Tissue-specific activation of a cloned alpha-fetoprotein gene during differentiation of a transfected embryonal carcinoma cell line

Regulated expression of a transfected human cardiac actin gene during differentiation of multipotential murine embryonal carcinoma cells

P19 embryonal carcinoma (EC) cells are multipotential stem cells which can be induced to differentiate in vitro into a variety of cell types, including cardiac muscle cells. A cloned human cardiac actin (CH-actin) gene was transfected into P19 cells, and stable transformants were isolated. Low levels of CH-actin mRNA were present in transformed EC cells, but a marked increase in the level of CH-actin mRNA was found as these cells differentiated into cardiac muscle. The accumulation of CH-actin mRNA paralleled that of the endogenous mouse cardiac actin mRNA. A chimeric gene, which consisted of the CH-actin promoter linked to the herpes simplex virus thymidine kinase coding region, was constructed and transfected into P19 cells. In these transformants, the thymidine kinase protein was located almost exclusively in cardiac muscle cells and was generally not detectable in EC or other nonmuscle cells. These results suggest that the transfected CH-actin promoter functions in the appropriate developmental and tissue-specific manner during the differentiation of multipotential EC cells in culture.

Detailed kinetics of adenovirus type-5 steady-state transcripts during early infection

The appearance and steady-state accumulation of specific viral RNAs during the early phase of adenovirus type 5 (Ad5) infection was examined. HeLa cells were synchronously infected and harvested at 30 min intervals throughout the first 12 h of infection. Total cytoplasmic RNA was extracted from infected cells and analyzed by hybridization-selection and translation to identify the viral mRNAs from each early region on the basis of the protein products they encode. The same RNA samples were used for S-1 nuclease and Northern blot analyses to quantitatively compare the levels of individual viral RNAs that accumulate within each early transcription region (E1A, E1B, L1, E2A, E3 and E4). The salient features of this analysis show that RNA accumulation occurs first from E1A followed by E2A, E3 and E4, E1B and lastly, L1. Although the profile of RNA accumulation was unique for each early region, overlapping RNAs within E1A, E3, and E4, respectively, remained generally parallel to one another throughout early infection, in contrast to RNAs from E1B and L1, respectively. Since both the appearance and quantitative accumulation of specific early viral mRNAs were examined at many time points, a number of subtleties associated with the complex dynamics of early Ad5 gene expression were revealed. In particular, the L1 region was shown to transcribe from the major late promoter two early RNAs of 3.81 Kb and 3.5 Kb, either or both of which encode the 52,55 kDa proteins; the auxiliary i leader sequence was found on the 3.81 Kb RNA but not on the 3.5 Kb RNA.

Proteoglycan-19, laminin and collagen type IV production is correlated with the levels of mRNA in F9 cell aggregates differentiating in the presence or absence of cyclic AMP

F9 embryonal carcinoma cells differentiate to embryoid bodies containing an outer epithelial layer of visceral endoderm cells when cultured as aggregates in medium containing retinoic acid (RA). Another pathway of differentiation to parietal endoderm is followed when dibutyryl cyclic AMP (cAMP) is added to the medium. We have measured the accumulated levels of RNA transcripts from a chondroitin sulfate proteoglycan gene (PG-19), the type IV collagen alpha 1, alpha 2 subunit genes, and laminin B1, B2 subunit genes during these differentiation processes. Laminin B2 gene is uniquely regulated among the extracellular matrix component genes studied. The level of laminin B2 RNA remains almost invariant during RA induction of differentiation but is induced 11-fold by cAMP with RA. In contrast, laminin B1, collagen IV alpha 1, and alpha 2 genes are induced in two stages with six- to sevenfold accumulation of RNA induced by RA and fourfold greater levels by cAMP (19- to 28-fold overall). All of these matrix-encoding genes except proteoglycan are expressed at low levels in unstimulated F9 cells, whereas PG-19 is completely undetectable and is observed only after 2 days of stimulation with RA. Its increased expression with RA and cAMP induction is at least 100-fold during F9 differentiation. Extracellular matrix transcripts are relatively stable and this accounts in part for high accumulated levels during differentiation. We conclude that several kinds of gene regulation occur among the matrix components and other differentiation markers, and this makes the F9 model system useful to study the differential effects of hormone treatments on cellular events leading to differentiation and loss of tumorigenicity.

Expression in transgenic mice of two genes of different tissue specificity integrated into a single chromosomal site

Transgenic mice were used to study the expression of pairs of genes with distinctly different tissue specificities, covalently linked and integrated into the same chromosomal site. A transgenic strain carrying, in close proximity and in the same orientation, the rat fast skeletal muscle myosin light-chain 2 (MLC2) gene and the mouse rearranged immunoglobulin kappa light-chain gene expressed the immunoglobulin gene specifically in the lymphoid tissues, whereas rat MLC2 transcripts were found in skeletal muscle but not in the spleen or the other tissues that were tested. In another transgenic strain, carrying the rat MLC2 gene and a modified rat skeletal muscle actin gene (actin-globin chimeric gene), transcripts of the rat MLC2 gene were detected in skeletal muscle only, whereas the actin-globin transcripts were detected in skeletal muscle as well as in the heart. Moreover, the expression of the chimeric gene was also developmentally regulated. Expression was higher in cardiac muscle than in the skeletal muscle of neonatal mice, whereas expression was higher in skeletal muscle in adult mice. This pattern is consistent with the regulation of the expression of the endogenous skeletal muscle actin gene. Thus, in those transgenic strains that expressed both genes, each gene retained its tissue specificity, in spite of their close proximity. These results indicate a high degree of autonomy of the control elements included in the cloned genomic DNA fragment and demonstrate that a single chromosomal site can be permissive for the proper expression of two genes with different tissue specificities.

