title: Cellular density and cell type are the key factors in growth inhibition induced by 2,5bis [1-aziridinyl]-1,4 benzoquinone (DZQ).
abstract: BACKGROUND
  Cell density regulates the expression of various antioxidant enzymes in cell culture. The aim of this study was to study the effect of 2,5 bis-[1-aziridinyl]-1,4 benzoquinone (DZQ), an antitumor quinone bioactivated by NQO1, on HeLa and HepG2 cells cultured at various cell densities.
  
  
  METHODS
  Quinone toxicity was determined by a colorimetric growth inhibition assay. NQO1 and catalase activities were measured spectrophotometrically in soluble fractions, and NQO1 polypeptide was quantified by immunostaining with a commercial polyclonal antiserum.
  
  
  RESULTS
  As reported previously, NQO1 activity was much higher in confluent HeLa cells than in sparse cells. However, HepG2 cultures showed an opposite pattern in the regulation of this antioxidant enzyme, sparse cell cultures showing higher NQO1 activity similar to that found in confluent HeLa cells. The expression pattern of catalase activity was similar to that of NQO1 in HeLa cells, but this activity was constant and cell density-independent in HepG2. The growth inhibition effect of DZQ, correlated with NQO1 activity within a given cell type, but HepG2 was always much more sensitive to DZQ than HeLa cells, even under conditions where NQO1 activity was high in HeLa but low in HepG2.
  
  
  CONCLUSIONS
  These results suggest that NQO1 activity is a major factor for DZQ bioactivation, but this enzyme is not likely the sole factor involved in the growth inhibition mediated by DZQ. Since part of the cytotoxic effect of DZQ is mediated by H2O2, other antioxidant enzymes, mainly catalase, could modulate the different growth inhibition found between HeLa and HepG2 cells. In confluent HeLa cells, the higher activity of NQO1 coincides with an increment of catalase activity, thus, reducing the oxidative stress produced by the H2O2 formed.
authors.affiliation: Departamento de Biología Celular, Fisiología e Inmunología, Facultad de Ciencias, Campus Universitario de Rabanales, Edificio Severo Ochoa, Universidad de Córdoba, 14014-Córdoba, Spain.
authors.forename: María del Carmen
authors.initials: Mdel C
authors.lastname: Córdoba-Pedregosa
authors.forename: José M
authors.initials: JM
authors.lastname: Villalba
authors.forename: David
authors.initials: D
authors.lastname: González-Aragón
authors.forename: Rosario I
authors.initials: RI
authors.lastname: Bello
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Alcaín
mesh_terms: D001388:Aziridines / Q000494:pharmacology*; D016227:Benzoquinones / Q000494:pharmacology*; D015153:Blotting, Western; D006528:Carcinoma, Hepatocellular / Q000378:metabolism / Q000473:pathology; D002374:Catalase / Q000378:metabolism; D002452:Cell Count; D049109:Cell Proliferation / Q000187:drug effects*; D003600:Cytosol / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006367:HeLa Cells / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism; D016660:NAD(P)H Dehydrogenase (Quinone) / Q000378:metabolism*; D018384:Oxidative Stress; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism
chemical_list: D001388:Aziridines; D016227:Benzoquinones; C009970:ethylenimine quinone; D006861:Hydrogen Peroxide; D002374:Catalase; D016660:NAD(P)H Dehydrogenase (Quinone); C447479:NQO1 protein, human