Liver sinusoidal endothelial cells and liver regeneration

Comprehensive Review of Future Liver Remnant (FLR) Assessment and Hypertrophy Techniques Before Major Hepatectomy: How to Assess and Manage the FLR

BACKGROUND

The regenerative capacities of the liver and improvements in surgical techniques have expanded the possibilities of resectability. Liver resection is often the only curative treatment for primary and secondary malignancies, despite the risk of post-hepatectomy liver failure (PHLF). This serious complication (with a 50% mortality rate) can be avoided by better assessment of liver volume and function of the future liver remnant (FLR).

OBJECTIVE

The aim of this review was to understand and assess clinical, biological, and imaging predictors of PHLF risk, as well as the various hypertrophy techniques, to achieve an adequate FLR before hepatectomy.

METHOD

We reviewed the state of the art in liver regeneration and FLR hypertrophy techniques.

RESULTS

The use of new biological scores (such as the aspartate aminotransferase/platelet ratio index + albumin-bilirubin [APRI+ALBI] score), concurrent utilization of 99mTc-mebrofenin scintigraphy (HBS), or dynamic hepatocyte contrast-enhanced MRI (DHCE-MRI) for liver volumetry helps predict the risk of PHLF. Besides portal vein embolization, there are other FLR optimization techniques that have their indications in case of risk of failure (e.g., associating liver partition and portal vein ligation for staged hepatectomy, liver venous deprivation) or in specific situations (transarterial radioembolization).

CONCLUSION

There is a need to standardize volumetry and function measurement techniques, as well as FLR hypertrophy techniques, to limit the risk of PHLF.

Single-cell analysis of the amphioxus hepatic caecum and vertebrate liver reveals genetic mechanisms of vertebrate liver evolution

The evolution of the vertebrate liver is a prime example of the evolution of complex organs, yet the driving genetic factors behind it remain unknown. Here we study the evolutionary genetics of liver by comparing the amphioxus hepatic caecum and the vertebrate liver, as well as examining the functional transition within vertebrates. Using in vivo and in vitro experiments, single-cell/nucleus RNA-seq data and gene knockout experiments, we confirm that the amphioxus hepatic caecum and vertebrate liver are homologous organs and show that the emergence of ohnologues from two rounds of whole-genome duplications greatly contributed to the functional complexity of the vertebrate liver. Two ohnologues, kdr and flt4, play an important role in the development of liver sinusoidal endothelial cells. In addition, we found that liver-related functions such as coagulation and bile production evolved in a step-by-step manner, with gene duplicates playing a crucial role. We reconstructed the genetic footprint of the transfer of haem detoxification from the liver to the spleen during vertebrate evolution. Together, these findings challenge the previous hypothesis that organ evolution is primarily driven by regulatory elements, underscoring the importance of gene duplicates in the emergence and diversification of a complex organ.

Sexually Dimorphic Response to Hepatic Injury in Newborn Suffering from Intrauterine Growth Restriction

Intrauterine growth restriction (IUGR), when a fetus does not grow as expected, is associated with a reduction in hepatic functionality and a higher risk for chronic liver disease in adulthood. Utilizing early developmental plasticity to reverse the outcome of poor fetal programming remains an unexplored area. Focusing on the biochemical profiles of neonates and previous transcriptome findings, piglets from the same fetus are selected as models for studying IUGR. The cellular landscape of the liver is created by scRNA-seq to reveal sex-dependent patterns in IUGR-induced hepatic injury. One week after birth, IUGR piglets experience hypoxic stress. IUGR females exhibit fibroblast-driven T cell conversion into an immune-adapted phenotype, which effectively alleviates inflammation and fosters hepatic regeneration. In contrast, males experience even more severe hepatic injury. Prolonged inflammation due to disrupted lipid metabolism hinders intercellular communication among non-immune cells, which ultimately impairs liver regeneration even into adulthood. Additionally, Apolipoprotein A4 (APOA4) is explored as a novel biomarker by reducing hepatic triglyceride deposition as a protective response against hypoxia in IUGR males. PPARα activation can mitigate hepatic damage and meanwhile restore over-expressed APOA4 to normal in IUGR males. The pioneering study offers valuable insights into the sexually dimorphic responses to hepatic injury during IUGR.

Depletion of Activated Hepatic Stellate Cells and Capillarized Liver Sinusoidal Endothelial Cells Using a Rationally Designed Protein for Nonalcoholic Steatohepatitis and Alcoholic Hepatitis Treatment

Nonalcoholic steatohepatitis (NASH) and alcoholic hepatitis (AH) affect a large part of the general population worldwide. Dysregulation of lipid metabolism and alcohol toxicity drive disease progression by the activation of hepatic stellate cells and the capillarization of liver sinusoidal endothelial cells. Collagen deposition, along with sinusoidal remodeling, alters sinusoid structure, resulting in hepatic inflammation, portal hypertension, liver failure, and other complications. Efforts were made to develop treatments for NASH and AH. However, the success of such treatments is limited and unpredictable. We report a strategy for NASH and AH treatment involving the induction of integrin αvβ3-mediated cell apoptosis using a rationally designed protein (ProAgio). Integrin αvβ3 is highly expressed in activated hepatic stellate cells (αHSCs), the angiogenic endothelium, and capillarized liver sinusoidal endothelial cells (caLSECs). ProAgio induces the apoptosis of these disease-driving cells, therefore decreasing collagen fibril, reversing sinusoid remodeling, and reducing immune cell infiltration. The reversal of sinusoid remodeling reduces the expression of leukocyte adhesion molecules on LSECs, thus decreasing leukocyte infiltration/activation in the diseased liver. Our studies present a novel and effective approach for NASH and AH treatment.

Endothelial POFUT1 controls injury-induced liver fibrosis by repressing fibrinogen synthesis

BACKGROUND & AIMS

NOTCH signaling in liver sinusoidal endothelial cells (LSECs) regulates liver fibrosis, a pathological feature of chronic liver diseases. POFUT1 is an essential regulator of NOTCH signaling. Here, we investigated the role of LSEC-expressed POFUT1 in liver fibrosis.

METHODS

Endothelial-specific Pofut1 knockout mice were generated and experimental liver fibrosis was induced by chronic carbon tetrachloride exposure or common bile duct ligation. Liver samples were assessed by ELISA, histology, electron microscopy, immunostaining and RNA in situ hybridization. LSECs and hepatic stellate cells (HSCs) were isolated for gene expression analysis by RNA sequencing, qPCR, and western blotting. Signaling crosstalk between LSECs and HSCs was investigated by treating HSCs with supernatant from LSEC cultures. Liver single-cell RNA sequencing datasets from patients with cirrhosis and healthy individuals were analyzed to evaluate the clinical relevance of gene expression changes observed in mouse studies.

RESULTS

POFUT1 loss promoted injury-induced LSEC capillarization and HSC activation, leading to aggravated liver fibrosis. RNA sequencing analysis revealed that POFUT1 deficiency upregulated fibrinogen expression in LSECs. Consistently, fibrinogen was elevated in LSECs of patients with cirrhosis. HSCs treated with supernatant from LSECs of Pofut1 null mice showed exacerbated activation compared to those treated with supernatant from control LSECs, and this effect was attenuated by knockdown of fibrinogen or by pharmacological inhibition of fibrinogen receptor signaling, altogether suggesting that LSEC-derived fibrinogen induced the activation of HSCs. Mechanistically, POFUT1 loss augmented fibrinogen expression by enhancing NOTCH/HES1/STAT3 signaling.

CONCLUSIONS

Endothelial POFUT1 prevents injury-induced liver fibrosis by repressing the expression of fibrinogen, which functions as a profibrotic paracrine signal to activate HSCs. Therapies targeting the POFUT1/fibrinogen axis offer a promising strategy for the prevention and treatment of fibrotic liver diseases.

IMPACT AND IMPLICATIONS

Paracrine signals produced by liver vasculature play a major role in the development of liver fibrosis, which is a pathological hallmark of most liver diseases. Identifying those paracrine signals is clinically relevant in that they may serve as therapeutic targets. In this study, we discovered that genetic deletion of Pofut1 aggravated experimental liver fibrosis in mouse models. Moreover, fibrinogen was identified as a downstream target repressed by Pofut1 in liver endothelial cells and functioned as a novel paracrine signal that drove liver fibrosis. In addition, fibrinogen was found to be relevant to cirrhosis and may serve as a potential therapeutic target for this devastating human disease.

