Effects of 5-azacytidine on expression of endogenous retrovirus-related sequences in C3H 10T1/2 cells

Endogenous Retroelements and the Viral Mimicry Response in Cancer Therapy and Cellular Homeostasis

Features of the cancer epigenome distinguish cancers from their respective cell of origin and establish therapeutic vulnerabilities that can be exploited through pharmacologic inhibition of DNA- or histone-modifying enzymes. Epigenetic therapies converge with cancer immunotherapies through "viral mimicry," a cellular state of active antiviral response triggered by endogenous nucleic acids often derived from aberrantly transcribed endogenous retrotransposons. This review describes the initial characterization and expansion of viral mimicry-inducing approaches as well as features that "prime" cancers for viral mimicry induction. Increased understanding of viral mimicry in therapeutic contexts suggests potential physiologic roles in cellular homeostasis. SIGNIFICANCE: Recent literature establishes elevated cytosolic double strand RNA (dsRNA) levels as a cancer-specific therapeutic vulnerability that can be elevated by viral mimicry-inducing therapies beyond tolerable thresholds to induce antiviral signaling and increase dependence on dsRNA stress responses mediated by ADAR1. Improved understanding of viral mimicry signaling and tolerance mechanisms reveals synergistic treatment combinations with epigenetic therapies that include inhibition of BCL2, ADAR1, and immune checkpoint blockade. Further characterization of viral mimicry tolerance may identify contexts that maximize efficacy of conventional cancer therapies.

Nuclear reprogramming and its role in vascular smooth muscle cells

In general terms, "nuclear reprogramming" refers to a change in gene expression profile that results in a significant switch in cellular phenotype. Nuclear reprogramming was first addressed by pioneering studies of cell differentiation during embryonic development. In recent years, nuclear reprogramming has been studied in great detail in the context of experimentally controlled dedifferentiation and transdifferentiation of mammalian cells for therapeutic purposes. In this review, we present a perspective on nuclear reprogramming in the context of spontaneous, pathophysiological phenotypic switch of vascular cells occurring in the atherosclerotic lesion. In particular, we focus on the current knowledge of epigenetic mechanisms participating in the extraordinary flexibility of the gene expression profile of vascular smooth muscle cells and other cell types participating in atherogenesis. Understanding how epigenetic changes participate in vascular cell plasticity may lead to effective therapies based on the remodelling of the vascular architecture.

Reprogramming chromatin

Cellular reprogramming involves the artificial dedifferentiation of somatic cells to a pluripotent state. When affected by overexpressing specific transcription factors, the process is highly inefficient, as only 0.1-1% of cells typically undergo the transformation. This low efficiency has been attributed to high kinetic barriers that affect all cells equally and can only be overcome by rare stochastic events. The barriers to reprogramming are likely to involve transformations of chromatin state because (i) inhibitors of chromatin-modifying enzymes can enhance the efficiency of reprogramming and (ii) knockdown or knock-out of chromatin-modifying enzymes can lower the efficiency of reprogramming. Here, we review the relationship between chromatin state transformations (chromatin reprogramming) and cellular reprogramming, with an emphasis on transcription factors, chromatin remodeling factors, histone modifications and DNA methylation.

TCUP: A Novel hAT Transposon Active in Maize Tissue Culture

Transposable elements (TEs) are capable of inducing heritable de novo genetic variation. The sequences capable of reactivation, and environmental factors that induce mobilization, remain poorly defined even in well-studied genomes such as maize. We treated maize tissue culture with the demethylating agent 5-aza-2-deoxcytidine and examined long-term tissue culture lines to discover silenced TEs that have the potential to induce heritable genetic variation. Through these screens we have identified a novel low copy number hAT transposon, Tissue Culture Up-Regulated (TCUP), which is transcribed at high levels in long-term maize black Mexican sweet (BMS) tissue culture and is transcribed in response to treatment with 5-aza-2-deoxycytidine. Analysis of the TIGR Maize Gene Index revealed that this element is the most frequently represented EST from the BMS cell culture library and is not represented in other tissue libraries, which is the basis for its name. A full-length sequence was assembled in inbred B73 that contains the putative functional motifs required for autonomous movement of a hAT transposon. Transposon display detected novel TCUP insertions in two long-term tissue-cultured cell lines of the genotype Hi-II A × B and BMS. This research implicates TCUP as a transposon that is capable of reactivation and which may also be particularly sensitive to the stress of the tissue culture environment. Our findings are consistent with the hypothesis that epigenetic alterations potentiate genomic responses to stress during clonal propagation of plants.

