UV Induces phosphorylation of protein kinase B (Akt) at Ser-473 and Thr-308 in mouse epidermal Cl 41 cells through hydrogen peroxide

Black Raspberry Extracts Inhibit Benzo(a)Pyrene Diol-Epoxide–Induced Activator Protein 1 Activation and VEGF Transcription by Targeting the Phosphotidylinositol 3-Kinase/Akt Pathway

Previous studies have shown that freeze-dried black raspberry extract fractions inhibit benzo(a)pyrene [B(a)P]-induced transformation of Syrian hamster embryo cells and benzo(a)pyrene diol-epoxide [B(a)PDE]-induced activator protein-1 (AP-1) activity in mouse epidermal Cl 41 cells. The phosphotidylinositol 3-kinase (PI-3K)/Akt pathway is critical for B(a)PDE-induced AP-1 activation in mouse epidermal Cl 41 cells. In the present study, we determined the potential involvement of PI-3K and its downstream kinases on the inhibition of AP-1 activation by black raspberry fractions, RO-FOO3, RO-FOO4, RO-ME, and RO-DM. In addition, we investigated the effects of these fractions on the expression of the AP-1 target genes, vascular endothelial growth factor (VEGF) and inducible nitric oxide synthase (iNOS). Pretreatment of Cl 41 cells with fractions RO-F003 and RO-ME reduced activation of AP-1 and the expression of VEGF, but not iNOS. In contrast, fractions RO-F004 and RO-DM had no effect on AP-1 activation or the expression of either VEGF or iNOS. Consistent with inhibition of AP-1 activation, the RO-ME fraction markedly inhibited activation of PI-3K, Akt, and p70 S6 kinase (p70(S6k)). In addition, overexpression of the dominant negative PI-3K mutant delta p85 reduced the induction of VEGF by B(a)PDE. It is likely that the inhibitory effects of fractions RO-FOO3 and RO-ME on B(a)PDE-induced AP-1 activation and VEGF expression are mediated by inhibition of the PI-3K/Akt pathway. In view of the important roles of AP-1 and VEGF in tumor development, one mechanism for the chemopreventive activity of black raspberries may be inhibition of the PI-3K/Akt/AP-1/VEGF pathway.

Essential roles of PI-3K/Akt/IKKbeta/NFkappaB pathway in cyclin D1 induction by arsenite in JB6 Cl41 cells

Skin is a major target of carcinogenic trivalent arsenic (arsenite, As3+). It has been thought that cell proliferation is one of the central events involved in the carcinogenic effect of arsenite. Cyclin D1, a nuclear protein playing a pivotal role in cell proliferation and cell cycle transition from G1 to S phases, has been reported to be induced in human fibroblast by arsenite via uncertain molecular mechanisms. In the present study, the potential roles of PI-3K/Akt/IKKbeta/NFkappaB signal pathway in cyclin D1 induction by arsenite were addressed in mouse epidermal Cl41 cells. We found that exposure of Cl41 cells to arsenite was able to induce cell proliferation, activate PI-3K-->Akt/p70(S6k) signal pathway and increase cyclin D1 expression at both transcription and protein levels. Pre-treatment of Cl41 cells with PI-3K inhibitor, wortmannin, significantly inhibited the phosphorylation of Akt and p70(S6k) and thereby dramatically impaired the cyclin D1 induction by arsenite, implicating the importance of the PI-3K signal pathway in the cyclin D1 induction by arsenite. Furthermore, inhibition of PI-3K/Akt by overexpression of Deltap85 or DN-Akt blocked arsenite-induced IKK phosphorylation, IkappaBalpha degradation and cyclin D1 expression, indicating that IKK/NFkappaB is the downstream transducer of arsenite-triggered PI-3K/Akt cascade. Moreover, inhibition of IKKbeta/NFkappaB signal pathway by overexpression of its dominant negative mutant, IKKbeta-KM, also significantly blocked arsenite-induced cyclin D1 expression. Overall, arsenite exposure triggered PI-3K/Akt/IKKbeta/NFkappaB signal cascade which in turn plays essential roles in inducing cyclin D1 expression.

