Chihiro Yamasaki

Near Completely Humanized Liver in Mice Shows Human-Type Metabolic Responses to Drugs

Human hepatocytes were transplanted into urokinase-type plasminogen activator-transgenic SCID mice (uPA/SCID mice), which are immunodeficient and undergo liver failure. The transplanted cells were characterized in terms of their in vivo growth potential and functions. The human hepatocytes progressively repopulated the murine host liver. However, the recipients died when the replacement index (RI) of the human hepatocytes exceeded 50%. The hosts (chimeric mice) survived at RI >50% when treated with a drug that has anti-human complement factor activity, and these mice developed livers with RI values as high as 96%. In total, 36 chimeric mice were generated, and the rate of successful engraftment was as high as 92%. The yield of chimeric mice with RI >70% was 32%. The human hepatocytes in the murine host liver expressed mRNAs for a variety of human cytochrome P450 (hCYP) subtypes, in a manner that was similar to the donor liver. The mRNAs for hCYP3A4 and hCYP1A1/2 were induced in the liver in a CYP type-specific manner when the mice were treated with rifampicin and 3-methylcholanthrene, respectively. These results indicate that human hepatocytes that propagate in mice retain their normal pharmacological responses. We conclude that the chimeric mouse developed in the present study is a useful model for assessing the functions and pharmacological responses of human hepatocytes.

Pleiotrophin/Heparin-Binding Growth-Associated Molecule as a Mitogen of Rat Hepatocytes and Its Role in Regeneration and Development of Liver

Previously pleiotrophin (PTN) was identified among proteins secreted by Swiss 3T3 cells as a mitogen for cultured adult rat hepatocytes. The present study showed that the growth of rat hepatocytes was enhanced when cultured with rat hepatic stellate cells (HSCs). HSCs expressed PTN mRNA and secreted its protein in the co-cultures. Recombinant PTN enhanced the growth of hepatocytes in culture, suggesting that HSCs stimulate the growth of hepatocytes through the action of PTN. To know the biological role of PTN in the growth of hepatocytes in vivo, we examined the expression of PTN in four regeneration models of adult liver and embryonic liver of rat. The expression of PTN mRNA in the liver was markedly up-regulated by the treatment with D-galactosamine (GalN) or with acetylaminofluorene followed by partial hepatectomy. HSCs expressed PTN mRNA in response to GalN treatment and its protein was found on hepatocytes. The mRNA expression of N-syndecan, a PTN receptor, was up-regulated in GalN-treated hepatocytes. The mesenchymal cells in the septum transversum enclosing the embryonic liver, but not embryonic HSCs, expressed PTN mRNA. We suggest that PTN is secreted from activated adult HSCs and embryonic mesenchymal cells as a mitogen of parenchymal cells in adult and embryonic liver, respectively.

Novel Robust in Vitro Hepatitis B Virus Infection Model Using Fresh Human Hepatocytes Isolated from Humanized Mice

The molecular mechanisms underlying the hepatitis B virus (HBV) life cycle are poorly understood because of the lack of appropriate in vitro infection models. Herein, we report a highly effective in vitro HBV infection system using fresh human hepatocytes (HHs) isolated from chimeric mice with humanized livers. After the inoculation of sera collected from HBV-infected chimeric mice or patients to HHs, we measured levels of HBV DNA, mRNA, covalently closed circular DNA, and viral protein expression in HHs. We investigated the neutralization activity of hepatitis B immune globulin and the effects of siRNA against sodium taurocholate-cotransporting polypeptide and clathrin heavy chain on HBV infection. We confirmed the expression of viral antigens in HHs and the presence of extracellular HBV DNA and hepatitis B surface antigen. The maximum infection rate was approximately 80%. Lamivudine and hepatitis B immune globulin treatment reduced HBV DNA levels in a dose-dependent manner. Knockdown of sodium taurocholate-cotransporting polypeptide and clathrin heavy chain significantly reduced the levels of hepatitis B surface antigen. Infection was successfully established using different donor HHs and inocula. Elevation of extracellular HBV DNA levels and the increase of HBV-positive HHs were blocked by continuous hepatitis B immune globulin treatment, indicating virus spread in this model. Chimeric mouse-derived HHs provide a robust in vitro infection model that can completely support the HBV life cycle.

