15-Lipoxygenase promotes resolution of inflammation in lymphedema by controlling Treg cell function through IFN-β

Lymphedema (LD) is characterized by the accumulation of interstitial fluid, lipids and inflammatory cell infiltrate in the limb. Here, we find that LD tissues from women who developed LD after breast cancer exhibit an inflamed gene expression profile. Lipidomic analysis reveals decrease in specialized pro-resolving mediators (SPM) generated by the 15-lipoxygenase (15-LO) in LD. In mice, the loss of SPM is associated with an increase in apoptotic regulatory T (Treg) cell number. In addition, the selective depletion of 15-LO in the lymphatic endothelium induces an aggravation of LD that can be rescued by Treg cell adoptive transfer or ALOX15-expressing lentivector injections. Mechanistically, exogenous injections of the pro-resolving cytokine IFN-β restores both 15-LO expression and Treg cell number in a mouse model of LD. These results provide evidence that lymphatic 15-LO may represent a therapeutic target for LD by serving as a mediator of Treg cell populations to resolve inflammation.

CXCL4L1-fibstatin cooperation inhibits tumor angiogenesis, lymphangiogenesis and metastasis

Anti-angiogenic and anti-lymphangiogenic drugs slow tumor progression and dissemination. However, an important difficulty is that a tumor reacts and compensates to obtain the blood supply needed for tumor growth and lymphatic vessels to escape to distant loci. Therefore, there is a growing consensus on the requirement of multiple anti-(lymph)angiogenic molecules to stop cell invasion efficiently. Here we studied the cooperation between endogenous anti-angiogenic molecules, endostatin and fibstatin, and a chemokine, the Platelet Factor-4 variant 1, CXCL4L1. Anti-angiogenic factors were co-expressed by IRES-based bicistronic vectors and their cooperation was analyzed either by local delivery following transduction of pancreatic adenocarcinoma cells with lentivectors, or by distant delivery resulting from intramuscular administration in vivo of adeno-associated virus derived vectors followed by tumor subcutaneous injection. In this study, fibstatin and CXCL4L1 cooperate to inhibit endothelial cell proliferation, migration and tubulogenesis in vitro. No synergistic effect was found for fibstatin-endostatin combination. Importantly, we demonstrated for the first time that fibstatin and CXCL4L1 not only inhibit in vivo angiogenesis, but also lymphangiogenesis and tumor spread to the lymph nodes, whereas no beneficial effect was found on tumor growth inhibition using molecule combinations compared to molecules alone. These data reveal the synergy of CXCL4L1 and fibstatin in inhibition of tumor angiogenesis, lymphangiogenesis and metastasis and highlight the potential of IRES-based vectors to develop anti-metastasis combined gene therapies.

The p53 isoform, Δ133p53α, stimulates angiogenesis and tumour progression

The tumour suppressor p53, involved in DNA repair, cell cycle arrest and apoptosis, also inhibits blood vessel formation, that is, angiogenesis, a process strongly contributing to tumour development. The p53 gene expresses 12 different proteins (isoforms), including TAp53 (p53 (or p53α), p53β and p53γ) and Δ133p53 isoforms (Δ133p53α, Δ133p53β and Δ133p53γ). The Δ133p53α isoform was shown to modulate p53 transcriptional activity and is overexpressed in various human tumours. However, its role in tumour progression is still unexplored. In the present study, we examined the involvement of Δ133p53 isoforms in tumoural angiogenesis and tumour growth in the highly angiogenic human glioblastoma U87. Our data show that conditioned media from U87 cells depleted for Δ133p53 isoforms block endothelial cell migration and tubulogenesis without affecting endothelial cell proliferation in vitro. The Δ133p53 depletion in U2OS osteosarcoma cells resulted in a similar angiogenesis blockade. Furthermore, using conditioned media from U87 cells ectopically expressing each Δ133p53 isoform, we determined that Δ133p53α and Δ133p53γ but not Δ133p53β, stimulate angiogenesis. Our in vivo data using the chicken chorio-allantoic membrane and mice xenografts establish that angiogenesis and growth of glioblastoma U87 tumours are inhibited upon depletion of Δ133p53 isoforms. By TaqMan low-density array, we show that alteration of expression ratio of Δ133p53 and TAp53 isoforms differentially regulates angiogenic gene expression with Δ133p53 isoforms inducing pro-angiogenic gene expression and repressing anti-angiogenic gene expression.

