Inhibition of cytotoxic fibrillation of human insulin using silver nanoparticles capped by aqueous extract of Echium amoenum

Iranian Borage (Echium amoenum) is a medicinal plant with a wide spectrum of therapeutic properties, including antioxidant, antimicrobial, anti-inflammatory, and immunomodulatory effects, which are commonly used in Iranian folk medicine for the treatment of sore throat, coughing, pneumonia, anxiety, and depression. In the present study, the capacity of aqueous extract of Iranian Borage (AEIB) to inhibit cytotoxic fibrillation of human insulin in vitro is investigated for the first time. Using LC-MS analysis, we identified caffeic acid and m-geranyl-p-dihydroxybenzoic acid as two main phenols and methylated derivatives of kaempferol and luteolin as the main flavonoids presented in AEIB. The impact of AEIB on the amyloid fibrillation and cytotoxicity of human insulin was examined using ThT fluorescence assay, circular dichroism spectroscopy, fluorescence microscopy, MTT-based cell viability assay, intracellular ROS content and mitochondrial membrane potential measurements, and hemolysis assay. The obtained results indicate that AEIB can significantly inhibit the amyloid assembly and cytotoxicity of human insulin. Based on the seeding experiments, we propose that binding to and stabilizing monomeric/partially unfolded species may be the main mechanism by which AEIB modulates fibrillogenesis of human insulin. Moreover, we found that green synthesis of metal-based nanoparticles is an effective strategy to improve the anti-amyloidogenic effects of AEIB. Molecular docking studies indicate the binding of two main constituents of AEIB to monomeric form of human insulin. Given significant neuroprotective effects of AEIB and its effectiveness in treatment of several neuropsychiatric disorders, and according to the obtained results, we believe that AEIB has the potency to be considered as a novel therapeutic drug for future research associated with neurodegenerative diseases.