Cell-specified expression of a selectable hybrid gene

Introduction to cancer gene therapy

Over the past decade, the unprecedented growth in science and technology has fueled the development of novel treatment strategies to combat disease. The creative and innovative efforts of scientists and clinicians to overcome the multitude of unforeseen obstacles to success is no better exemplified than in the field of cancer gene therapy. Since its inception, developers of cancer gene therapy have been charged with the challenge of altering basic tumor biology or, alternatively, the host responses for the purpose of tumor eradication and prevention. Several major therapeutic strategies have emerged from preclinical studies, and results from these early studies hold promise for altering the clinical outcome in a variety of malignancies. These strategies may be broadly subcategorized and range in intent from alteration of the tumor cell phenotype by replacement of defective cellular response genes (e.g., mutated or deleted tumor suppressor genes) to the enhancement of the immunological response to cancer (e.g., amplification of the cell surface antigen signature or modulation of the host response). Not surprisingly, the increasingly intricate nature of tumor biology revealed over the past several years has effectively raised the bar of success for those involved in the development of effective molecular and cancer gene therapy strategies. This, in turn, has led to the development of more complex therapies that frequently draw upon multiple disciplines in an effort to optimize treatment response.

Tissue-specific and developmental regulation of the rat insulin II gene enhancer, RIPE3, in transgenic mice

The rat insulin II gene enhancer, RIPE3 (-126 to -86), mediates beta-islet cell-specific activity in transfection assays. To investigate the in vivo activity of RIPE3, we generated mice carrying a transgene consisting of three copies of RIPE3 linked to a minimal chicken ovalbumin promoter in conjunction with sequences encoding the human growth hormone gene. 13 transgenic mice were obtained, 11 of which expressed the transgene, as determined by serum radioimmunoassay for human growth hormone. Expression of the transgene was assessed for cell specificity by immunocytochemistry. The pancreatic islet cells invariably stained for growth hormone, while the acinar and ductal cells did not. Staining of adjacent sections for insulin, glucagon, and somatostatin revealed that growth hormone was expressed in the beta-cell in all of the mice analyzed, but in some mice alpha-cells also contained growth hormone. RNase protection analysis revealed that the tissues that consistently express the transgene in these animals are the pancreas and brain. Developmental analysis revealed that the transgene was expressed in the pancreatic bud at embryonic day 9.5, corresponding to the temporal expression pattern of the insulin gene. These results signify that an element as small as 41 base pairs is capable of regulating pancreatic temporal and spatial gene expression in vivo.

Insulin gene expression in nonexpressing cells appears to be regulated by multiple distinct negative-acting control elements

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located between nucleotides -340 and -91 relative to the transcription start site. Transcription from the enhancer is controlled by both positive- and negative-acting cellular factors. Cell-type-specific expression is mediated principally by a single cis-acting enhancer element located between -100 and -91 in the rat insulin II gene (referred to as the insulin control element [ICE]), which is acted upon by both of these cellular activities. Analysis of the effect of 5' deletions within the insulin enhancer has identified a region between nucleotides -217 and -197 that is also a site of negative control. Deletion of these sequences from the 5' end of the enhancer leads to transcription of the enhancer in non-insulin-producing cells, even though the ICE is intact. Derepression of this ICE-mediated effect was shown to be due to the binding of a ubiquitously distributed cellular factor to a sequence element which resides just upstream of the ICE (i.e., between nucleotides -110 and -100). We discuss the possible relationship of these results to cell-type-specific regulation of the insulin gene.

Insulin gene expression in nonexpressing cells appears to be regulated by multiple distinct negative-acting control elements

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located between nucleotides -340 and -91 relative to the transcription start site. Transcription from the enhancer is controlled by both positive- and negative-acting cellular factors. Cell-type-specific expression is mediated principally by a single cis-acting enhancer element located between -100 and -91 in the rat insulin II gene (referred to as the insulin control element [ICE]), which is acted upon by both of these cellular activities. Analysis of the effect of 5' deletions within the insulin enhancer has identified a region between nucleotides -217 and -197 that is also a site of negative control. Deletion of these sequences from the 5' end of the enhancer leads to transcription of the enhancer in non-insulin-producing cells, even though the ICE is intact. Derepression of this ICE-mediated effect was shown to be due to the binding of a ubiquitously distributed cellular factor to a sequence element which resides just upstream of the ICE (i.e., between nucleotides -110 and -100). We discuss the possible relationship of these results to cell-type-specific regulation of the insulin gene.

Pancreatic beta-cell-type-specific transcription of the insulin gene is mediated by basic helix-loop-helix DNA-binding proteins

The pancreatic beta-cell-specific expression of the insulin gene is mediated, at least in part, by the interaction of unique trans-acting beta-cell factors with a cis-acting DNA element found within the insulin enhancer (5'-GC CATCTG-3'; referred to as the insulin control element [ICE]) present in the rat insulin II gene between positions -100 and -91. This sequence element contains the consensus binding site for a group of DNA-binding transcription factors called basic helix-loop-helix proteins (B-HLH). As a consequence of the similarity of the ICE with the DNA sequence motif associated with the cis-acting elements of the B-HLH class of binding proteins (CANNTG), the ability of this class of proteins to regulate cell-type-specific expression of the insulin gene was addressed. Cotransfection experiments indicated that overexpression of Id, a negative regulator of B-HLH protein function, inhibits ICE-mediated activity. Antibody to the E12/E47 B-HLH proteins attenuated the formation, in vitro, of a previously described (J. Whelan, S. R. Cordle, E. Henderson, P. A. Weil, and R. Stein, Mol. Cell. Biol. 10:1564-1572, 1990) beta-cell-specific activator factor(s)-ICE DNA complex. Both of these B-HLH proteins (E12 and E47) bound efficiently and specifically to the ICE sequences. The role of B-HLH proteins in mediating pancreatic beta-cell-specific transcription of the insulin gene is discussed.