Interaction of a liver-specific nuclear factor with the fibrinogen and alpha 1-antitrypsin promoters

The orderly and sequential activation of genes during development is hypothesized to be related to the selective expression of groups of regulatory proteins acting primarily at the level of transcription. A nuclear protein was found in hepatocytes, but not other cell types, that binds to a sequence required for hepatocyte-specific transcription of the gene for the beta chain of fibrinogen. This protein, hepatocyte nuclear factor 1 (HNF1), also interacts with homologous sequences required for optimal promoter function of the genes for the alpha chain of fibrinogen and alpha 1-antitrypsin. The promoter or enhancer regions for several viral and cellular genes not expressed in the liver did not compete for this binding. The restricted expression of HNF1 and its selective interaction with the control regions of several liver-specific genes indicate that it is involved in developmentally regulated gene expression in the liver.

Proviral sequences that restrict retroviral expression in mouse embryonal carcinoma cells

Embryonal carcinoma (EC) cells are nonpermissive for retrovirus replication. Restriction of retroviral expression in EC cells was studied by using DNA transfection techniques. To investigate the activity of the Moloney murine leukemia virus (M-MuLV)enhancer and promoter sequences, the M-MuLV long terminal repeat and the defined long terminal repeat deletions were linked to neo structural gene sequences that encode resistance to the neomycin analog G418. Transient expression data and drug resistance frequencies support the findings that the M-MuLV enhancer is not active in EC cells but that promoter sequences are functional. In addition, a proviral DNA fragment that encodes the leader RNA sequence of a M-MuLV recombinant retrovirus was found to restrict expression specifically in EC cells. Deletion analysis of the leader fragment localized the inhibitory sequences to a region that spans the M-MuLV tRNA primer binding site. It is not known whether restriction occurs at a transcriptional or posttranscriptional level, but steady-state RNA levels in transient expression assays were significantly reduced.

Expression of the human beta-globin gene in mouse teratocarcinoma cells

We have stably transformed PCC4 and F9 teratocarcinoma stem cells with the human beta-globin gene. The transformed PCC4 cells retained their ability to produce in host mice tumours containing various differentiated tissues. Three of the five clones examined had human beta-globin transcripts, with correct 5' termini; however, some transcripts were also initiated upstream from the natural cap site in two of the three clones. The expression of the human beta-globin gene was maintained when the stem cell phenotype was changed, either into endodermal cell type by retinoic acid treatment of the F9 clones, or into fibroblastic cell type by cell hybridization of transformed PCC4 cells with L fibroblasts. Thus, the human beta-globin gene introduced into early embryonic cells can be expressed constitutively and its expression is maintained when the pattern of gene expression in the cells is changed.

The regulation of albumin and alpha-fetoprotein gene expression in mammals

Albumin and alpha-fetoprotein (AFP) are two plasma proteins synthesized by the liver and the yolk sac. The production of these major proteins is subject to considerable and characteristic variations during both the course of development and hepatic carcinogenesis. It is therefore a system of choice for the analysis of genetic expression during normal differentiation and the cancerous state of eukaryotic cells. The knowledge of regulatory mechanisms at the cellular and molecular levels of the albumin and AFP genes has recently made great progress: 1) the cells which are responsible for the synthesis of albumin and AFP in the liver and other organs have been defined by conjointly using in vitro and in vivo molecular hybridization techniques; 2) the organization of these genes and their adjoining regions has been established in the rat, the mouse and man; 3) the level at which the synthesis of these two proteins is regulated has been determined; it is the transcriptional level. The transcriptional regulation of the albumin and AFP genes could be the result of genome and/or chromatin conformation level modifications. Different groups have shown that: 1) the global structure of the albumin and AFP genes does not change during the course of development and hepatic carcinogenesis; 2) modifications at the level of the methylation of certain specific cytosines could be associated with the variations in the transcription of these genes; 3) global or local (hypersensitive sites with DNase I) changes of chromatin conformation could be correlated to the potential or the overt activity of the transcription of these genes. Very recently certain 'regulatory' regions having cis 'enhancer' or 'silencer' properties have been detected upstream from the albumin and AFP genes. These regions are hypothesized to be DNA 'target' sequences on which trans-acting regulatory factors are fixed and which control the transcription of these genes. Starting from the framework of this recent work, a model of albumin and AFP gene regulation is proposed.