CpG hypomethylation in a large domain encompassing the embryonic beta-like globin genes in primitive erythrocytes

There is little evidence addressing the role of CpG methylation in transcriptional control of genes that do not contain CpG islands. This is reflected in the ongoing debate about whether CpG methylation merely suppresses retroelements or if it also plays a role in developmental and tissue-specific gene regulation. The genes of the beta-globin locus are an important model of mammalian developmental gene regulation and do not contain CpG islands. We have analyzed the methylation status of regions in the murine beta-like globin locus in uncultured primitive and definitive erythroblasts and other cultured primary and transformed cell types. A large ( approximately 20-kb) domain is hypomethylated only in primitive erythroid cells; it extends from the region just past the locus control region to before beta-major and encompasses the embryonic genes Ey, beta h1, and beta h0. Even retrotransposons in this region are hypomethylated in primitive erythroid cells. The existence of this large developmentally regulated domain of hypomethylation supports a mechanistic role for DNA methylation in developmental regulation of globin genes.

CpG methylation directly regulates transcriptional activity of the human endogenous retrovirus family HERV-K(HML-2)

A significant proportion of the human genome consists of stably inherited retroviral sequences. Most human endogenous retroviruses (HERVs) became defective over time. The HERV-K(HML-2) family is exceptional because of its coding capacity and the possible involvement in germ cell tumor (GCT) development. HERV-K(HML-2) transcription is strongly upregulated in GCTs. However, regulation of HERV-K(HML-2) transcription remains poorly understood. We investigated in detail the role of CpG methylation on the transcriptional activity of HERV-K(HML-2) long terminal repeats (LTRs). We find that CpG sites in various HERV-K(HML-2) proviral 5' LTRs are methylated at different levels in the cell line Tera-1. Methylation levels correlate with previously observed transcriptional activities of these proviruses. CpG-mediated silencing of HERV-K(HML-2) LTRs is further corroborated by transcriptional inactivity of in vitro-methylated 5' LTR reporter plasmids. However, CpG methylation levels do not solely regulate HERV-K(HML-2) 5' LTR activity, as evidenced by different LTR activities in the cell line T47D. A significant number of mutated CpG sites in evolutionary old HERV-K(HML-2) 5' LTRs suggests that CpG methylation had already silenced HERV-K(HML-2) proviruses millions of years ago. Direct silencing of HERV-K(HML-2) expression by CpG methylation enlightens upregulated HERV-K(HML-2) expression in usually hypomethylated GCT tissue.

Structure and expression of mobile ETnII retroelements and their coding-competent MusD relatives in the mouse