Pancreatic beta-cell-type-specific transcription of the insulin gene is mediated by basic helix-loop-helix DNA-binding proteins

The pancreatic beta-cell-specific expression of the insulin gene is mediated, at least in part, by the interaction of unique trans-acting beta-cell factors with a cis-acting DNA element found within the insulin enhancer (5'-GC CATCTG-3'; referred to as the insulin control element [ICE]) present in the rat insulin II gene between positions -100 and -91. This sequence element contains the consensus binding site for a group of DNA-binding transcription factors called basic helix-loop-helix proteins (B-HLH). As a consequence of the similarity of the ICE with the DNA sequence motif associated with the cis-acting elements of the B-HLH class of binding proteins (CANNTG), the ability of this class of proteins to regulate cell-type-specific expression of the insulin gene was addressed. Cotransfection experiments indicated that overexpression of Id, a negative regulator of B-HLH protein function, inhibits ICE-mediated activity. Antibody to the E12/E47 B-HLH proteins attenuated the formation, in vitro, of a previously described (J. Whelan, S. R. Cordle, E. Henderson, P. A. Weil, and R. Stein, Mol. Cell. Biol. 10:1564-1572, 1990) beta-cell-specific activator factor(s)-ICE DNA complex. Both of these B-HLH proteins (E12 and E47) bound efficiently and specifically to the ICE sequences. The role of B-HLH proteins in mediating pancreatic beta-cell-specific transcription of the insulin gene is discussed.

Retrovirus mediated transfer and expression of GM-CSF in haematopoietic cells

Two retrovirus vectors were compared for their ability to express granulocyte-macrophage colony stimulating factor (GM-CSF) in a haematopoietic cell line, FDCP1, which is dependent on GM-CSF for survival. Both a MoMLV-based vector pVneoGM, and a MPSV-based vector, M3neoGM, were found to be capable of transmitting and expressing both GM-CSF and neomycin sequences in the myeloid FDCP1 cell line. Our results also demonstrate that pVneoGM is more efficient at generating GM-CSF independent colonies than M3neoGM. Analysis of cell lines derived after infection confirmed pVneoGM expressed higher levels of GM-CSF. Cell lines generated by infection with pVneoGM responded to levels of exogenous recombinant GM-CSF which did not stimulate growth of the parental cell line, suggesting autocrine stimulation may convey a proliferative advantage under sub-optimal growth conditions. Finally the parental vectors pVneo and M3neo were shown to be capable of expressing the neomycin gene in both murine haematopoietic progenitor and stem cells.

Cooperativity of sequence elements mediates tissue specificity of the rat insulin II gene

The 5'-flanking region of the rat insulin II gene (-448 to +50) is sufficient for tissue-specific expression. To further determine the tissue-specific cis-acting element(s), important sequences defined by linker-scanning mutagenesis were placed upstream of a heterologous promoter and transfected into insulin-producing and -nonproducing cells. Rat insulin promoter element 3 (RIPE3), which spans from -125 to -86, was shown to confer beta-cell-specific expression in either orientation. However, two subregions of RIPE3, RIPE3a and RIPE3b (defined by linker-scanning mutations), displayed only marginal activities. These results suggest that the two subregions cooperate to confer tissue specificity, presumably via their cognate binding factors.

Identification of a pancreatic beta-cell insulin gene transcription factor that binds to and appears to activate cell-type-specific expression: its possible relationship to other cellular factors that bind to a common insulin gene sequence

The insulin gene is expressed almost exclusively in pancreatic beta-cells. Previous work in our laboratory has shown that pancreatic beta-cell-specific expression of the rat insulin II gene is controlled by a number of positive and negative cis-acting DNA elements within the enhancer. We have shown that one element within the enhancer, located between nucleotides -100 and -91 (GCCATCTGCT; referred to as the insulin control element [ICE]) relative to the transcription start site, is controlled by both positive- and negative-acting cellular transcription factors. The positive-acting factor appears to be uniquely active in beta-cells. To identify the nucleotides within the ICE that mediate positive cell-type-specific regulation, point mutations within this element were generated and assayed for their effects on expression. Base pairs -97, -94, -93, and -92 were found to be crucial for the activator function of this region, while mutations at base pairs -100, -96, and -91 had little or no effect on activity. The gel mobility shift assay was used to determine whether specific cellular factors associated directly with the ICE. Several specific protein-DNA complexes were detected in extracts prepared from insulin-producing and non-insulin-producing cells, including a complex unique to beta-cell extracts. The ability of unlabeled wild-type and point mutant versions of the ICE to compete for binding to these cellular factors demonstrated that the beta-cell-specific complex appears to contain the insulin gene activator protein(s). Interestingly, the adenovirus type 2 major late promoter upstream element (USE; GCCACGTGAC) also competed in the gel mobility shift assay for binding of cellular proteins to the ICE. These results suggested that the cellular factor that binds to the USE (i.e., USF) also interacts with the ICE. This was directly demonstrated by showing that ICE and USE sequences completed for the USF required for adenovirus type 2 major late promoter transcription in vitro and by showing that reticulocyte lysate-translated human USF products bound to the ICE. However, the USE sequences were unable to stimulate beta-cell-type-specific activity in vivo. We discuss the possible relationship of these observations to positive and negative control mediated by the ICE.

