TET3-mediated epigenetic regulation of hepatic glucose production

精确控制肝脏葡萄糖生成（HGP）是维持全身葡萄糖稳态的关键。进化上保守的转录因子HNF4α在肝脏分化和功能中的作用已被广泛研究。Hnf4α包含两个启动子P2和P1，它们以发育和组织特异性的方式通过选择性剪接驱动多个Hnf4α亚型。目前的教条是p2衍生的异构体在胎儿发育过程中占主导地位；然而，出生后，p1衍生的异构体接管，指导广泛的肝功能，包括糖异生。研究发现，HGP在成人肝脏中需要胎儿亚型的HNF4α。这种同型异构体在2型糖尿病（T2D）中在禁食期间急性诱导，并长期增加。P2异构体诱导发生在TET3上调的响应中，TET3是一类新的DNA去甲基化酶的成员，以前未发现与HGP有关。此外，肝脏特异性敲低TET3或P2异构体均可改善饮食性和遗传性T2D小鼠模型中的葡萄糖稳态。假设TET3是一种新的HGP调节因子，通过表观遗传诱导HNF4α胎儿亚型。为了验证这一假设，TET3特异性重新激活P2启动子的分子途径将被剖析。这些方法将涉及分子、生化和全基因组方法，以及小鼠和人类原代肝细胞和小鼠模型。在这些研究完成后，TET3特异性诱导HNF4α胎儿异构体增强HGP的机制将被确定。如果成功，这些研究有望发现新的T2D治疗靶点。

Precise control of hepatic glucose production (HGP) is pivotal to maintain whole-body glucose homeostasis. The evolutionarily conserved transcription factor HNF4α has been extensively studied for its role in hepatic differentiation and function. Hnf4α contains two promoters, P2 and P1, which drive multiple HNF4α isoforms via alternative splicing in a development- and tissue-specific manner. The current dogma is that the P2-derived isoform predominates during fetal development; however, after birth the P1-derived isoform takes over, directing a wide range of liver functions including gluconeogenesis. It is found that the fetal isoform of HNF4α is required for HGP in adult liver. This isoform is acutely induced during fasting and chronically increased in type-2 diabetes (T2D). P2 isoform induction occurs in response to upregulation of TET3, a member of a new class of DNA demethylases not previously shown to be involved in HGP. Furthermore, liver-specific knockdown of either TET3 or the P2 isoform alone improves glucose homeostasis in dietary and genetic mouse models of T2D. It is hypothesized that TET3 is a novel regulator of HGP by epigenetically inducing the HNF4α fetal isoform. To test the hypothesis, the molecular pathways by which TET3 specifically reactivates the P2 promoter will be dissected. The approaches will involve molecular, biochemical, and genome-wide approaches as well as mouse and human primary hepatocytes and mouse models. Upon completion of these studies the mechanisms by which TET3 specifically induces the HNF4α fetal isoform to enhance HGP will be defined. If successful, these studies hold the promise of discovering new therapeutic targets for T2D.