ETnII elements are mobile members of the repetitive early transposon family of mouse long terminal repeat (LTR) retroelements and have caused a number of mutations by inserting into genes. ETnII sequences lack retroviral genes, but the recent discovery of related MusD retroviral elements with regions similar to gag, pro, and pol suggests that MusD provides the proteins necessary for ETnII transposition in trans. For this study, we analyzed all ETnII elements in the draft sequence of the C57BL/6J genome and classified them into three subtypes (alpha, beta, and gamma) based on structural differences. We then used database searches and quantitative real-time PCR to determine the copy number and expression of ETnII and MusD elements in various mouse strains. In 7.5-day-old embryos of a mouse strain in which two mutations due to ETnII-beta insertions have been identified (SELH/Bc), we detected a three- to sixfold higher level of ETnII-beta and MusD transcripts than in control strains (C57BL/6J and LM/Bc). The increased ETnII transcription level can in part be attributed to a higher number of ETnII-beta elements, but 70% of the MusD transcripts appear to have been derived from one or a few MusD elements that are not detectable in C57BL/6J mice. This element belongs to a young MusD subgroup with intact open reading frames and identical LTRs, suggesting that the overexpressed element(s) in SELH/Bc mice might provide the proteins for the retrotransposition of ETnII and MusD elements. We also show that ETnII is expressed up to 30-fold more than MusD, which could explain why only ETnII, but not MusD, elements have been positively identified as new insertions.

Role of endogenous retroviruses in autoimmune diseases

Endogenous retroviruses (ERVs) correspond to the integrated proviral form of infectious retroviruses that are trapped within the genome by mutations. Endogenous retroviruses represent a key molecular link between the host genome and infectious viral particles. Proteins encoded by ERVs are recognized by antiviral immune responses and become targets of autoreactivity. Activation of ERVs, such as human ERV-K or a human T-cell lymphotropic virus-related endogenous sequence, may also mediate pathogenicity of Epstein-Barr virus. Endogenous retrovirus peptides can directly regulate immune responses. Thus, molecular mimicry and immunomodulation by ERVs may account for self-reactivity and abnormal T- and B-cell functions in autoimmune disorders.

Epigenetic regulation of an IAP retrotransposon in the aging mouse: progressive demethylation and de-silencing of the element by its repetitive induction

The recent insertion of a murine intracisternal A-particle (IAP) retrotransposon within one of the introns of a housekeeping gene, the circadian m.nocturnin gene, revealed a singular expression profile, both throughout the daytime and the mouse life span. Measurement of the levels of transcripts from this element by quantitative real-time RT-PCR, in organs of 1-24-month-old mice, disclosed that the inserted element--which is part of a large family of otherwise severely repressed mobile elements--becomes active upon aging, specifically in the liver where the m.nocturnin housekeeping gene is expressed in a circadian manner and induces a circadian expression of the IAP sequence. This age-dependent induction is cell-autonomous, as it persists in hepatocytes in primary culture. We further show, using methylation-sensitive enzymes, a correlation between the life-time kinetics of this process and a liver-specific demethylation of the IAP promoter. These results strongly support a model whereby the progressive demethylation and turning on of the IAP sequence is the sole result of the transient, daily activation-throughout the mouse life span--of its promoter. This phenomenon, which develops on a timescale of months to years in the aging mouse, might reveal a general epigenetic--and stochastic--process, which could account for a large series of events associated with cell and animal aging.

Isolation of the novel cDNA of a gene of which expression is induced by a demethylating stimulus

We have isolated a novel cDNA clone, named AZ2, from a cDNA library of mRNA prepared from C3H10T1/2 cells that had been transiently exposed to 5-azacytidine, a potent inhibitor of DNA methyltransferase. The elucidated nucleotide sequence revealed that the 5' region of the cDNA was rich in the CpG sequence. The AZ2 cDNA contained a 1215-nucleotide open reading frame, and the expected amino acid sequence had a molecular mass of 46090. The amount of the transcript increased on 5-azacytidine treatment of C3H10T1/2 cells, and the transcript was significantly expressed in mouse testis, brain, lung, kidney, heart and ovary. Specific antibodies raised against a fusion protein including glutathione S-transferase revealed a band of an approximately 48kDa translation product for testis, brain, lung, and cultured cells that ectopically expressed the AZ2 protein. The AZ2 protein was mainly localized in the cytoplasm. The amino-terminal part of the AZ2 protein was homologous to the previously reported TANK (Cheng and Baltimore, 1996. Genes Dev. 10, 963-973) and I-TRAF (Rothe, 1996. Proc. Natl. Acad. Sci. USA 93, 8241-8246), which participate in the signal transduction cascade from the tumor necrosis factor-receptor to the transcription factor, NFkappaB. Overexpression of AZ2 inhibited TNF alpha mediated NFkappaB activation. AZ2 could be a component of a regulator of the NFkappaB activation cascade.

