Activation of major histocompatibility complex class I mRNA containing an Alu-like repeat in polyoma virus-transformed rat cells

RNA polymerase III transcription--a battleground for tumour suppressors and oncogenes

This review provides a summary of the European Association for Cancer Research Award Lecture, presented at the ECCO12 meeting in Copenhagen in September 2003. It describes what we have learnt about the mechanisms responsible for deregulating RNA polymerase III transcription in transformed cells. A network has been discovered of unanticipated links to key tumour suppressors and oncogenes. Novel functions have been revealed for RB, p53 and c-Myc, that may help explain their profound biological effects.

Multiple mechanisms contribute to the activation of RNA polymerase III transcription in cells transformed by papovaviruses

RNA polymerase (pol) III transcription is abnormally active in fibroblasts transformed by polyomavirus (Py) or simian virus 40 (SV40). Several distinct mechanisms contribute to this effect. In untransformed fibroblasts, the basal pol III transcription factor (TF) IIIB is repressed through association with the retinoblastoma protein RB; this restraint is overcome by large T antigens of Py and SV40. Furthermore, cells transformed by these papovaviruses overexpress the BDP1 subunit of TFIIIB, at both the protein and mRNA levels. Despite the overexpression of BDP1, the abundance of the other TFIIIB components is unperturbed following papovavirus transformation. In contrast, mRNAs encoding all five subunits of the basal factor TFIIIC2 are found at elevated levels in fibroblasts transformed by Py or SV40. Thus, both papovaviruses stimulate pol III transcription by boosting production of selected components of the basal machinery. Py differs from SV40 in encoding a highly oncogenic middle T antigen that localizes outside the nucleus and activates several signal transduction pathways. Middle T can serve as a potent activator of a pol III reporter in transfected cells. Several distinct mechanisms therefore contribute to the high levels of pol III transcription that accompany transformation by Py and SV40.

RNA polymerase III transcription: its control by tumor suppressors and its deregulation by transforming agents

The level of RNA polymerase (pol) III transcription is tightly linked to the rate of growth; it is low in resting cells and increases following mitogenic stimulation. When mammalian cells begin to proliferate, maximal pol III activity is reached shortly before the G1/S transition; it then remains high throughout S and G2 phases. Recent data suggest that the retinoblastoma protein RB and its relatives p107 and p130 may be largely responsible for this pattern of expression. During G0 and early G1 phase, RB and p130 bind and repress the pol III-specific factor TFIIIB; shortly before S phase they dissociate from TFIIIB, allowing transcription to increase. At the end of interphase, when cells enter mitosis, pol III transcription is again suppressed; this mitotic repression is achieved through direct phosphorylation of TFIIIB. Thus, pol III transcription levels fluctuate as mammalian cells cycle, being high in S and G2 phases and low during mitosis and early G1. In addition to this cyclic regulation, TFIIIB can be bound and repressed by the tumor suppressor p53. Conversely, it is a target for activation by several viruses, including SV40, HBV, and HTLV-1. Some viruses also increase the activity of a second pol III-specific factor called TFIIIC. A large proportion of transformed and tumor cell types express abnormally high levels of pol III products. This may be explained, at least in part, by the very high frequency with which RB and p53 become inactivated during neoplastic transformation; loss of function of these cardinal tumor suppressors may release TFIIIB from key restraints that operate in normal cells.

Repression of RNA polymerase III transcription by the retinoblastoma protein

Transcription by RNA polymerase (pol) III is under cell-cycle control, being higher in S and G2 than in G0 and early G1 phases. Many transformed cell types have elevated pol III activity, presumably to sustain sufficient protein synthesis for unrestrained growth. The retinoblastoma tumour-suppressor protein (Rb) restricts cellular proliferation, and is often found mutated in transformed cells. Here we demonstrate that Rb can repress the level of transcription from pol III templates both in vitro and vivo. Analysis of Rb-deficient SAOS2 cells and primary fibroblasts from Rb-/- mice demonstrates elevated levels of pol III activity in the absence of functional Rb protein. Rb-induced repression of pol III activity is alleviated by mutations in the Rb pocket domain that occur naturally in tumours, and by viral transforming proteins that bind and inactivate Rb. These results implicate repression of pol III transcription as a mechanism for Rb-induced growth arrest, and suggest that restraining protein biosynthesis may be important in the prevention of tumour development.

