Extinction of growth hormone expression in somatic cell hybrids involves repression of the specific trans-activator GHF-1

Genome-wide analysis of hepatic gene silencing in mammalian cell hybrids

Silencing of tissue-specific gene expression in mammalian somatic cell hybrids is a well-documented epigenetic phenomenon which is both profound (involving a large number of genes) and enigmatic. Our aim was to utilize whole-genome microarray analyses to determine the true extent of gene silencing on a genomic level. By comparing gene expression profiles of hepatoma×fibroblast cell hybrids with those of parental cells, we have identified over 300 liver-enriched genes that are repressed at least 5-fold in the cell hybrids, the majority of which are repressed at least 10-fold. Also, we identify nearly 200 fibroblast-enriched genes that are repressed at least 5-fold. Silenced hepatic genes include several that encode transcription factors and proteins involved in signal transduction pathways. These data suggest that extensive reprogramming occurs in cell hybrids, leading to a nearly global (although not complete) loss of tissue-specific gene expression.

Dissociation of the Hepatic Phenotype from HNF4 and HNF1α Expression

Dedifferentiated cells have served as tools to understand the molecular consequences of the loss of tissue-specific pathways. Here we report the characterization of one of these cell lines, M29, which lacks the liver-enriched HNF4-HNF1α pathway, in order to determine if this class of variant cell lines could provide additional information regarding requirements for tissue-type expression. We report that although the liver-specific α1-antitrypsin (α1AT) gene remains silent despite reactivation of the HNF4/HNF1α pathway in the M29 cells, the frequency of activation of an integrated α1AT-APRT transgene is increased 1000-fold in response to these transcription factors. The human α1AT locus (introduced via chromosome transfer) also remained silent on these cells, despite HNF4 and HNF1α expression. Results from cell fusion experiments suggest that the defect in the M29 cells is recessive. Results suggest that the M29 cells contain a defect that represses liver gene expression despite the presence of the HNF4/HNF1α pathway.

Hepatocyte nuclear factor-4 prevents silencing of hepatocyte nuclear factor-1 expression in hepatoma x fibroblast cell hybrids

Hepatocyte nuclear factors-1alpha (HNF1alpha) and -4 (HNF4) are components of a liver-enriched transcription activation pathway which is thought to play a critical role in hepatocyte-specific gene expression, including activation of alpha1-antitrypsin gene expression. HNF1alpha, HNF4 and alpha1-antitrypsin (alpha1AT) genes are extinguished in hepatoma/fibroblast somatic cell hybrids, suggesting that fibroblasts contain a repressor-like activity. To determine the molecular basis for silencing of these genes in cell hybrids, ectopic expression of HNF1alpha and HNF4 was used. Results show that constitutive expression of HNF4 prevents extinction of HNF1alpha gene expression in hepatoma/fibroblast hybrids. In contrast, forced HNF1alpha expression failed to prevent extinction of the HNF4 locus in cell hybrids. Likewise, the alpha1AT gene remained silent in the presence of both HNF1alpha and HNF4. These results suggest that extinction of HNF1alpha is a simple lack-of-activation phenotype, whereas extinction of HNF4 andalpha1AT loci is more complex, perhaps involving negative regulation.

Thyroid hormone receptor beta2 promoter activity in pituitary cells is regulated by Pit-1

There are three known thyroid hormone receptor (TR) isoforms that arise from two distinct alpha and beta gene loci. TRalpha1 and TRbeta1 mRNAs are found in many tissues, whereas mRNA for the N-terminal TRbeta2 variant derived from the beta locus is readily detectable only in the pituitary gland and derived cell sources such as GH3 somatotropes and TtT-97 thyrotropes. We previously isolated the genomic region governing expression of the TRbeta2 isoform in thyrotropes and showed that transcription arose from multiple origins within a 400-base pair (bp) region. We now report that the region extending 500 bp upstream of the putative AUG codon (A is +1) contains six areas of interaction with the pituitary-specific transcription factor Pit-1. In addition there are separate areas that bind other factors present in thyrotrope cells. Promoter deletions revealed that removal of regions containing the Pit-1 sites at -456 to -432, -149 to -127, and -124 to -102 progressively decreased TRbeta2 promoter activity in thyrotropes. A more proximal footprinted area from -65 to -19, which accounted for the remaining promoter activity, contained sites that interacted with recombinant Pit-1; however, extracts of TtT-97 thyrotropes, which express Pit-1, footprinted this proximal region with a pattern of protection that differed from that produced by Pit-1. A comparative deletional analysis demonstrated that a shorter region extending only 204 bp from the AUG was sufficient to support TRbeta2 promoter activity in GH3 somatotropes. The more proximal Pit-1 sites, including the area from -53 to -19, whose pattern differed from Pit-1 in thyrotrope extracts, showed protection patterns with GH3 extracts that were indistinguishable from recombinant Pit-1. Site-directed mutagenesis that abrogated binding of both recombinant Pit-1 and Pit-1-containing nuclear extracts revealed that the two Pit-1 sites between -149 and -102 were important for TRbeta2 promoter activity with the more proximal being most critical. Finally, we showed that TRbeta2 promoter activity in alpha-TSH cells, which do not transcribe the endogenous TRbeta2 locus or produce Pit-1 protein, could be reconstituted to a level approaching that seen in expressing TtT-97 thyrotropes by cotransfecting a Pit-1 expression vector. Activation by Pit-1 was dependent on the same Pit-1 sites shown to be important for basal TRbeta2 promoter activity in thyrotropes as constructs lacking them by deletion or mutation were not stimulated by Pit-1.

The regulation of growth hormone secretion

The regulation of GH secretion involves finely balanced systems with multiple components. As our knowledge of the physiology of GH regulation expands, so does our understanding of the bases for GH diseases. We now can identify several cellular loci that cause GH deficiency or GH excess. In addition, the recent increased understanding of GH physiology has resulted in an increase in potential therapies for growth disorders.

