Expression mechanism of tryptophan hydroxylase 1 in mouse islets during pregnancy

Abstract

Serotonin signaling plays key roles in augmentation of pancreatic b-cell function during pregnancy. Increased expression of tryptophan hydroxylase 1 (Tph1), a rate-limiting enzyme for serotonin synthesis by lactogenic hormones, is involved in this phenomenon. To investigate its mechanisms, we here performed 5 0 -RACE and identified b-cell-specific transcription initiation sites for Tph1. Prolactin enhanced the expression of mRNA containing these exons; however, reporter gene plasmids containing the proximal 5 0 -flanking region of these exons did not show prolactin responsiveness in MIN6 cells.Prolactin-induced Tph1 expression was inhibited by a Jak2 inhibitor and was partially inhibited by an MEK1/2 or PI3K inhibitor. Therefore, we analyzed interferon g-activated sequences (GAS) and found GAS-A about 9-kbp upstream of the transcription start site. The reporter gene plasmid containing the GAS-A region linked to a heterologous promoter showed increased promoter activity by prolactin, which was inhibited by the forced expression of a dominant-negative mutant form of Stat5A and a Jak2 inhibitor. Chromatin immunoprecipitation analysis showed that prolactin treatment augmented Stat5 binding to the GAS-A region in MIN6 cells, as well as in isolated mouse islets, and that Stat5 recognized the GAS-A region in pregnant mouse islets. In addition, the transactivation activity of Stat5 was enhanced by prolactin through the Erk and PI3K pathways in MIN6 cells. Finally, serotonin expression was attenuated in islets of b-cell-specific Stat5-deficient mice compared with that of control littermates during pregnancy. Our findings suggest that prolactin-induced Tph1 expression is mediated by the activation of Jak2/Stat5, Erk, and PI3K pathways in b cells.Key Words

Serotonin competence of mouse beta cells during pregnancy

“…3). PRL signalling is triggered by PRL binding to preformed PRLR homodimers [7,24,30]. JAK2 is constitutively associated with the PRLR and is phosphorylated after PL binding.…”

“…Phosphorylated JAK2 recruits and phosphorylates STAT5 [31], which dimerises and migrates to the nucleus where it acts as a transcription factor. STAT5 recognises its target genes via an interferon γ-activated motif, which was recently identified in the Tph1 gene promoter [30].…”

“…This merits further investigation as serotonin is known to have potent vasoconstricting effects [39] and because pancreatic islets are known to have a very high local blood flow that may underlie part of the functional heterogeneity between islets [40]. Previous studies of blood flow in islets of pregnant rats demonstrated that islet blood flow was increased at day 15 of pregnancy but decreased at day 18 of pregnancy [30]. Regarding the heterogeneous pattern of the serotoninproducing cells throughout the islet, it could be that local production of serotonin at strategically interesting locations in the islet regulates arterioles in islets of pregnant mice.…”

Tryptophan Predicts the Risk for Future Type 2 Diabetes

“…Tryptophan, an aromatic amino acid metabolite, has expansive physiological functions in the regulation of growth and feed intake, mood and behavior, and immune responses[12, 13]. It is one of the essential amino acids for the human.…”

Biphasic changes in β-cell mass around parturition are accompanied by increased serotonin production

“…5A-H). Serotonin has been reported to regulate β-cell expansion during pregnancy in both humans and mice, and serotonin production is induced by PRL and placental lactogen 7,14 .…”