Role of DNA 5-methylcytosine transferase in cell transformation by fos

The Fos and Jun oncoproteins form dimeric complexes that stimulate transcription of genes containing activator protein-1 regulatory elements. We found, by representational difference analysis, that expression of DNA 5-methylcytosine transferase (dnmt1) in fos-transformed cells is three times the expression in normal fibroblasts and that fos-transformed cells contain about 20 percent more 5-methylcytosine than normal fibroblasts. Transfection of the gene encoding Dnmt1 induced morphological transformation, whereas inhibition of dnmt1 expression or activity resulted in reversion of fos transformation. Inhibition of histone deacetylase, which associates with methylated DNA, also caused reversion. These results suggest that fos may transform cells through alterations in DNA methylation and in histone deacetylation.

Genomic organization of the mouse AZ1 gene that encodes the protein localized to preacrosomes of spermatids

The AZ1 protein is localized to the preacrosome region of spermatids. Previous developmental studies suggested that AZ1 gene transcription begins in pachytene spermatocytes and that expression of the gene is induced in cultivated fibroblasts on treatment with 5-azacytidine, which is known to lead to the demethylation of genomic DNA. In the present study, we cloned the gene coding AZ1 cDNA and elucidated its genomic organization. AZ1 cDNA was encoded by 25 exons distributed within 25 kb of genomic DNA. The transcription started at nucleotide positions -312 and -311 of the cDNA, with the A of the ATG codon specifying the initiator methionine assigned as nucleotide position +1. Exon 1 and a part of intron 1 of the AZ1 gene constituted a typical CpG island, and an about 0.2-kb sequence of the 5'-proximal region exhibited a G + C content and CpG ratio higher than the averages for the mammalian genomic sequence.

Germ line-specific expression of intracisternal A-particle retrotransposons in transgenic mice

Intracisternal A-particle (IAP) sequences are endogenous retrovirus-like mobile elements, or retrotransposons, present at 1,000 copies in the mouse genome. These elements transpose in a replicative manner via an RNA intermediate and its reverse transcription, and their transposition should therefore be tightly controlled by their transcription level. To analyze the in vivo pattern of expression of these retrovirus-like elements, we constructed several independent transgenic mice with either a complete IAP element marked with an intron or with the IAP promoter, or long terminal repeat (LTR), alone controlling the expression of a lacZ reporter gene with a nuclear localization signal. For all transgenic lines analyzed, IAP expression as determined by reverse transcription-PCR analysis was found to be essentially restricted to the male germ line. Furthermore, in situ 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining of all organs disclosed specific beta-galactosidase-positive blue cells only within the testis, found as patches along the seminiferous tubules and often organized as assemblies of 2, 4, 8, or 16 cells. Histochemical analyses of tissues from 13.5-day-old embryos to adults demonstrated that this LTR activity is restricted to gonocytes and premeiotic undifferentiated spermatogonia. Finally, analysis of the methylation status of both transgenes and endogenous IAP LTRs demonstrated identical patterns, with methylation in somatic tissues and hypomethylation in the testis. Transgenic mice therefore reveal an intrinsic, highly restricted IAP expression which had escaped detection in previous global Northern (RNA) blot analyses and with possible strong biological relevance, as IAP activation specifically within the germ line might be a way to generate diversity at the evolutionary level without being deleterious to individuals.

