A 43-kDa protein related to c-Erb A alpha 1 is located in the mitochondrial matrix of rat liver

In order to characterize Sterling's triiodothyronine (T3) mitochondrial receptor using photoaffinity labeling, we observed two specific T3-binding proteins in the inner membrane (28 kDa) and in the matrix (43 kDa) of rat liver mitochondria. Western blots and immunoprecipitation using antibodies raised against the T3-binding domain of the T3 nuclear receptor c-Erb A alpha 1 indicated that at least the 43-kDa protein was c-Erb A alpha 1-related. In addition, gel mobility shift assays demonstrated the occurrence of a c-Erb A alpha 1-related mitochondrial protein that specifically binds to a natural or a palindromic thyroid-responsive element. Moreover, this protein specifically binds to a direct repeat 2 sequence located in the D-loop of the mitochondrial genome. Furthermore, electron microscopy studies allowed the direct observation of a c-Erb A-related protein in mitochondria. Lastly, the relative amounts of the 43-kDa protein related to c-Erb A alpha 1 were in good correlation with the known mitochondrial mass in three typical tissues. Interestingly, expression of a truncated form of the c-Erb A alpha 1 nuclear receptor in CV1 cells was associated with a mitochondrial localization and a stimulation of mitochondrial activity. These results supply evidence of the localization of a member of the nuclear receptor superfamily in the mitochondrial matrix involved in the regulation of mitochondrial activity that could act as a mitochondrial T3-dependent transcription factor.

Virofection: a new procedure to achieve stable expression of genes transferred into early embryos

A new procedure, virofection, designed to stabilize the expression of transfected DNA has been developed. It exploits the capacity of retroviruses to integrate their genome into the chromosomes of host cells. The co-transfection of two plasmids, one carrying the genome of a defective retrovirus vector, the other one encoding all the retroviral proteins, results in a transient production of infectious virus particles. These particles can infect the neighboring cells and this leads to the stable integration of the vector genome. This procedure is time-saving and appears to be quite efficient. When applied to chicken embryonic fibroblasts cultured in vitro, it resulted in the stable expression of the lacZ gene in more than 30% of the cells, and did not induce chronic viremia. Stable lacZ expression was also achieved in chicken embryos in ovo. Virofection appears to be a promising and generally applicable method for implementing stable, safe and efficient gene transfer in vitro and in vivo.

c-ErbA, but not v-ErbA, competes with a putative erythroid repressor for binding to the carbonic anhydrase II promoter

The carbonic anhydrase II (CAII) gene is the only known gene identified as direct target for v-ErbA-mediated repression in avian erythroleukemic cells transformed by Avian Erythroblastosis Virus (AEV). This gene is transcriptionally activated by thyroid hormone (T3) in normal erythrocytic cells. In this work we have analysed the molecular basis of the transcriptional control of the CAII gene by c-ErbA and v-ErbA. We show that several domains in the promoter control hormonal regulation of transcription. One domain proximal to the TATA box mediates T3 response but contains no identified binding site for c-ErbA. An other domain termed PAL2 is approximately 600 bp upstream the transcription initiation site and contains a c-ErbA binding site. We show that when it is associated to a heterologous promoter this site mediates transcriptional repression in erythrocytic cells but not in HeLa cells. Moreover, this site binds a nuclear erythrocyte-specific factor that we called NFX, which is different from c-ErbA. heterodimers between c-ErbA and the 9-cis retinoic acid receptor (RXR) compete with NFX for binding to PAL2. In contrast, v-ErbA alone or in association with RXR is a very poor competitor and is unable to chase NFX out of the PAL2 site. We propose that NFX is a transcription repressor whose activity is inhibited by c-ErbA but not v-ErbA. This mechanism might contribute to the overall regulation of the carbonic anhydrase II promoter. These data illustrate another possible mechanism through which v-ErbA might antagonize the function of c-ErbA in controlling gene expression.

