Cytotoxic
polyamine-derived amino
aldehydes, formed during cerebral ischaemia, damage adjacent tissue (the so-called 'penumbra') not subject to the initial ischaemic insult. One such product is
3-aminopropanal (3-AP), a potent
cytotoxin that accumulates in ischaemic brain, although the precise mechanisms responsible for its formation are still unclear. More relevant to the present investigations, the mechanisms by which such a small aldehydic compound might be cytotoxic are also not known, but we hypothesized that 3-AP, having the structure of a weak lysosomotropic base, might concentrate within lysosomes, making these organelles a probable focus of initial toxicity. Indeed, 3-AP leads to lysosomal
rupture of D384
glioma cells, a process which clearly precedes
caspase activation and apoptotic cell death. Immunohistochemistry reveals that 3-AP concentrates in the lysosomal compartment and prevention of this accumulation by the lysosomotropic base
ammonia, NH(3), protects against 3-AP cytotoxicity by increasing lysosomal pH. A
thiol compound, N-(2-mercaptopropionyl)glycine, reacts with and neutralizes 3-AP and significantly inhibits cytoxocity. Both amino and
aldehyde functions of 3-AP are necessary for toxicity: the amino group confers lysosomotropism and the
aldehyde is important for additional, presently unknown, reactions. We conclude that 3-AP exerts its toxic effects by accumulating intralysosomally, causing
rupture of these organelles and releasing lysosomal
enzymes which initiate
caspase activation and apoptosis (or
necrosis if the lysosomal
rupture is extensive). These results may have implications for the development of new
therapeutics designed to lessen secondary damage arising from focal cerebral ischaemia.