Binding preferences of the POU domain protein Brain-4: implications for autoregulation

The POU domain-containing transcription factor Brain-4 (Brn-4, RHS-2) was examined for its sites of expression and DNA binding preferences. In the rat, Brn-4 is expressed in 76 and 65% of vasopressin neurons in the paraventricular and supraoptic nuclei, respectively; but in only 10% of corticotropin-releasing factor neurons in the paraventricular nucleus of the hypothalamus. From these data we speculate that genes expressed within vasopressinergic neurons are more likely to be regulated by Brn-4 than those in corticotropin-releasing factor neurons. Random oligonucleotide site selection indicates Brn-4 prefers binding the DNA element CAATATGCTAAT and is inflexible in its spacing requirement between putative CAATAT and TAAT half sites, preferring 2 nucleotides between these elements. Electrophoretic mobility shift and DNase I footprinting analyses show five regions between nucleotides -457 and +22 of the Brn-4 promoter that are bound by Brn-4. Furthermore, Brn-4 can transactivate from this region of the Brn-4 promoter, suggesting that Brn-4 expression may be autoregulated.

Coordinate extinction of melanocyte-specific gene expression in hybrid cells

Whole cell hybrids and microcell hybrids between mouse fibroblasts and pigmented Syrian hamster melanoma cells were analyzed for coordinate regulation of melanocyte-specific gene products. Extinction of pigmentation was observed in whole-cell hybrids and in a microcell hybrid containing a single mouse chromosome (mouse chromosome 1). Analysis of melanocyte-specific transcripts using reverse transcription, combined with the polymerase chain reaction (RT-PCR), demonstrated that tyrosinase, TRP-1, TRP-2, and microphthalmia transcripts were all absent in unpigmented whole-cell hybrids and in the monochromosomal unpigmented microcell hybrid. A pigmented subclone of this microcell hybrid, however, re-expressed the tyrosinase, TRP-1, TRP-2, and microphthalmia genes. These data suggest that all of these genes are coordinately extinguished by a single fibroblast locus. Since the only fibroblast chromosome detected in the unpigmented microcell hybrid was mouse chromosome 1, these results also suggest that the extinguisher locus affecting the expression of the tyrosinase, TRP-1, TRP-2, and microphthalmia genes in hybrid cells is located on that mouse chromosome (or on a fragment of another chromosome present in the unpigmented monochromosomal microcell hybrid but undetected in our analyses). In contrast to the results with the melanocyte-specific genes mentioned above, transcripts for the melanocortin 1 receptor gene (MC1R) were present in the monochromosomal unpigmented microcell hybrid (although absent in the whole-cell hybrids). This suggests that regulation of MC1R gene expression is distinct from regulation of the other melanocyte-specific genes.

Control of growth hormone synthesis

A large body of research, primarily in the rodent and human species, has elucidated many of the details regarding the control of GH synthesis and release. Cell type-specific transcriptional control has been identified as the main mechanism of the somatotroph-specific expression of GH. The recent detailed analysis in rodents and humans of a highly specific transcriptional activator protein, PIT-1, has opened several new areas of study. This is especially true for research in the farm animal species, where PIT-1 has been cloned and its binding elements on the GH gene are being investigated in a number of economically important species. Genetic and biochemical analyses of PIT-1 and other GH regulators have shown the central role of PIT-1 not only in the cell-autonomous stimulation of GH gene transcription, but also in the participation of PIT-1 in the response at the GH gene to exogenous hormones such as RA and TH. PIT-1 has been implicated in the proliferative development of the pituitary itself, in the maintenance of anterior pituitary cell types once cell types are defined, and in the mechanism by which the hypothalamic signal for GH release is transduced. However, PIT-1 by itself does not activate the GH gene, so that additional unknown factors exist that need to be identified to fully understand the cell type-specific activation of the GH gene. In addition, GH gene regulatory elements acting through well-characterized systems such as TH have seemingly different effects; the specific context of the regulatory elements relative to the promoter elements appear to be crucial. These contextual details of GH gene regulation are not well understood for any species and need to be further studied to be able to make predictions for particular elements and regulatory mechanisms across species. The regulation of the pulsatile secretion of GH by GHRH and SRIH is reasonably well understood after the cloning and analysis of the two releasing factors and their receptors. Modification or manipulation of the pathways involved in the regulation of GH secretion is a potential means of enhancing the lean tissue growth of meat animals. However, further understanding of the systems controlling the in vivo release of GH is needed before such manipulations are likely to be productive. Several other research questions regarding the control of GH expression and release remain to be answered. What is the biochemical connection between exogenous signal transduction (i.e., GRH/GHRH-R, TR, ER, RAR) and PIT-1 at the GH gene? Are there additional coactivators or repressors of GH that respond to cAMP levels? Do ubiquitous regulatory factors such as GHF-3 and Zn-15, identified thus far only in the rat, exist in humans or livestock? Zn-15 is expected to be found in many mammalian species, because its recognition sequence between the PIT-1 binding sites is highly conserved across mammals (Figure 2). What is the mechanism causing GH levels to drop during aging? Does PIT-1 expression decrease during the lifespan of animals? Is it possible to increase GH gene expression within target tissues by directing the expression of PIT-1 to these tissues via transgenesis, or are other factors limiting in peripheral tissues so that the lack of PIT-1 expression is not the deciding factor? Finally, is there genetic variation in the expression of GHRH and/or SRIH or in their respective receptors? These questions are relevant to and could be investigated in several of the livestock species.

Negative regulation of expression of the pituitary-specific transcription factor GHF-1/Pit-1 by thyroid hormones through interference with promoter enhancer elements

Expression of the growth hormone gene is due to the presence of the pituitary-specific transcription factor GHF-1/Pit-1. The action of the thyroid hormone T3 is mediated by nuclear receptors that regulate transcription by interaction with DNA elements located near promoters of the regulated genes. In this study, we show that T3 inhibits expression of the GHF-1/Pit-1 gene in rat pituitary GH4C1 cells by a novel mechanism that involves transcriptional interference with other regulatory elements of the promoter. Sequences between bp -90 and -200 of the rat GHF-1/Pit-1 gene which do not contain a hormone response element but contain two cyclic AMP-responsive elements mediate most of the repressive effect of T3. The hormone reduces basal levels of GHF-1/Pit-1 promoter activity and antagonizes its response to cyclic AMP and the tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate). A similar repression is found with a heterologous promoter that contains four copies of the cyclic AMP-responsive element motif. This regulation provides a novel example of the cross-talk between the thyroid hormone receptor and the signal transduction pathways used by different hormones and growth factors. Additionally, T3 interferes with in vitro binding of GHF-1/Pit-1 to a positive autoregulatory element located at bp -45 to -63 and has a detectable inhibitory effect on the activity of a promoter construct which extends to bp -90 of 5'-flanking DNA. The regulation of the transcription factor provides a novel example of negative transcriptional regulation by thyroid hormones.

