SV40 enhancer and large-T antigen are instrumental in development of choroid plexus tumours in transgenic mice

Detection of simian virus 40 DNA sequence in human primary glioblastomas multiforme

OBJECT

Deoxyribonucleic acid oncoviruses can induce neoplastic transformation of cells because their viral proteins interfere with antiproliferative cellular proteins. Simian virus 40 (SV40) is a DNA virus that induces the emergence of ependymomas, choroid plexus tumors, mesotheliomas, osteosarcomas, sarcomas, and various tumors when injected into newborn hamsters. Recently, approximately 60% of human ependymomas, choroid plexus tumors, and mesotheliomas were reported to contain and express SV40 DNA sequences. In this study the presence of SV40 DNA sequences was investigated in human brain tumors.

METHODS

Three of 32 glioblastomas mutiforme (GBMs), but none of two ependymomas and five medulloblastomas, were found to possess SV40 DNA sequences when examined using polymerase chain reaction (PCR). The DNA sequence analysis of PCR-amplified fragments disclosed that the samples were identical to the regulatory region of SV40. All three GBMs, which arose in elderly patients with wild-type p53, were considered to be primary (de novo) tumors. Although each of the three tumors was immunohistochemically negative for SV40 T antigen, in situ hybridization successfully demonstrated the messenger RNA for SV40 T antigen.

CONCLUSIONS

The results of this study indicate that latent infection of SV40 in elderly people may be implicated in the tumorigenesis of certain primary GBMs.

Tumorigenesis in transgenic mice in which the SV40 T antigen is driven by the brain-specific FGF1 promoter

Gene expression can be manipulated by the introduction of a hybrid gene formed by linking a highly tissue-specific regulatory element to a gene whose expression might be expected to alter cellular function. Previously, we have shown that the human FGF1 gene contains four distinct tissue-specific promoters. In an effort to perturb the programming of proliferation and differentiation in a subset of neural cells, we have produced transgenic mice bearing the brain-specific promoter of the human FGF1 gene joined to the SV40 immediate early gene, which encodes the large T antigen. The resulting mice, and offspring from four individual lines, developed brain tumors that originated in the pontine gray, just rostral to the fourth ventricle. Tumors were moderately vascularized, as demonstrated by staining with both hematoxylin and eosin and antibodies to three different endothelial cell markers, but vessels were histologically normal. Scattered tumor foci were present as early as postnatal day 26; and affected animals died between 5 - 8 months of age. In mature animals, tumors lacked terminal differentiation markers for astrocytes (glial fibrillary acidic protein) or neurons (synaptophysin and neuron-specific enolase). However, they expressed high levels of proliferating cell nuclear antigen and vimentin, markers for proliferating cells. This immunophenotype is consistent with the tumor being at an early stage of differentiation. Therefore, these mice may provide a valuable tool for the study of tumorigenesis, replenishment and differentiation of neural stem cells.

Manipulating the germline: its impact on the study of carcinogenesis

Over the past two decades, the mouse has established itself as the primary organism in which to investigate the fundamental mechanisms of carcinogenesis and to model human neoplasia. The principal reason underlying such dominance almost certainly arises out of our ever increasing ability to manipulate the murine germline. Over the past 20 years we have moved from a position where animal models arose either spontaneously or were generated through exposure to carcinogen to a position in which it is possible to create and study precise mutations of choice. The most recent advances in inducible and conditional technologies now open the possibility for both temporal and tissue-specific gene manipulation. Each of these technological breakthroughs has facilitated significant steps forward in our understanding of the genetic basis of tumorigenesis. This review will highlight some of the major advances in the production and use of murine models of neoplasia over the last two decades.

Establishment of SV40-tsA58 transgenic rats as a source of conditionally immortalized cell lines

To isolate a variety of rat cell lines with differentiated functions, we established transgenic rat lines expressing the temperature-sensitive large T-antigen of simian virus 40 (SV40) tsA58 mutant under the control of the SV40 large T-antigen itself. We microinjected the DNA into 564 eggs of Wistar rat and 23 independent transgenic candidates were obtained. Ten pups died before weaning and eight transgenic rats could not transmit the transgene to the progeny. Finally, five lines of the transgenic rat were established. Although one line (#1511-6) had low reproductivity, the other four lines reproduced normally. Three out of the four lines (#1507-2, #1509-7, #1519-8) appeared normal but the other line had tumors in the brain and subcutaneous tissue at 3 weeks of age (#1511-6), and in the kidneys and subcutaneous tissue at 18 to 19-weeks of age (#1507-5). Fibroblast cells prepared from transgenic fetuses of lines #1507-5 and #1519-8 expressed the transgene and exhibited temperature-dependent growth. Both of the lines (#1507-5 and #1519-8) were successfully generated to be homozygous by sibling mating of transgenic offspring. These transgenic rat lines have bred through many generations and have been established to be a ready source of novel conditionally immortalized cell lines.

Immortalized kidney epithelial cells as tools for hormonally regulated ion transport studies

The development of transgenic mice carrying the simian virus-40 large T antigen gene or the temperature-sensitive simian virus-40 large T antigen gene, either alone or placed under the control of the 5'-regulatory regions of tissue-specific or ubiquitous genes, has permitted the production of differentiated, polarized kidney epithelial cells. This review covers the immortalized cell lines issued from the various parts of the renal tubule and, in particular, the recently established collecting duct cell lines that have been used as ex-vivo cell models to analyze the regulation of ion transport processes by hormones.

