Fractionation of rat hepatocyte subpopulations with varying metabolic potential, proliferative capacity, and retroviral gene transfer efficiency

Liver cell therapies: cellular sources and grafting strategies

The liver has a complex cellular composition and a remarkable regenerative capacity. The primary cell types in the liver are two parenchymal cell populations, hepatocytes and cholangiocytes, that perform most of the functions of the liver and that are helped through interactions with non-parenchymal cell types comprising stellate cells, endothelia and various hemopoietic cell populations. The regulation of the cells in the liver is mediated by an insoluble complex of proteins and carbohydrates, the extracellular matrix, working synergistically with soluble paracrine and systemic signals. In recent years, with the rapid development of genetic sequencing technologies, research on the liver's cellular composition and its regulatory mechanisms during various conditions has been extensively explored. Meanwhile breakthroughs in strategies for cell transplantation are enabling a future in which there can be a rescue of patients with end-stage liver diseases, offering potential solutions to the chronic shortage of livers and alternatives to liver transplantation. This review will focus on the cellular mechanisms of liver homeostasis and how to select ideal sources of cells to be transplanted to achieve liver regeneration and repair. Recent advances are summarized for promoting the treatment of end-stage liver diseases by forms of cell transplantation that now include grafting strategies.

Hepatocyte Transplantation Rebalances Cytokines for Hepatic Regeneration in Rats with Ataxia Telangiectasia Mutated Pathway-Related Acute Liver Failure

Inadequate DNA damage response related to ataxia telangiectasia mutated gene restricts hepatic regeneration in acute liver failure. Resolving mechanistic gaps in liver damage and repair requires additional animal models that are unconstrained by ultrarapid and unpredictable mortalities or substantial divergences from human pathology. This study used Fischer 344 rats primed with the antitubercular drug, rifampicin, plus phenobarbitone, and monocrotaline, a DNA adduct-forming alkaloid. Rifampicin and monocrotaline can cause liver failure in people. This regimen resulted in hepatic oxidative stress, necrosis, DNA double-strand breaks, liver test abnormalities, altered serum cytokine expression, and mortality. Healthy donor hepatocytes were transplanted ectopically in the peritoneal cavity to study whether they could supply metabolic support and rebalance inflammatory or protective cytokines affecting liver regeneration events. Hepatocyte transplantation increased candidate cytokine levels (granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, interferon-γ, IL-10, and IL-12), leading to Atm, Stat3, and Akt signaling in hepatocytes and nonparenchymal cells, lowering of inflammation, and improvements in intermediary metabolism, DNA repair, and hepatocyte proliferation. Such control of DNA damage and inflammation, along with stimulation of hepatic growth, offers paradigms for cell signaling to restore hepatic homeostasis and regeneration in acute liver failure. Further studies of molecular pathways of high pathobiological impact will advance the knowledge of liver regeneration.

TAZ inhibits glucocorticoid receptor and coordinates hepatic glucose homeostasis in normal physiological states

The elucidation of the mechanisms whereby the liver maintains glucose homeostasis is crucial for the understanding of physiological and pathological states. Here, we show a novel role of hepatic transcriptional co-activator with PDZ-binding motif (TAZ) in the inhibition of glucocorticoid receptor (GR). TAZ is abundantly expressed in pericentral hepatocytes and its expression is markedly reduced by fasting. TAZ interacts via its WW domain with the ligand-binding domain of GR to limit the binding of GR to the GR response element in gluconeogenic gene promoters. Therefore, liver-specific TAZ knockout mice show increases in glucose production and blood glucose concentration. Conversely, the overexpression of TAZ in mouse liver reduces the binding of GR to gluconeogenic gene promoters and glucose production. Thus, our findings demonstrate that hepatic TAZ inhibits GR transactivation of gluconeogenic genes and coordinates gluconeogenesis in response to physiological fasting and feeding.

