Revisiting proinsulin processing: Evidence that human β-cells process proinsulin with prohormone convertase (pc) 1/3 but not pc2

Abstract

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 sub-stantial evidence that mouse β-cells process proinsulin using prohormone convertase 1/3 (PC1/3) and then pro-hormone convertase 2 (PC2), this finding has not been verified in human β-cells. Immunofluorescence with val-idated antibodies revealed 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 experi-ments, PC2 immunoreactivity in neighboring α-cells acted as a positive control. In donors with type 2 diabe-tes, β-cells had elevated PC2 immunoreactivity, sug-gesting 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, chloro-quine, 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 proglu-cagon. 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 long-standing theory of pro-insulin processing.