Geoffrey Gloire

Photosynthesis and State Transitions in Mitochondrial Mutants of Chlamydomonas reinhardtii Affected in Respiration

Photosynthetic activities were analyzed in Chlamydomonas reinhardtii mitochondrial mutants affected in different complexes (I, III, IV, I + III, and I + IV) of the respiratory chain. Oxygen evolution curves showed a positive relationship between the apparent yield of photosynthetic linear electron transport and the number of active proton-pumping sites in mitochondria. Although no significant alterations of the quantitative relationships between major photosynthetic complexes were found in the mutants, 77 K fluorescence spectra showed a preferential excitation of photosystem I (PSI) compared with wild type, which was indicative of a shift toward state 2. This effect was correlated with high levels of phosphorylation of light-harvesting complex II polypeptides, indicating the preferential association of light-harvesting complex II with PSI. The transition to state 1 occurred in untreated wild-type cells exposed to PSI light or in 3-(3,4-dichlorophenyl)-1,1-dimethylureatreated cells exposed to white light. In mutants of the cytochrome pathway and in double mutants, this transition was only observed in white light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. This suggests higher rates of nonphotochemical plastoquinone reduction through the chlororespiratory pathway, which was confirmed by measurements of the complementary area above the fluorescence induction curve in dark-adapted cells. Photo-acoustic measurements of energy storage by PSI showed a stimulation of PSI-driven cyclic electron flow in the most affected mutants. The present results demonstrate that in C. reinhardtii mutants, permanent defects in the mitochondrial electron transport chain stabilize state 2, which favors cyclic over linear electron transport in the chloroplast.

Potentiation of tumor necrosis factor-induced NF-kappa B activation by deacetylase inhibitors is associated with a delayed cytoplasmic reappearance of I kappa B alpha.

ABSTRACT Previous studies have implicated acetylases and deacetylases in regulating the transcriptional activity of NF-κB. Here, we show that inhibitors of deacetylases such as trichostatin A (TSA) and sodium butyrate (NaBut) potentiated TNF-induced expression of several natural NF-κB-driven promoters. This transcriptional synergism observed between TNF and TSA (or NaBut) required intact κB sites in all promoters tested and was biologically relevant as demonstrated by RNase protection on two instances of endogenous NF-κB-regulated gene transcription. Importantly, TSA prolonged both TNF-induced DNA-binding activity and the presence of NF-κB in the nucleus. We showed that the p65 subunit of NF-κB was acetylated in vivo. However, this acetylation was weak, suggesting that other mechanisms could be implicated in the potentiated binding and transactivation activities of NF-κB after TNF plus TSA versus TNF treatment. Western blot and immunofluorescence confocal microscopy experiments revealed a delay in the cytoplasmic reappearance of the IκBα inhibitor that correlated temporally with the prolonged intranuclear binding and presence of NF-κB. This delay was due neither to a defect in IκBα mRNA production nor to a nuclear retention of IκBα but was rather due to a persistent proteasome-mediated degradation of IκBα. A prolongation of IκB kinase activity could explain, at least partially, the delayed IκBα cytoplasmic reappearance observed in presence of TNF plus TSA.

Restoration of SHIP-1 activity in human leukemic cells modifies NF-kappaB activation pathway and cellular survival upon oxidative stress.

Nuclear factor-kappa B (NF-κB) is an important prosurvival transcription factor activated in response to a large array of external stimuli, including reactive oxygen species (ROS). Previous works have shown that NF-κB activation by ROS involved tyrosine phosphorylation of the inhibitor IκBα through an IκB kinase (IKK)-independent mechanism. In the present work, we investigated with more details NF-κB redox regulation in human leukemic cells. By using different cell lines (CEM, Jurkat and the subclone Jurkat JR), we clearly showed that NF-κB activation by hydrogen peroxide (H2O2) is cell-type dependent: it activates NF-κB through tyrosine phosphorylation of IκBα in Jurkat cells, whereas it induces an IKK-mediated IκBα phosphorylation on S32 and 36 in CEM and Jurkat JR cells. We showed that this H2O2-induced IKK activation in CEM and Jurkat JR cells is mediated by SH2-containing inositol 5′-phosphatase 1 (SHIP-1), a lipid phosphatase that is absent in Jurkat cells. Indeed, the complementation of SHIP-1 in Jurkat cells made them shift to an IKK-dependent mechanism upon oxidative stress stimulation. We also showed that Jurkat cells expressing SHIP-1 are more resistant to H2O2-induced apoptosis than the parental cells, suggesting that SHIP-1 has an important role in leukemic cell responses to ROS in terms of signal transduction pathways and apoptosis resistance, which can be of interest in improving ROS-mediated chemotherapies.