Spatial Structure of Glycogen Molecules in Cells

Glycogen is a strongly branched polymer of α-D-glucose, with glucose residues in the linear chains linked by 1→4-bonds (~93% of the total number of bonds) and with branching after every 4-8 residues formed by 1→6-glycosidic bonds (~7% of the total number of bonds). It is thought currently that a fully formed glycogen molecule (β-particle) with the self-glycosylating protein glycogenin in the center has a spherical shape with diameter of ~42 nm and contains ~ 55,000 glucose residues. The glycogen molecule also includes numerous proteins involved in its synthesis and degradation, as well as proteins performing a carcass function. However, the type and force of bonds connecting these proteins to the polysaccharide moiety of glycogen are significantly different. This review presents the available data on the spatial structure of the glycogen molecule and its changes under various physiological and pathological conditions.

Glycogen content in hepatocytes is related with their size in normal rat liver but not in cirrhotic one

BACKGROUND & AIMS

Hepatocytes differ from one another by the degree of the ploidy, size, position in the liver lobule, and level of the DNA-synthetic processes. It is believed, that the cell size exerts substantial influence on the metabolism of the hepatocytes and the glycogen content in them. The aim of the present study was to test this hypothesis.

METHODS

Dry weight of hepatocytes, their ploidy and glycogen content were determined in the normal and the cirrhotic rat liver. Liver cirrhosis in rats was produced by chronic inhalation of CCl4 vapours in the course of 6 months. A combined cytophotometric method was used. Dry weight of the cell, its glycogen and DNA content were successively measured on a mapped preparation.

RESULT

Hepatocytes of each ploidy class in the normal and the cirrhotic rat liver accumulated glycogen at the same rate. In the normal liver, there was a distinct correlation between the size of hepatocytes and glycogen content in them. This correlation was observed in each ploidy class, and was especially pronounced in the class of mononucleate tetraploid hepatocytes. In the cirrhotic liver, there was no correlation between the size of the cells and their glycogen content.

CONCLUSIONS

The impairment of liver lobular structure probably explains the observed lack of correlation between hepatocyte size and their glycogen content in the cirrhotic liver. © 2016 International Society for Advancement of Cytometry.

Polyploidization in liver tissue

Polyploidy (alias whole genome amplification) refers to organisms containing more than two basic sets of chromosomes. Polyploidy was first observed in plants more than a century ago, and it is known that such processes occur in many eukaryotes under a variety of circumstances. In mammals, the development of polyploid cells can contribute to tissue differentiation and, therefore, possibly a gain of function; alternately, it can be associated with development of disease, such as cancer. Polyploidy can occur because of cell fusion or abnormal cell division (endoreplication, mitotic slippage, or cytokinesis failure). Polyploidy is a common characteristic of the mammalian liver. Polyploidization occurs mainly during liver development, but also in adults with increasing age or because of cellular stress (eg, surgical resection, toxic exposure, or viral infections). This review will explore the mechanisms that lead to the development of polyploid cells, our current state of understanding of how polyploidization is regulated during liver growth, and its consequence on liver function.

Physiological significance of polyploidization in mammalian cells

Programmed polyploidization occurs in all mammalian species during development and aging in selected tissues, but the biological properties of polyploid cells remain obscure. Spontaneous polyploidization arises during stress and has been observed in a variety of pathological conditions, such as cancer and degenerative diseases. A major challenge in the field is to test the predicted functions of polyploidization in vivo. However, recent genetic mouse models with diminished polyploidization phenotypes represent novel, powerful tools to unravel the biological function of polyploidization. Contrary to a longstanding hypothesis, polyploidization appears to not be required for differentiation and has no obvious impact on proliferation. Instead, polyploidization leads to increased cell size and genetic diversity, which could promote better adaptation to chronic injury or stress. We discuss here the consequences of reducing polyploidization in mice and review which stress responses and molecular signals trigger polyploidization during development and disease.

Construction of liver tissue in vivo with preparative partial hepatic irradiation and growth stimulus: investigations of less invasive techniques and progenitor cells

BACKGROUND

The selective proliferation of transplanted hepatocytes with a growth stimulus, such as partial hepatectomy or hepatocyte growth factor, concomitant with hepatic irradiation (HIR), which can suppress proliferation of host hepatocytes, has been reported. We have conducted experiments that focused on less invasive and clinically applicable techniques and progenitor cells.