The potential roles of endogenous retroviruses in autoimmunity

Endogenous retroviruses (ERVs) are estimated to comprise up to 1% of human DNA. While the genome of many ERVs is interrupted by termination codons, deletions or frame shift mutations, some ERVs are transcriptionally active and recent studies reveal protein expression or particle formation by human ERVs. ERVs have been implicated as aetiological agents of autoimmune disease, because of their structural and sequence similarities to exogenous retroviruses associated with immune dysregulation and their tissue-specific or differentiation-dependent expression. In fact, retrovirus-like particles distinct from those of known exogenous retroviruses and immune responses to ERV proteins have been observed in autoimmune disease. Quantitatively or structurally aberrant expression of normally cryptic ERVs, induced by environmental or endogenous factors, could initiate autoimmunity through direct or indirect mechanisms. ERVs may lead to immune dysregulation as insertional mutagens or cis-regulatory elements of cellular genes involved in immune function. ERVs may also encode elements like tax in human T-lymphotrophic virus type I (HTLV-I) or tat in human immunodeficiency virus-I (HIV-I) that are capable of transactivating cellular genes. More directly, human ERV gene products themselves may be immunologically active, by analogy with the superantigen activity in the long terminal repeat (LTR) of mouse mammary tumour viruses (MMTV) and the non-specific immunosuppressive activity in mammalian type C retrovirus env protein. Alternatively, increased expression of an ERV protein, or expression of a novel ERV protein not expressed in the thymus during acquisition of immune tolerance, may lead to its perception as a neoantigen. Paraneoplastic syndromes raise the possibility that novel ERV-encoded epitopes expressed by a tumour elicit immunity to cross-reactive epitopes in normal tissues. Recombination events between different but related ERVs, to whose products the host is immunologically tolerant, may also generate new antigenic determinants. Frequently reported humoral immunity to exogenous retrovirus proteins in autoimmune disease could be elicited by cross-reactive ERV proteins. A review of the evidence implicating ERVs in immune dysfunction leads to the conclusion that direct molecular studies are likely to establish a pathogenic role for ERVs in autoimmune disease.

Isolation of a novel cDNA that encodes a protein localized to the pre-acrosome region of spermatids

We have identified a novel cDNA clone, named AZ1, obtained from a cDNA library of mRNA prepared from C3H10T1/2 cells that had been transiently exposed to 5-azacytidine, a potent demethylating reagent. The amount of transcript increased with 5-azacytidine treatment of C3H10T1/2 cells and the transcript was highly expressed in mouse testis. As the mutant mouse jsd/jsd, which has a defect in germ cell maturation, barely expressed the transcript, the message was expected to be expressed specifically in spermatocytes. The mRNA was detected at significant levels in the testes from mice aged 16 days after birth, suggesting that its expression started at the pachytene spermatocyte stage. The elucidated nucleotide sequence contained a 2841-nucleotide open reading frame, and the expected amino acid sequence had a molecular mass of 107,254 Da. Specific antibodies raised against the fusion protein including glutathione S-transferase revealed an approximately 130-kDa band of a translation product in testis and in cultured cells transfected with AZ1 cDNA in the expression vector on Western-blot analysis. The protein was localized to the pre-acrosome region of round and elongated spermatids. However, it was not detected at a more advanced stage of spermatids, i.e. just before their release from Sertoli cells. This protein may play an important role in spermatogenesis.

MSX1 inhibits myoD expression in fibroblast x 10T1/2 cell hybrids

Transfer of human chromosome 11, which contains the myoD locus, from primary fibroblasts into 10T1/2 cells results in activation of myoD. In contrast, hybrids that retain human chromosome 11 and additional human chromosomes fail to activate myoD. We show that human chromosome 4 inhibits myoD activation. myoD enhancer/promoter reporter constructs show that repression is at the transcriptional level. Chromosome fragment-containing hybrids localize the repressing activity to the region of 4p that contains the homeobox gene MSX1. MSX1 is expressed in primary human fibroblasts and in 10T1/2 cells containing human chromosome 4, while parental 10T1/2 cells do not express Msx1. Forced expression of Msx1 represses myoD enhancer activity. Msx1 protein binds to the myoD enhancer and likely represses myoD transcription directly. Antisense MSX1 relieves repression mediated by chromosome 4. We conclude that MSX1 inhibits transcription of myoD and that myoD is a target for homeobox gene regulation.