Role of liver sinusoidal endothelial cell in metabolic dysfunction-associated fatty liver disease

Liver sinusoidal endothelial cells (LSECs) are highly specialized endothelial cells that represent the interface between blood cells on one side and hepatocytes on the other side. LSECs not only form a barrier within the hepatic sinus, but also play important physiological functions such as regulating hepatic vascular pressure, anti-inflammatory and anti-fibrotic. Pathologically, pathogenic factors can induce LSECs capillarization, that is, loss of fenestra and dysfunction, which are conducive to early steatosis, lay the foundation for the progression of metabolic dysfunction-associated fatty liver disease (MAFLD), and accelerate metabolic dysfunction-associated steatohepatitis (MASH) and liver fibrosis. The unique localization, phenotype, and function of LSECs make them potential candidates for reducing liver injury, inflammation, and preventing or reversing fibrosis in the future.

BMP9 is a key player in endothelial identity and its loss is sufficient to induce arteriovenous malformations

AIMS

BMP9 is a high affinity ligand of ALK1 and endoglin receptors that are mutated in the rare genetic vascular disorder hereditary hemorrhagic telangiectasia (HHT). We have previously shown that loss of Bmp9 in the 129/Ola genetic background leads to spontaneous liver fibrosis via capillarization of liver sinusoidal endothelial cells (LSEC) and kidney lesions. We aimed to decipher the molecular mechanisms downstream of BMP9 to better characterize its role in vascular homeostasis in different organs.

METHODS AND RESULTS

For this, we performed an RNA-seq analysis on LSEC from adult WT and Bmp9-KO mice and identified over 2000 differentially expressed genes. Gene ontology analysis showed that Bmp9 deletion led to a decrease in BMP and Notch signalling, but also LSEC capillary identity while increasing their cell cycle. The gene ontology term 'glomerulus development' was also negatively enriched in Bmp9-KO mice vs. WT supporting a role for BMP9 in kidney vascularization. Through different imaging approaches (electron microscopy, immunostainings), we found that loss of Bmp9 led to vascular enlargement of the glomeruli capillaries associated with alteration of podocytes. Importantly, we also showed for the first time that the loss of Bmp9 led to spontaneous arteriovenous malformations (AVMs) in the liver, gastrointestinal tract, and uterus.

CONCLUSION

Altogether, these results demonstrate that BMP9 plays an important role in vascular quiescence both locally in the liver by regulating endothelial capillary differentiation markers and cell cycle but also at distance in many organs via its presence in the circulation. It also reveals that loss of Bmp9 is sufficient to induce spontaneous AVMs, supporting a key role for BMP9 in the pathogenesis of HHT.

Liver sinusoidal endothelial cells contribute to portal hypertension through collagen type IV-driven sinusoidal remodeling

Portal hypertension (PHTN) is a severe complication of liver cirrhosis and is associated with intrahepatic sinusoidal remodeling induced by sinusoidal resistance and angiogenesis. Collagen type IV (COL4), a major component of basement membrane, forms in liver sinusoids upon chronic liver injury. However, the role, cellular source, and expression regulation of COL4 in liver diseases are unknown. Here, we examined how COL4 is produced and how it regulates sinusoidal remodeling in fibrosis and PHTN. Human cirrhotic liver sample RNA sequencing showed increased COL4 expression, which was further verified via immunofluorescence staining. Single-cell RNA sequencing identified liver sinusoidal endothelial cells (LSECs) as the predominant source of COL4 upregulation in mouse fibrotic liver. In addition, COL4 was upregulated in a TNF-α/NF-κB-dependent manner through an epigenetic mechanism in LSECs in vitro. Indeed, by utilizing a CRISPRi-dCas9-KRAB epigenome-editing approach, epigenetic repression of the enhancer-promoter interaction showed silencing of COL4 gene expression. LSEC-specific COL4 gene mutation or repression in vivo abrogated sinusoidal resistance and angiogenesis, which thereby alleviated sinusoidal remodeling and PHTN. Our findings reveal that LSECs promote sinusoidal remodeling and PHTN during liver fibrosis through COL4 deposition.

Molecular Pathways Governing the Termination of Liver Regeneration

The liver has the unique capacity to regenerate, and up to 70% of the liver can be removed without detrimental consequences to the organism. Liver regeneration is a complex process involving multiple signaling networks and organs. Liver regeneration proceeds through three phases: the initiation phase, the growth phase, and the termination phase. Termination of liver regeneration occurs when the liver reaches a liver-to-body weight that is required for homeostasis, the so-called "hepatostat." The initiation and growth phases have been the subject of many studies. The molecular pathways that govern the termination phase, however, remain to be fully elucidated. This review summarizes the pathways and molecules that signal the cessation of liver regrowth after partial hepatectomy and answers the question, "What factors drive the hepatostat?" SIGNIFICANCE STATEMENT: Unraveling the pathways underlying the cessation of liver regeneration enables the identification of druggable targets that will allow us to gain pharmacological control over liver regeneration. For these purposes, it would be useful to understand why the regenerative capacity of the liver is hampered under certain pathological circumstances so as to artificially modulate the regenerative processes (e.g., by blocking the cessation pathways) to improve clinical outcomes and safeguard the patient's life.

SENP1 attenuates hypoxia‑reoxygenation injury in liver sinusoid endothelial cells by relying on the HIF‑1α signaling pathway

Liver sinusoidal endothelial cells (LSECs) have an important role in hepatic ischemia‑reperfusion injury (I/R), but the specific molecular mechanism of action is unknown. LSEC proliferation is regulated and fenestration is maintained via the Sentrin/SUMO‑specific protease 1 (SENP1)/hypoxia‑inducible factor‑1α (HIF‑1α) signaling axis under hypoxic conditions. In the present study, a hypoxia‑reoxygenation (H‑R) injury model was established using mouse LSECs to explore the relationship between SENP1 and H‑R injury in vitro, and the specific underlying mechanism was identified, revealing new targets for the clinical attenuation of hepatic I/R injury. Following the culture of LSECs under H‑R conditions, it was demonstrated that the expression of SENP1 was upregulated by reverse transcription‑quantitative polymerase chain reaction and western blotting (WB). In addition, scanning electron microscopy indicated that fenestrae damage was increased, a Cell Counting Kit‑8 assay demonstrated that the proliferation of cells was impaired and flow cytometry showed that apoptosis was increased. After silencing SENP1 expression with short interfering RNA, the proliferation activity of LSECs decreased, the fenestrae damage increased, the apoptosis rate increased and the expression levels of SENP1, HIF‑1α, heme oxygenase and Bcl‑2 were downregulated (as demonstrated by WB), while the expression levels of apoptosis‑related proteins, cleaved‑caspase‑3 and Bax, were upregulated. Enzyme‑linked immunosorbent assay detection showed that the level of vascular endothelial growth factor in the supernatant decreased and the level of IL‑6 and TNF‑α increased. Following the administration of an HIF‑1α signaling pathway agonist, the situation was reversed. These results therefore suggested that SENP1 attenuated the reduction in proliferation, apoptosis and fenestration of LSECs observed following H‑R injury through the HIF‑1α signaling pathway. In conclusion, SENP1 may attenuate H‑R injury in LSECs in a HIF‑1α signaling pathway‑dependent manner.

Balance of Gata3 and Ramp2 in hepatocytes regulates hepatic vascular reconstitution in postoperative liver regeneration

BACKGROUND & AIMS

Post-hepatectomy liver failure (PHLF) leads to poor prognosis in patients undergoing hepatectomy, with hepatic vascular reconstitution playing a critical role. However, the regulators of hepatic vascular reconstitution remain unclear. In this study, we aimed to investigate the regulatory mechanisms of hepatic vascular reconstitution and identify biomarkers predicting PHLF in patients undergoing hepatectomy.

METHODS

Candidate genes that were associated with hepatic vascular reconstitution were screened using adeno-associated virus vectors in Alb-Cre-CRISPR/Cas9 mice subjected to partial hepatectomy. The biological activities of candidate genes were estimated using endothelial precursor transfusion and associating liver partition and portal vein ligation for staged hepatectomy (ALPPS) models. The level of candidates was detected in biopsies from patients undergoing ALPPS. Risk factors for PHLF were also screened using retrospective data.

RESULTS

Downregulation of Gata3 and upregulation of Ramp2 in hepatocytes promoted the proliferation of liver sinusoidal endothelial cells and hepatic revascularization. Pigment epithelium-derived factor (PEDF) and vascular endothelial growth factor A (VEGFA) played opposite roles in regulating the migration of endothelial precursors from bone marrow and the formation of new sinusoids after hepatectomy. Gata3 restricted endothelial cell function in patient-derived hepatic organoids, which was abrogated by a Gata3 inhibitor. Moreover, overexpression of Gata3 led to higher mortality in ALPPS mice, which was improved by a PEDF-neutralizing antibody. The expression of Gata3/RAMP and PEDF/VEGFA tended to have a negative correlation in patients undergoing ALPPS. A nomogram incorporating multiple factors, such as serum PEDF/VEGF index, was constructed and could efficiently predict the risk of PHLF.

