The 5' untranslated sequence of the c-sis/platelet-derived growth factor 2 transcript is a potent translational inhibitor

Effects of long 5' leader sequences on initiation by eukaryotic ribosomes in vitro

Lengthening the 5' noncoding sequence on SP6-derived transcripts can increase their translational efficiency by an order of magnitude under some conditions of translation in reticulocyte lysates. This effect was observed upon reiterating three different synthetic oligonucleotides, the sequences of which were designed simply to preclude secondary structure. It seems unlikely that such arbitrarily designed sequences are recognized by sequence-specific translational enhancer proteins. Rather, long 5' leader sequences appear to accumulate extra 40S ribosomal subunits, which may account for their translational advantage. The buildup of 40S subunits on long, unstructured leader sequences is predicted by the scanning model for initiation. Leader sequences such as these may be ideal for in vitro expression vectors.

Involvement of platelet-derived growth factor ligands and receptors in tumorigenesis

Platelet-derived growth factor (PDGF) isoforms and their receptors have important roles during embryogenesis, particularly in the development of various mesenchymal cell types in different organs. In the adult, PDGF stimulates wound healing and regulates tissue homeostasis. However, overactivity of PDGF signalling is associated with malignancies and other diseases characterized by excessive cell proliferation, such as fibrotic conditions and atherosclerosis. In certain tumours, genetic or epigenetic alterations of the genes for PDGF ligands and receptors drive tumour cell proliferation and survival. Examples include the rare skin tumour dermatofibrosarcoma protuberance, which is driven by autocrine PDGF stimulation due to translocation of a PDGF gene, and certain gastrointestinal stromal tumours and leukaemias, which are driven by constitute activation of PDGF receptors due to point mutations and formation of fusion proteins of the receptors, respectively. Moreover, PDGF stimulates cells in tumour stroma and promotes angiogenesis as well as the development of cancer-associated fibroblasts, both of which promote tumour progression. Inhibitors of PDGF signalling may thus be of clinical usefulness in the treatment of certain tumours.

Antisense oligonucleotides targeting translation inhibitory elements in 5' UTRs can selectively increase protein levels

A variety of diseases are caused by deficiencies in amounts or activity of key proteins. An approach that increases the amount of a specific protein might be of therapeutic benefit. We reasoned that translation could be specifically enhanced using trans-acting agents that counter the function of negative regulatory elements present in the 5' UTRs of some mRNAs. We recently showed that translation can be enhanced by antisense oligonucleotides (ASOs) that target upstream open reading frames. Here we report the amount of a protein can also be selectively increased using ASOs designed to hybridize to other translation inhibitory elements in 5' UTRs. Levels of human RNASEH1, LDLR, and ACP1 and of mouse ACP1 and ARF1 were increased up to 2.7-fold in different cell types and species upon treatment with chemically modified ASOs targeting 5' UTR inhibitory regions in the mRNAs encoding these proteins. The activities of ASOs in enhancing translation were sequence and position dependent and required helicase activity. The ASOs appear to improve the recruitment of translation initiation factors to the target mRNA. Importantly, ASOs targeting ACP1 mRNA significantly increased the level of ACP1 protein in mice, suggesting that this approach has therapeutic and research potentials.

Translational up-regulation and high-level protein expression from plasmid vectors by mTOR activation via different pathways in PC3 and 293T cells

BACKGROUND

Though 293T cells are widely used for expression of proteins from transfected plasmid vectors, the molecular basis for the high-level expression is yet to be understood. We recently identified the prostate carcinoma cell line PC3 to be as efficient as 293T in protein expression. This study was undertaken to decipher the molecular basis of high-level expression in these two cell lines.

METHODOLOGY/PRINCIPAL FINDINGS

In a survey of different cell lines for efficient expression of platelet-derived growth factor-B (PDGF-B), β-galactosidase (β-gal) and green fluorescent protein (GFP) from plasmid vectors, PC3 was found to express at 5-50-fold higher levels compared to the bone metastatic prostate carcinoma cell line PC3BM and many other cell lines. Further, the efficiency of transfection and level of expression of the reporters in PC3 were comparable to that in 293T. Comparative analyses revealed that the high level expression of the reporters in the two cell lines was due to increased translational efficiency. While phosphatidic acid (PA)-mediated activation of mTOR, as revealed by drastic reduction in reporter expression by n-butanol, primarily contributed to the high level expression in PC3, multiple pathways involving PA, PI3K/Akt and ERK1/2 appear to contribute to the abundant reporter expression in 293T. Thus the extent of translational up-regulation attained through the concerted activation of mTOR by multiple pathways in 293T could be achieved through its activation primarily by the PA pathway in PC3.

CONCLUSIONS/SIGNIFICANCE

Our studies reveal that the high-level expression of proteins from plasmid vectors is effected by translational up-regulation through mTOR activation via different signaling pathways in the two cell lines and that PC3 is as efficient as 293T for recombinant protein expression. Further, PC3 offers an advantage in that the level of expression of the protein can be regulated by simple addition of n-butanol to the culture medium.

Glioblastoma subclasses can be defined by activity among signal transduction pathways and associated genomic alterations

BACKGROUND

Glioblastoma multiforme (GBM) is an umbrella designation that includes a heterogeneous group of primary brain tumors. Several classification strategies of GBM have been reported, some by clinical course and others by resemblance to cell types either in the adult or during development. From a practical and therapeutic standpoint, classifying GBMs by signal transduction pathway activation and by mutation in pathway member genes may be particularly valuable for the development of targeted therapies.

METHODOLOGY/PRINCIPAL FINDINGS

We performed targeted proteomic analysis of 27 surgical glioma samples to identify patterns of coordinate activation among glioma-relevant signal transduction pathways, then compared these results with integrated analysis of genomic and expression data of 243 GBM samples from The Cancer Genome Atlas (TCGA). In the pattern of signaling, three subclasses of GBM emerge which appear to be associated with predominance of EGFR activation, PDGFR activation, or loss of the RAS regulator NF1. The EGFR signaling class has prominent Notch pathway activation measured by elevated expression of Notch ligands, cleaved Notch receptor, and downstream target Hes1. The PDGF class showed high levels of PDGFB ligand and phosphorylation of PDGFRbeta and NFKB. NF1-loss was associated with lower overall MAPK and PI3K activation and relative overexpression of the mesenchymal marker YKL40. These three signaling classes appear to correspond with distinct transcriptomal subclasses of primary GBM samples from TCGA for which copy number aberration and mutation of EGFR, PDGFRA, and NF1 are signature events.

CONCLUSIONS/SIGNIFICANCE

Proteomic analysis of GBM samples revealed three patterns of expression and activation of proteins in glioma-relevant signaling pathways. These three classes are comprised of roughly equal numbers showing either EGFR activation associated with amplification and mutation of the receptor, PDGF-pathway activation that is primarily ligand-driven, or loss of NF1 expression. The associated signaling activities correlating with these sentinel alterations provide insight into glioma biology and therapeutic strategies.

Thrombin-induced cell proliferation and platelet-derived growth factor-AB release from A172 human glioblastoma cells

BACKGROUND

In a previous study, we found that thrombin induced proliferation of TM-1 and T98G human glioma cells and that the mitogenic effect was abolished by hirudin.

OBJECTIVES

We investigated thrombin's effects on the proliferation of A172 human glioblastoma cells and the induction of growth factors. Furthermore, we examined whether or not the expression of heparin cofactor II (HCII) in A172 cells using adenovirus vector could suppress thrombin's effects.