Tissue-specific expression of the human type II collagen gene in mice

Type II collagen is crucial to the development of form in vertebrates as it is the major protein of cartilage. To study the factors regulating its expression we introduced a cosmid containing the human type II collagen gene, including 4.5 kilobases of 5' and 2.2 kilobases of 3' flanking DNA, into embryonic stem cells in vitro. The transformed cells contribute to all tissues in chimeric mice allowing the expression of the exogenous gene to be studied in vivo. Human type II collagen mRNA is restricted to tissues showing transcription from the endogenous gene and human type II collagen is found in extracellular matrix surrounding chondrocytes in cartilage. The results indicate that the cis-acting requirements for correct temporal and spatial regulation of the gene are contained within the introduced DNA.

Structure and expression of mos sequences in spontaneous and Moloney murine sarcoma virus-induced yolk sac carcinomas in rats

Facilitation of yolk-sac carcinoma (YSCa) development in fetectomized rats by the Moloney murine sarcoma virus/murine leukemia virus (Mo-MSV/MLV) complex was found to be closely associated with the presence of Mo-MSV sequences in the genomes of the YSCa cells. The virus-induced YSCas consisted of cells of mono- or oligoclonal origin which always contained in their genomes at least I randomly integrated Mo-MSV provirus. In YSCas which developed in the absence of virus, no rearrangement or amplification of c-mos could be detected. In addition, blot hybridization analysis of cellular RNA failed to detect mos-related RNA in cell lines derived from Mo-MSV-induced as well as from non-virally induced YSCas. The methylation level of c-mos DNA was low in all YSCa cell lines. In contrast, v-mos DNA in cell lines derived from Mo-MSV-induced YSCas was heavily methylated.

Human elongation factor 1 alpha: a polymorphic and conserved multigene family with multiple chromosomal localizations

One of the genes activated in human melanoma cells by the tumor-promoting phorbol ester is that of the elongation factor 1 alpha. A cDNA clone containing the complete 3'-end untranslated region and the nucleotide sequences coding for 227 carboxyterminal amino acids was isolated. Computer-assisted comparison with known sequences of elongation factors from other species revealed homologies up to 73% and 63% on amino acid and nucleotide sequences, respectively. Northern blot analysis of mRNA from unstimulated and phorbol ester-treated cells showed a 3- to 5-fold increase in cytoplasmic elongation factor 1 alpha mRNA after phorbol ester induction. When compared with the phorbol ester-inducible single-copy gene transcripts coding for the tissue-type plasminogen activator, the cellular mRNA content of elongation factor 1 alpha is 30 times higher. By Southern blot analysis experiments on human genomic DNA, a multi-gene family was found showing polymorphisms in restriction endonuclease fragment lengths (RFLP). Several polymorphisms were studied more extensively in the population on more than 100 DNA samples from normal individuals and in three-generation families. In situ hybridization of the cDNA probe to normal human metaphase chromosomes showed multiple chromosomal localizations of the elongation factor gene(s), with peak hybridization on the chromosomes 1, 2, 4, 5, 6, 7, and 15. The estimate of the gene copy number in humans is more than ten copies per (haploid) genome.

Specific sets of DNase I-hypersensitive sites are associated with the potential and overt expression of the rat albumin and alpha-fetoprotein genes

We have examined the chromatin structure of the 5'-flanking region of the albumin and alpha-fetoprotein (Afp) genes in different developing rat tissues and cloned cell lines that display various functional states of these genes. Nuclease-hypersensitive sites were probed with DNase I, using an indirect end-labeling technique. In albumin-producing rat cells two major DNase I-hypersensitive sites were found near the promoter region and one additional site was located approximately 3 kilobases (kb) upstream. Similarly, in Afp-producing rat tissues and cell lines we mapped one DNase I-hypersensitive region close to the promoter region and two cleavage sites further upstream at approximately 2.2 and approximately 3.8 kb from the cap site. The DNase I-hypersensitive sites of both genes were absent in nonhepatic rat cells and therefore appear to be tissue specific. Loss of specific sets of DNase I-hypersensitive sites accompanies the cessation of transcription for the Afp gene in adult rat liver and in a "dedifferentiated" hepatoma cell line. Likewise, specific sets of DNase I-hypersensitive sites disappear during the inactivation of the albumin gene in hepatoma cells. The distal upstream sites of the Afp and albumin genes display the same DNase I sensitivity in expressing and potentially expressible states. These findings suggest that reversible changes in short chromatin regions may be involved in the actual transcription of the albumin and Afp genes, while more permanent tissue-specific changes at other sites correlate with the capacity of these genes to be expressed during hepatic differentiation and neoplasia.