CONCLUSIONS

The balance of Gata3 and Ramp2 in hepatocytes regulates the proliferation of liver sinusoidal endothelial cells and hepatic revascularization via changes in the expression of PEDF and VEGFA, revealing potential targets for the prevention and treatment of PHLF.

IMPACT AND IMPLICATIONS

In this study, we show that the balance of Gata3 and Ramp2 in hepatocytes regulates hepatic vascular reconstitution by promoting a shift from pigment epithelium-derived factor (PEDF) to vascular endothelial growth factor A (VEGFA) expression during hepatectomy- or ALLPS (associating liver partition and portal vein ligation for staged hepatectomy)-induced liver regeneration. We also identified serum PEDF/VEGFA index as a potential predictor of post-hepatectomy liver failure in patients who underwent hepatectomy. This study improves our understanding of how hepatocytes contribute to liver regeneration and provides new targets for the prevention and treatment of post-hepatectomy liver failure.

The roles of liver sinusoidal endothelial cells in liver ischemia/reperfusion injury

Liver ischemia/reperfusion injury (IRI) is a major complication after partial hepatectomy and liver transplantation and during hypovolemic shock and hypoxia-related diseases. Liver IRI is a current research hotspot. The early stage of liver IRI is characterized by injury and dysfunction of liver sinusoidal endothelial cells (LSECs), which, along with hepatocytes, are the major cells involved in liver injury. In this review, we elaborate on the roles played by LSECs in liver IRI, including the pathological features of LSECs, LSECs exacerbation of the sterile inflammatory response, LSECs interactions with platelets and the promotion of liver regeneration, and the activation of LSECs autophagy. In addition, we discuss the study of LSECs as therapeutic targets for the treatment of liver IRI and the existing problems when applying LSECs in liver IRI research.

Interorgan communication with the liver: novel mechanisms and therapeutic targets

The liver is a multifunctional organ that plays crucial roles in numerous physiological processes, such as production of bile and proteins for blood plasma, regulation of blood levels of amino acids, processing of hemoglobin, clearance of metabolic waste, maintenance of glucose, etc. Therefore, the liver is essential for the homeostasis of organisms. With the development of research on the liver, there is growing concern about its effect on immune cells of innate and adaptive immunity. For example, the liver regulates the proliferation, differentiation, and effector functions of immune cells through various secreted proteins (also known as "hepatokines"). As a result, the liver is identified as an important regulator of the immune system. Furthermore, many diseases resulting from immune disorders are thought to be related to the dysfunction of the liver, including systemic lupus erythematosus, multiple sclerosis, and heart failure. Thus, the liver plays a role in remote immune regulation and is intricately linked with systemic immunity. This review provides a comprehensive overview of the liver remote regulation of the body's innate and adaptive immunity regarding to main areas: immune-related molecules secreted by the liver and the liver-resident cells. Additionally, we assessed the influence of the liver on various facets of systemic immune-related diseases, offering insights into the clinical application of target therapies for liver immune regulation, as well as future developmental trends.

In silico and in vivo anti-angiogenic validation on ethanolic extract of Curcuma longa and curcumin compound in hepatocellular carcinoma through mitogen activated protein kinase expression in male and female wistar rats

Hepatocellular carcinoma (HCC) is the most frequent primary malignancy of the liver. The aim of this study is to evaluate the comparative in silico and in vivo ameliorative potential of the ethanolic extract of Curcuma longa (EECL) in male and female Wistar rats administered N-nitrosodiethylamine-induced hepatocellular carcinoma. The MAPK compound was obtained from a protein data bank (PDB ID: 7AUV) for molecular docking. One hundred and twenty Wistar rats, were randomly selected into twelve groups (n = 5): Group A received regular diets as a basal control; groups B to G were administered 100 mg/kg NDEA twice in two weeks; while groups C to E received 200 mg/kg, 400 mg/kg, and 600 mg/kg of EECL; group F was treated with 200 mg/kg pure curcumin; and group G received 100 mg/kg Sylibon-140. Group H received only 200 mg/kg pure curcumin, and group I received 200 mg/kg of dimethylsulfoxide (DMSO). Groups J, K, and L received 200 mg/kg, 400 mg/kg and 600 mg/kg of EECL. MAPK and AFP mRNA in Wistar rats administered NDEA were upregulated as compared to EECL groups. In conclusion, the in silico and in vitro study validates the mitigating role of ethanolic extract of Curcuma longa and pure curcumin.

Single-cell RNA sequencing reveals the dynamics of hepatic non-parenchymal cells in autoprotection against acetaminophen-induced hepatotoxicity

Gaining a better understanding of autoprotection against drug-induced liver injury (DILI) may provide new strategies for its prevention and therapy. However, little is known about the underlying mechanisms of this phenomenon. We used single-cell RNA sequencing to characterize the dynamics and functions of hepatic non-parenchymal cells (NPCs) in autoprotection against DILI, using acetaminophen (APAP) as a model drug. Autoprotection was modeled through pretreatment with a mildly hepatotoxic dose of APAP in mice, followed by a higher dose in a secondary challenge. NPC subsets and dynamic changes were identified in the APAP (hepatotoxicity-sensitive) and APAP-resistant (hepatotoxicity-resistant) groups. A chemokine (C-C motif) ligand 2+ endothelial cell subset almost disappeared in the APAP-resistant group, and an R-spondin 3+ endothelial cell subset promoted hepatocyte proliferation and played an important role in APAP autoprotection. Moreover, the dendritic cell subset DC-3 may protect the liver from APAP hepatotoxicity by inducing low reactivity and suppressing the autoimmune response and occurrence of inflammation. DC-3 cells also promoted angiogenesis through crosstalk with endothelial cells via vascular endothelial growth factor-associated ligand-receptor pairs and facilitated liver tissue repair in the APAP-resistant group. In addition, the natural killer cell subsets NK-3 and NK-4 and the Sca-1-CD62L+ natural killer T cell subset may promote autoprotection through interferon-γ-dependent pathways. Furthermore, macrophage and neutrophil subpopulations with anti-inflammatory phenotypes promoted tolerance to APAP hepatotoxicity. Overall, this study reveals the dynamics of NPCs in the resistance to APAP hepatotoxicity and provides novel insights into the mechanism of autoprotection against DILI at a high resolution.

Vascular endothelial growth factor protein and gene delivery by novel nanomaterials for promoting liver regeneration after partial hepatectomy

Partial hepatectomy (PH) can lead to severe complications, including liver failure, due to the low regenerative capacity of the remaining liver, especially after extensive hepatectomy. Liver sinusoidal endothelial cells (LSECs), whose proliferation occurs more slowly and later than hepatocytes after PH, compose the lining of the hepatic sinusoids, which are the smallest blood vessels in the liver. Vascular endothelial growth factor (VEGF), secreted by hepatocytes, promotes LSEC proliferation. Supplementation of exogenous VEGF after hepatectomy also increases the number of LSECs in the remaining liver, thus promoting the reestablishment of the hepatic sinusoids and accelerating liver regeneration. At present, some shortcomings exist in the methods of supplementing exogenous VEGF, such as a low drug concentration in the liver and the reaching of other organs. More-over, VEGF should be administered multiple times and in large doses because of its short half-life. This review summarized the most recent findings on liver regeneration and new strategies for the localized delivery VEGF in the liver.

The pharmacology and mechanisms of traditional Chinese medicine in promoting liver regeneration: A new therapeutic option

BACKGROUND

The liver is renowned for its remarkable regenerative capacity to restore its structure, size and function after various types of liver injury. However, in patients with end-stage liver disease, the regenerative capacity is inhibited and liver transplantation is the only option. Considering the limitations of liver transplantation, promoting liver regeneration is suggested as a new therapeutic strategy for liver disease. Traditional Chinese medicine (TCM) has a long history of preventing and treating various liver diseases, and some of them have been proven to be effective in promoting liver regeneration, suggesting the therapeutic potential in liver diseases.

PURPOSE

This review aims to summarize the molecular mechanisms of liver regeneration and the pro-regenerative activity and mechanism of TCM formulas, extracts and active ingredients.

METHODS

We conducted a systematic search in PubMed, Web of Science and the Cochrane Library databases using "TCM", "liver regeneration" or their synonyms as keywords, and classified and summarized the retrieved literature. The PRISMA guidelines were followed.

RESULTS

Forty-one research articles met the themes of this review and previous critical studies were also reviewed to provide essential background information. Current evidences indicate that various TCM formulas, extracts and active ingredients have the effect on stimulating liver regeneration through modulating JAK/STAT, Hippo, PI3K/Akt and other signaling pathways. Besides, the mechanisms of liver regeneration, the limitation of existing studies and the application prospect of TCM to promote liver regeneration are also outlined and discussed in this review.

