Nucleolar disruption and apoptosis are distinct neuronal responses to etoposide-induced DNA damage

Although DNA damaging topoisomerase inhibitors induce apoptosis in developing neurons, their effects on adult neurons have not yet been characterized. We report a blockage of RNA-Polymerase-1-driven transcription and nucleolar stress in neocortical neurons of adult rats after intracarotid injection of the DNA-topoisomerase-2 inhibitor, etoposide. Intracerebroventricular injection of etoposide induced a similar response in neonatal rats. In contrast, etoposide triggered neuronal apoptosis in the neonates, but not the adults. Nucleolar disruption and apoptosis were also observed in etoposide-challenged cultured cortical neurons from newborn rats. In that system, activation of the DNA double strand break signaling kinase ataxia telangiectasia-mutated protein kinase, p53 and p53-dependent apoptosis required lower etoposide concentrations than did the p53-independent induction of nucleolar stress. These distinct responses may be coupled to different forms of etoposide-induced DNA damage. Indeed, double strand breaks by the over-expressed endonuclease I-Ppo1 were sufficient to induce p53-dependent apoptosis. Moreover, nucleolar transcription was insensitive to such damage implying single strand breaks and/or topoisomerase-2-DNA adducts as triggers of nucleolar stress. Because nucleolar stress is not age-restricted, it may underlie non-apoptotic neurotoxicity of chemotherapy- or neurodegeneration-associated DNA damage by reducing ribosomal biogenesis in adult brain. Conversely, nucleolar insensitivity to double strand breaks likely contributes to mature neuron tolerance of such lesions.