OBJECTIVE: Pregnancy and postpartum states drive dynamic expansion and regression of maternal β-cell mass. Little is known about what regulates postpartum regression. We recently profiled murine islets at different time points from late gestation to early postpartum to identify regulators of β-cell apoptosis or survival. One hit was c-Jun, a transcription factor which regulates proliferation, apoptosis, and survival in various tissues and cells. Here, we examine c-Jun regulation and function during gestation and postpartum and in murine and human islets. METHODS: To examine regulation of c-Jun within β-cells we used a mouse genetic model lacking β-cell prolactin receptor (PRLR) and stimulation of cultured islets with recombinant prolactin. We used chemical inhibitors of signal transduction pathways to examine signaling downstream of PRLR. TUNEL was used to detect endogenous or dexamethasone-induced apoptosis. Q-PCR, Western blotting, and immunostaining were used to assess gene and protein expression. Knockdown of c-Jun in MIN6 cells was accomplished using siRNA and lentiviral-shRNA. We examined both murine and human islets and tissues. RESULTS: We found that expression of c-Jun transcript in murine β-cells is temporally restricted to late gestation and early postpartum and requires prolactin signaling. Moreover, c-Jun protein expression was mutually exclusive with apoptotic β-cells identified by TUNEL staining. In both murine and human islets, prolactin treatment is sufficient to induce c-Jun expression and downstream MAPK/ERK signaling. Pharmacologic inhibition of c-Jun blocks prolactin-mediated survival of β-cells following pro-apoptotic stress, including failure to upregulate pro-survival factors Bcl2l1 (Bcl-xL) and Birc5 (Survivin). Finally, human islets during pregnancy also exhibit increased c-Jun expression in β-cells, but c-Jun induction is absent in β-cells from pregnant donors with gestational diabetes (GDM). CONCLUSIONS: c-Jun contributes to pro-survival effects of lactogens downstream of PRLR / MAPK signaling in β-cells. c-Jun regulation is conserved in human islets and pregnancy and dysregulated in GDM.