Transgene expression and repression in transgenic rats bearing the phosphoenolpyruvate carboxykinase-simian virus 40 T antigen or the phosphoenolpyruvate carboxykinase-transforming growth factor-alpha constructs

Transgenic Sprague-Dawley rats expressing either human transforming growth factor-alpha (TGFalpha) or simian virus 40 large and small T antigen (TAg), each under the control of the phosphoenolpyruvate carboxykinase (PEPCK) promoter, were developed as an approach to the study of the promotion of hepatocarcinogenesis in the presence of a transgene regulatable by diet and/or hormones. Five lines of PEPCK-TGFalpha transgenic rats were established, each genetic line containing from one to several copies of the transgene per haploid genome. Two PEPCK-TAg transgenic founder rats were obtained, each with multiple copies of the transgene. Expression of the transgene was undetectable in the TGFalpha transgenic rats and could not be induced when the animals were placed on a high-protein, low-carbohydrate diet. The transgene was found to be highly methylated in all of these lines. No pathological alterations in the liver and intestine were observed at any time (up to 2 years) during the lives of these rats. One line of transgenic rats expressing the PEPCK-TAg transgene developed pancreatic islet cell hyperplasias and carcinomas, with few normal islets evident in the pancreas. This transgene is integrated as a hypomethylated tandem array of 10 to 12 copies on chromosome 8q11. Expression of large T antigen is highest in pancreatic neoplasms, but is also detectable in the normal brain, kidney, and liver. Mortality is most rapid in males, starting at 5 months of age and reaching 100% by 8 months. Morphologically, islet cell differentiation in the tumors ranges from poor to well differentiated, with regions of necrosis and fibrosis. Spontaneous metastasis of TAg-positive tumor cells to regional lymph nodes was observed. These studies indicate the importance of DNA methylation in the repression of specific transgenes in the rat. However, the expression of the PEPCK-TAg induces neoplastic transformation in islet cells, probably late in neuroendocrine cell differentiation. T antigen expression during neoplastic development may result in a pervasive change in the islet cell growth properties with selection of a transformed phenotype as a possible requirement for cell viability.

Cytotoxic T-lymphocyte epitope immunodominance in the control of choroid plexus tumors in simian virus 40 large T antigen transgenic mice

The simian virus 40 (SV40) large tumor antigen (Tag) is a virus-encoded oncoprotein which is the target of a strong cytotoxic T-lymphocyte (CTL) response. Three immunodominant H-2(b)-restricted epitopes, designated epitopes I, II/III, and IV, have been defined. We investigated whether induction of CTLs directed against these Tag epitopes might control Tag-induced tumors in SV11(+) (H-2(b)) mice. SV11(+) mice develop spontaneous tumors of the choroid plexus due to expression of SV40 Tag as a transgene. We demonstrate that SV11(+) mice are functionally tolerant to the immunodominant Tag CTL epitopes. CTLs specific for the H-2Kb-restricted Tag epitope IV were induced in SV11(+) mice following adoptive transfer with unprimed C57BL/6 spleen cells and immunization with recombinant vaccinia viruses expressing either full-length Tag or the H-2Kb-restricted epitope IV as a minigene. In addition, irradiation of SV11(+) mice prior to adoptive transfer with unprimed C57BL/6 spleen cells led to the priming of epitope IV-specific CTLs by the endogenous Tag. Induction of epitope IV-specific CTLs in SV11(+) mice by either approach correlated with increased life span and control of the choroid plexus tumor progression, indicating that CTLs specific for the immunodominant Tag epitope IV control the progressive growth of spontaneous tumors induced by this DNA virus oncogene in transgenic mice.

Differential effects of cisplatin in proximal and distal renal tubule epithelial cell lines

Pathological studies suggest that cisplatin injures different portions of the nephron to different extents. To investigate this issue further, we examined the cytotoxicity and uptake of cisplatin in cell lines derived from S1 and S3 proximal tubule and distal convoluted tubule segments isolated from a mouse carrying the SV40 large T-antigen transgene. S1 cells displayed the highest sensitivity to cisplatin cytotoxicity, followed by S3 and distal convoluted tubule (DCT) cells. These differences in cytotoxicity did not correlate with differences in cisplatin uptake. Cytotoxic concentrations of cisplatin triggered apoptosis in all three cell lines. Although BAX and BCL-2 expression was similar among the three cell lines, the expression of the anti-apoptotic protein, BCL-X(L), was significantly lower in S1 cells than in S3 and DCT cells, and this may have contributed to the heightened sensitivity of S1 cells. Cisplatin transport characteristics demonstrated a saturable component of cisplatin uptake and differences in apparent K(M) and Vmax values among the three cell lines. The three cell lines were 43- to 176-fold more sensitive to cisplatin than to carboplatin. This distinction between the two drugs could not be fully explained by differences in the uptake rates of carboplatin and cisplatin. We conclude that cells from different portions of the nephron display different sensitivities to cisplatin, different transport characteristics for cisplatin and different levels of expression of BCL-X(L). In addition, the relative resistance of renal cells to carboplatin vs cisplatin is mostly due to the differential effects that follow internalization.

Role of a subdominant H-2Kd-restricted SV40 tumor antigen cytotoxic T lymphocyte epitope in tumor rejection

SV40-transformed mKSA cells (H-2d) readily induce progressively growing tumors in adult syngeneic BALB/c mice while expressing the full complement of H-2d MHC class I antigens. BALB/c mice previously immunized with SV40, soluble SV40 T antigen, or irradiated SV40-transformed syngeneic, allogeneic, or xenogeneic cells reject an mKSA tumor challenge even though these mice have been considered low- or nonresponders to T antigen due to difficulty in demonstrating SV40 T antigen-specific CTL. We have investigated the role of H-2d-restricted CTL in the rejection of SV40 tumors in BALB/c mice. Immunization of BALB/c mice with SV40 induced T antigen-specific CTL which were largely. H-2Ld-restricted. However, following repeated in vitro restimulation with mKSA cells, CTL emerged which recognized a subdominant H-2Kd-restricted epitope corresponding to T antigen residues 499-507. Immunization of BALB/c mice with a recombinant vaccinia virus expressing the T499-507 epitope provided partial protection against a challenge of syngeneic mKSA tumor cells and induced the generation of T499-507-specific CTL. These results indicate that a subdominant H-2Kd-restricted CTL epitope can participate in the rejection of SV40 tumors in BALB/c mice.

