Despite its toxicity, H(2)O(2) is produced as a signaling molecule that oxidizes critical cysteine residues of effectors such as protein tyrosine phosphatases in response to activation of cell surface receptors. It has remained unclear, however, how H(2)O(2) concentrations above the threshold required to modify effectors are achieved in the presence of the abundant detoxification enzymes peroxiredoxin (Prx) I and II. We now show that PrxI associated with membranes is transiently phosphorylated on tyrosine-194 and thereby inactivated both in cells stimulated via growth factor or immune receptors in vitro and in those at the margin of healing cutaneous wounds in mice. The localized inactivation of PrxI allows for the transient accumulation of H(2)O(2) around membranes, where signaling components are concentrated, while preventing the toxic accumulation of H(2)O(2) elsewhere. In contrast, PrxII was inactivated not by phosphorylation but rather by hyperoxidation of its catalytic cysteine during sustained oxidative stress.

Peroxiredoxins (Prxs) are a family of antioxidant proteins that reduce peroxide levels by using reducing agents such as thioredoxin. These proteins were characterized to have a number of cellular functions, including cell proliferation and differentiation and protection of specific proteins from oxidative damage. However, the physiological roles of the peroxiredoxins have not been determined. To clarify the physiological relevance of this protein type, we generated a mouse model deficient in Prx II, which is abundantly expressed in all types of cells. The Prx II ؊/؊ mice were healthy in appearance and fertile. However, they had splenomegaly caused by the congestion of red pulp with hemosiderin accumulation. Heinz bodies were detected in their peripheral blood, and morphologically abnormal cells were elevated in the dense red blood cell (RBC) fractions, which contained markedly higher levels of reactive oxygen species (ROS). The Prx II ؊/؊ mice had significantly decreased hematocrit levels, but increased reticulocyte counts and erythropoietin levels, indicative of a compensatory action to maintain hematologic homeostasis in the mice. In addition, a labeling experiment with the thiol-modifying reagent biotinylated iodoacetamide (BIAM) in Prx II ؊/؊ mice revealed that a variety of RBC proteins were highly oxidized.

The hepatitis B virus-X protein (HBx) regulates fundamental aspects of mitochondrial physiology. We show that HBx down-regulates mitochondrial enzymes involved in electron transport in oxidative phosphorylation (complexes I, III, IV, and V) and sensitizes the mitochondrial membrane potential in a hepatoma cell line. HBx also increases the level of mitochondrial reactive oxygen species and lipid peroxide production. HBx does not activate apoptotic signaling, although it sensitizes hepatoma cells to apoptotic signaling, which is dependent on reactive oxygen species. Increased intrahepatic lipid peroxidation in HBx transgenic mice demonstrated that oxidative injury occurs as a direct result of HBx expression. Therefore, we conclude that mitochondrial dysfunction is a crucial pathophysiological factor in HBx-expressing hepatoma cells and provides an experimental rationale in the investigation of mitochondrial function in rapidly renewed tissues, as in hepatocellular carcinomas.

CD36 is a membrane glycoprotein expressed on platelets, monocytes, macrophages, and several other cell types that was recently demonstrated to be involved in platelet activation in response to oxidized phospholipids, including oxidized LDL. Although the role of CD36 in other vascular cells has not been well defined, previous studies have demonstrated that cd36-knockout (cd36 -/-) mice have prolonged thrombosis times after vascular injury, which can be protective in the state of hyperlipidemia. Here, we found significantly less ROS in the vessel walls of cd36 -/-mice compared with WT after chemically induced arterial injury, suggesting that CD36 may contribute to ROS generation in the VSMCs themselves. Gene expression analysis revealed that the antioxidant enzymes peroxiredoxin-2 (Prdx2) and heme oxygenase-1 were upregulated in cd36 -/-VSMCs. Molecular dissection of the pathway in isolated mouse VSMCs revealed CD36 ligand-dependent induction of Fyn phosphorylation, with subsequent phosphorylation and degradation of the redox-sensitive transcription factor Nrf2. Chromatin immunoprecipitation experiments further showed that Nrf2 directly occupied the Prdx2 promoter. The importance of this pathway was evidenced by increased ROS generation in prdx2 -/-mice and decreased thrombosis times in both prdx2 -/-and nrf2 -/-mice after vascular injury. These data suggest that CD36-mediated downregulation of antioxidant systems in VSMCs may contribute to its prothrombotic, proinflammatory, and atherogenic effects. Introduction CD36, a class B scavenger receptor, is an 88-kDa membrane glycoprotein expressed on platelets, monocytes, macrophages, and several other cell types (1). It is a multifunctional receptor with independent capacity to bind 3 major classes of ligands: modified phospholipids, long-chain fatty acids, and proteins containing thrombospondin structural homology domains. Although CD36 was first identified on platelets, its role in platelet function remained undefined until recently, when our group showed that CD36 promotes platelet activation in response to oxidized phospholipids, including oxidized LDL (oxLDL) (2). We found that genetic deletion of CD36 in mice protected them from the prothrombotic state associated with hyperlipidemia (2) and oxidant stress and also significantly prolonged thrombotic occlusion times after vascular injury induced by ferric chloride (FeCl 3 ) (3). The mechanisms underlying this phenotype remain incompletely understood, but the antithrombotic effects of CD36 deficiency were dependent on the dose of FeCl 3 used to induce injury (3) and platelet transfusion studies revealed that they were in part platelet dependent. Furthermore, CD36-mediated signaling through specific Src family kinases and MAPKs was shown to promote platelet activation after vascular injury (4). The endogenous CD36 ligands generated during FeCl 3 -induced injury include endothelial cellderived microparticles (EMPs), which we showed bind specifically to platelet CD36 and enhance platelet reactivity (3).