Multiple regulatory elements in the intergenic region between the alpha-fetoprotein and albumin genes

Three enhancer elements spanning a distance of 7 kilobases have been found at the 5' end of the alpha-fetoprotein (AFP) gene. These elements were identified by transient expression assay after the introduction of a modified mouse AFP gene with variable amounts of 5' flanking sequence into a human hepatoma cell line, Hep G2. These regulatory elements function in a position-independent and orientation-independent manner that is typical of enhancers. All three elements will stimulate transcription from the promoter of the herpes simplex virus thymidine kinase gene. In Hep G2 cells, transcriptional activation from the heterologous promoter was approximately 25- to 50-fold higher than the basal levels obtained in the absence of AFP enhancer elements. In HeLa cells, the increase in thymidine kinase gene transcription varied from 6- to 14-fold, indicating that the enhancer elements exhibit some cell type specificity. Deletion analysis of the region proximal to the AFP transcription initiation site identified an essential region between 85 and 52 bases upstream of the site of initiation of transcription whose removal resulted in almost complete extinction of transcriptional activity. This region, which has been shown to be dispensable for transcription in HeLa cells, defines a second tissue-specific regulatory region in the gene.

Immunological characterization of a novel human colon-associated antigen (CAA) by a monoclonal antibody

We have generated a monoclonal antibody (MAb), designated anti-CAA (colon-associated antigen), using as immunogen a membrane-enriched fraction of a biopsy from a moderately-differentiated human colonic adenocarcinoma. The molecular weight of this reactive antigen was determined by Western blotting to be greater than 200 kDa. When immunohistochemical techniques were used, MAb anti-CAA reacted with epithelium in the majority of normal, dysplastic and malignant colon specimens tested (greater reactivity was observed in normal colon than in benign or malignant lesions). Cell sorter analyses demonstrated a heterogeneous distribution of CAA on the cell surface of the well-differentiated LS-174T cell line. Antigen positive and antigen-negative cells were separated by means of flow cytometric techniques. These two subpopulations were then inoculated into immunosuppressed rats, resulting in xenograft tumors which differed significantly in their degree of histologic differentiation. Antigen-positive cells developed into well-differentiated adenocarcinomas, while antigen-negative cells developed into poorly differentiated adenocarcinomas. These results, along with immunohistochemical studies, indicate that the antigen detected by MAb anti-CAA has characteristics of a colon-associated antigen whose expression correlates with cellular differentiation. Moreover, differences in molecular weight as well as tissue distribution indicate that CAA may be a novel antigen different from those previously described.

Diversity of alpha-fetoprotein gene expression in mice is generated by a combination of separate enhancer elements

The 5' flanking region of the mouse alpha-fetoprotein (AFP) gene contains a tissue-specific promoter and three upstream regulatory elements that behave as classical enhancers. At least one of these enhancers is now shown to be required for the tissue-specific expression of the AFP gene when it is introduced into the mouse genome by microinjection of cloned DNA fragments into fertilized eggs. Each enhancer can direct expression in the appropriate tissues, the visceral endoderm of the yolk sac, the fetal liver, and the gastrointestinal tract, but each exerts different influence in these three tissues. These differences may explain the tissue-specific diversity in the levels of expression characteristic of the AFP gene. The postnatal repression of transcription of the AFP gene in both liver and gut, as well as the reinitiation of its transcription during liver regeneration, is mimicked by the introduced gene when it is linked to the enhancer domains together or singly. Thus, the DNA sequence elements responsible for directing the activation of AFP transcription, its repression, and reinduction are contained in a limited segment of DNA within or 5' to the gene (or both) and are operative in the absence of the closely linked albumin gene.

On the dynamics of controlled metabolic network and cellular behaviour

The existence of elaborate control mechanisms for the various biochemical processes inside and within living cells is responsible for the coherent behaviour observed in its spatio-temporal organisation. Stability and sensitivity are both necessary properties of living systems and these are achieved through negative and positive feedback loops as in other control systems. We have studied a three-step reaction scheme involving a negative and a positive feedback loop in the form of end-product inhibition and allosteric activation. The variety of behaviour exhibited by this system, under different conditions, includes steady state, simple limit cycle oscillations, complex oscillations and period bifurcations leading to random oscillations or chaos. The system also shows the existence of two distinct chaotic regimes under the variation of a single parameter. These results, in comparison with single biochemical control loops, show that new behaviours can be exhibited in a more complex network which are not seen in the single control loops. The results are discussed in the light of a diverse variety of cellular functions in normal and altered cells indicating the role of controlled metabolic network as the underlying basis for cellular behaviour.

Two-level regulation of cardiac actin gene transcription: muscle-specific modulating factors can accumulate before gene activation

We have previously proposed that the upstream regions of the human cardiac actin gene contain sequences that interact with muscle-specific factors with direct high-level transcription of this gene in differentiated muscle cells. In this study we showed that these factors already accumulate in the dividing myoblasts of the mouse C2C12 cell line before differentiation of the cells. The endogenous cardiac actin gene in the C2C12 line is expressed only at a low level in myoblasts but at a high level when these cells differentiate into multinucleate myotubes. In contrast, human cardiac actin genes stably introduced into C2C12 cells show high-level expression in both myoblasts and myotubes, indicating that the endogenous cardiac actin gene is repressed in myoblasts by a mechanism which does not affect transfected genes. In a second muscle cell line (the rat L8 cell line), the level of expression of transfected cardiac actin genes increases when these cells differentiate into myotubes, paralleling the expression of the endogenous sarcomeric actin genes. We suggest that the level of transcriptional modulating factors is low in L8 myoblasts and increases when these cells differentiate into myotubes. Our results demonstrate that at least two steps are necessary for high-level cardiac actin gene expression: activation of the gene and subsequent modulation of its transcriptional activity. Furthermore, the results indicate that the two regulatory steps can be dissociated and that the factors involved in modulation are distinct from those involved in gene activation.

