title: Different ATM Signaling in Response to Chromium(VI) Metabolism via Ascorbate and Nonascorbate Reduction: Implications for in Vitro Models and Toxicogenomics.
abstract: BACKGROUND
  Carcinogenic hexavalent chromium [Cr(VI)] requires cellular reduction to generate DNA damage. Metabolism of Cr(VI) by its principal reducer ascorbate (Asc) lacks a Cr(V) intermediate, which is abundant in reactions with a minor reducing agent, glutathione. Cultured cells are widely used in mechanistic studies of Cr(VI) toxicity; however, they typically contain < 1% of normal Asc levels. Asc deficiency is also expected to diminish protection against reactive oxygen species.
  
  
  OBJECTIVE
  We assessed how the presence of Asc in cells affects their stress signaling and survival responses to chromate.
  
  
  METHODS
  We investigated the effects of Asc restoration in human lung H460 cells and normal human lung fibroblasts on the activation and functional role of ATM kinase, which controls DNA damage responses involving several hundreds of proteins.
  
  
  RESULTS
  Treatment of standard cultures with Cr(VI) strongly activated ATM, as indicated by its automodification at Ser1981 and by phosphorylation of checkpoint kinase 2 (CHK2) and chromatin/transcription regulator KRAB-associated protein 1 (KAP1). Confirming the importance of activated ATM, its inhibition impaired replication recovery and clonogenic survival. In contrast, fully Asc-restored cells lacked ATM activation by Cr(VI), and ATM silencing produced no significant effects on p53 stabilization, apoptosis, replication recovery, or clonogenic survival. Dose dependence studies found a close correlation between ATM activation and the extent of Cr(VI) reduction by glutathione.
  
  
  CONCLUSIONS
  Asc restoration in cultured cells dramatically altered their stress responses to Cr(VI) by preventing activation of the oxidant-sensitive ATM network. We suggest that toxicogenomic and other cell response-based approaches likely underestimate Cr(VI) genotoxicity when standard ATM-activating carcinogens are used as references.
  
  
  BACKGROUND
  Luczak MW, Green SE, Zhitkovich A. 2016. Different ATM signaling in response to chromium(VI) metabolism via ascorbate and nonascorbate reduction: implications for in vitro models and toxicogenomics. Environ Health Perspect 124:61-66; http://dx.doi.org/10.1289/ehp.1409434.
authors.affiliation: Department of Pathology and Laboratory Medicine, Brown University, Providence, Rhode Island, USA.
authors.forename: Michal W
authors.initials: MW
authors.lastname: Luczak
authors.forename: Samantha E
authors.initials: SE
authors.lastname: Green
authors.forename: Anatoly
authors.initials: A
authors.lastname: Zhitkovich
mesh_terms: D001205:Ascorbic Acid / Q000378:metabolism*; D064007:Ataxia Telangiectasia Mutated Proteins / Q000378:metabolism*; D002460:Cell Line; D064447:Checkpoint Kinase 2 / Q000378:metabolism; D002857:Chromium / Q000494:pharmacology*; D006801:Humans; D010084:Oxidation-Reduction / Q000187:drug effects; D010766:Phosphorylation / Q000187:drug effects; D015398:Signal Transduction / Q000187:drug effects; D043922:Toxicogenetics / Q000379:methods*
chemical_list: D002857:Chromium; C074702:chromium hexavalent ion; D064447:Checkpoint Kinase 2; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; D001205:Ascorbic Acid