Defective insulin maturation in patients with type 2 diabetes

Abstract

Objective: Progressive beta cell dysfunction is a hallmark of type 2 diabetes (T2D). Increasing evidence indicates that over-stimulating proinsulin synthesis causes proinsulin misfolding and impairs insulin maturation and storage in db/db mice. However, defective insulin maturation in patients with T2D remains unknown. Methods: We examined intra-islet and intra-cellular distributions of proinsulin and insulin and proinsulin to insulin ratio in the islets of patients with T2D. The expression of transcription factor NKX6.1 and dedifferentiation marker ALDH1A3, as well as glucagon were detected by immunofluorescence. Results: We identified a novel subgroup of beta cells expressing only proinsulin but not insulin. Importantly, significantly increased proinsulin positive and insulin negative (PI+/INS-) cells were evident in T2D, and this increase was strongly correlated with levels of Hemoglobin A1C (HbA1c) in T2D and prediabetes. The percentages of beta cells expressing prohormone convertase 1/3 and carboxypeptidase E were not reduced. Indeed, while proinsulin displayed higher degree of co-localization with the Golgi markers GM130/TGN46 in control beta cells, it appeared to be more diffused within the cytoplasm and less co-localized with GM130/TGN46 in PI+/INS- cells. Furthermore, the key functional transcription factor NKX6.1 markedly decreased in the islets of T2D, especially in the cells with PI+/INS-. The decreased NKX6.1+/PI+/INS+ was strongly correlated with levels of HbA1c in T2D. Almost all PI+/INS- cells showed absence of NKX6.1. Moreover, the percentages of PI+/INS- cells expressing ALDH1A3 were elevated along with an increased acquisition of glucagon immunostaining. Conclusion: Our data demonstrates defective insulin maturation in patients with T2D.

IER3IP1 is critical for maintaining glucose homeostasis through regulating the endoplasmic reticulum function and survival of β cells

“…Misfolded proinsulin is recognized by ER quality control machinery, preventing its export from the ER to the Golgi complex ( 14 , 16 ). We therefore examined localization of proinsulin and found that, consistent with our previous findings ( 14 , 33 ), proinsulin in wild-type β cells was mostly concentrated in the juxtanuclear region (suggesting successful proinsulin trafficking from the ER to the Golgi complex), whereas in IER3IP1-βKO β cells, proinsulin was more diffusely distributed throughout the cytoplasm (additionally, some proinsulin-positive cells lost detectable insulin, i.e., Pro + Ins − ), suggesting defective anterograde trafficking of proinsulin ( Fig. 4 G and H ).…”

“…Among 70 INS gene mutations associated with diabetes, the majority impair proinsulin oxidative folding with defective export from the endoplasmic reticulum (ER) ( 8 – 10 ) while also interacting (and impairing trafficking) of the coexpressed wild-type INS gene product, thereby decreasing insulin production ( 11 – 13 ). Proinsulin misfolding with defective formation and storage of mature insulin may also contribute to the development and progression of T2D ( 14 – 16 ).…”

Empagliflozin protects diabetic pancreatic tissue from damage by inhibiting the activation of the NLRP3/caspase-1/GSDMD pathway in pancreatic β cells: in vitro and in vivo studies

“…Immunofluorescence of the pancreatic β cells was performed as previously described [ 30 ]. Briefly, pancreatic β cells (2 × 10 4 ) were cultured in 24-well culture plates, fixed with 4% paraformaldehyde for 30 min, washed twice with PBS, and then permeabilized with methanol for 10 min at room temperature.…”

Pancreatic beta cell ER export in health and diabetes

“…These findings suggest that defective COPII dependent cargo export could be one of the underlying causes for the major beta cell defects in diabetes including impaired proinsulin maturation, loss of insulin content, abnormal insulin granule morphology, chronic ER stress and beta cell apoptosis in diabetes (1,4,73,103). In fact, defective insulin maturation with increased ER localization of proinsulin was reported in the beta cells of type 2 diabetes patients (104). It will be of importance to determine if ERES organization or COPII coat assembly is altered in human diabetic islets.…”