CRL4(DCAF8)泛素连接酶调控组蛋白H3泛素化在肝脏成熟和肝癌生成中的机制研究

Liver development follows a highly coordinated program of gene activation and silencing. In postnatal liver, expression of fetal genes such as AFP, GPC3 and IGF2 is progressively turned off, and the underlying mechanism is largely unknown, in contrast to the wealth of our knowledge about the early hepatic cell lineage specification and differentiation. We recently found that histone H3 is progressively and specifically ubiquitinated in mouse liver after birth, concomitant with declining expression of fetal liver genes. Inactivation of DDB1, an essential component of the CRL4 ubiquitin ligase complex, in adult mouse liver abolishes H3 ubiquitination and re-activates fetal liver gene expression. Additionally, we have screened and identified DCAF8 as a potential adaptor protein to bring H3 to CRL4. We therefore hypothesize that CRL4(DCAF8) ubiquitin ligase targets histone H3 for ubiquitination to silence fetal liver gene expression and promote liver maturation. In this proposal, we will: 1) demonstrate that CRL4(DCAF8) binds and ubiquitinates H3 in cell culture and mouse liver; 2) identify and validate the ubiquitination site(s) on H3 modified by CRL4(DCAF8); 3) determine the role of CRL4(DCAF8)-targeted H3 ubiquitination in regulating fetal liver gene expression in vitro and in vivo; 4) investigate the molecular mechanism underlying fetal liver gene silencing by CRL4(DCAF8)-targeted H3 ubiquitination. Since we also found that H3 ubiquitination is decreased in about 17% patient HCC samples compared to matching nontumor tissues, our study will shed light into how this specific epigenetic modification regulates liver malignancy in addition to normal liver maturation.

肝脏发育是一个基因激活和沉默高度协调的过程。在个体出生后，胚胎肝脏基因比如AFP,GPC3和IGF2表达逐渐下调，但调控机制尚不明确。我们发现出生后小鼠肝脏中组蛋白H3泛素化逐渐增强，同时胚胎肝脏基因的表达下调。在成年小鼠肝脏中敲除CRL4泛素连接酶会抑制H3泛素化，并激活胚胎肝脏基因的表达。此外，我们鉴定出DCAF8可能是CRL4泛素化H3的接头蛋白。我们因此提出假说：CRL4（DCAF8）泛素化组蛋白H3从而抑制胚胎肝脏基因的表达并促进肝脏发育成熟。我们将：1）证明CRL4（DCAF8）在细胞系和肝脏中结合并泛素化H3；2）鉴定并验证H3被CRL4（DCAF8）泛素化的位点；3）确定依赖于CRL4(DCAF8)的H3泛素化在调控胚胎肝脏基因表达中的作用；4）探索H3泛素化抑制胚胎肝脏基因表达的分子机制。该研究也会帮助阐明H3泛素化在肝癌生成过程中的作用。

组蛋白的翻译后修饰能调控染色体上基因的转录活性，组蛋白翻译后修饰的异常会导致发育障碍和疾病发生，但组蛋白是如何被泛素化调控的目前知之甚少。我们研究发现，在肝细胞中，CRL4（DDB1-Cul4-Rbx1）泛素连接酶能通过底物受体DCAF8多泛素化组蛋白H3的第79位赖氨酸位点（K79）位点。在肝癌细胞系中通过敲除DCAF8，或者过表达H3K79R突变的载体，或者在小鼠肝脏内敲除DDB1，都能够抑制组蛋白H3泛素化，进而改变H3K9的甲基化水平，重新激活胚胎肝脏相关基因的表达并最终导致细胞通过衰老而死亡。在组蛋白H3低泛素化的细胞系中增强CRL4（DACF8）活性能够增强其泛素化水平，并通过表观遗传学调控沉默基因的表达。我们的研究结果表明，组蛋白H3的多泛素化能够抑制肝脏在发育成熟过程中一系列基因的表达，而这个过程的紊乱可能对肝脏疾病，例如癌症有一定促进作用。我们的研究有助于更好地理解组蛋白的翻译后修饰，并为肝脏发育和肝癌发展过程的提供了新的生物标志物。我们已完成申请书上所列的研究目标，共发表标注本项目资助的期刊论文3篇，培养博士毕业生4人，硕士毕业生1人。