNA glycosylase 1 (Ogg1). Analysis with LightCycler quantitative 2-step RT-PCR demonstrated induction of cyclin D1 (CD1) and p21(WAF1/Cip1) mRNA expression on days 4 and 6, respectively, preceding marked elevation of PCNA and apoptotic indices. These results suggest that similar to genotoxic, non-genotoxic chemicals might induce reversible alteration to nuclear 8-OHdG in the rat liver after several days of continuous application; however, by a different mechanism. Increased 8-OHdG formation is caused by developing oxidative stress or apoptotic degradation of DNA and coordinated with enhanced expression of CD1 mRNA and cell proliferation, subsequent increase of p21(WAF1/Cip1) mRNA expression, cell-cycle arrest and apoptosis, while activation of 8-OHdG repair mechanisms contributes to protection of tissue against reactive oxygen species-induced cell death.

The roles of ATP and calcium in morphological changes and cytotoxicity induced by 1,4-benzoquinone in platelets

To understand the mechanism of 1,4-benzoquinone-induced cytotoxicity in platelets, the roles of ATP and calcium in platelet toxicity and morphological changes were investigated. Using scanning electron microscopy, morphological changes including membrane blebbing were observed in rat platelets 5 min after exposure to 1,4-benzoquinone, which were significantly different from shape changes (pseudopod formation) observed in response to physiological agonists. Benzoquinone-induced membrane blebbing of platelets was associated with rapid depletion of intracellular ATP and was independent of the presence of extracellular calcium. Benzoquinone-induced platelet lysis observed between 20 and 30 min was dependent on extracellular calcium and associated with increased cytosolic calcium. Cytotoxicity induced by 1,4-benzoquinone was inhibited by antagonists of calmodulin, suggesting that calmodulin could play an important role in platelet toxicity. These results suggested that the progression of events for benzoquinone-induced cytotoxicity in platelets was as follows: 1,4-benzoquinone depletes intracellular ATP; membrane blebbing occurs; calcium homeostasis is disrupted, activation of calmodulin-dependent processes results; finally cytotoxicity occurs.

Peroxynitrite-induced tyrosine nitration and inhibition of protein kinase C

Protein kinase C (PKC) is an important intracellular signaling molecule whose activity is essential for a number of aspects of neuronal function including synaptic plasticity. We investigated the regulation of PKC activity by reactive nitrogen species in order to examine whether such species regulate PKC in neurons. Neither autonomous nor cofactor-dependent PKC activity was altered when either hippocampal homogenates or rat brain purified PKC were incubated briefly with three different nitric oxide donor compounds. However, brief incubation of either hippocampal homogenates or purified PKC with peroxynitrite (ONOO(-)) inhibited cofactor-dependent PKC activity in a manner that correlated with the nitration of tyrosine residues on PKC, suggesting that this modification was responsible for the inhibition of PKC. Consistent with this idea, reducing agents had no effect on the inhibition of PKC activity caused by ONOO(-). Because there are numerous PKC isoforms that differ in the composition of the regulatory domain, we studied the effect of ONOO(-) on various PKC isoforms. ONOO(-) inhibited the cofactor-dependent activity of the alpha, betaII, epsilon, and zeta isoforms, indicating that inhibition of enzymatic activity by ONOO(-) was not PKC isoform-specific. We also were able to isolate nitrated PKCalpha and PKCbetaII from ONOO(-)-treated hippocampal homogenates via immunoprecipitation. Collectively, our findings support the hypothesis that ONOO(-) inhibits PKC activity via tyrosine nitration in neurons.

Induction of peroxiredoxin gene expression by oxygen in lungs of newborn primates

Peroxiredoxin (Prx) is an important antioxidant defense enzyme that reduces hydrogen peroxide to molecular oxygen by using reducing equivalents from thioredoxin. We report that lung Prx I messenger RNA (mRNA) is specifically upregulated by oxygen. Throughout the third trimester, mRNA for Prx I was expressed constitutively at low levels in fetal baboon lung. However, after premature birth (125 or 140 d gestation), lung Prx I mRNA increased rapidly with the onset of oxygen exposure. Premature animals (140 d) breathing 100% O(2) developed chronic lung disease within 7 to 14 d. These animals had greater lung Prx I mRNA after 1, 6, or 10 d of life than did fetal controls. In 140-d animals given lesser O(2) concentrations (as needed) that did not develop chronic lung disease, lung Prx I mRNA also was increased on Days 1 and 6, but not Day 10. In fetal distal lung explant culture, Prx I mRNA was elevated in 95% O(2), relative to 1% oxygen, and remained elevated at 24 h. Prx protein activity increased in 140-d premature baboons exposed to as-needed oxygen. By contrast, there was a decrease in Prx activity in 140-d premature baboons exposed to 100% oxygen. In the lung explants from prematures (140 d), there was no significant increase in Prx activity in response to 24 h exposure to hyperoxia, whereas exposure of explants to 48 h hyperoxia caused a nonsignificant decrease in Prx activity. Treatment of lung explants with actinomycin D inhibited Prx mRNA increases in 95% oxygen, indicating transcriptional regulation. In cellular signaling studies we demonstrated that protein kinase (PK) C activity increased when A549 cells were exposed to 95% oxygen, compared with 21% oxygen exposure. In lung explant cultures, specific PKC inhibitors calphostin C or GF109203X inhibited the increase in Prx I mRNA with 95% oxygen exposure, indicating PKC-mediated signaling. The acute increase in gene expression of Prx I in response to oxygen suggests an important role for this protein during the transition from relatively anaerobic fetal life to oxygen-breathing at birth.

