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Since caspase members have been identified as effectors of apoptosis, the role of CPP32/caspase-3 was further explored in cultured neurons from the embryonic rat forebrain submitted to a 6-h hypoxia which has previously been shown to induce apoptotic death within four days after reoxygenation, whereas a shorter aggression (i.e., for 3 h) leads by the same time to an increased number of living neurons, suggesting that sublethal hypoxia may promote neurogenesis. Neuronal expression of the active cleavage product of CPP32 (CPP32 p20) increased specifically after hypoxia for 6 h to finally reach 985% over control normoxic values at 96 h post-insult, while a 3-h hypoxia triggered the inducible stress protein HSP70 that has been shown to inhibit caspase-3. Proteolytic activity of caspase-3 was progressively stimulated by lethal hypoxia, as reflected by the degradation of two selective substrates, including poly (ADP-ribose) polymerase (PARP). Caspase-3 activity was blocked specifically and dose-dependently by the peptide inhibitor, DEVD-CHO, that reduced the number of apoptotic cells and prevented the hypoxia-induced decrease in cell viability, including when given 24 h post-insult. Interestingly, in these conditions, the inhibitory compounds enhanced the number of mitotic neurons. These data emphasize the critical role of caspase-3 in neuronal injury consecutive to hypoxia. Whereas caspase inhibitors may provide benefit over a broad therapeutic window, they might allow developing neurons to complete their cell cycle initiated in response to stress, as it is the case for sublethal hypoxia.

Molecular effects of pre-conditioning by 1-h hypoxia were investigated in cultured neurons from fetal rat forebrain, submitted the following day to a 6-h hypoxia that induces apoptosis. While preventing from apoptosis, pre-conditioning led to increased number of living neurons, DNA synthesis, with persistent overexpression of Bcl-2 and proliferating cell nuclear antigen (PCNA). Adaptative mechanisms would involve anti-apoptotic proteins and regulators of the cell cycle, to finally promote neuronal proliferation., (Copyright 1998 Elsevier Science B.V. All rights reserved.)