In humans, more than 50 diseases are related to protein fibrillation, including Alzheimer's and Parkinson's diseases. Inhibition of amyloid fibril formation using natural products is one of the main therapeutic strategies for preventing the progression of these diseases. In this context, phenolic extract fractions from Heliotropium bacciferum leaves were evaluated for their ability to inhibit the aggregation of human insulin (HI), an appropriate model protein for fibrillation under physiological conditions and agitation at 600 rpm. Our results exhibited HI fibrillation under studied conditions, with aggregation kinetic fitting a sigmoidal curve with and values of 44.4 h and 0.16 h−1, respectively. Structural changes preceding the onset of fibril formation were detected by 8-Anilino-1-naphthalene-sulphonic acid (ANS) probe, revealed the exposure of hydrophobic regions in the HI peptide to the solvent, with and values of 18.2 h 0.12 h−1, respectively. CD spectroscopy calculated the formed fibrils comprised of 51.6 % β-sheet structure and 43.0 % unordered structure. In vitro studies demonstrated a dose-dependent inhibitory effect of H. bacciferum extracts on HI fibrillation. The free phenolic fraction (FPF) at 200 μg/mL exhibited nearly complete inhibition, whereas the bound phenolic fraction (BPF) demonstrated a 52 % reduction in fibrillation. These findings were further validated using Rayleigh light scattering (RLS) and circular dichroism analyses. Transmission electron microscopy (TEM) validated the formation of insulin fibrillation and its inhibition by extract fractions. Moreover, MTT assay results on SH-SY5Y cells showed that both extract fractions attenuated HI fibril-induced neuronal toxicity in a dose-dependent manner. Furthermore, RT-PCR analysis revealed that co-treatment with a low concentration (7.8 μg/mL) of H. bacciferum extracts led to a significant reduction in the expression levels of pro-apoptotic genes (Casp3 and Bax) and an increase in the anti-apoptotic gene Bcl-2 in SH-SY5Y cells treated with fibrillated HI. This study highlights the potential of H. bacciferum extracts as therapeutic agents against protein fibrillation-related diseases and underscores the importance of polyphenols in preventing amyloid fibril formation.
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