MATERIALS AND METHODS

First, dipeptidyl-peptidase IV-F344 or jaundiced Gunn rats underwent partial HIR (only 30% of whole liver) and portal vein branch ligation (PVBL) of one lobe, followed by intrasplenic hepatocyte transplantation at 1 × 10(7). Second, after partial HIR and PVBL, two types of progenitor cells were transplanted (i.e., small hepatocytes (SHs) or adipose-derived mesenchymal stem cells.

RESULTS

Sixteen weeks after transplantation, the donor cells constituted > 70% of the hepatocytes of the irradiated lobe, showing connexin 32, phosphoenolpyruvate carboxykinase-1, and glycogen storage. Moreover, the serum bilirubin level had decreased significantly in the jaundiced Gunn rats and remained at this level throughout the 24 wk experimental period. The SHs grew more quickly than the hepatocytes. After 8 wk, around 40% of the host hepatocytes had been replaced by transplanted SHs. Although the donor adipose-derived mesenchymal cells were engrafted after 8 wk, their proliferation was not observed.

CONCLUSIONS

HIR, combined with PVBL, can be given to a selective liver lobe and is a less-invasive but effective method for proliferation of transplanted hepatocytes. Even a smaller number of SHs can construct liver tissue with their prevailing proliferative ability.

Liver hypertrophy: a review of adaptive (adverse and non-adverse) changes--conclusions from the 3rd International ESTP Expert Workshop

Preclinical toxicity studies have demonstrated that exposure of laboratory animals to liver enzyme inducers during preclinical safety assessment results in a signature of toxicological changes characterized by an increase in liver weight, hepatocellular hypertrophy, cell proliferation, and, frequently in long-term (life-time) studies, hepatocarcinogenesis. Recent advances over the last decade have revealed that for many xenobiotics, these changes may be induced through a common mechanism of action involving activation of the nuclear hormone receptors CAR, PXR, or PPARα. The generation of genetically engineered mice that express altered versions of these nuclear hormone receptors, together with other avenues of investigation, have now demonstrated that sensitivity to many of these effects is rodent-specific. These data are consistent with the available epidemiological and empirical human evidence and lend support to the scientific opinion that these changes have little relevance to man. The ESTP therefore convened an international panel of experts to debate the evidence in order to more clearly define for toxicologic pathologists what is considered adverse in the context of hepatocellular hypertrophy. The results of this workshop concluded that hepatomegaly as a consequence of hepatocellular hypertrophy without histologic or clinical pathology alterations indicative of liver toxicity was considered an adaptive and a non-adverse reaction. This conclusion should normally be reached by an integrative weight of evidence approach.

Intrasplenic transplantation of isolated adult rat hepatocytes: sex-reversal and/or suppression of the major constituent isoforms of cytochrome P450

Adult male and female rat hepatocytes were individually transplanted into the spleens of adult male and female rats. The recipients were euthanized at either eight, sixteen, thirty, or forty-five weeks following transplantation, at which time hepatic and splenic levels of liver-specific rat albumin mRNA as well as sex-dependent transcript levels of CYP2C11, -2C12, -2C7, -2A1, and -3A2-which accounts for > 60% of the total concentration of hepatic constituent cytochrome P450-were determined. Whereas the pre-infused hepatocytes expressed their expected cytochrome P450 sexual dimorphisms (female-specific CYP2C12, male-specific CYP3A2, and female-predominant CYP2A1), their post-transplantational competence now reflected the sexual dimorphisms of the recipient (as observed in the host's liver), which supports the concept that the sex-dependent growth hormone circulating profiles are the determinants regulating the expression levels of hepatic cytochrome P450. Also expressed at normal concentrations in the pre-infused hepatocytes, male-specific CYP2C11 and female-predominant CYP2C7 were inexplicably undetectable in the spleens of both recipient males and females, regardless of the sex of the donor hepatocytes, almost one year after transplantation.

