Decreased hepatic nitric oxide production contributes to the development of rat sinusoidal obstruction syndrome

[The role of activated hepatic stellate cells in liver fibrosis, portal hypertension and cancer angiogenesis]

Although hepatic stellate cells, which are liver specific pericytes, have been recognized within the vasculature of the sinusoid for more than one hundred years, the biology and function of these cells is unclear. Recent studies have highlighted the key role of stellate cells in a number of fundamental processes that include wound healing/fibrosis, vasoregulation, and vascular remodeling/angiogenesis. In the liver, these processes are particularly important in the development of cirrhosis, portal hypertension and cancer. This article highlights the recent advances in our understanding of the biology of hepatic stellate cells and discusses some of the recently-ascribed functions that are relevant to liver fibrosis, portal hypertension and cancer angiogenesis.

Sinusoidal endothelial cells prevent rat stellate cell activation and promote reversion to quiescence

Capillarization precedes hepatic fibrosis. We hypothesize that capillarization of sinusoidal endothelial cells (SEC) is permissive for hepatic stellate cell (HSC) activation and therefore permissive for fibrosis. We examined whether freshly isolated SECs prevent activation of HSCs and promote reversion to quiescence, and whether this effect was lost in capillarization. HSCs were cultured alone or co-cultured with differentiated or capillarized SECs.

RESULTS

Co-culture with freshly isolated SECs markedly decreased HSC activation after 3 days in culture, but co-culture with capillarized SEC had no effect. Inhibition of nitric oxide (NO) synthesis abolished SEC suppression of HSC activation. Activated HSCs reverted to quiescence when co-cultured with SEC plus vascular endothelial growth factor (VEGF) (that is, with SECs that maintained differentiation), but co-culture with capillarized SECs did not. Reversion of activated HSCs to quiescence in the presence of SECs plus VEGF was abolished by inhibition of NO synthesis. To establish whether there was indeed reversion, activated and quiescent HSCs were counted before and 3 days after adding freshly isolated SECs plus VEGF to activated HSCs, and proliferation was quantified in quiescent HSCs; the stoichiometry demonstrated reversion.

CONCLUSION

Differentiated SECs prevent HSC activation and promote reversion of activated HSCs to quiescence through VEGF-stimulated NO production. Capillarized SECs do not promote HSC quiescence, because of loss of VEGF-stimulated NO production.

Nitric oxide promotes caspase-independent hepatic stellate cell apoptosis through the generation of reactive oxygen species

Hepatic stellate cells (HSCs) contribute to portal hypertension through multiple mechanisms that include collagen deposition, vasoconstriction, and regulation of sinusoidal structure. Under normal physiologic conditions, endothelial nitric oxide (NO) synthase-derived NO exerts paracrine effects on HSCs; however, in cirrhosis, NO generation is impaired in association with concomitant HSC activation and changes in sinusoidal structure, events that contribute significantly to the development of portal hypertension. These concepts, in combination with recent evidence that induction of HSC-selective apoptosis may represent a useful target for treatment of chronic liver disease, led us to examine if NO may further limit HSC function through apoptosis. Indeed, both NO donors and endothelial NO synthase overexpression promoted HSC apoptotic pathways. HSC death conferred by NO occurred through mitochondrial membrane depolarization and through a caspase-independent pathway. Furthermore, NO-induced apoptosis of HSC did not occur through the canonical pathways of soluble guanylate cyclase or protein nitration, but rather through the generation of superoxide and hydroxyl radical intermediates. Lastly, HSC isolated from rats after bile duct ligation were more susceptible to NO-induced apoptosis. These data indicate that NO promotes HSC apoptosis through a signaling mechanism that involves mitochondria, is mediated by reactive oxygen species, and occurs independent of caspase activation.

CONCLUSION

We postulate that NO-dependent apoptosis of HSCs may maintain sinusoidal homeostasis, and may represent an additional beneficial effect of NO donors for therapy of portal hypertension.

