Pleiotrophin/heparin-binding growth-associated molecule as a mitogen of rat hepatocytes and its role in regeneration and development of liver

Pleiotrophin and metabolic disorders: insights into its role in metabolism

Pleiotrophin (PTN) is a cytokine which has been for long studied at the level of the central nervous system, however few studies focus on its role in the peripheral organs. The main aim of this review is to summarize the state of the art of what is known up to date about pleiotrophin and its implications in the main metabolic organs. In summary, pleiotrophin promotes the proliferation of preadipocytes, pancreatic β cells, as well as cells during the mammary gland development. Moreover, this cytokine is important for the structural integrity of the liver and the neuromuscular junction in the skeletal muscle. From a metabolic point of view, pleiotrophin plays a key role in the maintenance of glucose and lipid as well as whole-body insulin homeostasis and favors oxidative metabolism in the skeletal muscle. All in all, this review proposes pleiotrophin as a druggable target to prevent from the development of insulin-resistance-related pathologies.

Hepatic stellate cells maintain liver homeostasis through paracrine neurotrophin-3 signaling that induces hepatocyte proliferation

Organ size is maintained by the controlled proliferation of distinct cell populations. In the mouse liver, hepatocytes in the midlobular zone that are positive for cyclin D1 (CCND1) repopulate the parenchyma at a constant rate to preserve liver mass. Here, we investigated how hepatocyte proliferation is supported by hepatic stellate cells (HSCs), pericytes that are in close proximity to hepatocytes. We used T cells to ablate nearly all HSCs in the murine liver, enabling the unbiased characterization of HSC functions. In the normal liver, complete loss of HSCs persisted for up to 10 weeks and caused a gradual reduction in liver mass and in the number of CCND1+ hepatocytes. We identified neurotrophin-3 (Ntf-3) as an HSC-produced factor that induced the proliferation of midlobular hepatocytes through the activation of tropomyosin receptor kinase B (TrkB). Treating HSC-depleted mice with Ntf-3 restored CCND1+ hepatocytes in the midlobular region and increased liver mass. These findings establish that HSCs form the mitogenic niche for midlobular hepatocytes and identify Ntf-3 as a hepatocyte growth factor.

Deletion of pleiotrophin impairs glucose tolerance and liver metabolism in pregnant mice: Moonlighting role of glycerol kinase

Pleiotrophin is a pleiotropic cytokine that has been demonstrated to have a critical role in regulating energy metabolism, lipid turnover and plasticity of adipose tissue. Here, we hypothesize that this cytokine can be involved in regulatory processes of glucose and lipid homeostasis in the liver during pregnancy. Using 18-days pregnant Ptn-deficient mice, we evaluated the biochemical profile (circulating variables), tissue mRNA expression (qPCR) and protein levels of key enzymes and transcription factors involved in main metabolic pathways. Ptn deletion was associated with a reduction in body weight gain, hyperglycemia and glucose intolerance. Moreover, we observed an impairment in glucose synthesis and degradation during late pregnancy in Ptn^-/- mice. Hepatic lipid content was significantly lower (73.6%) in Ptn^-/- mice and was associated with a clear reduction in fatty acid, triacylglycerides and cholesterol synthesis. Ptn deletion was accompanying with a diabetogenic state in the mother and a decreased expression of key proteins involved in glucose and lipid uptake and metabolism. Moreover, Ptn^-/- pregnant mice have a decreased expression of transcription factors, such as PPAR-α, regulating lipid uptake and glucose and lipid utilization. Furthermore, the augmented expression and nuclear translocation of glycerol kinase, and the decrease in NUR77 protein levels in the knock-out animals can further explain the alterations observed in hepatic glucose metabolism. Our results point out for the first time that pleiotrophin is an important player in maintaining hepatic metabolic homeostasis during late gestation, and further highlighted the moonlighting role of glycerol kinase in the regulation of maternal glucose homeostasis during pregnancy.

Pleiotrophin Deficiency Induces Browning of Periovarian Adipose Tissue and Protects against High-Fat Diet-Induced Hepatic Steatosis

(1) Background: Pleiotrophin preserves insulin sensitivity, regulates adipose tissue lipid turnover and plasticity, energy metabolism and thermogenesis. The aim of this study was to determine the role of pleiotrophin in hepatic lipid metabolism and in the metabolic crosstalk between the liver and brown and white adipose tissue (AT) in a high-fat diet-induced (HFD) obesity mice model. (2) Methods: We analyzed circulating variables, lipid metabolism (hepatic lipid content and mRNA expression), brown AT thermogenesis (UCP-1 expression) and periovarian AT browning (brown adipocyte markers mRNA and immunodetection) in Ptn-/- mice either fed with standard-chow diet or with HFD and in their corresponding Ptn+/+ counterparts. (3) Results: HFD-Ptn-/- mice are protected against the development of HFD-induced insulin resistance, had lower liver lipid content and lower expression of the key enzymes involved in triacylglycerides and fatty acid synthesis in liver. HFD-Ptn-/- mice showed higher UCP-1 expression in brown AT. Moreover, Ptn deletion increased the expression of specific markers of brown/beige adipocytes and was associated with the immunodetection of UCP-1 enriched multilocular adipocytes in periovarian AT. (4) Conclusions: Ptn deletion protects against the development of HFD-induced insulin resistance and liver steatosis, by increasing UCP-1 expression in brown AT and promoting periovarian AT browning.

Hepatic stellate cells - from past till present: morphology, human markers, human cell lines, behavior in normal and liver pathology

Hepatic stellate cell (HSC), initially analyzed by von Kupffer, in 1876, revealed to be an extraordinary mesenchymal cell, essential for both hepatocellular function and lesions, being the hallmark of hepatic fibrogenesis and carcinogenesis. Apart from their implications in hepatic injury, HSCs play a vital role in liver development and regeneration, xenobiotic response, intermediate metabolism, and regulation of immune response. In this review, we discuss the current state of knowledge regarding HSCs morphology, human HSCs markers and human HSC cell lines. We also summarize the latest findings concerning their roles in normal and liver pathology, focusing on their impact in fibrogenesis, chronic viral hepatitis and liver tumors.

Single-cell RNA sequencing of human liver reveals hepatic stellate cell heterogeneity

Background & Aims

The multiple vital functions of the human liver are performed by highly specialised parenchymal and non-parenchymal cells organised in complex collaborative sinusoidal units. Although crucial for homeostasis, the cellular make-up of the human liver remains to be fully elucidated. Here, single-cell RNA-sequencing was used to unravel the heterogeneity of human liver cells, in particular of hepatocytes (HEPs) and hepatic stellate cells (HSCs).

Method

The transcriptome of ~25,000 freshly isolated human liver cells was profiled using droplet-based RNA-sequencing. Recently published data sets and RNA in situ hybridisation were integrated to validate and locate newly identified cell populations.

Results

In total, 22 cell populations were annotated that reflected the heterogeneity of human parenchymal and non-parenchymal liver cells. More than 20,000 HEPs were ordered along the portocentral axis to confirm known, and reveal previously undescribed, zonated liver functions. The existence of 2 subpopulations of human HSCs with unique gene expression signatures and distinct intralobular localisation was revealed (i.e. portal and central vein-concentrated GPC3⁺ HSCs and perisinusoidally located DBH⁺ HSCs). In particular, these data suggest that, although both subpopulations collaborate in the production and organisation of extracellular matrix, GPC3⁺ HSCs specifically express genes involved in the metabolism of glycosaminoglycans, whereas DBH⁺ HSCs display a gene signature that is reminiscent of antigen-presenting cells.

Conclusions

This study highlights metabolic zonation as a key determinant of HEP transcriptomic heterogeneity and, for the first time, outlines the existence of heterogeneous HSC subpopulations in the human liver. These findings call for further research on the functional implications of liver cell heterogeneity in health and disease.

Lay summary

This study resolves the cellular landscape of the human liver in an unbiased manner and at high resolution to provide new insights into human liver cell biology. The results highlight the physiological heterogeneity of human hepatic stellate cells.

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A cell atlas from the near-native transcriptome of >25,000 human liver cells is presented.

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Hepatocytes were ordered along the portocentral axis to reveal previously undescribed gene expression patterns and zonated liver functions.

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Two subpopulations of human hepatic stellate cells (HSCs) are reported, characterised by different spatial distribution in the native tissue.

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Characteristic gene signatures of HSC subpopulations are suggestive of far-reaching functional differences.

Myofibroblasts control the proliferation of fetal hepatoblasts and their differentiated cholangiocytes during the hepatoblast-to-cholangiocyte transition

Mesenchymal cells in the liver provide the microenvironment for hepatoblasts expansion and differentiation. We have previously demonstrated that myofibroblasts (MFs) promoted hepatoblasts differentiation into cholangiocytes, whereas its role in controlling the proliferation of hepatoblasts and their differentiated cholangiocytes remains elusive. Here, we investigated the role of MFs in regulating the proliferation of hepatoblasts and their differentiated cholangiocytes using an indirect coculture system. When cocultured with hepatoblasts, MFs promoted hepatoblasts differentiation into cholangiocytes and inhibited the proliferation and stemness of hepatoblasts. However, when hepatoblasts already differentiated into cholangiocytes, MFs promoted the differentiated cholangiocytes proliferation. In addition, hepatoblast proliferation genes such as hepatocyte growth factor (HGF), insulin-like growth factor-1 and 2 (IGF-1 and 2), midkine 1 (Mdk1), and pleiotrophin (Ptn) expression in MFs were down-regulated compared with their levels in fibroblasts. Our findings uncover the role of MFs in controlling the proliferation of hepatoblasts and their differentiated cholangiocytes, potentially providing a novel therapeutic strategy for cholangiocyte regeneration.

