Hepatocyte-specific Hepatocyte Nuclear Factor 4 alpha (HNF4) deletion decreases resting energy expenditure by disrupting lipid and carbohydrate homeostasis

Hepatocyte Nuclear Factor 4 alpha (HNF4α) is required for hepatocyte differentiation and regulates expression of genes involved in lipid and carbohydrate metabolism including those that control VLDL secretion and gluconeogenesis. Whereas previous studies have focused on specific genes regulated by HNF4α in metabolism, its overall role in whole body energy utilization has not been studied. In this study, we used indirect calorimetry to determine the effect of hepatocyte-specific HNF4α deletion (HNF4α-KO) in mice on whole body energy expenditure (EE) and substrate utilization in fed, fasted, and high fat diet (HFD) conditions. HNF4α-KO had reduced resting EE during fed conditions and higher rates of carbohydrate oxidation with fasting. HNF4α-KO mice exhibited decreased body mass caused by fat mass depletion despite no change in energy intake and evidence of positive energy balance. HNF4α-KO mice were able to upregulate lipid oxidation during HFD suggesting that their metabolic flexibility was intact. However, only hepatocyte specific HNF4α-KO mice exhibited significant reduction in basal metabolic rate and spontaneous activity during HFD. Consistent with previous studies, hepatic gene expression in HNF4α-KO supports decreased gluconeogenesis and decreased VLDL export and hepatic Beta-oxidation in HNF4α-KO livers across all feeding conditions. Together, our data suggest deletion of hepatic HNF4α increases dependence on dietary carbohydrates and endogenous lipids for energy during fed and fasted conditions by inhibiting hepatic gluconeogenesis, hepatic lipid export, and intestinal lipid absorption resulting in decreased whole body energy expenditure. These data clarify the role of hepatic HNF4α on systemic metabolism and energy homeostasis.

Wnt/β-catenin Signaling and Liver Regeneration: Circuit, Biology and Opportunities

The liver is uniquely bestowed with an ability to regenerate following a surgical or toxicant insult. One of the most researched models to demonstrate the regenerative potential of this organ is the partial hepatectomy model where two-thirds of the liver is surgically resected. The remnant liver replenishes the lost mass within 10-14 days in mice. The distinctive ability of the liver to regenerate has allowed living donor and split liver transplantation. One signaling pathway shown to be activated during the process of regeneration to contribute towards the mass and functional recovery of the liver is Wnt/β-catenin pathway. Very early after any insult to the liver, the cell-molecule circuitry of Wnt/β-catenin pathway is set into motion with the release of specific Wnt ligands from sinusoidal endothelial cells and macrophages, which in a paracrine manner, engage Frizzled and LDL-Related Protein-5/6 co-receptors on hepatocytes to stabilize β-catenin inducing its nuclear translocation. Nuclear β-catenin interacts with T-cell factor family of transcription factors to induce target genes including Cyclin D1 for proliferation, and others, for regulating hepatocyte function. Working in collaboration with other signaling pathways, Wnt/β-catenin signaling contributes to the restoration process without any compromise of function at any stage. Also, stimulation of this pathway through innovative means, induces liver regeneration when this process is exhausted or compromised, and thus has applications in the treatment of End Stage Liver Disease and in the field of liver transplantation. Thus, Wnt/β-catenin signaling pathway is highly relevant in the discipline of hepatic regenerative medicine.

Role of Non-coding RNA in Acetaminophen-induced Liver Injury

Genomic and transcriptomic analyses have well established that the major fraction of the mammalian genome is transcribed into different classes of RNAs ranging in size from a few nucleotides to hundreds of thousands of nucleotides, which do not encode any protein. Some of these non-coding RNAs (ncRNAs) are directly or indirectly linked to the regulation of expression or functions of ~25,000 proteins coded by <2% of the human genome. Among these regulatory RNAs, microRNAs are small (21-25 nucleotide) RNAs that are processed from precursor RNAs that have stem-loop structure, whereas non-coding RNAs >200 nucleotides are termed lncRNAs (long ncRNAs). Circular RNAs (circRNAs) are newly identified lncRNA members that are generated by back-splicing of primary transcripts. The functions of ncRNAs in modulating liver toxicity of xenobiotics are emerging only recently. Acetaminophen (N-acetyl-para-aminophenol, paracetamol or APAP) is a safe analgesic and antipyretic drug at the therapeutic dose. However, it can cause severe liver toxicity that may lead to liver failure if overdosed or combined with alcohol, herbs, or other xenobiotics. This review discusses the role of ncRNAs in acetaminophen metabolism, toxicity, and liver regeneration after APAP - induced liver injury (AILI).