Endocytosis via coated pits mediated by glycoprotein receptor in which the cytoplasmic tail is replaced by unrelated sequences

Rat 6 fibroblast cell lines expressing wild-type chicken liver glycoprotein receptor (CHL) or chimeric receptors with alternate cytoplasmic tails were produced to study the role of the cytoplasmic tail in mediating receptor localization in coated pits and endocytosis of ligand. Cells expressing CHL or cells expressing a hybrid receptor that contains the cytoplasmic tail of the asialoglycoprotein receptor display high-efficiency endocytosis of N-acetylglucosamine-conjugated bovine serum albumin in experiments designed to measure an initial internalization step, as well as in studies of continuous uptake and degradation. Substitution of the cytoplasmic tail by the equivalent domain of rat Na,K-ATPase beta subunit or by a stretch of Xenopus laevis globin beta chain does not abolish endocytosis but decreases the endocytosis rate constant from 15%-16%/min to 2.4% and 6.5%/min, respectively. Electron microscopy was used to visualize the glycoprotein binding sites at the surface of Rat 6 cells transfected with the various receptors. The percentage of receptors found in coated areas ranged from 32% for CHL to 9% for the Na,K-ATPase hybrid, indicating that clustering in coated pits correlates with efficiency of endocytosis. We concluded that replacement of the CHL cytoplasmic tail with unrelated sequences does not prevent, but decreases to varying extents, coated-pit localization and endocytosis efficiency. The construct with NH2-terminal globin tail lacks a signal for high-efficiency localization in coated pits but nevertheless is directed to the pits by an alternative mechanism.

Identification of a pancreatic beta-cell insulin gene transcription factor that binds to and appears to activate cell-type-specific expression: its possible relationship to other cellular factors that bind to a common insulin gene sequence

The insulin gene is expressed almost exclusively in pancreatic beta-cells. Previous work in our laboratory has shown that pancreatic beta-cell-specific expression of the rat insulin II gene is controlled by a number of positive and negative cis-acting DNA elements within the enhancer. We have shown that one element within the enhancer, located between nucleotides -100 and -91 (GCCATCTGCT; referred to as the insulin control element [ICE]) relative to the transcription start site, is controlled by both positive- and negative-acting cellular transcription factors. The positive-acting factor appears to be uniquely active in beta-cells. To identify the nucleotides within the ICE that mediate positive cell-type-specific regulation, point mutations within this element were generated and assayed for their effects on expression. Base pairs -97, -94, -93, and -92 were found to be crucial for the activator function of this region, while mutations at base pairs -100, -96, and -91 had little or no effect on activity. The gel mobility shift assay was used to determine whether specific cellular factors associated directly with the ICE. Several specific protein-DNA complexes were detected in extracts prepared from insulin-producing and non-insulin-producing cells, including a complex unique to beta-cell extracts. The ability of unlabeled wild-type and point mutant versions of the ICE to compete for binding to these cellular factors demonstrated that the beta-cell-specific complex appears to contain the insulin gene activator protein(s). Interestingly, the adenovirus type 2 major late promoter upstream element (USE; GCCACGTGAC) also competed in the gel mobility shift assay for binding of cellular proteins to the ICE. These results suggested that the cellular factor that binds to the USE (i.e., USF) also interacts with the ICE. This was directly demonstrated by showing that ICE and USE sequences completed for the USF required for adenovirus type 2 major late promoter transcription in vitro and by showing that reticulocyte lysate-translated human USF products bound to the ICE. However, the USE sequences were unable to stimulate beta-cell-type-specific activity in vivo. We discuss the possible relationship of these observations to positive and negative control mediated by the ICE.

Cooperativity of sequence elements mediates tissue specificity of the rat insulin II gene

The 5'-flanking region of the rat insulin II gene (-448 to +50) is sufficient for tissue-specific expression. To further determine the tissue-specific cis-acting element(s), important sequences defined by linker-scanning mutagenesis were placed upstream of a heterologous promoter and transfected into insulin-producing and -nonproducing cells. Rat insulin promoter element 3 (RIPE3), which spans from -125 to -86, was shown to confer beta-cell-specific expression in either orientation. However, two subregions of RIPE3, RIPE3a and RIPE3b (defined by linker-scanning mutations), displayed only marginal activities. These results suggest that the two subregions cooperate to confer tissue specificity, presumably via their cognate binding factors.

Insulin gene enhancer activity is inhibited by adenovirus 5 E1a gene products

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located within the 5'-flanking region of the insulin gene. Transcription from the enhancer is controlled by both positive- and negative-acting cellular transcription factors. It was previously shown that both the 243- and 289-amino-acid adenovirus type 5 E1a proteins can repress insulin gene transcription in vivo. To localize the insulin DNA sequences involved in this response, we examined the effects of a number of mutations within the 5'-flanking region of the rat insulin II gene on E1a-mediated repression of insulin gene transcription. We have found that E1a proteins inhibit enhancer-stimulated transcription of the insulin gene. The enhancer appears to contain at least two genetically separable and independent E1a target sequence elements. Interestingly, these same regions of the insulin enhancer have been shown to be negatively regulated by cellular transcription factors. These results suggest that E1a-like cellular factors may function in the pancreatic beta-cell-specific expression of the insulin gene.

Pancreatic beta-cell-type-specific expression of the rat insulin II gene is controlled by positive and negative cellular transcriptional elements

The insulin gene is expressed almost exclusively in pancreatic beta-cells. The DNA sequences that control cell-specific expression are located upstream of the transcription initiation site. To identify the cis-acting transcriptional control regions within the rat insulin II gene that are responsible for this tissue-specific expression pattern, we constructed a series of 5'-flanking deletion mutants and analyzed their expression in vivo in transfected insulin-producing and -nonproducing cell lines. Pancreatic beta-cell-specific expression was shown to be controlled by enhancer sequences lying between nucleotides -342 and -91 relative to the transcription start site. The rat insulin II enhancer appears to be a chimera, composed of a number of distinct cis-acting DNA elements. Both positive and negative transcriptional regulatory elements appear to be responsible for this cell-type-specific expression. We have shown that expression from one element within the enhancer, which is found between nucleotides -100 and -91, is regulated by both positive- and negative-acting cellular transcription factors. Expression from chimeras containing only the enhancer element sequences from -100 to -91 were active only in insulin-producing cells, indicating that the positive-acting factor(s) required for this activity may be active only in beta-cells. In contrast to the enhancer region, the rat insulin II gene promoter did not appear to require cell-specific transcription factors. Promoter mutants with 5'-flanking sequences extending to nucleotides -90 and -73 were constitutively active in both insulin-producing and -nonproducing cells. These results suggest that rat insulin II gene transcription in pancreatic beta-cells is imparted by a combination of both negative- and positive-acting cellular factors interacting with the gene enhancer.