METHODS

The effect of thrombin on cell proliferation was assessed using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay. The amount of growth factors in the conditioned medium was measured by enzyme-linked immunosorbent assay. The level of platelet-derived growth factor (PDGF)-B mRNA was assessed by reverse transcriptase-polymerase chain reaction analysis.

RESULTS

Thrombin-induced proliferation of A172 cells primarily depended on the enhanced secretion of PDGF-AB by thrombin. The action of thrombin depended on its proteolytic activity. However, thrombin-induced PDGF-AB secretion was not abolished by anti-protease-activated receptor (PAR) antibody. The PAR-1 agonist peptide had no effect on cell growth and PDGF-AB levels. Thrombin did not increase PDGF-B gene expression. Expression of HCII effectively suppressed thrombin-induced PDGF-AB release.

CONCLUSIONS

These results indicate that thrombin may play an important role in the proliferation of A172 cells by inducing PDGF-AB secretion and that thrombin's action is mediated by its proteolytic activity. Inhibition of thrombin's proteolytic activity may be a new therapeutic method for gliomas.

Structural and functional aspects of platelet-derived growth factor and its receptors

Platelet-derived growth factor (PDGF) is a dimeric molecule that exists as homodimers or heterodimers of related polypeptide chains (A and B). Two types of PDGF receptor have been identified. The PDGF alpha-receptor binds all three isoforms with high affinity whereas the beta-receptor binds only PDGF-BB with high affinity, PDGF-AB with low affinity and does not appear to bind PDGF-AA. The alpha- and beta-receptors are structurally related, each having an intracellular protein tyrosine kinase domain. Ligand-induced functional activation of the receptors appears to involve receptor dimerization. Binding of PDGF to its receptor is followed by internalization and degradation of the ligand-receptor complex. Experiments with mutant receptors have shown that ligand-induced internalization is not absolutely dependent on the kinase activity of the beta-receptor. The v-sis oncogene of simian sarcoma virus (SSV) is a retroviral version of the PDGF B chain gene and SSV-transformation is mediated by an autocrine PDGF-like growth factor. Formal evidence that the expression of the PDGF beta-receptor is sufficient to confer susceptibility to SSV-transformation has been obtained using porcine endothelial cells expressing a recombinant human beta-receptor. PDGF is a chemotactic agent for several cell types. Recent experiments have shown that the PDGF beta-receptor mediates a chemotactic response and that this effect requires an intact protein tyrosine kinase activity.

Sarcomas: genetics, signalling, and cellular origins. Part 2: TET-independent fusion proteins and receptor tyrosine kinase mutations

Although the mechanisms that underlie sarcoma development are still poorly understood, the identification of non-random chromosomal translocations and receptor tyrosine kinase mutations associated with defined sarcoma types has provided new insight into the pathogenesis of these tumours. In Part 1 of the review (J Pathol 2007;213:4-20), we addressed sarcomas that express fusion genes containing TET gene family products. Part 2 of the review summarizes our current understanding of the implications of fusion genes that do not contain TET family members in sarcoma development, as well as that of specific mutations in genes encoding receptor tyrosine kinases (RTKs). The final section will serve as a summary of both reviews and will attempt to provide a synthesis of some of the emerging principles of sarcomagenesis.

Dose-Dependent Effects of Platelet-Derived Growth Factor-B on Glial Tumorigenesis

Platelet-derived growth factor (PDGF) is expressed in many different tumors, but its precise roles in tumorigenesis remain to be fully defined. Here, we report on a mouse model that demonstrates dose-dependent effects of PDGF-B on glial tumorigenesis. By removing inhibitory regulatory elements in the PDGFB mRNA, we are able to substantially elevate its expression in tumor cells using a retroviral delivery system. This elevation in PDGF-B production results in tumors with shortened latency, increased cellularity, regions of necrosis, and general high-grade character. In addition, elevated PDGF-B in these tumors also mediates vascular smooth muscle cell recruitment that supports tumor angiogenesis. PDGF receptor (PDGFR) signaling appears to be required for the maintenance of these high-grade characteristics, because treatment of high-grade tumors with a small molecule inhibitor of PDGFR results in reversion to a lower grade tumor histology. Our data show that PDGFR signaling quantitatively regulates tumor grade and is required to sustain high-grade oligodendrogliomas.

Biology of platelet-derived growth factors in development

Platelet-derived growth factor (PDGF) was one of the first growth factors to be characterized, and the PDGF family of ligand and receptors has remained an archetype system for studies of the mechanisms of action of growth factors and receptor tyrosine kinases for more than two decades. The small size of the family has also facilitated genetic studies and, in particular, manipulations of the mouse PDGF and PDGF receptor genes have given important insights into the role of this family during mammalian development. These studies have shown that discrete populations of mesenchymal and neuroectodermal progenitor cells depend on PDGF signaling for their growth and distribution within developing organs. Other studies suggest that the same, or similar, cells may be targeted by exaggerated PDGF signaling in a number of pathological processes, including different types of cancer. The present review summarizes current views on the roles of PDGFs in developmental processes, and discusses the critical importance of the amount, spatial distribution, and bioavailability of the PDGF proteins for acquisition of the correct number and location of target cells.

Analysis of gene mutations in three cases of dermatofibrosarcoma protuberans (DFSP): ordinary DFSP, DFSP with fibrosarcomatous lesion (DFSP-FS) and lung metastasis of DFSP-FS

BACKGROUND

Fusion of the collagen type I alpha 1 (COL1A1) gene with the platelet-derived growth factor B-chain (PDGFB) gene has been pointed out in dermatofibrosarcoma protuberans (DFSP). Various exons of the COL1A1 gene have been shown to be involved in the fusion with exon 2 of the PDGFB gene.

OBJECTIVE

We studied the breakpoints of the COL1A1 gene using the tumor specimens from three patients with DFSP.

METHODS

Reverse transcriptase-polymerase chain reaction (PCR) was performed using cultured DFSP tumor cells or frozen tissue. Nucleotide sequence analysis was carried out using the PCR products to identify the breakpoints.

RESULTS

Cases 1, 2 and 3 were diagnosed as ordinary DFSP, DFSP with fibrosarcomatous lesion (DFSP-FS) and lung metastasis of DFSP-FS, respectively. The COL1A1-PDGFB fusion transcripts were detected from the tumor specimens. Sequence analysis revealed that the ends of exons 42, 29 and 38 in the COL1A1 gene were fused with the start of exon 2 in the PDGFB gene in case 1, 2 and 3, respectively.

CONCLUSION

This study identified a novel COL1A1 breakpoint, namely, exon 42 of the COL1A1 gene. Detection of the aberrant fusion transcript seems to be useful at differential diagnosis both in primary and metastatic lesions.