Role of protein kinase C, K(ATP) channels and DNA fragmentation in the infarct size-reducing effects of the free radical scavenger T-0970

1. In the present study, we investigated the effect of 1-(3-tert-butyl-2-hydroxy-5-methoxyphenyl)-3-(3-pyridylmethyl) urea hydrocloride (T-0970), a novel water-soluble low-molecular weight free radical scavenger, on the generation of hydroxyl radicals in vivo and on myocardial infarct size in an in vivo model of myocardial infarction in rabbits. 2. T-0970 scavenged hydroxyl radicals generated in the myocardium during reperfusion, as assessed by using a microdialysis technique and HPLC in an in vivo model with 30 min coronary occlusion and 30 min reperfusion in rabbits. 3. Another group of rabbits was subjected to 30 min coronary occlusion and 48 h reperfusion. The control group (n = 10) was infused with saline for 190 min from 10 min before occlusion to 180 min after reperfusion. The treatment group (T-0970 group; n = 10) was injected with a bolus 2.5 mg/kg T-0970 and then infused with T-0970 for 190 min from 10 min before reperfusion to 180 min after reperfusion at a rate of 100 microg/kg per min. The T-0970 + CHE group (n = 5) was given chelerythrine (CHE; a selective inhibitor of protein kinase C (PKC); 5 mg/kg, i.v.) 10 min before the administration of T-0970. The T-0970 + 5-HD group (n = 5) was given 5-hydroxydecanoate (5-HD; an inhibitor of mitochondrial K(ATP) channels; 5 mg/kg, i.v.) 10 min before the administration of T-0970. The CHE and 5-HD groups were given CHE (5 mg/kg, i.v.) and 5-HD (5 mg/kg, i.v.) 20 min before reperfusion, respectively. After 48 h reperfusion, infarct size was measured histologically and expressed as a percentage of the area at risk (AAR). In another series of experiments, the control (n = 5) and T-0970 (n = 5) groups were killed 4 h after reperfusion following 30 min coronary occlusion and DNA fragmentation in myocytes was assessed using in situ dUTP nick end-labelling (TUNEL) at the light microscopic level. 4. Infarct size, as a percentage of AAR, in the T-0970 group was significantly reduced compared with the control group (21+/-4 vs 41+/-4%, respectively; P<0.05). This reduction of infarct size by T-0970 was abolished by pretreatment with CHE and 5-HD. Neither CHE nor 5-HD alone had any effect on infarct size. The percentage of infarcted myocytes with DNA fragmentation by TUNEL in the T-0970 group was significantly reduced compared with the number in the control group (4.0+/-1.5 vs 10.7+/-1.9%, respectively; P<0.05). 5. T-0970, a free radical scavenger, improved reperfusion injury. This effect seemed to be mediated by activation of PKC, the opening of mitochondrial K(ATP) channels and inhibition of DNA fragmentation.

Role of oxidants in the signaling pathway of preconditioning

This review focuses on the possible role of reactive oxygen species in the pathogenesis of this phenomenon. Evidence in support of a role of oxidants in preconditioning has come from the observation that administration of oxygen radical scavengers during the reperfusion period following the initial "preconditioning" ischemia could prevent the phenomenon. In addition, a brief exposure to a low, nontoxic dose of oxygen radicals may reproduce the beneficial effects of ischemic preconditioning, thus suggesting that radicals can directly trigger the preconditioning pathway. To explain the effects of oxidants in this setting, it has been suggested that reperfusion after the initial, "preconditioning" ischemic episode results in the generation of relatively low amounts of oxygen radicals, which are insufficient to determine cell necrosis, but nevertheless could modify cellular activities that have been implicated as mediators of the preconditioning phenomenon. Recent evidence suggests that low levels of oxidants may have a modulatory role on several cell functions. Possible mechanisms of oxidant-mediated protection might be protein kinase C and other kinases, ATP-dependent potassium channels, or changes in sulfhydryl group redox state, while an effect on adenosine metabolism, or the induction of myocardial stunning presumably does not contribute to oxidant-mediated preconditioning. Finally, de novo protein synthesis and gene expression, and increased antioxidant defenses might be involved in the late phase of preconditioning. In summary, available data strongly suggest that oxygen radicals might be possible mediators of preconditioning. However, further investigation is required to clearly elucidate their exact role and mechanisms of action.

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

Superoxide-induced stimulation of protein kinase C via thiol modification and modulation of zinc content

We investigated the effects of mild oxidation on protein kinase C (PKC) using the xanthine/xanthine oxidase system of generating superoxide. Exposure of various PKC preparations to superoxide stimulated the autonomous activity of PKC. Similarly, thiol oxidation increased autonomous PKC activity, consistent with the notion that superoxide stimulates PKC via thiol oxidation. The superoxide-induced stimulation of PKC activity was partially reversed by reducing agents, suggesting that disulfide bond formation contributed to the oxidative stimulation of PKC. In addition, superoxide increased the autonomous activity of the alpha, beta(II), epsilon, and zeta PKC isoforms, all of which contain at least one cysteine-rich region. Taken together, our observations suggested that superoxide interacts with PKC at the cysteine-rich region, zinc finger motif of the enzyme. Therefore, we examined the effects of superoxide on this region by testing the hypothesis that superoxide stimulates PKC by promoting the release of zinc from PKC. We found that a zinc chelator stimulated the autonomous activity of PKC and that superoxide induced zinc release from an PKC-enriched enzyme preparation. In addition, oxidized PKC contained significantly less zinc than reduced PKC. Finally, we have isolated a persistent, autonomously active PKC by DEAE-cellulose column chromatography from hippocampal slices incubated with superoxide. Taken together, these data suggest that superoxide stimulates autonomous PKC activity via thiol oxidation and release of zinc from cysteine-rich region of PKC.

Hydrogen peroxide inhibits the immune response to lipopolysaccharide by attenuating signaling through c-Jun N-terminal kinase and p38 associated with protein kinase C

This study examined the immunomodulatory effects of hydrogen peroxide (H(2)O(2)) in B6C3F1 mouse splenic lymphocytes. H(2)O(2) produced a marked and dose-related inhibition of both lipopolysaccharide (LPS)-induced B-cell proliferation and concanavalin A (Con A)-induced T-cell proliferation. Unexpectedly, little effect was observed with H(2)O(2) on the antibody-forming cell (AFC) response to the polyclonal B-cell activator, LPS. It was also observed that H(2)O(2) did not have any detectable effect on forskolin-stimulated adenylate cyclase, indicating that cyclic AMP (cAMP) is not a mediator of H(2)O(2)-induced suppression of the immune response. Rather, LPS-induced activation of protein kinase C (PKC) was completely inhibited when cells were pretreated with H(2)O(2) for 18 h, although PKC activity was increased approximately twofold following treatment with H(2)O(2) for 10 min. In addition, H(2)O(2) pretreatment blocked the phosphorylation of two stress-activated mitogen-activated protein kinases (MAPKs), c-Jun N-terminal kinase (JNK) and p38 by LPS in a concentration-dependent fashion. Therefore, these data suggest that H(2)O(2) suppresses immune response through the desensitization of PKC, which subsequently results in inhibition of JNK and p38.

