Glioblastoma multiforme is the most severe form of
brain cancer. First line
therapy includes the methylating agent
temozolomide and/or the chloroethylating nitrosoureas [1-(2-chloroethyl)-1-nitrosourea;
CNU]
nimustine [1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea;
ACNU],
carmustine [1,3-bis(2-chloroethyl)-1-nitrosourea;
BCNU], or
lomustine [1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea;
CCNU]. The mechanism of cell death after
CNU treatment is largely unknown. Here we show that
ACNU and
BCNU induce apoptosis in U87MG [p53 wild-type (p53wt)] and U138MG [p53 mutant (p53mt)]
glioma cells. However, contrary to what we observed previously for
temozolomide, chloroethylating drugs are more toxic for p53-mutated
glioma cells and induce both apoptosis and
necrosis. Inactivation of p53 by
pifithrin-alpha or
siRNA down-regulation sensitized p53wt but not p53mt
glioma cells to
ACNU and
BCNU.
ACNU and
BCNU provoke the formation of
DNA double-strand breaks (
DSB) in
glioma cells that precede the onset of apoptosis and
necrosis. Although these DSBs are repaired in p53wt cells, they accumulate in p53mt cells. Therefore, functional p53 seems to stimulate the repair of
CNU-induced cross-links and/or DSBs generated from
CNU-induced lesions. Expression analysis revealed an up-regulation of xpc and ddb2
mRNA in response to
ACNU in U87MG but not U138MG cells, indicating p53 regulates a pathway that involves these DNA repair
proteins.
ACNU-induced apoptosis in p53wt
glioma cells is executed via both the extrinsic and intrinsic apoptotic pathway, whereas in p53mt
glioma cells, the mitochondrial pathway becomes activated. The data suggest that p53 has opposing effects in
gliomas treated with methylating or chloroethylating agents and, therefore, the p53 status should be taken into account when deciding which therapeutic
drug to use.