Transcripts of ceruloplasmin but not hepcidin, both major iron metabolism genes, exhibit a decreasing pattern along the portocentral axis of mouse liver

BACKGROUND/AIMS

During iron overload of dietary origin, iron accumulates predominantly in periportal hepatocytes. A gradient in the basal and normal transcriptional control of genes involved in iron metabolism along the portocentral axis of liver lobules could explain this feature. Therefore, we aimed at characterizing, by quantitative RT-PCR, the expression of iron metabolism genes in adult C57BL/6 mouse hepatocytes regarding lobular localisation, with special emphasis to cell ploidy, considering its possible relationship with lobular zonation.

METHODS

We used two methods to analyse separately periportal and perivenous liver cells: 1) a selective liver zonal destruction by digitonin prior to a classical collagenase dissociation, and 2) laser capture microdissection. We also developed a method to separate viable 4N and 8N polyploid hepatocytes by flow cytometer.

RESULTS

Transcripts of ceruloplasmin, involved in iron efflux, were overexpressed in periportal areas and the result was confirmed by in situ hybridization study. By contrast, hepcidin 1, hemojuvelin, ferroportin, transferrin receptor 2, hfe and L-ferritin mRNAs were not differentially expressed according to either lobular zonation or polyploidisation level.

CONCLUSIONS

At variance with glutamine or urea metabolism, iron metabolism is not featured by a metabolic zonation lying only on a basal transcriptional control. The preferential periportal expression of ceruloplasmin raises the issue of its special role in iron overload disorders involving a defect in cellular iron export.

Microarray analysis of gene expression of mouse hepatocytes of different ploidy

Polyploidisation in hepatocytes has been associated with many physiologic and pathologic processes such as proliferation, metabolism, regeneration, aging, and cancer. We studied gene expression patterns in hepatocytes of different ploidy. Primary hepatocytes were obtained from mice of different ages: young (4-6 weeks old), adult (8-10 weeks old), and older (22-24 weeks old). Diploid (2N), tetraploid (4N), and octoploid (8N) hepatocytes were isolated for studies using a high-density mouse genome microarray. No major changes of gene expression patterns between hepatocytes of different ploidy were found. Fifty genes were identified as differentially expressed in the diploid and tetraploid populations, but the changes were less than twofold either way. Four genes (Gas2, Igfbp2, Nr1i3, and Ccne2) were differentially expressed in tetraploid and octoploid cells. This was confirmed in two age groups, "adult" and "older," but once again the factors were less than twofold and the expressions of Gas2 and Igfbp2 were more different between age groups than between ploidy classes. Our results show that polyploid hepatocytes are stable and "normal" without aberrant gene expression, unlike what is thought for cancer cells. By contrast to megakaryocytes, hepatocyte polyploidisation is not a differentiation step associated with major changes in gene expression. Our data support the hypothesis that hepatocyte polyploidisation is a protective mechanism against oxidative stress that occurs via a controlled process throughout growth and aging where binucleation is important.

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.

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.

Xenobiotic stress induces hepatomegaly and liver tumors via the nuclear receptor constitutive androstane receptor

The constitutive androstane receptor (CAR, NR1I3) is a central regulator of xenobiotic metabolism. CAR activation induces hepatic expression of detoxification enzymes and transporters and increases liver size. Here we show that CAR-mediated hepatomegaly is a transient, adaptive response to acute xenobiotic stress. In contrast, chronic CAR activation results in hepatocarcinogenesis. In both acute and chronic xenobiotic responses, hepatocyte DNA replication is increased and apoptosis is decreased. These effects are absent in CAR null mice, which are completely resistant to tumorigenic effects of chronic xenobiotic stress. In the acute response, direct up-regulation of Mdm2 expression by CAR contributes to both increased DNA replication and inhibition of p53-mediated apoptosis. These results demonstrate an essential role for CAR in regulating both liver homeostasis and tumorigenesis in response to xenobiotic stresses, and they also identify a specific molecular mechanism linking chronic environmental stress and tumor formation.

