Abstract-Carbon monoxide (CO), a reaction product of the cytoprotective heme oxygenase (HO)-1, is antiapoptotic in a variety of models of cellular injury, but the precise mechanisms remain to be established. In human umbilical vein endothelial cells, exogenous CO activated Nrf2 through the phosphorylation of protein kinase R-like endoplasmic reticulum kinase (PERK), resulting in HO-1 expression. CO-induced activation of PERK was followed by the phosphorylation of eukaryotic translation initiation factor 2␣ and the expression of activating transcription factor 4. However, CO fails to induce X-box binding protein-1 expression and activating transcription factor 6 cleavage. CO had no significant effect on synthesis of endoplasmic reticulum (ER) chaperone proteins such as the 78-kDa glucoseregulated proteins 78 and 94. Instead, CO prevented X-box binding protein 1 expression and activating transcription factor 6 cleavage induced by ER-stress inducers such as thapsigargin, tunicamycin and homocysteine. CO also prevented endothelial apoptosis triggered by these ER inducers through suppression of C/EBP homologous protein expression, which was associated with its activation of p38 mitogen-activated protein kinase. Similarly, endogenous CO produced from endothelial HO-1 induced by either exogenous CO or a pharmacological inducer was also cytoprotective against ER stress through C/EBP homologous protein suppression. Our findings suggest that CO renders endothelial cells resistant to ER stress not only by downregulating C/EBP homologous protein expression via p38 mitogen-activated protein kinase activation but also by upregulating Nrf2-dependent HO-1 expression via PERK activation. Thus, the HO-1/CO system might be potential therapeutics in vascular diseases associated with ER stress.

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Sphingosine 1-phosphate (S1P) can prevent endothelial cell apoptosis. We investigated the molecular mechanisms and signaling pathways by which S1P protects endothelial cells from serum deprivation-induced apoptosis. We show here that human umbilical vein endothelial cells (HUVECs) undergo apoptosis associated with increased DEVDase activity, caspase-3 activation, cytochrome c release, and DNA fragmentation after 24 h of serum deprivation. These apoptotic markers were suppressed by the addition of S1P, the NO donor Snitroso-N-acetylpenicillamine (100 M), or caspase-3 inhibitor z-VAD-fmk. The protective effects of S1P were reversed by the nitric-oxide synthase (NOS) inhibitor N-monomethyl-L-arginine, but not by the soluble guanylyl cyclase inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]-quanoxaline-1-one, suggesting that NO, but not cGMP, is responsible for S1P protection from apoptosis. Furthermore, S1P increased NO production by enhancing Ca 2؉ -sensitive NOS activity without changes in the eNOS protein level. S1P-mediated cell survival and NO production were suppressed significantly by pretreatment with antisense oligonucleotide of EDG-1 and partially by EDG-3 antisense. S1P-mediated NO production was suppressed by the addition of pertussis toxin, an inhibitor of G i proteins, the specific inhibitor of phospholipase C (PLC), U73122, and the Ca 2؉ chelator BAPTA-AM. These findings indicate that S1P protects HUVECs from apoptosis through the activation of eNOS activity mainly through an EDG-1 and -3/G i /PLC/Ca 2؉ signaling pathway.Sphingosine 1-phosphate (S1P), 1 generated from sphingosine by sphingosine kinase, is a recently described lipid mediator regulating cellular responses, including cell proliferation (1), cell motility (2, 3), and morphological changes (2, 4) in endothelial cells. In addition, S1P was also demonstrated to protect endothelial cells from apoptosis induced by C 2 -ceramide, TNF␣, and anti-Fas antibody (1, 4, 5). All these effects of S1P on endothelial functions are thought to correlate with the intracellular signaling pathway linked to the EDG-1 (endothelial differentiation gene-1), EDG-3, and EDG-5 subtypes of G protein-coupled receptors (1-4, 6). Of these receptors, EDG-1 is exclusively expressed in HUVECs (2, 4), and S1P induces a robust calcium response mainly via the G i -coupled S1P receptors EDG-1 and -3 in HUVECs (4). Consistent with this view, we have recently observed that intracellular Ca 2ϩ mobilization evoked by the G i -PLC system is responsible for the signaling pathway of mitogen-activated protein kinases, focal adhesion kinase (p125 FAK ), and chemotaxis in response to S1P in HUVECs (6). It is likely that intracellular signaling events of S1P in endothelial cells may be directly associated with increase in [Ca 2ϩ ] i . Endogenous nitric oxide (NO) is synthesized from L-arginine by catalytic reaction of three isotypes of NO synthases (NOS), the neuronal or type I isoform (nNOS), the inducible or type II isoform (iNOS), and the endothelial or type III isoform (eNOS) (7,8). Endoth...

Nitric oxide (NO) functions not only as an important signaling molecule in the brain by producing cGMP, but also regulates neuronal cell apoptosis. The mechanism by which NO regulates apoptosis is unclear. In this study, we demonstrated that NO, produced either from the NO donor S-nitroso-N-acetyl-d,l-penicillamine (SNAP) or by transfection of neuronal NO synthase, suppressed 6-hydroxydopamine (6-OHDA)-induced apoptosis in PC12 cells by inhibiting mitochondrial cytochrome c release, caspase-3 and -9 activation, and DNA fragmentation. This protection was significantly reversed by the soluble guanylyl cyclase inhibitor 1H-(1,2,4)-oxadiazole[4,3-a]quinoxalon-1-one, indicating that cGMP is a key mediator in NO-mediated anti-apoptosis. Moreover, the membrane-permeable cGMP analog 8-Br-cGMP inhibited 6-OHDA-induced apoptosis. These anti-apoptotic effects of SNAP and 8-Br-cGMP were suppressed by cGMP-dependent protein kinase G (PKG) inhibitor KT5823, indicating that PKG is a downstream signal mediator in the suppression of apoptosis by NO and cGMP. Both SNAP and 8-Br-cGMP induced endogenous Akt activation and Bad phosphorylation, resulting in the inhibition of Bad translocation to mitochondria; these effects were inhibited by KT5823 and the phosphatidylinositol 3-kinase (PI3K) inhibitors LY294002 and Wortmannin. Our data suggest that the NO/cGMP pathway suppresses 6-OHDA-induced PC12 cell apoptosis by suppressing the mitochondrial apoptosis signal via PKG/PI3K/Akt-dependent Bad phosphorylation.