Proinsulin targeting to the regulated pathway is not impaired in carboxypeptidase E-deficient Cpe(fat)/Cpe(fat) mice

Sorting of proinsulin from the trans-Golgi network to secretory granules is critical for its conversion to insulin as well as for regulated insulin secretion. The proinsulin sorting mechanism is unknown. Recently, carboxypeptidase E (CPE) was proposed as a sorting receptor for prohormones. To know whether CPE is implicated in proinsulin sorting, pancreatic islets were isolated from CPE-deficient Cpe(fat)/Cpe(fat) mice and Cpe(fat)/+ controls, pulse-labeled ([H-3]leucine), and then chased in basal medium (90 min) to examine constitutive secretion followed by medium with secretagogues (60 min) to stimulate regulated secretion. Secretion of labeled proinsulin via the constitutive pathway was <2% even in Cpe(fat)/Cpe(fat) islets. After a 150-min chase, only 13% of radioactivity remained as proinsulin in Cpe(fat)/+ islets compared with 46% in Cpe(fat)/Cpe(fat) islets, reflecting slower conversion. Regulated secretion was stimulated to an equal extent from Cpe(fat)/+ and Cpe(fat)/Cpe(fat) mice with 20% of the total content of labeled (pro)insulin released during the 60-min stimulatory period. It is concluded that in CPE-deficient Cpe(fat)/ Cpe(fat) mice, proinsulin is efficiently routed to the regulated pathway and its release can be effectively stimulated by secretagogues. CPE is thus not essential for sorting proinsulin to granules.