Tight control of platelet-derived growth factor B/c-sis expression by interplay between the 5'-untranslated region sequence and the major upstream promoter

The long and GC-rich 5'-untranslated region (5'-UTR) of the known 3.8-kb platelet-derived growth factor B (PDGF-B)/c-sis mRNA is highly conserved and inhibits its own translation. It has been thought that this 5'-UTR functions by regulating translation possibly using an internal ribosome entry site (IRES)-mediated mechanism. However, in the present study we found no evidence that the 5'-UTR sequence of PDGF-B mRNA contains any IRES activity. Instead, we found that the 5'-UTR sequence of PDGF-B functions as a promoter both constitutively and upon induction in a variety of cell lines. The 5'-UTR sequence contains two promoters (termed P1 and P2) when only the 5'-UTR sequence is analyzed. In the presence of the upstream TATA-box-containing promoter (P0), P1 and P0 promoters are integrated into one promoter, whereas the P2 promoter still functions. The full promoter with combined P0, P1, and P2 produced two transcripts, with the major one having the full-length 5'-UTR and the minor one the short 5'-UTR. The integrated P0/P1 promoter and P2 promoter are likely responsible for producing the endogenous 3.8- and 2.8-kb PDGF-B mRNAs that are detected in cultured human renal microvascular endothelial cells, a few tumor cells, and rat brain tissues. Furthermore, we detected the 2.8-kb PDGF-B mRNA in erythroleukemia K562 cells upon 12-O-tetradecanoylphorbol-13-acetate-induced differentiation. Considering that the 5'-UTR in the 3.8-kb mRNA contains no IRES activity and inhibits cap-dependent translation, we believe that the endogenous 2.8-kb mRNA produced from the 5'-UTR promoter is likely the major template responsible for protein production both constitutively and upon induction. We also found that the transcription from the 5'-UTR P2 promoter might be coordinated by the major upstream P0 promoter upon stimulation. Based on these observations, we propose that the TATA-containing P0 promoter and the 5'-UTR promoter work together to tightly control the expression of PDGF-B.

IL-18 translational inhibition restricts IFN-gamma expression in crescentic glomerulonephritis

BACKGROUND

Interleukin-18 (IL-18), a potent inducer of interferon gamma (IFN-gamma) production, is a cytokine involved in the cell-mediated immune response that is expressed during inflammatory and pathologic conditions. IFN-gamma plays a role in the development of some models of glomerulonephritis (GN); however, the role of IL-18 in the production of IFN-gamma during these pathologies has not been studied.

METHODS

Rat IL-18 cDNA was isolated and the regulation of IL-18 gene expression was studied. IFN-gamma and IL-18 expression were determined in anti-glomerular basement membrane (GBM) antibody (Ab)-induced GN. Recombinant active IL-18 (rIL-18) was used to further identify its effect on IFN-gamma production during this GN. Glomerular injury and levels of IFN-gamma were assayed in Wistar Kyoto (WKY) rats with anti-GBM GN in the presence or absence of rIL-18.

RESULTS

Rat IL-18, similar to the mouse clone, requires processing by the IL-1beta converting enzyme to become activated. A rat IL-18 5'-untranslated region (UTR) translational inhibitor was identified that strongly inhibited the synthesis of IL-18. This translational inhibitor with different lengths (180 and 130 bp) was highly expressed during GN and correlated with minimal IFN-gamma mRNA expression. Injection of recombinant active IL-18 in WKY rats with anti-GBM GN was associated with an increase of glomerular IFN-gamma levels, proliferating cell nuclear antigen (PCNA)-ED1+ cells, and PCNA-CD8+ cells, with worsening of glomerular injury.

CONCLUSION

These data suggest that the translational control of IL-18 expression by its 5'-UTR limits the production of IL-18, resulting in restricted expression of mRNA and protein IFN-gamma in this model of GN. Furthermore, it was suggested that possible IL-18/IFN-gamma induction of local proliferation of macrophages and CD8+ cells might be an important mechanism for amplifying CD8+-mediated macrophage-dependent GN.

Dermatofibrosarcoma protuberans with COL1A1 (exon 18) -PDGFB (exon 2) fusion transcript

BACKGROUND

Fusion of the collagen type I alpha 1 (COL1A1) gene with the platelet-derived growth factor B-chain (PDGFB) gene has been described in dermatofibrosarcoma protuberans (DFSP). Various exons of the COL1A1 gene have been shown to be involved in the fusion with exon 2 of the PDGFB gene. Objectives We examined the breakpoint of the COL1A1 gene using the tumour specimen from the patient with DFSP.

METHODS

Reverse transcriptase-polymerase chain reaction (PCR) was performed using cultured DFSP tumour cells. Nucleotide sequence analysis was carried out using the PCR product to identify the breakpoint.

RESULTS

The COL1A1-PDGFB fusion transcript was detected from the tumour specimen. Sequence analysis revealed that exon 18 of the COL1A1 gene was fused with exon 2 of the PDGFB gene.

CONCLUSIONS

This study identified a novel COL1A1 breakpoint, namely, exon 18 of the COL1A1 gene.

Posttranscriptional control of the expression and function of diacylglycerol acyltransferase-1 in mouse adipocytes

Acyl-CoA:diacylglycerol acyltransferase-1 (DGAT1) catalyzes the final step of triglyceride synthesis in mammalian cells. Data obtained from DGAT1-knockout mice have indicated that this enzyme plays an important role in energy homeostasis. We investigated the regulation of the expression and function of DGAT1 in mouse 3T3-L1 cell as a model for mammalian adipocytes. We demonstrated that the DGAT1 protein level increased by approximately 90-fold following differentiation of 3T3-L1 into mature adipocytes, a change that was accompanied by approximately 7-fold increase in DGAT1 mRNA. On the other hand, forced overexpression of DGAT1 mRNA by >20-fold via a recombinant adenovirus only resulted in approximately 2-fold increase in DGAT1 protein in mature adipocytes and little increase in preadipocytes. These results indicated that gene expression of DGAT1 in adipocytes is subjected to rigorous posttranscriptional regulation, which is modulated significantly by the differentiation status of 3T3-L1 cells. Protein stability is not a significant factor in the control of DGAT1 expression. The steady-state levels of DGAT1 were unaffected by blockage of proteolytic pathways by ALLN. However, translational control was suggested by sequence analysis of the 5'-untranslated region of human DGAT1 (hDGAT1) mRNA. We found that the level of DGAT1 activity was predominantly a function of the steady-state level of DGAT1 protein. No significant functional changes were observed when the conserved tyrosine phosphorylation site in hDGAT1 was mutated by a single base pair substitution. Despite only a approximately 2-fold increase in DGAT1 protein caused by recombinant viral transduction, a proportionate increase in cellular triglyceride synthesis resulted without affecting the triglyceride lipolysis rate, leading to >2-fold increase in intracellular triglyceride accumulation. No change in adipocyte morphology or in the expression levels of lipoprotein lipase, proxisomal proliferation-activating receptor-gamma, and aP2 was evident secondary to DGAT1 overexpression at different stages in 3T3-L1 differentiation. These data suggest that dysregulation of DGAT1 may play a role in the development of obesity, and manipulation of the steady-state level of DGAT1 protein may offer a potential means to treat or prevent obesity.

Pushing the limits of the scanning mechanism for initiation of translation

Selection of the translational initiation site in most eukaryotic mRNAs appears to occur via a scanning mechanism which predicts that proximity to the 5' end plays a dominant role in identifying the start codon. This "position effect" is seen in cases where a mutation creates an AUG codon upstream from the normal start site and translation shifts to the upstream site. The position effect is evident also in cases where a silent internal AUG codon is activated upon being relocated closer to the 5' end. Two mechanisms for escaping the first-AUG rule--reinitiation and context-dependent leaky scanning--enable downstream AUG codons to be accessed in some mRNAs. Although these mechanisms are not new, many new examples of their use have emerged. Via these escape pathways, the scanning mechanism operates even in extreme cases, such as a plant virus mRNA in which translation initiates from three start sites over a distance of 900 nt. This depends on careful structural arrangements, however, which are rarely present in cellular mRNAs. Understanding the rules for initiation of translation enables understanding of human diseases in which the expression of a critical gene is reduced by mutations that add upstream AUG codons or change the context around the AUG(START) codon. The opposite problem occurs in the case of hereditary thrombocythemia: translational efficiency is increased by mutations that remove or restructure a small upstream open reading frame in thrombopoietin mRNA, and the resulting overproduction of the cytokine causes the disease. This and other examples support the idea that 5' leader sequences are sometimes structured deliberately in a way that constrains scanning in order to prevent harmful overproduction of potent regulatory proteins. The accumulated evidence reveals how the scanning mechanism dictates the pattern of transcription--forcing production of monocistronic mRNAs--and the pattern of translation of eukaryotic cellular and viral genes.

