Revisiting Proinsulin Processing: Evidence That Human β-Cells Process Proinsulin With Prohormone Convertase (PC) 1/3 but Not PC2

Insulin is first produced in pancreatic β-cells as the precursor prohormone proinsulin. Defective proinsulin processing has been implicated in the pathogenesis of both type 1 and type 2 diabetes. Though there is substantial evidence that mouse β-cells process proinsulin using prohormone convertase 1/3 (PC1/3) then prohormone convertase 2 (PC2), this finding has not been verified in human β-cells. Immunofluorescence with validated antibodies reveals that there was no detectable PC2 immunoreactivity in human β-cells and little PCSK2 mRNA by in situ hybridization. Similarly, rat β-cells were not immunoreactive for PC2. In all histological experiments, PC2 immunoreactivity in neighbouring α-cells acts as a positive control. In donors with type 2 diabetes, β-cells had elevated PC2 immunoreactivity, suggesting that aberrant PC2 expression may contribute to impaired proinsulin processing in β-cells of patients with diabetes. To support histological findings using a biochemical approach, human islets were used for pulse-chase experiments. Despite inhibition of PC2 function by temperature blockade, brefeldin-A, chloroquine, and multiple inhibitors that blocked production of mature glucagon from proglucagon, β-cells retained the ability to produce mature insulin. Conversely, suppression of PC1/3 blocked processing of proinsulin but not proglucagon. By demonstrating that healthy human β-cells process proinsulin by PC1/3 but not PC2 we suggest that there is a need to revise the longstanding theory of proinsulin processing.

Download Full-text

Revisiting proinsulin processing: Evidence that human β-cells process proinsulin with prohormone convertase (PC) 1/3 but not PC2

10.2337/db20-4567/suppl.12074721.v1 ◽

2020 ◽

Author(s):

Ada Admin ◽

Adam Ramzy ◽

Ali Asadi ◽

Timothy J Kieffer

Keyword(s):

Type 2 Diabetes ◽

Brefeldin A ◽

Human Islets ◽

Positive Control ◽

Prohormone Convertase ◽

Β Cells ◽

Healthy Human ◽

Proinsulin Processing ◽

Prohormone Convertase 2

Insulin is first produced in pancreatic β-cells as the precursor prohormone proinsulin. Defective proinsulin processing has been implicated in the pathogenesis of both type 1 and type 2 diabetes. Though there is substantial evidence that mouse β-cells process proinsulin using prohormone convertase 1/3 (PC1/3) then prohormone convertase 2 (PC2), this finding has not been verified in human β-cells. Immunofluorescence with validated antibodies reveals that there was no detectable PC2 immunoreactivity in human β-cells and little PCSK2 mRNA by in situ hybridization. Similarly, rat β-cells were not immunoreactive for PC2. In all histological experiments, PC2 immunoreactivity in neighbouring α-cells acts as a positive control. In donors with type 2 diabetes, β-cells had elevated PC2 immunoreactivity, suggesting that aberrant PC2 expression may contribute to impaired proinsulin processing in β-cells of patients with diabetes. To support histological findings using a biochemical approach, human islets were used for pulse-chase experiments. Despite inhibition of PC2 function by temperature blockade, brefeldin-A, chloroquine, and multiple inhibitors that blocked production of mature glucagon from proglucagon, β-cells retained the ability to produce mature insulin. Conversely, suppression of PC1/3 blocked processing of proinsulin but not proglucagon. By demonstrating that healthy human β-cells process proinsulin by PC1/3 but not PC2 we suggest that there is a need to revise the longstanding theory of proinsulin processing.

Download Full-text

Inside the Insulin Secretory Granule

Metabolites ◽

10.3390/metabo11080515 ◽

2021 ◽

Vol 11 (8) ◽

pp. 515

Author(s):

Mark Germanos ◽

Andy Gao ◽

Matthew Taper ◽

Belinda Yau ◽

Melkam A. Kebede

Keyword(s):

Secretory Granules ◽

Β Cells ◽

Β Cell ◽

Proinsulin Processing ◽

Membrane Bound ◽

Golgi Network ◽

Insulin Secretory Granule ◽

Insulin Storage ◽

Cellular Stimulation

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

Download Full-text

Paired box 6 (PAX6) regulates glucose metabolism via proinsulin processing mediated by prohormone convertase 1/3 (PC1/3)

Diabetologia ◽

10.1007/s00125-008-1210-x ◽

2008 ◽

Vol 52 (3) ◽

pp. 504-513 ◽

Cited By ~ 55

Author(s):

J. H. Wen ◽

Y. Y. Chen ◽

S. J. Song ◽

J. Ding ◽

Y. Gao ◽

...

Keyword(s):

Glucose Metabolism ◽

Prohormone Convertase ◽

Prohormone Convertase 1 ◽

Proinsulin Processing

Download Full-text

GLP-1 Analog Liraglutide Enhances Proinsulin Processing in Pancreatic β-Cells via a PKA-Dependent Pathway

Endocrinology ◽

10.1210/en.2014-1218 ◽

2014 ◽

Vol 155 (10) ◽

pp. 3817-3828 ◽

Cited By ~ 8

Author(s):

Liang Wang ◽

Ye Liu ◽

Jin Yang ◽

Hejun Zhao ◽

Jing Ke ◽

...

Keyword(s):

Type 2 Diabetes ◽

Protein Kinase ◽

Protein Kinase A ◽

Pancreatic Β Cell ◽

Β Cells ◽

Β Cell ◽

Pancreatic Β Cells ◽

Proinsulin Processing ◽

Kinase A

Abstract Hyperproinsulinemia has gained increasing attention in the development of type 2 diabetes. Clinical studies have demonstrated that glucagon-like peptide-1 (GLP-1)-based therapies significantly decrease plasma proinsulin/insulin ratio in patients with type 2 diabetes. However, the underlying mechanism remains unclear. Prohormone convertase (PC)-1/3 and PC2 are primarily responsible for processing proinsulin to insulin in pancreatic β-cells. We have recently reported that Pax6 mutation down-regulated PC1/3 and PC2 expression, resulting in defective proinsulin processing in Pax6 heterozygous mutant (Pax6m/+) mice. In this study, we investigated whether and how liraglutide, a novel GLP-1 analog, modulated proinsulin processing. Our results showed that liraglutide significantly up-regulated PC1/3 expression and decreased the proinsulin to insulin ratio in both Pax6m/+ and db/db diabetic mice. In the cultured mouse pancreatic β-cell line, Min6, liraglutide stimulated PC1/3 and PC2 expression and lowered the proinsulin to insulin ratio in a dose- and time-dependent manner. Moreover, the beneficial effects of liraglutide on PC1/3 and PC2 expression and proinsulin processing were dependent on the GLP-1 receptor-mediated cAMP/protein kinase A signaling pathway. The same mechanism was recapitulated in isolated mouse islets. In conclusion, liraglutide enhanced PC1/3- and PC2-dependent proinsulin processing in pancreatic β-cells through the activation of the GLP-1 receptor/cAMP/protein kinase A signaling pathway. Our study provides a new mechanism for improvement of pancreatic β-cell function by the GLP-1-based therapy.

Download Full-text