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

Twists and turns: a scientific journey

In this perspective I look back on the twists and turns that influenced the direction of my scientific career over the past 40 years. From my early ambition to be a chemist to my training in Philadelphia and Bethesda as a molecular biologist, I benefited enormously from generous and valuable mentoring. In my independent career in Philadelphia and Princeton, I was motivated by a keen interest in the changes in gene expression that direct the development of the mammalian embryo and inspired by the creativity and energy of my students, fellows, and research staff. After twelve years as President of Princeton University, I have happily returned to the faculty of the Department of Molecular Biology.

The oncofetal gene glypican 3 is regulated in the postnatal liver by zinc fingers and homeoboxes 2 and in the regenerating liver by alpha-fetoprotein regulator 2

The Glypican 3 (Gpc3) gene is expressed abundantly in the fetal liver, is inactive in the normal adult liver, and is frequently reactivated in hepatocellular carcinoma (HCC). This reactivation in HCC has led to considerable interest in Gpc3 as a diagnostic tumor marker and its possible role in tumorigenesis. Despite this interest, the basis for Gpc3 regulation is poorly understood. On the basis of the similarities between Gpc3 and alpha-fetoprotein expression in the liver, we reasoned that common factors might regulate these 2 genes. Here we identify zinc fingers and homeoboxes 2 (Zhx2) as a regulator of Gpc3. Mouse strain-specific differences in adult liver Gpc3 messenger RNA levels and transgenic mouse studies indicate that Zhx2 represses Gpc3 expression in the adult liver. We also demonstrate that Gpc3 is activated in the regenerating liver following a carbon tetrachloride treatment and that the level of Gpc3 induction is controlled by alpha-fetoprotein regulator 2 (Afr2).

CONCLUSION

We show that Zhx2 acts as a repressor of Gpc3 in the adult liver, and this raises the interesting possibility that Zhx2 might also be involved in Gpc3 reactivation in HCC. We also show that Gpc3 is activated in the regenerating liver in an Afr2-dependent manner. Zhx2 and Afr2 represent the first known regulators of Gpc3.

AUF1-like protein binds specifically to DAS cis-acting element that regulates mouse alpha-fetoprotein gene expression

Alpha-fetoprotein (AFP) is one of the major serum proteins in the early life of mammals. We have previously identified a novel cis-acting element designated as DAS at the 5'-flanking region of the AFP gene and demonstrated that the DAS sequence can be specifically recognized by nuclear protein DAP-II in AFP-producing hepatoma cells and retinoic acid (RA)-induced AFP-producing F9 cells. In this study, we used DNA affinity chromatography to purify the DAP-II proteins from the nuclear extracts (NE) of RA-treated F9 cells. The purified DAP-II complex mainly contained five proteins, with molecular weights of 45, 42, 32, 30, and 20 kDa, respectively. The identification of these proteins was determined by MALDI-TOF mass spectrometric analysis and a database search. These proteins were found to belong to the AUF1 RNA-binding protein family. Protein (30 kDa), one of five proteins in an isolated DAP-II complex, was matched with amino acid sequence highly similar to muAUF1-3. The expression of this protein is inducible by RA, and the pattern of the protein expression is the same as DAP-II proteins in F9 cells after treatment with RA during differentiation. Our results suggest that the 30-kDa protein is a novel isoform of AUF1 family and is the main component of the DAP-II complex that binds to the DAS sequence.

Adenovirus-mediated increase of HNF-3 levels stimulates expression of transthyretin and sonic hedgehog, which is associated with F9 cell differentiation toward the visceral endoderm lineage

Retinoic acid-induced differentiation of mouse F9 embryonal carcinoma cells toward the visceral endoderm lineage is accompanied by increased expression of the Forkhead Box (Fox) transcription factors hepatocyte nuclear factor 3a (HNF-3alpha) and HNF-3beta, suggesting that they play a crucial role in visceral endoderm development. Retinoic acid stimulation results in a cascade of HNF-3 induction in which HNF-3alpha is a primary target for retinoic acid action and its increase is required for subsequent induction of HNF-3beta expression. Increased expression of HNF-3beta precedes activation of its known target genes, including transthyretin (TTR), Sonic hedgehog (Shh), HNF-1alpha, HNF-1beta, and HNF-4alpha. In order to examine whether increased HNF-3 expression is sufficient to induce expression of its downstream target genes without retinoic acid stimulation, we have used adenovirus-based expression vectors to increase HNF-3 protein levels in F9 cells. We demonstrate that adenovirus-mediated increase of HNF-3alpha levels in F9 cells is sufficient to induce activation of endogenous HNF-3beta levels followed by increased TTR and Shh expression. Furthermore, we show that elevated HNF-3beta levels stimulate expression of endogenous TTR and Shh without retinoic acid stimulation. Moreover, ectopic HNF-3 levels in undifferentiated F9 cells are insufficient to induce HNF-3alpha, HNF-1alpha, HNF-1beta, and HNF-4alpha expression, suggesting that their transcriptional activation required other regulatory proteins induced by the retinoic acid differentiation program. Finally, our studies demonstrate the utility of cell infections with adenovirus expressing distinct transcription factors to identify endogenous target genes, which are assembled with the appropriate nucleosome structure.

Identification of an enhancer and an alternative promoter in the first intron of the alpha-fetoprotein gene

In this study we have characterized a positive regulatory region located in the first intron of the alpha-fetoprotein (AFP) gene. We show that the enhancer activity of the region depends on a 44 bp sequence centered on a CACCC motif. The sequence is the target of the two zinc fingers transcription factors BKLF and YY1. The introduction of a mutation destroying the CACCC box impairs the binding of BKLF but improves that of YY1. Moreover, the mutated sequence behaves as a negative control element, suggesting that BKLF behaves as a positive factor and that YY1 is a negative one. We also demonstrate the existence of a novel, tissue-specific AFP mRNA isoform present in the yolk sac and fetal liver which initiates from an alternative promoter located approximately 100 bp downstream of the enhancer element. The transcriptional start site controlled by this new promoter (called P2), was mapped to 66 bp downstream of a TATA box. A putative AUG translation site in-frame with exon 2 of the classical gene was found 295 bp downstream of the transcription start site. Like the traditional AFP promoter (P1), the P2 promoter is active in the yolk sac and fetal liver. Embryonic stem cells with an AFP knock-in gene containing either the P2 promoter or deleted for it were isolated and comparative analysis of embryonic bodies derived from these cells suggests that the P2 promoter contributes to early expression of the AFP gene.