Efficient gene transfer in skeletal muscle with AAV-derived bicistronic vector using the FGF-1 IRES

IRESs (internal ribosome entry sites) are RNA elements behaving as translational enhancers in conditions of global translation blockade. IRESs are also useful in biotechnological applications as they allow expression of several genes from a single mRNA. Up to now, most IRES-containing vectors use the IRES from encephalomyocarditis virus (EMCV), highly active in transiently transfected cells but long and not flexible in its positioning relative to the gene of interest. In contrast, several IRESs identified in cellular mRNAs are short and flexible and may therefore be advantageous in gene transfer vectors such as those derived from the adeno-associated virus (AAV), where the size of the transgene expression cassette is limited. Here, we have tested bicistronic AAV-derived vectors expressing two luciferase genes separated by the EMCV- or fibroblast growth factor 1 (FGF-1) IRES. We demonstrate that the AAV vector with the FGF-1 IRES, when administrated into the mouse muscle, leads to efficient expression of both transgenes with a stable stoechiometry, for at least 120 days. Interestingly, the bicistronic mRNA containing the FGF-1 IRES leads to transgene expression 10 times superior to that observed with EMCV, in vivo. AAV vectors featuring the FGF-1 IRES may thus be advantageous for gene therapy approaches in skeletal muscle involving coexpression of genes of interest.

The use of IRES-based bicistronic vectors allows the stable expression of recombinant G-protein coupled receptors such as NPY5 and histamine 4

Stable expression of G protein coupled receptors in cell lines is a crucial tool for the characterization of the molecular pharmacology of receptors and the screening for new antagonists. However, in some instances, many difficulties have been encountered to obtain stable cell lines expressing functional receptors. Here, we addressed the question of vector optimization to establish cell lines expressing the human neuropeptide Y receptor 5 (NPY5-R) or histamine receptor 4 (HH4R). We have compared bicistronic vectors containing viral or cellular internal ribosome entry sites (IRES), co-expressing the receptor and the neomycine resistance gene from a single mRNA, to a bigenic vector containing two distinct promoters upstream each different genes. This study is the first one to validate the use of three cellular IRESs for long-term transgene expression. Our results demonstrate for both NPY5-R and HH4R that the bicistronic vectors with EMCV, VEGF, FGF1A or FGF2 IRES provide clones expressing functional receptors with yields between 25% and 100%. In contrast, the bigenic vector provided no functional clones, related to a low expression of NPY5R mRNA. The cell lines expressing active receptor were stable after more than 50 passages. These data indicate that IRES-based bicistronic vectors are particularly appropriate to establish cell clones expressing active G-coupled protein receptors with a high yield. In the case of NPY5, it was a new way to produce such a stable cell line. Furthermore, the characteristics-presented herein-of this receptor pharmacological property are perfectly in line with those reported in the literature.

IRES-dependent regulation of FGF-2 mRNA translation in pathophysiological conditions in the mouse

The mRNA coding for FGF-2 (fibroblast growth factor 2), a major angiogenic factor, is translated by an IRES (internal ribosome entry site)-dependent mechanism. We have studied the role of the IRES in the regulation of FGF-2 expression in vivo, under pathophysiological conditions, by creating transgenic mice lines expressing bioluminescent bicistronic transgenes. Analysis of FGF-2 IRES activity indicates strong tissue specificity in adult brain and testis, suggesting a role of the IRES in the activation of FGF-2 expression in testis maturation and brain function. We have explored translational control of FGF-2 mRNA under diabetic hyperglycaemic conditions, as FGF-2 is implied in diabetes-related vascular complications. FGF-2 IRES is specifically activated in the aorta wall in streptozotocin-induced diabetic mice, in correlation with increased expression of endogenous FGF-2. Thus, under hyperglycaemic conditions, where cap-dependent translation is blocked, IRES activation participates in FGF-2 overexpression, which is one of the keys of diabetes-linked atherosclerosis aggravation. IRES activation under such pathophysiological conditions may involve ITAFs (IRES trans-acting factors), such as p53 or hnRNP AI (heterogeneous nuclear ribonucleoprotein AI), recently identified as inhibitory or activatory ITAFs respectively for FGF-2 IRES.

Role of Fibroblast Growth Factor-2 Isoforms in the Effect of Estradiol on Endothelial Cell Migration and Proliferation

Both 17beta-estradiol (E2) and fibroblast growth factor-2 (FGF2) stimulate angiogenesis and endothelial cell migration and proliferation. The first goal of this study was to explore the potential link between this hormone and this growth factor. E2-stimulated angiogenesis in SC Matrigel plugs in Fgf2+/+ mice, but not in Fgf2-/- mice. Cell cultures from subcutaneous Matrigel plugs demonstrated that E2 increased both migration and proliferation in endothelial cells from Fgf2+/+ mice, but not from in Fgf2-/- mice. Several isoforms of fibroblast growth factor-2 (FGF2) are expressed: the low molecular weight 18-kDa protein (FGF2lmw) is secreted and activates tyrosine kinase receptors (FGFRs), whereas the high molecular weight (21 and 22 kDa) isoforms (FGF2hmw) remains intranuclear, but their role is mainly unknown. The second goal of this study was to explore the respective roles of FGF2 isoforms in the effects of E2. We thus generated mice deficient only in the FGF2lmw (Fgf2lmw-/-). E2 stimulated in vivo angiogenesis and in vitro migration in endothelial cells from Fgf2lmw-/- as it did in Fgf2+/+ mice. E2 increased FGF2hmw protein abundance in endothelial cell cultures from Fgf2+/+ and Fgf2lmw-/- mice. As shown using siRNA transfection, these effects were FGFR independent but involved FGF2-Interacting Factor, an intracellular FGF2hmw partner. This is the first report for a physiological role for the intracellular FGF2hmw found to mediate the effect of E2 on endothelial cell migration via an intracrine action.