Reversibility of the differentiated state in somatic cells

Analysis of de novo gene activation in multinucleated heterokaryons has shown that the differentiated state, although stable, is not irreversible, and can be reprogrammed in the presence of appropriate combinations of trans-acting regulatory molecules. These properties have been exploited to design strategies for identifying novel regulators of cellular differentiation.

The octamer/mu E4 region of the immunoglobulin heavy-chain enhancer mediates gene repression in myeloma x T-lymphoma hybrids

We have shown previously that the immunoglobulin heavy-chain enhancer acts as a repressor of gene transcription in hybrids between immunoglobulin-producing myelomas and a T-lymphoma line. We have now mapped this repressive activity to a 51-bp enhancer subfragment which contains the octamer and mu E4 protein-binding motifs. Even a single copy of this subfragment will repress gene expression in hybrid cells. Mutational analyses of the repressor fragment suggest that in non-B cells, a strong transcriptional repressor(s) functions through the same motifs important for gene activation in B cells. Changes in chromatin structure that accompany reporter gene repression suggest a general mechanism for prohibiting immunoglobulin heavy-chain locus activation in inappropriate cell types.

The octamer/mu E4 region of the immunoglobulin heavy-chain enhancer mediates gene repression in myeloma x T-lymphoma hybrids

We have shown previously that the immunoglobulin heavy-chain enhancer acts as a repressor of gene transcription in hybrids between immunoglobulin-producing myelomas and a T-lymphoma line. We have now mapped this repressive activity to a 51-bp enhancer subfragment which contains the octamer and mu E4 protein-binding motifs. Even a single copy of this subfragment will repress gene expression in hybrid cells. Mutational analyses of the repressor fragment suggest that in non-B cells, a strong transcriptional repressor(s) functions through the same motifs important for gene activation in B cells. Changes in chromatin structure that accompany reporter gene repression suggest a general mechanism for prohibiting immunoglobulin heavy-chain locus activation in inappropriate cell types.

HNF4 and HNF1 as well as a panel of hepatic functions are extinguished and reexpressed in parallel in chromosomally reduced rat hepatoma-human fibroblast hybrids

Rat hepatoma-human fibroblast hybrids of two independent lineages containing only 8-11 human chromosomes show pleiotropic extinction of thirteen out of fifteen hepatic functions examined. Reexpression of the entire group of functions most often occurs in a block, and except for one discordant subclone, correlates with loss of human chromosome 2. The extinguished cells and their reexpressing derivatives have been examined for the expression of seven liver-enriched transcription factors. C/EBP, LAP, DBP, HNF3, and vHNF1 expression are not systematically extinguished in parallel with the hepatic functions. However, HNF1 and HNF4 show a perfect correlation with phenotype: these factors are expressed only in the cells showing pleiotropic reexpression. Since recent evidence indicates that HNF4 controls HNF1 expression, it can be proposed that the HNF4 gene is the primary target of the pleiotropic extinguisher.

Extinction of T cell receptor alpha-chain gene expression accompanied by loss of the lymphoid enhancer-binding factor 1 (LEF-1) in murine somatic cell hybrids

To investigate the presence of a negative regulatory factor(s) suppressing T-cell receptor alpha-chain (TCR alpha) gene expression in non-T cells, 10 independent cell hybrid clones were generated between mouse T-cell lymphoma EL4 cells (TCR alpha+/beta+) and mouse fibroblast B82 cells. These cell hybrids showed a typical fibroblastic morphology and retained an approximate sum of chromosome numbers derived from both parental cells. No transcripts of the TCR alpha gene were detected in the cell hybrids, although the presence of the rearranged TCR alpha allele from EL4 cells was confirmed. The possibility of involvement of nuclear proteins responsible for the activity of the TCR alpha gene enhancer in the extinction of TCR alpha gene expression in the cell hybrids was examined. Nuclear proteins which bind to the lymphoid enhancer-binding factor 1 (LEF-1) binding motif present in EL4 cells disappeared in the hybrid clones, whereas no significant change was observed in DNA-binding activity of nuclear proteins to a consensus cyclic AMP response element (CRE) and the Ets-1 binding motif between the parental cells and the cell hybrids. No transcripts of the LEF-1 gene were detected in the cell hybrids, despite the retention of the LEF-1 gene and murine chromosomes 3, on which the LEF-1 allele is located, from both parental cells. These results suggest that a trans-acting negative regulatory factor(s) present in fibroblasts suppresses LEF-1 gene expression and that suppression of LEF-1 may lead to the extinction of TCR alpha gene expression in the cell hybrids.

Extinction of T cell receptor alpha-chain gene expression accompanied by loss of the lymphoid enhancer-binding factor 1 (LEF-1) in murine somatic cell hybrids

To investigate the presence of a negative regulatory factor(s) suppressing T-cell receptor alpha-chain (TCR alpha) gene expression in non-T cells, 10 independent cell hybrid clones were generated between mouse T-cell lymphoma EL4 cells (TCR alpha+/beta+) and mouse fibroblast B82 cells. These cell hybrids showed a typical fibroblastic morphology and retained an approximate sum of chromosome numbers derived from both parental cells. No transcripts of the TCR alpha gene were detected in the cell hybrids, although the presence of the rearranged TCR alpha allele from EL4 cells was confirmed. The possibility of involvement of nuclear proteins responsible for the activity of the TCR alpha gene enhancer in the extinction of TCR alpha gene expression in the cell hybrids was examined. Nuclear proteins which bind to the lymphoid enhancer-binding factor 1 (LEF-1) binding motif present in EL4 cells disappeared in the hybrid clones, whereas no significant change was observed in DNA-binding activity of nuclear proteins to a consensus cyclic AMP response element (CRE) and the Ets-1 binding motif between the parental cells and the cell hybrids. No transcripts of the LEF-1 gene were detected in the cell hybrids, despite the retention of the LEF-1 gene and murine chromosomes 3, on which the LEF-1 allele is located, from both parental cells. These results suggest that a trans-acting negative regulatory factor(s) present in fibroblasts suppresses LEF-1 gene expression and that suppression of LEF-1 may lead to the extinction of TCR alpha gene expression in the cell hybrids.