Oxidative stress induces protein kinase D activation in intact cells. Involvement of Src and dependence on protein kinase C

Protein kinase D (PKD) is a protein serine kinase that is directly stimulated in vitro by phorbol esters and diacylglycerol in the presence of phospholipids, and activated by phorbol esters, neuropeptides, and platelet-derived growth factor via protein kinase C (PKC) in intact cells. Recently, oxidative stress was shown to activate transfected PKC isoforms via tyrosine phosphorylation, but PKD activation was not demonstrated. Here, we report that oxidative stress initiated by addition of H(2)O(2) (0.15-10 mm) to quiescent Swiss 3T3 fibroblasts activates PKD in a dose- and time- dependent manner, as measured by autophosphorylation and phosphorylation of an exogenous substrate, syntide-2. Oxidative stress also activated transfected PKD in COS-7 cells but not a kinase-deficient mutant PKD form or a PKD mutant with critical activating serine residues 744 and 748 mutated to alanines. Genistein, or the specific Src inhibitors PP-1 and PP-2 (1-10 micrometer) inhibited H(2)O(2)-mediated PKD activation by 45%, indicating that Src contributes to this signaling pathway. PKD activation by H(2)O(2) was also selectively potentiated by cotransfection of PKD together with an active form of Src (v-Src) in COS-7 cells, as compared with PDB-mediated activation. The specific phospholipase C inhibitor, partly blocked H(2)O(2)-mediated but not PDB-mediated PKD activation. In contrast, PKC inhibitors blocked H(2)O(2) or PDB-mediated PKD activation essentially completely, suggesting that whereas Src mediates part of its effects via phospholipase C activation, PKC acts more proximally as an upstream activator of PKD. Together, these studies reveal that oxidative stress activates PKD by initiating distinct Src-dependent and -independent pathways involving PKC.

Protein kinase C signaling and oxidative stress

Oxidative stress is involved in the pathogenesis of various degenerative diseases including cancer. It is now recognized that low levels of oxidants can modify cell-signaling proteins and that these modifications have functional consequences. Identifying the target proteins for redox modification is key to understanding how oxidants mediate pathological processes such as tumor promotion. These proteins are also likely to be important targets for chemopreventive antioxidants, which are known to block signaling induced by oxidants and to induce their own actions. Various antioxidant preventive agents also inhibit PKC-dependent cellular responses. Therefore, PKC is a logical candidate for redox modification by oxidants and antioxidants that may in part determine their cancer-promoting and anticancer activities, respectively. PKCs contain unique structural features that are susceptible to oxidative modification. The N-terminal regulatory domain contains zinc-binding, cysteine-rich motifs that are readily oxidized by peroxide. When oxidized, the autoinhibitory function of the regulatory domain is compromised and, consequently, cellular PKC activity is stimulated. The C-terminal catalytic domain contains several reactive cysteines that are targets for various chemopreventive antioxidants such as selenocompounds, polyphenolic agents such as curcumin, and vitamin E analogues. Modification of these cysteines decreases cellular PKC activity. Thus the two domains of PKC respond differently to two different type of agents: oxidants selectively react with the regulatory domain, stimulate cellular PKC, and signal for tumor promotion and cell growth. In contrast, antioxidant chemopreventive agents react with the catalytic domain, inhibit cellular PKC activity, and thus interfere with the action of tumor promoters.

The effect of protein kinase C activation on colonic epithelial cellular integrity

We have investigated whether activation of protein kinase C has a direct cytotoxic effect on colonic mucosal epithelial cells and whether oxidant-induced damage to colonocytes is mediated by activation of cellular protein kinase C. Incubation of freshly harvested cells from rat colon with the protein kinase C activator, phorbol 12-myristate, resulted in a concentration-dependent increase in the extent of cell injury. Phorbol 12-myristate acetate (0.1-10 microM) also increased cellular protein kinase C activity and this was reduced significantly by treating cells with the antagonists staurosporine or 2-[1-(3-dimethylaminopropyl)-indol-3-yl]3-(-indol-3-yl)maleimide (GF 109203X; 10 microM). Phorbol 12-myristate acetate treatment also resulted in increased translocation of proteins for protein kinase C isoforms alpha, delta and epsilon from cytosol to membrane particulate fractions. The antagonists reduced the extent of cell damage in response to phorbol 12-myristate acetate. Furthermore, cell injury in response to the phorbol acetate was also inhibited by the addition of the oxidant scavengers, superoxide dismutase or catalase to the cell suspension. Addition of H(2)O(2) to the incubation medium (0.1-100 microM) resulted in an increase in cellular protein kinase C activity, an increase in the expression of the alpha, beta and zeta isoforms and a reduction in cell integrity. The cellular damaging actions of H(2)O(2) were significantly reduced by the protein kinase C antagonists, staurosporine or 2-[1-(3-dimethylaminopropyl)-indol-3-yl]-3-(-indol-3-yl)maleimide (GF 109203X). These findings suggest that protein kinase C activation results in colonic cellular injury and this damage is mediated, at least in part, by release of reactive oxidants. Furthermore, oxidant-mediated damage to these cells also involves protein kinase C activation.

Activation of mitochondrial ATP-dependent potassium channels by nitric oxide

BACKGROUND

Nitric oxide (NO) has been implicated as a mediator of "second-window" ischemic preconditioning, and mitochondrial ATP-dependent K(+) (mitoK(ATP)) channels are the likely effectors. The links between NO and mitoK(ATP) channels are unknown.

METHODS AND RESULTS

We measured mitochondrial redox potential as an index of mitoK(ATP) channel opening in rabbit ventricular myocytes. The NO donor S-nitroso-N-acetyl-DL-penicillamine (SNAP, 0.1 to 1 mmol/L) oxidized the mitochondrial matrix dose-dependently without activating sarcolemmal K(ATP) channels. SNAP-induced oxidation was blocked by the selective mitoK(ATP) channel blocker 5-hydroxydecanoate and by the NO scavenger 2-(4-carboxyphenyl)-4,4', 5,5'-tetramethylimidazole-1-oxyl-3-oxide. SNAP-induced mitochondrial oxidation was detectable either by photomultiplier tube recordings of flavoprotein fluorescence or by confocal imaging. SNAP also enhanced the oxidative effects of diazoxide when both agents were applied together. Exposure to 1 mmol/L 8Br-cGMP failed to mimic the effects of SNAP.