A nontoxic and versatile protein salting-out method for isolation of DNA

A pivotal technique in basic and applied molecular biology is the isolation of DNA. However, the present DNA extraction methods are either toxic, expensive, time-consuming and laborious or restricted to certain applications. Here we describe a nontoxic and versatile protein salting-out method for convenient and rapid extraction of large as well as small DNA molecules from vertebrate cells and plasmid DNA from bacteria. Easy and relatively imprecise manipulations of a large number of samples result in high yields of pure mammalian and plasmid DNA that are suitable for transformation of bacteria, restriction enzyme analyses, Southern blotting, end labeling of DNA, PCR and sequencing.

v-erbA stimulates quail myoblast differentiation in a T3 independent, cell-specific manner

The v-erbA oncoprotein represents a mutated version of a thyroid hormone receptor, responsible for the induction of a differentiation arrest in chicken erythroid cells. We have studied the influence of v-erbA on proliferation and differentiation of avian myoblasts. Secondary quail myoblast cultures were infected either with an avian retrovirus carrying the v-erbA oncogene in association with the neomycin resistance gene, or with a control deleted v-erbA/neoR alpha retrovirus. We report here that v-erbA expression led to an increase in myoblast proliferation and to a surprising stimulation of quail myoblast terminal differentiation. In addition, these effects occurred in the presence or absence of T3, and v-erbA did not suppress T3 influence on myoblasts. Transient transfection assays demonstrated that, in contrast to its action in HeLa cells, v-erbA was unable to repress the transcriptional activation of a TRE-CAT reporter gene by liganded c-erbA alpha receptors in quail myoblasts. We also observed that the AP-1/c-erbA/v-erbA interactions are not functional in quail myoblasts. These data suggest that, in these cells, v-erbA action does not interfere with T3 induced mechanisms. They also demonstrate a cell specificity for the v-erbA pathway. Lastly, expression of c-erbA/v-erbA chimeric proteins and of the S61G v-erbA mutant indicates that the DNA binding domain of v-erbA, and more specifically serine 61, is directly involved in the enhancement of myoblast differentiation by the oncoprotein.

Contrasting patterns of retinoblastoma protein expression in mouse embryonic stem cells and embryonic fibroblasts

The expression of the retinoblastoma susceptibility (RB-1) gene was investigated in highly proliferating mouse embryonic stem (ES) cells and in slowly proliferating mouse embryonic fibroblasts. The RB protein was expressed at the same level in these two cell types. Mainly hyperphosphorylated RB was detected in exponentially-growing ES cells. Embryonic fibroblasts and embryonic stem cells were synchronized by colcemid block followed by mitotic shake-off. In embryonic fibroblasts, DNA replication started 10-15 h after exit from mitosis and RB was transiently dephosphorylated during the G1 phase as previously described. In ES cells, DNA replication started 2 h after release from the colcemid block but virtually no hypophosphorylated RB was observed after the release. Instead, there was a dramatic decrease in the total RB protein level between exit from mitosis and entry into S phase. These observations were made by using two different monoclonal antibodies, both in immunoblotting and immunoprecipitation experiments. Absence of hypophosphorylated RB and cell cycle-dependent change in total RB protein level may be relevant to the high proliferation rate and to the tumorigenic nature of mouse embryonic stem cells.

c-erbA alpha/T3R and RARs control commitment of hematopoietic self-renewing progenitor cells to apoptosis or differentiation and are antagonized by the v-erbA oncogene

In AEV-transformed erythroleukemic cells the v-erbA gene product is likely to antagonize the function of triiodothyronine (T3) and retinoic acid (RA) receptors and thereby to block cell differentiation. We have thus investigated the effects of T3 and RA on normal early erythrocytic progenitor cells. Here we show: (1) that either RA or T3 play an essential role during the early commitment to erythrocytic differentiation, (2) that both T3 and RA induce death by apoptosis and a strong inhibition of self-renewal in progenitor cells grown in the absence of differentiation-inducing agents and (3) that the v-erbA oncogene renders erythrocytic progenitor cells insensitive to apoptosis and to self-renewal inhibition induced by RA or T3. The behaviour of a non-transforming mutant of v-erbA suggests that this v-erbA-induced protection is related to its transforming potential.

v-jun cooperates with v-erbB to transform the thrombocytic/megakaryocytic lineage

