Apoptosis in gliomas: molecular mechanisms and therapeutic implications.

Understanding apoptosis is often considered a key to understand the genesis of tumors and to devise innovative strategies for their treatment. Similar to other types of cancer, essential pathways regulating apoptosis are also disrupted in malignant gliomas, notably the cell cycle control mechanisms regulated by the p53 and retinoblastoma (RB) proteins and their homologs. Moreover, cultured glioma cells appear not to activate the extrinsic death receptor-dependent apoptotic pathway in response to irradiation or cytotoxic drugs. A preferential expression of antiapoptotic rather than proapoptotic BCL-2 family proteins and high level expression of inhibitor-of-apoptosis proteins (IAP) may be responsible for the failure of glioma cells to activate caspases in response to apoptotic stimuli. Although apoptosis does occur spontaneously in malignant gliomas in vivo, there is little evidence that the current modes of non-surgical treatment, radiotherapy and chemotherapy, mediate their effects via induction of apoptosis, with the possible exception of anaplastic oligodendrogliomas which often show striking tumor regression on neuroimaging. Yet, the induction of apoptosis plays a conceptual role in the majority of novel experimental approaches to malignant glioma which are currently evaluated in cell culture and preclinical rodent models.