CONCLUSION

This review supports TCM as new potential therapeutic options for promoting liver regeneration and repair of the failing liver, although extensive pharmacokinetic and toxicological studies, as well as elaborate clinical trials, are still needed to demonstrate safety and efficacy.

Multiple Facets of Cellular Homeostasis and Regeneration of the Mammalian Liver

Liver regeneration occurs in response to diverse injuries and is capable of functionally reestablishing the lost parenchyma. This phenomenon has been known since antiquity, encapsulated in the Greek myth where Prometheus was to be punished by Zeus for sharing the gift of fire with humanity by having an eagle eat his liver daily, only to have the liver regrow back, thus ensuring eternal suffering and punishment. Today, this process is actively leveraged clinically during living donor liver transplantation whereby up to a two-thirds hepatectomy (resection or removal of part of the liver) on a donor is used for transplant to a recipient. The donor liver rapidly regenerates to recover the lost parenchymal mass to form a functional tissue. This astonishing regenerative process and unique capacity of the liver are examined in further detail in this review.

Constructing biomimetic liver models through biomaterials and vasculature engineering

The occurrence of various liver diseases can lead to organ failure of the liver, which is one of the leading causes of mortality worldwide. Liver tissue engineering see the potential for replacing liver transplantation and drug toxicity studies facing donor shortages. The basic elements in liver tissue engineering are cells and biomaterials. Both mature hepatocytes and differentiated stem cells can be used as the main source of cells to construct spheroids and organoids, achieving improved cell function. To mimic the extracellular matrix (ECM) environment, biomaterials need to be biocompatible and bioactive, which also help support cell proliferation and differentiation and allow ECM deposition and vascularized structures formation. In addition, advanced manufacturing approaches are required to construct the extracellular microenvironment, and it has been proved that the structured three-dimensional culture system can help to improve the activity of hepatocytes and the characterization of specific proteins. In summary, we review biomaterials for liver tissue engineering, including natural hydrogels and synthetic polymers, and advanced processing techniques for building vascularized microenvironments, including bioassembly, bioprinting and microfluidic methods. We then summarize the application fields including transplant and regeneration, disease models and drug cytotoxicity analysis. In the end, we put the challenges and prospects of vascularized liver tissue engineering.

Endothelial Cell Dysfunction and Nonalcoholic Fatty Liver Disease (NAFLD): A Concise Review

Nonalcoholic fatty liver disease (NAFLD) is one of the most common liver diseases worldwide. It is strongly associated with obesity, type 2 diabetes (T2DM), and other metabolic syndrome features. Reflecting the underlying pathogenesis and the cardiometabolic disorders associated with NAFLD, the term metabolic (dysfunction)-associated fatty liver disease (MAFLD) has recently been proposed. Indeed, over the past few years, growing evidence supports a strong correlation between NAFLD and increased cardiovascular disease (CVD) risk, independent of the presence of diabetes, hypertension, and obesity. This implies that NAFLD may also be directly involved in the pathogenesis of CVD. Notably, liver sinusoidal endothelial cell (LSEC) dysfunction appears to be implicated in the progression of NAFLD via numerous mechanisms, including the regulation of the inflammatory process, hepatic stellate activation, augmented vascular resistance, and the distortion of microcirculation, resulting in the progression of NAFLD. Vice versa, the liver secretes inflammatory molecules that are considered pro-atherogenic and may contribute to vascular endothelial dysfunction, resulting in atherosclerosis and CVD. In this review, we provide current evidence supporting the role of endothelial cell dysfunction in the pathogenesis of NAFLD and NAFLD-associated atherosclerosis. Endothelial cells could thus represent a "golden target" for the development of new treatment strategies for NAFLD and its comorbid CVD.

Cellular crosstalk during liver regeneration: unity in diversity

The liver is unique in its ability to regenerate from a wide range of injuries and diseases. Liver regeneration centers around hepatocyte proliferation and requires the coordinated actions of nonparenchymal cells, including biliary epithelial cells, liver sinusoidal endothelial cells, hepatic stellate cells and kupffer cells. Interactions among various hepatocyte and nonparenchymal cells populations constitute a sophisticated regulatory network that restores liver mass and function. In addition, there are two different ways of liver regeneration, self-replication of liver epithelial cells and transdifferentiation between liver epithelial cells. The interactions among cell populations and regenerative microenvironment in the two modes are distinct. Herein, we first review recent advances in the interactions between hepatocytes and surrounding cells and among nonparenchymal cells in the context of liver epithelial cell self-replication. Next, we discuss the crosstalk of several cell types in the context of liver epithelial transdifferentiation, which is also crucial for liver regeneration.

Coordinated activation of TGF-β and BMP pathways promotes autophagy and limits liver injury after acetaminophen intoxication

Ligands of the transforming growth factor-β (TGF-β) superfamily, including TGF-βs, activins, and bone morphogenetic proteins (BMPs), have been implicated in hepatic development, homeostasis, and pathophysiology. We explored the mechanisms by which hepatocytes decode and integrate injury-induced signaling from TGF-βs and activins (TGF-β/Activin) and BMPs. We mapped the spatiotemporal patterns of pathway activation during liver injury induced by acetaminophen (APAP) in dual reporter mice carrying a fluorescent reporter of TGF-β/Activin signaling and a fluorescent reporter of BMP signaling. APAP intoxication induced the expression of both reporters in a zone of cells near areas of tissue damage, which showed an increase in autophagy and demarcated the borders between healthy and injured tissues. Inhibition of TGF-β superfamily signaling by overexpressing the inhibitor Smad7 exacerbated acute liver histopathology but eventually accelerated tissue recovery. Transcriptomic analysis identified autophagy as a process stimulated by TGF-β1 and BMP4 in hepatocytes, with Trp53inp2, which encodes a rate-limiting factor for autophagy initiation, as the most highly induced autophagy-related gene. Collectively, these findings illustrate the functional interconnectivity of the TGF-β superfamily signaling system, implicate the coordinated activation of TGF-β/Activin and BMP pathways in balancing tissue reparatory and regenerative processes upon APAP-induced hepatotoxicity, and highlight opportunities and potential risks associated with targeting this signaling system for treating hepatic diseases.

Intercellular crosstalk of liver sinusoidal endothelial cells in liver fibrosis, cirrhosis and hepatocellular carcinoma

Intercellular crosstalk among various liver cells plays an important role in liver fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Capillarization of liver sinusoidal endothelial cells (LSECs) precedes fibrosis and accumulating evidence suggests that the crosstalk between LSECs and other liver cells is critical in the development and progression of liver fibrosis. LSECs dysfunction, a key event in the progression from fibrosis to cirrhosis, and subsequently obstruction of hepatic sinuses and increased intrahepatic vascular resistance (IHVR) contribute to development of portal hypertension (PHT) and cirrhosis. More importantly, immunosuppressive tumor microenvironment (TME), which is closely related to the crosstalk between LSECs and immune liver cells like CD8+ T cells, promotes advances tumorigenesis, especially HCC. However, the connections within the crosstalk between LSECs and other liver cells during the progression from liver fibrosis to cirrhosis to HCC have yet to be discussed. In this review, we first summarize the current knowledge of how different crosstalk between LSECs and other liver cells, including hepatocytes, hepatic stellate cells (HSCs), macrophoges, immune cells in liver and extra cellular matrix (ECM) contribute to the physiological function and the progrssion from liver fibrosis to cirrhosis, or even to HCC. Then we examine current treatment strategies for LSECs crosstalk in liver fibrosis, cirrhosis and HCC.

Oxidative Stress and Redox Signaling in the Pathophysiology of Liver Diseases

The increased production of derivatives of molecular oxygen and nitrogen in the form of reactive oxygen species (ROS) and reactive nitrogen species (RNS) lead to molecular damage called oxidative stress. Under normal physiological conditions, the ROS generation is tightly regulated in different cells and cellular compartments. Any disturbance in the balance between the cellular generation of ROS and antioxidant balance leads to oxidative stress. In this article, we discuss the sources of ROS (endogenous and exogenous) and antioxidant mechanisms. We also focus on the pathophysiological significance of oxidative stress in various cell types of the liver. Oxidative stress is implicated in the development and progression of various liver diseases. We narrate the master regulators of ROS-mediated signaling and their contribution to liver diseases. Nonalcoholic fatty liver diseases (NAFLD) are influenced by a "multiple parallel-hit model" in which oxidative stress plays a central role. We highlight the recent findings on the role of oxidative stress in the spectrum of NAFLD, including fibrosis and liver cancer. Finally, we provide a brief overview of oxidative stress biomarkers and their therapeutic applications in various liver-related disorders. Overall, the article sheds light on the significance of oxidative stress in the pathophysiology of the liver. © 2022 American Physiological Society. Compr Physiol 12:3167-3192, 2022.