The transforming properties of v-jun, the viral counterpart of the transcription factor AP1, were investigated in avian hematopoietic cells. Two retroviruses, called JB and JBN, expressing both v-jun and v-erbB, were constructed using an avian erythroblastosis-based vector. We show that the cooperative action of both oncogenes allowed the virus to efficiently transform bone marrow cells. No such transformation was obtained with either oncogene alone. JB-transformed bone marrow cells expressed GATA-1, TAL-1, and histone H5, suggesting that they belong to the erythrocytic/thrombocytic lineage. (Thrombocytes are the avian homologues of mammal megakaryocytes.) Moreover, after induction with phorbol 12-myristate 13-acetate JB-transformed bone marrow cells began to differentiate and synthesized high levels of platelet glycoproteins, indicating that they were of thrombocytic origin. These results were confirmed by c-ets1 analysis since this transcription factor, specifically found in cells with megakaryocytic but not erythrocytic features, was clearly detected in these cells.

Transforming growth factor beta 1-mediated growth inhibition in chick embryo fibroblasts: reversion by virally-expressed nuclear oncogenes

Transforming growth factor beta 1 (TGF-beta 1) inhibits growth of primary cultures of chick embryo fibroblasts by affecting G1 and strongly increasing the generation time. This inhibition is reversed by the nuclear oncogenes v-jun, v-fos, v-myc, but not v-erbA and v-ets. It is also reversed by v-myb from either avian myeloblastosis virus or avian E26 retrovirus. Taken together, these results strongly suggest that independent, functional interferences may take place between the TGF-beta 1-induced growth inhibitory pathway and the oncogen-driven stimulatory pathway(s) at the level of the AP-1, Myc, and Myb transcription factors.

Sequence analysis reveals that the BTG1 anti-proliferative gene is conserved throughout evolution in its coding and 3' non-coding regions

The human BTG1 gene (expressing an anti-proliferative function) is an evolutionarily conserved gene homologous to the murine PC3/TIS21 genes. Here, we report the cloning and sequencing of the murine BTG1 coding region and chicken BTG1 cDNA. The putative human and mouse BTG1 proteins are 100% identical; the chicken BTG1 cDNA contains an open reading frame of 170 amino acids with a 91% identity to its human and murine counterparts. The 3'-untranslated region of BTG1 is also highly conserved (82% homology between human and chicken), suggesting that it plays a key role in the regulation of BTG1 expression. These data confirm that BTG1 is phylogenetically highly conserved and that BTG1 and PC3/TIS21 may constitute the first members of a new family of functionally related genes.

Leukemogenicity of v-myb-transformed monoblasts cells can be modulated by normal bone marrow environment

The avian myeloblastosis virus (AMV) causes monoblastic leukemia in the chick. Two non-producer clones of AMV-transformed monoblasts, BM2/C3A and BM2L/A2B5, have been described (see Bottazzi et al., this issue). They differ in their growth requirements and in their ability to induce leukemia when injected into the chick embryo. We first genetically tagged these clones by retroviral infection with a vector expressing the bacterial lacZ gene. Then, we injected the lacZ-positive cells via the chorioallantoic vein into chick embryos. With BM2L/A2B5 cells, the bone marrow of the injected birds was rapidly invaded by lacZ-positive cells. In addition, these cells rapidly overgrew cultures of bone marrow cells derived from injected animals. Conversely, the growth of BM2/C3A was inhibited in the injected animals and only a few blue cells, with the morphology of macrophages, were detected in cultures of bone marrow cells. We developed an in vitro assay to mimic in vitro the differential growth of BM2/C3A and BM2L/A2B5 observed in vivo. These data strongly suggest that BM2/C3A cells retain their ability to differentiate into macrophages in the normal bone marrow environment and that BM2L/A2B5 cells differ from BMC/C3A in the loss of this capacity.

