EXPRESSION OF POLY (ADP-RIBOSE) POLYMERASE AND P53 IN CULTURED MAMMALIAN CELLS EXPOSED TO ACCELERATED HEAVY IONS (IRON OR ARGON)

Background – The possibility of using high-energy accelerated heavy-ion beams in radiotherapy of malignant tumors is an exciting development. Also, heavy ions comprise an important component of cosmic rays. The DNA molecule should be an important candidate target in heavy ion-induced cell lethality. Poly (ADP-ribose) polymerase (PARP) and tumor suppressor p53 are two important nuclear proteins that recognize heavy ions as a DNA damage signal. We studied the expression of PARP and p53 in cultured mammalian cells exposed to accelerated heavy ions (Fe or Ar). Methods – Chinese hamster ovary cells (CHO) and Mongolian gerbil fetal (MGF) cells were exposed to iron (Fe) or argon (Ar) ions, which are heavy ions accelerated by a ring cyclotron in Riken, Japan. We studied PARP and p53 expression as DNA strand-break recognition signal markers by immunohistochemistry assay in exposed cells to heavy ions and control cells. Results – The fraction of cells showing an elevated level of PARP in their nuclei compared to control cells after 1.5 hours of exposure to 4 Gy heavy ions was about 80%. The difference between PARP expression in CHO and MGF cells was negligible. In the immunohistochemistry assay for p53, which was performed in the Mongolian gerbil fetal cell line only, about 80% of cells showed an elevated level of p53 within the nucleus after 4 hours of exposure to 4 Gy heavy ions. However, the increase in PARP expression occurred earlier than p53 in nuclei of Mongolian gerbil fetal cells after exposure to heavy ions. About 20% of cells did not have increased PARP and p53 expression after exposure to either Fe or Ar ions. This 20% might represent a population of cells that did not receive nuclear DNA damage. Conclusion – These findings suggest that the probabilities of DNA damage after exposure to heavy ions could be evaluated based on PARP or p53 expression. The variation in expression can be related to the traversal of heavy ions through the cell nucleus and induce DNA strand-break as well as key structural components during apoptosis.