The HNF-3alpha transcription factor is a primary target for retinoic acid action

We have previously demonstrated that gene expression of the hepatocyte nuclear factor 3alpha (HNF-3alpha) transcription factor is activated during retinoic-acid-induced differentiation of F9 embryonal carcinoma cells (A. Jacob et al. (1994). Nucleic Acids Res. 22, 2126-2133). We have extended these studies and now show that HNF-3alpha mRNA is induced approximately 6 h after addition of retinoic acid to the cells, peaks at 1 day postdifferentiation, and then declines to undetectable levels. Furthermore, HNF-3alpha induction occurs in the absence of de novo protein synthesis, suggesting that it is a primary target for retinoic acid action. In order to corroborate this hypothesis, we have mapped the cis-acting HNF-3alpha promoter site that mediates the retinoic acid response. DNA sequence analysis indicates that the HNF-3alpha promoter contains an authentic retinoic acid response element (RARE) of the DR5 class. As expected, this element is able to confer retinoic acid responsiveness to a heterologous promoter. In addition, the HNF-3alpha-specific RARE is able to interact with various retinoic acid receptor heterodimers of the RAR/RXR type. Since HNF-3alpha is induced early during mammalian neurogenesis, our data shed new light on the connection between retinoic-acid-mediated HNF-3alpha activation and establishment of the neuronal phenotype.

Identification of a cis-acting element in the rat alpha-fetoprotein gene and its specific binding proteins in F9 cells during retinoic acid-induced differentiation

Mouse F9 embryonic teratocarcinoma stem cells can be induced to differentiate into visceral endoderm. Following retinoic acid (RA) treatment, alpha-fetoprotein (AFP), a differentiation marker, is expressed and secreted. The mechanism by which RA regulates AFP expression during differentiation is not clear. The relatively late induction of AFP indicates that the AFP gene may not be a primary target of RA activity during F9 cell differentiation. In this study, a CAT reporter plasmid containing the rat AFP 5'-regulatory region (-7040 to +7) adjacent to the CAT gene (pAFPCAT) was stably transfected into F9 cells and used to delineate a cis-acting element which associates with AFP gene activation. Similar spatial and temporal expression patterns between the transcriptional activity of the recombinant AFP gene and the endogenous AFP gene demonstrate that this stably transfected F9 system can be used to dissect both cis-elements and trans-acting factors responsible for RA-induced AFP expression. Using a series of deletion mutants of the pAFPCAT, the region between -2611 to -1855 was found to be important in AFP-induction. Subsequent analysis identified a functional sequence (-1905 to -1891, 5'-ACTAAAATGGAGACT-3') that differentially binds nuclear proteins from undifferentiated and differentiated F9 cells. This sequence, designed as differentiation-associated sequence (DAS) for its unique binding of a nuclear protein (DAP-II) that appears during RA-induced F9 differentiation, acts as a regulatory protein factor in AFP gene activation.

Differential induction of HNF-3 transcription factors during neuronal differentiation

We have investigated the regulation of transcription factors HNF-3alpha and HNF-3beta during the retinoic acid-mediated differentiation of mouse P19 cells. Retinoic acid treatment converts P19 stem cells into neurons and astrocytes and we have clearly shown that gene expression of both HNF-3alpha and HNF-3beta is activated during this process. HNF-3alpha transcription was detected 2 h after addition of retinoic acid and took place in the absence of de novo protein synthesis. This suggests that HNF-3alpha is a primary target for retinoic acid action. HNF-3alpha induction displays a biphasic profile and HNF-3alpha mRNA reaches maximal levels at 2 and 6 days postdifferentiation. Additional experiments strongly suggest that the second peak is due to HNF-3alpha induction in postmitotic neurons. P19 stem cells, on the other hand, do not contain any detectable HNF-3alpha mRNA. According to our studies, the retinoic acid-mediated induction of HNF-3alpha occurs at the level of transcriptional initiation and is conferred by distal promoter sequences. In comparison to HNF-3alpha, HNF-3beta induction is a subsequent event and detectable levels of HNF-3beta mRNA materialize approximately 1 day after addition of retinoic acid to P19 stem cells. Time course studies firmly demonstrate that HNF-3beta mRNA peaks at about 2 days postdifferentiation and then declines to virtually unreadable levels. This temporal pattern is consistent with HNF-3beta being a secondary target for retinoic acid. In analogy to HNF-3alpha, HNF-3beta activation also takes place at the level of transcriptional initiation. Recent studies implicate HNF-3alpha and HNF-3beta in early mammalian neurogenesis. The detection of HNF-3alpha/beta activation during P19 cell differentiation provides us with a convenient cell culture system to elucidate the induction mechanism and the precise role of both transcriptional regulators in the formation of neuronal cells.

RXR-mediated regulation of the alpha-fetoprotein gene through an upstream element

Retinoic acid (RA) is known to have potent effects on development and differentiation. RA exerts its effects on transcription through two distinct classes of nuclear receptors, the retinoic acid receptor (RAR) and the retinoid X receptor (RXR), that bind to specific RA-responsive elements (RARE) in target genes. alpha-Fetoprotein (AFP), a hepatocyte differentiation, maturation, and carcinogenesis marker, is transcriptionally upregulated by RA in McA-RH8994 hepatoma cells. Using deletion mapping analysis, we have identified a RARE-like sequence that is located between -2406 and -2378 of the transcription initiation site of the rat AFP gene. Sequence analysis demonstrated that this cis-acting element consists of three direct repeats and one inverted repeat of a GGGTCA-like half-site. The putative RARE can specifically bind to both RXR homodimers and RAR/RXR heterodimers as determined by gel mobility shift assays. A DR1 direct repeat was more efficient than a DR5 direct repeat oligonucleotide in competition for binding of the putative RARE to RXR and RAR/RXR. A mutagenesis study indicated that to have a full-strength induction, all the repeats were required. To further analyze the function of this element in vivo, a reporter gene construct of the putative RARE combined with the thymidine kinase promoter was cotransfected with RAR and RXR expression plasmids in CV1 cells. CAT assays demonstrated that overexpression of RXRalpha conferred the best RA response, consistent with our previous observation that 9-cis-RA is more potent than all-trans-RA for inducing the expression of the AFP gene. In addition, the RXR selective ligand LG100153 alone can stimulate the expression of the AFP gene. Our data suggest that an RXR-mediated pathway exists for modulation of AFP gene expression through a specific element.

