Pathophysiology and therapeutic options for cirrhotic portal hypertension

Portal hypertension represents the primary non-neoplastic complication of liver cirrhosis and has life-threatening consequences, such as oesophageal variceal bleeding, ascites, and hepatic encephalopathy. Portal hypertension occurs due to increased resistance of the cirrhotic liver vasculature to portal blood flow and is further aggravated by the hyperdynamic circulatory syndrome. Existing knowledge indicates that the profibrogenic phenotype acquired by sinusoidal cells is the initial factor leading to increased hepatic vascular tone and fibrosis, which cause increased vascular resistance and portal hypertension. Data also suggest that the phenotype of hepatic cells could be further impaired due to the altered mechanical properties of the cirrhotic liver itself, creating a deleterious cycle that worsens portal hypertension in the advanced stages of liver disease. In this Review, we discuss recent discoveries in the pathophysiology and treatment of cirrhotic portal hypertension, a condition with few pharmacological treatment options.

IRF3 regulates neuroinflammatory responses and the expression of genes associated with Alzheimer's disease

The pathological role of interferon signaling is emerging in neuroinflammatory disorders, yet, the specific role of Interferon Regulatory Factor 3 (IRF3) in neuroinflammation remains poorly understood. Here, we show that global IRF3 deficiency delays TLR4-mediated signaling in microglia and attenuates the hallmark features of LPS-induced inflammation such as cytokine release, microglial reactivity, astrocyte activation, myeloid cell infiltration, and inflammasome activation. Moreover, expression of a constitutively active IRF3 (S388D/S390D:IRF3-2D) in microglia induces a transcriptional program reminiscent of the Activated Response Microglia and the expression of genes associated with Alzheimer's Disease, notably apolipoprotein-e. Lastly, using bulk-RNAseq of IRF3-2D brain myeloid cells, we identified Z-DNA binding protein-1 as a target of IRF3 that is relevant across various neuroinflammatory disorders. Together, our results identify IRF3 as an important regulator of LPS-mediated neuroinflammatory responses and highlight IRF3 as a central regulator of disease-specific gene activation in different neuroinflammatory diseases.

Role of calcium integrin-binding protein 1 in the mechanobiology of the liver endothelium

Liver sinusoidal endothelial cells (LSECs) dysfunction is a key process in the development of chronic liver disease (CLD). Progressive scarring increases liver stiffness in a winch-like loop stimulating a dysfunctional liver cell phenotype. Cellular stretching is supported by biomechanically modulated molecular factors (BMMFs) that can translocate into the cytoplasm to support mechanotransduction through cytoskeleton remodeling and gene transcription. Currently, the molecular mechanisms of stiffness-induced LSECs dysfunction remain largely unclear. Here we propose calcium- and integrin-binding protein 1 (CIB1) as BMMF with crucial role in LSECs mechanobiology in CLD. CIB1 expression and translocation was characterized in healthy and cirrhotic human livers and in LSECs cultured on polyacrylamide gels with healthy and cirrhotic-like stiffnesses. Following the modulation of CIB1 with siRNA, the transcriptome was scrutinized to understand downstream effects of CIB1 downregulation. CIB1 expression is increased in LSECs in human cirrhosis. In vitro, CIB1 emerges as an endothelial BMMF. In human umbilical vein endothelial cells and LSECs, CIB1 expression and localization are modulated by stiffness-induced trafficking across the nuclear membrane. LSECs from cirrhotic liver tissue both in animal model and human disease exhibit an increased amount of CIB1 in cytoplasm. Knockdown of CIB1 in LSECs exposed to high stiffness improves LSECs phenotype by regulating the intracellular tension as well as the inflammatory response. Our results demonstrate that CIB1 is a key factor in sustaining cellular tension and stretching in response to high stiffness. CIB1 downregulation ameliorates LSECs dysfunction, enhancing their redifferentiation, and reducing the inflammatory response.