Interaction of hnRNP-C1/C2 proteins with RNA: analysis using the yeast three-hybrid system

Three-hybrid assays for the analysis of RNA-protein interactions in vivo are usually used, due to technical limitations, only for RNA baits that do not contain runs of four or more consecutive uridines. The present study provides the first example of a three-hybrid analysis of synthetic and natural uridine-rich RNA sequences. The use of the three-hybrid assay enabled us to demonstrate a functional difference between two closely related proteins, heterogeneous nuclear ribonucleoprotein C1 (hnRNP-C1) and hnRNP-C2. The hnRNP-C2 protein, an alternatively spliced variant of hnRNP-C1, contains an additional 13 amino acids between an RNA binding domain (RBD) and a basic leucine zipper-like motif (bZLM), also implied in RNA binding. This study shows that (i) for efficient binding of hnRNP-C1/C2 to RNA, the context of the U-stretch is more important than the stretch itself; (ii) both the RBD and the bZLM bind RNA independently; and (iii) the C2-related 13-amino acid insert enhances the specificity of either the RBD, the bZLM, or the full-length protein towards its ligand, allowing it to bind only the most high-affinity sequences while discriminating against those that do not perfectly match this category. The three-hybrid system is a powerful tool to work out the functional significance of peptide 'modules' within RNA binding proteins generated by alternative splicing.

Structural and functional analysis of a chimeric protein COL1A1-PDGFB generated by the translocation t(17;22)(q22;q13.1) in Dermatofibrosarcoma protuberans (DP)

Dermatofibrosarcoma protuberans (DP), an infiltrative skin tumour of intermediate malignancy, presents specific cytogenetic features such as reciprocal translocations t(17;22)(q22;q13.1) or supernumerary ring chromosomes derived from t(17;22). We have previously shown that both rings and translocated chromosomes derived from t(17;22) presented the same molecular rearrangement with fusion of the COL1A1 gene on chromosome 17 to the PDGFB gene on chromosome 22. To study the structure and function of the COL1A1-PDGFB chimeric protein, we used a tumour-derived chimeric COL1A1-PDGFB cDNA to perform stable and transient transfections in the Chinese hamster lung fibroblastic cell line PS200 and the human epithelial cell line HEK293. We demonstrated that the stably transfected clones that expressed the COL1A1-PDGFB chimeric protein became growth factors independent and tumorigenic in nude mice. In addition, COL1A1-PDGFB transfected cell supernatants significantly stimulated fibroblastic cell growth, through the activation of the PDGFB receptor pathway. By using anti-PDGFB and specific anti-COL1A1-PDGFB antibodies, we showed that, similar to PDGFB, the COL1A1-PDGFB chimeric proteins are processed in transfected cells into mature PDGFB dimers. These results strongly suggest that the COL1A1-PDGFB chimeric gene expression associated with DP, induces tumours formation through production of mature PDGFB, in an autocrine or paracrine way. Strikingly, mutagenesis experiments indicated that uncleaved COL1A1-PDGFB forms are mitogenic and therefore could contribute, as well as mature PDGFB, to the establishment of a transformed phenotype.

Translational pathophysiology: a novel molecular mechanism of human disease

In higher eukaryotes, the expression of about 1 gene in 10 is strongly regulated at the level of messenger RNA (mRNA) translation into protein. Negative regulatory effects are often mediated by the 5′-untranslated region (5′-UTR) and rely on the fact that the 40S ribosomal subunit first binds to the cap structure at the 5′-end of mRNA and then scans for the first AUG codon. Self-complementary sequences can form stable stem-loop structures that interfere with the assembly of the preinitiation complex and/or ribosomal scanning. These stem loops can be further stabilized by the interaction with RNA-binding proteins, as in the case of ferritin. The presence of AUG codons located upstream of the physiological start site can inhibit translation by causing premature initiation and thereby preventing the ribosome from reaching the physiological start codon, as in the case of thrombopoietin (TPO). Recently, mutations that cause disease through increased or decreased efficiency of mRNA translation have been discovered, defining translational pathophysiology as a novel mechanism of human disease. Hereditary hyperferritinemia/cataract syndrome arises from various point mutations or deletions within a protein-binding sequence in the 5′-UTR of the L-ferritin mRNA. Each unique mutation confers a characteristic degree of hyperferritinemia and severity of cataract in affected individuals. Hereditary thrombocythemia (sometimes called familial essential thrombocythemia or familial thrombocytosis) can be caused by mutations in upstream AUG codons in the 5′-UTR of the TPO mRNA that normally function as translational repressors. Their inactivation leads to excessive production of TPO and elevated platelet counts. Finally, predisposition to melanoma may originate from mutations that create translational repressors in the 5′-UTR of the cyclin-dependent kinase inhibitor–2A gene.

Translational pathophysiology: a novel molecular mechanism of human disease

In higher eukaryotes, the expression of about 1 gene in 10 is strongly regulated at the level of messenger RNA (mRNA) translation into protein. Negative regulatory effects are often mediated by the 5′-untranslated region (5′-UTR) and rely on the fact that the 40S ribosomal subunit first binds to the cap structure at the 5′-end of mRNA and then scans for the first AUG codon. Self-complementary sequences can form stable stem-loop structures that interfere with the assembly of the preinitiation complex and/or ribosomal scanning. These stem loops can be further stabilized by the interaction with RNA-binding proteins, as in the case of ferritin. The presence of AUG codons located upstream of the physiological start site can inhibit translation by causing premature initiation and thereby preventing the ribosome from reaching the physiological start codon, as in the case of thrombopoietin (TPO). Recently, mutations that cause disease through increased or decreased efficiency of mRNA translation have been discovered, defining translational pathophysiology as a novel mechanism of human disease. Hereditary hyperferritinemia/cataract syndrome arises from various point mutations or deletions within a protein-binding sequence in the 5′-UTR of the L-ferritin mRNA. Each unique mutation confers a characteristic degree of hyperferritinemia and severity of cataract in affected individuals. Hereditary thrombocythemia (sometimes called familial essential thrombocythemia or familial thrombocytosis) can be caused by mutations in upstream AUG codons in the 5′-UTR of the TPO mRNA that normally function as translational repressors. Their inactivation leads to excessive production of TPO and elevated platelet counts. Finally, predisposition to melanoma may originate from mutations that create translational repressors in the 5′-UTR of the cyclin-dependent kinase inhibitor–2A gene.