An endoplasmic reticulum-targeting signal sequence enhances the immunogenicity of an immunorecessive simian virus 40 large T antigen cytotoxic T-lymphocyte epitope

An immunological hierarchy among three H-2Db-restricted cytotoxic T lymphocyte (CTL) determinants in simian virus 40 (SV40) large T antigen (Tag) was described previously: determinants I and II/III are immunodominant, whereas determinant V is immunorecessive. To assess the immunogenicity of each determinant individually and define mechanisms that contribute to the immunorecessive nature of determinant V, we constructed a panel of recombinant vaccinia viruses (rVVs) expressing minigenes encoding these determinants in various polypeptide contexts. We found the following. (i) Immunization of mice with an rVV encoding full-length SV40 Tag resulted in priming for CTL responses to determinants I and II/III but not determinant V. (ii) rVVs encoding peptide I or II/III in the cytosol or targeted to the endoplasmic reticulum (ER) were highly antigenic and immunogenic. (iii) rVVs encoding peptide V minigenes were antigenic and immunogenic if the peptide was targeted to the ER, expressed in the cytosol with short flanking sequences, or expressed from within a self-protein, murine dihydrofolate reductase. (iv) Presentation of the nonflanked peptide V (preceded by a Met codon only) could be enhanced by using a potent inhibitor of the proteasome. (v) H-2Db-epitope V peptide complexes decayed more rapidly than complexes containing epitope I or II/III peptides. In brefeldin A blocking experiments, functional epitope V complexes were detected longer on targets expressing ER-targeted epitope V than on targets expressing forms of epitope V dependent on the transporter associated with antigen processing. Therefore, limited formation of relatively unstable cell surface H-2Db complexes most likely contributes to the immunorecessive nature of epitope V within SV40 Tag. Increasing the delivery of epitope V peptide to the major histocompatibility complex class I presentation pathway by ER targeting dramatically enhanced the immunogenicity of epitope V.

Adaptation to low-K+ media increases H(+)-K(+)-ATPase but not H(+)-ATPase-mediated pHi recovery in OMCD1 cells

Studies in rat and rabbit outer medullary collecting duct of inner stripe origin (OMCDis) suggest that both H(+)-ATPase and H(+)-K(+)-ATPase participate in H+ secretion. However, the relative contributions of these transporters, and, in particular, that of H(+)-K(+)-ATPase to K+ absorption have not been defined precisely. The present study was designed to delineate more clearly the response of these two transporters to hypokalemia and acidosis in a newly developed mouse OMCD1 cell line. In cells grown in normal K+ (5 mM) media, intracellular pH (pHi) recovery was similar either in the presence or absence of K+ in the perfusate (delta pHi/min = 0.014 +/- 0.001 vs. 0.017 +/- 0.003, not significant). The inhibitory effects of Sch-28080 (10 microM) and bafilomycin A1 (10 nM) on pHi recovery were evident only in the presence and absence of K+ in the perfusate, respectively. In cells grown in low-K+ (2.5 mM) media to simulate chronic hypokalemia, pHi recovery was significantly faster than in cells grown in normal K+ media (delta pHi/min = 0.045 +/- 0.01 vs. 0.014 +/- 0.001, P < 0.01) and was inhibited specifically by Sch-28080, not by bafilomycin A1. In contrast, in cells preconditioned to low pH (7.0) to simulate chronic acidosis, the enhanced pHi recovery was abolished by bafilomycin A1 but not by Sch-28080. 86Rb+ uptake, when used as a K+ congener, was inhibited by Sch-28080. The K(m) for 86Rb+ uptake (H(+)-K(+)-ATPase activity) and the 50% inhibitory concentration for Sch-28080 were 270 and 5.0 microM, respectively. These studies provide evidence that, in morphologically homogeneous OMCD1 cells, 1) both H(+)-K(+)-ATPase and H(+)-ATPase participate in pHi regulation, 2) the H(+)-K(+)-ATPase is selectively upregulated by preconditioning in low-K+ media, and 3) conversely, preconditioning in low-pH media stimulates only the H(+)-ATPase. Thus, in OMCDis, the H(+)-K(+)-ATPase and H(+)-ATPase respond selectively and independently to chronic hypokalemia and acidosis, respectively.

An in vitro tubulogenesis system using cell lines derived from the embryonic kidney shows dependence on multiple soluble growth factors

Interactions between the ureteric bud (UB) and metanephric mesenchyme are crucial for tubulogenesis during kidney development. Two immortalized cell lines derived from the day 11.5 embryonic kidney, UB cells, which appear to be epithelial (cytokeratin-positive, E-cadherin-positive, and ZO-1-positive by immunostaining) and BSN cells, which are largely mesenchymal (vimentin-positive, but negative for cytokeratin, cell surface E-cadherin, and cell surface ZO-1), were used to establish an in vitro tubulogenesis system. BSN cells expressed hepatocyte growth factor (HGF) and transforming growth factor-beta1 mRNAs, and its conditioned medium (BSN-CM) contained factors capable of activating the epidermal growth factor (EGF) receptor (EGFR). When UB cells were cultured in an extracellular matrix gel in the presence of the embryonic kidney or BSN-CM, the UB cells underwent morphogenetic changes characteristic of early in vitro branching tubulogenesis. These changes were largely inhibited by a combination of neutralizing anti-HGF antibodies and the EGFR inhibitor tyrphostin AG1478, suggesting that EGFR ligands, together with HGF, account for much of this early morphogenetic activity. Nevertheless, there was a significant fraction of tubulogenic activity that could not be inhibited, suggesting the existence of other soluble factors. Whereas HGF, EGF, transforming growth factor alpha, basic fibroblast growth factor (bFGF), and insulin-like growth factor 1 (IGF-1), or a mixture of these growth factors, induced epithelial processes for up to 3 days, only IGF-1, possibly bFGF, and the mixture were able to sustain morphogenesis for longer periods, though not nearly to the same degree as BSN-CM. Moreover, only BSN-CM induced branching tubular structures with clear lumens, consistent with the existence of other soluble factors crucial for the formation and/or maintenance of branching tubular structures with lumens in vitro.

Localization of large-T oncoprotein during the embryonic and fetal development of transgenic mice

Oncoproteins are not only involved in the development of tumors but also play a role in physiological regulation in embryonic growth and differentiation. The mechanisms by which regulation is accomplished in embryonic stages differ from postnatal or adult stages. Oncoproteins responsible for tumors in the adult, i.e., products of proto-oncogenes, are prevented from causing tumors in the embryo. If oncogenes are introduced artificially into the embryo. will they be governed by the embryonic regulation described above? To answer this question we used transgenic mice in which the hybrid construct MSV-SV40-Large-T, composed of the Simian-Virus-Oncogene. Large-T with its SV40-promotor and the Moloney Murine Sarcoma Virus (MSV)-enhancer, had been integrated. Under the influence of large-T expression, these animals develop either brain or endocrine pancreas tumors. In the present investigation, we localised large-T expression during development of mouse embryos and fetuses. Interestingly, we saw large-T positive reactions in organ anlagen other than those that later develop tumors. We found large-T antigens in cartilage anlagen, e.g., in ribs and vertebrae, particularly in fetuses of days 14 to 17, and also in a variety of epithelial cells such as in the lung or the choroid plexus. Our results indicate that, as for proto-oncogene products, the effect of an artificially introduced transgenic oncogene product can also be regulated by embryonic cells.