Mechanobiology of portal hypertension

The interplay between mechanical stimuli and cellular mechanobiology orchestrates the physiology of tissues and organs in a dynamic balance characterized by constant remodelling and adaptative processes. Environmental mechanical properties can be interpreted as a complex set of information and instructions that cells read continuously, and to which they respond. In cirrhosis, chronic inflammation and injury drive liver cells dysfunction, leading to excessive extracellular matrix deposition, sinusoidal pseudocapillarization, vascular occlusion and parenchymal extinction. These pathological events result in marked remodelling of the liver microarchitecture, which is cause and result of abnormal environmental mechanical forces, triggering and sustaining the long-standing and progressive process of liver fibrosis. Multiple mechanical forces such as strain, shear stress, and hydrostatic pressure can converge at different stages of the disease until reaching a point of no return where the fibrosis is considered non-reversible. Thereafter, reciprocal communication between cells and their niches becomes the driving force for disease progression. Accumulating evidence supports the idea that, rather than being a passive consequence of fibrosis and portal hypertension (PH), mechanical force-mediated pathways could themselves represent strategic targets for novel therapeutic approaches. In this manuscript, we aim to provide a comprehensive review of the mechanobiology of PH, by furnishing an introduction on the most important mechanisms, integrating these concepts into a discussion on the pathogenesis of PH, and exploring potential therapeutic strategies.

Neuroblastoma RAS viral oncogene homolog (N-RAS) deficiency aggravates liver injury and fibrosis

Progressive hepatic damage and fibrosis are major features of chronic liver diseases of different etiology, yet the underlying molecular mechanisms remain to be fully defined. N-RAS, a member of the RAS family of small guanine nucleotide-binding proteins also encompassing the highly homologous H-RAS and K-RAS isoforms, was previously reported to modulate cell death and renal fibrosis; however, its role in liver damage and fibrogenesis remains unknown. Here, we approached this question by using N-RAS deficient (N-RAS-/-) mice and two experimental models of liver injury and fibrosis, namely carbon tetrachloride (CCl4) intoxication and bile duct ligation (BDL). In wild-type (N-RAS+/+) mice both hepatotoxic procedures augmented N-RAS expression in the liver. Compared to N-RAS+/+ counterparts, N-RAS-/- mice subjected to either CCl4 or BDL showed exacerbated liver injury and fibrosis, which was associated with enhanced hepatic stellate cell (HSC) activation and leukocyte infiltration in the damaged liver. At the molecular level, after CCl4 or BDL, N-RAS-/- livers exhibited augmented expression of necroptotic death markers along with JNK1/2 hyperactivation. In line with this, N-RAS ablation in a human hepatocytic cell line resulted in enhanced activation of JNK and necroptosis mediators in response to cell death stimuli. Of note, loss of hepatic N-RAS expression was characteristic of chronic liver disease patients with fibrosis. Collectively, our study unveils a novel role for N-RAS as a negative controller of the progression of liver injury and fibrogenesis, by critically downregulating signaling pathways leading to hepatocyte necroptosis. Furthermore, it suggests that N-RAS may be of potential clinical value as prognostic biomarker of progressive fibrotic liver damage, or as a novel therapeutic target for the treatment of chronic liver disease.

Regression of portal hypertension: underlying mechanisms and therapeutic strategies

Portal hypertension is the main non-neoplastic complication of chronic liver disease, being the cause of important life-threatening events including the development of ascites or variceal bleeding. The primary factor in the development of portal hypertension is a pathological increase in the intrahepatic vascular resistance, due to liver microcirculatory dysfunction, which is subsequently aggravated by extra-hepatic vascular disturbances including elevation of portal blood inflow. Evidence from pre-clinical models of cirrhosis has demonstrated that portal hypertension and chronic liver disease can be reversible if the injurious etiological agent is removed and can be further promoted using pharmacological therapy. These important observations have been partially demonstrated in clinical studies. This paper aims at providing an updated review of the currently available data regarding spontaneous and drug-promoted regression of portal hypertension, paying special attention to the clinical evidence. It also considers pathophysiological caveats that highlight the need for caution in establishing a new dogma that human chronic liver disease and portal hypertension is reversible.