Identification of seven novel nucleotide variants in the hepatocyte nuclear factor-1alpha (TCF1) promoter region in MODY patients

Maturity onset diabetes of the young (MODY) is a heterogeneous subtype of type II diabetes mellitus. To date, five MODY genes have been identified. Mutations in the hepatocyte nuclear factor-1alpha (HNF-1alpha) gene are associated with MODY3. In the present work, we implemented the HNF-1alpha promoter region in the screening of MODY-suspect patients and identified seven variants not detected in control subjects. The family was available for the -119delG variant, and segregration between MODY and the variant is observed. Most of these variants are located in highly conserved regions and may alter HNF-1alpha expression through binding alteration of nuclear factors or other mechanisms. We demonstrate by functional studies that the transcriptional activity of the -283A>C and -218T>C variant promoters were 30% and 70% of the wild type activity, respectively. These data suggest that HNF-1alpha promoter variants could be diabetogenic mutations, and emphasize that the accurate HNF-1alpha expression is important for the maintenance of normal pancreatic beta cell function.

PDGF-A expression correlates with blood pressure and remodeling in 1K1C hypertensive rat arteries

We previously demonstrated remodeling of large and small arteries in angiotensin II-treated rats, paralleled by an increased expression of platelet-derived growth factor (PDGF)-A chain mRNA in large arteries. Both remodeling and PDGF-A expression were associated with elevation of blood pressure rather than a direct effect of angiotensin II. To further delineate the role of PDGF-A and elevated blood pressure, we assessed the level of PDGF-A and -B mRNA and protein in the wall of large as well as small arteries in the one-kidney, one-clip (1K1C) hypertensive rat, a non-renin-dependent model of hypertension. Fourteen days after renal artery stenosis, the thoracic aorta and both femoral arteries were collected from 1K1C rats (n = 8) and uninephrectomized controls (n = 8) and immediately processed for morphological measurement, immunohistochemistry, RT-PCR, and Western blotting. Systolic blood pressure was significantly elevated in hypertensive rats (202 +/- 26 mmHg) compared with control rats (122 +/- 7.9 mmHg) and was accompanied by arterial hypertrophy in both aorta and femoral arteries. The mRNA for PDGF-A chain was increased threefold in the thoracic aorta (P < 0.05) of 1K1C rats, whereas the message for PDGF-B was not significantly changed in hypertensive versus control animals. A higher staining of the intima-media was observed by using an anti-PDGF-A chain polyclonal antibody on paraffin-embedded sections. Western blot results indicated an approximately 2-fold increase in PDGF-A protein in aortic and femoral wall of the 1K1C rats. The results showed that both the mRNA and protein for PDGF-A chain are increased and well correlated with the blood pressure and wall area, suggesting a direct effect of elevated pressure on PDGF synthesis, which, in turn, may affect the onset and progression of vascular hypertrophy.

Control of translation initiation in animals

Regulation of translation initiation is a central control point in animal cells. We review our current understanding of the mechanisms of regulation, drawing particularly on examples in which the biological consequences of the regulation are clear. Specific mRNAs can be controlled via sequences in their 5' and 3' untranslated regions (UTRs) and by alterations in the translation machinery. The 5'UTR sequence can determine which initiation pathway is used to bring the ribosome to the initiation codon, how efficiently initiation occurs, and which initiation site is selected. 5'UTR-mediated control can also be accomplished via sequence-specific mRNA-binding proteins. Sequences in the 3' untranslated region and the poly(A) tail can have dramatic effects on initiation frequency, with particularly profound effects in oogenesis and early development. The mechanism by which 3'UTRs and poly(A) regulate initiation may involve contacts between proteins bound to these regions and the basal translation apparatus. mRNA localization signals in the 3'UTR can also dramatically influence translational activation and repression. Modulations of the initiation machinery, including phosphorylation of initiation factors and their regulated association with other proteins, can regulate both specific mRNAs and overall translation rates and thereby affect cell growth and phenotype.

The polyadenylation inhibitor cordycepin (3'dA) causes a decline in c-MYC mRNA levels without affecting c-MYC protein levels

Study of the distribution of the poly(A) tail length of c-myc mRNA in several cell lines revealed a distinct, prevailing population with short poly(A) tails, derived through sequential deadenylation. To elucidate the possible in vivo function of this distinct short tailed c-myc mRNA population, the polyadenylation inhibitor cordycepin was used. This resulted in a decline in steady state c-myc mRNA levels with the remaining messenger mostly oligoadenylated. However, c-MYC proteins did not follow the reduction of the c-myc mRNA. On the other hand, in cells exposed to physiological agents known to downregulate c-myc expression, the reduction of mRNA steady state levels, was reflected upon c-MYC protein levels. The dissociation between c-myc mRNA and protein levels caused by cordycepin was not due to the stabilization of the c-MYC proteins and was not an indiscriminate effect since in the presence of cordycepin, c-fos mRNA and protein levels concomitantly declined. Our data indicate that under these conditions, a long poly(A) tail is not instrumental for c-myc mRNA translation and furthermore, the discrepancy in the steady state of c-myc mRNA level: c-MYC protein ratio between control cells and cells treated with cordycepin indicates that c-myc mRNA is subjected to translational control.

Transfection of small numbers of human endothelial cells by electroporation and synthetic amphiphiles

OBJECTIVES

This study compared the efficiency of electroporation and synthetic amphiphiles. (SAINT-2pp/DOPE) in transfecting small numbers of human endothelial cells.

METHODS AND RESULTS

Optimal transfection conditions were tested and appeared to be 400 V and 960 microF for electroporation and a 10:1 ratio for concentrations of SAINT-2pp/DOPE: plasmid. Using these conditions, cell concentrations were lowered step-wise and we were able to transfect as few as one thousand cells with both methods. For detection of transfection of a small number of cells a sensitive assay was needed (Luciferase). A plasmid containing the neomycin resistance gene was used to determine the transfection rate expressed in colony forming units by counting colonies after selection. At low plasmid concentrations this transfection rate was within the same range for both electroporation and SAINT-2pp/DOPE transfection. Fluorescent in situ hybridisation of metaphase chromosomes of transfected endothelial cells using the plasmid as a probe showed that stable integration was possible with both methods.

CONCLUSIONS

Electroporation and a synthetic amphiphile, SAINT-2pp, provide the possibility of transfecting small numbers of cells resulting in stable integration of low plasmid concentrations. The availability of this technology is important in order to obtain functional endothelial cell lines from various human blood vessels for research purposes.

Translational control of growth factor and proto-oncogene expression

Control of translation is now understood to be one of the major regulatory events in eukaryotic gene expression. Moreover there is evidence which suggests that aberrant expression of growth-related genes by translational mechanisms makes a significant contribution to cell transformation. However, the mechanisms which regulate translation of specific growth-related mRNAs have yet to be fully elucidated. The majority of these mRNAs have long 5' untranslated regions (UTRs) and three features which are important in translational control have been identified, namely (i) structured regions which inhibit the scanning mechanisms of translation, (ii) regulatory upstream open reading frames and (iii) internal ribosome entry segments which are capable of initiating cap-independent translation. In this review the translational regulation of specific mRNAs encoding growth factors and proto-oncogenes by these three mechanisms will be discussed, together with examples of altered translational regulation in neoplasia.