Downregulation of ICAM-1 and VCAM-1 expression in endothelial cells treated by photodynamic therapy

Photodynamic therapy (PDT) is a treatment for cancer and several noncancerous proliferating cell diseases that depends on the uptake of a photosensitizing compound followed by selective irradiation with visible light. In the presence of oxygen, irradiation leads to the production of reactive oxygen species (ROS). A large production of ROS induces the death of cancer cells by apoptosis or necrosis. A small ROS production can activate various cellular pathways. Here, we show that PDT by pyropheophorbide-a methyl ester (PPME) induces the activation of nuclear factor kappa B (NF-κB) in HMEC-1 cells. NF-κB is active since it binds to the NF-κB sites of both ICAM-1 and vascular cell adhesion molecule-1 (VCAM-1) promoters and induces the transcription of several NF-κB target genes such as those of IL-6, ICAM-1, VCAM-1. In contrast, expression of ICAM-1 and VCAM-1 at the protein level was not observed, although we measured an IL-6 secretion. Using specific chemical inhibitors, we showed that the lack of ICAM-1 and VCAM-1 expression is the consequence of their degradation by lysosomal proteases. The proteasome and calpain pathways were not involved. All these observations were consistent with the fact that no adhesion of granulocytes was observed in these conditions.

Promoter-dependent Effect of IKKα on NF-κB/p65 DNA Binding

IKKα regulates many chromatin events in the nuclear phase of the NF-κB program, including phosphorylation of histone H3 and removal of co-repressors from NF-κB-dependent promoters. However, all of the nuclear functions of IKKα are not understood. In this study, using mouse embryonic fibroblasts IKKα knock-out and reexpressing IKKα after retroviral transduction, we demonstrate that IKKα contributes to NF-κB/p65 DNA binding activity on an exogenous κB element and on some, but not all, endogenous NF-κB-target promoters. Indeed, p65 chromatin immunoprecipitation assays revealed that IKKα is crucial for p65 binding on κB sites of icam-1 and mcp-1 promoters but not on iκbα promoter. The mutation of IKKα putative nuclear localization sequence, which prevents its nuclear translocation, or of crucial serines in the IKKα activation loop completely inhibits p65 binding on icam-1 and mcp-1 promoters and rather enhances p65 binding on the iκbα promoter. Further molecular studies demonstrated that the removal of chromatin-bound HDAC3, a histone deacetylase inhibiting p65 DNA binding, is differentially regulated by IKKα in a promoter-specific manner. Indeed, whereas the absence of IKKα induces HDAC3 recruitment and repression on the icam-1 promoter, it has an opposite effect on the iκbα promoter, where a better p65 binding occurs. We conclude that nuclear IKKα is required for p65 DNA binding in a gene-specific manner.

SHIP-1 inhibits CD95/APO-1/Fas-induced apoptosis in primary T lymphocytes and T leukemic cells by promoting CD95 glycosylation independently of its phosphatase activity.

SHIP-1 (SH2 (Src homology 2)-containing inositol 5′-phosphatase-1) functions as a negative regulator of immune responses by hydrolyzing phosphatidylinositol-3,4,5-triphosphate generated by phosphoinositide-3 (PI 3)-kinase activity. As a result, SHIP-1 deficiency in mice results in myeloproliferation and B-cell lymphoma. On the other hand, SHIP-1-deficient mice have a reduced T-cell population, but the underlying mechanisms are unknown. In this work, we hypothesized that SHIP-1 plays anti-apoptotic functions in T cells upon stimulation of the death receptor CD95/APO-1/Fas. Using primary T cells from SHIP-1−/− mice and T leukemic cell lines, we report that SHIP-1 is a potent inhibitor of CD95-induced death. We observed that a small fraction of the SHIP-1 pool is localized to the endoplasmic reticulum (ER), in which it promotes CD95 glycosylation. This post-translational modification requires an intact SH2 domain of SHIP-1, but is independent of its phosphatase activity. The glycosylated CD95 fails to oligomerize upon stimulation, resulting in impaired death-inducing signaling complex (DISC) formation and downstream apoptotic cascade. These results uncover an unanticipated inhibitory function for SHIP-1 and emphasize the role of glycosylation in the regulation of CD95 signaling in T cells. This work may also provide a new basis for therapeutic strategies using compounds inducing apoptosis through the CD95 pathway on SHIP-1-negative leukemic T cells.