Nuclear deformation mediates liver cell mechanosensing in cirrhosis

BACKGROUND & AIMS

Liver stiffness is increased in advanced chronic liver disease (ACLD) and accurately predicts prognosis in this population. Recent data suggest that extracellular matrix stiffness per se may modulate the phenotype of liver cells. We aimed at investigating the effect of matrix stiffness on the phenotype of liver cells of rats with cirrhosis, assessing its influence on their response to antifibrotic strategies and evaluating associated molecular mechanisms.

METHODS

Hepatocytes, hepatic stellate cells, and liver sinusoidal endothelial cells were isolated from healthy rats or rats with cirrhosis (carbon tetrachloride or thioacetamide), and cultured on polyacrylamide gels with different physiologically relevant stiffness for 72 h.

RESULTS

All cell types of rats with cirrhosis cultured at low stiffness showed a significant phenotype amelioration vs. rigid matrix (assessed by quantitative morphology, mRNA expression, protein synthesis, and electron microscopy imaging). Additionally, stiffness modified the antifibrotic effects of liraglutide in stellate cells of rats with cirrhosis. Finally, evaluation of nuclear morphology revealed that high stiffness induced nuclei deformation in all cell types, an observation confirmed in cells from human livers. Disconnecting the nucleus from the cytoskeleton by cytoskeleton disruption or a defective form of nesprin 1 significantly recovered spherical nuclear shape and quiescent phenotype of cells.

CONCLUSIONS

The environment's stiffness per se modulates the phenotype of healthy rats and liver cells of rats with cirrhosis by altering the nuclear morphology through cytoskeleton-derived mechanical forces. The reversibility of this mechanism suggests that targeting the stiffness-mediated intracellular mechanical tensions may represent a novel therapeutic strategy for ACLD.

LAY SUMMARY

During cirrhosis, the liver becomes scarred, stiff, and unable to perform its normal functions efficiently. In this study, we demonstrated that cells from diseased (stiff) livers recovered their functionality when placed in a soft environment (as that of a healthy liver). Furthermore, treatments aimed at tricking liver cells into believing they are in a healthy, soft liver improved their function and could potentially contribute to treat cirrhosis.

Exercise Attenuates the Transition from Fatty Liver to Steatohepatitis and Reduces Tumor Formation in Mice

Non-alcoholic fatty liver disease (NAFLD) leads to steatohepatitis (NASH), fibrosis, and hepatocellular carcinoma. For sedentary patients, lifestyle interventions combining exercise and dietary changes are a cornerstone of treatment. However, the benefit of exercise alone when dietary changes have failed is uncertain. We query whether exercise alone arrests the progression of NASH and tumorigenesis in a choline-deficient, high-fat diet (CD-HFD) murine model. Male C57Bl/6N mice received a control diet or CD-HFD for 12 weeks. CD-HFD mice were randomized further for 8 weeks of sedentariness (SED) or treadmill exercise (EXE). CD-HFD for 12 weeks produced NAFL. After 20 weeks, SED mice developed NASH and hepatic adenomas. Exercise attenuated the progression to NASH. EXE livers showed lower triglycerides and tumor necrosis factor-α expression, less fibrosis, less ballooning, and a lower NAFLD activity score than did SED livers. Plasma transaminases and triglycerides were lower. Exercise activated AMP-activated protein kinase (AMPK) with inhibition of mTORC1 and decreased S6 phosphorylation, reducing hepatocellular adenoma. Exercise activated autophagy with increased LC3-II/LC3-I and mitochondrial recruitment of phosphorylated PTEN-induced kinase. Therefore, exercise attenuates the transition from NAFL to NASH, improves biochemical and histological parameters of NAFLD, and impedes the progression of fibrosis and tumorigenesis associated with enhanced activation of AMPK signaling and favors liver autophagy. Our work supports the benefits of exercise independently of dietary changes.