The 12S adenoviral E1A protein immortalizes avian cells and interacts with the avian RB product

Quail cells were immortalized for the first time by using retroviruses expressing the 12S adenoviral E1A gene. In these cells, interaction between the 12S E1A product and the quail RB protein was shown, suggesting that the 12S adenoviral E1A product works in avian cells through similar biochemical pathways as in mammalian cells by interacting and inactivating host cellular proteins, including the RB product. These results confirm that the RB product exhibits a universal function among higher vertebrates in controlling cellular growth and tumor progression.

v-erbA cooperates with bFGF in neuroretina cell transformation

We have studied the transforming ability of the oncogenes erbA and/or erbB in chicken neuroretina (CNR) cells and the effect of basic fibroblast growth factor (bFGF) on the transformed phenotype. The erbA oncogene alone was only transforming in the presence of bFGF. In contrast, cells expressing erbB as well as erbA + erbB were transformed in a bFGF-independent manner and were unresponsive to the growth factor. We studied whether other oncogenes could also block the cooperation between erbA and bFGF. Cytoplasmic or membrane-bound oncogenes (src, ras, or mill raf) increased the transforming potential of erbA but rendered the cells unresponsive to bFGF. Conversely, the nuclear oncogenes tested (fos and myb-ets + myc) also cooperated with erbA in CNR cell transformation but the cells remained responsive to the growth factor. A likely explanation is that CNR cells carrying the cytoplasmic but not the nuclear oncogenes have already activated the bFGF signal transduction pathway.

Importance of 3' non-coding sequences for efficient retrovirus-mediated gene transfer in avian cells revealed by self-inactivating vectors

Avian leukosis virus-derived vectors were constructed with an internal transcriptional promoter and various 3' non-coding sequences. Deletions were introduced into the downstream U3 long terminal repeat (LTR) to obtain self-inactivation of LTR-mediated transcription after one round of replication. However, 3' non-coding sequences appeared to determine not only self-inactivation of the vectors but also gene transfer efficiency. Further analysis revealed the influence of these sequences on both internal gene expression and RNA packaging. One construct permitted gene transfer while inactivating 5' LTR-promoted transcription.

A hepatitis B virus pre-S-retinoic acid receptor beta chimera transforms erythrocytic progenitor cells in vitro

In this report, we investigated the transforming properties of retinoic acid receptor beta (RAR beta). The v-erbA protein, which is the viral oncogenic homologue of the thyroid hormone receptor, was replaced by either the complete RAR beta (beta R) or a hepatitis B virus pre-S-RAR beta (H beta R) hybrid product in an avian erythroblastosis virus-based vector. In chicken hematopoietic cells, the H beta R protein was able to transform erythroid progenitor cells, whereas no such transformation was observed with the wild-type beta R protein. Moreover, the fully transformed phenotype was observed even in the absence of v-erbB, and H beta R-transformed erythroid cells grew independently of growth factors and transforming growth factor alpha. The analysis of erythrocytic-specific proteins revealed that the transformed cells were blocked at the colony-forming unit-erythroid stage and that the expression of the carbonic anhydrase II gene, a gene normally regulated by thyroid hormones, was repressed by the H beta R protein. Finally, hepatocarcinomas rapidly developed in some chickens infected in ovo with viruses encoding either the normal or the hybrid H beta R, suggesting that an inappropriate expression of the RAR beta gene may represent an important event in oncogenesis.

The various domains of v-myb and v-ets oncogenes of E26 retrovirus contribute differently, but cooperatively, in transformation of hematopoietic lineages

The genome of the avian leukemia virus E26 is a unique example of association between two transcription factors which appear as a fused composite nuclear oncoprotein, P135gag-myb-ets. Previous studies with E26 have shown that v-myb and v-ets must cooperate to fully transform both erythrocytic and myelomonocytic precursor cells in vivo and in vitro. To analyse further the contribution of the individual domains involved in the transformation of various hematopoietic lineages, we have constructed several mutant viruses expressing a fusion protein with deletions in either v-myb or v-ets. We show here that integrity of the v-ets oncogene is necessary for transformation of the erythrocytic cells but that neither the DNA-binding domain nor the trans-activating domain of v-myb is required for this transformation. The DNA-binding domain of v-ets is necessary to transform myelomonocytic cells. Furthermore, we show that E26 onco-protein also transforms granulocytic cells. The v-ets DNA-binding domain is not necessary to transform them, whereas deleting the v-myb DNA-binding domain strongly reduces transformation of these cells. These data show that the v-myb and v-ets DNA-binding domains provide quite different contributions to the transformation of various hematopoietic lineages by E26.