Extinction of Oct-3/4 gene expression in embryonal carcinoma x fibroblast somatic cell hybrids is accompanied by changes in the methylation status, chromatin structure, and transcriptional activity of the Oct-3/4 upstream region

In this study we evaluate, for the first time, the molecular mechanism that underlies the extinction of a tissue-specific transcription factor, Oct-3/4, in somatic cell hybrids and compared it with its down-regulation in retinoic acid (RA)-treated embryonal carcinoma (EC) cells. The Oct-3/4 gene, which belongs to the POU family of transcription factors and is abundantly expressed in EC (OTF9-63) cells, provides an excellent model system with which to study the extinction phenomenon. Unlike other genes whose expression has been repressed in hybrid cells but not during in vivo differentiation, Oct-3/4 expression is dramatically repressed in OTF9-63 x fibroblast hybrids and also during embryogenesis. The ectopic expression of Oct-3/4 in hybrid cells under a constitutive promoter is sufficient for transcriptional activation of an octamer-dependent promoter. These results argue against the possibility that fibroblasts contain a direct repressor which binds directly to the octamer sequence and prevents Oct-3/4 protein from binding. The extinction of Oct-3/4 binding activity in the hybrid cells occurs at the level of mRNA transcription, similarly to the repression of Oct-3/4 transcription during in vivo differentiation. This shutdown of Oct-3/4 transcription in hybrid cells and in RA-treated EC cells is accompanied by de novo methylation of its 1.3-kb upstream region. In contrast to EC cells, in which this region is sensitive to MspI digestion, in hybrid cells and in RA-treated EC cells, the Oct-3/4 upstream region is resistant to MspI digestion, which suggests a change in its chromatin structure. Furthermore, extinction is not restricted to the endogenous Oct-3/4 gene but is also exerted upon a transiently transfected reporter gene driven by the Oct-3/4 upstream region. Thus, changes in the cellular activity of trans-acting factors acting on the upstream region also contribute to the inability of the hybrid and RA-treated EC cells to generate Oct-3/4 transcripts. In conclusion, this study draws a connection between the shutdown of Oct-3/4 expression in RA-differentiated EC cells and its extinction in hybrid cells. In both systems, repression of Oct-3/4 expression is achieved through changes in the methylation status, chromatin structure, and transcriptional activity of the Oct-3/4 upstream regulatory region.

Extinction of tyrosine aminotransferase gene activity in somatic cell hybrids involves modification and loss of several essential transcriptional activators

Extinction is defined as the loss of cell type-specific gene expression that occurs in somatic cell hybrids derived by fusion of cells with dissimilar phenotypes. To explore the basis of this dominant-negative regulation, we have studied the activities of the control elements of the liver-specific gene encoding tyrosine aminotransferase (TAT) in hepatoma/fibroblast hybrid crosses. We show that extinction in complete somatic cell hybrids is accompanied by the loss of activity of all known cell type-specific control elements of the TAT gene. This inactivity is the result of first, lack of expression of genes coding for the transcriptional activators HNF4 and HNF3 beta and HNF3 gamma, which bind to essential elements of the enhancers; and second, loss of in vivo binding and activity of ubiquitous factors to these enhancers, including CREB, which is the target for repression by the tissue-specific extinguisher locus TSE1. Complete extinction of TAT gene activity is therefore a multifactorial process affecting all three enhancers controlling liver-specific and hormone-inducible expression. It results from lack of activation, rather than active repression, and involves both post-translational modification and loss of essential transcriptional activators.

Extinction of Oct-3/4 gene expression in embryonal carcinoma x fibroblast somatic cell hybrids is accompanied by changes in the methylation status, chromatin structure, and transcriptional activity of the Oct-3/4 upstream region

In this study we evaluate, for the first time, the molecular mechanism that underlies the extinction of a tissue-specific transcription factor, Oct-3/4, in somatic cell hybrids and compared it with its down-regulation in retinoic acid (RA)-treated embryonal carcinoma (EC) cells. The Oct-3/4 gene, which belongs to the POU family of transcription factors and is abundantly expressed in EC (OTF9-63) cells, provides an excellent model system with which to study the extinction phenomenon. Unlike other genes whose expression has been repressed in hybrid cells but not during in vivo differentiation, Oct-3/4 expression is dramatically repressed in OTF9-63 x fibroblast hybrids and also during embryogenesis. The ectopic expression of Oct-3/4 in hybrid cells under a constitutive promoter is sufficient for transcriptional activation of an octamer-dependent promoter. These results argue against the possibility that fibroblasts contain a direct repressor which binds directly to the octamer sequence and prevents Oct-3/4 protein from binding. The extinction of Oct-3/4 binding activity in the hybrid cells occurs at the level of mRNA transcription, similarly to the repression of Oct-3/4 transcription during in vivo differentiation. This shutdown of Oct-3/4 transcription in hybrid cells and in RA-treated EC cells is accompanied by de novo methylation of its 1.3-kb upstream region. In contrast to EC cells, in which this region is sensitive to MspI digestion, in hybrid cells and in RA-treated EC cells, the Oct-3/4 upstream region is resistant to MspI digestion, which suggests a change in its chromatin structure. Furthermore, extinction is not restricted to the endogenous Oct-3/4 gene but is also exerted upon a transiently transfected reporter gene driven by the Oct-3/4 upstream region. Thus, changes in the cellular activity of trans-acting factors acting on the upstream region also contribute to the inability of the hybrid and RA-treated EC cells to generate Oct-3/4 transcripts. In conclusion, this study draws a connection between the shutdown of Oct-3/4 expression in RA-differentiated EC cells and its extinction in hybrid cells. In both systems, repression of Oct-3/4 expression is achieved through changes in the methylation status, chromatin structure, and transcriptional activity of the Oct-3/4 upstream regulatory region.

