Brazilin Inhibits Prostatic Acidic Phosphatase Fibrillogenesis and Decreases its Cytotoxicity.

A 39-amino acid peptide fragment that is derived from prostatic acidic phosphatase (PAP), PAP248-286 , is secreted in large amounts in human semen and forms amyloid fibrils. These fibrils can capture HIV virions and increase the attachment of virions to target cells; as such, they are called a "semen-derived enhancer of virus infection" (SEVI). Therefore, the inhibition of the formation of PAP248-286 amyloid fibrils is of great significance. Herein, we demonstrate that brazilin effectively inhibits PAP248-286 aggregation. The inhibitory effect increases with increasing brazilin concentration. Thioflavin T fluorescence assays and TEM observations confirmed that a few fibrils formed when brazilin was present with PAP248-286 in an equimolar concentration. Circular dichroism spectroscopy indicated that brazilin inhibited the secondary structural transitions from α-helices and random coils into ß-sheets. Cytotoxicity assays showed that brazilin significantly decreased the cytotoxicity of the fibrils at 0.01 mmol L-1 . Isothermal titration calorimetry revealed that hydrophobic interactions were the main driving force for the binding of brazilin to the PAP248-286 monomer (dissociation constant, 4.03 µmol L-1 ), and that the binding affinity of brazilin for the fibrils was at least three orders of magnitude lower than that for the monomer. These results indicate that brazilin holds great potential as a small-molecule agent against SEVIs.

Brazilin inhibits amyloid ß-protein fibrillogenesis, remodels amyloid fibrils and reduces amyloid cytotoxicity.

Soluble amyloid ß-protein (Aß) oligomers, the main neurotoxic species, are predominantly formed from monomers through a fibril-catalyzed secondary nucleation. Herein, we virtually screened an in-house library of natural compounds and discovered brazilin as a dual functional compound in both Aß42 fibrillogenesis inhibition and mature fibril remodeling, leading to significant reduction in Aß42 cytotoxicity. The potent inhibitory effect of brazilin was proven by an IC50 of 1.5 ± 0.3 µM, which was smaller than that of (-)-epigallocatechin gallate in Phase III clinical trials and about one order of magnitude smaller than those of curcumin and resveratrol. Most importantly, it was found that brazilin redirected Aß42 monomers and its mature fibrils into unstructured Aß aggregates with some ß-sheet structures, which could prevent both the primary nucleation and the fibril-catalyzed secondary nucleation. Molecular simulations demonstrated that brazilin inhibited Aß42 fibrillogenesis by directly binding to Aß42 species via hydrophobic interactions and hydrogen bonding and remodeled mature fibrils by disrupting the intermolecular salt bridge Asp23-Lys28 via hydrogen bonding. Both experimental and computational studies revealed a different working mechanism of brazilin from that of known inhibitors. These findings indicate that brazilin is of great potential as a neuroprotective and therapeutic agent for Alzheimer's disease.