Impaired endothelial autophagy promotes liver fibrosis by aggravating the oxidative stress response during acute liver injury

BACKGROUND & AIMS

Endothelial dysfunction plays an essential role in liver injury, yet the phenotypic regulation of liver sinusoidal endothelial cells (LSECs) remains unknown. Autophagy is an endogenous protective system whose loss could undermine LSEC integrity and phenotype. The aim of our study was to investigate the role of autophagy in the regulation of endothelial dysfunction and the impact of its manipulation during liver injury.

METHODS

We analyzed primary isolated LSECs from Atg7control and Atg7endo mice as well as rats after CCl4 induced liver injury. Liver tissue and primary isolated stellate cells were used to analyze liver fibrosis. Autophagy flux, microvascular function, nitric oxide bioavailability, cellular superoxide content and the antioxidant response were evaluated in endothelial cells.

RESULTS

Autophagy maintains LSEC homeostasis and is rapidly upregulated during capillarization in vitro and in vivo. Pharmacological and genetic downregulation of endothelial autophagy increases oxidative stress in vitro. During liver injury in vivo, the selective loss of endothelial autophagy leads to cellular dysfunction and reduced intrahepatic nitric oxide. The loss of autophagy also impairs LSECs ability to handle oxidative stress and aggravates fibrosis.

CONCLUSIONS

Autophagy contributes to maintaining endothelial phenotype and protecting LSECs from oxidative stress during early phases of liver disease. Selectively potentiating autophagy in LSECs during early stages of liver disease may be an attractive approach to modify the disease course and prevent fibrosis progression.

LAY SUMMARY

Liver endothelial cells are the first liver cell type affected after any kind of liver injury. The loss of their unique phenotype during injury amplifies liver damage by orchestrating the response of the liver microenvironment. Autophagy is a mechanism involved in the regulation of this initial response and its manipulation can modify the progression of liver damage.

Advances in therapeutic options for portal hypertension

Portal hypertension represents one of the major clinical consequences of chronic liver disease, having a deep impact on patients' prognosis and survival. Its pathophysiology defines a pathological increase in the intrahepatic vascular resistance as the primary factor in its development, being subsequently aggravated by a paradoxical increase in portal blood inflow. Although extensive preclinical and clinical research in the field has been developed in recent decades, no effective treatment targeting its primary mechanism has been defined. The present review critically summarizes the current knowledge in portal hypertension therapeutics, focusing on those strategies driven by the disease pathophysiology and underlying cellular mechanisms.

Cross-talk between autophagy and KLF2 determines endothelial cell phenotype and microvascular function in acute liver injury

BACKGROUND & AIMS

The transcription factor Krüppel-like factor 2 (KLF2), inducible by simvastatin, confers endothelial vasoprotection. Considering recent data suggesting activation of autophagy by statins, we aimed to: 1) characterize the relationship between autophagy and KLF2 in the endothelium, 2) assess this relationship in acute liver injury (cold ischemia/reperfusion) and 3) study the effects of modulating KLF2-autophagy in vitro and in vivo.

METHODS

Autophagic flux, the vasoprotective KLF2 pathway, cell viability and microvascular function were assessed in endothelial cells and in various pre-clinical models of acute liver injury (cold storage and warm reperfusion).

RESULTS

Positive feedback between autophagy and KLF2 was observed in the endothelium: KLF2 inducers, pharmacological (statins, resveratrol, GGTI-298), biomechanical (shear stress) or genetic (adenovirus containing KLF2), caused endothelial KLF2 overexpression through a Rac1-rab7-autophagy dependent mechanism, both in the specialized liver sinusoidal endothelial cells (LSEC) and in human umbilical vein endothelial cells. In turn, KLF2 induction promoted further activation of autophagy. Cold ischemia blunted autophagic flux. Upon reperfusion, LSEC stored in University of Wisconsin solution did not reactivate autophagy, which resulted in autophagosome accumulation probably due to impairment in autophagosome-lysosome fusion, ultimately leading to increased cell death and microvascular dysfunction. Simvastatin pretreatment maintained autophagy (through the upregulation of rab7), resulting in increased KLF2, improved cell viability, and ameliorated hepatic damage and microvascular function.

