Protective role of human insulin against the cytotoxicity associated with human mutant S20G islet amyloid polypeptide

AIMS/INTRODUCTION Islet amyloid polypeptide (IAPP) is a main component of islet amyloid in type 2 diabetes and cosecreted from β-cell with insulin. Clinical evidence from the patients with S20G mutation of the IAPP gene, as well as experimental evidence that insulin could inhibit amyloid formation of IAPP, suggests that a gradual reduction of insulin could be related to the cytotoxicity associated with S20G-IAPP through long-term deterioration of β-cells in type 2 diabetes. Our objective was to show an effect of human insulin on S20G-IAPP associated cytotoxicity. MATERIALS AND METHODS We analyzed the cytotoxicity associated with S20G-IAPP by controlling human insulin expression using adenovirus vectors with micro ribonucleic acid specifically against human insulin in endocrine AtT-20ins cells, which express human insulin permanently. Additionally, we carried out a follow-up study of circulating IAPP and insulin in type 2 diabetic patients. RESULTS S20G-IAPP expression was associated with a decrease in viability and an increase in terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling-positive cells in AtT-20ins cells. Furthermore, downregulation of human insulin enhanced the cytotoxicity associated with S20G-IAPP, and induced the cytotoxicity associated with wild-type (WT)-IAPP. Reduction of ubiquitin carboxy-terminal hydrolase L1 activity enhanced cytotoxicity under the downregulation of human insulin expression in both S20G- and WT-IAPP transduced cells. A 5-year follow up of type 2 diabetic patients showed a disproportionate increase of serum fasting IAPP-to-insulin ratio from baseline. CONCLUSIONS Human insulin plays a protective role against the cytotoxicity associated with S20G-IAPP, as well as WT-IAPP. The findings could suggest long-term deterioration of insulin secretion associates with IAPP linked cytotoxicity in type 2 diabetes.