Characterization of Novel Insulin Fibrils That Show Strong Cytotoxicity Under Physiological pH

Amyloid fibrils are β-sheet-rich protein aggregates that are associated with more than 20 diseases. Insulin is known to form amyloid fibrils under a variety of conditions in vitro. Insulin fibrillations have been generally performed under acidic conditions, which are conducive to the formation of fibrils. As insulin is found almost exclusively as a monomer in acidic solutions, insulin fibrillation under acidic conditions is proposed to occur via its monomer. However, insulin fibrils, which cause injection-localized amyloidosis, form under neutral pH conditions in vivo, because both subcutaneous tissue and almost all insulin formulations maintain a neutral pH. In this study, we induced fibrillation under conditions more closely resembling physiological conditions than those used in previous studies with the aim of better understanding the nature of injection-localized amyloidosis in vivo. The results of transmission electron microscopy, structural analyses, and MTT assay show that the fibrils formed under conditions more closely resembling physiological conditions have different properties from the fibrils described to date. The results of this study indicate that fibrils formed under conditions more closely resembling physiological conditions have different properties from insulin fibrils induced under the conditions reported in previous studies.