Interaction of SEWA sarcoma cell proteins with the intracisternal A-type particle long terminal repeat DNA sequence

Intracisternal A-type particle (IAP) transcripts are endogenous retrovirus-like sequences expressed during specific stages of normal development and in a variety of murine tumors. In this study, we have analyzed two cell lines derived originally from the SEWA murine osteosarcoma and grown either as ascites or as solid tumors, for proteins that might regulate IAP expression. We found that subline AA7-NA, originally derived from the ascites tumor, expressed about five times more IAP RNA than the AS12-AD subline, which was derived from a solid tumor. In view of this finding, we examined the binding of cellular proteins from the two cell lines to the 5' end of an IAP long terminal repeat sequence. Gel retardation assays of DNA-protein complexes and DNase I footprinting assays identified several DNA sequences within the long terminal repeat fragment that were protected by protein extracts from both SEWA sublines. Gel retardation assays using specific synthetic oligonucleotide sequences that correspond to two of these protected regions revealed different patterns of DNA-protein complexes with extracts from the two SEWA sublines. These data suggest that expression of IAP sequences is regulated by complex mechanisms involving several proteins that appear to differ between the two sublines.

Activation and demethylation of the intracisternal A particle genes by 5-azacytidine

Treatment of C3H 10T1/2 mouse embryo fibroblasts with the cytidine analogues 5-azacytidine and 5-aza-2-deoxycytidine causes altered gene expression and results in the manifestation of phenotypic changes and altered cell morphology. This includes the conversion of these cells to adipocytes, chondrocytes and myotubes. The effects of these analogues on intracisternal A particle (IAP) gene expression in mouse C3H 10T1/2 cells have been examined. Treatment with either 3 microM 5-azacytidine or 0.3 microM 5-aza-2-deoxycytidine for 24 h was associated with an immediate increase in IAP gene transcription, and with the subsequent appearance of IAPs in the cisternae of the cells 24 h after removal of the drug. Control cells contained no, or very few, IAPs and IAP mRNA. Analysis of the methylation status of the IAP genes, using the restriction endonucleases HpaII, MspI and HhaI, showed that these genes were already demethylated at the end of the 24-h treatment period. IAP gene transcripts were detectable even after a 16-h drug treatment period, at which stage the genes were not yet fully demethylated. After further growth in fresh medium for 90 h, the levels of IAP RNA started to decline, but the demethylated CpG sites were not yet remethylated. These results suggest the involvement of other factors, in addition to methylation, in the regulation of IAP gene expression. These drugs were found to have no stimulatory effect on several oncogenes examined in this study.

Gene structure and transcription in mouse cells with extensively demethylated DNA

In previous work, three clonal cell lines with extremely low DNA methylation levels were derived by multiple consecutive treatments of C3H 10T1/2 C18 (10T1/2) cells with 5-aza-2'-deoxycytidine (5-aza-CdR). In this study we examined the methylation status of genes in these three methyl-deficient clones to assess the specificity of the induced hypomethylation. Complete demethylation of virtually all 5'-CCGG-3' sites was observed in four genes examined, but some sites common to all three clones were persistently methylated even after further exhaustive 5-aza-CdR treatment. Thus, there is a subset of methylation sites within these cells which can never be stably demethylated. The extensive demethylation was not always associated with changes in the level of RNA expression of the genes examined but was strongly correlated with an altered chromatin structure of the unexpressed alpha 1-globin gene and the muscle determination gene MyoD1. These results provide a direct correlation between hypomethylation and the induction of a transcriptionally competent chromatin state.