Liver-specific expression of the mouse alpha-fetoprotein gene is mediated by cis-acting DNA elements

We have identified cis-acting regulatory elements in the 5' flanking region of the mouse alpha-fetoprotein (Afp) gene, using the expression of the bacterial gene for chloramphenicol acetyltransferase (CAT) in a transient expression assay. Tissue-specific enhancer activity was determined by transfection of mouse hepatoma (BWTG3) and fibroblast cells (C127, NIH 3T3) with various DNA fragments linked to the CAT gene. A 5.4-kilobase restriction fragment was shown to have characteristics typical of enhancers, including the ability to function independent of orientation and position and the ability to enhance transcription from a heterologous promoter. The enhancer activity was greatest in the hepatoma cells, which express Afp. By deletion analysis, it was demonstrated that enhancer activity is present in several subfragments, indicating the presence of more than one element in this fragment. An additional regulatory element within 950 base pairs of the Afp transcription initiation site has been identified and shown to confer tissue-specific expression on the CAT gene. This fragment, which lacks enhancer activity, contains the Afp promoter region and mediates the tissue-specific expression of the CAT gene when driven by nonspecific viral enhancers. We conclude from our studies that there are several types of regulatory elements in the 5' flanking region of the Afp gene that help mediate tissue-specific expression.

Class II histocompatibility antigen expression in human melanocytes transformed by Harvey murine sarcoma virus (Ha-MSV) and Kirsten MSV retroviruses

Human melanocytes infected with Ki-MSV or Ha-MSV, but not amphotropic MuLV, undergo a series of transformation-related changes that are characteristic of malignant melanoma. These are (a) expression of Ia antigens, in particular DP, DQ, and DR class II histocompatibility gene products, (b) a transformed morphology and ability to grow in soft agar, and (c) a 5-10-fold increase in the cell surface expression of GD3 ganglioside. However, other characteristics of melanoma, such as independence from specific growth factors and loss of adenosine deaminase binding protein were not observed. We conclude that viral ras oncogenes initiate early transformation events in melanocytes, and that Ia antigen expression is a transformation marker in this system.

Cell type-specific negative regulatory element in the control region of the rat alpha-fetoprotein gene

Albumin and alpha-fetoprotein are evolutionarily related genes that show differential and complex patterns of regulation during development. We investigated the role of the sequences flanking the transcription initiation site of the rat alpha-fetoprotein gene in transient transfection assays of hepatic and nonhepatic cell lines. Chimeric flanking regions and deletion analysis have defined the following three functionally different regions: a cell type-specific enhancer(s), encompassing 3 kilobases, located between -7 kilobase pairs and -4 kilobase pairs; a cell type-specific promoter, inactive in the absence of an enhancer and comprising at most the 180 base pairs upstream from the site of transcription initiation; and a 550-base-pair region, located between the promoter and the enhancer (at -3.5 kilobases), down-regulates transcription initiation in a cell type-specific manner and is also capable of repressing heterologous promoters. The implications of these findings with respect to the complex pattern of AFP regulation are discussed.

Regulated expression of a Drosophila melanogaster heat shock locus after stable integration in a Drosophila hydei cell line

DNA-mediated cotransformation has been used to transfer a Drosophila melanogaster heat shock locus into cultured Drosophila hydei cells by use of the copia-based selectable vector pCV2gpt and of pMH10A, a cloned 87A7 heat shock locus encoding a mutant heat shock protein (hsp). Transformed lines contain between 50 and 200 copies of both plasmids, each separately organized as a head-to-tail concatemer which is stably maintained in the transformed lines. Exposure of the cotransformants to heat shock temperatures induces the regulated expression of the hsp RNA and the mutant hsp in all the lines analyzed.

Differentiation, not determination, regulates muscle gene activation: transfection of troponin I genes into multipotential and muscle lineages of 10T1/2 cells

Transcription of quail skeletal muscle troponin I (TnI) genes was examined after stable transfection into multipotential 10T1/2 mouse cells and into determined myoblast lineages derived by 5-azacytidine conversion. Transfected TnI and endogenous mouse muscle genes were inactive both in multipotential 10T1/2 and in proliferating myoblasts but were activated coordinately and to high levels when myoblast lineages differentiated, regardless of whether TnI genes were transfected before or after myoblast lineage determination. We conclude that the TnI gene contains evolutionarily conserved control sequences that activate its transcription in response to differentiation-specific regulatory signals. Myoblast lineage determination, therefore, does not appear to act directly on TnI and other muscle genes but likely establishes a regulatory control system that mediates expression of differentiation-specific transcription signals.

