title: NAD(+) influx through connexin hemichannels prevents poly(ADP-ribose) polymerase-mediated astrocyte death.
abstract: OBJECTIVE
  Cell death induced by excessive activation of poly(ADP-ribose) polymerase (PARP) is inhibited by administration of NAD(+) extracellularly, but its preventive mechanism remains unclear. Here we investigated the involvement of NAD(+) and/or its metabolites, adenosine and nicotinamide, in the rescue of PARP-mediated astrocyte death by NAD(+) and explored the pathway through which intact NAD(+) could enter cells.
  
  
  METHODS
  PARP activation was induced by treatment with N-methyl-N'-nitro-N-nitrosoguanidine, a DNA-alkylating agent. The cellular NAD(+) content was determined by an enzymatic recycling assay, and cell viability was determined by measuring intracellular LDH activity.
  
  
  RESULTS
  NAD(+), but not adenosine and nicotinamide, could restore the cellular NAD(+) levels decreased by PARP activation. Pharmacological inhibition of the uptake of adenosine and nicotinamide had no effect on the prevention of PARP-triggered cell death by NAD(+), suggesting that unmetabolized NAD(+) remaining in the extracellular milieu might prevent PARP-mediated NAD(+) consumption and cell death. The increase in the cellular NAD(+) level caused by NAD(+) administration to PARP-activated cells was significantly inhibited by a connexin hemichannel blocker, carbenoxolone, but not by P2X7 receptor inhibition with selective antagonists and siRNA, or pannexin-selective blockers. Finally, pharmacological blockade of connexin hemichannels with 18β-glycyrrhetinic acid, octanol and carbenoxolone inhibited the NAD(+)-mediated cell rescue of PARP-triggered cell death.
  
  
  CONCLUSIONS
  These findings suggested that intact NAD(+) could get into astrocytes through connexin hemichannels, and that this process should play a key role in NAD(+)-mediated prevention of PARP-triggered astrocyte death.
authors: Department of Environmental Biochemistry, Kyoto Pharmaceutical University, 5 Nakauchi-cho, Misasagi, Kyoto 607-8414, Japan.
authors: Hiroto
authors: H
authors: Okuda
authors: Kentaro
authors: K
authors: Nishida
authors: Youichirou
authors: Y
authors: Higashi
authors: Kazuki
authors: K
authors: Nagasawa
mesh_terms: D000241:Adenosine / Q000378:metabolism; D000818:Animals; D001253:Astrocytes / Q000166:cytology* / Q000378:metabolism; D001692:Biological Transport / Q000187:drug effects; D016923:Cell Death / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D017630:Connexins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008769:Methylnitronitrosoguanidine / Q000378:metabolism; D051379:Mice; D009243:NAD / Q000378:metabolism*; D009536:Niacinamide / Q000378:metabolism; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism*; D058920:Purinergic P2X Receptor Antagonists / Q000494:pharmacology; D058486:Receptors, Purinergic P2X7 / Q000378:metabolism
chemical_list: D017630:Connexins; D058920:Purinergic P2X Receptor Antagonists; D058486:Receptors, Purinergic P2X7; D009243:NAD; D008769:Methylnitronitrosoguanidine; D009536:Niacinamide; D011065:Poly(ADP-ribose) Polymerases; D000241:Adenosine