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.

Hepatic hyperplasia associated with discordant xenogeneic parenchymal-nonparenchymal interactions in human hepatocyte-repopulated mice.

Liver mass is optimized in relation to body mass. Rat (r) and human (h) hepatocytes were transplanted into liver-injured immunodeficient mice and allowed to proliferate for 3 or 11 weeks, respectively, when the transplants stopped proliferating. Liver/body weight ratio was normal throughout in r-hepatocyte-bearing mice (r-hep-mice), but increased continuously in h-hepatocyte-bearing mice (h-hep-mice), until reaching approximately three times the normal m-liver size, which was considered to be hyperplasia of h-hepatocytes because there were no significant differences in cell size among host (mouse [m-]) and donor (r- and h-) hepatocytes. Transforming growth factor-beta (TGF-beta) type I receptor, TGF-beta type II receptor, and activin A type IIA receptor mRNAs in proliferating r-hepatocytes of r-hep-mice were lower than in resting r-hepatocytes (normal levels) and increased to normal levels during the termination phase. Concomitantly, m-hepatic stellate cells began to express TGF-beta proteins. In stark contrast, TGF-beta type II receptor and activin A type IIA receptor mRNAs in h-hepatocytes remained low throughout and m-hepatic stellate cells did not express TGF-beta in h-hep-mice. As expected, Smad2 and 3 translocated into nuclei in r-hep-mice but not in h-hep-mice. Histological analysis showed a paucity of m-stellate cells in h-hepatocyte colonies of h-hep-mouse liver. We conclude that m-stellate cells are able to normally interact with concordant r-hepatocytes but not with discordant h-hepatocytes, which seems to be at least partly responsible for the failure of the liver size optimization in h-hep-mice.

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 (Cmax, 12.2 ng/mL), and 10 mg/kg rifampicin (Cmax, 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.

Susceptibility of chimeric mice with livers repopulated by serially subcultured human hepatocytes to hepatitis B virus

We previously identified a small population of replicative hepatocytes in long‐term cultures of human adult parenchymal hepatocytes (PHs) at a frequency of 0.01%‐0.09%. These hepatocytes were able to grow continuously through serial subcultures as colony‐forming parenchymal hepatocytes (CFPHs). In the present study, we generated gene expression profiles for cultured CFPHs and found that they expressed cytokeratin 19, CD90 (Thy‐1), and CD44, but not mature hepatocyte markers such as tryptophan‐2,3‐dioxygenase (TO) and glucose‐6‐phosphatase (G6P), confirming that these cells are hepatic progenitor‐like cells. The cultured CFPHs were resistant to infection with human hepatitis B virus (HBV). To examine the growth and differentiation capacity of the cells in vivo, serially subcultured CFPHs were transplanted into the progeny of a cross between albumin promoter/enhancer‐driven urokinase plasminogen activator‐transgenic mice and severe combined immunodeficient (SCID) mice. The cells were engrafted into the liver and were able to grow for at least 10 weeks, ultimately reaching a maximum occupancy rate of 27%. The CFPHs in the host liver expressed differentiation markers such as TO, G6P, and cytochrome P450 subtypes and could be infected with HBV. CFPH‐chimeric mice with a relatively high replacement rate exhibited viremia and had high serum levels of hepatitis B surface antigen. Conclusion: Serially subcultured human hepatic progenitor‐like cells from postnatal livers successfully repopulated injured livers and exhibited several phenotypes of mature hepatocytes, including susceptibility to HBV. In vitro–expanded CFPHs can be used to characterize the differentiation state of human hepatic progenitor‐like cells. (HEPATOLOGY 2008.)