Adenovirus type 2 preferentially stimulates polymerase III transcription of Alu elements by relieving repression: a potential role for chromatin

The number of Alu transcripts that accumulate in HeLa and other human cells is normally very low; however, infection with adenovirus type 5 increases the expression of Alu elements dramatically, indicating that the potential for polymerase III (pol III)-dependent Alu transcription in vivo is far greater than generally observed (B. Panning and J.R. Smiley, Mol. Cell. Biol. 13:3231-3244, 1993). In this study, we employed nuclear run-on in combination with a novel RNase H-based assay to investigate transcription from uninfected and adenovirus type 2-infected nuclei, as well as genomic DNAs from uninfected and infected cells. When performed in the presence of excess uninfected nuclear extract, such assays revealed that (i) the vast majority of transcriptionally competent Alu elements in nuclei are masked from the pol III transcriptional machinery and (ii) the induction of Alu expression upon adenovirus infection can be largely accounted for by an increased availability of these elements to the pol III transcription machinery. We also investigated the role of H1 histone for silencing of Alu genes and, in comparison, mouse B2 repetitive elements. Depletion of H1 led to an approximately 17-fold activation of B2 repetitive elements but did not change Alu transcription relative to that of constitutively expressed 5S rRNA genes. These results are consistent with the view that Alu repeats are efficiently sequestered by chromatin proteins, that such masking cannot be accounted for by nonspecific H1-dependent repression, and that adenovirus infection at least partially overrides the repressive mechanism(s).

Coordinated posttranscriptional control of gene expression by modular elements including Alu-like repetitive sequences

We previously reported that in rat fibroblasts, accumulation of a set of mRNAs ("pIL genes") was modulated as a function of cell growth and transformation, at a posttranscriptional stage, and by a mechanism that depends on a short nucleotide sequence containing an ID repetitive element. In mouse fibroblasts, hybridization with rat pIL probes identified mRNAs with the same pattern of expression, which did not contain ID sequences but contained a different regulatory element, encompassing a repetitive sequence of the B1 family. Expression in mouse cells of a reporter beta-globin gene carrying this element inserted in its 3' noncoding region was growth- and transformation-dependent. The nucleotide sequences of two murine and of three rat pIL cDNAs showed clear similarities in the region immediately adjacent to the ID and B1 repeats. Both the repeat and the flanking sequence were required to confer on beta-globin constructs the pattern of expression characteristic of the pIL genes. The hypothesis is presented that repetitive sequences in the eukaryotic genome might be modular parts of complex regulatory elements ensuring the coordinated expression of various mRNA species.

De novo HLA class II and enhanced HLA class I molecule expression in SV40 transfected human thyroid epithelial cells

Human thyroid follicular cells normally synthesize and express HLA Class I molecules but in glands affected by autoimmune or neoplastic diseases they express Class II molecules. We have investigated the effect of SV40 virus transformation on the expression of MHC molecules in human thyrocytes. Primary cultures of human thyroid from two different glands were transfected with the plasmid pX-8, containing the 'early' region of SV40 virus, and two continuous lines of thyrocytes were obtained. The cell lines maintained features of parental thyroid epithelial cells showing the characteristic cytokeratin filament network, microvillar protrusion and tight junctions. In addition, the SV40-transfected cells responded to graded doses of TSH with increased c-AMP production. Thyrocytes from both cell lines hyperexpressed Class I molecules and a significant proportion of them also acquired constitutive Class II expression, as determined by indirect immunofluorescence (IFL), flow cytometry and Northern blotting hybridization using a DR beta probe. These cells were found to be normally up-regulated by interferon (IFN)-gamma. Indirect IFL and flow cytometry analysis were used to detect and quantify the expression of HLA-DR, DP and DQ subregions. A co-ordinated expression (DR greater than DP much greater than DQ), reminiscent of the inappropriate HLA expression found in thyroid autoimmune disease in vivo and of the in vitro regulation in normal thyrocytes, was observed. Clones derived from these cell lines differed in their level of constitutive Class II expression and in their sensitivity to Class II induction by IFN-gamma. In conclusion, these thyroid cell lines could provide a useful tool for further investigation of HLA gene regulation in thyroid cells and for elucidating on the mechanism involved in the inappropriate HLA expression described in autoimmune and neoplastic diseases of the thyroid.