Insertional mutation of the motor endplate disease (med) locus on mouse chromosome 15

Homozygous transgenic mice from line A4 have an early-onset progressive neuromuscular disorder characterized by paralysis of the rear limbs, muscle atrophy, and lethality by 4 weeks of age. The transgene insertion site was mapped to distal chromosome 15 close to the locus motor endplate disease (med). The sequence of mouse DNA flanking the insertion site junctions was determined. A small (< 20 kb) deletion was detected at the insertion site, with no evidence of additional rearrangement of the chromosomal DNA. Noncomplementation of the transgene-induced mutation and med was demonstrated in a cross with medJ/+mice. The new allele is designated medTgNA4Bs (medtg). The homologous human locus MED was assigned to chromosome 12. Synaptotagmin 1 and contactin 1 were eliminated as candidate genes for the med mutation. The transgene-induced allele provides molecular access to the med gene, whose function is required for synaptic transmission at the neuromuscular junction and long-term survival of cerebellar Purkinje cells.

Endogenous and transfected mouse alpha-fetoprotein genes in undifferentiated F9 cells are activated in transient heterokaryons

Mouse F9 teratocarcinoma cells provide a system to study developmentally regulated alpha-fetoprotein (AFP) gene expression. AFP is not expressed in undifferentiated F9 cells but is induced when cells differentiate as cell aggregates in the presence of retinoic acid. Previous studies have led to the suggestion that undifferentiated F9 cells contain negative regulators of AFP expression. To test this, we have used transient heterokaryons to ask whether inactive AFP genes in undifferentiated F9 cells are responsive to positively acting trans-acting factors. Our results indicate that silent endogenous and transfected AFP genes are activated when undifferentiated F9 cells are fused to human hepatoma HepG2 cells. This suggests that the lack of AFP expression in undifferentiated F9 cells is due to the absence or insufficient level of positive-acting transcription factors, rather than the presence of dominant negative regulators. We also demonstrate that stably transfected AFP genes, although unmethylated, are properly regulated in F9 cells.

Localization of alpha-fetoprotein during the formation of the neural tube and somites in chick embryo

Very little is known about the biological role of alpha-fetoprotein (AFP) in normal development. This study was undertaken to look for AFP-positive tissues involved in active morphogenetic and histogenetic events. Using a polyclonal antibody specific for AFP and the immunoperoxidase technique, we have studied the AFP localization during the formation of the neural tube and somites in chick embryo. Immunostaining of early whole embryos, cephalic fold stage, shows a strong immunoreaction for AFP in the cephalic and neural folds. In a more advanced stage of the development (6-somite stage), the AFP expression has followed the caudal direction of the neural fold forming process. Immunostaining of 6- and 9-somite embryo sections shows an increase of AFP expression from the most undifferentiated (the neural fold), to the most differentiated (the neural tube). AFP does not label the non-segmented paraxial mesoderm, from which the somite is derived. Instead, when the paraxial mesoderm is segmented and has formed a somite, AFP positive cells are detected in the somite. The morphological differentiation of somite is joined to one biochemical differentiation, since the myotome and sclerotome cells are AFP positive while the dermatome cells are AFP negative. The sclerotome cells become AFP negative when they surround the notochord to form the vertebral body. The results presented here strongly suggest a close association of AFP with cell proliferation and differentiation.

Analysis of the mechanism of glucocorticoid-mediated down regulation of the mouse alpha-fetoprotein gene

Regulation of alpha-fetoprotein gene expression by dexamethasone was examined in vivo and in vitro using primary mouse fetal liver cell cultures. Dexamethasone accelerates the developmental down regulation of AFP mRNA pools. However, treatment of primary fetal liver cells in culture does not reduce the AFP mRNA pool and may stabilize both AFP and albumin gene expression. These results indicate that in vivo the effect of dexamethasone may require interaction with another tissue or cell type. The mechanism of the dexamethasone mediated inhibition of AFP was examined by DNase I footprinting and transient expression assays. Two protein-binding regions of the proximal promoter (III and IV) show significant homology to the GRE consensus sequence. DNase I footprinting shows that only region IV can bind purified GR and competition with GRE oligonucleotides indicate that, using adult liver nuclear proteins, no GR is bound in either region. Nuclear protein from adrenalectomized mice show the same protection as controls. These results indicate that GR may not bind to the AFP proximal promoter in the adult. AFP promoter-CAT expression vectors were used to further examine the effect of dexamethasone on AFP expression. AFP promoter-CAT constructs were inhibited by 10(-6) M dexamethasone; while linking of an AFP enhancer to the promoter abolished the effect. We conclude that the in vitro effects on transiently expressed AFP directed expression vectors may be a function of vector structure and/or characteristics of the cells used whereas the in vivo effect may reflect normal regulatory mechanisms.

Retinoic acid-mediated activation of HNF-3 alpha during EC stem cell differentiation

We present evidence demonstrating that the liver-enriched transcription factor HNF-3 alpha is activated upon retinoic acid-induced differentiation of mouse F9 embryonal carcinoma cells. We have detected increases in the DNA binding activity and mRNA level of HNF-3 alpha. Both are reflections of the actual activation mechanism at the level of transcriptional initiation, which we showed with the help of HNF-3 alpha promoter constructs. Time course studies clearly show that HNF-3 alpha activation is a transient event. Employing Northern blots, HNF-3 alpha mRNA can be detected between 16 and 24 hours post-differentiation, reaches its zenith at approximately 1 day, and then declines to virtually undetectable levels. F9 cells can give rise to three distinct differentiated cell types; visceral endoderm, parietal endoderm, and primitive endoderm. We have clearly shown that HNF-3 alpha stimulation occurs upon primitive endoderm formation. In addition, the transcription factor is also activated during the induction of cell lineages that give rise to parietal and visceral endoderm. HNF-3 alpha stimulation upon visceral endoderm differentiation is accompanied by the activation of HNF-3 target genes such as transthyretin, suggesting that HNF-3 alpha is involved in the developmental activation of this gene. In contrast, HNF-3 alpha target genes in parietal and primitive endoderm have yet to be identified. However, the stimulation of HNF-3 alpha during primitive endoderm formation, which is an extremely early event during murine embryogenesis, points towards a role for the factor in crucial determination processes that occur early during development.

