Mitogenic effects of hepatic stimulator substance on cultured nonparenchymal liver epithelial cells

Hepatocyte Transplantation: An Alternative System for Evaluating Cell Survival and Immunoisolation

To evaluate systems for barrier immunoisolation of transplanted hepatocytes, we used transgenic mouse hepatocytes that secrete HBsAg. Hepatocytes were rapidly encapsulated in chitosan, a cationic polymer derived by deacetylation of chitin. Chitosan was allowed to electrostatically bond with anionic sodium alginate for creating an outer bipolymer membrane of the capsules. After encapsulation, hepatocyte viability remained unchanged for seven days in vitro with secretion of HBsAg into the culture medium throughout this period. Following intraperitoneal transplantation of encapsulated hepatocytes, HBsAg promptly appeared in blood of recipients. In congeneic recipients, serum HBsAg peaked at two weeks. Hepatocytes were present in recovered chitosan capsules and expressed HBsAg mRNA. In allogeneic recipients, however, serum HBsAg disappeared within one week and recovered chitosan capsules showed lymphomononuclear cells but not hepatocytes. Transplantation of chitosan encapsulatd HbsAg secreting hepatocytes failed to induce an anti-HBs response, suggesting modulation of the host immune response. These results indicate that transplantation systems using genetically modified hepatocytes which secrete gene products in the blood of recipients should facilitate evaluation of hepatocyte encapsulation.

BMP-4 induced proliferation and oriented differentiation of rat hepatic oval cells into hepatocytes

OBJECTIVE

To explore the role of bone morphogenetic protein 4 (BMP-4) in hepatic progenitor cells (HPCs).

METHODS

The effect of BMP-4 on rat hepatic oval cells was examined by using the WB-F344 rat hepatocytic epithelial stem-cell-like cell line. This hepatocytic cell line could exert various hepatocyte functions including the secretion of albumin and urea. Immunohistochemistry was used to examine the effects of BMP-4 and its antagonist, Noggin, on the proliferation and differentiation of these cells, cellular uptake and excretion of indocyanine green, the periodic acid-schiff (PAS) assay for glycogen storage and the expression of hepatic markers.

RESULTS

Our results showed for the first time that BMP-4 may acted as a potential inducer of hepatic differentiation in rat hepatic oval cells.

CONCLUSIONS

This cell source offers a much-needed attractive and expandable source for future investigations of drug screening, stem cell technologies and cellular transplantation, in a society with increasing levels of liver disease and damage.

Hepatic differentiation from human mesenchymal stem cells on a novel nanofiber scaffold

The emerging fields of tissue engineering and biomaterials have begun to provide potential treatment options for liver failure. The goal of the present study is to investigate the ability of a poly L-lactic acid (PLLA) nanofiber scaffold to support and enhance hepatic differentiation of human bone marrow-derived mesenchymal stem cells (hMSCs). A scaffold composed of poly L-lactic acid and collagen was fabricated by the electrospinning technique. After characterizing isolated hMSCs, they were seeded onto PLLA nanofiber scaffolds and induced to differentiate into a hepatocyte lineage. The mRNA levels and protein expression of several important hepatic genes were determined using RT-PCR, immunocytochemistry and ELISA. Flow cytometry revealed that the isolated bone marrow-derived stem cells were positive for hMSC-specific markers CD73, CD44, CD105 and CD166 and negative for hematopoietic markers CD34 and CD45. The differentiation of these stem cells into adipocytes and osteoblasts demonstrated their multipotency. Scanning electron microscopy showed adherence of cells in the nanofiber scaffold during differentiation towards hepatocytes. Our results showed that expression levels of liver-specific markers such as albumin, α-fetoprotein, and cytokeratins 8 and 18 were higher in differentiated cells on the nanofibers than when cultured on plates. Importantly, liver functioning serum proteins, albumin and α-1 antitrypsin were secreted into the culture medium at higher levels by the differentiated cells on the nanofibers than on the plates, demonstrating that our nanofibrous scaffolds promoted and enhanced hepatic differentiation under our culture conditions. Our results show that the engineered PLLA nanofibrous scaffold is a conducive matrix for the differentiation of MSCs into functional hepatocyte-like cells. This represents the first step for the use of this nanofibrous scaffold for culture and differentiation of stem cells that may be employed for tissue engineering and cell-based therapy applications.

