The aim of this study was to define metabolic signaling pathways that mediate DNA synthesis and cell cycle progression in adult rodent islets to devise strategies to enhance survival, growth, and proliferation. Since previous studies indicated that glucose-stimulated activation of mammalian target of rapamycin (mTOR) 3 H]thymidine incorporation without altering S phase accumulation under chronic elevated glucose, this increase in DNA synthesis also appears to be primarily related to an arrest in S phase and not cell proliferation.Both types 1 and 2 diabetes result from the inability of pancreatic -cells to secrete sufficient amounts of insulin to maintain normal glucose homeostasis due to an acquired secretory defect and/or inadequate -cell mass. Increased metabolic demands or stress responses that exert a positive effect on -cell mass include obesity, pregnancy, partial pancreatectomy, or chronic glucose exposure. -Cell mass is regulated by cellular mechanisms that include replication, neogenesis, hypertrophy, and apoptosis (1, 2). Recent studies have emphasized the importance of the proliferative capacity of existing adult -cells as a major source of new -cells during adult life that may significantly contribute to the maintenance of -cell mass (3).Mammalian target of rapamycin (mTOR) 2 is a serine/threonine protein kinase that integrates signals derived from growth factors and nutrients to regulate cell growth and proliferation through the regulatory proteins 70-kDa ribosomal protein S6 kinase (S6K1) and the eukaryotic initiation factor 4E-binding protein-1 (4EBP1). This signaling cascade stimulates protein translation and increases the capacity of the ribosomal protein machinery necessary for the onset of DNA synthesis (4, 5). Our previous studies have demonstrated that glucose robustly activates mTOR/S6K1/4EBP1 in an amino acid-dependent manner via its metabolism in both rodent and human islets. Glucose and amino acids, especially leucine and glutamate, are the most prominent activators of mTOR in islets, possibly through ATP production mediated by mitochondrial metabolism (6 -9). Insulin secreted by the -cell and growth factors also provide input to mTOR through the insulin signaling cascade to Akt (6). Akt may directly activate mTOR but also has been shown to inhibit the tumor suppressor proteins TSC1/2. These proteins are activated by AMP-dependent protein kinase that is regulated by the ATP/AMP ratio. Rapamycin specifically inhibits mTOR activation and signaling to 4EBP1 and S6K1. Recent reports have demonstrated a negative feedback pathway from chronically stimulated mTOR to IRS2 that inhibits the insulin signaling pathway (10). Apparently, this negative feedback to IRS2 does not reduce nutrient-stimulated mTOR activation in the -cell as S6K1 remains fully activated during a 4-or 6-day exposure to elevated glucose (7), although other IRS2-dependent pathways may be inhibited.Our previous studies demonstrated that the majority of glucose-stimulated [3 H]thymidine incorporation by rodent islets...