Tumorigenic and metastatic ability of SV40-transformed BALB/c cell lines and MHC antigen expression

Tumorigenic and metastatic potential were studied in relation to class I MHC expression in four different SV40-transformed BALB/c cell lines. All the lines studied, tumorigenic or not, expressed both H-2Kd and Dd, so MHC antigens did not seem to be involved in the control of SV40-transformed cells' growth in vivo. Lung metastases were observed in all tumour-bearing mice. Cells cultured after in vivo passage, obtained either from tumour tissue or from individual lung metastases, still expressed similar levels of H-2d antigens, thus suggesting that tumour growth and metastasis do not occur through the selection of variants with altered MHC expression.

Expression of class I and class II major histocompatibility antigens on human hepatocellular carcinoma

Previous reports indicate that human hepatocytes do not express class I and class II MHC antigens. Our analyses on 10 human hepatocellular carcinoma (HCC) cell lines by immunofluorescence tests and RIA, demonstrate that all the human HCC cell lines tested express class I MHC antigens and among them, three poorly differentiated human HCC cell lines also express class II MHC antigens. Results of immunoprecipitation and/or Western blotting experiments indicate similarity in the chemical nature of both the class I and class II MHC antigens expressed by the human HCC cell lines and by a human B lymphoblastoid cell line Raji. Furthermore, a new variant form of class I antigen was detected in some of these HCC cell lines. Immunohistochemical studies of HCC tissues using the peroxidase-antiperoxidase staining method indicated that class I and class II antigens were detectable in 7 out of 11 and 3 out of 11 HCC tissues from patients, respectively. The availability of MHC class I antigen-positive cultured HCC cell lines, including the poorly differentiated lines that also express MHC class II antigen, has provided us with interesting models to study the relationship between expression of MHC antigen and transformation and differentiation of human hepatocytes. These studies will also allow us some insight into the role of MHC class I and class II antigen in the immunosensitivity and immunogenicity of HCC cells to the host-immune response.

Induction of an Alu-sequence containing transcript by insulin in human hepatoma cells

A cDNA clone, designated AF19-1, was isolated from a cDNA library derived from insulin-stimulated hepatoma cells. The nucleotide sequences of AF19-1 showed 83% homology to Alu consensus sequence. It contained a full-length 300-bp Alu family sequence followed in direct tandem by a partial sequence of Alu left monomer. Primer extension analysis confirmed that this Alu transcript was induced even in short-term insulin stimulation. The increase in Alu-transcripts in the early phase of insulin stimulation in hepatoma cells suggests that the Alu sequences may play some important regulatory roles in gene expression.

Identification of a proliferation-related transcript with an elevated expression in the mid-gestation mouse embryo

Polyadenylated RNA enriched in transformation specific sequences from hamster embryo fibroblast cells transformed by HSV-2 was used to construct a cDNA library. A cDNA clone (pKG4) contained a sequence which was upregulated in HSV-2 transformed cells and also in fibroblastic cell lines transformed by SV40 and 3-methylcholanthrene. The expression of the KG4 sequences in HSV-2-transformed cells was found to be modulated by the growth state of the cells. In confluent cells its level was reduced 5-times compared to the homologous RNAs from exponentially growing cells. Expression of the KG4 sequence was also examined in mouse embryos from day 8 onwards and in adult tissues. During development, KG4 is expressed at all times examined. However, there is a dramatic increase in expression on day 11. In adult tissues, a low and variable level of expression was observed. These findings suggest that the KG4 sequence is related to cellular proliferation.