Protein kinase C negatively regulates Akt activity and modifies UVC-induced apoptosis in mouse keratinocytes

Skin keratinocytes are subject to frequent chemical and physical injury and have developed elaborate cell survival mechanisms to compensate. Among these, the Akt/protein kinase B (PKB) pathway protects keratinocytes from the toxic effects of ultraviolet light (UV). In contrast, the protein kinase C (PKC) family is involved in several keratinocyte death pathways. During an examination of potential interactions among these two pathways, we found that the insulin-like growth factor (IGF-1) activates both the PKC and the Akt signaling pathways in cultured primary mouse keratinocytes as indicated by increased phospho-PKC and phospho-Ser-473-Akt. IGF-1 also selectively induced translocation of PKCdelta and PKCepsilon from soluble to particulate fractions in mouse keratinocytes. Furthermore, the PKC-specific inhibitor, GF109203X, increased IGF-1-induced phospho-Ser-473-Akt and Akt kinase activity and enhanced IGF-1 protection from UVC-induced apoptosis. Selective activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) reduced phospho-Ser-473-Akt, suggesting that activation of PKC inhibits Akt activity. TPA also attenuated IGF-1 and epidermal growth factor-induced phospho-Ser-473-Akt, reduced Akt kinase activity, and blocked IGF-1 protection from UVC-induced apoptosis. The inhibition of Akt activity by TPA was reduced by inhibitors of protein phosphatase 2A, and TPA stimulated the association of phosphatase 2A with Akt. Individual PKC isoforms were overexpressed in cultured keratinocytes by transduction with adenoviral vectors or inhibited with PKC-selective inhibitors. These studies indicated that PKCdelta and PKCepsilon were selectively potent at causing dephosphorylation of Akt and modifying cell survival, whereas PKCalpha enhanced phosphorylation of Akt on Ser-473. Our results suggested that activation of PKCdelta and PKCepsilon provide a negative regulation for Akt phosphorylation and kinase activity in mouse keratinocytes and serve as modulators of cell survival pathways in response to external stimuli.

Role of Phagocyte Oxidase in UVA-Induced Oxidative Stress and Apoptosis in Keratinocytes

Chronic exposure to ultraviolet radiation including ultraviolet A (315-400 nm) (UVA) may cause photocarcinogenesis and photoaging. The UVA-induced production of reactive oxygen species (ROS) and the resultant oxidative stress exposure play an important role in these biological processes. Here we have investigated the role of phagocyte oxidase (PHOX, gp91phox) in the production of ROS, redox status change, and apoptosis after UVA exposure by using gp91phox-deficient (gp91phox-/-) primary keratinocytes. UVA radiation resulted in increased ROS production and oxidation of reduced glutathione (GSH) to its oxidized form (GSSG). The presence of diphenylene iodonium (DPI) inhibited ROS production by UVA. In comparison with wild-type cells, gp91phox-/- cells produced slightly less ROS and GSH oxidation. UVA radiation induced apoptosis in wild-type keratinocytes as detected by phosphatidylserine (PS) translocation, caspase activation, and DNA fragmentation. As compared with wild-type cells, UVA induced less PS translocation in gp91phox-deficient cells. No difference, however, was observed in caspase activation and DNA fragmentation after UVA exposure in wild-type and gp91phox-/- cells. These findings suggest that gp91phox plays a limited role in the UVA-induced ROS production, oxidative stress, and therefore the PS translocation, but has no effect on UVA-induced caspase activation and DNA fragmentation during apoptosis.

