Abstract
Ashwagandha (Withania somnifera) is a medicinal plant used in several traditional medical practices for treating a variety of ailments. With its multitude of polyphenols, ashwagandha possesses strong anticancer potential. In an effort to improve the anticancer efficacy and target specificity of ashwagandha, we coated gold nanoparticles with ashwagandha polyphenols, characterized the particles, evaluated their antiproliferative efficacy, and elucidated their mechanism of action in the cells. The Withania somnifera-coated gold nanoparticles (“Ws-GNPs”) were characterized using UV–visible spectroscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, dynamic light scattering, zeta potential analysis, and Fourier-transform infrared spectroscopy. The coating of ashwagandha polyphenols on the nanoparticles was confirmed by 1H nuclear magnetic resonance spectroscopy. The Ws-GNPs were then subjected to cellular-level analysesusing the triple-negative breast cancer cell line, MDA-MB-231, as the cell model. Ws-GNPs inhibited proliferation of these malignant breast cancer cells with an IC50 of 90 ± 10 μg/mL, which was half the IC50 of the ashwagandha-extract (Ws-EX)-treated cells (180 ± 23 μg/mL). The nanoparticles strongly inhibited the clonogenicity and migration of the cells, as evidenced from a clonogenic assay and wound-healing assay, respectively. Ws-GNPs showed concentration-dependent binding to purified tubulin as confirmed by an in vitro, spectrofluorometry-based tryptophan-quenching assay and an anilinonaphthalene sulfonate-binding assay. The binding perturbed the secondary structure of the protein, as evidenced by far-UV circular dichroism spectroscopy. In cells, the interactions between tubulin and the nanoparticles resulted in the suppressed dynamicity of the microtubules, the subsequent halting of the cell cycle progression through mitosis, and finally, apoptosis.
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