Cloning and genetic characterization of the human kinesin light-chain (KLC) gene

We report the isolation, sequence, and identification of a cDNA encoding the human kinesin light-chain (KLC) protein. The cDNA molecule consisted of 276 nucleotides of 5' untranslated region, the complete coding sequence of 1,710 nucleotides, and 322 nucleotides of 3' untranslated region. It encoded a polypeptide of 569 amino acids and a deduced molecular mass of 64,789 daltons. The predicted secondary internal structure of the KLC molecule consisted of about 27 contiguous repeats, each of approximately 21 amino acid residues, and could be divided into three domains. The amino-terminal domain consisted of heptad repeats typical of the rod domain of several cytoskeletal proteins. The central and carboxy-terminal domains consist of 21-mer repeats. KLC mRNA was expressed in most tissues analyzed. The gene, which was expressed in bacteria and Chinese hamster ovary cells, was provisionally assigned to the long arm of human chromosome 14.

Expression patterns of vHNF1 and HNF1 homeoproteins in early postimplantation embryos suggest distinct and sequential developmental roles

The homeoproteins HNF1 (LFB1/HNF1-A) and vHNF1 (LFB3/HNF1 beta) interact with an essential control element of a group of liver-specific genes. During development, these putative target genes are initially expressed in the visceral endoderm of the yolk sac and subsequently in fetal liver. To assess the possible involvement of HNF1 and/or vHNF1 as transcriptional regulators in the early steps of visceral endoderm differentiation, we have analyzed the expression pattern of both factors both in vitro during differentiation of murine F9 embryonal carcinoma cells and in vivo during early postimplantation mouse development. We show here that differentiation of F9 cells into either visceral or parietal endoderm is accompanied by a sharp induction in vHNF1 mRNA and protein. By contrast, only low levels of aberrantly sized HNF1 transcripts, but not DNA-binding protein, are found in F9 cells and its differentiated derivatives. At 6–7.5 days of gestation, high levels of vHNF1 mRNA are present in the visceral extraembryonic endoderm, which co-localize with transcripts of the transthyretin gene. HNF1 transcripts are first detected in the yolk sac roughly two embryonic days later, after the developmental onset of transcription of target genes. As development proceeds, discrepancies are observed between the level of transcripts of both vHNF1 and HNF1 and their respective nuclear binding proteins, notably in the yolk sac and embryonic kidney. In addition, we show that two alternative spliced isoforms of vHNF1 mRNA, vHNF-A and vHNF1-B, are expressed in both embryonic and adult tissues. Taken together, these data suggest that vHNF1 participates as a regulatory factor in the initial transcriptional activation of the target genes in the visceral endoderm of the yolk sac, whereas the later appearance of HNF1 could be required for maintenance of their expression. Our results also provide evidence of a posttranscriptional level of control of vHNF1 and HNF1 gene expression during development, in addition to the spatial restriction in transcription.

Derivation of a basic mechanism of control for embryonic genes as a specific subset

The alpha-fetoprotein gene is conceived as being methylated in the zygote and according to the model is in a heterochromatic state and is therefore in a non-functional condition. Specific DNA methylase genes would produce methylases capable of alkylating enhancer regions of alpha-fetoprotein and certain proteins that would alter the heterochromatin condition. Also involved is a gene for the synthesis of a conformational-inducer protein that is proposed to be capable of blocking genic regions from reheterochromatizing. One of the pivotal events is the accumulation of S-adenosyl-L-methionine that reaches intracellular pool concentrations allowing other redundant active S-adenosyltransferase genes to become active. During embryogenesis specific conformational-inducer proteins would block genes such as the gene for albumin from reheterochromatizing while alpha-fetoprotein gene becomes heterochromatized during subsequent cell cycles. This heterochromatin is formed with embryonic type proteins sensitive to ribosylation-induced conformational changes. The increase in synthesis of alpha-fetoprotein followed by a decrease as albumin synthesis increases during embryogenesis is predicted by the scheme.

α-Fetoprotein expression in fetal kidney cells does not require enhancers

Expression of the alpha-fetoprotein (AFP) and albumin genes was examined in fetal mouse kidney by analysis of tissue mRNA pool sizes during development and transient expression assays in primary kidney tissue culture cells. AFP is expressed at a much lower level in kidney than in liver but transcription of the gene is activated early during development and repressed after birth with a time-course similar to liver. However, albumin mRNA was not detected in fetal or new born mouse kidney. Transient expression assays using AFP- and albumin-CAT (chloramphenicol acetyl transferase) vectors were employed to characterize cis-acting elements active in the regulation of AFP expression in kidney. Primary fetal liver and kidney cells in culture were used for these assays. The AFP promoter is active in kidney cells and the information necessary for tissue specific expression and developmental repression are contained within the first 1.0 kb of 5' flanking sequences of the AFP gene. In addition, the AFP upstream enhancer elements are inactive in primary kidney cells. The mouse albumin promoter is shown to be inactive in kidney cells. The results obtained using transient expression assays are consistent with the observed low level of AFP expression, developmental repression of AFP, and the absence of expression of albumin in the mouse kidney.

Sequences and factors required for the F9 embryonal carcinoma stem cell E1a-like activity

F9 embryonal carcinoma (EC) stem cells contain an E1a-like activity that is absent from differentiated derivatives. We have previously characterized proteins present in F9 EC cell extracts that bind to the E1a-dependent E2A promoter and have shown that two of them, TF68 and DRTF1, are required for efficient transcription in vitro (N. B. La Thangue, B. Thimmapaya, and P. W. J. Rigby, Nucleic Acids Res. 18:2929-2938, 1990). We now show that the E1a-like activity is detectable in transient transfection assays. Deletion mutations show that a distal sequence element, which includes the ATF/CREB consensus, is required for expression in both cell types, although it does not mediate the down-regulation of promoter activity that accompanies differentiation. A series of point mutations generated by in vitro mutagenesis confirm this and show that sequences around -60 are necessary for efficient expression in stem cells but not in differentiated derivatives. These sequences bind DRTF1, the activity of which is strongly down-regulated during differentiation. Surprisingly, mutations in a previously uncharacterized region of the promoter restore activity to a promoter carrying the -60 mutation and lead to the formation of a new DNA-protein complex.

