Administration of hepatic stimulatory substance alone or with other liver growth factors does not ameliorate acetaminophen-induced liver failure

Fibrinolysis-Mediated Pathways in Acute Liver Injury

Acute liver injury (ALI), that is, the development of reduced liver function in patients without preexisting liver disease, can result from a wide range of causes, such as viral or bacterial infection, autoimmune disease, or adverse reaction to prescription and over-the-counter medications. ALI patients present with a complex coagulopathy, characterized by both hypercoagulable and hypocoagulable features. Similarly, ALI patients display a profound dysregulation of the fibrinolytic system with the vast majority of patients presenting with a hypofibrinolytic phenotype. Decades of research in experimental acute liver injury in mice suggest that fibrinolytic proteins, including plasmin(ogen), plasminogen activators, fibrinolysis inhibitors, and fibrin(ogen), can contribute to initial hepatotoxicity and/or stimulate liver repair. This review summarizes major experimental findings regarding the role of fibrinolytic factors in ALI from the last approximately 30 years and identifies unanswered questions, as well as highlighting areas for future research.

Liver Regeneration after Acetaminophen Hepatotoxicity: Mechanisms and Therapeutic Opportunities

Acetaminophen (N-acetyl-para-aminophenol; APAP) overdose is the most common cause of acute liver failure in the Western world, with limited treatment opportunities. For years, research on APAP overdose has been focused on investigating the mechanisms of hepatotoxicity, with limited success in advancing therapeutic strategies. Acute liver injury after any insult, including APAP overdose, is followed by compensatory liver regeneration, which promotes recovery and is a crucial determinant of the final outcome. Liver regeneration after APAP-induced liver injury is dose dependent and impaired after severe APAP overdose. Although robust regenerative response is associated with spontaneous recovery and survival, impaired regeneration results in faster progression of injury and death after APAP overdose. APAP hepatotoxicity-induced liver regeneration involves a complex time- and dose-dependent interplay of several signaling mediators, including growth factors, cytokines, angiogenic factors, and other mitogenic pathways. Compared with the liver injury, which is established before most patients seek medical attention and has proved difficult to manipulate, liver regeneration can be potentially modulated even in late-stage APAP-induced acute liver failure. Despite recent efforts to study the mechanisms of liver regeneration after APAP-induced liver injury, more comprehensive research in this area is required, especially regarding factors that contribute to impaired regenerative response, to develop novel regenerative therapies for APAP-induced acute liver failure.

Pro-regenerative signaling after acetaminophen-induced acute liver injury in mice identified using a novel incremental dose model

Acetaminophen (APAP) overdose results in acute liver failure and has limited treatment options. Previous studies show that stimulating liver regeneration is critical for survival after APAP overdose, but the mechanisms remain unclear. In this study, we identified major signaling pathways involved in liver regeneration after APAP-induced acute liver injury using a novel incremental dose model. Liver injury and regeneration were studied in C57BL/6 mice treated with either 300 mg/kg (APAP300) or 600 mg/kg (APAP600) APAP. Mice treated with APAP300 developed extensive liver injury and robust liver regeneration. In contrast, APAP600-treated mice exhibited significant liver injury but substantial inhibition of liver regeneration, resulting in sustained injury and decreased survival. The inhibition of liver regeneration in the APAP600 group was associated with cell cycle arrest and decreased cyclin D1 expression. Several known regenerative pathways, including the IL-6/STAT-3 and epidermal growth factor receptor/c-Met/mitogen-activated protein kinase pathways, were activated, even at APAP600, where regeneration was inhibited. However, canonical Wnt/β-catenin and NF-κB pathways were activated only in APAP300-treated mice, where liver regeneration was stimulated. Furthermore, overexpression of a stable form of β-catenin, where serine 45 is mutated to aspartic acid, in mice resulted in improved liver regeneration after APAP overdose. Taken together, our incremental dose model has identified a differential role of several signaling pathways in liver regeneration after APAP overdose and highlighted canonical Wnt signaling as a potential target for regenerative therapies for APAP-induced acute liver failure.

Transfection of hepatic stimulator substance gene desensitizes hepatoma cells to H2O2-induced cell apoptosis via preservation of mitochondria

Hepatic stimulator substance (HSS) protects liver cells from various toxins. However, the mechanism by which HSS protects hepatocytes remains unclear. In this study, we report that the HSS gene, after transfection into BEL-7402 hepatocma cells, is stably expressed in the mitochondria. Hydrogen peroxide (H(2)O(2))-induced cell apoptosis in the HSS-transfected cells is reduced, as shown by morphologic analysis. In the HSS-transfected cells, disruption of mitochondrial transmembrane potential (MTP) and cytochrome c leakage are reduced. The anti-apoptotic gene Bcl-2 is also highly expressed. In addition, ATP levels in the HSS-transfected cells are maintained. In conclusion, in hepatoma cells, HSS gene expression protects cells against H(2)O(2) injury, and this effect is likely to be associated with preservation of mitochondria.