Adenovirus type 12 early region 1A proteins repress class I HLA expression in transformed human cells

The adenovirus type 12 (Ad12) early region 1A (E1A) gene is thought to play a major role in repressing class I major histocompatibility complex expression in transformed rodent cells. However, since transformation by adenovirus requires both E1A and E1B genes, it has not been demonstrated whether the Ad12 E1A gene acts alone or synergistically with the E1B gene to accomplish this effect. Moreover, it is not known whether the repression of class I antigen synthesis by Ad12-transforming gene products occurs only in rodent cells. We show that the Ad12 E1A gene, in the absence of the E1B gene, is capable of greatly reducing the levels of class I HLA antigens and mRNAs in primary human cells transformed by the E1A gene of Ad12 and the large tumor antigen (T-antigen) gene of BK virus; control cells transformed by BK virus T-antigen gene alone or the highly related simian virus 40 T-antigen gene showed no apparent alteration in class I HLA expression. Human recombinant interferon gamma was able to restore synthesis of class I HLA antigens in transformed cells that produced Ad12 E1A proteins, indicating that these cells were not deficient for class I genes. These results strongly indicate that the Ad12 E1A proteins modulate class I gene expression by similar mechanisms in both transformed rodent and human cells.

Two-level regulation of cardiac actin gene transcription: muscle-specific modulating factors can accumulate before gene activation

We have previously proposed that the upstream regions of the human cardiac actin gene contain sequences that interact with muscle-specific factors with direct high-level transcription of this gene in differentiated muscle cells. In this study we showed that these factors already accumulate in the dividing myoblasts of the mouse C2C12 cell line before differentiation of the cells. The endogenous cardiac actin gene in the C2C12 line is expressed only at a low level in myoblasts but at a high level when these cells differentiate into multinucleate myotubes. In contrast, human cardiac actin genes stably introduced into C2C12 cells show high-level expression in both myoblasts and myotubes, indicating that the endogenous cardiac actin gene is repressed in myoblasts by a mechanism which does not affect transfected genes. In a second muscle cell line (the rat L8 cell line), the level of expression of transfected cardiac actin genes increases when these cells differentiate into myotubes, paralleling the expression of the endogenous sarcomeric actin genes. We suggest that the level of transcriptional modulating factors is low in L8 myoblasts and increases when these cells differentiate into myotubes. Our results demonstrate that at least two steps are necessary for high-level cardiac actin gene expression: activation of the gene and subsequent modulation of its transcriptional activity. Furthermore, the results indicate that the two regulatory steps can be dissociated and that the factors involved in modulation are distinct from those involved in gene activation.

Conditions affecting the differentiation of F9 teratocarcinoma cells: potentiation of response by cyclic AMP

F9 cells maintained in culture were shown to have a reduced ability to differentiate. The cells produced decreased amounts of alphafetoprotein when induced with retinoic acid. We show that consistent responses can be recovered after passage of F9 cells as a tumor. In addition, optimal differentiation of F9 cells to visceral endoderm may be achieved by the addition of very low concentrations of dibutyryl cyclic AMP (dbcAMP) to the medium.

Multiple regulatory elements in the intergenic region between the alpha-fetoprotein and albumin genes

Three enhancer elements spanning a distance of 7 kilobases have been found at the 5' end of the alpha-fetoprotein (AFP) gene. These elements were identified by transient expression assay after the introduction of a modified mouse AFP gene with variable amounts of 5' flanking sequence into a human hepatoma cell line, Hep G2. These regulatory elements function in a position-independent and orientation-independent manner that is typical of enhancers. All three elements will stimulate transcription from the promoter of the herpes simplex virus thymidine kinase gene. In Hep G2 cells, transcriptional activation from the heterologous promoter was approximately 25- to 50-fold higher than the basal levels obtained in the absence of AFP enhancer elements. In HeLa cells, the increase in thymidine kinase gene transcription varied from 6- to 14-fold, indicating that the enhancer elements exhibit some cell type specificity. Deletion analysis of the region proximal to the AFP transcription initiation site identified an essential region between 85 and 52 bases upstream of the site of initiation of transcription whose removal resulted in almost complete extinction of transcriptional activity. This region, which has been shown to be dispensable for transcription in HeLa cells, defines a second tissue-specific regulatory region in the gene.

Differential expression of alpha-fetoprotein genes on the inactive X chromosome in extraembryonic and somatic tissues of a transgenic mouse line

During development of the female mouse embryo, one of the two X chromosomes is inactivated in a random manner in most cell lineages. However, in the extraembryonic trophectoderm and primary endoderm lineages there is preferential inactivation of the paternally derived X chromosome. The inactivated X chromosomes of the extraembryonic and somatic tissues appear equally inactive at the level of the expression of X-linked genes. However, there are differences in the timing of their replication and the extent of DNA modification as determined by gene transfer. The identification of transgenic animals carrying X-linked modified alpha-fetoprotein (AFP) genes allowed us to examine whether the inactivation process extends to an autosomal gene which is normally expressed at high levels in specific extraembryonic and somatic cells, and if so, whether the inactivation process is different in these two tissues. Our results demonstrate that the X-linked AFP genes were expressed on the inactive X chromosome in the visceral endoderm of the yolk sac but not in fetal liver. Thus, the transcriptional activity of the AFP minigene on the inactive X chromosome is dependent on the tissue in which it resides, and most probably reflects differences in the nature of the maintenance of the inactive state of the extraembryonic and embryonic X chromosomes.