Expanding hepatocytes in vitro before cell transplantation: donor age-dependent proliferative capacity of cultured human hepatocytes

BACKGROUND

For hepatocyte transplantation as well as experimental purposes, it would be advantageous to be able to expand human hepatocytes in vitro. However, under serum-free conditions, even with supplements of HGF (hepatic growth factor) and EGF (epidermal growth factor), proliferation of human hepatocytes is hampered. The aim of this study was to identify differences in the proliferative capacity of cultured primary human hepatocytes related to the age of the liver donors.

METHODS

Proliferation was determined by BrdU-uptake, ploidy was measured using propidium iodide staining and flow cytometry, and the expression of cell cycle related proteins was determined by Western blotting.

RESULTS

During the initial culture, juvenile hepatocytes proliferated better than adult hepatocytes. The proliferation rate declined to barely detectable levels after 8 days in culture in both juvenile and adult hepatocytes. The higher proliferative capacity of juvenile hepatocytes was associated with a larger fraction of diploid cells and a higher viability. The expression of regulatory cell cycle related proteins was higher in juvenile than in adult hepatocytes.

CONCLUSIONS

The proliferation of human hepatocytes in vitro is critically related to a large fraction of diploid hepatocytes. The expression of regulatory cell cycle proteins reflects the proliferative capacity of cultured human hepatocytes. Juvenile as compared to adult human hepatocytes may be better suited for expansion in culture and could have a stronger repopulation capacity in vivo.

Hepatocyte ploidy in regenerating livers after partial hepatectomy, drug-induced necrosis, and cirrhosis

The hepatocyte ploidy was investigated by flow cytometry in regenerating Sprague-Dawley rat livers following either drug-induced acute necrosis (single sublethal doses of D-galactosamine or thioacetamide) or drug-induced chronic cirrhosis (repeated thioacetamide injections for 10-18 weeks) and in regenerating livers following 70% partial hepatectomy and was compared with that of normal hepatocytes. Twenty-four hours after partial hepatectomy, a significant decrease in 2n (1 diploid nucleus) hepatocytes and a significant increase in 8n (1 octoploid nucleus) hepatocytes occurred. In contrast, 24 h following induction of acute hepatic failure by single D-galactosamine or thioacetamide injections, a significant increase in 2n hepatocytes was observed, whereas the proportion of 8n hepatocytes remained unchanged. The liver ploidy returned to basal values within 21 days in all cases. In cirrhotic livers induced by chronic thioacetamide injections, the rate of 2n hepatocytes was about ten times that of the controls having the same age, while 4n (1 tetraploid nucleus) and 8n hepatocytes were one third of controls. The binucleation rate was also significantly decreased.

Cell proliferation and renewal of normal hepatocytes and bile duct cells in adult mouse liver

BACKGROUND/AIMS

The source of new cells in the normal adult liver has been controversial. Some investigators have hypothesized the streaming liver model. On the other hand, others reject this hypothesis. We examined hepatic cell kinetics by a special labeling method with [3H]thymidine.

METHODS

ICR mice received 112 repeated injections of [3H]thymidine at 6 h intervals for 28 days after birth and were killed immediately thereafter, or 100, 200 or 300 days after the last injection. Immediately after killing the animals, samples of the liver were taken and autoradiography was performed.

RESULTS

After continuous labeling, more than 90% of the cells in the liver were labeled. Mean grain counts of hepatocytes decreased to half over approximately 100 days. Those of bile duct cells decreased at a slower rate (50%) than hepatocytes. Mean grain counts of hepatocytes decreased over the regions, although those in perivenular region decreased more rapidly in comparison to those in periportal region.

CONCLUSIONS

The present study indicated that most cells in the liver arise postnatally. The changes in labeling of cells show that there is no special zone for proliferation of hepatocytes and they renew in all regions of the hepatic lobule, suggesting (i) that hepatocytes are supplied by postnatal replication and (ii) streaming of hepatocytes from periportal to pericentral regions does not occur in the adult mouse liver. The bile duct cells renewed more rapidly than hepatocytes.

