. Pancreatic islet-specific overexpression of Reg3 protein induced the expression of pro-islet genes and protected the mice against streptozotocin-induced diabetes mellitus. Am J Physiol Endocrinol Metab 300: E669 -E680, 2011. First published January 18, 2011; doi:10.1152/ajpendo.00600.2010.-Reg family proteins have been implicated in islet -cell proliferation, survival, and regeneration. The expression of Reg3 (pancreatitis-associated protein) is highly induced in experimental diabetes and acute pancreatitis, but its precise role has not been established. Through knockout studies, this protein was shown to be mitogenic, antiapoptotic, and anti-inflammatory in the liver and pancreatic acinars. To test whether it can promote islet cell growth or survival against experimental damage, we developed -cell-specific overexpression using rat insulin I promoter, evaluated the changes in normal islet function, gene expression profile, and the response to streptozotocin-induced diabetes. Significant and specific overexpression of Reg3 was achieved in the pancreatic islets of RIP-I/Reg3 mice, which exhibited normal islet histology, -cell mass, and in vivo and in vitro insulin secretion in response to high glucose yet were slightly hyperglycemic and low in islet GLUT2 level. Upon streptozotocin treatment, in contrast to wild-type littermates that became hyperglycemic in 3 days and lost 15% of their weight, RIP-I/Reg3 mice were significantly protected from hyperglycemia and weight loss. To identify specific targets affected by Reg3 overexpression, a whole genome DNA microarray on islet RNA isolated from the transgenic mice revealed more than 45 genes significantly either up-or downregulated. Among them, isletprotective osteopontin/SPP1 and acute responsive nuclear protein p8/NUPR1 were significantly induced, a result further confirmed by real-time PCR, Western blots, and immunohistochemistry. Our results suggest that Reg3 is unlikely an islet growth factor but a putative protector that prevents streptozotocin-induced damage by inducing the expression of specific genes.pancreatitis-associated protein; DNA microarray; real-time polymerase chain reaction; Western blot; immunofluorescence; Reg family proteins; transgenic mice A NORMAL -CELL MASS is a crucial element against the development of diabetes mellitus and is determined by at least four independent parameters, i.e., mitogenic replication, cell size expansion, neogenesis (from other pancreatic cells), and apoptosis (42). The net rate of -cell growth is the balance among the rates of -cell replication, hypertrophy, neogenesis, and apoptosis (10). Various growth factors such as hepatocyte growth factor and glucagon-like peptide-1 (GLP-1) have been reported to stimulate islet cell expansion (12, 13, 43). For a therapeutic purpose, -cell mass could also be supplemented by islet cell transplantation where growth factors were shown to promote the survival and/or expansion of transplanted cells in rodents (8,43). Unfortunately, very few of these factors have been pr...