Selective disruption of growth hormone transcription machinery by viral infection

Viruses that establish persistent infections may show selective and unique effects on the host's transcriptional machinery. Lymphocytic choriomeningitis virus (LCMV), a noncytolytic virus, can persistently infect a rat pituitary cell line. Although the infected cells remain free of structural damage, virus markedly interferes with growth hormone (GH) but only minimally interferes with prolactin transcription. The study of GH promoter-chloramphenicol acetyltransferase-transfected cells and GH promoter deletion mutants demonstrates that the viral effect is at the level of GH promoter and is due to interference with GH transactivator factor GHF1 (Pit1). Treatment of LCMV-infected cells with the antiviral agent ribavirin cures the infection and restores normal GH mRNA levels. These results illustrate a molecular mechanism by which a virus infection can disrupt synthesis of a cell's differentiated product without perturbing vital cellular functions.

Extinction of liver/bone/kidney alkaline phosphatase in osteosarcoma hybrid cells

We have constructed a series of interspecific somatic cell hybrids between the human osteoblast-like osteosarcoma, TE85, and a mouse fibrosarcoma, La-t-. In these whole-cell hybrids, we observed a 10-fold reduction of human liver/bone/kidney (L/B/K) alkaline phosphatase steady-state mRNA and alkaline phosphatase protein activity. The phenomenon of loss of tissue-specific gene expression has been termed extinction. Subclones of these hybrids were isolated, which reexpressed the alkaline phosphatase gene product. These late-passage hybrids had a reduced number of mouse fibroblast chromosomes when compared to earlier passages. This suggests that a trans-acting negative regulatory element, encoded in the fibroblast genome, regulates expression of L/B/K alkaline phosphatase. This is the first evidence that extinction plays a role in the regulation of osteoblast gene expression.

Interaction of basal positive and negative transcription elements controls repression of the proximal rat prolactin promoter in nonpituitary cells

The proximal rat prolactin (rPRL) promoter contains three cell-specific elements, designated footprints I, III, and IV, which restrict rPRL gene expression to anterior pituitary lactotroph cells. Footprint II (-130 to -120) binds a factor, which we have termed F2F, present in pituitary and nonpituitary cell types. Here we demonstrate that a key role of the footprint II site is to inhibit rPRL promoter activity in nonpituitary cells, specifically, by interfering with the basal activating function of a vicinal element. Gene transfer analysis revealed 20-fold activation of the rPRL promoter in nonpituitary cell types when footprint II was either deleted or specifically mutated. Similar activation of the intact rPRL promoter was obtained by in vivo F2F titration studies. In GH4 rat pituitary cells, the footprint II inhibitory activity was masked by the redundant, positively acting cell-specific elements and was inhibitory only if the two upstream sites, footprints III and IV, were deleted. Deletion of the -112 to -80 region in the footprint II site-specific mutant background resulted in complete loss of rPRL promoter activity in both pituitary and nonpituitary cell types, mapping a basal activating element that is operative irrespective of cell type to this region. While the basal activating element imparted an activating function in a heterologous promoter assay, the footprint II sequence did not display any inherent repressor function and actually induced several minimal heterologous promoters. However, the inhibitory activity of the footprint II site was detected only if it was in context with the basal activating element. These data underscore the importance of ubiquitous activating and inhibitory factors in establishing cell-specific gene expression and further emphasize the complexity of the molecular mechanisms which restrict gene expression to specific cell types. We provide a novel paradigm to study rPRL promoter function and hormone responsiveness independently of lactotroph cell-specific requirements.

Interaction of basal positive and negative transcription elements controls repression of the proximal rat prolactin promoter in nonpituitary cells

The proximal rat prolactin (rPRL) promoter contains three cell-specific elements, designated footprints I, III, and IV, which restrict rPRL gene expression to anterior pituitary lactotroph cells. Footprint II (-130 to -120) binds a factor, which we have termed F2F, present in pituitary and nonpituitary cell types. Here we demonstrate that a key role of the footprint II site is to inhibit rPRL promoter activity in nonpituitary cells, specifically, by interfering with the basal activating function of a vicinal element. Gene transfer analysis revealed 20-fold activation of the rPRL promoter in nonpituitary cell types when footprint II was either deleted or specifically mutated. Similar activation of the intact rPRL promoter was obtained by in vivo F2F titration studies. In GH4 rat pituitary cells, the footprint II inhibitory activity was masked by the redundant, positively acting cell-specific elements and was inhibitory only if the two upstream sites, footprints III and IV, were deleted. Deletion of the -112 to -80 region in the footprint II site-specific mutant background resulted in complete loss of rPRL promoter activity in both pituitary and nonpituitary cell types, mapping a basal activating element that is operative irrespective of cell type to this region. While the basal activating element imparted an activating function in a heterologous promoter assay, the footprint II sequence did not display any inherent repressor function and actually induced several minimal heterologous promoters. However, the inhibitory activity of the footprint II site was detected only if it was in context with the basal activating element. These data underscore the importance of ubiquitous activating and inhibitory factors in establishing cell-specific gene expression and further emphasize the complexity of the molecular mechanisms which restrict gene expression to specific cell types. We provide a novel paradigm to study rPRL promoter function and hormone responsiveness independently of lactotroph cell-specific requirements.

Extinction of alpha 1-antitrypsin gene expression in somatic cell hybrids: evidence for multiple controls

Expression of the liver-specific alpha 1-antitrypsin (alpha 1AT) gene is extinguished in hepatoma/fibroblast hybrids. To define the mechanism of extinction, we identified DNA sequences involved in this process by transiently transfecting mutant alpha 1AT promoters into parental and hybrid cells. The wild-type alpha 1AT promoter (-554 to +44 bp) was highly expressed in rat hepatoma cells, but activity was 100-fold less in fibroblasts or cell hybrids. Mutations in this region failed to activate alpha 1AT expression in nonhepatic cells, but mutations in the binding site for liver factor B1 (LF-B1) reduced hepatic-specific expression greater than 100-fold. Furthermore, the hybrid cells failed to express LF-B1-binding activity and mRNA. This suggested that alpha 1AT extinction in hybrids might be an indirect, lack-of-activation phenotype mediated primarily through repression of LF-B1. To test this possibility, we stably transfected an LF-B1 expression cassette into parental and hybrid cells and monitored expression of transfected and endogenous alpha 1AT genes. Surprisingly, although constitutive LF-B1 expression could activate alpha 1AT-CAT transgenes in these cells, it neither prevented nor reversed extinction of the chromosomal alpha 1AT genes. We conclude that although extinction of the LF-B1 trans-activator accompanies alpha 1AT extinction in cell hybrids, it does not play a causal role in this process.

