Tyrosine phosphorylation is a mandatory proximal step in radiation-induced activation of the protein kinase C signaling pathway in human B-lymphocyte precursors

The role of radiation induced oxidative stress as a regulator of radio-adaptive responses

Purpose: Various sources of radiation including radiofrequency, electromagnetic radiation (EMR), low- dose X-radiation, low-level microwave radiation and ionizing radiation (IR) are indispensable parts of modern life. In the current review, we discussed the adaptive responses of biological systems to radiation with a focus on the impacts of radiation-induced oxidative stress (RIOS) and its molecular downstream signaling pathways.Materials and methods: A comprehensive search was conducted in Web of Sciences, PubMed, Scopus, Google Scholar, Embase, and Cochrane Library. Keywords included Mesh terms of "radiation," "electromagnetic radiation," "adaptive immunity," "oxidative stress," and "immune checkpoints." Manuscripts published up until December 2019 were included.Results: RIOS induces various molecular adaptors connected with adaptive responses in radiation exposed cells. One of these adaptors includes p53 which promotes various cellular signaling pathways. RIOS also activates the intrinsic apoptotic pathway by depolarization of the mitochondrial membrane potential and activating the caspase apoptotic cascade. RIOS is also involved in radiation-induced proliferative responses through interaction with mitogen-activated protein kinases (MAPks) including p38 MAPK, ERK, and c-Jun N-terminal kinase (JNK). Protein kinase B (Akt)/phosphoinositide 3-kinase (PI3K) signaling pathway has also been reported to be involved in RIOS-induced proliferative responses. Furthermore, RIOS promotes genetic instability by introducing DNA structural and epigenetic alterations, as well as attenuating DNA repair mechanisms. Inflammatory transcription factors including macrophage migration inhibitory factor (MIF), nuclear factor κB (NF-κB), and signal transducer and activator of transcription-3 (STAT-3) paly major role in RIOS-induced inflammation.Conclusion: In conclusion, RIOS considerably contributes to radiation induced adaptive responses. Other possible molecular adaptors modulating RIOS-induced responses are yet to be divulged in future studies.

Sex and tissue-specific differences in low-dose radiation-induced oncogenic signaling

PURPOSE

The possible adverse health effects of low-dose radiation (LDR) exposure constitute a growing concern. Clinically and environmentally relevant exposures occur predominantly under chronic conditions, notwithstanding that most studies of LDR effects have been performed using a single acute exposure. Sex- and tissue-specificity of the LDR-induced changes have not been considered before. We investigated LDR-related expression patterns in muscle, liver and spleen of male and female mice subjected to acute and chronic LDR exposure. Genes involved in oncogenic signaling were of specific interest, as radiation is a well-known carcinogen.

MATERIALS AND METHODS

We analyzed the expression pattern of genes coding for growth factors and growth-factor receptors, cytoplasmic serine/threonine protein kinases, G-proteins and nuclear DNA-binding proteins, and other important components of oncogenic signaling.

RESULTS

We found sex- and tissue-specific changes in the expression of Ras superfamily members (Nras, Rab2, Rab34, Vav2), protein kinase C (PKC) isoforms (PKCbeta, PKCmu), AP-1 factor components (Jun, JunB and FosB), Wnt signaling pathway members as well as in a variety of other cellular proto-oncogenes and oncogenes. Importantly, Western blot analysis of JunB, PKCmu and Rab2 proteins supported the transcriptomic data.

CONCLUSIONS

Substantially different protein levels were observed in all three tissues (muscle, spleen and liver) of acutely and chronically irradiated female and male animals. Based on the obtained data and available literature, we discuss several possible mechanisms that may contribute to radiation-induced carcinogenesis in various tissues of males and females. From our results we could identify the genes that may serve as sex- and tissue-specific biomarkers of the LDR exposure.

Molecular mechanisms of MHC class I-antigen processing: redox considerations

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to the cell surface for screening by CD8(+) T cells. A number of ER-resident chaperones assist the assembly of peptides onto MHC class I molecules, a process that can be divided into several steps. Early folding of the MHC class I heavy chain is followed by its association with beta(2)-microglobulin (beta(2)m). The MHC class I heavy chain-beta(2)m heterodimer is incorporated into the peptide-loading complex, leading to peptide loading, release of the peptide-filled MHC class I molecules from the peptide-loading complex, and exit of the complete MHC class I complex from the ER. Because proper antigen presentation is vital for normal immune responses, the assembly of MHC class I molecules requires tight regulation. Emerging evidence indicates that thiol-based redox regulation plays critical roles in MHC class I-restricted antigen processing and presentation, establishing an unexpected link between redox biology and antigen processing. We review the influences of redox regulation on antigen processing and presentation. Because redox signaling pathways are a rich source of validated drug targets, newly discovered redox biology-mediated mechanisms of antigen processing may facilitate the development of more selective and therapeutic drugs or vaccines against immune diseases.

Hyperoxia-induced NF-kappaB activation occurs via a maturationally sensitive atypical pathway

NF-kappaB activation is exaggerated in neonatal organisms after oxidant and inflammatory insults, but the reason for this and the downstream effects are unclear. We hypothesized that specific phosphorylation patterns of IkappaBalpha could account for differences in NF-kappaB activation in hyperoxia-exposed fetal and adult lung fibroblasts. After exposure to hyperoxia (>95% O(2)), nuclear NF-kappaB binding increased in fetal, but not adult, lung fibroblasts. Unique to fetal cells, phosphorylation of IkappaBalpha on tyrosine 42, rather than serine 32/36 as seen in TNF-alpha-exposed cells, preceded NF-kappaB nuclear translocation. In fetal cells stably transfected with an NF-kappaB-driven luciferase reporter, hyperoxia significantly suppressed reporter activity, in contrast to increased reporter activity after TNF-alpha incubation. Targeted gene profiling analysis showed that hyperoxia resulted in decreased expression of multiple genes, including proapoptotic factors. Transfection with a dominant-negative IkappaBalpha (Y42F), which cannot be phosphorylated on tyrosine 42, resulted in upregulation of multiple proapoptotic genes. In support of this finding, caspase-3 activity and DNA laddering were specifically increased in fetal lung fibroblasts expressing Y42F after exposure to hyperoxia. These data demonstrate a unique pathway of NF-kappaB activation in fetal lung fibroblasts after exposure to hyperoxia, whereby these cells are protected against apoptosis. Activation of this pathway in fetal cells may prevent the normal pattern of fibroblast apoptosis necessary for normal lung development, resulting in aberrant lung morphology in vivo.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Many faces of NF-kappaB signaling induced by genotoxic stress

The nuclear factor-kappaB (NF-kappaB) family of dimeric transcription factors plays pivotal roles in physiologic and pathologic processes, including immune and inflammatory responses and development and progression of various human cancers. Inactive NF-kappaB dimers normally exist in the cytoplasm in association with inhibitor proteins belonging to the inhibitor of NF-kappaB (IkappaB) family of related proteins. Activation of NF-kappaB involves its release from IkappaB and subsequent nuclear translocation to induce expression of target genes. Intense research effort has revealed many distinct signaling pathways and mechanisms of NF-kappaB activation induced by immune and inflammatory stimuli. These aspects of NF-kappaB biology have been amply reviewed in the literature. However, those that involve DNA-damaging agents are less well understood, and multiple conflicting pathways and mechanisms have been described in the literature. In this review, we summarize the proposed mechanisms of NF-kappaB activation by various DNA-damaging agents, discuss the significance of such activation in the context of cancer treatment, and highlight some of the critical questions that remain to be addressed in future studies.

