Beta‐cell Prohormone Convertase 1/3 Plays a Larger Role in Glucose Homeostasis Than Beta‐cell Prohormone Convertase 2 in Mice

Abstract

Insulin is synthesized in pancreatic beta cells from its precursor proinsulin, which is cleaved by the prohormone convertases 1/3 (PCSK1) and 2 (PCSK2) to generate mature insulin. In patients with type 1 or type 2 diabetes, proinsulin processing to mature insulin is impaired and elevated circulating proinsulin:insulin ratios are found in both conditions. It is unknown whether the beta‐cell prohormone processing impairments observed in diabetes contribute to or are secondary consequences of beta‐cell failure. Given the key role of PCSK1 and PCSK2 in insulin synthesis and beta‐cell function, we hypothesized that beta‐cell PCSK1 or PCSK2 deficiency would result in beta‐cell dysfunction and glucose intolerance.Beta‐cell specific knockout mouse models of Pcsk1 (Pcsk1betaKO) and Pcsk2 (Pcsk2betaKO) were generated using cre‐lox recombination and monitored for changes in glycemia on chow and high‐fat (HF) diets. Neither Pcsk1betaKO or Pcsk2betaKO chow‐fed animals displayed hyperglycemia or impaired glucose tolerance in the first 14 weeks of life. By 26 weeks of age Pcsk1betaKO, but not Pcsk2betaKO, chow‐fed male mice displayed fasting hyperglycemia and impaired glucose tolerance. After 19 weeks of HF diet, 3 of 5 Pcsk1betaKO male mice developed diabetes (fasting glycemia > 16mM), while no hyperglycemia was observed in Pcsk2betaKO mice up to 30 weeks of diet duration. Pcsk1betaKO mice displayed elevated islet and plasma (pro)insulin levels, decreased islet mature insulin levels, translucent insulin granules, and trended toward increased beta‐cell area in the pancreas. Despite altered insulin biosynthesis and immature granules, Pcsk1betaKO islets displayed similar insulin secretion kinetics to Pcsk1 wild‐type islets ex vivo.Our data suggest that Pcsk1 plays a larger role in proinsulin processing and beta‐cell function in mice than Pcsk2. However, despite severe impairments in insulin biosynthesis, Pcsk1‐deficient beta cells compensate remarkably well at early ages and hyperglycemia is only observed in states of increased beta‐cell secretory demand (mild insulin resistance associated with aging or HF diet in mice). Thus, impaired Pcsk1‐mediated prohormone processing, but not Pcsk2‐mediated processing, in beta cells increases diabetes susceptibility in mice.Support or Funding InformationFunding was provided by the Canadian Institutes of Health Research and Canucks for Kids Fund Childhood Diabetes Laboratories