Polyploidy associated with oxidative injury attenuates proliferative potential of cells

Polyploid cells are encountered ubiquitously but the biological significance of polyploidy is unclear. In view of their extensive capacity for regeneration, hepatocytes offer excellent systems for analyzing growth control mechanisms. We isolated hepatocytes from adult rats with and without two-third partial hepatectomy, which induces hepatic polyploidy. Polyploid hepatocytes showed evidence for oxidative injury with antioxidant depletion, lipid peroxidation and 8-hydroxy-adducts of guanine in nuclear DNA. Liver repopulation assays in intact animals showed markedly decreased replication capacity in polyploid hepatocytes. Recapitulation of polyploidy in cultured hepatocytes established that mitogenic stimulation in the presence of oxidative DNA injury was capable of inducing polyploidy. The findings provide novel frameworks in the context of polyploidy for understanding tissue development, regeneration and oncogenesis.

Akt1/PKB upregulation leads to vascular smooth muscle cell hypertrophy and polyploidization

Vascular smooth muscle cells (VSMCs) at capacitance arteries of hypertensive individuals and animals undergo marked age- and blood pressure-dependent polyploidization and hypertrophy. We show here that VSMCs at capacitance arteries of rat models of hypertension display high levels of Akt1/PKB protein and activity. Gene transfer of Akt1 to VSMCs isolated from a normotensive rat strain was sufficient to abrogate the activity of the mitotic spindle cell-cycle checkpoint, promoting polyploidization and hypertrophy. Furthermore, the hypertrophic agent angiotensin II induced VSMC polyploidization in an Akt1-dependent manner. These results demonstrate that Akt1 regulates ploidy levels in VSMCs and contributes to vascular smooth muscle polyploidization and hypertrophy during hypertension.

Cell transplantation causes loss of gap junctions and activates GGT expression permanently in host liver

Cell transplantation into hepatic sinusoids, which is necessary for liver repopulation, could cause hepatic ischemia. To examine the effects of cell transplantation on host hepatocytes, we transplanted Fisher 344 rat hepatocytes into syngeneic dipeptidyl peptidase IV-deficient rats. Within 24 h of cell transplantation, areas of ischemic necrosis, along with transient disruption of gap junctions, appeared in the liver. Moreover, host hepatocytes expressed gamma-glutamyl transpeptidase (GGT) extensively, which was observed even 2 years after cell transplantation. GGT expression was not associated with alpha-fetoprotein activation, which is present in progenitor cells. Increased GGT expression was apparent after transplantation of nonparenchymal cells and latex beads but not after injection of saline, fragmented hepatocytes, hepatocyte growth factor, or turpentine. Some host hepatocytes exhibited apoptosis, as well as DNA synthesis, between 24 and 48 h after cell transplantation. Changes in gap junctions, GGT expression, DNA synthesis, and apoptosis after cell transplantation were prevented by vasodilators. The findings indicated the onset of ischemic liver injury after cell transplantation. These hepatic perturbations must be considered when transplanted cells are utilized as reporters for biological studies.

Hepatic polyploidy and liver growth control

The onset of cellular polyploidy is recognized in all differentiated mammalian tissues. Polyploidy has been noted frequently in the normal liver, as well as in pathophysiological states of the liver. As insights into the significance of polyploidy accumulate gradually, it is becoming clear that cells belonging to high ploidy classes exhibit advancement toward terminal differentiation and cellular senescence with greater probabilities of apoptosis. Involvement of specific genetic abnormalities, such as impaired DNA repair, may lead to hepatocellular polyploidy. Working models indicate that extensive polyploidy could lead to organ failure, as well as to oncogenesis with activation of precancerous cell clones.