Bottom-up signaling from HGF-containing surfaces promotes hepatic differentiation of mesenchymal stem cells

The capacity of stem cells to differentiate into specific cell types makes them very promising in tissue regeneration and repair. However, realizing this promise requires novel methods for guiding lineage-specific differentiation of stem cells. In this study, hepatocyte growth factor (HGF), an important morphogen in liver development, was co-printed with collagen I (Col) to create arrays of protein spots on glass. Human adipose stem cells (ASCs) were cultured on top of the HGF/Col spots for 2weeks. The effects of surface-immobilized HGF on hepatic differentiation of ASCs were analyzed using RT-PCR, ELISA and immunocytochemistry. Stimulation of stem cells with HGF from the bottom-up caused an upregulation in synthesis of α-fetoprotein and albumin, as determined by immunocytochemistry and ELISA. RT-PCR results showed that the mRNA levels for albumin, α-fetoprotein and α1-antitrypsin were 10- to 20-fold higher in stem cells cultured on the HGF/Col arrays compared to stem cells on Col only spots. Our results show that surfaces containing HGF co-printed with ECM proteins may be used to differentiate mesenchymal stem cells such as ASCs into hepatocyte-like cells. These results underscore the utility of growth factor-containing culture surfaces for stem cell differentiation.

Differentiation and characterization of metabolically functioning hepatocytes from human embryonic stem cells

Human embryonic stem cells (hESCs) may provide a cell source for functional hepatocytes for clinical applications and drug development. Initially, the hESC population was enriched to be more than 85% definitive endoderm (DE) as assessed by the expression of CXCR4, SOX17, and FOXA2. We then successfully converted DE into hepatic progenitors with 93% of the cells being positive for alpha-feto protein within 9 days. The percentage of albumin positive cells gradually increased to 90% at days 20-22 after differentiation. Moreover, our hESC-derived hepatocytes (hEH) developed a complete biotransformation system including phase I and II metabolizing enyzmes and phase III transporters. Nuclear receptors, which are critical in regulating the expression of metabolizing enzymes, were also expressed by our hEH. Using ultraperformance liquid chromatography-tandem mass spectrometry technology, we identified seven metabolic pathways of the drug bufuralol including four newly-reported ones in our hEH, which are the same as those in freshly isolated human primary hepatocytes (hPH). In addition, the results of the metabolism of four drugs indicate that our hEH have the capacity to metabolize these drugs at levels that are comparable to hPH. In conclusion, we have generated a relatively homogenous population of hepatocytes from hESCs, which appear to have complete metabolic function that is comparable to primary liver cells. These results represent a significant step towards the efficient differentiation of mature hepatocytes for cell-based therapeutics as well as for pharmacology and toxicology studies.

Differentiation and enrichment of hepatocyte-like cells from human embryonic stem cells in vitro and in vivo

Human embryonic stem cells (hESC) may provide a cell source for functional hepatocytes. The aim of this study is to establish a viable human hepatocyte-like cell line from hESC that can be used for cell-based therapies. The differentiated hESC were enriched by transducing with a lentivirus vector containing the green fluorescent protein (GFP) gene driven by the alpha1-antitrypsin promoter; the GFP gene is expressed in committed hepatocyte progenitors and hepatocytes. GFP+ hESC were purified by laser microdissection and pressure catapulting. In addition, differentiated hESC that were transduced with a lentivirus triple-fusion vector were transplanted into NOD-SCID mice, and the luciferase-induced bioluminescence in the livers was evaluated by a charge-coupled device camera. GFP+ hESC expressed a large series of liver-specific genes, and expression levels of these genes were significantly improved by purifying GFP+ hESC; our results demonstrated that purified differentiated hESC express nearly physiological levels of liver-specific genes and have liver-specific functions that are comparable to those of primary human hepatocytes. The differentiated hESC survived and engrafted in mouse livers, and human liver-specific mRNA and protein species were detected in the transplanted mouse liver and serum at 3 weeks after transplantation. This is the first time that human albumin generated by hESC-derived hepatocytes was detected in the serum of an animal model. This also represents the first successful transplantation of differentiated hESC in an animal liver and the first bioluminescence imaging of hESC in the liver. This study is an initial step in establishing a viable hepatocyte-like cell line from hESC. Disclosure of potential conflicts of interest is found at the end of this article.

Immunosuppression using the mTOR inhibition mechanism affects replacement of rat liver with transplanted cells

Successful grafting of tissues or cells from mismatched donors requires systemic immunosuppression. It is yet to be determined whether immunosuppressive manipulations perturb transplanted cell engraftment or proliferation. We used syngeneic and allogeneic cell transplantation assays based on F344 recipient rats lacking dipeptidyl peptidase IV enzyme activity to identify transplanted hepatocytes. Immunosuppressive drugs used were tacrolimus (a calcineurin inhibitor) and its synergistic partners, rapamycin (a regulator of the mammalian target of rapamycin [mTOR]) and mycophenolate mofetil (an inosine monophosphate dehydrogenase inhibitor). First, suitable drug doses capable of inducing long-term survival of allografted hepatocytes were identified. In pharmacologically effective doses, rapamycin enhanced cell engraftment by downregulating hepatic expression of selected inflammatory cytokines but profoundly impaired proliferation of transplanted cells, which was necessary for liver repopulation. In contrast, tacrolimus and/or mycophenolate mofetil perturbed neither transplanted cell engraftment nor their proliferation. Therefore, mTOR-dependent extracellular and intracellular mechanisms affected liver replacement with transplanted cells. In conclusion, insights into the biological effects of specific drugs on transplanted cells are critical in identifying suitable immunosuppressive strategies for cell therapy.