Insights on augmenter of liver regeneration cloning and function

Hepatic stimulator substance (HSS) has been referred to as a liver-specific but species non-specific growth factor. Gradient purification and sequence analysis of HSS protein indicated that it contained the augmenter of liver regeneration (ALR), also known as hepatopoietin (HPO). ALR, acting as a hepatotrophic growth factor, specifically stimulated proliferation of cultured hepatocytes as well as hepatoma cells in vitro, promoted liver regeneration and recovery of damaged hepatocytes and rescued acute hepatic failure in vivo. ALR belongs to the new Erv1/Alr protein family, members of which are found in lower and higher eukaryotes from yeast to man and even in some double-stranded DNA viruses. The present review article focuses on the molecular biology of ALR, examining the ALR gene and its expression from yeast to man and the biological function of ALR protein. ALR protein seems to be non-liver-specific as was previously believed, increasing the necessity to extend research on mammalian ALR protein in different tissues, organs and developmental stages in conditions of normal and abnormal cellular growth.

Intracellular signaling mechanisms of acetaminophen-induced liver cell death

Acetaminophen hepatotoxicity is the leading cause of drug-induced liver failure. Despite substantial efforts in the past, the mechanisms of acetaminophen-induced liver cell injury are still incompletely understood. Recent advances suggest that reactive metabolite formation, glutathione depletion, and alkylation of proteins, especially mitochondrial proteins, are critical initiating events for the toxicity. Bcl-2 family members Bax and Bid then form pores in the outer mitochondrial membrane and release intermembrane proteins, e.g., apoptosis-inducing factor (AIF) and endonuclease G, which then translocate to the nucleus and initiate chromatin condensation and DNA fragmentation, respectively. Mitochondrial dysfunction, due to covalent binding, leads to formation of reactive oxygen and peroxynitrite, which trigger the membrane permeability transition and the collapse of the mitochondrial membrane potential. In addition to the diminishing capacity to synthesize ATP, endonuclease G and AIF are further released. Endonuclease G, together with an activated nuclear Ca2+,Mg2+-dependent endonuclease, cause DNA degradation, thereby preventing cell recovery and regeneration. Disruption of the Ca2+ homeostasis also leads to activation of intracellular proteases, e.g., calpains, which can proteolytically cleave structural proteins. Thus, multiple events including massive mitochondrial dysfunction and ATP depletion, extensive DNA fragmentation, and modification of intracellular proteins contribute to the development of oncotic necrotic cell death in the liver after acetaminophen overdose. Based on the recognition of the temporal sequence and interdependency of these mechanisms, it appears most promising to therapeutically target either the initiating event (metabolic activation) or the central propagating event (mitochondrial dysfunction and peroxynitrite formation) to prevent acetaminophen-induced liver cell death.

Effect of hepatic stimulator substance administration on tissue regeneration due to thioacetamide-induced liver injury in rats

BACKGROUND

Hepatic stimulator substance (HSS) is a known hepatic growth factor that appears to be organ-specific but species-nonspecific. In the present study we investigated the effect of HSS administration in a rat model of liver injury and regeneration induced by thioacetamide (TAA) injection.

METHODS

TAA (300 mg/kg body weight) was injected intraperitoneally in male Wistar rats (group I). HSS (50 mg protein/kg body weight) was administered intraperitoneally either at 24 h (group II) or at 36 h (group III) after TAA treatment. The animals were killed at different time points after TAA injection, and the rate of tritiated thymidine incorporation into hepatic DNA, the activity of the enzyme thymidine kinase in liver, and the assessment of the mitotic index in hepatocytes were used to estimate liver regeneration.

RESULTS

The administration of TAA caused severe hepatic injury recognized histopathologically and by increased activities of the serum hepatic enzymes aspartate and alanine aminotransferases. The hepatic injury, which peaked at 24 h and 36 h after TAA injection, was followed by a regenerative process of hepatocytes which presented peaks after 48 h and 60 h (group I). The regenerative process of hepatocytes remained unaffected when HSS was administered 24 h after the injection of TAA (group II). In the case of HSS administration 36 h after the injection of TAA (group III) the examined indices of hepatocyte proliferation were statistically significantly increased at 48 h (P < 0.001), compared with those observed in group I.

CONCLUSIONS

The administration of HSS enhanced the hepatocyte proliferative capacity, induced by TAA treatment, depending on the time of its administration.