Redox regulation of signal transduction in smooth muscle cells: distinct effects of PKC-down regulation and PKC inhibitors on oxidant induced MAP kinase

Distinct roles of Ca2+, calmodulin, and protein kinase C in H2O2-induced activation of ERK1/2, p38 MAPK, and protein kinase B signaling in vascular smooth muscle cells

We have shown earlier that extracellular signal-regulated kinases 1 and 2 (ERK1/2) and protein kinase B (PKB), two key mediators of growth-promoting and proliferative responses, are activated by hydrogen peroxide (H(2)O(2)) in A10 vascular smooth muscle cells (VSMC). In the present studies, using a series of pharmacological inhibitors, we explored the upstream mechanisms responsible for their activation in response to H(2)O(2). H(2)O(2) treatment of VSMC stimulated ERK1/2, p38 mitogen-activated protein kinase (MAPK), and PKB phosphorylation in a dose- and time-dependent fashion. BAPTA-AM and EGTA, chelators of intracellular and extracellular Ca(2+), respectively, inhibited H(2)O(2)-stimulated ERK1/2, p38 MAPK, and PKB phosphorylation. Fluphenazine, an antagonist of the Ca(2+)-binding protein calmodulin, also suppressed the enhanced phosphorylation of ERK1/2, p38 MAPK, and PKB. In contrast, the protein kinase C (PKC) inhibitors Gö 6983 and Rö 31-8220 attenuated H(2)O(2)-induced ERK1/2 phosphorylation, but had no effect on p38 MAPK and PKB phosphorylation. Taken together, these data demonstrate that the activation of Ca(2+)/calmodulin-dependent pathways represents a key component mediating the stimulatory action of H(2)O(2) on ERK1/2, p38 MAPK, and PKB phosphorylation. On the other hand, PKC appears to be an upstream modulator of the increased ERK1/2 phosphorylation, but not of p38 MAPK and PKB in response to H(2)O(2) in VSMC.

Thiol oxidation by 2,2'-dithiodipyridine induced calcium mobilization in MG63 human osteosarcoma cells

2,2'-dithiodipyridine (2,2'-DTDP), a reactive disulphide that mobilizes Ca(2+) in muscle, induced an increase in cytoplasmic free Ca(2+)concentrations ([Ca(2+)](i)) in MG63 human osteosarcoma cells loaded with the Ca(2+)-sensitive dye fura-2. 2,2'-DTDP acted in a concentration-independent manner with an EC(50) of 50 microM. The Ca(2+) signal comprised an initial spike and a prolonged increase. Removing extracellular Ca(2+) did not alter the Ca(2+) signal, suggesting that the Ca(2+) signal was due to store Ca(2+) release. In Ca(2+)-free medium, the 2,2'-DTDP-induced [Ca(2+)](i) increase was not changed by depleting store Ca(2+) with 50 microM bredfeldin A (a Golgi apparatus permeabilizer), 2 microM carbonylcyanide m-chlorophenylhydrazone (CCCP, a mitochondrial uncoupler), 1 microM thapsigargin (an endoplasmic reticulum Ca(2+)pump inhibitor) or 5 microM ryanodine. Conversely, 2,2'-DTDP pretreatment abolished CCCP and thapsigargin-induced [Ca(2+)](i) increases. 2,2'-DTDP-induced Ca(2+) signals in Ca(2+)-containing medium were not affected by modulation of protein kinase C activity or suppression of phospholipase C activity. However, 2,2'-DTDP-induced Ca(2+) release was inhibited by a thiol-selective reducing reagent, dithiothreitol (5-25 microM) in a concentration-dependent manner. Collectively, this study shows that 2,2'-DTDP induced [Ca(2+)](i) increases in human osteosarcoma cells via releasing store Ca(2+)from multiple stores in a manner independent of protein kinase C or phospholipase C activity. The 2,2'-DTDP-induced store Ca(2+) release appeared to be dependent on oxidation of membranes.

Key role of PKC and Ca2+ in EGF protection of microtubules and intestinal barrier against oxidants

Using monolayers of human intestinal (Caco-2) cells, we showed that growth factors (GFs) protect microtubules and barrier integrity against oxidative injury. Studies in nongastrointestinal cell models suggest that protein kinase C (PKC) signaling is key in GF-induced effects and that cytosolic calcium concentration ([Ca2+](i)) is essential in cell integrity. We hypothesized that GF protection involves activating PKC and maintaining normal ([Ca2+](i)) Monolayers were pretreated with epidermal growth factor (EGF) or PKC or Ca2+ modulators before exposure to oxidants (H2O2 or HOCl). Oxidants disrupted microtubules and barrier integrity, and EGF protected from this damage. EGF caused rapid distribution of PKC-alpha, PKC-betaI, and PKC-zeta isoforms to cell membranes, enhancing PKC activity of membrane fractions while reducing PKC activity of cytosolic fractions. EGF enhanced (45)Ca2+ efflux and prevented oxidant-induced (sustained) rises in ([Ca2+](i)). PKC inhibitors abolished and PKC activators mimicked EGF protection. Oxidant damage was mimicked by and potentiated by a Ca2+ ionophore (A-23187), exacerbated by high-Ca2+ media, and prevented by calcium removal or chelation or by Ca2+ channel antagonists. PKC activators mimicked EGF on both (45)Ca2+ efflux and ([Ca2+](i)). Membrane Ca2+-ATPase pump inhibitors prevented protection by EGF or PKC activators. In conclusion, EGF protection of microtubules and the intestinal epithelial barrier requires activation of PKC signal transduction and normalization of ([Ca2+](i)).

