DNA-PK
The DNA-dependent protein kinase (DNA-PK) is a
DNA-end activated protein kinase that is required for
efficient repair of DNA double-strand breaks (DSBs)
and for normal resistance to ionizing radiation. DNA-PK is composed of a
DNA-binding subunit, Ku, and a catalytic subunit, DNA-PKcs
(PRKDC). We have previously shown that PRKDC is activated when the enzyme
interacts with the terminal nucleotides of a DSB. These nucleotides are often
damaged when DSBs are introduced by anticancer agents
and could therefore prevent recognition by DNA-PK. To determine whether DNA-PK
could recognize DNA strand breaks generated by agents used in the treatment of
cancer, we damaged plasmid DNA with anticancer drugs and ionizing radiation.
The DNA breaks were tested for the ability to activate purified DNA-PK. The
data indicate that DSBs produced by bleomycin, calicheamicin and two
types of ionizing radiation ((137)Cs gamma rays and
N(7+) ions: high and low linear energy transfer, respectively) activate DNA-PK
to levels matching the kinase activation obtained
with simple restriction endonuclease-induced DSBs. In contrast, the protein-linked DSBs
produced by etoposide and topoisomerase
II failed to bind and activate DNA-PK. Our findings indicate that DNA-PK
recognizes DSBs regardless of chemical complexity but
cannot recognize the protein-linked DSBs produced by etoposide and topoisomerase II.