Laurie D. DeLeve

Vascular disorders of the liver

This guideline has been approved by the American Association for the Study of Liver Diseases (AASLD) and represents the position of the association.

Glutathione metabolism and its role in hepatotoxicity

Glutathione (GSH) fulfills several essential functions: Detoxification of free radicals and toxic oxygen radicals, thiol-disulfide exchange and storage and transfer of cysteine. GSH is present in all mammalian cells, but may be especially important for organs with intense exposure to exogenous toxins such as the liver, kidney, lung and intestine. Within the cell mitochondrial GSH is the main defense against physiological oxidant stress generated by cellular respiration and may be a critical target for toxic oxygen and electrophilic metabolites. Glutathione homeostasis is a highly complex process, which is predominantly regulated by the liver, lung and kidney.

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. (HEPATOLOGY 2008.)

Hepatic Veno-occlusive Disease (Sinusoidal Obstruction Syndrome) After Hematopoietic Stem Cell Transplantation

Hepatic veno-occlusive disease (VOD), increasingly referred to as sinusoidal obstruction syndrome, is a well-recognized complication of hematopoietic stem cell transplantation and contributes to considerable morbidity and mortality. In the Western Hemisphere, VOD, classified as a conditioning-related toxicity, is most commonly caused by stem cell transplantation. VOD has been described after all types of stem cell transplantation, irrespective of the stem cell source, type of conditioning therapy, or underlying disease. Recognition of this disease in the posttransplantation setting remains a challenge in the absence of specific diagnostic features because many other more common conditions can mimic it. Limited therapeutic or preventive strategies are currently available for the management of VOD. In this review, we provide a comprehensive account of the pathophysiology of this disease as we understand it today, risk factors for its development, and the current state of knowledge regarding preventive and therapeutic options.

Role of Differentiation of Liver Sinusoidal Endothelial Cells in Progression and Regression of Hepatic Fibrosis in Rats

BACKGROUND & AIMS Capillarization, characterized by loss of differentiation of liver sinusoidal endothelial cells (LSECs), precedes the onset of hepatic fibrosis. We investigated whether restoration of LSEC differentiation would normalize crosstalk with activated hepatic stellate cells (HSC) and thereby promote quiescence of HSC and regression of fibrosis. METHODS Rat LSECs were cultured with inhibitors and/or agonists and examined by scanning electron microscopy for fenestrae in sieve plates. Cirrhosis was induced in rats using thioacetamide, followed by administration of BAY 60-2770, an activator of soluble guanylate cyclase (sGC). Fibrosis was assessed by Sirius red staining; expression of α-smooth muscle actin was measured by immunoblot analysis. RESULTS Maintenance of LSEC differentiation requires vascular endothelial growth factor-A stimulation of nitric oxide-dependent signaling (via sGC and cyclic guanosine monophosphate) and nitric oxide-independent signaling. In rats with thioacetamide-induced cirrhosis, BAY 60-2770 accelerated the complete reversal of capillarization (restored differentiation of LSECs) without directly affecting activation of HSCs or fibrosis. Restoration of differentiation to LSECs led to quiescence of HSCs and regression of fibrosis in the absence of further exposure to BAY 60-2770. Activation of sGC with BAY 60-2770 prevented progression of cirrhosis, despite continued administration of thioacetamide. CONCLUSIONS The state of LSEC differentiation plays a pivotal role in HSC activation and the fibrotic process.

Sinusoidal obstruction syndrome (veno-occlusive disease) in the rat is prevented by matrix metalloproteinase inhibition 1 ☆

BACKGROUND & AIMS The mechanical origins of the obstruction in sinusoidal obstruction syndrome are initiated by dehiscence of sinusoidal endothelial cells from the space of Disse. The biochemical changes that permit the dehiscence of the sinusoidal endothelial cells were investigated. METHODS In vitro and in vivo studies examined changes induced by monocrotaline, a pyrrolizidine alkaloid that induces sinusoidal obstruction syndrome in both humans and experimental animals. RESULTS In the monocrotaline-induced rat model of sinusoidal obstruction syndrome, there was an early increase of matrix metalloproteinase-9 and a later, lower-magnitude increase of matrix metalloproteinase-2 in the liver. In vitro studies of sinusoidal endothelial cells, hepatocytes, stellate cells, and Kupffer cells showed that sinusoidal endothelial cells are the major source of both basal and monocrotaline-induced matrix metalloproteinase-9/matrix metalloproteinase-2 activity. Monocrotaline caused depolymerization of F-actin in sinusoidal endothelial cells, and blocking of F-actin depolymerization prevented the increase in matrix metalloproteinase activity. Administration of matrix metalloproteinase inhibitors prevented the signs and histological changes associated with sinusoidal obstruction syndrome. CONCLUSIONS Monocrotaline causes depolymerization of F-actin in sinusoidal endothelial cells, which leads to increased expression of metalloproteinase-9 and matrix metalloproteinase-2 by sinusoidal endothelial cells. Inhibition of matrix metalloproteinase-9 and matrix metalloproteinase-2 prevents the development of sinusoidal obstruction syndrome, establishing that matrix metalloproteinase inhibitors may be a therapeutically viable strategy for prevention.

