Genetic responses against nitric oxide toxicity.
The threat of free radical damage is opposed by coordinated responses that modulate expression of sets of gene products. In mammalian cells, 12 proteins are induced by exposure to nitric oxide (NO) levels that are sub-toxic but exceed the level needed to activate guanylate cyclase. Heme oxygenase 1 (HO-1) synthesis increases substantially, due to a 30- to 70-fold increase in the level of HO-1 mRNA. HO-1 induction is cGMP-independent and occurs mainly through increased mRNA stability, which therefore indicates a new NO-signaling pathway. HO-1 induction contributes to dramatically increased NO resistance and, together with the other inducible functions, constitutes an adaptive resistance pathway that also defends against oxidants such as H2O2. In E. coli, an oxidative stress response, the soxRS regulon, is activated by direct exposure of E. coli to NO, or by NO generated in murine macrophages after phagocytosis of the bacteria. This response is governed by the SoxR protein, a homodimeric transcription factor (17-kDa subunits) containing [2Fe-2S] clusters essential for its activity. SoxR responds to superoxide stress through one-electron oxidation of the iron-sulfur centers, but such oxidation is not observed in reactions of NO with SoxR. Instead, NO nitrosylates the iron-sulfur centers of SoxR both in vitro and in intact cells, which yields a form of the protein with maximal transcriptional activity. Although nitrosylated SoxR is very stable in purified form, the spectroscopic signals for the nitrosylated iron-sulfur centers disappear rapidly in vivo, indicating an active process to reverse or eliminate them.
Demple
B
B
Department of Cancer Cell Biology and Division of Biological Sciences Graduate Program, Harvard School of Public Health, Boston, MA 02115, USA. bdemple@hsph.harvard.edu
D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D015152:Blotting, Northern; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance; D004790:Enzyme Induction / Q000187:drug effects; D005347:Fibroblasts / Q000187:drug effects / Q000201:enzymology; D015870:Gene Expression; D006367:HeLa Cells / Q000187:drug effects / Q000201:enzymology; D006419:Heme Oxygenase (Decyclizing) / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D051547:Heme Oxygenase-1; D006801:Humans; D008565:Membrane Proteins; D009046:Motor Neurons / Q000187:drug effects / Q000201:enzymology; D009569:Nitric Oxide / Q000633:toxicity*; D018384:Oxidative Stress; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D013997:Time Factors; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic
D001426:Bacterial Proteins; D008565:Membrane Proteins; D012333:RNA, Messenger; D014157:Transcription Factors; C070724:SoxR protein, Bacteria; D009569:Nitric Oxide; C496541:HMOX1 protein, human; D006419:Heme Oxygenase (Decyclizing); D051547:Heme Oxygenase-1