Browsing by Author "Lindberg, Iris"

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    Insulin regulates carboxypeptidase E by modulating translation initiation scaffolding protein eIF4G1 in pancreatic β cells
    (PNAS, 2014-06-03) Liew, Chong Wee; Assmann, Anke; Templin, Andrew T.; Raum, Jeffrey C.; Lipson, Kathryn L.; Rajan, Rajan; Qiang, Guifen; Hu, Jiang; Kawamori, Dan; Lindberg, Iris; Philipson, Louis H.; Sonenberg, Nahum; Goldfine, Allison B.; Stoffers, Doris A.; Mirmira, Raghavendra G.; Urano, Fumihiko; Kulkarni, Rohit N.; Department of Cellular & Integrative Physiology, IU School of Medicine
    Insulin resistance, hyperinsulinemia, and hyperproinsulinemia occur early in the pathogenesis of type 2 diabetes (T2D). Elevated levels of proinsulin and proinsulin intermediates are markers of β-cell dysfunction and are strongly associated with development of T2D in humans. However, the mechanism(s) underlying β-cell dysfunction leading to hyperproinsulinemia is poorly understood. Here, we show that disruption of insulin receptor (IR) expression in β cells has a direct impact on the expression of the convertase enzyme carboxypeptidase E (CPE) by inhibition of the eukaryotic translation initiation factor 4 gamma 1 translation initiation complex scaffolding protein that is mediated by the key transcription factors pancreatic and duodenal homeobox 1 and sterol regulatory element-binding protein 1, together leading to poor proinsulin processing. Reexpression of IR or restoring CPE expression each independently reverses the phenotype. Our results reveal the identity of key players that establish a previously unknown link between insulin signaling, translation initiation, and proinsulin processing, and provide previously unidentified mechanistic insight into the development of hyperproinsulinemia in insulin-resistant states.
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    SERCA2 regulates proinsulin processing and processing enzyme maturation in pancreatic beta cells
    (Springer, 2023) Iida, Hitoshi; Kono, Tatsuyoshi; Lee, Chih‑Chun; Krishnan, Preethi; Arvin, Matthew C.; Weaver, Staci A.; Jarvela, Timothy S.; Branco, Renato C. S.; McLaughlin, Madeline R.; Bone, Robert N.; Tong, Xin; Arvan, Peter; Lindberg, Iris; Evans‑Molina, Carmella; Medicine, School of Medicine
    Aims/hypothesis: Increased circulating levels of incompletely processed insulin (i.e. proinsulin) are observed clinically in type 1 and type 2 diabetes. Previous studies have suggested that Ca2+ signalling within beta cells regulates insulin processing and secretion; however, the mechanisms that link impaired Ca2+ signalling with defective insulin maturation remain incompletely understood. Methods: We generated mice with beta cell-specific sarcoendoplasmic reticulum Ca2+ ATPase-2 (SERCA2) deletion (βS2KO mice) and used an INS-1 cell line model of SERCA2 deficiency. Whole-body metabolic phenotyping, Ca2+ imaging, RNA-seq and protein processing assays were used to determine how loss of SERCA2 impacts beta cell function. To test key findings in human model systems, cadaveric islets were treated with diabetogenic stressors and prohormone convertase expression patterns were characterised. Results: βS2KO mice exhibited age-dependent glucose intolerance and increased plasma and pancreatic levels of proinsulin, while endoplasmic reticulum (ER) Ca2+ levels and glucose-stimulated Ca2+ synchronicity were reduced in βS2KO islets. Islets isolated from βS2KO mice and SERCA2-deficient INS-1 cells showed decreased expression of the active forms of the proinsulin processing enzymes PC1/3 and PC2. Additionally, immunofluorescence staining revealed mis-location and abnormal accumulation of proinsulin and proPC2 in the intermediate region between the ER and the Golgi (i.e. the ERGIC) and in the cis-Golgi in beta cells of βS2KO mice. Treatment of islets from human donors without diabetes with high glucose and palmitate concentrations led to reduced expression of the active forms of the proinsulin processing enzymes, thus phenocopying the findings observed in βS2KO islets and SERCA2-deficient INS-1 cells. Similar findings were observed in wild-type mouse islets treated with brefeldin A, a compound that perturbs ER-to-Golgi trafficking. Conclusions/interpretation: Taken together, these data highlight an important link between ER Ca2+ homeostasis and proinsulin processing in beta cells. Our findings suggest a model whereby chronic ER Ca2+ depletion due to SERCA2 deficiency impairs the spatial regulation of prohormone trafficking, processing and maturation within the secretory pathway. Data availability: RNA-seq data have been deposited in the Gene Expression Omnibus (GEO; accession no.: GSE207498).