Metabolic-vascular coupling in skeletal muscle: A potential role for capillary pericytes?

The matching of capillary blood flow to metabolic rate of the cells within organs and tissues is a critical microvascular function which ensures appropriate delivery of hormones and nutrients, and the removal of waste products. This relationship is particularly important in tissues where local metabolism, and hence capillary blood flow, must be regulated to avoid a mismatch between nutrient demand and supply that would compromise normal function. The consequences of a mismatch in microvascular blood flow and metabolism are acutely apparent in the brain and heart, where a sudden cessation of blood flow, for example following an embolism, acutely manifests as stroke or myocardial infarction. Even in more resilient tissues such as skeletal muscle, a short-term mismatch reduces muscle performance and exercise tolerance, and can cause intermittent claudication. In the longer-term, a microvascular-metabolic mismatch in skeletal muscle reduces insulin-mediated muscle glucose uptake, leading to disturbances in whole-body metabolic homeostasis. While the notion that capillary blood flow is fine-tuned to meet cellular metabolism is well accepted, the mechanisms that control this function and where and how different parts of the vascular tree contribute to capillary blood flow regulation remain poorly understood. Here, we discuss the emerging evidence implicating pericytes, mural cells that surround capillaries, as key mediators that match tissue metabolic demand with adequate capillary blood flow in a number of organs, including skeletal muscle.

Pericytes in the Liver

Liver pericytes, commonly named hepatic stellate cells (HSCs), reside in the space between liver sinusoidal endothelial cells (LSECs) and hepatocytes. They display important roles in health and disease. HSCs ensure the storage of the majority of vitamin A in a healthy body, and they represent the major source of fibrotic tissue in liver disease. Surrounding cells, such as LSECs, hepatocytes, and Kupffer cells, present a significant role in modulating HSC behavior. Therapeutic strategies against liver disease are being currently developed, where HSCs represent an ideal target. In this chapter, we will discuss HSC quiescence and activation in the context of healthy liver and diseases, such as fibrosis, steatohepatitis, and hepatocellular carcinoma.

Expression of the heparin-binding growth factors Midkine and pleiotrophin during ocular development

Midkine (MDK) and Pleiotrophin (PTN) belong to a group of heparin-binding growth factors that has been shown to have pleiotropic functions in various biological processes during development and disease. Development of the vertebrate eye is a multistep process that involves coordinated interactions between neuronal and non-neuronal cells, but very little is known about the potential function of MDK and PTN in these processes. In this study, we demonstrate by section in situ hybridization, the spatiotemporal expression of MDK and PTN during ocular development in chick and mouse. We show that MDK and PTN are expressed in dynamic patterns that overlap in a few non-neuronal tissues in the anterior eye and in neuronal cell layers of the posterior eye. We show that the expression patterns of MDK and PTN are only conserved in a few tissues in chick and mouse but they overlap with the expression of some of their receptors LRP1, RPTPZ, ALK, NOTCH2, ITGβ1, SDC1, and SDC3. The dynamic expression patterns of MDK, PTN and their receptors suggest that they function together during the multistep process of ocular development and they may play important roles in cell proliferation, adhesion, and migration of neuronal and non-neuronal cells.

Role of hepatic stellate cell (HSC)-derived cytokines in hepatic inflammation and immunity

In their quiescent state, Hepatic stellate cells (HSCs), are present in the sub-endothelial space of Disse and have minimal interaction with immune cells. However, upon activation following injury, HSCs directly or indirectly interact with various immune cells that enter the space of Disse and thereby regulate diverse hepatic function and immune physiology. Other than the normal physiological functions of HSCs such as hepatic homeostasis, maturation and differentiation, they also participate in hepatic inflammation by releasing a battery of inflammatory cytokines and chemokines and interacting with other liver cells. Here, we have reviewed the role of HSC in the pathogenesis of liver inflammation and some infectious diseases in order to understand how the interplay between immune cells and HSCs regulates the overall outcome and disease pathology.

Isolation of a unique hepatic stellate cell population expressing integrin α8 from embryonic mouse livers

BACKGROUND

Hepatic stellate cells (HSCs) play an important role in liver fibrogenesis. However, little is known about their phenotype and role in liver development. The aim of this study is to identify specific markers for embryonic HSCs.

RESULTS

Using antibodies against ALCAM and PDPN, we separated mesothelial cells (MCs) and HSCs from developing livers and identified integrin α8 (ITGA8) as a marker for embryonic desmin+ HSCs that are preferentially localized near the developing liver surface and α-smooth muscle actin+ perivascular mesenchymal cells around the vein. A cell lineage-tracing study revealed that upon differentiation, MC-derived HSCs or perivascular mesenchymal cells express ITGA8 during liver development. Using anti-ITGA8 antibodies, we succeeded in isolating MC-derived HSCs and perivascular mesenchymal cells from embryonic livers. In direct co-culture, ITGA8+ mesenchymal cells promoted the expression of hepatocyte and cholangiocyte markers in hepatoblasts. In the normal adult liver, expression of ITGA8 was restricted to portal fibroblasts in the portal triad. Upon liver injury, myofibroblasts increased the expression of ITGA8.

CONCLUSIONS

ITGA8 is a specific cell surface marker of MC-derived HSCs and perivascular mesenchymal cells in the developing liver. Our data suggest that ITGA8+ mesenchymal cells maintain the phenotype of hepatoblast in liver development. Developmental Dynamics 247:867-881, 2018. © 2018 Wiley Periodicals, Inc.

Pleiotrophin, a target of miR-384, promotes proliferation, metastasis and lipogenesis in HBV-related hepatocellular carcinoma

Hepatitis B virus (HBV) infection plays a crucial role and is a major cause of hepatocellular carcinoma (HCC) in China. microRNAs (miRNAs) have emerged as key players in hepatic steatosis and carcinogenesis. We found that down-regulation of miR-384 expression was a common event in HCC, especially HBV-related HCC. However, the possible function of miR-384 in HBV-related HCC remains unclear. The oncogene pleiotrophin (PTN) was a target of miR-384. HBx inhibited miR-384, increasing PTN expression. The PTN receptor N-syndecan was highly expressed in HCC. PTN induced by HBx acted as a growth factor via N-syndecan on hepatocytes and further promoted cell proliferation, metastasis and lipogenesis. PTN up-regulated sterol regulatory element-binding protein 1c (SREBP-1c) through the N-syndecan/PI3K/Akt/mTORC1 pathway and the expression of lipogenic genes, including fatty acid synthesis (FAS). PTN-mediated de novo lipid synthesis played an important role in HCC proliferation and metastasis. PI3K/AKT and an mTORC1 inhibitor diminished PTN-induced proliferation, metastasis and lipogenesis. Taken together, these data strongly suggest that the dysregulation of miR-384 could play a crucial role in HBV related to HCC, and the target gene of miR-384, PTN, represents a new potential therapeutic target for the prevention of hepatic steatosis and further progression to HCC after chronic HBV infection.

Hepatic Stellate Cells in Liver Fibrosis and siRNA-Based Therapy

Hepatic fibrosis is a reversible wound-healing response to either acute or chronic liver injury caused by hepatitis B or C, alcohol, and toxic agents. Hepatic fibrosis is characterized by excessive accumulation and reduced degradation of extracellular matrix (ECM). Excessive accumulation of ECM alters the hepatic architecture leading to liver fibrosis and cirrhosis. Cirrhosis results in failure of common functions of the liver. Hepatic stellate cells (HSC) play a major role in the development of liver fibrosis as HSC are the main source of the excessive production of ECM in an injured liver. RNA interference (RNAi) is a recently discovered therapeutic tool that may provide a solution to manage multiple diseases including liver fibrosis through silencing of specific gene expression in diseased cells. However, gene silencing using small interfering RNA (siRNA) is encountering many challenges in the body after systemic administration. Efficient and stable siRNA delivery to the target cells is a key issue for the development of siRNA therapeutic. For that reason, various viral and non-viral carriers for liver-targeted siRNA delivery have been developed. This review will cover the current strategies for the treatment of liver fibrosis as well as discussing non-viral approaches such as cationic polymers and lipid-based nanoparticles for targeted delivery of siRNA to the liver.