Estrogens and atherosclerosis

Numerous epidemiological as well as experimental studies have suggested that estradiol (E2) prevents atherosclerosis development. However two controlled prospective and randomized studies in women using hormone replacement therapy (HRT) did not confirm this beneficial effect. We then decided to use mouse models of atherosclerosis to define the possible mechanisms involved and the reasons for the discrepancy. We have shown that, although serum cholesterol decreases, this influence on lipid metabolism is negligible. Surprisingly, E2 induces an inflammatory-immune response towards a T helper cell (Th1) profile with increasing interferon-gamma production that could destabilize atheromatous plaques, and could account for the increase in the frequency of cardiovascular events in women undergoing HRT. At the level of the endothelium, E2 induces an increase in nitric oxide (NO) biodisponibility, but this phenomenon does not concern the development of fatty streaks. Nevertheless, the atheroprotective effect is apparently mediated at the level of the endothelium by a mechanism that has still to be characterized in molecular terms. These new acquisitions constitute a basis for new pharmacological developments allowing the prevention of deleterious effects and preserving the beneficial ones.

p53 directs conformational change and translation initiation blockade of human fibroblast growth factor 2 mRNA

Tumour suppressor p53 has been shown to inhibit fibroblast growth factor 2 expression post-transcriptionally in cultured cells. Here we have investigated the mechanism responsible for this post-transcriptional blockade. Deletion mutagenesis of the FGF-2 mRNA leader revealed the requirement of at least four RNA cis-acting elements to mediate the inhibitory effect of p53 in SK-Hep-1 transfected cells, suggesting the involvement of RNA secondary or tertiary structures. Recombinant wild-type, but not Ala(143) mutant p53, was able to specifically repress FGF-2 mRNA translation in rabbit reticulocyte lysate, in a dose dependent manner. Sucrose gradient experiments showed that p53 blocks translation initiation by preventing 80S ribosome formation on an mRNA bearing the FGF-2 mRNA leader sequence. Interaction of wild-type and mutant p53 with different RNAs showed no significant correlation between p53 RNA binding activity and its translational inhibiting effect. However, by checking the accessibility of the FGF-2 mRNA leader to complementary oligonucleotide probes, we showed that the binding to RNA of wild-type, but not mutant p53, induced RNA conformational changes that might be responsible for the translational blockade. This strongly suggests that p53 represses FGF-2 mRNA translation by a direct mechanism involving its nucleic acid unwinding-annealing activity.

Translation of the human c-myc P0 tricistronic mRNA involves two independent internal ribosome entry sites

The human c-myc proto-oncogene is transcribed from four alternative promoters (P0, P1, P2, and P3) giving rise to mRNAs having 5' leader sequences of various length. The c-myc P0 mRNA contains three open reading frames (ORFs), the last one encoding c-Myc1 and c-Myc2 proteins generated by alternative translation initiated at CUG and AUG codons. The middle ORF (MYCHEX1) and the 5' ORF (ORF1) code for proteins 188 and 114 amino acids in length, respectively. We and others previously identified an internal ribosome entry site (IRES) in P0 and P2 c-myc mRNAs, promoting the cap-independent translation of c-Myc1 and c-Myc2. Here, we report the presence of a second IRES (named IRES1) promoting the cap-independent translation of MYCHEX1 in c-myc P0 mRNA. Using deletion analysis, we mapped an 80-nt region essential for IRES1 activity. c-myc P0 mRNA is thus the first eukaryotic polycistronic mRNA described for which translation initiation of two different open reading frames (MYCHEX1 and c-Myc1/c-Myc2) involves internal ribosome entry.

Tumour suppressor p53 inhibits human fibroblast growth factor 2 expression by a post-transcriptional mechanism

Fibroblast growth factor-2 (FGF-2) is a powerful mitogen and angiogenic factor whose expression is strongly regulated at the translational level. The constitutive upregulation of FGF-2 isoforms in transformed cells prompted us to investigate the post-transcriptional effects of a tumour suppressor, p53, on FGF-2 expression. We show here in human primary skin fibroblasts that the cell density-dependent variation of FGF-2 mRNA translatability was inversely correlated with endogenous p53 expression. Transient cell transfection revealed an inhibitory effect of wild-type p53 on the expression of chimeric FGF--CAT proteins. RNAse mapping experiments ruled out any effect of p53 on FGF--CAT mRNA accumulation, suggesting a translational inhibition. This inhibition was mediated by the FGF-2 mRNA leader, but not by vascular endothelial growth factor or platelet derived growth factor mRNA leaders. Neither p53-like protein p73, nor p21/waf had any inhibitory activity. Furthermore a set of hot spot mutants of p53 bearing mutations in the DNA binding domain had no post-transcriptional inhibitory effect. In contrast a p53 mutant of the transactivating domain was still able to block FGF--CAT expression, indicating that the post-transcriptional activity of p53 described here was independent of the trans-activation of target genes. Such data reveal a novel mechanism by which p53 efficiently blocks the expression of a major proliferating, anti-apoptotic and angiogenic gene.