Ferroptosis and Acetaminophen Hepatotoxicity – Are We Going Down Another Rabbit Hole?

Acetaminophen (APAP) hepatotoxicity is the most frequent cause of acute liver failure in the US. The mechanisms of APAP-induced liver injury have been under extensive investigations for decades and many key events of this necrotic cell death are known today. Initially, two opposing hypotheses for cell death were proposed: Reactive metabolite and protein adducts formation versus reactive oxygen and lipid peroxidation (LPO). In the end, both mechanisms were reconciled, and it is now generally accepted that the toxicity starts with formation of reactive metabolites which, after glutathione depletion, bind to cellular proteins, especially on mitochondria. This results in a mitochondrial oxidant stress, which requires amplification through a mitogen activated protein kinase cascade leading ultimately to enough reactive oxygen and peroxynitrite formation to trigger the mitochondrial membrane permeability transition and cell death. However, the earlier rejected LPO hypothesis seems to make a come-back recently under a different name: ferroptosis. Therefore, the objective of this review was to critically evaluate the available information about intracellular signaling mechanisms of APAP-induced cell death and those of ferroptosis. Conclusion: Under pathophysiologically relevant conditions, there is no evidence for quantitatively enough LPO to cause cell death and thus APAP hepatotoxicity is not caused by ferroptosis. However, the role of mitochondria localized minor LPO remains to be further investigated.

Integrin Linked Kinase (ILK) and its Role in Liver Pathobiology

Integrin Linked Kinase is a vital signaling protein ubiquitously expressed throughout the body. It binds to intracellular integrins to help promote signaling related to cell adhesion, apoptosis, proliferation, migration, and a plethora of other common cellular functions. In this review, ILK’s role in the liver is detailed. Studies have shown ILK to be a major participant in hepatic ECM organization, liver regeneration, insulin resistance, and hepatocellular carcinoma.

Acetaminophen Test Battery (ATB): A Comprehensive Method to Study Acetaminophen-Induced Acute Liver Injury

Acetaminophen (APAP) overdose is the major cause of acute liver failure (ALF) in the Western world. Extensive research is ongoing to identify the mechanisms of APAP-induced ALF. APAP-induced acute liver injury is also one of the most commonly studied drug-induced liver injury models in the field of hepatotoxicity. APAP toxicity is triphasic and includes three mechanistically interlinked but temporally distinct phases of initiation, progression, and recovery/regeneration. Despite how commonly it is studied, the methods to study APAP toxicity differ significantly, often leading to confusing and contradictory data. There are number of reviews on mechanisms of APAP toxicity, but a detailed mechanism-based comprehensive method and list of assays that covers all phases of APAP hepatotoxicity are missing. The goal of this review is to provide a standard protocol and guidelines to study APAP toxicity in mice including a test battery that can help investigators to comprehensively analyze APAP toxicity in the specific context of their hypothesis. Further, we will identify the major roadblocks and common technical problems that can significantly affect the results. This acetaminophen test battery (ATB) will be an excellent guide for scientists studying this most common and clinically relevant drug-induced liver injury and will also be helpful as a roadmap for hypothesis development to study novel mechanisms.

Epidemiological Realities of Alcoholic Liver Disease: Global Burden, Research Trends, and Therapeutic Promise

Globally, alcohol consumption contributes to more than 3 million deaths each year. While much of its ramifications is preventable, a coherent public health discourse on how to limit alcohol-related harm has been overdue. By synthesizing information from national and global databases, we show in this analysis that alcohol consumption level and alcohol-attributable burden of diseases, particularly alcoholic liver disease (ALD), are intimately linked to national income distribution, cultural norms, religion, sex, age, and health status. Prevalence and burden of ALD are positively associated with economic standing in most countries, which necessitate active governmental control via cost-effective policies, such as the best buys proposed by the World Health Organization. To date, a number of critical questions remain unanswered over the molecular mechanisms underlying ALD pathophysiology; the insights gained thereof should provide new opportunities for the advancement of novel diagnostic and management strategies. In comparison with other prevailing liver diseases (e.g., viral hepatitis and nonalcoholic fatty liver disease), governmental support to ALD investigation has been sluggish in most Western countries and China, resulting in a dearth of breakthroughs on both the basic and clinical research fronts in the past decades. Emerging foci of clinical trials for ALD therapy include empirical use of probiotics, antioxidants, growth factors, monoclonal antibodies against key inflammatory mediators, and technology-enhanced behavioral interventions. In this article, we seek to provide a comprehensive analysis on the progress and challenges in tackling ALD as a global health problem, with particular emphasis on global disease burden, socioeconomic influences, research trends, government roles, and future therapies.

