Promutagen activation by freshly isolated and cryopreserved rat hepatocytes

Prolonged cultured human hepatocytes as an in vitro experimental system for the evaluation of potency and duration of activity of RNA therapeutics: Demonstration of prolonged duration of gene silencing effects of a GalNAc-conjugated human hypoxanthine phosphoribosyl transferase (HPRT1) siRNA

We report here the evaluation of a novel in vitro experimental model, prolonged cultured human hepatocytes (PCHC), as an experimental system to evaluate the potency and duration of effects of oligonucleotide therapeutics. A novel observation was made on the redifferentiation of PCHC upon prolonged culturing based on mRNA profiling of characteristic hepatic differentiation marker genes albumin, transferrin, and transthyretin. Consistent with the known de-differentiation of cultured human hepatocytes, decreases in marker gene expression were observed upon culturing of the hepatocytes for 2 days. A novel observation of re-differentiation was observed on day 7 as demonstrated by an increase in expression of the marker genes to levels similar to that observed on the first day of culture. The expression of the differentiation marker genes was highest on day 7, followed by a gradual decrease but remained higher than that on day 2 for up to the longest culture duration evaluated of 41 days. The redifferentiation phenomenon suggests that PCHC may be useful for the evaluation of the duration of effects of oligonucleotide therapeutics on gene expression in human hepatocytes. A proof of concept study was thereby conducted with PCHC with a GalNAc-conjugated siRNA targeting human hypoxanthine phosphoribosyl transferase1 (HPRT1). HPRT1 mRNA expression in siRNA-treated cultures decreased to 21% of that in untreated hepatocytes on day 1, <10% from days 2 to 12, <20% from days 16 to 33, and eventually recovered to 64% by day 41. Our results suggest that PCHC represent a clinically-relevant cost- and time-efficient experimental tool to aid in the evaluation of GalNAc-siRNA silencing activity, providing information on both efficacy and duration of efficacy. PCHC may be applicable in the drug development setting as a species- and cell type-relevant experimental tool to aid the development of oligonucleotide therapeutics.

Cryopreserved and hypothermically stored rat liver parenchymal cells as metabolizing system in the Salmonella mutagenicity assay

Freshly isolated and preserved rat liver parenchymal cells were used as metabolizing system in the Salmonella mutagenicity assay. The liver cells were isolated with EDTA perfusion without the addition of collagenase and had a viability of 96% as judged by trypan blue exclusion. When freshly isolated liver parenchymal were cryopreserved with a computer controlled freezing protocol and stored at -196 degrees C they had a mean viability of 89% after thawing. Furthermore, freshly isolated cells were stored at 0 degree C in University of Wisconsin organ transplantation solution. After 1 day of hypothermic storage they had a viability of 95%. Four different indirect mutagens, 2-aminoanthracene, benzo[a]pyrene, 7,12-dimetylbenz[a]anthracene and cyclophosphamide, were used with the liver cells as metabolizing system in the preincubation assay with Salmonella typhimurium TA100. After cryopreservation, liver parenchymal cells were able to activate all tested indirect mutagens to ultimate mutagens. However, the induction of revertants was lower with three of the four tested compounds. Only 2-aminoanthracene was activated to the same extent by freshly isolated and cryopreserved liver cells. 7-Hydroxymethyl-12-methylbenz[a]anthracene, which is activated to its ultimate mutagen by sulfotransferase, also induced a reduced mutagenic effect with cryopreserved liver cells in comparison to freshly isolated liver parenchymal cells. This indicates that phase I and phase II enzyme activities are effected by cryopreservation. However, identical mutation frequencies were obtained when freshly isolated liver parenchymal cells or 1 day hypothermically preserved liver parenchymal cells were used in the cell-mediated Salmonella mutagenicity test. The use of hypothermic short-time storage of liver parenchymal cells could help to make the liver cell-mediated genotoxicity test simpler and thereby more attractive.