The tissue-specific extinguisher locus TSE1 encodes a regulatory subunit of cAMP-dependent protein kinase

The tissue-specific extinguisher locus TSE1, a dominant negative regulator of transcription in somatic cell hybrids, acts via a cAMP response element (CRE) to repress activity of a hepatocyte-specific enhancer. Guided by the antagonism between TSE1 and cAMP-mediated signal transduction, we identified the regulatory subunit RI alpha of protein kinase A (PKA) as the product of the TSE1 locus. The evidence derives from concordant expression of RI alpha mRNA and TSE1 genetic activity, high resolution mapping of the RI alpha gene and TSE1 on human chromosome 17, and the ability of a transfected RI alpha cDNA to generate a phenocopy of TSE1-mediated extinction. The mechanism of TSE1/RI alpha-mediated extinction involves repression of basal PKA activity, reduced phosphorylation of CREB at Ser-133, and a corresponding reduction of in vivo protein binding at the target CRE.

Subtractive hybridization cloning of a tissue-specific extinguisher: TSE1 encodes a regulatory subunit of protein kinase A

Tissue-specific extinguisher 1 (TSE1) is a trans-acting locus on human chromosome 17 that down-regulates expression of seven liver genes in hepatoma x fibroblast hybrids. To study the mechanism by which TSE1 functions, we used subtractive cDNA hybridization to clone transcripts encoded within a 2-4 Mb segment of chromosome 17 that includes TSE1. High resolution mapping within this region indicated that 8 of 9 different human cDNAs so obtained were distinct from TSE1. The remaining cDNA clone mapped concordantly with TSE1 in a panel of fragment-containing hybrids. DNA sequencing indicated that this cDNA encoded regulatory subunit RI alpha of cAMP-dependent protein kinase, and RI alpha mRNA levels correlated with TSE1 activity in various hybrid lines. Stable transfection of wild-type or cAMP-binding mutant RI alpha alleles into hepatoma recipients produced an extinction phenotype indistinguishable from that encoded by human TSE1. We conclude that TSE1 encodes a regulatory subunit of protein kinase A whose activity differs in different cell types.

The pituitary-specific regulatory gene GHF1 contains a minimal cell type-specific promoter centered around its TATA box

GHF-1 is a pituitary-specific transcription factor responsible for activation of the growth hormone (GH) gene. The GHF1 gene is expressed exclusively in cells of the somatotrophic lineage, and its transcription is extinguished in somatic cell hybrids. The minimal sequences required for differential transcription of GHF1 in GH-expressing and -nonexpressing cell lines and somatic cell hybrids were localized to a 15-bp region surrounding and including its TATA box. This 15-bp fragment acts as a cell type-specific promoter element and is recognized by a transcription factor present in GH-expressing cell lines. Hence, in addition to enhancers and upstream promoter elements, the TATA element (TATA box plus surrounding sequences) can be, in certain cases, an important determinant of cell-type-specific transcription.

Extinction of insulin gene expression in hybrids between beta cells and fibroblasts is accompanied by loss of the putative beta-cell-specific transcription factor IEF1

Insulin-producing cells and fibroblasts were fused to produce hybrid lines. In hybrids derived from both hamster and rat insulinoma cells, no insulin mRNA could be detected in any of seven lines examined by Northern (RNA) analysis despite the presence in each line of the insulin genes of both parental cells. Hybrid cells were transfected with recombinant chloramphenicol acetyltransferase plasmids containing defined segments of the rat insulin I gene 5' flank. We observed no transcriptional activity of the intact insulin enhancer or of IEB2, a critical cis-acting element of the insulin enhancer. IEB2 has previously been shown to interact in vitro with IEF1, a DNA-binding activity observed selectively in insulin-producing cells. Hybrid cells showed no detectable IEF1 activity. Furthermore, the insulin enhancer was unable to reduce transcription directed by the Moloney sarcoma virus enhancer in a double-enhancer construct. Thus, extinction of insulin gene expression in the hybrids apparently does not operate through a direct action of repressors on the insulin enhancer; rather, extinction is accompanied by, and may be caused by, reduced DNA-binding activity of the putative transcriptional activator IEF1.

Extinction of insulin gene expression in hybrids between beta cells and fibroblasts is accompanied by loss of the putative beta-cell-specific transcription factor IEF1

Insulin-producing cells and fibroblasts were fused to produce hybrid lines. In hybrids derived from both hamster and rat insulinoma cells, no insulin mRNA could be detected in any of seven lines examined by Northern (RNA) analysis despite the presence in each line of the insulin genes of both parental cells. Hybrid cells were transfected with recombinant chloramphenicol acetyltransferase plasmids containing defined segments of the rat insulin I gene 5' flank. We observed no transcriptional activity of the intact insulin enhancer or of IEB2, a critical cis-acting element of the insulin enhancer. IEB2 has previously been shown to interact in vitro with IEF1, a DNA-binding activity observed selectively in insulin-producing cells. Hybrid cells showed no detectable IEF1 activity. Furthermore, the insulin enhancer was unable to reduce transcription directed by the Moloney sarcoma virus enhancer in a double-enhancer construct. Thus, extinction of insulin gene expression in the hybrids apparently does not operate through a direct action of repressors on the insulin enhancer; rather, extinction is accompanied by, and may be caused by, reduced DNA-binding activity of the putative transcriptional activator IEF1.