Promoter-dependent effect of IKK alpha on NF-kappa B/p65 DNA binding

IKKα regulates many chromatin events in the nuclear phase of the NF-κB program, including phosphorylation of histone H3 and removal of co-repressors from NF-κB-dependent promoters. However, all of the nuclear functions of IKKα are not understood. In this study, using mouse embryonic fibroblasts IKKα knock-out and reexpressing IKKα after retroviral transduction, we demonstrate that IKKα contributes to NF-κB/p65 DNA binding activity on an exogenous κB element and on some, but not all, endogenous NF-κB-target promoters. Indeed, p65 chromatin immunoprecipitation assays revealed that IKKα is crucial for p65 binding on κB sites of icam-1 and mcp-1 promoters but not on iκbα promoter. The mutation of IKKα putative nuclear localization sequence, which prevents its nuclear translocation, or of crucial serines in the IKKα activation loop completely inhibits p65 binding on icam-1 and mcp-1 promoters and rather enhances p65 binding on the iκbα promoter. Further molecular studies demonstrated that the removal of chromatin-bound HDAC3, a histone deacetylase inhibiting p65 DNA binding, is differentially regulated by IKKα in a promoter-specific manner. Indeed, whereas the absence of IKKα induces HDAC3 recruitment and repression on the icam-1 promoter, it has an opposite effect on the iκbα promoter, where a better p65 binding occurs. We conclude that nuclear IKKα is required for p65 DNA binding in a gene-specific manner.

Phosphorylation of p65(RelA) on Ser547 by ATM Represses NF-κB-Dependent Transcription of Specific Genes after Genotoxic Stress

The NF-κB pathway is involved in immune and inflammation responses, proliferation, differentiation and cell death or survival. It is activated by many external stimuli including genotoxic stress. DNA double-strand breaks activate NF-κB in an ATM-dependent manner. In this manuscript, a direct interaction between p65(RelA) and the N-terminal extremity of ATM is reported. We also report that only one of the five potential ATM-(S/T)Q target sites present in p65, namely Ser547, is specifically phosphorylated by ATM in vitro. A comparative transcriptomic analysis performed in HEK-293 cells expressing either wild-type HA-p65 or a non-phosphorylatable mutant HA-p65S547A identified several differentially transcribed genes after an etoposide treatment (e.g. IL8, A20, SELE). The transcription of these genes is increased in cells expressing the mutant. Substitution of Ser547 to alanine does not affect p65 binding abilities on the κB site of the IL8 promoter but reduces p65 interaction with HDAC1. Cells expressing p65S547A have a higher level of histone H3 acetylated on Lys9 at the IL8 promoter, which is in agreement with the higher gene induction observed. These results indicate that ATM regulates a sub-set of NF-κB dependent genes after a genotoxic stress by direct phosphorylation of p65.

Enzymatic and non-enzymatic activities of SHIP-1 in signal transduction and cancer

PI3K cascade is a central signaling pathway regulating cell proliferation, growth, differentiation, and survival. Tight regulation of the PI3K signaling pathway is necessary to avoid aberrant cell proliferation and cancer development. Together with SHIP-1, the inositol phosphatases PTEN and SHIP-2 are the gatekeepers of this pathway. In this review, we will focus on SHIP-1 functions. Negative regulation of immune cell activation by SHIP-1 is well characterized. Besides its catalytic activity, SHIP-1 also displays non-enzymatic activity playing role in several immune pathways. Indeed, SHIP-1 exhibits several domains that mediate protein-protein interaction. This review emphasizes the negative regulation of immune cell activation by SHIP-1 that is mediated by its protein-protein interaction.

Potentiation of Tumor Necrosis Factor-Induced NF-κB Activation by Deacetylase Inhibitors Is Associated with a Delayed Cytoplasmic Reappearance of IκBα

Previous studies have implicated acetylases and deacetylases in regulating the transcriptional activity of NF-B. Here, we show that inhibitors of deacetylases such as trichostatin A (TSA) and sodium butyrate (NaBut) potentiated TNF-induced expression of several natural NF-B-driven promoters. This transcriptional synergism observed between TNF and TSA (or NaBut) required intact B sites in all promoters tested and was biologically relevant as demonstrated by RNase protection on two instances of endogenous NF-B-regulated gene transcription. Importantly, TSA prolonged both TNF-induced DNA-binding activity and the presence of NF-B in the nucleus. We showed that the p65 subunit of NF-B was acetylated in vivo. However, this acetylation was weak, suggesting that other mechanisms could be implicated in the potentiated binding and transactivation activities of NF-B after TNF plus TSA versus TNF treatment. Western blot and immunofluorescence confocal microscopy experiments revealed a delay in the cytoplasmic reappearance of the IB inhibitor that correlated temporally with the prolonged intranuclear binding and presence of NF-B. This delay was due neither to a defect in IB mRNA production nor to a nuclear retention of IB but was rather due to a persistent proteasome-mediated degradation of IB. A prolongation of IB kinase activity could explain, at least partially, the delayed IB cytoplasmic reappearance observed in presence of TNF plus TSA.