Effects of shark hepatic stimulator substance on the function and antioxidant capacity of liver mitochondria in an animal model of acute liver injury

This study was carried out to investigate whether shark hepatic stimulator substance (HSS) can prevent acute liver injury and affect mitochondrial function and antioxidant defenses in a rat model of thioacetamide (TAA)-induced liver injury. The acute liver injury was induced by two intraperitoneal injections of TAA (400 mg/kg) in a 24 h interval. In the TAA plus shark HSS group, rats were treated with shark HSS (80 mg/kg) 1 h prior to each TAA injection. In this group, serum liver enzyme activities were significantly lower than those in the TAA group. The mitochondrial respiratory control ratio was improved, and the mitochondrial respiratory enzyme activities were increased in the TAA plus shark HSS group. The mitochondrial antioxidant enzyme activities and glutathione level were higher in the TAA plus shark HSS group than in the TAA group. These results suggest that the protective effect of shark HSS against TAA-induced acute liver injury may be a result of the restoration of the mitochondrial respiratory function and antioxidant defenses and decreased oxygen stress.

The hepatoprotective effect of hepatic stimulator substance (HSS) against liver regeneration arrest induced by acute ethanol intoxication

Male Wistar rats were randomized to receive ethanol (2.5 ml/kg by gastric intubation every 8 hr; group I), equal volumes of isocaloric to ethanol sucrose solution (group II), or ethanol and HSS (100 mg/kg intraperitoneally 10 and 16 hr after partial hepatectomy; groups III and IV, respectively) for up to 96 hr after partial hepatectomy, with ethanol administration starting 1 hr prior to partial hepatectomy. Animals were killed at 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 60, and 96 hr after partial hepatectomy. The rate of liver regeneration was evaluated by the mitotic index in H&E-stained sections, immunochemical detection of Ki67 nuclear antigen, rate of [3H]thymidine incorporation into hepatic DNA, and liver thymidine kinase enzymatic activity. The biological activity of HSS in groups I and II rats was evaluated using a bioassay. Ethanol administration arrested liver regeneration during the first 32 hr after partial hepatectomy and suppressed HSS activity throughout the period examined. Liver regeneration progressed after 32 hr despite the low levels of HSS activity. HSS administration at 10 and 16 hr reversed liver regeneration arrest induced by ethanol. Acute ethanol administration induces cell cycle arrest during the first 32 hr after partial hepatectomy and suppression of HSS biological activity seems to contribute to this effect. HSS administration reversed the inhibitory effect of ethanol on liver regeneration and caused synchronized entrance of hepatocytes in the S phase of the cell cycle. HSS seems to participate in the network of growth factors controlling the G1/S cell cycle checkpoint.

Analysis of the functional integrity of cryopreserved human liver cells including xenografting in immunodeficient mice to address suitability for clinical applications

BACKGROUND

The availability of well-characterized human liver cell populations that can be frozen and thawed will be critical for cell therapy. We addressed whether human hepatocytes can recover after cryopreservation and engraft in immunodeficient mice.

METHODS

We isolated cells from discarded human livers and studied the properties of cryopreserved cells. The viability of thawed cells was established with multiple in vitro assays, including analysis of liver gene expression, ureagenesis, cytochrome P450 activity, and growth factor-induced cell proliferation. The fate of transplanted cells was analysed in immunodeficient NOD-SCID mice.

RESULTS

After thawing, the viability of human hepatocytes exceeded 60%. Cells attached to culture dishes, proliferated following growth factor stimulation and exhibited liver-specific functions. After transplantation in NOD-SCID mice, cells engrafted in the peritoneal cavity, a heterologous site, as well as the liver itself, retained hepatic function and proliferated in response to liver injury. Transplanted hepatocytes were integrated in the liver parenchyma. Occasionally, transplanted cells were integrated in bile ducts.

CONCLUSIONS

Cryopreserved human liver cell showed the ability to retain functional integrity and to reconstitute both hepatic and biliary lineages in mice. These studies offer suitable paradigms aimed at characterizing liver cells prior to transplantation in people.