The Role of Mesothelial Cells in Liver Development, Injury, and Regeneration

Mesothelial cells (MCs) cover the surface of visceral organs and the parietal walls of cavities, and they synthesize lubricating fluids to create a slippery surface that facilitates movement between organs without friction. Recent studies have indicated that MCs play active roles in liver development, fibrosis, and regeneration. During liver development, the mesoderm produces MCs that form a single epithelial layer of the mesothelium. MCs exhibit an intermediate phenotype between epithelial cells and mesenchymal cells. Lineage tracing studies have indicated that during liver development, MCs act as mesenchymal progenitor cells that produce hepatic stellate cells, fibroblasts around blood vessels, and smooth muscle cells. Upon liver injury, MCs migrate inward from the liver surface and produce hepatic stellate cells or myofibroblast depending on the etiology, suggesting that MCs are the source of myofibroblasts in capsular fibrosis. Similar to the activation of hepatic stellate cells, transforming growth factor β induces the conversion of MCs into myofibroblasts. Further elucidation of the biological and molecular changes involved in MC activation and fibrogenesis will contribute to the development of novel approaches for the prevention and therapy of liver fibrosis.

Pleiotrophin regulates the ductular reaction by controlling the migration of cells in liver progenitor niches

OBJECTIVE

The ductular reaction (DR) involves mobilisation of reactive-appearing duct-like cells (RDC) along canals of Hering, and myofibroblastic (MF) differentiation of hepatic stellate cells (HSC) in the space of Disse. Perivascular cells in stem cell niches produce pleiotrophin (PTN) to inactivate the PTN receptor, protein tyrosine phosphatase receptor zeta-1 (PTPRZ1), thereby augmenting phosphoprotein-dependent signalling. We hypothesised that the DR is regulated by PTN/PTPRZ1 signalling.

DESIGN

PTN-GFP, PTN-knockout (KO), PTPRZ1-KO, and wild type (WT) mice were examined before and after bile duct ligation (BDL) for PTN, PTPRZ1 and the DR. RDC and HSC from WT, PTN-KO, and PTPRZ1-KO mice were also treated with PTN to determine effects on downstream signaling phosphoproteins, gene expression, growth, and migration. Liver biopsies from patients with DRs were also interrogated.

RESULTS

Although quiescent HSC and RDC lines expressed PTN and PTPRZ1 mRNAs, neither PTN nor PTPRZ1 protein was demonstrated in healthy liver. BDL induced PTN in MF-HSC and increased PTPRZ1 in MF-HSC and RDC. In WT mice, BDL triggered a DR characterised by periportal accumulation of collagen, RDC and MF-HSC. All aspects of this DR were increased in PTN-KO mice and suppressed in PTPRZ1-KO mice. In vitro studies revealed PTN-dependent accumulation of phosphoproteins that control cell-cell adhesion and migration, with resultant inhibition of cell migration. PTPRZ1-positive cells were prominent in the DRs of patients with ductal plate defects and adult cholestatic diseases.

CONCLUSIONS

PTN, and its receptor, PTPRZ1, regulate the DR to liver injury by controlling the migration of resident cells in adult liver progenitor niches.

Liver maturation deficiency in p57(Kip2)-/- mice occurs in a hepatocytic p57(Kip2) expression-independent manner

Fetal hepatic stem/progenitor cells, hepatoblasts, are highly proliferative cells and the source of both hepatocytes and cholangiocytes. In contrast, mature hepatocytes have a low proliferative potency and high metabolic functions. Cell proliferation is regulated by cell cycle-related molecules. However, the correlation between cell cycle regulation and hepatic maturation are still unknown. To address this issue, we revealed that the cell cycle inhibitor p57(Kip2) was expressed in the hepatoblasts and mesenchymal cells of fetal liver in a spatiotemporal manner. In addition, we found that hepatoblasts in p57(Kip2)-/- mice were highly proliferative and had deficient maturation compared with those in wild-type (WT) mice. However, there were no remarkable differences in the expression levels of cell cycle- and bipotency-related genes except for Ccnd2. Furthermore, p57(Kip2)-/- hepatoblasts could differentiate into mature hepatocytes in p57(Kip2)-/- and WT chimeric mice, suggesting that the intrinsic activity of p57(Kip2) does not simply regulate hepatoblast maturation.

Use of mouse liver mesothelial cells to prevent postoperative adhesion and promote liver regeneration after hepatectomy

BACKGROUND & AIMS

Repeated hepatectomy is widely accepted as one of the most effective curative treatment for recurrent hepatocellular carcinoma or liver metastasis from colorectal cancer. It has, however, two critical issues; postoperative adhesion and decrease of liver regenerative capacity. Postoperative adhesion makes surgical operations technically more demanding, leading to increased mortality and morbidity rates. Although the liver has a remarkable regenerative ability, volume and functional restoration after multiple repeated hepatectomy is not generally complete. So a new procedure that overcomes these two issues is required. We examined if a fetal liver mesothelial cells (FL-MCs) sheet could solve these two clinical issues simultaneously.

METHODS

We established a novel mouse hepatectomy model that reproduces postoperative adhesion on the resected liver surface. We isolated FL-MCs from mouse fetal liver and prepared a cell sheet. The FL-MCs sheet was then transplanted to the resected liver surface.

RESULTS

The FL-MCs sheet effectively prevented postoperative adhesion by expressing PCLP1, one of the transmembrane sialomucin family proteins and by activating the fibrinolytic system. Furthermore, the FL-MCs sheet facilitated liver regeneration by providing growth factors for hepatocytes, allowing quick recovery of liver weight and function. Additionally, we showed that an allogeneic FL-MCs sheet was as effective as a syngeneic cell sheet.

CONCLUSIONS

We demonstrate that the FL-MCs sheet is able to not only prevent postoperative adhesion but also promote liver regeneration in both syngeneic and allogeneic transplantation, and hence FL-MCs may serve as a potentially useful cell source for regenerative medicine after hepatectomy.

Hepatic stellate cells and liver fibrosis

Hepatic stellate cells are resident perisinusoidal cells distributed throughout the liver, with a remarkable range of functions in normal and injured liver. Derived embryologically from septum transversum mesenchyme, their precursors include submesothelial cells that invade the liver parenchyma from the hepatic capsule. In normal adult liver, their most characteristic feature is the presence of cytoplasmic perinuclear droplets that are laden with retinyl (vitamin A) esters. Normal stellate cells display several patterns of intermediate filaments expression (e.g., desmin, vimentin, and/or glial fibrillary acidic protein) suggesting that there are subpopulations within this parental cell type. In the normal liver, stellate cells participate in retinoid storage, vasoregulation through endothelial cell interactions, extracellular matrix homeostasis, drug detoxification, immunotolerance, and possibly the preservation of hepatocyte mass through secretion of mitogens including hepatocyte growth factor. During liver injury, stellate cells activate into alpha smooth muscle actin-expressing contractile myofibroblasts, which contribute to vascular distortion and increased vascular resistance, thereby promoting portal hypertension. Other features of stellate cell activation include mitogen-mediated proliferation, increased fibrogenesis driven by connective tissue growth factor, and transforming growth factor beta 1, amplified inflammation and immunoregulation, and altered matrix degradation. Evolving areas of interest in stellate cell biology seek to understand mechanisms of their clearance during fibrosis resolution by either apoptosis, senescence, or reversion, and their contribution to hepatic stem cell amplification, regeneration, and hepatocellular cancer.

Mesenchymal progenitor cells in mouse foetal liver regulate differentiation and proliferation of hepatoblasts

BACKGROUND & AIMS

Hepatoblasts are somatic progenitor cells of the foetal liver that possess high proliferative capacity and bi-potency for differentiation into both hepatocytes and cholangiocytes. Although mesenchymal cells are known to be important for liver ontogeny, current understanding of their interaction with hepatoblasts remains obscure. Mesenchymal cell populations in the developing liver were purified and their potential to support proliferation and differentiation of hepatoblasts was examined.

METHODS

Foetal liver cells were fractionated with a flow cytometer using antibodies against cell surface markers. Gene expression of mesenchymal-specific transcripts and morphological characteristics were analysed. The ability of the mesenchymal cells to support hepatoblast function was analysed using a transwell and direct coculture system.

RESULTS

CD45(-) Ter119(-) CD71(-) Dlk1(mid) PDGFRα(+) cells from the mid-foetal stage liver expressed the mesenchymal cell-specific transcription factors H2.0-like homeobox 1 and LIM homeobox 2 at high levels. Foetal mesenchymal cells make contact with hepatoblasts in vivo and possess the potential to differentiate into chondrocytes, osteocytes and adipocytes under appropriate cell culture conditions, indicating that these cells are possible candidates for mesenchymal stem/progenitor cells. Foetal mesenchymal cells expressed pleiotrophin, hepatocyte growth factor and midkine 1, which are involved in the growth of hepatoblasts. Using the coculture system with hepatoblasts and foetal mesenchymal cells, these cells were shown to support proliferation and maturation of hepatoblasts through indirect and direct interactions respectively.

CONCLUSIONS

Dlk1(mid) PDGFRα(+) cells in non-haematopoetic fraction derived from the foetal liver exhibit mesenchymal stem/progenitor cell characteristics and have abilities to support proliferation and differentiation of hepatoblasts.

