Apoptosis triggered by DNA damage O6-methylguanine in human lymphocytes requires DNA replication and is mediated by p53 and Fas/CD95/Apo-1

Various tumor-therapeutic drugs and environmental carcinogens alkylate DNA inducing O6-methylguanine (O6MeG) that provokes cell death by apoptosis. In rodent fibroblasts, apoptosis triggered by O6MeG is executed via the mitochondrial damage pathway. Conversion of O6MeG into critical downstream lesions requires mismatch repair (MMR). This is thought to signal apoptosis upon binding to O6MeG lesions mispaired with thymine. Alternatively, O6MeG lesions might be processed by MMR giving rise to DNA double-strand breaks (DSBs) during replication that finally provoke apoptosis. To test this, we examined apoptosis triggered by O6MeG in human peripheral lymphocytes in which O6-methylguanine-DNA methyltransferase (MGMT) had been inactivated by O6-benzylguanine (O6BG) and which were not proliferating or proliferating upon CD3/CD28 stimulation. Treatment with N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) or the anticancer drug temozolomide induced apoptosis only in proliferating, but not resting cells. With exceptional high alkylation doses (≥15 μM of MNNG), apoptosis was also observed in resting lymphocytes, albeit at a lower level than in proliferating cells. This response was not affected by O6BG, suggesting that replication-independent apoptosis at high dose levels is caused by lesions other than O6MeG. O6MeG-triggered apoptosis in proliferating lymphocytes was preceded by a wave of DSBs, which coincided with p53 and Fas receptor upregulation, while Fas ligand, Bax and Bcl-2 expression was not altered. Treatment with anti-Fas neutralizing antibody attenuated MNNG-induced apoptosis in MGMT-depleted proliferating lymphocytes. The data suggest that O6MeG is converted by MMR and DNA replication into DSBs that trigger apoptosis by p53 stabilization and Fas/CD95/Apo-1 upregulation. This is supported by the finding that ionizing radiation, inducing DSBs on its own, provokes apoptosis in lymphocytes in a replication-independent way. The strict proliferation dependence of apoptosis triggered by O6MeG may explain the specific killing response of MGMT-deficient proliferating cells, including tumors, to O6MeG generating anticancer drugs and suggests that tumor proliferation rate, Fas responsiveness, MGMT and MMR status are important prognosis parameters.