Insulin gene enhancer activity is inhibited by adenovirus 5 E1a gene products

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located within the 5'-flanking region of the insulin gene. Transcription from the enhancer is controlled by both positive- and negative-acting cellular transcription factors. It was previously shown that both the 243- and 289-amino-acid adenovirus type 5 E1a proteins can repress insulin gene transcription in vivo. To localize the insulin DNA sequences involved in this response, we examined the effects of a number of mutations within the 5'-flanking region of the rat insulin II gene on E1a-mediated repression of insulin gene transcription. We have found that E1a proteins inhibit enhancer-stimulated transcription of the insulin gene. The enhancer appears to contain at least two genetically separable and independent E1a target sequence elements. Interestingly, these same regions of the insulin enhancer have been shown to be negatively regulated by cellular transcription factors. These results suggest that E1a-like cellular factors may function in the pancreatic beta-cell-specific expression of the insulin gene.

Pancreatic beta-cell-type-specific expression of the rat insulin II gene is controlled by positive and negative cellular transcriptional elements

The insulin gene is expressed almost exclusively in pancreatic beta-cells. The DNA sequences that control cell-specific expression are located upstream of the transcription initiation site. To identify the cis-acting transcriptional control regions within the rat insulin II gene that are responsible for this tissue-specific expression pattern, we constructed a series of 5'-flanking deletion mutants and analyzed their expression in vivo in transfected insulin-producing and -nonproducing cell lines. Pancreatic beta-cell-specific expression was shown to be controlled by enhancer sequences lying between nucleotides -342 and -91 relative to the transcription start site. The rat insulin II enhancer appears to be a chimera, composed of a number of distinct cis-acting DNA elements. Both positive and negative transcriptional regulatory elements appear to be responsible for this cell-type-specific expression. We have shown that expression from one element within the enhancer, which is found between nucleotides -100 and -91, is regulated by both positive- and negative-acting cellular transcription factors. Expression from chimeras containing only the enhancer element sequences from -100 to -91 were active only in insulin-producing cells, indicating that the positive-acting factor(s) required for this activity may be active only in beta-cells. In contrast to the enhancer region, the rat insulin II gene promoter did not appear to require cell-specific transcription factors. Promoter mutants with 5'-flanking sequences extending to nucleotides -90 and -73 were constitutively active in both insulin-producing and -nonproducing cells. These results suggest that rat insulin II gene transcription in pancreatic beta-cells is imparted by a combination of both negative- and positive-acting cellular factors interacting with the gene enhancer.

The rat albumin gene promoter is appropriately regulated in transient but not in stable transfections

The tissue-specific expression of the liver-specific rat albumin gene promoter was examined after transfer to various hepatic and non-hepatic cell lines. A 402 base pair sequence from the albumin gene 5' flank enabled a fused reporter chloramphenicol acetyltransferase gene to be expressed in rat hepatoma cell lines but not in fibroblast lines or dedifferentiated hepatoma cells. However, when this same construct was analyzed in permanently transfected cell populations, it was expressed equally well in differentiated and dedifferentiated hepatoma cells and in two of three fibroblast lines tested. The inappropriate expression of the albumin promoter was also seen using the HSV tk gene and the E. coli gpt gene as reporters, and when assayed by colony formation in HAT medium (tk gene) or by S1 protection of transcripts in cotransfected populations (tk and gpt genes). These results show that gene regulatory elements can behave differently in transient vs. stable transfections, and suggest that chromosomal integration can provide long range positive influences on gene expression.

The COUP transcription factor binds to an upstream promoter element of the rat insulin II gene

Band-shifting and DNase I-footprinting assays have been used to study the trans-acting factor(s) binding to an important promoter element (-53 to -46 relative to the transcription start) of the rat insulin II gene. A binding activity which footprints a region between -60 and -40 was found in both HIT, a hamster insulinoma cell line, and HeLa cells. A mutation within this region which drastically decreases promoter activity in vivo also greatly reduces binding activity in vitro. This binding activity was purified from HeLa cells and identified by competition and renaturation analyses as being the same as the COUP (chicken ovalbumin upstream promoter) transcription factor, a DNA-binding protein required for efficient transcription of the ovalbumin gene in vitro. Interestingly, the binding sequences of the COUP transcription factor in the ovalbumin and the insulin promoters have only limited similarities.

The COUP transcription factor binds to an upstream promoter element of the rat insulin II gene

Band-shifting and DNase I-footprinting assays have been used to study the trans-acting factor(s) binding to an important promoter element (-53 to -46 relative to the transcription start) of the rat insulin II gene. A binding activity which footprints a region between -60 and -40 was found in both HIT, a hamster insulinoma cell line, and HeLa cells. A mutation within this region which drastically decreases promoter activity in vivo also greatly reduces binding activity in vitro. This binding activity was purified from HeLa cells and identified by competition and renaturation analyses as being the same as the COUP (chicken ovalbumin upstream promoter) transcription factor, a DNA-binding protein required for efficient transcription of the ovalbumin gene in vitro. Interestingly, the binding sequences of the COUP transcription factor in the ovalbumin and the insulin promoters have only limited similarities.