Isolation of human progenitor liver epithelial cells with extensive replication capacity and differentiation into mature hepatocytes

The liver can regenerate itself through the progenitor cells it harbors. Here we demonstrate isolation of epithelial progenitor/stem cells from the fetal human liver, which contains a large number of hepatoblasts. Progenitor liver cells displayed clonogenic capacity, expressed genes observed in hepatocytes, bile duct cells and oval cells, and incorporated genes transferred by adenoviral or lentiviral vectors. Under culture conditions,progenitor cells proliferated for several months, with each cell undergoing more than forty divisions, but they retained normal karyotypes. Progenitor cells differentiated into mature hepatocytes in mice with severe combined immunodeficiency, both when in an ectopic location and when in the liver itself. Cells integrated in the liver parenchyma and proliferated following liver injury. An abundance of progenitor cells in the fetal human liver is consistent with models indicating depletion of progenitor/stem cells during aging and maturation of organs. The studies indicate that isolation of progenitor cells from fetal organs will be appropriate for establishing novel systems to investigate basic mechanisms and for cell and gene therapy.

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.

KATP channels regulate mitogenically induced proliferation in primary rat hepatocytes and human liver cell lines. Implications for liver growth control and potential therapeutic targeting

To determine whether K(ATP) channels control liver growth, we used primary rat hepatocytes and several human cancer cell lines for assays. K(ATP) channel openers (minoxidil, cromakalim, and pinacidil) increased cellular DNA synthesis, whereas K(ATP) channel blockers (quinidine and glibenclamide) attenuated DNA synthesis. The channel inhibitor glibenclamide decreased the clonogenicity of HepG2 cells without inducing cytotoxicity or apoptosis. To demonstrate the specificity of drugs for K(+) channels, whole-cell patch-clamp recordings were made. Hepatocytes revealed K(+) currents with K(ATP) channel properties. These K(+) currents were augmented by minoxidil and pinacidil and attenuated by glibenclamide as well as tetraethylammonium, in agreement with established responses of K(ATP) channels. Reverse transcription of total cellular RNA followed by polymerase chain reaction showed expression of K(ATP) channel-specific subunits in rat hepatocytes and human liver cell lines. Calcium fluxes were unperturbed in glibenclamide-treated HepG2 cells and primary rat hepatocytes following induction with ATP and hepatocyte growth factor, respectively, suggesting that the effect of K(ATP) channel activity upon hepatocyte proliferation was not simply due to indirect modulation of intracellular calcium. The regulation of mitogen-related hepatocyte proliferation by K(ATP) channels advances our insights into liver growth control. The findings have implications in mechanisms concerning liver development, regeneration, and oncogenesis.

Hepatocyte transplantation improves survival in mice with liver toxicity induced by hepatic overexpression of Mad1 transcription factor

Hepatic overexpression of Mad1 with an adenoviral vector, AdMad, induced liver toxicity in immunodeficient mice. Transduction of cultured hepatocytes with AdMad inhibited cellular DNA synthesis and cell cycling, along with increased lactate dehydrogenase release, indicating cytotoxicity. When dipeptidyl peptidase IV-deficient F344 rat hepatocytes were transplanted into the liver of immunodeficient mice after treatment with AdMad, significant portions of the liver were repopulated. This was in agreement with corresponding losses of host hepatocytes, which showed increased apoptosis rates. Mortality in mice following AdMad treatment decreased significantly when animals were subjected to hepatocyte transplantation. The findings indicated that Mad1 overexpression perturbed hepatocyte survival. Investigation of pathophysiological mechanisms concerning specific cell cycle regulators in acute liver toxicity will thus be appropriate. Cell therapy has potential for treating acute liver injury under suitable circumstances.

Overexpression of Mad transcription factor inhibits proliferation of cultured human hepatocellular carcinoma cells along with tumor formation in immunodeficient animals

BACKGROUND

Dominant negative regulation of critical cell cycle molecules could perturb survival of cancer cells and help develop novel therapies.

METHODS

To perturb the activity of c-Myc, which regulates G0/G1 transitions, we overexpressed Mad1 protein with an adenoviral vector, AdMad. Studies were conducted with established cell lines, including HepG2, HuH-7 and PLC/PRF/5 liver cancer cells, RAT-1A embryonic fibroblasts and U373MG astrocytoma cells.

RESULTS

After AdMad-treatment, transduced cells exhibited decreased proliferation rates in culture conditions. RAT-1A embryonic fibroblasts and U373MG astrocytoma cells showed accumulations in G0/G1, whereas HepG2 and HuH-7 cells accumulated in G0/G1, and additionally in G2/M, albeit to a lesser extent. An in vitro assay using hepatocyte growth factor to stimulate proliferation in HuH-7 cells showed blunting of growth factor responsiveness, along with inhibition of cell cycle progression in AdMad-treated cells. No cytotoxicity was observed in AdMad-treated cells in culture, although cells lost clonogenic capacity in soft agar. In vivo assays using HepG2 cell tumors in immunodeficient mice showed that overexpression of AdMad prevented tumorigenesis.