The types of hepatic myofibroblasts contributing to liver fibrosis of different etiologies

Liver fibrosis results from dysregulation of normal wound healing, inflammation, activation of myofibroblasts, and deposition of extracellular matrix (ECM). Chronic liver injury causes death of hepatocytes and formation of apoptotic bodies, which in turn, release factors that recruit inflammatory cells (neutrophils, monocytes, macrophages, and lymphocytes) to the injured liver. Hepatic macrophages (Kupffer cells) produce TGFβ1 and other inflammatory cytokines that activate Collagen Type I producing myofibroblasts, which are not present in the normal liver. Secretion of TGFβ1 and activation of myofibroblasts play a critical role in the pathogenesis of liver fibrosis of different etiologies. Although the composition of fibrogenic myofibroblasts varies dependent on etiology of liver injury, liver resident hepatic stellate cells and portal fibroblasts are the major source of myofibroblasts in fibrotic liver in both experimental models of liver fibrosis and in patients with liver disease. Several studies have demonstrated that hepatic fibrosis can reverse upon cessation of liver injury. Regression of liver fibrosis is accompanied by the disappearance of fibrogenic myofibroblasts followed by resorption of the fibrous scar. Myofibroblasts either apoptose or inactivate into a quiescent-like state (e.g., stop collagen production and partially restore expression of lipogenic genes). Resolution of liver fibrosis is associated with recruitment of macrophages that secrete matrix-degrading enzymes (matrix metalloproteinase, collagenases) and are responsible for fibrosis resolution. However, prolonged/repeated liver injury may cause irreversible crosslinking of ECM and formation of uncleavable collagen fibers. Advanced fibrosis progresses to cirrhosis and hepatocellular carcinoma. The current review will summarize the role and contribution of different cell types to populations of fibrogenic myofibroblasts in fibrotic liver.

Elevated expression of pleiotrophin in lymphocytic leukemia CD19+ B cells

Pleiotrophin (PTN) has been demonstrated to be strongly expressed in many fetal tissues, but seldom in healthy adult tissues. While PTN has been reported to be expressed in many types of tumors as well as at high serum concentrations in patients with many types of cancer, to date, there has been no report that PTN is expressed in leukemia, especially in lymphocytic leukemia. We isolated the CD19(+) subset of B cells from peripheral blood from healthy adults, B-cell acute lymphocytic leukemia (B-ALL) patients, and B-cell chronic lymphocytic leukemia (B-CLL) patients and examined these cells for PTN mRNA and protein expression. We used immunocytochemistry, western blotting, and enzyme-linked immunosorbent assay to show that PTN protein is highly expressed in CD19(+) B cells from B-ALL and B-CLL patients, but barely expressed in B cells from healthy adults. We also examined PTN expression at the nucleic acid level using reverse transcription polymerase chain reaction (RT-PCR) and northern blotting and detected a high levels of PTN transcripts in the CD19(+) B cells from both groups of leukemia patients, but very few in the CD19(+) B cells from the healthy controls. Interestingly, the quantity of the PTN transcripts correlated with the severity of disease. Moreover, suppression of PTN activity with an anti-PTN antibody promoted apoptosis of cells from leukemia patients and cell lines SMS-SB and JVM-2. This effect of the anti-PTN antibody suggests that PTN may be a new target for the treatment of lymphocytic leukemia.

Chimeric mice with humanized livers: a unique tool for in vivo and in vitro enzyme induction studies

We performed in vivo and in vitro studies to determine the induction of human cytochrome P450 (CYP) using chimeric mice with humanized liver (PXB-mice^®) and human hepatocytes isolated from the PXB-mice (PXB-cells), which were derived from the same donor. For the in vivo study, PXB-mice were injected with 3-methylcholanthrene (3-MC, 2 or 20 mg/kg) or rifampicin (0.1 or 10 mg/kg) for four days. For the in vitro study, PXB-cells were incubated with 3-MC (10, 50, or 250 ng/mL) or with rifampicin (5 or 25 μg/mL). The CYP1A1 and 1A2, and CYP3A4 mRNA expression levels increased significantly in the PXB-mouse livers with 20 mg/kg of 3-MC (C_max, 12.2 ng/mL), and 10 mg/kg rifampicin (C_max, 6.9 μg/mL), respectively. The CYP1A1 mRNA expression level increased significantly in PXB-cells with 250 ng/mL of 3-MC, indicating lower sensitivity than in vivo. The CYP1A2 and CYP3A4 mRNA expression levels increased significantly with 50 ng/mL of 3-MC, and 5 μg/mL of rifampicin, respectively, which indicated that the sensitivities were similar between in vivo and in vitro studies. In conclusion, PXB-mice and PXB-cells provide a robust model as an intermediate between in vivo and in vitro human metabolic enzyme induction studies.

An in vitro expansion system for generation of human iPS cell-derived hepatic progenitor-like cells exhibiting a bipotent differentiation potential

Hepatoblasts, hepatic stem/progenitor cells in liver development, have a high proliferative potential and the ability to differentiate into both hepatocytes and cholangiocytes. In regenerative medicine and drug screening for the treatment of severe liver diseases, human induced pluripotent stem (iPS) cell-derived mature functional hepatocytes are considered to be a potentially good cell source. However, induction of proliferation of these cells is difficult ex vivo. To circumvent this problem, we generated hepatic progenitor-like cells from human iPS cells using serial cytokine treatments in vitro. Highly proliferative hepatic progenitor-like cells were purified by fluorescence-activated cell sorting using antibodies against CD13 and CD133 that are known cell surface markers of hepatic stem/progenitor cells in fetal and adult mouse livers. When the purified CD13^highCD133⁺ cells were cultured at a low density with feeder cells in the presence of suitable growth factors and signaling inhibitors (ALK inhibitor A-83-01 and ROCK inhibitor Y-27632), individual cells gave rise to relatively large colonies. These colonies consisted of two types of cells expressing hepatocytic marker genes (hepatocyte nuclear factor 4α and α-fetoprotein) and a cholangiocytic marker gene (cytokeratin 7), and continued to proliferate over long periods of time. In a spheroid formation assay, these cells were found to express genes required for mature liver function, such as cytochrome P450 enzymes, and secrete albumin. When these cells were cultured in a suitable extracellular matrix gel, they eventually formed a cholangiocytic cyst-like structure with epithelial polarity, suggesting that human iPS cell-derived hepatic progenitor-like cells have a bipotent differentiation ability. Collectively these data indicate that this novel procedure using an in vitro expansion system is useful for not only liver regeneration but also for the determination of molecular mechanisms that regulate liver development.

Hepatic stellate cells in liver development, regeneration, and cancer

Hepatic stellate cells are liver-specific mesenchymal cells that play vital roles in liver physiology and fibrogenesis. They are located in the space of Disse and maintain close interactions with sinusoidal endothelial cells and hepatic epithelial cells. It is becoming increasingly clear that hepatic stellate cells have a profound impact on the differentiation, proliferation, and morphogenesis of other hepatic cell types during liver development and regeneration. In this Review, we summarize and evaluate the recent advances in our understanding of the formation and characteristics of hepatic stellate cells, as well as their function in liver development, regeneration, and cancer. We also discuss how improved knowledge of these processes offers new perspectives for the treatment of patients with liver diseases.

Mesothelial cells give rise to hepatic stellate cells and myofibroblasts via mesothelial-mesenchymal transition in liver injury

In many organs, myofibroblasts play a major role in the scarring process in response to injury. In liver fibrogenesis, hepatic stellate cells (HSCs) are thought to transdifferentiate into myofibroblasts, but the origins of both HSCs and myofibroblasts remain elusive. In the developing liver, lung, and intestine, mesothelial cells (MCs) differentiate into specific mesenchymal cell types; however, the contribution of this differentiation to organ injury is unknown. In the present study, using mouse models, conditional cell lineage analysis has demonstrated that MCs expressing Wilms tumor 1 give rise to HSCs and myofibroblasts during liver fibrogenesis. Primary MCs, isolated from adult mouse liver using antibodies against glycoprotein M6a, undergo myofibroblastic transdifferentiation. Antagonism of TGF-β signaling suppresses transition of MCs to mesenchymal cells both in vitro and in vivo. These results indicate that MCs undergo mesothelial-mesenchymal transition and participate in liver injury via differentiation to HSCs and myofibroblasts.