In vivo system for characterizing clonal variation and tissue-specific gene regulatory factors based on function

The inducibility of stably transfected alpha-cardiac actin genes differs among L cell clones. We examined the ability of muscle-specific factors to induce the expression of the human muscle alpha-cardiac actin gene promoter when stably transfected into mouse fibroblast L cells. This promoter is transcriptionally active in L cells at a low level, 2-5% of that in transfected muscle cells. Upon fusion with muscle cells to form heterokaryons, expression of the transfected alpha-cardiac actin gene promoter can be induced. However, induction is observed with only 10% of transfected L cell clones and the magnitude of this induction varies between 5- and 50-fold. These properties of the transfected L cell appear to be stably inherited. Our results are consistent with the hypothesis that muscle cells contain factors capable of increasing the transcription of the transfected gene, but that differences among L cell clones, possibly in the site of integration in the genome, determine the extent to which the gene can respond. By fusion into heterokaryons, transfectants with responsive genes can be identified. Such clones should prove useful in determining the basis for clonal variation. In addition, they provide an in vivo system for isolating functionally active tissue-specific transcription factors and the genes that encode them.

A 29-nucleotide DNA segment containing an evolutionarily conserved motif is required in cis for cell-type-restricted repression of the chicken alpha-smooth muscle actin gene core promoter

A series of 5' deletion mutations of the upstream flanking sequences of the chicken alpha-smooth muscle (aortic) actin gene was prepared and inserted into the chloramphenicol acetyltransferase expression vector pSV0CAT. Deletion recombinants were transfected into fibroblasts, which actively express the alpha-smooth muscle actin gene, and primary myoblast cultures, which accumulate much lower quantities of alpha-smooth muscle actin mRNAs. The first 122 nucleotides of 5'-flanking DNA were found to contain a "core" promoter capable of accurately directing high levels of transcription in both fibroblasts and myotubes. The activity of this core promoter is modulated in fibroblasts by a "governor" element(s) located at least in part between nucleotides -257 and -123. This region contains sequences potentially conserved between mammalian and avian alpha-smooth muscle actin genes as well as one of a pair of 16-base-pair inverted CCAAT box-associated repeats which are conserved among all chordate muscle actin genes examined to date. A smaller DNA segment (-151 to -123) containing these upstream CCAAT box-associated repeats was sufficient to suppress expression of the core promoter in muscle cultures, suggesting that the upstream CCAAT box-associated repeats play a negative role in the alpha-smooth muscle actin gene promoter.

Transcription elements and factors of RNA polymerase B promoters of higher eukaryotes

The promoter for eukaryotic genes transcribed by RNA polymerase B can be divided into the TATA box (located at -30) and startsite (+1), the upstream element (situated between -40 and about -110), and the enhancer (no fixed position relative to the startsite). Trans-acting factors, which bind to these elements, have been identified and at least partially purified. The role of the TATA box is to bind factors which focus the transcription machinery to initiate at the startsite. The upstream element and the enhancer somehow modulate this interaction, possibly through direct protein-protein interactions. Another class of transcription factors, typified by viral proteins such as the adenovirus EIA products, do not appear to require binding to a particular DNA sequence to regulate transcription. The latest findings in these various subjects are discussed.

A 29-nucleotide DNA segment containing an evolutionarily conserved motif is required in cis for cell-type-restricted repression of the chicken alpha-smooth muscle actin gene core promoter

A series of 5' deletion mutations of the upstream flanking sequences of the chicken alpha-smooth muscle (aortic) actin gene was prepared and inserted into the chloramphenicol acetyltransferase expression vector pSV0CAT. Deletion recombinants were transfected into fibroblasts, which actively express the alpha-smooth muscle actin gene, and primary myoblast cultures, which accumulate much lower quantities of alpha-smooth muscle actin mRNAs. The first 122 nucleotides of 5'-flanking DNA were found to contain a "core" promoter capable of accurately directing high levels of transcription in both fibroblasts and myotubes. The activity of this core promoter is modulated in fibroblasts by a "governor" element(s) located at least in part between nucleotides -257 and -123. This region contains sequences potentially conserved between mammalian and avian alpha-smooth muscle actin genes as well as one of a pair of 16-base-pair inverted CCAAT box-associated repeats which are conserved among all chordate muscle actin genes examined to date. A smaller DNA segment (-151 to -123) containing these upstream CCAAT box-associated repeats was sufficient to suppress expression of the core promoter in muscle cultures, suggesting that the upstream CCAAT box-associated repeats play a negative role in the alpha-smooth muscle actin gene promoter.

A mutational analysis of the insulin gene transcription control region: expression in beta cells is dependent on two related sequences within the enhancer

Cell-specific expression of the insulin gene is controlled by cis-acting DNA sequences located within approximately equal to 350 base pairs of the 5' flanking DNA immediately upstream from the transcription start site. Using synthetic oligonucleotides, we have constructed a systematic series of block replacement mutants spanning this region. No single sequence appears to be absolutely required for expression. However, three of the mutants exhibit 5-10 times less activity and several others show 2-3 times less. Simultaneous mutation of two of the most mutationally sensitive regions leads to virtual abolition of activity. These two elements are structurally related and presumably represent key components of the machinery determining the cell-specific expression of the insulin gene.

Cloning vectors for expression of cDNA libraries in mammalian cells

We have constructed a series of compound cloning vectors (lambda ZD vectors), each consisting of phage lambda arms carrying a modified version of the retroviral expression vector pZIP-neoSV (x)1. cDNA, inserted into a cloning site present in the retroviral vector component, is cloned with high efficiency using the lambda system. A cDNA library in plasmids is then released by homologous recombination between the retroviral long terminal repeats. Retroviral transduction is achieved by transient expression of the released library in a cell line containing a packaging mutant of Moloney murine leukemia virus, followed by cocultivation of these producers with recipient cells. Transcription of cDNAs in the recipient cells is driven by the strong long terminal repeat promoter, and the transcripts, even from truncated cDNAs, can be expressed because translational start sites have been provided in all three reading frames (tri-initiator). Sequences conferring a recognizable phenotype can be rescued by cell fusion. The functionality of the tri-initiator and the rescue of a rare cDNA have been successfully tested using model systems.

