Enhancement of MTT, a tetrazolium salt, exocytosis by amyloid beta-protein and chloroquine in cultured rat astrocytes.

The effect of amyloid beta-protein (Abeta) on the cellular reducing activity has been a controversial issue. We determined the cellular reducing activity in cultured astrocytes using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and 2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl )-2H-tetrazolium (WST-8) reduction assays following Abeta treatment. MTT reduction was inhibited whereas WST-8 reduction was unaffected by the Abeta treatment. Furthermore, the early extracellular appearance of MTT formazan, a reduced product of MTT, was observed in association with the rapid disappearance of intracellular formazan granules. Notably, similar results were obtained in cultures treated with chloroquine, a perturbant of membrane trafficking. Our results suggest that MTT formazan exocytosis is enhanced in a similar manner by Abeta and chloroquine and that this biological activity of Abeta may underlie the pathogenesis of Alzheimer's disease.