CONCLUSIONS

NO directly activates mitoK(ATP) channels and potentiates the ability of diazoxide to open these channels. These results provide novel mechanistic links between NO-induced cardioprotection and mitoK(ATP) channels.

The involvement of protein kinase C in nitric oxide-induced damage to rat isolated colonic mucosal cells

1 The role of protein kinase C (PKC) in colonic cellular injury in response to high concentrations of nitric oxide (NO) released from the donor, S-nitroso-N-acetyl-DL-penicillamine (SNAP) was investigated. 2 Addition of SNAP (0.1-1000 microM) to the cellular suspension resulted in a dose-dependent increase in the extent of damage to isolated colonic mucosal cells as assessed by Trypan blue dye uptake and release of the lysosmal enzyme, N-acetyl-beta-glucosaminidase. SNAP treatment also resulted in an increase in cellular total PKC activity. These increases were reduced or eliminated by pretreatment of the cells with the PKC antagonists staurosporine or GF 109203X or the NO scavenger, phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl 3-oxide (PTIO). 3 PKC-alpha, PKC-delta, PKC-epsilon and PKC-zeta were detected in colonic cellular lysates by immunoblotting. However, only PKC-epsilon protein was increased in response to SNAP treatment. Furthermore, SNAP treatment resulted in activation of PKC-epsilon by causing translocation of the enzyme from the cytosolic to membrane fraction of the cell. This effect was eliminated if cells were preincubated with the NO scavenger, PTIO. 4 The extent of cellular damage in response to addition of SNAP to the incubation medium was enhanced by coincubation with the PKC activator, phorbol 12-myristate 13-acetate (PMA; 1 and 10 microM). 5 PKC activity and the extent of cell damage in response to SNAP were reduced by preincubation of the cells with the peroxyl scavenger, ebselen (0.01-10 microM). 6 These data suggest that the PKC-epsilon isoform of the enzyme mediates NO-induced damage to colonic mucosal cells. This response may occur, at least in part, due to peroxynitrite formation.

Monograph: reassessment of human cancer risk of aldrin/dieldrin

In 1987, the US Environmental Protection Agency (EPA) classified aldrin and dieldrin as category B2 carcinogens, i.e. probable human carcinogens, based largely on the increase in liver tumors in mice fed either organochlorine insecticide. At that date, the relevant epidemiology was deemed inadequate to influence the cancer risk assessment. More time has now elapsed since early exposures of manufacturing workers to aldrin/dieldrin; therefore, updated epidemiological data possess more power to detect exposure-related differences in cancer risk and mortality. Also, recent experimental studies provide a plausible mode of action to explain the mouse specificity of dieldrin-induced hepatocarcinogenesis and call into question the relevance of this activity to human cancer risk. This monograph places this new information within the historic and current perspectives of human cancer risk assessment, including EPA's 1996 Proposed Guidelines for Carcinogen Risk Assessment. Updated epidemiological studies of manufacturing workers in which lifetime exposures to aldrin/dieldrin have been quantified do not indicate increased mortality or cancer risk. In fact, at the middle range of exposures, there is evidence of a decrease in both mortality from all causes and cancer. Recent experimental studies indicate that dieldrin-induced hepatocarcinogenesis in mice occurs through a nongenotoxic mode of action, in which the slow oxidative metabolism of dieldrin is accompanied by an increased production of reactive oxygen species, depletion of hepatic antioxidant defenses (particularly alpha-tocopherol), and peroxidation of liver lipids. Dieldrin-induced oxidative stress or its sequelae apparently result in modulation of gene expression that favors expansion of initiated mouse, but not rat, liver cells; thus, dieldrin acts as a nongenotoxic promoter/accelerator of background liver tumorigenesis in the mouse. Within the framework of EPA's Proposed Guidelines for Carcinogen Risk Assessment, it is proposed that the most appropriate cancer risk descriptor for aldrin/dieldrin, relating to the mouse liver tumor response, is 'not likely a human carcinogen', a descriptor consistent with the example of phenobarbital cited by EPA.

Urate synthesis in the blood-sucking insect rhodnius prolixus. Stimulation by hemin is mediated by protein kinase C

Hemin is a catalyst of the formation of reactive oxygen species. We proposed that hematophagous insects are exposed to intense oxidative stress because of hemoglobin hydrolysis in their midgut (Petretsky, M. D., Ribeiro, J. M. C., Atella, G. C., Masuda, H., and Oliveira, P. L. (1995) J. Biol. Chem. 270, 10893-10896). We have shown that hemin stimulates urate synthesis in the blood-sucking insect Rhodnius prolixus (Graça-Souza, A. V., Petretsky, J. H., Demasi, M., Bechara, E. J. H., and Oliveira, P. L. (1997) Free Radical Biol. Med. 22, 209-214). Once released by fat body cells, urate accumulates in the hemolymph, where this radical scavenger constitutes an important defense against blood-feeding derived oxidative stress. Incubation of Rhodnius fat bodies with okadaic acid raises the level of urate synthesis, suggesting that urate production can be controlled by protein phosphorylation/dephosphorylation. Urate synthesis is stimulated by dibutyryl cAMP and inhibited by N(2((p-bromocinnamil)amino)ethyl)-5-isoquinolinesulfonamide (H-89), an inhibitor of protein kinase A, as well as activated by the protein kinase C activator phorbol 12-myristate 13-acetate. In the presence of hemin, however, inhibition of urate synthesis by H-89 does not occur, suggesting that the hemin stimulatory effect is not mediated by protein kinase A. Calphostin C completely inhibits the hemin-induced urate production, suggesting that the triggering of urate antioxidant response depends on protein kinase C activation. This conclusion is reinforced by the observation that in fat bodies exposed to hemin, both protein kinase C activity and phosphorylation of specific endogenous polypeptides are significantly increased.