Clostridium difficile toxin A regulates inducible cyclooxygenase-2 and prostaglandin E2 synthesis in colonocytes via reactive oxygen species and activation of p38 MAPK

Clostridium difficile toxin A induces acute colitis with neutrophil infiltration and up-regulation of numerous pro-inflammatory mediators, but the contribution of cyclooxygenase-2 (COX-2) induction in this infection is unknown. We report here that toxin A induces expression of COX-2 and secretion of prostaglandin E2 (PGE2) in a dose- and time-dependent manner in cultured NCM460 human colonocytes and in human intestinal xenografts. This induction was blocked by SB203580, a p38 MAPK inhibitor, which also decreased the phosphorylation of MSK-1, CREB/ATF-1, and COX-2 promoter activity following toxin A stimulation. Gel shift assays indicated that CREB/ATF-1 was the major proteins binding to the COX-2-CRE. Moreover, colonocytes exposed to toxin A produced reactive oxygen species (ROS), which activated p38 MAPK, MSK-1, and CREB/ATF-1, leading to subsequent COX-2 induction and PGE2 secretion. In intact mice, blockage of p38 MAPK inhibited toxin A-mediated induction of COX-2 in enterocytes as well as lamina propria cells, and significantly blocked the toxin A-induced ileal secretion of fluid and PGE2. Furthermore, a selective COX-2 inhibitor also diminished toxin A-associated ileal fluid and PGE2 secretion. The main signaling pathway for toxin A induction of human COX-2 involves ROS-mediated activation of p38 MAPK, MSK-1, CREB, and ATF-1. Toxin A triggers ileal inflammation and secretion of fluid via COX-2 induction and release of PGE2.

Mechanism of early biphasic activation of poly(ADP-ribose) polymerase-1 in response to ultraviolet B radiation

The damage to DNA caused by ultraviolet B radiation (280-320 nm) contributes significantly to development of sunlight-induced skin cancers. The susceptibility of mice to ultraviolet B-induced skin carcinogenesis is increased by an inhibitor of the DNA damage-activated nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP), hence PARP activation is likely to be associated with cellular responses that suppress carcinogenesis. To understand the role of activated PARP in these cellular functions, we need to first clearly identify the cause of PARP activation in ultraviolet B-irradiated cells. Ultraviolet B, like ultraviolet C, causes direct DNA damage of cyclobutane pyrimidine dimer and 6, 4-photoproduct types, which are subjected to the nucleotide excision repair. Moreover, ultraviolet B also causes oxidative DNA damage, which is subjected to base excision repair. To identify which of these two types of DNA damage activates PARP, we examined mechanism of early PARP activation in mouse fibroblasts exposed to ultraviolet B and C radiations. The ultraviolet B-irradiated cells rapidly activated PARP in two distinct phases, initially within the first 5 minutes and later between 60-120 minutes, whereas ultraviolet C-irradiated cells showed only the immediate PARP activation. Using antioxidants, local irradiation, chromatin immunoprecipitation and in vitro PARP assays, we identified that ultraviolet radiation-induced direct DNA damage, such as thymine dimers, cause the initial PARP activation, whereas ultraviolet B-induced oxidative damage cause the second PARP activation. Our results suggest that cells can selectively activate PARP for participation in different cellular responses associated with different DNA lesions.

Hydrogen peroxide mediates Rac1 activation of S6K1

We previously reported that hydrogen peroxide (H2O2) mediates mitogen activation of ribosomal protein S6 kinase 1 (S6K1) which plays an important role in cell proliferation and growth. In this study, we investigated a possible role of H2O2 as a molecular linker in Rac1 activation of S6K1. Overexpression of recombinant catalase in NIH-3T3 cells led to the drastic inhibition of H2O2 production by PDGF, which was accompanied by a decrease in S6K1 activity. Similarly, PDGF activation of S6K1 was significantly inhibited by transient transfection or stable transfection of the cells with a dominant-negative Rac1 (Rac1N17), while overexpression of constitutively active Rac1 (Rac1V12) in the cells led to an increase in basal activity of S6K1. In addition, stable transfection of Rat2 cells with Rac1N17 dramatically attenuated the H2O2 production by PDGF as compared with that in the control cells. In contrast, Rat2 cells stably transfected with Rac1V12 produced high level of H2O2 in the absence of PDGF, comparable to that in the control cells stimulated with PDGF. More importantly, elimination of H2O2 produced in Rat2 cells overexpressing Rac1V12 inhibited the Rac1V12 activation of S6K1, indicating the possible role of H2O2 as a mediator in the activation of S6K1 by Rac1. However, H2O2 could be also produced via other pathway, which is independent of Rac1 or PI3K, because in Rat2 cells stably transfected with Rac1N17, H2O2 could be produced by arsenite, which has been shown to be a stimulator of H2O2 production. Taken together, these results suggest that H2O2 plays a pivotal role as a mediator in Rac1 activation of S6K1.