CONCLUSIONS

These studies indicate roles of Mad in G2/M, as well as the potential of manipulating cell cycle controls for treating liver cancer.

Butyrate inhibits and Escherichia coli-derived mitogen(s) stimulate DNA synthesis in human hepatocytes in vitro

Bacterial constituents and products of the bacterial metabolism pass from the gut lumen to the portal vein and may influence the homeostasis of the liver. Our aim is to examine whether DNA synthesis of human hepatocyte cell lines is affected by constituents of Escherichia coli species as well as by intracolonic products of bacterial fermentation that reach the liver via the portal vein. Supernatant solutions and bacterial cell fractions (containing either whole dead bacteria, cell walls, cytosol or non-soluble intracellular components) of E. coli K12 and of E. coli species from rat fecal flora were separated by multi-step centrifugation, French press, and microfiltration. The supernatant solution and the cell fractions were incubated with a human hepatoma cell line (Hep-G2) and with a cell line derived from non-malignant human liver cells (Chang cells) for 24 h. The cells were labeled with tritiated thymidine before processing to autoradiography. DNA synthesis was estimated by the labeling index (LI%). DNA synthesis was also estimated following incubation of Hep-G2 cells with short chain fatty acids (acetic, propionic, butyric and succinic acid), acetaldehyde, and ammonium chloride. Epidermal growth factor and a water extract of Helicobacter pylori were used as references. The fractions of E. coli from rat fecal flora containing cytosol and non-soluble intracellular components significantly increased the labeling index in both Hep-G2 and Chang cells (p < 0.05). In addition, the supernatant solution significantly increased the LI in Chang cells (p < 0.05). Epidermal growth factor increased the LI of Hep-G2 cells dose-dependently (p < 0.05). Butyric acid reduced DNA synthesis at 10(-4) M (p < 0.05). The highest doses of acetaldehyde were cytotoxic and reduced the LI. Escherichia coli species contain mitogenic factors to human hepatocytes. The mitogen(s) are present in the supernatant solution, in the cytosol and in non-soluble intracellular components. Butyrate, which is a product of bacterial fermentation of colonic substrates inhibit DNA synthesis in the hepatocyte cell lines. Our findings suggest that soluble mitogen(s) that diffuse from the microorganism to the outer environment, intracellular bacterial constituents, and products of the bacterial metabolism that reach the liver via the portal vein may influence the cell kinetic steady-state of hepatic cells.

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.

Transplanted hepatocytes proliferate differently after CCl4 treatment and hepatocyte growth factor infusion

To understand regulation of transplanted hepatocyte proliferation in the normal liver, we used genetically marked rat or mouse cells. Hosts were subjected to liver injury by carbon tetrachloride (CCl4), to liver regeneration by a two-thirds partial hepatectomy, and to hepatocellular DNA synthesis by infusion of hepatocyte growth factor for comparative analysis. Transplanted hepatocytes were documented to integrate in periportal areas of the liver. In response to CCl4 treatments after cell transplantation, the transplanted hepatocyte mass increased incrementally, with the kinetics and magnitude of DNA synthesis being similar to those of host hepatocytes. In contrast, when cells were transplanted 24 h after CCl4 administration, transplanted hepatocytes appeared to be injured and most cells were rapidly cleared. When hepatocyte growth factor was infused into the portal circulation either subsequent to or before cell transplantation and engraftment, transplanted cell mass did not increase, although DNA synthesis rates increased in cultured primary hepatocytes as well as in intact mouse and rat livers. These data suggested that procedures causing selective ablation of host hepatocytes will be most effective in inducing transplanted cell proliferation in the normal liver. The number of transplanted hepatocytes was not increased in the liver by hepatocyte growth factor administration. Repopulation of the liver with genetically marked hepatocytes can provide effective reporters for studying liver growth control in the intact animal.