Active oxygen species in blue light mediated signal transduction in coleoptile tips

Coleoptile tip is a blue-light sensitive tissue possessing a "blue light receptor" which, upon activation, elicits a signal cascade resulting in phototropic curvature of the coleoptile. In this context, the nature of the photoreceptors and the exact mechanism through which the photoreceptors transduces the signal across the membrane are not clear. In this study, we attempted to examine whether the blue light receptor perturbs redox status of the coleoptile tip and sensitizes molecular oxygen as part of the signal reactions. Coleoptile tips of Sorghum bicolor and wheat (Triticum vulgare) grown in the dark showed pronounced ascorbate free radical signal, which diminished upon illumination with weak blue light for one minute. Concomitantly, the generation of superoxide radical by the coleoptile tip was augmented upon illumination with blue light. Various thiol blockers tested in this study caused powerful inhibition of blue light induced superoxide anion radical generation. Treatment with these thiol blockers, with the exception of NEM, resulted in marked increase in the levels of ascorbic acid free radical in the blue light irradiated coleoptiles. The blue light stimulated O*-2-generation by the coleoptile tip homogenate is also inhibited by the inhibitors of blue light responses viz phenylacetic acid, potassium iodide, and sodium azide. Based on our observations, we postulate that the activated blue light receptor present in the coleoptile tip sensitizes molecular oxygen to superoxide anion radical in the tip initializing the blue light signal cascade reactions.

Protein kinase C mediates experimental colitis in the rat

Protein kinase C (PKC) plays an important role in the cell signal transduction of many physiological processes. In contrast to these physiological responses, increases in PKC activity have also been associated with inflammatory disease states, including ulcerative colitis. The objective of this study was to examine the role of PKC as a causative mediator in initiation of experimentally induced colitis in the rat. Colitis was induced in rats by intrarectal (0.6 ml) instillation of 2,4,6-trinitrobenzenesulfonic acid (TNBS; 75 mg/kg in 50% ethanol) or the PKC activator phorbol 12-myristate 13-acetate (PMA; 1.5-3.0 mg/kg in 20% ethanol). Gross and histological mucosal damage, mucosal neutrophil infiltration, mucosal PKC activity, and PKC protein content for PKC isoforms alpha, beta, delta, and epsilon were assessed 2 h to 14 days after an inflammatory challenge. Both PKC activity and mucosal injury increased significantly within 4 h of TNBS treatment. PKC activity was maximal at 7 days and declined at 14 days, whereas mucosal damage became maximal at 1 day and declined after 7 days. In contrast, neutrophil infiltration as assessed by myeloperoxidase activity only increased 12 h after TNBS treatment, became maximal 1 day after TNBS administration, and declined thereafter. PKCbeta, -delta, and -epsilon were increased in response to TNBS, whereas PKCalpha protein content was decreased. The PKC antagonists staurosporine and GF-109203X (25 ng/kg iv) reduced TNBS-induced changes in mucosal PKC activity and the degree of mucosal damage. In contrast, neutropenia induced by antineutrophil serum treatment did not significantly affect the degree of injury or mucosal PKC activity. Furthermore, activation of mucosal PKC activity with PMA also induced mucosal damage, which was also inhibited by pretreatment with a PKC antagonist. In conclusion, these results suggest that increases in PKC activity play a causative role in TNBS-induced colitis. The PKC-mediated response to TNBS does not appear to involve neutrophil infiltration.

Redox regulation of cellular signalling

Extracellular stimuli elicit a variety of responses, such as cell proliferation and differentiation, through the cellular signalling system. Binding of growth factors to the respective receptor leads to the activation of receptor tyrosine kinases, which in turn stimulate downstream signalling systems such as mitogen-activated protein (MAP) kinases, phospholipase Cgamma (PLCgamma) and phosphatidylinositol 3-kinase. These biochemical reactions finally reach the nucleus, resulting in gene expression mediated by the activation of several transcription factors. Recent studies have revealed that cellular signalling pathways are regulated by the intracellular redox state. Generation of reactive oxygen species (ROS), such as H2O2, leads to the activation of protein tyrosine kinases followed by the stimulation of downstream signalling systems including MAP kinase and PLCgamma. The activation of PLCgamma by oxidative radical stress elevates the cellular Ca2+ levels by flux from the intracellular Ca2+ pool and from the extracellular space. Such reactions in the upstream signalling cascade, in concert, result in the activation of several transcription factors. On the other hand, reductants generally suppress the upstream signalling cascade resulting in the suppression of transcription factors. However, it is well known that cysteine residues in a reduced state are essential for the activity of many transcription factors. In fact, in vitro, oxidation of NFkappaB results in its activation, whereas reductants promote its activity. Thus, cellular signalling pathways are generally subjected to dual redox regulation in which redox has opposite effects on upstream signalling systems and downstream transcription factors. Not only are the cellular signalling pathways subjected to redox regulation, but also the signalling systems regulate the cellular redox state. When cells are activated by extracellular stimuli, the cells produce ROS, which in turn stimulate other cellular signalling pathways, indicating that ROS act as second messengers. It is thus evident that there is cross talk between the cellular signalling system and the cellular redox state. Cell death and life also are subjected to such dual redox regulation and cross talk. Death signals induce apoptosis through the activation of caspases in the cells. Oxidative radical stress induces the activation of caspases, whereas the oxidation of caspases results in their inactivation. Furthermore, some cell-death signals induce the production of ROS in the cells, and the ROS produced in turn stimulate the cell-death machinery. All this evidence shows that the cell's fate is determined by cross talk between the cellular signalling pathways and the cellular redox state through a complicated regulation mechanism.

Sodium hydrosulfite contractions of smooth muscle are calcium and myosin phosphorylation independent

In an effort to further understand the processes underlying hypoxic pulmonary vasoconstriction, we examined the mechanism by which sodium hydrosulfite (Na2S2O4), a potent reducing agent and oxygen scavenger, induces smooth muscle contraction. In rat pulmonary arterial strips, sodium hydrosulfite (10 mM) induced contractions that were 65.9 +/- 12.8% of the response to 60 mM KCl (n = 9 segments). Contractions were not inhibited by nisoldipine (5 microM) or by repeated stimulation with caffeine (10 mM), carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (10 microM), or cyclopiazonic acid (10 microM), all of which eliminated responses to contractile agonists. Maximum force generation after exposure to sodium hydrosulfite was 0.123 +/- 0.013 mN in the presence of 1.8 mM calcium and 0.127 +/- 0.015 mN in the absence of calcium. Sodium hydrosulfite contractions in pulmonary arterial segments were not due to the generation of H2O2 and occurred in the presence of chelerythrine (10 microM), which blocked phorbol ester contractions, and solution hyperoxygenation. Similar contractile responses were obtained in rat aortic and tracheal smooth muscles. Finally, contractions occurred in the complete absence of an increase in myosin light chain phosphorylation. Therefore sodium hydrosulfite-induced smooth muscle contraction is not specific to pulmonary arterial smooth muscle, is independent of calcium and myosin light chain phosphorylation, and is not mediated by either hypoxia or protein kinase C.

