Recombinant retroviral DNA yielding high expression of hepatitis B surface antigen

Expression pattern of the hepatitis B virus genome in transfected mouse fibroblasts

Permanent mouse fibroblast LTK- cells were transfected with dimeric hepatitis B virus (HBV) DNA linked to the simian virus 40 (SV40) early promoter/enhancer. Many clones stably expressed high levels of polyadenylated RNAs encoding hepatitis B surface (HBs) proteins (2.1 kb), HBe protein (3.6 kb), and HBx protein (0.6 kb). Although a chimeric RNA (4.0 kb) probably starting from the SV40 promoter was also synthesized, transcription of viral RNAs was predominantly directed by HBV promoters and its terminator. In contrast to HBV-transfected liver cells, the fibroblasts expressed only pregenomic 3.6-kb transcripts starting 5' to, but not within, the precore sequence. Thus, no normal core protein could be synthesized, but the cells expressed and secreted HBe protein of heterogeneous size. Small and middle HBs proteins were strongly expressed, while large HBs protein was almost absent. HBx mRNA expression was more efficient in mouse fibroblasts than in human hepatoma cells and 18-kDa HBx protein was exclusively detected in purified nuclei. Expression of HBe, small and middle HBs, and HBx proteins apparently does not require hepatic factors. Underexpression of HBc mRNA and large HBs mRNA suggests that activity of their promoters depends on cell-type-specific transcription factors.

Expression and Secretion of a Functional Scorpion Insecticidal Toxin in Cultured Mouse Cells

We have expressed a synthetic gene encoding the insecticidal neurotoxin of scorpion Androctonus australis (AaIT) in NIH/3T3 mouse fibroblast cells under the transcriptional control of a murine retroviral long terminal repeat. The secretion of the toxin into the culture medium was directed by the signal peptide of human interleukin-2. The recombinant AaIT produced was selectively toxic to yellow-fever mosquito larvae and harmless to mice.

A recombinant Chinese hamster ovary cell line containing a 300-fold amplified tetramer of the hepatitis B genome together with a double selection marker expresses high levels of viral protein

A new series of double-selection plasmids containing recombinant genes expressing the neomycin phosphotransferase (NEO) of transposon Tn5 and mouse dihydrofolate reductase (DHFR) in mammalian cells is described. Activity of the recombinant DHFR gene varied more than 50-fold, depending on the location of the simian virus 40 72 base-pair repeat or enhancer, which is part of the promoter of the NEO unit. A NEO-DHFR module with the enhancer located at the 3' end of the DHFR gene was inserted into a plasmid containing four tandem head-to-tail copies of the hepatitis B virus (HBV) genome and the new plasmid was used to transform DHFR- Chinese hamster ovary cells. In one of the cell lines obtained, an unrearranged copy of the HBV tetramer could be amplified 300-fold by increasing selective pressure with methotrexate, resulting in a proportional increase of the synthesis of HBV surface antigen. Four different mRNAs detected in the amplified cell line probably encode HBV core protein, pre-S and surface antigens, and the X protein. As a result of the DNA amplification, synthesis of HBV proteins is no longer restricted to resting cells. Integrated plasmid sequences appear to be stable during the amplification process.

The 89,000-Mr murine cytomegalovirus immediate-early protein activates gene transcription

To study trans-activation of gene expression by murine cytomegalovirus (MCMV) immediate-early (IE) proteins, the IE coding region 1 (ie1), which encodes the 89,000-Mr IE phosphoprotein (pp89), was stably introduced into L cells. A cell line was selected and characterized that efficiently expressed the authentic viral protein. The pp89 that was constitutively expressed in L cells stimulated the expression of transfected recombinant constructs containing the bacterial chloramphenicol acetyltransferase (CAT) gene under the control of viral promoters. The regulatory function of the ie1 product was confirmed by transient expression assays in which MCMV IE genes were cotransfected into L cells together with recombinant constructs of the CAT gene. For CAT activation by the ie1 product, a promoter region was required, but there was no preferential activation of a herpes simplex virus type 1 delayed-early promoter. All plasmid constructs that contained the intact coding sequences for pp89 induced gene expression in trans. The MCMV enhancer region was not essential for the expression of a functional IE gene product, and testing of the cis-regulatory activity of the MCMV enhancer revealed a low activity in L cells. Another region transcribed at IE times of infection, IE coding region 2, was unable to induce CAT expression and also did not augment the functional activity of ie1 after cotransfection.