Sequences and factors required for the F9 embryonal carcinoma stem cell E1a-like activity

F9 embryonal carcinoma (EC) stem cells contain an E1a-like activity that is absent from differentiated derivatives. We have previously characterized proteins present in F9 EC cell extracts that bind to the E1a-dependent E2A promoter and have shown that two of them, TF68 and DRTF1, are required for efficient transcription in vitro (N. B. La Thangue, B. Thimmapaya, and P. W. J. Rigby, Nucleic Acids Res. 18:2929-2938, 1990). We now show that the E1a-like activity is detectable in transient transfection assays. Deletion mutations show that a distal sequence element, which includes the ATF/CREB consensus, is required for expression in both cell types, although it does not mediate the down-regulation of promoter activity that accompanies differentiation. A series of point mutations generated by in vitro mutagenesis confirm this and show that sequences around -60 are necessary for efficient expression in stem cells but not in differentiated derivatives. These sequences bind DRTF1, the activity of which is strongly down-regulated during differentiation. Surprisingly, mutations in a previously uncharacterized region of the promoter restore activity to a promoter carrying the -60 mutation and lead to the formation of a new DNA-protein complex.

Role of alpha-fetoprotein regulatory elements in transcriptional activation in transient heterokaryons

The requirements for activation of the mouse alpha-fetoprotein (AFP) gene in transient heterokaryons were investigated. For this purpose, the 7-kilobases of DNA flanking the 5' end of the AFP gene were linked to a mouse major histocompatibility complex (MHC) class I structural gene. The fusion gene was stably integrated at different sites into mouse L-cells, which do not transcribe the AFP gene. Transient heterokaryon fusions demonstrated that the silent AFP-MHC gene and the endogenous AFP gene were activated by factors present in HepG2 cells, a liver-derived cell line, but not by those in HeLa cells. Activation was detected at the protein level in single heterokaryons by using monoclonal antibodies against the cell surface protein and at the mRNA level in populations of cells. The AFP promoter alone was sufficient for activation could be used for DNA transfer strategies to identify genes which can activate AFP promoter elements in trans.

Role of alpha-fetoprotein regulatory elements in transcriptional activation in transient heterokaryons

The requirements for activation of the mouse alpha-fetoprotein (AFP) gene in transient heterokaryons were investigated. For this purpose, the 7-kilobases of DNA flanking the 5' end of the AFP gene were linked to a mouse major histocompatibility complex (MHC) class I structural gene. The fusion gene was stably integrated at different sites into mouse L-cells, which do not transcribe the AFP gene. Transient heterokaryon fusions demonstrated that the silent AFP-MHC gene and the endogenous AFP gene were activated by factors present in HepG2 cells, a liver-derived cell line, but not by those in HeLa cells. Activation was detected at the protein level in single heterokaryons by using monoclonal antibodies against the cell surface protein and at the mRNA level in populations of cells. The AFP promoter alone was sufficient for activation could be used for DNA transfer strategies to identify genes which can activate AFP promoter elements in trans.

c-Ha-ras down regulates the alpha-fetoprotein gene but not the albumin gene in human hepatoma cells

We studied the effects of transfection of the normal c-Ha-ras gene, rasGly-12, and its oncogenic mutant, rasVal-12, on expression of the alpha-fetoprotein (AFP) and albumin genes in a human hepatoma cell line, HuH-7. The mutant and, to a lesser extent, the normal ras gene caused reduction of the AFP mRNA but not the albumin mRNA level in transfected HuH-7 cells. Cotransfection experiments with a rasVal-12 expression plasmid and a chloramphenicol acetyltransferase reporter gene fused to AFP regulatory sequences showed that rasVal-12 suppressed the activity of enhancer and promoter regions containing A + T-rich sequences (AT motif). In contrast, rasVal-12 did not affect the promoter activity of the albumin and human hepatitis B virus pre-S1 genes even though these promoters contain homologous A + T-rich elements. ras transfection appeared to induce phosphorylation of nuclear proteins that interact with the AFP AT motif, since gel mobility analysis revealed the formation of slow-moving complexes which was reversed by phosphatase treatment. However, similar changes in complex formation were observed with the albumin and hepatitis B surface antigen pre-S1 promoters. Therefore, this effect alone cannot explain the specific down regulation of the AFP promoter and enhancer activity. ras-mediated suppression of the AFP gene may reflect the process of developmental gene regulation in which AFP gene transcription is controlled by a G-protein-linked signal transduction cascade triggered by external growth stimuli.

c-Ha-ras down regulates the alpha-fetoprotein gene but not the albumin gene in human hepatoma cells

We studied the effects of transfection of the normal c-Ha-ras gene, rasGly-12, and its oncogenic mutant, rasVal-12, on expression of the alpha-fetoprotein (AFP) and albumin genes in a human hepatoma cell line, HuH-7. The mutant and, to a lesser extent, the normal ras gene caused reduction of the AFP mRNA but not the albumin mRNA level in transfected HuH-7 cells. Cotransfection experiments with a rasVal-12 expression plasmid and a chloramphenicol acetyltransferase reporter gene fused to AFP regulatory sequences showed that rasVal-12 suppressed the activity of enhancer and promoter regions containing A + T-rich sequences (AT motif). In contrast, rasVal-12 did not affect the promoter activity of the albumin and human hepatitis B virus pre-S1 genes even though these promoters contain homologous A + T-rich elements. ras transfection appeared to induce phosphorylation of nuclear proteins that interact with the AFP AT motif, since gel mobility analysis revealed the formation of slow-moving complexes which was reversed by phosphatase treatment. However, similar changes in complex formation were observed with the albumin and hepatitis B surface antigen pre-S1 promoters. Therefore, this effect alone cannot explain the specific down regulation of the AFP promoter and enhancer activity. ras-mediated suppression of the AFP gene may reflect the process of developmental gene regulation in which AFP gene transcription is controlled by a G-protein-linked signal transduction cascade triggered by external growth stimuli.

HNF-1 shares three sequence motifs with the POU domain proteins and is identical to LF-B1 and APF

The coordinate expression of genes during development and differentiation is thought to be accomplished by common transcription factors operating on the promoters of families of coexpressed genes. HNF-1 is a transcriptional factor involved in the expression of genes in the liver and was originally defined as playing a major role in coordinating the expression of the linked fibrinogen genes. We have isolated cDNA clones for HNF-1 using oligonucleotides prepared to the sequence of the purified protein. The sequence of HNF-1 shares homeo domain, as well as short acidic and basic sequences with the POU family of transcriptional activators. Peptides from the protein interacting with the albumin proximal element, or B box (APF), and the factor interacting with the alpha 1-antitrypsin promoter (LF-B1) are found in the predicted sequence of HNF-1. HNF-1 mRNA is not present in the dedifferentiated hepatoma variant, C2, but reappears upon selection for gluconeogenesis coincident with the re-expression of liver-specific genes. Finally, the mRNA is not present in somatic cell hybrids in which liver-specific gene expression is extinguished. In contrast to earlier published results, we find that in addition to being present in the liver, HNF is expressed in the kidney, intestine, and spleen, but not in other tissues. This pattern of expression mirrors the complex pattern of expression of many genes, such as alpha-fetoprotein, alpha 1-antitrypsin, and fibrinogen, whose promoters contain HNF-1 sites. These data indicate that HNF-1 is a more broadly acting transcription factor than has been indicated by previous work.