Hepatic veno-occlusive disease after hematopoietic stem cell transplantation: update on defibrotide and other current investigational therapies

Hepatic veno-occlusive disease (VOD), also known as sinusoidal obstruction syndrome (SOS), remains one of the most serious and common complications after myeloablative hematopoietic stem cell transplantation (HSCT). Clinical diagnosis of hepatic VOD is based on the clinical triad of (1) painful hepatomegaly, (2) hyperbilirubinemia and (3) unexplained fluid retention. While milder cases usually resolve spontaneously, severe VOD is associated with a grim prognosis. Defibrotide (DF), a polydisperse mixture of single-stranded oligonucleotide with antithrombotic and fibrinolytic effects on microvascular endothelium, has emerged as an effective and safe therapy for patients with severe VOD. Multiple studies, including a recent large international multicenter phase II clinical trial, have demonstrated 30-60% complete remission rates with DF, even among patients with severe VOD and multiorgan failure. This article will review our current understanding of hepatic VOD, and update the clinical trial experience with DF and other potential therapies for this feared transplant complication.

Hepatocyte survival in acute hepatitis is due to c-Jun/AP-1-dependent expression of inducible nitric oxide synthase

Analysis of the molecular factors determining hepatocyte survival or death in response to inflammatory stimuli is essential for understanding the pathogenesis of inflammatory liver disease and for identifying novel therapeutic approaches. c-Jun N-terminal kinase (JNK) is a major mediator of cytokine-induced cell death during hepatitis, but the signaling pathways downstream of JNK remain less well defined. Here we show that the transcription factor c-Jun/AP-1, a prototypic target of JNK, is strongly expressed in the liver of patients with acute liver injury. The molecular function of c-Jun in inflammatory liver disease was analyzed in mice by using the Con A model of T cell-mediated hepatitis. Mice lacking c-Jun in hepatocytes display increased liver cell death and mortality upon Con A injection. This phenotype is caused by impaired expression of inducible nitric oxide synthase (nos2), a direct transcriptional target of c-Jun, and reduced production of hepatoprotective nitric oxide (NO). Moreover, increased hepatotoxicity in mutant mice is likely caused by hypoxia and oxidative stress and can be rescued pharmacologically by liver-specific NO delivery. These findings demonstrate that c-Jun/AP-1 is hepatoprotective during acute hepatitis by regulating nos2/NO expression and thus functionally antagonizes the cell death-promoting functions of JNK.

Veno occlusive disease: update on clinical management

Hepatic veno-occlusive disease is a clinical syndrome characterized by hepatomegaly, ascites, weight gain and jaundice, due to sinusoidal congestion which can be caused by alkaloid ingestion, but the most frequent cause is haematopoietic stem cell transplantation (STC) and is also seen after solid organ transplantation. The incidence of veno occlusive disease (VOD) after STC ranges from 0 to 70%, but is decreasing. Survival is good when VOD is a mild form, but when it is severe and associated with an increase of hepatic venous pressure gradient > 20 mmHg, and mortality is about 90%. Prevention remains the best therapeutic strategy, by using non-myeloablative conditioning regimens before STC. Prophylactic administration of ursodeoxycholic acid, being an antioxidant and antiapoptotic agent, can have some benefit in reducing overall mortality. Defibrotide, which has pro-fibrinolytic and antithrombotic properties, is the most effective therapy; decompression of the sinusoids by a transjugular intrahepatic portosystemic shunt (TIPS) can be tried, especially to treat VOD after liver transplantation and when multiorgan failure (MOF) is not present. Liver transplantation can be the last option, but can not be considered a standard rescue therapy, because usually the concomitant presence of multiorgan failure contraindicates this procedure.

AUF-1 mediates inhibition by nitric oxide of lipopolysaccharide-induced matrix metalloproteinase-9 expression in cultured astrocytes

Neuroinflammatory diseases are associated with increased production of matrix metalloproteinase-9 (MMP-9) and excessive generation of nitric oxide (NO). NO has been reported to have variable effects on MMP-9 gene expression and activation in various cell types. In the present study, we investigated the effect of NOon MMP-9 expression in primary cortical astrocytes. Zymography and real-time PCR showed that lipopolysaccharide (LPS) dramatically increased latent MMP-9 gelatinolytic activity and MMP-9 mRNA expression. By using the NO donor DETA NONOate, we observed a dose-dependent inhibition of MMP-9 induction by LPS. Active forms of MMP-9 were not found by zymography after NO treatment. The MEK1/2 inhibitor U0126 completely inhibited LPS-induced MMP-9, which was partially inhibited by the p38 MAPK inhibitor SB203580. NO had no effect on LPS-stimulated ERK1/2 and p38 MAPK activation, suggesting that the inhibitory action of NO occurs downstream of MAPK cascades. Real-time PCR analysis showed that NO accelerated the degradation of MMP-9 mRNA after LPS induction. Western blotting and pull-down assay demonstrated that NO increased AUF-1 expression as well as its specific binding to the MMP-9 gene 3'-untranslated region. Knockdown of AUF-1 with siRNA partially reversed the inhibitory action of NO on LPS-stimulated MMP-9 induction. We conclude that NO does not activate MMP-9 in astrocyte cultures but reduces LPS-induced MMP-9 expression via accelerating MMP-9 mRNA degradation, which is partially mediated by AUF-1. Our results suggest that elevated NO concentrations may suppress MMP-9 and restrict the inflammatory response in neurodegenerative diseases.

