A hierarchical regulatory network ensures stable albumin transcription under various pathophysiological conditions

Physiologically, albumin is produced by hepatocytes. It remains largely unknown how patients are capable of maintaining essential albumin levels even in the condition of liver failure. Here, we delineate a hierarchical regulatory network that controls albumin transcription under different pathophysiological conditions. The ALB core promoter possesses a TATA box and nucleosome-free area, which allows constitutive binding of RNA Pol II and thus initiation of transcription. In normal conditions, HNF4α and C/EBPα facilitate albumin transcription through binding to its promoter. In severely damaged livers, hepatocellular HNF4α and C/EBPα expression is often inhibited. The absence of HNF4 and C/EBPα increases hedgehog ligand biosynthesis. Hedgehog upregulates FOXA2 expression through transcription factor GLI2 binding to the FOXA2 promoter. Subsequently, FOXA2 maintains albumin expression in the hepatocytes lacking HNF4α and C/EBPα. In patients with massive hepatocyte loss, the expression of albumin is activated in liver progenitor cells. Albumin transcription in these cells is regulated by HNF4α or FOXA2. Taken together, HNF4α, C/EBPα and FOXA2 form a hierarchical regulatory network that ensures stable albumin expression even in pathophysiological conditions. Mouse primary hepatocytes (MPH) were freshly isolated from three mice: Mouse1, Mouse2, Mouse3. RNA sequencing was performed in MPH with or without HNF4α and C/EBPα siRNA treatment for 48 hours.

从生理学角度而言，白蛋白（albumin）由肝细胞（hepatocytes）合成。目前学界仍未明确，即使在肝功能衰竭状态下，患者仍可维持必需的白蛋白水平的机制。本研究阐明了一种可在不同病理生理条件下调控白蛋白转录的层级调控网络。ALB基因的核心启动子包含TATA盒与核小体缺失区域，该区域可允许RNA聚合酶II（RNA Pol II）持续性结合，从而启动转录过程。在正常生理条件下，肝细胞核因子4α（HNF4α）与CCAAT增强子结合蛋白α（C/EBPα）可通过结合ALB基因启动子，促进白蛋白转录。在严重受损的肝脏中，肝细胞内的HNF4α与C/EBPα表达通常会受到抑制。HNF4α与C/EBPα的缺失会增加刺猬信号通路（Hedgehog）配体的生物合成。刺猬信号通路可通过转录因子GLI2结合FOXA2启动子，上调叉头框蛋白A2（FOXA2）的表达。随后，FOXA2可在缺失HNF4α与C/EBPα的肝细胞中维持白蛋白的表达。在发生大量肝细胞丢失的患者体内，肝祖细胞可激活白蛋白的表达，此类细胞中的白蛋白转录受HNF4α或FOXA2调控。综上，HNF4α、C/EBPα与FOXA2共同构成了层级调控网络，可确保即使在病理生理条件下，白蛋白的表达仍能保持稳定。研究人员从3只小鼠（小鼠1、小鼠2、小鼠3）中新鲜分离得到小鼠原代肝细胞（MPH），对经或未经HNF4α与C/EBPα小干扰RNA（siRNA）处理48小时的MPH进行了RNA测序。