Smad7 Foci Are Present in Micronuclei Induced by Heavy Particle Radiation

Mutat Res. 2013 Aug 30;756(1-2):108-14. doi: 10.1016/j.mrgentox.2013.04.011. Epub 2013 Apr 30.


DNA damage and reactive oxygen species (ROS) generated by ionizing radiation (IR) activate DNA damage response (DDR) and cytokine signaling pathways, including double strand break (DSB) repair and TGFβ/Smad signaling pathway. Proteins assembled at IR-induced DSB sites can be visualized as foci, including γH2AX, 53BP1, ATM and ATF2. Unrepaired DSBs are thought to be one origin of micronuclei (MN), an indicator of genotoxic stress and chromosomal instability. Studies have detected γH2AX in IR-induced MN, indicating the presence of DSB in MN. Previously we reported that TGFβ downstream proteins Smad7 and phospho-Smad2 (pSmad2) co-localized with DDR proteins following radiation. Here we studied the status of Smad7 and pSmad2 in MN post high linear energy transfer (LET) radiation in human normal and cancerous cells. We observed γH2AX foci in IR-induced MN, whereas 53BP1 and ATF2 were absent. Interestingly, Smad7 foci, but not pSmad2, were detectable in both spontaneous and IR-induced MN. We compared the effect of particle track structures on the yield of MN using 5.6MeV/u boron (B) and 600MeV/u iron (Fe) particles with similar LET (200 and 180keV/μm, respectively) in human fibroblasts. The frequency of MN induced by B was lower than that by Fe particles, albeit the proportion of Smad7-positive to Smad7-negative MN remained constant. An increased frequency of spontaneous MN, with slightly higher ratio of Smad7 or γH2AX positive, was found in human prostate cancer cells (PC3) compared to normal cells. 24h after 1Gy of Fe particles exposure, the yield of MN increased, and the majority (∼70%) carried γH2AX and Smad7. Phospho-ATM (Ser1981) foci were found in both spontaneous and IR-induced MN in PC3 cells, displaying a much lower frequency compared to γH2AX and Smad7. Our data suggest a unique role of Smad7 in IR-induced MN formation, which may associate with DNA repair, apoptosis and genomic instability.

Keywords: ATM; BNL; Brookhaven National Laboratory; DDR; DNA damage response; DNA repair; DSB; HZE; High LET radiation; IR; IR-induced foci; IRIF; LET; MN; Micronuclei; NASA space radiation laboratory; NSRL; ROS; Smad7; TGFβ; TGFβ/Smad pathway; ataxia telangiectasia mutated; double strand break; high charge and energy; ionizing radiation; linear energy transfer; micronuclei; pSmad2; phospho-Smad2; reactive oxygen species; transforming growth factor beta.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Activating Transcription Factor 2 / metabolism
  • Adult
  • Ataxia Telangiectasia Mutated Proteins
  • Cell Cycle Proteins / metabolism
  • Cell Nucleus / genetics*
  • Cell Nucleus / radiation effects
  • Cells, Cultured
  • Cesium Radioisotopes
  • DNA-Binding Proteins / metabolism
  • Esophagus / cytology
  • Esophagus / metabolism
  • Esophagus / radiation effects*
  • Fibroblasts / cytology
  • Fibroblasts / metabolism
  • Fibroblasts / radiation effects*
  • Fluorescent Antibody Technique
  • Gamma Rays*
  • Humans
  • Intracellular Signaling Peptides and Proteins / metabolism
  • Linear Energy Transfer
  • Male
  • Micronucleus Tests
  • Phosphorylation
  • Prostatic Neoplasms / metabolism
  • Prostatic Neoplasms / pathology*
  • Protein-Serine-Threonine Kinases / metabolism
  • Skin / cytology
  • Skin / metabolism
  • Skin / radiation effects*
  • Smad2 Protein / metabolism
  • Smad7 Protein / metabolism*
  • Transforming Growth Factor beta / metabolism
  • Tumor Suppressor Proteins / metabolism
  • Tumor Suppressor p53-Binding Protein 1


  • ATF2 protein, human
  • Activating Transcription Factor 2
  • Cell Cycle Proteins
  • Cesium Radioisotopes
  • DNA-Binding Proteins
  • Intracellular Signaling Peptides and Proteins
  • SMAD2 protein, human
  • SMAD7 protein, human
  • Smad2 Protein
  • Smad7 Protein
  • TP53BP1 protein, human
  • Transforming Growth Factor beta
  • Tumor Suppressor Proteins
  • Tumor Suppressor p53-Binding Protein 1
  • ATM protein, human
  • Ataxia Telangiectasia Mutated Proteins
  • Protein-Serine-Threonine Kinases