c-myc Internal ribosome entry site activity is developmentally controlled and subjected to a strong translational repression in adult transgenic mice

The expression of c-myc proto-oncogene, a key regulator of cell proliferation and apoptosis, is controlled at different transcriptional and posttranscriptional levels. In particular, the c-myc mRNA contains an internal ribosome entry site (IRES) able to promote translation initiation independently from the classical cap-dependent mechanism. We analyzed the variations of c-myc IRES activity ex vivo in different proliferating cell types, and in vivo in transgenic mice expressing a bicistronic dual luciferase construct. c-myc IRES efficiency was compared to that of encephalomyocarditis virus (EMCV) IRES under the same conditions. The c-myc IRES was active but with variable efficiency in all transiently transfected cell types; it was also active in the 11-day- old (E11) embryo and in some tissues of the E16 embryo. Strikingly, its activity was undetected or very low in all adult organs tested. In contrast, EMCV IRES was very active in most cell types ex vivo, as well as in embryonic and adult tissues. These data suggest a crucial role of IRES in the control of c-myc gene expression throughout development, either during embryogenesis where its activity might participate in cell proliferation or later on, where its silencing could contribute to the downregulation of c-myc expression, whose deregulation leads to tumor formation.

Pyrimidine tract binding protein and La autoantigen interact differently with the 5' untranslated regions of lentiviruses and oncoretrovirus mRNAs

Retrovirus genomic mRNA exhibits a several hundred nucleotides-long untranslated region (5' UTR) which encloses many control elements required for retrovirus replication. In addition, this 5' UTR contains translation regulatory elements, such as internal ribosome entry sites (IRESes) that have been described in oncoretroviruses, as well as in lentiviruses. UV cross-linking experiments suggested that the pyrimidine tract binding protein (PTB), a cellular protein known to regulate the activity of several picornaviral IRESes, binds to human T-cell leukemia virus (HTLV)-I RNA but not to lentiviral human immunodeficiency virus (HIV)-1, HIV-2 or simian immunodeficiency virus RNAs. To calculate the affinity of such RNA-protein interactions, we developed a new method based on the BIAcore technology. The absence of affinity of PTB for lentiviral RNAs was confirmed, whereas its affinity for HTLV-I RNAs was 1000-fold lower than for picornaviral RNAs. The BIAcore technology also revealed a significant affinity of the La autoantigen, previously described for its involvement in translational control of viral mRNAs, for HIV-1 and HTLV-I RNAs. Addition of recombinant PTB to in vitro translation experiments weakly enhanced translation initiation in the presence of HTLV-I IRES, suggesting that such an IRES requires additional trans-acting factors.

Alternative translation initiation of human fibroblast growth factor 2 mRNA controlled by its 3'-untranslated region involves a Poly(A) switch and a translational enhancer

Five fibroblast growth factor 2 (FGF-2) isoforms are synthesized from human FGF-2 mRNA by a process of alternative initiation of translation. The regulation of FGF-2 isoform expression by the mRNA 5823-nucleotide-long 3'-untranslated region containing eight alternative polyadenylation sites was examined. Because previous studies had shown that FGF-2 expression was regulated in primary cells but not in transformed cells, primary human skin fibroblasts were used in this study. Using an approach of cell transfection with synthetic reporter mRNAs, a novel translational enhancer (3'-TE) was identified in the 1370-nucleotide mRNA segment located upstream from the eighth poly(A) site. Deletion mutagenesis showed that the 3'-TE was composed of two domains with additive effects. The 3'-TE exhibited the unique feature of modulating the use of FGF-2 alternative initiation codons, which favored the relative expression of CUG-initiated isoforms. Interestingly, the use of an alternative polydenylation site removing the 3'-TE was detected in skin fibroblasts in response to heat shock and cell density variations. At high cell densities, 3'-TE removal was correlated with a loss of CUG-initiated FGF-2 expression. These data show that the FGF-2 mRNA 3'-untranslated region is able to modulate FGF-2 isoform expression by the coupled processes of translation activation and alternative polyadenylation.

Expression of human fibroblast growth factor 2 mRNA is post-transcriptionally controlled by a unique destabilizing element present in the 3'-untranslated region between alternative polyadenylation sites

Fibroblast growth factor 2 (FGF-2) belongs to a family of 18 genes coding for either mitogenic differentiating factors or oncogenic proteins, the expression of which must be tightly controlled. We looked for regulatory elements in the 5823-nucleotide-long 3'-untranslated region of the FGF-2 mRNA that contains eight potential alternative polyadenylation sites. Quantitative reverse transcription-polymerase chain reaction revealed that poly(A) site utilization was cell type-dependent, with the eighth poly(A) site being used (95%) in primary human skin fibroblasts, whereas proximal sites were used in the transformed cell lines studied here. We used a cell transfection approach with synthetic reporter mRNAs to localize a destabilizing element between the first and second poly(A) sites. Although AU-rich, the FGF-2-destabilizing element had unique features: it involved a 122-nucleotide direct repeat, with both elements of the repeat being required for the destabilizing activity. These data show that short stable FGF-2 mRNAs are present in transformed cells, whereas skin fibroblasts contain mostly long unstable mRNAs, suggesting that FGF-2 mRNA stability cannot be regulated in transformed cells. The results also provide evidence of a multilevel post-transcriptional control of FGF-2 expression; such a stringent control prevents FGF-2 overexpression and permits its expression to be enhanced only in relevant physiological situations.

