Claes B. and an increased nuclear pool of Ccnd2 via Prlr and Jak2. Consistent with these results, the loss of MafA resulted in impaired proliferation of -cells at 4 weeks of age. These results suggest that MafA regulates the postnatal proliferation of -cells via prolactin signaling. Introduction Accumulating evidence suggests that postnatal organ development and maturation are critical for future health, especially with respect to metabolic disease [1]. Pancreatic -cells vigorously proliferate postnatally to increase insulin secretion capacity [2], which is implicated in adult -cell mass [3]. Although the compensatory growth of -cell mass in insulin resistance has been intensively investigated [4], the signaling pathway that regulates postnatal proliferation of -cells is less well known [5]. Uncovering this mechanism will elucidate how -cell mass is Mmp16 regulated during development and how the insulin-expressing cells that differentiate from stem cells acquire the capacity to proliferate. During gestation, prolactin signaling is involved in the proliferation of -cells. Generally, placental lactogen or prolactin binds to prolactin receptor (Prlr), which phosphorylates Janus kinase 2 (Jak2) and signal transducer and activator of transcription 5B (Stat5B) [5]. Phosphorylated Stat5B translocates into the nucleus and activates the transcription of its target genes by binding to GAS motifs, the Stat5 binding sequences [6]. The downstream targets of Prlr/Jak2/Stat5B signaling in -cells include insulin, glucose transporter 2 (Glut2), glucokinase (Gck), tryptophan hydroxylase 1 (Tph1), cyclin D2 (Ccnd2) and Prlr [6], [7]. In addition, prolactin signaling may also be involved in the proliferation of -cells after birth, as knockout (KO) neonates have reduced -cell mass [8]. Maturation of -cells occurs concurrently with the expression of v-maf musculoaponeurotic fibrosarcoma oncogene family protein A (MafA) [9], a transcription factor that regulates the expression of insulin via the C1-A2 elements of the insulin promoter [10]. In the pancreas, MafA is expressed exclusively in mature -cells. Forced expression of MafA with Pdx1 and Ngn3 converts pancreatic acinar cells into NF 279 insulin-secreting cells [11]. MafA expression is reduced in the -cell with compromised function [12]. In the islets of the knockout (KO) mice, the ratio of the -cell mass to the -cell mass is normal at birth; however, this ratio is reduced during the neonatal period [13], suggesting that MafA may be involved in regulation of the postnatal -cell mass. Thus, the role of MafA in postnatal proliferation of -cells was investigated in this study. Materials and Methods Mice This study was carried out in strict accordance with the Fundamental Guidelines for Proper Conduct of Animal Experiment and Related Activities in Academic Research Institutions under the jurisdiction of Ministry of Health, Labour and Walfare. The protocol was approved by the Animal Care and Use Committee of the National Center for Global Health and Medicine (Permission Number: 13104). Islet isolation and pancreatic dissection were performed under deep anesthesia followed by cervical dislocation, NF 279 and all efforts were made to minimize suffering. The generation of KO mice was described previously [13]. Male mice were analyzed in this study. Mice were NF 279 genotyped by NaOH extraction methods as described previously [14]. The primers used in this analysis are listed in Table S2 in File S1. Construction of Mouse Prolactin Reporter Luciferase Vectors A reporter vector containing the human promoter ((promoter from high-quality mouse genomic DNA (Clontech) by PCR with the primers listed in Table S3 in File S1. An in-fusion cloning kit (Promega) was utilized to clone the amplified products into the pGL4.10 vector (Clontech, Palo Alto, CA), which NF 279 was digested with NheI and HindIII. The reporter vectors with deletions of the putative MafA binding regions,.