Gamma irradiation of Type B spermatogonia leads to heritable genomic instability in four generations of mice

Mice conceived 6 weeks after paternal exposure to ionizing radiation were fathered by sperm that were Type B spermatogonia at the time of irradiation. Previous studies of these offspring showed that this paternal F0 germ cell irradiation led to decreased embryonic cell proliferation rates, altered enzyme activities, protein levels and whole-body weights. In the present study, we examined four generations of CD1 mice following paternal F0 irradiation of the Type B spermatogonia (1.0 Gy, (137)Cs gamma rays) to determine the stability of the heritable effects. Offspring were evaluated for changes in protein kinase C and mitogen-activated protein kinase enzyme activities and Trp53 and p21(waf1) protein levels. Two or more endpoints were significantly altered in all four generations of offspring from the irradiated F0 sire (P <or= 0.05). To test the hypothesis that these heritable biochemical effects are random stochastic responses rather than some predictable uniform response, each endpoint was also evaluated in terms of a variability index (VI). Results of VI analyses show that the observed heritable phenotype is unpredictable in magnitude and direction of change for an endpoint between generations and within generations. These results indicate that irradiated spermatogonia develop a capacity to transmit a type of heritable genomic instability to four generations of offspring.

Inhibition of protein-tyrosine phosphatases by mild oxidative stresses is dependent on S-nitrosylation

Previous studies have shown that a Ca(2+)-dependent nitric-oxide synthase (NOS) is activated as part of a cellular response to low doses of ionizing radiation. Genetic and pharmacological inhibitor studies linked this NO signaling to the radiation-induced activation of ERK1/2. Herein, a mechanism for the radiation-induced activation of Tyr phosphorylation-dependent pathways (e.g. ERK1/2) involving the inhibition of protein-Tyr phosphatases (PTPs) by S-nitrosylation is tested. The basis for this mechanism resides in the redox-sensitive active site Cys in PTPs. These studies also examined oxidative stress induced by low concentrations of H(2)O(2). S-Nitrosylation of total cellular PTP and immunopurified SHP-1 and SHP-2 was detected as protection of PTP enzymatic activity from alkylation by N-ethylmaleimide and reversal by ascorbate. Both radiation and H(2)O(2) protected PTP activity from alkylation by a mechanism reversible by ascorbate and inhibited by NOS inhibitors or expression of a dominant negative mutant of NOS-1. Radiation and H(2)O(2) stimulated a transient increase in cytoplasmic free [Ca(2+)]. Radiation, H(2)O(2), and the Ca(2+) ionophore, ionomycin, also stimulated NOS activity, and this was associated with an enhanced S-nitrosylation of the active site Cys(453) determined by isolation of S-nitrosylated wild type but not active site Cys(453) --> Ser SHP-1 mutant by the "biotin-switch" method. Thus, one consequence of oxidative stimulation of NO generation is S-nitrosylation and inhibition of PTPs critical in cellular signal transduction pathways. These results support the conclusion that a mild oxidative signal is converted to a nitrosative one due to the better redox signaling properties of NO.

Activation of nuclear factor kappaB In vivo selectively protects the murine small intestine against ionizing radiation-induced damage

Exposure of mice to total body irradiation induces nuclear factor kappaB (NFkappaB) activation in a tissue-specific manner. In addition to the spleen, lymph nodes, and bone marrow, the tissues that exhibit NFkappaB activation now include the newly identified site of the intestinal epithelial cells. NFkappaB activated by total body irradiation mainly consists of NFkappaB p50/RelA heterodimers, and genetically targeted disruption of the NFkappaB p50 gene in mice significantly decreased the activation. By comparing tissue damage and lethality in wild-type and NFkappaB p50 knockout (p50-/-) mice after they were exposed to increasing doses of total body irradiation, we additionally examined the role of NFkappaB activation in total body irradiation-induced tissue damage. The results show that p50-/- mice are more sensitive to total body irradiation-induced lethality than wild-type mice (LD50/Day 7: wild-type = 13.12 Gy versus p50-/- = 7.75 Gy and LD50/Day 30: wild-type = 9.31 Gy versus p50-/- = 7.81 Gy). The increased radiosensitivity of p50-/- mice was associated with an elevated level of apoptosis in intestinal epithelial cells and decreased survival of the small intestinal crypts compared with wild-type mice (P < 0.01). In addition, RelA/TNFR1-deficient (RelA/TNFR1-/-) mice also exhibited a significant increase in intestinal epithelial cell apoptosis after they were exposed to total body irradiation as compared with TNFR1-deficient (TNFR1-/-) mice (P < 0.01). In contrast, no significant increase in total body irradiation-induced apoptosis or tissue injury was observed in bone marrow cells, spleen lymphocytes, and the liver, heart, lung, and kidney of p50-/- mice in comparison with wild-type mice. These findings indicate that activation of NFkappaB selectively protects the small intestine against ionizing radiation-induced damage.

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

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

Radiation and ceramide-induced apoptosis

Ceramide is a sphingolipid that acts as a second messenger in ubiquitous, evolutionarily conserved, signaling systems. Emerging data suggest that radiation acts directly on the plasma membrane of several cell types, activating acid sphingomyelinase, which generates ceramide by enzymatic hydrolysis of sphingomyelin. Ceramide then acts as a second messenger in initiating an apoptotic response via the mitochondrial system. Radiation-induced DNA damage can also initiate ceramide generation by activation of mitochondrial ceramide synthase and de novo synthesis of ceramide. In some cells and tissues, BAX is activated downstream of ceramide, regulating commitment to the apoptotic process via release of mitochondrial cytochrome c. Genetic and pharmacologic studies in vivo showed that radiation targets the acid sphingomyelinase apoptotic system of microvascular endothelial cells in the lungs, intestines and brain, as well as in oocytes, to initiate the pathogenesis of tissue damage. Regulated ceramide metabolism may produce metabolites, such as sphingosine 1-phosphate, shown to signal antiapoptosis, thus controlling the intensity of the apoptotic response and constituting a mechanism for radiation sensitivity or resistance. An improved understanding of this signaling system may offer new opportunities for the modulation of radiation effects in the treatment of cancer.

