Effect of β-amyloid on endothelial cells: lack of direct toxicity, enhancement of MTT-induced cell death and intracellular accumulation

Several cerebrovascular alterations have been described in Alzheimer’s disease (AD) including an accumulation of β-amyloid (βA) on the vascular walls in the brain. To investigate the potential toxic activity of βA on endothelial cells (EC), two endothelial murine cell lines derived from heart and brain were exposed to βA1–42 and the biologically active fragment βA25–35 in the range from 5 nM to 50 μM. In a low concentration range (50 nM to 2.5 μM) both peptides significantly reduced the 3-(4,5-dimethylthiazol-2y1)-2-5-diphenyltetrazolium bromide (MTT) signal in the endothelial cell lines exposed for 24 h. However, microscopic examination, lactate dehydrogenase (LDH) release determination and Neutral Red assay did not confirm any toxic effect associated with inhibition of MTT formazan reduction. The effect on MTT was not susceptible to anti-oxidant treatment and did not increase the sensitivity to oxidative stress. However, when the EC were exposed to βA and MTT for 1 h, cell viability, determined by LDH release, was strongly reduced, while in normal conditions MTT-induced cell death only after 2 h. An inhibitor of lysosomal ATPase activity, bafilomycin A1, completely antagonized this effect. The morphological examination showed that the functional activation by βA in EC enhanced the production of MTT formazan crystals. To verify the accumulation of βA in the lysosomal compartment we analyzed the subcellular distribution of βA1–42 at different exposure times of EC to the peptide. The peptide was found in several organelles and was absent in the cytoplasmic compartment; co-treatment with bafilomycin A1 did not reduce the intracellular presence of βA1–42. In our condition, the exposure of EC to βA induced an intracellular accumulation of the peptide and a vasoactive effect that did not appear associated with direct toxic activity.