Development of thymic carcinoma in transgenic mice expressing SV40 T antigen

We produced transgenic mice using SV40 Tag gene under the control of its own enhancer and promoter. Three transgenic lines (SNU-SVT125, 127, 248) consistently developed thymic carcinoma as well as choroid plexus carcinoma and dysplastic renal tubule. In SNU-SVT248 line, SV40 Tag transgene was expressed at thymus, spleen and kidney. Thymic epithelium showed high level expression of SV40 Tag in immunohistochemistry. Histopathological and electron microscopic analysis revealed that poorly differentiated carcinoma was derived from type 2 to 4 thymic epithelial cell. Our transgenic mice would provide a model for studies on the pathogenesis of thymic carcinoma and on the regulation of thymopoiesis by epithelial cells.

Establishment and characterization of choroid plexus carcinoma cell lines: connection between choroid plexus and immune systems

Murine choroid plexus cell lines were produced from choroid plexus carcinoma generated in transgenic mice harboring the viral oncogene simian virus 40 large tumor antigen under transcriptional control of an intronic enhancer region from the human immunoglobulin heavy chain (IgH) gene. Two morphologically distinct cell lines have been cloned. These established cell lines retained the characteristics of choroid plexus cells in that they expressed such choroid plexus cell marker or related proteins as transthyretin and alpha2-macroglobulin. They were tumorigenic in nude mice. In the cell lines, the muA and muB (HE2) motifs within the IgH intronic enhancer were active and we also demonstrated the existence of the proteins binding to these motifs, suggesting a potential link between the choroid plexus and immune systems. It is considered that these binding proteins act as trans-activators for the enhancer and may belong to the class of ETS-related proteins. These cell lines and xenografts should be useful materials for analyses of choroid plexus functions.

Alterations of the thymic selection process in transgenic mice expressing SV40 large T antigen

We generated SV40 T antigen transgenic mice (lines SVT125, SVT127, and SVT248) which developed unique thymic carcinomas originating from thymic cortical epithelial cells. In these mice we observed alterations in the thymic selection process not reported before in SV40 T antigen transgenic mice. Along with tumor cell growth, thymocytes increased in number and the proportion of CD4 or CD8 single positive cells rose to 10 times the normal level. Expression of SV40 T antigen was detectable by Northern analysis in thymic stromal cells but not in thymocytes. Thymic stromal cell lines, derived from the thymic tumor, produced high levels of cytokines which caused morphological transformation and growth stimulation in hematopoietic stem cells, including fetal liver cells and bone marrow cells. These observations suggest that the unusual multiplication of thymocytes and the alterations in thymic selection are the result of the activity of thymic stromal cells transformed by SV40 T antigen. The cell lines derived from the tumor can thus be used to study cytokines involved in thymic differentiation of T cells.

Xenobiotic transport differences in mouse mesangial cell clones expressing mdr1 and mdr3

P-glycoprotein (PGP), which confers multidrug resistance to cancer cells, is expressed in mouse kidney proximal tubule and mesangium. We report on the expression of PGP and its xenobiotic transport function in mesangial cells. Studies were performed in a mouse mesangial cell line (TKGM) and two cell clones. Ribonuclease protection assay and Western blot analysis demonstrated that TKGM cells expressed mdr1 and mdr3, the isoforms responsible for multidrug resistance. TKGM-F12 cells coexpressed mdr1 and mdr3 whereas TKGM-G2 cells expressed only mdr1. The drug transport function, measured by rhodamine 123 (R-123) efflux, was smaller in TKGM-F12 than in TKGM-G2 cells. The PGP substrates adriamycin, cyclosporin A, vinblastine, and verapamil inhibited R-123 transport in TKGM and TKGM-G2 cells. In the cells studied, PGP conferred some resistance to adriamycin; concomitant exposure to adriamycin with another PGP substrate impaired cell growth. The differential expression of mdr1 and mdr3 in mouse mesangial cell clones, the ability of mdr1 PGP to transport R-123, and the impairment of PGP-mediated transport in TKGM-F12 cells, coexpressing mdr1 and mdr3 products, are demonstrated. PGP may play a physiological role in mesangial cells.

Some characteristics of neoplastic cell transformation in transgenic mice

The role of the expression of different cellular genes and viral oncogenes in malignant cell transformation is discussed. We pay special attention to the role of the genes for growth factors and their receptors and homeobox genes in oncogenesis. Based on both the literature and our own data, specific features of tumors developed in transgenic mice are discussed. All of these data are used to analyze current theories of multistep oncogenesis and the stochastic component in this process. We suggest that all known evidence about the mechanisms of oncogenesis be used in studying the problem at various structural and functional levels in an organism. The chapter shows that transgenic mice are a most suitable model for studying various aspects of malignant transformation from the molecular to the organismal and populational levels.

Transgenic and gene disruption techniques in the study of neurocarcinogenesis

Transgenic technologies have come of age, and the field of carcinogenesis has profited extensively from the availability of these methods. Both the inappropriate expression of dominant oncogenes in specific tissues and the ability to "knock out" tumor suppressor genes in mammalian organisms have enabled substantial advancements of our understanding of development and progression of the neoplastic phenotype. In the first part of this article, we review the most popular techniques for modification of the mammalian genome in vivo, i.e. microinjection of fertilized eggs, retrovirus-mediated gene transfer, and targeted gene deletion through homologous recombination. Subsequently, we attempt a critical evaluation of the available models of neurocarcinogenesis, and discuss their impact and future potential for the study of cancer in the nervous system.

Relationship between expression of epidermal growth factor and simian virus 40 T antigen in a line of transgenic mice

The pattern of expression of the simian virus 40 (SV40) T antigen gene and resultant dysplasia were re-examined in a line of transgenic mice in which the T antigen gene was under the control of the SV40 early promoter. We found that T antigen expression in the kidney, and resulting dysplastic lesions, occurred exclusively in the distal convoluted tubules and the ascending limbs of Henle. Epidermal growth factor (EGF) expression in the kidney of normal mice was similarly immunolocalized. The correlation between high EGF immunoreactivity in normal mouse tissues and T antigen expression in the transgenic counterpart was also seen in the choroid plexus epithelium and in the submandibular glands of male mice. T antigen was not found in the submandibular gland of transgenic females. Similarly, EGF was only rarely detected in the normal female submandibular gland. In contrast to the correlation between T antigen expression in the transgenic mice and EGF expression in the corresponding tissues of the normal mice, within the dysplastic lesions of the transgenic mice EGF expression was severely diminished. Adenocarcinomas of the male submandibular gland from another line of transgenic mice that expresses the Int-1 transgene, showed similarly reduced levels of immunostaining for EGF. Thus, reduced expression of EGF might be a general feature of dysplasia and tumorigenesis in those tissues that normally express EGF.