Hydrogen peroxide-induced cytoskeletal rearrangement in cultured pulmonary endothelial cells

Although the signaling pathways leading to hydrogen peroxide (H2O2)-induced endothelial monolayer permeability remain ambiguous, cytoskeletal proteins are known to be essential for maintaining endothelial integrity and regulating solute flux through the monolayer. We have recently demonstrated that thrombin-induced actin reorganization in bovine pulmonary artery endothelial cells (BPAEC) requires activation of both myosin light chain kinase (MLCK) and protein kinase C (PKC). Therefore, the present study was designed to investigate the effects of H2O2 on actin reorganization in BPAEC. H2O2 initiated sustained recruitment of actin to the cytoskeleton and transient myosin recruitment in a time- and concentration-dependent manner. The H2O2-induced actin recruitment was significantly inhibited by the calmodulin antagonists, W7 and TFP, but not by the MLCK inhibitor, KT5926, nor the PKC inhibitors, H7 and calphostin C. H2O2 also caused actin filament rearrangement in BPAEC with disruption of the dense peripheral bands and formation of stress fibers. These alterations occurred prior to actin translocation to the cytoskeleton and are prevented by inhibition of either MLCK or PKC. High concentrations of H2O2 transiently attenuated PKC activity but slightly increased the phosphorylation of the prominent PKC substrate and actin-binding protein, myristoylated alanine-rich C kinase substrate (MARCKS), by 5 min. However, MARCKS phosphorylation was reduced to below basal levels by 30 min. On the other hand, H2O2 induced a time- and dose-dependent phosphorylation of myosin light chains which was eliminated by both MLCK and PKC inhibitors. These data suggest that MLCK contributes to H2O2-induced myosin light chain phosphorylation and actin rearrangement and that PKC may play a permissive role. Neither of these enzymes appears to be involved in the H2O2-induced recruitment of actin to the cytoskeleton.

Cancer-preventive selenocompounds induce a specific redox modification of cysteine-rich regions in Ca(2+)-dependent isoenzymes of protein kinase C

Since protein kinase C (PKC) serves as a receptor for phorbol ester type tumor promoters and oxidants and has unique redox-active cysteine-rich regions, we have determined whether various chemopreventive selenocompounds could affect this enzyme. At lower concentrations, selenite decreased the kinase activity (IC50 = 0.5 microM), while at higher concentrations it decreased phorbol ester binding. However, when the catalytic and regulatory domains of PKC were separated by proteolysis, the catalytic domain retained its sensitivity to selenite, while the regulatory domain lost its sensitivity. Cysteine residues were quantitated in PKC modified with selenite by using 5,5'-dithiobis(2-nitrobenzoic acid) and also by using 2-nitro-5-thiosulfobenzoic acid after sulfitolysis. At lower concentrations, selenite induced a modification of four cysteine residues resulting in the formation of two disulfides, while at higher concentrations it induced a modification of seven to eight cysteine residues resulting in the formation of three to four disulfides. Contrary to selenite, selenocystine and selenodiglutathione (GSSeSG) readily inactivated the kinase activity, but not the phorbol ester binding. These two agents induced a two-stage modification of PKC; a limited modification at low concentrations leads to a loss of affinity for ATP, while an excessive modification at high concentrations leads to a loss of Vmax. Selenocystine and GSSeSG were 100,000-fold more potent than GSSG in inactivating PKC. The isoenzymes alpha, beta, and gamma exhibited an identical susceptibility to these selenocompounds. These results suggested that the cysteine residues present within the catalytic domain of these isoenzymes, although apart in the sequence, may be clustered in the tertiary structure to react with selenite, as well as may be in close proximity to some of the cysteines in the regulatory domain. Selenite did not affect protein kinase A, whereas GSSeSG and selenocystine inactivated the catalytic subunit after dissociation from the regulatory subunit at concentrations 100- and 800-fold, respectively, higher than that required for PKC inactivation. All three selenocompounds did not affect the activities of phosphorylase kinase and protein phosphatase 2A. Taken together, these results suggest that the accessible redox-active cysteine residues present in the PKC catalytic domain can react with certain specificity with redox-active selenocompounds such as selenite, selenocystine, and GSSeSG relative to other protein kinases tested.

Activation of protein kinase C by tyrosine phosphorylation in response to H2O2

Protein kinase C (PKC) isoforms, alpha, betaI, and gamma of cPKC subgroup, delta and epsilon of nPKC subgroup, and zeta of aPKC subgroup, were tyrosine phosphorylated in COS-7 cells in response to H2O2. These isoforms isolated from the H2O2-treated cells showed enhanced enzyme activity to various extents. The enzymes, PKC alpha and delta, recovered from the cells were independent of lipid cofactors for their catalytic activity. Analysis of mutated molecules of PKC delta showed that tyrosine residues, which are conserved in the catalytic domain of the PKC family, are critical for PKC activation induced by H2O2. These results suggest that PKC isoforms can be activated through tyrosine phosphorylation in a manner unrelated to receptor-coupled hydrolysis of inositol phospholipids.

Activity of ubiquitin-dependent pathway in response to oxidative stress. Ubiquitin-activating enzyme is transiently up-regulated

Relations between the ubiquitin pathway and cellular stress have been noted, but data regarding responses of the ubiquitin pathway to oxidative stress are scanty. This paper documents the response of this pathway to oxidative stress in lens cells. A brief exposure of lens epithelial cells to physiologically relevant levels of H2O2 induces a transient increase in activity of the ubiquitin-dependent pathway. Ubiquitin conjugation activity was maximal and increased 3. 5-9.2-fold over the activity noted in untreated cells by 4 h after removal of H2O2. By 24 h after removal of H2O2, ubiquitin conjugation activity returned to the level noted in untreated cells. In parallel to the changes in ubiquitin conjugation activity, the activity of ubiquitin-activating enzyme (E1), as determined by thiol ester formation, increased 2-6.7-fold during recovery from oxidation. Addition of exogenous E1 resulted in an increase in ubiquitin conjugation activity and in the levels of ubiquitin carrier protein (E2)-ubiquitin thiol esters in both the untreated cells and the H2O2-treated cells. These data suggest that E1 is the rate-limiting enzyme in the ubiquitin conjugation process and that the increases in ubiquitin conjugation activity which are induced upon recovery from oxidation are primarily due to increased E1 activity. The oxidation- and recovery-induced up-regulation of E1 activity is primarily due to post-synthetic events. Substrate availability and up-regulation of E2 activities also appear to be related to the enhancement in ubiquitinylation upon recovery from oxidative stress. The oxidation-induced increases in ubiquitin conjugation activity were associated with an increase in intracellular proteolysis, suggesting that the transient increase in ubiquitinylation noted upon recovery from oxidative stress may play a role in removal of damaged proteins from the cells.