Gene structure and transcription in mouse cells with extensively demethylated DNA

In previous work, three clonal cell lines with extremely low DNA methylation levels were derived by multiple consecutive treatments of C3H 10T1/2 C18 (10T1/2) cells with 5-aza-2'-deoxycytidine (5-aza-CdR). In this study we examined the methylation status of genes in these three methyl-deficient clones to assess the specificity of the induced hypomethylation. Complete demethylation of virtually all 5'-CCGG-3' sites was observed in four genes examined, but some sites common to all three clones were persistently methylated even after further exhaustive 5-aza-CdR treatment. Thus, there is a subset of methylation sites within these cells which can never be stably demethylated. The extensive demethylation was not always associated with changes in the level of RNA expression of the genes examined but was strongly correlated with an altered chromatin structure of the unexpressed alpha 1-globin gene and the muscle determination gene MyoD1. These results provide a direct correlation between hypomethylation and the induction of a transcriptionally competent chromatin state.

DNA methylation and differentiation

The methylation of specific cytosine residues in DNA has been implicated in regulating gene expression and facilitating functional specialization of cellular phenotypes. Generally, the demethylation of certain CpG sites correlates with transcriptional activation of genes. 5-Azacytidine is an inhibitor of DNA methylation and has been widely used as a potent activator of suppressed genetic information. Treatment of cells with 5-azacytidine results in profound phenotypic alterations. The drug-induced hypomethylation of DNA apparently perturbs DNA-protein interactions that may consequently alter transcriptional activity and cell determination. The inhibitory effect of cytosine methylation may be exerted via altered DNA-protein interactions specifically or may be transduced by a change in the conformation of chromatin. Recent studies have demonstrated that cytosine methylation also plays a central role in parental imprinting, which in turn determines the differential expression of maternal and paternal genomes during embryogenesis. In other words, methylation is the mechanism whereby the embryo retains memory of the gametic origin of each component of genetic information. A memory of this type would probably persist during DNA replication and cell division as methylation patterns are stable and heritable.

Capture and characterization of 5-aza-2'-deoxycytidine-treated C3H/10T1/2 cells prior to transformation

A model system was developed to capture phenotypically normal cells committed to transformation to address two fundamental questions in cancer biology: (i) what are the earliest events in transformation; and (ii) what is the role of DNA methylation in carcinogenesis? Individual C3H/10T1/2 cells were treated with 5-aza-2'-deoxycytidine, which causes hypomethylation of DNA. Cells were grown to subconfluence, and individual microcolonies were trypsinized into two fractions. One fraction was cryopreserved, and the other was replated and maintained in culture. Ten percent of these replated colonies became morphologically transformed after 4-6 weeks. The cryopreserved ancestral cells of both transformed and nontransformed microcolonies were then cultured and compared to each other and to the transformants for phenotypic properties of cellular transformation. Pretransformed and nonpretransformed ancestral cells were initially morphologically indistinguishable, their early growth curves did not differ significantly, and they did not form colonies in soft agar. On continued growth in culture, however, the pretransformants displayed all of the phenotypic characteristics of transformation. Furthermore, transformation occurred in a given pretransformant in most or all of the cells of that clone. Thus, cells committed to transformation could be isolated in this way prior to phenotypic transformation. Studies of these pretransformed cells will permit examination of the earliest events in carcinogenesis and the role of DNA methylation in transformation.

Differences between normal and transformed murine fibroblasts in the expression of various promoter/enhancer-chloramphenicol acetyltransferase constructs