Two mammalian cell systems for propagation of the hepatitis B virus genome in extrachromosomal and chromosomally integrated states: production of the surface and e antigens

A recombinant plasmid consisting of (i) the entire genome of hepatitis B virus (HBV) DNA, (ii) the replication origin of SV40 virus, and (iii) a deletion derivative of pBR322 was introduced either into COS cells of monkey origin which constitutively express SV40 large T antigen, or into thymidine kinase(TK)-deficient mouse L cells together with the TK DNA of Herpes simplex virus. In the COS cell system, the transfecting recombinant DNA replicates via SV40 origin and is maintained in an autonomously replicating state. The cells carrying these extrachromosomal elements express the hepatitis B surface antigen gene at moderate rate, and release the products into the culture medium. However, neither core antigen nor e antigen expression was detected in this system. In the L cell system, the transformed L cells carry the recombinant DNA in a chromosomally integrated state. Such cells express the surface antigen gene at high rate, and release the products into the culture medium. This system also excretes the e antigen into the culture medium. The core antigen was not detected.

Co-transcribed 3' host sequences augment expression of integrated hepatitis B virus DNA

We have previously reported the cloning and structural analysis of integrated hepatitis B virus DNA copies from the human hepatoma cell line PLC/PRF/5. Here we show that the cloned DNA fragments of 10.7 kb and 10.5 kb contain intact coding sequences for HBsAg since Ltk- cells transfected with these DNAs secrete considerable amounts of HBsAg. We show for the 10.7-kb fragment that multiple readthrough messages composed of viral as well as cellular sequences are transcribed. These RNAs differ only in their 3' sequences. Furthermore, the 10.7-kb insert leads to a substantial increase in HBsAg produced compared with HBV DNA and with the 10.5-kb insert. We provide evidence that the different 3' sequences on the HBsAg transcripts account for the augmentation of expression.

A transformed Vero cell line stably producing the hepatitis B virus surface antigen

We have constructed plasmids in which transcription of the surface antigen gene of the human hepatitis B virus (HBsAg) is under the control of the SV40 early promoter. These plasmids have been used to analyze the expression of the HBsAg gene, both in mouse cells and in African green monkey kidney (Vero) cells. We have established a Vero cell line continuously expressing HBsAg that is indistinguishable from authentic 22 nm HBsAg particles. Post-transcriptional modification of HBsAg mRNA also appears to occur normally in the monkey cells. These cells might be useful as a source of antigen for the preparation of an antihepatitis vaccine.

A virus antigen (HBsAg) indicates metastases of human carcinoma cells in the nude mouse

The human hepatoma cell line PLC/PRF/5 produces tumours and lung metastases in nude athymic mice. Here we describe how detection of hepatitis B virus surface antigen, a marker of these cells, as led to identification of pancreatic and muscle metastases. The antigen was shown in the tissues, and antigen-producing PLC/PRF/5 cells were isolated from them. The number of cells detectable by this method is between 5 and 10 million. The principle used here may be adaptable be useful in studying micrometastasis formation in other systems.

RNA-tumorviruses, oncogenes, and their possible role in human carcinogenesis

The detection and characterization of oncogenes via RNA tumor viruses (or retroviruses) and the recognition of their location at breakpoints of chromosomal translocations which are frequently found in certain human neoplasms has promoted present understanding of molecular mechanisms underlying carcinogenesis. Oncogenes are cellular genes which can be transduced by RNA tumorviruses and induce malignant transformation under experimental conditions in vivo and in vitro. A role of retroviruses in human leukemogenesis is suggested by epidemiological observations and by the isolation of such viruses from several human T-cell leukemias and lymphomas (human T-cell leukemia/lymphoma virus or HTLV) as well as by biochemical association of retroviral markers with human leukemias. A role of HTLV has been suggested also in a human immune deficiency syndrome (AIDS). In view of the well known role of many factors in carcinogenesis the concept of carcinogenesis as a multistep process as well as the concept of cocarcinogenesis and the role of cofactors other than viruses, such as radiation and chemicals, aging, hormones, graft vs host reaction, environmental factors etc., will have to be carefully considered.

Signals regulating hepatitis B surface antigen transcription

About 200 million people are chronic carriers of hepatitis B surface antigen (HBsAg), but since hepatitis B virus (HBV) cannot be propagated in vitro, HBsAg transcription has been studied only in cell lines containing HBV DNA integrated into chromosomes, and HBsAg-related mRNAs 2.0 to 2.5 kilobases (kb) long have been described. We have analysed the transcripts produced in an infected chimpanzee liver and in a rat cell line containing HBV DNA. In contrast to previous suppositions we report here that the major S gene transcript initiates close to the S gene, that is, within the 'pre-S' region and is processed/polyadenylated at a site situated within the core gene. The efficiency of processing/polyadenylation at this site varies between the chimpanzee liver and the rat cell line studied. The S gene promoter does not contain a TATA box but instead has a sequence homologous to that which positions the 5' ends of the major simian virus 40 (SV40) late transcript.