Identification of a PKB/Akt hydrophobic motif Ser-473 kinase as DNA-dependent protein kinase

Full activation of protein kinase B (PKB)/Akt requires phosphorylation on Thr-308 and Ser-473 by 3-phosphoinositide-dependent kinase-1 (PDK1) and Ser-473 kinase (S473K), respectively. Although PDK1 has been well characterized, the identification of the S473K remains controversial. A major PKB Ser-473 kinase activity was purified from the membrane fraction of HEK293 cells and found to be DNA-dependent protein kinase (DNA-PK). DNA-PK co-localized and associated with PKB at the plasma membrane. In vitro, DNA-PK phosphorylated PKB on Ser-473, resulting in a approximately 10-fold enhancement of PKB activity. Knockdown of DNA-PK by small interfering RNA inhibited Ser-473 phosphorylation induced by insulin and pervanadate. DNA-PK-deficient glioblastoma cells did not respond to insulin at the level of Ser-473 phosphorylation; this effect was restored by complementation with the human PRKDC gene. We conclude that DNA-PK is a long sought after kinase responsible for the Ser-473 phosphorylation step in the activation of PKB.

Serine/threonine kinase AKT is frequently activated in human bile duct cancer and is associated with increased radioresistance

The prognosis for patients with bile duct cancer (BDC) remains poor. Although BDC cells are essentially radioresistant, recent reports have suggested that radiation therapy, in addition to its palliative role in the management of BDC, may improve patient survival. A better understanding of the mechanisms that lead to cellular radioresistance may assist in the development of more effective BDC therapies based on radiotherapy in combination with radiosensitizing agents. The serine/threonine kinase AKT/protein kinase B, a downstream effector of phosphatidylinositol 3'-kinase, is a well-characterized kinase that is known to play a critical role in antiapoptotic signaling pathways. In this investigation, we sought to clarify the role of AKT signaling in the radioresistance in BDC cells. First, to examine whether activated AKT is expressed in BDCs, tumor specimens were obtained from 19 consecutive BDC cases. Immunohistochemical staining using an anti-phosphorylated-AKT antibody showed that phosphorylated (activated) AKT was expressed in cancer cells but not in neighboring normal mucosa in 16 cases (84.2%). Next, to evaluate the role of AKT activation in the regulation of BDC cell radiosensitivity, clonogenic assays were performed using the phosphatidylinositol 3'-kinase inhibitor LY294002 with and without irradiation. LY294002 inhibited AKT activation in BDC cells and, on irradiation, decreased clonogenic survival in a radiation dose-dependent manner. Only a small decrease in cell viability was observed in cells exposed to LY294002. Expression of constitutively active AKT in BDC cells resulted in decreased radiosensitivity, whereas a dominant-negative AKT increased radiosensitivity. Furthermore, constitutively active AKT also inhibited radiation-induced apoptosis. Collectively, these results indicate that activated AKT in BDC cells is associated with radioresistance and suggest that pharmacological or genetic modulation of AKT activity may have important therapeutic implications in BDC patients treated with radiation.

A critical role of PI-3K/Akt/JNKs pathway in benzo[a]pyrene diol-epoxide (B[a]PDE)-induced AP-1 transactivation in mouse epidermal Cl41 cells

