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.

ABSTRACT:Recently, a chimeric mouse line in which the liver could be replaced by more than 80% with human hepatocytes was established in Japan. Because the chimeric mouse produces human albumin (hAlb), replacement by human hepatocytes could be estimated by the hAlb concentration in the blood of chimeric mice. In this study, we investigated human major cytochrome P450 (P450) in the livers of chimeric mice by mRNA, protein, and enzyme activity using real-time polymerase chain reaction, Western blot analysis, and high-performance liquid chromatography, respectively. Chimeric mice with humanized liver generated using hepatocytes from a Japanese and white donor were used. Human P450 mRNAs were expressed in the liver of chimeric mice, and major human P450 proteins such as CYP1A2, CYP2C9, and CYP3A4 were detected. The expression of P450 mRNA and protein was correlated with the hAlb concentration in the blood. The enzyme activities such as diclofenac 4-hydroxylase activity, dexamethasone 6-hydroxylase activity, and coumarin 7-hydroxylase activity, activities that are specific to human P450 but not to murine P450, were increased in a hAlb concentration-dependent manner. The chimeric mice with nearly 90% replacement by human hepatocytes demonstrated almost the same protein contents of human P450s and drug-metabolizing enzyme activity as those of the donor. It was confirmed that genomic DNA from the livers of the chimeric mice and that from the liver of the donor exhibited the same genotype. In conclusion, the chimeric mice exhibited a similarly efficient capacity of drug metabolism as humans, suggesting that they could be a useful animal model for drug development.

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.

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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.