Polybrominated biphenyl induction of cytochrome P450 mixed function oxidase activity in primary rat and human hepatocytes

Polybrominated biphenyl (PBB) is an industrial chemical and environmental contaminant with known incidence of significant human exposure. PBB has been studied in laboratory animals and found to have significant toxicological effects as well as being a potent inducer of hepatic cytochrome P450 mixed function oxidase (MFO) activity. As part of our program to compare the response of laboratory animals and humans to industrial and environmental toxicants, we studied the effect of a major component of commercial PBB mixtures, 2,2',4,4',5,5'-hexabromobiphenyl (HBB), on MFO induction in primary cultures of human and rat hepatocytes. MFO induction was evaluated by measuring the deethylation of 7-ethoxycoumarin by intact hepatocytes. Rat hepatocytes were found to be highly susceptible to HBB induction of 7-ethoxycoumarin O-deethylase (ECOD) activity, with significant induction observed at the lowest concentration tested of 10(-8) M. Human hepatocytes were found to have a higher threshold for HBB induction of ECOD activity than rat hepatocytes. The lowest concentration of HBB required for ECOD induction observed for human hepatocytes was 10- to 1000-fold higher (10(-7), 10(-6), 10(-5) M for the four human samples) than that found in rat hepatocytes. Future mechanistic investigation of this observed difference in sensitivity towards PBB between rat and human hepatocytes may aid the extrapolation of human health risk from toxicological data obtained from laboratory animals.

Cryopreservation and long-term storage of primary rat hepatocytes: effects on substrate-specific cytochrome P450-dependent activities and unscheduled DNA synthesis

The effects of cryopreservation and long-term storage on substrate-specific cytochrome P450-dependent activities and unscheduled DNA synthesis were studied in freshly isolated and cryopreserved hepatocytes derived from adult male Fischer 344 and Sprague-Dawley rats. Primary rat hepatocytes were isolated via an in situ collagenase perfusion technique, cryopreserved at -196 degrees C, and thawed at 5 weeks and 104 and 156 weeks post-freezing. In Fischer 344 and Sprague-Dawley rats, cryopreserved hepatocytes were equivalent or similar to freshly isolated hepatocytes in substrate-specific activities for 7-ethoxyresorufin-O-deethylase and dimethylnitrosamine-N-demethylase and unscheduled DNA synthesis responses. No significant differences in activities toward 7-ethoxyresorufin-O-deethylase and dimethylnitrosamine-N-demethylase, the substrate-specific activities for cytochromes P4501A1 and P4501A2 and cytochrome P4502E1, respectively, were observed between freshly isolated and cryopreserved hepatocytes. Similar unscheduled DNA synthesis responses, a measure of DNA damage and repair, were observed after exposure to the genotoxic carcinogens 2-acetylamino-fluorene, 7,12-dimethylbenz[a]anthracene, and dimethylnitrosamine; although some decreases were also observed in Fischer 344 hepatocytes after 104 weeks and Sprague-Dawley hepatocytes after 156 weeks in the highest concentrations tested. These results suggest that cryopreserved hepatocytes, stored for extended periods of time in liquid nitrogen, are metabolically equivalent to freshly isolated hepatocytes in their ability to activate precarcinogens.

Preservation of the rate and profile of xenobiotic metabolism in rat hepatocytes stored in liquid nitrogen

A simple procedure for cryopreservation of rat hepatocytes that allows recovery of viable cells retaining activities of phase I and phase II drug metabolism equivalent to freshly isolated cells is described. The cooling process was initiated 30 min after incubation of freshly isolated hepatocytes at 37 degrees in Krebs-Ringer bicarbonate buffer containing 15 mM glucose to allow for metabolic equilibration. At the end of this period, hepatocyte suspensions were supplemented with 1.7% albumin, 13.3% dimethyl sulfoxide, and the synthetic buffers, 3-[N-morpholino]propanesulfonic acid (MOPS) and N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES). Hepatocytes were cooled in a stepwise manner to -196 degrees by holding the cells for 1 hr at -20 degrees and then for 1 hr at -70 degrees before transfer into liquid nitrogen. After thawing and removal of damaged cells by centrifugation in Percoll, the total recovery of viable hepatocytes subjected to freezing was about 42%. The contents of ATP, ADP, and AMP were not altered significantly in cells stored in liquid nitrogen. The metabolic competence of cryopreserved hepatocytes was further confirmed by their ability to synthesize urea from NH4Cl and ornithine at the same high rate that was observed in freshly isolated cells (693 +/- 68 and 740 +/- 68 nmol.mg dry wt-1 x hr-1, respectively). Similarly, cryopreservation did not affect drug-metabolizing systems as indicated by the metabolism of benzo[a]pyrene and 7-ethoxycoumarin, two model substrates. In both freshly isolated and cryopreserved hepatocytes, 7-ethoxycoumarin was O-deethylated to 7-hydroxycoumarin at essentially the same rates (8.66 +/- 0.75 and 8.25 +/- 0.53 nmol.mg dry wt-1.hr-1, respectively) and 7-hydroxycoumarin accumulated in hepatocyte suspensions almost exclusively in the conjugated form. The storage of hepatocytes in liquid nitrogen also did not affect the complex metabolism of benzo[a]pyrene to total oxygenated metabolites and, more importantly, to metabolites conjugated with glutathione, glucuronic acid, and sulfuric acid. Thus, cryopreserved hepatocytes represent a valid and convenient model to study drug biotransformation in intact cells.