A new 34-kilodalton isoform of human fibroblast growth factor 2 is cap dependently synthesized by using a non-AUG start codon and behaves as a survival factor

Four isoforms of human fibroblast growth factor 2 (FGF-2) result from alternative initiations of translation at three CUG start codons and one AUG start codon. Here we characterize a new 34-kDa FGF-2 isoform whose expression is initiated at a fifth initiation codon. This 34-kDa FGF-2 was identified in HeLa cells by using an N-terminal directed antibody. Its initiation codon was identified by site-directed mutagenesis as being a CUG codon located at 86 nucleotides (nt) from the FGF-2 mRNA 5' end. Both in vitro translation and COS-7 cell transfection using bicistronic RNAs demonstrated that the 34-kDa FGF-2 was exclusively expressed in a cap-dependent manner. This contrasted with the expression of the other FGF-2 isoforms of 18, 22, 22.5, and 24 kDa, which is controlled by an internal ribosome entry site (IRES). Strikingly, expression of the other FGF-2 isoforms became partly cap dependent in vitro in the presence of the 5,823-nt-long 3' untranslated region of FGF-2 mRNA. Thus, the FGF-2 mRNA can be translated both by cap-dependent and IRES-driven mechanisms, the balance between these two mechanisms modulating the ratio of the different FGF-2 isoforms. The function of the new FGF-2 was also investigated. We found that the 34-kDa FGF-2, in contrast to the other isoforms, permitted NIH 3T3 cell survival in low-serum conditions. A new arginine-rich nuclear localization sequence (NLS) in the N-terminal region of the 34-kDa FGF-2 was characterized and found to be similar to the NLS of human immunodeficiency virus type 1 Rev protein. These data suggest that the function of the 34-kDa FGF-2 is mediated by nuclear targets.

Cell transformation results in the loss of the density-dependent translational regulation of the expression of fibroblast growth factor 2 isoforms

Alternative initiation of translation at three CUG and one AUG start codons leads to the synthesis of four isoforms of fibroblast growth factor 2 (FGF-2) that have distinct intracellular localizations and affect the cell phenotype differently. We show here that the expression of FGF-2 CUG-initiated isoforms decreases in a cell-density-dependent manner in normal human skin fibroblasts (HSFs) concomitantly with the FGF-2 mRNA level. In contrast, CUG-initiated FGF-2 expression is constitutive in SK-HEP-1 cells and in HSFs transformed with SV40 large T antigen. Cell transfection using a plasmid containing the FGF-2 mRNA leader fused to chloramphenicol acetyl transferase demonstrated that up-regulation of the CUG codons depends on cis-elements located in this leader. Furthermore, UV cross-linking experiments revealed a correlation between CUG codons utilization and the binding of several proteins to the mRNA leader. On the basis of the presence of an internal ribosome entry site (IRES) in the FGF-2 mRNA, we used bicistronic vectors to transfect normal and transformed cells. The density-dependent regulation in normal HSFs was cap-dependent, whereas the constitutive CUG-initiated FGF-2 expression in transformed cells occurred essentially by an IRES-dependent mechanism. Unexpectedly, the use of the AUG start codon occurred exclusively by internal entry, which suggests the presence of a second independent IRES in the FGF-2 mRNA that would be constitutive. A study of the eIF-4E levels and of the 4E-BP1 phosphorylation state at increasing cell densities showed a decrease of the eIF-4E level, concomitant with 4E-BP1 dephosphorylation in normal cells but not in transformed cells. These data point out a complex mechanism for the regulation of FGF-2 isoforms expression involving both the cap-dependent and the cap-independent initiation of translation and favor a positive role of CUG-initiated FGF-2 in cellular proliferation and transformation.

