Although mutations and deletions in the p53 tumor suppressor gene lead to resistance to low linear
energy transfer (LET) radiation, high-LET radiation efficiently induces cell lethality and apoptosis regardless
of the p53 gene status in cancer cells. Recently, it has been suggested that the induction of p53-independent
apoptosis takes place through the activation of Caspase-9 which results in the cleavage of
Caspase-3 and poly (ADP-ribose) polymerase (PARP). This study was designed to examine if high-LET
radiation depresses serine/threonine protein kinase B (PKB, also known as Akt) and Akt-related proteins.
Human gingival cancer cells (Ca9–22 cells) harboring a mutated p53 (mp53) gene were irradiated with
2 Gy of X-rays or Fe-ion beams. The cellular contents of Akt-related proteins participating in cell survival
signaling were analyzed with Western Blotting 1, 2, 3 and 6 h after irradiation. Cell cycle distributions
after irradiation were assayed with flow cytometric analysis. Akt-related protein levels decreased when
cells were irradiated with high-LET radiation. High-LET radiation increased G2/M phase arrests and suppressed
the progression of the cell cycle much more efficiently when compared to low-LET radiation.
These results suggest that high-LET radiation enhances apoptosis through the activation of Caspase-3
and Caspase-9, and suppresses cell growth by suppressing Akt-related signaling, even in mp53 bearing
cancer cells.