Amyloid beta protein inhibits cellular MTT reduction not by suppression of mitochondrial succinate dehydrogenase but by acceleration of MTT formazan exocytosis in cultured rat cortical astrocytes.

Alzheimer's disease amyloid beta protein (Abeta) inhibits cellular reduction of the dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Kaneko et al. have previously hypothesized that Abeta works by suppressing mitochondrial succinate dehydrogenase (SDH), but Liu and Schubert have recently demonstrated that Abeta decreases cellular MTT reduction by accelerating the exocytosis of MTT formazan in neuronal cells. To ask which is the case in astrocytes, we compared the effects of Abeta and 3-nitropropionic acid (3-NP), a specific SDH inhibitor, on MTT reduction in cultured rat cortical astrocytes. Treatment with 3-NP (10 mM) decreased cellular activity of MTT reduction, regardless of the time of incubation with MTT. On the other hand. Abeta-induced inhibition of cellular MTT reduction was dependent on the time of incubation with MTT. The cells treated with Abeta (0.1-1000 nM) exhibited normal capacity for MTT reduction at an early stage of incubation ( < 30 min), but ceased to reduce MTT at the late stage (> 1 h). Microscopic examination revealed that Abeta treatment accelerated the appearance of needle-like MTT formazan crystals at the cell surface. These observations support that Abeta accelerates the exocytosis of MTT formazan in astrocytes. In addition to inhibition of MTT reduction, Abeta is known to induce morphological changes in astrocytes. Following addition of Abeta (20 microM), polygonal astrocytes changed into process-bearing stellate cells. To explore a possible linkage between these two effects of Abeta, we tested if astrocyte stellation is induced by agents that mimic the effect of Abeta on MTT reduction. Cholesterol (5 5000 nM) and lysophosphatidic acid (0.2-20 microg/ml) were found to accelerate the exocytosis of MTT formazan in a similar manner to Abeta, but failed to induce astrocyte stellation. Therefore, Abeta-induced inhibition of MTT reduction is unlikely to be directly linked to its effect on astrocyte morphology.