Morphological and microarray analyses of human hepatocytes from xenogeneic host livers

We previously produced mice with human hepatocyte (h-hep) chimeric livers by transplanting h-heps into albumin enhancer/promoter-driven urokinase-type plasminogen activator-transgenic severe combined immunodeficient (SCID) mice with liver disease. The chimeric livers were constructed with h-heps, mouse hepatocytes, and mouse hepatic sinusoidal cells (m-HSCs). Here, we investigated the morphological features of the chimeric livers and the h-hep gene expression profiles in the xenogeneic animal body. To do so, we performed immunohistochemistry, morphometric analyses, and electron microscopic observations on chimeric mouse livers, and used microarray analyses to compare gene expression patterns in hepatocytes derived from chimeric mouse hepatocytes (c-heps) and h-heps. Morphometric analysis revealed that the ratio of hepatocytes to m-HSCs in the chimeric mouse livers were twofold higher than those in the SCID mouse livers, corresponding to twin-cell plates in the chimeric mouse liver. The h-heps in the chimeric mouse did not show hypoxia even in the twin-cell plate structure, probably because of low oxygen consumption by the h-heps relative to the mouse hepatocytes (m-heps). Immunohistochemical and electron microscopic examinations revealed that the sinusoids in the chimeric mouse livers were normally constructed with h-heps and m-HSCs. However, a number of microvilli projected into the intercellular clefts on the lateral aspects of the hepatocytes, features typical of a growth phase. Microarray profiles indicated that ∼82% of 16 605 probes were within a twofold range difference between h-heps and c-heps. Cluster and principal component analyses showed that the gene expression patterns of c-heps were extremely similar to those of h-heps. In conclusion, the chimeric mouse livers were normally reconstructed with h-heps and m-HSCs, and expressed most human genes at levels similar to those in human livers, although the chimeric livers showed morphological characteristics typical of growth.

Human hepatocyte propagation system in the mouse livers: functional maintenance of the production of coagulation and anticoagulation factors

We previously reported that cell-based therapies using isolated hepatocytes including hepatocyte transplantation and liver tissue engineering approaches provide therapeutic benefits to hemophilia. For clinical application of these approaches, it is important to establish an active hepatocyte proliferation system that enables providing a sufficient number of hepatocytes. We also reported that human hepatocytes, which were transplanted into the liver of urokinase-type plasminogen activator transgenic severe combined immunodeficiency (uPA/SCID) mice, were able to proliferate while retaining their ability to produce coagulation factor IX. The objective of this study was to explore the functionalities of other coagulation and anticoagulation factors of the propagated human hepatocytes in uPA/SCID mice. Human hepatocytes were transplanted into the liver of uPA/SCID mice, and the propagation status of human hepatocytes in the mice was monitored by the increase in serum human albumin levels and immunohistochemical evaluation on the liver sections. Using uPA/SCID livers with various stages of human hepatocyte propagation, we analyzed the gene expression levels of coagulation factors (prothrombin, factor VII, factor X, and factor VIII) and anticoagulation factors (protein C and protein S) by real-time polymerase chain reaction (PCR) using human-specific primers. As a result, the total amount of raw messenger RNA expression levels increased in all genes analyzed according to the progress of hepatocyte propagation and proliferation. Except for factor VIII, the gene expression levels of the highly repopulated uPA/SCID mouse livers with human hepatocyte showed higher levels than those of normal human livers, indicating that propagated human hepatocytes in the uPA/SCID system possess full functions to produce most of the coagulation-related factors. The current work demonstrated that human hepatocytes can be propagated in experimental animals while maintaining normal gene expression levels of coagulation-related factors. It could be speculated that the propagated cells serve as a cell source for the treatment of various types of coagulation factor deficiencies.

Hepatic stellate cell progenitor cells

Hepatic stellate cells (HSCs) are recognized as a major player in liver fibrogenesis. Upon liver injury, HSCs differentiate into myofibroblasts and participate in progression of fibrosis and cirrhosis. Additional cell types such as resident liver fibroblasts/myofibroblasts or bone marrow cells are also known to generate myofibroblasts. One of the major obstacles to understanding the mechanism of liver fibrogenesis is the lack of knowledge regarding the developmental origin of HSCs and other liver mesenchymal cells. Recent cell lineage analyses demonstrate that HSCs are derived from mesoderm during liver development. MesP1-expressing mesoderm gives rise to the septum transversum mesenchyme before liver formation and then to the liver mesothelium and mesenchymal cells, including HSCs and perivascular mesenchymal cells around the veins during liver development. During the growth of embryonic liver, the mesothelium, consisting of mesothelial cells and submesothelial cells, migrates inward from the liver surface and gives rise to HSCs and perivascular mesenchymal cells, including portal fibroblasts, smooth muscle cells around the portal vein, and fibroblasts around the central vein. Cell lineage analyses indicate that mesothelial cells are HSC progenitor cells capable of differentiating into HSCs and other liver mesenchymal cells during liver development.

Hepatic stellate cell-derived delta-like homolog 1 (DLK1) protein in liver regeneration

Hepatic stellate cells (HSCs) undergo myofibroblastic activation in liver fibrosis and regeneration. This phenotypic switch is mechanistically similar to dedifferentiation of adipocytes as such the necdin-Wnt pathway causes epigenetic repression of the master adipogenic gene Pparγ, to activate HSCs. Now we report that delta-like 1 homolog (DLK1) is expressed selectively in HSCs in the adult rodent liver and induced in liver fibrosis and regeneration. Dlk1 knockdown in activated HSCs, causes suppression of necdin and Wnt, epigenetic derepression of Pparγ, and morphologic and functional reversal to quiescent cells. Hepatic Dlk1 expression is induced 40-fold at 24 h after partial hepatectomy (PH) in mice. HSCs and hepatocytes (HCs) isolated from the regenerating liver show Dlk1 induction in both cell types. In HC and HSC co-culture, increased proliferation and Dlk1 expression by HCs from PH are abrogated with anti-DLK1 antibody (Ab). Dlk1 and Wnt10b expression by Sham HCs are increased by co-culture with PH HSCs, and these effects are abolished with anti-DLK Ab. A tail vein injection of anti-DLK1 Ab at 6 h after PH reduces early HC proliferation and liver growth, accompanied by decreased Wnt10b, nonphosphorylated β-catenin, p-β-catenin (Ser-552), cyclins (cyclin D and cyclin A), cyclin-dependent kinases (CDK4, and CDK1/2), p-ERK1/2, and p-AKT. In the mouse developing liver, HSC precursors and HSCs express high levels of Dlk1, concomitant with Dlk1 expression by hepatoblasts. These results suggest novel roles of HSC-derived DLK1 in activating HSCs via epigenetic Pparγ repression and participating in liver regeneration and development in a manner involving the mesenchymal-epithelial interaction.

Transcriptional ontogeny of the developing liver

Background

During embryogenesis the liver is derived from endodermal cells lining the digestive tract. These endodermal progenitor cells contribute to forming the parenchyma of a number of organs including the liver and pancreas. Early in organogenesis the fetal liver is populated by hematopoietic stem cells, the source for a number of blood cells including nucleated erythrocytes. A comprehensive analysis of the transcriptional changes that occur during the early stages of development to adulthood in the liver was carried out.

Results

We characterized gene expression changes in the developing mouse liver at gestational days (GD) 11.5, 12.5, 13.5, 14.5, 16.5, and 19 and in the neonate (postnatal day (PND) 7 and 32) compared to that in the adult liver (PND67) using full-genome microarrays. The fetal liver, and to a lesser extent the neonatal liver, exhibited dramatic differences in gene expression compared to adults. Canonical pathway analysis of the fetal liver signature demonstrated increases in functions important in cell replication and DNA fidelity whereas most metabolic pathways of intermediary metabolism were under expressed. Comparison of the dataset to a number of previously published microarray datasets revealed 1) a striking similarity between the fetal liver and that of the pancreas in both mice and humans, 2) a nucleated erythrocyte signature in the fetus and 3) under expression of most xenobiotic metabolism genes throughout development, with the exception of a number of transporters associated with either hematopoietic cells or cell proliferation in hepatocytes.

Conclusions

Overall, these findings reveal the complexity of gene expression changes during liver development and maturation, and provide a foundation to predict responses to chemical and drug exposure as a function of early life-stages.

Epigenetic cell fate regulation of hepatic stellate cells

Research in the past three decades has identified key mediators and signaling mechanisms responsible for myofibroblastic transdifferentiation (MTD) of hepatic stellate cells (HSC), the pivotal event in liver fibrogenesis. Yet, fundamental understanding of the MTD from the viewpoint of cell fate or lineage regulation has been elusive. Recent studies using genetic cell fate mapping techniques demonstrate HSC are derived from mesoderm and at least in part via septum transversum and mesothelium. HSC express markers for different cell types derived from multipotent mesenchymal progenitors. A regulatory commonality between differentiation of adipocytes and that of HSC is shown, and a shift from adipogenic to myogenic or neuronal phenotype characterizes HSC MTD. Central to this shift is a loss of expression of the master adipogenic regulator peroxisome proliferator activated receptor-γ (PPAR-γ). Restored expression of PPAR-γ and/or other adipogenic transcription factors reverses myofibroblastic HSC to differentiated cells. In MTD, Pparγ is epigenetically repressed by induction of methyl-CpG binding protein 2 and its enrichment to the promoter and polycomb repressive complex-facilitated histone H3 lysine 27 di/tri-methylation at the 3' exons. Blocking canonical wingless-related MMTV integration site (Wnt) signaling in myofibroblastic HSC with the co-receptor antagonist Dickkopf-1, abrogates these epigenetic mechanisms, restores PPAR-γ expression and HSC differentiation. Necdin, a melanoma antigen family protein, is identified as an upstream mediator for induction of the canonical Wnt10b and consequent Pparγ repression and HSC MTD. The identified morphogen-induced epigenetic regulation of Pparγ and HSC fate may serve as a novel target for manipulation of liver fibrosis and mesenchymal-epithelial interactions in liver regeneration.

