3,3',4',5'-Tetrahydroxyflavone induces formation of large aggregates of amyloid ß protein.

Amyloid ß protein (Aß) self-assembles into insoluble fibrils, and forms the senile plaques associated with Alzheimer's disease. 3,3',4',5'-Tetrahydroxyflavone, a synthetic analogue of the natural flavonoid fisetin, has been found to potently inhibit Aß fibril formation. In the present study, we investigated how inhibition of Aß fibril formation by this flavonoid affects Aß conformation and neurotoxicity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of Aß1-42 (20 µM) incubated with or without 3,3',4',5'-tetrahydroxyflavone demonstrated that 3,3',4',5'-tetrahydroxyflavone (100 µM) rapidly caused formation of atypical Aß conformers, which appeared as a very broad, smear-like band in the high molecular weight region and were distinguishable from soluble Aß oligomers or mature Aß fibrils. Transmission electron microscopy (TEM) revealed that large spherical Aß aggregates were preferentially formed in the presence of 3,3',4',5'-tetrahydroxyflavone. The SDS-resistant, smear-like band on SDS-PAGE and the large spherical aggregates in TEM both disappeared after heat treatment (100°C, 10 min). Furthermore, a neurotoxicity assay with cultured rat hippocampal neurons demonstrated that Aß incubated with 3,3',4',5'-tetrahydroxyflavone was significantly less toxic than Aß incubated without the flavonoid. These results suggest that the newly synthesized fisetin analogue 3,3',4',5'-tetrahydroxyflavone directly produces atypical, large Aß aggregates and reduces Aß toxicity.