2105-P: Beta Cell PCSK1 Deficiency Increases Diabetes Susceptibility in Male Mice

Proinsulin (PI) is processed to mature insulin by prohormone convertases 1/3 (Pcsk1) and 2 (Pcsk2). In individuals with T1D and T2D, there is an increase in the circulating PI:insulin ratio. We hypothesized that beta-cell Pcsk1 deletion would result in beta-cell dysfunction and hyperglycemia.

Male and female Pcsk1^flox/flox Ins1^cre/+ (Pcsk1^beta-KO) and Pcsk1^+/+ Ins1^cre/+ (Pcsk1^beta-WT) mice were fed either a chow or a high fat diet (HFD; 45% fat) and monitored until 30 weeks of age. Despite an approximately 150-fold increase in fasting plasma PI (p < 0.001) and only immature insulin granules detectable by electron microscopy, chow-fed Pcsk1^beta-KO male mice had normal glucose tolerance early in life and only developed glucose intolerance by 26 weeks of age (AUC: 934±294 vs. 597±223; p = 0.008). Histological analysis showed that male Pcsk1^beta-KO mice displayed a trend toward increased beta-cell area (1.3±0.6 vs. 0.8±0.4%; p = 0.08). The unexpectedly mild hyperglycemia in Pcsk1^beta-KO mice is likely due to incomplete disruption of proinsulin processing, as mass spectrometry and western blot analysis of Pcsk1^beta-KO islets confirmed markedly elevated proinsulin, but detectable mature insulin. In HFD-fed cohorts, 3/10 Pcsk1^beta-KO male mice developed sustained fasting hyperglycemia (>16 mM) after 19 weeks of diet. Chow and HFD-fed female Pcsk1^beta-KO mice displayed no differences in glycemia or body weight from Pcsk1^beta-WT controls. Deletion of both Pcsk1 and Pcsk2 in beta cells (Pcsk1^flox/flox Pcsk2^flox/flox Ins1^cre/+) resulted in undetectable mature insulin in islets (western blot), and impaired glucose tolerance at 10 weeks of age in both chow-fed male and female mice.

Our results suggest that beta-cell Pcsk1 deficiency is not sufficient to drive diabetes development, but increases diabetes susceptibility in conditions of increased insulin secretory demand. Our data in mice support the idea that beta-cell prohormone processing errors likely contribute to pathogenesis of diabetes but not obesity.