The Conundrum of the Pericentral Hepatic Niche: WNT/-Catenin Signaling, Metabolic Zonation, and Many Open Questions

WNT/-catenin signaling promotes stemness, proliferation, and cell fate decisions in various tissue stem cell compartments, which maintain organs with a high turnover of cells (e.g., skin, stomach, and gut). Thus, the -catenin target genes AXIN2 and LGR5 are widely considered as tissue stem cell markers. In contrast, AXIN2 and LGR5 are expressed in pericentral hepatocytes, which do not show overt proliferation during liver homeostasis. Given the low hepatocyte turnover, the liver does not require constant high rates of proliferation, whereas WNT/-catenin signaling is critical for metabolic zonation. Yet, WNT/-catenin pathway upregulation, including AXIN2 and LGR5 induction in hepatocytes throughout the liver, enables hepatocyte regeneration in response to various injuries. In this brief review, I discuss the role of WNT/-catenin signaling in controlling metabolic zonation and the conundrum around pericentral hepatocytes that have been proposed as liver stem cells.

Downregulation of p16 Decreases Biliary Damage and Liver Fibrosis in the Mdr2/ Mouse Model of Primary Sclerosing Cholangitis

Biliary senescence and hepatic fibrosis are hallmarks of cholangiopathies including primary sclerosing cholangitis (PSC). Senescent cholangiocytes display senescence-associated secretory phenotypes [SASPs, e.g., transforming growth factor-1 (TGF-1)] that further increase biliary senescence (by an autocrine loop) and trigger liver fibrosis by paracrine mechanisms. The aim of this study was to determine the effect of p16 inhibition and role of the TGF-1/microRNA (miR)-34a/sirtuin 1 (SIRT1) axis in biliary damage and liver fibrosis in the Mdr2/ mouse model of PSC. We treated (i) in vivo male wild-type (WT) and Mdr2/ mice with p16 Vivo-Morpholino or controls before measuring biliary mass [intrahepatic bile duct mass (IBDM)] and senescence, biliary SASP levels, and liver fibrosis, and (ii) in vitro intrahepatic murine cholangiocyte lines (IMCLs) with small interfering RNA against p16 before measuring the mRNA expression of proliferation, senescence, and fibrosis markers. p16 and miR-34a increased but SIRT1 decreased in Mdr2/ mice and PSC human liver samples compared to controls. p16 immunoreactivity and biliary senescence and SASP levels increased in Mdr2/ mice but decreased in Mdr2/ mice treated with p16 Vivo-Morpholino. The increase in IBDM and hepatic fibrosis (observed in Mdr2/ mice) returned to normal values in Mdr2/ mice treated with p16 Vivo-Morpholino. TGF-1 immunoreactivity and biliary SASPs levels were higher in Mdr2/ compared to those of WT mice but returned to normal values in Mdr2/ mice treated with p16 Vivo-Morpholino. The expression of fibrosis/senescence markers decreased in cholangiocytes from Mdr2/ mice treated with p16 Vivo-Morpholino (compared to Mdr2/ mice) and in IMCLs (after p16 silencing) compared to controls. Modulation of the TGF-1/miR-34a/SIRT1 axis may be important in the management of PSC phenotypes.

The Dynamic Interplay Between Mast Cells, Aging/Cellular Senescence, and Liver Disease

Mast cells are key players in acute immune responses that are evidenced by degranulation leading to a heightened allergic response. Activation of mast cells can trigger a number of different pathways contributing to metabolic conditions and disease progression. Aging results in irreversible physiological changes affecting all organs, including the liver. The liver undergoes senescence, changes in protein expression, and cell signaling phenotypes during aging, which regulate disease progression. Cellular senescence contributes to the age-related changes. Unsurprisingly, mast cells also undergo age-related changes in number, localization, and activation throughout their lifetime, which adversely affects the etiology and progression of many physiological conditions including liver diseases. In this review, we discuss the role of mast cells during aging, including features of aging (e.g., senescence) in the context of biliary diseases such as primary biliary cholangitis and primary sclerosing cholangitis and nonalcoholic fatty liver disease.