Background and aim: We have recently demonstrated that the transcription factor Pax8 is neither expressed in healthy human pancreatic islets nor in neuroendocrine tumors. Intriguingly, other studies have reported that Pax8 expression was increased in islets isolated from pregnant mice suggesting that this transcription factor could be stimulated under specific physiological conditions. To address this question, we investigated whether Pax8 was indeed induced in both human and mouse islets under gestational conditions and whether it correlated with islet mass expansion associated with pregnancy. Methods: In order to study the effect of pregnancy on Pax8 expression, we mimik gestation in vitro by culturing isolated human islets in the presence of 200ng/mL of human prolactin for 24, 48, 72 and 96 hours. RNA was extracted and quantitative RT-PCR (QT-RT-PCR) was performed to assess expression levels of Pax8, that were normalized to cyclophylin. As positive control for prolactin action, expression levels of TPH1 were also assessed..In parallel, cell proliferation was measured by BrdU incorporation. Additionally, Pax8 expression was also evaluated by QT-RTPCR in murine islets isolated from pregnant mice at gestional days 10.5, 14.5 18,5 as well as postpartumally. Immunohistochemistry analysis of Pax8 was performed on pancreas sections of pregnant mice to validate the results obtained at mRNA level. Results: Prolactin treatment evoked a transient increase in the proliferation of human islets reaching maximal levels of 2-fold at 48 hours. In parallel, TPH-1 expression was induced by approximately 2-fold at 48 hours emphasizing the ability of prolactin to partially recapitulate pregnancy conditions in vitro. More importantly, a 4-fold increase in Pax8 expression was observed after 72 hours of treatment. Similarly, Pax8 transcript levels in pregnant mice islets were also increased reaching maximal levels of 18-fold on gestional day 14.5 before returning to control levels postpartumally. Consistent with mRNA induction, Pax8 protein was also detected in pancreas sections of mice at gestional day 14.5. Interestingly, only a subpopulation of islet cells was Pax8 positive. Conclusion: A strong hormone-dependent up-regulation of Pax8 biosynthesis occurs in mice during pregnancy and most-likely in human. Intriguinly, expression of Pax8 was only present in a subpopulation of islet cells. Furthemore, induction of Pax8 did not correlate with onset of prolactin-induced proliferation in human raising the possibility that Pax8 may repress rather than stimulate islet cell replication.

This work is funded by the Spanish Ministry of Science and Innovation, Instituto de Salud Carlos III (PI10/00871) and by the Foundación Pública Adanluza Progreso y Salud.

Trabajo presentado en el XXIII Congreso Nacional de la Sociedad Española de Diabetes, celebrado en Vigo del 19 al 21 de abril de 2012.

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