Mouse skin tumorigenicity studies indicate that benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) contributes to carcinogenesis as both a tumor initiator and promoter. However, the mechanisms that mediate B[a]PDE tumor promotion effects remain unclear. Our results demonstrated that in mouse epidermal Cl41 cells, B[a]PDE treatment resulted in marked activation of AP-1 and its upstream MAPKs, including ERKs, JNKs and p38K. B[a]PDE exposure also led to activation of phosphotidylinositol 3-kinase (PI-3K), Akt and p70 S6 kinase (p70S6k). B[a]PDE-induced AP-1 transactivation was inhibited by pretreatment of cells with PI-3K inhibitors, wortmannin or Ly294002. In contrast, inhibition of p70S6k with rapamycin did not show any inhibitory effects. An overexpression of dominant-negative mutant of PI-3K, Deltap85, impaired B[a]PDE-induced activation of PI-3K, Akt and AP-1 transactivation. Furthermore, an overexpression of dominant-negative Akt mutant, Akt-T308A/S473A, blocked B[a]PDE-induced activation of Akt, AP-1 and JNKs, while it did not affect the activation of p70S6k, ERKs and p38 kinase. These results demonstrated that B[a]PDE was able to induce AP-1 transactivation and this AP-1 induction was specific through PI-3K/Akt/JNKs-dependent and p70S6k-independent pathways. This study also indicated that Akt-T308A/S473A blocks B[a]PDE-induced AP-1 activation specific through impairing JNK pathway. These findings will help us to understand the signal transduction pathways involved in the carcinogenic effects of B[a]PDE.

PI-3K and Akt are mediators of AP-1 induction by 5-MCDE in mouse epidermal Cl41 cells

5-Methylchrysene has been found to be a complete carcinogen in laboratory animals. However, the tumor promotion effects of (+/-)-anti-5-methylchrysene-1,2-diol-3,4-epoxide (5-MCDE) remain unclear. In the present work, we found that 5-MCDE induced marked activator protein-1 (AP-1) activation in Cl41 cells. 5-MCDE also induced a marked activation of phosphatidylinositol 3-kinase (PI-3K). Inhibition of PI-3K impaired 5-MCDE-induced AP-1 transactivation, suggesting that PI-3K is an upstream kinase involved in AP-1 activation by 5-MCDE. Furthermore, we found that Akt is a PI-3K downstream mediator for 5-MCDE-induced AP-1 transactivation, whereas another PI-3K downstream kinase, p70(S6K), was not involved in AP-1 activation by 5-MCDE. Moreover, inhibition of Akt activation blocked 5-MCDE-induced activation of extracellular signal-regulated protein kinases (ERKs) and c-Jun NH(2)-terminal kinases (JNKs), whereas it did not affect p38K activation. Consistently, overexpression of a dominant-negative mutant of ERK2 or JNK1 blocked the AP-1 activation by 5-MCDE. These results demonstrate that 5-MCDE is able to induce AP-1 activation, and the AP-1 induction is specifically through a PI-3K/Akt-dependent and p70(S6K)-independent pathway.

Oxidative stress activates the plasminogen activator inhibitor type 1 (PAI-1) promoter through an AP-1 response element and cooperates with insulin for additive effects on PAI-1 transcription

Oxidative stress is one of the characteristics of diabetes and is thought to be responsible for many of the pathophysiological changes caused by the disease. We previously identified an insulin response element in the promoter of plasminogen activator inhibitor 1 (PAI-1) that was activated by an unidentified member of the forkhead/winged helix (Fox) family of transcription factors. This element mediated a 5-7-fold increase in PAI-1 transcription because of insulin. Here we report that oxidative stress also caused a 3-fold increase in PAI-1 transcription and that the effect was additive with that of insulin. Antioxidants prevent this response. Mutational analysis of the PAI-1 promoter revealed that oxidative stress acted at an AP-1 site at -60/52 of the promoter. Gel mobility shift analysis demonstrated that binding to an AP-1 oligonucleotide was increased 4-fold by oxidative stress. Jun levels were increased by oxidants as assessed by reverse transcriptase-PCR. Western blotting demonstrated that a rapid and prolonged nuclear accumulation of phospho-c-Jun followed oxidant stimulation. The nuclear c-Jun phosphorylation was not observed in cells treated with reduced glutathione. Finally, JNK/SAPK activity was found to increase in response to oxidants, and inhibition of JNK/SAP blocked TBHQ-increased PAI-1-luciferase expression. Thus, oxidative stress stimulated AP-1 and activated the PAI-1 promoter.

