Staurosporine-induced apoptosis of cultured rat hippocampal neurons involves caspase-1-like proteases as upstream initiators and increased production of superoxide as a main downstream effector

Abstract

We induced apoptosis in cultured rat hippocampal neurons by exposure to the protein kinase inhibitor staurosporine (30 nM, 24 hr). Treatment with the antioxidant (±)-α-tocopherol (100 μM) or the superoxide dismutase-mimetic manganese tetrakis (4-benzoyl acid) porphyrin (1 μM) significantly reduced staurosporine-induced cell death. Using hydroethidine-based digital videomicroscopy, we observed a significant increase in intracellular superoxide production that peaked 6-8 hr into the staurosporine exposure. This increase occurred in the absence of gross mitochondrial depolarization monitored with the voltage-sensitive probe tetramethylrhodamine ethyl ester. We then prepared extracts from staurosporine-treated hippocampal neurons and monitored cleavage of acetyl-Tyr-Val-Ala-Aspaminomethyl-coumarin and acetyl- Asp-Glu-Val-Asp-AMC, fluorogenic substrates for caspase-1-like and caspase- 3-like proteases, respectively. Staurosporine caused a significant increase in caspase-1-like activity that preceded intracellular superoxide production and reached a maximum after 30 min. Caspase-3-like activity paralleled intracellular superoxide production, with peak activity seen after 8 hr. Treatment with the corresponding caspase-3-like protease inhibitor acetyl- Asp-Glu-Val-Asp-aldehyde (10 μM) prevented the increase in caspase-3-like activity and staurosporine-induced nuclear fragmentation, but failed to prevent the rise in superoxide production and subsequent cell death. In contrast, treatment with caspase-1-like protease inhibitors reduced both superoxide production and cell death. Of note, antioxidants prevented superoxide production, caspase-3-like protease activity, and cell death even when added 4 hr after the onset of the staurosporine exposure. These results suggest a scenario of an early, caspase-1-like activity followed by a delayed intracellular superoxide production that mediates staurosporine-induced cell death of cultured rat hippocampal neurons.