A cell type specific factor recognizes the rat thyroglobulin promoter

We have fused a 900 base pair long DNA segment containing the transcriptional start site of the rat thyroglobulin (Tg) gene to the bacterial gene for chloramphenicol acetyltransferase (cat). The fusion gene has been introduced into three different cell lines derived from the rat thyroid gland and into a rat liver cell line. Expression of the fusion gene was detected only in the one thyroid cell line that is able to express the endogenous Tg gene. The minimum DNA sequence required for the cell type specific expression was determined by deletion analysis; it extends 170 nucleotides upstream of the transcription initiation site. The Tg promoter contains a readily detectable binding sites for a factor present in salt extracts of thyroid cell nuclei. This binding site is not recognized by the nuclear extracts of any other cell type that we have tested, suggesting that it may help mediate the cell type specific expression of the Tg gene.

Repression of insulin gene expression by adenovirus type 5 E1a proteins

Insulin gene transcription relies on enhancer and promoter elements which are active in pancreatic beta cells. We showed that adenovirus type 5 infection of HIT T-15 cells, a transformed hamster beta cell line, represses insulin gene transcription and mRNA levels. Using expression plasmids transiently introduced into HIT T-15 cells, we showed that adenovirus type 5 E1a transcription regulatory proteins repress insulin enhancer-promoter element activity as assayed with a surrogate xanthine-guanine phosphoribosyltransferase gene. We relate E1a repression of the insulin gene to other examples of repression of enhancer-dependent genes by E1a and discuss the possible relationship of this repression to insulin gene regulation.

Human globin gene promoter sequences are sufficient for specific expression of a hybrid gene transfected into tissue culture cells

The contribution of the human globin gene promoters to tissue-specific transcription was studied by using globin promoters to transcribe the neo (G418 resistance) gene. After transfection into different cell types, neo gene expression was assayed by scoring colony formation in the presence of G418. In K562 human erythroleukemia cells, which express fetal and embryonic globin genes but not the adult beta-globin gene, the neo gene was expressed strongly from a fetal gamma- or embryonic zeta-globin gene promoter but only weakly from the beta promoter. In murine erythroleukemia cells which express the endogenous mouse beta genes, the neo gene was strongly expressed from both beta and gamma promoters. In two nonerythroid cell lines, human HeLa cells and mouse 3T3 fibroblasts, the globin gene promoters did not allow neo gene expression. Globin-neo genes were integrated in the erythroleukemia cell genomes mostly as a single copy per cell and were transcribed from the appropriate globin gene cap site. We conclude that globin gene promoter sequences extending from -373 to +48 base pairs (bp) (relative to the cap site) for the beta gene, -385 to +34 bp for the gamma gene, and -555 to +38 bp for the zeta gene are sufficient for tissue-specific and perhaps developmentally specific transcription.

Regulated expression of a complete human beta-globin gene encoded by a transmissible retrovirus vector

We introduced a human beta-globin gene into murine erythroleukemia (MEL) cells by infection with recombinant retroviruses containing the complete genomic globin sequence. The beta-globin gene was correctly regulated during differentiation, steady-state mRNA levels being induced 5- to 30-fold after treatment of the cells with the chemical inducer dimethyl sulfoxide. Studies using vectors which yield integrated proviruses lacking transcriptional enhancer sequences indicated that neither retroviral transcription nor the retroviral enhancer sequences themselves had any obvious effect on expression of the globin gene. Viral RNA expression also appeared inducible, being considerably depressed in uninduced MEL cells but approaching normal wild-type levels after dimethyl sulfoxide treatment. We provide data which suggest that the control point for both repression and subsequent activation of virus expression in MEL cells lies in the viral enhancer element.

Repression of insulin gene expression by adenovirus type 5 E1a proteins

Insulin gene transcription relies on enhancer and promoter elements which are active in pancreatic beta cells. We showed that adenovirus type 5 infection of HIT T-15 cells, a transformed hamster beta cell line, represses insulin gene transcription and mRNA levels. Using expression plasmids transiently introduced into HIT T-15 cells, we showed that adenovirus type 5 E1a transcription regulatory proteins repress insulin enhancer-promoter element activity as assayed with a surrogate xanthine-guanine phosphoribosyltransferase gene. We relate E1a repression of the insulin gene to other examples of repression of enhancer-dependent genes by E1a and discuss the possible relationship of this repression to insulin gene regulation.

Regulated expression of a complete human beta-globin gene encoded by a transmissible retrovirus vector

We introduced a human beta-globin gene into murine erythroleukemia (MEL) cells by infection with recombinant retroviruses containing the complete genomic globin sequence. The beta-globin gene was correctly regulated during differentiation, steady-state mRNA levels being induced 5- to 30-fold after treatment of the cells with the chemical inducer dimethyl sulfoxide. Studies using vectors which yield integrated proviruses lacking transcriptional enhancer sequences indicated that neither retroviral transcription nor the retroviral enhancer sequences themselves had any obvious effect on expression of the globin gene. Viral RNA expression also appeared inducible, being considerably depressed in uninduced MEL cells but approaching normal wild-type levels after dimethyl sulfoxide treatment. We provide data which suggest that the control point for both repression and subsequent activation of virus expression in MEL cells lies in the viral enhancer element.