Nickel Compounds Act through Phosphatidylinositol-3-kinase/Akt-Dependent, p70S6k-Independent Pathway to Induce Hypoxia Inducible Factor Transactivation and Cap43 Expression in Mouse Epidermal Cl41 Cells

Nickel compounds are a somewhat unique class of carcinogens. Previous studies have demonstrated that NiCl(2) exposure leads to marked induction of hypoxia inducible factor 1 (HIF-1) in human osteosarcoma and BALB/c 3T3 cells, a transcription factor that has been considered to play an important role in tumor promotion and progression. However, the signal transduction pathways leading to HIF-1 induction are not well understood. The present study indicated that exposure of mouse epidermal Cl41 cells to either Ni(3)S(2) or NiCl(2) resulted in activation of phosphatidylinositol 3-kinase (PI-3K), Akt, and p70 S6 kinase (p70(S6k)). Inhibition of PI-3K, Akt, and p70(S6k) by overexpression of a dominant-negative mutant of PI-3K (Deltap85) impaired nickel-induced HIF-1 transactivation. Furthermore, an overexpression of the dominant-negative Akt mutant (Akt-T308A/S473A) blocked nickel-induced Akt phosphorylation and HIF-1 transactivation, whereas inhibition of p70(S6k) activation by pretreatment of cells with rapamycin did not show significant inhibitory effects on HIF-1 transactivation induced by nickel compounds. Consistent with HIF-1 transactivation, inhibition of the PI-3K/Akt pathway by either overexpression of Deltap85 or Akt-T308A/S473A caused dramatic inhibition of Cap43 protein expression induced by nickel compounds, whereas pretreatment of cells with rapamycin did not exhibit inhibition of Cap43 induction. These results demonstrated that nickel compounds induce HIF-1 transactivation and Cap43 protein expression through a PI-3K/Akt-dependent and p70(S6k)-independent pathway. This study should help us understand the signal transduction pathways involved in the carcinogenic effects of nickel compounds.

Enhancement of catalase activity by repetitive low-grade H2O2 exposures protects fibroblasts from subsequent stress-induced apoptosis

Exposure of Chinese hamster V79 fibroblasts to mild and repetitive H2O2 doses in culture for 15 weeks produced no change in lipid peroxidation status, GSH/GSSG ratio and glutathione peroxidase activity of these cells (VST cells). In contrast, in VST cells catalase levels underwent a prominent increase which could be significantly inhibited and brought down to control levels after treatment with the catalase inhibitor 3-aminotriazole (3-AT). When control (VC) cells were exposed to UV radiation (UVC 5 J/m2) or H2O2 (7.5mM, 15 min), intracellular reactive oxygen species (ROS) levels rose prominently with significant activation of caspase-3. Marked nuclear fragmentation and lower cell viability were also noted in these cells. In contrast, VST cells demonstrated a significantly lower ROS level, an absence of nuclear fragmentation and an unchanged caspase-3 activity after exposure to UVC or H2O2. Cell viability was also significantly better preserved in VST cells than VC cells after UV or H2O2 exposures. Following 3-AT treatment of VST cells, UVC radiation or H2O2 brought about significantly higher elevations in intracellular ROS, increases in caspase-3 activity, significantly lowered cell viability and marked nuclear fragmentation, indicating the involvement of high catalase levels in the cytoprotective effects of repetitive stress. Therefore, upregulation of the antioxidant defense after repetitive oxidative stress imparted a superior ability to cope with subsequent acute stress and escape apoptotic death and loss of viability.

Elevated basal reactive oxygen species and phospho-Akt in murine keratinocytes resistant to ultraviolet B-induced apoptosis

Resistance to apoptosis may be a critical phenotype that cells must acquire during skin carcinogenesis. The Akt kinase is a known upstream regulator of apoptosis in many cell types and has been shown to be activated by increased reactive oxygen species (ROS). We have previously demonstrated that two malignant variants (6M90 and 6R90) of the mouse keratinocyte 308 cell line have elevated ROS because of loss of catalase activity, and that this elevated ROS confers a growth advantage. We report here that in addition to a growth advantage, chronically increased ROS in the variants resulted in an increase in resistance to ultraviolet (UV) B-induced apoptosis. This resistance was due to basal increases of Akt phosphorylation in the malignant variants compared to the 308 cells. Modulation of ROS in 6M90 and 6R90 cells by catalase overexpression or antioxidant treatment resulted in decreased levels of Akt phosphorylation and subsequent loss of resistance to UVB-induced apoptosis. Conversely, treatment of 308 cells with hydrogen peroxide caused increases in Akt phosphorylation and increased apoptosis resistance. These results indicate that the chronically elevated ROS often observed in tumors may contribute to a malignant phenotype by keeping Akt in a phosphorylated state, resulting in increased apoptosis resistance.

