A hierarchical regulatory network guarantees albumin synthesis under pathophysiological challenges

Introduction Albumin is indispensable for systemic homeostasis. In healthy humans, only hepatocytes synthesize albumin. Impressively, the serum albumin level in patients suffering from liver failure are nearly normal. In this study, we delineate a hierarchically transcriptional regulatory network that controls albumin expression in response to different pathophysiological challenges.

Methods Albumin levels in liver tissues and serum were examined in 157 patients (84 hepatocellular carcinoma, 38 decompensated cirrhosis and 35 acute liver failure). Transcription factors HNF4α, C/EBPα and FOXA2 were measured by immunohistochemistry (IHC). The mechanistic investigation was performed in human and mouse primary hepatocytes and liver progenitor cell (LPC) lines, HepaRG and BMOL. RNA sequencing was performed in mouse hepatocytes.

Results 62.4% of the examined patients maintain normal serum albumin levels. In liver tissues, IHC shows albumin expression in hepatocytes. However, in patients with massive hepatocyte loss (MHL), albumin express in activated LPC. In silico analyses imply that three hepatically enriched transcription factors, HNF4a, C/EBPα and FOXA2 possess binding sites in the ALB promoter. ChIP assays confirm binding for all the three factors to ALB promoter in both, hepatocytes and LPC. Knockdown of any factor by RNAi reduces albumin expression in both cell types. IHC analyses further show that HNF4a and C/EBPα are robustly expressed in hepatocytes of normal and non-cirrhotic livers, but are blunted in a large portion of cirrhotic livers. In these patients, hepatocytes instead express increased level of FOXA2. RNA-SEQ analyses show that hedgehog (HH) related gene transcription is induced with knockdown of C/EBPα. We findremarkable expression of Gli2 in hepatocytes lacking HNF4a and C/EBPα, but simultaneously expressing FOXA2. In vitro, disruption of HH signaling reduces FOXA2 expression in hepatocytes. In patients with MHL, active LPCs not only express albumin, but also display either HNF4a or FOXA2, indicating the two transcription factors are required for albumin expression in LPCs.

Conclusions HNF4a, C/EBPα and FOXA2 form a hierarchical regulatory network to guarantee essential albumin expression in response to variant pathophysiological challenges.

Publication History

Article published online:
07 September 2021