Insulin amyloid fibrils interact directly with the NLRP3, resulting in inflammasome activation and pyroptotic cell death.

Introduction: Nucleotide-binding oligomerization domain-like receptor family, pyrin domain containing 3 (NLRP3), an intracellular pattern recognition receptor, recognizes various pathogen-associated molecular pattern and/or damage-associated molecular pattern molecules to constitute inflammasome that act as an interleukin (IL)-1β processing platform. Injected insulin is reported to induce focal amyloidosis and the formation of subcutaneous lumps called insulin balls, but the formation of subcutaneous lumps and the underlying cytotoxic mechanism has not been elucidated.

Methods: Amyloid formation was evaluated by thioflavin T spectroscopic assay and scanning electron microscopy. Binding between insulin amyloid fibrils and NLRP3 was evaluated by immunoprecipitation followed by native polyacrylamide gel electrophoresis. Inflammasome activation was evaluated by immunofluorescence speck formation called "ASC speck" and Western blotting. IL-1β secretion in culture supernatants of peripheral blood mononuclear cells was evaluated by enzyme-linked immunosorbent assay. Cytotoxicity was measured by lactate dehydrogenase release assay.

Results: Insulin amyloid fibrils interact directly with NLRP3, resulting in NLRP3 inflammasome activation and pyroptotic cell death.

Conclusion: Insulin ball formation and cytotoxicity may be associated with NLRP3 inflammasome activation followed by pyroptotic cell death.