Mitomycin-Treated Endothelial and Smooth Muscle Cells Suitable for Safe Tissue Engineering Approaches

In our previous study, we showed that discarded cardiac tissue from the right atrial appendage and right ventricular myocardium is an available source of functional endothelial and smooth muscle cells for regenerative medicine and tissue engineering. In the study, we aimed to find out what benefits are given by vascular cells from cardiac explants used for seeding on vascular patches engrafted to repair vascular defects in vivo. Additionally, to make the application of these cells safer in regenerative medicine we tested an in vitro approach that arrested mitotic division to avoid the potential tumorigenic effect of dividing cells. A tissue-engineered construction in the form of a patch based on a polycaprolactone-gelatin scaffold and seeded with endothelial and smooth muscle cells was implanted into the abdominal aorta of immunodeficient SCID mice. Aortic patency was assessed using ultrasound, MRI, immunohistochemical and histological staining. Endothelial and smooth muscle cells were treated with mitomycin C at a therapeutic concentration of 10 μg/ml for 2 h with subsequent analysis of cell proliferation and function. The absence of the tumorigenic effect of mitomycin C-treated cells, as well as their angiogenic potential, was examined by injecting them into immunodeficient mice. Cell-containing patches engrafted in the abdominal aorta of immunodeficient mice form the vessel wall loaded with the appropriate cells and extracellular matrix, and do not interfere with normal patency. Endothelial and smooth muscle cells treated with mitomycin C show no tumorigenic effect in the SCID immunodeficient mouse model. During in vitro experiments, we have shown that treatment with mitomycin C does not lead to a decrease in cell viability. Despite the absence of proliferation, mitomycin C-treated vascular cells retain specific cell markers, produce specific extracellular matrix, and demonstrate the ability to stimulate angiogenesis in vivo. We pioneered an approach to arresting cell division with mitomycin C in endothelial and smooth muscle cells from cardiac explant, which prevents the risk of malignancy from dividing cells in vascular surgery. We believe that this approach to the fabrication of tissue-engineered constructs based on mitotically inactivated cells from waste postoperative material may be valuable to bring closer the development of safe cell products for regenerative medicine.

Inflammatory and fibrotic mechanisms in NAFLD-Implications for new treatment strategies

Non-alcoholic fatty liver disease is comprised of either simple steatosis (non-alcoholic fatty liver) or a more advanced inflammatory and fibrogenic stage (non-alcoholic steatohepatitis [NASH]). NASH affects a growing proportion of the global adult and pediatric population, leading to rising rates of liver fibrosis and hepatocellular carcinoma. NASH is a multifactorial disease that is part of a systemic metabolic disorder. Here, we provide an overview of the metabolic underpinnings of NASH pathogenesis and established drivers of inflammation and fibrosis. Clarification of underlying fibrogenic and inflammatory mechanisms will advance the development of novel treatment strategies as there are no approved therapies at present. We discuss emerging experimental approaches and potential novel investigational strategies derived from animal models including the inflammasome, epigenetic reprogramming, Hippo signaling, Notch signaling, engineered T cells to remove fibrogenic HSCs, and HSC-specific targeting therapies. Recently completed and ongoing clinical trials and antifibrotics are discussed, illuminating the growing expectation that one or more therapies will yield clinical benefit in NASH in the coming years.

Endothelial dysfunction in pathological processes of chronic liver disease during aging

Aging is associated with gradual changes in liver structure and physiological/pathological functions in hepatic cells including hepatocytes, cholangiocytes, Kupffer cells, hepatic stellate cells (HSCs), and liver sinusoidal endothelial cells (LSECs). LSECs are specialized hepatic endothelial cells that regulate liver homeostasis. These cells actively impact the hepatic microenvironment as they have fenestrations and a thin morphology to allow substance exchange between circulating blood and the liver tissue. As aging occurs, LSECs have a reduction in both the number and size of fenestrations, which is referred to as pseudocapillarization. This along with the aging of the liver leads to increased oxidative stress, decreased availability of nitric oxide, decreased hepatic blood flow, and increased inflammatory cytokines in LSECs. Vascular aging can also lead to hepatic hypoxia, HSC activation, and liver fibrosis. In this review, we described the basic structure of LSECs, and the effect of LSECs on hepatic inflammation and fibrosis during aging process. We briefly summarized the changes of hepatic microcirculation during liver inflammation, the effect of aging on the clearance function of LSECs, the interactions between LSECs and immunity, hepatocytes or other hepatic nonparenchymal cells, and the therapeutic intervention of liver diseases by targeting LSECs and vascular system. Since LSECs play an important role in the development of liver fibrosis and the changes of LSEC phenotype occur in the early stage of liver fibrosis, the study of LSECs in the fibrotic liver is valuable for the detection of early liver fibrosis and the early intervention of fibrotic response.

Hypoxia promotes the proliferation of mouse liver sinusoidal endothelial cells: miRNA-mRNA expression analysis

During the initial stage of liver regeneration (LR), hepatocytes and liver sinusoidal endothelial cells (LSECs) initiate regeneration in a hypoxic environment. However, the role of LSECs in liver regeneration in hypoxic environments and their specific molecular mechanism is unknown. Therefore, this study aimed to explore the miRNA-mRNA network that regulates the proliferation of LSECs during hypoxia. In this study, first, we found that the proliferation ability of primary LSECs treated with hypoxia was enhanced compared with the control group, and then whole transcriptome sequencing was performed to screen 1837 differentially expressed (DE) genes and 17 DE miRNAs. Subsequently, the bioinformatics method was used to predict the target genes of miRNAs, and 309 pairs of interacting miRNA-mRNA pairs were obtained. Furthermore, the miRNA-gene action network was established using the negative interacting miRNA-mRNA pairs. The selected mRNAs were analyzed by Gene Ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, and biological processes (BP) and signal pathways related to LSEC proliferation that were significantly enriched in GO-BP and KEGG were selected. Finally, 22 DE genes and 17 DE miRNAs were screened and the network was created. We also successfully verified the significant changes in the top six genes and miRNAs using qRT-PCR, and the results were consistent with the sequencing results. This study proposed that a specific miRNA-mRNA network is associated with hypoxia-induced proliferation of LSECs, which will assist in elucidating the potential mechanisms involved in hypoxia-promoting liver regeneration during LR.

Deciphering the Dynamic Complexities of the Liver Microenvironment - Toward a Better Understanding of Immune-Mediated liver Injury Caused by Immune Checkpoint Inhibitors (ILICI)

Immune checkpoint inhibitors (ICIs) represent a promising therapy for many types of cancer. However, only a portion of patients respond to this therapy and some patients develop clinically significant immune-mediated liver injury caused by immune checkpoint inhibitors (ILICI), an immune-related adverse event (irAE) that may require the interruption or termination of treatment and administration of systemic corticosteroids or other immunosuppressive agents. Although the incidence of ILICI is lower with monotherapy, the surge in combining ICIs with chemotherapy, targeted therapy, and combination of different ICIs has led to an increase in the incidence and severity of ILICI - a major challenge for development of effective and safe ICI therapy. In this review, we highlight the importance and contribution of the liver microenvironment to ILICI by focusing on the emerging roles of resident liver cells in modulating immune homeostasis and hepatocyte regeneration, two important decisive factors that dictate the initiation, progression, and recovery from ILICI. Based on the proposed contribution of the liver microenvironment on ICILI, we discuss the clinical characteristics of ILICI in patients with preexisting liver diseases, as well as the challenges of identifying prognostic biomarkers to guide the clinical management of severe ILICI. A better understanding of the liver microenvironment may lead to novel strategies and identification of novel biomarkers for effective management of ILICI.

Critical Role of LSEC in Post-Hepatectomy Liver Regeneration and Failure

Liver sinusoids are lined by liver sinusoidal endothelial cells (LSEC), which represent approximately 15 to 20% of the liver cells, but only 3% of the total liver volume. LSEC have unique functions, such as fluid filtration, blood vessel tone modulation, blood clotting, inflammatory cell recruitment, and metabolite and hormone trafficking. Different subtypes of liver endothelial cells are also known to control liver zonation and hepatocyte function. Here, we have reviewed the origin of LSEC, the different subtypes identified in the liver, as well as their renewal during homeostasis. The liver has the exceptional ability to regenerate from small remnants. The past decades have seen increasing awareness in the role of non-parenchymal cells in liver regeneration despite not being the most represented population. While a lot of knowledge has emerged, clarification is needed regarding the role of LSEC in sensing shear stress and on their participation in the inductive phase of regeneration by priming the hepatocytes and delivering mitogenic factors. It is also unclear if bone marrow-derived LSEC participate in the proliferative phase of liver regeneration. Similarly, data are scarce as to LSEC having a role in the termination phase of the regeneration process. Here, we review what is known about the interaction between LSEC and other liver cells during the different phases of liver regeneration. We next explain extended hepatectomy and small liver transplantation, which lead to "small for size syndrome" (SFSS), a lethal liver failure. SFSS is linked to endothelial denudation, necrosis, and lobular disturbance. Using the knowledge learned from partial hepatectomy studies on LSEC, we expose several techniques that are, or could be, used to avoid the "small for size syndrome" after extended hepatectomy or small liver transplantation.