Analysis of gene expression changes in relation to toxicity and tumorigenesis in the livers of Big Blue transgenic rats fed comfrey (Symphytum officinale)

Background

Comfrey is consumed by humans as a vegetable and a tea, and has been used as an herbal medicine for more than 2000 years. Comfrey, however, is hepatotoxic in livestock and humans and carcinogenic in experimental animals. Our previous study suggested that comfrey induces liver tumors by a genotoxic mechanism and that the pyrrolizidine alkaloids in the plant are responsible for mutation induction and tumor initiation in rat liver.

Results

In this study, we identified comfrey-induced gene expression profile in the livers of rats. Groups of 6 male transgenic Big Blue rats were fed a basal diet and a diet containing 8% comfrey roots, a dose that resulted in liver tumors in a previous carcinogenicity bioassay. The animals were treated for 12 weeks and sacrificed one day after the final treatment. We used a rat microarray containing 26,857 genes to perform genome-wide gene expression studies. Dietary comfrey resulted in marked changes in liver gene expression, as well as in significant decreases in the body weight and increases in liver mutant frequency. When a two-fold cutoff value and a P-value less than 0.01 were selected, 2,726 genes were identified as differentially expressed in comfrey-fed rats compared to control animals. Among these genes, there were 1,617 genes associated by Ingenuity Pathway Analysis with particular functions, and the differentially expressed genes in comfrey-fed rat livers were involved in metabolism, injury of endothelial cells, and liver injury and abnormalities, including liver fibrosis and cancer development.

Conclusion

The gene expression profile provides us a better understanding of underlying mechanisms for comfrey-induced hepatic toxicity. Integration of gene expression changes with known pathological changes can be used to formulate a mechanistic scheme for comfrey-induced liver toxicity and tumorigenesis.

Anticoagulation and inhibition of nitric oxide synthase influence hepatic hypoxia after monocrotaline exposure

Monocrotaline (MCT) is a pyrrolizidine alkaloid plant toxin that produces hepatotoxicity in humans and animals. Administration of MCT to rats causes rapid sinusoidal endothelial cell (SEC) injury, hemorrhage, pooling of blood and fibrin deposition in centrilobular regions of liver. These events precede hepatic parenchymal cell (HPC) injury and produce marked changes in the microvasculature of the liver, which could interrupt blood flow and produce hypoxia in affected regions. To test the hypothesis that hypoxia occurs in liver after MCT exposure, rats were treated with 300mgMCT/kg, and hypoxia was detected immunohistochemically. MCT produced significant hypoxia in centrilobular regions of livers by 8h after treatment. Inasmuch as fibrin deposition can impair oxygen delivery by reducing blood flow, the effect of anticoagulant treatment on MCT-induced hypoxia was determined. Administration of warfarin to MCT-treated rats reduced hypoxia in the liver by approximately 70%, suggesting that fibrin deposition plays a causal role in the development of hypoxia in the liver. Conversely, administration of l-NAME, a nonspecific inhibitor of nitric oxide synthases (NOSs), enhanced MCT-induced hypoxia and HPC injury. l-NAME did not, however, affect SEC injury or coagulation system activation. Results from these studies show that hypoxia occurs in the liver after MCT exposure. Furthermore, hypoxia precedes HPC injury, and manipulations that modify hypoxia also modulate HPC injury.