Changes in expression of transferrin, insulin-like growth factor 1, and interleukin 4 receptors after irradiation of cells of primary malignant brain tumor cell lines

Various immunotoxins have been developed for the treatment of cancer. The toxin is internalized by target cells through cell-surface receptors, and it is essential for these receptors to be expressed for the immunotoxin to have specific anti-tumor activity. Radiation therapy is one of the main treatment modalities for primary malignant brain tumors. The purpose of this study was to determine whether radiation influences the expression of cell-surface receptors. Cells of one human medulloblastoma (Daoy) and two glioblastoma (U373-MG and T98-G) cell lines were tested by exposing the cells to a single dose of 5 Gy gamma rays. Expression of transferrin receptors, type-1 insulin-like growth factor receptors (IGF1R), and interleukin 4 receptors (IL4R) was measured by flow cytometry analysis on unirradiated cells and on cells 3 to 120 h after irradiation. In Daoy cells, the absolute expression index of transferrin receptors increased during the 24 h after irradiation with the greatest change of 26% above control at 9 h. The absolute expression index of IGF1R increased 26.5% above control at 12 h. The absolute expression index of IL4R decreased 9 h after irradiation. In U373-MG cells the absolute expression index of transferrin receptors increased during the 24 h after irradiation, and the greatest increase was 45% above control at 9 h. The absolute expression index of IGF1R increased during the 12 h after irradiation with a maximum increase of 33% above control at 6 h. The absolute expression index of IL4R decreased with time after irradiation. In T98-G cells, the absolute expression index of both transferrin receptors and IL4R decreased after irradiation. The results suggest that the expression of growth factor receptors on brain tumor cells may be influenced by radiation. The effect of ionizing radiation on receptor expression should be considered when administration of targeted toxin is combined with radiation. Similar studies with other growth factor receptors used in targeted toxin therapy are recommended.

Radiosensitization by overexpression of the nonphosphorylation form of IkappaB-alpha in human glioma cells

To assess the role of NF-kappaB in cellular radiosensitivity, we constructed mutated IkappaB expression plasmids for SY-IkappaB (with mutations at residues of 32, 36 and 42) expression in human malignant glioma cells (radiosensitive MO54 and radioresistant T98 cells), giving respective cell types referred to as MO54-SY4 and T98-SY14. Both of the clones expressing SY-IkappaB became radiosensitive, compared with the parental MO54 and T98 cells. A treatment with herbimycin A or genistein did not change the radiosensitivity of cells expressing SY-IkappaB, but made both the MO54 and T98 parental cells more sensitive to ionizing radiation. A treatment with TNF-alpha induced DNA fragmentation and apoptosis in cells expressing SY-IkappaB, but not in MO54 and T98 cells. The survival after X-ray exposure of the parental MO54 cells was slightly increased by a TNF-alpha treatment, but that of the parental T98 cells did not change. The change in sensitivity to ultra-violet (UV) radiation and adriamycin in MO54-SY4 cells was very similar to that for X-ray sensitivity, but no change was observed in T98-SY14 cells. Significant sublethal damage repair was observed in T98 cells, whereas MO54 cells showed little repair activity. The expression of p53 was enhanced in the parental MO54 cells, while the p53 levels in the MO54-SY4, and in the parent and clonal T98 cells, did not change. Our data suggest that the serine and tyrosine phosphorylation of IkappaB-alpha may play a role in determining the radiosensitivity of malignant glioma cells.

Modulation of radiation-induced protein kinase C activity by phenolics

Natural phenolic compounds were tested in vitro for their effect on the activity of protein kinase C (PKC) isolated from the liver cytosol and the particulate fraction of unirradiated mice and mice irradiated at 5 Gy. Following irradiation, the PKC activity was found to be increased in both cytosolic and particulate fractions. Curcumin, ellagic acid and quercetin were effective in inhibiting radiation-induced PKC activity. Curcumin and ellagic acid were found to be more inhibitory towards radiation-induced PKC activity, while quercetin was the least effective. Curcumin was found to inhibit the activated cytosolic and particulate PKC at very low concentrations. Activation of PKC is one of the means of conferring radioresistance on a tumour cell. Suppression of PKC activity by phenolics may be one of the means of preventing the development of radioresistance following radiotherapy.

Alterations in hepatic kinase activity following whole body gamma-irradiation of mice

The chronological activation of the signaling molecules following whole body gamma-irradiation was investigated in mouse liver. The activity of two kinases, tyrosine kinase and protein kinase C (PKC), was found to respond differently to gamma-irradiation. Tyrosine kinase was found to respond to much lower doses of irradiation (10 cGy), whereas PKC was found to be activated at comparatively higher doses (3 Gy). Tyrosine kinase showed a sharp activation at 30 min and then a decline to normal values at 1 h. Activation of PKC was apparent at as early as 15 min of irradiation and showed a maximal increase at 30 min. This was followed by a decline to normal values at 1 h. The response of the whole organ was found to be different from that of reported effects on a single cell. These results suggest that the data obtained from the single-cell studies would have limited application in the experiments involving the whole animal. Interruption of these signals at various steps is currently being used to manipulate tumor response to radiotherapy. In such cases, the difference in response of a single cell and a whole animal must be considered.

Regulation of FOS by different compartmental stresses induced by low levels of ionizing radiation

We irradiated different cellular compartments and measured changes in expression of the FOS gene at the mRNA and protein levels. [(3)H]Thymidine and tritiated water were used to irradiate the nucleus and the whole cell, respectively. (125)I-Concanavalin A binding was used to irradiate the cell membrane differentially. Changes in FOS mRNA and protein levels were measured using semi-quantitative RT-PCR and SDS-PAGE Western blotting, respectively. Irradiation of the nucleus or the whole cell at a dose rate of 0.075 Gy/h caused no change in the level of FOS mRNA expression, but modestly (1.5-fold) induced FOS protein after 0.5 h. Irradiation of the nucleus at a dose rate of 0.43 Gy/h induced FOS mRNA by 1.5-fold after 0.5 h, but there was no significant effect after whole-cell irradiation. FOS protein was transiently induced 2.5-fold above control levels 0.5 h after a 0. 43-Gy/h exposure of the nucleus or the whole cell. Irradiation of the cell membrane at a dose rate of 1.8 Gy/h for up to 2 h caused no change in the levels of expression of FOS mRNA or protein, but a dose rate of 6.8 Gy/h transiently increased the level of FOS mRNA 3-fold after 0.5 h. These data demonstrate the complexity of the cellular response to radiation-induced damage at low doses. The lack of quantitative agreement between the transcript and protein levels for FOS suggests a role for post-transcriptional regulation.