Culturing of primary hepatocytes as entrapped aggregates in a packed bed bioreactor: a potential bioartificial liver

Conventional culture systems for hepatocytes generally involve cells cultured as flat, monolayer cells, with limited cell-cell contact, in a static pool of medium, unlike the liver in vivo where the parenchymal cells are cuboidal, with extensive cell-cell contact, and are continuously perfused with blood. We report here a novel bioreactor system for the culturing of primary hepatocytes with cuboidal cell shape, extensive cell-cell contact, and perfusing medium. The hepatocytes were inoculated into the bioreactor and allowed to recirculate at a rate optimal for them to collide and form aggregates. These newly-formed aggregates were subsequently entrapped in a packed bed of glass beads. The bioreactor was perfused with oxygenated nutrient medium, with controlled oxygen tension, pH, and medium perfusion rate. The hepatocytes were viable for up to the longest time point studied of 15 days in culture based on urea synthesis, albumin synthesis and cell morphology. Light microscopy studies of hepatocytes cultured for 15 days in the bioreactor showed interconnecting three-dimensional structures resembling the hepatic cell plate in the liver organ. Electron microscopy studies on the same cells revealed ultrastructure similar to the hepatocytes in vivo, including the presence of plentiful mitochondria, rough and smooth endoplasmic reticulum, glycogen granules, peroxisomes, and desmosomes. We believe that our hepatocyte bioreactor is a major improvement over conventional culture systems, with important industrial applications including toxicology, drug metabolism, and protein/peptide synthesis. The hepatocyte bioreactor concept may also be used as the basis for the development of a bioartificial liver to provide extracorporeal hepatic support to patients with hepatic failure.

A simplified method for the culturing of primary adult rat and human hepatocytes as multicellular spheroids

A simple and highly reproducible method was established for the culturing of adult rat and human hepatocytes as multicellular aggregates (spheroids). Purified rat and human liver parenchymal cells were cultured on nontissue culture (bacteriological) polystyrene petri dishes on a rotating platform. After an overnight incubation, the cells were found to form multicellular aggregates. The aggregates became spheroidal in shape after several days in culture. Histological sections of the spheroids showed an organized structure consisting of squamated cells on the outermost layer and cuboidal cells in the interior. Cellular structures characteristic of hepatocytes in the liver in vivo including bile canaliculi, peroxisomes, Golgi bodies, abundant mitochondria, and rough and smooth endoplasmic reticulum were observed with electron microscopy. The spheroids were found to be viable up to the longest time studied of approx. 1 month in culture as demonstrated by their adherence and growth on collagen-coated substratum. The morphological resemblance between hepatocytes cultured as spheroids and the liver in vivo suggests that the spheroids may be a useful in vitro experimental model of the liver. Our simple method should allow hepatocytes to be cultured as spheroids easily in any laboratory equipped for cell culture. Our study here also is the first to report the culturing of human hepatocytes as spheroids.

Gene transfer in primary cultures of human hepatocytes

Using liposomes as the mediator of DNA transfer, we were successful in the transfection of human hepatocytes isolated from surgical samples with an E. coli beta-galactosidase gene (beta-gal). A comparison of transfection efficiency showed that of the four promoters used, cytomegalovirus (CMV) promoter yielded higher transfection efficiencies than Rous sarcoma virus (RSV), Simian virus-40 (SV-40) and human alpha-1 antitrypsin (AAT) promoters. These studies represent the first report on the successful transfection of primary cultures of human hepatocytes.