Expression and function of P-glycoprotein in a mouse kidney cell line

P-glycoprotein (PGP), a transporter conferring multidrug resistance to cancer cells, is expressed in the kidney. C219 monoclonal antibody binding revealed PGP in proximal tubules and mesangium of mouse kidneys. A cell line (TKPTS) expressing PGP was developed from proximal tubules of the 8Tg(SV40E)Bri7 mouse. Northern blot analysis demonstrated a 5.0-kb message identified as mdr1 by ribonuclease protection assay. Cyclosporin A (CSA) at 0.15 and 10 microM increased cellular accumulation of verapamil (VRP) by 32 and 121%, respectively (P < 0.001). VRP at 5 microM increased steady-state cellular accumulation of CSA by 46% (P = 0.02). Basal-to-apical transport of the PGP substrate vinblastine was inhibited by VRP. Rhodamine-123 (R-123) influx was rapid and independent of PGP. R-123 efflux was inhibited by VRP and CSA. Inhibition of PGP transport by VRP, CSA, and PSC-833 decreased the 50% effective dose of adriamycin. The concomitant administration of VRP and CSA was not deleterious and coincided with preferential accumulation of VRP over CSA. Inhibition of PGP-mediated transport is demonstrated as a mechanism of renal cell toxicity.

Transcriptional activation by viral enhancers: critical dependence on extracellular matrix-cell interactions in mammary epithelial cells

Extracellular matrix (ECM)-cell interactions are essential for the regulation of many genes in differentiated cell types. A number of expression vectors that work well in cells cultured on tissue-culture plastic appear to be inactive or sporadically active in vivo. We reasoned that these responses also may be influenced by the ECM. We therefore examined three commonly used viral enhancers and found that they all responded either positively or negatively to the presence of exogenous ECM. Using mouse mammary epithelial cells, we found that a mouse mammary tumor virus enhancer linked to its own promoter or to a truncated (and by itself inactive) beta-casein promoter drove transcription efficiently only when the cells were in contact with an ECM (more than a 100-fold induction over tissue-culture plastic). Similarly, the cytomegalovirus enhancer was more active in cells in contact with ECM. In contrast, the simian virus 40 enhancer, linked to the beta-casein promoter, was 12-fold more active in cells on tissue-culture plastic. This activity was strongly reduced when the cells interacted with ECM. Thus, we conclude that different enhancers can respond to ECM by either activating or suppressing transcription. This observation has important implications for understanding the mechanisms of promoter action and for designing expression systems for use in gene therapy.

Animal models for brain tumors: historical perspectives and future directions

The scientific understanding of the biology of human brain tumors has advanced in large part through the use of animal models. For most of this century, investigators have been evaluating the inciting factors in brain tumor development, and applying this knowledge to direct tumor growth in laboratory animals. Virus-induced, carcinogen-induced, and transplant-based models have been vigorously investigated. As knowledge of the molecular biology of neoplasia has advanced, transgenic technology has been introduced. The authors review the development of animal models for brain tumor, and focus on the role of transgenic models in elucidating the complex process of central nervous system neoplasia.

The fodrin-ankyrin cytoskeleton of choroid plexus preferentially colocalizes with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2

A unique feature of the choroid plexus as a single-layer epithelium is its localization of Na+K(+)-ATPase at its apical (lumenal) surface. In contrast, a band 3 (AE1)-related anion exchanger protein has been localized to the basolateral surface of the choroid plexus. Both Na+K(+)-ATPase and AE1 in other tissues have been shown to bind via ankyrin to the spectrin-actin-based membrane cytoskeleton. Since linkage of integral membrane proteins to the membrane cytoskeleton is important for their restriction to specialized domains of the cell surface, we investigated the polarity of the choroid plexus membrane cytoskeleton. We developed isoform-specific antibodies to confirm the identity of choroid plexus band 3-related polypeptide as AE2. We demonstrated that ankyrin, fodrin/spectrin, actin, myosin, and alpha-actinin are predominantly apical in choroid plexus and preferentially colocalize with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2. Colchicine administration did not alter the polarity of apical cytoskeletal and transport proteins or basolateral AE2 in choroid plexus, suggesting that biosynthetic targeting of these proteins is not microtubule dependent. In choroid plexus papilloma, Na+K(+)-ATPase and AE2 were decreased in amount and failed to preserve their polarized distributions.

Expression of viral T-antigen in pathological tissues from transgenic mice carrying JC-SV40 chimeric DNAs

Immunostaining methods were used to detect viral T-antigen and the cellular protein p53 in pathological tissues obtained from transgenic mice carrying JC-SV40 hybrid viral DNAs. A transgenic mouse carrying the SV40 regulatory region and JC virus (JCV) T-antigen-coding sequences exhibited an SV40-characteristic choroid plexus papilloma that expressed JCV T-antigen and p53. JCV-associated pathology was observed in two other mice in which the JCV regulatory signals directed SV40 T-antigen-induced adrenal neuroblastomas and brain neoplastic cells. However, these mice also exhibited an SV40-characteristic osteosarcoma and abdominal lymphoma that contained SV40 T-antigen and p53-positive cells. Contrasting thymic pathology was observed in the two types of mice where the SV40 regulatory region directed a JCV T-antigen-induced thymoma in one mouse, and the JCV regulatory region directed SV40 T-antigen-induced thymic hypoplasia in two other mice.