CONCLUSIONS

We herein describe for the first time the complex autophagy-KLF2 relationship, modulating the phenotype and survival of the endothelium. These results help understanding the mechanisms of protection conferred by KLF2-inducers, such as simvastatin, in hepatic vascular disorders.

LAY SUMMARY

Autophagy and the transcription factor KLF2 share a common activation pathway in the endothelium, being able to regulate each other. Statins maintain microvascular function through the inhibition of Rac1, which consequently liberates Rab7, activates autophagy and increments the expression of KLF2.

Resistin and visfatin in steatotic and non-steatotic livers in the setting of partial hepatectomy under ischemia-reperfusion

BACKGROUND & AIMS

This study examined whether the regulation of resistin and visfatin could reduce damage and improve regeneration in both steatotic and non-steatotic livers undergoing partial hepatectomy under ischemia-reperfusion, a procedure commonly applied in clinical practice to reduce bleeding.

METHODS

Resistin and visfatin were pharmacologically modulated in lean and obese animals undergoing partial hepatectomy under ischemia-reperfusion.

RESULTS

No evident role for these adipocytokines was observed in non-steatotic livers. However, obese animals undergoing liver surgery showed increased resistin in liver and plasma, without changes in adipose tissue, together with visfatin downregulation in liver and increment in plasma and adipose tissue. Endogenous resistin maintains low levels of visfatin in the liver by blocking its hepatic uptake from the circulation, thus regulating the visfatin detrimental effects on hepatic damage and regenerative failure. Indeed, the administration of anti-resistin antibodies increased hepatic accumulation of adipocyte-derived visfatin, exacerbating damage and regenerative failure. Interestingly, treatment with anti-visfatin antibodies protected steatotic livers, and similar results were obtained with the concomitant inhibition of resistin and visfatin. Thus, when visfatin was inhibited, the injurious effects of anti-resistin antibodies disappeared. Herein we show that upregulation of visfatin increased NAD levels in the remnant steatotic liver, whereas visfatin inhibition decreased them. These later observations suggest that visfatin may favour synthesis of NAD instead of DNA and induces alterations in amino acid metabolism-urea cycle and NO production, overall negatively affecting liver viability.

CONCLUSIONS

Our results indicate the clinical potential of visfatin blocking-based therapies in steatotic livers undergoing partial hepatectomy with ischemia-reperfusion.

Simvastatin maintains function and viability of steatotic rat livers procured for transplantation

BACKGROUND & AIMS

Liver grafts obtained from healthy rat donors develop acute microcirculatory dysfunction due to cold-storage and warm-reperfusion injuries. These detrimental effects are avoided adding simvastatin to the cold-storage solution. Considering the importance of increasing organ donor pool for transplantation, we characterized whether simvastatin pretreatment can protect steatotic grafts from cold-storage and warm-reperfusion injuries.

METHODS

Rats fed with high-fat diet received a single dose of simvastatin, or its vehicle, 30 min before liver procurement. Grafts were then cold stored for 0 h (control group) or 16 h and warm reperfused. At the end of the reperfusion period, hepatic vascular resistance, endothelial function, nitric oxide pathway, cell death, oxidative stress, autophagy, and liver injury were evaluated. Hepatic vascular resistance and endothelial function were determined in a group of simvastatin-treated livers in the presence of the nitric oxide synthase inhibitor L-NNA.

RESULTS

Cold-stored rat steatotic livers exhibit increased hepatic vascular resistance and marked endothelial dysfunction, together with liver damage, oxidative stress, and low nitric oxide. Simvastatin markedly improved liver injury and prevented hepatic endothelial dysfunction. The beneficial effects of simvastatin were associated with cell death diminution, autophagy induction, and nitric oxide release. Statin-derived liver microcirculation protection was not observed when nitric oxide production was blunted.

CONCLUSIONS

Pretreatment of steatotic liver donors with simvastatin shortly before procurement of the liver graft strongly protects both parenchymal and endothelial components of the liver after warm reperfusion. Our data reinforce the use of statins to protect liver grafts undergoing transplantation.