Two independent internal ribosome entry sites are involved in translation initiation of vascular endothelial growth factor mRNA

The mRNA of vascular endothelial growth factor (VEGF), the major angiogenic growth factor, contains an unusually long (1,038 nucleotides) and structured 5' untranslated region (UTR). According to the classical translation initiation model of ribosome scanning, such a 5' UTR is expected to be a strong translation inhibitor. In vitro and bicistronic strategies were used to show that the VEGF mRNA translation was cap independent and occurred by an internal ribosome entry process. For the first time, we demonstrate that two independent internal ribosome entry sites (IRESs) are present in this 5' UTR. IRES A is located within the 300 nucleotides upstream from the AUG start codon. RNA secondary structure prediction and site-directed mutagenesis allowed the identification of a 49-nucleotide structural domain (D4) essential to IRES A activity. UV cross-linking experiments revealed that IRES A activity was correlated with binding of a 100-kDa protein to the D4 domain. IRES B is located in the first half of the 5' UTR. An element between nucleotides 379 and 483 is required for its activity. Immunoprecipitation experiments demonstrated that a main IRES B-bound protein was the polypyrimidine tract binding protein (PTB), a well-known regulator of picornavirus IRESs. However, we showed that binding of the PTB on IRES B does not seem to be correlated with its activity. Evidence is provided of an original cumulative effect of two IRESs, probably controlled by different factors, to promote an efficient initiation of translation at the same AUG codon.

Alternative translation of the proto-oncogene c-myc by an internal ribosome entry site

The human proto-oncogene c-myc encodes two proteins, c-Myc1 and c-Myc2, from two initiation codons, CUG and AUG, respectively. It is also transcribed from four alternative promoters (P0, P1, P2, and P3), giving rise to different RNA 5'-leader sequences, the long sizes of which suggest that they must be inefficiently translated by the classical ribosome scanning mechanism. Here we have examined the influence of three c-myc mRNA 5'-leaders on the translation of chimeric myc-CAT mRNAs. We observed that in the reticulocyte rabbit lysate, these 5'-leaders lead to cap-independent translation initiation. To determine whether this kind of initiation resulted from the presence of an internal ribosome entry site (IRES), COS-7 cells were transfected with bicistronic vectors containing the different c-myc 5'-leaders in the intercistronic region. An IRES was identified, requiring elements located within the P2 leader, between nucleotides -363 and -94 upstream from the CUG start codon. This is the first demonstration of the existence of IRES-dependent translation for a proto-oncogene. This IRES could be a translation enhancer, allowing activation of c-myc expression under the control of trans-acting factors and in response to specific cell stimuli.

Translation of CUG- but not AUG-initiated forms of human fibroblast growth factor 2 is activated in transformed and stressed cells

Four isoforms of the human fibroblast growth factor 2 (FGF-2), with different intracellular localizations and distinct effects on cell phenotype, result from alternative initiations of translation at three CUG and one AUG start codons. We showed here by Western immunoblotting and immunoprecipitation that the CUG-initiated forms of FGF-2 were synthesized in transformed cells, whereas "normal" cells almost exclusively produced the AUG-initiated form. CUG-initiated FGF-2 was induced in primary skin fibroblasts in response to heat shock and oxidative stress. In transformed cells and in stressed fibroblasts, CUG expression was dependent on cis-elements within the 5' region of FGF-2 mRNA and was not correlated to mRNA level, indicating a translational regulation. UV cross-linking experiments revealed that CUG expression was linked to the binding of several cellular proteins to FGF-2 mRNA 5' region. Since translation of FGF-2 mRNA was previously shown to occur by internal ribosome entry, a nonclassical mechanism already described for picornaviruses, the cross-linking patterns of FGF-2 and picornavirus mRNAs were compared. Comigration of several proteins, including a p60, was observed. However, this p60 was shown to be different from the p57/PTB internal entry factor, suggesting a specificity towards FGF-2 mRNA. We report here a process of translational activation of the FGF-2 CUG-initiated forms in direct relation with trans-acting factors specific to transformed and stressed cells. These data favor a critical role of CUG-initiated FGF-2 in cell transformation and in the stress response.

Relief of ornithine decarboxylase messenger RNA translational repression induced by alternative splicing of its 5' untranslated region

The ornithine decarboxylase enzyme (ODC) is the key regulator of polyamine synthesis and is a member of the cellular proto-oncogene family. Its expression becomes constitutively activated by carcinogens, viruses, and oncogenes. ODC mRNA has a long 5' untranslated region that could be important in the regulation of enzyme levels by affecting translation. To test this hypothesis, we have determined the role of this region on the constitutive ODC hyperexpression measured in AR4-2J cells, an azaserine-induced, tumor-derived pancreatic acinar cell line. Construction of expression vectors in which ODC 5' leader sequence was placed flanking the chloramphenicol acetyltransferase reporter gene allowed us to identify three AR4-2J specific, different alternatively spliced ODC 5' leaders. The 5' ends of exons 2 and 3 were lengthened by 17 and 13 bases, respectively. Translation performed in a cell-free system as well as in COS7 transient transfection experiments demonstrated that AR4-2J isoforms induce a strong increase in the rate of translation. These results provide evidence that alternative splicing observed in tumoral cells, coupled with translation regulation, relieves the translation repression mediated by the long and structured 5' untranslated region of the ODC proto-oncogene.

