278 Jak/Stat3 SIGNALING PROMOTES SOMATIC CELL REPROGRAMMING BY EPIGENETIC REGULATION

Abstract

Although leukemia inhibitory factor (LIF) maintains the ground state pluripotency of mouse embryonic stem cells and induced pluripotent stem cells (iPSC) by activating the Janus kinase/signal transducer and activator of transcription 3 (Jak/Stat3) pathway, the mechanism remains unclear. Stat3 has only been shown to promote complete reprogramming of epiblast and neural stem cells, and the partially reprogrammed cells (pre-iPSC). We investigated if and how Jak/Stat3 activation promotes reprogramming of terminally differentiated mouse embryonic fibroblasts (MEF). We demonstrated that activated Stat3 not only promotes but is essential for the pluripotency establishment in MEF cell reprogramming. We further demonstrated that, during reprogramming, inhibiting Jak/Stat3 activity blocks demethylation of Oct4 and Nanog regulatory DNA sequences in induced cells, which are marked by suppressed endogenous pluripotent gene expression. These are correlated with significant upregulation of DNA methyltransferase (Dnmt) 1 and histone deacetylases (HDAC) expression, as well as the increased expression of lysine-specific histone demethylase 2 and methyl-CpG binding protein 2. Inhibiting Jak/Stat3 also blocks the expression of Dnmt3L, which is correlated with the failure of retroviral transgene silencing. Furthermore, Dnmt or HDAC inhibitor but not overexpression of Nanog rescues the reprogramming arrested by Jak/Stat3 inhibition or LIF deprivation. Finally, we demonstrated that LIF/Stat3 signal also represents the prerequisite for complete reprogramming of pre-iPSC. We conclude that Jak/Stat3 activity plays a fundamental role in promoting the establishment of pluripotency at the epigenetic level, by facilitating DNA demethylation/de novo methylation and open-chromatin formation during late stage reprogramming.