Prolactin-deficient GH3B3 cells are defective in the utilization of the endogenous prolactin promoter yet are fully competent to initiate transcription from a transfected prolactin promoter

Transcription of the prolactin (PRL) gene has been analyzed in wild-type D6, PRL-deficient B3, and revertant r16 GH3 cells. Levels of processed nuclear transcripts from the PRL gene were substantially reduced in the deficient line compared to wild-type cells and returned to greater than wild-type levels in the revertant line. Rare PRL transcripts in the deficient line contained the same 5' end found on transcripts in wild-type and revertant cells as judged by primer extension and S1 nuclease protection assays, implying that the cells are deficient in utilization of the normal wild-type promoter. Deficient cells also contained wild-type levels of the PRL- and growth hormone-specific transcription factor pit-1/GHF-1, and no difference was found in the ability of extracts from wild-type and deficient cells to retard various restriction fragments from both the proximal and the distal PRL promoter regions. The deficient and wild-type cells were equally competent in initiating transcription from a transfected rat PRL promoter containing both the distal and proximal promoter elements. These observations imply that PRL-deficient cells are not defective in a trans-activating factor functioning on these PRL promoter fragments (trans model). Rather, inefficient use of the PRL promoter in the variant cells may reflect an increased methylation state of the PRL gene itself (cis model).

Extinction of the human insulin gene expression in insulinoma x fibroblast somatic cell hybrids involves cis-acting DNA elements

Insulin gene expression in rat insulinoma (RIN) cells is extinct in RIN x fibroblast hybrids and can reappear upon loss of DNA contributed by the fibroblast parent. (Besnard et al., Exp. Cell Res. 185:101-108, 1989). In the present study, we looked for the role of 5'-flanking sequences of the human insulin gene in the negative control observed in the hybrids. RIN cells were transformed with composite genes which consisted of the coding sequence of the gpt gene placed under the control of 5'-flanking regions of the human insulin gene (Ins.gpt gene). Upon hybridization of these cells with mouse fibroblasts, the expression of both Ins.gpt and endogenous rat insulin genes were suppressed together. The results obtained indicate that cis-acting DNA elements are involved in the negative control of the gene. These elements are located in a fragment spread from -258 to +241 of the transcription origin of the human insulin gene.

A comparison of transcriptional regulatory element activities in transformed and non-transformed rat anterior pituitary cells

Transformed (GH-3) and non-transformed (P3) rat anterior pituitary cells were compared in their ability to direct expression of plasmids containing a variety of eukaryotic transcriptional regulatory elements (TREs). These include the herpes simplex virus thymidine kinase (HSV-TK), Rous sarcoma virus long terminal repeat (RSV-LTR), simian virus 40 early (SV-40E), human cytomegalovirus immediate-early (CMV-IE) and mouse metallothionein 1 (mMT-1) TREs. Chloramphenicol acetyl transferase (CAT) gene expression served as a reporter in this study. Following transient transfection, the cell lines exhibited similar profiles of TRE utilization. In each cell line. CMV-IE was most efficient in directing reporter gene expression, although 2-fold greater activity was observed in GH-3 versus P3 cells. RSV-LTR directed gene expression was lower than that of CMV-IE while both HSV-TK and SV-40E were inactive in each cell line. Also, the mMT-1 promoter was inducible by addition of ZnCl2 to the culture media, though the level required for maximal activation differed between the two cell lines. Transfected GH-3 and P3 cells, therefore, displayed similar TRE utilization profiles yet significant differences were observed in the ability of these cell lines to respond to specific regulatory elements.

Constitutive production of granulocyte colony-stimulating factor by hybrids of a SV40-transformed mouse macrophage and a renal adenocarcinoma cell line

Mouse macrophage BAM3 cells produced colony-stimulating factors (CSFs) after stimulation with bacterial lipopolysaccharide (LPS). By assaying the CSF using various interleukin 3-dependent cell lines, it was shown that most of the CSFs produced by BAM3 cells were granulocyte CSF (G-CSF). The granulocyte-macrophage CSF (GM-CSF) gene was also expressed in BAM3 cells after stimulation with LPS. When BAM3 cells were fused with the mouse renal adenocarcinoma cell line RAG which does not produce G-CSF, two of four hybrid cell lines constitutively produced large quantities of G-CSF. About 300 bp of the promoter region of mouse G-CSF chromosomal gene was inserted upstream of the Escherichia coli chloramphenicol acetyltransferase gene, and introduced into BAM3, RAG and hybrid cells. The G-CSF promoter was activated by stimulation with LPS, in BAM3 cells, but was inert in RAG cells. On the other hand, there was significant constitutive CAT activity in the hybrid cells.

Extinction of retinol-binding protein gene expression in somatic cell-hybrids: identification of the target sequences

The Retinol-Binding Protein (RBP) is expressed primarily in the liver. The regulatory elements involved in its tissue-specific expression have been identified and mapped to the 5' flanking region of the RBP gene. In this paper heterokaryons and somatic cell-hybrids have been produced and analysed in order to demonstrate that the RBP gene is subject to extinction and to identify the target sequences of this phenomenon. We show here that the gene is extinguished in fusions of hepatoma with a variety of cells of different species and embryonic lineages. The repression is not due to loss of the gene and occurs also when chromosome 10, where the gene is located, is inherited from the expressing parental cell-type. Hybrid clones were transfected with constructs carrying DNA segments of different lengths from the 5' flanking region of the RBP gene fused to a reporter gene. We demonstrate that extinction takes place also on an exogenous RBP-CAT gene, mimicking the phenomenon observed with the endogenous gene in its chromosomal location. Moreover, we identify and map the target sequences of the putative extinguishing function. Our data thus show that extinction of RBP is mediated through the DNA segment that is involved in its tissue-specific expression.

A single amino acid change in CUP2 alters its mode of DNA binding

CUP2 is a copper-dependent transcriptional activator of the yeast CUP1 metallothionein gene. In the presence of Cu+ and Ag+) ions its DNA-binding domain is thought to fold as a cysteine-coordinated Cu cluster which recognizes the palindromic CUP1 upstream activation sequence (UASc). Using mobility shift, methylation interference, and DNase I and hydroxyl radical footprinting assays, we examined the interaction of wild-type and variant CUP2 proteins produced in Escherichia coli with the UASc. Our results suggest that CUP2 has a complex Cu-coordinated DNA-binding domain containing different parts that function as DNA-binding elements recognizing distinct sequence motifs embedded within the UASc. A single-amino-acid substitution of cysteine 11 with a tyrosine results in decreased Cu binding, apparent inactivation of one of the DNA-binding elements and a dramatic change in the recognition properties of CUP2. This variant protein interacts with only one part of the wild-type site and prefers to bind to a different half-site from the wild-type protein. Although the variant has about 10% of wild-type DNA-binding activity, it appears to be completely incapable of activating transcription.