Human globin gene promoter sequences are sufficient for specific expression of a hybrid gene transfected into tissue culture cells

The contribution of the human globin gene promoters to tissue-specific transcription was studied by using globin promoters to transcribe the neo (G418 resistance) gene. After transfection into different cell types, neo gene expression was assayed by scoring colony formation in the presence of G418. In K562 human erythroleukemia cells, which express fetal and embryonic globin genes but not the adult beta-globin gene, the neo gene was expressed strongly from a fetal gamma- or embryonic zeta-globin gene promoter but only weakly from the beta promoter. In murine erythroleukemia cells which express the endogenous mouse beta genes, the neo gene was strongly expressed from both beta and gamma promoters. In two nonerythroid cell lines, human HeLa cells and mouse 3T3 fibroblasts, the globin gene promoters did not allow neo gene expression. Globin-neo genes were integrated in the erythroleukemia cell genomes mostly as a single copy per cell and were transcribed from the appropriate globin gene cap site. We conclude that globin gene promoter sequences extending from -373 to +48 base pairs (bp) (relative to the cap site) for the beta gene, -385 to +34 bp for the gamma gene, and -555 to +38 bp for the zeta gene are sufficient for tissue-specific and perhaps developmentally specific transcription.

Comparison of promoter suppression in avian and murine retrovirus vectors

Previously, we described "promoter suppression" in infectious retrovirus vectors with two genes and an internal promoter. Here, we examined several parameters of promoter suppression and found that the amount of suppression in an integrated retrovirus vector was dependent both on whether the vector was derived from spleen necrosis virus or murine leukemia virus and on which internal promoter was present in the vector. Murine leukemia virus vectors showed less suppression than analogous spleen necrosis virus vectors. Furthermore, the amount of suppression was not dependent on either the relative strengths of the promoters nor the distance between the promoters. Moreover, we found that in vectors in which one promoter was suppressed, there was an inverse correlation between the DNaseI sensitivity of the chromatin surrounding a promoter and the suppression of its expression.

RNA-mediated gene duplication: the rat preproinsulin I gene is a functional retroposon

Rats and mice have two, equally expressed, nonallelic genes encoding preproinsulin (genes I and II). Cytological hybridization with metaphase chromosomes indicated that both genes reside on rat chromosome I but are approximately 100,000 kilobases apart. In mice the two genes reside on two different chromosomes. DNA sequence comparisons of the gene-flanking regions in rats and mice indicated that the preproinsulin gene I has lost one of the two introns present in gene II, is flanked by a long (41-base) direct repeat, and has a remnant of a polydeoxyadenylate acid tract preceding the downstream direct repeat. These structural features indicated that gene I was generated by an RNA-mediated duplication-transposition event involving a transcript of gene II which was initiated upstream from the normal capping site. Sequence divergence analysis indicated that the pair of the original gene and its retroposed, but functional, counterpart (which appeared about 35 million years ago) is maintained by strong negative selection operating primarily on the segments encoding the chains of the mature hormone, whereas the segments encoding the parts of the polypeptide that are eliminated during processing and also the introns and the flanking regions are evolving neutrally.

Enhancer-dependent expression of the rat preproinsulin gene in bovine papillomavirus type 1 vectors

The effect of position in a bovine papillomavirus type 1 (BPV-1) vector on foreign gene expression was assessed with the rat preproinsulin (rI1) gene. The rI1 gene was inserted at each of the BPV-1/pML2d junctions in either transcriptional orientation in derivatives of the pdBPV-1(142-6) vector which consists of the BamHI linear genome of BPV-1 DNA cloned into pML2d. Transformed lines of C127 cells were established and assayed for rI1 gene expression. Cells containing the rI1 gene at the 3' end of the BPV-1 transforming region expressed rat proinsulin, whereas cells with the gene at the 5' end of the nontransforming region did not. Variability in the plasmid copy number or in the extent of DNA rearrangement could not account for this difference. We conclude that the expression of the rat preproinsulin gene (which is normally tissue specific for pancreatic islet cells) in C127 cells depends on the transcriptional activation afforded by viral enhancer sequences located at the 3' end of the transforming region. Intervening BPV-1 or pML2d sequences appear to block this enhancer-mediated gene activation. In agreement with enhancer-dependent activation, a rat preproinsulin gene located in a blocked position (i.e., not adjacent to the BPV-1 enhancer) could be activated by the insertion of a DNA fragment containing the simian virus 40, Moloney murine sarcoma virus, or BPV-1 enhancer element adjacent to the rI1 gene. Thus, a gene which is normally not expressed in a particular cell may be activated when placed adjacent to a viral enhancer in a BPV-1 vector.

Tissue-specific expression of the rat albumin gene: genetic control of its extinction in microcell hybrids

Numerous studies of cell hybrids have indicated that somatic cells produce negative regulators (extinguishers) that prevent the expression of functions foreign to their own differentiation. Here, we report genetic evidence of such control. In microcell hybrids between well-differentiated rat hepatoma cells and microcells of mouse fibroblast L cells, the extinction of albumin synthesis is directly related to the presence of a single specific chromosome of the mouse fibroblast parent. The expression of several other hepatic functions is not affected. Transfection of these hybrids with a recombinant plasmid, containing a tissue-specific control element of the upstream region of the rat albumin gene linked to coding sequences of the chloramphenicol acetyltransferase gene, reveals that extinction acts on or via this cis-control element.

Factors involved in production of helper virus-free retrovirus vectors

Retrovirus vectors allow efficient transfer of genetic material into cells. We describe an improved method for making cell lines which secrete broad host range retrovirus vectors in the absence of helper virus. This method was used to make virus-producing cell lines from several retrovirus vector constructions that encode dominant selectable markers. Virus titers from such lines exceeded 10(6) colony-forming units per milliliter of medium exposed to the cells. Cell lines that secreted certain vectors remained free of helper virus, while cell lines made using other vectors always secreted helper virus. Secretion of helper virus apparently depended on recombination between vector and the retrovirus packaging system, and factors involved in this event were investigated.