Altered HLA antigens expressed on T and B lymphocytes of adult T-cell leukemia/lymphoma patients and their relatives

The HLA types of peripheral blood lymphocytes (PBL) of 36 adult T-cell leukemia/lymphoma (ATLL) patients were examined and compared with those of 45 healthy relatives of these patients, and with those of 10 non-ATLL families including 80 healthy members. Thirty-one percent of ATLL patients showed either a gain or a loss of HLA antigens determined by the presence of alien HLA antigens or the absence of inherent HLA antigens deduced from familial haplotype analysis. The antigen specificity of HLA gained or lost was variable and differed from case to case among ATLL patients. Although the gain of HLA was detected only in ATLL patients, the loss of HLA was found both in ATLL patients and in asymptomatic healthy relatives. The rate of HLA loss in ATLL patients (8.4%) and their relatives (17.8%) was much higher than in relatives of non-ATLL patients (1.1%). The HLA gain and loss revealed in the PBL of ATLL patients were confirmed by altered HLA phenotypes in the cloned T and B cells established from ATLL patients. Our results suggest that latently infecting HTLV-I may induce altered HLA phenotypes in T and B cells, primarily with loss of HLA antigens in a population of asymptomatic virus carriers, and secondarily with a gain of HLA antigens after the development of ATLL.

Cloning and expression of human tissue factor cDNA

Tissue factor is a membrane protein that plays an essential role in the initiation of blood coagulation. When exposed to the circulation, tissue factor interacts with the serine protease factor VII, and the complex triggers fibrin clot formation by activating both factors IX and X of the coagulation cascade. This report describes the cloning and expression of the complementary DNA (cDNA) for human tissue factor. The cDNA encodes a protein of 263 amino acids preceded by a 32 amino acid signal peptide. The predicted protein sequence contains a potential hydrophobic membrane anchoring domain at its carboxy terminus, and bears no significant homology to any other known protein. Tissue factor mRNA of 2400 nucleotides was detected in adipose, adrenal, small intestine and a number of other tissues by Northern blot hybridization analysis. In order to confirm the identity of the cDNA, an expression vector containing the cloned cDNA was used to transfect cultured mammalian cells. These cells produced active tissue factor which was assayed using purified factors VII and X.

The transcription of B2 repeated sequences is regulated during the transition from quiescent to proliferative state in cultured rodent cells

The RNA polymerase III-dependent transcription of B2 repeated sequences has been monitored during the transition from the quiescent to proliferative state in cultured rodent cells and after polyomavirus-induced transformation. The level of RNAs containing B2 sequences was found to be higher in both the proliferative state of normal cells and in polyomavirus-transformed cells. In both systems, nuclear run-off transcription assays indicated that high levels of B2 RNAs are due to an enhanced transcription rate. These results suggest the presence of a B2-specific RNA pol III transcription factor(s) whose activity is sensitive to cell cycle progression and oncogenic transformation.

A systematic survey of repetitive sequences abundantly expressed in rat tumors

A cDNA library made from poly(A)+ RNA of a rat ascites hepatoma cell line, AH60C, was screened with respect to transcribed repetitive sequences by probing with AH60C genomic DNA. Thirty clones, comprising 1.2% of the library, were found to contain repetitive elements. The sequences of 12 clones among them were abundantly expressed in poly(A)+ RNA of AH60C, yet sparse in that of rat normal liver. This differential expression was also noted in case of other tumors and normal tissues tested. These 12 clones could be divided into two classes, depending on whether their sequences were expressed in cytoplasmic poly(A)+ RNA (class I) or not (class II). Rat Alu-like type II (B2) repeats and an unknown repetitive sequence were found in the clones of class I. Adjacent to each of them, we detected two non-repeated sequences, the expressions of which were also activated. The presence of another unknown repetitive sequence was observed in the clones of class II. It is noted that more than one-third of the transcribed repetitive sequences were involved in the predominant expression in tumor cells, whereas the majority of overall transcripts have been found to be common to both transformed cells and their normal counterparts.

Regulation of viral and cellular promoter activity by polyomavirus early proteins

The chloramphenicol-acetyl-transferase (CAT) expression system has been utilized to study the ability of the polyomavirus (Py) early proteins, the 100K large T, the 55K middle T and 22K small T-antigens, to activate a variety of eukaryotic promoters (the SV40 early, the alpha 2(1) collagen, the rabbit beta-globin, the polyomavirus early and the H-2 class I) in both transient and stable expression assays. We have found that either the complete polyomavirus early region or a plasmid expressing only the 55K middle T-antigen are capable of stimulating the expression of all the promoter-CAT plasmids in transient co-transfection experiments in both NIH-3T3 and Rat-2 cells. Conversely, the Py early proteins do not stimulate the transcription of most of the promoter-CAT genes stably introduced in the cell chromosomes, with the exception of H-2 class I promoter, when stimulation of transcription has been observed upon infection with recombinant retrovirus encoding the Py middle T-antigen.