eIF4E expression in tumors: its possible role in progression of malignancies

A central issue in the study of neoplastic transformation is to understand how proto-oncogene products deregulate normal processes of cell growth and differentiation: an intrinsic aspect of this is to probe the sequence of events leading to altered expression of proto-oncogenes. In the past few years, studies aimed at understanding the regulation and function of protein synthesis initiation factors, eIF4E initially, culminated in the unexpected finding that a moderate overexpression of this factor results in dramatic phenotypic changes, including rapid proliferation and malignant transformation. Conversely, the tumorigenic properties of cancer cells can be strongly inhibited by antisense-RNA against eIF4E, or overexpression of the inhibitory proteins: 4E-BPs. Furthermore, eIF4E is elevated in carcinomas of the breast, head and neck (HNSCC) and prostate, but not in typical benign lesions. This is a strong indication that elevated eIF4E expression may mark a critical transition in cancer progression. Establishing a greater protein synthesis output may be a necessary step for cancer cells in order to sustain their rapid proliferation. However, analysis of cells transformed by eIF4E revealed that the synthesis of only a few proteins was greatly enhanced, while synthesis of most was minimally increased. One possible explanation is that eIF4E causes these effects by specifically increasing the translational efficiency of several oncogene transcripts, leading to overexpression of their products. The feasibility of this hypothesis was confirmed experimentally with the identification of several important products that are specifically upregulated in eIF4E-overexpressing cells. These include: c-Myc, cyclin DI and ODC, which control cycle progression and tumorigenesis; basic fibroblast growth factor (FGF-2) and vascular endothelial growth factor (VEGF), which are powerful promoters of cell growth and angiogenesis. A deeper understanding of the mRNAs that are strongly dependent on excess eIF4E/F for efficient translation will eventually result in fuller understanding of the fundamental role of translational control in different pathophysiological conditions, including malignancy.

Eukaryotic mRNAs encoding abundant and scarce proteins are statistically dissimilar in many structural features

It is well known that non-coding mRNA sequences are dissimilar in many structural features. For individual mRNAs correlations were found for some of these features and their translational efficiency. However, no systematic statistical analysis was undertaken to relate protein abundance and structural characteristics of mRNA encoding the given protein. We have demonstrated that structural and contextual features of eukaryotic mRNAs encoding high- and low-abundant proteins differ in the 5' untranslated regions (UTR). Statistically, 5' UTRs of low-expression mRNAs are longer, their guanine plus cytosine content is higher, they have a less optimal context of the translation initiation codons of the main open reading frames and contain more frequently upstream AUG than 5' UTRs of high-expression mRNAs. Apart from the differences in 5' UTRs, high-expression mRNAs contain stronger termination signals. Structural features of low- and high-expression mRNAs are likely to contribute to the yield of their protein products.

Expression of platelet-derived growth factor B-chain and platelet-derived growth factor beta-receptor in fibroblasts of scleroderma

Scleroderma is a fibrotic disease occurring in a localized or systemic form. The pathogenesis of scleroderma remains poorly understood. Recent studies revealed that various cytokines and growth factors were involved in the development of scleroderma fibrosis. Platelet-derived growth factor (PDGF) is a potent growth factor for mesenchymal cells, especially fibroblasts. It can promote fibroblasts proliferation, enhance extracellular matrix synthesis. It is also a chemoattractant to fibroblasts. To better understand the role of PDGF in pathogenesis of scleroderma, we performed both in vivo studies on the expression of PDGF beta-receptor protein in scleroderma tissue and in vitro studies on the expression of PDGF B-chain and PDGF beta-receptor mRNA in cultured fibroblasts derived from both lesions of scleroderma and normal skin. Immunohistochemistry staining showed that PDGF beta-receptor expression was greatly elevated in the dermis of scleroderma lesion whereas PDGF beta-receptor were expressed at low levels in normal skin. Northern blot analysis showed that cultured fibroblasts from scleroderma had higher expression of PDGF B-chain and PDGF beta-receptor mRNA than those from normal control. Two PDGF B-chain mRNA transcripts, 2.8 and 4.0 kb, were expressed. The 2.8 kb transcripts which had more efficient translation ability was the more predominantly expressed one. These results indicate that PDGF B-chain/PDGF beta-receptor signal pathway might be involved in the development of fibrosis in scleroderma, and that the 2.8 kb PDGF B-chain mRNA transcript may be the main modulation gene.

Human Integrin β3 Gene Expression: Evidence for a Megakaryocytic Cell-Specific cis-Acting Element

The human integrin β3 participates in a wide range of adhesive biologic functions and is expressed in a selected subset of tissues, but little is known about the cis-acting DNA elements or trans-acting factors responsible for this regulation. Using cell lines characterized for β3 expression, a number of upstream regulatory regions in the β3 gene were identified. (1) The three regions from −1159 to −584, −290 to −146, and −126 to −115 demonstrated positive, negative, and negative activity, respectively. (2) The region from −115 to +29 of the β3 gene was sufficient for cell-specific activity. Deletion of the sequence from −115 to −89 produced a 6- to 40-fold reduction in reporter gene activity in β3-expressing megakaryocytic cell lines (K562, Dami, and HEL), but only a 1.7- and 2.7-fold reduction, respectively, in β3-expressing endothelial and melanoma cell lines, and 1.3- and 2.8-fold reduction, respectively, in non–β3-expressing Chinese hamster ovary and 293 cell lines. This sequence also bound nuclear proteins in a cell-specific manner in electrophoretic mobility shift assays. Mutational analysis indicated that the sequence GAGGGG (positions −113 to −108) is a megakaryocytic cell line-specificcis-acting element. (3) The region from −89 to +29 promoted lower activity in all cell lines. We also provide evidence that a CCCACCC sequence at position −70 has transcriptional activity, most likely through the Sp1 transcription factor. These data supply the first detailed map of the transcriptional regulatory elements of the 5′ region of the β3 gene, define positive regulatory sequences with potent megakaryocyte preferential activity, and indicate that the ubiquitous transcription factor, Sp1, may augment β3 gene expression.

© 1998 by The American Society of Hematology.

Human Integrin β3 Gene Expression: Evidence for a Megakaryocytic Cell-Specific cis-Acting Element

The human integrin β3 participates in a wide range of adhesive biologic functions and is expressed in a selected subset of tissues, but little is known about the cis-acting DNA elements or trans-acting factors responsible for this regulation. Using cell lines characterized for β3 expression, a number of upstream regulatory regions in the β3 gene were identified. (1) The three regions from −1159 to −584, −290 to −146, and −126 to −115 demonstrated positive, negative, and negative activity, respectively. (2) The region from −115 to +29 of the β3 gene was sufficient for cell-specific activity. Deletion of the sequence from −115 to −89 produced a 6- to 40-fold reduction in reporter gene activity in β3-expressing megakaryocytic cell lines (K562, Dami, and HEL), but only a 1.7- and 2.7-fold reduction, respectively, in β3-expressing endothelial and melanoma cell lines, and 1.3- and 2.8-fold reduction, respectively, in non–β3-expressing Chinese hamster ovary and 293 cell lines. This sequence also bound nuclear proteins in a cell-specific manner in electrophoretic mobility shift assays. Mutational analysis indicated that the sequence GAGGGG (positions −113 to −108) is a megakaryocytic cell line-specificcis-acting element. (3) The region from −89 to +29 promoted lower activity in all cell lines. We also provide evidence that a CCCACCC sequence at position −70 has transcriptional activity, most likely through the Sp1 transcription factor. These data supply the first detailed map of the transcriptional regulatory elements of the 5′ region of the β3 gene, define positive regulatory sequences with potent megakaryocyte preferential activity, and indicate that the ubiquitous transcription factor, Sp1, may augment β3 gene expression.

© 1998 by The American Society of Hematology.