The transcription control region of the rat alpha-fetoprotein gene. DNA sequence and homology studies

The alpha-fetoprotein (AFP) gene, an important system for studying developmental and tissue-specific gene expression, is regulated mostly through the control of transcription. The promoter and cis-acting DNA elements which regulate the rat gene lie within a 7 kbp region upstream of the cap site. We have determined the sequence of this entire region. It contains several repetitive elements and a species-specific distribution of DNA methylation sites. We aligned our rat AFP sequence with fragmentary mouse and human AFP sequences to define blocks of highly conserved sequence, which we then analyzed for homology to known transcription regulatory sequences. Our analysis demonstrates that the regulatory region of the rat AFP gene is unusually complex.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: XXIII. Enhancer theory

Embryonic gene enhancers may be controlled by deheterochromatization of genes of trans-acting factors caused by altered methylation states of hypersensitive chromatin. Such specific chromatin sites would have a certain required helical pitch (X-type DNA). Non-core regions surrounding the enhancer motif would allow interactions with specific cell type promoters. Differences in embryonic and adult type genes may be reduced to these enhancer activities and certain trans-acting proteins effecting these genic control elements. Furthermore all spurious embryonic gene activities of neoplasms may also be due to such factors. Even pleotrophic and cascade phenomena in dysdifferentiation processes may be described in terms of enhancer mechanisms. Duplications of enhancer sequences may be fundamental to the potential of hyperactivity by embryonic type genes. Examples of this would be alpha-fetoprotein correlations with enhancer activity and possibly increased nucleolar organizer activity of embryonic cells. The manner by which chemical carcinogens would be involved with enhancer processes would be via "key" mechanisms that have been presented previously.

Differentiation-responsive elements in the 5' region of the mouse tissue plasminogen activator gene confer two-stage regulation by retinoic acid and cyclic AMP in teratocarcinoma cells

F9 cells induced to differentiate with retinoic acid (RA) increase transcription of the tissue plasminogen activator (t-PA) gene. Further treatment of these cells with cyclic AMP (cAMP) results in an additional stimulation of t-PA gene transcription. To investigate the mechanism of this two-stage regulation, 4 kilobase pairs (kbp) of 5'-flanking sequence from the murine t-PA gene was isolated. Two major start sites for transcription were found, neither of which depended on a classical TATA motif for correct initiation. By using transient transfection assays, it was determined that 4-kbp of flanking sequence could confer on reporter genes the same two-stage differentiation-specific expression as was observed for the endogenous t-PA gene. Deletion analyses of this 4-kbp fragment showed that 190 bp of flanking sequence was sufficient to bestow the same degree of two-stage regulation on reporter gene constructs. Within this region of DNA, sequence analysis revealed a possible cAMP regulatory element, a CTF/NF-1 recognition sequence, two potential Sp1 sites, and five potential binding sites for transcription factor AP-2. The deletion experiments, coupled with the positions of these potential cis-acting elements, suggest that multiple transcription factors, including those that bind to cAMP regulatory element, CTF/NF-1, Sp1, and AP-2 sites, may be involved in regulation of the t-PA gene during F9 cell differentiation.

Interaction of a hepatoma-specific nuclear factor with transcription-regulatory sequences of the human alpha-fetoprotein and albumin genes

DNase I footprinting and gel mobility shift analysis showed that an HuH-7 hepatoma nuclear protein, termed AFP1, binds specifically to an AT-rich sequence, TGATTAATAATTACA, in domain B of the human alpha-fetoprotein enhancer. No such binding activity was found in HeLa cell nuclei. Transient transfection studies showed that a 54-base-pair region corresponding to the AFP1-binding site could stimulate the simian virus 40 early promoter to express a linked chloramphenicol acetyltransferase gene in an orientation-independent and cell-specific manner. The correlation between the binding of AFP1 and the stimulation of chloramphenicol acetyltransferase gene expression strongly suggests that specific interaction of AFP1 with the AT motif is important for cell-specific transcriptional enhancement. Competition gel mobility shift analysis revealed that similar AT-rich sequences with high affinities to AFP1 were also present in the promoters of the alpha-fetoprotein and albumin genes. These results suggest that AFP1 may function as a common regulatory factor in the transcription of the alpha-fetoprotein and albumin genes.

Postnatal repression of the alpha-fetoprotein gene is enhancer independent

The mammalian liver undergoes a number of dramatic changes in gene expression during development. One of these is typified by the alpha-fetoprotein (AFP) gene, which is activated in the fetal liver but undergoes a transcriptional decline at birth. In contrast, although activated at the same time during fetal development, albumin gene transcription is maintained at high levels in adult animals. To determine whether the postnatal decline in AFP gene transcription is mediated through its distal enhancers or through more proximal elements surrounding the promoter or structural gene, chimeric genes bearing substitutions of albumin gene cis-acting elements for the equivalent AFP gene elements were introduced into the germ line of mice. The expression of the transgenes was then analyzed at various stages of development. Our results indicate that the AFP gene enhancers are not involved in the postnatal decline in AFP transcription. Rather, a region within the first kilobase of DNA upstream of the AFP gene, including its promoter, and/or portions of the structural gene is sufficient to direct postnatal repression of the gene.

Differentiation-responsive elements in the 5' region of the mouse tissue plasminogen activator gene confer two-stage regulation by retinoic acid and cyclic AMP in teratocarcinoma cells

F9 cells induced to differentiate with retinoic acid (RA) increase transcription of the tissue plasminogen activator (t-PA) gene. Further treatment of these cells with cyclic AMP (cAMP) results in an additional stimulation of t-PA gene transcription. To investigate the mechanism of this two-stage regulation, 4 kilobase pairs (kbp) of 5'-flanking sequence from the murine t-PA gene was isolated. Two major start sites for transcription were found, neither of which depended on a classical TATA motif for correct initiation. By using transient transfection assays, it was determined that 4-kbp of flanking sequence could confer on reporter genes the same two-stage differentiation-specific expression as was observed for the endogenous t-PA gene. Deletion analyses of this 4-kbp fragment showed that 190 bp of flanking sequence was sufficient to bestow the same degree of two-stage regulation on reporter gene constructs. Within this region of DNA, sequence analysis revealed a possible cAMP regulatory element, a CTF/NF-1 recognition sequence, two potential Sp1 sites, and five potential binding sites for transcription factor AP-2. The deletion experiments, coupled with the positions of these potential cis-acting elements, suggest that multiple transcription factors, including those that bind to cAMP regulatory element, CTF/NF-1, Sp1, and AP-2 sites, may be involved in regulation of the t-PA gene during F9 cell differentiation.