Rhabdomyosarcoma arising in transgenic mice harboring the beta-globin locus control region fused with simian virus 40 large T antigen gene

The beta-globin locus control region (LCR) confers a high level of erythroid-specific and copy-number-dependent expression to human globin genes in transgenic mice. Simian virus 40 T (tumor) antigen (Tag) with its own natural enhancer causes choroid plexus tumors in mice. We investigated the effect of the LCR on Tag gene expression, reasoning that mice harboring a LCR-Tag fusion gene might develop hematopoietic malignancies. To test this hypothesis we introduced an enhancerless Tag gene downstream of a LCR cassette into the germ lines of mice. The phenotypes of the transgenic mice depended on the copy number of the transgene. While mice with 1-2 copies matured normally, those with 3-7 copies developed rhabdomyosarcomas in different anatomic sites at high frequency and showed hyperplasia of the pancreatic islet cells which progressed to pancreatic islet tumors. In addition, the mice bearing 7 copies of the transgene had hypoglycemia and were stunted in growth. Mice with more than 10 copies were markedly stunted in growth and died within 2-4 weeks. Tag expression was detected at high levels in the mouse tumors but not in any other tissues, including the hematopoietic cells.

Expression of c-fos and c-myc protooncogenes in an immortalized hepatocyte line harbouring SV40 T antigen and hGH as transgenes

A clonal hepatocyte line (FMH-202-2), derived from livers of fetal transgenic mice harbouring human growth hormone (hGH) and SV40 T antigen as transgenes, was used in the investigation of protooncogene expression involved in liver-specific growth control and/or in hepatocellular transformation. In this model system, representing an immortalized, yet untransformed phenotype, the transgenes hGH and SV40 T antigen were expressed constitutively. The c-fos protooncogene was induced by incubation with insulin, epidermal growth factor (EGF) and insulin-like growth factor (IGF-I) in a transient manner comparable to its expression in primary murine hepatocytes. Elucidation of second messenger mechanisms demonstrated that c-fos induction by hepatotrophic growth factors was not mediated by protein kinase C. In contrast to primary hepatocytes, the c-myc protooncogene exhibited a constitutive expression pattern which was independent of growth factor stimulation. These results indicate that apart from hGH and SV40 T antigen, c-myc may play a role in cellular immortalization, but that constitutive expression of these genes, even in combined coexpression, does not suffice to induce the transformed phenotype.

A mouse mammary tumor virus mammary gland enhancer confers tissue-specific but not lactation-dependent expression in transgenic mice

The long terminal repeat (LTR) of mouse mammary tumor virus (MMTV) is known to contain a number of transcriptional regulatory elements, including glucocorticoid response elements. In this study, we showed that a mammary gland/salivary gland enhancer found in the LTR of this virus directs expression of a heterologous promoter to both virgin and lactating mammary glands in transgenic mice. Using transgenic mice containing hybrid gene constructs with various deletions of the LTR sequences linked to marker genes, we also showed that the dramatic increase in MMTV expression that occurs during lactation is due to the glucocorticoid response elements. Thus, the MMTV LTR encodes two distinct elements, both of which are required for a high level of expression in lactating mammary glands.

Antibody response to human papovavirus JC (JCV) and simian virus 40 (SV40) T antigens in SV40 T antigen-transgenic mice

Human papovavirus JC (JCV) and simian virus 40 (SV40) genomes share approximately 69% homology; and there is antigenic cross-reactivity between JCV and SV40 tumor or T antigens. In order to determine whether a selective immune response to JCV T antigen could be demonstrated, transgenic mice (SV11+) that express SV40 T antigen in the choroid plexus and are partially tolerant to antigenic determinants on SV40 T antigen were immunized with SV40 or JCV T antigens and their antibody responses were analyzed. The results show that SV11+ mice responded as well as their nontransgenic litter mates to JCV T antigen. Monoclonal antibodies were derived from hybridomas generated from immunized mice which reacted specifically with epitopes in the amino and carboxy terminal halves on JCV T antigen. These studies show that transgenic mice expressing SV40 T antigen are capable of responding to determinants not shared between JCV and SV40 T antigen.

SV40 small t deletion mutants preferentially transform mononuclear phagocytes and B lymphocytes in vivo

Hamsters injected intracardiacally with wild-type SV40 developed diverse tumors, with no one type predominating. However, hamsters injected with small t deletion mutants of SV40 uniformly developed true histiocytic or B-cell lymphomas, both of which arise from rapidly dividing cell populations, with very few tumors of other types. Our results are consistent with in vitro studies which suggest a requirement for the small t function in SV40 transformation of nondividing cells.

Hepatitis B virus infection and primary hepatocellular carcinoma

For many years, epidemiological studies have demonstrated a strong link between chronic hepatitis B virus (HBV) infection and the development of primary hepatocellular carcinoma (PHC). Other hepatocarcinogens such as hepatitis C virus and aflatoxin also contribute to hepatocarcinogenesis either in conjunction with HBV infection or alone. Cellular and molecular biological studies are providing explanations for the HBV-PHC relationship, and models are now being formulated to further test the relative importance of various factors such as viral DNA integration, activation of oncogenes, genetic instability, loss of tumor suppressor genes, and trans-activating properties of HBV to the pathogenesis of PHC. Further research will probably define more than a single mechanism whereby chronic HBV infection results in PHC.

Unregulated and basal transcriptional activities of the regulatory sequence of the type 18 human papillomavirus genome in transgenic mice

Type 18 human papillomavirus (HPV18) is a genital virus closely associated with cervical carcinoma. To analyze the transcriptional activities of the long control region (LCR) of the HPV18 genome, we have produced 12 transgenic mice harboring the HPV18/LCR sequence fused to a promoterless SV40 T-antigen (TAg) gene. The mice were small in body size, generally very weak, and none lived longer than 110 days. Three mice with the longest life span (58-110 days) developed hyperplastic thymus and/or lymph node and were further analyzed. In these mice, Northern hybridization failed to detect TAg transcripts in any of the 25 organs studied. However, spliced TAg RNA was detected by polymerase chain reaction in the hyperplastic thymus and lymph node and in the normal submaxillary gland, stomach, large intestine, urinary bladder, and the cerebrum, indicating the presence of very low cellular levels of TAg RNA in these organs. When immunostaining was performed on the hyperplastic thymus, TAg protein was detected only in the ductal epithelial cells. Our results appear to indicate that the HPV18/LCR sequence was able to express only unregulated and basal levels of transcriptional activity in transgenic mice. Such a mode of transcription has become a major hindrance in the use of transgenic mouse system for the studies of the biology of the human papillomavirus.