Alternative translation initiation of the Moloney murine leukemia virus mRNA controlled by internal ribosome entry involving the p57/PTB splicing factor

Moloney murine leukemia virus (Mo-MuLV) genomic mRNA codes for two gag precursors by alternative initiations of translation. An AUG codon governs the synthesis of the retroviral capsid proteins precursor, whereas a CUG codon directs the synthesis of a glycosylated cell surface antigen, the gross cell surface antigen. Control of the relative synthesis of the two precursors is crucial for MuLV infectivity and pathology. Furthermore, the MuLV mRNA leader sequence is very long and should inhibit translation according to the classical scanning model. This suggests a different translation initiation mechanism allowing gag efficient expression. We demonstrate, by using bicistronic vectors expressed in COS-7 cells, that the Mo-MuLV mRNA leader drives translation initiation by internal ribosome entry. We have localized the internal ribosome entry site (IRES) between the two initiation codons. This 126 nucleotide long IRES implies an oligopyrimidine tract located 45 nucleotides upstream of AUG codon. UV cross-linking and affinity chromatography experiments show that the PTB/p57 splicing factor specifically interacts with this oligopyrimidine tract. The MuLV IRES controls alternative translation initiation by activating the capsid protein precursor expression. This gag translational enhancer could exist in other retroviruses.

Alternative translation of human fibroblast growth factor 2 mRNA occurs by internal entry of ribosomes

Alternative initiations of translation of the human fibroblast growth factor 2 (FGF-2) mRNA, at three CUG start codons and one AUG start codon, result in the synthesis of four isoforms of FGF-2. This process has important consequences on the fate of FGF-2: the CUG-initiated products are nuclear and their constitutive expression is able to induce cell immortalization, whereas the AUG-initiated product, mostly cytoplasmic, can generate cell transformation. Thus, the different isoforms probably have distinct targets in the cell. We show here that translation initiation of the FGF-2 mRNA breaks the rule of the cap-dependent ribosome scanning mechanism. First, translation of the FGF-2 mRNA was shown to be cap independent in vitro. This cap-independent translation required a sequence located between nucleotides (nt) 192 and 256 from the 5' end of the 318-nt-long 5' untranslated region. Second, expression of bicistronic vectors in COS-7 cells indicated that the FGF-2 mRNA is translated through a process of internal ribosome entry mediated by the mRNA leader sequence. By introducing additional AUG codons into the RNA leader sequence, we localized an internal ribosome entry site to between nt 154 and 318 of the 5' untranslated region, just upstream of the first CUG. The presence of an internal ribosome entry site in the FGF-2 mRNA suggests that the process of internal translation initiation, by controlling the expression of a growth factor, could have a crucial role in the control of cell proliferation and differentiation.

A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag- mutation in the Friend murine leukemia virus (F-MuLV). F-MuLV induces severe early hemolytic anemia and subsequent erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

Antiestrogens inhibit the replication of the retroviral Moloney murine leukemia virus in vitro

Widely used in breast cancer therapy, tamoxifen exhibits in vitro and in vivo pleiotropic activities that are generally attributed to its binding to the estrogen receptor. However, several reports have shown that the antiestrogen binding site (ABS) is also an intracellular target of the drug. This dual affinity determines at least two modes of action for the triphenylethylenic antiestrogens; one would be estrogen reversible and the other irreversible. Here, tamoxifen is shown to inhibit the production of Moloney murine leukemia virus virions by fibroblastic A9 cells, in which estrogen receptor is not detectable either by binding or by radioimmunoassay. Moreover, a specific ligand of the ABS induces effects equivalent to those of tamoxifen, suggesting that tamoxifen inhibits Moloney murine leukemia virus replication through an estrogen-independent pathway involving the ABS.

cis-acting elements involved in the alternative translation initiation process of human basic fibroblast growth factor mRNA

Four forms of basic fibroblast growth factor (bFGF) are synthesized from the same mRNA, resulting from alternative initiations of translation at three CUG start codons and one AUG start codon. The CUG- and AUG-initiated forms have distinct intracellular localizations and can modify cell phenotypes differently, indicating that control of the alternative expression of the different forms of bFGF has an important impact on the cell. In this study, we investigated the roles of the mRNA 5' untranslated region and the alternatively translated region located between the CUG and AUG codons in the regulation of alternative translation of the different forms of bFGF. Deletions and site-directed mutagenesis were carried out in bFGF mRNA leader, and translation was studied in vitro and in vivo. The results enabled us to identify five cis-acting RNA elements (two in the 5' untranslated region and three in the alternatively translated region) involved in the regulation of either global or alternative initiation of translation. Each of these elements had a specific effect on the level of synthesis of the different forms of bFGF. Furthermore, we showed that the 5' untranslated region regulatory elements had different effects on bFGF translation, depending on the translation system used. These results suggest that bFGF translation is modulated by cis-acting elements corresponding to secondary or tertiary RNA structures, which could be the targets of cell-specific trans-acting factors.