Chimeric Mice with Hepatocyte-humanized Liver as an Appropriate Model to Study Human Peroxisome Proliferator–activated Receptor-α

Peroxisome proliferator (PP)–activated receptor-α (PPARα) agonists exhibit species-specific effects on livers of the rodent and human (h), which has been considered to reside in the difference of PPARα gene structures. However, the contribution of h-hepatocytes (heps) to the species-specificity remains to be clarified. In this study, the effects of fenofibrate were investigated using a hepatocyte-humanized chimeric mouse (m) model whose livers were replaced with h-heps at >70%. Fenofibrate induced hepatocellular hypertrophy, cell proliferation, and peroxisome proliferation in livers of severe combined immunodeficiency (SCID) mice, but not in the h-hep of chimeric mouse livers. Fenofibrate increased the expression of the enzymes of β- and ω-hydroxylation and deoxygenation of lipids at both gene and protein levels in SCID mouse livers, but not in the h-heps of chimeric mouse livers, supporting the studies with h-PPARα-transgenic mice, a hitherto reliable model for studying the regulation of h-PPARα in the h-liver in most respects, except the induction of the peroxisome proliferation. This study indicates the importance of not only h-PPARα gene but also h-heps themselves to correctly predict effects of fibrates on h-livers, and, therefore, suggests that the chimeric mouse is a currently available, consistent, and reliable model to obtain pharmaceutical data concerning the effects of fibrates on h-livers.

Transmission of HBV DNA Mediated by Ceramide-Triggered Extracellular Vesicles

Background & Aims An extracellular vesicle (EV) is a nanovesicle that shuttles proteins, nucleic acids, and lipids, thereby influencing cell behavior. A recent crop of reports have shown that EVs are involved in infectious biology, influencing host immunity and playing a role in the viral life cycle. In the present work, we investigated the EV-mediated transmission of hepatitis B virus (HBV) infection. Methods We investigated the EV-mediated transmission of HBV infection by using a HBV infectious culture system that uses primary human hepatocytes derived from humanized chimeric mice (PXB-cells). Purified EVs were isolated by ultracentrifugation. To analyze the EVs and virions, we used stimulated emission depletion microscopy. Results Purified EVs from HBV-infected PXB-cells were shown to contain HBV DNA and to be capable of transmitting HBV DNA to naive PXB-cells. These HBV-DNA–transmitting EVs were shown to be generated through a ceramide-triggered EV production pathway. Furthermore, we showed that these HBV-DNA–transmitting EVs were resistant to antibody neutralization; stimulated emission depletion microscopy showed that EVs lacked hepatitis B surface antigen, the target of neutralizing antibodies. Conclusions These findings suggest that EVs harbor a DNA cargo capable of transmitting viral DNA into hepatocytes during HBV infection, representing an additional antibody-neutralization–resistant route of HBV infection.

A long-term culture system based on a collagen vitrigel membrane chamber that supports liver-specific functions of hepatocytes isolated from mice with humanized livers

During drug discovery, in vitro models are used to predict the in vivo pharmacokinetic and toxicological properties of drug candidates in humans. However, the conventional method of culturing human hepatocytes as monolayers does not necessarily replicate biologic reactions and does not support liver-specific functions, such as cytochrome P450 (CYP) activities, for prolonged periods. To remedy these problems and thus increase and prolong hepatic functions, we developed a culture system comprising a collagen vitrigel membrane (CVM) chamber and PXB-cells®, fresh hepatocytes isolated from liver-humanized chimeric mice (PXB-mice®). To quantitatively assess our new system, we evaluated the activities of 5 major CYP isoforms (CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A), albumin secretion, and urea synthesis. First, between Days 14 and 21, the activities of all CYP isoforms tested in vitrigel culture were equal to or higher than in conventional monolayer culture system. Second, the activities of CYP3A, CYP2C9, and CYP2C19 during Days 10 through 17 were higher in vitrigel culture than in suspended PXB-cells prepared on Day 0 (suspension assay). Third, albumin secretion and urea synthesis were higher in vitrigel culture than in conventional monolayer culture. Fourth, the vitrigel-cultured PXB-cells showed the characteristic morphology of parenchymal hepatocytes and were almost all alive in monolayer. These results indicate that our vitrigel culture method is superior to the conventional monolayer method in terms of diverse liver-specific functions, including CYP activity. Our findings suggest that the vitrigel culture method could be a powerful in vitro tool for predicting the pharmacokinetic and toxicological properties of drug candidates in humans.