title: DNA double-strand break repair, DNA-PK, and DNA ligases in two human squamous carcinoma cell lines with different radiosensitivity.
abstract: OBJECTIVE
  Variation in sensitivity to radiotherapy among tumors has been related to the capacity of cells to repair radiation-induced DNA double-strand breaks (DSBs). DNA-dependent protein kinase (DNA-PK) and DNA ligases may affect DNA dsb rejoining. This study was performed to compare rate of rejoining of radiation-induced DSBs, DNA-PK, and DNA ligase activities in two human squamous carcinoma cell lines with different sensitivity to ionizing radiation.
  
  
  METHODS
  Cell survival of two human squamous carcinoma cell lines, UM-SCC-1 and UM-SCC-14A, was determined by an in vitro clonogenic assay. DSB rejoining was studied using pulsed field gel electrophoresis (PFGE). DNA-PK activity was determined using BIOTRAK DNA-PK enzyme assay system (Amersham). DNA ligase activity in crude cell extracts was measured using [5'-33P] Poly (dA) x (oligo (dT) as a substrate. Proteolytic degradation of proteins was analyzed by means of Western blotting.
  
  
  RESULTS
  Applying the commonly used linear-quadratic equation to describe cell survival, S = e-alphaD-betaD2, the two cell lines roughly have the same alpha value (approximately 0.40 Gy(-1)) whereas the beta value was considerably higher in UM-SCC-14A (0.067 Gy(-2)+/-0.007 Gy(-2) [SEM]) as compared to UM-SCC-1 (0.013 Gy(-2)+/-0.004 Gy(-2) [SEM]). Furthermore, UM-SCC-1 was more proficient in rejoining of X-ray-induced DSBs as compared to UM-SCC-14A as quantified by PFGE. The constitutive level of DNA-PK activity was 1.6 times higher in UM-SCC-1 as compared to UM-SCC-14A ( < 0.05). The constitutive level of DNA ligase activity was similar in the two cell lines.
  
  
  CONCLUSIONS
  The results suggest that the proficiency in rejoining of DSBs is associated with DNA-PK activity but not with total DNA ligase activity.
authors.affiliation: Biomedicine Unit, Swedish Radiation Protection Institute, Stockholm.
authors.forename: A G
authors.initials: AG
authors.lastname: Polischouk
authors.forename: B
authors.initials: B
authors.lastname: Cedervall
authors.forename: S
authors.initials: S
authors.lastname: Ljungquist
authors.forename: J
authors.initials: J
authors.lastname: Flygare
authors.forename: D
authors.initials: D
authors.lastname: Hellgren
authors.forename: R
authors.initials: R
authors.lastname: Grénman
authors.forename: R
authors.initials: R
authors.lastname: Lewensohn
mesh_terms: D002294:Carcinoma, Squamous Cell / Q000235:genetics*; D002470:Cell Survival / Q000235:genetics; D004247:DNA / Q000235:genetics* / Q000528:radiation effects; D004249:DNA Damage; D011088:DNA Ligases / Q000502:physiology* / Q000528:radiation effects; D004260:DNA Repair; D051747:DNA-Activated Protein Kinase; D004268:DNA-Binding Proteins; D006801:Humans; D009687:Nuclear Proteins; D017346:Protein Serine-Threonine Kinases / Q000502:physiology* / Q000528:radiation effects; D011829:Radiation Dosage; D011836:Radiation Tolerance; D016196:S Phase; D014407:Tumor Cells, Cultured / Q000528:radiation effects