Regulation of a promoter of the fibroblast growth factor 1 gene in prostate and breast cancer cells

FGF-1 mRNA is expressed in the prostate cancer cell lines LNCaP and PC-3 and in the breast carcinoma cell line MDA-MB-231. Levels of FGF-1 mRNA have been shown to be up-regulated by serum, phorbol esters, and combinations of growth factors. It was shown that the major FGF-1 mRNA species expressed following serum stimulation in MDA-MB-231 cells is FGF-1.C. To better understand the potential role of FGF-1 in human prostate and breast cancer, we began an analysis of the cis- and trans-acting elements of one of its promoters required for the serum, PMA, and androgen regulation in breast and prostate cancer cell lines. We show that FGF-1.C steady-state mRNA levels are increased following serum or PMA stimulation of PC-3 cells. Further, we determine the FGF-1.C transcription start site in PC-3 cells. By sequence analysis, we show that consensus AP1, AP2, and Sp1 sites and ARE- and CRE-near consensus elements are present in the immediate 5' region of the FGF-1.C transcription start site. Gel-shift assays show that oligonucleotides containing FGF-1.C AP1, AP2, or Spl sequences form specific DNA-protein complexes with nuclear extracts from PC-3 cells. To determine if these or other cis-acting sequences are responsible for the serum, androgen, or growth factor regulation of FGF-1 expression, fragments of the FGF-1.C promoter region were cloned upstream of the luciferase reporter gene. We show that FGF-1 synergizes with androgen to enhance FGF-1.C transcription in LNCaP cells. We further show that the DNA fragment containing sequence up to 1614 nucleotides upstream of the FGF-1.C transcription start site is sufficient for stimulating promoter activity following serum treatment of MDA-MB-231 cells. Thus, FGF-1.C promoter contains sequences that are important for androgen or serum stimulation in prostate and breast cancer cells.

Production and oestrogen regulation of FGF1 in normal and cancer breast cells

To investigate the relationship between the FGF1 oestrogen regulation and the normal/cancer status of breast cells, we have studied FGF1 17beta-oestradiol regulation in normal, transformed and cancerous cells. Normal (NMEC), transformed (HBL-100) and cancerous (MCF-7, MDA-MB-231) human mammary epithelial cells express different levels of FGF1 mRNAs. Western blot analysis allowed us to characterize FGF1 as an 18 kDa form of this polypeptide. Using a neutralizing anti-FGF1 antibody we demonstrated that FGF1 is secreted by all mammary cells studied and stimulates their proliferation in an autocrine manner. We have examined the regulation of FGF1 mRNAs in response to 17beta-oestradiol. FGF1 mRNAs were upregulated in hormone-dependent NMEC but was not upregulated either in hormone-sensitive HBL-100 cells or in the hormone-dependent cancerous cell line MCF-7. However, enzyme linked immunosorbent assay indicated an increase of FGF1 protein in NMEC, HBL-100 and MCF-7 cells. We have also examined the 17beta-oestradiol regulation of the four alternatively spliced FGF1 mRNAs: 1.A, 1.B, 1. C and 1.D. Only 1.B transcripts were downregulated by 17beta-oestradiol in normal cells. These results show that 17beta-oestradiol regulates FGF1 mRNAs in a cell-specific manner, and that this regulation may be transcriptional or translational depending on cell phenotype. The specificity of oestradiol effects was checked using its receptor antagonist tamoxifen.

Translational control elements in the major human transforming growth factor-beta 1 mRNA

Polysome analysis indicates that the major 2.4 kb transforming growth factor-beta 1 (TGF-beta 1) transcript is poorly translated, both in cultured cells, and in vivo in mouse liver. In contrast, the TGF-beta 2 transcripts are efficiently translated. The contribution of the 5'- and 3'-untranslated regions (UTRs) to the translational inhibition of the full-length TGF-beta 1 transcript was studied by deletion analysis. Despite their high G + C content, both UTRs stimulated translation in vitro. However, polysome analysis of synthetic TGF-beta 1 mRNAs transfected into MCF-7 cells suggests that the cell contains a limited pool of trans-acting factors that interact with the 5'UTR to make it inhibitory in vivo. Further deletion analysis in vitro revealed multiple stimulatory and inhibitory regions in the 5'UTR. This has important implications for the translatability of the naturally occurring shorter TGF-beta 1 transcripts and provides a framework for higher resolution mapping studies. Overall, the poor translational efficiency of the major TGF-beta 1 mRNA in vivo appears to be due to a combination of poor initiation sequence context, and inhibitory interactions of limiting transacting factors with cis-inhibitory elements embedded in an otherwise stimulatory 5'UTR.

Effect of phenotype on the transcription of the genes for platelet-derived growth factor (PDGF) isoforms in human smooth muscle cells, monocyte-derived macrophages, and endothelial cells in vitro

Proliferation of arterial smooth muscle cells (ASMCs) contributes considerably to enlargement of the arterial wall during atherosclerosis. The platelet-derived growth factor (PDGF) is a well-known mitogen and chemoattractant for ASMCs. Quantitative reverse transcription-polymerase chain reaction showed that cells appearing in atherosclerotic lesions, such as ASMCs, endothelial cells, and monocytes/macrophages, expressed mRNAs for both PDGF A and B chains in vitro, with the highest expression in endothelial cells. On proliferation, ASMCs and endothelial cells upregulated PDGF A mRNA. Differentiation of macrophages increased the amount of both mRNAs. Thus, the regulation of PDGF A- and B-chain expression depends on cell types and phenotypic states of the cells, which have also been found in vivo in human atherosclerotic lesions. PDGF A can be produced as short and long isoforms. The latter binds with high affinity to glycosaminoglycans. Irrespective of phenotype, only the minor part of total PDGF A mRNA consisted of the long variant in ASMCs, while endothelial cells produced 40% of total PDGF A as the long form. The differentiation of macrophages increased the production of the long PDGF A mRNA from 10% to 40%. Thus, increasing numbers of stimulated cells in the atherosclerotic lesion may increase the transcription of PDGF isoforms, and particularly of the long PDGF A isoform. Together with increasing amounts of ASMC-derived proteoglycans in developing lesions, this may contribute to accumulation of PDGF in the arterial wall matrix, resulting in prolonged stimulation of ASMCs.

PDGF2/c-sis mRNA leader contains a differentiation-linked internal ribosomal entry site (D-IRES)

It has become clear that a given cell type can qualitatively and quantitatively affect the expression of the platelet-derived growth factor B (PDGF2/c-sis) gene at multiple levels. In a previous report, we showed that PDGF2/c-sis 5'-untranslated region has a translational modulating activity during megakaryocytic differentiation of K562 cells. This study points to the mechanism used for this translational modulation. The unusual mRNA leader, which imposes a major barrier to conventional ribosomal scanning, was found to contain an internal ribosomal entry site that becomes more potent in differentiating cells and was termed differentiation-linked internal ribosomal entry site (D-IRES). The D-IRES element defines a functional role for the cumbersome 1022-nucleotide-long mRNA leader and accounts for its uncommon, evolutionary conserved architecture. The differentiation-linked enhancement of internal translation, which provides an additional step to the fine tuning of PDGF2/c-sis gene expression, might be employed by numerous critical regulatory genes with unusual mRNA leaders and might have widespread implications for cellular growth and development.