Evaluation of extracellular lipid peroxidation in brain cortex of anaesthetized rats by microdialysis perfusion and high-performance liquid chromatography with fluorimetric detection

A method for in vivo evaluation of lipid peroxidation in the extracellular space of anaesthetized rat brain cortex was developed. This method involved the use of microdialysis perfusion and high-performance liquid chromatography. The microdialysates, eluted from implanted probes, were reacted with thiobarbituric acid (TBA) prior to analysis by an HPLC system equipped with a fluorescence detector (excitation and emission wavelengths were 515 and 550 nm, respectively). Lipid peroxidation in the extracellular space was evaluated as the concentration of malondialdehyde, a lipid peroxidation end product which reacts with TBA to form a fluorescent conjugate. Significantly increased production of malondialdehyde following hydrogen peroxide perfusion (0.03%, 0.3% at a flow-rate of 1 microl/min) was observed in the brain cortex of anaesthetized rats.

Oxygen radicals can induce preconditioning in rabbit hearts

Indirect evidence suggests that oxygen radicals may contribute to ischemic preconditioning. We directly investigated whether exposure to oxygen radicals per se, in the absence of ischemia, could reproduce the beneficial effects of ischemic preconditioning on infarct size and on postischemic contractile dysfunction. In one branch of the study, isolated rabbit hearts underwent 30 minutes of total global ischemia and 45 minutes of reperfusion (n=6, control group). A second group, before ischemia/reperfusion, was exposed for 5 minutes to a low flux of oxygen radicals generated by purine/xanthine oxidase (P/XO), followed by a 15-minute washout (n=6). Oxygen radical pretreatment significantly improved postischemic recovery of contractile function. We then investigated in another branch of the study whether this preconditioning effect would also reduce infarct size and whether it was mediated by protein kinase C activation. Control hearts were subjected to coronary artery occlusion for 30 minutes, followed by 2.5 hours of reperfusion (n=6). A second group, before coronary occlusion, was exposed to oxygen radicals and washout as described (n=8). A third group was subjected to oxygen radical infusion, but an inhibitor of protein kinase C (polymyxin B, 50 micromol/L) was administered throughout subsequent ischemia (n=7). A fourth group was exposed to oxygen radicals in the presence of scavengers (superoxide dismutase, 250 U/mL; catalase 500, U/mL; n=8). Pretreatment with oxygen radicals markedly reduced infarct size, from 65+/-19% of risk region in controls to 12+/-4% (P<.05). Protein kinase C inhibition significantly attenuated this effect (infarct size, 37+/-9% of risk region; P<.05 versus P/XO; P=NS versus controls). Oxygen radical-induced preconditioning was prevented by scavengers (infarct size, 55+/-14% of risk region; P<.05 versus P/XO; P=NS versus P/XO+polymyxin B). Our data show that in the absence of ischemia, exposure to low concentrations of oxygen radicals can reproduce the beneficial effects of ischemic preconditioning on infarct size and postischemic recovery of left ventricular function. Thus, oxygen radicals might be potential contributors to ischemic preconditioning.

Comparative in vitro and in vivo protein kinase C activation by selected pesticides and transition metal salts

Various pesticides and transition metals induce oxidative deterioration of biological macromolecules. Protein kinase C (PKC) may mediate these effects. However, no information is available regarding whether these xenobiotics can modulate PKC which is a critical event signaling the increase in endothelial permeability and cell proliferation. Female Sprague-Dawley rats were treated p.o. with two 0.25 LD50 doses of selected pesticides and transition metal salts at 0 and 21 h, and killed at 24 h. PKC activities were measured in liver and brain tissues. Cultured PC-12 cells were incubated for 24 h with 50, 100 or 200 nM concentrations of these pesticides, while 0.20, 0.40 or 0.60 microM concentrations of cadmium chloride (Cd(II)) and sodium dichromate (Cr(VI)) salts were employed. PKC activations were observed in the hepatic and brain cytosol fractions by all xenobiotics. Approximately 1.4- to 2.0-fold and 1.6- to 3.5-fold increases in PKC activity in the hepatic and brain cytosol fractions were observed, respectively. In the hepatic tissues, the greatest increases in activities were observed with TCDD, chlorpyrifos, endrin and Cd(II), while chlorpyrifos and fenthion exerted the greatest increases in the brain tissues. In cultured PC-12 cells, the greatest activation of PKC was observed primarily with 100-nM concentrations of the pesticides. The maximum effects were induced by chlorpyrifos, fenthion, Cd(II) and Cr(VI) salt. The results clearly indicate that pesticides as well as Cd(II) and Cr(VI) salts can modulate a vital component of the cell signaling pathway, namely PKC activity. PKC may be a target of free radicals and oxidative stress, leading to altered cell proliferation and differentiation.

Activation of hypoxia-inducible transcription factor depends primarily upon redox-sensitive stabilization of its alpha subunit

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor that is critical for hypoxic induction of a number of physiologically important genes. We present evidence that regulation of HIF-1 activity is primarily determined by the stability of the HIF-1alpha protein. Both HIF-1alpha and HIF-1beta mRNAs were constitutively expressed in HeLa and Hep3B cells with no significant induction by hypoxia. However, the HIF-1alpha protein was barely detectable in normoxic cells, even when HIF-1alpha was overexpressed, but was highly induced in hypoxic cells, whereas HIF-1beta protein levels remained constant, regardless of pO2. Hypoxia-induced HIF-1 binding as well as the HIF-1alpha protein were rapidly and drastically decreased in vivo following an abrupt increase to normal oxygen tension. Moreover, short pre-exposure of cells to hydrogen peroxide selectively prevented hypoxia-induced HIF-1 binding via blocking accumulation of HIF-1alpha protein, whereas treatment of hypoxic cell extracts with H2O2 had no effect on HIF-1 binding. These observations suggest that an intact redox-dependent signaling pathway is required for destablization of the HIF-1alpha protein. In hypoxic cell extracts, HIF-1 DNA binding was reversibly abolished by sulfhydryl oxidation. Furthermore, the addition of reduced thioredoxin to cell extracts enhanced HIF-1 DNA binding. Consistent with these results, overexpression of thioredoxin and Ref-1 significantly potentiated hypoxia-induced expression of a reporter construct containing the wild-type HIF-1 binding site. These experiments indicate that activation of HIF-1 involves redox-dependent stabilization of HIF-1alpha protein.