Metabolism of a liver-selective nitric oxide-releasing agent, V-PYRRO/NO, by human microsomal cytochromes P450

Endogenously generated nitric oxide (NO) mediates a host of important physiological functions, playing roles in the vascular, immunological, and neurological systems. As a result, exogenous agents that release NO have become important therapeutic interventions and research tools. O(2)-Vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (V-PYRRO/NO) is a prodrug designed with the hypothesis that it might release nitric oxide via epoxidation of the vinyl group by cytochrome P450, followed by enzymatic and/or spontaneous epoxide hydration to release the ultimate NO-donating moiety, 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (PYRRO/NO) ion. In this study, we investigated this hypothetical activation mechanism quantitatively for V-PYRRO/NO using cDNA-expressed human cytochrome P450 (CYP)2E1. Incubation with CYP2E1 and an NADPH-regenerating system resulted in a time-dependent decomposition of V-PYRRO/NO, with a turnover rate of 2.0 nmol/min/pmol CYP2E1. Nitrate and nitrite were detected in high yield as metabolites of NO. The predicted organic metabolites pyrrolidine and glycolaldehyde were also detected in near-quantitative yields. The enzymatic decomposition of V-PYRRO/NO was also catalyzed, albeit at lower rates, by CYP2A6 and CYP2B6. We conclude that the initial step in the metabolism of V-PYRRO/NO to NO in the liver is catalyzed efficiently but not exclusively by the alcohol-inducible form of cytochrome P450 (CYP2E1). The results confirm the proposed activation mechanism involving enzymatic oxidation of the vinyl group in V-PYRRO/NO followed by epoxide hydration and hydrolytic decomposition of the resulting PYRRO/NO ion to generate nitric oxide.

Drug-induced liver injury: summary of a single topic clinical research conference

Idiosyncratic drug induced liver injury (DILI) remains poorly understood. It is assumed that the affected individuals possess a rare combination of genetic and non genetic factors that, if identified, would greatly improve understanding of the underlying mechanisms. This single topic conference brought together basic scientists, translational investigators, and clinicians with an interest in DILI. The goal was to define high priority areas of investigation that will soon be made possible by The Drug-Induced Liver Injury Network (DILIN). Since 2004 DILIN has been collecting clinical data, genomic DNA and some tissues from patients who have experienced bone fide DILI. The presentations spanned many different areas of DILI, and included novel data concerning mechanisms of hepatotoxicity, new "omics" approaches, and the challenges of improving causation assessment.

Pivotal role of PAI-1 in a murine model of hepatic vein thrombosis

Hepatic veno-occlusive disease (VOD) is a common complication of high-dose chemotherapy associated with bone marrow transplantation. While the pathogenesis of VOD is uncertain, plasminogen activator inhibitor-1 (PAI-1) has emerged as a diagnostic marker and predictor of VOD in humans. In this study, we investigated the role of PAI-1 in a murine model of VOD produced by long-term nitric oxide synthase inhibition using L-NAME. After 6 weeks, wild-type (WT) mice developed extensive fibrinoid hepatic venous thrombi and biochemical evidence of hepatic injury and dysfunction. In contrast, PAI-1-deficient mice were largely protected from the development of hepatic vein thrombosis. Furthermore, WT mice that received tiplaxtinin, an antagonist of PAI-1, were effectively protected from L-NAME-induced thrombosis. Taken together, these data indicate that NO and PAI-1 play pivotal and antagonistic roles in hepatic vein thrombosis and that PAI-1 is a potential target in the prevention and treatment of VOD in humans.

Supportive care

To optimize treatment outcomes for hematologic malignancies, minimizing the consequences of treatment complications requires as much skill as the choice of the treatment itself. Myelosuppression and immunosuppression are frequent complications and have potentially serious infectious consequences. Invasive fungal infections and infections from respiratory viruses are increasing in frequency and have life-threatening potential. Damage to vital organs, especially the liver, is another important concern. In this chapter, the scope of invasive fungal and respiratory viral infections, recent insights into the pathogenesis of hepatic sinusoidal injury, and recent developments that impact prevention and treatment approaches for these complications are described. In Section I, Dr. John Wingard describes the advantages and disadvantages of various treatment options for invasive infections by the two chief fungal pathogens, Candida and Aspergillus. Adjunctive therapies and practical considerations that clinicians should weigh in choosing one or another of the various agents are discussed. The studies that have evaluated antifungal prophylaxis and empirical treatment strategies are reviewed. Finally, new approaches such as combination therapy, new diagnostics, and efforts to bolster host immunity are considered. In Section II, Dr. W. Garrett Nichols describes the epidemiology of community-acquired respiratory viruses (CRV) in patients with hematologic malignancies. Risk factors, clinical syndromes, and possible indirect effects of CRV infections are discussed. Treatment and prevention options are reviewed. In Section III, Dr. George McDonald describes sinusoidal obstruction syndrome (once known as hepatic veno-occlusive disease). Recent insights into pathogenesis are described. Diagnostic criteria and the advantages and disadvantages of various diagnostic methods are reviewed and prognosis is considered. Prevention and treatment options are discussed.