Paclitaxel shows cytotoxic activity in human hepatocellular carcinoma cell lines

Paclitaxel stabilizes microtubules with inhibition of mitotic spindle formation and has been found effective in several solid cancers. To test whether paclitaxel could be cytotoxic in human HCC cell lines, we used established HuH-7 and HepG2 cell lines. Changes in cell number, DNA synthesis rates and cell viability were determined. We tested whether paclitaxel-treated cells underwent apoptosis, microtubular reorganization, and cell cycle restriction. Studies also examined whether chemosensitization with verapamil enhanced the antitumor activity of paclitaxel. The cell viability was impaired at greater than 0.01 microM paclitaxel concentrations (LD50, 0.8 microM), with flow cytometry indicating accumulation of cells in G2/M, and immunostaining showing polymerized microtubules with characteristic banding patterns. This G2/M restriction was further characterized by flow cytometry, which revealed cyclin A and cdc2 kinase accumulation in paclitaxel-treated cells. Exposure to paclitaxel decreased [3H]thymidine incorporation into DNA in cells at 24 h but this significantly increased at 72 h, most likely due to DNA repair mechanisms related to cell cycle restriction. The cell death was via both apoptotic and non-apoptotic mechanisms. Finally, co-administration of the chemosensitizer verapamil in doses as little as 1 microM increased the antitumor efficacy of paclitaxel by up to five-fold and changed the LD50 of paclitaxel to 0.1 microM. The findings indicate that paclitaxel is cytotoxic to cultured hepatocellular carcinoma cells. Clinical studies of paclitaxel in patients with hepatocellular carcinoma may help determine additional therapies.

Differentiation-specific regulation of transgene expression in a diploid epithelial cell line derived from the normal F344 rat liver

To establish the differentiation potential of progenitor cells, non-parenchymal epithelial cells from the F344 rat liver (FNRL cells) were studied. These cells reacted with the OV-6 antibody marker of oval cells, but were negative for hepatocyte markers (albumin, transferrin, glycogen, glucose-6-phosphatase, H4 antigen), biliary markers (gamma glutamyl transpeptidase, cytokeratin-19), and alpha-fetoprotein, although exposure to sodium butyrate induced nascent albumin and alpha-fetoprotein mRNA transcription. When stably transduced, FNRL cells expressed a retroviral promotor-driven lacZ reporter in vitro, similar to transgene expression in hepatocyte-derived HepG2 cells. However, lacZ expression in FNRL cells was rapidly extinguished in intact animals, whereas the reporter remained active in HepG2 cells. Transplanted FNRL cells showed copious glucose-6-phosphatase expression; however, the cell differentiation programme remained incomplete, despite two-thirds partial hepatectomy, D-galactosamine treatment or bile duct ligation. Interestingly, lacZ expression resumed in cultures of FNRL cells explanted from recipients. Moreover, lacZ expression was down-regulated by gamma-interferon in FNRL cells, without affecting lacZ activity in HepG2 cells. The data indicate that although subpopulations of oval cells may not fully differentiate into mature hepatocytes, these cells might serve critical functions, such as glucose utilization, and help survival after liver injury. Also, introduced genes may be regulated in progenitor cells at multiple levels, including by interactions between regulatory sequences, differentiation-specific cellular factors, and extracellular signals; in vivo studies are thus especially important for analysing gene regulation in progenitor cells.

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

The liver contains hepatocytes with varying ploidy and gene expression. To isolate cells on the basis of ploidy for analyzing mechanisms concerning cell proliferation and differentiation, we used Percoll gradients to separate F344 rat hepatocyte subpopulations. Specific fractions were enriched in polyploid (H2 fraction) or diploid (H3 and H4 fractions) hepatocytes containing glycogen and glucose-6-phosphatase. H4 cells were relatively smaller with greater nuclear/cytoplasmic ratios, less complex cytoplasm, and higher serum albumin or ceruloplasmin biosynthetic rates. H2 fraction cells were larger with lesser nuclear/cytoplasmic ratio, more complex cytoplasm, and more cytochrome P450 activity. Phenotypic marking showed that H4 cells originated in zone one and H2 cells in zones two or three of the liver lobule. H4 cells showed much greater mitogenic responsiveness to human hepatocyte growth factor. Retroviral gene transfer, which requires both viral receptors and cellular DNA synthesis, was significantly more efficient in H4 cells. The findings indicated that small diploid and large polyploid hepatocytes show unique biological differences. The ability to isolate hepatocytes of varying maturity is relevant for mechanisms concerning liver growth control and hepatic gene expression.

Simultaneous up-regulation of viral receptor expression and DNA synthesis is required for increasing efficiency of retroviral hepatic gene transfer