Role of PRKCM (PKCmu) in radiation-induced increase of JUN proto-oncogene mRNA levels in B-lineage lymphoid cells

Exposure of cells to ionizing radiation results in both activation of protein kinase C (PRKC, also known as PKC) and induction of transcription of the JUN proto-oncogene. PRKC plays a pivotal role in radiation-induced JUN expression, since inhibition of PRKC abrogates the JUN signal. However, the specific PRKC isoforms involved in radiation-induced elevation of JUN mRNA levels have not been identified. Here we demonstrate that in DT40 B-lineage lymphoid cells, the mu isoform of PRKC (PRKCM) is critical for the response of JUN to ionizing radiation. The zinc chelator, 1, 10-phenanthroline, abrogated induction of JUN after exposure to ionizing radiation, indicating that this PRKCM-mediated response is also dependent on zinc.

Expression of class A scavenger receptor inhibits apoptosis of macrophages triggered by oxidized low density lipoprotein and oxysterol

The class A macrophage scavenger receptor (MSR-A) is a multifunctional trimeric glycoprotein involved in innate immune response as well as the development of lipid-laden foam cells during atherosclerosis. The MSR ligand, oxidized low density lipoprotein (oxLDL), is known to be cytotoxic to macrophages and other cell types. This study examined whether MSR mediates or modulates oxLDL-induced apoptosis. Treatment with oxLDL and its cytotoxic oxysterol, 7-ketocholesterol (7-KC), reduced viability and increased DNA fragmentation in human THP-1 cells, Chinese hamster ovary cells, and mouse peritoneal macrophages. However, cell death and DNA fragmentation were markedly diminished in the phorbol ester-differentiated MSR-expressing THP-1 cells and Chinese hamster ovary cells, with stable expression of MSR-AI after cDNA transfection when exposed to the same concentrations of oxLDL and 7-KC. Moreover, treatment with oxLDL and 7-KC induced much greater death and DNA fragmentation in MSR-A-deficient peritoneal macrophages compared with wild-type macrophages. Thus, MSR-A does not act as a receptor responsible for the apoptotic effect of oxLDL, and instead, expression of this receptor confers resistance of macrophages to the apoptotic stimulation by oxLDL and its cytotoxic lipid component. These results suggest that by preventing apoptosis, MSR-A may contribute to the long-term survival of macrophages and macrophage-derived lipid-laden foam cells in atherosclerotic lesions.

Ceramide mediates radiation-induced death of endothelium

The sphingomyelin (SM) pathway is an ubiquitous, evolutionarily conserved signaling system, analogous to conventional systems such as the cAMP and phosphoinositide pathways. Ceramide is generated from SM by the action of a neutral or acid SMase, or by de novo synthesis coordinated through the enzyme ceramide synthase. Once generated, ceramide may serve as a second messenger in signaling responses to physiologic or environmental stimuli, or may be converted to a variety of structural or effector molecules. In the radiation response, ceramide serves as a second messenger in initiating apoptosis, while some of its metabolites block apoptosis. In certain cells, such as endothelial, lymphoid and haematopoietic cells, ceramide mediates apoptosis while in others ceramide may serve only as a co-signal for or play no role in the death response. Regulated ceramide metabolism may determine the balance between pro- and anti-apoptotic signals, and hence, the intensity of the apoptotic response, thus constituting a mechanism of radiation sensitivity or resistance. This paradigm may offer new opportunities for modulation of the radiation effects in the treatment of cancer. Chemical modifiers of ceramide metabolism may be useful to enhance the therapeutic effects or reduce the toxicity of radiation treatment.

Activation of a rel-A/CEBP-beta-related transcription factor heteromer by PGG-glucan in a murine monocytic cell line

PGG-Glucan is a soluble beta-glucan immunomodulator that enhances a variety of leukocyte microbicidal activities without activating inflammatory cytokines. Although several different cell surface receptors for soluble (and particulate) beta-glucans have been described, the signal transduction pathway(s) used by these soluble ligands have not been elucidated. Previously we reported that PGG-Glucan treatment of mouse BMC2.3 macrophage cells activates a nuclear factor kappa-B-like (NF-kappaB) transcription factor complex containing subunit p65 (rel-A) attached to an unidentified cohort. In this study, we identify the cohort to be a non-rel family member: a CCAAT enhancer-binding protein-beta (C/EBP-beta)-related molecule with an apparent size of 48 kDa, which is a different protein than the previously identified C/EBP-beta p34 also present in these cells. C/EBP-beta is a member of the bZIP family whose members have previously been shown to interact with rel family members. This rel/bZIP heteromer complex activated by PGG-Glucan is different from the p65/p50 rel/rel complex induced in these cells by lipopolysaccharide (LPS). Thus, our data demonstrate that PGG-Glucan uses signal transduction pathways different from those used by LPS, which activates leukocyte microbicidal activities and inflammatory cytokines. We further show that heteromer activation appears to use protein kinase C (PKC) and protein tyrosine kinase (PTK) pathways, but not mitogen-activated protein kinase p38. Inhibitor kappa-B-alpha (IkappaB-alpha) is associated with the heteromer; this association decreases after PGG-Glucan treatment. These data are consistent with a model whereby treatment of BMC2.3 cells with PGG-Glucan activates IkappaB-alpha via PKC and/or PTK pathways, permitting translocation of the rel-A/CEBP-beta heteromer complex to the nucleus and increases its DNA-binding affinity.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.

Inhibition of I kappaB-alpha phosphorylation at serine and tyrosine acts independently on sensitization to DNA damaging agents in human glioma cells

Molecular mechanisms and/or intrinsic factors controlling cellular radiosensitivity are not fully understood in mammalian cells. The recent studies have suggested that nuclear factor kappaB (NF-kappaB) is one of such factors. The activation and regulation of NF-kappaB are tightly controlled by IkappaB-alpha, a cellular inhibitory protein of NF-kappaB. Most importantly, phosphorylation regulates activity of the inhibitor IkappaB-alpha, which sequesters NF-kappaB in the cytosol. Two different pathways for the phosphorylation of IkappaB-alpha are demonstrated, such as serine (at residues 32 and 36) and tyrosine (at residue 42) phosphorylations. To assess a role of the transcription factor, NF-kappaB, on cellular sensitivity to DNA damaging agents, we constructed three different types of expression plasmids, i.e. S-IkappaB (mutations at residues 32 and 36), Y-IkappaB (mutation at residue 42) and SY-IkappaB (mutations at residues 32, 36 and 42). The cell clones expressing S-IkappaB and Y-IkappaB proteins became sensitive to X-rays as compared with the parental and vector-transfected cells. The cell clones expressing SY-IkappaB were further radiosensitive. By the treatment with herbimycin A, an inhibitor of phosphorylation, the X-ray sensitivity of cells expressing SY-IkappaB did not change, while that of the cells expressing S-IkappaB and Y-IkappaB and the parental cells was enhanced. Change in the sensitivity to adriamycin and UV in those clones was very similar to that in the X-ray sensitivity. The inhibition of IkappaB-alpha phosphorylation at serine and tyrosine acts independently on the sensitization to X-rays, adriamycin and UV. These findings suggest that the transcriptional activation induced by NF-kappaB may play a role in the DNA damage repair. The present study proposes a possibility that the inactivation of NF-kappaB by inhibition of both serine and tyrosine phosphorylations may be useful for the treatment of cancer in radio- and chemotherapies.