Growth hormone-dependent pathogenesis of human hepatic steatosis in a novel mouse model bearing a human hepatocyte-repopulated liver

Clinical studies have shown a close association between nonalcoholic fatty liver disease and adult-onset GH deficiency, but the relevant molecular mechanisms are still unclear. No mouse model has been suitable to study the etiological relationship of human nonalcoholic fatty liver disease and human adult-onset GH deficiency under conditions similar to the human liver in vivo. We generated human (h-)hepatocyte chimeric mice with livers that were predominantly repopulated with h-hepatocytes in a h-GH-deficient state. The chimeric mouse liver was mostly repopulated with h-hepatocytes about 50 d after transplantation and spontaneously became fatty in the h-hepatocyte regions after about 70 d. Infusion of the chimeric mouse with h-GH drastically decreased steatosis, showing the direct cause of h-GH deficiency in the generation of hepatic steatosis. Using microarray profiles aided by real-time quantitative RT-PCR, comparison between h-hepatocytes from h-GH-untreated and -treated mice identified 14 GH-up-regulated and four GH-down-regulated genes, including IGF-I, SOCS2, NNMT, IGFLS, P4AH1, SLC16A1, SRD5A1, FADS1, and AKR1B10, respectively. These GH-up- and -down-regulated genes were expressed in the chimeric mouse liver at lower and higher levels than in human livers, respectively. Treatment of the chimeric mice with h-GH ameliorated their altered expression. h-Hepatocytes were separated from chimeric mouse livers for testing in vitro effects of h-GH or h-IGF-I on gene expression, and results showed that GH directly regulated the expression of IGF-I, SOCS2, NNMT, IGFALS, P4AH1, FADS1, and AKR1B10. In conclusion, the chimeric mouse is a novel h-GH-deficient animal model for studying in vivo h-GH-dependent human liver dysfunctions.

Septum transversum-derived mesothelium gives rise to hepatic stellate cells and perivascular mesenchymal cells in developing mouse liver

The septum transversum mesenchyme (STM) signals to induce hepatogenesis from the foregut endoderm. Hepatic stellate cells (HSCs) are sinusoidal pericytes assumed to originate from the STM and participate in mesenchymal-epithelial interaction in embryonic and adult livers. However, the developmental origin of HSCs remains elusive due to the lack of markers for STM and HSCs. We previously identified submesothelial cells (SubMCs) beneath mesothelial cells (MCs) as a potential precursor for HSCs in developing livers. In the present study, we reveal that both STM in embryonic day (E) 9.5 and MC/SubMCs in E12.5 share the expression of activated leukocyte cell adhesion molecule (Alcam), desmin, and Wilms tumor 1 homolog (Wt1). A cell lineage analysis using MesP1(Cre) /Rosa26lacZ(flox) mice identifies the mesodermal origin of the STM, HSCs, and perivascular mesenchymal cells (PMCs). A conditional cell lineage analysis using the Wt1(CreERT2) mice demonstrates that Wt1(+) STM gives rise to MCs, SubMCs, HSCs, and PMCs during liver development. Furthermore, we find that Wt1(+) MC/SubMCs migrate inward from the liver surface to generate HSCs and PMCs including portal fibroblasts, smooth muscle cells, and fibroblasts around the central veins. On the other hand, the Wt1(+) STM and MC/SubMCs do not contribute to sinusoidal endothelial cells, Kupffer cells, and hepatoblasts.

CONCLUSION

our results demonstrate that HSCs and PMCs are derived from MC/SubMCs, which are traced back to mesodermal STM during liver development.

Analysis of gene expression profiles of liver stellate cells during liver regeneration in rats

This study performed a large-scale, high-throughput analysis of transcriptional profiling of liver stellate cells (LSCs) at the cellular level to investigate changes in the biological activity of LSCs during rat liver regeneration (LR) and the relation of these changes to LR. First, a rat liver regeneration model was established by partial hepatectomy (PH). Stellate cells were isolated in high purity and yield from the regenerating rat liver by Percoll density gradient centrifugation and immunomagnetic bead sorting. The changes in gene expression of LSCs after PH were examined using a rat genome 230 2.0 array composed of 24622 genes. The results indicated that 10241 of the 24622 genes investigated on the array were differentially expressed in LSCs. Of the 10241 genes, 1563 known genes were related to LR, which were grouped into three major gene expression clusters according to three-fold cut-off threshold: the upregulated gene cluster, the down-regulated gene cluster, and the cluster composed of genes showing complex changes in expression. Additionally, the genes were grouped into those involved in transcription regulation, signal transduction, transport, cellular metabolism, inflammation and immunity by functional analysis. When gene expression profiles were combined with the results of gene functional analysis, most of the genes involved in cytokine secretion and retinol metabolism in LSCs were significantly enriched in the cluster characterized by decreased expression, whereas genes involved in lipid metabolism were mostly enriched in the cluster showing increased expression. Based on further analysis of genes expressed in a phase-dependent manner during LR, it was suggested that lipid metabolism in LSCs was enhanced in the whole regeneration process, and that immune response and cytokine secretion were impaired during all three regenerative phases.

Characterization and functional analyses of hepatic mesothelial cells in mouse liver development

BACKGROUND & AIMS

At the onset of liver development, cardiac mesoderm, septum transversum mesenchyme, and endothelial cells are involved in the specification and/or proliferation of hepatoblasts. After this initial stage, however, it is unclear which cells support the proliferation and differentiation of hepatocytes. Here we characterized the nature of mouse hepatic mesothelial cells (MCs) and investigated their role in organogenesis.

METHODS

Using anti-podocalyxin-like protein 1 (PCLP1) and anti-mesothelin antibodies, we characterized MCs during liver development by immunohistochemistry, flow cytometry, and gene expression analysis. The possible role of MCs in hepatogenesis was investigated by in vitro culture and analysis of Wilms' tumor 1 homologue (WT1) knockout mice.

RESULTS

PCLP1 was highly expressed in immature MCs, covering the surface of lobes. PCLP1 expression in MCs was down-regulated along with development, whereas mesothelin expression was up-regulated, indicating that these molecules distinguished developmental stages of MCs. The proliferation potential of MCs was high in the fetus and declined after birth. Fetal MCs expressed various growth factors and strongly enhanced the expansion of fetal hepatocytes in vitro, whereas differentiated MCs exhibited less growth factor expression, and differentiated MCs failed to enhance hepatocyte proliferation in vitro. In WT1-deficient embryos, hepatocyte proliferation was impaired due to defective MCs.

CONCLUSIONS

The mesothelium is not only an inert protective sheet covering the parenchyma but also changes its characteristics dynamically during development and plays an active role in organogenesis by promoting expansion of parenchymal cells.

Global gene expression profiling reveals a key role of CD44 in hepatic oval-cell reaction after 2-AAF/CCl4 injury in rodents

Liver progenitors, so-called oval cells, proliferate remarkably from periportal areas after severe liver injury when hepatocyte regeneration is compromised. These cells invade far into the liver parenchyma. Molecular mechanisms underlying these behaviors of oval cells remain poorly understood. In this study, we treated rats with 2-acetylaminofluorene/carbon tetrachloride to induce hepatic oval cells. By expression microarray analysis, we investigated global gene expression profiles in liver tissue, with an emphasis on adhesion molecules, extracellular matrix proteins, matrix metalloproteinases (MMPs), growth factors/cytokines, and receptors that might contribute to the distinct behaviors of oval cells. Genes upregulated at least twofold were selected. We then performed immunostaining to verify the microarray results and identified expression of MMP-7 and CD44 in oval cells. Staining of cytokeratin (CK)-19, an oval-cell marker, was similar between oval cells located next to periportal areas and those located far within the parenchyma. In contrast, CD44 staining was more intense in the parenchyma than in periportal areas, suggesting a role of CD44 in oval-cell invasion. Moreover, newly differentiated CK-19+ hepatocytes within foci did not show CD44 staining, suggesting that CD44 is related to the undifferentiated oval-cell phenotype. We then investigated oval-cell reactivity in CD44-deficient mice fed an oval cell-inducing diet of 3,5-diethoxycarbonyl-1,4-dihydrocollidine. Results showed significantly reduced oval-cell reactivity in CD44-deficient mice. Thus, oval cells express MMP-7 and CD44, and CD44 appears to play critical roles in the proliferation, invasion, and differentiation of hepatic oval cells in rodents.