Several studies have shown physiological functions of interleukin (IL)-32, a novel cytokine. However, the role of IL-32 in cancer development has not been reported. In this study, we showed that IL-32γ inhibited tumor growth in IL-32γ-overexpressing transgenic mice inoculated with melanoma as well as colon tumor growth in xenograft nude mice inoculated with IL-32γ-transfected colon cancer cells (SW620). The inhibitory effect of IL-32γ on tumor growth was associated with the inhibition of constitutive activated nuclear transcription factor-κB (NF-κB) and of signal transducer and activator of transcription 3 (STAT3). The expression of antiapoptotic, cell proliferation and tumor-promoting genes (bcl-2, X-chromosome inhibitor of apoptosis protein (IAP), cellular IAP and cellular FADD-like IL-1β-converting enzyme-inhibitory protein, cyclin D), cyclin-dependent kinase 4, cycolooxygenase-2 and inducible nitric oxide synthase was decreased, whereas the expression of apoptotic target genes (caspase-3 and -9, bax) increased. In tumor, spleen and blood, the number of cytotoxic CD8+ T cells and CD57+ natural killer cells and the levels of IL-10 increased, but that of tumor necrosis factor-α (TNF-α), IL-1β and IL-6 decreased. We also found that forced overexpression of IL-32γ inhibited colon cancer cell (SW620 and HCT116) growth accompanied with the inhibition of activated NF-κB and STAT3 in vitro. In addition, when IL-32γ was knocked down by small interfering RNA (siRNA) or neutralized with an anti-IL-32γ antibody, IL-32γ-induced colon cancer cell growth inhibition, the IL-32γ-induced decrease of TNF-α, IL-1 and IL-6 production, and the increase of IL-10 production were abolished. However, siRNA of NF-κB and STAT3 augmented IL-32γ-induced colon cancer cell growth inhibition. These findings indicate significant pathophysiological roles of IL-32γ in cancer development.

The receptor-type protein tyrosine phosphatases (RPTPs) have been linked to signal transduction, cell adhesion, and neurite extension. PTPRT/RPTPq is exclusively expressed in the central nervous system and regulates synapse formation by interacting with cell adhesion molecules and Fyn protein tyrosine kinase. Overexpression of PTPRT in cultured neurons increased the number of excitatory and inhibitory synapses by recruiting neuroligins that interact with PTPRT through their ecto-domains. In contrast, knockdown of PTPRT inhibited synapse formation and withered dendrites. Incubation of cultured neurons with recombinant proteins containing the extracellular region of PTPRT reduced the number of synapses by inhibiting the interaction between ecto-domains. Synapse formation by PTPRT was inhibited by phosphorylation of tyrosine 912 within the membrane-proximal catalytic domain of PTPRT by Fyn. This tyrosine phosphorylation reduced phosphatase activity of PTPRT and reinforced homophilic interactions of PTPRT, thereby preventing the heterophilic interaction between PTPRT and neuroligins. These results suggest that brain-specific PTPRT regulates synapse formation through interaction with cell adhesion molecules, and this function and the phosphatase activity are attenuated through tyrosine phosphorylation by the synaptic tyrosine kinase Fyn.

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Cellular antioxidant enzymes play crucial roles in aerobic organisms by eliminating detrimental oxidants and maintaining the intracellular redox homeostasis. Therefore, the function of antioxidant enzymes is inextricably linked to the redox-dependent activities of multiple proteins and signaling pathways. Here, we report that the VEGFR2 RTK has an oxidation-sensitive cysteine residue whose reduced state is preserved specifically by peroxiredoxin II (PrxII) in vascular endothelial cells. In the absence of PrxII, the cellular H(2)O(2) level is markedly increased and the VEGFR2 becomes inactive, no longer responding to VEGF stimulation. Such VEGFR2 inactivation is due to the formation of intramolecular disulfide linkage between Cys1199 and Cys1206 in the C-terminal tail. Interestingly, the PrxII-mediated VEGFR2 protection is achieved by association of two proteins in the caveolae. Furthermore, PrxII deficiency suppresses tumor angiogenesis in vivo. This study thus demonstrates a physiological function of PrxII as the residential antioxidant safeguard specific to the redox-sensitive VEGFR2.