Atrial natriuretic peptide synthesis in atrial tumors of transgenic mice

Transgenic mice harboring a chimeric gene linking mouse protamine 1 5'-flanking sequence to the coding sequence of the simian virus 40 T-antigen develop spontaneous rhabdomyosarcomas of the right atria. The presence of the tumors is accompanied by dramatic elevations in plasma atrial natriuretic peptide (ANP) immunoreactivity (1,698 +/- 993 vs. 60 +/- 18 fmol/ml for controls) and hematocrit (56 +/- 8 vs. 51 +/- 2 for controls). The immunoreactive ANP (irANP) present in the tumors is similar in size to irANP found in normal mouse atria. ANP mRNA transcripts present in the tumors also appear to be very similar in overall size and 5'-termini to those produced in normal cardiac tissue. Microscopically, the tumors are composed of a disorganized array of densely packed abnormal-appearing cells. Immunocytochemistry and in situ hybridization analysis reveal considerable heterogeneity in ANP gene expression. ANP peptide and mRNA are detectable throughout the parenchyma of the tumors, but absolute levels of expression vary widely among different cells in the population. These tumors represent a potentially valuable model for the study of inappropriate ANP secretion and may provide a tissue source for the development of an ANP-producing atrial cell line.

Photoreceptor degeneration induced by the expression of simian virus 40 large tumor antigen in the retina of transgenic mice

Expression of the viral oncogene encoding the simian virus 40 (SV40) large tumor antigen (T antigen) typically promotes tumorigenesis in mammalian cells. To generate transgenic mice that express T antigen in rod photoreceptors, a chimeric construct consisting of a mouse opsin promoter fragment fused to the coding region of SV40 T antigen was generated. Expression of T antigen in the transgenic retina began at early stages of postnatal development concomitant with expression of endogenous opsin. Instead of inducing hyperplasia or tumor formation, T-antigen expression caused a rapidly progressing photoreceptor degeneration. The degeneration was accompanied by sustained DNA synthesis in photoreceptor cells, as evidenced by incorporation of [3H]thymidine and by the appearance of mitotic figures at postnatal day 10, a stage when nontransgenic photoreceptor cells are postmitotic and quiescent. Although transgenic photoreceptor cells undergo S phase and enter mitosis, the consequences of T-antigen expression are not proliferation and tumorigenesis but proliferation and cell death.

Localization of overexpressed c-myc mRNA in polycystic kidneys of the cpk mouse

The C57BL/6J-cpk mouse has a form of autosomal-recessive polycystic kidney disease characterized by the rapid growth of large collecting duct cysts and the development of severe renal failure usually by three to four weeks of age. Previous studies had shown higher steady-state levels of proto-oncogene mRNA in these cystic kidneys. It is now shown using nuclear run-on transcription that the c-fos and c-myc proto-oncogenes are transcribed at higher rates in cystic kidneys, and thus that increased transcription, in part, may account for the increased mRNA levels. c-myc mRNA was detected by in situ hybridization in nephron anlagen and elongating tubules of normal and cystic kidneys during late fetal and early neonatal kidney development. Localization of c-myc expression in the normal kidney decreased with age over the three-week postnatal period. By contrast, c-myc mRNA was found in cysts as early as three days of age, with increased levels at two and three weeks. c-myc expression was also elevated in apparently normal, non-dividing proximal tubules in three-week-old cystic animals. On the basis of these findings, we suggest that c-myc expression is linked to the proliferation of cells engaged in the primary cystogenic process, and that expression of this gene in proximal tubule cells of severely azotemic animals reflects the compensatory response of residual tubular epithelial cells to progressive renal dysfunction.

Uniform cell-autonomous tumorigenesis of the choroid plexus by papovavirus large T antigens

The simian virus 40 (SV40) large tumor antigen (T antigen) under its natural regulatory elements induces choroid plexus papillomas in transgenic mice. Because these tumors develop focally after several months, it has been suggested that secondary cellular alterations are required to induce a tumor in this tissue. In contrast to SV40, the related lymphotropic papovavirus early region induces rapid nonfocal choroid plexus neoplasia in transgenic mice. Here, using hybrid gene constructs, we showed that T antigen from either virus in in fact sufficient to induce these tumors. Their abilities to induce proliferative abnormalities in other tissues, such as kidney and thymus, were also indistinguishable. Differences in the rate of choroid plexus tumorigenesis reflected differences in the control regions of the two viruses, rather than differences in T antigen per se. Under SV40 regulation, expression was limited to a fraction of the choroid plexus cells prior to the formation of focal tumors. When SV40 T antigen was placed under lymphotropic papovavirus control, in contrast, expression was generally uniform in the choroid plexus and rapid expansion of the tissue ensued. We found a direct relationship between T-antigen expression, morphological transformation, and proliferation of the choroid plexus epithelial cells. Analysis of mosaic transgenic mice indicated further that T antigen exerts its mitogenic effect cell autonomously. These studies form the foundation for elucidating the role of various T-antigen subactivities in tumorigenesis.

Uniform cell-autonomous tumorigenesis of the choroid plexus by papovavirus large T antigens

The simian virus 40 (SV40) large tumor antigen (T antigen) under its natural regulatory elements induces choroid plexus papillomas in transgenic mice. Because these tumors develop focally after several months, it has been suggested that secondary cellular alterations are required to induce a tumor in this tissue. In contrast to SV40, the related lymphotropic papovavirus early region induces rapid nonfocal choroid plexus neoplasia in transgenic mice. Here, using hybrid gene constructs, we showed that T antigen from either virus in in fact sufficient to induce these tumors. Their abilities to induce proliferative abnormalities in other tissues, such as kidney and thymus, were also indistinguishable. Differences in the rate of choroid plexus tumorigenesis reflected differences in the control regions of the two viruses, rather than differences in T antigen per se. Under SV40 regulation, expression was limited to a fraction of the choroid plexus cells prior to the formation of focal tumors. When SV40 T antigen was placed under lymphotropic papovavirus control, in contrast, expression was generally uniform in the choroid plexus and rapid expansion of the tissue ensued. We found a direct relationship between T-antigen expression, morphological transformation, and proliferation of the choroid plexus epithelial cells. Analysis of mosaic transgenic mice indicated further that T antigen exerts its mitogenic effect cell autonomously. These studies form the foundation for elucidating the role of various T-antigen subactivities in tumorigenesis.

