BETA-CELL LINES DERIVED FROM TRANSGENIC CPE(FAT) CPE(FAT) MICE ARE DEFECTIVE IN CARBOXYPEPTIDASE-E AND PROINSULIN PROCESSING/

A spontaneous point mutation in the coding region of the carboxypeptid ase E (CPE) gene in Cpe(fat)/Cpe(fat) mice affects proinsulin processi ng. Cell lines derived from the pancreatic beta-cells of Cpe(fat)/ Cpe (fat) mice were generated by crossing C57BLKS/J-Cpe(fat)/+ mice with N OD mice expressing the simian virus 40 large T oncogene under the cont rol of the rat insulin II promoter. Two cell lines, designated NIT-2 a nd NIT-3, were cultured from adenomatous islets obtained from F2 litte rmates and were compared with the NIT-1 cell line previously developed from mice with wild-type CPE. Electron microscopy of the cultured NIT -2 and -3 cells showed increased numbers of enlarged and electron-luce nt granules compared with NIT-1 cells. Pro-CPE, but not the mature for m of CPE, is present in NIT-P and -3 cells, and neither pro-CPE nor CP E are secreted into the medium. Immunocytochemistry shows the pro-CPE to be localized to an endoplasmic reticulum-like structure in NIT-3 ce lls. Proinsulin is less extensively processed in NIT-2 and -3 cells th an in NIT-1 cells, indicating that the Cpe(fat) mutation affects both the endopeptidase and carboxypeptidase reactions. The secretion of ins ulin/proinsulin from NIT-2 and -3 cells is significantly elevated by s ecretagogues, indicating that CPE is not required for sorting proinsul in into the regulated pathway.