Viral RNA annealing activities of the nucleocapsid protein of Moloney murine leukemia virus are zinc independent

The zinc fingers of retroviral gag nucleocapsid proteins (NC) are required for the specific packaging of the dimeric RNA genome into virions. In vitro, NC proteins activate both dimerization of viral RNA and annealing of the replication primer tRNA onto viral RNA, two reactions necessary for the production of infectious virions. In this study the role of the zinc finger of Moloney murine leukemia virus (MoMuLV) NCp10 in RNA binding and annealing activities was investigated through modification or replacement of residues involved in zinc coordination. These alterations did not affect the ability of NCp10 to bind RNA and promote RNA annealing in vitro, despite a complete loss of zinc affinity. However mutation of two conserved lysine residues adjacent to the finger motif reduced both RNA binding and annealing activities of NCp10. These findings suggest that the complexed NC zinc finger is not directly involved in RNA-protein interactions but more probably in a zinc dependent conformation of NC protein modulating viral protein-protein interactions, essential to the process of viral RNA selection and virion assembly. Then the NC zinc finger may cooperate to select the viral RNA genome to be packaged into virions.

cis elements and trans-acting factors involved in dimer formation of murine leukemia virus RNA

The genetic material of all retroviruses examined so far consists of two identical RNA molecules joined at their 5' ends by the dimer linkage structure (DLS). Since the precise location of the DLS as well as the mechanism and role(s) of RNA dimerization remain unclear, we analyzed the dimerization process of Moloney murine leukemia virus (MoMuLV) genomic RNA. For this purpose we derived an in vitro model for RNA dimerization. By using this model, murine leukemia virus RNA was shown to form dimeric molecules. Deletion mutagenesis in the 620-nucleotide leader of MoMuLV RNA showed that the dimer promoting sequences are located within the encapsidation element Psi between positions 215 and 420. Furthermore, hybridization assays in which DNA oligomers were used to probe monomer and dimer forms of MoMuLV RNA indicated that the DLS probably maps between positions 280 and 330 from the RNA 5' end. Also, retroviral nucleocapsid protein was shown to catalyze dimerization of MoMuLV RNA and to be tightly bound to genomic dimer RNA in virions. These results suggest that MoMuLV RNA dimerization and encapsidation are probably controlled by the same cis element, Psi, and trans-acting factor, nucleocapsid protein, and thus might be linked during virion formation.

High molecular mass forms of basic fibroblast growth factor are initiated by alternative CUG codons

A 6.75-kilobase human hepatoma-derived basic fibroblast growth factor (bFGF) cDNA was cloned and sequenced. An amino-terminal sequence generated from a purified hepatoma bFGF was found to correspond to the nucleotide sequence and to begin 8 amino acids upstream from the putative methionine start codon thought to initiate a 154-amino acid bFGF translation product. This sequence suggests that a form of bFGF of at least 163 amino acids exists. The hepatoma cDNA was transcribed in vitro into RNA; in vitro translation of this RNA generated three forms of bFGF with molecular masses of 18, 21, and 22.5 kDa. By use of in vitro mutagenesis, it was found that the 22.5-kDa bFGF and possibly the 21-kDa form were initiated with CUG start codons. The 18-kDa bFGF was initiated with an AUG codon. By transfecting into COS cells human hepatoma bFGF cDNA and a construct from which the AUG initiator was eliminated, it was found that the higher molecular mass forms of bFGF were as biologically active as the 18-kDa form.

CUG initiation codon used for the synthesis of a cell surface antigen coded by the murine leukemia virus

Murine leukemia virus (MuLV) codes for two precursors of the group-specific antigens, Pr65gag and Pr75gag, in vivo. While Pr65gag is the precursor to the virion structural proteins, Pr75gag undergoes glycosylation and is found on the surface of the infected cell as gp85gag, and it is thought to play a role in virus maturation and spread. Pr65gag synthesis starts at an AUG codon within a favourable initiation context (AAUAUGG at positions 618 to 624). The gp85gag start codon is upstream but its precise location is not known. To map the initiation codon of gp85gag, we used deletion and site-directed mutagenesis of the leader sequence of MuLV RNA and in vitro translation of the RNAs. Synthesis of the MuLV gp85gag protein appears to be initiated at a CUG codon located within a favourable context (ACCCUGG at positions 354 to 359 for Moloney-MuLV). The possible function of gp85gag was investigated by expressing Moloney-MuLV and Friend-MuLV proviral DNA and mutants deficient for gp85gag synthesis in mouse and rat cells. The results indicate that the gp85gag protein probably facilitates the spread of virus infection in tissue culture.

Small finger protein of avian and murine retroviruses has nucleic acid annealing activity and positions the replication primer tRNA onto genomic RNA

Retrovirus virions carry a diploid genome associated with a large number of small viral finger protein molecules which are required for encapsidation. Our present results show that finger protein p12 of Rous sarcoma virus (RSV) and p10 of murine leukaemia virus (MuLV) positions replication primer tRNA on the replication initiation site (PBS) at the 5' end of the RNA genome. An RSV mutant with a Val-Pro insertion in the finger motif of p12 is able to partially encapsidate genomic RNA but is not infectious because mutated p12 is incapable of positioning the replication primer, tRNATrp. Since all known replication competent retroviruses, and the plant virus CaMV, code for finger proteins analogous to RSV p12 or MuLV p10, the initial stage of reverse transcription in avian, mammalian and human retroviruses and in CaMV is probably controlled in an analogous way.