The molecular determinants of beta-cell mass expansion remain poorly understood. Cyclin D2 is the major D-type cyclin expressed in beta-cells, essential for adult beta-cell growth. We hypothesized that cyclin D2 could be actively regulated in beta-cells, which could allow mitogenic stimuli to influence beta-cell expansion. Cyclin D2 protein was sharply increased after partial pancreatectomy, but cyclin D2 mRNA was unchanged, suggesting posttranscriptional regulatory mechanisms influence cyclin D2 expression in beta-cells. Consistent with this hypothesis, cyclin D2 protein stability is powerfully regulated in fibroblasts. Threonine 280 of cyclin D2 is phosphorylated, and this residue critically limits D2 stability. We derived transgenic (tg) mice with threonine 280 of cyclin D2 mutated to alanine (T280A) or wild-type cyclin D2 under the control of the insulin promoter. Cyclin D2 T280A protein was expressed at much higher levels than wild-type cyclin D2 protein in beta-cells, despite equivalent expression of tg mRNAs. Cyclin D2 T280A tg mice exhibited a constitutively nuclear cyclin D2 localization in beta-cells, and increased cyclin D2 stability in islets. Interestingly, threonine 280-mutant cyclin D2 tg mice had greatly reduced beta-cell apoptosis, with suppressed expression of proapoptotic genes. Suppressed beta-cell apoptosis in threonine 280-mutant cyclin D2 tg mice resulted in greatly increased beta-cell area in aged mice. Taken together, these data indicate that cyclin D2 is regulated by protein stability in pancreatic beta-cells, that signals that act upon threonine 280 limit cyclin D2 stability in beta-cells, and that threonine 280-mutant cyclin D2 overexpression prolongs beta-cell survival and augments beta-cell mass expansion.