Increased circulating levels of proinsulin (PI) and proinsulin-to-insulin ratio (PI/INS) is a well-known abnormality in type 2 diabetes (T2D). The exact mechanism behind this increase in T2D, as well as in T1D, is unknown. The aim of this study was to analyze proinsulin and insulin expression in pancreatic islets of tissue biopsies of patients undergoing partial pancreatectomy (PP) with normal glucose tolerance (NGT), impaired glucose tolerance (IGT) and T2D, in order to explore the alterations that occur in islets during metabolic stress. Frozen sections of pancreatic tissue biopsies (n=17) classified following OGTT into n=5 NGT, n=9 IGT and n=3 T2D were stained for INS and PI through double immunofluorescence staining. β-cell glucose sensitivity (calculated as the ratio of insulin secretion and glucose increments), basal insulin secretion, and glucose levels 2-hours following OGTT were analyzed. Image analysis was performed on each individual islet through Volocity software. In-situ staining measurements were correlated with patients’ clinical parameters. PI-INS colocalization as well as the area (µm2) of PI positivity and PI/INS ratio gradually increased from NGT to IGT and T2D pancreatic islets (p<0.0001), suggesting an altered localization and processing of PI in INS which potentially cause the release of immature granules with a higher relative content of PI. We demonstrated that the increase of PI/INS ratio was associated with higher basal INS (r=0.65 p<0.004) and glucose levels 2-hours following OGTT (r=0.75 p<0.0008). Finally, we also observed that the reduction of β-cell glucose sensitivity is linked to increase of in-situ PI-INS ratio (r=-0.6 p<0.02).
In conclusion, our results suggest that the increase of PI/INS ratio in pancreatic islets is directly related to metabolic defects in dysfunctional β cells, highlighting the importance of correct PI processing and folding in the maintenance of glucose homeostasis.
N. Brusco: None. G. Sebastiani: None. G. Licata: None. G.E. Grieco: None. L. Nigi: None. D. Fignani: None. S. Moffa: None. G. Sorice: None. G. Di Giuseppe: None. C. Cefalo: None. A. Mari: Consultant; Self; Lilly Diabetes. Research Support; Self; Boehringer Ingelheim International GmbH. F. Dotta: None. A. Giaccari: Speaker’s Bureau; Self; Amgen, AstraZeneca, Eli Lilly and Company, Merck Sharp & Dohme Corp., Mundipharma, Novo Nordisk A/S, Sanofi-Aventis. T. Mezza: None.
European Foundation for the Study of Diabetes; AstraZeneca