Ionizing radiation stimulates existing signal transduction pathways involving the activation of epidermal growth factor receptor and ERBB-3, and changes of intracellular calcium in A431 human squamous carcinoma cells

Previous studies demonstrated that ionizing radiation activates the epidermal growth factor receptor (EGFR), as measured by Tyr autophosphorylation, and induces transient increases in cytosolic free [Ca2+], [Ca2+]f. The mechanistic linkage between these events has been investigated in A431 squamous carcinoma cells with the EGFR Tyr kinase inhibitor, AG1478. EGFR autophosphorylation induced by radiation at doses of 0.5-5 Gy or EGF concentrations of 1-10 ng/ml is inhibited by >75% at 100 nM AG1478. Activation of EGFR enhances IP3 production as a result of phospholipase C (PLC) activation. At the doses used, radiation stimulates Tyr phosphorylation of both, PLCgamma and erbB-3, and also mediates the association between erbB-3 and PLCgamma not previously described. The increased erbB-3 Tyr phosphorylation is to a significant extent due to transactivation by EGFR as >70% of radiation- and EGF-induced erbB-3 Tyr phosphorylation is inhibited by AG 1478. The radiation-induced changes in [Ca2+]f are dependent upon EGFR, erbB-3 and PLCgamma activation since radiation stimulated IP3 formation and Ca2+ oscillations are inhibited by AG1478, the PLCgamma inhibitor U73122 or neutralizing antibody against an extracellular epitope of erbB-3. These results demonstrate that radiation induces qualitatively and quantitatively similar responses to EGF in stimulation of the plasma membrane-associated receptor Tyr kinases and immediate downstream effectors, such as PLCgamma and Ca2+.

Ionizing radiation stimulates octamer factor DNA binding activity in human carcinoma cells

In mammalian cells, the octamer motif (ATGCAAAT) binding proteins, Oct-1 and Oct-2, play an important role in the transcriptional transactivation of several ubiquitously expressed genes as well as cell-specifically expressed genes. To date, a role of the octamer binding proteins in damage-stimulated response is not known. In this report, we demonstrate that DNA-binding activity of Oct-1, as demonstrated by the electrophoretic mobility shift assay, is significantly induced in a dose-dependent manner upon treatment of human head and neck squamous carcinoma cells (PCI-04A) with ionizing radiation (5 Gy: 5-fold; 15 Gy: 11-fold). By comparison, activities of other transcription factors were modestly increased (15 Gy: AP-1, 2.5-fold; NF-kappaB, 2.6-fold; SP-1, 5-fold). Radiation stimulation of Oct-1 activity was also noted in two other human cancer cell lines, albeit to a lesser extent (MDA-MB231 breast carcinoma cells and PC-3 prostate carcinoma cells (5 Gy: approximately 2-fold). These data represent the first report of the activation of an octamer factor DNA binding activity in response to environmental cues and suggest a novel role of Oct-1 in the radiation signaling cascade in these cancer cells.

Mechanism of radiation-induced diacylglycerol production in primary cultured rat hepatocytes

Protein kinase C (PKC) is known to be a key enzyme in radiation-induced signal transduction pathways. We have previously demonstrated that gamma-irradiation induces PKC activation and translocation from cytosol to membranes as a consequence of membrane lipid peroxidation in cultured rat hepatocytes (Int. J. Radiat. Biol. 70, 473-480, 1996). The present study was undertaken to investigate production of diacylglycerol, an endogenous activator of PKC, following gamma-irradiation of hepatocytes. Diacylglycerol content increased 3 min after irradiation, then decreased at 15 min and increased again at 30 min, indicating a biphasic pattern. This result implies participation of diacylglycerol in the radiation-induced activation of PKC in hepatocytes. In order to clarify the mechanism of the initial process of radiation-induced diacylglycerol production, the effects of reactive oxygens were investigated. Treatment of cells with hydroxyl radical, a major oxygen radical produced by radiation, induced diacylglycerol production without any change in the content of phosphatidylcholine, showing a peak at 1 min after treatment. No change in the diacylglycerol content was observed at that time by hydrogen peroxide treatment. Furthermore, the diacylglycerol production by hydroxyl radical was inhibited by pretreatment with neomycin sulfate, a phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitor. These results suggest that radiation exerts PI-PLC activation through hydroxyl radical generation, followed by diacylglycerol production and PKC activation.

Molecular mechanisms of ionizing radiation-induced apoptosis

Ionizing radiation activates not only signalling pathways in the nucleus as a result of DNA damage, but also signalling pathways initiated at the level of the plasma membrane. Proteins involved in DNA damage recognition include poly(ADP ribose) polymerase (PARP), DNA-dependent protein kinase, p53 and ataxia- telangiectasia mutated (ATM). Many of these proteins are inactivated by caspases during the execution phase of apoptosis. Signalling pathways outside the nucleus involve tyrosine kinases such as stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), protein kinase C, ceramide and reactive oxygen species. Recent evidence shows that tumour cells resistant to ionizing radiation-induced apoptosis have defective ceramide signalling. How these signalling pathways converge to activate the caspases is presently unknown, although in some cell types a role for calpain has been suggested.

Genistein is neuroprotective in murine models of familial amyotrophic lateral sclerosis and stroke

Amyotrophic lateral sclerosis (ALS), whether sporadic or familial (FALS), is a progressive, fatal neurodegenerative disorder involving the motor neurons of the cortex, brain stem, and spinal cord. In some studies, the male/female ratio of ALS patients was as high as 2 to 1. In FALS mice, disease onset and mortality were earlier among males than among females. This sexual dimorphism was due to estrogen, as treatment with genistein, a phytoestrogen, eliminated the observed sexual dimorphism in FALS mice. Genistein treatment also protected against oxygen singlet-induced cerebral damage in vivo. However, sexual dimorphism was not observed in this model of stroke; and genistein was equally effective in males and females. These data suggest that genistein has both estrogen-dependent and estrogen-independent neuroprotective activities and it should be investigated as a prophylactic agent against pathologic conditions such as ALS and stroke.