Localization of small RNAs hybridizable to a B2 clone in the nuclear fraction of mouse cell lines

An RNA polymerase III transcript, 7SK nuclear RNA, was found to bear sequence homology to the B2 class of highly repeated elements in the mouse genome. Northern blot hybridizations between the small RNAs and two B2 clones showed that only one of them (p49C8) hybridized to 7SK RNA. Both clones, however, hybridized to 4.5SI RNA as well as to a third class of nuclear RNA transcripts around 170 nucleotides long, whose levels were found to be greatly increased upon induction of transformation in a mouse 3T3 cell line transformed with a temperature-sensitive mutant of simian virus 40.

Induction of specific transcription by RNA polymerase III in transformed cells

RNA polymerase III (pol III) transcripts of the highly repeated mouse B2 gene family are increased in many oncogenically transformed murine cell lines. In cells transformed by simian virus 40, the small, cytoplasmic B2 RNAs are present at 20-fold-higher levels than in normal cells (M. R. D. Scott, K. Westphal, and P. W. J. Rigby, Cell 34:557-567, 1983; K. Singh, M. Carey, S. Saragosti, and M. Botchan, Nature [London] 314:553-556). We found that transcripts of the highly repeated B1 gene family are also increased 20-fold upon simian virus 40 transformation and showed that these RNAs result from pol III transcription. In contrast, transcripts from less highly repeated pol III templates such as the 5S, 7SL, 7SK, 4.5SI, tRNAMet, and tRNAPro genes are unaffected. The expression of the B2 RNAs in isolated nuclei shows that the augmentation is due mainly to an increased rate of transcription by pol III. There is thus specific transformation-inducible pol III transcription. We developed an in vitro transcription assay which utilizes genomic DNA as a template to study the transcription of all members of a repetitive gene family in their native context. This assay reproduces the low cytoplasmic levels of B1 compared with B2 RNAs suggesting that this ratio is dictated by intrinsic signals in the DNA.

Analysis of the expression of H-2K, D and L locus mRNAs by use of an oligonucleotide primer

By employing an oligonucleotide-dependent cDNA synthesis/HaeIII digestion procedure followed by gel electrophoretic separation of HaeIII digested cDNAs, we have developed a method for analyzing the expression of H-2 mRNAs encoding the K, D and L locus histocompatibility antigens. This method provides unambiguous data as the individual K, D or L specific cDNA bands can be isolated from the gel and partially sequenced. The method is readily applicable to analyzing the relative expression of K, D and L locus mRNAs in normal mouse tissues and their tumors.

Nucleotide sequence of the BamHI repetitive sequence, including the HindIII fundamental unit, as a possible mobile element from the Japanese monkey Macaca fuscata

Clustered repeat units produced by BamHI digestion of genomic DNA from the Japanese monkey Macaca fuscata [JMr(BamHI)] were sequenced by dideoxy DNA sequencing. The nucleotide sequences of several individual repeats showed that the BamHI repeat contains the 170-bp HindIII element as an integral part, and that it has more than 90% homology with the HindIII repeat element [AGMr(HindIII)] found in the genomic DNA of the African green monkey. In the JMr(BamHI) repeat unit, the 170-bp HindIII element is flanked by a 6-bp inverted repeat, which is part of a 22-bp direct repeat. This latter repeat of 22-bp asymmetrically overlaps the border between the internal AGMr(HindIII)-like region and adjacent regions of the JMr(BamHI) repeat. A similar structural feature of the BamHI repeat unit has been found in the genomic DNA of the baboon, but not in that of the African green monkey. These results show clearly that the BamHI repeat of the modern Japanese monkey originated as a result of insertion of an AGMr(HindIII) element into a certain site(s) of the genomic DNA of an ancestor of the modern Japanese monkey before Macaca-Cercocebus divergence.