A single amino acid change in CUP2 alters its mode of DNA binding

CUP2 is a copper-dependent transcriptional activator of the yeast CUP1 metallothionein gene. In the presence of Cu+ and Ag+) ions its DNA-binding domain is thought to fold as a cysteine-coordinated Cu cluster which recognizes the palindromic CUP1 upstream activation sequence (UASc). Using mobility shift, methylation interference, and DNase I and hydroxyl radical footprinting assays, we examined the interaction of wild-type and variant CUP2 proteins produced in Escherichia coli with the UASc. Our results suggest that CUP2 has a complex Cu-coordinated DNA-binding domain containing different parts that function as DNA-binding elements recognizing distinct sequence motifs embedded within the UASc. A single-amino-acid substitution of cysteine 11 with a tyrosine results in decreased Cu binding, apparent inactivation of one of the DNA-binding elements and a dramatic change in the recognition properties of CUP2. This variant protein interacts with only one part of the wild-type site and prefers to bind to a different half-site from the wild-type protein. Although the variant has about 10% of wild-type DNA-binding activity, it appears to be completely incapable of activating transcription.

Regulation of the pituitary-specific homeobox gene GHF1 by cell-autonomous and environmental cues

Homeodomain proteins function in determination of mating type in yeast, segmentation in fruit flies and cell-type specific gene expression in mammals. In Drosophila, expression of homeobox genes is controlled by cell-autonomous interactions between regulatory proteins and environmental clues. Similar controls may operate during mammalian limb development and frog embryogenesis. But, the exact way in which expression of homeodomain proteins is regulated in these systems is not clear and requires biochemical analysis of homeobox gene transcription. We now describe such an analysis of the GHF1 gene, which encodes a mammalian homeodomain protein specifying expression of the growth hormone (GH) gene in anterior pituitary somatotrophs. GHF1 is transcribed in a highly restricted manner and the presence of GHF1 protein is correlated both temporally and spatially with activation of the GH gene during pituitary development. Analysis of the GHF1 promoter indicates that transcription is also controlled by cell-autonomous interactions involving positive autoregulation by GHF1, and environmental cues that modulate the intracellular level of cyclic AMP and thereby the activity of cAMP response element binding protein (CREB), a ubiquitous transactivator that binds to the GHF1 promoter.

A cyclic AMP response element mediates repression of tyrosine aminotransferase gene transcription by the tissue-specific extinguisher locus Tse-1

Tyrosine aminotransferase (TAT) gene expression is liver specific and inducible by glucocorticoids and via the cAMP signaling pathway. In fibroblasts and other nonliver cells the gene is subject to negative control by the trans-dominant tissue-specific extinguisher locus Tse-1. We identified a hepatocyte-specific enhancer that is repressed by Tse-1. Two distinct sequence motifs are absolutely essential for function of this enhancer: a cAMP response element (CRE), which is the target for repression by Tse-1, and a hepatocyte-specific element. The specificity of the enhancer is generated by the combination of these two essential elements, which are fully interdependent. In vivo footprinting indicates that Tse-1 acts by affecting protein binding at the CRE. A direct antagonism between Tse-1 and the cAMP signaling pathway suggests that Tse-1 plays a role in control of developmental activation of the TAT gene.

Two genetically defined trans-acting loci coordinately regulate overlapping sets of liver-specific genes

Mice homozygous for deletions around the albino locus fail to activate expression of a set of neonatal liver functions and die shortly after birth. This phenotype is thought to result from the loss of a positive transacting factor, denoted alf, in deletion homozygotes. Using differential cDNA screening, we isolated and characterized genes whose cell type-specific transcription is affected by alf and found as a common feature that expression of these genes is induced by glucocorticoids and cAMP. Surprisingly, a subset of these alf-responsive genes is negatively controlled by the tissue-specific extinguisher locus Tse-1. Administration of glucocorticoids and cAMP leads to reversal of Tse-1-mediated extinction of these genes. These results show that two trans-acting factors coordinately regulate expression of overlapping sets of liver-specific genes. We suggest that both the lethal phenotype and the extinguished state result from interference with hormone signal transduction.

Extinction of an immunoglobulin kappa promoter in cell hybrids is mediated by the octamer motif and correlates with suppression of Oct-2 expression

When immunoglobulin-expressing B cells are fused with fibroblasts, immunoglobulin expression is rapidly and selectively suppressed. here we demonstrate that the conserved octamer motif of a kappa light chain gene promoter plays a crucial role in mediating this "extinction" phenomenon. Replacement of this octamer site by an Sp1 or NF1 binding site is sufficient to bypass extinction. Furthermore, in early cell hybrids, immunoglobulin suppression is correlated with absence of the cell-specific transcription factor Oct-2 and its transcripts. Such hybrids cannot support transcription of a transiently introduced reporter plasmid, driven by an octamer-containing promoter, unless an expression vector encoding Oct-2 is cotransfected. Transfection of the same Oct-2 expression vector into hybrid cells is also sufficient to "reactivate" an integrated kappa promoter construct. Thus, our data further establish the role of Oct-2 for immunoglobulin transcription and show that in B cell x fibroblast hybrids, the lack of a necessary cell-specific transcription factor is involved in the extinction of immunoglobulin expression.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.

Expression of GHF-1 protein in mouse pituitaries correlates both temporally and spatially with the onset of growth hormone gene activity

The relationship between expression of the pituitary-specific transcription factor, GHF-1, and activation of the growth hormone and prolactin genes during mouse anterior pituitary development was investigated. While GHF-1 transcripts were detected within 24 hr of the first observable events in anterior pituitary differentiation, no GHF-1 protein could be detected until about 3 days later. The appearance of GHF-1 protein showed good temporal and spatial correlation with activation of the growth hormone gene. Prolactin gene expression, on the other hand, was observed transiently during embryonic day 16 in two different populations of cells, of which the major one does not contain GHF-1 or growth hormone. These results suggest that expression of GHF-1 is controlled both transcriptionally and posttranscriptionally. The spatial and temporal correlation between the appearance of GHF-1 protein and growth hormone gene activation suggests that GHF-1 is responsible for this very last step in the specialization of somatotrophic cells.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.