Enhancer-dependent expression of the rat preproinsulin gene in bovine papillomavirus type 1 vectors

The effect of position in a bovine papillomavirus type 1 (BPV-1) vector on foreign gene expression was assessed with the rat preproinsulin (rI1) gene. The rI1 gene was inserted at each of the BPV-1/pML2d junctions in either transcriptional orientation in derivatives of the pdBPV-1(142-6) vector which consists of the BamHI linear genome of BPV-1 DNA cloned into pML2d. Transformed lines of C127 cells were established and assayed for rI1 gene expression. Cells containing the rI1 gene at the 3' end of the BPV-1 transforming region expressed rat proinsulin, whereas cells with the gene at the 5' end of the nontransforming region did not. Variability in the plasmid copy number or in the extent of DNA rearrangement could not account for this difference. We conclude that the expression of the rat preproinsulin gene (which is normally tissue specific for pancreatic islet cells) in C127 cells depends on the transcriptional activation afforded by viral enhancer sequences located at the 3' end of the transforming region. Intervening BPV-1 or pML2d sequences appear to block this enhancer-mediated gene activation. In agreement with enhancer-dependent activation, a rat preproinsulin gene located in a blocked position (i.e., not adjacent to the BPV-1 enhancer) could be activated by the insertion of a DNA fragment containing the simian virus 40, Moloney murine sarcoma virus, or BPV-1 enhancer element adjacent to the rI1 gene. Thus, a gene which is normally not expressed in a particular cell may be activated when placed adjacent to a viral enhancer in a BPV-1 vector.

Plasticity of the differentiated state

Heterokaryons provide a model system in which to examine how tissue-specific phenotypes arise and are maintained. When muscle cells are fused with nonmuscle cells, muscle gene expression is activated in the nonmuscle cell type. Gene expression was studied either at a single cell level with monoclonal antibodies or in mass cultures at a biochemical and molecular level. In all of the nonmuscle cell types tested, including representatives of different embryonic lineages, phenotypes, and developmental stages, muscle gene expression was induced. Differences among cell types in the kinetics, frequency, and gene dosage requirements for gene expression provide clues to the underlying regulatory mechanisms. These results show that the expression of genes in the nuclei of differentiated cells is remarkably plastic and susceptible to modulation by the cytoplasm. The isolation of the genes encoding the tissue-specific trans-acting regulators responsible for muscle gene activation should now be possible.

Rat LINE1: the origin and evolution of a family of long interspersed middle repetitive DNA elements

We present approximately 7.0 kb of composite DNA sequence of a long interspersed middle repetitive element (LINE1) present in high copy number in the rat genome. The family of these repeats, which includes transcribing members, is the rat homologue of the mouse MIF-Bam-R and human Kpn I LINEs. Sequence alignments between specimens from these three species define the length of a putative unidentified open reading frame, and document extensive recombination events that, in conjunction with retroposition, have generated this large family of pseudogenes and pseudogene fragments. Comparative mapping of truncated elements indicates that a specific endonucleolytic activity might be involved in illegitimate (nonhomologous) recombination events. Sequence divergence analyses provide insights into the origin and molecular evolution of these elements.

RNA-mediated gene duplication: the rat preproinsulin I gene is a functional retroposon

Rats and mice have two, equally expressed, nonallelic genes encoding preproinsulin (genes I and II). Cytological hybridization with metaphase chromosomes indicated that both genes reside on rat chromosome I but are approximately 100,000 kilobases apart. In mice the two genes reside on two different chromosomes. DNA sequence comparisons of the gene-flanking regions in rats and mice indicated that the preproinsulin gene I has lost one of the two introns present in gene II, is flanked by a long (41-base) direct repeat, and has a remnant of a polydeoxyadenylate acid tract preceding the downstream direct repeat. These structural features indicated that gene I was generated by an RNA-mediated duplication-transposition event involving a transcript of gene II which was initiated upstream from the normal capping site. Sequence divergence analysis indicated that the pair of the original gene and its retroposed, but functional, counterpart (which appeared about 35 million years ago) is maintained by strong negative selection operating primarily on the segments encoding the chains of the mature hormone, whereas the segments encoding the parts of the polypeptide that are eliminated during processing and also the introns and the flanking regions are evolving neutrally.

Generation of helper-free amphotropic retroviruses that transduce a dominant-acting, methotrexate-resistant dihydrofolate reductase gene

We constructed several retroviruses which transduced a mutant dihydrofolate reductase gene that was resistant to methotrexate inhibition and functioned as a dominant selectable marker. The titer of dihydrofolate reductase-transducing virus produced by virus-producing cells could be increased to very high levels by selection of the cells in increasing concentrations of methotrexate. Helper virus-free dihydrofolate reductase-transducing virus was also generated by using a broad-host-range amphotropic retroviral packaging system. Cell lines producing helper-free dihydrofolate reductase-transducing virus with a titer of 4 X 10(6) per ml were generated. These retroviral vectors should have general utility for high-efficiency transduction of genes in cultured cells and in animals.

Functional analysis of the transcription control region located within the avian retroviral long terminal repeat

We used several quantitative assays of in vivo transient gene expression to dissect the elements within the Rous sarcoma virus long terminal repeat (LTR) which constitute the retroviral transcription control region. Site-directed deletion mutagenesis was used to locate and define the enhancer and promoter elements within the LTR. In addition, we inserted exogenous DNA fragments into the LTR to examine the effects of position and sequence on the activity of these LTR transcriptional elements. The Rous sarcoma virus enhancer element, which we propose is located entirely within the LTR, was shown to activate both the beta-globin and retroviral LTR promoters when located in cis. We observed a striking correlation between the degree of activation and the distance between the retroviral promoter and enhancer elements. The LTR promoter element mediated the activation effect of the enhancer element, as LTR deletion mutants containing only the enhancer and TATA box region expressed little activity. The promoter region encoded a low but significant level of transcriptional activity even in the absence of an enhancer. Overall LTR transcriptional activity declined sharply with increasing distance between the LTR promoter and initiator elements. These results shed light on both the importance of the spatial arrangement of the sequence elements within this eucaryotic transcription control region and on the functional interrelationship between these elements.