Emerging Importance of Chemokine Receptor CXCR4 and Its Ligand in Liver Disease

Chemokine receptors are members of the G protein-coupled receptor superfamily, which together with chemokine ligands form chemokine networks to regulate various cellular functions, immune and physiological processes. These receptors are closely related to cell movement and thus play a vital role in several physiological and pathological processes that require regulation of cell migration. CXCR4, one of the most intensively studied chemokine receptors, is involved in many functions in addition to immune cells recruitment and plays a pivotal role in the pathogenesis of liver disease. Aberrant CXCR4 expression pattern is related to the migration and movement of liver specific cells in liver disease through its cross-talk with a variety of significant cell signaling pathways. An in-depth understanding of CXCR4-mediated signaling pathway and its role in liver disease is critical to identifying potential therapeutic strategies. Current therapeutic strategies for liver disease mainly focus on regulating the key functions of specific cells in the liver, in which the CXCR4 pathway plays a crucial role. Multiple challenges remain to be overcome in order to more effectively target CXCR4 pathway and identify novel combination therapies with existing strategies. This review emphasizes the role of CXCR4 and its important cell signaling pathways in the pathogenesis of liver disease and summarizes the targeted therapeutic studies conducted to date.

F4/80+ Kupffer Cell-Derived Oncostatin M Sustains the Progression Phase of Liver Regeneration through Inhibition of TGF-β2 Pathway

The role of Kupffer cells (KCs) in liver regeneration is complicated and controversial. To investigate the distinct role of F4/80⁺ KCs at the different stages of the regeneration process, two-thirds partial hepatectomy (PHx) was performed in mice to induce physiological liver regeneration. In pre- or post-PHx, the clearance of KCs by intraperitoneal injection of the anti-F4/80 antibody (α-F4/80) was performed to study the distinct role of F4/80⁺ KCs during the regenerative process. In RNA sequencing of isolated F4/80⁺ KCs, the initiation phase was compared with the progression phase. Immunohistochemistry and immunofluorescence staining of Ki67, HNF-4α, CD-31, and F4/80 and Western blot of the TGF-β2 pathway were performed. Depletion of F4/80⁺ KCs in pre-PHx delayed the peak of hepatocyte proliferation from 48 h to 120 h, whereas depletion in post-PHx unexpectedly led to persistent inhibition of hepatocyte proliferation, indicating the distinct role of F4/80⁺ KCs in the initiation and progression phases of liver regeneration. F4/80⁺ KC depletion in post-PHx could significantly increase TGF-β2 serum levels, while TGF-βRI partially rescued the impaired proliferation of hepatocytes. Additionally, F4/80⁺ KC depletion in post-PHx significantly lowered the expression of oncostatin M (OSM), a key downstream mediator of interleukin-6, which is required for hepatocyte proliferation during liver regeneration. In vivo, recombinant OSM (r-OSM) treatment alleviated the inhibitory effect of α-F4/80 on the regenerative progression. Collectively, F4/80⁺ KCs release OSM to inhibit TGF-β2 activation, sustaining hepatocyte proliferation by releasing a proliferative brake.

Biomaterial-based cell delivery strategies to promote liver regeneration

Chronic liver disease and cirrhosis is a widespread and untreatable condition that leads to lifelong impairment and eventual death. The scarcity of liver transplantation options requires the development of new strategies to attenuate disease progression and reestablish liver function by promoting regeneration. Biomaterials are becoming an increasingly promising option to both culture and deliver cells to support in vivo viability and long-term function. There is a wide variety of both natural and synthetic biomaterials that are becoming established as delivery vehicles with their own unique advantages and disadvantages for liver regeneration. We review the latest developments in cell transplantation strategies to promote liver regeneration, with a focus on the use of both natural and synthetic biomaterials for cell culture and delivery. We conclude that future work will need to refine the use of these biomaterials and combine them with novel strategies that recapitulate liver organization and function in order to translate this strategy to clinical use.

Formyl Peptide Receptor 2 Alleviates Hepatic Fibrosis in Liver Cirrhosis by Vascular Remodeling

Hexapeptide WKYMVm (Trp-Lys-Tyr-Met-Val-D-Met), a ligand of formyl peptide receptor 2, exhibits anti-inflammatory and angiogenic properties in disease models. However, the therapeutic effects of WKYMVm on hepatic fibrosis have not been evaluated to date. Therefore, we investigated whether WKYMVm exerts antifibrotic effects and induces vascular regeneration in a rat model of bile duct ligation (BDL). The antifibrotic and angiogenic effects of WKYMVm on liver regeneration in the BDL rat model were analyzed using biochemical assays, qRT-PCR, western blotting, immunofluorescence, and immunohistochemistry. To determine the effects of WKYMVm on hepatic fibrosis and angiogenesis in vitro, we measured the expression levels of fibrotic factors in hepatic stellate cells (HSCs) and angiogenic factors in human umbilical vein endothelial cells (HUVECs). WKYMVm attenuated the expression of collagen type I (Col I) and α-smooth muscle actin (α-SMA) and significantly increased the levels of angiogenetic factors in the BDL model (p < 0.05). WKYMVm reduced fibrotic marker expression in transforming growth factor (TGF)-β-induced HSCs and promoted angiogenic activity through tube formation in 5-Fluorouracil (FU)-treated HUVECs (p < 0.05). Also, WKYMVm administration enhanced hepatocyte proliferation in BDL rats (p < 0.05). The WKYMVm alleviates hepatic fibrosis by inhibiting HSC activation and promotes hepatic regeneration via vascular remodeling. These data suggest that the WKYMVm may be a new therapeutic agent for liver fibrosis.

The Power of Plasticity-Metabolic Regulation of Hepatic Stellate Cells

Hepatic stellate cells (HSCs) are resident non-parenchymal liver pericytes whose plasticity enables them to regulate a remarkable range of physiologic and pathologic responses. To support their functions in health and disease, HSCs engage pathways regulating carbohydrate, mitochondrial, lipid, and retinoid homeostasis. In chronic liver injury, HSCs drive hepatic fibrosis and are implicated in inflammation and cancer. To do so, the cells activate, or transdifferentiate, from a quiescent state into proliferative, motile myofibroblasts that secrete extracellular matrix, which demands rapid adaptation to meet a heightened energy need. Adaptations include reprogramming of central carbon metabolism, enhanced mitochondrial number and activity, endoplasmic reticulum stress, and liberation of free fatty acids through autophagy-dependent hydrolysis of retinyl esters that are stored in cytoplasmic droplets. As an archetype for pericytes in other tissues, recognition of the HSC's metabolic drivers and vulnerabilities offer the potential to target these pathways therapeutically to enhance parenchymal growth and modulate repair.

The vascular endothelial growth factor signaling pathway regulates liver sinusoidal endothelial cells during liver regeneration after partial hepatectomy

INTRODUCTION

Liver regeneration after partial hepatectomy is a very complex and well-regulated procedure. It utilizes all liver cell types, which are associated with signaling pathways involving growth factors, cytokines, and stimulatory and inhibitory feedback of several growth-related signals. Liver sinusoidal endothelial cells (LSECs) contribute to liver regeneration after partial hepatectomy. Vascular endothelial growth factor (VEGF) has various functions in LSECs. In this review, we summarize the relationship between VEGF and LSECs involving VEGF regulatory activity in the vascular endothelium.

AREAS COVERED

Maintenance of the fenestrated LSEC phenotype requires two VEGF pathways: VEGF stimulated-NO acting through the cGMP pathway and VEGF independent of nitric oxide (NO). The results suggest that VEGF is a key regenerating mediator of LSECs in the partial hepatectomy model. NO-independent pathway was also essential to the maintenance of the LSEC in liver regeneration.

EXPERT OPINION

Liver regeneration remains a fascinating and significative research field in recent years. The liver involved of molecular pathways except for LSEC-VEGF pathways that make the field of liver further depth studies should be put into effect to elaborate the undetermined confusions, which will be better to understand liver regeneration.

