Stimulation and inhibition of 1,3-bis(2-chloroethyl)-1-nitrosourea-induced strand breaks and interstrand cross-linking in Col E1 plasmid deoxyribonucleic acid by polyamines and inorganic cations

Pharmaceutical-mediated inactivation of p53 sensitizes U87MG glioma cells to BCNU and temozolomide

Pifithrin-alpha (PFTalpha) is a small molecule inhibitor of p53. By reversibly blocking apoptosis in response to DNA damage, PFTalpha protects normal cells from lethal doses of gamma-radiation (Komarov et al., Science, 1999;285:1733-7). We examined the effect of PFTalpha on the chemosensitivity of a human cancer in which cell cycle arrest, not apoptosis, is the principle cellular consequence of p53 activation. This was of interest because E6 silencing of p53 sensitizes U87MG astrocytic glioma cells to BCNU and temozolomide (TMZ), cytotoxic drugs that are modestly helpful in the treatment of aggressive astrocytic gliomas. We observed that exposure of U87MG cells to PFTalpha before cytotoxic chemotherapy attenuated p53-mediated induction of p21WAF1 protein levels, sensitizing U87MG cells to BCNU and TMZ. Sensitization of U87MG cells was associated with G1 arrest, delayed entry into S-phase and decreased repair of DNA damage by BCNU. Our findings suggest that in addition to protecting normal cells from the toxic effects of radiation and chemotherapy, small molecule inhibitors of p53, like PFTalpha, might play a role in clinical oncology by sensitizing certain resistant cancers to cytotoxic chemotherapies.

Human Dkk-1, a gene encoding a Wnt antagonist, responds to DNA damage and its overexpression sensitizes brain tumor cells to apoptosis following alkylation damage of DNA

The human Dkk-1 (hDkk-1) gene, a transcriptional target of the p53 tumor suppressor, encodes a powerful inhibitor of the Wnt signaling pathway and regulates the spatial patterning/morphogenesis of the mammalian central nervous system. We investigated the p53-related functions of the hDkk-1 gene by studying its response to DNA damage and its modulation of apoptosis in human glioma cells. Various chemotherapeutic and other agents that induce DNA adducts and compromise its integrity (1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), cisplatin, H(2)O(2) and UV rays) enhanced the expression of hDkk-1 significantly. The damage-induced increase in hDkk-1 mRNA levels occurred in many human tumor cell lines, irrespective of their p53 gene status. The human glioblastoma cell line, U87MG, which had undetectable hDkk-1 expression, was engineered to express moderate levels of the hDkk protein by stable transfection. The engineered cells did not show any morphological changes, but underwent marked apoptosis after ceramide treatment. Further, the DNA cross-linking drugs BCNU and cisplatin, but not the microtubule poison vincristine, induced significant cell death in U87MG/hDkk cells, and this was accompanied by altered Bcl-2/Bax expression and a reduction in the amount of telomere DNA as visualized by fluorescence in situ hybridization. These results show that hDkk-1 is a pro-apoptotic gene and suggest that it may play important roles in linking the oncogenic Wnt and p53 tumor suppressor pathways.