Evidence for an active type of cell death with ultrastructural features distinct from apoptosis: The effects of 3-acetylpyridine neurotoxicity

Abstract

3-Acetylpyridine is a niacinamide antagonist with potent neurotoxic properties in vitro and in vivo. 3-Acetylpyridine neurotoxicity was associated with positive DNA end-labelling and displayed features of active cell death without the ultrastructural changes of apoptotic cell death. After systemic administration in rats (70 mg/kg), we detected labelled nuclei in the inferior olive using in situ DNA end-labelling. However, the conventional chromatin stain did not show chromatin condensation or fragmentation and electron microscopy studies failed to reveal features of apoptosis. Although areas of condensed chromatin were present in some nuclei, cytoplasmic damage with extensive organelle swelling was the most prominent finding. In vitro, 3-acetylpyridine (0.1–1 mM) induced degeneration of cerebellar granule neurons in a concentration- and time-dependent manner. The protein synthesis inhibitor cycloheximide (10 μg/ml) and the transcriptional inhibitor actinomycin D (10 μM) protected against 3-acetylpyridine toxicity. In contrast, neither the free radical scavenger α-phenyl-N-tertbutylnitron (100 μM), nor glutathione ethyl ester (10–100 μM), N-acetyl-cysteine (10–200 μM) or 3-aminobenzamide (0.1–4 mM), an inhibitor of poly(ADP-ribose) synthesis, were effective. 3-Acetylpyridine-induced neuronal death in vitro was associated with positive in situ DNA labelling. However, DNA fragmentation could not be demonstrated prior to neuronal cell loss and no DNA “laddering” was detected by DNA gel electrophoresis. Correspondingly, no apoptotic nuclei were revealed upon electron microscopy but organelle swelling and extensive vacuolization, changes similar to autophagocytosis. In conclusion, 3-acetylpyridine induces an active form of cell death that required de novo protein synthesis but is distinct from apoptosis. A loss of glutathione accompanies, but does not precede, cell death.