Comparison of the 5' regions of human and mouse carbonic anhydrase II genes and identification of possible regulatory elements

The nucleotide sequence of the 5' region of the human carbonic anhydrase II gene has been determined. This sequence begins 643 base pairs upstream from the ATG start site and continues through exon 1, intron 1, exon 2 and the adjoining 125 nucleotides of intron 2. The human sequence is compared with homologous regions of the mouse (YBR strain) carbonic anhydrase II gene by aligning the two sequences for optimal homology. In addition to a TATA box and a putative CCAAT box (CCACC in human and CCACT in mouse), three conserved tandem-repeat elements in mouse and two in human (consensus: cCNGTCACCTCCgC) are located 15 and 22 base pairs upstream, respectively, from the CCAAT boxes in the human and mouse sequences. This repeat element is similar to a tandem repeat sequence located at about the same position in mammalian beta-globin genes, and may represent regulatory elements common to both the carbonic anhydrase and beta-globin genes. The regions surrounding exon 1 are extremely G + C-rich in both human and mouse genes. In addition, several CCGCCC or GGGCGG sequences which may be important for transcriptional efficiency are found in the 5' flanking regions of the human and mouse genes.

Regulated expression of a Drosophila melanogaster heat shock locus after stable integration in a Drosophila hydei cell line

DNA-mediated cotransformation has been used to transfer a Drosophila melanogaster heat shock locus into cultured Drosophila hydei cells by use of the copia-based selectable vector pCV2gpt and of pMH10A, a cloned 87A7 heat shock locus encoding a mutant heat shock protein (hsp). Transformed lines contain between 50 and 200 copies of both plasmids, each separately organized as a head-to-tail concatemer which is stably maintained in the transformed lines. Exposure of the cotransformants to heat shock temperatures induces the regulated expression of the hsp RNA and the mutant hsp in all the lines analyzed.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: XIV. Mutational and non-mutational mechanisms as subsets of a more general mechanism. Part C. A defined cancer mutation

The results of applying a mechanism of ethionine-induced embryonic gene derepressions to explain similar features found in hereditary tyrosinemia have been extended to a well defined cancer mutation. In all three cases, the described mechanism is compatible with the explanation for the etiology of embryonic like phenotypic expressions in cells and potentially for the carcinogenic process. The essence of the formulated process for a human bladder carcinoma mutation in the ras gene for a protein phosphokinase states that a specific proto-oncogene is mutated to an oncogene by various known processes. The protein phosphokinase that has an altered specificity resulting in anomalous phosphorylation of important regulating proteins by a non-mutation mechanism, i.e. by ethionine, would produce the same effect in a hypomethylated state of deoxyribonucleic acid causing an embryonic type protein phosphokinase gene to become activated. These embryonic oncogenes are supersensitive to methylation control mechanisms--thus the link between non-mutation and mutation type mechanisms.

Developmental regulation of alpha-fetoprotein genes in transgenic mice

The mouse alpha-fetoprotein gene is activated in embryonic development in the visceral endoderm of the extraembryonic yolk sac and the fetal liver and gut. Transcription of the gene is subsequently repressed in the neonatal liver. To ask whether the DNA sequence elements required for tissue-specific activation are the same or different from those required for postnatal developmental regulation of the gene, modified copies of the alpha-fetoprotein gene were microinjected into fertilized mouse eggs. Those animals which developed to term and carried integrated copies of the modified gene were analyzed for expression. In approximately 50% of such animals, the introduced gene was active only in the three cell lineages which expressed the authentic alpha-fetoprotein gene. Furthermore, its expression was repressed in the neonatal liver. Thus, we conclude that the modified genes, which included either 7 or 14 kilobase pairs of 5'-flanking DNA, contained the DNA sequence information to direct both tissue-specific expression and developmental regulation. The observation that 50% of the mice which carried the modified gene did not express it in any tissue, combined with the fact that the level of expression was highly variable between expressing transgenic animals, suggested that the gene was susceptible to its site of integration in the mouse genome.

Differentiation, not determination, regulates muscle gene activation: transfection of troponin I genes into multipotential and muscle lineages of 10T1/2 cells

Transcription of quail skeletal muscle troponin I (TnI) genes was examined after stable transfection into multipotential 10T1/2 mouse cells and into determined myoblast lineages derived by 5-azacytidine conversion. Transfected TnI and endogenous mouse muscle genes were inactive both in multipotential 10T1/2 and in proliferating myoblasts but were activated coordinately and to high levels when myoblast lineages differentiated, regardless of whether TnI genes were transfected before or after myoblast lineage determination. We conclude that the TnI gene contains evolutionarily conserved control sequences that activate its transcription in response to differentiation-specific regulatory signals. Myoblast lineage determination, therefore, does not appear to act directly on TnI and other muscle genes but likely establishes a regulatory control system that mediates expression of differentiation-specific transcription signals.