Pleiotrophin is expressed in avian somites and tendon anlagen

Pleiotrophin (Ptn) is a secreted, developmentally regulated growth factor associated with the extracellular matrix. During mammalian embryogenesis, Ptn has been suggested to play a role in the development of various embryonic structures including nervous system and skeleton. In the avian embryo, Ptn has been proposed to be involved in limb cartilage development, but embryonic Ptn expression has not been comprehensively studied. We isolated a cDNA fragment containing the full-length coding sequence of chick Ptn and studied the expression of Ptn in detail until embryonic day 10. We, furthermore, isolated a 6,385-bp phage clone containing the Ptn cDNA of 2,551 bp and additional 3,787 bp downstream of the published Ptn cDNA sequence classifying a yet Ptn-unrelated chEST clone as the 3' untranslated region of Ptn. Our studies revealed novel expression domains in developing somites and during limb formation. We found prominent expression in the somitocoel cells of epithelial somites, and in a sclerotomal subcompartment, the syndetome, which gives rise to the axial tendons in the vertebral motion segment. In the limbs, Ptn was markedly expressed in tendon anlagen and in phalangeal joints. Our results introduce Ptn as a novel marker gene in avian somite and tendon development.

Hepatocytes from Fibrotic Liver Possess High Growth Potential in Vivo

Hepatocyte transplantation is effective for treating liver failure, but healthy donors as a source of hepatocytes are quite limited. The livers of patients with hepatic fibrosis could be an alternative source; however, few reports have examined the nature of hepatocytes from fibrotic livers (f-hepatocytes). In this study, we compared the growth of f-hepatocytes and hepatocytes from normal livers (n-hepatocytes). Hepatocytes were isolated from normal and CCl4-treated wild-type Fischer rats that express dipeptidyl dipeptidase IV (DPPIV) gene (DPPIV+). The n- and f-hepatocytes proliferated in culture at similar rates. Both types of hepatocytes were transplanted into DPPIV- mutant Fischer rats that had been treated with retrorsine to injure the liver and were partially hepatectomized (PHx) before transplantation. Both n- and f-DPPIV+-hepatocytes proliferated and formed colonies. The colony sizes of f-hepatocytes 21 days posttransplantation were approximately three times those of n-hepatocytes. The hepatocytes were analyzed using a fluorescence activated cell sorter (FACS). The FACS profile differed between f- and n-hepatocytes: f-hepatocytes were less granular, less autofluorescent, and smaller than n-hepatocytes. These characteristics of f-hepatocytes resembled those reported for small-sized n-hepatocytes (SHs), which are highly proliferative and preferentially express a unique set of 10 SH genes. However, f-hepatocytes preferentially expressed only five of the SH genes. The expression profile of f-hepatocytes was rather similar to that of proliferating n-hepatocytes in the regenerating liver after PHx. The f-hepatocytes were morphologically normal and did not show any preneoplastic phenotype. These normal and proliferative natures of f-hepatocytes in vivo suggest the fibrotic liver as a source of hepatocytes for transplantation.

Mesenchymal origin of hepatic stellate cells, submesothelial cells, and perivascular mesenchymal cells during mouse liver development

The knowledge concerning fetal hepatic stellate cells (HSCs) is scarce, and their cell lineage and functions are largely unknown. The current study isolated fetal liver mesenchymal cells from a mouse expressing beta-galactosidase under the control of Msx2 promoter by fluorescence-activated cell sorting (FACS) and surveyed marker genes by microarray analysis. Based on the location and immunostaining with conventional and newly disclosed markers, we have identified three distinct populations of fetal liver mesenchymal cells expressing both desmin and p75 neurotrophin receptor (p75NTR): HSCs in the liver parenchyma; perivascular mesenchymal cells expressing alpha-smooth muscle actin (alpha-SMA); and submesothelial cells associated with the basal lamina beneath mesothelial cells and expressing activated leukocyte cell adhesion molecule (ALCAM) and platelet-derived growth factor receptor alpha. A transitional cell type from the submesothelial cell phenotype to fetal HSCs was also identified near the liver surface. Mesothelial cells expressed podoplanin and ALCAM. Ki-67 staining showed that proliferative activity of the submesothelial cells is higher than that of mesothelial cells and transitional cells. Using anti-ALCAM antibodies, submesothelial and mesothelial cells were isolated by FACS. The ALCAM(+) cells expressed hepatocyte growth factor and pleiotrophin. In culture, the ALCAM(+) cells rapidly acquired myofibroblastic morphology and alpha-SMA expression. The ALCAM(+) cells formed intracellular lipid droplets when embedded in collagen gel and treated with retinol, suggesting the potential for ALCAM(+) cells to differentiate to HSCs. Finally, we demonstrated that fetal HSCs, submesothelial cells, and perivascular mesenchymal cells are all derived from mesoderm by using MesP1-Cre and ROSA26 reporter mice.

CONCLUSION

Fetal HSCs, submesothelial cells, and perivascular mesenchymal cells are mesodermal in origin, and ALCAM(+) submesothelial cells may be a precursor for HSCs in developing liver.

Pleiotrophin inhibits transforming growth factor beta1-induced apoptosis in hepatoma cell lines

Pleiotrophin (PTN) is a hepatocyte growth factor and considered to play roles in liver fibrogenesis and hepatocarcinogenesis. In this study we examined the mechanism of the action of PTN in these pathological processes. First, we confirmed that hepatic stellate cells (HSCs) and Kupffer cells, and also later hepatocytes in hyperplastic nodules increased PTN mRNA expressions during carbon tetrachloride-induced liver fibrosis. Then, the relationship between PTN and transforming growth factor beta1 (TGFbeta1), a known potent pro-fibrogenetic cytokine, in carcinogenesis was investigated using hepatoma cell lines. Huh-7 human hepatoma cells weakly expressed PTN, but HepG2 human hepatoma cells and FaO rat hepatoma cells did not. Recombinant (r) TGFbeta1 induced the cultured Huh-7 cells to undergo apoptosis, which was inhibited by rPTN. Huh-7 cells became resistant to TGFbeta1-, but not mitomycin C-induced apoptosis when transfected with PTN gene, indicating the specificity of the PTN anti-apoptotic activity. Poly ADP ribose polymerase, procaspase-8 and procaspase-3 were not cleaved in the TGFbeta1-reluctant cells. The TGFbeta1-induced caspase-3 activation was also suppressed in Huh-7 and FaO cells both transduced with PTN gene-bearing adenoviruses. In summary, PTN was expressed in HSCs, Kupffer cells, and hepatocytes in fibrotic liver. We propose that PTN specifically antagonizes the TGFbeta1 activity during liver fibrosis.

Fate-mapping evidence that hepatic stellate cells are epithelial progenitors in adult mouse livers

Liver injury activates quiescent hepatic stellate cells (Q-HSC) to proliferative myofibroblasts. Accumulation of myofibroblastic hepatic stellate cells (MF-HSC) sometimes causes cirrhosis and liver failure. However, MF-HSC also promote liver regeneration by producing growth factors for oval cells, bipotent progenitors of hepatocytes and cholangiocytes. Genes that are expressed by primary hepatic stellate cell (HSC) isolates overlap those expressed by oval cells, and hepatocytic and ductular cells emerge when HSC are cultured under certain conditions. We evaluated the hypothesis that HSC are a type of oval cell and, thus, capable of generating hepatocytes to regenerate injured livers. Because Q-HSC express glial fibrillary acidic protein (GFAP), we crossed mice in which GFAP promoter elements regulated Cre-recombinase with ROSA-loxP-stop-loxP-green fluorescent protein (GFP) mice to generate GFAP-Cre/GFP double-transgenic mice. These mice were fed methionine choline-deficient, ethionine-supplemented diets to activate and expand HSC and oval cell populations. GFP(+) progeny of GFAP-expressing precursors were characterized by immunohistochemistry. Basal expression of mesenchymal markers was negligible in GFAP(+)Q-HSC. When activated by liver injury or culture, HSC downregulated expression of GFAP but remained GFP(+); they became highly proliferative and began to coexpress markers of mesenchyme and oval cells. These transitional cells disappeared as GFP-expressing hepatocytes emerged, began to express albumin, and eventually repopulated large areas of the hepatic parenchyma. Ductular cells also expressed GFAP and GFP, but their proliferative activity did not increase in this model. These findings suggest that HSC are a type of oval cell that transitions through a mesenchymal phase before differentiating into hepatocytes during liver regeneration. Disclosure of potential conflicts of interest is found at the end of this article.

Hepatic stellate cells: protean, multifunctional, and enigmatic cells of the liver

The hepatic stellate cell has surprised and engaged physiologists, pathologists, and hepatologists for over 130 years, yet clear evidence of its role in hepatic injury and fibrosis only emerged following the refinement of methods for its isolation and characterization. The paradigm in liver injury of activation of quiescent vitamin A-rich stellate cells into proliferative, contractile, and fibrogenic myofibroblasts has launched an era of astonishing progress in understanding the mechanistic basis of hepatic fibrosis progression and regression. But this simple paradigm has now yielded to a remarkably broad appreciation of the cell's functions not only in liver injury, but also in hepatic development, regeneration, xenobiotic responses, intermediary metabolism, and immunoregulation. Among the most exciting prospects is that stellate cells are essential for hepatic progenitor cell amplification and differentiation. Equally intriguing is the remarkable plasticity of stellate cells, not only in their variable intermediate filament phenotype, but also in their functions. Stellate cells can be viewed as the nexus in a complex sinusoidal milieu that requires tightly regulated autocrine and paracrine cross-talk, rapid responses to evolving extracellular matrix content, and exquisite responsiveness to the metabolic needs imposed by liver growth and repair. Moreover, roles vital to systemic homeostasis include their storage and mobilization of retinoids, their emerging capacity for antigen presentation and induction of tolerance, as well as their emerging relationship to bone marrow-derived cells. As interest in this cell type intensifies, more surprises and mysteries are sure to unfold that will ultimately benefit our understanding of liver physiology and the diagnosis and treatment of liver disease.