Two regulatory domains flank the mouse H19 gene

The mouse H19 gene was identified by virtue of its coordinate regulation with the mouse alpha-fetoprotein gene. Both genes are expressed in the fetal liver, gut, and visceral endoderm of the yolk sac and are repressed shortly after birth in the liver and gut. They are both under the control of two trans-acting loci: raf, which affects the adult basal levels of the two mRNAs, and Rif, which affects their inducibility during liver regeneration. One crucial difference between the two genes is the activation of the H19 gene in mesoderm derivatives, skeletal and cardiac muscle. As a strategy for explaining both the similarities and differences in their modes of expression, the regulatory domains responsible for the expression of the H19 gene in liver were identified by transiently introducing the gene into a human hepatoma cell line. Two regions necessary for high-level expression of the gene could be identified, a promoter-proximal domain immediately preceding the start of transcription and an enhancer domain which lies between 5 and 6.5 kilobases 3' of the polyadenylation site. The 3' domain consists of two separable enhancer elements, each of which exhibits the properties of tissue-specific enhancers. Nucleotide sequence comparisons between the two H19 and three alpha-fetoprotein enhancers revealed limited similarities which are candidates for binding of common regulatory factors. Sequences which lie 3' of the gene are also required for the expression of the H19 gene following differentiation of teratocarcinoma cells into visceral endoderm.

Repression of the IgH enhancer in teratocarcinoma cells associated with a novel octamer factor

Embryonal carcinoma (EC) cell lines are models for early cells in mouse embryogenesis. A 300-base pair fragment of the heavy chain enhancer was inactive in F9 EC cells, unlike in other nonlymphoid cells where it has significant activity. Alterations of the octamer motif increased enhancer activity. Nuclear extracts from F9 cells contained an octamer binding protein (NF-A3) that was unique to EC cells; the amount of NF-A3 decreased upon differentiation. It is proposed that NF-A3 represses specific regulatory sequences that contain the octamer motif. Thus, the same DNA sequence mediates either negative or positive transcriptional effects, depending on the cell type.

Cre-stimulated recombination at loxP-containing DNA sequences placed into the mammalian genome

The cre gene of coliphage P1 encodes a 38 kDa protein which efficiently promotes both intra- and intermolecular recombination at specific 34 bp sites called loxP. To demonstrate that the Cre protein can promote DNA recombination at loxP sites resident on a mammalian chromosome, a mouse cell line was constructed containing two directly repeated loxP sites flanking a 2.5 kb yeast DNA fragment and inserted between the SV40 promoter and the neo structural gene to disrupt expression of the neo gene. Expression of the cre gene in this cell line results in excision of the intervening yeast DNA and thus permits sufficient expression of the neo gene to allow cell growth in high concentrations of G418. Southern analysis indicated that Cre-mediated excision occurred at the loxP sites. In the absence of the cre gene such excisive events are quite rare. Cre-mediated recombination should thus be quite useful in effecting a variety of genomic rearrangements in eukaryotic cells.

Interaction of a hepatoma-specific nuclear factor with transcription-regulatory sequences of the human alpha-fetoprotein and albumin genes

DNase I footprinting and gel mobility shift analysis showed that an HuH-7 hepatoma nuclear protein, termed AFP1, binds specifically to an AT-rich sequence, TGATTAATAATTACA, in domain B of the human alpha-fetoprotein enhancer. No such binding activity was found in HeLa cell nuclei. Transient transfection studies showed that a 54-base-pair region corresponding to the AFP1-binding site could stimulate the simian virus 40 early promoter to express a linked chloramphenicol acetyltransferase gene in an orientation-independent and cell-specific manner. The correlation between the binding of AFP1 and the stimulation of chloramphenicol acetyltransferase gene expression strongly suggests that specific interaction of AFP1 with the AT motif is important for cell-specific transcriptional enhancement. Competition gel mobility shift analysis revealed that similar AT-rich sequences with high affinities to AFP1 were also present in the promoters of the alpha-fetoprotein and albumin genes. These results suggest that AFP1 may function as a common regulatory factor in the transcription of the alpha-fetoprotein and albumin genes.

Two regulatory domains flank the mouse H19 gene

The mouse H19 gene was identified by virtue of its coordinate regulation with the mouse alpha-fetoprotein gene. Both genes are expressed in the fetal liver, gut, and visceral endoderm of the yolk sac and are repressed shortly after birth in the liver and gut. They are both under the control of two trans-acting loci: raf, which affects the adult basal levels of the two mRNAs, and Rif, which affects their inducibility during liver regeneration. One crucial difference between the two genes is the activation of the H19 gene in mesoderm derivatives, skeletal and cardiac muscle. As a strategy for explaining both the similarities and differences in their modes of expression, the regulatory domains responsible for the expression of the H19 gene in liver were identified by transiently introducing the gene into a human hepatoma cell line. Two regions necessary for high-level expression of the gene could be identified, a promoter-proximal domain immediately preceding the start of transcription and an enhancer domain which lies between 5 and 6.5 kilobases 3' of the polyadenylation site. The 3' domain consists of two separable enhancer elements, each of which exhibits the properties of tissue-specific enhancers. Nucleotide sequence comparisons between the two H19 and three alpha-fetoprotein enhancers revealed limited similarities which are candidates for binding of common regulatory factors. Sequences which lie 3' of the gene are also required for the expression of the H19 gene following differentiation of teratocarcinoma cells into visceral endoderm.

Negative regulation of retrovirus expression in embryonal carcinoma cells mediated by an intragenic domain

An intragenic region spanning the tRNA primer binding site of a Moloney murine leukemia virus recombinant retrovirus was found to restrict expression specifically in embryonal carcinoma (EC) cells. When the inhibitory domain was present, the levels of steady-state RNA synthesized from integrated recombinant templates in stable cotransformation assays were reduced 20-fold in EC cells but not in C2 myoblast cells. Transient-cotransfection assays showed that repression of a template containing the EC-specific inhibitory component was relieved by an excess of specific competitor DNA. In addition, repression mediated by the inhibitory component was orientation independent. This evidence demonstrates the presence of a saturable, trans-acting negative regulatory factor(s) in EC cells and suggests that the interaction of the factor(s) with the intragenic inhibitory component occurs at the DNA level.

