Immuno-overlay: a method for identification of hepatoma cell colonies that secrete albumin

Establishment and characterization of differentiated, nontransformed hepatocyte cell lines derived from mice transgenic for transforming growth factor alpha

Hepatocytes are extensively used in studies of gene regulation but cannot be maintained in long-term culture as replicating, differentiated cells while remaining nontumorigenic. We have derived two hepatocyte lines from livers of transgenic mice overexpressing transforming growth factor alpha, a potent hepatocyte mitogen, which overcome these limitations. The transgenic hepatocytes were maintained for > or = 2 months in serum-supplemented primary culture and gave rise to cell lines, of which two (AML12 and AML14) have been cultured for > 1.5 years (> 80 passages). Both lines have typical hepatocyte features such as peroxisomes and bile canalicular-like structures, do not grow in soft agar, and are nontumorigenic in nude mice. Like normal hepatocytes, AML cells express high levels of mRNA for serum (albumin, alpha 1-antitrypsin, and transferrin) and gap junction (connexins 26 and 32) proteins, secrete albumin, and contain solely isozyme 5 of lactate dehydrogenase. After extensive passaging, AML12 cells continue to strongly coexpress hepatocyte connexin mRNAs but do not display nonparenchymal cell markers. Although mRNA levels for some serum proteins progressively fall, high expression in late AML12 cultures may be regained by passage in serum-free medium. The AML14 line loses expression of both differentiated markers and transgene mRNA with extended passaging, and hepatocytic traits are only partially restored by passage in serum-free medium. These differentiated, nontumorigenic cell lines should serve as models in which to study hepatocyte growth and differentiation.

Transformation of differentiated rat hepatocytes with adenovirus and adenovirus DNA

Primary cultures of hepatocytes isolated by collagenase perfusion of adult rats were transformed by infection with adenovirus type 5 or transfection with adenovirus DNA. Total virion DNA or recombinant plasmid DNA containing the adenovirus E1A and E1B genes transformed hepatocytes at comparable frequencies. No foci of replicating hepatocytes were detected after transfection with a plasmid containing the E1A gene alone. The frequency of transformation by the adenovirus E1A and E1B genes was dependent on the composition of the culture medium. Transformation occurred at a low frequency when the transfected hepatocytes were maintained in a chemically defined medium (CDM), but the frequency was enhanced 8- to 10-fold when the cells were maintained in (i) serum-supplemented medium or (ii) CDM supplemented with epidermal growth factor. Cell lines derived from the adenovirus-transformed colonies of hepatocytes expressed adenovirus E1A and E1B RNAs. When hepatocytes were maintained in CDM supplemented with dimethyl sulfoxide and transfected with plasmids containing the E1A and E1B genes, it was possible to derive cell lines that retained the ability to express several liver-specific genes, including albumin, transferrin, hemopexin, and the third component of complement. The amount of albumin secreted per cell varied from 1 to 5 pg per cell per 24 h, and in one cell line it was below detectable levels by passage 9. Adenovirus-transformed hepatocytes were not tumorigenic when inoculated subcutaneously into neonatal syngeneic rats. We conclude that the adenovirus E1A and E1B genes are capable of transforming adult rat hepatocytes, a differentiated epithelial cell type.

Use of fluorescence-activated cell sorter to isolate mutant mammalian cells deficient in an internal protein, dihydrofolate reductase

Dihydrofolate reductase-deficient mutants of Chinese hamster ovary cells have been selected based on their inability to bind a fluorescent derivative of methotrexate, a substrate analog. Nonfluorescent mutant cells were isolated from mutagenized populations using a fluorescence-activated cell sorter. After multiple rounds of sorting plus regrowth, the mutant cell frequency was increased from an initial 10(-5) to greater than 0.9. This use of the cell sorter to isolate mutants deficient in an internal protein should be applicable to any gene product that is able to bind a fluorescent ligand tightly. The method has the advantage of allowing the screening of large numbers of cells and of selecting for partially expressed phenotypes.

Quantitative assay for albumin-producing liver cells after simian virus 40 transformation of rat hepatocytes maintained in chemically defined medium

Transformation of rat hepatocytes by simian virus 40 in chemically defined medium was examined. When hepatocytes plated on collagen-coated plates were infected with simian virus 40, colonies of replicating cells appeared as early as 40 days after infection, whereas no colonies appeared in control cultures. Medium from 85% of the transformed cultures contained albumin. When collagen was eliminated and hepatocytes were plated on Primaria plastic cell culture dishes, transformation occurred; medium from 86% of the transformed cultures contained albumin but the maximum albumin level secreted per culture was only 62% of that produced by cultures on collagen-coated plates. Quantitative assays for transformation were established. Transformation was linear after infection with 2-50 plaque-forming units of virus per hepatocyte, and the transformation frequency was the same on the two plating surfaces. An immuno-overlay technique made it possible to identify, purify, and determine the morphology of the albumin-producing cells. When ornithine was substituted for arginine in the medium, the transformation frequency decreased markedly while the percentage of colonies producing albumin increased from 30% to 100%. We conclude that we have defined an assay for quantifying transformation of a normal hepatocyte population and for identifying and enumerating epithelial liver cell transformants that produce albumin.

Generation and chracterization of variants of mouse hepatoma cells with defects in hepato-specific gene expression. I. Albumin synthesis variants

Clonal variants of mouse hepatoma cells that either fail to produce albumin (variant 19/2) or show significantly reduced levels (100-fold less) of albumin production (variant 1/c/1) were isolated from the parental line. Hepa la, after a single exposure to N-methyl-N'-nitrosoguanidine (MNNG). Intracellular levels of albumin in both variants were below detection by our assay. Analyses by cDNA-RNA reassociation kinetics indicate that there are approximately 3900 molecules of cytoplasmic albumin mRNA per cell in the parent and less than 10 molecules per cell in both variants. Southern blotting of the Eco RI restriction fragments of cellular DNA from the parent and variants did not indicate any major deletions in the albumin gene DNA sequences. We conclude that in the two variants studied, processes that regulate albumin production via alterations in the level of cytoplasmic albumin mRNA have been affected. Our analyses have also shown that alpha-fetoprotein (AFP) production is lacking in one variant (19/2) and is slightly reduced in the other (1/c/1). Transferrin secretion is lower than the parental line in both variants. Thus multiple nonlethal defects in hepatic gene expression can be obtained in Hepa la cells in culture that will be useful in determining the number and kinds of genes that control the expression of liver-specific loci.