Fibroblast growth factor 10 is critical for liver growth during embryogenesis and controls hepatoblast survival via beta-catenin activation

Fibroblast growth factor (FGF) signaling and beta-catenin activation have been shown to be crucial for early embryonic liver development. This study determined the significance of FGF10-mediated signaling in a murine embryonic liver progenitor cell population as well as its relation to beta-catenin activation. We observed that Fgf10(-/-) and Fgfr2b(-/-) mouse embryonic livers are smaller than wild-type livers; Fgf10(-/-) livers exhibit diminished proliferation of hepatoblasts. A comparison of beta-galactosidase activity as a readout of Fgf10 expression in Fgf10(+/LacZ) mice and of beta-catenin activation in TOPGAL mice, demonstrated peak Fgf10 expression from E9 to E13.5 coinciding with peak beta-catenin activation. Flow cytometric isolation and marker gene expression analysis of LacZ(+) cells from E13.5 Fgf10(+/LacZ) and TOPGAL livers, respectively, revealed that Fgf10 expression and beta-catenin signaling occur distinctly in stellate/myofibroblastic cells and hepatoblasts, respectively. Moreover, hepatoblasts express Fgfr2b, which strongly suggests they can respond to recombinant FGF10 produced by stellate cells. Fgfr2b(-/-)/TOPGAL(+/+) embryonic livers displayed less beta-galactosidase activity than livers of Fgfr2b(+/+)/TOPGAL(+/+) littermates. In addition, cultures of whole liver explants in Matrigel or cell in suspension from E12.5 TOPGAL(+/+)mice displayed a marked increase in beta-galactosidase activity and cell survival upon treatment with recombinant FGF10, indicating that FGFR (most likely FGFR2B) activation is upstream of beta-catenin signaling and promote hepatoblast survival.

CONCLUSION

Embryonic stellate/myofibroblastic cells promote beta-catenin activation in and survival of hepatoblasts via FGF10-mediated signaling. We suggest a role for stellate/myofibroblastic FGF10 within the liver stem cell niche in supporting the proliferating hepatoblast.

The presence of pleiotrophin in the human intervertebral disc is associated with increased vascularization: an immunohistologic study

STUDY DESIGN

An immunohistological study of surgical specimens of human intervertebral disc.

OBJECTIVE

To examine the presence of pleiotrophin in diseased or damaged intervertebral disc tissue and the association between its presence and the extent of tissue vascularization and innervation.

SUMMARY OF BACKGROUND DATA

Increased levels of pleiotrophin, a growth and differentiation factor that is active in various pathophysiologic processes, including angiogenesis, has been associated with osteoarthritic changes of human articular cartilage. The association between pleiotrophin expression and pathologic conditions of the human intervertebral disc is unknown.

METHODS

Specimens of human lumbar intervertebral discs, obtained following surgical discectomy, were divided into 3 groups: non-degenerated discs (n = 7), degenerated discs (n = 6), and prolapsed discs (n = 11). Serial tissue sections of each specimen were immunostained to determine the presence of pleiotrophin, blood vessels (CD34-positive endothelial cells), and nerves (neurofilament 200 kDa [NF200]-positive nerve fibers).

RESULTS

Pleiotrophin immunoreactivity was seen in disc cells, endothelial cells, and in the extracellular matrix in most specimens of intervertebral disc but was most prevalent in vascularized tissue in prolapsed discs. There was a significant correlation between the presence of pleiotrophin-positive disc cells and that of CD34-positive blood vessels. NF200-positive nerves were seen in vascularized areas of more degenerated discs, but nerves did not appear to codistribute with blood vessels or pleiotrophin positivity in prolapsed discs.

CONCLUSIONS

Pleiotrophin is present in pathologic human intervertebral discs, and its prevalence and distribution suggest that it may play a role in neovascularization of diseased or damaged disc tissue.

Growth potential of adult hepatocytes in mammals: Highly replicative small hepatocytes with liver progenitor‐like traits

The liver is one of the few organs that is capable of completely regenerating itself without using a stem cell population. When damaged, growth factors and cytokines are released, stimulating terminally differentiated adult hepatocytes and making them re-enter the cell cycle. We have been developing a series of studies on the growth potential of rat and human hepatocytes to identify a population of hepatocytes that is responsible for the regeneration of the injured liver. For this purpose, we established an appropriate culture method for hepatocytes by which growth and differentiation capacities are practically examined under various experimental conditions. This in vitro assay system allows us to identify small hepatocytes that are prominently replicative compared to large hepatocytes. Non-parenchymal cells play critical roles in the proliferation of small hepatocytes. These hepatocytes are present in both rat and human liver and are located in portal regions there. Phenotypic features were examined at morphological and gene/protein levels in detail, which showed the phenotypic plasticity in vitro. Mammalian liver includes a population of small hepatocytes in normal adults with a minute occupancy rate. We speculate that small hepatocytes play a role in regenerating the injured liver and in compensating for apoptotic hepatocytes in the physiological turnover of hepatocytes.

Identification of vitamin A-free cells in a stellate cell-enriched fraction of normal rat liver as myofibroblasts

Myofibroblasts (MFs) as well as hepatic stellate cells (HSCs) are known to be involved in liver fibrogenesis. Quiescent HSCs (qHSCs) in culture have been thought to differentiate to replicative activated HSCs (aHSCs). In this study a qHSC-enriched fraction isolated by Nycodenz-isodensity centrifugation was separated with a fluorescence-activated cell sorter, which revealed the presence of a small fraction (occupancy rate=0.4%) of cells that did not show vitamin A-autofluorescence under ultraviolet (UV)-irradiation (UV- cells). The remaining vitamin A-containing cells were autofluorescent (UV+) and originally expressed markers of qHSCs, and, in culture, did not grow, lost vitamin A, and expressed markers of aHSCs. UV- cells showed morphology of MFs, and, in culture, grew to form colonies and expressed markers of MFs. These results indicated that UV+ and UV- cells represent qHSCs and MFs, respectively, and that aHSCs have no growth potential and are a cell-type distinct from proliferative MFs. Gene expression profiles of UV- cells (MFs) newly identified gremlin as one of MF-preferential genes and its proteins were localized around fibrotic septa in rat and human livers. In addition, we suggested that the qHSC-enriched fraction included approximately 6% of liver MFs.

Multiplicative mononuclear small hepatocytes in adult rat liver: Their isolation as a homogeneous population and localization to periportal zone

Adult rat liver contains a minor population of hepatocytes called small hepatocytes (SHs) that are smaller in size and show a higher replicative potential than conventional parenchymal hepatocytes (PHs). However, SHs have been hitherto characterized using a "SH-fraction" that was contaminated with PHs. In the present study, we isolated a PH-free SH-fraction from the adult rat liver using fluorescence-activated cell sorter combined with centrifugal elutriation and characterized the hepatocytes in the fraction. These hepatocytes were designated R3Hs in this study. R3Hs were mononuclear and of lower ploidy. They expressed at high levels genes of Cdc2, connexin 26, hydroxysteroid sulfotransferase, pancreatic secretory trypsin inhibitor, and prostaglandin E2 receptor EP3 subtype. We conclude that SHs dominate the periportal zone in the adult liver, because mRNA or proteins of these genes were exclusively expressed by periportal hepatocytes.

Growth and differentiation of colony-forming human hepatocytes in vitro

BACKGROUND/AIMS

Parenchymal hepatocytes (PHs) of rat contain colony-forming parenchymal hepatocytes (CF-PHs) as a small fraction. We aimed to demonstrate the presence of CF-PHs in humans and characterize them with respect to growth and differentiation potential.

METHODS

Human PHs were co-cultured with Swiss 3T3 cells in the medium containing human serum, EGF, nicontinamide, and ascorbic acid 2-phosphate. To examine differentiation potential hepatocytes were cultured on gels of Matrigel Matrix.

RESULTS

Few PHs formed colonies, the colony-forming efficiency being as low as 0.01-0.09%. The CF-PHs could be subcultured up to 7 passages. They showed a liver epithelial cell-like morphology, and immunocytochemically positive for albumin (ALB), cytokeratin (CK) 7, 8, 18, and 19 in a pre- and early phase-confluence, whereas they showed a typical differentiated hepatocyte-like morphology, and positive for alpha(1)-antitrypsin, but negative for CK7 and 19 in condensed regions at confluence. The CF-PHs at late confluence expressed mRNAs of ALB, HNF4, and isoforms of cytochrome P450 at low levels. However, when cultured on Matrigel, these cells expressed them at high levels comparable to those of original PHs.

CONCLUSIONS

We concluded that the human liver contains highly replicative hepatic progenitor-like cells as a minute population that retain a normal differentiation potential.