Signal transduction of stress via ceramide

The sphingomyelin (SM) pathway is a ubiquitous, evolutionarily conserved signalling system analogous to conventional systems such as the cAMP and phosphoinositide pathways. Ceramide, which serves as second messenger in this pathway, is generated from SM by the action of a neutral or acidic SMase, or by de novo synthesis co-ordinated through the enzyme ceramide synthase. A number of direct targets for ceramide action have now been identified, including ceramide-activated protein kinase, ceramide-activated protein phosphatase and protein kinase Czeta, which couple the SM pathway to well defined intracellular signalling cascades. The SM pathway induces differentiation, proliferation or growth arrest, depending on the cell type. Very often, however, the outcome of signalling through this pathway is apoptosis. Mammalian systems respond to diverse stresses with ceramide generation, and recent studies show that yeast manifest a form of this response. Thus ceramide signalling is an older stress response system than the caspase/apoptotic death pathway, and hence these two pathways must have become linked later in evolution. Signalling of the stress response through ceramide appears to play a role in the development of human diseases, including ischaemia/reperfusion injury, insulin resistance and diabetes, atherogenesis, septic shock and ovarian failure. Further, ceramide signalling mediates the therapeutic effects of chemotherapy and radiation in some cells. An understanding of the mechanisms by which ceramide regulates physiological and pathological events in specific cells may provide new targets for pharmacological intervention.

Ionizing radiation and short wavelength UV activate NF-kappaB through two distinct mechanisms

We examined the mechanisms by which two different types of photonic radiation, short wavelength UV (UV-C) and gamma radiation, activate transcription factor NF-kappaB. Exposure of mammalian cells to either form of radiation resulted in induction with similar kinetics of NF-kappaB DNA binding activity, nuclear translocation of its p65(RelA) subunit, and degradation of the major NF-kappaB inhibitor IkappaBalpha. In both cases, induction of NF-kappaB activity was attenuated by proteasome inhibitors and a mutation in ubiquitin-activating enzyme, suggesting that both UV-C and gamma radiation induce degradation of IkappaBs by means of the ubiquitin/proteasome pathway. However, although the induction of IkappaBalpha degradation by gamma rays was dependent on its phosphorylation at Ser-32 and Ser-36, UV-C-induced IkappaBalpha degradation was not dependent on phosphorylation of these residues. Even the "super repressor" IkappaBalpha mutant, which contains alanines at positions 32 and 36, was still susceptible to UV-C-induced degradation. Correspondingly, we found that gamma radiation led to activation of IKK, the protein kinase that phosphorylates IkappaBalpha at Ser-32 and Ser-36, whereas UV-C radiation did not. Furthermore, expression of a catalytically inactive IKKbeta mutant prevented NF-kappaB activation by gamma radiation, but not by UV-C. These results indicate that gamma radiation and UV-C activate NF-kappaB through two distinct mechanisms.

p53-dependent X-ray-induced modulation of cytokine mRNA levels in vivo

In vitro studies have shown that ionizing radiation can cause increases in some cytokine mRNA levels and activation of the nuclear NF-kappa B and/or AP1 transcription factors which have been implicated in the transcriptional regulation of many cytokine genes. Thus, radiation-induced upregulation of cytokine mRNAs appeared to be in part a direct consequence of transcription factor activation. To test this in vitro model in vivo, the effects of whole-body X-irradiation (0-10 Gy) on cytokine and other gene mRNA levels have been examined in mice. Increases and decreases in cytokine mRNA levels were detected in tissues which underwent an early wave of apoptosis (bone marrow and/or spleen), but not in more radioresistant tissues (kidney, liver, brain, and heart). Some mouse strain-specific differences were observed, but none of the changes in mRNA level was detected in p53-/- mice. As activation of the NF-kappa B and AP1 transcription factors was not detected in early-(spleen) or late-(liver) responding tissues in 10 Gy X-irradiated p53+/+ mice in vivo, it is concluded that the modulation of cytokine gene expression in vivo is p53-dependent and indirectly associated with apoptosis.

Expression of inducible nitric oxide synthase activity in human colon epithelial cells: modulation by T lymphocyte derived cytokines

BACKGROUND

Nitric oxide (NO) synthesis and inducible nitric oxide synthase (iNOS) expression are increased in colonic biopsy specimens from patients with ulcerative colitis, but the cellular source of NO production is not known.

AIMS

To examine the distribution of iNOS in human colonic mucosa and to explore the ability of T lymphocyte derived cytokines to regulate iNOS expression and activity in human colonic epithelial cells.

METHODS

iNOS expression was examined using immunohistochemistry in colonic biopsy samples from 12 patients with ulcerative colitis and three with infectious colitis and compared with 10 normal controls. In vitro iNOS expression and activity were determined in HT-29 cell cultures; nitrite levels were measured using a fluorescent substrate, iNOS mRNA expression by northern blot analysis, and iNOS protein expression by western blot analysis.

RESULTS

No iNOS expression was detected (10 of 10) in non-inflamed mucosa derived from normal controls. In 11 of 12 cases of newly diagnosed ulcerative colitis, iNOS protein was expressed in the epithelial cells, while no other positive cells were found in the lamina propria. Similar iNOS labelling was found in colonic biopsy samples from patients with infectious colitis in the acute phase, but when re-examined in samples from patients in total remission, no iNOS staining was observed. Both interleukin (IL)-13 and IL-4, but not IL-10, are potent inhibitors of iNOS expression and activity induced by an optimal combination of cytokines, namely IL-1 alpha, tumour necrosis factor alpha and interferon gamma.

CONCLUSIONS

The data suggest that the epithelium is the major source of iNOS activity in ulcerative colitis and that IL-13 and IL-4 may act as intrinsic regulators of NO generation in intestinal inflammation.

Free radical stimulation of tyrosine kinase and phosphatase activity in human peripheral blood mononuclear cells

Human lymphocytes were challenged with reactive oxygen species (ROS) generated by xanthine/xanthine oxidase leading to an increase in tyrosine phosphorylation, together with an increase in tyrosine phosphatase activity. In the presence of 50 microM vanadate and xanthine/xanthine oxidase, tyrosine phosphatase activity was inhibited and a marked increase in tyrosine phosphorylation was observed. The addition of catalase abolished the increase in tyrosine phosphorylation while the addition of superoxide dismutase had no effect. This suggests that vanadate together with hydrogen peroxide derived from xanthine/xanthine oxidase activity, interact to produce an agent that is an effective inhibitor of tyrosine phosphatase activity. When human lymphocytes were challenged with xanthine/xanthine oxidase in the presence of 50 microM CuCl2, an increase in both tyrosine phosphatase and kinase activity was observed. Cupric ions inhibited xanthine oxidase activity by 84%; neither superoxide or hydroxyl radicals could be detected, but traces of hydrogen peroxide were detected in the medium. We conclude that unbound metals can interact with ROS and readily influence signalling mechanisms in human lymphocytes.