Isolation of a novel human cDNA (rhoHP1) homologous to rho genes

A novel full-length cDNA showing homology with rho genes was isolated from a human placenta cDNA library. Sequencing of a total of 1086 nucleotides of this clone revealed an open reading frame of 630 nucleotides (210 amino acids). In view of its degree of homology to members of the Rho family of molecules (50-54% identical amino acids, 60-63% identical nucleotides within the coding region), the predicted product was designated RhoHP1(Rho-related protein HP1). Northern analysis indicated that a message about 1.2-kb long is expressed in human heart, placenta, liver, skeletal muscle, and pancreas and, with weaker intensity, in several other tissues.

Deregulation of the platelet-derived growth factor B-chain gene via fusion with collagen gene COL1A1 in dermatofibrosarcoma protuberans and giant-cell fibroblastoma

Dermatofibrosarcoma protuberans (DP), an infiltrative skin tumour of intermediate malignancy, presents specific features such as reciprocal translocations t(17;22)(q22;q13) and supernumerary ring chromosomes derived from the t(17;22). In this report, the breakpoints from translocations and rings in DP and its juvenile form, giant cell fibroblastoma (GCF), were characterised on the genomic and RNA level. These rearrangements fuse the platelet-derived growth factor B-chain (PDGFB, c-sis proto-oncogene) and the collagen type I alpha 1 (COL1A1) genes. PDGFB has transforming activity and is a potent mitogen for a number of cell types, but its role in oncogenic processes is not fully understood. COL1A1 is a major constituent of the connective tissue matrix. Neither PDGFB nor COL1A1 have so far been implicated in any tumour translocations. These gene fusions delete exon 1 of PDGFB, and release this growth factor from its normal regulation.

Distribution and functions of platelet-derived growth factors and their receptors during embryogenesis

Platelet-derived growth factors (PDGFs) are soluble proteins that mediate intercellular signaling via receptor tyrosine kinases. The patterns of PDGF and PDGF receptor expression during embryogenesis are complex and dynamic and suggest that signaling can be autocrine or paracrine, depending on the particular tissue and the stage of development. Mesenchymal cells throughout the embryo and within some developing organs produce PDGF receptors, whereas their ligands are often produced by adjacent epithelial or endothelial cells. Disruption of PDGF signaling in the embryo leads to morphogenetic defects and embryonic or perinatal lethality. Tissues that are particularly susceptible to the absence of PDGF signaling are migrating mesoderm cells during gastrulation, nonneuronal neural crest cell derivatives, and kidney mesangial cells. These tissues share the common feature of undergoing epithelial-mesenchymal transitions. We review current knowledge of the distribution of PDGF ligands and receptors and discuss how this distribution may relate to several roles for PDGF during embryogenesis, particularly the regulation of mesenchymal cell behavior.

Cloning of multiple chicken FGF1 mRNAs and their differential expression during development of whole embryo and of the lens

Five different 5' untranslated regions (5' UTRs) of FGF1 mRNAs were cloned in chicken. The structure of these transcripts suggests that, as in mammals, distinct 5' untranslated exons are spliced to the first coding exon via alternative splicing and alternative promoter usage. In an attempt to correlate the expression of specific transcripts to distinct biological activities, the distribution of these transcripts in different tissues and during the development of both the whole embryo and the lens was studied. In tissues, we have shown a differential, but not exclusive, expression of these transcripts. In the whole embryo, the expression of one transcript correlates with later developmental processes. In the lens, only two transcripts were detected that are both differently expressed and distributed. These results suggest that the biological properties of FGF1 depend on the expression of specific FGF1 mRNAs. Because these transcripts only differ in their 5' UTRs, they could be involved in distinct translational controls.

Analysis of the tumor vasculature and metastatic behavior of xenografts of human melanoma cell lines transfected with vascular permeability factor

Vascular permeability factor (VPF) is an important mediator of vascular development in tumors. Some human melanoma cell lines have a low VPF expression level in culture, but this level is upregulated when growing as a tumor in nude mice. Other melanoma lines have a constitutively high VPF expression. To compare the biological behavior of tumors with these two expression patterns, a human melanoma cell line with an inducible VPF expression was transfected with VPF expression constructs. In this way, several lines were obtained that constitutively produce either the soluble VPF121 or the matrix-associated VPF189 variant at levels of 4 to 30 times the VPF level in mature tumors derived from the parental line. The recombinant VPF RNA, which lacks most of the 5' noncoding sequences present in the endogenous VPF mRNA, was much more efficiently translated than the endogenous messenger. Upon injection in nude mice, all VPF-transfected lines developed tumors with aberrations in vascularization and in distribution of matrix components. In these tumors the blood vessels were hyperpermeable for an i.v. injected protein tracer. Transfection did not influence the in vitro growth rate of the cell lines, but the tumors from the VPF-transfected lines had higher growth rates in vivo than tumors from the parental line or the vector-transfected line. Although the incidence of lung metastasis was similar in all lines, the number of metastases per affected lung was significantly increased in mice carrying VPF-transfectant tumors. We conclude that the pattern and the level of VPF expression in a tumor are important determinants of the architecture and functionality of the vascular bed, but that overexpression of VPF does not necessarily lead to an increase of microvascular density or metastatic spread. The role of VPF in melanoma progression is obviously complex and may be difficult to derive in its generality from a single experimental model.

Interleukin 6 induces the expression of vascular endothelial growth factor

Angiogenesis, the formation of new blood vessels, is induced by various growth factors and cytokines that act either directly or indirectly. Vascular endothelial growth factor (VEGF) is a specific mitogen for vascular endothelial cells and therefore has a central role in physiological events of angiogenesis. Interleukin-6 (IL-6) expression on the other hand is elevated in tissues that undergo active angiogenesis but does not induce proliferation of endothelial cells. We demonstrate using Northern analysis that treatment of various cell lines with IL-6 for 6-48 h results in a significant induction of VEGF mRNA. The level of induction is comparable to the documented induction of VEGF mRNA by hypoxia or cobalt chloride, an activator of hypoxia-induced genes. In addition, it is demonstrated by transient transfection assays that the effect of IL-6 is mediated not only by DNA elements at the promoter region but also through specific motif(s) located in the 5'-untranslated region (5'-UTR) of VEGF mRNA. Our results imply that IL-6 may induce angiogenesis indirectly by inducing VEGF expression. It is also shown that the 5'-UTR is important for the expression of VEGF. The 5'-UTR of VEGF is exceptionally long (1038 base pairs) and very rich in G + C. This suggests that secondary structures in the 5'-UTR might be essential for VEGF expression through transcriptional and post-transcriptional control mechanisms.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

The human insulin-like growth factor II leader 1 contains an internal ribosomal entry site

Insulin-like growth factor II is a small peptide growth hormone, encoded by four mRNAs with unique 5' untranslated regions and identical coding regions. The 5' untranslated region transcribed from promoter 1 is 598 nt (leader 1). The properties of this leader 1 suggest a strong regulation of translation; the high G + C-content, the presence of an upstream open reading frame, and the length of the 5' UTR are 3 elements which prohibit efficient translation and which may modulate expression. In this paper we show that the human IGFII leader 1 harbours sequence elements that allow translation initiation to occur by internal initiation on the IGF sequence. This mode of initiation was described first for picornaviral mRNAs, that are naturally uncapped. The IGFII leader 1-dependent expression in HeLa cells was resistant to infection with poliovirus; abrogation of cap-dependent initiation by poliovirus had apparently no effect on IGFII expression. Moreover, a downstream CAT-cistron in a bicistronic construct was translated upon insertion of the leader 1 sequence. The translational properties of the IGFII leader 1 suggest that internal initiation on this leader may be modulated during proliferation or differentiation, enabling cell-stage dependent expression of IGFII.