Induction of specific transcription by RNA polymerase III in transformed cells

RNA polymerase III (pol III) transcripts of the highly repeated mouse B2 gene family are increased in many oncogenically transformed murine cell lines. In cells transformed by simian virus 40, the small, cytoplasmic B2 RNAs are present at 20-fold-higher levels than in normal cells (M. R. D. Scott, K. Westphal, and P. W. J. Rigby, Cell 34:557-567, 1983; K. Singh, M. Carey, S. Saragosti, and M. Botchan, Nature [London] 314:553-556). We found that transcripts of the highly repeated B1 gene family are also increased 20-fold upon simian virus 40 transformation and showed that these RNAs result from pol III transcription. In contrast, transcripts from less highly repeated pol III templates such as the 5S, 7SL, 7SK, 4.5SI, tRNAMet, and tRNAPro genes are unaffected. The expression of the B2 RNAs in isolated nuclei shows that the augmentation is due mainly to an increased rate of transcription by pol III. There is thus specific transformation-inducible pol III transcription. We developed an in vitro transcription assay which utilizes genomic DNA as a template to study the transcription of all members of a repetitive gene family in their native context. This assay reproduces the low cytoplasmic levels of B1 compared with B2 RNAs suggesting that this ratio is dictated by intrinsic signals in the DNA.

Keratin gene expression in simian virus 40-transformed human keratinocytes

Previous reports from this laboratory indicate that cultured simian virus 40 (SV40)-transformed human keratinocytes express keratin proteins characteristic of simple epithelia that are not found in their untransformed counterparts. In this study we show by in vitro translation and RNA transfer blot analysis that the altered keratin synthesis reflects changes in the abundance of specific keratin mRNAs. SV40-transformed keratinocytes have a reduced abundance of transcripts for 58-, 56-, 52-, 50-, 48-, and 46-kDa keratin species, compared with uninfected cultured keratinocytes, but express significant levels of transcripts for 52-, 45-, and 40-kDa keratins, typical of simple epithelia. The SV40-transformed cells also express mRNA for a 48-kDa keratin that is unique to SV40-transformed keratinocytes. Analysis of the keratin genome with keratin-specific cDNAs as probes indicates that the changes in keratin transcription are not correlated with gross rearrangements of the keratin genome. These results suggest that analysis of viral transformation of cultured keratinocytes affords a novel approach to study mechanisms regulating keratin gene expression.

Histocompatibility antigens on murine tumors

Recent advances in tumor immunology suggest that the expression of the histocompatibility antigens, encoded by the major histocompatibility complex, is important in controlling the metastatic growth of certain murine tumors. The anomalous expression of histocompatibility antigens in many neoplasms appears to be associated with the ability of these cells to evade the immune system and progress to metastasis. This review examines some of the underlying molecular and immunobiological interactions that might determine the metastatic outcome of cellular transformation.

The class I major histocompatibility antigen gene activated in a line of SV40-transformed mouse cells is H-2Dd, not Qa/Tla

We have previously described several complementary DNA clones isolated because they correspond to messenger RNAs present at higher levels in the simian virus 40 (SV40)-transformed BALB/c 3T3 cell line SV3T3 Cl38 than in the normal, parental BALB/c 3T3 line. One of these clones, pAG64, hybridizes to RNAs which, while present in BALB/c 3T3 cells, are 10-20-fold more abundant in SV3T3 Cl38 and are found at high levels in a wide variety of transformed cell lines. Nucleotide sequence analysis showed that pAG64 encodes a class I antigen of the major histocompatibility complex. To ascertain the identity of pAG64, we compared its sequence with the available sequences of d haplotype class I antigen genes [K locus, L locus, D locus and the Qa gene defined by genomic clone 27.1] and found that it showed multiple clustered differences from each of these sequences. We therefore concluded that it was not derived from the H-2Kd, H-2Ld or H-2Dd genes and thus must correspond to one of the other class I antigen genes, namely those of the Qa/Tla complex, although it was clearly not the Qa gene defined by the genomic clone 27.1. We now report subsequent findings which indicate that pAG64 in fact corresponds to the H-2Dd gene and not to a Qa/Tla gene.