Expression of histocompatibility antigens H-2K, -D, and -L is reduced in adenovirus-12-transformed mouse cells and is restored by interferon gamma

Primary mouse cells transformed by adenovirus type 12 (Ad12) expressed negligible amounts of class I antigens H-2K, -D, and -L on the cell surface and were capable of forming tumors in syngeneic animals, whereas cells transformed by Ad5 continued to express class I antigens and were nontumorigenic. Cells from a tumor, generated by injection of Ad12-transformed mouse cells into a syngeneic mouse, also expressed low levels of H-2 antigens, indicating that this phenotype is maintained in vivo. In all Ad12-transformed cells, synthesis of the H-2 heavy chain was not detected whereas the beta 2-microglobulin light chain was synthesized. Furthermore, the level of cytoplasmic H-2 mRNA in the Ad12 lines was greatly reduced. Reduction of H-2 expression is instructed solely by the transforming region of the viral genome, since this repression occurred in cells transformed by a DNA fragment containing only Ad12 E1A and E1B genes. Addition of recombinant murine interferon gamma strongly stimulated expression of class I antigens in the Ad12 transformants as well as in cells from the Ad12 tumor. This result indicates that Ad12 does not preferentially transform cells that are deficient for class I genes and that Ad12 does not mutate the class I genes in cells it transforms. The correlation between tumorigenicity and loss of H-2 expression in Ad12-transformed cells is discussed.

Developmental regulation of alpha-fetoprotein genes in transgenic mice

The mouse alpha-fetoprotein gene is activated in embryonic development in the visceral endoderm of the extraembryonic yolk sac and the fetal liver and gut. Transcription of the gene is subsequently repressed in the neonatal liver. To ask whether the DNA sequence elements required for tissue-specific activation are the same or different from those required for postnatal developmental regulation of the gene, modified copies of the alpha-fetoprotein gene were microinjected into fertilized mouse eggs. Those animals which developed to term and carried integrated copies of the modified gene were analyzed for expression. In approximately 50% of such animals, the introduced gene was active only in the three cell lineages which expressed the authentic alpha-fetoprotein gene. Furthermore, its expression was repressed in the neonatal liver. Thus, we conclude that the modified genes, which included either 7 or 14 kilobase pairs of 5'-flanking DNA, contained the DNA sequence information to direct both tissue-specific expression and developmental regulation. The observation that 50% of the mice which carried the modified gene did not express it in any tissue, combined with the fact that the level of expression was highly variable between expressing transgenic animals, suggested that the gene was susceptible to its site of integration in the mouse genome.

The location of cis-acting regulatory sequences in the human T cell lymphotropic virus type III (HTLV-III/LAV) long terminal repeat

The location of cis-acting regulatory sequences within the long terminal repeat (LTR) of the human T cell lymphotropic virus type III (HTLV-III/LAV) was determined. An enhancer element capable of increasing the rate of transcription from a heterologous promoter, irrespective of distance and orientation, is located between nucleotides -137 and -17 (cap site +1). The promoter sequences present near the TATA box respond to heterologous enhancers. The sequences present between nucleotides -17 and +80 are responsive to HTLV-III-associated trans-acting regulatory factors. Activation of these sequences by the viral regulatory factors requires the presence of a functional enhancer. The enhancer requirement is nonspecific, as the enhancer sequences of RSV, HTLV-I, and SV40 can functionally replace the HTLV-III enhancer. These findings define a new type of regulatory element, provide insight into the mechanisms that regulate HTLV-III gene expression, and may help to explain the effects of this virus on infected cells.

Functional analysis of the mouse H-2Kb gene promoter in embryonal carcinoma cells

Mouse embryonal carcinoma (EC) cells do not express the major H-2 class I transplantation antigens. The latter, however, become detectable upon in vitro differentiation of EC cells. Neither class I H-2 genes nor the gene coding for beta-2 microglobulin (beta 2m) is transcribed in EC cells. We have constructed two hybrid plasmids containing the 5' flanking region of an H-2Kb gene followed by the coding regions of either the herpes simplex virus thymidine kinase (H-2 tk) or the chloramphenicol acetyl transferase (H-2 CAT) genes. Upon transfer into EC cells, the H-2 tk hybrid gene is expressed in F9 tk- cell lines which thus acquire a stable tk+ phenotype. When such transformed clones are induced to differentiate in vitro, tk activity shows a moderate increase, which reflects an increase in transcription of the hybrid gene. In transient transformation experiments, EC cells were found to express the H-2 CAT hybrid gene as well. We conclude that the 2 kilobase pair region of the H-2Kb gene which we used contains an active promoter region, but does not include all the elements required for the correct regulation of the H-2Kb gene.

The use of cloned gene probes to study differentiation in teratocarcinomas

The molecular control of early embryonic gene expression can be studied in teratocarcinoma derived cell lines. Several genes known to be regulated during in vitro differentiation have been cloned and used to measure mRNA levels. In all cases studied there is good agreement between protein levels and mRNA levels, strongly implying transcriptional regulation. Two non-exclusive hypotheses have been proposed to explain transcriptional control: methylation of DNA and the interactive effects of cis-acting enhancer sequences and trans-acting proteins. Experiments with oncoviruses suggest both mechanisms may play a role.