Stem cell therapy and tissue engineering strategies using cell aggregates and decellularized scaffolds for the rescue of liver failure

Liver failure is a lethal condition with hepatocellular dysfunction, and liver transplantation is presently the only effective treatment. However, due to the limited availability of donors and the potential immune rejection, novel therapeutic strategies are actively sought to restore the normal hepatic architectures and functions, especially for livers with inherited metabolic dysfunctions or chronic diseases. Although the conventional cell therapy has shown promising results, the direct infusion of hepatocytes is hampered by limited hepatocyte sources, poor cell viability, and engraftment. Hence, this review mainly highlights the role of stem cells and progenitors as the alternative cell source and summarizes the potential approaches based on tissue engineering to improve the delivery efficiency of cells. Particularly, the underlying mechanisms for cell therapy using stem cells and progenitors are discussed in two main aspects: paracrine effect and cell differentiation. Moreover, tissue-engineering approaches using cell aggregates and decellularized liver scaffolds for bioengineering of functional hepatic constructs are discussed and compared in terms of the potential to replicate liver physiological structures. In the end, a potentially effective strategy combining the premium advantages of stem cell aggregates and decellularized liver scaffolds is proposed as the future direction of liver tissue engineering and regeneration.

Hypoxia maintains the fenestration of liver sinusoidal endothelial cells and promotes their proliferation through the SENP1/HIF-1α/VEGF signaling axis

Liver sinusoidal endothelial cells (LSECs), which play a very critical role in liver regeneration, function in hypoxic environments, but few studies have elucidated the specific mechanism. As a hypoxia-sensitive gene, Sentrin/SUMO-specific protease 1(SENP1) is upregulated in solid tumors due to hypoxia and promotes tumor proliferation. We speculate that LSECs may upregulate SENP1 in hypoxic environments and that SENP1 may act on downstream genes to allow the cells to adapt to the hypoxic environment. To elucidate the reasons for the survival of LSECs under hypoxia, we designed experiments to explore the possible mechanism. First, we cultured murine LSECs in hypoxic conditions for a certain time (24 h and 72 h), and then, we observed that the proliferation ability of the hypoxia group was higher than that of the normoxia group, and the number of unique fenestrae of the LSECs in the hypoxia group was more than that of the LSECs in the normoxia group. Then, we divided the LSECs into several groups for hypoxic culture for time points (6 h, 12 h, 24 h, 36 h, and 72 h), and we found that the expression of SENP1, HIF-1α and VEGF was significantly upregulated. Then, we silenced SENP1 and HIF-1α with si-SENP1 and si-HIF-1α, respectively. SENP1, HIF-1α and VEGF were significantly downregulated, as determined by RT-PCR, WB and ELISA. Unexpectedly, the proliferation activity of the LSECs decreased and the fenestrae disappeared more in the si-SENP1 and si-HIF-1α groups than in the control group. It is concluded that LSECs cultured under hypoxic conditions may maintain fenestrae and promote proliferation through the SENP1/HIF-1α/VEGF signaling axis, thereby adapting to the hypoxic environment.

Sinusoidal Endothelial Cell Progenitor Cells Promote Tumour Progression in Patients with Hepatocellular Carcinoma

Objective

As sinusoidal endothelial cell progenitor cells (SEPCs) play a significant role in liver regeneration, it is necessary to elucidate whether SEPCs participate in tumour progression of hepatocellular carcinoma (HCC).

Methods

A total of 45 patients with primary HCC who underwent liver resection were included in this study. The liver tumours were removed from the patients, and partial tissues were prepared to identify SEPCs through double staining of CD133/CD45 and CD133/CD31 at the same location. Blood samples were collected to examine liver function parameters and tumour markers. The demographics and clinicopathological characteristics of the patients were collected for correlation analysis with SEPCs.

Results

SEPCs were observed in several blood vessels within the HCC nodules of all 45 patients, but no SEPCs were detected in the tumour-adjacent tissues. The number of SEPCs was correlated with the expression levels of HCC tumour markers α-fetoprotein (AFP) and CA199. There was a positive correlation between the expression of SEPC markers and diameter of HCC tumours in differently differentiated specimens (P < 0.01). The expression levels of SEPC markers were significantly higher in patients with poorly differentiated HCC than in patients with moderately and highly differentiated HCC (P < 0.05).

Conclusions

SEPCs are closely associated with HCC progression; therefore, SEPCs may be considered potential prognostic and metastatic biomarkers and therapeutic candidates for HCC.

Transient Support from Fibroblasts is Sufficient to Drive Functional Vascularization in Engineered Tissues

Formation of capillary blood vasculature is a critical requirement for native as well as engineered organs and can be induced in vitro by co-culturing endothelial cells with fibroblasts. However, whether these fibroblasts are required only in the initial morphogenesis of endothelial cells or needed throughout is unknown, and the ability to remove these stromal cells after assembly could be useful for clinical translation. In this study, we introduce a technique termed CAMEO (Controlled Apoptosis in Multicellular Tissues for Engineered Organogenesis), whereby fibroblasts are selectively ablated on demand, and utilize it to probe the dispensability of fibroblasts in vascular morphogenesis. The presence of fibroblasts is shown to be necessary only during the first few days of endothelial cell morphogenesis, after which they can be ablated without significantly affecting the structural and functional features of the developed vasculature. Furthermore, we demonstrate the use of CAMEO to vascularize a construct containing primary human hepatocytes that improved tissue function. In conclusion, this study suggests that transient, initial support from fibroblasts is sufficient to drive vascular morphogenesis in engineered tissues, and this strategy of engineering-via-elimination may provide a new general approach for achieving desired functions and cell compositions in engineered organs.

Hepatic sinusoids versus central veins: Structures, markers, angiocrines, and roles in liver regeneration and homeostasis

The blood circulates through the hepatic sinusoids delivering nutrients and oxygen to the liver parenchyma and drains into the hepatic central vein, yet the structures and phenotypes of these vessels are distinctively different. Sinusoidal endothelial cells are uniquely fenestrated, lack basal lamina and possess organelles involved in endocytosis, pinocytosis, degradation, synthesis and secretion. Hepatic central veins are nonfenestrated but are also active in synthesis and secretion. Endothelial cells of sinusoids and central veins secrete angiocrines that play respective roles in hepatic regeneration and metabolic homeostasis. The list of markers for identifying sinusoidal endothelial cells is long and their terminologies are complex. Further, their uses vary in different investigations and, in some instances, could be confusing. Central vein markers are fewer but more distinctive. Here we analyze and categorize the molecular pathways/modules associated with the sinusoid-mediated liver regeneration in response to partial hepatectomy and chemical-induced acute or chronic injury. Similarly, we highlight the findings that central vein-derived angiocrines interact with Wnt/β-catenin in perivenous hepatocytes to direct gene expression and maintain pericentral metabolic zonation. The proposal that perivenous hepatocytes behave as stem/progenitor cells to provoke hepatic homeostatic cell renewal is reevaluated and newer concepts of broad zonal distribution of hepatocyte proliferation in liver homeostasis and regeneration are updated. Thus, this review integrates the structures, biology and physiology of liver sinusoids and central veins in mediating hepatic regeneration and metabolic homeostasis.

The effect of aging on VEGF/VEGFR2 signal pathway genes expression in rat liver sinusoidal endothelial cell

Liver sinusoidal endothelial cells (LSECs) play a key role in the initiation and neoangiogenesis of liver regeneration. We presume that the abnormity of the VEGF/VEGFR2 and its pathway gene Id1, Wnt2 and HGF expression in aged LSECs may be an important mechanism to affect liver regeneration of the elderly. LSECs from two different groups (adult and old) were isolated in a rodent model, and observed by SEM and TEM. The adult and old rats were underwent 70% partial hepatectomy. The proliferation of hepatocytes and LSECs were analyzed by Immunofluorescence staining. The expression of VEGF/VEGFR2 and its pathway gene in isolated LSECs and liver tissue after hepatectomy were detected by qRT-PCR and Western blot. There is a decreased number of endothelial fenestrae in the LSECs of the old group, compared to the adult group. The old group had a lower expression of VEGF/VEGFR2 and its pathway gene than the adult groups (p < 0.01). The results of western blot were consistent with those of qRT-PCR. The hepatocytes had a high proliferation rate at first 4 days after hepatectomy, and a significantly higher proliferation rate in the adult group. The LSECs began to proliferate after 4 days of hepatectomy, and showed a quantity advantage in the adult group. The adult group had a significantly higher expression of VEGF/VEGFR2 and its pathway gene after hepatectomy than the old group (p < 0.01). LSCEs turn to be defenestration in structure and have a low expression of VEGF/VEGFR2 and its pathway gene with aging.