Protection of epidermal cells against UVB injury by the antioxidant selenium-containing single-chain Fv catalytic antibody

The antioxidant effect of selenium-containing single-chain Fv catalytic antibody (Se-scFv2F3), a new mimic of glutathione peroxidase, was confirmed using a model system in which cultured rat skin epidermal cells were injured by ultraviolet B (UVB). The cell damage was characterized in terms of lipid peroxidation of the cells, cell viability, and cell membrane integrity. The injury effects of UVB and protection effects of Se-scFv2F3 on the cells were studied using the model system. UVB can damage the cells severely. Upon precultivation of the cells with 0.4U/ml Se-scFv2F3, however, the damage was significantly reduced as shown by the increase in cell viability, the decrease in the malondialdehyde and hydrogen peroxide levels, and the normalization of lactate dehydrogenase activity. In addition, a novel finding that Se-scFv2F3 can stimulate cultured epidermal cells to proliferate under certain conditions was observed.

Pepstatin A attenuates the inhibitory effect of N-acetyl-L-cysteine on proliferation of hepatic myofibroblasts (stellate cells)

The pharmacological interaction between pepstatin A, a specific inhibitor of cathepsin D, and N-acetyl-L-cysteine was analyzed using hepatic stellate cells in primary culture. Isolated rat stellate cells were cultured on plastic dishes in Dulbecco's modified Eagle's medium (DMEM). Proteins and phospho-proteins were detected by Western blot. DNA synthesis was determined by [3H]thymidine uptake. Pepstatin A restored DNA synthesis of stellate cells stimulated by either platelet-derived growth factor-BB (PDGF-BB) or insulin-like growth factor-I (IGF-I), an effect that was attenuated by N-acetyl-L-cysteine. This agent induced the recovery of both the expression of PDGF receptor beta and IGF-I receptor beta and the phosphorylation of p42/44 mitogen-activated protein kinase (MAPK) and Akt under stimulation with either PDGF-BB or IGF-I, which were downregulated by N-acetyl-L-cysteine. Expression of cathepsin D was induced in activated stellate cells. These results indicate that pepstatin A hampers the inhibitory effect of N-acetyl-L-cysteine on stellate cell growth by ameliorating growth factor receptor downregulation.

The transcriptional response after oxidative stress is defective in Cockayne syndrome group B cells

Cockayne syndrome (CS) is a human hereditary disease belonging to the group of segmental progerias, and the clinical phenotype is characterized by postnatal growth failure, neurological dysfunction, cachetic dwarfism, photosensitivity, sensorineural hearing loss, and retinal degradation. CS-B cells are defective in transcription-coupled DNA repair, base excision repair, transcription, and chromatin structural organization. Using array analysis, we have examined the expression profile in CS complementation group B (CS-B) fibroblasts after exposure to oxidative stress (H2O2) before and after complete complementation with the CSB gene. The following isogenic cell lines were compared: CS-B cells (CS-B null), CS-B cells complemented with wild-type CSB (CS-B wt), and a stably transformed cell line with a point mutation in the ATPase domain of CSB (CS-B ATPase mutant). In the wt rescued cells, we detected significant induction (two-fold) of 112 genes out of the 6912 analysed. The patterns suggested an induction or upregulation of genes involved in several DNA metabolic processes including DNA repair, transcription, and signal transduction. In both CS-B mutant cell lines, we found a general deficiency in transcription after oxidative stress, suggesting that the CSB protein influenced the regulation of transcription of certain genes. Of the 6912 genes, 122 were differentially regulated by more than two-fold. Evidently, the ATPase function of CSB is biologically important as the deficiencies seen in the ATPase mutant cells are very similar to those observed in the CS-B-null cells. Some major defects are in the transcription of genes involved in DNA repair, signal transduction, and ribosomal functions.