Adaptive response to DNA-damaging agents: a review of potential mechanisms

The study of the adaptive response, i.e. a reduced effect from a higher challenging dose of a stressor when a smaller inducing dose had been applied a few hours earlier, has opened many new vistas into the mechanisms by which cells can adapt to hazardous environments. Although the entire chain from the initial event, supposedly the presence of DNA damage, to the end effect, presumably improved DNA repair, has not been fully elucidated, many individual links have been postulated. Initial elements--following the still unknown signal for the presence of radiation damage--are various kinases (protein kinase C and stress-activated protein kinases), which, in turn, induce early response genes whose products initiate a cascade of protein-DNA interactions that regulate gene transcription and ultimately result in specific biological responses. These responses include the activation of later genes that can promote production of growth factors and cytokines, trigger DNA repair, and regulate progress through the cell cycle. Indeed, there appears to be a relation between the induction of the adaptive response and the effects of radiation and cytostatic agents on the cell cycle, although these effects, especially the G1 delay, occur at much higher doses than the adaptive response, and one may not indiscriminately extrapolate mechanisms responsible for cell cycle changes observed at high doses, e.g. for radiation in the order of grays, to those involved in the adaptive responses at much lower doses, i.e. some tens of milligrays.

Stimulation of Src family protein-tyrosine kinases as a proximal and mandatory step for SYK kinase-dependent phospholipase Cgamma2 activation in lymphoma B cells exposed to low energy electromagnetic fields

Here, we present evidence that exposure of DT40 lymphoma B cells to low energy electromagnetic field (EMF) results in a tyrosine kinase-dependent activation of phospholipase Cgamma2 (PLC-gamma2) leading to increased inositol phospholipid turnover. B cells rendered PLC-gamma2-deficient by targeted disruption of the PLC-gamma2 gene as well as PLC-gamma2-deficient cells reconstituted with Src homology domain 2 (SH2) domain mutant PLC-gamma2 did not show any increase in inositol-1,4,5-trisphosphate levels after EMF exposure, providing direct evidence that PLC-gamma2 is responsible for EMF-induced stimulation of inositol phospholipid turnover, and its SH2 domains are essential for this function. B cells rendered SYK-deficient by targeted disruption of the syk gene did not show PLC-gamma2 activation in response to EMF exposure. The C-terminal SH2 domain of SYK kinase is essential for its ability to activate PLC-gamma2. SYK-deficient cells reconstituted with a C-terminal SH2 domain mutant syk gene failed to elicit increased inositol phospholipid turnover after EMF exposure, whereas SYK-deficient cells reconstituted with an N-terminal SH2 domain mutant syk gene showed a normal EMF response. LYN kinase is essential for the initiation of this biochemical signaling cascade. Lymphoma B cells rendered LYN-deficient through targeted disruption of the lyn gene did not elicit enhanced inositol phospholipid turnover after EMF exposure. Introduction of the wild-type (but not a kinase domain mutant) mouse fyn gene into LYN-deficient B cells restored their EMF responsiveness. B cells reconstituted with a SH2 domain mutant fyn gene showed a normal EMF response, whereas no increase in inositol phospholipid turnover in response to EMF was noticed in LYN-deficient cells reconstituted with a SH3 domain mutant fyn gene. Taken together, these results indicate that EMF-induced PLC-gamma2 activation is mediated by LYN-regulated stimulation of SYK, which acts downstream of LYN kinase and upstream of PLC-gamma2.

Contribution of protein kinase C to p53-dependent WAF1 induction pathway after heat treatment in human glioblastoma cell lines

To examine whether protein kinase C (PKC) contributes to p53-dependent WAF1 induction after heat treatment, the effects of calphostin C (CAL), a specific inhibitor of PKC, on WAF1 induction were analyzed by PKC activity and gel mobility-shift assays and Western blot analysis in human glioblastoma cell lines. Heat-induced accumulation of WAF1 in A-172 cells carrying wild-type p53 (wtp53) was suppressed by CAL in a dose-dependent manner. In T98G cells carrying mutant p53 (mp53), no significant accumulation of WAF1 was observed after heat treatment and CAL exerted no significant effects on this response of T98G cells. In accordance with the accumulation of WAF1, heat-induced activation of the binding ability of p53 to p53 consensus sequence (p53 CON) was suppressed by CAL in A-172 cells but no DNA-binding activity was observed in the mp53 in T98G cells. PKC in A-172 cells was activated rapidly (within 5 min) after heat treatment in the membrane fraction but not in the cytosolic fraction. When the cell lines were treated with the PKC activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), WAF1 was accumulated in A-172 cells in a dose-dependent manner but not in T98G cells. In addition, the cellular contents of WAF1 after heating did not increase in A-172 cells transformed with mp53. These results suggest that PKC contributes to heat-induced signal transduction leading to p53-dependent WAF1 induction in a way that PKC is involved in the specific DNA-binding activation of p53.

Differential inhibition of radiation-induced apoptosis

The most common mechanism by which radiation kills cells is the induction of DNA double-strand breaks that results in the loss of cell proliferation. Even though apoptosis is increasingly identified in experimental systems in vitro and in vivo, it is still generally regarded as a rare mode of radiation-induced cell kill with minor relevance for the clinical effects of radiation. This review will focus on pro- and antiapoptotic signaling that affects the apoptotic outcome in irradiated mammalian cells. In particular, we will concentrate on the sphingomyelin/ceramide signal transduction pathway which is involved in initiation of stress-induced apoptosis in a variety of normal and neoplastic cells. We will also discuss the crosstalk between the sphingomyelin/ceramide pathway and the protein kinase C pathway which constitutes an antiapoptotic pathway, and the potential for pharmacological modulation to increase the fraction of apoptotic cells undergoing apoptosis after radiation exposure.

Mechanisms of radiation-induced gene responses

While identifying genes differentially expressed in cells exposed to ultraviolet radiation, we identified a transcript with a 25-nucleotide region that is highly conserved among a variety of species, including Bacillus circulans, pumpkin, yeast, Drosophila, mouse, and man. In the 5' untranslated region of a gene, the sequence is predominantly in a +/+ orientation with respect to the coding DNA strand; while in the coding region and the 3' untranslated region, the sequence is most frequently in a -/+ orientation. The element is found in many different genes that have diverse functions. Gel mobility shift assays demonstrated the presence of a protein in HeLa cell extracts that binds to the sense and antisense single-stranded consensus oligomers, as well as to double-stranded oligonucleotide. When double-stranded oligomer was used, the size shift demonstrated an additional protein-oligomer complex larger than the one bound to either sense or antisense single-stranded consensus oligomers alone. This element may bind to protein(s) that maintain DNA in a single-stranded orientation for transcription, or be important in the transcription-coupled DNA repair process.