To understand the relative contribution of viral receptor expression and cell proliferation in retroviral gene transfer, we created human hepatocyte-derived HuH-7.MCAT-1 cell lines. These cells constitutively express the murine ecotropic retroviral receptor MCAT-1 without changes in morphology or proliferation states. The MCAT-1 receptor is also a cationic amino acid transporter, and the HuH-7.MCAT-1.7 cells showed increased Vmax of uptake and steady-state accumulation of the cationic amino acids L-arginine and L-lysine. In HuH-7.MCAT-1 cells, L-arginine uptake was significantly up-regulated by norepinephrine and dexamethasone, and hepatocyte growth factor also increased L-arginine uptake along with cellular DNA synthesis. Gene transfer was also markedly increased in HuH-7. MCAT-1.7 cells incubated with an ecotropic LacZ retrovirus, and this further increased with hormones and hepatocyte growth factor. To define whether viral receptor up-regulation by itself increased gene transfer, cell cycling was inhibited by a recombinant adenovirus expressing the Mad transcription factor (AdMad), which is a dominant-negative c-Myc regulator. This restricted cells in G0/G1, without attenuating MCAT-1 activity, as shown by flow cytometry and L-arginine uptake analysis, respectively. When asynchronously cycling HuH-7.MCAT-1.7 cells were first infected with the AdMad virus and then exposed to the ecotropic LacZ virus, gene transfer was virtually abolished. The data indicate that while up-regulation of viral receptors can greatly enhance retrovirally mediated gene transfer, DNA synthesis remains an absolute requirement for hepatic gene therapy with this approach.

Cell type-specific mechanisms regulate hepatitis B virus transgene expression in liver and other organs

Intracellular expression of hepatitis B virus (HBV) was analysed in transgenic HBV mouse lines designated G7 and G26, the former lacking hepatitis B surface antigen (HBsAg) promoters. HBsAg mRNA expression was greater in the G26 line than in the G7 line, although in situ hybridization showed a qualitatively similar expression pattern in specific cell types. HBsAg mRNA was most abundant in hepatocytes, followed in magnitude by proximal renal tubular epithelial cells, pancreatic acinar cells, and epithelial cells of the gastric, small intestinal, and bronchiolar mucosae. In biliary epithelial cells, brain, spleen, large intestine, testis, heart, and skeletal muscle, HBsAg mRNA was undetectable. In cell transfection assays, the HBV enhancer/preS1 promoter efficiently expressed a luciferase reporter with appropriate upregulation by HNF-3 alpha and C/EBP alpha transcription factors in hepatocyte-derived cells but not in non-parenchymal epithelial liver cells or fibroblasts. These results suggest that cell-type specificity of HBV expression is regulated by interactions between viral elements and cellular transactivators. Variable expression of G7 and G26 HBV transgenes in epithelial cells combined with differences in transgene expression in similar sets of cells suggests at least two levels of regulation: one directing cell specificity of HBV expression and the other governing quantitative expression of HBV mRNA.

Integration of transplanted hepatocytes into host liver plates demonstrated with dipeptidyl peptidase IV-deficient rats

To analyze mechanisms of liver repopulation, we transplanted normal hepatocytes into syngeneic rats deficient in dipeptidyl peptidase IV activity. When isolated hepatocytes were injected into splenic pulp, cells promptly migrated into hepatic sinusoids. To examine whether transplanted hepatocytes entered liver plates and integrated with host hepatocytes, we analyzed sharing of hepatocyte-specific gap junctions and bile canaliculi. Colocalization studies showed gap junctions uniting adjacent transplanted and host hepatocytes in liver plates. Visualization of bile canalicular domains in transplanted and host hepatocytes with dipeptidyl peptidase IV and ATPase activities, respectively, demonstrated hybrid bile canaliculi, which excreted a fluorescent conjugated bile acid analogue. These results indicate that transplanted hepatocytes swiftly overcome mechanical barriers in hepatic sinusoids to enter liver plates and join host cells. Integration into liver parenchyma should physiologically regulate the function or disposition of transplanted hepatocytes and benefit applications such as gene therapy.

Studies on the biological characterization and mitogenic interactions between hepatic stimulator substance and acidic fibroblast growth factor

During liver regeneration, hepatic stimulator substance (HSS) and acidic fibroblast growth factor (FGF-1) are produced in the liver. These growth factors may be involved in liver growth control but an understanding of their regulatory interactions is limited. To further characterize the mitogenic activity of HSS, we compared its effects with FGF-1 in cells of hepatocyte, non-parenchymal liver epithelial and non-hepatic lineages. Our studies with these cell types demonstrated differences in the mitogenic specificities of HSS and FGF-1. Whereas exposure of primary hepatocytes to epidermal growth factor and HSS synergistically increased DNA synthesis, simultaneous exposure to HSS and FGF-1 resulted in no such effect. Receptor-binding assays showed that HSS did not compete with FGF-1 in binding to FGF-1 receptors on rat primary hepatocytes. Additional immunoblot analysis demonstrated no cross-reactivity between FGF-1 antibodies and HSS. Distinct mitogenic and immunologic properties of HSS and FGF-1 should facilitate further analysis of liver regeneration and hepatic oncogenesis.

