Induction of cell death by L-alpha-aminoadipic acid exposure in cultured rat astrocytes: Relationship to protein synthesis

The excitotoxin, L-alpha-aminoadipic acid (L-AAA), kills primary astrocytes in the brain. The mechanism underlying the induction of cell death is not well understood although many possible mechanisms are theorized. Previous s tudies have reported that astrocytes die after prolonged exposure to L-AAA suggesting a delayed programmed cell death and apoptosis. In this study rat cortical astrocytes exposed to continuous 1 mM L-AAA exposure for 24-, 48- , or 72 hours demonstrated increased DNA laddering, a characteristic of apo ptosis. Unexpectedly, this was not ameliorated by the presence of cyclohexi mide at 0.1 mu g/ml medium. Because of our interest in cytoprotective heat shock proteins induced by excitoxic stress, we studied the effect of prolon ged exposure of L-AAA on the synthesis of stress proteins and protein synth esis in rat cortical astrocytes. Protein synthesis as measured by [S-35]-me thionine labeling showed a marked and significant decrease in incorporation of radiolabel after 24 hours of exposure to L-AAA and prior to induction o f significant cell death noted at 48- and 72 hours of L-AAA exposure. The i nhibition of protein synthesis was partially reversible at 24 hours if cell s were labeled in medium without L-AAA during the radiolabeling period. Hea t shock or stress proteins, HSP70 and heme oxygenase-1 (HO-1), were analyze d after a 24 hour exposure to L-AAA and showed no significant induction of HSP70 or HO-1. The findings suggest that the prolonged inhibition of protei n synthesis and associated lack of induction of HSP70 and HO-1 synthesis co ntributed to apoptotic cell death induced by the excitoxin L-AAA. (C) 2000 Intox Press, Inc.