UV-B induced keratinocyte apoptosis is blocked by 2-selenium-bridged beta-cyclodextrin, a GPX mimic

Cell proliferation and cell death of keratinocytes are tightly regulated to ensure epidermal homeostasis. UV-B induces keratinocyte apoptosis. UV-B also induces lipid peroxidation of keratinocytes to increase their amount of malondialdehyde (MDA). These phenomena can be explained by the production of reactive oxygen species (ROS) induced by UV-B radiation. We synthesized 2-selenium-bridged beta-cyclodextrin (2-SeCD) to imitate glutathione peroxidase (GPX), an important antioxidant and established a damage system, in which keratinocytes can be damaged by Ultraviolet B (UV-B) radiation. Using this damage system we studied 2-SeCD protection of keratinocytes against injury induced by UV-B. Experimental results showed that 2-SeCD could protect keratinocytes from apoptosis. Moreover, 2-SeCD inhibits lipid peroxidation of keratinocytes and scavenges ROS. 2-SeCD inhibits the UV-B induced apoptotic signal transduction. This antiapoptotic mechanism may be partly related to the elimination of hydrogen peroxide.

Cellular response to oxidative stress: signaling for suicide and survival

Reactive oxygen species (ROS), whether produced endogenously as a consequence of normal cell functions or derived from external sources, pose a constant threat to cells living in an aerobic environment as they can result in severe damage to DNA, protein, and lipids. The importance of oxidative damage to the pathogenesis of many diseases as well as to degenerative processes of aging has becoming increasingly apparent over the past few years. Cells contain a number of antioxidant defenses to minimize fluctuations in ROS, but ROS generation often exceeds the cell's antioxidant capacity, resulting in a condition termed oxidative stress. Host survival depends upon the ability of cells and tissues to adapt to or resist the stress, and repair or remove damaged molecules or cells. Numerous stress response mechanisms have evolved for these purposes, and they are rapidly activated in response to oxidative insults. Some of the pathways are preferentially linked to enhanced survival, while others are more frequently associated with cell death. Still others have been implicated in both extremes depending on the particular circumstances. In this review, we discuss the various signaling pathways known to be activated in response to oxidative stress in mammalian cells, the mechanisms leading to their activation, and their roles in influencing cell survival. These pathways constitute important avenues for therapeutic interventions aimed at limiting oxidative damage or attenuating its sequelae.

Regulation of Fas expression by STAT3 and c-Jun is mediated by phosphatidylinositol 3-kinase-AKT signaling

Cooperation between STAT3 and c-Jun results in suppression of Fas Receptor (FasR) transcription, which is often seen in advanced human tumors. To identify requirements for STAT3-Jun cooperation, we elucidated the role of protein kinases that affect both transcription factors. The phosphatidylinositol 3-kinase (PI3K)-AKT signaling pathway was found capable of down-regulating both STAT3- and c-Jun-dependent transcription, resulting in derepression of FasR transcription. Conversely, inhibition of PI3K-AKT signaling via the specific pharmacological inhibitor LY294002 up-regulated AP1/Jun- and STAT-dependent transcriptional activities, resulting in suppression of the FasR promoter activities and decreased FasR surface expression. PI3K-AKT's ability to affect FasR transcription was not observed in c-jun null fibroblasts, suggesting that c-Jun is required for PI3K/AKT-mediated regulation of FasR transcription. Interestingly, the dominant negative form of Rac1 (RacN17) was also efficient in relieving FasR expression, suggesting that the increase in FasR expression following AKT stimuli could be mediated via AKT ability to elicit suppression of Rac1, which in turn decreases JNK activities and c-Jun phosphorylation. Overall, our findings demonstrate that through its negative effects on both c-Jun and STAT3, the PI3K-AKT pathway disrupts cooperation between c-Jun and STAT3, which is required for silencing the FasR promoter, resulting in increased expression of surface FasR and concomitant sensitization to FasL-mediated programmed cell death.