High levels of poly(A)+ RNAs homologous to certain endogenous retrovirus-related DNA sequences are frequently seen in carcinogen-transformed rodent cells. To explore the underlying mechanism, transient expression assays were done to determine whether carcinogen- or radiation-transformed C3H 10T 1/2 cell lines differ from normal 10T 1/2 cells in terms of their ability to express the chloramphenicol acetyltransferase (CAT) gene when it is linked to various promoter/enhancer sequences, including two independently isolated intracisternal A particle (IAP) long terminal repeat sequences designated prcm and pMIA6. We found that with several constructs, CAT activity was always 3- to 10-fold higher in the transformed 10T 1/2 cell lines than in the normal 10T 1/2 cells. The prcm-CAT construct displayed the highest CAT activity in both the normal and transformed C3H 10T 1/2 cells. Studies with 32P-labeled prcm-CAT DNA and Southern blot analyses indicated that the differences in CAT activity between normal and transformed cells were not due to greater uptake or retention of the transfected DNA by the transformed cells. Competition studies provided evidence that factors required for the expression of the prcm-CAT construct are present in limited amounts in normal 10T 1/2 cells and in excess amounts in transformed 10T 1/2 cells. These putative factors may play a role in the increased expression of endogenous retrovirus-related sequences in the transformed cells.

Expression of a single transfected cDNA converts fibroblasts to myoblasts

5-azacytidine treatment of mouse C3H10T1/2 embryonic fibroblasts converts them to myoblasts at a frequency suggesting alteration of one or only a few closely linked regulatory loci. Assuming such loci to be differentially expressed as poly(A)+ RNA in proliferating myoblasts, we prepared proliferating myoblast-specific, subtracted cDNA probes to screen a myocyte cDNA library. Based on a number of criteria, three cDNAs were selected and characterized. We show that expression of one of these cDNAs transfected into C3H10T1/2 fibroblasts, where it is not normally expressed, is sufficient to convert them to stable myoblasts. Myogenesis also occurs, but to a lesser extent, when this cDNA is expressed in a number of other cell lines. The major open reading frame encoded by this cDNA contains a short protein segment similar to a sequence present in the myc protein family.

mRNA from human colon tumor and mucosa related to the pol gene of an endogenous A-type retrovirus

The expression of endogenous retroviral genes in mammals may be etiologically related to genetic diseases including cancer. Recently, A-type human endogenous retroviral genomes have been cloned using the reverse transcriptase (pol) genes of rodent intracisternal A-particles (IAP). In this report, RNA from human colon adenocarcinoma and surrounding mucosa was hybridized to mouse IAP pol and gag genes to examine the expression of human endogenous A-type retroviruses. Abundant, heterogeneous size, polyadenylated transcripts homologous to the mouse IAP pol gene were detected in both tissues. Transcripts homologous to the mouse IAP gag region were not found.

Expression of type C-related endogenous retroviral sequences in human colon tumors and colon cancer cell lines

Type C-related human endogenous retroviral sequences have been previously discovered and characterized [Martin, M. A., Bryan, T., Rasheed, S. & Khan, A.S. (1981) Proc. Natl. Acad. Sci. USA 78, 4892-4896]. We investigated the transcriptional pattern of these sequences to determine whether and to what extent their expression is altered in colon tumors and colon cancer cell lines as compared to normal colon mucosa (NCM). Of two long terminal repeat (LTR)-specific transcripts [3.6 and 2.1 kilobases (kb)], the 3.6-kb RNA was particularly abundant in NCM but strikingly decreased in most primary colon cancers tested. In NCM we identified three envelope gene (env)-related transcripts--namely, 3.0, 1.7, and 0.6 kb. Colon tumors appeared to express higher levels of these transcripts, especially the 1.7-kb species. In one case of dysplasia and in one benign tumor, this 1.7-kb transcript was clearly increased. We also examined the pattern of transcription in colon cancer cell lines HCT and Caco2. The LTR-homologous 3.6-kb transcript, very abundant in NCM, was decreased in primary tumors and in HCT cells and virtually absent in Caco2 cells. The latter, however, appeared to produce the transcript when growing exponentially, indicating that Caco2 cultures provide an inducible system susceptible to in vitro manipulation. Both cell lines also contained higher amounts of the 1.7-kb env-related transcript. The decrease of the 3.6-kb RNA in colon tumors versus NCM may be the result of an altered pattern of differentiation, whereas the increase of the 1.7-kb RNA in tumors may represent an early marker of colon neoplasia.