v-erbB oncogene expression accounts for most variations in protein synthesis after avian erythroblastosis virus infection of chicken embryo fibroblasts: a two-dimensional electrophoresis study

The effect of the v-erbA and/or v-erbB oncogenes on cellular gene expression was investigated after separation by two-dimensional polyacrylamide gel electrophoresis of [35S]methionine-labelled proteins from chicken embryo fibroblasts (CEF), infected by either the avian erythroblastosis virus (AEV) carrying both oncogenes, or by viruses carrying only one of them. We observed significant changes in the synthesis of 34 proteins in AEV-transformed CEF as compared with control cells. The synthesis of 24 of them was increased while the synthesis of the other 10 proteins was decreased. The expression of v-erbB alone is necessary and sufficient to induce changes in the synthesis of 27 proteins while the 7 remaining modifications are observed only in cells expressing v-erbB together with v-erbA. Moreover, the deregulation of protein synthesis by v-erbB-expressing viruses was correlated with the morphological transformation state of cells.

BTG1, a member of a new family of antiproliferative genes

The BTG1 gene locus has been shown to be involved in a t(8;12)(q24;q22) chromosomal translocation in a case of B-cell chronic lymphocytic leukemia. We report here the cloning and sequencing of the human BTG1 cDNA and establish the genomic organization of this gene. The full-length cDNA isolated from a lymphoblastoid cell line contains an open reading frame of 171 amino acids. BTG1 expression is maximal in the G0/G1 phases of the cell cycle and is down-regulated when cells progress throughout G1. Furthermore, transfection experiments of NIH3T3 cells indicate that BTG1 negatively regulates cell proliferation. The BTG1 open reading frame is 60% homologous to PC3, an immediate early gene induced by nerve growth factor in rat PC12 cells. Sequence and Northern blot analyses indicate that BTG1 and PC3 are not cognate genes. We then postulate that these two genes are the first members of a new family of antiproliferative genes.

A novel mechanism of action for v-ErbA: abrogation of the inactivation of transcription factor AP-1 by retinoic acid and thyroid hormone receptors

Ligand-activated retinoic acid receptor alpha (RAR alpha) and c-ErbA alpha repress the AP-1-mediated transcriptional activation of the interstitial collagenase gene promoter by specifically decreasing the activity of the AP-1 transcription factor. On the other hand, the v-ErbA oncoprotein fails to repress the AP-1 activity and acts as a dominant negative oncoprotein by overcoming the repression of the AP-1 activity induced by RAR alpha and c-ErbA alpha. This maintenance by v-ErbA of a fully active AP-1 complex is correlated with the abrogation by this same oncogene product of the growth-inhibitory response of chicken embryo fibroblasts to retinoic acid treatment. This new mechanism of action of v-ErbA together with its previously discovered dominant repressor effect on transcription of thyroid hormone-activated target genes may explain the contribution of the v-erbA oncogene to sarcomatogenic and leukemogenic transformation.

v-erbA oncogene abrogates growth inhibition of chicken embryo fibroblasts induced by retinoic acid

Retinoic acid inhibits chicken embryo fibroblast (CEF) proliferation by altering the G1 phase of the cell cycle with induction of a strong increase in the generation time. This growth-inhibitory response to retinoic acid is abrogated by expression of the v-erbA oncogene, suggesting an interference between retinoic acid receptors and the v-ErbA oncoprotein. Moreover, CEF expressing either the v-src, v-jun or v-fos oncogenes are also insensitive to retinoic acid treatment. In contrast, CEF expressing either the v-myc, v-myb-ets, v-mil, v-sea or v-erbB oncogenes are still sensitive to retinoic acid. These data strongly suggest functional interferences between the retinoic acid receptors and the AP-1 transcription factor complex in the control of expression of genes involved in CEF proliferation.