The Role of Sinusoidal Endothelial Cells in the Axis of Inflammation and Cancer Within the Liver

Liver sinusoidal endothelial cells (LSEC) form a unique barrier between the liver sinusoids and the underlying parenchyma, and thus play a crucial role in maintaining metabolic and immune homeostasis, as well as actively contributing to disease pathophysiology. Whilst their endocytic and scavenging function is integral for nutrient exchange and clearance of waste products, their capillarisation and dysfunction precedes fibrogenesis. Furthermore, their ability to promote immune tolerance and recruit distinct immunosuppressive leukocyte subsets can allow persistence of chronic viral infections and facilitate tumour development. In this review, we present the immunological and barrier functions of LSEC, along with their role in orchestrating fibrotic processes which precede tumourigenesis. We also summarise the role of LSEC in modulating the tumour microenvironment, and promoting development of a pre-metastatic niche, which can drive formation of secondary liver tumours. Finally, we summarise closely inter-linked disease pathways which collectively perpetuate pathogenesis, highlighting LSEC as novel targets for therapeutic intervention.

The Role of Lysophosphatidic Acid in Adult Stem Cells

Stem cells are undifferentiated multipotent precursor cells that are capable both of perpetuating themselves as stem cells (self-renewal) and of undergoing differentiation into one or more specialized types of cells. And these stem cells have been reported to reside within distinct anatomic locations termed “niches”. The long-term goals of stem cell biology range from an understanding of cell-lineage determination and tissue organization to cellular therapeutics for degenerative diseases. Stem cells maintain tissue function throughout an organism’s lifespan by replacing differentiated cells. To perform this function, stem cells provide a unique combination of multilineage developmental potential and the capacity to undergo self-renewing divisions. The loss of self-renewal capacity in stem cells underlies certain degenerative diseases and the aging process. This self-renewal regulation must balance the regenerative needs of tissues that persist throughout life. Recent evidence suggests lysophosphatidic acid (LPA) signaling pathway plays an important role in the regulation of a variety of stem cells. In this review, we summarize the evidence linking between LPA and stem cell regulation. The LPA-induced signaling pathway regulates the proliferation and survival of stem cells and progenitors, and thus are likely to play a role in the maintenance of stem cell population in the body. This lipid mediator regulatory system can be a novel potential therapeutics for stem cell maintenance.

Liver structural transformation after partial hepatectomy and repeated partial hepatectomy in rats: A renewed view on liver regeneration

BACKGROUND

The phenomenon of liver regeneration after partial hepatectomy (PH) is still a subject of considerable interest due to the increasing frequency of half liver transplantation on the one hand, and on the other hand, new surgical approaches which allow removal of massive space-occupying hepatic tumors, which earlier was considered as inoperable. Interestingly, the mechanisms of liver regeneration are extensively studied after PH but less attention is paid to the architectonics of the regenerated organ. Because of this, the question “How does the structure of regenerated liver differ from normal, regular liver?” has not been fully answered yet. Furthermore, almost without any attention is left the liver's structural transformation after repeated hepatectomy (of the re-regenereted liver).

AIM

To compare the architectonics of the lobules and circulatory bed of normal, re-generated and re-regenerated livers.

METHODS

The livers of 40 adult, male, albino Wistar rats were studied. 14 rats were subjected to PH - the 1^st study group (SG1); 10 rats underwent repeated PH – the 2^nd study group (SG2); 16 rats were subjected to sham operation - control group (CG); The livers were studied after 9 months from PH, and after 6 months from repeated PH. Cytological (Schiff reaction for the determination of DNA concen-tration), histological (H&E, Masson trichrome, CK8 Immunohistochemical marker, transparent slides after Indian Ink injection, ), morphometrical (hepatocytes areas, perimeters and ploidy) and Electron Microscopical (Scanning Electron Microscopy of corrosion casts) methods were used.

RESULTS

In the SG1 and SG2, the area of hepatocytes and their perimeter are increased compared to the CG (P < 0.05). However, the areas and perimeters of the hepatocytes of the SG₁ and SG₂ groups reveal a lesser difference. In regenerated (SG₁) and re-regenerated (SG₂) livers, the hepatocytes form the remodeled lobules, which size (300-1200 µm) exceeds the sizes of the lobules from CG (300-600 µm). The remodeled lobules (especially the “mega-lobules” with the sizes 1000-1200 µm) contain the transformed meshworks of the sinusoids, the part of which is dilated asymmetrically. This meshwork might have originated from the several portal venules (interlobular and/or inlet). The boundaries between the adjacent lobules (including mega-lobules) are widened and filled by connective tissue fibers, which gives the liver parenchyma a nodular look. In SG₂ the unevenness of sinusoid diameters, as well as the boundaries between the lobules (including the mega-lobules) are more vividly expressed in comparison with SG₁. The liver tissue of both SG₁ and SG₂ is featured by the slightly expressed ductular reaction.

CONCLUSION

Regenerated and re-regenerated livers in comparison with normal liver contain hypertrophied hepatocytes with increased ploidy which together with transformed sinusoidal and biliary meshworks form the remodeled lobulli.

Generation of Functional Liver Sinusoidal Endothelial Cells from Human Pluripotent Stem-Cell-Derived Venous Angioblasts

Liver sinusoidal endothelial cells (LSECs) form a highly specialized microvasculature that plays a critical role in liver function and disease. To better understand this role, we developed a strategy to generate LSECs from human pluripotent stem cells (hPSCs) by first optimizing the specification of arterial and venous angioblasts and derivative endothelial populations. Induction of a LSEC-like fate by hypoxia, cyclic AMP (cAMP) agonism, and transforming growth factor β (TGF-β) inhibition revealed that venous endothelial cells responded more rapidly and robustly than the arterial cells to upregulate LSEC markers and functions in vitro. Upon intrahepatic transplantation in neonates, venous angioblasts engrafted the liver and generated mature, fenestrated LSECs with scavenger functions and molecular profiles of primary human LSECs. When transplanted into the liver of adult mice, angioblasts efficiently gave rise to mature LSECs with robust factor VIII (FVIII) production. Humanization of the murine liver with hPSC-derived LSECs provides a tractable system for studying the biology of this key liver cell type.

Interleukin 22 in Liver Injury, Inflammation and Cancer

Interleukin 22(IL-22), a member of the IL-10 cytokine family and is an emerging CD4+Th cytokine that plays an important role in anti-microbial defense, homeostasis and tissue repair. We are interested in IL-22 as it has the double function of suppressing or encouraging inflammation in various disease models including hepatic inflammation. As a survival factor for hepatocytes, IL-22 plays a protective role in many kinds of liver diseases, such as hepatitis, liver fibrosis, or hepatocellular carcinoma (HCC) by binding to the receptors IL-22R1 and IL-10R2. Overexpression of IL-22 reduces liver fibrosis by attenuating the activation of hepatic stellate cell (the main cell types involved in hepatic fibrosis), and down-regulating the levels of inflammatory cytokines. Administration of exogenous IL-22 increases the replication of hepatocytes by inhibiting cell apoptosis and promoting mitosis, ultimately plays a contributing role in liver regeneration. Furthermore, treatment with IL-22 activates hepatic signal transducer and activator of transcription 3 (STAT3), ameliorates hepatic oxidative stress and alcoholic fatty liver, effectively alleviate the liver damage caused by alcohol and toxicant. In conclusion, the hepatoprotective functions and liver regeneration promoting effect of IL-22 suggests the therapeutic potential of IL-22 in the treatment of human hepatic diseases.

TLR3 promotes hepatocyte proliferation after partial hepatectomy by stimulating uPA expression and the release of tissue-bound HGF

TLR3 is implicated in anti-viral immune responses, but may also act as a sensor of tissue damage in the absence of infection. Here, we provide evidence for an essential role of TLR3 in liver regeneration after an acute loss of tissue due to partial hepatectomy. Mice lacking TLR3 had a severe and sustained defect in the restoration of liver tissue with reduced liver-to-body weight ratios even after an extended recovery period of 2 weeks. Hepatocyte cell cycle progression into S phase was impaired in TLR3-deficient mice. Mechanistic analyses revealed that TLR3-deficient mice had markedly reduced systemic levels of active HGF, but had increased amounts of inactive tissue-bound HGF. Importantly, expression of uPA, which orchestrates the processing and release of HGF from the hepatic extracellular matrix, was reduced in regenerating livers of TLR3-deficient mice. In addition, expression of the HGF maturation factor HGFAC was transiently diminished in TLR3-deficient mice. In vitro, engagement of TLR3 directly stimulated expression of uPA by hepatic stellate cells. Thus, TLR3 supports liver regeneration through upregulation of uPA, which promotes the release of preformed HGF from extracellular matrix stores.