George Acs

Long-term culture and passage of human fetal liver cells that synthesize albumin.

SummaryLong-term cultures of hepatocytes were established from livers of human fetuses obtained by abortion at 18 to 23 wk of gestation. Cells obtained by collagenase dissociation of liver were maintained in defined serum-free medium on a substratum of positively charged plastic. Under these conditions, the cells divide and form a confluent monolayer. After multiple passages over a period of 3 mo., the cells retained an epithelioid morphology and continued to synthesize and secrete albumin.

Identification of protein-binding sites in the hepatitis B virus enhancer and core promoter domains

We have investigated the role of liver-specific trans-acting factor(s) in the regulation of hepatitis B virus (HBV) gene expression. A recorder plasmid (pEcoAluCAT; HBV nucleotides 1 through 1878) was constructed containing the HBV enhancer and the promoter region of the pregenomic RNA, which was ligated to the bacterial chloramphenicol acetyltransferase (CAT) gene. Upon transfecting this plasmid into various cell lines, the CAT gene was expressed only in cells of liver origin. Moreover, competition cotransfections with pEcoAluCAT and plasmids containing HBV enhancer sequences in human hepatoblastoma-derived HepG2 cells indicated the presence of titratable trans-acting factor(s) in these cells. Gel mobility shift assays using HBV enhancer and core promoter domains confirmed the existence of sequence-specific DNA-binding proteins in liver cell nuclear extract which bound to these regions. These binding sites encompass 17- and 12-nucleotide palindromes in the HBV enhancer and core promoter domains, respectively, when mapped by the methylation interference assay.

A nude mouse model for the in vivo production of hepatitis B virus

Hepatitis B virus genome-transfected HepG2 cells (2.2.15 cells) inoculated into nude mice produced tumors within 2-8 wk. Dane particles, hepatitis B virus deoxyribonucleic acid polymerase activity, hepatitis B surface antigen, and hepatitis B e antigen were detected in the serum, and 36% of mice developed antibodies to hepatitis B core antigen. In the tumors, hepatitis B surface, core, and e antigens were observed by electron microscopy and immunoenzymatic techniques. In-situ hybridization and Southern blot analysis showed hepatitis B virus deoxyribonucleic acid in the tumor. Tumors could be propagated by injection of minced tumor tissue or of a tumor-derived cell line. Liver of tumor-bearing mice as well as sera and tissues of mice inoculated with control cell lines did not show hepatitis B virus genome or viral markers. Tumors induced by both 2.2.15 and nontransfected HepG2 cells exhibited myc oncogene protein and various hepatoma-associated antigens (alpha-fetoprotein, alpha-1-antitrypsin, alpha-1-antichymotrypsin, carcinoembryonic antigen, cytokeratin), suggesting that viral formation does not interfere with expression of these antigens. This experimental model will be helpful to study the effect of drugs on in-vivo hepatitis B virus replication and viral antigen expression.

Selective inhibition of hepatitis B virus and human immunodeficiency virus sequence-promoted gene expression by cotransfected poly(I):poly(C)

The transient expression of hepatitis B virus (HBV) surface and eJ antigens caused by transfection of human hepatoblastoma HepG2 cells with HBV DNA was markedly inhibited by cotransfection with poly(I):poly(C). Cotransfection with poly(I):poly(C) also inhibited the expression of bacterial chloramphenicol acetyltransferase (CAT) gene which was under the control of either the HBV core promoter or the human immunodeficiency virus (HIV-1) long terminal repeat. This inhibition was much more pronounced on the expression of HBV-promoted CAT than HIV-promoted CAT. The uptake of reporter plasmid was not affected by cotransfected poly(I):poly(C). The inhibition was found to be at the steady-state CAT mRNA level and appeared to be specific for HBV and HIV regulatory sequences since CAT expression directed by other viral and cellular regulatory sequences was not inhibited. Cotransfection with a mixture of equal amounts of poly(I) and poly(C) had similar inhibitory effects whereas cotransfection with poly(l) or poly(C) alone, or other double-stranded ribo- or deoxyribonucleotides, did not have such strong effects. The addition of poly(l):poly(C) to the culture medium of cells transfected with these reporter plasmids caused little inhibition. Transfection with poly(l):poly(C) induced a minimal amount of intracellular interferon-alpha in HepG2 cells which may be involved in selective inhibition of HBV-and HIV-1-directed gene expression. 2-Aminopurine, an inhibitor of double-stranded RNA activated protein kinase known to block interferon gene induction by poly(l):poly(C), partially reversed the poly(l):poly(C)-induced inhibitory effect on HBV-CAT expression.

Characterization of RNA transcripts and virally coded proteins synthesized in mouse fibroblasts transfected with hepatitis B DNA: HBeAg synthesis in HBcAg-negative cells with active core-antigen genes

A line of mouse cells expressing hepatitis B virus (HBV) surface and ‘e’ antigens identical in their physico-chemical properties to antigens from patients infected with HBV was isolated after transfection of 3T3 ceils with cloned HBV DNA. The studies reported here indicate that the ceils contain uninterrupted copies of the entire HBV genome which are unmethylated on CCGG sites and have no gross deletions or rearrangements. The entire core region is transcribed into polyadenylated RNAs large enough to serve as messengers for production of viral core antigen (HBcAg) yet no HBcAg can be detected. This suggests that the ceils produce a primary translation product copied from the HBcAg messenger which either cannot assume the proper configuration for display of HBcAg determinants or is rapidly converted to HBeAg by proteolysis.

Inhibition of development of characteristic indices of myeloid differentiation by a factor released from a tetraploid variant of the human promyelocytic cell line, HL-60☆

HL-60TR, a tetraploid variant of the human promyeloid cell line HL-60, was obtained by culturing HL-60 cells for one week with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) at 400 pM. HL-60TR cells have lost the ability to develop several common markers of maturation in response to compounds that induce monocytoid or myelocytoid differentiation of HL-60 cells. In addition, they release a factor which inhibits induction of the same markers in HL-60 cells. Medium conditioned by HL-60TR cells also inhibits colony formation by normal mouse bone marrow cells. These properties have been maintained by HL-60TR cells through more than one year of constant subculture in the absence of TPA, a finding which suggests the possibility that TPA may promote tumor formation not only through direct effects on the phenotype of initiated cells but also through induction of continued production of factors that affect differentiation of normal stem cells.