Specific induction of protein kinase C delta subspecies after transient middle cerebral artery occlusion in the rat brain: inhibition by MK-801

Protein kinase C (PKC) consists of a family of closely related Ca2+/phospholipid-dependent phosphotransferase isozymes, most of which are present in the brain and are differentially activated by second messengers. Calcium-dependent PKC activity may cause neuronal degeneration after ischemic insult. PKC is also involved in trophic-factor signaling, indicating that activity of some PKC subspecies may be beneficial to the injured brain. Therefore, we screened long-term changes in the expression of multiple PKC subspecies after focal brain ischemia. Middle cerebral artery occlusion was produced by using an intraluminal suture for 30 min of 90 min. In in situ hybridization experiments, mRNA levels of PKC alpha, -beta, -gamma, -delta, -epsilon, and -zeta were decreased in the infarct core 4 hr after ischemia and were lost completely 12 hr after ischemia. In areas surrounding the core, PKC delta mRNA was specifically induced 4, 12, and 24 hr after ischemia in the cortex. At 3 and 7 d, the core and a rim around it showed increased mRNA levels of PKC delta. No other subspecies were induced. At 2 d, immunoblotting demonstrated increased levels of PKC delta protein in the perifocal tissue, and immunocytochemistry revealed an increased number of PKC delta-positive neurons in the perifocal cortex. In the core, PKC delta-positive macrophages and endothelial cells were seen. Pretreatment with MK-801, an NMDA antagonist, inhibited cortical PKC delta mRNA induction. The data show that focal brain ischemia induces PKC delta mRNA and protein but not other PKC subspecies through the activation of NMDA receptors and that the upregulation lasts for several days in neurons of the perifocal zone.

Inhibition of protein kinase C and proto-oncogene expression by crocetin in NIH/3T3 cells

Crocetin, a carotenoid isolated from the seeds of Gardenia jasminoides, was found to be a potent inhibitor of tumor promotion induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse skin. When mouse fibroblast NIH/3T3 cells were treated with TPA alone, protein kinase C (PKC) translocated from the cytosolic fraction to the particulate fraction. Pretreatment with 60 and 120 microM crocetin for 15 min inhibited the TPA-induced PKC activity in the particulate fraction by 50% and 66%, respectively, but did not affect the level of PKC protein. Crocetin also reduced the level of TPA-stimulated phosphorylation of cellular proteins. Cells pretreated with crocetin (120 microM) had 55% less PKC [3H]phorbol dibutyrate-binding capacity. Suppression of TPA (100 ng/mL)-induced c-jun and c-fos gene expression was also observed in the mouse fibroblast cells pretreated with crocetin (30, 60, and 120 microM). Our results provided a basis for understanding the inhibitory effect of crocetin on TPA-mediated tumor promotion.

Arsenite induces apoptosis in Chinese hamster ovary cells by generation of reactive oxygen species

Arsenic, a human carcinogen, possesses a serious environmental threat but the mechanism of its toxicity remains unclear. Knowledge of how arsenic induces cell death and how cells escape the death path may help to understand arsenic carcinogenesis. We have investigated the nature of sodium arsenite-induced cell death in Chinese hamster ovary K1 cells. Following phosphate-citric acid buffer extraction, apoptotic cells with lower DNA content than the G1 cells were detected by flow cytometry. Immediately after 4 h of 40 microM arsenite treatment, no appreciable fraction of cells with sub-G1 DNA content was detected; however, the sub-G1 cell fraction increased with postarsenite incubation time, and detectable increase started at 8 h of incubation, whereas the intracellular peroxide level as measured by the fluorescent intensity of 2',7'-dichlorofluorescein increased immediately following a 4-h arsenite treatment. Simultaneous treatment with arsenite plus antioxidant (N-acetyl-cysteine, Trolox, and Tempo); copper ion chelator (neocuproine); protein kinase inhibitor (H-7) or protein synthesis inhibitor (cycloheximide) reduced the fraction of sub-G1 cell and internucleosomal DNA degradation. Trolox, neocuproine, or cycloheximide given after arsenite treatment also effectively reduced apoptosis. These results lead to a working hypothesis that arsenite-induced apoptosis in CHO-K1 cells is triggered by the generation of hydrogen peroxide, followed by a copper-mediated Fenton reaction that catalyzes the production of hydroxyl radicals, which selectively activates protein kinase through de novo synthesis of macromolecules.

Inhibitions of protein kinase C and proto-oncogene expressions in NIH 3T3 cells by apigenin

Apigenin, a low-toxic and non-mutagenic plant flavonoid, suppresses 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-mediated tumour promotion of mouse skin. TPA has the ability to activate protein kinase C (PKC) and induce proto-oncogene expression. Our study shows that apigenin inhibits PKC by competing with ATP, and exhibits an IC50 value of 10 +/- 0.5 microM. Apigenin also reduces the level of TPA-stimulated phosphorylation of cellular proteins. Of the protein tyrosine kinases tested, the fibroblast growth factor (FGF) receptor was most strongly affected by apigenin (IC50 20 microM), and pp60v-src most weakly affected (IC50 > 200 microM). Treatment of NIH 3T3 cells with 100 ng/ml TPA and 10, 50 and 100 microM apigenin resulted in 50, 80 and 100% suppression of TPA-induced C-JUN expression, respectively. Treatment of TPA with 10 microM apigenin inhibited TPA-induced C-FOS expression. TPA-stimulated cell growth was suppressed by 25 microM apigenin. Our results provide some evidence for understanding apigenin's inhibitory effects of TPA-mediated tumour promotion.