A tubule cell line established from transgenic mice harboring temperature-sensitive simian virus 40 large T-antigen gene

Immortalization of cell lines with specific functions is important for examination of organ-specific functions in vitro. We established a kidney tubule cell line (TKC2) exhibiting a specific physiological response to hormone from the primary culture of kidneys of adult transgenic mice harboring temperature-sensitive SV40 large T-antigen gene. TKC2 cells showed temperature-sensitive growth in culture and exhibited characters of distal tubule cells such as dome formation at confluent culture and stimulation of cAMP synthesis by arginine vasopressin. These phenotypes are maintained after long passages in culture and may provide a new experimental system for studying renal physiology.

The use of transgenic animals to study the role of growth factors in endocrinology

Transgenesis is identified as being of special interest in the study of growth factors where their multicellular origins and complex interactions make them particularly difficult to characterize using classical experimental approaches developed to investigate hormones originating in specialized cells in discrete glands. Through allowing molecular 'tinkering' in intact animals, transgenesis enables specific growth factors to be 'ablated or replaced' from specific tissues and organs and target cell response and impact of modulatory factors such as binding proteins to be explored in the intact animal. To the endocrinologist, the potential applications of such technology are legend. This chapter provides a brief overview of the technique and provides linkages to the rapidly developing body of literature in establishing transgenesis in growth factor research.

Transgenic models of tumor development

Numerous cancer-prone strains of mice have been created by the introduction of candidate tumor-promoting genes into fertilized eggs. Each transgenic strain is predisposed to develop specific types of tumors, but they usually arise stochastically because of the need for spontaneous mutation of genes that collaborate with the introduced oncogene. These mice are providing insights into the effects of individual oncogenes on cellular proliferation, differentiation, and viability, as well as on oncogene cooperativity. Their predisposed state imposes sensitivity to viral and chemical carcinogenesis, and the mice should prove valuable in tests of potential carcinogens, therapies, and preventive measures.

Hepatocyte cell lines established from transgenic mice harboring temperature-sensitive simian virus 40 large T-antigen gene

To establish cell lines exhibiting differentiation phenotypes, the immortalized cell lines were rapidly established from the primary culture of different tissues of transgenic mice harboring SV40 temperature-sensitive large T-antigen gene. The established cell lines grew at permissive temperature (33 degrees C), but not at nonpermissive temperature (39 degrees C). Several different cell types could be rapidly immortalized and cloned from the adult transgenic mice tissues. Among those cell lines, the established hepatocyte cell lines (TLR cell lines) exhibited liver-specific morphological and biochemical properties, but their properties were not coupled with the growth condition modified by temperature. The hepatocyte cell lines showed an inducibility of P450IA1 by 3-methylcholanthrene as observed in rat livers and this liver-specific function was stable even after 6 months of culture by continuous passages.

Identification of cis sequences controlling efficient position-independent tissue-specific expression of human major histocompatibility complex class I genes in transgenic mice

We previously reported that genomic major histocompatibility complex class I human leukocyte antigen (HLA)-B7 gene constructs with as little as 0.66 kb of 5'- and 2.0 kb of 3'-flanking DNA were expressed efficiently and appropriately in transgenic mice. To identify and characterize the relevant cis-acting regulatory elements in more detail, we have generated and analyzed a series of transgenic mice carrying native HLA-B7 genes with further 5' truncations or intronic deletions and hybrid constructs linking the 5'-flanking region of B7 to a reporter gene. We were unable to detect a specific requirement for sequence information within introns 2 to 7 for either appropriate constitutive or inducible class I expression in adult animals. The results revealed the presence of cis-acting regulatory sequences between -0.075 kb and -0.66 kb involved in driving efficient copy number-dependent constitutive and gamma interferon-enhanced tissue-specific expression. The region from -0.11 to -0.66 kb is also sufficient to prevent integration site-specific "position effects," because in its absence HLA-B7 expression is frequently detected at significant levels at inappropriate sites. Conserved sequence elements homologous to the H-2 class I regulatory element, or enhancer A, and the interferon response sequence are located between about -151 and -228 bp of the B7 gene. Our results also indicate the existence of sequences downstream of -0.11 kb which can influence the pattern of tissue-specific expression of the HLA-B7 gene and the ability of this gene to respond to gamma interferon.

Identification of cis sequences controlling efficient position-independent tissue-specific expression of human major histocompatibility complex class I genes in transgenic mice

We previously reported that genomic major histocompatibility complex class I human leukocyte antigen (HLA)-B7 gene constructs with as little as 0.66 kb of 5'- and 2.0 kb of 3'-flanking DNA were expressed efficiently and appropriately in transgenic mice. To identify and characterize the relevant cis-acting regulatory elements in more detail, we have generated and analyzed a series of transgenic mice carrying native HLA-B7 genes with further 5' truncations or intronic deletions and hybrid constructs linking the 5'-flanking region of B7 to a reporter gene. We were unable to detect a specific requirement for sequence information within introns 2 to 7 for either appropriate constitutive or inducible class I expression in adult animals. The results revealed the presence of cis-acting regulatory sequences between -0.075 kb and -0.66 kb involved in driving efficient copy number-dependent constitutive and gamma interferon-enhanced tissue-specific expression. The region from -0.11 to -0.66 kb is also sufficient to prevent integration site-specific "position effects," because in its absence HLA-B7 expression is frequently detected at significant levels at inappropriate sites. Conserved sequence elements homologous to the H-2 class I regulatory element, or enhancer A, and the interferon response sequence are located between about -151 and -228 bp of the B7 gene. Our results also indicate the existence of sequences downstream of -0.11 kb which can influence the pattern of tissue-specific expression of the HLA-B7 gene and the ability of this gene to respond to gamma interferon.

Characterization of trans-immortalized hepatic cell lines established from transgenic mice

Hepato-specific regulatory (promoter/enhancer) DNA sequences were used for targeting the expression of onc genes, such as murine c-myc and Simian Virus 40 T Antigen, to hepatocytes of transgenic mice which subsequently developed hepatocellular carcinomas after a variable period of time (depending on the type of onc gene employed). Several trans-immortalized cell lines were established and compared with respect to the expression of adult hepatic markers and response to growth factors. Despite the morphological differences observed between trans-hepatomas, owing to the expression of the two different onc genes, all tumor-derived cell lines behaved in a comparable fashion during long-term culture displaying an adult hepatic phenotype for at least 40 passages. They differed, however, in response to epidermal growth factor. When the gene coding for human alpha 1-antitrypsin was placed under the control of the same hepato-specific promoter/enhancer, high levels of the human recombinant protein could be harvested from the supernatants of trans-hepatoma-derived cell lines.