Reactive oxygen species regulate signaling pathways induced by M1 muscarinic receptors in PC12M1 cells

Activation of the m1 muscarinic receptor subtype in rat pheochromocytoma (PC12) cells stably expressing cloned m1 muscarinic acetylcholine receptors was previously shown to induce morphological changes and growth arrest. However, the signaling pathways which lead to these effects were not identified. In an attempt to characterize the intracellular signaling that might be involved in the muscarinic-induced effects, we investigated the role of reactive oxygen species in the regulation of these processes. Stimulation of the muscarinic receptor in these cells increased the intracellular concentrations of reactive oxygen species. Muscarinic activation induced intracellular signaling pathways that involve activation of Ras, extracellular signal-regulated kinase (ERK), and p38. These pathways were partially blocked when reactive oxygen species (ROS) production was prevented by the antioxidant N-acetylcysteine. Other muscarinic-induced signals, such as activation of c-Jun NH(2)-terminal kinase (JNK) or an increase in the binding activity of the transcription factors nuclear factor-kappa B and activator protein-1, were inhibited by the antioxidant dicoumarol. N-Acetylcysteine also blocked the growth arrest and changes in cell shape induced by stimulation of the muscarinic receptor in PC12M1 cells. These findings suggest that ROS act as second messengers in muscarinic-induced cellular signaling. Moreover, generation of ROS appears to be an early and critical intermediary event, which occurs immediately after stimulation of the muscarinic receptor and affects in a variety of mechanisms the muscarinic-mediated cellular signaling.

Pulsatile stretch-induced extracellular signal-regulated kinase 1/2 activation in organ culture of rabbit aorta involves reactive oxygen species

Increased steady intraluminal pressure in blood vessels activates the extracellular signal-regulated kinase (ERK)1/2 pathway. However, signal transduction of pulsatile stretch has not been elucidated. Using an organ culture model of rabbit aorta, we studied ERK1/2 activation by pulsatility in vessels maintained at 80 mm Hg for 24 hours. ERK1/2 activity was evaluated by in-gel kinase assays and by Western blot. Compared with control aortas without pulsatility, aortas submitted to a pulsatile 10% variation in vessel diameter displayed a significant increase in ERK1/2 activity (207+/-12%, P<0.001), which remained high after removal of the endothelium. Unlike steady overstretch, pulsatile stretch-induced activation of ERK1/2 was not modified by herbimycin A, a Src family tyrosine kinase inhibitor, but was reduced by other tyrosine kinase inhibitors, tyrphostin A48 and genistein (162+/-27% and 144+/-14%, respectively). Conversely, ERK1/2 activity was markedly decreased in pulsatile vessels treated with staurosporine (114+/-18%) although neither of the more specific protein kinase C inhibitors, Ro-31-8220 or Gö-6976, blocked ERK1/2 activation (209+/-24% and 238+/-34%, respectively), whereas staurosporine had no effect on steady overstretch-induced ERK1/2 activation. Pulsatility induced superoxide anion generation, which was prevented by the NADPH oxidase inhibitor diphenyleneiodonium. Furthermore, polyethylene glycol-superoxide dismutase completely abolished ERK1/2 activation by pulsatility (114+/-12%). Finally, ERK1/2 and O(2)(-) levels in freshly isolated vessels were equivalent to the levels found in pulsatile vessels. In conclusion, pulsatile stretch activates ERK1/2 in the arterial wall via pathways different from those induced by steady overstretch. Pulsatility might be considered a physiological stimulus that maintains a certain degree of ERK1/2 activation via oxygen-derived free radical production.

Hydrogen peroxide inhibits the immune response to lipopolysaccharide by attenuating signaling through c-Jun N-terminal kinase and p38 associated with protein kinase C

This study examined the immunomodulatory effects of hydrogen peroxide (H(2)O(2)) in B6C3F1 mouse splenic lymphocytes. H(2)O(2) produced a marked and dose-related inhibition of both lipopolysaccharide (LPS)-induced B-cell proliferation and concanavalin A (Con A)-induced T-cell proliferation. Unexpectedly, little effect was observed with H(2)O(2) on the antibody-forming cell (AFC) response to the polyclonal B-cell activator, LPS. It was also observed that H(2)O(2) did not have any detectable effect on forskolin-stimulated adenylate cyclase, indicating that cyclic AMP (cAMP) is not a mediator of H(2)O(2)-induced suppression of the immune response. Rather, LPS-induced activation of protein kinase C (PKC) was completely inhibited when cells were pretreated with H(2)O(2) for 18 h, although PKC activity was increased approximately twofold following treatment with H(2)O(2) for 10 min. In addition, H(2)O(2) pretreatment blocked the phosphorylation of two stress-activated mitogen-activated protein kinases (MAPKs), c-Jun N-terminal kinase (JNK) and p38 by LPS in a concentration-dependent fashion. Therefore, these data suggest that H(2)O(2) suppresses immune response through the desensitization of PKC, which subsequently results in inhibition of JNK and p38.

Role of redox potential and reactive oxygen species in stress signaling

Stress-activated signaling cascades are affected by altered redox potential. Key contributors to altered redox potential are reactive oxygen species (ROS) which are formed, in most cases, by exogenous genotoxic agents including irradiation, inflammatory cytokines and chemical carcinogens. ROS and altered redox potential can be considered as the primary intracellular changes which regulate protein kinases, thereby serving as an important cellular component linking external stimuli with signal transduction in stress response. The mechanisms, which underlie the ROS-mediated response, involve direct alteration of kinases and transcription factors, and indirect modulation of cysteine-rich redox-sensitive proteins exemplified by thioredoxin and glutathione S-transferase. This review summarizes the current understanding of the mechanisms contributing to ROS-related changes in key stress activated signaling cascades.