Site-specific DNA recombination in mammalian cells by the Cre recombinase of bacteriophage P1

The Cre protein encoded by the coliphage P1 is a 38-kDa protein that efficiently promotes both intra- and intermolecular synapsis and recombination of DNA both in Escherichia coli and in vitro. Recombination occurs at a specific site, called lox, and does not require any other protein factors. The Cre protein is shown here also to be able to cause synapsis of DNA and site-specific recombination in a mammalian cell line. A stable mouse cell line was established that expresses the Cre protein under the control of the Cd2+-inducible metallothionein I gene promoter. DNA recombination was monitored with DNA substrates containing two directly repeated lox sites. One such substrate is a circular plasmid with two directly repeated lox sites (lox2) flanking a marker gene and was introduced into cells by Ca3(PO4)2 transformation. As a second substrate we used a pseudorabies virus (a herpesvirus) containing a lox2 insertion designed to provide a sensitive detection system for recombination. In both cases, site-specific recombination in vivo is dependent on the presence of the Cre protein and occurs specifically at the 34-base-pair lox sites. These results demonstrate the controlled site-specific synapsis of DNA and recombination by a prokaryotic protein in mammalian cells and suggest that Cre-mediated site-specific recombination may be a useful tool for understanding and modulating genome rearrangements in eukaryotes.

Expression vectors permitting cDNA cloning and enrichment for specific sequences by hybridization/selection

A set of vectors is described which allow the efficient cloning of full-length cDNAs, using a modification of the method of Okayama and Berg [Mol. Cell Biol. 2 (1982) 161-170], and enrichment of specific sequences directly from cDNA libraries by hybridization/selection. The vectors pcDpolyB+ and pcDpolyB- are derived from an expression vector described previously [Okayama and Berg, Mol. Cell Biol. 3 (1983) 280-289] and allow expression of cloned cDNAs in eukaryotic cells from the simian virus 40 early region promoter. The vectors BSB+ and BSB- contain convenient priming sites for sequence analysis and the T3 and T7 RNA polymerase promoters, allowing synthesis of transcripts homologous to either strand of the cDNA. Each of these vectors also contains the intergenic region from the bacteriophage f1 permitting synthesis of single-stranded (ss) copies of the cDNA libraries. Enrichment for cDNAs containing sequences homologous to the hypoxanthine phosphoribosyl transferase gene from an ss copy of a cDNA library by hybridization/selection is demonstrated. Levels of enrichment sufficient for the direct cloning of specific sequences without requiring colony or plaque hybridizations were obtained. Libraries constructed from different cell types can be screened against each other to create sublibraries highly enriched in sequences specific to a single cell type. The availability of cDNA expression libraries enriched for cell-type-specific cDNAs should greatly enhance the efficiency with which cDNAs can be identified on the basis of functional assays.

Fine-structure mapping of the three mouse alpha-fetoprotein gene enhancers

Multiple cellular enhancers have been identified previously in the 5'-flanking region of the mouse alpha-fetoprotein gene by transient expression assay. In this report the enhancers have been localized to three regions 200 to 300 base pairs in length at 2.5, 5.0, and 6.5 kilobases of DNA upstream of the transcriptional start site. Nucleotide sequence analysis of the three enhancers revealed areas of homology among them, the most significant of which were two regions of 10 and 18 nucleotides in length. Two of the enhancers were analyzed in detail and shown to be composed of multiple nonidentical domains, none of which was sufficient for full enhancer activity; rather, they acted in an additive fashion in generating the full activity of the enhancer. The tissue-specific activity of the enhancer at -2.5 kilobases was assessed by comparing the activities of subdomains in liver- and non-liver-derived cell lines and was found to be the result of both positive elements within the enhancer and at least one negative element to its 5' end. In contrast, the tissue specificity of the enhancer at -5.0 kilobases was maintained when the minimal essential region was tested alone. The nucleotide sequence similarities, as well as the differences among the enhancers, may explain their differing biological activities both in tissue culture and in vivo.

Fine-structure mapping of the three mouse alpha-fetoprotein gene enhancers

Multiple cellular enhancers have been identified previously in the 5'-flanking region of the mouse alpha-fetoprotein gene by transient expression assay. In this report the enhancers have been localized to three regions 200 to 300 base pairs in length at 2.5, 5.0, and 6.5 kilobases of DNA upstream of the transcriptional start site. Nucleotide sequence analysis of the three enhancers revealed areas of homology among them, the most significant of which were two regions of 10 and 18 nucleotides in length. Two of the enhancers were analyzed in detail and shown to be composed of multiple nonidentical domains, none of which was sufficient for full enhancer activity; rather, they acted in an additive fashion in generating the full activity of the enhancer. The tissue-specific activity of the enhancer at -2.5 kilobases was assessed by comparing the activities of subdomains in liver- and non-liver-derived cell lines and was found to be the result of both positive elements within the enhancer and at least one negative element to its 5' end. In contrast, the tissue specificity of the enhancer at -5.0 kilobases was maintained when the minimal essential region was tested alone. The nucleotide sequence similarities, as well as the differences among the enhancers, may explain their differing biological activities both in tissue culture and in vivo.

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.

Differential requirements for cellular enhancers in stem and differentiated cells

Multiple cis-acting regulatory elements consisting of three cellular enhancers and a proximal promoter element have been identified in the region upstream of the mouse alpha-fetoprotein (AFP) gene. We examined the role of these sequences during differentiation by the introduction of modified AFP genes into cells at different stages of commitment to its expression. Modified AFP genes introduced stably into F9 embryonal carcinoma stem cells by DNA transfection were silent until activated by treatment with retinoic acid to form visceral endoderm. Their activation required the presence of both the enhancer and proximal promoter domains. The introduced genes activated simultaneously with the endogenous AFP genes, but reached maximal levels of expression more rapidly, suggesting a greater initial accessibility to transcription factors. In contrast, when modified AFP genes were stably introduced into HepG2 cells, a human hepatoma cell line that constitutively expresses the AFP gene, the proximal promoter sequences were sufficient to direct a low level of expression. The absolute requirement for the AFP enhancers in F9 cells but not in HepG2 cells supports a model by which there is an obligate requirement for enhancers during differentiation in addition to their role in enhancing gene expression after differentiation.

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.