(-)-Epigallocatechin-3-Gallate (EGCG) Maintains κ-Casein in Its Pre-Fibrillar State without Redirecting Its Aggregation Pathway

The polyphenol (-)-epigallocatechin-3-gallate (EGCG) has recently attracted much research interest in the field of protein-misfolding diseases because of its potent anti-amyloid activity against amyloid-β, α-synuclein and huntingtin, the amyloid-fibril-forming proteins involved in Alzheimer's, Parkinson's and Huntington's diseases, respectively. EGCG redirects the aggregation of these polypeptides to a disordered off-folding pathway that results in the formation of non-toxic amorphous aggregates. Whether this anti-fibril activity is specific to these disease-related target proteins or is more generic remains to be established. In addition, the mechanism by which EGCG exerts its effects, as with all anti-amyloidogenic polyphenols, remains unclear. To address these aspects, we have investigated the ability of EGCG to inhibit amyloidogenesis of the generic model fibril-forming protein RCMκ-CN (reduced and carboxymethylated κ-casein) and thereby protect pheochromocytoma-12 cells from RCMκ-CN amyloid-induced toxicity. We found that EGCG potently inhibits in vitro fibril formation by RCMκ-CN [the IC₅₀ for 50 μM RCMκ-CN is 13 ± 1 μM]. Biophysical studies reveal that EGCG prevents RCMκ-CN fibril formation by stabilising RCMκ-CN in its native-like state rather than by redirecting its aggregation to the disordered, amorphous aggregation pathway. Thus, while it appears that EGCG is a generic inhibitor of amyloid-fibril formation, the mechanism by which it achieves this inhibition is specific to the target fibril-forming polypeptide. It is proposed that EGCG is directed to the amyloidogenic sheet-turn-sheet motif of monomeric RCMκ-CN with high affinity by strong non-specific hydrophobic associations. Additional non-covalent π-π stacking interactions between the polyphenolic and aromatic residues common to the amyloidogenic sequence are also implicated.