Liver sinusoidal endothelial cells and liver regeneration.

Liver sinusoidal endothelial cells (LSECs) have long been noted to contribute to liver regeneration after liver injury. In normal liver, the major cellular source of HGF is the hepatic stellate cell, but after liver injury, HGF expression has been thought to increase markedly in proliferating LSECs. However, emerging data suggest that even after injury, LSEC expression of HGF does not increase greatly. In contrast, bone marrow progenitor cells of LSECs (BM SPCs), which are rich in HGF, are recruited to the liver after injury. This Review examines liver regeneration from the perspective that BM SPCs that have been recruited to the liver, rather than mature LSECs, drive liver regeneration.

Liver sinusoidal endothelial cells in hepatic fibrosis

Capillarization, lack of liver sinusoidal endothelial cell (LSEC) fenestration, and formation of an organized basement membrane not only precedes fibrosis, but is also permissive for hepatic stellate cell activation and fibrosis. Thus, dysregulation of the LSEC phenotype is a critical step in the fibrotic process. Both a vascular endothelial growth factor (VEGF)‐stimulated, nitric oxide (NO)‐independent pathway and a VEGF‐stimulated NO‐dependent pathway are necessary to maintain the differentiated LSEC phenotype. The NO‐dependent pathway is impaired in capillarization and activation of this pathway downstream from NO restores LSEC differentiation in vivo. Restoration of LSEC differentiation in vivo promotes HSC quiescence, enhances regression of fibrosis, and prevents progression of cirrhosis. (Hepatology 2015;61:1740–1746)

Liver sinusoidal endothelial cell progenitor cells promote liver regeneration in rats.

The ability of the liver to regenerate is crucial to protect liver function after injury and during chronic disease. Increases in hepatocyte growth factor (HGF) in liver sinusoidal endothelial cells (LSECs) are thought to drive liver regeneration. However, in contrast to endothelial progenitor cells, mature LSECs express little HGF. Therefore, we sought to establish in rats whether liver injury causes BM LSEC progenitor cells to engraft in the liver and provide increased levels of HGF and to examine the relative contribution of resident and BM LSEC progenitors. LSEC label-retaining cells and progenitors were identified in liver and LSEC progenitors in BM. BM LSEC progenitors did not contribute to normal LSEC turnover in the liver. However, after partial hepatectomy, BM LSEC progenitor proliferation and mobilization to the circulation doubled. In the liver, one-quarter of the LSECs were BM derived, and BM LSEC progenitors differentiated into fenestrated LSECs. When irradiated rats underwent partial hepatectomy, liver regeneration was compromised, but infusion of LSEC progenitors rescued the defect. Further analysis revealed that BM LSEC progenitors expressed substantially more HGF and were more proliferative than resident LSEC progenitors after partial hepatectomy. Resident LSEC progenitors within their niche may play a smaller role in recovery from partial hepatectomy than BM LSEC progenitors, but, when infused after injury, these progenitors engrafted and expanded markedly over a 2-month period. In conclusion, LSEC progenitor cells are present in liver and BM, and recruitment of BM LSEC progenitors is necessary for normal liver regeneration.

Sinusoidal endothelial cells as a target for acetaminophen toxicity: Direct action versus requirement for hepatocyte activation in different mouse strains

Hepatic congestion occurs early in acetaminophen poisoning. This study examines whether acetaminophen is toxic to sinusoidal endothelial cells (SEC), which might lead to microcirculatory disruption. Acetaminophen toxicity was examined in vivo and in vitro in SEC and hepatocytes from C3H-HEN and Swiss Webster mice. In both strains, there was significantly more toxicity to SEC than to hepatocytes; in SEC from C3H-HEN mice, acetaminophen was directly toxic, but the presence of hepatocytes was required for toxicity to Swiss SEC. Acetaminophen, 750 mg/kg, by gavage caused toxicity with variability within and between strains, but all animals died between 3.5 and 6 hr with zone 3 hemorrhagic necrosis. Pretreatment of C3H-HEN SEC with aminobenzotriazole, a suicide inhibitor of P450, abolished toxicity. Baseline glutathione (GSH) levels were comparable, but a 12-hr incubation with acetaminophen decreased GSH by 60 and 8%, respectively, in C3H-HEN and Swiss SEC in single cell type culture. In co-culture, under conditions where Swiss SEC viability declined by 73%, hepatocyte viability and GSH only decreased by 21 and 20%, respectively. In conclusion, acetaminophen was toxic to SEC. It was directly toxic to SEC in one mouse strain and required hepatocyte activation in another strain. The lack of direct toxicity to Swiss SEC may be due to the lack of an activating P450 isozyme. Zone 3 hemorrhagic necrosis in vivo was comparable in both strains, despite differences in the pathways leading to SEC toxicity in vitro. We propose that toxicity to SEC may contribute to hepatic congestion in acetaminophen intoxication.