EGF receptor phosphorylation is affected by ionizing radiation

Eukaryotic cells respond to ionizing radiation with cell cycle arrest, activation of DNA repair mechanisms, and lethality. However, little is known about the molecular mechanisms that constitute these responses. Here we report that ionizing radiation enhances epidermal growth factor (EGF) receptor tyrosine phosphorylation in intact cells as well as in isolated membranes of A431 cells. Phosphoamino acid analysis revealed that ionizing radiation preferentially enhances tyrosine phosphorylation, while EGF enhances the phosphorylation of all three phosphoamino acids (serine, threonine and tyrosine) of the EGF receptor. In addition, radiation reduces the turnover rate of the EGF receptor, while EGF increases the rate of the receptor turnover and down-regulation. Moreover, the confined radiation-induced phosphorylation of tyrosine residues is inhibited by genistein, indicating that this phosphorylation of EGF receptor is due to protein tyrosine kinase activation. These studies provide novel insights into the capacity of radiation to modulate EGF receptor phosphorylation and function. The radiation-induced elevation in the EGF receptor tyrosine phosphorylation and the receptor's slower rate of turnover are discussed in terms of their possible role in cell growth and apoptosis modulation.

Tyrosine phosphorylation in DNA damage and cell death in hypoxic injury to LLC-PK1 cells

Hypoxia is classically considered to result in a necrotic form of cell injury. We have recently demonstrated a role of endonuclease activation, considered a feature of apoptosis, in DNA damage and cell death in chemical hypoxic injury to renal tubular epithelial cells (LLC-PK1 cells). Tyrosine phosphorylation has been implicated to be involved in cell signaling pathway leading to cell growth, proliferation, and apoptotic death. However, a role of tyrosine phosphorylation as a signal transduction pathway involved in DNA damage and cell death has not been previously examined in hypoxic injury in any tissue. In the present study, we have demonstrated that chemical hypoxia with a combination of antimycin A, a mitochondrial respiration inhibitor, and substrate deprivation resulted in rapid increase in protein tyrosine kinases activity and protein tyrosine phosphorylation prior to any evidence of cell death in LLC-PK1 cells. The inhibitors of protein tyrosine kinases, genistein, lavendustin A, tyrphostin, and herbimycin A provided a marked protection against chemical hypoxia-induced DNA damage (as measured by alkaline unwinding assay) and cell death (as measured by trypan blue exclusion assay). In a separate study, we confirmed the ability of the inhibitors, lavendustin A and herbimycin A to prevent chemical hypoxia-induced increase in protein tyrosine kinases activity and protein tyrosine phosphorylation. In addition, the inhibitors used did not affect ATP depletion induced by antimycin A, suggesting that the inhibitors do not alter cellular uptake of antimycin A. Taken together, our data provide a strong evidence that tyrosine phosphorylation plays as important role in DNA damage and cell death in chemical hypoxic injury to renal tubular epithelial cells.

Finkel-Biskis-Reilly mouse osteosarcoma virus v-fos inhibits the cellular response to ionizing radiation in a myristoylation-dependent manner

DNA damage is recognized as a central component of carcinogenesis. DNA-damaging agents activate a number of signal transduction pathways that lead to repair of the DNA, apoptosis, or cell cycle arrest. It is reasoned that a cell deficient in DNA repair is more likely to acquire other cancer-promoting mutations. Despite the recent interest in the link between DNA damage and carcinogenesis, retroviral oncogenes have not yet been shown to affect the DNA damage-signaling pathway. In this report, we show that Finkel-Biskis-Reilly mouse osteosarcoma virus (FBR) v-fos, the retroviral homologue of the c-fos proto-oncogene, inhibits the cellular response to ionizing radiation. Cells that express FBR v-Fos show a decreased ability to repair DNA damage caused by ionizing radiation, and these cells show decreased survival in response to ionizing radiation. In addition, FBR v-Fos inhibits DNA-dependent protein kinase, a kinase specifically activated upon exposure to ionizing radiation. These effects were specific to ionizing radiation, as no effect of FBR v-Fos on the UV light signaling pathway was seen. Last, these effects were dependent on a lipid modification required for FBR v-Fos tumorigenesis, that of myristoylation of FBR v-Fos. A non-myristoylated mutant FBR v-Fos caused none of these effects. This study suggests that a retroviral oncogene can lead to an increased genomic instability, which can ultimately increase the carcinogenic potential of a cell.

Redox regulation of cellular activation

Growing evidence has indicated that cellular reduction/oxidation (redox) status regulates various aspects of cellular function. Oxidative stress can elicit positive responses such as cellular proliferation or activation, as well as negative responses such as growth inhibition or cell death. Cellular redox status is maintained by intracellular redox-regulating molecules, including thioredoxin (TRX). TRX is a small multifunctional protein that has a redox-active disulfide/dithiol within the conserved active site sequence: Cys-Gly-Pro-Cys. Adult T cell leukemia-derived factor (ADF), which we originally defined as an IL-2 receptor alpha-chain/Tac inducer produced by human T cell lymphotrophic virus-I (HTLV-I)-transformed T cells, has been identified as human TRX. TRX/ADF is a stress-inducible protein secreted from cells. TRX/ADF has both intracellular and extracellular functions as one of the key regulators of signaling in the cellular responses against various stresses. Extracellularly, TRX/ADF shows a cytoprotective activity against oxidative stress-induced apoptosis and a growth-promoting effect as an autocrine growth factor. Intracellularly, TRX/ADF is involved in the regulation of protein-protein or protein-nucleic acid interactions through the reduction/oxidation of protein cysteine residues. For example, TRX/ADF translocates from the cytosol into the nucleus by a variety of cellular stresses, to regulate the expression of various genes through the redox factor-1 (Ref-1)/APEX. Further studies to clarify the regulatory roles of TRX/ADF and its target molecules may elucidate the intracellular signaling pathways in the responses against various stresses. The concept of "redox regulation" is emerging as an understanding of the novel mechanisms in the pathogenesis of several disorders, including viral infections, immunodeficiency, malignant transformation, and degenerative disease.

Role of protein kinase activity in apoptosis

The transmission of signals from the plasma membrane to the nucleus involves a number of different pathways all of which have in common protein modification. The modification is primarily in the form of phosphorylation which leads to the activation of a series of protein kinases. It is now evident that these pathways are common to stimuli that lead to mitogenic and apoptotic responses. Even the same stimuli under different physiological conditions can cause either cell proliferation or apoptosis. Activation of specific protein kinases can in some circumstances protect against cell death, while in others it protects the cell against apoptosis. Some of the pathways involved lead to activation of transcription factors and the subsequent induction of genes involved in the process of cell death or proliferation. In other cases, such as for the tumour suppressor gene product p53, activation may be initiated both at the level of gene expression or through pre-existing proteins. Yet in others, while the initial steps in the pathway are ill-defined, it is clear that downstream activation of a series of cystein proteases is instrumental in pushing the cell towards apoptosis. In this report we review the involvement of protein kinases at several different levels in the control of cell behaviour.