Heterogeneity of growth potential of adult rat hepatocytes in vitro

Nearly pure populations of small hepatocytes (SHs), parenchymal hepatocytes (PHs), and nonparenchymal cells (NPCs) were prepared from the adult rat, and cocultures of hepatocytes and NPCs were reconstituted from them first to obtain the direct evidence that NPCs promote the growth of hepatocytes and second to compare the growth potential between SHs and PHs. SHs and PHs underwent multiple divisions when cocultured with NPCs, whereas neither SHs nor PHs formed colonies at 10 days when cultured alone. Stellate cells in the NPCs were shown to be responsible for this growth promotion. SHs showed a higher growth capacity than PHs. To clearly show the relationship between the growth potential and the size of hepatocytes, SHs and PHs were further fractionated by a fluorescence-activated cell sorter, because the size distribution of SHs and PHs was half overlapped. SHs produced 2 cell populations, SH-R2 and SH-R3. The former showed a greater extent of granularity and autofluorescence than the latter. In contrast, PHs produced only 1 population (PH-R2), which corresponded to the SH-R2. The size of hepatocytes of SH-R3 was smaller (17.1 +/- 0.2 microm) than those of SH-R2 (22.6 +/- 0.5 microm) and PH-R2 (24.1 +/- 0.1 microm) and there was not a significant overlap in the size distribution between the 2 groups. The hepatocytes of SH-R3 were highly replicative and 4 or 5 times higher in their growth potential than those of SH-R2 and PH-R2. We concluded that the growth potential of hepatocytes is heterogeneous and is correlated with their size and the extent of their granularity and autofluorescence.

Partial hepatectomy-induced polyploidy attenuates hepatocyte replication and activates cell aging events

In understanding mechanisms of liver repopulation with transplanted hepatocytes, we studied the consequences of hepatic polyploidization in the two-thirds partial hepatectomy model of liver regeneration. Liver repopulation studies using genetically marked rodent hepatocytes showed that the number of previously transplanted hepatocytes did not increase in the liver with subsequential partial hepatectomy. In contrast, recipients undergoing partial hepatectomy before cells were transplanted showed proliferation in transplanted hepatocytes, with kinetics of DNA synthesis differing in transplanted and host hepatocytes. Also, partial hepatectomy caused multiple changes in the rat liver, including accumulation of polyploid hepatocytes along with prolonged depletion of diploid hepatocytes, as well as increased senescence-associated beta-galactosidase and p21 expression. Remnant hepatocytes in the partially hepatectomized liver showed increased autofluorescence and cytoplasmic complexity on flow cytometry, which are associated with lipofuscin accumulation during cell aging, and underwent apoptosis more frequently. Moreover, hepatocytes from the partially hepatectomized liver showed attenuated proliferative capacity in cell culture. These findings were compatible with decreased proliferative potential of hepatocytes experiencing partial hepatectomy compared with hepatocytes from the unperturbed liver. Attenuation of proliferative capacity and other changes in hepatocytes experiencing partial hepatectomy offer novel perspectives concerning liver regeneration in the context of cell ploidy.

Position-specific gene expression in the liver lobule is directed by the microenvironment and not by the previous cell differentiation state

Mechanisms directing position-specific liver gene regulation are incompletely understood. To establish whether this aspect of hepatic gene expression is an inveterate phenomenon, we used transplanted hepatocytes as reporters in dipeptidyl peptidase IV-deficient F344 rats. After integration in liver parenchyma, the position of transplanted cells was shifted from periportal to perivenous areas by targeted hepatic ablations with carbon tetrachloride. In controls, transplanted cells showed greater glucose-6-phosphatase and lesser glycogen content in periportal areas. This pattern was reversed when transplanted cells shifted from periportal to perivenous areas. Transplanted hepatocytes in perivenous areas exhibited inducible cytochrome P450 activity, which was deficient in periportal hepatocytes. Moreover, cytochrome P450 activity was rapidly extinguished in activated hepatocytes when these cells were transplanted into the nonpermissive liver of suckling rat pups. In cells isolated from the normal F344 rat liver, cytochrome P450 inducibility was originally greater in perivenous hepatocytes; however, periportal cells rapidly acquired this facility in culture conditions. These findings indicate that the liver microenvironment exerts supremacy over prior differentiation state of cells in directing position-specific gene expression. Therefore, persistence of specialized hepatocellular function will require interactions with regulatory signals and substrate availability, which bears upon further analysis of liver gene regulation, including in progenitor and/or stem cells.