Natural lipopeptide surfactin inhibits insulin aggregation and prevents amyloid-induced cytotoxicity and inflammation

Despite the centennial of the pioneering discovery of insulin, challenges regarding insulin aggregation at the site of repeated injection, as well as during industrial production, storage, and transportation, have cast a gloom over this remarkable success story. Aggregation substantially impacts the quality, potency, and effectiveness of insulin formulation, decreasing bioavailability, intensifying the risk of immunogenicity, and endangering patient health. Herein, the suitability of Bacillus-derived amphiphilic biosurfactant surfactin was demonstrated to evade temperature-induced fibrillation of insulin. Fluorometric assessment employing thioflavin-T and intrinsic tyrosine fluorescence indicated the dose-dependent reduction in insulin unfolding and subsequent formation of fibrils. Decreased thioflavin fluorescence at the plateau stage and about a 79% increase in fibrillation lag time was observed for samples containing an equimolar ratio of insulin to surfactin. Circular dichroism analysis further indicated about a 30% increase in α-helix content in samples containing an equimolar ratio of insulin and surfactin, as compared to controls. The findings were further bolstered by dynamic light scattering, electron microscopic analysis, and native polyacrylamide gel electrophoresis. Molecular docking and simulation studies did not indicate any structural destabilization of insulin with surfactin, having major interactions at the hydrophobic and potential amyloidogenic regions of insulin B-chain: B12-B19 (VEALYLVC) and B23-B27 (GFFYT) through hydrophobic and hydrogen bonds. Notably, an almost 22% increase in cell viability was observed for cells incubated with aggregated insulin containing equimolar concentrations of SFN. Surfactin also subdued insulin amyloid-induced upregulation of the pro-inflammatory genes. Thus, surfactin may be contemplated as an additive in biopharmaceutical formulations to imbue stability and may also be evaluated as anti-amyloid therapeutics for other pathological proteinopathies.