Butyrate synchronization of hepatocytes: modulation of cycling and cell cycle regulated gene expression

To develop a model for studies of liver growth control, we characterized cell cycle synchronization of liver-derived cells with sodium butyrate. Exposure of cultured HTC (rat hepatoma) cells to 5 mM butyrate arrested cell growth in a reversible manner. Flow cytometric analysis revealed that butyrate-treated HTC cells were restricted in G0/G1, as well as S/G2M phases. After release from butyrate arrest, HTC cells underwent synchronous cycles of DNA synthesis and transited through S phase. Inhibition of cell growth by butyrate was associated with a complex pattern of cell cycle regulated gene expression, including a decoupling of c-fos and c-jun gene expression. Transcription of c-fos, as well as c-jun increased with butyrate arrest, whereas steady rate mRNA levels of c-jun only were increased, suggesting additional regulation of c-fos. In addition, butyrate-arrested cells exhibited a transcriptionally determined accumulation of H3 histone, C-Ha-ras and ornithine decarboxylase mRNAs, suggesting that cell cycle-related check points following the onset of S phase were modulated. An increase in c-myc mRNA levels in butyrate-arrested cells was post-transcriptionally regulated. After release from butyrate-arrest, the abundance of immediate early, as well as S phase regulated, gene expression changed coordinately with S phase cell transitions. Thus, exposure of HTC cells to butyrate modulates cell cycle regulated gene expression, inhibits cycling, and results in accumulation of cells in specific compartments. Synchronization of liver cells with butyrate should, therefore, provide a useful model for defining cell cycle-related events in response to various mitogenic stimuli.

Studies on the safety of intrasplenic hepatocyte transplantation: relevance to ex vivo gene therapy and liver repopulation in acute hepatic failure

Hepatocytes transplanted into the host liver engraft promptly, retain normal function, and survive indefinitely. Although intrasplenic transplantation is effective in delivering hepatocytes to the liver, to define potentially limiting complications, we studied its safety in normal, cirrhotic, and partial portal vein-ligated rats. In normal rats, portal pressures increased severalfold after hepatocyte transplantation but returned to normal within 3 weeks. In contrast, in portal hypertensive rats with partial portal vein ligation or cirrhosis, portal pressures were either unchanged or increased less after hepatocyte transplantation. However, more transplanted cells migrated to the lungs along with a rise in right atrial pressures in portal hypertensive rats. Further quantitative studies using 111Indium-labeled hepatocytes showed that intrasplenic retention of transplanted hepatocytes was similar in all animal groups. Intrahepatic cell translocation was comparable in normal and cirrhotic rats, whereas fewer cells migrated to the liver in partial portal vein-ligated rats. The most remarkable difference, however, was significantly greater intrapulmonary translocation of hepatocytes in portal hypertensive rats, which was presumably related to portosystemic shunting. These results indicate that because intrasplenic hepatocyte transplantation induces only temporary portal hypertension in normal subjects, potential strategies to augment liver repopulation could include repeated cell transplantation. This should be useful for optimizing the results of ex vivo gene therapy, or other hepatocyte-based therapies. However, the hepatic and portal hemodynamic status requires careful evaluation in portal hypertensive or cirrhotic subjects if serious complications are to be avoided.

Analysis of hepatocellular proliferation: study of archival liver tissue is facilitated by an endogenous marker of DNA replication

Assessment of liver regeneration with endogenous genes that are expressed during DNA replication is physiological, specific and direct. To determine whether H3 histone messenger RNA expression (which is tightly coupled with DNA synthesis) could be used for this purpose, we initially examined liver regeneration in a mouse model. After partial hepatectomy, RNA transblot studies showed induction of H3 histone messenger RNA expression in regenerating mouse livers. In situ molecular hybridization demonstrated that the overall pattern of H3 histone messenger RNA expression correlated with [3H]thymidine labeling of hepatocytes. After partial hepatectomy, H3 histone messenger RNA expression in hepatocytes peaked at 48 hr (greater than 60 times greater than at 24 hr; p less than 0.001) and then rapidly declined. Although hepatocyte labeling with [3H]thymidine showed similar kinetics of liver regeneration, use of this parameter resulted in overestimation of the proliferative compartment when it was compared with H3 histone messenger RNA expression. Next we determined whether H3 histone messenger RNA expression could be used to study hepatocellular proliferation in archival human material. H3 histone messenger RNA-expressing hepatocytes were identified on in situ hybridization in patients with acute or chronic active hepatitis and active cirrhosis, but not inactive cirrhosis. These studies demonstrate that H3 histone messenger RNA is expressed in a phasic manner during liver regeneration. Use of H3 histone messenger RNA expression to evaluate hepatocellular proliferation should facilitate clinical studies and greatly advance our understanding of the pathophysiology of liver regeneration.