HNF4α isoforms: the fraternal twin master regulators of liver function

Circadian Gating of Thyroid Hormone Action in Hepatocytes

Thyroid hormones, thyroxin (T4) and the biologically active triiodothyronine (T3), play important roles in liver metabolic regulation, including fatty acid biosynthesis, beta-oxidation, and cholesterol homeostasis. These functions position TH signaling as a potential target for the treatment of metabolic dysfunction-associated steatotic liver disease (MASLD). Elevated T3 levels in the circulation are associated with increased hepatic lipid turnover, which is also under the control of the circadian clock system. In this study, we developed a cell system to study the impact of hepatocyte circadian rhythms on the metabolic response to T3 treatment under control and steatotic conditions. Synchronized AML-12 circadian reporter hepatocytes were treated with T3 at different circadian phases and metabolic conditions. T3 treatment increased metabolic activity in a dose-independent fashion and had no significant effect on circadian rhythms in AML-12 cells. T3 had marked time-of-treatment-dependent effects on metabolic transcript expression. Steatosis induction altered metabolic transcript expression in AML-12 cells. In this condition, the circadian rhythm period was lengthened, and this effect was independent of T3. Under steatotic conditions, T3 had marked time-of-treatment dependent effects on metabolic transcript expression, which differed from those observed under control conditions. These findings reveal a time-of-day-dependent response of hepatocytes to T3, which is further modulated by the metabolic state. Our data suggest that time has a strong influence on liver TH action, which might be considered when treating MASLD.

Patent Highlights June-July 2023

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

Hepatocyte nuclear factor 4 a (HNF4α): A perspective in cancer

HNF4α, a transcription factor, plays a vital role in regulating functional genes and biological processes. Its alternative splicing leads to various transcript variants encoding different isoforms. The spotlight has shifted towards the extensive discussion on tumors interplayed withHNF4α abnormalities. Aberrant HNF4α expression has emerged as sentinel markers of epigenetic shifts, casting reverberations upon downstream target genes and intricate signaling pathways, most notably with cancer. This review provides a comprehensive overview of HNF4α's involvement in tumor progression and metastasis, elucidating its role and underlying mechanisms.

HNF4α isoforms regulate the circadian balance between carbohydrate and lipid metabolism in the liver

Hepatocyte Nuclear Factor 4α (HNF4α), a master regulator of hepatocyte differentiation, is regulated by two promoters (P1 and P2) which drive the expression of different isoforms. P1-HNF4α is the major isoform in the adult liver while P2-HNF4α is thought to be expressed only in fetal liver and liver cancer. Here, we show that P2-HNF4α is indeed expressed in the normal adult liver at Zeitgeber time (ZT)9 and ZT21. Using exon swap mice that express only P2-HNF4α we show that this isoform orchestrates a distinct transcriptome and metabolome via unique chromatin and protein-protein interactions, including with different clock proteins at different times of the day leading to subtle differences in circadian gene regulation. Furthermore, deletion of the Clock gene alters the circadian oscillation of P2- (but not P1-)HNF4α RNA, revealing a complex feedback loop between the HNF4α isoforms and the hepatic clock. Finally, we demonstrate that while P1-HNF4α drives gluconeogenesis, P2-HNF4α drives ketogenesis and is required for elevated levels of ketone bodies in female mice. Taken together, we propose that the highly conserved two-promoter structure of the Hnf4a gene is an evolutionarily conserved mechanism to maintain the balance between gluconeogenesis and ketogenesis in the liver in a circadian fashion.

Mechanism and Effect of HNF4α Decrease in a Rat Model of Cirrhosis and Liver Failure

BACKGROUND & AIMS

HNF4α, a master regulator of liver development and the mature hepatocyte phenotype, is down-regulated in chronic and inflammatory liver disease. We used contemporary transcriptomics and epigenomics to study the cause and effects of this down-regulation and characterized a multicellular etiology.

METHODS

Progressive changes in the rat carbon tetrachloride model were studied by deep RNA sequencing and genome-wide chromatin immunoprecipitation sequencing analysis of transcription factor (TF) binding and chromatin modification. Studies compared decompensated cirrhosis with liver failure after 26 weeks of treatment with earlier compensated cirrhosis and with additional rat models of chronic fibrosis. Finally, to resolve cell-specific responses and intercellular signaling, we compared transcriptomes of liver, nonparenchymal, and inflammatory cells.

RESULTS

HNF4α was significantly lower in 26-week cirrhosis, part of a general reduction of TFs that regulate metabolism. Nevertheless, increased binding of HNF4α contributed to strong activation of major phenotypic genes, whereas reduced binding to other genes had a moderate phenotypic effect. Decreased Hnf4a expression was the combined effect of STAT3 and nuclear factor kappa B (NFκB) activation, which similarly reduced expression of other metabolic TFs. STAT/NFκB also induced de novo expression of Osmr by hepatocytes to complement induced expression of Osm by nonparenchymal cells.

CONCLUSIONS

Liver decompensation by inflammatory STAT3 and NFκB signaling was not a direct consequence of progressive cirrhosis. Despite significant reduction of Hnf4a expression, residual levels of this abundant TF still stimulated strong new gene expression. Reduction of HNF4α was part of a broad hepatocyte transcriptional response to inflammation.